This AOP is licensed under the BY-SA license. This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.

AOP: 294

Title

A descriptive phrase which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the AO. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Increased reactive oxygen and nitrogen species (RONS) leading to increased risk of breast cancer

Short name
A name that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
RONS leading to breast cancer

Graphical Representation

A graphical representation of the AOP.This graphic should list all KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs. More help
Click to download graphical representation template Explore AOP in a Third Party Tool

Authors

The names and affiliations of the individual(s)/organisation(s) that created/developed the AOP. More help

Jessica S. Helm* and Ruthann A. Rudel*

*Silent Spring Institute, Newton, MA 02460

Point of Contact

The user responsible for managing the AOP entry in the AOP-KB and controlling write access to the page by defining the contributors as described in the next section.   More help
Evgeniia Kazymova   (email point of contact)

Contributors

Users with write access to the AOP page.  Entries in this field are controlled by the Point of Contact. More help
  • Jessica Helm
  • Evgeniia Kazymova

Coaches

This field is used to identify coaches who supported the development of the AOP. Each coach selected must be a registered author. More help

Status

Provides users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. OECD Status - Tracks the level of review/endorsement the AOP has been subjected to. OECD Project Number - Project number is designated and updated by the OECD. SAAOP Status - Status managed and updated by SAAOP curators. More help
Handbook Version OECD status OECD project
v2.0 Under Development 1.80
This AOP was last modified on April 29, 2023 13:02

Revision dates for related pages

Page Revision Date/Time
Increase in reactive oxygen and nitrogen species (RONS) May 08, 2019 12:30
Increase, DNA damage May 08, 2019 12:28
Increase, Mutations January 10, 2023 19:00
N/A, Breast Cancer December 20, 2022 09:12
Increase, Cell Proliferation (Epithelial Cells) May 08, 2019 12:41
Tissue resident cell activation March 22, 2023 16:03
Increased Pro-inflammatory mediators March 21, 2023 15:50
Leukocyte recruitment/activation December 01, 2017 09:33
Increased, Ductal Hyperplasia September 16, 2017 10:17
Increase in RONS leads to Increase, DNA Damage May 08, 2019 14:43
Increase, DNA Damage leads to Increase, Mutations May 08, 2019 14:41
Increase, Mutations leads to Increase, Cell Proliferation (Epithelial Cells) May 08, 2019 14:54
Increase, Cell Proliferation (Epithelial Cells) leads to Increase, Mutations May 08, 2019 15:02
Increase, Cell Proliferation (Epithelial Cells) leads to Increased, Ductal Hyperplasia December 03, 2016 16:38
Increased, Ductal Hyperplasia leads to N/A, Breast Cancer May 08, 2019 15:09
Increase in RONS leads to Tissue resident cell activation May 09, 2019 15:47
Increase, DNA Damage leads to Tissue resident cell activation May 10, 2019 17:15
Tissue resident cell activation leads to Increased pro-inflammatory mediators August 02, 2018 03:37
Increased pro-inflammatory mediators leads to Leukocyte recruitment/activation November 12, 2018 10:55
Leukocyte recruitment/activation leads to Increase in RONS May 09, 2019 15:59
Increased pro-inflammatory mediators leads to Increase in RONS May 09, 2019 16:00
Increased pro-inflammatory mediators leads to Increase, Cell Proliferation (Epithelial Cells) May 10, 2019 13:30
Increased pro-inflammatory mediators leads to N/A, Breast Cancer May 10, 2019 13:41
Ionizing Radiation May 07, 2019 12:12
Other DNA damaging agents May 10, 2019 14:46

Abstract

A concise and informative summation of the AOP under development that can stand-alone from the AOP page. The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance. More help

Knowledge about established breast carcinogens can support improved 21st century toxicological testing methods by identifying key mechanistic events. Ionizing radiation (IR) increases the risk of breast cancer, especially for women and for exposure at younger ages. We used the Adverse Outcome Pathway (AOP) framework to outline and evaluate the evidence linking ionizing radiation with breast cancer from molecular initiating events (MIE) to the adverse outcome (AO) through intermediate key events (KE). We identified prospective key events using recent literature on ionizing radiation and carcinogenesis, focusing on review articles. We searched PubMed for each key event and ionizing radiation, and used references cited in the resulting papers and targeted searches with related key words to identify additional papers. We manually curated publications and evaluated data quality. The AOP specifies that ionizing radiation directly and indirectly causes DNA damage and increases production of reactive oxygen and nitrogen species (RONS), and these are designated as MIEs.  RONS lead to DNA damage (MIE) which leads to mutations (KE).  Proliferation (KE) amplifies the effects of DNA damage and mutations leading to the AO of breast cancer. Separately, RONS (and DNA damage) also increase inflammation (KE). Inflammation contributes to direct and indirect effects (effects in cells not directly reached by IR) via positive feedback to RONS and DNA damage, and separately increases proliferation and the AO through pro-carcinogenic effects on cells and tissue. These MIEs and KEs overlap at multiple points with events characteristic of “background” induction of breast carcinogenesis, including hormone-responsive proliferation, oxidative activity, and DNA damage. These overlaps make the breast particularly susceptible to ionizing radiation and reinforce the importance of these MIEs and KEs as part of toxicological panels for carcinogenicity. The AOP identifies areas for additional research, including better description of the time and dose-dependence of MIEs and KEs in mammary tissues directly and indirectly exposed to IR.

This AOP extends the characteristics of mammary carcinogens beyond DNA damage, highlighting the important role in breast cancer of chemicals that increase RONS, cell proliferation, and inflammation. Chemicals that increase these biological processes should be considered potential breast carcinogens, and predictive methods should be developed to identify chemicals that increase these processes. Ultimately, this AOP will improve methods that predict chemical breast carcinogens so that exposure can be reduced.

AOP Development Strategy

Context

Used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development.The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. More help

Breast cancer imposes a significant burden on women worldwide and is an important target for prevention. It is the most common invasive cancer in women with the highest rates found in North America and Europe (Ervik, Lam et al. 2016), and incidence is increasing globally (Forouzanfar, Foreman et al. 2011). In the US, the National Cancer Institute estimates that the total number of new breast cancers will increase from 283,000 to 441,000 between 2011 and 2030 (Rosenberg, Barker et al. 2015). Twin studies suggest that heritable factors explain at most a third of breast cancers and around 60% of all cancers are related to avoidable factors (Ronckers, Erdmann et al. 2005; Colditz and Wei 2012; Moller, Mucci et al. 2016), leaving significant room for prevention efforts focused on environmental factors to reduce new cases. Well-documented risk factors include tobacco and alcohol use as well as obesity, physical activity, and exposure to carcinogens (Colditz and Wei 2012).

Breast cancer incidence and risk varies with age, and hormonal and reproductive factors. Incidence increases with age, with rates among women increasing rapidly after age 30 and peaking around 75 years of age (NCI SEER 2016). Incidence is strongly influenced by the reproductive hormones estrogen and progesterone and by childbirth, which influence the proliferation and number of cells in the breast (Gertig, Stillman et al. 1999; Ronckers, Erdmann et al. 2005; Bijwaard, Brenner et al. 2010; Dall, Risbridger et al. 2017). Breast cancer risk increases with earlier puberty or later menopause (CGHFBC 2012; Bodicoat, Schoemaker et al. 2014), factors that increase cumulative estrogen and progesterone exposure and the number of proliferative menstrual cycles in the breast. Conversely, risk decreases in women with ovariectomies (Olson, Sellers et al. 2004; Press, Sullivan-Halley et al. 2011) and with menopause (CGHFBC 2012). Risk also decreases with number of pregnancies, breastfeeding, and increasing time since childbirth. This decrease in risk is thought to be related to the differentiation of stem cells in the breast during pregnancy and lactation and the decline in epithelial cell number after childbirth (Gertig, Stillman et al. 1999; Dall, Risbridger et al. 2017). Breast cancer incidence in men is less than 1% that of women, a difference attributed to low levels of estrogen and progesterone and few breast epithelial cells (Stang and Thomssen 2008).

Bijwaard, H., A. Brenner, et al. (2010). "Breast cancer risk from different mammography screening practices." Radiation research174(3): 367-376.

Bodicoat, D. H., M. J. Schoemaker, et al. (2014). "Timing of pubertal stages and breast cancer risk: the Breakthrough Generations Study." Breast cancer research : BCR 16(1): R18.

CGHFBC (Collaborative Group on Hormonal Factors in Breast Cancer) (2012). "Menarche, menopause, and breast cancer risk: individual participant meta-analysis, including 118 964 women with breast cancer from 117 epidemiological studies." The Lancet. Oncology 13(11): 1141-1151.

Colditz, G. A. and E. K. Wei (2012). "Preventability of cancer: the relative contributions of biologic and social and physical environmental determinants of cancer mortality." Annu Rev Public Health 33: 137-156.

Dall, G., G. Risbridger, et al. (2017). "Mammary stem cells and parity-induced breast cancer protection- new insights." The Journal of steroid biochemistry and molecular biology 170: 54-60.

Ervik, M., F. Lam, et al. (2016). "Cancer Today."   Retrieved 03/23/2018, 2018, from http://gco.iarc.fr/today.

Forouzanfar, M. H., K. J. Foreman, et al. (2011). "Breast and cervical cancer in 187 countries between 1980 and 2010: a systematic analysis." The Lancet 378(9801): 1461-1484.

Gertig, D. M., I. E. Stillman, et al. (1999). "Association of age and reproductive factors with benign breast tissue composition." Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology 8(10): 873-879.

Moller, S., L. A. Mucci, et al. (2016). "The Heritability of Breast Cancer among Women in the Nordic Twin Study of Cancer." Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology 25(1): 145-150.

NCI SEER (National Cancer Institute Surveillance, E., and End Results Program), (2016). Cancer of the Breast (Invasive): SEER Incidence and U.S. Death Rates, Age-Adjusted and Age-Specific Rates, by Race and Sex. SEER Cancer Statistics Review (1975-2014), National Cancer Institute: Table 4-11.

Olson, J. E., T. A. Sellers, et al. (2004). "Bilateral oophorectomy and breast cancer risk reduction among women with a family history." Cancer detection and prevention 28(5): 357-360.

Press, D. J., J. Sullivan-Halley, et al. (2011). "Breast cancer risk and ovariectomy, hysterectomy, and tubal sterilization in the women's contraceptive and reproductive experiences study." American journal of epidemiology 173(1): 38-47.

Ronckers, C. M., C. A. Erdmann, et al. (2005). "Radiation and breast cancer: a review of current evidence." Breast cancer research : BCR 7(1): 21-32.

Rosenberg, P. S., K. A. Barker, et al. (2015). "Estrogen Receptor Status and the Future Burden of Invasive and In Situ Breast Cancers in the United States." Journal of the National Cancer Institute 107(9).

Stang, A. and C. Thomssen (2008). "Decline in breast cancer incidence in the United States: what about male breast cancer?" Breast cancer research and treatment 112(3): 595-596

Strategy

Provides a description of the approaches to the identification, screening and quality assessment of the data relevant to identification of the key events and key event relationships included in the AOP or AOP network.This information is important as a basis to support the objective/envisaged application of the AOP by the regulatory community and to facilitate the reuse of its components.  Suggested content includes a rationale for and description of the scope and focus of the data search and identification strategy/ies including the nature of preliminary scoping and/or expert input, the overall literature screening strategy and more focused literature surveys to identify additional information (including e.g., key search terms, databases and time period searched, any tools used). More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help

Events:

Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a prototypical stressor and the biological system) of an AOP. More help
Key Events (KE)
A measurable event within a specific biological level of organisation. More help
Adverse Outcomes (AO)
An AO is a specialized KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help
Type Event ID Title Short name
KE 1182 Increase, Cell Proliferation (Epithelial Cells) Increase, Cell Proliferation (Epithelial Cells)
KE 1492 Tissue resident cell activation Tissue resident cell activation
KE 1493 Increased Pro-inflammatory mediators Increased pro-inflammatory mediators
KE 1494 Leukocyte recruitment/activation Leukocyte recruitment/activation
AO 1194 Increase, DNA damage Increase, DNA Damage
AO 185 Increase, Mutations Increase, Mutations
AO 1192 Increased, Ductal Hyperplasia Increased, Ductal Hyperplasia
AO 1193 N/A, Breast Cancer N/A, Breast Cancer

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarizes all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP.Each table entry acts as a link to the individual KER description page. More help

Network View

This network graphic is automatically generated based on the information provided in the MIE(s), KEs, AO(s), KERs and Weight of Evidence (WoE) summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help

Prototypical Stressors

A structured data field that can be used to identify one or more “prototypical” stressors that act through this AOP. Prototypical stressors are stressors for which responses at multiple key events have been well documented. More help

Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected.In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available. More help

Sex Applicability

The sex for which the AOP is known to be applicable. More help

Overall Assessment of the AOP

Addressess the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and Weight of Evidence (WoE) for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). More help
Attached file:

See Annex I for the assessment of the relative level of confidence in the overall AOP based on rank ordered weight of evidence elements.

See Appendix 2 (KEs and KERs) for the evidence supporting each key event and key event relationship.

Domain of Applicability

Addressess the relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context. More help

While the key events described here are likely relevant to all tissues after exposure to IR, it is particularly relevant to the female mammary gland. While ionizing radiation causes many kinds of cancers including leukemia, lung, bladder, and thyroid cancers (BEIR 2006; Preston, Ron et al. 2007), breast cancers are among the cancers most increased by exposure to ionizing radiation (Preston, Ron et al. 2007).

The lengthy and hormone-dependent developmental trajectory of the mammary gland is likely to be a major factor in its susceptibility to breast cancer. Numerous epidemiological and laboratory studies support the requirement for ovarian hormones in the risk of breast cancer from ionizing radiation (Grant, Cologne et al. 2018). Although at first examination breast cancer from ionizing radiation and hormones involve very different processes, in fact the hormone-dependent and ionizing radiation pathways of carcinogenesis intersect at multiple points that are part of breast development leaving the hormone-exposed breast more vulnerable to radiation. Two studies in humans and rats also suggest that IR can increase long term concentrations of circulating estrogen which would further amplify any additive effects, although additional evidence is needed (Suman, Johnson et al. 2012; Grant, Cologne et al. 2018).

One major mechanism promoting breast cancer from ionizing radiation is the proliferation of breast stem cells. Stem cells are considered to be important to initiation because of their long life and capacity to pass on mutations to many progeny. Breast tissue is responsive to estrogen and progesterone, reproductive hormones that rise at puberty and stimulate cellular proliferation with each reproductive cycle and in pregnancy. These hormonal proliferative cycles increase the risk of cancer in breast tissue (Brisken, Hess et al. 2015). IR increases the long term proliferation of stem cells in pubertal but not adult mammary gland (Nguyen, Oketch-Rabah et al. 2011; Datta, Hyduke et al. 2012; Snijders, Marchetti et al. 2012; Suman, Johnson et al. 2012; Tang, Fernandez-Garcia et al. 2014). Replication of stem cells in the IR-exposed breast is therefore particularly elevated during puberty, likely contributing to the increased susceptibility to breast cancer from IR at this age.

Another vulnerability of the breast to IR is a byproduct of proliferation: mutations. Replication itself increases the likelihood of mutations, which add to mutations arising from IR and increase the likelihood of oncogenic transformation (Atashgaran, Wrin et al. 2016). Furthermore, the high replication rate of mammary gland epithelial cells during puberty and pregnancy increases reliance on homologous recombination pathways (Kass, Lim et al. 2016). Disruption of these HR processes by IR-induced mutation or increased demand for repair can increase mutation rates and increase tumorigenesis (Mahdi, Huo et al. 2018). This disruption is particularly relevant for mammary stem cells which are highly replicating and dependent on HR but shift to NHEJ to respond to DNA damage from IR (Chang, Zhang et al. 2015). The consequence of mutations in stem cells is significant, since these cells can clonally expand to generate many mutated progeny. However, errors in stem cell division may not be the sole or primary factor driving cancer from radiation, since excess cancer risk for solid cancers at different sites from the atomic bomb are not clearly related to the number of stem cell divisions at that site (Tomasetti, Li et al. 2017).

The elevated estrogen associated with development and the estrous cycle may also have direct effects that further complement the carcinogenic effects of IR. Estrogen directly increases oxidative stress in virgin (but not parous) mice (Yuan, Dietrich et al. 2016), interferes with DNA repair (Pedram, Razandi et al. 2009; Li, Chen et al. 2014) increases mutations (Mailander, Meza et al. 2006), and increases TGF-b (Jerry, Dunphy et al. 2010). Each of these effects would increase the impact of the same events arising from IR alone.

Inflammation from the estrous cycle may also contribute to tumorigenesis following IR. Cytokines and macrophages play an integral role in mammary gland development and ductal elaboration, with alternating inflammatory, immune surveillance, and phagocytic activity occurring over each estrous cycle (Hodson, Chua et al. 2013; Atashgaran, Wrin et al. 2016; Brady, Chuntova et al. 2016). This inflammation could potentially increase IR-induced DNA damage and mutations and promote tumorigenic and invasive characteristics.

The enhancement of IR induced tumorigenesis by the estrous cycle may be replicated or further enhanced by exogenous endocrine disrupting chemicals. Indeed, evidence suggests that BPA (and presumably other estrogenic chemicals) exposure in utero can increase the mammary gland’s response to progesterone during puberty (Brisken, Hess et al. 2015). This enhancement would presumably also increase the risk of breast cancer from ionizing radiation, since that risk increases with estrogen exposure and the number of menstrual cycles.

Uncertainty arising from extrapolating from rodent and human in vitro studies to human biology

Uncertainty in this pathway arises from inconsistencies in carcinogenesis between rodent and mouse species and strains and from incomplete information about the same mechanisms operating in humans. This raises questions about whether all evidence should be weighted equally.

Almost half of the data included here is from in vitro experiments on human primary or cultured cells, which should have a high degree of relevance for this pathway in humans.  However, most of the human cells are not from mammary gland, and most of the mammary gland derived cells are cancer or immortalized cells that will not respond in exactly the same way as primary cells. Even this human data should therefore be interpreted with some caution.

Most of the remaining data in this AOP is from mice, with a relatively small number of rat studies. As a breast cancer model, mice share important characteristics with humans (Medina 2007; Imaoka, Nishimura et al. 2009). Mice and humans share similar epithelial cell types (Lim, Wu et al. 2010) and a similar developmental regime with the bulk of epithelial development occurring postnatally and accelerating during puberty, with differentiation during pregnancy (Medina 2007). Tumors in humans originate in the terminal ductal lobular unit, a structure that includes the lobule with secretory alveoli and the start of the collecting duct. The developmental terminal end bud structure is thought to be particularly vulnerable to carcinogens because of the presence of stem cells and proliferation, although it is not the only possible site of initiation. Similarly, tumors in mice originate in predominantly in alveoli as well as terminal end buds and small ducts (Medina 2007). Humans are more susceptible to carcinogens around puberty, and pregnancy is protective. Evidence on the role of development and reproduction in mammary carcinogenesis in mice is limited compared with rats but is consistent with sensitivity to radiation around puberty (Imaoka, Nishimura et al. 2009), and parity is protective for chemical carcinogens (Medina 2007). In addition, proliferation contributes to carcinogenesis in both mice and humans (Medina 2007).

However, mice differ from humans in some notable ways (Medina 2007). Mammary tumors are not common in mice, so susceptible strains or tumor-promoting viruses are used to increase spontaneous incidence and response to carcinogenic stimuli. This difference may be partially attributable to hormone responsiveness of tumors. Although tumors in mice depend on hormones for development, breast cancers in rats and humans are frequently hormone receptor positive, while mammary tumors in mice are not (Nandi, Guzman et al. 1995; Medina 2007; Imaoka, Nishimura et al. 2009).

Atashgaran, V., J. Wrin, et al. (2016). "Dissecting the Biology of Menstrual Cycle-Associated Breast Cancer Risk." Front Oncol 6: 267.

Brady, N. J., P. Chuntova, et al. (2016). "Macrophages: Regulators of the Inflammatory Microenvironment during Mammary Gland Development and Breast Cancer." Mediators Inflamm 2016: 4549676.

Brisken, C., K. Hess, et al. (2015). "Progesterone and Overlooked Endocrine Pathways in Breast Cancer Pathogenesis." Endocrinology 156(10): 3442-3450.

Chang, C. H., M. Zhang, et al. (2015). "Mammary Stem Cells and Tumor-Initiating Cells Are More Resistant to Apoptosis and Exhibit Increased DNA Repair Activity in Response to DNA Damage." Stem Cell Reports 5(3): 378-391.

Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation (2006). Health risks from exposure to low levels of ionizing radiation : BEIR VII, Phase 2, National Research Council of the National Academies.

Datta, K., D. R. Hyduke, et al. (2012). "Exposure to ionizing radiation induced persistent gene expression changes in mouse mammary gland." Radiat Oncol 7: 205.

Grant, E. J., J. B. Cologne, et al. (2018). "Bioavailable serum estradiol may alter radiation risk of postmenopausal breast cancer: a nested case-control study." International journal of radiation biology 94(2): 97-105.

Hodson, L. J., A. C. Chua, et al. (2013). "Macrophage phenotype in the mammary gland fluctuates over the course of the estrous cycle and is regulated by ovarian steroid hormones." Biol Reprod 89(3): 65.

Imaoka, T., M. Nishimura, et al. (2009). "Radiation-induced mammary carcinogenesis in rodent models: what's different from chemical carcinogenesis?" J Radiat Res 50(4): 281-293.

Jerry, D. J., K. A. Dunphy, et al. (2010). "Estrogens, regulation of p53 and breast cancer risk: a balancing act." Cellular and molecular life sciences : CMLS 67(7): 1017-1023.

Kass, E. M., P. X. Lim, et al. (2016). "Robust homology-directed repair within mouse mammary tissue is not specifically affected by Brca2 mutation." Nat Commun 7: 13241.

Li, Z., K. Chen, et al. (2014). "Cyclin D1 integrates estrogen-mediated DNA damage repair signaling." Cancer Res 74(14): 3959-3970.

Lim, E., D. Wu, et al. (2010). "Transcriptome analyses of mouse and human mammary cell subpopulations reveal multiple conserved genes and pathways." Breast cancer research : BCR 12(2): R21.

Mahdi, A. H., Y. Huo, et al. (2018). "Evidence of Intertissue Differences in the DNA Damage Response and the Pro-oncogenic Role of NF-kappaB in Mice with Disengaged BRCA1-PALB2 Interaction." Cancer Res 78(14): 3969-3981.

Mailander, P. C., J. L. Meza, et al. (2006). "Induction of A.T to G.C mutations by erroneous repair of depurinated DNA following estrogen treatment of the mammary gland of ACI rats." The Journal of steroid biochemistry and molecular biology 101(4-5): 204-215.

Medina, D. (2007). "Chemical carcinogenesis of rat and mouse mammary glands." Breast Dis 28: 63-68.

Nandi, S., R. C. Guzman, et al. (1995). "Hormones and mammary carcinogenesis in mice, rats, and humans: a unifying hypothesis." Proceedings of the National Academy of Sciences of the United States of America 92(9): 3650-3657.

Nguyen, D. H., H. A. Oketch-Rabah, et al. (2011). "Radiation acts on the microenvironment to affect breast carcinogenesis by distinct mechanisms that decrease cancer latency and affect tumor type." Cancer Cell 19(5): 640-651.

Pedram, A., M. Razandi, et al. (2009). "Estrogen inhibits ATR signaling to cell cycle checkpoints and DNA repair." Mol Biol Cell 20(14): 3374-3389.

Preston, D. L., E. Ron, et al. (2007). "Solid cancer incidence in atomic bomb survivors: 1958-1998." Radiation research 168(1): 1-64.

Snijders, A. M., F. Marchetti, et al. (2012). "Genetic differences in transcript responses to low-dose ionizing radiation identify tissue functions associated with breast cancer susceptibility." PLoS One 7(10): e45394.

Suman, S., M. D. Johnson, et al. (2012). "Exposure to ionizing radiation causes long-term increase in serum estradiol and activation of PI3K-Akt signaling pathway in mouse mammary gland." International journal of radiation oncology, biology, physics 84(2): 500-507.

Tang, J., I. Fernandez-Garcia, et al. (2014). "Irradiation of juvenile, but not adult, mammary gland increases stem cell self-renewal and estrogen receptor negative tumors." Stem Cells 32(3): 649-661.

Tomasetti, C., L. Li, et al. (2017). "Stem cell divisions, somatic mutations, cancer etiology, and cancer prevention." Science 355(6331): 1330-1334.

Yuan, L., A. K. Dietrich, et al. (2016). "17beta-Estradiol alters oxidative damage and oxidative stress response protein expression in the mouse mammary gland." Mol Cell Endocrinol 426: 11-21.

Essentiality of the Key Events

The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently, evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence. The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs. More help

IR appears to be a “complete” carcinogen in the mammary gland in that the toxin acts as an initiator through the formation of oxidative stress and pro-mutagenic DNA damage and (the MIEs) and as a promoter through increasing inflammation and proliferation, similar to many chemical carcinogens (Russo and Russo 1996). We have high confidence in the evidence linking stressor (IR) with adverse outcome (breast cancer).  The weight of evidence for the first pathway from RONS and DNA damage to Mutation and Proliferation is High while the weight of evidence for the second pathway from RONS to Inflammation to Proliferation and Breast Cancer is Moderate. These evaluations are based on the supporting evidence for all KEs and the considerations in Annex 1, and based on the need for additional evidence in the essentiality of Inflammation for the genesis of breast cancer.

This AOP could not address the large number of related topics that interact with the key events described here. These topics include events following IR that may interact with these key events such as immune surveillance (which may change with the inflammatory environment after IR (Schreiber, Old et al. 2011; Barcellos-Hoff 2013; Lumniczky and Safrany 2015); IR effect on survival/apoptosis and interactions of apoptosis with inflammation, mutation, compensatory proliferation, and selection process; changes to DNA repair; and the role of epigenetics in carcinogenesis from IR (Daino, Nishimura et al. 2018). This AOP also does not address other influences on these key events beyond reproductive hormones and typical breast development. Subsequent contributions to this AOP should elaborate on these points.

Defining question

High (Strong)

Moderate

Low (Weak)

2. Support for essentiality of KEs

Are downstream KEs and/or the AO prevented if an upstream KE is blocked?

Direct evidence from specifically designed experimental studies illustrating essentiality for at least one of the important KEs

Indirect evidence that sufficient modification of an expected modulating factor attenuates or augments a KE

No or contradictory experimental evidence of the essentiality of any of the KEs.

MIE: Increase in reactive oxygen and nitrogen species (RONS)

Essentiality is High. The most significant support comes from the relatively large number of studies using antioxidants or other interventions to reduce RONS, which show a reduction in DNA damage and mutations. Additional support comes from experiments increasing external oxidants like H2O2, which show that RONS are independently capable of causing DNA damage and mutations. Uncertainties arise from the smaller effects of RONS on DNA damage compared with ionizing radiation. Mammary gland relevance is less certain due to the relatively few experiments in breast tissue.

KE/AO: Increase in DNA damage

Essentiality is High. The essentiality of this MIE to cancer is generally accepted. Supporting evidence comes from application of mutagenic agents: the increase in DNA damage precedes mutations, proliferation, and tumorigenesis. Further indirect evidence comes from evidence for MIE1, in which antioxidants that reduce DNA damage also reduce mutations and chromosomal damage. Finally, mutations in DNA repair genes increase the risk of tumors.

KE/AO: Increase in mutation

Essentiality is High. The contribution of this MIE to cancer is generally accepted. Evidence comes from knock-out and knock-in experiments, which find that mutations in certain key genes increase tumorigenesis. However, an ongoing debate pits the singular importance of mutations against a significant role for the tissue microenvironment. This debate is fueled by transplant studies that show the importance of tissue environment for tumorigenesis and suggesting that mutations may not be sufficient for tumorigenesis.

KE: Increase, Cell Proliferation (epithelial cells)

Essentiality is High. Cellular proliferation is a key characteristic of cancer cells and can lead to hyperplasia, an intermediate phase in the development of tumorigenesis. Proliferation also increases the number of cells with mutations, which can further promote proliferation and/or changes to the local microenvironment.

KE/AO: Increase, Ductal Hyperplasia

Essentiality is High. Evidence comes from transplant experiments showing that non-proliferating tissue is less tumorigenic than proliferating lesions, and from interventions that reduce both proliferation and tumors. Further evidence comes from animals that are resistant to both mammary gland proliferation and tumors from ionizing radiation. Uncertainty arises from conflicting evidence on the tumorigenicity of hyperplasia, the absence of hyperplasia observed before some tumors, and spontaneous regression of tumors.

KEs: Tissue Resident Cell Activation, Increased Pro-inflammatory mediators, Leukocyte recruitment/activation

Essentiality is Moderate. These key events were reviewed as a group. Evidence comes from using genetic modifications, antibodies, and antioxidants to reduce inflammatory and anti-inflammatory factors. These interventions reduce DNA damage, mutations, and mechanisms contributing to tumorigenesis and invasion. Uncertainty arises from conflicting effects in different genetic backgrounds and in different organs.

MIE1: Increase in RONS

Essentiality is High. The most significant support comes from the relatively large number of studies using antioxidants or other interventions to reduce RONS, which show a reduction in DNA damage and mutations. Additional support comes from experiments increasing external oxidants like H2O2, which show that RONS are independently capable of causing DNA damage and mutations. Uncertainties arise from the smaller effects of RONS on DNA damage compared with ionizing radiation. Mammary gland relevance is less certain due to the relatively few experiments in breast tissue.

Multiple studies support the hypothesis that elevated RONS is a key part of the adverse outcome pathway for breast cancer from ionizing radiation. The strongest evidence comes from studies showing that reducing RONS also reduces DNA damage in irradiated cells and bystander cells, including genomic instability observed at later time points after IR. Free radical and NADPH oxidase inhibitors reduce the effect of IR on DNA nucleotide damage, double strand breaks, chromosomal damage, and mutations in isolated DNA and cultured cells (Winyard, Faux et al. 1992; Douki, Ravanat et al. 2006; Choi, Kang et al. 2007; Jones, Riggs et al. 2007; Ameziane-El-Hassani, Boufraqech et al. 2010; Ameziane-El-Hassani, Talbot et al. 2015; Manna, Das et al. 2015) and on nucleotide damage and double strand breaks in vivo (Pazhanisamy, Li et al. 2011; Ozyurt, Cevik et al. 2014). RONS reduction after ionizing radiation also reduces genomic instability in animals and in cloned cell lines (Dayal, Martin et al. 2008; Dayal, Martin et al. 2009; Pazhanisamy, Li et al. 2011; Bensimon, Biard et al. 2016). RONS are similarly implicated in IR effects in bystander cells. Antioxidants (including a nitric oxide scavenger) and oxidase inhibitors added before or after radiation reduce micronuclei and gamma-H2AX formation in bystander cells (Azzam, De Toledo et al. 2002; Yang, Asaad et al. 2005; Yang, Anzenberg et al. 2007). Antioxidant activity also reduces the inflammatory response to IR in animals and cultured skin cells (Berruyer, Martin et al. 2004; Das, Manna et al. 2014; Ozyurt, Cevik et al. 2014; Haddadi, Rezaeyan et al. 2017; Zhang, Zhu et al. 2017).

RONS are sufficient to trigger subsequent key events in this AOP. Extracellularly applied or intracellularly generated ROS (which also facilitates the formation of RNS) are capable of creating DNA damage in vitro including base damage, single and double strand breaks, and chromosomal damage (Oya, Yamamoto et al. 1986; Dahm-Daphi, Sass et al. 2000; Nakamura, Purvis et al. 2003; Gradzka and Iwanenko 2005; Ismail, Nystrom et al. 2005; Driessens, Versteyhe et al. 2009; Berdelle, Nikolova et al. 2011; Lorat, Brunner et al. 2015; Stanicka, Russell et al. 2015) and mutations (Sandhu and Birnboim 1997; Ameziane-El-Hassani, Boufraqech et al. 2010; Seager, Shah et al. 2012; Sharma, Collins et al. 2016). Similarly, decreased antioxidant activity and higher RONS is observed in cells with genomic instability (Dayal, Martin et al. 2008; Buonanno, de Toledo et al. 2011). To our knowledge, no experiments have tested whether elevating intracellular RONS alone in one group of cells can cause bystander effects in another.

Evidence in Mammary Gland

The increase of RONS following IR has been shown in a wide range of cells, in vivo and in vitro, including epithelial cells, and in two studies in mammary epithelial cells (Jones, Riggs et al. 2007; Bensimon, Biard et al. 2016). Both mammary cell studies also show increased RONS and DNA damage over a day after IR in vitro and link DNA damage with elevated RONS.

Uncertainties or Inconsistencies

The mitigating effects of antioxidants on IR-generated DNA damage support the essentiality of RONS in producing DNA damage and mutations. However, externally applied RONS is less effective than IR at generating double strand breaks and mutations (Sandhu and Birnboim 1997; Dahm-Daphi, Sass et al. 2000; Gradzka and Iwanenko 2005; Ismail, Nystrom et al. 2005). One possible explanation for this discrepancy is that IR may elicit a higher concentration of localized RONS than can be achieved with external application of H2O2. IR deposits energy and oxidizes molecules within a relatively small area over a rapid timescale potentially permitting a very high local concentration which could precede or overwhelm local buffering capacity. In contrast, extracellularly applied H2O2 would interact with many antioxidants and other molecules on its way to the nucleus, where the concentration would slowly reach a lower steady state.

As expected for RONS as a key event for DNA damage from IR, DNA damage from IR and H2O2 are additive in cells (Dahm-Daphi, Sass et al. 2000; Driessens, Versteyhe et al. 2009). Unexpectedly however, inhibiting glutathione (which should increase or sustain the effects of RONS), increases DNA damage from H2O2 but not IR. This lack of effect of glutathione inhibition on IR conflicts with multiple studies showing decreased DNA damage from IR with anti-oxidants. One possible explanation is that the concentration or reaction rate of glutathione is already inadequate to buffer the elevated RONS from IR, so further inhibition has no measurable effect.

KE/AO: Increase in DNA damage

Essentiality is High. The essentiality of this MIE to cancer is generally accepted. Supporting evidence comes from application of mutagenic agents: the increase in DNA damage precedes mutations, proliferation, and tumorigenesis. Further indirect evidence comes from evidence for MIE1, in which antioxidants that reduce DNA damage also reduce mutations and chromosomal damage. Finally, mutations in DNA repair genes increase the risk of tumors.

Increases or decreases in DNA damage are associated with corresponding increases or decreases in downstream key events in the pathway to breast cancer. An external agent (ionizing radiation) that increases DNA damage (Padula, Ponzinibbio et al. 2016) also causes chromosomal damage and increased mutations (Sandhu and Birnboim 1997; Jones, Riggs et al. 2007; Denissova, Nasello et al. 2012; Fibach and Rachmilewitz 2015), transforms cells (Yang, Craise et al. 1992; Yang, Georgy et al. 1997; Unger, Wienberg et al. 2010), and causes tumors (Poirier and Beland 1994; Little 2009). Polymorphisms or mutations in DNA repair genes affect tumor formation after ionizing radiation in animals (Yu, Okayasu et al. 2001; Umesako, Fujisawa et al. 2005) and in people (Millikan, Player et al. 2005; Andrieu, Easton et al. 2006; Broeks, Braaf et al. 2007; Bernstein, Haile et al. 2010; Brooks, Teraoka et al. 2012; Pijpe, Andrieu et al. 2012; Bernstein, Thomas et al. 2013). Consistent with these findings, antioxidants that reduce DNA damage from stressors like IR also reduce chromosomal aberrations and micronuclei arising from those stressors (Azzam, De Toledo et al. 2002; Choi, Kang et al. 2007; Jones, Riggs et al. 2007).

Evidence in mammary gland

The majority of research on the effects of IR on DNA damage has been performed in tissues other than mammary gland, but several studies suggest that effects in the mammary gland (and its consequences) would be consistent with other tissues. Oxidative DNA damage in mammary cells increases immediately after exposure to IR (Haegele, Wolfe et al. 1998), and double stranded breaks, micronuclei, and (later) chromosomal aberrations appear two hours to six days after IR exposure in vivo and in vitro (Soler, Pampalona et al. 2009; Snijders, Marchetti et al. 2012; Hernandez, Terradas et al. 2013). Genomic instability was reported in genetically susceptible cells after a month of higher doses of IR (4 doses of 1.8 Gy but not 0.75 Gy) (Snijders, Marchetti et al. 2012).

KE/AO: Increase in mutation

Essentiality is High. The contribution of this MIE to cancer is generally accepted. Evidence comes from knock-out and knock-in experiments, which find that mutations in certain key genes increase tumorigenesis. However, an ongoing debate pits the singular importance of mutations against a significant role for the tissue microenvironment. This debate is fueled by transplant studies that show the importance of tissue environment for tumorigenesis and suggesting that mutations may not be sufficient for tumorigenesis.

Mutations increase transformation in culture (Wang, Su et al. 2011) and proliferation and tumors in mice (Radice, Ferreira-Cornwell et al. 1997; Umesako, Fujisawa et al. 2005; de Ostrovich, Lambertz et al. 2008; Podsypanina, Politi et al. 2008; Francis, Bergsied et al. 2009; Gustin, Karakas et al. 2009; Francis, Chakrabarti et al. 2011; Tao, Xiang et al. 2017). Restoring function in mutated genes regresses tumors in animals (Martins, Brown-Swigart et al. 2006; Podsypanina, Politi et al. 2008). Mutations are common in tumors (Haag, Hsu et al. 1996; Greenman, Stephens et al. 2007; Stratton, Campbell et al. 2009; CGAN (Cancer Genome Atlas Network) 2012; Vandin, Upfal et al. 2012; Garraway and Lander 2013; Vogelstein, Papadopoulos et al. 2013; Yang, Killian et al. 2015) and tumors are largely clonal, suggesting that individual mutations offer the tumor evolutionary advantages (Wang, Waters et al. 2014; Yates, Gerstung et al. 2015; Begg, Ostrovnaya et al. 2016).

Evidence in mammary gland

Many of the studies in support of the proliferative and tumorigenic role of mutations are in mammary gland or breast cancers. Further support for including DNA damage and mutation in the mechanistic pathway linking ionizing radiation with breast cancer comes from the observation that variants in DNA repair genes increase the risk of mammary tumors in animals after IR (Yu, Okayasu et al. 2001; Umesako, Fujisawa et al. 2005) and increase breast cancer after IR (Millikan, Player et al. 2005; Andrieu, Easton et al. 2006; Broeks, Braaf et al. 2007; Bernstein, Haile et al. 2010; Brooks, Teraoka et al. 2012; Pijpe, Andrieu et al. 2012; Bernstein, Thomas et al. 2013). BRCA is perhaps the best known DNA repair gene linked with breast cancer risk, and several studies of these studies have suggested a link between BRCA mutation status and increased susceptibility to breast cancer following ionizing radiation, particularly in women exposed at younger ages (Pijpe, Andrieu et al. 2012).

Uncertainties or Inconsistencies

Mutations alone are not sufficient or even essential for tumor growth in mammary glands. Mammary tumor incidence following ionizing radiation varies significantly by sex and depends on the presence of ovarian hormones (Cronkite, Shellabarger et al. 1960; Segaloff and Maxfield 1971; Shellabarger, Stone et al. 1976; Holtzman, Stone et al. 1979; Holtzman, Stone et al. 1981; Welsch, Goodrich-Smith et al. 1981; Clifton, Yasukawa-Barnes et al. 1985; Solleveld, van Zwieten et al. 1986; Broerse, Hennen et al. 1987; Lemon, Kumar et al. 1989; Inano, Suzuki et al. 1991; Inano, Suzuki et al. 1996; Peterson, Servinsky et al. 2005). Tumor growth from transplanted tumor cells varies with age, parity, and lactational status (Maffini, Calabro et al. 2005; McDaniel, Rumer et al. 2006), and stroma treated with carcinogens or IR supports tumors from pre-malignant epithelial cells (Barcellos-Hoff and Ravani 2000; Maffini, Soto et al. 2004; Nguyen, Oketch-Rabah et al. 2011). While the mechanisms underlying these contextual factors have not been clearly identified, the proliferative effect of hormones on the mammary gland may serve to amplify damaged and mutated cells and modify the stromal environment to increase the likelihood of cellular transformation. Inflammatory responses including the release of cytokines and the activation of inflammatory and anti-inflammatory signaling pathways likely also amplify the effects of DNA damage and mutations through many of the same mechanisms.

KE: Increase in proliferation

Essentiality is High. Cellular proliferation is a key characteristic of cancer cells (Hanahan and Weinberg 2011) and can lead to hyperplasia, an intermediate phase in the development of tumorigenesis. Proliferation also increases the number of cells with mutations, which can further promote proliferation and/or changes to the local microenvironment.

Evidence in mammary gland

Multiple studies show that mammary gland proliferates after IR or chemical carcinogen treatment prior to the appearance of mammary tumors. Epithelial cells proliferate following IR in vitro (Mukhopadhyay, Costes et al. 2010) and in vivo (Nguyen, Oketch-Rabah et al. 2011; Snijders, Marchetti et al. 2012; Suman, Johnson et al. 2012; Tang, Fernandez-Garcia et al. 2014). Increasing proliferation leads to hyperplasia (Korkaya, Paulson et al. 2009). Proliferative nodules and hyperplasia appear in mammary terminal end bud, alveolae, and ducts of rats and mice after exposure to chemical carcinogens (Beuving, Bern et al. 1967; Beuving, Faulkin et al. 1967; Russo, Saby et al. 1977; Purnell 1980) and ionizing radiation (Faulkin, Shellabarger et al. 1967; Ullrich and Preston 1991; Imaoka, Nishimura et al. 2006). Proliferating foci precede the development of tumors (Haslam and Bern 1977; Purnell 1980) and form tumors more effectively than non-proliferating tissue (Deome, Faulkin et al. 1959; Beuving 1968; Rivera, Hill et al. 1981).

Supporting the essentiality of these proliferative processes to tumorigenesis, ACI rats that exhibit no mammary proliferation or hyperplasia following IR are resistant to tumors following IR (Kutanzi, Koturbash et al. 2010). Interventions reducing proliferation in susceptible PyVT and BALB/c mice also reduce mammary tumors (Luo, Fan et al. 2009; Connelly, Barham et al. 2011; Tang, Fernandez-Garcia et al. 2014).

Uncertainties or Inconsistencies

Some studies report carcinogenesis in the absence of hyperplasia (Sinha and Dao 1974) and others do not find increased tumorigenesis from transplanted hyperplasia (Beuving, Bern et al. 1967; Haslam and Bern 1977; Sinha and Dao 1977). The failure of some proliferative foci to form tumors and the regression of some tumors when formed (Haslam and Bern 1977; Purnell 1980; Korkola and Archer 1999) suggests that proliferation may not be sufficient for sustained tumorigenesis in mammary gland.

KE/AO: Increase, ductal hyperplasia

Essentiality is High. Evidence comes from transplant experiments showing that non-proliferating tissue is less tumorigenic than proliferating lesions, and from interventions that reduce both proliferation and tumors. Further evidence comes from animals that are resistant to both mammary gland proliferation and tumors from ionizing radiation. Uncertainty arises from conflicting evidence on the tumorigenicity of hyperplasia, the absence of hyperplasia observed before some tumors, and spontaneous regression of tumors.

Hyperplasia signals the presence of excess proliferation (a key characteristic of cancer cells (Hanahan and Weinberg 2011)) and represents an intermediate phase in the development of tumorigenesis.

Evidence in mammary gland

Multiple studies show that mammary gland proliferates after IR or chemical carcinogen treatment prior to the appearance of mammary tumors. Proliferative nodules and hyperplasia appear in mammary terminal end bud, alveolae, and ducts of rats and mice after exposure to chemical carcinogens (Beuving, Bern et al. 1967; Beuving, Faulkin et al. 1967; Russo, Saby et al. 1977; Purnell 1980) and ionizing radiation (Faulkin, Shellabarger et al. 1967; Ullrich and Preston 1991; Imaoka, Nishimura et al. 2006). Proliferating foci precede the development of tumors (Haslam and Bern 1977; Purnell 1980) and form tumors more effectively than non-proliferating tissue (Deome, Faulkin et al. 1959; Beuving 1968; Rivera, Hill et al. 1981). Adenocarcinomas in rats appear to preferentially form from terminal end bud hyperplasia (Haslam and Bern 1977; Russo, Saby et al. 1977; Purnell 1980), similar to the origin of many breast cancers for humans and for some mice after IR (Medina and Thompson 2000).

Supporting the essentiality of these proliferative processes to tumorigenesis, ACI rats that exhibit no mammary proliferation or hyperplasia following IR are resistant to tumors following IR (Kutanzi, Koturbash et al. 2010). Interventions reducing proliferation in susceptible PyVT and BALB/c mice also reduce mammary tumors (Luo, Fan et al. 2009; Connelly, Barham et al. 2011).

Uncertainties or Inconsistencies

Some studies report carcinogenesis in the absence of hyperplasia (Sinha and Dao 1974) and others do not find increased tumorigenesis from transplanted hyperplasia (Beuving, Bern et al. 1967; Haslam and Bern 1977; Sinha and Dao 1977). The failure of some lesions to form tumors and the regression of some tumors when formed (Haslam and Bern 1977; Purnell 1980; Korkola and Archer 1999) suggests that hyperplasia alone may not be sufficient for sustained tumorigenesis in mammary gland.

 

KEs: Tissue resident cell activation, Increase, Pro-inflammatory mediators, Leukocyte Recruitment/Activation

Essentiality is Moderate. These key events were reviewed as a group. Evidence comes from using genetic modifications, antibodies, and antioxidants to reduce inflammatory and anti-inflammatory factors. These interventions reduce DNA damage, mutations, and mechanisms contributing to tumorigenesis and invasion. Uncertainty arises from conflicting effects in different genetic backgrounds and in different organs.

Tumors and tumor cells exhibit features of inflammation, and inflammation is generally understood to promote transformation and tumor progression by supporting multiple hallmarks of cancer including oxidative activity and DNA damage, survival and proliferation, angiogenesis, and invasion and metastasis (Iliopoulos, Hirsch et al. 2009; Hanahan and Weinberg 2011; Esquivel-Velazquez, Ostoa-Saloma et al. 2015).

Many of these cancer promoting effects of inflammation can be seen following exposure to ionizing radiation (Bisht, Bradbury et al. 2003; Elahi, Suraweera et al. 2009; Nguyen, Oketch-Rabah et al. 2011; Bouchard, Bouvette et al. 2013; Nguyen, Fredlund et al. 2013; Illa-Bochaca, Ouyang et al. 2014). Inflammatory pathways are commonly activated in breast and mammary cancers following IR (Nguyen, Oketch-Rabah et al. 2011; Nguyen, Fredlund et al. 2013; Illa-Bochaca, Ouyang et al. 2014). Polymorphisms in inflammation genes are associated with breast cancer risk from IR in radiation technologists (Schonfeld, Bhatti et al. 2010) and with susceptibility to intestinal adenoma following IR in mice (Elahi, Suraweera et al. 2009). Cytokines TGF-β and IL6 transform  primary human mammospheres and pre-malignant mammary epithelial cell lines in vitro and make them tumorigenic in vivo (Sansone, Storci et al. 2007; Iliopoulos, Hirsch et al. 2009; Nguyen, Oketch-Rabah et al. 2011), and inflammation related factors COX2 and TGF-β are required for the full effect of IR on DNA damage and transformation in vitro and mammary tumor growth and invasion in vivo (Bisht, Bradbury et al. 2003; Nguyen, Oketch-Rabah et al. 2011).

One mechanism of cancer promotion involves oxidative activity and DNA damage: inflammation in response to IR increases oxidative activity in a positive feedback loop leading to increased DNA lesions and mutations. Oxidative activity mediates the increase in inflammatory markers (TNF-a and neutrophil markers) in bladder and kidney (Ozyurt, Cevik et al. 2014), and TNF-a and neutrophils increase oxidative activity (Jackson, Gajewski et al. 1989; Stevens, Bucurenci et al. 1992; Zhang, Zhu et al. 2017). Inflammatory activity from neutrophils and TNF-a and NF-kB-dependent COX2 and NO damage DNA and increase mutations by increasing oxidative activity (Jackson, Gajewski et al. 1989; Zhou, Ivanov et al. 2005). The mutations can be reduced by blocking the inflammatory factors NF-kB, COX2, TNF-a, or nitric oxide, or with antioxidants (Jackson, Gajewski et al. 1989; Zhou, Ivanov et al. 2005; Zhou, Ivanov et al. 2008; Zhang, Zhu et al. 2017). Antibodies to TNF-a or TGF-β reduce DNA damage in bone marrow (Burr, Robinson et al. 2010; Rastogi, Coates et al. 2012) and CHO cells (Han, Chen et al. 2010). Inhibiting TNF-a also reduces genomic instability in directly irradiated (but not bystander) lymphocytes (Moore, Marsden et al. 2005) and in bone marrow of CBA/Ca mice susceptible to IR-induced leukemia but not resistant C57BL/6 mice (Lorimore, Mukherjee et al. 2011). Inhibiting inflammatory factors NF-kB or iNOS reduces IR-induced bystander mutations in lung fibroblasts (Zhou, Ivanov et al. 2008).

Inflammatory pathways activated by IR are also capable of promoting tumor growth and metastasis. Exposure to IR or RONS sensitizes mammary epithelial cells to respond to TGF-β - which is widely activated by IR (Ehrhart, Segarini et al. 1997). IR and TGF-β signaling leads to an epithelial to mesenchymal (EMT)-like transition, which disrupts the expression and distribution of cell adhesion molecules and multicellular organization and promotes invasion (Park, Henshall-Powell et al. 2003; Andarawewa, Erickson et al. 2007; Andarawewa, Costes et al. 2011; Iizuka, Sasatani et al. 2017). This mechanism resembles wound healing (Koh and DiPietro 2011; Perez, Vago et al. 2014; Landen, Li et al. 2016), but also resembles malignancy - invasive breast cancer cell lines overexpress TGF-β and respond to TGF-β with increased invasion (Kim, Kim et al. 2004; Gomes, Terra et al. 2012). 

The response to TGF-β likely involves an increase in senescence in fibroblasts. IR-induced senescence releases a suite of signaling molecules including pro-inflammatory IL6 and proteases (MMPs) (Tsai, Chuang et al. 2005; Liakou, Mavrogonatou et al. 2016; Perrott, Wiley et al. 2017). The signaling molecules released by IR-senescent fibroblasts promote the disorganized tissue structure of mammary epithelial cells and the growth, EMT, and invasion of breast cancer epithelial cells or mutant epithelial cells (Tsai, Chuang et al. 2005; Liakou, Mavrogonatou et al. 2016; Perrott, Wiley et al. 2017) and 3D mammary tumor models (Sourisseau, Harrington et al. 2011). The induction of senescence in fibroblasts by IR requires TGF-β (Liakou, Mavrogonatou et al. 2016), and the release of the pro-invasive signaling molecules involves an IL-1 dependent activation of NF-kB (Perrott, Wiley et al. 2017). Senescence following IR also selects for a post-senescent variant of epithelial cell that is more conducive to tumorigenesis (Mukhopadhyay, Costes et al. 2010).

Il6 may play an important function in the carcinogenic response to IR. IL6 is expressed in mouse mammary gland after IR (Bouchard, Bouvette et al. 2013). IL6 is produced by IR-senescent fibroblasts, but may also be expressed by epithelial cells after IR since primary human mammospheres and pre-malignant mammary epithelial cell lines respond to IL6 with increased IL6 expression (Sansone, Storci et al. 2007; Iliopoulos, Hirsch et al. 2009). IL6 promotes the mobility and tumorigenesis of normal and breast cancer epithelial cells (Sansone, Storci et al. 2007; Sasser, Sullivan et al. 2007; Studebaker, Storci et al. 2008; Iliopoulos, Hirsch et al. 2009; Iliopoulos, Jaeger et al. 2010). This activity depends on transcription factor NOTCH3, which supports the renewal of stem-like cell populations (Sansone, Storci et al. 2007), and NOTCH has been implicated in multiple other studies in the proliferative response to IR in mammary epithelia (Nguyen, Oketch-Rabah et al. 2011; Marusyk, Tabassum et al. 2014; Tang, Fernandez-Garcia et al. 2014). The NF-kB/IL6/STAT3 signaling pathway generates cancer stem cells in multiple types of breast cancer cells (Iliopoulos, Hirsch et al. 2009; Iliopoulos, Jaeger et al. 2010; Iliopoulos, Hirsch et al. 2011) and is also implicated in colon and other cancers (Iliopoulos, Jaeger et al. 2010). The inflammation related transcription factor NF-kB also contributes to mammary tumorigenesis and metastasis in PyVt mice, in which mammary tumors are induced by expression of an MMTV-driven oncogene (Connelly, Barham et al. 2011). Interestingly, breast cancer fibroblasts and fibroblasts from common sites of breast cancer metastasis (bone, lung) express IL6. IL6 is required for the growth and tumor promoting effects of these fibroblasts on ER-positive cancer cells in vitro and in vivo. ER-negative breast epithelial cells release autocrine IL6 and may therefore be less dependent on IL6 from fibroblasts, although IL6 also transforms these cells (Sasser, Sullivan et al. 2007; Studebaker, Storci et al. 2008; Iliopoulos, Hirsch et al. 2009).

Inflammation is suspected to play a role in the indirect effects of radiation, in which cells not directly targeted by radiation exhibit effects including DNA damage and RONS (Lorimore and Wright 2003; Mukherjee, Coates et al. 2014; Sprung, Ivashkevich et al. 2015). In addition to the IR-induced release of inflammatory signals that are diffusible and can trigger systemic immune responses, inflammatory factors COX2 and TGF-β are produced in bystander cells that are not directly irradiated but are exposed to irradiated cells or media (Zhou, Ivanov et al. 2005; Zhou, Ivanov et al. 2008; Chai, Calaf et al. 2013; Chai, Lam et al. 2013; Wang, Wu et al. 2015).

Inflammatory factors TGF-β, TNF-a, COX2, and NO are implicated in the RONS (Shao, Folkard et al. 2008; Zhou, Ivanov et al. 2008; Wang, Wu et al. 2015), DNA damage (Dickey, Baird et al. 2009; Han, Chen et al. 2010; Dickey, Baird et al. 2012; Chai, Calaf et al. 2013; Chai, Lam et al. 2013; Wang, Wu et al. 2015) and mutations (Zhou, Ivanov et al. 2005; Zhou, Ivanov et al. 2008) observed in bystander cells and in the appearance of genomic instability (Moore, Marsden et al. 2005; Natarajan, Gibbons et al. 2007; Lorimore, Chrystal et al. 2008; Lorimore, Mukherjee et al. 2011) after IR. Further evidence for inflammation in indirect effects of IR come from tumors arising from mammary epithelial cells transplanted into IR exposed cleared fat pads: inflammation-related genes and pathways are upregulated or enriched in the gene expression patters of these indirectly IR-induced tumors (Nguyen, Oketch-Rabah et al. 2011; Nguyen, Fredlund et al. 2013; Illa-Bochaca, Ouyang et al. 2014).

Evidence in mammary gland

Many of the studies above that link inflammatory signals with increased oxidative activity, senescence, EMT, bystander effects, genomic instability, and tumorigenesis, and metastasis use mammary tissue. Since inflammation-related signals are reported after IR in mammary gland (Barcellos-Hoff, Derynck et al. 1994; Dickey, Baird et al. 2009; Datta, Hyduke et al. 2012; Snijders, Marchetti et al. 2012; Bouchard, Bouvette et al. 2013; Wang, Wu et al. 2015) inflammation likely contributes to many of the effects of IR in this tissue.

Uncertainties or inconsistencies

The effects of inflammation can be both pro and anti-tumorigenic. For example, in addition to TGF-β’s role in EMT, in mammary epithelial cells TGF-β is essential to apoptosis of DNA damaged cells including damage following ionizing radiation (Ewan, Henshall-Powell et al. 2002), thus limiting genomic instability (Maxwell, Fleisch et al. 2008). Inflammatory factors TNF-a and COX2 play a similar role in bone marrow of C57BL/6 mice (Lorimore, Rastogi et al. 2013). By eliminating cells with severe DNA damage and curtailing genomic instability, apoptosis (and therefore TGF-β or TNF-a) limits the appearance of major (possibly carcinogenic) mutations following ionizing radiation. However, apoptosis (and thus TGF-β or TNF-a) can indirectly promote tumorigenesis through compensatory proliferation (Loree, Koturbash et al. 2006; Fogarty and Bergmann 2017).

Genetic background also influences the interaction between inflammation and tumorigenesis. Polymorphisms in inflammatory genes influence susceptibility to intestinal cancer following IR (Elahi, Suraweera et al. 2009). In the SPRET outbred mouse higher baseline TGF-β during development decreases tumor incidence following lower doses of IR (0.1 Gy), possibly by reducing ductal branching and susceptibility (Zhang, Lo et al. 2015). Conversely, the BALB/c mouse susceptible to mammary tumors after IR has a lower baseline TGF-β (and a polymorphism in a DNA damage repair-related gene). Early (4 hours) after low dose (0.075 Gy) IR BALB/c mice have suppressed immune pathways and macrophage response but increased IL6, COX2, and TGF-β pathway activation in mammary gland compared to the tumor-resistant C57BL/6 mouse (Snijders, Marchetti et al. 2012; Bouchard, Bouvette et al. 2013).  By 1 week after IR, the BALB/c mice show TGF-β -dependent inflammation in the mammary gland, and by 1 month after IR, their mammary glands show proliferation (Nguyen, Martinez-Ruiz et al. 2011; Snijders, Marchetti et al. 2012), suggesting that TGF-β is associated with inflammation, proliferation, and mammary tumorigenesis in these mice. Consistent with this pattern, BALB/c mice that are heterozygous for TGF-β are more resistant to mammary tumorigenesis following IR (Nguyen, Oketch-Rabah et al. 2011). However, the BALB/c mouse also has a polymorphism in a DNA repair gene associated with IR-induced genomic instability (Yu, Okayasu et al. 2001), making it difficult to distinguish potentially overlapping mechanisms.

While inflammatory signals are associated with bystander effects including DNA damage, genomic instability, and mutation, these effects vary between organs in vivo (Chai, Calaf et al. 2013; Chai, Lam et al. 2013), by genotype (Coates, Rundle et al. 2008; Lorimore, Chrystal et al. 2008; Lorimore, Mukherjee et al. 2011), and by cell type (Chai, Calaf et al. 2013). Further research will be required to identify all the underlying factors determining differences in bystander effects, but one variable is the appearance of a protective apoptotic response to cytokines under some conditions (Lorimore, Mukherjee et al. 2011; Lorimore, Rastogi et al. 2013).

One major piece of conflicting evidence comes from a direct test of the essentiality of inflammation to IR-induced carcinogenesis. In a mouse model of lymphoma, a mutation preventing the PIDD/NEMO dependent activation of NF-kB blocks early IR-induced activation of NF-kB (4-24 h) and production of TNF-a (5-48 h) but not lymphoma, suggesting that activation of these inflammatory factors is not essential in this time period (Bock, Krumschnabel et al. 2013). However, this study examined only day one post-IR time points for NF-kB activity, and did not block production of IL6. Later activation of NF-kB or activation of other inflammation-related factors including IL6 and TGF-β could therefore potentially have contributed to lymphoma.

Evidence Assessment

Addressess the biological plausibility, empirical support, and quantitative understanding from each KER in an AOP. More help

Support for biological plausibility of KERs 

Defining question

High (Strong)

Moderate

Low (Weak)

a. Is there a mechanistic relationship between KEup and KEdown consistent with established biological knowledge?

Extensive understanding of the KER based on extensive previous documentation and broad acceptance

KER is plausible based on analogy to accepted biological relationships, but scientific understanding is incomplete

Empirical support for association between KEs, but the structural or functional relationship between them is not understood.

MIE1 => MIE2 

Increase in RONS leads to increase in DNA damage

High. Reactive oxygen and nitrogen species from oxygen and respiratory activity are generally acknowledged to damage DNA under a range of cellular conditions.

MIE2 => KE1

DNA damage leads to mutations                     

High. DNA damage in the form of nucleotide damage, single strand and double strand breaks, and complex damage can generate mutations, particularly when a damaged cell undergoes replication.

KE1 => KE2 

Mutations can promote proliferation

High. Multiple mechanisms limit the proliferation of cells in normal biological systems. Mutations in many of the genes controlling these mechanisms promote proliferation.

KE2 => KE1

Proliferation leads to mutation

High. Proliferation is generally acknowledged to increase mutations through incorporating or amplifying the impact of unrepaired DNA damage as mutations.

KE2 => AO

Proliferation promotes breast cancer and invasion

 High. It is generally accepted that proliferation contributes to cancer. Proliferation increases the number of cells with mutations, which can further promote proliferation and/or changes to the local microenvironment.

MIE1 => KE3

Increase in RONS leads to inflammation

 Moderate. Damage from RONS can activate some inflammatory and anti-inflammatory pathways (TLR, TGF-β), and RONS are an essential part of the primary signaling pathways of multiple inflammatory and anti-inflammatory pathways (TLR4, TNF-a, TGF-β, NFkB).

KE3 => MIE1

Inflammation leads to an increase in RONS

High. Inflammation is commonly understood to generate RONS via inflammatory signaling and activated immune cells.

KE3 => KE2

Inflammation leads to proliferation

High. Inflammation is generally understood to lead to proliferation during recovery from inflammation.

KE3 => AO 

Inflammation promotes breast cancer and invasion

Moderate. Tissue environment is known to be a major factor in carcinogenesis, and inflammatory processes are implicated in the development and invasiveness of breast and other cancers.

Empirical support for KERs

Defining questions

High (Strong)

Moderate

Low (Weak)

Empirical support for KERs

Does empirical evidence support that a change in KEup leads to an appropriate change in KEdown? Does KEup occur at lower doses and earlier time points than KE down and is the incidence of KEup > than that for KEdown? Inconsistencies?

Multiple studies showing dependent change in both events following exposure to a wide range of specific stressors. No or few critical data gaps or conflicting data.

Demonstrated dependent change in both events following exposure to a small number of stressors. Some inconsistencies with expected pattern that can be explained by various factors.

Limited or no studies reporting dependent change in both events following exposure to a specific stressor; and/or significant inconsistencies in empirical support across taxa and species that don’t align with hypothesized AOP

MIE1 => MIE2                      Increase in RONS leads to increase in DNA damage

High. Multiple studies show an increase in DNA damage with RONS treatment as well as dependent changes in both RONS and DNA damage in response to stressors. DNA damage increases with RONS dose, and temporal concordance between RONS and DNA damage events following ionizing radiation is consistent with a causative relationship, although few studies examine multiple doses and time points. A small number of studies do not find double strand breaks at physiological doses, or report an increase in one key event but not the other.

MIE2 => KE1

DNA damage leads to mutations                     

High. It is generally accepted that DNA damage leads to mutations. Empirical support comes in part from the observation that agents which increase DNA damage also cause mutations, that DNA damage precedes the appearance of mutations, and that interventions to reduce DNA damage also reduce mutations. None of the identified studies measure both outcomes over the same range of time points. This constitutes a readily addressable data gap.

KE1 => KE2                     Mutations can promote proliferation

Moderate. Mutations that promote proliferation are frequently found in cancers, and both mutation and proliferation occur in response to tumorigenic stressors like ionizing radiation. Although not measured together after stressors, mutations appear over the same time frame or prior to the appearance of proliferation. Multiple uncertainties and conflicting evidence weaken this key event relationship. The two key events differ in their dose response- mutation but not proliferation increases with ionizing radiation dose. Furthermore, a single mutation is not necessarily sufficient to increase proliferation- proliferation typically requires multiple mutations or a change in the surrounding environment. In mammary tissue, stromal state strongly influences the proliferative nature of epithelial cells – even epithelial cells with mutated tumor suppressors may be unable to form tumors in the absence of stromal changes.

KE2 => KE1                     Proliferation leads to mutation

High. We did not evaluate the empirical support for this KER in response to IR. However proliferation or mitosis is required for some types of DNA damage to be made permanent and heritable, and further DNA damage including mutation promoting double strand breaks can occur when cells divide before DNA repair is complete.

KE2 => AO                      Proliferation promotes breast cancer and invasion

High. Carcinogenic agents increase proliferation and hyperplasia as well as tumors. Proliferation and hyperplasia appear prior to or at the same time as tumors, grow into carcinomas, and form mammary tumors more effectively than non-proliferating tissue. Disruption of proliferation is associated with decreased tumor growth, and tumor resistant rats do not show proliferation. However, the discrepancy between the non-linear proliferative and linear mammary tumor response to carcinogen dose coupled with evidence of independent occurrences of proliferation and tumorigenesis suggests that while proliferation and hyperplasia likely promote carcinogenesis, additional factors also contribute to carcinogenesis.

MIE1 => KE3                      Increase in RONS leads to inflammation

Moderate. Both RONS and inflammation increase in response to agents that increase either RONS or inflammation. Multiple studies show dose-dependent changes in both RONS and inflammation in response to stressors including ionizing radiation and antioxidants. RONS have been measured at the same or earlier time points as inflammatory markers, but additional studies are needed to characterize the inflammatory response at the earliest time points to support causation. Uncertainties come from the positive feedback from inflammation to RONS potentially interfering with attempts to establish causality, and from the large number of inflammation related factors with differing responses to stressors and experimental variation.

KE3 => MIE1

Inflammation leads to an increase in RONS

High. Signals arising from inflammation can be both pro- and anti-inflammatory, and both can have effects on RONS and downstream key events. Multiple inflammation-related factors increase RONS or oxidative damage, and ionizing radiation increases both inflammation-related signaling and RONS or oxidative damage over the same time points. Interventions to reduce inflammation also reduce RONS. The dose-dependence of the response to stressors is generally consistent between the two key events, although this is based on a small number of studies with some conflicting evidence.

KE3 => KE2

Inflammation leads to proliferation

High. We did not evaluate the empirical support for this KER in response to IR. However, inflammation is generally understood to promote proliferation and survival

KE3 => AO                      Inflammation promotes breast cancer and invasion

Moderate. Interventions to increase inflammatory factors increase the carcinogenic potential of targeted and non-targeted cells. Inflammation is documented at earlier time points than tumorigenesis or invasion- within minutes or hours compared to days to months for carcinogenesis, consistent with an inflammatory mechanism of tumorigenesis and invasion. Inhibition of cytokines, inflammatory signaling pathways, and downstream effectors of inflammation activity prevent transformation, tumorigenesis, and invasion following IR or stimulation of inflammatory pathways. However, the key event and the adverse outcome differ in their dose-response to ionizing radiation: inflammation always does not increase linearly with dose, while breast cancer and invasion does. Uncertainty arises from the multifunctional nature of inflammation-related pathways which may be pro- or anti-inflammatory and pro- or anti-carcinogenic based on context. Both pro- and anti-inflammatory factors may contribute to carcinogenesis- further research will be required to identify the context of each.

Known Modulating Factors

Modulating factors (MFs) may alter the shape of the response-response function that describes the quantitative relationship between two KES, thus having an impact on the progression of the pathway or the severity of the AO.The evidence supporting the influence of various modulating factors is assembled within the individual KERs. More help

Quantitative Understanding

Optional field to provide quantitative weight of evidence descriptors.  More help

Considerations for Potential Applications of the AOP (optional)

Addressess potential applications of an AOP to support regulatory decision-making.This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. More help

Because of the long latency of mammary tumors, the two-year rodent carcinogenicity bioassay is the primary assay for the adverse outcome of breast cancer. The assay is included in the OECD Test No. 451 and 453 for carcinogenicity and combined toxicity and carcinogenicity.  Mammary tumors are also reported in short term, sub-chronic, and chronic toxicity tests, but these tests are less sensitive due to their shorter duration.

This AOP is relevant to guideline tests addressing DNA damage and mutation. MIE2: Increase in DNA damage is relevant to OECD Test Nos. 473, 475, 483, 487, and 489, which detect DNA damage in the form of single and double strand breaks, chromosomal damage and micronuclei, as well as some forms of nucleotide damage. KE1: Increase in mutation is relevant to OECD Test Nos. 471, 476, 488, and 490 for in vitro and in vivo mutations. To our knowledge no guideline tests address increases in RONS, proliferation, or inflammation, although some in vitro tests in ToxCast or in development elsewhere may reflect changes in these key events.

References

List of the literature that was cited for this AOP. More help

Ameziane-El-Hassani, R., M. Boufraqech, et al. (2010). "Role of H2O2 in RET/PTC1 chromosomal rearrangement produced by ionizing radiation in human thyroid cells." Cancer Res 70(10): 4123-4132.

Ameziane-El-Hassani, R., M. Talbot, et al. (2015). "NADPH oxidase DUOX1 promotes long-term persistence of oxidative stress after an exposure to irradiation." Proceedings of the National Academy of Sciences of the United States of America 112(16): 5051-5056.

Andarawewa, K. L., S. V. Costes, et al. (2011). "Lack of radiation dose or quality dependence of epithelial-to-mesenchymal transition (EMT) mediated by transforming growth factor beta." International journal of radiation oncology, biology, physics 79(5): 1523-1531.

Andarawewa, K. L., A. C. Erickson, et al. (2007). "Ionizing radiation predisposes nonmalignant human mammary epithelial cells to undergo transforming growth factor beta induced epithelial to mesenchymal transition." Cancer Res 67(18): 8662-8670.

Andrieu, N., D. F. Easton, et al. (2006). "Effect of chest X-rays on the risk of breast cancer among BRCA1/2 mutation carriers in the international BRCA1/2 carrier cohort study: a report from the EMBRACE, GENEPSO, GEO-HEBON, and IBCCS Collaborators' Group." Journal of clinical oncology : official journal of the American Society of Clinical Oncology 24(21): 3361-3366.

Azzam, E. I., S. M. De Toledo, et al. (2002). "Oxidative metabolism modulates signal transduction and micronucleus formation in bystander cells from alpha-particle-irradiated normal human fibroblast cultures." Cancer research 62(19): 5436-5442.

Barcellos-Hoff, M. H. (2013). "Does microenvironment contribute to the etiology of estrogen receptor-negative breast cancer?" Clin Cancer Res 19(3): 541-548.

Barcellos-Hoff, M. H., R. Derynck, et al. (1994). "Transforming growth factor-beta activation in irradiated murine mammary gland." J Clin Invest 93(2): 892-899.

Barcellos-Hoff, M. H. and S. A. Ravani (2000). "Irradiated mammary gland stroma promotes the expression of tumorigenic potential by unirradiated epithelial cells." Cancer Res 60(5): 1254-1260.

Begg, C. B., I. Ostrovnaya, et al. (2016). "Clonal relationships between lobular carcinoma in situ and other breast malignancies." Breast cancer research : BCR 18(1): 66.

Bensimon, J., D. Biard, et al. (2016). "Forced extinction of CD24 stem-like breast cancer marker alone promotes radiation resistance through the control of oxidative stress." Mol Carcinog 55(3): 245-254.

Berdelle, N., T. Nikolova, et al. (2011). "Artesunate induces oxidative DNA damage, sustained DNA double-strand breaks, and the ATM/ATR damage response in cancer cells." Molecular cancer therapeutics 10(12): 2224-2233.

Bernstein, J. L., R. W. Haile, et al. (2010). "Radiation exposure, the ATM Gene, and contralateral breast cancer in the women's environmental cancer and radiation epidemiology study." Journal of the National Cancer Institute 102(7): 475-483.

Bernstein, J. L., D. C. Thomas, et al. (2013). "Contralateral breast cancer after radiotherapy among BRCA1 and BRCA2 mutation carriers: a WECARE study report." European journal of cancer 49(14): 2979-2985.

Berruyer, C., F. M. Martin, et al. (2004). "Vanin-1-/- mice exhibit a glutathione-mediated tissue resistance to oxidative stress." Mol Cell Biol 24(16): 7214-7224.

Beuving, L. J. (1968). "Mammary tumor formation within outgrowths of transplanted hyperplastic nodules from carcinogen-treated rats." Journal of the National Cancer Institute 40(6): 1287-1291.

Beuving, L. J., H. A. Bern, et al. (1967). "Occurrence and Transplantation of Carcinogen-Induced Hyperplastic Nodules in Fischer Rats2." JNCI: Journal of the National Cancer Institute 39(3): 431-447.

Beuving, L. J., J. L. J. Faulkin, et al. (1967). "Hyperplastic Lesions in the Mammary Glands of Sprague-Dawley Rats After 7,12-Dimethylbenz[a]anthracene Treatment2." JNCI: Journal of the National Cancer Institute 39(3): 423-429.

Bisht, K. S., C. M. Bradbury, et al. (2003). "Inhibition of cyclooxygenase-2 with NS-398 and the prevention of radiation-induced transformation, micronuclei formation and clonogenic cell death in C3H 10T1/2 cells." Int J Radiat Biol 79(11): 879-888.

Bock, F. J., G. Krumschnabel, et al. (2013). "Loss of PIDD limits NF-kappaB activation and cytokine production but not cell survival or transformation after DNA damage." Cell death and differentiation 20(4): 546-557.

Bouchard, G., G. Bouvette, et al. (2013). "Pre-irradiation of mouse mammary gland stimulates cancer cell migration and development of lung metastases." British journal of cancer 109(7): 1829-1838.

Broeks, A., L. M. Braaf, et al. (2007). "Identification of women with an increased risk of developing radiation-induced breast cancer: a case only study." Breast cancer research : BCR 9(2): R26.

Broerse, J. J., L. A. Hennen, et al. (1987). "Mammary carcinogenesis in different rat strains after irradiation and hormone administration." Int J Radiat Biol Relat Stud Phys Chem Med 51(6): 1091-1100.

Brooks, J. D., S. N. Teraoka, et al. (2012). "Variants in activators and downstream targets of ATM, radiation exposure, and contralateral breast cancer risk in the WECARE study." Human mutation 33(1): 158-164.

Buonanno, M., S. M. de Toledo, et al. (2011). "Long-term consequences of radiation-induced bystander effects depend on radiation quality and dose and correlate with oxidative stress." Radiation research 175(4): 405-415.

Burr, K. L., J. I. Robinson, et al. (2010). "Radiation-induced delayed bystander-type effects mediated by hemopoietic cells." Radiation research 173(6): 760-768.

CGAN (Cancer Genome Atlas Network) (2012). "Comprehensive molecular portraits of human breast tumours." Nature 490(7418): 61-70.

Chai, Y., G. M. Calaf, et al. (2013). "Radiation induced COX-2 expression and mutagenesis at non-targeted lung tissues of gpt delta transgenic mice." British journal of cancer 108(1): 91-98.

Chai, Y., R. K. Lam, et al. (2013). "Radiation-induced non-targeted response in vivo: role of the TGFbeta-TGFBR1-COX-2 signalling pathway." Br J Cancer 108(5): 1106-1112.

Choi, K. M., C. M. Kang, et al. (2007). "Ionizing radiation-induced micronucleus formation is mediated by reactive oxygen species that are produced in a manner dependent on mitochondria, Nox1, and JNK." Oncol Rep 17(5): 1183-1188.

Clifton, K. H., J. Yasukawa-Barnes, et al. (1985). "Irradiation and prolactin effects on rat mammary carcinogenesis: intrasplenic pituitary and estrone capsule implants." Journal of the National Cancer Institute 75(1): 167-175.

Coates, P. J., J. K. Rundle, et al. (2008). "Indirect macrophage responses to ionizing radiation: implications for genotype-dependent bystander signaling." Cancer Res 68(2): 450-456.

Connelly, L., W. Barham, et al. (2011). "Inhibition of NF-kappa B activity in mammary epithelium increases tumor latency and decreases tumor burden." Oncogene 30(12): 1402-1412.

Cronkite, E. P., C. J. Shellabarger, et al. (1960). "Studies on radiation-induced mammary gland neoplasia in the rat. I. The role of the ovary in the neoplastic response of the breast tissue to total- or partial-body x-irradiation." Radiation research 12: 81-93.

Dahm-Daphi, J., C. Sass, et al. (2000). "Comparison of biological effects of DNA damage induced by ionizing radiation and hydrogen peroxide in CHO cells." International journal of radiation biology 76(1): 67-75.

Daino, K., M. Nishimura, et al. (2018). "Epigenetic dysregulation of key developmental genes in radiation-induced rat mammary carcinomas." Int J Cancer 143(2): 343-354.

Das, U., K. Manna, et al. (2014). "Role of ferulic acid in the amelioration of ionizing radiation induced inflammation: a murine model." PLoS One 9(5): e97599.

Datta, K., D. R. Hyduke, et al. (2012). "Exposure to ionizing radiation induced persistent gene expression changes in mouse mammary gland." Radiat Oncol 7: 205.

Dayal, D., S. M. Martin, et al. (2008). "Hydrogen peroxide mediates the radiation-induced mutator phenotype in mammalian cells." Biochem J 413(1): 185-191.

Dayal, D., S. M. Martin, et al. (2009). "Mitochondrial complex II dysfunction can contribute significantly to genomic instability after exposure to ionizing radiation." Radiation research 172(6): 737-745.

de Ostrovich, K. K., I. Lambertz, et al. (2008). "Paracrine overexpression of insulin-like growth factor-1 enhances mammary tumorigenesis in vivo." The American journal of pathology 173(3): 824-834.

Denissova, N. G., C. M. Nasello, et al. (2012). "Resveratrol protects mouse embryonic stem cells from ionizing radiation by accelerating recovery from DNA strand breakage." Carcinogenesis 33(1): 149-155.

Deome, K. B., L. J. Faulkin, Jr., et al. (1959). "Development of mammary tumors from hyperplastic alveolar nodules transplanted into gland-free mammary fat pads of female C3H mice." Cancer Res 19(5): 515-520.

Dickey, J. S., B. J. Baird, et al. (2012). "Susceptibility to bystander DNA damage is influenced by replication and transcriptional activity." Nucleic acids research 40(20): 10274-10286.

Dickey, J. S., B. J. Baird, et al. (2009). "Intercellular communication of cellular stress monitored by gamma-H2AX induction." Carcinogenesis 30(10): 1686-1695.

Douki, T., J. L. Ravanat, et al. (2006). "Minor contribution of direct ionization to DNA base damage inducedby heavy ions." International journal of radiation biology 82(2): 119-127.

Driessens, N., S. Versteyhe, et al. (2009). "Hydrogen peroxide induces DNA single- and double-strand breaks in thyroid cells and is therefore a potential mutagen for this organ." Endocrine-related cancer 16(3): 845-856.

Ehrhart, E. J., P. Segarini, et al. (1997). "Latent transforming growth factor beta1 activation in situ: quantitative and functional evidence after low-dose gamma-irradiation." FASEB journal : official publication of the Federation of American Societies for Experimental Biology 11(12): 991-1002.

Elahi, E., N. Suraweera, et al. (2009). "Five quantitative trait loci control radiation-induced adenoma multiplicity in Mom1R Apc Min/+ mice." PLoS One 4(2): e4388.

Esquivel-Velazquez, M., P. Ostoa-Saloma, et al. (2015). "The role of cytokines in breast cancer development and progression." J Interferon Cytokine Res 35(1): 1-16.

Ewan, K. B., R. L. Henshall-Powell, et al. (2002). "Transforming growth factor-beta1 mediates cellular response to DNA damage in situ." Cancer Res 62(20): 5627-5631.

Faulkin, J. L. J., C. J. Shellabarger, et al. (1967). "Hyperplastic Lesions of Sprague-Dawley Rat Mammary Glands After X Irradiation2." JNCI: Journal of the National Cancer Institute 39(3): 449-459.

Fibach, E. and E. A. Rachmilewitz (2015). "The Effect of Fermented Papaya Preparation on Radioactive Exposure." Radiation research 184(3): 304-313.

Fogarty, C. E. and A. Bergmann (2017). "Killers creating new life: caspases drive apoptosis-induced proliferation in tissue repair and disease." Cell death and differentiation 24(8): 1390-1400.

Francis, S. M., J. Bergsied, et al. (2009). "A functional connection between pRB and transforming growth factor beta in growth inhibition and mammary gland development." Molecular and cellular biology 29(16): 4455-4466.

Francis, S. M., S. Chakrabarti, et al. (2011). "A context-specific role for retinoblastoma protein-dependent negative growth control in suppressing mammary tumorigenesis." PLoS One 6(2): e16434.

Garraway, L. A. and E. S. Lander (2013). "Lessons from the cancer genome." Cell 153(1): 17-37.

Gomes, L. R., L. F. Terra, et al. (2012). "TGF-beta1 modulates the homeostasis between MMPs and MMP inhibitors through p38 MAPK and ERK1/2 in highly invasive breast cancer cells." BMC Cancer 12: 26.

Gradzka, I. and T. Iwanenko (2005). "A non-radioactive, PFGE-based assay for low levels of DNA double-strand breaks in mammalian cells." DNA repair 4(10): 1129-1139.

Greenman, C., P. Stephens, et al. (2007). "Patterns of somatic mutation in human cancer genomes." Nature 446(7132): 153-158.

Gustin, J. P., B. Karakas, et al. (2009). "Knockin of mutant PIK3CA activates multiple oncogenic pathways." Proceedings of the National Academy of Sciences of the United States of America 106(8): 2835-2840.

Haag, J. D., L. C. Hsu, et al. (1996). "Allelic imbalance in mammary carcinomas induced by either 7,12-dimethylbenz[a]anthracene or ionizing radiation in rats carrying genes conferring differential susceptibilities to mammary carcinogenesis." Mol Carcinog 17(3): 134-143.

Haddadi, G. H., A. Rezaeyan, et al. (2017). "Hesperidin as Radioprotector against Radiation-induced Lung Damage in Rat: A Histopathological Study." J Med Phys 42(1): 25-32.

Haegele, A. D., P. Wolfe, et al. (1998). "X-radiation induces 8-hydroxy-2'-deoxyguanosine formation in vivo in rat mammary gland DNA." Carcinogenesis 19(7): 1319-1321.

Han, W., S. Chen, et al. (2010). "Nitric oxide mediated DNA double strand breaks induced in proliferating bystander cells after alpha-particle irradiation." Mutation research 684(1-2): 81-89.

Hanahan, D. and R. A. Weinberg (2011). "Hallmarks of cancer: the next generation." Cell 144(5): 646-674.

Haslam, S. Z. and H. A. Bern (1977). "Histopathogenesis of 7,12-diemthylbenz(a)anthracene-induced rat mammary tumors." Proceedings of the National Academy of Sciences of the United States of America 74(9): 4020-4024.

Hernandez, L., M. Terradas, et al. (2013). "Increased mammogram-induced DNA damage in mammary epithelial cells aged in vitro." PLoS One 8(5): e63052.

Holtzman, S., J. P. Stone, et al. (1979). "Synergism of diethylstilbestrol and radiation in mammary carcinogenesis in female F344 rats." Journal of the National Cancer Institute 63(4): 1071-1074.

Holtzman, S., J. P. Stone, et al. (1981). "Synergism of estrogens and X-rays in mammary carcinogenesis in female ACI rats." Journal of the National Cancer Institute 67(2): 455-459.

Iizuka, D., M. Sasatani, et al. (2017). "Hydrogen Peroxide Enhances TGFbeta-mediated Epithelial-to-Mesenchymal Transition in Human Mammary Epithelial MCF-10A Cells." Anticancer Res 37(3): 987-995.

Iliopoulos, D., H. A. Hirsch, et al. (2009). "An epigenetic switch involving NF-kappaB, Lin28, Let-7 MicroRNA, and IL6 links inflammation to cell transformation." Cell 139(4): 693-706.

Iliopoulos, D., H. A. Hirsch, et al. (2011). "Inducible formation of breast cancer stem cells and their dynamic equilibrium with non-stem cancer cells via IL6 secretion." Proceedings of the National Academy of Sciences of the United States of America 108(4): 1397-1402.

Iliopoulos, D., S. A. Jaeger, et al. (2010). "STAT3 activation of miR-21 and miR-181b-1 via PTEN and CYLD are part of the epigenetic switch linking inflammation to cancer." Mol Cell 39(4): 493-506.

Illa-Bochaca, I., H. Ouyang, et al. (2014). "Densely ionizing radiation acts via the microenvironment to promote aggressive Trp53-null mammary carcinomas." Cancer Res 74(23): 7137-7148.

Imaoka, T., M. Nishimura, et al. (2006). "Persistent cell proliferation of terminal end buds precedes radiation-induced rat mammary carcinogenesis." In Vivo 20(3): 353-358.

Inano, H., K. Suzuki, et al. (1991). "Pregnancy-dependent initiation in tumorigenesis of Wistar rat mammary glands by 60Co-irradiation." Carcinogenesis 12(6): 1085-1090.

Inano, H., K. Suzuki, et al. (1996). "Relationship between induction of mammary tumors and change of testicular functions in male rats following gamma-ray irradiation and/or diethylstilbestrol." Carcinogenesis 17(2): 355-360.

Ismail, I. H., S. Nystrom, et al. (2005). "Activation of ataxia telangiectasia mutated by DNA strand break-inducing agents correlates closely with the number of DNA double strand breaks." J Biol Chem 280(6): 4649-4655.

Jackson, J. H., E. Gajewski, et al. (1989). "Damage to the bases in DNA induced by stimulated human neutrophils." J Clin Invest 84(5): 1644-1649.

Jones, J. A., P. K. Riggs, et al. (2007). "Ionizing radiation-induced bioeffects in space and strategies to reduce cellular injury and carcinogenesis." Aviat Space Environ Med 78(4 Suppl): A67-78.

Kim, E. S., M. S. Kim, et al. (2004). "TGF-beta-induced upregulation of MMP-2 and MMP-9 depends on p38 MAPK, but not ERK signaling in MCF10A human breast epithelial cells." Int J Oncol 25(5): 1375-1382.

Koh, T. J. and L. A. DiPietro (2011). "Inflammation and wound healing: the role of the macrophage." Expert Rev Mol Med 13: e23.

Korkaya, H., A. Paulson, et al. (2009). "Regulation of mammary stem/progenitor cells by PTEN/Akt/beta-catenin signaling." PLoS biology 7(6): e1000121.

Korkola, J. E. and M. C. Archer (1999). "Resistance to mammary tumorigenesis in Copenhagen rats is associated with the loss of preneoplastic lesions." Carcinogenesis 20(2): 221-227.

Kutanzi, K. R., I. Koturbash, et al. (2010). "Imbalance between apoptosis and cell proliferation during early stages of mammary gland carcinogenesis in ACI rats." Mutation research 694(1-2): 1-6.

Landen, N. X., D. Li, et al. (2016). "Transition from inflammation to proliferation: a critical step during wound healing." Cellular and molecular life sciences : CMLS 73(20): 3861-3885.

Lemon, H. M., P. F. Kumar, et al. (1989). "Inhibition of radiogenic mammary carcinoma in rats by estriol or tamoxifen." Cancer 63(9): 1685-1692.

Liakou, E., E. Mavrogonatou, et al. (2016). "Ionizing radiation-mediated premature senescence and paracrine interactions with cancer cells enhance the expression of syndecan 1 in human breast stromal fibroblasts: the role of TGF-beta." Aging (Albany NY) 8(8): 1650-1669.

Little, M. P. (2009). "Heterogeneity of variation of relative risk by age at exposure in the Japanese atomic bomb survivors." Radiation and environmental biophysics 48(3): 253-262.

Lorat, Y., C. U. Brunner, et al. (2015). "Nanoscale analysis of clustered DNA damage after high-LET irradiation by quantitative electron microscopy--the heavy burden to repair." DNA repair 28: 93-106.

Loree, J., I. Koturbash, et al. (2006). "Radiation-induced molecular changes in rat mammary tissue: possible implications for radiation-induced carcinogenesis." International journal of radiation biology 82(11): 805-815.

Lorimore, S. A., J. A. Chrystal, et al. (2008). "Chromosomal instability in unirradiated hemaopoietic cells induced by macrophages exposed in vivo to ionizing radiation." Cancer Res 68(19): 8122-8126.

Lorimore, S. A., D. Mukherjee, et al. (2011). "Long-lived inflammatory signaling in irradiated bone marrow is genome dependent." Cancer Res 71(20): 6485-6491.

Lorimore, S. A., S. Rastogi, et al. (2013). "The influence of p53 functions on radiation-induced inflammatory bystander-type signaling in murine bone marrow." Radiation research 179(4): 406-415.

Lorimore, S. A. and E. G. Wright (2003). "Radiation-induced genomic instability and bystander effects: related inflammatory-type responses to radiation-induced stress and injury? A review." Int J Radiat Biol 79(1): 15-25.

Lumniczky, K. and G. Safrany (2015). "The impact of radiation therapy on the antitumor immunity: local effects and systemic consequences." Cancer Lett 356(1): 114-125.

Luo, M., H. Fan, et al. (2009). "Mammary epithelial-specific ablation of the focal adhesion kinase suppresses mammary tumorigenesis by affecting mammary cancer stem/progenitor cells." Cancer research 69(2): 466-474.

Maffini, M. V., J. M. Calabro, et al. (2005). "Stromal regulation of neoplastic development: age-dependent normalization of neoplastic mammary cells by mammary stroma." Am J Pathol 167(5): 1405-1410.

Maffini, M. V., A. M. Soto, et al. (2004). "The stroma as a crucial target in rat mammary gland carcinogenesis." J Cell Sci 117(Pt 8): 1495-1502.

Manna, K., U. Das, et al. (2015). "Naringin inhibits gamma radiation-induced oxidative DNA damage and inflammation, by modulating p53 and NF-kappaB signaling pathways in murine splenocytes." Free Radic Res 49(4): 422-439.

Martins, C. P., L. Brown-Swigart, et al. (2006). "Modeling the therapeutic efficacy of p53 restoration in tumors." Cell 127(7): 1323-1334.

Marusyk, A., D. P. Tabassum, et al. (2014). "Non-cell-autonomous driving of tumour growth supports sub-clonal heterogeneity." Nature 514(7520): 54-58.

Maxwell, C. A., M. C. Fleisch, et al. (2008). "Targeted and nontargeted effects of ionizing radiation that impact genomic instability." Cancer Res 68(20): 8304-8311.

McDaniel, S. M., K. K. Rumer, et al. (2006). "Remodeling of the mammary microenvironment after lactation promotes breast tumor cell metastasis." Am J Pathol 168(2): 608-620.

Medina, D. and H. J. Thompson (2000). A Comparison of the Salient Features of Mouse, Rat, and Human Mammary Tumorigenesis. Methods in Mammary Gland Biology and Breast Cancer Research. M. M. Ip and B. B. Asch. Boston, MA, Springer US: 31-36.

Millikan, R. C., J. S. Player, et al. (2005). "Polymorphisms in DNA repair genes, medical exposure to ionizing radiation, and breast cancer risk." Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology 14(10): 2326-2334.

Moore, S. R., S. Marsden, et al. (2005). "Genomic instability in human lymphocytes irradiated with individual charged particles: involvement of tumor necrosis factor alpha in irradiated cells but not bystander cells." Radiation research 163(2): 183-190.

Mukherjee, D., P. J. Coates, et al. (2014). "Responses to ionizing radiation mediated by inflammatory mechanisms." J Pathol 232(3): 289-299.

Mukhopadhyay, R., S. V. Costes, et al. (2010). "Promotion of variant human mammary epithelial cell outgrowth by ionizing radiation: an agent-based model supported by in vitro studies." Breast cancer research : BCR 12(1): R11.

Nakamura, J., E. R. Purvis, et al. (2003). "Micromolar concentrations of hydrogen peroxide induce oxidative DNA lesions more efficiently than millimolar concentrations in mammalian cells." Nucleic acids research 31(6): 1790-1795.

Natarajan, M., C. F. Gibbons, et al. (2007). "Oxidative stress signalling: a potential mediator of tumour necrosis factor alpha-induced genomic instability in primary vascular endothelial cells." Br J Radiol 80 Spec No 1: S13-22.

Nguyen, D. H., E. Fredlund, et al. (2013). "Murine microenvironment metaprofiles associate with human cancer etiology and intrinsic subtypes." Clin Cancer Res 19(6): 1353-1362.

Nguyen, D. H., H. Martinez-Ruiz, et al. (2011). "Consequences of epithelial or stromal TGFbeta1 depletion in the mammary gland." J Mammary Gland Biol Neoplasia 16(2): 147-155.

Nguyen, D. H., H. A. Oketch-Rabah, et al. (2011). "Radiation acts on the microenvironment to affect breast carcinogenesis by distinct mechanisms that decrease cancer latency and affect tumor type." Cancer Cell 19(5): 640-651.

Oya, Y., K. Yamamoto, et al. (1986). "The biological activity of hydrogen peroxide. I. Induction of chromosome-type aberrations susceptible to inhibition by scavengers of hydroxyl radicals in human embryonic fibroblasts." Mutation research 172(3): 245-253.

Ozyurt, H., O. Cevik, et al. (2014). "Quercetin protects radiation-induced DNA damage and apoptosis in kidney and bladder tissues of rats." Free Radic Res 48(10): 1247-1255.

Padula, G., M. V. Ponzinibbio, et al. (2016). "Possible radioprotective effect of folic acid supplementation on low dose ionizing radiation-induced genomic instability in vitro." Indian J Exp Biol 54(8): 537-543.

Park, C. C., R. L. Henshall-Powell, et al. (2003). "Ionizing radiation induces heritable disruption of epithelial cell interactions." Proc Natl Acad Sci U S A 100(19): 10728-10733.

Pazhanisamy, S. K., H. Li, et al. (2011). "NADPH oxidase inhibition attenuates total body irradiation-induced haematopoietic genomic instability." Mutagenesis 26(3): 431-435.

Perez, D. A., J. P. Vago, et al. (2014). "Switching off key signaling survival molecules to switch on the resolution of inflammation." Mediators Inflamm 2014: 829851.

Perrott, K. M., C. D. Wiley, et al. (2017). "Apigenin suppresses the senescence-associated secretory phenotype and paracrine effects on breast cancer cells." Geroscience 39(2): 161-173.

Peterson, N. C., M. D. Servinsky, et al. (2005). "Tamoxifen resistance and Her2/neu expression in an aged, irradiated rat breast carcinoma model." Carcinogenesis 26(9): 1542-1552.

Pijpe, A., N. Andrieu, et al. (2012). "Exposure to diagnostic radiation and risk of breast cancer among carriers of BRCA1/2 mutations: retrospective cohort study (GENE-RAD-RISK)." BMJ 345: e5660.

Podsypanina, K., K. Politi, et al. (2008). "Oncogene cooperation in tumor maintenance and tumor recurrence in mouse mammary tumors induced by Myc and mutant Kras." Proceedings of the National Academy of Sciences of the United States of America 105(13): 5242-5247.

Poirier, M. C. and F. A. Beland (1994). "DNA adduct measurements and tumor incidence during chronic carcinogen exposure in rodents." Environmental health perspectives 102 Suppl 6: 161-165.

Purnell, D. M. (1980). "The relationship of terminal duct hyperplasia to mammary carcinoma in 7,12-dimethylbenz(alpha)anthracene-treated LEW/Mai rats." The American journal of pathology 98(2): 311-324.

Radice, G. L., M. C. Ferreira-Cornwell, et al. (1997). "Precocious mammary gland development in P-cadherin-deficient mice." The Journal of cell biology 139(4): 1025-1032.

Rastogi, S., P. J. Coates, et al. (2012). "Bystander-type effects mediated by long-lived inflammatory signaling in irradiated bone marrow." Radiation research 177(3): 244-250.

Rivera, E. M., S. D. Hill, et al. (1981). "Organ culture passage enhances the oncogenicity of carcinogen-induced hyperplastic mammary nodules." In vitro 17(2): 159-166.

Russo, I. H. and J. Russo (1996). "Mammary gland neoplasia in long-term rodent studies." Environmental health perspectives 104(9): 938-967.

Russo, J., J. Saby, et al. (1977). "Pathogenesis of Mammary Carcinomas Induced in Rats by 7, 12-Dimethylbenz[a]anthracene2." JNCI: Journal of the National Cancer Institute 59(2): 435-445.

Sandhu, J. K. and H. C. Birnboim (1997). "Mutagenicity and cytotoxicity of reactive oxygen and nitrogen species in the MN-11 murine tumor cell line." Mutation research 379(2): 241-252.

Sansone, P., G. Storci, et al. (2007). "IL-6 triggers malignant features in mammospheres from human ductal breast carcinoma and normal mammary gland." J Clin Invest 117(12): 3988-4002.

Sasser, A. K., N. J. Sullivan, et al. (2007). "Interleukin-6 is a potent growth factor for ER-alpha-positive human breast cancer." FASEB journal : official publication of the Federation of American Societies for Experimental Biology 21(13): 3763-3770.

Schonfeld, S. J., P. Bhatti, et al. (2010). "Polymorphisms in oxidative stress and inflammation pathway genes, low-dose ionizing radiation, and the risk of breast cancer among US radiologic technologists." Cancer Causes Control 21(11): 1857-1866.

Schreiber, R. D., L. J. Old, et al. (2011). "Cancer immunoediting: integrating immunity's roles in cancer suppression and promotion." Science 331(6024): 1565-1570.

Seager, A. L., U. K. Shah, et al. (2012). "Pro-oxidant induced DNA damage in human lymphoblastoid cells: homeostatic mechanisms of genotoxic tolerance." Toxicological sciences : an official journal of the Society of Toxicology 128(2): 387-397.

Segaloff, A. and W. S. Maxfield (1971). "The synergism between radiation and estrogen in the production of mammary cancer in the rat." Cancer Res 31(2): 166-168.

Shao, C., M. Folkard, et al. (2008). "Role of TGF-beta1 and nitric oxide in the bystander response of irradiated glioma cells." Oncogene 27(4): 434-440.

Sharma, V., L. B. Collins, et al. (2016). "Oxidative stress at low levels can induce clustered DNA lesions leading to NHEJ mediated mutations." Oncotarget 7(18): 25377-25390.

Shellabarger, C. J., J. P. Stone, et al. (1976). "Synergism between neutron radiation and diethylstilbestrol in the production of mammary adenocarcinomas in the rat." Cancer research 36(3): 1019-1022.

Sinha, D. and T. L. Dao (1974). "A Direct Mechanism of Mammary Carcinogenesis Induced by 7,12-Dimethylbenz[a]anthracene2." JNCI: Journal of the National Cancer Institute 53(3): 841-846.

Sinha, D. and T. L. Dao (1977). "Hyperplastic alveolar nodules of the rat mammary gland: tumor-producing capability in vivo and in vitro." Cancer letters 2(3): 153-160.

Snijders, A. M., F. Marchetti, et al. (2012). "Genetic differences in transcript responses to low-dose ionizing radiation identify tissue functions associated with breast cancer susceptibility." PLoS One 7(10): e45394.

Soler, D., J. Pampalona, et al. (2009). "Radiation sensitivity increases with proliferation-associated telomere dysfunction in nontransformed human epithelial cells." Aging Cell 8(4): 414-425.

Solleveld, H. A., M. J. van Zwieten, et al. (1986). "Effects of X-irradiation, ovariohysterectomy and estradiol-17 beta on incidence, benign/malignant ratio and multiplicity of rat mammary neoplasms--a preliminary report." Leuk Res 10(7): 755-759.

Sourisseau, T., K. J. Harrington, et al. (2011). "Changes in tumor tissue organization in collagen-I sensitize cells to ionizing radiation in an ex vivo model of solid mammary tumor growth and local invasion." Cell Cycle 10(22): 3979-3981.

Sprung, C. N., A. Ivashkevich, et al. (2015). "Oxidative DNA damage caused by inflammation may link to stress-induced non-targeted effects." Cancer Lett 356(1): 72-81.

Stanicka, J., E. G. Russell, et al. (2015). "NADPH oxidase-generated hydrogen peroxide induces DNA damage in mutant FLT3-expressing leukemia cells." The Journal of biological chemistry 290(15): 9348-9361.

Stevens, C. R., N. Bucurenci, et al. (1992). "Application of methionine as a detector molecule for the assessment of oxygen radical generation by human neutrophils and endothelial cells." Free Radic Res Commun 17(2): 143-154.

Stratton, M. R., P. J. Campbell, et al. (2009). "The cancer genome." Nature 458(7239): 719-724.

Studebaker, A. W., G. Storci, et al. (2008). "Fibroblasts isolated from common sites of breast cancer metastasis enhance cancer cell growth rates and invasiveness in an interleukin-6-dependent manner." Cancer Res 68(21): 9087-9095.

Suman, S., M. D. Johnson, et al. (2012). "Exposure to ionizing radiation causes long-term increase in serum estradiol and activation of PI3K-Akt signaling pathway in mouse mammary gland." International journal of radiation oncology, biology, physics 84(2): 500-507.

Tang, J., I. Fernandez-Garcia, et al. (2014). "Irradiation of juvenile, but not adult, mammary gland increases stem cell self-renewal and estrogen receptor negative tumors." Stem Cells 32(3): 649-661.

Tao, L., D. Xiang, et al. (2017). "Induced p53 loss in mouse luminal cells causes clonal expansion and development of mammary tumours." Nat Commun 8: 14431.

Tsai, K. K., E. Y. Chuang, et al. (2005). "Cellular mechanisms for low-dose ionizing radiation-induced perturbation of the breast tissue microenvironment." Cancer Res 65(15): 6734-6744.

Ullrich, R. L. and R. J. Preston (1991). "Radiation induced mammary cancer." Journal of radiation research 32 Suppl 2: 104-109.

Umesako, S., K. Fujisawa, et al. (2005). "Atm heterozygous deficiency enhances development of mammary carcinomas in p53 heterozygous knockout mice." Breast cancer research : BCR 7(1): R164-170.

Unger, K., J. Wienberg, et al. (2010). "Novel gene rearrangements in transformed breast cells identified by high-resolution breakpoint analysis of chromosomal aberrations." Endocrine-related cancer 17(1): 87-98.

Vandin, F., E. Upfal, et al. (2012). "De novo discovery of mutated driver pathways in cancer." Genome research 22(2): 375-385.

Vogelstein, B., N. Papadopoulos, et al. (2013). "Cancer genome landscapes." Science 339(6127): 1546-1558.

Wang, J., F. Su, et al. (2011). "Mechanisms of increased risk of tumorigenesis in Atm and Brca1 double heterozygosity." Radiat Oncol 6: 96.

Wang, T. J., C. C. Wu, et al. (2015). "Induction of Non-Targeted Stress Responses in Mammary Tissues by Heavy Ions." PLoS One 10(8): e0136307.

Wang, Y., J. Waters, et al. (2014). "Clonal evolution in breast cancer revealed by single nucleus genome sequencing." Nature 512(7513): 155-160.

Welsch, C. W., M. Goodrich-Smith, et al. (1981). "Effect of an estrogen antagonist (tamoxifen) on the initiation and progression of gamma-irradiation-induced mammary tumors in female Sprague-Dawley rats." European journal of cancer & clinical oncology 17(12): 1255-1258.

Winyard, P. G., S. P. Faux, et al. (1992). "Bleomycin-induced unscheduled DNA synthesis in non-permeabilized human and rat hepatocytes is not paralleled by 8-oxo-7,8-dihydrodeoxyguanosine formation." Biochem Pharmacol 44(7): 1255-1260.

Yang, H., V. Anzenberg, et al. (2007). "The time dependence of bystander responses induced by iron-ion radiation in normal human skin fibroblasts." Radiation research 168(3): 292-298.

Yang, H., N. Asaad, et al. (2005). "Medium-mediated intercellular communication is involved in bystander responses of X-ray-irradiated normal human fibroblasts." Oncogene 24(12): 2096-2103.

Yang, T.-H., L. M. Craise, et al. (1992). "Chromosomal changes in cultured human epithelial cells transformed by low- and high-LET radiation." Adv Space Res 12(2-3): 127-136.

Yang, T. C., K. A. Georgy, et al. (1997). "Initiation of oncogenic transformation in human mammary epithelial cells by charged particles." Radiat Oncol Investig 5(3): 134-138.

Yang, X. R., J. K. Killian, et al. (2015). "Characterization of genomic alterations in radiation-associated breast cancer among childhood cancer survivors, using comparative genomic hybridization (CGH) arrays." PLoS One 10(3): e0116078.

Yates, L. R., M. Gerstung, et al. (2015). "Subclonal diversification of primary breast cancer revealed by multiregion sequencing." Nat Med 21(7): 751-759.

Yu, Y., R. Okayasu, et al. (2001). "Elevated breast cancer risk in irradiated BALB/c mice associates with unique functional polymorphism of the Prkdc (DNA-dependent protein kinase catalytic subunit) gene." Cancer Res 61(5): 1820-1824.

Zhang, P., A. Lo, et al. (2015). "Identification of genetic loci that control mammary tumor susceptibility through the host microenvironment." Sci Rep 5: 8919.

Zhang, Q., L. Zhu, et al. (2017). "Ionizing radiation promotes CCL27 secretion from keratinocytes through the cross talk between TNF-alpha and ROS." J Biochem Mol Toxicol 31(3).

Zhou, H., V. N. Ivanov, et al. (2005). "Mechanism of radiation-induced bystander effect: role of the cyclooxygenase-2 signaling pathway." Proceedings of the National Academy of Sciences of the United States of America 102(41): 14641-14646.

Zhou, H., V. N. Ivanov, et al. (2008). "Mitochondrial function and nuclear factor-kappaB-mediated signaling in radiation-induced bystander effects." Cancer Res 68(7): 2233-2240.