To the extent possible under law, AOP-Wiki has waived all copyright and related or neighboring rights to KE:850

Event: 850

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Induction, CYP1A2/CYP1A5

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Induction, CYP1A2/CYP1A5

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help
Level of Biological Organization

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help
Process Object Action
gene expression cytochrome P450 1A2 increased
gene expression cytochrome P450 1A5 (chicken) increased

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
AHR activation-uroporphyria KeyEvent Allie Always (send email) Open for citation & comment WPHA/WNT Endorsed


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. 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 in relation to this KE. More help
Term Scientific Term Evidence Link
human Homo sapiens High NCBI
mouse Mus musculus High NCBI
rat Rattus norvegicus High NCBI
chicken Gallus gallus High NCBI
zebrafish Danio rerio High NCBI
Haliaeetus leucocephalus Haliaeetus leucocephalus High NCBI
Ardea herodias Ardea herodias High NCBI
Double-crested cormorant Double-crested cormorant High NCBI
Nycticorax nycticorax Nycticorax nycticorax High NCBI
osprey Pandion haliaetus High NCBI

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help
Life stage Evidence
All life stages High

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help
Term Evidence
Unspecific High

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

The Cyp1A2/Cyp1A5 gene encodes a member of the cytochrome P450 superfamily of enzymes. The cytochrome P450 proteins are monooxygenases which catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids. The protein encoded by this gene localizes to the endoplasmic reticulum and its expression is induced by some polycyclic aromatic hydrocarbons (PAHs), some of which are found in cigarette smoke. The enzyme's endogenous substrate is unknown; however, it is able to metabolize some PAHs to carcinogenic intermediates. Other xenobiotic substrates for this enzyme include caffeine, aflatoxin B1, and acetaminophen. [4]

The CYP1A subfamily of enzymes is very well studied and is often used as a biomarker of Dioxin-like compound (DLC) exposure and toxicity[5][6][7][8]. CYP1A5 is the avian isoform and is orthologous to the mammalian CYP1A2[9]. CYP1A5 is expressed in avian heart, liver and kidney tissues[10][11], and has been measured in avian hepatocyte and cardiomyocyte cultures[12][13][10][14]. Mouse CYP1A2 is only constitutively expressed in the liver, but is inducible in the liver, lung, and duodenum[20].

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?

Enzyme activity

There are a number of substrates that are preferentially metabolized by Cyp1A2 and CYP1A5 allowing for CYP1A activity to be measured as a function metabolite formation. Methoxyresorufin O-demethylation (MROD) is a classic marker of Cyp1A2/5 activity[21] and is often used due to the ease of fluorometric techniques; however, Burke et al.[21] suggest that a ratio of MROD to ethoxyresorufin O-demethylation (EROD) is a better measure of CYP1A2 activity due to the contribution of CYP1A1 to MROD. CYP1A2/5 activity can also be measured as the metabolic rate of arachidonic acid[11], oroporphyrinogen[22], acetanilide 4-hydroxylase and caffeine[23]. Caffeine metabolism has been used in clinical studies as a biomarker for CYP1A2 activity in humans[24].

Quantitative polymerase chain reaction (QPCR)

Levels of CYP1A2/5 messenger RNA can be measured using QPCR. This technique monitors the amplification of a targeted gene during PCR as accumulative fluorescence [25]. For example, Head and Kennedy[26] developed a multiplex QPCR assay utilizing dual-labeled fluorescent probes to measure CYP1A4 and CYP1A5 mRNA levels simultaneously from samples already analyzed for EROD activity. QPCR has high throughput capability and a low detection limit relative to other methods.

Luciferase reporter gene (LRG) assay

An LRG assay can be used to measure AHR1-mediated transactivation of a target gene. This assay is particularly useful as it can measures CYP1A4/5 induction exclusively caused by activation of the AHR, through which many DLCs exert their toxic effects. This assay is easily modified to measure AHR1-mediated transactivation in various species, simply by transfecting the desired AHR cDNA clone and reporter gene construct (containing the appropriate reporter gene) into the chosen cell line. This has been demonstrated to be an efficient high throughput method in various avian and mammalian studies.[27][28][29]


The European Union Reference Laboratory for Alternatives to Animal Testing (EURL-ECVAM) is working on a human hepatic in vitro metabolically competent test systems to evaluate CYPs induction. Cryopreserved human HepaRG® or cryopreserved human primary hepatocytes are incubated in presence of a potential CYP1A2 inducer and the identity and abundance of CYP1A2 product is evaluated using analytical HPLC (High Performance Liquid Chromatography) coupled with mass spectrometry (MS). HPLC is applied for concentration and purification of the product to be detected, whereas MS is applied for its specific quantification [31].

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help

CYP1A expression has been measured in chicken[11] as well as in wild bird species, including bald eagles (Haliaeetus leucocephalus)[15], great blue herons (Ardea herodias)[16], double-crested cormorants (Phalacrocorax auritus)[17], black-crowned night herons (Nycticorax nycticorax)[18] and ospreys (Pandion haliaetus)[19]. It's also been measured in a number of mammalian and piscine species including humans, rats[21], mice[20] and zebrafish[30].


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help
  1. 1.0 1.1 Fujii-Kuriyama, Y., and Kawajiri, K. (2010). Molecular mechanisms of the physiological functions of the aryl hydrocarbon (dioxin) receptor, a multifunctional regulator that senses and responds to environmental stimuli. Proc.Jpn.Acad.Ser.B Phys.Biol.Sci. 86, 40-53.
  2. Giesy, J. P., Kannan, K., Blankenship, A. L., Jones, P. D., and Newsted, J. L. (2006). Toxicology of PCBs and related compounds. In Endocrine Disruption Biological Bases for Health Effects in Wildlife and Humans (D. O. Norris, and J. A. Carr, Eds.), pp. 245-331. Oxford University Press, New York.
  3. 3.0 3.1 Mimura, J., and Fujii-Kuriyama, Y. (2003). Functional role of AhR in the expression of toxic effects by TCDD. Biochimica et Biophysica Acta - General Subjects 1619, 263-268.
  4. [1]"Entrez Gene: cytochrome P450; Gene ID: 1544."
  5. Harris, M. L., and Elliott, J. E. (2011). Effects of Polychlorinated Biphenyls, Dibenzo-p-Dioxins and Dibenzofurans, and Polybrominated Diphenyl Ethers in Wild Birds. In Environmental Contaminants in Biota (J. P. Meador, Ed.), pp. 477-528. CRC Press.
  6. Head, J. A., Farmahin, R., Kehoe, A. S., O'Brien, J. M., Shutt, J. L., and Kennedy, S. W. (2010). Characterization of the avian aryl hydrocarbon receptor 1 from blood using non-lethal sampling methods. Ecotoxicology 19, 1560-1566.
  7. Rifkind, A. B. (2006). CYP1A in TCDD toxicity and in physiology - With particular reference to CYP dependent arachidonic acid metabolism and other endogenous substrates. Drug Metabolism Reviews 38, 291-335.
  8. Safe, S. (1987). Determination of 2,3,7,8-TCDD toxic equivalent factors (TEFs): Support for the use of the in vitro AHH induction assay. Chemosphere 16, 791-802.
  9. Goldstone, H. M. H., and Stegeman, J. J. (2006). A revised evolutionary history of the CYP1A subfamily: Gene duplication, gene conversion, and positive selection. Journal of Molecular Evolution 62, 708-717.
  10. 10.0 10.1 Jones, S. P., and Kennedy, S. W. (2009). Chicken embryo cardiomyocyte cultures--a new approach for studying effects of halogenated aromatic hydrocarbons in the avian heart. Toxicol.Sci 109, 66-74.
  11. 11.0 11.1 Rifkind, A. B., Kanetoshi, A., Orlinick, J., Capdevila, J. H., and Lee, C. A. (1994). Purification and biochemical characterization of two major cytochrome P-450 isoforms induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin in chick embryo liver. J.Biol.Chem. 269, 3387-3396.
  12. Farmahin, R., Crump, D., Jones, S. P., Mundy, L. J., and Kennedy, S. W. (2013a). Cytochrome P4501A induction in primary cultures of embryonic European starling hepatocytes exposed to TCDD, PeCDF and TCDF. Ecotoxicology.
  13. Hervé, J. C., Crump, D., Jones, S. P., Mundy, L. J., Giesy, J. P., Zwiernik, M. J., Bursian, S. J., Jones, P. D., Wiseman, S. B., Wan, Y., and Kennedy, S. W. (2010a). Cytochrome P4501A induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin and two chlorinated dibenzofurans in primary hepatocyte cultures of three avian species. Toxicol.Sci. 113, 380-391.
  14. Manning, G. E., Mundy, L. J., Crump, D., Jones, S. P., Chiu, S., Klein, J., Konstantinov, A., Potter, D., and Kennedy, S. W. (2013). Cytochrome P4501A induction in avian hepatocyte cultures exposed to polychlorinated biphenyls: comparisons with AHR1-mediated reporter gene activity and in ovo toxicity. Toxicol.Appl.Pharmacol. 266, 38-47.
  15. Elliott, J. E., Norstrom, R. J., Lorenzen, A., Hart, L. E., Philibert, H., Kennedy, S. W., Stegeman, J. J., Bellward, G. D., and Cheng, K. M. (1996). Biological effects of polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls in bald eagle (Haliaeetus leucocephalus) chicks. Environ.Toxicol.Chem. 15, 782-793.
  16. Bellward, G. D., Norstrom, R. J., Whitehead, P. E., Elliott, J. E., Bandiera, S. M., Dworschak, C., Chang, T., Forbes, S., Cadario, B., Hart, L. E., and . (1990). Comparison of polychlorinated dibenzodioxin levels with hepatic mixed-function oxidase induction in great blue herons. J.Toxicol.Environ.Health 30, 33-52.
  17. Sanderson, J. T., Norstrom, R. J., Elliott, J. E., Hart, L. E., Cheng, K. M., and Bellward, G. D. (1994). Biological effects of polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls in double-crested cormorant chicks (Phalacrocorax auritus). J.Toxicol.Environ.Health 41, 247-265.
  18. Rattner, B. A., Hatfield, J. S., Melancon, M. J., Custer, T. W., and Tillitt, D. E. (1994). Relation among cytochrome-P450, Ah-active PCB congeners and dioxin equivalents in pipping black-crowned night-heron embryos. Environ.Toxicol.Chem. 13, 1805-1812.
  19. Elliott, J. E., Wilson, L. K., Henny, C. J., Trudeau, S. F., Leighton, F. A., Kennedy, S. W., and Cheng, K. M. (2001). Assessment of biological effects of chlorinated hydrocarbons in osprey chicks. Environ.Toxicol.Chem. 20, 866-879.
  20. Dey, A., Jones, J. E., and Nebert, D. W. (1999). Tissue- and cell type-specific expression of cytochrome P450 1A1 and cytochrome P450 1A2 mRNA in the mouse localized in situ hybridization. Biochem. Pharmacol. 58 (3), 525-537.
  21. 21.0 21.1 Burke, M. D., Thompson, S., Weaver, R. J., Wolf, C. R., and Mayer, R. T. (1994). Cytochrome P450 specificities of alkoxyresorufin O-dealkylation in human and rat liver. Biochem. Pharmacol. 48 (5), 923-936.
  22. Sinclair, P. R., Gorman, N., Walton, H. S., Sinclair, J. F., Lee, C. A., and Rifkind, A. B. (1997). Identification of CYP1A5 as the CYP1A enzyme mainly responsible for uroporphyrinogen oxidation induced by AH receptor ligands in chicken liver and kidney. Drug Metab. Dispos. 25 (7), 779-783.
  23. Staskal, D. F., Diliberto, J. J., DeVito, M. J., and Birnbaum, L. S. (2005). Inhibition of human and rat CYP1A2 by TCDD and dioxin-like chemicals. Toxicol. Sci. 84 (2), 225-231.
  24. Kalow, W., and Tang, B. K. (1991). Use of caffeine metabolite ratios to explore CYP1A2 and xanthine oxidase activities. Clin Pharmacol. Ther. 50 (5 Pt 1), 508-519.
  25. [2]"Real-time polymerase chain reaction"
  26. Head, J. A., and Kennedy, S. W. (2007). Same-sample analysis of ethoxyresorufin-O-deethylase activity and cytochrome P4501A mRNA abundance in chicken embryo hepatocytes. Anal. Biochem. 360 (2), 294-302.
  27. Farmahin, R., Manning, G. E., Crump, D., Wu, D., Mundy, L. J., Jones, S. P., Hahn, M. E., Karchner, S. I., Giesy, J. P., Bursian, S. J., Zwiernik, M. J., Fredricks, T. B., and Kennedy, S. W. (2013). Amino acid sequence of the ligand-binding domain of the aryl hydrocarbon receptor 1 predicts sensitivity of wild birds to effects of dioxin-like compounds. Toxicol. Sci. 131 (1), 139-152.
  28. Farmahin, R., Wu, D., Crump, D., Hervé, J. C., Jones, S. P., Hahn, M. E., Karchner, S. I., Giesy, J. P., Bursian, S. J., Zwiernik, M. J., and Kennedy, S. W. (2012). Sequence and in vitro function of chicken, ring-necked pheasant, and Japanese quail AHR1 predict in vivo sensitivity to dioxins. Environ. Sci. Technol. 46 (5), 2967-2975.
  29. Garrison, P. M., Tullis, K., Aarts, J. M., Brouwer, A., Giesy, J. P., and Denison, M. S. (1996). Species-specific recombinant cell lines as bioassay systems for the detection of 2,3,7,8-tetrachlorodibenzo-p-dioxin-like chemicals. Fundam. Appl. Toxicol. 30 (2), 194-203.
  30. Prasch, A. L., Teraoka, H., Carney, S. A., Dong, W., Hiraga, T., Stegeman, J. J., Heideman, W., and Peterson, R. E. (2003). Aryl hydrocarbon receptor 2 mediates 2,3,7,8-tetrachlorodibenzo-p-dioxin developmental toxicity in zebrafish. Toxicol. Sci. 76(1), 138-150.
  31. European Union Reference Laboratory for Alternatives to Animal Testing (EURL-ECVAM), Multi‐study validation trial for cytochrome P450 induction providing a reliable human metabolically competent standard model or method using the human cryopreserved primary hepatocytes and the human cryopreserved HepaRG® cell line.Draft guideline, OECD.