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Event: 167

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

Activation, LXR

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
Activation, LXR

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
Molecular

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
Cell term
hepatocyte

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
signaling oxysterols receptor LXR-beta increased
signaling oxysterols receptor LXR-alpha 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
LXR Activation to Liver Steatosis MolecularInitiatingEvent Agnes Aggy (send email) Not under active development
NR1I3 suppression to steatosis MolecularInitiatingEvent Allie Always (send email) Under Development: Contributions and Comments Welcome

Stressors

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

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

Sex Applicability

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

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 LXR receptor

Liver X receptors are ligand-activated transcription factors of the nuclear receptor superfamily first identified in 1994 in rat liver (Apfel et al. 1994, Song 1994). There are two LXR isoforms termed a and ß (NR1H3 and NR1H2) which upon activation form heterodimers with retinoid X receptor (RXR) and bind to the LXR response element found in the promoter region of the target genes (Baranowski 2008). LXRs were shown to function as sterol sensors protecting the cells from cholesterol overload by stimulating reverse cholesterol transport and activating its conversion to bile acids in the liver (Baranowski 2008).

LXRa expression is restricted to liver, kidney, intestine, fat tissue, macrophages, lung, and spleen and is highest in liver, hence the name liver X receptor a (LXRa). LXRβ is expressed in almost all tissues and organs, hence the early name UR (ubiquitous receptor) (Ory 2004). The different pattern of expression suggests that LXRa and LXRβ have different roles in regulating physiological function. This is also supported from the observation that LXRa deficient mice do not develop hepatic steatosis when treated with LXR agonist that activates both types (Lund et al. 2006) and consequently the role of the two isoforms in relation to adverse effects could be different.

The molecular initiating event

Generally speaking chemicals that are able to act through NRs are usually specific ligands. These chemicals are mainly lipophilic and they mimic the action of natural hormones. However, in some cases hydrophilic chemicals (like phthalates) are also capable to act as ligands in NRs due to the molecular structure of the proteins and the pocket sites of the receptors.

The molecular initiating event in the presented MoA is the binding to the LXR or the permissive RXR of the LXR-RXR dimer leading to activation. LXR activation can be achieved via a wide range of endogenous neutral and acidic ligands as shown by crystallographic analysis (Williams et al. 2003). There are known endogenous but also synthetic ligands that can act as agonists. Endogenous agonists for this receptor are the oxysterols (oxidized cholesterol derivatives like 22(R)-hydroxycholesterol, 24(S)-hydroxycholesterol, 27-hydroxycholesterol, and cholestenoic acid) mainly with similar affinity for the two isoforms (Baranowski 2008). Oxysterols bind directly to the typical hydrophobic pocket in the C-terminal domain (Williams et al. 2003). Other endogenous ligands are the D-glucose and D-Glucose-6-phosphate (Mitro 2007). However, the hydrophilic nature of glucose and its low affinity for LXR present a challenge to the central dogma about the nature of the NR-ligand interaction (Lazar & Wilson 2007). Unsaturated fatty acids have also been shown to bind and regulate LXRa activity in cells. However, in contrast to the role of oxysterols, the biological relevance of this observation has not been established in vivo (Pawar et al. 2003). The function of LXRs is also modulated by many currently used drugs such as statins, fibrates, and thazolidinedione derivatives (Jamroz-Wiśniewska et al. 2007). Some synthetic LXR agonists have been developed like the non-steroidal agonists T0901317 and GW3965 (Schultz et al 2000, Collins et al. 2002). LXR forms a permissive dimer with the RXR which means that chemicals that can activate this receptor can trigger the same pathway as the LXR agonists. The endogenous RXR agonist is 9-cis-retinoic acid (Heyman et al. 1992) while synthetic agonists include LGD1069 and LG100268 (Boehm et al. 1994 and 1995).

In addition to the agonist binding in the LXR there are other mechanisms for its control. LXRa gene promoter contains also functional peroxisome proliferator response element (PPRE) and peroxisome proliferator-activated receptor (PPAR) a and γ agonists were shown to stimulate LXRa expression in human and rodent (Baranowski 2008). Control of the LXRa expression is also dependent on insulin and post-translationally by protein kinase A that phosphorylates receptor protein at two sites thereby impairing its dimerization and DNA-binding (Baranowski 2008).

Identification of the site of action

As already mentioned above LXR isoforms are expressed in various tissues but in relation to the presented MoA we refer to LXRs that are expressed in the hepatocytes.

Nuclear receptors may be classified into two broad classes according to their sub-cellular distribution in the absence of ligand. Type I NRs (like ER and AhR) are located in the cytosol (and they are translocated into the nucleus after ligand binding) while type II NRs like LXRs (but also PXR, PPARa and PPARγ) are located in the nucleus of the cell.

The specific site of binding and the affinity of a ligand for the LXRs depend on the structure of the ligand.

Binding in the LXREs and target genes transcription

Upon ligand-induced activation both isoforms form obligate heterodimers with the retinoid X receptor (RXR) and regulate gene expression through binding to LXR response elements (LXREs) in the promoter regions of the target genes (Fig. 1). The LXRE consists of two idealized hexanucleotide sequences (AGGTCA) separated by four bases (DR-4 element).

Mechanism-of-transcriptional-regulation-mediated-by-LXRs.png

Figure 1. Mechanism of transcriptional regulation mediated by LXRs. RXR - retinoid X receptor, LXRE - LXR response element (Baranowski 2008)

Target genes of LXRs are involved in cholesterol and lipid metabolism regulation ([1], [2]) including:

  • ABC - ATP Binding Cassette transporter isoforms A1, G1, G5, and G8
  • ApoE - Apolipoprotein E
  • CETP - Cholesterylester Transfer Protein
  • CYP7A1 - Cytochrome P450 isoform 7A1 - cholesterol 7a-hydroxylase
  • FAS - Fatty Acid Synthase
  • LPL - Lipoprotein Lipase
  • LXR-a - Liver X Receptor-a
  • SREBP-1c - Sterol Response Element Binding Protein 1c
  • ChREBP - Carbohydrate Response Element Binding Protein
  • FAT/CD36 – Fatty acid uptake transporter (liver)

Auto-regulation of the LXRa

Human specific auto-regulated expression specifically of the LXRa has been demonstrated from several studies (Laffitte et al. 2001, Whitney et al. 2001, Li et al. 2002, Kase et al. 2007). Human LXRa gene promoter has a functional LXRE activated by both LXRa and β. In addition human liver LXRa expression is induced by both natural and synthetic LXR agonists.

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). ?

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

Evidence for Perturbation by Stressor

Overview for Molecular Initiating Event

When a specific MIE can be defined (i.e., the molecular target and nature of interaction is known), in addition to describing the biological state associated with the MIE, how it can be measured, and its taxonomic, life stage, and sex applicability, it is useful to list stressors known to trigger the MIE and provide evidence supporting that initiation. This will often be a list of prototypical compounds demonstrated to interact with the target molecule in the manner detailed in the MIE description to initiate a given pathway (e.g., 2,3,7,8-TCDD as a prototypical AhR agonist; 17α-ethynyl estradiol as a prototypical ER agonist). Depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). Known stressors should be included in the MIE description, but it is not expected to include a comprehensive list. Rather initially, stressors identified will be exemplary and the stressor list will be expanded over time. For more information on MIE, please see pages 32-33 in the User Handbook.

References

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 (https://www.oecd.org/about/publishing/OECD-Style-Guide-Third-Edition.pdf) (OECD, 2015). More help
  1. Peet 1998
  2. Edwardsa et al. 2002