Key Event Title
Key Event Component
|peroxisome proliferator activated receptor signaling pathway||peroxisome proliferator-activated receptor gamma||increased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|LXR activation leading to hepatic steatosis||MolecularInitiatingEvent|
|NR1I3 (CAR) suppression leading to hepatic steatosis||MolecularInitiatingEvent|
|Epigenetic modification of PPARG leading to adipogenesis||MolecularInitiatingEvent|
Level of Biological Organization
How This Key Event Works
The Peroxisome Proliferator Activated receptor γ (PPARγ) belongs to Peroxisome Proliferator Activated receptors (PPARs; NR1C) steroid/thyroid/retinoid receptor superfamily of transcription factors, which respond to specific ligands by altering gene expression in a cell-specific manner. The PPARγ gene contains three promoters that yield three isoforms, namely, PPAR-γ1, 2 and 3. PPAR-γ1 and γ3 RNA transcripts translate into the identical PPAR-γ1 protein.
PPARγ is abundantly expressed in adipose tissue, promoting adipocyte differentiation, but is also present in various cells and tissues, for review see (Braissant et al. 1996). PPARγ expression is tissue dependent (L Fajas et al. 1997), (Lluis Fajas, Fruchart, and Auwerx 1998). PPARγ is most highly expressed in white adipose tissue and brown adipose tissue, where it is a master regulator of adipogenesis as well as a potent modulator of whole-body lipid metabolism and insulin sensitivity (Evans, Barish, and Wang 2004), (Tontonoz and Spiegelman 2008). Whereas PPARγ1 is expressed in many tissues, the expression of PPARγ2 is restricted to adipose tissue under physiological conditions but can be induced in other tissues by a high-fat diet (Saraf et al. 2012).
General role in biology
PPARγ is activated after the binding of natural ligands such as polyunsaturated fatty acids and prostaglandin metabolites. It can also be activated by synthetic ligands such as thiazolidinediones (TZDs) (rosiglitazone, pioglitazone or troglitazone) (Lehmann et al., 1995). PPARγ controls many vital processes such as glucose metabolism and inflammation as well as variety of developmental programs(Wahli & Desvergne, 1999), (Rotman et al., 2008), (Wahli & Michalik, 2012). This receptor itself is essential for developmental processes since targeted disruption of this gene results in embryo lethality, due in part to defective placental development, therefore modulation of PPARγ activity may impact endocrine regulated processes during development as well as later in life.
How It Is Measured or Detected
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?
Binding of ligands to PPARγ is measured using binding assays in vitro and in silico, whereas the information about functional activation is derived from the transactivation using e.g. reporter assay with a reporter gene that demonstrates functional activation of a nuclear receptor by a specific compound. Binding of agonists within the ligand-binding site of PPARs causes a conformational change promoting binding to transcriptional coactivators. Conversely, binding of antagonists results in a conformation that favours the binding of corepressors (Yu & Reddy, 2007) (Viswakarma et al., 2010. Transactivation assays are performed using the transient or stably transfected cells with the PPARγ expression plasmid and a reporter plasmid, correspondingly. There are also other methods that have been used to measure PPARγ activity, such as the Electrophoretic Mobility Shift Assay (EMSA) or commercially available PPARγ transcription factor assay kits, see Table 1. The transactivation (stable transfection) assay provides the most applicable OECD Level 2 assay aimed at identifying the initiating event leading to adverse outcome (LeBlanc, Norris, & Kloas, 2011). Currently no internationally validated assays are available.
|Key event||PPARγ activation|
|What is measured?||Ligand Binding||Transcriptional activity|
|Method/test category||molecular modelling||binding assay||transactivation reporter gene assay||transcription factor assay|
|Method/test name||molecular modelling; docking||Scintillation proximity binding assay||luciferase reporter gene assay||PPARγ (mouse/rat) Reporter Assay Kit||Electrophoretic Mobility Shift Assay (EMSA)|
|Test environment||In silico||In vitro||In vitro||In vitro, ex vivo|
|Test principle||Computational simulation of a candidate ligand binding to a receptor, Predicts the strength of association or binding affinity.||direct binding indicating the mode of action for PPARα/γ||Quantifying changes in luciferase expression in the treated reporter cells provides a sensitive surrogate measure of the changes in PPAR functional activity.||PPARγ once activated by a ligand, the receptor binds to a promoter element in the gene for target gene and activates its transcription. The bound (activated) to DNA PPAR is measured.|
|Test outcome||A binding interaction between a small molecule ligand and an enzyme protein may result in activation or inhibition of the enzyme. If the protein is a receptor, ligand binding may result in agonism or antagonism||Assess the ability of compounds to bind to PPARγ. Identifies the modulators of PPARγ.||The changes in activity of reporter gene levels functionally linked to a PPAR-responsive element/promoter gives information about the activity of the PPAR activation.||Protein: DNA binding, DNA binding activity|
|Test background||Predicts the preferred orientation of one molecule to a second when bound to each other to form a stable complex. Knowledge of the preferred orientation in turn may be used to predict the strength of association or binding affinity between two molecules using, for example, scoring functions.||This assay determines whether compounds interact directly with PPARγ.||PPARγ COS-1cell transactivation assay (transient transfection with human or mouse PPARγ expression plasmid and pHD(x3)-Luc reporter plasmid||(PPRE)3- luciferase reporter construct C2C12||Proprietary rodent cell line expressing the mouse/rat PPARγ||Transcriptional activity of PPARγ can be assessed using commercially available kits like e.g. PPARγ transcription factor assay kit (Abcam, Cambridge, USA or Cayman Chemical, USA).||Gene regulation and determining protein: DNA interactions are the detected by the EMSA. EMSA can be used qualitatively to identify sequence-specific DNA-binding proteins (such as transcription factors) in crude lysates and, in conjunction with mutagenesis, to identify the important binding sequences within a given genes upstream regulatory region. EMSA can also be utilized quantitatively to measure thermodynamic and kinetic parameters.|
|Application domain||Virtual screening||In vitro screening||In vitro Screening, functional studies activity (reported use: agonist)||In vitro Screening functional activity (antagonist/agonist)||Functional studies||Functional studies|
|Ref||(Feige et al., 2007), (Kaya, Mohr, Waxman, & Vajda, 2006)||(Lapinskas et al., 2005), (Wu, Gao, & Wang, 2005)||(Maloney & Waxman, 1999)||(Feige et al., 2007)||Cayman, (Gijsbers et al. 2013)||Abcam|
Table 1 Summary of the chosen methods to measure the PPARγ activation.
Evidence Supporting Taxonomic Applicability
PPARγ have been identified in frog (Xenopus laevis), mouse, human, rat, fish, hamster and chicken (Wahli & Desvergne, 1999).
Evidence for Perturbation by Stressor
Overview for Molecular Initiating Event
1.1 Binding and activation of receptor
PPARγ ligands thiazolidinediones (Rosiglitazone, Pioglitazone, Troglitazone) (Lehmann et al. 1995), (Forman et al. 1995), (Willson et al. 2000).
MEHP (CAS 4376-20-9) directly binds to PPARγ (Lapinskas et al. 2005), (ToxCastTM Data)in vitro and in silico (Feige et al. 2007), (Rotman et al. 2008), (Kaya et al. 2006) and activates this receptor in transactivation assays (Maloney & Waxman 1999), (Hurst & Waxman 2003b), (Venkata et al. 2006), (ToxCastTM Data). In summary, there is experimental in vitro evidence for binding and transcriptional activation of PPARγ. DEHP (CAS 117-81-7) was not found to bind and activate PPARγ (Lapinskas et al. 2005), (Maloney & Waxman 1999). However recent studies show activation of PPARγ by DEHP(ToxCastTM Data), (Pereira-Fernandes et al. 2013). DEHP was also found to increase the levels of PPARγ in vitro (Lin et al. 2011). Notably, PPARγ is responsive to DEHP in vitro and is translocated to the nucleus (in primary Sertoli cells) (Dufour et al. 2003), (Bhattacharya et al. 2005).
Butylparaben was not found to bind to the PPARγ (ToxCastTM Data), but activated the human PPARγ (ToxCastTM Data), (Pereira-Fernandes et al. 2013) and mPPARγ in reporter gene assay (Taxvig et al., 2012).
Bisphenol A was not found to bind to the PPARγ (ToxCastTM Data), but activated the human PPARγ (ToxCastTM Data), (Pereira-Fernandes et al. 2013) but not mouse PPARγ (Taxvig et al., 2012) in reporter gene assay. BPA was also reported to increase PPARγ (mRNA) in ovarian granulosa cell line and human luteinized granulosa cells (Kwintkiewicz, Nishi, Yanase, & Giudice, 2010).
Tributyltin (TBT) activates all three heterodimers of PPAR with RXR, primarily through its interaction with RXR (le Maire et al. 2009).
1.2 Activation of target genes
MEHP activation of endogenous PPARγ target genes was evidenced by the stimulation of PPARγ-dependent adipogenesis in the 3T3-L1 cell differentiation model (Hurst & Waxman, 2003).
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