Key Event Overview
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AOPs Including This Key Event
|AOP Name||Event Type||Essentiality|
|Antagonist binding to PPARalpha leading to starvation-like body-weight loss||KE||Strong|
Level of Biological Organization
How this Key Event works
The transcription co-repressors, silencing mediator for retinoid and thyroid hormone receptors (SMRT) and nuclear receptor co-repressor (N-CoR) have been observed to compete with transcriptional co-activators for binding to nuclear receptors (including PPARα) thus suppressing basal transcriptional activity (Nagy et al 1999, Xu et al 2002). Binding motifs for the co-repressors and co-activators to PPARα have been observed to be conserved, however the co-repressor blocks the AF-2 helix from adopting the active conformation, as demonstrated in x-ray crystallography results presented in Xu et al (2002). PPARα agonists and antagonists have been developed to selectively enhance co-activator or co-repressor binding, respectively (Xu et al 2001, Xu et al 2002). Regarding the present key event, the prior binding of a PPARα antagonists such as GW6471 can stabilize the binding of the co-repressors to the PPARα signaling complex suppressing nuclear signaling and thus downstream transcription of PPARα-regulated genes.
Wilbanks et al. (2014) and Gust et al (2015) demonstrated inhibition of human PPARα nuclear signaling in in vitro nuclear signaling bioassays in response to 2,4-dinitrotoluene(2,4-DNT) and 2-amino-4,6-dinitrotoluene (2A-DNT), respectively. However, it is unknown if this response was manifested through the co-repressor binding stabilization that was identified in (Xu et al 2002).
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?
In Xu et al (2002), X-ray crystallography was used to characterize the suppressed PPARα signaling complex (PPARα / GW6471 / SMRT) and was compared against the activated PPARα complex which included binding of PPARα with the agonist GW409544 and the co-activator, steroid receptor coactivator-1 (SRC-1).
The effects of 2,4-DNT and 2A-DNT on PPAR signaling was investigated using nuclear receptor reporter assays were conducted for (PPARalpha, PPARgamma, PPARsigma, and RXRalpha human cell-based assays, (Indigo Biosciences).
Evidence Supporting Taxonomic Applicability
The fundamental mechanics for co-repressor antagonism of nuclear signaling by SMRT and N-CoR have been observed to be equivalent when comparing humans and yeast (Nagy et al 1999). Therefore, the taxonomic applicability should be broad across eukaryotes. However, knowledge of the specific amino acid sequence for PPARα in the species of interest relative to human (for which the present evidence is based) is valuable for understanding the confidence in extrapolating relationships across species.
Gust KA, Nanduri B, Rawat A, Wilbanks MS, Ang CY, Johnson DR, Pendarvis K, Chen X, Quinn Jr. MJ, Johnson MS, Burgess SC, Perkins EJ. 2015. Systems Toxicology Identifies Mechanistic Impacts of 2-amino-4,6-dinitrotoluene (2A-DNT) Exposure in Northern Bobwhite. BMC Genomics. 16:587 doi:10.1186/s12864-015-1798-4
Nagy L, Kao H-Y, Love JD, Li C, Banayo E, Gooch JT, Krishna V, Chatterjee K, Evans RM, Schwabe JWR: Mechanism of corepressor binding and release from nuclear hormone receptors. Genes Dev 1999, 13(24):3209-3216.
Wilbanks, M., Gust, K.A., Atwa, S., Sunesara, I., Johnson, D., Ang, C.Y., Meyer., S.A., and Perkins, E.J. 2014. Validation of a genomics-based hypothetical adverse outcome pathway: 2,4-dinitrotoluene perturbs PPAR signaling thus impairing energy metabolism and exercise endurance. Toxicological Sciences. 141(1):44-58.
Xu HE, Lambert MH, Montana VG, Plunket KD, Moore LB, Collins JL, Oplinger JA, Kliewer SA, Gampe RT, McKee DD et al: Structural determinants of ligand binding selectivity between the peroxisome proliferator-activated receptors. Proceedings of the National Academy of Sciences 2001, 98(24):13919-13924.
Xu HE, Stanley TB, Montana VG, Lambert MH, Shearer BG, Cobb JE, McKee DD, Galardi CM, Plunket KD, Nolte RT et al: Structural basis for antagonist-mediated recruitment of nuclear co-repressors by PPAR[alpha]. Nature 2002, 415(6873):813-817.