Key Event Title
Key Event Component
|receptor transactivation||peroxisome proliferator-activated receptor alpha||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|Antagonist binding to PPARα leading to body-weight loss||KeyEvent|
Level of Biological Organization
|Homo sapiens||Homo sapiens||Strong||NCBI|
|Mus musculus||Mus musculus||Strong||NCBI|
Life Stage Applicability
|Not Otherwise Specified||Not Specified|
How This Key Event Works
PPARα acts as a nuclear signaling element that controls the transcription of a variety of genes involved in lipid catabolism and energy production pathways (Desvergne and Wahli 1999, Kersten 2014). Fatty acids serve as the ligands that stimulate PPARα nuclear signaling where the fatty acids (likely in association with fatty acid binding proteins) bind to the ligand binding domain of PPARα along with co-activators to the PPARα regulatory complex at promoter regions of PPARα-regulated genes (termed PPAR response elements, PPREs) initiating the transcription of genes that metabolize the fatty acids (Ahmed et al 2007, Wolfrum et al. 2001, Desvergne and Wahli 1999, Kersten 2014, Xu et al 2001).
Specifically, PPARα contains both a ligand-binding domain that binds fatty acids and a DNA-binding domain that initiates binding to PPREs in the promoter regions of PPARα-regulated genes (Ahmed et al 2007, Hihi et al 2002). Binding of the fatty acid ligands to PPARα facilitates heterodimeric binding with another ligand-activated nuclear receptor, the retinoid X receptor (RXR), forming an activated PPAR-RXR transcriptional regulator complex (DiRenzo et al 1997, Ahmed et al 2007). PPAR competes for binding to RXR with retinoic acid receptors (RARs) where the RAR/RXR heterodimer inhibits transcription of genes downstream of PPREs (DiRenzo et al 1997). Transcriptional regulation activity of the PPAR/RXR complex is also influenced by the binding and release of accessory molecules that act as coactivators such as steroid receptor co-activator 1 (SRC-1) or as corepressors such as nuclear receptor corepressor (N-CoR, DiRenzo et al 1997, Ahmed et al 2007, Xu et al 2002). Such binding of the co-repressor N-CoR to the PPARα/RXRα complex has been demonstrated to inhibit the potential for transcriptional transactivation (Xu et al 2002). The exact mechanisms by which the PPAR/RXR complex facilitate transcription are still not well understood. It has been observed that RXR contains a highly conserved motif at the C-terminal end of the ligand-binding domain known as activating function 2 (AF2) which undergoes conformational changes allowing interaction with coactivators / corepressors, the former of which is hypothesized to recruit the components of the transcriptional machinery necessary to transcribe the downstream gene (DiRenzo et al 1997). Even the basal transcriptional machinery itself is recognized to vary across cell types and the prototypical preinitiation complex (PIC) is inherently highly flexible, confirmationally diverse including multi-faceted interactions of activators, core promotion factors, the RNA polymerase II enzyme, elongation factors, and chromatin remodeling complexes all combined at the promoter to facilitate gene transcription (Levine et al 2014). Given the current KE (KE2, PPARalpha transactivation of gene expression, Decreased), a variety of upstream influences my impair the function of the PPARα-RXRα heterodimer and/or affect coactivator / corepressor binding leading to decreased PIC competence resulting in impaired transcription of downstream genes.
PPARα regulates expression of genes encoding nearly every enzymatic step of fatty acid catabolism including fatty acid uptake into cells, fatty acid activation to acyl-CoAs, and the release of cellular energy from fatty acids through the oxidative breakdown of acyl-CoAs to acetyl-CoA , and in starvation conditions, the repackaging of Acetyl-CoA substrates into ketone bodies via ketogenesis pathways (Kersten 2014, Desvergne and Wahli 1999, Evans et al 2004). A pathway-level schematic for PPARα transactivation is illustrated in KEGG Pathway map03320 providing the specific gene targets and associated functional responses that are transcriptionally regulated by PPARα.
How It Is Measured or Detected
X-ray crystallography was used to describe the ligand binding domain (fatty acid binding domain) of PPARα and demonstrate the binding complex of PPARα, the artificial ligand (GW6471), and the co-repressor silencing mediator for retinoid and thyroid hormone receptors (SMRT, Xu et al 2001, 2002). Fold activation of the PPARα-GW6471-SMRT transcriptional regulatory complex was measured in mammalian two-hybrid assays (Xu et al 2001). PPAR-RXR and RAR-RXR heterodimerization and activity were quantified using expression vectors for murine PPARα, RAR and RXR in CV-1 cells transfected with 1ug of reporter plasmid and 50-200ng of expression plasmid (DiRenzo et al 1997). DNA-dependent radioligand binding assays were conducted to quantify ligand binding in the assays described in DiRenzo et al (1997).versions of each individual. X-ray crystallography has been used with limited success to describe the PIC while high-resolution electron microscopy is providing additional insights into the fully functionalized PIC when bound to the promotor and including all accessory molecules (Levine et al 2014). Effects of PPARα transactivation on expression of downstream genes been examined by a variety of methods, especially RT-qPCR (Kersten et al 2014). As a recommendation for investigating specific genes regulated by PPARα, as part of this KE, see the KEGG pathway for PPAR Signaling (map03320).
Evidence Supporting Taxonomic Applicability
Mus musculus (Kersten 2014), Homo sapiens in clinical observations (Kersten 2014) and in in vitro assays (reviewed in Kersten 2014).
Evidence for Perturbation by Stressor
Ahmed, W., Ziouzenkova, O., Brown, J., Devchand, P., Francis, S., Kadakia, M., Kanda, T., Orasanu, G., Sharlach, M., Zandbergen, F., Plutzky, J., 2007. PPARs and their metabolic modulation: new mechanisms for transcriptional regulation? J. Intern. Med. 262, 184-198.
Desvergne B, Wahli W (1999) Peroxisome proliferator-activated receptors: nuclear control of metabolism. Endocrine Reviews 20(5): 649-688.
DiRenzo, J., Soderstrom, M., Kurokawa, R., Ogliastro, M.H., Ricote, M., Ingrey, S., Horlein, A., Rosenfeld, M.G., Glass, C.K., 1997. Peroxisome proliferator-activated receptors and retinoic acid receptors differentially control the interactions of retinoid X receptor heterodimers with ligands, coactivators, and corepressors. Mol. Cell. Biol. 17, 2166-2176.
Evans RM, Barish GD, Wang YX: PPARs and the complex journey to obesity. Nat Med 2004, 10(4):355-361.
Hihi, A.K., Michalik, L., Wahli, W., 2002. PPARs: transcriptional effectors of fatty acids and their derivatives. Cell Molec Life Sci 59, 790-798.
Kersten S. 2014. Integrated physiology and systems biology of PPARalpha. Molecular Metabolism 2014, 3(4):354-371.
Levine, M., Cattoglio, C., Tjian, R., 2014. Looping Back to Leap Forward: Transcription Enters a New Era. Cell 157, 13-25.
Wolfrum C, Borrmann CM, Borchers T, Spener F (2001) Fatty acids and hypolipidemic drugs regulate peroxisome proliferator-activated receptors alpha - and gamma-mediated gene expression via liver fatty acid binding protein: a signaling path to the nucleus. Proc Natl Acad Sci USA 98(5):2323-2328.
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.