| The peroxisome proliferator activated receptors α, β, and γ belong to the NR1 sub-family of nuclear receptors. They are ligand inducible transcription factors, which can be activated by endogenous fatty acids, synthetic fatty acid analogues, and xenobiotics. The three isoforms of PPAR demonstrate differential tissue distribution and ligand specificity. While PPARβ is the least understood isoform, PPARα and PPARγ have been studied extensively. PPARα mediates the carcinogenic effects of peroxisome proliferators in rodents and mediates fatty acid catabolism. PPARγ is a key mediator of adipogenesis and plays a role in the treatment of metabolic disorders termed syndrome “X”, which is characterized by obesity, diabeties, and artherosclerosis. PPARβ is involved in the keratinocyte proliferation and differentiation and is known to modulate skin and colon carcinogenesis.; Many NR3 subfamily of receptors (e.g. Glucocorticoid receptor, Estrogen receptor) interact with the cytosolic heat shock protein 90 (hsp90) molecular chaperone complex, which aids in the folding of the ligand binding domain of the receptor to acquire a proper ligand binding conformation. Unlike NR3 subfamily, NR1 subfamily receptors, which include thyroid hormone receptor and Vitamin D3 receptor, are not known to be stably associated with cytosolic molecular chaperone complexes. However, since hsp90 is present ubiquitously in eukaryotic cytosol, there is a possibility of involvement of this chaperone with PPARα complex. The initial observation that PPARα co-migrates with hsp90 in a sucrose density gradient also lead us to further test this hypothesis. In order to investigate whether PPARα interacts with hsp90, PPARα from mouse liver cytosol was immunoprecipitated using PPARα specific monoclonal antibody, 3B6/PPAR. Despite belonging to NR1 of nuclear receptors, our results revealed that PPARα interacts with hsp90. In addition, the hepatitis B virus X-associated protein 2 (XAP2) co-immunoprecipitated with PPARα. Complex formation between XAP2 and PPARα was also demonstrated in an in vitro translation binding assay. Transient expression of XAP2 with PPARα resulted in down regulation of a peroxisome proliferator response element (PPRE)-driven reporter gene activity suggesting a repressive effect of XAP2 in the PPARα complex. Our results are the first demonstration that PPARα is stably associated with other proteins in tissue extracts and the first nuclear receptor shown to functionally interact with XAP2. (Abstract shortened by UMI.)... |
