Heat shock protein-90 (Hsp90) acts as a repressor of peroxisome proliferator-activated receptor-alpha (PPARalpha) and PPARbeta activity.

Abstract

The nuclear receptor (NR) peroxisome proliferator-activated receptor-alpha (PPARalpha) mediates the effects of several hypolipidemic drugs, endogenous fatty acids, and peroxisome proliferators. Despite belonging to a class of NR not known to interact with cytosolic chaperone complexes, we have recently shown that PPARalpha interacts with heat shock protein 90 (Hsp90), although the biological consequence of this association was unknown. In the present study, PPARalpha directly associated with Hsp90 in vitro to a much greater extent than either PPARbeta or PPARgamma. This interaction is similar to other NR-Hsp90 complexes with association occurring between the middle of Hsp90 and the hinge (D) and ligand binding domain (EF) of PPARalpha. Using several different approaches to disrupt Hsp90 complexes within the cell, we demonstrate that Hsp90 is a repressor of both PPARalpha and PPARbeta activity. Treatment with geldanamycin (GA) increased the activity of PPARalpha and in the presence of ligand in transient transfection assays. PPARalpha-response element (PPRE)-reporter assays in a stable cell line treated with GA resulted in enhanced expression of a known target gene, acyl-CoA oxidase. Similarly, overexpression of the tetratricopeptide repeat (TPR) of protein phosphatase 5 (PP5) increased PPARalpha or PPARbeta activity in a PPRE-reporter assay and decreased the interaction between PPARalpha or PPARbeta and Hsp90 in a mammalian two-hybrid assay. Finally, cotransfection with the C-terminal hsp-interacting protein (CHIP) construct, a TPR-containing ubiquitin ligase that interacts with hsp90, increased PPARalpha's and decreased PPARbeta's ability to regulate PPRE-reporter activity upon ligand activation. All three methods to disrupt Hsp90 function (GA, PP5-TPR, CHIP) resulted in an alteration in PPARalpha or PPARbeta activity to a much greater extent than PPARgamma. While FKBP52 had no effect on PPARalpha activity, p23 greatly enhanced constitutive and Wy14 643 induced PPRE-reporter activity. Thus, we describe the chaperone complex as being a regulator of PPARalpha and PPARbeta activity and have identified a novel, subtype-specific, inhibitory role for Hsp90.