Signal transduction pathways of peroxisome proliferator-activated receptor-alpha$

The overall goal of the present study was to dissect the molecular mechanisms that lead to transcriptional activity of the peroxisome proliferator-activated receptor alpha (PPARα). Peroxisome proliferators (PPs) are structurally similar rodent hepatocarcinogens that elevate the transcriptional activity PPARα, a steroid hormone receptor (SHR). The regulation of gene expression by PPARα occurs in a PP-dependent manner by binding to response elements in promoter regulatory regions of target genes as a heterodimer of PPAR and retinoid X receptor alpha (RXRα). In addition to PPs\u27 affect on PPARα activity, several growth factors are known modulators of PPARα, although the mechanism is unknown. In an attempt to characterize PPARα\u27s signal transduction pathway, the ability of the mitogen-activated protein kinase (MAPK) pathway to regulate receptor activation was examined. Geldanamycin (GA), a heat shock protein 90 (Hsp90) binding agent which inhibits MAPK by disrupting the interaction between Hsp90 and raf-1, enhanced activation of PPARα by the potent PP Wy-14,643. However, PD098059, a specific inhibitor of MAPK/Erk kinase (MEK), suppressed Wy-14,643-mediated activation of PPARα. Overexpression of constitutively active or dominant negative mutants of MEK in transient transfection assays confirmed that inhibition of the MAPK pathway abolished PPARα transcriptional activity. In addition, MAPK was able to phosphorylate PPARα in vitro. Since GA enhanced ligand-dependent activation of PPAR$ independent of the MAPK pathway, alternative pathways were examined, including the possibility that PPAR$ directly associated with Hsp90. As with other SHRs, PPARα co-fractionated with Hsp90 in sucrose gradients when the receptor was translated in vitro. However, in pull-down assays Hsp90 did not stoichiometrically associate with PPARα in vitro. The identification of novel PPARα-associated proteins did not shed light on the mechanism by which GA enhances PPARα activity. It is therefore likely that GA effect on PPARα activity is due to its ability to alter Hsp90-associated, MEK-independent signal transduction pathways. These results indicate that growth regulatory pathways regulate PPARα transcriptional activity. Cross-talk between growth factor and PP-mediated signal transduction pathways undoubtedly play a major role in the complex regulation of gene expression and possibly in the tumorigenic effects of PPs