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Activation of PPARα has been demonstrated to inhibit tumor growth and angiogenesis,yet the mechanisms behind these actions remain to be elucidated.The present study examined the effects of PPARα activation on the HIF-1α signaling pathway in human breast (MCF-7) and ovarian (A2780) cancer cells.Incubation of cancer cells under 1% oxygen for 16 hours significantly induced HIF-1α expression and activity as assayed with Western blot and reporter gene analysis.Treatment of the cells with PPARα agonists, but not a PPARγ agonist, prior to hypoxia diminished hypoxia-induced HIF-1α expression and activity, and addition of a PPARα antagonist attenuated the suppression of the HIF-1α signaling.An HA-tagged HIF-1α and HIF-1α-P402/P564A mutant was over-expressed in MCF-7 cells, and activation of PPARα attenuated hypoxia-induced HA-tagged HIF-1α protein expression without affecting the HA-tagged HIF-1α mutant protein level,indicating that PPARα activation promotes HIF-1α degradation in our model system.This was further confirmed by the use of MG-132, a proteasome inhibitor, which reversed PPARα-mediated suppression of the HIF-1α expression, and by RT-PCR analysis of HIF-1α mRNA, which was unchanged by PPARα activation.Using the co-immunoprecipitation technique, we found that activation of PPARα enhances the binding of HIF-1 α to pVHL, a protein known to mediate HIF-1α degradation through the ubiquitin-proteasome pathway.Following PPARα-mediated suppression of HIF-1α signaling, vascular endothelial growth factor secretion from the cancer cells was significantly reduced, and tube formation by the endothelial line EA.hy926 was dramatically impaired.Taken together, these findings demonstrate for the first time that activation of PPARα suppresses hypoxia-induced HIF-1α signaling in cancer cells, providing novel insight into the anti-cancer properties of PPARα agonists.