Hypoxia-Inducible Factor 1-Mediated Inhibition of Peroxisome Proliferator-Activated Receptor α Expression During Hypoxia

Abstract

Peroxisome proliferator-activated receptors (PPARs) are nuclear hormone-binding proteins that regulate transcriptional responses to peroxisome proliferators and structurally diverse fatty acids. PPARs have been implicated in a wide variety of functions, including lipid homeostasis and inflammatory responses. In this study, we examined the expression of PPAR-α in response to ambient hypoxia. Initial studies using microarray analysis of intestinal epithelial mRNA revealed that hypoxia rapidly down-regulates PPAR-α mRNA and protein in epithelial cells in vitro and in vivo. Subsequent studies revealed that the PPAR-α gene bears a DNA consensus motif for the transcription factor hypoxia-inducible factor 1 (HIF-1). EMSA analysis revealed that ambient hypoxia induces HIF-1α binding to the HIF-1 consensus domain of PPAR-α in parallel to HIF-1 nuclear accumulation, and antisense depletion of HIF-1α resulted in a loss of PPAR-α down-regulation. The PPAR-α ligand pirinixic acid (WY14643) functionally promoted IFN-γ-induced ICAM-1 expression in normoxic epithelia, and this response was lost in cells pre-exposed to ambient hypoxia. Such results indicate that HIF-1-dependent down-regulation of PPAR-α may provide an adaptive response to proinflammatory stimuli during cellular hypoxia. These studies provide unique insight into the regulation of PPAR-α expression and, importantly, provide an example of a down-regulatory pathway mediated by HIF-1.