Comprehensive Analysis of PPARa-Dependent Regulation of Hepatic Lipid Metabolism by Expression Profiling

PPARa is a ligand-activated transcription factor involved in the regulation of nutrient metabolism and inflammation. Although much is already known about the function of PPARa in hepatic lipid metabolism, many PPARa-dependent pathways and genes have yet to be discovered. In order to obtain an overview of PPARa-regulated genes relevant to lipid metabolism, and to probe for novel candidate PPAR¿ target genes, livers from several animal studies in which PPARa was activated and/or disabled were analyzed by Affymetrix GeneChips. Numerous novel PPARa-regulated genes relevant to lipid metabolism were identified. Out of this set of genes, eight genes were singled out for study of PPARa-dependent regulation in mouse liver and in mouse, rat, and human primary hepatocytes, including thioredoxin interacting protein (Txnip), electron-transferring-flavoprotein ß polypeptide (Etfb), electron-transferring-flavoprotein dehydrogenase (Etfdh), phosphatidylcholine transfer protein (Pctp), endothelial lipase (EL, Lipg), adipose triglyceride lipase (Pnpla2), hormone-sensitive lipase (HSL, Lipe), and monoglyceride lipase (Mgll). Using an in silico screening approach, one or more PPAR response elements (PPREs) were identified in each of these genes. Regulation of Pnpla2, Lipe, and Mgll, which are involved in triglyceride hydrolysis, was studied under conditions of elevated hepatic lipids. In wild-type mice fed a high fat diet, the decrease in hepatic lipids following treatment with the PPARa agonist Wy14643 was paralleled by significant up-regulation of Pnpla2, Lipe, and Mgll, suggesting that induction of triglyceride hydrolysis may contribute to the anti-steatotic role of PPARa. Our study illustrates the power of transcriptional profiling to uncover novel PPARa-regulated genes and pathways in liver