PPARα is a ligand-activated transcription factor involved in the regulation of nutrient metabolism and inflammation. Although much is already known about the function of PPARα in hepatic lipid metabolism, many PPARα-dependent pathways and genes have yet to be discovered. In order to obtain an overview of PPARα-regulated genes relevant to lipid metabolism, and to probe for novel candidate PPARα target genes, livers from several animal studies in which PPARα was activated and/or disabled were analyzed by Affymetrix GeneChips. Numerous novel PPARα-regulated genes relevant to lipid metabolism were identified. Out of this set of genes, eight genes were singled out for study of PPARα-dependent regulation in mouse liver and in mouse, rat, and human primary hepatocytes, including thioredoxin interacting protein (Txnip), electron-transferring-flavoprotein β 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 PPARα 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 PPARα. Our study illustrates the power of transcriptional profiling to uncover novel PPARα-regulated genes and pathways in liver.

Peroxisome proliferator-activated receptor ␣ (PPAR␣) is an important transcription factor in liver that can be activated physiologically by fasting or pharmacologically by using high-affinity synthetic agonists. Here we initially set out to elucidate the similarities in gene induction between Wy14643 and fasting. Numerous genes were commonly regulated in liver between the two treatments, including many classical PPAR␣ target genes, such as Aldh3a2 and Cpt2. Remarkably, several genes induced by Wy14643 were upregulated by fasting independently of PPAR␣, including Lpin2 and St3gal5, suggesting involvement of another transcription factor. Using chromatin immunoprecipitation, Lpin2 and St3gal5 were shown to be direct targets of PPAR␤/␦ during fasting, whereas Aldh3a2 and Cpt2 were exclusive targets of PPAR␣. Binding of PPAR␤/␦ to the Lpin2 and St3gal5 genes followed the plasma free fatty acid (FFA) concentration, consistent with activation of PPAR␤/␦ by plasma FFAs. Subsequent experiments using transgenic and knockout mice for Angptl4, a potent stimulant of adipose tissue lipolysis, confirmed the stimulatory effect of plasma FFAs on Lpin2 and St3gal5 expression levels via PPAR␤/␦. In contrast, the data did not support activation of PPAR␣ by plasma FFAs. The results identify Lpin2 and St3gal5 as novel PPAR␤/␦ target genes and show that upregulation of gene expression by PPAR␤/␦ is sensitive to plasma FFA levels. In contrast, this is not the case for PPAR␣, revealing a novel mechanism for functional differentiation between PPARs.Hepatic lipid metabolism is governed by a complex interplay between hormones, transcription factors, and energy substrates, allowing for rapid adaptations to changes in metabolic needs (21). According to the traditional view, energy substrates such as fatty acids influence lipid metabolism by promoting flux through a particular pathway via mass action. However, it has become clear that energy substrates can also directly govern the transcription of enzymes involved in lipid metabolism via mechanisms analogous to those of many hormones. Indeed, it is now evident that glucose and fatty acids play a major regulatory role in hepatic lipid metabolism via direct activation or inhibition of specific transcription factors, including carbohydrate response element binding protein (6, 63), sterol response element binding protein 1 (SREBP1) (2, 41, 58, 61, 62), and peroxisome proliferator-activated receptor ␣ (PPAR␣) (38).Although numerous transcription factors have been shown to be activated by fatty acids in vitro, recent data suggest that PPAR␣ is dominant in mediating the effects of dietary fatty acids on gene expression in liver (48). PPAR␣ is a member of the superfamily of nuclear receptors and is closely related to the other PPAR isoforms, ␤/␦ and ␥ (32). Similar to several other nuclear receptors, PPARs function as heterodimers with the retinoid X receptor and bind to specific sequences on the DNA referred to as PPAR response elements (PPREs) (8,11,26). Numerous studies have shown that fatty acids...

Little is known about the role of the transcription factor peroxisome proliferator-activated receptor (PPAR) β/δ in liver. Here we set out to better elucidate the function of PPARβ/δ in liver by comparing the effect of PPARα and PPARβ/δ deletion using whole genome transcriptional profiling and analysis of plasma and liver metabolites. In fed state, the number of genes altered by PPARα and PPARβ/δ deletion was similar, whereas in fasted state the effect of PPARα deletion was much more pronounced, consistent with the pattern of gene expression of PPARα and PPARβ/δ. Minor overlap was found between PPARα- and PPARβ/δ-dependent gene regulation in liver. Pathways upregulated by PPARβ/δ deletion were connected to innate immunity and inflammation. Pathways downregulated by PPARβ/δ deletion included lipoprotein metabolism and various pathways related to glucose utilization, which correlated with elevated plasma glucose and triglycerides and reduced plasma cholesterol in PPARβ/δ−/− mice. Downregulated genes that may underlie these metabolic alterations included Pklr, Fbp1, Apoa4, Vldlr, Lipg, and Pcsk9, which may represent novel PPARβ/δ target genes. In contrast to PPARα−/− mice, no changes in plasma free fatty acid, plasma β-hydroxybutyrate, liver triglycerides, and liver glycogen were observed in PPARβ/δ−/− mice. Our data indicate that PPARβ/δ governs glucose utilization and lipoprotein metabolism and has an important anti-inflammatory role in liver. Overall, our analysis reveals divergent roles of PPARα and PPARβ/δ in regulation of gene expression in mouse liver.