SUMMARY De novo lipogenesis in adipocytes, especially with high fat feeding, is poorly understood. We demonstrate that an adipocyte lipogenic pathway encompassing fatty acid synthase (FAS) and PexRAP (Peroxisomal Reductase Activating PPARγ) modulates endogenous PPARγ activation and adiposity. Mice lacking FAS in adult adipose tissue manifested increased energy expenditure, increased brown fat-like adipocytes in subcutaneous adipose tissue, and resistance to diet-induced obesity. FAS knockdown in embryonic fibroblasts decreased PPARγ transcriptional activity and adipogenesis. FAS-dependent alkyl ether phosphatidylcholine species were associated with PPARγ and treatment of 3T3-L1 cells with one such ether lipid increased PPARγ transcriptional activity. PexRAP, a protein required for alkyl ether lipid synthesis, was associated with peroxisomes and induced during adipogenesis. PexRAP knockdown in cells decreased PPARγ transcriptional activity and adipogenesis. PexRAP knockdown in mice decreased expression of PPARγ–dependent genes and reduced diet-induced adiposity. These findings suggest that inhibiting PexRAP or related lipogenic enzymes could treat obesity and diabetes.

Fatty acid synthase (FAS) catalyzes the de novo synthesis of fatty acids. In the liver, FAS has long been categorized as a housekeeping protein, producing fat for storage of energy when nutrients are present in excess. Most previous studies of FAS regulation have focused on the control of gene expression. However, recent findings suggest that hepatic FAS may also be involved in signaling processes that include activation of peroxisome proliferator-activated receptor α (PPARα). Moreover, reports of rapid alterations in FAS activity and as well as findings of post-translational modifications of the FAS protein support the notion that dynamic events in addition to transcription impact FAS regulation. These results indicate that FAS enzyme activity can impact liver physiology through signaling as well as energy storage and that its regulation may be complex.

This article is available online at http://www.jlr.org targeted to treat lipid disorders, diabetes, and obesity, is highly expressed in the liver. Its induction by fasting promotes lipid uptake, fatty acid ␤ -oxidation, ketogenesis, and gluconeogenesis ( 1, 2 ). Ligand binding to PPAR ␣ causes it to heterodimerize with retinoid X receptor (RXR) ␣ , allowing activation of gene transcription at peroxisome proliferator response elements (PPRE) ( 3, 4 ). Synthetic PPAR ␣ ligands, such as fi brates, used for human lipid disorders ( 5 ) have been known for decades, but potential endogenous ligands were identifi ed only recently ( 6, 7 ). Mice with liver-specifi c deletion of the lipogenic enzyme fatty acid synthase (FAS) have impaired PPAR ␣ activity ( 8 ), and FAS activates PPAR ␣ by producing an endogenous phospholipid ligand ( 6 ). FAS also activates PPAR ␣ in brain and macrophages ( 9, 10 ).Mammalian FAS synthesizes long-chain fatty acids, primarily palmitate, through the activities of seven functional domains: acyl carrier, acyl transferase, ␤ -ketoacyl synthase, ␤ -ketoacyl reductase, ␤ -hydroxyacyl dehydratase, enoyl reductase, and thioesterase ( 11 ). Like PPAR ␣ , FAS is highly expressed in liver ( 12 ). In times of nutrient excess, hepatic FAS converts carbohydrate to lipid that is stored in lipid droplets or secreted in the form of VLDL ( 13 ). Nutrient excess is associated with elevated levels of insulin, known to induce FAS expression.These accepted physiological roles for PPAR ␣ and FAS appear to confl ict with the observation that inactivation of FAS impairs PPAR ␣ activation. How might FAS activate a process stimulated by feeding such as insulin-responsive lipogenesis and also activate a process stimulated by fasting such as the induction of PPAR ␣ -dependent gene expression?We hypothesized that distinct subcellular pools of FAS mediate these disparate effects. Compartmentalization would permit regulation of an FAS pool generating lipids for signaling that would be distinct from an FAS pool generating lipids for energy storage. In support of this hypothesis, Abstract Peroxisome proliferator-activated receptor (PPAR) ␣ is a nuclear receptor that coordinates liver metabolism during fasting. Fatty acid synthase (FAS) is an enzyme that stores excess calories as fat during feeding, but it also activates hepatic PPAR ␣ by promoting synthesis of an endogenous ligand. Here we show that the mechanism underlying this paradoxical relationship involves the differential regulation of FAS in at least two distinct subcellular pools: cytoplasmic and membrane-associated. In mouse liver and cultured hepatoma cells, the ratio of cytoplasmic to membrane FAS-specifi c activity was increased with fasting, indicating higher cytoplasmic FAS activity under conditions associated with PPAR ␣ activation. This effect was due to a nutrient-dependent and compartment-selective covalent modifi cation of FAS. Cytoplasmic FAS was preferentially phosphorylated during feeding or insulin treatment at Thr-1029 and Thr-1033, which fl ank a d...

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