Nutrient-dependent phosphorylation channels lipid synthesis to regulate PPARα

Abstract

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...

Thyroid hormone responsive protein Spot14 enhances catalysis of fatty acid synthase in lactating mammary epithelium

“…IVB). This observation indicates that the regulation of FASN by mTORC1 may not be a feature in lactating MECs as was observed in liver ( 12 ). Site-specifi c antibodies are needed to properly analyze the phosphorylation status of FASN as it pertains to enzyme activity.…”

“…It has been reported that mTORC mediates threonine phosphorylation at positions 1029 and 1033, which negatively regulates FASN in the mouse liver ( 12 ). Currently, no commercial antibodies directed against the phosphorylated epitopes of FASN exist; its analysis requires immunoprecipitation of FASN and use of phosphorylated threonine-specifi c antibodies ( 12 ). No difference was observed in the phosphorylated enzyme responsible for releasing MCFA products from FASN during lactation ( 55 ), was maintained at the mRNA level (supplementary Fig.…”

“…Regulation of FASN activity by phosphorylation is beginning to emerge. It has been reported that mTORC mediates threonine phosphorylation at positions 1029 and 1033, which negatively regulates FASN in the mouse liver ( 12 ). Currently, no commercial antibodies directed against the phosphorylated epitopes of FASN exist; its analysis requires immunoprecipitation of FASN and use of phosphorylated threonine-specifi c antibodies ( 12 ).…”

De Novo Palmitate Synthesis Supports Oncogenic Signalling and Tumor Growth Through Diverse Mechanisms: Implications for FASN-Targeted Therapeutics

“…Do tumor cells, at least certain subtypes with definable features, rely on de novo synthesized saturated fatty acids for vital growth, proliferation, and survival functions? Currently available data support the concept that tumor cells require what can be thought of as privileged pools of fatty acids and lipids that can be satisfied only through FASN-mediated de novo palmitate synthesis [40]. The requirement for these privileged pools and the detailed function that they provide remains to be established unequivocally; this is an area deserving active investigation.…”

Crystal Structure and Substrate Specificity of Human Thioesterase 2

“…The expression of FASN is up-regulated in cancer cells by gene amplification and a variety of transcriptional and translational mechanisms (3)(4)(5)(6)(7). The enzymatic activity of FASN can also be modulated by phosphorylation (8). In these scenarios either FASN levels are affected or activity level is modulated.…”