We establish in this study that PLA2GXIIB is an HNF-4α target gene. We demonstrate that HNF-4α binds to a response element on the PLA2GXIIB promoter. Moreover, HNF-4α agonists induce PLA2GXIIB expression in human hepatocarcinoma cells. Importantly, PLA2GXIIB-null mice accumulate triglyceride, cholesterol, and fatty acids in the liver and develop severe hepatosteatosis resembling some of the phenotypes of liver-specific HNF-4α–null mice. These defects are in part due to compromised hepatic very low-density
lipoprotein secretion. Finally, adenovirus-mediated overexpression of HNF-4α elevates Selleck BMS-907351 serum triglyceride level in wild-type but not PLA2GXIIB-null mice. Conclusion: Collectively, these evidences suggest that HNF-4α is a key physiological PLA2GXIIB transcriptional regulator and that PLA2GXIIB is a novel mediator of triglyceride metabolism in the liver. (HEPATOLOGY 2011;53:458-466) Hepatocyte nuclear factor-4 alpha (HNF-4α) was initially identified as a transcriptional factor required for liver-specific gene expression.1 Coenzyme A (CoA) derivatives of certain fatty acids can bind to the ligand-binding domain of HNF-4α and activate the receptor,2 suggesting that HNF-4α activity is subjected to modulation by metabolic and nutritional
signals. HNF-4α regulates the expression of gluconeogenic genes, including phosphoenolpyruvate carboxykinase (PEPCK)3 and glucose-6-phosphatase (G6P),4 through binding to hormone response elements on their promoters and recruiting coactivators such as peroxisome proliferator-activated Navitoclax cell line receptor γ coactivator-1α (PGC-1α) to their promoters.5 In addition, HNF-4α and PGC-1α regulate the expression of lipoproteins and packaging enzymes such as microsomal triglyceride transfer protein (MTP) that are involved in very low-density lipoprotein much (VLDL) secretion. Significantly, liver-specific HNF-4α-null (HNF4αLivKO) mice suffer from severe defects in lipid homeostasis
with hepatosteatosis and reduced serum triglyceride (TG) and cholesterol levels.6 Phospholipases A2 are enzymes that catalyze the hydrolysis of glycerophospholipids at the Sn2 position to release free fatty acids such as arachidonic acid and lysophospholipids that are precursors of signaling molecules.7 In particular, arachidonic acid and its metabolites leukotrienes and prostaglandins are key inflammatory regulators. On the basis of their protein structures and biochemical properties, the superfamily of PLA2 can be divided into five principal kinds of enzyme: cytosolic PLA2s (cPLA2s), Ca2+-independent PLA2s (iPLA2s), lysosomal PLA2s, platelet activating factor acetylhydrolases, and secreted PLA2s (sPLA2s). Secreted PLA2s have relatively low molecular masses of 14-19 kDa, a large number of disulfides, and similar Ca2+-dependent catalytic mechanism. Mammalian sPLA2s include GIB, GIIA, GIIC, GIID, GIIE, GIIF, GIII, GV, GX, GXIA, GXIB, GXIIA, and GXIIB.