Regulation of Hepatic SphK2 by Bile Acids: Effects on Lipid Metabolism

描述（由申请人提供）：胆汁酸对肝脏脂质稳态至关重要。在过去的十年里，研究人员发现胆汁酸不仅可以作为清洁剂，还可以作为重要的信号分子。它们已被证明通过激活特定的核受体、G蛋白偶联受体（GPCR、TGR5）和肝脏和胃肠道细胞中的多种信号通路来调节参与胆汁酸、葡萄糖、脂肪酸和脂蛋白的代谢/合成以及能量代谢的许多编码酶/蛋白质的基因的表达。我们之前报道过偶联胆汁酸通过G¿i蛋白偶联受体激活AKT和ERK1/2通路。我们最近的研究表明，胆汁酸上调鞘氨醇激酶2 （SphK2）的表达（mRNA和蛋白水平）和酶活性，并增加血清鞘氨醇-1-磷酸（S1P）水平。ERK1/2激活SphK2增加核S1P水平，这已被证明是特异性组蛋白去乙酰化酶（HDAC 1和2）的抑制剂。组蛋白乙酰化的增加通常与生理上转录活性的增加有关

DESCRIPTION (provided by applicant): Bile acids are crucial for lipid homeostasis in the liver. During the last decade, researchers have discovered that bile acids not only act as detergents, but also as important signaling molecules. They have been shown to regulate the expression of numerous genes encoding enzymes/proteins involved in the metabolism/synthesis of bile acids, glucose, fatty acids, and lipoproteins as well as energy metabolism by activating specific nuclear receptors, a G protein coupled receptor (GPCR, TGR5), and multiple signaling pathways in cells of the liver and gastrointestinal tract. We have previously reported that conjugated bile acids activate the AKT and ERK1/2 pathways via G¿i protein coupled receptor(s). Our most recent studies show that bile acids upregulate sphingosine kinase 2 (SphK2) expression (at both mRNA and protein levels) and enzyme activity and increase serum sphingosine-1-phosphate (S1P) levels. Activation of SphK2 by ERK1/2 increases nuclear S1P levels, which has been shown to be an inhibitor of specific histone deacetylases (HDAC 1 and 2). Increased histone acetylation is often associated with an increase in the transcriptional activity of physiologically linked genes. Our preliminary data strongly indicates that conjugated bile acids along with S1P activate the S1P receptor 2 (S1P2), which further activates the downstream ERK1/2 and AKT signaling pathways in hepatocytes. We recently discovered that in SphK2-/- mice the mRNA levels of key transcriptional factors and key enzymes involved in hepatic lipid metabolism were significantly down-regulated. The SphK2-/- mouse was also found to have an ~18-fold increase in serum triglycerides and a 2 to 5-fold decrease in both HDL and LDL as compared to wild-type control mice. Moreover, tauroursodeoxycholate (TUDCA)-mediated inhibition of lipid accumulation was reversed in the primary hepatocytes of S1P2-/- and SphK2-/- mice. However, the physiological link between bile acids, S1P, S1P receptors and their roles in regulating hepatic sterol/lipid metabolism have not been revealed. Based on the recent studies and our preliminary results, we hypothesize that bile acid-mediated activation of S1P2 plays a critical role in regulating hepatic lipid metabolism by phosphorylation and activation of SphK2 in the nucleus. The following three specific aims are proposed to test our central hypothesis. Aim 1: Characterize the activation of S1P2 by different bile acids and determine the structural-functional relationship of S1P2 and bile acids; Aim 2: Elucidate the role of S1P2 and SphK2 in bile acid-mediated regulation of hepatic sterol/lipid metabolism using S1P2-/- and SphK2-/- mice models; Aim 3: Determine the effects of administration of different bile acids (TUDCA, glycocholate and deoxycholate) and S1P2 antagonist on SphK2 expression and activity as well as activities of HDAC1 and HDAC2 in the liver using an in vivo rat model of bile acid depletion. Completion of this study will not only establish a novel theory in bile acid biology, but will also provide new mechanistic insights into the pathophysiology of dyslipidemia and other metabolic diseases including non-alcoholic fatty liver disease (NAFLD). Also, this study has potential impact on the identification and development of novel therapeutic targets for effective treatment of NAFLD and other related metabolic diseases.