Sterol regulatory element-binding protein-1 (SREBP-1) is a pivotal activator of lipogenic enzymes involved in fatty acid synthesis. Transcription and proteolytic cleavage of SREBP-1 is tightly regulated by nutritional and hormonal factors under physiological and pathological conditions. Insulin dramatically increases the transcription of SREBP-1 mRNA primarily by increasing the activity of LXRs, and promotes post-translational processing of the protein. In ob/ob diabetic mouse livers, nuclear SREBP-1c is highly induced and contributes to the elevated hepatic fatty acid biosynthesis and steatosis. Besides sterols, other lipids have also been shown to regulate SREBP-1 processing, including unsaturated fatty acids (FAs) and phospholipids. The mechanisms by which phospholipids affect SREBP-1c expression remain to be resolved. Recent work from our lab has identified an LXR-Lpcat3 pathway that dynamically modulates membrane phospholipid (phosphatidylcholine and Phosphatidylethanolamine) composition in response to changes in cellular lipid metabolism. Activation of LXRs preferentially drives the incorporation of polyunsaturated fatty acids into phospholipids through induction of the remodeling enzyme Lpcat3. Loss of Lpcat3 in liver selectively reduces arachidonoyl PC in liver membranes, and results in decreased membrane fluidity. Following on our initial work, we discovered that Lpcat3 regulates the LXR-mediated expression of genes involved in fatty acid biosynthesis. Our preliminary data suggest that Lpcat3 regulates the expression of lipogenic genes mainly through manipulating proteolytic cleavage of SREBP- 1. In this proposal, we aim to 1) Define the mechanisms by which Lpcat3 regulates SREBP-1 processing using Lpcat3 liver specific knockout primary hepatocytes and mice. 2) Determine whether Lpcat3 and phospholipid remodeling are involved in the regulation of SREBP-1c expression in liver under physiological or pathological conditions. We will assess if Lpcat3 mediates insulin-induced SREBP-1 expression in hepatocytes and in WT and LKO mouse livers during fasting/refeeding. We will examine the effect of Lpcat3 inhibition on SREBP-1 processing in ob/ob mice. Finally we will determine the involvement of Lpcat3 and phospholipid remodeling in human nonalcoholic fatty liver diseases (NAFLD). Given the close association between dysregulation of de novo lipogenesis and major human diseases, understanding the regulation of SREBP-1 expression is expect to provide novel insights into the molecular control of the development of fatty liver and dyslipidemia, and may suggest potential therapeutic opportunities for these diseases.
(Relevance) The rapidly increasing prevalence of obesity and its related metabolic conditions such as dyslipidemia, diabetes, heart diseases, and cancer etc., pose a major healthcare problem in western world. The proposed research will help us understand the mechanisms by which Lysophosphatidylcholine acyltransferase 3 (Lpcat3) and its product arachidonoyl PC regulate SREBP-1 processing and hepatic lipogenesis. It may unravel potential target for more effective treatment of these metabolic disorders.
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