Mechanism underlying the regulation of SREBP-1 activation by glycerolipids

Abstract

Fatty acids are an indispensable energy reservoir of cells and are stored in their neutral lipid form as triglycerides inside lipid droplets. Fatty acids are also integral building blocks of cellular membranes and are required for the synthesis of phospholipids such as phosphatidylcholine and phosphatidylethanolamine. Fatty acids that are derived from de-novo lipid synthesis Alberts, Majerus undergo activation by fatty acyl- coenzyme A synthetase (ACSL) enzymes which catalyze the conversion of a fatty acid to its activated, acyl-coenzyme A (CoA) form. Activated fatty acids can undergo esterification with either a glycerol or a cholesterol molecule to be stored as triglycerides and cholesterol esters inside lipid droplets and are utilized from this energy source in states of physiological fasting. The de-novo synthesis pathways of both sterols and fatty acids originate from a common molecule acetyl-CoA, and are regulated by 2 isoforms of the SREBP transcription factor family. While the cellular mechanisms that govern cholesterol biosynthesis by SREBP-2 are well characterized, the mechanisms that govern fatty acid synthesis by SREBP-1 are not well understood. Specifically, how cells maintain the availability of sufficient fatty acids to fulfill biological need, and at the same time prevent the excessive synthesis and overaccumulation of fatty-acids is not known. In this dissertation, we studied how the processes of fatty acid synthesis and fatty acid storage are related to each other. Specifically, we examined how the activity of diacylglycerol acyltransferases DGAT-1 and DGAT-2 -- enzymes that esterify fatty acids into triglycerides – is coordinated with the SREBP-1 based transcription of de-novo fatty acid synthesis enzymes. By using a DGAT-2 genetic deletion mouse model and pharmacological inhibitors of DGAT-1 and DGAT-2, we discover that the proteolytic activation of SREBP-1 is coordinated with the storage of fatty acids into triglycerides. This coordination is a result of the retention of SREBP cleavage activating protein (SCAP) in the endoplasmic reticulum membrane, resulting in reduced SREBP-1 proteolytic cleavage. Further, we identify diacylglycerols as being the regulatory lipid mediators of this coordination between SREBP-1 and DGAT-2. The results from our work elucidate a previously unknown biological pathway regulating lipid synthesis. In addition, our data have significant clinical relevance for the identification of novel therapeutic approaches against non-alcoholic fatty liver disease. Our findings shed light on the mechanism of action of DGAT inhibitors which show promising efficacy for the treatment of non-alcoholic fatty liver disease and are currently under clinical development