The role of metabolic enzymes in lipid droplet dynamics

Abstract

Lipid droplets (LDs) are important metabolic and dynamic organelles existing in most cell types and organisms. Each LD contains a neutral lipid core, triacylglycerols (TAGs) and/or cholesteryl esters (CEs). Changes in the cellular dynamics of LDs are associated with human diseases. However, the molecular mechanisms governing the biogenesis and expansion of LDs are still poorly understood. In this thesis, I investigated the role of two metabolic enzymes, CDP-diacylglycerol (CDP-DAG) synthases (CDSs) and Acyl-CoA: cholesterol acyltransferases (ACATs) in LD growth. CDSs and ACATs are responsible for the synthesis of TAG and CE, respectively. CDSs, encoded by CDS1 and CDS2 genes in mammals, catalyse the formation of CDP-DAG from phosphatidic acid (PA). PA is the precursor for TAG and phospholipid synthesis. Depleting CDS1 or CDS2 caused supersized lipid droplet (sLD) formation. My data suggested that CDS1 and CDS2 functioned differently during LD formation in both the early stage (initial LD formation) and the late stage (sLD formation). The mechanisms underlying sLD expansion resulting from CDS1 or CDS2 deficiency were investigated. The fusion event mediated by CIDEa, b, and especially CIDEc, was responsible for sLD formation caused by CDS1 depletion, while the enhanced TAG synthesis resulted in the formation of sLDs caused by CDS2 deficiency. This promoted the TAG-synthesising enzymes DGAT2/GPAT4 to accumulate on the LD surface. ACATs are known to catalyse the conversion of free cholesterol (FC) into CEs. In mammals, there are two ACAT isoenzymes. ACAT1 is present ubiquitously in many cell types; ACAT2 is enriched mainly in intestinal enterocytes. Adipose tissue is the major storage site of TAG and has the largest FC pool in the body. However, no ACAT activity can be detected in adipocytes. Through generating an ACAT1/2 overexpressing system in adipocytes, our data showed that the overexpression of ACAT1/2, especially ACAT2, severely impaired differentiation and LD formation during adipogenesis. Also, the overexpression of ACATs promoted the accumulation of FC on the LD surface through impacting cholesterol homeostasis. Moreover, overexpressing ACAT1/2 affected metabolic functions in adipocytes. In conclusion, work in this thesis has provided mechanistic insights into the roles of CDS enzymes and ACAT enzymes in LD dynamics.