Lipid droplets (LDs) are found in nearly all eukaryotic cells, and each consists of a neutral lipid core enveloped by a phospholipid monolayer and surface proteins ( 1 ). The main lipids found in the cores of LDs are triacylglycerols (TGs) and sterol esters (SEs). Whether TGs are required for the formation of LDs is unknown. The major known enzymes that catalyze TG synthesis in mammals are the acyl CoA:diacylglycerol acyltransferases (DGAT; Fig. 1A ) ( 2 ), which catalyze the covalent addition of a fatty acyl chain to diacylglycerol. Genetic deletion of DGAT1 in mice revealed that this enzyme is not essential and that DGAT1 knockout (DGAT1 KO) mice have reductions in TG levels in many tissues, including adipose tissue, when fed a high-fat diet ( 3 ). Deletion of DGAT2 revealed that this enzyme is essential: mice lacking DGAT2 have severe reductions in TG levels and die shortly after birth ( 4 ). Nevertheless, newborn DGAT2 KO mice do have some TG, which may be due to DGAT1 activity. Given that enzymes in both the DGAT1 (MBOAT, 16 family members) and DGAT2 (7 members) families possess many different lipid acyltransferase activities ( 5, 6 ), and that several of these Abstract The total contribution of the acyl CoA:diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, to mammalian triacylglycerol (TG) synthesis has not been determined. Similarly, whether DGAT enzymes are required for lipid droplet (LD) formation is unknown. In this study, we examined the requirement for DGAT enzymes in TG synthesis and LDs in differentiated adipocytes with genetic deletions of DGAT1 and DGAT2. Adipocytes with a single deletion of either enzyme were capable of TG synthesis and LD formation. In contrast, adipocytes with deletions of both DGATs were severely lacking in TG and did not have LDs, indicating that DGAT1 and DGAT2 account for nearly all TG synthesis in adipocytes and appear to be required for LD formation during adipogenesis. DGAT enzymes were not absolutely required for LD formation in mammalian cells, however; macrophages defi cient in both DGAT enzymes were able to form LDs when incubated with cholesterolrich lipoproteins. Although adipocytes lacking both DGATs had no TG or LDs, they were fully differentiated by multiple criteria.Our fi ndings show that DGAT1 and DGAT2 account for the vast majority of TG synthesis in mice, and DGAT function is required for LDs in adipocytes, but not in all cell

The niacin receptor GPR109A is a Gi-protein coupled receptor which mediates the effects of niacin on inhibiting intracellular triglyceride lipolysis in adipocytes. However, the role of GPR109A in mediating the effects of niacin on high density lipoprotein (HDL) metabolism is unclear. We found niacin has no effect on HDL-C in GPR109A knockout mice. Furthermore, niacin lowered intracellular cAMP in primary hepatocytes mediated by GPR109A. We used an adeno-associated viral (AAV) serotype 8 vector encoding GPR109A under the control of the hepatic-specific thyroxine-binding globulin promoter to specifically overexpress GPR109A in mouse liver. Plasma HDL-C, hepatic ABCA1 and the HDL cholesterol production rate were significantly reduced in mice overexpressing GPR109A. Overexpression of GPR109A reduced primary hepatocyte free cholesterol efflux to apoA-I; conversely, GPR109A deficient hepatocytes had increased ABCA1-mediated cholesterol efflux. These data support the concept that the HDL-C lowering effect of niacin in wild-type mice is mediated through stimulation of GPR109A in hepatocytes; such an effect then leads to reduced hepatocyte ABCA1 expression and activity, decreased cholesterol efflux to nascent apoA-I, and reduced HDL-C levels. These results indicate that niacin-mediated activation of GP109A in liver lowers ABCA1 expression leading to reduced hepatic cholesterol efflux to HDL.

The modification of cholesterol efflux capacity (CEC) by current medications and interventions has been investigated in both large randomized control trials and smaller observational cohorts. This review serves to compile the results of these studies and evaluate CEC modulation by commonly used medications. Altering CEC could be a novel therapeutic approach to improving cardiovascular risk profiles.