Niacin is a widely used lipid-regulating agent in dyslipidemic patients. Previously, we have shown that niacin inhibits triacylglycerol synthesis. In this report, using HepG2 cells, we have examined the effect of niacin on the mRNA expression and microsomal activity of diacylglycerol acyltransferase 1 and 2 (DGAT1 and DGAT2), the last committed but distinctly different enzymes for triglyceride synthesis. Addition of niacin to the DGAT assay reaction mixture dose-dependently (0-3 mM) inhibited DGAT activity by 35-50%, and the IC 50 was found to be 0.1 mM. Enzyme kinetic studies showed apparent K m values of 8.3 M and 100 M using [ 14 C]oleoyl-CoA and sn -1,2-dioleoylglycerol as substrates, respectively. A decrease in apparent V max was observed with niacin, whereas the apparent K m remained constant. A Lineweaver-Burk plot of DGAT inhibition by niacin showed a noncompetitive type of inhibition. Niacin selectively inhibited DGAT2 but not DGAT1 activity. Niacin inhibited overt DGAT activity. Niacin had no effect on the expression of DGAT1 and DGAT2 mRNA. These data suggest that niacin directly and noncompetitively inhibits DGAT2 but not DGAT1, resulting in decreased triglyceride synthesis and hepatic atherogenic lipoprotein secretion, thus indicating a major target site for its mechanism of action. Niacin is an effective, unique lipid-regulating agent that beneficially reduces plasma triglycerides (TGs), cholesterol, and atherogenic apolipoprotein B (apoB)-containing lipoproteins (VLDL, LDL, and lipoprotein [a]) and increases antiatherogenic apoA-I-containing HDL levels (1-3). Several clinical trials have demonstrated that treatment with niacin significantly reduces total mortality and coronary events and retards the progression or induces the regression of coronary atherosclerosis (4-6). Despite its wide usage as a broad-spectrum lipid-regulating agent, the cellular and molecular mechanisms by which niacin modulates the hepatic lipid metabolism and the production of VLDL and LDL particles are incompletely understood.Using HepG2 cells as an in vitro model system, we have previously shown that niacin inhibited the incorporation of radiolabeled oleic acid or glycerol into TGs, suggesting decreased de novo synthesis of TGs by niacin (7). Additionally, we have shown that niacin also increased intracellular apoB degradation in HepG2 cells (7). Because the synthesis and availability of TG play a critical role in intracellular apoB processing and secretion of apoB-containing lipoproteins (8-10), our previous studies suggested that niacin, by inhibiting TG synthesis, increased intracellular apoB degradation, resulting in reduced secretion of apoB-containing particles (7). In support of these in vitro studies, earlier turnover studies in humans suggested that niacin decreased the production (transport) rate of TG from radiolabeled fatty acids, thus decreasing TG-rich lipoproteins (e.g., VLDL) and their product LDL (11).Modulation in the TG synthetic pathway and associated key enzyme systems may provide important to...
