Triglycerides (or triacylglycerols) represent the major form of stored energy in eukaryotes. Triglyceride synthesis has been assumed to occur primarily through acyl CoA:diacylglycerol transferase (Dgat), a microsomal enzyme that catalyses the final and only committed step in the glycerol phosphate pathway. Therefore, Dgat has been considered necessary for adipose tissue formation and essential for survival. Here we show that Dgat-deficient (Dgat-/-) mice are viable and can still synthesize triglycerides. Moreover, these mice are lean and resistant to diet-induced obesity. The obesity resistance involves increased energy expenditure and increased activity. Dgat deficiency also alters triglyceride metabolism in other tissues, including the mammary gland, where lactation is defective in Dgat-/- females. Our findings indicate that multiple mechanisms exist for triglyceride synthesis and suggest that the selective inhibition of Dgat-mediated triglyceride synthesis may be useful for treating obesity.

Studies involving the cloning and disruption of the gene for acyl-CoA:diacylglycerol acyltransferase (DGAT) have shown that alternative mechanisms exist for triglyceride synthesis. In this study, we cloned and characterized a second mammalian DGAT, DGAT2, which was identified by its homology to a DGAT in the fungus Mortierella rammaniana. DGAT2 is a member of a gene family that has no homology with DGAT1 and includes several mouse and human homologues that are candidates for additional DGAT genes. The expression of DGAT2 in insect cells stimulated triglyceride synthesis 6-fold in assays with cellular membranes, and DGAT2 activity was dependent on the presence of fatty acyl-CoA and diacylglycerol, indicating that this protein is a DGAT. Activity was not observed for acyl acceptors other than diacylglycerol. DGAT2 activity was inhibited by a high concentration (100 mM) of MgCl 2 in an in vitro assay, a characteristic that distinguishes DGAT2 from DGAT1. DGAT2 is expressed in many tissues with high expression levels in the liver and white adipose tissue, suggesting that it may play a significant role in mammalian triglyceride metabolism.

Deficiency of acyl CoA:cholesterol acyltransferase 2 (ACAT2) in mice results in a reduction in cholesterol ester synthesis in the small intestine and liver, which in turn limits intestinal cholesterol absorption, hepatic cholesterol gallstone formation, and the accumulation of cholesterol esters in the plasma lipoproteins. Here we examined the contribution of ACAT2-derived cholesterol esters to atherosclerosis by crossing ACAT2-deficient (ACAT2 -/-) mice with apolipoprotein (apo) E-deficient (ApoE -/-) mice, an atherosclerosissusceptible strain that has impaired apoE-mediated clearance of apoB-containing lipoproteins. ACAT2 -/-ApoE -/-mice and ACAT2 ؉/؉ ApoE -/-(control) mice had similar elevations of plasma apoB and total plasma lipids; however, the lipid cores of the apoB-containing lipoproteins in ACAT2 -/-ApoE -/-mice contained primarily triglycerides rather than cholesterol esters. At 30 wk of age, only the control mice had significant atherosclerosis, which was nearly absent in ACAT2 -/-ApoE -/-mice. ACAT2 deficiency in the apoE-deficient background also led to a compensatory increase in the activity of lecithin͞cholesterol acyltransferase, the major plasma cholesterol esterification enzyme, which increased highdensity lipoprotein cholesterol esters. Our results demonstrate the crucial role of ACAT2-derived cholesterol esters in the development of atherosclerosis in mice and suggest that triglyceride-rich apoBcontaining lipoproteins are not as atherogenic as those containing cholesterol esters. Our results also support the rationale of pharmacological inhibition of ACAT2 as a therapy for atherosclerosis.