SUMMARY Lipid droplets (LDs) store metabolic energy and membrane lipid precursors. With excess metabolic energy, cells synthesize triacylglycerol (TG) and form LDs that grow dramatically. It is unclear how TG synthesis relates to LD formation and growth. Here, we identify two LD subpopulations: smaller LDs of relatively constant size, and LDs that grow larger. The latter population contains isoenzymes for each step of TG synthesis. Glycerol-3-phosphate acyltransferase 4 (GPAT4), which catalyzes the first and rate-limiting step, relocalizes from the endoplasmic reticulum (ER) to a subset of forming LDs, where it becomes stably associated. ER-to-LD targeting of GPAT4 and other LD-localized TG synthesis isozymes is required for LD growth. Key features of GPAT4 ER-to-LD targeting and function in LD growth are conserved between Drosophila and mammalian cells. Our results explain how TG synthesis is coupled with LD growth and identify two distinct LD subpopulations based on their capacity for localized TG synthesis.

Liver fatty acid-binding protein (L-Fabp

The importance of cholesterol ester synthesis by acyl CoA:cholesterol acyltransferase (ACAT) enzymes in intestinal and hepatic cholesterol metabolism has been unclear. We now demonstrate that ACAT2 is the major ACAT in mouse small intestine and liver, and suggest that ACAT2 deficiency has profound effects on cholesterol metabolism in mice fed a cholesterol-rich diet, including complete resistance to diet-induced hypercholesterolemia and cholesterol gallstone formation. The underlying mechanism involves the lack of cholesterol ester synthesis in the intestine and a resultant reduced capacity to absorb cholesterol. Our results indicate that ACAT2 has an important role in the response to dietary cholesterol, and suggest that ACAT2 inhibition may be a useful strategy for treating hypercholesterolemia or cholesterol gallstones.

Dietary triacylglycerols are a major source of energy for animals. The absorption of dietary triacylglycerols involves their hydrolysis to free fatty acids and monoacylglycerols in the intestinal lumen, the uptake of these products into enterocytes, the resynthesis of triacylgylcerols, and the incorporation of newly synthesized triacylglycerols into nascent chylomicrons for secretion. In enterocytes, the final step in triacylglycerol synthesis is believed to be catalyzed primarily through the actions of acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. In this study, we analyzed intestinal triacylglycerol absorption and chylomicron synthesis and secretion in DGAT1-deficient (Dgat1 ؊/؊ ) mice. Surprisingly, DGAT1 was not essential for quantitative dietary triacylglycerol absorption, even in mice fed a high fat diet, or for the synthesis of chylomicrons. However, Dgat1 ؊/؊ mice had reduced postabsorptive chylomicronemia (1 h after a high fat challenge) and accumulated neutrallipid droplets in the cytoplasm of enterocytes when chronically fed a high fat diet. These results suggest a reduced rate of triacylglycerol absorption in Dgat1 mice. Analysis of intestine from Dgat1؊/؊ mice revealed activity for two other enzymes, DGAT2 and diacylglycerol transacylase, that catalyze triacylglycerol synthesis and apparently help to compensate for the absence of DGAT1. Our findings indicate that multiple mechanisms for triacylglycerol synthesis in the intestine facilitate triacylglycerol absorption.The absorption of triacylglycerols by the intestine is highly efficient, and more than 95% of dietary triacylglycerols is absorbed, even if the diet is rich in fat. By comparison, only 30 -70% of dietary cholesterol is absorbed in most animals (1). The high efficiency of triacylglycerol absorption is likely due to an evolutionary pressure that maximized the ability to absorb rich sources of energy (such as fat) when food sources were scarce.Intestinal triacylglycerol absorption occurs by a series of steps in which dietary triacylglycerols are first hydrolyzed in the intestinal lumen and then resynthesized within enterocytes. In the lumen, dietary triacylglycerols are hydrolyzed by lipases to generate free fatty acids and monoacylglycerols. These molecules are taken up by enterocytes and then enter the triacylglycerol biosynthesis pathways. The triacylglycerol products are incorporated into nascent chylomicrons, which are subsequently secreted from enterocytes and enter the lymphatic system.Triacylglycerol biosynthesis in the intestine is believed to occur mainly through the monoacylglycerol pathway. In this pathway, monoacylglycerol and fatty acyl-CoA are covalently joined to form diacylglycerol in a reaction catalyzed by monoacylglycerol acyltransferase (MGAT) 1 (2). Diacylglycerol and fatty acyl-CoA are then used to synthesize triacylglycerol in a reaction catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes. High levels of DGAT activity are present in the small intestine (3-5), and both known DGAT genes, Dgat1...