Yao-Wu Zheng

Triacylglycerols are quantitatively the most important storage form of energy for eukaryotic cells. Acyl CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the terminal and only committed step in triacylglycerol synthesis, by using diacylglycerol and fatty acyl CoA as substrates. DGAT plays a fundamental role in the metabolism of cellular diacylglycerol and is important in higher eukaryotes for physiologic processes involving triacylglycerol metabolism such as intestinal fat absorption, lipoprotein assembly, adipose tissue formation, and lactation. DGAT is an integral membrane protein that has never been purified to homogeneity, nor has its gene been cloned. We identified an expressed sequence tag clone that shared regions of similarity with acyl CoA:cholesterol acyltransferase, an enzyme that also uses fatty acyl CoA as a substrate. Expression of a mouse cDNA for this expressed sequence tag in insect cells resulted in high levels of DGAT activity in cell membranes. No other acyltransferase activity was detected when a variety of substrates, including cholesterol, were used as acyl acceptors. The gene was expressed in all tissues examined; during differentiation of NIH 3T3-L1 cells into adipocytes, its expression increased markedly in parallel with increases in DGAT activity. The identification of this cDNA encoding a DGAT will greatly facilitate studies of cellular glycerolipid metabolism and its regulation.

The synthesis of cholesterol esters by acyl-CoA:cholesterol acyltransferase (ACAT, EC 2.3.1.26) is an important component of cellular cholesterol homeostasis. Cholesterol ester formation also is hypothesized to be important in several physiologic processes, including intestinal cholesterol absorption, hepatic lipoprotein production, and macrophage foam cell formation in atherosclerotic lesions. Mouse tissue expression studies and the disruption of the mouse ACAT gene (Acact) have indicated that more than one ACAT exists in mammals and specifically that another enzyme is important in mouse liver and intestine. We now describe a second mammalian ACAT enzyme, designated ACAT-2, that is 44% identical to the first cloned mouse ACAT (henceforth designated ACAT-1). Infection of H5 insect cells with an ACAT-2 recombinant baculovirus resulted in expression of a approximately 46-kDa protein in cell membranes that was associated with high levels of cholesterol esterification activity. Both ACAT-1 and ACAT-2 also catalyzed the esterification of the 3beta-hydroxyl group of a variety of oxysterols. Cholesterol esterification activities for ACAT-1 and ACAT-2 exhibited different IC50 values when assayed in the presence of several ACAT-specific inhibitors, demonstrating that ACAT inhibitors can selectively target specific forms of ACAT. ACAT-2 was expressed primarily in mouse liver and small intestine, supporting the hypothesis that ACAT-2 contributes to cholesterol esterification in these tissues. The mouse ACAT-2 gene (Acact2) maps to chromosome 15 in a region containing a quantitative trait locus influencing plasma cholesterol levels. The identification and cloning of ACAT-2 will facilitate molecular approaches to understanding the role of ACAT enzymes in mammalian biology.