ACAT-2, A 2ND MAMMALIAN ACYL-COA-CHOLESTEROL ACYLTRANSFERASE - ITS CLONING, EXPRESSION, AND CHARACTERIZATION

The synthesis of cholesterol esters by acyl-CoA:cholesterol acyltransf erase (ACAT, EC 2.3.1.26) is an important component of cellular choles terol homeostasis, Cholesterol ester formation also is hypothesized to be important in several physiologic processes, including intestinal c holesterol absorption, hepatic lipoprotein production, and macrophage foam cell formation in atherosclerotic lesions. Mouse tissue expressio n studies and the disruption of the mouse ACAT gene (Acact) have indic ated that more than one ACAT exists in mammals and specifically that a nother enzyme is important in mouse liver and intestine. We now descri be a second mammalian ACAT enzyme, designated ACAT-2, that is 44% iden tical to the first cloned mouse ACAT (henceforth designated ACAT-1), I nfection of H5 insect cells with an ACAT-2 recombinant baculovirus res ulted in expression of a similar to 46-kDa protein in cell membranes t hat was associated with high levels of cholesterol esterification acti vity. Both ACAT-1 and ACAT-2 also catalyzed the esterification of the SP-hydroxyl group of a variety of oxysterols, Cholesterol esterificati on activities for ACAT-1 and ACAT-2 exhibited different IC50 values wh en assayed in the presence of several ACAT-specific inhibitors, demons trating that ACAT inhibitors can selectively target specific forms of ACAT, ACAT-2 was expressed primarily in mouse liver and small intestin e, supporting the hypothesis that ACAT-2 contributes to cholesterol es terification in these tissues. The mouse ACAT-S gene (Acact2) maps to chromosome 15 in a region containing a quantitative trait locus influe ncing plasma cholesterol levels. The identification and cloning of ACA T-S will facilitate molecular approaches to understanding the role of ACAT enzymes in mammalian biology.