Regulation of acyl-coenzyme A:cholesterol acyltransferase (ACAT) synthesis, degradation, and translocation by high-density lipoprotein2 at a low concentration

Although plasma HDL2 cholesterol concentration stands in inverse relation to risk for atherosclerotic disease, little is known about the mechanism of the apparent cardioprotection. In mouse P388D1 macrophages, HDL2 at a low concentration (less than or equal to 40 mug/mL) inhibits macrophage acyl-coenzyme A:cholesterol acyltransferase (ACAT), the enzyme that catalyzes esterification of intracellular cholesterol. The effects of HDL2 on ACAT synthesis, degradation, and intracellular translocation were investigated in mouse P388D1 macrophages. HDL2 at a low concentration enhanced ACAT synthesis but not total ACAT mass. Immunocytochemical studies showed that in the absence of lipoproteins, ACAT associated primarily with the perinuclear region of the cell. The addition of HDL2, however, induced the transfer of ACAT to vesicular structures and the cell periphery adjacent to the plasma membrane. Subfractionation combined with immunoprecipitation complemented these observations and showed that HDL2 promoted the transfer of ACAT to the plasma membrane fraction. Brefeldin A, which inhibits vesicular protein transport from the endoplasmic reticulum to the Golgi compartment in mammalian cells, blocked ACAT translocation and partially restored ACAT activity. These results suggest that HDL2 is an initiating factor in a signal transduction pathway that leads to intracellular ACAT translocation and inactivation.