Regulation of acyl-coenzyme A : cholesterol acyltransferase (ACAT) synthesis, degradation, and translocation by high-density lipoprotein(2) at a low concentration

Although plasma HDL2 cholesterol concentration stands in inverse relation t o risk for atherosclerotic disease, little is known about the mechanism of the apparent cardioprotection. In mouse P388D1 macrophages, HDL2 at a low c oncentration (less than or equal to 40 mug/mL) inhibits macrophage acyl-coe nzyme A:cholesterol acyltransferase (ACAT), the enzyme that catalyzes ester ification of intracellular cholesterol. The effects of HDL2 on ACAT synthes is, degradation, and intracellular translocation were investigated in mouse P388D1 macrophages. HDL2 at a low concentration enhanced ACAT synthesis bu t not total ACAT mass. Immunocytochemical studies showed that in the absenc e of lipoproteins, ACAT associated primarily with the perinuclear region of the cell. The addition of HDL2, however, induced the transfer of ACAT to v esicular structures and the cell periphery adjacent to the plasma membrane. Subfractionation combined with immunoprecipitation complemented these obse rvations and showed that HDL2 promoted the transfer of ACAT to the plasma m embrane fraction. Brefeldin A, which inhibits vesicular protein transport f rom the endoplasmic reticulum to the Golgi compartment in mammalian cells, blocked ACAT translocation and partially restored ACAT activity. These resu lts suggest that HDL2 is an initiating factor in a signal transduction path way that leads to intracellular ACAT translocation and inactivation.