THE ACTIN CYTOSKELETON IS IMPORTANT FOR THE STIMULATION OF CHOLESTEROL ESTERIFICATION BY ATHEROGENIC LIPOPROTEINS IN MACROPHAGES

Stimulation of intracellular cholesterol esterification, which is cata lyzed by the enzyme acyl-coenzyme A: cholesterol O-acyltransferase (AC AT), by atherogenic lipoproteins in macrophages is a key step in the a bility of these cells to store lipoprotein-cholesterol and in the even tual development of atheroma foam cells. Herein, we provide evidence t hat the actin cytoskeleton plays an important role in the stimulation of cholesterol esterification by atherogenic lipoproteins in macrophag es. When the actin cytoskeleton of cultured mouse peritoneal macrophag es was disrupted by treatment with cytochalasin D or Clostridial C2 to xin, the ability of beta very low density lipoprotein (beta-VLDL) to s timulate cholesterol esterification was decreased 3-6-fold, even under conditions in which beta-VLDL protein degradation, cholesteryl ester hydrolysis, or net cholesterol delivery to the cells was matched. Este rification of cellular phospholipids and triglycerides was not affecte d by this treatment. Cytochalasin D treatment of macrophages also inhi bited the ability of acetyl-low density lipoprotein, another foam cell -forming lipoprotein, to stimulate cholesterol esterification, but sti mulation of cholesterol esterification by 25-hydroxycholesterol was no t inhibited by cytochalasin D. Cytochalasin D was found to inhibit nei ther the exit of beta-VLDL derived cholesterol from lysosomes nor the ability of beta-VLDL to down-regulate endogenous cholesterol synthesis . From these data we conclude that an intact actin cytoskeleton is nec essary for efficient stimulation of cholesterol esterification by athe rogenic lipoproteins in macrophages. Although the exact function of ac tin in the cholesterol esterification pathway remains to be determined , our data rule out a general role for actin in intracellular choleste rol trafficking or maintenance of ACAT enzyme activity. Rather, we spe culate that actin filaments play a role in specific cellular entry pro cesses of atherogenic lipoproteins and/or in establishing transport or contact between the plasma membrane cholesterol substrate pool and th e ACAT enzyme in macrophages.