Apolipoprotein A-I stimulates the transport of intracellular cholesterol to cell-surface cholesterol-rich domains (caveolae)

We have studied the effect of lipid-free human plasma apolipoprotein A-1 (apoA-1) on the transport of newly synthesized cholesterol to cell-surface cholesterol-rich domains, which in human skin fibroblasts are mainly represented by caveolae. Changes in transport of newly synthesized cholesterol were assessed after labelling cells with [C-14]acetate at 15 degreesC and warming cells to permit the transfer of cholesterol, followed by the selective oxidation of cholesterol in cholesterol-rich domains (caveolae) in the plasma membrane before their partial purification, ApoA-1, but not BSA added in an equimolar concentration, enhanced the transport of cholesterol to the caveolae up to 5-fold in a dose- and time-dependent manner. The effect of apoA-1 on cholesterol transport exceeded its effect on cholesterol efflux, resulting in an accumulation of intracellular cholesterol in caveolae. Methyl-beta -cyclodextrin, added at a concentration promoting cholesterol efflux to the same extent as apoA-1, also stimulated cholesterol trafficking, but was 3-fold less effective than apoA-1. Progesterone inhibited the transport of newly synthesized cholesterol to the caveolac. Treatment of cells with apoA-1 stimulated the expression of caveolin, increasing the amount of caveolin protein and mRNA by approx. 2-fold. We conclude that apoA-1 induces the transport of intracellular cholesterol to cell-surface caveolac, possibly in part through the stimulation of caveolin expression.