Cholesterol efflux to apolipoprotein AI involves endocytosis and resecretion in a calcium-dependent pathway

We previously have described the cAMP-mediated induction of cholesterol and phospholipid efflux from the murine macrophage RAW264 cell line to lipid-f ree apolipoprotein accepters. This induction of cholesterol efflux is assoc iated with increased binding and association of apolipoprotein to the cells . In the present study, using primarily apolipoprotein AI (apeAI) as the ac ceptor, cAMP-dependent cholesterol efflux to apolipoprotein accepters was a ssociated with apoAI binding to coated pits, cellular uptake, and resecreti on. After cell association and washing, 58% of the apoAI was resecreted dur ing a 90-min chase period. In addition, after apoAI uptake and washing, cho lesterol efflux was observed during a chase period without additional accep ters. Cholesterol efflux was partially blocked by chlorpromazine and hypert onic media, two inhibitors of coated pit endocytosis. Cholesterol efflux to apoAI was found to depend on extracellular calcium. By temporally separati ng the cAMP induction phase from the apoAI chase phase, calcium was found t o be required during the apoAI chase phase rather than during the cAMP indu ction period. In the absence of calcium the 8-Br-cAMP-mediated induction of apoAI binding was maintained, but the specific apoAI cellular association was inhibited. The data are consistent with a model for cholesterol efflux to apolipoproteins that involves a calcium-dependent endocytic pathway, fol lowed by recycling and the subsequent release of the nascent lipoprotein pa rticle from the cell.