Quantitation of pre beta-HDL-dependent and nonspecific components of the total efflux of cellular cholesterol and phospholipid.

Both receptor-mediated and diffusional processes have been proposed as mechanisms for the efflux of cellular cholesterol to plasma. The depletion of a minor high-density lipoprotein subfraction (pre beta-1-HDL) from plasma by incubation was associated with a proportional reduction in up to 58% of cholesterol and lecithin efflux from cultured fibroblasts. Pre beta-HDL-dependent efflux was blocked by protease pretreatment of the cells, while residual ("nonspecific") efflux was protease-insensitive. The whole of cholesterol efflux from blood erythrocytes was both pre beta-1-HDL-and protease-independent. These data suggest that two distinct pathways contribute to total efflux from fibroblast monolayers; one of these is directly proportional to plasma pre beta-1-HDL concentration and may involve a cell-surface protein.