Efflux of phospholipid from fibroblasts with normal and elevated levels of cholesterol

To address the hypothesis that phospholipid efflux from cells contributes to lipoprotein structure, we have examined the efflux of biosynthetically labeled [32P]phospholipid from cells to lipoproteins. With normal human skin fibroblasts in monolayer culture, high density lipoprotein (HDL3) promoted the efflux of phospholipid in a concentration-dependent manner. As analyzed by TLC, the major phospholipids released from fibroblasts were phosphatidylcholine, sphingomyelin, lysophosphatidylcholine and phosphatidylethanolamine. At identical concentrations, HDL3 and dimethylsuberimidate treated-HDL3 promoted similar efflux, suggesting that efflux did not depend on specific binding of HDL3 to the cell surface. When the content of cholesterol in fibroblasts was doubled by pre-incubation with LDL and cholesterol-rich liposomes, the fractional efflux of phospholipid to HDL3 and other acceptors was stimulated about 2-fold. Most of this stimulation was due to enhanced release of phosphatidylcholine. Similar effects of enrichment were found with Fu5AH rat hepatoma cells, but not with J774 mouse macrophages. The results support the hypothesis that the efflux of phospholipid from cells contributes to the phospholipid content of HDL. This may enhance the ability of HDL to remove cholesterol from cells, the initial step in reverse cholesterol transport.