Sphingomyelin biosynthesis and efflux correlates with cholesterol metabolism and is higher in vascular endothelial cells than in smooth muscle cells.

Abstract

We compared the metabolism of cellular phospholipids in bovine aortic endothelial and smooth muscle cells in culture. [3H]Choline was incorporated in both cell types into phosphatidylcholine (86-90%) and sphingomyelin (10-14%). Endothelial cells demonstrated preferential efflux of sphingomyelin which represented 22.5% of the radiolabelled phospholipids in the incubation medium while in smooth muscle cells it represented 10%, so that after 7 days, the sphingomyelin in the medium represented 40% and 16% of total synthesized sphingomyelin in endothelial and smooth muscle cells, respectively. Incorporation of [3H]choline by endothelial and smooth muscle cells was reduced in the presence of serum, but not in the presence of lipoprotein deficient serum, indicating that cells can acquire phosphatidylcholine and sphingomyelin from lipoproteins. Lipoproteins were shown also to support the efflux of cellular radiolabelled phospholipids from both cell types, but at a higher degree from endothelial cells than from smooth muscle cells. Exposure of these cultures to cholesterol rich serum increased the synthesis of phosphatidylcholine, and to a higher extent of sphingomyelin, with concomitant decrease in the efflux of these two phospholipids. These results demonstrate the role of cholesterol in the regulation of phosphatidylcholine and sphingomyelin biosynthesis and efflux in vascular cells. Furthermore, the higher efflux of sphingomyelin in endothelial cells than in smooth muscle cells may support the extensive efflux of cholesterol observed in endothelial cells and indicate biochemical differences in lipid metabolism between vascular endothelial and smooth muscle cells.