Cholesterol ester formation in cultured human fibroblasts. Stimulation by oxygenated sterols

Incubation of monolayers of cultured human fibroblasts with oxygenated sterols (25-hydroxycholesterol, 7-ketocholesterol, or 6-ketocholestanol) markedly enhanced the rate at which the cells esterified their endogenous cholesterol and produced an increase in the cellular content of cholesterol esters. The enhanced esterification capacity was associated with an increase in the activity of a membrane-bound fatty acyl-CoA:cholesteryl acyltransferase. Incubation of cells for 5 hours with 5 mug/ml of 25-hydroxycholesterol produced an 8-fold increase in the specific activity of this enzyme when assayed in cell-free extracts. Since the oxygenated sterols that elevated the activity of fatty acyl-CoA:cholesteryl acyl-transferase also suppressed the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the data suggest that the processes of cholesterol ester formation and cholesterol synthesis in human fibroblasts are regulated in a reciprocal manner by coordinate changes in the activities of these two membrane-bound enzymes.