Intracellular processing of exogenously derived non-lipoprotein [3H]cholesterol in normal and mutant human skin fibroblasts deficient in acid sterol ester hydrolase.

By studying the incorporation and esterification of non-lipoprotein, free [3H]cholesterol in normal and acid sterol ester hydrolase-deficient human fibroblasts, it was examined whether the esterification reaction of the lysosomal acid sterol ester hydrolase contributed to the formation of cellular [3H]cholesteryl esters. Both the normal and the acid sterol ester hydrolase-deficient cells incorporated exogenous, vesicle-derived free [3H]cholesterol linearly as a function of time. Also, the rate of [3H]cholesteryl ester formation was almost the same in normal and mutant fibroblasts, indicating that the apparent esterification activity of the acid sterol ester hydrolase in normal fibroblasts did not contribute to the formation of [3H]cholesteryl esters in intact cells. To examine whether the incorporated [3H]cholesterol was transported into the endoplasmic reticulum and esterified by the acyl-CoA: cholesterol acyltransferase, the rate of [3H]cholesteryl ester formation was measured in the presence or absence of the acyl-CoA: cholesterol acyltransferase-inhibitor 58-035 (Sandoz Inc.). Results showed that the formation of [3H]cholesteryl esters was reduced markedly when cells were co-incubated with the acyltransferase inhibitor. Maximal inhibition (i.e., 75%) was obtained at an inhibitor concentration of 1 microgram/ml. Since the inhibitor 58-035 is very specific for acyl-CoA: cholesterol acyltransferase, this finding clearly shows that exogenous, exchangeable [3H]cholesterol can reach and mix with the intracellular substrate pool of the enzyme.