Impairment of exogenous sphingomyelin degradation in cultured fibroblasts from familial hypercholesterolemia

The degradation of exogenous sphingomyelin was investigated in cultured fibroblasts from normal subjects and subjects with familial hypercholesterolemia, either in whole medium or in lipoprotein-deficient medium. When introduced in whole medium, sphingomyelin degradation was significantly decreased (about 1.5-fold) in heterozygotes, and dramatically (about 4-fold) in homozygotes from familial hypercholesterolemia. Sphingomyelinase activity, measured in vitro by conventional methods, was not altered in fibroblasts from familial hypercholesterolemia. The sphingomyelin uptake was notably lower in familial hypercholesterolemia than in controls. The decrease in exogenous sphingomyelin degradation was also found in lipoprotein-deficient medium, suggesting that it is not related to the low density lipoprotein receptor impairment which exists in familial hypercholesterolemia. These results are discussed in relation to sphingomyelin and cholesterol metabolism, and possible abnormalities of the cell membrane in familial hypercholesterolemia are suggested.