Regulation of cellular sterol flux and synthesis by human serum lipoproteins

Abstract

The effects of human serum and serum lipoproteins on cellular sterol flux and synthesis were measured in an L-cell mouse fibroblast tissue culture system. Since L cells synthesize desmosterol as their major sterol, endogenous cellular sterol (desmosterol) can easily be differentiated from exogenous serum lipoprotein sterol (cholesterol). Taking advantage of this unique property, influx, efflux and synthesis of sterol were measured. Therefore, following incubation with serum or serum lipoproteins, cellular incorporation of cholesterol was a measure of influx, and desmosterol released from the cells into the media was a measure of cellular efflux. The effect of sterol flux on cellular desmosterol synthesis was determined using the incorporation of [ 14C] acetate into sterol and measurement of net desmosterol synthesis. Time-course experiments demonstrated that after the addition of human serum, sterol flux was rapid, with influx and efflux approaching maximum values within 8 h, and cellular sterol synthesis was reduced by 50% within 24 h. Comparative determinations made at 4 and 24 h after the addition to the media of human high-density lipoproteins (HDL), low-density lipoproteins (LDL), or serum yielded the following results: (a) HDL at both sampling times promoted the greatest efflux of desmosterol from the cell; (b) influx of cholesterol was similar in the presence of both HDL and LDL; (c) in all instances an increase in cellular sterol occurred; (d) sterol synthesis was not related solely to cellular sterol content, and was most reduced in the presence of LDL.