Cholesterol flux between high density lipoproteins and cultured rat luteal cells

Although high density lipoprotein (HDL) is a promoter of cholesterol efflux from peripheral cells, in steroidogenic tissues it has been shown to provide cholesterol for steroid synthesis. The present study examines the dynamics of cholesterol exchange between HDL particles and rat luteal cells. Cholesterol influx was measured by following the uptake of label from HDL particles labeled with [3H]cholesterol or [3H]cholesteryl linoleate. The efflux of cholesterol was simultaneously measured by incubating [3H]cholesterol-labeled luteal cells with unlabeled HDL. Conversion of endogenous and lipoprotein-derived steroids to progestins was also determined. The results showed that HDL promotes both influx and efflux of cholesterol in those cells. The amount of influx exceeds efflux, thus resulting in a net uptake of cholesterol from HDL by rat luteal cells. The relative utilization of endogenous vs. HDL-derived cholesterol for steroid synthesis was also examined. The results show that only a fraction of the HDL-derived cholesterol was converted directly to steroids. The fraction that was converted depended on the HDL concentration and had no apparent relation to the incubation time. These results show that although cholesterol flux in luteal cells is bidirectional in the presence of HDL, the influx exceeds the amount of efflux, and the internalized cholesterol is diluted with the endogenous cholesterol pool before it is converted to steroids.