The hormonal stimulation of high-density lipoprotein binding, internalization and degradation by cultured rat adrenal cortical cells

Several studies have shown that high-density lipoprotein stimulates steroidogenesis in rat tissues which have been treated with pituitary hormones. To determine whether these hormones can directly affect receptors for high-density lipoprotein, we have incubated cultured rat adrenal cortical cells with 125I-labeled human HDL3 and studied the effect of corticotrophin on the binding, internalization and degradation of this lipoprotein. ACTH stimulated all these parameters of HDL metabolism in a dose-dependent manner with maximal stimulation occurring between 10 and 20 mU/ml. The effect was temperature-dependent and showed target cell specificity. Although the hormone stimulated binding and internalization 5-6-fold, degradation of HDL3 was substantially less (2-fold) than anticipated. This suggests that the lipoprotein was taken up by vesicles resembling receptosomes which escape fusion with lysosomes; thus degradation of entrapped particles does not occur, while transfer of cholesterol for steroidogenesis is unaffected.