Regulation of high density lipoprotein receptor activity in cultured human skin fibroblasts and human arterial smooth muscle cells

Abstract

Cultured human skin fibroblasts and human arterial smooth muscle cells possess high-affinity binding sites specific for high density lipoproteins (HDL). Results from the present study demonstrate that binding of HDL to these sites is up-regulated in response to cholesterol loading of cells. When fibroblasts or smooth muscle cells were preincubated with nonlipoprotein cholesterol, cellular binding of 125I-HDL3 was enhanced severalfold. This enlargement was sustained in the presence of cholesterol but was readily reversed when cells were exposed to cholesterol-free medium. The stimulatory effect of cholesterol treatment was prevented by cycloheximide, suggesting the involvement of protein synthesis. Kinetic analysis of HDL3 binding showed that prior exposure to cholesterol led to an induction of high-affinity binding sites on the cell surface. In the up-regulated state, the apparent dissociation constant (K(d)) of these sites was ~2 μg protein/ml. Competition studies indicated that the HDL binding sites recognized either HDL3 or HDL2 but interacted weakly with low density lipoprotein (LDL). Exposure of cells to lipoprotein cholesterol in the form of LDL also enhanced HDL binding by a process related to delivery of sterol onto cells via the LDL receptor pathway. Enhancement of HDL binding to fibroblasts by either nonlipoprotein cholesterol or LDL was associated with an increased cell cholesterol content, a suppressed rate of cholesterol synthesis, decreased LDL recpetor activity, and an enhanced rate of cholesterol ester formation. A comparison of HDL3 binding with the effects of HDL3 on cholesterol transport from cells revealed similar saturation profiles, implying a link between the two processes. Thus, cultured human fibroblasts and human arterial smooth muscle cells appear to possess specific receptors for HDL that may function to facilitate cholesterol removal from cells.