Expression and microvillar localization of scavenger receptor class B, type I (SR-BI) and selective cholesteryl ester...

This study investigates the relationship between the high density lipoprotein (HDL) receptor (scavenger receptors, SR-BI and SR-BII), selective lipoprotein-cholesteryl ester uptake, and testosterone production in Leydig cells of control, hypocholesterolemic and gonadotrophic hormone (hCG) treated rats. Leydig cells from mature control rats show poor efficiency in incorporation of labeled HDL-cholesteryl esters into testosterone, poor selective uptake of lipoprotein lipids overall, and a dramatic reduction of circulating levels of lipoproteins has no apparent effect on testosterone production or expression of intracellular enzymes synthesizing cholesterol. Leydig cells from control rats show minimal levels of SR-BI and SR-BII. However, similarly aged rats treated with hCG for several days undergo changes consistent with hormone-desensitization. Despite the resulting low levels of testosterone production, SR-BI levels are dramatically increased, Leydig cells now efficiently internalize HDL-supplied cholesteryl esters by the selective cholesterol uptake process, and various other cholesterol-sensitive genes of the cells are up-regulated. Only SR-BII expression remains negligible and unchanged throughout this period. It is of interest that Leydig cell SR-BI of hCG-treated rats is localized in surface microvilli, but is present also in an elaborate and complex channel system within the cytoplasm of the cells. In summary, Leydig cells differ from other rat steroidogenic cells in not depending on exogenous lipoprotein-cholesterol during periods of normal steroid hormone production. However, trophic hormone desensitization is accompanied by increased Leydig cell SR-BI expression and increased selective HDL-cholesteryl ester uptake, presumably in preparation for renewed testosterone production.