Regulation of high density lipoprotein receptor messenger ribonucleic acidexpression and cholesterol transport in theca-interstitial cells by insulin and human chorionic gonadotropin

The synthesis of androgens by theca-interstitial cells is stimulated by LH and the insulin/insulin-like growth factor I (IGF-I) system. An essential e lement of the steroidogenesis is the uptake of plasma cholesterol and trans formation to steroid hormones. In the rat, the uptake of cholesterol by the theca-interstitial cells is mediated by the high density lipoprotein recep tor. The goal of the present study was to examine whether insulin has any e ffect on cholesterol delivery into theca-interstitial cells. The effects of insulin and hCG on the expression of the high density lipoprotein receptor (SR-BI) messenger RNA (mRNA) and intracellular cholesterol levels were exa mined in rat theca-interstitial cells under in vivo and in vitro conditions . Twenty-five-day-old rats were treated with insulin, hCG, or insulin follo wed by hCG. The expression of SR-BI mRNA was then examined in ovaries enric hed in theca-interstitial cell population by Northern blot analysis. Treatm ent with insulin increased the expression of SR-BI mRNA over that in contro ls treated with saline, hCG administration also increased the expression of SR-BI mRNA. A combination of insulin followed by hCG produced an even grea ter increase in SR-BI mRNA expression. Measurements of cellular cholesterol in the ovarian tissue showed an increase in total and free cholesterol lev els in response to insulin treatment. As expected, administration of hCG pr oduced a depletion of cellular cholesterol, and the depletion was even more pronounced in response to treatment with insulin and hCG. The effect of in sulin and hCG on SR-SBI mRNA expression was then examined under in vitro co nditions using primary cultures of theca-interstitial cells. Treatment with insulin produced an increase in SR-BI mRNA expression. As the cultured the ca-interstitial cells were not able to maintain hCG receptors, hCG addition produced no increase in SR-BI mRNA expression. However, in the presence of insulin, these cells were able to maintain hCG receptors and readily respo nded to hCG to increase SR-BI mRNA expression. Although insulin alone produ ced a modest increase in total and free cholesterol levels, in the presence of insulin, hCG produced the expected depletion of cellular cholesterol co ntent. The present study shows that insulin has a stimulatory effect on the expression of high density lipoprotein receptors in theca-interstitial cel ls, suggesting that one of the actions of insulin is to increase intracellu lar cholesterol, which is subsequently mobilized for androgen biosynthesis in theca-interstitial cells.