Growth hormone and bile acid synthesis. Key role for the activity of hepatic microsomal cholesterol 7alpha-hydroxylase in the rat.

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Abstract
Growth hormone (GH) has an important role in the regulation of hepatic LDL receptor expression and plasma lipoprotein levels. This investigation was undertaken to characterize the effects of GH on hepatic cholesterol and bile acid metabolism in the rat. In hypophysectomized (Hx) rats, the activities of the rate-limiting enzymes in cholesterol and bile acid biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) and cholesterol 7alpha-hydroxylase (C7alphaOH), were reduced by 71 and 64%, respectively. HMG CoA reductase mRNA levels were reduced by 37%, whereas C7alphaOH mRNA was increased by 81%. LDL receptor expression was reduced by 18% in Hx rats, without any change in the LDL receptor mRNA levels. Although the normal diurnal variation of C7alphaOH activity was preserved in Hx rats, the activity of C7alphaOH was much reduced both at midday and midnight. Total hepatic cholesterol was increased by 14% in Hx animals whereas microsomal cholesterol was unchanged. The rate of cholesterol esterification was enhanced (by 38%) in liver microsomes from Hx rats. Stepwise hormonal substitution of Hx rats showed that GH, but not thyroid hormone or cortisone, was essential to normalize the enzymatic activity of C7alphaOH. GH also normalized the altered plasma lipoprotein pattern in Hx rats, and increased the fecal output of bile acids. The latter effect was particularly evident when GH was combined with cortisone and thyroid hormone. Also in normal rats, GH stimulated C7alphaOH activity. In conclusion, GH has an essential role to maintain a normal enzymatic activity of C7alphaOH, and this, at least in part, explains the effects of GH on hepatic cholesterol metabolism. GH is also of critical importance to normalize the altered plasma lipoprotein pattern in Hx rats.

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