Hormonal regulation of cholesterol 7alpha-hydroxylase specific activity, mRNA levels, and transcriptional activity in vivo in the rat.

In primary cultures of rat hepatocytes, transcription of the cholesterol 7alpha-hydroxylase gene is induced synergistically by glucocorticoid and thyroid hormones. The objective of the present study was to evaluate the role of endogenous glucocorticoid and thyroid hormones in the maintenance of cholesterol 7alpha-hydroxylase gene expression in vivo. Male Sprague-Dawley rats underwent adrenalectomy (A), thyroidectomy (T), adrenalectomy + thyroidectomy (A + T), hypophysectomy (H), or sham surgery (paired controls). Ten days post surgery, livers were harvested and choles terol 7alpha-hydroxylase specific activity, steady-state mRNA levels, and transcriptional activity were determined. Serum corticosterone levels were <2% of paired controls in A, A + T, and H rats. Free thyroxine index was <32% of paired controls in rats with T and H. When compared to sham-operated controls, A + T and H led to decreases in cholesterol 7alpha-hydroxylase specific activities of 44 +/- 8% and 57 +/- 3%, respectively (P < 0.03 and < 0.05). Similar changes were observed in cholesterol 7alpha-hydroxylase steady-state mRNA levels, which decreased by 43 +/- 10% (P < 0.001) and 56 +/- 19% (P < 0.05), respectively. Cholesterol 7alpha-hydroxylase transcriptional activity in A + T and H rats decreased by 34 +/- 11% (P < 0.01) and 61 +/- 4% (P < 0.001), respectively. The observed decreases were greater after H than after A + T, suggesting the possibility that another pituitary hormone plays a role in regulation of cholesterol 7alpha-hydroxylase. Thyroidectomy alone led to a decrease in cholesterol 7alpha-hydroxylase specific activity of 37 +/- 7% (P < 0.05) and a trend toward decreased steady-state mRNA levels (21 +/- 12%; P = ns). Adrenalectomy did not significantly decrease cholesterol 7alpha-hydroxylase specific activity or mRNA levels. Neither thyroidectomy nor adrenalectomy alone affected transcriptional activity. We conclude that under physiologic circumstances, full expression of the cholesterol 7alpha-hydroxylase gene requires synergistic action of glucocorticoids and thyroid hormone.