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The present study examines the feedback control governing human cholesterol 7alpha-hydroxylase mRNA expression in the human hepatoblastoma cell line, Hep G2. Glycochenodeoxycholate (GCDC) and glycodeoxycholate, hydrophobic bile salts, decreased cholesterol 7alpha-hydroxylase mRNA levels and bile acid synthesis in a concentration-dependent (76 +/- 8%, P<0.001, and 48 +/- 3%, P<0.01, respectively) and time-dependent manner. Cholesterol 7alpha-hydroxylase mRNA levels were repressed with a half-maximal inhibitory concentration of <12.5 microM by GCDC and a half-life of 30 min by 100 microM of the bile acid. The addition of actinomycin D (10 microgram/ml) alone or in combination with GCDC (100 microM) led to similar concentration-and time-dependent suppression of cholesterol 7alpha-hydroxylase mRNA. Glycocholate (100 microM), not internalized based on lack of uptake of a fluorescent cholate analogue, had no effect on cholesterol 7alpha-hydroxylase mRNA or total bile acid synthesis. In cultures transfected with a rat cholesterol 7alpha-hydroxylase promoter construct, reporter gene activity was decreased (31%, P<0.01) by GCDC (100 microM). Hep G2 cells maintain the intracellular machinery to express and rapidly regulate human cholesterol 7alpha-hydroxylase by hydrophobic bile acids. These data suggest that Hep G2 cells will support functional studies of the human cholesterol 7alpha-hydroxylase gene.

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