We have recently reported that coordinate down‐regulation of cholesterol 7α‐hydroxylase and sterol 27‐hydroxylase by bile acids results in suppression of bile acid synthesis in cultured rat hepatocytes [Twisk, J., De Wit, E. & Princen, H. M. G. (1995) Biochem. J. 305, 505–511]. In the current study, we have assessed the effects of a large group of different bile acids, both naturally occurring and synthetic, on these two key enzymes, to elucidate structural features which render bile acids potent as regulators of bile acid synthesis.

Addition of 50 μM deoxycholate or cholate, two relatively hydrophobic bile acids, to the culture medium of hepatocytes resulted in strong suppression of cholesterol 7α‐hydroxylase (suppression of 75% and 88%, respectively) and sterol 27‐hydroxylase activity (suppression of 76% and 72%, respectively). These effects were also reflected in the mRNA levels and the transcriptional activities of the two enzymes, showing a parallel suppression of both parameters in response to cholate (suppression of 78% and 43% for cholesterol 7α‐hydroxylase mRNA and transcription, respectively, and suppression of 76% and 42% for sterol 27‐hydroxylase mRNA and transcription, respectively). In contrast, no effects were observed with the two hydrophilic bile acids, β‐muricholate and ursocholate. Transient expression analysis in cultured rat hepatocytes, using a promoter‐reporter construct containing the proximal part of the cholesterol 7α‐hydroxylase promoter, demonstrated a reduction of transcriptional activity by cholate (reduction of 72%), but not by ursocholate.

Assessment of the effects of 27 different bile acids, varying in the number, position and orientation (α/β) of hydroxyl groups on the steroid nucleus of the molecule, on cholesterol 7α‐hydroxylase mRNA showed only a moderate correlation with the hydrophobicity index of the bile acid involved (r= 0.61; P < 0.0001). Analysis of the three‐dimensional structure of a number of these bile acids suggests that hydroxyl groups situated in close proximity to each other within the molecule, creating a hydrophilic environment, as in the case of cholate, may be a prerequisite for a strong inhibitory potency. Deviation from this situation leads to a markedly lesser effect on suppression of cholesterol 7α‐hydroxylase and sterol 27‐hydroxylase.