STRUCTURAL ASPECTS OF BILE-ACIDS INVOLVED IN THE REGULATION OF CHOLESTEROL 7-ALPHA-HYDROXYLASE AND STEROL 27-HYDROXYLASE

We have recently reported that coordinate down-regulation of cholester ol 7 alpha-hydroxylase and sterol 27-hydroxylase by bile acids results in suppression of bile acid synthesis in cultured rat hepatocytes [Tw isk, J., De Wit, E. and Princen, H. M. G. (1995) Biochem. J. 305, 505- 511]. In the current study, we have assessed the effects of a large gr oup of different bile acids, both naturally occurring and synthetic, o n these two key enzymes, to elucidate structural features which render bile acids potent as regulators of bile acid synthesis.Addition of 50 mu M deoxycholate or cholate, two relatively hydrophobic bile acids, to the culture medium of hepatocytes resulted in strong suppression of cholesterol 7 alpha-hydroxylase (suppression of 75% and 88%, respecti vely) and sterol 27-hydroxylase activity (suppression of 76% and 72%, respectively). These effects were also reflected in the mRNA levels an d the transcriptional activities of the two enzymes, showing a paralle l suppression of both parameters in response to cholate (suppression o f 78% and 43% for cholesterol 7 alpha-hydroxylase mRNA and transcripti on, respectively, and suppression of 76% and 42% for sterol 27-hydroxy lase mRNA and transcription, respectively). In contrast, no effects we re observed with the two hydrophilic bile acids, beta-muricholate and ursocholate. Transient expression analysis in cultured rat hepatocytes , using a promoter-reporter construct containing the proximal part of the cholesterol 7 alpha-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, va rying in the number, position and orientation (alpha/beta) of hydroxyl groups on the steroid nucleus of the molecule, on cholesterol 7 alpha -hydroxylase mRNA showed only a moderate correlation with the hydropho bicity index of the bile acid involved (r = 0.61; P < 0.0001). Analysi s of the three-dimensional structure of a number of these bile acids s uggests that hydroxyl groups situated in close proximity to each other within the molecule, creating a hydrophilic environment, as in the ca se of cholate, may be a prerequisite for a strong inhibitory potency. Deviation from this situation leads to a markedly lesser effect on sup pression of cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase.