Regulation of
bile acid synthesis, a key determinant of
cholesterol homeostasis, is still incompletely understood. To elucidate the feedback control exerted on
bile acid biosynthesis in humans with obstructive
cholestasis, 16 patients with
bile duct obstruction were studied. In vivo 7alpha-hydroxylation, reflecting
bile acid synthesis, was assayed in 13 of them by
tritium release analysis. Serum
27-hydroxycholesterol was determined by gas chromatography-mass spectrometry. In a subgroup, hepatic
cholesterol 7alpha-hydroxylase mRNA was assayed by real-time polymerase chain reaction (PCR),
enzyme activity was determined by
isotope incorporation, and microsomal
cholesterol content was assayed by gas chromatography-mass spectrometry. Age-matched control subjects were studied in parallel. Hydroxylation rates were lower in cholestatic patients (108 +/- 33 mg of
cholesterol per day, mean +/- SEM; controls: 297 +/- 40 mg/d; P <.01). The reduction was proportional to the severity of
cholestasis, and synthetic rates were normalized in 4 subjects restudied after resolution of biliary obstruction. Consistent findings were obtained by analysis of serum
7alpha-hydroxycholesterol levels. On the other hand, hepatic
cholesterol 7alpha-hydroxylase mRNA, microsomal
enzyme activity, and
cholesterol content tended to be increased in
cholestasis. Finally, serum
27-hydroxycholesterol levels were slightly reduced in cholestatic subjects and were not related with the severity of the disease. Suppression of in vivo
bile acid synthesis with no corresponding reduction in tissue 7alpha-hydroxylase expression and activity is consistent with nontranscriptional, posttranslational levels of regulation; these may play a role in the feedback control of
bile acid synthesis in particular conditions. Alteration of the alternate biosynthetic pathway seems unlikely according to the present data.