Regulation of sterol 27-hydroxylase and an alternative pathway of bile acid biosynthesis in primary cultures of rat hepatocytes.
In man, hepatic mitochondrial sterol 27-hydroxylase and microsomal cholesterol 7alpha-hydroxylase initiate distinct pathways of bile acid biosynthesis from cholesterol, the "acidic" and "neutral" pathways, respectively. A similar acidic pathway in the rat has been hypothesized, but its quantitative importance and ability to be regulated at the level of sterol 27-hydroxylase are uncertain. In this study, we explored the molecular regulation of sterol 27-hydroxylase and the acidic pathway of bile acid biosynthesis in primary cultures of adult rat hepatocytes. mRNA and protein turnover rates were approximately 10-fold slower for sterol 27-hydroxylase than for cholesterol 7alpha-hydroxylase. Sterol 27-hydroxylase mRNA was not spontaneously expressed in culture. The sole requirement for preserving sterol 27-hydroxylase mRNA at the level of freshly isolated hepatocytes (0 h) after 72 h was the addition of dexamethasone (0.1 microM; > 7-fold induction). Sterol 27-hydroxylase mRNA, mass and specific activity were not affected by thyroxine (1.0 microM), dibutyryl-cAMP (5O microM), nor squalestatin 1 (15O nM-1.0 microM), an inhibitor of cholesterol biosynthesis. Taurocholate (50 microM), however, repressed sterol 27-hydroxylase mRNA levels by 55%. Sterol 27-hydroxylase specific activity in isolated mitochondria was increased > 10-fold by the addition of 2-hydroxypropyl-beta-cyclodextrin. Under culture conditions designed to maximally repress cholesterol 7alpha-hydroxylase and bile acid synthesis from the neutral pathway but maintain sterol 27-hydroxylase mRNA and activity near 0 h levels, bile acid synthesis from [14C]cholesterol remained relatively high and consisted of beta-muricholate, the product of chenodeoxycholate in the rat. We conclude that rat liver harbors a quantitatively important alternative pathway of bile acid biosynthesis and that its initiating enzyme, sterol 27-hydroxylase, may be slowly regulated by glucocorticoids and bile acids.
Stravitz
R T
RT
Department of Medicine, Division of Gastroenterology, Medical College of Virginia, Richmond 23298, USA.
Vlahcevic
Z R
ZR
Russell
T L
TL
Heizer
M L
ML
Avadhani
N G
NG
Hylemon
P B
PB
D000818:Animals; D001647:Bile Acids and Salts / Q000096:biosynthesis* / Q000502:physiology; D002478:Cells, Cultured; D053493:Cholestanetriol 26-Monooxygenase; D002784:Cholesterol / Q000502:physiology; D002790:Cholesterol 7-alpha-Hydroxylase / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D015870:Gene Expression / Q000187:drug effects; D008099:Liver / Q000378:metabolism*; D008297:Male; D008930:Mitochondria, Liver / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D017207:Rats, Sprague-Dawley; D013250:Steroid Hydroxylases / Q000378:metabolism*
D001647:Bile Acids and Salts; D012333:RNA, Messenger; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D013250:Steroid Hydroxylases; D002790:Cholesterol 7-alpha-Hydroxylase; D053493:Cholestanetriol 26-Monooxygenase