Lipoprotein cholesterol uptake mediates up-regulation of bile-acid synthesis by increasing cholesterol 7alpha-hydroxylase but not sterol 27-hydroxylase gene expression in cultured rat hepatocytes.
Lipoproteins may supply substrate for the formation of bile acids, and the amount of hepatic cholesterol can regulate bile-acid synthesis and increase cholesterol 7alpha-hydroxylase expression. However, the effect of lipoprotein cholesterol on sterol 27-hydroxylase expression and the role of different lipoproteins in regulating both enzymes are not well established. We studied the effect of different rabbit lipoproteins on cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase in cultured rat hepatocytes. beta-Migrating very-low-density lipoprotein (betaVLDL) and intermediate-density lipoprotein (IDL) caused a significant increase in the intracellular cholesteryl ester content of cells (2. 3- and 2-fold, respectively) at a concentration of 200 microgram of cholesterol/ml, whereas high-density lipoprotein (HDL, 50% v/v), containing no apolipoprotein E (apo E), showed no effect after a 24-h incubation. betaVLDL and IDL increased bile-acid synthesis (1. 9- and 1.6-fold, respectively) by up-regulation of cholesterol 7alpha-hydroxylase activity (1.7- and 1.5-fold, respectively). Dose- and time-dependent changes in cholesterol 7alpha-hydroxylase mRNA levels and gene expression underlie the increase in enzyme activity. Incubation of cells with HDL showed no effect. Sterol 27-hydroxylase gene expression was not affected by any of the lipoproteins added. Transient-expression experiments in hepatocytes, transfected with a promoter-reporter construct containing the proximal 348 nucleotides of the rat cholesterol 7alpha-hydroxylase promoter, showed an enhanced gene transcription (2-fold) with betaVLDL, indicating that a sequence important for a cholesterol-induced transcriptional response is located in this part of the cholesterol 7alpha-hydroxylase gene. The extent of stimulation of cholesterol 7alpha-hydroxylase is associated with the apo E content of the lipoprotein particle, which is important in the uptake of lipoprotein cholesterol. We conclude that physiological concentrations of cholesterol in apo E-containing lipoproteins increase bile-acid synthesis by stimulating cholesterol 7alpha-hydroxylase gene transcription, whereas HDL has no effect and sterol 27-hydroxylase is not affected.
Gaubius Laboratory, TNO-PG, Zernikedreef 9, 2333 CK, Leiden, The Netherlands.
S M
SM
Post
J
J
Twisk
L
L
van der Fits
E C
EC
de Wit
M F
MF
Hoekman
W H
WH
Mager
H M
HM
Princen
D000818:Animals; D001647:Bile Acids and Salts / Q000096:biosynthesis* / Q000235:genetics; D002478:Cells, Cultured; D053493:Cholestanetriol 26-Monooxygenase; D002784:Cholesterol / Q000494:pharmacology*; D002790:Cholesterol 7-alpha-Hydroxylase / Q000235:genetics / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000187:drug effects; D008074:Lipoproteins / Q000494:pharmacology*; D008099:Liver / Q000201:enzymology*; D008297:Male; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar; D013250:Steroid Hydroxylases / Q000235:genetics / Q000378:metabolism*
D001647:Bile Acids and Salts; D008074:Lipoproteins; C030104:lipoprotein cholesterol; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D013250:Steroid Hydroxylases; D002790:Cholesterol 7-alpha-Hydroxylase; D053493:Cholestanetriol 26-Monooxygenase