Cholesterol and bile acids regulate cholesterol 7 alpha-hydroxylase expression at the transcriptional level in culture and in transgenic mice.
Cholesterol 7 alpha-hydroxylase (7 alpha-hydroxylase) is the rate-limiting enzyme in bile acid biosynthesis. It is subject to a feedback control, whereby high levels of bile acids suppress its activity, and cholesterol exerts a positive control. It has been suggested that posttranscriptional control plays a major part in that regulation. We have studied the mechanisms by which cholesterol and bile acids regulate expression of the 7 alpha-hydroxylase gene and found it to be solely at the transcriptional level by using two different approaches. First, using a tissue culture system, we localized a liver-specific enhancer located 7 kb upstream of the transcriptional initiation site. We also showed that low-density lipoprotein mediates transcriptional activation of chimeric genes, containing either the 7 alpha-hydroxylase or the albumin enhancer in front of the 7 alpha-hydroxylase proximal promoter, to the same extent as the in vivo cholesterol-mediated regulation of 7 alpha-hydroxylase mRNA. In a second approach, using transgenic mice, we have found that expression of an albumin enhancer-7 alpha-hydroxylase-lacZ fusion gene is restricted to the liver and is regulated by cholesterol and bile acids in a manner quantitatively similar to that of the endogenous gene. We also found, that a liver-specific enhancer is necessary for expression of the rat 7 alpha-hydroxylase gene, in agreement with the tissue culture experiments. Together, these results demonstrate that cholesterol and bile acids regulate the expression of the 7 alpha-hydroxylase gene solely at the transcriptional level.
Ramirez
M I
MI
Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, Worcester 01655.
Karaoglu
D
D
Haro
D
D
Barillas
C
C
Bashirzadeh
R
R
Gil
G
G
D000818:Animals; D001483:Base Sequence; D001647:Bile Acids and Salts / Q000502:physiology*; D002460:Cell Line; D002478:Cells, Cultured; D002784:Cholesterol / Q000502:physiology*; D002790:Cholesterol 7-alpha-Hydroxylase / Q000096:biosynthesis* / Q000235:genetics; D004742:Enhancer Elements, Genetic / Q000187:drug effects; D005246:Feedback; D005260:Female; D015971:Gene Expression Regulation, Enzymologic; D008077:Lipoproteins, LDL / Q000494:pharmacology; D008099:Liver / Q000201:enzymology*; D008297:Male; D051379:Mice; D008822:Mice, Transgenic; D008969:Molecular Sequence Data; D009928:Organ Specificity; D011401:Promoter Regions, Genetic; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D015183:Restriction Mapping; D012709:Serum Albumin / Q000096:biosynthesis / Q000235:genetics; D012737:Sex Factors; D014158:Transcription, Genetic; D014162:Transfection; D001616:beta-Galactosidase / Q000096:biosynthesis / Q000378:metabolism
D001647:Bile Acids and Salts; D008077:Lipoproteins, LDL; D011993:Recombinant Fusion Proteins; D012709:Serum Albumin; D002784:Cholesterol; D002790:Cholesterol 7-alpha-Hydroxylase; D001616:beta-Galactosidase