Regulation of cholesterol 7 alpha-hydroxylase in the liver. Cloning, sequencing, and regulation of cholesterol 7 alpha-hydroxylase mRNA.

Monospecific antibody against purified rat liver cholesterol 7 alpha-hydroxylase cytochrome P-450 was used to screen a lambda gt11 cDNA library constructed from immuno-enriched polysomal RNA of cholestyramine-treated female rat liver. Two types of cDNA clones differing in the length of the 3'-untranslated region were identified, and DNA sequences were determined. The full length clone contains 3561 base pairs plus a long poly(A) tail. The amino acid sequence deduced from the open reading frame revealed a unique P-450 protein containing 503 amino acid residues which belonged to a new gene family designated family VII or CYP7. Southern blot hybridization experiments indicated that the minimal size of P-450 VII gene was 11 kilobase pairs (kb), and there was probably only one gene in this new family. Northern blot hybridization using specific cDNA probes revealed at least two major mRNA species of about 4.0 kb and 2.1 kb, respectively. These two mRNA species may be derived from the use of different polyadenylation signals and reverse-transcribed to two types of cDNA clones. Cholesterol 7 alpha-hydroxylase mRNAs were induced 2- to 3-fold in rat liver by cholestyramine treatment. The mRNA level was rapidly reduced upon the removal of the inducer. Similarly, cholesterol feeding induced enzyme activity, protein, and mRNA levels in the rat by 2-fold, suggesting that cholesterol is an important regulator of cholesterol 7 alpha-hydroxylase in the liver. On the other hand, dexamethasone and pregnenolone-16 alpha-carbonitrile drastically reduced the activity, protein, and mRNA levels. These experiments suggest that the induction of cholesterol 7 alpha-hydroxylase activity by cholestyramine or cholesterol and inhibition of cholesterol 7 alpha-hydroxylase activity by bile acid feedback are results of the rapid turnover of cholesterol 7 alpha-hydroxylase enzyme and mRNA levels.