title: Cholesterol 7 alpha-hydroxylase is up-regulated by the competitive inhibitor 7-oxocholesterol in rat liver.
abstract: Rats of the Sprague-Dawley strain were infused intravenously with a fat emulsion (Intralipid, trademark of Kabi Pharmacia, Uppsala, Sweden) containing 7-oxocholesterol. This resulted in an increased cholesterol 7 alpha-hydroxylase activity in liver microsomes as compared to controls and was accompanied by increased levels of cholesterol 7 alpha-hydroxylase mRNA and microsomal cholesterol 7 alpha-hydroxylase protein. Rats were also fed a cholestyramine-supplemented diet and infused with 7-oxocholesterol. These animals excreted about half as much bile acids in faeces as cholestyramine-fed controls. Addition of 7-oxocholesterol to liver microsomes from normal rats in amounts corresponding to those present in microsomes from 7-oxocholesterol-treated rats inhibited the cholesterol 7 alpha-hydroxylase activity by about 75%. Cholesterol induced a type-I binding spectrum when added to a purified bacterial-expressed cholesterol 7 alpha-hydroxylase (P-450c7 delta 2-24). 7-Oxocholesterol competitively inhibited the cholesterol binding spectrum, while 7 beta-hydroxycholesterol did not interfere with binding of cholesterol to the enzyme. It is concluded that treatment with the competitive inhibitor 7-oxocholesterol leads to a reduced bile acid biosynthesis and, as a consequence of reduced bile acid inhibition, a compensatory increase in cholesterol 7 alpha-hydroxylase synthesis. The high enzyme activity measured in microsomal preparations from 7-oxocholesterol-treated rats may be due to a continuous conversion of 7-oxocholesterol into less inhibitory metabolites, e.g. 7 beta-hydroxycholesterol. The latter compound was found in high concentrations in liver microsomes from rats treated with 7-oxocholesterol. The physiological importance of these results is discussed in relation to the previous findings that 7-oxocholesterol is accumulated in liver after cholesterol feeding and that 7-oxocholesterol is formed from cholesterol during lipid peroxidation.
authors.affiliation: Karolinska Institutet, Department of Clinical Chemistry, Huddinge University Hospital, Sweden.
authors.forename: O
authors.initials: O
authors.lastname: Breuer
authors.forename: E
authors.initials: E
authors.lastname: Sudjana-Sugiaman
authors.forename: G
authors.initials: G
authors.lastname: Eggertsen
authors.forename: J Y
authors.initials: JY
authors.lastname: Chiang
authors.forename: I
authors.initials: I
authors.lastname: Björkhem
mesh_terms: D000818:Animals; D001647:Bile Acids and Salts / Q000096:biosynthesis; D002784:Cholesterol / Q000378:metabolism; D002790:Cholesterol 7-alpha-Hydroxylase / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D007262:Infusions, Intravenous; D007653:Ketocholesterols / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D015854:Up-Regulation
chemical_list: D001647:Bile Acids and Salts; D007653:Ketocholesterols; D012333:RNA, Messenger; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D002790:Cholesterol 7-alpha-Hydroxylase; C003001:7-ketocholesterol