title: Expression of 7 alpha-hydroxylase in non-hepatic cells results in liver phenotypic resistance of the low density lipoprotein receptor to cholesterol repression.
abstract: The goal of this study was to understand why the expression of low density lipoprotein (LDL) receptors by the liver is poorly down-regulated by cholesterol. We examined the hypothesis that 7 alpha-hydroxylase may indirectly induce the expression of the LDL receptor by metabolizing, i.e. inactivating oxysterol repressors. Non-hepatic Chinese hamster ovary cells, transfected with a plasmid encoding 7 alpha-hydroxylase, expressed both the mRNA and functional activity of this liver-specific enzyme. In the presence of 5% serum, expression of the LDL receptor by transfected cells was > 20 times that of non-transfected cells despite a 50% increased content of cholesterol ester. Both cell types displayed an almost complete repression of the LDL receptor by the oxysterol 25-hydroxycholesterol, suggesting that transcriptional control of the LDL receptor gene remained intact in the transfected cells. However, only cells expressing 7 alpha-hydroxylase showed a derepression of the LDL receptor with time. This transient sensitivity to 25-hydroxycholesterol repression was attributed to a 3-fold greater rate of metabolism of [3H]25-hydroxycholesterol. The paradoxical induction of LDL receptor mRNA in transfected cells having greater amounts of cholesterol esters suggests that 7 alpha-hydroxylase may preferentially use oxysterols rather than cholesterol as substrates. The combined data are consistent with the proposal that 7 alpha-hydroxylase indirectly induces the LDL receptor gene by metabolizing (inactivating) oxysterol repressors. Liver-specific expression of 7 alpha-hydroxylase can account for the relative resistance of hepatic LDL receptors to down-regulation.
authors.affiliation: Department of Biology, San Diego State University, California 92182.
authors.forename: S
authors.initials: S
authors.lastname: Dueland
authors.forename: J D
authors.initials: JD
authors.lastname: Trawick
authors.forename: M S
authors.initials: MS
authors.lastname: Nenseter
authors.forename: A A
authors.initials: AA
authors.lastname: MacPhee
authors.forename: R A
authors.initials: RA
authors.lastname: Davis
mesh_terms: D000199:Actins / Q000235:genetics; D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D016466:CHO Cells; D002784:Cholesterol / Q000378:metabolism* / Q000494:pharmacology; D002788:Cholesterol Esters / Q000378:metabolism; D006224:Cricetinae; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics / Q000378:metabolism*; D004351:Drug Resistance; D006888:Hydroxycholesterols / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism*; D010641:Phenotype; D012333:RNA, Messenger / Q000032:analysis / Q000235:genetics / Q000378:metabolism; D051381:Rats; D011973:Receptors, LDL / Q000096:biosynthesis / Q000187:drug effects* / Q000235:genetics; D013250:Steroid Hydroxylases / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
chemical_list: D000199:Actins; D002788:Cholesterol Esters; D006888:Hydroxycholesterols; D012333:RNA, Messenger; D011973:Receptors, LDL; C007997:25-hydroxycholesterol; D003577:Cytochrome P-450 Enzyme System; D002784:Cholesterol; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C029009:steroid hormone 7-alpha-hydroxylase