title: Increased production of apolipoprotein B-containing lipoproteins in the absence of hyperlipidemia in transgenic mice expressing cholesterol 7alpha-hydroxylase.
abstract: The finding that expression of a cholesterol 7alpha-hydroxylase (CYP7A1) transgene in cultured rat hepatoma cells caused a coordinate increase in lipogenesis and secretion of apoB-containing lipoproteins led to the hypothesis that hepatic production of apoB-containing lipoproteins may be linked to the expression of CYP7A1 (Wang, S.-L., Du, E., Martin, T. D., and Davis, R. A. (1997) J. Biol. Chem. 272, 19351-19358). To examine this hypothesis in vivo, a transgene encoding CYP7A1 driven by the constitutive liver-specific enhancer of the human apoE gene was expressed in C56BL/6 mice. The expression of CYP7A1 mRNA (20-fold), protein ( approximately 10-fold), and enzyme activity (5-fold) was markedly increased in transgenic mice compared with non-transgenic littermates. The bile acid pool of CYP7A1 transgenic mice was doubled mainly due to increased hydrophobic dihydroxy bile acids. In CYP7A1 transgenic mice, livers contained approximately 3-fold more sterol response element-binding protein-2 mRNA. Hepatic expression of mRNAs encoding lipogenic enzymes (i.e. fatty-acid synthase, acetyl-CoA carboxylase, stearoyl-CoA desaturase, squalene synthase, farnesyl-pyrophosphate synthase, 3-hydroxy-3-methylglutaryl-CoA reductase, and low density lipoprotein receptor) as well as microsomal triglyceride transfer protein were elevated approximately 3-5-fold in transgenic mice. CYP7A1 transgenic mice also displayed a >2-fold increase in hepatic production and secretion of triglyceride-rich apoB-containing lipoproteins. Despite the increased hepatic secretion of apoB-containing lipoproteins in CYP7A1 mice, plasma levels of triglycerides and cholesterol were not significantly increased. These data suggest that the 5-fold increased expression of the low density lipoprotein receptor displayed by the livers of CYP7A1 transgenic mice was sufficient to compensate for the 2-fold increase production of apoB-containing lipoproteins. These findings emphasize the important homeostatic role that CYP7A1 plays in balancing the anabolic lipoprotein assembly/secretion pathway with the cholesterol catabolic bile acid synthetic pathway.
authors.affiliation: Mammalian Cell and Molecular Biology Laboratory, San Diego State University, San Diego, California 92182-4614, USA.
authors.forename: J H
authors.initials: JH
authors.lastname: Miyake
authors.forename: X D
authors.initials: XD
authors.lastname: Doung
authors.forename: W
authors.initials: W
authors.lastname: Strauss
authors.forename: G L
authors.initials: GL
authors.lastname: Moore
authors.forename: L W
authors.initials: LW
authors.lastname: Castellani
authors.forename: L K
authors.initials: LK
authors.lastname: Curtiss
authors.forename: J M
authors.initials: JM
authors.lastname: Taylor
authors.forename: R A
authors.initials: RA
authors.lastname: Davis
mesh_terms: D000818:Animals; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B / Q000096:biosynthesis* / Q000097:blood / Q000378:metabolism; D001647:Bile Acids and Salts / Q000378:metabolism; D022762:CCAAT-Enhancer-Binding Proteins / Q000096:biosynthesis / Q000235:genetics; D002352:Carrier Proteins / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D002790:Cholesterol 7-alpha-Hydroxylase / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000096:biosynthesis / Q000235:genetics; D006949:Hyperlipidemias / Q000097:blood; D050356:Lipid Metabolism; D008055:Lipids / Q000097:blood; D008099:Liver / Q000201:enzymology* / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D012333:RNA, Messenger / Q000096:biosynthesis; D011973:Receptors, LDL / Q000096:biosynthesis / Q000235:genetics; D051780:Sterol Regulatory Element Binding Protein 1; D051782:Sterol Regulatory Element Binding Protein 2; D013655:Taurochenodeoxycholic Acid / Q000378:metabolism; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics; D014280:Triglycerides / Q000097:blood
chemical_list: D053299:Apolipoprotein B-100; D001055:Apolipoproteins B; D001647:Bile Acids and Salts; D022762:CCAAT-Enhancer-Binding Proteins; D002352:Carrier Proteins; D004268:DNA-Binding Proteins; D008055:Lipids; D012333:RNA, Messenger; D011973:Receptors, LDL; C497388:Srebf1 protein, mouse; C497391:Srebf2 protein, mouse; D051780:Sterol Regulatory Element Binding Protein 1; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors; D014280:Triglycerides; C106092:microsomal triglyceride transfer protein; D013655:Taurochenodeoxycholic Acid; D002784:Cholesterol; D002790:Cholesterol 7-alpha-Hydroxylase