Coordinate regulation of lipogenesis, the assembly and secretion of apolipoprotein B-containing lipoproteins by sterol response element binding protein 1.
Stable plasmid-driven expression of the liver-specific gene product cholesterol 7alpha-hydroxylase (7alpha-hydroxylase) was used to alter the cellular content of transcriptionally active sterol response element binding protein 1 (SREBP1). As a result of stable expression of 7alpha-hydroxylase, individual single cell clones expressed varying amounts of mature SREBP1 protein. These single cell clones provided an opportunity to identify SREBP1-regulated genes that may influence the assembly and secretion of apoB-containing lipoproteins. Our results show that in McArdle rat hepatoma cells, which normally do not express 7alpha-hydroxylase, plasmid-driven expression of 7alpha-hydroxylase results in the following: 1) a linear relationship between (i) the cellular content of mature SREBP1 and 7alpha-hydroxylase protein, (ii) the relative expression of 7alpha-hydroxylase mRNA and the mRNA's encoding the enzymes regulating fatty acid, i.e. acetyl-CoA carboxylase and sterol synthesis, i.e. HMG-CoA reductase, (iii) the relative expression of 7alpha-hydroxylase mRNA and microsomal triglyceride transfer protein mRNA, a gene product that is essential for the assembly and secretion of apoB-containing lipoproteins; 2) increased synthesis of all lipoprotein lipids (cholesterol, cholesterol esters, triglycerides, and phospholipids); and 3) increased secretion of apoB100 without any change in apoB mRNA. Cells expressing 7alpha-hydroxylase contained significantly less cholesterol (both free and esterified). The increased cellular content of mature SREBP1 and increased secretion of apoB100 were concomitantly reversed by 25-hydroxycholesterol, suggesting that the content of mature SREBP1, known to be decreased by 25-hydroxycholesterol, mediates the changes in the lipoprotein assembly and secretion pathway that are caused by 7alpha-hydroxylase. These data suggest that several steps in the assembly and secretion of apoB-containing lipoproteins by McArdle hepatoma cells may be coordinately linked through the cellular content of mature SREBP1.
Wang
S L
SL
Mammalian Cell and Molecular Biology Laboratory, Department of Biology and Molecular Biology Institute, San Diego State University, San Diego, California 92182-0057, USA.
Du
E Z
EZ
Martin
T D
TD
Davis
R A
RA
D000103:Acetyl-CoA Carboxylase / Q000235:genetics; D000818:Animals; D001055:Apolipoproteins B / Q000378:metabolism*; D001189:Aryl Hydrocarbon Hydroxylases; D022762:CCAAT-Enhancer-Binding Proteins; D003577:Cytochrome P-450 Enzyme System / Q000032:analysis / Q000235:genetics; D004268:DNA-Binding Proteins / Q000502:physiology*; D006903:Hydroxymethylglutaryl CoA Reductases / Q000235:genetics; D008055:Lipids / Q000096:biosynthesis*; D008079:Lipoproteins, VLDL / Q000096:biosynthesis; D009687:Nuclear Proteins / Q000502:physiology*; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D013250:Steroid Hydroxylases / Q000032:analysis / Q000235:genetics; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors
D001055:Apolipoproteins B; D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D008055:Lipids; D008079:Lipoproteins, VLDL; D009687:Nuclear Proteins; D012333:RNA, Messenger; C497389:Srebf1 protein, rat; D051780:Sterol Regulatory Element Binding Protein 1; D014157:Transcription Factors; D003577:Cytochrome P-450 Enzyme System; D006903:Hydroxymethylglutaryl CoA Reductases; D013250:Steroid Hydroxylases; D001189:Aryl Hydrocarbon Hydroxylases; C029009:steroid hormone 7-alpha-hydroxylase; D000103:Acetyl-CoA Carboxylase