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 ch olesterol 7 alpha-hydroxylase (7 alpha-hydroxylase) was used to alter the cellular content of transcriptionally active sterol response eleme nt binding protein 1 (SREBP1), As a result of stable expression of 7 a lpha-hydroxylase, individual single cell clones expressed varying amou nts of mature SREBP1 protein, These single cell clones provided an opp ortunity to identify SREBP1-regulated genes that may influence the ass embly and secretion of apoB-containing lipoproteins, Our results show that in McArdle rat hepatoma cells, which normally do not express 7 al pha-hydroxylase, plasmid-driven expression of 7 alpha-hydroxylase resu lts in the following: 1) a linear relationship between (i) the cellula r content of mature SREBP1 and 7 alpha-hydroxylase protein, (ii) the r elative expression of 7 alpha-hydroxylase mRNA and the mRNA's encoding the enzymes regulating fatty acid, i.e. acetyl-CoA carboxylase and st erol synthesis, i.e. HMG-CoA reductase, (iii) the relative expression of 7 alpha-hydroxylase mRNA and microsomal triglyceride transfer prote in mRNA, a gene product that is essential for the assembly and secreti on of apoB-containing lipoproteins; 2) increased synthesis of all lipo protein lipids (cholesterol, cholesterol esters, triglycerides, and ph ospholipids); and 3) increased secretion of apoB100 without any change in apoB mRNA, Cells expressing 7 alpha-hydroxylase contained signific antly less cholesterol (both free and esterified), The increased cellu lar content of mature SREBP1 and increased secretion of apoB100 were c oncomitantly reversed by 25-hydroxycholesterol, suggesting that the co ntent of mature SREBP1, known to be decreased by 25-hydroxycholesterol , mediates the changes in the lipoprotein assembly and secretion pathw ay that are caused by 7 alpha-hydroxylase. These data suggest that sev eral steps in the assembly and secretion of apoB-containing lipoprotei ns by McArdle hepatoma cells may be coordinately linked through the ce llular content of mature SREBP1.