Insulin and sterol-regulatory element-binding protein-1c (SREBP-1C) regulation of gene expression in 3T3-L1 adipocytes -: Identification of CCAAT/enhancer-binding protein β as an SREBP-1c target

We evaluated the hypothesis of sterol-regulatory element-binding protein (SREBP)-1c being a general mediator of the transcriptional effects of insulin, with a focus on adipocytes, in which insulin profoundly influences specific gene expression. Using real time quantitative reverse transcriptase-PCR to monitor changes in the expression of about 50 genes that cover a wide range of adipocyte functions, we have compared the impact of insulin treatment with that of adenoviral overexpression of either dominant positive or dominant negative SREBP-1c mutants in 3T3-L1 adipocytes. As expected, insulin up-regulated, dominant positive stimulated, and dominant negative decreased previously characterized direct SREBP targets (FAS, SCD-1, and low density lipoprotein receptor). We also identified three novel SREBP-1c transcriptional targets in adipocytes, which were confirmed by run-on assays: plasminogen activator inhibitor 1, CCAAT/enhancer-binding protein delta (C/EBPdelta), and C/EBPbeta. Because most insulin-regulated genes were also modulated by SREBP-1c mutants, our data establish that 1) SREBP-1c is an important mediator of insulin transcriptional effects in adipocytes, and 2) C/EBPbeta is under the direct control of SREBP-1c, as demonstrated by the ability of SREBP-1c to activate the transcription from C/EBPbeta promoter through canonical SREBP binding sites. Thus, some of the effects of insulin and/or SREBP-1c in mature fat cells might require C/EBPbeta or C/EBPdelta as transcriptional relays.