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Modulation of the murine peroxisome proliferator-activated receptor gamma 2 promoter activity by CCAAT/enhancer-binding proteins.
Peroxisome proliferator-activated receptor gamma (PPARgamma) and CCAAT/enhancer-binding proteins (C/EBPs) are transcriptional regulators essential for adipocyte differentiation and function. Previous findings indicate that PPARgamma2 transcription is regulated by members of the C/EBP family. We demonstrate here that C/EBPalpha and C/EBPdelta, but not C/EBPbeta, induce the activity of the PPARgamma2 promoter in transiently transfected 3T3-L1 preadipocytes and bind to two juxtaposed low affinity C/EBP binding sites. Results obtained with chimeras containing interchanged C/EBPalpha-C/EBPbeta N-terminal transactivation domain and C-terminal DNA binding dimerization domain indicate that the N-terminal part of C/EBPbeta prevents it from binding to the PPARgamma2 promoter. Indeed, deletion mutants of C/EBPbeta lacking the N-terminal part of the molecule are able to bind to the PPARgamma2 promoter. We further demonstrate that deletion of a region located between amino acids 184-212, upstream of the DNA binding domain, permits C/EBPbeta binding to the PPARgamma2 promoter, implicating an inhibitory region in C/EBPbeta for modulating DNA binding specificity to the PPARgamma2 promoter. In summary, this study indicates that C/EBPbeta but not C/EBPalpha or C/EBPdelta is unable to bind to C/EBP binding sites in the mouse PPARgamma2 promoter. The lack of binding is due to a region N-terminal of the C/EBPbeta DNA binding domain. Our findings illustrate a mechanism by which C/EBP isoforms differentially modulate the transactivation of the PPARgamma2 promoter.
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