CTBP1

Abstract: The C-terminal binding protein (CtBP) family of proteins has been linked to multiple biological processes through their association with numerous transcription factors. We generated mice harboring mutations in both Ctbp1 and Ctbp2 to address the in vivo function of CtBPs during vertebrate development. Ctbp1 mutant mice are small but viable and fertile, whereas Ctbp2-null mice show defects in axial patterning and die by E10.5 due to aberrant extraembryonic development. Mice harboring various combinations of Ctbp1 and Ctbp2 mutant alleles exhibit dosage-sensitive defects in a wide range of developmental processes. The strong genetic interaction, as well as transcription assays with CtBP-deficient cells, indicates that CtBPs have overlapping roles in regulating gene expression. We suggest that the observed phenotypes reflect the large number of transcription factors whose activities are compromised in the absence of CtBP.

Abstract: White adipose tissue (WAT) stores energy in the form of triglycerides, whereas brown tissue (BAT) expends energy, primarily by oxidizing lipids. WAT also secretes many cytokines and acute-phase proteins that contribute to insulin resistance in obese subjects. In this study, we have investigated the mechanisms by which activation of peroxisome proliferator-activated receptor gamma (PPARgamma) with synthetic agonists induces a brown phenotype in white adipocytes in vivo and in vitro. We demonstrate that this phenotypic conversion is characterized by repression of a set of white fat genes ("visceral white"), including the resistin, angiotensinogen, and chemerin genes, in addition to induction of brown-specific genes, such as Ucp-1. Importantly, the level of expression of the "visceral white" genes is high in mesenteric and gonadal WAT depots but low in the subcutaneous WAT depot and in BAT. Mutation of critical amino acids within helix 7 of the ligand-binding domain of PPARgamma prevents inhibition of visceral white gene expression by the synthetic agonists and therefore shows a direct role for PPARgamma in the repression process. Inhibition of the white adipocyte genes also depends on the expression of C/EBPalpha and the corepressors, carboxy-terminal binding proteins 1 and 2 (CtBP1/2). The data further show that repression of resistin and angiotensinogen expression involves recruitment of CtBP1/2, directed by C/EBPalpha, to the minimal promoter of the corresponding genes in response to the PPARgamma ligand. Developing strategies to enhance the brown phenotype in white adipocytes while reducing secretion of stress-related cytokines from visceral WAT is a means to combat obesity-associated disorders.