Abstract 3514: Activation of Smad Transcriptional Corepressor TGIF by Insulin Is Required for Adipocyte Differentiation.
Adipogenesis is a well-controlled process that is regulated by transcription factors that act sequentially to trigger early and terminal adipocyte differentiation. Since the accumulation of visceral adipose tissue is closely associated with insulin resistance, a hallmark of metabolic syndrome, it is important to identify genes that are required for adipocyte differentiation. It has been previously shown that transgenic overexpression of TGF-β1 in mouse adipose tissue leads to reduced total body fat. The cell-surface availability of TGF-β receptors strongly decreases during adipocyte differentiation and interferes with the activation of Smad2/3, downstream effectors of TGF-β. Nevertheless, there may be alternative mechanisms that perturb TGF-β-dependent signaling during adipocyte differentiation. To identify genes that are required for the differentiation of 3T3-L1 preadipocytes into mature adipocytes, we used retrovirus insertion-mediated random mutagenesis to generate 3T3-L1 cell lines that lose their ability to differentiate into mature adipocytes. One of the genes identified was TG-interacting factor (TGIF), a DNA-binding homeodomain protein that has been demonstrated to suppress Smad-mediated activation of TGF-β-regulated transcription. In a TGIF-disrupted clone of 3T3-L1 preadipocytes, the rate of differentiation into mature adipocyte was clearly reduced compared with that in the wild-type clone. Suppression of TGIF by lenti virus-mediated RNAi also inhibited the differentiation of 3T3-L1 cells. Insulin specifically increased the abundance of TGIF protein, primarily by enhancing its stability. In addition, insulin caused the rapid accumulation of TGIF in the nuclei and increased binding to DNA. Forced expression of TGIF repressed both endogenous and exogenous Smad2/3-mediated induction of TGF-β-dependent promoter activity in 3T3-L1 cells. These findings demonstrate that the Smad transcriptional corepressor TGIF is required for insulin-dependent differentiation of preadipocytes into mature adipocytes. Our results also provide a novel mechanism by which insulin antagonizes TGF-β signaling during adipocyte differentiation.