Adipocyte-specific Expression of Murine Resistin Is Mediated by Synergism between Peroxisome Proliferator-activated Receptor γ and CCAAT/Enhancer-binding Proteins

Takuya Tomaru,David J Steger,Martina I Lefterova,Michael Schupp,Mitchell A Lazar

doi:10.1074/jbc.m808407200

Takuya Tomaru, David J Steger + Show 3 more

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https://doi.org/10.1074/jbc.m808407200

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Abstract

Resistin antagonizes insulin action in mouse, making it a potential therapeutic target for treating metabolic diseases such as diabetes. To better understand how mouse resistin gene (Retn) expression is restricted to fat tissue, we identified an adipocyte-specific enhancer located approximately 8.8-kb upstream of the transcription start site. This region contains a binding site for the master adipogenic regulator peroxisome proliferator-activated receptor gamma (PPARgamma), and binds endogenous PPARgamma together with its partner retinoid-X receptor alpha (RXRalpha). It also contains three binding sites for CCAAT/enhancer-binding protein (C/EBP), and is bound by endogenous C/EBPalpha and C/EBPbeta in adipocytes. Exogenous expression of PPARgamma/RXRalpha and C/EBPalpha in non-adipocyte cells synergistically drives robust expression from the enhancer. Although PPARgamma ligands repress Retn transcription in adipocytes, rosiglitazone paradoxically stimulates the enhancer activity, suggesting that the enhancer is not directly involved in negative regulation. Unlike expression of Retn in mouse, human resistin (RETN) is expressed primarily in macrophages. Interestingly, the region homologous to the mouse Retn enhancer in the human gene contains all three C/EBP elements, but is not conserved for the sequence bound by PPARgamma. Furthermore, it displays little or no binding by PPARgamma in vitro. Taken together, the data suggest that a composite enhancer binding both PPARgamma and C/EBP factors confers adipocyte-specific expression to Retn in mouse, and its absence from the human gene may explain the lack of adipocyte expression in humans.

Highlights

Obese individuals have increased risk of insulin resistance, which leads to type 2 diabetes, hypertension, and cardiovascular disease [1]
We report a novel adipocyte-specific enhancer at ϳ8.8-kb upstream of the mouse Retn gene, where PPAR␥ and
We discovered the enhancer by ChIP-chip mapping of chromatin marks, and it was independently confirmed by functional dissection of the 5Ј-flanking sequence by reporter assay, and genome-wide ChIP-chip analysis for PPAR␥- and CCAAT/enhancer-binding proteins (C/EBPs)␣-binding sites

Summary

Introduction

Obese individuals have increased risk of insulin resistance, which leads to type 2 diabetes, hypertension, and cardiovascular disease [1]. Various mouse models, including infusion of recombinant resistin, overexpression of resistin in liver, or deletion of Retn, have consistently shown that resistin antagonizes insulin action (8 –10) These reports clearly indicate that resistin plays major roles in normal glucose homeostasis as well as the pathophysiology of insulin resistance in rodents, it is controversial whether these conclusions translate to the human situation [11]. C/EBP␤ and C/EBP␦ are rapidly induced in the early stage of adipogenesis [30, 31] These transcription factors subsequently drive C/EBP␣ and PPAR␥ expression [32,33,34]. Adipocyte Expression of Murine Resistin pocyte-specific genes including adipocyte P2 (aP2) [37,38,39] and leptin [40, 41] Based on these reports, we thought C/EBPs and PPAR␥ might be possible candidates that explain adipocytespecific expression of Retn

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