Previously, we have isolated and characterized an enhancer from the 5'-flanking region of the adipocyte P2 (aP2) gene that directs high-level adipocyte-specific gene expression in both cultured cells and transgenic mice. The key regulator of this enhancer is a cell type-restricted nuclear factor termed ARF6. Target sequences for ARF6 in the aP2 enhancer exhibit homology to a direct repeat of hormone response elements (HREs) spaced by one nucleotide; this motif (DR-1) has been demonstrated previously to be the preferred binding site for heterodimers of the retinoid X receptor (RXR) and the peroxisome proliferator-activated receptor (PPAR). We have cloned a novel member of the peroxisome proliferator-activated receptor family designated mPPAR gamma 2, and we demonstrate that a heterodimeric complex of mPPAR gamma 2 and RXR alpha constitute a functional ARF6 complex. Expression of mPPAR gamma 2 is induced very early during the differentiation of several cultured adipocyte cell lines and is strikingly adipose-specific in vivo. mPPAR gamma 2 and RXR alpha form heterodimers on ARF6-binding sites in vitro, and antiserum to RXR alpha specifically inhibits ARF6 activity in adipocyte nuclear extracts. Moreover, forced expression of mPPAR gamma 2 and RXR alpha activates the adipocyte-specific aP2 enhancer in cultured fibroblasts, and this activation is potentiated by peroxisome proliferators, fatty acids, and 9-cis retinoic acid. These results identify mPPAR gamma 2 as the first adipocyte-specific transcription factor and suggest mechanisms whereby fatty acids, peroxisome proliferators, 9-cis retinoic acid, and other lipids may regulate adipocyte gene expression and differentiation.

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mPPAR gamma 2: tissue-specific regulator of an adipocyte enhancer.

Essential fatty acids in health and chronic disease

The organization, promoter analysis, and expression of the human PPARgamma gene.

Role of the peroxisome proliferator-activated receptor (PPAR) in mediating the effects of fibrates and fatty acids on gene expression.

Increased activity of lipoprotein lipase (LPL) may explain the hypotriglyceridemic effects of fibrates, thiazolidinediones and fatty acids, which are known activators (and/or ligands) of the various peroxisome proliferator-activated receptors (PPARs). Treatment with compounds which activate preferentially PPARalpha, such as fenofibrate, induced LPL expression exclusively in rat liver. In contrast, the antidiabetic thiazolidinedione BRL 49653, a high affinity ligand for PPARgamma, had no effect on liver, but induced LPL expression in rat adipose tissue. In the hepatocyte cell line AML-12, fenofibric acid, but not BRL 49653, induced LPL mRNA, whereas in 3T3-L1 preadipocytes, the PPARgamma ligand induced LPL mRNA levels much quicker and to a higher extent than fenofibric acid. In both the in vivo and in vitro studies, inducibility by either PPARalpha or gamma activators, correlated with the tissue distribution of the respective PPARs: an adipocyte-restricted expression of PPARgamma, whereas PPARalpha was expressed predominantly in liver. A sequence element was identified in the human LPL promoter that mediates the functional responsiveness to fibrates and thiazolidinediones. Methylation interference and gel retardation assays demonstrated that a PPARalpha or gamma and the 9-cis retinoic acid receptor (RXR) heterodimers bind to this sequence -169 TGCCCTTTCCCCC -157. These data provide evidence that transcriptional activation of the LPL gene by fibrates and thiazolidinediones is mediated by PPAR-RXR heterodimers and contributes significantly to their hypotriglyceridemic effects in vivo. Whereas thiazolidinediones predominantly affect adipocyte LPL production through activation of PPARgamma, fibrates exert their effects mainly in the liver via activation of PPARalpha.

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BinCdicte Bertrand

Papers

Regulation of adipose cell differentiation. I. Fatty acids are inducers of the aP2 gene expression