Differential Regulation of Peroxisome Proliferator Activated Receptor g1 (PPARg1) and PPARg2 Messenger RNA Expression in the Early Stages of Adipogenesis

Adipocyte differentiation is driven by the expression and activation of three transcription factor families: the differentially expressed CAAT/enhancer binding proteins (C/EBPs) a, b, and d; the helix-loop-helix adipocyte differentiation and determination factor-1; and peroxisome proliferator activated receptor g (PPARg), expressed as two isoforms, PPARg1 and the adipocyte-specific PPARg2. Overexpression of PPARg can induce adipocyte differentiation; therefore, we analyzed the expression of the two PPARg isoforms during early stages of differentiation to determine whether one was preferentially induced as an early determining event. Surprisingly, in the first 24 h, a 3–6fold increase of PPARg2 mRNA was observed, whereas PPARg1 mRNA remained unchanged. PPARg1 was induced 1 day later. Overexpression of C/EBPb has also been shown to induce adipocyte differentiation. A C/EBP site was identified only in the human PPARg2 promoter. Its deletion blunted the response of PPARg2 promoter to cotransfected C/EBPb or methylisobutylxanthine treatment. We hypothesize that PPARg2 initiates adipocyte differentiation.