Regulation of Peroxisome Proliferator–Activated Receptor-γ Activity by Mammalian Target of Rapamycin and Amino Acids in Adipogenesis

Regulation of Peroxisome Proliferator–Activated Receptor-γ Activity by Mammalian Target of Rapamycin and Amino Acids in Adipogenesis Jae Eun Kim and Jie Chen From the Department of Cell and Structural Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois Address correspondence and reprint requests to Jie Chen, Department of Cell and Structural Biology, University of Illinois at Urbana-Champaign, 601 S. Goodwin Ave., B107 Urbana, IL 61801. E-mail: jiechen{at}uiuc.edu Abstract Adipocyte differentiation is a developmental process that is critical for metabolic homeostasis and nutrient signaling. The mammalian target of rapamycin (mTOR) mediates nutrient signaling to regulate cell growth, proliferation, and diverse cellular differentiation. It has been reported that rapamycin, the inhibitor of mTOR and an immunosuppressant, blocks adipocyte differentiation, but the mechanism underlying this phenomenon remains unknown. Here we show that mTOR plays a critical role in 3T3-L1 preadipocyte differentiation and that mTOR kinase activity is required for this process. Rapamycin specifically disrupted the positive transcriptional feedback loop between CCAAT/enhancer-binding protein-α and peroxisome proliferator–activated receptor-γ (PPAR-γ), two key transcription factors in adipogenesis, by directly targeting the transactivation activity of PPAR-γ. In addition, we demonstrate for the first time that PPAR-γ activity is dependent on amino acid sufficiency, revealing a molecular link between nutrient status and adipogenesis. The results of our further investigation have led us to propose a model in which the mTOR pathway and the phosphatidylinositol 3-kinase/Akt pathway act in parallel to regulate PPAR-γ activation during adipogenesis by mediating nutrient availability and insulin signals, respectively. It is interesting that troglitazone (a thiazolidinedione drug) reversed the inhibitory effects of rapamycin and amino acid deprivation, implicating therapeutic values of thiazolidinedione drugs to counter certain side effects of rapamycin as an immunosuppressant. 4E-BP1, eukaryotic initiation factor 4E-binding protein 1 CMV, cytomegalovirus DEX, dexamethasone DMEM, Dulbecco’s modified Eagle’s medium; MIX, isobutylmethylxanthine mTOR, mammalian target of rapamycin PI3K, phosphatidylinositol 3-kinase PPAR, peroxisome proliferator–activated receptor; PPRE, PPAR-γ responsive element RXR, retinoid X receptor S6K1, ribosomal S6 kinase 1 TZD, thiazolidinedione Foot