Regulation of the Growth Arrest and DNA Damage-Inducible Gene 45 (GADD45) by Peroxisome Proliferator-Activated Receptor γ in Vascular Smooth Muscle Cells
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Peroxisome proliferator-activated receptor (PPAR) γ is activated by thiazolidinediones (TZDs), widely used as insulin-sensitizing agents for the treatment of type 2 diabetes. TZDs have been shown to induce apoptosis in a variety of mammalian cells. In vascular smooth muscle cells (VSMCs), proliferation and apoptosis may be competing processes during the formation of restenotic and atherosclerotic lesions. The precise molecular mechanisms by which TZDs induce apoptosis in VSMCs, however, remain unclear. In the present study, we demonstrate that the TZDs rosiglitazone (RSG), troglitazone (TRO), and a novel non-TZD partial PPARγ agonist (nTZDpa) induce caspase-mediated apoptosis of human coronary VSMCs. Induction of VSMC apoptosis correlated closely with an upregulation of growth arrest and DNA damage-inducible gene 45 (GADD45) mRNA expression and transcription, a well-recognized modulator of cell cycle arrest and apoptosis. Using adenoviral-mediated overexpression of a constitutively active PPARγ mutant and the irreversible PPARγ antagonist GW9662, we provide evidence that PPARγ ligands induce caspase-mediated apoptosis and GADD45 expression through a receptor-dependent pathway. Deletion analysis of the GADD45 promoter revealed that a 153-bp region between −234 and −81 bp proximal to the transcription start site, containing an Oct-1 element, was crucial for the PPARγ ligand–mediated induction of the GADD45 promoter. PPARγ activation induced Oct-1 protein expression and DNA binding and stimulated activity of a reporter plasmid driven by multiple Oct-1 elements. These findings suggest that activation of PPARγ can lead to apoptosis and growth arrest in VSMCs, at least in part, by inducing Oct-1–mediated transcription of GADD45. The full text of this article is available online at http://www.circresaha.org.