The insulin receptor:: a new anticancer target for peroxisome proliferator-activated receptor-γ (PPARγ) and thiazolidinedione-PPARγ agonists

The peroxisome proliferator-activated receptor-gamma (PPAR gamma) is a member of the nuclear hormone receptor superfamily. Ligand activation of PPAR gamma is associated with differentiation and inhibition of proliferation in the normal and malignant cells. Herein, we studied the effects of PPAR gamma and the PPAR gamma agonists thiazolidinediones (TZDs) on the insulin receptor (IR), a cell membrane tyrosine kinase receptor protein, whose role is of paramount importance in mediating the metabolic and growth-promoting effects of the peptide hormone insulin. Overexpression of the PPAR gamma 1 in human hepatocellular (HepG2) cells was associated with decreased IR gene transcription and protein expression levels, and these reductions were more evident in the presence of TZDs. Since no PPAR gamma response elements were identified on the IR promoter, we postulated that PPAR gamma adversely affects the IR gene transcription by perturbing the assembly and stability of the transcriptionally active multiprotein-DNA complex identified previously, which includes the high-mobility group All protein, the ubiquitously expressed transcription factor (Sp1), the CAAT enhancer-binding protein (C/EBP beta), and, in some cell lines, the developmentally regulated activator protein-2 (AP-2) transcription factor. Using glutathione S-transferase pull-down assays combined with electrophoretic mobility shift assay and chromatin immunoprecipitation, we demonstrated that by interacting with Spl, C/EBP beta, and AP-2, PPAR gamma can prevent Sp1/AP-2 protein-protein association and inhibit binding of Spl and C/EBP to DNA, thus reducing IR gene transcription. Our results demonstrate that IR is a new target gene of PPAR gamma, and support a potential use of TZDs as anti-proliferative agents in selected neoplastic tissues overexpressing IRS.