Peroxisome Proliferator-Activated Receptor γ-Dependent Activation of p21 in Panc-28 Pancreatic Cancer Cells Involves Sp1 and Sp4 Proteins

1,1-Bis(3'-indolyl)-1-(p-trifluoromethylphenyl)methane (DIM-C-pPhCF3) and troglitazone activate peroxisome proliferator-activated receptor gamma (PPARgamma) in Panc-28 pancreatic cancer cells and also inhibit cell proliferation. DIM-C-pPhCF3 was more active than troglitazone and was used as a model to investigate the mechanism of PPARgamma-dependent inhibition of Panc-28 cell growth. DIM-C-pPhCF3 significantly inhibited G0/G1-->S phase progression, as determined by FACS analysis, and this was associated with decreased retinoblastoma protein phosphorylation and increased p21 protein and mRNA expression, but no change in p27 or cyclin D1. PPARgamma antagonists blocked DIM-C-pPhCF3-induced growth inhibition and induction of p21 protein, and similar inhibitory effects were observed in Panc-28 cells transfected with a construct (pWWP) containing a -2325 to +8 p21 promoter insert. Deletion analysis of the p21 promoter indicated that PPARgamma-dependent activation of p21 promoter constructs by DIM-C-pPhCF3 required GC-rich sites 3 and 4 in the proximal region (-124 to -60) of the p21 promoter. The results of RNA interference and protein expression/DNA binding assays suggest that DIM-C-pPhCF3 induced p21 expression through a novel mechanism that involves PPARgamma interactions with both Sp1 and Sp4 proteins bound to the proximal GC-rich region of the p21 promoter.