Although ligands for the peroxisome proliferator activated receptor gamma (PPARγ) are known to inhibit proliferation of human prostate cancer cells, the mechanisms underlying PPARγ agonist-induced reductions in prostate cancer cell proliferation are not fully understood. In MCF-7 human breast cancer cells, PPARγ ligands inhibit cell proliferation by arresting the cells in the G1 phase of the cell cycle. This G1 arrest is due, in part, to the down-regulation of cyclin D1 and other proteins that regulate the G1-S transition. To determine whether PPARγ agonists alter expression of G1-associated cell cycle proteins in human prostate cancer cells, we tested the effects of the PPARγ ligand ciglitazone on the PC-3 and C4-2 androgen-independent human prostate cancer cell lines. Western blot analysis revealed that ciglitazone concentrations (30-45 μM) that inhibit PC-3 and C4-2 cell proliferation also reduced expression of cyclin D1. Cyclin D1 protein levels were reduced by greater than 50% in both cell lines after twenty-four hours of ciglitazone exposure, and cyclin D1 levels remained depressed after three days of treatment. Transient transfection of cyclin D1-specific siRNA into PC-3 cells not only lowered cyclin D1 protein levels, but also decreased PC-3 cell proliferation, as measured by a reduction in cell number and [3H]-thymidine incorporation. In addition, luciferase-based reporter assays indicated that PPARγ transcriptional activity was increased in PC-3 cells transfected with cyclin D1 siRNA. These data suggest that down-regulation of cyclin D1 by ciglitazone resulted not only in an increase in PPARγ activity but also a decrease in PC-3 cell proliferation. However, cyclin D1 was not the only cell cycle-related protein whose expression was altered by ciglitazone. Ciglitazone reduced protein levels of cyclin D3 within both PC-3 and C4-2 cells. Expression of the cyclin dependent kinase inhibitor p21 was also induced by ciglitazone in the PC-3 cell line. Therefore, while decreases in cyclin D1 appear to play a significant role in ciglitazone-induced inhibition of human prostate cancer cell proliferation, alterations in p21 and cyclin D3 protein expression may also contribute to the net anti-proliferative effect produced by ciglitazone.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]