Down-regulation of peroxisome proliferator-activated receptor gamma in human cervical carcinoma

Objective Peroxisome proliferator-activated receptor gamma (PPARγ) is a member of the nuclear hormone receptor superfamily. Treatment of PPARγ ligands has been shown to inhibit the growth of various human cancer cells. However, it has not been reported whether human cervical carcinoma cells express PPARγ. In this study, we investigated the expression of PPARγ in human normal cervix and cervical carcinoma tissues, and as well as the effect of PPARγ ligands on cervical cancer cells survival. Methods Fresh cervical tissues from a study group of 10 study patients diagnosed with cervical carcinoma were analyzed for the expression of PPARγ using real-time RT-PCR and Western blot analysis. Immunohistochemical staining for PPARγ was also performed on the serial sections of 40 cervical carcinomas. In addition, we evaluated the feasibility of PPARγ ligands, as a potential therapeutic drug against cervical cancer cells using MTT assay and FACS analysis. Results We found that there were lower expression levels of PPARγ mRNA and protein in cervical carcinoma tissues than in normal cervical tissues. The extent and intensity of immunoreactive PPARγ in normal cervix tissues were statistically much greater than those of carcinoma tissues. In order to study effects of PPAR ligand on cell proliferation, we chose ciglitizone that showed very potent growth inhibitory effects on the proliferation of two human cervical cancer cell lines (C-33-A and C-4II). C-4II cells express high expression of PPARγ, while C-33A cells express low level of PPARγ. Treatment with ciglitizone inhibited the growth of C-4II cells in a dose-dependent manner, while the growth inhibitory effect of ciglitizone was much less in C-33A cells. In order to test whether ciglitizone-induced growth suppressive effects on cervical cancer cell lines is PPAR-dependent, we treated cervical cancer cells with ciglitizone and/or GW9662 (a PPARγ antagonist). No significant difference in cell survival was found in cells treated with ciglitizone alone vs. co-treated with ciglitizone and GW9662. GW9662 alone did not induce any cell growth arrest in the cells that we used (data not shown). Thus, we concluded that growth suppressive effects by ciglitizone may not be dependent upon status of PPAR expression. To clarify the mechanism by which ciglitizone inhibits the growth of cervical carcinoma cells, flow cytometry and Western blotting assay were performed. As results, we demonstrated that a large portion of C-4II cells (but not in C-33A) after ciglitizone treatment were arrest at G1 phase with the induction of p21 Cip1/Waf1 and p27 kip1 protein. Conclusions These results suggest that PPARγ is down-regulated in multiple human cervical cancer tissues and cell lines. Ciglitizone may suppress human cervical cancer cells in PPAR-independent manner.