Overexpression of FOXGIcontributes to TGF-β resistance through inhibition of p2 WAFI/CIPI expression in ovarian cancer

BACKGROUND: Loss of growth inhibitory response to transforming growth factor-β (TGF-β) is a common feature of epithelial cancers. Recent studies have reported that genetic lesions and overexpression of oncoproteins in TGF-β/Smads signalling cascade contribute to the TGF-β resistance. Here, we showed that the overexpressed FOXG 1 was involved in attenuating the anti-proliferative control of TGF-β/Smads signalling in ovarian cancer. METHODS: FOXG and p21 WAFI/CIPI expressions were evaluated by real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR), western blot and immunohistochemical analyses. The effect of FOXG 1 on p21 WAFI/CIPI transcriptional activity was examined by luciferase reporter assays. Cell lines stably expressing or short hairpin RNA interference-mediated knockdown FOXG I were established for studying the gain-or-loss functional effects of FOXG I XTT cell proliferation assay was used to measure cell growth of ovarian cancer cells. RESULTS: Quantitative RT-PCR and western blot analyses showed that FOXGI was upregulated and inversely associated with the expression levels of p21 WAFI/CIPI in ovarian cancer. The overexpression of FOXGI I was significantly correlated with high-grade ovarian cancer (P=0.025). Immunohistochemical analysis on ovarian cancer tissue array was further evidenced that FOXG was highly expressed and significantly correlated with high-grade ovarian cancer (P=0.048). Functionally, enforced expression of FOXGI I selectively blocked the TGF-β-induced p21 WAFI/CIPI expressions and increased cell proliferation in ovarian cancer cells. Conversely, FOXG I knockdown resulted in a 20-26% decrease in cell proliferation together with 16-33% increase in p2 WAFI/CIPI expression. Notably, FOXG I was able to inhibit the p2 WAFI/CIPI promoter activity in a p53-independent manner by transient reporter assays. CONCLUSION: Our results suggest that FOXG I acts as an oncoprotein inhibiting TGF-β-mediated anti-proliferative responses in ovarian cancer cells through suppressing p2 WAF/CIPI transcription.