Background: Loss of growth inhibitory response to transforming growth factor-beta (TGF-beta) is a common feature of epithelial cancers. Recent studies have reported that genetic lesions and overexpression of oncoproteins in TGF-beta/Smads signalling cascade contribute to the TGF-beta resistance. Here, we showed that the overexpressed FOXG1 was involved in attenuating the anti-proliferative control of TGF-beta/Smads signalling in ovarian cancer.
Methods: FOXG1 and p21(WAF1/CIP1) expressions were evaluated by real-time quantitative reverse-transcription polymerase chain reaction (RT-PCR), western blot and immunohistochemical analyses. The effect of FOXG1 on p21(WAF1/CIP1) transcriptional activity was examined by luciferase reporter assays. Cell lines stably expressing or short hairpin RNA interference-mediated knockdown FOXG1 were established for studying the gain-or-loss functional effects of FOXG1. XTT cell proliferation assay was used to measure cell growth of ovarian cancer cells.
Results: Quantitative RT-PCR and western blot analyses showed that FOXG1 was upregulated and inversely associated with the expression levels of p21(WAF1/CIP1) in ovarian cancer. The overexpression of FOXG1 was significantly correlated with high-grade ovarian cancer (P=0.025). Immunohistochemical analysis on ovarian cancer tissue array was further evidenced that FOXG1 was highly expressed and significantly correlated with high-grade ovarian cancer (P=0.048). Functionally, enforced expression of FOXG1 selectively blocked the TGF-beta-induced p21(WAF1/CIP1) expressions and increased cell proliferation in ovarian cancer cells. Conversely, FOXG1 knockdown resulted in a 20-26% decrease in cell proliferation together with 16-33% increase in p21(WAF1/CIP1) expression. Notably, FOXG1 was able to inhibit the p21(WAF1/CIP1) promoter activity in a p53-independent manner by transient reporter assays.
Conclusion: Our results suggest that FOXG1 acts as an oncoprotein inhibiting TGF-beta-mediated anti-proliferative responses in ovarian cancer cells through suppressing p21(WAF1/CIP1) transcription.