Small G-protein RhoE is underexpressed in prostate cancer and induces cell cycle arrest and apoptosis

BACKGROUND. RhoE/Rnd3, a recently described novel member of the Rho GTPases family, was discussed as a possible antagonist of the RhoA protein that stimulates cell cycle progression and is overexpressed and/or overactivated in Prostate cancer. We investigated the expression of RhoE and its role in cell cycle regulation and apoptosis in the human prostate. METHODS. RhoE expression in cell lines and tissue specimens was assessed by immunoblot analysis, real-time PCR (RT-PCR), and immunohistochemistry. To elucidate RhoE effects on the prostate, RhoE was cloned and overexpressed in DU-145 prostate cancer. Cell cycle modulation and apoptosis was investigated by immunoblot and FACS analysis. RESULTS. Immunoblot analysis showed a strong RhoE signal in both, benign epithelial and stromal cells. In contrast, almost no protein was detected in various prostate cancer cells. On RTPCR and microarray analysis, RhoE mRNA expression was significantly reduced in malignant tissue when compared to benign samples. RhoE immunostaining was strong in benign tissue, especially in prostate epithelial cells, whereas it was minimal or absent in malignant tissue. Forced RhoE overexpression in a prostate cancer cell line inhibits the expression of two proteins essential for G2/M transition, namely CDC2 and cyclin B1, and induces G2/M arrest. In addition, apoptotic cell death as measured by a cleavage product of caspase 3 is significantly increased in RhoE-overexpressing cells. CONCLUSION. In conclusion, our findings suggest RhoE as a tumor suppressor gene that is downregulatated early in the development of prostate cancer.