Regulation of androgen receptor transactivity and mTOR-S6 kinase pathway by Rheb in prostate cancer cell proliferation

BACKGROUND Ras homolog‐enriched in brain (Rheb), a small GTP‐binding protein, is associated with prostate carcinogenesis through activating mammalian target of rapamycin (mTOR) signaling pathway. This study aimed to elucidate whether Rheb promotes proliferation of prostate cancer cells and can act as a potent therapeutic target in prostate cancer. METHODS Prostate cancer cell lines and human prostatic tissues were examined for the expression of Rheb. The effects of forced expression or knockdown of Rheb on cell proliferation were also examined. Semi‐quantitative and quantitative RT‐PCR were performed to evaluate mRNA expression. Western blotting was used to examine protein expression. Cell count and WST‐1 assay were used to measure cell proliferation. Fluorescence‐activated cell sorting was used to assess the cell cycle. RESULTS Rheb mRNA and protein expression was higher in more aggressive, androgen‐independent prostate cancer cell lines PC3, DU145, and C4‐2, compared with the less aggressive LNCaP. Rheb expression was higher in cancer tissues than in benign prostatic epithelia. Forced expression of Rheb in LNCaP cells accelerated proliferation without enhancing androgen receptor transactivity. Attenuation of Rheb expression or treatment with the mTOR inhibitor rapamycin decreased proliferation of PC3 and DU145 cells, with a decrease in the activated form of p70S6 kinase, one of the main targets of mTOR. CONCLUSIONS Rheb potentiates proliferation of prostate cancer cells and inhibition of Rheb or mTOR can lead to suppressed proliferation of aggressive prostate cancer cell lines in vitro. Rheb and the mTOR pathway are therefore probable targets for suppressing prostate cancer. Prostate 70: 866–874, 2010. © 2010 Wiley‐Liss, Inc.