Inhibition of mTORC1 kinase activates Smads 1 and 5 but not Smad8 in human prostate cancer cells, mediating cytostatic response to rapamycin.

Although hyperactivated mTOR is well recognized as being pivotal to prostate cancer growth and progression, the underlying mechanisms by which it promotes such responses remain incompletely understood. Here, we show that rapamycin activates Smads 1 and 5 in human prostate cancer cells and tissues through blocking mTORC1 kinase. Small hairpin RNA-based gene silencing and gene overexpression approaches reveal that Smads 1 and 5 mediate, whereas Smad8 represses, rapamycin-induced cell death and expression of the bone morphogenetic protein (BMP) transcriptional target Id1 in human prostate cancer cell lines. Moreover, such phospho-Smad1/5-mediated rapamycin responses were blocked by LDN-193189 (a BMPRI kinase inhibitor) or Noggin (a BMP antagonist) in LNCaP prostate cancer cells. Likewise, the mTOR kinase inhibitors Ku-0063794 and WYE-354 each enhanced phosphorylation of Smad1/5. Intriguingly, silencing raptor alone enhanced, whereas silencing rictor repressed, the phosphorylation of Smad1/5, indicating that mTORC1 represses, whereas mTORC2 activates, BMP signaling. Immunohistochemical analysis showed increased levels of phospho-Smad1/5 concomitant with suppression of phospho-S6 and survivin levels in PC3 human prostate cancer xenografts in athymic mice administered rapamycin (intraperitoneally, 5 mg/kg/d, 2-6 days). Moreover, we show that compared with prostate tumor tissue from untreated patients, levels of phospho-Smad1/5 were significantly elevated in the prostate tumor tissue of patients with high-risk prostate cancer who received 8 weeks of the rapalog everolimus as part of a neoadjuvant clinical trial before undergoing local definitive therapy by radical prostatectomy. Taken together, our data implicate Smads 1, 5 and 8 as potential prognostic markers and therapeutic targets for mTOR inhibition therapy of prostate cancer.