Inhibition of mTORC1 Kinase Activates Smads 1 and 5 but Not Smad8 in Human Prostate Cancer Cells, Mediating Cytostatic Response to Rapamycin

Abstract

Although hyper-activated 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. ShRNA-based gene silencing and gene overexpression approaches reveal that Smads 1 and 5 mediate while Smad8 represses rapamycin-induced cell death and expression of the 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 while silencing Rictor repressed the phosphorylation of Smad1/5, indicating that mTORC1 represses while 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 (i.p., 5 mg/kg/day, 2 to 6 days). Moreover, we show that compared to prostate tumor tissue from untreated patients, levels of phospho-Smad1/5 were significantly elevated in the prostate tumor tissue of high-risk prostate cancer patients who received 8 weeks of the rapalog everolimus as part of a neoadjuvant clinical trial prior to 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.

Critical Role of a Survivin/TGF-β/mTORC1 Axis in IGF-I-Mediated Growth of Prostate Epithelial Cells

“…The molecular mechanism behind suppression of BMP signaling by TGF-β is under investigation in our group. Our results that sh-mTOR and sh-Raptor activate Smad1/5/8 are consistent with our recent article demonstrating that mTORC1 kinase represses P-Smad1/5, whereas mTORC2 activates P-Smad1/5 in human PCa cell lines [63]. Despite the activation of BMP Smad signaling, Survivin levels remain elevated.…”

Identification and validation of a prognostic 5-protein signature for biochemical recurrence following radical prostatectomy in prostate cancer

“…Potential molecular mechanism may be explained that upregulated YB-1 contributed to reduced intracellular androgen accumulation, thus weaning PCa off androgen dependency and upregulating tumor survival [23]. In accordance with the present study, SMAD1 is reported to be signi cantly elevated in patients with high-risk PCa [24]. The same conclusion is also drawn by increasing evidence which show the downregulation of SMAD1 contributed to the PCa proliferation, migration and invasion [25].…”

Rapamycin rescues BMP mediated midline craniosynostosis phenotype through reduction of mTOR signaling in a mouse model

“…It has been reported that rapamycin treatment activates the BMP-Smad pathway (van der Poel, Hanrahan, Zhong, & Simons, 2003;Wahdan-Alaswad et al, 2012) due to the dissociation of FKBP12 from the BMP type 1 receptor (Wang et al, 1996). Previous studies have alluded to the differential impact of rapamycin on mTORC1 and Akt signaling and rapamycin's contributions to apoptosis and cell survival.…”