Phosphodiesterase 5/PKG Signal Governs Stemness Of Prostate Cancer Stem Cells Through Hippo/TAZ Pathway

The Hippo signaling appears to be highly conserved in evolution, which plays important biological roles in organ growth and development, cancer initiation and development. Cancer stem cells have been identified in various tumors, which highly hypothesized to contribute to cancer initiation, chemoresistance, metastasis and relapse; however, the molecular mechanisms governing the sternness of cancer stem cells still remain elusive, especially to prostate cancer stem cells. Because of the properties of androgen receptor-negative and undifferentiated, PC3 cell is an ideal in vitro model for PCSC biological study. We successfully isolated PC3-derived clone (Prostate cancer stem cells, PCSC) with the molecular profile FAM65Bhigh, MFI2low, LEF1low, which possessed properties of cancer stem cells. TAZ, Nanog and SOX2 which are sternness-related transcription factors, which were highly elevated in PCSC.TAZ knockdown significantly reversed expression patterns of PCSC markers, attenuated colony formation, and decreased expression of Nanog and SOX2, decreased tumorigenesis and tumor growth of PCSC. Therefore, TAZ is vital to maintain stemness of PCSC.Several studies have hinted at the possible effect of NO/cGMP on ES cell differentiatioa We found that NO/cGMP/PKG signaling was largely suppressed in PCSC; compared to PC3 cells, genes expression of iNOS, eNOS and pkgl/2 were significantly decreased in PCSC, while pde5a was significantly increased, these were also confirmed in protein expression level; meanwhile, PCSC dramatic deceased endogenous NO production compared to PC3 cells. However, the contribution of NO/cGMP/PKG signal to self-maintenance of PCSC remains elusive. Our data shown that PDE5 inhibitor dramatically decreased protein level of TAZ and its nucleus transfocation, then further suppressed the activity of TEAD4 luciferase reporter; meanwhile, these were also confirmed by experiment results of 8-br-cGMP (PKG activator) or KT5823 (PKG inhibitor). We next found that PKG bound to MST, which in turn induced phosphorylation of MST and activated hippo signaling cascade, leaded to phosphorylation of TAZ atSer89 site, resulting in its ubiquitinated degradation and dysfunction. Therefore, PCSC elevated TAZ to maintain the stemness by inhibiting NO/cGMP/PKG/Hippo signaling cascade.Our data further shown that PDE5 inhibitor as well as TAZ knockdown, significantly reversed expression patterns of PCSC markers, attenuated colony formation, decreased expression of Nanog and SOX2, and decreased tumor growth of PCSC. Therefore, PDE5 inhibitor or TAZ knockdown induced trans-diflerentiation in PCSC. What’s more, the combination of PDE5 inhibitor and cisplatin significantly fecititated cisplatin-induced cytotoxicity in PCSC in vitro study, and also significantly facilitated cisplatin-induced suppression of tumor growth in PCSC xenografls. Taken together, our findings presented an alternative interpretation for prostate cancer initiation and development, chemoresistance, relapse and metastasis; besides, our findings also presented an alternative rationale for the combination of PDE5 inhibitor and cisplatin in prostate cancer therapy.