Targeting Stat3 signaling inhibits human prostate cancer cell growth in vitro and in vivo.

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Patients with advanced prostate cancer initially respond to androgen deprivation therapy but usually relapse with androgen-independent disease (AIPC). Conventional chemotherapy has not been shown an increase in survival, though there is considerable palliative benefit of cytotoxic agents. Interleukin-6 (IL-6) a multifunctional cytokine, promotes androgen independent cell growth by activating androgen receptor and protecting cells from apoptotic cell death. IL-6 activates several major signaling pathways including JAK-STAT, PI3-AKT, and MAPK in human prostate cancer cells. Studies have shown that Stat3 is constitutively active in prostate cancer. Additionally, Stat3 is a major mediator of IL-6 induced signaling in prostate cancer cells and IL-6 induced AR-mediated gene activation requires activated Stat3. Thus IL-6/Stat3 signaling pathways are potential new molecular targets in prostate cancer. We evaluated two Stat3 signaling inhibitors: AG490 and JSI-124 (Cucurbitacin I). AG490 is a tyrosine kinase inhibitor selective for the JAK family kinases. The effects of AG490 on the growth of prostate cancer cells that express activated Stat3 in vitro and in vivo were evaluated. We chose to study the following human: LNCaP, DU145, PC3, CWR22Rv1 and Dunning rat: G, AT1, AT2, AT6.1, AT3.1 prostate cell lines. AG490 abrogated Stat3 activity and inhibited the growth of human and prostate cancer cell lines with IC₅₀ from 10 to 40 uM. DU145 tumor model was used to study the effects in vivo. Once tumors reached a size of 0.3 cm³ they either received AG490 (0.5 mg/mice i.p daily for 14 days) or vehicle (DMSO/RPMI) only. AG490 suppressed tumor growth by 50% (p<0.01). The effects of JSI-124 in human prostate cancer cell lines: LNCaP, DU145, PC3 and CWR22Rv1 were evaluated. The IC₅₀ of JSI-124 was found to range from 0.5 to 1.0 uM. In addition, we evaluated the potential use of RNA interference to block Stat3 expression and activation and the effect on the growth of human prostate cancer cells. Blockade of Stat3 activation by the Stat3 siRNA suppresses the growth of human prostate cancer cells and Stat3-mediated gene expression and induces apoptotic cell death. The Stat3 siRNA does not inhibit the proliferation nor induces apoptosis of Stat3-inactive human prostate cancer cells. In addition, the Stat3 siRNA inhibits the levels of androgen-regulated prostate specific antigen (PSA) expression in prostate cancer cells. These results demonstrated that targeting Stat3 signaling using siRNA technique may serve as a novel therapeutic strategy for treatment of prostate cancer expressing constitutively activated Stat3. Collectively, our data suggest that targeting IL-6/Stat3 signaling may represent an opportunity in the development of new treatments for AIPC.