Accumulating studies provide strong evidence that Stat3 activation plays an important role in the development and progression of prostate cancer. We previously demonstrated that increased Stat3 activation frequently occurs in prostate cancer and that constitutively activated Stat3 promotes prostate cancer cell tumor growth [3
]. Blockade of Stat3 activation by a dominant negative Stat3 mutant resulted in suppression of prostate cancer growth both in vitro and in vivo [7
]. Numerous studies also demonstrate that Stat3 activates AR-mediated gene expression and prevents apoptosis [4
]. Collectively, these findings indicate that targeting Stat3 signaling may represent a novel approach to treat prostate cancer.
In this study, we have identified that sanguinarine inhibits constitutively activated Stat3 and suppresses cell growth and invasion. Sanguinarine has the ability to inhibit Stat3 phosphorylation at both tyrosine 705 and serine 727 sites, suggesting that sanguinarine has the potential to completely block Stat3 activation since maximal activation of Stat3 requires phosphorylation at both tyrosine 705 and serine 727. To understand how sanguinarine inhibits Stat3 activation, we examined the effect of sanguinarine on Jak2, a kinase that activates Stat3, and found that sanguinarine effectively inhibits Jak2 phosphorylation, suggesting that inhibition of Stat3 activation by sanguinarine is through suppression of Jak2 expression. In addition to inhibition of Jak2, sanguinarine can reduce Src phosphorylation, another kinase that activates Stat3.
The oncogenic effect of Stat3 is mostly through dys-regulation of its target genes such as c-myc and survivin. We have previously demonstrated that sanguinarine inhibits survivin expression in prostate cancer cells [23
]. In this study, we showed that sanguinarine can also inhibit c-myc expression. Furthermore, analyzing the effect of sanguinarine on Stat3 responsive elements, we found that sanguinarine can inhibit transactivation of Stat3 responsive elements, suggesting that sanguinarine is capable of inhibition of Stat3 target gene expression.
Stat3 plays an important role in cancer cell motility, migration, and invasion in many types of cancer such as breast, bladder, ovarian, and brain [42
]. In prostate cancer, Stat3 activation has been found to be associated with lymph node and bone metastases of clinical human prostate cancer, and Stat3 promotes human prostate cancer cell migration and metastases [2
]. Introduction of an activating mutant form of Stat3 (Stat3c) into immortalized prostate epithelial cells resulted in tumorigenesis and enhanced migration through integrin beta 6 [10
]. Inhibition of Stat3 activation by Stat3 specific shRNA or AG490 suppressed migration of prostate cancer cells that express constitutively activated Stat3 [2
]. We showed that sanguinarine inhibited cell motility in DU145 cells expressing constitutively activated Stat3 and LN-s17 cells expressing activated Stat3 induced by IL-6. Furthermore, invasion ability of DU145 cells was also inhibited by sanguinarine. The ability of inhibition of migration and invasion by sanguinarine is comparable to that of the Stat3F, the dominant negative Stat3. These results further confirm that inhibition of Stat3 activation by sanguinarine results in suppression of cell migration and invasion. In addition to the newly identified target of Stat3, sanguinarine has been shown to possess anti-microbial, anti-oxidant, anti-inflammatory, and anti-tumor properties and is widely used in toothpaste and mouthwash to prevent/treat gingivitis and other inflammatory conditions of the mouth [24
In conclusion, we have identified that sanguinarine inhibits constitutively activated Stat3 and suppresses cell proliferation, migration, and invasion. Since sanguinarine has been approved and used clinically, our study holds therapeutic promise for repositioning of sanguinarine for treatment of advanced prostate cancer by targeting constitutively activated Stat3. We are currently testing the efficacy of sanguinarine in mouse models of prostate cancer with constitutively activated Stat3.