Chelidonine Interferes with IL-6R/STAT3 Signaling in Uveal Melanoma Cells

There is increasing evidence suggesting the importance of IL-6 in oncogenesis: it stimulates tumor cells proliferation and promotes cell survival through the inhibition of apoptosis. IL-6 acts on a receptor complex consisting of the cytokine-specific IL-6Rα chain and the signal-transducing gp130 subunit. Binding of IL-6 to IL-6Rα induces dimerization of gp130 which initiates multiple signaling cascades, including STAT3 activation. Chelidonine, the major alkaloid component of C. majus, provokes cell death in a variety of tumor cells, possibly through the antiapoptotic Bcl-2 protein. Expression of Bcl-2 is upregulated by STAT3 activation, which is thought to be responsible for IL-6-mediated survival of tumor cells. Herein we aimed to study the effect of chelidonine on the viability of human uveal melanoma cells as wells as its interference with the IL-6R/STAT3 signaling pathway. Antiproliferative and cell death-inducing effects of chelidonine were assessed by flow cytometry. The apoptotic potential of chelidonine was followed by DNA fragmentation and PI exclusion/annexin V binding assays. Expression of STAT3, Bcl-2 and IL-6Rα and the efficiency of STAT3 activation was also studied by flow cytometry. Combined analysis of cell death experiments revealed chelidonine-induced apoptosis of UM cells. Moreover, alkaloid treatment also resulted in necrotic cell death. Pretreatment of cells even with sublethal doses of chelidonine led to the appearance of a subpopulation with abrogated STAT3 activation upon IL-6 stimulation and modified Bcl-2 expression levels. We detected cells with reduced expression of STAT3 and IL-6Rα; however, the amount of these cells was significantly lower than that of cells with abolished STAT3 signaling. According to our results chelidonine exerts its effect via a STAT3-dependent mechanism. Our findings imply the possible use of chelidonine in cancer therapy: it can either provoke cell death or weaken the antiapoptotic machinery of tumor cells fueled by IL-6.