Inhibition of Src activity enhances the tumor-suppressive effect of the connexin 32 gene in Caki-1 renal cancer cells

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Connexin (Cx) genes exert negative growth effects on tumor cells with certain cell specificity. We have recently reported that Cx32 acts as a tumor suppressor gene in a metastatic renal cancer cell line (Caki-1) due to the inactivation of Src. In line with the previous study, here we investigated if an Src family inhibitor (PP1) could enhance the tumor-suppressive effect of Cx32 in Caki-1 cells from human metastatic renal cell carcinoma. We examined the difference in the cytotoxic effect of PP1 on two cell clones, Cx32-transfected Caki-1 cells (Caki-1T) and mock-transfected Caki-1 cells (Caki-1W), in vitro as well as in vivo. PP1 showed more cytotoxic effect on Caki-1T than on Caki-1W at lower doses. This reinforcement was also observed in a xenograft model of nude mice. The in vitro reinforcement of the cytotoxic effect depended not only on control of cell-cycle transition but also on the induction of apoptosis, and the occurrence of the event was mostly caused by potential inhibition of Src activity in Caki-1T. Also, under a hypoxic condition, which is a typical environment of tumor tissue, Cx32 suppressed hypoxia-induced Src activation, and PP1 enhanced cytotoxicity in Caki-1T. These results suggest that, in addition to the Cx32-dependent tumor-suppressive effect, the concomitant inhibition of Src by PP1 is an effective procedure to induce a cytotoxic effect in Caki-1 cells.