Overexpression of a dominant-negative signal transducer and activator of transcription 3 variant in tumor cells leads to production of soluble factors that induce apoptosis and cell cycle arrest

Gene therapy of B16 tumors with a dominant-negative signal transducer and a ctivator of transcription (Stat3) variant, designated Sta3 beta, results in inhibition of tumor growth and tumor regression. Although only 10-15% of t he tumor cells are transfected in vivo, the Stat3 beta -induced antitumor e ffect is associated with massive apoptosis of B16 tumor cells, indicative o f a potent bystander effect. Here, me provide evidence that blocking Stat3 signaling in B16 cells results in release of soluble factors that are capab le of inducing apoptosis and cell cycle arrest of nontransfected B16 cells, RNase protection assays using multi-template probes specific for hey physi ological regulators of apoptosis reveal that overexpression of Stat3 beta i n B16 tumor cells induces the expression of the apoptotic effector, tumor n ecrosis factor-related apoptosis-inducing ligand, These in vitro results su ggest that the observed in vivo bystander effect leading to tumor cell grow th inhibition is mediated, at least in part, by soluble factors produced as a result of overexpression of Stat3 beta in tumor cells.