Stat3-mediated transformation of NIH-3T3 cells by the constitutively active Q205L Galphao protein.

Expression of Q205L Galphao (Galphao*), an alpha subunit of heterotrimeric guanine nucleotide-binding proteins (G proteins) that lacks guanosine triphosphatase (GTPase) activity in NIH-3T3 cells, results in transformation. Expression of Galphao* in NIH-3T3 cells activated signal transducer and activator of transcription 3 (Stat3) but not mitogen-activated protein (MAP) kinases 1 or 2. Coexpression of dominant negative Stat3 inhibited Galphao*-induced transformation of NIH-3T3 cells and activation of endogenous Stat3. Furthermore, Galphao* expression increased activity of the tyrosine kinase c-Src, and the Galphao*-induced activation of Stat3 was blocked by expression of Csk (carboxyl-terminal Src kinase), which inactivates c-Src. The results indicate that Stat3 can function as a downstream effector for Galphao* and mediate its biological effects.