G alpha(12) stimulates c-Jun NH2-terminal kinase through the small G proteins Ras and Rac

The pertussis toxin (PTX) insensitive heterotrimeric G protein G(12) has been implicated in mitogenesis and transformation, but its direct effecters remain unknown, To define potential signaling pathways utilized by G(12), we expressed an activated mutant of its alpha subunit, G alpha(12)(Q229L), in HEK293 cells and examined its effects on Ras and mitogen-activated protein kinases (MAPKs), Transient expression of activated G alpha(12) increased the percentage of has in the active, GTP-bound state, stimulated c-Jun NH2-terminal kinase (JNK) activity, and enhanced the transcriptional activity of c-Jun, Dominant negative Ras (N17Ras) inhibited G alpha(12)-mediated JNK activation in NIH3T3 cells but failed to do so in HEK293 cells, In contrast, dominant negative Rac (N17Rac1) inhibited JNK activation by G alpha(12), in HEK293 cells as well as three other cell lines. In 1321N1 cells, where thrombin stimulates G(12)-dependent mitogenesis, coexpression of N17Rac1 or a dominant negative mutant of MEKK1 (MEKK Delta(K432M)) inhibits c-Jun/AP-1 sensitive reporter gene expression stimulated by thrombin or G alpha(12). These data demonstrate that the alpha subunit of the heterotrimeric G protein G(12), like tyrosine kinase growth factor receptors, activates has and recruits a signal transduction pathway involving the small GTP-binding protein Rac that leads to JNK activation.