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) h as been implicated in mitogenesis and transformation, but its direct e ffecters remain unknown, To define potential signaling pathways utiliz ed 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 act ivated 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 R ac (N17Rac1) inhibited JNK activation by G alpha(12), in HEK293 cells as well as three other cell lines. In 1321N1 cells, where thrombin sti mulates G(12)-dependent mitogenesis, coexpression of N17Rac1 or a domi nant negative mutant of MEKK1 (MEKK Delta(K432M)) inhibits c-Jun/AP-1 sensitive reporter gene expression stimulated by thrombin or G alpha(1 2). These data demonstrate that the alpha subunit of the heterotrimeri c G protein G(12), like tyrosine kinase growth factor receptors, activ ates has and recruits a signal transduction pathway involving the smal l GTP-binding protein Rac that leads to JNK activation.