DEPENDENCE OF ACTIVATED G-ALPHA-12-INDUCED G(1) TO S-PHASE CELL-CYCLEPROGRESSION ON BOTH RAS MITOGEN-ACTIVATED PROTEIN-KINASE AND RAS/RAS1/JUN N-TERMINAL KINASE CASCADES IN NIH3T3 FIBROBLASTS/

We evaluated the roles of mitogen-activated protein kinase (MAPK) and Jun N-terminal kinase (JNK) signaling cascades in G alpha 12-induced G (1) to S phase cell cycle progression in NIH3T3(M17) fibroblasts. Tran sient expression of a constitutively active mutant of G alpha 12, G al pha 12(R203C), resulted in a a-fold increase in the number of bromodeo xyuridine-positive S phase cells over vector control level under serum -deprived conditions. Consistent with the ability of G alpha 12(R203C) to induce G(1)/S transition, its expression led to a a-fold increase in cyclin A promoter activity, which showed a marked synergism with a low concentration of serum, resulting in up to a 15-fold elevation ove r the basal level. In addition, G alpha 12(R203C) caused a a-fold stim ulation in E2F-mediated transactivation. Wild type G alpha 12 showed s imilar stimulatory effects on cyclin A promoter activity and E2F-media ted transactivation, although of lesser magnitude. We observed a modes t but constitutive activation of MAPK in cells transfected with G alph a 12(R203C), which was abolished by a dominant negative form of Ras, G alpha 12(R203C) also induced a 3-fold increase in JNK activity, which was abolished by dominant negative forms of either Rad or Ras. The ex pression of dominant negative forms of Ras, MAPK, Rac1, or JNK inhibit ed G alpha 12(R203C)-induced increases in bromodeoxyuridine-positive c ells. Also, the dominant negative forms of Ras, MAPK, and JNK strongly inhibited G alpha 12(R203C)-induced stimulation of cyclin A promoter activity. These results demonstrate that both the Ras/MAPK and Ras/Rac 1/JNK pathways convey necessary, if not sufficient, mitogenic signals induced by G alpha 12 activation.