Dependence of activated Galpha12-induced G1 to S phase cell cycle progression on both Ras/mitogen-activated protein kinase and Ras/Rac1/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 Galpha12-induced G1 to S phase cell cycle progression in NIH3T3(M17) fibroblasts. Transient expression of a constitutively active mutant of Galpha12, Galpha12(R203C), resulted in a 2-fold increase in the number of bromodeoxyuridine-positive S phase cells over vector control level under serum-deprived conditions. Consistent with the ability of Galpha12(R203C) to induce G1/S transition, its expression led to a 2-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 over the basal level. In addition, Galpha12(R203C) caused a 2-fold stimulation in E2F-mediated transactivation. Wild type Galpha12 showed similar stimulatory effects on cyclin A promoter activity and E2F-mediated transactivation, although of lesser magnitude. We observed a modest but constitutive activation of MAPK in cells transfected with Galpha12(R203C), which was abolished by a dominant negative form of Ras. Galpha12(R203C) also induced a 3-fold increase in JNK activity, which was abolished by dominant negative forms of either Rac1 or Ras. The expression of dominant negative forms of Ras, MAPK, Rac1, or JNK inhibited Galpha12(R203C)-induced increases in bromodeoxyuridine-positive cells. Also, the dominant negative forms of Ras, MAPK, and JNK strongly inhibited Galpha12(R203C)-induced stimulation of cyclin A promoter activity. These results demonstrate that both the Ras/MAPK and Ras/Rac1/JNK pathways convey necessary, if not sufficient, mitogenic signals induced by Galpha12 activation.