Activated G_q-α potentiates platelet-derived growth factor-stimulated mitogenesis in confluent cell cultures

We studied the effects of activation of the G_q-α signaling pathway on mitogenesis by expressing a mutant (Q209L), activated -subunit of G_q (α_q*) in NIH-3T3 cells. A clonal NIH-3T3 cell line expressing α_q* in an inducible manner was isolated. Expression of α_q* is induced with dexamethasone, allowing the use of non-induced cells as controls for the effects of α_q* expression. We found that, by itself, expression of α_q* did not increase either DNA synthesis or mitogen-activated protein (MAP) kinase activity in serum-starved cells. Because α_q* transforms cells grown in the presence of serum (De Vivo M., Chen, J., Codina, J., and Iyengar, R. (1992) J. Biol. Chem. 267, 18263-18266), we tested whether growth factor-stimulated signaling and mitogenesis were affected by expression of α_q*. Platelet-derived growth factor (PDGF) stimulated thymidine incorporation modestly (50%) in contact-inhibited, confluent cell cultures. In cells expressing α_q*, PDGF stimulated DNA synthesis up to 3-fold over basal. Concomitant with the potentiation of PDGF-stimulated DNA synthesis, expression of α_q* potentiated PDGF-stimulated p44 MAP kinase activity. PDGF was much more effective in stimulating both DNA synthesis and p44 MAP kinase activity in subconfluent cell cultures and expression of α_q* exerted little or no effect on PDGF-stimulated effects in subconfluent cells. These data show that cooperation between signaling pathways may occur in a cell state-specific fashion. Such cooperation in part may be responsible for the triggering of complex cellular responses such as cell transformation.