ACTIVATED G(Q)-ALPHA POTENTIATES PLATELET-DERIVED GROWTH FACTOR-STIMULATED MITOGENESIS IN CONFLUENT CELL-CULTURES

We studied the effects of activation of the G(q)-alpha signaling pathw ay on mitogenesis by expressing a mutant (Q209L), activated alpha-subu nit of G(q) (alpha(q)) in NM-3T3 cells. A clonal NIH-3T3 cell line ex pressing alpha(q) in an inducible manner was isolated. Expression of alpha(q) is induced with dexamethasone, allowing the use of non-induc ed cells as controls for the effects of alpha(q) expression. We found that, by itself, expression of alpha(q) did not increase either DNA synthesis or mitogen-activated protein (MAP) kinase activity in serum- starved cells. Because alpha(q) transforms cells grown in the presenc e of serum (De Vivo M., Chen, J., Codina, J., and Iyengar, R. (1992) J . Biol. Chem. 267, 18263-18266), we tested whether growth factor-stimu lated signaling and mitogenesis were affected by expression of alpha(q ). Platelet-derived growth factor (PDGF) stimulated thymidine incorpo ration modestly (50%) in contact-inhibited, confluent cell cultures. I n cells expressing alpha(q), PDGF stimulated DNA synthesis up to 3-fo ld over basal. Concomitant with the potentiation of PDGF-stimulated DN A synthesis, expression of alpha(q) potentiated PDGF-stimulated p44 M AP kinase activity. PDGF was much more effective in stimulating both D NA synthesis and p44 MAP kinase activity in subconfluent cell cultures and expression of alpha(q) exerted little or no effect on PDGF-stimu lated 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 com plex cellular responses such as cell transformation.