Evidence for the involvement of two pathways in activation of extracellular signal-regulated kinase (Erk) and cell proliferation by Gi and Gq protein-coupled receptors in osteoblast-like cells

The mechanisms by which Gi and Gq protein-coupled receptors mediate mitogenic signaling in osteoblast-like cells are unknown and were investigated in MC3T3-E1 cells using specific receptor agonists such as lysophosphatidic acid (LPA) and prostaglandin F(2α) (PGF(2α)). In contrast to their implication in epidermal growth factor (EGF) receptor tyrosine kinase signaling, the adaptor protein Shc, the Grb2/Sos complex, and the small G protein Ras were not involved in the activation of Erk induced by either LPA or PGF(2α) in MC3T3-E1 cells, suggesting that activation of Erk by Gi and Gq protein-coupled receptors is Ras independent in these cells. Using specific kinase inhibitors and kinetic analyses, we provide evidence for two distinct components in the activation of Erk by Gi and Gq protein-coupled receptors in MC3T3-E1 cells including an Src-like kinase-dependent pathway and a protein kinase C (PKC)-dependent mechanism. Functional analyses suggested that these two components are required for optimal DNA synthesis in response to LPA and PGF(2α). These results suggest the implication of two pathways in the stimulation of Erk and cell replication by growth factors acting through Gi and Gq protein-coupled receptors in bone-forming cells.