Mitogenic signaling by transfected neuromedin B receptors in Rat-1 cells.

The rat neuromedin B (NMB) receptor was expressed in Rat-1 fibroblasts to elucidate the signaling pathways and mitogenic effects mediated by this seven-transmembrane domain receptor. Receptor expression was verified by ligand binding and Ca2+ mobilization, which were blocked by the NMB receptor antagonist D-Nal-Cys-Tyr-D-Trp-Orn-Val-Cys-Nal-NH2. NMB acted as a potent growth factor promoting DNA synthesis and cell proliferation in serum-free medium in Rat-1 cells transfected with the NMB receptor. Prior to DNA synthesis, NMB stimulated phosphorylation of 80K/MARCKS, a major substrate of protein kinase C, which could be prevented by the selective protein kinase C inhibitor GF 109203X. Furthermore, NMB induced a rapid p42MAPK activation and tyrosine phosphorylation of multiple proteins including p125FAK and paxillin. The half-maximal concentrations (EC50) of NMB required to induce DNA synthesis (0.7-0.9 nM) and cell proliferation (0.7-1 nM) paralleled the Kd for 125I-[D-Tyr0]NMB binding and the EC50 values for the induction of the early signaling events. Thus, NMB can activate multiple signal transduction pathways and act as a sole mitogen through its receptor expressed in Rat-1 fibroblasts.