Bombesin and neuromedin B stimulate the activation of p42(mapk) and p74(raf-1) via a protein kinase C-independent pathway in Rat-1 cells.

The mechanisms by which seven transmembrane receptors activate p42-(mapk)/p44(mapk) are not well defined although p21ras- and protein kinase C (PKC)-dependent pathways have been implicated, typically for Gi- and Gq-coupled receptors, respectively. Here, we demonstrate that in Rat-1 cells transfected with the Gq-coupled bombesin/gastrin releasing peptide receptor, bombesin stimulated activation of p42(mapk) that was not inhibited by the specific PKC inhibitor GF 109203X or by down regulation of phorbol ester-sensitive PKC isoforms. In addition, bombesin rapidly stimulated p74(raf-1) activity that was also independent of PKC activity and insensitive to inhibition by pertussis toxin. Furthermore, addition of neuromedin B to Rat-1 cells transfected with the neuromedin B preferring receptor also activated p42(mapk) and p74(raf-1) in a PKC-independent and pertussis toxin-insensitive manner. Finally we show that addition of bombesin to Rat-1 cells stimulated the GTP loading of p21ras. Our results reveal a novel PKC-independent pathway in the action of Gq-coupled receptors and stress the importance of cell context in defining the signal transduction pathway(s) that link specific receptors to the activation of the mitogen-activated protein kinase cascade.