G(o)-protein alpha-subunits activate mitogen-activated protein kinase via a novel protein kinase C-dependent mechanism

Mitogen-activated protein kinase (MAPK) is activated in response to both receptor tyrosine kinases and G-protein-coupled receptors, Recently, G(i)-coupled receptors, such as the alpha(2A) adrenergic receptor, were shown to mediate Ras-dependent MAPK activation via a pathway requiring G-protein beta gamma subunits (G(beta gamma)) and many of the same intermediates involved in receptor tyrosine kinase signaling. In contrast, G(q)-coupled receptors, such as the M(1) muscarinic acetylcholine receptor (M(1)AChR), activate MAPK via a pathway that is Ras-independent but requires the activity of protein kinase C(PKC). Here we show that, in Chinese hamster ovary cells, the M(1)AChR and platelet-activating factor receptor (PAFR) mediate MAPK activation via the alpha-subunit of the G(o) protein. G(o)-mediated MAPK activation was sensitive to treatment with pertussis toxin but insensitive to inhibition by a G beta gamma-sequestering peptide (beta ARK1ct), M(1)AChR and PAFR catalyzed G(o) alpha-subunit GTP exchange, and MAPK activation could be partially rescued by a pertussis toxin-insensitive mutant of G(o alpha) but not by similar mutants of G(i). G(o)-mediated MAPK activation was insensitive to inhibition by a dominant negative mutant of Ras(N17Ras) but was completely blocked by cellular depletion of PKC. Thus, M(1)AChR and PAFR, which have previously been shown to couple to G(q), are also coupled to G(o) to activate a novel PKC-dependent mitogenic signaling pathway.