G(O)-PROTEIN ALPHA-SUBUNITS ACTIVATE MITOGEN-ACTIVATED PROTEIN-KINASEVIA A NOVEL PROTEIN-KINASE C-DEPENDENT MECHANISM

Mitogen-activated protein kinase (MAPK) is activated in response to bo th receptor tyrosine kinases and G-protein-coupled receptors, Recently , G(i)-coupled receptors, such as the alpha(2A) adrenergic receptor, w ere shown to mediate Ras-dependent MAPK activation via a pathway requi ring G-protein beta gamma subunits (G(beta gamma)) and many of the sam e intermediates involved in receptor tyrosine kinase signaling. In con trast, G(q)-coupled receptors, such as the M(1) muscarinic acetylcholi ne receptor (M(1)AChR), activate MAPK via a pathway that is Ras-indepe ndent but requires the activity of protein kinase C(PKC). Here we show that, in Chinese hamster ovary cells, the M(1)AChR and platelet-activ ating factor receptor (PAFR) mediate MAPK activation via the alpha-sub unit 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 cata lyzed G(o) alpha-subunit GTP exchange, and MAPK activation could be pa rtially 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)AC hR and PAFR, which have previously been shown to couple to G(q), are a lso coupled to G(o) to activate a novel PKC-dependent mitogenic signal ing pathway.