Gi-mediated activation of mitogen-activated protein kinase (MAPK) pathway by receptor mimetic basic secretagogues of connective tissue-type mast cells: bifurcation of arachidonic acid-induced release upstream of MAPK.

The family of basic secretagogues of connective tissue mast cells act as receptor mimetic agents, which trigger exocytosis by directly activating G proteins. We now demonstrate that pertussis toxin (Ptx)-sensitive Gi proteins, activated by compound 48/80 (c48/80), a potent member of this family, also activate the p42/p44 MAP kinases (MAPKs). This activation was potentiated by the protein tyrosine phosphatase inhibitor vanadate, whereas the tyrphostin AG-18, a competitive inhibitor of protein tyrosine kinases (PTKs); the protein kinase C inhibitors K252a and GF109203X; the phosphatidylinositol-3-kinase (PI-3K) inhibitors wortmannin and LY294002; and EGTA have abolished this activation. These results suggest that c48/80 activated the p42/p44 MAPKs via a mechanism that involves PTKs, protein kinase C, phosphatidylinositol-3-kinase and Ca2+ as mediators. Protein tyrosine phosphorylation and activation of the p42/p44 MAPKs were closely correlated with stimulation of arachidonic acid (AA) release by c48/80 but not with histamine secretion. However, whereas PD98059, the inhibitor of the MAPK kinase has abrogated MAPK activation, this inhibitor failed to effect release of AA. We therefore conclude that by activating Ptx-sensitive Gi protein(s), the basic secretagogues of mast cells stimulate multiple signaling pathways, which diverge to regulate the production and release of the different inflammatory mediators. Whereas the signaling pathway responsible for triggering histamine release is PTK independent, the pathway responsible for the stimulation of AA release bifurcates downstream to PTKs but upstream to the activation of MAPKs.