Platelet-derived growth factor activates a mammalian Ste20 coupled mitogen-activated protein kinase in airway smooth muscle

We have investigated the mechanisms regulating p38MAPK in airway smooth muscle cells. Incubation of cells with platelet-derived growth factor (PDGF) stimulated MAPKA kinase-2 activity, a kinase immediately down-stream of P38MAPK. Preincubation of the cells with forskolin (10 microM), which stimulated a 20-fold increase in intracellular cAMP formation, inhibited this response. An antibody raised against subdomain VI of yeast Ste20 kinase co-immunoprecipitated p38MAPK from cell lysates. The immunoprecipitated kinase(s) was shown to catalyse the phosphorylation of myelin basic protein (MBP) and to activate purified MAPKAP kinase-2. Incubation of cells with PDGF did not increase the amount of p38MAPK isolated in the anti-Ste20 immunoprecipitate. However, the kinase phosphorylated MBP and stimulated purified MAPKAP kinase-2 activity more effectively than kinase from control cells. The preincubation of cells with forskolin (10 microM) reduced the amount of P38MAPK in the immunoprecipitate and this correlated with a decrease in kinase activity. We conclude the PDGF induces the activation of p38MAPK, whereas forskolin elicits its dissociation from the complex with Ste20. Therefore, Ste20/p38MAPK may form part of a signal transduction pathway linked to activation of MAPKAP kinase-2 in ASM cells.