Abstract

Using cultured airway smooth muscle cells, we showed previously that the platelet-derived growth factor (PDGF) receptor uses the G-protein, G_i, to stimulate Grb-2-associated phosphoinositide 3-kinase (PI3K) activity. We also showed that this was an intermediate step in the activation of p42/p44 mitogen-activated protein kinase (p42/p44 MAPK) by PDGF. We now present two lines of evidence that provide further support for this model. First, we report that PDGF stimulates the G_i-mediated tyrosine phosphorylation of the Grb-2 adaptor protein, Gab1. This phosphorylation appears to be necessary for association of PI3K1a with the Gab1-Grb-2 complex. Second, PI3K appears to promote the subsequent association of dynamin II (which is involved in clathrin-mediated endocytic processing) with the complex. Furthermore, inhibitors of PI3K and clathrin-mediated endocytosis reduced the PDGF-dependent activation of p42/p44 MAPK, suggesting a role for PI3K in the endocytic signaling process leading to stimulation of p42/p44 MAPK. Together, these results begin to define a common signaling model for certain growth factor receptors (e.g., PDGF, insulin, insulin-like growth factor-1, and fibroblast growth factor) which use G_i to transmit signals to p42/p44 MAPK.