Pleiotropic coupling of G protein-coupled receptors to the mitogen-activated protein kinase cascade - Role of focal adhesions and receptor tyrosine kinases

G protein-coupled receptors (GPCRs)initiate Ras-dependent activation of the Erk 1/2 mitogen-activated protein kinase cascade by stimulating recruitmen t of Ras guanine nucleotide exchange factors to the plasma membrane. Both i ntegrin-based focal adhesion complexes and receptor tyrosine kinases have b een proposed as scaffolds upon which the GPCR-induced Ras activation comple x may assemble. Using specific inhibitors of focal adhesion complex assembl y and receptor tyrosine kinase activation, we have determined the relative contribution of each to activation of the Erk 1/2 cascade following stimula tion of endogenous GPCRs in three different cell types. The tetrapeptide RG DS, which inhibits integrin dimerization, and cytochalasin D, which depolym erizes the actin cytoskeleton, disrupt the assembly of focal adhesions. In PC12 rat pheochromocytoma cells, both agents block lysophosphatidic acid (L PA)- and bradykinin-stimulated Erk 1/2 phosphorylation, suggesting that int act focal adhesion complexes are required for GPCR-induced mitogen-activate d protein kinase activation in these cells, In Rat 1 fibroblasts, Erk 1/2 a ctivation via LPA and thrombin receptors is completely insensitive to both agents. Conversely, the epidermal growth factor receptor-specific tyrphosti n AG1478 inhibits GPCR-mediated Erk 1/2 activation in Rat 1 cells but has n o effect in PC12 cells. In HEH-293 human embryonic kidney cells, LPA and th rombin receptor-mediated Erk 1/2 activation is partially sensitive to both the RGDS peptide and tyrphostin AG1478, suggesting that both focal adhesion and receptor tyrosine kinase scaffolds are employed in these cells. The de pendence of GPCR-mediated Erk 1/2 activation on intact focal adhesions corr elates with expression of the calcium-regulated focal adhesion kinase, Pyk2 , In all three cell types, GPCR-stimulated Erk 1/2 activation is significan tly inhibited by the Src kinase inhibitors, herbimycin A and 4-amino-5-(4-m ethylphenyl)-7-(t-butyl)pyra-zolo-D-3,4-pyrimidine (PP1), suggesting that S rc family nonreceptor tyrosine kinases represent a point of convergence for signals originating from either scaffold.