Essential role for G protein-coupled receptor endocytosis in the activation of mitogen-activated protein kinase

The classical paradigm for G protein-coupled receptor (GPCR) signal transduction involves the agonist dependent interaction of GPCRs with heterotrimeric G proteins at the plasma membrane and the subsequent generation, by membrane-localized effecters, of soluble second messengers or ion currents, Termination of GPCR signals follows G protein-coupled receptor kinase (GRK)- and beta-arrestin-mediated receptor uncoupling and internalization. Here we show that these paradigms are inadequate to account for GPCR-mediated, Ras-dependent activation of the mitogen-activated protein (MAP) kinases Erk1 and -2, In HEK293 cells expressing dominant suppressor mutants of beta-arrestin or dynamin, beta(2)-adrenergic receptor-mediated activation of MAP kinase is inhibited, The inhibitors of receptor internalization specifically blocked Raf-mediated activation of MEK. Plasma membrane-delimited steps in the GPCR-mediated activation of the MAP kinase pathway, such as tyrosine phosphorylation of Shc and Raf kinase activation by Pas, are unaffected by inhibitors of receptor internalization. Thus, GRKs and beta-arrestins, which uncouple GPCRs and target them for internalization, function as essential elements in the GPCR-mediated MAP kinase signaling cascade.