G protein-coupled receptor-mediated mitogen-activated protein kinase activation through cooperation of Gαq, and Gαi signals

G protein-coupled receptors (GPCRs) have been shown to stimulate extracellular regulated kinases (ERKs) through a number of linear pathways that are initiated by G(q/11) or G(i) proteins. We studied signaling to the ERK cascade by receptors that simultaneously activate both G protein subfamilies. In HEK293T cells, bradykinin B-2 receptor (B2R)-induced stimulation of ERK2 and transcriptional activity of Elk1 are dependent on G alpha(q)-mediated protein kinase C (PKC) and on Ga-i-induced Ras activation, while they are independent of G beta gamma subunits, phosphatidylinositol 3-kinase, and tyrosine kinases. Similar results were obtained with m(1) and m(3) muscarinic receptors in HEK293T cells and with the B2R in human and mouse fibroblasts, indicating a general mechanism in signaling toward the ERK cascade. Furthermore, the bradykinin-induced activation of ERK is strongly reduced in G alpha(q/11)-deficient fibroblasts. In addition, we found that constitutively active mutants of G alpha(q/11) or G alpha(i) proteins alone poorly stimulate ERK, whereas a combination of both led to synergistic effects. We conclude that dually coupled GPCRs require a cooperation of G alpha(i)- and G(q/11)-mediated pathways for efficient stimulation of the ERK cascade. Cooperative signaling by multiple G proteins thus might represent a novel concept implicated in the regulation of cellular responses by GPCRs.