G Protein-Coupled Receptor-Mediated Mitogen-Activated Protein Kinase Activation through Cooperation of Gαq and Gαi Signals

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Abstract
G protein-coupled receptors (GPCRs) have been shown to stimulate extracellular regulated kinases (ERKs) through a number of linear pathways that are initiated by Gq/11 or Giproteins. We studied signaling to the ERK cascade by receptors that simultaneously activate both G protein subfamilies. In HEK293T cells, bradykinin B2 receptor (B2R)-induced stimulation of ERK2 and transcriptional activity of Elk1 are dependent on Gαq-mediated protein kinase C (PKC) and on Gαi-induced Ras activation, while they are independent of Gβγ subunits, phosphatidylinositol 3-kinase, and tyrosine kinases. Similar results were obtained with m1 and m3muscarinic 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 Gαq/11-deficient fibroblasts. In addition, we found that constitutively active mutants of Gαq/11 or Gαi proteins alone poorly stimulate ERK2, whereas a combination of both led to synergistic effects. We conclude that dually coupled GPCRs require a cooperation of Gαi- and Gq/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.