Epidermal growth factor differentially augments Gi-mediated stimulation of c-Jun N-terminal kinase activity

1 Signaling networks involving different receptor systems allow extracellular signals to be integrated and transformed into various biological activities. In this report, we studied the activity of the c-Jun N-terminal kinase (JNK) subgroup of mitogen-activated protein kinases (MAPKs), in response to stimulation by G protein-coupled receptors (GPCRs) and co-activation with epithermal growth factor receptor ( EGFR). 2 Stimulation of exogenous GPCRs in Cos-7 cells induced JNK activation of different magnitudes depending on their G-protein coupling specificities (G(q)>G(i)>G(s)), and a moderate JNK activation was linked to stimulation of endogenous EGFR by EGF. 3 Co-stimulation with GPCR agonists and EGF resulted in differential augmentation of JNK activities, with G(i)-coupled receptors associated with a synergistic JNK activation upon co-stimulation with EGF, while G(q)- and G(s)- coupled receptors were incapable of triggering this effect. 4 This G(i)/EGF-induced synergistic JNK activation was inhibited by pertussis toxin and AG1478, and may involve Src family tyrosine kinases, PI3 K, Ca2+/calmodulin and small GTPases as important intermediates, while Ca2+ mobilization was triggered by the stimulation of G(q)-coupled receptor or EGF treatment, but not by the G(i)- or G(s)-coupled receptors. 5 Transient expression of Gbetagamma subunits with EGF treatment, or co-activation of exogenous G(i)-coupled receptor with thapsigargin also resulted in a synergistic JNK activation. Activation of G(i)-coupled receptor accompanied with EGF treatment enhanced the expression level and activity of MAPK phosphatase type I, which occurred after the maximal synergistic JNK activation. 6 Our results support a mechanistic model where EGF signaling may differentially regulate the JNK activities triggered by GPCRs of different coupling specificities. British Journal of Pharmacology (2004).