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

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, Ca(2+)/calmodulin and small GTPases as important intermediates, while Ca(2+) 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.