G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) are two major signal transduction systems that allow extracellular signals to be integrated and transformed into various biological activities. Recent studies have shown that these two receptor systems utilize similar intracellular signaling pathways to exhibit their biological functions. One of these pathways is the mitogen-activated protein kinase (MAPK) cascade that includes the subgroup termed c-Jun N-terminal kinase (JNK). By expressing GPCRs which specifically coupled to different G protein families (G_s, G_i and G_q) in Cos-7 cells, the present study demonstrated that stimulation of these receptors induced JNK activation of different magnitudes depending on their G protein coupling specificities (G_qG_iG_s). Diffe...[ Read more ]

G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs) are two major signal transduction systems that allow extracellular signals to be integrated and transformed into various biological activities. Recent studies have shown that these two receptor systems utilize similar intracellular signaling pathways to exhibit their biological functions. One of these pathways is the mitogen-activated protein kinase (MAPK) cascade that includes the subgroup termed c-Jun N-terminal kinase (JNK). By expressing GPCRs which specifically coupled to different G protein families (G_s, G_i and G_q) in Cos-7 cells, the present study demonstrated that stimulation of these receptors induced JNK activation of different magnitudes depending on their G protein coupling specificities (G_q>G_i>G_s). Different G protein families may utilize both common as well as specific signaling intermediates and second messengers to trigger the subsequent JNK activation. Co-stimulation of GPCRs with different coupling specificities was associated with a "fine-tuning" character on the JNK activity, in which the Gβγ subunits and second messengers seem to play critical roles in this event. A moderate increase of JNK activity was linked to activation of epithermal growth factor receptor (EGFR) by EGF, and co-stimulation with GPCR agonists and EGF resulted in differential augmentation of JNK activities, with no further enhancement for G_q- and G_s-coupled receptors, while G_i-coupled receptors were associated with a synergistic JNK activation upon co-stimulation with EGF. This synergistic JNK activation may be caused by the signal integration between Gβγ-mediated Src activity, and the Ca²⁺/calmodulin (CaM)-regulated event, converging at the levels of guanine nucleotide exchange factors (GEFs) and small guanine nucleotide triphosphate hydrolases (GTPases) of the Ras and Rho-families. The MAPK phosphatase type I (MKP-1) had no contribution towards this synergistic response. My present study proposes a signaling network model which links up GPCRs with different coupling preferences and the EGFR, to the subsequent JNK regulatory cascade.