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G_q-Mediated Activation of c-Jun N-Terminal Kinase by the Gastrin-Releasing Peptide-Preferring Bombesin Receptor Is Inhibited upon Costimulation of the G_s-Coupled Dopamine D₁ Receptor in COS-7 Cells

Department of Biochemistry, the Biotechnology Research Institute, and the Molecular Neuroscience Center, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

G protein-coupled receptors (GPCRs) of G_i- or G_q-coupling specificity are effectively linked to activation of the c-Jun N-terminal kinase (JNK) cascade. However, little is known with regard to the regulation of JNK by G_s-coupled receptors. In this report, we used COS-7 cells transfected with the dopamine D₁ receptor (D₁R) to illustrate the signaling mechanism for G_s-mediated JNK activation. Stimulation of D₁R triggered a weak but significant elevation of JNK activity in a time- and dose-dependent manner. This D₁R-mediated JNK activation required the participation of G, Src-like kinases, and small GTPases, whereas disruptions of cAMP-, phosphoinositide-3-kinase-, and epidermal growth factor receptor-mediated signaling had no effect. Costimulation of D₁R with GPCRs of other coupling specificities resulted in differential activation profiles of JNK. Activation of G_s-coupled D₁R weakly potentiated the JNK activation induced by the G_i-coupled opioid receptor-like receptor, but it exhibited a significant inhibitory effect on the kinase activity triggered by the G_q-coupled gastrin-releasing peptide-preferring bombesin receptor (GRPR). Administration of Spadenosine-3',5'-cyclic monophosphorothioate triethylamine (a cAMP analog that mimics the G_s/cAMP signal) also suppressed the JNK activation mediated by G_q-coupled GRPR, as well as the Ca²⁺-induced kinase activation upon thapsigargin treatment. Moreover, the Ca²⁺ signal from GRPR synergistically potentiated the D₁R-triggered cAMP elevation when the two receptors were stimulated simultaneously. Taken together, our results demonstrated that stimulation of G_s-coupled receptors in COS-7 cells not only enhanced the JNK activity, but also exhibited a "tuning" effect on the kinase activation mediated by GPCRs of other coupling specificities.

Address correspondence to: Dr. Yung H Wong, Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. E-mail: boyung{at}ust.hk

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