D4 Dopamine Receptor Differentially Regulates Akt/Nuclear Factor-κB and Extracellular Signal-Regulated Kinase Pathways in D4MN9D Cells

The present study was designed to investigate the role of D 4 dopamine receptors in regulating the Akt/nuclear factor-κB (NF-κB) and extracellular signal-regulated kinase (ERK) signaling pathways. The D 4 dopamine receptor agonist PD168077 induced time- and dose-dependent activation of Akt and ERK in D 4 MN9D cells that stably express D 4 dopamine receptors. Maximal Akt and ERK stimulation was achieved at 1 μM PD168077. The agonist-mediated stimulations of Akt and ERK were abolished when cells were preincubated with 50 ng/ml PTX or with 1 μM L745,870, a D 4 dopamine receptor antagonist, indicating that activation of the Akt or ERK pathways is mediated by D 4 dopamine receptors and require a pertussis toxin-sensitive G protein. We also detected a time- and dose-dependent activation of NF-κB. Activation of NF-κB by 1 μM PD168077 was attenuated in D 4 MN9D cells that were transfected with a kinase-deficient Akt but not in cells transfected with a dominant negative Ras (N17Ras), suggesting that NF-κB activation requires Akt but is independent of Ras. In contrast, the transfection of N17Ras into D 4 MN9D cells blunted D 4 dopamine receptor-mediated ERK activation, indicating a Ras-dependent mechanism. Moreover, PP2 (20 nM), an inhibitor of Src, blocked D 4 receptor-mediated SHC phosphorylation and ERK activation. In contrast, transfection of a kinase-dead Akt did not alter D 4 receptor-stimulated ERK. However, PP2 and the mitogen activated protein kinase kinase inhibitor PD98059 did not change D 4 receptor-mediated Akt/NF-κB activation. All these indicate that distinct mechanisms mediate ERK and Akt/NF-κB activation by D 4 dopamine receptor stimulation. We also demonstrated that D 4 receptor-stimulated cell proliferation is mediated by the Src/SHC/Ras/ERK pathway.