Comparative studies of molecular mechanisms of dopamine D2 and D3 receptors for the activation of extracellular signal-regulated kinase

Dopamine D-2 and D-3 receptors (D2R/D3R), which have similar structural architecture as well as functional similarities, are expressed in the same brain dopaminergic neurons. It is intriguing that two receptor proteins with virtually the same functional roles are expressed in the same neuron. Recently we have shown that D2R and D3R possess different regulatory processes including intracellular trafficking properties, which implies that they might employ different signaling mechanisms for regulation of the same cellular processes. Here we studied the signaling pathways of ERK activation mediated by D2R and D3R in HEK-293 cells and corroborated them with concomitant studies in COS-7 cells and C6 cells. Our results show that Src, phosphatidylinositol 3-kinase, and atypical protein kinase C were commonly involved in D2R-/D3R-mediated ERK activation. However, beta-arrestin and sequestration of D2R/D3R were found not to be involved. ERK activations mediated by D3R, but not D2R, were blocked by betaARK-CT, AG1478 epidermal growth factor receptor ( EGFR) inhibitor, and by dominant negative mutants of Ras and Raf, suggesting the involvement of the Gbeta(gammai) pathway. The alpha-subunit of G(o) (Galpha(o)) was able to couple with D3R to mediate ERK activation. We conclude that D3R mainly utilizes the betagamma pathway of G(i) protein, which involves the transactivation of EGFR in HEK-293 cells. In contrast, the alpha-subunit of the G(i) protein plays a main role in D2R-mediated ERK activation. Our study suggests one example of intricate cellular regulations in the brain, that is, dopaminergic neurons could regulate ERK activity more flexibly through alternative usage of either the D2R or D3R pathway depending on the cellular situation.