Comparative study of the molecular mechanisms underlying the G protein and beta-arrestin-dependent pathways that lead to ERKs activation upon stimulation by dopamine D-2 receptor

Dopamine D-2 receptor (D2R) has been shown to activate extracellular signal-regulated kinases (ERKs) via distinct pathways dependent on either G-protein or beta-arrestin. However, there has not been a systematic study of the regulatory process of D2R-mediated ERKs activation by G protein-versus beta-arrestin-dependent signaling since D2R stimulation of ERKs reflects the simultaneous action of both pathways. Here, we investigated that differential regulation of D2R-mediated ERKs activation via these two pathways. Our results showed that G protein-dependent ERKs activation was transient, rapid, reached maximum level at around 2 min, and importantly, the activated ERKs were entirely confined to the cytoplasm. In contrast, beta-arrestin-dependent ERKs activation was more sustained, slower, reached maximum level at around 10 min, and phosphorylated ERKs translocated into the nucleus. Src was found to be commonly involved in both the G protein- and beta-arrestin-dependent pathway-mediated ERKs activation. Pertussis toxin Gi/o inhibitor, GRK2-CT, AG1478 epidermal growth factor receptor inhibitor, and wortmannin phosphoinositide 3-kinase inhibitor all blocked G protein-dependent ERKs activation. In contrast, GRK2 and beta-Arr2 played a main role in beta-arrestin-dependent ERKs activation. Receptor endocytosis showed minimal effect on the activation of ERKs mediated by both pathways. Furthermore, we found that the formation of a complex composed of phospho-ERKs, beta-Arr2, and importin beta 1 promoted the nuclear translocation of activated ERKs. The differential regulation of various cellular components, as well as temporal and spatial patterns of ERKs activation via these two pathways, suggest the existence of distinct physiological outcomes.