Distinct Roles for Gα(i)2 and Gβγ in Signaling to DNA Synthesis and Gα(i)3 in Cellular Transformation by Dopamine D2S Receptor Activation in BALB/c 3T3 Cells

Control of cell proliferation depends on intracellular mediators that determine the cellular response to external cues. In neuroendocrine cells, the dopamine D2 receptor short form (D2S receptor) inhibits cell proliferation, whereas in mesenchymal cells the same receptor enhances cell proliferation. Nontransformed BALB/c 3T3 fibroblast cells were stably transfected with the D2S receptor cDNA to study the G proteins that direct D2S signaling to stimulate cell proliferation. Pertussis toxin inactivates G(i) and G(o) proteins and blocks signaling of the D2S receptor in these cells. D2S receptor signaling was reconstituted by individually transfecting pertussis toxin-resistant Gα(i/o) subunit mutants and measuring D2-induced responses in pertussis toxin-treated cells. This approach identified Gα(i)2 and Gα(i)3 as mediators of the D2S receptor-mediated inhibition of forskolin-stimulated adenylyl cyclase activity; Gα(i)2-mediated D2S-induced stimulation of p42 and p44 mitogen-activated kinase (MAPK) and DNA synthesis, whereas Gα(i)3 was required for formation of transformed foci. Transfection of toxin-resistant Gα(i)1 cDNA induced abnormal cell growth independent of D2S receptor activation, while Gα(o) inhibited dopamine-induced transformation. The role of Gβγ subunits was assessed by ectopic expression of the carboxyl-terminal domain of G protein receptor kinase to selectively antagonize Gβγ activity. Mobilization of Gβγ subunits was required for D2S-induced calcium mobilization, MAPK activation, and DNA synthesis. These findings reveal a remarkable and distinct G protein specificity for D2S receptor-mediated signaling to initiate DNA synthesis (Gα(i)2 and Gβγ) and oncogenic transformation (Gα(i)3), and they indicate that acute activation of MAPK correlates with enhanced DNA synthesis but not with transformation.