Localization of the GTP-binding protein g_iα in myelomonocytic progenitor cells is regulated by proliferation (GM-CSF, IL-3) and differentiation (TNF) signals

We have examined the role of G_iα in haemopoietic cells using the myelomonocytic progenitor cell lines FDC-P1 and WEHI-3B (JCS). During growth factor-dependent proliferation of FDC-PI cells G_Iα was found predominantly in the nucleus and associated with the plasma membrane. Following removal of growth factor, G_iα accumulated in the cytoplasm and at the plasma membrane. Treatment of FDC-PI cells with pertussis toxin (PT) completely inhibited translocation of G_iα to the nucleus and reduced the sensitivity of FDC-P1 cells to the proliferative effects of growth factors, indicating that translocalion of G_iα plays a regulatory role in, but may not be essential for, cell division. G_iα initially associated with DNA during S/G2 of the FDC-P1 cell cycle but separated from condensing chromosomes during mitosis. In contrast to FDC-P1 cells, WEHI-3B (JCS) cells proliferate in the absence of added growth factors but can be induced to differentiate by TNF-α. In proliferating JCS cells G_iα was again associated with the nucleus but when proliferation was inhibited by TNF-α G_iα accumulated in the cytoplasm with none detected in the nucleus. Thus the cytokine regulated accumulation of G_iα at different intracellular sites correlated with the ability of the cell to progress through the proliferative cycle. When the tyrosine kinase inhibitor genistein was added to FDC-P1 cells prior to stimulation with IL-3 or GM-CSF, proliferation was almost completely inhibited but translocation of G_iα was not affected, suggesting that tyrosine phosphorylation was not involved in G protein translocation but was essential for cytokine induced cell division. Cholera toxin (CT) also inhibited proliferation of FDC-P1 cells but had no effect on translocation of G_iα to the nucleus. The near complete inhibition of cell division by genistein and CT without a corresponding effect on G_iα movement indicates that G_iα can be regulated independently of tyrosine kinase and adenylyl cyclase activities, respectively.