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

We have examined the role of Gi alpha in haemopoietic cells using the myelomonocytic progenitor cell lines FDC-P1 and WEHI-3B (JCS). During growth factor-dependent proliferation of FDC-P1 cells Gi alpha was found predominantly in the nucleus and associated with the plasma membrane. Following removal of growth factor, Gi alpha accumulated in the cytoplasm and at the plasma membrane. Treatment of FDC-P1 cells with pertussis toxin (PT) completely inhibited translocation of Gi alpha to the nucleus and reduced the sensitivity of FDC-P1 cells to the proliferative effects of growth factors, indicating that translocation of Gi alpha plays a regulatory role in, but may not be essential for, cell division. Gi alpha 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-alpha. In proliferating JCS cells Gi alpha was again associated with the nucleus but when proliferation was inhibited by TNF-alpha, Gi alpha accumulated in the cytoplasm with none detected in the nucleus. Thus the cytokine regulated accumulation of Gi alpha 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 Gi alpha 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 Gi alpha to the nucleus. The near complete inhibition of cell division by genistein and CT without a corresponding effect on Gi alpha movement indicates that Gi alpha can be regulated independently of tyrosine kinase and adenylyl cyclase activities, respectively.