Colony-stimulating factor (CSF) controls proliferation of CSF-dependent cells by acting during the G1 phase of the cell cycle.

Proliferation of granulocyte/macrophage (GM) progenitor cells in soft agar cultures is dependent on the continuous presence of colony-stimulating factors (CSF). To elucidate this dependency we studied the effect of deprivation and readdition of CSF on the cell cycle kinetics of a GM-CSF-dependent murine mast/basophil cell line (PT-18). Flow cytometry and [3H]thymidine incorporation have been used to analyze the complete cell cycle. Removal of CSF from the culture medium resulted in accumulation of the cells in the G1 phase. Eighteen hours after removal of CSF, 85% of the cells were arrested in G1 phase. Readdition of GM-CSF to such quiescent cells was followed by progression of the cells from G1 into S phase with a lag period of 10 hr. A similar lag period was observed when cells released from G2+M arrest progressed, in the presence of CSF, to S phase via the G1 phase. These findings indicate that deprivation of GM-CSF does not move PT-18 cells out of the cycle to a G0 phase but rather arrests them at early G1 phase. Finally, we demonstrate that, for the cells to progress through the cell cycle, the requirement for the presence of GM-CSF is limited to the first 6 hr of the 10-hr duration of the G1 phase.