Hematopoietic cell development is regulated by a series of growth factors that are progressively restricted in their biological activity. IL-3 is a multi-lineage growth factor that supports the growth and differentiation of progenitor cells belonging to multiple lineages. However, the mechanism by which IL-3 induces proliferation and differentiation of these cells is not completely understood. In this report, we have used two IL-3-dependent cell lines, FDC-P1 (a myeloid progenitor) and F15.12 (a lymphoid progenitor) to investigate IL-3-mediated growth and differentiation. When either FDC-P1 or FL5.12 cells are deprived of IL-3, greater than 90% of all cells accumulate in the G0 phase of the cell cycle. Upon readdition of IL-3, the cells will reenter the active phases of the cell cycle. Therefore, IL-3 can act as both a competence (G0----G1) factor, and a progression (G1----M) factor for hematopoietic precursor clones. FDC-P1 cells can also proliferate in response to granulocyte/macrophage colony-stimulating factor (G/M-CSF) and IL-4 (B cell stimulatory factor 1 [BSF-1]). However, resting (G0) FDC-P1 cells have lost their ability to grow in response to both G/M-CSF and IL-4, even though both factors can induce a G0----G1 transition. Therefore, G/M-CSF or IL-4 behave as progression factors among certain IL-3-responsive clones, and in those cases only in defined points in the cell cycle. Both IL-4 and G/M-CSF can maintain long-term growth of FDC-P1 cells. Upon removal of factor for 24 h, these clones accumulate in the G1 phase of the cell cycle and do not appear to enter G0 even after 36 h of factor deprivation. Therefore, cells maintained in G/M-CSF or IL-4 have altered growth requirements compared with the IL-3-dependent lines from which they were derived. The ability of various hematopoietic growth factors to regulate cell cycle progression in IL-3-dependent cell lines is dependent not only upon the lineage from which these cells were derived, but also the phase of the cell cycle in which those cells reside. The consequences of these interactions dictate the manner by which various clones will respond to CSFs and whether the cells will grow and/or differentiate.

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