Interleukin 3 and cell cycle progression

Interleukin 3 (IL‐3) is a regulatory glycoprotein required for the proliferation and differentiation of cells from many if not all hemopoietic lineages. With the emergence of the competence‐progression model of cell proliferation, which predicts that growth factors function at specific stages of the cell cycle, we examined the possibility that IL‐3 functions at a specific stage of the cell cycle. C‐63 cells were developed as a cell line from normal murine bone marrow. They have a mast cell phenotype and require pokeweed‐stimulated spleen cell‐conditioned medium (CM), a rich source of IL‐3, for their continued growth. Exponentially growing cells were transferred from growth medium, which contains CM, to medium lacking CM or IL‐3. After 24 hours, cell viability had decreased 40–50%. The remaining viable cells did not incorporate ³H‐thymidine, and displayed a single peak at G1 in a DNA histogram. Restimulation of these cells with CM or IL‐3 resulted in a dramatic rise in ³H‐thymidine uptake 20–24 hours after restimulation. DNA histograms of restimulated cultures indicated that the cells were progressing in a wave‐like fashion throughout the remainder of the cell cycle. The length of time necessary for cells to be in contact with CM or IL‐3 before they could progress into the remainder of the cell cycle was also examined. Cells incubated with CM or IL‐3 for less than 16 hours could not progress into S phase, whereas cells incubated for 16 hours or longer could progress into S phase and through the remainder of the cell cycle. These data suggest that IL‐3 exerts its function at a specific stage of the cell cycle.