Changes in signal transduction downstream from the granulocyte-macrophage colony-stimulating factor receptor during differentiation of primary hemopoietic cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a multifuctional cytokine, having different effects on primitive hemopoietic cells and terminally differentiated end-cells of the myeloid lineage. Human primitive hemopoietic cells (CD34+) were obtained from the peripheral blood after mobilization and induced to proliferate and then differentiate with a combination of cytokines in vitro. Cells at different time points were then used to analyze the expression of the GM-CSF receptor and GM-CSF mediated activation of the JAK 2-STAT 5 and MAP kinase pathways. Scatchard analysis as measured by radioligand binding revealed that freshly purified CD34+ cells expressed 36 ± 1 high affinity receptors per cell (mean ± SE, n = 3) and the level of expression was not significantly different after 3 days in culture, but rose five- to tenfold by day 8. The day 0 CD34+ cells were hyporesponsive to GM-CSF, but by 3 days in culture the cells were still morphologically immature but were actively proliferating and exhibited maximal GM-CSF induced JAK 2-STAT 5 and MAP kinase activation at the optimal time point. Further culture of the CD34+ cells resulted in myeloid differentiation associated with prolongation of MAP kinase activation but not JAK 2-STAT 5 activation. These data indicate that the JAK 2-STAT 5 and MAP kinase pathways are independently regulated and that changes in these signaling pathways occur with differentiation.