ACTIVATION OF 70-KDA S6 KINASE, INDUCED BY THE CYTOKINES INTERLEUKIN-3 AND ERYTHROPOIETIN AND INHIBITED BY RAPAMYCIN, IS NOT AN ABSOLUTE REQUIREMENT FOR CELL-PROLIFERATION

The cytokines interleukin (IL)-3 and erythropoietin (EPO) are critical regulators of the proliferation and differentiation of cells of the h ematopoietic system, but their intracellular mechanisms of action are not fully understood. Binding of IL-3 to the IL-3 receptor (IL-3R) and binding of EPO to the EPOR both induce changes in intracellular tyros ine and serine/threonine phosphorylation; the phosphorylation of a num ber of polypeptides appears to be a shared response upon cytokine stim ulation. We have previously shown that binding of IL-2 to the IL-2R ac tivates the 70-kDa (p70) 86 kinase, a serine/threonine kinase whose ac tivity is regulated by serine/threonine phosphorylation; the immunosup pressant rapamycin inhibits IL-2-dependent proliferation and IL-2-trig gered activation of p70 S6 kinase. We, therefore, sought to examine wh ether induction of p70 86 kinase activity is a conserved response upon cytokine triggering, and whether this activity is essential for cell proliferation. Proliferation of the IL-3-dependent pro-B cell line Ba/ F3 transfected with the EPOR (Ba/F3-EPOR) can be supported by either I L-3 or EPO. In this cell line, both IL-3 and EPO induced p70 86 kinase activity; rapamycin inhibited both the IL-3 and EPO-induced activatio n of the 70-kDa S6 kinase as well as cellular proliferation. Thus: p70 S6 kinase activation appears to be a common intermediate triggered by the stimulation of IL-3, EPO, and IL-2 receptors. The Friend spleen f ocus-forming virus gp55 renders the EPOR constitutively active, and co nfers growth factor independence on cells expressing EPOR. Ba/F3-EPOR cotransfected with gp55 (Ba/F3-EpoRgp55) and the erythroleukemia cell line MEL, which also expresses both the EPOR and gp55, were analyzed. Rapamycin inhibited the activation of p70 S6 kinase in both cell lines . However, rapamycin inhibited proliferation of Ba/F3-EpoRgp55 but not of MEL cells despite inhibition of p70 86 kinase activity in both cel ls. Thus, p70 86 kinase activation is not an absolute requirement for cell proliferation. These results are discussed in relation to the rol e of the activation of the 70-kDa 86 kinase activation pathway in the regulation of cell cycle progression.