INTERLEUKIN-3 (IL-3) INHIBITS ERYTHROPOIETIN-INDUCED DIFFERENTIATION IN BA F3 CELLS VIA THE IL-3 RECEPTOR-ALPHA SUBONIT/

Introduction of erythropoietin receptors (EpoRs) into the interleukin- 3 (IL-3)-dependent murine hemopoietic cell line, Ba/F3, enables these cells to not only proliferate, after an initial lag in G(1), but also to increase beta-globin mRNA levels in response to erythropoietin (Epo ), With IL-3 and Epo costimulation, IL-3-induced signaling appears to be dominant since no increase in beta-globin mRNA occurs, Differentiat ion and proliferation signals may be uncoupled since EpoRs lacking all eight intracellular tyrosines were compromised in proliferative signa ling but retained erythroid differentiation ability, intriguingly, a c himeric receptor of the extracellular domain of the EpoR and the trans membrane and intracellular domains of IL-3R beta(IL-3) chain (EpoR/IL- 3R beta(IL-3)) was capable of Epo-induced proliferative and differenti ating signaling suggesting either the existence of a second EpoR subun it responsible for differentiation or that the a! subunit of the IL-3 receptor (IL-3R) prevents it. Arguing against the former, a truncated EpoR lacking an intracellular domain was incapable of promoting prolif eration or differentiation. An EpoR/IL-3R alpha chimera, in contrast, was capable of transmitting a weak Epo-induced proliferative signal bu t failed to stimulate accumulation of beta-globin mRNA, Most significa ntly, coexpression of the EpoR/IL-3R alpha chimera with either EpoR/IL -3R beta or wild-type EpoRs suppressed Epo-induced beta-globin mRNA ac cumulation, Taken together, these results suggest an active role for t he IL-3R alpha subunit in inhibiting EpoR-specific differentiating sig nals.