Epo regulates erythroid proliferation and differentiation through distinctsignaling pathways: implication for erythropoiesis and Friend virus-induced erythroleukemia

We have recently isolated the erythroleukemic cell line, HB60-5 that prolif erates in the presence of erythropoietin (Epo) and stem cell factor (SCF), but undergoes terminal differentiation in the presence of Epo alone. Ectopi c expression of the ets related transcription factor Fli-1 in these cells r esulted in the establishment of the Epo-dependent cell line HB60-ED that pr oliferates in the presence of Epo, In this study, me utilized these two cel l lines to examine the signal transduction pathways that are activated in r esponse to Epo and SCF stimulation, We demonstrate that Epo, but not SCF, p hosphorylates STAT-5 in both HB60-5 and HB60-ED cells. Interestingly, SCF a ctivates the Shc/ras pathway in HB60-5 cells while Epo does not. However, b oth Epo and SCF are capable of activating the Shc/ras pathway in HB60-ED ce lls, Furthermore, enforced expression of gp55 in HB60-5 cells by means of i nfection with the Spleen Focus Forming virus-P (SFFV-P), confers Epo indepe ndent growth, which is associated with the up-regulation of Fli-1, Activati on of the Shc/ras pathway is readily detected in gp55 expressing cells in r esponse to both Epo and SCF, and is associated with a block in STAT-SE tyro sine phosphorylation, These results suggest that STAT-5 activation, in the absence of Shc/ras activation, plays a role in erythroid differentiation. M oreover, Fli-1 is capable of switching Epo-induced differentiation to Epo-i nduced proliferation, suggesting that this ets factor regulated genes whose products modulate the Epo-Epo-R signal transduction pathway.