Epo regulates erythroid proliferation and differentiation through distinct signaling pathways: implication for...

We have recently isolated the erythroleukemic cell line, HB60-5, that proliferates in the presence of erythropoietin (Epo) and stem cell factor (SCF), but undergoes terminal differentiation in the presence of Epo alone. Ectopic expression of the ets related transcription factor Fli-1 in these cells resulted in the establishment of the Epo-dependent cell line HB60-ED that proliferates in the presence of Epo. In this study, we utilized these two cell lines to examine the signal transduction pathways that are activated in response to Epo and SCF stimulation. We demonstrate that Epo, but not SCF, phosphorylates STAT-5 in both HB60-5 and HB60-ED cells. Interestingly, SCF activates the Shc/ras pathway in HB60-5 cells while Epo does not. However, both Epo and SCF are capable of activating the Shc/ras pathway in HB60ED cells. Furthermore, enforced expression of gp55 in HB60-5 cells by means of infection with the Spleen Focus Forming virus-P (SFFV-P), confers Epo independent growth, which is associated with the up-regulation of Fli-1. Activation of the Shc/ras pathway is readily detected in gp55 expressing cells in response to both Epo and SCF, and is associated with a block in STAT-5B tyrosine phosphorylation. These results suggest that STAT-5 activation, in the absence of Shc/ras activation, plays a role in erythroid differentiation. Moreover, Fli-1 is capable of switching Epo-induced differentiation to Epo-induced proliferation, suggesting that this ets factor regulated genes whose products modulate the Epo-Epo-R signal transduction pathway.