Erythropoietin Receptor and STAT5-Specific Pathways Promote SKT6 Cell Hemoglobinization

Lyn deficiency reduces GATA-1, EKLF and STAT5, and induces extramedullary stress erythropoiesis

In vitro studies have implicated the Lyn tyrosine kinase in erythropoietin signaling. In this study, we show that J2E erythroid cells lacking Lyn have impaired signaling and reduced levels of transcription factors STAT5a, EKLF and GATA-1. Since mice lacking STAT5, EKLF or GATA-1 have red cell abnormalities, this study also examined the erythroid compartment of Lyn(-/-) mice. Significantly, STAT5, EKLF and GATA-1 levels were appreciably lower in Lyn(-/-) erythroblasts, and the phenotype of Lyn(-/-) animals was remarkably similar to GATA-1(low) animals. Although young adult Lyn-deficient mice had normal hematocrits, older mice developed anemia. Grossly enlarged erythroblasts and florid erythrophagocytosis were detected in the bone marrow of mice lacking Lyn. Markedly elevated erythroid progenitors and precursor levels were observed in the spleens, but not bone marrow, of Lyn(-/-) animals indicating that extramedullary erythropoiesis was occurring. These data indicate that Lyn(-/-) mice display extramedullary stress erythropoiesis to compensate for intrinsic and extrinsic erythroid defects.

Erythropoietin receptor-dependent erythroid colony-forming unit development: capacities of Y343 and phosphotyrosine-null receptor forms

Red cell development depends on the binding of erythropoietin (EPO) to receptors expressed by erythroid colony-forming units (CFUe) and the subsequent activation of receptor-bound Janus kinase (Jak2). Jak2 then mediates the phosphorylation of receptor tyrosine sites and the recruitment of 25 or more Src homology 2 domain-encoding proteins and associated factors. Previous studies have shown that an EPO receptor form containing Jak2-binding domains plus a single phosphotyrosine343 (PY343)–STAT5-binding site provides all signals needed for erythroid cell development. However, roles for PY343 and STAT5 remain controversial, and findings regarding PY-null receptor activities and erythropoiesis in STAT5-deficient mice are disparate. To study activities of a PY-null EPO receptor in primary cells while avoiding compensatory mechanisms, a form retaining domains for Jak2 binding and activation, but lacking all cytoplasmic tyrosine sites, was expressed in transgenic mice from aGATA1 gene-derived vector as a human epidermal growth factor receptor- murine EPO receptor chimera (EE-T-Y343F). The bio-signaling capacities of this receptor form were investigated in CFUe from thiamphenicol-treated mice. Interestingly, this PY-null EPO receptor form supported CFUe development (in the absence of detectable STAT5 activation) at efficiencies within 3-fold of those levels mediated by either an EE-T-Y343 form or the endogenous EPO receptor. However, EE-T-Y343F–dependent Ter119+ erythroblast maturation was attenuated. In tests of cosignaling with c-Kit, EE-T-Y343F nonetheless retained full capacity to synergize with c-Kit in promoting erythroid progenitor cell proliferation. Thus, EPO receptor PY-dependent events can assist late erythropoiesis but may be nonessential for EPO receptor–c-Kit synergy.

Control of myeloid differentiation and survival by Stats

Hematopoiesis involves a complex array of growth factors that regulate the survival and proliferation of immature progenitors, influence differentiation commitment, and modulate end-stage cell functions. This mini-review is focused on the role of Stat activation in the development of myeloid cells in response to hematopoietic cytokines. Much of the evidence implicating Stats in these cellular processes comes from studies of mutant cytokine receptors selectively uncoupled from Stat activation, dominant-inhibitory Stat mutants, and mice with targeted disruptions of Stat genes. Together these approaches provide strong evidence that Stat activation, particularly of Stat3 and Stat5, plays an important role in myeloid differentiation and survival. Oncogene (2000).

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

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.