A Minimal Cytoplasmic Subdomain of the Erythropoietin Receptor Mediates Erythroid and Megakaryocytic Cell Development

In vivo regulation of erythropoiesis by chemically inducible dimerization of the erythropoietin receptor intracellular domain

Erythropoietin (Epo) and its receptor (EpoR) are required for the regulation of erythropoiesis. Epo binds to the EpoR homodimer on the surface of erythroid progenitors and erythroblasts, and positions the intracellular domains of the homodimer to be in close proximity with each other. This conformational change is sufficient for the initiation of Epo-EpoR signal transduction. Here, we established a system of chemically regulated erythropoiesis in transgenic mice expressing a modified EpoR intracellular domain (amino acids 247-406) in which dimerization is induced using a specific compound (chemical inducer of dimerization, CID). Erythropoiesis is reversibly induced by oral administration of the CID to the transgenic mice. Because transgene expression is limited to hematopoietic cells by the Gata1 gene regulatory region, the effect of the CID is limited to erythropoiesis without adverse effects. Additionally, we show that the 160 amino acid sequence is the minimal essential domain of EpoR for intracellular signaling of chemically inducible erythropoiesis in vivo. We propose that the CID-dependent dimerization system combined with the EpoR intracellular domain and the Gata1 gene regulatory region generates a novel peroral strategy for the treatment of anemia.

JAK2/STAT5/Bcl-xL signalling is essential for erythropoietin-mediated protection against apoptosis induced in PC12 cells by the amyloid β-peptide Aβ25-35

BACKGROUND AND PURPOSE

Erythropoietin (EPO) exerts neuroprotective actions in the CNS, including protection against apoptosis induced by the amyloid β-peptide Aβ25-35 . However, it remains unclear which signalling pathway activated by EPO is involved in this neuroprotection. Here, we have investigated whether JAK2/STAT5/Bcl-xL and ERK1/2 signalling pathways are essential for EPO-mediated protection against apoptosis induced by Aβ25-35 .

EXPERIMENTAL APPROACH

EPO was added to cultures of PC12 cells, 1 h before Aβ25-35 . For kinase inhibitor studies, AG490 and PD98059 were added to PC12 cells, 0.5 h before the addition of EPO. Transfection with siRNA was used to knockdown STAT5. Activation of JAK2/STAT5/Bcl-xL and ERK1/2 signalling pathways were investigated by Western blotting. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyl-tetrazolium bromide assay and apoptosis was detected by TUNEL and acridine orange-ethidium bromide double staining.

KEY RESULTS

EPO increased phosphorylation of JAK2 and STAT5 in PC12 cells treated with Aβ25-35 . Furthermore, EPO modulated the nuclear translocation of phospho-STAT5, which increased expression of Bcl-xL and decreased levels of caspase-3. These beneficial effects were blocked by the JAK2 inhibitor, AG490 or STAT5 knockdown. However, the ERK1/2 pathway did not play a crucial role in our model.

CONCLUSIONS AND IMPLICATIONS

EPO protected PC12 cells against Aβ25-35 -induced neurotoxicity. Activation of JAK2/STAT5/Bcl-xL pathway was important in EPO-mediated neuroprotection. EPO may serve as a novel protective agent against Aβ25-35 -induced cytotoxicity in, for instance, Alzheimer's disease.

In vivo knockdown of adipocyte erythropoietin receptor does not alter glucose or energy homeostasis

The growing prevalence of obesity and diabetes necessitate a better understanding of the role of adipocyte biology in metabolism. Increasingly, erythropoietin (EPO) has been shown to have extraerythropoietic and cytoprotective roles. Exogenous administration has recently been shown to have beneficial effects on obesity and diabetes in mouse models and EPO can modulate adipogenesis and insulin signaling in 3T3-L1 adipocytes. However, its physiological role in adipocytes has not been identified. Using male and female mice with adipose tissue-specific knockdown of the EPO receptor, we determine that adipocyte EPO signaling is not essential for the maintenance of energy homeostasis or glucose metabolism. Adipose tissue-specific disruption of EPO receptor did not alter adipose tissue expansion, adipocyte morphology, insulin resistance, inflammation, or angiogenesis in vivo. In contrast to the pharmacological effects of EPO, we demonstrate that EPO signaling at physiological levels is not essential for adipose tissue regulation of metabolism.

Novel pathways to erythropoiesis induced by dimerization of intracellular C-Mpl in human hematopoietic progenitors

The cytokine thrombopoietin (Tpo) plays a critical role in hematopoiesis by binding to the extracellular domain and inducing homodimerization of the intracellular signaling domain of its receptor, c-Mpl. Mpl homodimerization can also be accomplished by binding of a synthetic ligand to a constitutively expressed fusion protein F36VMpl consisting of a ligand binding domain (F36V) and the intracellular signaling domain of Mpl. Unexpectedly, in contrast to Tpo stimulation, robust erythropoiesis is induced after dimerization of F36VMpl in human CD34+ progenitor cells. The goal of this study was to define the hematopoietic progenitor stages at which dimerization of intracellular Mpl induces erythropoiesis and the downstream molecular events that mediate this unanticipated effect. Dimerization (in the absence of erythropoietin and other cytokines) in human common myeloid progenitors and megakaryocytic erythroid progenitors caused a significant increase in CD34+ cells (p < .01) and induced all stages of erythropoiesis including production of enucleated red blood cells. In contrast, erythropoiesis was not seen with Tpo stimulation. CD34+ cell expansion was the result of increased cell cycling and survival (p < .05). Microarray profiling of CD34+ cells demonstrated that a unique transcriptional pattern is activated in progenitors by F36VMpl dimerization. Ligand-inducible dimerization of intracellular Mpl in human myeloerythroid progenitors induces progenitor expansion and erythropoiesis through molecular mechanisms that are not shared by Tpo stimulation of endogenous Mpl.

Attenuation of EPO-dependent erythroblast formation by death-associated protein kinase-2

The adult erythron is maintained via dynamic modulation of erythroblast survival potentials. Toward identifying novel regulators of this process, murine splenic erythroblasts at 3 developmental stages were prepared, purified and profiled. Stage-to-stage modulated genes were then functionally categorized, with a focus on apoptotic factors. In parallel with BCL-X and NIX, death-associated protein kinase-2 (DAPK2) was substantially up-modulated during late erythropoiesis. Among hematopoietic lineages, DAPK2 was expressed predominantly in erythroid cells. In a Gata1-IE3.9int-DAPK2 transgenic mouse model, effects on steady-state reticulocyte and red blood cell (RBC) levels were limited. During hemolytic anemia, however, erythropoiesis was markedly deficient. Ex vivo ana-lyses revealed heightened apoptosis due to DAPK2 at a Kit(-)CD71(high)Ter119(-) stage, together with a subsequent multifold defect in late-stage Kit(-)CD71(high)Ter119(+) cell formation. In UT7epo cells, siRNA knock-down of DAPK2 enhanced survival due to cytokine withdrawal, and DAPK2's phosphorylation and kinase activity also were erythropoietin (EPO)-modulated. DAPK2 therefore comprises a new candidate attenuator of stress erythropoiesis.

The erythropoietin receptor in normal and cancer tissues

The hormone erythropoietin (EPO) is essential for the survival, proliferation and differentiation of the erythrocytic progenitors. The EPO receptor (EPO-R) of erythrocytic cells belongs to the cytokine class I receptor family and signals through various protein kinases and STAT transcription factors. The EPO-R is also expressed in many organs outside the bone marrow, suggesting that EPO is a pleiotropic anti-apoptotic factor. The controversial issue as to whether the EPO-R is functional in tumor tissue is critically reviewed. Importantly, most studies of EPO-R detection in tumor tissue have provided falsely positive results because of the lack of EPO-R specific antibodies. However, endogenous EPO appears to be necessary to maintain the viability of endothelial cells and to promote tumor angiogenesis. Although there is no clinical proof that the administration of erythropoiesis stimulating agents (ESAs) promotes tumor growth and mortality, present recommendations are that (i) ESAs should be administered at the lowest dose sufficient to avoid the need for red blood cell transfusions, (ii) ESAs should not be used in patients with active malignant disease not receiving chemotherapy or radiotherapy, (iii) ESAs should be discontinued following the completion of a chemotherapy course, (iv) the target Hb should be 12 g/dL and not higher and (v) the risks of shortened survival and tumor progression have not been excluded when ESAs are dosed to target Hb <12 g/dL.

Transgene insertion in proximity to the c-myb gene disrupts erythroid-megakaryocytic lineage bifurcation

The nuclear proto-oncogene c-myb plays crucial roles in the growth, survival, and differentiation of hematopoietic cells. We established three lines of erythropoietin receptor-transgenic mice and found that one of them exhibited anemia, thrombocythemia, and splenomegaly. These abnormalities were independent of the function of the transgenic erythropoietin receptor and were observed exclusively in mice harboring the transgene homozygously, suggesting transgenic disruption of a certain gene. The transgene was inserted 77 kb upstream of the c-myb gene, and c-Myb expression was markedly decreased in megakaryocyte/erythrocyte lineage-restricted progenitors (MEPs) of the homozygous mutant mice. In the bone marrows and spleens of the mutant mice, numbers of megakaryocytes were increased and numbers of erythroid progenitors were decreased. These abnormalities were reproducible in vitro in a coculture assay of MEPs with OP9 cells but eliminated by the retroviral expression of c-Myb in MEPs. The erythroid/megakaryocytic abnormalities were reconstituted in mice in vivo by transplantation of mutant mouse bone marrow cells. These results demonstrate that the transgene insertion into the c-myb gene far upstream regulatory region affects the gene expression at the stage of MEPs, leading to an imbalance between erythroid and megakaryocytic cells, and suggest that c-Myb is an essential regulator of the erythroid-megakaryocytic lineage bifurcation.

Lyn kinase promotes erythroblast expansion and late-stage development

Lyn kinase is known to modulate the formation and function of B cells, monocytes, and mast cells. However, Lyn-/- mice also develop erythrosplenomegaly, and cases for both negative and positive erythropoietic actions of Lyn recently have been outlined. In phenylhydrazine-treated Lyn-/- mice, extramedullary splenic erythropoiesis was hyperactivated, but this did not lead to accelerated recovery from anemia. Furthermore, ex vivo analyses of the development of bone marrow-derived Lyn-/- erythroblasts in unique primary culture systems indicated positive roles for Lyn at 2 stages. Late-stage Lyn-/- erythroblasts exhibited deficit Ter119(pos) cell formation, and this was paralleled by increased apoptosis (and decreased Bcl-xL expression). During early development, Lyn-/- erythroblasts accumulated at a Kit(pos)CD71(high) stage, possessed decreased proliferative capacity, and were attenuated in entering an apparent G1/S cell-cycle phase. In proposed compensatory responses, Lyn-/- erythroblasts expressed increased levels of activated Akt and p60-Src and decreased levels of death-associated protein kinase-2. Stat5 activation and Bcl-xL expression, in contrast, were significantly decreased in keeping with decreased survival and developmental potentials. Lyn, therefore, is proposed to function via erythroid cell-intrinsic mechanisms to promote progenitor cell expansion beyond a Kit(pos)CD71(high) stage and to support subsequent late-stage development.

Co-operative signalling mechanisms required for erythroid precursor expansion in response to erythropoietin and stem cell factor

The regeneration of circulating red blood cells in response to anaemia associated with blood loss or haemolysis involves an increased rate of erythropoiesis and expansion of proerythroblasts, the bone marrow precursor cells that terminally differentiate into mature erythrocytes. This study investigated the mechanisms by which erythropoietin (Epo) and stem cell factor (Scf) modulate the expansion of proerythroblasts. Homogenous populations of primary human proerythroblasts were generated in liquid cultures of CD34(+) cells. In serum-free cultures, proerythroblasts failed to survive in the presence of Epo or Scf alone, but exhibited synergistic proliferation in response to combined Epo and Scf treatment, exhibiting one-log expansion in 5 d. Intracellular signal transduction in response to Epo and Scf revealed that tyrosine phosphorylation of signal transducers and activators of transcription (Stat) 5, a downstream target for the non-receptor tyrosine kinase, Janus kinase 2 (Jak2), was mediated by Epo but not Scf. The mitogen-activated protein kinases (MAPKs) extracellular regulated kinase (Erk) 1-2 were phosphorylated in response to either Epo or Scf. Phosphorylation of Akt, a signalling molecule downstream of phosphatidylinositol 3-kinase (PI3K), was observed following Scf but not Epo treatment. To determine the contribution of specific signalling pathways to synergistic expansion of proerythroblasts in response to co-operative effects of Epo and Scf, cells were treated with kinase inhibitors targeting Jak2, PI3K and MAPK kinase. There was a significant, dose-dependent inhibition of proerythroblast expansion in response to all three kinase inhibitors. In conclusion, Epo- and Scf-mediated co-operative, synergistic expansion of primary erythroid precursors requires selective activation of multiple signalling pathways, including the Jak-Stat, PI3K and MAPK pathways.

A systematic scan of interactions with tyrosine motifs in the erythropoietin receptor using a mammalian 2-hybrid approach

Signaling via the erythropoietin receptor (EpoR) depends on the interaction of several proteins with phosphorylated tyrosine-containing motifs in its cytosolic domain. Detailed mapping of these interactions is required for an accurate insight into Epo signaling. We recently developed a mammalian protein-protein interaction trap (MAPPIT), a cytokine receptor-based 2-hybrid method that operates in intact Hek293-T mammalian cells. As baits, we used intracellular segments of the EpoR containing 1 or 2 tyrosines. Several known signaling molecules, including cytokine-inducible SH2-containing protein (CIS), suppressor of cytokine signaling-2 (SOCS2), phosphatidylinositol 3′-kinase (PI3-K), phospholipase C-γ (PLC-γ), and signal transducer and activator of transcription 5 (STAT5) were used as prey. We also extended the MAPPIT method to enable interaction analysis with wild-type EpoR. In this relay MAPPIT approach, instead of using isolated EpoR fragments as bait, we used the full-length EpoR itself as a “receptor bait.” Finally, we introduced MAPPIT in the erythroleukemic TF-1 cell line, which is a more natural setting of the EpoR. With these strategies several known interactions with the EpoR were analyzed and evidence for new interactions was obtained.

Erythropoietin as an antiapoptotic, tissue-protective cytokine

Erythropoietin (EPO) increases the number of circulating erythrocytes primarily by preventing apoptosis of erythroid progenitors. In addition to this proerythroid action, results of recent studies show that systemically administered EPO is protective in vivo, in several animal models of neuronal injury. In vitro, EPO prevents neuronal apoptosis induced by a variety of stimuli. This review summarizes the neuroprotective actions of EPO and discusses the underlying mechanisms in terms of signal transduction pathways involved. The understanding of these mechanisms will help differentiate the neuroprotective actions of EPO from its role in the bone marrow.

Normal and Oncogenic Forms of the Receptor Tyrosine Kinase Kit

Kit is a receptor tyrosine kinase (RTK) that binds stem cell factor. This receptor ligand combination is important for normal hematopoiesis, as well as pigmentation, gut function, and reproduction. Structurally, Kit has both an extracellular and intracellular region. Theintra-cellular region is comprised of a juxtamembrane domain (JMD), a kinase domain, a kinase insert, and a carboxyl tail. Inappropriate expression or activation of Kit is associated with a variety of diseases in humans. Activating mutations in Kit have been identified primarily in the JMD and the second part of the kinase domain and have been associated with gastrointestinal stromal cell tumors and mastocytosis, respectively. There are also reports of activating mutations in some forms of germ cell tumors and core binding factor leukemias. Since the cloning of the Kit ligand in the early 1990s, there has been an explosion of information relating to the mechanism of action of normal forms of Kit as well as activated mutants. This is important because understanding this RTK at the biochemical level could assist in the development of therapeutics to treat primary and secondary defects in the tissues that require Kit. Furthermore, understanding the mechanisms mediating transformation of cells by activated Kit mutants will help in the design of interventions for human disease associated with these mutations. The objective of this review is to summarize what is known about normal and oncogenic forms of Kit. We will place particular emphasis on recent developments in understanding the mechanisms of action of normal and activated forms of this RTK and its association with human disease, particularly in hematopoietic cells.

Attenuated signaling by a phosphotyrosine-null Epo receptor form in primary erythroid progenitor cells

Signals provided by the erythropoieitin receptor (EpoR) are required for erythroid development beyond the erythroid colony-forming unit (CFU-e) stage and are propagated via the EpoR-tethered Janus kinase, JAK2. JAK2 functions, in part, to phosphorylate 8 conserved EpoR phosphotyrosine (PY) sites for the binding of a diverse set of signaling factors. However, recent studies in transgenic and knock-in mice have demonstrated substantial bioactivity for PY-null EpoR forms. Presently, the activities of a PY-null EpoR-HM form in primary progenitor cells from knock-in mice were further assessed using optimized Epo dose-dependent proliferation, survival, and differentiation assays. As compared with the wild-type (wt)-EpoR, EpoR-HM activity was compromised several-fold in each context when Epo was limited to physiologic concentrations. Possible compensatory increases in serum growth factor levels also were investigated, and as assayed using embryonic stem (ES) cell-derived erythroid G1E2 cells, activities in serum from EpoR-HM mice were substantially elevated. In addition, when challenged with phenylhydrazine-induced anemia, EpoR-HM mice failed to respond with efficient splenic stress erythropoiesis. Thus, the function of this JAK2-coupled but minimal PY-null EpoR-HM form appears to be attenuated in several contexts and to be assisted in vivo by compensatory mechanisms. Roles normally played by EpoR PY sites and distal domains therefore should receive continued attention.

Novel role of 1,25(OH)(2)D(3) in induction of erythroid progenitor cell proliferation

OBJECTIVE

Burst-forming unit erythroid and colony-forming unit erythroid growth in vitro is lower in studies of continuous ambulatory peritoneal dialysis patients than healthy controls. Burst-forming unit erythroid growth was potentiated by addition of 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and normalized by erythropoietin (Epo) therapy, suggesting an interaction between Epo and 1,25(OH)(2)D(3) at the stem cell level. The objective of this study was to determine the mechanism by which 1,25(OH)(2)D(3) enhances the stimulatory effect of Epo on the growth of erythroid precursor cells.

MATERIALS AND METHODS

We examined the effect of 1,25(OH)(2)D(3) and Epo on stem cell proliferation. Proliferation of TF1 cells of erythroid origin was measured by the XTT method, 3[H] thymidine incorporation, and cell counting by trypan blue exclusion; cord blood (CB) stem cells were counted. Epo receptor (EpoR) quantitation was evaluated by 125I-Epo binding and Scatchard analysis, immunoprecipitation, and Western blotting. Expression of EpoR mRNA was measured by reverse transcriptase polymerase chain reaction.

RESULTS

The stem cell factor-dependent CB stem cells and the TF1 cells responded to Epo and 1,25(OH)(2)D(3) by increased proliferation, while their simultaneous addition potentiated cell proliferation in a synergistic manner (25.67% +/- 4.8% of Epo proliferation at day 10 for CB cells; p < 0.005). 1,25(OH)(2)D(3) produced an up-regulation of EpoR number in TF1 cells and increased the expression of EpoR mRNA (p < 0.01).

CONCLUSIONS

The increase in EpoR expression induced by 1,25(OH)(2)D(3) might explain the synergistic interaction between Epo and 1,25(OH)(2)D(3) in stem cells.

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.

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 phosphotyrosine(343) (PY(343))-STAT5-binding site provides all signals needed for erythroid cell development. However, roles for PY(343) 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 a GATA1 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.

An optimized system for studies of EPO-dependent murine pro-erythroblast development

OBJECTIVE

Objectives were to develop new means to isolate useful numbers of primary progenitor cells and to quantitatively assay the stepwise maturation of erythroblasts.

METHODS

Approaches involved dosing mice with thiamphenicol (TAP) to yield staged cohorts of pro-erythroid cells; optimizing conditions for their EPO-dependent in vitro growth and survival; developing assays for CFU-E maturation; analyzing stage-specific transcript expression; and expressing a heterologous, erythroid-specific tag (EE372) in transgenic mice.

RESULTS

Per TAP-treated mouse, 3 x 10(7) highly EPO-responsive erythroid progenitor cells were generated that represented up to 30% of total splenocytes and showed strict dependence on EPO for survival, growth, and immediate response gene expression. In this developing cohort, a tightly programmed sequence of gene expression was observed, and maximal expression of c-kit, EPO receptor, and beta-globin transcripts occurred at 72, 96, and 120 hours post-TAP withdrawal, respectively. Also, the newly discovered erythroid-specific dual-specificity kinase, DYRK3, was revealed to be expressed at a late CFU-E stage. In vitro, these progenitor cells matured stepwise from high FALS Ter119- cells (24-hour culture) to high FALS Ter119+ cells (24-36 hours) to low FALS Ter119+ maturing erythroblasts (40-48 hours) and sharp differences in their morphologies were observed. Finally, a MACS-based procedure for the purification of erythroid progenitor cells from TAP-treated EE372 transgenic mice also was developed.

CONCLUSIONS

A comprehensive new system for isolating large numbers of primary murine erythroid progenitor cells and quantitatively monitoring their development is established that should serve well in investigations of endogenous and pharmacological regulators of red blood cell development.

A minimal cytoplasmic subdomain of the erythropoietin receptor mediates p70 S6 kinase phosphorylation

Erythropoietin (EPO) is a lineage-restricted growth factor that is required for erythroid proliferation and differentiation. EPO stimulates the phosphorylation and activation of p70 S6 kinase (p70 S6K), which is required for cell cycle progression. Here, the minimal cytoplasmic domains of the EPO receptor (EPO-R) required for p70 S6K activation were determined.Ba/F3 cells were stably transfected with wild-type (WT) EPO-R or EPO-R carboxyl-terminal deletion mutants, designated by the number of amino acids deleted from the cytoplasmic tail (-99, -131, -221). Transfected cells were growth factor deprived and then stimulated with EPO. p70 S6K, JAK2, IRS-2, and ERK1/2 phosphorylation/activation were examined. The ability of transfected 3-phosphoinositide-dependent protein kinase 1 (PDK1) to reconstitute p70 S6K phosphorylation in EPO-R mutants also was determined. Phosphorylation and activation of p70 S6K, JAK2, IRS-2, and ERK1/2 in Ba/F3 cells transfected with EPO-R-99 or EPO-R-99Y343F were similar to WT EPO-R. In contrast, EPO-dependent p70 S6K phosphorylation/activation, as well as IRS-2 and ERK1/2 phosphorylation, were minimal or absent in cells transfected with EPO-R-131 or EPO-R-221. JAK2 phosphorylation was reduced significantly in cells transfected with EPO-R-131 and abolished with EPO-R-221. To examine the role of PDK1, a kinase known to phosphorylate p70 S6K, Ba/F3 EPO-R-131 cells were transiently transfected with PDK1. WT constitutively active PDK1 restored p70 S6K phosphorylation in Ba/F3 EPO-R-131 cells but not in Ba/F3 EPO-R-221 cells. The results demonstrate that a minimal cytoplasmic subdomain of the EPO-R extending between -99 and -131 is required for p70 S6K phosphorylation and activation. The results also demonstrate that PDK1 is a critical component in this signaling pathway, which requires the presence of domains between -131 and -221 for its activation of p70 S6K.

Integrative signaling by minimal erythropoietin receptor forms and c-Kit

Erythroid homeostasis depends critically upon erythropoietin (Epo) and stem cell factor cosignaling in late progenitor cells. Epo bioresponses are relayed efficiently by minimal receptor forms that retain a single Tyr-343 site for STAT5 binding, while forms that lack all cytoplasmic Tyr(P) sites activate JAK2 and the transcription of c-Myc plus presumed additional target genes. In FDCER cell lines, which express endogenous c-Kit, the signaling capacities of such minimal Epo receptor forms (ER-HY343 and ER-HY343F) have been dissected to reveal: 1) that Epo-dependent mitogenesis, survival, and bcl-x gene expression via ER-HY343 depend upon the intactness of the Tyr-343 STAT5 binding site; 2) that ER-HY343-dependent bcl-x(L) gene transcription is enhanced markedly via c-Kit; 3) that socs-3, plfap, dpp-1, and cacy-bp gene transcription is induced via ER-HY343, whereas dpp-1 and cacy-bp gene expression is also supported by ER-HY343F; 4) that ectopically expressed SOCS-3 suppresses proliferative signaling by not only ER-HY343 but also c-Kit; and 5) that in FDCER and primary erythroid cells, c-Kit appears to provide the primary route to MAPK activation. Thus, integration circuits exist in only select downstream pathways within Epo and stem call factor receptor signaling.

mDYRK3 kinase is expressed selectively in late erythroid progenitor cells and attenuates colony-forming unit-erythroid development

DYRKs are a new subfamily of dual-specificity kinases that was originally discovered on the basis of homology to Yak1, an inhibitor of cell cycle progression in yeast. At present, mDYRK-3 and mDYRK-2 have been cloned, and mDYRK-3 has been characterized with respect to kinase activity, expression among tissues and hematopoietic cells, and possible function during erythropoiesis. In sequence, mDYRK-3 diverges markedly in noncatalytic domains from mDYRK-2 and mDYRK-1a, but is 91.3% identical overall to hDYRK-3. Catalytically, mDYRK-3 readily phosphorylated myelin basic protein (but not histone 2B) and also appeared to autophosphorylate in vitro. Expression of mDYRK-1a, mDYRK-2, and mDYRK-3 was high in testes, but unlike mDYRK1a and mDYRK 2, mDYRK-3 was not expressed at appreciable levels in other tissues examined. Among hematopoietic cells, however, mDYRK-3 expression was selectively elevated in erythroid cell lines and primary pro-erythroid cells. In developmentally synchronized erythroid progenitor cells, expression peaked sharply following exposure to erythropoietin plus stem cell factor (SCF) (but not SCF alone), and in situ hybridizations of sectioned embryos revealed selective expression of mDYRK-3 in fetal liver. Interestingly, antisense oligonucleotides to mDYRK-3 were shown to significantly and specifically enhance colony-forming unit-erythroid colony formation. Thus, it is proposed that mDYRK-3 kinase functions as a lineage-restricted, stage-specific suppressor of red cell development. (Blood. 2001;97:901-910)

Mouse model of congenital polycythemia: Homologous replacement of murine gene by mutant human erythropoietin receptor gene

Mutations causing truncations of the cytoplasmic domain of the human erythropoietin receptor (EPOR) result in a dominantly inherited disorder-primary familial congenital polycythemia. This disorder is characterized by increased numbers of erythrocytes (polycythemia) and by in vitro hypersensitivity of erythroid precursors to erythropoietin. The consequences of EPOR truncation in nonerythroid tissues are unknown. We replaced the murine EPOR gene with a wild-type human EPOR gene and a mutant human EPOR gene that we initially identified in a patient with polycythemia. This mutation leads to an EPOR truncated after the first tyrosine residue of the intracellular domain. Mice heterozygous for this mutant allele and a wild-type human EPOR allele mimicked the human disorder. Interestingly, mice that were homozygous for the mutant human allele were severely polycythemic but viable. Our results provide a model for functional studies of EPOR-triggered signaling pathways in erythropoiesis. These animals can now be used to investigate the molecular pathophysiology of this gain-of-function EPOR mutation in erythroid tissue and in those nonerythroid tissues that express EPOR.

Intron 1 elements promote erythroid-specific GATA-1 gene expression

The zinc finger protein GATA-1 functions in a concentration-dependent fashion to activate the transcription of erythroid and megakaryocytic genes. Less is understood, however, regarding factors that regulate the GATA-1 gene. Presently elements within intron 1 are shown to markedly affect its erythroid-restricted transcription. Within a full-length 6. 8-kilobase GATA-1 gene construct (G6.8-Luc) the deletion of a central subdomain of intron 1 inhibited transcription >/=10-fold in transiently transfected erythroid SKT6 cells, and likewise inhibited high-level transcription in erythroid FDCW2ER-GATA1 cells. In parental myeloid FDCER cells, however, low-level transcription was largely unaffected by intron 1 deletions. Within intron 1, repeated GATA and Ap1 consensus elements in a central region are described which when linked directly to reporter cassettes promote transcription in erythroid SKT6 and FDCER-GATA1 cells at high rates. Moreover, GATA-1 activated transcription from this subdomain in 293 cells, and in SKT6 cells this subdomain footprinted in vivo. For stably integrated GFP reporter constructs in erythroid SKT6 cells, corroborating results were obtained. Deletion of intronic GATA and Ap1 motifs abrogated the activity of G6.8-pEGFP; activity was decreased by 43 and 56%, respectively, by the deletion of either motif; and the above 1800-base pair region of intron 1 per se was transcribed at rates uniformly greater than G6.8-pEGFP. Also described is the differential utilization of exons 1a and 1b among primary erythromegakaryocytic and myeloid cells.

Subtraction cloning and initial characterization of novel epo-immediate response genes

Recent studies of erythropoietin (Epo) receptor signalling suggest that signals for mitogenesis, survival and differentiation are relayed efficiently by receptor forms lacking at least seven of eight cytoplasmic (phospho)tyrosine [(P)Y] sites for effector recruitment. While such receptor forms are known to activate Jak2 and a limited set of known immediate response genes (IRGs), the complex activities they exert predict the existence of additional target genes. To identify such targets, a minimal Epo receptor chimera was expressed in Epo-responsive erythroid SKT6 cells, and genes whose transcription is induced via this active receptor form were cloned by subtractive hybridization. Several known genes not previously linked to Epo signalling were discovered to be Epo IRGs including two which may further propagate Epo signals [Prl1 tyrosine phosphatase and receptor activator of of NFkappaB (Rank)], and three regulators of protein synthesis (EF1alpha, eIF3-p66 and Nat1). Several Epo IRGs were novel murine clones including FM2 and FM6 which proved to represent broadly expressed IRGs, and FM3 and FL10 which were induced primarily in haematopoietic cells. Interestingly, FL10 proved to correspond to a recently discovered regulator of yeast mating-type switching, and was induced by Epo in vivo. Thus, several new Epo signalling targets are described, which may modulate haematopoietic cell development.

Role of thrombopoietin in hematopoietic stem cell and progenitor regulation

Thrombopoietin performs an essential role during hematopoiesis by regulating the expansion and maturation of megakaryocytes. In keeping with this function, megakaryocytes, platelets, and their precursors all express the thrombopoietin receptor, Mpl, on their cell surface. However, Mpl is also expressed on primitive, pluripotent hematopoietic progenitors and plays an important role in the regulation of lineages other than megakaryocytes as well as primitive progenitors. Recently, the ability of thrombopoietin to maintain and expand repopulating stem cells has been demonstrated. Thus, thrombopoietin is unique among the hematopoietic cytokines because it is necessary both for terminal maturation and regulation of lineage-specific megakaryocytes and also for maintenance of the most primitive hematopoietic stem cells. Many new strategies are evolving to exploit the activity of thrombopoietin on primitive progenitors. This may lead to faster hematopoietic recovery from marrow-suppressive therapy, effective methods of ex vivo expansion of hematopoietic stem cells, and retroviral transduction of stem cells to facilitate gene therapy.

Signal transduction in the erythropoietin receptor system

Events relayed via the single transmembrane receptor for erythropoietin (Epo) are essential for the development of committed erythroid progenitor cells beyond the colony-forming unit-erythroid stage, and this clearly involves Epo's inhibition of programmed cell death (PCD). Less well resolved, however, are issues regarding the precise nature of Epo-dependent antiapoptotic mechanisms, the extent to which Epo might also promote mitogenesis and/or terminal erythroid differentiation, and the essential vs modulatory nature of certain Epo receptor cytoplasmic subdomains, signal transducing factors, and downstream pathways. Accordingly, this review focuses on the following aspects of Epo signal transduction: (1) Epo receptor/Jak2 activation mechanisms; (2) the critical vs dispensable nature of (P)Y sites and SH2 domain-encoding effectors in survival, growth, and differentiation responses; (3) primary mechanisms by which Epo inhibits PCD; (4) the integration of signals relayed by coexpressed and possibly directly interacting cytokine receptors; and (5) predictions regarding effector function which are provided by the association of certain primary and familial polycythemias with mutated human Epo receptor forms.