Expression of a constitutively active erythropoietin receptor in primary hematopoietic progenitors abrogates erythropoietin dependence and enhances erythroid colony-forming unit, erythroid burst-forming unit, and granulocyte/macrophage progenitor growth

Unpaired Extracellular Cysteine Mutations of CSF3R Mediate Gain or Loss of Function

Exclusive of membrane-proximal mutations seen commonly in chronic neutrophilic leukemia (e.g., T618I), functionally defective mutations in the extracellular domain of the G-CSF receptor (CSF3R) have been reported only in severe congenital and idiopathic neutropenia patients. Here, we describe the first activating mutation in the fibronectin-like type III domain of the extracellular region of CSF3R (W341C) in a leukemia patient. This mutation transformed cells via cysteine-mediated intermolecular disulfide bonds, leading to receptor dimerization. Interestingly, a CSF3R cytoplasmic truncation mutation (W791X) found on the same allele as the extracellular mutation and the expansion of the compound mutation was associated with increased leukocytosis and disease progression of the patient. Notably, the primary patient sample and cells transformed by W341C and W341C/W791X exhibited sensitivity to JAK inhibitors. We further showed that disruption of original cysteine pairs in the CSF3R extracellular domain resulted in either gain- or loss-of-function changes, part of which was attributable to cysteine-mediated dimer formation. This, therefore, represents the first characterization of unpaired cysteines that mediate both gain- and loss-of-function phenotypes. Overall, our results show the structural and functional importance of conserved extracellular cysteine pairs in CSF3R and suggest the necessity for broader screening of CSF3R extracellular domain in leukemia patients. Cancer Res; 77(16); 4258-67. ©2017 AACR.

JAK2 V617F stimulates proliferation of erythropoietin-dependent erythroid progenitors and delays their differentiation by activating Stat1 and other nonerythroid signaling pathways

JAK2 V617F is a mutant-activated JAK2 kinase found in most polycythemia vera (PV) patients; it skews normal proliferation and differentiation of hematopoietic stem and progenitor cells and simulates aberrant expansion of erythroid progenitors. JAK2 V617F is known to activate some signaling pathways not normally activated in mature erythroblasts, but there has been no systematic study of signal transduction pathways or gene expression in erythroid cells expressing JAK2 V617F undergoing erythropoietin (Epo)-dependent terminal differentiation. Here we report that expression of JAK2 V617F in murine fetal liver Epo-dependent progenitors allows them to divide approximately six rather than the normal approximately four times in the presence of Epo, delaying their exit from the cell cycle. Over time, the number of red cells formed from each Epo-dependent progenitor increases fourfold, and these cells eventually differentiate into normal enucleated reticulocytes. We report that purified fetal liver Epo-dependent progenitors express many cytokine receptors additional to the EpoR. Expression of JAK2 V617F triggers activation of Stat5, the only STAT normally activated by Epo, as well as activation of Stat1 and Stat3. Expression of JAK2 V617F also leads to transient induction of many genes not normally activated in terminally differentiating erythroid cells and that are characteristic of other hematopoietic lineages. Inhibition of Stat1 activation blocks JAK2 V617F hyperproliferation of erythroid progenitors, and we conclude that Stat1-mediated activation of nonerythroid signaling pathways delays terminal erythroid differentiation and permits extended cell divisions.

Large-scale production of red blood cells from stem cells: what are the technical challenges ahead?

Blood-transfusion centers regularly face the challenge of donor blood shortages, especially for rare blood groups. The possibility of producing universal red blood cells from stem cells industrially has become a possible alternative since the successful injection of blood generated in vitro into a human being in 2011. Although there remains many biological and regulatory issues concerning the efficacy and safety of this new product, the major challenge today for future clinical applications is switching from the current limited 2-dimensional production techniques to large-scale 3-dimensional bioreactors. In addition to requiring technological breakthroughs, the whole process also has to become at least five-fold more cost-efficient to match the current prices of high-quality blood products. The current review sums up the main biological advances of the past decade, outlines the key biotechnological challenges for the large-scale cost-effective production of red blood cells, proposes solutions based on strategies used in the bioindustry and presents the state-of-the-art of large-scale blood production.

Erythropoietin receptor contributes to melanoma cell survival in vivo

Erythropoietin (Epo) is widely used clinically to treat anemia associated with various clinical conditions including cancer. Data from several clinical trials suggest significant adverse effect of Epo treatment on cancer patient survival. However, controversy exists whether Epo receptor (EpoR) is functional in cancer cells. In this study, we demonstrated that EpoR mRNA expression was detectable in 90.1% of 65 melanoma cell lines, and increased copy number of the Epo and EpoR loci occurred in 30 and 24.6% of 130 primary melanomas, respectively. EpoR knockdown in melanoma cells resulted in diminished ERK phosphorylation in response to Epo stimulation, decreased cell proliferation and increased response to the inhibitory effect of hypoxia and cisplatin in vitro. EpoR knockdown significantly decreased melanoma xenograft size and tumor invasion in vivo. On the contrary, constitutive activation of EpoR activated cell proliferation pathways in melanoma cells and resulted in increased cell proliferation and resistance to hypoxia and cisplatin treatment in vitro. EpoR activation resulted in significantly larger xenografts with increased tumor invasion of surrounding tissue in vivo. Daily administration of recombinant Epo fails to stimulate melanoma growth in vivo, but the treatment increased vascular size in the xenografts. Increased local recurrence after excision of the primary tumors was observed after Epo treatment. Epo induced angiogenesis in Matrigel plug assays, and neutralization of Epo secreted by melanoma cells results in decreased angiogenesis. These data support that EpoR is functional in melanoma and EpoR activation may promote melanoma progression, and suggest that Epo may stimulate angiogenesis and increase survival of melanoma cells under hypoxic condition in vivo.

The erythropoietin receptor: molecular structure and hematopoietic signaling pathways

The process of erythropoiesis in the fetal liver and adult bone marrow is regulated by the hormone erythropoietin (Epo), which is produced in the kidney at low levels under homeostatic conditions. Defects in Epo production result in severe anemia; use of recombinant hormone has improved the lives of patients with renal failure or anemia because of bone marrow suppression. Deletion of the Epo gene in mice leads to embryonic lethality at days 13 to 15, coincident with the establishment of definitive (adult-type) erythropoiesis and underscoring the absolute necessity of Epo function in vivo. Epo has proven to be a successful pharmaceutical agent, one of the early triumphs of recombinant protein technology. Because of its clinical importance, a great deal of attention has focused on the molecular mechanisms of Epo-regulated erythropoiesis. This review highlights the basic concepts of Epo signal transduction within the hematopoietic system, the major site of Epo action in vivo.

The erythropoietin receptor: molecular structure and hematopoietic signaling pathways

The process of erythropoiesis in the fetal liver and adult bone marrow is regulated by the hormone erythropoietin (Epo), which is produced in the kidney at low levels under homeostatic conditions. Defects in Epo production result in severe anemia; use of recombinant hormone has improved the lives of patients with renal failure or anemia because of bone marrow suppression. Deletion of the Epo gene in mice leads to embryonic lethality at days 13 to 15, coincident with the establishment of definitive (adult-type) erythropoiesis and underscoring the absolute necessity of Epo function in vivo. Epo has proven to be a successful pharmaceutical agent, one of the early triumphs of recombinant protein technology. Because of its clinical importance, a great deal of attention has focused on the molecular mechanisms of Epo-regulated erythropoiesis. This review highlights the basic concepts of Epo signal transduction within the hematopoietic system, the major site of Epo action in vivo.

Manufactured RBC--rivers of blood, or an oasis in the desert?

Red blood cell (RBC) transfusion is an essential practice in modern medicine, one that is entirely dependent on the availability of donor blood. Constraints in donor supply have led to proposals that transfusible RBC could be manufactured from stem cells. While it is possible to generate small amounts of RBC in vitro, very large numbers of cells are required to be of clinical significance. We explore the challenges facing large scale manufacture of RBC and technological developments required for such a scenario to be realised.

Preclinical evaluation of nilotinib efficacy in an imatinib-resistant KIT-driven tumor model

The novel KIT inhibitor nilotinib is currently being evaluated for its clinical utility in the treatment of gastrointestinal stromal tumor. However, the effects of nilotinib in cells expressing commonly occurring KIT mutations remain to be fully defined. The aim of this study was therefore to investigate the efficacy of nilotinib against cells expressing imatinib-sensitive or imatinib-resistant KIT mutations and to evaluate [(18)F] fluorodeoxyglucose-positron emission tomography (FDG-PET) imaging as a biomarker of nilotinib response in vivo. Nilotinib inhibited the proliferation of imatinib-responsive V560G-KIT FDC-P1 and imatinib-resistant D816V-KIT FDC-P1 cells with a GI(50) of 4.9 and 630 nmol/L, respectively, whereas apoptosis studies revealed that nilotinib and imatinib were equipotent against the V560G cell line. In contrast, although 10 micromol/L nilotinib induced >50% apoptosis in the D816V cells at 16 hours, 10 micromol/L imatinib had no effect on cell survival at 24 hours. Syngeneic DBA2/J mice bearing FDC-P1-KIT tumors were evaluated for response to nilotinib by FDG-PET. V560G-KIT FDC-P1 tumor FDG uptake was significantly reduced compared with baseline levels following 2 days of nilotinib treatment. In contrast, no effect of nilotinib was observed on tumor growth or FDG-PET uptake into D816V tumors despite intratumoral drug levels reaching in excess of 10 micromol/L at 4 hours after dosing. Biomarker analysis revealed the inhibition of KIT phosphorylation in V560G but not D816V tumors. These findings show the in vivo activity of nilotinib in the treatment of tumors bearing V560G-KIT but not D816V-KIT and the utility of FDG-PET imaging to assess tumor response to this agent.

PU.1 immortalizes hematopoietic progenitors in a GM-CSF-dependent manner

OBJECTIVE

The Ets family transcription factor PU.1 is essential for both myeloid and lymphoid development. PU.1 was discovered because of its involvement in murine erythroleukemia. We previously described that infection with a retroviral vector encoding PU.1 immortalizes fetal liver progenitor cells in response to granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling. In this study, we sought to characterize PU.1-immortalized progenitor (PIP) cells.

METHODS

PIP cells were characterized using microscopy, reverse-transcriptase polymerase chain reaction analysis, and flow cytometric analysis. In addition, progenitors were immortalized with a retrovirus containing a PU.1 cDNA flanked by loxP sites. The differentiation potential of immortalized progenitors was tested by Cre-mediated excision of the proviral PU.1 cDNA.

RESULTS

PIP cells are blastlike in morphology and express cell surface markers indicative of myeloid development. Immortalization of progenitor cells requires both an acidic activation domain and an intact DNA-binding domain of PU.1. Gene expression analysis of PIP cells demonstrated the expression of genes of both myeloid and erythroid lineages. Proliferation of PIP cells was GM-CSF dependent and restricted. Upon Cre-mediated excision of proviral PU.1 cDNA, increased expression of myeloid and erythroid-specific genes was observed; as well as the appearance of both macrophages and erythrocytes in culture.

CONCLUSION

We demonstrate that ectopic expression of PU.1 is sufficient to immortalize a hematopoietic progenitor with myeloid and erythroid differentiation potential in response to GM-CSF. These data highlight the importance of the level of PU.1 expression at critical stages of hematopoiesis.

Erythropoietin blockade inhibits the induction of tumor angiogenesis and progression

Background

The induction of tumor angiogenesis, a pathologic process critical for tumor progression, is mediated by multiple regulatory factors released by tumor and host cells. We investigated the role of the hematopoietic cytokine erythropoietin as an angiogenic factor that modulates tumor progression.

Methodology/Principal Findings

Fluorescently-labeled rodent mammary carcinoma cells were injected into dorsal skin-fold window chambers in mice, an angiogenesis model that allows direct, non-invasive, serial visualization and real-time assessment of tumor cells and neovascularization simultaneously using intravital microscopy and computerized image analysis during the initial stages of tumorigenesis. Erythropoietin or its antagonist proteins were co-injected with tumor cells into window chambers. In vivo growth of cells engineered to stably express a constitutively active erythropoietin receptor EPOR-R129C or the erythropoietin antagonist R103A-EPO were analyzed in window chambers and in the mammary fat pads of athymic nude mice. Co-injection of erythropoietin with tumor cells or expression of EPOR-R129C in tumor cells significantly stimulated tumor neovascularization and growth in window chambers. Co-injection of erythropoietin antagonist proteins (soluble EPOR or anti-EPO antibody) with tumor cells or stable expression of antagonist R103A-EPO protein secreted from tumor cells inhibited angiogenesis and impaired tumor growth. In orthotopic tumor xenograft studies, EPOR-R129C expression significantly promoted tumor growth associated with increased expression of Ki67 proliferation antigen, enhanced microvessel density, decreased tumor hypoxia, and increased phosphorylation of extracellular-regulated kinases ERK1/2. R103A-EPO antagonist expression in mammary carcinoma cells was associated with near-complete disruption of primary tumor formation in the mammary fat pad.

Conclusions/Significance

These data indicate that erythropoietin is an important angiogenic factor that regulates the induction of tumor cell-induced neovascularization and growth during the initial stages of tumorigenesis. The suppression of tumor angiogenesis and progression by erythropoietin blockade suggests that erythropoietin may constitute a potential target for the therapeutic modulation of angiogenesis in cancer.

Hematopoietic growth factors, signaling and the chronic myeloproliferative disorders

The chronic myeloproliferative diseases (CMDs) are a group of conditions characterized by unregulated blood cell production, that due either to excessive numbers of erythrocytes, leukocytes or platelets, or their defective function cause symptoms and signs of fatigue, headache, ruddy cyanosis, hemorrhage, abdominal distension, and the complications of vascular thrombosis. In the late 19th century Vaquez provided the first description of polycythemia vera (PV) and Hueck defined idiopathic myelofibrosis (IMF). In 1920, di Guglielmo established criteria for patients with essential thrombocythemia (ET). In 1951, Dameshek argued that these disorders, along with chronic myelogenous leukemia (CML) display many similar clinical and laboratory features [Dameshek W. Some speculations on the myeloproliferative syndromes. Blood 1951;6:372-5], and grouped them. In 2002, the World Health Organization expanded the definition of CMDs to also include chronic neutrophilic leukemia (CNL), chronic eosinophilic leukemia/hypereosinophilic syndrome (CEL/HES) and systemic mast cell disorder (SMCD) [Vardiman JW, Harris NL, Brunning RD. The World Health Organization (WHO) classification of the myeloid neoplasms. Blood 2002;100:2292-302]. While the molecular pathogenesis of CML is well known [Melo JV, Deininger MW. Biology of chronic myelogenous leukemia-signaling pathways of initiation and transformation. Hematol Oncol Clin North Am 2004;18:545-68], and the causes of CEL/HES and SMCD have been identified in about half of all cases [Gotlib J, Cools J, Malone III JM, Schrier SL, Gilliland DG, Coutre SE. The FIP1L1-PDGFRalpha fusion tyrosine kinase in hypereosinophilic syndrome and chronic eosinophilic leukemia: implications for diagnosis, classification, and management. Blood 2004; 103:2879-91; Valent P, Akin C, Sperr WR, Horny HP, Metcalfe DD. Mast cell proliferative disorders: current view on variants recognized by the World Health Organization. Hematol Oncol Clin North Am 2003; 17:1227-41], until very recently the etiologies of the three classically defined CMDs, PV, IMF and ET, were poorly understood. Each of these disorders is characterized by excessive hematopoiesis, a process usually dependent on one or more hematopoietic growth factors (HGFs). This review will focus on how our knowledge of the molecular mechanisms by which HGFs are produced, bind cell surface receptors and transduce survival and proliferative signals have provided the platform on which the multiple origins of CMDs can be understood and novel therapeutic interventions designed.

Characterization of purified intraembryonic hematopoietic stem cells as a tool to define their site of origin

Little is known about hematopoietic stem cell (HSC) development from mesoderm. To gain more information on the intraembryonic HSC site of origin, we purified multipotent hematopoietic progenitors from the aorta-gonads-mesonephros (AGM) of mice. This population, expressing c-Kit, AA4.1, CD31, and CD41, but not Flk1, and mainly negative for CD45, proved capable of long-term reconstitution in sublethally irradiated Rag2gammac(-/-) recipients. We assigned the expression of GATA-2, GATA-3, and lmo2 to AGM-HSC, whereas erythromyeloid progenitors express only GATA-2. This unique combination of surface markers and transcription factors could be allocated in the AGM to the intraaortic clusters and the subaortic patches underlying aortic endothelial cells. Taken together, those data indicate that embryonic HSCs (i) differ from their fetal liver and adult counterpart by the low expression of CD45, (ii) do not colocalize with aortic endothelial cells as previously thought, and (iii) are localized, at 10.5 days postcoitum, in the splanchnic mesoderm underlying aortic endothelial cells, within GATA-3(+)CD31(+) cell clusters.

Enforced granulocyte/macrophage colony-stimulating factor signals do not support lymphopoiesis, but instruct lymphoid to myelomonocytic lineage conversion

We evaluated the effects of ectopic granulocyte/macrophage colony-stimulating factor (GM-CSF) signals on hematopoietic commitment and differentiation. Lineage-restricted progenitors purified from mice with the ubiquitous transgenic human GM-CSF receptor (hGM-CSFR) were used for the analysis. In cultures with hGM-CSF alone, hGM-CSFR-expressing (hGM-CSFR+) granulocyte/monocyte progenitors (GMPs) and megakaryocyte/erythrocyte progenitors (MEPs) exclusively gave rise to granulocyte/monocyte (GM) and megakaryocyte/erythroid (MegE) colonies, respectively, providing formal proof that GM-CSF signals support the GM and MegE lineage differentiation without affecting the physiological myeloid fate. hGM-CSFR transgenic mice were crossed with mice deficient in interleukin (IL)-7, an essential cytokine for T and B cell development. Administration of hGM-CSF in these mice could not restore T or B lymphopoiesis, indicating that enforced GM-CSF signals cannot substitute for IL-7 to promote lymphopoiesis. Strikingly, >50% hGM-CSFR+ common lymphoid progenitors (CLPs) and >20% hGM-CSFR+ pro-T cells gave rise to granulocyte, monocyte, and/or myeloid dendritic cells, but not MegE lineage cells in the presence of hGM-CSF. Injection of hGM-CSF into mice transplanted with hGM-CSFR+ CLPs blocked their lymphoid differentiation, but induced development of GM cells in vivo. Thus, hGM-CSF transduces permissive signals for myeloerythroid differentiation, whereas it transmits potent instructive signals for the GM differentiation to CLPs and early T cell progenitors. These data suggest that a majority of CLPs and a fraction of pro-T cells possess plasticity for myelomonocytic differentiation that can be activated by ectopic GM-CSF signals, supporting the hypothesis that the down-regulation of GM-CSFR is a critical event in producing cells with a lymphoid-restricted lineage potential.

Erythropoietin receptors that signal through Stat5 or Stat3 support fetal liver and adult erythropoiesis: lack of specificity of stat signals during red blood cell development

Erythropoietin (Epo) is essential for formation of mature red blood cells (RBC). However, the function of Epo receptor (EpoR)-dependent signaling pathways in the regulation of erythropoiesis remains unclear. To determine whether specific Stat signals are required for RBC development, we changed the Stat signaling specificity of the EpoR. The wild-type EpoR activates only Stat5. Thus, we substituted the major Stat5 binding sites (residues 343 and 401) in the EpoR cytoplasmic region with the Stat3 binding/activation motif from gp130. We demonstrated that activated EpoRs containing a single substitution stimulate Stat5 and Stat3, whereas an EpoR with both substitutions stimulates Stat3 but not Stat5. We then determined the ability of these receptors to support fetal liver and adult erythropoiesis. Our results show that erythropoiesis is stimulated by EpoRs that activate Stat5, both Stat5 and Stat3, or Stat3 in place of Stat5. These findings demonstrate that the specificity of EpoR Stat signaling is not essential for RBC development.

BCR-ABL and v-SRC tyrosine kinase oncoproteins support normal erythroid development in erythropoietin receptor-deficient progenitor cells

Erythropoietin (Epo)-independent differentiation of erythroid progenitors is a major characteristic of myeloproliferative disorders, including chronic myeloid leukemia. Epo receptor (EpoR) signaling is crucial for normal erythroid development, as evidenced by the properties of Epo(-/-) and EpoR(-/-) mice, which contain a normal number of fetal liver erythroid progenitors but die in utero from a severe anemia attributable to the absence of red cell maturation. Here we show that two constitutively active cytoplasmic protein tyrosine kinases, P210(BCR-ABL) and v-SRC, can functionally replace the EpoR and support full proliferation, differentiation, and maturation of fetal liver erythroid progenitors from EpoR(-/-) mice. These protein tyrosine kinases can also partially complement the myeloid growth factors IL-3, IL-6, and Steel factor, which are normally required in addition to Epo for erythroid development. Additionally, BCR-ABL mutants that lack residues necessary for transformation of fibroblasts or bone marrow cells can fully support normal erythroid development. These results demonstrate that activated tyrosine kinase oncoproteins implicated in tumorigenesis and human leukemia can functionally complement for cytokine receptor signaling pathways to support normal erythropoiesis in EpoR-deficient cells. Moreover, terminal differentiation of erythroid cells requires generic signals provided by activated protein tyrosine kinases and does not require a specific signal unique to a cytokine receptor.

Activation of erythropoietin signaling by receptor dimerization

The hormone erythropoietin (Epo) is essential for red blood cell development. Epo binds a high affinity receptor on the surface of erythroid progenitor cells, stimulating receptor dimerization and activation of the intracellular signal transduction pathways that support erythroid cell survival, proliferation and differentiation. Biochemical and structural analysis of the erythropoietin receptor (EpoR) is revealing the molecular mechanisms of EpoR function, leading the way to the development of small molecule Epo mimetics. This review focuses on the role EpoR dimerization plays in receptor function.

Abnormalities of cytokine receptor signalling contributing to diseases of red blood cell production

The production of erythroid cells is a dynamic and exquisitely regulated process. The mature red cell is only the final phase of a complex but orderly series of genetic events that are initiated at the time a multipotent stem cell becomes committed to expressing the erythroid programme. Aberrations either in the intrinsic generation and/or amplification of functional erythroid cells or in the regulatory influences of microenvironment or cytokines form the basis for a number of blood diseases. In this review we focus upon abnormalities in red blood cell production and discuss how alterations in cytokine regulation of red blood cell production may contribute to these disease processes. We discuss clinical states in which blood red cell numbers are altered, including primary familial and congenital polycythaemia, the myeloproliferative disorder polycythaemia vera, erythroleukaemia, and Diamond-Blackfan anaemia. These disorders are briefly described and evidence supporting a potential role of specific cytokine receptor signalling defects as contributing to these phenotypes is discussed.

Oligomerization and scaffolding functions of the erythropoietin receptor cytoplasmic tail

Signal transduction by the erythropoietin receptor (EPOR) is activated by ligand-mediated receptor homodimerization. However, the relationship between extracellular and intracellular domain oligomerization remains poorly understood. To assess the requirements for dimerization of receptor cytoplasmic sequences for signaling, we overexpressed mutant EPORs in combination with wild-type (WT) EPOR to drive formation of heterodimeric (i.e. WT-mutant) receptor complexes. Dimerization of the membrane-proximal portion of the EPOR cytoplasmic region was found to be critical for the initiation of mitogenic signaling. However, dimerization of the entire EPOR cytoplasmic region was not required. To examine this process more closely, we generated chimeras between the intracellular and transmembrane portions of the EPOR and the extracellular domains of the interleukin-2 receptor beta and gammac chains. These chimeras allowed us to assess more precisely the signaling role of each receptor chain because only heterodimers of WT and mutant receptor chimeras form in the presence of interleukin-2. Coexpression studies demonstrated that a functional receptor complex requires the membrane-proximal region of each receptor subunit in the oligomer to permit activation of JAK2 but only one membrane-distal tail to activate STAT5 and to support cell proliferation. Thus, this study defines key relationships involved in the assembly and activation of the EPOR signal transduction complex which may be applicable to other homodimeric cytokine receptors.

Dysregulated hematopoiesis and a progressive neurological disorder induced by expression of an activated form of the human common beta chain in transgenic mice

Previously we described activating mutations of hbetac, the common signaling subunit of the receptors for the hematopoietic and inflammatory cytokines, GM-CSF, IL-3, and IL-5. The activated mutant, hbetacFIDelta, is able to confer growth factor-independent proliferation on the murine myeloid cell line FDC-P1, and on primary committed myeloid progenitors. We have used this activating mutation to study the effects of chronic cytokine receptor stimulation. Transgenic mice were produced carrying the hbetacFIDelta cDNA linked to the constitutive promoter derived from the phosphoglycerate kinase gene, PGK-1. Transgene expression was demonstrated in several tissues and functional activity of the mutant receptor was confirmed in hematopoietic tissues by the presence of granulocyte macrophage and macrophage colony-forming cells (CFU-GM and CFU-M) in the absence of added cytokines. All transgenic mice display a myeloproliferative disorder characterized by splenomegaly, erythrocytosis, and granulocytic and megakaryocytic hyperplasia. This disorder resembles the human disease polycythemia vera, suggesting that activating mutations in hbetac may play a role in the pathogenesis of this myeloproliferative disorder. In addition, these transgenic mice develop a sporadic, progressive neurological disease and display bilateral, symmetrical foci of necrosis in the white matter of brain stem associated with an accumulation of macrophages. Thus, chronic hbetac activation has the potential to contribute to pathological events in the central nervous system.

Absence of cytokine receptor-dependent specificity in red blood cell differentiation in vivo

Erythropoietin (EPO) is required for red blood cell development, but whether EPO-specific signals directly instruct erythroid differentiation is unknown. We used a dominant system in which constitutively active variants of the EPO receptor were introduced into erythroid progenitors in mice. Chimeric receptors were constructed by replacing the cytoplasmic tail of constitutively active variants of the EPO receptor with tails of diverse cytokine receptors. Receptors linked to granulocyte or platelet production supported complete erythroid development in vitro and in vivo, as did the growth hormone receptor, a nonhematopoietic receptor. Therefore, EPOR-specific signals are not required for terminal differentiation of erythrocytes. Furthermore, we found that cellular context can influence cytokine receptor signaling.

Redundant and Selective Roles for Erythropoietin Receptor Tyrosines in Erythropoiesis In Vivo

Cytokine receptors have been shown in cell culture systems to use phosphotyrosine residues as docking sites for certain signal transduction intermediates. Studies using various cellular backgrounds have yielded conflicting information about the importance of such residues. The present studies were undertaken to determine whether or not tyrosine residues within the erythropoietin receptor (EPOR) are essential for biologic activity during hematopoiesis in vivo. A variant of the EPOR was constructed that contains both a substitution (R129C) causing constitutive receptor activation as well as replacement of all eight cytoplasmic tyrosines by phenylalanines (cEPORYF). A comparison between animals exposed to recombinant retroviruses expressing cEPOR and cEPORYF showed that efficient red blood cell (RBC) development in vivo is dependent on the presence of tyrosine residues in the cytoplasmic domain of the EPOR. In addition, an inefficient EPOR tyrosine independent pathway supporting RBC development was detected. Tyrosine add-back mutants showed that multiple individual tyrosines have the capacity to restore full erythropoietic potential to the EPOR as determined in whole animals. The analysis of primary erythroid progenitors transduced with the various cEPOR tyrosine mutants and tyrosine add-backs showed that only tyrosine 343 (Y1) and tyrosine 479 (Y8) were capable of supporting immature burst-forming unit–erythroid progenitor development. Thus, this receptor is characterized by striking functional redundancy of tyrosines in a biologically relevant context. However, selective tyrosine residues may be uniquely important for early signals supporting erythroid development.

Redundant and Selective Roles for Erythropoietin Receptor Tyrosines in Erythropoiesis In Vivo

Cytokine receptors have been shown in cell culture systems to use phosphotyrosine residues as docking sites for certain signal transduction intermediates. Studies using various cellular backgrounds have yielded conflicting information about the importance of such residues. The present studies were undertaken to determine whether or not tyrosine residues within the erythropoietin receptor (EPOR) are essential for biologic activity during hematopoiesis in vivo. A variant of the EPOR was constructed that contains both a substitution (R129C) causing constitutive receptor activation as well as replacement of all eight cytoplasmic tyrosines by phenylalanines (cEPORYF). A comparison between animals exposed to recombinant retroviruses expressing cEPOR and cEPORYF showed that efficient red blood cell (RBC) development in vivo is dependent on the presence of tyrosine residues in the cytoplasmic domain of the EPOR. In addition, an inefficient EPOR tyrosine independent pathway supporting RBC development was detected. Tyrosine add-back mutants showed that multiple individual tyrosines have the capacity to restore full erythropoietic potential to the EPOR as determined in whole animals. The analysis of primary erythroid progenitors transduced with the various cEPOR tyrosine mutants and tyrosine add-backs showed that only tyrosine 343 (Y1) and tyrosine 479 (Y8) were capable of supporting immature burst-forming unit–erythroid progenitor development. Thus, this receptor is characterized by striking functional redundancy of tyrosines in a biologically relevant context. However, selective tyrosine residues may be uniquely important for early signals supporting erythroid development.

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF)-dependent in vitro and in vivo proliferation and differentiation of all hematopoietic progenitor cells in hGM-CSF receptor transgenic mice

To examine the relation between receptor expression and differentiation of hematopoietic cells, we produced transgenic mice that constitutively expressed the human granulocyte-macrophage colony stimulating factor (hGM-CSF) receptor at almost all stages of hematopoietic cell development. The high-affinity GM-CSF receptor is species specific, allowing analysis of the specific effects of hGM-CSF in our mouse model. Proliferation and differentiation of hematopoietic progenitor cells from transgenic mice were analyzed by means of methylcellulose colony-forming assay and in vivo treatment with hGM-CSF, respectively. We found that hGM-CSF supported various types of colonies, including granulocyte-macrophage, mast cell, megakaryocyte, blast cell, and mixed hematopoietic colonies, whereas mouse GM-CSF supported only granulocyte-macrophage colonies. In addition, hGM-CSF generated erythrocyte colonies in the absence of erythropoietin. Furthermore, in vivo administration of hGM-CSF to transgenic mice resulted in a dose-dependent increase in reticulocytes and white blood cells in the peripheral blood. The spleens of the mice showed gross enlargement, mainly caused by an increase of erythroid cells and their progenitors. Taken together, these results indicate that hGM-CSF receptor-mediated signals can support the growth of cells of all hematopoietic cell lineages if this receptor is present on the cell surface. This implies that the differentiation of hematopoietic progenitor cells is not determined by exogenous cytokine stimulation (instruction model) but by an intrinsic cell program in which cytokines simply select cells that express the appropriate receptor (stochastic model).

Cooperation of Spi-1/PU.1 with an activated erythropoietin receptor inhibits apoptosis and Epo-dependent differentiation in primary erythroblasts and induces their Kit ligand-dependent proliferation

Spi-1/PU.1 is a myeloid- and B-cell specific transcription factor which is also involved in Friend virus-induced murine erythroleukemia. The pre-leukemic phase of Friend erythroleukemia results from activation of the erythropoietin receptor (EpoR) by the spleen focus forming virus (SFFV) envelope glycoprotein, followed by the emergence of leukemic clones characterized by overexpression of Spi-1 and mutation of the p53 tumor suppressor gene. We developed a heterologous system to analyze the contribution of these alterations to the induction of primary erythroblast transformation. Avian erythroblasts expressing the activated mouse EpoR(R129C) differentiated into erythrocytes in response to hEpo. Expression of Spi-1 in these cells inhibited this ability to differentiate and rescued the cells from the apoptotic cell death program normally induced upon hEpo withdrawal. Although devoid of any effect by itself, a mutant p53 cooperated with Spi-1 and EpoR(R129C) to reinforce both phenotypes. Analysis of erythroblasts co-expressing Spi-1 and the wild-type mouse EpoR showed that differentiation arrest and inhibition of apoptosis depended on specific cooperation between Spi-1 and EpoR(R129C). This cooperation was also required to induce the sustained proliferation of differentiation-blocked erythroblasts in response to ligand activation of the endogenous tyrosine kinase receptor c-Kit. These results show that Spi-1/PU.1 requires signals emanating from specific cytokine and growth factor receptors to affect the survival, proliferation and differentiation control of primary erythroblasts. They also suggest that the function of Spi-1/PU.1 in the late phase of Friend leukemia requires specific signaling from the gp55-modified EpoR generated during the early phase of the disease.

Hematopoietic and Lymphopoietic Responses in Human Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF ) Receptor Transgenic Mice Injected With Human GM-CSF

Using a clonal assay of bone marrow (BM) cells from transgenic mice (Tg-mice) expressing the human granulocyte-macrophage colony-stimulating factor receptor (hGM-CSFR), we found in earlier studies that hGM-CSF alone supported the development not only of granulocyte-macrophage colonies, but also of erythrocytes, megakaryocytes, mast cells, blast cells, and mixed hematopoietic colonies. In this report, we evaluated the in vivo effects of hGM-CSF on hematopoietic and lymphopoietic responses in the hGM-CSFR Tg-mice. Administration of this factor to Tg-mice resulted in dose-dependent increases in numbers of reticulocytes and white blood cells (WBCs) in the peripheral blood. Morphological analysis of WBCs showed that the numbers of all types of the cell, including neutrophils, eosinophils, monocytes, and lymphocytes increased; the most remarkable being in lymphocytes that contained a number of large granular lymphocytes (LGLs) in addition to mature T and B cells. However, total cellularity of the BM of the Tg-mice decreased in a dose-dependent manner when hGM-CSF was injected. In sharp contrast to the BM, spleens of the Tg-mice were grossly enlarged. Although all types of blood cells and hematopoietic progenitors increased in the spleen, erythroid cells and their progenitors showed the most significant increase. Increased numbers of megakaryocytes and LGLs were also observed in spleen and liver of the treated Tg-mice. Flow cytometric analysis showed that LGLs expanded in Tg-mice expressed Mac-1+CD3−NK1.1+. The thymus of Tg-mice treated with hGM-CSF exhibited a dose-dependent shrinkage and a remarkable decrease in CD4+CD8+ cells. Thus, hGM-CSF stimulated not only myelopoiesis but also erythropoiesis and megakaryopoiesis of hGM-CSFR Tg-mice in vivo, in accordance with our reported in vitro findings. In addition, hGM-CSF affected the development of lymphoid cells, including natural killer cells of these Tg-mice.

Hematopoietic and Lymphopoietic Responses in Human Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF ) Receptor Transgenic Mice Injected With Human GM-CSF

Using a clonal assay of bone marrow (BM) cells from transgenic mice (Tg-mice) expressing the human granulocyte-macrophage colony-stimulating factor receptor (hGM-CSFR), we found in earlier studies that hGM-CSF alone supported the development not only of granulocyte-macrophage colonies, but also of erythrocytes, megakaryocytes, mast cells, blast cells, and mixed hematopoietic colonies. In this report, we evaluated the in vivo effects of hGM-CSF on hematopoietic and lymphopoietic responses in the hGM-CSFR Tg-mice. Administration of this factor to Tg-mice resulted in dose-dependent increases in numbers of reticulocytes and white blood cells (WBCs) in the peripheral blood. Morphological analysis of WBCs showed that the numbers of all types of the cell, including neutrophils, eosinophils, monocytes, and lymphocytes increased; the most remarkable being in lymphocytes that contained a number of large granular lymphocytes (LGLs) in addition to mature T and B cells. However, total cellularity of the BM of the Tg-mice decreased in a dose-dependent manner when hGM-CSF was injected. In sharp contrast to the BM, spleens of the Tg-mice were grossly enlarged. Although all types of blood cells and hematopoietic progenitors increased in the spleen, erythroid cells and their progenitors showed the most significant increase. Increased numbers of megakaryocytes and LGLs were also observed in spleen and liver of the treated Tg-mice. Flow cytometric analysis showed that LGLs expanded in Tg-mice expressed Mac-1+CD3−NK1.1+. The thymus of Tg-mice treated with hGM-CSF exhibited a dose-dependent shrinkage and a remarkable decrease in CD4+CD8+ cells. Thus, hGM-CSF stimulated not only myelopoiesis but also erythropoiesis and megakaryopoiesis of hGM-CSFR Tg-mice in vivo, in accordance with our reported in vitro findings. In addition, hGM-CSF affected the development of lymphoid cells, including natural killer cells of these Tg-mice.

Selective Expression of mRNA Coding for the Truncated Form of Erythropoietin Receptor in Hematopoietic Cells and Its Decrease in Patients With Polycythemia Vera

The mRNA encoding full-length erythropoietin (EPO) receptor (EPOR-F ) comprises exons I through VIII. Another membrane-bound EPOR (EPOR-T) isoform has a truncated cytoplasmic region and is encoded by the mRNA containing unspliced intron VII (EPOR-T mRNA). EPOR-T is believed to have a dominantly negative function against EPOR-F. We show that EPOR-T mRNA is markedly decreased in the blood cells of patients with polycythemia vera (PV). We also show that EPOR-T mRNA is not detected in erythroid/megakaryocytic leukemia cell lines, but is expressed in nonerythroid/nonmegakaryocytic lines, suggesting the presence of a cell type–specific system by which intron VII of the EPOR transcript is spliced. Deregulation of this splicing system in early hematopoietic progenitors possibly explains the profound decrease in EPOR-T mRNA and consequent pathophysiology of PV.

Selective Expression of mRNA Coding for the Truncated Form of Erythropoietin Receptor in Hematopoietic Cells and Its Decrease in Patients With Polycythemia Vera

The mRNA encoding full-length erythropoietin (EPO) receptor (EPOR-F ) comprises exons I through VIII. Another membrane-bound EPOR (EPOR-T) isoform has a truncated cytoplasmic region and is encoded by the mRNA containing unspliced intron VII (EPOR-T mRNA). EPOR-T is believed to have a dominantly negative function against EPOR-F. We show that EPOR-T mRNA is markedly decreased in the blood cells of patients with polycythemia vera (PV). We also show that EPOR-T mRNA is not detected in erythroid/megakaryocytic leukemia cell lines, but is expressed in nonerythroid/nonmegakaryocytic lines, suggesting the presence of a cell type–specific system by which intron VII of the EPOR transcript is spliced. Deregulation of this splicing system in early hematopoietic progenitors possibly explains the profound decrease in EPOR-T mRNA and consequent pathophysiology of PV.

The prolactin receptor and severely truncated erythropoietin receptors support differentiation of erythroid progenitors

Activation of the erythropoietin receptor is essential for the survival, proliferation, and differentiation of erythroid progenitors. To understand the role of erythropoietin receptor (EpoR) activation in erythroid differentiation, we infected primary erythroid progenitors with high-titer retrovirus encoding the non-hematopoietic prolactin receptor. The infected progenitors responded to prolactin in the absence of Epo by generating fully differentiated erythroid colonies. Therefore, differentiation of erythroid progenitors does not require an intracellular signal generated uniquely by the EpoR; the EpoR does not have an instructive role in erythroid differentiation. We also infected primary erythroid progenitors with retrovirus encoding chimeric receptors containing the extracellular domain of PrlR and the intracellular domain of either the wild-type or truncated EpoRs. A chimeric receptor containing only the membrane-proximal 136 amino acids of the EpoR cytoplasmic domain efficiently supported prolactin-dependent differentiation of erythroid progenitors. Substitution of the single cytoplasmic domain tyrosine in this receptor with phenylalanine (Y343F) eliminated its ability to support differentiation. The minimal EpoR cytoplasmic domain required for erythroid differentiation is therefore the same as that previously reported to be sufficient to support cell proliferation (D'Andrea, A. D., Yoshimura, A., Youssoufian, H., Zon, L. I., Koo, J. W., and Lodish, H. F. (1991) Mol. Cell. Biol. 11, 1980-1987; Miura, O., D'Andrea, A. D., Kabat, D., and Ihle, J. N. (1991) Mol. Cell. Biol. 11, 4895-4902; He, T.-C., Jiang, N., Zhuang, H., Quelle, D. E., and Wojchowski, D. M. (1994) J. Biol. Chem. 269, 18291-18294).

Cell surface organization of the erythropoietin receptor complex differs depending on its mode of activation

During erythroid development erythropoietin (EPO) binds specifically to a receptor primarily present on committed erythroid progenitors, stimulating mitogenic, survival, and differentiative growth response pathways. Other modes of erythropoietin receptor (EPO-R) activation, such as interaction with the env gene Friend virus envelope glycoprotein (F-gp55) of spleen focus-forming virus or specific mutations in the extracellular domain of the EPO-R, give rise to pathological consequences, in vivo and EPO-independent proliferation and differentiation of cultured cells. Activating extracellular receptor mutations result in covalently linked receptor homodimers. These observations and others have led to the proposal that EPO activates the EPO-R by inducing dimer formation on the cell surface. It has been assumed that F-gp55 also induces dimer formation of the EPO-R; however, clear evidence of this is lacking. In addition, EPO and F-gp55 stimulation of the EPO-R elicit different biological responses. To probe whether the cell surface EPO-R is structurally different with these activators, we contrasted the cell surface EPO-R complex formed following receptor activation by EPO, F-gp55, and mutations in the extracellular domain of the receptor. Our results indicate that cell surface forms of activated EPO-R differ, as judged by their differential association with F-gp55 and pattern of associated cell surface proteins. Interestingly, we find that the env gene of an anemic strain of Friend virus, Rauscher virus envelope glycoprotein, does not interact with the EPO-R at the cell surface. Thus, the mode of Rauscher virus envelope glycoprotein-induced erythroblastosis may be distinct from F-gp55-induced erythroblastosis and possibly not involve the EPO-R.

Homodimerization of Erythropoietin Receptor by a Bivalent Monoclonal Antibody Triggers Cell Proliferation and Differentiation of Erythroid Precursors

Erythropoietin (EPO) stimulates proliferation and differentiation of erythroid progenitor cells. Several lines of evidence indicate that the most likely mechanism of EPO receptor (EPO-R) activation by EPO is homodimerization of the receptor on the surface of erythrocyte precursors. Therefore, we argued that it should be possible to raise EPO-R monoclonal antibodies (MoAbs) that would activate the receptor by dimerization and thus mimic EPO action. We have identified such an agonist MoAb (MoAb34) directed against the extracellular EPO binding domain of the EPO-R. This bivalent IgG antibody triggers the proliferation of EPO-dependent cell lines and induces differentiation of erythroid precursors in vitro. In contrast, the monovalent Fab fragment, which cannot dimerize the receptor, is completely inactive. The mechanism of receptor activation by homodimerization implies that at high ligand concentrations the formation of 1:1 receptor/ligand complexes is favored over 2:1 complexes, thereby turning the ligand agonist into an antagonist. Thus, EPO and MoAb34 should self-antagonize at high concentrations in both cell proliferation and differentiation assays. Our data indeed demonstrate that EPO and MoAb34 antagonize ligand-dependent cell proliferation with IC50 values of approximately 20 and 2 μmol/L, respectively. Erythroid colony formation (BFUe) is inhibited at MoAb34 concentrations above 1 μmol/L. Furthermore, we analyzed the MoAb34:EPO-R interaction using a mathematic model describing antibody-mediated receptor dimerization. The data for proliferation and differentiation activity were consistent with the receptor dimer formation on the cell surface predicted by the model.

Cytokine receptor signal transduction and the control of hematopoietic cell development

The cytokine receptor superfamily is characterized by structural motifs in the exoplasmic domain and by the absence of catalytic activity in the cytosolic segment. Activated by ligand-triggered multimerization, these receptors in turn activate a number of cytosolic signal transduction proteins, including protein tyrosine kinases and phosphatases, and affect an array of cellular functions that include proliferation and differentiation. Molecular study of these receptors is revealing the roles they play in the control of normal hematopoiesis and in the development of disease.

Signaling by chimeric erythropoietin-TGF-beta receptors: homodimerization of the cytoplasmic domain of the type I TGF-beta receptor and heterodimerization with the type II receptor are both required for intracellular signal transduction

Transforming growth factor-beta (TGF-beta) affects multiple cellular functions through the type I and type II receptor Ser/Thr kinases (TbetaRI and TbetaRII). Analysis of TGF-beta signaling pathways has been hampered by the lack of cell lines in which both TbetaRI and TbetaRII are deleted, and by the inability to study signal transduction by TbetaRI independently of TbetaRII since TbetaRI does not bind TGF-beta directly. To overcome these problems, we constructed and expressed chimeric receptors with the extracellular domain of the erythropoietin receptor (EpoR) and the cytoplasmic domains of TbetaRI or TbetaRII. When expressed in Ba/F3 cells, which do not express EpoR, Epo induces the formation of a heteromeric complex between cell surface EpoR-TbetaRI and EpoR-TbetaRII chimeras. Neither the EpoR-TbetaRI nor the EpoR-TbetaRII chimera interacts with endogenous TGF-beta receptors. Ba/F3 cells expressing both EpoR-TbetaRI and EpoR-TbetaRII chimeras, but not EpoR-TbetaRI or EpoR-TbetaRII alone, undergo Epo-induced growth arrest. When expressed in Ba/F3 cells in the absence of the EpoR-TbetaRII chimera, EpoR-TbetaRI(T204D), a chimeric receptor with a point mutation in the GS domain of TbetaRI that is autophosphorylated constitutively, triggers growth inhibition in response to Epo. Thus, both homo- and heterodimerization of the cytoplasmic domain of the type I TGF-beta receptor are required for intracellular signal transduction leading to inhibition of cell proliferation. These chimeric receptors provide a unique system to study the function and signal transduction of individual TGF-beta receptor subunits independently of endogenous TGF-beta receptors.

Constitutive activation of a variant of the env-mpl oncogene product by disulfide-linked homodimerization

The myeloproliferative leukemia retrovirus (MPLV) has the v-mpl cellular sequences transduced in frame with the deleted and rearranged Friend murine leukemia virus env gene. The resulting env-mpl fusion oncogene is responsible for an acute myeloproliferative disorder induced in mice by MPLV. v-mpl is a truncated form of the c-mpl gene which encodes the receptor for thrombopoietin. We investigated the contribution of the Env-Mpl extracellular domain in the constitutive activation of this truncated cytokine receptor and found that the rearrangement of the env sequences in the env-mpl fusion gene was not required for oncogenicity. A pathogenic variant, DEL3MPLV, was generated, which differs from MPLV by the deletions of 22 amino acids of the Env signal peptide, all of the mature Env sequences, and 18 N-terminal amino acids of the v-Mpl extracellular domain. The resulting del3-mpl oncogene product conserves in its extracellular region the first 12 amino acids of the Env signal sequence including a cysteine residue, and 25 amino acids of the v-Mpl. We show here that a mutation converting this cysteine to a glycine completely abolishes del3-mpl oncogenicity and that the del3-mpl oncogene product is constitutively activated by disulfide-linked homodimerization.

Expression of an activated erythropoietin or a colony-stimulating factor 1 receptor by pluripotent progenitors enhances colony formation but does not induce differentiation

Whether the presence of specific receptors on the surface of developing cells is the cause or consequence of lineage restriction is not known. If activation of specific receptors is the driving event in differentiation, the premature expression of specific receptors would promote differentiation along that pathway. In this study pluripotent progenitors, obtained from blast cell colonies (pooled or individual) of 5-fluorouracil-treated mice, were infected with retroviral vectors containing either an activated receptor for erythropoietin (EPO), an erythroid progenitor growth factor, or the receptor for colony-stimulating factor 1 (CSF-1), a macrophage growth factor. These receptors exhibit expression patterns restricted to committed progenitors. The developmental potential of infected pluripotent progenitors was not changed, although they expressed the exogenous genes, suggesting that in these cells activation of lineage-specific receptors does not induce differentiation. Acquisition of a constitutively activated EPO receptor allowed erythroid development in mixed colonies in the absence of EPO, as expected. Infection of progenitors with a virus containing the CSF-1 receptor promoted the development of granulocyte/macrophage (GM) colonies but did not alter the differentiation potential of either colony-forming unit (CFU)-GM or CFU-mix.

Murine pluripotent hematopoietic progenitors constitutively expressing a normal erythropoietin receptor proliferate in response to erythropoietin without preferential erythroid cell differentiation

Erythropoietin (EPO) is a prime regulator of the growth and differentiation of erythroid blood cells. The EPO receptor (EPO-R) is expressed in late erythroid progenitors (mature BFU-E and CFU-E), and EPO induces proliferation and differentiation of these cells. By introducing, with a retroviral vector, a normal EPO-R cDNA into murine adult bone marrow cells, we showed that EPO is also able to induce proliferation in pluripotent progenitor cells. After 7 days of coculture with virus-producing cells, bone marrow cells were plated in methylcellulose culture in the presence of EPO, interleukin-3, or Steel factor alone or in combination. In the presence of EPO alone, EPO-R virus-infected bone marrow cells gave rise to mixed colonies comprising erythrocytes, granulocytes, macrophages and megakaryocytes. The addition of interleukin-3 or Steel factor to methylcellulose cultures containing EPO did not significantly modify the number of mixed colonies. The cells which generate these mixed colonies have a high proliferative potential as shown by the size and the ability of the mixed colonies to give rise to secondary colonies. Thus, it appears that EPO has the same effect on EPO-R-expressing multipotent cell proliferation as would a combination of several growth factors. Finally, our results demonstrate that inducing pluripotent progenitor cells to proliferate via the EPO signaling pathway has no major influence on their commitment.

Murine pluripotent hematopoietic progenitors constitutively expressing a normal erythropoietin receptor proliferate in response to erythropoietin without preferential erythroid cell differentiation

Erythropoietin (EPO) is a prime regulator of the growth and differentiation of erythroid blood cells. The EPO receptor (EPO-R) is expressed in late erythroid progenitors (mature BFU-E and CFU-E), and EPO induces proliferation and differentiation of these cells. By introducing, with a retroviral vector, a normal EPO-R cDNA into murine adult bone marrow cells, we showed that EPO is also able to induce proliferation in pluripotent progenitor cells. After 7 days of coculture with virus-producing cells, bone marrow cells were plated in methylcellulose culture in the presence of EPO, interleukin-3, or Steel factor alone or in combination. In the presence of EPO alone, EPO-R virus-infected bone marrow cells gave rise to mixed colonies comprising erythrocytes, granulocytes, macrophages and megakaryocytes. The addition of interleukin-3 or Steel factor to methylcellulose cultures containing EPO did not significantly modify the number of mixed colonies. The cells which generate these mixed colonies have a high proliferative potential as shown by the size and the ability of the mixed colonies to give rise to secondary colonies. Thus, it appears that EPO has the same effect on EPO-R-expressing multipotent cell proliferation as would a combination of several growth factors. Finally, our results demonstrate that inducing pluripotent progenitor cells to proliferate via the EPO signaling pathway has no major influence on their commitment.

Activation and inhibition of erythropoietin receptor function: role of receptor dimerization

Members of the cytokine receptor superfamily have structurally similar extracellular ligand-binding domains yet diverse cytoplasmic regions lacking any obvious catalytic domains. Many of these receptors form ligand-induced oligomers which are likely to participate in transmembrane signaling. A constitutively active (factor-independent) mutant of the erythropoietin receptor (EPO-R), R129C in the exoplasmic domain, forms disulfide-linked homodimers, suggesting that the wild-type EPO-R is activated by ligand-induced homodimerization. Here, we have taken two approaches to probe the role EPO-R dimerization plays in signal transduction. First, on the basis of the crystal structure of the ligand-bound, homodimeric growth hormone receptor (GH-R) and sequence alignment between the GH-R and EPO-R, we identified residues of the EPO-R which may be involved in intersubunit contacts in an EPO-R homodimer. Residue 129 of the EPO-R corresponds to a residue localized to the GH-R dimer interface region. Alanine or cysteine substitutions were introduced at four other residues of the EPO-R predicted to be in the dimer interface region. Substitution of residue E-132 or E-133 with cysteine renders the EPO-R constitutively active. Like the arginine-to-cysteine mutation at position 129 in the exoplasmic domain (R129C), E132C and E133C form disulfide-linked homodimers, suggesting that constitutive activity is due to covalent dimerization. In the second approach, we have coexpressed the wild-type EPO-R with inactive mutants of the receptor missing all or part of the cytosolic domain. These truncated receptors have a dominant inhibitory effect on the proliferative action of the wild-type receptor. Taken together, these results strengthen the hypothesis that an initial step in EPO- and EPO-R-mediated signal transduction is ligand-induced receptor dimerization.

Activation and inhibition of erythropoietin receptor function: role of receptor dimerization

Members of the cytokine receptor superfamily have structurally similar extracellular ligand-binding domains yet diverse cytoplasmic regions lacking any obvious catalytic domains. Many of these receptors form ligand-induced oligomers which are likely to participate in transmembrane signaling. A constitutively active (factor-independent) mutant of the erythropoietin receptor (EPO-R), R129C in the exoplasmic domain, forms disulfide-linked homodimers, suggesting that the wild-type EPO-R is activated by ligand-induced homodimerization. Here, we have taken two approaches to probe the role EPO-R dimerization plays in signal transduction. First, on the basis of the crystal structure of the ligand-bound, homodimeric growth hormone receptor (GH-R) and sequence alignment between the GH-R and EPO-R, we identified residues of the EPO-R which may be involved in intersubunit contacts in an EPO-R homodimer. Residue 129 of the EPO-R corresponds to a residue localized to the GH-R dimer interface region. Alanine or cysteine substitutions were introduced at four other residues of the EPO-R predicted to be in the dimer interface region. Substitution of residue E-132 or E-133 with cysteine renders the EPO-R constitutively active. Like the arginine-to-cysteine mutation at position 129 in the exoplasmic domain (R129C), E132C and E133C form disulfide-linked homodimers, suggesting that constitutive activity is due to covalent dimerization. In the second approach, we have coexpressed the wild-type EPO-R with inactive mutants of the receptor missing all or part of the cytosolic domain. These truncated receptors have a dominant inhibitory effect on the proliferative action of the wild-type receptor. Taken together, these results strengthen the hypothesis that an initial step in EPO- and EPO-R-mediated signal transduction is ligand-induced receptor dimerization.

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.

A constitutively activated erythropoietin receptor stimulates proliferation and contributes to transformation of multipotent, committed nonerythroid and erythroid progenitor cells

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin receptor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induces transient thrombocytosis and erythrocytosis in infected mice. Clonogenic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate proliferation of committed megakaryocytic and erythroid progenitors as well as nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R-infected mice, eight multiphenotypic immortal cell lines were isolated and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contained a mutant form of the p53 gene. However, in contrast to infection by SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led to increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of nonerythroid as well as very immature erythroid progenitor cells. The sites of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.

High efficiency retroviral mediated gene transduction into single isolated immature and replatable CD34(3+) hematopoietic stem/progenitor cells from human umbilical cord blood

Umbilical cord blood is rich in hematopoietic stem and progenitor cells and has recently been used successfully in the clinic as an alternative source of engrafting and marrow repopulating cells. With the likelihood that cord blood stem/progenitor cells will be used for gene therapy to correct genetic disorders, we evaluated if a TK-neo gene could be directly transduced in a stable manner into single isolated subsets of purified immature hematopoietic cells that demonstrate self-renewed ability as estimated by colony replating capacity. Sorted CD34(3+) cells from cord blood were prestimulated with erythropoietin (Epo), steel factor (SLF), interleukin (IL)-3, and granulocyte-macrophage colony stimulating factor (GM-CSF) and transduced with the gene in two ways. CD34(3+) cells were incubated with retroviral-containing supernatant from TK-neo vector-producing cells, washed, and plated directly or resorted as CD34(3+) cells into single wells containing a single cell or 10 cells. Alternatively, CD34(3+) cells were sorted as a single cell/well and then incubated with viral supernatant. These cells were cultured with Epo, SLF, IL-3, and GM-CSF +/- G418. The TK-neo gene was introduced at very high efficiency into low numbers of or isolated single purified CD34(3+) immature hematopoietic cells without stromal cells as a source of virus or accessory cells. Proviral integration was detected in primary G418-resistant(R) colonies derived from single immature hematopoietic cells, and in cells from replated colonies derived from G418R-colony forming unit-granulocyte erythroid macrophage megakaryocyte (CFU-GEMM) and -high proliferative potential colony forming cells (HPP-CFC). This demonstrates stable expression of the transduced gene into single purified stem/progenitor cells with replating capacity, results that should be applicable for future clinical studies that may utilize selected subsets of stem/progenitor cells for gene therapy.

Signal transduction in the erythropoietin receptor system

Developing erythroid cells require the glycoprotein hormone, erythropoietin (EPO) as an activator of the rapid proliferation of early proerythroblasts (colony forming units-erythroid [CFU-e]), and subsequently as an activator of late erythroid gene expression. Activation of these growth and differentiation events proceeds from the binding of EPO at its transmembrane receptor (Class I cytokine receptor), to the engagement of a complex set of signaling pathways. Studies of reconstituted activities of the cloned EPO receptor in transfected hematopoietic cell lines have served well in identifying receptor domains and downstream mediators involved in proliferative signaling. Extracellular domains have been defined which contribute to ligand binding, receptor processing and transport, and possible dimerization. Cytosolic regions have been delineated which mediate induced mitogenesis, early gene transcription, activated protein tyrosine phosphorylation, down modulation of EPO- and granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced proliferation, and direct association with PI3- and JAK-2 kinases. These newly defined properties begin to align the EPO receptor mechanistically with growth factor receptors (GFR) which encode, or likewise associate with, regulated protein tyrosine kinases including the Class II cytokine receptors for interferons alpha/beta and gamma. An improved understanding of factors which mediate EPO-induced late erythroid gene activation also is emerging. These factors and pathways may be distinct from those associated with EPO-induced proliferation and may involve induced increases in cellular Ca++, cAMP and arachidonic acid, as well as the modulation of GATA-1, and/or SCL. Attributes of model systems used in studies of the role of EPO in late erythroid differentiation also are considered.