Generation of committed erythroid BFU-E and CFU-E progenitors does not require erythropoietin or the erythropoietin receptor

Phosphatidylinositol 4-phosphatase type II is an erythropoietin-responsive gene

The erythroleukemia developed by spi-1/PU.1 transgenic mice is a multistep process. At disease onset, preleukemic cells are arrested in differentiation at the proerythroblast stage (HS1 stage) and their survival and growth are under the tight control of erythropoietin (Epo). During disease progression, malignant proerythroblasts characterized by Epo autonomous growth and in vivo tumorigenicity can be isolated (HS2 stage). During analysis of transcriptional profiling representive of discrete stages of leukemic progression, we found that the phosphatidylinositol 4-phosphatase type II gene was turned off in malignant cells. PI-4-phosphatase II is an enzyme that hydrolyses the 4-phosphate position of phosphatidylinositol-3-4-bisphosphate (PtdIns(3,4)P(2)) to form PtdIns(3)P. Using malignant cells engineered to stably express PI-4-phosphatase II, we showed that PI-4-phosphatase II reduced Akt activation level. Moreover, stimulation of malignant cells with Epo-induced PI-4-phosphatase II transcription pointing this gene as an Epo-responsive gene. This study provides first insight for a physiological role of PI-4-phosphatase II in the proerythroblast by controlling Epo responsiveness through a negative regulation of the PI3K/Akt pathway.

Phosphatidylserine-dependent engulfment by macrophages of nuclei from erythroid precursor cells

Definitive erythropoiesis usually occurs in the bone marrow or fetal liver, where erythroblasts are associated with a central macrophage in anatomical units called 'blood islands'. Late in erythropoiesis, nuclei are expelled from the erythroid precursor cells and engulfed by the macrophages in the blood island. Here we show that the nuclei are engulfed by macrophages only after they are disconnected from reticulocytes, and that phosphatidylserine, which is often used as an 'eat me' signal for apoptotic cells, is also used for the engulfment of nuclei expelled from erythroblasts. We investigated the mechanism behind the enucleation and engulfment processes by isolating late-stage erythroblasts from the spleens of phlebotomized mice. When these erythroblasts were cultured, the nuclei protruded spontaneously from the erythroblasts. A weak physical force could disconnect the nuclei from the reticulocytes. The released nuclei contained an undetectable level of ATP, and quickly exposed phosphatidylserine on their surface. Fetal liver macrophages efficiently engulfed the nuclei; masking the phosphatidylserine on the nuclei with the dominant-negative form of milk-fat-globule EGF8 (MFG-E8) prevented this engulfment.

Erythropoietin stimulates phosphorylation and activation of GATA-1 via the PI3-kinase/AKT signaling pathway

Erythropoietin (Epo) stimulation of its receptor's downstream signaling pathways and optimum function of GATA-1 transcription factor are both essential for normal erythroid cell development. Epo-receptor (EpoR) signaling and GATA-1 regulate proliferation, survival, differentiation, and maturation of erythroid cells. Whether any signal that is generated by EpoR targets GATA-1 or affects GATA-1 transcriptional activity is not known. Here, we demonstrate that stimulation of EpoR results in phosphorylation of GATA-1 at serine 310 (S310) in primary fetal liver erythroid progenitors and in cultured erythroid cells. We show that phosphorylation of GATA-1 is important for Epo-induced maturation of fetal liver erythroid progenitor cells. The PI3-kinase/AKT signaling pathway is identified as a mediator of Epo-induced phosphorylation of GATA-1. AKT serine threonine kinase phosphorylates GATA-1S310 in vitro and in erythroid cells and enhances GATA-1 transcriptional activity. These data demonstrate that EpoR signaling phosphorylates GATA-1 and modulates its activity via the PI3-kinase/AKT signaling pathway.

Identification of K-ras as the major regulator for cytokine-dependent Akt activation in erythroid progenitors in vivo

Despite intensive investigation, controversial results have been obtained concerning the precise signaling pathway(s) regulated by K-ras in different cell types. We show that in primary fetal liver erythroid progenitors, erythropoietin activates all three Ras isoforms, but preferentially N- and K-ras. In K-ras(-/-) fetal liver cells (FLC), erythropoietin- or stem cell factor-dependent Akt activation is greatly reduced, whereas other pathways including Stat5 and p44/p42 MAP kinase are activated normally. We further studied the effects of reduced cytokine-dependent Akt activation in erythroid differentiation. We find that freshly isolated K-ras(-/-) FLC show an approximately 7-fold increase of apoptosis and delayed erythroid differentiation, but only at the stage of erythroid progenitors and very early erythroblasts. When K-ras(-/-) erythroid progenitors are cultured in vitro, there is a significant delay in erythroid differentiation but little increase in apoptosis. Furthermore, we show that partial pharmacologic inhibition of the phosphatidylinositol 3-kinase/Akt pathway in wild-type erythroid progenitors leads to a delay in erythroid differentiation similar to that observed in K-ras(-/-) FLC. Taken together, our data identify K-ras as the major regulator for cytokine-dependent Akt activation, which is important for erythroid differentiation in vivo.

Role of Tyrosine Kinases and Phosphatases in Polycythemia Vera

Protein tyrosine kinases (PTKs) and phosphatases (PTPs) play a crucial role in normal cell development, and dysfunction of these enzymes has been implicated in human cancers. Polycythemia vera (PV) is a clonal hematologic disease characterized by hypersensitivity of hematopoietic progenitor cells to growth factors and cytokines. Recently, a unique and clonal mutation in the JAK homology 2 (JH2) domain of JAK2 that results in a valine to phenylalanine substitution at position 617 (V617F) was found in the majority of PV patients. This mutation leads to constitutive JAK2 activation and abnormal signaling and induces erythrocytosis in an animal model. The mutation is also found in a significant percentage of patients with idiopathic myelofibrosis (50%) and essential thrombocythemia (30%). Thus, it seems probable that this mutation associates with other molecular genetic events to cause different myeloproliferative disorders (MPDs). One of these secondary events is the transition to homozygosity of the mutated gene in 30% of the PV patients. Other events may include defects in PTPs, but these remain to be characterized. Recent studies represent a great step forward in the molecular pathogenesis in PV and the development of targeted new drugs to treat the disease.

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.

Phosphatidylinositol 3-kinase/Akt induced by erythropoietin renders the erythroid differentiation factor GATA-1 competent for TIMP-1 gene transactivation

The contribution of erythropoietin to the differentiation of the red blood cell lineage remains elusive, and the demonstration of a molecular link between erythropoietin and the transcription of genes associated with erythroid differentiation is lacking. In erythroid cells, expression of the tissue inhibitor of matrix metalloproteinase (TIMP-1) is strictly dependent on erythropoietin. We report here that erythropoietin regulates the transcription of the TIMP-1 gene upon binding to its receptor in erythroid cells by triggering the activation of phosphatidylinositol 3-kinase (PI3K)/Akt. We found that Akt directly phosphorylates the transcription factor GATA-1 at serine 310 and that this site-specific phosphorylation is required for the transcriptional activation of the TIMP-1 promoter. This chain of events can be recapitulated in nonerythroid cells by transfection of the implicated molecular partners, resulting in the expression of the normally silent endogenous TIMP-1 gene. Conversely, TIMP-1 secretion is profoundly decreased in erythroid cells from fetal livers of transgenic knock-in mice homozygous for a GATA(S310A) gene, which encodes a GATA-1 mutant that cannot be phosphorylated at Ser(310). Furthermore, retrovirus-mediated expression of GATA(S310A) into GATA-1(null)-derived embryonic stem cells decreases the rate of hemoglobinization by more than 50% compared to expressed wild-type GATA-1. These findings provide the first example of a chain of coupling mechanisms between the binding of erythropoietin to its receptor and GATA-1-dependent gene expression.

Enhancement by growth factors of cardiac myocyte differentiation from embryonic stem cells: A promising foundation for cardiac regeneration

Cell transplantation is a promising, still novel, potentially therapeutic approach for the treatment of heart diseases. Clinical applications require generation of large number of donor cells. Embryonic stem (ES) cells are capable of self-renewal apparently in an unlimited fashion, in vitro. Theoretically, they can differentiate into any cell type required for cell transplantation, including cardiac myocytes. Diverse growth factors have been implicated in programming diverse cellular processes, including development of the embryonic heart, ES cell self-renewal, and cardiac myocyte differentiation from ES cells. This review addresses the current understanding of the role of growth factors in the differentiation of cardiac myocytes from ES-embryoid body cell systems in vitro as well as cardiac regeneration in vivo.

Erythropoietin receptor spliced forms differentially expressed in blind subterranean mole rats

Erythropoietin (Epo) is the primary regulator of erythropoiesis, controlling the proliferation, maturation, and survival of erythroid progenitor cells. The functions of Epo are mediated through its specific receptor (EpoR) expressed mainly on the surface of erythroid progenitor cells, and the expression of both responds to hypoxia. The subterranean mole rat (Spalax) is a unique model system to study the molecular mechanisms for adaptation to hypoxia. Here, we cloned two forms of Spalax EpoR: a complete EpoR cDNA as well as a novel truncated bone marrow specific EpoR form. In the full-length Spalax EpoR (sEpoR), two out of the eight conserved tyrosine- phosphorylation sites were substituted (Y481F and Y499G), suggesting that Spalax Epo signaling pathways may be modulated. The level of the sEpoR mRNA in the spleen and in bone marrow was relatively low and similar in Spalax newborns and adults, with no significant response to hypoxia. The truncated sEpoR was not detected in the spleen and comprised only approximately 1% of the sEpoR expressed in the bone marrow. In Rattus, the truncated EpoR form was approximately 15% of the total expressed receptor. The level of Rattus EpoR in newborn spleens was three- to fourfold higher than in Spalax newborns and decreased toward adulthood. Severe hypoxia induces a significant increase in adult Rattus EpoR. Our data provide further insight into the adaptive mechanisms of Spalax to the extreme conditions of hypoxia in its subterranean environment.

MEK kinase 1 activity is required for definitive erythropoiesis in the mouse fetal liver

Mitogen-activated protein kinase/extracellular signal to regulated kinase (MEK) kinase 1 (MEKK1) is a c-Jun N-terminal kinase (JNK) activating kinase known to be implicated in proinflammatory responses and cell motility. Using mice deficient for MEKK1 kinase activity (Mekk1(DeltaKD)) we show a role for MEKK1 in definitive mouse erythropoiesis. Although Mekk1(DeltaKD) mice are alive and fertile on a 129 x C57/BL6 background, the frequency of Mekk1(DeltaKD) embryos that develop past embryonic day (E) 14.5 is dramatically reduced when backcrossed into the C57/BL6 background. At E13.5, Mekk1(DeltaKD) embryos have normal morphology but are anemic due to failure of definitive erythropoiesis. When Mekk1(DeltaKD) fetal liver cells were transferred to lethally irradiated wild-type hosts, mature red blood cells were generated from the mutant cells, suggesting that MEKK1 functions in a non-cell-autonomous manner. Based on immunohistochemical and hemoglobin chain transcription analysis, we propose that the failure of definitive erythropoiesis is due to a deficiency in enucleation activity caused by insufficient macrophage-mediated nuclear DNA destruction.

Recombinant human erythropoietin in the treatment of nonrenal anemia

Recombinant human erythropoietins (rhEPO) reliably increase hemoglobin levels in cancer patients experiencing chemotherapy-associated anemia. However, in patients with "anemia of cancer" not being treated with chemotherapy, rhEPO appears less effective. Recently, two studies have been broadly discussed which have raised concern on the concomitant use of erythropoietin and chemo- or radiation therapy in cancer patients. In addition, use of rhEPO is generally not considered cost-effective. Thus, the application of rhEPO should be limited to indications with proven clinical benefit. This review will provide an overview of the state of the art use of rhEPO in anemic patients and will discuss future developments.

Role of erythropoietin for angiogenesis and vasculogenesis: from embryonic development through adulthood

Erythropoietin (EPO), a stimulator of erythropoiesis, was previously shown to stimulate angiogenesis and proliferation of endothelial cells. Here, we investigated and compared the influence of EPO on cell number, proliferation, apoptosis, migration, and differentiation of endothelial cells in intact mouse embryoid bodies (EB), isolated endothelial cells from EB (EBEC), and adult human endothelial progenitor cells (hEPC). EB were treated with EPO (0.5 U/ml) immediately after plating was completed (day 5+0) or 3 days later. EPO treatment was continued until days 5+3 or 5+6. Cultured EBEC were treated 3 days after being plated, and primary hEPC from young healthy adults were treated 5 days after being plated with EPO for 48 h. Immunohistochemistry was performed with anti-PECAM (CD31), anti-Ki67, anti-CD34, anti-CD133, anti-EphB4, and anti-ephrinB2 antibodies. In all, mouse EB and EBEC and hEPC, EPO-treatment resulted in increased number of endothelial cells, increased proliferation, decreased apoptosis, and enhanced migration. In EB, this EPO effect was most pronounced when treatment was begun early (day 5+0) and was accompanied by an enhanced endothelial tube formation. In EBEC and hEPC, EPO shifted the phenotypic differentiation toward an increased ratio of EphB4-positive cells, i.e., toward a venous phenotype. These results are consistent with an important role of EPO for the number, proliferation, apoptosis, function, and phenotypical development of immature endothelial cells, which persists from early development through adulthood. They provide additional and further evidence for a strong interrelation between hematopoiesis and vasculogenesis/angiogenesis (sharing the same pathways), which may be important in many physiological and pathophysiological conditions.

Turning cells red: signal transduction mediated by erythropoietin

Erythropoietin (EPO) is the crucial cytokine regulator of red blood-cell production. Since the discovery of EPO in 1985 and the isolation of its cognate receptor four years later, there has been significant interest in understanding the unique ability of this ligand-receptor pair to promote erythroid mitogenesis, survival and differentiation. The development of knockout mice has elucidated the precise role of the ligand, receptor and downstream players in murine erythroid development. In this review, we summarize EPO-mediated signaling pathways and examine their significance in vivo.

Identification of an acquired JAK2 mutation in polycythemia vera

Polycythemia vera (PV) is a human clonal hematological disorder. The molecular etiology of the disease has not been identified. PV hematopoietic progenitor cells exhibit hypersensitivity to growth factors and cytokines, suggesting possible abnormalities in protein-tyrosine kinases and phosphatases. By sequencing the entire coding regions of cDNAs of candidate enzymes, we identified a G:C--> T:A point mutation of the JAK2 tyrosine kinase in 20 of 24 PV blood samples but none in 12 normal samples. The mutation has varying degrees of heterozygosity and is apparently acquired. It changes conserved Val(617) to Phe in the pseudokinase domain of JAK2 that is known to have an inhibitory role. The mutant JAK2 has enhanced kinase activity, and when overexpressed together with the erythropoietin receptor in cells, it caused hyperactivation of erythropoietin-induced cell signaling. This gain-of-function mutation of JAK may explain the hypersensitivity of PV progenitor cells to growth factors and cytokines. Our study thus defines a molecular defect of PV.

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.

Bcl-x(L) prevents apoptosis of late-stage erythroblasts but does not mediate the antiapoptotic effect of erythropoietin

The long form of B-cell lymphoma-x (Bcl-x(L)), an outer mitochondrial membrane protein, has been proposed to mediate the antiapoptotic action of erythropoietin on erythroid progenitor cells and to be necessary for heme synthesis in erythroblasts. Mice with conditional knockout of Bcl-x(L) (conditional bcl-x(-/-) mice) develop severe anemia that has been attributed to hemolysis and is accompanied by splenomegaly. We characterized further the anemia of conditional bcl-x(-/-) mice and investigated the role of Bcl-x(L) in the action of erythropoietin and in heme synthesis. We analyzed peripheral blood cells and cultured splenic erythroblasts of conditional bcl-x(-/-) mice and littermates that were rendered anemic by bleeding. Although they had massive splenic erythroblastosis, conditional bcl-x(-/-) mice had decreased circulating reticulocytes compared to littermates even prior to bleeding the littermates. Compared to erythroblasts of bled littermates, bcl-x(-/-) erythroblasts cultured with erythropoietin underwent apoptosis during the later, hemoglobin-synthesizing stages of differentiation. The bcl-x(-/-) erythroblasts synthesized heme, but at reduced rates compared to bled littermate erythroblasts. When cultured without erythropoietin, bcl-x(-/-) erythroblasts underwent apoptosis at early stages of differentiation, prior to hemoglobin synthesis. Bcl-x(L) is not required for heme synthesis and does not mediate the antiapoptotic effects of erythropoietin, but it prevents ineffective erythropoiesis due to apoptosis in late-stage, hemoglobin-synthesizing erythroblasts.

HIF-2α regulates murine hematopoietic development in an erythropoietin-dependent manner

Erythropoiesis in the adult mammal depends critically on erythropoietin, an inducible cytokine with pluripotent effects. Erythropoietin gene expression increases under conditions associated with lowered oxygen content such as anemia and hypoxia. HIF-1α, the founding member of the hypoxia-inducible factor (HIF) alpha class, was identified by its ability to bind and activate the hypoxia-responsive enhancer in the erythropoietin regulatory region in vitro. The existence of multiple HIF alpha members raises the question of which HIF alpha member or members regulates erythropoietin expression in vivo. We previously reported that mice lacking wild-type HIF-2α, encoded by the EPAS1 gene, exhibit pancytopenia. In this study, we have characterized the etiology of this hematopoietic phenotype. Molecular studies of EPAS1-null kidneys reveal dramatically decreased erythropoietin gene expression. EPAS1-null as well as heterozygous mice have impaired renal erythropoietin induction in response to hypoxia. Treatment of EPAS1-null mice with exogenous erythropoietin reverses the hematopoietic and other defects. We propose that HIF-2α is an essential regulator of murine erythropoietin production. Impairments in HIF signaling, involving either HIF-1α or HIF-2α, may play a prominent role in conditions involving altered hematopoietic or erythropoietin homeostasis.

Erythropoietin hypersensitivity in primary familial and congenital polycythemia: Role of tyrosines Y285 and Y344 in erythropoietin receptor cytoplasmic domain

Erythropoietin receptor (EPOR) gene mutations leading to truncations of the cytoplasmic, carboxy-terminal region of EPOR have been described in some patients with primary familial and congenital polycythemia (PFCP), a disorder characterized by isolated erythrocytosis and increased sensitivity of erythroid progenitors to Epo. We studied the role of EPOR in the pathogenesis of PFCP and the requirement for intracytoplasmic tyrosine residues Y285 and Y344 in generation of Epo hypersensitivity phenotype. Interleukin-3-dependent hematopoietic cells were engineered to express variant human EPORs using retrovirus-mediated gene transfer. We introduced tyrosine to phenylalanine substitutions in EPOR-ME, a naturally occurring, mutant human EPOR (G5881T), truncated by 110 carboxy-terminal amino acids and associated with autosomal dominantly inherited PFCP. Cells expressing EPOR-ME exhibited increased Epo sensitivity compared to cells expressing wild type EPOR. Mutation of Y285 alone had a relatively minor effect on Epo hypersensitivity whereas mutation of Y344 resulted in loss of increased Epo sensitivity. Expression of a tyrosine-null truncated EPOR conferred further decrease of Epo-mediated proliferation suggesting that both Y285 and Y344 may contribute to proliferation signals. In the context of EPOR-ME, Y344 was required for Epo-induced Stat5 tyrosine phosphorylation. The positive effect of either Y285 or Y344 on cellular proliferation was associated with Epo-induced tyrosine phosphorylation of Stat1. These findings suggest that both tyrosine residues Y285 and Y344 in the cytoplasmic domain of EPOR-ME may contribute to increased Epo sensitivity that is characteristic of PFCP phenotype.

Structural Requirements of the Extracellular to Transmembrane Domain Junction for Erythropoietin Receptor Function

The erythropoietin receptor (EpoR) is crucial for erythrocyte formation. The x-ray crystal structures of the EpoR extracellular domain lack the juxtamembrane (JM) region and the junction to the transmembrane (TM) domain. Yet the JM-TM regions are important for transmitting the conformational change imposed on the receptor dimer by Epo binding. Cysteine-scanning mutagenesis of the JM-TM regions identified three novel constitutively active mutants, demonstrating close disulfide-bonded juxtapositioning of these residues in the JM (L223C) and N-terminal TM domain (L226C, I227C). Chemical cross-linking defined the interface of the active helical TM dimer and revealed that the JM-TM segment encompassing Leu(226)-Leu(230) is non-helical. Molecular dynamics and NMR studies indicated that the TM-JM junction forms an N-terminal helix cap. This structure is important for EpoR function because replacement of this motif by consecutive leucines rendered the receptor constitutively active.

Retinoic acid, hypoxia, and GATA factors cooperatively control the onset of fetal liver erythropoietin expression and erythropoietic differentiation

The cytokine erythropoietin (Epo) is an essential factor promoting the survival, proliferation, and differentiation of erythroid progenitor cells. Epo expression and the initial phase of definitive erythropoietic differentiation in the fetal liver (E9-E12) are compromised in mouse embryos lacking the retinoic acid receptor RXRalpha. Our previous work demonstrated that the Epo gene is a direct target of retinoic acid action, via a retinoic acid receptor binding site in the Epo gene enhancer. However, Epo expression and erythropoietic differentiation become normalized in RXRalpha mutants from E12. In this study, we have investigated the molecular mechanisms underlying the transition in Epo gene regulation from RXRalpha-dependence to RXRalpha-independence. We find that three independent regulatory components are required for high level Epo expression in the early fetal liver: ligand-activated retinoic acid receptors, the hypoxia-regulated factor HIF1, and GATA factors. By E11.5, the fetal liver is no longer hypoxic, and retinoic acid signaling is no longer active; Epo expression from E11.5 onward is enhancer-independent, and is driven instead by basal promoter elements that provide a sufficient level of expression to support further erythropoietic differentiation.

Role of c-Kit and erythropoietin receptor in erythropoiesis

Erythropoiesis is regulated by a number of growth factors, among which stem cell factor (SCF) and erythropoietin (Epo) play a non-redundant function. Viable mice with mutations in the SCF gene (encoded by the Steel (Sl) locus), or its receptor gene c-Kit (encoded by the White spotting (W) locus) develop a hypoplastic macrocytic anemia. Mutants of W or Sl that are completely devoid of c-Kit or SCF expression die in utero of anemia between days 14 and 16 of gestation and contain reduced numbers of erythroid progenitors in the fetal liver. Likewise, Epo and Epo receptor (Epo-R)-deficient mice die in utero due to a marked reduction in the number of committed fetal liver derived erythroid progenitors. Thus, committed erythroid progenitors require both c-Kit and Epo-R signal transduction pathways for their survival, proliferation and differentiation. In vitro, Epo alone is capable of generating mature erythroid progenitors; however, a combined treatment of Epo and SCF results in synergistic proliferation and expansion of developing erythroid progenitors. This review summarizes recent advances made towards understanding the signaling mechanisms by which Epo-R and c-Kit regulate growth, survival, and differentiation of erythroid progenitors alone and cooperatively.

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

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

Expression of angiopoietin-1 and -2, and its clinical significance in human bladder cancer

OBJECTIVE

To investigate the relationship between angiopoietin-1 and -2 expression and the clinicopathological variables and clinical outcome in patients with bladder cancer treated by surgical resection, as both have been recently identified as antagonistic angiogenic factors which regulate tumour growth.

MATERIALS AND METHODS

The expression of angiopoietin-1 and -2 were assessed by immunohistochemistry in tissue sections from 52 transitional cell carcinomas of the bladder (33 grade 1, 15 grade 2, four grade 3, including two associated with carcinoma in situ; 22 were stage Ta, 19 T1 and 11 T2 tumours). Normal bladder specimens were also resected during each operation as controls. The expression angiopoietins were related to the clinicopathological variables of the tumours.

RESULTS

Positive immunostaining was detected in 18 samples (35%) for angiopoietin-1 and in 23 (44) for angiopoietin-2. There was no significant difference in survival according to tumour angiopoietin-1 status in the patients, but in contrast the overall survival of patients with angiopoietin-2-positive tumours was significantly lower than for those with angiopoietin-2-negative tumours (P < 0.05). Positive angiopoietin-2 expression was significantly correlated with histological grade (P = 0.026), histological stage (P = 0.009) and poor prognosis (P < 0.05). On multivariate analysis, positive angiopoietin-2 expression was an independent negative predictor for survival (P = 0.042).

CONCLUSIONS

These results suggest that angiopoietin-2 overexpression is associated with tumour progression, thereby indicating a poor prognosis for some patients treated by surgical resection for bladder carcinoma.

Generation of a conditional knockout allele for the Janus kinase 2 (Jak2) gene in mice

To study biologically relevant functions of the Janus kinase 2 (Jak2) in multiple cytokine and hormone receptor signal transduction pathways, we generated a conditional knockout (floxed) allele of this gene by placing loxP sites around the first coding exon of Jak2. Homozygous floxed animals developed normally and exhibited no phenotypic abnormalities. The conversion of the floxed allele into a null mutation was achieved by transmitting the targeted allele through the female germline of MMTV-Cre (line A) mice. Embryos that carry two Jak2 null alleles died around midgestation and exhibited impaired definitive erythropoiesis, which is a hallmark of Jak2 deficiency reported previously in conventional knockouts. This observation suggested that the Cre-mediated deletion of the first coding exon results in a true null mutation that is incapable of mediating signals through the erythropoietin receptor. Using mouse embryonic fibroblasts derived from Jak2 null embryos and their wildtype littermate controls, we demonstrated that Jak2-deficiency decouples growth hormone-receptor signaling from its downstream mediators, the signal transducer and activator of transcription (Stat) 5a and 5b.

Lnk inhibits erythropoiesis and Epo-dependent JAK2 activation and downstream signaling pathways

Erythropoietin (Epo), along with its receptor EpoR, is the principal regulator of red cell development. Upon Epo addition, the EpoR signaling through the Janus kinase 2 (JAK2) activates multiple pathways including Stat5, phosphoinositide-3 kinase (PI-3K)/Akt, and p42/44 mitogen-activated protein kinase (MAPK). The adaptor protein Lnk is implicated in cytokine receptor signaling. Here, we showed that Lnk-deficient mice have elevated numbers of erythroid progenitors, and that splenic erythroid colony-forming unit (CFU-e) progenitors are hypersensitive to Epo. Lnk(-/-) mice also exhibit superior recovery after erythropoietic stress. In addition, Lnk deficiency resulted in enhanced Epo-induced signaling pathways in splenic erythroid progenitors. Conversely, Lnk overexpression inhibits Epo-induced cell growth in 32D/EpoR cells. In primary culture of fetal liver cells, Lnk overexpression inhibits Epo-dependent erythroblast differentiation and induces apoptosis. Lnk blocks 3 major signaling pathways, Stat5, Akt, and MAPK, induced by Epo in primary erythroblasts. In addition, the Lnk Src homology 2 (SH2) domain is essential for its inhibitory function, whereas the conserved tyrosine near the C-terminus and the pleckstrin homology (PH) domain of Lnk are not critical. Furthermore, wild-type Lnk, but not the Lnk SH2 mutant, becomes tyrosine-phosphorylated following Epo administration and inhibits EpoR phosphorylation and JAK2 activation. Hence, Lnk, through its SH2 domain, negatively modulates EpoR signaling by attenuating JAK2 activation, and regulates Epo-mediated erythropoiesis.

Malarial anaemia: mechanisms and implications of insufficient erythropoiesis during blood-stage malaria

It has been proposed that the basis of severe malarial anaemia, a major cause of morbidity and mortality in endemic areas, is multifactorial. Inappropriately low reticulocytosis is observed in malaria patients suggesting that insufficient erythropoiesis is a major factor. Clinical studies provide conflicting data concerning the production of adequate levels of erythropoietin (EPO) during malaria. Plasmodium chabaudi AS causes non-lethal infection in resistant C57BL/6 mice, and lethal infection in susceptible A/J mice. In P. chabaudi AS infected C57BL/6 and A/J mice, which experience varying degrees of severity of anaemia, kidney EPO production is appropriate to the severity of anaemia and is regulated by haematocrit level. Neutralisation of endogenous EPO during infection leads to lethal anaemia while timely administration of exogenous EPO rescues mice although reticulocytosis is suppressed in proportion to the parasitemia level. Characterisation of alterations in splenic erythroid compartments in naive and P. chabaudi AS infected A/J mice revealed that infection, with or without EPO treatment, leads to sub-optimal increases in TER119+ erythroblasts compared to EPO-treated naive mice. A lower percentage of TER119+ erythroblasts in infected mice undergo terminal differentiation to become mature haemoglobin-producing cells. Furthermore, there is a shift in transferrin receptor (CD71) expression from TER119+ cells to a non-erythroid population. Deficiencies in the number and maturation of TER119+ erythroblasts during infection coincide with blunted proliferation to EPO stimulation in vitro by splenocytes, although a high frequency express EPO receptor (EPOR). Together, these data suggest that during malaria, EPO-induced proliferation of early EPOR+ erythroid progenitors is suppressed, leading to sub-optimal generation of TER119+ erythroblasts. Moreover, a shift in CD71 expression may result in impaired terminal maturation of erythroblasts. Thus, suppressed proliferation, differentiation, and maturation of erythroid precursors in association with inadequate reticulocytosis may be the basis of insufficient erythropoiesis during malaria.

Differential STAT5 signaling by ligand-dependent and constitutively active cytokine receptors

Many leukemia and cancer cells exhibit constitutive activation of STAT5, which was suggested to provide an anti-apoptotic advantage. Transformation of cytokine-dependent hematopoietic cells, such as Ba/F3 cells to autonomous growth and tumorigenicity equally results in selection for constitutive activation of STAT5. We compared STAT5 signaling between erythropoietin(Epo)-dependent cells and cells that were transformed by oncogenic activation of the erythropoietin receptor (EpoR) by coexpression of the gp55-P envelope protein of the spleen focus forming virus or by expression of the R129C constitutively active EpoR mutant. In transformed cells it was mainly STAT5B that was constitutively activated. In contrast, Epo stimulation activated both STAT5A and STAT5B. In transformed cells, chromatin immunoprecipitation (ChIP) showed STAT5 to be physically bound to promoters of STAT5 target genes, such as Bcl(XL), and to be able to promote transactivation of the Bcl(XL) promoter in a constitutive fashion. Sequencing of native sequences after ChIP with anti-STAT5 antibodies in Epo-dependent and -transformed cells indicated that in gp55-transformed cells, STAT5B bound in the chromatin not only to N3 high affinity, but also to low affinity N4 GAS sites. Transactivation for N3 GAS sites in luciferase reporters was specific to gp55 transformation. Because we also found preferential constitutive STAT5B activation after transformation of cells by a truncated form of the G-CSF-R that produces severe neutropenia (Kostmann syndrome) and favors leukemia in humans, we discuss the potential role of STAT5B in oncogenic transformation of hematopoietic cells.

Phosphoprotein profiling of erythropoietin receptor- dependent pathways using different proteomic strategies

Proteomic techniques provide new tools for the global analysis of protein profiles but also for the investigation of specific protein functions. The analysis of signaling cascades has traditionally been performed by the determination of enzymatic or transcription factor activities representing a certain pathway. Functional proteomics now allows more comprehensive approaches to study cellular responses induced during ligand/receptor interactions. In this study we evaluated proteomic strategies for the investigation of structure-function relationships in the erythropoietin receptor signalling complex. After expression of epidermal growth factor/erythropoietin receptor mutant molecules in an identical cellular background we characterized their potential to induce cellular activities. Using this system we focused our efforts on post-translational modifications of signalling proteins reflecting a substantial part of receptor-dependent signaling events. Although tyrosine phosphorylated proteins were enriched by immunoprecipitation the analysis using the classical approach combining two-dimensional gel electrophoresis and identification by matrix assisted laser desorption/ionization-time of flight-mass spectrometry revealed that low expressed signaling proteins cannot be detected by this technique. An alternative strategy using one-dimensional gel separation of phosphoproteins and liquid chromatography-tandem mass spectrometry, however, allowed us to identify multiple proteins involved in intracellular signalling representing already established pathways but also proteins which have not been linked to EPO-induced signaling so far. This approach offers the potential to extend functional proteomic studies to complex signaling processes.

Stage-specific repression by the EKLF transcriptional activator

Dynamic changes in transcription factor function can be mediated by switching its interaction with coactivators and corepressors. Erythroid Kruppel-like factor (EKLF) is an erythroid cell-specific transcription factor that plays a critical role in beta-globin gene activation via its interactions with CBP/p300 and SWI/SNF proteins. Unexpectedly, it also interacts with Sin3A and histone deacetylase 1 (HDAC1) corepressors via its zinc finger domain. We now find that selected point mutants can uncouple activation and repression and that an intact finger structure is not required for interactions with Sin3A/HDAC1 or for transrepression. Most intriguingly, EKLF repression exhibits stage specificity, with reversible EKLF-Sin3A interactions playing a key role in this process. Finally, we have located a key lysine residue that is both a substrate for CBP acetylation and required for Sin3A interaction. These data suggest a model whereby the stage of the erythroid cell alters the acetylation status of EKLF and plays a critical role in directing its coactivator-corepressor interactions and downstream transcriptional effects.

A unique activating mutation in JAK2 (V617F) is at the origin of polycythemia vera and allows a new classification of myeloproliferative diseases

Myeloproliferative disorders (MPDs) are heterogeneous diseases that occur at the level of a multipotent hematopoietic stem cell. They are characterized by increased blood cell production related to cytokine hypersensitivity and virtually normal cell maturation. The molecular pathogenesis of the MPDs has been poorly understood, except for chronic myeloid leukemia (CML), where the Bcr-Abl fusion protein exhibits constitutive kinase activity. Since some rare MPDs are also related to a dysregulated kinase activity, a similar mechanism was thought to be likely responsible for the more frequent MPDs. We investigated the mechanisms of endogenous erythroid colony formation (EEC) by polycythemia vera (PV) erythroid progenitor cells and found that EEC formation was abolished by a pharmacological inhibitor of JAK2 as well as an siRNA against JAK2. JAK2 sequencing revealed a unique mutation in the JH2 domain leading to a V617F substitution in more than 80% of the PV samples. This mutation in the pseudokinase autoinhibitory domain results in constitutive kinase activity and induces cytokine hypersensitivity or independence of factor-dependent cell lines. Retroviral transduction of the mutant JAK2 into murine HSC leads to the development of an MPD with polycythemia. The same mutation was found in about 50% of patients with idiopathic myelofibrosis (IMF) and 30% of patients with essential thrombocythemia (ET). Using different approaches, four other teams have obtained similar results. The identification of the JAK2 mutation represents a major advance in our understanding of the molecular pathogenesis of MPDs that will likely permit a new classification of these diseases and the development of novel therapeutic approaches.

Relative quantification of erythropoietin receptor-dependent phosphoproteins using in-gel 18O-labeling and tandem mass spectrometry

On examining different proteomics approaches for the investigation of structure-function relationships of erythropoietin (EPO) receptor signaling, it was found that two-dimensional gel electrophoresis/mass spectrometry procedures are clearly limited in their ability to detect low-expressed signaling proteins. Instead it was found that a strategy involving anti-phosphotyrosine immunoprecipitation, one-dimensional gel electrophoresis (1DE), and capillary liquid chromatography/tandem mass spectrometry (LC/MS/MS) provides the sensitivity required for identification of signaling proteins. In the present work the immunoprecipitation/1DE/LC/MS approach was combined with an in-gel 18O-labeling technique to analyze EPO receptor-dependent proteins. Identification and relative quantification of more than 180 EPO receptor-dependent proteins were achieved directly based on the in-gel 18O-labeling approach.

Differentiating factors between erythropoiesis-stimulating agents: a guide to selection for anaemia of chronic kidney disease

Endogenous erythropoietin (EPO) consists of a central polypeptide core covered by post-translationally linked carbohydrates. Three of the four currently available erythropoiesis stimulating agents (ESA)--epoetin-alpha, epoetin-beta and epoetin-omega- are composed of an identical amino acid sequence, but glycosylation varies as a result of type- and host cell-specific differences in the production process. Epoetin-alpha and epoetin-beta resemble each other with respect to molecular characteristics and pharmacokinetic data, although epoetin-beta has a higher molecular weight, a lower number of sialylated glycan residues and possibly slight pharmacokinetic advantages such as a longer terminal elimination half-life. A serious adverse effect of long-term administration of ESA is pure red cell aplasia. This effect has been observed predominantly with subcutaneous use of epoetin-alpha produced outside the US after albumin was removed from the formulation. In comparison with the intravenous route, subcutaneous administration of epoetin has been reported to have a dose-sparing effect in some studies. Epoetin-beta has been the subject of studies aimed at proving efficacy with a reduced administration frequency but results are not unequivocal. Epoetin-omega is produced in a different host cell than all other erythropoietic agents, hence glycosylation and pharmacokinetics are different. Small-scale clinical studies found epoetin-omega to be slightly more potent than epoetin-alpha. Epoetin-delta is a recently approved agent produced by human cells that are genetically engineered to transcribe and translate the EPO gene under the control of a newly introduced regulatory DNA sequence. However, epoetin-delta is not yet on the market and few data are available. The erythropoietin analogue darbepoetin-alpha carries two additional glycosylation sites that permit a higher degree of glycosylation. Consequently, in comparison with the other epoetins, darbepoetin-alpha has a longer serum half-life and a higher relative potency, which further increases with extension of the administration interval. Dosage requirements of darbepoetin-alpha do not appear to differ between the intravenous and subcutaneous routes of administration. The less frequent administration of darbepoetin-alpha in comparison to the other epoetins may reduce drug costs in the long term, but the variability in dosage or dosage frequency required within a single patient is high. Further studies should be aimed at defining predictors of the individual demand for erythropoietic agents, thereby allowing nephrologists to prescribe a cost-effective, individualised regimen.

Activated Fps/Fes tyrosine kinase regulates erythroid differentiation and survival

OBJECTIVE

A substantial body of evidence implicates the cytoplasmic protein tyrosine kinase Fps/Fes in regulation of myeloid differentiation and survival. In this study we wished to determine if Fps/Fes also plays a role in the regulation of erythropoiesis.

METHODS

Mice tissue-specifically expressing a "gain-of-function" mutant fps/fes transgene (fps(MF)) encoding an activated variant of Fps/Fes (MFps), were used to explore the in vivo biological role of Fps/Fes. Erythropoiesis in these mice was assessed by hematological analysis, lineage marker analysis, bone-marrow colony assays, and biochemical approaches.

RESULTS

fps(MF) mice displayed reductions in peripheral red cell counts. However, there was an accumulation of immature erythroid precursors, which displayed increased survival. Fps/Fes and the related Fer kinase were both detected in early erythroid progenitors/blasts and in mature red cells. Fps/Fes was also activated in response to erythropoietin (EPO) and stem cell factor (SCF), two critical factors in erythroid development. In addition, increased Stat5A/B activation and reduced Erk1/2 phosphorylation was observed in fps(MF) primary erythroid cells in response to EPO or SCF, respectively.

CONCLUSIONS

These data support a role for Fps/Fes in regulating the survival and differentiation of erythroid cells through modulation of Stat5A/B and Erk kinase pathways induced by EPO and SCF. The increased numbers and survival of erythroid progenitors from fps(MF) mice, and their differential responsiveness to SCF and EPO, implicates Fps/Fes in the commitment of multilineage progenitors to the erythroid lineage. The anemic phenotype in fps(MF) mice suggests that downregulation of Fps/Fes activity might be required for terminal erythroid differentiation.

The role of erythropoietin in regulating angiogenesis

Erythropoietin (EPO) is an essential growth factor that regulates erythrocyte production in mammals. In this study, we demonstrate a novel role of EPO in regulating angiogenesis in vivo. Epo and Epo receptor (EpoR) are expressed in the vasculature during embryogenesis. Deletion of Epo or EpoR leads to angiogenic defects starting at E10.5, 2 days before ventricular hypoplasia and 3 days before the onset of the embryonic lethal phenotype. Overall, angiogenesis was severely affected in the mutant embryos: vascular anomalies included decreased complexity of the vessel networks. However, de novo vasculogenesis remained intact, consistent with the differential expression of Epo and EpoR during the early stages of embryonic development. The aforementioned angiogenesis defect can be partially rescued by expressing human EPO during embryogenesis. Moreover, Ang-1 expression is regulated by EPO/EPOR under normoxic conditions. Taken together, our results suggest important roles of EPO and EPOR in angiogenesis.

Upregulation of erythropoietin receptor during postnatal and postpneumonectomy lung growth

Circulating erythropoietin (EPO) stimulates erythrocytosis, whereas organ-specific local EPO receptor (EPOR) expression has been linked to angiogenesis, tissue growth, and development. On the basis of the observation of concurrent enhancement of lung growth and erythrocyte production during exposure to chronic hypoxia, we hypothesized that a paracrine EPO system is involved in mediating lung growth. We analyzed EPOR protein expression in normal dog lung tissue during postnatal maturation and during compensatory lung growth after right pneumonectomy (PNX). Membrane-bound EPOR was significantly more abundant in the immature lung compared with mature lung and in the remaining lung 3 wk after PNX compared with matched sham controls. COOH-terminal cytosolic EPOR peptides, which were even more abundant than membrane-bound EPOR, were also upregulated in immature lung but differentially processed after PNX. Apoptosis was enhanced during both types of lung growth in direct relationship to cellular proliferation and EPOR expression. We conclude that both developmental and compensatory lung growth involve paracrine EPO signaling with parallel upregulation but differential processing of EPOR.

Prospective isolation and global gene expression analysis of the erythrocyte colony-forming unit (CFU-E)

The erythrocyte colony-forming unit (CFU-E) is a rare bone marrow (BM) progenitor that generates erythrocyte colonies in 48 hours. The existence of CFU-Es is based on these colonies, but CFU-Es have not been purified prospectively by phenotype. We have separated the "nonstem," "nonlymphoid" compartment (lineage marker [lin]-c-Kit+Sca-1-IL-7Ralpha-) into interleukin 3 receptor alpha negative (IL-3Ralpha-) and IL-3Ralpha+ subsets. Within IL-3Ralpha- but not IL-3Ralpha+ cells we have identified TER119-CD41-CD71+ erythrocyte-committed progenitors (EPs). EPs generate CFU-E colonies at about 70% efficiency and generate reticulocytes in vivo. Depletion of EPs from BM strongly reduces CFU-E frequencies. EPs lack potential for erythrocyte burst-forming unit, megakaryocyte, granulocyte (G), and monocyte (M) colonies, and for spleen colony-forming units. Chronically suppressed erythropoiesis in interferon consensus sequence-binding protein (ICSBP)-deficient BM is associated with reduced frequencies of both the EP population and CFU-E colonies. During phenylhydrazine-induced acute anemia, numbers of both the EP population and CFU-E colonies increase. Collectively, EPs (lin-c-Kit+Sca-1-IL-7Ralpha-IL-3Ralpha-CD41-CD71+) account for most, if not all, CFU-E activity in BM. As a first molecular characterization, we have compared global gene expression in EPs and nonerythroid GM progenitors. These analyses define an erythroid progenitor-specific gene expression pattern. The prospective isolation of EPs is an important step to analyze physiologic and pathologic erythropoiesis.

New insights into erythropoiesis: the roles of folate, vitamin B12, and iron

Erythropoiesis is the process in which new erythrocytes are produced. These new erythrocytes replace the oldest erythrocytes (normally about one percent) that are phagocytosed and destroyed each day. Folate, vitamin B12, and iron have crucial roles in erythropoiesis. Erythroblasts require folate and vitamin B12 for proliferation during their differentiation. Deficiency of folate or vitamin B12 inhibits purine and thymidylate syntheses, impairs DNA synthesis, and causes erythroblast apoptosis, resulting in anemia from ineffective erythropoiesis. Erythroblasts require large amounts of iron for hemoglobin synthesis. Large amounts of iron are recycled daily with hemoglobin breakdown from destroyed old erythrocytes. Many recently identified proteins are involved in absorption, storage, and cellular export of nonheme iron and in erythroblast uptake and utilization of iron. Erythroblast heme levels regulate uptake of iron and globin synthesis such that iron deficiency causes anemia by retarded production rates with smaller, less hemoglobinized erythrocytes.

The origin of hematopoietic cell type diversity

The hematopoietic system remains robust with regards to extrinsic perturbations, in sharp contrast with the stochastic behavior of hematopoeitic stem cells (HSCs) at the single cell level, suggesting that stability may be achieved within a stem cell system that undergoes constant self-renewal, commitment to differentiation and generates cell type diversification. Converging evidence at the interface of cellular, molecular and numerical studies suggests that diversity is generated by the chaotic dynamics of transcription factor networks within a cell and of the combination of growth factors and cytokines in the environment, both involving cooperation and competition. Current evidence indicates that HSCs are primed for multilineage gene expression. A subtle shift in transcription factor dosage is sufficient to perturb this equilibrium and to drive lineage commitment that involves a resolution of complexity at the molecular level and a transition towards less chaotic behavior. This dynamical instability establishes a state of responsiveness to extrinsic signals. Evolutionary conserved environmental cues that drive pattern formation or migratory behavior during embryonic development operate in the adult to influence the decision between self-renewal and differentiation in HSCs, as exemplified by the role of Notch1, Wnt proteins, BMPs and VEGF. In contrast, a network of cytokines uniquely present in mammalians influences later developmental stages, from progenitors with more restricted potentials (tri-, bi- or unipotent) to mature functional cells. These cytokines have co-opted the ancient Jak-STAT pathway but also appear to trigger lineage-affiliated transcription factors, thus linking environmental signaling to cell fate decisions.

Erythroblasts derived in vitro from embryonic stem cells in the presence of erythropoietin do not express the TER-119 antigen

OBJECTIVE

In this study, we analyzed murine primitive erythropoiesis by coculturing Flk-1+ ES-derived cells with OP9 to find efficient culture conditions for erythroid cell induction. We utilized a nonserum culture system and EPO (erythropoietin) and found that this cytokine had unique properties.

MATERIALS AND METHODS

ES cells (E14.1) were first differentiated to Flk-1+ cells and then cocultured with OP9 stromal cells. BIT9500 was used as a serum replacement. The erythroid morphology, hemoglobin types, and TER-119 expression levels were analyzed.

RESULTS

Primitive erythroid cells with embryonic hemoglobin were generated very efficiently when the serum-containing culture was converted to the nonserum system. In this serum-free culture, TER-119+ erythroblasts appeared first on day 2 and maturation proceeded until day 7. When EPO was added to this coculture, the number of induced floating cells increased twofold to threefold. Unexpectedly, the erythroid-specific antigen TER-119 expression of these cells was drastically reduced. Since reduced TER-119 expression is usually interpreted as maturation arrest, we examined the phenotypic features of the EPO-treated cells. We found, however, no evidence of maturation arrest in the aspects of morphology and hemoglobin content. EPO did not suppress TER-119 expression of erythroblasts derived from fetal liver or adult bone marrow.

CONCLUSIONS

Our results showed that EPO had the unusual property of inducing TER-119- erythroblasts in ES-derived primitive erythropoiesis. It is likely that this effect is unique to primitive erythropoiesis.

Identification of a cytokine-induced antiapoptotic molecule anamorsin essential for definitive hematopoiesis

Many growth factors and cytokines prevent apoptosis. Using an expression cloning method, we identified a novel antiapoptotic molecule named Anamorsin, which does not show any homology to known apoptosis regulatory molecules such as Bcl-2 family, caspase family, or signal transduction molecules. The expression of Anamorsin was completely dependent on stimulation with growth factors such as interleukin 3, stem cell factor, and thrombopoietin in factor-dependent hematopoietic cell lines, and forced expression of Anamorsin conferred resistance to apoptosis caused by growth factor deprivation in vitro. Furthermore, Anamorsin was found to act as an antiapoptotic molecule in vivo because Anamorsin^−/− mice die in late gestation due to defective definitive hematopoiesis in the fetal liver (FL). Although the number of hematopoietic stem/progenitor cells in the FL did not decrease in these mice, myeloid, and particularly erythroid colony formation in response to cytokines, was severely disrupted. Also, Anamorsin^−/− erythroid cells initiated apoptosis during terminal maturation. As for the mechanism of Anamorsin-mediated cell survival, a microarray analysis revealed that the expression of Bcl-xL and Jak2 was severely impaired in the FL of Anamorsin^−/− mice. Thus, Anamorsin is considered to be a necessary molecule for hematopoiesis that mediates antiapoptotic effects of various cytokines.

EVI1 induces myelodysplastic syndrome in mice

Myelodysplasia is a hematological disease in which genomic abnormalities accumulate in a hematopoietic stem cell leading to severe pancytopenia, multilineage differentiation impairment, and bone marrow (BM) apoptosis. Mortality in the disease results from pancytopenia or transformation to acute myeloid leukemia. There are frequent cytogenetic abnormalities, including deletions of chromosomes 5, 7, or both. Recurring chromosomal translocations in myelodysplasia are rare, but the most frequent are the t(3;3)(q21;q26) and the inv(3)(q21q26), which lead to the inappropriate activation of the EVI1 gene located at 3q26. To better understand the role of EVI1 in this disease, we have generated a murine model of EVI1-positive myelodysplasia by BM infection and transplantation. We find that EVI1 induces a fatal disease of several stages that is characterized by severe pancytopenia. The disease does not progress to acute myeloid leukemia. Comparison of in vitro and in vivo results suggests that EVI1 acts at two levels. The immediate effects of EVI1 are hyperproliferation of BM cells and downregulation of EpoR and c-Mpl, which are important for terminal erythroid differentiation and platelet formation. These defects are not fatal, and the mice survive for about 10 months with compensated hematopoiesis. Over this time, compensation fails, and the mice succumb to fatal peripheral cytopenia.

Two-phase culture in Diamond Blackfan anemia: localization of erythroid defect

The erythroid defect in Diamond Blackfan anemia (DBA) is known to be intrinsic to the stem cell, but its molecular pathophysiology remains obscure. Using a 2-phase liquid erythroid culture system, we have demonstrated a consistent defect in DBA, regardless of clinical severity, including 3 first-degree relatives with normal hemoglobin levels but increased erythrocyte adenosine deaminase activity. DBA cultures were indistinguishable from controls until the end of erythropoietin (Epo)-free phase 1, but failed to demonstrate the normal synchronized wave of erythroid expansion and terminal differentiation on exposure to Epo. Dexamethasone increased Epo sensitivity of erythroid progenitor cells, and enhanced erythroid expansion in phase 2 in both normal and DBA cultures. In DBA cultures treated with dexamethasone, Epo sensitivity was comparable to normal, but erythroid expansion remained subnormal. In clonogenic phase 2 cultures, the number of colonies did not significantly differ between normal cultures and DBA, in the presence or absence of dexamethasone, and at both low and high Epo concentrations. However, colonies were markedly smaller in DBA under all conditions. This suggests that the Epo-triggered onset of terminal maturation is intact in DBA, and the defect lies down-stream of the Epo receptor, influencing survival and/or proliferation of erythroid progenitors.

Protein inhibitor of activated STAT Y (PIASy) and a splice variant lacking exon 6 enhance sumoylation but are not essential for embryogenesis and adult life

Protein inhibitor of activated STAT Y (PIASy) is the shortest member of the PIAS family and has been reported to modulate the transcriptional activities of STAT1, lymphoid enhancer factor 1 (LEF-1), and the androgen receptor. PIAS proteins have also been identified as E3 ligases for the small ubiquitin-like modifier (SUMO) proteins. PIASy in particular has been reported to mediate SUMO-2/3 modification of LEF-1, sequestering it into nuclear bodies, and SUMO-1 ligation to c-Myb, modulating its transcriptional activation properties. We have cloned murine Piasy and a splice variant which omits exon 6, containing the nuclear retention PINIT motif. Cell culture studies indicate that both the full length and the splice variant are localized in the nucleus but differentially enhance SUMO ligation. To further understand the functions of PIASy, we have generated PIASy-deficient mice. Surprisingly, Piasy(-/-) mice appear phenotypically normal. Activation of STAT1 is not significantly perturbed in Piasy(-/-) cells, and sumoylation patterns for SUMO-1 or SUMO-3 modification are similar when comparing tissues and embryonic fibroblasts from wild-type and knockout mice. Our study demonstrates that at steady state, PIASy is either dispensable or compensated for by other PIAS family members or by other mechanisms when deleted.

A novel role for STAT1 in regulating murine erythropoiesis: deletion of STAT1 results in overall reduction of erythroid progenitors and alters their distribution

Erythropoietin (EPO) activates many distinct signal transduction cascades on engagement of its receptor. Deletion of the EPO, EPO receptor (EPO-R), or JAK2 genes in mice results in embryonic lethality due to a fatal anemia. EPO activates signal transducer and activator of transcription 1 (STAT1), STAT3, and STAT5a/b transcription factors in erythroid cell lines. Studies have focused on STAT5 as the primary target of EPO-dependent JAK2 activation. However, STAT5a/b(-/-) mice are viable, displaying a nonfatal anemia during embryogenesis, and delayed differentiation in adult erythropoiesis. Importantly, EPO-R cytoplasmic tyrosines are dispensable for viability in vivo. Interestingly, no cytoplasmic tyrosines are required for phosphorylation of STAT1. This led us to examine whether STAT1-deficient mice have altered erythropoiesis. A shift in erythropoiesis was observed in STAT1(-/-) mice, with reduced bone marrow-derived erythroid colony-forming units (CFU-Es) and a compensatory increase in splenic burst-forming units (BFU-Es) and CFU-Es. Both types of splenic-derived cells displayed EPO hyperresponsiveness. A 1.6-fold reduction in total CFU-Es was observed in STAT1-deficient mice, whereas total BFU-Es were comparable. Flow cytometry of STAT1-deficient erythroid cells revealed a less differentiated phenotype, associated with increased apoptosis of early erythroblasts. STAT1-deficient erythroblasts from phenylhydrazine-primed mice displayed enhanced phosphorylation of STAT5a/b, Erk1/2, and protein kinase B (PKB)/Akt. These results illustrate that STAT1 plays an important role in the regulation of erythropoiesis.

Apoptotic mechanisms in the control of erythropoiesis

Erythropoiesis is a complex multistep process encompassing the differentiation of hemopoietic stem cells to mature erythrocytes. The steps involved in this complex differentiation process are numerous and involve first the differentiation to early erythoid progenitors (burst-forming units-erythroid, BFU-E), then to late erythroid progenitors (colony-forming units-erythroid) and finally to morphologically recognizable erythroid precursors. A key event of late stages of erythropoiesis is nuclear condensation, followed by extrusion of the nucleus to produce enucleated reticulocytes and finally mature erythrocytes. During the differentiation process, the cells became progressively sensitive to erythropoietin that controls both the survival and proliferation of erythroid cells. A normal homeostasis of the erythropoietic system requires an appropriate balance between the rate of erythroid cell production and red blood cell destruction. Growing evidences outlined in the present review indicate that apoptotic mechanism play a relevant role in the control of erythropoiesis under physiologic and pathologic conditions. Withdrawal of erythropoietin or stimulation of death receptors such as Fas or TRAIL-Rs leads to activation of a subset of caspase-3, -7 and -8, which then cleave the transcription factors GATA-1 and TAL-1 and trigger apoptosis. In addition, there is evidence that a number of caspases are physiologically activated during erythroid differentiation and are functionally required for erythroid maturation. Several caspase substrates are cleaved in differentiating cells, including the protein acinus whose activation by cleavage is required for chromatin condensation. The studies on normal erythropoiesis have clearly indicated that immature erythroid precursors are sensitive to apoptotic triggering mediated by activation of the intrinsic and extrinsic apoptotic pathways. These apoptotic mechanisms are frequently exacerbated in some pathologic conditions, associated with the development of anemia (ie, thalassemias, multiple myeloma, myelodysplasia, aplastic anemia). The considerable progress in our understanding of the apoptotic mechanisms underlying normal and pathologic erythropoiesis may offer the way to improve the treatment of several pathologic conditions associated with the development of anemia.

Oxygen-dependent and tissue-specific regulation of erythropoietin gene expression

Hypoxia-inducible expression of the gene encoding for the glycoprotein hormone erythropoietin (EPO) is the paradigm of oxygen-regulated gene expression. EPO is the main regulator of red blood cell production and more than 100 years of research on the regulation of EPO production have led to the identification of a widespread cellular oxygen sensing mechanism. Central to this signaling cascade is the transcription factor complex hypoxia-inducible factor-1 (HIF-1). Meanwhile, it is known that HIF-1 controls more than 50 oxygen-dependent genes and is now recognized as the main regulator of oxygen homoeostasis in the body. In addition to hypoxic induction, expression of the EPO gene is tightly regulated in a tissue-specific manner. During ontogeny, production of EPO required for erythropoiesis is switched from the fetal liver to the kidneys. Here EPO is mainly synthesized in adulthood. Production of EPO has also been found in organs where it has nonerythropoietic functions: EPO is important for development of the brain and is neuroprotective, whereas it stimulates angiogenesis in the reproductive tract and possibly in other organs. Understanding oxygen and tissue-specific regulation of EPO production is of high relevance for physiology. Moreover, this knowledge might be useful for new therapies to treat human diseases.

Rescue of lethal c-KitW/W mice by erythropoietin

Homozygous natural white-spotted (W) mutations in the gene encoding the receptor tyrosine kinase c-Kit are associated with hypoplastic bone marrow, severe macrocytic anemia, and lethality during early postnatal life. c-Kit(W/W) mice can be rescued by wild-type hematopoietic stem cells (HSCs), but it is not known whether the lethality of c-Kit(W/W) mice is the result of HSC failure or defects specific for erythropoiesis. Here we show that transgenic expression of erythropoietin (EPO) can overcome the lethality caused by the c-Kit(W/W) mutation. In W mutant mice rescued by EPO, termed WEPO, erythrocyte colony-forming units (CFU-Es) are rescued to normal frequencies. Hence, Epo receptor signals can partially bypass the strict requirement for c-Kit signaling in erythropoiesis in the absence of c-Kit in vivo. Using a series of W and rescue mouse strains, we define here the erythropoietic threshold permitting survival in vivo. The lethality of c-Kit(W/W) mice has precluded analysis of this crucial receptor-ligand pair in adult stem/progenitor cells. Our strategy to generate viable c-Kit(W/W) mice will be useful to analyze the role of this important receptor tyrosine kinase in adult life in vivo.

Development of primitive and definitive hematopoiesis from nonhuman primate embryonic stem cells in vitro

Although information about the development of primitive and definitive hematopoiesis has been elucidated in murine embryos and embryonic stem (ES) cells, there have been few in vitro studies of these processes in primates. In this study, we investigated hematopoietic differentiation from cynomolgus monkey ES cells grown on OP9, a stromal cell line deficient in macrophage colony-stimulating factor. Primitive erythrocytes (EryP) and definitive erythrocytes (EryD) developed sequentially from ES cells in the culture system; this was confirmed by immunostaining and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of embryonic, fetal and adult globin genes. EryP were detected on day 8 without exogenous erythropoietin (EPO), whereas EryD appeared on day 16 and had an indispensable requirement for exogenous EPO. RT-PCR analysis of the cultures revealed a sequential expression of genes associated with primitive and definitive hematopoietic development that was equivalent to that seen during primate ontogeny in vivo. Vascular endothelial growth factor (VEGF) increased, in a dose-dependent manner, not only the number of floating hematopoietic cells, but also the number of adherent hematopoietic cell clusters containing CD34-positive immature progenitors. In colony assays, exogenous VEGF also had a dose-dependent stimulatory effect on the generation of primitive erythroid colonies. More efficient primitive and definitive erythropoiesis was induced by re-plating sorted CD34-positive cells. Thus, this system reproduces early hematopoietic development in vitro and can serve as a model for analyzing the mechanisms of hematopoietic development in primates.

PI3 kinase is important for Ras, MEK and Erk activation of Epo-stimulated human erythroid progenitors

Background

Erythropoietin is a multifunctional cytokine which regulates the number of erythrocytes circulating in mammalian blood. This is crucial in order to maintain an appropriate oxygen supply throughout the body. Stimulation of primary human erythroid progenitors (PEPs) with erythropoietin (Epo) leads to the activation of the mitogenic kinases (MEKs and Erks). How this is accomplished mechanistically remained unclear.

Results

Biochemical studies with human cord blood-derived PEPs now show that Ras and the class Ib enzyme of the phosphatidylinositol-3 kinase (PI3K) family, PI3K gamma, are activated in response to minimal Epo concentrations. Surprisingly, three structurally different PI3K inhibitors block Ras, MEK and Erk activation in PEPs by Epo. Furthermore, Erk activation in PEPs is insensitive to the inhibition of Raf kinases but suppressed upon PKC inhibition. In contrast, Erk activation induced by stem cell factor, which activates c-Kit in the same cells, is sensitive to Raf inhibition and insensitive to PI3K and PKC inhibitors.

Conclusions

These unexpected findings contrast with previous results in human primary cells using Epo at supraphysiological concentrations and open new doors to eventually understanding how low Epo concentrations mediate the moderate proliferation of erythroid progenitors under homeostatic blood oxygen levels. They indicate that the basal activation of MEKs and Erks in PEPs by minimal concentrations of Epo does not occur through the classical cascade Shc/Grb2/Sos/Ras/Raf/MEK/Erk. Instead, MEKs and Erks are signal mediators of PI3K, probably the recently described PI3K gamma, through a Raf-independent signaling pathway which requires PKC activity. It is likely that higher concentrations of Epo that are induced by hypoxia, for example, following blood loss, lead to additional mitogenic signals which greatly accelerate erythroid progenitor proliferation.