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

Erythroid Kruppel like factor: from fishing expedition to gourmet meal

Erythroid Kruppel like factor (EKLF) is the founding member of a family of transcription factors which are defined by the presence of three C-terminal C2H2-type zinc fingers. Since its discovery 6 years ago, the study of EKLF has been intense. In this review I will revisit the discovery of EKLF, and highlight recent advances in our understanding of how it interacts with other proteins to regulate erythroid gene transcription. The current knowledge of the biological role/s of EKLF in erythroid cell differentiation and globin gene switching are summarized.

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.

Multilineage embryonic hematopoiesis requires hypoxic ARNT activity

Although most cells undergo growth arrest during hypoxia, endothelial cells and placental cytotrophoblasts proliferate in response to low O(2). We demonstrate that proliferation of embryonic multilineage hematopoietic progenitors is also regulated by a hypoxia-mediated signaling pathway. This pathway requires HIF-1 (HIF-1alpha/ARNT heterodimers) because Arnt(-/-) embryoid bodies fail to exhibit hypoxia-mediated progenitor proliferation. Furthermore, Arnt(-/-) embryos exhibit decreased numbers of yolk sac hematopoietic progenitors. This defect is cell extrinsic, is accompanied by a decrease in ARNT-dependent VEGF expression, and is rescued by exogenous VEGF. Therefore, "physiologic hypoxia" encountered by embryos is essential for the proliferation or survival of hematopoietic precursors during development.

SOCS3 is essential in the regulation of fetal liver erythropoiesis

SOCS3 (CIS3/JAB2) is an SH2-containing protein that binds to the activation loop of Janus kinases, inhibiting kinase activity, and thereby suppressing cytokine signaling. During embryonic development, SOCS3 is highly expressed in erythroid lineage cells and is Epo independent. Transgene-mediated expression blocks fetal erythropoiesis, resulting in embryonic lethality. SOCS3 deletion results in an embryonic lethality at 12-16 days associated with marked erythrocytosis. Moreover, the in vitro proliferative capacity of progenitors is greatly increased. SOCS3-deficient fetal liver stem cells can reconstitute hematopoiesis in lethally irradiated adults, indicating that its absence does not disturb bone marrow erythropoiesis. Reconstitution of lymphoid lineages in JAK3-deficient mice also occurs normally. The results demonstrate that SOCS3 is critical in negatively regulating fetal liver hematopoiesis.

Increased expression of the distal, but not of the proximal, Gata1 transcripts during differentiation of primary erythroid cells

Gata1 is expressed from either one of two alternative promoters, the erythroid (proximal to the AUG) and the testis (distal to the AUG) promoter, both used by hemopoietic cells. To clarify the role of the distal and proximal Gata1 transcripts in erythroid differentiation, we determined by specific reverse transcriptase-polymerase chain reactions their relative levels of expression during the differentiation of erythroid precursors purified from the spleen of mice treated with phenylhydrazine (PHZ) or infected with the anemia-inducing strain of the Friend virus (FVA cells). PHZ cells are erythroid precursors that progress in vivo to erythroblasts in 3 days. Both PHZ and FVA cells synchronously proliferate and differentiate in vitro in the presence of erythropoietin (EPO). The levels of total and of distal, but not of proximal, Gata1 transcripts increased by five- to eightfold during in vivo and in vitro differentiation of FVA and PHZ cells. The increase in expression was temporally associated with an increase in the expression of Eklf, Scl, and Nfe2, three genes required for erythroid differentiation, and preceded by 24 h the repression of Gata2 and Myb expression. The day 1 PHZ cells that survived 18 h in the absence of EPO do not express globin genes and express detectable levels of distal but not of proximal Gata1 transcripts. These cells activate the expression of the globin genes within 2 h when exposed to EPO. Therefore, during erythroid differentiation of primary cells, increased expression of distal Gata1 transcripts underlies the increase in the expression of total Gata1 associated with the establishment of the erythroid differentiation program.

Purification, amplification and characterization of a population of human erythroid progenitors

In humans, studies of the erythroid cell lineage are hampered by difficulties in obtaining sufficient numbers of erythroid progenitors. In fact, these progenitors in bone marrow or peripheral blood are scarce and no specific antibodies are available. We describe a new method which allows proliferation in liquid culture of large numbers of pure normal human erythroid progenitors. CD34+ cells were cultured for 7 d in serum-free conditions with the cytokine mixture interleukin (IL)-3/IL-6/stem cell factor (SCF). This resulted in cell expansion and the appearance of a high proportion of CD36+ cells which were purified on day 7. Methylcellulose clones from these cells were composed of 96.6% late BFU-E and 3.4% CFU-GM. These CD36+ cells could be recultured with the same cytokine mixture plus or minus erythropoietin (Epo) for a further 2-7 d. In both conditions further amplification of CD36+ cells was observed, but Epo induced a more dramatic cell expansion. Glycophorin-positive mature cells appeared only in the presence of Epo, and terminal red cell differentiation was observed after 7 d of secondary culture. Cells obtained from adult CD34+ progenitors mostly contained adult haemoglobin, whereas cord blood-derived cells contained equal proportions of adult and fetal haemoglobin. Activation of STAT5 and tyrosine phosphorylation of the Epo receptor and JAK2 were observed after Epo stimulation of these cells. This new method represents a straightforward alternative to the procedures previously described for the purification of normal erythroid progenitors and is useful in the study of erythropoietic regulation.

A Critical Role for PU.1 in Homing and Long-Term Engraftment by Hematopoietic Stem Cells in the Bone Marrow

We have previously demonstrated that PU.1 is required for the production of lymphoid and myeloid, but not of erythroid progenitors in the fetal liver. In this study, competitive reconstitution assays show that E14.5 PU.1−/− hematopoietic progenitors (HPC) fail to sustain definitive/adult erythropoiesis or to contribute to the lymphoid and myeloid lineages. PU.1−/−HPC are unable to respond synergistically to erythropoietin plus stem cell factor and have reduced expression of c-kit, which may explain the erythroid defect. Fluorescently labeled,PU.1−/−, AA4.1+, fetal liver HPC were transferred into irradiated recipients, where they demonstrated a severely impaired ability to home to and colonize the bone marrow.PU.1−/− HPC were found to lack integrins 4 (VLA-4/CD49d), 5 (VLA-5/CD49e), and CD11b (M). Collectively, this study has shown that PU.1 plays an important role in controlling migration of hematopoietic progenitors to the bone marrow and the establishment of long-term multilineage hematopoiesis.

Inactivation of erythropoietin leads to defects in cardiac morphogenesis

Erythropoietin is an essential growth factor that promotes survival, proliferation, and differentiation of mammalian erythroid progenitor cells. Erythropoietin(−/−) and erythropoietin receptor(−/−) mouse embryos die around embryonic day 13.5 due, in part, to failure of erythropoiesis in the fetal liver. In this study, we demonstrated a novel role of erythropoietin and erythropoietin receptor in cardiac development in vivo. We found that erythropoietin receptor is expressed in the developing murine heart in a temporal and cell type-specific manner: it is initially detected by embryonic day 10.5 and persists until day 14.5. Both erythropoietin(−/−) and erythropoietin receptor(−/−) embryos suffered from ventricular hypoplasia at day 12–13 of gestation. This defect appears to be independent from the general state of hypoxia and is likely due to a reduction in the number of proliferating cardiac myocytes in the ventricular myocardium. Cell proliferation assays revealed that erythropoietin acts as a mitogen in cells isolated from erythropoietin(−/−) mice, while it has no effect in hearts from erythropoietin receptor(−/−) animals. Erythropoietin(−/−) and erythropoietin receptor(−/−) embryos also suffered from epicardium detachment and abnormalities in the vascular network. Finally, through a series of chimeric analysis, we provided evidence that erythropoietin acts in a manner which is non-cell-autonomous. Our results elucidate a novel role of erythropoietin in cardiac morphogenesis and suggest a combination of anemia and cardiac failure as the cause of embryonic lethality in the erythropoietin(−/−) and erythropoietin receptor(−/−) animals.

A Critical Role for PU.1 in Homing and Long-Term Engraftment by Hematopoietic Stem Cells in the Bone Marrow

We have previously demonstrated that PU.1 is required for the production of lymphoid and myeloid, but not of erythroid progenitors in the fetal liver. In this study, competitive reconstitution assays show that E14.5 PU.1−/− hematopoietic progenitors (HPC) fail to sustain definitive/adult erythropoiesis or to contribute to the lymphoid and myeloid lineages. PU.1−/−HPC are unable to respond synergistically to erythropoietin plus stem cell factor and have reduced expression of c-kit, which may explain the erythroid defect. Fluorescently labeled,PU.1−/−, AA4.1+, fetal liver HPC were transferred into irradiated recipients, where they demonstrated a severely impaired ability to home to and colonize the bone marrow.PU.1−/− HPC were found to lack integrins 4 (VLA-4/CD49d), 5 (VLA-5/CD49e), and CD11b (M). Collectively, this study has shown that PU.1 plays an important role in controlling migration of hematopoietic progenitors to the bone marrow and the establishment of long-term multilineage hematopoiesis.

A role for G-CSF receptor signaling in the regulation of hematopoietic cell function but not lineage commitment or differentiation

To investigate the specificity of cytokine signals in hematopoietic differentiation, we generated mice with a targeted mutation of their G-CSF receptor (G-CSFR) such that the cytoplasmic (signaling) domain of the G-CSFR is replaced with the cytoplasmic domain of the erythropoietin receptor. In homozygous mutant mice, expression of this chimeric receptor had no apparent affect on lineage commitment and was able to support the production of morphologically mature neutrophils. However, mutant neutrophils displayed reduced chemotaxis, and G-CSF-stimulated mobilization of neutrophils and hematopoietic progenitors from the bone marrow to blood was markedly impaired. Thus, the G-CSFR is generating unique signals that are required for certain specialized hematopoietic cell functions but are not required for granulocytic differentiation or lineage commitment.

Ineffective erythropoiesis in myelodysplastic syndromes: correlation with Fas expression but not with lack of erythropoietin receptor signal transduction

Ineffective erythropoiesis in myelodysplasia is characterized by a defect in erythroid progenitor growth and by abnormal erythroid differentiation. Increased apoptosis of erythroid, granulocytic and megakaryocytic lineages is thought to account for cytopenias. Erythropoietin (Epo)-induced BFU-E and CFU-E growth was studied in 25 myelodysplastic syndrome (MDS) marrow specimens and found to be drastically diminished. To investigate the functionality of Epo-R in MDS marrow, we focused on Epo-induced STAT5 activation. Epo was able to stimulate STAT5 DNA binding activity in all normal and 12/24 MDS marrows tested, with no correlation between the level of STAT5 activation and the development of erythroid colonies in response to Epo. In contrast, impaired proliferation of erythroid progenitors was related to an increased expression of the transmembrane mediator of apoptotic cell death Fas/CD95 on the glycophorin A+ subpopulation. Therefore we conclude that the stimulation of pro-apoptotic signals rather than the defect of anti-apoptotic pathways resulting from Epo-stimulated Jak2-STAT5 pathway, predominantly accounts for ineffective erythropoiesis in myelodysplasia.

Fetal anemia and apoptosis of red cell progenitors in Stat5a-/-5b-/- mice: a direct role for Stat5 in Bcl-X(L) induction

The erythropoietin receptor (EpoR) is essential for production of red blood cells; a principal function of EpoR is to rescue committed erythroid progenitors from apoptosis. Stat5 is rapidly activated following EpoR stimulation, but its function in erythropoiesis has been unclear since adult Stat5a-/-5b-/- mice have normal steady-state hematocrit. Here we show that Stat5 is essential for the high erythropoietic rate during fetal development. Stat5a-/-5b-/- embryos are severely anemic; erythroid progenitors are present in low numbers, show higher levels of apoptosis, and are less responsive to Epo. These findings are explained by a crucial role for Stat5 in EpoR's antiapoptotic signaling: it mediates the immediate-early induction of Bcl-X(L) in erythroid cells through direct binding to the Bcl-X promoter.

The Glucocorticoid Receptor Cooperates With the Erythropoietin Receptor and c-Kit to Enhance and Sustain Proliferation of Erythroid Progenitors In Vitro

Although erythropoietin (Epo) is essential for the production of mature red blood cells, the cooperation with other factors is required for a proper balance between progenitor proliferation and differentiation. In avian erythroid progenitors, steroid hormones cooperate with tyrosine kinase receptors to induce renewal of erythroid progenitors. We examined the role of corticosteroids in the in vitro expansion of primary human erythroid cells in liquid cultures and colony assays. Dexamethasone (Dex), a synthetic glucocorticoid hormone, cooperated with Epo and stem cell factor to induce erythroid progenitors to undergo 15 to 22 cell divisions, corresponding to a 105- to 106-fold amplification of erythroid cells. Dex acted directly on erythroid progenitors and maintained the colony-forming capacity of the progenitor cells expanded in liquid cultures. The hormone delayed terminal differentiation into erythrocytes, which was assayed by morphology, hemoglobin accumulation, and the expression of genes characteristic for immature cells. Sustained proliferation of erythroid progenitors could be induced equally well from purified erythroid burst-forming units (BFU-E), from CD34+ blast cells, and from bone marrow depleted from CD34+ cells.

The Glucocorticoid Receptor Cooperates With the Erythropoietin Receptor and c-Kit to Enhance and Sustain Proliferation of Erythroid Progenitors In Vitro

Although erythropoietin (Epo) is essential for the production of mature red blood cells, the cooperation with other factors is required for a proper balance between progenitor proliferation and differentiation. In avian erythroid progenitors, steroid hormones cooperate with tyrosine kinase receptors to induce renewal of erythroid progenitors. We examined the role of corticosteroids in the in vitro expansion of primary human erythroid cells in liquid cultures and colony assays. Dexamethasone (Dex), a synthetic glucocorticoid hormone, cooperated with Epo and stem cell factor to induce erythroid progenitors to undergo 15 to 22 cell divisions, corresponding to a 105- to 106-fold amplification of erythroid cells. Dex acted directly on erythroid progenitors and maintained the colony-forming capacity of the progenitor cells expanded in liquid cultures. The hormone delayed terminal differentiation into erythrocytes, which was assayed by morphology, hemoglobin accumulation, and the expression of genes characteristic for immature cells. Sustained proliferation of erythroid progenitors could be induced equally well from purified erythroid burst-forming units (BFU-E), from CD34+ blast cells, and from bone marrow depleted from CD34+ cells.

Gene expression in proliferating human erythroid cells

A complete understanding of human erythropoiesis will require a robust description of transcriptional activity in hematopoietic cells that proliferate and differentiate in response to erythropoietin (EPO). For this purpose, we cultured peripheral blood mononuclear cells in the presence or in the absence of EPO and examined the transcriptional profile of those cells arising only in response to EPO. A distinct population of CD71( +) cells that demonstrated an average of six additional doublings in suspension culture and erythroid colony formation in methylcellulose was isolated. Suppression subtractive hybridization of mRNA isolated from those cells permitted the identification of transcribed genes. A summary of 719 expressed sequence tags (ESTs) describing 505 independent transcripts is provided here with a full analysis of each EST available at http://hembase.niddk.nih.gov. Several transcripts that matched genes previously reported in the context of erythroid differentiation including 4 cell surface proteins were expressed at this developmental stage. Active chromatin remodeling was suggested by the identification of 4 histone proteins, 4 high-mobility group proteins, 13 transcription factors, and 6 genes involved in DNA recombination and repair. Numerous genes associated with leukemic translocations were also recognized including topoisomerases I and II, nucleophosmin, Translin, EGR1, dek, pim-1, TFG, and MLL. In addition to known transcripts, 44 novel EST were discovered. This transcriptional profile provides the first genomic-scale description of gene activity in erythroid progenitor cells.

Genetic Evidence for an Additional Factor Required for Erythropoietin-Induced Signal Transduction

Erythropoietin (EPO) and its receptor (EPOR) are required for the development of mature erythrocytes. After binding of ligand, the EPOR activates a variety of signaling pathways that ultimately control cellular proliferation, survival, and specific gene expression. Although erythroid progenitors appear to be the principal EPO-responsive cell type in vivo due to the restricted expression of the EPOR, many growth factor–dependent cell lines expressing the EPOR can respond to EPO by activating many or all of these pathways. In the present study, we have identified a cellular context (the interleukin-2 [IL-2]–dependent HT-2 line) in which the EPO stimulation of the EPOR fails to support cellular proliferation, STAT-5 induction, or MAPK activation, despite efficient phosphorylation of the EPOR and JAK2 and inhibition of apoptosis after withdrawal of IL-2. Interestingly, when we fused HT-2 cells expressing the EPOR with Ba/F3 cells in a complementation assay, the resulting hybridomas proliferated and potently activated STAT-5 and MAPK in response to EPO. These data indicate that an unidentified cellular factor is needed to mediate signaling by the EPOR. Moreover, Ba/F3 cells apparently express this factor(s) and somatic fusions can, therefore, confer EPO-responsiveness to HT-2 cells that lack this factor.

GATA-1 and Erythropoietin Cooperate to Promote Erythroid Cell Survival by Regulating bcl-xL Expression

The transcription factor GATA-1 is essential for normal erythropoiesis. By examining in vitro–differentiated embryonic stem cells, we showed previously that in the absence of GATA-1, committed erythroid precursors fail to complete maturation and instead undergo apoptosis. The mechanisms by which GATA-1 controls cell survival are unknown. Here we report that in erythroid cells, GATA-1 strongly induces the expression of the anti-apoptotic protein bcl-xL, but not the related proteins bcl-2 and mcl-1. Consistent with a role for bcl-xL in mediating GATA-1–induced erythroid cell survival, in vitro–differentiated bcl-xL−/− embryonic stem cells fail to generate viable mature definitive erythroid cells, a phenotype resembling that of GATA-1 gene disruption. In addition, we show that erythropoietin, which is also required for erythroid cell survival, cooperates with GATA-1 to stimulate bcl-xL gene expression and to maintain erythroid cell viability during terminal maturation. Together, our data show that bcl-xL is essential for normal erythroid development and suggest a regulatory hierarchy in which bcl-xL is a critical downstream effector of GATA-1 and erythropoietin-mediated signals.

Activation of the erythropoietin receptor by the gp55-P viral envelope protein is determined by a single amino acid in its transmembrane domain

The spleen focus forming virus (SFFV) gp55-P envelope glycoprotein specifically binds to and activates murine erythropoietin receptors (EpoRs) coexpressed in the same cell, triggering proliferation of erythroid progenitors and inducing erythroleukemia. Here we demonstrate specific interactions between the single transmembrane domains of the two proteins that are essential for receptor activation. The human EpoR is not activated by gp55-P but by mutation of a single amino acid, L238, in its transmembrane sequence to its murine counterpart serine, resulting in its ability to be activated. The converse mutation in the murine EpoR (S238L) abolishes activation by gp55-P. Computational searches of interactions between the membrane-spanning segments of murine EpoR and gp55-P provide a possible explanation: the face of the EpoR transmembrane domain containing S238 is predicted to interact specifically with gp55-P but not gp55-A, a variant which is much less effective in activating the murine EpoR. Mutational studies on gp55-P M390, which is predicted to interact with S238, provide additional support for this model. Mutation of M390 to isoleucine, the corresponding residue in gp55-A, abolishes activation, but the gp55-P M390L mutation is fully functional. gp55-P is thought to activate signaling by the EpoR by inducing receptor oligomerization through interactions involving specific transmembrane residues.

bcl-x prevents apoptotic cell death of both primitive and definitive erythrocytes at the end of maturation

bcl-x is a member of the bcl-2 gene family, which regulates apoptotic cell death in various cell lineages. There is circumstantial evidence suggesting that bcl-x might play a role in the apoptosis of erythroid lineage cells, although there is no direct evidence. In this study, we used Bcl-X null mouse embryonic stem (ES) cells, and showed that Bcl-X is indispensable for the production of both embryonic primitive erythrocytes (EryP) and adult definitive erythrocytes (EryD) at the end of their maturation. In vivo, bcl-x-/- ES cells did not contribute to circulating EryD in adult chimeric mice that were produced by blastocyst microinjection of the bcl-x-/- ES cells. bcl-x-/- EryP and EryD were produced by in vitro differentiation induction of ES cells on macrophage colony-stimulating factor-deficient stromal cell line OP9, and further analysis was carried out. The emergence of immature EryP and EryD from bcl-x-/- ES cells was similar to that from bcl-x+/+ ES cells. However, prominent cell death of bcl-x-/- EryP and EryD occurred when the cells matured. The data show that the antiapoptotic function of bcl-x acts at the very end of erythroid maturation.

Fli-1, an Ets-related transcription factor, regulates erythropoietin-induced erythroid proliferation and differentiation: evidence for direct transcriptional repression of the Rb gene during differentiation

Erythropoietin (Epo) is a major regulator of erythropoiesis that alters the survival, proliferation, and differentiation of erythroid progenitor cells. The mechanism by which these events are regulated has not yet been determined. Using HB60, a newly established erythroblastic cell line, we show here that Epo-induced terminal erythroid differentiation is associated with a transient downregulation in the expression of the Ets-related transcription factor Fli-1. Constitutive expression of Fli-1 in HB60 cells, similar to retroviral insertional activation of Fli-1 observed in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia, blocks Epo-induced differentiation while promoting Epo-induced proliferation. These results suggest that Fli-1 modulates the response of erythroid cells to Epo. To understand the mechanism by which Fli-1 regulates erythropoiesis, we searched for downstream target genes whose expression is regulated by this transcription factor. Here we show that the retinoblastoma (Rb) gene, which was previously shown to be involved in the development of mature erythrocytes, contains a Fli-1 consensus binding site within its promoter. Fli-1 binds to this cryptic Ets consensus site within the Rb promoter and transcriptionally represses Rb expression. Both the expression level and the phosphorylation status of Rb are consistent with the response of HB60 cells to Epo-induced terminal differentiation. We suggest that the negative regulation of Rb by Fli-1 could be one of the critical determinants in erythroid progenitor cell differentiation that is specifically deregulated during F-MuLV-induced erythroleukemia.

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.

Protein Kinase B (c-Akt), Phosphatidylinositol 3-Kinase, and STAT5 Are Activated by Erythropoietin (EPO) in HCD57 Erythroid Cells But Are Constitutively Active in an EPO-Independent, Apoptosis-Resistant Subclone (HCD57-SREI Cells)

We found that erythropoietin (EPO) and stem cell factor (SCF) activated protein kinase B (PKB/Akt) in EPO-dependent HCD57 erythroid cells. To better understand signals controlling proliferation and viability, erythroid cells that resist apoptosis in the absence of EPO were subcloned and characterized (HCD57-SREI cells). Constitutive activations of PKB/Akt, STAT5a, and STAT5b were noted in these EPO-independent cells. PI3-kinase activity was an upstream activator of PKB/Akt because the PI3-kinase inhibitor LY294002 blocked both constitutive PKB/Akt and factor-dependent PKB/Akt activity. The LY294002 study showed that proliferation and viability of both HCD57-SREI and HCD57 cells correlated with the activity of PKB/Akt; however, PKB/Akt activity alone did not protect these cells from apoptosis. Treatment of HCD57 cells with SCF also activated PKB/Akt, but did not protect from apoptosis. This result suggested that PKB/PI3-kinase activity is necessary but not sufficient to promote viability and/or proliferation. Constitutive STAT5 activity, activated through an unknown pathway not including JAK2 or EPOR, may act in concert with the constitutive PI3-kinase/PKB/Akt pathway to protect the EPO-independent HCD57-SREI cells from apoptosis and promote limited proliferation.

Protein Kinase B (c-Akt), Phosphatidylinositol 3-Kinase, and STAT5 Are Activated by Erythropoietin (EPO) in HCD57 Erythroid Cells But Are Constitutively Active in an EPO-Independent, Apoptosis-Resistant Subclone (HCD57-SREI Cells)

We found that erythropoietin (EPO) and stem cell factor (SCF) activated protein kinase B (PKB/Akt) in EPO-dependent HCD57 erythroid cells. To better understand signals controlling proliferation and viability, erythroid cells that resist apoptosis in the absence of EPO were subcloned and characterized (HCD57-SREI cells). Constitutive activations of PKB/Akt, STAT5a, and STAT5b were noted in these EPO-independent cells. PI3-kinase activity was an upstream activator of PKB/Akt because the PI3-kinase inhibitor LY294002 blocked both constitutive PKB/Akt and factor-dependent PKB/Akt activity. The LY294002 study showed that proliferation and viability of both HCD57-SREI and HCD57 cells correlated with the activity of PKB/Akt; however, PKB/Akt activity alone did not protect these cells from apoptosis. Treatment of HCD57 cells with SCF also activated PKB/Akt, but did not protect from apoptosis. This result suggested that PKB/PI3-kinase activity is necessary but not sufficient to promote viability and/or proliferation. Constitutive STAT5 activity, activated through an unknown pathway not including JAK2 or EPOR, may act in concert with the constitutive PI3-kinase/PKB/Akt pathway to protect the EPO-independent HCD57-SREI cells from apoptosis and promote limited proliferation.

Functions of c-Jun in liver and heart development

Mice lacking the AP-1 transcription factor c-Jun die around embryonic day E13.0 but little is known about the cell types affected as well as the cause of embryonic lethality. Here we show that a fraction of mutant E13.0 fetal livers exhibits extensive apoptosis of both hematopoietic cells and hepatoblasts, whereas the expression of 15 mRNAs, including those of albumin, keratin 18, hepatocyte nuclear factor 1, beta-globin, and erythropoietin, some of which are putative AP-1 target genes, is not affected. Apoptosis of hematopoietic cells in mutant livers is most likely not due to a cell-autonomous defect, since c-jun-/- fetal liver cells are able to reconstitute all hematopoietic compartments of lethally irradiated recipient mice. A developmental analysis of chimeras showed contribution of c-jun-/- ES cell derivatives to fetal, but not to adult livers, suggesting a role of c-Jun in hepatocyte turnover. This is in agreement with the reduced mitotic and increased apoptotic rates found in primary liver cell cultures derived from c-jun-/- fetuses. Furthermore, a novel function for c-Jun was found in heart development. The heart outflow tract of c-jun-/- fetuses show malformations that resemble the human disease of a truncus arteriosus persistens. Therefore, the lethality of c-jun mutant fetuses is most likely due to pleiotropic defects reflecting the diversity of functions of c-Jun in development, such as a role in neural crest cell function, in the maintenance of hepatic hematopoiesis and in the regulation of apoptosis.

The Notch/Jagged pathway inhibits proliferation of human hematopoietic progenitors in vitro

The cell surface receptor Notch1 is expressed on CD34+ hematopoietic precursors, whereas one of its ligands, Jagged1, is expressed on bone marrow stromal cells. To examine the role of Notch signaling in early hematopoiesis, human CD34+ cells were cultured in the presence or absence of exogenous cytokines on feeder layers that either did or did not express Jagged1. In the absence of recombinant growth factors, Jagged1 decreased myeloid colony formation by CD34+ cells, as well as 3H-thymidine incorporation and entry into S phase. In the presence of a strong cytokine signal to proliferate and mature, (interleukin 3 [IL-3] and IL-6, stem cell factor [SCF], and G-CSF), Jagged1 did not significantly alter either the fold expansion or the types of colonies formed by CD34+ cells. However, in the presence of SCF alone, Jagged1 increased erythroid colony formation twofold. These results demonstrate that Notch can modulate a growth factor signal, and that in the absence of growth factor stimulation, the Jagged1-Notch pathway preserves CD34+ cells in an immature state.

TGF-beta cooperates with TGF-alpha to induce the self-renewal of normal erythrocytic progenitors: evidence for an autocrine mechanism

Simultaneous addition of both TGF-alpha and TGF-beta induces the sustained, long-term outgrowth of chicken erythrocytic progenitor cells, referred to as T2ECs from both chick bone marrow and 2-day-old chicken embryos. By analysis for differentiation antigens and gene expression, these cells were shown to represent very immature haematopoietic progenitors committed to the erythrocytic lineage. T2ECs differentiate into almost pure populations of fully mature erythrocytes within 6 days, when TGF-alpha and TGF-beta are withdrawn and the cells exposed to anaemic chicken serum plus insulin. Outgrowth of these cells from various sources invariably required both TGF-alpha and TGF-beta, as well as glucocorticoids. Proliferating, established T2ECs still require TGF-alpha, but are independent of exogenous TGF-beta. Using a TGF-beta-neutralizing antibody or expressing a dominant-negative TGF-beta receptor II, we demonstrate that T2ECs generate an autocrine loop involving TGF-beta during their establishment, which is required for sustained proliferation. Using specific inhibitors, we also show that signalling via Mek-1 is specifically required for induction and maintenance of cell proliferation driven by cooperation between the TGF-alpha and -beta receptors. These results establish a novel mechanism by which self-renewal of erythrocytic progenitors is induced and establish avian T2ECs as a new, quasi-optimal model system to study erythrocytic progenitors.

Regulated Expression and Functional Role of the Transcription Factor CHOP (GADD153) in Erythroid Growth and Differentiation

The hematopoietic growth factor erythropoietin (Epo) triggers changes in the expression of genes that encode important regulators of erythroid cell growth and differentiation. We now report that Epo markedly upregulates chop (gadd153) expression and that this transcription factor plays a role in erythropoiesis. Using a differential hybridization assay, we isolated a full-length cDNA ofchop as an Epo upregulated gene in Rauscher murine erythroleukemia cells. RNase protection assays demonstrated that Epo or dimethyl sulfoxide induction increased steady-state mRNA levels 10- to 20-fold after 24 to 48 hours. Western blot analysis confirmed a marked increase in CHOP protein. Among the other c/ebp family members, only c/ebp β was also upregulated during erythroid differentiation. Among normal hematopoietic cells examined, steady-state mRNA levels were highest in erythroid cells, with levels peaking during terminal differentiation. Transient overexpression ofchop in Rauscher cells resulted in a significant increase in Epo- or dimethyl sulfoxide (DMSO)-induced hemoglobinization, further linking chop upregulation to erythroid differentiation. Artificial downregulation of chop in normal murine bone marrow cells with antisense oligodeoxynucleotides inhibited colony-forming unit-erythroid (CFU-E)–derived colony growth in a concentration-dependent manner. Burst-forming unit-erythroid (BFU-E)–derived colony growth was not affected. Using a Far Western type of analysis, we detected several potential CHOP binding partners among the nuclear proteins of Rauscher cells. Importantly, the number and relative abundance of these proteins changed with differentiation. The results strongly suggest that CHOP plays a role in erythropoiesis, possibly through interactions with both C/EBP and non-C/EBP family members.

Erythropoietin Induces the Tyrosine Phosphorylation of GAB1 and Its Association With SHC, SHP2, SHIP, and Phosphatidylinositol 3-Kinase

Five tyrosine-phosphorylated proteins with molecular masses of 180, 145, 116, 100, and 70 kD are associated with phosphatidylinositol 3-kinase (PI 3-kinase) in erythropoietin (Epo)-stimulated UT-7 cells. The 180- and 70-kD proteins have been previously shown to be IRS2 and the Epo receptor. In this report, we show that the 116-kD protein is the IRS2-related molecular adapter, GAB1. Indeed, Epo induced the transient tyrosine phosphorylation of GAB1 in UT-7 cells. Both kinetics and Epo dose-response experiments showed that GAB1 tyrosine phosphorylation was a direct consequence of Epo receptor activation. After tyrosine phosphorylation, GAB1 associated with the PI 3-kinase, the phosphotyrosine phosphatase SHP2, the phosphatidylinositol 3,4,5 trisphosphate 5-phosphatase SHIP, and the molecular adapter SHC. GAB1 was also associated with the molecular adapter GRB2 in unstimulated cells, and this association dramatically increased after Epo stimulation. Thus, GAB1 could be a scaffold protein able to couple the Epo receptor activation with the stimulation of several intracellular signaling pathways. Epo-induced tyrosine phosphorylation of GAB1 was also observed in normal human erythroid progenitors isolated from cord blood. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and thrombopoietin (TPO) also induced the tyrosine phosphorylation of GAB1 in UT-7 cells, indicating that this molecule participates in the signal transduction of several cytokine receptors.

Erythropoietin Induces the Tyrosine Phosphorylation of GAB1 and Its Association With SHC, SHP2, SHIP, and Phosphatidylinositol 3-Kinase

Five tyrosine-phosphorylated proteins with molecular masses of 180, 145, 116, 100, and 70 kD are associated with phosphatidylinositol 3-kinase (PI 3-kinase) in erythropoietin (Epo)-stimulated UT-7 cells. The 180- and 70-kD proteins have been previously shown to be IRS2 and the Epo receptor. In this report, we show that the 116-kD protein is the IRS2-related molecular adapter, GAB1. Indeed, Epo induced the transient tyrosine phosphorylation of GAB1 in UT-7 cells. Both kinetics and Epo dose-response experiments showed that GAB1 tyrosine phosphorylation was a direct consequence of Epo receptor activation. After tyrosine phosphorylation, GAB1 associated with the PI 3-kinase, the phosphotyrosine phosphatase SHP2, the phosphatidylinositol 3,4,5 trisphosphate 5-phosphatase SHIP, and the molecular adapter SHC. GAB1 was also associated with the molecular adapter GRB2 in unstimulated cells, and this association dramatically increased after Epo stimulation. Thus, GAB1 could be a scaffold protein able to couple the Epo receptor activation with the stimulation of several intracellular signaling pathways. Epo-induced tyrosine phosphorylation of GAB1 was also observed in normal human erythroid progenitors isolated from cord blood. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and thrombopoietin (TPO) also induced the tyrosine phosphorylation of GAB1 in UT-7 cells, indicating that this molecule participates in the signal transduction of several cytokine receptors.

Expression of the erythropoietin receptor by trophoblast cellsin the human placenta

Nonclassical sites of erythropoietin (EPO) and erythropoietin receptor (EPO-R) expression have been described that suggest new physiological roles for this hormone unrelated to erythropoiesis. The recent finding of EPO expression by trophoblast cells in the human placenta prompted us to consider whether these cells also express EPO-R. With use of immunocytochemistry, EPO-R was identified in villous and extravillous cytotrophoblast cells, as well as in the syncytiotrophoblast at all gestational ages. EPO-R was also expressed by cells within the villous core, including endothelial cells of fetoplacental blood vessels. Placental tissues and isolated and immunopurified trophoblast cells, as well as trophoblast-derived choriocarcinoma Jar cells, expressed immunoreactive EPO-R on Western blot. EPO-R mRNA was also detected in the same placental tissues and trophoblast cells by nested-primer reverse transcription-polymerase chain reaction. Finally, EPO-R was functional insofar as the receptor was phosphorylated on tyrosine residues in response to exogenous EPO treatment of cultured trophoblast or Jar cells. Thus, the present findings support the hypothesis that trophoblast cells of the human placenta express EPO-R. In view of these results, taken together with previous work demonstrating EPO expression by the same cells, an autocrine role for this hormone in the survival, proliferation, or differentiation of placental trophoblast cells is proposed.

Flt3 ligand can promote survival and macrophage development without proliferation in myeloid progenitor cells

Flt3 ligand elicits a variety of effects on early hemopoietic progenitors by occupying its cognate receptor, Flt3, a member of the type III tyrosine kinase receptor family. The cytokines macrophage colony-stimulating factor (M-CSF) and stem cell factor (SCF) bind to related members of this tyrosine kinase receptors family, c-fms and c-kit, respectively. The relative effects of the cytokines M-CSF, SCF, and Flt3L on the proliferation and development of the late myeloid progenitors granulocyte-macrophage colony-forming cells (GM-CFC) were investigated. Distinct biologic responses were stimulated by ligand binding to these different tyrosine kinase receptors in enriched GM-CFC. M-CSF stimulated GM-CFC to proliferate and develop into macrophages. SCF, on the other hand, stimulated GM-CFC to develop into neutrophils. Flt3 ligand had a relatively small proliferative effect on enriched GM-CFC compared to SCF and M-CSF and had no ability to either stimulate colony formation or synergize with these two cytokines in promoting DNA synthesis, colony formation, or expansion in liquid culture. Flt3 ligand, however, was capable of maintaining the clonogenic potential of GM-CFC and acted as an anti-apoptotic agent as assessed using the Annexin-V apoptosis assay. GM-CFC cultured in Flt3 ligand eventually formed macrophages and neutrophils in liquid culture. Labeling with the membrane-associated cell tracker dye PKH26 indicated that the majority of the enriched GM-CFC responded to Flt3 ligand by undergoing limited proliferation and macrophage development, whereas other cells survived but did not proliferate and differentiate into macrophages. Thus, Flt3 ligand promoted survival and stimulated development without proliferation in primary-enriched myeloid progenitor cells.

Role of Cytokine Signaling Molecules in Erythroid Differentiation of Mouse Fetal Liver Hematopoietic Cells: Functional Analysis of Signaling Molecules by Retrovirus-Mediated Expression

Erythropoietin (EPO) and its cell surface receptor (EPOR) play a central role in proliferation, differentiation, and survival of erythroid progenitors. Signals induced by EPO have been studied extensively by using erythroid as well as nonerythroid cell lines, and various controversial results have been reported as to the role of signaling molecules in erythroid differentiation. Here we describe a novel approach to analyze the EPO signaling by using primary mouse fetal liver hematopoietic cells to avoid possible artifacts due to established cell lines. Our strategy is based on high-titer retrovirus vectors with a bicistronic expression system consisting of an internal ribosome entry site (IRES) and green fluorescent protein (GFP). By placing the cDNA for a signaling molecule in front of IRES-GFP, virus-infected cells can be viably sorted by fluorescence-activated cell sorter, and the effect of expression of the signaling molecule can be assessed. By using this system, expression of cell-survival genes such as Bcl-2 and Bcl-XL was found to enhance erythroid colony formation from colony-forming unit–erythroid (CFU-E) in response to EPO. However, their expression was not sufficient for erythroid colony formation from CFU-E alone, indicating that EPO induces signals for erythroid differentiation. To examine the role of EPOR tyrosine residues in erythroid differentiation, we introduced a chimeric EGFR-EPOR receptor, which has the extracellular domain of the EGF receptor and the intracellular domain of the EPOR, as well as a mutant EGFR-EPOR in which all the cytoplasmic tyrosine residues are replaced with phenylalanine, and found that tyrosine residues of EPOR are essential for erythroid colony formation from CFU-E. We further analyzed the function of the downstream signaling molecules by expressing modified signaling molecules and found that both JAK2/STAT5 and Ras, two major signaling pathways activated by EPOR, are involved in full erythroid differentiation.

Role of Cytokine Signaling Molecules in Erythroid Differentiation of Mouse Fetal Liver Hematopoietic Cells: Functional Analysis of Signaling Molecules by Retrovirus-Mediated Expression

Erythropoietin (EPO) and its cell surface receptor (EPOR) play a central role in proliferation, differentiation, and survival of erythroid progenitors. Signals induced by EPO have been studied extensively by using erythroid as well as nonerythroid cell lines, and various controversial results have been reported as to the role of signaling molecules in erythroid differentiation. Here we describe a novel approach to analyze the EPO signaling by using primary mouse fetal liver hematopoietic cells to avoid possible artifacts due to established cell lines. Our strategy is based on high-titer retrovirus vectors with a bicistronic expression system consisting of an internal ribosome entry site (IRES) and green fluorescent protein (GFP). By placing the cDNA for a signaling molecule in front of IRES-GFP, virus-infected cells can be viably sorted by fluorescence-activated cell sorter, and the effect of expression of the signaling molecule can be assessed. By using this system, expression of cell-survival genes such as Bcl-2 and Bcl-XL was found to enhance erythroid colony formation from colony-forming unit–erythroid (CFU-E) in response to EPO. However, their expression was not sufficient for erythroid colony formation from CFU-E alone, indicating that EPO induces signals for erythroid differentiation. To examine the role of EPOR tyrosine residues in erythroid differentiation, we introduced a chimeric EGFR-EPOR receptor, which has the extracellular domain of the EGF receptor and the intracellular domain of the EPOR, as well as a mutant EGFR-EPOR in which all the cytoplasmic tyrosine residues are replaced with phenylalanine, and found that tyrosine residues of EPOR are essential for erythroid colony formation from CFU-E. We further analyzed the function of the downstream signaling molecules by expressing modified signaling molecules and found that both JAK2/STAT5 and Ras, two major signaling pathways activated by EPOR, are involved in full erythroid differentiation.

Severe leukopenia and dysregulated erythropoiesis in SCID mice persistently infected with the parvovirus minute virus of mice

Parvovirus minute virus of mice strain i (MVMi) infects committed granulocyte-macrophage CFU and erythroid burst-forming unit (CFU-GM and BFU-E, respectively) and pluripotent (CFU-S) mouse hematopoietic progenitors in vitro. To study the effects of MVMi infection on mouse hemopoiesis in the absence of a specific immune response, adult SCID mice were inoculated by the natural intranasal route of infection and monitored for hematopoietic and viral multiplication parameters. Infected animals developed a very severe viral-dose-dependent leukopenia by 30 days postinfection (d.p.i.) that led to death within 100 days, even though the number of circulating platelets and erythrocytes remained unaltered throughout the disease. In the bone marrow of every lethally inoculated mouse, a deep suppression of CFU-GM and BFU-E clonogenic progenitors occurring during the 20- to 35-d.p.i. interval corresponded with the maximal MVMi production, as determined by the accumulation of virus DNA replicative intermediates and the yield of infectious virus. Viral productive infection was limited to a small subset of primitive cells expressing the major replicative viral antigen (NS-1 protein), the numbers of which declined with the disease. However, the infection induced a sharp and lasting unbalance of the marrow hemopoiesis, denoted by a marked depletion of granulomacrophagic cells (GR-1(+) and MAC-1(+)) concomitant with a twofold absolute increase in erythroid cells (TER-119(+)). A stimulated definitive erythropoiesis in the infected mice was further evidenced by a 12-fold increase per femur of recognizable proerythroblasts, a quantitative apoptosis confined to uninfected TER-119(+) cells, as well as by a 4-fold elevation in the number of circulating reticulocytes. Therefore, MVMi targets and suppresses primitive hemopoietic progenitors leading to a very severe leukopenia, but compensatory mechanisms are mounted specifically by the erythroid lineage that maintain an effective erythropoiesis. The results show that infection of SCID mice with the parvovirus MVMi causes a novel dysregulation of murine hemopoiesis in vivo.

Erythroid gene expression is differentially regulated by erythropoietin, haemin and delta-aminolaevulinic acid in UT-7 cells

Erythropoietin (Epo) is essential for the later stages of erythropoiesis, acting to promote cell survival and proliferation, but its role in differentiation remains to be defined. The UT-7 cell line exhibits both erythroid and megakaryocytic characteristics and can be induced to differentiate along the erythroid pathway by Epo or the megakaryocytic pathway by phorbol myristic acetate. We have compared the effects of Epo and the chemical inducers, delta-aminolaevulinic acid (delta-ALA) and haemin on the differentiation capacity of UT-7 cells. Epo alone promoted relatively early events in erythroid maturation, without significant changes in haemoglobin production or morphology. GATA-2 and c-myb were down-regulated by Epo, and GATA-2 was further down-modulated by the inducers. Conversely, SCL expression was up-regulated by Epo and further increased by haemin and delta-ALA. Epo caused an increase in the proportion of cells expressing cell surface glycophorin A (GPA) and up-regulated beta- and gamma-globin by several fold. Both haemin and delta-ALA caused a de novo increase in alpha-globin expression as well as enhancing Epo-induced beta-globin expression, leading to a marked increase in haemoglobin production. These results suggest that haemoglobin production in UT-7 cells is limited by a deficiency of erythroid-specific aminolaevulinic acid synthase (ALAS-E) activity or globin synthesis as a consequence of their immaturity as a multipotential cell line.

Apoptosis and polycythemia vera

Polycythemia vera is an acquired clonal myeloproliferative disorder characterized by increased numbers of erythroid cells, often with a concomitant rise in neutrophils and/or megakaryocytes. Normally, erythropoietin is essential for the survival and proliferation of erythroid progenitors; however in polycythemia vera the erythroid progenitor cells can survive and develop in the absence of erythropoietin. Members of the Bcl-2 family of apoptosis regulators have been shown to mediate the erythropoietin-dependent survival of erythroid cells. In this article, recent advances in understanding the mechanisms used by erythroid progenitors from patients with polycythemia vera to control apoptosis, are 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.

A novel way to induce erythroid progenitor self renewal: cooperation of c-Kit with the erythropoietin receptor

Red blood cells are of vital importance for oxygen transport in vertebrates. Thus, their formation during development and homeostasis requires tight control of both progenitor proliferation and terminal red cell differentiation. Self renewal (i.e. long-term proliferation without differentiation) of committed erythroid progenitors has recently been shown to contribute to this regulation. Avian erythroid progenitors expressing the EGF receptor/c-ErbB (SCF/TGFalpha progenitors) can be induced to long-term proliferation by the c-ErbB ligand transforming growth factor alpha and the steroids estradiol and dexamethasone. These progenitors have not yet been described in mammals and their factor requirements are untypical for adult erythroid progenitors. Here we describe a second, distinct type of erythroid progenitor (EpoR progenitors) which can be established from freshly isolated bone marrow and is induced to self renew by ligands relevant for erythropoiesis, i.e. erythropoietin, stem cell factor, the ligand for c-Kit and the glucocorticoid receptor ligand dexamethasone. Limiting dilution cloning indicates that these EpoR progenitors are derived from normal BFU-E/CFU-E. For a detailed study, mEpoR progenitors were generated by retroviral expression of the murine Epo receptor in bone marrow erythroblasts. These progenitors carry out the normal erythroid differentiation program in recombinant differentiation factors only. We show that mEpoR progenitors are more mature than SCF/TGFalpha progenitors and also do no longer respond to transforming growth factor alpha and estradiol. In contrast they are now highly sensitive to low levels of thyroid hormone, facilitating their terminal maturation into erythrocytes.

Oxygen tension modulates beta-globin switching in embryoid bodies

Little is known about the factors influencing the hemoglobin switch in vertebrates during development. Inasmuch as the mammalian conceptus is exposed to changing oxygen tensions in utero, we examined the effect of different oxygen concentrations on beta-globin switching. We used an in vitro model of mouse embryogenesis based on the differentiation of blastocyst-derived embryonic stem cells to embryoid bodies (EBs). Cultivation of EBs at increasing oxygen concentrations (starting at 1% O2) did not influence the temporal expression pattern of embryonic (betaH1) globin compared to the normoxic controls (20% O2). In contrast, when compared to normoxically grown EBs, expression of fetal/adult (betamaj) globin in EBs cultured at varying oxygen concentrations was delayed by about 2 days and persisted throughout differentiation. Quantitation of hemoglobin in EBs using a 2,7-diaminofluorene-based colorimetric assay revealed the appearence of hemoglobin in two waves, an early and a late one. This observation was verified by spectrophotometric analysis of hemoglobin within single EBs. These two waves might reflect the switch of erythropoiesis from yolk sac to fetal liver. Reduced oxygenation is known to activate the hypoxia-inducible factor-1 (HIF-1), which in turn specifically induces expression of a variety of genes among them erythropoietin (EPO). Although EBs increased EPO expression upon hypoxic exposure, the altered beta-globin appearance was not related to EPO levels as determined in EBs overexpressing EPO. Since mRNA from both mouse HIF-1alpha isoforms was detected in all EBs tested at different differentiation stages, we propose that HIF-1 modulates beta-globin expression during development.

Specificity of signaling by hematopoietic cytokine receptors: instructive versus permissive effects

The helical cytokines constitute a family of proteins with a common three-dimensional structure. They exert a wide variety of biological effects with a preference for the hematopoietic system. The effects of helical cytokines are mediated by cell surface receptors, which belong to the cytokine receptor superfamily and signal by activating cytoplasmic tyrosine kinases of the Janus kinase (Jak) family and other downstream signaling pathways. The relevance of each of these pathways for eliciting a specific cellular response remains to be determined. This review will focus on cytokine receptors which play a role in the regulation of hematopoiesis and summarize data the address the question how specificity of signaling is achieved.

Analysis of erythropoietin and erythropoietin receptor genes expression in cattle during acute infection with Trypanosoma congolense

Acute Trypanosoma congolense infection induced moderate, transient anemia in N'Dama cattle (trypanotolerant) and severe anemia in Boran cattle (trypanosusceptible). Erythropoietin receptor (EpoR) was cloned and sequenced from the two breeds of cattle. A single position mutation of Tyr in the Boran to His in the N'Dama predicted amino acid sequence was revealed. The mRNA transcription of erythropoietin (Epo) in kidneys and EpoR in the bone marrow of infected cattle was determined by competitive reverse transcription and the polymerase chain reaction (RT-PCR). Though Epo mRNA transcription increased in the kidneys during infection, the increase was not significantly different (p>0.05) between the two breeds of infected cattle. The level of EpoR transcripts in the bone marrow of infected N'Damas was significantly higher (p<0.05) than that detected in the marrows from infected Boran cattle. While infection seem to increase levels of transcription of IL-1alpha and beta, and TNFalpha in kidneys from both Boran and N'Dama cattle, no significant difference was detected in the level of mRNAs of these cytokines in the kidney from the two breed of cattle. The amount of IFNgamma mRNA transcripts were not changed with infection in N'Dama cattle, while on the contrary a significant higher levels of IFNgamma was found in kidneys from infected Boran cattle as compared to the other groups. A significant (p<0.05) increase in the levels of IL-1alpha and beta, and IFNgamma mRNA transcripts were detected in the marrows of infected Borans as compared to the infected N'Dama cattle. In this study the increase in the level of TNFalpha mRNA in the marrows of the two infected breeds was not different. This implies there is no negative effect of TNFalpha on hematopoiesis during acute infection. These findings suggest that the levels of Epo and EpoR in the infected Boran cattle were inadequate for their degree of anemia, which might be due in part to high expression of IFNgamma during acute infection with T. congolense.

Signaling by the cytokine receptor superfamily

A variety of cytokines that regulate functions of multiple lineages share the utilization of receptors that are structurally and functionally related and are referred to as the cytokine receptor superfamily. These receptors associate with one or more of the four mammalian Janus kinases (Jaks) and ligand-induced receptor aggregation results in their activation. Critical roles for Jak3 and Jak2 are demonstrated by the phenotypes of mice that lack each gene. Among the substrates of the Jaks are one or more of the seven members of the signal transducers and activators of transcription (Stats). Each Stat family member plays a critical role in the biological functions of specific cytokines as demonstrated by the phenotype of mice lacking one or more of these genes.

Hemopoietic growth factor receptor abnormalities in leukemia

Growth factor and cytokine control of hemopoiesis, the process of blood cell development, is mediated by specific interactions with cell-surface receptors. Hemopoietic growth factor receptors belong to two major families, the transmembrane protein tyrosine kinases and the hemopoietin receptors. Ligand binding stimulates receptor aggregation and activation resulting in transduction of signals that induce diverse cellular responses including proliferation, maturation, prevention of apoptosis and/or functional activation. Deregulation of hemopoiesis can result in leukemia, the malignant transformation of blood cells, or the development of other hemoproliferative disorders. As hemopoietic growth factor receptors are integral to blood cell regulation, it is feasible that receptor abnormalities may contribute to leukemia by circumventing normal growth factor control or altering the balance of proliferation and differentiation. Although considerable experimental evidence has clearly established the leukemogenic potential of mutated growth factor receptors, studies to date suggest that such abnormalities contribute only rarely to human disease.

Activation of the Jak-STAT-signaling pathway in embryonic lens cells

Previous studies showed that lens epithelial cells proliferate rapidly in the embryo and that a lens mitogen, most likely derived from the blood, is present in the anterior chamber of the embryonic eye (Hyatt, G. A., and Beebe, D. C., Development 117, 701-709, 1993). Messenger RNAs for several growth factor receptors have been identified in embryonic lens epithelial cells. We tested several growth factors that are ligands for these receptors for their ability to maintain lens cell proliferation. Embryo serum, PDGF, GM-CSF, and G-CSF maintained lens cell proliferation, but NGF, VEGF, and HGF did not. This and a previous study (Potts, J. D., Harocopos, G. J., and Beebe, D. C., Curr. Eye Res. 12, 759-763, 1993) detected members of the Janus kinase family (Jaks) in the developing lens. Because Jaks are central players in the Jak-STAT-signaling pathway, we identified STAT proteins in the lens and tested whether they were phosphorylated in response to mitogens. STAT1 and STAT3, but not STAT 5 were detected in chicken embryo lens epithelial cells. Only STAT3 was found in terminally differentiated lens fiber cells. STAT1 and STAT3 were phosphorylated in lens cells analyzed immediately after removal from the embryo and when lens epithelial explants were treated with embryo serum, PDGF, or GM-CSF, but not with NGF. Chicken embryo vitreous humor or IGF-1, factors that stimulate lens cell differentiation, but not proliferation, did not cause STAT phosphorylation. When lens epithelial cells were cultured for 4 h in unsupplemented medium, STAT1 and STAT3 declined to nearly undetectable levels. Treatment with PDGF or embryo serum for an additional 15 min restored STAT1 and -3 levels. This recovery was blocked by cycloheximide, but not actinomycin D, suggesting that STAT levels are regulated at the level of translation. STAT levels were maintained in epithelial explants by lens mitogens, but not by factors that stimulated lens fiber differentiation. Both factors that stimulated lens cell proliferation and those that caused fiber differentiation protected cultured lens epithelial cells from apoptosis. These data suggest that the factor(s) responsible for lens cell proliferation in vivo activates the Jak-STAT-signaling pathway. They also indicate that growth factors maintain STAT protein levels in lens epithelial cells by promoting the translation of STAT mRNA, an aspect of STAT regulation that has not been described previously. Signaling by most of the growth factors and cytokines known to activate the Jak-STAT pathway has been disrupted in mice by mutation or targeted deletion. Consideration of the phenotypes of these mice suggests that the factor responsible for lens cell proliferation in vivo may be a growth factor or cytokine that has not yet been described.