Erythroid Progenitor Cell Development Research Articles

Arsenite and monomethylarsonous acid disrupt erythropoiesis through combined effects on differentiation and survival pathways in early erythroid progenitors

Strong epidemiological evidence demonstrates an association between chronic arsenic exposure and anemia. We recently found that As+3 impairs erythropoiesis by disrupting the function of GATA-1; however the downstream pathways impacted by the loss of GATA-1 function have not been evaluated. Additionally, our previous findings indicate that the predominant arsenical in the bone marrow of mice exposed to As+3 in their drinking water for 30 days was MMA+3, but the impacts of this arsenical on erythorpoisis also remain largely unknown. The goal of this study was to address these critical knowledge gaps by evaluating the comparative effects of arsenite (As+3) and the As+3 metabolite, monomethyarsonous acid (MMA+3) on two critical regulatory pathways that control the differentiation and survival of early erythroid progenitor cells. We found that 500 nM As+3 and 100 and 500 nM MMA+3 suppress erythropoiesis by impairing the differentiation of early stage erythroid progenitors. The suppression of early erythroid progenitor cell development was attributed to combined effects on differentiation and survival pathways mediated by disruption of GATA-1 and STAT5. Our results show that As+3 primarily disrupted GATA-1 function; whereas, MMA+3 suppressed both GATA-1 and STAT5 activity. Collectively, these findings provide novel mechanistic insights into arsenic-induced dyserythropoiesis and suggest that MMA+3 may be more toxic than As+3 to early developing erythroid cells.

Persistent injury-associated anemia: the role of the bone marrow microenvironment

BackgroundThe regulation of erythropoiesis involves hematopoietic progenitor cells, bone marrow stroma, and the microenvironment. Following severe injury, a hypercatecholamine state develops that is associated with increased mobilization of hematopoietic progenitor cells to peripheral blood and decreased growth of bone marrow erythroid progenitor cells that manifests clinically as a persistent injury-associated anemia. Changes within the bone marrow microenvironment influence the development of erythroid progenitor cells. Therefore, we sought to determine the effects of lung contusion, hemorrhagic shock, and chronic stress on the hematopoietic cytokine response. Materials and methodsBone marrow was obtained from male Sprague–Dawley rats (n = 6/group) killed 7 d after lung contusion followed by hemorrhagic shock (LCHS) or LCHS followed by daily chronic restraint stress (LCHS/CS). End point polymerase chain reaction was performed for interleukin-1β, interleukin-10, stem cell factor, transforming growth factor-β, high-mobility group box-1 (HMGB-1), and B-cell lymphoma-extra large. ResultsSeven days following LCHS and LCHS/CS, bone marrow expression of prohematopoietic cytokines (interleukin-1β, interleukin-10, stem cell factor, and transforming growth factor-β) was significantly decreased, and bone marrow expression of HMGB-1 was significantly increased. B-cell lymphoma-extra large bone marrow expression was not affected by LCHS or LCHS/CS (naïve: 44 ± 12, LCHS: 44 ± 12, LCHS/CS: 37 ± 1, all P > 0.05). ConclusionsThe bone marrow microenvironment was significantly altered following severe trauma in a rodent model. Prohematopoietic cytokines were downregulated, and the proinflammatory cytokine HMGB-1 had increased bone marrow expression. Modulation of the bone marrow microenvironment may represent a therapeutic strategy following severe trauma to alleviate persistent injury-associated anemia.

Low level arsenite exposures suppress the development of bone marrow erythroid progenitors and result in anemia in adult male mice

Epidemiological studies report an association between chronic arsenic (As) exposure and anemia in men, and women who are predisposed to anemia. The purpose of these studies was to determine whether a 60 d drinking water exposure of adult male C57BL/6J mice to 0, 100, and 500ppb arsenite (As+3) results in anemia due to alterations in erythroid progenitor cell development in the bone marrow. Exposure to 500ppb As+3 for 60 d resulted in a reduction of mean corpuscular hemoglobin (MCH) levels, but did not significantly alter red blood cell (RBC) counts, hemoglobin (Hgb) levels, mean corpuscular Hgb concentrations (MCHC), or mean corpuscular volumes (MCV). Attenuation of burst-forming unit-erythroid (BFU-E) colony formation was observed in bone marrow cells of mice exposed to 500ppb As+3. The differentiation of late-stage bone marrow erythroblasts as defined by CD71 and Ter119 surface marker expression was reduced with the 500ppb As+3 exposure. Mice exposed to 500ppb As+3 also had elevated serum levels of erythropoietin (EPO). Collectively, these results show that exposure to low levels of As+3 attenuate the development of early BFU-E cells and reduce the differentiation of late-stage erythroblasts. This suppression of bone marrow erythropoiesis may be a contributing factor to the mild hypochromic anemia observed in 500ppb As+3 exposed mice.

Disrupted Expression of the Splicing Regulators Tra2-Beta and SRSF1 and Subsequent Aberrant Splicing of FLVCR1 Transcript As an Underlying Mechanism for the Erythropoietic Failure Observed in Diamond-Blackfan Anemia

Diamond Blackfan Anemia (DBA) is a rare inherited anemia caused by disruption of erythroid progenitor cell development at the burst-forming and colony-forming units. Approximately 50% of DBA patients have additional non-hematopoietic abnormalities including heart defects and short stature. Sixteen genes encoding specific ribosomal proteins (RPs) and the transcription factor GATA1 have been found mutated in approximately 65% of DBA patients, with the RPS19 gene being mutated in 25% of patients. Because RPs are components of the ribosome that is critical for protein synthesis, the identification of RP mutations suggest a translation deficiency as a mechanism for erythroid progenitor failure. However, erythropoietic failure in DBA may arise by causes other non-translational functions of the DBA genes. We have previously reported that inhibiting the heme exporter protein FLVCR1 in human hematopoietic stem cells and progenitors (HSC/Ps), is sufficient to specifically block erythropoiesis, which mimics the hematological features observed in DBA. Moreover, we reported aberrant splicing of exons (E) 2 and 3 of the FLVCR1 transcript in DBA erythroid cells that resulted in disruption of FLVCR1 protein expression and function. We observed aberrant FLVCR1 splicing in erythroid cells from patients with RPS19 and RPL5 mutations and from typical DBA patients whose genetic background has not been known at that time; thus, suggesting FLVCR1 dysfunction as central for the DBA erythropoietic failure.In the present work, we obtained evidence suggesting that DBA pathogenesis is caused by abnormality in the cellular splicing machinery. We performed In silico analysis of the binding motifs of seven SR proteins on each of the FLVCR1 exons. We found binding motifs for TRA2-β, SRSF1 and 9G8 on E2 and/or E3, the two FLVCR1 exons that are skipped in DBA erythroid cells and in RPS19-disrupted K562 cells. We focused on expression analysis of TRA2B and SRSF1 in RPS19-reduced K562 cells. Expression of both TRA2B and SRSF1 mRNA were significantly disrupted in RPS19-disrupted K562 cells, which correlated with disrupted expression of their respective proteins. To test the specificity of disrupted SR splicing regulators and aberrant FLVCR1 splicing, we analyzed TRA2-β, and SRSF1 protein expression, and FLVCR1 aberrant splicing, in K562 cells disrupted in SBDS gene. The SBDS gene product is critical for ribosome biogenesis and the gene is mutated in 90% of patients with Shwachman-Diamond Syndrome. Interestingly, K562 cells down-regulated in SBDS showed normal levels of TRA2-β and SRSF1 expression, and no FLVCR1 aberrant splicing was observed. Importantly, erythroid differentiation potential of the DBA cell model can be rescued by expression of exogenous Tra2-β cDNA or FLVCR1 cDNA.Altogether our data suggests that mutations in the ribosome protein genes in DBA result in down-regulation of the splicing factors, TRA2-β and SRSF1, which impair normal FLVCR1 splicing and consequently erythropoiesis failure. DisclosuresNo relevant conflicts of interest to declare.

Critical Role of the Matricellular Protein SPARC in Mediating Erythroid Progenitor Cell Development in Zebrafish

Sparc (osteonectin) is a multifunctional matricellular glycoprotein expressed by many differentiated cells. Members of this family mediate cell-matrix interactions rather than acting as structural components of the extracellular matrix (ECM); therefore, they can influence many remodelling events, including haematopoiesis. We have investigated the role of sparc in embryonic haematopoiesis using a morpholino antisense oligonucleotide-based knockdown approach. Knockdown of sparc function resulted in specific erythroid progenitor cell differentiation defects that were highlighted by changes in gene expression and morphology, which could be rescued by injection of sparc mRNA. Furthermore, a comparison of blood phenotypes of sparc and fgfs knockdowns with similar defects and the sparc rescue of the fgf21 blood phenotype places sparc downstream of fgf21 in the genetic network regulating haematopoiesis in zebrafish. These results establish a role for an ECM protein (Sparc) as an important regulator of embryonic haematopoiesis during early development in zebrafish.

Transcriptome- Based Analysis of EPO Dependent Cfu-e to Erythroblast Development

AbstractAbstract 3188To provide molecular insight into erythroid developmental programs, including EPO- regulated aspects, we have employed transcriptome-based approaches to analyze the stage-wise development of purified murine bone marrow- derived CFUe, proerythroblasts and maturing erythroblasts. In vivo and ex vivo, these progenitors develop as KitposCD71highTer119neg; KitnegCD71highTer119neg; and KitnegCD71highTer119pos cohorts (designated as E1, E2, E3 stages, respectively). In the context of EPO- modulation, stage E1 cells exhibited ∼250 EPO- regulated target genes. In stage-E2 proerythroblasts, in contrast, 750 transcripts proved to be EPO- regulated while stage-E3 erythroblasts exhibited only select EPO- modulated genes (<50). At E1 and E2 stages, EPO- regulated targets included overlapping yet distinct sets, and this was reflected in functional sets of GO terms. Major EPO- targets at stage E1 included cell cycle and cell biogenesis genes, while in E2 proerythroblasts, negative regulators of protein kinases (and kinase activity) constituted a major EPO/EPOR target set. As E1 CFUe transitioned to E2 proerythroblasts, an unexpected transient narrowing in the complexity of overall expressed genes was exhibited. Stage- modulated global sets of transcripts included myeloid cell differentiation factors, cell number homeostasis factors and heme biosynthetic processes. As E2 proerythroblasts transitioned to E3 erythroblasts, functional GO sets were associated predominantly with dynamics in organelle and cellular compartments. In addition, HomoloGene and Connectivity Mapping approaches were applied to compare transcriptomes of stage E1, E2 and E3 murine bone marrow erythroid progenitors with four recently studied stages of human erythroid progenitor cell development (here, termed H1, H2, H3 and H4). High correlation of stage E1 m-CFUe with not only human H1 CFUe but also H2 proerythroblasts was observed (0.59 and 0.57 correlations). Stage E2 murine proerythroblasts best corresponded to H2 human proerythroblasts (0.36 correlation score), while E3 murine erythroblasts aligned closely with human H4 late stage erythroblasts (0.58 correlation score). These latter analyses provide novel transcriptome- based comparisons, and transcript specific insight, into conserved vs distinct features of murine and human erythroid development at CFUe to maturing erythroblast stages. Disclosures:No relevant conflicts of interest to declare.

Isolation of hematopoietic stem cells and the effect of CD38 expression during the early erythroid progenitor cell development process

The aim of this study was to investigate changes in primitive hematopoietic cells through CD38 expression, identify the stage at which erythrocyte differentiation CD38 gains activity and the effects of serum factors on this expression by establishing a hematopoietic stem cell system in the erythroid development process. Using an immunomagnetic labeling and separation technique, CD34(+) cells were selected from cord blood. The CD34(+) cells were cultured in a 2 mM L-glutamine-enriched medium containing erythropoietin (Epo), penicillin-streptomycin and stem cell factor (SCF), and were incubated in 5% CO(2) at 37°C. In erythroid development pathways following CD38 expression, primitive/progenitor human hematopoietic cells obtained from cord blood were assessed through the erythroid development process in a serum-free medium in the presence of proper SCF and Epo. At the end of the 26-day process, using staining with a Megacult-c staining kit, it was determined that progenitor cells nucleate and differentiate into erythroid cell lines of 8-10 μm. During the course of this process, we analyzed increases over time in NAD glycohydrolase activity rates using the supernatant liquid samples. Results of co-culture experiments in cell culture studies showed that the stimulating effects of CD38 expression originate from specific serum factors. CD38 expression has been shown to occur at hematopoietic cell sources as well as at a number of differentiation levels. In the proliferation process the possible induction of CD38 through specific serum factors leads us to conclude that it may be involved in proliferation with a physiological task or that it may be involved in an event, such as an apoptotic process.

CpG-ODN 2006 and human parvovirus B19 genome consensus sequences selectively inhibit growth and development of erythroid progenitor cells

AbstractRecent studies have shown that anemia is commonly observed after exposure to pathogens or pathogen-derived products, which are recognized via Toll-like receptor 9 (TLR9). In the current study, we demonstrate that CpG oligodeoxynucleotide-2006, a TLR9 ligand with phosphodiester (PO; 2006-PO) but not with the phosphorothioate backbone, selectively inhibits the erythroid growth derived from human CD34+ cells. The 2006-PO was internalized by the erythroid progenitors within 30 minutes; however, expression of TLR9 mRNA was not detected in these cells. The 2006-PO directly inhibited burst-forming unit-erythroid growth, resulted in the accumulation of cells in S and G2/M phases, and increased cell size and frequency of apoptotic cells. These features were similar to those observed in erythroid progenitors infected with human parvovirus B19 that causes pure red cell aplasia. The consensus sequence of 2006-PO was defined as 5′-GTTTTGT-3′, which was located in the P6-promoter region of B19 and inhibited erythroid growth in a sequence-specific manner and down-regulated expression of erythropoietin receptor (EPOR) mRNA and EPOR. B19 genome extracted from serum also inhibited erythroid growth and down-regulated expression of EPOR on glycophorin A+ cells. These results provide a possible insight into our understanding of the mechanisms of human parvovirus B19-mediated inhibition of erythropoiesis.

Non-Redundant Roles for an Epo/EpoR/STAT5 Regulated Intracellular Serpin, Serpina-3G, during Epo- and Anemia- Induced Erythropoiesis

Interest in advancing a more sophisticated knowledge of EPOR action mechanisms remains intense based on the emergence of novel ESAs; cytoprotective effects of EPO and ESAs in ischemically injured tissues; and adverse effects of EPO on cancer progression. By employing primary bone marrow erythroblasts and global transcriptome analyses, we recently have discovered several previously undescribed EPO/EPOR response factors that regulate erythroid progenitor cell development. These include Cyclin-G2, the pseudokinase Tribbles-3 (TRB3), and a strongly EPO-induced intracellular serpin, Serpina-3G (S3G). Here we report on EPOR- mediated routes to S3G expression, and on phenotypes resulting from S3G's reinforced, or disrupted expression. As determined using early-stage erythroid progenitors with minimal knocked-in EPOR alleles, S3G expression proved to depend upon an EPOR-PY343/STAT5 axis. At the protein level, S3G was cytoplasmically retained (as was S3G-flag in stably transduced G1E2 cells, and in transgenic Gata1-IE3.9int-S3G mice). Ectopic expression of S3G further revealed advantaged expansion capacities. To critically assess S3G's function, S3G knock-out mice were prepared. Consistent with a hypothesis that S3G might affect erythropoiesis selectively during anemia, BFUe, CFUe and red cell levels in S3G-null mice at steady-state were not significantly perturbed. However, S3G's deletion markedly impaired erythropoiesis during hemolytic anemia, and inhibited EPO- induced erythropoiesis per se. In particular, hematocrits during anemia in S3G-null mice fell to 23.0 +/− 2.2% as compared to 38.7 +/− 1.8% for S3G+/+ controls. In response to EPO, an increase in hematocrit of only 1.75 +/− 0.2% was exhibited for S3G-null mice as compared to 4.9 +/− 0.5% for S3G+/+ controls. Investigations therefore establish S3G as an important non-redundant mediator of EPO- and anemia- induced erythropoiesis. They also raise interesting new questions concerning the nature of S3G's action pathways, and S3G's candidate pro-erythroblast intrinsic target protease.

Enhanced alternative splicing of the FLVCR1 gene in Diamond Blackfan anemia disrupts FLVCR1 expression and function that are critical for erythropoiesis

Diamond-Blackfan anemia is a fatal congenital anemia characterized by a specific disruption in erythroid progenitor cell development. Approximately 25% of patients have mutations in the ribosomal protein RPS19 suggesting that Diamond-Blackfan anemia may be caused by a defect in ribosome biogenesis and translation. However, it is unclear how these defects specifically disrupt early erythropoiesis. Recent studies have shown that the retroviral receptor/heme exporter FLVCR1 is critical for early erythropoiesis. FLVCR1 null mice, despite dying in utero and having reduced myeloid and lymphoid cell growth, show a disruption in early erythropoiesis and have craniofacial and limb deformities similar to those found in some Diamond-Blackfan anemia patients. In this study, we recapitulated the Diamond-Blackfan anemia hematologic features of reduced erythropoiesis but normal myelopoiesis by disrupting FLVCR1 in human hematopoietic stem cells. We found that CD71(high) cells, which are enriched for immature erythroid cells, from Diamond-Blackfan anemia patients negative for RPS19 gene mutations express alternatively spliced isoforms of FLVCR1 transcript which encode proteins whose expression and function are disrupted. More importantly, our results suggest alternative splicing of FLVCR1 is significantly enhanced in Diamond-Blackfan anemia immature erythroid cells. Furthermore, we also observed enhanced FLVCR1 alternative splicing and a dramatic reduction of FLVCR1 protein expression in RPS19 down-regulated human K562 cells, which were used as a model to represent RPS19 gene mutated Diamond-Blackfan anemia. Taken together, our results suggest enhanced alternative splicing of FLVCR1 transcripts and subsequent FLVCR1 insufficiency as an additional contributing factor to the erythropoietic defect observed in Diamond-Blackfan anemia.

Profiling of Early Gene Expression Induced by Erythropoietin Receptor Structural Variants

The development of erythroid progenitor cells is triggered via the expression of the erythropoietin receptor (EPOR) and its activation by erythropoietin. The function of the resulting receptor complex depends critically on the presence of activated JAK2, and the complex contains a large number of signaling molecules recruited to eight phosphorylated tyrosine residues. Studies using mutant receptor forms have demonstrated that truncated receptors lacking all tyrosines are able to support red blood cell development with low efficiency, whereas add-back mutants containing either Tyr343 or Tyr479 reconstitute EPOR signaling and erythropoiesis in vivo. To study the contribution of tyrosines to receptor function, we analyzed the activation of essential signaling pathways and early gene induction promoted by different receptor structural variants using human epidermal growth factor receptor/murine EPOR hybrids. In our experiments, receptors lacking all tyrosine residues or the JAK2-binding site did not induce mitogenic and anti-apoptotic signaling, whereas add-back mutant receptors containing single tyrosine residues (Try343 and Tyr479) supported the activation of these functions efficiently. Profiling of early gene expression using cDNA array hybridization revealed that (i) the high redundancy in the activation of signaling pathways is continued at the level of transcription; (ii) the expression of many genes targeted by the wild-type receptor is not supported by add-back mutants; and (iii) a small set of genes are exclusively induced by add-back receptors. We report the identification of several early genes that have not been implicated in the EPOR-dependent response so far.

Lyn Kinase Supports Primary Erythroid Progenitor Cell Development.

Several lines of investigation have implicated Lyn as an important positive effector of red cell development: Deregulated Src kinases promote erythroleukemia, and Lyn is the predominant Src kinase of erythroid cells; as critical erythropoietic factors, Kit and the Epo receptor each stimulate Lyn kinase; and in an insightful set of investigations in J2E cells, Lyn has been shown to be required for Epo-dependent late erythroid development. Based on these considerations, adult bone marrow-derived primary erythroid progenitor cells from Lyn −/− mice presently were assessed for their ex vivo growth, survival and differentiation potentials. Lyn −/− erythroid progenitors expanded efficiently in serum-free media, and showed essentially wild-type Epo dose-dependent proliferative responsiveness. When transferred to BSA/insulin/transferrin differentiation medium, however, Lyn −/− erythroid progenitor cells clearly faltered in their development to Ter119-high, CD71-low erythroblasts. For these Lyn −/− cells, annexin-V binding studies revealed that this defect was associated, in part, with a stage-specific loss in survival potential. Interestingly, however, this defect was not Epo-dose dependent. In addition, MACS-isolated Kit-positive early erythroid progenitor cells prepared from Lyn −/− mice (unlike preparations from wild-type mice) failed to support synergistic effects of SCF-plus-Epo in 3HdT incorporation assays. In response to phenylhydrazine, Lyn −/− mice exhibited expanded erythroid progenitor cell pools (including BFUe and CFUe), and this hyper-expansion may occur in response to the compromised survival of late Lyn −/− erythroblasts. Analyses of pp60-Src expression revealed elevated levels of activated PY416-Src specifically in Lyn −/− EPC, a finding that is consistent with the activation of apparent compensatory mechanisms. In contrast, no significant changes in the levels of GATA1 or other assessed erythroid defining factors were detected. In response to phenylhydrazine, Lyn −/− mice showed ≥2-fold enhanced splenomegaly, as well as enhanced frequencies of BFUe, CFUe and Ter119(+) cells. Overall, these studies in primary erythroid progenitor cells from Lyn −/− mice reveal a previously undiscovered positive role for Lyn as a late-stage specific positive effector of erythroid cell survival, and regulator of Epo receptor and Kit co-signaling.

Potential role of protein kinase C on the differentiation of erythroid progenitor cells

The effect of protein kinase C inhibitors, staurosporine and 1-(5-isoquinolinyl sulfonyl)-2-methyl piperazine(H7) onin vitro differentiation of erythroid progenitor cells which were isolated from spleens of mice infected with the anemia-inducing strain of Friend virus were examined. Erythropoietin-mediated differentiation of erythroid progenitor cells, as determined by the incorporation of59Fe into protoporphyrin, was inhibited by staurosporine and H7 in a concentration-dependent manner. Scatchard analysis of the3H-phorbol-12, 13-dibutyrate binding to erythroid progenitor cells revealed that at the high affinity sites the dissociation constant was 22nM and the maximum number of3H-phorbol-12,13-dibutyrate binding sites per cell was approximately 3.7×105. Cytosolic protein kinase C was isolated from erythroid progenitor cells and then purified by sequential column chromatography. Two isoforms of protein kinase C were found. Photoaffinity labeling of the purified protein kinase C samples with3H-phorbol 12-myristate 13-acetate followed by analysis of SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and autofluorography showed radiolabeled 82-KDa peptides. Radiolabeling of the 82-kDa peptides with3H-phorbol 12-myristate 13-acetate was almost completely blocked by excess unlabeled phorbol 12-myristate 13-acetate. Results of phorbol 12-myristate 13-acetate-promoted phosphorylation with the purified protein kinase C samples showed that the phosphorylation of 82-kDa peptides was increased as the concentration of phorbol 12-myristate 13-acetate was increased from 10−8 M to 10−4M. In light of the findings that erythroid progenitor cells possessed an abundance of protein kinase C and that staurosporine and H7 inhibited erythroid differentiation, it seemed likely that protein kinase C would play a role in the erythroid progenitor cell development.

Interferon‐γ exerts its negative regulatory effect primarily on the earliest stages of murine erythroid progenitor cell development

Interferon-gamma (INF-gamma) has been shown to suppress erythropoiesis and perhaps to contribute to the anemia of chronic disease. In this study we demonstrated that the concentration of INF gamma required to suppress murine burst forming unit-erythroid (BFU-E) growth was significantly less than that required to suppress colony forming unit-erythroid (CFU-E) growth. INF gamma acted at the most primitive step in erythroid progenitor cell differentiation and proliferation, as inhibition was maximal when added at the time of BFU-E culture initiation. Inhibition was progressively less if INF gamma addition was delayed after culture initiation. The effects of INF gamma on BFU-E did not require the presence of interleukin-1 alpha (IL-1 alpha), tumor necrosis factor-alpha (TNF alpha), or granulocyte macrophage colony stimulating factor (GM-CSF), as its effects were not neutralized by monoclonal antibodies against IL-1 alpha, TNF alpha, or GM-CSF. This applied whether INF gamma was added to culture with individual antibodies or with a combination of all three antibodies. INF gamma was not required for IL-1 alpha- or TNF alpha-induced suppression of BFU-E, as their effects were not neutralized by a monoclonal anti-INF gamma antibody. In contrast, GM-CSF-induced suppression of BFU-E was negated by the simultaneous addition of anti-INF gamma. We have previously shown that the addition of TNF alpha does not suppress BFU-E growth in cultures from marrow depleted of macrophages. Suppression did occur, however, if a small concentration of INF gamma that does not inhibit and increasing concentration of TNF alpha were added to culture, suggesting a synergistic effect between INF-gamma and TNF alpha. These observations suggest that INF gamma is a potent direct inhibitor of erythroid colony growth in vitro. It exerts its negative regulatory effect primarily on the earliest stages of erythroid progenitor cell differentiation and proliferation, as much higher doses are required to suppress late erythroid cell development. INF gamma is also involved in GM-CSF-induced inhibition of BFU-E colony growth.

The extended box 2 subdomain of erythropoietin receptor is nonessential for Jak2 activation yet critical for efficient mitogenesis in FDC-ER cells

The development of erythroid progenitor cells depends upon exposure to the glycoprotein hormone, erythropoietin (EPO). Binding of EPO to its transmembrane receptor leads to the rapid tyrosine phosphorylation of several cellular targets including Shc, Raf-1, Gap120, the cloned EPO receptor (EPOR), pp100/97, and a M(r) 130,000 EPO-activated receptor-associated Janus protein tyrosine kinase, Jak2. A membrane-proximal cytosolic region of the EPOR recently has been shown to be essential for the activation of Jak2 and sufficient for EPO-induced mitogenesis. This cytosolic region includes 8-12 amino acid box 1 and box 2 subdomains, which are conserved in certain class I receptors as well as a more distal 10-40 amino acid subdomain (extended box 2 subdomain, ExBx2), which likewise is implicated in mitogenic signaling. Through the expression of EPOR carboxyl-terminal truncation mutants in FDC-P1 cells, we presently show that an EPOR form truncated within the ExBx2 domain efficiently activates Jak2, yet is deficient in mitogenesis. Efficient expression of this mutant receptor at the cell surface and its ability to activate Jak2 indicate that poor mitogenic activity does not result from aberrant transport or folding. Rather, failure of this mutant to support proliferation above nominal rates underlines an apparent role for the EPOR ExBx2 subdomain in the activation of a distinct primary mitogenic effector.

Distinct roles of erythropoietin, insulin-like growth factor I, and stem cell factor in the development of erythroid progenitor cells.

Erythropoietin (EP), insulin-like growth factor I (IGF-I) and stem cell factor (SCF) each reduce apoptosis of human erythroid progenitor cells. To determine if these growth factors have additional roles in stimulating erythropoiesis, the proliferation, maturation, and survival of highly purified human erythroid colony-forming cells (ECFCs) were studied during the application of different combinations of these growth factors in a serum-free liquid culture. EP maintained cell viability and supported heme synthesis during erythroid maturation, with little increase in viable cell number or stimulation of DNA synthesis. The addition of SCF with EP resulted in a substantial increase in DNA synthesis, which was greater than that seen with the addition of EP and was associated with a large expansion in the number of ECFCs. Thus EP, by itself, produces little increase in cell proliferation, and expansion of the number of erythroid cells depends upon the presence of SCF with EP. The addition of IGF-I with EP led to enhanced heme synthesis and moderate cellular proliferation, but also greatly enhanced nuclear condensation and enucleation in the late erythroblasts. Thus EP, by itself, is not sufficient for complete end-terminal nuclear condensation/enucleation and the presence of IGF-I is necessary for this complete process. While EP greatly reduced apoptosis during 16 h of incubation at 37 degrees C, the addition of SCF and IGF-I with EP had little additional effect, but these additions enhanced DNA synthesis > 3.4-fold. Thus SCF may have an additional role in directly stimulating proliferation through a process that is distinct from apoptosis. Our observations indicate that EP prevents apoptosis and maintains erythroid cell viability and development. IGF-I enhances erythroid maturation and proliferation, but the proliferation of erythroid progenitors is mainly controlled by the addition of SCF with EP, independent of an effect on apoptosis.

Mutations in the WSAWSE and cytosolic domains of the erythropoietin receptor affect signal transduction and ligand binding and internalization.

The terminal development of erythroid progenitor cells is promoted in part through the interaction of erythropoietin (EPO) with its cell surface receptor. This receptor and a growing family of related cytokine receptors share homologous extracellular features, including a well-conserved WSXWS motif. To explore the functional significance of this motif in the murine EPO receptor, five WSAWSE mutants were prepared and their signal-transducing, ligand binding, and endocytotic properties were compared. EPO receptors mutated at tryptophan residues (W-232, W-235----G; W-235----G; W-235----F) failed to mediate EPO-induced growth or pp100 phosphorylation, while S-236----T and E-237----K mutants exhibited partial to full activity (50 to 100% of wild-type growth and induced phosphorylation). Ligand affinity was reduced for mutant receptors (two- to fivefold), yet expression at the cell surface for all receptors was nearly equivalent. Also, the ability of mutated receptors to internalize ligand was either markedly reduced or abolished (W-235----F), indicating a role for the WSAWSE region in hormone internalization. Interestingly, receptor forms lacking 97% of the cytosolic domain (no signal-transducing capacity; binding affinity reduced two- to threefold) internalized EPO efficiently. This and all WSAWSE receptor forms studied also mediated specific cross-linking of 125I-EPO to three accessory membrane proteins (M(r)s, 120,000, 105,000, and 93,000). These findings suggest that the WSAWSE domain of the EPO receptor is important for EPO-induced signal transduction and ligand internalization. In contrast, although the cytosolic domain is required for growth signaling, it appears nonessential for efficient endocytosis.

Mutations in the WSAWSE and cytosolic domains of the erythropoietin receptor affect signal transduction and ligand binding and internalization.

The terminal development of erythroid progenitor cells is promoted in part through the interaction of erythropoietin (EPO) with its cell surface receptor. This receptor and a growing family of related cytokine receptors share homologous extracellular features, including a well-conserved WSXWS motif. To explore the functional significance of this motif in the murine EPO receptor, five WSAWSE mutants were prepared and their signal-transducing, ligand binding, and endocytotic properties were compared. EPO receptors mutated at tryptophan residues (W-232, W-235----G; W-235----G; W-235----F) failed to mediate EPO-induced growth or pp100 phosphorylation, while S-236----T and E-237----K mutants exhibited partial to full activity (50 to 100% of wild-type growth and induced phosphorylation). Ligand affinity was reduced for mutant receptors (two- to fivefold), yet expression at the cell surface for all receptors was nearly equivalent. Also, the ability of mutated receptors to internalize ligand was either markedly reduced or abolished (W-235----F), indicating a role for the WSAWSE region in hormone internalization. Interestingly, receptor forms lacking 97% of the cytosolic domain (no signal-transducing capacity; binding affinity reduced two- to threefold) internalized EPO efficiently. This and all WSAWSE receptor forms studied also mediated specific cross-linking of 125I-EPO to three accessory membrane proteins (M(r)s, 120,000, 105,000, and 93,000). These findings suggest that the WSAWSE domain of the EPO receptor is important for EPO-induced signal transduction and ligand internalization. In contrast, although the cytosolic domain is required for growth signaling, it appears nonessential for efficient endocytosis.