C-myc expression affects proliferation but not terminal differentiation or survival of explanted erythroid progenitor cells

DKC1 is a transcriptional target of GATA1 and drives upregulation of telomerase activity in normal human erythroblasts

Telomerase is a ribonucleoprotein complex that maintains the length and integrity of telomeres, and thereby enables cellular proliferation. Understanding the regulation of telomerase in hematopoietic cells is relevant to the pathogenesis of leukemia, in which telomerase is constitutively activated, as well as bone marrow failure syndromes that feature telomerase insufficiency. Past studies showing high levels of telomerase in human erythroblasts and a prevalence of anemia in disorders of telomerase insufficiency provide the rationale for investigating telomerase regulation in erythroid cells. Here it is shown for the first time that the telomerase RNA-binding protein dyskerin (encoded by DKC1) is dramatically upregulated as human hematopoietic stem and progenitor cells commit to the erythroid lineage, driving an increase in telomerase activity in the presence of limiting amounts of TERT mRNA. It is also shown that upregulation of DKC1 was necessary for expansion of glycophorin A⁺ erythroblasts and sufficient to extend telomeres in erythroleukemia cells. Chromatin immunoprecipitation and reporter assays implicated GATA1-mediated transcriptional regulation of DKC1 in the modulation of telomerase in erythroid lineage cells. Together these results describe a novel mechanism of telomerase regulation in erythroid cells which contrasts with mechanisms centered on transcriptional regulation of TERT that are known to operate in other cell types. This is the first study to reveal a biological context in which telomerase is upregulated by DKC1 and to implicate GATA1 in telomerase regulation. The results from this study are relevant to hematopoietic disorders involving DKC1 mutations, GATA1 deregulation and/or telomerase insufficiency.

Erythropoietin drives breast cancer progression by activation of its receptor EPOR

Breast cancer is a leading cause of cancer-related deaths. Anemia is common in breast cancer patients and can be treated with blood transfusions or with recombinant erythropoietin (EPO) to stimulate red blood cell production. Clinical studies have indicated decreased survival in some groups of cancer patients treated with EPO. Numerous tumor cells express the EPO receptor (EPOR), posing a risk that EPO treatment would enhance tumor growth, but the mechanisms involved in breast tumor progression are poorly understood.

Here, we have examined the functional role of the EPO-EPOR axis in pre-clinical models of breast cancer. EPO induced the activation of PI3K/AKT and MAPK pathways in human breast cancer cell lines. EPOR knockdown abrogated human tumor cell growth, induced apoptosis through Bim, reduced invasiveness, and caused downregulation of MYC expression. EPO-induced MYC expression is mediated through the PI3K/AKT and MAPK pathways, and overexpression of MYC partially rescued loss of cell proliferation caused by EPOR downregulation. In a xenotransplantation model, designed to simulate recombinant EPO therapy in breast cancer patients, knockdown of EPOR markedly reduced tumor growth.

Thus, our experiments in vitro and in vivo demonstrate that functional EPOR signaling is essential for the tumor-promoting effects of EPO and underline the importance of the EPO-EPOR axis in breast tumor progression.

Extensive ex vivo expansion of functional human erythroid precursors established from umbilical cord blood cells by defined factors

There is a constant shortage of red blood cells (RBCs) from sufficiently matched donors for patients who need chronic transfusion. Ex vivo expansion and maturation of human erythroid precursors (erythroblasts) from the patients or optimally matched donors could represent a potential solution. Proliferating erythroblasts can be expanded from umbilical cord blood mononuclear cells (CB MNCs) ex vivo for 10(6)-10(7)-fold (in ~50 days) before proliferation arrest and reaching sufficient number for broad application. Here, we report that ectopic expression of three genetic factors (Sox2, c-Myc, and an shRNA against TP53 gene) associated with iPSC derivation enables CB-derived erythroblasts to undergo extended expansion (~10(68)-fold in ~12 months) in a serum-free culture condition without change of cell identity or function. These expanding erythroblasts maintain immature erythroblast phenotypes and morphology, a normal diploid karyotype and dependence on a specific combination of growth factors for proliferation throughout expansion period. When being switched to a terminal differentiation condition, these immortalized erythroblasts gradually exit cell cycle, decrease cell size, accumulate hemoglobin, condense nuclei and eventually give rise to enucleated hemoglobin-containing erythrocytes that can bind and release oxygen. Our result may ultimately lead to an alternative approach to generate unlimited numbers of RBCs for personalized transfusion medicine.

Differential gene expression during terminal erythroid differentiation

Terminal erythroid differentiation in mammals is the process whereby nucleated precursor cells accumulate erythroid-specific proteins such as hemoglobin, undergo extensive cellular and nuclear remodeling, and ultimately shed their nuclei to form reticulocytes, which then become mature erythrocytes in the circulation. Little is known about the mechanisms that enable erythroblasts to undergo such a transformation. We hypothesized that genes involved in these mechanisms were likely expressed at restricted times during the differentiation process and used differential display reverse transcriptase polymerase chain reaction as a first step in identifying such genes. We identified three differentially expressed cDNAs that we termed late erythroblast (LEB) 1-3. None of these cDNAs were previously identified as being expressed in erythroblasts and their patterns of expression indicated they are likely to be involved in the differentiation process. LEB-1 cDNA was derived from the gene A330102K04Rik (approved gene symbol Apoll1), and shares homology with members of the apolipoprotein L family in humans. LEB-3 cDNA was derived from the novel gene D930015E06Rik, that has no known function. LEB-2 cDNA was derived from the gene ranBP16 (approved gene symbol Xpo7), a nuclear exportin. D930015E06Rik mRNA is also strongly expressed in the testis and was localized to a region of the seminiferous tubule where secondary spermatocytes and early spermatids are found, suggesting a role for D930015E06Rik in spermatogenesis as well as terminal erythroid differentiation. We have thus identified three genes not previously described as being expressed in erythroblasts that could be relevant in elucidating mechanisms involved in terminal erythroid differentiation.

Role of NF-kappa B in regulation of apoptosis of erythroid progenitor cells

Erythropoietin (EPO) and interferon-gamma (IFN-gamma) added to human erythroid progenitor cells purified from peripheral blood (erythroid colony-forming cells; ECFC) significantly reduces apoptosis as assessed by flow cytometry (FCM) using annexin V. To clarify the role of NF-kappaB in the regulation of the apoptosis of erythroid progenitor cells, cyclosporin A (CsA), which blocks dissociation of the NF-kappaB complex, was added to serum-free cultures of ECFC. CsA induced the apoptosis of ECFCs in the presence of EPO or IFN-gamma, but at different magnitudes. In the presence of a relatively low concentration of CsA (10 microm), apoptosis was induced only in cultures with EPO. The direct involvement of NF-kappaB was then assessed by Western blotting and confocal microscopy. In the presence of EPO, NF-kappaB was abundant both in the cytoplasm and in the nucleus, and nuclear expression was diminished after adding CsA. In contrast, NF-kappaB was undetectable in the nucleus in the presence of IFN-gamma. The effect of CsA on mitochondrial function was investigated by determining the DeltaPsim and reactive oxygen species production. CsA disturbed the transmembrane potential in the presence of either EPO or IFN-gamma, although the viability of the cells was maintained in the presence of IFN-gamma plus CsA. These results indicate that IFN-gamma reduced the apoptosis of erythroid progenitor cells through a unique signaling pathway that is independent of NF-kappaB translocation, and which is not mediated by modulating mitochondrial function, whereas EPO reduced apoptosis through NF-kappaB translocation to the nucleus.

Role of the c-myc proto-oncogene in the proliferation of hereditary gingival fibromatosis fibroblasts

BACKGROUND

Hereditary gingival fibromatosis (HGF) is a fibrotic gingival enlargement. In previous work, HGF fibroblasts grew faster and produced more collagen and fibronectin (FN) than normal gingival (GN) fibroblasts. HGF FN and collagen production, but not proliferation, were under autocrine transforming growth factor (TGF)-beta control, suggesting other means of activation of HGF proliferation. Elevated/prolonged expression of the proto-oncogene c-myc is implicated in disregulation of cell growth. The objectives of this study were to: 1) determine if c-myc expression is abnormal in quiescent and serum-stimulated HGF and GN fibroblasts and 2) determine the relationship between c-myc expression and fibroblast proliferation using a c-myc antisense oligonucleotide (ODN).

METHODS

Proliferation was determined by enzyme-linked immunosorbent assay (ELISA), measuring incorporation of bromodeoxyuridine into DNA. Expression of c-myc was determined by quantitative polymerase chain reaction (PCR), using incorporation of fluorescent dCTP and detection via electrophoresis.

RESULTS

Proliferation was minimal until 24 hours or more after serum stimulation, when HGF proliferation was greater than GN (P < or = 0.02). All cells expressed c-myc mRNA at quiescence and > or = 1 hour after serum stimulation. Expression of c-myc in quiescent HGF fibroblasts was elevated, and it peaked and remained higher after serum stimulation than in GN cells. Proliferation of an HGF cell line was inhibited by 4 microM c-myc antisense ODN (14% decrease; P < or = 0.006) and 8 microM c-myc antisense ODN (approximately 80% decrease; P < or = 0.0001), but generally not by c-myc sense ODN. This effect was reversed by hybridizing the c-myc antisense and sense ODNs (P = 0.007).

CONCLUSION

Data suggest that elevated proliferation of an HGF fibroblast cell line is related to elevated c-myc expression.

Opposite effects of inhibitors of mitogen-activated protein kinase pathways on the egr-1 and β-globin expression in erythropoietin-responsive murine erythroleukemia cells

The effect of erythropoietin (Epo) on the expression of mitogen-activated protein kinase (MAPK) target genes egr-1 and c-fos was investigated in Epo-responsive murine erythroblastic cell line ELM-I-1. Epo induced a transient rise in egr-1 mRNA without a similar effect on c-fos expression. The induction of egr-1 correlated with a rapid ERK1/2 phosphorylation and was prevented with MEK1/2 inhibitors PD 98059 and UO126. The p38 inhibitor SB 203580 enhanced ERK1/2 phosphorylation and egr-1 mRNA levels. Longer incubations of ELM-I-1 cells with Epo revealed a second later phase of increase in egr-1 expression which was also prevented by MEK1/2 inhibitors, whereas SB 203580 had a stimulatory effect. In contrast, the beta-globin mRNA production was enhanced in the presence of PD 98059 and UO126 and reduced by SB 203580. The results suggest a regulatory role of egr-1 expression in Epo signal transduction and provide pharmacological evidence for the negative modulation of differentiation-specific gene expression by the ERK1/2 pathway in murine erythroleukemia cells.

Regulation of α7 Integrin Expression during Muscle Differentiation

Expression of the laminin-binding alpha7 integrin is tightly regulated during myogenic differentiation, reflecting required functions that range from cell motility to formation of stable myotendinous junctions. However, the exact mechanism controlling alpha7 expression in a tissue- and differentiation-specific manner is poorly understood. This report provides evidence that alpha7 gene expression during muscle differentiation is regulated by the c-Myc transcription factor. In myoblasts, alpha7 is expressed at basal levels, but following conversion to myotubes the expression of the integrin is strongly elevated. The increased alpha7 mRNA and protein levels following myogenic differentiation are inversely correlated with c-Myc expression. Transfection of myoblasts with the c-Myc transcription factor down-regulated alpha7 expression, whereas overexpression of Madmyc, a dominant-negative c-Myc chimera, induced elevated alpha7 expression. Functional analysis with site-specific deletions identified a specific double E-box sequence in the upstream promoter region (-2.0 to -2.6 kb) that is responsible for c-Myc-induced suppression of alpha7 expression. DNA-protein binding assays and supershift analysis revealed that c-Myc forms a complex with this double E-box sequence. Our results suggest that the interaction of c-Myc with this promoter region is an important regulatory element controlling alpha7 integrin expression during muscle development and myotendinous junction formation.

Studies with chimeric Mpl/JAK2 receptors indicate that both JAK2 and the membrane-proximal domain of Mpl are required for cellular proliferation

The thrombopoietin (TPO) receptor c-Mpl, like other members of the cytokine receptor superfamily, requires the association and activation of Janus kinases (JAKs) for normal signal transduction. The membrane-proximal portion of the signaling domain, containing conserved box1 and box2 motifs, is sufficient to support the proliferation of cytokine-dependent cell lines and basal megakaryocytopoiesis in vivo. We hypothesized that activation of the JAK2 kinase alone might be sufficient for proliferative signaling. To test this premise, we constructed chimeric receptors in which the extracellular and transmembrane portions of Mpl were fused to the pseudokinase and kinase domains of murine JAK2 kinase. When expressed in the interleukin-3-dependent cell line Ba/F3, the chimeric receptors were appropriately expressed on the cell surface and were able to initiate tyrosine kinase activity upon exposure to TPO. However, chimeric receptors lacking an intact box2 domain of Mpl were unable to support proliferation at any concentration of TPO. Only chimeric receptors containing both JAK2 kinase activity and the box2 region initiated proliferative signaling. Within the box2 motif, we determined that the sequence Glu(56)-Ile(57)-Leu(58) of the Mpl cytoplasmic domain is critical for proliferation of the chimeric receptors. Furthermore, TPO-dependent induction of c-myc transcription is also dependent on this motif. These results indicate that JAK2 activation alone is not sufficient for TPO-induced proliferation and that one or more essential signaling pathways must arise from the cytoplasmic domain of Mpl that includes box2. Although the nature of the signal transduction pathway is not yet known, this second proliferative event is likely to regulate c-myc expression.

Identification of the human erythropoietin receptor region required for Stat1 and Stat3 activation

Signal transducers and activators of transcription (Stat) proteins play important roles in the regulation of hematopoiesis as downstream molecules of cytokine signal transduction. It was previously demonstrated that erythropoietin (EPO), a major regulator of erythropoiesis, activates 3 different Stat members, Stat1, Stat3, and Stat5, in a human EPO-dependent cell line, UT-7/EPO. To clarify the mechanism by which EPO activates Stat1 and Stat3 via the EPO receptor (EPOR), a series of chimeric receptors was constructed bearing the extracellular domain of the granulocyte colony-stimulating factor receptor linked to the transmembrane domain of EPOR and the full length or several mutants of the cytoplasmic domain of EPOR, and these chimeric receptor complementary DNAs were introduced into UT-7/EPO cells. Tyr432 on human EPOR was important for activation of Stat1 and Stat3 and c-myc gene induction. In addition, Jak2 and Fes tyrosine kinases were involved in EPO-induced activation of Stat1 and Stat3. These results indicate that Stat1 and Stat3 are activated by EPO via distinct mechanisms from Stat5.

Erythropoietin activates two distinct signaling pathways required for the initiation and the elongation of c-myc

Erythropoietin (Epo) stimulation of erythroid cells results in the activation of several kinases and a rapid induction of c-myc expression. Protein kinase C is necessary for Epo up-regulation of c-myc by promoting elongation at the 3'-end of exon 1. PKCepsilon mediates this signal. We now show that Epo triggers two signaling pathways to c-myc. Epo rapidly up-regulated Myc protein in BaF3-EpoR cells. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 blocked Myc up-regulation in a concentration-dependent manner but had no effect on the Epo-induced phosphorylation of ERK1 and ERK2. LY294002 also had no effect on Epo up-regulation of c-fos. MEK1 inhibitor PD98059 blocked both the c-myc and the c-fos responses to Epo. PD98059 and the PKC inhibitor H7 also blocked the phosphorylation of ERK1 and ERK2. PD98059 but not LY294002 inhibited Epo induction of ERK1 and ERK2 phosphorylation in normal erythroid cells. LY294002 blocked transcription of c-myc at exon 1. PD98059 had no effect on transcription from exon 1 but, rather, blocked Epo-induced c-myc elongation at the 3'-end of exon 1. These results identify two Epo signaling pathways to c-myc, one of which is PI3K-dependent operating on transcriptional initiation, whereas the other is mitogen-activated protein kinase-dependent operating on elongation.

Molecular mechanisms of erythropoietin signaling

Erythropoietin is an obligatory growth factor for red blood cell production. The receptor for erythropoietin contains a single membrane-spanning domain with no intrinsic tyrosine kinase motifs. On binding to erythropoietin, the receptor dimerizes and activates multiple intracellular signaling molecules, including but not limited to JAK2, STAT5, PI 3-kinase, IRS-2, RAS, and Ca2+ channels. This review focuses on cytoplasmic signaling cascades involved in erythropoietin action.

Prevention of beta cell dysfunction and apoptosis activation in human islets by adenoviral gene transfer of the insulin-like growth factor I

Interleukin-1beta is a potent pro-inflammatory cytokine that has been shown to inhibit islet beta cell function as well as to activate Fas-mediated apoptosis in a nitric oxide-dependent manner. Furthermore, this cytokine is effective in recruiting lymphocytes that mediate beta cell destruction in IDDM onset. The insulin-like growth factor I (IGF-I) has been shown to block IL-1beta actions in vitro. We hypothesized that gene transfer of the insulin-like growth factor I to intact human islets could prevent IL-1beta-induced beta cell dysfunction and sensitization to Fas-triggered apoptosis activation. Intact human islets were infected with adenoviral vectors encoding IGF-I as well as beta-galactosidase and enhanced green fluorescent protein as controls. Adenoviral gene transfer of human IGF-I prevented IL-1beta-mediated nitric oxide production from human islets in vitro as well as the suppression of beta cell function as determined by glucose-stimulated insulin production. Moreover, IGF-I gene transfer prevented IL-1beta-induced, Fas-mediated apoptosis. These results suggest that locally produced IGF-I from cultured islets may be beneficial in maintaining beta cell function and promoting islet survival before and following islet transplantation as a potential therapy for type I diabetes.

Signaling induced by erythropoietin and stem cell factor in UT-7/Epo cells: transient versus sustained proliferation

UT-7/Epo cells are human factor-dependent erythroleukemic cells, requiring erythropoietin (Epo) for long-term growth. Stem cell factor (SCF) stimulates proliferation of UT-7/Epo only transiently, for three to five days. An investigation of the signal transduction pathways activated by these cytokines in UT-7/Epo cells may identify those signals specifically required for sustained growth. Proliferation assays demonstrate that SCF generates a substantial growth response in UT-7/Epo cells; however, the cells do not multiply or survive past five to seven days. While Epo induces the activation of JAK2 and STAT5, SCF stimulation shows no activation of JAK2 or STATs 1, 3, or 5. The activation of MAPK (p42/44) by SCF was transient, lasting only 30 min, in contrast to Epo, which stimulated phosphorylation of p42/44 for up to 2 h. The expression of the early response genes c-fos, egr1, and cytokine-inducible SH2 protein (CIS) in response to SCF or Epo stimulation demonstrated that the transient expression of p42/44 correlated with the transient expression of c-fos and egr1. In addition, CIS was activated by Epo but not SCF. These data indicate that EpoR, JAK2, and STAT5 activation are not required for the initiation of proliferation of these erythroid cells, that the transient activation of p42/44 correlates with the transient gene expression of c-fos and egr1, and sustained expression of c-fos and egr1 as seen in UT-7/Epo cells continuously grown in Epo may be necessary for long-term proliferation.

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.

NF-κB Transcription Factors Are Involved in Normal Erythropoiesis

NF-κB/Rel designates a widely distributed family of transcription factors involved in immune and acute phase responses. Here, the expression and function of NF-κB factors in erythroid proliferation and differentiation were explored. In an erythroleukemia cell line, TF-1, high levels of p105/p50, p100/p52, p65, and IκBα were detected 24 hours after growth factor deprivation. In response to granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulation, significant induction of p52 expression was observed. GM-CSF also induced nuclear translocation of both p52 and p65. No induction of NF-κB factors was observed with erythropoietin stimulation of TF-1 cells. Overexpression of p52 and p65 in TF-1 cells by transient transfection resulted in significant induction of a κB-TATA-luciferase reporter plasmid, showing that these factors are functional in vivo in erythroid cells. To determine whether NF-κB factors may play a role in normal erythropoiesis, levels of these factors were determined in burst-forming unit-erythroid (BFU-E)–derived cells at different stages of differentiation. The NF-κB factors p105/p50, p100/p52, and p65 were highly expressed in early BFU-E–derived precursors, which are rapidly proliferating, and declined during maturation. Furthermore, nuclear levels of NF-κB factors p50, p52, and p65 were higher in less mature precursors (day 10 BFU-E–derived cells) compared with more differentiated (day 14) erythroblasts. In nuclear extracts from day 10 BFU-E–derived cells, p50, p52, and p65 were able to form complexes, which bound to κB sites in the promoters of both the c-myb and c-mycgenes, suggesting that c-myb and c-myc may be among the κB-containing genes regulated by NF-κB factors in normal erythroid cells. Taken together, these data show that NF-κB factors are modulated by GM-CSF and suggest they function to regulate specific κB containing genes involved in erythropoiesis.

NF-κB Transcription Factors Are Involved in Normal Erythropoiesis

NF-κB/Rel designates a widely distributed family of transcription factors involved in immune and acute phase responses. Here, the expression and function of NF-κB factors in erythroid proliferation and differentiation were explored. In an erythroleukemia cell line, TF-1, high levels of p105/p50, p100/p52, p65, and IκBα were detected 24 hours after growth factor deprivation. In response to granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulation, significant induction of p52 expression was observed. GM-CSF also induced nuclear translocation of both p52 and p65. No induction of NF-κB factors was observed with erythropoietin stimulation of TF-1 cells. Overexpression of p52 and p65 in TF-1 cells by transient transfection resulted in significant induction of a κB-TATA-luciferase reporter plasmid, showing that these factors are functional in vivo in erythroid cells. To determine whether NF-κB factors may play a role in normal erythropoiesis, levels of these factors were determined in burst-forming unit-erythroid (BFU-E)–derived cells at different stages of differentiation. The NF-κB factors p105/p50, p100/p52, and p65 were highly expressed in early BFU-E–derived precursors, which are rapidly proliferating, and declined during maturation. Furthermore, nuclear levels of NF-κB factors p50, p52, and p65 were higher in less mature precursors (day 10 BFU-E–derived cells) compared with more differentiated (day 14) erythroblasts. In nuclear extracts from day 10 BFU-E–derived cells, p50, p52, and p65 were able to form complexes, which bound to κB sites in the promoters of both the c-myb and c-mycgenes, suggesting that c-myb and c-myc may be among the κB-containing genes regulated by NF-κB factors in normal erythroid cells. Taken together, these data show that NF-κB factors are modulated by GM-CSF and suggest they function to regulate specific κB containing genes involved in erythropoiesis.

The many roles of c-Myc in apoptosis

The proto-oncogene c-myc encodes a transcription factor c-Myc, which is of great importance in controlling cell growth and vitality. The quantity of c-Myc is carefully controlled by many mechanisms, and its actions to induce and repress genes are modulated by interactions with other regulatory proteins. Understanding the kinetic and quantitative relationships that determine how and what genes c-Myc regulates is essential to understanding how Myc is involved in apoptosis. Reduction of c-myc expression and its inappropriate expression can be associated with cellular apoptosis. This review outlines the nature and regulation of the c-myc gene and of c-Myc and presents the systems and conditions in which Myc-related apoptotic events occur. Hypotheses of the mechanisms by which expression and repression of c-myc lead to apoptosis are discussed.

Both the polycythemia- and anemia-inducing strains of Friend spleen focus-forming virus induce constitutive activation of the Raf-1/mitogen-activated protein kinase signal transduction pathway

The erythroleukemia-inducing Friend spleen focus-forming virus (SFFV) encodes a unique envelope glycoprotein which allows erythroid cells to proliferate and differentiate in the absence of erythropoietin (Epo). In an attempt to understand how the virus causes Epo independence, we have been studying signal transduction pathways activated by Epo to determine if SFFV exerts its biological effects by constitutively activating any of these pathways in the absence of Epo. We previously demonstrated that Stat proteins, the downstream components of the Epo-induced Jak-Stat pathway, are constitutively activated in SFFV-infected cells. In this study, we demonstrate that SFFV also activates Raf-1, MEK and mitogen-activated protein (MAP) kinase, the downstream components of the Raf-1/MAP kinase pathway. This pathway was activated in cells infected with the polycythemia-inducing strain of SFFV, which induces both proliferation and differentiation of erythroid cells in the absence of Epo, as well as in cells infected with the anemia-inducing strain of the virus, which still require Epo for differentiation. Inhibition of Raf-1 by using antisense oligonucleotides led to a partial inhibition of the Epo-independent proliferation of SFFV-infected cells. Expression of the transcription factors c-Jun and JunB, but not c-Fos, was induced in SFFV-infected cells in the absence of Epo, suggesting that constitutive activation of the Raf-1/MAP kinase pathway by the virus may result in deregulation of AP-1 activity. We conclude from our studies that infection of erythroid cells with SFFV leads to the constitutive activation of signal transduction molecules in both the Jak-Stat and Raf-1/MAP kinase pathways and that both of these pathways must be activated to achieve maximum proliferation and differentiation of erythroid cells in the absence of Epo.

The Roles of Bcl-XL and Apopain in the Control of Erythropoiesis by Erythropoietin

Erythropoietin (EP) is required by late-stage erythroid progenitor cells to prevent apoptosis. Several lines of evidence suggest that it is this action of EP that regulates erythrocyte production in vivo. To study the control of apoptosis in mouse and human erythroblasts, the expression of members of the Bcl-2 family of proteins and the expression and activation of the apoptosis-linked cysteine protease Yama/CPP32/apopain were examined. These proteins have been implicated as regulators of apoptosis in several cell models. The Bcl-2 family members analyzed were Bcl-2, Bcl-X, Bax, Bad, Bak, A1, and Mcl-1. Bcl-X expression in proerythroblasts was highly EP-dependent. Bcl-X was strongly increased during the terminal differentiation stages of human and mouse erythroblasts, reaching maximum transcript and protein levels at the time of maximum hemoglobin synthesis. This increase in Bcl-X expression led to an apparent level of approximately 50 times the level in proerythroblasts. In contrast, neither mouse nor human erythroblasts expressed Bcl-2 transcript or protein. Bax and Bad proteins remained relatively constant throughout differentiation, but diminished near the time of enucleation. Bak protein was present in early erythroblasts, but diminished progressively during differentiation. EP deprivation in both mouse and human erythroblasts led to activation of the cysteine protease, apopain, as was indicated by cleavage of the proenzyme into its proteolytically active fragments. Apopain activation was detectable within 2 hours of EP deprivation in mouse erythroblasts. These findings suggest an important role for Bcl-X in late erythroid differentiation and for apopain in apoptosis of erythroblasts caused by deprivation of EP.

The Roles of Bcl-XL and Apopain in the Control of Erythropoiesis by Erythropoietin

Erythropoietin (EP) is required by late-stage erythroid progenitor cells to prevent apoptosis. Several lines of evidence suggest that it is this action of EP that regulates erythrocyte production in vivo. To study the control of apoptosis in mouse and human erythroblasts, the expression of members of the Bcl-2 family of proteins and the expression and activation of the apoptosis-linked cysteine protease Yama/CPP32/apopain were examined. These proteins have been implicated as regulators of apoptosis in several cell models. The Bcl-2 family members analyzed were Bcl-2, Bcl-X, Bax, Bad, Bak, A1, and Mcl-1. Bcl-X expression in proerythroblasts was highly EP-dependent. Bcl-X was strongly increased during the terminal differentiation stages of human and mouse erythroblasts, reaching maximum transcript and protein levels at the time of maximum hemoglobin synthesis. This increase in Bcl-X expression led to an apparent level of approximately 50 times the level in proerythroblasts. In contrast, neither mouse nor human erythroblasts expressed Bcl-2 transcript or protein. Bax and Bad proteins remained relatively constant throughout differentiation, but diminished near the time of enucleation. Bak protein was present in early erythroblasts, but diminished progressively during differentiation. EP deprivation in both mouse and human erythroblasts led to activation of the cysteine protease, apopain, as was indicated by cleavage of the proenzyme into its proteolytically active fragments. Apopain activation was detectable within 2 hours of EP deprivation in mouse erythroblasts. These findings suggest an important role for Bcl-X in late erythroid differentiation and for apopain in apoptosis of erythroblasts caused by deprivation of EP.