Changes in c-Kit expression and effects of SCF during differentiation of human erythroid progenitor cells

N6-methyl-2'-deoxyadenosine promotes self-renewal of BFU-E progenitor in erythropoiesis

Red blood cells supply the oxygen required for all human cells and are in demand for emerging blood-loss therapy. Here we identified N6-methyl-2'-deoxyadenosine (6mdA) as an agonist that promotes the hyperproliferation of burst-forming unit erythroid (BFU-E) progenitor cells. In addition, 6mdA represses the apoptosis of erythroid progenitor cells (EPCs). Combined use of with SCF and EPO enabled cultures of isolated BFU-E to be expanded up to 5,000-fold. Transcriptome analysis showed that 6mdA upregulates the expression of the EPC-associated factors c-Kit, Myb, and Gata2 and downregulates that of the erythroid maturation-related transcription factors Gata1, Spi1, and Klf1. Mechanistic studies suggested that 6mdA enhances and prolongs the activation of erythropoiesis-associated master gene c-Kit and its downstream signaling, leading to expansion and accumulation of EPCs. Collectively, we demonstrate that 6mdA can efficiently stimulate the EPC hyperproliferation and provide a new regenerative medicine recipe to improve ex vivo generation of red blood cells.

Role of Caspase-10-P13tBID axis in erythropoiesis regulation

Red blood cell production is negatively controlled by the rate of apoptosis at the stage of CFU-E/pro-erythroblast differentiation, depending on the balance between erythropoietin (EPO) levels and activation of the Fas/FasL pathway. At this stage, activation of transient caspases through depolarization via mitochondrial outer membrane permeabilization (MOMP) is also required for terminal erythroid differentiation. Molecular mechanisms regulating the differential levels of MOMP during differentiation and apoptosis, however, remain poorly understood. Here we show a novel and essential role for the caspase-10-P13-tBID axis in erythroid terminal differentiation. Caspase-10 (but not caspase-8, which is activated during apoptosis) is activated at the early stages of erythroid terminal differentiation leading to the cleavage of P22-BID into P18-tBID, and later into P13-tBID. Erythropoietin (EPO) by inducing casein kinase I alpha (CKIα) expression, which in turn phosphorylates P18-tBID, prevents the generation of MYR-P15-tBID (leading to apoptosis) and allows the generation of P13-tBID by caspase-10. Unlike P15-tBID, P13-tBID is not myristoylated and as such, does not irreversibly anchor the mitochondrial membrane resulting in a transient MOMP. Likewise, transduction of a P13-tBID fragment induces rapid and strong erythroid terminal differentiation. Thus, EPO modulates the pattern of BID cleavage to control the level of MOMP and determines the fate of erythroblasts between apoptosis and differentiation. This pathway is impaired in 5q- myelodysplastic syndromes because of CK1α haplo-insufficiency and may contribute to erythroid differentiation arrest and high sensitivity of this disease to lenalidomide (LEN).

SBR-Blood: systems biology repository for hematopoietic cells

Extensive research into hematopoiesis (the development of blood cells) over several decades has generated large sets of expression and epigenetic profiles in multiple human and mouse blood cell types. However, there is no single location to analyze how gene regulatory processes lead to different mature blood cells. We have developed a new database framework called hematopoietic Systems Biology Repository (SBR-Blood), available online at http://sbrblood.nhgri.nih.gov, which allows user-initiated analyses for cell type correlations or gene-specific behavior during differentiation using publicly available datasets for array- and sequencing-based platforms from mouse hematopoietic cells. SBR-Blood organizes information by both cell identity and by hematopoietic lineage. The validity and usability of SBR-Blood has been established through the reproduction of workflows relevant to expression data, DNA methylation, histone modifications and transcription factor occupancy profiles.

ZNF16 (HZF1) promotes erythropoiesis and megakaryocytopoiesis via regulation of the c-KIT gene

We previously characterized the zinc finger protein gene HZF1 [also known as ZNF16 (zinc finger protein 16)] and demonstrated its important roles in erythroid and megakaryocytic differentiation of K562 cells. In the present study, we investigated its effect on erythroid and megakaryocytic differentiation of HSPCs (haemopoietic stem/progenitor cells). We observed up-regulation of ZNF16 during erythroid and megakaryocytic differentiation of the CD34+ HSPCs, and demonstrated that ZNF16 promotes erythroid and megakaryocytic differentiation by gain-of-function and loss-of-function experiments. Using a luciferase reporter and ChIP assays ZNF16 was demonstrated to bind to the c-KIT gene promoter and inhibit its expression in K562 cells. Enforced expression and knockdown of ZNF16 down-regulated and up-regulated the expression of the c-KIT gene in K562 cells and HSPCs respectively. Significantly decreased levels of the c-Kit protein were observed following erythroid and megakaryocytic differentiation of K562 and CD34+ cells. The knockdown of c-KIT partially rescued the differentiation inhibition caused by ZNF16 knockdown. The knockdown of c-KIT also blocked the activity of the c-Raf/MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase]/ERK/c-Jun signal pathway and reduced further the level of HEY1 (hes-related family bHLH transcription factor with YRPW motif 1), a repressor of GATA1 (GATA-binding protein 1) transcription, which finally up-regulated the expression of GATA1, a central regulator of erythroid and megakaryocytic differentiation. In conclusion the results of the present study demonstrate that ZNF16 plays an important role in erythropoiesis and megakaryocytopoiesis via its regulation of the c-Kit/c-Raf/MEK/ERK/c-Jun/HEY1/GATA1 cascade.

Novel evolutionary-conserved role for the activity-dependent neuroprotective protein (ADNP) family that is important for erythropoiesis

BACKGROUND

ADNP is vital for embryonic development. Is this function conserved for the homologous protein ADNP2?

RESULTS

Down-regulation/silencing of ADNP or ADNP2 in zebrafish embryos or mouse erythroleukemia cells inhibited erythroid maturation, with ADNP directly associating with the β-globin locus control region.

CONCLUSION

ADNPs are novel molecular regulators of erythropoiesis.

SIGNIFICANCE

New regulators of globin synthesis are suggested. Activity-dependent neuroprotective protein (ADNP) and its homologue ADNP2 belong to a homeodomain, the zinc finger-containing protein family. ADNP is essential for mouse embryonic brain formation. ADNP2 is associated with cell survival, but its role in embryogenesis has not been evaluated. Here, we describe the use of the zebrafish model to elucidate the developmental roles of ADNP and ADNP2. Although we expected brain defects, we were astonished to discover that the knockdown zebrafish embryos were actually lacking blood and suffered from defective hemoglobin production. Evolutionary conservation was established using mouse erythroleukemia (MEL) cells, a well studied erythropoiesis model, in which silencing of ADNP or ADNP2 produced similar results as in zebrafish. Exogenous RNA encoding ADNP/ADNP2 rescued the MEL cell undifferentiated state, demonstrating phenotype specificity. Brg1, an ADNP-interacting chromatin-remodeling protein involved in erythropoiesis through regulation of the globin locus, was shown here to interact also with ADNP2. Furthermore, chromatin immunoprecipitation revealed recruitment of ADNP, similar to Brg1, to the mouse β-globin locus control region in MEL cells. This recruitment was apparently diminished upon dimethyl sulfoxide (DMSO)-induced erythrocyte differentiation compared with the nondifferentiated state. Importantly, exogenous RNA encoding ADNP/ADNP2 significantly increased β-globin expression in MEL cells in the absence of any other differentiation factors. Taken together, our results reveal an ancestral role for the ADNP protein family in maturation and differentiation of the erythroid lineage, associated with direct regulation of β-globin expression.

Erythropoiesis-stimulating agents

Erythropoiesis is the process whereby erythroid progenitor cells differentiate and divide, resulting in increased numbers of red blood cells (RBCs). RBCs contain hemoglobin, the main oxygen carrying component in blood. The large number of RBCs found in blood is required to support the prodigious consumption of oxygen by tissues as they undergo oxygen-dependent processes. Erythropoietin is a hormone that when it binds and activates Epo receptors resident on the surface of cells results in stimulation of erythropoiesis. Successful cloning of the EPO gene allowed for the first time production of recombinant human erythropoietin and other erythropoiesis stimulating agents (ESAs), which are used to treat anemia in patients. In this chapter, the control of Epo levels and erythropoiesis, the various forms of ESAs used commercially, and their physical and biological properties are discussed.

Caspase-activated ROCK-1 allows erythroblast terminal maturation independently of cytokine-induced Rho signaling

Stem cell factor (SCF) and erythropoietin are strictly required for preventing apoptosis and stimulating proliferation, allowing the differentiation of erythroid precursors from colony-forming unit-E to the polychromatophilic stage. In contrast, terminal maturation to generate reticulocytes occurs independently of cytokine signaling by a mechanism not fully understood. Terminal differentiation is characterized by a sequence of morphological changes including a progressive decrease in cell size, chromatin condensation in the nucleus and disappearance of organelles, which requires transient caspase activation. These events are followed by nucleus extrusion as a consequence of plasma membrane and cytoskeleton reorganization. Here, we show that in early step, SCF stimulates the Rho/ROCK pathway until the basophilic stage. Thereafter, ROCK-1 is activated independently of Rho signaling by caspase-3-mediated cleavage, allowing terminal maturation at least in part through phosphorylation of the light chain of myosin II. Therefore, in this differentiation system, final maturation occurs independently of SCF signaling through caspase-induced ROCK-1 kinase activation.

Mechanism of human Hb switching: a possible role of the kit receptor/miR 221-222 complex

BACKGROUND

The human hemoglobin switch (HbF-->HbA) takes place in the peri/post-natal period. In adult life, however, the residual HbF (<1%) may be partially reactivated by chemical inducers and/or cytokines such as the kit ligand (KL). MicroRNAs (miRs) play a pivotal role in normal hematopoiesis: downmodulation of miR-221/222 stimulates human erythropoietic proliferation through upmodulation of the kit receptor.

DESIGN AND METHODS

We have explored the possible role of kit/KL in perinatal Hb switching by evaluating: i) the expression levels of both kit and kit ligand on CD34(+) cells and in plasma isolated from pre-, mid- and full-term cord blood samples; ii) the reactivation of HbF synthesis in KL-treated unilineage erythroid cell cultures; iii) the functional role of miR-221/222 in HbF production.

RESULTS

In perinatal life, kit expression showed a gradual decline directly correlated to the decrease of HbF (from 80-90% to <30%). Moreover, in full-term cord blood erythroid cultures, kit ligand induced a marked increase of HbF (up to 80%) specifically abrogated by addition of the kit inhibitor imatinib, thus reversing the Hb switch. MiR-221/222 expression exhibited rising levels during peri/post-natal development. In functional studies, overexpression of these miRs in cord blood progenitors caused a remarkable decrease in kit expression, erythroblast proliferation and HbF content, whereas their suppression induced opposite effects.

CONCLUSIONS

Our studies indicate that human perinatal Hb switching is under control of the kit receptor/miR 221-222 complex. We do not exclude, however, that other mechanisms (i.e. glucocorticoids and the HbF inhibitor BCL11A) may also contribute to the peri/post-natal Hb switch.

Erythropoietin down-regulates stem cell factor receptor (Kit) expression in the leukemic proerythroblast: role of Lyn kinase

Overexpression of the transcription factor Spi-1/PU.1 by transgenesis in mice induces a maturation arrest at the proerythroblastic stage of differentiation. We have previously isolated a panel of spi-1 transgenic erythroleukemic cell lines that proliferated in the presence of either erythropoietin (Epo) or stem cell factor (SCF). Using these cell lines, we observed that EpoR stimulation by Epo down-regulated expression of the SCF receptor Kit and induced expression of the Src kinase Lyn. Furthermore, enforced expression of Lyn in the cell lines increased cell proliferation in response to Epo, but reduced cell growth in response to SCF in accordance with Lyn ability to down-regulate Kit expression. Together, the data suggest that Epo-R/Lyn signaling pathway is essential for extinction of SCF signaling leading the proerythroblast to strict Epo dependency. These results highlight a new role for Lyn as an effector of EpoR in controlling Kit expression. They suggest that Lyn may play a central role in during erythroid differentiation at the switch between proliferation and maturation.

Evidence for downregulation of erythropoietin receptor in bone marrow erythroid cells of patients with chronic idiopathic neutropenia

OBJECTIVE

The aim of this study is to probe the mechanisms underlying anemia in patients with chronic idiopathic neutropenia (CIN) by evaluating parameters of bone marrow (BM) erythropoiesis.

PATIENTS AND METHODS

Ten CIN patients fulfilling the criteria of anemia of chronic disease, 27 nonanemic CIN patients, and 30 healthy volunteers were enrolled in the study. Reserves and survival characteristics of BM erythroid cells were evaluated using flow cytometry and clonogenic assays. Serum erythropoietin (EPO) was measured with ELISA. Expression of EPO receptors (EPORs) on BM erythroid cells was evaluated by flow cytometry and reverse-transcription polymerase chain reaction.

RESULTS

CIN patients display defective erythropoiesis in addition to previously reported impaired granulopoiesis. Patients have low number of CD34(+)/CD71(+) progenitor and CD36(-)/ Glycophorin A(+) (GlycoA(+)) precursor BM cells, and increased proportion of apoptotic cells within the CD34(+)/CD71(+) and CD36(+)/GlycoA(+) compartments. Burst-forming units erythroid (BFU-Es) in BM mononuclear or purified CD34(+) cells were significantly reduced in the patients. Patient BFU-Es increased significantly following in vitro treatment with tumor necrosis factor-alpha (TNF-alpha) and/or interferon gamma (IFN-gamma)-neutralizing antibodies. Local TNF-alpha and IFN-gamma production was higher in anemic than in nonanemic patients. EPO production was appropriate in the patients, but EPOR expression was significantly reduced in patient GlycoA(+) cells, especially in anemic patients.

CONCLUSION

Impaired BM erythropoiesis in CIN patients is probably the result of increased local production of TNF-alpha and IFN-gamma that induce apoptosis, cell growth inhibition, and downregulation of EPOR expression on erythroid cells. We suggest that anemia in CIN patients displays overlapping pathophysiologic features with anemia of chronic disease.

A Brg1 mutation that uncouples ATPase activity from chromatin remodeling reveals an essential role for SWI/SNF-related complexes in beta-globin expression and erythroid development

The Brg1 catalytic subunit of SWI/SNF-related complexes has been implicated in many developmental and physiological processes, but null homozygotes die as blastocysts prior to implantation. To circumvent this early embryonic lethality, we performed an ENU mutagenesis screen and generated a Brg1 hypomorph mutation in the ATPase domain. The mutant Brg1 protein is stable, assembles into SWI/SNF-related complexes, and exhibits normal ATPase activity but is unable to establish DNase I hypersensitivity sites characteristic of open chromatin. Mutant embryos develop normally until midgestation but then exhibit a distinct block in the development of the erythroid lineage, leading to anemia and death. The mutant Brg1 protein is recruited to the beta-globin locus, but chromatin remodeling and transcription are perturbed. Histone acetylation and DNA methylation are also affected. To our knowledge, Brg1 is the first chromatin-modifying factor shown to be required for beta-globin regulation and erythropoiesis in vivo. Not only does this mutation establish a role for Brg1 during organogenesis, it also demonstrates that ATPase activity can be uncoupled from chromatin remodeling.

Defective fetal liver erythropoiesis and T lymphopoiesis in mice lacking the phosphatidylserine receptor

Clearance of apoptotic cells by macrophages is considered important for prevention of inflammatory responses leading to tissue damage. The phosphatidylserine receptor (PSR), which specifically binds to phosphatidylserine (PS) exposed on the surface of apoptotic cells, mediates uptake of apoptotic cells in vitro, yet the physiologic relevance of PSR remains unknown. This issue was addressed by generating PSR-deficient (PSR-/-) mice. PSR-/- mice exhibited severe anemia and died during the perinatal period. In the PSR-/- fetal livers, erythroid differentiation was blocked at an early erythroblast stage. In addition, PSR-/- embryos exhibited thymus atrophy owing to a developmental defect of T-lymphoid cells. Clearance of apoptotic cells by macrophages was impaired in both liver and thymus of PSR-/- embryos. However, this did not induce up-regulation of inflammatory cytokines. These results indicate that during embryonic development, PSR-mediated apoptotic cell uptake is required for definitive erythropoiesis and T lymphopoiesis, independently of the prevention of inflammatory responses. (Blood. 2004;103:3362-3364)

Differential regulation of SOCS genes in normal and transformed erythroid cells

The SOCS family of genes are negative regulators of cytokine signalling with SOCS-1 displaying tumor suppressor activity. SOCS-1, CIS and SOCS-3 have been implicated in the regulation of red blood cell production. In this study, a detailed examination was conducted on the expression patterns of these three SOCS family members in normal erythroid progenitors and a panel of erythroleukemic cell lines. Unexpectedly, differences in SOCS gene expression were observed during maturation of normal red cell progenitors, viz changes to CIS were inversely related to the alterations of SOCS-1 and SOCS-3. Similarly, these SOCS genes were differentially expressed in transformed erythoid cells - erythroleukemic cells immortalized at an immature stage of differentiation expressed SOCS-1 and SOCS-3 mRNA constitutively, whereas in more mature cell lines SOCS-1 and CIS were induced only after exposure to erythropoietin (Epo). Significantly, when ectopic expression of the tyrosine kinase Lyn was used to promote differentiation of immature cell lines, constitutive expression of SOCS-1 and SOCS-3 was completely suppressed. Modulation of intracellular signalling via mutated Epo receptors in mature erythroleukemic lines also highlighted different responses by the three SOCS family members. Close scrutiny of SOCS-1 revealed that, despite large increases in mRNA levels, the activity of the promoter did not alter after erythropoietin stimulation; in addition, erythroid cells from SOCS-1-/- mice displayed increased sensitivity to Epo. These observations indicate complex, stage-specific regulation of SOCS genes during normal erythroid maturation and in erythroleukemic cells.

Spontaneous circulation of myeloid-lymphoid-initiating cells and SCID-repopulating cells in sickle cell crisis

The only curative therapy for sickle cell disease (SCD) is allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy approaches for autologous HSC transplantation are being developed. Although earlier engraftment is seen when cells from GCSF-mobilized blood are transplanted than when bone marrow is transplanted, administration of GCSF to patients with SCD can cause significant morbidity. We tested whether primitive hematopoietic progenitors are spontaneously mobilized in the blood of patients with SCD during acute crisis (AC-SCD patients). The frequency of myeloid-lymphoid-initiating cells (ML-ICs) and SCID-repopulating cells (SRCs) was significantly higher in blood from AC-SCD patients than in blood from patients with steady-state SCD or from normal donors. The presence of SRCs in peripheral blood was not associated with detection of long-term culture-initiating cells, consistent with the notion that SRCs are more primitive than long-term culture-initiating cells. As ML-ICs and SRCs were both detected in blood of AC-SCD patients only, these assays may both measure primitive progenitors. The frequency of ML-ICs also correlated with increases in stem cell factor, GCSF, and IL-8 levels in AC-SCD compared with steady-state SCD and normal-donor sera. Because significant numbers of ML-ICs and SRCs are mobilized in the blood without exogenous cytokine treatment during acute crisis of SCD, collection of peripheral blood progenitors during crisis may yield a source of autologous HSCs suitable for ex-vivo correction by gene therapy approaches and subsequent transplantation.

W/Wv marrow stromal cells engraft and enhance early erythropoietic progenitors in unconditioned Sl/Sld murine recipients

Transplantation of marrow stromal cells may provide a means of modulating hematopoiesis and serve as a form of cell therapy. We employed a murine transplant model using Sl/Sl(d) mice, which have macrocytic anemia due to defective expression of stem cell factor (SCF) on bone marrow stromal cells. Donor cells were derived from the complementary mutant strain W/W(v), which also exhibit anemia, due to mutations in c-kit, the SCF receptor expressed on hematopoietic stem cells. The strength of this model is that any correction of the Sl/Sl(d) anemia from the infusion of W/W(v) stromal cells can be attributed to the effect of the stromal cells and not to contaminating W/W(v) hematopoietic stem cells, a major concern in experiments involving wild-type animals. Cultured stromal cells were infused into unconditioned non-splenectomized Sl/Sl(d) mice. Engraftment of donor stromal cells reached levels of up to 1.0% of total marrow cells 4 months post transplant. However, stromal engraftment was not detectable in the spleen. Recipients of W/W(v) stroma showed a significant increase in the committed erythroid progenitors compared with those receiving Sl/Sl(d) stromal cells: 109 +/- 26 vs 68 +/- 5 CFU-E per 10(5) BMC, P = 0.002; 25 +/- 10 vs 15 +/- 5 BFU-E per 10(5) BMC, P = 0.037, for W/W(v) and Sl/Sl(d) stroma recipients, respectively. Despite this increase in erythroid progenitors, the anemia was not corrected. Our data suggest that in this murine model, splenic erythropoiesis may influence stromal cell therapy, and that higher levels of marrow engraftment may be necessary to obtain a clinically significant effect.

Enforced expression of HOX11 is associated with an immature phenotype in J2E erythroid cells

The HOX11 gene encodes a homeodomain transcription factor that is essential for spleen development during embryogenesis. HOX11 is also leukaemogenic, both through its clinical association with childhood T-cell acute lymphoblastic leukaemia, and its ability to immortalize other haematopoietic cell lineages experimentally. To examine the pathological role of HOX11 in tumorigenesis, we constitutively expressed HOX11 cDNA in J2E murine erythroleukaemic cells, which are capable of terminal differentiation. Enforced HOX11 expression was found to induce a profound alteration in J2E cellular morphology and differentiation status. Our analyses revealed that HOX11 produced clones with a preponderance of less differentiated cells that were highly adherent to plastic. Morphologically, the cells overexpressing HOX11 were larger and had decreased globin levels, as well as a reduction in haemoglobin synthesis in response to erythropoietin (EPO). Immunocytochemical analysis confirmed the immature erythroid phenotype imposed by HOX11, with clones transfected with HOX11 demonstrating expression of the c-Kit stem cell marker, while retaining EPO receptor expression. Taken together, these results show that HOX11 alters erythroid differentiation, favouring a less mature progenitor-like stage. This supports the notion that disrupted haematopoietic cell differentiation is responsible for pre-leukaemic immortalization by the HOX11 oncoprotein.

Basic fibroblast growth factor modulates the expression of glycophorin A and c-kit and inhibits erythroid differentiation in K562 cells

Basic fibroblast growth factor (bFGF) is produced by bone marrow stromal cells as well as by normal and leukemic hematopoietic cells. In this study, we examine the direct effects of bFGF on erythroid differentiation in K562 cells in order to determine whether bFGF can promote the expression of a primitive phenotype. Low levels of bFGF inhibited erythroid differentiation as evidenced by decreased expression of glycophorin A and increased expression of c-kit. bFGF also increased both the numbers and the sizes of colonies of K562 cells in soft agar assays. The addition of TGF-beta to these cells induced erythroid differentiation which resulted in an increase in glycophorin A and a decrease in c-kit. The simultaneous addition of bFGF and TGF-beta to K562 cells prevented both the TGF-beta-mediated increase in glycophorin A expression and the decrease in c-kit expression associated with erythroid differentiation. bFGF antagonised the TGF-beta-mediated promotion of erythroid differentiation in K562 cells in a dose dependent manner and these two cytokines counteracted each other on an approximately molar basis. These results indicate that bFGF alone increases expression of c-kit and promotes a primitive phenotype in K562 cells. In addition, bFGF counteracts the effects of differentiation-inducing cytokines, such as TGF-beta, on hematopoietic cells. It is therefore possible that enhanced production of bFGF by leukemic cells could contribute to their neoplastic phenotype by opposing the effects of negative regulators or cytokines that induce differentiation.

Stem cell factor protects c-kit+ human primary erythroid cells from apoptosis

OBJECTIVE

It has been reported that stem cell factor (SCF) promotes cell survival in primary cultured human erythroid colony-forming cells (ECFC). Given the heterogeneous nature of ECFC, which may affect interpretation of the data, we purified c-kit+ ECFC and investigated the specificity and mechanisms of the anti-apoptotic effects of SCF on these cells.

MATERIALS AND METHODS

Glycophorin A+ (GPA+) c-kit+ cells were purified from primary cultured ECFC derived from purified human CD34+ cells. The GPA+c-kit- and nonerythroid cells were generated from the same CD34+ cells. Apoptosis of ECFC was investigated in the absence or presence of SCF and erythropoietin (EPO) in serum-free medium. DNA fragmentation was measured with enzyme linked immunosorbent assay for oligonucleosome-sized DNA, gel electrophoresis, and annexin V labeling. Characterization of expanded cells and enriched cells was performed using multiparameter flow cytometry. For Akt assay, cells were lysed and the cleared lysates subjected to SDS-PAGE followed by Western blotting.

RESULTS

In GPA+c-kit+ cells, deprivation of cytokine caused rapid DNA fragmentation within 4 hours that reached a maximum at 6 hours. This was partially but clearly prevented by SCF or EPO. In contrast, no significant DNA fragmentation was seen in GPA+c-kit- and nonerythroid cells within 24 hours. PP2, a specific Src family kinase inhibitor, but not its inactive analogue PP3, reversed the anti-apoptotic effects of SCF. PP2 also inhibited SCF-induced phosphorylation of Akt.

CONCLUSION

These data indicate that SCF protects purified human GPA+c-kit+ cells from apoptosis and suggest that kit-mediated Src kinase activation is involved in Akt activation and cell survival.

Stem cell factor prevents Fas-mediated apoptosis of human erythroid precursor cells with Src-family kinase dependency

The Fas ligand (Fas-L) expressed on mature erythroblasts may induce apoptosis of more immature erythroid cells that express Fas, whereas stem cell factor (SCF) may prevent Fas-mediated cell death in hematopoietic progenitor cells. The manner in which SCF prevents Fas-mediated cell death still is unclear. Given the essential role of SCF and the potentially important involvement of the Fas/Fas-L system in the development of erythrocytes, we studied mechanisms related to SCF prevention of Fas-mediated apoptosis. We used primary cultured human erythroid colony-forming cells (ECFC) derived from CD34+ cells and enriched glycophorin A positive (GPA+) c-kit+ cells in ECFC. Apoptosis of ECFC was induced by an Fas-L mimetic monoclonal antibody CH11. DNA fragmentation and the activation of caspase-3 and caspase-8 were measured using commercially available kits. Characterization of expanded cells was performed using multiparameter flow cytometry. Lyn kinase activity was measured by enolase kinase assays. SCF inhibited the CH11-induced DNA fragmentation of ECFC as well as enriched GPA+ c-kit+ cells in ECFC, but not those of GPA+ c-kit- cells. SCF also inhibited the activation of caspase-3 and caspase-8, without downregulation of the surface expression of Fas, suggesting that SCF prevents apoptosis through uncoupling of Fas ligation from subsequent caspase activation. PP2, a specific inhibitor of Src-family kinases, antagonized the effects of SCF in preventing Fas-mediated apoptosis. We propose that SCF prevents Fas-mediated apoptosis of erythroid progenitor cells in a manner dependent on the activity of Src-family tyrosine kinases. We also identified active Lyn in erythroid cells. These data suggest the presence of a novel Src-family-dependent function of SCF in the development of erythrocytes.

Pluripotent hemopoietic stem cells in mice and humans

Although it has been reported previously that pluripotent hemopoietic stem cells (P-HSCs) express c-kit, the receptor for stem cell factor (steel factor), we and other groups have recently shown that P-HSCs do not express c-kit. In this review, we provide evidence that c-kit<low P-HSCs in mice have long-term-reconstituting activity (LTRA > 2 years) and the capacity to form colony-forming units in spleen (CFU-S) on Day 16, although c-kit(low) HSCs or c-kit+ HSCs have LTRA less than 1.5 years and the capacity to form CFU-S on Day 14 or on Day 10, respectively. In addition, we have found that there is a major histocompatibility complex (MHC) restriction between P-HSCs and stromal cells; normal P-HSCs can proliferate and differentiate efficiently in collaboration with MHC class I-compatible (but not MHC class I-incompatible) stromal cells. In humans, we also show that c-kit<low P-HSCs can differentiate into c-kit(low) cells, then c-kit+ cells in vitro.

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.

Expression of the c-kit (CD117) molecule in normal and malignant hematopoiesis

The c-kit proto-oncogen (CD 117) has been shown to be present in several cell types including normal and neoplastic hemopoietic cells. Among normal BM cells, CD117 expression has been found in about half of the CD34+ precursors including progenitors committed to the erythroid, granulo-monocytic, and megakaryocytic cell lineages. In addition, strong CD117 expression is detected in bone marrow mast cells as well as in a small subset of NK cells displaying strong reactivity for CD56, and in a relatively important proportion of CD3 /CD4 /CD8 prothymocytes. These results suggest that CD117 expression can be detected in both myeloid and lymphoid lineages although for the lymphoid lineage it would be restricted to a small NK-cell subset and early T-cell precursors. In acute leukemias CD117 expression was initially associated with AML. Nevertheless, at present it is well established that CD 117 expression may also be found in a relatively important proportion of T-ALL while it is usually absent in B-lineage ALL. Moreover, recent studies have shown that in about one-third of multiple myeloma cases and patients with monoclonal gammopathy of undetermined significance plasma cells display reactivity for CD1117. The prognostic influence of CD117 expression has not yet been clearly established. The analysis of this marker may also be of value for the investigation of minimal residual disease (MRD). It has been suggested that CD117 in combination with other antigens may be of great help for the identification of leukemia-associated phenotypes that could be used to monitor MRD in both acute myeloid leukemias and multiple myeloma patients achieving morphological complete remission.

In vitro effect of stem cell factor on human clonogenic marrow progenitors after myeloablative treatments

Abnormal hematopoiesis, including a deficiency of marrow progenitors and particularly of erythroid progenitors, has been described after autologous stem cell transplantation (ASCT), persisting for several years. In order to explain this deficiency, a resistance of marrow progenitors to stem cell factor (SCF) after ASCT was investigated. Marrow samples were harvested from pregraft patients at graft collection prior to ASCT, transplanted patients 6-24 months after high-dose therapy and control patients. CD34+ cells were cultured in a serum-free clonogenic assay with increasing doses of SCF. The clonogenic efficiency without SCF was lower for BFU-E in treated groups than in controls, whereas it was not different for CFU-GM. With increasing doses of SCF a dose-dependent effect was found on the numbers of both CFU-GM and BFU-E in all groups, although the maximal number of BFU-E remained lower in treated groups. However, the SCF dose that induced 50% of maximal BFU-E growth (D50) was similar in all groups. Furthermore, a dose-dependent effect on the size of BFU-E was found in all groups, with no difference in the proportion of large colonies. Thus, clonogenic erythroid progenitors from patients who have received myelotoxic treatments remain sensitive to SCF, with no evidence for a chemotherapy-related resistance.

Induction of the erythropoietin receptor gene and acquisition of responsiveness to erythropoietin by stem cell factor in HML/SE, a human leukemic cell line

HML/SE is a cytokine-dependent cell line established from childhood acute megakaryoblastic leukemia. Granulocyte-macrophage colony-stimulating factor or stem cell factor (SCF) alone could stimulate proliferation of HML/SE cells, however interleukin-3, interleukin-6, granulocyte colony-stimulating factor and thrombopoietin could not. Although erythropoietin (EPO) alone stimulated neither proliferation nor differentiation of HML/SE cells, it did stimulate proliferation of HML/SE cells and production of hemoglobin in the presence of SCF. SCF activated the human EPO receptor promoter and induced EPO receptor gene expression. Given these results, we speculate that HML/SE cells acquired responsiveness to EPO via the EPO receptor induced by SCF. Mutation analysis of putative transcription factor binding sites in the human EPO receptor promoter suggested that Sp1, rather than the GATA-1 binding site, contributed to the induction of the hEPOR gene. Although it is well documented that hematopoietic stem cells and primitive progenitors require both an early-acting cytokine and a lineage-specific cytokine to differentiate to a certain lineage, related mechanisms are not well understood. HML/SE may serve as an excellent model system to analyze functions of early-acting cytokine SCF and lineage-specific cytokine EPO related to proliferation and differentiation of hematopoietic stem cells.

Induction of c-kit molecules on human CD34+/c-kit < low cells: evidence for CD34+/c-kit < low cells as primitive hematopoietic stem cells

c-kit, a receptor for stem cell factor, has been widely accepted as a distinctive marker for hematopoietic stem cells. However, the level of c-kit expression on pluripotent hematopoietic stem cells is still controversial in mice and humans. We purified CD34+/c-kit < low cells (phenotypically c-kit-negative but only detectable at the message level) from human cord blood and examined their maturational steps in relation to the expression of c-kit molecules. When the CD34+/c-kit < low cells were cultured with cytokines (flt 3 ligand, interleukin 6 and interleukin 7) plus immobilized anti-CD34 monoclonal antibody (to crosslink CD34 molecules), c-kit molecules were clearly induced within 24 h. The c-kit expression gradually increased until day 8. When CD34+/c-kit(low) or CD34+/c-kit+ cells that had been induced from CD34+/c-kit < low cells were resorted and recultured using a methylcellulose culture system, they showed the same colony-forming ability as the freshly isolated CD34+/c-kit(low) or CD34+/c-kit+ cells, respectively. Furthermore, CD34+/c-kit < low cells have a similar hematopoietic potential to CD34+/c-kit(low) cells in assays for long-term culture initiating cell and colony-forming unit culture generated from long-term cultures. These findings suggest that CD34+/c-kit < low cells mature into CD34+/c-kit(low) and CD34+/c-kit+ cells, and acquire the reactivity to various humoral hematopoietic stimuli. Moreover, CD34+/c-kit < low cells showed a low level of rhodamine 123 retention, suggesting that CD34+/c-kit < low cells have multidrug resistance. Therefore, the CD34+/c-kit < low cells without colony-forming unit-granulocyte-erythroid-macrophage-megakaryocyte activity are also a pluripotent hematopoietic stem cell population, and the expression of c-kit on c-kit < low cells is the first maturational step of hematopoiesis.

In Vivo Effects of Flt3/Flk2 Ligand on Mobilization of Hematopoietic Progenitors in Primates and Potent Synergistic Enhancement With Granulocyte Colony-Stimulating Factor

The Flt3 receptor is expressed in primitive hematopoietic cells and its ligand exerts proliferative effects on these cells in vitro in synergy with other cytokines. To expand on the functional properties of Flt3 ligand (FL) in vivo we treated nonhuman primates with FL and tested its ability to mobilize stem/progenitor cells when given alone or in combination with granulocyte colony-stimulating factor (G-CSF ) treatment. FL alone (200 μg/kg/day) mobilizes progenitors with slow kinetics and with a peak effect at the end of 2 weeks of treatment. The spectrum of mobilized progenitors includes myeloid, lymphoid, megakaryocytic, and osteoclastogenic but a low proportion of burst-forming unit (BFU)e. Bone marrow (BM) studies before and during the treatment suggested that proliferative effects in BM may have preceded effects on peripheral blood mobilization. To assess the synergy of FL with G-CSF in mobilization of progenitors we used two schemes: one in which G-CSF was used for the last 5 days of a 12-day treatment with FL; the other in which both cytokines were given concurrently for 5 days only (FL, 200 μg/kg; G-CSF, 100 μg/kg). Both schemes yielded much higher progenitor mobilization levels (peak levels of colony-forming cells [CFSs] 41,000 to 95,000/mL blood) than observed with either FL (CFC 4,600 to 7,300/mL) or G-CSF (8,405 ± 3,024/mL) used alone at the same doses. Furthermore, there was a progressive and significant expansion of progenitors in vitro during 2 weeks in suspension cultures of mononuclear cells or of CD34+ cells only in the animal with the combined treatment. Likewise, substantial mobilization of osteoclastogenic progenitors was documented only with the combined treatment. Given the functional properties of FL, its synergistic mobilization with G-CSF, and its anticipated good tolerance (because of the absence of an effect on mast cell activation), a clinical use is projected for this cytokine in peripheral blood transplantation settings, as well as in experiments with ex vivo gene transfer.

In Vivo Effects of Flt3/Flk2 Ligand on Mobilization of Hematopoietic Progenitors in Primates and Potent Synergistic Enhancement With Granulocyte Colony-Stimulating Factor

The Flt3 receptor is expressed in primitive hematopoietic cells and its ligand exerts proliferative effects on these cells in vitro in synergy with other cytokines. To expand on the functional properties of Flt3 ligand (FL) in vivo we treated nonhuman primates with FL and tested its ability to mobilize stem/progenitor cells when given alone or in combination with granulocyte colony-stimulating factor (G-CSF ) treatment. FL alone (200 μg/kg/day) mobilizes progenitors with slow kinetics and with a peak effect at the end of 2 weeks of treatment. The spectrum of mobilized progenitors includes myeloid, lymphoid, megakaryocytic, and osteoclastogenic but a low proportion of burst-forming unit (BFU)e. Bone marrow (BM) studies before and during the treatment suggested that proliferative effects in BM may have preceded effects on peripheral blood mobilization. To assess the synergy of FL with G-CSF in mobilization of progenitors we used two schemes: one in which G-CSF was used for the last 5 days of a 12-day treatment with FL; the other in which both cytokines were given concurrently for 5 days only (FL, 200 μg/kg; G-CSF, 100 μg/kg). Both schemes yielded much higher progenitor mobilization levels (peak levels of colony-forming cells [CFSs] 41,000 to 95,000/mL blood) than observed with either FL (CFC 4,600 to 7,300/mL) or G-CSF (8,405 ± 3,024/mL) used alone at the same doses. Furthermore, there was a progressive and significant expansion of progenitors in vitro during 2 weeks in suspension cultures of mononuclear cells or of CD34+ cells only in the animal with the combined treatment. Likewise, substantial mobilization of osteoclastogenic progenitors was documented only with the combined treatment. Given the functional properties of FL, its synergistic mobilization with G-CSF, and its anticipated good tolerance (because of the absence of an effect on mast cell activation), a clinical use is projected for this cytokine in peripheral blood transplantation settings, as well as in experiments with ex vivo gene transfer.

Effects of recombinant human thrombopoietin alone and in combination with erythropoietin and early-acting cytokines on human mobilized purified CD34+ progenitor cells cultured in serum-depleted medium

The effects of recombinant thrombopoietin (TPO) alone and in combination with erythropoietin (EPO) and early-acting cytokines such as interleukin 3 (IL-3), stem cell factor (SCF) and GM-CSF on highly purified mobilized human CD34+ progenitor cells were studied in a serum-depleted culture system. Eight leukapheresis samples were cultured for seven days and analyzed; aliquots were replated and re-evaluated on day 12. Three-color flow cytometry was used together with morphologic analysis to determine proliferation and megakaryocytic or erythroid maturation. TPO alone was sufficient for cell survival and proliferation in serum-depleted medium. In the absence of other growth factors, almost all CD34+ cells differentiated along the megakaryocytic pathway within 12 days. Concomitantly, the progenitor cells gradually acquired the morphologic features of mature megakaryocytes. After exposure to TPO for one week, 50% of the cells still expressed CD34; by day 12 the remaining CD34+ cells (11%) were all coexpressing CD41. TPO alone did not support proliferation of glycophorin-A-positive cells. The addition of TPO to early-acting cytokines (EPO, GM-CSF, SCF and/or IL-3) not only increased the overall megakaryocyte expansion, but also generated a different maturation pattern of the CD41+ megakaryocyte progenitors. It further doubled the number of erythroid cells and c-kit+ cells in the second week of culture. Interestingly, the overall number of CD34+ cells was increased about fivefold when TPO was added to the early-acting cytokines, with a marked expansion of the CD34+/CD41+ and CD34+/CD117+ subpopulations. TPO can augment the pool of committed progenitors, thereby increasing the number of its own target cells and the number of EPO-responsive cells. These properties make TPO an interesting cytokine for the ex vivo expansion of human progenitor cells.

The use of recombinant SCF protein for rapid determination of c-kit expression in normal and abnormal erythropoiesis

Stem cell factor (SCF) is the ligand for the dimeric c-kit tyrosine kinase receptor. Binding of SCF to c-kit is a crucial element in the developmental stimulus of late stem cells and early progenitor cells. In the erythroid lineage the SCF stimulus is important not only for proliferation and differentiation, but is also known to enhance later haemoglobin production. In an earlier report we described a rapid non-radioactive technique using the extended ester-attached labelled SCF protein itself for detecting c-kit expression in marrow and peripheral blood mononuclear populations. In the present study we have taken this a step further to analyse c-kit expression in developing erythroid cells in vitro, principally using normal donor samples. This was designed for use as a foundation for the comparison of haematological disorders. In this case we tested 4 patients with the congenital disorder of erythropoiesis, Diamond-Blackfan anaemia (DBA), finding that in all cases DBA c-kit expression was elevated over normal, in 1 case as high as 348% of the normal average. This may be indicative of the reduced state of progenitor development in these patients. These results show that the described technique is beneficial for analysis in the stem and progenitor compartment.