Distinct signaling from stem cell factor and erythropoietin in HCD57 cells

Integrative proteomics reveals principles of dynamic phosphosignaling networks in human erythropoiesis

Human erythropoiesis is an exquisitely controlled multistep developmental process, and its dysregulation leads to numerous human diseases. Transcriptome and epigenome studies provided insights into system-wide regulation, but we currently lack a global mechanistic view on the dynamics of proteome and post-translational regulation coordinating erythroid maturation. We established a mass spectrometry (MS)-based proteomics workflow to quantify and dynamically track 7,400 proteins and 27,000 phosphorylation sites of five distinct maturation stages of in vitro reconstituted erythropoiesis of CD34+ HSPCs. Our data reveal developmental regulation through drastic proteome remodeling across stages of erythroid maturation encompassing most protein classes. This includes various orchestrated changes in solute carriers indicating adjustments to altered metabolic requirements. To define the distinct proteome of each maturation stage, we developed a computational deconvolution approach which revealed stage-specific marker proteins. The dynamic phosphoproteomes combined with a kinome-targeted CRISPR/Cas9 screen uncovered coordinated networks of erythropoietic kinases and pinpointed downregulation of c-Kit/MAPK signaling axis as key driver of maturation. Our system-wide view establishes the functional dynamic of complex phosphosignaling networks and regulation through proteome remodeling in erythropoiesis.

SRC-Family Kinases in Acute Myeloid Leukaemia and Mastocytosis

Protein tyrosine kinases have been recognized as important actors of cell transformation and cancer progression, since their discovery as products of viral oncogenes. SRC-family kinases (SFKs) play crucial roles in normal hematopoiesis. Not surprisingly, they are hyperactivated and are essential for membrane receptor downstream signaling in hematological malignancies such as acute myeloid leukemia (AML) and mastocytosis. The precise roles of SFKs are difficult to delineate due to the number of substrates, the functional redundancy among members, and the use of tools that are not selective. Yet, a large num ber of studies have accumulated evidence to support that SFKs are rational therapeutic targets in AML and mastocytosis. These two pathologies are regulated by two related receptor tyrosine kinases, which are well known in the field of hematology: FLT3 and KIT. FLT3 is one of the most frequently mutated genes in AML, while KIT oncogenic mutations occur in 80-90% of mastocytosis. Studies on oncogenic FLT3 and KIT signaling have shed light on specific roles for members of the SFK family. This review highlights the central roles of SFKs in AML and mastocytosis, and their interconnection with FLT3 and KIT oncoproteins.

Erythropoietin acts as an anti-inflammatory signal on murine mast cells

Recently it was found that the erythropoietin receptor (EpoR) is expressed on innate immune cells, such as dendritic cells and macrophages. We found that murine bone marrow-derived mast cells express the EpoR and that its expression is increased under hypoxic conditions. Interestingly, Epo stimulation of the cells did not activate signal transducer and activator of transcription molecules, nor did we find differences in the expression of typical STAT-dependent genes, the proliferation rate, and the ability to differentiate or to protect the cells from apoptosis. Instead, we demonstrate that stimulation of mast cells with Epo leads to phosphorylation of the receptor tyrosine kinase c-kit. We hypothesize that this is due to the formation of a receptor complex between the EpoR and c-kit. The common beta chain of the IL-3 receptor family, which was described as part of the tissue protective receptor (TPR) on other non-erythroid cells, however is not activated. To investigate whether the EpoR/c-kit complex has tissue protective properties, cells were treated with the Toll-like receptor ligand LPS. Combined Epo and LPS treatment downregulated the inflammatory response of the cells as detected by a decrease in IL-6 and TNF-α secretion.

Neocytolysis: none, one or many? A reappraisal and future perspectives

Neocytolysis is the hypothesis formulated to explain experimental evidence of selective lysis of young red blood cells (RBCs) (neocytes) associated with decreased plasma levels of erythropoietin (EPO). In humans, it appears to take place whenever a fast RBC mass reduction is required, i.e., in astronauts during the first days of spaceflight under weightlessness, where a fast reduction in plasma volume and increase in haematocrit occur. EPO plasma levels then decline and a decrease in RBC mass takes place, apparently because of the selective lysis of the youngest, recently generated RBCs (neocytes). The same process seems to occur in people descending to sea level after acclimatization at high altitude. After descent, the polycythaemia developed at high altitude must be abrogated, and a rapid reduction in the number of circulating RBCs is obtained by a decrease in EPO synthesis and the lysis of what seem to be young RBCs. In vivo, neocytolysis seems to be abolished by EPO administration. More recent research has ascribed to neocytolysis the RBC destruction that occurs under such disparate pathophysiologic conditions as nephropathy, severe obstructive pulmonary disease, blood doping, and even malaria anaemia. According to the theory, EPO's central role would be not only to stimulate the production of new RBCs in conditions of anaemia, as maintained by the orthodox view, but also that of a cytoprotective factor for circulating young RBCs. Why neocytes are specifically destroyed and how is this related to decreased EPO levels has not yet been elucidated. Changes in membrane molecules of young RBCs isolated from astronauts or mountain climbers upon return to normal conditions seem to indicate a higher susceptibility of neocytes to ingestion by macrophages. By limiting the context to space missions and high altitude expeditions, this review will address unresolved and critical issues that in our opinion have not been sufficiently highlighted in previous works.

Progesterone receptor induces bcl-x expression through intragenic binding sites favoring RNA polymerase II elongation

Steroid receptors were classically described for regulating transcription by binding to target gene promoters. However, genome-wide studies reveal that steroid receptors-binding sites are mainly located at intragenic regions. To determine the role of these sites, we examined the effect of progestins on the transcription of the bcl-x gene, where only intragenic progesterone receptor-binding sites (PRbs) were identified. We found that in response to hormone treatment, the PR is recruited to these sites along with two histone acetyltransferases CREB-binding protein (CBP) and GCN5, leading to an increase in histone H3 and H4 acetylation and to the binding of the SWI/SNF complex. Concomitant, a more relaxed chromatin was detected along bcl-x gene mainly in the regions surrounding the intragenic PRbs. PR also mediated the recruitment of the positive elongation factor pTEFb, favoring RNA polymerase II (Pol II) elongation activity. Together these events promoted the re-distribution of the active Pol II toward the 3′-end of the gene and a decrease in the ratio between proximal and distal transcription. These results suggest a novel mechanism by which PR regulates gene expression by facilitating the proper passage of the polymerase along hormone-dependent genes.

Distinct functions of erythropoietin and stem cell factor are linked to activation of mTOR kinase signaling pathway in human erythroid progenitors

Erythropoietin (EPO) and Stem Cell Factor (SCF) have partially distinct functions in erythroid cell development. The primary functions of EPO are to prevent apoptosis and promote differentiation, with a minor role as a mitogen. On the other hand SCF acts primarily as a mitogenic factor promoting erythroid cell proliferation with a minor role in inhibition of apoptosis. The concerted effects of these two growth factors are responsible for guiding initial commitment, expansion and differentiation of progenitors. The aim of the study was to identify signaling elements pertinent to translational control and elucidate whether both cytokines can contribute to protein translation providing some functional redundancy as seen with respect to apoptosis. The current study focused on non-apoptotic functions of SCF mediated through mTOR/p70S6 leading to protein translation and cell proliferation. We utilized a human primary erythroid progenitors and erythroblasts that are responsive to EPO and SCF to investigate the activation of mTOR/p70S6 kinases and their downstream effectors, the pathway primarily responsible for protein translation. We showed that mTOR, p70S6 kinases and their downstream signaling elements 4EBP1 and S6 ribosomal protein are all activated by SCF but not by EPO in primary erythroid progenitors. We also found that SCF is the sole contributor to activation of the protein translational machinery and activation of mTOR/p70S6 pathway is confined to the proliferative phase of erythroid differentiation program. Altogether these results demonstrate that unlike the survival function which is supported by both EPO and SCF protein translation essential for proliferation is governed by only SCF.

Phosphorylation of Bcl-associated death protein (Bad) by erythropoietin-activated c-Jun N-terminal protein kinase 1 contributes to survival of erythropoietin-dependent cells

The glycoprotein erythropoietin (Epo) is a hematopoietic cytokine necessary for the survival of erythrocytes from immature erythroid cells. The mitogen-activated c-Jun N-terminal kinase 1 (JNK1) plays an important role in the proliferation and survival of erythroid cells in response to Epo. However, the precise mechanism of JNK1 activation promoting erythroid cell survival is incompletely understood. Here, we reported that JNK1 is required for Epo-mediated cell survival through phosphorylation and inactivation of the pro-apoptotic, Bcl-2 homology domain 3 (BH3)-only Bcl-associated death protein (Bad). Upon Epo withdrawal, HCD57 cells, a murine Epo-dependent cell line, displayed increased apoptotic cell death that was associated with decreased JNK1 activity. Epo withdrawal-induced apoptosis was promoted by inhibition of JNK1 activity but suppressed by expression of a constitutively active JNK1. Furthermore, Epo-activated JNK1 phosphorylated Bad at threonine 201, thereby inhibiting the association of Bad with the anti-apoptotic molecule B-cell lymphoma-extra large (Bcl-X(L)). Replacement of threonine 201 by alanine in Bad promoted Epo withdrawal-induced apoptosis. Thus, our results provide a molecular mechanism by which JNK1 contributes to the survival of erythroid cells.

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.

Expression of stem cell factor and its receptor c-Kit during the development of intrahepatic cholangiocarcinoma

Stem cell factor (SCF) and its receptor, c-Kit, constitute an important signal transduction system with proliferative and anti-apoptotic functions. Besides regulating hemopoietic stem cell proliferation and liver regeneration, it has been implicated in the regulation of human malignancies. However, the cellular expression of the SCF-c-Kit gene system in the liver during cholangiocarcinogenesis has not been studied to date. The protein- and mRNA-expression levels of SCF and c-Kit genes were examined in normal rat liver, in isolated normal rat liver cells and in a thioacetamide-induced rat model of intrahepatic cholangiocarcinoma (CC). Immunohistochemical analysis of the normal liver showed that SCF is expressed in the wall of the hepatic artery and in some cells, which were located along the sinusoids, although it was absent from hepatocytes and biliary epithelial cells. The mRNA analysis of isolated normal liver cell populations revealed a co-expression of SCF- and c-Kit-mRNA in sinusoidal endothelial cells and in Kupffer cells, whereas passaged and cultured liver myofibroblasts (MFs) expressed only SCF. Low levels of the SCF- and c-Kit-mRNA expression could be detected in isolated hepatocytes of the normal liver. Immunohistochemical analysis of the CC tissue showed SCF positivity in proliferating biliary cells (CK-19(+)), in macrophages (ED-1(+)) and in MFs (alpha-smooth-muscle-actin, alpha-SMA(+)) of the tumoral microenvironment. c-Kit-positivity could be detected on hepatocytes of the regenerating nodules and on the proliferating bile ducts of CC. Compared with the normal liver tissue, SCF-mRNA from the CC tissue was upregulated up to 20-fold, whereas c-Kit-mRNA was upregulated up to fivefold. These data indicate that several cell populations may become able to express SCF and/or c-Kit during cholangiocarcinogenesis. Therefore, the SCF-c-Kit system may contribute to tumor development, for instance, by inducing proliferation of hepatocytes and of biliary cells and by acting as a surviving factor for CC cells.

Erythropoietin-induced phosphorylation/degradation of BIM contributes to survival of erythroid cells

OBJECTIVE

A proapoptotic BH3-only protein BIM (BCL-2 interacting mediator of cell death) can link cytokine receptor signaling with the apoptotic machinery in hematopoietic cells. We investigated here the role of BIM in erythropoietin (EPO)-mediated survival in erythroid cells.

MATERIALS AND METHODS

We downregulated BIM in EPO-dependent HCD57 erythroid cells with short hairpin RNA (shRNA), and used real-time polymerase chain reaction, Western blots, and flow cytometry to characterize BIM expression and apoptosis. Hematologic analyses of BIM-deficient (Bim(-/-)) mice were conducted.

RESULTS

BIM expression increases in primary murine erythroid cells and HCD57 cells deprived of EPO. Whereas Bim mRNA increased less than twofold, BIM protein increased more than 10-fold after EPO withdrawal, suggesting posttranscriptional regulation of BIM. EPO treatment resulted in rapid phosphorylation of BIM at Serine 65 and phosphorylation correlated with degradation of BIM. Inhibition of extracellular signal-regulated kinase (ERK) by a MEK/ERK inhibitor, U0126, blocked both phosphorylation and degradation of BIM, resulting in apoptosis. Treatment with a proteasome inhibitor, MG-132, also blocked degradation of phosphorylated BIM. Downregulation of BIM with the shRNA resulted in HCD57 cells more resistant to apoptosis induced by either EPO withdrawal or ERK inhibition. Although we observed no significant changes in the number of erythrocytes or reticulocytes in the circulation of Bim(-/-) mice, erythroid progenitors from bone marrow in Bim(-/-) mice were reduced in number and more resistant to apoptosis induced by U0126 MEK/ERK inhibitor.

CONCLUSION

EPO protects erythroid cells from apoptosis in part through ERK-mediated phosphorylation followed by proteasomal degradation of BIM.

Phase II open label trial of imatinib in polycythemia rubra vera

Polycythemia rubra vera is a chronic myeloproliferative disorder characterized by panmyelosis with the resultant potential for thrombosis, myelofibrosis, and acute leukemia. Treatment has rested on phlebotomy and hydroxyurea. In 2002, we reported two patients who were unable to tolerate hydroxyurea but responded to imatinib mesylate (Gleevec). These patients have remained in complete hematologic remission on imatinib since 1999. As a result we began a phase II, open label trial of imatinib in patients with polycythemia vera. Patients meeting the Polycythemia Vera Study group criteria for the diagnosis of polycythemia vera, either naïve or intolerant to prior treatment were allowed to enroll. Initial therapy was begun with imatinib mesylate at 400 mg a day and two dose escalations, one to 600 and second to 800 mg a day, were allowed for patients not achieving a target hematocrit of 44 or less; or a platelet count of less than 600,000/mm(3). Twenty patients were enrolled, 15 achieved complete hematologic remission within 12 weeks and ten remain on study. Six patients remain in remission on 400 mg a day and four on 500 mg a day. Gastrointestinal or cutaneous toxicities were primarily grade I or II. All patients were negative for bcr/abl. Imatinib mesylate is capable of producing hematologic remission in the majority of patients with polycythemia vera and provides another option for patient management, particularly in those intolerant to hydroxyurea.

Erythropoietin enhancement of rat pancreatic tumor cell proliferation requires the activation of ERK and JNK signals

Erythropoietin (EPO) regulates the proliferation and differentiation of erythroid cells by binding to its specific transmembrane receptor EPOR. Recent studies, however, have shown that the EPOR is additionally present in various cancer cells and EPO induces the proliferation of these cells, suggesting a different function for EPO other than erythropoiesis. Therefore, the purpose of the present study was to examine EPOR expression and the role of EPO in the proliferation and signaling cascades involved in this process, using the rat pancreatic tumor cell line AR42J. Our results showed that AR42J cells expressed EPOR, and EPO significantly enhanced their proliferation. Cell cycle analysis of EPO-treated cells indicated an increased percentage of cells in the S phase, whereas cell numbers in G0/G1 phase were significantly reduced. Phosphorylation of extracellular regulatory kinase 1/2 (ERK1/2) and c-Jun NH2terminal kinase 1/2 (JNK1/2) was rapidly stimulated and sustained after EPO addition. Treatment of cells with mitogen-activated protein/ERK kinase (MEK) inhibitor PD98059 or JNK inhibitor SP600125 significantly inhibited EPO-enhanced proliferation and also increased the fraction of cells in G0/G1 phase. Furthermore, the inhibition of JNK using small interference RNA (siRNA) suppressed EPO-enhanced proliferation of AR42J cells. Taken together, our results indicate that AR42J cells express EPOR and that the activation of both ERK1/2 and JNK1/2 by EPO is essential in regulating proliferation and the cell cycle. Thus both appear to play a key role in EPO-enhanced proliferation and suggest that the presence of both is required for EPO-mediated proliferation of AR42J cells.

Stem cell factor and its receptor, c-kit, are important for hepatocyte proliferation in wild-type and tumor necrosis factor receptor-1 knockout mice after 70% hepatectomy

BACKGROUND

Stem cell factor (SCF) has well-known proliferative effects on hematopoietic cells. SCF also has effects on differentiation and proliferation in other cell types. Interleukin-6 (IL-6) and tumor necrosis factor (TNF)-alpha have proliferative effects in the liver. Recent studies in our laboratory have linked SCF's hepatoproliferative actions to those of IL-6, demonstrating that IL-6-induced hepatocyte proliferation depends, at least in part, on SCF. We now hypothesize that TNF-alpha's hepatoproliferative effects are also dependent on SCF.

METHODS AND RESULTS

In vitro studies using primary mouse hepatocytes show that SCF is induced by TNF-alpha; anti-SCF antibody treatment in this system inhibits TNF-alpha-induced hepatocyte proliferation, suggesting that TNF-alpha-induced hepatocyte proliferation is also SCF dependent. Additional in vivo experiments were performed in which wild type and/or TNF-alpha receptor-1 knockout mice (TNFR1(-/-)) were subjected to 70% hepatectomy or sham laparotomy. TNFR1(-/-) mice are known to have delayed hepatic regeneration after partial hepatectomy. Initial experiments demonstrated that the SCF receptor, c-kit, is upregulated after partial hepatectomy in wild-type mice, further emphasizing the importance of this system in the restoration of hepatic mass. SCF administration to TNFR1(-/-) mice in the context of partial hepatectomy restores hepatocyte proliferation to normal. Further, SCF administration to TNFR1(-/-) mice before hepatectomy increases phosphotyrosine signal transducer and activator (p-stat-3) levels, suggesting that SCF-induced increases in hepatocyte proliferation may also be stat-3 mediated.

CONCLUSIONS

These data suggest that TNF-alpha-induced hepatocyte proliferation depends, at least in part, on SCF and that SCF and its receptor, c-kit, are important for the liver's regenerative processes.

Osteopontin regulates actin cytoskeleton and contributes to cell proliferation in primary erythroblasts

Erythropoietin and stem cell factor are the key cytokines that regulate early stages of erythroid differentiation. However, it remains undetermined whether additional cytokines also play a role in the differentiation program. Here, we report that osteopontin (OPN) is highly expressed and secreted by erythroblasts during differentiation. We also demonstrate that OPN-deficient human and mouse erythroblasts exhibit defects in F-actin filaments, and addition of exogenous OPN to OPN-deficient erythroblasts restored the F-actin filaments in these cells. Furthermore, our studies demonstrate that OPN contributes to erythroblast proliferation. OPN knock-out male mice exhibit lower hematocrit and hemoglobin levels compared with their wild-type counterparts. We also show that OPN mediates phosphorylation or activation of multiple proteins including Rac-1 GTPase and the actin-binding protein, adducin, in human erythroblasts. In addition, we show that the OPN effects include regulation of intracellular calcium in human erythroblasts. Finally, we demonstrate that human erythroblasts express CD44 and integrins beta1 and alpha4, three known receptors for OPN, and that the integrin beta1 receptor is involved in transmitting the proliferative signal. Together these results provide evidence for signal transduction by OPN and contribution to multiple functions during the erythroid differentiation program in human and mouse.

Vitamin E and mast cells

Mast cells play an important role in the immune system by interacting with B and T cells and by releasing several mediators involved in activating other cells. Hyperreactivity of mast cells and their uncontrolled accumulation in tissues lead to increased release of inflammatory mediators contributing to the pathogenesis of several diseases such as rheumatoid arthritis, atherosclerosis, multiple sclerosis, and allergic disorders such as asthma and allergic rhinitis. Interference with mast cell proliferation, survival, degranulation, and migration by synthetic or natural compounds may represent a preventive strategy for the management of these diseases. Natural vitamin E covers a group of eight analogues-the alpha-, beta-, gamma-, and delta-tocopherols and the alpha-, beta-, gamma-, and delta-tocotrienols, but only alpha-tocopherol is efficiently retained by the liver and distributed to peripheral tissues. Mast cells preferentially locate in the proximity of tissues that interface with the external environment (the epithelial surface of the skin, the gastrointestinal mucosa, and the respiratory system), what may render them accessible to treatments with inefficiently retained natural vitamin E analogues and synthetic derivatives. In addition to scavenging free radicals, the natural vitamin E analogues differently modulate signal transduction and gene expression in several cell lines; in mast cells, protein kinase C, protein phosphatase 2A, and protein kinase B are affected by vitamin E, leading to the modulation of proliferation, apoptosis, secretion, and migration. In this chapter, the possibility that vitamin E can prevent diseases with mast cells involvement by modulating signal transduction and gene expression is evaluated.

Direct interaction between Kit and the interleukin-7 receptor

In vivo analyses of thymopoiesis in mice defective in signaling through Kit and gammac or Kit and IL-7Ralpha demonstrate synergy and partial complementation of gammac or IL-7-mediated signaling by the Kit signaling pathway. Our molecular analysis in T-lymphoid cells as well as in nonhematopoietic cells shows that Kit and IL-7R signaling pathways directly interact. KL-mediated activation of Kit induced strong tyrosine phosphorylation of gammac and IL-7Ralpha in the absence of IL-7. Activated Kit formed a complex with either IL-7Ralpha or gammac, and tyrosine phosphorylation of both subunits occurred independently of Jak3, suggesting that gammac and IL-7Ralpha are each direct substrates of Kit. Kit activated Jak3 in an IL-7R-dependent manner. Moreover, deficient Stat5 activation of the Kit mutant YY567/569FF lacking intrinsic Src activation capacity was partially reconstituted in the presence of IL-7R and Jak3. Based on the molecular data, we propose a model of Kit-mediated functional activation of gammac-containing receptors such as IL-7R, similar to the interaction between Kit and Epo-R. Such indirect activation of the Jak-Stat pathway induced by the interaction between an RTK and type I cytokine receptor could be the underlying mechanism for a context-specific signaling repertoire of a pleiotropic RTK-like Kit.

Newly identified c-KIT receptor tyrosine kinase ITD in childhood AML induces ligand-independent growth and is responsive to a synergistic effect of imatinib and rapamycin

Activating mutations of c-KIT lead to ligand-independent growth. Internal tandem duplications (ITDs) of exon 11, which encodes the juxtamembrane domain (JMD), are constitutively activating mutations found in 7% of gastrointestinal stromal tumors (GISTs) but have not been described in childhood acute myeloid leukemia (AML). DNA and cDNA from 60 children with AML were screened by polymerase chain reaction (PCR) for mutations of the JMD. A complex ITD (kit cITD) involving exon 11 and exon 12 was identified with a relative frequency of 7% (4/60). The human kit cITDs were inserted into the murine c-Kit backbone and expressed in Ba/F3 cells. KIT cITD induced factorindependent growth and apoptosis resistance, and exhibited constitutive autophosphorylation. KIT cITD constitutively activated the PI3K/AKT pathway and phosphorylated STAT1, STAT3, STAT5, and SHP-2. Imatinib (IM) or rapamycin (Rap) led to complete inhibition of growth, with IC50 values at nanomolar levels. IM and Rap synergistically inhibited growth and surmounted KIT cITD-induced apoptosis resistance. IM but not LY294002 inhibited phosphorylation of STAT3 and STAT5, suggesting aberrant cross talk between PI3K- and STAT-activating pathways. The findings presented may have immediate therapeutic impact for a subgroup of childhood AML-expressing c-KIT mutations.

Signals for stress erythropoiesis are integrated via an erythropoietin receptor-phosphotyrosine-343-Stat5 axis

Anemia due to chronic disease or chemotherapy often is ameliorated by erythropoietin (Epo). Present studies reveal that, unlike steady-state erythropoiesis, erythropoiesis during anemia depends sharply on an Epo receptor-phosphotyrosine-343-Stat5 signaling axis. In mice expressing a phosphotyrosine-null (PY-null) Epo receptor allele (EpoR-HM), severe and persistent anemia was induced by hemolysis or 5-fluorouracil. In short-term transplantation experiments, donor EpoR-HM bone marrow cells also failed to efficiently repopulate the erythroid compartment. In each context, stress erythropoiesis was rescued to WT levels upon the selective restoration of an EpoR PY343 Stat5-binding site (EpoR-H allele). As studied using a unique primary culture system, EpoR-HM erythroblasts exhibited marked stage-specific losses in Epo-dependent growth and survival. EpoR-H PY343 signals restored efficient erythroblast expansion, and the selective Epo induction of the Stat5 target genes proviral integration site-1 (Pim-1) and oncostatin-M. Bcl2-like 1 (Bcl-x), in contrast, was not significantly induced via WT-EpoR, EpoR-HM, or EpoR-H alleles. In Kit+ CD71+ erythroblasts, EpoR-PY343 signals furthermore enhanced SCF growth effects, and SCF modulation of Pim-1 kinase and oncostatin-M expression. In maturing Kit- CD71+ erythroblasts, oncostatin-M exerted antiapoptotic effects that likewise depended on EpoR PY343-mediated events. Stress erythropoiesis, therefore, requires stage-specific EpoR-PY343-Stat5 signals, some of which selectively bolster SCF and oncostatin-M action.

Stimulation of cell growth by erythropoietin in RAW264.7 cells: Association with AP-1 activation

Erythropoietin (EPO), a hematopoietic factor, is required for normal erythrocyte developments, but it has been demonstrated to have many other functions, and its receptor is localized in other tissues. In the present study, we investigated whether EPO can promote other cell proliferation and possible molecular mechanisms. EPO restored the inhibition of the RAW264.7 and PC12 cell growth by fetal bovine serum (FBS) withdrawal in a dose dependent manner, but not that of other cell types tested. The restoring effect of EPO was completed when the RAW264.7 cells were cultured in the medium containing as low as 3% of FBS, and 10 U/mL EPO could replace FBS. The restoring effect of EPO in the RAW264.7 cells was associated with the increased of c-Fos and c-Jun expression as well as AP-1 activation. These data demonstrate that EPO can stimulate RAW264. 7 cell as well as PC12 cell growth even when the cells were cultured without FBS or in the presence of small amounts of FBS in the medium, and this stimulating effect is associated with the activation of AP-1 transcription factor.

Stem cell factor and its receptor c-Kit as targets for inflammatory diseases

Stem cell factor (SCF), the ligand of the c-Kit receptor, is expressed by various structural and inflammatory cells in the airways. Binding of SCF to c-Kit leads to activation of multiple pathways, including phosphatidyl-inositol-3 (PI3)-kinase, phospholipase C (PLC)-gamma, Src kinase, Janus kinase (JAK)/Signal Transducers and Activators of Transcription (STAT) and mitogen activated protein (MAP) kinase pathways. SCF is an important growth factor for mast cells, promoting their generation from CD34+ progenitor cells. In vitro, SCF induces mast cells survival, adhesion to extracellular matrix and degranulation, leading to expression and release of histamine, pro-inflammatory cytokines and chemokines. SCF also induces eosinophil adhesion and activation. SCF is upregulated in inflammatory conditions both in vitro and in vivo, in human and mice. Inhibition of the SCF/c-Kit pathway leads to significant decrease of histamine levels, mast cells and eosinophil infiltration, interleukin (IL)-4 production and airway hyperresponsiveness in vivo. Taken together, these data suggest that SCF/c-Kit may be a potential therapeutic target for the control of mast cell and eosinophil number and activation in inflammatory diseases.

Systemic mastocytosis

Systemic mastocytosis is a fascinating disease with diverse clinical features. There have been numerous advances in understanding the basis of clinical manifestations of this disease and of its molecular pathogenesis in the last several decades. The development of methods to study mast cell biology using cell culture and murine models has proven invaluable in this regard. Clarification of the roles of mast cells in various biological processes has expanded our understanding of their importance in innate immunity, as well as allergy. New diagnostic methods have allowed the design of detailed criteria to assist in distinguishing reactive mast cell hyperplasia from systemic mastocytosis. Variants and subvariants of systemic mastocytosis have been defined to assist in determining prognosis and in management of the disease. Elucidation of the roles of the Kit receptor tyrosine kinase and signal transduction pathway activation has contributed to development of potential targeted therapeutic approaches that may prove useful in the future.

Co-operative signalling mechanisms required for erythroid precursor expansion in response to erythropoietin and stem cell factor

The regeneration of circulating red blood cells in response to anaemia associated with blood loss or haemolysis involves an increased rate of erythropoiesis and expansion of proerythroblasts, the bone marrow precursor cells that terminally differentiate into mature erythrocytes. This study investigated the mechanisms by which erythropoietin (Epo) and stem cell factor (Scf) modulate the expansion of proerythroblasts. Homogenous populations of primary human proerythroblasts were generated in liquid cultures of CD34(+) cells. In serum-free cultures, proerythroblasts failed to survive in the presence of Epo or Scf alone, but exhibited synergistic proliferation in response to combined Epo and Scf treatment, exhibiting one-log expansion in 5 d. Intracellular signal transduction in response to Epo and Scf revealed that tyrosine phosphorylation of signal transducers and activators of transcription (Stat) 5, a downstream target for the non-receptor tyrosine kinase, Janus kinase 2 (Jak2), was mediated by Epo but not Scf. The mitogen-activated protein kinases (MAPKs) extracellular regulated kinase (Erk) 1-2 were phosphorylated in response to either Epo or Scf. Phosphorylation of Akt, a signalling molecule downstream of phosphatidylinositol 3-kinase (PI3K), was observed following Scf but not Epo treatment. To determine the contribution of specific signalling pathways to synergistic expansion of proerythroblasts in response to co-operative effects of Epo and Scf, cells were treated with kinase inhibitors targeting Jak2, PI3K and MAPK kinase. There was a significant, dose-dependent inhibition of proerythroblast expansion in response to all three kinase inhibitors. In conclusion, Epo- and Scf-mediated co-operative, synergistic expansion of primary erythroid precursors requires selective activation of multiple signalling pathways, including the Jak-Stat, PI3K and MAPK pathways.

Stem cell factor synergistically enhances thrombopoietin-induced STAT5 signaling in megakaryocyte progenitors through JAK2 and Src kinase

Stem cell factor (SCF) has a potent synergistic effect during megakaryopoiesis when administered in combination with the major megakaryocytic cytokine, thrombopoietin (TPO). In this study we analyzed the underlying mechanisms with regard to STAT5 activity. TPO stimulation of MO7e cells resulted in STAT5 transactivation, which could be enhanced 1.6-fold by costimulation with SCF, whereas SCF alone did not induce STAT5 transcriptional activity. This costimulatory effect of SCF was reflected in an increase in TPO-induced STAT5 DNA binding and increased and prolonged STAT5 tyrosine phosphorylation in both MO7e cells and primary human megakaryocyte progenitors. In contrast, serine phosphorylation of STAT5 was constitutive and associated with an inhibitory effect on STAT5 transactivation. Signal transduction pathways that might synergize in TPO-mediated STAT5 transactivation were analyzed using specific pharmacological inhibitors and indicated an essential role for Janus-activated kinase 2 (JAK2) and a partial role for Src-family kinases. Costimulation with SCF was found to increase and prolong tyrosine phosphorylation of JAK2 and the TPO receptor c-mpl. In addition, the Src kinase inhibitor SU6656 partially downregulated the additional effect of SCF costimulation on STAT5 tyrosine phosphorylation. SCF-induced enhancement of JAK2 phosphorylation was not affected by inhibition of Src kinase, suggesting that both JAK2 and Src kinase mediate STAT5 tyrosine phosphorylation. Synergistic activation of JAK2 and Src kinase may thus contribute to the enhanced STAT5 signaling in the presence of TPO and SCF.

The anaemia of cancer: death by a thousand cuts

Cancer has a negative systemic impact on its host in addition to its local or metastatic effects, and no cancer complication is more ubiquitous than anaemia, a condition for which there is now a specific remedy, the recombinant growth factor erythropoietin. This is not a trivial therapeutic consideration, because cancer-associated anaemia has an adverse influence on survival regardless of tumour type. However, the pharmacological correction of anaemia with recombinant erythropoietin could promote tumour growth, whereas the use of tumour-necrosis factor-alpha (TNFalpha) and TNF-related apoptosis-inducing ligand as antitumour agents could exacerbate anaemia, thereby perpetuating tissue hypoxia and tumour progression.

Normal and Oncogenic Forms of the Receptor Tyrosine Kinase Kit

Kit is a receptor tyrosine kinase (RTK) that binds stem cell factor. This receptor ligand combination is important for normal hematopoiesis, as well as pigmentation, gut function, and reproduction. Structurally, Kit has both an extracellular and intracellular region. Theintra-cellular region is comprised of a juxtamembrane domain (JMD), a kinase domain, a kinase insert, and a carboxyl tail. Inappropriate expression or activation of Kit is associated with a variety of diseases in humans. Activating mutations in Kit have been identified primarily in the JMD and the second part of the kinase domain and have been associated with gastrointestinal stromal cell tumors and mastocytosis, respectively. There are also reports of activating mutations in some forms of germ cell tumors and core binding factor leukemias. Since the cloning of the Kit ligand in the early 1990s, there has been an explosion of information relating to the mechanism of action of normal forms of Kit as well as activated mutants. This is important because understanding this RTK at the biochemical level could assist in the development of therapeutics to treat primary and secondary defects in the tissues that require Kit. Furthermore, understanding the mechanisms mediating transformation of cells by activated Kit mutants will help in the design of interventions for human disease associated with these mutations. The objective of this review is to summarize what is known about normal and oncogenic forms of Kit. We will place particular emphasis on recent developments in understanding the mechanisms of action of normal and activated forms of this RTK and its association with human disease, particularly in hematopoietic cells.

Inhibition of HMC-1 mast cell proliferation by vitamin E: involvement of the protein kinase B pathway

The effects of four natural tocopherols on the proliferation and signaling pathways were examined in the human mastocytoma cell line (HMC-1). The four tocopherols inhibited HMC-1 cell proliferation with different potency (delta > alpha = gamma > beta). Growth inhibition correlated with the reduction of PKB (protein kinase B) phosphorylation by the different tocopherols. The reduction of PKB phosphorylation led to a decrease of its activity, as judged from a parallel reduction of GSKalpha/beta phosphorylation. The translocation of PKB to the membrane, as a response to receptor stimulation by NGFbeta, is also prevented by treatment with tocopherols. In the presence of PKC or PP2A inhibitors, the reduction of PKB phosphorylation by tocopherols was still observed, thus excluding the direct involvement of these enzymes. Other pathways, such as the Ras-stimulated ERK1/2 (extracellular signal responsive kinase) pathway, were not affected by tocopherol treatment. The tocopherols did not significantly change oxidative stress in HMC-1 cells, suggesting that the observed effects are not the result of a general reduction of oxidative stress. Thus, the tocopherols interfere with PKB phosphorylation and reduce proliferation of HMC-1 cells, possibly by modulating either phosphatidylinositol 3-kinase, a kinase phosphorylating PKB (PDK1/2), or a phosphatase that dephosphorylates it. Inhibition of proliferation and PKB signaling in HMC-1 cells by vitamin E suggests a role in preventing diseases with mast cell involvement, such as allergies, atherosclerosis, and tumorigenesis.

Erythropoietin-dependent autocrine secretion of tumor necrosis factor-alpha in hematopoietic cells modulates proliferation via MAP kinase–ERK-1/2 and does not require tyrosine docking sites in the EPO receptor

Primary erythroid cells and erythroid cell lines may synthesize and secrete tumor necrosis factor-alpha (TNF-alpha) following stimulation with erythropoietin (EPO). The effect of triggering TNF-alpha synthesis and secretion was investigated in erythroleukemia and myeloid cell lines: HCD57, DA3-EPOR, and BAF3-EPOR. The EPO-induced, membrane-bound form of autocrine TNF-alpha seemed to enhance proliferation of HCD57 and DA3-EPOR cells; however, the concentration of secreted autocrine/paracrine TNF-alpha was never sufficient to have an effect. Autocrine TNF-alpha acts through TNFRII receptors to stimulate proliferation. Modulation of mitogen-activated protein kinase (MAPK)/extracellular signal-related kinase (ERK-1/2) activity by the membrane-bound form of autocrine TNF-alpha apparently played a central role in the control of EPO-dependent proliferation of HCD57 and DA3-EPOR cells. Primary erythroid cells and DA3-EPOR cells were found to express similar, high levels of both TNFRI and TNFRII, showing that differential expression of TNF-alpha receptors does not explain why primary cells are inhibited and DA3-EPOR cells are stimulated by autocrine TNF-alpha. BAF3 cells expressing a mutant EPOR with no cytoplasmic tyrosine residues were capable of triggering EPO-dependent TNF-alpha synthesis and secretion, indicating that tyrosine-docking sites in the EPOR were not required for EPO-dependent TNF-alpha secretion.

JAK2 contributes to the intrinsic capacity of primary hematopoietic cells to respond to stem cell factor

OBJECTIVE

Stem cell factor (SCF) is the ligand for the receptor tyrosine kinase (RTK) Kit. The literature contains conflicting reports regarding the capacity of SCF to activate JAK2. Previous work has addressed this controversial issue using biochemical approaches. Here we use a genetic approach to determine the direct role of JAK2 in SCF-mediated growth and differentiation of primary hematopoietic cells.

MATERIALS AND METHODS

Fetal liver cells were isolated from JAK2-deficient murine embryos at day 12 of development. SCF-induced growth and differentiation of this unfractionated population of cells were determined by 3H-thymidine incorporation in bulk cultures, single-cell colony assays, and cytochemistry. In addition, Kit+ cells were isolated from fetal liver by fluorescence-activated cell sorting (FACS) and assessed for growth using 3H-thymidine and colony assays.

RESULTS

SCF-induced growth of unfractionated JAK2-deficient fetal liver cells was reduced by 70% compared to cells from wild-type fetal liver in single-cell assays. This was of particular note because there were three-fold more Kit+ cells in JAK2-deficient fetal liver. Reductions in SCF-induced growth were not observed in bulk cultures of JAK2-deficient fetal liver, suggesting that additional factors cooperate with SCF to overcome the absence of JAK2 in this heterogeneous population of cells. SCF-induced 3H-thymidine incorporation of FACS-purified Kit+ fetal liver deficient for JAK2 was impaired by approximately 50%, whereas colony formation in methylcellulose was reduced 95%. JAK2 also was required for differentiation of this purified population of progenitors into mast cells.

CONCLUSION

JAK2 contributes to the intrinsic capacity of fetal liver hematopoietic progenitor cells to proliferate and differentiate in response to SCF.

Jun N-terminal kinase promotes proliferation of immature erythroid cells and erythropoietin-dependent cell lines

Erythropoietin (EPO) is the hormone necessary for development of erythrocytes from immature erythroid cells. EPO activates Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase (MAPK) family in the EPO-dependent murine erythroid HCD57 cells. Therefore, we tested if JNK activity supported proliferation and/or survival of these cells. Treatment with the JNK inhibitor SP600125 inhibited JNK activity and EPO-dependent proliferation of HCD57 cells and the human EPO-dependent cell lines TF-1 and UT7-EPO. SP600125 also increased the fraction of cells in G2/M. Introduction of a dominant-negative form of JNK1 inhibited EPO-dependent proliferation in HCD57 cells but did not increase the fraction of cells in G2/M. Constitutive JNK activity was observed in primary murine erythroid progenitors. Treatment of primary mouse bone marrow cells with the SP600125 inhibitor reduced the number of erythroid burst-forming units (BFU-e's) but not the more differentiated erythroid colony-forming units (CFU-e's), and SP600125 protected the BFU-e's from apoptosis induced by cytosine arabinoside, demonstrating that the SP600125 inhibited proliferation of the BFU-e's. Therefore, JNK activity appears to be an important regulator of proliferation in immature, primary erythroid cells and 3 erythroid cell lines but may not be required for the survival or proliferation of CFU-e's or proerythroblasts.

Differentiation stage-specific activation of p38 mitogen-activated protein kinase isoforms in primary human erythroid cells

p38alpha, p38beta, p38gamma, and p38delta are four isoforms of p38 mitogen-activated protein (MAP) kinase (MAPK) involved in multiple cellular functions such as cell proliferation, differentiation, apoptosis, and inflammation response. In the present study, we examined the mRNA expression pattern of each of the four isoforms during erythroid differentiation of primary erythroid progenitors. We show that p38alpha and p38gamma transcripts are expressed in early hematopoietic progenitors as well as in late differentiating erythroblasts, whereas p38delta mRNA is only expressed and active during the terminal phase of erythroid differentiation. On the other hand, p38beta is minimally expressed in early CD34(+) hematopoietic progenitors but not expressed in lineage-committed erythroid progenitors. We also determined the phosphorylation/activation of p38alpha, MAPK kinase 3/6, and MAPKAP-2 in response to erythropoietin and stem cell factor. We found that phosphorylation of p38alpha, MAPK kinase kinase 3/6 and MAPKAP-2 occurs only upon growth factor withdrawal in primary erythroid progenitors. Moreover, our data indicate that activation of p38alpha does not induce apoptosis or promote proliferation of erythroid progenitors. On the other hand, under steady-state culture conditions, both p38alpha and p38delta isoforms are increasingly phosphorylated activated in the terminal phase of differentiation. This increased phosphorylation/activity was accompanied by up-regulation of heat shock protein 27 phosphorylation. Finally, we demonstrate that tumor necrosis factor alpha, an inflammatory cytokine that is modulated by p38alpha, is expressed by differentiating erythroblasts and inhibition of p38alpha or tumor necrosis factor alpha results in reduction in differentiation. Taken together, our data demonstrate that both p38alpha and delta isoforms function to promote the late-stage differentiation of primary erythroid progenitors and are likely to be involved in functions related to erythrocyte membrane remodeling and enucleation.

Roles for an Epo receptor Tyr-343 Stat5 pathway in proliferative co-signaling with kit

Erythroid progenitor cell expansion depends upon co-signaling by Epo receptor (EpoR) and Kit, but underlying mechanisms are incompletely understood. To quantitatively analyze EpoR contributions to co-signaling, phosphotyrosine (Tyr(P)) mutants were expressed as human epidermal growth factor (hEGF) receptor-mEpoR EE chimeras at matched and physiological levels in FDCW2 hematopoietic progenitor cells and were assayed for proliferative activities in the absence or presence of endogenous Kit stimulation. Two Tyr(P)-null (but Jak2-coupled) EpoR forms each retained <or=25% of the wild-type activity, whereas the add-back of single Tyr(P) sites in the EpoR forms EE-T-Y343 (Stat5 binding site), EE-Y479 (p85/phosphatidylinositol 3-kinase binding site), or EE-Y464 (Src kinase binding site) significantly enhanced activities (to 100, 95, and 50% of EE-WT (wild type) levels, respectively). EE-Y343&Y401 and EEF343&F401 double add-back and deletion constructs were also prepared and were shown to possess 90 and <or=50% of wild-type activity. In contrast, efficient Kit co-signaling activity was retained only by EE-T-Y343 and EE-Y343&Y401 EpoR forms. EE-T-Y343 together with EE-T-Y343F and EE-WT EpoR forms were also analyzed in embryonic stem cell-derived erythroid G1E-2 cells with highly comparable outcomes, including the ability of EE-T-Y343 (but not EE-T-Y-343F) to synergize with Kit. Despite specific connection of EE-T-Y343 to Stat5, the contributions of Kit to EpoR-dependent proliferation did not involve Kit effects on Stat5 activation (but was limited by the mutation of Kit Tyr(P)-567 and Tyr(P)-569 Src kinase recruitment sites). Instead, co-signaling appears to depend upon the downstream integration of Kit signals with the targets of an EpoR/Jak2/Y343/Stat 5 response axis.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

Stem cell factor enhances erythropoietin-mediated transactivation of signal transducer and activator of transcription 5 (STAT5) via the PKA/CREB pathway

OBJECTIVE

To define whether the observed synergistic effects of erythropoietin (EPO) and stem cell factor (SCF) on erythroid cells can, in part, be mediated by the signal transducer and activator of transcription 5 (STAT5).

METHODS

STAT5 activation was examined in erythroid cell lines by analyzing the effects of EPO and SCF on STAT5 tyrosine phosphorylation, serine phosphorylation, DNA binding, and STAT5-mediated gene transactivation.

RESULTS

EPO induced a 5.0-fold+/-0.4-fold increase in STAT5 transactivation, which could be further enhanced by SCF. SCF pretreatment followed by EPO stimulation resulted in a 9.0-fold+/-0.9-fold increase in STAT5 transactivation, while SCF alone did not increase STAT5 transactivation. This costimulatory effect of SCF was not mediated by increased STAT5 tyrosine or serine phosphorylation or increased STAT5 DNA binding. In addition, enhanced STAT5 transactivation was independent of the phosphatidyl inositol 3-kinase and MAPK(p42/p44) pathways. Instead, the protein kinase A (PKA) inhibitor protein PKI and the PKA inhibitor H89 prevented the costimulatory SCF effect. Furthermore, the PKA target CREB showed a strongly increased and prolonged serine-133 phosphorylation after costimulation with SCF + EPO. The involvement of CREB in STAT5 transactivation was demonstrated by overexpression of serine-133-mutated CREB, which completely blocked the SCF effect. In addition, the CREB-binding protein CBP/p300 was shown to be essential for EPO- and SCF-mediated STAT5 transactivation.

CONCLUSION

SCF enhances the EPO-mediated STAT5 transactivation by triggering a PKA/CREB-dependent pathway.

Induction of globin mRNA expression by interleukin-3 in a stem cell factor-dependent SV-40 T-antigen-immortalized multipotent hematopoietic cell line

Erythropoiesis requires the stepwise action on immature progenitors of several growth factors, including stem cell factor (SCF), interleukin 3 (IL-3), and erythropoietin (Epo). Epo is required to sustain proliferation and survival of committed progenitors and might further modulate the level of expression of several erythroid genes, including globin genes. Here we report a new SCF-dependent immortalized mouse progenitor cell line (GATA-1 ts SCF) that can also grow in either Epo or IL-3 as the sole growth factor. When grown in SCF, these cells show an "open" chromatin structure of the beta-globin LCR, but do not significantly express globin. However, Epo or IL-3 induce globin expression and are required for its maintainance. This effect of IL-3 is unexpected as IL-3 was previously reported either to be unable to induce hemoglobinization, or even to antagonize it. This suggests that GATA-1 ts SCF cells may have progressed to a stage in which globin genes are already poised for expression and only require signal(s) that can be elicited by either Epo or IL-3. Through the use of inhibitors, we suggest that p38 may be one of the molecules modulating induction and maintenance of globin expression.

Functional and biochemical consequences of abrogating the activation of multiple diverse early signaling pathways in Kit. Role for Src kinase pathway in Kit-induced cooperation with erythropoietin receptor

Kit receptor tyrosine kinase and erythropoietin receptor (Epo-R) cooperate in regulating blood cell development. Mice that lack the expression of Kit or Epo-R die in utero of severe anemia. Stimulation of Kit by its ligand, stem cell factor activates several distinct early signaling pathways, including phospholipase C gamma, phosphatidylinositol 3-kinase, Src kinase, Grb2, and Grb7. The role of these pathways in Kit-induced growth, proliferation, or cooperation with Epo-R is not known. We demonstrate that inactivation of any one of these early signaling pathways in Kit significantly impairs growth and proliferation. However, inactivation of the Src pathway demonstrated the most profound defect. Combined stimulation with Epo also resulted in impaired cooperation between Src-defective Kit mutant and Epo-R and, to a lesser extent, with Kit mutants defective in the activation of phosphatidylinositol 3-kinase or Grb2. The impaired cooperation between the Src-defective Kit mutant and Epo-R was associated with reduced transphosphorylation of Epo-R and expression of c-Myc. Remarkably, restoration of only the Src pathway in a Kit receptor defective in the activation of all early signaling pathways demonstrated a 50% correction in proliferation in response to Kit stimulation and completely restored the cooperation with Epo-R. These data demonstrate an essential role for Src pathway in regulating growth, proliferation, and cooperation with Epo-R downstream from Kit.

Stem cell factor increases the expression of FLIP that inhibits IFNgamma -induced apoptosis in human erythroid progenitor cells

Interferon gamma (IFNgamma) acts on human erythroid colony-forming cells (ECFCs) to up-regulate Fas, without a demonstrable change of Fas ligand (FasL) or Fas-associated DD-containing protein (FADD) expression and activates caspase-8 plus caspase-3, which produce apoptosis. Our previous data showed that stem cell factor (SCF) reduced the inhibitory effect of IFNgamma on human ECFCs when both factors were present in the cultures. However, the mechanism by which SCF prevents IFNgamma-induced apoptosis in ECFCs is unclear. In this study we used highly purified human ECFCs to investigate the mechanism of the effect of SCF on IFNgamma-induced apoptosis. Because the binding of FasL to Fas is the first step of the apoptosis cascade and IFNgamma strongly up-regulates Fas expression, we added FasL (50 ng/mL) to the cultures with IFNgamma to accentuate the IFNgamma-induced activation of caspase-8 and caspase-3 plus subsequent apoptosis. SCF (100 ng/mL) clearly inhibited the activation of caspase-8 and caspase-3 induced by IFNgamma and/or FasL, and it also reduced apoptosis as measured by the terminal dUTP nick-end labeling (TUNEL) assay. SCF did not decrease the surface expression of Fas on the ECFCs. FADD-like interleukin 1 beta (IL-1beta)-converting enzyme (FLICE)-inhibitory protein (FLIP) has been reported to interact with FADD and/or caspase-8 at the death-inducing signaling complex (DISC) level following Fas stimulation and acts as a dominant-negative caspase-8. SCF increased FLIP mRNA and protein expression, concomitant with reduced apoptosis, whereas IFNgamma and/or FasL did not change FLIP expression. Reduction of FLIP expression with antisense oligonucleotides decreased the capacity of SCF to inhibit IFNgamma-induced apoptosis, demonstrating a definite role for FLIP in the SCF-induced protection of ECFCs from IFNgamma-initiated apoptosis.

Stem cell factor attenuates liver damage in a murine model of acetaminophen-induced hepatic injury

Acute liver injury is a common cause of intensive care unit visits. In these studies, we used a murine model of acetaminophen poisoning to examine the role of stem cell factor (SCF) on liver damage. In the initial studies, we identified that the liver produces relatively high constitutive levels of SCF. Upon administration of acetaminophen, the levels of SCF fell dramatically, correlating to damage within the liver. When the liver was allowed to regenerate, the levels of SCF again correlated with the liver regeneration. We next treated mice with anti-SCF before sublethal doses of acetaminophen and significantly increased lethality in anti-SCF-treated animals. When exogenous SCF was given to mice, the lethality was significantly reduced compared with the control acetaminophen-treated animals and the damage within the liver tissue was attenuated. The administration of rSCF reduced the level of steady-state mRNA for cytochrome P450 cyp2E1 enzyme both in vitro and in vivo. These data suggest that SCF functions as an important factor that protects livers from acute damage.

Tumor necrosis factor-alpha expressed constitutively in erythroid cells or induced by erythropoietin has negative and stimulatory roles in normal erythropoiesis and erythroleukemia

Binding of erythropoietin (EPO) to its receptor (EPOR) on erythroid cells induces the activation of numerous signal transduction pathways, including the mitogen-activated protein kinase Jun-N-terminal kinase (JNK). In an effort to understand the regulation of EPO-induced proliferation and JNK activation, we have examined the role of potential autocrine factors in the proliferation of the murine erythroleukemia cell line HCD57. We report here that treatment of these cells with EPO induced the expression and secretion of tumor necrosis factor alpha (TNF-alpha). EPO-dependent proliferation was reduced by the addition of neutralizing antibodies to TNF-alpha, and exogenously added TNF-alpha induced proliferation of HCD57 cells. EPO also could induce TNF-alpha expression in BAF3 and DA3 myeloid cells ectopically expressing EPOR. Addition of TNF-alpha activated JNK in HCD57 cells, and the activity of JNK was partially inhibited by addition of a TNF-alpha neutralizing antibody. Primary human and murine erythroid progenitors expressed TNF-alpha in either an EPO-dependent or constitutive manner. However, TNF-alpha had an inhibitory effect on both immature primary human and murine cells, suggestive that the proliferative effects of TNF-alpha may be limited to erythroleukemic cells. This study suggests a novel role for autocrine TNF-alpha expression in the proliferation of erythroleukemia cells that is distinct from the effect of TNF-alpha in normal erythropoiesis.

Erythropoietin: physiology and pharmacology update

This minireview is an update of a 1997 review on erythropoietin (EPO) in this journal. EPO is a 30,400-dalton glycoprotein that regulates red cell production. In the human, EPO is produced by peritubular cells in the kidneys of the adult and in hepatocytes in the fetus. Small amounts of extra-renal EPO are produced by the liver in adult human subjects. EPO binds to an erythroid progenitor cell surface receptor that includes a p66 chain, and, when activated, the p66 protein becomes dimerized. EPO receptor activation induces a JAK2 tyrosine kinase, which leads to tyrosine phosphorylation of the EPO receptor and several proteins. EPO receptor binding leads to intracellular activation of the Ras/mitogen-activated kinase pathway, which is involved with cell proliferation, phosphatidylinositol 3-kinase, and STATS 1, 3, 5A, and 5B transcriptional factors. EPO acts primarily to rescue erythroid cells from apoptosis (programmed cell death) to increase their survival. EPO acts synergistically with several growth factors (SCF, GM-CSF, 1L-3, and IGF-1) to cause maturation and proliferation of erythroid progenitor cells (primarily colony-forming unit-E). Oxygen-dependent regulation of EPO gene expression is postulated to be controlled by a hypoxia-inducible transcription factor (HIF-1alpha). Hypoxia-inducible EPO production is controlled by a 50-bp hypoxia-inducible enhancer that is approximately 120 bp 3' to the polyadenylation site. Hypoxia signal transduction pathways involve kinases A and C, phospholipase A(2), and transcription factors ATF-1 and CREB-1. A model has been proposed for adenosine activation of EPO production that involves protein kinases A and C and the phospholipase A(2) pathway. Other effects of EPO include a hematocrit-independent, vasoconstriction-dependent hypertension, increased endothelin production, upregulation of tissue renin, change in vascular tissue prostaglandins production, stimulation of angiogenesis, and stimulation of endothelial and vascular smooth muscle cell proliferation. Recombinant human EPO (rHuEPO) is currently being used to treat patients with anemias associated with chronic renal failure, AIDS patients with anemia due to treatment with zidovudine, nonmyeloid malignancies in patients treated with chemotherapeutic agents, perioperative surgical patients, and autologous blood donation. A novel erythropoiesis-stimulating factor (NESP, darbepoetin) has been synthesized and when compared with rHuEPO, NESP has a higher carbohydrate content (52% vs 40%), a longer plasma half-life, the amino acid sequence differs from that of native human EPO at five positions, and has been reported to maintain hemoglobin levels just as effectively in patients with chronic renal failure as rHuEPO at less frequent dosing. The use of rHuEPO and darbepoetin to enhance athletic performance is officially banned by most sports-governing bodies because the excessive erythrocytosis can lead to increased thrombogenicity and can cause deep vein, coronary, and cerebral thromboses.

Erythropoietin protects neurons against chemical hypoxia and cerebral ischemic injury by up-regulating Bcl-xL expression

Erythropoietin (EPO) promotes neuronal survival after cerebral ischemia in vivo and after hypoxia in vitro. However, the mechanisms underlying the protective effects of EPO on ischemic/hypoxic neurons are not fully understood. The present in vitro experiments showed that EPO attenuated neuronal damage caused by chemical hypoxia at lower extracellular concentrations (10(- 4)-10(-2) U/ml) than were previously considered. Moreover, EPO at a concentration of 10(-3) U/ml up-regulated Bcl-xL mRNA and protein expressions in cultured neurons. Subsequent in vivo study focused on whether EPO rescued hippocampal CA1 neurons from lethal ischemic damage and up-regulated the expressions of Bcl-xL mRNA and protein in the hippocampal CA1 field of ischemic gerbils. EPO was infused into the cerebroventricles of gerbils immediately after 3 min of ischemia for 28 days. Infusion of EPO at a dose of 5 U/day prevented the occurrence of ischemia-induced learning disability. Subsequent light microscopic examinations showed that pyramidal neurons in the hippocampal CA1 field were significantly more numerous in ischemic gerbils infused with EPO (5 U/day) than in those receiving vehicle infusion. The same dose of EPO infusion caused significantly more intense expressions of Bcl-xL mRNA and protein in the hippocampal CA1 field of ischemic gerbils than did vehicle infusion. These findings suggest that EPO prevents delayed neuronal death in the hippocampal CA1 field, possibly through up-regulation of Bcl-xL, which is known to facilitate neuron survival.

Role of JunB in erythroid differentiation

The role of junB as a regulator of erythroid cell survival, proliferation, and differentiation was tested by controlled expression of JunB in the erythropoietin (EPO)-dependent erythroleukemia cell line HCD57. JunB induced erythroid differentiation as evidenced by increased expression of the erythroid-specific proteins beta-globin, spectrin-alpha, and TER-119. Expression of JunB for at least 48 h was required for the differentiated phenotype to emerge. Differentiation was accompanied by a slower rate of proliferation and an increase in the expression of the cell cycle inhibitory protein p27. p27 protein expression increased due to reduced turnover without changes in transcription, indicating global changes in cell physiology following JunB induction. JunB expression was also studied in mouse and human primary erythroid cells. JunB expression increased immediately in both primary mouse cells and HCD57 cells treated with EPO and quickly returned to base-line levels, followed by a secondary rise in JunB in primary erythroid cells, but not in HCD57 cells, 36-48 h later. This result suggested that the initial EPO-dependent JunB induction was not sufficient to induce differentiation, but that the late EPO-independent JunB expression in primary erythroid cells was necessary for differentiation. This study suggests that JunB is an important regulator of erythroid differentiation.

Stem cell factor regulation of Fas-mediated apoptosis of human erythroid precursor cells

Multiple cytokines regulate the development of erythrocytes. Increasing attention has been directed to the possible role of Fas and its cognate ligand (Fas-L), a subject of wide interest. Documentation of in vitro data supports the role of Fas and Fas-L in erythropoiesis. Several laboratories, including ours, investigated the opposing actions of erythropoietin (EPO) and stem cell factor (SCF) on Fas-mediated cell death of the erythroid cells. Only circumstantial in vivo evidence has accumulated concerning the issue. There are several reports suggesting that Fas-mediated cell death may have a role in some pathological conditions. Results of the accumulating findings and possible implications in clinical hematology are summarized in this review.

Phosphatidylinositol 3 kinase contributes to the transformation of hematopoietic cells by the D816V c-Kit mutant

Stem cell factor (SCF) binds the receptor tyrosine kinase c-Kit and is critical for normal hematopoiesis. Substitution of valine for aspartic acid 816 (D816V) constitutively actives human c-Kit, and this mutation is found in patients with mastocytosis, leukemia, and germ cell tumors. Immortalized murine progenitor cells (MIHCs) transduced with wild-type c-Kit proliferate in response to SCF, whereas cells expressing D816V c-Kit (MIHC-D816V) are factor-independent and tumorigenic. However, the mechanisms mediating transformation by D816V c-Kit are unknown. The objective of this study was to identify signaling components that contribute to D816V c-Kit-mediated transformation. SCF stimulates association of p85PI3K with phosphorylated tyrosine 721 of wild-type c-Kit. Phosphatidylinositol 3 kinase (PI3K) subsequently contributes to the activation of Akt and Jnks. In contrast, these studies demonstrated that the D816V c-Kit mutant was constitutively associated with phosphorylated p85PI3K, and, downstream of PI3K, Jnk 1 and Jnk 2 were activated but Akt was not. Interestingly, Erks 1 and 2 were not constitutively activated by D816V c-Kit. Thus, D816V c-Kit maintains the activity of PI3K but not of all signaling pathways activated by wild-type c-Kit. Further, all pathways downstream of PI3K are not constitutively active in MIHC-D816V cells. Studies with a PI3K inhibitor and D816V/Y721F c-Kit, a mutant incapable of recruiting PI3K, indicate that constitutive activation of PI3K through direct recruitment by D816V c-Kit plays a role in factor-independent growth of MIHC and is critical for tumorigenicity.

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.

Promoter choice influences alternative splicing and determines the balance of isoforms expressed from the mouse bcl-X gene

Differential splicing from the bcl-X gene generates several isoforms with opposite effects on the apoptotic response. To explore the mechanism controlling the balance between the various isoforms, we have characterized the 5' region of the mouse bcl-X gene. We identified three new promoters in addition to the two previously described (Grillot, D. A., M., G.-G., Ekhterae, D., Duan, L., Inohara, N., Ohta, S., Seldin, M. F., and Núñez, G. (1997) J. Immunol. 158, 4750-4757). These five promoters (P1-P5) would give rise to at least five mRNAs with different 5'-untranslated region, all sharing the same translation initiation site. Except for the product of the most proximal promoter (P1), the other mRNAs are generated by alternative splicing of noncoding exons to a common acceptor site located in the first translated exon. Reverse transcriptase-polymerase chain reaction, primer extension, and RNase protection assays demonstrate a tissue-specific pattern of promoter usage. P1 and P2 are active in all tissues analyzed, whereas the other three promoter show tissue-specific activities. P3 is active in spleen, liver, and kidney, P4 is active in uterus and spleen, and P5 is active in spleen, liver, brain, and thymus. We present evidence suggesting that promoter selection influences the outcome of the splice process. Transcripts from P1 generate mainly the mRNA for the long isoform Bcl-X(L), whereas transcripts from P2 generate mRNAs for the isoforms Bcl-X(L), Bcl-X(S), and Bcl-X(gamma) and transcripts from P3 yield mainly mRNAs for the isoform Bcl-X(gamma). Our results suggest a key role of promoter choice in determining alternative splicing and, thus, the balance of Bcl-X isoforms.

Signal transducer and activator of transcription 3 activation is required for Asp816 mutant c-Kit–mediated cytokine-independent survival and proliferation in human leukemia cells

Activating mutations of c-kit at codon 816 (Asp816) have been implicated in a variety of malignancies, including acute myeloid leukemia (AML). The mutant c-Kit receptor confers cytokine-independent survival of leukemia cells and induces tumorigenicity. Changes in the signal transduction pathways responsible for Asp816 mutant c-Kit–mediated biologic effects are largely undefined. The results of this study show that Asp816 mutant c-Kit induces constitutive activation of signal transducer and activator of transcription 3 (STAT3) and STAT1, and up-regulates STAT3 downstream targets, Bcl-xL and c-myc. The phosphatidylinositol-3-kinase (PI-3K)/Akt pathway, but not the Ras-mediated mitogen-activated protein (MAP) kinase pathway, is also constitutively activated by Asp816 mutant c-Kit. Suppression of STAT3 activation by a dominant negative molecule in MO7e leukemia cells transduced with mutant c-kit inhibits stem cell factor (SCF)-independent survival and proliferation, accompanied by the down-regulation of Bcl-xL and c-myc. However, activated STAT3 does not appear to be the sole mediator that is responsible for the phenotypic changes induced by Asp816 mutant c-Kit, because expression of constitutively activated STAT3 in MO7e cells does not completely reconstitute cytokine independence. Activation of other signaling components by mutant c-Kit, such as those in the PI-3K/Akt pathway, is demonstrated and may also be needed for the mutant c-Kit–mediated biologic effects. The investigation of altered signal transduction pathways and the resulting functional consequences mediated by Asp816 mutant c-Kit should provide important information for the characterization of subsets of leukemia and potential molecular pathways for therapeutic targeting.

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.