The cytoplasmic region of the erythropoietin receptor contains nonoverlapping positive and negative growth-regulatory domains

Tamoxifen for the treatment of myeloproliferative neoplasms: A Phase II clinical trial and exploratory analysis

Current therapies for myeloproliferative neoplasms (MPNs) improve symptoms but have limited effect on tumor size. In preclinical studies, tamoxifen restored normal apoptosis in mutated hematopoietic stem/progenitor cells (HSPCs). TAMARIN Phase-II, multicenter, single-arm clinical trial assessed tamoxifen's safety and activity in patients with stable MPNs, no prior thrombotic events and mutated JAK2V617F, CALRins5 or CALRdel52 peripheral blood allele burden ≥20% (EudraCT 2015-005497-38). 38 patients were recruited over 112w and 32 completed 24w-treatment. The study's A'herns success criteria were met as the primary outcome ( ≥ 50% reduction in mutant allele burden at 24w) was observed in 3/38 patients. Secondary outcomes included ≥25% reduction at 24w (5/38), ≥50% reduction at 12w (0/38), thrombotic events (2/38), toxicities, hematological response, proportion of patients in each IWG-MRT response category and ELN response criteria. As exploratory outcomes, baseline analysis of HSPC transcriptome segregates responders and non-responders, suggesting a predictive signature. In responder HSPCs, longitudinal analysis shows high baseline expression of JAK-STAT signaling and oxidative phosphorylation genes, which are downregulated by tamoxifen. We further demonstrate in preclinical studies that in JAK2V617F+ cells, 4-hydroxytamoxifen inhibits mitochondrial complex-I, activates integrated stress response and decreases pathogenic JAK2-signaling. These results warrant further investigation of tamoxifen in MPN, with careful consideration of thrombotic risk.

New pathogenic mechanisms induced by germline erythropoietin receptor mutations in primary erythrocytosis

Primary familial and congenital polycythemia is characterized by erythropoietin hypersensitivity of erythroid progenitors due to germline nonsense or frameshift mutations in the erythropoietin receptor gene. All mutations so far described lead to the truncation of the C-terminal receptor sequence that contains negative regulatory domains. Their removal is presented as sufficient to cause the erythropoietin hypersensitivity phenotype. Here we provide evidence for a new mechanism whereby the presence of novel sequences generated by frameshift mutations is required for the phenotype rather than just extensive truncation resulting from nonsense mutations. We show that the erythropoietin hypersensitivity induced by a new erythropoietin receptor mutant, p.Gln434Profs*11, could not be explained by the loss of negative signaling and of the internalization domains, but rather by the appearance of a new C-terminal tail. The latter, by increasing erythropoietin receptor dimerization, stability and cell-surface localization, causes pre-activation of erythropoietin receptor and JAK2, constitutive signaling and hypersensitivity to erythropoietin. Similar results were obtained with another mutant, p.Pro438Metfs*6, which shares the same last five amino acid residues (MDTVP) with erythropoietin receptor p.Gln434Profs*11, confirming the involvement of the new peptide sequence in the erythropoietin hypersensitivity phenotype. These results suggest a new mechanism that might be common to erythropoietin receptor frameshift mutations. In summary, we show that primary familial and congenital polycythemia is more complex than expected since distinct mechanisms are involved in the erythropoietin hypersensitivity phenotype, according to the type of erythropoietin receptor mutation.

Functional characterization of a chimeric soluble Fas ligand polymer with in vivo anti-tumor activity

Binding of ligand FasL to its receptor Fas triggers apoptosis via the caspase cascade. FasL itself is homotrimeric, and a productive apoptotic signal requires that FasL be oligomerized beyond the homotrimeric state. We generated a series of FasL chimeras by fusing FasL to domains of the Leukemia Inhibitory Factor receptor gp190 which confer homotypic oligomerization, and analyzed the capacity of these soluble chimeras to trigger cell death. We observed that the most efficient FasL chimera, called pFasL, was also the most polymeric, as it reached the size of a dodecamer. Using a cellular model, we investigated the structure-function relationships of the FasL/Fas interactions for our chimeras, and we demonstrated that the Fas-mediated apoptotic signal did not solely rely on ligand-mediated receptor aggregation, but also required a conformational adaptation of the Fas receptor. When injected into mice, pFasL did not trigger liver injury at a dose which displayed anti-tumor activity in a model of human tumor transplanted to immunodeficient animals, suggesting a potential therapeutic use. Therefore, the optimization of the FasL conformation has to be considered for the development of efficient FasL-derived anti-cancer drugs targeting Fas.

Stat5 signaling specifies basal versus stress erythropoietic responses through distinct binary and graded dynamic modalities

Stat5 signaling in erythroblasts can assume either a binary, low-intensity form, essential for basal erythropoiesis, or a graded, high-intensity response, restricted to early erythroblasts and to erythropoietic stress.

Erythropoietin (Epo)-induced Stat5 phosphorylation (p-Stat5) is essential for both basal erythropoiesis and for its acceleration during hypoxic stress. A key challenge lies in understanding how Stat5 signaling elicits distinct functions during basal and stress erythropoiesis. Here we asked whether these distinct functions might be specified by the dynamic behavior of the Stat5 signal. We used flow cytometry to analyze Stat5 phosphorylation dynamics in primary erythropoietic tissue in vivo and in vitro, identifying two signaling modalities. In later (basophilic) erythroblasts, Epo stimulation triggers a low intensity but decisive, binary (digital) p-Stat5 signal. In early erythroblasts the binary signal is superseded by a high-intensity graded (analog) p-Stat5 response. We elucidated the biological functions of binary and graded Stat5 signaling using the EpoR-HM mice, which express a “knocked-in” EpoR mutant lacking cytoplasmic phosphotyrosines. Strikingly, EpoR-HM mice are restricted to the binary signaling mode, which rescues these mice from fatal perinatal anemia by promoting binary survival decisions in erythroblasts. However, the absence of the graded p-Stat5 response in the EpoR-HM mice prevents them from accelerating red cell production in response to stress, including a failure to upregulate the transferrin receptor, which we show is a novel stress target. We found that Stat5 protein levels decline with erythroblast differentiation, governing the transition from high-intensity graded signaling in early erythroblasts to low-intensity binary signaling in later erythroblasts. Thus, using exogenous Stat5, we converted later erythroblasts into high-intensity graded signal transducers capable of eliciting a downstream stress response. Unlike the Stat5 protein, EpoR expression in erythroblasts does not limit the Stat5 signaling response, a non-Michaelian paradigm with therapeutic implications in myeloproliferative disease. Our findings show how the binary and graded modalities combine to generate high-fidelity Stat5 signaling over the entire basal and stress Epo range. They suggest that dynamic behavior may encode information during STAT signal transduction.

Hormone signaling through the erythropoietin (Epo) pathway is required both for the continuous replacement of red blood cells (RBCs) that are lost through aging (a process known as "basal erythropoiesis") and to boost tissue oxygen when bleeding, in anemia or at high altitude ("stress erythropoiesis"). A key challenge lies in understanding how extracellular Epo concentration is translated into different intracellular signals that promote transcription of proteins that are specific to basal versus stress erythropoiesis. Binding of Epo to its receptor EpoR on the surface of an erythroblast (the precursors of RBCs) triggers the addition of phosphates to a target protein Stat5; the phosphorylated Stat5 becomes activated and induces transcription. We show that the dynamic properties of the Stat5 activation signal convey additional information that specifies either basal or stress responses. During basal conditions, the Stat5 signal is low and binary in nature—an on/off switch-like response. Stress, on the other hand, triggers a distinct Stat5 response consisting of a highintensity signal that increases in a graded fashion with rising Epo concentration. We found that a mouse bearing a truncated EpoR is restricted to the low-intensity binary Stat5 signal and correspondingly fails to initiate stress erythropoiesis. Ultimately, it is the Stat5 protein level in erythroblasts that determines their ability to generate the high-intensity graded Stat5 signal in response to high Epo. These findings have therapeutic potential: targeting Stat5's high-intensity graded signal may inhibit its aberrant function in blood cell cancers without affecting its important binary response in normal cells.

The erythropoietin receptor: molecular structure and hematopoietic signaling pathways

The process of erythropoiesis in the fetal liver and adult bone marrow is regulated by the hormone erythropoietin (Epo), which is produced in the kidney at low levels under homeostatic conditions. Defects in Epo production result in severe anemia; use of recombinant hormone has improved the lives of patients with renal failure or anemia because of bone marrow suppression. Deletion of the Epo gene in mice leads to embryonic lethality at days 13 to 15, coincident with the establishment of definitive (adult-type) erythropoiesis and underscoring the absolute necessity of Epo function in vivo. Epo has proven to be a successful pharmaceutical agent, one of the early triumphs of recombinant protein technology. Because of its clinical importance, a great deal of attention has focused on the molecular mechanisms of Epo-regulated erythropoiesis. This review highlights the basic concepts of Epo signal transduction within the hematopoietic system, the major site of Epo action in vivo.

Analysis of genomic aberrations and gene expression profiling identifies novel lesions and pathways in myeloproliferative neoplasms

Polycythemia vera (PV), essential thrombocythemia and primary myelofibrosis, are myeloproliferative neoplasms (MPNs) with distinct clinical features and are associated with the JAK2V617F mutation. To identify genomic anomalies involved in the pathogenesis of these disorders, we profiled 87 MPN patients using Affymetrix 250K single-nucleotide polymorphism (SNP) arrays. Aberrations affecting chr9 were the most frequently observed and included 9pLOH (n=16), trisomy 9 (n=6) and amplifications of 9p13.3–23.3 (n=1), 9q33.1–34.13 (n=1) and 9q34.13 (n=6). Patients with trisomy 9 were associated with elevated JAK2V617F mutant allele burden, suggesting that gain of chr9 represents an alternative mechanism for increasing JAK2V617F dosage. Gene expression profiling of patients with and without chr9 abnormalities (+9, 9pLOH), identified genes potentially involved in disease pathogenesis including JAK2, STAT5B and MAPK14. We also observed recurrent gains of 1p36.31–36.33 (n=6), 17q21.2–q21.31 (n=5) and 17q25.1–25.3 (n=5) and deletions affecting 18p11.31–11.32 (n=8). Combined SNP and gene expression analysis identified aberrations affecting components of a non-canonical PRC2 complex (EZH1, SUZ12 and JARID2) and genes comprising a ‘HSC signature' (MLLT3, SMARCA2 and PBX1). We show that NFIB, which is amplified in 7/87 MPN patients and upregulated in PV CD34+ cells, protects cells from apoptosis induced by cytokine withdrawal.

Erythropoietin receptor response circuits

PURPOSE OF REVIEW

In 1985-1989, erythropoietin (EPO), its receptor (EPOR), and janus kinase 2 were cloned; established to be essential for definitive erythropoiesis; and initially intensely studied. Recently, new impetus, tools, and model systems have emerged to re-examine EPO/EPOR actions, and are addressed in this review. Impetus includes indications that EPO affects significantly more than standard erythroblast survival pathways, the development of novel erythropoiesis-stimulating agents, increasing evidence for EPO/EPOR cytoprotection of ischemically injured tissues, and potential EPO-mediated worsening of tumorigenesis.

RECENT FINDINGS

New findings are reviewed in four functional contexts: (pro)erythroblast survival mechanisms, new candidate EPO/EPOR effects on erythroid cell development and new EPOR responses, EPOR downmodulation and trafficking, and novel erythropoiesis-stimulating agents.

SUMMARY

As Current Opinion, this monograph seeks to summarize, and provoke, new EPO/EPOR action concepts. Specific problems addressed include: beyond (and before) BCL-XL, what key survival factors are deployed in early-stage proerythroblasts? Are distinct EPO/EPOR signals transduced in stage-selective fashions? Is erythroblast proliferation also modulated by EPO/EPOR signals? What functions are subserved by new noncanonical EPO/EPOR response factors (e.g. podocalyxin like-1, tribbles 3, reactive oxygen species, and nuclear factor kappa B)? What key regulators mediate EPOR inhibition and trafficking? And for emerging erythropoiesis-stimulating agents, to what extent do activities parallel EPOs (or differ in advantageous, potentially complicating ways, or both)?

Apoptosis induced by JAK2 inhibition is mediated by Bim and enhanced by the BH3 mimetic ABT-737 in JAK2 mutant human erythroid cells

The activating mutation JAK2 V617F plays a central role in the pathogenesis of polycythemia vera, essential thrombocythemia, and primary myelofibrosis. Inhibition of JAK2 activity leads to growth inhibition and apoptosis in cells with mutated JAK2. However, the proapoptotic proteins involved in JAK2 inhibition-induced apoptosis remain unclear. In this study, we show that JAK2 inhibition-induced apoptosis correlated with up-regulation of the nonphosphorylated form of the BH3-only protein Bim in hematopoietic cell lines bearing JAK2 mutations. Knockdown of Bim dramatically inhibited apoptosis induced by JAK2 inhibition, which was reversed by the BH3 mimetic agent ABT-737. In addition, ABT-737 enhanced the apoptosis induced by JAK2 inhibition in JAK2 V617F(+) HEL and SET-2 cells. The combination of JAK inhibitor I and ABT-737 reduced the number of erythroid colonies derived from CD34(+) cells isolated from JAK2 V617F(+) polycythemia vera patients more efficiently than either drug alone. These data suggest that Bim is a key effector molecule in JAK2 inhibition-induced apoptosis and that targeting this apoptotic pathway could be a novel therapeutic strategy for patients with activating JAK2 mutations.

Efficacy of the JAK2 inhibitor INCB16562 in a murine model of MPLW515L-induced thrombocytosis and myelofibrosis

The discovery of JAK2 and MPL mutations in patients with myeloproliferative neoplasms (MPNs) provided important insight into the genetic basis of these disorders and led to the development of JAK2 kinase inhibitors for MPN therapy. Although recent studies have shown that JAK2 kinase inhibitors demonstrate efficacy in a JAK2V617F murine bone marrow transplantation model, the effects of JAK2 inhibitors on MPLW515L-mediated myeloproliferation have not been investigated. In this report, we describe the in vitro and in vivo effects of INCB16562, a small-molecule JAK2 inhibitor. INCB16562 inhibited proliferation and signaling in cell lines transformed by JAK2 and MPL mutations. Compared with vehicle treatment, INCB16562 treatment improved survival, normalized white blood cell counts and platelet counts, and markedly reduced extramedullary hematopoeisis and bone marrow fibrosis. We observed inhibition of STAT3 and STAT5 phosphorylation in vivo consistent with potent inhibition of JAK-STAT signaling. These data suggest JAK2 inhibitor therapy may be of value in the treatment of JAK2V617F-negative MPNs. However, we did not observe a decrease in the size of the malignant clone in the bone marrow of treated mice at the end of therapy, which suggests that JAK2 inhibitor therapy, by itself, was not curative in this MPN model.

Erythropoietin signaling promotes transplanted progenitor cell survival

We examine the potential for erythropoietin signaling to promote donor cell survival in a model of myoblast transplantation. Expression of a truncated erythropoietin receptor in hematopoietic stem cells has been shown to promote selective engraftment in mice. We previously demonstrated expression of endogenous erythropoietin receptor on murine myoblasts, and erythropoietin treatment can stimulate myoblast proliferation and delay differentiation. Here, we report that enhanced erythropoietin receptor expression, as well as exogenous erythropoietin treatment in myoblasts, provided a survival advantage and protection against apoptosis under serum-starvation conditions. When cultured in differentiation medium, expression of the myogenic regulatory proteins shifted toward early differentiation with increased erythropoietin receptor. Expression of early myogenic differentiation proteins Myf-5 and MyoD increased, while later stage protein myogenin decreased. Transplantation of C2C12 myoblasts overexpressing truncated erythropoietin receptor showed more transplanted cell incorporation into muscle fibers in muscular dystrophy mdx mice. These cells also restored dystrophin protein expression in mdx mice at 6 wk after cell treatment that was further increased with exogenous erythropoietin administration. In summary, enhanced erythropoietin receptor expression promotes transplanted cell survival in a mouse model for myoblast transplantation and provides dystrophin expression in mice with muscular dystrophy.

Survival and proliferative roles of erythropoietin beyond the erythroid lineage

Since the isolation and purification of erythropoietin (EPO) in 1977, the essential role of EPO for mature red blood cell production has been well established. The cloning of the EPO gene and production of recombinant human EPO led to the widespread use of EPO in treating patients with anaemia. However, the biological activity of EPO is not restricted to regulation of erythropoiesis. EPO receptor (EPOR) expression is also found in endothelial, brain, cardiovascular and other tissues, although at levels considerably lower than that of erythroid progenitor cells. This review discusses the survival and proliferative activity of EPO that extends beyond erythroid progenitor cells. Loss of EpoR expression in mouse models provides evidence for the role of endogenous EPO signalling in nonhaematopoietic tissue during development or for tissue maintenance and/or repair. Determining the extent and distribution of receptor expression provides insights into the potential protective activity of EPO in brain, heart and other nonhaematopoietic tissues.

An extracellular region of the erythropoietin receptor of the subterranean blind mole rat Spalax enhances receptor maturation

Erythropoietic functions of erythropoietin (EPO) are mediated by its receptor (EPO-R), which is present on the cell surface of erythroid progenitors and induced by hypoxia. We focused on EPO-R from Spalax galili (sEPO-R), one of the four Israeli species of the subterranean blind mole rat, Spalax ehrenbergi superspecies, as a special natural animal model of high tolerance to hypoxia. Led by the intriguing observation that most of the mouse EPO-R (mEPO-R) is retained in the endoplasmic reticulum (ER), we hypothesized that sEPO-R is expressed at higher levels on the cell surface, thus maximizing the response to elevated EPO, which has been reported in this species. Indeed, we found increased cell-surface levels of sEPO-R as compared with mEPO-R by using flow cytometry analysis of BOSC cells transiently expressing HA-tagged EPO-Rs (full length or truncated). We then postulated that unique extracellular sEPO-R sequence features contribute to its processing and cell-surface expression. To map these domains of the sEPO-R that augment receptor maturation, we generated EPO-R derivatives in which parts of the extracellular region of mEPO-R were replaced with the corresponding fragments of sEPO-R. We found that an extracellular portion of sEPO-R, harboring the N-glycosylation site, conferred enhanced maturation and increased transport to the cell surface of the respective chimeric receptor. Taken together, we demonstrate higher surface expression of sEPO-R, attributed at least in part to increased ER exit, mediated by an extracellular region of this receptor. We speculate that these sEPO-R sequence features play a role in the adaptation of Spalax to extreme hypoxia.

JAK2 exon 12 mutations in polycythemia vera and idiopathic erythrocytosis

BACKGROUND

The V617F mutation, which causes the substitution of phenylalanine for valine at position 617 of the Janus kinase (JAK) 2 gene (JAK2), is often present in patients with polycythemia vera, essential thrombocythemia, and idiopathic myelofibrosis. However, the molecular basis of these myeloproliferative disorders in patients without the V617F mutation is unclear.

METHODS

We searched for new mutations in members of the JAK and signal transducer and activator of transcription (STAT) gene families in patients with V617F-negative polycythemia vera or idiopathic erythrocytosis. The mutations were characterized biochemically and in a murine model of bone marrow transplantation.

RESULTS

We identified four somatic gain-of-function mutations affecting JAK2 exon 12 in 10 V617F-negative patients. Those with a JAK2 exon 12 mutation presented with an isolated erythrocytosis and distinctive bone marrow morphology, and several also had reduced serum erythropoietin levels. Erythroid colonies could be grown from their blood samples in the absence of exogenous erythropoietin. All such erythroid colonies were heterozygous for the mutation, whereas colonies homozygous for the mutation occur in most patients with V617F-positive polycythemia vera. BaF3 cells expressing the murine erythropoietin receptor and also carrying exon 12 mutations could proliferate without added interleukin-3. They also exhibited increased phosphorylation of JAK2 and extracellular regulated kinase 1 and 2, as compared with cells transduced by wild-type JAK2 or V617F JAK2. Three of the exon 12 mutations included a substitution of leucine for lysine at position 539 of JAK2. This mutation resulted in a myeloproliferative phenotype, including erythrocytosis, in a murine model of retroviral bone marrow transplantation.

CONCLUSIONS

JAK2 exon 12 mutations define a distinctive myeloproliferative syndrome that affects patients who currently receive a diagnosis of polycythemia vera or idiopathic erythrocytosis.

JAK2T875N is a novel activating mutation that results in myeloproliferative disease with features of megakaryoblastic leukemia in a murine bone marrow transplantation model

Acute megakaryoblastic leukemia (AMKL) is a subtype of acute myeloid leukemia associated with a poor prognosis. However, there are relatively few insights into the genetic etiology of AMKL. We developed a screening assay for mutations that cause AMKL, based on the hypothesis that constitutive activation of STAT5 would be a biochemical indicator of mutation in an upstream effector tyrosine kinase. We screened human AMKL cell lines for constitutive STAT5 activation, and then used an approach combining mass spectrometry identification of tyrosine phosphorylated proteins and growth inhibition in the presence of selective small molecule tyrosine kinase inhibitors that would inform DNA sequence analysis of candidate tyrosine kinases. Using this strategy, we identified a new JAK2T875N mutation in the AMKL cell line CHRF-288-11. JAK2T875N is a constitutively activated tyrosine kinase that activates downstream effectors including STAT5 in hematopoietic cells in vitro. In a murine transplant model, JAK2T875N induced a myeloproliferative disease characterized by features of AMKL, including megakaryocytic hyperplasia in the spleen; impaired megakaryocyte polyploidization; and increased reticulin fibrosis of the bone marrow and spleen. These findings provide new insights into pathways and therapeutic targets that contribute to the pathogenesis of AMKL.

A unique role for Stat5 in recovery from acute anemia

The precise role of erythropoietin receptor-activated (EpoR-activated) Stat5 in the regulation of erythropoiesis remains unclear. In this issue of the JCI, Menon and colleagues present new experimental data that indicate a distinct role for Stat5 in the regulation of stress-induced erythropoiesis, such as during acute anemic states (see the related article beginning on page 683). A critical function for Stat5 is to promote cell survival, possibly through transcriptional induction of the antiapoptotic protein Bcl-x. In the present experimental system, erythropoietin-Stat5 signals did not induce Bcl-x expression but did induce oncostatin-M. Moreover, oncostatin-M was found to enhance survival of erythroid progenitors. This work differentiates between steady-state (or homeostatic) erythropoiesis and stress-induced erythropoiesis at the level of EpoR signaling.

Expression of a homodimeric type I cytokine receptor is required for JAK2V617F-mediated transformation

A recurrent somatic activating mutation in the nonreceptor tyrosine kinase JAK2 (JAK2V617F) occurs in the majority of patients with the myeloproliferative disorders polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and, less commonly, chronic myelomonocytic leukemia. We do not understand the basis for the specificity of the JAK2V617F mutation in clonal disorders of the myeloid, but not lymphoid, lineage, nor has the basis for the pleiotropic phenotype of JAK2V617F-associated myeloproliferative disorders been delineated. However, the presence of the identical mutation in patients with related, but clinicopathologically distinct, myeloid disorders suggests that interactions between the JAK2V617F kinase and other signaling molecules may influence the phenotype of hematopoietic progenitors expressing JAK2V617F. Here, we show that coexpression of the JAK2V617F mutant kinase with a homodimeric Type I cytokine receptor, the erythropoietin receptor (EpoR), the thrombopoietin receptor, or the granulocyte colony-stimulating-factor receptor, is necessary for transformation of hematopoietic cells to growth-factor independence and for hormone-independent activation of JAK-STAT signaling. Furthermore, EpoR mutations that impair erythropoietin-mediated JAK2 or STAT5 activation also impair transformation mediated by the JAK2V617F kinase, indicating that JAK2V617F requires a cytokine receptor scaffold for its transforming and signaling activities. Our results reveal the molecular basis for the prevalence of JAK2V617F in diseases of myeloid lineage cells that express these Type I cytokine receptors but not in lymphoid lineage cells that do not.

Analysis of the biochemical mechanisms for the endocrine actions of fibroblast growth factor-23

Fibroblast growth factor (FGF)-23 has emerged as an endocrine regulator of phosphate and of vitamin D metabolism. It is produced in bone and, unlike other FGFs, circulates in the bloodstream to ultimately regulate phosphate handling and vitamin D production in the kidney. Presently, it is unknown which of the seven principal FGF receptors (FGFRs) transmits FGF23 biological activity. Furthermore, the molecular basis for the endocrine mode of FGF23 action is unclear. Herein, we performed surface plasmon resonance and mitogenesis experiments to comprehensively characterize receptor binding specificity. Our data demonstrate that FGF23 binds and activates the c splice isoforms of FGFR1-3, as well as FGFR4, but not the b splice isoforms of FGFR1-3. Interestingly, highly sulfated and longer glycosaminoglycan (GAG) species were capable of promoting FGF23 mitogenic activity. We also show that FGF23 induces tyrosine phosphorylation and inhibits sodium-phosphate cotransporter Npt2a mRNA expression using opossum kidney cells, a model kidney proximal tubule cell line. Removal of cell surface GAGs abolishes the effects of FGF23, and exogenous highly sulfated GAG is capable of restoring FGF23 activity, suggesting that proximal tubule cells naturally express GAGs that are permissive for FGF23 action. We propose that FGF23 signals through multiple FGFRs and that the unique endocrine actions of FGF23 involve escape from FGF23-producing cells and circulation to the kidney, where highly sulfated GAGs most likely act as cofactors for FGF23 activity. Our biochemical findings provide important insights into the molecular mechanisms by which dysregulated FGF23 signaling leads to disorders of hyper- and hypophosphatemia.

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.

Dna damage-induced G(1) arrest in hematopoietic cells is overridden following phosphatidylinositol 3-kinase-dependent activation of cyclin-dependent kinase 2

Exposure of hematopoietic cells to DNA-damaging agents induces p53-independent cell cycle arrest at a G(1) checkpoint. Previously, we have shown that this growth arrest can be overridden by cytokine growth factors, such as erythropoietin or interleukin-3, through activation of a phosphatidylinositol 3-kinase (PI 3-kinase)/Akt-dependent signaling pathway. Here, we show that gamma-irradiated murine myeloid 32D cells arrest in G(1) with active cyclin D-cyclin-dependent kinase 4 (Cdk4) but with inactive cyclin E-Cdk2 kinases. The arrest was associated with elevated levels of the Cdk inhibitors p21(Cip1) and p27(Kip1), yet neither was associated with Cdk2. Instead, irradiation-induced inhibition of cyclin E-Cdk2 correlated with absence of the activating threonine-160 phosphorylation on Cdk2. Cytokine treatment of irradiated cells induced Cdk2 phosphorylation and activation, and cells entered into S phase despite sustained high-level expression of p21 and p27. Notably, the PI 3-kinase inhibitor, LY294002, completely blocked cytokine-induced Cdk2 activation and cell growth in irradiated 32D cells but not in nonirradiated cells. Together, these findings demonstrate a novel mechanism underlying the DNA damage-induced G(1) arrest of hematopoietic cells, that is, inhibition of Cdk2 phosphorylation and activation. These observations link PI 3-kinase signaling pathways with the regulation of Cdk2 activity.

The distal region and receptor tyrosines of the Epo receptor are non-essential for in vivo erythropoiesis

The erythropoietin receptor (EpoR) is required for the proliferation and survival of committed erythroid lineage cells. Previous studies have utilized receptor mutations to show the requirement for the distal half of the cytoplasmic domain of the EpoR and receptor tyrosines for activation of signaling pathways potentially critical to Epo function. To extend these studies to in vivo erythropoiesis, we have created two mutant strains of mice. One strain (H) contains a truncation of the distal half of the cytoplasmic domain, while the second strain (HM) contains the same truncation as well as the mutation of the residual tyrosine (Y(343)) to a phenylalanine. Strikingly, both strains of mice are viable, with only slight alterations in constitutive erythropoiesis or in in vitro assays of red cell lineage function. Challenging H mutant mice with continuous injections of Epo results in an erythrocytosis that is not seen in HM mice. The results demonstrate that neither the distal region nor receptor tyrosines are essential for in vivo EpoR function, but contribute to receptor function in a subtle manner.

Mouse model of congenital polycythemia: Homologous replacement of murine gene by mutant human erythropoietin receptor gene

Mutations causing truncations of the cytoplasmic domain of the human erythropoietin receptor (EPOR) result in a dominantly inherited disorder-primary familial congenital polycythemia. This disorder is characterized by increased numbers of erythrocytes (polycythemia) and by in vitro hypersensitivity of erythroid precursors to erythropoietin. The consequences of EPOR truncation in nonerythroid tissues are unknown. We replaced the murine EPOR gene with a wild-type human EPOR gene and a mutant human EPOR gene that we initially identified in a patient with polycythemia. This mutation leads to an EPOR truncated after the first tyrosine residue of the intracellular domain. Mice heterozygous for this mutant allele and a wild-type human EPOR allele mimicked the human disorder. Interestingly, mice that were homozygous for the mutant human allele were severely polycythemic but viable. Our results provide a model for functional studies of EPOR-triggered signaling pathways in erythropoiesis. These animals can now be used to investigate the molecular pathophysiology of this gain-of-function EPOR mutation in erythroid tissue and in those nonerythroid tissues that express EPOR.

Dominant action of mutated erythropoietin receptors on differentiation in vitro and erythroleukemia development in vivo

J2E cells produce rapid, fatal erythroleukemias in vivo but still respond to erythropoietin (epo) in vitro by differentiating, proliferating and remaining viable in the absence of serum. Mutant epo receptors were introduced into these cells to determine whether they could influence the different biological responses to epo in vitro and the development of erythroleukemias. Three mutant receptors were used as cytoplasmic truncation mutants Delta257 and Delta321 (above box 1 and below box 2 respectively), and the cytoplasmic point mutant W282R (defective for JAK2 activation). Strikingly, the Delta321 mutation produced a hyper-sensitive response in vitro to epo-induced differentiation and viability, but not to proliferation. In contrast with the Delta321 receptor, the Delta257 and W282R mutants inhibited all biological responses to epo due to impaired JAK2 phosphorylation. Significantly, erythroleukemias took almost twice as long to develop with cells containing the W282R mutation, indicating that JAK2 plays an important role in the emergence of these leukemias. These data demonstrate that mutant epo receptors dominantly altered responses of J2E cells to epo in culture and the development of erythroleukemias. Oncogene (2000) 19, 953 - 960.

Clonal selection and in vivo quantitation of protein interactions with protein-fragment complementation assays

Two strategies are described for detecting constitutive or induced protein-protein interactions in intact mammalian cells; these strategies are based on oligomerization domain-assisted complementation of rationally designed fragments of the murine enzyme dihydrofolate reductase (DHFR; EC 1.5.1.3). We describe a dominant clonal-selection assay of stably transfected cells expressing partner proteins FKBP (FK506 binding protein) and FRAP (FKBP-rapamycin binding protein) fused to DHFR fragments and show a rapamycin dose-dependent survival of clones that requires approximately 25 molecules of reconstituted DHFR per cell. A fluorescence assay also is described, based on stoichiometric binding of fluorescein-methotrexate to reconstituted DHFR in vivo. Formation of the FKBP-rapamycin-FRAP complex is detected in stably and transiently transfected cells. Quantitative rapamycin dose-dependence of this complex is shown to be consistent with in vitro binding and distinguishable from a known constitutive interaction of FKBP and FRAP. We also show that this strategy can be applied to study membrane protein receptors, demonstrating dose-dependent activation of the erythropoietin receptor by ligands. The combination of these clonal-selection and fluorescence assays in intact mammalian cells makes possible selection by simple survival, flow cytometry, or both. High-throughput drug screening and quantitative analysis of induction or disruption of protein-protein interactions are also made possible.

Oligomerization and scaffolding functions of the erythropoietin receptor cytoplasmic tail

Signal transduction by the erythropoietin receptor (EPOR) is activated by ligand-mediated receptor homodimerization. However, the relationship between extracellular and intracellular domain oligomerization remains poorly understood. To assess the requirements for dimerization of receptor cytoplasmic sequences for signaling, we overexpressed mutant EPORs in combination with wild-type (WT) EPOR to drive formation of heterodimeric (i.e. WT-mutant) receptor complexes. Dimerization of the membrane-proximal portion of the EPOR cytoplasmic region was found to be critical for the initiation of mitogenic signaling. However, dimerization of the entire EPOR cytoplasmic region was not required. To examine this process more closely, we generated chimeras between the intracellular and transmembrane portions of the EPOR and the extracellular domains of the interleukin-2 receptor beta and gammac chains. These chimeras allowed us to assess more precisely the signaling role of each receptor chain because only heterodimers of WT and mutant receptor chimeras form in the presence of interleukin-2. Coexpression studies demonstrated that a functional receptor complex requires the membrane-proximal region of each receptor subunit in the oligomer to permit activation of JAK2 but only one membrane-distal tail to activate STAT5 and to support cell proliferation. Thus, this study defines key relationships involved in the assembly and activation of the EPOR signal transduction complex which may be applicable to other homodimeric cytokine receptors.

Absence of polycythemia in a child with a unique erythropoietin receptor mutation in a family with autosomal dominant primary polycythemia

Primary familial and congenital polycythemia (PFCP or familial erythrocytosis) is a rare proliferative disorder of erythroid progenitor cells, characterized by elevated erythrocyte mass and hemoglobin concentration, hypersensitivity of erythroid progenitors to erythropoietin (EPO), and autosomal dominant inheritance or sporadic occurrence. A number of EPO receptor (EPOR) mutations were found in subjects with PFCP; most of these mutations resulted in the truncation of the COOH-terminal of the EPOR protein. We studied a family with autosomal dominant inheritance of PFCP in which four subjects were affected in three generations. We screened the affected individuals for EPOR gene mutations using SSCP analysis and found a C5964G mutation in exon VIII that changes tyrosine codon 426 to a translation termination codon resulting in an EPOR protein truncated by 83 amino acids. The mutant C5964G-EPOR exhibited hypersensitive EPO-dependent proliferation compared to the wild-type EPOR when tested in a murine interleukin-3-dependent myeloid cell line (FDC-P1). We also examined the segregation of the mutation with PFCP in the family and found that a child in the third generation inherited the mutation without having laboratory evidence of polycythemia. Further in vitro analysis of the erythroid progenitor cells of this affected child revealed that the progenitor cells were hypersensitive to EPO (a hallmark of PFCP) suggesting the presence of the disease at the level of progenitor cells. Failure of this child to develop polycythemia suggests the existence of as yet unidentified environmental or genetic factors that may suppress disease development.

Stoichiometric structure-function analysis of the prolactin receptor signaling domain by receptor chimeras

The intracellular domain of the prolactin (PRL) receptor (PRLr) is required for PRL-induced signaling and proliferation. To identify and test the functional stoichiometry of those PRLr motifs required for transduction and growth, chimeras consisting of the extracellular domain of either the alpha or beta subunit of human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GM-CSFr) and the intracellular domain of the rat PRLr were synthesized. Because the high-affinity binding of GM-CSF results from the specific pairing of one alpha- and one beta-GM-CSFr, use of GM-CSFr/PRLr chimera enabled targeted dimerization of the PRLr intracellular domain. To that end, the extracellular domains of the alpha- and beta-GM-CSFr were conjugated to one of the following mutations: (i) PRLr C-terminal truncations, termed alpha278, alpha294, alpha300, alpha322, or beta322; (ii) PRLr tyrosine replacements, termed Y309F, Y382F, or Y309+382F; or, (iii) PRLr wild-type short, intermediate, or long isoforms. These chimeras were cotransfected into the cytokine-responsive Ba/F3 line, and their expression was confirmed by ligand binding and Northern and Western blot analyses. Data from these studies revealed that heterodimeric complexes of the wild type with C-terminal truncation mutants of the PRLr intracellular domain were incapable of ligand-induced signaling or proliferation. Replacement of any single tyrosine residue (Y309F or Y382F) in the dimerized PRLr complex resulted in a moderate reduction of receptor-associated Jak2 activation and proliferation. In contrast, trans replacement of these residues (i.e., alphaY309F and betaY382F) markedly reduced ligand-driven Jak2 activation and proliferation, while cis replacement of both tyrosine residues in a single intracellular domain (i.e., alphaY309+382F) produced an inactive signaling complex. Analysis of these GM-CSFr-PRLr complexes revealed equivalent levels of Jak2 in association with the mutant receptor chains, suggesting that the tyrosine residues at 309 and 382 do not contribute to Jak association, but instead to its activation. Heterodimeric pairings of the intracellular domains from the known PRLr receptor isoforms (short-intermediate, short-long, and intermediate-long) also yielded inactive receptor complexes. These data demonstrate that the tyrosine residues at 309 and 382, as well as additional residues within the C terminus of the dimerized PRLr complex, contribute to PRL-driven signaling and proliferation. Furthermore, these findings indicate a functional requirement for the pairing of Y309 and Y382 in trans within the dimerized receptor complex.

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

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

A chimeric receptor/oncogene that can be regulated by a ligand in vitro and in vivo

The BCR/ABL oncogene encodes an activated tyrosine kinase that causes human chronic myelogenous leukemia. The mechanism of transformation, however, is complex and not well understood. One of the important contributions of BCR to transformation is believed to be dimerization or oligomerization of ABL, thereby activating ABL tyrosine kinase activity. We reasoned that if ABL was dimerized through other mechanisms, activation of the tyrosine kinase activity should also result, and the activated kinase may also be transforming. Erythropoietin is known to activate its receptor by causing dimerization, and therefore a synthetic oncogene was created by linking the extracytoplasmic and transmembrane domains of the EPO receptor with c-ABL. This chimeric receptor was stably expressed in Ba/F3 cells and, in the absence of EPO, had no detectable biological effect on the cells. EPO, however, induced a rapid, dose-dependent activation of ABL tyrosine kinase activity and phosphorylation of several cellular proteins. The major target proteins have been identified, and are very similar to the known substrates of BCR/ABL, including Shc, CBL, CRKL, and several proteins in the cytoskeleton. EPO treatment also resulted in biological effects that were remarkably similar to those of BCR/ABL, including improved viability, altered integrin function, and a weak mitogenic signal. The biological effects were in part dose-dependent, in that low EPO concentrations enhanced viability but did not cause proliferation. At high EPO doses, kinase activation was maximal, and a mitogenic effect was also revealed. In nude mice, Ba/F3 cells expressing this chimeric receptor did not cause detectable disease without administration of pharmacologic doses of EPO. If EPO was given intraperitoneally 5 days a week, however, a dose-dependent lethal leukemia resulted. This ligand-regulatable oncogene mimics some of the biological effects of BCR/ABL, and analysis of ABL mutants in this system will be useful to dissect the signaling pathways that cause CML.

A proliferation switch for genetically modified cells

Receptor dimerization is the key signaling event for many cytokines, including erythropoietin. A system has been recently developed that permits intracellular protein dimerization to be reversibly activated in response to a lipid-soluble dimeric form of the drug FK506, called FK1012. FK1012 is used as a pharmacological mediator of dimerization to bring together FK506 binding domains, taken from the endogenous protein FKBP12. In experiments reported herein, FK1012-induced dimerization of a fusion protein containing the intracellular portion of the erythropoietin receptor allowed cells normally dependent on interleukin 3 to proliferate in its absence. FK506 competitively reversed the proliferative effect of FK1012 but had no influence on the proliferative effect of interleukin 3. Signaling pathways activated by FK1012 mimicked those activated by erythropoietin, because both JAK2 and STAT5 were phosphorylated in response to FK1012. This approach may provide a means to specifically and reversibly stimulate the proliferation of genetically modified cell populations in vitro or in vivo.

Dissecting the thrombopoietin receptor: functional elements of the Mpl cytoplasmic domain

Thrombopoietin (TPO) acts through its receptor, Mpl, to stimulate the proliferation and maturation of megakaryocytes and their progenitors. The Mpl cytoplasmic domain controls this process through assembly of an active signaling complex using various receptor docking sites. In this report, eight carboxyl truncations of the 121-aa murine Mpl cytoplasmic domain were tested for the ability to support growth of a cytokine-dependent cell line (Ba/F3) and for their capacity to induce TPO-stimulated tyrosine phosphorylation of specific signaling proteins. Point mutations of the five tyrosine residues in the cytoplasmic domain of the receptor were subsequently used to confirm our conclusions. From these studies we demonstrate that: (i) TPO-induced proliferation is moderately reduced by truncation of as many as 53 C-terminal amino acids of Mpl, including the sites of receptor tyrosine phosphorylation; (ii) truncation/mutation of residues 69-83 of the Mpl cytoplasmic domain enhances proliferative signaling, perhaps mediated by a decrease in receptor-driven cellular differentiation; (iii) Mpl can be phosphorylated at either Y112 or Y117 but not at the three proximal cytoplasmic tyrosine residues (Y8, Y29, and Y78); (iv) Y112 of Mpl is necessary for tyrosine phosphorylation of Shc and Shc-associated p145 (SHIP); and (v) unlike STAT3, STAT5 is partially phosphorylated in the absence of any tyrosine residues in the Mpl cytoplasmic domain. These studies identify subdomains of Mpl necessary for activation of several critical signaling pathways and point to two potentially novel mechanisms of TPO-induced signal transduction, an indirect pathway to STAT5 activation and a differentiation domain that acts by limiting proliferation.

Proliferation of multipotent hematopoietic cells controlled by a truncated erythropoietin receptor transgene

The long-term efficacy of gene therapy using bone marrow transplantation requires the engraftment of genetically altered totipotent hematopoietic stem cells (THSCs). Ex vivo expansion of corrected THSCs is one way to increase the efficiency of the procedure. Similarly, selective in vivo expansion of the therapeutic THSCs rather than the endogenous THSCs could favor the transplant. To test whether a conferred proliferative advantage gene can facilitate the in vitro and in vivo expansion of hematopoietic stem cells, we have generated transgenic mice expressing a truncated receptor for the growth factor erythropoietin. These mice are phenotypically normal, but when treated in vivo with exogenous erythropoietin they exhibit a marked increase in multipotent, clonogenic hematopoietic cells [colony-forming units in the spleen (CFU-S) and CFUs that give rise to granulocytes, erythroid cells, macrophages, and megakaryocytes within the same colony (CFU-GEMM)] in comparison with the wild-type mice. In addition, long-term in vitro culture of tEpoR transgenic bone marrow in the presence of erythropoietin induces exponential expansion of trilineage hematopoietic stem cells not seen with wild-type bone marrow. Thus, the truncated erythropoietin receptor gene shows promise as a means for obtaining cytokine-inducible hematopoietic stem cell proliferation to facilitate the direct targeting of THSCs and to provide a competitive repopulation advantage for transplanted therapeutic stem cells.

The murine DUB-1 gene is specifically induced by the betac subunit of interleukin-3 receptor

Cytokines regulate cell growth and differentiation by inducing the expression of specific target genes. We have recently isolated a cytokine-inducible, immediate-early cDNA, DUB-1, that encodes a deubiquitinating enzyme. The DUB-1 mRNA was specifically induced by the receptors for interleukin-3, granulocyte-macrophage colony-stimulating factor, and interleukin-5, suggesting a role for the beta common (betac subunit known to be shared by these receptors. In order to identify the mechanism of cytokine induction, we isolated a murine genomic clone for DUB-1 containing a functional promoter region. The DUB-1 gene contains two exons, and the nucleotide sequence of its coding region is identical to the sequence of DUB-1 cDNA. Various regions of the 5' flanking region of the DUB-1 gene were assayed for cytokine-inducible activity. An enhancer region that retains the beta c-specific inducible activity of the DUB-1 gene was identified. Enhancer activity was localized to a 112-bp fragment located 1.4 kb upstream from the ATG start codon. Gel mobility shift assays revealed two specific protein complexes that bound to this minimal enhancer region. One complex was induced by betac signaling, while the other was noninducible. Finally, the membrane-proximal region of human betac was required for DUB-1 induction. In conclusion, DUB-1 is the first example of an immediate-early gene that is induced by a specific subunit of a cytokine receptor. Further analysis of the DUB-1 enhancer element may reveal specific determinants of a betac-specific signaling pathway.

DUB-1, a deubiquitinating enzyme with growth-suppressing activity

Cytokines regulate cell growth by inducing the expression of specific target genes. Using the differential display method, we have cloned a cytokine-inducible immediate early gene, DUB-1 (for deubiquitinating enzyme). DUB-1 is related to members of the UBP superfamily of deubiquitinating enzymes, which includes the oncoprotein Tre-2. A glutathione S-transferase-DUB-1 fusion protein cleaved ubiquitin from a ubiquitin-beta-galactosidase protein. When a conserved cysteine residue of DUB-1, required for ubiquitin-specific thiol protease activity, was mutated to serine (C60S), deubiquitinating activity was abolished. Continuous expression of DUB-1 from a steroid-inducible promoter induced growth arrest in the G1 phase of the cell cycle. Cells arrested by DUB-1 expression remained viable and resumed proliferation upon steroid withdrawal. Our results suggest that DUB-1 regulates cellular growth by modulating either the ubiquitin-dependent proteolysis or the ubiquitination state of an unknown growth regulatory factor(s).

Erythropoietin induces activation of Stat5 through association with specific tyrosines on the receptor that are not required for a mitogenic response

The cytoplasmic domain of the erythropoietin receptor (EpoR) contains a membrane-distal region that is dispensable for mitogenesis but is required for the recruitment and tyrosine phosphorylation of a variety of signaling proteins. The membrane-proximal region of 96 amino acids is necessary and sufficient for mitogenesis as well as Jak2 activation, induction of c-fos, c-myc, cis, the T-cell receptor gamma locus (TCR-gamma), and c-pim-1. The studies presented here demonstrate that this region is also necessary and sufficient for the activation of Stat5A and Stat5B. The membrane-proximal domain contains a single tyrosine, Y-343, which when mutated eliminates the ability of the receptor to couple Epo binding to the activation of Stat5. Furthermore, peptide competitions demonstrate that this site, when phosphorylated, can disrupt Stat5 DNA binding activity, consistent with a role of Y-343 as a site of recruitment to the receptor. Cells expressing the truncated, Y343F mutant (a mutant with a Y-to-F alteration at position 343) proliferate in response to Epo in a manner comparable to that of the controls. However, in these cells, Epo stimulation does not induce the appearance of transcripts for cis, TCR-gamma, or c-fos, suggesting a role for Stat5 in their regulation.

Functional regions of the mouse interleukin-10 receptor cytoplasmic domain

The functions of wild-type and mutant mouse interleukin-10 receptors (mIL-10R) expressed in murine Ba/F3 cells were studied. As observed previously, IL-10 stimulates proliferation of IL-10R-expressing Ba/F3 cells. Accumulation of viable cells in the proliferation assay is to a significant extent balanced by concomitant cell death. Moreover, growth in IL-10 also induces a previously unrecognized response, differentiation of the cells, as evidenced both by formation of large clusters of cells in cultures with IL-10 and by induction or enhancement of expression of several cell surface antigens, including CD32/16, CD2, LECAM-1 (v-selectin), and heat-stable antigen. Two distinct functional regions near the C terminus of the mIL-10R cytoplasmic domain which mediate proliferation were identified; one of these regions also mediates the differentiation response. A third region proximal to the transmembrane domain was identified; removal of this region renders the cell 10- to 100-fold more sensitive to IL-10 in the proliferation assay. In cells expressing both wild-type and mutant IL-10R, stimulation with IL-10 leads to tyrosine phosphorylation of the kinases JAK1 and TYK2 but not JAK2 or JAK3 under the conditions tested.

Erythropoietin-induced cellular differentiation requires prolongation of the G1 phase of the cell cycle

Erythropoietin (EPO), like many other hematopoietic growth factors, can induce either growth or differentiation of hematopoietic cells. Little is known about the molecular basis of this cellular decision, in part because of a paucity of cell lines in which these two phenomena can be dissociated. Ectopic expression of the EPO receptor (EPO-R) in Ba/F3, a murine interleukin 3 (IL-3)-dependent progenitor cell line, confers EPO-dependent cell growth. In these cells (Ba/F3-EPO-R), EPO also induces beta-globin mRNA, a specific marker of erythroid differentiation. Here we show that the induction of erythroid differentiation by EPO requires a delay in cell growth and a prolongation of the (G1) phase of the cell cycle. Interestingly, this effect on G1 prolongation was concentration dependent. At low EPO concentrations (0.05-0.1 unit of EPO per ml; 1 pM EPO = 0.01 unit of EPO per ml), EPO prolonged G1 and induced differentiation; at high concentrations (0.5-10.0 units per ml), EPO shortened G1 and preferentially stimulated growth. IL-3 stimulated Ba/F3 growth but not differentiation at all growth factor concentrations ranging from 0.1 to 500 pM. Moreover, IL-3 suppressed EPO-induced beta-globin induction in a dose-dependent manner. This suppression correlated with the shortening of G1 by IL-3. Taken together, these data demonstrate distinct effects of EPO and IL-3 and a balance between erythroid growth and differentiation that is cell cycle dependent.

A mouse erythroleukemia cell line possessing friend spleen focus-forming virus gp55 transgene and temperature-sensitive mutant p53 gene

Two different erythroleukemia cell lines have been established from the splenic lesions of transgenic mice possessing the Friend spleen focus-forming virus (F-SFFV) gp55 gene. One showed a near-diploid karyotype and a temperature-sensitive (ts) p53 mutation, and the other, a hyper-triploid karyotype with double p53 mutations found by single-strand conformation polymorphism (SSCP) analysis. The cell lines both retained No.11 chromosomes on which p53 genes are localized. Another p53 allele in the cell line with the ts-p53 mutation appeared intact in the SSCP analysis of the genomic exon 5. The cells with the ts-mutant p53 gene showed no apparent change with temperature shift in their growth or dimethylsulfoxide-induced differentiation, although the wild-type p53 gene on the other allele was not expressing. This ts-p53Val-135 gene made p53-deficient fibroblasts anchorage-independent at 37 degrees C but not at 32 degrees C. This non-virus-producing, mouse erythroleukemia cell line will be useful for the study of mutated p53 function during the induction of erythrodifferentiation or apoptotic change.

Cell surface activation of the erythropoietin receptor by Friend spleen focus-forming virus gp55

The leukemogenic membrane glycoprotein gp55, encoded by Friend spleen focus-forming virus (SFFV), induces erythroid cell proliferation through its interaction with the erythropoietin receptor (EPO-R). There are two forms of gp55 in SFFV-infected cells: an intracellular form (more than 95% of the total protein), which is localized within the endoplasmic reticulum (ER) membranes, and a cell surface form (about 3 to 5%). Because both forms of the viral proteins bind to EPO-R, it is not clear whether the viral protein induces mitogenesis intracellularly or at the cell surface. To address this question, we constructed an EPO-R mutant that contained a 6-amino-acid (DEKKMP) C-terminus ER retention signal. Biochemical and functional analyses with this mutant indicated that it was completely retained in the ER and not expressed at the cell surface. Further analysis showed that the mutant, like the wild-type EPO-R, interacted with SFFV gp55. However, this apparent intracellular interaction between the two proteins failed to induce growth factor-independent proliferation of Ba/F3 cells. Furthermore, spontaneous variants of the ER-retained EPO-R selected on the basis of their ability to induce cell proliferation when coexpressed with gp55 were exclusively expressed at the cell surface. Thus, our results support the hypothesis that the mitogenic activation of the EPO-R by gp55 requires the interaction of the two proteins at the cell surface.

Deletions in one domain of the Friend virus-encoded membrane glycoprotein overcome host range restrictions for erythroleukemia

Although the Friend virus-encoded membrane glycoprotein (gp55) activates erythropoietin receptors (EpoR) to cause erythroblastosis only in certain inbred strains of mice but not in other species, mutant viruses can overcome aspects of mouse resistance. Thus, mice homozygous for the resistance allele of the Fv-2 gene are unaffected by gp55 but are susceptible to mutant glycoproteins that have partial deletions in their ecotropic domains. These and other results have suggested that proteins coded for by polymorphic Fv-2 alleles might directly or indirectly interact with EpoR and that changes in gp55 can overcome this defense. A new viral mutant with an exceptionally large deletion in its ecotropic domain is now also shown to overcome Fv-2rr resistance. In all cases, the glycoproteins that activate EpoR are processed to cell surfaces as disulfide-bonded dimers. To initiate analysis of nonmurine resistances, we expressed human EpoR and mouse EpoR in the interleukin 3-dependent mouse cell line BaF3 and compared the abilities of Friend virus-encoded glycoproteins to convert these cells to growth factor independence. Human EpoR was activated in these cells by erythropoietin but was resistant to gp55. However, human EpoR was efficiently activated in these cells by the same viral mutants that overcome Fv-2rr resistance in mice. By construction and analysis of human-mouse EpoR chimeras, we obtained evidence that the cytosolic domain of human EpoR contributes to its resistance to gp55 and that this resistance is mediated by accessory cellular factors. Aspects of host resistance in both murine and nonmurine species are targeted specifically against the ecotropic domain of gp55.

Selectable retrovirus vectors encoding Friend virus gp55 or erythropoietin induce polycythemia with different phenotypic expression and disease progression

The Friend spleen focus-forming virus induces a massive expansion of erythroid progenitor cells resulting in polycythemia and splenomegaly. The pathogenic agent is the membrane glycoprotein gp55, encoded by the env gene. Recent evidence indicates that gp55 binds to and activates the erythropoietin (Epo) receptor. It is not clear, however, whether gp55 completely mimics the natural receptor ligand (Epo). To directly compare both effectors, we constructed selectable retroviral vectors which carry either the env or the Epo gene. The selection marker allowed for clonal analysis of infected cells. After infection of DBA/2J mice, the spleen weight, hematological indices, and Epo titer of peripheral blood were monitored. Although both viruses induced an acute erythrocytosis, there were significant differences in disease phenotype and progression. The Epo virus caused an enhanced increase of hematocrit and erythrocytes, whereas with the env virus the pool of late progenitors (CFU-erythroid) was dramatically expanded, resulting in a more severe splenomegaly. The distribution of cytologically recognizable erythroid precursors was shifted towards immature cell types by the env vector compared with Epo. These data suggest that Epo and gp55 differentially affect proliferation and differentiation. Gp55 appears to promote proliferation over differentiation, whereas Epo preferentially drives differentiation.

Activated Ki-Ras complements erythropoietin signaling in CTLL-2 cells, inducing tyrosine phosphorylation of a 160-kDa protein

We have previously shown that expression of erythropoietin (EPO) receptor (EPOR) alone failed to confer EPO responsiveness on the interleukin 2-dependent T-cell line CTLL-2, whereas the introduction of the EPOR into interleukin 3-dependent pro-B-cell lines, such as BAF-B03, allowed the cells to proliferate in response to EPO. Here, we report that additional expression of v-Ki-Ras conferred EPO-dependent growth on CTLL-2 cells expressing the EPOR, with additional formation of a high-affinity EPOR. To investigate possible mechanisms of EPOR downstream signaling induced by v-Ki-Ras expression in these CTLL-2-derived cells, we carried out anti-phosphotyrosine immunoblot analysis of the EPOR complex immunoprecipitated with anti-EPOR antibody from lysates of cells with and without cytokine stimulation, revealing two 160-kDa and 130-kDa phosphotyrosyl proteins. An anti-JAK2 antibody did not react with these proteins, suggesting that they may represent cellular components involved in an EPO-EPOR signaling pathway induced by v-Ki-Ras. Similar phosphotyrosyl proteins were present among Friend erythroleukemia cell lines, in which the Friend virus gp55/EPOR complex on the cell surface constitutively sends signals for cell growth.

The Fanconi anemia polypeptide FACC is localized to the cytoplasm

Fanconi anemia (FA) is an autosomal recessive disease characterized by congenital anomalies, aplastic anemia, and chromosomal instability. A cDNA encoding the FA complementation group C (FACC) polypeptide was recently cloned [Strathdee, C. A., Gavish, H., Shannon, W. R. & Buchwald, M. (1992) Nature (London) 356, 763-767]. To further characterize this polypeptide, we generated a rabbit polyclonal antiserum against its carboxyl terminus. We used this antiserum to analyze the FACC polypeptide from normal or mutant (FA) lymphoblast cell lines. By immunoprecipitation, the wild-type FACC was a 60-kDa protein, consistent with its predicted molecular mass. FA group C cell lines expressed full-length FACC, truncated FACC, or no detectable FACC polypeptide. In addition, the antiserum specifically immunoprecipitated a 50-kDa and a 150-kDa FACC-related protein (FRP-50 and FRP-150). Unexpectedly, cell fractionation and immunofluorescence studies demonstrated that the FACC polypeptide localizes to the cytoplasm. In conclusion, we have generated an antiserum specific for the human FACC polypeptide. The antiserum should be useful for screening FA cells for mutant FACC polypeptides and for identifying and cloning FACC-related proteins.

Granulocyte colony-stimulating factor receptor signaling involves the formation of a three-component complex with Lyn and Syk protein-tyrosine kinases

Granulocyte colony-stimulating factor (G-CSF) is a glycoprotein that critically regulates the viability, proliferation, and differentiation of granulocytic precursors and the function of neutrophils by signaling through its receptor. Cloning of the human G-CSF receptor (G-CSFR) cDNA has demonstrated sequence homology with other members of the hematopoietic/cytokine receptor superfamily. G-CSF stimulates the appearance of phosphotyrosine proteins in several types of human and murine myeloid cells. Since the receptor does not possess intrinsic tyrosine kinase activity, we hypothesized that G-CSFR interacts with and activates cytosolic protein-tyrosine kinases (PTKs). In vitro protein kinase assay of human G-CSFR immunoprecipitates demonstrated at least two tyrosine phosphoproteins, pp55 and pp70. We observed that G-CSF activated p53/p56lyn, a Src-related PTK, and p72syk, a non-Src-related PTK. Lyn and Syk were recovered in anti-G-CSFR immunoprecipitates; Lyn was detected in the absence of ligand. In addition, upon G-CSF stimulation, Lyn coimmunoprecipitated with Syk. Analysis of the G-CSFR amino acid sequence revealed a potential receptor activation motif for Syk. On the basis of immunoprecipitation and sequence analysis data, we propose that the human G-CSFR forms a three-component signaling complex with Lyn and Syk. Their sequential recruitment into the G-CSFR signaling complex demonstrates the coordinated involvement of two PTKs with a member of the hematopoietic/cytokine receptor superfamily.

Identification of a nonsense mutation in the granulocyte-colony-stimulating factor receptor in severe congenital neutropenia

Severe congenital neutropenia (Kostmann syndrome) is characterized by profound absolute neutropenia and a maturation arrest of marrow progenitor cells at the promyelocyte-myelocyte stage. Marrow cells from such patients frequently display a reduced responsiveness to granulocyte-colony-stimulating factor (G-CSF). G-CSF binds to and activates a specific receptor which transduces signals critical for the proliferation and maturation of granulocytic progenitor cells. Here we report the identification of a somatic point mutation in one allele of the G-CSF receptor gene in a patient with severe congenital neutropenia. The mutation results in a cytoplasmic truncation of the receptor. When expressed in murine myeloid cells, the mutant receptor transduced a strong growth signal but, in contrast to the wild-type G-CSF receptor, was defective in maturation induction. The mutant receptor chain may act in a dominant negative manner to block granulocytic maturation.

A dominant negative erythropoietin (EPO) receptor inhibits EPO-dependent growth and blocks F-gp55-dependent transformation

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated to signal cell growth by binding either EPO or F-gp55, the Friend spleen focus-forming virus glycoprotein. Activation by F-gp55 results in constitutive EPO-R signalling and the first stage of Friend virus-induced erythroleukemia. We have generated a truncated form of the EPO-R polypeptide [EPO-R(T)] which lacks the critical cytoplasmic signal-transducing domain of the EPO-R required for EPO- or F-gp55-induced cell growth. EPO-R(T) specifically inhibited the EPO-dependent growth of EPO-R-expressing Ba/F3 cells without changing the interleukin-3-dependent growth of these cells. In addition, Ba/F3 cells that coexpressed wild-type EPO-R and EPO-R(T) were resistant to transformation by F-gp55 despite efficient expression of the F-gp55 transforming oncoprotein in infected cells. EPO-R(T) inhibited the EPO-dependent tyrosine phosphorylation of wild-type EPO-R, the tyrosine kinase (JAK2), and the SH2 adaptor protein (Shc). In conclusion, the EPO-R(T) polypeptide is a dominant negative polypeptide which specifically interferes with the early stages of EPO-R-mediated signal transduction and which prevents Friend virus transformation of erythroblasts.

A dominant negative erythropoietin (EPO) receptor inhibits EPO-dependent growth and blocks F-gp55-dependent transformation

The erythropoietin receptor (EPO-R), a member of the cytokine receptor superfamily, can be activated to signal cell growth by binding either EPO or F-gp55, the Friend spleen focus-forming virus glycoprotein. Activation by F-gp55 results in constitutive EPO-R signalling and the first stage of Friend virus-induced erythroleukemia. We have generated a truncated form of the EPO-R polypeptide [EPO-R(T)] which lacks the critical cytoplasmic signal-transducing domain of the EPO-R required for EPO- or F-gp55-induced cell growth. EPO-R(T) specifically inhibited the EPO-dependent growth of EPO-R-expressing Ba/F3 cells without changing the interleukin-3-dependent growth of these cells. In addition, Ba/F3 cells that coexpressed wild-type EPO-R and EPO-R(T) were resistant to transformation by F-gp55 despite efficient expression of the F-gp55 transforming oncoprotein in infected cells. EPO-R(T) inhibited the EPO-dependent tyrosine phosphorylation of wild-type EPO-R, the tyrosine kinase (JAK2), and the SH2 adaptor protein (Shc). In conclusion, the EPO-R(T) polypeptide is a dominant negative polypeptide which specifically interferes with the early stages of EPO-R-mediated signal transduction and which prevents Friend virus transformation of erythroblasts.

Mutational analysis of a critical signaling domain of the human interleukin 4 receptor

The human interleukin 4 receptor (hIL-4R) is a member of a superfamily of cytokine receptors defined by conserved features of their extracellular domains. The intracellular domains of the hIL-4R and of other members of this family lack any recognizable enzymatic motifs, though ligand-dependent tyrosine phosphorylation of these receptors has been observed. Recent studies have suggested that serine-rich and acidic domains within the cytoplasmic portions of cytokine receptors might be required for signal transduction. Using deletion and truncation mutants of the hIL-4R, we have explored an essential 39-amino acid signaling domain that is rich in acidic amino acid residues and in serine residues that form consensus phosphorylation sites for known serine/threonine kinases. To assess the contribution of these motifs to signaling, we engineered site-directed mutants of these residues. Surprisingly, cells expressing mutant hIL-4R lacking either the serine or the acidic amino acids retain the ability of cells expressing the wild-type receptor to proliferate in hIL-4. Furthermore, receptors in which all six cytoplasmic tyrosines are absent can function, suggesting that tyrosine phosphorylation of the receptor may be an epiphenomenon rather than a requisite event in signaling.

Multiple regions within the cytoplasmic domains of the leukemia inhibitory factor receptor and gp130 cooperate in signal transduction in hepatic and neuronal cells

The receptor for leukemia inhibitory factor (LIFR), in combination with the signal-transducing subunit for interleukin-6-type cytokine receptors, gp130, and LIF, activates transcription of acute-phase plasma protein genes in human and rat hepatoma cells and the vasoactive intestinal peptide gene in a human neuroblastoma cell line. To identify the regions within the cytoplasmic domain of LIFR that initiate signal transduction independently of gp130, we constructed a chimeric receptor by linking the extracellular domain of the granulocyte colony-stimulating factor receptor (G-CSFR) to the transmembrane and cytoplasmic domain of human LIFR. The function of the chimeric receptor protein in transcriptional activation was assessed by G-CSF-mediated stimulation of cotransfected cytokine-responsive reporter gene constructs in hepatoma and neuroblastoma cells. By using the full-length cytoplasmic domain and mutants with progressive carboxy-terminal deletions, internal deletions, or point mutations, we identified the first 150 amino acid residues of LIFR as the minimal region necessary for signaling. The signaling reaction appears to involve a cooperativity between the first 70-amino-acid region containing the two sequence motifs conserved among hematopoietin receptors (box 1 and box 2) and a critical sequence between residues 141 and 150 (box 3). Analogous analyses of the cytoplasmic domains of G-CSFR and gp130 indicated similar arrangements of functional domains in these receptor subunits and the requirement of a box 3-related motif for signaling.

Ligand-induced activation of chimeric receptors between the erythropoietin receptor and receptor tyrosine kinases

Ligand-induced dimerization is a key step in the activation of receptor tyrosine kinases, including the epidermal growth factor receptor, stem cell factor receptor (c-kit), and colony-stimulating factor 1 receptor (c-fms). The erythropoietin receptor (EPOR), a member of the cytokine receptor family, contains no kinase motif and its activation mechanism remains unclear. Here we show that chimeric receptors carrying the extracellular domain of the epidermal growth factor receptor or c-kit linked to the cytoplasmic domain of the EPOR, transmitted epidermal growth factor or stem cell factor-dependent proliferation signals in an interleukin 3-dependent cell line. The chimeric receptors as well as the wild-type EPOR also mediated the ligand-induced tyrosine phosphorylation of a set of similar proteins. Moreover, erythropoietin triggered mitogenic signals of chimeric receptors carrying the extracellular domain of the EPOR linked to the tyrosine kinase of c-fms. These data demonstrate the interchangeability of domains between two distinct receptor families and suggest that ligand-induced dimerization is a key step in activating the EPOR.

Hematopoietic cell phosphatase associates with the interleukin-3 (IL-3) receptor beta chain and down-regulates IL-3-induced tyrosine phosphorylation and mitogenesis

Hematopoietic cell phosphatase (HCP) is a tyrosine phosphatase with two Src homology 2 (SH2) domains that is predominantly expressed in hematopoietic cells, including cells whose growth is regulated by interleukin-3 (IL-3). The potential effects of HCP on IL-3-induced protein tyrosine phosphorylation and growth regulation were examined to assess the role of HCP in hematopoiesis. Our studies demonstrate that, following ligand binding, HCP specifically associates with the beta chain of the IL-3 receptor through the amino-terminal SH2 domain of HCP, both in vivo and in vitro, and can dephosphorylate the receptor chain in vitro. The effects of increasing or decreasing HCP levels in IL-3-dependent cells were assessed with dexamethasone-inducible constructs containing an HCP cDNA in sense and antisense orientations. Increased HCP levels were found to reduce the levels of IL-3-induced tyrosine phosphorylation of the receptor and to dramatically suppress cell growth. Conversely, decreasing the levels of HCP increased IL-3-induced tyrosine phosphorylation of the receptor and marginally increased growth rate. These results support a role for HCP in the regulation of hematopoietic cell growth and begin to provide a mechanistic explanation for the dramatic effects that the genetic loss of HCP, which occurs in motheaten (me) and viable motheaten (mev) mice, has on hematopoiesis.