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

Amplified EPOR/JAK2 Genes Define a Unique Subtype of Acute Erythroid Leukemia

Acute erythroid leukemia (AEL) is a unique subtype of acute myeloid leukemia characterized by prominent erythroid proliferation whose molecular basis is poorly understood. To elucidate the underlying mechanism of erythroid proliferation, we analyzed 121 AEL using whole-genome, whole-exome, and/or targeted-capture sequencing, together with transcriptome analysis of 21 AEL samples. Combining publicly available sequencing data, we found a high frequency of gains and amplifications involving EPOR/JAK2 in TP53-mutated cases, particularly those having >80% erythroblasts designated as pure erythroid leukemia (10/13). These cases were frequently accompanied by gains and amplifications of ERG/ETS2 and associated with a very poor prognosis, even compared with other TP53-mutated AEL. In addition to activation of the STAT5 pathway, a common feature across all AEL cases, these AEL cases exhibited enhanced cell proliferation and heme metabolism and often showed high sensitivity to ruxolitinib in vitro and in xenograft models, highlighting a potential role of JAK2 inhibition in therapeutics of AEL.

SIGNIFICANCE

This study reveals the major role of gains, amplifications, and mutations of EPOR and JAK2 in the pathogenesis of pure erythroleukemia. Their frequent response to ruxolitinib in patient-derived xenograft and cell culture models highlights a possible therapeutic role of JAK2 inhibition for erythroleukemia with EPOR/JAK2-involving lesions. This article is highlighted in the In This Issue feature, p. 369.

Cytosolic lysine residues enhance anterograde transport and activation of the erythropoietin receptor

Lysine residues are key residues in many cellular processes, in part due to their ability to accept a wide variety of post-translational modifications. In the present study, we identify the EPO-R [EPO (erythropoietin) receptor] cytosolic lysine residues as enhancers of receptor function. EPO-R drives survival, proliferation and differentiation of erythroid progenitor cells via binding of its ligand EPO. We mutated the five EPO-R cytosolic lysine residues to arginine residues (5KR EPO-R), eliminating putative lysine-dependent modifications. Overexpressed 5KR EPO-R displayed impaired ubiquitination and improved stability compared with wt (wild-type) EPO-R. Unexpectedly, fusion proteins consisting of VSVGtsO45 (vesicular stomatitis virus glycoprotein temperature-sensitive folding mutant) with wt or 5KR EPO-R cytosolic domains demonstrated delayed glycan maturation kinetics upon substitution of the lysine residues. Moreover, VSVG-wt EPO-R, but not VSVG-5KR EPO-R, displayed endoplasmic reticulum-associated ubiquitination. Despite similar cell-surface EPO-binding levels of both receptors and the lack of EPO-induced ubiquitination by 5KR EPO-R, the lysine-less mutant produced weaker receptor activation and signalling than the wt receptor. We thus propose that EPO-R cytosolic lysine residues enhance receptor function, most probably through ubiquitination and/or other post-translational modifications.

Calmodulin physically interacts with the erythropoietin receptor and enhances Jak2-mediated signaling

Stimulation of the erythropoietin receptor (EpoR) induces a transient increase in intracellular Ca2+ level as well as activation of the Jak2 tyrosine kinase to stimulate various downstream signaling pathways. Here, we demonstrate that the universal Ca2+ receptor calmodulin (CaM) binds EpoR in a Ca2+-dependent manner in vitro. Binding studies using various EpoR mutants in hematopoietic cells showed that CaM binds the membrane-proximal 65-amino-acid cytoplasmic region (amino acids 258-312) of EpoR that is critical for activation of Jak2-mediated EpoR signaling. Structurally unrelated CaM antagonists, W-13 and CMZ, inhibited activation of Jak2-mediated EpoR signaling pathways, whereas W-12, a W-13 analog, did not show any significant inhibitory effect. Moreover, overexpression of CaM augmented Epo-induced tyrosine phosphorylation of the EpoR. W-13, but not W-12, also inhibited Epo-induced proliferation and survival. Together, these results indicate that CaM binds to the membrane-proximal EpoR cytoplasmic region and plays an essential role in activation of Jak2-mediated EpoR signaling.

Contribution of the Box 1 and Box 2 motifs of cytokine receptors to Jak1 association and activation

Kinases of the Jak family (Jak1/2/3 and Tyk2) interact with the membrane proximal domain of different cytokine receptors and play a critical role in the activation of cytokine and growth factor signaling pathways. In this report we demonstrate that both the Box 1 and Box 2 motif collaborate in the association and activation of Jak1 by type I interferons. Mutational analysis of the beta chain of type I interferon receptor (IFNalphaRbetaL/IFNAR2) revealed that Box 1 plays a more significant role in activation than in the association with Jak1. On the contrary, the Box 2 motif contributes more to the association with Jak1 than to kinase activation. Additionally, the study of the Jak1 binding sites on the IL2 receptor beta (IL2Rbeta), IFNgammaRalpha/IFNGR1, and IL10Ralpha/IL10R1 chains suggests that cytokine receptors have two different kinds of interaction with Jak1. One form of interaction involves the Box 1 and the previously described Box 2 motif, which we now designate as Box 2A, characterized by the VEVI and LEVL sequences present in IFNalphaRbetaL/IFNAR2 and IL2Rbeta subunits, respectively. The second form of interaction requires a motif termed Box 2B, which is present in the IFNgammaRalpha/IFNGR1 (SILLPKS) and IL10Ralpha/IL10R1 (SVLLFKK) chains. Interestingly, Box 2B localizes close to the membrane region (8-10 amino acids from the membrane) similar to Box 1, whereas Box 2A is more distal (38-58 amino acids from the membrane).

Two distinct domains within the N-terminal region of Janus kinase 1 interact with cytokine receptors

The interaction between receptors and kinases of the Janus kinase (Jak) family is critical for signaling by growth factors, cytokines, and IFNs. Therefore, the characterization of the domains involved in these interactions is pivotal not only in understanding kinase activation but also in the development of drugs that mimic or inhibit signaling. In this report, we have characterized the domains of Jak1 required to associate with distinct cytokine receptor subunits: IFN-alpha R beta L, IFN-gamma R alpha, IL-10R alpha, IL-2R beta, and IL-4R alpha. We demonstrate that two regions of Jak1 are necessary for the interaction with cytokine receptors. First, a common N-terminal region that includes Jak homology (JH) domain 7 and the first 19 aa of JH6, and, second, a C-terminal region (JH6-3) that was different for distinct receptors. The contribution of the two different regions of Jak1 to cytokine receptor binding was also variable. Deletion of JH7-6 impaired the association of IL-2R beta and IL-4R alpha chains with Jak1 but did not have a major impact on the binding of Jak1 to IFN-alpha R beta L or IL-10R alpha. Interestingly, regardless of the effect on receptor binding, removal of JH7-6 completely abrogated kinase activation, indicating that this domain is required for ligand-driven kinase activation and, thus, for proper signaling through cytokine receptors.

Dissection of c-Mpl and thrombopoietin function: studies of knockout mice and receptor signal transduction

The physiological roles and mechanisms of action of thrombopoietin (TPO) and its receptor c-Mpl have been studied through the analysis of mice genetically deficient in these molecules, as well as through the dissection of signaling events utilizing chimeric receptors. The evidence clearly demonstrates that the TPO/c-Mpl system provides dominant control in the regulation of megakaryocytopoiesis. The signaling mechanisms that underlie this process appear to be similar to those noted with other members of the hematopoietic cytokine and cytokine receptor families.

Molecular cloning of a novel type 1 cytokine receptor similar to the common gamma chain

In a complementary DNA (cDNA) screening of murine Th2-skewed lymphocytes with our recently developed signal sequence trap method termed SST-REX, a novel type 1 cytokine receptor, Delta1 (δ1), was identified. Although δ1 is ubiquitously expressed in multiple tissues, the expression level is higher in Th2-skewed lymphocytes than in Th1-skewed ones. The δ1 cDNA encodes a 359–amino acid type 1 membrane protein. The extracellular domain of 206 amino acids showed 24% identity with the murine common γ receptor that is shared among the receptors for interleukin(IL)-2, IL-4, IL-7, IL-9, and IL-15. The membrane-proximal region of δ1 includes a box1 motif, which is important for association with Janus kinases (JAKs), and showed a significant homology with that of the mouse erythropoietin receptor (EPOR). A box2 motif was also found in close proximity to the box1 region. Dimerization of the cytoplasmic region of δ1 alone did not transduce proliferative signals in IL-3–dependent cell lines. However, the membrane-proximal region of δ1 could substitute for that of human EPOR in transmitting proliferative signals and activating JAK2. These results suggest that δ1 is a subunit of cytokine receptor that may be involved in multiple receptor systems and play a regulatory role in the immune system and hematopoiesis.

The intron 5-inserted form of rat erythropoietin receptor is expressed as a membrane-bound form

The cDNA encoding an intron 5-inserted form of the erythropoietin receptor (I5Epo-R) has been cloned from rat. DNA sequence analysis reveals that the insertion of intron 5, which consists of 79 bp, causes a shift in reading frame and results in termination in the region of exon 7. The deduced amino acid sequence is composed of 316 amino acid residues, which is a molecular weight of 34220. To study the function of rat I5Epo-R, we established a Chinese hamster ovary cell line expressing rat I5Epo-R. Western blot analysis and binding studies with 125I-recombinant human erythropoietin showed that the transfected cells expressed rat I5Epo-R with a molecular size of 36 kDa as a membrane-bound form, but not as a soluble form, and had a single class of binding sites with a Kd of 700 pM.

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.

Structural and mechanistic aspects of Janus kinases: how the two-faced god wields a double-edged sword

The Janus family of protein-tyrosine kinases has long been known to function in signal transduction pathways initiated by a host of cytokines. A brief overview of the role of Janus kinases (Jaks) in both cytokine and noncytokine signaling pathways highlights the broad physiologic importance of this kinase family. New insights into the structural and mechanistic regulatory aspects of Janus kinases are rapidly emerging. Recent mutational analyses allow the dissection of Jaks into three distinct structural domains governing receptor affiliation, autoregulation, and catalysis. A fourth domain determining substrate specificity is as yet poorly defined and is, therefore, discussed in the context of known substrates and inhibitors, a collection of molecules that have been expanded recently to include Stam and Jab. The proposed mechanism of the interconversion of Janus kinases from inactive to fully active enzymes involves three states of enzymatic activity. Additional layers of regulation can be independently superimposed on this multistate model, providing a simplified description of the behavior of Janus kinases under normal and pathologic circumstances.

Erythropoietin induces the tyrosine phosphorylation of insulin receptor substrate-2. An alternate pathway for erythropoietin-induced phosphatidylinositol 3-kinase activation

In this report, we demonstrate that insulin receptor substrate-2 (IRS-2) is phosphorylated on tyrosine following treatment of UT-7 cells with erythropoietin. We have investigated the expression of IRS-1 and IRS-2 in several cell lines with erythroid and/or megakaryocytic features, and we observed that IRS-2 was expressed in all cell lines tested. In contrast, we did not detect the expression of IRS-1 in these cells. In response to erythropoietin, IRS-2 was immediately phosphorylated on tyrosine, with maximal phosphorylation between 1 and 5 min. Tyrosine-phosphorylated IRS-2 was associated with phosphatidylinositol 3-kinase and with a 140-kDa protein that comigrated with the phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase, SHIP. Moreover, IRS-2 was constitutively associated with the erythropoietin receptor. We did not observe the association of IRS-2 with JAK2, Grb2, or PTP1D. Using BaF3 cells transfected with mutated erythropoietin receptors, we demonstrate that neither the tyrosine residues of the intracellular domain nor the last 109 amino acids of the erythropoietin receptor are required for erythropoietin-induced IRS-2 tyrosine phosphorylation. Altogether, our results indicate that erythropoietin-induced IRS-2 tyrosine phosphorylation could account for the previously reported activation of phosphatidylinositol 3-kinase mediated by erythropoietin receptors mutated in the phosphatidylinositol 3-kinase-binding site (Damen, J., Cutler, R. L., Jiao, H., Yi, T., and Krystal, G. (1995) J. Biol. Chem. 270, 23402-23406; Gobert, S., Porteu, F., Pallu, S., Muller, O., Sabbah, M., Dusanter-Fourt, I., Courtois, G., Lacombe, C., Gisselbrecht, S., and Mayeux, P. (1995) Blood 86, 598-606).

A region of the beta subunit of the interferon alpha receptor different from box 1 interacts with Jak1 and is sufficient to activate the Jak-Stat pathway and induce an antiviral state

Coexpression of the alpha and betaL subunits of the human interferon alpha (IFNalpha) receptor is required for the induction of an antiviral state by human IFNalpha. To explore the role of the different domains of the betaL subunit in IFNalpha signaling, we coexpressed wild-type alpha subunit and truncated forms of the betaL chain in L-929 cells. Our results demonstrated that the first 82 amino acids (AAs) (AAs 265-346) of the cytoplasmic domain of the betaL chain are sufficient to activate the Jak-Stat pathway and trigger an antiviral state after IFNalpha2 binding to the receptor. This region of the betaL chain, required for Jak1 binding and activation, contains the Box 1 motif that is important for the interaction of some cytokine receptors with Jak kinases. However, using glutathione S-transferase fusion proteins containing amino- and carboxyl-terminal deletions of the betaL cytoplasmic domain, we demonstrate that the main Jak1-binding region (corresponding to AAs 300-346 on the beta subunit) is distinct from the Box 1 domain (AAs 287-295).

Cytokine receptor signal transduction through Jak tyrosine kinases and Stat transcription factors

Cytokines are the principal regulators of cell proliferation and differentiation of hematopoietic cells and these responses are initiated through activation of hematopoietic cytokine receptors. Although the receptor intracellular domains lack any kinase domains, activation of cytokine receptors lead to rapid induction of tyrosine phosphorylation. Recently, cytokine receptors have been shown to associate with and activate members of the cytoplasmic Jak tyrosine kinase family. Activation of Jak kinases leads to phosphorylation of several signaling proteins and thereby couples ligand-mediated receptor stimulation to activation of intracellular signaling pathways. The best characterized substrates for Jaks are the Stat transcription factors, which are crucial mediators of cytokine-mediated gene responses, and, particularly, central determinants for the specificity in cytokine responses.

The prolactin receptor and severely truncated erythropoietin receptors support differentiation of erythroid progenitors

Activation of the erythropoietin receptor is essential for the survival, proliferation, and differentiation of erythroid progenitors. To understand the role of erythropoietin receptor (EpoR) activation in erythroid differentiation, we infected primary erythroid progenitors with high-titer retrovirus encoding the non-hematopoietic prolactin receptor. The infected progenitors responded to prolactin in the absence of Epo by generating fully differentiated erythroid colonies. Therefore, differentiation of erythroid progenitors does not require an intracellular signal generated uniquely by the EpoR; the EpoR does not have an instructive role in erythroid differentiation. We also infected primary erythroid progenitors with retrovirus encoding chimeric receptors containing the extracellular domain of PrlR and the intracellular domain of either the wild-type or truncated EpoRs. A chimeric receptor containing only the membrane-proximal 136 amino acids of the EpoR cytoplasmic domain efficiently supported prolactin-dependent differentiation of erythroid progenitors. Substitution of the single cytoplasmic domain tyrosine in this receptor with phenylalanine (Y343F) eliminated its ability to support differentiation. The minimal EpoR cytoplasmic domain required for erythroid differentiation is therefore the same as that previously reported to be sufficient to support cell proliferation (D'Andrea, A. D., Yoshimura, A., Youssoufian, H., Zon, L. I., Koo, J. W., and Lodish, H. F. (1991) Mol. Cell. Biol. 11, 1980-1987; Miura, O., D'Andrea, A. D., Kabat, D., and Ihle, J. N. (1991) Mol. Cell. Biol. 11, 4895-4902; He, T.-C., Jiang, N., Zhuang, H., Quelle, D. E., and Wojchowski, D. M. (1994) J. Biol. Chem. 269, 18291-18294).

Mitogenic signaling and inhibition of apoptosis via the erythropoietin receptor Box-1 domain

Studies of proliferative signaling via type 1 cytokine receptors have revealed a three-step activation mechanism. Cytokine-induced receptor dimerization mediates the trans-phosphorylation of Jak kinases, Jaks phosphorylate receptors at tyrosine sites, and SH2 domain-encoding effectors then are recruited to these sites. Signaling factors that associate with activated erythropoietin (Epo) receptor complexes include phospholipase C-gamma, phosphatidylinositol 3-kinase, SHIP, Shc, Grb2, Cbl, Crk-l, HCP, Syp, and STAT5. While at least certain of these factors modulate proliferative signaling, mutated Epo receptor forms lacking Tyr(P) sites retain substantial mitogenic activity. Presently we show that a highly truncated Epo receptor form that retains box-1, yet lacks the conserved box-2 domain (and all Tyr(P) sites) nonetheless effectively promotes mitogenesis, survival, and Myc and Pim-1 expression. In addition, mitogenesis and Myc expression are shown to be supported by a direct Epo receptor-Jak2 kinase domain chimera. Thus, Epo-dependent mitogenesis and inhibition of apoptosis each depend critically upon only the Epo receptor box-1 domain, with no essential role exerted in these response pathways by the box-2 domain.

Distinct signaling from stem cell factor and erythropoietin in HCD57 cells

A recent report (Wu, H., Klingmuller, U., Besmer, P., and Lodish, H. F. (1995) Nature 377, 242-246) documents the interaction of the erythropoietin (EPO) receptor (EPOR) with the stem cell factor (SCF) receptor (c-KIT) and suggests that SCF acts through the EPOR. To elucidate the ability of SCF to affect the erythropoietin signaling pathway, we studied the effect of SCF on EPOR phosphorylation, SHC/ERK-1 activity, and cell proliferation and apoptosis in EPO-dependent HCD57 cells. Treatment of these cells with SCF resulted in phosphorylation of the EPOR. However, SCF-dependent phosphorylation of the EPOR did not initiate an EPO-like intracellular signal. SCF induced proliferation, SHC phosphorylation, and activation of ERK-1 but did not activate the JAK/STAT pathway. SCF stimulated SHC phosphorylation and ERK-1 activation independent of the EPOR in cells where the EPOR was down-regulated; the presence of the EPOR appeared to facilitate SCF activation of SHC and ERK-1. Furthermore, treatment of HCD57 cells with SCF increased cell number over a 3-day treatment, but apoptosis was observed in these cells. These data may illustrate two distinct pathways for erythroid cell proliferation and prevention of apoptosis in response to EPO, thereby providing a system to discriminate these intracellular signals.

Association of JAK2 and STAT5 with erythropoietin receptors. Role of receptor phosphorylation in erythropoietin signal transduction

Cytokine receptors act at least partially by associating with Janus tyrosine protein kinases at the conserved box one motif of the receptor. These receptor-associated kinases then activate STAT transcription factors through phosphorylation. We found that the 78-kDa erythropoietin receptor (EPOR), a highly modified form of the 66-kDa receptor which is abundant in HCD57 cells, was phosphorylated on serine residues without EPO stimulation. Coprecipitation experiments showed the 78-kDa EPOR but not the more abundant 66-kDa EPOR was associated with JAK2, a Janus protein kinase, in both the presence and absence of EPO. Solubilized 78-kDa EPOR bound to purified, genetically engineered JAK2 better than the 62-76-kDa receptor proteins, and additional phosphorylation of tyrosine residues further increased the binding of the 78-kDa EPOR to JAK2-agarose beads. STAT5 DNA binding was activated by 10-100-fold lower concentrations of EPO in HCD57 cells than in primary erythroid cells, and STAT5 associated with the EPOR in an EPO-dependent manner. These data suggest that phosphorylation of either serine or tyrosine residues of the EPOR can enhance the association of the receptor with JAK2, possibly increasing the sensitivity to EPO.

The box1 domain of the erythropoietin receptor specifies Janus kinase 2 activation and functions mitogenically within an interleukin 2 beta-receptor chimera

Several distinct classes of cytokine receptors engage Jak kinases as primary effectors. Among type 1 receptors, Janus-activated kinase (Jak) recruitment is mediated by membrane-proximal cytoplasmic domains, which typically contain conserved box motifs. In the erythropoietin receptor (Epo-R), two such motifs (box1 and box2) have been suggested to be essential for the activation of Jak2 and mitogenesis. Presently, an Epo-R chimera containing the extracellular and box1 domains of the Epo-R (Jak2-associated receptor) and the box2 and carboxyl-terminal domains of the interleukin 2 beta-receptor (IL2beta-R; a Jak1-associated subunit) is shown to activate Jak2. Interestingly, Jak2 also was activated in FDC-P1 cells by a control Epo-R chimera containing the complete IL2beta-R cytoplasmic domain, and mitogenesis was supported by each of these above chimeras. By comparison, in BaF3 cells expressing IL2 receptor alpha and gamma subunits, an ectopically expressed IL2beta-R chimera containing the box1 domain of the Epo-R, activated Jak2 and Jak3 and was as mitogenically active as the wild-type IL2beta-R (Jak1 and Jak3 activation). Thus, the box1 domain of the Epo-R specifies Jak2 activation and functions efficiently within a heterologous IL2 receptor complex that normally activates Jak1 and Jak3.

Inhibition of proliferation but not erythroid differentiation of J2E cells by rapamycin

During erythropoiesis, replication and maturation are tightly coupled processes. Here, we show that the immunosuppressant rapamycin inhibited basal- as well as erythropoietin-stimulated proliferation of the erythroid cell line J2E. In addition, it enhanced the antiproliferative effect of sodium butyrate. Although rapamycin suppressed erythroid cell division, it did not affect terminal differentiation induced by erythropoietin or sodium butyrate. The proliferative status of J2E cells correlated well with the activity of the ribosomal S6 kinase p70S6k, an enzyme effectively blocked by rapamycin. It was concluded from this study that erythroid maturation proceeded normally despite the rapamycin-induced inhibition of mitosis and of p70S6k activity. These data provide further evidence that separate signalling pathways for proliferation and differentiation exist in erythroid cells.

Disrupted signaling in a mutant J2E cell line that shows enhanced viability, but does not proliferate or differentiate, with erythropoietin

The immature erythroid J2E cell line proliferates and terminally differentiates following erythropoietin stimulation. In contrast, the mutant J2E-NR clone does not respond to erythropoietin by either proliferating or differentiating. Here we show that erythropoietin can act as a viability factor for both the J2E and J2E-NR lines, indicating that erythropoietin-initiated maturation is separable from the prevention of cell death. The inability of J2E-NR cells to mature in response to erythropoietin was not due to a defect in the erythropoietin receptor sequence, although surface receptor numbers were reduced. Both the receptor and Janus kinase 2 were phosphorylated after erythropoietin stimulation of J2E-NR cells. However, protein interactions with the erythropoietin receptor and Grb2 were restricted in the mutant cells. Subsequent investigation of several other signaling molecules exposed numerous alterations in J2E-NR cells; phosphorylation changes to phosphatidylinositol 3-kinase, phospholipase Cgamma, p120 GAP, and mitogen-activated protein kinases (p42 and p44) observed in erythropoietin-stimulated J2E cells were not seen in the J2E-NR line. These data indicate that some pathways activated during erythropoietin-induced differentiation may not be essential for the prevention of apoptosis.

The signal transduction pathway of erythropoietin involves three forms of mitogen-activated protein (MAP) kinase in UT7 erythroleukemia cells

The survival and proliferation of the UT-7 human leukemic cell line is strictly dependent on the presence of either interleukin 3, granulocyte-macrophage colony-stimulating factor or erythropoietin. In these cells, erythropoietin stimulation led to the rapid phosphorylation of several proteins including the erythropoietin receptor and proteins with molecular masses around 45 kDa which could be mitogen-activated protein (MAP) kinases. Separation of cytosol from resting or erythropoietin-stimulated UT-7 cells by anion-exchange chromatography revealed two peaks of myelin basic protein kinase activity. The kinase activity of the first peak was independent of erythropoietin treatment of the cells and corresponded to an unidentified 50-kDa kinase, whereas the second peak was only present in erythropoietin-stimulated cells and corresponded to three forms of MAP kinases with molecular masses of 45, 44 and 42 kDa. The three forms were separated by hydrophobic chromatography and were shown to be activated in erythropoietin-stimulated cells. The 44-kDa and 42-kDa forms corresponded to extracellular signal-regulated kinase (ERK)-1 and ERK-2, respectively. Evidence was obtained showing that the 45-kDa form is not a shifted form of ERK-1 but corresponded to a less well defined form of MAP kinase which may be the previously described ERK-4. MAP kinase activation was detected after 1 min erythropoietin stimulation and remained detectable after more than 1 hour. A role for MAP kinase activation in erythropoietin-stimulated cell proliferation was suggested by the simultaneous inhibition of erythropoietin-induced MAP kinase stimulation and cell proliferation. The potential activator of MAP kinase, RAF-1, was hyperphosphorylated in erythropoietin-stimulated cells and its autophosphorylation activity was strongly increased. The protein adaptor Shc was heavily phosphorylated in UT-7 erythropoietin-stimulated cells and associated strongly with a unidentified 145-kDa protein. However, Shc bound poorly to the activated erythropoietin receptor and most Shc proteins were cytosolic in both unstimulated and erythropoietin-stimulated cells. In contrast, Grb2 associated efficiently with the activated erythropoietin receptor and a significant part of Grb2 was associated to a particulate subcellular fraction upon erythropoietin stimulation.

Interaction of the erythropoietin and stem-cell-factor receptors

Mutations in the KIT transmembrane protein-tyrosine kinase receptor affect erythropoiesis, resulting in fewer committed late progenitors (colony-forming unit erythroid, CFU-E) in the fetal liver. As the survival and proliferation of CFU-Es depend absolutely on erythropoietin (EPO), these results suggest that CFU-Es cannot proliferate or mature further unless both the KIT and EPO receptor signalling pathways are functional. How KIT affects proliferation or differentiation of CFU-Es is not clear. Here we show that the KIT ligand SCF (for stem-cell factor) can replace EPO in supporting the growth and survival of HCD57 cells, an EPO-dependent erythroid-progenitor cell line expressing high levels of KIT. SCF supports the proliferation of 32D cells that express KIT only if they also express the EPO receptor. In HCD57 cells, SCF rapidly induces tyrosine phosphorylation of the EPO receptor, and KIT physically associates with the extended box 2 region in the cytoplasmic domain of the EPO receptor. Our results indicate that KIT may activate the EPO receptor by tyrosine phosphorylation to induce further proliferation and maturation of CFU-Es.

Mapping of the cytoplasmic domain of the human growth hormone receptor required for the activation of Jak2 and Stat proteins

Incubation of cells with growth hormone (GH) stimulates both tyrosine phosphorylation of the Jak2 tyrosine kinase and, in some cells, the transcription factor Stat1 alpha (1-4). When the promyeloid cell line FDC-P1 is transfected with the human growth hormone receptor, these cells can grow in the presence of GH and in the absence of interleukin-3. Growth hormone treatment of cells expressing the human growth hormone receptor did not activate Stat1 alpha. However, a complex is present in extracts prepared from growth hormone-treated cells that binds to the gamma response region, an enhancer present in the promoter of the high affinity Fc gamma R1 receptor to which cytokine-activated Stat complexes bind. When truncations of the cytoplasmic domain of the receptor are expressed in FDC-P1 cells only the membrane-proximal 80 amino acids (containing box 1 and box 2) are required for activation of both a GH-stimulated binding activity (GHSF) and tyrosine phosphorylation of Jak2. Activation of GHSF can be inhibited in a cell-free system by the addition of a glutathione S-transferase fusion protein containing these 80 amino acids. Replacement of the one tyrosine in this region of the receptor with a phenylalanine does not alter the activation of either GHSF or Jak2, suggesting that tyrosine phosphorylation of the receptor is not required for GH activation of GHSF. Moreover, a cell line expressing a receptor with only the 54 membrane-proximal amino acids of the intracellular domain (including box 1) shows constitutively tyrosine-phosphorylated Jak2 as well as GHSF binding. With this truncated receptor, there is little if any additional GH-induced tyrosine phosphorylation of Jak2 or induced binding to the gamma response region. These results define the importance of the membrane-proximal 80 amino acids of the GH receptor (with the conserved box 1 and box 2 domains) with regard to GH activation of both Jak2 and Stat(s). They also suggest that within these domains there may be positive and negative elements that regulate Jak2 function.

Signal transduction by a CD16/CD7/Jak2 fusion protein

The addition of interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) to hormone-dependent cells induces tyrosine phosphorylation of Janus protein kinase 2 (Jak2) and activates its in vitro kinase activity. To explore the role of Jak2 in IL-3/GM-CSF-mediated signal transduction, we constructed a CD16/CD7/Jak2 (CD16/Jak2) fusion gene containing the external domain of CD16 and the entire Jak2 molecule and expressed this fusion protein using a recombinant vaccinia virus. The clustering of CD16/Jak2 fusion protein by cross-linking with an anti-CD16 antibody induced autophosphorylation of the fusion protein but did not induce the phosphorylation of either the endogenous Jak2 or the beta chain. Cross-linking of CD16/Jak2 stimulates the tyrosine phosphorylation of a large group of proteins that are also phosphorylated after the addition of IL-3 or GM-CSF and include proteins of 145, 97, 67, 52, and 42 kDa. Closer analysis demonstrated that the CD16/Jak2 phosphorylates Shc, a 52-kDa protein, and the 145-kDa protein associated tightly with Shc, as well as mitogen-associated protein kinase (pp42). Electrophoretic mobility shift assays demonstrate that CD16/Jak2 activates the ability of signal transduction and activation of transcription (STAT) proteins to bind to an interferon-gamma-activated sequence oligonucleotide in a manner similar to that seen after IL-3 treatment. Cross-linking of the CD16/Jak2 protein stimulated increases in c-fos and junB similar to IL-3 but did not cause major changes in the levels of the c-myc message, which normally increases after IL-3 treatment. Thus, a transmembrane CD16/Jak2 fusion is capable of activating protein phosphorylation and mRNA transcription in a manner similar but not identical to hematopoietic growth factors.

Erythropoietin-dependent inhibition of apoptosis is supported by carboxyl-truncated receptor forms and blocked by dominant-negative forms of Jak2

Apoptosis, or programmed cell death (PCD), recently has emerged as an important homeostatic mechanism within several hematopoietic lineages. This process is subject to both positive and negative modulation by cytokines and within the erythroid lineage is inhibited by interleukin-3, stem cell factor, and erythropoietin (Epo). Through the expression of carboxyl-truncated Epo receptor mutants in FDC-P1 cells, a receptor form possessing 80 membrane-proximal cytoplasmic residues is shown to efficiently mediate Epo-dependent inhibition of PCD. This is in contrast to previous studies that attributed this activity to a distal carboxyl-terminal receptor subdomain (and/or heterodimerization of wild type Epo receptors with a truncated non-functional receptor form). Epo-dependent inhibition of PCD also is shown to be blocked by ectopic expression of kinase-deficient dominant-negative forms of Jak2 (Jak2 delta VIII and Jak2-829), further underlining a role of this membrane-proximal subdomain of the Epo receptor in the inhibition of PCD. To our knowledge, this comprises the first direct evidence for an essential role for a Jak tyrosine kinase (Jak2) in this apoptotic response pathway.

Erythropoietin-induced recruitment of Shc via a receptor phosphotyrosine-independent, Jak2-associated pathway

Based on the recently implicated role of Shc as a signaling effector for type I cytokine receptors, factors which mediate the recruitment and phosphorylation of Shc in the erythropoietin receptor (EPOR) system have been studied. FDC-P1 cells stably expressing the wild type murine EPOR supported the EPO-induced association of Shc with Jak2 and its rapid tyrosine phosphorylation. However, this did not depend upon the presence of phosphotyrosine sites within the EPOR and was mediated by a mitogenically deficient receptor form (EPOR329) lacking cytoplasmic tyrosine residues. This was shown both by Western blotting of Shc and Jak2 co-immunoprecipitates and through the development of an in vitro assay for cytokine-induced Shc phosphorylation. The direct association of Shc with Jak2 also was observed and was shown to depend upon EPO-exposure and the SH2 subdomain of Shc. Together, these studies indicate that Jak2, in part, may mediate the EPO-induced phosphorylation of Shc.

Protein tyrosine phosphorylation as a mechanism which regulates cytokine activation of early response genes

Two well-defined rapid responses which occur as a consequence of growth factors binding to their cell surface receptors involve tyrosine phosphorylation of cellular proteins and the induction of the transcription of cellular genes. Recent advances have been made in purification and cloning of Src homology 2 and 3 (SH2/SH3) domain-containing transcription factors which are required for the activation of early response genes by interferons. These transcription factors are covalently modified by tyrosine phosphorylation such that they interact with enhancers needed for interferon-stimulated gene expression. The Jak family of tyrosine kinases are also an integral component in these signalling cascades. The information gained concerning interferon signalling has now been extended to include a broad network of cytokine-regulated signalling systems which use tyrosine phosphorylation of a family of structurally related proteins to activate transcription of early response genes.

The Janus protein tyrosine kinase family and its role in cytokine signaling

During the past 2 years, research from quite divergent areas has converged to provide the first insights into the mechanisms by which cytokines that utilize receptors of the cytokine receptor superfamily function. On the one hand, the obscure Jak family of cytoplasmic protein tyrosine kinases was independently implicated in IFN and hematopoietic growth factor signaling. Recent studies have expanded these initial observations to demonstrate that Jaks are critical to the functioning of all the receptors of the cytokine receptor superfamily. A variety of questions remain to be explored regarding the structure and function of Jaks and their interaction with receptors. It will also be important to pursue additional approaches to determine if the Jaks are necessary for various biological responses, particularly for mitogenic responses. The second major area of convergence has been the demonstration that members of the Stat family of transcription factors, initially identified in IFN-regulated gene expression, are generally involved in cytokine signaling. Clearly, a number of Stat-like activities remain to be cloned and it can be anticipated that the family contains additional members. Although a variety of genes are known to be regulated by the Stats association with IFN responses, much less is known concerning the genes regulated by the new Stats in cytokine signaling. Of particular importance is information relating to their potential contribution to mitogenic responses. From a biochemical standpoint, the Stats represent a remarkable family of proteins with regard to the ability of the modification of a single tyrosine residue to so dramatically affect cellular localization and DNA binding activity. Studies to identify the domains involved, and associated proteins that might contribute to either property, will be of considerable interest. More generally, it can hypothesized that Jaks and Stats, if important for proliferation and differentiation, may be the targets for malignant transformation. Although none of the genes map to chromosomal breakpoints that have been implicated in transformation, gain of function mutations is a likely mechanism that needs to be explored. Similarly, the Jak-Stat pathway would appear to be an excellent target for the development of drugs that affect a variety of cytokine functions.