Signal transduction in the erythropoietin receptor system

Janus Kinase Inhibitors Differentially Inhibit Specific Cytokine Signals in the Mesenteric Lymph Node Cells of Inflammatory Bowel Disease Patients

BACKGROUND

Janus kinase [JAK] inhibitors [JAKinibs] are effective small molecule therapies for treating Crohn's disease [CD] and ulcerative colitis [UC], collectively known as inflammatory bowel disease [IBD]. By preventing JAKs from phosphorylating signal transducer and activator of transcription proteins, JAKinibs disrupt cytokine signalling pathways that promote inflammation. Despite considerable overlap in the JAKs they target, first- and second-generation JAKinibs display different clinical efficacies in CD and UC.

METHODS

We conducted a comparative phosflow study of four JAKinibs [filgotinib, upadacitinib, tofacitinib, and deucravacitinib] to observe subtle mechanistic differences that may dictate their clinical behaviour. Resected mesenteric lymph node [MLN] cells from 19 patients [9 CD, 10 UC] were analysed by flow cytometry in the presence or absence of different cytokine stimuli and titrated JAKinibs.

RESULTS

We found a higher potency of the JAK 1/3-preferential inhibitor, tofacitinib, for JAK 3-dependent cytokine signalling pathways in comparison to filgotinib, but a higher potency of the JAK 1-preferential inhibitors, filgotinib and upadacitinib, for JAK 3-independent cytokine signalling pathways. Deucravacitinib, a TYK2-preferential inhibitor, demonstrated a much narrower selectivity by inhibiting only IL-10 and IFN-β pathways, albeit more potently than the other JAKinibs. Additionally, we found some differences in the sensitivity of immune cells from CD versus UC, and patients with versus without a CD-associated NOD2 polymorphism, to phosphorylate signal transducer and activator of transcriptions in response to specific cytokine stimulation.

CONCLUSIONS

Despite their similarities, differences exist in the relative potencies of different JAKinibs against distinct cytokine families, to explain their clinical efficacy.

Tyrosine-phosphorylated SOCS3 negatively regulates cellular transformation mediated by the myeloproliferative neoplasm-associated JAK2 V617F mutant

In the majority of myeloproliferative neoplasms (MPNs) patients, a point mutation, V617F has been found in Janus kinase 2 (JAK2) gene, and this JAK2 mutant provoked aberrant signaling pathway. In the current study, we found that suppressor of cytokine signaling proteins 3 (SOCS3) possessed the tumor suppressive activity against the JAK2 V617F mutant-provoked cellular transformation. The knockdown of SOCS3 increased the expression level of the JAK2 V617F mutant, which enhanced the activation of signaling mediators, including signal transducer and activator of transcription 3 and 5 (STAT3, STAT5) and extracellular signal-regulated kinase (ERK), and also increased of the proliferation rate and tumorigenesis activity of Ba/F3 cells expressing the JAK2 V617F mutant and erythropoietin receptor (EpoR). In contrast, the enforced expression of SOCS3 significantly inhibited the JAK2 V617F mutant-induced activation of downstream signaling molecules, cell proliferation, and tumorigenesis by down-regulating the expression level of the JAK2 V617F mutant. SOCS3 interacted with the JAK2V617F mutant through its SH2 domain and was phosphorylated at Tyr-204 and Tyr-221 in its SOCS box by the JAK2V617F mutant. SOCS3 mutants carrying a mutation in the SH2 domain (R71E) and a substitution at Tyr-221 (Y221F) failed to exert inhibitory effects on JAK2V617F mutant-induced cellular transformation and tumorigenesis. Collectively, these results imply that SOCS3 plays a negative role in the JAK2 V617F mutant-induced oncogenic signaling pathway through its SH2 domain and the phosphorylation of Tyr-221 in its SOCS box.

Three Tyrosine Residues in the Erythropoietin Receptor Are Essential for Janus Kinase 2 V617F Mutant-induced Tumorigenesis

The erythropoietin receptor (EpoR) regulates development of blood cells, and its full activation normally requires the cytokine erythropoietin (Epo). In the case of myeloproliferative neoplasms (MPN), Epo-independent signaling through EpoR can be caused by a point mutation, V617F, in the EpoR-interacting tyrosine kinase Janus kinase 2 (JAK2). In cells expressing the JAK2 V617F mutant, eight tyrosine residues in the intracellular domain of EpoR are phosphorylated, but the functional role of these phosphorylations in oncogenic signaling is incompletely understood. Here, to evaluate the functional consequences of the phosphorylation of these tyrosine residues, we constructed an EpoR-8YF mutant in which we substituted all eight tyrosine residues with phenylalanine. Co-expression of EpoR-8YF with the JAK2 V617F mutant failed to induce cytokine-independent cell proliferation and tumorigenesis, indicating that JAK2-mediated EpoR phosphorylation is the reason for JAK2 V617F mutant-induced oncogenic signaling. An exhaustive mutational analysis of the eight EpoR tyrosine residues indicated that three of these residues, Tyr-343, Tyr-460, and Tyr-464, are required for the JAK2 V617F mutant to exhibit its oncogenic activity. We also showed that phosphorylation at these three residues was necessary for full activation of the transcription factor STAT5, which is a critical downstream factor of JAK2 V617F-induced oncogenic signaling. In contrast, Epo stimulation could moderately stimulate the proliferation of cells expressing wild type JAK2 and EpoR-8YF, suggesting that the requirement of the phosphorylation of these three tyrosine residues seems to be specific for the oncogenic proliferation provoked by V617F mutation. Collectively, these results have revealed that phosphorylation of Tyr-343, Tyr-460, and Tyr-464 in EpoR underlies JAK2 V617F mutant-induced tumorigenesis. We propose that the targeted disruption of this pathway has therapeutic utility for managing MPN.

Interleukin-4 and granulocyte-macrophage colony-stimulating factor mediates the upregulation of soluble vascular endothelial growth factor receptor-1 in RAW264.7 cells-a process in which p38 mitogen-activated protein kinase signaling has an important role

BACKGROUND/PURPOSE

Soluble vascular endothelial growth factor receptor-1 (sVEGFR1) antagonizes angiogenesis by inhibiting the biological function of vascular endothelial growth factor (VEGF). Immature dendritic cells (imDCs) express high levels of sVEGFR1 during development and are antiangiogenic. This study aimed to investigate the changes in VEGFR1, sVEGFR1, and VEGF levels during the development of imDCs and explore the underlying signaling mechanisms.

METHODS

To model the differentiation of imDCs from monocytes, RAW264.7 cells, a murine monocyte/macrophage cell line, were stimulated by interleukin-4 (IL-4; 10 ng/mL, 20 ng/mL, and 40 ng/mL) and/or by granulocyte-macrophage colony-stimulating factor (GM-CSF; 10 ng/mL, 20 ng/mL, and 50 ng/mL) and/or pretreated by the p38 inhibitor SB203580. The levels of VEGFR1, sVEGFR1, and VEGF were detected by reverse transcription polymerase chain reaction (RT-PCR), Western blot, and enzyme-linked immunosorbent assay (ELISA).

RESULTS

IL-4 increased the VEGFR1 mRNA and sVEGFR1 levels in RAW264.7 (p < 0.05). This increase was inhibited by SB203580. Granulocyte-macrophage colony-stimulating factor increased the sVEGFR1 levels, but it had no significant effect on VEGFR1 mRNA levels. SB203580 decreased the expression of VEGFR1 mRNA induced by GM-CSF, whereas sVEGFR1 was unaffected. IL-4 had a greater effect on sVEGFR1 levels, compared to GM-CSF.

CONCLUSION

IL-4 and GM-CSF increased sVEGFR1 levels, but did not significantly effect VEGF expression, and led to the antiangiogenesis properties of monocytes. p38 Mitogen-activated protein kinase signaling has an important role in the process.

Use of high-dose erythropoietin for repair after injury: A comparison of outcomes in heart and kidney

CONTEXT

There is a need to define the exact benefits and contraindications of use of high-dose recombinant human erythropoietin (EPO) for its non-hematopoietic function as a cytokine that enhances tissue repair after injury. This review compares the outcomes from use of EPO in the injured heart and kidney, two organs that are thought, traditionally, to have intrinsically-different repair mechanisms.

EVIDENCE ACQUISITIONS

Directory of Open Access Journals (DOAJ), Google Scholar, Pubmed (NLM), LISTA (EBSCO) and Web of Science have been searched.

RESULTS

Ongoing work by us on EPO protection of ischemia-reperfusion-injured kidneys indicated, first, that EPO acutely enhanced kidney repair via anti-apoptotic, pro-regenerative mechanisms, and second, that EPO may promote chronic fibrosis in the long term. Work by others on the ischaemia-injured heart has also indicated that EPO promotes repair. Although myocardial infarcts are made up mostly of necrotic tissue, many publications state EPO is anti-apoptotic in the heart, as well as promoting healing via cell differentiation and stimulation of granulation tissue. In the case of the heart, promotion of fibrosis may be advantageous where an infarct has destroyed a zone of cardiomyocytes, but if EPO stimulates progressive fibrosis in the heart, this may promote cardiac failure.

CONCLUSIONS

A major concern in relation to the use of EPO in a cytoprotective role is its stimulation of long-term inflammation and fibrosis. EPO usage for cytoprotection is undoubtedly advantageous, but it may need to be offset with an anti-inflammatory agent in some organs, like kidney and heart, where progression to chronic fibrosis after acute injury is often recorded.

Potential of Novel EPO Derivatives in Limb Ischemia

Erythropoietin (EPO) has tissue-protective properties, but it increases the risk of thromboembolism by raising the haemoglobin concentration. New generation of EPO derivatives is tissue protective without the haematopoietic side effects. Preclinical studies have demonstrated their effectiveness and safety. This paper summarizes the development in EPO derivatives with emphasis on their potential use in critical limb ischaemia.

Erythropoietin in the intensive care unit: beyond treatment of anemia

Erythropoietin (EPO) is the major hormone stimulating the production and differentiation of red blood cells. EPO is used widely for treating anemia of critical illness or anemia induced by chemotherapy. EPO at pharmacological doses is used in this setting to raise hemoglobin levels (by preventing the apoptosis of erythroid progenitor cells) and is designed to reduce patient exposure to allogenic blood through transfusions. Stroke, heart failure, and acute kidney injury are a frequently encountered clinical problem. Unfortunately, in the intensive care unit advances in supportive interventions have done little to reduce the high mortality associated with these conditions. Tissue protection with EPO at high, nonpharmacological doses after injury has been found in the brain, heart, and kidney of several animal models. It is now well known that EPO has anti-apoptotic effects in cells other than erythroid progenitor cells, which is considered to be independent of EPOs erythropoietic activities. This review article summarizes what is known in preclinical models of critical illness and discusses why this does not correlate with randomized, controlled clinical trials.

Erythropoietin molecules to treat acute ischemic stroke: a translational dilemma!

IMPORTANCE OF THE FIELD

Since the realization that erythropoietin (EPO) molecules have 'neuroprotective' properties, they have been investigated as treatments for acute ischemic stroke (AIS), but not systematically. The results of the 2009 clinical trial showed that EPO was ineffective as a stroke treatment, and moreover, increased mortality when combined with tissue plasminogen activator. Currently, CEPO, an EPO analog, is entering into a safety, tolerability and pharmacokinetic clinical trial for the treatment of AIS.

AREAS COVERED IN THIS REVIEW

This review covers translational and clinical studies carried out over the period 1998 - 2010.

WHAT THE READER WILL GAIN

The primary aim of this article is to review the information available regarding the pharmacological and biological characteristics of EPO molecules. Second, based upon the translational research with EPO molecules in preclinical stroke models, a recommendation is made regarding the continued development of EPO molecules as an option to treat AIS.

TAKE HOME MESSAGE

EPO, CEPO and helix B peptide EPO analogs have significant neuroprotective activity is preclinical stroke models. However, given the detrimental effect of EPO in a recent clinical trial, preclinical safety studies of EPO molecules in embolic stroke models that parallel acute ischemic stroke in humans are warrented.

Pregnancy alters the proliferation and apoptosis of mouse splenic erythroid lineage cells and leukocytes

Pregnancy induces dynamic changes in the maternal environment that include reversible modifications in response to systemic mediators and local signals. The spleen can be used to determine the effects of pregnancy on multiple cellular populations, including those of the erythroid lineage and the immune system. Current evidence suggests that the transient increase in the size of the spleen during pregnancy is due to the expansion of erythroid precursors. However, it is unclear what factors contribute to this increase. Moreover, the additional erythroid cells may compete with neighboring leukocytes for growth factors or space, and this may in turn alter the function of these populations. Therefore, we assessed proliferation and apoptosis throughout gestation using in vivo bromodeoxyuridine incorporation and the TUNEL assay, respectively. Here, we show that erythroid-lineage TER-119(+) cells expanded significantly in midgestation because of enhanced proliferation and diminished apoptosis. This correlated with increased expression of the erythropoietin receptor (Epor) and decreased expression of the death receptor Fas, respectively. Leukocytes demonstrated population-specific responses. Natural killer cells proliferated in early pregnancy. Both lymphocytes and CD11B(+) cells underwent enhanced proliferation during midgestation. In contrast, neutrophils exhibited augmented proliferation throughout pregnancy. These subset-specific alterations in proliferation and death in the spleen suggest that complex regulation of population dynamics exists during pregnancy.

Advances in hemorrhagic stroke therapy: conventional and novel approaches

Treatments for spontaneous intracerebral, thrombolytic-induced and intraventricular hemorrhages (IVH) are still at the preclinical or early clinical investigational stages. There has been some renewed interest in the use of surgical evacuation surgery or thrombolytics to remove hematomas, but these techniques can be used only for specific types of brain bleeding. The STICH (Surgical Trial in Intracerebral Haemorrhage) clinical trials should provide some insight into the potential for such techniques to counteract hematoma-induced damage and subsequently, morbidity and mortality. More recently, clinical trials (ATACH [Antihypertensive Treatment in Acute Cerebral Hemorrhage] and INTERACT [Intensive Blood Pressure Reduction in Acute Cerebral Hemorrhage Trial]) have begun testing whether or not regulating blood pressure affects the well-being of hemorrhage patients, but the findings thus far have not conclusively demonstrated a positive result. More promising trials, such as the early stage CHANT (Cerebral Hemorrhagic And NXY-059 Treatment) and the late stage FAST (Factor VIIa for Acute Hemorrhagic Stroke Treatment), have addressed whether or not manipulating oxidative stress and components of the blood coagulation cascade can achieve an improved prognosis following spontaneous hemorrhages. However, CHANT was halted prematurely because although it showed that the spin trap agent NXY-059 was safe, it also demonstrated that the drug was ineffective in treating acute ischemic stroke. In addition, the recombinant activated factor VII FAST trial recently concluded with only modestly positive results. Despite a beneficial effect on the primary end point of reducing hemorrhage volume, controlling the coagulation cascade with recombinant factor VIIa did not decrease the mortality rate. Consequently, Novo Nordisk has abandoned further development of the drug for the treatment of intracerebral hemorrhaging. Even though progress in hemorrhage therapy that successfully reduces the escalating morbidity and mortality rate associated with brain bleeding is slow, perseverance and applied translational drug development will eventually be productive. The urgent need for such therapy becomes more evident in light of concerns related to uncontrolled high blood pressure in the general population, increased use of blood thinners by the elderly (e.g., warfarin) and thrombolytics by acute ischemic stroke patients, respectively. The future of drug development for hemorrhage may require a multifaceted approach, such as combining drugs with diverse mechanisms of action. Because of the substantial benefit of factor VIIa in reducing hemorrhage volume, it should be considered as a prime drug candidate included in combination therapy as an off-label use if the FAST trial proves that the risk of thromboembolic events is not increased with drug administration. Other promising drugs that may be considered in combination include uncompetitive NMDA receptor antagonists (such as memantine), antioxidants, metalloprotease inhibitors, statins and erythropoietin analogs, all of which have been shown to reduce hemorrhage and behavioral deficits in one or more animal models.

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

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

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

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

GM-CSF induces expression of soluble VEGF receptor-1 from human monocytes and inhibits angiogenesis in mice

GM-CSF promotes homeostasis of myeloid cells. We report that GM-CSF upregulates mRNA and protein production of the soluble form of membrane bound VEGF receptor-1 (sVEGFR-1) in human monocytes. This sVEGFR-1 was biologically active, as cell-free supernatants from GM-CSF-stimulated monocytes blocked detection of endogenously expressed VEGF and inhibited endothelial cell migration and tube formation, even in the presence of exogenous rhVEGF. VEGF activity was recovered by neutralizing sVEGFR-1. To determine whether these events were important in vivo, Matrigel plugs were incubated with rhVEGF, rhGM-CSF, or rhGM-CSF/rhVEGF and injected into mice. Plugs containing GM-CSF or GM-CSF/VEGF had less endothelial cell invasion than plugs containing rhVEGF and were similar to plugs incubated with PBS alone. Neutralizing antibodies specific for sVEGFR-1 injected in these plugs reversed the effects of GM-CSF or GM-CSF/VEGF, while an isogenic antibody did not. Thus, GM-CSF and monocytes play a vital role in angiogenesis through the regulation of VEGF and sVEGFR-1.

Signalling mechanisms and the role of calcineurin in erythropoiesis

Erythropoietin (Epo) is the principal regulator of the production of circulating erythrocytes by controlling the proliferation, the differentiation and the survival of the erythroid progenitor cells. Early down-regulation of c-myb expression in erythroleukemia cells is a common feature of the action of Epo and chemical inducers of differentiation such as DMSO. Previously we have shown that in our Epo-responsive murine erythroleukemia cell line ELM-I-1, [Ca2+]i increasing agents can mimic the effect of Epo on c-myb expression and activate nuclear signal transduction processes involved in the induction of hemoglobin synthesis. These results also indicated that the Ca2+-induced down-regulation of c-myb expression and hemoglobin synthesis are mediated by the Ca2+/calmodulin dependent serine/threonine-specific protein phosphatase PP2B, calcineurin, but the Epo induced processes are not mediated by PP2B. In spite of this, we demonstrated in this paper that in ELM-I-1 cells the Epo-induced down-regulation of c-myb expression and hemoglobin production can be effectively enhanced by the simultaneously added [Ca2+]i-increasing agent, cyclopiazonic acid (CPA). This observation further supports the existence of at least two independent signalling pathways in the mechanism of Epo and [Ca2+]i increasing agents and the strong correlation between c-myb expression and hemoglobin production in differentiating cells. Although the c-AMP-response element binding protein (CREB) could be the common target of both calcium-dependent and -independent dephosphorylation, our results do not support the involvement of CREB in the regulation of c-myb gene expression. In addition to the calcineurin mediated down-regulation of c-myb expression, we have found a negative regulatory effect in the Ca2+-mediated transcriptional activation of certain genes. In response to [Ca2+]i-increasing agents in ELM-I-1 cells, both, egr-1 and c-fos mRNA expression increased significantly after the inhibition of calcineurin by cyclosporine A. Cyclosporin A exerted stimulatory effects on the egr-1 and c-fos expression also at lower (150-400 nM) intracellular Ca2+ levels. This potential co-regulation of c-myb, egr-1 and c-fos expression by calcineurin suggests that the negative modulation of egr-1 and c-fos expression may also be important for the induction of erythroid differentiation by [Ca2+]i-increasing agents. This negative modulation may also contribute to the Epo-induced differentiation in the case of a moderate increase of [Ca2+]i.

Granulocyte-macrophage colony-stimulating factor-activated signaling pathways in human neutrophils. Involvement of Jak2 in the stimulation of phosphatidylinositol 3-kinase

Granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates many of the biological activities of human neutrophils. The signaling pathways via which these effects are mediated are not fully understood. We have shown previously that GM-CSF treatment of human neutrophils activates the Janus kinase/signal transducers and activators of transcription (Jak/STAT) pathway and, more specifically, Jak2, STAT3, and STAT5B in neutrophils. GM-CSF also stimulates the activity of the phosphatidylinositol 3-kinase (PI3-kinase) in a tyrosine kinase-dependent manner. Here we report that pretreating the cells with a Jak2 inhibitor (AG-490) abolishes tyrosine phosphorylation of the p85 subunit of PI3-kinase induced by GM-CSF. Furthermore, p85 was found to associate with Jak2, but not with Lyn, in stimulated cells in situ and with its autophosphorylated form in vitro; however, Jak2 did not bind to either of the two Src homology 2 (SH2) domains of the p85 subunit of PI3-kinase. Although STAT5B bound to the carboxyl-terminal SH2 domain of p85, it was absent from the complex containing PI3-kinase and Jak2. These results suggest that stimulation of the activity of PI3-kinase induced by GM-CSF is mediated by Jak2 and that the association between Jak2 and p85 depends on an adaptor protein yet to be identified.

An antagonist peptide-EPO receptor complex suggests that receptor dimerization is not sufficient for activation

Dimerization of the erythropoietin (EPO) receptor (EPOR), in the presence of either natural (EPO) or synthetic (EPO-mimetic peptides, EMPs) ligands is the principal extracellular event that leads to receptor activation. The crystal structure of the extracellular domain of EPOR bound to an inactive (antagonist) peptide at 2.7 A resolution has unexpectedly revealed that dimerization still occurs, but the orientation between receptor molecules is altered relative to active (agonist) peptide complexes. Comparison of the biological properties of agonist and antagonist EMPs with EPO suggests that the extracellular domain orientation is tightly coupled to the cytoplasmic signaling events and, hence, provides valuable new insights into the design of synthetic ligands for EPOR and other cytokine receptors.

Granulocyte-macrophage colony-stimulating factor-activated signaling pathways in human neutrophils. Selective activation of Jak2, Stat3, and Stat5b

Granulocyte-macrophage colony stimulating factor (GM-CSF) regulates many of the biological functions of human neutrophils. This includes the stimulation of protein synthesis and the tyrosine phosphorylation of various proteins among which is JAK2. The present study was aimed at characterizing in detail the pattern of activation by GM-CSF of the JAK/STAT pathway in human neutrophils. The results obtained show that the stimulation of human neutrophils by GM-CSF specifically led to tyrosine phosphorylation of JAK2 and had no effect on JAK1, JAK3, or TYK2. Furthermore, GM-CSF induced the tyrosine phosphorylation of STAT3 and STAT5 but not of STAT1, STAT2, STAT4, or STAT6. Tyrosine phosphorylation of STAT3 was transient reaching its maximum at 15 min. STAT5 presented a different pattern of tyrosine phosphorylation. The anti-STAT5 antibodies identified two proteins at 94 and 92 kDa. The 94-kDa STAT5 was constitutively tyrosine phosphorylated and showed no change upon GM-CSF stimulation. On the other hand, the 92-kDa STAT5 was tyrosine phosphorylated within 1 min of GM-CSF treatment and this was maintained for at least 30 min. By the use of specific antibodies, it was determined that only STAT5B, and not STAT5A, was tyrosine phosphorylated in GM-CSF-treated neutrophils. Furthermore, GM-CSF treatment induced an increase in the ability of STAT3 and STAT5B, but not STAT5A, to bind DNA probes. The specificity of the pattern of activation of the JAK/STAT pathway suggests that it may be directly linked to the modulation of the functions of mature nondividing, human neutrophils by GM-CSF.

Molecular approaches to receptors as targets for drug discovery

The cloning of a great number of receptors and channels has revealed that many of these targets for drug discovery can be grouped into superfamilies based on sequence and structural similarities. This review presents an overview of how molecular biological approaches have revealed a plethora of receptor subtypes, led to new definitions of subtypes and isoforms, and played a role in the development of high selective drugs. Moreover, the diversity of subtypes has molded current views of the structure and function of receptor families. Practical difficulties and limitations inherent in the characterization of the ligand binding and signaling properties of expressed recombinant receptors are discussed. The importance of evaluating drug-receptor interactions that differ with temporally transient and distinct receptor conformational states is emphasized. Structural motifs and signal transduction features are presented for the following major receptor superfamilies: ligand-gated ion channel, voltage-dependent ion channel, G-protein coupled, receptor tyrosine-kinase, receptor protein tyrosine-phosphatase, cytokine and nuclear hormone. In addition, a prototypic receptor is analyzed to illustrate functional properties of a given family. The review concludes with a discussion of future directions in receptor research that will impact drug discovery, with a specific focus on orphan receptors as targets for drug discovery. Methods for classifying orphan receptors based upon homologies with members of existing superfamilies are presented together with molecular approaches to the greater challenge of defining their physiological roles. Besides revealing new orphan receptors, the human genome sequencing project will result in the identification of an abundance of novel receptors that will be molecular targets for the development of highly selective drugs. These findings will spur the discovery and development of an exciting new generation of receptor-subtype specific drugs with enhanced therapeutic specificity.

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.

The effect of human recombinant erythropoietin on the growth of a human neuroblastoma cell line

Erythropoietin is a growth factor. Cancer can be described as disturbance of the fine balance of positive and negative growth control mechanisms. The effect of human recombinant erythropoietin (EPO) was studied on the cell growth and differentiation of a human neuroblastoma cell line (h-NMB). Cell growth curves, trypan blue staining and thymidine uptake were used to assess cell proliferation and death. To assess cell differentiation, neutral endopeptidase (cell membrane enzyme marker), creatine kinase (cytosolic enzyme marker), dopamine uptake (dopamine transporter marker) and cell morphology were determined. Specific EPO receptor mRNA, by RT-PCR technique, was demonstrated. The incubation of erythropoietin with the tumor cell line resulted in inhibition of cell proliferation as evidenced in a diminished cell growth. EPO was shown to have induced a differentiation process as seen from the two different enzymatic markers, membranal and cytosolic, and from the cells dopamine uptake studies. However, the morphological changes did not document a full differentiation effect. EPO specific antibodies blocked the effects of EPO on cell proliferation and creatine kinase activity. In this study, EPO did not produce any sign of proliferation in the nervous tumor cell line used.

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.

Erythropoietin-induced transcription at the murine beta maj-globin promoter. A central role for GATA-1

Using J2E cells and the murine beta maj-globin promoter as a model, we have performed the first direct analyses of erythropoietin (EPO)-activated transcription from defined templates. The -346 to +26 beta maj promoter was shown to comprise a target for maximal activation. This included a positive role for a -346 to -107-base pair (bp) domain in J2E cells, but not in F-MEL cells. Mutagenesis of a -215-bp AGATAA element within this domain showed that this effect did not require GATA-1 binding. In contrast, a critical role for GATA-1 at a -60-bp (G)GATAG element was defined by mutagenesis (GGg-TAG and TGATAG), complementation with a synthetic TGATAA element, and the demonstrated specific binding of GATA-1. Proximal CCAAT (-75) and CACCC (-90) elements also were shown to contribute to transcriptional activation in J2E cells, yet exerted quantitatively distinct effects in the F-MEL system. Based on these results, minimal [TGATAA]4-TATA and TGATAA-CACCC-TATA promoters were constructed and assayed in each system. Remarkably, the [TGATAA]4-TATA promoter, but not the TGATAA-CACCC-TATA promoter, was induced efficiently by EPO in J2E cells, whereas the TGATAA-CACCC-TATA promoter was highly induced by Me2SO in F-MEL cells. These findings suggest that mechanisms of EPO-induced transcription in J2E cells involve GATA-1 and differ from chemically activated mechanisms studied previously in F-MEL cells. Globin induction in J2E cells was not associated with effects of EPO on levels or nuclear translocation of GATA-1. However, hemoglobinization was induced by okadaic acid, 8-Br-cAMP, and forskolin, a finding consistent with induction mechanisms that may involve modulated serine/threonine phosphorylation.