Characterization of the Role of the Human Granulocyte-Macrophage Colony-Stimulating Factor Receptor α Subunit in the Activation of JAK2 and STAT5

Mouse Models of CMML

Chronic myelomonocytic leukemia (CMML) is a rare and challenging type of myeloproliferative neoplasm. Poor prognosis and high mortality, associated predominantly with progression to secondary acute myeloid leukemia (sAML), is still an unsolved problem. Despite a growing body of knowledge about the molecular repertoire of this disease, at present, the prognostic significance of CMML-associated mutations is controversial. The absence of available CMML cell lines and the small number of patients with CMML make pre-clinical testing and clinical trials complicated. Currently, specific therapy for CMML has not been approved; most of the currently available therapeutic approaches are based on myelodysplastic syndrome (MDS) and other myeloproliferative neoplasm (MNP) studies. In this regard, the development of the robust CMML animal models is currently the focus of interest. This review describes important studies concerning animal models of CMML, examples of methodological approaches, and the obtained hematologic phenotypes.

Myelo-erythroid commitment after burn injury is under β-adrenergic control via MafB regulation

Severely injured burn patients receive multiple blood transfusions for anemia of critical illness despite the adverse consequences. One limiting factor to consider alternate treatment strategies is the lack of a reliable test platform to study molecular mechanisms of impaired erythropoiesis. This study illustrates how conditions resulting in a high catecholamine microenvironment such as burns can instigate myelo-erythroid reprioritization influenced by β-adrenergic stimulation leading to anemia. In a mouse model of scald burn injury, we observed, along with a threefold increase in bone marrow LSK cells (lin^neg Sca1⁺cKit⁺), that the myeloid shift is accompanied with a significant reduction in megakaryocyte erythrocyte progenitors (MEPs). β-Blocker administration (propranolol) for 6 days after burn, not only reduced the number of LSKs and MafB⁺ cells in multipotent progenitors, but also influenced myelo-erythroid bifurcation by increasing the MEPs and reducing the granulocyte monocyte progenitors in the bone marrow of burn mice. Furthermore, similar results were observed in burn patients' peripheral blood mononuclear cell-derived ex vivo culture system, demonstrating that commitment stage of erythropoiesis is impaired in burn patients and intervention with propranolol (nonselective β1,2-adrenergic blocker) increases MEPs. Also, MafB⁺ cells that were significantly increased following standard burn care could be mitigated when propranolol was administered to burn patients, establishing the mechanistic regulation of erythroid commitment by myeloid regulatory transcription factor MafB. Overall, results demonstrate that β-adrenergic blockers following burn injury can redirect the hematopoietic commitment toward erythroid lineage by lowering MafB expression in multipotent progenitors and be of potential therapeutic value to increase erythropoietin responsiveness in burn patients.

NF-κB inhibitors impair lung epithelial tight junctions in the absence of inflammation

NF-κB (p50/p65) is the best characterized transcription factor known to regulate cell responses to inflammation. However, NF-κB is also constitutively expressed. We used inhibitors of the classical NF-κB signaling pathway to determine whether this transcription factor has a role in regulating alveolar epithelial tight junctions. Primary rat type II alveolar epithelial cells were isolated and cultured on Transwell permeable supports coated with collagen for 5 d to generate a model type I cell monolayer. Treatment of alveolar epithelial monolayers overnight with one of 2 different IκB kinase inhibitors (BAY 11-7082 or BMS-345541) resulted in a dose-dependent decrease in TER at concentrations that did not affect cell viability. In response to BMS-345541 treatment there was an increase in total claudin-4 and claudin-5 along with a decrease in claudin-18, as determined by immunoblot. However, there was little effect on the total amount of cell-associated claudin-7, occludin, junctional adhesion molecule A (JAM-A), zonula occludens (ZO)-1 or ZO-2. Moreover, treatment with BMS-345541 resulted in altered tight junction morphology as assessed by immunofluorescence microscopy. Cells treated with BMS-345541 had an increase in claudin-18 containing projections emanating from tight junctions ("spikes") that were less prominent in control cells. There also were several areas of cell-cell contact which lacked ZO-1 and ZO-2 localization as well as rearrangements to the actin cytoskeleton in response to BMS-345541. Consistent with an anti-inflammatory effect, BMS-345541 antagonized the deleterious effects of lipopolysaccharide (LPS) on alveolar epithelial barrier function. However, BMS-345541 also inhibited the ability of GM-CSF to increase alveolar epithelial TER. These data suggest a dual role for NF-κB in regulating alveolar barrier function and that constitutive NF-κB function is required for the integrity of alveolar epithelial tight junctions.

Upregulation of GM-CSF by TGF-β1 in epithelial mesenchymal transition of human HERS/ERM cells

Hertwig's epithelial root sheath/epithelial rests of Malassez (HERS/ERM) have been suggested to play an important role in tooth root formation, particularly in periodontal development. Epithelial mesenchymal transition (EMT) has been suggested to contribute to root development in tooth. However, the mechanism of interaction between HERS/ERM cells and dental mesenchymal cells has not been fully understood. In this study, we investigated the effect of exogenous transforming growth factor beta 1 (TGF-β1) in human HERS/ERM cells in order to verify the role of granulocyte macrophage colony-stimulating factor (GM-CSF) in EMT process. Antibody array was used to screen secretion factors by exogenous TGF-β1. Secretion of GM-CSF was increased by exogenous TGF-β1. Expression levels of EMT markers, vimentin, ZEB1 (zinc finger E-box binding homeobox 1), and E-cadherin, were confirmed using reverse transcription polymerase chain reaction and immunocytochemistry. Treatment with GM-CSF increased the expression of vimentin and ZEB1, similar to TGF-β1 treatment, and decreased the expression of E-cadherin. Our results suggest that GM-CSF could induce EMT in human HERS/ERM cells.

The role of JAK-STAT signaling in adipose tissue function

Adipocytes play important roles in lipid storage, energy homeostasis and whole body insulin sensitivity. The JAK-STAT (Janus Kinase-Signal Transducer and Activator of Transcription) pathway mediates a variety of physiological processes including development, hematopoiesis, and inflammation. Although the JAK-STAT signaling pathway occurs in all cells, this pathway can mediate cell specific responses. Studies in the last two decades have identified hormones and cytokines that activate the JAK-STAT signaling pathway. These cytokines and hormones have profound effects on adipocytes. The content of this review will introduce the types of adipocytes and immune cells that make up adipose tissue, the impact of obesity on adipose cellular composition and function, and the general constituents of the JAK-STAT pathway and how its activators regulate adipose tissue development and physiology. A summary of the identification of STAT target genes in adipocytes reveals how these transcription factors impact various areas of adipocyte metabolism including insulin action, modulation of lipid stores, and glucose homeostasis. Lastly, we will evaluate exciting new data linking the JAK-STAT pathway and brown adipose tissue and consider the future outlook in this area of investigation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Bcl3 prevents acute inflammatory lung injury in mice by restraining emergency granulopoiesis

Granulocytes are pivotal regulators of tissue injury. However, the transcriptional mechanisms that regulate granulopoiesis under inflammatory conditions are poorly understood. Here we show that the transcriptional coregulator B cell leukemia/lymphoma 3 (Bcl3) limits granulopoiesis under emergency (i.e., inflammatory) conditions, but not homeostatic conditions. Treatment of mouse myeloid progenitors with G-CSF--serum concentrations of which rise under inflammatory conditions--rapidly increased Bcl3 transcript accumulation in a STAT3-dependent manner. Bcl3-deficient myeloid progenitors demonstrated an enhanced capacity to proliferate and differentiate into granulocytes following G-CSF stimulation, whereas the accumulation of Bcl3 protein attenuated granulopoiesis in an NF-κB p50-dependent manner. In a clinically relevant model of transplant-mediated lung ischemia reperfusion injury, expression of Bcl3 in recipients inhibited emergency granulopoiesis and limited acute graft damage. These data demonstrate a critical role for Bcl3 in regulating emergency granulopoiesis and suggest that targeting the differentiation of myeloid progenitors may be a therapeutic strategy for preventing inflammatory lung injury.

Alternative modes of GM-CSF receptor activation revealed using activated mutants of the common beta-subunit

Granulocyte/macrophage colony-stimulating factor promotes growth, survival, differentiation, and activation of normal myeloid cells and plays an important role in myeloid leukemias. The GM-CSF receptor (GMR) shares a signaling subunit, beta(c), with interleukin-3 and interleukin-5 receptors and has recently been shown to induce activation of Janus kinase 2 (JAK2) and downstream signaling via formation of a unique dodecameric receptor complex. In this study we use 2 activated beta(c) mutants that display distinct signaling capacity and have differential requirements for the GMR alpha-subunit (GMR-alpha) to dissect the signaling pathways associated with the GM-CSF response. The V449E transmembrane mutant selectively activates JAK2/signal transducer and activator of transcription 5 and extracellular signal-regulated kinase (ERK) pathways, resulting in a high level of sensitivity to JAK and ERK inhibitors, whereas the extracellular mutant (FIDelta) selectively activates the phosphoinositide 3-kinase/Akt and IkappaKbeta/nuclear factorkappaB pathways. We also demonstrate a novel and direct interaction between the SH3 domains of Lyn and Src with a conserved proline-rich motif in GMR-alpha and show a selective requirement for Src family kinases by the FIDelta mutant. We relate the nonoverlapping nature of signaling by the activated mutants to the structure of the unique GMR complex and propose alternative modes of receptor activation acting synergistically in the mature liganded receptor complex.

Granulocyte Macrophage-Colony Stimulating Factor receptor expression on human cardiomyocytes from end-stage heart failure patients

BACKGROUND

In remodelling ventricles, the progression of heart failure is associated with structural changes involving the extra-cellular matrix (ECM) and the cytoskeleton of cardiomyocytes, associated with fibrosis, cellular damage and death. The role of some cytokines and haematopoietic growth factors in the mechanism of both damage and regeneration of cardiac tissue during acute myocardial infarction has been demonstrated. Following heart damage, the development of scarred tissue was considered to be the only outcome, since myocytes were considered to be terminally differentiated cells. However, recent studies in animal models and adult human hearts have shown that myocytes can proliferate under the modulation of several factors.

AIMS

To assess Granulocyte Macrophage-Colony Stimulating Factor (GM-CSF) receptor expression in healthy and diseased human hearts, and to evaluate the possible role of GM-CSF and its receptor in the regeneration of cardiac tissue in chronic cardiomyopathy.

METHODS AND RESULTS

GM-CSFR expression in human cardiac tissue from explanted hearts of ten patients with end-stage heart failure and in cardiac biopsies from eight normal human hearts was studied by immunohistochemistry, and cellular and molecular biology assays. Our results demonstrated an increase in GM-CSFR in cardiomyocytes from end-stage heart failure tissues as compared to normal control tissues.

CONCLUSIONS

We hypothesize that GM-CSF plays a role in apoptotic and/or ECM deposition processes as well as in cytoskeleton modification in the myocardium.

The alpha subunit of the granulocyte-macrophage colony-stimulating factor receptor interacts with c-Kit and inhibits c-Kit signaling

The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) regulates hematopoiesis and the function of mature host defense cells through the GM-CSF receptor (GMR), which is composed of alpha (alphaGMR) and beta (betaGMR) subunits. Stem cell factor is another important hematopoietic cytokine that signals through c-Kit, a receptor tyrosine kinase, and regulates hematopoietic stem cell maintenance and erythroid development. Like other cytokine receptors, GMR and c-Kit are generally deemed as independent adaptor molecules capable of transducing cytokine-specific signals. We found that the alphaGMR directly interacts with c-Kit and that the interaction is mediated by the cytoplasmic domains. Furthermore, alphaGMR inhibited c-Kit auto-phosphorylation induced by the ligand stem cell factor. Consistent with the inhibitory effect, the expression of alphaGMR was suppressed in cells whose viability was dependent on c-Kit signaling. In contrast, the alternatively spliced alpha2 isoform of the alphaGMR could not inhibit c-Kit signaling, providing a rationale for the existence of the alpha2 isoform. Our results suggest that in addition to having the commonly appreciated roles in cytokine signal transduction, the receptors alphaGMR and c-Kit could interact to coordinate their signal initiation.

PI3-kinase activation by GM-CSF in endothelium is upstream of Jak/Stat pathway: Role of αGMR

GM-CSF has been identified as a growth factor for endothelial cells. In this study, we investigated the role of PI3-kinase pathway in mediating GM-CSF induced angiogenesis. GM-CSF induced tube formation in human umbilical vein endothelial cells, as examined using Matrigel assay, was inhibited by specific inhibitors of PI3-kinase, wortmannin, and LY294002. The regulatory subunit of PI3-kinase (p85) interacted with alphaGMR via its C-SH2 domain in a GM-CSF-dependent fashion with concomitant phosphorylation of p85 and activation of PI3-kinase pathway. p85 binding site on the alphaGMR was essential to induce GM-CSF receptor-dependent Stat activation. Furthermore, inhibition of PI3-kinase activity also abrogated GM-CSF induced Stat activation. These studies underscore the significance of the GM-CSF mediated PI3-kinase activation and its role in angiogenesis.

Cell biology of IL-7, a key lymphotrophin

IL-7 is essential for the development and survival of T lymphocytes. This review is primarily from the perspective of the cell biology of the responding T cell. Beginning with IL-7 receptor structure and regulation, the major signaling pathways appear to be via PI3K and Stat5, although the requirement for either has yet to be verified by published knockout experiments. The proliferation pathway induced by IL-7 differs from conventional growth factors and is primarily through posttranslational regulation of p27, a Cdk inhibitor, and Cdc25a, a Cdk-activating phosphatase. The survival function of IL-7 is largely through maintaining a favorable balance of bcl-2 family members including Bcl-2 itself and Mcl-1 on the positive side, and Bax, Bad and Bim on the negative side. There are also some remarkable metabolic effects of IL-7 withdrawal. Studies of IL-7 receptor signaling have yet to turn up unique pathways, despite the unique requirement for IL-7 in T cell biology. There remain significant questions regarding IL-7 production and the major producing cells have yet to be fully characterized.

Pulmonary alveolar proteinosis

Pulmonary alveolar proteinosis (PAP) has been recognized for almost half a century. At least three separate pathophysiologic mechanisms may lead to the characteristic feature of PAP: the excessive accumulation of surfactant lipoprotein in pulmonary alveoli, with associated disturbance of pulmonary gas exchange. The prognosis for adult patients with PAP varies, but disease-specific survival rate exceeds 80% at 5 years. The survival rates for adult PAP patients seem to have increased progressively in the four decades since the initial clinical description of this condition. The last decade has brought new advances in laboratory and clinical research that are lifting a veil not only on PAP but also on general aspects of pulmonary surfactant biology and innate immune defense.

Overlapping motifs in the membrane-proximal region of cytokine receptor accessory and signaling subunits

The membrane-proximal cytoplasmic region of cytokine receptors (CRs) is highly conserved and essential for receptor activation. In particular this region is essential for the activation of members of the Janus family of protein kinases (JAK) which results in initiation of receptor signaling. We have examined the sequence of this region in a number of CR signaling and accessory subunits with a view to better delineating motifs that play an important role in initiating receptor activity. Here, we have delineated two distinct proline-rich motifs in the membrane-proximal domains of cytokine receptors. Their configuration and distribution among CR subunits strongly suggest a model in which the two motifs act in a concerted manner to induce full receptor and JAK activation.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

The laminin receptor modulates granulocyte-macrophage colony-stimulating factor receptor complex formation and modulates its signaling

Basement membrane matrix proteins are known to up-regulate granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling in neutrophils and mononuclear phagocytes, but the mechanisms involved are poorly understood. We used the intracellular portion of the alpha subunit of the GM-CSF receptor (alphaGMR) to search for interacting proteins and identified the 67-kDa laminin receptor (LR), a nonintegrin matrix protein receptor expressed in several types of host defense cells and certain tumors, as a binding partner. LR was found to interact with the beta subunit of the GMR (betaGMR) as well. Whereas GM-CSF functions by engaging the alphaGMR and betaGMR into receptor complexes, LR inhibited GM-CSF-induced receptor complex formation. Laminin and fibronectin binding to LR was found to prevent the binding of betaGMR to LR and relieved the LR inhibition of GMR. These findings provide a mechanistic basis for enhancing host defense cell responsiveness to GM-CSF at transendothelial migration sites while suppressing it in circulation.

GMCSF activates NF-kappaB via direct interaction of the GMCSF receptor with IkappaB kinase beta

Granulocyte-macrophage colony-stimulating factor (GMCSF) has a central role in proliferation and differentiation of hematopoetic cells. Furthermore, it influences the proliferation and migration of endothelial cells. GMCSF elicits these functions by activating a receptor consisting of a ligand-specific alpha-chain and a beta-chain, which is common for GMCSF, interleukin-3 (IL-3), and IL-5. It is known that various signaling molecules such as Janus kinase 2 or transcription factors of the signal transducer and activator of transcription (STAT) family bind to the common beta-chain and initiate signaling cascades. However, alpha-chain-specific signal transduction adapters have to be postulated given that IL-3, IL-5, and GMCSF induce partly distinct biologic responses. Using a yeast 2-hybrid system, we identified the alpha-chain of the GMCSF receptor (GMRalpha) as putative interaction partner of IkappaB kinase beta, one of the central signaling kinases activating the transcription factor nuclear factor-kappaB (NF-kappaB). Using endogenous protein levels of endothelial cell extracts, we could verify the interaction by coimmunoprecipitation experiments. Fluorescence resonance energy transfer (FRET) microscopy confirmed the direct interaction of CFP-IKKbeta and YFPGMRalpha in living cells. Functional studies demonstrated GMCSF-dependent activation of IkappaB kinase activity in endothelial cells, degradation of IkappaB, and activation of NF-kappaB. Further biologic studies using GMCSF-dependent TF-1 cells indicated that GMCSF-triggered activation of NF-kappaB is important for cell survival and proliferation.

Targeting Raf-1 gene expression by a DNA enzyme inhibits juvenile myelomonocytic leukemia cell growth

Juvenile myelomonocytic leukemia (JMML) is an aggressive childhood disorder with few therapeutic options. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor-alpha (TNF-alpha) promote JMML cell growth. A hyperactive function of the ras oncogene is a hallmark of JMML. We therefore targeted the protein kinase Raf-1 downstream of Ras using a DNA enzyme that degrades mRNA-Raf-1. Western blots of JMML cell lysates revealed phosphorylated Raf-1 protein, indicating constitutive activation. Addition of GM-CSF, but not TNF-alpha, increased phosphorylation of both Raf-1 and the mitogen-activated protein kinases (MAPKs) JNK-1 and ERK-1. Depletion of Raf-1 protein markedly impaired activation of MAPKs, induced substantial inhibition of JMML cell colony formation, and virtually abolished GM-CSF hypersensitivity in JMML cells. Exogenous TNF-alpha, but not GM-CSF, restored colony formation of JMML cells pretreated with the enzyme. We could not detect any effect of the enzyme on the proliferation of normal bone marrow cells, indicating its specificity and potential safety. When immunodeficient mice engrafted with JMML cells were treated continuously with the enzyme via a peritoneal osmotic mini-pump for 4 weeks, a profound reduction in the JMML cell numbers in the recipient murine bone marrows was found. We conclude that GM-CSF is a chief regulator of JMML growth and exerts its proleukemic effects primarily via the Ras/Raf-1 signaling cascade. TNF-alpha plays a permissive role, being dependent upon GM-CSF to induce JMML cell proliferation. The DNA enzyme efficiently catabolized mRNA-Raf-1 with subsequent inhibition of JMML cell growth, suggesting its potential as a mechanism-based therapy in this fatal leukemia.

Uncoupling of proliferation and Stat5 activation in thymic stromal lymphopoietin-mediated signal transduction

Thymic stromal lymphopoietin (TSLP) is a cytokine that facilitates B lymphocyte differentiation and costimulates T cells. Previous studies have demonstrated that a functional TSLP receptor complex is a heterodimer consisting of the TSLP receptor and the IL-7R alpha-chain. TSLP-mediated signaling is unique among members of the cytokine receptor family in that activation of the transcription factor Stat5 occurs without detectable Janus kinase activation. Using a variety of biological systems we demonstrate here that TSLP-mediated Stat5 activation can be uncoupled from proliferation. We also show that the single tyrosine residue in the cytoplasmic domain of the TSLP receptor is critical for TSLP-mediated proliferation, but is dispensable for Stat5 activation. Our data demonstrate that TSLP-mediated Stat5 activation is insufficient for cell proliferation and identifies residues within the TSLP receptor complex required to mediate these downstream events.

In vivo activation of JAK2/STAT-3 pathway during angiogenesis induced by GM-CSF

Besides the regulation of hematopoiesis, granulocyte-macrophage colony-stimulating factor (GM-CSF) induces the expression of a functional program in cultured endothelial cells (ECs) related to angiogenesis and to the their survival in bone marrow microenvironment. ECs express the specific GM-CSF receptor that signals through the recruitment and the activation of Janus kinase (JAK)2 (Soldi et al., Blood 89, 863-872, 1987). We now report that GM-CSF in vivo induces angiogenesis and activates JAK-2 and signal transducers and activators of transcription (STAT)-3. This cytokine has an angiogenetic activity in chick chorioallantoic membrane (CAM) without recruitment of inflammatory cells and induces vessel sprouting from chicken aorta rings. When added to CAM, subnanomolar concentrations of GM-CSF cause a rapid phosphorylation in tyrosine residues of JAK-2 persisting at least for 10 min. Furthermore, we show that signal transducers and activators of transcription (STAT)-3, but not STAT-5, also are phosphorylated for 30 min after GM-CSF stimulation. AG-490, a JAK-2 inhibitor, reduced in a dose-dependent manner the angiogenic effect of GM-CSF in CAM. These findings provide the first evidence that the JAK-2/STAT-3 pathway is activated in vivo and participates in vessel formation triggered by GM-CSF.

IL-3 signaling and the role of Src kinases, JAKs and STATs: a covert liaison unveiled

Hematopoiesis is the cumulative result of intricately regulated signal transduction cascades that are mediated by cytokines and their cognate receptors. Proper culmination of these diverse signaling pathways forms the basis for an orderly generation of different cell types and aberrations in these pathways is an underlying cause for diseases such as cancer. Over the past several years, downstream events initiated upon cytokine/growth factor stimulation have been a major focus of biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of novel transcription factors, termed signal transducers and activators of transcription (STATs) appear to orchestrate the downstream events propagated by cytokine/growth factor interactions with their cognate receptors. Until recently, the JAK proteins were considered to be the tyrosine kinases, which dictated the levels of phosphorylation and activation of STAT proteins, forming the basis of the JAK-STAT model. However, over the past few years, increasing evidence has accumulated which indicates that at least some of the STAT protein activation may be mediated by members of the Src gene family following cytokine/growth factor stimulation. Studies have demonstrated that the Src-family of tyrosine kinases can phosphorylate and activate certain STAT proteins, in lieu of JAK kinases. In such a scenario, JAK kinases may be more crucial to phosphorylation of the cytokine/growth factor receptors and in the process create docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Tyrosine phosphorylation and activation of STAT proteins can be achieved either by JAKs or Src-kinases depending on the nature of STAT that is being activated. This forms the basis for the JAK-Src-STAT model proposed in this review. The concerted action of JAK kinases, members of the Src-kinase family and STAT proteins, leads to cell proliferation and cell survival, the end-point of the cytokine/growth factor stimulus. Oncogene (2000).

Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activates several kinases and transcription factors through interaction with a heterodimeric receptor complex. We previously demonstrated that phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein, CREB, occurs through a protein kinase A-independent pathway and is required for GM-CSF–induced transcriptional activation of the immediate early gene, early growth response-1 (egr-1). Recent reports indicate that receptor tyrosine kinases can induce CREB phosphorylation through activation of pp90RSK. We performed immune complex kinase assays in the human myeloid leukemic cell line, TF-1, which revealed that GM-CSF induced pp90RSK activation and phosphorylation of CREB within 5 minutes of stimulation. Transfection with the kinase-defective pp90RSK expression plasmid demonstrated a statistically significant decrease in transcriptional activation of a −116 CAT/egr-1 promoter construct in response to GM-CSF. Furthermore, activation of pp90RSK, CREB and egr-1in GM-CSF–treated cells was inhibited by the presence of the inhibitor, PD98059. In this study, we report that GM-CSF induces CREB phosphorylation and egr-1 transcription by activating pp90RSK through an MEK-dependent signaling pathway.

Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activates several kinases and transcription factors through interaction with a heterodimeric receptor complex. We previously demonstrated that phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein, CREB, occurs through a protein kinase A-independent pathway and is required for GM-CSF–induced transcriptional activation of the immediate early gene, early growth response-1 (egr-1). Recent reports indicate that receptor tyrosine kinases can induce CREB phosphorylation through activation of pp90RSK. We performed immune complex kinase assays in the human myeloid leukemic cell line, TF-1, which revealed that GM-CSF induced pp90RSK activation and phosphorylation of CREB within 5 minutes of stimulation. Transfection with the kinase-defective pp90RSK expression plasmid demonstrated a statistically significant decrease in transcriptional activation of a −116 CAT/egr-1 promoter construct in response to GM-CSF. Furthermore, activation of pp90RSK, CREB and egr-1in GM-CSF–treated cells was inhibited by the presence of the inhibitor, PD98059. In this study, we report that GM-CSF induces CREB phosphorylation and egr-1 transcription by activating pp90RSK through an MEK-dependent signaling pathway.

Analysis of signals and functions of the chimeric human granulocyte-macrophage colony-stimulating factor receptor in BA/F3 cells and transgenic mice

Receptors for GM-CSF, IL-3, and IL-5 are composed of two subunits: alpha, which is specific for each cytokine, and betac, which is shared by all. Although the role of betac in signal transduction has been extensively studied, the role of the alpha subunit has remained to be clarified. To analyze the role of the human (h) GM-CSF receptor alpha subunit, we constructed a chimeric receptor subunit composed of extracellular and transmembrane regions of alpha fused with the cytoplasmic region of betac, designated alpha/beta. In BA/F3 cells, chimeric receptor composed of alpha/beta,beta can transduce signals for mitogen-activated protein kinase cascade activation and proliferation in response to hGM-CSF. Although phosphorylation of Jak1 but not of Jak2 occurred with stimulation of hGM-CSF, the dominant-negative Jak2 but not the dominant-negative Jak1 suppresses c-fos promoter activation. To determine whether the chimeric receptor alpha/beta,beta is functional in vivo, we developed transgenic mice expressing the chimeric receptor alpha/beta,beta. Bone marrow cells from the transgenic mice expressing the alpha/beta,beta receptor form not only GM colonies but also various lineages of colonies in response to GM-CSF. In addition, mast cells were produced when bone marrow cells of the transgenic mouse were cultured with hGM-CSF. Thus, it appears that the cytoplasmic region of the alpha subunit is not required for hGM-CSF promoting activities, even in bone marrow cells.

A model for assembly and activation of the GM-CSF, IL-3 and IL-5 receptors: insights from activated mutants of the common beta subunit

Granulocyte-macrophage colony stimulating factor (GM-CSF), Interleukin-3 (IL-3) and Interleukin-5 (IL-5) have overlapping, pleiotropic effects on hematopoietic cells, including neutrophils, eosinophils, monocytes and early progenitor cells. The high-affinity receptors for human GM-CSF, IL-3, and IL-5 share a common beta-subunit (hbeta(c)), which is essential for signalling and plays a major role in recruiting intracellular signalling molecules. While activation of the cytoplasmic tyrosine kinase JAK2 appears to be the initiating event for signalling, the immediate events that trigger this are still unclear. We have isolated a number of activated mutants of hbeta(c), which can be grouped into classes defined by their state of receptor phosphorylation, their requirement for alpha subunit as a cofactor, and their activities in primary cells and cell lines. We discuss these findings with regard to the stoichiometry, activation, and signalling of the normal GM-CSF/IL-3/IL-5 receptor complexes. Specifically, this work has implications for the role of the ligand-specific alpha-subunits in initiating the signalling through the beta-subunit, the role of beta subunit dimerization as a receptor trigger, and the function of receptor tyrosine phosphorylation in generating growth and survival signals. Based on the properties of the activated mutants and the recent structures of erythropoietin receptor (Epo-R) complexes, we propose a model in which (1) activation of hbeta(c) can occur via alternative states that differ with respect to stoichiometry and subunit assembly, but which all mediate proliferative responses, and (2) each of the different classes of activated mutants mimics one of these alternative states.

Nf1 and Gmcsf interact in myeloid leukemogenesis

The NF1 tumor suppressor gene encodes neurofibromin, a GTPase-activating protein (GAP) for p21ras (Ras). Children with NF1 are predisposed to juvenile myelomonocytic leukemia (JMML). Some heterozygous Nf1 mutant mice develop a similar myeloproliferative disorder (MPD), and adoptive transfer of Nf1-deficient fetal liver cells consistently induces this MPD. Human JMML and murine Nf1-deficient cells are hypersensitive to granulocyte-macrophage colony-stimulating factor (GM-CSF) in methylcellulose cultures. We generated hematopoietic cells deficient in both Nf1 and Gmcsf to test whether GM-CSF is required to drive excessive proliferation of Nf1-/- cells in vivo. Here we show that GM-CSF play a central role in establishing and maintaining the MPD and that recipients engrafted with Nf1-/- Gmcsf-/- hematopoietic cells are hypersensitive to exogenous GM-CSF.

Functional analysis of a single chain chimeric alpha/beta-granulocyte-macrophage colony-stimulating factor receptor. Importance of a glutamate residue in the transmembrane region

To analyze the function of each subunit of the receptor for granulocyte-macrophage colony-stimulating factor (GM-CSF), GMR, we previously generated a single-chain chimeric receptor by fusion of the extracellular and transmembrane domain from the alpha-subunit (alpha-GMR) to the intracellular part of the beta-subunit (beta-GMR) introducing an additional glutamate residue at the fusion site (alpha/beta-GMR). We demonstrated the capacity of alpha/beta-GMR to bind GM-CSF with low affinity and to induce GM-CSF-dependent activation of tyrosine kinase activity and proliferation in transfected Ba/F3 cells. To further compare the functions of wild type and chimeric receptors, we now report that this alpha/beta-GMR is sufficient to mediate morphological changes, expression of alpha(4)- and beta(1)-integrin receptor subunits, and serine-phosphorylation of Akt kinase. To analyze the function of the glutamate residue at the fusion region of alpha/beta-GMR various point mutants changing this amino acid and its position were expressed in Ba/F3 cells. None of these mutants was capable of supporting GM-CSF-dependent proliferation; however, when beta-GMR was coexpressed, GM-CSF mediated short and long term proliferation. Interestingly, some mutants but not alpha/beta-GMR can induce proliferation in the presence of an anti-alpha-GMR antibody. These data demonstrate the significance of a glutamate residue in the transmembrane region of alpha/beta-GMR for ligand-induced receptor activation.

Heterodimerization of the  and β Chains of the Interleukin-3 (IL-3) Receptor Is Necessary and Sufficient for IL-3–Induced Mitogenesis

The high-affinity receptor for interleukin-3 (IL-3) is a complex of the IL-3–binding subunit (IL-3) and a larger β chain—βc, or, in the mouse, βc or its close relative βIL-3. There is evidence that the critical event that initiates signaling is not the approximation of the cytoplasmic domains of IL-3 and βIL-3, but is, rather, the formation of a β-β homodimer. Many of these studies involved the analyses of receptor chimeras where the cytoplasmic domains were derived from IL-3, βc or βIL-3, and the extracellular domains were derived from other cytokine receptors, such as the erythropoietin receptor (EpoR). However, evidence that the EpoR may also associate with other receptors clouds the interpretation of these experiments. Therefore, we reevaluated the structure of the functional IL-3R using chimeric receptors with extracellular domains derived not from members of the cytokine-receptor family, but from CD8 or CD16. We show, by expression of these chimeras in Ba/F3 or CTLL-2 cells, that mitogenic signals were only generated by heterodimerization of the cytoplasmic domains of IL-3 and βIL-3. Homodimers of either IL-3 or βIL-3, alone or in combination, were nonfunctional. Furthermore, the ability of heterodimers to stimulate mitogenesis correlated with their ability to induce tyrosine phosphorylation of JAK-2. These data suggest that the physiological activation of the IL-3R involves the generation of simple heterodimers of IL-3 and βIL-3.

Activation of Granulocyte-Macrophage Colony-Stimulating Factor and Interleukin-3 Receptor Subunits in a Multipotential Hematopoietic Progenitor Cell Line Leads to Differential Effects on Development

Activation of specific cytokine receptors promotes survival and proliferation of hematopoietic progenitor cells but their role in the control of differentiation is unclear. To address this issue, the effects of human interleukin-3 (hIL-3) and human granulocyte-macrophage colony-stimulating factor (hGM-CSF) on hematopoietic development were investigated in hematopoietic progenitor cells. Murine multipotent factor-dependent cell-Paterson (FDCP)-mix cells, which can self-renew or differentiate, were transfected with the genes encoding the unique  and/or shared βc human hIL-3 receptor (hIL-3 R) or hGM-CSF receptor (hGM R) subunits by retroviral gene transfer. Selective activation of hIL-3 R,βc or hGM R,βc transfects by hIL-3 and hGM-CSF promoted self-renewal and myeloid differentiation, respectively, over a range of cytokine (0.1 to 100 ng/mL) concentrations. These qualitatively distinct developmental outcomes were associated with different patterns of protein tyrosine phosphorylation and, thus, differential signaling pathway activation. The cell lines generated provide a model to investigate molecular events underlying self-renewal and differentiation and indicate that the  subunits act in combination with the hβc to govern developmental decisions. The role of the  subunit in conferring specificity was studied by using a chimeric receptor composed of the extracellular hIL-3 R and intracellular hGM R subunit domains. This receptor promoted differentiation in response to hIL-3. Thus, the  subunit cytosolic domain is an essential component in determining cell fate via specific signaling events.

Activation of Granulocyte-Macrophage Colony-Stimulating Factor and Interleukin-3 Receptor Subunits in a Multipotential Hematopoietic Progenitor Cell Line Leads to Differential Effects on Development

Activation of specific cytokine receptors promotes survival and proliferation of hematopoietic progenitor cells but their role in the control of differentiation is unclear. To address this issue, the effects of human interleukin-3 (hIL-3) and human granulocyte-macrophage colony-stimulating factor (hGM-CSF) on hematopoietic development were investigated in hematopoietic progenitor cells. Murine multipotent factor-dependent cell-Paterson (FDCP)-mix cells, which can self-renew or differentiate, were transfected with the genes encoding the unique  and/or shared βc human hIL-3 receptor (hIL-3 R) or hGM-CSF receptor (hGM R) subunits by retroviral gene transfer. Selective activation of hIL-3 R,βc or hGM R,βc transfects by hIL-3 and hGM-CSF promoted self-renewal and myeloid differentiation, respectively, over a range of cytokine (0.1 to 100 ng/mL) concentrations. These qualitatively distinct developmental outcomes were associated with different patterns of protein tyrosine phosphorylation and, thus, differential signaling pathway activation. The cell lines generated provide a model to investigate molecular events underlying self-renewal and differentiation and indicate that the  subunits act in combination with the hβc to govern developmental decisions. The role of the  subunit in conferring specificity was studied by using a chimeric receptor composed of the extracellular hIL-3 R and intracellular hGM R subunit domains. This receptor promoted differentiation in response to hIL-3. Thus, the  subunit cytosolic domain is an essential component in determining cell fate via specific signaling events.

Heterodimerization of the  and β Chains of the Interleukin-3 (IL-3) Receptor Is Necessary and Sufficient for IL-3–Induced Mitogenesis

The high-affinity receptor for interleukin-3 (IL-3) is a complex of the IL-3–binding subunit (IL-3) and a larger β chain—βc, or, in the mouse, βc or its close relative βIL-3. There is evidence that the critical event that initiates signaling is not the approximation of the cytoplasmic domains of IL-3 and βIL-3, but is, rather, the formation of a β-β homodimer. Many of these studies involved the analyses of receptor chimeras where the cytoplasmic domains were derived from IL-3, βc or βIL-3, and the extracellular domains were derived from other cytokine receptors, such as the erythropoietin receptor (EpoR). However, evidence that the EpoR may also associate with other receptors clouds the interpretation of these experiments. Therefore, we reevaluated the structure of the functional IL-3R using chimeric receptors with extracellular domains derived not from members of the cytokine-receptor family, but from CD8 or CD16. We show, by expression of these chimeras in Ba/F3 or CTLL-2 cells, that mitogenic signals were only generated by heterodimerization of the cytoplasmic domains of IL-3 and βIL-3. Homodimers of either IL-3 or βIL-3, alone or in combination, were nonfunctional. Furthermore, the ability of heterodimers to stimulate mitogenesis correlated with their ability to induce tyrosine phosphorylation of JAK-2. These data suggest that the physiological activation of the IL-3R involves the generation of simple heterodimers of IL-3 and βIL-3.

Characterization of the cytoplasmic domain of interleukin-13 receptor-alpha

Interleukin (IL)-13 and IL-4 are pleiotropic immunoregulatory cytokines that share many overlapping biological properties reflecting the fact that both can utilize a receptor complex composed of the IL-4 receptor-alpha (IL-4Ralpha) chain and the IL-13Ralpha chain. The cytoplasmic domain of the IL-13Ralpha is 60 amino acids long and is essential for IL-13-dependent growth. It contains a Pro-rich domain in the membrane-proximal region and two Tyr residues. Here we show that a truncated IL-13Ralpha, lacking the 38 carboxyl-terminal residues but retaining the Pro-rich region, can support IL-13-dependent proliferation, although with reduced efficiency. A Y402F mutant of the cytoplasmic domain of IL-13Ralpha supported normal IL-13-induced growth. However, tyrosine phosphorylation of signal transducer and activator of transcription 3 (STAT3), which we show is induced by IL-13 and IL-4 in cells that express the IL-13Ralpha, was significantly reduced. The cytoplasmic domain of IL-13Ralpha was constitutively associated with STAT3, Tyk2, and Janus kinase 1 (JAK1). IL-13-induced tyrosine phosphorylation of IL-13Ralpha in vivo could not be detected using anti-Tyr(P) antibodies. A glutathione S-transferase fusion protein of the cytoplasmic domain of IL-13Ralpha was phosphorylated on tyrosine in vitro by JAK1, JAK3, and Tyk2, although the tyrosine phosphorylation events mediated by Tyk2 and JAK3 were not detectable using anti-phosphotyrosine antibodies. These data, together with the demonstration that IL-13Ralpha associates constitutively with Tyk2 and that Tyr-402 is involved in IL-13-induced phosphorylation of STAT3, suggest that the latter is mediated by Tyk2. Tyrosine phosphorylation of STAT3, which was not necessary for IL-13-induced proliferation, may account for some of the effects of IL-4 and IL-13 on the function of their targets.