Granulocyte-macrophage colony-stimulating factor provokes RAS activation and transcription of c-fos through different modes of signaling

Two Cases of Juvenile Myelomonocytic Leukemia and Myelin Oligodendrocyte Glycoprotein Antibody-Associated Disease

BACKGROUND

Myelin oligodendrocyte glycoprotein (MOG) antibody-associated disease (MOGAD) is an autoimmune demyelinating disorder that often manifests after infections or vaccinations. We report two patients who developed MOGAD out of eight patients with juvenile myelomonocytic leukemia (JMML) that has never been reported.

METHODS

We investigated two patients with JMML who developed MOGAD among 127 patients with leukemia from 2012 to 2021.

RESULTS

Patient 1 was treated for JMML and developed fever and impaired consciousness at two years and one month of age. Magnetic resonance imaging revealed high-intensity lesions in the left frontal and left occipital white matter. The serum anti-MOG antibody test was positive, while the test was negative in the stored serum 45 days before the onset of encephalopathy. He had relapse of MOGAD after steroid therapy and plasmapheresis. Patient 2, who was treated for JMML, became apathetic and mute at three years and seven months of age. Magnetic resonance imaging revealed left frontoparietal subcortical high-intensity lesions. Anti-MOG antibody at the onset of encephalopathy was positive, while it was negative in stored serum 57 days before and 47 days after the onset.

CONCLUSION

We treated two patients who developed MOGAD out of eight patients with JMML and none with MOGAD out of 119 patients with acute lymphocytic leukemia, acute myelocytic leukemia, or chronic myelocytic leukemia. The activated autoimmune process via the RAS pathway abnormality may have led to the formation of the anti-MOG antibody and the onset of MOGAD. MOGAD can occur in children with JMML, and abnormalities of the RAS pathway possibly contribute to its onset.

IL-3-Induced Immediate Expression of c-fos and c-jun Is Modulated by the IKK2-JNK Axis

Interleukin (IL)-3 is a pleiotropic cytokine that regulates the survival, proliferation, and differentiation of hematopoietic cells. The binding of IL-3 to its receptor activates intracellular signaling, inducing transcription of immediate early genes (IEGs) such as c-fos, c-jun, and c-myc; however, transcriptional regulation under IL-3 signaling is not fully understood. This study assessed the role of the inhibitor of nuclear factor-κB kinases (IKKs) in inducing IL-3-mediated expression of IEGs. We show that IKK1 and IKK2 are required for the IL-3-induced immediate expression of c-fos and c-jun in murine hematopoietic Ba/F3 cells. Although IKK2 is well-known for its pivotal role as a regulator of the canonical nuclear factor-κB (NF-κB) pathway, activation of IKKs did not induce the nuclear translocation of the NF-κB transcription factor. We further revealed the important role of IKK2 in the activation of c-Jun N-terminal kinase (JNK), which mediates the IL-3-induced expression of c-fos and c-jun. These findings indicate that the IKK2-JNK axis modulates the IL-3-induced expression of IEGs in a canonical NF-κB-independent manner.

SHP-2 in Lymphocytes' Cytokine and Inhibitory Receptor Signaling

Somewhat counterintuitively, the tyrosine phosphatase SHP-2 (SH2 domain-containing protein tyrosine phosphatase-2) is crucial for the activation of extracellular signal-regulated kinase (ERK) downstream of various growth factor receptors, thereby exerting essential developmental functions. This phosphatase also deploys proto-oncogenic functions and specific inhibitors have recently been developed. With respect to the immune system, the role of SHP-2 in the signaling of cytokines relevant for myelopoiesis and myeloid malignancies has been intensively studied. The function of this phosphatase downstream of cytokines important for lymphocytes is less understood, though multiple lines of evidence suggest its importance. In addition, SHP-2 has been proposed to mediate the suppressive effects of inhibitory receptors (IRs) that sustain a dysfunctional state in anticancer T cells. Molecules involved in IR signaling are of potential pharmaceutical interest as blockade of these inhibitory circuits leads to remarkable clinical benefit. Here, we discuss the dichotomy in the functions ascribed to SHP-2 downstream of cytokine receptors and IRs, with a focus on T and NK lymphocytes. Further, we highlight the importance of broadening our understanding of SHP-2′s relevance in lymphocytes, an essential step to inform on side effects and unanticipated benefits of its therapeutic blockade.

The role of IL-3 in bone

In the recent past, there has been a burgeoning interest in targeting cytokines such as IL-3 for specific disease conditions of bone such as rheumatoid arthritis and multiple myeloma. Unlike other cytokines, IL-3 is a cytokine with a multilineage potential and broad spectrum of target cells and it plays a vital role in hematopoiesis. Due to its common receptor subunit, the action of IL-3 shows functional redundancy with other cytokines such as the granulocyte-macrophage colony-stimulating factor and IL-5. IL-3 has been successfully used in ameliorating radiation-induced bone marrow aplasia and similar conditions. However, the role of IL-3 in bone cells has not been fully unraveled yet; therefore, the aim of this overview is to present the effects of IL-3 in bone with a special emphasis on osteoclasts and osteoblasts in a concise manner.

Regulation of T-Cell Signaling by Post-Translational Modifications in Autoimmune Disease

The adaptive immune system involves antigen-specific host defense mechanisms mediated by T and B cells. In particular, CD4⁺ T cells play a central role in the elimination of pathogens. Immunological tolerance in the thymus regulates T lymphocytes to avoid self-components, including induction of cell death in immature T cells expressing the self-reactive T-cell receptor repertoire. In the periphery, mature T cells are also regulated by tolerance, e.g., via induction of anergy or regulatory T cells. Thus, T cells strictly control intrinsic signal transduction to prevent excessive responses or self-reactions. If the inhibitory effects of T cells on these mechanisms are disrupted, T cells may incorrectly attack self-components, which can lead to autoimmune disease. The functions of T cells are supported by post-translational modifications, particularly phosphorylation, of signaling molecules, the proper regulation of which is controlled by endogenous mechanisms within the T cells themselves. In recent years, molecular targeted agents against kinases have been developed for treatment of autoimmune diseases. In this review, we discuss T-cell signal transduction in autoimmune disease and provide an overview of acetylation-mediated regulation of T-cell signaling pathways.

Interleukins, from 1 to 37, and interferon-γ: receptors, functions, and roles in diseases

Advancing our understanding of mechanisms of immune regulation in allergy, asthma, autoimmune diseases, tumor development, organ transplantation, and chronic infections could lead to effective and targeted therapies. Subsets of immune and inflammatory cells interact via ILs and IFNs; reciprocal regulation and counter balance among T(h) and regulatory T cells, as well as subsets of B cells, offer opportunities for immune interventions. Here, we review current knowledge about ILs 1 to 37 and IFN-γ. Our understanding of the effects of ILs has greatly increased since the discoveries of monocyte IL (called IL-1) and lymphocyte IL (called IL-2); more than 40 cytokines are now designated as ILs. Studies of transgenic or knockout mice with altered expression of these cytokines or their receptors and analyses of mutations and polymorphisms in human genes that encode these products have provided important information about IL and IFN functions. We discuss their signaling pathways, cellular sources, targets, roles in immune regulation and cellular networks, roles in allergy and asthma, and roles in defense against infections.

The role of interchain heterodisulfide formation in activation of the human common beta and mouse betaIL-3 receptors

The cytokines, interleukin-3 (IL-3), interleukin-5 (IL-5), and granulocyte-macrophage colony-stimulating factor (GM-CSF), exhibit overlapping activities in the regulation of hematopoietic cells. In humans, the common beta (betac) receptor is shared by the three cytokines and functions together with cytokine-specific alpha subunits in signaling. A widely accepted hypothesis is that receptor activation requires heterodisulfide formation between the domain 1 D-E loop disulfide in human betac (hbetac) and unidentified cysteine residues in the N-terminal domains of the alpha receptors. Since the development of this hypothesis, new data have been obtained showing that domain 1 of hbetac is part of the cytokine binding epitope of this receptor and that an IL-3Ralpha isoform lacking the N-terminal Ig-like domain (the "SP2" isoform) is competent for signaling. We therefore investigated whether distortion of the domain 1-domain 4 ligand-binding epitope in hbetac and the related mouse receptor, beta(IL-3), could account for the loss of receptor signaling when the domain 1 D-E loop disulfide is disrupted. Indeed, mutation of the disulfide in hbetac led to both a complete loss of high affinity binding with the human IL-3Ralpha SP2 isoform and of downstream signaling. Mutation of the orthologous residues in the mouse IL-3-specific receptor, beta(IL-3), not only precluded direct binding of mouse IL-3 but also resulted in complete loss of high affinity binding and signaling with the mouse IL-3Ralpha SP2 isoform. Our data are most consistent with a role for the domain 1 D-E loop disulfide of hbetac and beta(IL-3) in maintaining the precise positions of ligand-binding residues necessary for normal high affinity binding and signaling.

The Ig-like domain of human GM-CSF receptor α plays a critical role in cytokine binding and receptor activation

GM-CSF (granulocyte/macrophage colony-stimulating factor) is an important mediator of inducible haemopoiesis and inflammation, and has a critical role in the function of alveolar macrophages. Its clinical applications include the mobilization of haemopoietic progenitors, and a role as an immune stimulant and vaccine adjuvant in cancer patients. GM-CSF signals via a specific α receptor (GM-CSFRα) and the shared hβc (human common β-subunit). The present study has investigated the role of the Ig-like domain of GM-CSFRα in GM-CSF binding and signalling. Deletion of the Ig-like domain abolished direct GM-CSF binding and decreased growth signalling in the presence of hβc. To locate the specific residues in the Ig-like domain of GM-CSFRα involved in GM-CSF binding, a structural alignment was made with a related receptor, IL-13Rα1 (interleukin-13 receptor α1), whose structure and mode of interaction with its ligand has recently been elucidated. Mutagenesis of candidate residues in the predicted region of interaction identified Val51 and Cys60 as having critical roles in binding to the α receptor, with Arg54 and Leu55 also being important. High-affinity binding in the presence of hβc was strongly affected by mutation of Cys60 and was also reduced by mutation of Val51, Arg54 and Leu55. Of the four key residues, growth signalling was most severely affected by mutation of Cys60. The results indicate a previously unrecognized role for the Ig-like domain, and in particular Cys60, of GM-CSFRα in the binding of GM-CSF and subsequent activation of cellular signalling.

Cross-talk between ICAM-1 and granulocyte-macrophage colony-stimulating factor receptor signaling modulates eosinophil survival and activation

Reversal of eosinophilic inflammation has been an elusive therapeutic goal in the management of asthma pathogenesis. In this regard, GM-CSF is a primary candidate cytokine regulating eosinophil activation and survival in the lung; however, its molecular mechanism of propagation and maintenance of stimulated eosinophil activation is not well understood. In this study, we elucidate those late interactions occurring between the GM-CSF receptor and activated eosinophil signaling molecules. Using coimmunoprecipitation with GM-CSF-stimulated eosinophils, we have identified that the GM-CSF receptor beta-chain (GMRbeta) interacted with ICAM-1 and Shp2 phosphatase, as well as Slp76 and ADAP adaptor proteins. Separate experiments using affinity binding with a tyrosine-phosphorylated peptide containing an ITIM (ICAM-1 residues 480-488) showed binding to Shp2 phosphatase and GMRbeta. However, the interaction of GMRbeta with the phosphorylated ICAM-1-derived peptide was observed only with stimulated eosinophil lysates, suggesting that the interaction of GMRbeta with ICAM-1 required phosphorylated Shp2 and/or phosphorylated GMRbeta. Importantly, we found that inhibition of ICAM-1 in activated eosinophils blocked GM-CSF-induced expression of c-fos, c-myc, IL-8, and TNF-alpha. Moreover, inhibition of ICAM-1 expression with either antisense oligonucleotide or an ICAM-1-blocking Ab effectively inhibited ERK activation and eosinophil survival. We concluded that the interaction between ICAM-1 and the GM-CSF receptor was essential for GM-CSF-induced eosinophil activation and survival. Taken together, these results provide novel mechanistic insights defining the interaction between ICAM-1 and the GM-CSF receptor and highlight the importance of targeting ICAM-1 and GM-CSF/IL-5/IL-3 receptor systems as a therapeutic strategy to counter eosinophilia in asthma.

The Shc-binding site of the betac subunit of the GM-CSF/IL-3/IL-5 receptors is a negative regulator of hematopoiesis

Tyrosine and serine phosphorylation of the common beta chain (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is widely viewed as a general mechanism that provides positive inputs by coupling the receptor to signaling pathways that stimulate several cellular functions. We show here that despite the known action of Tyr577 in beta(c) to recruit Shc-PI-3 kinase (PI3K) pathway members, Tyr577 plays, surprisingly, a negative regulatory role in cell function, and that this is mediated, at least in part, through the uncoupling of SH2-containing inositol 5'-phosphatase (SHIP) from beta(c). Fetal liver cells from beta(c)/beta(IL-3)(-/-) mice expressing human GM-CSF receptor alpha chain and beta(c) Tyr577Phe mutant showed enhanced colony formation and expansion of progenitor cells in response to GM-CSF. Dissection of these activities revealed that basal survival was increased, as well as cytokine-stimulated proliferation. As expected, the recruitment and activation of Shc was abolished, but interestingly, Gab-2 and Akt phosphorylation increased. Significantly, the activation of PI3K was enhanced and prolonged, accompanied by loss of SHIP activity. These results reveal a previously unrecognized negative signaling role for Tyr577 in beta(c) and demonstrate that uncoupling Shc from cytokine receptors enhances PI3K signaling as well as survival and proliferation.

The molecular basis of IL-21-mediated proliferation

Interleukin-21 (IL-21) is a type I cytokine that modulates functions of T, B, natural killer (NK), and myeloid cells. The IL-21 receptor (IL-21R) is closely related to the IL-2 receptor beta chain and is capable of transducing signals through its dimerization with the common cytokine receptor gamma chain (gamma(c)), the protein whose expression is defective in humans with X-linked severe combined immunodeficiency. To clarify the molecular basis of IL-21 actions, we investigated the role of tyrosine residues in the IL-21R cytoplasmic domain. Simultaneous mutation of all 6 tyrosines greatly diminished IL-21-mediated proliferation, whereas retention of tyrosine 510 (Y510) allowed full proliferation. Y510 efficiently mediated IL-21-induced phosphorylation of Stat1 and Stat3, but not of Stat5, and CD8(+) T cells from Stat1/Stat3 double knock-out mice exhibited decreased proliferation in response to IL-21 + IL-15. In addition, IL-21 weakly induced phosphorylation of Shc and Akt, and consistent with this, specific inhibitors of the MAPK and PI3K pathways inhibited IL-21-mediated proliferation. Collectively, these data indicate the involvement of the Jak-STAT, MAPK, and PI3K pathways in IL-21 signaling.

Ral is both necessary and sufficient for the inhibition of myeloid differentiation mediated by Ras

Hyperactivation of Ras is one of the most common abnormalities in acute myeloid leukemia. In experimental models, Ras inhibits myeloid differentiation, which is characteristic of leukemia; however, the mechanism through which it disrupts hematopoiesis is poorly understood. In multipotent FDCP-mix cells, Ras inhibits terminal neutrophil differentiation, thereby indefinitely extending their proliferative potential. Ras also strongly promotes the sensitivity of these cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). Using this model, we have dissected the signaling elements downstream of Ras to determine their relative contribution to the dysregulation of hematopoiesis. Cells expressing Ras mutants selectively activating Raf (Ras*T35S) or phosphatidylinositol 3-kinase (Ras*Y40C) did not significantly affect differentiation or proliferative capacity, whereas Ras*E37G (which selectively activates RalGEFs) perpetuated proliferation and blocked neutrophil development in a manner similar to that of Ras. Correspondingly, expression of constitutively active versions of these effectors confirmed the overriding importance of Ral guanine nucleotide exchange factors. Cells expressing Ras demonstrated hyperactivation of Ral, which itself was able to exactly mimic the phenotype of Ras, including hypersensitivity to GM-CSF. Conversely, dominant negative Ral promoted spontaneous neutrophil development. Ral, in turn, appears to influence differentiation through multiple effectors. These data show, for the first time, the importance of Ral in regulating differentiation and self-renewal in hematopoietic cells.

Constitutive mutants of the GM-CSF receptor reveal multiple pathways leading to myeloid cell survival, proliferation, and granulocyte-macrophage differentiation

Activation of the granulocyte-macrophage colony-stimulating factor (GM-CSF) family of receptors promotes the survival, proliferation, and differentiation of cells of the myeloid compartment. Several signaling pathways are activated downstream of the receptor, however it is not clear how these induce specific biologic outcomes. We have previously identified 2 classes of constitutively active mutants of the shared signaling subunit, human (h) betac, of the human GM-CSF/interleukin-3 (IL-3)/IL-5 receptors that exhibit different modes of signaling. In a factor-dependent bipotential myeloid cell line, FDB1, an activated mutant containing a substitution in the transmembrane domain (V449E) induces factor-independent proliferation and survival, while mutants in the extracellular domain induce factor-independent granulocyte-macrophage differentiation. Here we have used further mutational analysis to demonstrate that there are nonredundant functions for several regions of the cytoplasmic domain with regard to mediating proliferation, viability, and differentiation, which have not been revealed by previous studies with the wild-type GM-CSF receptor. This unique lack of redundancy has revealed an association of a conserved membrane-proximal region with viability signaling and a critical but distinct role for tyrosine 577 in the activities of each class of mutant.

Cooperation of cytokine signaling with chimeric transcription factors in leukemogenesis: PML-retinoic acid receptor alpha blocks growth factor-mediated differentiation

We utilized a mouse model of acute promyelocytic leukemia (APL) to investigate how aberrant activation of cytokine signaling pathways interacts with chimeric transcription factors to generate acute myeloid leukemia. Expression in mice of the APL-associated fusion, PML-RARA, initially has only modest effects on myelopoiesis. Whereas treatment of control animals with interleukin-3 (IL-3) resulted in expanded myelopoiesis without a block in differentiation, PML-RARA abrogated differentiation that normally characterizes the response to IL-3. Retroviral transduction of bone marrow with an IL-3-expressing retrovirus revealed that IL-3 and promyelocytic leukemia-retinoic acid receptor alpha (PML-RARalpha) combined to generate a lethal leukemia-like syndrome in <21 days. We also observed that a constitutively activated mutant IL-3 receptor, beta(c)V449E, cooperated with PML-RARalpha in leukemogenesis, whereas a different activated mutant, beta(c)I374N, did not. Analysis of additional mutations introduced into beta(c)V449E showed that, although tyrosine phosphorylation of beta(c) is necessary for cooperation, the Src homology 2 domain-containing transforming protein binding site is dispensable. Our results indicate that chimeric transcription factors can block the differentiative effects of growth factors. This combination can be potently leukemogenic, but the particular manner in which these types of mutations interact determines the ability of such combinations to generate acute myeloid leukemia.

A novel functional epitope formed by domains 1 and 4 of the human common beta-subunit is involved in receptor activation by granulocyte macrophage colony-stimulating factor and interleukin 5

The receptors for human interleukins 3 and 5 and granulocyte macrophage colony-stimulating factor are composed of ligand-specific alpha-subunits and a common beta-subunit (betac), the major signaling entity. The way in which betac interacts with ligands in the respective activation complexes has remained poorly understood. The recently determined crystal structure of the extracellular domain of betac revealed a possible ligand-binding interface composed of domain 1 of one chain of the betac dimer and the adjacent domain 4 of the symmetry-related chain. We have used site-directed mutagenesis, in conjunction with ligand binding and proliferation studies, to demonstrate the critical requirement of the domain 1 residues, Tyr(15) (A-B loop) and Phe(79) (E-F loop), in high affinity complex formation and receptor activation. The novel ligand-receptor interface formed between domains 1 and 4 represents the first example of a class I cytokine receptor interface to be composed of two noncontiguous fibronectin III domains.

Biosensor analysis of dynamics of interleukin 5 receptor subunit beta(c) interaction with IL5:IL5R(alpha) complexes

To gain insight into IL5 receptor subunit recruitment mechanism, and in particular the experimentally elusive pathway for assembly of signaling subunit beta(c), we constructed a soluble beta(c) ectodomain (s(beta)(c)) and developed an optical biosensor assay to measure its binding kinetics. Functionally active s(beta)(c) was anchored via a C-terminal His tag to immobilized anti-His monoclonal antibodies on the sensor surface. Using this surface, we quantitated for the first time direct binding of s(beta)(c) to IL5R(alpha) complexed to either wild-type or single-chain IL5. Binding was much weaker if at all with either R(alpha) or IL5 alone. Kinetic evaluation revealed a moderate affinity (0.2-1 microM) and relatively fast off rate for the s(beta)(c) interaction with IL5:R(alpha) complexes. The data support a model in which beta(c) recruitment occurs with preformed IL5:R(alpha) complex. Dissociation kinetics analysis suggests that the IL5-alpha-beta(c) complex is relatively short-lived. Overall, this study solidifies a model of sequential recruitment of receptor subunits by IL5, provides a novel biosensor binding assay of beta(c) recruitment dynamics, and sets the stage for more advanced characterization of the roles of structural elements within R(alpha), beta(c), and cytokines of the IL5/IL3/GM-CSF family in receptor recruitment and activation.

Human GM-CSF induces HIV-1 LTR by multiple signalling pathways

Human immunodeficiency virus type-1 (HIV-1) gene expression is known to be affected by numerous cytokines or growth factors. However, the effect of granulocyte-macrophage colony-stimulating factor (GM-CSF) on long terminal repeat (LTR)-mediated transcription of HIV-1 still remains unknown. By transient transfection experiments with HIV-1 LTR reporter constructs, we showed that strong LTR-mediated activation was induced by GM-CSF in mouse Ba/F3 cells expressing human GM-CSF receptors (GM-CSFR). Mutational analysis of the HIV-1 LTR reporters revealed that both NF-kappaB and Sp1 binding sites play important roles as positive regulatory elements. Analysis of various mutants of the cytoplasmic region of GM-CSFR indicated that both the conserved membrane proximal region and tyrosine residues located in the distal part of the beta subunit were required for HIV-1 LTR activation. Possible involvement of MAPK and PI3-K signalling pathways was suggested by the partial inhibition by wortmannin, a specific inhibitor of the PI3-K pathway, and enhancement by constitutively active MEK1, of HIV-1 LTR activation. However, the MEK1 pathway is not essential since MEK1 inhibitor PD98059 did not suppress GM-CSF-induced HIV-1-LTR activation. Further analyses of GM-CSFR mutants suggested that some other unknown signalling pathway also participates in GM-CSF-induced HIV-1 LTR activation. Taken together, the data suggest that GM-CSF could upregulate the LTR-driven transcription of HIV-1 through modulation of NF-kappaB and SP1 by multiple signalling pathways.

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.

JAKs, STATs and Src kinases in hematopoiesis

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 leukemias and other myeloproliferative and lymphoproliferative disorders. Over the past decade, downstream signal transduction events initiated upon cytokine/growth factor stimulation have been a major focus of basic and applied biomedical research. As a result, several key concepts have emerged allowing a better understanding of the complex signaling processes. A group of 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. Similarly, cytoplasmic Janus protein tyrosine kinases (JAKs) and Src family of kinases seem to play a critical role in diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Accumulating evidence suggests that STAT protein activation may be mediated by members of both JAK and Src family members following cytokine/growth factor stimulation. In addition, JAK kinases appear to be essential for the phosphorylation of the cytokine receptors which results in the creation of docking sites on the receptors for binding of SH2-containing proteins such as STATs, Src-kinases and other signaling intermediates. Cell and tissue-specificity of cytokine action appears to be determined by the nature of signal transduction pathways activated by cytokine/receptor interactions. The integration of these diverse signaling cues from active JAK kinases, members of the Src-family kinases and STAT proteins, leads to cell proliferation, cell survival and differentiation, the end-point of the cytokine/growth factor stimulus.

Identification and characterization of Magmas, a novel mitochondria-associated protein involved in granulocyte-macrophage colony-stimulating factor signal transduction

OBJECTIVE

The aim of this study was to identify granulocyte-macrophage colony-stimulating factor (GM-CSF) responsive genes.

MATERIALS AND METHODS

Potential GM-CSF responsive genes were identified by comparing the mRNA expression pattern of the murine myeloid cell line PGMD1 grown in either interleukin-3 (IL-3) or GM-CSF by differential display. Human and murine cDNA clones of one of the bands having increased expression in GM-CSF were isolated. mRNA expression of the gene was examined by Northern blot. Immunohistochemistry and studies with a green fluorescent fusion protein were used to determine its intracellular location. Growth factor-stimulated proliferation of PGMD1 cells transfected with constitutively expressed sense and anti-sense cDNA constructs of the gene was measured by 3H-thymidine incorporation.

RESULTS

A gene, named Magmas (mitochondria-associated granulocyte macrophage CSF signaling molecule), was shown to be rapidly induced when cells were switched from IL-3 to GM-CSF. Analysis of the amino acid sequence of Magmas showed it contained a mitochondrial signal peptide, but not any other known functional domains. The human and murine clones encode nearly identical 13-kDa proteins that localized to the mitochondria. Magmas mRNA expression was observed in all tissues examined. PGMD1 cells that overexpressed Magmas proliferated similarly to untransfected cells when cultured in IL-3 or GM-CSF. In contrast, cells with reduced protein levels grew normally in IL-3, but had impaired proliferation in GM-CSF.

CONCLUSION

Magmas is a mitochondrial protein involved in GM-CSF signal transduction.

Involvement of Hgs/Hrs in signaling for cytokine-mediated c-fos induction through interaction with TAK1 and Pak1

Hgs/Hrs is a tyrosine-phosphorylated FYVE finger protein that is induced by stimulation with various cytokines and growth factors. Here we show that Hgs plays critical roles in the signaling pathway for the interleukin-2-induced activation of the serum-response element and cyclic AMP-response element of the c-fos promoter. We found that Hgs associated physically with transforming growth factor-beta-activated kinase 1 (TAK1) and p21-activated kinase 1 (Pak1), which mediate the activation of c-Jun N-terminal kinase and serum response factor, respectively, leading to transactivation via the serum-response element and cyclic AMP-response element. These results suggest that Hgs is involved in the TAK1-JNK and Pak1-serum response factor pathways for the c-fos induction that is initiated by cytokines.

Induction of human matrix metalloproteinase-12 gene transcriptional activity by GM-CSF requires the AP-1 binding site in human U937 monocytic cells

Matrix metalloproteinase-12 (MMP-12) is critical for the migration of monocytes/macrophages into inflammatory sites through the basement membranes. We previously reported that MMP-12 expression was initially induced by granulocyte macrophage colony-stimulating factor (GM-CSF) in human peripheral blood monocytes and U937 monocytic cells. To further elucidate the molecular mechanism for the regulation of MMP-12 expression by GM-CSF in monocytes, we determined the sequence requirements for the MMP-12 gene transcriptional response of U937 monocytic cells to GM-CSF by using luciferase reporter and electrophoretic mobility shift assays. A series of 5'-deletion and site-directed mutation of the human MMP-12 promoter demonstrated that an AP-1 site spanning the -81 to -75-bp region is critical for the induction of MMP-12 promoter activity by GM-CSF. The electrophoretic mobility shift assay revealed that AP-1 binding activity was increased by GM-CSF treatment and that the AP-1 complex induced by GM-CSF consisted of multiple Jun and Fos isoforms. These results indicate that MMP-12 expression in U937 monocytes was initially induced by GM-CSF through the AP-1 binding activity.

Structural and functional hot spots in cytokine receptors

The activation of cytokine receptors is a stepwise process that depends on their specific interaction with cognate cytokines, the formation of oligomeric receptor complexes, and the initiation of cytoplasmic phosphorylation events. The recent determination of the structure of extracellular domains of several cytokine receptors allows comparison of their cytokine-binding surfaces. This comparison reveals a common structural framework that supports considerable diversity and adaptability of the binding surfaces that determine both the specificity and the orientation of subunits in the active receptor complex. These regions of the cytokine receptors have been targeted for the development of specific agonists and antagonists. The physical coupling of signaling intermediates to the intracellular domains of their receptors plays a major role in determining biological responses to cytokines. In this review, we focus principally on the receptors for cytokines of the granulocyte-macrophage colony-stimulating factor (GM-CSF) family and, where appropriate, compare them with related cytokine receptors. Several paradigms are beginning to emerge that focus on the ability of the extracellular portion of the cytokine receptor to recognize the appropriate cytokine and on a phosphorylated motif in the intracellular region of the GM-CSF receptor that couples to a specific signaling pathway.

Initiation of polyoma virus origin-dependent DNA replication through STAT5 activation by human granulocyte-macrophage colony-stimulating factor

Several lines of evidence indicate that transcriptional activation is coupled with DNA replication initiation, but the nature of initiation of DNA replication in mammalian cells is unclear. Polyoma virus replicon is an excellent system to analyze the initiation of DNA replication in murine cells because its replication requires an enhancer, and all components of replication machinery, except for DNA helicase large T antigen, are supplied by host cells. This system was used to examine the role of signal transducer and activator of transcription (STAT5) in replication initiation of polyoma replicon in the mouse lymphoid cell line BA/F3. The plasmid with tandem repeats of consensus STAT5 binding sites followed by polyoma replication origin was replicated by stimulation with human granulocyte-macrophage colony-stimulating factor (hGM-CSF) in the presence of polyoma large T antigen in BA/F3 cells. Mutation analysis of the hGM-CSF receptor beta subunit revealed that only the box1 region is essential, and the C-terminal tyrosine residues are dispensable for the activity. Addition of the tyrosine kinase inhibitor genistein suppressed this replication without affecting transcriptional activation of STAT5. Because deletion analysis of STAT5 indicates the importance of the C-terminal transcriptional activation domain of STAT5 for the initiation of replication, the role of this region in the activation of replication was examined with a GAL4-STAT5 fusion protein. GAL4-STAT5 activated replication of the plasmid containing tandem repeats of GAL4 binding sites and polyoma replication origin in BA/F3 cells. Mutation analysis of GAL4-STAT5 indicated that multiple serine residues coordinately have a role in activating replication. This is the first direct evidence indicating the potential involvement of STAT5 in replication.

Janus kinases: components of multiple signaling pathways

Cytoplasmic Janus protein tyrosine kinases (JAKs) are crucial components of diverse signal transduction pathways that govern cellular survival, proliferation, differentiation and apoptosis. Evidence to date, indicates that JAK kinase function may integrate components of diverse signaling cascades. While it is likely that activation of STAT proteins may be an important function attributed to the JAK kinases, it is certainly not the only function performed by this key family of cytoplasmic tyrosine kinases. Emerging evidence indicates that phosphorylation of cytokine and growth factor receptors may be the primary functional attribute of JAK kinases. The JAK-triggered receptor phosphorylation can potentially be a rate-limiting event for a successful culmination of downstream signaling events. In support of this hypothesis, it has been found that JAK kinase function is required for optimal activation of the Src-kinase cascade, the Ras-MAP kinase pathway, the PI3K-AKT pathway and STAT signaling following the interaction of cytokine/interferon receptors with their ligands. Aberrations in JAK kinase activity, that may lead to derailment of one or more of the above mentioned pathways could disrupt normal cellular responses and result in disease states. Thus, over-activation of JAK kinases has been implicated in tumorigenesis. In contrast, loss of JAK kinase function has been found to result in disease states such as severe-combined immunodeficiency. In summary, optimal JAK kinase activity is a critical determinant of normal transmission of cytokine and growth factor signals.

In vitro analysis of STAT5 activation by granulocyte-macrophage colony-stimulating factor

BACKGROUND

The granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor activates multiple and complex signalling pathways in response to GM-CSF stimulation. Biochemical studies suggested that signalling pathways are transmitted through protein/protein interactions, but how these biochemical cascades are initiated and transmitted in response to cytokine stimulation is largely unknown.

RESULTS

To investigate these events biochemically, we established an in vitro system leading to the GM-CSF-dependent activation of Janus kinase (JAK) 2/signal transducer and activator of transcription (STAT) 5 in cell homogenates prepared from BA/F3 cells expressing the GM-CSF receptor. Activation of STAT5 DNA binding ability requires both membrane and cytoplasmic fractions while phosphorylation of JAK2 requires only the membrane fraction. Since the addition of anti-betac or phosphotyrosine antibodies inhibited GM-CSF induced STAT5 DNA binding activity, we examined the role of tyrosine residues of betac for in vitro activation of STAT5. Addition of synthetic tyrosine-phosphorylated peptides derived from betac cytoplasmic tyrosines prior to GM-CSF stimulation inhibited the in vitro activation of STAT5. The association between these tyrosine-phosphorylated peptides and STAT5 was observed by using peptide-coupling beads and BA/F3 lysates.

CONCLUSIONS

We established a GM-CSF-dependent in vitro system. In cases of STAT5 activation, each phosphorylated tyrosine residue of betac can act as a docking site and enhance STAT5 activation.

New role for Shc in activation of the phosphatidylinositol 3-kinase/Akt pathway

Most, if not all, cytokines activate phosphatidylinositol 3-kinase (PI-3K). Although many cytokine receptors have direct binding sites for the p85 subunit of PI-3K, others, such as the interleukin-3 (IL-3) receptor beta common chain (betac) and the IL-2 receptor beta chain (IL-2Rbeta), lack such sites, leaving the mechanism by which they activate PI-3K unclear. Here, we show that the protooncoprotein Shc, which promotes Ras activation by recruiting the Grb2-Sos complex in response to stimulation of cytokine stimulation, also signals to the PI-3K/Akt pathway. Analysis of Y-->F and "add-back" mutants of betac shows that Y577, the Shc binding site, is the major site required for Gab2 phosphorylation in response to cytokine stimulation. When fused directly to a mutant form of IL-2Rbeta that lacks other cytoplasmic tyrosines, Shc can promote Gab2 tyrosyl phosphorylation. Mutation of the three tyrosyl phosphorylation sites of Shc, which bind Grb2, blocks the ability of the Shc chimera to evoke Gab2 tyrosyl phosphorylation. Overexpression of mutants of Grb2 with inactive SH2 or SH3 domains also blocks cytokine-stimulated Gab2 phosphorylation. The majority of cytokine-stimulated PI-3K activity associates with Gab2, and inducible expression of a Gab2 mutant unable to bind PI-3K markedly impairs IL-3-induced Akt activation and cell growth. Experiments with the chimeric receptors indicate that Shc also signals to the PI-3K/Akt pathway in response to IL-2. Our results suggest that cytokine receptors lacking direct PI-3K binding sites activate Akt via a Shc/Grb2/Gab2/PI-3K pathway, thereby regulating cell survival and/or proliferation.

Analysis of antiapoptosis activity of human GM-CSF receptor

Human GM-CSF (hGM-CSF) induces proliferation and sustains the viability of a mouse IL-3-dependent lymphoid cell line BA/F3 that expresses the functional hGM-CSF receptor (hGMR). To reveal an antiapoptotic mechanism of hGM-CSF, we analyzed various apoptotic markers of BA/F3 cells in various conditions. Within 24 hours of factor depletion, caspase 3-like, but not caspase 1-like, enzyme activity and DNA fragmentation were augmented. Analysis with the tyrosine kinase inhibitor (genistein) and an MEK1 inhibitor (PD98059) on antiapoptosis activity indicates that the activation of either the genistein-sensitive signaling pathway or the PD98059-sensitive signaling pathway of the betac subunit may be sufficient to suppress apoptosis through hGMR. Because hGMR mutants (which activate JAK2 but neither STAT5 nor the MAPK cascade) have antiapoptotic activity in BA/F3 cells, the involvement of JAK2, excluding the molecules mentioned earlier, for antiapoptosis activity seems likely. Because the JAK2 inhibitor AG-490 suppressed the antiapoptotic activity of hGM-CSF, the essential role for JAK2 activation to maintain the viability is considered. Interestingly, hGMR mutants, which lack MAPK cascade activation, require a higher dose of hGM-CSF than that for wild-type hGMR. Because the expression level and affinity to hGM-CSF among wild-type hGMR and mutant hGMR are the same, we speculated that biologic response is determined by a combination of strength of various signaling events.

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).

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.

Shc associates with the IL-3 receptor beta subunit, SHIP and Gab2 following IL-3 stimulation. Contribution of Shc PTB and SH2 domains

p46(Shc) and p52(Shc) become heavily tyrosine phosphorylated in response to interleukin 3 (IL-3) treatment. We have investigated the potential of Shc to integrate IL-3 signalling pathways and demonstrate that Shc associates with the beta subunits of the human (betac) and murine (Aic2A) IL-3 receptors, SHIP and Gab2 following IL-3 stimulation. The interaction between Shc and the IL-3 receptor beta chains was direct, mediated by both the SH2 and PTB domains. Interaction with SHIP was via the Shc PTB domain and the Shc SH2 domain mediated the interaction with Gab2. Phosphopeptide competition studies suggest that the SH2 domain interacts primarily with tyrosine 612 of betac (610 of Aic2A), and the PTB domain with tyrosine 577 of betac (575 of Aic2A). PTB binding to IL-3R beta chains was of highest affinity, and appeared to play the primary role in binding. These findings suggest that Shc may play an important role in coordinately integrating IL-3 signalling pathways.

The role of disulfide-linked dimerization in interleukin-3 receptor signaling and biological activity

Cysteine residues 86 and 91 of the beta subunit of the human interleukin (hIL)-3 receptor (hbetac) participate in disulfide-linked receptor subunit heterodimerization. This linkage is essential for receptor tyrosine phosphorylation, since the Cys-86 --> Ala (Mc4) and Cys-91 --> Ala (Mc5) mutations abolished both events. Here, we used these mutants to examine whether disulfide-linked receptor dimerization affects the biological and biochemical activities of the IL-3 receptor. Murine T cells expressing hIL-3Ralpha and Mc4 or Mc5 did not proliferate in hIL-3, whereas cells expressing wild-type hbetac exhibited rapid proliferation. However, a small subpopulation of cells expressing each mutant could be selected for growth in IL-3, and these proliferated similarly to cells expressing wild-type hbetac, despite failing to undergo IL-3-stimulated hbetac tyrosine phosphorylation. The Mc4 and Mc5 mutations substantially reduced, but did not abrogate, IL-3-mediated anti-apoptotic activity in the unselected populations. Moreover, the mutations abolished IL-3-induced JAK2, STAT, and AKT activation in the unselected cells, whereas activation of these molecules in IL-3-selected cells was normal. In contrast, Mc4 and Mc5 showed a limited effect on activation of Erk1 and -2 in unselected cells. These data suggest that whereas disulfide-mediated cross-linking and hbetac tyrosine phosphorylation are normally important for receptor activation, alternative mechanisms can bypass these requirements.

Two distinct signaling pathways downstream of Janus kinase 2 play redundant roles for antiapoptotic activity of granulocyte-macrophage colony-stimulating factor

Human granulocyte-macrophage colony-stimulating factor (hGM-CSF) induces proliferation and sustains the viability of the mouse interleukin-3-dependent cell line BA/F3 expressing the hGM-CSF receptor. Analysis of the antiapoptosis activity of GM-CSF receptor betac mutants showed that box1 but not the C-terminal region containing tyrosine residues is essential for GM-CSF-dependent antiapoptotic activity. Because betac mutants, which activate Janus kinase 2 but neither signal transducer and activator of transcription 5 nor the MAPK cascade sustain antiapoptosis activity, involvement of Janus kinase 2, excluding the above molecules, in antiapoptosis activity seems likely. GM-CSF activates phosphoinositide-3-OH kinase as well as Akt, and activation of both was suppressed by addition of wortmannin. Interestingly, wortmannin did not affect GM-CSF-dependent antiapoptosis, thus indicating that the phosphoinositide-3-OH kinase pathway is not essential for cell surivival. Analysis using the tyrosine kinase inhibitor genistein and a MAPK/extracellular signal-regulated kinase (ERK) kinase 1 inhibitor, PD98059, indicates that activation of either the genistein-sensitive signaling pathway or the PD98059-sensitive signaling pathway from betac may be sufficient to suppress apoptosis. Wild-type and a betac mutant lacking tyrosine residues can induce expression of c-myc and bcl-x(L) genes; however, drug sensitivities for activation of these genes differ from those for antiapoptosis activity of GM-CSF, which means that these gene products may be involved yet are inadequate to promote cell survival.

Mutant RAS inhibits neutrophil but not macrophage differentiation and allows continued growth of neutrophil precursors

Mutational activation of RAS is the most common molecular abnormality in myeloid leukemias. In order to better understand its role in leukemogenesis, we have devised a model based on the multipotent cell line, FDCP-mix. We show that expression of mutant RAS in FDCP-mix strongly inhibits terminal neutrophil differentiation under the influence of G-CSF plus GM-CSF at the metamyelocyte stage, whereas macrophage differentiation was unaffected. In addition, whereas control cultures differentiated and became postmitotic under these conditions, FDCP-mix cells expressing mutant RAS continued to proliferate indefinitely while maintaining a metamyelocytic phenotype. Labeling of these cultures with the fluorescent tracking dye, PKH26, showed that this extended proliferative capacity resulted from continued division of metamyelocytes in the culture. Dissection of the growth factor response of these cells demonstrated that GM-CSF was critical in maintaining proliferation and inhibiting the differentiation of these cells. We further show the block in neutrophil differentiation could be partially overcome by treatment with low-dose Ara C, suggesting that maintenance of cell cycle progression may be partly responsible for the anti-differentiation effect of this oncogene. These findings suggest that activation of RAS is able to specifically inhibit terminal neutrophil differentiation and in so doing promotes continued division of metamyelocyte cells.

Identification of a 14-3-3 Binding Sequence in the Common β Chain of the Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), Interleukin-3 (IL-3), and IL-5 Receptors That Is Serine-Phosphorylated by GM-CSF

The common β chain (βc) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is the major signaling subunit of these receptors coupling ligand binding to multiple biological activities. It is thought that these multiple functions arise as a consequence of the recruitment of specific signaling molecules to tyrosine-phosphorylated residues in the cytoplasmic domain of βc. However, the contribution of serine phosphorylation in βc to the recruitment of signaling molecules is not known. We show here the identification of a phosphoserine motif in the cytoplasmic domain of βc that interacts with the adaptor protein 14-3-3ζ. Coimmunoprecipitation and pull-down experiments with a glutathione S-transferase (GST):14-3-3ζ fusion protein showed that 14-3-3 directly associates with βc but not the GM-CSF receptor  chain. C-terminal truncation mutants of βcfurther showed that a region between amino acids 544 and 626 in βc was required for its association with 14-3-3ζ. This region contains the sequence 582HSRSLP587, which closely resembles the RSXSXP (where S is phosphorylated) consensus 14-3-3 binding site identified in a number of signaling molecules, including Raf-1. Significantly, substitution of582HSRSLP587 for EFAAAA completely abolished interaction of βc with GST–14-3-3ζ. Furthermore, the interaction of βc with GST–14-3-3 was greatly reduced in the presence of a peptide containing the 14-3-3 binding site, but only when 585Ser was phosphorylated. Direct binding experiments showed that the peptide containing phosphorylated 585Ser bound 14-3-3ζ with an affinity of 150 nmol/L. To study the regulation of 585S phosphorylation in vivo, we raised antibodies that specifically recognized 585Ser-phosphorylated βc. Using these antibodies, we showed that GM-CSF stimulation strongly upregulated 585Ser phosphorylation in M1 myeloid leukemic cells. The proximity of the SHC-binding site (577Tyr) to the 14-3-3–binding site (582HSRSLP587) and their conservation between mouse, rat, and human βc but not in other cytokine receptors suggest that they form a distinct motif that may subserve specialized functions associated with the GM-CSF, IL-3, and IL-5 receptors.

Identification of a 14-3-3 Binding Sequence in the Common β Chain of the Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF), Interleukin-3 (IL-3), and IL-5 Receptors That Is Serine-Phosphorylated by GM-CSF

The common β chain (βc) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is the major signaling subunit of these receptors coupling ligand binding to multiple biological activities. It is thought that these multiple functions arise as a consequence of the recruitment of specific signaling molecules to tyrosine-phosphorylated residues in the cytoplasmic domain of βc. However, the contribution of serine phosphorylation in βc to the recruitment of signaling molecules is not known. We show here the identification of a phosphoserine motif in the cytoplasmic domain of βc that interacts with the adaptor protein 14-3-3ζ. Coimmunoprecipitation and pull-down experiments with a glutathione S-transferase (GST):14-3-3ζ fusion protein showed that 14-3-3 directly associates with βc but not the GM-CSF receptor  chain. C-terminal truncation mutants of βcfurther showed that a region between amino acids 544 and 626 in βc was required for its association with 14-3-3ζ. This region contains the sequence 582HSRSLP587, which closely resembles the RSXSXP (where S is phosphorylated) consensus 14-3-3 binding site identified in a number of signaling molecules, including Raf-1. Significantly, substitution of582HSRSLP587 for EFAAAA completely abolished interaction of βc with GST–14-3-3ζ. Furthermore, the interaction of βc with GST–14-3-3 was greatly reduced in the presence of a peptide containing the 14-3-3 binding site, but only when 585Ser was phosphorylated. Direct binding experiments showed that the peptide containing phosphorylated 585Ser bound 14-3-3ζ with an affinity of 150 nmol/L. To study the regulation of 585S phosphorylation in vivo, we raised antibodies that specifically recognized 585Ser-phosphorylated βc. Using these antibodies, we showed that GM-CSF stimulation strongly upregulated 585Ser phosphorylation in M1 myeloid leukemic cells. The proximity of the SHC-binding site (577Tyr) to the 14-3-3–binding site (582HSRSLP587) and their conservation between mouse, rat, and human βc but not in other cytokine receptors suggest that they form a distinct motif that may subserve specialized functions associated with the GM-CSF, IL-3, and IL-5 receptors.

Regulation and function of protein kinase B and MAP kinase activation by the IL-5/GM-CSF/IL-3 receptor

Interleukin (IL)-3, IL-5 and granulocyte-macrophage colony-stimulating factor (GM-CSF) regulate proliferation, differentiation and apoptosis of target cells. Receptors for these cytokines consist of a cytokine-specific alpha subunit and a common shared beta c subunit. Tyrosine phosphorylation of the beta c is thought to play a critical role in mediating signal transduction events. We have examined the effect of mutation of beta c tyrosines on the activation of multiple signal transduction pathways. Activation of protein kinase B (PKB) required JAK2 and was inhibited by dominant-negative phosphatidylinositol 3-kinase (P13K). Overexpression of JAK2 was sufficient to activate both protein kinase B (PKB) and extracellular regulated kinase-1 (ERK1). Tyrosine 577 and 612 were found to be critical for the activation of PKB and ERK1, but not activation of STAT transcription factors. Activation of both PKB and ERK have been implicated in the regulation of proliferation and apoptosis. We generated GM-CSFR stable cell lines expressing receptor mutants to evaluate their effect on these processes. Activation of both PKB and ERK was perturbed, while STAT activation remained unaffected. Tyrosines 577 and 612 were necessary for optimal proliferation, however, mutation of these tyrosine residues did not affect GM-CSF mediated rescue from apoptosis. These data demonstrate that while phosphorylation of beta c tyrosine residues 577 and 612 are important for optimal cell proliferation, rescue from apoptosis can be mediated by alternative signalling routes apparently independent of PKB or ERK activation.

Gab-Family Adapter Proteins Act Downstream of Cytokine and Growth Factor Receptors and T- and B-Cell Antigen Receptors

We previously found that the adapter protein Gab1 (110 kD) is tyrosine-phosphorylated and forms a complex with SHP-2 and PI-3 kinase upon stimulation through either the interleukin-3 receptor (IL-3R) or gp130, the common receptor subunit of IL-6–family cytokines. In this report, we identified another adapter molecule (100 kD) interacting with SHP-2 and PI-3 kinase in response to various stimuli. The molecule displays striking homology to Gab1 at the amino acid level; thus, we named it Gab2. It contains a PH domain, proline-rich sequences, and tyrosine residues that bind to SH2 domains when they are phosphorylated. Gab1 is phosphorylated on tyrosine upon stimulation through the thrombopoietin receptor (TPOR), stem cell factor receptor (SCFR), and T-cell and B-cell antigen receptors (TCR and BCR, respectively), in addition to IL-3R and gp130. Tyrosine phosphorylation of Gab2 was induced by stimulation through gp130, IL-2R, IL-3R, TPOR, SCFR, and TCR. Gab1 and Gab2 were shown to be substrates for SHP-2 in vitro. Overexpression of Gab2 enhanced the gp130 or Src-related kinases–mediated ERK2 activation as that of Gab1 did. These data indicate that Gab-family molecules act as adapters for transmitting various signals.

Constitutive activation of JAK2 confers murine interleukin-3-independent survival and proliferation of BA/F3 cells

The Janus tyrosine kinase 2 (JAK2) plays an essential role of cytokine receptor signaling, including that of the human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor. We reported earlier that the activation of JAK2 is essential for all the examined signals induced by human GM-CSF through the box1 region of betac, such as promotion of cell survival and proliferation. To elucidate the role of JAK2 in cell survival and proliferation, we generated an artificial activation system by constructing a chimeric molecule (beta/JAK2) consisting of betac extracellular and transmembrane regions fused with JAK2, and we analyzed various signaling events in interleukin-3-dependent mouse pro-B cell, BA/F3. The beta/JAK2 was constitutively phosphorylated in the absence of human GM-CSF and murine interleukin-3, and this led to proliferation and cell survival. Western blot analysis showed that STAT5, Shc, and SHP-2 were not phosphorylated in the cells, and the consistent activation of beta-casein and c-fos promoters was not enhanced. In contrast, c-myc transcription was constitutively activated. We propose that the activation of beta/JAK2 suffices for survival and proliferation and that the activation of STAT5 and mitogen-activated protein kinase cascade is not required for these activities in BA/F3 cells.

The Mapping of the Lyn Kinase Binding Site of the Common β Subunit of IL-3/Granulocyte-Macrophage Colony- Stimulating Factor/IL-5 Receptor

It has been shown that a membrane-proximal region within common β (βc) receptor of IL-3/granulocyte-macrophage CSF/IL-5 (amino acids 450–517) is important for Lyn binding. We have shown previously that Lyn kinase is physically associated with the IL-5R βc subunit in unstimulated cells. The result suggests that this association involves binding modules that are not activation or phosphorylation dependent. The objective of this study was to map the exact Lyn binding site on βc. Using overlapping and/or sequential peptides derived from βc 450–517, we narrowed down the Lyn binding site to nine amino acid residues, βc 457–465. The P→A mutation in this region abrogated the binding to Lyn, indicating a critical role of proline residues. We created a cell-permeable Lyn-binding peptide by N-stearation. This cell-permeable peptide blocked the association of Lyn, but not Jak2 with βc in situ. We also investigated the βc binding site of Lyn kinase. Our results suggest that the N-terminal unique domain of Lyn kinase is important for binding to βc receptor. To our knowledge, this is the first molecular identification of the Lyn binding site of βc receptor. This finding may help develop specific inhibitors of Lyn-coupled signaling pathways.

Cloning of p97/Gab2, the major SHP2-binding protein in hematopoietic cells, reveals a novel pathway for cytokine-induced gene activation

Several components in cytokine signaling remain unidentified. We report the cloning and initial characterization of one such component, p97, a widely expressed scaffolding protein distantly related to Drosophila DOS and mammalian Gab1. Upon cytokine, growth factor, or antigen receptor stimulation, p97 becomes tyrosyl phosphorylated and associates with several SH2 domain-containing proteins, including SHP2. Expression of p97 mutants unable to bind SHP2 blocks cytokine-induced c-fos promoter activation, inhibiting Elk1-mediated and STAT5-mediated transactivation. Surprisingly, such mutants do not inhibit MAPK activation. Our results identify p97 as an important regulator of receptor signaling that controls a novel pathway to immediate-early gene activation and suggest multiple functions for SHP2 in cytokine receptor signaling.

Activation and functional analysis of Janus kinase 2 in BA/F3 cells using the coumermycin/gyrase B system

Janus kinase 2 (Jak2) protein tyrosine kinase plays an important role in interleukin-3- or granulocyte-macrophage colony-stimulating factor-mediated signal transduction pathways leading to cell proliferation, activation of early response genes, and inhibition of apoptosis. However, it is unclear whether Jak2 can activate these signaling pathways directly without the involvement of cytokine receptor phosphorylation. To investigate the specific role of Jak2 in the regulation of signal transduction pathways, we generated gyrase B (GyrB)-Jak2 fusion proteins, dimerized through the addition of coumermycin. Coumermycin induced autophosphorylation of GyrB-Jak2 fusion proteins, thus bypassing receptor activation. Using different types of chimeric Jak2 molecules, we observed that although the kinase domain of Jak2 is sufficient for autophosphorylation, the N-terminal regions are essential for the phosphorylation of Stat5 and for the induction of short-term cell proliferation. Moreover, coumermycin-induced activation of Jak2 can also lead to increased levels of c-myc and CIS mRNAs in BA/F3 cells stably expressing the Jak2 fusion protein with the intact N-terminal region. Conversely, activation of the chimeric Jak2 induced neither phosphorylation of Shc or SHP-2 nor activation of the c-fos promoter. Here, we showed that the GyrB-Jak2 system can serve as an excellent model to dissect signals of receptor-dependent and -independent events. We also obtained evidence indicating a role for the N-terminal region of Jak2 in downstream signaling events.

Functional Analysis of Mature Hematopoietic Cells From Mice Lacking the βc Chain of the Granulocyte-Macrophage Colony-Stimulating Factor Receptor

Mice with a null mutation of the βc chain of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors (βc-null mice) develop an alveolar proteinosis-like lung disease. The pathogenesis of this disease is uncertain and, although a defect in alveolar macrophage function has been postulated, no previous analysis of mature hematopoietic cells in mice with alveolar proteinosis has been reported. Therefore, we undertook a functional analysis of the mature hematopoietic cell compartment in βc-null mice. In addition, we reexamined the roles of the GM-CSF receptor  chain and the βc chain in signaling by GM-CSF. Neutrophils and macrophages from βc-null mice were capable of normal survival and phagocytosis in the absence of stimulus and of similar levels of nitric oxide production in response to interferon-γ and lipopolysaccharide. GM-CSF–mediated augmentation of survival, phagocytosis, and hydrogen-ion production were absent in neutrophils from βc-null mice. Interestingly, we were unable to show any ability of the GM-CSF receptor -chain alone to mediate glucose transport in these cells. In keeping with the βc-null mice lung pathology, examination of lavage fluid from the lungs of βc-null mice showed increased cellularity. This was caused by an increase in the number of lymphocytes, neutrophils, and macrophages. Large foamy cells in the lavage fluid from βc-null mice were identified as macrophages using immunohistochemistry. Functional analysis showed that these βc-null alveolar macrophages were capable of phagocytosis but uptake of colloidal carbon and cellular adhesion were reduced. In summary, mature hematopoietic cells with a null mutation of the βc receptor were unable to perform GM-CSF–mediated hematopoietic cell functions including glucose transport, but responded normally to a range of other ligands.

Functional Analysis of Mature Hematopoietic Cells From Mice Lacking the βc Chain of the Granulocyte-Macrophage Colony-Stimulating Factor Receptor

Mice with a null mutation of the βc chain of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors (βc-null mice) develop an alveolar proteinosis-like lung disease. The pathogenesis of this disease is uncertain and, although a defect in alveolar macrophage function has been postulated, no previous analysis of mature hematopoietic cells in mice with alveolar proteinosis has been reported. Therefore, we undertook a functional analysis of the mature hematopoietic cell compartment in βc-null mice. In addition, we reexamined the roles of the GM-CSF receptor  chain and the βc chain in signaling by GM-CSF. Neutrophils and macrophages from βc-null mice were capable of normal survival and phagocytosis in the absence of stimulus and of similar levels of nitric oxide production in response to interferon-γ and lipopolysaccharide. GM-CSF–mediated augmentation of survival, phagocytosis, and hydrogen-ion production were absent in neutrophils from βc-null mice. Interestingly, we were unable to show any ability of the GM-CSF receptor -chain alone to mediate glucose transport in these cells. In keeping with the βc-null mice lung pathology, examination of lavage fluid from the lungs of βc-null mice showed increased cellularity. This was caused by an increase in the number of lymphocytes, neutrophils, and macrophages. Large foamy cells in the lavage fluid from βc-null mice were identified as macrophages using immunohistochemistry. Functional analysis showed that these βc-null alveolar macrophages were capable of phagocytosis but uptake of colloidal carbon and cellular adhesion were reduced. In summary, mature hematopoietic cells with a null mutation of the βc receptor were unable to perform GM-CSF–mediated hematopoietic cell functions including glucose transport, but responded normally to a range of other ligands.

Mechanism of activation of the GM-CSF, IL-3, and IL-5 family of receptors

The process of ligand binding leading to receptor activation is an ordered and sequential one. High-affinity binding of GM-CSF, interleukin 3 (IL-3), and IL-5 to their receptors induces a number of key events at the cell surface and within the cytoplasm that are necessary for receptor activation. These include receptor oligomerization, activation of tyrosine kinase activity, phosphorylation of the receptor, and the recruitment of SH2 (src-homology) and PTB (phosphotyrosine binding) domain proteins to the receptor. Such a sequence of events represents a recurrent theme among cytokine, growth factor, and hormone receptors; however, a number of very recent and interesting findings have identified unique features in this receptor system in terms of: A) how GM-CSF/IL-3/IL-5 bind, oligomerize, and activate their cognate receptors; B) how multiple biological responses such as proliferation, survival, and differentiation can be transduced from activated GM-CSF, IL-3, or IL-5 receptors, and C) how the presence of novel phosphotyrosine-independent signaling motifs within a specific cytoplasmic domain of betaC may be important for mediating survival and differentiation by these cytokines. This review does not attempt to be all-encompassing but rather to focus on the most recent and significant discoveries that distinguish the GM-CSF/IL-3/IL-5 receptor subfamily from other cytokine receptors.