Signal transduction by the high-affinity GM-CSF receptor: two distinct cytoplasmic regions of the common beta subunit responsible for different signaling

Mechanism of Emodin in the Treatment of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic, systemic, and autoimmune disease, and its main pathological changes are inflammatory cell infiltration accompanied by the secretion and accumulation of a variety of related cytokines, which induce the destruction of cartilage and bone tissue. Therefore, the modulation of inflammatory cells and cytokines is a key therapeutic target for controlling inflammation in RA. This review details the effects of emodin on the differentiation and maturation of T lymphocytes, dendritic cells, and regulatory T cells. In addition, the systematic introduction of emodin directly or indirectly affects proinflammatory cytokines (TNF-α, IL-6, IL-1, IL-1β, IL-17, IL-19, and M-CSF) and anti-inflammatory cytokines (the secretion of IL-4, IL-10, IL-13, and TGF-β) through the coregulation of a variety of inflammatory cytokines to inhibit inflammation in RA and promote recovery. Understanding the potential mechanism of emodin in the treatment of RA in detail provides a systematic theoretical basis for the clinical application of emodin in the future.

Neutrophil Functional Heterogeneity and Implications for Viral Infections and Treatments

Evidence suggests that neutrophils exert specialized effector functions during infection and inflammation, and that these cells can affect the duration, severity, and outcome of the infection. These functions are related to variations in phenotypes that have implications in immunoregulation during viral infections. Although the complexity of the heterogeneity of neutrophils is still in the process of being uncovered, evidence indicates that they display phenotypes and functions that can assist in viral clearance or augment and amplify the immunopathology of viruses. Therefore, deciphering and understanding neutrophil subsets and their polarization in viral infections is of importance. In this review, the different phenotypes of neutrophils and the roles they play in viral infections are discussed. We also examine the possible ways to target neutrophil subsets during viral infections as potential anti-viral treatments.

Cytokine Networks in the Pathogenesis of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic systemic inflammation causing progressive joint damage that can lead to lifelong disability. The pathogenesis of RA involves a complex network of various cytokines and cells that trigger synovial cell proliferation and cause damage to both cartilage and bone. Involvement of the cytokines tumor necrosis factor (TNF)-α and interleukin (IL)-6 is central to the pathogenesis of RA, but recent research has revealed that other cytokines such as IL-7, IL-17, IL-21, IL-23, granulocyte macrophage colony-stimulating factor (GM-CSF), IL-1β, IL-18, IL-33, and IL-2 also play a role. Clarification of RA pathology has led to the development of therapeutic agents such as biological disease-modifying anti-rheumatic drugs (DMARDs) and Janus kinase (JAK) inhibitors, and further details of the immunological background to RA are emerging. This review covers existing knowledge regarding the roles of cytokines, related immune cells and the immune system in RA, manipulation of which may offer the potential for even safer and more effective treatments in the future.

Targeting Granulocyte-Monocyte Colony-Stimulating Factor Signaling in Rheumatoid Arthritis: Future Prospects

Rheumatoid arthritis (RA) is a systemic, autoimmune disease that affects joints and extra-articular structures. In the last decade, the management of this chronic disease has dramatically changed with the introduction of several targeted mechanisms of action, such as tumor necrosis factor-α inhibition, T-cell costimulation inhibition, B-cell depletion, interleukin-6 blockade, and Janus kinase inhibition. Beyond its well-known hematopoietic role on the proliferation and differentiation of myeloid cells, granulocyte-monocyte colony-stimulating factor (GM-CSF) is a proinflammatory mediator acting as a cytokine, with a proven pathogenetic role in autoimmune disorders such as RA. In vitro studies clearly demonstrated the effect of GM-CSF in the communication between resident tissue cells and activated macrophages at chronic inflammation sites, and confirmed the elevation of GM-CSF levels in inflamed synovial tissue of RA subjects compared with healthy controls. Moreover, a pivotal role of GM-CSF in the perception of pain has been clearly confirmed. Therefore, blockade of the GM-CSF pathway by monoclonal antibodies directed against the cytokine itself or its receptor has been investigated in refractory RA patients. Overall, the safety profile of GM-CSF inhibitors seems to be very favorable, with a particularly low incidence of infectious complications. The efficacy of this new mechanism of action is comparable with main competitors, even though the response rates reported in phase II randomized controlled trials (RCTs) appear to be numerically lower than the response rates observed with other biological disease-modifying antirheumatic drugs already licensed for RA. Mainly because of this reason, nowadays the development program of most GM-CSF blockers for RA has been discontinued, with the exception of otilimab, which is under evaluation in two phase III RCTs with a head-to head non-inferiority design against tofacitinib. These studies will likely be useful for better defining the potential role of GM-CSF inhibition in the therapeutic algorithm of RA. On the other hand, the potential role of GM-CSF blockade in the treatment of other rheumatic diseases is now under investigation. Phase II trials are ongoing with the aim of evaluating mavrilimumab for the treatment of giant cell arteritis, and namilumab for the treatment of spondyloarthritis. Moreover, GM-CSF inhibitors have been tested in osteoarthritis and diffuse subtype of systemic sclerosis. This review aims to describe in detail the available evidence on the GM-CSF blocking pathway in RA management, paving the way to a possible alternative treatment for RA patients. Novel insights regarding the potential use of GM-CSF blockers for alternative indications will be also addressed.

Synergy of Interleukin (IL)-5 and IL-18 in eosinophil mediated pathogenesis of allergic diseases

Eosinophils are circulating granulocytes that have pleiotropic effects in response to inflammatory signals in the body. In response to allergens or pathogens, exposure eosinophils are recruited in various organs that execute pathological immune responses. IL-5 plays a key role in the differentiation, development, and survival of eosinophils. Eosinophils are involved in a variety of allergic diseases including asthma, dermatitis and various gastrointestinal disorders (EGID). IL-5 signal transduction involves JAK-STAT-p38MAPK-NFκB activation and executes extracellular matrix remodeling, EMT transition and immune responses in allergic diseases. IL-18 is a classical cytokine also involved in immune responses and has a critical role in inflammasome pathway. We recently identified the IL-18 role in the generation, transformation, and maturation of (CD101+CD274+) pathogenic eosinophils. In, addition, several other cytokines like IL-2, IL-4, IL-13, IL-21, and IL-33 also contribute in advancing eosinophils associated immune responses in innate and adaptive immunity. This review discusses with a major focus (1) Eosinophils and its constituents, (2) Role of IL-5 and IL-18 in eosinophils development, transformation, maturation, signal transduction of IL-5 and IL-18, (3) The role of eosinophils in allergic disorders and (4) The role of several other associated cytokines in promoting eosinophils mediated allergic diseases.

Computational design and experimental characterization of a novel β-common receptor inhibitory peptide

In short-term animal models of ischemia, erythropoietin (EPO) signaling through the heterodimeric EPO receptor (EPOR)/β-common receptor (βCR) is believed to elicit tissue protective effects. However, large, randomized, controlled trials demonstrate that targeting a higher hemoglobin level by administering higher doses of EPO, which are more likely to activate the heterodimeric EPOR/βCR, is associated with an increase in adverse cardiovascular events. Thus, inhibition of long-term activation of the βCR may have therapeutic implications. This study aimed to design and evaluate the efficacy of novel computationally designed βCR inhibitory peptides (βIP). These novel βIPs were designed based on a truncated portion of Helix-A from EPO, specifically residues 11-26 (VLERYLLEAKEAEKIT). Seven novel peptides (P1 to P7) were designed. Peptide 7 (P7), VLERYLHEAKHAEKIT, demonstrated the most robust inhibitory activity. We also report here the ability of P7 to inhibit βCR-induced nitric oxide (NO) production and angiogenesis in human umbilical vein endothelial cells (HUVECs). Specifically, we found that P7 βIP completely abolished EPO-induced NO production. The inhibitory effect could be overcome with super physiological doses of EPO, suggesting a competitive inhibition. βCR-induced angiogenesis in HUVEC's was also abolished with treatment of P7 βIP, but P7 βIP did not inhibit vascular endothelial growth factor (VEGF)-induced angiogenesis. In addition, we demonstrate that the novel P7 βIP does not inhibit EPO-induced erythropoiesis with use of peripheral blood mononuclear cells (PBMCs). These results, for the first time, describe a novel, potent βCR peptide inhibitor that inhibit the actions of the βCR without affecting erythropoiesis.

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.

Future therapeutic targets in rheumatoid arthritis?

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by persistent joint inflammation. Without adequate treatment, patients with RA will develop joint deformity and progressive functional impairment. With the implementation of treat-to-target strategies and availability of biologic therapies, the outcomes for patients with RA have significantly improved. However, the unmet need in the treatment of RA remains high as some patients do not respond sufficiently to the currently available agents, remission is not always achieved and refractory disease is not uncommon. With better understanding of the pathophysiology of RA, new therapeutic approaches are emerging. Apart from more selective Janus kinase inhibition, there is a great interest in the granulocyte macrophage-colony stimulating factor pathway, Bruton's tyrosine kinase pathway, phosphoinositide-3-kinase pathway, neural stimulation and dendritic cell-based therapeutics. In this review, we will discuss the therapeutic potential of these novel approaches.

JAK2/STAT5/Bcl-xL signalling is essential for erythropoietin-mediated protection against apoptosis induced in PC12 cells by the amyloid β-peptide Aβ25-35

BACKGROUND AND PURPOSE

Erythropoietin (EPO) exerts neuroprotective actions in the CNS, including protection against apoptosis induced by the amyloid β-peptide Aβ25-35 . However, it remains unclear which signalling pathway activated by EPO is involved in this neuroprotection. Here, we have investigated whether JAK2/STAT5/Bcl-xL and ERK1/2 signalling pathways are essential for EPO-mediated protection against apoptosis induced by Aβ25-35 .

EXPERIMENTAL APPROACH

EPO was added to cultures of PC12 cells, 1 h before Aβ25-35 . For kinase inhibitor studies, AG490 and PD98059 were added to PC12 cells, 0.5 h before the addition of EPO. Transfection with siRNA was used to knockdown STAT5. Activation of JAK2/STAT5/Bcl-xL and ERK1/2 signalling pathways were investigated by Western blotting. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl) 2,5-diphenyl-tetrazolium bromide assay and apoptosis was detected by TUNEL and acridine orange-ethidium bromide double staining.

KEY RESULTS

EPO increased phosphorylation of JAK2 and STAT5 in PC12 cells treated with Aβ25-35 . Furthermore, EPO modulated the nuclear translocation of phospho-STAT5, which increased expression of Bcl-xL and decreased levels of caspase-3. These beneficial effects were blocked by the JAK2 inhibitor, AG490 or STAT5 knockdown. However, the ERK1/2 pathway did not play a crucial role in our model.

CONCLUSIONS AND IMPLICATIONS

EPO protected PC12 cells against Aβ25-35 -induced neurotoxicity. Activation of JAK2/STAT5/Bcl-xL pathway was important in EPO-mediated neuroprotection. EPO may serve as a novel protective agent against Aβ25-35 -induced cytotoxicity in, for instance, Alzheimer's disease.

Reprint of Neutrophil cell surface receptors and their intracellular signal transduction pathways

Neutrophils play a critical role in the host defense against bacterial and fungal infections, but their inappropriate activation also contributes to tissue damage during autoimmune and inflammatory diseases. Neutrophils express a large number of cell surface receptors for the recognition of pathogen invasion and the inflammatory environment. Those include G-protein-coupled chemokine and chemoattractant receptors, Fc-receptors, adhesion receptors such as selectins/selectin ligands and integrins, various cytokine receptors, as well as innate immune receptors such as Toll-like receptors and C-type lectins. The various cell surface receptors trigger very diverse signal transduction pathways including activation of heterotrimeric and monomeric G-proteins, receptor-induced and store-operated Ca(2+) signals, protein and lipid kinases, adapter proteins and cytoskeletal rearrangement. Here we provide an overview of the receptors involved in neutrophil activation and the intracellular signal transduction processes they trigger. This knowledge is crucial for understanding how neutrophils participate in antimicrobial host defense and inflammatory tissue damage and may also point to possible future targets of the pharmacological therapy of neutrophil-mediated autoimmune or inflammatory diseases.

Neutrophil cell surface receptors and their intracellular signal transduction pathways

Neutrophils play a critical role in the host defense against bacterial and fungal infections, but their inappropriate activation also contributes to tissue damage during autoimmune and inflammatory diseases. Neutrophils express a large number of cell surface receptors for the recognition of pathogen invasion and the inflammatory environment. Those include G-protein-coupled chemokine and chemoattractant receptors, Fc-receptors, adhesion receptors such as selectins/selectin ligands and integrins, various cytokine receptors, as well as innate immune receptors such as Toll-like receptors and C-type lectins. The various cell surface receptors trigger very diverse signal transduction pathways including activation of heterotrimeric and monomeric G-proteins, receptor-induced and store-operated Ca^2 + signals, protein and lipid kinases, adapter proteins and cytoskeletal rearrangement. Here we provide an overview of the receptors involved in neutrophil activation and the intracellular signal transduction processes they trigger. This knowledge is crucial for understanding how neutrophils participate in antimicrobial host defense and inflammatory tissue damage and may also point to possible future targets of the pharmacological therapy of neutrophil-mediated autoimmune or inflammatory diseases.

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Neutrophils are crucial players in innate and adaptive immunity.

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Neutrophils also participate in autoimmune and inflammatory diseases.

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Various neutrophil receptors recognize pathogens and the inflammatory environment.

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The various cell surface receptors trigger diverse intracellular signaling.

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Neutrophil receptors and signaling are potential targets in inflammatory diseases.

GM-CSF as a therapeutic target in inflammatory diseases

GM-CSF is a well-known haemopoietic growth factor that is used in the clinic to correct neutropaenia, usually as a result of chemotherapy. GM-CSF also has many pro-inflammatory functions and recent data implicates GM-CSF as a key factor in Th17 driven autoimmune inflammatory conditions. In this review we summarize the findings that have led to the development of GM-CSF antagonists for the treatment of autoimmune diseases like rheumatoid arthritis (RA) and discuss some results of recent clinical trials of these agents.

GM-CSF increases LPS-induced production of proinflammatory mediators via upregulation of TLR4 and CD14 in murine microglia

BACKGROUND

Microglia are resident macrophage-like cells in the central nervous system (CNS) and cause innate immune responses via the LPS receptors, Toll-like receptor (TLR) 4 and CD14, in a variety of neuroinflammatory disorders including bacterial infection, Alzheimer's disease, and amyotrophic lateral sclerosis. Granulocyte macrophage-colony stimulating factor (GM-CSF) activates microglia and induces inflammatory responses via binding to GM-CSF receptor complex composed of two different subunit GM-CSF receptor α (GM-CSFRα) and common β chain (βc). GM-CSF has been shown to be associated with neuroinflammatory responses in multiple sclerosis and Alzheimer's disease. However, the mechanisms how GM-CSF promotes neuroinflammation still remain unclear.

METHODS

Microglia were stimulated with 20 ng/ml GM-CSF and the levels of TLR4 and CD14 expression were evaluated by RT-PCR and flowcytometry. LPS binding was analyzed by flowcytometry. GM-CSF receptor complex was analyzed by immunocytochemistry. The levels of IL-1β, IL-6 and TNF-α in culture supernatant of GM-CSF-stimulated microglia and NF-κB nuclear translocation were determined by ELISA. Production of nitric oxide (NO) was measured by the Griess method. The levels of p-ERK1/2, ERK1/2, p-p38 and p38 were assessed by Western blotting. Statistically significant differences between experimental groups were determined by one-way ANOVA followed by Tukey test for multiple comparisons.

RESULTS

GM-CSF receptor complex was expressed in microglia. GM-CSF enhanced TLR4 and CD14 expressions in microglia and subsequent LPS-binding to the cell surface. In addition, GM-CSF priming increased LPS-induced NF-κB nuclear translocation and production of IL-1β, IL-6, TNF-α and NO by microglia. GM-CSF upregulated the levels of p-ERK1/2 and p-p38, suggesting that induction of TLR4 and CD14 expression by GM-CSF was mediated through ERK1/2 and p38, respectively.

CONCLUSIONS

These results suggest that GM-CSF upregulates TLR4 and CD14 expression in microglia through ERK1/2 and p38, respectively, and thus promotes the LPS receptor-mediated inflammation in the CNS.

Molecular and clinical rationale for therapeutic targeting of interleukin-5 and its receptor

Interleukin-5 is a Th2 homodimeric cytokine involved in the differentiation, maturation, migration, development, survival, trafficking and effector function of blood and local tissue eosinophils, in addition to basophils and mast cells. The IL-5 receptor (IL-5R) consists of an IL-5-specific α subunit that interacts in conformationally dynamic ways with the receptor's βc subunit, an aggregate of domains it shares with binding sites of IL-3 and granulocyte-macrophage colony-stimulating factor. IL-5 and IL-5R drive allergic and inflammatory immune responses characterizing numerous diseases, such as asthma, atopic dermatitis, chronic obstructive pulmonary disease, eosinophilic gastrointestinal diseases, hyper-eosinophilic syndrome, Churg-Strauss syndrome and eosinophilic nasal polyposis. Although corticosteroid therapy is the primary treatment for these diseases, a substantial number of patients exhibit incomplete responses and suffer side-effects. Two monoclonal antibodies have been designed to neutralize IL-5 (mepolizumab and reslizumab). Both antibodies have demonstrated the ability to reduce blood and tissue eosinophil counts. One additional monoclonal antibody, benralizumab (MEDI-563), has been developed to target IL-5R and attenuate eosinophilia through antibody-dependent cellular cytotoxicity. All three monoclonal antibodies are being clinically evaluated. Antisense oligonucleotide technology targeting the common βc IL-5R subunit is also being used therapeutically to inhibit IL-5-mediated effects (TPI ASM8). Small interfering RNA technology has also been used therapeutically to inhibit the expression of IL-5 in animal models. This review summarizes the structural interactions between IL-5 and IL-5R and the functional consequences of such interactions, and describes the pre-clinical and clinical evidence supporting IL-5R as a therapeutic target.

Deficiency of SHP1 leads to sustained and increased ERK activation in mast cells, thereby inhibiting IL-3-dependent proliferation and cell death

SHP-1 plays an important role for the regulation of signaling from various hematopoietic cell receptors. In this study, we examined IL-3-induced cell proliferation and IL-3 depletion-induced apoptosis in bone marrow-derived mast cells (BMMC) established from motheaten (me) that lack SHP-1 expression, viable motheaten (me(v)) expressing phosphatase-deficient SHP-1, and wild-type (WT) mice. When BMMC were stimulated with IL-3, increased ERK activation was evident in resting state and sustained in me-BMMC relative to WT-BMMC. ERK is known to be involved in the regulation of cell proliferation and apoptosis in some cells. In accordance with sustained ERK activation, apoptosis was decreased in me- and me(v)-BMMC compared with WT-BMMC. In contrast to the predicted role of ERK as a pro-survival molecule, IL-3-induced cell proliferation was much lower in me- and me(v)-BMMC than WT-BMMC. Stimulation with lower concentration of IL-3 or addition of PD98059, a MEK inhibitor, to the culture resulted in the suppression of decreased apoptosis and cell proliferation in me- and me(v)-BMMC. Collectively, these results suggest that SHP-1 positively regulates IL-3-dependent mast cell proliferation and apoptosis by inhibiting ERK activity through its phosphatase activity. Furthermore, our results indicate that ERK would act as a negative regulator for cell proliferation and induce apoptosis when its activity is highly increased.

IL-5- and eosinophil-mediated inflammation: from discovery to therapy

IL-5 was originally defined as a T-cell-derived cytokine that triggers activated B cells for terminal differentiation into antibody-secreting plasma cells, at least in mice. Concurrently, IL-5 was recognized as the major maturation and differentiation factor for eosinophils in mice and humans. Over-expression of IL-5 significantly increases eosinophil numbers and antibody levels in vivo. Conversely, mice lacking a functional gene for IL-5 or the IL-5 receptor alpha chain (IL-5Ralpha) display a number of developmental and functional impairments in B-cell and eosinophil lineages. In addition to the Janus kinase-signal transducer and activator of transcription pathway, the tyrosine kinases Lyn and Btk (Bruton agammaglobulinemia tyrosine kinase) are involved, and Ras GTPase-extracellular signal-regulated kinase (Ras-ERK) signals are important for IL-5-dependent cell proliferation and survival. IL-5 critically regulates expression of genes involved in proliferation, cell survival and maturation and effector functions of B cells and eosinophils. Thus, IL-5 plays a pivotal role in innate and acquired immune responses and eosinophilia. In humans, the biologic effects of IL-5 are best characterized for eosinophils. The recent expansion in our understanding of the mechanisms of eosinophil development and activation in the context of IL-5 has led to advances in therapeutic options. A new therapy currently in clinical trials uses humanized mAbs against IL-5 or the IL-5R.

Novel reporter cell line to analyze cytokine-mediated expression regulation of c-myc gene

Growth-promoting cytokines induce expression of nuclear proto-oncogenes that play critical roles in the regulation of cell proliferation. c-myc gene is one of those nuclear proto-oncogenes, whose regulation mechanisms of cytokine-mediated expression are not fully understood. Here, I generated a green fluorescent protein (GFP) reporter system that faithfully reflects interleukin-3 (IL-3)-induced c-myc gene expression. Flowcytometric analysis revealed cytokine-specific expression of reporter GFP. Kinetics of GFP mRNA expression was similar to that of endogenous c-myc mRNA. The reporter cell line will be a useful tool for studies of cell proliferation regulation through analysis of cytokine-induced c-myc gene expression.

Interleukin 5 in the link between the innate and acquired immune response

Interleukin-5 (IL-5) is an interdigitating homodimeric glycoprotein that is initially identified by its ability to support the in vitro growth and differentiation of mouse B cells and eosinophils. IL-5 transgenic mouse shows two predominant features, remarkable increase in B-1 cells resulting in enhanced serum antibody levels, predominantly IgM, IgA, and IgE classes and in expansion of eosinophil numbers in the blood and eosinophil infiltration into various tissues. Conversely, mice lacking a functional gene for IL-5 or IL-5 receptor alpha chain (IL-5Ralpha) display a number of developmental and functional impairments in B cells and eosinophils. IL-5 receptor (IL-5R) comprises alpha and betac chains. IL-5 specifically binds to IL-5Ralpha and induces the recruitment of betac to IL-5R. Although precise mechanisms on cell-lineage-specific IL-5Ralpha expression remain elusive, several transcription factors including Sp1, E12/E47, Oct-2, and c/EBPbeta have been shown to regulate its expression in B cells and eosinophils. JAK2 and JAK1 tyrosine kinase are constitutively associated with IL-5Ralpha and betac, respectively, and are activated by IL-5 stimulation. IL-5 activates at least three different signaling pathways including JAK2/STAT5 pathway, Btk pathway, and Ras/ERK pathway. IL-5 is one of key cytokines for mouse B cell differentiation in general, particularly for fate-determination of terminal B cell differentiation to antibody-secreting plasma cells. IL-5 critically regulates homeostatic proliferation and survival of and natural antibody production by B-1 cells, and enhances the AID and Blimp-1 expression in activated B-2 cells leading to induce mu to gamma1 class switch recombination and terminal differentiation to IgM- and IgG1-secreting plasma cells, respectively. In humans, major target cells of IL-5 are eosinophils. IL-5 appears to play important roles in pathogenesis of asthma, hypereosinophilic syndromes, and eosinophil-dependent inflammatory diseases. Clinical studies will provide a strong impetus for investigating the means of modulating IL-5 effects. We will discuss the role of IL-5 in the link between innate and acquired immune response, particularly emphasis of the molecular basis of IL-5-dependent B cell activation, allergen-induced chronic inflammation and hypereosinophilic syndromes on a novel target for therapy.

A dual-color fluorescence-based platform to identify selective inhibitors of Akt signaling

BACKGROUND

Inhibition of Akt signaling is considered one of the most promising therapeutic strategies for many cancers. However, rational target-orientated approaches to cell based drug screens for anti-cancer agents have historically been compromised by the notorious absence of suitable control cells.

METHODOLOGY/PRINCIPAL FINDINGS

In order to address this fundamental problem, we have developed BaFiso, a live-cell screening platform to identify specific inhibitors of this pathway. BaFiso relies on the co-culture of isogenic cell lines that have been engineered to sustain interleukin-3 independent survival of the parental Ba/F3 cells, and that are individually tagged with different fluorescent proteins. Whilst in the first of these two lines cell survival in the absence of IL-3 is dependent on the expression of activated Akt, the cells expressing constitutively-activated Stat5 signaling display IL-3 independent growth and survival in an Akt-independent manner. Small molecules can then be screened in these lines to identify inhibitors that rescue IL-3 dependence.

CONCLUSIONS/SIGNIFICANCE

BaFiso measures differential cell survival using multiparametric live cell imaging and permits selective inhibitors of Akt signaling to be identified. BaFiso is a platform technology suitable for the identification of small molecule inhibitors of IL-3 mediated survival signaling.

Hematopoietic cytokines

The production of hematopoietic cells is under the tight control of a group of hematopoietic cytokines. Each cytokine has multiple actions mediated by receptors whose cytoplasmic domains contain specialized regions initiating the various responses-survival, proliferation, differentiation commitment, maturation, and functional activation. Individual cytokines can be lineage specific or can regulate cells in multiple lineages, and for some cell types, such as stem cells or megakaryocyte progenitors, the simultaneous action of multiple cytokines is required for proliferative responses. The same cytokines control basal and emergency hematopoietic cell proliferation. Three cytokines, erythropoietin, granulocyte colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor, have now been in routine clinical use to stimulate cell production and in total have been used in the management of many millions of patients. In this little review, discussion will be restricted to those cytokines well established as influencing the production of hematopoietic cells and will exclude newer candidate regulators and those active on lymphoid cells. As requested, this account will describe the cytokines in a historical manner, using a sequential format of discovery, understanding, validation, and puzzlement, a sequence that reflects the evolving views on these cytokines over the past 50 years.

CBAP interacts with the un-liganded common β-subunit of the GM-CSF/IL-3/IL-5 receptor and induces apoptosis via mitochondrial dysfunction

The cytoplasmic domain of the common beta-chain (betac) of the granulocyte-macrophage-colony-stimulating factor (GM-CSF)/interleukin-3 (IL-3)/IL-5 receptor contains a membrane proximal region that is sufficient to mediate ligand-dependent mitogenic activity. Within this region two motifs, designated as box 1 and box 2, are highly conserved among members of the cytokine receptor superfamily. Whereas box 1 is required for the recruitment and phosphorylation of Janus kinase-2, the function of box 2 remains largely unknown. Here, we report the identification of a novel transmembrane protein (common beta-chain associated protein (CBAP)) which directly associated with betac via the box 2 motif. Interestingly, such an association only occurred in the absence of GM-CSF in vivo. Ectopic overexpression of CBAP triggered apoptosis of factor-dependent cells via mitochondrial dysfunction, which could be inhibited by Bcl-2 overexpression. Reduced expression of endogenous CBAP by small interfering RNA did not interfere GM-CSF-activated signaling molecules, but such treatment significantly inhibited apoptosis induced by GM-CSF deprivation, but not other death stimuli. Domain mapping studies indicated that one apoptogenic domain of CBAP correlated with its ability to interact with betac. Taken together, these results suggest that CBAP modulates GM-CSF-deprivation-induced apoptosis possibly via a novel mechanism involving interaction with an un-liganded betac molecule.

Interleukin-5 regulates genes involved in B-cell terminal maturation

Interleukin (IL)-5 induces CD38-activated splenic B cells to differentiate into immunoglobulin M-secreting cells and undergo micro to gamma 1 class switch recombination (CSR) at the DNA level, resulting in immunoglobulin G1 (IgG1) production. Interestingly, IL-4, a well-known IgG1-inducing factor does not induce immunoglobulin production or micro to gamma 1 CSR in CD38-activated B cells. In the present study, we implemented complementary DNA microarrays to investigate the contribution of IL-5-induced gene expression in CD38-stimulated B cells to immunoglobulin-secreting cell differentiation and micro to gamma 1 CSR. IL-5 and IL-4 stimulation of CD38-activated B cells induced the expression of 418 and 289 genes, respectively, that consisted of several clusters. Surprisingly, IL-5-inducible 78 genes were redundantly regulated by IL-4. IL-5 and IL-4 also suppressed the gene expression of 319 and 325 genes, respectively, 97 of which were overlapped. Genes critically regulated by IL-5 include immunoglobulin-related genes such as J chain and immunoglobulinkappa, and genes involved in B-cell maturation such as BCL6, activation-induced cytidine deaminase (Aid) and B lymphocyte-induced maturation protein-1 (Blimp-1) and tend to be induced slowly after IL-5 stimulation. Intriguingly, among genes, the retroviral induction of Blimp-1 and Aid in CD38-activated B cells could induce IL-4-dependent maturation to Syndecan-1+ antibody-secreting cells and micro to gamma 1 CSR, respectively, in CD38-activated B cells. Taken together, preferential Aid and Blimp-1 expression plays a critical role in IL-5-induced immunoglobulin-secreting cell differentiation and micro to gamma 1 CSR in CD38-activated B cells.

Granulocyte macrophage colony-stimulating factor enhances retinoic acid-induced gene expression

We reported previously that treatment of human myeloblastic leukemia ML-1 cells with all-trans retinoic acid (ATRA) in combination with GM-CSF enhances the granulocytic differentiation, which is induced only slightly by ATRA alone. To investigate the mechanism underlying this differentiation and the synergistic effect of ATRA and GM-CSF, we used cDNA microarray to examine gene expression profiles of ML-1 cells treated with ATRA and/or GM-CSF. We identified 22 up-regulated genes in ML-1 cells treated with both reagents and examined the expression of these genes in cells treated with ATRA and/or GM-CSF by Northern blot analysis. Comparison of cells treated with both reagents and cells treated with ATRA or GM-CSF alone revealed that expression of nine of the 19 genes was induced synergistically by combined treatment with ATRA and GM-CSF. Expression of most of these genes was increased only slightly by ATRA alone, and this induction was enhanced by the addition of GM-CSF. These results indicate that GM-CSF enhances ATRA-induced gene expression. Moreover, studies with inhibitors of signaling molecules suggested that activation of JAK2 is associated with the synergistic induction of several genes by ATRA and GM-CSF. JAK2 inhibitor suppressed induction of NBT-reducing activity in ML-1 cells treated with both reagents. It is likely that the enhancer effect of GM-CSF on ATRA-induced gene expression leads to the differentiation induced synergistically by ATRA combined with GM-CSF. Further studies of the mechanism underlying this effect may identify better approaches for the treatment of RA-insensitive leukemia.

Therapeutic potential of treating chronic obstructive pulmonary disease (COPD) by neutralising granulocyte macrophage-colony stimulating factor (GM-CSF)

Chronic obstructive pulmonary disease (COPD) is a major incurable global health burden and will become the third largest cause of death in the world by 2020. It is currently believed that an exaggerated inflammatory response to inhaled irritants, in particular cigarette smoke, causes progressive airflow limitation. This inflammation, where macrophages and neutrophils are prominent, leads to oxidative stress, emphysema (loss of lung structure), small airways fibrosis and mucus hypersecretion. However, COPD responds poorly to current anti-inflammatory treatments including potent glucocorticosteroids, which produce little or no benefit. In this review we consider the therapeutic potential of targeting granulocyte macrophage-colony stimulating factor (GM-CSF) for the treatment of COPD. GM-CSF is a major regulator of both macrophage and neutrophil activation and survival in the lung-these cells are intimately linked to COPD. Animal data indicates that neutralisation of GM-CSF ameliorates experimental COPD and predicts therapeutic utility in treating stable COPD and treating exacerbations. As such, GM-CSF represents an attractive therapeutic target for the treatment of COPD.

Transcription Factors Mediating Interleukin‐3 Survival Signals

Interleukin-3 (IL-3) is one of the major hematopoietic cytokines that regulate the survival of hematopoietic cells of various lineages. Although the mechanism underlying the survival effect of IL-3 has been investigated intensively for more than a decade, our knowledge of the survival-signaling network remains incomplete. Binding of IL-3 to its cognate receptors initiates rapid tyrosine phosphorylation of Janus kinases (JAKs) and of signal transducer and activator of transcription (STAT) proteins, as well as activation of the phosphatidylinositol-3 kinase (PI-3K)/Akt and Ras/Raf/MAPK kinase (MEK)/mitogen-activated protein kinase (MAPK) pathways. These signals culminate in induction of a constellation of antiapoptotic genes and prevent cell death from occurring. Thus IL-3 signaling has substantial effects on kinase activation and gene transcription. Previous articles have summarized the roles of these kinase pathways in cell proliferation and survival. In this chapter, we will focus on the role of several newly characterized transcriptional factors, which are targets of these initial kinase cascades and bridge the gap between kinases and survival effector genes, in transducing the IL-3 survival signal. The biological significance of the existence of these multiple survival-specific transcription pathways will also be discussed.

Simultaneous activation of JAK1 and JAK2 confers IL-3 independent growth on Ba/F3 pro-B cells

JAK1 and JAK2 are tyrosine kinases involved in the regulation of cell proliferation, differentiation, and survival. These proteins may play a key role in mediating the effects of the cytokine IL-3 on hematopoietic cells. IL-3 induces tyrosine phosphorylation of both JAK1 and JAK2. However, it is not clear whether the activation of JAK1, JAK2, or both is sufficient to confer factor-independent growth in IL-3 dependent cells. To address this issue, fusion proteins CD16/CD7/JAK (CDJAK), comprised of a CD16 extracellular domain, a CD7 transmembrane domain, and a JAK cytoplasmic region (either a wild-type JAK or a dominant negative mutant of JAK) were constructed. We established several Ba/F3 derivatives that stably overexpress the conditionally active forms of either CDJAK1, CDJAK2, or both these fusion proteins. In this study, the autophosphorylation of CDJAK1 or CDJAK2 was induced by crosslinking with anti-CD16 antibody. We demonstrated that, like their wild-type counterparts, CDJAK1 and CDJAK2 were preassociated with the IL-3 receptor beta and alpha subunits, respectively. Furthermore, the simultaneous activation of both CDJAK1 and CDJAK2 fusion proteins, but not either one alone, led to the tyrosine phosphorylation of the IL-3 receptor beta subunit, the activation of downstream signaling molecules, including STAT5, Akt, and MAPK, and the conferring of factor-independent growth to IL-3-dependent Ba/F3 cells. Coexpression of dominant negative mutants CDJAK1KE or CDJAK2KE with wild type CDJAK2 or CDJAK1, respectively, inhibited these activation activities. These results suggest that JAK1 and JAK2 must work cooperatively and not independently and that their actions are dependent on having normal kinase activity to trigger downstream signals leading to IL-3 independent proliferation and survival of Ba/F3 cells.

Accumulation of an intron-retained mRNA for granulocyte macrophage-colony stimulating factor receptor common beta chain in neutrophils of myelodysplastic syndromes

We recently identified a reduction in the neutrophil surface expression of common beta chain (beta c) of the receptor for granulocyte macrophage-colony stimulating factor (GM-CSF) in the patients with myelodysplastic syndromes (MDS). To determine the etiology of the impaired beta c expression, beta c mRNA from neutrophilic granulocytes of MDS patients and healthy controls was analyzed by a combination of direct reverse transcriptase-polymerase chain reaction-based single-strand conformational polymorphism and sequencing. Nine different beta c transcripts were detected, but none was specific for MDS. However, one of the transcripts (beta c79) containing a 79-base intron insertion between exons V and VI was significantly increased in MDS. This 27-kd isoform consisted of the beta c N-terminal 182 amino acids followed by a new 84-amino-acid sequence. beta c79 was overexpressed in all MDS subtypes. No genomic mutations were detected within the intron or at the intron/exon boundaries. The isoform is predominantly located in the cytoplasm by Western blot analysis and was unable to generate high-affinity binding sites or transduce a signal for proliferation when coexpressed with the receptor for human GM-CSF alpha chain. Our study suggests that the accumulation of the abnormal beta c transcripts with intron V retention results in the reduction in cell-surface expression of beta c observed in MDS.

Normal neutrophil maturation is associated with selective loss of MAP kinase activation by G-CSF

Although both GM-CSF and G-CSF activate p42/44 MAPK in neutrophil progenitors, the ability of G-CSF to cause MAPK activation is lost in mature neutrophils, while GM-CSF exposure still causes activation. The mechanism of this differential effect related to maturation status has not been explored. We verified that G-CSF and GM-CSF receptors remain functional on purified mature neutrophils by demonstrating that both cytokines caused phosphorylation of STAT3. However, only GM-CSF was capable of activating MAPK as assessed by gel shift and in vitro kinase assay. Both G-CSF and GM-CSF caused activation of p21 ras in neutrophils, demonstrating that early events in the ras-MAPK pathway remain functional after stimulation by either cytokine. Inhibition of tyrosine phosphatase activity by pervanadate restored the ability of G-CSF to activate MAPK in mature neutrophils. Specific inhibition of the SHP-1 phosphatase, known to be activated by G-CSF but not GM-CSF also restored the ability of G-CSF to activate MAPK in neutrophils. These studies suggest that G-CSF activation of SHP-1 may be an important regulatory step for permitting optimal terminal differentiation during neutrophil production and add to our knowledge of the instructional role of G-CSF and GM-CSF for balancing proliferation and differentiation of neutrophil progenitor cells. This information may prove useful for the understanding of conditions in which neutrophil proliferative/differentiative balancing is dysregulated, such as myeloid leukemia and myelodysplastic disorders.

Interleukin-5 receptors on human lung eosinophils after segmental allergen challenge

BACKGROUND

IL-5 is a specific cytokine for eosinophil accumulation, activation and prolongation of survival and can be recovered in elevated concentrations from the bronchoalveolar compartment in atopic asthma following allergen challenge.

OBJECTIVE

The action of IL-5 is mediated via the specific IL-5 receptor-alpha (IL-5Ralpha). Although in vitro data suggest that IL-5R expression is regulated by cytokines such as IL-3, IL-5 and GM-CSF, IL-5R regulation in vivo and its kinetics following allergen provocation are incompletely understood.

METHODS

We investigated IL-5R regulation in vivo following segmental allergen provocation (SAP) with an individually standardized dose of allergen in 12 patients with atopic asthma. Lavage was performed 10 min and 18 h (eight patients) and 10 min and 42 h (eight patients) after allergen challenge. In addition to differential cell counts, IL-5Ralpha was measured by flow cytometry and IL-5 concentrations in bronchoalveolar lavage (BAL) fluid were determined by ELISA.

RESULTS

IL-5Ralpha expression decreased significantly on peripheral blood and on BAL eosinophils 18 and 42 h after SAP. In contrast, IL-5 concentrations increased significantly in BAL fluid 18 and 42 h after SAP. In four and two patients, respectively, there were detectable IL-5 concentrations in serum 18 or 42 h after allergen exposure.

CONCLUSIONS

Although there was no correlation between IL-5 concentrations and IL-5Ralpha expression on eosinophils in BAL, our data support previous in vitro and in vivo findings of a negative feedback mechanism between IL-5 concentrations and IL-5Ralpha expression on eosinophils.

Cytokine-induced phosphoinositide 3-kinase activity promotes Cdk2 activation in factor-dependent hematopoietic cells

Cytokine growth factors regulate the proliferation of hematopoietic cells through activation of several distinct signaling pathways. We have assessed the contribution of phosphoinositide 3-kinase (PI3K) pathways to erythropoietin (Epo) and interleukin (IL)-3-induced proliferation of factor-dependent hematopoietic cells. Lack of cytokine-induced PI3K activation caused by receptor mutation or treatment with a specific inhibitor (LY294002) did not prevent proliferation but resulted in an increase in the G1 phase content and doubling time of cell cultures. The reduced proliferation of cells lacking cytokine-induced PI3K activity could be partially restored by overexpressing constitutively active Akt. Inhibition of PI3K activity decreased the proportion of cytokine-treated cells entering S phase and was associated with a significant reduction in cytokine-induced phosphorylation and activation of Cdk2. By contrast, Cdk4 activity and p27(Kip1) expression were not significantly altered by inhibition of PI3K. Together, these observations identify a mechanism through which cytokine-activated PI3K contributes to G1 to S phase progression in factor-dependent hematopoietic cells by enhancing the phosphorylation and activation of Cdk2.

Effects of a conditionally active v-ErbB and an EGF-R inhibitor on transformation of NIH-3T3 cells and abrogation of cytokine dependency of hematopoietic cells

Epidermal growth factor (EGF) and its cognate receptor (EGF-R) are often dysregulated in human neoplasia. Moreover, EGF-R-transformed cell lines have constitutive EGF-R activity, which makes elucidation of its effects difficult to determine. In the following studies, the effects of a novel conditionally activated form of EGF-R, v-ErbB:ER, on the morphological transformation of NIH-3T3 cells and the abrogation of hematopoietic cell cytokine dependence were investigated. The v-ErbB ES-4 oncogene was fused to the hormone binding domain of the estrogen receptor (ER). This construct, v-ErbB:ER, requires beta-estradiol or 4-OH tamoxifen for activation. v-ErbB:ER conditionally transformed NIH-3T3 cells and abrogated cytokine dependence of hematopoietic cells. Stimulation of v-ErbB:ER activity resulted in the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt and Raf/MEK/ERK kinase cascades. To determine the importance of these signal transduction pathways, the conditionally transformed hematopoietic cells were treated with EGF-R, PI3K and MEK inhibitors. The EGF-R inhibitor AG1478 effectively inhibited MEK, ERK and Akt activation, and induced apoptosis when the cells were grown in response to v-ErbB:ER. Apoptosis was observed at 100- to 1000-fold lower concentrations of AG1478 when the cells were grown in response to v-ErbB:ER as opposed to IL-3. Furthermore, the parental, BCR-ABL- and Raf-transformed cells were only susceptible to the apoptosis-inducing effects of AG1478 at the highest concentrations demonstrating the specificity of these inhibitors. MEK or PI3K inhibitors suppressed ERK or Akt activation, respectively, and induced apoptosis in the v-ErbB:ER-responsive cells. However, MEK and PI3K inhibitors only induced apoptosis at 1000-fold higher concentrations than the EGFR inhibitor. This novel v-ErbB:ER construct and these conditionally transformed cell lines will be useful to further elucidate ErbB-mediated signal transduction and to determine the effectiveness of various inhibitors in targeting different aspects of EGF-R-mediated signal transduction and malignant transformation.

Signal transduction and functional changes in neutrophils with aging

It is well known that the immune response decreases during aging, leading to a higher susceptibility to infections, cancers and autoimmune disorders. Most widely studied have been alterations in the adaptive immune response. Recently, the role of the innate immune response as a first-line defence against bacterial invasion and as a modulator of the adaptive immune response has become more widely recognized. One of the most important cell components of the innate response is neutrophils and it is therefore important to elucidate their function during aging. With aging there is an alteration of the receptor-driven functions of human neutrophils, such as superoxide anion production, chemotaxis and apoptosis. One of the alterations underlying these functional changes is a decrease in signalling elicited by specific receptors. Alterations were also found in the neutrophil membrane lipid rafts. These alterations in neutrophil functions and signal transduction that occur during aging might contribute to the significant increase in infections in old age.

Signaling through JAK2-STAT5 pathway is essential for IL-3-induced activation of microglia

Microglia, the resident macrophage of the brain, mediates immune and inflammatory responses in the central nervous system (CNS). Activation of microglia and secretion of inflammatory cytokines associate with the pathogenesis of CNS diseases, including multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease, prion disease, and AIDS dementia. Microbial pathogens, cytokines, chemokines, and costimulatory molecules are potent inducers of microglial activation in the CNS. Signaling through its receptor, IL-3 induces the activation of JAK-STAT and MAP kinase pathways in microglial cells. In this study, we found that in vitro treatment of EOC-20 microglial cells with tyrphostin AG490 blocked IL-3-induced tyrosine phosphorylation of JAK2, STAT5A, and STAT5B signaling proteins. Stable transfection of EOC-20 cells with a dominant negative JAK2 mutant also blocked IL-3-induced tyrosine phosphorylation of JAK2, STAT5A, and STAT5B in microglia. The blockade of JAK2-STAT5 pathway resulted in a decrease in IL-3-induced proliferation and expression of CD40 and major histocompatibility complex class II molecules in microglia. These findings highlight the fact that JAK2-STAT5 signaling pathway plays a critical role in mediating IL-3-induced activation of microglia.

Synergism of cytoplasmic kinases in IL6-induced ligand-independent activation of androgen receptor in prostate cancer cells

IL6 is a pleiotropic cytokine which has been implicated in ligand-independent activation of androgen receptor in prostate cancer cells. Here, we present the evidence that two cytoplasmic kinases Pim1 and Etk are involved in this process. We showed that Pim1 is expressed in all prostate cancer cell lines examined. Both the expression level and the kinase activity of Pim1 are regulated by IL6 in these cells. Furthermore, we showed that IL6 downstream tyrosine kinase Etk can induce tyrosine phosphorylation of Pim1 which is correlated with its kinase activity. Mutation of the conserved Tyrosine 218 in the activation loop results in reduced kinase activity of Pim1. Interestingly, Etk can also be activated by Pim1 when they are coexpressed in prostate cancer cells, suggesting a possible positive feedback loop between Etk and Pim1. It appears that both Pim1 and Etk are required for IL6-induced activation of androgen receptor-mediated transcription in prostate cancer cells because overexpression of the kinase-deficient form of either Pim1 or Etk dramatically blocks the IL6 effect. Coexpression of the two kinases together but neither one alone is sufficient to activate ARE-containing promoter. Taken together, our data suggest a synergism of Ser/Thr kinase Pim1 and tyrosine kinase Etk in IL6 signaling and provide new insights into ligand-independent activation of androgen receptor in prostate cancer cells.

Granulocyte-macrophage colony-stimulating factor and interleukin-3 induce cell cycle progression through the synthesis of c-Myc protein by internal ribosome entry site-mediated translation via phosphatidylinositol 3-kinase pathway in human factor-dependent leukemic cells

To investigate the roles of c-myc during hematopoietic proliferation induced by growth factors, we used factor-dependent human leukemic cell lines (MO7e and F36P) in which proliferation, cell cycle progression, and c-Myc expression were strictly regulated by granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3). In these cell lines, both c-myc mRNA and c-Myc protein stability were not affected by GM-CSF and IL-3, suggesting a regulation of c-Myc protein at the translational level. However, rapamycin, an inhibitor of cap-dependent translation, did not block c-myc induction by GM-CSF and IL-3. Thus, we studied the cap-independent translation, the internal ribosome entry site (IRES), during c-Myc protein synthesis using dicistronic reporter gene plasmids and found that GM-CSF and IL-3 activated c-myc IRES to initiate translation. c-myc IRES activation, c-Myc protein expression, and cell cycle progression were all blocked by a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002. In another factor-dependent cell line, UT7, we observed the cell cycle progression and up-regulation of c-Myc protein, c-myc mRNA, and c-myc IRES simultaneously, which were all inhibited by LY294002. Results indicate that hematopoietic growth factors induce cell cycle progression via IRES-mediated translation of c-myc though the PI3K pathway in human factor-dependent leukemic 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.

Erythropoietin receptors associate with a ubiquitin ligase, p33RUL, and require its activity for erythropoietin-induced proliferation

The proliferation and survival of hematopoietic cells is strictly regulated by cytokine growth factors that act through receptors of the Type I cytokine receptor family, including erythropoietin (Epo) and its receptor, EpoR. Mitogenic signaling by these receptors depends on activation of Jak tyrosine kinases. However, other required components of this pathway have not been fully identified. In a screen for proteins that interact with EpoR and Jak2, we identified a novel member of the U-box family of ubiquitin ligases. This receptor-associated ubiquitin ligase, RUL, co-precipitated with EpoR from mammalian cells and mediated ubiquitination of EpoR. Also, endogenously expressed RUL was rapidly and transiently phosphorylated on serine after cytokine treatment of factor-dependent hematopoietic cells. Expression of ubiquitin ligase-deficient mutants of RUL inhibited Epo-induced expression of c-myc and bcl-2, two immediate-early genes normally associated with Epo-induced cell growth. Consistent with that finding, expression of mutant RUL also inhibited Epo-dependent proliferation and survival of factor-dependent cells. Together, these observations suggest that RUL is a required component of mitogenic signaling by EpoR. We also show that RUL is phosphorylated in response to growth factors that act through non-cytokine receptors, suggesting that RUL may function as a common regulator of mitogenesis.

Rapamycin specifically interferes with GM-CSF signaling in human dendritic cells, leading to apoptosis via increased p27KIP1 expression

The longevity of dendritic cells (DCs) is a critical regulatory factor influencing the outcome of immune responses. Recently, we demonstrated that the immunosuppressive drug rapamycin (Rapa) specifically induces apoptosis in DCs but not in other myeloid cell types. The present study unraveled the mechanism used by Rapa to induce apoptosis in human monocyte-derived DCs. Our data demonstrate that granulocyte-macrophage colony-stimulating factor (GM-CSF) preserves DC survival specifically via the phosphatidylinositol-3 lipid kinase/mammalian target of rapamycin (PI3K/mTOR) signaling pathway, which is abrogated by Rapa at the level of mTOR. Disruption of this GM-CSF signaling pathway induced loss of mitochondrial membrane potential, phosphatidyl-serine exposure, and nuclear changes. Apoptosis of these nonproliferating DCs was preceded by an up-regulation of the cell cycle inhibitor p27(KIP1). Overexpression of p27(KIP1) in DCs using adenoviral gene transduction revealed that apoptosis is directly regulated by p27(KIP1). Furthermore, both overexpression of p27(KIP1) and disruption of the GM-CSF/PI3K/mTOR signaling pathway decreased the expression of the antiapoptotic protein mcl-1. This mTOR/p27(KIP1)/mcl-1 survival seems unique for DCs and may provide novel opportunities to influence immune responses by specific interference with the life span of these cells.

Characterization of p21Ras-mediated apoptosis induced by protein kinase C inhibition and application to human tumor cell lines

Suppression of PKC activity can selectively induce apoptosis in cells expressing a constitutively activated p21Ras protein. We demonstrate that continued expression of p21Ras activity is required in PKC-mediated apoptosis because farnesyltransferase inhibitors abrogated the loss of viability in p21Ras-transformed cells occurring following PKC inhibition. Studies utilizing gene transfer or viral vectors demonstrate that transient expression of oncogenic p21Ras activity is sufficient for induction of apoptosis by PKC inhibition, whereas physiologic activation of p21Ras by growth factor is not sufficient to induce apoptosis. Mechanistically, the p21Ras-mediated apoptosis induced by PKC inhibition is dependent upon mitochondrial dysregulation, with a concurrent loss of mitochondrial membrane potential (psim). Cyclosporine A, which prevented the loss of psim, also inhibited HMG-induced DNA fragmentation in cells expressing an activated p21Ras. Induction of apoptosis by PKC inhibition in human tumors with oncogenic p21Ras mutations was demonstrated. Inhibition of PKC caused increased apoptosis in MIA-PaCa-2, a human pancreatic tumor line containing a mutated Ki-ras allele, when compared to HS766T, a human pancreatic tumor line with normal Ki-ras alleles. Furthermore, PKC inhibition induced apoptosis in HCT116, a human colorectal tumor line containing an oncogenic Ki-ras allele but not in a subline (Hke3) in which the mutated Ki-ras allele had been disrupted. The PKC inhibitor 1-O-hexadecyl-2-O-methyl-rac-glycerol (HMG), significantly reduced p21Ras-mediated tumor growth in vivo in a nude mouse MIA-PaCa-2 xenograft model. Collectively these studies suggest the therapeutic feasibility of targeting PKC activity in tumors expressing an activated p21Ras oncoprotein.

E4BP4/NFIL3, a PAR-related bZIP factor with many roles

E4BP4, a mammalian basic leucine zipper (bZIP) transcription factor, was first identified through its ability to bind and repress viral promoter sequences. Subsequently, E4BP4 and homologues in other species have been implicated in a diverse range of processes including commitment to cell survival versus apoptosis, the anti-inflammatory response and, most recently, in the mammalian circadian oscillatory mechanism. In some of these cases at least, E4BP4 appears to act antagonistically with members of the related PAR family of transcription factors with which it shares DNA-binding specificity. This diversity of function is mirrored by the regulatory pathways impinging on E4BP4, which include regulation by ras via the lymphokine IL-3 in murine B-cells, by thyroid hormone during Xenopus tail resorption, by glucocorticoids in murine fibroblasts and by calcium in rat smooth muscle cells. This article will cover the unfolding role/s of and regulation of E4BP4, E4BP4-like proteins and PAR factors in species as diverse as mouse and C. elegans.

Role of p38 and ERK MAP kinase in proliferation of erythroid progenitors in response to stimulation by soluble and membrane isoforms of stem cell factor

Two alternatively spliced stem cell factor (SCF) transcripts encode protein products, which differ in the duration of membrane presentation. One form, soluble SCF (S-SCF) gets rapidly processed to yield predominantly secreted protein. The other form, membrane-associated SCF (MA-SCF) lacks the primary proteolytic cleavage site but is cleaved slowly from an alternate site, and thus represents a more stable membrane form of SCF. Mutants of SCF that lack the expression of MA-SCF (Steel-dickie) or possess a defect in its presentation (Steel17H) manifest deficiencies in erythroid cell development. In this study, we have compared the consequence(s) of activating Kit, the receptor for SCF by MA-SCF with S-SCF, and an obligate membrane-restricted (MR) form of SCF (MR-SCF) on erythroid cell survival, proliferation, cell cycle progression, and the activation of p38 and ERK MAP kinase pathways. Activation of Kit by MR-SCF was associated with a significantly lower incidence of apoptosis and cell death in erythroid cells compared to either other isoform. MR- or MA-SCF–induced stimulation of erythroid cells resulted in similar and significantly greater proliferation and cell cycle progression compared to soluble SCF. The increase in proliferation and cell cycle progression via MA- or MR-SCF stimulation correlated with sustained and enhanced activation of p38 and ERK MAP kinase pathways. In addition, MR- or MA-SCF–induced proliferation was more sensitive to the inhibitory effects of ERK inhibitor compared to S-SCF–induced proliferation. In contrast, soluble SCF-induced proliferation was more sensitive to the inhibitory effects of p38 inhibitor compared with MR- or MA-SCF. These results suggest that different isoforms of SCF may use different biochemical pathways in stimulation of survival and/or proliferation of erythroid cells.

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.

Modulation by growth factors of the expression of interleukin 3 and granulocyte-macrophage colony-stimulating factor receptor common chain beta c

Interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 5 (IL-5) exert their biological activities through interaction with cell-surface receptors that consist of two subunits, a specific a subunit and a common beta transducing subunit (beta c). We have evaluated the effect of growth factors on the expression of beta c in normal monocytes. Addition of either GM-CSF or M-CSF to monocytes elicited a marked increase of beta c chain expression, a phenomenon seemingly related to a stimulation of the transcriptional activity of this gene mediated through an enhancement of the PU.1 DNA binding activity. Interestingly, during the activation of beta c chain expression by growth factors a switch from the synthesis of the truncated betaIT to the full-length beta c was observed. Similar observations have been made also in the growth factor-dependent erythroleukemic cell line TF-1, showing that GM-CSF deprivation elicited a marked decrease of beta c chain expression.

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.

c-myc is required for osteoclast differentiation

The role of the receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL)-a tumor necrosis factor (TNF)-related cytokine-in osteoclast formation has been established clearly. However, the downstream signaling pathways activated by this cytokine remain largely unknown. To identify genes that play a role in osteoclastogenesis, we used RAW 264.7 mouse monocytes as a model system for the differentiation of multinucleated osteoclasts from mononucleated precursors. RAW 264.7 cells were induced with RANKL to form multinucleated giant osteoclast-like cells (OCLs) that expressed a number of osteoclast-specific markers and were able to form resorption pits on both calcium phosphate films and bone slices. This system was used to identify genes that are regulated by RANKL and may play a role in osteoclast differentiation. The proto-oncogene c-myc was strongly up-regulated in RANKL-induced OCLs but was absent in undifferentiated cells. Expression of Myc partners Max and Mad, on the other hand, was constant during OCL differentiation. We expressed a dominant negative Myc in RAW 264.7 cells and were able to block RANKL-induced OCL formation. Northern Blot analysis revealed a delay and a significant reduction in the level of messenger RNA (mRNA) for tartrate-resistant acid phosphatase (TRAP) and cathepsin K. We conclude that c-myc is a downstream target of RANKL and its expression is required for RANKL-induced osteoclastogenesis.