Engagement of the CrkL adapter in interleukin-5 signaling in eosinophils

Transcriptional Regulation of the Human IL5RA Gene through Alternative Promoter Usage during Eosinophil Development

Regulation of the IL-5 receptor alpha (IL5RA) gene is complicated, with two known promoters (P1 and P2) driving transcription, and two known isoforms (transmembrane and soluble) dichotomously affecting the signaling potential of the protein products. Here, we sought to determine the patterns of P1 and P2 promoter usage and transcription factor occupancy during primary human eosinophil development from CD34⁺ hematopoietic stem cell progenitors. We found that during eosinophilopoiesis, both promoters were active but subject to distinct temporal regulation, coincident with combinatorial interactions of transcription factors, including GATA-1, PU.1, and C/EBP family members. P1 displayed a relatively constant level of activity throughout eosinophil development, while P2 activity peaked early and waned thereafter. The soluble IL-5Rα mRNA peaked early and showed the greatest magnitude fold-induction, while the signaling-competent transmembrane isoform peaked moderately. Two human eosinophilic cell lines whose relative use of P1 and P2 were similar to eosinophils differentiated in culture were used to functionally test putative transcription factor binding sites. Transcription factor occupancy was then validated in primary cultures by ChIP. We conclude that IL-5-dependent generation of eosinophils from CD34⁺ precursors involves complex and dynamic activity including both promoters, several interacting transcription factors, and both signaling and antagonistic protein products.

IL-7-induced phosphorylation of the adaptor Crk-like and other targets

IL-7 is required for T cell differentiation and mature T cell homeostasis and promotes pro-B cell proliferation and survival. Tyrosine phosphorylation plays a central role in IL-7 signaling. We identified by two-dimensional electrophoresis followed by anti-phosphotyrosine immunoblotting and mass spectrometry sixteen tyrosine phosphorylated proteins from the IL-7-dependent cell line D1. IL-7 stimulation induced the phosphorylation of the proteins STI1, ATIC and hnRNPH, involved in pathways related to survival, proliferation and gene expression, respectively, and increased the phosphorylation of CrkL, a member of a family of adaptors including the highly homologous Crk isoforms CrkII and CrkI, important in multiple signaling pathways. We observed an increased phosphorylation of CrkL in murine pro-B cells and in murine and human T cells. In addition, IL-7 increased the association of CrkL with the transcription factor Stat5, essential for IL-7 pro-survival activity. The selective tyrosine kinase inhibitor Imatinib. counteracted the IL-7 pro-survival effect in D1 cells and decreased CrkL phosphorylation. These data suggested that CrkL could play a pro-survival role in IL-7-mediated signaling. We observed that pro-B cells also expressed, in addition to CrkL, the Crk isoforms CrkII and CrkI and therefore utilized pro-B cells conditionally deficient in all three to evaluate the role of these proteins. The observation that the IL-7 pro-survival effect was reduced in Crk/CrkL conditionally-deficient pro-B cells further pointed to a pro-survival role of these adaptors. To further evaluate the role of these proteins, gene expression studies were performed in Crk/CrkL conditionally-deficient pro-B cells. IL-7 decreased the transcription of the receptor LAIR1, which inhibits B cell proliferation, in a Crk/CrkL-dependent manner, suggesting that the Crk family of proteins may promote pro-B cell proliferation. Our data contribute to the understanding of IL-7 signaling and suggest the involvement of Crk family proteins in pathways promoting survival and proliferation.

p130Cas scaffolds the signalosome to direct adaptor-effector cross talk during Kaposi's sarcoma-associated herpesvirus trafficking in human microvascular dermal endothelial cells

Kaposi's sarcoma-associated herpesvirus (KSHV) interacts with cell surface receptors, such as heparan sulfate, integrins (α3β1, αVβ3, and αVβ5), and EphrinA2 (EphA2), and activates focal adhesion kinase (FAK), Src, phosphoinositol 3-kinase (PI3-K), c-Cbl, and RhoA GTPase signal molecules early during lipid raft (LR)-dependent productive macropinocytic entry into human dermal microvascular endothelial cells. Our recent studies have identified CIB1 as a signal amplifier facilitating EphA2 phosphorylation and subsequent cytoskeletal cross talk during KSHV macropinocytosis. Although CIB1 lacks an enzymatic activity and traditional adaptor domain or known interacting sequence, it associated with the KSHV entry signal complex and the CIB1-KSHV association was sustained over 30 min postinfection. To identify factors scaffolding the EphA2-CIB1 signal axis, the role of major cellular scaffold protein p130Cas (Crk-associated substrate of Src) was investigated. Inhibitor and small interfering RNA (siRNA) studies demonstrated that KSHV induced p130Cas in an EphA2-, CIB1-, and Src-dependent manner. p130Cas and Crk were associated with KSHV, LRs, EphA2, and CIB1 early during infection. Live-cell microscopy and biochemical studies demonstrated that p130Cas knockdown did not affect KSHV entry but significantly reduced productive nuclear trafficking of viral DNA and routed KSHV to lysosomal degradation. p130Cas aided in scaffolding adaptor Crk to downstream guanine nucleotide exchange factor phospho-C3G possibly to coordinate GTPase signaling during KSHV trafficking. Collectively, these studies demonstrate that p130Cas acts as a bridging molecule between the KSHV-induced entry signal complex and the downstream trafficking signalosome in endothelial cells and suggest that simultaneous targeting of KSHV entry receptors with p130Cas would be an attractive potential avenue for therapeutic intervention in KSHV infection.

IMPORTANCE

Eukaryotic cell adaptor molecules, without any intrinsic enzymatic activity, are well known to allow a great diversity of specific and coordinated protein-protein interactions imparting signal amplification to different networks for physiological and pathological signaling. They are involved in integrating signals from growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. The present study identifies human microvascular dermal endothelial (HMVEC-d) cellular scaffold protein p130Cas (Crk-associated substrate) as a platform to promote Kaposi's sarcoma-associated herpesvirus (KSHV) trafficking. Early during KSHV de novo infection, p130Cas associates with lipid rafts and scaffolds EphrinA2 (EphA2)-associated critical adaptor members to downstream effector molecules, promoting successful nuclear delivery of the KSHV genome. Hence, simultaneous targeting of the receptor EphA2 and scaffolding action of p130Cas can potentially uncouple the signal cross talk of the KSHV entry-associated upstream signal complex from the immediate downstream trafficking-associated signalosome, consequently routing KSHV toward lysosomal degradation and eventually blocking KSHV infection and associated malignancies.

Developmental, malignancy-related, and cross-species analysis of eosinophil, mast cell, and basophil siglec-8 expression

OBJECTIVE

The aim of this study is to determine when during hematopoiesis Siglec-8 gets expressed, whether it is expressed on hematologic malignancies, and if there are other non-human species that express Siglec-8.

METHODS

Siglec-8 mRNA and cell surface expression was monitored during in vitro maturation of human eosinophils and mast cells. Flow cytometry was performed on human blood and bone marrow samples, and on blood samples from dogs, baboons, and rhesus and cynomolgus monkeys.

RESULTS

Siglec-8 is a late maturation marker. It is detectable on eosinophils and basophils from subjects with chronic eosinophilic leukemia, chronic myelogenous leukemia, and on malignant and non-malignant bone marrow mast cells, as well as the HMC-1.2 cell line. None of the Siglec-8 monoclonal antibodies tested recognized leukocytes from dogs, baboons, and rhesus and cynomolgus monkeys.

CONCLUSIONS

Siglec-8-based therapies should not target immature human leukocytes but should recognize mature and malignant eosinophils, mast cells, and basophils. So far, there is no suitable species for preclinical testing of Siglec-8 monoclonal antibodies.

Signalling to actin: role of C3G, a multitasking guanine-nucleotide-exchange factor

C3G (Crk SH3-domain-binding guanine-nucleotide-releasing factor) is a ubiquitously expressed member of a class of molecules called GEFs (guanine-nucleotide-exchange factor) that activate small GTPases and is involved in pathways triggered by a variety of signals. It is essential for mammalian embryonic development and many cellular functions in adult tissues. C3G participates in regulating functions that require cytoskeletal remodelling such as adhesion, migration, maintenance of cell junctions, neurite growth and vesicle traffic. C3G is spatially and temporally regulated to act on Ras family GTPases Rap1, Rap2, R-Ras, TC21 and Rho family member TC10. Increased C3G protein levels are associated with differentiation of various cell types, indicating an important role for C3G in cellular differentiation. In signalling pathways, C3G serves functions dependent on catalytic activity as well as protein interaction and can therefore integrate signals necessary for the execution of more than one cellular function. This review summarizes our current knowledge of the biology of C3G with emphasis on its role as a transducer of signals to the actin cytoskeleton. Deregulated C3G may also contribute to pathogenesis of human disorders and therefore could be a potential therapeutic target.

Combined activities of secretory phospholipases and eosinophil lysophospholipases induce pulmonary surfactant dysfunction by phospholipid hydrolysis

BACKGROUND

Surfactant dysfunction is implicated in small airway closure in asthma. Increased activity of secretory phospholipase A(2) (sPLA(2)) in the airways is associated with asthma exacerbations. Phosphatidylcholine, the principal component of pulmonary surfactant that maintains small airway patency, is hydrolyzed by sPLA(2). The lysophosphatidylcholine product is the substrate for eosinophil lysophospholipases.

OBJECTIVE

To determine whether surfactant phospholipid hydrolysis by the combined activities of sPLA(2)s and eosinophil lysophospholipases induces surfactant dysfunction.

METHODS

The effect of these enzymes on surfactant function was determined by capillary surfactometry. Thin layer chromatography was used to correlate enzyme-induced changes in surfactant phospholipid composition and function. Phosphatidylcholine and its hydrolytic products were measured by using mass spectrometry.

RESULTS

Eosinophils express a 25-kd lysophospholipase and group IIA sPLA(2). Phospholipase A(2) alone induced only a small decrease in surfactant function, and 25-kd lysophospholipase alone degraded lysophosphatidylcholine but had no effect on surfactant function. The combined actions of sPLA(2) and lysophospholipase produced dose-dependent and time-dependent losses of surfactant function, concomitant with hydrolysis of phosphatidylcholine and lysophosphatidylcholine. Lysates of AML14.3D10 eosinophils induced surfactant dysfunction, indicating these cells express all the necessary lipolytic activities. In contrast, lysates of blood eosinophils required exogenous phospholipase A(2) to induce maximal surfactant dysfunction.

CONCLUSION

The combined activities of sPLA(2)s and eosinophil lysophospholipases are necessary to degrade surfactant phospholipids sufficiently to induce functional losses in surfactant activity as reported in asthma.

CLINICAL IMPLICATIONS

The phospholipases and lysophospholipases expressed by eosinophils or other airway cells may represent novel therapeutic targets for blocking surfactant degradation, dysfunction, and peripheral airway closure in asthma.

Thiocyanate-dependent induction of endothelial cell adhesion molecule expression by phagocyte peroxidases: a novel HOSCN-specific oxidant mechanism to amplify inflammation

Both eosinophil peroxidase (EPO) and neutrophil myeloperoxidase (MPO) preferentially oxidize SCN(-) to generate HOSCN, a weak, sulfhydryl-reactive oxidant, as a major physiologic product. We here show that HOSCN is a uniquely potent phagocyte oxidant inducer of E-selectin, ICAM-1, and VCAM-1 expression in HUVEC as detected by Western blot and flow cytometry. EMSA and inhibitor studies show that HOSCN up-regulation of these adhesion molecules is transcriptionally mediated through a mechanism that is dependent upon activation of the NF-kappaB p65/p50 transcription factor and constitutively suppressed by PI3K-Akt pathway activity. HUVEC monolayers exposed to HOSCN bind 8-fold more neutrophils and 3- to 4-fold more Aml14.3D10 cells (a differentiated cell line model of mature eosinophils) than control monolayers. Blocking Ab studies confirm the involvement of E-selectin and ICAM-1 but not VCAM-1 in neutrophil adhesion and of all three in Aml14.3D10 adhesion. Intraperitoneal injection of HOSCN evoked an 8-fold increase in neutrophil peritoneal extravasation. In addition to NF-kappaB, HOSCN also activates the potentially proinflammatory transcription factors Stat4, CDP, GRE, CBF, Ets-1/PEA3, and TFIID, a pattern easily distinguishable from that induced by LPS. These results suggest that phagocyte peroxidases function to amplify inflammation through a novel, HOSCN-specific oxidant mechanism.

Systematic Evaluation of Transcellular Activities of Secretory Phospholipases A2

The mechanisms by which secretory phospholipase A2 (PLA2) exerts cellular effects are not fully understood. To elucidate these mechanisms, we systematically and quantitatively assessed the activities of human group IIA, V, and X PLA2s on originating and neighboring cells using orthogonal fluorogenic substrates in various mixed cell systems. When HEK293 cells stably expressing each of these PLA2s were mixed with non-transfected HEK293 cells, group V and X PLA2s showed strong transcellular lipolytic activity, whereas group IIA PLA2 exhibited much lower transcellular activity. The transcellular activity of group V PLA2 was highly dependent on the presence of cell surface heparan sulfate proteoglycans of acceptor cells. Activation of RBL-2H3 and DLD-1 cells that express endogenous group V PLA2 led to the secretion of group V PLA2 and its transcellular action on neighboring human neutrophils and eosinophils, respectively. Similarly, activation of human bronchial epithelial cells, BEAS-2B, caused large increases in arachidonic acid and leukotriene C4 release from neighboring human eosinophils. Collectively, these studies show that group V and X PLA2s can act transcellularly on mammalian cells and suggest that group V PLA2 released from neighboring cells may function in triggering the activation of inflammatory cells under physiological conditions.

Eosinophil-fibroblast interactions induce fibroblast IL-6 secretion and extracellular matrix gene expression: implications in fibrogenesis

BACKGROUND

Eosinophils are frequently associated with tissue remodeling and fibrosis in allergic and other diseases and animal models. Their close physical proximity to fibroblasts at sites of tissue remodeling strongly implicates them in fibrogenesis, including subepithelial fibrosis and airway remodeling characteristic of asthma.

OBJECTIVE

To identify the mediators and characterize the mechanisms underlying the fibrogenic activities of eosinophils.

METHODS

A coculture system of blood eosinophils or eosinophil cell lines with normal fibroblasts was used to assess their ability to induce a fibrogenic fibroblast phenotype, including IL-6 secretion and mRNA expression, and induction of genes involved in extracellular matrix production and homeostasis. The mediators of these responses were identified by using transwell barrier cocultures, eosinophil-conditioned media, and cytokine-specific antibody neutralization.

RESULTS

Eosinophil-fibroblast coculture induced potent fibroblast IL-6 secretion and mRNA expression, responses further enhanced by IL-5. The soluble nature of the eosinophil-derived mediators was demonstrated by using eosinophil-fibroblast coculture in the presence of permeable transwell barriers, and fibroblast culture in eosinophil-conditioned media, indicating that cell contact was not required. Induction of fibroblast IL-6 expression was accompanied by increased expression of fibronectin and the extracellular matrix regulatory genes plasminogen activator inhibitor 1 and tissue inhibitor of metalloproteinase 1. Antibody neutralization identified the principal eosinophil-derived mediator of fibroblast IL-6 expression as IL-1beta (>60%), with lesser contributions from IL-1alpha, IL-4, and TGF-beta (10% to 20%).

CONCLUSION

Eosinophils express at least 2 potent mediators (IL-1beta and TGF-beta) that induce a fibrogenic fibroblast phenotype, strongly supporting a role for the eosinophil in the dysregulation of extracellular matrix homeostasis and consequent tissue remodeling and fibrosis in eosinophil-associated diseases.

Comparative proteomic analysis of peripheral blood eosinophils from healthy donors and atopic dermatitis patients with eosinophilia

Atopic dermatitis (AD) is an allergic disease that has recently shown a dramatic increase of incidence in developed countries. Eosinophilia, the accumulation of eosinophils, occurs in AD patients through an anti-apoptotic mechanism. To understand the target proteins involved in the anti-apoptotic signaling of eosinophilia, we used a proteomic approach to analyze eosinophil proteins from AD patients with eosinophilia and healthy donors. Protein spots in two-dimensional electrophoresis (2-DE) gels were identified with peptide mass fingerprinting (PMF) based on matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) and database searching. More spots were observed in the 2-DE proteome map from AD patient samples (1310 +/- 58 spots) than in those from healthy donors (1121 +/- 40 spots). We identified 51 proteins affected by eosinophilia: 19 related to signaling, 8 involved in regulation of metabolism, 4 related to apoptosis, and 3 involved in inflammation. The other identified proteins were associated with transcription, RNA processing, translation, the cytoskeleton, and unknown functions. Among the identified proteins, we observed prominent increases in the expressions of cyclinA2, voltage-dependent anion channel protein 2, and 38 kDa FK506 binding protein 8 in eosinophils from AD patients in comparison to healthy donors. PMF and immunoblotting of a single spot that was expressed in eosinophils from healthy individuals but not in AD patients identified the protein as phosphorylated growth receptor binding 7 (Grb7) adaptor protein. Increased phosphorylation of Grb7 and its upstream signaling protein, focal adhesion kinase (FAK), was detected in low viability eosinophils such as those from healthy donors or in cultured eosinophils (AML14.3D10 cells) treated with dexamethasone. These results suggest that phosphorylation of Grb7 and the expressions of cyclinA2, voltage-dependent anion channel protein 2, and 38 kDa FK506 binding protein 8 may be related with the anti-apoptosis mechanism of eosinophilia.

Hydrogen peroxide generated extracellularly by receptor-ligand interaction facilitates cell signaling

Reactive oxygen species (ROS) are key components of postreceptor intracellular signaling pathways; however, the role of ROS in signal initiation is uncertain. We discovered that receptor-ligand interaction caused the generation of hydrogen peroxide (H2O2). Using members of the hematopoietin receptor superfamily, as well as EGF receptor, we show that H2O2 is generated by specific receptor-ligand interaction in cells and in cell-free systems. With cognate ligand, the extracellular domain of the receptor was sufficient for H2O2 generation. We also found that production of H2O2 was diminished in a granulocyte-macrophage colony-stimulating factor receptor mutant unable to bind ligand. Exogenously added H2O2 induced signaling in the absence of ligand, whereas catalase and a membrane-bound peroxiredoxin inhibited ligand-dependent signaling. Our results suggest that H2O2 produced by receptor-ligand interaction is involved as a chemical mediator that facilitates cell signaling.

Differential effect of dexamethasone on cell death and STAT5 activation during in vitro eosinopoiesis

Glucorticoids reduce eosinophilia through a direct effect on eosinophils or indirectly on cells producing cytokines and chemokines. Conflicting data have been previously reported concerning glucocorticoid effects on eosinopoiesis. To elucidate this point, dexamethasone was added during eosinophil differentiation of CD34+ cells. Dexamethasone enhanced proliferation and differentiation through an early effect on immature cells. Dexamethasone inhibited apoptosis during early differentiation, whereas death of mature cells was increased. Signal transducer and activator of transcription 5 (STAT5) is a transcription factor involved in the proliferation, differentiation and survival of haematopoietic cells, which interacts with glucocorticoid receptor. Activation of STAT5 by interleukin-5 was investigated during eosinophil differentiation. Long isoforms of STAT5 were activated during the entire period in the culture as well as in blood eosinophils, while short isoforms were only activated during early differentiation. Short isoforms were less activated in the presence of dexamethasone. This suggests that dexamethasone could act on proliferation, differentiation and apoptosis during eosinophil differentiation through an association of STAT5 with the glucocorticoid receptor.

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

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

The p38 mitogen-activated protein kinase pathway and its role in interferon signaling

Interferons (IFNs) are pleiotropic cytokines that exhibit multiple biological effects on cells and tissues. IFN receptors are expressed widely in mammalian cells and virtually all different cell types express them on their surface. The Type I IFN receptor has a multichain structure, composed of at least two distinct receptor subunits, IFNalphaR1 and IFNalphaR2. Two Jak-kinases, Tyk-2 and Jak-1, associate with the different receptor subunits and are activated in response to IFNalpha or IFNbeta to regulate engagement of multiple downstream signaling cascades. These include the Stat-pathway, whose function is essential for transcriptional activation of IFN-sensitive genes, and the insulin receptor substrate pathway, which regulates downstream activation of the phosphatidyl-inositol-3' kinase. Members of the Map family of kinases are also activated by the Type I IFN receptor and participate in the generation of IFN signals. The p38 Map kinase pathway appears to play a very important role in the induction of IFN responses. p38 is rapidly activated during engagement of the Type I IFN receptor, and such an activation is regulated by the small G-protein Rac1, which functions as its upstream effector in a tyrosine kinase-dependent manner. The activated form of p38 regulates downstream activation of other serine kinases, notably MapKapK-2 and MapKapK-3, indicating the existence of Type I IFN-dependent signaling cascades activated downstream of p38. Extensive studies have shown that p38 plays a critical role in Type I IFN-dependent transcriptional regulation, without modifying activation of the Stat-pathway. It is now well established that the function of p38 is essential for gene transcription via ISRE or GAS elements, but has no effects on the phosphorylation of Stat-proteins, the formation of Stat-complexes, and their binding to the promoters of IFN-sensitive genes. As Type I IFNs regulate gene expression for proteins with antiviral properties, it is not surprising that pharmacological inhibition of the p38 pathway blocks induction of IFNalpha-antiviral responses. In addition, pharmacological inhibition of p38 abrogates the suppressive effects of Type I IFNs on normal human hematopoietic progenitors, indicating a critical role for this signaling cascade in the induction of the regulatory effects of Type I IFNs on hematopoiesis. p38 is also activated during IFNalpha-treatment of primary leukemia cells from patients with chronic myelogenous leukemia. Such activation is required for IFNalpha-dependent suppression of leukemic cell progenitor growth, indicating that this pathway plays a critical role in the induction of the antileukemic effects of IFNalpha.

Glucocorticoid regulation of human eosinophil gene expression

Molecular analysis of steroid-regulated gene expression in freshly isolated human eosinophils is difficult due to the inherent high rate of spontaneous apoptosis and elevated levels of endogenous ribonucleases. To circumvent these limitations, we determined if the human eosinophilic cell line EoL-1 could serve as an in vitro model of glucocorticoid signaling. We found by optimizing growth conditions in low serum-containing media that dexamethasone (Dex) treatment of EoL-1 cells induced an apoptotic pathway that was inhibited by interleukin-5 (IL-5). Moreover, gene expression profiling using RNA from untreated EoL-1 cells and from freshly isolated human eosinophils identified 380 commonly expressed genes, including the eosinophil markers granule major basic protein, prostaglandin-endoperoxide synthase 1 and arachidonate 15-lipoxygenase. Expression profiling was performed using EoL-1 cells that had been treated with dexamethasone for 0, 4, 12, 24 and 48h identifying 162 genes as differentially expressed. Two of the most highly upregulated genes based on expression profiling were the transcription factor Ets-2 and the MHC Class II genes (Q, R, and P). Expression of these genes in EoL-1 cells was shown to be dexamethasone-induced at the RNA and protein levels which is consistent with the known function of Ets-2 in controlling cell cycle progression and the role of MHC Class II antigens in mediating eosinophil functions.

Analysis of gene expression in peripheral blood eosinophils from patients with atopic dermatitis and in vitro cytokine-stimulated blood eosinophils

Investigation of differentially expressed genes in eosinophils of patients with allergic diseases such as atopic dermatitis (AD) will provide important information for elucidating possible mechanisms of pathology. To identify novel genes that are expressed in AD, we compared gene expression in samples of peripheral blood eosinophils from AD patients and healthy volunteers. RNA was extracted from peripheral blood eosinophils. The expression of various genes, such as those for cytokine receptors, eosinophil activation marker, platelet activating factor (PAF) receptor, eosinophil-specific granular proteins and apoptosis-related genes, was confirmed using real-time reverse transcription-polymerase chain reaction (RT-PCR). Peripheral blood eosinophils of healthy volunteers were also isolated and stimulated for introduction of various cytokines. RNA was extracted and gene expression was monitored. Several genes, such as those for cytokine receptors (granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor alpha and beta chain and interleukin (IL)-3 receptor alpha chain), CD44 and PAF receptor were expressed at significantly higher levels in AD patients than in healthy volunteers. In addition, the anti-apoptotic genes, bcl-2 and bcl-xL, were expressed at increased levels in AD patients. No single gene expression correlated with clinical markers, such as eosinophil count or IgE levels. Expression of GM-CSF receptor beta chain and IL-3 receptor alpha chain in isolated blood eosinophils of healthy volunteers was stimulated by IL-5, IL-4, interferon (IFN)-gamma and GM-CSF. Expression of bcl-2 and bcl-xL was also increased after stimulation with IL-5, IL-4 or IFN-gamma. The in vitro enhancement of cytokine-stimulated gene expression correlated well with the enhancement observed in clinical samples of eosinophils, suggesting that cytokines may affect gene expression in vivo in eosinophils of patients with AD.

A growing family of guanine nucleotide exchange factors is responsible for activation of Ras-family GTPases

GTPases of the Ras subfamily regulate a diverse array of cellular-signaling pathways, coupling extracellular signals to the intracellular response machinery. Guanine nucleotide exchange factors (GEFs) are primarily responsible for linking cell-surface receptors to Ras protein activation. They do this by catalyzing the dissociation of GDP from the inactive Ras proteins. GTP can then bind and induce a conformational change that permits interaction with downstream effectors. Over the past 5 years, approximately 20 novel Ras-family GEFs have been identified and characterized. These data indicate that a variety of different signaling mechanisms can be induced to activate Ras, enabling tyrosine kinases, G-protein-coupled receptors, adhesion molecules, second messengers, and various protein-interaction modules to relocate and/or activate GEFs and elevate intracellular Ras-GTP levels. This review discusses the structure and function of the catalytic or CDC25 homology domain common to almost all Ras-family GEFs. It also details our current knowledge about the regulation and function of this rapidly growing family of enzymes that include Sos1 and 2, GRF1 and 2, CalDAG-GEF/GRP1-4, C3G, cAMP-GEF/Epac 1 and 2, PDZ-GEFs, MR-GEF, RalGDS family members, RalGPS, BCAR3, Smg GDS, and phospholipase C(epsilon).

Novel combinatorial interactions of GATA-1, PU.1, and C/EBPepsilon isoforms regulate transcription of the gene encoding eosinophil granule major basic protein

GATA-1 and the ets factor PU.1 have been reported to functionally antagonize one another in the regulation of erythroid versus myeloid gene transcription and development. The CCAAT enhancer binding protein epsilon (C/EBPepsilon) is expressed as multiple isoforms and has been shown to be essential to myeloid (granulocyte) terminal differentiation. We have defined a novel synergistic, as opposed to antagonistic, combinatorial interaction between GATA-1 and PU.1, and a unique repressor role for certain C/EBPepsilon isoforms in the transcriptional regulation of a model eosinophil granulocyte gene, the major basic protein (MBP). The eosinophil-specific P2 promoter of the MBP gene contains GATA-1, C/EBP, and PU.1 consensus sites that bind these factors in nuclear extracts of the eosinophil myelocyte cell line, AML14.3D10. The promoter is transactivated by GATA-1 alone but is synergistically transactivated by low levels of PU.1 in the context of optimal levels of GATA-1. The C/EBPepsilon(27) isoform strongly represses GATA-1 activity and completely blocks GATA-1/PU.1 synergy. In vitro mutational analyses of the MBP-P2 promoter showed that both the GATA-1/PU.1 synergy, and repressor activity of C/EBPepsilon(27) are mediated via protein-protein interactions through the C/EBP and/or GATA-binding sites but not the PU.1 sites. Co-immunoprecipitations using lysates of AML14.3D10 eosinophils show that both C/EBPepsilon(32/30) and epsilon(27) physically interact in vivo with PU.1 and GATA-1, demonstrating functional interactions among these factors in eosinophil progenitors. Our findings identify novel combinatorial protein-protein interactions for GATA-1, PU.1, and C/EBPepsilon isoforms in eosinophil gene transcription that include GATA-1/PU.1 synergy and repressor activity for C/EBPepsilon(27).

Vitamin C inhibits granulocyte macrophage-colony-stimulating factor-induced signaling pathways

Vitamin C is present in the cytosol as ascorbic acid, functioning primarily as a cofactor for enzymatic reactions and as an antioxidant to scavenge free radicals. Human granulocyte macrophage-colony-stimulating factor (GM-CSF) induces an increase in reactive oxygen species (ROS) and uses ROS for some signaling functions. We therefore investigated the effect of vitamin C on GM-CSF-mediated responses. Loading U937 cells with vitamin C decreased intracellular levels of ROS and inhibited the production of ROS induced by GM-CSF. Vitamin C suppressed GM-CSF-dependent phosphorylation of the signal transducer and activator of transcription 5 (Stat-5) and mitogen-activated protein (MAP) kinase (Erk1 and Erk2) in a dose-dependent manner as was phosphorylation of MAP kinase induced by both interleukin 3 (IL-3) and GM-CSF in HL-60 cells. In 293T cells transfected with alpha and beta GM-CSF receptor subunits (alphaGMR and betaGMR), GM-CSF-induced phosphorylation of betaGMR and Jak-2 activation was suppressed by vitamin C loading. GM-CSF-mediated transcriptional activation of a luciferase reporter construct containing STAT-binding sites was also inhibited by vitamin C. These results substantiate the importance of ROS in GM-CSF signaling and indicate a role for vitamin C in downmodulating GM-CSF signaling responses. Our findings point to vitamin C as a regulator of cytokine redox-signal transduction in host defense cells and a possible role in controlling inflammatory responses.

Signaling via the interferon-alpha receptor in chronic myelogenous leukemia cells

It is well established that IFNalpha has significant clinical activity in the treatment of chronic myelogenous leukemia (CML). This cytokine has been used for many years in the management of patients in the chronic phase of the disease, but the mechanisms by which it induces growth inhibitory effects in CML-cells have not been elucidated. Understanding the signaling mechanisms by which the Type I IFN receptor transduces growth inhibitory signals in BCR-ABL expressing cells should prove very valuable, as it may result in the design of new, more specific pharmacological compounds that target the same cellular cascades. Recent evidence indicates that, in addition to the classic IFN-activated Jak-Stat pathway, the Type I IFN receptor engages in its signaling cascade the CrkL-adapter protein, which is also a substrate for the kinase activity of the BCR-ABL oncogene. In addition, it appears that activation of a member of the Map kinase (MAPK) family of proteins, the p38 MAPK, is essential for the generation of the antileukemic effects of IFNalpha. This review summarizes the recent advances in the-field of interferon signaling in CML cells and discusses the implications of identifying signaling proteins that mediate IFNalpha-induced growth inhibition.

The CrkL adapter protein is required for type I interferon-dependent gene transcription and activation of the small G-protein Rap1

We sought to determine the functional role of the CrkL adapter protein and downstream pathways in interferon signaling. In experiments using CrkL(--) mouse embryonic fibroblasts, we found that CrkL is required for IFN alpha-dependent gene transcription via GAS elements, apparently via the formation of DNA-binding complexes with Stat5. On the other hand, gene transcription via ISRE elements is intact in the absence of CrkL, indicating that the regulatory effects on gene transcription are mediated only via the formation of CrkL:Stat5 complexes. Our studies also indicate that activation of the small GTPase Rap1 by IFN alpha is defective in cells lacking CrkL, indicating that the protein plays a critical role in regulating activation of the growth inhibitory C3G/Rap1 pathway. The IFN alpha-inducible activation of the small GTPase Rap1 requires a functional N-terminus SH3 domain in the CrkL protein, while the C-terminus SH3 domain does not appear to play a role in such a CrkL-function. We also demonstrate that both the Tyk-2 and Jak-1 kinases are required for activation of the CrkL/Rap1 pathway, as the Type I IFN-dependent GTP-bound form of Rap1 is inhibited by overexpression of dominant-negative Tyk-2 or Jak-1 mutants and is defective in cells lacking Tyk-2 or Jak-1. Taken altogether, these findings indicate that CrkL provides an important link between Jak-kinases and downstream cascades that play critical roles in IFN-dependent transcriptional regulation and induction of growth inhibitory responses.

Crk family adaptors-signalling complex formation and biological roles

Crk family adaptors are widely expressed and mediate the timely formation of signal transduction protein complexes upon a variety of extracellular stimuli, including various growth and differentiation factors. Selective formation of multi-protein complexes by the Crk and Crk-like (CRKL) proteins depends on specific motifs recognized by their SH2 and SH3 domains. In the case of the first SH3 domains [SH3(1)] a P-x-x-P-x-K motif is crucial for highly selective binding, while the SH2 domains prefer motifs which conform to the consensus pY-x-x-P. Crk family proteins are involved in the relocalization and activation of several different effector proteins which include guanine nucleotide releasing proteins like C3G, protein kinases of the Abl- and GCK-families and small GTPases like Rap1 and Rac. Crk-type proteins have been found not only in vertebrates but also in flies and nematodes. Major insight into the function of Crk within organisms came from the genetic model organism C. elegans, where the Crk-homologue CED-2 regulates cell engulfment and phagocytosis. Other biological outcomes of the Crk-activated signal transduction cascades include the modulation of cell adhesion, cell migration and immune cell responses. Crk family adaptors also appear to play a role in mediating the action of human oncogenes like the leukaemia-inducing Bcr-Abl protein. This review summarizes some key findings and highlights recent insights and open questions.

Thrombopoietin and interleukin-2 induce association of CRK with STAT5

Crk (Crk I and II) proteins and closely related CrkL are adapters which are commonly involved in various signaling processes in various cells, and these proteins share many ligands. Whether they have redundant or distinct physiologic roles is unclear. By coprecipitation and far Western blotting analysis, we demonstrate that Crk (I/II) binds to tyrosine phosphorylated STAT5 in cells stimulated by cytokines such as thrombopoietin (TPO) and interleukin-2 (IL-2). The association did not require nuclear elements and can be observed in primary cells as this was also demonstrated in TPO-stimulated platelets. Using a beta-casein promoter STAT5 binding site as a probe, we have also demonstrated that CrkL (a close relative of Crk) antiserum, but not Crk antiserum, supershifted the STAT5-DNA complex by an electrophoretic mobility shift assay, suggesting that CrkL, but not Crk, is the major component of the complex. Thus, Crk and CrkL may have distinct roles in the regulation of STAT5.