Regulation of Src homology 2-containing tyrosine phosphatase 1 during activation of human neutrophils. Role of protein kinase C

Inhibition of SHP-1 activity by PKC-θ regulates NK cell activation threshold and cytotoxicity

Natural killer (NK) cells play a crucial role in immunity, killing virally infected and cancerous cells. The balance of signals initiated upon engagement of activating and inhibitory NK receptors with cognate ligands determines killing or tolerance. Nevertheless, the molecular mechanisms regulating rapid NK cell discrimination between healthy and malignant cells in a heterogeneous tissue environment are incompletely understood. The SHP-1 tyrosine phosphatase is the central negative NK cell regulator that dephosphorylates key activating signaling proteins. Though the mechanism by which SHP-1 mediates NK cell inhibition has been partially elucidated, the pathways by which SHP-1 is itself regulated remain unclear. Here, we show that phosphorylation of SHP-1 in NK cells on the S591 residue by PKC-θ promotes the inhibited SHP-1 ‘folded’ state. Silencing PKC-θ maintains SHP-1 in the active conformation, reduces NK cell activation and cytotoxicity, and promotes tumor progression in vivo. This study reveals a molecular pathway that sustains the NK cell activation threshold through suppression of SHP-1 activity.

Shp1 function in myeloid cells

The motheaten mouse was first described in 1975 as a model of systemic inflammation and autoimmunity, as a result of immune system dysregulation. The phenotype was later ascribed to mutations in the cytoplasmic tyrosine phosphatase Shp1. This phosphatase is expressed widely throughout the hematopoietic system and has been shown to impact a multitude of cell signaling pathways. The determination of which cell types contribute to the different aspects of the phenotype caused by global Shp1 loss or mutation and which pathways within these cell types are regulated by Shp1 is important to further our understanding of immune system regulation. In this review, we focus on the role of Shp1 in myeloid cells and how its dysregulation affects immune function, which can impact human disease.

Tumor antigen ROR1 targeted drug delivery mediated selective leukemic but not normal B-cell cytotoxicity in chronic lymphocytic leukemia

Selective cytotoxicity to cancer cells without compromising their normal counterparts pose a huge challenge for traditional drug design. Here we developed a tumor antigen-targeted delivery of immunonanoparticle carrying a novel non-immunosuppressive FTY720 derivative OSU-2S with potent cytotoxicity against leukemic B cells. OSU-2S induces activation of protein phosphatase 2A (PP2A), phosphorylation and nuclear translocation of SHP1(S591) and deregulation of multiple cellular processes in chronic lymphocytic leukemia (CLL) resulting in potent cytotoxicity. To preclude OSU-2S-mediated effects on these ubiquitous phosphatases in unintended cells and avoid potential adverse effects, we developed an OSU-2S-targeted delivery of immunonanoparticles (2A2-OSU-2S-ILP), that mediated selective cytotoxicity of CLL but not normal B cells through targeting receptor tyrosine kinase ROR1 expressed in leukemic but not normal B cells. Developing a novel spontaneous CLL mouse model expressing human ROR1 (hROR1) in all leukemic B cells, we demonstrate the therapeutic benefit of enhanced survival with 2A2-OSU-2S-ILP in vivo. The newly developed non-immunosuppressive OSU-2S, its delivery using human CLL directed immunonanoparticles and the novel transgenic (Tg) mouse model of CLL that expresses hROR1 exclusively in leukemic B cell surface are highly innovative and can be applied to CLL and other ROR1+ malignancies including mantle cell lymphoma and acute lymphoblastic leukemia.

Inflammation-induced proteolytic processing of the SIRPα cytoplasmic ITIM in neutrophils propagates a proinflammatory state

Signal regulatory protein α (SIRPα), an immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptor, is an essential negative regulator of leukocyte inflammatory responses. Here we report that SIRPα cytoplasmic signalling ITIMs in neutrophils are cleaved during active inflammation and that the loss of SIRPα ITIMs enhances the polymorphonuclear leukocyte (PMN) inflammatory response. Using human leukocytes and two inflammatory models in mice, we show that the cleavage of SIRPα ITIMs in PMNs but not monocytes occurs at the post-acute stage of inflammation and correlates with increased PMN recruitment to inflammatory loci. Enhanced transmigration of PMNs and PMN-associated tissue damage are confirmed in mutant mice expressing SIRPα but lacking the ITIMs. Moreover, the loss of SIRPα ITIMs in PMNs during colitis is blocked by an anti-interleukin-17 (IL-17) antibody. These results demonstrate a SIRPα-based mechanism that dynamically regulates PMN inflammatory responses by generating a CD47-binding but non-signalling SIRPα 'decoy'.

DCIR-mediated enhancement of HIV-1 infection requires the ITIM-associated signal transduction pathway

Dendritic cell immunoreceptor (DCIR) is a C-type lectin receptor expressed at high levels on dendritic cells (DCs). This surface molecule acts as an attachment factor for HIV-1 on DCs and contributes to trans- and cis-infection pathways. Moreover, DICR is induced by HIV-1 in CD4+ T cells and promotes virus replication in this cell type. Nothing is known hitherto about the DCIR-dependent signaling, which is induced following HIV-1 ligation. First, specific pharmacologic inhibitors were tested on HIV-1 binding/entry and, second, specific antisense oligonucleotides targeted, more specifically kinases and phosphatases, were used. Our results show that SHP-1, SHP-2, Syk, and Src kinases (ie, Src, Fyn, and Hck) as well as PKC-α and MAP kinases (ie, Erk1/2 and p38) are all involved in the DCIR-mediated signal transduction pathway triggered by HIV-1. By mutagenesis and through the use of intracellular phosphorylated peptides, we show as well a pivotal role for the tyrosine and threonine residues of the DCIR immunoreceptor tyrosine-based inhibitory motif (ITIM). Our data suggest for the first time an involvement of ITIM domain in HIV-1–mediated signaling events and a relationship between phosphorylation events and DCIR function with respect to HIV-1 biology.

IMPDHII protein inhibits Toll-like receptor 2-mediated activation of NF-kappaB

Toll-like receptor 2 (TLR2) plays an essential role in innate immunity by the recognition of a large variety of pathogen-associated molecular patterns. It induces its recruitment to lipid rafts induces the formation of a membranous activation cluster necessary to enhance, amplify, and control downstream signaling. However, the exact composition of the TLR2-mediated molecular complex is unknown. We performed a proteomic analysis in lipopeptide-stimulated THP1 and found IMPDHII protein rapidly recruited to lipid raft. Whereas IMPDHII is essential for lymphocyte proliferation, its biologic function within innate immune signal pathways has not been established yet. We report here that IMPDHII plays an important role in the negative regulation of TLR2 signaling by modulating PI3K activity. Indeed, IMPDHII increases the phosphatase activity of SHP1, which participates to the inactivation of PI3K.

Protein kinase C isoforms in neutrophil adhesion and activation

Neutrophils are the first line of defense against bacterial and mycotic pathogens. In order to reach the pathogens, neutrophils need to transmigrate through the vascular endothelium and migrate to the site of infection. Defense strategies against pathogens include phagocytosis, production and release of oxygen radicals through the oxidative burst, and degranulation of antimicrobial and inflammatory molecules. Protein kinase C (PKC)-δ is required for full assembly of NADPH oxidase and activation of the respiratory burst. Neutrophils also express PKC-α and -β, which may be involved in adhesion, degranulation and phagocytosis, but the evidence is not conclusive yet. This review focuses on the potential impact of protein kinase C isoforms on neutrophil adhesion and activation.

Identification of phosphoproteins associated with human neutrophil granules following chemotactic peptide stimulation

Regulated exocytosis of neutrophil intracellular storage granules is necessary for neutrophil participation in the inflammatory response. The signal transduction pathways that participate in neutrophil exocytosis are complex and poorly defined. Several protein kinases, including p38 MAPK and the nonreceptor tyrosine kinases, Hck and Fgr, participate in this response. However, the downstream targets of these kinases that regulate exocytosis are unknown. The present study combined a novel inhibitor of neutrophil exocytosis with proteomic techniques to identify phosphopeptides and phosphoproteins from a population of gelatinase and specific granules isolated from unstimulated and fMLF-stimulated neutrophils. To prevent loss of granule-associated phosphoproteins upon exocytosis, neutrophils were pretreated with a TAT-fusion protein containing a SNARE domain from SNAP-23 (TAT-SNAP-23), which inhibited fMLF-stimulated CD66b-containing granule exocytosis by 100±10%. Following TAT-SNAP-23 pretreatment, neutrophils were stimulated with the chemotactic peptide fMLF for 0 min, 1 min, and 2 min. Granules were isolated by gradient centrifugation and subjected to proteolytic digestion with trypsin or chymotrypsin to obtain peptides from the outer surface of the granule. Phosphopeptides were enriched by gallium or TiO2 affinity chromatography, and phosphopeptides and phosphorylation sites were identified by reversed phase high performance liquid chromatography-electrospray ionization-tandem MS. This resulted in the identification of 243 unique phosphopeptides corresponding to 235 proteins, including known regulators of vesicle trafficking. The analysis identified 79 phosphoproteins from resting neutrophils, 81 following 1 min of fMLF stimulation, and 118 following 2 min of stimulation. Bioinformatic analysis identified a potential Src tyrosine kinase motif from a phosphopeptide corresponding to G protein coupled receptor kinase 5 (GRK5). Phosphorylation of GRK5 by Src was confirmed by an in vitro kinase reaction and by precursor ion scanning for phospho-tyrosine specific immonium ions containing Tyr251 and Tyr253. Immunoprecipitation of phosphorylated GRK5 from intact cells was reduced by a Src inhibitor. In conclusion, targets of signal transduction pathways were identified that are candidates to regulate neutrophil granule exocytosis.

Siglec-9 and SHP-1 are differentially expressed in neonatal and adult neutrophils

Neonatal PMN (polymorphonuclear neutrophils) exhibit altered inflammatory responsiveness and greater longevity compared with adult PMN; however, the involved mechanisms are incompletely defined. Receptors containing immunoreceptor tyrosine-based inhibitory motif (ITIM) domains promote apoptosis by activating inhibitory phosphatases, such as Src homology domain 2-containing tyrosine phosphatase-1 (SHP-1), that block survival signals. Sialic acid-binding immunoglobulin-like lectin (Siglec)-9, an immune inhibitory receptor with an ITIM domain, has been shown to induce cell death in adult PMN in association with SHP-1. To test our hypothesis that neonatal PMN inflammatory function may be modulated by unique Siglec-9 and SHP-1 interactions, we compared expression of these proteins in adult and neonatal PMN. Neonatal PMN exhibited diminished cellular expression of Siglec-9, which was phosphorylated in the basal state. Granulocyte-macrophage colony-stimulating factor (GM-CSF) treatment decreased Siglec-9 phosphorylation levels in neonatal PMN but promoted its phosphorylation in adult PMN, observations associated with altered survival signaling. Although SHP-1 expression was also diminished in neonatal PMN, GM-CSF treatment had minimal effect on phosphorylation status. Further analysis revealed that Siglec-9 and SHP-1 physically interact, as has been observed in other immune cells. Our data suggest that age-specific interactions between Siglec-9 and SHP-1 may influence the altered inflammatory responsiveness and longevity of neonatal PMN.

Inhibition of the human neutrophil NADPH oxidase by Coxiella burnetii

Coxiella burnetii is an obligate intracellular Gram-negative pathogen. A notable feature of C. burnetii is its ability to replicate within acidic phagolysosomes; however, the mechanisms utilized in evading host defenses are not well defined. Here, we investigated human neutrophil phagocytosis of C. burnetii (Nine Mile, phase II; NMII) and the effect of phagocytosed organisms on neutrophil reactive oxygen species (ROS) production. We found that opsonization with immune serum substantially enhanced phagocytosis of NMII. Human neutrophils phagocytosing opsonized NMII generated very little ROS compared to cells phagocytosing opsonized Staphylococcus aureus, Escherichia coli, or zymosan. However, phagocytosis of NMII did not affect the subsequent ROS response to a soluble agonist, indicating inhibition was localized to the phagolysosome and was not a global effect. Indeed, analysis of NADPH oxidase assembly in neutrophils after phagocytosis showed that translocation of cytosolic NADPH oxidase proteins, p47(phox) and p67(phox), to the membrane was absent in cells phagocytosing NMII, as compared to cells phagocytosing S. aureus or activated by phorbol myristate acetate. Thus, phagocytosed NMII is able to disrupt assembly of the human neutrophil NADPH oxidase, which represents a novel virulence mechanism for this organism and appears to be a common mechanism of virulence for many intracellular pathogens.

Regulation of hematopoietic cell function by inhibitory immunoglobulin G receptors and their inositol lipid phosphatase effectors

Numerous autoimmune and inflammatory disorders stem from the dysregulation of hematopoietic cell activation. The activity of inositol lipid and protein tyrosine phosphatases, and the receptors that recruit them, is critical for prevention of these disorders. Balanced signaling by inhibitory and activating receptors is now recognized to be an important factor in tuning cell function and inflammatory potential. In this review, we provide an overview of current knowledge of membrane proximal events in signaling by inhibitory/regulatory receptors focusing on structural and functional characteristics of receptors and their effectors Src homology 2 (SH2) domain-containing tyrosine phosphatase 1 and SH2 domain-containing inositol 5-phosphatase-1. We review use of new strategies to identify novel regulatory receptors and effectors. Finally, we discuss complementary actions of paired inhibitory and activating receptors, using Fc gammaRIIA and Fc gammaRIIB regulation human basophil activation as a prototype.

Age-related impairment of GM-CSF-induced signalling in neutrophils: role of SHP-1 and SOCS proteins

Functional activities of mature human neutrophils are strongly influenced by the pro-inflammatory cytokine granulocyte macrophage-colony stimulating factor (GM-CSF). Accordingly, a defective response to GM-CSF might have dramatic consequences for neutrophil functions and the host defence against infections. Such an event is most likely to occur in senescence. A number of studies have, in fact, reported an impairment of the GM-CSF capacity to prime and/or to activate respiratory burst, as well as to delay apoptotic events, in neutrophils from elderly individuals. In the last 2 decades many efforts have been made to explore at molecular levels the mechanism underlying these defects. Recent studies let us depict a scenario in which an increased activity of inhibitory molecules, such as Src homology domain-containing protein tyrosine phosphatase-1 (SHP-1) and suppressors of cytokine signalling (SOCS), is responsible for the age-related failure of GM-CSF to stimulate neutrophil functions via inhibition of Lyn-, phosphoinositide 3-kinase (PI3-K)/extracellular signal-regulated kinase (ERK)- and signal transducers and activators of transcription (STAT)-dependent pathways. The control of SHP-1 and/or SOCS activity might therefore be an important therapeutic target for the restoration of normal immune responses during senescence.

Rapid T cell receptor-mediated SHP-1 S591 phosphorylation regulates SHP-1 cellular localization and phosphatase activity

Since the tyrosine phosphatase SHP-1 plays a major role in regulating T cell signaling, we investigated regulation thereof by Ser/Thr phosphorylation. We found that T cell receptor (TCR) stimulation induced fast (<or=1 min) and transient phosphorylation of SHP-1 S591 in both Jurkat and human peripheral blood T-cells (PBT). Phosphorylation of S591 in T-cells could be mediated artificially by a constitutive active PKC-theta construct, but the dose dependence of inhibition by PKC inhibitors indicated that PKCs were not the relevant basophilic kinase in the physiological response. S591 phosphorylation inhibited phosphatase function since a S591D mutant had lower activity than the S591A mutant. Additional evidence that S591 phosphorylation alters SHP-1 function was provided by studies of Jurkat cells stably expressing SHP-1 wild type or mutants. In those cells, S591D mutation reduced the capacity of transfected SHP-1 to inhibit TCR-induced phosphorylation of PLC-gamma1. Interestingly, SHP-1 Y536 phosphorylation (previously shown to augment phosphatase activity) was also induced in PBT by TCR signal but at a much later time compared with S591 ( approximately 30 min). S591 phosphorylation also altered cellular distribution of SHP-1 because: 1) SHP-1 in lipid rafts and a sheared membrane fraction was hypophosphorylated; 2) In stably transfected Jurkat cell lines, S591D mutant protein had reduced presence in both lipid raft and the sheared membrane fraction; 3) S591 phosphorylation prevented nuclear localization of a C-terminal GFP tagged SHP-1 construct. Our studies also shed light on an additional mechanism regulating SHP-1 nuclear localization, namely conformational autoinhibition. These findings highlight elegant regulation of SHP-1 by sequential phosphorylation of serine then tyrosine.

Reactive oxygen and nitrogen species as signaling molecules regulating neutrophil function

As a cornerstone of the innate immune response, neutrophils are the archetypical phagocytic cell; they actively seek out, ingest, and destroy pathogenic microorganisms. To achieve this essential role in host defense, neutrophils deploy a potent antimicrobial arsenal that includes oxidants, proteinases, and antimicrobial peptides. Importantly, oxidants produced by neutrophils, referred to in this article as reactive oxygen (ROS) and reactive nitrogen (RNS) species, have a dual function. On one hand they function as potent antimicrobial agents by virtue of their ability to kill microbial pathogens directly. On the other hand, they participate as signaling molecules that regulate diverse physiological signaling pathways in neutrophils. In the latter role, ROS and RNS serve as modulators of protein and lipid kinases and phosphatases, membrane receptors, ion channels, and transcription factors, including NF-kappaB. The latter regulates expression of key cytokines and chemokines that further modulate the inflammatory response. During the inflammatory response, ROS and RNS modulate phagocytosis, secretion, gene expression, and apoptosis. Under pathological circumstances such as acute lung injury and sepsis, excess production of ROS may influence vicinal cells such as endothelium or epithelium, contributing to inflammatory tissue injury. A better understanding of these pathways will help identify novel targets for amelioration of the untoward effects of inflammation.

Impairment of SHP-1 down-regulation in the lipid rafts of human neutrophils under GM-CSF stimulation contributes to their age-related, altered functions

It has been shown that the functions and the rescue from apoptosis by proinflammatory mediators of polymorphonuclear leukocytes (PMN) tend to diminish with aging. Here, we investigated the role of protein tyrosine phosphatases (PTP), especially Src homology domain-containing protein tyrosine phosphatase-1 (SHP-1), in the age-related, altered PMN functions under granulocyte macrophage-colony stimulating factor (GM-CSF) stimulation. The inhibition of PTP suggested a differential effect of GM-CSF on phosphatase activity in modulating PMN functions with aging. The down-regulation of phosphatase activity of immunopurified SHP-1 from lipid rafts of PMN of young donors was found significantly altered at 1 min of stimulation with aging. In young donors, SHP-1 is displaced from lipid rafts at 1 min of stimulation, whereas in the elderly, SHP-1 is constantly present. We assessed in PMN lipid rafts the phosphorylation of tyrosine and serine residues of SHP-1, which regulates its activity. We observed an alteration in the phosphorylation of tyrosine and serine residues of SHP-1 in PMN of elderly subjects, suggesting that GM-CSF was unable to inhibit SHP-1 activity by serine phosphorylation. GM-CSF activates Lyn rapidly, and we found alterations in its activation and translocation to the lipid rafts with aging. We also demonstrate that SHP-1 in the PMN of elderly is constantly recruited to Lyn, which cannot be relieved by GM-CSF. In contrast, in the young, the resting recruitment could be relieved by GM-CSF. Our results suggest an alteration of the SHP-1 modulation by GM-CSF in lipid rafts of PMN with aging. These alterations could contribute to the decreased GM-CSF effects on PMN.

Activation of protein kinase CbetaI constitutes a new neurotrophic pathway for deafferented spiral ganglion neurons

In mammals, degeneration of peripheral auditory neurons constitutes one of the main causes of sensorineural hearing loss. Unfortunately, to date, pharmacological interventions aimed at counteracting this condition have not presented complete effectiveness in protecting the integrity of cochlear neural elements. In this context, the protein kinase C (PKC) family of enzymes are important signalling molecules that play a role in preventing neurodegeneration after nervous system injury. The present study demonstrates, for the first time, that the PKC signalling pathway is directly neurotrophic to axotomised spiral ganglion neurons (SGNs). We found that PKCbetaI was strictly expressed by postnatal and adult SGNs both in situ and in vitro. In cultures of SGNs, we observed that activators of PKC, such as phorbol esters and bryostatin 1, induced neuronal survival and neurite regrowth in a manner dependent on the activation of PKCbetaI. The neuroprotective effects of PKC activators were suppressed by pre-treatment with LY294002 (a PI3K inhibitor) and with U0126 (a MEK inhibitor), indicating that PKC activators promote the survival and neurite outgrowth of SGNs by both PI3K/Akt and MEK/ERK-dependent mechanisms. In addition, whereas combining the neurotrophins brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) was shown to provide only an additive effect on SGN survival, the interaction between PKC and neurotrophin signalling gave rise to a synergistic increase in SGN survival. Taken together, the data indicate that PKCbetaI activation represents a key factor for the protection of the integrity of neural elements in the cochlea.

A SHPing tale: perspectives on the regulation of SHP-1 and SHP-2 tyrosine phosphatases by the C-terminal tail

Protein tyrosine phosphorylation is a ubiquitous signalling mechanism and is regulated by a balance between the action of kinases and phosphatases. The SH2 domain-containing phosphatases SHP-1 and SHP-2 are the best studied of the classical non-receptor tyrosine phosphatases, but it is intriguing that despite their close sequence and structural homology these two phosphatases play quite different cellular roles. In particular, whereas SHP-1 plays a largely negative signalling role suppressing cellular activation, SHP-2 plays a largely positive signalling role. Major sequence differences between the two molecules are apparent in the approximately 100 amino acid residues at the extreme C-terminus of the proteins, beyond the phosphatase catalytic domain. Here we review how the differences in the tails of these proteins may regulate their activities and explain some of their functional differences.

Increased chemoattractant induced neutrophil oxidative burst, accelerated apoptosis, and dysregulated tyrosine phosphorylation associated with lifelong bacterial infections

A boy with lifelong recurrent bacterial infection at cutaneous and mucosal sites was investigated. PMN oxidative burst to phorbol myristate acetate (PMA) and zymosan was normal but was increased 20- to 50-fold upon C5a or formyl-met-leu-phe (fMLP) chemoattractant stimulation, accompanied by accelerated PMN apoptosis. His PMNs showed increased constitutive tyrosine phosphorylation of 21-, 25-, and 44-kDa proteins, and of src-family kinases (p59(hck), p58(fgr), and p53/56(lyn)). Phosphorylation was abnormally enhanced following fMLP stimulation. Expression and activity of the major PMN tyrosine phosphatases, i.e., CD45, CD148, and SHP-1 and -2, was normal. However, dephosphorylation of phospho-p58(fgr) and phospho-p53/56(lyn) by lysates of patient's PMNs was enhanced. Thus, another phosphatase may be overactive, perhaps dephosphorylating a regulatory (inhibitory) site on a protein tyrosine kinase, accounting for the abnormal PMN tyrosine phosphorylation and function. With age (now 13 years), T-cell lymphopenia and loss of T-cell responses developed. This appears to be a unique primary immunodeficiency with abnormal PMN oxidative and apoptotic responses to chemoattractants, dysregulated protein tyrosine phosphorylation, serious bacterial infection, and T-lymphocyte attrition.

Siglec-9 transduces apoptotic and nonapoptotic death signals into neutrophils depending on the proinflammatory cytokine environment

We report about new apoptotic and non-apoptotic death pathways in neutrophils that are initiated via the surface molecule sialic acid-binding immunoglobulin-like lectin (Siglec)-9. In normal neutrophils, Siglec-9 ligation induced apoptosis. Inflammatory neutrophils obtained from patients with acute septic shock or rheumatoid arthritis demonstrated increased Siglec-9, but normal Fas receptor-mediated cytotoxic responses when compared with normal blood neutrophils. The increased Siglec-9-mediated death was mimicked in vitro by short-term preincubation of normal neutrophils with proinflammatory cytokines, such as granulocyte/macrophage colony-stimulating factor (GM-CSF), interferon-alpha (IFN-alpha), and IFN-gamma, and was demonstrated to be caspase independent. Experiments using scavengers of reactive oxygen species (ROS) or neutrophils unable to generate ROS indicated that both Siglec-9-mediated caspase-dependent and caspase-independent forms of neutrophil death depend on ROS. Interestingly, the caspase-independent form of neutrophil death was characterized by cytoplasmic vacuolization and several other nonapoptotic morphologic features, which were also seen in neutrophils present in joint fluids from rheumatoid arthritis patients. Taken together, these data suggest that apoptotic (ROS- and caspase-dependent) and nonapoptotic (ROS-dependent) death pathways are initiated in neutrophils via Siglec-9. The new insights have important implications for the pathogenesis, diagnosis, and treatment of inflammatory diseases such as sepsis and rheumatoid arthritis.

IFN-alpha priming results in a gain of proinflammatory function by IL-10: implications for systemic lupus erythematosus pathogenesis

Interleukin-10 is a predominantly anti-inflammatory cytokine that inhibits macrophage and dendritic cell function, but can acquire proinflammatory activity during immune responses. We investigated whether type I IFNs, which are elevated during infections and in autoimmune diseases, modulate the activity of IL-10. Priming of primary human macrophages with low concentrations of IFN-alpha diminished the ability of IL-10 to suppress TNF-alpha production. IFN-alpha conferred a proinflammatory gain of function on IL-10, leading to IL-10 activation of expression of IFN-gamma-inducible, STAT1-dependent genes such as IFN regulatory factor 1, IFN-gamma-inducible protein-10 (CXCL10), and monokine induced by IFN-gamma (CXCL9). IFN-alpha priming resulted in greatly enhanced STAT1 activation in response to IL-10, and STAT1 was required for IL-10 activation of IFN-gamma-inducible protein-10 and monokine induced by IFN-gamma expression in IFN-alpha-primed cells. In control, unprimed cells, IL-10 activation of STAT1 was suppressed by constitutive activity of protein kinase C and Src homology 2 domain-containing phosphatase 1. These results demonstrate that type I IFNs regulate the balance between IL-10 anti- and proinflammatory activity, and provide insight into molecular mechanisms that regulate IL-10 function. Gain of IL-10 proinflammatory functions may contribute to its pathogenic role in autoimmune diseases characterized by elevated type I IFN levels, such as systemic lupus erythematosus.

Regulation of SHP-1 Tyrosine Phosphatase in Human Platelets by Serine Phosphorylation at Its C Terminus

SHP-1 is a Src homology 2 (SH2) domain-containing tyrosine phosphatase that plays an essential role in negative regulation of immune cell activity. We describe here a new model for regulation of SHP-1 involving phosphorylation of its C-terminal Ser591 by associated protein kinase Calpha. In human platelets, SHP-1 was found to constitutively associate with its substrate Vav1 and, through its SH2 domains, with protein kinase Calpha. Upon activation of either PAR1 or PAR4 thrombin receptors, the association between the three proteins was retained, and Vav1 became phosphorylated on tyrosine and SHP-1 became phosphorylated on Ser591. Phosphorylation of SHP-1 was mediated by protein kinase C and negatively regulated the activity of SHP-1 as demonstrated by a decrease in the in vitro ability of SHP-1 to dephosphorylate Vav1 on tyrosine. Protein kinase Calpha therefore critically and negatively regulates SHP-1 function, forming part of a mechanism to retain SHP-1 in a basal active state through interaction with its SH2 domains, and phosphorylating its C-terminal Ser591 upon cellular activation leading to inhibition of SHP-1 activity and an increase in the tyrosine phosphorylation status of its substrates.

SHP-1: a regulator of neutrophil apoptosis

The Src homology domain 2 (SH2)-containing tyrosine phosphatase-1 (SHP-1) has been implicated in the regulation of differentiation, proliferation, and activation of hematopoietic cells. In this review, we discuss the potential role of SHP-1 in modulating apoptosis pathways in neutrophils. Low enzymatic SHP-1 was associated with increased neutrophil survival in vitro and SHP-1-deficient mice were reported to develop severe neutrophilic inflammatory responses. In contrast, high expression of this phosphatase was observed in neutropenic patients. Moreover, when neutrophils were exposed to Fas ligand, TNF-alpha, or TRAIL, the anti-apoptotic effects of granulocyte-macrophage colony-stimulating factor (GM-CSF), granulocyte colony-stimulating factor (G-CSF), or IFN-gamma were blocked, most likely by SHP-1-mediated inactivation of anti-apoptotic signaling molecules. In summary, the current available data point to an important role of SHP-1 in the regulation of neutrophil apoptosis.

Somatostatin receptors

In 1972, Brazeau et al. isolated somatostatin (somatotropin release-inhibiting factor, SRIF), a cyclic polypeptide with two biologically active isoforms (SRIF-14 and SRIF-28). This event prompted the successful quest for SRIF receptors. Then, nearly a quarter of a century later, it was announced that a neuropeptide, to be named cortistatin (CST), had been cloned, bearing strong resemblance to SRIF. Evidence of special CST receptors never emerged, however. CST rather competed with both SRIF isoforms for specific receptor binding. And binding to the known subtypes with affinities in the nanomolar range, it has therefore been acknowledged to be a third endogenous ligand at SRIF receptors. This review goes through mechanisms of signal transduction, pharmacology, and anatomical distribution of SRIF receptors. Structurally, SRIF receptors belong to the superfamily of G protein-coupled (GPC) receptors, sharing the characteristic seven-transmembrane-segment (STMS) topography. Years of intensive research have resulted in cloning of five receptor subtypes (sst(1)-sst(5)), one of which is represented by two splice variants (sst(2A) and sst(2B)). The individual subtypes, functionally coupled to the effectors of signal transduction, are differentially expressed throughout the mammalian organism, with corresponding differences in physiological impact. It is evident that receptor function, from a physiological point of view, cannot simply be reduced to the accumulated operations of individual receptors. Far from being isolated functional units, receptors co-operate. The total receptor apparatus of individual cell types is composed of different-ligand receptors (e.g. SRIF and non-SRIF receptors) and co-expressed receptor subtypes (e.g. sst(2) and sst(5) receptors) in characteristic proportions. In other words, levels of individual receptor subtypes are highly cell-specific and vary with the co-expression of different-ligand receptors. However, the question is how to quantify the relative contributions of individual receptor subtypes to the integration of transduced signals, ultimately the result of collective receptor activity. The generation of knock-out (KO) mice, intended as a means to define the contributions made by individual receptor subtypes, necessarily marks but an approximation. Furthermore, we must now take into account the stunning complexity of receptor co-operation indicated by the observation of receptor homo- and heterodimerisation, let alone oligomerisation. Theoretically, this phenomenon adds a novel series of functional megareceptors/super-receptors, with varied pharmacological profiles, to the catalogue of monomeric receptor subtypes isolated and cloned in the past. SRIF analogues include both peptides and non-peptides, receptor agonists and antagonists. Relatively long half lives, as compared to those of the endogenous ligands, have been paramount from the outset. Motivated by theoretical puzzles or the shortcomings of present-day diagnostics and therapy, investigators have also aimed to produce subtype-selective analogues. Several have become available.

Neutrophil apoptosis pathways and their modifications in inflammation

Neutrophils are constantly produced in large numbers in the bone marrow, and the same numbers of cells need to die within a defined time period in order to keep cellular homeostasis under physiologic conditions. Changing the rate of apoptosis rapidly changes cell numbers in such systems. For instance, in many bacterial and autoimmune inflammatory diseases, delayed apoptosis is one important mechanism for neutrophil accumulation. Excessive production of granulocyte/macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF), two important neutrophil survival factors, is often observed in such inflammatory responses. Cytokine withdrawal, as it occurs in the resolution phase of inflammation, leads to the induction of apoptosis. Moreover, neutrophil apoptosis can be accelerated both in the presence and in the absence of survival factors by activation of distinct members of the tumor necrosis factor/nerve growth factor receptor family. This review focuses on recently published work regarding signaling pathways that regulate neutrophil apoptosis.

Signal regulatory protein (SIRPalpha), a cellular ligand for CD47, regulates neutrophil transmigration

Recent studies have demonstrated that CD47 plays an important role in regulating human neutrophil (PMN) chemotaxis. Two ligands for CD47, thrombospondin and SIRPalpha, have been described. However, it is not known if SIRP-CD47 interactions play a role in regulating PMN migration. In this study, we show that SIRPalpha1 directly binds to the immunoglobulin variable domain loop of purified human CD47 and that such SIRP-CD47 interactions regulate PMN transmigration. Specifically, PMN migration across both human epithelial monolayers and collagen-coated filters was partially inhibited by anti-SIRP monoclonal antibodies. Similar kinetics of inhibition were observed for PMN transmigration in the presence of soluble, recombinant CD47 consisting of the SIRP-binding loop. In contrast, anti-CD47 monoclonal antibodies inhibited PMN transmigration by markedly different kinetics. Results of signal transduction experiments suggested differential regulation of PMN migration by SIRP versus CD47 by phosphatidylinositol 3-kinase and tyrosine kinases, respectively. Immunoprecipitation followed by Western blotting after SDS-PAGE under nonreducing conditions suggested that several SIRP protein species may be present in PMN. Stimulation of PMN with fMLP resulted in increased surface expression of these SIRP proteins, consistent with the existence of intracellular pools. Taken together, these results demonstrate that PMN migration is regulated by CD47 through SIRPalpha-dependent and SIRPalpha-independent mechanisms.

Preservation of the pattern of tyrosine phosphorylation in human neutrophil lysates. II. A sequential lysis protocol for the analysis of tyrosine phosphorylation-dependent signalling

In stimulated neutrophils, the majority of tyrosine phosphorylated proteins are concentrated in Triton X-100 or NP-40 insoluble fractions. Most immunobiochemical studies, whose objective is to study the functional relevance of tyrosine phosphorylation are, however, performed using the supernatants of cells lysed in non-ionic detergent-containing buffers (RIPA lysis buffers). This observation prompted us to develop an alternative lysis protocol. We established a procedure involving the sequential lysis of neutrophils in buffers of increasing tonicities that not only preserved and solubilized tyrosine phosphorylated proteins but also retained their enzymatic activities. The sequential lysis of neutrophils in hypotonic, isotonic and hypertonic buffers containing non-ionic detergents resulted in the solubilisation of a significant fraction of tyrosine phosphorylated proteins. Furthermore, we observed that in monosodium urate crystals-stimulated neutrophils, Lyn activity was enhanced in the soluble fraction recovered from the hypertonic fraction, but not from that of the first hypotonic lysis. The distribution of tyrosine phosphorylated proteins between the NP-40 soluble and insoluble fractions was both substrate- and agonist-dependent. In neutrophils stimulated with fMet-Leu-Phe, MSU crystals or by CD32 ligation, the tyrosine phosphorylated proteins were mostly insoluble. On the other hand, in GM-CSF-treated cells, the phosphoproteins were more equally distributed between the two fractions. The results of this study provide a new experimental procedure for the investigation of tyrosine phosphorylation pathways in activated human neutrophils which may also be applicable to other cell types.

Redox regulation of Fcgamma receptor-mediated phagocytosis: implications for host defense and tissue injury

Recent advances in our understanding of the mechanisms that regulate acute and chronic inflammatory responses have revealed a key role for reactive oxygen intermediates in modulating the activation of neutrophils. Opsonized microbes and immune complexes initiate the oxidative burst by the engagement of receptors for immunoglobulin G, termed Fcgamma receptors. The regulation of phagocytic cell function by oxidant-sensitive signaling pathways optimizes host defense capabilities, but it also amplifies tissue damage. This review will focus on the cross-talk between Fcgamma receptors and reactive oxygen intermediates at sites of inflammation and its role in microbial immunity.

Protein-tyrosine phosphatase MEG2 is expressed by human neutrophils. Localization to the phagosome and activation by polyphosphoinositides

Signaling pathways involving reversible tyrosine phosphorylation are essential for neutrophil antimicrobial responses. Using reverse transcriptase PCR, expression of the protein-tyrosine phosphatase MEG2 by peripheral neutrophilic polymorphonuclear leukocytes (PMN) was identified. Polyclonal antibodies against MEG2 were developed that confirmed expression of MEG2 protein by PMN. Through a combination of immunofluorescence and cell fractionation followed by immunoblotting, we determined that MEG2 is predominantly cytosolic with components present in secondary and tertiary granules and secretory vesicles. MEG2 activity, as determined by immunoprecipitation and in vitro phosphatase assays, is inhibited after exposure of cells to the particulate stimulant opsonized zymosan or to phorbol 12-myristate 13-acetate but largely unaffected by the chemoattractant N-formyl-methionyl-leucyl-phenyalanine. Studies using bacterially expressed glutathione S-transferase MEG2 fusion protein indicate that cysteine 515 is essential for catalytic activity, whereas the noncatalytic (N-terminal) domain of MEG2 negatively regulates the enzymatic activity of the C-terminal phosphatase domain. The activity of MEG2 is enhanced by specific polyphosphoinositides with the order of potency being phosphatidylinositol (PI) 4,5-diphosphate > PI 3,4,5-triphosphate > PI 4-phosphate. MEG2 associates at an early stage with nascent phagosomes. Taken together, our results indicate that MEG2 is a polyphosphoinositide-activated tyrosine phosphatase that may be involved in signaling events regulating phagocytosis, an essential antimicrobial function in the innate immune response.

The Protein-tyrosine-phosphatase SHP2 is phosphorylated on serine residues 576 and 591 by protein kinase C isoforms alpha, beta 1, beta 2, and eta

To study whether protein kinase C (PKC) isoforms can interact with protein-tyrosine-phosphatases (PTPs) which are connected to the insulin signaling pathway, we co-overexpressed PKC isoforms together with insulin receptor, docking proteins, and the PTPs SHP1 and SHP2 in human embryonic kidney (HEK) 293 cells. After phorbol ester induced activation of PKC isoforms alpha, beta 1, beta 2, and eta, we could show a defined gel mobility shift of SHP2, indicating phosphorylation on serine/threonine residues. This phosphorylation was not dependent on insulin receptor or insulin receptor substrate-1 (IRS-1) overexpression and did not occur for the closely related phosphatase SHP1. Furthermore, PKC phosphorylation of SHP2 was completely blocked by the PKC inhibitor bisindolylmaleimide and was not detectable when SHP2 was co-overexpressed with kinase negative mutants of PKC beta 1 and -beta 2. The phosphorylation also occurred on endogenous SHP2 in Chinese hamster ovary (CHO) cells stably overexpressing PKC beta 2. Using point mutants of SHP2, we identified serine residues 576 and 591 as phosphorylation sites for PKC. However, no change of phosphatase activity by TPA treatment was detected in an in vitro assay. In summary, SHP2 is phosphorylated on serine residues 576 and 591 by PKC isoforms alpha, beta 1, beta 2, and eta.

Protein kinase C isoforms in normal and leukemic neutrophils: altered levels in leukemic neutrophils and changes during myeloid maturation in chronic myeloid leukemia

Protein kinase C (PKC) is reported to play a role in maturation of the myeloid cell and functions of the mature neutrophil. The neutrophils in chronic myeloid leukemia (CML) exhibit defects in several functions. As a step towards understanding the role of PKC in the defects in function of the leukemic cells, this study investigates the expression of PKC isoforms, their subcellular distribution, levels and kinase activity in the normal and leukemic neutrophils. It also investigates changes in representative PKC isoforms during myeloid maturation. This study confirms the presence of PKC alpha, beta and delta and shows, for the first time, the presence of non conventional PKC isoform theta, atypical PKC isoform lambda/iota and PKC isoform mu in normal human neutrophils. In unstimulated cells all the detected PKC isoforms showed a predominantly cytosolic localisation in normal and CML neutrophils. Cytosol-membrane distribution of PKC alpha and delta were significantly altered in leukemic neutrophils as compared to normal cells. Cytosolic levels of all PKC isoforms were reduced in CML neutrophils with PKC alpha, beta, iota, theta, and mu showing a significant decrease. Cytosolic levels of PKC delta contrary to the trend observed for other PKC isoforms showed a slight increase in CML cells, while its membrane levels were significantly reduced in CML neutrophils. Total PKC kinase activity in CML neutrophil cytosol was significantly reduced, while specific kinase activity of two representative isoforms, PKC alpha and delta, from normal and CML neutrophils were similar, thereby increasing the significance of the altered levels of PKC isoforms in CML, and highlighting their role in the defects in function exhibited by the leukemic neutrophils. The levels of PKC delta and iota increased and decreased respectively as the leukemic myeloid cell matured from the blast to the neutrophil, while the levels of PKC alpha and beta were not altered. This suggests a role for PKC delta and iota in the maturation of the leukemic myeloid cell.

Alpha-tocopherol modulates tyrosine phosphorylation in human neutrophils by inhibition of protein kinase C activity and activation of tyrosine phosphatases

Alpha-tocopherol augmentation in human neutrophils was investigated for effects on neutrophil activation and tyrosine phosphorylation of proteins, through its modulation of protein kinase C (PKC) and tyrosine phosphatase activities. Incubation of neutrophils with alpha-tocopherol succinate (TS) resulted in a dose-dependent incorporation into cell membranes, up to 2.5 nmol/2x10(6) cells. A saturating dose of TS (40 micromol/l) inhibited oxidant production by neutrophils stimulated with phorbol myristate acetate (PMA) or opsonized zymosan (OZ) by 86 and 57%, as measured by luminol-amplified chemiluminescence (CL). With PMA, TS inhibited CL generation to a similar extent to staurosporine (10 nmol/l) or genistein (100 micromol/l), and much more than Trolox (40 micromol/l). With OZ, TS inhibited CL to a similar extent to Trolox. Neutrophil PKC activity was inhibited 50% or more by TS or staurosporine. The enzyme activity was unaffected by genistein or Trolox, indicating a specific interaction of alpha-tocopherol. TS or Trolox increased protein tyrosine phosphorylation in resting neutrophils, and as with staurosporine further increased tyrosine phosphorylation in PMA-stimulated neutrophils, while the tyrosine kinase (TK) inhibitor genistein diminished phosphorylation. These effects in resting or PMA-stimulated neutrophils were unrelated to protein tyrosine phosphatase (PTP) activities, which were maintained or increased by TS or Trolox. In OZ-stimulated neutrophils, on the other hand, all four compounds inhibited the increase in tyrosine-phosphorylated proteins. In this case, the effects of pre-incubation with TS or Trolox corresponded with partial inhibition of the marked (85%) decrease in PTP activity induced by OZ. These results indicate that alpha-tocopherol inhibits PMA-activation of human neutrophils by inhibition of PKC activity, and inhibits tyrosine phosphorylation and activation of OZ-stimulated neutrophils also through inhibition of phosphatase inactivation.

A functional nuclear localization sequence in the C-terminal domain of SHP-1

The Src homology 2 domain-containing protein tyrosine phosphatases SHP-1 and SHP-2 play an important role in many intracellular signaling pathways. Both SHP-1 and SHP-2 have been shown to interact with a diverse range of cytosolic and membrane-bound signaling proteins. Generally, SHP-1 and SHP-2 perform opposing roles in signaling processes; SHP-1 acts as a negative regulator of transduction in hemopoietic cells, whereas SHP-2 acts as a positive regulator. Intriguingly, SHP-1 has been proposed to play a positive regulating role in nonhemopoietic cells, although the mechanisms for this are not understood. Here we show that green fluorescent protein-tagged SHP-1 is unexpectedly localized within the nucleus of transfected HEK293 cells. In contrast, the highly related SHP-2 protein is more abundant within the cytoplasm of transfected cells. In accordance with this, endogenous SHP-1 is localized within the nucleus of several other nonhemopoietic cell types, whereas SHP-2 is distributed throughout the cytoplasm. In contrast, SHP-1 is confined to the cytoplasm of hemopoietic cells, with very little nuclear SHP-1 evident. Using chimeric SHP proteins and mutagenesis studies, the nuclear localization signal of SHP-1 was identified within the C-terminal domain of SHP-1 and found to consist of a short cluster of basic amino acids (KRK). Although the KRK motif resembles half of a bipartite nuclear localization signal, it appears to function independently and is absolutely required for nuclear import. Our findings show that SHP-1 and SHP-2 are distinctly localized within nonhemopoietic cells, with the localization of SHP-1 differing dramatically between nonhemopoietic and hemopoietic cell lineages. This implies that SHP-1 nuclear import is a tightly regulated process and indicates that SHP-1 may possess novel nuclear targets.

Neutrophil-mediated biodegradation of medical implant materials

During the acute inflammatory response to implanted medical devices, human neutrophils (PMN) release oxidative and hydrolytic activities which may ultimately contribute to the degradation of the biomaterial. In this study, the biological activities secreted by live PMNs which may contribute to biodegradation were investigated using a 14C label in the monomer unit of a poly(ester-urea-urethane) (PEUU) substrate. By using specific inhibitors, it was possible to propose a mechanism for PMN-mediated biodegradation. PMN, labeled with 3H-arachidonic acid, released significantly more 3H when adherent to PEUU than when adherent to tissue culture grade polystyrene (P<0.05). The phospholipase A2 (PLA2) inhibitors, aristolochic acid (ARIST) and quinacrine (QUIN), decreased the release of 3H and inhibited PEUU biodegradation (>50%, P<0.05). ARIST had no effect on cell viability, whereas QUIN significantly decreased it. The serine protease inhibitor, phenylmethylsulfonylfluoride inhibited biodegradation, but did not decrease cell survival. There is evidence to suggest that activation via the PLA2 pathway caused the release of hydrolytic activities which were able to elicit 14C release from PEUU. The role of oxidative compounds which were released via activation by phorbol myristate acetate (PMA), was not apparent, since PMA inhibited biodegradation and cell survival (>40%, P<0.05). This study has shown that it is possible to find out the differences in PMN activation through the PLA2 pathway when exposed to different material surfaces, making this a model system worthy of further investigation.

Deficiency of Src homology 2-containing phosphatase 1 results in abnormalities in murine neutrophil function: studies in motheaten mice

Neutrophils, an essential component of the innate immune system, are regulated in part by signaling pathways involving protein tyrosine phosphorylation. While protein tyrosine kinase functions in regulating neutrophil behavior have been extensively investigated, little is known about the role for specific protein tyrosine phosphatases (PTP) in modulating neutrophil signaling cascades. A key role for Src homology 2 domain-containing phosphatase 1 (SHP-1), a PTP, in neutrophil physiology is, however, implied by the overexpansion and inappropriate activation of granulocyte populations in SHP-1-deficient motheaten (me/me) and motheaten viable (me(v)/me(v)) mice. To directly investigate the importance of SHP-1 to phagocytic cell function, bone marrow neutrophils were isolated from both me/me and me(v)/me(v) mice and examined with respect to their responses to various stimuli. The results of these studies revealed that both quiescent and activated neutrophils from motheaten mice manifested enhanced tyrosine phosphorylation of cellular proteins in the 60- to 80-kDa range relative to that detected in wild-type congenic control neutrophils. MOTHEATEN: neutrophils also demonstrated increased oxidant production, surface expression of CD18, and adhesion to protein-coated plastic. Chemotaxis, however, was severely diminished in the SHP-deficient neutrophils relative to control neutrophils, which was possibly attributable to a combination of defective deadhesion and altered actin assembly. Taken together, these results indicate a significant role for SHP-1 in modulating the tyrosine phosphorylation-dependent signaling pathways that regulate neutrophil microbicidal functions.

Differential role of angiotensin II receptor subtypes on endothelial superoxide formation

The physiological role of the angiotensin II AT2 receptor subtype is not fully characterized. We studied whether AT2 receptor could antagonize AT1 mediated superoxide formation in endothelial cells. In quiescent human umbilical vein endothelial cells (HUVEC) superoxide formation was measured after long-term incubation (6 h) with angiotensin II in the presence or absence of its receptor blocker candesartan (AT1) or PD123319 (AT2) using the cytochrome c assay. In separate experiments, the effects of AT2 mediated effects on activities of cellular phosphates including the src homology 2 domain containing phosphatases (SHP-1) was studied. The basal superoxide formation (0.19+/-0.03 nmol superoxide mg protein(-1) min(-1)) in HUVEC was increased by 37.1% after exposure to angiotensin II (100 nM,) which was due to an activation of a NAD(P)H oxidase. This was abolished by candesartan (1 microM) as well as the tyrosine kinase inhibitor genistein. In contrast, blockade of AT2 receptors by PD123319 enhanced the superoxide formation by 73.7% in intact cells. Stimulation of AT2 went along with an increased activity of tyrosine phosphatases in total cell lysates (29.8%) and, in particular, a marked stimulation of src homology 2 domain containing phosphatases (SHP-1, by 293.4%). The tyrosine phosphatase inhibitor vanadate, in turn, prevented the AT2 mediated effects on superoxide formation. The expression of both angiotensin II receptor subtypes AT1 and AT2 was confirmed by RT - PCR analysis. It is concluded that AT2 functionally antagonizes the AT1 induced endothelial superoxide formation by a pathway involving tyrosine phosphatases.

Perinuclear localization of the protein-tyrosine phosphatase SHP-1 and inhibition of epidermal growth factor-stimulated STAT1/3 activation in A431 cells

The SH2 domain protein-tyrosine phosphatase SHP-1 has been shown earlier to bind to the epidermal growth factor receptor and to have the capacity for receptor dephosphorylation. New bi- and tricistronic expression vectors (pNRTIS-21 and pNRTIS-33, respectively) based on the tetracycline system were constructed and employed to generate stable cell lines with inducible expression of SHP-1. Inducible overexpression of SHP-1 in A431 cells led to attenuation of epidermal growth factor (EGF) receptor autophosphorylation and of EGF-induced DNA binding of 'signal transducers and activators of transcription' (STAT) 1 and 3. SHP-1 was localized in the cytoplasm with an enrichment in the perinuclear compartment. Association of SHP-1 with perinuclear structures may form the basis for a partial cofractionation with nuclei observed in different types of transfected cells and also with endogenous SHP-1 in U-937 cells. Treatment of SHP-1-overexpressing A431 cells or of HaCaT human keratinocytes expressing SHP-1 endogenously with the Ca2+-ionophore A23187 resulted in partial nuclear accumulation of SHP-1. Thus, SHP-1 may interact with substrates or regulatory proteins in perinuclear or nuclear structures.

Modulation of human neutrophil responses to CD32 cross-linking by serine/threonine phosphatase inhibitors: cross-talk between serine/threonine and tyrosine phosphorylation

The interplay between serine/threonine and tyrosine phosphorylation was studied in human neutrophils. The direct effects of calyculin and okadaic acid, potent inhibitors of PP1 and PP2A serine/threonine phosphatases, on the patterns of neutrophil phosphorylation, and their effects on the responses of neutrophils to CD32 cross-linking were monitored. After a 2-min incubation with 10-6 M calyculin, a transient tyrosine phosphorylation of a subset of proteins, among which Cbl and Syk, was observed. After a longer incubation (>5 min) with calyculin, concomitant with an accumulation of serine and threonine phosphorylation, neutrophil responses to CD32 cross-linking were selectively altered. Tyrosine phosphorylation of Cbl in response to CD32 cross-linking was inhibited by calyculin, and this inhibition was linked with a slower electrophoretic mobility of Cbl as a consequence of its phosphorylation on serine/threonine residues. However, tyrosine phosphorylation of Syk and of the receptor itself were not affected. Furthermore, the mobilization of intracellular calcium stimulated by CD32 cross-linking was totally abrogated by calyculin. Finally, the stimulation of superoxide production observed in response to CD32 cross-linking was enhanced in calyculin-treated cells. These results suggest that serine/threonine phosphorylation events regulate the signaling pathways activated by CD32 cross-linking in neutrophils and identify a novel mechanism of modulation of the functional responsiveness of human neutrophils to CD32 cross-linking.

Protein Kinase C (PKC) Isoforms Translocate to Triton-Insoluble Fractions in Stimulated Human Neutrophils: Correlation of Conventional PKC with Activation of NADPH Oxidase

The responses of human neutrophils (PMN) involve reorganization and phosphorylation of cytoskeletal components. We investigated the translocation of protein kinase C (PKC) isoforms to PMN cytoskeletal (Triton-insoluble) fractions, in conjunction with activation of the respiratory burst enzyme NADPH oxidase. In resting PMN, PKC-δ (29%) and small amounts of PKC-α (0.6%), but not PKC-βII, were present in cytoskeletal fractions. Upon stimulation with the PKC agonist PMA, the levels of PKC-α, PKC-βII, and PKC-δ increased in the cytoskeletal fraction, concomitant with a decrease in the noncytoskeletal (Triton-soluble) fractions. PKC-δ maximally associated with cytoskeletal fractions at 160 nM PMA and then declined, while PKC-α and PKC-βII plateaued at 300 nM PMA. Translocation of PKC-δ was maximal by 2 min and sustained for at least 10 min. Translocation of PKC-α and PKC-βII was biphasic, plateauing at 2–3 min and then increasing up to 10 min. Under maximal stimulation conditions, PKC isoforms were entirely cytoskeletal associated. Translocation of the NADPH oxidase component p47phox to the cytoskeletal fraction correlated with translocation of PKC-α and PKC-βII, but not with translocation of PKC-δ. Oxidase activity in cytoskeletal fractions paralleled translocation of PKC-α, PKC-βII, and p47phox. Stimulation with 1,2-dioctanoylglycerol resulted in little translocation of PKC isoforms or p47phox, and in minimal oxidase activity. We conclude that conventional PKC isoforms (PKC-α and/or PKC-βII) may regulate PMA-stimulated cytoskeletal association and activation of NADPH oxidase. PKC-δ may modulate other PMN responses that involve cytoskeletal components.

Genetic evidence that stress-activated p38 MAP kinase is necessary but not sufficient for UV activation of HIV gene expression

We have examined the role of stress-activated p38 MAP kinase in regulating human immunodeficiency virus (HIV) gene expression in response to ultraviolet light (UV). We found that UV activated p38 in HeLa cells harboring stably integrated copies of an HIVcat plasmid to levels similar to those obtained by hyperosmotic shock. However, hyperosmotic shock resulted in one order of magnitude smaller increase in CAT activity than treatment with UV. The specific p38 inhibitor SB203580 significantly decreased (>80%) UV activation of HIV gene expression whereas PD98059, a specific MEK-1 inhibitor did not, suggesting that p38 is specifically involved in the HIV UV response and little to no contribution is provided by MEK-1 and the p42/p44 MAP kinase pathway. Whereas increased binding of NF-kappaB to an oligonucleotide spanning the HIV enhancer was observed after UV, as expected, this binding was not affected by SB203580. Furthermore, UV activation of HIV gene expression in cells having the cat reporter gene under control of an HIV promoter deleted of the enhancer (-69/+80) produced results indistinguishable from those using HIVcat/HeLa cells with an intact HIV promoter (-485/+80), suggesting that SB203580 acts through the basal transcription machinery. Northern blot analysis of steady-state RNA from HIVcat/HeLa cells revealed an almost complete inhibition of UV activation with SB203580 at the RNA level. Similarly, the UV response was almost completely obliterated at the CAT and RNA levels in HIVcat/HeLa cells stably transfected with a plasmid expressing a kinase-inactive mutant of p38 (isoform alpha), without affecting NF-kappaB activation, providing strong genetic evidence that p38, at least the alpha isoform, is necessary for UV activation of HIV gene expression and that NF-kappaB activation alone is insufficient. These results firmly establish p38 MAP kinase as a key modulator of HIV gene expression in response to UV that acts independently of NF-kappaB.

Tyrosine phosphatase SHP-1 is involved in CD66-mediated phagocytosis of Opa52-expressing Neisseria gonorrhoeae

Opa proteins of Neisseria gonorrhoeae bind to CD66 receptors on human phagocytes, thereby inducing efficient uptake of the bacteria in the absence of opsonins. The interaction of Opa proteins and CD66 receptors leads to activation of Src family tyrosine kinases, a process that is of critical importance for the efficient, CD66-mediated internalization. Here we show that during Opa-mediated stimulation of CD66 the activity of the host cell tyrosine phosphatase SHP-1 is strongly downregulated, concomitant with increases in the tyrosine phosphorylation of several cellular proteins. Since the SHP-1 tyrosine phosphorylation level itself is influenced by Opa-induced events, this phosphatase comprises an important regulatory checkpoint of the pathogen-triggered signaling cascade in human phagocytes.

Binding of phosphatidic acid to the protein-tyrosine phosphatase SHP-1 as a basis for activity modulation

Activation of the SH2 domain-possessing protein-tyrosine phosphatase SHP-1 by acidic phospholipids as phosphatidic acid (PA) has been described earlier and suggested to participate in regulation of SHP-1 activity toward cellular substrates. The mechanism of this activation is poorly understood. Direct binding of phosphatidic acid to recombinant SHP-1 could be demonstrated by measuring the extent of [(14)C]PA binding in a chromatographic assay, by measuring the extent of binding of SHP-1 to PA-coated ELISA plates or silica beads (TRANSIL), and by spectroscopic assays employing fluorescently labeled PA liposomes. In addition to PA, phosphatidylinositol 3,4, 5-trisphosphate (PIP3), dipalmitoylphosphatidylglycerol, phosphatidylinositol 4,5-bisphosphate, and phosphatidylserine (PS) were found to bind to SHP-1, albeit to a lesser extent. A high-affinity binding site for PA and PIP3 was mapped to the 41 C-terminal amino acids of SHP-1. This site was absent from the related protein-tyrosine phosphatase SHP-2 and conferred activation of SHP-1 by PA toward two different substrates at low lipid concentrations. A SHP-1 mutant missing this binding site could, however, still be activated toward phosphorylated myelin basic protein as a substrate at high PA concentrations. This activation is likely to be mediated by a second, low-affinity binding site for PA in the N-terminal part of SHP-1 within the SH2 domains. High-affinity phospholipid binding to the C-terminus of SHP-1 may present a specific mechanism of regulating activity and/or cellular localization.

Reactive Oxygen Intermediates Enhance Fcγ Receptor Signaling and Amplify Phagocytic Capacity

Receptors for the Fc region of IgG (FcγR) mediate internalization of opsonized particles by human neutrophils (PMN) and mononuclear phagocytes. Cross-linking of FcγR leads to activation of protein tyrosine kinases and phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within FcγR subunits, both obligatory early signals for phagocytosis. Human PMN constitutively express two structurally distinct FcγR, FcγRIIa and FcγRIIIb, and can be induced to express FcγRI by IFN-γ. We have previously shown that stimulation of PMN through FcγRIIIb results in enhanced FcγRIIa-mediated phagocytic activity that is inhibited by catalase. In the present study, we have tested the hypothesis that reactive oxygen intermediates (ROI) have the capacity to regulate FcγR responses and defined a mechanism for this effect. We show that H2O2 augmented phagocytosis mediated by FcγRIIa and FcγRI in PMN and amplified receptor-triggered tyrosine phosphorylation of FcγR-associated ITAMs and signaling elements. Generation of endogenous oxidants in PMN by cross-linking FcγRIIIb similarly enhanced phosphorylation of FcγRIIa and Syk, a tyrosine kinase required for phagocytic function, in a catalase-sensitive manner. Our results provide a mechanism for priming phagocytes for enhanced responses to receptor-driven effects. ROI generated in an inflammatory milieu may stimulate quiescent cells to rapidly increase the magnitude of their effector function. Indeed, human monocytes incubated in the presence of stimulated PMN showed oxidant-induced increases in FcγRIIa-mediated phagocytosis. Definition of the role of oxidants as amplifiers of FcγR signaling identifies a target for therapeutic intervention in immune complex-mediated tissue injury.

Protein kinase C regulates integrin-induced activation of the extracellular regulated kinase pathway upstream of Shc

Adhesion of fibroblasts to extracellular matrices via integrin receptors is accompanied by extensive cytoskeletal rearrangements and intracellular signaling events. The protein kinase C (PKC) family of serine/threonine kinases has been implicated in several integrin-mediated events including focal adhesion formation, cell spreading, cell migration, and cytoskeletal rearrangements. However, the mechanism by which PKC regulates integrin function is not known. To characterize the role of PKC family kinases in mediating integrin-induced signaling, we monitored the effects of PKC inhibition on fibronectin-induced signaling events in Cos7 cells using pharmacological and genetic approaches. We found that inhibition of classical and novel isoforms of PKC by down-regulation with 12-0-tetradeconoyl-phorbol-13-acetate or overexpression of dominant-negative mutants of PKC significantly reduced extracellular regulated kinase 2 (Erk2) activation by fibronectin receptors in Cos7 cells. Furthermore, overexpression of constitutively active PKCalpha, PKCdelta, or PKCepsilon was sufficient to rescue 12-0-tetradeconoyl-phorbol-13-acetate-mediated down-regulation of Erk2 activation, and all three of these PKC isoforms were activated following adhesion. PKC was required for maximal activation of mitogen-activated kinase kinase 1, Raf-1, and Ras, tyrosine phosphorylation of Shc, and Shc association with Grb2. PKC inhibition does not appear to have a generalized effect on integrin signaling, because it does not block integrin-induced focal adhesion kinase or paxillin tyrosine phosphorylation. These results indicate that PKC activity enhances Erk2 activation in response to fibronectin by stimulating the Erk/mitogen-activated protein kinase pathway at an early step upstream of Shc.

Negative Regulation of Myeloid Cell Proliferation and Function by the SH2 Domain-Containing Tyrosine Phosphatase-1

The SH2 domain containing tyrosine phosphatase SHP-1 has been implicated in the regulation of a multiplicity of signaling pathways involved in hemopoietic cell growth, differentiation, and activation. A pivotal contribution of SHP-1 in the modulation of myeloid cell signaling cascades has been revealed by the demonstration that SHP-1 gene mutation is responsible for the overexpansion and inappropriate activation of myelomonocytic populations in motheaten mice. To investigate the role of SHP-1 in regulation of myeloid leukocytes, an HA epitope-tagged dominant negative (interfering) SHP-1 (SHP-1C453S) was expressed in the myelo-monocytic cell line U937 using the pcDNA3 vector. Overexpression of this protein in SHP-1C453S transfectants was demonstrated by Western blot analysis and by detection of decreased specific activity. Growth, proliferation, and IL-3-induced proliferative responses were substantially increased in the SHP-1C453S-overexpressing cells relative to those in control cells. The results of cell cycle analysis also revealed that the proportion of cells overexpressing SHP-1C453S in S phase was greater than that of control cells. The SHP-1C453S-expressing cells also displayed diminished rates of apoptosis as detected by flow cytometric analysis of propidium iodide-stained cells and terminal deoxynucleotidyltransferase-mediated fluorescein-dUTP nick end-labeling assay. While motility and phagocytosis were not affected by SHP-1C453S overexpression, adhesion and the oxidative burst in response to PMA were enhanced in the SHP-1C453S compared with those in the vector alone transfectants. Taken together, these results suggest that SHP-1 exerts an important negative regulatory influence on cell proliferation and activation while promoting spontaneous cell death in myeloid cells.

Microvesicle release is associated with extensive protein tyrosine dephosphorylation in platelets stimulated by A23187 or a mixture of thrombin and collagen

Phosphatidylserine exposure and microvesicle release give rise to procoagulant activity during platelet activation. We have previously shown that whereas the Ca2+ ionophore A23187 and 2,5-di-(t-butyl)-1, 4-benzohydroquinone, a Ca2+-ATPase inhibitor, induce phosphatidylserine exposure, only the former triggers microvesicle release. We now report that microvesicle formation with ionophore A23187 is specifically associated with mu-calpain activation, increased protein tyrosine phosphatase (PTP) activity and decreased tyrosine phosphorylation. The degree to which calpain and individual PTPs were activated in response to A23187 depended on the extent of bivalent cation chelation in the external medium. EGTA (2 mM) blocked or severely retarded their activation, and addition of extracellular Ca2+ in excess (2 mM) resulted in virtually immediate tyrosine dephosphorylation. Dephosphorylation was correlated with an increase in total PTP activity in platelet lysates. In platelets stimulated by a combination of thrombin and collagen, only the subpopulation undergoing microvesicle release and isolated by their binding to annexin-V-coated magnetic beads exhibited protein tyrosine dephosphorylation. Detection of PTP activity in an 'in-gel' assay showed the Ca2+-dependent appearance of active low-molecular-mass bands at 38, 36 and 27 kDa. Individual PTPs varied in their protease sensitivity to changes in intracellular Ca2+ levels. For example, PTP1B was a more sensitive substrate than SH2-domain-containing tyrosine phosphatase-1 for mu-calpain cleavage. Incubation of platelets with the PTP inhibitors, phenylarsine oxide and benzylphosphonic acid acetoxymethyl ester, led to increased tyrosine phosphorylation and the surface expression of aminophospholipids but little microvesicle formation. Furthermore, microvesicle release in response to ionophore A23187 was inhibited. We conclude that platelet microvesicle formation is associated with extensive protein tyrosine dephosphorylation.

The role of actin-binding protein 280 in integrin-dependent mechanoprotection

To survive in a mechanically active environment, cells must adapt to variations of applied membrane tension. A collagen-coated magnetic bead model was used to apply forces directly to the actin cytoskeleton through integrin receptors. We demonstrate here that by a calcium-dependent mechanism, human fibroblasts reinforce locally their connection with extracellular adhesion sites by inducing actin assembly and by recruiting actin-binding protein 280 (ABP-280) into cortical adhesion complexes. ABP-280 was phosphorylated on serine residues as a result of force application. This phosphorylation and the force-induced actin reorganization were largely abrogated by inhibitors of protein kinase C. In a human melanoma cell line that does not express ABP-280, actin accumulation could not be induced by force, whereas in stable transfectants expressing ABP-280, force-induced actin accumulation was similar to human fibroblasts. Cortical actin assembly played a role in regulating the activity of stretch-activated, calcium-permeable channels (SAC) since sustained force application desensitized SAC to subsequent force applications, and the decrease in stretch sensitivity was reversed after treatment with cytochalasin D. ABP-280-deficient cells showed a > 90% increase in cell death compared with ABP-280 +ve cells after force application. We conclude that ABP-280 plays an important role in mechanoprotection by reinforcing the membrane cortex and desensitizing SACs.

Lipoarabinomannan of Mycobacterium tuberculosis promotes protein tyrosine dephosphorylation and inhibition of mitogen-activated protein kinase in human mononuclear phagocytes. Role of the Src homology 2 containing tyrosine phosphatase 1

Lipoarabinomannan (LAM) is a putative virulence factor of Mycobacterium tuberculosis that inhibits monocyte functions, and this may involve antagonism of cell signaling pathways. The effects of LAM on protein tyrosine phosphorylation in cells of the human monocytic cell line THP-1 were examined. LAM promoted tyrosine dephosphorylation of multiple cell proteins and attenuated phorbol 12-myristate 13-acetate-induced activation of mitogen-activated protein kinase. To examine whether these effects of LAM could be related to activation of a phosphatase, fractions from LAM-treated cells were analyzed for dephosphorylation of para-nitrophenol phosphate. The data show that LAM induced increased phosphatase activity associated with the membrane fraction. The Src homology 2 containing tyrosine phosphatase 1 (SHP-1) is important for signal termination and was examined as a potential target of LAM. Exposure of cells to LAM brought about (i) an increase in tyrosine phosphorylation of SHP-1, and (ii) translocation of the phosphatase to the membrane. Phosphatase assay of SHP-1 immunoprecipitated from LAM-treated cells, using phosphorylated mitogen-activated protein kinase as substrate, indicated that LAM promoted increased activity of SHP-1 in vivo. LAM also activated SHP-1 directly in vitro. Exposure of cells to LAM also attenuated the expression of tumor necrosis factor-alpha, interleukin-12, and major histocompatibility class II molecules. These results suggest that one mechanism by which LAM deactivates monocytes involves activation of SHP-1.

The marine toxin okadaic acid reduces O2- generation and tyrosine phosphorylation in LPS-primed rat neutrophils

Contrasting effects of okadaic acid (OKA) on neutrophil (PMN) superoxide anion (O2-) generation have been reported. In this study, we examined the effect of OKA on phorbol myristate acetate (PMA)-stimulated O2- generation in rat PMNs primed with LPS in vivo (LPS-PMN) and saline-treated rat PMNs (SAL-PMN). The following results were observed: (1) OKA, but neither genistein nor vanadate, markedly reduced O2- generation in a dose and time-dependent manner; (2) genistein, a tyrosine kinase inhibitor, as well as OKA, reduced tyrosine phosphorylation; (3) sodium orthovanadate, a tyrosine phosphatase inhibitor, potently enhanced tyrosine phosphorylation. Our studies suggest that OKA might reduce tyrosine phosphorylation by affecting the activity of tyrosine phosphatases regulated by serine-threonine phosphorylation.