Effect of inhibitors of distinct signalling pathways on neutrophil Q2- generation in response to tumor necrosis factor-alpha, and antibodies against CD18 and CD11a: evidence for a common and unique pattern of sensitivity to wortmannin and protein tyrosine kinase inhibitors

Inhibition of phospholipase A(2) abrogates intracellular processing of NADPH-oxidase derived reactive oxygen species in human neutrophils

Upon activation of human neutrophils, superoxide can be produced at two cellular sites; either in the plasma membrane, giving extracellular release of oxidants, or in intracellular organelles, resulting in oxidants being retained in the cell. The involvement of phospholipase A(2) (PLA(2)) in phorbol myristate acetate (PMA)-induced activation of the two pools of NADPH-oxidase was investigated using a variety of PLA(2) inhibitors and the oxidase activity was measured by luminol/isoluminol-amplified chemiluminescence (CL). Two of the seven inhibitors were without effect, two inhibitors inhibited both intra- and extracellular ROS production equally, and three inhibitors inhibited intracellular but not extracellular CL. Using another technique to measure ROS, PHPA oxidation, we found that intracellular ROS production was unaltered with the three last inhibitors, indicating that PLA(2) is not involved in the NADPH-oxidase activity per se, but in the intracellular processing of the radicals necessary for the CL reaction to take place. The PLA(2) inhibitors did not abolish the activity of myeloperoxidase (MPO), an enzyme necessary for intracellular CL to occur. Instead, we suggest that these PLA(2) inhibitors block heterotypic granule fusion and prohibit the colocalization of ROS and MPO needed for intracellular CL activity.

Involvement of Src family tyrosine kinase in apoptosis of human neutrophils induced by protozoan parasite Entamoeba histolytica

Tyrosine kinases are one of the most important regulators for intracellular signal transduction related to inflammatory responses. However, there are no reports describing the effects of tyrosine kinases on neutrophil apoptosis induced by Entamoeba histolytica. In this study, isolated human neutrophils from peripheral blood were incubated with live trophozoites in the presence or absence of tyrosine kinase inhibitors. Entamoeba-induced receptor shedding of CD16 and PS externalization in neutrophils were inhibited by pre-incubation of neutrophils with the broad-spectrum tyrosine kinase inhibitor genistein or the Src family kinase inhibitor PP2. Entamoeba-induced ROS production was also inhibited by genistein or PP2. Moreover, genistein and PP2 blocked the phosphorylation of ERK and p38 MAPK in neutrophils induced by E. histolytica. These results suggest that Src tyrosine kinases may participate in the signaling event for ROS-dependent activation of MAPKs during neutrophil apoptosis induced by E. histolytica.

c-Abl kinase is required for beta 2 integrin-mediated neutrophil adhesion

Integrin regulation in neutrophil adhesion is essential for innate immune response. c-Abl kinase is a nonreceptor tyrosine kinase and is critical for signaling transduction from various receptors in leukocytes. Using neutrophils and dHL-60 (neutrophil-like differentiation of HL-60) cells, we show that c-Abl kinase is activated by beta(2) integrin engagement and is required for beta(2) integrin-dependent neutrophil sustained adhesion and spreading. The expression of beta(2) integrin on neutrophils induced by TNF-alpha is not affected by c-Abl kinase inhibitor STI571, suggesting that c-Abl kinase is not involved in TNF-alpha-induced integrin activation. The recruitment of c-Abl kinase to beta(2) integrin is dependent on talin head domain, which constitutively interacts with beta(2) integrin cytoplasmic domain. After activated, c-Abl kinase increases the tyrosine phosphorylation of Vav. The SH3 domain of c-Abl kinase is involved in its interaction with talin and Vav. Thus, c-Abl kinase plays an essential role in the activation of Vav induced by beta(2) integrin ligation and in regulating neutrophil-sustained adhesion and spreading.

Fc gamma R-stimulated activation of the NADPH oxidase: phosphoinositide-binding protein p40phox regulates NADPH oxidase activity after enzyme assembly on the phagosome

The phagocyte NADPH oxidase generates superoxide for microbial killing, and includes a membrane-bound flavocytochrome b(558) and cytosolic p67(phox), p47(phox), and p40(phox) subunits that undergo membrane translocation upon cellular activation. The function of p40(phox), which binds p67(phox) in resting cells, is incompletely understood. Recent studies showed that phagocytosis-induced superoxide production is stimulated by p40(phox) and its binding to phosphatidylinositol-3-phosphate (PI3P), a phosphoinositide enriched in membranes of internalized phagosomes. To better define the role of p40(phox) in FcgammaR-induced oxidase activation, we used immunofluorescence and real-time imaging of FcgammaR-induced phagocytosis. YFP-tagged p67(phox) and p40(phox) translocated to granulocyte phagosomes before phagosome internalization and accumulation of a probe for PI3P. p67(phox) and p47(phox) accumulation on nascent and internalized phagosomes did not require p40(phox) or PI3 kinase activity, although superoxide production before and after phagosome sealing was decreased by mutation of the p40(phox) PI3P-binding domain or wortmannin. Translocation of p40(phox) to nascent phagosomes required binding to p67(phox) but not PI3P, although the loss of PI3P binding reduced p40(phox) retention after phagosome internalization. We conclude that p40(phox) functions primarily to regulate FcgammaR-induced NADPH oxidase activity rather than assembly, and stimulates superoxide production via a PI3P signal that increases after phagosome internalization.

Involvement of β2-integrin in ROS-mediated neutrophil apoptosis induced by Entamoeba histolytica

Cellular adhesion through beta 2-integrin (CD18) is an important step in signal transduction leading to apoptosis of human neutrophils, and NADPH oxidase-derived reactive oxygen species (ROS) are essential for neutrophil apoptosis induced by Entamoeba histolytica. Therefore, we investigated the role of beta 2-integrin-mediated signals in ROS-dependent neutrophil apoptosis induced by E. histolytica. Entamoeba-induced apoptosis was inhibited by pre-incubation of cells with mAb to CD18, but not CD29, suggesting that beta )-integrin plays an important role in this response. Moreover, Entamoeba-induced ROS generation in neutrophils was inhibited by mAbs against CD18 or CD11b, but not by mAbs against CD11a, CD11c, or CD29. A combination of d-galactose plus anti-CD18 mAb had a larger inhibitory effect than d-galactose alone on Entamoeba-induced apoptosis and ROS generation. Furthermore, Entamoeba-induced apoptosis and ROS generation were inhibited by pre-treatment of cells with an inhibitor of phosphatidylinositol-3-kinase (PI-3-kinase). These results indicate that beta 2-integrin and PI-3-kinase are crucial signaling molecules in ROS-dependent apoptosis of neutrophils induced by E. histolytica.

Induction of reactive oxygen species from isolated rat glomeruli by protein kinase C activation and TNF-alpha stimulation, and effects of a phosphodiesterase inhibitor

Diabetic nephropathy is a major complication of diabetes leading to end-stage renal disease, which requires hemodialysis. Although the mechanism by which it progresses is largely unknown, the role of hyperglycemia-derived oxidative stress has recently been the focus of attention as the cause of diabetic complications. Constituent cells of the renal glomeruli have the capacity to release reactive oxygen species (ROS) upon stimulation of NADPH oxidase activated by protein kinase C (PKC). Hyperglycemia and insulin resistance in the diabetic state are often associated with activation of PKC and tumor necrosis factor (TNF)-alpha, respectively. The aim of this study is to clarify the signaling pathway leading to ROS production by PKC and TNF-alpha in rat glomeruli. Isolated rat glomeruli were stimulated with phorbol 12-myristate 13-acetate (PMA) and TNF-alpha, and the amount of ROS was measured using a chemiluminescence method. Stimulation with PMA (10 ng/ml) generated ROS with a peak value of 136+/-1.2 cpm/mg protein (mean+/-SEM). The PKC inhibitor H-7, the NADPH oxidase inhibitor diphenylene iodonium and the phosphatidylinositol-3 (PI-3) kinase inhibitor wortmannin inhibited PMA-induced ROS production by 100%, 100% and 80%, respectively. In addition, TNF-alpha stimulated ROS production (283+/-5.8/mg protein/20 min). The phosphodiesterase inhibitor cilostazol activates protein kinase A and is reported to improve albuminuria in diabetic rats. Cilostazol (100 microg/ml) inhibited PMA, and TNF-alpha-induced ROS production by 78+/-1.8, and 19+/-2.7%, respectively. The effects of cilostazol were not additive with wortmannin. Cilostazol arrests oxidative stress induced by PKC activation by inhibiting the PI-3 kinase-dependent pathway, and may thus prevent the development of diabetic nephropathy.

Beta-glucan is a fungal determinant for adhesion-dependent human neutrophil functions

Candida albicans is a common cause of nosocomial infections whose virulence depends on the reversible switch from blastoconidia to hyphal forms. Neutrophils (or polymorphonuclear leukocytes (PMNs)) readily clear blastoconidia by phagocytosis, but filaments are too long to be ingested. Mechanisms regulating immune recognition and response to filamentous fungal pathogens are not well understood, although known risk factors for developing life-threatening infections are neutropenia or defects in the NADPH oxidase system. We show human PMNs generate a respiratory burst response to unopsonized hyphae. Ab specific for beta-glucan, a major component of yeast cell walls, blocks this response, establishing beta-glucan as a key molecular pattern recognized by PMNs in response to C. albicans. This study also elucidates recognition and signaling mechanisms used by PMNs in response to beta-glucan under conditions where phagocytosis cannot occur. Human PMNs adhered to immobilized beta-glucan and released an efficient plasma membrane respiratory burst. Ab blockade of the integrin complement receptor 3 (CD11b/CD18) significantly inhibited both of these functions. Furthermore, we show a role for p38 MAPK and actin but not protein kinase C zeta in generating the respiratory burst to beta-glucan. Taken together, results show that beta-glucan in C. albicans hyphae is accessible to PMNs and sufficient to support an innate immune response.

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.

Blockade of class IA phosphoinositide 3-kinase in neutrophils prevents NADPH oxidase activation- and adhesion-dependent inflammation

We examined the role of class IA phosphoinositide 3-kinase (PI3K) in the regulation of activation of NADPH oxidase in PMNs and the mechanism of PMN-dependent lung inflammation and microvessel injury induced by the pro-inflammatory cytokine TNF-alpha. TNF-alpha stimulation of PMNs resulted in superoxide production that was dependent on CD11b/CD18-mediated PMN adhesion. Additionally, TNF-alpha induced the association of CD11b/CD18 with the NADPH oxidase subunit Nox2 (gp91(phox)) and phosphorylation of p47(phox), indicating the CD11b/CD18 dependence of NADPH oxidase activation. Transduction of wild-type PMNs with Deltap85 protein, a dominant-negative form of the class IA PI3K regulatory subunit, p85alpha, fused to HIV-TAT (TAT-Deltap85) prevented (i) CD11b/CD18-dependent PMN adhesion, (ii) interaction of CD11b/CD18 with Nox2 and phosphorylation of p47(phox), and (iii) PMN oxidant production. Furthermore, studies in mice showed that i.v. infusion of TAT-Deltap85 significantly reduced the recruitment of PMNs in lungs and increase in lung microvascular permeability induced by TNF-alpha. We conclude that class IA PI3K serves as a nodal point regulating CD11b/CD18-integrin-dependent PMN adhesion and activation of NADPH oxidase, and leads to oxidant production at sites of PMN adhesion, and the resultant lung microvascular injury in mice.

Activation of human neutrophils by granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor alpha: role of phosphatidylinositol 3-kinase

Stimulation of human neutrophils with granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or tumor necrosis factor alpha (TNF) resulted in phosphorylation of Akt, the potency being GM-CSF > G-CSF = TNF, which was inhibited by wortmannin. The findings indicated that phosphatidylinositol 3-kinase (PI3K) is activated by these cytokines. The possible participation of PI3K in activation of neutrophil functions induced by these cytokines was explored with PI3K inhibitors (wortmannin and LY294002). Superoxide release and adherence induced by GM-CSF or TNF were inhibited by PI3K inhibitors. Actin reorganization and morphological changes induced by G-CSF or GM-CSF were also inhibited by wortmannin, whereas these responses induced by TNF were unaffected by wortmannin. These findings suggested that PI3K is differentially involved in cytokine-mediated activation of neutrophil functions depending on the cytokines used. The results also showed that activation of extracellular signal-regulated kinase, but not p38 mitogen-activated protein kinase, induced by these cytokines is partly mediated by PI3K activation.

Responses of neutrophils to anti-integrin antibodies depends on costimulation through low affinity Fc gamma Rs: full activation requires both integrin and nonintegrin signals

The relative contribution of integrin and nonintegrin signals to neutrophil activation is incompletely understood. Immobilized anti-integrin Abs were previously shown to induce robust activation of neutrophils without any additional stimulus, suggesting that cross-linking of integrins is sufficient for full activation of the cells. However, the possible contribution from other receptors has not been tested in this system. In this study, we show that neutrophil responses to anti-integrin Abs requires costimulation through low-affinity Fc gamma Rs. Murine neutrophils lacking the FcR gamma-chain or Fc gamma RIII failed to respond to immobilized Abs against beta(1), beta(2), or beta(3) integrins and the activation of wild-type cells could be prevented by blocking Abs against Fc gamma RII/III. Plate-bound anti-CD18 Abs initiated a respiratory burst from human neutrophils, but this response was abrogated when the F(ab')(2) of the same Abs were used or the cells were preincubated with Fc gamma RIIA-blocking Abs. Lack of Fc gamma RIII or administration of Fc gamma R-blocking Abs had no effect on responses of TNF-stimulated cells plated on fibrinogen or rICAM-1. TNF restored the respiratory burst of Fc gamma RIII-deficient neutrophils plated on anti-CD18 mAbs. The p38 MAPK inhibitor SB203580 attenuated the responses of neutrophils to anti-CD18 mAbs or TNF stimulation on a fibrinogen surface. Taken together, these results indicate that activation of low-affinity Fc gamma Rs is required for neutrophil responses induced by anti-integrin Abs and suggest that a second coactivation signal (e.g., through TNF or FcR ligation) is indispensable for full integrin-mediated activation of neutrophils. These second signals are interchangeable and they may converge on the p38 MAPK.

Role of protein tyrosine kinase p53/56lyn in diminished lipopolysaccharide priming of formylmethionylleucyl- phenylalanine-induced superoxide production in human newborn neutrophils

Human newborns are more susceptible than adults to bacterial infection. With gram-negative bacteria, this may be due to a diminished response of newborn leukocytes to lipopolysaccharide (LPS). Since protein tyrosine kinase inhibition abolishes LPS priming in adult cells, we hypothesized that protein tyrosine kinases may have a critical role in LPS priming of polymorphonuclear neutrophils (PMNs) and that newborn PMNs may have altered protein tyrosine kinase activities. In the present study, we investigated the role of src family protein tyrosine kinases in the LPS response of newborn PMNs compared to adult cells. In a respiratory assay, the LPS-primed increase in formylmethionylleucylphenylalanine (fMLP)-triggered O2- release by adult PMNs was greatly decreased by PP1 [4-amino-5-(4-methyphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine], a src kinase inhibitor, to the level of untreated newborn PMNs, in which LPS failed to prime. LPS activated the src-like kinases p59hck (HCK) and p58fgr (FGR) in both adult and newborn PMNs but increased the activation of p53/56lyn (LYN) only in adult cells. In newborn PMNs, LYN was highly phosphorylated independent of LPS. We evaluated subcellular fractions of PMNs and found that the phosphorylated form of LYN was mainly in the Triton-extractable, cytosolic fraction in adult PMNs, while in newborn cells it was located mainly in Triton-insoluble, granule- and membrane-associated fractions. In contrast, the phosphorylated mitogen-activated protein kinases ERK1/2 and p38 were mainly detected in the cytosol in both adult and newborn PMNs. These data indicate a role for LYN in the regulation of LPS priming. The trapping of phosphorylated LYN in the membrane-granule fraction in newborn PMNs may contribute to the deficiency of newborn cells in responding to LPS stimulation.

Selective role of PI3K delta in neutrophil inflammatory responses

Although members of the class I phosphoinositide 3-kinases (PI3Ks) have been implicated in neutrophil inflammatory responses, the contribution of the individual PI3K isoforms in neutrophil activation has not been tractable with the non-selective inhibitors, LY294002 and wortmannin. We have developed a novel series of PI3K inhibitors that is selective for PI3K delta, an isoform expressed predominantly in hematopoietic cells. In addition to being selective between members of class I PI3Ks, representatives of these inhibitors such as IC980033 and IC87114 did not inhibit any protein kinases tested. Utilizing these inhibitors we report here a novel role for PI3K delta in neutrophil activation. Inhibition of PI3K delta with IC980033 and IC87114 blocked both fMLP- and TNF1 alpha-induced neutrophil superoxide generation and elastase exocytosis. The PI3K delta inhibitor IC87114 also blocked TNF1 alpha-stimulated elastase exocytosis from neutrophils in a mouse model of inflammation. To our knowledge, this is the first in vivo efficacy demonstration of a PI3K delta inhibitor in an animal model. Inhibition of PI3K delta, however, had no effect on in vitro neutrophil bactericidal activity and Fc gamma R-stimulated superoxide generation. Thus, PI3K delta plays an essential role in certain signaling pathways of neutrophil activation and appears to be an attractive target for the development of an anti-inflammatory therapeutic.

Down-regulation of Rac activity during beta 2 integrin-mediated adhesion of human neutrophils

In human neutrophils, beta2 integrin engagement mediated a decrease in GTP-bound Rac1 and Rac2. Pretreatment of neutrophils with LY294002 or PP1 (inhibiting phosphatidylinositol 3-kinase (PI 3-kinase) and Src kinases, respectively) partly reversed the beta2 integrin-induced down-regulation of Rac activities. In contrast, beta2 integrins induced stimulation of Cdc42 that was independent of Src family members. The PI 3-kinase dependence of the beta2 integrin-mediated decrease in GTP-bound Rac could be explained by an enhanced Rac-GAP activity, since this activity was blocked by LY204002, whereas PP1 only had a minor effect. The fact that only Rac1 but not Rac2 (the dominating Rac) redistributed to the detergent-insoluble fraction and that it was independent of GTP loading excludes the possibility that down-regulation of Rac activities was due to depletion of GTP-bound Rac from the detergent-soluble fraction. The beta2 integrin-triggered relocalization of Rac1 to the cytoskeleton was enabled by a PI 3-kinase-induced dissociation of Rac1 from LyGDI. The dissociations of Rac1 and Rac2 from LyGDI also explained the PI 3-kinase-dependent translocations of Rac GTPases to the plasma membrane. However, these accumulations of Rac in the membrane, as well as that of p47phox and p67phox, were also regulated by Src tyrosine kinases. Inasmuch as Rac GTPases are part of the NADPH oxidase and the respiratory burst is elicited in neutrophils adherent by beta2 integrins, our results indicate that activation of the NADPH oxidase does not depend on the levels of Rac-GTP but instead requires a beta2 integrin-induced targeting of the Rac GTPases as well as p47phox and p67phox to the plasma membrane.

Serine protease inhibitors inhibit superoxide release and adherence in human neutrophils stimulated by granulocyte-macrophage colony-stimulating factor and tumor necrosis factor-alpha

Stimulation of human neutrophils with granulocyte-macrophage colony-stimulating factor (GM-CSF) or tumor necrosis factor-alpha (TNF) results in increased superoxide (O2-) release and adherence. O2- release and adherence are dependent on activation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). Possible participation of serine proteases in GM-CSF- or TNF-induced activation of human neutrophils was explored with various serine protease inhibitors, including phenylmethylsulfonyl fluoride, L-1-tosylamido-2-phenylethyl-chloromethyl ketone and N-alpha-p-tosyl-L-lysine-chloromethyl ketone. GM-CSF- or TNF-induced O2- release and adherence were inhibited in parallel by pretreatment of neutrophils with these inhibitors. On the other hand, GM-CSF- or TNF-induced phosphorylation of ERK and p38 MAPK was unaffected by these inhibitors at the concentrations effective for the inhibition of O2- release and adherence. These findings suggest that serine proteases are involved in GM-CSF- and TNF-induced O2- release and adherence in human neutrophils and that serine proteases function downstream or independently of the activation of ERK and p38 MAPK.

Cross-linking of beta2 integrins caused diminished responses of neutrophils to priming agents like lipopolysaccharide or tumor necrosis factor-alpha: possible involvement of tyrosine kinase Syk

Neutrophils up-regulate beta2 integrins like CD11b/CD18 in response to lipopolysaccharide (LPS). Up-regulation of beta2 integrins causes neutrophils to adhere to surfaces, and to release superoxide anion (O2-). When neutrophils are exposed to LPS plus plasma under conditions not favorable for adherence (absence of Mg2+), the cells do not spontaneously release O2-, but instead they are primed for enhanced release of O2- after subsequent triggering by fMLP. In the presence of Mg2+, neutrophils adhere in response to LPS but fMLP-triggered O2- release by LPS-primed neutrophils is diminished. To understand why adherence interferes with the response of neutrophils to N-formyl-methionyl-leucyl-phenylalanine (fMLP), beta2 integrins were cross-linked by mouse monoclonal antibodies that had been immobilized by surface-bound anti-mouse antibody. When unprimed neutrophils were trapped on the surface by these cross-linked monoclonal antibodies, O2- release was triggered, and priming by LPS for fMLP-triggered O2- release was diminished, indicating that this cross-linking of beta2 integrins mimicked adherence. Alkaline phosphatase is up-regulated by LPS or tumor necrosis factor-alpha, and this response was also diminished by the cross-linking antibodies. The diminished alkaline phosphatase up-regulation was reversed by genistein, a general inhibitor of tyrosine kinases, and by piceatannol, an inhibitor for Syk kinase. Piceatannol also inhibited the phosphorylation of Syk caused by cross-linking of beta2 integrins. These results suggested that adherence-induced triggering and Syk kinase activation might be responsible for the diminished response of LPS-primed neutrophils to fMLP when neutrophils were adherent.

TNFalpha elicits association of PI 3-kinase with the p60TNFR and activation of PI 3-kinase in adherent neutrophils

Tumor necrosis factor (TNFalpha) is an incomplete secretagogue in neutrophils and requires the engagement of beta integrins to trigger secretion of superoxide anion (O(-)(2)). The p60 TNF receptor (p60TNFR) is responsible for signal transduction for activation of O(-)(2) generation. Activation of TNFalpha-triggered O(-)(2) generation in neutrophils adherent to fibrinogen-coated surfaces involves the beta2 integrin receptor CD11b/CD18. Phosphoinositide 3-kinase (PI 3-kinase) is essential for activation of O(-)(2) generation; wortmannin, an inhibitor of PI 3-kinase, inhibited TNFalpha-elicited O(-)(2) generation. p60TNFR immunoprecipitated from neutrophils was associated with immunoreactivity to PI 3-kinase in adherent neutrophils exposed to TNFalpha, but not in TNFalpha-treated neutrophils in suspension. In addition, PI 3-kinase immunoprecipitated from TNFalpha-activated neutrophils showed enhanced activity in adherent but not in nonadherent neutrophils. These findings suggest that synergism between CD11b/CD18 and p60TNFR in the presence of TNFalpha is required to elicit assembly of a signaling complex involving association of p60TNFR with PI 3-kinase, activation of PI 3-kinase, and generation of O(-)(2).

Role of intracellular chloride in the reversible activation of neutrophil beta 2 integrins: a lesson from TNF stimulation

The process of beta(2) integrin activation, which enhances the interaction of these heterodimers with ligands, plays a crucial role in the adherence-dependent neutrophilic polymorphonuclear leukocytes' (PMN) responses to TNF. Our previous observation, showing that a marked decrease of the high basal Cl(-) content (Cl(-)(i)) is an essential step in the TNF-induced activation of PMN, stimulated this study, which investigates the role of alterations of Cl(-)(i) in the activation of beta(2) integrins triggered by TNF. Here we show that TNF enhances the expression of activation-specific neoepitopes of beta(2) integrins, namely, epitope 24, a unique epitope present on all three leukocyte integrin alpha subunits, and epitope CBRM1/5, localized to the I domain on the alpha-chain of Mac-1 (CD11bCD18). Moreover, we demonstrate that the conformational changes underlying the expression of the neoepitopes are dependent on a drop in Cl(-)(i) because 1) inhibition of Cl(-)(i) decrease is invariably accompanied by inhibition of beta(2) integrin activation, 2) Cl(-)(i) decrease induced by means other than agonist stimulation, i.e., by placing PMN in Cl(-)-free buffers, activates beta(2) integrins, and 3) restoration of the original Cl(-)(i) levels is accompanied by deactivation of beta(2) integrins. We also show that Cl(-)(i) decrease is required for TNF-induced cytoplasmic alkalinization, but such a rise in pH(i) does not seem to be relevant for beta(2) integrin activation. The results of our study emphasize the role of Cl(-) as a new PMN "second messenger."

Analysis of the functional characteristics of L-selectin and its expression on normal and CD18-deficient bovine neutrophils

In vivo responsiveness to epinephrine, expression of L-selectin on neutrophils, changes in intracellular calcium ([Ca2+]i), sulfatide-induced superoxide production and tyrosine phosphorylation in neutrophils were evaluated to elucidate the role of L-selectin-associated functions of normal and CD18-deficient bovine neutrophils. The number of neutrophils in peripheral blood was significantly increased (P < 0.05) in four normal calves at 5-20 min after in vivo administration of epinephrine; however, no significant increase of neutrophils was found in three calves with bovine leucocyte adhesion deficiency (BLAD). Expression of L-selectin on neutrophils from three calves with BLAD was 61-77% of that of normal calves. Pretreatment of neutrophils with phorbol myristate acetate caused a marked decrease in the expression of L-selectin on neutrophils from both normal and BLAD calves. The sulfatide-induced sustained phase of [Ca2+]i concentration in neutrophils from calves with BLAD was significantly (P < 0.05) decreased. Following stimulation with aggregated IgG, the transient phase of [Ca2+]i in neutrophils from normal and BLAD calves was increased; however, the sustained phase of [Ca2+]i in BLAD neutrophils was significantly lower (P < 0.05) than that of controls. Sulfatide-induced O2- production and chemiluminescent response in neutrophils from calves with BLAD were 48-51% of those of normal calves and were inhibited by genistein and wortmannin, respectively, in a dose-dependent manner. The amount of tyrosine phosphorylated 100 kDa protein in neutrophils from BLAD calves stimulated with sulfatides was 57% of that of controls. The degree of L-selectin expression on neutrophils was correlated with the intracellular signalling events and the related superoxide production.

Kinase pathways in chemoattractant-induced degranulation of neutrophils: the role of p38 mitogen-activated protein kinase activated by Src family kinases

The aim of the present study was to investigate the role of tyrosine phosphorylation pathways in fMLP-induced exocytosis of the different secretory compartments (primary and secondary granules, as well as secretory vesicles) of neutrophils. Genistein, a broad specificity tyrosine kinase inhibitor, blocked the exocytosis of primary and secondary granules, but had only a marginal effect on the release of secretory vesicles. Genistein also inhibited the phosphorylation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinases (MAPK), raising the possibility that inhibition of ERK and/or p38 MAPK might be responsible for the effect of the drug on the degranulation response. Indeed, SB203580, an inhibitor of p38 MAPK, decreased the release of primary and secondary granules, but not that of secretory vesicles. However, blocking the ERK pathway with PD98059 had no effect on any of the exocytic responses tested. PP1, an inhibitor of Src family kinases, also attenuated the release of primary and secondary granules, and neutrophils from mice deficient in the Src family kinases Hck, Fgr, and Lyn were also defective in secondary granule release. Furthermore, activation of p38 MAPK was blocked by both PP1 and the hck-/-fgr-/-lyn-/- mutation. Taken together, our data indicate that fMLP-induced degranulation of primary and secondary granules of neutrophils is mediated by p38 MAPK activated via Src family tyrosine kinases. Although piceatannol, a reportedly selective inhibitor of Syk, also prevented degranulation and activation of p38 MAPK, no fMLP-induced phosphorylation of Syk could be observed, raising doubts about the specificity of the inhibitor.

Redox regulation of beta2-integrin CD11b/CD18 activation

Although the central role of beta2-integrin CD11b / CD18 in neutrophil functions is well recognized, signaling pathway that regulate integrin activation remain to be elucidated. We analyzed the contribution of oxido-reduction mechanisms in this signaling. Exogenously added H(2)O(2) induced CD11b/CD18-dependent neutrophil adhesion and expression of an integrin activation neoepitope recognized by monoclonal antibody (mAb) clone 24. H(2)O(2)-triggered beta2-integrin activation was inhibited by tyrosine kinase inhibitors and by complexing sulfhydryl groups with phenylarsine oxide (PAO). CD11b/CD18-dependent adhesion and mAb 24 antigen expression triggered by physiological agonists such as TNF-alpha were inhibited by diphenylene iodonium (DPI, an inhibitor of flavoprotein oxidoreductase), by free radical scavengers, by tyrosine kinase inhibitors and by PAO. No inhibition was observed when adhesion was induced by the integrin-activating KIM 185 mAb. Taken together, these results emphasize the importance of an oxidative S-thiolation step(s) in the tyrosine kinase-dependent signaling pathway leading to beta2-integrin activation. H(2)O(2) would directly mediate this oxidative reaction and bypass the initial agonist/receptor pathway to promote integrin-dependent adhesion. The putative oxidase(s) involved in this process is not NADPH oxidase, since adhesion of neutrophils from patients with chronic granulomatous disease was normal and inhibited by scavengers and DPI. These data shed a new light on the regulation of integrin activation required for cell migration into inflamed organs.

Integrin signalling in neutrophils and macrophages

Integrins have been characterized extensively as adhesion receptors capable of transducing signals inside the cell. In myelomonocytic cells, integrin-mediated adhesive interactions regulate different selective cell responses, such as transmigration into the inflammatory site, cytokine secretion, production or reactive oxygen intermediates, degranulation and phagocytosis. In the last few years, great progress has been made in elucidating mechanisms of signal transduction by integrins in neutrophils and macrophages. This review summarises the current information on the role of integrins in regulating myelomonocytic cell functions and highlights the signalling pathways activated by integrin engagement in these cells. Also, exploiting the current knowledge of mechanisms of integrin signal transduction in other cell types, we propose a model to explain how integrins transduce signals inside neutrophils and macrophages, and how signaling pathways leading to regulation of selective cell functions may be coordinated.

Role of the tyrosine kinase pyk2 in the integrin-dependent activation of human neutrophils by TNF

Secretion of inflammatory products from neutrophils can be induced by a combination of signals from ligated integrins and receptors for soluble, physiological agonists such as TNF. Here we identify pyk2 in primary human neutrophils; localize it to focal adhesions and podosomes; and demonstrate its tyrosine phosphorylation, activation, and association with paxillin during stimulation of adherent cells by TNF. Tyrphostin A9 emerged as the most potent and selective of 51 tyrosine kinase inhibitors tested against the TNF-induced respiratory burst. Tyrphostin A9 inhibited TNF-induced tyrosine phosphorylation of pyk2 without blocking the cells' bactericidal activity. Wortmannin, an inhibitor of phosphatidylinositol-3-kinase, potently blocked the TNF-induced respiratory burst and selectively inhibited tyrosine phosphorylation of pyk2. Thus, pyk2 appears to play an essential role in the ability of neutrophils to integrate signals from beta(2) integrins and TNF receptors.

A novel protein kinase target for the lipid second messenger phosphatidic acid

Activation of phospholipase D occurs in response to a wide variety of hormones, growth factors, and other extracellular signals. The initial product of phospholipase D, phosphatidic acid (PA), is thought to serve a signaling function, but the intracellular targets for this lipid second messenger are not clearly identified. The production of PA in human neutrophils is closely correlated with the activation of NADPH oxidase, the enzyme responsible for the respiratory burst. We have developed a cell-free system, in which the activation of NADPH oxidase is induced by the addition of PA. Characterization of this system revealed that a multi-functional cytosolic protein kinase was a target for PA, and that two NADPH oxidase components were substrates for the enzyme. Partial purification of the PA-activated protein kinase separated the enzyme from known protein kinase targets of PA. The partially purified enzyme was selectively activated by PA, compared to other phospholipids, and phosphorylated the oxidase component p47-phox on both serine and tyrosine residues. PA-activated protein kinase activity was present in a variety of hematopoietic cells and cell lines and in rat brain, suggesting it has widespread distribution. We conclude that this protein kinase may be a novel target for the second messenger function of PA.

Involvement of both the tyrosine kinase and the phosphatidylinositol-3' kinase signal transduction pathways in the regulation of lipoprotein lipase expression in J774.2 macrophages by cytokines and lipopolysaccharide

The regulation of macrophage lipoprotein lipase (LPL) by cytokines and lipopolysaccharide (LPS) is of potentially crucial importance in the pathogenesis of atherosclerosis and in the responses to endotoxin challenge. We show here that the reduction of LPL activity in J774.2 macrophages observed in the presence of interleukin (IL-1) and IL-11 was sensitive to herbimycin A, with the effect of LPS, interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) on LPL activity being sensitive to both herbimycin A and wortmannin. The action of the inhibitors on the IFN-gamma-dependent reduction of LPL activity was mediated at the level of LPL mRNA metabolism, with translational and/or post-translational levels of regulation being involved in the action of all the other mediators tested. These observations suggest that both the tyrosine kinase and the phosphatidylinositol-3'-kinase signalling pathways are involved in the suppression of macrophage LPL expression by LPS and cytokines.

Src-family kinase-p53/ Lyn p56 plays an important role in TNF-alpha-stimulated production of O2- by human neutrophils adherent to fibrinogen

Stimulation of neutrophil function by TNFalpha is largely dependent on beta2 integrins. It has also been proposed that src-family kinases are involved in this process. However, the functions of src-like kinases in human neutrophils still remain to be determined. In the present study, we used the new src-family kinase specific inhibitor PP1 [4-Amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] to investigate the role src-kinases play in TNFalpha stimulation of neutrophil function. Our results demonstrated that, in neutrophils adherent to fibrinogen, PP1 inhibited TNFalpha-stimulated superoxide production and protein tyrosine phosphorylation in a dose-dependent manner. In in vitro kinase assays, PP1 profoundly inhibited the activation of p53/56lyn but not p59hck or p72syk. Only slight inhibition was found of p58c-fgr. These data indicate that p53/56lyn plays an important role in TNFalpha-mediated stimulation of PMN function.

Adhesion-Dependent Degranulation of Neutrophils Requires the Src Family Kinases Fgr and Hck

Polymorphonuclear neutrophils (PMN) adherent to integrin ligands respond to inflammatory mediators by reorganizing their cytoskeleton and releasing reactive oxygen intermediates. As Src family tyrosine kinases are implicated in these responses, we investigated their possible role in regulating degranulation. Human PMN incubated on fibrinogen released lactoferrin in response to TNF-α and this response was inhibited by PP1, a Src family tyrosine kinase inhibitor. This drug had no effect on lactoferrin secretion induced by PMA, an adhesion-independent agonist of PMN degranulation. However, PP1 blocked secretion in PMN plated on plain tissue culture plastic, a surface inducing PMN spreading in the absence of any stimulus. Double knockout hck−/−fgr−/− PMN adherent to collagen or fibrinogen failed to release lactoferrin in response to TNF-α but responded to PMA as wild-type PMN. Degranulation induced by spreading over tissue culture plastic was also defective in hck−/−fgr−/− PMN. Defective adhesion-dependent degranulation required the absence of both kinases, because single knockout fgr−/− or hck−/− PMN responded as wild-type cells. Analysis of lactoferrin secretion in hck−/−fgr−/− or PP1-treated, suspended PMN showed that Src kinases are not implicated in degranulation dependent on activation of protein kinase C or increase in intracellular free Ca2+ but may play a role in the response to FMLP of cytochalasin B-treated PMN. These findings identify a role for Src family kinases in a signaling pathway leading to granule-plasma membrane fusion and suggest that Fgr and Hck would be targets for pharmacological control of adhesion-dependent degranulation in the inflammatory site.

Triggering of Chloride Ion Efflux from Human Neutrophils as a Novel Function of Leukocyte β2 Integrins: Relationship with Spreading and Activation of the Respiratory Burst

PMN residing on immobilized fibronectin have been shown to respond to TNF with an intense and long lasting Cl− efflux that leads to a marked decrease of the unusually high basal Cl− content of these phagocytes. The finding that this Cl− efflux depends, at least in part, on β2 integrin engagement stimulated the present investigation, which addresses the question as to whether β2 integrins per se, in the absence of PMN agonists, are able to generate signals triggering Cl− efflux. We induced β2 integrin cross-linking by plating PMN onto surface-bound mAbs directed against either the common β-chain (CD18) or the individual α-chains (CD11a, CD11b, CD11c) of LFA-1, CR3, and gp150/95. Anti-CD18 mAbs triggered a marked release of Cl− ions, which was accompanied by spreading and activation of the respiratory burst. Cross-linking of gp150/95 and LFA-1 generated the most powerful signals for the activation of Cl− efflux. The results of three independent experimental approaches, i.e., kinetic studies, use of Cl− transport inhibitors, and modulation of Cl− efflux with different amounts of anti-β2 integrin mAbs, indicated that Cl− efflux regulates both spreading and respiratory burst triggered by β2 integrin cross-linking. Cl− efflux appears to be independent on either alterations of [Ca2+]i or changes in the plasma membrane potential and shows sensitivity to a raise in pHi. This study uncovers a new signaling ability of β2 integrins and contributes to highlight the role of Cl− efflux in the outside-in signal transduction pathway regulating adherence-dependent PMN responses.

Resistance to endotoxic shock and reduced neutrophil migration in mice deficient for the Src-family kinases Hck and Fgr

Signal transduction through the leukocyte integrins is required for the processes of firm adhesion, activation, and chemotaxis of neutrophils during inflammatory reactions. Neutrophils isolated from knockout mice that are deficient in the expression of p59/61(hck) (Hck) and p58(c-fgr) (Fgr), members of the Src-family of protein tyrosine kinases, have been shown to be defective in adhesion mediated activation. Cells from these animals have impaired induction of respiratory burst and granule secretion following plating on surfaces that crosslink beta2 and beta3 integrins. To determine if the defective function of hck-/-fgr-/- neutrophils observed in vitro also results in impaired inflammatory responses in vivo, we examined responses induced by lipopolysaccharide (LPS) injection in these animals. The hck-/-fgr-/- mice showed marked resistance to the lethal effects of high-dose LPS injection despite the fact that high levels of serum tumor necrosis factor alpha and interleukin 1alpha were detected. Serum chemistry analysis revealed a marked reduction in liver and renal damage in mutant mice treated with LPS, whereas blood counts showed a marked neutrophilia that was not seen in wild-type animals. Direct examination of liver sections from mutant mice revealed reduced neutrophil migration into the tissue. These data demonstrate that defective integrin signaling in neutrophils, caused by loss of Hck and Fgr tyrosine kinase activity, results in impaired inflammation-dependent tissue injury in vivo.

Syk Activation Is Required for Spreading and H2O2 Release in Adherent Human Neutrophils

Chemoattractant-stimulated polymorphonuclear leukocytes (PMNs) that are adherent to extracellular matrix proteins exhibit a massive, sustained respiratory burst that requires cell spreading. However, the signaling pathways culminating in PMN spreading are not well characterized. Studies showing that protein tyrosine phosphorylation increases with PMN spreading suggest that phosphorylation is critical for this process. In the present study, we observed increased tyrosine phosphorylation of both focal adhesion kinase and Syk in FMLP-activated PMNs that had been plated onto fibrinogen; an increase in Syk activity, but not focal adhesion kinase activity, was apparent. The time course of Syk phosphorylation correlated with the initiation of cell spreading and H2O2 release. Pretreatment of PMNs with piceatannol, a Syk-selective inhibitor, blocked Syk activity, cell spreading, and H2O2 release, indicating that Syk activity was required for the activation of adherent PMNs. Paxillin is a cytoskeletally associated protein that is also tyrosine phosphorylated during PMN spreading and H2O2 release. Paxillin phosphorylation is kinetically slower than Syk phosphorylation and is inhibited with piceatannol, suggesting that paxillin is a substrate for Syk. An analysis of Syk immunoprecipitates indicated that Syk and paxillin associate during PMN spreading. This interaction is not mediated by the src kinases Lyn and Fgr, since neither kinase coprecipitated with Syk. Syk from FMLP-activated, adherent PMNs phosphorylated paxillin-glutathione S-transferase, suggesting that paxillin is a substrate for Syk in vivo. These results indicate that PMN spreading and H2O2 release require a Syk-dependent signaling pathway leading to paxillin phosphorylation.

CD43 (sialophorin, leukosialin) shedding is an initial event during neutrophil migration, which could be closely related to the spreading of adherent cells

Leukosialin is a negatively-charged mucin-like membrane protein of leukocytes. This anti-adhesive molecule prevents uncontrolled cellular interactions and is proteolytically cleaved during neutrophil activation. CD43 is shed in vivo during neutrophil migration to the inflammatory site. We have analysed the decrease in CD43 expression during in vitro adherence of TNF-alpha activated PMN. CD43 was quantitated by flow cytometry on TNF-alpha-activated PMN either maintained in suspension or allowed to adhere then detached with EDTA. Although TNF did not induce significant modification of CD43 expression on suspended cells, we showed that 40% of membrane CD43 is released during neutrophil TNF-induced adhesion to serum-coated plates or endothelial cells, and that migration through the endothelial monolayer did not result in further shedding. Adhesion-blocking anti-beta 2 integrin mAbs prevented CD43 shedding. beta 2 integrin "activation" by anti-CD 18 mAbs or Mn ions did not decrease CD43 expression if adhesion was prevented by stirring. Inhibitors of signal transduction or of cytoskeleton association, which allowed cells to adhere but not to spread, inhibited the shedding of CD43 during adhesion. We conclude that CD43 shedding is not promoted by beta 2 integrins engagement or adhesion but is concomitant with spreading of adherent cells.

Inhibition of neutrophil oxidative burst and granule secretion by wortmannin: potential role of MAP kinase and renaturable kinases

Exposure of neutrophils to a variety of agonists including soluble chemoattractant peptides and cytokines results in degranulation and activation of the oxidative burst (effector functions) that are required for bacterial killing. At present, the signaling pathways regulating these important functions are incompletely characterized. Mitogen-activated protein (MAP) kinases (MAPK) as well as members of a family of "renaturable kinases" are rapidly activated in neutrophils in response to diverse physiological agonists, suggesting that they may regulate cell activation. Antagonists of phosphatidyl inositol-3-(OH) kinase (PI3-kinase) such as wortmannin (Wtmn) inhibit these effector responses as well as certain of the above-mentioned kinases, leading to the suggestion that these enzymes lie downstream of PI3-kinase in the pathway regulating the oxidative burst and granule secretion. However, an apparent discrepancy exists in that, while virtually obliterating activity of PI3-kinase and the oxidase at low concentrations (ID50 < 20 nM), Wtmn has only variable inhibitory effects on MAPK even at substantially higher concentrations (75-100 nM). This raises the possibility that the inhibitory effects of Wtmn are mediated via other enzyme systems. The purpose of the current study was therefore to compare the effects of Wtmn on PI3-kinase activity and on the chemoattractant-activated kinases, and to determine the potential relationship of these pathways to microbicidal responses. In human neutrophils, both the oxidative burst and granule secretion induced by fMLP were inhibited by Wtmn but at markedly different concentrations: the oxidative burst was inhibited with an ID50 of < 5 nM while granule secretion was only partially inhibited at concentrations exceeding 75 nM. Activation of both MEK-1 and MAPK in response to fMLP was only partially inhibited by high doses of Wtmn (ID50 of > 100 nM and approximately 75 nM, respectively). In contrast, Wtmn potently inhibited fMLP-induced activation of the 63 and 69 kDa renaturable kinases (ID50 approximately 5-10 nM). We speculate that the renaturable kinases may be involved in the regulation of the oxidative burst, whereas the MAPK pathway may play a role in other neutrophil functions such as granule secretion.

Alteration by flutamide of neutrophil response to stimulation. Implications for tissue injury

When activated, inflammatory cells such as polymorphonuclear leukocytes (PMNs) can damage isolated hepatocytes in vitro. These studies were performed to determine if flutamide activates PMNs. Flutamide (Eulexin) is an orally active, nonsteroidal antiandrogen that can cause liver injury associated with inflammation. Activation of PMNs was assessed from the production of superoxide anion and the release of myeloperoxidase in the presence or absence of flutamide and phorbol myristate acetate (PMA) or f-methionyl-leucyl-phenylalanine (fmlp). In addition, hepatocytes were cocultured with PMNs stimulated with PMA or fmlp in the presence or absence of flutamide, and cytotoxicity to hepatocytes was evaluated from the release of alanine aminotransferase into the medium. Flutamide alone did not stimulate the generation of superoxide anion by PMNs but potentiated its production in response to PMA. At lower concentrations of flutamide (i.e. 25 microM), there was a tendency toward increased release of myeloperoxidase, whereas at higher concentrations (i.e. 75-100 microM) flutamide inhibited degranulation in response to fmlp. In coculture with hepatocytes, PMNs exposed to either flutamide, fmlp, or PMA alone caused a significant increase in release of alanine aminotransferase. Hepatocellular toxicity caused by PMNs incubated with flutamide and PMA was additive and was not affected by the addition of superoxide dismutase and catalase. Flutamide had no significant effect on fmlp-induced injury in cocultures. These data indicate that flutamide alters the activation of PMNs by subsequent stimuli in vitro. In addition, in the presence of flutamide, minor PMN-mediated injury to isolated hepatocytes was observed.

Activation of MEKK by formyl-methionyl-leucyl-phenylalanine in human neutrophils. Mapping pathways for mitogen-activated protein kinase activation

Mechanisms of neutrophil activation in response to chemoattractants remain incompletely understood. We have recently reported a Ras-mediated c-Raf pathway leading to the activation of mitogen-activated protein (MAP) kinase in human neutrophils stimulated with the chemoattractant formyl-Met-Leu-Phe (FMLP). However, concern that Raf activation may not fully account for the early FMLP-mediated human neutrophil responses prompted us to investigate the activation of MAP kinase/ERK kinase (MEK) by MEK kinase (MEKK). In cell lysates we identified protein species at 180, 160, 110, 72, and 54 kDa with a monoclonal antibody to MEKK. Activation of MEKK was determined on immunoprecipitates from FMLP-stimulated neutrophils by in vitro kinase assay, which utilized both MEK1 and MEK2 as substrates. It was rapid, detectable at 30 s and reaching a plateau at 5 min, and it was inhibited in a dose-dependent fashion by a specific phosphatidylinositol 3-kinase inhibitor, wortmannin. Partial inhibition by pertussis toxin was observed. We were unable to show inhibition of the MEKK response by GF 109203X, a protein kinase C-specific inhibitor. These data indicate that in neutrophils activation of MEKK in addition to Raf may underlie stimulation of MAP kinase and other MAP kinase homologues by FMLP.

Chloride ion efflux regulates adherence, spreading, and respiratory burst of neutrophils stimulated by tumor necrosis factor-alpha (TNF) on biologic surfaces

Chloride ion efflux is an early event occurring after exposure of neutrophilic polymorphonuclear leukocytes (PMN) in suspension to several agonists, including cytokines such as tumor necrosis factor-alpha (TNF) and granulocyte/macrophage-colony stimulating factor (Shimizu, Y., R.H. Daniels, M.A. Elmore, M.J. Finnen, M.E. Hill, and J.M. Lackie. 1993. Biochem. Pharmacol. 9:1743-1751). We have studied TNF-induced Cl- movements in PMN residing on fibronectin (FN) (FN-PMN) and their relationships to adherence, spreading, and activation of the respiratory burst. Occupancy of the TNF-R55 and engagement of beta 2 integrins cosignaled for an early, marked, and prolonged Cl- efflux that was accompanied by a fall in intracellular chloride levels (Cl-i). A possible causal relationship between Cl- efflux, adherence, and respiratory burst was first suggested by kinetic studies, showing that TNF-induced Cl- efflux preceded both the adhesive and metabolic response, and was then confirmed by inhibition of all three responses by pretreating PMN with inhibitors of Cl- efflux, such as ethacrynic acid. Moreover, Cl- efflux induced by means other than TNF treatment, i.e., by using Cl(-)-free media, was followed by increased adherence, spreading, and metabolic activation, thus mimicking TNF effects. These studies provide the first evidence that a drastic decrease of Cl-i in FN-PMN may represent an essential step in the cascade of events leading to activation of proadhesive molecules, reorganization of the cytoskeleton network, and assembly of the O2(-)-forming NADPH oxidase.

Regulation of Src family tyrosine kinase activities in adherent human neutrophils. Evidence that reactive oxygen intermediates produced by adherent neutrophils increase the activity of the p58c-fgr and p53/56lyn tyrosine kinases

Src family tyrosine kinases have been implicated in the adhesion-dependent activation of neutrophil functions (Yan, S. R., Fumagalli, L., and Berton, G. (1995) J. Inflamm. 45, 297-312; Lowell, C. A., Fumagalli, L., and Berton, G. (1996) J. Cell Biol. 133, 895-910). Because the activity of tyrosine kinases can be affected by oxidants, we investigated whether reactive oxygen intermediates (ROI) produced by adherent neutrophils regulate Src family kinase activities. Inhibition of ROI production by diphenylene iodonium, an inhibitor of NADPH oxidase, or degradation of H2O2 by exogenously added catalase inhibited the adhesion-stimulated activities of p58(c-fgr) and p53/56(lyn). In addition, adhesion-stimulated p58(c-fgr) and p53/56(lyn) activities were greatly reduced in neutrophils from patients with chronic granulomatous disease (CGD) that are deficient in the production of ROI. Exogenously added H2O2 increased p58(c-fgr) and p53/56(lyn) activities in nonadherent neutrophils. Although ROI regulated the activities of p58(c-fgr) and p53/56(lyn), they did not affect the redistribution of the two kinases to a Triton X-100-insoluble, cytoskeletal fraction that occurs in adherent neutrophils. Tyrosine phosphorylation of proteins in adherent, CGD neutrophils was only partially inhibited, suggesting that the full activation of p58(c-fgr) and p53/56(lyn), which depends on endogenously produced ROI, does not represent an absolute requirement for protein tyrosine phosphorylation. The adhesion-stimulated activity of the tyrosine kinase p72(syk) was not affected by catalase in normal neutrophils, and it was comparable in normal and CGD neutrophils. These findings suggest that ROI endogenously produced by adherent neutrophils regulate Src family kinases activity selectively and establish the existence of a cross-talk between reorganization of the cytoskeleton, production of ROI, and Src family tyrosine kinase activities in signaling by adhesion.

Neutrophil activation by adhesion: mechanisms and pathophysiological implications

Neutrophil adhesion plays an essential role in the formation of an inflammatory exudate. Moreover, adhesion activates selective neutrophil functions and regulates the cell response to additional stimuli. In this review we summarize the information available on adhesion molecules involved in neutrophil adhesion to endothelial cells and extracellular matrix proteins and the experimental approaches which have been developed to block neutrophil adhesion and neutrophil mediated tissue damage. We also address the mechanisms of activation of selective neutrophil functions by adhesion molecules and, in particular the mechanisms of signal transduction by neutrophil integrins. On the basis of recent results obtained in our and other laboratories we propose a model hypothesizing mechanisms of signaling by neutrophil integrins involved in regulation of selective functions.

Antibody-induced engagement of beta 2 integrins on adherent human neutrophils triggers activation of p21ras through tyrosine phosphorylation of the protooncogene product Vav

It is known that beta 2 integrins are crucial for leukocyte cell-cell and cell-matrix interactions, and accumulating evidence now suggests that integrins serve not only as a structural link but also as a signal-transducing unit that controls adhesion-induced changes in cell functions. In the present study, we plated human neutrophils on surface-bound anti-beta 2 (CD18) antibodies and found that the small GTP-binding protein p21ras is activated by beta 2 integrins. Pretreatment of the cells with genistein, a tyrosine kinase inhibitor, led to a complete block of p21ras activation, an effect that was not achieved with either U73122, which abolishes the beta 2 integrin-induced Ca2+ signal, or wortmannin, which totally inhibits the phosphatidylinositol 3-kinase activity. Western blot analysis revealed that antibody-induced engagement of beta 2 integrins causes tyrosine phosphorylation of several proteins in the cells. One of these tyrosine-phosphorylated proteins had an apparent molecular mass of 95 kDa and was identified as the protooncogene product Vav, a p21ras guanine nucleotide exchange factor that is specifically expressed in cells of hematopoietic lineage. A role for Vav in the activation of p21ras is supported by the observations that antibody-induced engagement of beta 2 integrins causes an association of Vav with p21ras and that the effect of genistein on p21ras activation coincided with its ability to inhibit both the tyrosine phosphorylation of Vav and the Vav-p21ras association. Taken together, these results indicate that antibody-induced engagement of beta 2 integrins on neutrophils triggers tyrosine phosphorylation of Vav and, possibly through its association, a downstream activation of p21ras.

Deficiency of Src family kinases p59/61hck and p58c-fgr results in defective adhesion-dependent neutrophil functions

Cross-linking of the neutrophil-beta 2- or beta 3-related leukocyte response integrins by extracellular matrix (ECM) proteins or monoclonal antibodies (mAb) stimulates cytoskeletal rearrangement leading to cell spreading and respiratory burst. Tyrosin phosphorylation of a variety of proteins and activation of the Src family kinases within polymorphonuclear leukocytes (PMN) have recently been implicated in the intracellular signaling pathways generated by leukocyte integrins (Yan, S.R., L. Fumagalli, and G Berton. 1995. J. Inflammation. 45:217-311.) To directly test whether these functional responses are dependent on the Src family kinases p59/61hck and p58c-fgr, we examined adhesion-dependent respiratory burst in PMNs isolated from hck -/-, fgr -/-, and hck -/- fgr -/- knockout mice. Purified bone marrow PMNS from wild-type mice released significant amounts of O2- when adherent to fibrinogen-, fibronectin-, or collagen-coated surfaces, in the presence of activating agents such as tumor necrosis factor (TNF) or formyl-methionyl-leucyl-phenylalanine, as described for human PMNs. PMNs from hck-/-fgr-/- double-mutant mic, however, failed to respond. This defect was specific for integrin signaling, since respiratory burst was normal in hck-/-fgr-/-PMNs stimulated by immune complexes or PMA. Stimulation of respiratory burst was observed in TNF-primed wild-type PMN plated on surfaces coated with murine intracellular adhesion molecule-1 (ICAM-1), while hck-/-fgr-/- PMNs, failed to respond. Direct cross-linking of the subunits of beta 2 and beta 2 integrins by surface-bound mAbs was elicited O2- production by wild-type PMNs, while the double-mutant hck-/-fgr-/- cells failed to respond. Photomicroscopy and cell adhesion assays revealed that the impaired functional responses of hck-/-fgr-/- PMNs were caused by defective spreading and tight adhesion on either ECM protein- or mAb-coated surfaces. In contrast, hck-/-or fgr-/-single mutant cells produced O2- at levels equivalent to wild-type cells on ECM protein, murine ICAM-1, and antiintegrin mAb-coated surfaces. Hence, either p59/61 hck and p 58c-fgr is required for signaling through leukocyte beta 2 and beta 3 integrins leading to PMN spreading and respiratory burst. This is the first direct genetic evidence of the importance of Src family kinases in integrin signaling within leukocytes, and it is also the best example of overlapping function between members of this gene family within a defined signal transduction pathway.

Involvement of tyrosine phosphorylation in endothelial adhesion molecule induction

Induction of endothelial adhesion molecules by the cytokine tumor necrosis factor-alpha (TNF) can occur independently of protein kinase C and activation of a protein tyrosine kinase (PTK) has recently been implicated in the upregulation of vascular cell adhesion molecule 1 (VCAM-1) by interleukin-4 (IL-4) on endothelial cells. We demonstrate that the PTK inhibitors herbimycin A or genistein suppress induction of endothelial VCAM-1 and E-selectin, as well as subsequent monocytic cell adhesion to endothelial cells stimulated by TNF. Inhibition studies indicate that specific tyrosine phosphorylation following PTK activation is involved in the mobilization of the transcription factor, nuclear factor kappa B, and VCAM-1 mRNA expression. This may have implications for pathophysiological conditions that involve the upregulation of these molecules (e.g. inflammation and atherosclerosis).

The effect of cetirizine on the integrin-dependent respiratory burst of normodense eosinophils

It has been proposed that cetirizine inhibits eosinophil migration and adherence. We evaluated the possible effect of cetirizine on integrin-induced eosinophil proinflammatory activation. Normodense eosinophils were triggered with monoclonal antibodies to integrins in the presence of different concentrations of certirizine. Proinflammatory activation was measured by evaluation of O2- production. Only at high concentrations (250 micrograms/ml) and in the first 15 min did certirizine significantly inhibit (p < 0.02) the eosinophil respiratory burst. No effect was shown for lower concentrations (50 and 100 micrograms/ml) or after 15 min. These data suggest that, only at very high concentrations, cetirizine may induce a transient inhibition of the integrin-induced eosinophil respiratory burst.

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.

Beta 2 integrin-dependent protein tyrosine phosphorylation and activation of the FGR protein tyrosine kinase in human neutrophils

Stimulation of adherent human neutrophils (PMN) with tumor necrosis factor (TNF) triggers protein tyrosine phosphorylation (Fuortes, M., W. W. Jin, and C. Nathan. 1993. J. Cell Biol. 120:777-784). We investigated the dependence of this response on beta 2 integrins by using PMN isolated from a leukocyte adhesion deficiency (LAD) patient, which do not express beta 2 integrins, and by plating PMN on surface bound anti-beta 2 (CD18) antibodies. Protein tyrosine phosphorylation increased in PMN plated on fibrinogen and this phosphorylation was enhanced by TNF. Triggering of protein tyrosine phosphorylation did not occur in LAD PMN plated on fibrinogen either in the absence or the presence of TNF. Surface bound anti-CD18, but not isotype-matched anti-Class I major histocompatibility complex (MHC) antigens, antibodies triggered tyrosine phosphorylation in normal, but not in LAD PMN. As the major tyrosine phosphorylated proteins we found in our assay conditions migrated with an apparent molecular mass of 56-60 kD, we investigated whether beta 2 integrins are implicated in activation of members of the src family of intracellular protein-tyrosine kinases. We found that the fgr protein-tyrosine kinase (p58fgr) activity, and its extent of phosphorylation in tyrosine, in PMN adherent to fibrinogen, was enhanced by TNF. Activation of p58fgr in response to TNF was evident within 10 min of treatment and increased with times up to 30 min. Also other activators of beta 2 integrins such as phorbol-12-myristate 13-acetate (PMA), and formyl methionyl-leucyl-phenylalanine (FMLP), induced activation of p58fgr kinase activity. Activation of p58fgr kinase activity, and phosphorylation in tyrosine, did not occur in PMN of a LAD patient in response to TNF. Soluble anti-CD18, but not anti-Class I MHC antigens, antibodies inhibited activation of p58fgr kinase activity in PMN adherent to fibrinogen in response to TNF, PMA, and FMLP. These findings demonstrate that, in PMN, beta 2 integrins are implicated in triggering of protein tyrosine phosphorylation, and establish a link between beta 2 integrin-dependent adhesion and the protein tyrosine kinase fgr in cell signaling.

Wortmannin binds specifically to 1-phosphatidylinositol 3-kinase while inhibiting guanine nucleotide-binding protein-coupled receptor signaling in neutrophil leukocytes

Wortmannin (WT) and its derivative 17-hydroxywortmannin (HWT) inhibit at nanomolar concentrations superoxide formation and exocytosis in neutrophils stimulated with chemotactic agonists. Treatment of neutrophils with radiolabeled [3H]HWT resulted in specific and saturable binding that paralleled the inhibition of the respiratory burst. Both half-maximal binding and half-maximal inhibition were observed at 5 nM, and > 90% of maximal binding and inhibition was observed at 20 nM HWT. Fluorography of subcellular fractions that were separated on NaDodSO4/PAGE showed that [3H]HWT binds covalently to a 110-kDa cytosolic protein. The WT-binding protein was purified from human neutrophils and bovine brain homogenates by column chromatography. The pure protein was eluted from gel filtration columns with an apparent molecular mass of 200 kDa and showed a heterodimeric structure on Coomassie-stained NaDodSO4/PAGE. In addition to the 110 kDa wortmannin binding protein an equally intense band was seen migrating at 85 kDa. This band was identified on Western blots as p85 alpha, the regulatory subunit of phosphatidylinositol (PI) 3-kinase (ATP:1-phosphatidyl-1D-myo-inositol 3-phosphotransferase, EC 2.7.1.137). The purified protein contained PI 3-kinase activity that was enriched > 20,000-fold from human neutrophil cytosol during preparation. The data impose a key role for PI 3-kinase-mediated signal transduction through guanine nucleotide-binding protein-coupled receptors and suggest that 3-phosphorylated inositol phospholipids are important second messengers for immediate responses in neutrophils. Furthermore, the results show that WT is a powerful and selective tool to study the function of PI 3-kinase.

Phospholipases and protein kinases during phagocyte activation

Phospholipases and protein kinases are critical for the intracellular transmission and amplification of signals induced by extracellular ligands. Chemotactic activation of phagocytes through G protein coupled receptors leads to inflammatory responses of the immune cells. Downstream of G proteins, phospholipases generate precursors for eicosanoid synthesis and are involved in the functional responses. Recently, the molecular characterization of specific enzymes of the signalling cascades has gained much attention in research.