Regulation of polymorphonuclear leukocyte phagocytosis by myosin light chain kinase after activation of mitogen-activated protein kinase

Unconventional myosin VIIa and vezatin, two proteins crucial for Listeria entry into epithelial cells

Listeria monocytogenes is a bacterial pathogen with the capacity to invade non-phagocytic cells. This dynamic process involves coordinated membrane remodelling and actin cytoskeleton rearrangements. Although some of the molecular factors promoting these events have been identified, the driving force allowing internalization is unknown. One of the receptors for L. monocytogenes on epithelial cells is E-cadherin, a transmembrane protein normally involved in homophilic interactions that allow cell-cell contacts at the adherens junctions. E-cadherin has to be connected to the actin cytoskeleton to mediate strong cell-cell adhesion and to trigger Listeria entry; alpha- and beta-catenins play key roles in these processes. We have recently identified an unconventional myosin, myosin VIIa and its ligand vezatin, at the adherens junctions of polarized epithelial cells. Here, we demonstrate by pharmacological and genetic approaches that both myosin VIIa and vezatin are crucial for Listeria internalization. These results provide the first evidence for the role of an unconventional myosin in bacterial internalization and a novel example of the exploitation of mammalian proteins, by a pathogen, to establish a successful infection.

Induction of mast cell interactions with blood vessel wall components by direct contact with intact T cells or T cell membranes in vitro

BACKGROUND

Mast cells exert profound pleiotropic effects on immune cell reactions at inflammatory sites, where they are most likely influenced not only by the extracellular matrix (ECM) and inflammatory mediators but also by the proximity of activated T lymphocytes. We recently reported that activated T cells induce mast cell degranulation with the release of TNF-alpha, and that this activation pathway is mediated by lymphocyte function-associated antigen-1 (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) binding.

OBJECTIVE

To determine how this contact between the two cell types can modulate mast cell behaviour in an inflammatory milieu by examining the adhesion of mast cells to endothelial cells and ECM ligands in an integrin-dependent manner.

METHODS

Human mast cells (HMC-1) were co-cultured with resting or activated T cells followed by testing their adhesion to endothelial cell and ECM ligands, stromal derived factor-1alpha (SDF-1alpha)-induced migration, and western blotting.

RESULTS

Co-culturing HMC-1 with activated, but not with resting T cells resulted in marked stimulation of mast cell adhesion to vascular cell adhesion molecule-1 and ICAM-1 in a very late antigen-4- and LFA-1-dependent fashion. In addition, activated T cells or T cell membranes promoted HMC-1 adhesion to fibronectin (FN) and laminin. This effect was accompanied by the phosphorylation of extracellular regulated kinase and p38, but not of c-Jun N-terminal kinase. Importantly, the adhesive property of mast cells depended exclusively on the direct contact between the two cell types, since neither supernatants from activated T cells nor separation of the two cell populations with a porous membrane affected mast cell adhesion to FN. Furthermore, similar results were obtained when mast cells were incubated with purified membranes from activated T cells. These results suggest that, in addition to stimulating mast cell degranulation, the proximity of activated T lymphocytes to mast cells can mediate the adhesion of mast cell precursors to the endothelial ligands and ECM. Activated T cells also stimulated SDF-1alpha-induced mast cell migration.

CONCLUSION

This symbiotic relationship between the two types of immune cells may serve to direct mast cells to specific sites of inflammation where their effector functions are required.

Positive Regulation of Phagocytosis by SIRPβ and Its Signaling Mechanism in Macrophages

SIRPbeta (signal-regulatory protein beta) is a transmembrane protein that is expressed in hematopoietic cells but whose functions are unknown. We have now cloned mouse SIRPbeta cDNA and have shown that the gene is expressed in various tissues in addition to cells of the macrophage lineage. Engagement of SIRPbeta by specific monoclonal antibodies promoted Fcgamma receptor-dependent or -independent phagocytosis in mouse peritoneal macrophages. It also induced marked activation of MAPK and the upstream kinase MEK but weak activation of Akt. MEK inhibitors markedly blocked the promotion of phagocytosis by SIRPbeta, whereas an inhibitor of phosphoinositide 3-kinase partly blocked such response. In addition, inhibitors of myosin light chain kinase or of myosin ATPase blocked the promotion of phagocytosis by SIRPbeta. Furthermore, SIRPbeta induced the formation of filopodia and lamellipodia in macrophages as well as the translocation of activated MAPK to these structures. It also elicited tyrosine phosphorylation of DAP12, Syk, and SLP-76, and a Syk inhibitor blocked the promotion of phagocytosis and activation of MAPK by SIRPbeta. Our results suggest that engagement of SIRPbeta promotes phagocytosis in macrophages by inducing the tyrosine phosphorylation of DAP12, Syk, and SLP-76 and the subsequent activation of a MEK-MAPK-myosin light chain kinase cascade.

Fcγ receptor signaling in primary human microglia: differential roles of PI-3K and Ras/ERK MAPK pathways in phagocytosis and chemokine induction

Cryptococcus neoformans monoclonal antibody immune complex (IC) induces beta-chemokines and phagocytosis in primary human microglia via activation of Fc receptor for immunoglobulin G (FcgammaR). In this report, we investigated microglial FcgammaR signal-transduction pathways by using adenoviral-mediated gene transfer and specific inhibitors of cell-signaling pathways. We found that Src inhibitor PP2 and Syk inhibitor piceatannol inhibited phagocytosis, macrophage-inflammatory protein-1alpha (MIP-1alpha) release, as well as phosphorylation of extracellular-regulated kinase (ERK) and Akt, consistent with Src/Syk involvement early in FcgammaR signaling. Constitutively active mitogen-activated protein kinase kinase (MEK) induced MIP-1alpha, and Ras dominant-negative (DN) inhibited IC-induced ERK phosphorylation and MIP-1alpha production. These results suggest that the Ras/MEK/ERK pathway is necessary and sufficient in IC-induced MIP-1alpha expression. Neither Ras DN nor the MEK inhibitor U0126 inhibited phagocytosis. In contrast, phosphatidylinositol-3 kinase (PI-3K) inhibitors Wortmannin and LY294002 inhibited phagocytosis without affecting ERK phosphorylation or MIP-1alpha production. Conversely, Ras DN or U0126 did not affect Akt phosphorylation. Together, these results demonstrate distinct roles played by the PI-3K and Ras/MEK/ERK pathways in phagocytosis and MIP-1alpha induction, respectively. Our results demonstrating activation of functionally distinct pathways following microglial FcgammaR engagement may have implications for human central nervous system diseases.

Rho is involved in superoxide formation during phagocytosis of opsonized zymosans

Phagocytosis is accompanied by the production of superoxide by the NADPH oxidase complex, for which GTP-bound Rac is essential. We wanted to determine whether Rho is also involved in the production of superoxide during phagocytosis. Inhibition of Rho by Tat-C3 exoenzyme (Tat-C3) blocked superoxide formation and curtailed the phagocytosis of serum- (SOZ), C3bi- (COZ), and IgG-opsonized zymosan (IOZ) particles. Tat-C3 did not affect superoxide formation in response to phorbol myristate acetate (PMA), formyl Met-Leu-Phe (fMLP), or macrophage colony-stimulating factor (M-CSF). Superoxide formation was also reduced in J774 cells transfected with a cDNA expressing dominant-negative form of RhoA (N19RhoA). However, purified prenylated recombinant RhoA did not activate NADPH oxidase in vitro, suggesting that Rho does not interact directly with NADPH oxidase. Tat-C3 inhibited the activity of RhoA, but did not affect that of Rac in vitro or in vivo. It also inhibited the phosphorylation of p47(PHOX), one of the cytosolic components of NADPH oxidase. Taken together, these results suggest that Rho plays an important role in superoxide formation during phagocytosis of SOZ, COZ, and IOZ via phosphorylation of p47(PHOX).

Actin reorganization and morphological changes in human neutrophils stimulated by TNF, GM-CSF, and G-CSF: the role of MAP kinases

Stimulation of human neutrophils with tumor necrosis factor-alpha (TNF), granulocyte-macrophage colony-stimulating factor (GM-CSF), or granulocyte CSF (G-CSF) resulted in decreased fluorescence intensity of FITC-phalloidin (actin depolymerization) and morphological changes. Cytokine-induced actin depolymerization was dependent on the concentration of cytokines used as stimuli. The maximal changes were detected at 10 min after stimulation with TNF or GM-CSF and at 20 min after stimulation with G-CSF. Cytokine-induced actin depolymerization was sustained for at least 30 min after stimulation. In contrast, N-formyl-methionyl-leucyl-phenylalanine (FMLP) rapidly (within 45 s) induced an increase in the fluorescence intensity of FITC-phalloidin (actin polymerization) and morphological changes. TNF- and GM-CSF-induced actin depolymerization and morphological changes, but not FMLP-induced responses, were partially inhibited by either PD-98059, an inhibitor of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) kinase, or SB-203580, an inhibitor of p38 MAPK, and were almost completely abolished by these inhibitors in combination. G-CSF-induced responses were almost completely abolished by PD-98059 and were unaffected by SB-203580. These findings are consistent with the ability of these cytokines to activate the distinct MAPK subtype cascade in human neutrophils. Phosphorylated ERK and p38 MAPK were not colocalized with F-actin in neutrophils stimulated by cytokines or FMLP. Furthermore, FMLP-induced polarization and actin polymerization were prevented by cytokine pretreatment. These findings suggest that TNF, GM-CSF, and G-CSF induce actin depolymerization and morphological changes through activation of ERK and/or p38 MAPK and that cytokine-induced actin reorganization may be partly responsible for the inhibitory effect of these cytokines on neutrophil chemotaxis.

Distribution and expression of non-muscle myosin light chain kinase in rabbit livers

AIM: To study the distribution and expression of non-muscle myosin light chain kinase (nmMLCK) in rabbit livers.

METHODS: Human nmMLCK N-terminal cDNA was amplified by polymerase chain reaction (PCR) and was inserted into pBKcmv to construct expression vectors. The recombinant plasmid was transformed into XL1-blue. Expression protein was induced by IPTG and then purified by SDS-PAGE and electroelution, which was used to prepare the polycolonal antibody to detect the distribution and expression of nmMLCK in rabbit livers with immunofluorescene techniques.

RESULTS: The polyclonal antibody was prepared, by which nmMLCK expression was detected and distributed mainly in peripheral hepatocytes.

CONCLUSION: nmMLCK can express in hepatocytes peripherally, and may play certain roles in the regulation of hepatic functions.

Annexin 1 regulates cell proliferation by disruption of cell morphology and inhibition of cyclin D1 expression through sustained activation of the ERK1/2 MAPK signal

Cellular proliferation is controlled by the integration and coordination of extracellular signals. This study explores the role of the protein annexin 1 (ANXA1) in the regulation of such events. We show that ANXA1 has a cell-type independent, anti-proliferative function through sustained activation of the ERK signaling cascade. Moreover, ANXA1 reduces proliferation by ERK-mediated disruption of the actin cytoskeleton and ablation of cyclin D1 protein expression and not by ERK-mediated induction of the cyclin-dependent kinase, CDK2, inhibitor p21(cip/waf). Finally, ANXA1 regulates the ERK pathway at a proximal location, by SH2 domain-independent association with the adapter protein Grb-2. In summary, overexpression of ANXA1 mediates the disruption of normal cell morphology and inhibits cyclin D1 expression, therefore reducing cell proliferation through proximal modulation of the ERK signal transduction pathway.

Engulfment of apoptotic cells is negatively regulated by Rho-mediated signaling

The rapid and efficient phagocytosis of apoptotic cells plays a critical role in preventing secondary necrosis, inflammation as well as in tissue remodeling and regulating immune responses. However, the molecular details of engulfment are just beginning to be elucidated. Among the Rho family GTPases, previous studies have implicated a role for Rac and Cdc42 in the uptake of apoptotic cells by phagocytes, yet the role of Rho has remained unclear. Here, we present evidence that Rho-GTP levels decrease during engulfment. RhoA seems to negatively affect basal engulfment, such that inhibition of Rho-mediated signaling in phagocytes enhanced the uptake of apoptotic targets. Activation of endogenous Rho or overexpression of constitutively active forms of Rho also inhibited engulfment. By testing mutants of RhoA that selectively activate downstream effectors, the Rho-kinase seemed to be primarily responsible for this inhibitory effect. Taken together, these data suggest that inhibition of Rho- and Rho-kinase-mediated signaling might be important during engulfment, which could have important implications for several clinical trials involving inhibition of the Rho kinase.

Asp1424Asn MYH9 mutation results in an unstable protein responsible for the phenotypes in May-Hegglin anomaly/Fechtner syndrome

May-Hegglin anomaly (MHA), Fechtner syndrome (FTNS), Sebastian syndrome (SBS), and Epstein syndrome (EPS) are a group of rare, autosomal dominant disorders characterized by thrombocytopenia, giant platelets, and Döhle-like inclusion bodies, together with variable manifestations of Alport-like symptoms that include high-tone sensorineural deafness, cataracts, and nephritis. These disorders result from mutations in the MYH9 gene, which encodes for the nonmuscle myosin heavy chain A protein (also known as NMMHC-A). To date 20 different mutations have been characterized for this gene, but no clear phenotype-genotype correlation has been established, and very little is known regarding the molecular pathogenesis of this group of diseases. Here, we describe 2 new families with MHA/FTNS phenotypes that have been characterized in terms of their mutations, protein localization in megakaryocytes, protein expression, and mRNA stability. Our findings suggest that, at least for the Asp1424Asn mutation in the MYH9 gene, the phenotypes result from a highly unstable protein. No abnormalities in protein localization or mRNA stability were observed. We hypothesize that haploinsufficiency of the MYH9 results in a failure to properly reorganize the cytoskeleton in megakaryocytes as required for efficient platelet production.

Low density lipoprotein receptor-related protein (LRP) is required for lactoferrin-enhanced collagen gel contractile activity of human fibroblasts

Fibroblasts plated on a type I collagen gel can reduce the size of the gel in a way that mimics the reorganization of the collagen matrix that accompanies the wound healing process. We demonstrated previously that lactoferrin (Lf) specifically binds to WI-38 human fibroblasts and enhances their collagen gel contractile activity. The effect of Lf correlated with the phosphorylation of myosin light chain (MLC), suggesting that Lf promotes fibroblast contractile activity by regulating MLC phosphorylation. We found here that the binding of Lf to WI-38 cells was inhibited by recombinant receptor-associated protein (RAP), a universal competitor for ligand binding to LRP (LDL receptor-related protein), and RAP can also promote the collagen gel contractile activity. These observations suggest that LRP is a receptor that mediates the Lf-induced enhancement of collagen gel contractile activity in WI-38 fibroblasts. To confirm the hypothesis, we utilized LRP antisense oligonucleotide, which was modified by morpholino linkage. Suppression of LRP expression abrogated the Lf-induced enhancement the contractile activity in fibroblasts. Treatment of fibroblasts with Lf enhanced the phosphorylation of ERK1/2 and the activation of MLC kinase (MLCK). These effects were attenuated by suppression of LRP expression. These findings suggest that LRP is involved in the Lf-enhanced collagen gel contractile activity of WI-38 fibroblasts by converting the Lf binding signal into the activation of ERK1/2 and MLCK.

ML-7 inhibits exocytosis of superoxide-producing intracellular compartments in human neutrophils stimulated with phorbol myristate acetate in a myosin light chain kinase-independent manner

ML-7, (5-iodonaphthalene-1-sulfonyl) homopiperazine, is commonly employed as a myosin light chain kinase (MLCK) inhibitor. In the present study, we demonstrated that ML-7 affects the superoxide (O(2)(-))-producing system of human neutrophils in an MLCK-independent manner. Human neutrophils were stimulated with phorbol myristate acetate (PMA), which does not activate MLCK. ML-7 inhibited extracellular release, but not intracellular production of O(2)(-) in the stimulated cells. Fluorescence microscopy revealed the generation of O(2)(-) at intracellular compartments in the stimulated cells exposed to ML-7. At the electron microscopic level, the reaction product of NADPH oxidase activity was found in intracellular compartments. ML-7 strongly inhibited the association of the oxidant-producing intracellular compartments with the plasma membrane. Furthermore, the upregulation of alkaline phosphatase activity, a marker enzyme of the oxidant-producing intracellular compartments, was also inhibited by ML-7. These findings indicate that ML-7 inhibits the fusion of the oxidant-producing intracellular compartments to the plasma membrane resulting in the inhibition of the extracellular release of O(2)(-) in PMA-stimulated human neutrophils in an MLCK-independent manner.

Phosphoinositide-3-kinase-independent contractile activities associated with Fcgamma-receptor-mediated phagocytosis and macropinocytosis in macrophages

Previous studies have shown that Fcgamma receptor (FcR)-mediated phagocytosis and macropinocytosis in macrophages consist of two dissociable activities: a phosphoinositide 3-kinase (PI3K)-independent extension of phagocytic cups and a PI3K-dependent contractile mechanism that closes phagosomes and ruffles into intracellular organelles. Here, we identify an additional contractile activity that persists in the presence of the PI3K inhibitor wortmannin. ML-7, an inhibitor of myosin-light-chain kinase (MLCK), inhibited FcR-mediated phagocytosis, macropinocytosis and cell movements associated with ruffling. Scanning electron microscopy demonstrated a striking difference in morphology between phagocytic cups in the different inhibitors: whereas phagocytic cups of control cells and wortmannin-treated cells conformed closely to particles and appeared to have constricted them, the phagocytic cups in cells treated with ML-7 were more open. Video microscopy of macrophages expressing green-fluorescent-protein (GFP)-actin fusions revealed that bound IgG-opsonized erythrocytes were squeezed during phagosome formation and closure. In ML-7, GFP-actin-rich protrusions extended outward but failed to squeeze particles. Moreover, in contrast to the effects of PI3K inhibitors, ML-7 markedly reduced ruffle movement, and perturbed circular ruffle formation. These PI3K-independent myosin-II-based contractile activities that squeeze phagocytic cups and curve ruffles therefore represent a third component activity of the actin cytoskeleton during phagocytosis and macropinocytosis.

Myosin light chain kinase mediates eosinophil chemotaxis in a mitogen-activated protein kinase-dependent manner

BACKGROUND

Eosinophil migration in the tissue is one characteristic feature of allergic diseases. The CC chemokine eotaxin plays a pivotal role in local accumulation of eosinophils. Myosin light chain kinase (MLCK) is known to regulate cytoskeletal rearrangement and cell motility by means of phosphorylation of myosin light chain (MLC).

OBJECTIVE

We have previously shown that mitogen-activated protein (MAP) kinases are important for eosinophil migration. In the present study we hypothesized that MLCK is downstream of MAP kinases, thereby linking the MAP kinase pathway to the activation of cytoskeletal components required for eosinophil chemotaxis.

METHODS

Blood eosinophils were purified by using Percoll and anti-CD16 antibody-coated magnetic beads. We investigated the phosphorylation of MLCK and MLC by using the phosphorous 32-orthophosphates-labeled eosinophils. The kinase activity of MLCK was determined by measuring the phosphotransferase activity for the MLCK-specific peptide substrate. The chemotaxis assay was performed in a 48-well Boyden microchamber.

RESULTS

The phosphotransferase activity of MLCK for a substrate peptide was enhanced in eotaxin-stimulated eosinophils. We also found that eotaxin induced phosphorylation of MLCK in vivo in phosphorous 32-orthophosphate-labeled eosinophils. PD98059 (MAP/extracellular signal-regulated kinase inhibitor) or SB202190 (p38 MAP kinase inhibitor) abrogated the eotaxin-induced phosphorylation of MLCK. The phosphorylation of MLC was upregulated by eotaxin. Eosinophil chemotaxis was inhibited by means of pretreatment of the MLCK inhibitor ML-7.

CONCLUSION

These results suggest that eotaxin regulates MLCK through both extracellular signal-regulated kinase 1/2 and p38 MAP kinase. MLCK activation is a critical step in the cytoskeletal rearrangements leading to eosinophil migration.

The bovine lactoferrin region responsible for promoting the collagen gel contractile activity of human fibroblasts

We have reported that bovine lactoferrin (bLf) promotes the contractile activity of collagen gels by WI-38 human fibroblasts via the phosphorylation of myosin light chain (MLC). To identify the region of bLf that is responsible for this activity, we prepared bLf fragments by limited proteolysis using trypsin and investigated the effects of each fragment on gel contractile activity. Lf consists of a single polypeptide chain containing two lobes that are independent globular structures termed the N- and C-lobes. The fragment corresponding to the C-lobe of bLf (amino acids 341-689) had a more prominent effect on collagen gel contractile activity than did that of either native bLf or its N-lobe (1-284). Further hydrolysis of the C-lobe with either pepsin or trypsin resulted in a loss of this activity. The effect of the C-lobe on collagen gel contraction by fibroblasts was dose-dependent and was associated with the elevation of MLC phosphorylation.

The leukocyte cytoskeleton in cell migration and immune interactions

Leukocyte migration is crucial during the development of the immune system and in the responses to infection, inflammation, and tumor rejection. The migratory behavior of leukocytes under physiological and pathological conditions as well as the extracellular cues and intracellular machinery that control and guide migration have been studied thoroughly. The cytoskeleton of leukocytes is extremely versatile, bearing characteristic features that enable these cells to migrate under conditions of flow through narrow spaces and onto target tissues. What makes the cytoskeleton machinery so extraordinary is not so much its molecular composition, but its flexibility which allows it to display a unique combination of responses to the extracellular medium and a rapid regulation of the architecture of its components. This review focuses on the cytoskeleton of the leukocyte. Its molecular components and the regulation of their assembly and organization are discussed. Furthermore, it highlights aspects of the regulation of the leukocyte cytoskeleton that confer flexibility to these cells in order to perform their specific tasks. Finally, different subcellular structures such as the immunological synapse, the uropod of migrating leukocytes, and the phagosome displayed by phagocytic cells are discussed in detail. The relationship of the leukocyte with its environment occurs through different kinds of receptors that interact with ligands that are soluble, fixed on the membrane of other cells, or immobilized on the extracellular matrix. The impact of receptor-ligand binding on the functional responses and the rearrangement of the cytoskeleton is also examined.

The CY domain of the Fcgamma RIa alpha-chain (CD64) alters gamma-chain tyrosine-based signaling and phagocytosis

Although the cytoplasmic domain of the human FcgammaRIa alpha-chain lacks tyrosine-based phosphorylation motifs, it modulates receptor cycling and receptor-specific cytokine production. The cytoplasmic domain of FcgammaRIa is constitutively phosphorylated, and the inhibition of dephosphorylation with okadaic acid, an inhibitor of type 1 and type 2A protein serine/threonine phosphatase, inhibits both receptor-induced activation of the early tyrosine phosphorylation cascade and receptor-specific phagocytosis. To explore the basis for these effects of the cytoplasmic domain of FcgammaRIa, we developed a series of human FcgammaRIa molecular variants, expressed in the murine macrophage cell line P388D1, and demonstrate that serine phosphorylation of the cytoplasmic domain is an important regulatory mechanism. Truncation of the cytoplasmic domain and mutation of the cytoplasmic domain serine residues to alanine abolish the okadaic acid inhibition of phagocytic function. In contrast, the serine mutants did not recapitulate the selective effects of cytoplasmic domain truncation on cytokine production. These results demonstrate for the first time a direct functional role for serine phosphorylation in the alpha-chain of FcgammaRIa and suggest that the cytoplasmic domain of FcgammaRI regulates the different functional capacities of the FcgammaRIa-receptor complex.

Insulin inhibits PDGF-directed VSMC migration via NO/ cGMP increase of MKP-1 and its inactivation of MAPKs

In this study, we examined the role of insulin in the control of vascular smooth muscle cell (VSMC) migration in the normal vasculature. Platelet-derived growth factor (PDGF) increased VSMC migration, which was inhibited by pretreatment with insulin in a dose-dependent manner. Insulin also caused a 60% decrease in PDGF-stimulated mitogen-activated protein kinase (MAPK) phosphorylation and activation. Insulin inhibition of MAPK was accompanied by a rapid induction of MAPK phosphatase (MKP-1), which inactivates MAPKs by dephosphorylation. Pretreatment with inhibitors of the nitric oxide (NO)/cGMP pathway, blocked insulin-induced MKP-1 expression and restored PDGF-stimulated MAPK activation and migration. In contrast, adenoviral infection of VSMCs with MKP-1 or cGMP-dependent protein kinase Ialpha (cGK Ialpha), the downstream effector of cGMP signaling, blocked the activation of MAPK and prevented PDGF-directed VSMC migration. Expression of antisense MKP-1 RNA prevented insulin's inhibitory effect and restored PDGF-directed VSMC migration and MAPK phosphorylation. We conclude that insulin inhibition of VSMC migration may be mediated in part by NO/cGMP/cGK Ialpha induction of MKP-1 and consequent inactivation of MAPKs.

Rho-kinase and myosin-II control phagocytic cup formation during CR, but not FcgammaR, phagocytosis

Phagocytosis through Fcgamma receptor (FcgammaR) or complement receptor 3 (CR) requires Arp2/3 complex-mediated actin polymerization, although each receptor uses a distinct signaling pathway. Rac and Cdc42 are required for actin and Arp2/3 complex recruitment during FcgammaR phagocytosis, while Rho controls actin assembly at CR phagosomes. To better understand the role of Rho in CR phagocytosis, we tested the idea that a known target of Rho, Rho-kinase (ROK), might control phagocytic cup formation and/or engulfment of particles. Inhibitors of ROK (dominant-negative ROK and Y-27632) and of the downstream target of ROK, myosin-II (ML7, BDM, and dominant-negative myosin-II), were used to test this idea. We found that inhibition of the Rho --> ROK --> myosin-II pathway caused a decreased accumulation of Arp2/3 complex and F-actin around bound particles, which led to a reduction in CR-mediated phagocytic engulfment. FcgammaR-mediated phagocytosis, in contrast, was independent of Rho or ROK activity and was only dependent on myosin-II for particle internalization, not for actin cup formation. While myosins have been previously implicated in FcgammaR phagocytosis, to our knowledge, this is the first demonstration of a role for myosin-II in CR phagocytosis.

Myosin X is a downstream effector of PI(3)K during phagocytosis

Phagocytosis is a phosphatidylinositol-3-OH-kinase (PI(3)K)-dependent process in macrophages. We identified Myo10 (Myosin-X), an unconventional myosin with pleckstrin homology (PH) domains, as a potential downstream target of PI(3)K. Myo10 was recruited to phagocytic cups in a wortmannin-sensitive manner. Expression of a truncation construct of Myo10 (Myo10 tail) in a macrophage cell line or cytosolic loading of anti-Myo10 antibodies in bovine alveolar macrophages inhibited phagocytosis. In contrast, expression of a Myo10 tail construct containing a point mutation in one of its PH domains failed to inhibit phagocytosis. Expression of Myo10 tail inhibited spreading, but not adhesion, on IgG-coated substrates, consistent with a function for Myo10 in pseudopod extension. We propose that Myo10 provides a molecular link between PI(3)K and pseudopod extension during phagocytosis.

Phagocytosis of microbes: complexity in action

The phagocytic response of innate immune cells such as macrophages is defined by the activation of complex signaling networks that are stimulated by microbial contact. Many individual proteins have been demonstrated to participate in phagocytosis, and the application of high-throughput tools has indicated that many more remain to be described. In this review, we examine this complexity and describe how during recognition, multiple receptors are simultaneously engaged to mediate internalization, activate microbial killing, and induce the production of inflammatory cytokines and chemokines. Many signaling molecules perform multiple functions during phagocytosis, and these molecules are likely to be key regulators of the process. Indeed, pathogenic microorganisms target many of these molecules in their attempts to evade destruction.

Tumor necrosis factor-alpha-stimulated polymorphonuclear leukocytes suppress migration and bactericidal activity of polymorphonuclear leukocytes in a paracrine manner

OBJECTIVE

Polymorphonuclear leukocytes (PMN) and tumor necrosis factor-alpha (TNF-alpha) play prominent roles in acute respiratory distress syndrome, ischemia-reperfusion injury, trauma, and sepsis. Whereas direct effects of TNF-alpha on PMN function and viability are well documented, little data are available addressing the ability of PMN to communicate with each other in response to cytokine stimulation. Therefore, the aim of this study was to determine whether TNF-alpha can modulate PMN function by inducing PMN to secrete products upon stimulation, which would affect other PMN in vitro in a manner independent of cell contact.

METHODS

PMN were purified daily from blood obtained from a pool of 22 healthy volunteers. Conditioned media (CM-TNF) was prepared by incubating PMN in Hanks' balanced salt solution plus TNF-alpha for 1-4 hrs. Freshly isolated PMN were resuspended in CM-TNF and analyzed for 1) phagocytosis of opsonized Escherichia coli, 2) oxidative metabolism as measured as an index of DCF-DA activation, and 3) migration to chemoattractants through Transwell inserts.

RESULTS

CM-TNF decreased PMN phagocytotic activity by 8% to 15% and completely suppressed oxidative metabolism but did not modulate the expression of receptors associated with these functions. CM-TNF suppressed the migration of PMN to two biologically relevant agents, N-formyl-methionyl-leucyl-phenylalanine and leukotriene B4, by approximately 65%, but had no effect on PMN migration to interleukin-8. This suppression was observed for migration across plastic filters as well as extracellular matrix proteins.

CONCLUSION

Our data demonstrate that PMN stimulated with TNF-alpha suppress the immunologic function and migration of other PMN independent of cell-cell contact and suggest that TNF-alpha may participate in a negative feedback loop by inducing a PMN-derived factor that counteracts its activity.

Regulation of polymorphonuclear leukocyte degranulation and oxidant production by ceramide through inhibition of phospholipase D

Exogenous C(2)-ceramide has been shown to inhibit polymorphonuclear leukocyte (PMN) phagocytosis through inhibition of phospholipase D (PLD) and downstream events, including activation of extracellular signal-regulated kinases 1 and 2, leading to the hyphothesis that the sphingomyelinase pathway is involved in termination of phagocytosis. Here it is postulated that increased PLD activity generating phosphatidic acid and diacylglycerol (DAG) is essential for superoxide release and degranulation and that ceramide, previously shown to be generated during PMN activation, inhibits PLD activation, thereby leading to inhibition of PMN function. When PMNs were primed with granulocyte colony-stimulating factor (G-CSF) and then activated with N-formyl-methionyl-leucyl-phenylalanine (FMLP), C(2)-ceramide (10 microM) completely inhibited release of superoxide, lactoferrin, and gelatinase; the DAG analog sn-1,2-didecanoylglycerol (DiC10) (10 microM) restored oxidase activation and degranulation in the ceramide-treated cells. Similarly, C(2)-ceramide inhibited oxidase activity and degranulation of PMNs treated with cytochalasin B followed by FMLP, and DiC10 restored function. In contrast, C(2)-ceramide did not inhibit phosphorylation of p47phox or p38 mitogen-activated protein kinase, or translocation of p47phox, PLD-containing organelles, adenosine diphosphate-ribosylation factor 1, RhoA, protein kinase C (PKC)-beta or PKC-alpha to the plasma membrane in G-CSF or cytochalasin B-treated, FMLP-activated PMNs. PLD activity increased by 3-fold in G-CSF-primed PMNs stimulated by FMLP and by 30-fold in cytochalasin B-treated PMNs stimulated by FMLP. Both PLD activities were completely inhibited by 10 microM C(2)-ceramide. In conclusion, superoxide, gelatinase, and lactoferrin release require activation of the PLD pathway in primed PMNs and cytochalasin B-treated PMNs. Ceramide may affect protein interactions with PLD in the plasma membrane, thereby attenuating PMN activation.

Phagocytosis and innate immunity

Phagocytosis is an evolutionarily conserved process utilized by many cells to ingest microbial pathogens, and apoptotic and necrotic corpses. Recent investigation has revealed a fundamental requirement for two co-ordinated cellular processes--cytoskeletal alterations and membrane trafficking--in the phagocytic event. Some elements of this machinery are co-opted by certain pathogens to gain entry into host cells.

Preliminary research on myosin light chain kinase in rabbit liver

AIM: To study preliminarily the properties of myosin light chain kinase (MLCK) in rabbit liver.

METHODS: The expression of MLCK was detected by reverse transcription-polymerase chain reaction (RT-PCR); the MLCK was obtained from rabbit liver, and its activity was analyzed by γ-³² P incorporation technique to detect the phosphorylation of myosin light chain.

RESULTS: MLCK was expressed in rabbit liver, and the activity of the enzyme was similar to rabbit smooth muscle MLCK, and calmodulin- dependent. When the concentration was 0.65 mg •L¯¹, the activity was at the highest level.

CONCLUSION: MLCK expressed in rabbit liver may catalyze the phosphorylation of myosin light chain, which may play important roles in the regulation of hepatic cell functions.

Phagocytic signaling strategies: Fc(gamma)receptor-mediated phagocytosis as a model system

Phagocytosis is a phylogenetically ancient process by which eukaryotic cells engulf insoluble substances whose size exceeds approximately 0.5 microm. The engulfment process requires the concerted action of several fundamental cellular pathways and is governed by multiple transmembrane signaling events. Here we focus on phagocytosis mediated by a well-studied class of phagocytic receptors that recognize the Fc portion of IgG (Fc(gamma)Rs ).

Effects of lactoferrin on collagen gel contractile activity and myosin light chain phosphorylation in human fibroblasts

When fibroblasts are plated on a type I collagen gel they reduce the size of the gel and the extent of collagen gel contraction reflects the motile activity of the fibroblasts. We found that both bovine and human lactoferrin (Lf) enhanced the collagen gel contractile activity of WI-38 human fibroblasts. Rho inhibitor (exoenzyme C3), Rho kinase inhibitor (Y-27632), myosin light chain kinase inhibitor (ML-7), MEK inhibitor (PD98059) and Src family tyrosine kinase inhibitor inhibited the Lf-enhanced collagen gel contraction. Treatment of fibroblasts with Lf induced the phosphorylation of myosin light chain (MLC) within 30 min. Lf-enhanced MLC phosphorylation was inhibited by Y-27632 and ML-7. These results suggest that Lf promotes the motility of fibroblasts by regulating MLC phosphorylation.

Evaluation of the process for superoxide production by NADPH oxidase in human neutrophils: evidence for cytoplasmic origin of superoxide

We present an up-to-date insight into the function of NADPH oxidase in human neutrophils, the signalling pathways involved in activation of this enzyme and the process of association of its components with the cytoskeleton. We also discuss the functional implications of morphological studies revealing localization of the sites of NADPH oxidase activity. An original model of the process of superoxide (O2*-) production in human neutrophils is shown. Organization of NADPH oxidase is associated with several components. Upon stimulation, tri-phox cytosolic components of NADPH oxidase (p40-phox, p47-phox and p67-phox) bind to actin filaments. This process involves other actin-binding proteins, such as cofilin and coronin. Activated protein kinase C, translocated from the plasma membrane, phosphorylates cytosolic components at a scaffold of cytoskeleton. Subsequently, p40-phox, responsible for maintaining the resting state of NADPH oxidase, is separated from other two cytosolic phox proteins following an attachment of the active form of small GTP-binding protein Rac to p67-phox. Cytosolic duo-phox proteins (p47-phox and p67-phox) conjugate with membrane components (gp91-phox, p22-phox and Rapla) of NADPH oxidase residing within membranes of intracellular compartments. This chain of events triggers production of O2*-. Then, oxidant-producing intracellular compartments associate with the plasma membrane. Eventually, intracellularly produced O2*- is released to the extracellular environment through the orifice formed by fusion of oxidant-producing compartments with the plasma membrane. Intracellular movement of the oxidant-producing compartments may be regulated by myosin light chain kinase. The review emphasizes that functional assembly of NADPH oxidase and, therefore, generation of O2*- is accomplished essentially within the intracellular compartments. Upon neutrophil stimulation, intracellularly generated O2*- is transported to the plasma membrane to be released and to ensure host defense against infection.

Myosin light chain kinase regulates capacitative ca(2+) entry in human monocytes/macrophages

Monocytes/macrophages are present in all stages of atherosclerosis. Although many of their activities depend to various extents on changes in intracellular Ca(2+) concentration ([Ca(2+)](i)), mechanisms regulating [Ca(2+)](i) in these cells remain unclear. We aimed to explore the role of myosin light chain kinase (MLCK) in Ca(2+) signaling in freshly isolated human monocytes/macrophages. Large capacitative Ca(2+) entry (CCE) was observed under fura 2 fluoroscopy in human monocytes/macrophages treated with thapsigargin and cyclopiazonic acid. ML-9 and wortmannin, 2 structurally different inhibitors of MLCK, dose-dependently (1 to 100 micromol/L) prevented CCE and completely did so at 100 micromol/L, whereas inhibitors of tyrosine kinase and protein kinase C had only partial effects. Western blotting showed that thapsigargin significantly caused myosin light chain phosphorylation, which was almost completely blocked by ML-9 (100 micromol/L) and wortmannin (100 micromol/L). ML-9 also dose-dependently (1 to 100 micromol/L) inhibited this phosphorylation, which was well correlated with its inhibition of CCE. Transfection with MLCK antisense completely prevented CCE in response to thapsigargin and cyclopiazonic acid, whereas MLCK sense had no effect. These data strongly indicate that MLCK regulates CCE in human monocytes/macrophages. The study suggests a possible involvement of MLCK in many Ca(2+)-dependent activities of monocytes/macrophages.

Phagocytosis and the actin cytoskeleton

The process of engulfing a foreign particle - phagocytosis - is of fundamental importance for a wide diversity of organisms. From simple unicellular organisms that use phagocytosis to obtain their next meal, to complex metazoans in which phagocytic cells represent an essential branch of the immune system, evolution has armed cells with a fantastic repertoire of molecules that serve to bring about this complex event. Regardless of the organism or specific molecules concerned, however, all phagocytic processes are driven by a finely controlled rearrangement of the actin cytoskeleton. A variety of signals can converge to locally reorganise the actin cytoskeleton at a phagosome, and there are significant similarities and differences between different organisms and between different engulfment processes within the same organism. Recent advances have demonstrated the complexity of phagocytic signalling, such as the involvement of phosphoinostide lipids and multicomponent signalling complexes in transducing signals from phagocytic receptors to the cytoskeleton. Similarly, a wide diversity of ‘effector molecules’ are now implicated in actin-remodelling downstream of these receptors.

Mechanism in the sequential control of cell morphology and S phase entry by epidermal growth factor involves distinct MEK/ERK activations

Cell shape plays a role in cell growth, differentiation, and death. Herein, we used the hepatocyte, a normal, highly differentiated cell characterized by a long G1 phase, to understand the mechanisms that link cell shape to growth. First, evidence was provided that the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) cascade is a key transduction pathway controlling the hepatocyte morphology. MEK2/ERK2 activation in early G1 phase did not lead to cell proliferation but induced cell shape spreading and demonstration was provided that this MAPK-dependent spreading was required for reaching G1/S transition and DNA replication. Moreover, epidermal growth factor (EGF) was found to control this morphogenic signal in addition to its mitogenic effect. Thus, blockade of cell spreading by cytochalasin D or PD98059 treatment resulted in inhibition of EGF-dependent DNA replication. Our data led us to assess the first third of G1, is exclusively devoted to the growth factor-dependent morphogenic events, whereas the mitogenic signal occurred at only approximately mid-G1 phase. Moreover, these two growth factor-related sequential signaling events involved successively activation of MEK2-ERK2 and then MEK1/2-ERK1/2 isoforms. In addition, we demonstrated that inhibition of extracellular matrix receptor, such as integrin beta1 subunit, leads to cell arrest in G1, whereas EGF was found to up-regulated integrin beta1 and fibronectin in a MEK-ERK-dependent manner. This process in relation to cytoskeletal reorganization could induce hepatocyte spreading, making them permissive for DNA replication. Our results provide new insight into the mechanisms by which a growth factor can temporally control dual morphogenic and mitogenic signals during the G1 phase.

Cleavage of mitogen-activated protein kinases in human neutrophils undergoing apoptosis: role in decreased responsiveness to inflammatory cytokines

Extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) are major signaling molecules activated in human neutrophils stimulated by cytokines. Both molecules were cleaved at the N-terminal portion in neutrophils undergoing apoptosis induced by in vitro culture alone or treatment with TNF and/or cycloheximide. The cleavage of both molecules was inhibited by G-CSF and benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone, a caspase inhibitor, both of which can inhibit neutrophil apoptosis. In a cell-free system, ERK and p38 MAPK were not cleaved by recombinant caspase-3 or caspase-8 while gelsolin was cleaved by caspase-3 under the same condition. The cleavage of both molecules appears to be specific to mature neutrophils, since it was not detected in immature cells (HL-60 and Jurkat) undergoing apoptosis, indicating that proteases responsible for the cleavage of both molecules may develop during differentiation into mature neutrophils. Concomitant with the cleavage of ERK and p38 MAPK, GM-CSF- and TNF-induced superoxide release, adherence, and phosphorylation of ERK and p38 MAPK were decreased in neutrophils undergoing apoptosis. In addition, GM-CSF- and TNF-induced superoxide release and adherence were inhibited by PD98059 MAPK/ERK kinase inhibitor) as well as SB203580 (p38 MAPK inhibitor), suggesting possible involvement of ERK and p38 MAPK in superoxide release and adherence induced by these cytokines. These findings indicate that ERK and p38 MAPK are cleaved and degraded in neutrophils undergoing apoptosis in a caspase-dependent manner and the cleavage of both molecules may be partly responsible for decreased functional responsiveness to inflammatory cytokines.

Priming of the neutrophil respiratory burst involves p38 mitogen-activated protein kinase-dependent exocytosis of flavocytochrome b558-containing granules

The respiratory burst of human neutrophils is primed by a number of pro-inflammatory stimuli, including tumor necrosis factor-alpha (TNFalpha) and lipopolysaccharide (LPS); however, the mechanism of priming remains unknown. LPS has been shown previously to increase membrane expression of flavocytochrome b(558), a component of the NADPH oxidase. This study shows that TNFalpha also increases membrane expression of flavocytochrome b(558). Mitogen-activated protein kinase (MAPK) modules have been implicated in the action of priming agents. Pharmacologic inhibitors of MAPKs, SB203580 and PD098059, revealed that priming of the respiratory burst and up-regulation of flavocytochrome b(558) are dependent on p38 MAPK but not on extracellular-signal regulated kinase (ERK). TNFalpha and LPS primed respiratory burst activity and increased membrane expression of CD35 and CD66b, specific markers of secretory vesicles and specific granules that contain flavocytochrome b(558), with similar time courses and concentration dependences. These processes also required p38 MAPK but were independent of ERK. TNFalpha failed to prime respiratory burst activity or to increase membrane CD35 expression in enucleated neutrophil cytoplasts. These data suggest that one mechanism by which TNFalpha and LPS prime neutrophil respiratory burst activity is by increasing membrane expression of flavocytochrome b(558) through exocytosis of intracellular granules in a process regulated by p38 MAPK.