Phosphoinositide 3-kinase-dependent and -independent activation of the small GTPase Rac2 in human neutrophils

Targeting the myeloid microenvironment in neuroblastoma

Myeloid cells (granulocytes and monocytes/macrophages) play an important role in neuroblastoma. By inducing a complex immunosuppressive network, myeloid cells pose a challenge for the adaptive immune system to eliminate tumor cells, especially in high-risk neuroblastoma. This review first summarizes the pro- and anti-tumorigenic functions of myeloid cells, including granulocytes, monocytes, macrophages, and myeloid-derived suppressor cells (MDSC) during the development and progression of neuroblastoma. Secondly, we discuss how myeloid cells are engaged in the current treatment regimen and explore novel strategies to target these cells in neuroblastoma. These strategies include: (1) engaging myeloid cells as effector cells, (2) ablating myeloid cells or blocking the recruitment of myeloid cells to the tumor microenvironment and (3) reprogramming myeloid cells. Here we describe that despite their immunosuppressive traits, tumor-associated myeloid cells can still be engaged as effector cells, which is clear in anti-GD2 immunotherapy. However, their full potential is not yet reached, and myeloid cell engagement can be enhanced, for example by targeting the CD47/SIRPα axis. Though depletion of myeloid cells or blocking myeloid cell infiltration has been proven effective, this strategy also depletes possible effector cells for immunotherapy from the tumor microenvironment. Therefore, reprogramming of suppressive myeloid cells might be the optimal strategy, which reverses immunosuppressive traits, preserves myeloid cells as effectors of immunotherapy, and subsequently reactivates tumor-infiltrating T cells.

Phosphoinositide-3 kinase gamma regulates caspase-1 activation and leukocyte recruitment in acute murine gout

This study investigates the participation of PI3Kγ in the development of joint inflammation and dysfunction in an experimental model of acute gout in mice. Acute gout was induced by injection of monosodium urate (MSU) crystals into the tibiofemoral joint of mice. The involvement of PI3Kγ was evaluated using a selective inhibitor and mice deficient for PI3Kγ (PI3Kγ^-/- ) or with loss of kinase activity. Neutrophils recovered from the inflamed joint were quantified and stained for phosphorylated Akt (pAkt) and production of reactive oxygen species (ROS). The adherence of leukocytes to the joint microvasculature was assessed by intravital microscopy and cleaved caspase-1 by Western blot. Injection of MSU crystals induced massive accumulation of neutrophils expressing phosphorylated Akt. In the absence of PI3Kγ, there was reduction of pAkt expression, chemokine production, and neutrophil recruitment. Genetic or pharmacological inhibition of PI3Kγ reduced the adherence of leukocytes to the joint microvasculature, even in joints with established inflammation. Neutrophils from PI3Kγ^-/- mice produced less ROS than wild-type neutrophils. There was decreased joint damage and dysfunction in the absence of PI3Kγ. In addition, in the absence of PI3Kγ activity, there was reduction of cleaved caspase-1 and IL-1β production in synovial tissue after injection of MSU crystals and leukotriene B₄ . Our studies suggest that PI3Kγ is crucial for MSU crystal-induced acute joint inflammation. It is necessary for regulating caspase-1 activation and for mediating neutrophil migration and activation. Drugs that impair PI3Kγ function may be useful to control acute gout inflammation.

Neither eosinophils nor neutrophils require ATG5-dependent autophagy for extracellular DNA trap formation

The importance of extracellular traps (ETs) in innate immunity is well established, but the molecular mechanisms responsible for their formation remain unclear and in scientific dispute. ETs have been defined as extracellular DNA scaffolds associated with the granule proteins of eosinophils or neutrophils. They are capable of killing bacteria extracellularly. Based mainly on results with phosphoinositide 3-kinase (PI3K) inhibitors such as 3-methyladenine (3-MA) and wortmannin, which are commonly used to inhibit autophagy, several groups have reported that autophagy is required for neutrophil extracellular trap (NET) formation. We decided to investigate this apparent dependence on autophagy for ET release and generated genetically modified mice that lack, specifically in eosinophils or neutrophils, autophagy-related 5 (Atg5), a gene encoding a protein essential for autophagosome formation. Interestingly, neither eosinophils nor neutrophils from Atg5-deficient mice exhibited abnormalities in ET formation upon physiological activation or exposure to low concentrations of PMA, although we could confirm that human and mouse eosinophils and neutrophils, after pre-treatment with inhibitors of class III PI3K, show a block both in reactive oxygen species (ROS) production and in ET formation. The so-called late autophagy inhibitors bafilomycin A1 and chloroquine, on the other hand, were without effect. These data indicate that ET formation occurs independently of autophagy and that the inhibition of ROS production and ET formation in the presence of 3-MA and wortmannin is probably owing to their additional ability to block the class I PI3Ks, which are involved in signalling cascades initiated by triggers of ET formation.

Titanium Dioxide Nanoparticles Increase Superoxide Anion Production by Acting on NADPH Oxidase

Titanium dioxide (TiO₂) anatase nanoparticles (NPs) are metal oxide NPs commercialized for several uses of everyday life. However their toxicity has been poorly investigated. Cellular internalization of NPs has been shown to activate macrophages and neutrophils that contribute to superoxide anion production by the NADPH oxidase complex. Transmission electron micrososcopy images showed that the membrane fractions were close to the NPs while fluorescence indicated an interaction between NPs and cytosolic proteins. Using a cell-free system, we have investigated the influence of TiO₂ NPs on the behavior of the NADPH oxidase. In the absence of the classical activator molecules of the enzyme (arachidonic acid) but in the presence of TiO₂ NPs, no production of superoxide ions could be detected indicating that TiO₂ NPs were unable to activate by themselves the complex. However once the NADPH oxidase was activated (i.e., by arachidonic acid), the rate of superoxide anion production went up to 140% of its value without NPs, this effect being dependent on their concentration. In the presence of TiO₂ nanoparticles, the NADPH oxidase produces more superoxide ions, hence induces higher oxidative stress. This hyper-activation and the subsequent increase in ROS production by TiO₂ NPs could participate to the oxidative stress development.

Cholesterol: A modulator of the phagocyte NADPH oxidase activity - A cell-free study

The NADPH oxidase Nox2, a multi-subunit enzyme complex comprising membrane and cytosolic proteins, catalyzes a very intense production of superoxide ions O₂^•−, which are transformed into other reactive oxygen species (ROS). In vitro, it has to be activated by addition of amphiphiles like arachidonic acid (AA). It has been shown that the membrane part of phagocyte NADPH oxidase is present in lipid rafts rich in cholesterol. Cholesterol plays a significant role in the development of cardio-vascular diseases that are always accompanied by oxidative stress. Our aim was to investigate the influence of cholesterol on the activation process of NADPH oxidase. Our results clearly show that, in a cell-free system, cholesterol is not an efficient activator of NADPH oxidase like arachidonic acid (AA), however it triggers a basal low superoxide production at concentrations similar to what found in neutrophile. A higher concentration, if present during the assembly process of the enzyme, has an inhibitory role on the production of O₂^•−. Added cholesterol acts on both cytosolic and membrane components, leading to imperfect assembly and decreasing the affinity of cytosolic subunits to the membrane ones. Added to the cytosolic proteins, it retains their conformations but still allows some conformational change induced by AA addition, indispensable to activation of NADPH oxidase.

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Natural cholesterol is important for the NADPH oxidase function.

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Added cholesterol alone activates slightly the NADPH oxidase.

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Cholesterol addition lowers the AA dependent activity of NADPH oxidase.

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Added cholesterol acts on both cytosolic and membrane components.

DOCK2 and DOCK5 act additively in neutrophils to regulate chemotaxis, superoxide production, and extracellular trap formation

Neutrophils are highly motile leukocytes that play important roles in the innate immune response to invading pathogens. Neutrophils rapidly migrate to the site of infections and kill pathogens by producing reactive oxygen species (ROS). Neutrophil chemotaxis and ROS production require activation of Rac small GTPase. DOCK2, an atypical guanine nucleotide exchange factor (GEF), is one of the major regulators of Rac in neutrophils. However, because DOCK2 deficiency does not completely abolish fMLF-induced Rac activation, other Rac GEFs may also participate in this process. In this study, we show that DOCK5 acts with DOCK2 in neutrophils to regulate multiple cellular functions. We found that fMLF- and PMA-induced Rac activation were almost completely lost in mouse neutrophils lacking both DOCK2 and DOCK5. Although β2 integrin-mediated adhesion occurred normally even in the absence of DOCK2 and DOCK5, mouse neutrophils lacking DOCK2 and DOCK5 exhibited a severe defect in chemotaxis and ROS production. Similar results were obtained when human neutrophils were treated with CPYPP, a small-molecule inhibitor of these DOCK GEFs. Additionally, we found that DOCK2 and DOCK5 regulate formation of neutrophil extracellular traps (NETs). Because NETs are involved in vascular inflammation and autoimmune responses, DOCK2 and DOCK5 would be a therapeutic target for controlling NET-mediated inflammatory disorders.

Rho family and Rap GTPase activation assays

The detection of Ras superfamily GTPase activity in innate immune cells is important when studying signaling events elicited by various ligands and cellular processes. The development of high-affinity probes detecting the activated, GTP-bound form of small GTPases has significantly enhanced our understanding of initiation and termination of GTPase-regulated signaling pathways. These probes are created by fusing a high-affinity GTPase-binding domain derived from a specific downstream effector protein to glutathione S-transferase (GST). Such domains bind preferentially to the GTP-bound form of the upstream Rho or Ras GTPase. Coupling these probes to beads enables extraction of the complex and subsequent quantification of the active GTP-binding protein by immunoblotting. Although effector domains that discriminate efficiently between GDP- and GTP-bound states and highly specific antibodies are not yet available for every small GTPase, analysis of certain members of the Rho and Ras GTPase family is now routinely performed. Here, we describe affinity-based pulldown assays for detection of Rho GTPase (Rac1/2, Cdc42, RhoA/B) and Rap1/2 activity in stimulated neutrophils or macrophages.

Arachidonic acid induces direct interaction of the p67(phox)-Rac complex with the phagocyte oxidase Nox2, leading to superoxide production

The phagocyte NADPH oxidase Nox2, heterodimerized with p22(phox) in the membrane, is dormant in resting cells but becomes activated upon cell stimulation to produce superoxide, a precursor of microbicidal oxidants. Nox2 activation requires two switches to be turned on simultaneously: a conformational change of the cytosolic protein p47(phox) and GDP/GTP exchange on the small GTPase Rac. These proteins, in an active form, bind to their respective targets, p22(phox) and p67(phox), leading to productive oxidase assembly at the membrane. Although arachidonic acid (AA) efficiently activates Nox2 both in vivo and in vitro, the mechanism has not been fully understood, except that AA induces p47(phox) conformational change. Here we show that AA elicits GDP-to-GTP exchange on Rac at the cellular level, consistent with its role as a potent Nox2 activator. However, even when constitutively active forms of p47(phox) and Rac1 are both expressed in HeLa cells, superoxide production by Nox2 is scarcely induced in the absence of AA. These active proteins also fail to effectively activate Nox2 in a cell-free reconstituted system without AA. Without affecting Rac-GTP binding to p67(phox), AA induces the direct interaction of Rac-GTP-bound p67(phox) with the C-terminal cytosolic region of Nox2. p67(phox)-Rac-Nox2 assembly and superoxide production are both abrogated by alanine substitution for Tyr-198, Leu-199, and Val-204 in the p67(phox) activation domain that localizes the C-terminal to the Rac-binding domain. Thus the "third" switch (AA-inducible interaction of p67(phox)·Rac-GTP with Nox2) is required to be turned on at the same time for Nox2 activation.

Phosphorylation of Noxo1 at threonine 341 regulates its interaction with Noxa1 and the superoxide-producing activity of Nox1

Superoxide production by Nox1, a member of the Nox family NAPDH oxidases, requires expression of its regulatory soluble proteins Noxo1 (Nox organizer 1) and Noxa1 (Nox activator 1) and is markedly enhanced upon cell stimulation with phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC). The mechanism underlying PMA-induced enhancement of Nox1 activity, however, remains to be elucidated. Here we show that, in response to PMA, Noxo1 undergoes phosphorylation at multiple sites, which is inhibited by the PKC inhibitor GF109203X. Among them, Thr341 in Noxo1 is directly phosphorylated by PKC in vitro, and alanine substitution for this residue reduces not only PMA-induced Noxo1 phosphorylation but also PMA-dependent enhancement of Nox1-catalyzed superoxide production. Phosphorylation of Thr341 allows Noxo1 to sufficiently interact with Noxa1, an interaction that participates in Nox1 activation. Thus phosphorylation of Noxo1 at Thr341 appears to play a crucial role in PMA-elicited activation of Nox1, providing a molecular link between PKC-mediated signal transduction and Nox1-catalyzed superoxide production. Furthermore, Ser154 in Noxo1 is phosphorylated in both resting and PMA-stimulated cells, and the phosphorylation probably participates in a PMA-independent constitutive activity of Nox1. Ser154 may also be involved in protein kinase A (PKA) mediated regulation of Nox1; this serine is the major residue that is phosphorylated by PKA in vitro. Thus phosphorylation of Noxo1 at Thr341 and at Ser154 appears to regulate Nox1 activity in different manners.

STRUCTURED DIGITAL ABSTRACT

Noxo1 binds to p22phox by pull down (1, 2, 3) Noxo1 binds to Noxo1 by pull down (View interaction) Noxa1 binds to Noxo1 by pull down (1, 2, 3, 4, 5).

FERM domain containing protein 7 interacts with the Rho GDP dissociation inhibitor and specifically activates Rac1 signaling

The FERM domain containing protein 7 gene (FRMD7) associated with the X-linked disorder idiopathic congenital nystagmus (ICN) is involved in the regulation of neurite elongation during neuronal development. Members of the Rho family of small G-proteins (Rho GTPases) are key regulators of the actin cytoskeleton and are implicated in the control of neuronal morphology. The Rho GDP dissociation inhibitor alpha, RhoGDIα, the main regulator of Rho GTPases, can form a complex with the GDP-bound form of Rho GTPases and inhibit their activation. Here, we demonstrate that the full length of the mouse FRMD7, rather than the N-terminus or the C-terminus alone, directly interacts with RhoGDIα and specifically initiates Rac1 signaling in mouse neuroblastoma cell line (neuro-2a). Moreover, we show that wild-type human FRMD7 protein is able to activate Rac1 signaling by interacting with RhoGDIα and releasing Rac1 from Rac1-RhoGDIα complex. However, two missense mutations (c.781C>G and c.886G>C) of human FRMD7 proteins weaken the ability to interact with RhoGDIα and release less Rac1, that induce the activation of Rac1 to a lesser degree; while an additional mutant, c.1003C>T, which results in a C-terminal truncated protein, almost fails to interact with RhoGDIα and to activate Rac1 signaling. Collectively, these results suggest that FRMD7 interacts with one of the Rho GTPase regulators, RhoGDIα, and activates the Rho subfamily member Rac1, which regulates reorganization of actin filaments and controls neuronal outgrowth. We predict that human mutant FRMD7 thus influences Rac1 signaling activation, which can lead to abnormal neuronal outgrowth and cause the X-linked ICN.

The hematopoietic cell-specific Rho GTPase inhibitor ARHGDIB/D4GDI limits HIV type 1 replication

Rho GTPases are able to influence the replication of human immunodeficiency virus type 1 (HIV-1). However, little is known about the regulation of HIV-1 replication by guanine nucleotide dissociation inhibitors (GDIs), one of the three major regulators of the Rho GTPase activation cycle. From a T cell-based cDNA library screening, ARHGDIB/RhoGDIβ, a hematopoietic lineage-specific GDI family protein, was identified as a negative regulator of HIV-1 replication. Up-regulation of ARHGDIB attenuated the replication of HIV-1 in multiple T cell lines. The results showed that (1) a significant portion of RhoA and Rac1, but not Cdc42, exists in the GTP-bound active form under steady-state conditions, (2) ectopic ARHGDIB expression reduced the F-actin content and the active forms of both RhoA and Rac1, and (3) HIV-1 infection was attenuated by either ectopic expression of ARHGDIB or inhibition of the RhoA signal cascade at the HIV-1 Env-dependent early phase of the viral life cycle. This is in good agreement with the previous finding that RhoA and Rac1 promote HIV-1 entry by increasing the efficiency of receptor clustering and virus-cell membrane fusion. In conclusion, the ARHGDIB is a lymphoid-specific intrinsic negative regulator of HIV-1 replication that acts by simultaneously inhibiting RhoA and Rac1 functions.

TNF-α-decreased thrombomodulin expression in monocytes is inhibited by propofol through regulation of tristetraprolin and human antigen R activities

Thrombomodulin (TM) is expressed on the surface of monocytes and is a key regulator of actual immune capacity. Propofol is an anesthetic agent that exerts anti-inflammatory effects. The objective of this study was to determine whether propofol could modulate TM in TNF-α-stimulated monocytes. THP-1 cells and male New Zealand rabbits were used in this study. The results showed that TNF-α decreases the TM expression by mediating posttranscriptional modification, and this inhibition may be repressed by treatment with propofol. Immunofluorescence, immunoprecipitation, and pull-down assays were used to demonstrate that Rac1-dependent nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, Cdc42, and p38 mitogen-activated protein kinase activation, as well as tristetraprolin (TTP) expression, all contributed to the downregulation of TM in TNF-α-treated cells. Propofol reversed the effects of TNF-α on TM downregulation. Propofol mediated the expression of intracellular TTP and the distribution of cytosolic human antigen R (HuR) and changed their interactions with the 3'-untranslated region of TM mRNA regulating by Cdc42 and Rac1. In addition, the animal study showed that propofol regulates TM, TTP, and HuR expression on monocytes in TNF-α-treated rabbits. In conclusion, the inhibition of TM expression in TNF-α-treated monocytes was mediated by the activation of NADPH oxidase and the expression of TTP. Propofol may inhibit the downregulation of TM by mediating NADPH oxidase and TTP inactivation and through the activation of HuR in vitro and in vivo. Utilizing TTP and HuR to control TM expression may be a promising approach for controlling systemic inflammation, and propofol may possess potential implications for the clinical immunity of monocytes after anesthesia or surgery.

Activation and function of small GTPases Rho, Rac, and Cdc42 during gastrulation

Gastrulation is comprised of a series of cell polarization and directional cell migration events that establish the physical body plan of the embryo. One of the major ligand-based pathways that has emerged to play crucial roles in the regulation of gastrulation is the non-canonical Wnt signaling pathway. This aspect of Wnt signaling is comprised of a number of signaling branches that are subsequently integrated for the regulation of changes to the actin cytoskeleton during cell polarization and cell migration during vertebrate gastrulation. The Rho family of small GTPases are activated and required for non-canonical Wnt signaling during gastrulation, and in this chapter, we describe biochemical assays for the detection of Wnt-mediated activation of Rho, Rac, and Cdc42 in both mammalian cells and Xenopus embryo explants.

Assessment of the role for Rho family GTPases in NADPH oxidase activation

Rac, a member of the Rho family small GTPases, plays a crucial role in activation of Nox family NADPH oxidases in animals, enzymes dedicated to production of reactive oxygen species such as superoxide. The phagocyte oxidase Nox2, crucial for microbicidal activity during phagocytosis, is activated in a manner completely dependent on Rac. Rac in the GTP-bound form directly binds to the oxidase activator p67( phox ), which in turn interacts with Nox2, leading to superoxide production. Rac also participates in activation of the nonphagocytic oxidase Nox1; in this case, GTP-bound Rac functions by interacting with Noxa1, a p67( phox )-related protein that is required for Nox1 activation. On the other hand, in the presence of either p67( phox ) or Noxa1, Rac facilitates superoxide production by Nox3, which is responsible in the inner ear for formation of otoconia, tiny mineralized structures that are required for sensing balance and gravity. All the three mammalian homologs of Rac (Rac1, Rac2, and Rac3), but not Cdc42 or RhoA, are capable of serving as an activator of Nox1-3. Here, we describe methods for the assay of Rac binding to p67( phox ) and Noxa1 and for the reconstitution of Rac-dependent Nox activity in cell-free and whole-cell systems.

Pathophysiological roles of NADPH oxidase/nox family proteins in the vascular system. -Review and perspective-

It has been established that oxidative stress plays a crucial role in the development and progression of vascular diseases. Besides the mitochondria, the NADPH oxidase/Nox family proteins are now thought to be important origins of the reactive oxygen species that underlie various vascular disease states, such as hypertension, atherosclerosis, angiogenesis, and ischemia/reperfusion injury. This review summarizes the basis of vascular Nox proteins and discusses their pathophysiological roles in the vascular system.

Sphingosine 1-phosphate regulates matrix metalloproteinase-9 expression and breast cell invasion through S1P3-Gαq coupling

Recent evidence suggests that inflammation is involved in malignant progression of breast cancer. Sphingosine 1-phosphate (S1P), acting on the G-protein-coupled receptors, is known as a potent inflammatory mediator. In this study, the effect of the inflammatory lipid S1P on the regulation of invasive/migratory phenotypes of MCF10A human breast epithelial cells was investigated to elucidate a causal relationship between inflammation and the control of invasiveness of breast cells. We show that S1P causes induction of matrix metalloproteinase-9 (MMP-9) in vitro and in vivo, and thus enhances invasion and migration. We also show that fos plays a crucial role in the transcriptional activation of MMP-9 by S1P. In addition, activation of extracellular-signal-regulated kinases 1 and 2 (ERK1/2), p38 and alpha serine/threonine-protein kinase (Akt) are involved in the process of S1P-mediated induction of MMP-9 expression and invasion. Activation of the S1P receptor S1P₃ and G(αq) are required for S1P-induced invasive/migratory responses, suggesting that the enhancement of S1P-mediated invasiveness is triggered by the specific coupling of S1P₃ to the heterotrimeric G(αq) subunit. Activation of phospholipase C-β₄ and intracellular Ca²⁺ release are required for S1P-induced MMP-9 upregulation. Taken together, this study demonstrated that S1P regulates MMP-9 induction and invasiveness through coupling of S1P₃ and G(αq) in MCF10A cells, thus providing a molecular basis for the crucial role of S1P in promoting breast cell invasion.

A novel TNFR1-triggered apoptosis pathway mediated by class IA PI3Ks in neutrophils

The most common form of neutrophil death is apoptosis. In the present study, we report surprising differences in the molecular mechanisms used for caspase activation between FAS/CD95-stimulated and TNF receptor 1 (TNFR1)-stimulated neutrophils. Whereas FAS-induced apoptosis was followed by caspase-8 activation and required Bid to initiate the mitochondrial amplification loop, TNF-α-induced apoptosis involved class IA PI3Ks, which were activated by MAPK p38. TNF-α-induced PI3K activation resulted in the generation of reactive oxygen species, which activated caspase-3, a mechanism that did not operate in neutrophils without active NADPH oxidase. We conclude that in neutrophils, proapoptotic pathways after TNFR1 stimulation are initiated by p38 and PI3K, but not by caspase-8, a finding that should be considered in anti-inflammatory drug-development strategies.

Leukotriene B4 augments and restores Fc gammaRs-dependent phagocytosis in macrophages

Phagocytosis by macrophages is essential for host defense, i.e. preventing invasion of pathogens and foreign materials. Macrophages engulf immunoglobulin G (IgG)-opsonized particles through the action of the receptors for the Fc of IgG (FcγRs). Leukotriene B(4) (LTB(4)) is a classical lipid chemoattractant derived from arachidonic acid. Leukotriene B(4) receptor 1 (BLT1), a high affinity LTB(4) receptor, is expressed in a variety of immune cells such as neutrophils, macrophages, and dendritic cells. Although LTB(4) has been shown to enhance macrophage phagocytosis, few studies have investigated the intracellular mechanisms involved in this in detail. Furthermore, there have been no reports of the direct cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling. Here, we show that FcγRs-dependent phagocytosis was attenuated in BLT1-deficient macrophages as compared with wild-type (WT) cells. Moreover, cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling was identified at the level of phosphatidylinositol 3-OH kinase (PI3K) and Rac, downstream of Syk. In addition, the trimeric G(i) protein (G(i)) was found to be essential for BLT1-dependent phagocytosis. Surprisingly, we found that LTB(4)-BLT1 signaling restores phagocytosis in the absence of FcγRs signaling. These data indicate that LTB(4)-BLT1 signaling plays a pivotal role in macrophage phagocytosis and innate immunity.

Blockade of LTB4-induced chemotaxis by bioactive molecules interfering with the BLT2-Galphai interaction

BLT2, a low-affinity leukotriene B4 (LTB4) receptor, is a member of the G-protein coupled receptor (GPCR) family and is involved in the pathogenesis of inflammatory diseases such as asthma. Despite its clinical implications, however, no pharmacological inhibitors are available. In the present study, we screened for small molecules that interfere with the interaction between the third intracellular loop region of BLT2 (BLT2iL3) and the Galphai3 protein subunit (Galphai3), using a high-throughput screening (HTS) assay with a library of 1040 FDA-approved drugs and bioactive compounds. We identified two small molecules-purpurin [1,2,4-trihydroxy-9,10-anthraquinone; IC50 = 1.6 microM for BLT2] and chloranil [tetrachloro-1,4-benzoquinone; IC50 = 0.42 microM for BLT2]-as specific BLT2-blocking agents. We found that blockade of the BLT2iL3-Galphai3 interaction by these small molecules inhibited the BLT2-downstream signaling cascade. For example, BLT2-signaling to phosphoinositide-3 kinase (PI3K)/Akt phosphorylation was completely abolished by these molecules. Furthermore, we observed that these small molecules blocked LTB4-induced chemotaxis by inhibiting the BLT2-PI3K/Akt-downstream, Rac1-reactive oxygen species-dependent pathway. Taken together, our results show that purpurin and chloranil interfere with the interaction between BLT2iL3 and Galphai3 and thus block the biological functions of BLT2 (e.g., chemotaxis). The present findings suggest a potential application of purpurin and chloranil as pharmacological therapeutic agents against BLT2-associated inflammatory human diseases.

Rac1 targeting suppresses p53 deficiency-mediated lymphomagenesis

Mutation of the p53 tumor suppressor is associated with disease progression, therapeutic resistance, and poor prognosis in patients with lymphoid malignancies and can occur in approximately 50% of Burkitt lymphomas. Thus, new therapies are needed to specifically target p53-deficient lymphomas with increased efficacy. In the current study, the specific impact of inhibition of the small GTPase Rac1 on p53-deficient B- and T-lymphoma cells was investigated. p53 deficiency resulted in increased Rac1 activity in both B-cell and T-cell lines, and its suppression was able to abrogate p53 deficiency-mediated lymphoma cell proliferation. Further, Rac targeting resulted in increased apoptosis via a p53-independent mechanism. By probing multiple signaling axes and performing rescue studies, we show that the antiproliferative effect of Rac1 targeting in lymphoma cells may involve the PAK and Akt signaling pathway, but not the mitogen-activated protein (MAP) kinase pathway. The effects of inhibition of Rac1 were extended in vivo where Rac1 targeting was able to specifically impair p53-deficient lymphoma cell growth in mouse xenografts and postpone lymphomagenesis onset in murine transplantation models. Because the Rac1 signaling axis is a critical determinant of apoptosis and tumorigenesis, it may represent an important basis for therapy in the treatment of p53-deficient lymphomas.

Differential regulation of phospholipase C-beta2 activity and membrane interaction by Galphaq, Gbeta1gamma2, and Rac2

We combined fluorescence recovery after photobleaching (FRAP) beam-size analysis with biochemical assays to investigate the mechanisms of membrane recruitment and activation of phospholipase C-beta(2) (PLCbeta(2)) by G protein alpha(q) and betagamma dimers. We show that activation by alpha(q) and betagamma differ from activation by Rac2 and from each other. Stimulation by alpha(q) enhanced the plasma membrane association of PLCbeta(2), but not of PLCbeta(2)Delta, which lacks the alpha(q)-interacting region. Although alpha(q) resembled Rac2 in increasing the contribution of exchange to the FRAP of PLCbeta(2) and in enhancing its membrane association, the latter effect was weaker than with Rac2. Moreover, the membrane recruitment of PLCbeta(2) by alpha(q) occurred by enhancing PLCbeta(2) association with fast-diffusing (lipid-like) membrane components, whereas stimulation by Rac2 led to interactions with slow diffusing membrane sites. On the other hand, activation by betagamma shifted the FRAP of PLCbeta(2) and PLCbeta(2)Delta to pure lateral diffusion 3- to 5-fold faster than lipids, suggesting surfing-like diffusion along the membrane. We propose that these different modes of PLCbeta(2) membrane recruitment may accommodate contrasting functional needs to hydrolyze phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) in localized versus dispersed populations. PLCbeta(2) activation by Rac2, which leads to slow lateral diffusion and much faster exchange, recruits PLCbeta(2) to act locally on PtdInsP(2) at specific domains. Activation by alpha(q) leads to lipid-like diffusion of PLCbeta(2) accompanied by exchange, enabling the sampling of larger, yet limited, areas prior to dissociation. Finally, activation by betagamma recruits PLCbeta(2) to the membrane by transient interactions, leading to fast "surfing" diffusion along the membrane, sampling large regions for dispersed PtdInsP(2) populations.

Characterization of P-Rex1 for its role in fMet-Leu-Phe-induced superoxide production in reconstituted COS(phox) cells

P-Rex1 (phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1) is a Rac-specific guanine nucleotide exchange factor activated by Gbetagamma subunits and by PtdIns((3,4,5))P(3). Recent studies indicate that P-Rex1 plays an important role in signaling downstream of neutrophil chemoattractant receptors. Here we report that heterologous expression of P-Rex1, but not Vav1, reconstitutes formyl peptide receptor 1 (FPR1)-mediated NADPH oxidase activation in the transgenic COS(phox) cells expressing gp91(phox), p22(phox), p67(phox) and p47(phox). A successful reconstitution requires the expression of a full-length P-Rex1 with intact DH and PH domains, and is accompanied by P-Rex1 membrane localization as well as Rac1 activation. P-Rex1-dependent superoxide generation in the reconstituted COS(phox) cells was further enhanced by expression of the novel PKC isoform PKCdelta and by overexpression of Akt. Heterologous expression of P-Rex1 in COS(phox) cells potentiated fMet-Leu-Phe-induced Akt phosphorylation, whereas expression of a constitutively active form of Akt enhanced Rac1 activation. In contrast, a dominant negative Akt mutant reduced the fMet-Leu-Phe stimulated superoxide generation as well as Rac1 activation. These results demonstrate that in COS(phox) cells, P-Rex1 is a critical component for FPR1-mediated signaling leading to NADPH oxidase activation, and there is a crosstalk between the P-Rex1-Rac pathway and Akt in superoxide generation.

Vang-like protein 2 and Rac1 interact to regulate adherens junctions

The Wnt planar cell polarity (Wnt/PCP) pathway signals through small Rho-like GTPases to regulate the cytoskeleton. The core PCP proteins have been mapped to the Wnt/PCP pathway genetically, but the molecular mechanism of their action remains unknown. Here, we investigate the function of the mammalian PCP protein Vang-like protein 2 (Vangl2). RNAi knockdown of Vangl2 impaired cell-cell adhesion and cytoskeletal integrity in the epithelial cell lines HEK293T and MDCK. Similar effects were observed when Vangl2 was overexpressed in HEK293T, MDCK or C17.2 cells. The effects of Vangl2 overexpression could be blocked by knockdown of the small GTPase Rac1 or by dominant-negative Rac1. In itself, knockdown of Rac1 impaired cytoskeletal integrity and reduced cell-cell adhesion. We found that Vangl2 bound and re-distributed Rac1 within the cells but did not alter Rac1 activity. Moreover, both transgenic mouse embryos overexpressing Vangl2 in neural stem cells and loop-tail Vangl2 loss-of-function embryos displayed impaired adherens junctions, a cytoskeletal unit essential for neural tube rigidity and neural tube closure. In vivo, Rac1 was re-distributed within the cells in a similar way to that observed by us in vitro. We propose that Vangl2 affects cell adhesion and the cytoskeleton by recruiting Rac1 and targeting its activity in the cell to adherens junctions.

Regulation of TNF-induced oxygen radical production in human neutrophils: role of delta-PKC

In human neutrophils, TNF-elicited O(2)(-) production requires adherence and integrin activation. How this cooperative signaling between TNFRs and integrins regulates O(2)(-) generation has yet to be fully elucidated. Previously, we identified delta-PKC as a critical early regulator of TNF signaling in adherent neutrophils. In this study, we demonstrate that inhibition of delta-PKC with a dominant-negative delta-PKC TAT peptide resulted in a significant delay in the onset time of TNF-elicited O(2)(-) generation but had no effect on Vmax, indicating an involvement of delta-PKC in the initiation of O(2)(-) production. In contrast, fMLP-elicited O(2)(-) production in adherent and nonadherent neutrophils was delta-PKC-independent, suggesting differential regulation of O(2)(-) production. An important step in activation of the NADPH oxidase is phosphorylation of the cytosolic p47phox component. In adherent neutrophils, TNF triggered a time-dependent association of delta-PKC with p47phox, which was associated with p47phox phosphorylation, indicating a role for delta-PKC in regulating O(2)(-) production at the level of p47phox. Activation of ERK and p38 MAPK is also required for TNF-elicited O(2)(-) generation. TNF-mediated ERK but not p38 MAPK recruitment to p47phox was delta-PKC-dependent. delta-PKC activity is controlled through serine/threonine phosphorylation, and phosphorylation of delta-PKC (Ser643) and delta-PKC (Thr505) was increased significantly by TNF in adherent cells via a PI3K-dependent process. Thus, signaling for TNF-elicited O(2)(-) generation is regulated by delta-PKC. Adherence-dependent cooperative signaling activates PI3K signaling, delta-PKC phosphorylation, and delta-PKC recruitment to p47phox. delta-PKC activates p47phox by serine phosphorylation or indirectly through control of ERK recruitment to p47phox.

OAG induces an additional PKC-, PI3K-, and Rac2-mediated signaling pathway up-regulating NOX2 activity, independently of Ca2+ entry

The requirement of calcium ion (Ca(2)(+)) entry for neutrophil NADPH oxidase (NOX2) regulation is clearly established. However, its role in the signaling pathway leading to NOX2 activation is still elusive. 1-oleoyl-2-acetyl-sn-glycerol (OAG) causes an increase in NOX2 activity and has been shown to directly modulate Ca(2)(+) channels unrelated to the well-known store-operated Ca(2)(+) entry (SOCE) mechanism. In our study, we have investigated the potential role of OAG in Ca(2)(+) influx-mediated NOX2 activity in neutrophil-like-differentiated HL-60 cells to further characterize second signals involved in the regulation of NOX2. OAG inhibited fMLF- and thapsigargin-induced Ca(2)(+) entry, a phenomenon that was not restored by protein kinase C (PKC) or PI3K blockade. Addition of OAG resulted in a rapid decrease of maximal intracellular Ca(2)(+) concentration induced by thapsigargin. Both results suggest that OAG has an inhibitory effect, independent of PI3K and PKC, on the regulation of SOCE. In contrast to SOCE inhibition, OAG-induced NOX2 activation was mediated by PKC and PI3K. Our data establish that both kinases exert their effects through the regulation of Rac2 activity. In addition, OAG potentiated the effect of fMLF on the activation of NOX2 and led to a discernible activity of NOX2 upon thapsigargin stimulation. In conclusion, our results demonstrate that an additional PKC- and/or PI3K-dependent signal may act in synergy with Ca(2)(+) influx to trigger NOX2 activation.

Gene expression analysis of human promyelocytic leukemia HL-60 cell differentiation and cytotoxicity induced by natural and synthetic retinoids

AIMS

This study analyzed gene expression profiles of human promyelocytic leukemia HL-60 cells treated with natural and synthetic retinoids (ATRA, RII and R9158), in an attempt to investigate the structure-function relationship of the retinoids in inducing cell differentiation and cytotoxicity.

MAIN METHODS

Flow cytometry was used to determine cell cycle changes in HL-60 cells following treatment (1.0 muM) with natural and synthetic retinoids (ATRA, RII and R9158), and cDNA microarrays were used to monitor the gene expression profiles of HL-60 cells treated with the various retinoids.

KEY FINDINGS

Consistent with retinoid-induced cell differentiation, treatment with these three retinoids correlated with an increase in the percentage of cells arrested in the G1/G0 phase of the cell cycle. Microarray analysis showed upregulation of known differentiation genes, adhesion molecules, and the oxidase activation pathway following retinoid treatment. Differential expression of several genes was observed in HL-60 cells treated with the three retinoids. For example, tissue remodeling protein genes, ubiquitin genes, and signal transduction genes were highly expressed in ATRA- and R9158-treated HL-60 cells, but remained unchanged in HL-60 cells treated with RII.

SIGNIFICANCE

The above findings suggest that the differentiation of HL-60 cells induced by the three retinoids occurs through similar pathways, and that there exists a structure-function relationship regarding retinoids and the induction of cell differentiation and cytotoxicity.

TNF-alpha-induced up-regulation of intercellular adhesion molecule-1 is regulated by a Rac-ROS-dependent cascade in human airway epithelial cells

Up-regulation of intercellular adhesion molecule-1 (ICAM-1) in the lung airway epithelium is associated with the epithelium-leukocyte interaction, critical for the pathogenesis of various lung airway inflammatory diseases such as asthma. However, little is known about how ICAM-1 is up-regulated in human airway epithelial cells. In this study, we show that tumor TNF-alpha induces monocyte adhesion to A549 human lung airway epithelium and also up-regulation of ICAM-1 expression. These effects were significantly diminished by pre-treatment with diphenyliodonium (DPI), an inhibitor of NADPH oxidase-like flavoenzyme. In addition, the level of reactive oxygen species (ROS) was increased in response to TNF-alpha in A549 cells, suggesting a potential role of ROS in the TNF-alpha-induced signaling to ICAM-1 expression and monocyte adhesion to airway epithelium. Further, we found out that expression of RacN17, a dominant negative mutant of Rac1, suppressed TNF-alpha-induced ROS generation, ICAM-1 expression, and monocyte adhesion to airway epithelium. These findings suggest that Rac1 lies upstream of ROS generation in the TNF-alpha-induced signaling to ICAM-1 expression in airway epithelium. Finally, pretreatment with pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-kappaB, reduced TNF-alpha-induced ICAM-1 expression and both DPI and RacN17 significantly diminished NF-kappaB activation in response to TNF-alpha. Together, we propose that Rac1-ROS-linked cascade mediate TNF-alpha-induced ICAM-1 up-regulation in the airway epithelium via NF-kappaB-dependent manner.

Evolutionary changes of astroglia in Elasmobranchii comparing to amniotes: a study based on three immunohistochemical markers (GFAP, S-100, and glutamine synthetase)

This paper supplements former studies on elasmobranch species with an immunohistochemical investigation into glutamine synthetase and S-100 protein, in addition to GFAP, and extends its scope to the representatives of almost every group of Elasmobranchii: squalomorph sharks, galeomorph sharks, skates (Rajiformes) and rays (Torpediniformes and Myliobatifomes). More glial elements were labeled by S-100 protein, and even more so by using glutamine synthetase immunostaining than by GFAP: more astrocytes (mainly non-perivascular ones) were detected in the telencephalon of sharks, skates and rays. Only the markers S-100 and glutamine synthetase, but not GFAP, characterized the Bergmann-glia of skates and rays and astrocyte-like non-ependymal cells in Squalus acanthias. Another squalomorph shark species, Pristiophorus cirratus, however, had GFAP immunopositive astrocytes. Of all the species studied, the greatest number of GFAP positive astrocytes could be observed in Mobula japanica (order Myliobatiformes), in each major brain part. According to anatomical location, perivascular glia comprised varied types, including even a location in Mobula, which can also be found in mammals. Remnants of radial glia were found in confined areas of skates, less so in rays. In the rhombencephalon and in the spinal cord modified ependymoglia predominated in every group. In conclusion, there was no meaningful difference between the astroglial architectures of squalomorph and galeomorph sharks. The difference in the astroglial structure between sharks and batoids, however, was confined to the telencephalon and mesencephalon, and did not take place in the rhombencephalon, the latter structure being quite similar in all the species studied. The appearance of astrocytes in the relatively thin-walled shark telencephalon, however, indicates that the brain thickening promoted the preponderance of astrocytes rather than their appearance itself. Although the evolutionary changes of astroglia had some similarities in Elasmobranchii and Amniota, there was one meaningful difference: in Elasmobranchii astrocytes did not prevail in conservative brain regions as they did in the progressive brain regions.

Phosphoinositide 3-Kinase γ Inhibition Plays a Crucial Role in Early Steps of Inflammation by Blocking Neutrophil Recruitment

Leukocyte trafficking to inflammatory sites is a gradual process, which is dominated in its early phases by chemokine- and cytokine-mediated neutrophil recruitment. The chemokine regulated on activation normal T cell expressed and secreted (RANTES) has been shown to be highly expressed in the joints of patient with rheumatoid arthritis and to promote leukocyte trafficking into the synovial tissue. In this study, we investigated the effect of RANTES in a murine model of peritoneal chemotaxis, and we found that RANTES dose-dependently induces neutrophil recruitment. Then, through morphological and histological analyses, we observed that activated neutrophils represent the major infiltrating population in response to RANTES chemotactic stimulus. Furthermore, we demonstrated that oral administration of either nonisoform-specific phosphoinositide 3-kinase (PI3K) inhibitor LY294002 (morpholin-4-yl-8-phenylchromen-4-one) or selective PI3Kgamma inhibitor AS041164 (5-benzo[1,3]dioxol-5-ylmethylene-thiazolidine-2,4-dione) blocks RANTES-induced chemotaxis and reduces the level of AKT phosphorylation. Because the two compounds showed a similar pharmacokinetic profile in terms of bioavailability and half-life after oral route administration, the selective inhibition of the PI3Kgamma-isoform pathway through AS041164 was three times more potent in reducing neutrophil recruitment. Finally, to confirm the blockade of neutrophil infiltration that occurs in the early phase of the inflammatory response, AS041164 was also tested in a model of carrageenan-induced paw edema in rats. Therefore, the PI3Kgamma pathway plays an important role in controlling neutrophil chemotaxis during early steps of inflammation.

Role of the small GTPase Rac in p22phox-dependent NADPH oxidases

The superoxide-producing phagocyte NADPH oxidase gp91(phox)/Nox2 and the non-phagocytic oxidases Nox1 and Nox3 each form a complex in the membrane with p22(phox), which provides both stabilization and a docking site for organizer proteins. The p22(phox)-complexed Nox2 and Nox1 are dormant on their own, and their activation requires soluble supportive proteins such as a Nox organizer (p47(phox) or Noxo1) and a Nox activator (p67(phox) or Noxa1). The small GTPase Rac directly binds to the activators, and thus plays an essential role in the Nox2-based oxidase containing p47(phox) and p67(phox) or a positive role in Nox1 activity supported by Noxo1 and Noxa1. Although Nox3 complexed with p22(phox) constitutively produce superoxide, the production can be enhanced by supportive proteins. Here we compare the roles of Rac in these p22(phox)-dependent oxidases using the organizer and activator in different combinations. Expression of constitutively active Rac1(Q61L) is essential for activation of the Nox2- or Nox1-based oxidase containing the organizer p47(phox) and either p67(phox) or Noxa1. When these oxidases use Noxo1 as an organizer instead of p47(phox), they produce a small but significant amount of superoxide without expression of Rac1(Q61L), although the production is enhanced by Rac1(Q61L). Thus p47(phox) is likely related to strict dependence on Rac. The Nox3-based oxidase has a similar tendency in the change of the dependence: Rac plays a positive role in Nox3 activation in the presence of p47(phox) and either p67(phox) or Noxa1, whereas Rac fails to upregulate Nox3 activity when p47(phox) is replaced with Noxo1. We also demonstrate that, in the Nox3-based oxidase containing solely p67(phox) as supportive protein, expression of Rac1(Q61L) enhances not only superoxide production but also membrane translocation of p67(phox). Since the enhancements are not observed with a mutant p67(phox) defective in binding to Rac, this GTPase appear to directly recruit p67(phox) to the membrane.

Caldesmon is a cytoskeletal target for PKC in endothelium

We have previously shown that treatment of bovine endothelial cell (EC) monolayers with phorbol myristate acetate (PMA) leads to the thinning of cortical actin ring and rearrangement of the cytoskeleton into a grid-like structure, concomitant with the loss of endothelial barrier function. In the current work, we focused on caldesmon, a cytoskeletal protein, regulating actomyosin interaction. We hypothesized that protein kinase C (PKC) activation by PMA leads to the changes in caldesmon properties such as phosphorylation and cellular localization. We demonstrate here that PMA induces both myosin and caldesmon redistribution from cortical ring into the grid-like network. However, the initial step of PMA-induced actin and myosin redistribution is not followed by caldesmon redistribution. Co-immunoprecipitation experiments revealed that short-term PMA (5 min) treatment leads to the weakening of caldesmon ability to bind actin and, to the lesser extent, myosin. Prolonged incubation (15-60 min) with PMA, however, strengthens caldesmon complexes with actin and myosin, which correlates with the grid-like actin network formation. PMA stimulation leads to an immediate increase in caldesmon Ser/Thr phosphorylation. This process occurs at sites distinct from the sites specific for ERK1/2 phosphorylation and correlates with caldesmon dissociation from the actomyosin complex. Inhibition of ERK-kinase MEK fails to abolish grid-like structure formation, although reducing PMA-induced weakening of the cortical actin ring, whereas inhibition of PKC reverses PMA-induced cytoskeletal rearrangement. Our results suggest that PKC-dependent phosphorylation of caldesmon is involved in PMA-mediated complex cytoskeletal changes leading to the EC barrier compromise.

Rac and Rap GTPase activation assays

The detection of Ras superfamily GTPase activity in neutrophils is important when studying signaling events elicited by various ligands and cellular processes. Substantial progress in monitoring GTPase activation has been made in recent years by the development of high-affinity probes for small GTPases. These probes are created by fusing a high-affinity GTPase-binding domain derived from a specific downstream effector protein to glutathione-S-transferase (GST). Such domains bind preferentially to the GTP-bound form of specific Rho or Ras GTPases. Coupling these probes to beads enables extraction of the complex and subsequent quantification of active GTP-binding protein by immunoblotting. Although effector domains that discriminate efficiently between GDP- and GTP-bound states and highly specific antibodies are not yet available for many small GTPases, analysis of certain members of the Rho and Ras GTPase family are now routinely performed. Here, we describe affinity-based pull-down assays for detection of Rac/Cdc42 and Rap activity in stimulated neutrophils.

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.

Phosphoinositides and Rho proteins spatially regulate actin polymerization to initiate and maintain directed movement in a one-dimensional model of a motile cell

Gradient sensing, polarization, and chemotaxis of motile cells involve the actin cytoskeleton, and regulatory modules, including the phosphoinositides (PIs), their kinases/phosphatases, and small GTPases (Rho proteins). Here we model their individual components (PIP1, PIP2, PIP3; PTEN, PI3K, PI5K; Cdc42, Rac, Rho; Arp2/3, and actin), their interconversions, interactions, and modular functions in the context of a one-dimensional dynamic model for protrusive cell motility, with parameter values derived from in vitro and in vivo studies. In response to a spatially graded stimulus, the model produces stable amplified internal profiles of regulatory components, and initiates persistent motility (consistent with experimental observations). By connecting the modules, we find that Rho GTPases work as a spatial switch, and that the PIs filter noise, and define the front versus back. Relatively fast PI diffusion also leads to selection of a unique pattern of Rho distribution from a collection of possible patterns. We use the model to explore the importance of specific hypothesized interactions, to explore mutant phenotypes, and to study the role of actin polymerization in the maintenance of the PI asymmetry. We also suggest a mechanism to explain the spatial exclusion of Cdc42 and PTEN and the inability of cells lacking active Cdc42 to properly detect chemoattractant gradients.

Involvement of p38 MAP Kinase in Not Only Activation of the Phagocyte NADPH Oxidase Induced by Formyl-methionyl-leucyl-phenylalanine but Also Determination of the Extent of the Activity

Activated NADPH oxidase in neutrophils produces superoxide. We investigated the role of p38 MAP kinase in activating NADPH oxidase stimulated by the bacteria-derived peptide fMLP. fMLP-stimulated superoxide production was completely abolished by SB203580, a p38 MAP kinase inhibitor, whereas anisomycin, a p38 MAP kinase activator, did not induce superoxide production, indicating that p38 MAP kinase was essential, but not sufficient, for NADPH oxidase activation. Anisomycin pretreatment strongly activated p38 MAP kinase in fMLP-stimulated cells, accompanied by greatly increased superoxide production, suggesting that p38 MAP kinase determines the extent of the fMLP-stimulated NADPH oxidase activity. Furthermore, superoxide production was remarkably reactivated by cytochalasin B addition after fMLP-stimulated production had disappeared, and this was correlated with highly activated p38 MAP kinase. These results suggest that p38 MAP kinase is involved not only in activating NADPH oxidase stimulated by fMLP but also in determining the extent of its activity.

NADPH oxidases in the gastrointestinal tract: a potential role of Nox1 in innate immune response and carcinogenesis

The gastrointestinal epithelium functions as physical and innate immune barriers against commensal or pathogenic microbes. NADPH oxidase 1 (Nox1) and dual oxidase 2 (Duox2), highly expressed in the colon, are suggested to play a potential role in host defense. Guinea-pig gastric pit cells and human colonic epithelial cells (T84 cells) express Nox1. With regard to activation of Nox1, the gastric epithelial cells are primed with Helicobacter pylori lipopolysaccharide, whereas T84 cells preferentially use the Toll-like receptor (TLR) 5, rather than TLR4, against Salmonella enteritidis infection. Thus, gastric and colonic epithelial cells may use different TLR members to discern pathogenicities among bacteria, depending on their environments and to activate Nox1 appropriately for host defense. Nox1-derived reactive oxygen species (ROS) have been implicated in the pathogenesis of inflammation-associated tumor development. The human stomach does not express Nox1. Helicobacter pylori infection alone does not induce it, whereas Nox1 is specifically expressed in gastric adenocarcinomas. In the human colon, Nox1 is differentiation-dependently expressed, and its expression is upregulated in adenomas and well-differentiated adenocarcinomas. Although Nox1 expression may not be directly linked to mitogenic activity, Nox1-derived ROS may exert a cancer-promoting effect by increasing resistance to programmed cell death of tumor cells.

Direct involvement of the small GTPase Rac in activation of the superoxide-producing NADPH oxidase Nox1

Activation of the non-phagocytic superoxide-producing NADPH oxidase Nox1, complexed with p22(phox) at the membrane, requires its regulatory soluble proteins Noxo1 and Noxa1. However, the role of the small GTPase Rac remained to be clarified. Here we show that Rac directly participates in Nox1 activation via interacting with Noxa1. Electropermeabilized HeLa cells, ectopically expressing Nox1, Noxo1, and Noxa1, produce superoxide in a GTP-dependent manner, which is abrogated by expression of a mutant Noxa1(R103E), defective in Rac binding. Superoxide production in Nox1-expressing HeLa and Caco-2 cells is decreased by depletion or sequestration of Rac; on the other hand, it is enhanced by expression of the constitutively active Rac1(Q61L), but not by that of a mutant Rac1 with the A27K substitution, deficient in binding to Noxa1. We also demonstrate that Nox1 activation requires membrane recruitment of Noxa1, which is normally mediated via Noxa1 binding to Noxo1, a protein tethered to the Nox1 partner p22(phox): the Noxa1-Noxo1 and Noxo1-p22(phox) interactions are both essential for Nox1 activity. Rac likely facilitates the membrane localization of Noxa1: although Noxa1(W436R), defective in Noxo1 binding, neither associates with the membrane nor activates Nox1, the effects of the W436R substitution are restored by expression of Rac1(Q61L). The Rac-Noxa1 interaction also serves at a step different from the Noxa1 localization, because the binding-defective Noxa1(R103E), albeit targeted to the membrane, does not support superoxide production by Nox1. Furthermore, a mutant Noxa1 carrying the substitution of Ala for Val-205 in the activation domain, which is expected to undergo a conformational change upon Rac binding, fully localizes to the membrane but fails to activate Nox1.

Activation of phosphoinositide 3-kinase and Src family kinase is required for respiratory burst in rat neutrophils stimulated with artocarpol A

Artocarpol A (ART), a natural product isolated from Artocarpus rigida, stimulated superoxide anion (O2*-) generation, which was inhibited by 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY 294002), a phosphoinositide 3-kinase (PI3K) inhibitor, in rat neutrophils. ART stimulated phosphorylation of protein kinase B (PKB/Akt) on both T308 and S473 residues, and LY 294002 inhibited these effects. Rat neutrophils expressed both class IA PI3K subunits (p85, p110alpha, p110beta, and p110delta) and a class IB PI3K subunit (p110gamma) as assessed by a combination of Western blotting and reverse transcription-polymerase chain reaction (RT-PCR) approaches. Stimulation of neutrophils with ART evoked phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) formation, which reached a maximal level at 2 min and was attenuated by LY 294002, as evidenced by immunofluorescence microscopy and by flow cytometry. Detectable membrane-association of class IA PI3Ks, class IB PI3K and Ras was seen as early as 1.5, 0.5 and 1.5 min, respectively, after stimulation with ART. The kinetics of ART-induced Ras activation paralleled the kinetics of class IA PI3Ks recruitment to membrane caused by ART, and the p85 and p110gamma immunoprecipitates contain Ras. ART stimulated Src family kinase activation, which was detectable within 1.5 min of incubation with ART. Both Src kinase activity and PtdIns(3,4,5)P3 formation in ART-stimulated neutrophils were inhibited by 4-amino-1-tert-butyl-3-(1'-naphthyl)pyrazolo[3,4-d]pyrimidine (PP1 analog). PP1 analog also attenuated the ART-stimulated O2*- generation in rat neutrophils. These results indicate that the stimulation of respiratory burst by ART in neutrophils implicates PI3K signaling.

The superoxide-producing NAD(P)H oxidase Nox4 in the nucleus of human vascular endothelial cells

The superoxide-producing NAD(P)H oxidase Nox4 was initially identified as an enzyme that is highly expressed in the kidney and is possibly involved in oxygen sensing and cellular senescence. Although the oxidase is also abundant in vascular endothelial cells, its role remains to be elucidated. Here we show that Nox4 preferentially localizes to the nucleus of human umbilical vein endothelial cells (HUVECs), by immunocytochemistry and immunoelectron microscopy using three kinds of affinity-purified antibodies raised against distinct immunogens from human Nox4. Silencing of Nox4 by RNA interference (RNAi) abrogates nuclear signals given with the antibodies, confirming the nuclear localization of Nox4. The nuclear fraction of HUVECs exhibits an NAD(P)H-dependent superoxide-producing activity in a manner dependent on Nox4, which activity can be enhanced upon cell stimulation with phorbol 12-myristate 13-acetate. This stimulant also facilitates gene expression as estimated in the present transfection assay of HUVECs using a reporter regulated by the Maf-recognition element MARE, a DNA sequence that constitutes a part of oxidative stress response. Both basal and stimulated transcriptional activities are impaired by RNAi-mediated Nox4 silencing. Thus Nox4 appears to produce superoxide in the nucleus of HUVECs, thereby regulating gene expression via a mechanism for oxidative stress response.

Effect of Agkistroden blomhoffi (mamushi) on superoxide generation in human neutrophils

BACKGROUND

The purpose of this study is to investigate the effect of Agkistroden blomhoffi (mamushi) aqueous extract on stimulus-induced superoxide generation in human neutrophils.

METHODS

Superoxide generation was assayed by measuring the reduction of ferricytochrome C (cyt c) using a dual-beam spectrophotometer. The phosphorylation of neutrophil proteins and the translocation of cytosolic compounds to the cell membrane were detected using specific monoclonal antibodies.

RESULTS

Mamushi aqueous extract markedly suppressed fMLP-induced superoxide generation in a concentration-dependent manner. fMLP-induced translocation of cytosolic compounds p47phox, p67phox and Rac to the cell membrane was suppressed in parallel to the suppression of superoxide generation. The fractions with MW below 10 kDa dose-dependently suppressed tyrosyl phosphorylation of 45.0, 77.6 and 99.5 kDa proteins in fMLP-treated human neutrophils; the inhibition of tyrosyl phosphorylation was in parallel to that of the fMLP-induced translocation of p47phox to the cell membrane and superoxide generation.

CONCLUSIONS

Mamushi aqueous extract markedly suppressed fMLP-induced superoxide generation in human neutrophils and also suppressed the translocation of cytosolic compounds to the cell membrane and tyrosyl phosphorylation of proteins. The efficacious components are the fractions with MW below 10 kDa. O-phosphoserylethanolamine, a new dipeptide isolated from mamushi, also suppressed fMLP-induced superoxide generation in human neutrophils in a dose-dependent manner.

WASP family proteins act between cytoskeleton and cellular signaling pathways

This review considers the proteins of the WASP (Wiskott-Aldrich syndrome protein) family and their role in the regulation of actin-based motility. It contains detailed classification of the WASP family proteins and data on their subcellular localization. Impairments of expression of the WASP family proteins cause certain cell pathologies. The review also deals with domain organization of these proteins and proteins interacting with various domains of the WASP proteins. Special attention is given to analysis of the role of the WASP family proteins in initiating directed actin assembly in the leading edge of the migrating cell and on the surface of some bacteria. Putative pathways of regulation of WASP proteins by various protein ligands and their links with cell signaling systems are considered.

Ca2+-dependent regulation of rho GTPases triggers turning of nerve growth cones

Cytoplasmic Ca2+ elevation and changes in Rho GTPase activity are both known to mediate axon guidance by extracellular factors, but the causal relationship between these two events has been unclear. Here we show that direct elevation of cytoplasmic Ca2+ by extracellular application of a low concentration of ryanodine, which activated Ca2+ release from intracellular stores, upregulated Cdc42/Rac, but downregulated RhoA, in cultured cerebellar granule cells and human embryonic kidney 293T cells. Chemoattractive turning of the growth cone triggered by a gradient of ryanodine was blocked by overexpression of mutant forms of Cdc42 but not of RhoA in Xenopus spinal cord neurons. Furthermore, Ca2+-induced GTPase activity correlated with activation of protein kinase C and required a basal activity of Ca2+/calmodulin-dependent protein kinase II. Thus, Rho GTPases may mediate axon guidance by linking upstream Ca2+ signals triggered by guidance factors to downstream cytoskeletal rearrangements.

Activation of Rho and Rac by Wnt/frizzled signaling

Wnt/Frizzled (Fz) signaling controls developmental, physiological, and pathological processes by means of several distinct pathways. Wnt/Fz activation of the small GTPases Rho and Rac has emerged as a key mechanism that regulates cell polarity and movements during vertebrate gastrulation. In this chapter, we describe biochemical assays for the detection of Wnt/Fz activation of Rho and Rac in both mammalian cells and Xenopus embryo explants.

ARAP3 is essential for formation of lamellipodia after growth factor stimulation

Rho and Arf family small GTPases control dynamic actin rearrangements and vesicular trafficking events. ARAP3 is a dual GAP for RhoA and Arf6 that is regulated by phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P3], a product of the phosphoinositide 3-kinase (PI3K) signalling pathway. To investigate the physiological function of ARAP3, we used an RNAi-based approach in an endothelial cell model. ARAP3-deficient cells showed increased activities of RhoA and Arf6. Phenotypically, they were more rounded than control counterparts and displayed very fine stress fibres. ARAP3-deficient cells were not capable of producing lamellipodia upon growth factor stimulation, a process known to depend on PI3K and Rac activities. Rac was transiently activated in stimulated ARAP3 RNAi cells although its cellular localisation was altered, a likely consequence of increased Arf6 activity. We conclude that ARAP3 recruitment to sites of elevated PtdIns(3,4,5)P3 is crucial to allow localised inactivation of RhoA and cycling of Arf6, both of which are necessary to allow growth factor-stimulated formation of lamellipodia.

G protein-coupled receptors serve as mechanosensors for fluid shear stress in neutrophils

Many cells respond to fluid shear stress but in a cell type-specific fashion. Fluid shear stress applied to leukocytes serves to control pseudopod formation, migration, and other functions. Specifically, fresh neutrophils or neutrophilic leukocytes derived from differentiated HL60 cells respond to fluid shear stress by cytoplasmic pseudopod retraction. The membrane elements that sense fluid shear and induce such a specific response are still unknown, however. We hypothesized that membrane receptors may serve as fluid shear sensors. We found that fluid shear decreased the constitutive activity of G protein-coupled receptors (GPCRs). Inhibition of GPCR constitutive activity by inverse agonists abolished fluid shear stress-induced cell area reduction. Among the GPCRs in neutrophils, the formyl peptide receptor (FPR) exhibits relatively high constitutive activity. Undifferentiated HL60 cells that lacked FPR formed few pseudopods and showed no detectable response to fluid shear stress, whereas expression of FPR in undifferentiated HL60 cells caused pseudopod projection and robust pseudopod retraction during fluid shear. FPR small interfering RNA-transfected differentiated HL60 cells exhibited no response to fluid shear stress. These results suggest that GPCRs serve as mechanosensors for fluid shear stress in neutrophils by decreasing its constitutive activity and reducing pseudopod projection.

Hem-1 complexes are essential for Rac activation, actin polymerization, and myosin regulation during neutrophil chemotaxis

Migrating cells need to make different actin assemblies at the cell's leading and trailing edges and to maintain physical separation of signals for these assemblies. This asymmetric control of activities represents one important form of cell polarity. There are significant gaps in our understanding of the components involved in generating and maintaining polarity during chemotaxis. Here we characterize a family of complexes (which we term leading edge complexes), scaffolded by hematopoietic protein 1 (Hem-1), that organize the neutrophil's leading edge. The Wiskott-Aldrich syndrome protein family Verprolin-homologous protein (WAVE)2 complex, which mediates activation of actin polymerization by Rac, is only one member of this family. A subset of these leading edge complexes are biochemically separable from the WAVE2 complex and contain a diverse set of potential polarity-regulating proteins. RNA interference-mediated knockdown of Hem-1-containing complexes in neutrophil-like cells: (a) dramatically impairs attractant-induced actin polymerization, polarity, and chemotaxis; (b) substantially weakens Rac activation and phosphatidylinositol-(3,4,5)-tris-phosphate production, disrupting the (phosphatidylinositol-(3,4,5)-tris-phosphate)/Rac/F-actin-mediated feedback circuit that organizes the leading edge; and (c) prevents exclusion of activated myosin from the leading edge, perhaps by misregulating leading edge complexes that contain inhibitors of the Rho-actomyosin pathway. Taken together, these observations show that versatile Hem-1-containing complexes coordinate diverse regulatory signals at the leading edge of polarized neutrophils, including but not confined to those involving WAVE2-dependent actin polymerization.