Function of PI3Kgamma in thymocyte development, T cell activation, and neutrophil migration

Role of defective ERK phosphorylation in the impaired GM-CSF-induced oxidative response of neutrophils in elderly humans

GM-CSF-induced oxidative responses are defective in neutrophils of elderly humans. In the present study we evaluated whether this phenomenon might be related to alterations in cytokine-dependent MAPK signalling. Neutrophils obtained from elderly humans and stimulated with GM-CSF showed a significant reduction in phosphorylated ERK1/2 levels and an even higher decrease in ERK1/2 activation with respect to baseline. No changes in GM-CSF-induced p38 MAPK phosphorylation were observed. Cell pretreatment with the MEK inhibitor PD98059 determined a marked suppression of GM-CSF-induced O2- release. Interestingly, under the above experimental condition, there was no longer any difference in O2- production observed between elderly and young subjects. Furthermore, despite the fact that the p38 MAPK pathway was activated less strongly by GM-CSF, the p38 MAPK inhibitor SB203580 reduced GM-CSF-induced O2- production in the neutrophils of the elderly to levels similar to those obtained with PD98059. TNF-alpha-triggered O2- production was not altered by ageing and in fact, a similar ERK1/2 or p38 MAPK activation was found in TNF-alpha-stimulated neutrophils from elderly and young individuals. In accordance with the different potency of TNF-alpha in activating ERK1/2 and p38 MAPK, the TNF-alpha-induced oxidative responses were more sensitive to the inhibitory effects of SB203580 than to those of PD98059 in young as well as elderly subjects. These results suggest that, along the GM-CSF-dependent ERK signalling pathway, a step proximal to MEK1/2 but distal to the connection with the p38 MAPK module likely becomes defective as a feature of age. The consequent decline in ERK1/2 activation could potentially account for the GM-CSF-dependent impairment of the neutrophil respiratory burst that occurs with ageing.

Involvement of phosphoinositide 3-kinase gamma, Rac, and PAK signaling in chemokine-induced macrophage migration

In macrophages, chemotactic stimuli cause the activation of Rac and PAK, but little is known about the signaling pathways involved and their role in chemotactic gradient sensing. Herein, we report that in macrophages, the chemokine RANTES (regulated on activation normal T cell expressed and secreted)/CCL5 activates the small GTPase Rac and its downstream target PAK2 within seconds. This response depends on Gi activation and largely on the subsequent triggering of phosphoinositide 3-kinase gamma (PI3Kgamma) and Rac. Retroviral transduction of tagged Rac1 and -2 indicates that RANTES/CCL5-mediated activation of PI3Kgamma triggers Rac1 but not Rac2. In agreement, silencing of Rac1 by shRNA blocks PAK2 activity and inhibits RANTES/CCL5-induced macrophage polarization and directional migration. On the other hand, the tyrosine kinase receptor agonist CSF-1 activates PAK2 independently of PI3Kgamma and Rac. Our results thus demonstrate a chemokine-specific signaling pathway in which Gi and PI3Kgamma coordinate to drive Rac1 and PAK2 activation that eventually controls the chemotactic response.

Exclusion of exogenous phosphatidylinositol-3,4,5-trisphosphate from neutrophil-polarizing pseudopodia: stabilization of the uropod and cell polarity

Although there is accumulating evidence that the generation and localization of phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) have important functions in neutrophil polarization and chemotaxis, the mechanism of this linkage has yet to be established. Here, using exogenous fluorescent PtdIns(3,4,5)P(3) introduced into the inner leaflet of the neutrophil plasma membrane by a cationic carrier, we show that: first, PtdIns(3,4,5)P(3) uniformly delivered to the neutrophil plasma membrane is excluded from newly forming pseudopodia; second, PtdIns(3,4,5)P(3) translocates to and is immobilized at the pole opposite a stable polarizing pseudopod; third, asymmetric delivery of PtdIns(3,4,5)P(3) to the neutrophil triggers the generation of polarizing pseudopodia at the opposite pole; and finally, PtdIns(3,4,5)P(3) triggers repetitive Ca(2+) signals, the onset of which precedes morphological polarization. These data suggest that translocation and immobilization of PtdIns(3,4,5)P(3) or a 3,x-phosphorylated metabolite in the uropod functions as an important polarization cue that defines neutrophil polarity and stabilizes the generation of pseudopodia at the opposite pole.

Roles of Rho-family GTPases in cell polarisation and directional migration

Polarised cell migration is a tightly regulated process that occurs in tissue development, chemotaxis and wound healing. Rho-family GTPases, including Cdc42, Rac1 and RhoA, play a central role in establishing cell polarisation, which requires asymmetric and ordered distribution of the signalling molecules and the cytoskeleton. Recent advances reveal that Rho GTPases, together with phosphatidylinositol 3-kinase, contribute to asymmetric phosphatidylinositol 3,4,5-trisphosphate distribution via a positive-feedback loop. Phosphatidylinositol 3,4,5-trisphosphate thereby activates the signalling cascades to the cytoskeleton as a second messenger. Rho GTPases also capture and stabilise microtubules through their effectors (e.g. IQGAP1, mDia and Par6) near the cell cortex, leading to polarised cell morphology and directional cell migration. Thus, elucidation of the signal transduction cascades from receptors to Rho GTPases and, subsequently, from Rho GTPases to microtubules has begun.

Activation of Phosphoinositide 3-Kinases by the CCR4 Ligand Macrophage-Derived Chemokine Is a Dispensable Signal for T Lymphocyte Chemotaxis

Macrophage-derived chemokine (MDC/CC chemokine ligand 22 (CCL22)) mediates its cellular effects principally by binding to its receptor CCR4, and together they constitute a multifunctional chemokine/receptor system with homeostatic and inflammatory roles in the body. We report the CCL22-induced accumulation of phosphatidylinositol-(3,4,5)-trisphosphate (PI(3,4,5)P(3)) in the leukemic T cell line CEM. CCL22 also had the ability to chemoattract human Th2 cells and CEM cells in a pertussis toxin-sensitive manner. Although the PI(3,4,5)P(3) accumulation along with the pertussis toxin-susceptible phosphorylation of protein kinase B were sensitive to the two phosphoinositide 3-kinase inhibitors, LY294002 and wortmannin, cell migration was unaffected. However, cell migration was abrogated with the Rho-dependent kinase inhibitor, Y-27632. These data demonstrate that although there is PI(3,4,5)P(3) accumulation downstream of CCR4, phosphoinositide 3-kinase activity is a dispensable signal for CCR4-stimulated chemotaxis of Th2 cells and the CEM T cell line.

Mouse models to study G-protein-mediated signaling

The G-protein-mediated signaling system has evolved as one of the most widely used transmembrane signaling mechanisms in eukaryotic organisms. Mammalian cells express many G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. This review focuses on recent data from studies in mutant mice, which have elucidated some of the roles of G-protein-mediated signaling in physiology and pathophysiology.

Phosphoinositide 3-kinase regulates the phosphorylation of NADPH oxidase component p47(phox) by controlling cPKC/PKCdelta but not Akt

Superoxide production by NADPH oxidase is essential for the bactericidal properties of phagocytes. Phosphorylation of p47(phox), one of the cytosolic components of NADPH oxidase, is a crucial step of the oxidase activation. Some evidences suggest that phosphoinositide 3-kinase (PI3K) is involved in p47(phox) phosphorylation, but it has not been fully understood how PI3K regulates it. The aim of this study was to examine the mechanism underlying the PI3K regulation of p47(phox) phosphorylation. Pharmacological inhibition of PI3K attenuated both fMLP-stimulated p47(phox) phosphorylation and NADPH oxidase activity in HL-60 cells differentiated to a neutrophil-like phenotype. Although fMLP elicited Akt activation in a PI3K-dependent manner, an Akt inhibitor had no effect on the oxidase activity triggered by fMLP. In vitro kinase assay revealed that Akt was unable to catalyze p47(phox) phosphorylation. Interestingly, the activation of cPKC and PKCdelta after fMLP stimulation was dependent on PI3K. Furthermore, PI3K inhibitors reduced the activation of phospholipase Cgamma2 without affecting tyrosine phosphorylation on it. These results suggest that PI3K regulates the phosphorylation of NADPH oxidase component p47(phox) by controlling diacylglycerol-dependent PKCs but not Akt.

Deficient phospholipase C activity in blood polimorphonuclear neutrophils from patients with liver cirrhosis

BACKGROUND/AIMS

Circulating neutrophils from cirrhotic patients have a reduced capacity to generate superoxide anion (O(2)(-)), which might contribute to frequent bacterial infections in these patients. We studied the signal transduction pathways involved in the generation of O(2)(-) in neutrophils from 98 cirrhotic patients and 46 healthy controls.

METHODS

We measured O(2)(-) production in neutrophils induced by fMLP, opsonized zymosan, TNF alpha, NaF, AlF(4)(-), A23187 and phorbol myristate acetate. Furthermore, we measured phospholipase C activity in neutrophils from healthy controls and end-stage cirrhotic patients.

RESULTS

O(2)(-) production was decreased in neutrophils from patients in response to fMLP, opsonized zymosan and TNF alpha. Likewise, response of these cells to G-protein stimulation by fluorides was also decreased. These reduced responses correlated significantly with the degree of liver dysfunction. On the contrary, neutrophils from patients responded normally to A23187 and phorbol esters stimulation indicating that Ca(2+)- and PKC-dependent pathways are intact in these cells. Finally, phospholipase C activity was markedly reduced in neutrophils from end-stage liver cirrhosis.

CONCLUSIONS

These data confirm that O(2)(-) generation by neutrophils is decreased in patients with cirrhosis, particularly in those with more severe liver dysfunction, and suggest that this defect involves phosphatidylinositol specific phospholipase C activity.

Interleukin-7 inactivates the pro-apoptotic protein Bad promoting T cell survival

Interleukin-7 (IL-7) is a cytokine that is required for T cell development and survival. The anti-apoptotic function of IL-7 is partly through induction of Bcl-2 synthesis and cytosolic retention of Bax. Here we show that the Bcl-2 homology 3 domain-only protein, Bad, is involved in cell death following IL-7 withdrawal from D1 cells, an IL-7-dependent murine thymocyte cell line. IL-7 stimulation resulted in the inactivation of Bad by phosphorylation at Ser-112, -136, and -155. The phosphoinositide 3-kinase (PI3K)/Akt pathway has been implicated previously in Bad phosphorylation. In response to IL-7, the PI3K/Akt pathway induced phosphorylation at Ser-136 and -155, but Ser-112 was partly independent of the PI3K/Akt pathway, indicating an as yet unknown pathway in this response. Following IL-7 withdrawal, dephosphorylated Bad translocated from cytosol to mitochondria, bound to Bcl-2, and accelerated cell death. Thus, the inactivation of Bad contributes to the survival function of IL-7.

Therapy for chronic obstructive pulmonary disease in the 21st century

Chronic obstructive pulmonary disease (COPD) is a common, smoking-related, severe respiratory condition characterised by progressive, irreversible airflow limitation. Current treatment of COPD is symptomatic, with no drugs capable of halting the relentless progression of airflow obstruction. Better understanding of the airway inflammation, oxidative stress and alveolar destruction that characterise COPD has delineated new disease targets, with consequent identification of novel compounds with therapeutic potential. These new drugs include aids to smoking cessation (e.g. bupropion) and improvements to existing therapies, for example long-acting rather than short-acting bronchodilators, as well as combination therapy. New antiproteases include acyl-enzyme and transition state inhibitors of neutrophil elastase (e.g. sivelestat and ONO-6818), matrix metalloprotease inhibitors (e.g. batimastat), cathepsin inhibitors and peptide protease inhibitors (e.g. DX-890 [EPI-HNE-4] and trappin-2). New antioxidants include superoxide dismutase mimetics (e.g. AEOL-10113) and spin trap compounds (e.g. N-tert-butyl-alpha-phenylnitrone). New anti-inflammatory interventions include phosphodiesterase-4 inhibitors (e.g. cilomilast), inhibitors of tumour necrosis factor-alpha (e.g. humanised monoclonal antibodies), adenosine A(2a) receptor agonists (e.g. CGS-21680), adhesion molecule inhibitors (e.g. bimosiamose [TBC1269]), inhibitors of nuclear factor-kappaB (e.g. the naturally occurring compounds hypoestoxide and (-)-epigallocatechin-3-gallate) and activators of histone deacetylase (e.g. theophylline). There are also selective inhibitors of specific extracellular mediators such as chemokines (e.g. CXCR2 and CCR2 antagonists) and leukotriene B(4) (e.g. SB201146), and of intracellular signal transduction molecules such as p38 mitogen activated protein kinase (e.g. RWJ67657) and phosphoinositide 3-kinase. Retinoids may be one of the few potential treatments capable of reversing alveolar destruction in COPD, and a number of compounds are in clinical trial (e.g. all-trans-retinoic acid). Talniflumate (MSI-1995), an inhibitor of human calcium-activated chloride channels, has been developed to treat mucous hypersecretion. In addition, the purinoceptor P2Y(2) receptor agonist diquafosol (INS365) is undergoing clinical trials to increase mucus clearance. The challenge to transferral of these new compounds from preclinical research to disease management is the design of effective clinical trials. The current scarcity of well characterised surrogate markers predicts that long-term studies in large numbers of patients will be needed to monitor changes in disease progression.

Granulocyte-macrophage colony-stimulating factor is a chemoattractant cytokine for human neutrophils: involvement of the ribosomal p70 S6 kinase signaling pathway

GM-CSF stimulates proliferation of myeloid precursors in bone marrow and primes mature leukocytes for enhanced functionality. We demonstrate that GM-CSF is a powerful chemotactic and chemokinetic agent for human neutrophils. GM-CSF-induced chemotaxis is time dependent and is specifically neutralized with Abs directed to either the ligand itself or its receptor. Maximal chemotactic response was achieved at approximately 7 nM GM-CSF, and the EC(50) was approximately 0.9 nM. Both concentrations are similar to the effective concentrations of IL-8 and less than the effective concentrations of other neutrophil chemoattractants such as neutrophil-activating peptide-78, granulocyte chemotactic protein-2, leukotriene B(4), and FMLP. GM-CSF also acts as a chemoattractant for native cells bearing the GM-CSF receptor, such as monocytes, as well as for GM-CSF receptor-bearing myeloid cell lines, HL60 (promyelomonocyte leukemic cell line) and MPD (myeloproliferative disorder cell line), following differentiation induction. GM-CSF induced a rapid, transient increase in F-actin polymerization and the formation of focal contact rings in neutrophils, which are prerequisites for cell migration. The mechanism of GM-CSF-induced chemotaxis appears to involve the cell signaling molecule, ribosomal p70 S6 kinase (p70S6K). Both p70S6K enzymatic activity and T(421)/S(424) and T(389) phosphorylation are markedly increased with GM-CSF. In addition, the p70S6K inhibitor hamartin transduced into cells as active protein, interfered with GM-CSF-dependent migration, and attenuated p70S6K phosphorylation. These data indicate that GM-CSF exhibits chemotactic functionality and suggest new avenues for the investigation of the molecular basis of chemotaxis as it relates to inflammation and tissue injury.

CCL19/CCL21-triggered signal transduction and migration of dendritic cells requires prostaglandin E2

The control of dendritic cell (DC) migration is pivotal for the initiation of cellular immune responses. When activated with inflammatory stimuli, the chemokine receptor CCR7 is up-regulated on DCs. Activated DCs home to lymphoid organs, where the CCR7 ligands CCL19 and CCL21 are expressed. We previously found that human monocyte-derived DCs (MoDCs) exclusively migrated to CCL19 and CCL21 when matured in the presence of prostaglandin (PG) E2. Because PGE2 did not alter CCR7 cell surface expression, we examined whether PGE2 may exert its effect by coupling CCR7 to signal transduction modules. Indeed, stimulation with CCR7 ligands led to enhanced phosphatidylinositol-3-kinase–mediated phosphorylation of protein kinase B when MoDCs were matured in the presence of PGE2. Moreover, CCL19/CCL21-induced intracellular calcium mobilization in MoDCs occurred only when PGE2 was present during maturation. MoDC migration to CCL19 and CCL21 was dependent on phospholipase C and intracellular calcium flux but not on phosphatidylinositol-3 kinase. Hence, our data provide insight into CCL19/CCL21-triggered signal transduction pathways and identify a novel function for PGE2 in controlling the migration of mature MoDCs by facilitating CCR7 signal transduction.

Glucagon-like peptide-1 regulates proliferation and apoptosis via activation of protein kinase B in pancreatic INS-1 beta cells

AIMS/HYPOTHESIS

The incretin hormone glucagon-like peptide-1 augments islet cell mass in vivo by increasing proliferation and decreasing apoptosis of the beta cells. However, the signalling pathways that mediate these effects are mostly unknown. Using a clonal rat pancreatic beta cell line (INS-1), we examined the role of protein kinase B in mediating beta-cell growth and survival stimulated by glucagon-like peptide-1.

METHODS

Immunoblot analysis was used to detect active (phospho-) and total protein kinase B. Proliferation was assessed using (3)H-thymidine incorporation, while apoptosis was quantitated using 4'-6-diamidino-2-phenylindole staining and APO percentage apoptosis assay. Kinase-dead and wild-type protein kinase B was introduced into cells using adenoviral vectors.

RESULTS

Glucagon-like peptide-1 rapidly activated protein kinase B in INS-1 cells (by 2.7+/-0.7-fold, p<0.05). This effect was completely abrogated by inhibition, with wortmannin, of the upstream activator of protein kinase B, phosphatidylinositol-3-kinase. Glucagon-like peptide-1 also stimulated INS-1 cell proliferation in a dose-dependent manner (by 1.8+/-0.5-fold at 10(-7) mol/l, p<0.01), and inhibited staurosporine-induced apoptosis (by 69+/-12%, p<0.05). Both of these effects were also prevented by wortmannin treatment. Ablation of protein kinase B by adenovirus-mediated overexpression of the kinase-dead form of protein kinase Balpha prevented protein kinase B phosphorylation and completely abrogated both cellular proliferation ( p<0.05) and protection from drug-induced cellular death ( p<0.01) induced by glucagon-like peptide-1.

CONCLUSIONS/INTERPRETATION

These results identify protein kinase B as an essential mediator linking the glucagon-like peptide-1 signal to the intracellular machinery that modulates beta-cell growth and survival.

Cell migration: Rho GTPases lead the way

Rho GTPases control signal transduction pathways that link cell surface receptors to a variety of intracellular responses. They are best known as regulators of the actin cytoskeleton, but in addition they control cell polarity, gene expression, microtubule dynamics and vesicular trafficking. Through these diverse functions, Rho GTPases influence many aspects of cell behavior. This review will focus specifically on their role in cell migration.

Do phosphoinositide 3-kinases direct lymphocyte navigation?

Phosphoinositide 3-kinase (PI3K)-dependent signalling pathways have been suggested to have pivotal roles in determining the polarity of leukocytes moving toward a chemotactic gradient, a process termed chemotaxis. Current perceptions concerning the role of PI3K in leukocyte migration are based predominantly around evidence derived from single-cell organisms and analysis of neutrophil migration from mice deficient in the gamma-isoform of the p110 catalytic subunit. With regard to directed T-lymphocyte migration, there is convincing evidence for the activation of PI3K isoforms in T lymphocytes by several chemokines. However, there is a growing body of evidence, which now indicates that in T lymphocytes at least, PI3K activation can be a dispensable signal for directed cell migration in certain settings. In fact, evidence is emerging that during directed cell migration, T lymphocytes use biochemical pathways distinct from those adopted by monocytes. The non-universal role of PI3K in directional cell migration and the existence of cell-specific signalling pathways for chemotactic responses has important implications for the validation of effective new targets for inflammation, where one aim is to block migration of leukocytes to the site of inflammatory lesion.

Phosphatidylinositide 3-kinase gamma regulates key pathologic responses to cholecystokinin in pancreatic acinar cells

BACKGROUND & AIMS

Early events in the pancreatic acinar cell critical for development of pancreatitis include activation of the transcription factor nuclear factor kappa B (NF-kappa B), abnormal Ca(2+) responses, and trypsinogen activation. Mechanisms underlying these responses, which can be studied in isolated pancreatic acini stimulated with supraphysiologic doses of cholecystokinin (CCK-8), remain poorly understood. We here report that these responses are regulated by phosphatidylinositide 3-kinase (PI3K) gamma.

METHODS

To inactivate PI3K, we used mice deficient in the catalytic PI3K gamma subunit p110 gamma as well as the PI3K inhibitors LY294002 and wortmannin. We measured Ca(2+) responses by using Fura-2, NF-kappa B-binding activity by electromobility shift assay, I kappa B degradation by Western blotting, and trypsinogen activation by fluorogenic assay.

RESULTS

CCK-induced intracellular Ca(2+) mobilization, Ca(2+) influx, trypsinogen, and NF-kappa B activation were all diminished in pancreatic acini isolated from p110 gamma(-/-) mice. Both in mouse and rat acini, these responses were inhibited by the PI3K inhibitors. The Ca(2+) signal and trypsinogen activation were similarly reduced in acini isolated from p110 gamma(-/-) and p110 gamma(+/-) mice compared with wild-type mice. By contrast, NF-kappa B activation was inhibited in p110 gamma(-/-) acini but not in p110 gamma(+/-) acini. These differences indicate that the mechanism of NF-kappa B regulation by PI3K gamma differs from those for the Ca(2+) and trypsinogen responses. CCK-induced responses in p110 gamma(-/-) acini were all further inhibited by LY294002, indicating the involvement of other PI3K isoform(s), in addition to PI3K gamma.

CONCLUSIONS

The results show that key pathologic responses of the pancreatic acinar cell are regulated by PI3K gamma and suggest an important role for this PI3K isoform in pancreatitis.

Mechanisms and implications of phosphoinositide 3-kinase delta in promoting neutrophil trafficking into inflamed tissue

The phosphoinositide 3-kinase (PI3K) catalytic subunit p110 delta is expressed in neutrophils and is thought to play a role in their accumulation at sites of inflammation by contributing to chemoattractant-directed migration. We report here that p110 delta is present in endothelial cells and participates in neutrophil trafficking by modulating the proadhesive state of these cells in response to tumor necrosis factor alpha (TNF alpha). Specifically, administration of the selective inhibitor of PI3K delta, IC87114, to animals reduced neutrophil tethering to and increased rolling velocities on cytokine-activated microvessels in a manner similar to that observed in mice deficient in p110 delta. These results were confirmed in vitro as inhibition of this isoform in endothelium, but not neutrophils, diminished cell attachment in flow. A role for PI3K delta in TNF alpha-induced signaling is demonstrated by a reduction in Akt-phosphorylation and phosphatidylinositol-dependent kinase 1 (PDK1) enzyme activity upon treatment of this cell type with IC87114. p110 delta expressed in neutrophils also contributes to trafficking as demonstrated by the impaired movement of these cells across inflamed venules in animals in which this catalytic subunit was blocked or genetically deleted, results corroborated in transwell migration assays. Thus, PI3K delta may be a reasonable therapeutic target in specific inflammatory conditions as blockade of its activity reduces neutrophil influx into tissues by diminishing their attachment to and migration across vascular endothelium.

Blockade of inflammation and airway hyperresponsiveness in immune-sensitized mice by dominant-negative phosphoinositide 3-kinase-TAT

Phosphoinositide 3-kinase (PI3K) is thought to contribute to the pathogenesis of asthma by effecting the recruitment, activation, and apoptosis of inflammatory cells. We examined the role of class IA PI3K in antigen-induced airway inflammation and hyperresponsiveness by i.p. administration into mice of Δp85 protein, a dominant negative form of the class IA PI3K regulatory subunit, p85α, which was fused to HIV-TAT (TAT-Δp85). Intraperitoneal administration of TAT-Δp85 caused time-dependent transduction into blood leukocytes, and inhibited activated phosphorylation of protein kinase B (PKB), a downstream target of PI3K, in lung tissues in mice receiving intranasal FMLP. Antigen challenge elicited pulmonary infiltration of lymphocytes, eosinophils and neutrophils, increase in mucus-containing epithelial cells, and airway hyperresponsiveness to methacholine. Except for modest airway neutrophilia, these effects all were blocked by treatment with 3–10 mg/kg of TAT-Δp85. There was also significant reduction in IL-5 and IL-4 secretion into the BAL. Intranasal administration of IL-5 caused eosinophil migration into the airway lumen, which was attenuated by systemic pretreatment with TAT-Δp85. We conclude that PI3K has a regulatory role in Th2-cell cytokine secretion, airway inflammation, and airway hyperresponsiveness in mice.

Activation of group IV cytosolic phospholipase A2 in human eosinophils by phosphoinositide 3-kinase through a mitogen-activated protein kinase-independent pathway

Activation of group IV cytosolic phospholipase A(2) (gIV-PLA(2)) is the essential first step in the synthesis of inflammatory eicosanoids and in integrin-mediated adhesion of leukocytes. Prior investigations have demonstrated that phosphorylation of gIV-PLA(2) results from activation of at least two isoforms of mitogen-activated protein kinase (MAPK). We investigated the potential role of phosphoinositide 3-kinase (PI3K) in the activation of gIV-PLA(2) and the hydrolysis of membrane phosphatidylcholine in fMLP-stimulated human blood eosinophils. Transduction into eosinophils of Deltap85, a dominant negative form of class IA PI3K adaptor subunit, fused to an HIV-TAT protein transduction domain (TAT-Deltap85) concentration dependently inhibited fMLP-stimulated phosphorylation of protein kinase B, a downstream target of PI3K. FMLP caused increased arachidonic acid (AA) release and secretion of leukotriene C(4) (LTC(4)). TAT-Deltap85 and LY294002, a PI3K inhibitor, blocked the phosphorylation of gIV-PLA(2) at Ser(505) caused by fMLP, thus inhibiting gIV-PLA(2) hydrolysis and production of AA and LTC(4) in eosinophils. FMLP also caused extracellular signal-related kinases 1 and 2 and p38 MAPK phosphorylation in eosinophils; however, neither phosphorylation of extracellular signal-related kinases 1 and 2 nor p38 was inhibited by TAT-Deltap85 or LY294002. Inhibition of 1) p70 S6 kinase by rapamycin, 2) protein kinase B by Akt inhibitor, or 3) protein kinase C by Ro-31-8220, the potential downstream targets of PI3K for activation of gIV-PLA(2), had no effect on AA release or LTC(4) secretion caused by fMLP. We find that PI3K is required for gIV-PLA(2) activation and hydrolytic production of AA in activated eosinophils. Our data suggest that this essential PI3K independently activates gIV-PLA(2) through a pathway that does not involve MAPK.

The expanding role of PI3-kinase in bone

Phosphatidylinositol-3-kinases (PI3-Ks) play an important role in signal transduction and have been implicated in mediating a broad range of cellular responses. There are three classes of PI3-Ks [I (a and b subclasses), II, and III] with different substrate specificities and different modes of regulation. In osteoclasts, PI3-K has been shown to be a critical downstream effector from at least three cell-surface receptors, c-fms [the receptor for colony-stimulating factor 1 (CSF-1)], alphaVB3 integrin, and RANK [receptor activator of nuclear factor-kB (NF-kB)]. Furthermore, PI3-K is known to partner with the cytoplasmic tyrosine kinase c-src in mediating the effects of activated c-fms. The effector actions of PI3-K are diverse, including influencing osteoclast survival and activity, mediating actin remodeling and motility, and regulation of attachment structures. Less is known about the roles of PI3-K in osteoblasts. However, recent evidence suggests a role for PI3-K in osteoblast differentiation and survival. The classification, structure, function, and regulation of PI3-Ks will be reviewed here, with particular emphasis on the role of PI3-K in bone.

Roles of PI3K in neutrophil function

Neutrophils are terminally differentiated cells that play a vital role in host defense. It has recently become evident that phospholipid regulation plays an import role in many neutrophil functions. We review the regulation of neutrophil functions such as chemotaxis, superoxide production, and phagocytosis by phosphatidylinositol-3,4,5-trisphosphate (PIP3), which is generated in neutrophils by PI3K(gamma). Several lines of evidence are presented demonstrating the importance of this kinase in regulating chemotaxis, in particular the directionality of chemotactic migration. Evidence suggesting that this kinase is important for phagocytosis, especially during engulfment and the internalization of large particles, is also reviewed. Finally, it is suggested that PI3K is important for superoxide production and neutrophil priming. The common link between these seemingly diverse functions is that PI3K(gamma), via its phospholipid products, appears to be providing spatial-temporal cues for the binding of actin-organizing proteins.

Cytoskeletal remodeling in leukocyte function

PURPOSE OF REVIEW

This review focuses on recent developments in understanding the roles and regulation of the cytoskeleton in the function of leukocytes.

RECENT FINDINGS

New studies have shed light on the regulation and dynamics of actin and microtubules in leukocytes relevant both to cell motility generally and to immune function specifically. The roles of cytoskeletal dynamics in processes such as cell activation, cell migration, and phagocytosis are being elucidated. Dramatic progress has been made recently in understanding the mechanisms of leukocyte directional sensing, polarization, and chemotaxis.

SUMMARY

Leukocytes need to be activated, polarize, change shape, move, or phagocytose in response to their environment. Leukocytes accomplish these processes by remodeling their cytoskeleton, the active musculoskeletal system of the cell that is not just the ultimate effector of motile responses but is also a dynamic framework for subcellular organization and regional signaling. Active areas of research include the direct and indirect reciprocal interactions between the cytoskeleton and the membrane and among cytoskeletal elements. The pervasive and multi-layered roles played by small GTPases of the Rho family and phosphoinositides in leukocyte function are also becoming clearer.

Regulation of the Actin Cytoskeleton by PI(4,5)P2 and PI(3,4,5)P3

The actin cytoskeleton is fundamental for various motile and morphogenetic processes in cells. The structure and dynamics of the actin cytoskeleton are regulated by a wide array of actin-binding proteins, whose activities are controlled by various signal transduction pathways. Recent studies have shown that certain membrane phospholipids, especially PI(4,5)P2 and PI(3,4,5)P3, regulate actin filament assembly in cells and in cell extracts. PI(4,5)P2 appears to be a general regulator of actin polymerization at the plasma membrane or at membrane microdomains, whereas PI(3,4,5)P3 promotes the assembly of specialized actin filament structures in response to some growth factors. Biochemical studies have demonstrated that the activities of many proteins promoting actin assembly are upregulated by PI(4,5)P2, whereas proteins that inhibit actin assembly or promote filament disassembly are down-regulated by PI(4,5)P2. PI(3,4,5)P3 promotes its effects on the actin cytoskeleton mainly through activation of the Rho family of small GTPases. In addition to their effects on actin dynamics, both PI(4,5)P2 and PI(3,4,5)P3 promote the formation of specific actin filament structures through activation/inactivation of actin filament cross-linking proteins and proteins that mediate cytoskeleton-plasma membrane interactions.

Platelet-activating factor increases pH(i) in bovine neutrophils through the PI3K-ERK1/2 pathway

1. Platelet-activating factor (PAF) is known to stimulate a variety of neutrophil activities, including chemotaxis, phagocytosis, degranulation, reactive oxygen species production and intracellular pH increase. The purpose of this study was to investigate the effect of PAF on pH((i)), specifically if these changes in pH are the result of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathway activation in bovine neutrophils. 2. PAF caused intracellular alkalinization in 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester-loaded bovine neutrophils. This phenomenon seems to be mediated by amiloride-sensitive Na(+)/H(+) exchange, and is inhibited by WEB2086 (a selective PAF receptor antagonist), genistein (a tyrosine kinase inhibitor), wortmannin and LY294002 (PI3K inhibitors), and PD98059 and UO126 (MEK inhibitors). 3. PAF 100 nm induced an increase in tyrosine phosphorylation of proteins 62, 44 and 21 kDa with a maximum response at 2 min of incubation. 4. Unlike human neutrophils, bovine neutrophils are strongly stimulated by PAF via phosphorylation of ERK1/2 (extracellular-signal-regulated protein kinase) with an EC(50) of 30 and 13 nm, respectively. 5. PAF MAPK activation was also inhibited by WEB2086, pertussis toxin (PTX), genistein, wortmannin, LY294002, PD98059 and UO126 in bovine neutrophils. The ERK1/2 activation is dependent on PI3K pathway, because protein kinase B was phosphorylated by PAF and inhibited by wortmannin and LY294002, but not by U0126. 6. Our results suggest that PAF induces intracellular alkalinization via PI3K-MAPK activation. This effect is upstream regulated by PAF receptor, PTX-sensitive G protein, tyrosine kinase, PI3K and MEK1/2 in bovine neutrophils.

A versatile high-throughput screen for inhibitors of lipid kinase activity: development of an immobilized phospholipid plate assay for phosphoinositide 3-kinase gamma

The family of phosphoinositide 3-kinases (PI3K) regulates fundamental cellular responses such as proliferation, apoptosis, motility, and adhesion. In particular, the PI3K gamma isoform plays a critical role in the control of cell migration. Despite the attractiveness of PI3-kinases as drug targets, drug discovery efforts have been hampered by the lack of appropriate lipid kinase assay formats suitable for high-throughput screening. The authors report the development of a simple and robust 384-well plate assay that is based on(33) P-phosphate transfer from radiolabeled [gamma(33) P]ATP to phosphatidylinositol immobilized on Maxisorp plates. The established assay format for PI3K gamma was easily adapted to the automated screening platform and was successfully employed for high-throughput screening. Enzymatic and inhibition characteristics of recombinant human PI3K gamma determined with the plate assay are in very good agreement with previously reported values determined in other assay formats. Maximal catalytic activity of PI3K gamma was observed at pH 7.0. The apparent K(m) value for ATP using a 1:1 mixture of phosphatidylinositol and phosphatidylserine was determined to be 7.3 microM (6.0-8.6 microM, 95% confidence interval [CI]). IC(50) values for known PI3-kinase inhibitors were determined to be 1.45 nM (1.17-1.80 nM, 95% CI) for wortmannin and estimated from partial inhibition data to be 1400, 2830, and 21,400 nM for quercetin, LY294002, and staurosporine, respectively. This novel assay approach allows for screening of inhibitors of lipid kinases in high-throughput mode and thereby may facilitate the identification of novel inhibitory structures for drug development.

Signaling to migration in neutrophils: importance of localized pathways

Neutrophils, a major type of blood leukocytes, are indispensable for host defense of bacterial infections. Directed migration in a gradient of chemotactic stimuli enables these cells to rapidly find the site of infection and destroy the invading pathogens. Chemotactic factors bind to seven-transmembrane-domain receptors and activate heterotrimeric G-proteins. Downstream of these proteins a complex interrelated signaling network is activated in human neutrophils. Stimulation of phospholipase C beta results in activation of protein kinase C isoforms and increases in cytosolic calcium. Activation of the enzyme phosphoinositide 3-kinase results in increased production of phosphatidylinositol 3,4,5-trisphosphate and phosphatidyl 3,4-bisphosphate. In addition, small GTP-binding proteins of the Rho family, the mitogen-activated protein kinase cascade, tyrosine kinases and protein phosphatases are activated. The enzyme phosphoinositide 3-kinase and the small cytosolic GTP-binding proteins Rho and Rac emerge as key regulators of neutrophil migration. A steep internal gradient of phosphatidylinositol 3,4,5-trisphosphate, with a high concentration in the leading lamellae, is thought to regulate polarized actin polymerization and formation of protrusions, together with Rac which may be more directly involved in initiating actin reorganization. Rho may regulate localized myosin activation, tail retraction, cell body traction and dynamics of adhesion. The impact of these different signaling pathways on reversible actin polymerization, development of polarity, reversible adhesion and migration, and the putative targets of these pathways in neutrophils, are reviewed in this article. Insight into mechanisms regulating migration of neutrophils could potentially lead to novel therapeutic strategies for counteracting chronic activation of neutrophils which leads to tissue damage.

Abnormal PTEN expression in portal hypertensive gastric mucosa: a key to impaired PI 3-kinase/Akt activation and delayed injury healing?

Phosphatase and tensin homologue deleted on chromosome ten (PTEN) is a dual-specificity phosphatase that has activity toward both phosphorylated peptides and phospholipids. PTEN inhibits activation of Akt, the downstream effector of PI 3-kinase, which is integral to cell proliferation, migration, survival, and angiogenesis essential for tissue injury healing. PTEN expression and activation during injury healing remain unexplored. Portal hypertensive (PHT) gastric mucosa has impaired injury healing, but the underlying mechanisms remain unknown. We investigated whether impaired healing of injured PHT gastric mucosa is due to abnormal PTEN expression/activation that leads to decreased Akt activation. We also investigated the possible involvement of Egr-1, which regulates PTEN in some cells (e.g., fetal kidney epithelial cells), and TNF-alpha, which can induce Egr-1 expression. In PHT gastric mucosa 6 h after injury, PTEN protein levels were increased by 2.7-fold; unphosphorylated PTEN (reflecting activated PTEN) was increased by 2.4-fold; Akt phosphorylation (reflecting Akt activation) was reduced by 2-fold; and Egr-1 expression was increased by 3.3-fold vs. normal gastric mucosa. TNF-alpha neutralization reversed all of the above abnormalities in PHT gastric mucosa, reduced mucosal injury, and enhanced healing. We conclude that, in injured PHT gastric mucosa, overexpressed/activated PTEN leads to the reduced activation of the PI 3-kinase/Akt pathway that results in impaired injury healing.

Neutrophil cell signaling in infection: role of phosphatidylinositide 3-kinase

Neutrophils play a pivotal role in the innate immune response to microbial pathogens. They are uniquely suited to this role by virtue of specialized antimicrobial capabilities that include the capacity to sense minute amounts of microbial products and inflammatory mediators, to move to the site of infection, and finally to bind, internalize and kill the pathogens. To optimize host defense capabilities while minimizing damage to host tissues ('collateral damage'), these microbicidal responses must be tightly regulated. Additionally, neutrophils clear inflammatory debris, a process that is necessary for restoration of the native architecture and function of the tissue. This review highlights some recent advances in our knowledge of cell signaling as it pertains to neutrophil function, with specific emphasis on the role of the phosphatidylinositide 3-kinase in antimicrobial function.

Signaling pathways at the leading edge of chemotaxing cells

Chemotaxis, or directed cell movement towards small molecule ligands, is a central function of many cell types and plays a key role in diverse biological processes. This review summarizes our present understanding of the signaling pathways that control the ability of cells to sense the chemoattractant gradient and respond by converting a shallow extracellular gradient into a steep intracellular gradient that leads to formation of a pseudopod in the direction of the chemoattractant gradient and contraction of the cell's posterior. The review focuses on the phosphatidylinositol 3-kinase pathway in Dictyostelium and our understanding of parallel pathways in leukocytes.

Two phases of actin polymerization display different dependencies on PI(3,4,5)P3 accumulation and have unique roles during chemotaxis

The directional movement of cells in chemoattractant gradients requires sophisticated control of the actin cytoskeleton. Uniform exposure of Dictyostelium discoideum amoebae as well as mammalian leukocytes to chemoattractant triggers two phases of actin polymerization. In the initial rapid phase, motility stops and the cell rounds up. During the second slow phase, pseudopodia are extended from local regions of the cell perimeter. These responses are highly correlated with temporal and spatial accumulations of PI(3,4,5)P3/PI(3,4)P2 reflected by the translocation of specific PH domains to the membrane. The slower phase of PI accumulation and actin polymerization is more prominent in less differentiated, unpolarized cells, is selectively increased by disruption of PTEN, and is relatively more sensitive to perturbations of PI3K. Optimal levels of the second responses allow the cell to respond rapidly to switches in gradient direction by extending lateral pseudopods. Consequently, PI3K inhibitors impair chemotaxis in wild-type cells but partially restore polarity and chemotactic response in pten- cells. Surprisingly, the fast phase of PI(3,4,5)P3 accumulation and actin polymerization, which is relatively resistant to PI3K inhibition, can support inefficient but reasonably accurate chemotaxis.

Phosphoinositide 3-kinase gamma-deficient mice are protected from isoproterenol-induced heart failure

BACKGROUND

We have recently shown that genetic inactivation of phosphoinositide 3-kinase gamma (PI3Kgamma), the isoform linked to G-protein-coupled receptors, results in increased cardiac contractility with no effect on basal cell size. Signaling via the G-protein-coupled beta-adrenergic receptors has been implicated in cardiac hypertrophy and heart failure, suggesting that PI3Kgamma might play a role in the pathogenesis of heart disease.

METHODS AND RESULTS

To determine the role for PI3Kgamma in hypertrophy induced by G-protein-coupled receptors and cardiomyopathy, we infused isoproterenol, a beta-adrenergic receptor agonist, into PI3Kgamma-deficient mice. Compared with controls, isoproterenol infusion in PI3Kgamma-deficient mice resulted in an attenuated cardiac hypertrophic response and markedly reduced interstitial fibrosis. Intriguingly, chronic beta-adrenergic receptor stimulation triggered impaired heart functions in wild-type mice, whereas PI3Kgamma-deficient mice retained their increased heart function and did not develop heart failure. The lack of PI3Kgamma attenuated the activation of Akt/protein kinase B and extracellular signal-regulated kinase 1/2 signaling pathways in cardiac myocytes in response to isoproterenol. beta1- and beta2-adrenergic receptor densities were decreased by similar amounts in PI3Kgamma-deficient and control mice, suggesting that PI3Kgamma isoform plays no role in the downregulation of beta-adrenergic receptors after chronic beta-adrenergic stimulation.

CONCLUSIONS

Our data show that PI3Kgamma is critical for the induction of hypertrophy, fibrosis, and cardiac dysfunction function in response to beta-adrenergic receptor stimulation in vivo. Thus, PI3Kgamma may represent a novel therapeutic target for the treatment of decreased cardiac function in heart failure.

Dual Phosphorylation of Phosphoinositide 3-Kinase Adaptor Grb2-Associated Binder 2 Is Responsible for Superoxide Formation Synergistically Stimulated by Fcγ and Formyl-Methionyl-Leucyl-Phenylalanine Receptors in Differentiated THP-1 Cells

The class Ia phosphoinositide (PI) 3-kinase consisting of p110 catalytic and p85 regulatory subunits is activated by Tyr kinase-linked membrane receptors such as FcgammaRII through the association of p85 with the phosphorylated receptors or adaptors. The heterodimeric PI 3-kinase is also activated by G protein-coupled chemotactic fMLP receptors, and activation of the lipid kinase plays an important role in various immune responses, including superoxide formation in neutrophils. Although fMLP-induced superoxide formation is markedly enhanced in FcgammaRII-primed neutrophils, the molecular mechanisms remain poorly characterized. In this study, we identified two Tyr-phosphorylated proteins, c-Cbl (Casitas B-lineage lymphoma) and Grb2-associated binder 2 (Gab2), as PI 3-kinase adaptors that are Tyr phosphorylated upon the stimulation of FcgammaRII in differentiated neutrophil-like THP-1 cells. Interestingly, Gab2 was, but c-Cbl was not, further Ser/Thr phosphorylated by fMLP. Thus, the adaptor Gab2 appeared to be dually phosphorylated at the Ser/Thr and Tyr residues through the two different types of membrane receptors. The Ser/Thr phosphorylation of Gab2 required the activation of extracellular signal-regulated kinase, and fMLP receptor stimulation indeed activated extracellular signal-regulated kinase in the cells. Enhanced superoxide formation in response to Fcgamma and fMLP was markedly attenuated when the Gab2 Ser/Thr phosphorylation was inhibited. These results show the importance of the dual phosphorylation of PI 3-kinase adaptor Gab2 for the enhanced superoxide formation in neutrophil-type cells.

Class I phosphoinositide 3-kinase p110beta is required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by macrophages

Phosphoinositide 3-kinases (PI3Ks) play an important role in a variety of cellular functions, including phagocytosis. PI3Ks are activated during phagocytosis induced by several receptors and have been shown to be required for phagocytosis through the use of inhibitors such as wortmannin and LY294002. Mammalian cells have multiple isoforms of PI3K, and the role of the individual isoforms during phagocytosis has not been addressed. The class I PI3Ks consist of a catalytic p110 isoform associated with a regulatory subunit. Mammals have three genes for the class IA p110 subunits encoding p110alpha, p110beta, and p110delta and one gene for the class IB p110 subunit encoding p110gamma. Here we report a specific recruitment of p110beta and p110delta (but not p110alpha) isoforms to the nascent phagosome during apoptotic cell phagocytosis by fibroblasts. By microinjecting inhibitory antibodies specific to class IA p110 subunits, we have shown that p110beta is the major isoform required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by primary mouse macrophages. Macrophages from mice expressing a catalytically inactive form of p110delta showed no defect in the phagocytosis of apoptotic cells and IgG-opsonized particles, confirming the lack of a major role for p110delta in this process. Similarly, p110gamma-deficient macrophages phagocytosed apoptotic cells normally. Our findings demonstrate that p110beta is the major class I catalytic isoform required for apoptotic cell and Fcgamma receptor-mediated phagocytosis by primary macrophages.

Mechanisms of leukotriene B4--triggered monocyte adhesion

OBJECTIVE

Leukotriene B4 (LTB4) has been implicated in the trafficking of monocytes to inflammatory pathologic conditions, such as transplant rejection and atherosclerosis. The aim of this study was to determine the mechanisms by which LTB4 contributes to monocyte capture from the circulation.

METHODS AND RESULTS

In in vitro and in vivo vascular models, the lipid chemoattractant LTB4 was an equipotent agonist of monocyte adhesion compared with the chemokine monocyte chemoattractant protein-1 (MCP-1). Adenoviral gene transfer of specific endothelial adhesion molecules and blocking monoclonal antibody studies demonstrated that LTB4 triggers both beta1- and beta2-integrin-dependent adhesion. Flow cytometry studies suggested that changes in integrin avidity or affinity, rather than alterations of integrin surface expression, were responsible for the chemoattractant-triggered arrest. Surprisingly, in contrast to the peptide chemokine MCP-1, LTB4 did not activate the phosphoinositide 3-kinase pathway, which is a functionally critical step in chemokine-triggered effector functions.

CONCLUSIONS

LTB4 is a potent trigger of monocyte adhesion under flow yet mediates its effects via pathways that appear to differ from peptide chemoattractants. A better understanding of the mechanisms of LTB4-induced monocyte trafficking might shed insight into disease pathogenesis and pinpoint critical steps for therapeutic intervention for multiple human inflammatory pathologic conditions.

G-proteins as transducers in transmembrane signalling

The G-protein-mediated signalling system has evolved as one of the most widely used transmembrane signalling mechanisms in mammalian organisms. All mammalian cells express G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. G-protein-mediated signalling is involved in many physiological and pathological processes. This review summarizes some general aspects of G-protein-mediated signalling and focusses on recent data especially from studies in mutant mice which have elucidated some of the cellular and biological functions of heterotrimeric G-prtoteins.

Requirement for PI 3-kinase γ in macrophage migration to MCP-1 and CSF-1

Phosphoinositide 3-kinases (PI3Ks) are important regulators of cell migration. The PI3K isoform gamma is primarily expressed in haematopoietic cells, and is activated by G protein-coupled receptors (GPCRs). Here, we investigate the contribution of PI3Kgamma to macrophage responses to chemoattractants, using bone marrow-derived macrophages from wild-type and PI3Kgamma-null mice. We observe that early membrane ruffling induced by MCP-1, which activates a GPCR, or by CSF-1, which activates a tyrosine kinase receptor, is unaltered in PI3Kgamma(-/-) mice, although by 30 min MCP-1-induced cell polarization was strongly reduced in PI3Kgamma(-/-) compared to wild-type macrophages. The migration behaviour of the macrophages was analysed by time-lapse microscopy in Dunn chemotaxis chambers. PI3Kgamma(-/-) macrophages showed reduced migration speed and translocation, and no chemotaxis to MCP-1. Interestingly, there was also a reduction in migration efficiency in PI3Kgamma(-/-) macrophages stimulated with CSF-1 although early CSF-1R signalling was normal. These results indicate that the initial actin reorganization induced by either a GPCR or tyrosine kinase receptor agonist is not dependent on PI3Kgamma, whereas PI3Kgamma is needed for optimal migration of macrophages to either agonist.

The human formyl peptide receptor as model system for constitutively active G-protein-coupled receptors

According to the two-state model of G-protein-coupled receptor (GPCR) activation, GPCRs isomerize from an inactive (R) state to an active (R*) state. In the R* state, GPCRs activate G-proteins. Agonist-independent R/R* isomerization is referred to as constitutive activity and results in an increase in basal G-protein activity, i.e. GDP/GTP exchange. Agonists stabilize the R* state and further increase, whereas inverse agonists stabilize the R state and decrease, basal G-protein activity. Constitutive activity is observed in numerous wild-type GPCRs and disease-causing GPCR mutants with increased constitutive activity. The human formyl peptide receptor (FPR) exists in several isoforms (FPR-26, FPR-98 and FPR-G6) and activates chemotaxis and cytotoxic cell functions of phagocytes through G(i)-proteins. Studies in HL-60 leukemia cell membranes demonstrated inhibitory effects of Na(+) and pertussis toxin on basal G(i)-protein activity, suggesting that the FPR is constitutively active. However, since HL-60 cells express several constitutively active chemoattractant receptors, analysis of constitutive FPR activity was difficult. Sf9 insect cells do not express chemoattractant receptors and G(i)-proteins and provide a sensitive reconstitution system for FPR/G(i)-protein coupling. Such expression studies showed that FPR-26 is much more constitutively active than FPR-98 and FPR-G6 as assessed by the relative inhibitory effects of Na(+) and of the inverse agonist cyclosporin H on basal G(i)-protein activity. Site-directed mutagenesis studies suggest that the E346A exchange in the C-terminus critically determines dimerization and constitutive activity of FPR. Moreover, N-glycosylation of the N-terminus seems to be important for constitutive FPR activity. Finally, we discuss some future directions of research.

Eosinophil Major Basic Protein Stimulates Neutrophil Superoxide Production by a Class IAPhosphoinositide 3-Kinase and Protein Kinase C-ζ-Dependent Pathway

Eosinophil major basic protein (MBP) is an effective stimulus for neutrophil superoxide (O(2)(-)) production, degranulation, and IL-8 production. In this study we evaluated the participation of phosphoinositide 3-kinase (PI3K) and PI3K-associated signaling events in neutrophil activation by MBP. Inhibition of PI3K activity blocked MBP-stimulated O(2)(-) production, but not degranulation or IL-8 production. Measurement of Akt phosphorylation at Ser(473) and Thr(308) confirmed that MBP stimulated PI3K activity and also demonstrated indirectly activation of phosphoinositide-dependent kinase-1 by MBP. Genistein and the Src kinase family inhibitor, 4-amino-5-(4-methyphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, inhibited MBP-stimulated phosphorylation of Akt. 4-Amino-5-(4-methyphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine also inhibited MBP-stimulated O(2)(-) production. MBP stimulated phosphorylation and translocation of the p85 subunit of class I(A) PI3K, but not translocation of the p110gamma subunit of class I(B) PI3K, to the neutrophil membrane. Inhibition of protein kinase Czeta (PKCzeta) inhibited MBP-stimulated O(2)(-) production. Measurement of phosphorylated PKCzeta (Thr(410)) and PKCdelta (Thr(505)) confirmed that PKCzeta, but not PKCdelta, is activated in MBP-stimulated neutrophils. The time courses for phosphorylation and translocation of the p85 subunit of class I(A) PI3K, activation of Akt, and activation of PKCzeta were similar. Moreover, inhibition of PI3K activity inhibited MBP-induced activation of PKCzeta. We conclude that MBP stimulates a Src kinase-dependent activation of class I(A) PI3K and, in turn, activation of PKCzeta in neutrophils, which contributes to the activation of NADPH oxidase and the resultant O(2)(-) production in response to MBP stimulation.

CXCR3-mediated chemotaxis of human T cells is regulated by a Gi- and phospholipase C-dependent pathway and not via activation of MEK/p44/p42 MAPK nor Akt/PI-3 kinase

The chemokines CXCL9, 10, and 11 exert their action via CXC chemokine receptor-3 (CXCR3), a receptor highly expressed on activated T cells. These interferon gamma (IFNgamma)-induced chemokines are thought to be crucial in directing activated T cells to sites of inflammation. As such, they play an important role in several chronic inflammatory diseases including ulcerative colitis, multiple sclerosis, artherosclerosis, and delayed-type hypersensitivity reactions of the skin. In this study, we first demonstrate that in COS-7 cells heterologously expressing CXCR3, CXCL11 is a potent activator of the pertussis toxin (PTX)-sensitive p44/p42 mitogen-activated protein kinase (MAPK) and Akt/phosphatidylinositol 3 kinase (PI3K) pathways. Next, we show that these signal transduction pathways are also operative and PTX sensitive in primary human T cells expressing CXCR3. Importantly, abrogation of these signaling cascades by specific inhibitors did not block the migration of T cells toward CXCR3 ligands, suggesting that MAPK and Akt activation is not crucial for CXCR3-mediated chemotaxis of T cells. Finally, we demonstrate that CXCR3-targeting chemokines control T-cell migration via PTX-sensitive, phospholipase C pathways and phosphatidylinositol kinases other than class I PI3Kgamma.

Novel pathways of F-actin polymerization in the human neutrophil

Recently we demonstrated the existence of a phosphatidylinositol 3-kinase (PI3K)-independent F-actin polymerization during neutrophil pseudopod extension. Here we examine the use of the PI3K-dependent and PI3K-independent pathways of activation by the N-formyl peptide receptor and the chemokine receptors, and the priming of the 2 pathways by granulocyte-macrophage colony-stimulating factor (GM-CSF) and insulin. The inhibition of PI3K activity with wortmannin showed that rate of pseudopod extension stimulated with N-formyl-Met-Leu-Phe (fMLP was mostly dependent on PI3K, while the rate of interleukin-8 (IL-8)-stimulated pseudopod extension was less dependent on PI3K. The incubation of cells with either GM-CSF or insulin increased the rate of pseudopod extension by 50% when the cells were stimulated with IL-8 but not with fMLP. The stimulation with IL-8 phosphorylated the PI3K regulatory subunit. This phosphorylation was enhanced by GM-CSF, which increased PI3K activity and total phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) production. The effect of GM-CSF was blocked with wortmannin. In contrast, insulin did not increase p85 phosphorylation and did not enhance PI3K activity or PtdIns(3,4,5)P3 production. The effect of insulin was insensitive to wortmannin; however, it was blocked by an Src homology 2 (SH2)-binding peptide. These data indicate that priming of IL-8 activation with GM-CSF was mediated via the PI3Ks of class IA, while priming with insulin used a PI3K-independent pathway.

Selective role of PI3K delta in neutrophil inflammatory responses

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

Role of phosphoinositide 3-kinase signaling in mast cells: new insights from knockout mouse studies

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases essential for diverse physiological reactions. In recent years a series of gene-targeted mice lacking different types of PI3Ks and related molecules have been generated which enable us to understand the role of PI3K pathways, particularly class I members, in vivo. Analyses of such gene-targeted mice have led to major discoveries in the physiological roles of PI3K signaling in mast cell biology. In particular the role of PI3Ks has been extensively studied in signaling through the high-affinity IgE receptor (FcepsilonRI), since mast cells are the main effector cells in type I allergic reaction associated with IgE-dependent mechanisms. Furthermore, the knockout mice have provided significant information concerning the role of PI3K signals in mast cell differentiation. This review presents several new insights into mast cell biology, which have been elucidated by the analyses of these knockout mice.

Differential signaling of formyl peptide receptor-like 1 by Trp-Lys-Tyr-Met-Val-Met-CONH2 or lipoxin A4 in human neutrophils

Classical chemoattractant receptors are of fundamental importance to immune responses. The two major roles of such receptors are the modulation of chemotaxis and the generation of reactive oxygen species. The formyl peptide receptor-like 1 (FPRL1) can be stimulated by two different ligands, Trp-Lys-Tyr-Met-Val-Met-CONH2 (WKYMVM) and lipoxin A4 (LXA4). Although leukocyte chemotaxis mediated by activated FPRL1 has been reported, the role of FPRL1 in superoxide generation remains to be studied. In this study, we examined the effect of WKYMVM or LXA4 on chemotactic migration and superoxide generation in human neutrophils. WKYMVM and LXA4 stimulated neutrophil chemotaxis via tyrosine phosphorylation events. In terms of reactive oxygen species generation, WKYMVM but not LXA4 stimulated superoxide generation in neutrophils. To understand this difference on superoxide generation via the same receptor, FPRL1, we compared the signaling pathways downstream of FPRL1 by the two different ligands. At first, we confirmed that both WKYMVM and LXA4 caused intracellular calcium ([Ca2+]i) increase in a pertussis toxin-sensitive manner and that these ligands competitively inhibited each other with respect to [Ca2+]i increase in neutrophils. This result suggests that WKYMVM and LXA4 share the same receptor, FPRL1. By investigating cellular signaling by WKYMVM and LXA4, we found that WKYMVM but not LXA4 induced extracellular signal-regulated protein kinases (ERKs), c-Jun NH2-terminal kinase, and phospholipase A2 (PLA2) activation. We also found that ERK-mediated cytosolic PLA2 activity is essential for superoxide generation. These results indicate that the activation of FPRL1 by the two different ligands can induce differential cellular signaling and unique functional consequences in human neutrophils.

Leading the way: Directional sensing through phosphatidylinositol 3-kinase and other signaling pathways

Chemoattractant-responsive cells are able to translate a shallow extracellular chemical gradient into a steep intracellular gradient resulting in the localization of F-actin assembly at the front and an actomyosin network at the rear that moves the cell forward. Recent evidence suggests that one of the first asymmetric cellular responses is the localized accumulation of PtdIns(3,4,5)P3, the product of class I phosphoinositide 3-kinase (PI3K) at the site of the new leading edge. The strong accumulation of PtdIns(3,4,5)P3 results from the localized activation of PI3K and also from feedback loops that amplify PtdIns(3,4,5)P3 synthesis at the front and control its degradation at the side and back of cells. These different pathways are temporally and spatially regulated and integrate with other signaling pathways during directional sensing and chemotaxis.

Rapamycin inhibits GM-CSF-induced neutrophil migration

The molecular mechanisms that govern cell movement are the subject of intense study, as they impact biologically and medically important processes such as leukocyte chemotaxis and angiogenesis, among others. We demonstrate that leukocyte chemotaxis is prevented by the macrolide immunosuppressant rapamycin, a specific inhibitor of the mammalian target of rapamycin (mTOR)/ribosomal p70-S6 kinase (p70S6K) pathway. Both neutrophil chemotaxis and chemokinesis elicited by granulocyte-macrophage colony-stimulating factor (GM-CSF) were strongly inhibited by rapamycin with an IC(50) of 0.3 nM. Inhibition, although at a higher dose, was also observed when the chemoattractant was interleukin-8. As for the mechanism, rapamycin targeted the increase of phosphorylation of p70S6K due to GM-CSF treatment, as demonstrated with specific anti-p70S6K immunoprecipitation and subsequent immunoblotting with anti-T(421)/S(424) antibodies. Rapamycin also inhibited GM-CSF-induced actin polymerization, a hallmark of leukocyte migration. The specificity of the effect of rapamycin was confirmed by the use of the structural analog FK506, which did not have a significant effect on chemotaxis but effectively rescued rapamycin-induced p70S6K inhibition. This was expected from a competitive effect of both molecules on FK506-binding proteins (FKBP). Additionally, GM-CSF-induced chemotaxis was completely (>90%) blocked by a combination of rapamycin and the MAPK kinase (MEK) inhibitor PD-98059. In summary, the results presented here indicate for the first time that rapamycin, at sub-nanomolar concentrations, inhibits GM-CSF-induced chemotaxis and chemokinesis. This serves to underscore the relevance of the mTOR/S6K pathway in neutrophil migration.

Phosphoinositide 3 kinase mediates Toll-like receptor 4-induced activation of NF-kappa B in endothelial cells

Many of the proinflammatory effects of gram-negative bacteria are elicited by the interaction of bacterial lipopolysaccharide (LPS) with Toll-like receptor 4 (TLR4) expressed on host cells. TLR4 signaling leads to activation of NF-kappa B and transcription of many genes involved in the inflammatory response. In this study, we examined the signaling pathways involved in NF-kappa B activation by TLR4 signaling in human microvascular endothelial cells. Akt is a major downstream target of phosphoinositide 3 kinase (PI3-kinase), and PI3-kinase activation is necessary and sufficient for Akt phosphorylation. Consequently, Akt kinase activation was used as a measure of PI3-kinase activity. In a stable transfection system, dominant-negative mutants of myeloid differentiation factor 88 (MyD88) and interleukin-1 (IL-1) receptor-associated kinase 1 (IRAK-1) (MyD88-TIR and IRAK-DD, respectively) blocked Akt kinase activity in response to LPS and IL-1 beta. A dominant-negative mutant (Mal-P/H) of MyD88 adapter-like protein (Mal), a protein with homology to MyD88, failed to inhibit LPS- or IL-1 beta-induced Akt activity. Moreover, a dominant-negative mutant of p85 (p85-DN) inhibited the NF-kappa B luciferase activity, IL-6 production, and I kappa B alpha degradation elicited by LPS and IL-1 beta but not that stimulated by tumor necrosis factor alpha. The dominant-negative mutant of Akt partially inhibited the NF-kappa B luciferase activity evoked by LPS and IL-1 beta. However, expression of a constitutively activated Akt failed to induce NF-kappa B luciferase activity. These findings indicate that TLR4- and IL-1R-induced PI3-kinase activity is mediated by the adapter proteins MyD88 and IRAK-1 but not Mal. Further, these studies suggest that PI3-kinase is an important mediator of LPS and IL-1 beta signaling leading to NF-kappa B activation in endothelial cells and that Akt is necessary but not sufficient for NF-kappa B activation by TLR4.

Radiation sensitization of human cancer cells in vivo by inhibiting the activity of PI3K using LY294002

Multiple genetic alterations such as in Ras or EGFR can result in sustained signaling through PI3K. Our previous experiments have shown that resistance to radiation results from PI3K activity in cells in culture. Here we examined whether inhibition of PI3K in vivo would sensitize tumors to radiation. The human bladder cancer cell line T24 has amplified and mutated H-Ras resulting in sustained PI3K activity and phosphorylation of the downstream target of PI3K, Akt. Nude mice bearing T24 tumor cell xenografts were randomly assigned to one of four groups: control, radiation alone, the PI3K inhibitor LY294002 alone, or combined LY294002 and radiation. The LY294002 was delivered intraperitoneally to the mice. Downregulation of Akt was documented by Western blot analysis of tumor lysates. In vivo sensitization was measured using clonogenic assays or regrowth assays.A dose of 100 mg/kg of LY294002, but not 50 mg/kg, consistently eliminated the phosphorylation of Akt. This inhibition was transient, and Akt activity returned after 30 min. This dose resulted in severe respiratory depression and lethargy resolving without lethality. It is not possible to tell whether these side effects of LY294002 were mechanism-based or idiosyncratic. The PI3K inhibitor LY294002 by itself had minimal antitumor effect. The combination of LY294002 and radiation resulted in significant and synergistic reduction in clonogenicity and growth delay. Inhibition of PI3K by LY294002 can synergistically enhance radiation efficacy. This acts as a proof of principle that inhibition of the Ras to PI3K pathway could be useful clinically.

PI3K and negative regulation of TLR signaling

Excessive immune responses are detrimental to the host and negative feedback regulation is crucial for the maintenance of immune-system integrity. Recent studies have shown that phosphoinositide 3-kinase (PI3K) is an endogenous suppressor of interleukin-12 (IL-12) production triggered by Toll-like receptor (TLR) signaling and limits excessive Th1 polarization. Unlike IRAK-M (IL-1 receptor-associated kinase-M) and SOCS-1 (suppressor of cytokine signaling-1) that are induced by TLR signaling and function during the second or continuous exposure to stimulation, PI3K functions at the early phase of TLR signaling and modulates the magnitude of the primary activation. Thus, PI3K, IRAK-M and SOCS-1 have unique roles in the gate-keeping system, preventing excessive innate immune responses.