Is there a relationship between phosphatidylinositol trisphosphate and F-actin polymerization in human neutrophils?

Recent Advances in the Development of Anticancer Drugs that Act against Signalling Pathways

Cancer can be considered a disease of deranged intracellular signalling. The intracellular signalling pathways that mediate the effects of oncogenes on cell growth and transformation present attractive targets for the development of new classes of drugs for the prevention and treatment of cancer. This is a new approach to developing anticancer drugs and the potential, as well as some of the problems, inherent in the approach are discussed. Anticancer drugs that produce their effects by disrupting signalling pathways are already in clinical trial. Some properties of these drugs, as well as other inhibitors of signalling pathways under development as potential anticancer drugs, are reviewed.

U-73122 reduces the cell growth in cultured MG-63 ostesarcoma cell line involving Phosphoinositide-specific Phospholipases C

The definition of the number and nature of the signal transduction pathways involved in the pathogenesis and the identification of the molecules promoting metastasis spread might improve the knowledge of the natural history of osteosarcoma, also allowing refine the prognosis and opening the way to novel therapeutic strategies. Phosphatydil inositol (4,5) bisphosphate (PIP2), belonging to the Phosphoinositide (PI) signal transduction pathway, was related to the regulation of ezrin, an ezrin-radixin-moesin protein involved in metastatic osteosarcoma spread. The levels of PIP2 are regulated by means of the PI-specific Phospholipase C (PLC) enzymes. Recent literature data suggested that in osteosarcoma the panel of expression of PLC isoforms varies in a complex and unclear manner and is related to ezrin, probably networking with Ras GTPases, such as RhoA and Rac1. We analyzed the expression and the subcellular localization of PLC enzymes in cultured human osteosarcoma MG-63 cells, commonly used as an experimental model for human osteoblasts, using U-73122 PLC inhibitor, U-73343 inactive analogue, and by silencing ezrin. The treatment with U-73122 significantly reduces the number of MG-63 viable cells and contemporarily modifies the expression and the subcellular localization of selected PLC isoforms. U-73122 reduces the cell growth in cultured MG-63 ostesarcoma cell line involving PI-specific Phospholipases C.

Relationship of light scatter change and Cdc42-regulated actin status

BACKGROUND

Cdc42 GTPase has important roles in regulating intracellular actin reorganization. The current methods to monitor actin changes are typically complex and point by point.

METHODS

The effects of Cdc42 inhibitors on the side scatter changes were tested in a newly developed continuous assay using the flow cytometer. Staining with fluorescently labeled phalloidin was used for comparison.

RESULTS

Cdc42-specific inhibitors caused dose-dependent changes of both the right-angle side scatter and the phalloidin-stained actin.

CONCLUSIONS

The right-angle light scatter change can be used as a method to circumvent phalloidin staining and be an early convenient step in screening Cdc42 inhibitors. © 2015 International Clinical Cytometry Society.

Ezrin silencing remodulates the expression of Phosphoinositide-specific Phospholipase C enzymes in human osteosarcoma cell lines

Ezrin, a protein belonging to the Ezrin, radixin and moesin (ERM) family, was engaged in the metastatic spread of osteosarcoma. The Protein 4.1, Ezrin, radixin, moesin (FERM) domain of Ezrin binds the membrane Phosphatydil inositol (4,5) bisphosphate (PIP2), a crucial molecule belonging to the Phosphoinositide (PI) signal transduction pathway. The cytoskeleton cross-linker function of Ezrin largely depends on membrane PIP2 levels, and thus upon the activity of related enzymes belonging to the PI-specific phospholipase C (PI-PLC) family. Based on the role of Ezrin in tumour progression and metastasis, we silenced the expression of Vil2 (OMIM *123900), the gene which codifies for Ezrin, in cultured human osteosarcoma 143B and Hs888 cell lines. After Ezrin silencing, the growth rate of both cell lines was significantly reduced and morphogical changes were observed. We also observed moderate variations both of selected PI-PLC enzymes within the cell and of expression of the corresponding PLC genes. In 143B cell line the transcription of PLCB1 decreased, of PLCG2 increased and of PLCE differed in a time-dependent manner. In Hs888, the expression of PLCB1 and of PLCD4 significantly increased, of PLCE moderately increased in a time dependent manner; the expression of PLCG2 was up-regulated. These observations indicate that Ezrin silencing affects the transcription of selected PLC genes, suggesting that Ezrin might influence the expression regulation of PI-PLC enzymes.

PI3K mediated electrotaxis of embryonic and adult neural progenitor cells in the presence of growth factors

Correct guidance of the migration of neural progenitor cells (NPCs) is essential for the development and repair of the central nervous system (CNS). Electric field (EF)-guided migration, electrotaxis, has been observed in many cell types. We report here that, in applied EFs of physiological magnitude, embryonic and adult NPCs show marked electrotaxis, which is dependent on the PI3K/Akt pathway. The electrotaxis was also evidenced by ex vivo investigation that transplanted NPCs migrated directionally towards cathode in organotypic spinal cord slice model when treated with EFs. Genetic disruption or pharmacological inhibition of phosphoinositide 3-kinase (PI3K) impaired electrotaxis, whereas EF exposure increased Akt phosphorylation in a growth factor-dependent manner and increased phosphatidylinositol-3,4,5-trisphosphate (PIP3) levels. EF treatments also induced asymmetric redistribution of PIP3, growth factor receptors, and actin cytoskeleton. Electrotaxis in both embryonic and adult NPCs requires epidermal growth factor (EGF) and fibroblast growth factor (FGF). Our results demonstrate the importance of the PI3K/Akt pathway in directed migration of NPCs driven by EFs and growth factors and highlight the potential of EFs to enhance the guidance of various NPC populations in CNS repair therapies.

Gene delivery of l-caldesmon protects cytoskeletal cell membrane integrity against adenovirus infection independently of myosin ATPase and actin assembly

The cytoskeleton is critical to the viral life cycle. Agents like cytochalasin inhibit viral infections but cannot be used for antiviral therapy because of their toxicity. We report the efficacy, safety, and mechanisms by which gene delivery of human wild-type low-molecular-weight caldesmon (l-CaD) protects cell membrane integrity from adenovirus infection in a DF-1 cell line, an immortalized avian fibroblast that is null for l-CaD. Transfection with an adenovirus (Ad)-controlled construct mediated a dose-dependent decline in transcellular resistance. In accordance with a computational model of cytoskeletal membrane properties, Ad disturbed cell-cell and cell-matrix adhesion and membrane capacitance. Transfection with the Ad-l-CaD construct attenuated adenovirus-mediated loss in transcellular resistance. Quantitation of vinculin-stained plaques revealed an increase in total focal contact mass in monolayers transfected with the Ad-l-CaD construct. Expression of l-CaD protected transcellular resistance through primary effects on membrane capacitance and independently of actin solubility and effects on prestress, as measured by the decline in isometric tension in response to cytochalasin D. Expression of l-CaD exhibited less Trypan blue cell toxicity than cytochalasin, and, unlike cytochalasin, it did not interfere with wound closure or adversely effect transcellular resistance. These findings demonstrate the gene delivery of wild-type human l-CaD as a potentially efficacious and safe agent that inhibits some of the cytopathic effects of adenovirus.

Retinoic acid signaling through PI 3-kinase induces differentiation of human endometrial adenocarcinoma cells

The specific signals required for actin polymerization in response to extracellular factors remain unknown. However, in many cell types, there is a correlation between actin polymerization, activation of phosphatidylinositol 3-kinase (PI 3-kinase), and the production of the second messenger phosphatidylinositol-3,4,5-triphosphate. Increased levels of PI 3-kinase have been detected during cell growth and transformation. However, PI 3-kinase is also activated during differentiation, suggesting that PI 3-kinase and its lipid products also play a role in the regulation of cellular differentiation. The newly characterized CAC-1 cell line established from a poorly differentiated human endometrial adenocarcinoma (Exp. Mol. Pathol. 69 (2000), 175) was used as a model to investigate the role of PI 3-kinase in differentiation induction. CAC-1 cells differentiated upon treatment with pharmacological doses of retinoids (1 micro M of 13-cis or all-trans), evidenced by actin filament reorganization, and cell enlargement. PI 3-kinase staining is primarily localized to perinuclear regions in untreated cells. However, retinoic acid treatment induced PI 3-kinase to relocalize throughout the cytoplasm. Subcellular fractionation and Western blotting confirmed that PI 3-kinase decreased in the particulate fraction, concurrent with retinoid-induced differentiation. Interestingly, pretreatment with the PI 3-kinase inhibitor wortmannin (100 nM) prior to retinoic acid treatment prevented retinoic acid-induced actin reorganization and cell enlargement. To distinuish whether retinoid regulation of PI 3-kinase is mediated through traditional nuclear retinoic acid receptors, the levels of retinoic acid receptor-beta (RAR-beta) protein were evaluated. Retinoid treatment did not alter RAR-beta protein levels compared to controls. These data suggest that PI 3-kinase activity and cytoplasmic relocalization are required for retinoid-induced differentiation of poorly differentiated human endometrial adenocarcinoma cells.

Chemotactic signaling pathways in neutrophils: from receptor to actin assembly

In this review, we present an overview of the signaling elements between neutrophil chemotactic receptors and the actin cytoskeleton that drives cell motility. From receptor-ligand interactions, activation of heterotrimeric G-proteins, their downstream effectors PLC and PI-3 kinase, the activation of small GTPases of the Rho family, and their regulation of particular cytoskeletal regulatory proteins, we describe pathways specific to the chemotaxing neutrophil and elements documented to be important for neutrophil function.

Nucleotides induce chemotaxis and actin polymerization in immature but not mature human dendritic cells via activation of pertussis toxin-sensitive P2y receptors

Dendritic cells (DCs) are considered the principal initiators of immune response because of their ability to migrate into peripheral tissues and lymphoid organs, process antigens, and activate naive T cells. There is evidence that extracellular nucleotides regulate certain functions of DCs via G-protein-coupled P2Y receptors (P2YR) and ion-channel-gated P2X receptors (P2XR). Here we investigated the chemotactic activity and analyzed the migration-associated intracellular signaling events such as actin reorganization and Ca(++) transients induced by common P2R agonists such as adenosine 5'-triphosphate (ATP) and 2-methylthioadenosine triphosphate, the P2YR agonists UTP and adenosine 5'-diphosphate (ADP), or the P2XR agonists alphabeta-methylenadenosine-5'-triphosphate and 2',3'-(4-benzoyl)benzoyl-ATP. The common P2R agonists and the selective P2YR agonists turned out to be potent chemotactic stimuli for immature DCs, but not for mature DCs. In contrast, P2XR agonists had only marginal chemotactic activity in both DC types. Chemotaxis was paralleled by a rise in the intracellular Ca(++) concentration and by actin polymerization. Studies with pertussis toxin implicated that intracellular signaling events such as actin polymerization, mobilization of intracellular Ca(++), and migration induced by nucleotides was mediated via G(i/o) protein-coupled P2YR. Moreover, functional studies revealed selective down-regulation of this G(i/o) protein-coupled chemotactic P2YR responsiveness during maturation, although immature and mature DCs expressed similar amounts of mRNA for the P2R subtypes (P2Y(2)R, P2Y(4)R, P2Y(5)R, P2Y(7)R, P2Y(11)R and P2X(1)R, P2X(4)R, P2X(7)R), and no major differences in respect to the mRNA expression of these receptors could be observed by semiquantitative reverse transcription and polymerase chain reaction (RT-PCR). In summary, our data describe a differential chemotactic response of immature and mature DCs to nucleotides, and lend further support to the hypothesis that P2R are a novel class of immunomodulatory plasma membrane receptors suitable for pharmacological intervention.

Eukaryotic cell locomotion depends on the propagation of self-organized reaction-diffusion waves and oscillations of actin filament assembly

Actin filament (F-actin) assembly kinetics determines the locomotion and shape of crawling eukaryotic cells, but the nature of these kinetics and their determining reactions are unclear. Live BHK21 fibroblasts, mouse melanoma cells, and Dictyostelium amoebae, locomoting on glass and expressing Green Fluorescent Protein-actin fusion proteins, were examined by confocal microscopy. The cells demonstrated three-dimensional bands of F-actin, which propagated throughout the cytoplasm at rates usually ranging between 2 and 5 microm/min in each cell type and produced lamellipodia or pseudopodia at the cell boundary. F-actin's dynamic behavior and supramolecular spatial patterns resembled in detail self-organized chemical waves in dissipative, physico-chemical systems. On this basis, the present observations provide the first evidence of self-organized, and probably autocatalytic, chemical reaction-diffusion waves of reversible actin filament assembly in vertebrate cells and a comprehensive record of wave and locomotory dynamics in vegetative-stage Dictyostelium cells. The intensity and frequency of F-actin wavefronts determine locomotory cell projections and the rotating oscillatory waves, which structure the cell surface. F-actin assembly waves thus provide a fundamental, deterministic, and nonlinear mechanism of cell locomotion and shape, which complements mechanisms based exclusively on stochastic molecular reaction kinetics.

Phosphoinositides regulate membrane-dependent actin assembly by latex bead phagosomes

Actin assembly on membrane surfaces is an elusive process in which several phosphoinositides (PIPs) have been implicated. We have reconstituted actin assembly using a defined membrane surface, the latex bead phagosome (LBP), and shown that the PI(4,5)P(2)-binding proteins ezrin and/or moesin were essential for this process (). Here, we provide several lines of evidence that both preexisting and newly synthesized PI(4,5)P(2), and probably PI(4)P, are essential for phagosomal actin assembly; only these PIPs were routinely synthesized from ATP during in vitro actin assembly. Treatment of LBP with phospholipase C or with adenosine, an inhibitor of type II PI 4-kinase, as well as preincubation with anti-PI(4)P or anti-PI(4,5)P(2) antibodies all inhibited this process. Incorporation of extra PI(4)P or PI(4,5)P(2) into the LBP membrane led to a fivefold increase in the number of phagosomes that assemble actin. An ezrin mutant mutated in the PI(4,5)P(2)-binding sites was less efficient in binding to LBPs and in reconstituting actin assembly than wild-type ezrin. Our data show that PI 4- and PI 5-kinase, and under some conditions also PI 3-kinase, activities are present on LBPs and can be activated by ATP, even in the absence of GTP or cytosolic components. However, PI 3-kinase activity is not required for actin assembly, because the process was not affected by PI 3-kinase inhibitors. We suggest that the ezrin-dependent actin assembly on the LBP membrane may require active turnover of D4 and D5 PIPs on the organelle membrane.

Phosphatidylinositol 3′-Kinase Blocks CD95 Aggregation and Caspase-8 Cleavage at the Death-Inducing Signaling Complex by Modulating Lateral Diffusion of CD95

Activation of phosphatidylinositol 3'-kinase (PI 3'-K) after ligation of CD3 protects Th2 cells from CD95-mediated apoptosis. Here we show that protection is achieved by inhibition of the formation of CD95 aggregates and consequent activation of caspase-8. Inhibition of aggregate formation is mediated by changes in the actin cytoskeleton, which in turn inhibit lateral diffusion of CD95, reducing its diffusion coefficient, D, 10-fold. After cytochalasin D treatment of stimulated cells, the lateral diffusion of CD95 increases to the value measured on unstimulated cells, and CD95 molecules aggregate to process caspase-8 and mediate apoptosis. Regulation of functional receptor formation by modulating lateral diffusion is a novel mechanism for controlling receptor activity.

Evidence for the involvement of phosphatidylinositol 3-kinase in fMLP-stimulated neutrophil adhesion to ICAM-1-transfected cells

Phosphatidylinositol 3-kinase (PI-3K) controls important intracellular steps involved in inflammation, immunity, and cell growth. PI-3K also modulates leukocyte integrin adhesiveness. In this study we evaluated the role of PI-3K on neutrophil adhesion to intercellular adhesion molecule-1 (ICAM-1)-transfected cells. N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated neutrophil adhesion was inhibited by wortmannin and LY294002, two unrelated PI-3K inhibitors, whereas phorbol myristate acetate (PMA)-induced neutrophil adhesion was not inhibited by them. After fMLP stimulation, a rapid activation of AKT and ERK was observed. However, only activation of AKT was reversed by the PI-3K inhibitors. Neutrophil expression of the beta2-integrins Mac-1, lymphocyte function-associated antigen-1(LFA-1), and gp150.95 was not affected by wortmannin, nor was expression of the activation epitope recognized by MAB24. We conclude that (a) PI-3K is involved in fMLP-activated neutrophil adhesion to ICAM-1-transfected cells, (b) the mechanism involved is not mediated by the modulation of beta2-integrin expression or activation, and (c) another mechanism seems to involve the adhesion to ICAM-1 when a cellular system of adhesion is used.

Stimulation of human neutrophils by chemotactic factors is associated with the activation of phosphatidylinositol 3-kinase gamma

The activation of human polymorphonuclear neutrophil leukocytes (neutrophils) is associated with an increased synthesis of the highly phosphorylated phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)). The aims of the present investigation were to determine whether the newly described, G protein-dependent phosphatidylinositol 3-kinase (PI3K), p110gamma, was involved in the responses to chemotactic factors interacting with G protein-coupled receptors. The presence of p110gamma in neutrophils was first established both at the protein and the mRNA level. Stimulation of the cells with fMet-Leu-Phe or interleukin-8 increased the PI3K activity in p110gamma, but not p85, immunoprecipitates. The time course of this effect (threshold within less than 5 s, maximal activation at 10-15 s) was consistent with that of the generation of PtdIns(3,4,5)P(3). Wortmannin, a PI3K inhibitor, abrogated the effects of fMet-Leu-Phe, which were also significantly inhibited by pertussis toxin. Finally, fMet-Leu-Phe also induced a significant translocation of p110gamma to a particulate fraction derived from these cells. These data indicate that p110gamma represent the major PI3K activated by fMet-Leu-Phe and interleukin-8 at very early time points following the stimulation of human neutrophils.

A membrane-permeant ester of phosphatidylinositol 3,4, 5-trisphosphate (PIP(3)) is an activator of human neutrophil migration

Activity of phosphatidylinositol (PI) 3-kinase is required for optimal migration of human neutrophils [Niggli and Keller (1999) Eur. J. Pharmacol. 335, 43-52]. We have tested the direct effect of a product of PI 3-kinase, phosphatidylinositol 3,4,5-trisphosphate (PIP(3)), on neutrophil migration. To this end, a membrane-permeant ester of PIP(3), dilauroyl phosphatidylinositol 3,4, 5-trisphosphate-heptakis-(acetooxymethyl)ester (PIP(3)/AM) was used. PIP(3)/AM (ED(50): 10-17 microM) induced development of polarity and accumulation of F-actin in the leading lamellae in up to 70% of the cells. These cells exhibited stimulated random migration, comparable to that observed in uniform concentrations of chemotactic peptide. Evidence is provided for a role of Rho-kinase and for activation of PI 3-kinase in a positive feedback loop in PIP(3)/AM-induced motility.

Clustering of beta(2)-integrins on human neutrophils activates dual signaling pathways to PtdIns 3-kinase

The beta(2)-integrins on leukocytes can serve as a signaling unit during cell adhesion and locomotion, and to further clarify this important property we investigated the possible mechanisms of beta(2)-integrin-induced activation of PtdIns 3-kinase. It has previously been demonstrated that clustering of beta(2)-integrins activates p21(ras) by a tyrosine kinase-dependent pathway, and here we show that active p21(ras) interacts with its downstream target, PtdIns 3-kinase. Engagement of beta(2)-integrins also activates the tyrosine kinases p58(c-fgr) and p59/61(hck) and causes them to associate with the p85 subunit of PtdIns 3-kinase. These findings suggest a mechanism whereby p58(c-fgr) and p59/61(hck) are directly involved in the activation of PtdIns 3-kinase. No coupling between p58(c-fgr) and p59/61(hck) could be detected; hence these kinases probably trigger independent but parallel signals to PtdIns 3-kinase. The effect of beta(2)-integrin clustering on PtdIns 3-kinase activity was monitored as the activation of protein kinase B (PKB). Stimulation of PKB by beta(2)-integrins was abolished by genistein and wortmannin but not by using methyl transferase inhibitors to abrogate the influence of p21(ras)-related proteins. Thus, even if PtdIns 3-kinase is not activated by p21(ras), it can maintain full enzyme activity due to the mentioned interaction with p58(c-fgr) or p59/61(hck). These tyrosine kinases apparently activate similar pathways that operate in parallel and therefore have the potential to substitute for each other in mediating adhesion and regulating cell locomotion.

Interleukin-6 activates phosphatidylinositol-3 kinase, which inhibits apoptosis in human prostate cancer cell lines

BACKGROUND

A number of recent studies have identified interleukin (IL)-6 as an important regulator of prostate cancer growth. Here, we investigate the potential interaction of IL-6 with phosphatidylinositol (PI)-3 kinase, a key growth regulatory enzyme, in prostate cancer cell lines.

METHODS

Tyrosine phosphorylation of p85, the regulatory subunit of PI-3 kinase, in the human prostate cancer cell lines LNCaP and PC-3 was assessed by sequential immunoprecipitation with anti-p85 antibody and immunoblotting with anti-phosphotyrosine. The effects of wortmannin, an inhibitor of PI-3 kinase, and/or IL-6 on cell growth were assessed by MTT assays. DNA laddering experiments were performed to assay for programmed cell death.

RESULTS

Tyrosine phosphorylation of p85 is upregulated by IL-6 in both LNCaP and PC-3. IL-6 promotes coprecipitation of p85 with gp130, the signal-transducing component of the IL-6 receptor. Inhibition of PI-3 kinase with wortmannin induces programmed cell death in PC-3 cells. In contrast, wortmannin has no effect on LNCaP growth when used alone; however, combined with IL-6, wortmannin promotes apoptosis in these cells.

CONCLUSIONS

PI-3 kinase is involved in IL-6 signal transduction and delivers an antiapoptotic signal in human prostate cancer cell lines.

Both wortmannin and simvastatin inhibit the adipogenesis in 3T3-L1 cells during the late phase of differentiation

As we reported previously, both wortmannin and lovastatin inhibit the differentiation in 3T3-L1 cells when these drugs were applied during the insulin-induced cell differentiation. In the present study, 3T3-L1 cells were treated with wortmannin and simvastatin after the completion of the insulin-stimulation, and differentiation was found to be significantly decreased by these drugs. This suggests intracellular signaling pathways play roles in the differentiation of 3T3-L1 cells even after the completion of the insulin-stimulation and also suggests that not only the early phase (day 0 to day 2) but also the late phase (day 2 to day 4) of differentiation is important for the differentiation of the cells.

Mitogen-activated protein kinase activation regulates intestinal epithelial differentiation

BACKGROUND & AIMS

The human colon cancer-derived cell line HT29 displays a multipotent phenotype. A subclone of HT29 cells containing numerous mucous granules and termed HT29-18-N2 was studied to determine the cellular mechanisms underlying a switch to the differentiated phenotype.

METHODS

Northern (RNA) blotting, immunoblotting, and immunocytochemistry of HT29-N2 cells, grown under glucose-containing and glucose-free conditions with or without the use of the mitogen-activated protein (MAP) kinase kinase (MEK) inhibitor PD98059, were performed.

RESULTS

Loss of activation of the MAP kinases ERK 1 and ERK 2 in HT29-N2 cells upon a change to glucose-free growth medium preceded the change in phenotype and up-regulation of the goblet cell gene product intestinal trefoil factor (ITF). Long-term pharmacological MAP kinase inhibition with the MEK inhibitor PD98059 induced expression of the terminal differentiation markers ITF, sucrase-isomaltase, and the mucin gene MUC2. This was accompanied by morphological evidence of gland formation and mucin secretion and the appearance of discrete goblet cell and enterocyte populations. Induction of ITF and sucrase-isomaltase after MEK inhibition in HT29-N2 cells did not involve loss of MAP kinase responsiveness and was not mediated by receptor tyrosine kinases.

CONCLUSIONS

Regulation of ERK activation may be a key biochemical switch responsible for terminal differentiation of components of the crypt-villus unit.

A novel monoclonal antibody against rat LFA-1: blockade of LFA-1 and CD4 augments class II MHC expression on T cells

Previously, we have shown that myelin basic protein (MBP)-specific Lewis rat GP2.E5/R1 (R1) T cells cultured with antigen and irradiated syngeneic splenocytes (IrrSPL) in the presence of anti-CD4 and LRTC1 monoclonal antibodies (MAbs) become highly effective antigen presenting cells (APC). The purpose of these studies was to identify the ligand for the LRTC1 MAb and to determine whether this MAb affected MBP-stimulated IL-2 production and expression of MHC class II molecules by T cells. In the current studies, we show that the LRTC1 MAb specifically immunoprecipitated molecular species of approximately 95, 150, and 180 kD. Commercially available anti-CD18 (beta2 integrin, beta-chain of LFA-1, MAC-1, and p150, 95) and LRTC1 MAb immunoprecipitated proteins with identical mobilities on 1-D and 2-D SDS-PAGE gels. Moreover, anti-CD18 and LRTC1 immunoprecipitates also showed identical mobilities on 1-D gels after enzymatic cleavage of N-linked oligosaccharides and thereby had the same patterns of differential glycosylation. Anti-CD4 MAb W3/25 and LRTC1 MAb synergistically inhibited T-cell IL-2 mRNA and IL-2 bioactivity, but augmented antigen-stimulated surface I-A on R1 T cells. In conclusion, these studies describe the characteristics of a novel anti-LFA-1 MAb, LRTC1, which should prove useful in studying costimulatory and adhesion pathways among rat leukocytes.

Intracellular Signal-transducing elements involved in transendothelial migration of lymphoma cells

To investigate the molecular mechanisms underlying transendothelial migration of tumor cells, an essential process for their hematogenous dissemination, we developed an in vitro model system that allows the separate monitoring of cell adhesion and transmigration processes. This system uses a human pre-B lymphoma cell line, Nalm-6, and a cultured mouse endothelial cell line, KOP2.16. Nalm-6 cells rapidly adhered to KOP2.16 and subsequently transmigrated underneath them. Using this model, we examined the effects on transendothelial migration, of various reagents which specifically interfere with the function of intracellular signal transduction molecules. Treatment of Nalm-6 cells with wortmannin (WMN), herbimycin A, pertussis toxin, or C3 exoenzyme of Clostridium botulinum, which specifically inhibit P13 kinase and/or myosin light chain kinase, herbimycin-sensitive tyrosine kinases, heterotrimeric G proteins, and the small G proteins, and the small G proteins rho/rac, respectively, reduced transmigration in a dose-dependent manner, Pretreatment of KOP2.16 endothelial cells with WMN also reduced transmigration in a dose-dependent manner. Binding of Nalm-6 binding to KOp2.16 was not affected, even when Nalm-6 or KOP2.16 cells were pretreated with these inhibitors, indicating that the reduction of transmigration was not due to a reduction of Nalm-6 to KOP2.16. These results also indicate that the signal transduction pathway(s) involved in transmigration can be dissociated from that of adhesion. Our results support the notion that endothelial cells are not a passive barrier in lymphoma extravasation, but that they assist lymphoma cell extravasation.

The small GTP-binding protein Rac promotes the dissociation of gelsolin from actin filaments in neutrophils

Gelsolin is an actin filament-capping protein that has been shown to play a key role in cell migration. Here we have studied the involvement of phosphoinositide 3-kinase (PI 3-kinase) and GTP-binding proteins (G-proteins) in the regulation of gelsolin-actin interactions in neutrophils. Inhibition of PI 3-kinase activity in vivo by wortmannin did not affect the dissociation of actin-gelsolin (1:1) complexes induced by neutrophil stimulation with N-formyl-Met-Leu-Phe. Guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) indirectly promoted the dissociation of actin-gelsolin complexes in a cell-free system using neutrophil cytosol, and this effect was blocked by the GDP dissociation inhibitor for Rho (Rho-GDI). The GTPgammaS-loaded ialpha2 and the beta1gamma2 subunits of heterotrimeric G-proteins (Gialpha2 and Gbeta1gamma2) also triggered actin-gelsolin dissociation in a Rho-GDI-sensitive manner. GTP-loaded activated Rac, but not activated Rho, induced the dissociation of cytosolic actin-gelsolin complexes. The guanine nucleotide exchange on Rac was increased by addition of GTPgammaS-loaded Gialpha2 or Gbeta1gamma2 to neutrophil cytosol. These findings suggest that activation of Rac by G-protein-coupled receptors in neutrophils triggers uncapping of actin filaments, independently of PI 3-kinase.

Phosphatidylinositol 3-kinase association with the osteoclast cytoskeleton, and its involvement in osteoclast attachment and spreading

Osteoclast activation involves attachment to the mineralized bone matrix and reorganization of the cytoskeleton, leading to polarization of the cell. Signaling molecules, PI3-kinase, rho A, and pp60c-src, were shown to be essential for osteoclastic bone resorption. In this study we have focused on the involvement of these signaling molecules in the early event of osteoclast activation: attachment, spreading, and organization of the cytoskeleton. Highly purified osteoclasts were fractionated into Triton X-100-soluble or cytosolic and Triton X-100-insoluble or cytoskeletal fractions, and the distribution of above-mentioned signaling molecules between the two fractions was examined. PI3-kinase, rho A, and pp60c-src all showed translocation to the cytoskeletal fraction upon osteoclast attachment to plastic. However, PI3-kinase and rho A, but not pp60c-src, showed further translocation of 2.4- and 3.2-fold, respectively, upon attachment of osteoclasts to bone. PI3-kinase translocation to the cytoskeleton was inhibited by either cytochalasin B or colchicine. Furthermore, treatment of osteoclasts with the PI3-kinase inhibitor wortmannin decreased its translocation, suggesting that PI3-kinase activity was needed for its translocation. Moreover, wortmannin inhibited osteoclast attachment to both bone and plastic and caused drastic changes in osteoclast morphology resulting in rounding of the cells, disappearance of F-actin structures or podosomes, and appearance of punctate or vesicular structures inside the cells. Osteoblastic MB1.8 cells and IC-21 macrophages did not show additional translocation of PI3-kinase or rho A upon attachment to bone or changes in attachment or morphology in response to wortmannin. Finally, PI3-kinase coimmunoprecipitated with alpha v beta 3 integrin from osteoclasts.

The N-formyl peptide receptor: a model for the study of chemoattractant receptor structure and function

N-formyl peptides, such as fMet-Leu-Phe, are one of the most potent chemoattractants for phagocytic leukocytes. The interaction of N-formyl peptides with their specific cell surface receptors has been studied extensively and used as a model system for the characterization of G-protein-coupled signal transduction in phagocytes. The cloning of the N-formyl peptide receptor cDNA from several species and the identification of homologous genes have allowed detailed studies of structural and functional aspects of the receptor. Recent findings that the receptor is expressed in nonhematopoietic cells and that nonformylated peptides can activate the receptor suggest potentially novel functions and the existence of additional ligands for this receptor.

The phosphatidylinositol 3-kinase inhibitor wortmannin markedly reduces chemotactic peptide-induced locomotion and increases in cytoskeletal actin in human neutrophils

To define a possible role of the enzyme phosphatidylinositol 3-kinase (PI 3-kinase) in motile functions of neutrophils, we have used a potent inhibitor of this enzyme, [1S-(1alpha,6b alpha,9a beta,11alpha,11bbeta)]-1-(acetyloxy)-1,6b,7,8,9a,10,11 ,11b-octahydro-1-(methoxymethyl)-9a,11b-dimethyl-3H-furo[4,3,2-de]indeno [4,5-h]-2-benzopyran-3,6,9-trione (wortmannin). Wortmannin markedly attenuated chemotactic peptide-induced development of polarity, locomotion and increases in cytoskeletal actin and alpha-actinin in human neutrophils at low, nM, concentrations (ED50 = 4-40 nM; 0.4-3 pmol/10(6) cells). The increase in cytoskeletal actin induced by phorbol-12-myristate-13-acetate in contrast was not affected by wortmannin (18 pmol/10[6] cells). Moreover, the increase in total F-actin induced by an incubation for 1 min with chemotactic peptide was much less sensitive to wortmannin than increases in cytoskeletal actin; 80 pmol/10(6) cells were necessary for half-maximal inhibition. Wortmannin thus appears to primarily affect F-actin organization, rather than polymerization. Inhibition of development of polarity by wortmannin correlated with inhibition of production of phosphatidylinositol 3,4,5-trisphosphate. According to our findings, activation of a wortmannin-sensitive target, very likely PI 3-kinase, is required for optimal chemotactic peptide-induced neutrophil motility.

Down-regulation of LFA-1-mediated T cell adhesion induced by the HIV envelope glycoprotein gp160 requires phosphatidylinositol-3-kinase activity

Human immunodeficiency virus binds to CD4+ T lymphocyte by the interaction, in part, between its gp120 envelope glycoprotein and the CD4 molecule. We and others have reported that the lipid kinase phosphatidylinositol-3-kinase (PI3-kinase) is associated with the CD4-p56lck complex and can be activated by various CD4 ligands. In a previous report we showed that the gp160 envelope down-regulates lymphocyte function-associated antigen-1 (LFA-1)-dependent adhesion between CD4+ T cells and B cells. This down-regulation was shown to be p56lck-dependent. Here we investigate the role of PI3-kinase in the inhibition of adhesion induced by gp160 binding to CD4. We found that gp160 activates the PI3-kinase of HUT78 CD4+ T cell lines in a way dependent on CD4-p56lck association, since no activation was detected when the interaction between CD4 and p56lck was disrupted. It was also shown, using different inhibitors of the PI3-kinase (wortmannin, Ly294002 and antisense oligonucleotides), that this lipid kinase was necessary for the down-regulation of LFA-1-mediated adhesion induced by gp160. These results strongly suggest that PI3-kinase activation induced by gp160 leads to down-regulation of LFA-1-mediated T cell adhesion to B cells. Inhibition by gp160 of cytoskeleton rearrangement-dependent, anti-CD3-mediated T cell adhesion to B cells was blocked by neutralization of PI3-kinase activity, while inhibition of cytoskeleton rearrangement-independent, Mg(2+)-induced T cell adhesion was not. These results emphasize the role of PI3-kinase in the regulation of cytoskeleton structure. It is proposed that gp160 activates both p56lck and PI3-kinase which lead to a cytoskeleton organization unfavorable for LFA-1 function.

Gelsolin binding to phosphatidylinositol 4,5-bisphosphate is modulated by calcium and pH

The actin cytoskeleton of nonmuscle cells undergoes extensive remodeling during agonist stimulation. Lamellipodial extension is initiated by uncapping of actin nuclei at the cortical cytoplasm to allow filament elongation. Many actin filament capping proteins are regulated by phosphatidylinositol 4,5-bisphosphate (PIP2), which is hydrolyzed by phospholipase C. It is hypothesized that PIP2 dissociates capping proteins from filament ends to promote actin assembly. However, since actin polymerization often occurs at a time when PIP2 concentration is decreased rather than increased, capping protein interactions with PIP2 may not be regulated solely by the bulk PIP2 concentration. We present evidence that PIP2 binding to the gelsolin family of capping proteins is enhanced by Ca2+. Binding was examined by equilibrium and nonequilibrium gel filtration and by monitoring intrinsic tryptophan fluorescence. Gelsolin and CapG affinity for PIP2 were increased 8- and 4-fold, respectively, by microM Ca2+, and the Ca2+ requirement was reduced by lowering the pH from 7.5 to 7.0. Studies with the NH2- and COOH-terminal halves of gelsolin showed that PIP2 binding occurred primarily at the NH2-terminal half, and Ca2+ exposed its PIP2 binding sites through a change in the COOH-terminal half. Mild acidification promotes PIP2 binding by directly affecting the NH2-terminal sites. Our findings can explain increased PIP2-induced uncapping even as the PIP2 concentration drops during cell activation. The change in gelsolin family PIP2 binding affinity during cell activation can impact divergent PIP2-dependent processes by altering PIP2 availability. Cross-talk between these proteins provides a multilayered mechanism for positive and negative modulation of signal transduction from the plasma membrane to the cytoskeleton.

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

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

Dual action of cAMP-dependent protein kinase on granulocyte movement

Cell locomotion is a continuous cycle of integrin-dependent attachments and detachments along chemotactic gradients, driven by dynamic modulations of the actin network. Cyclic AMP (cAMP), which is known to be generated by N-formyl-l-methionyl-l-leucyl-l-phenylalanine (fMLP) receptors but not by beta2 integrins, was investigated as a coordinator of granulocyte locomotion. Elevation of cAMP by exposure to forskolin (100 microM) and 1-isobutyl-methylxanthine (IBMX; 100 microM) caused a marked reduction in beta2 integrin-induced polymerisation of actin, but had a less pronounced effect on the fMLP-induced actin response. Pretreatment of cells with rp-adenosine-3',5'-cyclic monophosphorothioate (rp-cAMPS; 50 microM), an inhibitor of the cAMP-dependent protein kinase (cAPK), resulted in a significant increase in the fMLP-induced actin polymerisation response. In agreement with the effect on filamentous actin (F-actin) forskolin and IBMX markedly suppressed the migration of granulocytes towards fMLP. Surprisingly enough, pretreatment of cells with rp-cAMPS inhibited cell movement to the same extent as forskolin and IBMX did. This dual action of cAMP on granulocyte migration suggest an important regulatory mechanism whereby the balance of this intracellular signal results in an optimal locomotory response.

Effect of wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) on N-formyl-methionyl-leucyl-phenylalanine-induced phospholipase D activation in differentiated HL60 cells: possible involvement of phosphatidylinositol 3-kinase in phospholipase D activation

Phospholipase D (PLD) plays an important role in neutrophil activation. However, despite various proposed mechanisms, its detailed regulatory mechanism is not fully understood. The functional coupling between phosphatidylinositol 3-kinase (PI 3-kinase) and PLD was investigated in N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated human promyelocytic leukemia HL60 cells, using wortmannin, a fungal metabolite that is known as a selective inhibitor for phosphatidylinositol 3-kinase. Treatment of cells with this drug inhibited the formation of both phosphatidylinositol 3,4,5-trisphosphate (PIP3), a product of PI 3-kinase, and phosphatidylbutanol (PBut), the specific product of transphosphatidylation due to PLD in the presence of butanol, with similar concentration dependence (IC50 = 30-70 nM). Another PI 3-kinase inhibitor, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002) also inhibited PBut formation in a concentration-dependent manner. However, wortmannin failed to inhibit phorbol 12-myristate 13-acetate-induced PLD activation in whole cells and membrane PLD activity in an in vitro assay system, indicating that inhibition of fMLP-induced PLD activation by wortmannin was not due to its direct effect on PLD activity. These results suggest that a major part of inhibition of PLD activation by wortmannin might be mediated through its effect on PI 3-kinase.

Phosphatidylinositol 3-kinase is a negative regulator of cellular differentiation

Phosphatidylinositol 3-kinase (PI3K) has been shown to be an important mediator of intracellular signal transduction in mammalian cells. We show here, for the first time, that the blockade of PI3K activity in human fetal undifferentiated cells induced morphological and functional endocrine differentiation. This was associated with an increase in mRNA levels of insulin, glucagon, and somatostatin, as well as an increase in the insulin protein content and secretion in response to secretagogues. Blockade of PI3K also increased the proportion of pluripotent precursor cells coexpressing multiple hormones and the total number of terminally differentiated cells originating from these precursor cells. We examined whether any of the recently described modulators of endocrine differentiation could participate in regulating PI3K activity in fetal islet cells. The activity of PI3K was inversely correlated with the hepatocyte growth factor/scatter factor-induced downregulation or nicotinamideinduced upregulation of islet-specific gene expression, giving support to the role of PI3K, as a negative regulator of endocrine differentiation. In conclusion, our results provide a mechanism for the regulation of hormone-specific gene expression during human fetal neogenesis. They also suggest a novel function for PI3K, as a negative regulator of cellular differentiation.

Evidence for the involvement of phosphatidylinositol 4,5-bisphosphate 3-kinase in CD18-mediated adhesion of human neutrophils to fibrinogen

The CD18 integrin mediates chemotaxin-induced adhesion of neutrophils to fibrinogen. In neutrophils chemotaxins activate different intracellular pathways, which metabolize phosphatidylinositol 4,5-bisphosphate. To analyse the functional role of phosphatidylinositol 4,5-bisphosphate 3-kinase in the adhesion response, studies with the fungal metabolite wortmannin and the chemically unrelated compound LY 294002 were performed. These compounds inhibited with similar concentration dependency chemotaxin-induced formation of [32P]phosphatidylinositol 3,4,5-trisphosphate and CD18-mediated adhesion of neutrophils to fibrinogen, but did not influence expression of CD18 molecules at the cell surface. These data suggest involvement of phosphatidylinositol 4,5-bisphosphate 3-kinase in chemokine-induced avidity changes of CD18 integrins.

Chemotactic peptide N-formyl-met-leu-phe activation of p38 mitogen-activated protein kinase (MAPK) and MAPK-activated protein kinase-2 in human neutrophils

Activation of polymorphonuclear leukocytes (PMN) by chemotactic peptides initiates a series of functional responses that serve to eliminate pathogens. The intermediate steps that link engagement of the chemoattractant receptor to the microbicidal responses involve protein kinases that have yet to be identified. In this study we detected in human PMN the presence of p38 mitogen-activated protein kinase (MAPK), which became rapidly tyrosine phosphorylated and activated in response to the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP). Pretreatment of PMN with wortmannin, a phosphatidylinositol 3-kinase inhibitor, or bis-indolylmaleimide, a protein kinase C antagonist, resulted in partial inhibition of p38 phosphorylation upon fMLP stimulation. Similarly, phosphorylation of p38 was only partially inhibited when the fMLP-induced cytosolic calcium transient was prevented. Stimulation of PMN by the chemoattractant also resulted in the rapid phosphorylation and activation of MAPK-activated protein kinase-2 (MAPKAPK-2), which was completely inhibited by the specific p38 inhibitor, SB203580. The physical interaction of p38 with MAPKAPK-2 was studied by coimmunoprecipitation. These two kinases were found to be associated in unstimulated PMN but dissociated upon activation of the cells by fMLP. Together these findings demonstrate the activation of p38 by chemotactic peptides in human PMN by a process involving phosphatidylinositol 3-kinase, protein kinase C, and calcium. p38, in turn, is an upstream activator of MAPKAPK-2.

Selective recognition of phosphatidylinositol 3,4,5-trisphosphate by a synthetic peptide

The present study takes a novel approach to explore the mode of action of phosphoinositide 3-kinase lipid products by identifying a synthetic peptide W-NG(28-43) (WAAKIQASFRGHMARKK) that displays discriminative affinity with phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3). This PtdIns(3,4,5)P3-binding peptide was discovered by a gel filtration-based binding assay and exhibits a high degree of stereochemical selectivity in phosphoinositide recognition. It forms a 1:1 complex with PtdIns(3,4,5)P3 with Kd of 2 microM, but binds phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) with substantially lower affinity (5- and 40-fold, respectively) despite the largely shared structural motifs with PtdIns(3,4,5)P3. Other phospholipids examined, including phosphatidylserine, phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine, show low or negligible affinity with the peptide. Several lines of evidence indicate that this phosphoinositide-peptide interaction is not due to nonspecific electrostatic interactions or phospholipid aggregation, and requires a cooperative action among the hydrophobic and basic residues to exert the selective recognition. CD data suggest that the peptide acquires an ordered structure upon binding to PtdIns(3,4,5)P3. Further, we demonstrate that PtdIns(3,4,5)P3 enhances the phosphorylation rate of this binding peptide by protein kinase C (PKC)-alpha in a dose-dependent manner. In view of the findings that this stimulatory effect is not noted with other PKC peptide substrates lacking affinity with PtdIns(3,4,5)P3 and that PKC-alpha is not susceptible to PtdIns(3,4,5)P3 activation, the activity enhancement is thought to result from the substrate-concentrating effect of the D-3 phosphoinositide, i.e. the presence of PtdIns(3,4,5)P3 allows the peptide to bind to the same vesicles/micelles to which PKC is bound. Moreover, it is noteworthy that neurogranin, the full-length protein of W-NG(28-43) and a relevant PKC substrate in the forebrain, binds PtdIns(3,4,5)P3 with high affinity. Taken together, it is plausible that, in addition to PKC activation, PtdIns(3,4,5)P3 provides an alternative mechanism to regulate PKC activity in vivo by recruiting and concentrating its target proteins at the interface to facilitate the subsequent PKC phosphorylation.

Profilin and gelsolin stimulate phosphatidylinositol 3-kinase activity

Actin-binding proteins such as profilin and gelsolin bind to phosphatidylinositol (PI) 4,5-bisphosphate (PI 4,5-P2) and regulate the concentration of monomeric actin. We report here that profilin and gelsolin stimulate PI 3-kinase-mediated phosphorylation of PI 4,5-P2 (lipid kinase activity) in a concentration-dependent manner. This effect is specific to profilin and gelsolin because other cytoskeletal proteins such as tau or actin do not affect PI 3-kinase activity. In addition to lipid kinase activity, PI 3-kinase also has protein kinase activity: it phosphorylates proteins (p85 subunit of PI 3-kinase). However, the protein kinase activity of PI 3-kinase was not affected in the presence of profilin. Kinetic analysis, as a function of varying concentrations of ATP and PI 4,5-P2, showed that profilin affects the Vmax of PI 3-kinase without affecting k(m). Profilin may also affect PI 3-kinase activity by its direct association to the enzyme because dot-blot analysis using antibody to glutathione S-transferase (GST) suggested that GST-85 kDa, a fusion protein of PI 3-kinase, binds to profilin. However, PI 3-kinase did not affect the actin-sequestering ability of profilin (determined by pyrene-labeled actin), which indicates that actin and p85 do not share a common binding site on profilin. These studies suggest that profilin and gelsolin may control the generation of 3-OH phosphorylated phosphoinositides, which in turn may regulate the actin polymerization.

TOR2 is required for organization of the actin cytoskeleton in yeast

The Saccharomyces cerevisiae gene TOR2 encodes a putative phosphatidylinositol kinase that has two essential functions. One function is redundant with TOR1, a TOR2 homolog, and is required for signaling translation initiation and early G1 progression. The second essential function is unique to TOR2. Here we report that loss of the TOR2-unique function disrupts polarized distribution of the actin cytoskeleton. A screen for dosage suppressors of a dominant negative TOR2 allele identified TCP20/CCT6, encoding a subunit of the cytosolic chaperonin TCP-1 that is involved in the biogenesis of actin structures. Overexpression of TCP20 restores growth and polarized distribution of the actin cytoskeleton in a tor2 mutant. TCP20 overexpression does not restore growth in a tor1 tor2 double mutant. We suggest that the unique function of the phosphatidylinositol kinase homolog TOR2 is required for signaling organization of the actin cytoskeleton during the cell cycle. TOR2, via its two functions, may thus integrate temporal and spatial control of cell growth.

Lipid products of phosphoinositide 3-kinase bind human profilin with high affinity

To gain insight into the physiological function of phosphoinositide 3-kinase (PI 3-kinase) lipid products, this study examines the interactions of the D-3 phosphoinositides with profilin and the consequent effects on actin dynamics and phosphoinositide turnover. Profilin, a ubiquitous actin-regulating protein, plays a putative role in regulating actin assembly and PLC-gamma 1 signaling in light of its unique interactions with actin and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Here we raise evidence that the affinity of profilin with the D-3 phosphoinositides is substantially higher than that of PtdIns(4,5)P2. The dissociation constants (Kd) are estimated to be 1.1 microM, 5.7 microM, and 11 microM for phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P2], phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3], and PtdIns(4,5)P2, respectively. Spectroscopic data show that while all these phosphoinositides alter the tryptophan fluorescence of profilin in a similar fashion, the respective conformational effect on profilin is vastly different. Based on CD data, the alpha-helical contents of profilin in the presence of 8 molar equiv of PtdIns(4,5)P2, PtdIns(3,4,5)P3, and PtdIns(3,4)P2 are 17.4%, 11.5%, and 1.4%, respectively, vis-a-vis 9.4% for profilin alone. In contrast, no appreciable change in the fluorescence and CD spectra is observed when related inositol phosphates such as Ins(1,4,5)P3, Ins(1,3,4,5)P4, or Ins(1,3,4)P3 at comparable concentrations are tested. Evidence suggests that this differential recognition bears functional significance concerning the intricate roles of profilin and inositol lipids in modulating actin polymerization and PtdIns(4,5)P2 turnover. The relative potency of individual phosphoinositides in offsetting the inhibitory effect of profilin on actin assembly is PtdIns(3,4)P2 > PtdIns(3,4,5)P3 > PtdIns(4,5)P2, consistent with their relative binding affinity with profilin. Moreover, the inhibitory effect of profilin on PLC-gamma 1-mediated PtdIns(4,5)P2 hydrolysis is overcome by PtdIns(3,4)P2 and PtdIns(3,4,5)P3 through a combined effect of PLC-gamma 1 activation and preferential profilin binding. This D-3 phosphoinositide-mediated regulation may represent a new mechanism for controlling PtdIns(4,5)P2 turnover by PLC-gamma 1.

Evidence of the involvement of phosphatidylinositol 3-kinase in the migration, actin stress fiber formation, and alpha v beta 3-integrin-mediated adherence of human melanoma cells

Tumor invasion and formation of metastases are major obstacles for a successful therapy of melanomas. Metastasis is thought to require multiple steps such as alpha v beta 3-integrin-mediated adhesion, proteolytic digestion of extracellular matrix by metalloproteinase-2, and reorganization of the actin cytoskeleton. To analyze the functional role of phosphatidylinositol 3-kinase in these processes, melanoma cells were treated with the fungal metabolite wortmannin. Wortmannin inhibited phosphatidylinositol 3-kinase activity in melanoma cells and migration in an equally concentration-dependent fashion. Flow cytometric analysis of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)phallacidin-stained actin network indicated reduction of actin filaments by wortmannin. Fluorescence laser confocal microscopy experiments revealed breakdown of actin stress fibers. In addition, wortmannin inhibited alpha v beta 3-integrin-mediated adhesion of melanoma cells to vitronectin. Since flow cytometric measurements did not show altered expression of the alpha v beta 3-integrin at the cell surface, avidity changes of the alpha v beta 3-integrin by wortmannin are suggested. In contrast to the actin analysis and adhesion assays, wortmannin had no influence on mRNA expression or on protein secretion of metalloproteinase-2. These data provide evidence that phosphatidylinositol 3-kinase is an essential signal transduction protein required for migration of melanoma cells, regulating formation of the actin stress fiber as well as alpha v beta 3-integrin-mediated adhesion.

Synthesis of Photoactivatable 1,2-O-Diacyl-sn-glycerol Derivatives of 1-L-Phosphatidyl-D-myo-inositol 4,5-Bisphosphate (PtdInsP(2)) and 3,4,5-Trisphosphate (PtdInsP(3))

Photoactivatable analogues of 1-L-phosphatidyl-D-myo-inositol 4,5-bisphosphate (PtdIns(4,5)P(2) or PtdInsP(2)) and the corresponding 3,4,5-trisphosphate (PtdIns(3,4,5)P(3) or PtdInsP(3)) were prepared from the two chiral precursors, methyl alpha-D-glucopyranoside and 1,2-isopropylidene-sn-glycerol. Two key synthetic transformations included the Ferrier rearrangement reaction to construct the optically-pure inositol skeleton and the sequential acylation of the primary and secondary hydroxyl groups on the glycerol derivatives. The sn-1-O-(6-aminohexanoyl) PtdInsP(2) and PtdInsP(3) derivatives were further modified to contain benzophenone photophores in unlabeled and high specific activity tritium-labeled forms.

Insulin and ATP stimulate actin polymerization in U937 cells by a wortmannin-sensitive mechanism

ATP and insulin stimulate increases in phosphatidylinositol (3,4,5)-trisphosphate levels in myeloid-derived U937 cells. Quantification of FITC-phalloidin binding by fluorescence-activated cell sorting reveals that both ATP and insulin stimulate actin polymerization with distinctive kinetics in U937 cells. The response to ATP is rapid and dose-dependent with an EC50 of 200 nM, and is abolished by pre-incubation with the Ca2+ chelator BAPTA-AM. At 800 nM concentration, wortmannin, a potent inhibitor of phosphoinositide 3-kinase (PI3K), blocks the late, but not the early phase of actin polymerization stimulated by 100 nM ATP. Responses elicited by 10 micrograms/ml insulin are slower, smaller and more transient than responses to ATP, and are inhibited by preincubation with 100 nM wortmannin. Actin polymerization can also be stimulated by thapsigargin, but not by phorbol ester, providing further evidence for a role for Ca2+ in actin polymerization. These data implicate distinct Ca2+ and PI3K-mediated pathways in the regulation of actin polymerization.

Purification and characterization of the phosphatidylinositol-3,4,5-trisphosphate phosphatase in bovine thymus

Using phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] prepared from phosphatidylinositol 4,5-bisphosphate and inositolphospholipid 3-kinase, we identified in bovine thymus extracts the enzyme activity which catalyzed dephosphorylation of PtdIns(3,4,5)P3, to produce phosphatidylinositol biphosphate. Since bovine thymus exhibited the highest level of activity among tissues screened, we tried to purify this enzyme PtdINs(3,4,5)P3 phosphatase from bovine thymus. After sequential chromatographies using S-Sepharose, heparin-Sepharose, blue Sepharose, and Toyopearl HW55, the enzyme was purified 1875-fold with a yield of 10%. SDS/PAGE analysis revealed that a 120-kDA protein band copurified with the enzyme activity. The apparent molecular mass of the active protein was 120 kDa on size-exclusion chromatography, suggesting that the 120-kDa band on SDS/PAGE is the PtdIns(3,4,5)P3 phosphatase. Since PtdIns(3,4,5)P3 phosphatase seemed to be the only activity that metabolized PtdIns(3,4,5)P3, and the enzyme did not hydrolyze phosphatidylinositol 4,5-biphosphate, the enzyme may play a critical role in the inositolphospholipid 3-kinase signalling.

Brush border phosphatidylinositol 3-kinase mediates epidermal growth factor stimulation of intestinal NaCl absorption and Na+/H+ exchange

In terminally differentiated ileal villus Na+-absorptive cells, epidermal growth factor (EGF) stimulates NaCl absorption and its component brush border Na+/H+ exchanger, acting via basolateral membrane receptors, and as we confirm here, a brush border tyrosine kinase. In the present study we show that brush border phosphatidylinositol 3-kinase (PI 3-kinase) is involved in EGF stimulation of NaCl absorption and brush border Na+/H+ exchange. In rabbit ileum studied with the Ussing chamber-voltage clamp technique, EGF stimulation of active NaCl absorption is inhibited by the selective PI 3-kinase inhibitor wortmannin. PI 3-kinase, a largely cytosolic enzyme, translocates specifically to the brush border of ileal absorptive cells following EGF treatment. This translocation occurs as early as 1 min after EGF treatment and remains increased at the brush border for at least 15 min. EGF also causes a rapid (1 min) and large (4-5-fold) increase in brush border PI 3-kinase activity. Involvement of PI 3-kinase activity in intestinal Na+ absorption is established further by studies done in the human colon cancer cell line, Caco-2, stably transfected with the intestinal brush border isoform of the Na+/H+ exchanger, NHE3 (Caco-2/NHE3 cells). Brush border Na+/H+ exchange activity was measured using the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)5-(6)-carboxyfluorescein. EGF added to the basolateral surface but not apical surface of Caco-2/NHE3 cells increased brush border Na+/H+ exchange activity. The EGF-induced increase in brush border Na+/H+ exchange activity was completely abolished in cells pretreated with wortmannin. EGF treatment caused increased tyrosine phosphorylation of PI 3-kinase in both ileal brush border membranes and Caco-2/NHE3 cells, suggesting that a tyrosine kinase upstream of the PI 3-kinase is involved in the EGF effects on Na+ absorption. In conclusion, the present study provides evidence in two separate intestinal models, the ileum and a human colon cancer cell line, that PI 3-kinase is an intermediate in EGF stimulation of intestinal Na+ absorption.

Chemotactic 5-oxo-icosatetraenoic acids activate a unique pattern of neutrophil responses. Analysis of phospholipid metabolism, intracellular Ca2+ transients, actin reorganization, superoxide-anion production and receptor up-regulation

Neutrophil cell responses and signal pathways elicited by the chemotactic arachidonic acid metabolites (6E, 8Z, 11Z, 14Z)-5-oxo-icosatetraenoic acid and (6E, 8Z, 11Z, 13E)-5-oxo-15-hydroxy-icosatetraenoic acid were studied and compared with those of other chemotaxins. Polyphosphoinositol lipid analysis revealed activation of phosphatidylinositol-biphosphate 3-kinase by both agonists. Experiments with Fura-2 in the presence of EGTA indicated Ca2+ mobilization from intracellular stores by both 5-oxo-icosanoids. A transient actin response and production of small amounts of superoxide anions upon stimulation with both agents was detected. The changes induced by 5-oxo-icosanoids were more moderate and transient than those obtained by other chemotaxins. Desensitization studies indicated cross-desensitization between both 5-oxo-icosanoids, but no interference with the response of other chemotaxins. All cell responses elicited by 5-oxo-icosanoids at concentrations 500-fold higher than the ED50 of other functions did not induce up-regulation of CD11b and N-formyl-peptide receptors at the cell surface, and failed to potentiate N-formyl-peptide-induced superoxide anion production. These results indicate that 5-oxo-icosanoids trigger a unique pattern of neutrophil responses.

Integrin alphavbeta3 mediates chemotactic and haptotactic motility in human melanoma cells through different signaling pathways

Distinctions between chemotaxis and haptotaxis of cells to extracellular matrix proteins have not been defined in terms of mechanisms or signaling pathways. Migration of A2058 human melanoma cells to soluble (chemotaxis) and substratum-bound (haptotaxis) vitronectin, mediated by alphav beta3, provided a system with which to address these questions. Both chemotaxis and haptotaxis were completely inhibited by treatment with RGD-containing peptides. Chemotaxis was abolished by a blocking antibody to alphavbeta3 (LM609), whereas haptotaxis was inhibited only by approximately 50%, suggesting involvement of multiple receptors and/or signaling pathways. However, blocking antibodies to alphavbeta5, also present on A2058 cells, did not inhibit. Pertussis toxin treatment of cells inhibited chemotaxis by >80%, but did not inhibit haptotaxis. Adhesion and spreading over vitronectin induced the phosphorylation of paxillin on tyrosine. In cells migrating over substratum-bound vitronectin, tyrosine phosphorylation of paxillin increased 5-fold between 45 min and 5 h. Dilutions of anti- alphavbeta3 that inhibited haptotaxis also inhibited phosphorylation of paxillin (by approximately 50%) and modestly reduced cell spreading. In contrast, soluble vitronectin (50-100 microg/ml) did not induce tyrosine phosphorylation of paxillin. The data suggest that soluble vitronectin stimulates chemotaxis predominantly through a G protein-mediated pathway that is functionally linked to alphavbeta3. Haptotaxis is analogous to directional cell spreading and requires alphavbeta3-mediated tyrosine phosphorylation of paxillin.

Cytokinesis arrest and redistribution of actin-cytoskeleton regulatory components in cells expressing the Rho GTPase CDC42Hs

In mammalian cells, Rho GTPases control the reorganisation of the actin cytoskeleton in response to growth factors. In the cytoplasm, the polymerisation of actin filaments and their organisation into complex architectures is orchestrated by numerous proteins which act either directly, by interacting with actin, or by producing secondary messengers which serve as mediators between signal transduction pathways and the microfilament organisation. We sought to determine whether the intracellular distribution of some of these regulatory components may be controlled by the Rho GTPase CDC42Hs. With this aim, we have established HeLa-derived human cell lines in which expression of a constitutively activated mutant of CDC42Hs is inducible. Morphological analysis by immunofluorescence labelling and confocal laser scanning microscopy revealed a massive reorganisation of F-actin in cortical microspikes as well as podosome-like structures located at the ventral face of the cells. Concomitantly, the cells became giant and multinucleate indicating that cytokinesis was impaired. The actin bundling protein T-plastin, the vasodilatator-stimulated phosphoprotein (VASP), a profilin ligand, as well as the 85 kDa regulatory subunit of the phosphoinosite 3-kinase redistributed with F-actin into the CDC42Hs-induced structures.

The 3-phosphorylated phosphoinositide response of 3T3-L1 preadipose cells exposed to insulin, insulin-like growth factor-1, or platelet-derived growth factor

We compared the pattern of 3-phosphorylated phosphoinositides produced by confluent 3T3-L1 preadipose cells upon exposure to growth factors that either induce differentiation (insulin, insulin-like growth factor-1) or do not (platelet-derived growth factor). Following addition of insulin or insulin-like growth factor-1, PI(3,4,5)P3 rapidly rose, on average, to levels tenfold over basal. PI(3,4)P2 either did not change (after insulin) or slightly increased (1.5 fold). Time course studies with insulin demonstrated that the rise in PI(3,4,5)P3 peaked by 1 minute, and levels then remained steady over 30 minutes. Dose-response experiments showed that insulin at a concentration of 1 nM was sufficient for the PI(3,4,5)P3 response. Insulin failed to increase PI(3,4)P2 at any of the time points or at any of the doses used. In contrast, after addition of platelet-derived growth factor, both PI(3,4)P2 and PI(3,4,5)P3 rose concurrently and to comparable extents. These data suggest that one possible mechanism contributing to insulin/insulin-like growth factor-1-induced 3T3-L1 preadipose cell differentiation is a distinct pattern of 3-phosphorylated phosphoinositide accumulation, defined by a prominent increase in PI(3,4,5)P3 with no (in the case of insulin), or a minimal (in the case of IGF-1), rise in PI(3,4)P2.

The phosphatidylinositol 4-phosphate 5-kinase family

The existence of a PIP5K family of enzymes has been suggested by Western blotting and purification of numerous PIP5Ks from various tissues and cell types. The erythrocyte has at least two PIP5Ks, named PIP5KI and PIP5KII, while the brain appears to have even more isoforms. The cloning of the first PIP5K, the PIP5KII alpha, is just the beginning of the molecular classification of this protein family. The PIP5KII alpha sequence has shown that these enzymes lack obvious homology to protein, sugar and other lipid kinases. The identification of two S. cerevisiae homologues, Mss4p and Fab1p, confirms that this family of kinases is widely distributed in eukaryotes. Not surprisingly, cloning experiments have identified additional isoforms. By cloning additional isoforms, insights into the structure and functions of this family of enzymes will be gained. One reason for a large family of PIP5Ks is that many forms of regulation and cellular functions have been ascribed to PIP5Ks, as summarized in Figure 10. Some of these functional links result from PtdIns[4,5]P2 being required for a given process, but the direct involvement of specific PIP5Ks is not well defined. Which PIP5K isoforms are regulated by a specific mechanism or are involved in a cellular process often is not clear. For example, which PIP5Ks produce PtdIns[4,5]P2 that is hydrolyzed by PLC or phosphorylated by the PI 3-kinase is not known. A few exceptions are PIP5KII not being able to phosphorylate PtdIns[4,5]P2 in native membranes, and PIP5KIs being stimulated by PtdA, required for secretion, and possibly regulated by G proteins of the Rho subfamily. The multiplicity of regulation and functions of each PIP5K isoform remains to be elucidated. Another factor governing the number of isoforms may be presence of multiple pools of polyphosphoinositides and the localizing of PIP5K function within cells. The polyphosphoinositides appear to be compartmentalized within cells and each pool appears to be sensitive to specific signals. These polyphosphoinositide pools may include those in the plasma membrane that are used by PLC, nuclear pools that appear to turn over separately from cytoplasmic pools and a small pool at sites of vesicle fusion with the plasma membrane. Each pool may be controlled by a specific PIP5K isoform. This would explain the diversity of PIP5K cellular roles. Another possibility is that the PIP5Ks are localized to certain areas of the cell by being part of a protein or proteolipid complex. Furthermore, the presence of PITP or PLC in the complex would potentially impart specificity and speed on the use of PtdIns[4]P and PtdIns[4,5]P2 because these lipids could be channeled quickly from one enzyme to the next. The concept of localized complexes containing particular PIP5K isoforms that control the composition of different polyphosphoinositide pools will likely be important as the family of PIP5K isoforms grows.