PI 3-kinase: a pivotal pathway in T-cell activation?

RSK-mediated nuclear accumulation of the cold-shock Y-box protein-1 controls proliferation of T cells and T-ALL blasts

Deregulated proliferation is key to tumor progression. Although unrestricted proliferation of solid tumor cells correlates with the cold-shock protein Y-box (YB)-binding protein-1 accumulation in the nuclei, little is known about its expression and function in hematopoietic malignancies, such as T-cell acute lymphoblastic leukemia (T-ALL). Here we show that YB-1 protein is highly enriched in the nuclei of activated T cells and malignant human T-ALL cell lines but not in resting T cells. YB-1 S¹⁰² mutations that either mimic (S102D) or prevent phosphorylation (S102N) led to accumulation of YB-1 in the nucleus of T cells or strictly excluded it, respectively. Inactivation of ribosomal S6 kinase (RSK) was sufficient to abrogate T-cell and T-ALL cell proliferation, suggesting that RSK mediates cell-cycle progression, possibly dependent on YB-1-phosphorylation. Indeed, phosphomimetic YB-1^S102D enhanced proliferation implying that S¹⁰² phosphorylation is a prerequisite for malignant T-cell proliferation. At initial diagnosis of T-ALL, YB-1 localization was significantly altered in the nuclei of tumor blasts derived from bone marrow or peripheral blood. Our data show deregulated YB-1 in the nucleus as a yet unreported characteristic of T-ALL blasts and may refine strategies to restrict progression of hematopoietic tumors.

Role of two adaptor molecules SLP-76 and LAT in the PI3K signaling pathway in activated T cells

Previously, we identified p85, a subunit of PI3K, as one of the molecules that interacts with the N-terminal region of Src homology 2 domain-containing leukocyte protein of 76 kDa (SLP-76). We also demonstrated that tyrosine phosphorylation either at the 113 and/or 128 position is sufficient for the association of SLP-76 with the Src homology 2 domain near the N terminus of p85. The present study further examines the role of the association of these two molecules on the activation of PI3K signaling cascade. Experiments were done to determine the role of SLP-76, either wild-type, tyrosine mutants, or membrane-targeted forms of various SLP-76 constructs, on the membrane localization and phosphorylation of Akt, which is an event downstream of PI3K activation. Reconstitution studies with these various SLP-76 constructs in a Jurkat variant cell line that lacks SLP-76 or linker for activation of T cells (LAT) show that the activation of PI3K pathway following TCR ligation requires both SLP-76 and LAT adaptor proteins. The results suggest that SLP-76 associates with p85 after T cell activation and that LAT recruits this complex to the membrane, leading to Akt activation.

Enamel matrix derivative protein stimulated wound healing via phosphoinositide 3-kinase

BACKGROUND

Enamel matrix derivative (EMD) protein has been clinically used for periodontal regeneration, but the molecular mechanisms are not clear. Previous studies suggested that the activation of phosphoinositide 3-kinase (PI 3-kinase) plays a key role in facilitating cell migration. Given that the migration of osteoblasts is one of the key steps in the wound-healing processes, we hypothesized that EMD protein would stimulate osteoblast migration by activating PI 3-kinase. In this study, we tested this hypothesis using MG-63 cells as model systems to evaluate mechanisms of migration by stimulation with EMD protein.

METHODS

Confluent MG-63 cells were mechanically scratched using a sterilized 1-mm pipette tip that removed the cells within a circular area. The wells were incubated for 24 hours in various stimulation conditions (25, 50, or 100 microg/ml EMD protein) with or without the PI 3-kinase inhibitor wortmannin (1, 10, and 100 nM) or LY294002 (1, 10, and 100 microM). Migrated cells in the wound section were counted by randomly selecting three areas from one well. The activation of PI 3-kinase by EMD protein was evaluated by the phosphorylation of Akt using Western blot analysis.

RESULTS

Although EMD protein did not affect proliferation, it enhanced migration into wounds on MG-63 cells. We showed that EMD protein enhanced the phosphorylation of Akt in a dose-dependent manner. We demonstrated that the PI 3-kinase inhibitors wortmannin and LY294002 blocked migration into wounds and the phosphorylation of Akt enhanced by EMD protein in MG-63 cells.

CONCLUSION

These results demonstrated that the activation of PI 3-kinase plays a key role in the EMD protein-stimulated migration of osteoblasts.

Mechanisms of chemokine and antigen-dependent T-lymphocyte navigation

T-lymphocyte trafficking is targeted to specific organs by selective molecular interactions depending on their differentiation and functional properties. Specific chemokine receptors have been associated with organ-specific trafficking of memory and effector T-cells, as well as the recirculation of naïve T-cells to secondary lymphoid organs. In addition to the acquisition of tissue-selective integrins and chemokine receptors, an additional level of specificity for T-cell trafficking into the tissue is provided by specific recognition of antigen displayed by the endothelium involving the TCRs (T-cell antigen receptors) and co-stimulatory receptors. Activation of PI3K (phosphoinositide 3-kinase) is a robust signalling event shared by most chemokine receptors as well as the TCR and co-stimulatory receptors, contributing to several aspects of T-lymphocyte homing as well as actin reorganization and other components of the general migratory machinery. Accordingly, inhibition of PI3K has been considered seriously as a potential therapeutic strategy by which to combat various T-lymphocyte-dependent pathologies, including autoimmune and inflammatory diseases, as well as to prevent transplant rejection. However, there is substantial evidence for PI3K-independent mechanisms that facilitate T-lymphocyte migration. In this regard, several other signalling-pathway components, including small GTPases, PLC (phospholipase C) and PKC (protein kinase C) isoforms, have also been implicated in T-lymphocyte migration in response to chemokine stimulation. The present review will therefore examine the PI3K-dependent and -independent signal-transduction pathways involved in T-cell migration during distinct modes of T-cell trafficking in response to either chemokines or the TCR and co-stimulatory molecules.

Antigen-independent adhesion and cell spreading by inducible costimulator engagement inhibits T cell migration in a PI-3K-dependent manner

Engagement of the costimulatory protein ICOS activates effector/memory T cells in tissue by enhancing TCR-mediated proliferation and cytokine production. We now report that in an antigen-independent manner, ICOS also induces adhesion and spreading in human effector/memory T cells, consequently inhibiting cell migration. T cell spreading and elongation after ICOS ligation are accompanied by the formation of two types of actin-rich membrane protrusions: thin, finger-like structures similar to filopodia and short, discrete microspikes. Although filopodia/microspike formation occurs independently of the PI-3K signaling cascade, ICOS-mediated T cell elongation depends on PI-3K activity, which inhibits the accumulation of GTP-bound RhoA. Further inhibition of RhoA activation exacerbates the ICOS-mediated, elongated phenotype. We propose that in inflamed tissue, ICOS engagement by ICOS ligand on a professional or nonprofessional APC prevents the forward motility of the T cell by inhibiting RhoA-dependent uropod retraction. The resulting ICOS-induced T cell spreading and filopodia/microspike formation may promote antigen recognition by enhancing a T cell's scanning potential of an adherent APC surface.

Group B streptococcus exploits lipid rafts and phosphoinositide 3-kinase/Akt signaling pathway to invade human endometrial cells

OBJECTIVE

We sought to determine the role lipid rafts and phosphoinositide 3-kinase (PI3K) in invasiveness of group B streptococci (GBS) to endometrial cells.

STUDY DESIGN

Antibiotic protection assay and electron microscopy were used to evaluate the invasion of GBS to human endometrial Ishikawa cells cholesterol-depleted by using methyl-beta-cyclodextrin or treated with PI3K inhibitors: wortmannin or LY294002. Immunoblotting analysis of Akt phosphorylation and cellular imaging of GFP-Akt-PH probe were used to assess PI3Ks activation in infected cells.

RESULTS

Infected Ishikawa cells streptococci are associated to membrane ruffles with morphological features of undergoing internalization. GBS remained attached but completely failed to invade to cholesterol-depleted human endometrial cells or displayed decreased invasiveness in the presence of PI3K inhibitors. Cholesterol depletion resulted in loss of membrane ruffling and dispersion of raft-associated molecules: monosialoganglioside GM1 and PI3K.

CONCLUSION

This work provides the evidence that lipid rafts and raft-associated PI3K are implicated in GBS invasion to human endometrial cells.

Nonadherent cells switch to a Rac-mediated, SHIP regulated, Akt activation mode for survival

Constitutively active Rac stimulates Akt activity in T lymphocytes cultured in suspension. This regulation contrasts with findings obtained in fibroblasts, endothelial or neuronal cells grown on substrate, where Akt stimulation occurs independently of Rac. We now show that V12Rac-mediated stimulation of Akt is not restricted to the hematopoietic lineage but is dependent on the adherence status of the cell. V12Rac-mediated stimulation of Akt as well as molecular association between Rac and Akt occurred exclusively in cells kept in suspension. Stimulation and complex formation are dependent on SHIP but in a manner that differs from its role in dephosphorylation of phosphoinositide lipids. Adherent cells lacking SHIP, but not those lacking PTEN, are able to activate Akt through the Rac pathway. Our data reveal the existence of a bona fide Rac to Akt signaling pathway, tightly regulated by SHIP and operational in suspended cells only. This pathway may point to an alternative survival signal that is called into action when cells lose contact with the substrate and/or with other cells.

Mechanisms of immune suppression by interleukin-10 and transforming growth factor-beta: the role of T regulatory cells

Specific immune suppression and induction of tolerance are essential processes in the regulation and circumvention of immune defence. The balance between allergen-specific type 1 regulatory (Tr1) cells and T helper (Th) 2 cells appears to be decisive in the development of allergy. Tr1 cells consistently represent the dominant subset specific for common environmental allergens in healthy individuals. In contrast, there is a high frequency of allergen-specific interleukin-4 (IL-4)-secreting T cells in allergic individuals. Allergen-specific immunotherapy can induce specific Tr1 cells that abolish allergen-induced proliferation of Th1 and Th2 cells, as well as their cytokine production. Tr1 cells utilize multiple suppressor mechanisms, such as IL-10 and transforming growth factor-beta (TGF-beta) as secreted cytokines and various surface molecules, such as cytotoxic T-lymphocyte antigen 4 and programmed death-1. IL-10 only inhibits T cells stimulated by low numbers of triggered T-cell receptors, which depend on CD28 costimulation. IL-10 inhibits CD28 tyrosine phosphorylation, preventing the binding of phosphatidylinositol 3-kinase p85 and consequently inhibiting the CD28 signalling pathway. In addition, IL-10 and TGF-beta secreted by Tr1 cells skew the antibody production from immunoglobulin E (IgE) towards the non-inflammatory isotypes IgG4 and IgA, respectively. Induction of antigen-specific Tr1 cells can thus re-direct an inappropriate immune response against allergens or auto-antigens using a broad range of suppressor mechanisms.

Factors controlling fibroblast growth factor receptor-1's cytoplasmic trafficking and its regulation as revealed by FRAP analysis

Biochemical and microscopic studies have indicated that FGFR1 is a transmembrane and soluble protein present in the cytosol and nucleus. How FGFR1 enters the cytosol and subsequently the nucleus to control cell development and associated gene activities has become a compelling question. Analyses of protein synthesis, cytoplasmic subcompartmental distribution and movement of FGFR1-EGFP and FGFR1 mutants showed that FGFR1 exists as three separate populations (a) a newly synthesized, highly mobile, nonglycosylated, cytosolic receptor that is depleted by brefeldin A and resides outside the ER-Golgi lumen, (b) a slowly diffusing membrane receptor population, and (c) an immobile membrane pool increased by brefeldin A. RSK1 increases the highly mobile cytosolic FGFR1 population and its overall diffusion rate leading to increased FGFR1 nuclear accumulation, which coaccumulates with RSK1. A model is proposed in which newly synthesized FGFR1 can enter the (a) "nuclear pathway," where the nonglycosylated receptor is extruded from the pre-Golgi producing highly mobile cytosolic receptor molecules that rapidly accumulate in the nucleus or (b) "membrane pathway," in which FGFR1 is processed through the Golgi, where its movement is spatially restricted to trans-Golgi membranes with limited lateral mobility. Entrance into the nuclear pathway is favored by FGFR1's interaction with kinase active RSK1.

Key role of the p110δ isoform of PI3K in B-cell antigen and IL-4 receptor signaling: comparative analysis of genetic and pharmacologic interference with p110δ function in B cells

Mouse gene–targeting studies have documented a central role of the p110δ isoform of phosphoinositide 3-kinase (PI3K) in B-cell development and function. A defect in B-cell antigen receptor (BCR) signaling is key to this B-cell phenotype. Here we further characterize this signaling defect and report that a p110δ-selective small molecule inhibitor mirrors the effect of genetic inactivation of p110δ in BCR signaling. p110δ activity is indispensable for BCR-induced DNA synthesis and phosphorylation of Akt/protein kinase B (PKB), forkhead transcription factor/forkhead box O3a (FOXO3a), and p70 S6 kinase (p70 S6K), with modest effects on the phosphorylation of glycogen synthase kinase 3 α/β (GSK3α/β) and extracellular signal-regulated kinase (Erk). The PI3K-dependent component of intracellular calcium mobilization also completely relies on p110δ catalytic activity. Resting B cells with inactive p110δ fail to enter the cell cycle, correlating with an incapacity to up-regulate the expression of cyclins D2, A, and E, and to phosphorylate the retinoblastoma protein (Rb). p110δ is also critical for interleukin 4 (IL-4)–induced phosphorylation of Akt/PKB and FOXO3a, and protection from apoptosis. Taken together, these data show that defects observed in p110δ mutant mice are not merely a consequence of altered B-cell differentiation, and emphasize the potential utility of p110δ as a drug target in autoimmune diseases in which B cells play a crucial role.

Ligation of CD28 Stimulates the Formation of a Multimeric Signaling Complex Involving Grb-2-Associated Binder 2 (Gab2), Src Homology Phosphatase-2, and Phosphatidylinositol 3-Kinase: Evidence That Negative Regulation of CD28 Signaling Requires the Gab2 Pleckstrin Homology Domain

Grb-2-associated binder (Gab)2 is a scaffolding adaptor protein that has been reported to promote growth factor and cytokine receptor signal transduction, but inhibit TCR-mediated signaling events. In this study, we show that ligation of CD28 by its natural ligand B7-1/CD80, induces tyrosine phosphorylation of Gab2 and its coassociation with Src homology phosphatase (SHP)-2 and class IA PI3K in Jurkat cells. Overexpression of wild-type Gab2 revealed a negative role in regulation of CD3/CD28 induction of the transcription factors NF-kappaB and AP-1. To characterize this inhibitory function further, we used Gab2 mutants unable to bind either PI3K or SHP-2 and a PH domain deletion mutant. Although PI3K has previously been implicated as necessary for Gab2-mediated inhibition of TCR signaling, Gab2 mutants defective in their ability to bind PI3K or SHP-2 retained their inhibitory function, whereas deletion of the PH domain ablated the inhibitory effect of Gab2. Together, these data demonstrate that CD28 stimulation of T cells is sufficient to induce an inhibitory multimeric signaling complex involving Gab2, SHP-2, and PI3K. Furthermore, the inhibitory capacity of Gab2 is strictly dependent upon the integrity of its PH domain, suggesting phosphoinositide-mediated membrane recruitment is important to Gab2 function in T cells.

Fructose-induced increases in neonatal rat intestinal fructose transport involve the PI3-kinase/Akt signaling pathway

Expression of rat glucose transporter-5 (GLUT5) is tightly regulated during development. Expression and activity are low throughout the suckling and weaning stages, but perfusion of the small intestinal lumen with fructose solutions during weaning precociously enhances GLUT5 activity and expression. Little is known, however, about the signal transduction pathways involved in the substrate-induced precocious GLUT5 development. We found that wortmannin and LY-294002, inhibitors of phosphatidylinositol 3-kinase (PI3-kinase) specifically inhibited the increase in fructose uptake rate and brush-border GLUT5 protein abundance but not GLUT5 mRNA abundance. Perfusion of EGF, an activator of PI3-kinase, also resulted in a marked wortmannin-inhibitable increase in fructose uptake. Perfusion of fructose for 4 h increased cytosolic immunostaining of phosphatidylinositol-3,4,5-triphosphate (PIP(3)), the primary product of PI3-kinase, mainly in the mid- to upper-villus regions in which the brush-border membrane also stained strongly with GLUT5. Perfusion of glucose for 4 h had little effect on fructose or glucose uptake and PIP(3) or GLUT5 staining. SH-5, an Akt inhibitor, prevented the increase in fructose uptake and GLUT5 protein induced by fructose solutions, and had no effect on glucose uptake. The PI3-kinase/Akt signaling pathway may be involved in the synthesis and/or recruitment to the brush border of GLUT5 transporters by luminal fructose in the small intestine of weaning rats. Increases in fructose transport during the critical weaning period when rats are shifting to a new diet may be modulated by several signaling pathways whose cross talk during development still needs to be elucidated.

Phosphoinositide-3 kinases critically regulate the recruitment and survival of eosinophils in vivo: importance for the resolution of allergic inflammation

The phosphatidylinositol-3 kinase (PI3K) family of signaling enzymes plays a crucial role in leukocyte recruitment and activation and hence, likely regulates the induction and propagation phases of inflammation. However, little data have emerged showing a role for these processes in the resolution phase in models of in vivo inflammation. Here, we have evaluated the role of PI3K for the migration and survival of eosinophils in a model of allergic pleurisy in mice. Eosinophil accumulation in PI3Kgamma-deficient mice was inhibited at 48 h, as compared with wild-type mice but not at earlier time-points (6 and 24 h). Experiments with adoptive transfer of bone marrow showed that PI3Kgamma in eosinophils but not in non-bone marrow-derived cells was required for their accumulation. Systemic treatment with PI3K inhibitors before antigen challenge prevented the recruitment of eosinophils. This was associated with decreased Akt phosphorylation, interleukin-5 production, and eosinophil release from the bone marrow. Treatment with PI3K inhibitors 24 h after antigen challenge markedly cleared the accumulated eosinophils, an effect associated with inhibition of Akt phosphorylation and an increased number of apoptotic events. Altogether, our data demonstrate an important role of PI3Kgamma for the maintenance of eosinophilic inflammation in vivo, whereas other isoforms of PI3K may be relevant for the recruitment process.

Ras initiates phosphatidyl-inositol-3-kinase (PI3K)/PKB mediated signalling pathways in untransformed human peripheral blood T lymphocytes

Activation of T lymphocytes through costimulation of the T cell receptor/CD3 complex (TCR/CD3) and coreceptors (e.g. CD2 or CD28) leads to production of the growth factor interleukin-2 (IL-2) and subsequent proliferation. For these activation processes, remodelling of the actin cytoskeleton plays an important functional role. We have shown that the activity of the actin-remodelling protein cofilin is crucially involved in T lymphocyte activation processes. In unstimulated human peripheral blood T lymphocytes (PB-T) cofilin exists in its inactive ser-3-phosphorylated form. T lymphocyte activation through costimulation of TCR plus the coreceptors CD28 or CD2, respectively, induces the dephosphorylation of cofilin. Concomitantly, cofilin associates with the actin cytoskeleton. The functional importance of cofilin for T lymphocyte activation was shown employing cell permeable peptides which block binding of cofilin to actin. In human PB-T these peptides impair the formation of the immunological synapse and inhibit the induction of T lymphocyte proliferation and cytokine production. The serine phosphatases PP1 and PP2A dephosphorylate cofilin in T lymphocytes. Importantly, a PKC-Ras-MEK/PI3K-cascade links costimulation of PB-T through TCR/CD3 and CD28 to activation of cofilin through dephosphorylation. Notably, the induction of cofilin dephosphorylation requires the combined activities of two Ras-effectors, namely MEK and PI3K. With respect to PI3K, this result was unexpected since so far it was generally assumed that-unlike in other cell types-Ras is not able to activate PI3K in T lymphocytes, as concluded from experiments performed with the human T-lymphoma line Jurkat. This discrepancy implied that the signalling events upstream of PI3K differ between PB-T and Jurkat cells. In line with this, we found that in PB-T the PI3K-inhibitors wortmannin and LY294002 block activation induced cofilin dephosphorylation and its association with the actin cytoskeleton. In Jurkat cells, however, where cofilin is present mainly in its non-phosphorylated form and permanently associated with the actin cytoskeleton, wortmannin and LY294002 do not block these events. Studies by others employing these PI3K-inhibitors have also led to such contradictory results: While in stimulated PB-T these inhibitors repress expression of IL-2, they even enhance IL-2 expression in Jurkat cells. These findings show that signalling events in Jurkat cells are not representative for signalling processes in untransformed human T lymphocytes. Importantly, our data demonstrate that-rebutting a persistent dogma-a T-cell specific uncoupling of PI3K from Ras does not exist.

CD28 Costimulation Controls Histone Hyperacetylation of the Interleukin 5 Gene Locus in Developing Th2 Cells

Interleukin 5 (IL-5) plays a unique role in allergic inflammatory responses, and the understanding of molecular mechanisms underlying the generation of IL-5-producing cells is crucial for the regulation of allergic disorders. Differentiation of naive CD4 T cells into type-2 helper (Th2) cells is accompanied by chromatin remodeling including hyperacetylation of histones H3 and H4 in the nucleosomes associated with the IL-4, IL-13, and IL-5 genes. Histone hyperacetylation of the IL-5 gene displayed a delayed kinetics compared with that of the IL-4 and IL-13 genes, suggesting a distinct remodeling mechanism for the IL-5-gene locus. Here we studied the role of CD28 costimulation in the generation of IL-5-producing cells and the histone hyperacetylation of the IL-5 gene locus. CD28-costimulation selectively enhanced histone hyperacetylation of the IL-5 gene locus that appeared to be mediated through NF-kappaB activation and subsequent up-regulation of GATA3. The CD28 costimulation-sensitive histone hyperacetylation spanned almost the entire intergenic region between the IL-5 and RAD50 accompanied with intergenic transcript. Thus, this is the first demonstration that CD28 costimulation controls a chromatin-remodeling process during Th2 cell differentiation.

IN VITRO METABOLISM OF THE PHOSPHATIDYLINOSITOL 3-KINASE INHIBITOR, WORTMANNIN, BY CARBONYL REDUCTASE

The phosphatidylinositol 3-kinase inhibitor, wortmannin, is extensively used in molecular signaling studies and has been proposed as a potential antineoplastic agent. The failure to detect wortmannin in mouse plasma after i.v. administration prompted in vitro studies of wortmannin metabolism. Wortmannin was incubated with mouse tissue homogenates, homogenate fractions, or purified, recombinant human carbonyl reductase in the presence of specified cofactors and inhibitors. Reaction products were characterized and quantified with liquid chromatography (LC)/mass spectrometry. Reaction rates were characterized using Michaelis-Menten kinetics. Wortmannin was metabolized to a material 2 atomic mass units greater than wortmannin. Liver homogenate had the highest metabolic activity. Some metabolism occurred in kidney and lung homogenates. Very little metabolism occurred in brain or red blood cell homogenates. Liver S9 fraction and cytosol metabolized wortmannin in the presence of NADPH and, to a much lesser extent, in the presence of NADH. Microsomal metabolism of wortmannin was minimal. Purified, recombinant human carbonyl reductase metabolized wortmannin. Quercetin, a carbonyl reductase inhibitor, greatly decreased wortmannin metabolism by S9, cytosol, and carbonyl reductase. The K(M) for wortmannin metabolism by purified, recombinant human carbonyl reductase was 119 +/- 9 microM, and the V(max) was 58 +/- 9 nmol/min/mg of protein. LC-tandem mass spectrometry spectra indicated that carbonyl reductase metabolized wortmannin to 17-OH-wortmannin. Wortmannin reduction by carbonyl reductase may partly explain why wortmannin is not detected in plasma after being administered to mice. Metabolism of wortmannin to 17-OH-wortmannin has mechanistic, and possibly toxicologic, implications because 17-OH-wortmannin is 10-fold more potent an inhibitor of phosphatidylinositol 3-kinase than is wortmannin.

Phosphatidylinositol-3 kinase inhibitors reproduce the selective antiproliferative effects of imatinib on chronic myeloid leukaemia progenitor cells

We investigated the role of the phosphatidylinositol-3 kinase (PI-3K) pathway in regulating the proliferation of primary chronic myeloid leukaemia (CML) progenitor cells by using imatinib to inhibit the activity of p210(Bcr-Abl). The effect of imatinib on the expression of PI-3K pathway proteins was investigated by kinase assays and Western blotting; PI-3K was inhibited by wortmannin or LY294002, Jak2 by AG490 and farnesylation by FTI II; progenitor cell proliferation (self-renewal) was measured by growing myeloid colonies in vitro, then replating them to observe secondary colony formation. Suppression of p210(Bcr-Abl) with imatinib indirectly suppressed the activity of PI-3K and its downstream targets (Erk, Akt and p70S6 kinase), thereby implicating the PI-3K pathway in p210(Bcr-Abl)-mediated signalling in primary CML progenitor cells. The PI-3K inhibitors, wortmannin and LY294002 reproduced the differential effects of imatinib on normal and CML progenitor cell proliferation in vitro by increasing normal cell (P = 0.001) and reducing CML cell proliferation (P = 0.0003). This differential effect was attributable to dysregulated signalling by granulocyte colony-stimulating factor in CML. The responses of individual patient's cells to wortmannin correlated with their responses to imatinib (P = 0.004) but not their responses to AG490 (Jak2 kinase inhibitor) or FTI II (farnesyltransferase inhibitor). Individual responses to wortmannin also correlated with responses to interferon alpha (IFNalpha) (P = 0.016). Imatinib-resistant K562 cells were sensitive to LY294002. Inhibition of the PI-3K pathway may be common to imatinib and IFNalpha and reflect dysregulated cytokine signalling. As imatinib-resistant cells remained sensitive to wortmannin and LY294002, targeting the PI-3K pathway may provide an alternative therapy for imatinib-resistant patients.

Opposing effects of PI3 kinase pathway activation on human myeloid and erythroid progenitor cell proliferation and differentiation in vitro

OBJECTIVE

To investigate 1) the effects of lineage-specific cytokines (G-CSF and EPO) combined with ligands for different classes of cytokine receptors (common beta chain, gp130, and tyrosine kinase) on proliferation by human myeloid and erythroid progenitor cells; and 2) the signal transduction pathways associated with combinatorial cytokine actions.

PATIENTS AND METHODS

CFU-GM and BFU-E were cloned in vitro. Secondary colony formation by replated CFU-GM and subcolony formation by BFU-E provided measures of progenitor cell proliferation. Studies were performed in the presence of cytokine combinations with and without signal transduction inhibitors.

RESULTS

Proliferation by CFU-GM and BFU-E was enhanced synergistically when common beta chain receptor cytokines (IL-3 or GM-CSF) were combined with G-CSF or EPO, but not with gp130 receptor cytokines (LIF or IL-6) or tyrosine kinase receptor cytokines (SCF, HGF, Flt-3 ligand, or PDGF). Delayed addition studies with G-CSF+IL-3 and EPO+IL-3 demonstrated that synergy required the presence of both cytokines from the initiation of the culture. The Jak2-specific inhibitor, AG490, abrogated the effect of combining IL-3 with EPO but had no effect on the enhanced CFU-GM proliferation stimulated by IL-3+G-CSF. The PI3 kinase inhibitors LY294002 and wortmannin substituted for G-CSF in combination with IL-3 since proliferation in the presence of LY294002/wortmannin+IL-3 was enhanced to the same extent as in the presence of G-CSF+IL-3. In contrast, LY294002 and wortmannin inhibited proliferation in the presence of EPO and in the presence of EPO+IL-3.

CONCLUSION

1) IL-3 may activate different signal transduction pathways when combined with G-CSF and when combined with EPO; 2) different signal transducing intermediates regulate erythroid and myeloid progenitor cell proliferation; and 3) inhibition of the PI3 kinase pathway suppresses myeloid progenitor cell differentiation and thereby increases proliferation.

Use of high-performance liquid chromatography to characterize the rapid decomposition of wortmannin in tissue culture media

Although wortmannin is extensively used in molecular signaling studies, its stability in tissue culture medium has not been assessed precisely. Therefore, we used high-performance liquid chromatography (HPLC) and mass spectrometry (MS) to characterize the decomposition of wortmannin in five commonly used media. Wortmannin was added to medium alone or to medium supplemented with 10% unheated or heat-inactivated fetal bovine serum and incubated at 37 degrees C. After 0, 5, 10, 20, 35, and 60 min, wortmannin remaining in the medium was quantified, and its decay constant and half-life were calculated. In all media, wortmannin decomposed monoexponentially, with half-lives between 8 and 13 min. HPLC/MS indicated that wortmannin decomposed to materials with m/z 447, 433, 373, and 313. Acidification of material produced by incubation of wortmannin in tissue culture medium or 1 microM NaOH converted the material with m/z 447 back to one that cochromatographed with and had an m/z (429) identical to that of wortmannin. Therefore wortmannin is much less stable in tissue culture medium than previously thought although some apparent loss of wortmannin reflects reversible, pH-dependent opening of the lactone ring of wortmannin. This rapid and complex decomposition of wortmannin argues for care being taken in how it is used in in vitro studies.

Phosphatidylinositide 3-kinase priming couples c-FLIP to T cell activation

Cellular FLICE (FADD-like interleukin-1-beta-converting enzyme)-inhibitory protein (c-FLIP) inhibits death receptor-induced apoptosis by binding to FADD (Fas-associated death domain protein) and pro-caspase-8. c-FLIP has also been shown to transmit activation signals and to enhance interleukin (IL)-2 production. However, c-FLIP-mediated T cell activation is difficult to detect in most cells. We found that in DO11.10 T cells, c-FLIP expression led to inhibition of IL-2 production, in contrast to the readily detectable c-FLIP-induced activation in Jurkat cells. A direct comparison revealed that distinct signal pathways were regulated by c-FLIP in Jurkat cells and DO11.10 cells. We investigated whether constitutively activated phosphatidylinositide 3-kinase (PI3K) in Jurkat cells stimulated c-FLIP. Inhibition of PI3K in Jurkat cells abrogated a c-FLIP-mediated increase in IL-2 production. In addition, c-FLIP coordinated with active PI3K for ERK activation. Furthermore, introduction of PTEN back into Jurkat cells eliminated the stimulatory effect of c-FLIP on IL-2 production and ERK activation. Our results suggest that priming with PI3K promotes the coupling of c-FLIP to T cell activation.

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.

Survival signals in leukemic large granular lymphocytes

The central hypothesis of our laboratory research program in large granular lymphocyte (LGL) leukemia is that leukemic LGL represent antigen-driven cytotoxic T lymphocytes (CTL) with characteristics of dysregulated apoptosis. The clinical features of LGL leukemia highlight the association of autoimmune diseases such as rheumatoid arthritis with the T-cell form of LGL leukemia. We therefore used LGL leukemia as a model disease of dysregulated apoptosis leading to both malignant and autoimmune diseases. Here, we review our understanding of survival signals activated in leukemic LGL in the context of knowledge concerning apoptotic pathways in activated normal lymphocytes.

PI3K in lymphocyte development, differentiation and activation

Phosphoinositide 3-kinases (PI3Ks) regulate numerous biological processes, including cell growth, differentiation, survival, proliferation, migration and metabolism. In the immune system, impaired PI3K signalling leads to immunodeficiency, whereas unrestrained PI3K signalling contributes to autoimmunity and leukaemia. New insights into the role of PI3Ks in lymphocyte biology have been derived from gene-targeting studies, which have identified the PI3K subunits that are involved in B-cell and T-cell signalling. In particular, the catalytic subunit p110delta seems to be adapted to transmit antigen-receptor signalling in B and T cells. Additional recent work has provided new insights into the molecular interactions that lead to PI3K activation and the signalling pathways that are regulated by PI3K.

The PI-3 kinase/Akt pathway and T cell activation: pleiotropic pathways downstream of PIP3

Ligation of the T cell receptor for antigen (TCR) and/or costimulatory receptor CD28 results in rapid activation of phosphoinositide-3 kinase (PI-3 kinase). It remains unclear, however, precisely how this activation occurs and also how the newly generated phospholipid products trigger the various events associated with T cell activation. Here we discuss the current understanding of how PI-3 kinase is activated by the TCR and CD28 and what roles its products play in T cell activation. We also review recent advances in understanding the function of Akt in particular, especially its role in CD28 costimulation. Several functional targets of Akt are discussed in this regard: inducible transcription, cell survival, glucose metabolism, and the cellular translational machinery. These pathways have been associated with TCR/CD28 costimulation, and they have also been implicated as targets of Akt.

Therapeutic potential of phosphoinositide 3-kinase inhibitors

At least one Holy Grail for many academic researchers and pharmaceutical research divisions alike has been to identify therapeutically useful selective PI3K inhibitors. There are several different but closely related PI3Ks which are thought to have distinct biological roles. Until now, however, researchers have been frustrated by poor selectivity of the available pharmacological inhibitors, which are unable to distinguish the different isoforms of PI3K adequately. Fortunately, recently published work gives cause for optimism; there are now several patent specifications published that describe new PI3K inhibitors, including some that are more selective for the delta isoform of PI3K. Given the involvement of PI3Ks in a plethora of biological settings, such isoform-selective inhibitors may have immense potential use for the treatment of patients with inflammatory and autoimmune disorders as well as cancer and cardiovascular diseases.

Chemokines: attractive mediators of the immune response

An effective inflammatory immune response first requires the recruitment of cells to the site of inflammation and then their appropriate activation and regulation. Chemokines are critical in this response since they are both chemotactic and immunoregulatory molecules. In this regard, the interaction between CCL5 and CCR5 may be critical in regulating T cell functions, by mediating their recruitment and polarization, activation, and differentiation. Various tyrosine phosphorylation signaling cascades can be engaged following chemokine receptor aggregation on T cells, including the Jak-Stat pathway, FAK activation, the MAP kinase pathway, PI3-kinase activation, and transactivation of the T cell receptor. This review will address specific aspects related to chemokine-T cell interactions and the molecular signaling mechanisms that influence T cell function in an inflammatory immune response.

Loss of a single allele of SHIP exacerbates the immunopathology of Pten heterozygous mice

Phosphatidylinositol 3-kinase (PI3K) has emerged as a critical component of multiple immune system intracellular signalling pathways. The levels and relative ratios of PI3K products, phosphatidylinositol (3,4) bisphosphate (PI(3,4)P(2)) and phosphatidylinositol (3,4,5) trisphosphate (PIP(3)), are regulated by inositol phosphatases such as Pten and SHIP. Interestingly, mice heterozygous for Pten, a 3'-inositol phosphatase, develop a progressive lymphoproliferative syndrome with autoimmune features. Given the importance of PIP(3) species in regulating immune responses, we hypothesized that heterozygosity for the 5'-inositol phosphatase SHIP might exacerbate the autoimmune phenotype of Pten(+/-) mice. In keeping with this, mice heterozygous for both Pten and SHIP developed lymphoproliferation, hypergammaglobulinaemia, autoantibody titres and renal pathology that were more severe than that of Pten(+/-) mice. These results suggest that the relative levels of phosphatidylinositol phosphatases are likely critical to immune system homeostasis and they also highlight the potential for gene dosage effects in regulating susceptibility and/or severity of autoimmunity.

Novel agents for the prevention of breast cancer: targeting transcription factors and signal transduction pathways

Transformation of breast cells occurs through loss or mutation of tumor suppressor genes, or activation or amplification of oncogenes, leading to deregulation of signal transduction pathways, abnormal amplification of growth signals, and aberrant expression of genes that ultimately transform the cells into invasive cancer. The goal of cancer preventive therapy, or "chemoprevention," is to eliminate premalignant cells or to block the progression of normal cells into cancer. Multiple alterations in signal pathways and transcription factors are observed in mammary gland tumorigenesis. In particular, estrogen receptor (ER) deregulation plays a critical role in breast cancer development and progress, and targeting ER with selective ER modulators (SERMs) has achieved significant reduction of breast cancer incidence in women at high risk for breast cancer. However, not all breast cancer is prevented by SERMs, because 30-40% of the tumors are ER-negative. Other receptors for retinoids, vitamin D analogs and peroxisome proliferator-activiator, along with transcription factors such as AP-1, NF-kappaB, and STATs (signal transducers and activators of transcription) affect breast tumorigenesis. This is also true for the signal transduction pathways, for example cyclooxygenase 2 (Cox-2), HER2/neu, mitogen-activated protein kinase (MAPK), and PI3K/Akt. Therefore, proteins in pathways that are altered during the process of mammary tumorigenesis may be promising targets of future chemopreventive drugs. Many newly-developed synthetic or natural compounds/agents are now under testing in preclinical studies and clinical trials. Receptor selective retinoids, receptor tyrosine kinase inhibitors (TKIs), SERMs, Cox-2 inhibitors, and others are some of the promising novel agents for the prevention of breast cancer. The chemopreventive activity of these agents and other novel signal transduction inhibitors are discussed in this chapter.

TGFbeta induces GDNF responsiveness in neurons by recruitment of GFRalpha1 to the plasma membrane

We have previously shown that the neurotrophic effect of glial cell line-derived neurotrophic factor (GDNF) in vitro and in vivo requires the presence of transforming growth factor (TGF)beta. Using primary neurons (chick E8 ciliary) we show that the combination of GDNF plus TGFbeta promotes survival, whereas the single factors do not. This cooperative effect is inhibited by blocking the extracellular signal-regulated kinase (ERK)/MAPK pathway, but not by interfering with the PI3 kinase signaling cascade. Although there is no functional GDNF signaling in the absence of TGFbeta, pretreatment with TGFbeta confers GDNF responsiveness to the cells. This is not due to upregulation of GDNF receptors mRNA and protein, but to TGFbeta-induced recruitment of the glycosyl-phosphatidylinositol-anchored GDNF receptor (GFR)alpha1 to the plasma membrane. This is supported by the fact that GDNF in the presence of a soluble GFRalpha1 can promote survival in the absence of TGFbeta. Our data suggest that TGFbeta is involved in GFRalpha1 membrane translocation, thereby permitting GDNF signaling and neurotrophic effects.

T-cell activation through the antigen receptor. Part 1: signaling components, signaling pathways, and signal integration at the T-cell antigen receptor synapse

Part 1 of this review will highlight the basic components and signaling pathways by which the T-cell antigen receptor (TCR) activates mature extrathymic T cells. TCR signaling commences with an early wave of protein tyrosine kinase activation, which is mediated by the Src kinases Lck and Fyn, the 70-kd zeta-associated protein kinase, and members of the Tec kinase family. This early wave of protein tyrosine phosphorylation leads to the activation of downstream signaling pathways, including an increase in intracellular free calcium, protein kinase C, nuclear factor kappaB and Ras-mitogen-activated protein kinase activation. These pathways activate transcription factors, such as activator protein 1, nuclear factor of activated T cells, and Rel proteins, which ultimately lead to the expression of genes that control cellular proliferation, differentiation, anergy, or apoptosis. This review also describes how costimulatory receptors assist in signal transduction and assembly of macromolecular complexes at the TCR contact site with the antigen-presenting cell, also known as the immune synapse. These basic concepts of TCR signal transduction will be used in part 2 to explain how T-cell function can be altered by therapeutic targeting of TCR signaling components, as well as to explain modification of TCR signaling during T(H)1/T(H)2 differentiation, tolerance, and immune senescence.

Ordered just so: lipid rafts and lymphocyte function

Immunologists have long been occupied with the description of cellular activation signaling events that originate with the stimulation of multichain immunoreceptors at the cell surface. These signals are transmitted by a protein-partner-signaling cascade through the cytoplasm to the nucleus, where they culminate in changes in gene expression, metabolic state, and entry into cell cycle. For T cells and B cells, these signaling cascades start with the ligation of the T cell receptor (TCR) and B cell receptor (BCR), respectively, and result in the recruitment and activation of related families of signaling molecules at the cell surface. Until recently, this gathering of signaling proteins was thought to occur within the featureless plasma membrane, a cellular organ that was envisioned as a boundary between the inner and outer components of the cell, but which contributed little to the signaling process. However, the past few years have seen the gradual realization that activation of signaling in lymphocytes takes place in and around specialized membrane subdomains called lipid rafts (also known as DIGs and GEMs). Here, we provide a brief overview of the analogous structures and compositions of lipid raft-associated signaling complexes in T cells and B cells, and the ways in which lymphocytes--and their pathogen adversaries--use lipid rafts to their benefit.

Inhibition of lymphocyte kinase Lck and phosphatidylinositol 3-kinase by a novel immunosuppressant, lymphostin

Lck is a Src-family tyrosine kinase that is expressed predominantly in T cells, where it plays important roles in T-cell activation. Lymphostin was isolated from Streptomyces sp. as an inhibitor of Lck. As previously reported, lymphostin inhibited Lck (IC50 0.05 microM) and the mixed lymphocyte reaction (IC50 0.009 microM). We have now examined the mechanism of inhibition by lymphostin. Lymphostin inhibited protein-tyrosine kinase activity in Jurkat T cells, demonstrating the effectiveness of the compound at the cellular level. Furthermore, lymphostin suppressed delayed-type hypersensitivity in mice. However, the inhibitory activity against Lck at the cellular level was weaker than that against the mixed lymphocyte reaction. Thus, we examined the effects of lymphostin on other kinases. Interestingly, lymphostin also inhibited phosphatidylinositol 3-kinase (IC50 0.001 microM). Consequently, we conclude that lymphostin inhibits the mixed lymphocyte reaction and delayed-type hypersensitivity not only through the blockade of Lck, but through the blockade of phosphatidylinositol 3-kinase as well.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Phosphatidylinositol 3-kinase p85 adaptor function in T-cells. Co-stimulation and regulation of cytokine transcription independent of associated p110

Phosphatidylinositol 3-kinase (PI3K) is a key regulator of a variety of cellular functions from cytoskeletal organization, vesicular trafficking, and cell proliferation to apoptosis. The enzyme complex is comprised of an 85-kDa adaptor (p85) coupled to a 110-kDa catalytic subunit (p110). While the function of PI3K has been largely attributed to the generation of D-3 lipids, an unanswered question has been whether p85 with a number of motifs (SH2, SH3, BcR homology (BH) region) can generate independent intracellular signals. In this study, we demonstrate that p85 lacking p110 (Deltap85) can activate NFAT transcription in T-cell hybridomas and normal splenocytes. This up-regulatory effect was unaffected by inhibition of PI 3-kinase, and cooperated specifically with Rac1, but not related family members. Stimulation correlated with Rac1 binding and was lost with the deletion of the BH domain. Lastly, the CD28-Deltap85 chimera also cooperated with TcR/CD3 to provide co-signals that enhanced IL-2 transcription. Our findings identify for the first time p85 as an adaptor that operates independently of the classic PI 3-kinase catalytic pathway and further shows that this pathway can provide co-signals in the regulation of T-cell function.

Role of phosphoinositide 3-kinase in TCR-signaled regulation of CD8-mediated adhesion to class I MHC protein

CD8 must be activated by signaling through the TCR in order to mediate CTL adhesion. Up-regulation of adhesion to class I protein is shown to be blocked by specific inhibitors of phosphoinositide 3-OH kinase (PI3-K), indicating a critical role for this enzyme in signaling for activation of CD8. A minimal TCR stimulus that activates CD8 does not result in a detectable increase in total cellular PI3-K activity, but an increase in PI3-K activity associated with p59(fyn) kinase can be detected. Genistein blocks this increase concomitantly with blocking the activation of adhesion, suggesting that activation of fyn-associated PI3-K is downstream of TCR-dependent activation of protein tyrosine kinase(s) in the signaling pathway that leads to up-regulation of CD8-dependent adhesion. Treatment of cells with phorbol ester also blocks the TCR-dependent increase in fyn-associated PI3-K and inhibits CD8-dependent adhesion. This suggests a feedback model for deactivation of CD8 adhesion to allow target cell release by CTL and recycling to kill additional targets. In contrast, phorbol ester treatment up-regulates integrin-mediated adhesions, suggesting complex cross-talk between the TCR and the different adhesion/cosignaling receptors during the binding and killing of antigen-bearing targets.

Role of IL-10 in allergen-specific immunotherapy and normal response to allergens

Induction of specific unresponsiveness (tolerance/anergy) in peripheral T cells by interleukin-10 (IL-10) and recovery by cytokines from the tissue microenvironment represent two key steps in specific immunotherapy of allergy and in natural exposure to allergens in healthy individuals. IL-10 elicits anergy in T cells by selective inhibition of the CD28 costimulatory pathway and controls suppression and development of antigen-specific immunity.

The EphA8 receptor regulates integrin activity through p110gamma phosphatidylinositol-3 kinase in a tyrosine kinase activity-independent manner

Recent genetic studies suggest that ephrins may function in a kinase-independent Eph receptor pathway. Here we report that expression of EphA8 in either NIH 3T3 or HEK293 cells enhanced cell adhesion to fibronectin via alpha(5)beta(1)- or beta(3) integrins. Interestingly, a kinase-inactive EphA8 mutant also markedly promoted cell attachment to fibronectin in these cell lines. Using a panel of EphA8 point mutants, we have demonstrated that EphA8 kinase activity does not correlate with its ability to promote cell attachment to fibronectin. Analysis using EphA8 extracellular and intracellular domain mutants has revealed that enhanced cell adhesion is dependent on ephrin A binding to the extracellular domain and the juxtamembrane segment of the cytoplasmic domain of the receptor. EphA8-promoted adhesion was efficiently inhibited by wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. Additionally, we found that EphA8 had associated PI 3-kinase activity and that the p110gamma isoform of PI 3-kinase is associated with EphA8. In vitro binding experiments revealed that the EphA8 juxtamembrane segment was sufficient for the formation of a stable complex with p110gamma. Similar results were obtained in assay using cells stripped of endogenous ephrin A ligands by treatment with preclustered ephrin A5-Fc proteins. In addition, a membrane-targeted lipid kinase-inactive p110gamma mutant was demonstrated to stably associate with EphA8 and suppress EphA8-promoted cell adhesion to fibronectin. Taken together, these results suggest the presence of a novel mechanism by which the EphA8 receptor localizes p110gamma PI 3-kinase to the plasma membrane in a tyrosine kinase-independent fashion, thereby allowing access to lipid substrates to enable the signals required for integrin-mediated cell adhesion.

Phosphoinositide 3-kinases in T lymphocyte activation

Biochemical experiments have established that the metabolism of inositol phospholipids by phosphoinositide 3-kinases (PI3Ks) and lipid-phosphatases is triggered by many receptors that control T lymphocyte function, including antigen-receptors, costimulatory molecules, cytokines and chemokines. Novel effectors of PI3K have been identified in the immune system and shown to be important in the control of lymphocyte activation. Moreover, key lipid-phosphatases have been identified that act to terminate or modulate PI3K signalling in cells of the immune system.

Mechanisms of interleukin-10-mediated immune suppression

Specific immune suppression and induction of anergy are essential processes in the regulation and circumvention of immune defence. Interleukin-10 (IL-10), a suppressor cytokine of T-cell proliferative and cytokine responses, plays a key regulatory role in tolerizing exogenous antigens during specific immunotherapy (SIT) of allergy and natural exposure to antigens. Specific T-cell tolerance is directed against the T-cell epitopes of an antigen and characterized by suppressed proliferative and T helper type 1 (Th1) and type 2 (Th2) cytokine responses. IL-10 elicits tolerance in T cells by selective inhibition of the CD28 co-stimulatory pathway and thereby controls suppression and development of antigen-specific immunity. IL-10 only inhibits T cells stimulated by low numbers of triggered T-cell receptors and which therefore depend on CD28 co-stimulation. T cells receiving a strong signal from the T-cell receptor alone, and thus not requiring CD28 co-stimulation, are not affected by IL-10. IL-10 inhibits CD28 tyrosine phosphorylation, the initial step of the CD28 signalling pathway, and consequently the phosphatidylinositol 3-kinase p85 binding to CD28. Together these results demonstrate that IL-10-induced selective inhibition of the CD28 co-stimulatory pathway acts as a decisive mechanism in determining whether a T cell will contribute to an immune response or become anergic.

Prostaglandin E2 increases growth and motility of colorectal carcinoma cells

Chronic use of nonsteroidal anti-inflammatory drugs results in a significant reduction of risk and mortality from colorectal cancer in humans. All of the mechanism(s) by which nonsteroidal anti-inflammatory drugs exert their protective effects are not completely understood, but they are known to inhibit cyclooxygenase activity. The cyclooxygenase enzymes catalyze a key reaction in the conversion of arachidonic acid to prostaglandins, such as prostaglandin E(2) (PGE(2)). Here we demonstrate that PGE(2) treatment of LS-174 human colorectal carcinoma cells leads to increased motility and changes in cell shape. The prostaglandin EP(4) receptor signaling pathway appears to play a role in transducing signals which regulate these effects. PGE(2) treatment results in an activation of phosphatidylinositol 3-kinase/protein kinase B pathway that is required for the PGE(2)-induced changes in carcinoma cell motility and colony morphology. Our results suggest that PGE(2) might enhance the invasive potential of colorectal carcinoma cells via activation of major intracellular signal transduction pathways not previously reported to be regulated by prostaglandins.

Akt/PKB activity is required for Ha-Ras-mediated transformation of intestinal epithelial cells

Phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB/Akt) is thought to serve as an oncogenic signaling pathway which can be activated by Ras. The role of PI3K/Akt in Ras-mediated transformation of intestinal epithelial cells is currently not clear. Here we demonstrate that inducible expression of oncogenic Ha-Ras results in activation of PKB/Akt in rat intestinal epithelial cells (RIE-iHa-Ras), which was blocked by treatment with inhibitors of PI3K activity. The PI3K inhibitor, LY-294002, partially reversed the morphological transformation induced by Ha-Ras and resulted in a modest stimulation of apoptosis. The most pronounced phenotypic alteration following inhibition of PI3K was induction of G(1) phase cell cycle arrest. LY-294002 blocked the Ha-Ras-induced expression of cyclin D1, cyclin-dependent kinase (CDK) 2, and increased the levels of p27(kip). Both LY-294002 and wortmannin significantly reduced anchorage-independent growth of RIE-iHa-Ras cells. Forced expression of both the constitutively active forms of Raf (DeltaRaf-22W or Raf BXB) and Akt (Akt-myr) resulted in transformation of RIE cells that was not achieved by transfection with either the Raf mutant construct or Akt-myr alone. These findings delineate an important role for PI3K/Akt in Ras-mediated transformation of intestinal epithelial cells.

Selective pharmacological inhibitors reveal differences between Thy-1- and T cell receptor-mediated signal transduction in mouse T lymphocytes

A compelling body of evidence suggests a role for Thy-1 (CD90), a cell surface glycoprotein of mouse T lymphocytes, in signal transduction resulting in T cell activation. Despite more than 3 decades of investigation, intracellular biochemical events governing the Thy-1 signaling cascade are only vaguely understood. We have employed selective pharmacological inhibitors of signaling molecules to compare downstream elements participating in the Thy-1 signal transduction pathway with those involved in the T cell receptor (TCR)/CD3-associated signaling pathway. Mitogenic anti-Thy-1 or anti-CD3 monoclonal antibody (mAb) were used to cause T cells from C57BL/6 mice to proliferate in the presence or absence of different pharmacological inhibitors. Cyclosporine A, herbimycin A, LY294002, calphostin C and PD98059 all inhibited anti-Thy-1-induced T lymphocyte proliferation, indicating the involvement of calcineurin, protein tyrosine kinases, phosphatidylinositol 3-kinase, protein kinase C, and MEK1 (MAPK kinase 1), respectively, in Thy-1 signaling. Similar results were obtained when T cells were stimulated through the TCR with anti-CD3 monoclonal antibody in the presence or absence of the different inhibitors. Interestingly, the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 augmented anti-Thy-1-induced T cell proliferation, whereas anti-CD3-induced proliferative response was partially suppressed by the same inhibitor. The Thy-1 signal transduction pathway, therefore, shares a requirement for calcineurin and several major kinase families with the TCR signaling pathway. However, Thy-1 and TCR-associated signaling pathways are differentially regulated by p38 MAPK.

Integrating signals from T-cell receptor and serum by T cells enhance translation of tumour necrosis factor-alpha

Tumour necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine produced by several cell types, including T cells upon antigen stimulation. Its production is crucial for the development of an early defence against many pathogens, but its beneficial effects are dependent on the strength and duration of its expression. In this paper we present evidence indicating that serum increases translational efficiency of TNF-alpha in human peripheral blood mononuclear cells stimulated with superantigen. The increase in translation of TNF-alpha due to serum could be inhibited by the phosphatidylinositol (PI) 3-K inhibitors, wortmannin and LY294002, suggesting that PI 3-K is involved in the translational control of TNF-alpha by serum. Similarly to primary T cells, stimulation of Jurkat T cells with superantigen led to TNF-alpha secretion and this was up-regulated by serum. Transfection of Jurkat cells with a constitutively active form of PI 3-Kalpha increased the production of TNF-alpha in cells stimulated with superantigen. Additionally, we used the specific inhibitors targeting ERK kinase and p38 mitogen-activated protein kinase (MAPK), potentially downstream of PI 3-kinase, PD98059 and SB203580. Differently from with PI 3-K inhibitors, the accumulation of TNF-alpha mRNA was inhibited by PD98059 or SB203580. These results suggest that, in T cells, activation of PI 3-K is an important step in controlling TNF-alpha protein synthesis in response to growth factors.

Costimulation of resting B lymphocytes alters the IL-4-activated IRS2 signaling pathway in a STAT6 independent manner: implications for cell survival and proliferation

IL-4 is an important B cell survival and growth factor. IL-4 induced the tyrosine phosphorylation of IRS2 in resting B lymphocytes and in LPS- or CD40L-activated blasts. Phosphorylated IRS2 coprecipitated with the p85 subunit of PI 3' kinase in both resting and activated cells. By contrast, association of phosphorylated IRS2 with GRB2 was not detected in resting B cells after IL-4 treatment although both proteins were expressed. However, IL-4 induced association of IRS2 with GRB2 in B cell blasts. The pattern of IL-4-induced recruitment of p85 and GRB2 to IRS2 observed in B cells derived from STAT6 null mice was identical to that observed for normal mice. While IL-4 alone does not induce activation of MEK, a MEK1 inhibitor suppressed the IL-4-induced proliferative response of LPS-activated B cell blasts. These results demonstrate that costimulation of splenic B cells alters IL-4-induced signal transduction independent of STAT6 leading to proliferation. Furthermore, proliferation induced by IL-4 in LPS-activated blasts is dependent upon the MAP kinase pathway.

Akt provides the CD28 costimulatory signal for up-regulation of IL-2 and IFN-gamma but not TH2 cytokines

A region of the interleukin-2 (IL-2) promoter known as the RE/AP element is activated in concert by signals that originate from the T cell antigen receptor and the CD28 coreceptor. We show here that the serine-threonine kinase Akt can provide a costimulatory signal for RE/AP activation that is indistinguishable from the signal provided by CD28. This includes the ability of Akt, like antibodies to CD28, to synergize with protein kinase C theta (PKC-theta) in the induction of RE/AP. Retrovirus-mediated expression of activated Akt in primary T cells from CD28-deficient mice is capable of selectively restoring production of IL-2 and interferon gamma, but not IL-4 or IL-5. Our results provide evidence that CD28 costimulation of different cytokines is mediated by discrete signaling pathways, one of which includes Akt.

The T cell antigen receptor activates phosphatidylinositol 3-kinase-regulated serine kinases protein kinase B and ribosomal S6 kinase 1

The present study has explored T cell antigen receptor-regulated serine kinases in human T cells. The results identify two phosphatidylinositol 3-kinase (PI3K)-controlled serine kinases operating downstream of the T cell receptor (TCR) in primary T cells: (i) protein kinase B whose activation regulates the phosphorylation of glycogen synthase kinase 3 and (ii) ribosomal S6 kinase 1, a kinase with a critical role in the regulation of protein synthesis and cell growth. T cells express two isoforms of S6k1: a 70 kDa cytoplasmic kinase and an 85 kDa isoform that has a classic nuclear localisation. TCR ligation triggers a parallel engagement of both the 70 and 85 kDa isoforms of S6k1 in a response that requires PI3K function.