Synthesis and function of 3-phosphorylated inositol lipids

Stable gene silencing in human monocytic cell lines using lentiviral-delivered small interference RNA. Silencing of the p110alpha isoform of phosphoinositide 3-kinase reveals differential regulation of adherence induced by 1alpha,25-dihydroxycholecalciferol and bacterial lipopolysaccharide

Studying mononuclear phagocyte cell biology through genetic manipulation by non-viral transfection methods has been challenging due to the dual problems of low transfection efficiency and the difficulty in obtaining stable transfection. To overcome this problem, we developed a system for mediating RNA interference in monocytic cells. The p110alpha isoform of phosphoinositide 3-kinases (PI3Ks) was silenced using a lentiviral vector expressing short hairpin RNA (shRNA). This resulted in the generation of stable THP-1 and U-937 monocytic cell lines deficient in p110alpha. Notably, p110alpha was silenced without affecting levels of either the other class I(A) PI3K catalytic subunits p110beta and p110delta, or the p85alpha regulatory subunit. The role of p110alpha in mediating cell adherence was examined. Monocyte adherence induced in response to either lipopolysaccharide (LPS) or 1alpha,25-dihydroxycholecalciferol (D(3)) was blocked by the PI3K inhibitor LY294002. However, although adherence induced in response to D(3) was sensitive to silencing of p110alpha, LPS-induced adherence was not. Expression of the monocyte differentiation marker CD11b was also induced by D(3) in a PI3K-dependent manner and gene silencing using shRNA showed that p110alpha was also required for this effect. Taken together, these findings demonstrate that LPS and D(3) use distinct isoforms of class I(A) PI3K to induce functional responses and that lentiviral-mediated delivery of shRNA is a powerful approach to study monocyte biology.

Ras, Akt, and Mechanotransduction in the Cardiac Myocyte

The Ras subfamily of 21-kDa ("small") guanine nucleotide binding proteins [which includes Ha-Ras, Ki(A)-Ras, Ki(B)-Ras, and N-Ras] is universally important in regulating intracellular signaling events in mammalian cells and controls their growth, proliferation, senescence, differentiation, and survival. These Ras isoforms act as membrane-associated biological switches that transduce signals from transmembrane receptors, thus potentially activating a variety of downstream signaling proteins. These include ultimately two Ser/Thr protein kinase families, the extracellular signal-regulated kinases 1/2 (ERK1/2) and Akt (or protein kinase B). Activation of ERK1/2 has been associated with cardiac myocyte hypertrophy (ie, increased cell size and myofibrillogenesis, with concurrent transcriptional changes to a fetal pattern of gene expression), whereas activation of Akt is associated with the increased protein accretion in hypertrophy. Both ERK1/2 and Akt may promote myocyte survival. In the intact heart in vivo and in primary cultures of cardiac myocytes, mechanical strain induces hypertrophy, a process known as mechanotransduction, which may involve Ras, ERK1/2, and Akt. In this study, general and cardiospecific aspects of the regulation of Ras and Akt will be described. The various mechanisms through which mechanical strain might initiate Ras- or Akt-dependent signaling will be discussed. The overall conclusion is that although an involvement of Ras and Akt in mechanotransduction is likely, more work (particularly focusing on mechanoreception) needs to be undertaken before it is unequivocally established.

Interplay between growth factor and nutrient signaling: lessons from Drosophila TOR

During normal development, cellular and organismal growth is coordinately regulated. Each cell and each individual organ integrates information about nutrient availability, hormonal signals, and intrinsic growth programs. Describing the signaling pathways involved in these processes and how they are integrated is important to understand how growth is controlled during development and may also permit the development of means to curb uncontrolled growth in disease. In recent years, the biochemical analysis of cellular growth in cultured cells and the genetic dissection of growth control in model organisms has identified two conserved signaling pathways dedicated to cellular growth. The target of rapamycin (TOR) pathway regulates growth in response to nutrients, and the insulin/IGF pathways are involved in coordinating cellular growth in response to endocrine signals. This review discusses recent advances in the understanding of the interaction between these pathways, with a special focus on the contribution of the genetic analysis of these pathways in Drosophila.

Trk receptors: roles in neuronal signal transduction

Trk receptors are a family of three receptor tyrosine kinases, each of which can be activated by one or more of four neurotrophins-nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophins 3 and 4 (NT3 and NT4). Neurotrophin signaling through these receptors regulates cell survival, proliferation, the fate of neural precursors, axon and dendrite growth and patterning, and the expression and activity of functionally important proteins, such as ion channels and neurotransmitter receptors. In the adult nervous system, the Trk receptors regulate synaptic strength and plasticity. The cytoplasmic domains of Trk receptors contain several sites of tyrosine phosphorylation that recruit intermediates in intracellular signaling cascades. As a result, Trk receptor signaling activates several small G proteins, including Ras, Rap-1, and the Cdc-42-Rac-Rho family, as well as pathways regulated by MAP kinase, PI 3-kinase and phospholipase-C-gamma (PLC-gamma). Trk receptor activation has different consequences in different cells, and the specificity of downstream Trk receptor-mediated signaling is controlled through expression of intermediates in these signaling pathways and membrane trafficking that regulates localization of different signaling constituents. Perhaps the most fascinating aspect of Trk receptor-mediated signaling is its interplay with signaling promoted by the pan-neurotrophin receptor p75NTR. p75NTR activates a distinct set of signaling pathways within cells that are in some instances synergistic and in other instances antagonistic to those activated by Trk receptors. Several of these are proapoptotic but are suppressed by Trk receptor-initiated signaling. p75NTR also influences the conformations of Trk receptors; this modifies ligand-binding specificity and affinity with important developmental consequences.

Gβγ subunits stimulate p21-activated kinase 1 (PAK1) through activation of PI3-kinase and Akt but act independently of Rac1/Cdc42

The p21-activated kinase (PAK) family is homologous to the yeast sterile 20 (Ste20) and regulates a wide variety of cellular responses, including cell morphology, proliferation, and survival. In this study we examined the activation of PAK1 by Gbetagamma subunits. Co-transfection of COS7 cells with Gbeta1gamma2 or Gbeta1gamma5 was sufficient to induce agonist-independent activation of PAK1. Expression of dominant/negative Rac, Cdc42, or Ras did not inhibit this Gbetagamma-dependent activation. Wortmannin, which inhibits phosphoinositide 3-kinase (PI3-kinase) activity, and expression of a dominant/negative form of Akt were sufficient to abrogate the activation of PAK1 that was induced by Gbetagamma. These results reveal that stimulation of PAK1 by Gbetagamma can occur via a PI3-kinase and Akt pathway that does not require Rac1 or Cdc42.

Direct interaction of major histocompatibility complex class II-derived peptides with class Ia phosphoinositide 3-kinase results in dose-dependent stimulatory effects

Peptides corresponding to residues 65-79 of human lymphocyte antigen class II sequence (DQA*03011) are cell-permeable and at high concentrations block activation of protein kinase B/Akt and p70-S6 kinase in T-cells, effects attributed to inhibition of phosphoinositide (PI) 3-kinase activity. To understand the molecular basis of this, we analyzed the effect this peptide had on activity of class I PI 3-kinases. Although there was no effect on the activity of class Ib PI 3-kinase or on the protein kinase activity of class I PI 3-kinases, there was a biphasic effect on lipid kinase activity of the class Ia enzymes. There was an inhibition of activity at higher peptide concentrations because of a formation of insoluble complexes between peptide and enzyme. Conversely, at lower peptide concentrations there was a profound activation of PI 3-kinase activity of class Ia PI 3-kinases. Studies of peptide variants revealed that all active peptides conform to heptad repeat motifs characteristic of coiled-coil helices. Surface plasmon resonance studies confirmed direct sequence-specific binding of active peptide to the p85alpha adapter subunit of class Ia PI 3-kinase. Active peptides also activated protein kinase B and extracellular signal-regulated kinase (ERK) in vivo in a wortmannin-sensitive manner while reducing recoverable cellular p85 levels. These results indicate that the human lymphocyte antigen class II-derived peptides regulate PI 3-kinase by direct interaction, probably via the coiled-coil domain. These peptides define a novel mechanism of regulating PI 3-kinase and will provide a useful tool for specifically dissecting the function of class Ia PI 3-kinase in cells and for probing structure-function relationships in the class Ia PI 3-kinase heterodimers.

Interaction of the protein tyrosine phosphatase PTPL1 with the PtdIns(3,4)P2-binding adaptor protein TAPP1

It has been postulated that PtdIns(3,4) P (2), one of the immediate breakdown products of PtdIns(3,4,5) P (3), functions as a signalling molecule in insulin- and growth-factor-stimulated pathways. To date, the t andem- P H-domain-containing p rotein- 1 (TAPP1) and related TAPP2 are still the only known PH-domain-containing proteins that interact strongly and specifically with PtdIns(3,4) P (2). In this study we demonstrate that endogenously expressed TAPP1, is constitutively associated with the protein-tyrosine-phosphatase-like protein-1 (PTPL1 also known as FAP-1). We show that PTPL1 binds to TAPP1 and TAPP2, principally though its first PDZ domain [where PDZ is postsynaptic density protein ( P SD-95)/ Drosophila disc large tumour suppressor ( d lg)/tight junction protein ( Z O1)] and show that this renders PTPL1 capable of associating with PtdIns(3,4) P (2) in vitro. Our data suggest that the binding of TAPP1 to PTPL1 does not influence PTPL1 phosphatase activity, but instead functions to maintain PTPL1 in the cytoplasm. Following stimulation of cells with hydrogen peroxide to induce PtdIns(3,4) P (2) production, PTPL1, complexed to TAPP1, translocates to the plasma membrane. This study provides the first evidence that TAPP1 and PtdIns(3,4) P (2) could function to regulate the membrane localization of PTPL1. We speculate that if PTPL1 was recruited to the plasma membrane by increasing levels of PtdIns(3,4) P (2), it could trigger a negative feedback loop in which phosphoinositide-3-kinase-dependent or other signalling pathways could be switched off by the phosphatase-catalysed dephosphorylation of receptor tyrosine kinases or tyrosine phosphorylated adaptor proteins such as IRS1 or IRS2. Consistent with this notion we observed RNA-interference-mediated knock-down of TAPP1 in HEK-293 cells, enhanced activation and phosphorylation of PKB following IGF1 stimulation.

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.

Centaurin-alpha1 is an in vivo phosphatidylinositol 3,4,5-trisphosphate-dependent GTPase-activating protein for ARF6 that is involved in actin cytoskeleton organization

The ADP-ribosylation factor (ARF) 6 small GTPase regulates vesicle trafficking and cytoskeletal actin reorganization. The GTPase-activating proteins (GAPs) catalyze the formation of inactive ARF6GDP. Centaurin-alpha1 contains an ARF GAP and two pleckstrin homology (PH) domains, which bind the second messenger phosphatidylinositol 3,4,5-trisphosphate (PIP3). Here, we show that centaurin-alpha1 specifically inhibits in vivo GTP loading of ARF6 and redistribution of ARF6 from the endosomal compartment to the plasma membrane, which are indicative of its activation. Centaurin-alpha1 also inhibited cortical actin formation in a PIP3-dependent manner. Moreover, the constitutively active mutant of ARF6, but not that of ARF1, reverses the inhibition of cortical actin formation by centaurin-alpha1. An artificially plasma membrane-targeted centaurin-alpha1 bypasses the requirement of PIP3 for its involvement in ARF6 inactivation, suggesting that PIP3 is required for recruitment of centaurin-alpha1 to the plasma membrane but not for its activity. Together, these data suggest that centaurin-alpha1 negatively regulates ARF6 activity by functioning as an in vivo PIP3-dependent ARF6 GAP.

Norepinephrine activates extracellular-regulated kinase in cortical neurons

BACKGROUND

Previous studies demonstrate that indirect activation of monoamine receptors by antidepressant treatment increases neurotrophic factors that activate the mitogen-activated protein kinase cascade; however, it is also possible that these monoamine receptors influence the mitogen-activated protein kinase pathway independent of neurotrophic factors. The influence of norepinephrine on the phosphorylation of extracellular-regulated protein kinase is characterized.

METHODS

Primary cerebral cortical cultures were prepared from embryonic day 18 rat brains and were subsequently incubated with norepinephrine in the absence or presence of agents acting as noradrenergic receptors or as intracellular signaling proteins. Levels of phosphorylated extracellular-regulated protein kinase were determined by immunoblot.

RESULTS

The results demonstrate that incubation with norepinephrine produces a time- and dose-dependent activation of phosphorylated extracellular-regulated protein kinase and that this increase is dependent on activation of alpha(2)- and beta-adrenergic receptor subtypes. In addition, the results demonstrate that norepinephrine activation of phosphorylated extracellular-regulated protein kinase is dependent on a pertussis toxin-sensitive G protein, a receptor tyrosine kinase, and activation of phosphatidylinositol 3-kinase.

CONCLUSIONS

The findings suggest that activation of the mitogen-activated protein kinase cascade by norepinephrine can occur via a tyrosine kinase-dependent signaling pathway but independent of classical second-messenger or Src-dependent kinases.

Inhibition of receptor-localized PI3K preserves cardiac beta-adrenergic receptor function and ameliorates pressure overload heart failure

beta-Adrenergic receptor (betaAR) downregulation and desensitization are hallmarks of the failing heart. However, whether abnormalities in betaAR function are mechanistically linked to the cause of heart failure is not known. We hypothesized that downregulation of cardiac betaARs can be prevented through inhibition of PI3K activity within the receptor complex, because PI3K is necessary for betaAR internalization. Here we show that in genetically modified mice, disrupting the recruitment of PI3K to agonist-activated betaARs in vivo prevents receptor downregulation in response to chronic catecholamine administration and ameliorates the development of heart failure with pressure overload. Disruption of PI3K/betaAR colocalization is required to preserve betaAR signaling, since deletion of a single PI3K isoform (PI3Kgamma knockout) is insufficient to prevent the recruitment of other PI3K isoforms and subsequent betaAR downregulation with catecholamine stress. These data demonstrate a specific role for receptor-localized PI3K in the regulation of betaAR turnover and show that abnormalities in betaAR function are associated with the development of heart failure. Thus, a strategy that blocks the membrane translocation of PI3K and leads to the inhibition of betaAR-localized PI3K activity represents a novel therapeutic approach to restore normal betaAR signaling and preserve cardiac function in the pressure overloaded failing heart.

Binding of phosphatidylinositol 3,4,5-trisphosphate to the pleckstrin homology domain of protein kinase B induces a conformational change

Protein kinase B (PKB/Akt) is a key regulator of cell growth, proliferation and metabolism. It possesses an N-terminal pleckstrin homology (PH) domain that interacts with equal affinity with the second messengers PtdIns(3,4,5)P3 and PtdIns(3,4)P2, generated through insulin and growth factor-mediated activation of phosphoinositide 3-kinase (PI3K). The binding of PKB to PtdIns(3,4,5)P3/PtdIns(3,4)P2 recruits PKB from the cytosol to the plasma membrane and is also thought to induce a conformational change that converts PKB into a substrate that can be activated by the phosphoinositide-dependent kinase 1 (PDK1). In this study we describe two high-resolution crystal structures of the PH domain of PKBalpha in a noncomplexed form and compare this to a new atomic resolution (0.98 A, where 1 A=0.1 nm) structure of the PH domain of PKBalpha complexed to Ins(1,3,4,5)P4, the head group of PtdIns(3,4,5)P3. Remarkably, in contrast to all other PH domains crystallized so far, our data suggest that binding of Ins(1,3,4,5)P4 to the PH domain of PKB, induces a large conformational change. This is characterized by marked changes in certain residues making up the phosphoinositide-binding site, formation of a short a-helix in variable loop 2, and a movement of variable loop 3 away from the lipid-binding site. Solution studies with CD also provided evidence of conformational changes taking place upon binding of Ins(1,3,4,5)P4 to the PH domain of PKB. Our data provides the first structural insight into the mechanism by which the interaction of PKB with PtdIns(3,4,5)P3/PtdIns(3,4)P2 induces conformational changes that could enable PKB to be activated by PDK1.

Nuclear magnetic resonance structure of the P395S mutant of the N-SH2 domain of the p85 subunit of PI3 kinase: an SH2 domain with altered specificity

Understanding the specificity of Src homology 2 (SH2) domains is important because of their critical role in cell signaling. Previous genetic analysis has characterized mutants of the N-terminal src homology 2 (SH2) domain of the p85 subunit of phosphoinositide 3-kinase (PI3K). The P395S mutant exhibits a specificity for phosphopeptide binding different from that of the wild-type SH2. The P395S mutant has an increased affinity for the platelet-derived growth factor receptor (PDGFr) compared to polyomavirus middle T antigen (MT). Solution structures of the P395S mutant of the p85 N-SH2 alone and complexed to a PDGFr phosphopeptide were determined to explain the change in specificity. Chemical shift perturbations caused by different peptides were compared for mutant and wild-type structures. The results show that the single P395S mutation has broad effects on the structure. Furthermore, they provide a rationale for the observed changes in binding preference.

Phosphoinositide 3-OH kinase inhibition prevents ventilation-induced lung cell activation

In acute respiratory distress syndrome patients, protective ventilation strategies reduce mortality and proinflammatory mediator levels. It has been suggested that some of the side effects of mechanical ventilation are caused by the excessive release of mediators capable of causing pulmonary inflammation and tissue destruction (biotrauma). Selective inhibition of this process might be used to minimize the side effects of artificial mechanical ventilation. This study was designed to identify the cell types and specific signaling mechanisms that are activated by ventilation with increased pressure/volume (overventilation). In isolated perfused mouse lungs, overventilation caused nuclear translocation of nuclear factor-kappaB (NF-kappaB) and enhanced expression of interleukin-6 mRNA in alveolar macrophages and alveolar epithelial type II cells. The phosphoinositide 3-OH kinase inhibitor Ly294002 prevented nuclear translocation of NF-kappaB and the subsequent release of interleukin-6 and macrophage inflammatory protein-2alpha in overventilated but not in endotoxic lungs. Similar results were obtained in rats in vivo, where Ly294002 prevented NF-kappaB activation by overventilation but not by endotoxin. These findings show that alveolar macrophages and alveolar epithelial type II cells contribute to the ventilation-induced release of proinflammatory mediators and that selective inhibition of this process is possible without inhibiting the activation of NF-kappaB by endotoxin.

Rho GTPases and phosphoinositide 3-kinase organize formation of branched dendrites

Neurons receive information from other neurons via their dendritic tree. Dendrites and their branches result from alternating outgrowth and retraction. The Rho GTPases Rac and Cdc42 (cell division cycle 42) facilitate the outgrowth of branches, whereas Rho attenuates it. The mechanism of neurite retraction is unknown. Because the adenylyl cyclase activator forskolin causes numerous branched extensions in NG108-15 cells, we have investigated the underlying mechanism in this cell line. In additional studies, we used cultured hippocampal neurons in which forskolin induces branched dendrites. In both cell types, forskolin enhanced the activity of Cdc42, but not that of Rac, although both GTPases were necessary for the formation of branched extensions. Time lapse microscopy showed that forskolin did not increase the rate of addition of new extensions or branches, but it reduced the rate of the retraction so that more branched extensions persisted. Inhibition of phosphoinositide 3-kinase activity by wortmannin or LY294002 also reduced the rate of retraction and thus facilitated dendritic arborization. Forskolin diminished the activity of phosphoinositide 3-kinases. Inhibitors of phosphoinositide 3-kinases not only reduced the retraction but also the addition of new dendrites and branches. This reduction was no longer present when Rho kinase was simultaneously inactivated, suggesting an interaction of phosphoinositide 3-kinases and Rho kinase. The present results show a central role of phosphoinositide 3-kinases in dendrite formation. In neuronal cells, increased levels of cAMP can support dendritic arborization by modulating the activity of the lipid kinase.

Role of the pleckstrin homology domain of PLCgamma1 in its interaction with the insulin receptor

A thiol-reactive membrane-associated protein (TRAP) binds covalently to the cytoplasmic domain of the human insulin receptor (IR) β-subunit when cells are treated with the homobifunctional cross-linker reagent 1,6-bismaleimidohexane. Here, TRAP was found to be phospholipase C γ1 (PLCγ1) by mass spectrometry analysis. PLCγ1 associated with the IR both in cultured cell lines and in a primary culture of rat hepatocytes. Insulin increased PLCγ1 tyrosine phosphorylation at Tyr-783 and its colocalization with the IR in punctated structures enriched in cortical actin at the dorsal plasma membrane. This association was found to be independent of PLCγ1 Src homology 2 domains, and instead required the pleckstrin homology (PH)–EF-hand domain. Expression of the PH–EF construct blocked endogenous PLCγ1 binding to the IR and inhibited insulin-dependent phosphorylation of mitogen-activated protein kinase (MAPK), but not AKT. Silencing PLCγ1 expression using small interfering RNA markedly reduced insulin-dependent MAPK regulation in HepG2 cells. Conversely, reconstitution of PLCγ1 in PLCγ1^−/− fibroblasts improved MAPK activation by insulin. Our results show that PLCγ1 is a thiol-reactive protein whose association with the IR could contribute to the activation of MAPK signaling by insulin.

Topographical expression of class IA and class II phosphoinositide 3-kinase enzymes in normal human tissues is consistent with a role in differentiation

Background

Growth factor, cytokine and chemokine-induced activation of PI3K enzymes constitutes the start of a complex signalling cascade, which ultimately mediates cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. The PI3K enzyme family is divided into 3 classes; class I (subdivided into IA and IB), class II (PI3K-C2α, PI3K-C2β and PI3K-C2γ) and class III PI3K. Expression of these enzymes in human tissue has not been clearly defined.

Methods

In this study, we analysed the immunohistochemical topographical expression profile of class IA (anti-p85 adaptor) and class II PI3K (PI3K-C2α and PI3K-C2β) enzymes in 104 formalin-fixed, paraffin embedded normal adult human (age 33–71 years, median 44 years) tissue specimens including those from the gastrointestinal, genitourinary, hepatobiliary, endocrine, integument and lymphoid systems. Antibody specificity was verified by Western blotting of cell lysates and peptide blocking studies. Immunohistochemistry intensity was scored from undetectable to strong.

Results

PI3K enzymes were expressed in selected cell populations of epithelial or mesenchymal origin. Columnar epithelium and transitional epithelia were reactive but mucous secreting and stratified squamous epithelia were not. Mesenchymal elements (smooth muscle and endothelial cells) and glomerular epithelium were only expressed PI3K-C2α while ganglion cells expressed p85 and PI3K-C2β. All three enzymes were detected in macrophages, which served as an internal positive control. None of the three PI3K isozymes was detected in the stem cell/progenitor compartments or in B lymphocyte aggregates.

Conclusions

Taken together, these data suggest that PI3K enzyme distribution is not ubiquitous but expressed selectively in fully differentiated, non-proliferating cells. Identification of the normal in vivo expression pattern of class IA and class II PI3K paves the way for further analyses which will clarify the role played by these enzymes in inflammatory, neoplastic and other human disease conditions.

SHIP-2 and PTEN are expressed and active in vascular smooth muscle cell nuclei, but only SHIP-2 is associated with nuclear speckles

Recently, the control of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3)-dependant signaling by phosphatases has emerged, but there is a shortage of information on intranuclear PtdIns(3,4,5)P3 phosphatases. Therefore, we investigated the dephosphorylation of [32P]PtdIns(3,4,5)P3 specifically labeled on the D-3 position of the inositol ring in membrane-free nuclei isolated from pig aorta vascular smooth muscle cells (VSMCs). In vitro PtdIns(3,4,5)P3 phosphatase assays revealed the production of both [32P]PtdIns(3,4)P2 and inorganic phosphate, demonstrating the presence of PtdIns(3,4,5)P3 5- and 3-phosphatase activities inside the VSMC nucleus, respectively. Both activities presented the same potency in cellular lysates, whereas the nuclear PtdIns(3,4,5)P3 5-phosphatase activity appeared to be the most efficient. Immunoblot experiments showed for the first time the expression of the 5-phosphatase SHIP-2 (src homology 2 domain-containing inositol phosphatase) as well as the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) in VSMC nuclei. In addition, immunoprecipitations from nuclear fractions indicated a [32P]PtdIns(3,4,5)P3 dephosphorylation by both SHIP-2 and PTEN. Moreover, confocal microscopy analyses demonstrated that SHIP-2 but not PTEN colocalized with a speckle-specific component, the SC35 splicing factor. These results suggest that SHIP-2 may be the primary enzyme for metabolizing PtdIns(3,4,5)P3 into PtdIns(3,4)P2 within the nucleus, thus producing another second messenger, whereas PTEN could down-regulate nuclear phosphoinositide 3-kinase signaling. Finally, intranuclear PtdIns(3,4,5)P3 phosphatases might be involved in the control of VSMC proliferation and the pathogenesis of vascular proliferative disorders.

Identification of mammalian Vps24p as an effector of phosphatidylinositol 3,5-bisphosphate-dependent endosome compartmentalization

Phosphatidylinositol 3,5-bisphosphate is a membrane lipid found in all eukaryotes so far studied but downstream effector proteins of this lipid have yet to be identified. Here we report the use of cDNA phage libraries in conjunction with synthetic biotinylated derivatives of phosphatidylinositol 3,5-bisphosphate in the identification of a mammalian phosphatidylinositol 3,5-bisphosphate-binding protein, mVps24p. This protein is orthologous to the Saccharomyces cerevisiae protein, Vps24p, a class-E vacuolar protein-sorting protein. Using in vitro liposome binding and competition assays, we demonstrate that mVps24p selectively binds to phosphatidylinositol 3,5-bisphosphate and phosphatidylinositol 3,4-bisphosphate in preference to other phosphoinositides tested. When expressed in cultured mammalian cells, full-length mVps24p is cytosolic. However, when cells expressing the full-length mVps24p are co-transfected with a mutated form of mVps4p (which is defective in ATP hydrolysis), or when a N-terminal construct of mVps24p is expressed, the class-E cellular phenotype with swollen vacuoles is induced and mVps24p is membrane-associated. Furthermore, the accumulation of the N-terminal mVps24p construct on the swollen endosomal membranes is abrogated when phosphatidylinositol 3,5-bisphosphate synthesis is blocked with wortmannin. These data provide the first direct link between phosphatidylinositol 3,5-bisphosphate and the protein machinery involved in the production of the class-E cellular phenotype. We hypothesize that accumulation of Vps24 on membranes occurs when membrane association (dependent on interaction of phosphatidylinositol 3,5-bisphosphate with the N-terminal domain of the protein) is uncoupled from membrane disassociation (driven by Vps4p).

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

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

Induction of cIAP-2 in human colon cancer cells through PKC delta/NF-kappa B

Activation of protein kinase C (PKC) prevents apoptosis in certain cells; however, the mechanisms are largely unknown. Inhibitors of apoptosis (IAP) family members, including NAIP, cIAP-1, cIAP-2, XIAP/hILP, survivin, and BRUCE, block apoptosis by binding and potently inhibiting caspases. Activation of NF-kappa B contributes to cIAP-2 induction; however, the cellular mechanisms regulating cIAP-2 expression have not been entirely defined. In this study, we examined the role of the PKC and NF-kappa B pathways in the regulation of cIAP-2 in human colon cancers. We found that cIAP-2 mRNA levels were markedly increased in human colon cancer cells by treatment with the phorbol ester, phorbol-12-myristate-13-acetate (PMA), or bryostatin 1. Inhibitors of the Ca2+-independent, novel PKC isoforms, but not inhibitors of MAPK, PI3-kinase, or PKA, blocked PMA-stimulated cIAP-2 mRNA expression, suggesting a role of PKC in PMA-mediated cIAP-2 induction. Pretreatment with the PKC delta-selective inhibitor rottlerin or transfection with an antisense PKC delta oligonucleotide inhibited PMA-induced cIAP-2 expression, whereas cotransfection with a PKC delta plasmid induced cIAP-2 promoter activity, which, taken together, identifies a role for PKC delta in cIAP-2 induction. Treatment with the proteasome inhibitor, MG132 or inhibitors of NF-kappa B (e.g. PDTC and gliotoxin), decreased PMA-induced up-regulation of cIAP-2. PMA-induced NF-kappa B activation was blocked by either GF109203x, MG132, PDTC, or gliotoxin. Moreover, overexpression of PKC delta-induced cIAP-2 promoter activity and increased NF-kappa B transactivation, suggesting regulation of cIAP-2 expression by a PKC delta/NF-kappa B pathway. In conclusion, our findings demonstrate a role for a PKC/NF-kappa B-dependent pathway in the regulation of cIAP-2 expression in human colon cancer cells. These data suggest a novel mechanism for the anti-apoptotic function mediated by the PKC delta/NF-kappa B/cIAP-2 pathway in certain cancers.

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3beta, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85alpha. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.

G-proteins as transducers in transmembrane signalling

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

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

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

Kinetic analysis of platelet-derived growth factor receptor/phosphoinositide 3-kinase/Akt signaling in fibroblasts

Isoforms of the serine-threonine kinase Akt coordinate multiple cell survival pathways in response to stimuli such as platelet-derived growth factor (PDGF). Activation of Akt is a multistep process, which relies on the production of 3'-phosphorylated phosphoinositide (PI) lipids by PI 3-kinases. To quantitatively assess the kinetics of PDGF receptor/PI 3-kinase/Akt signaling in fibroblasts, a systematic study of this pathway was performed, and a mechanistic mathematical model that describes its operation was formulated. We find that PDGF receptor phosphorylation exhibits positive cooperativity with respect to PDGF concentration, and its kinetics are quantitatively consistent with a mechanism in which receptor dimerization is initially mediated by the association of two 1:1 PDGF/PDGF receptor complexes. Receptor phosphorylation is transient at high concentrations of PDGF, consistent with the loss of activated receptors upon endocytosis. By comparison, Akt activation responds to lower PDGF concentrations and exhibits more sustained kinetics. Further analysis and modeling suggest that the pathway is saturated at the level of PI 3-kinase activation, and that the p110alpha catalytic subunit of PI 3-kinase contributes most to PDGF-stimulated 3'-PI production. Thus, at high concentrations of PDGF the kinetics of 3'-PI production are limited by the turnover rate of these lipids, while the Akt response is additionally influenced by the rate of Akt deactivation.

Deficiency of PDK1 in cardiac muscle results in heart failure and increased sensitivity to hypoxia

We employed Cre/loxP technology to generate mPDK1(-/-) mice, which lack PDK1 in cardiac muscle. Insulin did not activate PKB and S6K, nor did it stimulate 6-phosphofructo-2-kinase and production of fructose 2,6-bisphosphate, in the hearts of mPDK1(-/-) mice, consistent with PDK1 mediating these processes. All mPDK1(-/-) mice died suddenly between 5 and 11 weeks of age. The mPDK1(-/-) animals had thinner ventricular walls, enlarged atria and right ventricles. Moreover, mPDK1(-/-) muscle mass was markedly reduced due to a reduction in cardiomyocyte volume rather than cardiomyocyte cell number, and markers of heart failure were elevated. These results suggested mPDK1(-/-) mice died of heart failure, a conclusion supported by echocardiographic analysis. By employing a single-cell assay we found that cardiomyocytes from mPDK1(-/-) mice are markedly more sensitive to hypoxia. These results establish that the PDK1 signalling network plays an important role in regulating cardiac viability and preventing heart failure. They also suggest that a deficiency of the PDK1 pathway might contribute to development of cardiac disease in humans.

Regulation of endogenous SH2 domain-containing inositol 5-phosphatase (SHIP2) in 3T3-L1 and human preadipocytes

The role of the inositol lipid 5-phosphatase (SHIP2) in preadipocyte signaling is not known. Although overexpression of SHIP2 inhibited proliferation and (3)H-thymidine incorporation in 3T3-L1 preadipocytes, there was no effect on insulin-induced adipogenesis. Insulin promoted SHIP2 tyrosine phosphorylation in differentiated 3T3-L1 adipocytes, but did not do so in preadipocytes. The absence of SHIP2 tyrosine phosphorylation suggests a potential explanation for the isolated rise in PI(3,4,5)P3, without any changes in PI(3,4)P2, previously observed following insulin treatment of these cells. Lack of SHIP2 tyrosine phosphorylation by insulin was also observed in primary cultures of human abdominal subcutaneous preadipocytes. These cells also produced PI(3,4,5)P3, but not PI(3,4)P2, in response to insulin. Comparison of insulin vs. PDGF treatment on SHIP2 tyrosine phosphorylation in 3T3-L1 and human preadipocytes revealed that only PDGF, which stimulates the accumulation of PI(3,4,5)P3 as well as PI(3,4)P2, was active in this regard, and only PDGF promoted the association of 52 kDa form of Shc with SHIP2. Nevertheless, both insulin and PDGF were equally effective in translocating SHIP2 to the plasma membrane in 3T3-L1 preadipocytes. Lack of SHIP2 tyrosine phosphorylation may account for the insulin-specific inositol phospholipid pattern of accumulation in preadipocytes.

Novel pathways of F-actin polymerization in the human neutrophil

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

Evidence for a role of SNX16 in regulating traffic between the early and later endosomal compartments

Sorting nexins (SNXs) are a growing family of proteins characterized by the presence of a PX domain. The PX domain mediates membrane association by interaction with phosphoinositides. The SNXs are generally believed to participate in membrane trafficking, but information regarding the function of individual proteins is limited. In this report, we describe the major characteristics of one member, SNX16. SNX16 is a novel 343-amino acid protein consisting of a central PX domain followed by a potential coiled-coil domain and a C-terminal region. Like other sorting nexins, SNX16 associates with the membrane via the PX domain which interacts with the phospholipid phosphatidylinositol 3-phosphate. We show via biochemical and cellular studies that SNX16 is distributed in both early and late endosome/lysosome structures. The coiled-coil domain is necessary for localization to the later endosomal structures, as mutant SNX16 lacking this domain was found only in early endosomes. Trafficking of internalized epidermal growth factor was also delayed by this SNX16 mutant, as these cells showed a delay in the segregation of epidermal growth factor in the early endosome for its delivery to later compartments. In addition, the coiled-coil domain is shown here to be important for homo-oligomerization of SNX16. Taken together, these results suggest that SNX16 is a sorting nexin that may function in the trafficking of proteins between the early and late endosomal compartments.

Genetic analysis of the myotubularin family of phosphatases in Caenorhabditis elegans

Myotubularins (MTMs) constitute a large family of lipid phosphatases that specifically dephosphorylate phosphatidylinositol (3)P. MTM1 and MTM2 are mutated in X-linked myotubular myopathy and Charcot-Marie-Tooth disease (type 4B), respectively, although the mechanisms whereby MTM dysfunction leads to these diseases is unknown. To gain insight into MTM function, we undertook the study of MTMs in the nematode Caenorhabditis elegans, which possesses representative homologues of the four major subgroups of MTMs identified in mammals. As in mammals, we found that C. elegans MTMs mediate distinct functions. let-512 (vps34) encodes the C. elegans homologue of the yeast and mammalian homologue of the phosphatidylinositol 3-kinase Vps34. We found that reduction of mtm-6 (F53A2.8) function by RNA inhibition rescued the larval lethality of let-512 (vps34) mutants and that the reduction of mtm-1 (Y110A7A.5) activity by RNA inhibition rescued the endocytosis defect of let-512 animals. Together, these observations provide genetic evidence that MTMs negatively regulate phosphatidylinositol (3)P levels. Analysis of MTM expression patterns using transcriptional green fluorescence protein reporters demonstrated that these two MTMs exhibit mostly non-overlapping expression patterns and that MTM-green fluorescence protein fusion proteins are localized to different subcellular locations. These observations suggest that some of the different functions of MTMs might, in part, be a consequence of unique expression and localization patterns. However, our finding that at least three C. elegans MTMs play essential roles in coelomocyte endocytosis, a process that also requires VPS34, indicates that MTMs do not simply turn off VPS34 but unexpectedly also function as positive regulators of biological processes.

Oligomerization controls in tissue-specific manner ligand binding of native, affinity-purified p42IP4/centaurin α1 and cytohesins—proteins with high affinity for the messengers d-inositol 1,3,4,5-tetrakisphosphate/phosphatidylinositol 3,4,5-trisphosphate

Several distinct receptor proteins for the second messengers Ins(1,3,4,5)P(4) and PtdIns(3,4,5)P(3) are already known, such as the brain-specific p42(IP4), which we have previously cloned from different species, and cytohesins. However, it is still unclear whether proteins interacting with phosphoinositide and inositolpolyphosphate second messengers are regulated differently in different tissues. Here, we investigated these native proteins for comparison also from rat lung cytosol and purified them by PtdIns(3,4,5)P(3) affinity chromatography. Proteins selectively binding Ins(1,3,4,5)P(4) with high affinity also showed high affinity and specificity towards PtdIns(3,4,5)P(3). In lung cytosol, two prominent protein bands were found in the eluate from a PtdIns(3,4,5)P(3) affinity column. We identified these proteins by mass spectrometry as the cytohesin family of Arf guanosine nucleotide exchange factors (cytohesin 1, ARNO, GRP-1) and as Bruton's tyrosine kinase. Western blot analysis indicated that p42(IP4) was present in lung only at very low concentrations. Applying the affinity purification scheme established for rat lung cytosol to cytosol from rat brain, however, yielded only p42(IP4). We identified cytohesins in rat brain by Western blotting and PCR, but cytohesins surprisingly did not bind to the PtdIns(3,4,5)P(3)-affinity column. Gel filtration experiments of brain cytosol revealed that brain cytohesins are bound to large molecular weight complexes (150 to more than 500 kDa). Thus, we hypothesize that this finding explains why brain cytohesins apparently do not bind the inositolphosphate ligand. In lung cytosol, on the other hand, cytohesins occur as dimers. Gel filtration also showed that p42(IP4) in brain cytosol occurs as a monomer. Thus, oligomerization (homomeric or heteromeric) of InsP(4)/PtdInsP(3) binding proteins can modulate their function in a tissue-dependent manner because it can modify their ability to interact with the ligands.

Transformation of mouse fibroblasts by Jaagsiekte sheep retrovirus envelope does not require phosphatidylinositol 3-kinase

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma, a transmissible lung cancer of sheep. The envelope of JSRV may have oncogenic properties, since it can morphologically transform mouse NIH 3T3 cells and other fibroblast lines. Recently, we found that the cytoplasmic tail of the envelope transmembrane (TM) protein is necessary for transformation, and in particular a consensus binding motif (YXXM) for phosphatidylinositol 3-kinase (PI3K) is important. Moreover, JSRV-transformed cells show phosphorylation (activation) of Akt/protein kinase B, a downstream target of PI3K. In these studies, we directly tested for the involvement of PI3K in transformation by JSRV. Contrary to expectations, four different experiments indicated that PI3K is not necessary for JSRV-induced transformation: (i) cotransfection with a dominant negative truncated form of the PI3K regulatory subunit (Deltap85) did not affect transformation frequency, (ii) cells stably expressing Deltap85 showed the same frequencies of transformation as parental NIH 3T3 cells, (iii) fibroblasts established from double-knockout mice lacking PI3K p85alpha and p85beta could be transformed with JSRV envelope, and (iv) incubation of cells with the PI3K inhibitor LY294002 did not specifically inhibit transformation, nor did the drug reverse transformation of JSRV-transformed cells. One alternate explanation for the lack of transformation by YXXM mutants could be that they were defective in intracellular trafficking. However, confocal microscopy of epitope-tagged envelope proteins of both wild-type and nontransforming YXXM mutants showed a cell surface or plasma membrane localization. While PI3K is not required for JSRV-induced transformation of NIH 3T3 cells, the downstream target Akt kinase was found to be activated (phosphorylated) in JSRV-transformed PI3K-negative cells. Therefore, JSRV envelope can induce PI3K-independent phosphorylation of Akt.

Insulin-induced Drosophila S6 kinase activation requires phosphoinositide 3-kinase and protein kinase B

An important mechanism by which insulin regulates cell growth and protein synthesis is through activation of the p70 ribosomal S6 protein kinase (S6K). In mammalian cells, insulin-induced PI3K (phosphoinositide 3-kinase) activation, generates the lipid second messenger PtdIns(3,4,5) P (3), which is thought to play a key role in triggering the activation of S6K. Although the major components of the insulin-signalling pathway are conserved in Drosophila, recent studies suggested that S6K activation does not require PI3K in this system. To investigate further the role of dPI3K (Drosophila PI3K) in dS6K (Drosophila S6K) activation, we examined the effect of two structurally distinct PI3K inhibitors on insulin-induced dS6K activation in Kc167 and S2 Drosophila cell lines. We found that both inhibitors prevented insulin-stimulated phosphorylation and activation of dS6K. To investigate further the role of the dPI3K pathway in regulating dS6K activation, we also used dsRNAi (double-stranded RNA-mediated interference) to decrease expression of dPI3K and the PtdIns(3,4,5) P (3) phosphatase dPTEN ( Drosophila phosphatase and tensin homologue deleted on chromosome 10) in Kc167 and S2 cells. Knock-down of dPI3K prevented dS6K activation, whereas knock-down of dPTEN, which would be expected to increase PtdIns(3,4,5) P (3) levels, stimulated dS6K activity. Moreover, when the expression of the dPI3K target, dPKB (Drosophila protein kinase B), was decreased to undetectable levels, we found that insulin could no longer trigger dS6K activation. This observation provides the first direct demonstration that dPKB is required for insulin-stimulated dS6K activation. We also present evidence that the amino-acid-induced activation of dS6K in the absence of insulin, thought to be mediated by dTOR (Drosophila target of rapamycin), which is unaffected by the inhibition of dPI3K by wortmannin. The results of the present study support the view that, in Drosophila cells, dPI3K and dPKB, as well dTOR, are required for the activation of dS6K by insulin.

A new selective AKT pharmacological inhibitor reduces resistance to chemotherapeutic drugs, TRAIL, all-trans-retinoic acid, and ionizing radiation of human leukemia cells

It is now well established that the reduced capacity of tumor cells of undergoing cell death through apoptosis plays a key role both in the pathogenesis of cancer and in therapeutic treatment failure. Indeed, tumor cells frequently display multiple alterations in signal transduction pathways leading to either cell survival or apoptosis. In mammals, the pathway based on phosphoinositide 3-kinase (PI3K)/Akt conveys survival signals of extreme importance and its downregulation, by means of pharmacological inhibitors of PI3K, considerably lowers resistance to various types of therapy in solid tumors. We recently described an HL60 leukemia cell clone (HL60AR cells) with a constitutively active PI3K/Akt pathway. These cells were resistant to multiple chemotherapeutic drugs, all-trans-retinoic acid (ATRA), and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Treatment with two pharmacological inhibitors of PI3K, wortmannin and Ly294002, restored sensitivity of HL60AR cells to the aforementioned treatments. However, these inhibitors have some drawbacks that may severely limit or impede their clinical use. Here, we have tested whether or not a new selective Akt inhibitor, 1L-6-hydroxymethyl-chiro-inositol 2(R)-2-O-methyl-3-O-octadecylcarbonate (Akt inhibitor), was as effective as Ly294002 in lowering the sensitivity threshold of HL60 cells to chemotherapeutic drugs, TRAIL, ATRA, and ionizing radiation. Our findings demonstrate that, at a concentration which does not affect PI3K activity, the Akt inhibitor markedly reduced resistance of HL60AR cells to etoposide, cytarabine, TRAIL, ATRA, and ionizing radiation. This effect was likely achieved through downregulation of expression of antiapoptotic proteins such as c-IAP1, c-IAP2, cFLIP(L), and of Bad phosphorylation on Ser 136. The Akt inhibitor did not influence PTEN activity. At variance with Ly294002, the Akt inhibitor did not negatively affect phosphorylation of protein kinase C-zeta and it was less effective in downregulating p70S6 kinase (p70S6K) activity. The Akt inhibitor increased sensitivity to apoptotic inducers of K562 and U937, but not of MOLT-4, leukemia cells. Overall, our results indicate that selective Akt pharmacological inhibitors might be used in the future for enhancing the sensitivity of leukemia cells to therapeutic treatments that induce apoptosis or for overcoming resistance to these treatments.

AngII induces transient phospholipase D activity in the H295R glomerulosa cell model

In this report we demonstrate that in human adrenocortical carcinoma NCI H295R cells, a model for adrenal glomerulosa cells, PLD was activated both by AngII and protein kinase C (PKC)-activating phorbol 12-myristate 13-acetate (PMA). However, while PMA triggered sustained PLD activation, AngII induced transient PLD activation, in contrast to results in bovine glomerulosa cells in primary culture. Despite the transient effect of AngII on PLD activity, PLD-derived lipid signals were required for maximal AngII-elicited aldosterone secretion. AngII-induced PLD activation was inhibited by PKC inhibitors, but not by tyrosine kinase or calcium/calmodulin-dependent kinase inhibitors or a calmodulin antagonist. Both AngII- and PMA-stimulated PLD activity was enhanced by phosphoinositide 3-kinase (PI3K) inhibitors. Akt, a downstream protein kinase activated by the products of PI3K, was constitutively active in H295R cells, and this activity was blocked by PI3K inhibitors. These results suggested that in H295R adrenocortical carcinoma cells, AngII-induced PLD activation was promoted by PKC and inhibited by the constitutively active PI3K pathway.

Tuberculosis toxin blocking phagosome maturation inhibits a novel Ca2+/calmodulin-PI3K hVPS34 cascade

The capacity of Mycobacterium tuberculosis to infect latently over one billion people and cause two million fatalities annually rests with its ability to block phagosomal maturation into the phagolysosome in infected macrophages. Here we describe how M. tuberculosis toxin lipoarabinomannan (LAM) causes phagosome maturation arrest, interfering with a new pathway connecting intracellular signaling and membrane trafficking. LAM from virulent M. tuberculosis, but not from avirulent mycobacteria, blocked cytosolic Ca2+ increase. Ca2+ and calmodulin were required for a newly uncovered Ca2+/calmodulin phosphatidylinositol (PI)3 kinase hVPS34 cascade, essential for production of PI 3 phosphate (PI3P) on liposomes in vitro and on phagosomes in vivo. The interference of the trafficking toxin LAM with the calmodulin-dependent production of PI3P described here ensures long-term M. tuberculosis residence in vacuoles sequestered away from the bactericidal and antigen-processing organelles in infected macrophages.

Insulin induces phosphatidylinositol-3-phosphate formation through TC10 activation

Phosphatidylinositol-3-phosphate (PtdIns-3-P) is considered as a lipid constitutively present on endosomes; it does not seem to have a dynamic role in signalling. In contrast, phosphatidylinositol-3,4,5-trisphosphate (PtdIns-3,4,5-P(3)) plays a crucial role in different signalling pathways including translocation of the glucose transporter protein GLUT4 to the plasma membrane upon insulin receptor activation. GLUT4 translocation requires activation of two distinct pathways involving phosphatidylinositol 3-kinase (PI 3-K) and the small GTP-binding protein TC10, respectively. The contribution of each pathway remains to be elucidated. Here we show that insulin specifically induces the formation of PtdIns-3-P in insulin- responsive cells. The insulin-mediated formation of PtdIns-3-P occurs through the activation of TC10 at the lipid rafts subdomain of the plasma membrane. Exogenous PtdIns-3-P induces the plasma membrane translocation of both overexpressed and endogenous GLUT4. These data indicate that PtdIns-3-P is specifically produced downstream from insulin-mediated activation of TC10 to promote the plasma membrane translocation of GLUT4. These results give a new insight into the intracellular role of PtdIns-3-P and shed light on some aspects of insulin signalling so far not completely understood.

Phosphoinositide 3-kinase is required for process outgrowth and cell polarization of gastrulating mesendodermal cells

BACKGROUND

During vertebrate gastrulation, cell polarization and migration are core components in the cellular rearrangements that lead to the formation of the three germ layers, ectoderm, mesoderm, and endoderm. Previous studies have implicated the Wnt/planar cell polarity (PCP) signaling pathway in controlling cell morphology and movement during gastrulation. However, cell polarization and directed cell migration are reduced but not completely abolished in the absence of Wnt/PCP signals; this observation indicates that other signaling pathways must be involved.

RESULTS

We show that Phosphoinositide 3-Kinases (PI3Ks) are required at the onset of zebrafish gastrulation in mesendodermal cells for process formation and cell polarization. Platelet Derived Growth Factor (PDGF) functions upstream of PI3K, while Protein Kinase B (PKB), a downstream effector of PI3K activity, localizes to the leading edge of migrating mesendodermal cells. In the absence of PI3K activity, PKB localization and cell polarization are strongly reduced in mesendodermal cells and are followed by slower but still highly coordinated and directed movements of these cells.

CONCLUSIONS

We have identified a novel role of a signaling pathway comprised of PDGF, PI3K, and PKB in the control of morphogenetic cell movements during gastrulation. Furthermore, our findings provide insight into the relationship between cell polarization and directed cell migration at the onset of zebrafish gastrulation.

SHIP-2 forms a tetrameric complex with filamin, actin, and GPIb-IX-V: localization of SHIP-2 to the activated platelet actin cytoskeleton

The platelet receptor for the von Willebrand factor (VWF) glycoprotein Ib-IX-V (GPIb-IX-V) complex mediates platelet adhesion at sites of vascular injury. The cytoplasmic tail of the GPIbalpha subunit interacts with the actin-binding protein, filamin, anchoring the receptor in the cytoskeleton. In motile cells, the second messenger phosphatidylinositol 3,4,5 trisphosphate (PtdIns(3,4,5)P3) induces submembraneous actin remodeling. The inositol polyphosphate 5-phosphatase, Src homology 2 domain-containing inositol polyphosphate 5-phosphatase-2 (SHIP-2), hydrolyzes PtdIns(3,4,5)P3 forming phosphatidylinositol 3,4 bisphosphate (PtdIns(3,4)P2) and regulates membrane ruffling via complex formation with filamin. In this study we investigate the intracellular location and association of SHIP-2 with filamin, actin, and the GPIb-IX-V complex in platelets. Immunoprecipitation of SHIP-2 from the Triton-soluble fraction of unstimulated platelets demonstrated association between SHIP-2, filamin, actin, and GPIb-IX-V. SHIP-2 associated with filamin or GPIb-IX-V was active and demonstrated PtdIns(3,4,5)P3 5-phosphatase activity. Following thrombin or VWF-induced platelet activation, detection of the SHIP-2, filamin, and receptor complex decreased in the Triton-soluble fraction, although in control studies the level of SHIP-2, filamin, or GPIb-IX-V immunoprecipitated by their respective antibodies did not change following platelet activation. In activated platelets spreading on a VWF matrix, SHIP-2 localized intensely with actin at the central actin ring and colocalized with actin and filamin at filopodia and lamellipodia. In spread platelets, GPIb-IX-V localized to the center of the platelet and showed little colocalization with filamin at the plasma membrane. These studies demonstrate a functionally active complex between SHIP-2, filamin, actin, and GPIb-IX-V that may orchestrate the localized hydrolysis of PtdIns(3,4,5)P3 and thereby regulate cortical and submembraneous actin.

Molecular and physiologic actions of insulin related to production of nitric oxide in vascular endothelium

Insulin has important vascular actions that regulate blood flow, in addition to its classical actions to coordinate glucose homeostasis. Insulin-stimulated production of nitric oxide in vascular endothelium results in capillary recruitment and vasodilation that diverts and increases blood flow to skeletal muscle and consequently increases glucose disposal. Thus, vascular actions of insulin may be essential for coupling hemodynamic and metabolic homeostasis. A complete biochemical signaling pathway linking the insulin receptor to activation of endothelial nitric oxide synthase in vascular endothelium has recently been elucidated. Moreover, the time course and dose response for capillary recruitment in response to physiologic concentrations of insulin parallels that of insulin-mediated glucose uptake in vivo. Taken together, these observations suggest a molecular mechanism that may help to explain how insulin resistance contributes to cardiovascular components of the metabolic syndrome and vascular complications of diabetes.

SH2 and PTB domains in tyrosine kinase signaling

Intracellular signaling pathways that involve protein tyrosine kinases (PTKs) are critical for the control of most cellular processes. Dysfunctions in PTKs, or in the signaling pathways that they regulate, result in a variety of diseases such as cancer, diabetes, immune deficiency, and many others. SH2 (Src homology region 2) and PTB (phosphotyrosine-binding) domains are small protein modules that mediate protein-protein interactions involved in many signal transduction pathways. Both domains were initially identified as modules that recognize phosphorylated tyrosines in receptor tyrosine kinases and other signaling proteins. Subsequent studies have shown that, while binding of SH2 domains to their target proteins is strictly regulated by tyrosine phosphorylation, most PTB domains actually bind to their (nonphosphorylated) targets constitutively. The functions of SH2 and PTB domains include targeting of their host proteins to different cellular compartments, assembly of key components of signaling pathways in response to extracellular signals, and the control of autoinhibition, activation and dimerization of their host proteins. The information flow from the cell surface to different cellular compartments to regulate the cell cycle, cell shape and movement, cell proliferation, differentiation and cell survival are all controlled in part by SH2 and PTB domains that can recognize phosphotyrosine or particular amino acid sequence motifs in a wide variety of target molecules.

Bcl-xL mediates a survival mechanism independent of the phosphoinositide 3-kinase/Akt pathway in prostate cancer cells

Among various molecular strategies by which prostate cancer cells evade apoptosis, phosphoinositide 3-kinase (PI3K)/Akt signaling represents a dominant survival pathway. However, different prostate cancer cell lines such as LNCaP and PC-3 display differential sensitivity to the apoptotic effect of PI3K inhibition in serum-free media, reflecting the heterogeneous nature of prostate cancer in apoptosis regulation. Whereas both cell lines are equally susceptible to LY294002-mediated Akt dephosphorylation, only LNCaP cells default to apoptosis, as evidenced by DNA fragmentation and cytochrome c release. In PC-3 cells, Akt deactivation does not lead to cytochrome c release, suggesting that the intermediary signaling pathway is short-circuited by an antiapoptotic factor. This study presents evidence that Bcl-xL overexpression provides a distinct survival mechanism that protects PC-3 cells from apoptotic signals emanating from PI3K inhibition. First, the Bcl-xL/BAD ratio in PC-3 cells is at least an order of magnitude greater than that of LNCaP cells. Second, ectopic expression of Bcl-xL protects LNCaP cells against LY294002-induced apoptosis. Third, antisense down-regulation of Bcl-xL sensitizes PC-3 cells to the apoptotic effect of LY294002. The physiological relevance of this Bcl-xL-mediated survival mechanism is further underscored by the protective effect of serum on LY294002-induced cell death in LNCaP cells, which is correlated with a multifold increase in Bcl-xL expression. In contrast to Bcl-xL, Bcl-2 expression levels are similar in both cells lines, and do not respond to serum stimulation, suggesting that Bcl-2 may not play a physiological role in antagonizing apoptosis signals pertinent to BAD activation in prostate cancer cells.

Class I phosphoinositide 3-kinases

Phosphoinositide 3-kinases (PI3-kinases) are a family of enzymes that 3'-phosphorylate the inositol head group of membrane phosphoinositides. They are subdivided into three major classes (I, II and III) based on their structural homology, regulation and substrate specificity. It is now becoming clear that PI3-kinase isoforms are subject to differential regulation and may play distinct roles within the cell. PI3-kinases and their second messenger lipid products have been implicated in a plethora of cellular responses with increasing evidence for involvement in the pathogenesis of human diseases. The future development of specific PI3-kinase isoform inhibitors may offer therapeutic benefit in a broad range of clinical settings, related to cancer, inflammatory and immunological diseases.

Threonine 308 phosphorylated form of Akt translocates to the nucleus of PC12 cells under nerve growth factor stimulation and associates with the nuclear matrix protein nucleolin

We have examined the issue of whether or not in PC12 cells it may be observed a nerve growth factor (NGF) nuclear translocation of an active (phosphorylated) Akt. Western blot analysis with antibodies to either total or phosphorylated Akt showed a maximal nuclear translocation after 15 min of NGF stimulation. NGF increased rapidly and transiently the enzymatic activity of immunoprecipitable nuclear Akt and after 45 min the values returned to a level close to the basal one. Enzyme translocation was blocked by the selective phosphoinositide 3-kinase inhibitor, LY294002. Confocal microscopy of samples stained with antibody to Akt showed an evident increase in immunostaining intensity in the nuclear interior after NGF treatment. Treatment of cells with inhibitors of protein phosphatase PP2A, calyculin A, or okadaic acid, maintained the phosphorylation levels of nuclear Akt. Immunoprecipitation experiments revealed an association between Akt and PP2A that was maximal when nuclear Akt activity was decreased. Both total and active Akt associated with the nuclear matrix and, in particular, with the protein nucleolin, with which Akt co-immunoprecipitated. These findings strongly suggest that the intranuclear translocation of active Akt is an important step in the signaling pathways elicited by the neurotrophin NGF and that the intranuclear control of Akt is achieved through the action of PP2A.

Phosphoinositide binding inhibits alpha-actinin bundling activity

alpha-Actinin is an abundant actin-bundling and adhesion protein that directly links actin filaments to integrin receptors. Previously, in platelet-derived growth factor-treated fibroblasts, we demonstrated that phosphoinositides bind to alpha-actinin, regulating its localization (Greenwood, J. A., Theibert, A. B., Prestwich, G. D., and Murphy-Ullrich, J. E. (2000) J. Cell Biol. 150, 627- 642). In this study, phosphoinositide binding and regulation of alpha-actinin function is further characterized. Phosphoinositide binding specificity, determined using a protein-lipid overlay procedure, suggests that alpha-actinin interacts with phosphates on the 4th and 5th position of the inositol head group. Binding assays and mutational analyses demonstrate that phosphoinositides bind to the calponin homology domain 2 of alpha-actinin. Phosphoinositide binding inhibited the bundling activity of alpha-actinin by blocking the interaction of the actin-binding domain with actin filaments. Consistent with these results, excessive bundling of actin filaments was observed in fibroblasts expressing an alpha-actinin mutant with decreased phosphoinositide affinity. We conclude that the interaction of alpha-actinin with phosphoinositides regulates actin stress fibers in the cell by controlling the extent to which microfilaments are bundled.

Protein tyrosine phosphatase RQ is a phosphatidylinositol phosphatase that can regulate cell survival and proliferation

Protein tyrosine phosphatase RQ (PTPRQ) was initially identified as a protein tyrosine phosphatase (PTPase)-like protein that is upregulated in a model of renal injury. Here we present evidence that, like PTEN, the biologically important enzymatic activity of PTPRQ is as a phosphatidylinositol phosphatase (PIPase). The PIPase specificity of PTPRQ is broader than that of PTEN and depends on different amino acid residues in the catalytic domain. In vitro, the recombinant catalytic domain of PTPRQ has low PTPase activity against tyrosine-phosphorylated peptide and protein substrates but can dephosphorylate a broad range of phosphatidylinositol phosphates, including phosphatidylinositol 3,4,5-trisphosphate and most phosphatidylinositol monophosphates and diphosphates. Phosphate can be hydrolyzed from the D3 and D5 positions in the inositol ring. PTPRQ does not have either of the basic amino acids in the catalytic domain that are important for the PIPase activity of PTEN or the sequence motifs that are characteristic of type II phosphatidylinositol 5-phosphatases. Instead, the PIPase activity depends on the WPE sequence present in the catalytic cleft of PTPRQ, and in the "inactive" D2 domains of many dual-domain PTPases, in place of the WPD motif present in standard active PTPases. Overexpression of PTPRQ in cultured cells inhibits proliferation and induces apoptosis. An E2171D mutation that retains or increases PTPase activity but eliminates PIPase activity, eliminates the inhibitory effects on proliferation and apoptosis. These results indicate that PTPRQ represents a subtype of the PTPases whose biological activities result from its PIPase activity rather than its PTPase activity.

Specific interaction of protein tyrosine phosphatase-MEG2 with phosphatidylserine

Protein tyrosine phosphatase (PTP)-MEG2 is an intracellular tyrosine phosphatase that contains a Sec14 homology domain. We have purified the full-length and truncated forms of the enzyme from recombinant adenovirus-infected human 293 cells. By using lipid-membrane overlay and liposome binding assays, we demonstrated that PTP-MEG2 specifically binds phosphatidylserine among over 20 lipid compounds tested. The binding is mediated by its N-terminal Sec14 domain. In intact cells, the Sec14 domain is responsible for localization of PTP-MEG2 to the perinuclear region, and uploading of PS into the cell membrane causes translocation of PTP-MEG2 to the plasma membrane. Phosphatidylserine is a relatively abundant cell membrane phospholipid non-symmetrically distributed in the outer layer and inner layer of cell membranes. It has recently been defined as an important ligand for clearance of apoptotic cells. By specifically binding phosphatidylserine, PTP-MEG2 may play an important role in regulating signaling processes associated with phagocytosis of apoptotic cells.