Intracellular movement of green fluorescent protein-tagged phosphatidylinositol 3-kinase in response to growth factor receptor signaling

Non-catalytic role of phosphoinositide 3-kinase in mesenchymal cell migration through non-canonical induction of p85β/AP2-mediated endocytosis

Class IA phosphoinositide 3-kinase (PI3K) galvanizes fundamental cellular processes such as migration, proliferation, and differentiation. To enable these multifaceted roles, the catalytic subunit p110 utilizes the multi-domain, regulatory subunit p85 through its inter SH2 domain (iSH2). In cell migration, its product PI(3,4,5)P3 generates locomotive activity. While non-catalytic roles are also implicated, underlying mechanisms and their relationship to PI(3,4,5)P3 signaling remain elusive. Here, we report that a disordered region of iSH2 contains AP2 binding motifs which can trigger clathrin and dynamin-mediated endocytosis independent of PI3K catalytic activity. The AP2 binding motif mutants of p85 aberrantly accumulate at focal adhesions and increase both velocity and persistency in fibroblast migration. We thus propose the dual functionality of PI3K in the control of cell motility, catalytic and non-catalytic, arising distinctly from juxtaposed regions within iSH2.

Non-catalytic role of phosphoinositide 3-kinase in mesenchymal cell migration through non-canonical induction of p85β/AP-2-mediated endocytosis

Class IA phosphoinositide 3-kinase (PI3K) galvanizes fundamental cellular processes such as migration, proliferation, and differentiation. To enable multifaceted roles, the catalytic subunit p110 utilizes a multi-domain, regulatory subunit p85 through its inter SH2 domain (iSH2). In cell migration, their product PI(3,4,5)P3 generates locomotive activity. While non-catalytic roles are also implicated, underlying mechanisms and its relationship to PI(3,4,5)P3 signaling remain elusive. Here, we report that a disordered region of iSH2 contains previously uncharacterized AP-2 binding motifs which can trigger clathrin and dynamin-mediated endocytosis independent of PI3K catalytic activity. The AP-2 binding motif mutants of p85 aberrantly accumulate at focal adhesions and upregulate both velocity and persistency in fibroblast migration. We thus propose the dual functionality of PI3K in the control of cell motility, catalytic and non-catalytic, arising distinctly from juxtaposed regions within iSH2.

Non-catalytic role of phosphoinositide 3-kinase in mesenchymal cell migration through non-canonical induction of p85β/AP-2-mediated endocytosis

Class IA phosphoinositide 3-kinase (PI3K) galvanizes fundamental cellular processes such as migration, proliferation, and differentiation. To enable multifaceted roles, the catalytic subunit p110 utilizes a multidomain, regulatory subunit p85 through its inter SH2 domain (iSH2). In cell migration, their product PI(3,4,5)P3 generates locomotive activity. While non-catalytic roles are also implicated, underlying mechanisms and its relationship to PI(3,4,5)P3 signaling remain elusive. Here, we report that a disordered region of iSH2 contains previously uncharacterized AP-2 binding motifs which can trigger clathrin and dynamin-mediated endocytosis independent of PI3K catalytic activity. The AP-2 binding motif mutants of p85 aberrantly accumulate at focal adhesions and upregulate both velocity and persistency in fibroblast migration. We thus propose the dual functionality of PI3K in the control of cell motility, catalytic and non-catalytic, arising distinctly from juxtaposed regions within iSH2.

MYO6 Regulates Spatial Organization of Signaling Endosomes Driving AKT Activation and Actin Dynamics

APPL1- and RAB5-positive signaling endosomes play a crucial role in the activation of AKT in response to extracellular stimuli. Myosin VI (MYO6) and two of its cargo adaptor proteins, GIPC and TOM1/TOM1L2, localize to these peripheral endosomes and mediate endosome association with cortical actin filaments. Loss of MYO6 leads to the displacement of these endosomes from the cell cortex and accumulation in the perinuclear space. Depletion of this myosin not only affects endosome positioning, but also induces actin and lipid remodeling consistent with endosome maturation, including accumulation of F-actin and the endosomal lipid PI(3)P. These processes acutely perturb endosome function, as both AKT phosphorylation and RAC-dependent membrane ruffling were markedly reduced by depletion of either APPL1 or MYO6. These results place MYO6 and its binding partners at a central nexus in cellular signaling linking actin dynamics at the cell surface and endosomal signaling in the cell cortex.

Maximizing the Efficacy of MAPK-Targeted Treatment in PTENLOF/BRAFMUT Melanoma through PI3K and IGF1R Inhibition

The introduction of MAPK pathway inhibitors paved the road for significant advancements in the treatment of BRAF-mutant (BRAF(MUT)) melanoma. However, even BRAF/MEK inhibitor combination therapy has failed to offer a curative treatment option, most likely because these pathways constitute a codependent signaling network. Concomitant PTEN loss of function (PTEN(LOF)) occurs in approximately 40% of BRAF(MUT) melanomas. In this study, we sought to identify the nodes of the PTEN/PI3K pathway that would be amenable to combined therapy with MAPK pathway inhibitors for the treatment of PTEN(LOF)/BRAF(MUT) melanoma. Large-scale compound sensitivity profiling revealed that PTEN(LOF) melanoma cell lines were sensitive to PI3Kβ inhibitors, albeit only partially. An unbiased shRNA screen (7,500 genes and 20 shRNAs/genes) across 11 cell lines in the presence of a PI3Kβ inhibitor identified an adaptive response involving the IGF1R-PI3Kα axis. Combined inhibition of the MAPK pathway, PI3Kβ, and PI3Kα or insulin-like growth factor receptor 1 (IGF1R) synergistically sustained pathway blockade, induced apoptosis, and inhibited tumor growth in PTEN(LOF)/BRAF(MUT) melanoma models. Notably, combined treatment with the IGF1R inhibitor, but not the PI3Kα inhibitor, failed to elevate glucose or insulin signaling. Taken together, our findings provide a strong rationale for testing combinations of panPI3K, PI3Kβ + IGF1R, and MAPK pathway inhibitors in PTEN(LOF)/BRAF(MUT) melanoma patients to achieve maximal response.

Epidermal growth factor-stimulated Akt phosphorylation requires clathrin or ErbB2 but not receptor endocytosis

Upon ligand binding, the epidermal growth factor receptor (EGFR) activates signaling and undergoes endocytosis. EGFR signaling leading to Akt activation is impaired by perturbation of clathrin but not by inhibition of internalization through perturbation of dynamin. Clathrin may thus directly regulate receptor signaling at the cell surface.

Epidermal growth factor (EGF) binding to its receptor (EGFR) activates several signaling intermediates, including Akt, leading to control of cell survival and metabolism. Concomitantly, ligand-bound EGFR is incorporated into clathrin-coated pits—membrane structures containing clathrin and other proteins—eventually leading to receptor internalization. Whether clathrin might regulate EGFR signaling at the plasma membrane before vesicle scission is poorly understood. We compared the effect of clathrin perturbation (preventing formation of, or receptor recruitment to, clathrin structures) to that of dynamin2 (allowing formation of clathrin structures but preventing EGFR internalization) under conditions in which EGFR endocytosis is clathrin dependent. Clathrin perturbation by siRNA gene silencing, with the clathrin inhibitor pitstop2, or knocksideways silencing inhibited EGF-simulated Gab1 and Akt phosphorylation in ARPE-19 cells. In contrast, perturbation of dynamin2 with inhibitors or by siRNA gene silencing did not affect EGF-stimulated Gab1 or Akt phosphorylation. EGF stimulation enriched Gab1 and phospho-Gab1 within clathrin structures. ARPE-19 cells have low ErbB2 expression, and overexpression and knockdown experiments revealed that robust ErbB2 expression bypassed the requirement for clathrin for EGF-stimulated Akt phosphorylation. Thus clathrin scaffolds may represent unique plasma membrane signaling microdomains required for signaling by certain receptors, a function that can be separated from vesicle formation.

Deconstructing breast cancer cell biology and the mechanisms of multidrug resistance

Cancer complexity constantly challenges the way that clinicians manage breast cancer therapy. Tumor heterogeneity and intratumoral stroma characteristics allow cells with different phenotypes and deregulated apoptotic, proliferative and migration abilities to co-exist contributing to a disappointing therapeutic response. While new approaches are being associated with conventional chemotherapy, such as hormonal therapy or target monoclonal antibodies, recurrence and metastasization are still observed. Membrane transporters are the cell's first line of contact with anticancer drugs having a major role in multidrug resistance events. This structural-based activity enables the cell to be drug-resistant by decreasing drug intracellular concentration through an efflux-transport mechanism, mainly associated with overexpression of ATP-binding cassette (ABC) proteins. This review focuses on some of the important structural and biological properties of the malignant cell and tumor microenvironment, addressing the role of the membrane ABC transporters in therapeutic outcomes, and highlighting related molecular pathways that may represent meaningful target therapies.

Live-cell fluorescence imaging reveals high stoichiometry of Grb2 binding to the EGF receptor sustained during endocytosis

Activation of epidermal growth factor (EGF) receptor (EGFR) leads to its interaction with Grb2, a dual-function adapter mediating both signaling through Ras and receptor endocytosis. We used time-lapse three-dimensional imaging by spinning disk confocal microscopy to analyze trafficking of EGFR and Grb2 in living HeLa cells stimulated with low, physiological concentrations of EGFR ligands. Endogenous Grb2 was replaced in these cells by Grb2 fused to yellow fluorescent protein (YFP). After transient residence in the plasma membrane, Rhodamine-conjugated EGF (EGF-Rh) and Grb2-YFP were rapidly internalized and accumulated in endosomes. Quantitative image analysis revealed that on average two Grb2-YFP molecules were colocalized with one EGF-Rh in cells stimulated with 2 ng/ml EGF-Rh, and the excess of Grb2-YFP over EGF-Rh was even higher when a receptor-saturating concentration of EGF-Rh was used. Therefore, we hypothesize that a single EGFR molecule can be simultaneously associated with functionally distinct Grb2 interaction partners during and after endocytosis. Continuous presence of Grb2-YFP in endosomes was also observed when EGFR was activated by transforming growth factor-α and amphiregulin, suggesting that endosomal EGFRs remain ligand occupied and signaling competent, despite the fact that these growth factors are thought to dissociate from the receptor at acidic pH. The prolonged localization and activity of EGFR-Grb2 complexes in endosomes correlated with the sustained activation of extracellular stimulus-regulated kinase 1/2, suggesting that endosomal EGFRs contribute significantly to this signaling pathway. We propose that endosomal EGFRs function to extend signaling in time and space to compensate for rapid downregulation of surface EGFRs in cells with low receptor expression levels.

Does the PI3K pathway play a role in basal breast cancer?

Many cell kinases exert their proliferative and pro-survival effects through activation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway. Basal-like breast cancer is a subtype that can be identified by molecular analysis and often includes tumors lacking expression of estrogen receptor/progesterone receptor or human epidermal growth factor receptor, also known as triple-negative breast cancers. Triple-negative cancers comprise < 20% of all breast cancers and have no obvious mechanism driving proliferation, yet these tumors demonstrate higher levels of Akt activation compared with non-triple-negative breast cancers. This suggests a possible role for targeting the PI3K pathway for the treatment of this subset of aggressive cancers. Most clinical trials which have attempted targeting the PI3K/Akt pathway in triple-negative breast cancer have involved the use of EGFR inhibitors with limited success. Novel agents targeting PI3K are under development in early-phase clinical trials and may demonstrate benefit in combination with chemotherapy or other targeted agents such as mitogen-activated protein kinase inhibitors for the treatment of triple-negative or basal-like breast cancer.

Mal connects TLR2 to PI3Kinase activation and phagocyte polarization

The recognition of bacterial lipoproteins by toll-like receptor (TLR) 2 is pivotal for inflammation initiation and control in many bacterial infections. TLR2-dependent signalling is currently believed to essentially require both adaptor proteins MyD88 (myeloid differentiation primary response gene 88) and Mal/TIRAP (MyD88-adapter-like/TIR-domain-containing adaptor protein). TLR2-dependent, but MyD88-independent responses have not been described yet. We report here on a novel-signalling pathway downstream of TLR2, which does not adhere to the established model. On stimulation of the TLR2/6 heterodimer with diacylated bacterial lipoproteins, Mal directly interacts with the regulatory subunit of phosphoinositide 3-kinase (PI3K), p85alpha, in an inducible fashion. The Mal-p85alpha interaction drives PI3K-dependent phosphorylation of Akt, phosphatidylinositol(3,4,5)P3 (PIP(3)) generation and macrophage polarization. MyD88 is not essential for PI3K activation and Akt phosphorylation; however, cooperates with Mal for PIP(3) formation and accumulation at the leading edge. In contrast to TLR2/6, TLR2/1 does not require Mal or MyD88 for Akt phosphorylation. Hence, Mal specifically connects TLR2/6 to PI3K activation, PIP(3) generation and macrophage polarization.

Signal amplification between Gbetagamma release and PI3Kgamma-mediated PI(3,4,5)P3 formation monitored by a fluorescent Gbetagamma biosensor protein and repetitive two component total internal reflection/fluorescence redistribution after photobleaching analysis

Phosphoinositide 3-kinase gamma (PI3Kgamma) is activated by Gbetagamma release after stimulation of Galpha i -coupled receptors, involving a recruitment of the enzyme to the plasma membrane via interaction of the regulatory subunit p101 or p87 with Gbetagamma. The receptor-mediated release of Gbetagamma was, however, insufficient to elicit a translocation of p101 observable by classical fluorescence microscopy approaches. Since the mobilities of plasma membrane-associated and cytosolic proteins differ strongly, small changes in the amount of plasma membrane association should be detectable by an altered diffusional behavior. Here, changes in mobility were monitored by fluorescence redistribution after photobleaching (FRAP) which was repetitively applied before and after stimulation of cells. To combine the advantages of total internal reflection (TIR) illumination, which preferentially excites fluorophors located at or near the plasma membrane, with that provided by the mobility information, we developed a combined TIR/FRAP setup which enabled us to point bleach parts of an image that was observed under TIR illumination. For FRAP data analysis, we introduce a convolution-based method and a global two component model. Using this TIR/FRAP approach, an increased plasma membrane association of the fluorescent Gbetagamma-binding domain of p101 after Gbetagamma release by G protein-coupled receptor stimulation could be detected and quantified. By comparing the translocation efficiency of this domain with that of YFP-GRP1(PH), a biosensor for the PI3Kgamma product PI(3,4,5)P3, we evaluate the signal amplification between Gbetagamma release and PI(3,4,5)P3 formation after activation of Galpha i -coupled receptors.

Escherichia coli DraE adhesin-associated bacterial internalization by epithelial cells is promoted independently by decay-accelerating factor and carcinoembryonic antigen-related cell adhesion molecule binding and does not require the DraD invasin

The Dr family of Escherichia coli adhesins are virulence factors associated with diarrhea and urinary tract infections. Dr fimbriae are comprised of two subunits. DraE/AfaE represents the major structural, antigenic, and adhesive subunit, which recognizes decay-accelerating factor (DAF) and carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) CEA, CEACAM1, CEACAM3, and CEACAM6 as binding receptors. The DraD/AfaD subunit caps fimbriae and has been implicated in the entry of Dr-fimbriated E. coli into host cells. In this study, we demonstrate that DAF or CEACAM receptors independently promote DraE-mediated internalization of E. coli by CHO cell transfectants expressing these receptors. We also found that DraE-positive recombinant bacteria adhere to and are internalized by primary human bladder epithelial cells which express DAF and CEACAMs. DraE-mediated bacterial internalization by bladder cells was inhibited by agents which disrupt lipid rafts, microtubules, and phosphatidylinositol 3-kinase (PI3K) activity. Immunofluorescence confocal microscopic examination of epithelial cells detected considerable recruitment of caveolin, beta(1) integrin, phosphorylated ezrin, phosphorylated PI3K, and tubulin, but not F-actin, by cell-associated bacteria. Finally, we demonstrate that the DraD subunit, previously implicated as an "invasin," is not required for beta(1) integrin recruitment or bacterial internalization.

Galphaq binds two effectors separately in cells: evidence for predetermined signaling pathways

G-proteins transduce signals along diverse pathways, but the factors involved in pathway selection are largely unknown. Here, we have studied the ability of Galpha(q) to select between two effectors-mammalian inositide-specific phospholipase Cbeta (PLCbeta) and phosphoinositide-3-kinase (PI3K)-in human embryonic kidney 293 cells. These studies were carried out by measuring interactions between eCFP- and eYFP-tagged proteins using Forster resonance energy transfer in the basal state and during stimulation. Instead of association of Galpha(q) with effectors through diffusion and exchange, we found separate and stable pools of Galpha(q)-PLCbeta and Galpha(q)-PI3K complexes existing throughout the stimulation cycle. These separate complexes existed despite the ability of Galpha(q) to simultaneously bind both effectors as determined by in vitro measurements using purified proteins. Preformed G-protein/effector complexes will limit the number of pathways that a given signal will take, which may simplify predictive models.

Phosphorylation regulates tau interactions with Src homology 3 domains of phosphatidylinositol 3-kinase, phospholipase Cgamma1, Grb2, and Src family kinases

The microtubule-associated protein tau can associate with various other proteins in addition to tubulin, including the SH3 domains of Src family tyrosine kinases. Tau is well known to aggregate to form hyperphosphorylated filamentous deposits in several neurodegenerative diseases (tauopathies) including Alzheimer disease. We now report that tau can bind to SH3 domains derived from the p85alpha subunit of phosphatidylinositol 3-kinase, phospholipase Cgamma1, and the N-terminal (but not the C-terminal) SH3 of Grb2 as well as to the kinases Fyn, cSrc, and Fgr. However, the short inserts found in neuron-specific isoforms of Src prevented the binding of tau. The experimentally determined binding of tau peptides is well accounted for when modeled into the peptide binding cleft in the SH3 domain of Fyn. After phosphorylation in vitro or in transfected cells, tau showed reduced binding to SH3 domains; no binding was detected with hyperphosphorylated tau isolated from Alzheimer brain, but SH3 binding was restored by phosphatase treatment. Tau mutants with serines and threonines replaced by glutamate, to mimic phosphorylation, showed reduced SH3 binding. These results strongly suggest that tau has a potential role in cell signaling in addition to its accepted role in cytoskeletal assembly, with regulation by phosphorylation that may be disrupted in the tauopathies including Alzheimer disease.

Mapping ErbB receptors on breast cancer cell membranes during signal transduction

Distributions of ErbB receptors on membranes of SKBR3 breast cancer cells were mapped by immunoelectron microscopy. The most abundant receptor, ErbB2, is phosphorylated, clustered and active. Kinase inhibitors ablate ErbB2 phosphorylation without dispersing clusters. Modest co-clustering of ErbB2 and EGFR, even after EGF treatment, suggests that both are predominantly involved in homointeractions. Heregulin leads to dramatic clusters of ErbB3 that contain some ErbB2 and EGFR and abundant PI 3-kinase. Other docking proteins, such as Shc and STAT5, respond differently to receptor activation. Levels of Shc at the membrane increase two- to five-fold with EGF, whereas pre-associated STAT5 becomes strongly phosphorylated. These data suggest that the distinct topography of receptors and their docking partners modulates signaling activities.

Differential association of phosphatidylinositol 3-kinase, SHIP-1, and PTEN with forming phagosomes

In macrophages, enzymes that synthesize or hydrolyze phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P(3)] regulate Fcgamma receptor-mediated phagocytosis. Inhibition of phosphatidylinositol 3-kinase (PI3K) or overexpression of the lipid phosphatases phosphatase and tensin homologue (PTEN) and Src homology 2 domain-containing inositol phosphatase (SHIP-1), which hydrolyze PI(3,4,5)P(3) to phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4-bisphosphate [PI(3,4)P(2)], respectively, inhibit phagocytosis in macrophages. To examine how these enzymes regulate phagosome formation, the distributions of yellow fluorescent protein (YFP) chimeras of enzymes and pleckstrin homology (PH) domains specific for their substrates and products were analyzed quantitatively. PTEN-YFP did not localize to phagosomes, suggesting that PTEN regulates phagocytosis globally within the macrophage. SHIP1-YFP and p85-YFP were recruited to forming phagosomes. SHIP1-YFP sequestered to the leading edge and dissociated from phagocytic cups earlier than did p85-cyan fluorescent protein, indicating that SHIP-1 inhibitory activities are restricted to the early stages of phagocytosis. PH domain chimeras indicated that early during phagocytosis, PI(3,4,5)P(3) was slightly more abundant than PI(3,4)P(2) at the leading edge of the forming cup. These results support a model in which phagosomal PI3K generates PI(3,4,5)P(3) necessary for later stages of phagocytosis, PTEN determines whether those late stages can occur, and SHIP-1 regulates when and where they occur by transiently suppressing PI(3,4,5)P(3)-dependent activities necessary for completion of phagocytosis.

Insights into the role of heparan sulphate in fibroblast growth factor signalling

Signalling from the FGFs (fibroblast growth factors) is crucial for the correct development and homoeostasis of a wide range of cells and tissues. The FGF/FGFR (FGF receptor) signalling system forms an important paradigm for HS (heparan sulphate)-binding proteins, as both the growth factor and receptor bind to HS, and HS or heparin is an absolute requirement for full signalling. The FGF signalling system has been extremely well structurally characterized, and details of each interaction involved in forming a ternary complex of FGF-FGFR-heparin have been elucidated. Recent work has focused on a more thorough understanding of the nature of the FGF-heparin complex in particular, demonstrating that FGFs preferentially bind to similar sites on the co-receptor, and that FGF-FGFR pairs show greater specificity for heparin sulphation patterns than individual FGFs. Further work has suggested that FGF-FGFR-heparin signalling complexes contain one molecule of heparin only, and that when longer fragments of heparin are used to form FGF-FGFR-heparin complexes, multiple complexes form upon the saccharide. These observations form the basis of a model where the range of interactions that FGFs and FGFRs can form with one another and with HS may lead to the formation of complexes with more than two FGFR units. Therefore HS will be crucial to FGF signalling from the initial signalling event to the formation of large receptor clusters.

Phosphatidylinositol 3-kinase gamma signaling through protein kinase Czeta induces NADPH oxidase-mediated oxidant generation and NF-kappaB activation in endothelial cells

We addressed the role of class 1B phosphatidylinositol 3-kinase (PI3K) isoform PI3Kgamma in mediating NADPH oxidase activation and reactive oxidant species (ROS) generation in endothelial cells (ECs) and of PI3Kgamma-mediated oxidant signaling in the mechanism of NF-kappaB activation and intercellular adhesion molecule (ICAM)-1 expression. We used lung microvascular ECs isolated from mice with targeted deletion of the p110gamma catalytic subunit of PI3Kgamma. Tumor necrosis factor (TNF) alpha challenge of wild type ECs caused p110gamma translocation to the plasma membrane and phosphatidylinositol 1,4,5-trisphosphate production coupled to ROS production; however, this response was blocked in p110gamma-/- ECs. ROS production was the result of TNFalpha activation of Ser phosphorylation of NADPH oxidase subunit p47(phox) and its translocation to EC membranes. NADPH oxidase activation failed to occur in p110gamma-/- ECs. Additionally, the TNFalpha-activated NF-kappaB binding to the ICAM-1 promoter, ICAM-1 protein expression, and PMN adhesion to ECs required functional PI3Kgamma. TNFalpha challenge of p110gamma-/- ECs failed to induce phosphorylation of PDK1 and activation of the atypical PKC isoform, PKCzeta. Thus, PI3Kgamma lies upstream of PKCzeta in the endothelium, and its activation is crucial in signaling NADPH oxidase-dependent oxidant production and subsequent NF-kappaB activation and ICAM-1 expression.

Multimers of the fibroblast growth factor (FGF)-FGF receptor-saccharide complex are formed on long oligomers of heparin

The minimal signalling unit for tyrosine kinase receptors is two protomers dimerized by one or more ligands. However, it is clear that maximal signalling requires the formation of larger complexes of many receptors at discrete foci on the cell surface. The biological interactions that lead to this are likely to be diverse and have system specific components. In the present study, we demonstrate that, in the FGF (fibroblast growth factor)-FGFR (FGF receptor) system, multimers of the minimal complex composed of two FGF1 and two FGFR2 protomers can form on a single chain of the co-receptor heparin. Using size-exclusion chromatography, we show that two complexes can form on heparin chains as small as 16 saccharide units. We also show by MS that discrete complexes containing exactly two copies of the minimal signalling unit are formed. However, the doublet of complexes appears to be less co-operative than the formation of the 2:2:1 FGF1:FGFR2:heparin complex, suggesting that this mechanism is one of a number of weaker interactions that might be involved in the formation of a focal complex on the cell surface.

The p85 regulatory subunit of phosphoinositide 3-kinase down-regulates IRS-1 signaling via the formation of a sequestration complex

Phosphoinositide (PI) 3-kinase is required for most insulin and insulin-like growth factor (IGF) 1–dependent cellular responses. The p85 regulatory subunit of PI 3-kinase is required to mediate the insulin-dependent recruitment of PI 3-kinase to the plasma membrane, yet mice with reduced p85 expression have increased insulin sensitivity. To further understand the role of p85, we examined IGF-1–dependent translocation of p85α by using a green fluorescence protein (GFP)–tagged p85α (EGFP–p85α). In response to IGF-1, but not to PDGF signaling, EGFP–p85α translocates to discrete foci in the cell. These foci contain the insulin receptor substrate (IRS) 1 adaptor molecule, and their formation requires the binding of p85 to IRS-1. Surprisingly, monomeric p85 is preferentially localized to these foci compared with the p85–p110 dimer, and these foci are not sites of phosphatidylinositol-3,4,5-trisphosphate production. Ultrastructural analysis reveals that p85–IRS-1 foci are cytosolic protein complexes devoid of membrane. These results suggest a mechanism of signal down-regulation of IRS-1 that is mediated by monomeric p85 through the formation of a sequestration complex between p85 and IRS-1.

Phosphoinositide Binding Regulates α-Actinin Dynamics

The active association-dissociation of dynamic protein-protein interactions is critical for the ability of the actin cytoskeleton to remodel. To determine the influence of phosphoinositide binding on the dynamic interaction of alpha-actinin with actin filaments and integrin adhesion receptors, fluorescence recovery after photobleaching (FRAP) microscopy was carried out comparing wild-type green fluorescent protein (GFP)-alpha-actinin and a GFP-alpha-actinin mutant with a decreased affinity for phosphoinositides (Fraley, T. S., Tran, T. C., Corgan, A. M., Nash, C. A., Hao, J., Critchley, D. R., and Greenwood, J. A. (2003) J. Biol. Chem. 278, 24039-24045). In fibroblasts, recovery of the mutant alpha-actinin protein was 2.2 times slower than the wild type along actin stress fibers and 1.5 times slower within focal adhesions. FRAP was also measured in U87MG glioblastoma cells, which have higher levels of 3-phosphorylated phosphoinositides. As expected, alpha-actinin turnover for both the stress fiber and focal adhesion populations was faster in U87MG cells compared with fibroblasts with recovery of the mutant protein slower than the wild type along actin stress fibers. To understand the influence of alpha-actinin turnover on the modulation of the actin cytoskeleton, wild-type or mutant alpha-actinin was co-expressed with constitutively active phosphoinositide (PI) 3-kinase. Co-expression with the alpha-actinin mutant inhibited actin reorganization with the appearance of enlarged alpha-actinin containing focal adhesions. These results demonstrate that the binding of phosphoinositides regulates the association-dissociation rate of alpha-actinin with actin filaments and integrin adhesion receptors and that the dynamics of alpha-actinin is important for PI 3-kinase-induced reorganization of the actin cytoskeleton. In conclusion, phosphoinositide regulation of alpha-actinin dynamics modulates the plasticity of the actin cytoskeleton influencing remodeling.

Membrane localization of all class I PI 3-kinase isoforms suppresses c-Myc-induced apoptosis in Rat1 fibroblasts via Akt

Phosphoinositide 3'-kinases (PI3Ks) constitute a family of lipid kinases implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. PI3Ks are heterodimers made up of four different 110-kDa catalytic subunits (p110alpha, p110beta, p110gamma, and p110delta) and a smaller regulatory subunit. Despite a clear implication of PI3Ks in survival signaling, the contribution of the individual PI3K isoforms has not been elucidated. To address this issue, we generated Rat1 fibroblasts that co-express c-Myc and membrane targeted derivates of the different p110 isoforms. Here we present data for the first time showing that activation of PI3-kinase signaling through membrane localization of p110beta, p110gamma, and p110delta protects c-Myc overexpressing Rat1 fibroblasts from apoptosis caused by serum deprivation like it has been described for p110alpha. Expression of each p110 isoform reduces significantly caspase-3 like activity in this apoptosis model. Decreased caspase-3 activity correlates with the increase in Akt phosphorylation in cells that contain one of the myristoylated p110 isoforms. p110 isoform-mediated protection from cell death was abrogated upon expression of a kinase-negative version of Akt.

Assigning functional domains within the p101 regulatory subunit of phosphoinositide 3-kinase gamma

Phosphoinositide 3-Kinase (PI3K) gamma is a lipid kinase that is regulated by G-protein-coupled receptors. It plays a crucial role in inflammatory and allergic processes. Activation of PI3Kgamma is primarily mediated by Gbetagamma subunits. The regulatory p101 subunit of PI3Kgamma binds to Gbetagamma and, thereby, recruits the catalytic p110gamma subunit to the plasma membrane. Despite its crucial role in the activation of PI3Kgamma, the structural organization of p101 is still largely elusive. Employing fluorescence resonance energy transfer measurements, coimmunoprecipitation and colocalization studies with p101 deletion mutants, we show here that distinct regions within the p101 primary structure are responsible for interaction with p110gamma and Gbetagamma. The p110gamma binding site is confined to the N terminus, whereas binding to Gbetagamma is mediated by a C-terminal domain of p101. These domains appear to be highly conserved among various species ranging from Xenopus to men. In addition to establishing a domain structure for p101, our results point to the existence of a previously unknown, p101-related regulatory subunit for PI3Kgamma.

Simulating complex intracellular processes using object-oriented computational modelling

The aim of this paper is to give an overview of computer modelling and simulation in cellular biology, in particular as applied to complex biochemical processes within the cell. This is illustrated by the use of the techniques of object-oriented modelling, where the computer is used to construct abstractions of objects in the domain being modelled, and these objects then interact within the computer to simulate the system and allow emergent properties to be observed. The paper also discusses the role of computer simulation in understanding complexity in biological systems, and the kinds of information which can be obtained about biology via simulation.

On the cross-regulation of protein tyrosine phosphatases and receptor tyrosine kinases in intracellular signaling

Intracellular signaling proteins are very often regulated by site-specific phosphorylation. For example, growth factor receptors in eukaryotic cells contain intrinsic tyrosine kinase activity and use inter- and intra-molecular interactions to recruit and orient potential protein substrates for phosphorylation. Equally important in determining the magnitude and kinetics of such a response is protein dephosphorylation, catalysed by phosphatase enzymes. A growing body of evidence indicates that certain protein tyrosine phosphatases (PTPs), like tyrosine kinases, are affected by intermolecular interactions that alter the specific activity or localization of their catalytic domains. Using a detailed kinetic modeling framework, we theoretically explore the regulation of PTPs through their association with receptor tyrosine kinases, as noted for the Src homology 2-domain-containing PTPs, SHP-1 and -2. Receptor-PTP binding, in turn, is expected to influence the phosphorylation pattern of those receptors and proteins they associate with, and we show how PTPs might serve to co- or counter-regulate parallel pathways in a signaling network.

Mechanical constraint imposed on plasma membrane through transverse phospholipid imbalance induces reversible actin polymerization via phosphoinositide 3-kinase activation

Platelets were used to explore the effect of membrane curvature induced by phospholipid excess on cell shape and on organization of the actin cytoskeleton. We showed that the addition of short chain analogues of phospholipids to the outer leaflet of plasma membrane of resting platelets immediately induced a shape change with long filopodia formation containing newly polymerized actin. Cells recovered rapidly their discoid shape and their initial F-actin content only with the phosphatidylserine analogue, which was transported to the inner leaflet by aminophospholipid translocase. Filopodia formation and actin polymerization were inhibited in platelets pre-incubated with cytochalasin D. Both wortmannin and LY294002, two unrelated inhibitors of phosphoinositide 3-kinase, considerably reduced actin polymerization and filopodia formation. Phospholipid imbalance was accompanied by a reversible translocation of phosphoinositide 3-kinase from cytoplasm to plasma membrane. In agreement with a role for PI 3-kinase, when phospholipids were added to platelets, PtdIns(3,4)P2 increased two-fold and Akt protein was partly phosphorylated. A similar shape change was also observed in nocodazole-treated L929 fibroblasts which were incubated with the similar phospholipid analogues. In those nucleated cells, where the microtubule cytoskeleton was disrupted, a major actin-dependent membrane extension was induced by addition of short chain phospholipids that required the functional integrity of PI 3-kinase. We conclude that any physical constraint acting on plasma membrane and resulting on local changes in membrane curvature is sufficient to initiate transient actin polymerization via phosphoinositide 3-kinase activation.

Identification of a novel domain in two mammalian inositol-polyphosphate 5-phosphatases that mediates membrane ruffle localization. The inositol 5-phosphatase skip localizes to the endoplasmic reticulum and translocates to membrane ruffles following epidermal growth factor stimulation

SKIP (skeletal muscle and kidney enriched inositol phosphatase) is a recently identified phosphatidylinositol 3,4,5-trisphosphate- and phosphatidylinositol 4,5-bisphosphate-specific 5-phosphatase. In this study, we investigated the intracellular localization of SKIP. Indirect immunofluorescence and subcellular fractionation showed that, in serum-starved cells, both endogenous and recombinant SKIP colocalized with markers of the endoplasmic reticulum (ER). Following epidermal growth factor (EGF) stimulation, SKIP transiently translocated to plasma membrane ruffles and colocalized with submembranous actin. Data base searching demonstrated a novel 128-amino acid domain in the C terminus of SKIP, designated SKICH for SKIP carboxyl homology, which is also found in the 107-kDa 5-phosphatase PIPP and in members of the TRAF6-binding protein family. Recombinant SKIP lacking the SKICH domain localized to the ER, but did not translocate to membrane ruffles following EGF stimulation. The SKIP SKICH domain showed perinuclear localization and mediated EGF-stimulated plasma membrane ruffle localization. The SKICH domain of the 5-phosphatase PIPP also mediated plasma membrane ruffle localization. Mutational analysis identified the core sequence within the SKICH domain that mediated constitutive membrane association and C-terminal sequences unique to SKIP that contributed to ER localization. Collectively, these studies demonstrate a novel membrane-targeting domain that serves to recruit SKIP and PIPP to membrane ruffles.

Roles of G beta gamma in membrane recruitment and activation of p110 gamma/p101 phosphoinositide 3-kinase gamma

Receptor-regulated class I phosphoinositide 3-kinases (PI3K) phosphorylate the membrane lipid phosphatidylinositol (PtdIns)-4,5-P2 to PtdIns-3,4,5-P3. This, in turn, recruits and activates cytosolic effectors with PtdIns-3,4,5-P3-binding pleckstrin homology (PH) domains, thereby controlling important cellular functions such as proliferation, survival, or chemotaxis. The class IB p110 gamma/p101 PI3K gamma is activated by G beta gamma on stimulation of G protein-coupled receptors. It is currently unknown whether in living cells G beta gamma acts as a membrane anchor or an allosteric activator of PI3K gamma, and which role its noncatalytic p101 subunit plays in its activation by G beta gamma. Using GFP-tagged PI3K gamma subunits expressed in HEK cells, we show that G beta gamma recruits the enzyme from the cytosol to the membrane by interaction with its p101 subunit. Accordingly, p101 was found to be required for G protein-mediated activation of PI3K gamma in living cells, as assessed by use of GFP-tagged PtdIns-3,4,5-P3-binding PH domains. Furthermore, membrane-targeted p110 gamma displayed basal enzymatic activity, but was further stimulated by G beta gamma, even in the absence of p101. Therefore, we conclude that in vivo, G beta gamma activates PI3K gamma by a mechanism assigning specific roles for both PI3K gamma subunits, i.e., membrane recruitment is mediated via the noncatalytic p101 subunit, and direct stimulation of G beta gamma with p110 gamma contributes to activation of PI3K gamma.

v-Src's hold over actin and cell adhesions

The oncoprotein v-Src and its cellular homologue (c-Src) are tyrosine kinases that modulate the actin cytoskeleton and cell adhesions. Through the concerted action of their protein-interaction and kinase domains, they are targeted to cell matrix integrin adhesions or cadherin-dependent junctions between epithelial cells, where they phosphorylate substrates that induce adhesion turnover and actin re-modelling. Recent experiments have defined some of the key targets and effector pathways that mediate the pleiotropic oncogenic effects of v-Src.

Activation of IRS-2-mediated signal transduction by IGF-1, but not TGF-alpha or EGF, augments pancreatic beta-cell proliferation

Transforming growth factor (TGF)-alpha- and epidermal growth factor (EGF)-induced signal transduction was directly compared with that of glucose and insulin-like growth factor-1 (IGF-1) in INS-1 cells. TGF-alpha/EGF transiently (<20 min) induced phosphorylation of extracellular-regulated kinase (Erk)-1/2 (>20-fold), glycogen synthase kinase (GSK)-3 (>10-fold), and protein kinase B (PKB) (Ser(473) and Thr(308)), but did not increase [(3)H]thymidine incorporation. In contrast, phosphorylation of Erk1/2, GSK-3, and PKB in response to glucose and IGF-1 was more prolonged (>24 h) and, though not as robust as TGF-alpha/EGF, did increase beta-cell proliferation. Phosphorylation of p70(S6K) was also increased by IGF-1/glucose, but not by TGF-alpha/EGF, despite upstream PKB activation. It was found that IGF-1 induced phosphatidylinositol 3-kinase (PI3K) association with insulin receptor substrate (IRS)-1 and -2 in a glucose-dependent manner, whereas TGF-alpha/EGF did not. The importance of specific IRS-2-mediated signaling events was emphasized in that adenoviral-mediated overexpression of IRS-2 further increased glucose/IGF-1-induced beta-cell proliferation (more than twofold; P < 0.05) compared with control or adenoviral-mediated IRS-1 overexpressing INS-1 cells. Neither IRS-1 nor IRS-2 overexpression induced a beta-cell proliferative response to TGF-alpha/EGF. Thus, a prolonged activation of Erk1/2 and PI3K signaling pathways is important in committing a beta-cell to a mitogenic event, and it is likely that this sustained activation is instigated by signal transduction occurring specifically through IRS-2.

E-cadherin homophilic ligation directly signals through Rac and phosphatidylinositol 3-kinase to regulate adhesive contacts

Classical cadherins mediate cell recognition and cohesion in many tissues of the body. It is increasingly apparent that dynamic cadherin contacts play key roles during morphogenesis and that a range of cell signals are activated as cells form contacts with one another. It has been difficult, however, to determine whether these signals represent direct downstream consequences of cadherin ligation or are juxtacrine signals that are activated when cadherin adhesion brings cell surfaces together but are not direct downstream targets of cadherin signaling. In this study, we used a functional cadherin ligand (hE/Fc) to directly test whether E-cadherin ligation regulates phosphatidylinositol 3-kinase (PI 3-kinase) and Rac signaling. We report that homophilic cadherin ligation recruits Rac to nascent adhesive contacts and specifically stimulates Rac signaling. Adhesion to hE/Fc also recruits PI 3-kinase to the cadherin complex, leading to the production of phosphatidylinositol 3,4,5-trisphosphate in nascent cadherin contacts. Rac activation involved an early phase, which was PI 3-kinase-independent, and a later amplification phase, which was inhibited by wortmannin. PI 3-kinase and Rac activity were necessary for productive adhesive contacts to form following initial homophilic ligation. We conclude that E-cadherin is a cellular receptor that is activated upon homophilic ligation to signal through PI 3-kinase and Rac. We propose that a key function of these cadherin-activated signals is to control adhesive contacts, probably via regulation of the actin cytoskeleton, which ultimately serves to mediate adhesive cell-cell recognition.

Vav3 modulates B cell receptor responses by regulating phosphoinositide 3-kinase activation

To elucidate the mechanism(s) by which Vav3, a new member of the Vav family proteins, participates in B cell antigen receptor (BCR) signaling, we have generated a B cell line deficient in Vav3. Here we report that Vav3 influences phosphoinositide 3-kinase (PI3K) function through Rac1 in that phosphatidylinositol-3,4,5-trisphosphate (PIP3) generation was attenuated by loss of Vav3 or by expression of a dominant negative form of Rac1. The functional interaction between PI3K and Rac1 was also demonstrated by increased PI3K activity in the presence of GTP-bound Rac1. In addition, we show that defects of calcium mobilization and c-Jun NH2-terminal kinase (JNK) activation in Vav3-deficient cells are relieved by deletion of a PIP3 hydrolyzing enzyme, SH2 domain-containing inositol polyphosphate 5'-phosphatase (SHIP). Hence, our results suggest a role for Vav3 in regulating the B cell responses by promoting the sustained production of PIP3 and thereby calcium flux.

Recruitment of the class II phosphoinositide 3-kinase C2beta to the epidermal growth factor receptor: role of Grb2

Previously we demonstrated that the class II phosphoinositide 3-kinase C2beta (PI3K-C2beta) is rapidly recruited to a phosphotyrosine signaling complex containing the activated receptor for epidermal growth factor (EGF). Although this association was shown to be dependent upon specific phosphotyrosine residues present on the EGF receptor, the underlying mechanism remained unclear. In this study the interaction between PI3K-C2beta and the EGF receptor is competitively attenuated by synthetic peptides derived from each of three proline-rich motifs present within the N-terminal region of the PI3K. Further, a series of N-terminal PI3K-C2beta fragments, truncated prior to each proline-rich region, bound the receptor with decreased efficiency. A single proline-rich region was unable to mediate receptor association. Finally, an equivalent N-terminal fragment of PI3K-C2alpha that lacks similar proline-rich motifs was unable to affinity purify the activated EGF receptor from cell lysates. Since these findings revealed that the interaction between the EGF receptor and PI3K-C2beta is indirect, we sought to identify an adaptor molecule that could mediate their association. In addition to the EGF receptor, PI3K-C2beta(2-298) also isolated both Shc and Grb2 from A431 cell lysates. Recombinant Grb2 directly bound PI3K-C2beta in vitro, and this effect was reproduced using either SH3 domain expressed as a glutathione S-transferase (GST) fusion. Interaction with Grb2 dramatically increased the catalytic activity of this PI3K. The relevance of this association was confirmed when PI3K-C2beta was isolated by coimmunoprecipitation with anti-Grb2 antibody from numerous cell lines. Using immobilized, phosphorylated EGF receptor, recombinant PI3K-C2beta was only purified in the presence of Grb2. We conclude that proline-rich motifs within the N terminus of PI3K-C2beta mediate the association of this enzyme with activated EGF receptor and that this interaction involves the Grb2 adaptor.

Cooperative control of Akt phosphorylation, bcl-2 expression, and apoptosis by cytoskeletal microfilaments and microtubules in capillary endothelial cells

Capillary endothelial cells can be switched between growth and apoptosis by modulating their shape with the use of micropatterned adhesive islands. The present study was carried out to examine whether cytoskeletal filaments contribute to this response. Disruption of microfilaments or microtubules with the use of cytochalasin D or nocodazole, respectively, led to levels of apoptosis in capillary cells equivalent to that previously demonstrated by inducing cell rounding with the use of micropatterned culture surfaces containing small (<20 microm in diameter) circular adhesive islands coated with fibronectin. Simultaneous disruption of microfilaments and microtubules led to more pronounced cell rounding and to enhanced levels of apoptosis approaching that observed during anoikis in fully detached (suspended) cells, indicating that these two cytoskeletal filament systems can cooperate to promote cell survival. Western blot analysis revealed that the protein kinase Akt, which is known to be critical for control of cell survival became dephosphorylated during cell rounding induced by disruption of the cytoskeleton, and that this was accompanied by a decrease in bcl-2 expression as well as a subsequent increase in caspase activation. This ability of the cytoskeleton to control capillary endothelial cell survival may be important for understanding the relationship among extracellular matrix turnover, cell shape changes, and apoptosis during angiogenesis inhibition.

Golgi clusters and vesicles mediate mitotic inheritance independently of the endoplasmic reticulum

We have examined the fate of Golgi membranes during mitotic inheritance in animal cells using four-dimensional fluorescence microscopy, serial section reconstruction of electron micrographs, and peroxidase cytochemistry to track the fate of a Golgi enzyme fused to horseradish peroxidase. All three approaches show that partitioning of Golgi membranes is mediated by Golgi clusters that persist throughout mitosis, together with shed vesicles that are often found associated with spindle microtubules. We have been unable to find evidence that Golgi membranes fuse during the later phases of mitosis with the endoplasmic reticulum (ER) as a strategy for Golgi partitioning (Zaal, K.J., C.L. Smith, R.S. Polishchuk, N. Altan, N.B. Cole, J. Ellenberg, K. Hirschberg, J.F. Presley, T.H. Roberts, E. Siggia, et al. 1999. Cell. 99:589-601) and suggest that these results, in part, are the consequence of slow or abortive folding of GFP-Golgi chimeras in the ER. Furthermore, we show that accurate partitioning is accomplished early in mitosis, by a process of cytoplasmic redistribution of Golgi fragments and vesicles yielding a balance of Golgi membranes on either side of the metaphase plate before cell division.

EGF-dependent translocation of green fluorescent protein-tagged PLC-gamma1 to the plasma membrane and endosomes

Growth factor-dependent translocation of phospholipase C-gamma1 (PLC-gamma1) was investigated using a green fluorescent protein-tagged PLC-gamma1 (PLC-gamma1-GFP) expressed in human epidermoid carcinoma A-431 cells. In the absence of growth factors, PLC-gamma1-GFP was present throughout the cytoplasm of A-431 cells. Treatment of the cells with epidermal growth factor (EGF) produced a very rapid redistribution of PLC-gamma1-GFP to the plasma membrane in a nonuniform manner. This translocation to the plasma membrane was insensitive to an inhibitor of phosphatidylinositol 3-kinase and was independent of cell adhesion. However, the translocation was disrupted by an agent which depolymerizes the actin cytoskeleton. At later times following the addition of EGF, PLC-gamma1-GFP appeared associated with intracellular vesicles. Stimulation of A-431 cells by Texas red-conjugated EGF for more than 10 min resulted in punctate intracellular PLC-gamma1-GFP distribution that colocalized with Texas red-conjugated EGF. This suggests that PLC-gamma1 is translocated to endosomes after EGF treatment, probably by associating with the internalized and autophosphorylated EGF receptor. Fractionation studies demonstrated that the EGF-induced plasma membrane-localized PLC-gamma1 is concentrated in caveolae microdomains. Disruption of caveolae with methyl-beta-cyclodextrin resulted in the ablation of EGF-induced, but not bradykinin-induced, mobilization of intracellular Ca(2+). This treatment, however, only partially decreased PLC-gamma1 membrane translocation.

Integrin-specific activation of Rac controls progression through the G(1) phase of the cell cycle

Adhesion to fibronectin through the alpha5beta1 integrin enables endothelial cells to proliferate in response to growth factors, whereas adhesion to laminin through alpha2beta1 results in growth arrest under the same conditions. On laminin, endothelial cells fail to translate Cyclin D1 mRNA and activate CDK4 and CDK6. Activated Rac, but not MEK1, PI-3K, or Akt, rescues biosynthesis of cyclin D1 and progression through the G(1) phase. Conversely, dominant negative Rac prevents these events on fibronectin. Mitogens promote activation of Rac on fibronectin but not laminin. This process is mediated by SOS and PI-3K and requires coordinate upstream signals through Shc and FAK. These results indicate that Rac is a crucial mediator of the integrin-specific control of cell cycle in endothelial cells.

Tumor invasion as dysregulated cell motility

Investigations across a range of disciplines over the past decade have brought the study of cell motility and its role in invasion to an exciting threshold. The biophysical forces proximally involved in generating cell locomotion, as well as the underlying signaling and genomic regulatory processes, are gradually becoming elucidated. We now appreciate the intricacies of the many cellular and extracellular events that modulate cell migration. This has enabled the demonstration of a causal role of cell motility in tumor progression, with various points of 'dysregulation' of motility being responsible for promoting invasion. In this paper, we describe key fundamental principles governing cell motility and branch out to describe the essence of the data that describe these principles. It has become evident that many proposed models may indeed be converging into a tightly-woven tapestry of coordinated events which employ various growth factors and their receptors, adhesion receptors (integrins), downstream molecules, cytoskeletal components, and altered genomic regulation to accomplish cell motility. Tumor invasion occurs in response to dysregulation of many of these modulatory points; specific examples include increased signaling from the EGF receptor and through PLC gamma, altered localization and expression of integrins, changes in actin modifying proteins and increased transcription from specific promoter sites. This diversity of alterations all leading to tumor invasion point to the difficulty of correcting causal events leading to tumor invasion and rather suggest that the underlying common processes required for motility be targeted for therapeutic intervention.

BCAP: the tyrosine kinase substrate that connects B cell receptor to phosphoinositide 3-kinase activation

Tyrosine phosphorylation of adaptor proteins permits the B cell antigen receptor (BCR)-associated protein tyrosine kinases to regulate downstream effector molecules. Here, we report the identification of a novel B cell adaptor for phosphoinositide 3-kinase (PI3K), termed BCAP. Tyrosine phosphorylation of BCAP is mediated by Syk and Btk, thereby providing binding site(s) for the p85 subunit of PI3K. Disruption of the BCAP gene in the DT40 B cell line inhibits BCR-mediated phosphatidylinositol 3,4,5-trisphosphate generation, leading to impaired Akt response. Moreover, recruitment of PI3K to glycolipid-enriched microdomains (GEMs) is significantly attenuated in the absence of BCAP. Hence, these data suggest that BCAP bridges BCR-associated kinases to the PI3K pathway by regulating PI3K localization.

Role of the PI3K regulatory subunit in the control of actin organization and cell migration

Cell migration represents an important cellular response that utilizes cytoskeletal reorganization as its driving force. Here, we describe a new signaling cascade linking PDGF receptor stimulation to actin rearrangements and cell migration. We demonstrate that PDGF activates Cdc42 and its downstream effector N-WASP to mediate filopodia formation, actin stress fiber disassembly, and a reduction in focal adhesion complexes. Induction of the Cdc42 pathway is independent of phosphoinositide 3-kinase (PI3K) enzymatic activity, but it is dependent on the p85alpha regulatory subunit of PI3K. Finally, data are provided showing that activation of this pathway is required for PDGF-induced cell migration on collagen. These observations show the essential role of the PI3K regulatory subunit p85alpha in controlling PDGF receptor-induced cytoskeletal changes and cell migration, illustrating a novel signaling pathway that links receptor stimulation at the cell membrane with actin dynamics.

Restructuring of focal adhesion plaques by PI 3-kinase. Regulation by PtdIns (3,4,5)-p(3) binding to alpha-actinin

Focal adhesions are an elaborate network of interconnecting proteins linking actin stress fibers to the extracellular matrix substrate. Modulation of the focal adhesion plaque provides a mechanism for the regulation of cellular adhesive strength. Using interference reflection microscopy, we found that activation of phosphoinositide 3-kinase (PI 3-kinase) by PDGF induces the dissipation of focal adhesions. Loss of this close apposition between the cell membrane and the extracellular matrix coincided with a redistribution of alpha-actinin and vinculin from the focal adhesion complex to the Triton X-100-soluble fraction. In contrast, talin and paxillin remained localized to focal adhesions, suggesting that activation of PI 3-kinase induced a restructuring of the plaque rather than complete dispersion. Furthermore, phosphatidylinositol (3,4, 5)-trisphosphate (PtdIns (3,4,5)-P(3)), a lipid product of PI 3-kinase, was sufficient to induce restructuring of the focal adhesion plaque. We also found that PtdIns (3,4,5)-P(3) binds to alpha-actinin in PDGF-treated cells. Further evidence demonstrated that activation of PI 3-kinase by PDGF induced a decrease in the association of alpha-actinin with the integrin beta subunit, and that PtdIns (3,4,5)-P(3) could disrupt this interaction in vitro. Modification of focal adhesion structure by PI 3-kinase and its lipid product, PtdIns (3,4,5)-P(3), has important implications for the regulation of cellular adhesive strength and motility.

Adhesion-linked kinases in cancer; emphasis on src, focal adhesion kinase and PI 3-kinase

Our understanding of the complex signal transduction pathways involved in signalling within cancer cells, between cancer cells and between cancer cells and their environment has increased dramatically in recent years. Here we concentrate on three non-receptor kinases: Src, focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI 3-kinase). These form part of a complex network of intracellular signals which is thought to be important in regulating cancer cells.