Identification of two SH3-binding motifs in the regulatory subunit of phosphatidylinositol 3-kinase

Functional selection in SH3-mediated activation of the PI3 kinase

The phosphoinositide-3 kinase (PI3K), a heterodimeric enzyme, plays a pivotal role in cellular metabolism and survival. Its deregulation is associated with major human diseases, particularly cancer. The p85 regulatory subunit of PI3K binds to the catalytic p110 subunit via its C-terminal domains, stabilising it in an inhibited state. Certain Src homology 3 (SH3) domains can activate p110 by binding to the proline-rich (PR) 1 motif located at the N-terminus of p85. However, the mechanism by which this N-terminal interaction activates the C-terminally bound p110 remains elusive. Moreover, the intrinsically poor ligand selectivity of SH3 domains raises the question of how they can control PI3K. Combining structural, biophysical, and functional methods, we demonstrate that the answers to both these unknown issues are linked: PI3K-activating SH3 domains engage in additional "tertiary" interactions with the C-terminal domains of p85, thereby relieving their inhibition of p110. SH3 domains lacking these tertiary interactions may still bind to p85 but cannot activate PI3K. Thus, p85 uses a functional selection mechanism that precludes nonspecific activation rather than nonspecific binding. This separation of binding and activation may provide a general mechanism for how biological activities can be controlled by promiscuous protein-protein interaction domains.

Neurotransmitter Receptor HTR2B Regulates Lipid Metabolism to Inhibit Ferroptosis in Gastric Cancer

Nerves can support tumor development by secreting neurotransmitters that promote cancer cell proliferation and invasion. 5-Hydroxytryptamine (5-HT) is a critical neurotransmitter in the gastrointestinal nervous system, and 5-HT signaling has been shown to play a role in tumorigenesis. Here, we found that expression of the 5-HT receptor HTR2B was significantly elevated in human gastric adenocarcinoma tissues compared with nontumor tissues, and high HTR2B expression corresponded to shorter patient survival. Both 5-HT and a specific HTR2B agonist enhanced gastric adenocarcinoma cell viability under metabolic stress, reduced cellular and lipid reactive oxygen species, and suppressed ferroptosis; conversely, HTR2B loss or inhibition with a selective HTR2B antagonist yielded the inverse tumor suppressive effects. In a patient-derived xenograft tumor model, HTR2B-positive tumors displayed accelerated growth, which was inhibited by HTR2B antagonists. Single-cell analysis of human gastric adenocarcinoma tissues revealed enrichment of PI3K/Akt/mTOR and fatty acid metabolism-related gene clusters in cells expressing HTR2B compared with HTR2B-negative cells. Mechanistically, HTR2B cooperated with Fyn to directly regulate p85 activity and trigger the PI3K/Akt/mTOR signaling pathway, which led to increased expression of HIF1α and ABCD1 along with decreased levels of lipid peroxidation and ferroptosis. Together, these findings demonstrate that HTR2B activity modulates PI3K/Akt/mTOR signaling to stimulate gastric cancer cell survival and indicate that HTR2B expression could be a potential prognostic biomarker in patients with gastric cancer.

SIGNIFICANCE

Nerve cancer cross-talk mediated by HTR2B inhibits lipid peroxidation and ferroptosis in gastric cancer cells and promotes viability under metabolic stress, resulting in increased tumor growth and decreased patient survival.

How the Discovery of the CD4/CD8-p56lck Complexes Changed Immunology and Immunotherapy

The past 25 years have seen enormous progress in uncovering the receptors and signaling mechanisms on T-cells that activate their various effecter functions. Until the late 1980s, most studies on T-cells had focused on the influx of calcium and the levels of cAMP/GMP in T-cells. My laboratory then uncovered the interaction of CD4 and CD8 co-receptors with the protein-tyrosine kinase p56lck which are now widely accepted as the initiators of the tyrosine phosphorylation cascade leading to T-cell activation. The finding explained how immune recognition receptors expressed by many immune cells, which lack intrinsic catalytic activity, can transduce activation signals via non-covalent association with non-receptor tyrosine kinases. The discovery also established the concept that a protein tyrosine phosphorylation cascade operated in T-cells. In this vein, we and others then showed that the CD4- and CD8-p56lck complexes phosphorylate the TCR complexes which led to the identification of other protein-tyrosine kinases such as ZAP-70 and an array of substrates that are now central to studies in T-cell immunity. Other receptors such as B-cell receptor, Fc receptors and others were also subsequently found to use src kinases to control cell growth. In T-cells, p56lck driven phosphorylation targets include co-receptors such as CD28 and CTLA-4 and immune cell-specific adaptor proteins such as LAT and SLP-76 which act to integrate signals proximal to surface receptors. CD4/CD8-p56lck regulated events in T-cells include intracellular calcium mobilization, integrin activation and the induction of transcription factors for gene expression. Lastly, the identification of the targets of p56lck in the TCR and CD28 provided the framework for the development of chimeric antigen receptor (CAR) therapy in the treatment of cancer. In this review, I outline a history of the development of events that led to the development of the "TCR signaling paradigm" and its implications to immunology and immunotherapy.

Metabolic signaling in T cells

The maintenance of organismal homeostasis requires partitioning and transport of biochemical molecules between organ systems, their composite cells, and subcellular organelles. Although transcriptional programming undeniably defines the functional state of cells and tissues, underlying biochemical networks are intricately intertwined with transcriptional, translational, and post-translational regulation. Studies of the metabolic regulation of immunity have elegantly illustrated this phenomenon. The cells of the immune system interface with a diverse set of environmental conditions. Circulating immune cells perfuse peripheral organs in the blood and lymph, patrolling for pathogen invasion. Resident immune cells remain in tissues and play more newly appreciated roles in tissue homeostasis and immunity. Each of these cell populations interacts with unique and dynamic tissue environments, which vary greatly in biochemical composition. Furthermore, the effector response of immune cells to a diverse set of activating cues requires unique cellular adaptations to supply the requisite biochemical landscape. In this review, we examine the role of spatial partitioning of metabolic processes in immune function. We focus on studies of lymphocyte metabolism, with reference to the greater immunometabolism literature when appropriate to illustrate this concept.

PI3Kβ-A Versatile Transducer for GPCR, RTK, and Small GTPase Signaling

The phosphoinositide 3-kinase (PI3K) family includes eight distinct catalytic subunits and seven regulatory subunits. Only two PI3Ks are directly regulated downstream from G protein-coupled receptors (GPCRs): the class I enzymes PI3Kβ and PI3Kγ. Both enzymes produce phosphatidylinositol 3,4,5-trisposphate in vivo and are regulated by both heterotrimeric G proteins and small GTPases from the Ras or Rho families. However, PI3Kβ is also regulated by direct interactions with receptor tyrosine kinases (RTKs) and their tyrosine phosphorylated substrates, and similar to the class II and III PI3Ks, it binds activated Rab5. The unusually complex regulation of PI3Kβ by small and trimeric G proteins and RTKs leads to a rich landscape of signaling responses at the cellular and organismic levels. This review focuses first on the regulation of PI3Kβ activity in vitro and in cells, and then summarizes the biology of PI3Kβ signaling in distinct tissues and in human disease.

Targeting phosphoinositide 3-kinase (PI3K) in head and neck squamous cell carcinoma (HNSCC)

The landscape of head and neck squamous cell carcinoma (HNSCC) has been changing rapidly due to growing proportion of HPV-related disease and development of new therapeutic agents. At the same time, there has been a constant need for individually tailored treatment based on genetic biomarkers in order to optimize patient survival and alleviate treatment-related toxicities. In this regard, aberrations of PI3K pathway have important clinical implications in the treatment of HNSCC. They frequently constitute ‘gain of function’ mutations which trigger oncogenesis, and PI3K mutations can also lead to emergence of drug resistance after treatment with EGFR inhibitors. In this article, we review PI3K pathway as a target of treatment for HNSCC and summarize PI3K/mTOR inhibitors that are currently under clinical trials. In light of recent advancement of immune checkpoint inhibitors, consideration of PI3K inhibitors as potential immune modulators is also suggested.

P85 regulates neuronal migration through affecting neuronal morphology during mouse corticogenesis

In mammalian developing embryonic cortex, projection neurons migrate from the ventricular zone to the cortical plate, guided by radial glial cells with a transformation between bipolar and multipolar morphology. Previous studies have demonstrated that the PI3K-Akt-mTOR signal plays a critical role in brain development. However, the function of P85 in cortical development is still unclear. In the present study, we found that overexpression of P85 impaired cortical neuronal migration. Using in utero electroporation, we revealed that the length of the leading process in P85 overexpressed neurons became shorter than that in the control group but with more branches. Using markers for new-born neurons, we further found that overexpression of P85 did not affect the ultimate fate of these cortical neurons. These findings indicated that the P85 subunit plays an essential role in neuronal migration and neuronal morphology during mouse corticogenesis.

Signaling in Effector Lymphocytes: Insights toward Safer Immunotherapy

Receptors on T and NK cells systematically propagate highly complex signaling cascades that direct immune effector functions, leading to protective immunity. While extensive studies have delineated hundreds of signaling events that take place upon receptor engagement, the precise molecular mechanism that differentially regulates the induction or repression of a unique effector function is yet to be fully defined. Such knowledge can potentiate the tailoring of signal transductions and transform cancer immunotherapies. Targeted manipulations of signaling cascades can augment one effector function such as antitumor cytotoxicity while contain the overt generation of pro-inflammatory cytokines that contribute to treatment-related toxicity such as “cytokine storm” and “cytokine-release syndrome” or lead to autoimmune diseases. Here, we summarize how individual signaling molecules or nodes may be optimally targeted to permit selective ablation of toxic immune side effects.

Α-Dystrobrevin-1 recruits Grb2 and α-catulin to organize neurotransmitter receptors at the neuromuscular junction

Neuromuscular junctions (NMJs), the synapses made by motor neurons on muscle fibers, form during embryonic development but undergo substantial remodeling postnatally. Several lines of evidence suggest that α-dystrobrevin, a component of the dystrophin-associated glycoprotein complex (DGC), is a crucial regulator of the remodeling process and that tyrosine phosphorylation of one isoform, α-dystrobrevin-1, is required for its function at synapses. We identified a functionally important phosphorylation site on α-dystrobrevin-1, generated phosphorylation-specific antibodies to it and used them to demonstrate dramatic increases in phosphorylation during the remodeling period, as well as in nerve-dependent regulation in adults. We then identified proteins that bind to this site in a phosphorylation-dependent manner and others that bind to α-dystrobrevin-1 in a phosphorylation-independent manner. They include multiple members of the DGC, as well as α-catulin, liprin-α1, Usp9x, PI3K, Arhgef5 and Grb2. Finally, we show that two interactors, α-catulin (phosphorylation independent) and Grb2 (phosphorylation dependent) are localized to NMJs in vivo, and that they are required for proper organization of neurotransmitter receptors on myotubes.

Suppression of phospho-p85α-GTP-Rac1 lipid raft interaction by bichalcone analog attenuates cancer cell invasion

The p85α subunit of phosphatidylinositol 3-kinase (PI3K) acts as a key regulator of cell proliferation and motility, which mediates signals that confer chemoresistance to many human cancer cells. Using small interfering RNAs against matrix metalloproteinase-2 (MMP-2) and the MMP-2 promoter-driven luciferase assay, we showed that the new synthetic bichalcone analog TSWU-CD4 inhibits the invasion of human cancer cells by down-regulating MMP-2 expression. Treatment with TSWU-CD4 inhibited MMP-2 expression and cell invasion, which were restored by ectopic wild type (wt) p85α or a constitutively active form of MAPK kinase 3 (CA MKK3), CA MKK6, or CA p38α mitogen-activated protein kinase (MAPK). The attenuated formation of lipid raft-associated phospho (p)-p85α-GTP-Rac1 complexes, protein kinase B (Akt) Ser 473 phosphorylation, and cell invasion by TSWU-CD4 was reversed by overexpression of wt p85α or the p85α Brc-homology (BH) domain. The ectopic expression of CA Rac1^L61 (but not wt Rac1) could overcome the suppression of Ser 473 phosphorylation, lipid raft association of Akt, the interaction between GTP-bound Rac1 and p85α in lipid rafts, and cell invasion by TSWU-CD4. The involvement of Akt activity in the functions of NF-κB-mediated MMP-2 was further confirmed through the attenuation of Akt phosphorylation signaling using the Akt-specific inhibitor MK-2206 and ectopic expression of NF-κB p65. Collectively, the inhibitory effect of TSWU-CD4 on cancer cell invasion was likely to suppress the p-p85α-GTP-Rac1 interaction in lipid rafts by targeting the p85α BH domain, which resulted in the suppression of MMP-2 expression via the PI3K-Akt-mediated ERK-MKK3/MKK6-p38 MAPK-NF-κB signaling pathway. © 2016 Wiley Periodicals, Inc.

Assembly and Molecular Architecture of the Phosphoinositide 3-Kinase p85α Homodimer

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are activated by growth factor and G-protein-coupled receptors and propagate intracellular signals for growth, survival, proliferation, and metabolism. p85α, a modular protein consisting of five domains, binds and inhibits the enzymatic activity of class IA PI3K catalytic subunits. Here, we describe the structural states of the p85α dimer, based on data from in vivo and in vitro solution characterization. Our in vitro assembly and structural analyses have been enabled by the creation of cysteine-free p85α that is functionally equivalent to native p85α. Analytical ultracentrifugation studies showed that p85α undergoes rapidly reversible monomer-dimer assembly that is highly exothermic in nature. In addition to the documented SH3-PR1 dimerization interaction, we identified a second intermolecular interaction mediated by cSH2 domains at the C-terminal end of the polypeptide. We have demonstrated in vivo concentration-dependent dimerization of p85α using fluorescence fluctuation spectroscopy. Finally, we have defined solution conditions under which the protein is predominantly monomeric or dimeric, providing the basis for small angle x-ray scattering and chemical cross-linking structural analysis of the discrete dimer. These experimental data have been used for the integrative structure determination of the p85α dimer. Our study provides new insight into the structure and assembly of the p85α homodimer and suggests that this protein is a highly dynamic molecule whose conformational flexibility allows it to transiently associate with multiple binding proteins.

Absence of both Sos-1 and Sos-2 in peripheral CD4(+) T cells leads to PI3K pathway activation and defects in migration

Sos-1 and Sos-2 are ubiquitously expressed Ras-guanine exchange factors involved in Erk-MAP kinase pathway activation. Using mice lacking genes encoding Sos-1 and Sos-2, we evaluated the role of these proteins in peripheral T-cell signaling and function. Our results confirmed that TCR-mediated Erk activation in peripheral CD4(+) T cells does not depend on Sos-1 and Sos-2, although IL-2-mediated Erk activation does. Unexpectedly, however, we show an increase in AKT phosphorylation in Sos-1/2dKO CD4(+) T cells upon TCR and IL-2 stimulation. Activation of AKT was likely a consequence of increased recruitment of PI3K to Grb2 upon TCR and/or IL-2 stimulation in Sos-1/2dKO CD4(+) T cells. The increased activity of the PI3K/AKT pathway led to downregulation of the surface receptor CD62L in Sos-1/2dKO T cells and a subsequent impairment in T-cell migration.

Signaling by Fyn-ADAP via the Carma1-Bcl-10-MAP3K7 signalosome exclusively regulates inflammatory cytokine production in NK cells

Inflammation is a critical component of the immune response. However, acute or chronic inflammation can be highly destructive. Uncontrolled inflammation forms the basis for allergy, asthma, and multiple autoimmune disorders. Here, we identify a signaling pathway that is exclusively responsible for inflammatory cytokine production but not for cytotoxicity. Recognition of H60⁺ or CD137L⁺ tumor cells by murine NK cells led to efficient cytotoxicity and inflammatory cytokine production. Both of these effector functions required Lck, Fyn, PI(3)K-p85α, PI(3)K-p110δ, and PLC-γ2. However, the complex of Fyn and the adapter ADAP exclusively regulated inflammatory cytokine production but not cytotoxicity in NK cells. This unique function of ADAP required a Carma1-Bcl10-MAP3K7 signaling axis. Our results identify molecules that can be targeted to regulate inflammation without compromising NK cell cytotoxicity.

Requirement and redundancy of the Src family kinases Fyn and Lyn in perforin-dependent killing of Cryptococcus neoformans by NK cells

Natural killer (NK) cells directly recognize and kill fungi, such as the pathogenic fungus Cryptococcus neoformans, via cytolytic mechanisms. However, the precise signaling pathways governing this NK cell microbicidal activity and the implications for fungal recognition are still unknown. Previously, it was reported that NK cell anticryptococcal activity is mediated through a conserved phosphatidylinositol 3-kinase-extracellular signal-regulated kinase 1/2 (PI3K-ERK1/2) pathway. Using YT (a human NK-like cell line) and primary human NK cells, we sought to identify the upstream, receptor-proximal signaling elements that led to fungal cytolysis. We demonstrate that Src family kinases were activated in response to C. neoformans. Furthermore, pharmacologic inhibition with an Src kinase inhibitor blocked C. neoformans-induced downstream activation of PI3K and ERK1/2 and abrogated cryptococcal killing. At the same time, the inhibitor disrupted the polarization of perforin-containing granules toward the NK cell-cryptococcal synapse but had no effect on conjugate formation between the organism and the NK cell. Finally, small interfering RNA (siRNA) double (but not single) knockdown of two Src family kinases, Fyn and Lyn, blocked cryptococcal killing. Together these data demonstrate a mechanism whereby the Src family kinases, Fyn and Lyn, redundantly mediate anticryptococcal activity through the activation of PI3K and ERK1/2, which in turn facilitates killing by inducing the polarization of perforin-containing granules to the NK cell-cryptococcal synapse.

Stem Cell Factor Receptor/c-Kit: From Basic Science to Clinical Implications

Stem cell factor (SCF) is a dimeric molecule that exerts its biological functions by binding to and activating the receptor tyrosine kinase c-Kit. Activation of c-Kit leads to its autophosphorylation and initiation of signal transduction. Signaling proteins are recruited to activated c-Kit by certain interaction domains (e.g., SH2 and PTB) that specifically bind to phosphorylated tyrosine residues in the intracellular region of c-Kit. Activation of c-Kit signaling has been found to mediate cell survival, migration, and proliferation depending on the cell type. Signaling from c-Kit is crucial for normal hematopoiesis, pigmentation, fertility, gut movement, and some aspects of the nervous system. Deregulated c-Kit kinase activity has been found in a number of pathological conditions, including cancer and allergy. The observation that gain-of-function mutations in c-Kit can promote tumor formation and progression has stimulated the development of therapeutics agents targeting this receptor, e.g., the clinically used inhibitor imatinib mesylate. Also other clinically used multiselective kinase inhibitors, for instance, sorafenib and sunitinib, have c-Kit included in their range of targets. Furthermore, loss-of-function mutations in c-Kit have been observed and shown to give rise to a condition called piebaldism. This review provides a summary of our current knowledge regarding structural and functional aspects of c-Kit signaling both under normal and pathological conditions, as well as advances in the development of low-molecular-weight molecules inhibiting c-Kit function.

SH3 domains: modules of protein-protein interactions

Src homology 3 (SH3) domains are involved in the regulation of important cellular pathways, such as cell proliferation, migration and cytoskeletal modifications. Recognition of polyproline and a number of noncanonical sequences by SH3 domains has been extensively studied by crystallography, nuclear magnetic resonance and other methods. High-affinity peptides that bind SH3 domains are used in drug development as candidates for anticancer treatment. This review summarizes the latest achievements in deciphering structural determinants of SH3 function.

PI3K signalling in B- and T-lymphocytes: new developments and therapeutic advances

Activation of PI3K (phosphoinositide 3-kinase) is a shared response to engagement of diverse types of transmembrane receptors. Depending on the cell type and stimulus, PI3K activation can promote different fates including proliferation, survival, migration and differentiation. The diverse roles of PI3K signalling are well illustrated by studies of lymphocytes, the cells that mediate adaptive immunity. Genetic and pharmacological experiments have shown that PI3K activation regulates many steps in the development, activation and differentiation of both B- and T-cells. These findings have prompted the development of PI3K inhibitors for the treatment of autoimmunity and inflammatory diseases. PI3K activation, however, has both positive and negative roles in immune system activation. Consequently, although PI3K suppression can attenuate immune responses it can also enhance inflammation, disrupt peripheral tolerance and promote autoimmunity. An exciting discovery is that a selective inhibitor of the p110δ catalytic isoform of PI3K, CAL-101, achieves impressive clinical efficacy in certain B-cell malignancies. A model is emerging in which p110δ inhibition disrupts signals from the lymphoid microenvironment, leading to release of leukaemia and lymphoma cells from their protective niche. These encouraging findings have given further momentum to PI3K drug development efforts in both cancer and immune diseases.

Multiple roles for the p85α isoform in the regulation and function of PI3K signalling and receptor trafficking

The p85α protein is best known as the regulatory subunit of class 1A PI3Ks (phosphoinositide 3-kinases) through its interaction, stabilization and repression of p110-PI3K catalytic subunits. PI3Ks play multiple roles in the regulation of cell survival, signalling, proliferation, migration and vesicle trafficking. The present review will focus on p85α, with special emphasis on its important roles in the regulation of PTEN (phosphatase and tensin homologue deleted on chromosome 10) and Rab5 functions. The phosphatidylinositol-3-phosphatase PTEN directly counteracts PI3K signalling through dephosphorylation of PI3K lipid products. Thus the balance of p85α–p110 and p85α–PTEN complexes determines the signalling output of the PI3K/PTEN pathway, and under conditions of reduced p85α levels, the p85α–PTEN complex is selectively reduced, promoting PI3K signalling. Rab5 GTPases are important during the endocytosis, intracellular trafficking and degradation of activated receptor complexes. The p85α protein helps switch off Rab5, and if defective in this p85α function, results in sustained activated receptor tyrosine kinase signalling and cell transformation through disrupted receptor trafficking. The central role for p85α in the regulation of PTEN and Rab5 has widened the scope of p85α functions to include integration of PI3K activation (p110-mediated), deactivation (PTEN-mediated) and receptor trafficking/signalling (Rab5-mediated) functions, all with key roles in maintaining cellular homoeostasis.

Hyperglycemia-induced p66shc inhibits insulin-like growth factor I-dependent cell survival via impairment of Src kinase-mediated phosphoinositide-3 kinase/AKT activation in vascular smooth muscle cells

Hyperglycemia has been shown to induce the p66shc expression leading to increased reactive oxygen species (ROS) generation and apoptosis. In the present study, we demonstrated that hyperglycemia induced p66shc expression in vascular smooth muscle cells. This induction was associated with an increase in apoptosis as assessed by the increase of capspase-3 enzymatic activity, cleaved caspase-3 protein, and the number of dead cells. The ability of IGF-I to inhibit apoptosis was also attenuated. Further studies showed that hyperglycemia-induced p66shc inhibited IGF-I-stimulated phosphoinositide (PI)-3 kinase and AKT activation. Mechanistic studies showed that knockdown of p66shc enhanced IGF-I-stimulated SHPS-1/p85, p85/SHP-2, and p85/Grb2 association, all of which are required for PI-3 kinase/AKT activation. These responses were attenuated by overexpression of p66shc. IGF-I-stimulated p85 and AKT recruitment to the cell membrane fraction was altered in the same manner. Disruption of p66shc-Src interaction using either a blocking peptide or by expressing a p66shc mutant that did not bind to Src rescued IGF-I-stimulated PI-3 kinase/AKT activation as well as IGF-I-dependent cell survival. Although the highest absolute level of ROS was detected in p66shc-overexpressing cells, the relative increase in ROS induced by hyperglycemia was independent of p66shc expression. Taken together, our data suggest that the increase in p66shc that occurs in response to hyperglycemia is functioning to inhibit IGF-I-stimulated signaling and that the incremental increase in SMC sensitivity to IGF-I stimulation that occurs in response to p66shc induction of ROS is not sufficient to overcome the inhibitory effect of p66shc on Src kinase activation.

Baculovirus production of fully-active phosphoinositide 3-kinase alpha as a p85alpha-p110alpha fusion for X-ray crystallographic analysis with ATP competitive enzyme inhibitors

Phosphoinositide 3-kinases have been targeted for therapeutic research because they are key components of a cell signaling cascade controlling proliferation, growth, and survival. Direct activation of the PI3Kalpha pathway contributes to the development and progression of solid tumors in breast, endometrial, colon, ovarian, and gastric cancers. In the context of a drug discovery effort, the availability of a robust crystallographic system is a means to understand the subtle differences between ATP competitive inhibitor interactions with the active site and their selectivity against other PI3Kinase enzymes. To generate a suitable recombinant design for this purpose, a p85alpha-p110alpha fusion system was developed which enabled the expression and purification of a stoichiometrically homogeneous, constitutively active enzyme for structure determination with potent ATP competitive inhibitors (Raha et al., in preparation) [56]. This approach has yielded preparations with activity and inhibition characteristics comparable to those of the full-length PI3Kalpha from which X-ray diffracting crystals were grown with inhibitors bound in the active site.

The regulation of class IA PI 3-kinases by inter-subunit interactions

Phosphoinositide 3-kinases (PI 3-kinases) are activated by growth factor and hormone receptors, and regulate cell growth, survival, motility, and responses to changes in nutritional conditions (Engelman et al. 2006). PI 3-kinases have been classified according to their subunit composition and their substrate specificity for phosphoinositides (Vanhaesebroeck et al. 2001). The class IA PI 3-kinase is a heterodimer consisting of one regulatory subunit (p85α, p85β, p55α, p50α, or p55γ) and one 110-kDa catalytic subunit (p110α, β or δ). The Class IB PI 3-kinase is also a dimer, composed of one regulatory subunit (p101 or p87) and one catalytic subunit (p110γ) (Wymann et al. 2003). Class I enzymes will utilize PI, PI[4]P, or PI[4,5]P2 as substrates in vitro, but are thought to primarily produce PI[3,4,5]P3 in cells.The crystal structure of the Class IB PI 3-kinase catalytic subunit p110γ was solved in 1999 (Walker et al. 1999), and crystal or NMR structures of the Class IA p110α catalytic subunit and all of the individual domains of the Class IA p85α regulatory subunit have been solved (Booker et al. 1992; Günther et al. 1996; Hoedemaeker et al. 1999; Huang et al. 2007; Koyama et al. 1993; Miled et al. 2007; Musacchio et al. 1996; Nolte et al. 1996; Siegal et al. 1998). However, a structure of an intact PI 3-kinase enzyme has remained elusive. In spite of this, studies over the past 10 years have lead to important insights into how the enzyme is regulated under physiological conditions. This chapter will specifically discuss the regulation of Class IA PI 3-kinase enzymatic activity, focusing on regulatory interactions between the p85 and p110 subunits and the modulation of these interactions by physiological activators and oncogenic mutations. The complex web of signaling downstream from Class IA PI 3-kinases will be discussed in other chapters in this volume.

Dietary salt activates an endothelial proline-rich tyrosine kinase 2/c-Src/phosphatidylinositol 3-kinase complex to promote endothelial nitric oxide synthase phosphorylation

Although many laboratories have shown that dietary NaCl (salt) intake increases NO production in rodents and humans, the mechanism has not been uncovered. In the present study, pharmacological and dominant-negative strategies were used to show that feeding a formulated diet containing increased amounts of salt to young male Sprague-Dawley rats induced the formation of an endothelial cell-signaling complex that contained proline-rich tyrosine kinase 2, c-Src (also known as pp60(c-src)), and phosphatidylinositol 3-kinase. In the setting of a high-salt diet, proline-rich tyrosine kinase 2 served as the scaffold for c-Src-mediated phosphatidylinositol 3-kinase activation. Phosphatidylinositol 3-kinase was the upstream activator of protein kinase B (Akt), which was responsible for phosphorylation of the rat endothelial isoform of NO synthase at S1176 and thereby promoted the increase in NO production. The combined findings illustrated the crucial role for a proline-rich tyrosine kinase 2-signaling complex in the endothelial response to salt intake.

The p85alpha subunit of class IA phosphatidylinositol 3-kinase regulates the expression of multiple genes involved in osteoclast maturation and migration

Intracellular signals involved in the maturation and function of osteoclasts are poorly understood. Here, we demonstrate that osteoclasts express multiple regulatory subunits of class I(A) phosphatidylinositol 3-kinase (PI3-K) although the expression of the full-length form of p85alpha is most abundant. In vivo, deficiency of p85alpha results in a significantly greater number of trabeculae and significantly lower spacing between trabeculae as well as increased bone mass in both males and females compared to their sex-matched wild-type controls. Consistently, p85alpha(-/-) osteoclast progenitors show impaired growth and differentiation, which is associated with reduced activation of Akt and mitogen-activated protein kinase extracellular signal-regulated kinase 1 (Erk1)/Erk2 in vitro. Furthermore, a significant reduction in the ability of p85alpha(-/-) osteoclasts to adhere to as well as to migrate via integrin alphavbeta3 was observed, which was associated with reduced bone resorption. Microarray as well as quantitative real-time PCR analysis of p85alpha(-/-) osteoclasts revealed a significant reduction in the expression of several genes associated with the maturation and migration of osteoclasts, including microphathalmia-associated transcription factor, tartrate-resistant acid phosphatase, cathepsin K, and beta3 integrin. Restoring the expression of the full-length form of p85alpha but not the version with a deletion of the Src homology-3 domain restored the maturation of p85alpha(-/-) osteoclasts to wild-type levels. These results highlight the importance of the full-length version of the p85alpha subunit of class I(A) PI3-K in controlling multiple aspects of osteoclast functions.

Insulin-like growth factor-I stimulates Shc-dependent phosphatidylinositol 3-kinase activation via Grb2-associated p85 in vascular smooth muscle cells

Insulin-like growth factor-I (IGF-I) stimulates vascular smooth muscle cell proliferation and migration by activating both MAPK and phosphatidylinositol 3-kinase (PI3K). Vascular smooth muscle cells (VSMCs) maintained in 25 mm glucose sustain MAPK activation via increased Shc phosphorylation and Grb2 association resulting in an enhanced mitogenic response compared with cells grown in 5 mm glucose. PI3K plays a major role in IGF-I-stimulated VSMC migration, and hyperglycemia augments this response. In contrast to MAPK activation the role of Shc in modulating PI3K in response to IGF-I has not been determined. In this study we show that impaired Shc association with Grb2 results in decreased Grb2-p85 association, SHPS-1-p85 recruitment, and PI3K activation in response to IGF-I. Exposure of VSMCs to cell-permeable peptides, which contained polyproline sequences from p85 proposed to mediate Grb2 association, resulted in inhibition of Grb2-p85 binding and AKT phosphorylation. Transfected cells that expressed p85 mutant that had specific prolines mutated to alanines resulted in less Grb2-p85 association, and a Grb2 mutant (W36A/W193A) that attenuated p85 binding showed decreased association of p85 with SHPS-1, PI3K activation, AKT phosphorylation, cell proliferation, and migration in response to IGF-I. Cellular exposure to 25 mm glucose, which is required for Shc phosphorylation in response to IGF-I, resulted in enhanced Grb2 binding to p85, activation of PI3K activity, and increased AKT phosphorylation as compared with cells exposed to 5 mm glucose. We conclude that in VSMCs exposed to hyperglycemia, IGF-I stimulation of Shc facilitates the transfer of Grb2 to p85 resulting in enhanced PI3K activation and AKT phosphorylation leading to enhanced cell proliferation and migration.

Computational models of tandem SRC homology 2 domain interactions and application to phosphoinositide 3-kinase

Intracellular signal transduction proteins typically utilize multiple interaction domains for proper targeting, and thus a broad diversity of distinct signaling complexes may be assembled. Considering the coordination of only two such domains, as in tandem Src homology 2 (SH2) domain constructs, gives rise to a kinetic scheme that is not adequately described by simple models used routinely to interpret in vitro binding measurements. To analyze the interactions between tandem SH2 domains and bisphosphorylated peptides, we formulated detailed kinetic models and applied them to the phosphoinositide 3-kinase p85 regulatory subunit/platelet-derived growth factor beta-receptor system. Data for this system from different in vitro assay platforms, including surface plasmon resonance, competition binding, and isothermal titration calorimetry, were reconciled to estimate the magnitude of the cooperativity characterizing the sequential binding of the high and low affinity SH2 domains (C-SH2 and N-SH2, respectively). Compared with values based on an effective volume approximation, the estimated cooperativity is 3 orders of magnitude lower, indicative of significant structural constraints. Homodimerization of full-length p85 was found to be an alternative mechanism for high avidity binding to phosphorylated platelet-derived growth factor receptors, which would render the N-SH2 domain dispensable for receptor binding.

The SH3 domain of a M7 interacts with its C-terminal proline-rich region

Myosins play essential roles in migration, cytokinesis, endocytosis, and adhesion. They are composed of a large N-terminal motor domain with ATPase and actin binding sites and C-terminal neck and tail regions, whose functional roles and structural context in the protein are less well characterized. The tail regions of myosins I, IV, VII, XII, and XV each contain a putative SH3 domain that may be involved in protein-protein interactions. SH3 domains are reported to bind proline-rich motifs, especially "PxxP" sequences, and such interactions serve regulatory functions. The activity of Src, PI3, and Itk kinases, for example, is regulated by intramolecular interactions between their SH3 domain and internal proline-rich sequences. Here, we use NMR spectroscopy to reveal the structure of a protein construct from Dictyostelium myosin VII (DdM7) spanning A1620-T1706, which contains its SH3 domain and adjacent proline-rich region. The SH3 domain forms the signature beta-barrel architecture found in other SH3 domains, with conserved tryptophan and tyrosine residues forming a hydrophobic pocket known to bind "PxxP" motifs. In addition, acidic residues in the RT or n-Src loops are available to interact with the basic anchoring residues that are typically found in ligands or proteins that bind SH3 domains. The DdM7 SH3 differs in the hydrophobicity of the second pocket formed by the 3(10) helix and following beta-strand, which contains polar rather than hydrophobic side chains. Most unusual, however, is that this domain binds its adjacent proline-rich region at a surface remote from the region previously identified to bind "PxxP" motifs. The interaction may affect the orientation of the tail without sacrificing the availability of the canonical "PxxP"-binding surface.

Tryptase activates phosphatidylinositol 3-kinases proteolytically independently from proteinase-activated receptor-2 in cultured dog airway smooth muscle cells

Mast cell tryptase is a potent mitogen for many cells in the airways and lung, but the cellular mechanisms for its growth stimulatory effects are poorly understood. Our major goal was to determine whether tryptase activates phosphatidylinositol 3-kinases (PI 3-kinases) in cultured dog tracheal smooth muscle cells to induce its mitogenic effects. After exposure to tryptase, cells were lysed. Immunocomplexes prepared from the lysates using an antibody to the p85 subunit of PI 3-kinase, but not using anti-phosphotyrosine antibodies, possessed increased capacity to phosphorylate inositol on its D3 hydroxyl group. Tryptase also increased phosphorylation of Akt, a downstream target of PI 3-kinases. This effect was abolished by one PI 3-kinase inhibitor, wortmannin, and attenuated by another, LY-294004, which also blocked tryptase's mitogenic effects. Treatment of tryptase with p-amidino phenylmethanesulfonyl fluoride, to abolish its proteolytic activity irreversibly, inhibited its stimulatory effects on Akt phosphorylation. Proteinase-activated receptor-2 (PAR-2)-activating peptides failed to increase Akt phosphorylation in cultured dog tracheal smooth muscle cells, but the PAR-2-activating peptides did induce brisk increases in Akt phosphorylation in Madin-Darby canine kidney cells. We concluded that tryptase activates PI 3-kinases in cultured dog tracheal smooth muscle cells to induce its potent mitogenic effects. These effects of tryptase on PI 3-kinases appear to occur via novel proteolytic mechanisms independent from PAR-2. Also, tryptase, although comparable in mitogenic potency to platelet-derived growth factor (PDGF), induces considerably less tyrosine phosphorylation on proteins than occur in response to PDGF.

Corneal cell survival in adenovirus type 19 infection requires phosphoinositide 3-kinase/Akt activation

Adenovirus type 19 is a major cause of epidemic keratoconjunctivitis, the only ocular adenoviral infection associated with prolonged corneal inflammation. In this study, we investigated the role of phosphoinositide 3-kinase (PI3K) and Akt and their downstream targets in adenovirus infection, and here we report the novel finding that adenovirus type 19 utilizes the PI3K/Akt pathway to maintain corneal fibroblast viability in acute infection. We demonstrate phosphorylation of GSK-3beta and nuclear translocation of the p65 subunit of NF-kappaB, both downstream targets of the PI3K/Akt pathway, in adenovirus-infected corneal fibroblasts in a PI3K-dependent manner. Inhibition of PI3K had no effect on early viral gene expression, suggesting normal viral internalization, but pretreatment with the PI3K inhibitor LY294002 or overexpression of dominant negative Akt induced early cytopathic effect and caspase-mediated cell death in adenovirus-infected cells. Early cell death could be circumvented despite LY294002 by overexpression of constitutively active Akt. Furthermore, we show an interaction between cSrc and the p85 regulatory subunit of PI3K in infected cells through a phosphorylation-dependent mechanism. The results presented in this paper provide the first direct evidence that PI3K-mediated Akt activation in adenovirus-infected corneal cells may contribute to viral pathogenesis by the prolongation of cell viability.

Recruitment of phosphoinositide 3-kinase defines a positive contribution of tyrosine kinase signaling to E-cadherin function

Classical cadherin adhesion molecules can function as adhesion-activated cell-signaling receptors. One key target for cadherin signaling is the lipid kinase phosphoinositide (PI) 3-kinase, which is recruited to cell-cell contacts and activated by E-cadherin. In this study, we sought to identify upstream factors necessary for E-cadherin to activate PI 3-kinase signaling. We found that inhibition of tyrosine kinase signaling blocked recruitment of PI 3-kinase to E-cadherin contacts and abolished the ability of E-cadherin to activate PI 3-kinase signaling. Tyrosine kinase inhibitors further perturbed several parameters of cadherin function, including cell adhesion and the ability of cells to productively extend nascent cadherin-adhesive contacts. Notably, the functional effects of tyrosine kinase blockade were rescued by expression of a constitutively active form of PI 3-kinase that restores PI 3-kinase signaling. Finally, using dominant negative Src mutants and Src-null cells, we identified Src as one key upstream kinase in the E-cadherin/PI 3-kinase-signaling pathway. Taken together, our findings indicate that tyrosine kinase activity, notably Src signaling, can contribute positively to cadherin function by supporting E-cadherin signaling to PI 3-kinase.

Calmodulin-mediated activation of Akt regulates survival of c-Myc-overexpressing mouse mammary carcinoma cells

c-Myc-overexpressing mammary epithelial cells are proapoptotic; their survival is strongly promoted by epidermal growth factor (EGF). We now demonstrate that EGF-induced Akt activation and survival in transgenic mouse mammary tumor virus-c-Myc mouse mammary carcinoma cells are both calcium/calmodulin-dependent. Akt activation is abolished by the phospholipase C-gamma inhibitor U-73122, by the intracellular calcium chelator BAPTA-AM, and by the specific calmodulin antagonist W-7. These results implicate calcium/calmodulin in the activation of Akt in these cells. In addition, Akt activation by serum and insulin is also inhibited by W-7. EGF-induced and calcium/calmodulin-mediated Akt activation occurs in both tumorigenic and non-tumorigenic mouse and human mammary epithelial cells, independent of their overexpression of c-Myc. These results imply that calcium/calmodulin may be a common regulator of Akt activation, irrespective of upstream receptor activator, mammalian species, and transformation status in mammary epithelial cells. However, only c-Myc-overexpressing mouse mammary carcinoma cells (but not normal mouse mammary epithelial cells) undergo apoptosis in the presence of the calmodulin antagonist W-7, indicating the vital selective role of calmodulin for survival of these cells. Calcium/calmodulin-regulated Akt activation is mediated directly by neither calmodulin kinases nor phosphatidylinositol 3-kinase (PI-3 kinase). Pharmacological inhibitors of calmodulin kinase kinase and calmodulin kinases II and III do not inhibit EGF-induced Akt activation, and calmodulin antagonist W-7 does not inhibit phosphotyrosine-associated PI-3 kinase activation. Akt is, however, co-immunoprecipitated with calmodulin in an EGF-dependent manner, which is inhibited by calmodulin antagonist W-7. We conclude that calmodulin may serve a vital regulatory function to direct the localization of Akt to the plasma membrane for its activation by PI-3 kinase.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Regulation of phosphoinositide 3-kinase by its intrinsic serine kinase activity in vivo

One potentially important mechanism for regulating class Ia phosphoinositide 3-kinase (PI 3-kinase) activity is autophosphorylation of the p85 alpha adapter subunit on Ser608 by the intrinsic protein kinase activity of the p110 catalytic subunit, as this downregulates the lipid kinase activity in vitro. Here we investigate whether this phosphorylation can occur in vivo. We find that p110 alpha phosphorylates p85 alpha Ser608 in vivo with significant stoichiometry. However, p110 beta is far less efficient at phosphorylating p85 alpha Ser608, identifying a potential difference in the mechanisms by which these two isoforms are regulated. The p85 alpha Ser608 phosphorylation was increased by treatment with insulin, platelet-derived growth factor, and the phosphatase inhibitor okadaic acid. The functional effects of this phosphorylation are highlighted by mutation of Ser608, which results in reduced lipid kinase activity and reduced association of the p110 alpha catalytic subunit with p85 alpha. The importance of this phosphorylation was further highlighted by the finding that autophosphorylation on Ser608 was impaired, while lipid kinase activity was increased, in a p85 alpha mutant recently discovered in human tumors. These results provide the first evidence that phosphorylation of Ser608 plays a role as a shutoff switch in growth factor signaling and contributes to the differences in functional properties of different PI 3-kinase isoforms in vivo.

Apoptosis signaling by the novel compound 3-Cl-AHPC involves increased EGFR proteolysis and accompanying decreased phosphatidylinositol 3-kinase and AKT kinase activities

The threonine and serine protein kinase AKT plays a major role in inhibiting apoptosis in a number of malignant cell types including prostate and breast carcinoma. Activation of AKT is a complex process involving translocation to the plasma membrane and phosphorylation of serine and threonine amino-acid residues. We now report that the novel compound 4-[3-(1-adamantyl)-4-hydroxyphenyl]-3-chlorocinnamic acid (3-Cl-AHPC), induces apoptosis in breast and prostate carcinoma cells and inhibits AKT activity in these cells. Overexpression of a constitutively activated AKT inhibits 3-Cl-AHPC-mediated apoptosis. Decrease in AKT activity occurs through 3-Cl-AHPC inhibition of phosphatidylinositol 3 kinase (PI3-K) activity. 3-Cl-AHPC inhibits PI3-K activity by enhancing epidermal growth factor receptor (EGFR) proteolysis and thus inhibiting EGFR association with the p85 subunit of PI3-K. 3-Cl-AHPC-mediated decrease in PI3-K activity results in the reduced synthesis of phosphatidylinositol 3,4 bisphosphate and phosphatidylinositol 3,4,5 triphosphate with the subsequent inhibition of integrin-linked kinase activity and serine-473 phosphorylation of AKT. Overexpression of EGFR results in increased AKT activity and inhibition of 3-Cl-AHPC-mediated decrease in AKT activation, AKT activity and 3-Cl-AHPC-mediated apoptosis. Inhibition of AKT activity by this compound results in the inability of AKT to phosphorylate and inactivate the proapoptotic forkhead transcription factor.

Src kinase mediates phosphatidylinositol 3-kinase/Akt-dependent rapid endothelial nitric-oxide synthase activation by estrogen

17beta-Estradiol activates endothelial nitric oxide synthase (eNOS), enhancing nitric oxide (NO) release from endothelial cells via the phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway. The upstream regulators of this pathway are unknown. We now demonstrate that 17beta-estradiol rapidly activates eNOS through Src kinase in human endothelial cells. The Src family kinase specific-inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) abrogates 17beta-estradiol- but not ionomycin-stimulated NO release. Consistent with these results, PP2 blocked 17beta-estradiol-induced Akt phosphorylation but did not inhibit NO release from cells transduced with a constitutively active Akt. PP2 abrogated 17beta-estradiol-induced activation of PI3-kinase, indicating that the PP2-inhibitable kinase is upstream of PI3-kinase and Akt. A 17beta-estradiol-induced estrogen receptor/c-Src association correlated with rapid c-Src phosphorylation. Moreover, transfection of kinase-dead c-Src inhibited 17beta-estradiol-induced Akt phosphorylation, whereas constitutively active c-Src increased basal Akt phosphorylation. Estrogen stimulation of murine embryonic fibroblasts with homozygous deletions of the c-src, fyn, and yes genes failed to induce Akt phosphorylation, whereas cells maintaining c-Src expression demonstrated estrogen-induced Akt activation. Estrogen rapidly activated c-Src inducing an estrogen receptor, c-Src, and P85 (regulatory subunit of PI3-kinase) complex formation. This complex formation results in the successive activation of PI3-kinase, Akt, and eNOS with consequent enhanced NO release, implicating c-Src as a critical upstream regulator of the estrogen-stimulated PI3-kinase/Akt/eNOS pathway.

Role of phosphatidylinositol 3-kinase in the binding of Bordetella pertussis to human monocytes

Bordetella pertussis, the causative agent of whooping cough, adheres to human monocytes by means of filamentous haemagglutinin (FHA), a bacterial surface protein that is recognized by complement receptor type 3 (CR3, alphaMbeta2 integrin). Previous work has shown that an FHA Arg-Gly-Asp (RGD, residues 1097-1099) site interacts with a complex composed of leucocyte response integrin (LRI, alphavbeta3 integrin) and integrin-associated protein (IAP, CD47) on human monocytes, resulting in enhancement of CR3-mediated bacterial binding. However, the pathway that mediates alphavbeta3-alphaMbeta2 integrin signalling remains to be characterized. Here we describe the involvement of phosphatidylinositol 3-kinase (PI3-K) in this pathway. Wortmannin and LY294002, inhibitors of PI3-K, reduced alphavbeta3/IAP-upregulated, CR3-associated bacterial binding to human monocytes. B. pertussis infection of human monocytes resulted in a marked recruitment of cellular PI3-K to the sites of B. pertussis contact. In contrast, cells infected with an isogenic strain carrying a G1098A mutation at the FHA RGD site did not show any recruitment of PI3-K. We found that ligation of FHA by alphavbeta3/IAP induced RGD-dependent tyrosine phosphorylation of a 60 kDa protein, which associated with IAP and PI3-K in human monocytes. These results suggest that PI3-K and a tyrosine phosphorylated 60 kDa protein may be involved in this biologically important integrin signalling pathway.

Phosphatidylinositol-3 kinase p85 enhances expression from the myelin basic protein promoter in oligodendrocytes

Phosphatidylinositol-3 kinase (PI3K) is a family of enzymes that phosphorylates the D3 position of phosphoinositides in membranes which can then act as a second messenger and affect many essential cellular processes such as survival, proliferation and differentiation. Class IA PI3K is composed of two subunits: a regulatory subunit, p85, and a catalytic subunit, p110. The p85 subunit is composed of several adapter domains which, upon interaction with the appropriate molecules, transmit the signal to activate p110. We have used the spontaneously immortalized oligodendrocyte cell line, CG4, to examine the role of PI3K in maturation of the oligodendrocyte. We show that overexpression of the p85 subunit enhances expression of myelin basic protein (MBP) upon differentiation of CG4 cells and primary oligodendrocytes. In experiments in CG4 cells, neither cotransfection with the tumor suppressor PTEN, which dephosphorylates the D3 position of phosphoinositides, nor inhibition of PI3K activity with wortmannin mimics this effect. Further, we have shown that this effect is dependent on the coexpression of the two SH2 domains within p85. Thus, the p85-mediated enhancement of MBP promoter activity in oligodendrocytes appears to be independent of PI3K activity and dependent on the adapter functions of the p85 subunit's SH2 domains.

Inhibition of Src family kinases blocks epidermal growth factor (EGF)-induced activation of Akt, phosphorylation of c-Cbl, and ubiquitination of the EGF receptor

Stimulation of T47D cells with epidermal growth factor (EGF) results in the activation of the intrinsic tyrosine kinases of the receptor and the phosphorylation of multiple cellular proteins including the receptor, scaffold molecules such as c-Cbl, adapter molecules such as Shc, and the serine/threonine protein kinase Akt. We demonstrate that EGF stimulation of T47D cells results in the activation of the Src protein-tyrosine kinase and that the Src kinase inhibitor PP1 blocks the EGF-induced phosphorylation of c-Cbl but not the activation/phosphorylation of the EGF receptor itself. PP1 also blocks EGF-induced ubiquitination of the EGF receptor, which is presumably mediated by phosphorylated c-Cbl. Src is associated with c-Cbl, and we have previously demonstrated that the Src-like kinase Fyn can phosphorylate c-Cbl at a preferred binding site for the p85 subunit of phosphatidylinositol 3'-kinase. PP1 treatment blocks EGF-induced activation of the anti-apoptotic protein kinase Akt suggesting that Src may regulate activation of Akt, perhaps by a Src --> c-Cbl --> phosphatidylinositol 3'-kinase --> Akt pathway.

Target-assisted iterative screening reveals novel interactors for PSD95, Nedd4, Src, Abl and Crk proteins

A new in vitro screening method has been developed and applied to a commercial phage-displayed cDNA library to search for novel protein-protein interactions. PDZ, WW and SH3 domains from PSD95, Nedd4, Src, Abl and Crk proteins were used as targets. 12 novel putative and 2 previously reported interactions were identified in test screens. The novel screening format, dubbed TAIS (target-assisted iterative screening), is discussed as an alternative platform to existing technologies for a pair-wise characterization of protein-protein interactions.

Phosphoinositide 3-kinase in immunological systems

Phosphoinositide 3-kinases (PI3Ks) are an evolutionarily conserved family of signal transducing enzymes. A great variety of stimuli activate PI3K, leading to the transient accumulation of its lipid products in cell membranes. These lipids serve as second messengers to regulate the location and activity of an array of downstream effector molecules. In cells of the mammalian immune system, PI3K is activated by receptors for antigen, cytokines, costimulatory molecules, immunoglobulins and chemoattractants. Signaling via PI3K regulates immune cell proliferation, survival, differentiation, chemotaxis, phagocytosis, degranulation, and respiratory burst. Here we review our current understanding of PI3K signaling in leukocytes.

Heregulin-dependent activation of phosphoinositide 3-kinase and Akt via the ErbB2/ErbB3 co-receptor

The ErbB2/ErbB3 heregulin co-receptor has been shown to couple to phosphoinositide (PI) 3-kinase in a heregulin-dependent manner. The recruitment and activation of PI 3-kinase by this co-receptor is presumed to occur via its interaction with phosphorylated Tyr-Xaa-Xaa-Met (YXXM) motifs occurring in the ErbB3 C terminus. In this study, mutant ErbB3 receptor proteins expressed in COS7 cells were used to investigate PI 3-kinase-dependent signaling pathways activated by the ErbB2/ErbB3 co-receptor. We observed that a mutant ErbB3 protein with each of its six YXXM motifs containing a Tyr --> Phe substitution was unable to bind either the p85 regulatory or p110 catalytic subunit of PI 3-kinase. However, restoration of a single YXXM motif was sufficient to mediate association with the PI 3-kinase holoenzyme, although at a lower level than wild-type ErbB3. When ErbB3 YXXM motifs were restored in pairs, evidence for cooperativity between two, those incorporating Tyr-1273 and Tyr-1286, was observed. Interestingly, we have shown that an apparent association of PI 3-kinase activity with ErbB2/Neu was due to the residual presence of ErbB3 in ErbB2 immunoprecipitates. The necessity of ErbB3 association with PI 3-kinase for downstream signaling to the effector kinase Akt was also investigated. Here, the heregulin-dependent translocation of Akt to the plasma membrane and its subsequent activation was observed in intact NIH-3T3 fibroblasts. Recruitment of PI 3-kinase to ErbB3 was required for both activities, and it appeared that ErbB2 activation alone was not sufficient to activate PI 3-kinase signaling in these cells.

Phosphoinositide 3-kinase inhibition in cancer treatment

Over the past ten years, our knowledge of the integral role that the phospho-inositide 3-kinases (PI3Ks) and their 3'-phosphorylated lipid products (3'-phosphorylated phosphoinositides; 3P-PIs) play in the mediation of signal transduction, cytoskeletal rearrangements and membrane trafficking has expanded considerably. They are now known to be involved in the regulation of cell growth, differentiation, mobility, proliferation and survival and hence they have become a potential target for the control of the growth and spread of cancer cells. More recently, the correlation of the multiplicity of isomers (both catalytic and regulatory) within the different classes of the PI3Ks with their functional relevance has become possible. This, combined with our further understanding of the protein recognition patterns for their different 3P-PIs and the newly-described pathways in the control of the levels of these by dephosphorylation, has provided new aspects and areas for interference in these multiple PI3K signalling pathways. However, in the search for effective, non-toxic, drugs for use in the treatment of cancers, these individual targets for PI3K inhibition need to be further correlated with the specific in vivo effects on cell survival, invasivity and metastatic potential. Here, the range of PI3K inhibition targets are discussed in the light of recent experimental findings, with a view to the exploitation of their specificities in new approaches to effective cancer treatments based on PI3K activity inhibition.

N-terminal domains of the class ia phosphoinositide 3-kinase regulatory subunit play a role in cytoskeletal but not mitogenic signaling

Phosphoinositide (PI) 3-kinases are required for the acute regulation of the cytoskeleton by growth factors. We have shown previously that in the MTLn3 rat adenocarcinoma cells line, the p85/p110alpha PI 3-kinase is required for epidermal growth factor (EGF)-stimulated lamellipod extension and formation of new actin barbed ends at the leading edge of the cell. We have now examined the role of the p85alpha regulatory subunit in greater detail. Microinjection of recombinant p85alpha into MTLn3 cells blocked both EGF-stimulated mitogenic signaling and lamellipod extension. In contrast, a truncated p85(1-333), which lacks the SH2 and iSH2 domains and does not bind p110, had no effect on EGF-stimulated mitogenesis but still blocked EGF-stimulated lamellipod extension. Additional deletional analysis showed that the SH3 domain was not required for inhibition of lamellipod extension, as a construct containing only the proline-rich and breakpoint cluster region (BCR) homology domains was sufficient for inhibition. Although the BCR domain of p85 binds Rac, the effects of the p85 constructs were not because of a general inhibition of Rac signaling, because sorbitol-induced JNK activation in MTLn3 cells was not inhibited. These data show that the proline-rich and BCR homology domains of p85 are involved in the coupling of p85/p110 PI 3-kinases to regulation of the actin cytoskeleton. These data provide evidence of a distinct cellular function for the N-terminal domains of p85.

Pike. A nuclear gtpase that enhances PI3kinase activity and is regulated by protein 4.1N

While cytoplasmic PI3Kinase (PI3K) is well characterized, regulation of nuclear PI3K has been obscure. A novel protein, PIKE (PI3Kinase Enhancer), interacts with nuclear PI3K to stimulate its lipid kinase activity. PIKE encodes a 753 amino acid nuclear GTPase. Dominant-negative PIKE prevents the NGF enhancement of PI3K and upregulation of cyclin D1. NGF treatment also leads to PIKE interactions with 4.1N, which has translocated to the nucleus, fitting with the initial identification of PIKE based on its binding 4.1N in a yeast two-hybrid screen. Overexpression of 4.1N abolishes PIKE effects on PI3K. Activation of nuclear PI3K by PIKE is inhibited by the NGF-stimulated 4.1N translocation to the nucleus. Thus, PIKE physiologically modulates the activation by NGF of nuclear PI3K.

Negative regulation of PI 3-kinase by Ruk, a novel adaptor protein

Class I(A) phosphatidylinositol 3-kinase (PI 3-kinase) is a key component of important intracellular signalling cascades. We have identified an adaptor protein, Ruk(l), which forms complexes with the PI 3-kinase holoenzyme in vitro and in vivo. This interaction involves the proline-rich region of Ruk and the SH3 domain of the p85 alpha regulatory subunit of the class I(A) PI 3-kinase. In contrast to many other adaptor proteins that activate PI 3-kinase, interaction with Ruk(l) substantially inhibits the lipid kinase activity of the enzyme. Overexpression of Ruk(l) in cultured primary neurons induces apoptosis, an effect that could be reversed by co-expression of constitutively activated forms of the p110 alpha catalytic subunit of PI 3-kinase or its downstream effector PKB/Akt. Our data provide evidence for the existence of a negative regulator of the PI 3-kinase signalling pathway that is essential for maintaining cellular homeostasis. Structural similarities between Ruk, CIN85 and CD2AP/CMS suggest that these proteins form a novel family of adaptor molecules that are involved in various intracellular signalling pathways.

SH3 domains and drug design: ligands, structure, and biological function

The ligand binding preferences, structural features, and biological function of SH3 (Src homology 3) domains are discussed. SH3 domains bind "core" Pro-rich peptide ligands (7-9 amino acids in length) in a polyproline II helical conformation in a highly conserved aromatic rich patch on the protein surface (approximately 390 A2). The ligands can interact with the protein in one of two orientations, depending on the position (N- vs C-terminal) of ligand residues binding to the SH3 selectivity pocket. Core SH3 ligands are characterized by relatively weak interactions (KD = 5-100 microM) that show little binding selectivity within SH3 families. Higher affinity, more selective contiguous ligands require additional flanking residues that bind to less conserved portions of the SH3 surface, with corresponding increase in ligand size and complexity. In contrast to peptide ligands, protein ligands of SH3 domains can exploit multiple discontiguous interactions to enhance affinity and selectivity. A protein-SH3 interaction that utilizes unique interactions may permit the design of small high affinity SH3 ligands. At present, the extended nature of the binding site and homologous nature of the core binding region among SH3 domains present key challenges for structure-based drug design.

Signal transduction by the TCR for antigen

The past several years have seen the beginning of a shift in the way that TCR signal transduction is studied. Although many investigators continue to identify new molecules, particularly adaptor proteins, others have attempted to look at signaling events in a larger cellular context. Thus the identification of distinct formations of signaling molecules at junctions between T cells and antigen-presenting cells, the role of the cytoskeleton and the partitioning of molecules into specialized lipid subdomains have been the subjects of many publications. Such concepts are helping to assemble a blueprint of how the myriad adaptors and kinases fit together to effect T cell activation.

Activation of phosphatidylinositol 3-kinase by a complex of p59fyn and the receptor tyrosine kinase Xmrk is involved in malignant transformation of pigment cells

Malignant melanoma in the fish Xiphophorus is induced by overexpression of the Xmrk-oncogene, encoding a subclass I receptor tyrosine kinase. The mutationally activated Xmrk protein triggers constitutive mitogenic signalling in fish melanoma cells. In recent studies we showed that in melanoma cells phosphatidylinositol (PtdIns) 3-kinase, as well as p59fyn, has elevated levels of kinase activity. Both bind directly to different phosphotyrosine residues in the Xmrk receptor C-terminus through their SH2 domains. To analyse the mechanism of regulation of these Xmrk-associated kinases in melanoma we characterized the protein-protein interactions between PtdIns 3-kinase, p59fyn and the Xmrk receptor in detail. A ternary complex in which the p85 subunit of PtdIns 3-kinase is associated with p59fyn as well as with Xmrk was identified. Contrary to complexes described for other receptors, the adaptor protein p120Cbl was not involved in these interactions. Thus, we describe here a new mechanism of activation of PtdIns 3-kinase by a receptor of the epidermal growth factor receptor family in which p59fyn acts as an adaptor as well as an activator of PtdIns 3-kinase. Activation of PtdIns 3-kinase activity by fyn was also found in vivo. The fact that this was only detectable in highly transformed Xmrk overexpressing melanomas but not in benign lesions points to the essential role of the Xmrk receptor in this mechanism of regulation.

Evidence for a functional association between phosphatidylinositol 3-kinase and c-src in the spreading response of osteoclasts to colony-stimulating factor-1

Osteoclasts are bone-resorbing cells whose normal function depends in part upon their ability to migrate over the bone surface to initiate new sites of bone resorption. The growth factor/cytokine, colony-stimulating factor-1 (CSF-1), potently stimulates osteoclast motility, in a c-src-dependent fashion. The intracellular signaling molecules that participate with c-src in CSF-1-induced remodeling of the osteoclast cytoskeleton have not been identified. Here we demonstrate, using the inhibitors wortmannin and LY294002, that activation of phosphatidylinositol 3-kinase (PI3-K) is required for CSF-1-induced spreading in osteoclasts. After CSF-1 treatment of osteoclast-like cells, PI3-K activity associated with the CSF-1 receptor c-fms, is increased, and the 85-kDa regulatory subunit of PI3-K and c-src coimmunoprecipitate. CSF-1 induces redistribution of PI3-K to the periphery of the cell. The association between p85 and c-src is due in part to a direct interaction between the proline-rich sequences of p85 and the SH3 domain of c-src. In vitro, the c-src SH3 domain stimulates PI3-K activity. Taken together, the current data suggest that c-src, via its SH3 domain, may participate in CSF-1-induced activation of PI3-K and that PI3-K and c-src are in the signaling pathway that subserves CSF-1-induced cytoskeletal changes in osteoclasts.