William E. Hughes

Ezrin is a downstream effector of trafficking PKC-integrin complexes involved in the control of cell motility

Protein kinase C (PKC) α has been implicated in β1 integrin‐mediated cell migration. Stable expression of PKCα is shown here to enhance wound closure. This PKC‐driven migratory response directly correlates with increased C‐terminal threonine phosphorylation of ezrin/moesin/radixin (ERM) at the wound edge. Both the wound migratory response and ERM phosphorylation are dependent upon the catalytic function of PKC and are susceptible to inhibition by phosphatidylinositol 3‐kinase blockade. Upon phorbol 12,13‐dibutyrate stimulation, green fluorescent protein–PKCα and β1 integrins co‐sediment with ERM proteins in low‐density sucrose gradient fractions that are enriched in transferrin receptors. Using fluorescence lifetime imaging microscopy, PKCα is shown to form a molecular complex with ezrin, and the PKC‐co‐precipitated endogenous ERM is hyperphosphorylated at the C‐terminal threonine residue, i.e. activated. Electron microscopy showed an enrichment of both proteins in plasma membrane protrusions. Finally, overexpression of the C‐terminal threonine phosphorylation site mutant of ezrin has a dominant inhibitory effect on PKCα‐induced cell migration. We provide the first evidence that PKCα or a PKCα‐associated serine/threonine kinase can phosphorylate the ERM C‐terminal threonine residue within a kinase–ezrin molecular complex in vivo.

The Role of Phosphoinositide 3-Kinase C2α in Insulin Signaling

The members of the class II phosphoinositide 3-kinase (PI3K) family can be activated by several stimuli, indicating that these enzymes can regulate many intracellular processes. Nevertheless, to date, there has been no definitive identification of their in vivo product, their mechanism(s) of activation, or their precise intracellular roles. By metabolic labeling, we here identify phosphatidylinositol 3-phosphate as the sole in vivo product of the insulin-dependent activation of PI3K-C2α, confirming the emerging role of such a phosphoinositide in signaling. We demonstrate that activation of PI3K-C2α involves its recruitment to the plasma membrane and that activation is mediated by the GTPase TC10. This is the first report showing a membrane targeting-mediated mechanism of activation for PI3K-C2α and that a small GTP-binding protein can activate a class II PI3K isoform. We also demonstrate that PI3K-C2α contributes to maximal insulin-induced translocation of the glucose transporter GLUT4 to the plasma membrane and subsequent glucose uptake, definitely assessing the role of this enzyme in insulin signaling.

SAC1 Encodes a Regulated Lipid Phosphoinositide Phosphatase, Defects in Which Can Be Suppressed by the Homologous Inp52p and Inp53p Phosphatases

The yeast protein Sac1p is involved in a range of cellular functions, including inositol metabolism, actin cytoskeletal organization, endoplasmic reticulum ATP transport, phosphatidylinositol-phosphatidylcholine transfer protein function, and multiple-drug sensitivity. The activity of Sac1p and its relationship to these phenotypes are unresolved. We show here that the regulation of lipid phosphoinositides in sac1 mutants is defective, resulting in altered levels of all lipid phos- phoinositides, particularly phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. We have identified two proteins with homology to Sac1p that can suppress drug sensitivity and also restore the levels of the phosphoinositides in sac1mutants. Overexpression of truncated forms of these suppressor genes confirmed that suppression was due to phosphoinositide phosphatase activity within these proteins. We have now demonstrated this activity for Sac1p and have characterized its specificity. The in vitro phosphatase activity and specificity of Sac1p were not altered by some mutations. Indeed, in vivo mutant Sac1p phosphatase activity also appeared unchanged under conditions in which cells were drug-resistant. However, under different growth conditions, both drug sensitivity and the phosphatase defect were manifest. It is concluded that SAC1 encodes a novel lipid phosphoinositide phosphatase in which specific mutations can cause the sac1phenotypes by altering the in vivo regulation of the protein rather than by destroying phosphatase activity.

The Role of Phosphoinositide 3-Kinase C2 in Insulin Signaling *□

The members of the class II phosphoinositide 3-kinase (PI3K) family can be activated by several stimuli, indicating that these enzymes can regulate many intracellular processes. Nevertheless, to date, there has been no definitive identification of their in vivo product, their mechanism(s) of activation, or their precise intracellular roles. By metabolic labeling, we here identify phosphatidylinositol 3-phosphate as the sole in vivo product of the insulin-dependent activation of PI3K-C2α, confirming the emerging role of such a phosphoinositide in signaling. We demonstrate that activation of PI3K-C2α involves its recruitment to the plasma membrane and that activation is mediated by the GTPase TC10. This is the first report showing a membrane targeting-mediated mechanism of activation for PI3K-C2α and that a small GTP-binding protein can activate a class II PI3K isoform. We also demonstrate that PI3K-C2α contributes to maximal insulin-induced translocation of the glucose transporter GLUT4 to the plasma membrane and subsequent glucose uptake, definitely assessing the role of this enzyme in insulin signaling.

Site-Directed Perturbation of Protein Kinase C- Integrin Interaction Blocks Carcinoma Cell Chemotaxis

ABSTRACT Polarized cell movement is an essential requisite for cancer metastasis; thus, interference with the tumor cell motility machinery would significantly modify its metastatic behavior. Protein kinase Cα (PKCα) has been implicated in the promotion of a migratory cell phenotype. We report that the phorbol ester-induced cell polarization and directional motility in breast carcinoma cells is determined by a 12-amino-acid motif (amino acids 313 to 325) within the PKCα V3 hinge domain. This motif is also required for a direct association between PKCα and β1 integrin. Efficient binding of β1 integrin to PKCα requires the presence of both NPXY motifs (Cyto-2 and Cyto-3) in the integrin distal cytoplasmic domains. A cell-permeant inhibitor based on the PKC-binding sequence of β1 integrin was shown to block both PKCα-driven and epidermal growth factor (EGF)-induced chemotaxis. When introduced as a minigene by retroviral transduction into human breast carcinoma cells, this inhibitor caused a striking reduction in chemotaxis towards an EGF gradient. Taken together, these findings identify a direct link between PKCα and β1 integrin that is critical for directed tumor cell migration. Importantly, our findings outline a new concept as to how carcinoma cell chemotaxis is enhanced and provide a conceptual basis for interfering with tumor cell dissemination.

Phospholipid signalling through phospholipase D and phosphatidic acid

Phospholipase D (PLD) hydrolyzes the phosphodiester bond of the predominant membrane phospholipid, phosphatidylcholine producing phosphatidic acid and free choline. This activity can participate in signal transduction pathways and impact on vesicle trafficking for secretion and endocytosis, as well as receptor signalling. Phospholipids can regulate PLD activity directly, through specific intermolecular interactions, or indirectly, through their effect on the localization or activity of PLDs protein effectors. This short review highlights these various phospholipid inputs into the regulation of PLD activity and also reviews potential roles for PLD‐generated phosphatidic acid, particularly a mechanism by which the phospholipid may participate in the process of vesicular trafficking. iubmb Life, 58: 457 ‐ 461, 2006

Annexin A6 stimulates the membrane recruitment of p120GAP to modulate Ras and Raf-1 activity

Annexin A6 is a calcium-dependent membrane-binding protein that interacts with signalling proteins, including the GTPase-activating protein p120GAP, one of the most important inactivators of Ras. Since we have demonstrated that annexin A6 inhibits EGF- and TPA-induced Ras signalling, we investigated whether modulation of Ras activity by annexin A6 was mediated via altered subcellular localization of p120GAP. First, we exploited our observation that high-density lipoproteins (HDL) can activate the Ras/MAP kinase pathway. Expression of annexin A6 caused a significant reduction in HDL-induced activation of Ras and Raf-1. Annexin A6 promoted membrane binding of p120GAP in vitro, and plasma membrane targeting of p120GAP in living cells, both in a Ca2+-dependent manner, which is consistent with annexin A6 promoting the Ca2+-dependent assembly of p120GAP-Ras at the plasma membrane. We then extended these studies to other cell types and stimuli. Expression of annexin A6 in A431 cells reduced, while RNAi-mediated suppression of annexin A6 in HeLa cells enhanced EGF-induced Ras and Erk activation. Importantly, the enhancement of Ras activation following RNAi-mediated reduction in p120GAP levels was more marked in annexin A6-expressing A431 cells than controls, indicating that the effect of annexin A6 on Ras was mediated via p120GAP. Finally, we demonstrated that annexin A6 promotes plasma membrane targeting of p120GAP in A431 cells in response to a variety of stimuli, resulting in colocalization with H-Ras. These findings demonstrate an important role for annexin A6 in regulating plasma membrane localization of p120GAP and hence Ras activity.

A Subset of Interleukin-21+ Chemokine Receptor CCR9+ T Helper Cells Target Accessory Organs of the Digestive System in Autoimmunity

This study describes a CD4+ T helper (Th) cell subset marked by coexpression of the cytokine interleukin 21 (IL-21) and the gut-homing chemokine receptor CCR9. Although CCR9+ Th cells were observed in healthy mice and humans, they were enriched in the inflamed pancreas and salivary glands of NOD mice and in the circulation of Sjögrens syndrome patients. CCR9+ Th cells expressed large amounts of IL-21, inducible T cell costimulator (ICOS), and the transcription factors Bcl6 and Maf, and also supported antibody production from B cells, thereby resembling T follicular B helper (Tfh) cells. However, in contrast to Tfh cells, CCR9+ Th cells displayed limited expression of CXCR5 and the targets of CCR9+ Th cells were CD8+ T cells whose responsiveness to IL-21 was necessary for the development of diabetes. Thus, CCR9+ Th cells are a subset of IL-21-producing T helper cells that influence regional specification of autoimmune diseases that affect accessory organs of the digestive system.

Identification of a distal GLUT4 trafficking event controlled by actin polymerization.

The insulin-stimulated trafficking of GLUT4 to the plasma membrane in muscle and fat tissue constitutes a central process in blood glucose homeostasis. The tethering, docking, and fusion of GLUT4 vesicles with the plasma membrane (PM) represent the most distal steps in this pathway and have been recently shown to be key targets of insulin action. However, it remains unclear how insulin influences these processes to promote the insertion of the glucose transporter into the PM. In this study we have identified a previously uncharacterized role for cortical actin in the distal trafficking of GLUT4. Using high-frequency total internal reflection fluorescence microscopy (TIRFM) imaging, we show that insulin increases actin polymerization near the PM and that disruption of this process inhibited GLUT4 exocytosis. Using TIRFM in combination with probes that could distinguish between vesicle transport and fusion, we found that defective actin remodeling was accompanied by normal insulin-regulated accumulation of GLUT4 vesicles close to the PM, but the final exocytotic fusion step was impaired. These data clearly resolve multiple steps of the final stages of GLUT4 trafficking, demonstrating a crucial role for actin in the final stage of this process.

Three-dimensional cancer models mimic cell–matrix interactions in the tumour microenvironment

Basic in vitro systems can be used to model and assess complex diseases, such as cancer. Recent advances in this field include the incorporation of multiple cell types and extracellular matrix proteins into three-dimensional (3D) models to recapitulate the structure, organization and functionality of live tissue in situ. Cells within such a 3D environment behave very differently from cells on two-dimensional (2D) substrates, as cell-matrix interactions trigger signalling pathways and cellular responses in 3D, which may not be observed in 2D. Thus, the use of 3D systems can be advantageous for the assessment of disease progression over 2D set-ups alone. Here, we highlight the current advantages and challenges of employing 3D systems in the study of cancer and provide an overview to guide the appropriate use of distinct models in cancer research.