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

Mechanisms of MAdCAM-1 gene expression in human intestinal microvascular endothelial cells

Mucosal addressin cell adhesion molecule-1 (MAdCAM-1) is a homing receptor preferentially expressed on gut-associated endothelial cells that plays a central role in leukocyte traffic into the mucosal immune compartment. Although the molecular mechanisms underlying endothelial ICAM-1 or E-selectin expression have been intensively investigated, the mechanisms that regulate human MAdCAM-1 expression have not been defined. We report MAdCAM-1 gene and protein expression in primary cultures of human intestinal microvascular endothelial cells (HIMEC) that was not demonstrated in human umbilical vein endothelial cells. Similar to ICAM-1 and E-selectin expression, MAdCAM-1 gene expression in HIMEC was inducible with TNF-alpha, IL-1beta, or LPS activation. However, in striking contrast to ICAM-1 and E-selectin expression, MAdCAM-1 mRNA and protein expression in HIMEC was heavily dependent on culture duration and/or cellular density, suggesting a prominent role for cell-cell interaction among these endothelial cells in the expression of the mucosal addressin. MAdCAM-1 expression was inhibited by both SN-50 (NF-kappaB inhibitor) and LY-294002 [phosphatidylinositol 3-kinase (PI3-K) inhibitor], whereas ICAM-1 and E-selectin expression was inhibited by SN-50 but not by LY-294002. The Akt phosphorylation by TNF-alpha or LPS was greater at higher cell density, demonstrating a pattern similar to that of MAdCAM-1 expression. NF-kappaB activation was not affected by cellular density in HIMEC. MAdCAM-1 expression in human gut endothelial cells is regulated by distinct signaling mechanisms involving both NF-kappaB and PI3-K/Akt. These data also suggest that PI3-K/Akt is involved in the gut-specific differentiation of HIMEC, which results in expression of the mucosal addressin MAdCAM-1.

PI3K signaling controls cell fate at many points in B lymphocyte development and activation

Many receptors on diverse cell types activate phosphoinositide 3-kinase (PI3K). The lipid products of PI3K, termed 3-phosphoinositides, regulate numerous cellular processes by recruiting specific proteins to membrane signaling complexes. In the B lymphocyte lineage, PI3K activation is a critical control point at various stages of development, proliferation and differentiation. PI3K signaling is promoted by stimulatory receptors such as surface immunoglobulin, CD40, Toll-like receptors and cytokine receptors, and opposed by the inhibitory receptor FcgammaRIIB1. Genetic dissection of the PI3K pathway in mice has indicated that certain B cell functions are regulated by a limited set of PI3K isoforms and downstream effectors. Here we review our current understanding of how signals are relayed to and from PI3K in B cells.

Phosphoinositides: Older Than We First Thought?

Despite nearly 50 years of study, it is good to see that phosphoinositides are still capable of springing the odd surprise. Signaling by the second messenger phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) was thought to be absent in yeast, but a recent paper now describes the presence of PtdIns(3,4,5)P(3) in Schizosaccharomyces pombe.

PKN3 is required for malignant prostate cell growth downstream of activated PI 3-kinase

Chronic activation of the phosphoinositide 3-kinase (PI3K)/PTEN signal transduction pathway contributes to metastatic cell growth, but up to now effectors mediating this response are poorly defined. By simulating chronic activation of PI3K signaling experimentally, combined with three-dimensional (3D) culture conditions and gene expression profiling, we aimed to identify novel effectors that contribute to malignant cell growth. Using this approach we identified and validated PKN3, a barely characterized protein kinase C-related molecule, as a novel effector mediating malignant cell growth downstream of activated PI3K. PKN3 is required for invasive prostate cell growth as assessed by 3D cell culture assays and in an orthotopic mouse tumor model by inducible expression of short hairpin RNA (shRNA). We demonstrate that PKN3 is regulated by PI3K at both the expression level and the catalytic activity level. Therefore, PKN3 might represent a preferred target for therapeutic intervention in cancers that lack tumor suppressor PTEN function or depend on chronic activation of PI3K.

Microglia release activators of neuronal proliferation mediated by activation of mitogen-activated protein kinase, phosphatidylinositol-3-kinase/Akt and delta-Notch signalling cascades

Microglia, the resident macrophage of the brain, can release substances that aid neuronal development, differentiation and survival. We have investigated the effects of non-activated microglia on the survival of cultured rat cerebellar granule neurones. Microglial-conditioned medium, collected from primary rat microglial cultures, was used to treat 7-day-in-vitro neurones, and neuronal viability and proliferation was assessed following a further 1 or 7 days in culture. Microglial-conditioned medium enhanced neuronal survival by up to 50% compared with untreated neurones and this effect was completely abated by pretreatment of the microglia with l-leucine methyl ester. The expression of the proliferation marker Ki-67 increased in neuronal cultures treated with microglial-conditioned medium suggesting enhanced proliferation of precursor neurones. Microglial-induced neuronal proliferation could be attenuated by specific inhibition of mitogen-activated protein kinase or phosphatidylinositol-3-kinase/Akt signalling pathways, and by selective fractionation and immunodepletion of the microglial-conditioned medium. Activation of the Notch pathway was enhanced as antibody against the Notch ligand, delta-1, prevented the microglial-induced neuronal proliferation. These results show that microglia release stable neurotrophic factors that can promote neuronal precursor cell proliferation.

Involvement of phosphoinositide 3-kinase gamma, Rac, and PAK signaling in chemokine-induced macrophage migration

In macrophages, chemotactic stimuli cause the activation of Rac and PAK, but little is known about the signaling pathways involved and their role in chemotactic gradient sensing. Herein, we report that in macrophages, the chemokine RANTES (regulated on activation normal T cell expressed and secreted)/CCL5 activates the small GTPase Rac and its downstream target PAK2 within seconds. This response depends on Gi activation and largely on the subsequent triggering of phosphoinositide 3-kinase gamma (PI3Kgamma) and Rac. Retroviral transduction of tagged Rac1 and -2 indicates that RANTES/CCL5-mediated activation of PI3Kgamma triggers Rac1 but not Rac2. In agreement, silencing of Rac1 by shRNA blocks PAK2 activity and inhibits RANTES/CCL5-induced macrophage polarization and directional migration. On the other hand, the tyrosine kinase receptor agonist CSF-1 activates PAK2 independently of PI3Kgamma and Rac. Our results thus demonstrate a chemokine-specific signaling pathway in which Gi and PI3Kgamma coordinate to drive Rac1 and PAK2 activation that eventually controls the chemotactic response.

Methyl Selenium-Induced Vascular Endothelial Apoptosis Is Executed by Caspases and Principally Mediated by P38 MAPK Pathway

The induction of vascular endothelial cell apoptosis and inhibition of tumor-associated angiogenesis by selenium may contribute to its cancer chemopreventive effects. Here we examined the stress-activated/mitogen-activated protein kinases (p38 MAPK, ERK1/2) and protein kinase B/AKT as potential signaling mediators for apoptosis induction by a methylselenol precursor methylseleninic acid (MSeA) in human umbilical vein endothelial cells (HUVEC). Time course experiments showed that p38 MAPK hyperphosphorylation and ERK1/2 dephosphorylation occurred before the cleavage of procaspase-3 and poly(ADP-ribose) polymerase (PARP), whereas AKT dephosphorylation occurred after caspase activation. The p38 MAPK inhibitor SB202190 attenuated the MSeA-induced morphological changes and decreased DNA fragmentation and the cleavage of procaspase-3 and PARP in concordant proportions. The general caspase inhibitor zVADfmk completely blocked the MSeA-induced PARP cleavage and DNA fragmentation, whereas zDEVDfmk, an inhibitor for caspase-3-like activities, was nearly as effective for inhibiting apoptosis. In comparison, apoptosis induced by selenite in HUVECs was observed in the complete absence of an activation of the major caspases. Taken together, the data support p38 MAPK as a key upstream mediator for the methylselenol-specific induction of vascular endothelial caspase-dependent apoptosis, which is principally executed by caspase-3-like activities.

Knockdown of MTP18, a Novel Phosphatidylinositol 3-Kinase-dependent Protein, Affects Mitochondrial Morphology and Induces Apoptosis

We identified a novel human cDNA encoding a mitochondrial protein, MTP18 (mitochondrial protein, 18 kDa) as a transcriptional downstream target of phosphatidylinositol (PI) 3-kinase signaling. We demonstrate that MTP18 mRNA as well as protein expression is dependent on PI 3-kinase activity. Confocal microscopy and biochemical fractionation revealed a mitochondrial localization of MTP18. Loss-of-function analysis employing antisense molecules revealed that MTP18 is essential for cell viability in PC-3 and HaCaT cells. We show that knockdown of MTP18 protein level results in a cytochrome c release from mitochondria and consequently leads to apoptosis. In addition, HaCaT cells with reduced levels of MTP18 become more sensitive to apoptotic stimuli. This effect is accompanied by dramatic subcellular alterations. Reduction of MTP18 impairs mitochondrial morphology resulting in the formation of a highly interconnected mitochondrial reticulum in COS-7 cells. Conversely, overexpression of MTP18 induces a punctuate morphology of mitochondria suggesting also a functional role of MTP18 in maintaining the mitochondrial integrity. Hence, our data indicate an unexpected connection of PI 3-kinase signaling, apoptosis and the morphology of mammalian mitochondria.

Signal transduction pathway in endothelial dysfunction

BACKGROUND

Endothelial dysfunction is an important feature of sepsis, acute respiratory distress syndrome (ARDS), and other infectious conditions. Previously, we reported an in vitro model to study endothelial dysfunction, in which endothelial cells are induced to form capillary tube networks by culturing on a basement membrane matrix (Matrigel). In this study, we defined the signal transduction pathways that lead to endothelial cell function and capillary disruption characteristic of sepsis and other infectious conditions.

METHODS

Human aortic endothelial cells (HAEC) were cultured on a laminin-rich matrix to form capillary-like networks. The HAECs were treated with a protein tyrosine phosphatase inhibitor (sodium orthovanadate), a phosphoinositon-3-phosphate inhibitor (wortmannin), or a protein kinase C inhibitor (bisindolylmaleimide) before capillary tubes had formed or after the capillary tubes had matured. The degree of capillary tube formation was quantified by counting the intersection of capillary networks in triplicate wells. Statistical significance was determined by analysis of variance.

RESULTS

Endothelial dysfunction occurred after inhibition of protein tyrosine phosphatase or protein kinase C. Whereas inhibition of phosphoinositon-3-phosphate did not cause endothelial dysfunction, sodium orthovanadate (2-20 microM) and bisindolylmaleimide (2-10 microM) significantly reduced capillary networks. The mean +/- SD of the number of capillary tubes in the control, sodium orthovanadate-treated, and bisindolylmaleimide-treated groups were 251.0 +/- 7.0, 65.6 +/- 9.9 (p < 0.001), and 181.7 +/- 0.1 (p < 0.001), respectively. Sodium orthovanadate (20-200 microM) and bisindolylmaleimide (10-100 microM) inhibited capillary tube formation. At higher concentrations, sodium orthovanadate (> 200 microM) and bisindolylmaleimide (>100 microM) disrupted mature capillary tubes.

CONCLUSIONS

Our results suggest that PKC and protein tyrosine phosphatase play a role in endothelial dysfunction by interfering with the phosphorylation signals within endothelial cells. These mechanisms may be important in the endothelial dysfunction in sepsis and other infectious conditions.

Signaling specificity by Ras family GTPases is determined by the full spectrum of effectors they regulate

Ras family GTPases (RFGs) regulate signaling pathways that control multiple biological processes. How signaling specificity among the closely related family members is achieved is poorly understood. We have taken a proteomics approach to signaling by RFGs, and we have analyzed interactions of a panel of RFGs with a comprehensive group of known and potential effectors. We have found remarkable differences in the ability of RFGs to regulate the various isoforms of known effector families. We have also identified several proteins as novel effectors of RFGs with differential binding specificities to the various RFGs. We propose that specificity among RFGs is achieved by the differential regulation of combinations of effector families as well as by the selective regulation of different isoforms within an effector family. An understanding of this new level of complexity in the signaling pathways regulated by RFGs is necessary to understand how they carry out their many cellular functions. It will also likely have critical implications in the treatment of human diseases such as cancer.

PI3 kinase is important for Ras, MEK and Erk activation of Epo-stimulated human erythroid progenitors

Background

Erythropoietin is a multifunctional cytokine which regulates the number of erythrocytes circulating in mammalian blood. This is crucial in order to maintain an appropriate oxygen supply throughout the body. Stimulation of primary human erythroid progenitors (PEPs) with erythropoietin (Epo) leads to the activation of the mitogenic kinases (MEKs and Erks). How this is accomplished mechanistically remained unclear.

Results

Biochemical studies with human cord blood-derived PEPs now show that Ras and the class Ib enzyme of the phosphatidylinositol-3 kinase (PI3K) family, PI3K gamma, are activated in response to minimal Epo concentrations. Surprisingly, three structurally different PI3K inhibitors block Ras, MEK and Erk activation in PEPs by Epo. Furthermore, Erk activation in PEPs is insensitive to the inhibition of Raf kinases but suppressed upon PKC inhibition. In contrast, Erk activation induced by stem cell factor, which activates c-Kit in the same cells, is sensitive to Raf inhibition and insensitive to PI3K and PKC inhibitors.

Conclusions

These unexpected findings contrast with previous results in human primary cells using Epo at supraphysiological concentrations and open new doors to eventually understanding how low Epo concentrations mediate the moderate proliferation of erythroid progenitors under homeostatic blood oxygen levels. They indicate that the basal activation of MEKs and Erks in PEPs by minimal concentrations of Epo does not occur through the classical cascade Shc/Grb2/Sos/Ras/Raf/MEK/Erk. Instead, MEKs and Erks are signal mediators of PI3K, probably the recently described PI3K gamma, through a Raf-independent signaling pathway which requires PKC activity. It is likely that higher concentrations of Epo that are induced by hypoxia, for example, following blood loss, lead to additional mitogenic signals which greatly accelerate erythroid progenitor proliferation.

Participation of PI3K and ERK1/2 pathways are required for human brain vascular smooth muscle cell migration

Human brain vascular smooth muscle cell (HBVSMC) migration contributes to angiogenesis and several pathological processes in the brain. However, the molecular mechanism of angiogenesis, in which smooth muscle cell contributes, remains unclear. Our study investigates the role of vascular endothelial growth factor (VEGF) in the HBVSMC migration and elucidates the chemotactic signaling pathway mediating this action. We used the in vitro 'scratch' wound method to detect the HBVSMC migration. VEGF(165) (1-40ng/ml) induced the HBVSMC migration in a dose-dependent manner (P<0.05). VEGF(165) does not induce HBVSMC proliferation. Wortmannin, a specific phosphatidylinositol 3-kinase (PI3K) inhibitor, significantly inhibited serine/threonine kinase Akt/protein kinase B (PKB) phosphorylation and reduced HBVSMC migration into the wound edge following VEGF(165) stimulation (P<0.05). PD98059, an extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor, also significantly inhibited ERK1/2 phosphorylation and reduced the numbers of SMC migration. Parallel distance measurement showed that VEGF(165) induced HBVSMC migration significantly reduced due to inhibition of PI3K or ERK1/2 phosphorylation (P<0.05). Our results demonstrate that VEGF(165) could induce HBVSMC migration but not proliferation in vitro. Inhibiting Akt/PKB or ERK1/2 phosphorylation could reduce VEGF(165) induced HBVSMC migration. We provide the first evidence that activation of PI3K or ERK1/2 pathways are a crucial event in VEGF(165) mediated signal transduction leading to HBVSMC migration.

Impaired platelet responses to thrombin and collagen in AKT-1-deficient mice

We investigated the role of Akt-1, one of the major downstream effectors of phosphoinositide 3-kinase (PI3K), in platelet function using mice in which the gene for Akt-1 had been inactivated. Using ex vivo techniques, we showed that Akt-1-deficient mice exhibited impaired platelet aggregation and spreading in response to various agonists. These differences were most apparent in platelets activated with low concentrations of thrombin. Although Akt-1 is not the predominant Akt isoform in mouse platelets, its absence diminished the amount of total phospho-Akt and inhibited increases in intracellular Ca(2+) concentration in response to thrombin. Moreover, thrombin-induced platelet alpha-granule release as well as release of adenosine triphosphate from dense granules was also defective in Akt-1-null platelets. Although the absence of Akt-1 did not influence expression of the major platelet receptors for thrombin and collagen, fibrinogen binding in response to these agonists was significantly reduced. As a consequence of impaired alpha(IIb)beta(3) activation and platelet aggregation, Akt-1 null mice showed significantly longer bleeding times than wild-type mice.

Protein kinase inhibitors: insights into drug design from structure

Protein kinases are targets for treatment of a number of diseases. This review focuses on kinase inhibitors that are in the clinic or in clinical trials and for which structural information is available. Structures have informed drug design and have illuminated the mechanism of inhibition. We review progress with the receptor tyrosine kinases (growth factor receptors EGFR, VEGFR, and FGFR) and nonreceptor tyrosine kinases (Bcr-Abl), where advances have been made with cancer therapeutic agents such as Herceptin and Gleevec. Among the serine-threonine kinases, p38, Rho-kinase, cyclin-dependent kinases, and Chk1 have been targeted with productive results for inflammation and cancer. Structures have provided insights into targeting the inactive or active form of the kinase, for targeting the global constellation of residues at the ATP site or less conserved additional pockets or single residues, and into targeting noncatalytic domains.

Bruton's tyrosine kinase (Btk) enhances transcriptional co-activation activity of BAM11, a Btk-associated molecule of a subunit of SWI/SNF complexes

Bruton's tyrosine kinase (Btk) is required for B cell development and signal transduction through cell-surface molecules such as BCR and IL-5 receptor. We have identified a Btk-associated molecule, BAM11 (hereafter referred to as BAM) that binds to the pleckstrin homology (PH) domain of Btk, and inhibits Btk activity both in vivo and in vitro. In this study, we demonstrate BAM's transcriptional co-activation activity and its functional interaction with Btk. By using transient transcription assays, we demonstrate that the enforced expression of BAM enhances transcriptional activity of the synthetic reporter gene. The C-terminus of BAM is essential for the transcriptional co-activation activity. The ectopic expression of Btk together with BAM enhances BAM's transcriptional co-activation activity. BAM's transcriptional co-activation activity is enhanced through interaction with Btk, and requires both its intact PH domain and functional kinase activity. We also show that enforced expression of TFII-I, another Btk-binding protein with transcriptional activity, together with BAM and Btk, further augments BAM- and Btk-dependent transcriptional co-activation. Furthermore, BAM can be co-immunoprecipitated with the INI1/SNF5 protein, a member of the SWI/SNF complex that remodels chromatin and activates transcription. We propose a model in which Btk regulates gene transcription in B cells by activating BAM and the SWI/SNF transcriptional complex via TFII-I activation.

Two distinct waves of membrane-proximal B cell antigen receptor signaling differentially regulated by Src homology 2-containing inositol polyphosphate 5-phosphatase

The phosphatidylinositol 3-kinase (PI3K) pathway plays a critical role in B cell activation and differentiation. Recruitment of pleckstrin homology (PH) domain-containing signal transduction proteins to the plasma membrane through binding to 3-phosphoinositide second messengers represents a major effector mechanism for PI3Ks. We have found that the PH domains of Bam32 and tandem PH domain-containing protein 2 (TAPP2) specify a temporally distinct wave of membrane recruitment compared with that of Bruton's tyrosine kinase (Btk), with recruitment of these two adaptors representing a later stage of the response. In this study we provide direct evidence that PH domain-dependent recruitment of Btk to the membrane is blocked by coligation of the inhibitory receptor FcgammaRII in human B lymphoma cells. In contrast, recruitment specified by the Bam32 or TAPP2 PH domains is completely insensitive to FcgammaRII inhibition. This differential regulation can be accounted for by Src homology 2-containing inositol polyphosphate 5-phosphatase (SHIP) activity alone, as expression of membrane-targeted SHIP completely abrogated Btk recruitment, but had no inhibitory effect on Bam32 or TAPP2 recruitment. Strikingly, kinetic analysis revealed that membrane recruitment of Bam32 and TAPP2 is actually more rapid under "inhibitory" signaling conditions. Analysis of 3-phosphoinositide generation under activating and inhibitory signaling conditions indicated that recruitment of Bam32 and TAPP2 is inversely correlated with the SHIP substrate/product ratio (phosphatidylinositol 3,4,5-trisphosphate/phosphatidylinositol 3,4-bisphosphate). Overexpression of TAPP2 in B cells led to an increase in the sustained phase of the calcium response and increased NF-AT-dependent transcriptional activation after B cell Ag receptor ligation. Together, these results suggest that Bam32 and TAPP2 adaptors define a novel group of SHIP-activated targets of PI3K that regulate B cell Ag receptor signaling.

Leptin increases cardiomyocyte hyperplasia via extracellular signal-regulated kinase- and phosphatidylinositol 3-kinase-dependent signaling pathways

Obesity is a major risk factor for the development of heart failure. Importantly, it is now appreciated that a change in the number of myocytes is one of multiple structural and functional alterations (remodeling) leading to heart failure. Here we investigate the effect of leptin, the product of the obese (ob) gene, on proliferation of human and murine cardiomyocytes. Leptin caused a time- and dose-dependent significant increase in proliferation of HL-1 cells that was inhibited by preincubation with PD98059 and LY294002, suggesting that leptin mediated proliferation via extracellular signal-regulated kinase-1/2- and phosphatidylinositol-3-kinase-dependent signaling pathways. We confirmed that leptin activates both extracellular signal-regulated kinase-1/2 phosphorylation and association of phosphatidylinositol-3-kinase (regulatory p85 subunit) with phosphotyrosine immunoprecipitates. We also examined bromodeoxyuridine incorporation as a measure of new DNA synthesis and demonstrated a stimulatory effect of leptin in both HL-1 cells and human cardiomyocytes. Bromodeoxyuridine incorporation in HL-1 cells was inhibited by PD98059 and LY294002. Our results establish a mitogenic effect of leptin in cardiomyocytes and provide additional evidence for a potential direct link between leptin and cardiac remodeling in obesity.

Development by Self-Digestion

Autophagy is the major cellular pathway for the degradation of long-lived proteins and cytoplasmic organelles. It involves the rearrangement of subcellular membranes to sequester cargo for delivery to the lysosome where the sequestered material is degraded and recycled. For many decades, it has been known that autophagy occurs in a wide range of eukaryotic organisms and in multiple different cell types during starvation, cellular and tissue remodeling, and cell death. However, until recently, the functions of autophagy in normal development were largely unknown. The identification of a set of evolutionarily conserved genes that are essential for autophagy has opened up new frontiers for deciphering the role of autophagy in diverse biological processes. In this review, we summarize our current knowledge about the molecular machinery of autophagy and the role of the autophagic machinery in eukaryotic development.

Anemia, thrombocytopenia, leukocytosis, extramedullary hematopoiesis, and impaired progenitor function in Pten+/-SHIP-/- mice: a novel model of myelodysplasia

The myeloproliferative disorder of mice lacking the Src homology 2 (SH2)-containing 5' phosphoinositol phosphatase, SHIP, underscores the need for closely regulating phosphatidylinositol 3-kinase (PI3K) pathway activity, and hence levels of phosphatidylinositol species during hematopoiesis. The role of the 3' phosphoinositol phosphatase Pten in this process is less clear, as its absence leads to embryonic lethality. Despite Pten heterozygosity being associated with a lymphoproliferative disorder, we found no evidence of a hematopoietic defect in Pten(+/-) mice. Since SHIP shares the same substrate (PIP(3)) with Pten, we hypothesized that the former might compensate for Pten haploinsufficiency in the marrow. Thus, we examined the effect of Pten heterozygosity in SHIP(-/-) mice, predicting that further dysregulation of PIP(3) metabolism would exacerbate the pheno-type of the latter. Indeed, compared with SHIP(-/-) mice, Pten(+/-)SHIP(-/-) animals developed a myelodysplastic phenotype characterized by increased hepatosplenomegaly, extramedullary hematopoiesis, anemia, and thrombocytopenia. Consistent with a marrow defect, clonogenic assays demonstrated reductions in committed myeloid and megakaryocytic progenitors in these animals. Providing further evidence of a Pten(+/-)SHIP(-/-) progenitor abnormality, reconstitution of irradiated mice with marrows from these mice led to a marked defect in short-term repopulation of peripheral blood by donor cells. These studies suggest that the regulation of the levels and/or ratios of PI3K-derived phosphoinositol species by these 2 phosphatases is critical to normal hematopoiesis.

Functional folding of a cytoplasmic single-chain variable fragment and its use as elutable protein purification tag

The variable fragment (Fv) of the monoclonal B1-8 antibody recognizes 3-nitro-4-hydroxy-phenylacetate (NP) and 5-iodo-NP (NIP) allowing for the affinity purification of the respective B cell antigen receptor with NP-sepharose and its specific elution with NIP-capronic acid (NIPcap). We generated an intracellular single-chain B1-8 Fv (iscFv), fused it to the N-terminus of the regulatory subunit (p85alpha) of phosphatidylinositol-3-kinase (PI3K) (isc-p85alpha), and examined the potential of this iscFv to serve as an intracellular elutable protein purification tag. The isc-p85alpha fusion protein could be specifically affinity-purified from the lysates of transfected Drosophila S2 cells with NP-sepharose and eluted with NIPcap, indicating the functional folding of the iscFv in the reducing environment of the cytosol. Furthermore, co-purification of the catalytic subunit of PI3K (p110) was achieved from lysates of co-transfected S2 cells as well as RBL-2H3 mast cells stably expressing isc-p85alpha. This indicates that the iscFv part of isc-p85alpha does not negatively influence p85alpha folding and interaction with p110. Moreover, successful incorporation of the p85alpha-moiety of isc-p85alpha into endogenous protein complexes in mast cells suggests the use of isc-containing fusion proteins for the native purification, elution, and analysis of intracellular signaling complexes.

Inhibiting Src family tyrosine kinase activity blocks glutamate signalling to ERK1/2 and Akt/PKB but not JNK in cultured striatal neurones

Glutamate receptor activation of mitogen-activated protein (MAP) kinase signalling cascades has been implicated in diverse neuronal functions such as synaptic plasticity, development and excitotoxicity. We have previously shown that Ca2+-influx through NMDA receptors in cultured striatal neurones mediates the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt/protein kinase B (PKB) through a phosphatidylinositol 3-kinase (PI 3-kinase)-dependent pathway. Exposing neurones to the Src family tyrosine kinase inhibitor PP2, but not the inactive analogue PP3, inhibited NMDA receptor-induced phosphorylation of ERK1/2 and Akt/PKB in a concentration-dependent manner, and reduced cAMP response element-binding protein (CREB) phosphorylation. To establish a link between Src family tyrosine kinase-mediated phosphorylation and PI 3-kinase signalling, affinity precipitation experiments were performed with the SH2 domains of the PI 3-kinase regulatory subunit p85. This revealed a Src-dependent phosphorylation of a focal adhesion kinase (FAK)-p85 complex on glutamate stimulation. Demonstrating that PI3-kinase is not ubiquitously involved in NMDA receptor signal transduction, the PI 3-kinase inhibitors wortmannin and LY294002 did not prevent NMDA receptor Ca2+-dependent phosphorylation of c-Jun N-terminal kinase 1/2 (JNK1/2). Further, inhibiting Src family kinases increased NMDA receptor-dependent JNK1/2 phosphorylation, suggesting that Src family kinase-dependent cascades may physiologically limit signalling to JNK. These results demonstrate that Src family tyrosine kinases and PI3-kinase are pivotal regulators of NMDA receptor signalling to ERK/Akt and JNK in striatal neurones.

Role for the pleckstrin homology domain-containing protein CKIP-1 in phosphatidylinositol 3-kinase-regulated muscle differentiation

In this work, we report the implication of the pleckstrin homology (PH) domain-containing protein CKIP-1 in phosphatidylinositol 3-kinase (PI3-K)-regulated muscle differentiation. CKIP-1 is upregulated during muscle differentiation in C2C12 cells. We show that CKIP-1 binds to phosphatidylinositol 3-phosphate through its PH domain and localizes to the plasma membrane in a PI3-K-dependent manner. Activation of PI3-K by insulin or expression of an active form of PI3-K p110 induces a rapid translocation of CKIP-1 to the plasma membrane. Conversely, expression of the 3-phosphoinositide phosphatase myotubularin or PI3-K inhibition by LY294002, wortmannin, or mutant p85 abolishes CKIP-1 binding to the membrane. Upon induction of differentiation in low-serum medium, CKIP-1 overexpression in C2C12 myoblasts first promotes proliferation and then stimulates the expression of myogenin and cell fusion in a manner reminiscent of the dual positive effect of insulin-like growth factors on muscle cells. Interference with the PI3-K pathway impedes the effect of CKIP-1 on C2C12 cell differentiation. Finally, silencing of CKIP-1 by RNA interference abolishes proliferation and delays myogenin expression. Altogether, these data strongly implicate CKIP-1 as a new component of PI3-K signaling in muscle differentiation.

A Ratiometric Expressible FRET Sensor for Phosphoinositides Displays a Signal Change in Highly Dynamic Membrane Structures in Fibroblasts

Phosphoinositides are important signal transduction intermediates in cell growth, survival, and motility. We have invented a fluorescence sensor for polyphosphorylated phosphoinositides based on a peptide derived from the Listeria protein ActA that undergoes a random coil to helix transition upon lipid binding. The sensor, termed CAY, is a fusion protein of cyan and yellow fluorescent proteins flanking the peptide at its N- and C-termini, respectively. CAY displays fluorescence resonance energy transfer in vitro in the absence of phosphorylated phosphoinositides, and this energy transfer is lost upon interaction with these phospholipids. These results demonstrate that a short peptide undergoing a coil to helix transition can be sufficient for the engineering of a FRET-based biosensor. CAY is predominantly localized to the cytoplasm in fibroblasts expressing the sensor but shows loss of fluorescence resonance energy transfer in regions of active actin dynamics such as ruffles that have previously been demonstrated to contain high levels of phosphoinositides.

The B cell SH2/PH domain-containing adaptor Bam32/DAPP1 is required for T cell-independent II antigen responses

BACKGROUND

Bam32/DAPP1 is a B cell adaptor composed of both a PH and an SH2 domain. Previous studies in cell culture and chicken DT40 cells have indicated that Bam32 is critical for normal signaling downstream of the B cell receptor (BCR).

RESULTS

We now study the function of Bam32 in mice in which Bam32 has been disrupted by a viral gene trap approach. Although B and T cell development is normal in Bam32(-/-) mice, B cell proliferation is reduced by about 50% after BCR crosslinking when compared with Bam32(+/+) mice. Differences in the activation of Erk, Jnk and p38 Map kinases, PLCgamma, and Ca(2+) flux do not account for the defect in proliferation as activation was similar in Bam32(+/+) and Bam32(-/-) B cells. Interestingly, whereas antibody response to T-dependent (TD) and T-independent (TI)-I antigens was similar between Bam32(+/+) and Bam32(-/-) mice, TI-II responses were defective in Bam32(-/-) mice; Bam32(-/-) mice failed to undergo isotype class switch recombination (CSR) to produce IgG3 antibodies due to a cell-autonomous defect in generation of IgG3 germline transcripts. The defect in TI-II antigen response led to an impaired antibody response to immunization with type 3 Streptococcus pneumoniae capsular polyschaccharide (PS), resulting in a markedly increased susceptibility to infection by Streptococcus pneumoniae.

CONCLUSIONS

These findings indicate that Bam32 specifically couples an upstream signal to the IgG3 isotype heavy chain CSR and suggest that defects in Bam32 may account for the increased susceptibility to encapusulated organisms in a subset of immunodeficient patients.

Prostaglandin E2 synergistically enhances receptor tyrosine kinase-dependent signaling system in colon cancer cells

Cyclooxygenase (COX)-generated prostaglandin E(2) (PGE(2)) plays critical roles in colorectal carcinogenesis. Recently, we have shown that PGE(2) and transforming growth factor-alpha synergistically induces the expression of amphiregulin (AR) in colon cancer cells (Shao, J., Evers, B. M., and Sheng, H. (2003) Cancer Res. 63, 5218-5223). In this study, we demonstrated synergistic actions of PGE(2) and the receptor tyrosine kinase signaling system in AR expression and in tumorigenic potential of colon cancer cells. Activation of the Ras/Raf/MAPK pathway induced AR transcription in colon cancer LS-174 cells that was enhanced by PGE(2) in a synergistic fashion. The cAMP-responsive element within the AR promoter was required for the synergistic activation of AR transcription. An Sp1 element was responsible for the basal transcription of AR and significantly enhanced the synergy between PGE(2) and the epidermal growth factor receptor (EGFR) signaling system. Furthermore, activation of both PGE(2) and EGFR signaling pathways synergistically promoted the growth and migration of colon cancer cells. Our results suggest that COX-2/PGE(2) may exert pro-oncogenic effects through synergistic induction of receptor tyrosine kinase-dependent signaling pathway, thus, provide a novel mechanism for the combinatorial treatment of colonic neoplasms targeting both COX-2/PGE(2) and the EGFR system that has demonstrated remarkable advantages.

Redox regulation of PI 3-kinase signalling via inactivation of PTEN

The tumour suppressor PTEN is a PtdIns(3,4,5)P(3) phosphatase that regulates many cellular processes through direct antagonism of PI 3-kinase signalling. Here we show that oxidative stress activates PI 3-kinase-dependent signalling via the inactivation of PTEN. We use two assay systems to show that cellular PTEN phosphatase activity is inhibited by oxidative stress induced by 1 mM hydrogen peroxide. PTEN inactivation by oxidative stress also causes an increase in cellular PtdIns(3,4,5)P(3) levels and activation of the downstream PtdIns(3,4,5)P(3) target, PKB/Akt, that does not occur in cells lacking PTEN. We then show that endogenous oxidant production in RAW264.7 macrophages inactivates a fraction of the cellular PTEN, and that this is associated with an oxidant-dependent activation of downstream signalling. These results show that oxidants, including those produced by cells, can activate downstream signalling via the inactivation of PTEN. This demonstrates a novel mechanism of regulation of the activity of this important tumour suppressor and the signalling pathways it regulates. These results may have significant implications for the many cellular processes in which PtdIns(3,4,5)P(3) and oxidants are produced concurrently.

PI3K inhibitors block skeletogenesis but not patterning in sea urchin embryos

Skeletogenesis in the sea urchin embryo is a simple model of biomineralization, pattern formation, and cell-cell communication during embryonic development. The calcium carbonate skeletal spicules are secreted by primary mesenchyme cells (PMCs), but the skeletal pattern is dictated by the embryonic ectoderm. Although the process of skeletogenesis is well characterized, there is little molecular understanding of the basis of patterning within this system. In this study, we examined the contribution of phosphatidylinositide 3-kinase (PI3K)-mediated signaling to the skeletogenic process in sea urchin embryos by using the well-established PI3K inhibitors LY294002 and wortmannin. Our results show that PI3K inhibitors specifically and reversibly block skeletogenesis, and that this blockade occurs within the PMCs rather than in the ectoderm, because the inhibitors block spiculogenesis in cultured micromeres. Our results are consistent with a model in which PI3K signaling is required, not for pattern sensing or interpretation but rather for the biomineralization process itself in the sea urchin embryo.

Retroviral oncogenes and TOR

Retroviruses have recruited the catalytic subunit of PI 3-kinase and its downstream target, Akt, as oncogenes. These viruses cause tumors in animals and induce oncogenic transformation in cell culture. The oncogenicity of these viruses is specifically inhibited by rapamycin; retroviruses carrying other oncogenes are insensitive to this macrolide antibiotic. Rapamycin is an inhibitor of the TOR (target of rapamycin) kinase whose downstream targets include p70 S6 kinase and the negative regulator of translation initiation 4E-BP. Emerging evidence suggests that the TOR signals transmitted to the translational machinery are essential for oncogenic transformation by the PI 3-kinase pathway.