SH2-containing inositol 5'-phosphatase SHIP2 associates with the p130(Cas) adapter protein and regulates cellular adhesion and spreading

Silencer of death domains (SODD) inhibits skeletal muscle and kidney enriched inositol 5-phosphatase (SKIP) and regulates phosphoinositide 3-kinase (PI3K)/Akt signaling to the actin cytoskeleton

Phosphoinositide 3-kinase (PI3K) regulates cell polarity and migration by generating phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) at the leading edge of migrating cells. The serine-threonine protein kinase Akt binds to PI(3,4,5)P(3), resulting in its activation. Active Akt promotes spatially regulated actin cytoskeletal remodeling and thereby directed cell migration. The inositol polyphosphate 5-phosphatases (5-ptases) degrade PI(3,4,5)P(3) to form PI(3,4)P(2), which leads to diminished Akt activation. Several 5-ptases, including SKIP and SHIP2, inhibit actin cytoskeletal reorganization by opposing PI3K/Akt signaling. In this current study, we identify a molecular co-chaperone termed silencer of death domains (SODD/BAG4) that forms a complex with several 5-ptase family members, including SKIP, SHIP1, and SHIP2. The interaction between SODD and SKIP exerts an inhibitory effect on SKIP PI(3,4,5)P(3) 5-ptase catalytic activity and consequently enhances the recruitment of PI(3,4,5)P(3)-effectors to the plasma membrane. In contrast, SODD(-/-) mouse embryonic fibroblasts exhibit reduced Akt-Ser(473) and -Thr(308) phosphorylation following EGF stimulation, associated with increased SKIP PI(3,4,5)P(3)-5-ptase activity. SODD(-/-) mouse embryonic fibroblasts exhibit decreased EGF-stimulated F-actin stress fibers, lamellipodia, and focal adhesion complexity, a phenotype that is rescued by the expression of constitutively active Akt1. Furthermore, reduced cell migration was observed in SODD(-/-) macrophages, which express the three 5-ptases shown to interact with SODD (SKIP, SHIP1, and SHIP2). Therefore, this study identifies SODD as a novel regulator of PI3K/Akt signaling to the actin cytoskeleton.

LyGDI, a novel SHIP-interacting protein, is a negative regulator of FcγR-mediated phagocytosis

SHIP and SHIP-2 are inositol phosphatases that regulate FcγR-mediated phagocytosis through catalytic as well as non-catalytic mechanisms. In this study we have used two-dimensional fluorescence difference gel electrophoresis (DIGE) analysis to identify downstream signaling proteins that uniquely associate with SHIP or SHIP-2 upon FcγR clustering in human monocytes. We identified LyGDI as a binding partner of SHIP, associating inducibly with the SHIP/Grb2/Shc complex. Immunodepletion and competition experiments with recombinant SHIP domains revealed that Grb2 and the proline-rich domain of SHIP were necessary for SHIP-LyGDI association. Functional studies in primary human monocytes showed that LyGDI sequesters Rac in the cytosol, preventing it from localizing to the membrane. Consistent with this, suppression of LyGDI expression resulted in significantly enhanced FcγR-mediated phagocytosis.

The host phosphoinositide 5-phosphatase SHIP2 regulates dissemination of vaccinia virus

After fusing with the plasma membrane, enveloped poxvirus virions form actin-filled membranous protrusions, called tails, beneath themselves and move toward adjacent uninfected cells. While much is known about the host and viral proteins that mediate formation of actin tails, much less is known about the factors controlling release. We found that the phosphoinositide 5-phosphatase SHIP2 localizes to actin tails. Localization requires phosphotyrosine, Abl and Src family tyrosine kinases, and neural Wiskott-Aldrich syndrome protein (N-WASP) but not the Arp2/Arp3 complex or actin. Cells lacking SHIP2 have normal actin tails but release more virus. Moreover, cells infected with viral strains with mutations in the release inhibitor A34 release more virus but recruit less SHIP2 to tails. Thus, the inhibitory effects of A34 on virus release are mediated by SHIP2. Together, these data suggest that SHIP2 and A34 may act as gatekeepers to regulate dissemination of poxviruses when environmental conditions are conducive.

SHIP2 and its involvement in various diseases

IMPORTANCE OF THE FIELD

Inositol polyphosphate 5-phosphatase (SHIP2) is an important negative regulator of intracellular phosphatidylinositol phosphate, a key second messenger of various intracellular signaling pathways. The functional upregulation of SHIP2 results in signaling blockade, leading to related disorders.

AREAS COVERED IN THIS REVIEW

We first summarize the role of SHIP2 in the regulation of insulin signaling and type 2 diabetes, including remarkable advances in pharmacological approaches. In addition, this review highlights new findings regarding the involvement of SHIP2 in a number of diseases, including cancer, neurodegenerative diseases, and atherosclerosis.

WHAT THE READER WILL GAIN

Recently identified small-molecule inhibitors of SHIP2 phosphatase activity emphasize the potential therapeutic value of SHIP2. In addition, currently available evidence demonstrates the importance of the scaffolding-type protein function of SHIP2. Understanding this interesting function will help clarify the complicated involvement of SHIP2 in various disorders.

TAKE HOME MESSAGE

Recent studies have demonstrated that SHIP2 is a promising therapeutic target for not only type 2 diabetes, but also cancer, neurodegenerative diseases, and atherosclerosis. Targeting SHIP2 through specific small-molecule inhibitors will have beneficial effects on these diseases.

Stretchy proteins on stretchy substrates: the important elements of integrin-mediated rigidity sensing

Matrix and tissue rigidity guides many cellular processes, including the differentiation of stem cells and the migration of cells in health and disease. Cells actively and transiently test rigidity using mechanisms limited by inherent physical parameters that include the strength of extracellular attachments, the pulling capacity on these attachments, and the sensitivity of the mechanotransduction system. Here, we focus on rigidity sensing mediated through the integrin family of extracellular matrix receptors and linked proteins and discuss the evidence supporting these proteins as mechanosensors.

p130Cas, Crk-associated substrate plays essential roles in liver development by regulating sinusoidal endothelial cell fenestration

p130Cas, Crk-associated substrate (Cas), is an adaptor/scaffold protein that plays a central role in actin cytoskeletal reorganization. We previously showed that mice in which Cas was deleted (Cas(-/-)) died in utero because of early cardiovascular maldevelopment. To further investigate the in vivo roles of Cas, we generated mice with a hypomorphic Cas allele lacking the exon 2-derived region (Cas(Deltaex2/Deltaex2)), which encodes Src homology domain 3 (SH3) of Cas. Cas(Deltaex2/Deltaex2) mice again died as embryos, but they particularly showed progressive liver degeneration with hepatocyte apoptosis. Because Cas expression in the liver is preferentially detected in sinusoidal endothelial cells (SECs), the observed hepatocyte apoptosis was most likely ascribable to impaired function of SECs. To address this possibility, we stably introduced a Cas mutant lacking the SH3 domain (Cas DeltaSH3) into an SEC line (NP31). Intriguingly, the introduction of Cas DeltaSH3 induced a loss of fenestrae, the characteristic cell-penetrating pores in SECs that serve as a critical route for supplying oxygen and nutrients to hepatocytes. The disappearance of fenestrae in Cas DeltaSH3-expressing cells was associated with an attenuation of actin stress fiber formation, a marked reduction in tyrosine phosphorylation of Cas, and defective binding of Cas to CrkII.

CONCLUSION

Cas plays pivotal roles in liver development through the reorganization of the actin cytoskeleton and formation of fenestrae in SECs.

The inositol 5-phosphatase SHIP2 regulates endocytic clathrin-coated pit dynamics

SHIP2 is recruited early to clathrin-coated pits by the scaffold protein intersectin and dissociates before fission.

Phosphatidylinositol (PI) 4,5-bisphosphate (PI(4,5)P₂) and its phosphorylated product PI 3,4,5-triphosphate (PI(3,4,5)P₃) are two major phosphoinositides concentrated at the plasma membrane. Their levels, which are tightly controlled by kinases, phospholipases, and phosphatases, regulate a variety of cellular functions, including clathrin-mediated endocytosis and receptor signaling. In this study, we show that the inositol 5-phosphatase SHIP2, a negative regulator of PI(3,4,5)P₃-dependent signaling, also negatively regulates PI(4,5)P₂ levels and is concentrated at endocytic clathrin-coated pits (CCPs) via interactions with the scaffold protein intersectin. SHIP2 is recruited early at the pits and dissociates before fission. Both knockdown of SHIP2 expression and acute production of PI(3,4,5)P₃ shorten CCP lifetime by enhancing the rate of pit maturation, which is consistent with a positive role of both SHIP2 substrates, PI(4,5)P₂ and PI(3,4,5)P₃, on coat assembly. Because SHIP2 is a negative regulator of insulin signaling, our findings suggest the importance of the phosphoinositide metabolism at CCPs in the regulation of insulin signal output.

LL5beta directs the translocation of filamin A and SHIP2 to sites of phosphatidylinositol 3,4,5-triphosphate (PtdIns(3,4,5)P3) accumulation, and PtdIns(3,4,5)P3 localization is mutually modified by co-recruited SHIP2

Phosphatidylinositol 3,4,5-triphosphate (PtdIns(3,4,5)P(3)) accumulates at the leading edge of migrating cells and works, at least partially, as both a compass to indicate directionality and a hub for subsequent intracellular events. However, how PtdIns(3,4,5)P(3) regulates the migratory machinery has not been fully elucidated. Here, we demonstrate a novel mechanism for efficient lamellipodium formation that depends on PtdIns(3,4,5)P(3) and the reciprocal regulation of PtdIns(3,4,5)P(3) itself. LL5beta, whose subcellular localization is directed by membrane PtdIns(3,4,5)P(3), recruits the actin-cross-linking protein Filamin A to the plasma membrane, where PtdIns(3,4,5)P(3) accumulates, with the Filamin A-binding Src homology 2 domain-containing inositol polyphosphate 5-phosphatase 2 (SHIP2). A large and dynamic lamellipodium was formed in the presence of Filamin A and LL5beta by the application of epidermal growth factor. Conversely, depletion of either Filamin A or LL5beta or the overexpression of either an F-actin-cross-linking mutant of Filamin A or a mutant of LL5beta without its PtdIns(3,4,5)P(3)-interacting region inhibited such events in COS-7 cells. Because F-actin initially polymerizes near the plasma membrane, it is likely that membrane-recruited Filamin A efficiently cross-links newly polymerized F-actin, leading to enhanced lamellipodium formation at the site of PtdIns(3,4,5)P(3) accumulation. Moreover, we demonstrate that co-recruited SHIP2 dephosphorylates PtdIns(3,4,5)P(3) at the same location.

The SH2 domains of inositol polyphosphate 5-phosphatases SHIP1 and SHIP2 have similar ligand specificity but different binding kinetics

SH2 domain-containing inositol 5-phosphatases 1 (SHIP1) and 2 (SHIP2) are structurally similar proteins that catalyze the degradation of lipid secondary messenger phosphatidylinositol 3,4,5-triphosphate to produce phosphatidylinositol 3,4-diphosphate. Despite their high sequence identity (51%), SHIP1 and SHIP2 share little overlap in their in vivo functions. In this work, the sequence specificity of the SHIP2 SH2 domain was systematically defined through the screening of a combinatorial pY peptide library. Comparison of its specificity profile with that of the SHIP1 SH2 domain showed that the two SH2 domains have similar specificities, both recognizing pY peptides of the consensus sequence pY[S/Y][L/Y/M][L/M/I/V], although there are also subtle differences such as the tolerance of an arginine at the pY + 1 position by the SHIP2 but not SHIP1 SH2 domain. Surface plasmon resonance analysis of their interaction with various pY peptides suggested that the two domains have similar binding affinities but dramatically different binding kinetics, with the SHIP1 SH2 domain having fast association and dissociation rates while the SHIP2 domain showing apparent slow-binding behavior. Site-directed mutagenesis and kinetic studies indicated that the SHIP2 SH2 domain exists as a mixture of two conformational isomers. The major, inactive isomer apparently contains two cis peptidyl-prolyl bonds at positions 88 and 105, whereas the minor, active isomer has both proline residues in their trans configuration. Cis-trans isomerization of the peptidyl-prolyl bonds may provide a potential mechanism for regulating the interaction between SHIP2 and pY proteins. These data suggest that a combination of tissue distribution, specificity, and kinetic differences is likely responsible for their in vivo functional differences.

SHIP2 (SH2 domain-containing inositol phosphatase 2) SH2 domain negatively controls SHIP2 monoubiquitination in response to epidermal growth factor

The SH2 domain containing inositol 5-phosphatase SHIP2 contains several interacting domains that are important for scaffolding properties. We and others have previously reported that SHIP2 interacts with the E3 ubiquitin ligase c-Cbl. Here, we identified human SHIP2 monoubiquitination on lysine 315. SHIP2 could also be polyubiquitinated but was not degraded by the 26 S proteasome. Furthermore, we identified a ubiquitin-interacting motif at the C-terminal end of SHIP2 that confers ubiquitin binding capacity. However, this ubiquitin-interacting motif is dispensable for its monoubiquitination. We showed that neither c-Cbl nor Nedd4-1 play the role of ubiquitin ligase for SHIP2. Strikingly, monoubiquitination of the DeltaSH2-SHIP2 mutant (lacking the N-terminal SH2 domain) is strongly increased, suggesting an intrinsic inhibitory effect of the SHIP2 SH2 domain on its monoubiquitination. Moreover, SHIP2 monoubiquitination was increased upon 30 min of epidermal growth factor stimulation. This correlates with the loss of interaction between the SHIP2 SH2 domain and c-Cbl. In this model, c-Cbl could mask the monoubiquitination site and thereby prevent SHIP2 monoubiquitination. The present study thus reveals an unexpected and novel role of SHIP2 SH2 domain in the regulation of its newly identified monoubiquitination.

Protein phosphatase 2A PR130/B''alpha1 subunit binds to the SH2 domain-containing inositol polyphosphate 5-phosphatase 2 and prevents epidermal growth factor (EGF)-induced EGF receptor degradation sustaining EGF-mediated signaling

To elucidate novel cell biological functions of specific protein phosphatase 2A (PP2A) holoenzymes, we identified and biochemically characterized a complex between the Src homology 2 (SH2) domain-containing inositol polyphosphate 5-phosphatase 2 (SHIP2) and a PP2A holoenzyme comprising PR130/B''alpha1 as a regulatory subunit (PP2A(T130)) in several mammalian cell lines. PR130/B''alpha1 and SHIP2 partially colocalize in untreated HeLa cells, and both translocate to the cell membrane on epidermal growth factor (EGF) stimulation. Concomitantly, a transient EGF-dependent interaction of PR130/B''alpha1 with the EGF receptor (EGFR) was observed, whereas the SHIP2-PR130 interaction remained constitutive. As previously reported for SHIP2, RNA interference-mediated knockdown of PR130 in COS-7 cells resulted in increased EGF-induced proteasome-dependent EGFR degradation, and an increased interaction of EGFR with the E3 ligase c-Cbl. In concordance with faster EGFR clearance or desensitization, intrinsic EGFR kinase activity (phospho-Tyr-1068) and downstream protein kinase B and extracellular signal-regulated kinase/mitogen-activated protein kinase pathways were more rapidly inactivated in PR130-knockdown cells. Notably, these effects could be rescued by reintroduction of RNA interference-resistant Myc-PR130, excluding any off-target effect. These data highlight a novel biological role of the PP2A(T130) holoenzyme in EGF signaling through interaction with EGFR and the phosphatidylinositol (3,4,5)-trisphosphate 5-phosphatase SHIP2. This interaction may be of clinical relevance as dysfunction of EGF-mediated signaling has been linked to various human cancers.

The role of the inositol polyphosphate 5-phosphatases in cellular function and human disease

Phosphoinositides are membrane-bound signalling molecules that regulate cell proliferation and survival, cytoskeletal reorganization and vesicular trafficking by recruiting effector proteins to cellular membranes. Growth factor or insulin stimulation induces a canonical cascade resulting in the transient phosphorylation of PtdIns(4,5)P(2) by PI3K (phosphoinositide 3-kinase) to form PtdIns(3,4,5)P(3), which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) back to PtdIns(4,5)P(2), or by the 5-ptases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). The 5-ptases also hydrolyse PtdIns(4,5)P(2), forming PtdIns4P. Ten mammalian 5-ptases have been identified, which share a catalytic mechanism similar to that of the apurinic/apyrimidinic endonucleases. Gene-targeted deletion of 5-ptases in mice has revealed that these enzymes regulate haemopoietic cell proliferation, synaptic vesicle recycling, insulin signalling, endocytosis, vesicular trafficking and actin polymerization. Several studies have revealed that the molecular basis of Lowe's syndrome is due to mutations in the 5-ptase OCRL (oculocerebrorenal syndrome of Lowe). Futhermore, the 5-ptases SHIP [SH2 (Src homology 2)-domain-containing inositol phosphatase] 2, SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) and 72-5ptase (72 kDa 5-ptase)/Type IV/Inpp5e (inositol polyphosphate 5-phosphatase E) are implicated in negatively regulating insulin signalling and glucose homoeostasis in specific tissues. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. Gene profiling studies have identified changes in the expression of various 5-ptases in specific cancers. In addition, 5-ptases such as SHIP1, SHIP2 and 72-5ptase/Type IV/Inpp5e regulate macrophage phagocytosis, and SHIP1 also controls haemopoietic cell proliferation. Therefore the 5-ptases are a significant family of signal-modulating enzymes that govern a plethora of cellular functions by regulating the levels of specific phosphoinositides. Emerging studies have implicated their loss or gain of function in human disease.

FRET imaging and in silico simulation: analysis of the signaling network of nerve growth factor-induced neuritogenesis

Genetically encoded probes based on Förster resonance energy transfer (FRET) enable us to decipher spatiotemporal information encoded in complex tissues such as the brain. Firstly, this review focuses on FRET probes wherein both the donor and acceptor are fluorescence proteins and are incorporated into a single molecule, i.e. unimolecular probes. Advantages of these probes lie in their easy loading into cells, the simple acquisition of FRET images, and the clear evaluation of data. Next, we introduce our recent study which encompasses FRET imaging and in silico simulation. In nerve growth factor-induced neurite outgrowth in PC12 cells, we found positive and negative signaling feedback loops. We propose that these feedback loops determine neurite-budding sites. We would like to emphasize that it is now time to accelerate crossover research in neuroscience, optics, and computational biology.

Potential molecular mechanism for c-Src kinase-mediated regulation of intestinal cell migration

The ubiquitously expressed Src tyrosine kinases (c-Src, c-Yes, and c-Fyn) regulate intestinal cell growth and differentiation. Src activity is also elevated in the majority of malignant and premalignant tumors of the colon. The development of fibroblasts with the three ubiquitously expressed kinases deleted (SYF cells) has identified the role of Src proteins in the regulation of actin dynamics associated with increased cell migration and invasion. Despite this, unexpectedly nothing is known about the role of the individual Src kinases on intestinal cell cytoskeleton and/or cell migration. We have previously reported that villin, an epithelial cell-specific actin-modifying protein that regulates actin reorganization, cell morphology, cell migration, cell invasion, and apoptosis, is tyrosine-phosphorylated. In this report using the SYF cells reconstituted individually with c-Src, c-Yes, c-Fyn, and wild type or phosphorylation site mutants of villin, we demonstrate for the first time the absolute requirement for c-Src in villin-induced regulation of cell migration. The other major finding of our study is that contrary to previous reports, the nonreceptor tyrosine kinase, Jak3 (Janus kinase 3), does not regulate phosphorylation of villin or villin-induced cell migration and is, in fact, not expressed in intestinal epithelial cells. Further, we identify SHP-2 and PTP-PEST (protein-tyrosine phosphatase proline-, glutamate-, serine-, and threonine-rich sequence) as negative regulators of c-Src kinase and demonstrate a new function for these phosphatases in intestinal cell migration. Together, these data suggest that in colorectal carcinogenesis, elevation of c-Src or down-regulation of SHP-2 and/or PTP-PEST may promote cancer metastases and invasion by regulating villin-induced cell migration and cell invasion.

The docking properties of SHIP2 influence both JIP1 tyrosine phosphorylation and JNK activity

SHIP2 (SH2-containing inositol polyphosphate 5-phosphatase 2) is an ubiquitously expressed phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P(3)) 5-phosphatase which contains various motifs susceptible to mediate protein-protein interaction. In cell models, evidence has been provided that SHIP2 plays a role in insulin and growth factor signaling, cytoskeletal organization, cell adhesion and migration. Herein we describe the c-Jun NH2-terminal kinase (JNK)-interacting protein 1 (JIP1) as a new protein partner of SHIP2. The interaction between SHIP2 and JIP1 was confirmed in both overexpression systems and native cells. Without modifying the association of JIP1 with the MAPKs in the scaffold complex and with no apparent change of Akt phosphorylation, SHIP2 positively modulated the MLK3/JIP1-mediated JNK1 activation. Moreover, SHIP2 positively regulated the tyrosine phosphorylation of JIP1. This up-regulation was prevented by inhibitors of the Src family and Abl kinases, PP2 and Glivec. The effects of SHIP2 on JNK activity and JIP1 tyrosine phosphorylation were independent of the SHIP2 phosphoinositide 5-phosphatase activity, as similar results were obtained when using a SHIP2 catalytic inactive mutant instead of wild-type SHIP2. Together, these data suggest that by its docking properties, SHIP2 can modulate JIP1-mediated JNK pathway signaling.

Functional analysis of Src homology 3-encoding exon (exon 2) of p130Cas in primary fibroblasts derived from exon 2-specific knockout mice

p130Cas (Cas, Crk-associated substrate) is an adaptor molecule composed of a Src homology 3 (SH3) domain, a substrate domain (SD) and a Src binding domain (SBD). The SH3 domain of Cas associates with focal adhesion kinase (FAK), but its role in cellular function has not fully been understood. To address this issue, we established and analyzed primary fibroblasts derived from mice expressing a truncated Cas lacking exon 2, which encodes the SH3 domain (Cas Deltaexon 2). In comparison to wild-type cells, Cas exon 2(Delta/Delta) cells showed reduced motility, which could be due to impaired tyrosine-phosphorylation of FAK and Cas, reduced FAK/Cas/Src/CrkII binding, and also impaired localization of Cas Deltaexon 2 to focal adhesions on fibronectin. In addition, to analyze downstream signaling pathways regulated by Cas exon 2, we performed microarray analyses. Interestingly, we found that a deficiency of Cas exon 2 up-regulated expression of CXC Chemokine Receptor-4 and CC Chemokine Receptor-5, which may be regulated by IkappaBalpha phosphorylation. These results indicate that the SH3-encoding exon of Cas participates in cell motility, tyrosine-phosphorylation of FAK and Cas, FAK/Cas/Src/CrkII complex formation, recruitment of Cas to focal adhesions and regulation of cell motility-associated gene expression in primary fibroblasts.

The association between the SH2-containing inositol polyphosphate 5-Phosphatase 2 (SHIP2) and the adaptor protein APS has an impact on biochemical properties of both partners

SHIP2 (SH2-containing inositol polyphosphate 5-phosphatase 2) is a phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P(3)) 5-phosphatase containing various motifs susceptible to mediate protein-protein interaction. In cell models, SHIP2 negatively regulates insulin signalling through its catalytic PtdIns(3,4,5)P(3) 5-phosphatase activity. We have previously reported that SHIP2 interacts with the c-Cbl associated protein (CAP) and c-Cbl, proteins implicated in the insulin cellular response regulating the small G protein TC10. The first steps of the TC10 pathway are the recruitment and tyrosine phosphorylation by the insulin receptor of the adaptor protein with Pleckstrin Homology and Src Homology 2 domains (APS). Herein, we show that SHIP2 can directly interact with APS in 3T3-L1 adipocytes and in transfected CHO-IR cells (Chinese hamster ovary cells stably transfected with the insulin receptor). Upon insulin stimulation, APS and SHIP2 are recruited to cell membranes as seen by immunofluorescence studies, which is consistent with their interaction. We also observed that SHIP2 negatively regulates APS insulin-induced tyrosine phosphorylation and consequently inhibits APS association with c-Cbl. APS, which specifically interacts with SHIP2, but not PTEN, in turn, increases the PtdIns(3,4,5)P(3) 5-phosphatase activity of SHIP2 in an inositol phosphatase assay. Co-transfection of SHIP2 and APS in CHO-IR cells further increases the inhibitory effect of SHIP2 on Akt insulin-induced phosphorylation. Therefore, the interaction between APS and SHIP2 provides to both proteins potential negative regulatory mechanisms to act on the insulin cascade.

Nonadherent cells switch to a Rac-mediated, SHIP regulated, Akt activation mode for survival

Constitutively active Rac stimulates Akt activity in T lymphocytes cultured in suspension. This regulation contrasts with findings obtained in fibroblasts, endothelial or neuronal cells grown on substrate, where Akt stimulation occurs independently of Rac. We now show that V12Rac-mediated stimulation of Akt is not restricted to the hematopoietic lineage but is dependent on the adherence status of the cell. V12Rac-mediated stimulation of Akt as well as molecular association between Rac and Akt occurred exclusively in cells kept in suspension. Stimulation and complex formation are dependent on SHIP but in a manner that differs from its role in dephosphorylation of phosphoinositide lipids. Adherent cells lacking SHIP, but not those lacking PTEN, are able to activate Akt through the Rac pathway. Our data reveal the existence of a bona fide Rac to Akt signaling pathway, tightly regulated by SHIP and operational in suspended cells only. This pathway may point to an alternative survival signal that is called into action when cells lose contact with the substrate and/or with other cells.

SHIP2 controls PtdIns(3,4,5)P3 levels and PKB activity in response to oxidative stress

Reactive oxygen species (ROS) are known to be involved in redox signalling pathways that may contribute to normal cell function as well as disease progression. The tumour suppressor PTEN and the inositol 5-phosphatase SHIP2 are critical enzymes in the control of PtdIns(3,4,5)P(3) level. It has been reported that oxidants, including those produced in cells such as macrophages, can activate downstream signalling via the inactivation of PTEN. The present study evaluates the potential impact of SHIP2 on phosphoinositides in cells exposed to sodium peroxide. We used a model of SHIP2 deficient mouse embryonic fibroblasts (MEFs) stimulated by H(2)O(2): at 15 min, PtdIns(3,4,5)P(3) was markedly increased in SHIP2 -/- cells as compared to +/+ cells. In contrast, no significant increase in PtdIns(3,4)P(2) could be detected at 15 or 120 min incubation of the cells with H(2)O(2) (0.6 mM). PKB activity was also upregulated in SHIP2 -/- cells as compared to +/+ cells in response to H(2)O(2). SHIP2 add back experiments in SHIP2 -/- cells confirm its critical role as a lipid phosphatase in the control of PtdIns(3,4,5)P(3) level in response to H(2)O(2). We conclude that SHIP2 lipid phosphatase activity plays an important role in the metabolism PtdIns(3,4,5)P(3) which is demonstrated in oxygen stressed cells.

Phosphoinositol phosphatase SHIP2 promotes cancer development and metastasis coupled with alterations in EGF receptor turnover

Phosphoinositol phosphatases are important regulators of signaling pathways relevant to both diabetes and cancer. A 3'-phosphoinositol phosphatase, phosphatase homologous to tensin (PTEN), is both a tumor suppressor and a negative regulator of insulin action. A 5'-phosphoinositol phosphatase, SH2-containing 5'-inositol phosphatase (SHIP2), regulates insulin signaling and its genetic knockout prevents high-fat diet-induced obesity in mice. SHIP2 also regulates cytoskeleton remodeling and receptor endocytosis. This and the fact that both PTEN and SHIP2 act on the same substrate suggest a potential role for SHIP2 in cancer. Here we report that, in direct contrast to PTEN, SHIP2 protein expression is elevated in a number of breast cancer cell lines. RNA interference-mediated silencing of SHIP2 in MDA-231 cells suppresses epidermal growth factor receptor (EGFR) levels by means of enhanced receptor degradation. Furthermore, endogenous SHIP2 in MDA-231 breast cancer cells supports in vitro cell proliferation, increases cellular sensitivity to drugs targeting the EGFR and supports cancer development and metastasis in nude mice. In addition, significantly high proportions (44%; P = 0.0001) of clinical specimens of breast cancer tissues in comparison with non-cancerous breast tissues contain elevated expression of SHIP2 protein. Taken together, our results demonstrate that SHIP2 is a clinically relevant novel anticancer target that links perturbed metabolism to cancer development.

The inositol 5'-phosphatase SHIP-2 negatively regulates IgE-induced mast cell degranulation and cytokine production

Aggregation of the high-affinity IgE receptor (FcepsilonRI) on mast cells initiates signaling pathways leading to degranulation and cytokine release. It has been reported that SHIP-1 negatively regulates FcepsilonRI-triggered pathways but it is unknown whether its homologous protein SHIP-2 has the same function. We have used a lentiviral-based RNA interference technique to obtain SHIP-2 knockdown bone marrow-derived mast cells (BMMCs) and have found that elimination of SHIP-2 results in both increased mast cell degranulation and cytokine (IL-4 and IL-13) gene expression upon FcepsilonRI stimulation. Elimination of SHIP-2 from BMMCs has no effect on FcepsilonRI-triggered calcium flux, tyrosine phosphorylation of MAPKs or in actin depolymerization following activation. Rather, we observe that absence of SHIP-2 results in increased activation of the small GTPase Rac-1 and in enhanced microtubule polymerization upon FcepsilonRI engagement. Coimmunoprecipitation experiments in rat basophilic leukemia (RBL 2H3) cells show that SHIP-2 interacts with the FcepsilonRI beta-chain, Gab2 and Lyn and that unlike SHIP-1, it does not associate with SHC in mast cells. Our results report a negative regulatory role of SHIP-2 on mast cell activation that is calcium independent and distinct from the regulation by SHIP-1.

The PI3K effector Arap3 interacts with the PI(3,4,5)P3 phosphatase SHIP2 in a SAM domain-dependent manner

Arap3 is a phosphoinositide (PI) 3 kinase effector that serves as a GTPase activating protein (GAP) for both Arf and Rho G-proteins. The protein has multiple pleckstrin homology (PH) domains that bind preferentially phosphatidyl-inositol-3,4,5-trisphosphate (PI(3,4,5,)P3) to induce translocation of Arap3 to the plasma membrane upon PI3K activation. Arap3 also contains a Ras association (RA) domain that interacts with the small G-protein Rap1 and a sterile alpha motif (SAM) domain of unknown function. In a yeast two-hybrid screen for new interaction partners of Arap3, we identified the PI 5'-phosphatase SHIP2 as an interaction partner of Arap3. The interaction between Arap3 and SHIP2 was observed with endogenous proteins and shown to be mediated by the SAM domain of Arap3 and SHIP2. In vitro, these two domains show specificity for a heterodimeric interaction. Since it was shown previously that Arap3 has a higher affinity for PI(3,4,5,)P3 than for PI(3,4)P2, we propose that the SAM domain of Arap3 can function to recruit a negative regulator of PI3K signaling into the effector complex.

An essential role for the SHIP2-dependent negative feedback loop in neuritogenesis of nerve growth factor-stimulated PC12 cells

The local accumulation of phosphatidylinositol (3,4,5) trisphosphate (PIP(3)) and resulting activation of Rac1/Cdc42 play a critical role in nerve growth factor (NGF)-induced neurite outgrowth. To further explore the mechanism, we visualized PIP(3), phosphatidylinositol (3,4) bisphosphate, and Rac1/Cdc42 activities by fluorescence resonance energy transfer (FRET) imaging in NGF-stimulated PC12 cells. Based on the obtained FRET images, and with the help of in silico kinetic reaction model, we predicted that PI-5-phosphatase negatively regulates PIP(3) upon NGF stimulation. In agreement with this model, depletion of Src homology 2 domain-containing inositol polyphosphate 5-phosphatase 2 (SHIP2) markedly potentiated NGF-induced Rac1/Cdc42 activation and PIP(3) accumulation and considerably increased the number and the length of neurites in phosphate and tensin homologue-depleted PC12 cells. Further refinement of the computational model predicted Rac1 regulation of PI3-kinase and SHIP2, which was also validated experimentally. We propose that the SHIP2-mediated negative feedback on PIP(3) coordinately works with the PI3-kinase-mediated positive feedback to form an initial protrusive pattern and, later, to punctuate the PIP(3) accumulation to maintain proper neurite outgrowth.

Transcriptional profiling of C2C12 myotubes in response to SHIP2 depletion and insulin stimulation

Phosphoinositide lipids generated at the cell membrane are a key component of a variety of signaling pathways. Among several inositol phosphatases that regulate the availability of signaling phosphoinositide lipids, the type II SH2-domain-containing inositol 5-phosphatase (SHIP2; approved gene symbol Inppl1) is believed to have multiple functions, including the regulation of insulin signaling and cytoskeletal functions. To understand the function of SHIP2 in C2C12 muscle cells, we depleted SHIP2 through the use of RNA interference and analyzed the global effect of SHIP2 depletion on gene expression using Affymetrix microarrays containing approximately 45,000 mouse probe sets. Expression of SHIP2-targeting small-hairpin RNA in differentiated C2C12 muscle cells led to >80% decrease in SHIP2 mRNA and 60-80% decrease in SHIP2 protein, which resulted in significant gene expression changes linked to cytoskeletal functions, including altered expression of adducin-alpha, pallidin, stathmin-like-2, and synaptojanin-2 binding protein. Insulin treatment of C2C12 muscle cells caused transcriptional changes associated with known signaling pathways. However, SHIP2 depletion had no discernible effect on insulin-regulated gene expression. Taken together, our results suggest that SHIP2 is involved in the regulation of cytoskeletal functions, but a large reduction of SHIP2 in C2C12 muscle cells is not sufficient to affect insulin-mediated gene expression.

Regulation of PDGF-stimulated SHIP2 tyrosine phosphorylation and association with Shc in 3T3-L1 preadipocytes

In 3T3-L1 and human preadipocytes, insulin results in the isolated rise in phosphatidylinositol (PI)-3,4,5-P3, whereas PDGF produces PI(3,4)P2 in addition to PI(3,4,5)P3. SH2 domain-containing inositol 5-phosphatase 2 (SHIP2) converts PI(3,4,5)P3 into PI(3,4)P2. PDGF, but not insulin, stimulates SHIP2 tyrosine phosphorylation and its association with Shc in human and 3T3-L1 preadipocytes. We now demonstrate that SHIP2 tyrosine phosphorylation and association with Shc in PDGF-treated 3T3-L1 preadipocytes was reduced by bisindolylmaleimide I (BisI), an inhibitor of conventional/novel protein kinase C (PKC). However, the production of PI(3,4)P2 and PI(3,4,5)P3 by PDGF was unaffected by BisI. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) was not sufficient to induce SHIP2 tyrosine phosphorylation. Furthermore, we identified threonine 958 (T958) as a novel PDGF-responsive SHIP2 phosphorylation site. Mutation of T958 to alanine reduced PDGF-stimulated SHIP2 tyrosine phosphorylation and association with Shc, but did not alter its anti-proliferative effect on preadipocytes. This study demonstrates that SHIP2 tyrosine phosphorylation and Shc association can be regulated by serine/threonine signaling pathways, either indirectly (via PKC), or directly (via T958). Interestingly, the anti-proliferative effect of SHIP2 T958A, as well as another SHIP2 mutant (Y986F, Y987F) that also displays defective tyrosine phosphorylation and Shc association, does not depend on these molecular events.

Lipid phosphatases as a possible therapeutic target in cases of type 2 diabetes and obesity

Phosphatidyl inositol 3-kinase (PI3-kinase) functions as a lipid kinase to produce PI(3,4,5)P(3) from PI(4,5)P(2) in vivo. PI(3,4,5)P(3) is crucial as a lipid second messenger in various metabolic effects of insulin. Lipid phosphatases, src homology 2 domain containing inositol 5'-phosphatase 2 (SHIP2) and skeletal muscle and kidney-enriched inositol phosphatase (SKIP) hydrolyze PI(3,4,5)P(3) to PI(3,4)P(2) and phosphatase and tensin homolog deleted on chromosome ten (PTEN) hydrolyzes PI(3,4,5)P(3) to PI(4,5)P(2). SHIP2 negatively regulates insulin signaling relatively specifically via its 5'-phosphatase activity. Targeted disruption of the SHIP2 gene in mice resulted in increased insulin sensitivity and conferred protection from obesity induced by a high-fat diet. Polymorphisms in the human SHIP2 gene are associated, at least in part, with the insulin resistance of type 2 diabetes. Importantly, inhibition of endogenous SHIP2 through the liver-specific expression of a dominant-negative SHIP2 improves glucose metabolism and insulin resistance in diabetic db/db mice. Overexpression of PTEN and SKIP also inhibited insulin-induced phosphorylation of Akt and the uptake of glucose in cultured cells. Although a homozygous disruption of the PTEN gene in mice results in embryonic lethality, either skeletal muscle or adipose tissue-specific disruption of PTEN ameliorated glucose metabolism without formation of tumors in animal models of diabetes. The role of SKIP in glucose metabolism remains to be further clarified in vivo. Taken together, inhibition of endogenous SHIP2 in the whole body appears to be effective at improving the insulin resistance associated with type 2 diabetes and/or obesity. Inhibition of PTEN in the tissues specifically targeted, including skeletal muscle and fat, may result in an amelioration of insulin resistance in type 2 diabetes, although caution against the formation of tumors is needed.

Lipid phosphatases as drug discovery targets for type 2 diabetes

The soaring incidence of type 2 diabetes has created pressure for new pharmaceutical strategies to treat this devastating disease. With much of the focus on overcoming insulin resistance, investigation has focused on finding ways to restore activation of the phosphatidylinositol 3'-kinase pathway, which is diminished in many patients with type 2 diabetes. Here we review the evidence that lipid phosphatases, specifically PTEN and SHIP2, attenuate this important insulin signalling pathway. Both in vivo and in vitro studies indicate their role in regulating whole-body energy metabolism, and possibly weight gain as well. The promise and challenges presented by this new class of drug discovery targets will also be discussed.

SHIP2 interaction with the cytoskeletal protein Vinexin

The src homology 2 (SH2) domain-containing inositol 5-phosphatase 2 (SHIP2) catalyses the dephosphorylation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] to phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P2]. We report the identification of the cytoskeletal protein Vinexin as a protein interacting with SHIP2. This was achieved by yeast two-hybrid screening using the C-terminal region of SHIP2 as bait. Vinexin has previously been identified as a vinculin-binding protein that plays a key role in cell spreading and cytoskeletal organization. The interaction between SHIP2 and Vinexin was confirmed in lysates of both COS-7 cells and mouse embryonic fibroblasts (MEF). The C-terminus was involved in the interaction, as shown by the transfection of a truncated C-terminus mutant of SHIP2. In addition, we showed the colocalization between Vinexin alpha and SHIP2 at the periphery of transfected COS-7 cells. When added in vitro to SHIP2, Vinexin did not affect the PtdIns(3,4,5)P3 5-phosphatase activity of SHIP2. Enhanced cell adhesion to collagen-I-coated dishes was shown upon transfection of either SHIP2 or Vinexin to COS-7 cells. This effect was no longer observed with either a catalytic mutant or the C-terminus mutant of SHIP2. It also appears SHIP2 specific; this was not seen with SHIP1. Adhesion to the same matrix was decreased in SHIP2-/- MEF cells compared with MEF+/+ cells. Our data suggest that SHIP2 interaction with Vinexin promotes the localization of SHIP2 at the periphery of the cells leaving its catalytic site intact. The complex formation between Vinexin and SHIP2 may increase cellular adhesion. The data reinforce the concept that SHIP2 is active both as a PtdIns(3,4,5)P3 5-phosphatase and as a modulator of focal contact formation.

The SH2 domain containing inositol polyphosphate 5-phosphatase-2: SHIP2

Phosphoinositides are membrane-bound signaling molecules that recruit, activate and localize target effectors to intracellular membranes regulating apoptosis, cell proliferation, insulin signaling and membrane trafficking. The SH2 domain containing inositol polyphosphate 5-phosphatase-2 (SHIP2) hydrolyzes phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) generating phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2). Overexpression of SHIP2 inhibits insulin-stimulated phosphoinositide 3-kinase (PI3K) dependent signaling events. Analysis of diabetic human subjects has revealed an association between SHIP2 gene polymorphisms and type 2 diabetes mellitus. Genetic ablation of SHIP2 in mice has generated conflicting results. SHIP2 knockout mice were originally reported to show lethal neonatal hypoglycemia resulting from insulin hypersensitivity, but in addition to inactivating the SHIP2 gene, the Phox2a gene was also inadvertently deleted. Another SHIP2 knockout mouse has now been generated which inactivates the SHIP2 gene but leaves Phox2a intact. These animals show normal insulin and glucose tolerance but are highly resistant to weight gain on high fat diets, exhibiting an obesity-resistant phenotype. Therefore, SHIP2 remains a significant therapeutic target for the treatment of both obesity and type 2 diabetes.

Crk-associated substrate tyrosine phosphorylation sites are critical for invasion and metastasis of SRC-transformed cells

Crk-associated substrate (CAS, p130Cas) is a major tyrosine phosphorylated protein in cells transformed by v-crk and v-src oncogenes. We recently reported that reexpression of CAS in CAS-deficient mouse embryo fibroblasts transformed by oncogenic Src promoted an invasive phenotype associated with enhanced cell migration through Matrigel, organization of actin into large podosome ring and belt structures, activation of matrix metalloproteinase-2, and elevated tyrosine phosphorylation of the focal adhesion proteins FAK and paxillin. We have now extended these studies to examine the mechanism by which CAS achieves these changes and to evaluate the potential role for CAS in promoting in vivo tumor growth and metastasis. Whereas the presence or absence of CAS did not alter the primary growth of subcutaneous-injected Src-transformed mouse embryo fibroblasts, CAS expression was required to promote lung metastasis following removal of the primary tumor. The substrate domain YxxP tyrosines, the major sites of CAS phosphorylation by Src that mediate interactions with Crk, were found to be critical for promoting both invasive and metastatic properties of the cells. The ability of CAS to promote Matrigel invasion, formation of large podosome structures, and tyrosine phosphorylation of Src substrates, including FAK, paxillin, and cortactin, was also strictly dependent on the YxxP tyrosines. In contrast, matrix metalloproteinase-2 activation was most dependent on the CAS SH3 domain, whereas the substrate domain YxxP sites also contributed to this property. Thus multiple CAS-mediated signaling events are implicated in promoting invasive and metastatic properties of Src-transformed cells.

Anti-adipogenic effect of PDGF is reversed by PKC inhibition

Healthy adipose tissue function depends on adipogenesis. The capacity to form new adipocytes prevents the emergence of insulin-resistant hypertrophied adipocytes, as well as the deleterious lipid deposition in muscle, liver, and pancreas. It is therefore important to understand how adipogenesis is modulated. Platelet-derived growth factor (PDGF) is anti-adipogenic, but the stage of differentiation that it targets, and the signaling pathways that it triggers, are not defined. We have studied the inhibitory effect of PDGF on murine 3T3-L1 preadipocyte and human preadipocyte differentiation. There was a significant attenuation in the protein expression of the adipogenic transcription factors, PPARgamma and C/EBPalpha, as well as in the levels of later differentiation markers, including adiponectin, aP2, and fatty acid synthase. PDGF treatment resulted in the persistence of PDGF receptor and PKCalpha expression, in contrast to the expected downregulation of both proteins that occurs during differentiation. Inactivation of conventional PKC isoforms, by bisindolylmaleimide I or PKC pseudosubstrate M20-28, partially reversed the inhibition of 3T3-L1 and human preadipocyte differentiation by PDGF, as assessed by fatty acid synthase expression and morphological appearance.

The 1.1 A resolution crystal structure of the p130cas SH3 domain and ramifications for ligand selectivity

The Crk-associated tyrosine kinase substrate p130cas (CAS) is a docking protein containing an SH3 domain near its N terminus, followed by a short proline-rich segment, a large central substrate domain composed of 15 repeats of the four amino acid sequence YxxP, a serine-rich region and a carboxy-terminal domain, which possesses consensus binding sites for the SH2 and SH3 domains of Src (YDYV and RPLPSPP, respectively). The SH3 domain of CAS mediates its interaction with several proteins involved in signaling pathways such as focal adhesion kinase (FAK), tyrosine phosphatases PTP1B and PTP-PEST, and the guanine nucleotide exchange factor C3G. As a homolog of the corresponding Src docking domain, the CAS SH3 domain binds to proline-rich sequences (PxxP) of its interacting partners that can adopt a polyproline type II helix. We have determined a high-resolution X-ray structure of the recombinant human CAS SH3 domain. The domain, residues 1-69, crystallized in two related space groups, P2(1) and C222(1), that provided diffraction data to 1.1 A and 2.1 A, respectively. The crystal structure shows, in addition to the conserved SH3 domain architecture, the way in which the CAS characteristic amino acids form an atypically charged ligand-binding surface. This arrangement provides a rationale for the unusual ligand recognition motif exhibited by the CAS SH3 domain. The structure enables modelling of the docking interactions to its ligands, for example from focal adhesion kinase, and supports structure-based drug design of inhibitors of the CAS-FAK interaction.

Phosphatidylinositol 3,4,5-trisphosphate modulation in SHIP2-deficient mouse embryonic fibroblasts

SHIP2, the ubiquitous SH2 domain containing inositol 5-phosphatase, includes a series of protein interacting domains and has the ability to dephosphorylate phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)]in vitro. The present study, which was undertaken to evaluate the impact of SHIP2 on PtdIns(3,4,5)P(3) levels, was performed in a mouse embryonic fibroblast (MEF) model using SHIP2 deficient (-/-) MEF cells derived from knockout mice. PtdIns(3,4,5)P(3) was upregulated in serum stimulated -/- MEF cells as compared to +/+ MEF cells. Although the absence of SHIP2 had no effect on basal PtdIns(3,4,5)P(3) levels, we show here that this lipid was significantly upregulated in SHIP2 -/- cells but only after short-term (i.e. 5-10 min) incubation with serum. The difference in PtdIns(3,4,5)P(3) levels in heterozygous fibroblast cells was intermediate between the +/+ and the -/- cells. In our model, insulin-like growth factor-1 stimulation did not show this upregulation. Serum stimulated phosphoinositide 3-kinase (PI 3-kinase) activity appeared to be comparable between +/+ and -/- cells. Moreover, protein kinase B, but not mitogen activated protein kinase activity, was also potentiated in SHIP2 deficient cells stimulated by serum. The upregulation of protein kinase B activity in serum stimulated cells was totally reversed in the presence of the PI 3-kinase inhibitor LY-294002, in both +/+ and -/- cells. Altogether, these data establish a link between SHIP2 and the acute control of PtdIns(3,4,5)P(3) levels in intact cells.

The SH2-domian-containing inositol 5-phosphatase (SHIP)-2 binds to c-Met directly via tyrosine residue 1356 and involves hepatocyte growth factor (HGF)-induced lamellipodium formation, cell scattering and cell spreading

Recently, evidence has been accumulating that inositol and phosphatidylinositol polyphosphate play important roles in a variety of signal transduction systems including membrane traffic, actin cytoskeleton rearrangement and cell motility. In this paper, we show for the first time that the SH2-domain-containing inositol 5-phosphatase (SHIP)-2 binds directly to the hepatocyte growth factor (HGF/SF) receptor, c-Met, via phosphotyrosine 1356. HGF induces the breakdown of cell junctions and the dispersion of colonies of epithelial cells including MDCK cells. Whereas only few lamellipodia are observed in MDCK cells 2 min after stimulation with HGF, both SHIP-2- and SHIP-1-overexpressing cells form large, broad lamellipodia. The number of lamellipodia is 2-4-fold greater than that of mock-transfected MDCK cells in the same time period and SHIP is found to colocalize with actin at the leading edge. Furthermore, overexpression of a catalytic inactive mutant of SHIP-2 suppresses HGF-potentiated cell scattering and cell spreading, although these mutant-expressing cells form enhanced number of lamellipodia 2 min after HGF stimulation. Interestingly, cells expressing a mutant lacking the proline-rich domain of SHIP-2 at the C-terminal form few lamellipodia, but still spread and scatter upon stimulation with HGF at a reduced rate. These data suggest that phosphatase activity is required for HGF-mediated cell spreading and scattering but not for alteration of lamellipodium formation, while the proline-rich region influences lamellipodium formation. Furthermore, treatment with 10 microM of phosphatidylinositol 3 (PI3) kinase inhibitor, LY294002, abrogates HGF-induced cell scattering of SHIP-2-overexpressing cells but not parental HEK293 cells, suggesting that a balance between PI3 kinase and SHIP is important for cell motility.

SH2-containing 5'-inositol phosphatase, SHIP2, regulates cytoskeleton organization and ligand-dependent down-regulation of the epidermal growth factor receptor

Phosphoinositide lipid second messengers are integral components of signaling pathways mediated by insulin, growth factors, and integrins. SHIP2 dephosphorylates phosphatidylinositol 3,4,5-trisphosphate generated by the activated phosphatidylinositol 3'-kinase. SHIP2 down-regulates insulin signaling and is present at higher levels in diabetes and obesity. SHIP2 associates with p130Cas and filamin, regulators of cell adhesion/migration and cytoskeleton, influencing cell adhesion/spreading. Type I collagen specifically induces Src-mediated tyrosine phosphorylation of SHIP2. To better understand SHIP2 function, we employed RNA interference (RNAi) approach to silence the expression of the endogenous SHIP2 in HeLa cells. Suppression of SHIP2 levels caused severe F-actin deformities characterized by weak cortical actin and peripheral actin spikes. SHIP2 RNAi cells displayed cell-spreading defects involving a notable absence of focal contact structures and the formation of multiple slender membrane protrusions capped by actin spikes. Furthermore, decreased SHIP2 levels altered distribution of early endocytic antigen 1 (EEA1)-positive endocytic vesicles and of vesicles containing internalized epidermal growth factor (EGF) and transferrin. EGF treatment of SHIP2 RNAi cells led to the following: enhanced EGF receptor (EGFR) degradation; increased EGFR ubiquitination; and increased association of EGFR with c-Cbl ubiquitin ligase. Taken together, these experiments demonstrate that SHIP2 functions in the maintenance and dynamic remodeling of actin structures as well as in endocytosis, having a major impact on ligand-induced EGFR internalization and degradation. Accordingly, we suggest that, in HeLa cells, SHIP2 plays a distinct role in signaling pathways mediated by integrins and growth factor receptors.

Regulation of integrin-mediated cellular responses through assembly of a CAS/Crk scaffold

The molecular coupling of CAS and Crk in response to integrin activation is an evolutionary conserved signaling module that controls cell proliferation, survival and migration. However, when deregulated, CAS/Crk signaling also contributes to cancer progression and developmental defects in humans. Here we highlight recent advances in our understanding of how CAS/Crk complexes assemble in cells to modulate the actin cytoskeleton, and the molecular mechanisms that regulate this process. We discuss in detail the spatiotemporal dynamics of CAS/Crk assembly and how this scaffold recruits specific effector proteins that couple integrin signaling networks to the migration machinery of cells. We also highlight the importance of CAS/Crk signaling in the dual regulation of cell migration and survival mechanisms that operate in invasive cells during development and pathological conditions associated with cancer metastasis.

SHIP2 Is Recruited to the Cell Membrane upon Macrophage Colony-Stimulating Factor (M-CSF) Stimulation and Regulates M-CSF-Induced Signaling

The Src homology 2-containing inositol phosphatase SHIP1 functions in hemopoietic cells to limit activation events mediated by PI3K products, including Akt activation and cell survival. In contrast to the limited cellular expression of SHIP1, the related isoform SHIP2, is widely expressed in both parenchymal and hemopoietic cells. The goal of this study was to determine how SHIP2 functions to regulate M-CSF signaling. We report that 1) SHIP2 was tyrosine-phosphorylated in M-CSF-stimulated human alveolar macrophages, human THP-1 cells, murine macrophages, and the murine macrophage cell line RAW264; 2) SHIP2 associated with the M-CSF receptor after M-CSF stimulation; and 3) SHIP2 associated with the actin-binding protein filamin and localization to the cell membrane, requiring the proline-rich domain, but not on the Src homology 2 domain of SHIP2. Analyzing the function of SHIP2 in M-CSF-stimulated cells by expressing either wild-type SHIP2 or an Src homology 2 domain mutant of SHIP2 reduced Akt activation in response to M-CSF stimulation. In contrast, the expression of a catalytically deficient mutant of SHIP2 or the proline-rich domain of SHIP2 enhanced Akt activation. Similarly, the expression of wild-type SHIP2 inhibited NF-kappaB-mediated gene transcription. Finally, fetal liver-derived macrophages from SHIP2 gene knockout mice enhanced activation of Akt in response to M-CSF treatment. These data suggest a novel regulatory role for SHIP2 in M-CSF-stimulated myeloid cells.

TGF-beta1 (transforming growth factor-beta1)-mediated adhesion of gastric carcinoma cells involves a decrease in Ras/ERKs (extracellular-signal-regulated kinases) cascade activity dependent on c-Src activity

Signalling by integrin-mediated cell anchorage to extracellular matrix proteins is co-operative with other receptor-mediated signalling pathways to regulate cell adhesion, spreading, proliferation, survival, migration, differentiation and gene expression. It was observed that an anchorage-independent gastric carcinoma cell line (SNU16) became adherent on TGF-beta1 (transforming growth factor beta1) treatment. To understand how a signal cross-talk between integrin and TGF-beta1 pathways forms the basis for TGF-beta1 effects, cell adhesion and signalling activities were studied using an adherent subline (SNU16Ad, an adherent variant cell line derived from SNU16) derived from the SNU16 cells. SNU16 and SNU16Ad cells, but not integrin alpha5-expressing SNU16 cells, showed an increase in adhesion on extracellular matrix proteins after TGF-beta1 treatment. This increase was shown to be mediated by an integrin alpha3 subunit, which was up-regulated in adherent SNU16Ad cells and in TGF-beta1-treated SNU16 cells, compared with the parental SNU16 cells. After TGF-beta1 treatment of SNU16Ad cells on fibronectin, Tyr-416 phosphorylation of c-Src was increased, but Ras-GTP loading and ERK1/ERK2 (extracellular-signal-regulated kinases 1 and 2) activity were decreased, which showed a dependence on c-Src family kinase activity. Studies on adhesion and signalling activities using pharmacological inhibitors or by transient-transfection approaches showed that inhibition of ERK1/ERK2 activity increased TGF-beta1-mediated cell adhesion slightly, but not the basal cell adhesion significantly, and that c-Src family kinase activity and decrease in Ras/ERKs cascade activity were required for the TGF-beta1 effects. Altogether, the present study indicates that TGF-beta1 treatment causes anchorage-independent gastric carcinoma cells to adhere by an increase in integrin alpha3 level and a c-Src family kinase activity-dependent decrease in Ras/ERKs cascade activity.

SH2-containing inositol 5-phosphatases 1 and 2 in blood platelets: their interactions and roles in the control of phosphatidylinositol 3,4,5-trisphosphate levels

Src homology domain 2-containing inositol 5-phosphatases 1 and 2 (SHIP1 and SHIP2) are capable of dephosphorylating the second messenger PtdIns(3,4,5) P3 (phosphatidylinositol 3,4,5-trisphosphate) and interacting with several signalling proteins. SHIP1 is essentially expressed in haematopoietic cells, whereas SHIP2, a closely related enzyme, is ubiquitous. In the present study, we show that SHIP1 and SHIP2 are expressed as functional PtdIns(3,4,5) P3 5-phosphatases in human blood platelets and are capable of interacting when these two lipid phosphatases are co-expressed, either naturally (platelets and A20 B lymphoma cells) or artificially (COS-7 cells). Using COS-7 cells transfected with deletion mutants of SHIP2, we demonstrate that the Src homology domain 2 of SHIP2 is the minimal and sufficient protein motif responsible for the interaction between the two phosphatases. These results prompted us to investigate the relative importance of SHIP1 and SHIP2 in the control of PtdIns(3,4,5) P3 levels in platelets using homozygous or heterozygous SHIP1- or SHIP2-deficient mice. Our results strongly suggest that SHIP1, rather than SHIP2, plays a major role in controlling PtdIns(3,4,5) P3 levels in response to thrombin or collagen activation of mouse blood platelets.

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

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

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

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

The c-Cbl-associated protein and c-Cbl are two new partners of the SH2-containing inositol polyphosphate 5-phosphatase SHIP2

SHIP2 is a phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)) 5-phosphatase which contains motifs susceptible to mediate protein-protein interaction. Using yeast two-hybrid, GST-pulldown, and coimmunoprecipitation studies, we isolated the CAP cDNA as a specific partner of SHIP2 proline-rich domain and showed by GST-pulldown experiments that the interaction took place with the SH3C of CAP. The interaction was not modulated in COS-7 cells stimulated by EGF neither in CHO cells overexpressing the insulin receptor in the presence or absence of insulin stimulation. We also showed that SHIP2 was able to coimmunoprecipitate with endogenous c-Cbl protein in the absence of CAP and with the insulin receptor in CHO-IR cell extracts. The presence of SHIP2 in a complex around the insulin receptor could account for the very specific increase in insulin sensitivity of SHIP2 knock-out mice.

Src family tyrosine kinases regulate adhesion-dependent tyrosine phosphorylation of 5'-inositol phosphatase SHIP2 during cell attachment and spreading on collagen I

Inositol phosphatases play an important role in regulation of cellular levels of lipid second messengers. Recently we have reported a novel function for SHIP2 in cell adhesion and spreading. In this study, we further characterize the adhesion-dependent tyrosine phosphorylation of SHIP2 and examine the role of Src family tyrosine kinases in the regulation of SHIP2 function. SHIP2 was tyrosine phosphorylated during cell attachment and spreading on collagen I, but not on fibronectin, collagen IV, laminin or poly-L-lysine. SHIP2 tyrosine phosphorylation, induced by plating on a collagen-I-coated surface but not by epidermal growth factor or insulin treatment of cells, was completely blocked by small molecule inhibitors of Src family kinases. SHIP2 could be phosphorylated in vitro by recombinant Src kinase and tyrosines 986-987 in the NPXY motif of SHIP2 appear to be the major sites of phosphorylation for Src both in vitro and in vivo. An activated form of Src induced strong tyrosine phosphorylation of SHIP2 while a dominant-negative form decreased collagen-I-dependent SHIP2 phosphorylation. SHIP2 associated with the adapter protein Shc via its NPXY motif during cell spreading on collagen I in a Src activity-dependent manner. Expression of SHIP2 with mutated NPXY motif caused deregulation of lamellipodia formation during spreading on collagen I. These observations indicate that SHIP2 is regulated by Src family kinases during cell attachment and spreading on collagen I and suggest an important role for SHIP2 as a part of a signaling pathway that regulates actin cytoskeleton remodeling.

SHIP2 overexpression strongly reduces the proliferation rate of K562 erythroleukemia cell line

SHIP2 belongs to the inositol 5-phosphatase family and is characterized by a phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) 5-phosphatase activity. Evidence based on mice lacking the SHIP2 gene has demonstrated its predominant role in the control of insulin sensitivity. However, SHIP2 expression in both hematopoietic and non-hematopoietic cells suggests additional functions. SHIP2 was previously identified in chronic myelogenous progenitor cells, in which its constitutive tyrosine phosphorylation was reported by Wisniewski et al., [Blood 93 (1999) 2707-2720]. Here, we further investigated the function of SHIP2 in this hematopoietic and malignant context. A detailed analysis of the substrate specificity of SHIP2 indicated that this phosphatase is primarily directed towards PI(3,4,5)P(3) both in vitro and in K562 chronic myeloid leukemia cells. The SHIP2-mediated decrease in PI(3,4,5)P(3) levels and increase in phosphatidylinositol 3,4-bisphosphate (PI(3,4)P(2)) was accompanied by a reduction of cell proliferation, characterized by an accumulation of the cells in the G2/M phase of the cell cycle. Thus, in addition to its role in the control of insulin sensitivity, SHIP2 may also play a role in cell proliferation, at least in chronic myelogenous progenitor cells.

PTEN, but not SHIP and SHIP2, suppresses the PI3K/Akt pathway and induces growth inhibition and apoptosis of myeloma cells

Expression of PTEN tumor suppressor gene has been known to dephosphorylate the phosphatidylinositol 3' kinase (PI3K) products on the 3 prime inositol ring, resulting in reduced Akt activation. Loss of PTEN expression in OPM2 and delta47 human myeloma lines led to high Akt activity toward insulin-like growth factor I (IGF-I). In contrast, mouse plasma cell tumor (PCT) lines, expressing wild type PTEN, did not respond to IGF-I for Akt activation. We demonstrated here that endogenous PTEN played a negative role in controlling Akt activity in both mouse PCT and NIH3T3 fibroblast lines by using anti-sense oligonucleotides against PTEN. To determine the role of src-homology 2-containing inositol 5' phosphatase (SHIP) in regulating the PI3K/Akt pathway, we manipulated its expression by down-regulation and overexpression in myeloma, PCT and NIH3T3 lines and analysed Akt activation. Our results showed that SHIP, unlike PTEN, did not affect Akt activity in all systems analysed, despite its ability to dephosphorylate a PI3K product. Although SHIP2 expression resulted in suppression of interleukin-6-mediated mitogen-activated protein kinase activation, expression of SHIP and SHIP2 in a PTEN-null myeloma line did not suppress Akt activity. Biologically, expression of only PTEN, but not SHIP and SHIP2, resulted in growth inhibition and increased apoptosis in OPM2 myeloma line. Together, our results have established the role of PTEN, but not SHIP and SHIP2, in negatively regulating the PI3K/Akt cascade and in myeloma leukemogenesis.

Differential regulation of cell migration, actin stress fiber organization, and cell transformation by functional domains of Crk-associated substrate

The Crk-associated substrate (Cas) is a unique docking protein that possesses a repetitive stretch of tyrosine-containing motifs and an Src homology 3 (SH3) domain. Embryonic fibroblasts lacking Cas demonstrated resistance to Src-induced transformation along with impaired actin bundling and cell motility, indicating critical roles of Cas in actin cytoskeleton organization, cell migration, and oncogenesis. To gain further insight into roles of each domain of Cas in these processes, a compensation assay was performed by expressing a series of Cas mutants in Cas-deficient fibroblasts. The results showed that motifs containing YDxP were indispensable for actin cytoskeleton organization and cell migration, suggesting that CrkII-mediated signaling regulates these biological processes. The C-terminal Src-binding domain played essential roles in cell migration and membrane localization of Cas, although it was dispensable in the organization of actin stress fibers. Furthermore, the Src-binding domain was also a prerequisite for Src transformation possibly, because of its crucial role in the phosphorylation of Cas during transformation. Overall, differential uses of the Cas domains in individual biological processes were demonstrated.