5' phospholipid phosphatase SHIP-2 causes protein kinase B inactivation and cell cycle arrest in glioblastoma cells

PI3K/Akt and apoptosis: size matters

Recent research has examined Akt and Akt-related serine-threonine kinases in signaling cascades that regulate cell survival and are important in the pathogenesis of degenerative diseases and in cancer. We seek to recapitulate the research that has helped to define the current understanding of the role of the Akt pathway under normal and pathologic conditions, also in view of genetic models of Akt function. In particular, we will evaluate the mechanisms of Akt regulation and the role of Akt substrates in Akt-dependent biologic responses in the decisions of cell death and cell survival. Here, we hope to establish the mechanisms of apoptosis suppression by Akt kinase as a framework for a more general understanding of growth factor-dependent regulation of cell survival.

Inhibition of phosphatidylinositol 3-kinase dephosphorylates BAD and promotes apoptosis in myeloid leukemias

The phosphatidylinositol 3-kinase (PI3K)/AKT protein kinase pathway is involved in cell growth, proliferation, and apoptosis. The functional activation of PI3K/AKT provides survival signals and blockade of this pathway may facilitate cell death. Downstream targets of PI3K-AKT include the proapoptotic protein BAD, caspase-9, NF-kappaB, and Forkhead. We have previously reported that BAD is constitutively phosphorylated in primary acute myeloid leukemia (AML) cells, a post-transcriptional modification, which inactivates its proapoptotic function. In this study, we tested the hypothesis that the inhibition of PI3K by LY294002 results in the dephosphorylation of AKT and BAD, and thus promote leukemia cell apoptosis. We investigated the effects of LY294002 in megakaryocytic leukemia-derived MO7E cells, primary AML and normal bone marrow progenitor cells. In MO7E cells, LY294002 reduced AKT kinase activity, induced dephosphorylation of AKT and BAD, and increased apoptosis. Concomitant inhibition of mitogen-activated protein kinase signaling or combination with all-trans retinoic acid further enhanced apoptosis of leukemic cells. In primary AML samples, clonogenic cell growth was significantly reduced. Normal hematopoietic progenitors were less affected, suggesting preferential targeting of leukemia cells. In conclusion, the data suggest that the inhibition of the PI3K/AKT signaling pathway restores apoptosis in AML and may be explored as a novel target for molecular therapeutics in AML.

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.

Adenovirus-mediated transfer of the PTEN gene inhibits human colorectal cancer growth in vitro and in vivo

The tumor-suppressor gene PTEN encodes a multifunctional phosphatase that is mutated in a variety of human cancers. PTEN inhibits the phosphatidylinositol 3-kinase pathway and downstream functions, including activation of Akt/protein kinase B (PKB), cell survival, and cell proliferation in tumor cells carrying mutant- or deletion-type PTEN. In such tumor cells, enforced expression of PTEN decreases cell proliferation through cell-cycle arrest at G1 phase accompanied, in some cases, by induction of apoptosis. More recently, the tumor-suppressive effect of PTEN has been reported in ovarian and thyroid tumors that are wild type for PTEN. In the present study, we examined the tumor-suppressive effect of PTEN in human colorectal cancer cells that are wild type for PTEN. Adenoviral-mediated transfer of PTEN (Ad-PTEN) suppressed cell growth and induced apoptosis significantly in colorectal cancer cells (DLD-1, HT29, and SW480) carrying wtPTEN than in normal colon fibroblast cells (CCD-18Co) carrying wtPTEN. This suppression was induced through downregulation of the Akt/PKB pathway, dephosphorylation of focal adhesion kinase (FAK) and mitogen-activated protein kinase (MAPK) and cell-cycle arrest at the G2/M phase, but not the G1 phase. Furthermore, treatment of human colorectal tumor xenografts (HT-29, and SW480) with Ad-PTEN resulted in significant (P=0.01) suppression of tumor growth. These results indicate that Ad-PTEN exerts its tumor-suppressive effect on colorectal cancer cells through inhibition of cell-cycle progression and induction of cell death. Thus Ad-PTEN may be a potential therapeutic for treatment of colorectal cancers.

SHIP-2 and PTEN are expressed and active in vascular smooth muscle cell nuclei, but only SHIP-2 is associated with nuclear speckles

Recently, the control of phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3)-dependant signaling by phosphatases has emerged, but there is a shortage of information on intranuclear PtdIns(3,4,5)P3 phosphatases. Therefore, we investigated the dephosphorylation of [32P]PtdIns(3,4,5)P3 specifically labeled on the D-3 position of the inositol ring in membrane-free nuclei isolated from pig aorta vascular smooth muscle cells (VSMCs). In vitro PtdIns(3,4,5)P3 phosphatase assays revealed the production of both [32P]PtdIns(3,4)P2 and inorganic phosphate, demonstrating the presence of PtdIns(3,4,5)P3 5- and 3-phosphatase activities inside the VSMC nucleus, respectively. Both activities presented the same potency in cellular lysates, whereas the nuclear PtdIns(3,4,5)P3 5-phosphatase activity appeared to be the most efficient. Immunoblot experiments showed for the first time the expression of the 5-phosphatase SHIP-2 (src homology 2 domain-containing inositol phosphatase) as well as the 3-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) in VSMC nuclei. In addition, immunoprecipitations from nuclear fractions indicated a [32P]PtdIns(3,4,5)P3 dephosphorylation by both SHIP-2 and PTEN. Moreover, confocal microscopy analyses demonstrated that SHIP-2 but not PTEN colocalized with a speckle-specific component, the SC35 splicing factor. These results suggest that SHIP-2 may be the primary enzyme for metabolizing PtdIns(3,4,5)P3 into PtdIns(3,4)P2 within the nucleus, thus producing another second messenger, whereas PTEN could down-regulate nuclear phosphoinositide 3-kinase signaling. Finally, intranuclear PtdIns(3,4,5)P3 phosphatases might be involved in the control of VSMC proliferation and the pathogenesis of vascular proliferative disorders.

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.

Dual role of SRC homology domain 2-containing inositol phosphatase 2 in the regulation of platelet-derived growth factor and insulin-like growth factor I signaling in rat vascular smooth muscle cells

Src homology domain 2 (SH2)-containing inositol phosphatase 2 (SHIP2) possesses 5-phosphatase activity and an SH2 domain. The role of SHIP2 in platelet-derived growth factor (PDGF) and IGF-I signaling was studied by expressing wild-type (WT-) and a catalytically defective (Delta IP-) SHIP2 into rat aortic smooth muscle cells by adenovirus-mediated gene transfer. PDGF- and IGF-I-induced tyrosine phosphorylation of their respective receptors and phosphatidylinositol 3-kinase (PI3-kinase) activity were not affected by the expression of either WT- or Delta IP-SHIP2. SHIP2 possessed 5'-phosphatase activity to hydrolyze the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate in vivo. Akt and glycogen synthase kinase 3beta are known to be downstream molecules of PI3-kinase, leading to the antiapoptotic effect. Overexpression of WT-SHIP2 inhibited PDGF- and IGF-I-induced phosphorylation of these molecules and the protective effect of poly(ADP-ribose) polymerase degradation, whereas these phosphorylations and the protective effect were enhanced by the expression of Delta IP-SHIP2, which functions in a dominant negative fashion. Regarding the Ras-MAPK pathway, PDGF- and IGF-I-induced tyrosine phosphorylation of Shc was not affected by the expression of either WT- or Delta IP-SHIP2, whereas both expressed SHIP2 associated with Shc. Importantly, PDGF and IGF-I stimulation of Shc/Grb2 binding, MAPK activation, and 5-bromo-2'-deoxyuridine incorporation were all decreased in both WT- and Delta IP-SHIP2 expression. These results indicate that SHIP2 plays a negative regulatory role in PDGF and IGF-I signaling in vascular smooth muscle cells. As the bifunctional role, our results suggest that SHIP2 regulates PDGF- and IGF-I-mediated signaling downstream of PI3-kinase, leading to the antiapoptotic effect via 5-phosphatase activity, and that SHIP2 regulates the growth factor-induced Ras-MAPK pathway mainly via the SH2 domain.

The phosphatidylinositol (PI)-5-phosphate 4-kinase type II enzyme controls insulin signaling by regulating PI-3,4,5-trisphosphate degradation

Phosphatidylinositol-5-phosphate (PI-5-P) is a newly identified phosphoinositide with characteristics of a signaling lipid but no known cellular function. PI-5-P levels are controlled by the type II PI-5-P 4-kinases (PIP4K IIs), a family of kinases that converts PI-5-P into phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2). The PI-5-P pathway is an alternative route for PI-4,5-P2 synthesis as the bulk of this lipid is generated by the canonical pathway in which phosphatidylinositol-4-phosphate (PI-4-P) is the intermediate. Here we examined the effect of activation of the PI-5-P pathway on phosphoinositide 3-kinase (PI3K) signaling by expressing PIP4K II beta in cells that lack this enzyme. Although PIP4K II generates PI-4,5-P2, a substrate for PI3K, expression of this enzyme reduced rather than increased phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P3) levels in cells stimulated with insulin or cells expressing activated PI3K. This reduction in PI-3,4,5-P3 levels resulted in decreased activation of the downstream protein kinase, Akt/PKB. Consistent with these results, expression of IpgD, a bacterial phosphatase that converts PI-4,5-P2 to PI-5-P, resulted in Akt activation, and this effect was partially reversed by PIP4K II beta. PIP4K II beta expression did not impair insulin-dependent association of PI3K with insulin receptor substrate 1 (IRS1) but abbreviated Akt activation, indicating that PIP4K II regulates PI-3,4,5-P3 degradation rather than synthesis. These data support a model in which the PI-5-P pathway controls insulin signaling that leads to Akt activation by regulating a PI-3,4,5-P3 phosphatase.

SHIP-2 inositol phosphatase is inducibly expressed in human monocytes and serves to regulate Fcgamma receptor-mediated signaling

SHIP-2, a recently identified inositol 5'-phosphatase, shares high level homology with SHIP-1. Although the role of SHIP-1 has been extensively studied, the role of SHIP-2 in myeloid cell functions is not known. Here, we have analyzed the expression patterns, molecular mechanism of activation, and function of SHIP-2 in human myeloid cell Fcgamma receptor (FcgammaR) signaling. We report that SHIP-2 is expressed in transformed myeloid cells and in primary macrophages, but not in peripheral blood monocytes. Treatment of peripheral blood monocytes with bacterial lipopolysaccharide induced expression of SHIP-2 in a dose-dependent manner. FcgammaRIIa clustering in THP-1 cells induced SHIP-2 tyrosine phosphorylation, suggesting a role for SHIP-2 in modulating FcgammaR-mediated function. Consistent with this notion, overexpression of wild-type SHIP-2 (but not catalytically deficient SHIP-2) in THP-1 cells almost completely abrogated NFkappaB-mediated gene transcription in response to FcgammaRIIa clustering. Furthermore, FcgammaRIIa-induced Akt activation was blocked by wild-type SHIP-2, but not by a catalytically deficient mutant of SHIP-2. Additional experiments analyzing the molecular mechanism of SHIP-2 induction by FcgammaRIIa revealed that SHIP-2 associated with the phosphorylated FcgammaRIIa immunoreceptor tyrosine-based activation motif via the SHIP-2 SH2 domain. Thus, an SH2 domain mutant of SHIP-2 failed to associate with FcgammaRIIa or to become tyrosine-phosphorylated upon FcgammaRIIa clustering. Finally, we also demonstrate that SHIP-2 phosphorylation was induced by FcgammaRI clustering in THP-1 cells. These findings unravel a novel level of regulation of FcgammaR-mediated activation of human myeloid cells by the expression and function of the inositol phosphatase SHIP-2.

Reversal of denervation-induced insulin resistance by SHIP2 protein synthesis blockade

Short-term muscle denervation is a reproducible model of tissue-specific insulin resistance. To investigate the molecular basis of insulin resistance in denervated muscle, the downstream signaling molecules of the insulin-signaling pathway were examined in intact and denervated soleus muscle of rats. Short-term denervation induced a significant fall in glucose clearance rates (62% of control, P < 0.05) as detected by euglycemic hyperinsulinemic clamp and was associated with a significant decrease in insulin-stimulated tyrosine phosphorylation of the insulin receptor (IR; 73% of control, P < 0.05), IR substrate 1 (IRS1; 69% of control, P < 0.05), and IRS2 (82% of control, P < 0.05) and serine phosphorylation of Akt (39% of control, P < 0.05). Moreover, denervation reduced insulin-induced association between IRS1/IRS2 and p85/phosphatidylinositol (PI) 3-kinase. Nevertheless, denervation caused an increase in PI 3-kinase activity associated with IRS1 (275%, P < 0.05) and IRS2 (180%, P < 0.05), but the contents of phosphorylated PI detected by HPLC were significantly reduced in lipid fractions. In the face of the apparent discrepancy, we evaluated the expression and activity of the 5-inositol, lipid phosphatase SH2 domain-containing inositol phosphatase (SHIP2), and the serine phosphorylation of p85/PI 3-kinase. No major differences in SHIP2 expression were detected between intact and denervated muscle. However, serine phosphorylation of p85/PI 3-kinase was reduced in denervated muscle, whereas the blockade of SHIP2 expression by antisense oligonucleotide treatment led to partial restoration of phosphorylated PI contents and to improved glucose uptake. Thus modulation of the functional status of SHIP2 may be a major mechanism of insulin resistance induced by denervation.

Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy

The PI3K/Akt signal transduction cascade has been investigated extensively for its roles in oncogenic transformation. Initial studies implicated both PI3K and Akt in prevention of apoptosis. However, more recent evidence has also associated this pathway with regulation of cell cycle progression. Uncovering the signaling network spanning from extracellular environment to the nucleus should illuminate biochemical events contributing to malignant transformation. Here, we discuss PI3K/Akt-mediated signal transduction including its mechanisms of activation, signal transducing molecules, and effects on gene expression that contribute to tumorigenesis. Effects of PI3K/Akt signaling on important proteins controlling cellular proliferation are emphasized. These targets include cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. Furthermore, strategies used to inhibit the PI3K/Akt pathway are presented. The potential for cancer treatment with agents inhibiting this pathway is also addressed.

Profiling of tyrosine phosphorylation pathways in human cells using mass spectrometry

The reversible phosphorylation of tyrosine residues is an important mechanism for modulating biological processes such as cellular signaling, differentiation, and growth, and if deregulated, can result in various types of cancer. Therefore, an understanding of these dynamic cellular processes at the molecular level requires the ability to assess changes in the sites of tyrosine phosphorylation across numerous proteins simultaneously as well as over time. Here we describe a sensitive approach based on multidimensional liquid chromatography/mass spectrometry that enables the rapid identification of numerous sites of tyrosine phosphorylation on a number of different proteins from human whole cell lysates. We used this methodology to follow changes in tyrosine phosphorylation patterns that occur over time during either the activation of human T cells or the inhibition of the oncogenic BCR-ABL fusion product in chronic myelogenous leukemia cells in response to treatment with STI571 (Gleevec). Together, these experiments rapidly identified 64 unique sites of tyrosine phosphorylation on 32 different proteins. Half of these sites have been documented in the literature, validating the merits of our approach, whereas motif analysis suggests that a number of the undocumented sites are also potentially involved in biological pathways. This methodology should enable the rapid generation of new insights into signaling pathways as they occur in states of health and disease.

Growth inhibition of human malignant glioma cells induced by the PI3-K-specific inhibitor

OBJECT

The phosphatase and tensin homolog deleted from chromosome 10 (PTEN) functions as a tumor suppressor by negatively regulating the growth/survival signals of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway. The PI3-K/Akt pathway in PTEN-deficient tumors may be one of the key targets for anticancer therapy. The authors examined the effects of the PI3-K inhibitor 2-(4-morpholinyl)-8-phenylchromone (LY294002) on human malignant glioma cells, and compared these effects on PTEN-deficient cells with those on PTEN-wild-type (PTEN-wt) cells.

METHODS

Using human malignant glioma cell lines, including the PTEN-deficient cells A172 and U87MG and the PTEN-wt cells LN18 and LN229, the effects of LY294002 on cell growth, apoptosis, and chemotherapeutic agent-induced cytotoxicity were evaluated. The LY294002 inhibited the growth of U87MG cells associated with reduced phosphatidylinositol 3,4,5,-trisphosphate and phosphorylated Akt, and also induced growth inhibition in three other cell lines. Although LY294002 caused apoptosis in all four cell lines, apoptosis seemed to contribute to only a small portion of growth inhibition induced by LY294002. There was no link between the status of PTEN and the median inhibitory concentration values for LY294002 or between the gene status and the extent of LY294002-induced apoptosis. The LY294002 significantly augmented the cytotoxicity induced by etoposide in PTEN-deficient cells, but not in PTEN-wt cells. Enhancement of 1,3-bis(2-chloroethyl)-1-nitrosourea- and cisplatin-induced cytotoxicity by LY294002 was not linked to the status of PTEN. No marked difference in the amounts of phosphorylated Akt was found between PTEN-deficient and PTEN-wt cells.

CONCLUSIONS

The findings show that PI3-K is a possible target for therapy in patients with gliomas, and PI3-K inhibitors in combination with chemotherapeutic agents could be potent therapeutic modalities for patients with malignant gliomas.

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.

Association of SH2-containing inositol phosphatase 2 with the insulin resistance of diabetic db/db mice

SH-2-containing inositol 5'-phosphatase 2 (SHIP-2) is a physiologically important lipid phosphatase that functions to hydrolyze phosphatidylinositol (PI) 3-kinase product PI(3,4,5)P3 to PI(3,4)P2 in the negative regulation of insulin signaling. We investigated whether SHIP-2 is associated with the insulin resistance of diabetic db/db mice. The amount of SHIP-2 protein was elevated in quadriceps muscle and epididymal fat tissue, but not in the liver, of db/db mice relative to that in control db/+m mice. In accordance with the enhanced expression of SHIP-2, its localization at the membrane preparation was increased in the skeletal muscle and fat tissue of db/db mice. Insulin stimulation of PI 3-kinase activity was modestly decreased in skeletal muscle, fat tissue, and liver of db/db mice compared with that of db/+m mice. In addition to the modest decrease at the level of PI 3-kinase, the activity of Akt and protein kinase C (PKC)-zeta/lambda, which are downstream molecules of PI 3-kinase, was more severely reduced in the skeletal muscle and fat tissue, but not in liver of db/db mice. Treatment with the insulin-sensitizing agent rosiglitazone decreased the elevated expression of SHIP-2 in the skeletal muscle and fat tissue of db/db mice. Insulin-induced Akt activation and PKC-zeta/lambda phosphorylation were restored to the control level, although insulin-stimulated PI 3-kinase activation was minimally affected in the skeletal muscle and fat tissue of db/db mice. These results indicate that SHIP-2 is a novel molecule associated with insulin resistance in the skeletal muscle and fat tissue, and that insulin-induced activity of the downstream molecules of PI 3-kinase is decreased, at least in part, by the elevated expression of SHIP-2 in diabetic db/db mice.

The protein kinase B/Akt signalling pathway in human malignancy

Protein kinase B or Akt (PKB/Akt) is a serine/threonine kinase, which in mammals comprises three highly homologous members known as PKBalpha (Akt1), PKBbeta (Akt2), and PKBgamma (Akt3). PKB/Akt is activated in cells exposed to diverse stimuli such as hormones, growth factors, and extracellular matrix components. The activation mechanism remains to be fully characterised but occurs downstream of phosphoinositide 3-kinase (PI-3K). PI-3K generates phosphatidylinositol-3,4,5-trisphosphate (PIP(3)), a lipid second messenger essential for the translocation of PKB/Akt to the plasma membrane where it is phosphorylated and activated by phosphoinositide-dependent kinase-1 (PDK-1) and possibly other kinases. PKB/Akt phosphorylates and regulates the function of many cellular proteins involved in processes that include metabolism, apoptosis, and proliferation. Recent evidence indicates that PKB/Akt is frequently constitutively active in many types of human cancer. Constitutive PKB/Akt activation can occur due to amplification of PKB/Akt genes or as a result of mutations in components of the signalling pathway that activates PKB/Akt. Although the mechanisms have not yet been fully characterised, constitutive PKB/Akt signalling is believed to promote proliferation and increased cell survival and thereby contributing to cancer progression. This review surveys recent developments in understanding the mechanisms and consequences of PKB/Akt activation in human malignancy.

PTEN: The down side of PI 3-kinase signalling

The PTEN tumour suppressor protein is a phosphoinositide 3-phosphatase that, by metabolising phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3)), acts in direct antagonism to growth factor stimulated PI 3-kinases. A wealth of data has now illuminated pathways that can be controlled by PTEN through PtdIns(3,4,5)P(3), some of which, when deregulated, give a selective advantage to tumour cells. Early studies of PTEN showed that its activity was able to promote cell cycle arrest and apoptosis and inhibit cell motility, but more recent data have identified other functional consequences of PTEN action, such as effects on the regulation of angiogenesis. The structure of PTEN includes several features not seen in related protein phosphatases, which adapt the enzyme to act efficiently as a lipid phosphatase, including a C2 domain tightly associated with the phosphatase domain, and a broader and deeper active site pocket. Several pieces of data indicate that PTEN is a principal regulator of the cellular levels of PtdIns(3,4,5)P(3), but work is only just beginning to uncover mechanisms by which the cellular activity of PTEN can be controlled. There also remains the vexing question of whether any of PTEN's cellular functions reflect its evolutionary roots as a member of the protein tyrosine phosphatase superfamily.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

Phosphoinositide-specific inositol polyphosphate 5-phosphatase IV inhibits Akt/protein kinase B phosphorylation and leads to apoptotic cell death

Phosphoinositide-specific inositol polyphosphate 5- phosphatase IV has the affinity for PI(3,4,5)P(3) (K(m) = 0.65 microM) that is approximately 10-fold greater than the other inositol polyphosphate 5-phosphatases, which use this substrate including SHIP, OCRL, and 5ptase II, suggesting that it may be important in controlling intracellular levels of this metabolite. We created cell lines stably expressing the enzyme to study its effect on cell function. We found that overexpression of 5ptase IV in 293 cells caused the rapid depletion of both PI(4,5)P(2) and PI(3,4,5)P(3) in cells with corresponding increases in the products, PI(4)P and PI(3,4)P(2), changing the balance of two phosphoinositol products of phosphoinositide 3-kinase, PI(3,4)P(2) and PI(3,4,5)P(3), in the cell. One of the targets of these phosphoinositides is the serine/threonine kinase Akt, which plays an important role in the control of apoptosis. We were able to address the relative roles of PI(3,4)P(2) and PI(3,4,5)P(3) in the activation of Akt by selective depletion of these phosphoinositides in cells stably transfected with 5ptase IV and inositol polyphosphate 4-phosphatase (4ptase I). In cells transfected with 4ptase I, the level of PI(3,4)P(2) was reduced, and PI(3,4,5)P(3) was increased. Expression of the two enzymes had the opposite effect on the phosphorylation of Akt in response to stimulation with growth factors or heat shock. Akt phosphorylation was inhibited in cells expressing 5ptase IV but increased in 4ptase I cells and correlated with the intracellular level of PI(3,4,5)P(3) and not that of PI(3,4)P(2). The inhibition of Akt phosphorylation in cells expressing 5ptase IV makes them highly susceptible to FAS-induced apoptosis, whereas overexpressing of the 4ptase I protects cells from apoptosis. Our results place 5ptase IV as a relevant biological regulator of PI3K/Akt pathway in cells.

Phosphatidylinositol 3-kinase is essential for the proliferation of lymphoblastoid cells

B-cell lymphoma, which is increasing world wide, includes such varied conditions as post-transplant lymphoproliferative disease (PTLD) and Burkitt's lymphoma. This study has characterized a role for the signalling molecule phosphatidylinositol 3-kinase, PI3K, in the regulation of growth and survival of immortalized B-lymphocytes. Burkitt's lymphoma cells die rapidly following inhibition of PI3K with LY294002, a chemical inhibitor. Furthermore, Epstein-Barr virus (EBV) immortalized B-cells, lymphoblastoid cell lines, which are a model of PTLD, do not die but are growth inhibited. This growth inhibition is due to an accumulation at G1 phase of the cell cycle and is paralleled by a loss of E2F transcriptional activity, which is essential for cell cycle entry. An active form of PI3K promotes E2F transcriptional activity in lymphoblastoid cell lines. Treatment of LCL with LY294002 causes a reduction of the expression of both cyclin D2 and cyclin D3, two key cyclins required for cell cycle progression but does not affect the expression of the EBV latent genes, EBNA2A or LMP-1. LY294002 also causes an increase in p27kip1, a cyclin dependent kinase inhibitor and results in the dephosphorylation of members of the pocket protein family. These data describe a mechanism by which PI3K plays a role in B-lymphocyte growth and suggests that a pathway from PI3K to D-type cyclin expression may provide diagnostic or treatment opportunities.

Thrombopoietin: a pan-hematopoietic cytokine

The recent discovery of thrombopoietin has enhanced our understanding of both hematopoiesis and platelet production. Thrombopoietin supports hematopoietic stem cell survival and expansion as well as promoting all aspects of megakaryocyte development. The hormone displays many structural similarities to other members of the hematopoietic cytokine family and some notable differences, and regulation of its expression requires both receptor-mediated removal and other mechanisms. Thrombopoietin induces receptor dimerization and tyrosine phosphorylation, and a series of signaling events including activation of JAK/STAT, Shc/Ras/MAPK and PI3K/Akt; these pathways overlap with those induced by other cytokines, but the differences that lead to the unique biological effects of the hormone are gradually being uncovered. Our growing appreciation of how cytokine signaling pathways are translated into megakaryocyte development is discussed.

Tyrosine phosphorylation mapping of the epidermal growth factor receptor signaling pathway

Phosphorylation is one of the most common forms of protein modification. The most frequent targets for protein phosphorylation in eukaryotes are serine and threonine residues, although tyrosine residues also undergo phosphorylation. Many of the currently applied methods for the detection and localization of protein phosphorylation sites are mass spectrometry-based and are biased against the analysis of tyrosine-phosphorylated residues because of the stability and low reactivity of phosphotyrosines. To overcome this lack of sensitive methods for the detection of phosphotyrosine-containing peptides, we have recently developed a method that is not affected by the more predominant threonine or serine phosphorylation within cells. It is based on the specific detection of immonium ion of phosphotyrosine at 216.043 Da and does not require prior knowledge of the protein sequence. In this report, we describe the first application of this new method in a proteomic strategy. Using anti-phosphotyrosine antibodies for immunoprecipitation and one-dimensional gel electrophoresis, we have identified 10 proteins in the epidermal growth factor receptor signaling pathway, of which 8 have been shown previously to be involved in epidermal growth factor signaling. Most importantly, in addition to several known tyrosine phosphorylation sites, we have identified five novel sites on SHIP-2, Hrs, Cbl, STAM, and STAM2, most of which were not predicted to be phosphorylated. Because of its sensitivity and selectivity, this approach will be useful in proteomic approaches to study tyrosine phosphorylation in a number of signal transduction pathways.

The SH2-containing inositol polyphosphate 5-phosphatase, SHIP-2, binds filamin and regulates submembraneous actin

SHIP-2 is a phosphoinositidylinositol 3,4,5 trisphosphate (PtdIns[3,4,5]P3) 5-phosphatase that contains an NH2-terminal SH2 domain, a central 5-phosphatase domain, and a COOH-terminal proline-rich domain. SHIP-2 negatively regulates insulin signaling. In unstimulated cells, SHIP-2 localized in a perinuclear cytosolic distribution and at the leading edge of the cell. Endogenous and recombinant SHIP-2 localized to membrane ruffles, which were mediated by the COOH-terminal proline-rich domain. To identify proteins that bind to the SHIP-2 proline-rich domain, yeast two-hybrid screening was performed, which isolated actin-binding protein filamin C. In addition, both filamin A and B specifically interacted with SHIP-2 in this assay. SHIP-2 coimmunoprecipitated with filamin from COS-7 cells, and association between these species did not change after epidermal growth factor stimulation. SHIP-2 colocalized with filamin at Z-lines and the sarcolemma in striated muscle sections and at membrane ruffles in COS-7 cells, although the membrane ruffling response was reduced in cells overexpressing SHIP-2. SHIP-2 membrane ruffle localization was dependent on filamin binding, as SHIP-2 was expressed exclusively in the cytosol of filamin-deficient cells. Recombinant SHIP-2 regulated PtdIns(3,4,5)P3 levels and submembraneous actin at membrane ruffles after growth factor stimulation, dependent on SHIP-2 catalytic activity. Collectively these studies demonstrate that filamin-dependent SHIP-2 localization critically regulates phosphatidylinositol 3 kinase signaling to the actin cytoskeleton.

PTEN does not modulate GLUT4 translocation in rat adipose cells under physiological conditions

PTEN is a 3'-inositol lipid phosphatase that dephosphorylates products of PI 3-kinase. Since PI 3-kinase is required for many metabolic actions of insulin, we investigated the role of PTEN in insulin-stimulated translocation of GLUT4. In control rat adipose cells, we observed a approximately 2-fold increase in cell surface GLUT4 upon maximal insulin stimulation. Overexpression of wild-type PTEN abolished this response to insulin. Translocation of GLUT4 in cells overexpressing PTEN mutants without lipid phosphatase activity was similar to that observed in control cells. Overexpression of PTEN-CBR3 (mutant with disrupted membrane association domain) partially impaired translocation of GLUT4. In Cos-7 cells, overexpression of wild-type PTEN had no effect on ERK2 phosphorylation in response to acute insulin stimulation. However, Elk-1 phosphorylation in response to chronic insulin treatment was significantly decreased. Thus, when PTEN is overexpressed, both its lipid phosphatase activity and subcellular localization play a role in antagonizing metabolic actions of insulin that are dependent on PI 3-kinase but independent of MAP kinase. However, because translocation of GLUT4 in cells overexpressing a dominant inhibitory PTEN mutant (C124S) was similar to that of control cells, we conclude that endogenous PTEN may not modulate metabolic functions of insulin under normal physiological conditions.

PKB/AKT: functional insights from genetic models

Since its discovery 10 years ago, the potential functions of protein kinase B (PKB)/AKT have been catalogued with increasing efficiency. The physiological relevance of some of the proposed mechanisms by which PKB/AKT mediates many of its effects has been questioned, and recent work using new reagents and approaches has revealed some cracks in our understanding of this important molecule, and also hinted that these effects may involve other players.

Targeting mutants of PTEN reveal distinct subsets of tumour suppressor functions

The tumour suppressor protein PTEN (phosphatase and tensin homolog deleted on chromosome 10) is a lipid phosphatase which can antagonize the phosphoinositide 3-kinase (PI 3-kinase) signalling pathway, promoting apoptosis and inhibiting cell-cycle progression and cell motility. We show that very little cellular PTEN is associated with the plasma membrane, but that artificial membrane-targeting of PTEN enhances its inhibition of signalling to protein kinase B (PKB). Evidence for potential targeting of PTEN to the membrane through PDZ domain-mediated protein-protein interactions led us to use a PTEN enzyme with a deletion of the C-terminal PDZ-binding sequence, that retains full phosphatase activity against soluble substrates, and to analyse the efficiency of this mutant in different cellular assays. The extreme C-terminal PDZ-binding sequence was dispensable for the efficient down-regulation of cellular PtdIns(3,4,5)P3 levels and a number of PI 3-kinase-dependent signalling activities, including PKB and p70S6K. However, the PDZ-binding sequence was required for the efficient inhibition of cell spreading. The data show that a PTEN mutation, similar to those found in some tumours, affects some functions of the protein but not others, and implicate the deregulation of PTEN-dependent processes other than PKB activation in the development of some tumours. Significantly, this hypothesis is supported by data showing low levels of PKB phosphorylation in a glioblastoma sample carrying a mutation in the extreme C-terminus of PTEN compared with tumours carrying phosphatase-inactivating mutations of the enzyme. Our data show that deregulation of PKB is not a universal feature of tumours carrying PTEN mutations and implicate other processes that may be deregulated in these tumours.

Partially distinct molecular mechanisms mediate inhibitory FcgammaRIIB signaling in resting and activated B cells

FcgammaRIIB functions as an inhibitory receptor to dampen B cell Ag receptor signals and immune responses. Accumulating evidence indicates that ex vivo B cells require the inositol 5-phosphatase, Src homology domain 2-containing inositol 5-phosphatase (SHIP), for FcgammaRIIB-mediated inhibitory signaling. However, we report here that LPS-activated primary B cells do not require SHIP and thus differ from resting B cells. SHIP-deficient B cell blasts display efficient FcgammaRIIB-dependent inhibition of calcium mobilization as well as Akt and extracellular signal-related protein kinase phosphorylation. Surprisingly, FcgammaRIIB-dependent degradation of phosphatidylinositol 3,4,5-trisphosphate and conversion into phosphatidylinositol 3,4-bisphosphate occur in SHIP-deficient B cell blasts, demonstrating the function of an additional inositol 5-phosphatase. Further analysis reveals that while resting cells express only SHIP, B cell blasts also express the recently described inositol 5-phosphatase, SHIP-2. Finally, data suggest that both SHIP-2 and SHIP can mediate downstream biologic consequences of FcgammaRIIB signaling, including inhibition of the proliferative response.

Tumor suppressor PTEN: modulator of cell signaling, growth, migration and apoptosis

PTEN (also known as MMAC-1 or TEP-1) is one of the most frequently mutated tumor suppressors in human cancer. It is also essential for embryonic development. PTEN functions primarily as a lipid phosphatase to regulate crucial signal transduction pathways; a key target is phosphatidylinositol 3,4,5-trisphosphate. In addition, it displays weak tyrosine phosphatase activity, which may downmodulate signaling pathways that involve focal adhesion kinase (FAK) or Shc. Levels of PTEN are regulated in embryos and adult organisms, and gene-targeting studies demonstrate that it has a crucial role in normal development. Functions for PTEN have been identified in the regulation of many normal cell processes, including growth, adhesion, migration, invasion and apoptosis. PTEN appears to play particularly important roles in regulating anoikis (apoptosis of cells after loss of contact with extracellular matrix) and cell migration. Gene targeting and transient expression studies have provided insight into the specific signaling pathways that regulate these processes. Characterization of the diverse signaling networks modulated by PTEN, as well as the regulation of PTEN concentration, enzymatic activity, and coordination with other phosphatases, should provide intriguing new insight into the biology of normal and malignant cells.

The Src Homology 2 Domain Containing Inositol 5-Phosphatase SHIP2 Is Recruited to the Epidermal Growth Factor (EGF) Receptor and Dephosphorylates Phosphatidylinositol 3,4,5-Trisphosphate in EGF-stimulated COS-7 Cells

The lipid phosphatase SHIP2 (Src homology 2 domain containing inositol 5-phosphatase 2) has been shown to be expressed in nonhemopoietic and hemopoietic cells. It has been implicated in signaling events initiated by several extracellular signals, such as epidermal growth factor (EGF) and insulin. In COS-7 cells, SHIP2 was tyrosine-phosphorylated at least at two separated tyrosine phosphorylation sites in response to EGF. SHIP2 was coimmunoprecipitated with the EGF receptor (EGFR) and also with the adaptor protein Shc. A C-terminal truncated form of SHIP2 that lacks the 366 last amino acids, referred to as tSHIP2, was also precipitated with the EGFR when transfected in COS-7 cells. The Src homology 2 domain of SHIP2 was unable to precipitate the EGFR in EGF-stimulated cells. Moreover, when transfected in COS-7 cells, it could not be detected in immunoprecipitates of the EGFR. When the His-tagged full-length enzyme was expressed in COS-7 cells and stained with anti-His6 monoclonal antibody, a signal was observed at plasma membranes in EGF-stimulated cells that colocalize with the EGFR by double staining. Upon stimulation by EGF, phosphatidylinositol 3,4,5-trisphosphate and protein kinase B activity were decreased in SHIP2-transfected COS-7 cells as compared with the vector alone. SHIP2 appears therefore in a tyrosine-phosphorylated complex with at least two other proteins, the EGFR and Shc.

The SH2 domain containing inositol 5-phosphatase SHIP2 controls phosphatidylinositol 3,4,5-trisphosphate levels in CHO-IR cells stimulated by insulin

The lipid phosphatase SHIP2 (SH2 domain containing inositol 5-phosphatase 2) has recently been shown to be a potent negative regulator of insulin signaling and insulin sensitivity in vivo. We show here that SHIP2 is expressed in Chinese hamster ovary cells overexpressing the insulin receptor (CHO-IR cells) and tyrosine phosphorylated upon insulin stimulation. We show that SHIP2, which is recruited in anti-phosphotyrosine immunoprecipitates in insulin-stimulated cells, accounts for the insulin sensitivity or apparent increase in activity reported by Guilherme et al. (J. Biol. Chem. 271, 29533-29536, 1996). Overexpression of SHIP2 led to a decrease of the insulin-dependent PIP3 production as well as Akt/PKB activation and MAPK stimulation.

Overexpression of SH2-containing inositol phosphatase 2 results in negative regulation of insulin-induced metabolic actions in 3T3-L1 adipocytes via its 5'-phosphatase catalytic activity

Phosphatidylinositol (PI) 3-kinase plays an important role in various metabolic actions of insulin including glucose uptake and glycogen synthesis. Although PI 3-kinase primarily functions as a lipid kinase which preferentially phosphorylates the D-3 position of phospholipids, the effect of hydrolysis of the key PI 3-kinase product PI 3,4,5-triphosphate [PI(3,4,5)P3] on these biological responses is unknown. We recently cloned rat SH2-containing inositol phosphatase 2 (SHIP2) cDNA which possesses the 5'-phosphatase activity to hydrolyze PI(3,4,5)P3 to PI 3,4-bisphosphate [PI(3,4)P2] and which is mainly expressed in the target tissues of insulin. To study the role of SHIP2 in insulin signaling, wild-type SHIP2 (WT-SHIP2) and 5'-phosphatase-defective SHIP2 (Delta IP-SHIP2) were overexpressed in 3T3-L1 adipocytes by means of adenovirus-mediated gene transfer. Early events of insulin signaling including insulin-induced tyrosine phosphorylation of the insulin receptor beta subunit and IRS-1, IRS-1 association with the p85 subunit, and PI 3-kinase activity were not affected by expression of either WT-SHIP2 or Delta IP-SHIP2. Because WT-SHIP2 possesses the 5'-phosphatase catalytic region, its overexpression marked by decreased insulin-induced PI(3,4,5)P3 production, as expected. In contrast, the amount of PI(3,4,5)P3 was increased by the expression of Delta IP-SHIP2, indicating that Delta IP-SHIP2 functions in a dominant-negative manner in 3T3-L1 adipocytes. Both PI(3,4,5)P3 and PI(3,4)P2 were known to possibly activate downstream targets Akt and protein kinase C lambda in vitro. Importantly, expression of WT-SHIP2 inhibited insulin-induced activation of Akt and protein kinase C lambda, whereas these activations were increased by expression of Delta IP-SHIP2 in vivo. Consistent with the regulation of downstream molecules of PI 3-kinase, insulin-induced 2-deoxyglucose uptake and Glut4 translocation were decreased by expression of WT-SHIP2 and increased by expression of Delta IP-SHIP2. In addition, insulin-induced phosphorylation of GSK-3beta and activation of PP1 followed by activation of glycogen synthase and glycogen synthesis were decreased by expression of WT-SHIP2 and increased by the expression of Delta IP-SHIP2. These results indicate that SHIP2 negatively regulates metabolic signaling of insulin via the 5'-phosphatase activity and that PI(3,4,5)P3 rather than PI(3,4)P2 is important for in vivo regulation of insulin-induced activation of downstream molecules of PI 3-kinase leading to glucose uptake and glycogen synthesis.