Embryonic and hematopoietic stem cells express a novel SH2-containing inositol 5'-phosphatase isoform that partners with the Grb2 adapter protein

Pathological Roles of INPP5D in Alzheimer's Disease

Current hypothesis of Alzheimer's disease (AD) postulates that amyloid β (Aβ) deposition in the brain causes tau inclusion in neurons and leads to cognitive decline. The discovery of the genetic association between triggering receptor expressed on myeloid cells 2 (TREM2) with increased AD risk points to a causal link between microglia and AD pathogenesis, and revealed a crucial role of TREM2-dependent clustering of microglia around amyloid plaques that prevents Aβ toxicity to facilitate tau deposition near the plaques. Here we review the physiological and pathological roles of another AD risk gene expressed in microglia, inositol polyphosphate-5-polyphosphatase D (INPP5D), which encodes a phosphoinositide phosphatase. Evidence suggests that its risk polymorphisms alter the expression level and/or function of INPP5D, while concomitantly affecting tau levels in cerebrospinal fluids. In β-amyloidosis mice, INPP5D was upregulated upon Aβ deposition and negatively regulated the microglial clustering toward amyloid plaques. INPP5D seems to exert its function by acting antagonistically at downstream of the TREM2 signaling pathway, suggesting that it is a novel regulator of the protective barrier by microglia. Further studies to elucidate INPP5D's role in AD may help in developing new therapeutic targets for AD treatment.

Dickkopf-3: An Update on a Potential Regulator of the Tumor Microenvironment

Dickkopf-3 (Dkk-3) is a member of the Dickkopf family protein of secreted Wingless-related integration site (Wnt) antagonists that appears to modulate regulators of the host microenvironment. In contrast to the clear anti-tumorigenic effects of Dkk-3-based gene therapies, the role of endogenous Dkk-3 in cancer is context-dependent, with elevated expression associated with tumor promotion and suppression in different settings. The receptors and effectors that mediate the diverse effects of Dkk-3 have not been characterized in detail, contributing to an ongoing mystery of its mechanism of action. This review compares the various functions of Dkk-3 in the tumor microenvironment, where Dkk-3 has been found to be expressed by subpopulations of fibroblasts, endothelial, and immune cells, in addition to epithelial cells. We also discuss how the activation or inhibition of Dkk-3, depending on tumor type and context, might be used to treat different types of cancers.

Striking a balance: PIP2 and PIP3 signaling in neuronal health and disease

Phosphoinositides are membrane phospholipids involved in a variety of cellular processes like growth, development, metabolism, and transport. This review focuses on the maintenance of cellular homeostasis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and phosphatidylinositol 3,4,5-trisphosphate (PIP₃). The critical balance of these PIPs is crucial for regulation of neuronal form and function. The activity of PIP₂ and PIP₃ can be regulated through kinases, phosphatases, phospholipases and cholesterol microdomains. PIP₂ and PIP₃ carry out their functions either indirectly through their effectors activating integral signaling pathways, or through direct regulation of membrane channels, transporters, and cytoskeletal proteins. Any perturbations to the balance between PIP₂ and PIP₃ signaling result in neurodevelopmental and neurodegenerative disorders. This review will discuss the upstream modulators and downstream effectors of the PIP₂ and PIP₃ signaling, in the context of neuronal health and disease.

Targeting SHIP1 and SHIP2 in Cancer

Simple Summary

Phosphoinositol signaling pathways and their dysregulation have been shown to have a fundamental role in health and disease, respectively. The SH2-containing 5′ inositol phosphatases, SHIP1 and SHIP2, are regulators of the PI3K/AKT pathway that have crucial roles in cancer progression. This review aims to summarize the role of SHIP1 and SHIP2 in cancer signaling and the immune response to cancer, the discovery and use of SHIP inhibitors and agonists as possible cancer therapeutics.

Abstract

Membrane-anchored and soluble inositol phospholipid species are critical mediators of intracellular cell signaling cascades. Alterations in their normal production or degradation are implicated in the pathology of a number of disorders including cancer and pro-inflammatory conditions. The SH2-containing 5′ inositol phosphatases, SHIP1 and SHIP2, play a fundamental role in these processes by depleting PI(3,4,5)P₃, but also by producing PI(3,4)P₂ at the inner leaflet of the plasma membrane. With the intent of targeting SHIP1 or SHIP2 selectively, or both paralogs simultaneously, small molecule inhibitors and agonists have been developed and tested in vitro and in vivo over the last decade in various disease models. These studies have shown promising results in various pre-clinical models of disease including cancer and tumor immunotherapy. In this review the potential use of SHIP inhibitors in cancer is discussed with particular attention to the molecular structure, binding site and efficacy of these SHIP inhibitors.

Mammary development in the embryo and adult: new insights into the journey of morphogenesis and commitment

The mammary gland is a unique tissue and the defining feature of the class Mammalia. It is a late-evolving epidermal appendage that has the primary function of providing nutrition for the young, although recent studies have highlighted additional benefits of milk including the provision of passive immunity and a microbiome and, in humans, the psychosocial benefits of breastfeeding. In this Review, we outline the various stages of mammary gland development in the mouse, with a particular focus on lineage specification and the new insights that have been gained by the application of recent technological advances in imaging in both real-time and three-dimensions, and in single cell RNA sequencing. These studies have revealed the complexity of subpopulations of cells that contribute to the mammary stem and progenitor cell hierarchy and we suggest a new terminology to distinguish these cells.

s-SHIP Promoter Expression Identifies Mouse Mammary Cancer Stem Cells

During normal mammary gland development, s-SHIP promoter expression marks a distinct type of mammary stem cells, at two different stages, puberty and early mid-pregnancy. To determine whether s-SHIP is a marker of mammary cancer stem cells (CSCs), we generated bitransgenic mice by crossing the C3(1)-SV40 T-antigen transgenic mouse model of breast cancer, and a transgenic mouse (11.5kb-GFP) expressing green fluorescent protein from the s-SHIP promoter. Here we show that in mammary tumors originating in these bitransgenic mice, s-SHIP promoter expression enriches a rare cell population with CSC activity as demonstrated by sphere-forming assays in vitro and limiting dilution transplantation in vivo. These s-SHIP-positive CSCs are characterized by lower expression of Delta-like non-canonical Notch ligand 1 (DLK1), a negative regulator of the Notch pathway. Inactivation of Dlk1 in s-SHIP-negative tumor cells increases their tumorigenic potential, suggesting a role for DLK1 in mammary cancer stemness.

Phosphatidylinositol Kinases and Phosphatases in Entamoeba histolytica

Phosphatidylinositol (PtdIns) metabolism is indispensable in eukaryotes. Phosphoinositides (PIs) are phosphorylated derivatives of PtdIns and consist of seven species generated by reversible phosphorylation of the inositol moieties at the positions 3, 4, and 5. Each of the seven PIs has a unique subcellular and membrane domain distribution. In the enteric protozoan parasite Entamoeba histolytica, it has been previously shown that the PIs phosphatidylinositol 3-phosphate (PtdIns3P), PtdIns(4,5)P₂, and PtdIns(3,4,5)P₃ are localized to phagosomes/phagocytic cups, plasma membrane, and phagocytic cups, respectively. The localization of these PIs in E. histolytica is similar to that in mammalian cells, suggesting that PIs have orthologous functions in E. histolytica. In contrast, the conservation of the enzymes that metabolize PIs in this organism has not been well-documented. In this review, we summarized the full repertoire of the PI kinases and PI phosphatases found in E. histolytica via a genome-wide survey of the current genomic information. E. histolytica appears to have 10 PI kinases and 23 PI phosphatases. It has a panel of evolutionarily conserved enzymes that generate all the seven PI species. However, class II PI 3-kinases, type II PI 4-kinases, type III PI 5-phosphatases, and PI 4P-specific phosphatases are not present. Additionally, regulatory subunits of class I PI 3-kinases and type III PI 4-kinases have not been identified. Instead, homologs of class I PI 3-kinases and PTEN, a PI 3-phosphatase, exist as multiple isoforms, which likely reflects that elaborate signaling cascades mediated by PtdIns(3,4,5)P₃ are present in this organism. There are several enzymes that have the nuclear localization signal: one phosphatidylinositol phosphate (PIP) kinase, two PI 3-phosphatases, and one PI 5-phosphatase; this suggests that PI metabolism also has conserved roles related to nuclear functions in E. histolytica, as it does in model organisms.

Regulation of hematopoietic cell signaling by SHIP-1 inositol phosphatase: growth factors and beyond

SHIP-1 is a hematopoietic-specific inositol phosphatase activated downstream of a multitude of receptors including those for growth factors, cytokines, antigen, immunoglobulin and toll-like receptor agonists where it exerts inhibitory control. While it is constitutively expressed in all immune cells, SHIP-1 expression is negatively regulated by the inflammatory and oncogenic micro-RNA miR-155. Knockout mouse studies have shown the importance of SHIP-1 in various immune cell subsets and have revealed a range of immune-mediated pathologies that are engendered due to loss of SHIP-1's regulatory activity, impelling investigations into the role of SHIP-1 in human disease. In this review, we provide an overview of the literature relating to the role of SHIP-1 in hematopoietic cell signaling and function, we summarize recent reports that highlight the dysregulation of the SHIP-1 pathway in cancers, autoimmune disorders and inflammatory diseases, and lastly we discuss the importance of SHIP-1 in restraining myeloid growth factor signaling.

Protective effect of stromal Dickkopf-3 in prostate cancer: opposing roles for TGFBI and ECM-1

Aberrant transforming growth factor-β (TGF-β) signaling is a hallmark of the stromal microenvironment in cancer. Dickkopf-3 (Dkk-3), shown to inhibit TGF-β signaling, is downregulated in prostate cancer and upregulated in the stroma in benign prostatic hyperplasia, but the function of stromal Dkk-3 is unclear. Here we show that DKK3 silencing in WPMY-1 prostate stromal cells increases TGF-β signaling activity and that stromal cell-conditioned media inhibit prostate cancer cell invasion in a Dkk-3-dependent manner. DKK3 silencing increased the level of the cell-adhesion regulator TGF-β-induced protein (TGFBI) in stromal and epithelial cell-conditioned media, and recombinant TGFBI increased prostate cancer cell invasion. Reduced expression of Dkk-3 in patient tumors was associated with increased expression of TGFBI. DKK3 silencing reduced the level of extracellular matrix protein-1 (ECM-1) in prostate stromal cell-conditioned media but increased it in epithelial cell-conditioned media, and recombinant ECM-1 inhibited TGFBI-induced prostate cancer cell invasion. Increased ECM1 and DKK3 mRNA expression in prostate tumors was associated with increased relapse-free survival. These observations are consistent with a model in which the loss of Dkk-3 in prostate cancer leads to increased secretion of TGFBI and ECM-1, which have tumor-promoting and tumor-protective roles, respectively. Determining how the balance between the opposing roles of extracellular factors influences prostate carcinogenesis will be key to developing therapies that target the tumor microenvironment.

s-SHIP expression identifies a subset of murine basal prostate cells as neonatal stem cells

Isolation of prostate stem cells (PSCs) is crucial for understanding their biology during normal development and tumorigenesis. In this aim, we used a transgenic mouse model expressing GFP from the stem cell-specific s-SHIP promoter to mark putative stem cells during postnatal prostate development. Here we show that cells identified by GFP expression are present transiently during early prostate development and localize to the basal cell layer of the epithelium. These prostate GFP+ cells are a subpopulation of the Lin- CD24+ Sca-1+ CD49f+ cells and are capable of self-renewal together with enhanced growth potential in sphere-forming assay in vitro, a phenotype consistent with that of a PSC population. Transplantation assays of prostate GFP+ cells demonstrate reconstitution of prostate ducts containing both basal and luminal cells in renal grafts. Altogether, these results demonstrate that s-SHIP promoter expression is a new marker for neonatal basal prostate cells exhibiting stem cell properties that enables PSCs in situ identification and isolation via a single consistent parameter. Transcriptional profiling of these GFP+ neonatal stem cells showed an increased expression of several components of the Wnt signaling pathway. It also identified stem cell regulators with potential applications for further analyses of normal and cancer stem cells.

Regulation of immune cell signaling by SHIP1: A phosphatase, scaffold protein, and potential therapeutic target

The phosphoinositide phosphatase SHIP is a critical regulator of immune cell activation. Despite considerable study, the mechanisms controlling SHIP activity to ensure balanced cell activation remain incompletely understood. SHIP dampens BCR signaling in part through its association with the inhibitory coreceptor Fc gamma receptor IIB, and serves as an effector for other inhibitory receptors in various immune cell types. The established paradigm emphasizes SHIP's inhibitory receptor-dependent function in regulating phosphoinositide 3-kinase signaling by dephosphorylating the phosphoinositide PI(3,4,5)P₃ ; however, substantial evidence indicates that SHIP can be activated independently of inhibitory receptors and can function as an intrinsic brake on activation signaling. Here, we integrate historical and recent reports addressing the regulation and function of SHIP in immune cells, which together indicate that SHIP acts as a multifunctional protein controlled by multiple regulatory inputs, and influences downstream signaling via both phosphatase-dependent and -independent means. We further summarize accumulated evidence regarding the functions of SHIP in B cells, T cells, NK cells, dendritic cells, mast cells, and macrophages, and data suggesting defective expression or activity of SHIP in autoimmune and malignant disorders. Lastly, we discuss the biological activities, therapeutic promise, and limitations of small molecule modulators of SHIP enzymatic activity.

Synthesis and initial evaluation of quinoline-based inhibitors of the SH2-containing inositol 5'-phosphatase (SHIP)

Recently, inhibition of the SH2-containing inositol 5'-phosphatase 1 (SHIP1) has become an attractive strategy for facilitating engraftment of MHC-I mismatched bone marrow grafts, increasing the number of adult stem cells in vivo, and inducing mobilization of hematopoietic stem cells. Utilizing high-throughput screening, two quinoline small molecules (NSC13480 and NSC305787) that inhibit SHIP1 enzymatic activity were discovered. New syntheses of these inhibitors have been developed which verified the relative stereochemistry of these structures. Utilizing this synthetic route, some analogs of these quinolines have been prepared and tested for their ability to inhibit SHIP. These structure activity studies determined that an amine tethered to the quinoline core is required for SHIP inhibition. SHIP inhibition may explain the antitumor effects of similar quinoline amino alcohols and provides an impetus for further synthetic studies in this class of compounds.

Role of SHIP1 in Invariant NKT Cell Development and Functions

SHIP1 is a 5'-inositol phosphatase known to negatively regulate the signaling product of the PI3K pathway, phosphatidylinositol (3-5)-trisphosphate. SHIP1 is recruited to a large number of inhibitory receptors expressed on invariant NK (iNKT) cells. We hypothesized that SHIP1 deletion would have major effects on iNKT cell development by altering the thresholds for positive and negative selection. Germline SHIP1 deletion has been shown to affect T cells as well as other immune cell populations. However, the role of SHIP1 on T cell function has been controversial, and its participation on iNKT cell development and function has not been examined. We evaluated the consequences of SHIP1 deletion on iNKT cells using germline-deficient mice, chimeric mice, and conditionally deficient mice. We found that T cell and iNKT cell development are impaired in germline-deficient animals. However, this phenotype can be rescued by extrinsic expression of SHIP1. In contrast, SHIP1 is required cell autonomously for optimal iNKT cell cytokine secretion. This suggests that SHIP1 calibrates the threshold of iNKT cell reactivity. These data further our understanding of how iNKT cell activation is regulated and provide insights into the biology of this unique cell lineage.

Hypoxia-induced p53 modulates both apoptosis and radiosensitivity via AKT

Restoration of hypoxia-induced apoptosis in tumors harboring p53 mutations has been proposed as a potential therapeutic strategy; however, the transcriptional targets that mediate hypoxia-induced p53-dependent apoptosis remain elusive. Here, we demonstrated that hypoxia-induced p53-dependent apoptosis is reliant on the DNA-binding and transactivation domains of p53 but not on the acetylation sites K120 and K164, which, in contrast, are essential for DNA damage-induced, p53-dependent apoptosis. Evaluation of hypoxia-induced transcripts in multiple cell lines identified a group of genes that are hypoxia-inducible proapoptotic targets of p53, including inositol polyphosphate-5-phosphatase (INPP5D), pleckstrin domain-containing A3 (PHLDA3), sulfatase 2 (SULF2), B cell translocation gene 2 (BTG2), cytoplasmic FMR1-interacting protein 2 (CYFIP2), and KN motif and ankyrin repeat domains 3 (KANK3). These targets were also regulated by p53 in human cancers, including breast, brain, colorectal, kidney, bladder, and melanoma cancers. Downregulation of these hypoxia-inducible targets associated with poor prognosis, suggesting that hypoxia-induced apoptosis contributes to p53-mediated tumor suppression and treatment response. Induction of p53 targets, PHLDA3, and a specific INPP5D transcript mediated apoptosis in response to hypoxia through AKT inhibition. Moreover, pharmacological inhibition of AKT led to apoptosis in the hypoxic regions of p53-deficient tumors and consequently increased radiosensitivity. Together, these results identify mediators of hypoxia-induced p53-dependent apoptosis and suggest AKT inhibition may improve radiotherapy response in p53-deficient tumors.

Enrichment of Human Stem-Like Prostate Cells with s-SHIP Promoter Activity Uncovers a Role in Stemness for the Long Noncoding RNA H19

Understanding normal and cancer stem cells should provide insights into the origin of prostate cancer and their mechanisms of resistance to current treatment strategies. In this study, we isolated and characterized stem-like cells present in the immortalized human prostate cell line, RWPE-1. We used a reporter system with green fluorescent protein (GFP) driven by the promoter of s-SHIP (for stem-SH2-domain-containing 5'-inositol phosphatase) whose stem cell-specific expression has been previously shown. We observed that s-SHIP-GFP-expressing RWPE-1 cells showed stem cell characteristics such as increased expression of stem cell surface markers (CD44, CD166, TROP2) and pluripotency transcription factors (Oct4, Sox2), and enhanced sphere-forming capacity and resistance to arsenite-induced cell death. Concomitant increased expression of the long noncoding RNA H19 was observed, which prompted us to investigate a putative role in stemness for this oncofetal gene. Targeted suppression of H19 with siRNA decreased Oct4 and Sox2 gene expression and colony-forming potential in RWPE-1 cells. Conversely, overexpression of H19 significantly increased gene expression of these two transcription factors and the sphere-forming capacity of RWPE-1 cells. Analysis of H19 expression in various prostate and mammary human cell lines revealed similarities with Sox2 expression, suggesting that a functional relationship may exist between H19 and Sox2. Collectively, we provide the first evidence that s-SHIP-GFP promoter reporter offers a unique marker for the enrichment of human stem-like cell populations and highlight a role in stemness for the long noncoding RNA H19.

Molecular control of PtdIns(3,4,5)P3 signaling in neutrophils

Neutrophils play critical roles in innate immunity and host defense. However, excessive neutrophil accumulation or hyper-responsiveness of neutrophils can be detrimental to the host system. Thus, the response of neutrophils to inflammatory stimuli needs to be tightly controlled. Many cellular processes in neutrophils are mediated by localized formation of an inositol phospholipid, phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3), at the plasma membrane. The PtdIns(3,4,5)P3 signaling pathway is negatively regulated by lipid phosphatases and inositol phosphates, which consequently play a critical role in controlling neutrophil function and would be expected to act as ideal therapeutic targets for enhancing or suppressing innate immune responses. Here, we comprehensively review current understanding about the action of lipid phosphatases and inositol phosphates in the control of neutrophil function in infection and inflammation.

PI3K/AKT signaling pathway and cancer: an updated review

Despite development of novel agents targeting oncogenic pathways, matching targeted therapies to the genetic status of individual tumors is proving to be a daunting task for clinicians. To improve the clinical efficacy and to reduce the toxic side effects of treatments, a deep characterization of genetic alterations in different tumors is required. The mutational profile often evidences a gain of function or hyperactivity of phosphoinositide 3-kinases (PI3Ks) in tumors. These enzymes are activated downstream tyrosine kinase receptors (RTKs) and/or G proteins coupled receptors (GPCRs) and, via AKT, are able to induce mammalian target of rapamycin (mTOR) stimulation. Here, we elucidate the impact of class I (p110α, β, γ, and δ) catalytic subunit mutations on AKT-mediated cellular processes that control crucial mechanisms in tumor development. Moreover, the interrelation of PI3K signaling with mTOR, ERK, and RAS pathways will be discussed, exploiting the potential benefits of PI3K signaling inhibitors in clinical use.

SHIP1 regulates MSC numbers and their osteolineage commitment by limiting induction of the PI3K/Akt/β-catenin/Id2 axis

Here, we show that Src homology 2-domain-containing inositol 5'-phosphatase 1 (SHIP1) is required for the efficient development of osteoblasts from mesenchymal stem cells (MSCs) such that bone growth and density are reduced in mice that lack SHIP1 expression in MSCs. We find that SHIP1 promotes the osteogenic output of MSCs by limiting activation of the PI3K/Akt/β-catenin pathway required for induction of the MSC stemness factor Id2. In parallel, we demonstrate that mice with myeloid-restricted ablation of SHIP1, including osteoclasts (OCs), show no reduction in bone mass or density. Hence, diminished bone mass and density in the SHIP1-deficient mice results from SHIP deficiency in MSC and osteolineage progenitors. Intriguingly, mice with a SHIP-deficient MSC compartment also exhibit decreased OC numbers. In agreement with our genetic findings we also show that treatment of mice with an SHIP1 inhibitor (SHIPi) significantly reduces bone mass. These findings demonstrate a novel role for SHIP1 in MSC fate determination and bone growth. Further, SHIPi may represent a novel therapeutic approach to limit bone development in osteopetrotic and sclerotic bone diseases.

The dynamics of murine mammary stem/progenitor cells

The stem/progenitor cells in the murine mammary gland are a highly dynamic population of cells that are responsible for ductal elongation in puberty, homeostasis maintenance in adult, and lobulo-alveolar genesis during pregnancy. In recent years understanding the epithelial cell hierarchy within the mammary gland is becoming particularly important as these different stem/progenitor cells were perceived to be the cells of origin for various subtypes of breast cancer. Although significant advances have been made in enrichment and isolation of stem/progenitor cells by combinations of antibodies against cell surface proteins together with flow cytometry, and in identification of stem/progenitor cells with multi-lineage differentiation and self-renewal using mammary fat pad reconstitution assay and in vivo genetic labeling technique, a clear understanding of how these different stem/progenitors are orchestrated in the mammary gland is still lacking. Here we discuss the different in vivo and in vitro methods currently available for stem/progenitor identification, their associated caveats, and a possible new hierarchy model to reconcile various putative stem/progenitor cell populations identified by different research groups.

SHIP-1 deficiency in the myeloid compartment is insufficient to induce myeloid expansion or chronic inflammation

SHIP-1 has an important role in controlling immune cell function through its ability to downmodulate PI3K signaling pathways that regulate cell survival and responses to stimulation. Mice deficient in SHIP-1 display several chronic inflammatory phenotypes including antibody-mediated autoimmune disease, Crohn's disease-like ileitis and a lung disease reminiscent of chronic obstructive pulmonary disease. The ileum and lungs of SHIP-1-deficient mice are infiltrated at an early age with abundant myeloid cells and the mice have a limited lifespan primarily thought to be due to the consolidation of lungs with spontaneously activated macrophages. To determine whether the myeloid compartment is the key initiator of inflammatory disease in SHIP-1-deficient mice, we examined two independent strains of mice harboring myeloid-restricted deletion of SHIP-1. Contrary to expectations, conditional deletion of SHIP-1 in myeloid cells did not result in consolidating pneumonia or segmental ileitis typical of germline SHIP-1 deficiency. In addition, other myeloid cell abnormalities characteristic of germline loss of SHIP-1, including flagrant splenomegaly and enhanced myelopoiesis, were absent in mice lacking SHIP-1 in myeloid cells. This study indicates that the spontaneous inflammatory disease characteristic of germline SHIP-1 deficiency is not initiated solely by LysM-positive myeloid cells but requires the simultaneous loss of SHIP-1 in other hematolymphoid lineages.

Discovery and development of small molecule SHIP phosphatase modulators

Inositol phospholipids play an important role in the transfer of signaling information across the cell membrane in eukaryotes. These signals are often governed by the phosphorylation patterns on the inositols, which are mediated by a number of inositol kinases and phosphatases. The src homology 2 (SH2) containing inositol 5-phosphatase (SHIP) plays a central role in these processes, influencing signals delivered through the PI3K/Akt/mTOR pathway. SHIP modulation by small molecules has been implicated as a treatment in a number of human disease states, including cancer, inflammatory diseases, diabetes, atherosclerosis, and Alzheimer's disease. In addition, alteration of SHIP phosphatase activity may provide a means to facilitate bone marrow transplantation and increase blood cell production. This review discusses the cellular signaling pathways and protein-protein interactions that provide the molecular basis for targeting the SHIP enzyme in these disease states. In addition, a comprehensive survey of small molecule modulators of SHIP1 and SHIP2 is provided, with a focus on the structure, potency, selectivity, and solubility properties of these compounds.

Inositol polyphosphate phosphatases in human disease

Phosphoinositide signalling molecules interact with a plethora of effector proteins to regulate cell proliferation and survival, vesicular trafficking, metabolism, actin dynamics and many other cellular functions. The generation of specific phosphoinositide species is achieved by the activity of phosphoinositide kinases and phosphatases, which phosphorylate and dephosphorylate, respectively, the inositol headgroup of phosphoinositide molecules. The phosphoinositide phosphatases can be classified as 3-, 4- and 5-phosphatases based on their specificity for dephosphorylating phosphates from specific positions on the inositol head group. The SAC phosphatases show less specificity for the position of the phosphate on the inositol ring. The phosphoinositide phosphatases regulate PI3K/Akt signalling, insulin signalling, endocytosis, vesicle trafficking, cell migration, proliferation and apoptosis. Mouse knockout models of several of the phosphoinositide phosphatases have revealed significant physiological roles for these enzymes, including the regulation of embryonic development, fertility, neurological function, the immune system and insulin sensitivity. Importantly, several phosphoinositide phosphatases have been directly associated with a range of human diseases. Genetic mutations in the 5-phosphatase INPP5E are causative of the ciliopathy syndromes Joubert and MORM, and mutations in the 5-phosphatase OCRL result in Lowe's syndrome and Dent 2 disease. Additionally, polymorphisms in the 5-phosphatase SHIP2 confer diabetes susceptibility in specific populations, whereas reduced protein expression of SHIP1 is reported in several human leukaemias. The 4-phosphatase, INPP4B, has recently been identified as a tumour suppressor in human breast and prostate cancer. Mutations in one SAC phosphatase, SAC3/FIG4, results in the degenerative neuropathy, Charcot-Marie-Tooth disease. Indeed, an understanding of the precise functions of phosphoinositide phosphatases is not only important in the context of normal human physiology, but to reveal the mechanisms by which these enzyme families are implicated in an increasing repertoire of human diseases.

Molecular hierarchy of mammary differentiation yields refined markers of mammary stem cells

The partial purification of mouse mammary gland stem cells (MaSCs) using combinatorial cell surface markers (Lin(-)CD24(+)CD29(h)CD49f(h)) has improved our understanding of their role in normal development and breast tumorigenesis. Despite the significant improvement in MaSC enrichment, there is presently no methodology that adequately isolates pure MaSCs. Seeking new markers of MaSCs, we characterized the stem-like properties and expression signature of label-retaining cells from the mammary gland of mice expressing a controllable H2b-GFP transgene. In this system, the transgene expression can be repressed in a doxycycline-dependent fashion, allowing isolation of slowly dividing cells with retained nuclear GFP signal. Here, we show that H2b-GFP(h) cells reside within the predicted MaSC compartment and display greater mammary reconstitution unit frequency compared with H2b-GFP(neg) MaSCs. According to their transcriptome profile, H2b-GFP(h) MaSCs are enriched for pathways thought to play important roles in adult stem cells. We found Cd1d, a glycoprotein expressed on the surface of antigen-presenting cells, to be highly expressed by H2b-GFP(h) MaSCs, and isolation of Cd1d(+) MaSCs further improved the mammary reconstitution unit enrichment frequency to nearly a single-cell level. Additionally, we functionally characterized a set of MaSC-enriched genes, discovering factors controlling MaSC survival. Collectively, our data provide tools for isolating a more precisely defined population of MaSCs and point to potentially critical factors for MaSC maintenance.

Role of inositol phospholipid signaling in natural killer cell biology

Natural killer (NK) cells are important for host defense against malignancy and infection. At a cellular level NK cells are activated when signals from activating receptors exceed signaling from inhibitory receptors. At a molecular level NK cells undergo an education process to both prevent autoimmunity and acquire lytic capacity. Mouse models have shown important roles for inositol phospholipid signaling in lymphocytes. NK cells from mice with deletion in different members of the inositol phospholipid signaling pathway exhibit defects in development, NK cell repertoire expression and effector function. Here we review the current state of knowledge concerning the function of inositol phospholipid signaling components in NK cell biology.

Mouse natural killer cell development and maturation are differentially regulated by SHIP-1

The SH2-containing inositol phosphatase-1 (SHIP-1) is a 5' inositol phosphatase known to negatively regulate the product of phosphoinositide-3 kinase (PI3K), phosphatidylinositol-3.4,5-trisphosphate. SHIP-1 can be recruited to a large number of inhibitory receptors expressed on natural killer (NK) cells. However, its role in NK cell development, maturation, and functions is not well defined. In this study, we found that the absence of SHIP-1 results in a loss of peripheral NK cells. However, using chimeric mice we demonstrated that SHIP-1 expression is not required intrinsically for NK cell lineage development. In contrast, SHIP-1 is required cell autonomously for NK cell terminal differentiation. These findings reveal both a direct and indirect role for SHIP-1 at different NK cell development checkpoints. Notably, SHIP-1-deficient NK cells display an impaired ability to secrete IFN-γ during cytokine receptor-mediated responses, whereas immunoreceptor tyrosine-based activation motif containing receptor-mediated responses is not affected. Taken together, our results provide novel insights on how SHIP-1 participates in the development, maturation, and effector functions of NK cells.

PI3K keeps the balance between metabolism and cancer

Epidemiological studies have established a positive correlation between cancer and metabolic disorders, suggesting that aberrant cell metabolism is a common feature of nearly all tumors. To meet their demand of building block molecules, cancer cells switch to a heavily glucose-dependent metabolism. As insulin triggers glucose uptake, most tumors are or become insulin-dependent. However, the effects of insulin and of other similar growth factors are not only limited to metabolic control but also favor tumor growth by stimulating proliferation and survival. A key signaling event mediating these metabolic and proliferative responses is the activation of the phosphatidylinositol-3 kinases (PI3K) pathway. In this review, we will thus discuss the current concepts of tumor metabolism and the opportunity of PI3K-targeted therapies to exploit the "sweet tooth" of cancer cells.

Reversible Ser/Thr SHIP phosphorylation: a new paradigm in phosphoinositide signalling?: Targeting of SHIP1/2 phosphatases may be controlled by phosphorylation on Ser and Thr residues

Phosphoinositide (PI) phosphatases such as the SH2 domain-containing inositol 5-phosphatases 1/2 (SHIP1 and 2) are important signalling enzymes in human physiopathology. SHIP1/2 interact with a large number of immune and growth factor receptors. Tyrosine phosphorylation of SHIP1/2 has been considered to be the determining regulatory modification. However, here we present a hypothesis, based on recent key publications, highlighting the determining role of Ser/Thr phosphorylation in regulating several key properties of SHIP1/2. Since a subunit of the Ser/Thr phosphatase PP2A has been shown to interact with SHIP2, a putative mechanism for reversing SHIP2 Ser/Thr phosphorylation can be anticipated. PI phosphatases are potential target molecules in human diseases, particularly, but not exclusively, in cancer and diabetes. Therefore, this novel regulatory mechanism deserves further attention in the hunt for discovering novel or complementary therapeutic strategies. This mechanism may be more broadly involved in regulating PI signalling in the case of synaptojanin1 or the phosphatase, tensin homolog, deleted on chromosome TEN.

Phosphoinositide lipid phosphatase SHIP1 and PTEN coordinate to regulate cell migration and adhesion

SHIP1 regulates PtdIns(3,4,5)P3 production in response to cell adhesion. Loss of SHIP1 leads to elevated PtdIns(3,4,5)P3 and Akt activation upon adhesion. SHIP1^−/− neutrophils lose polarity upon cell adhesion. They are extremely adherent, which impairs chemotaxis. Chemotaxis in SHIP1^−/− neutrophils can be rescued by reducing cell adhesion.

The second messenger phosphatidylinositol(3,4,5)P₃ (PtdIns(3,4,5)P₃) is formed by stimulation of various receptors, including G protein–coupled receptors and integrins. The lipid phosphatases PTEN and SHIP1 are critical in regulating the level of PtdIns(3,4,5)P₃ during chemotaxis. Observations that loss of PTEN had minor and loss of SHIP1 resulted in a severe chemotaxis defect in neutrophils led to the belief that SHIP1 rather than PTEN acts as a predominant phospholipid phosphatase in establishing a PtdIns(3,4,5)P₃ compass. In this study, we show that SHIP1 regulates PtdIns(3,4,5)P₃ production in response to cell adhesion and plays a limited role when cells are in suspension. SHIP1^−/− neutrophils lose their polarity upon cell adhesion and are extremely adherent, which impairs chemotaxis. However, chemo­taxis can be restored by reducing adhesion. Loss of SHIP1 elevates Akt activation following cell adhesion due to increased PtdIns(3,4,5)P₃ production. From our observations, we conclude that SHIP1 prevents formation of top-down PtdIns(3,4,5)P₃ polarity to facilitate proper cell attachment and detachment during chemotaxis.

Phosphoinositide phosphatases: just as important as the kinases

Phosphoinositide phosphatases comprise several large enzyme families with over 35 mammalian enzymes identified to date that degrade many phosphoinositide signals. Growth factor or insulin stimulation activates the phosphoinositide 3-kinase that phosphorylates phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] to form phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)], which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) to PtdIns(4,5)P(2), or by the 5-phosphatases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). 5-phosphatases also hydrolyze PtdIns(4,5)P(2) forming PtdIns(4)P. Ten mammalian 5-phosphatases have been identified, which regulate hematopoietic cell proliferation, synaptic vesicle recycling, insulin signaling, and embryonic development. Two 5-phosphatase genes, OCRL and INPP5E are mutated in Lowe and Joubert syndrome respectively. SHIP [SH2 (Src homology 2)-domain inositol phosphatase] 2, and SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) negatively regulate insulin signaling and glucose homeostasis. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. SHIP1 controls hematopoietic cell proliferation and is mutated in some leukemias. The inositol polyphosphate 4-phosphatases, INPP4A and INPP4B degrade PtdIns(3,4)P(2) to PtdIns(3)P and regulate neuroexcitatory cell death, or act as a tumor suppressor in breast cancer respectively. The Sac phosphatases degrade multiple phosphoinositides, such as PtdIns(3)P, PtdIns(4)P, PtdIns(5)P and PtdIns(3,5)P(2) to form PtdIns. Mutation in the Sac phosphatase gene, FIG4, leads to a degenerative neuropathy. Therefore the phosphatases, like the lipid kinases, play major roles in regulating cellular functions and their mutation or altered expression leads to many human diseases.

The SH2-domain of SHIP1 interacts with the SHIP1 C-terminus: impact on SHIP1/Ig-α interaction

The SH2-containing inositol 5'-phosphatase, SHIP1, negatively regulates signal transduction from the B cell antigen receptor (BCR). The mode of coupling between SHIP1 and the BCR has not been elucidated so far. In comparison to wild-type cells, B cells expressing a mutant IgD- or IgM-BCR containing a C-terminally truncated Ig-α respond to pervanadate stimulation with markedly reduced tyrosine phosphorylation of SHIP1 and augmented activation of protein kinase B. This indicates that SHIP1 is capable of interacting with the C-terminus of Ig-α. Employing a system of fluorescence resonance energy transfer in S2 cells, we can clearly demonstrate interaction between the SH2-domain of SHIP1 and Ig-α. Furthermore, a fluorescently labeled SH2-domain of SHIP1 translocates to the plasma membrane in an Ig-α-dependent manner. Interestingly, whereas the SHIP1 SH2-domain can be pulled-down with phospho-peptides corresponding to the immunoreceptor tyrosine-based activation motif (ITAM) of Ig-α from detergent lysates, no interaction between full-length SHIP1 and the phosphorylated Ig-α ITAM can be observed. Further studies show that the SH2-domain of SHIP1 can bind to the C-terminus of the SHIP1 molecule, most probably by inter- as well as intra-molecular means, and that this interaction regulates the association between different forms of SHIP1 and Ig-α.

Phosphatases: the new brakes for cancer development?

The phosphatidylinositol 3-kinase (PI3K) pathway plays a pivotal role in the maintenance of processes such as cell growth, proliferation, survival, and metabolism in all cells and tissues. Dysregulation of the PI3K/Akt signaling pathway occurs in patients with many cancers and other disorders. This aberrant activation of PI3K/Akt pathway is primarily caused by loss of function of all negative controllers known as inositol polyphosphate phosphatases and phosphoprotein phosphatases. Recent studies provided evidence of distinct functions of the four main phosphatases—phosphatase and tensin homologue deleted on chromosome 10 (PTEN), Src homology 2-containing inositol 5′-phosphatase (SHIP), inositol polyphosphate 4-phosphatase type II (INPP4B), and protein phosphatase 2A (PP2A)—in different tissues with respect to regulation of cancer development. We will review the structures and functions of PTEN, SHIP, INPP4B, and PP2A phosphatases in suppressing cancer progression and their deregulation in cancer and highlight recent advances in our understanding of the PI3K/Akt signaling axis.

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.

Enzymatic and non-enzymatic activities of SHIP-1 in signal transduction and cancer

PI3K cascade is a central signaling pathway regulating cell proliferation, growth, differentiation, and survival. Tight regulation of the PI3K signaling pathway is necessary to avoid aberrant cell proliferation and cancer development. Together with SHIP-1, the inositol phosphatases PTEN and SHIP-2 are the gatekeepers of this pathway. In this review, we will focus on SHIP-1 functions. Negative regulation of immune cell activation by SHIP-1 is well characterized. Besides its catalytic activity, SHIP-1 also displays non-enzymatic activity playing role in several immune pathways. Indeed, SHIP-1 exhibits several domains that mediate protein-protein interaction. This review emphasizes the negative regulation of immune cell activation by SHIP-1 that is mediated by its protein-protein interaction.

Inositol phospholipid signaling and the biology of natural killer cells

A family of phosphoinositide-3 kinase (PI3K) isoenzymes catalyzes the production of second messengers that recruit critical regulators of cell growth, survival, proliferation and motility. Conversely, 3'-(phosphatase and tensin homolog) and 5'-inositol polyphosphatases (SH2-containing inositol phosphatases 1/2, SHIP1/2) are recruited to sites of PI3K signaling at the plasma membrane to oppose or, in some cases, to modify and enhance PI3K signaling. A substantial and growing body of literature demonstrates that these enzymes which mediate interchange of phosphates on inositol phospholipid species at the plasma membrane have prominent roles in natural killer cell biology, including development, effector functions and trafficking. Here, we review the salient points of these recent papers with a special emphasis on the role of p110δ and SHIP1 in natural killer cells.

An ENU-induced mouse mutant of SHIP1 reveals a critical role of the stem cell isoform for suppression of macrophage activation

In a recessive ENU mutagenesis screen for embryonic lethality, we identified a mouse pedigree with a missense mutation of SHIP1 (SHIP1(el20)) leading to an amino acid substitution I641T in the inositol-5'-phosphatase domain that represses phosphatidylinositol-3-kinase signaling. Despite detectable expression of functional SHIP1 protein, the phenotype of homozygous SHIP1(el20/el20) mice was more severe than gene-targeted SHIP1-null (SHIP1(-/-)) mice. Compared with age-matched SHIP1(-/-) mice, 5-week-old SHIP1(el20/el20) mice had increased myeloid cells, serum IL-6 levels, marked reductions in lymphoid cells, and died by 7 weeks of age with infiltration of the lungs by activated macrophages. Bone marrow transplantation demonstrated that these defects were hematopoietic-cell-autonomous. We show that the el20 mutation reduces expression in SHIP1(el20/el20) macrophages of both SHIP1 and s-SHIP, an isoform of SHIP1 generated by an internal promoter. In contrast, SHIP1(-/-) macrophages express normal levels of s-SHIP. Compound heterozygous mice (SHIP1(-/el20)) had the same phenotype as SHIP1(-/-) mice, thus providing genetic proof that the more severe phenotype of SHIP1(el20/el20) mice is probably the result of concomitant loss of SHIP1 and s-SHIP. Our results suggest that s-SHIP synergizes with SHIP1 for suppression of macrophage activation, thus providing the first evidence for a role of s-SHIP in adult hematopoiesis.

Inhibitor and activator: dual functions for SHIP in immunity and cancer

SHIP1 is at the nexus of intracellular signaling pathways in immune cells that mediate bone marrow (BM) graft rejection, production of inflammatory and immunosuppressive cytokines, immunoregulatory cell formation, the BM niche that supports development of the immune system, and immune cancers. This review summarizes how SHIP participates in normal immune physiology or the pathologies that result when SHIP is mutated. This review also proposes that SHIP can have either inhibitory or activating roles in cell signaling that are determined by whether signaling pathways distal to PI3K are promoted by SHIP's substrate (PI(3,4,5)P(3) ) or its product (PI(3,4)P(2) ). This review also proposes the "two PIP hypothesis" that postulates that both SHIP's product and its substrate are necessary for a cancer cell to achieve and sustain a malignant state. Finally, due to the recent discovery of small molecule antagonists and agonists for SHIP, this review discusses potential therapeutic settings where chemical modulation of SHIP might be of benefit.

Role of SHIP in cancer

The SH2-containing inositol-5'-phosphatase, SHIP (or SHIP1), is a hematopoietic-restricted phosphatidylinositide phosphatase that translocates to the plasma membrane after extracellular stimulation and hydrolyzes the phosphatidylinositol-3-kinase-generated second messenger PI-3,4,5-P(3) to PI-3,4-P(2). As a result, SHIP dampens down PI-3,4,5-P(3)-mediated signaling and represses the proliferation, differentiation, survival, activation, and migration of hematopoietic cells. There are multiple lines of evidence suggesting that SHIP may act as a tumor suppressor during leukemogenesis and lymphomagenesis. Because of its ability to skew macrophage progenitors toward M1 macrophages and naïve T cells toward T helper 1 and T helper 17 cells, SHIP may play a critical role in activating the immune system to eradicate solid tumors. In this review, we will discuss the role of SHIP in hematopoietic cells and its therapeutic potential in terms of suppressing leukemias and lymphomas and manipulating the immune system to combat cancer.

s-SHIP promoter expression marks activated stem cells in developing mouse mammary tissue

Mammary stem cells (MaSCs) play critical roles in normal development and perhaps tumorigenesis of the mammary gland. Using combined cell markers, adult MaSCs have been enriched in a basal cell population, but the exact identity of MaSCs remains unknown. We used the s-SHIP promoter to tag presumptive stem cells with GFP in the embryos of a transgenic mouse model. Here we show, in postnatal mammary gland development, that GFP(+) cap cells in puberty and basal alveolar bud cells in pregnancy each exhibit self-renewal and regenerative capabilities for all mammary epithelial cells of a new functional mammary gland upon transplantation. Single GFP(+) cells can regenerate the mammary epithelial network. GFP(+) mammary epithelial cells are p63(+), CD24(mod), CD49f(high), and CD29(high); are actively proliferating; and express s-SHIP mRNA. Overall, our results identify the activated MaSC population in vivo at the forefront of rapidly developing terminal end buds (puberty) and alveolar buds (pregnancy) in the mammary gland. In addition, GFP(+) basal cells are expanded in MMTV-Wnt1 breast tumors but not in ErbB2 tumors. These results enable MaSC in situ identification and isolation via a consistent single parameter using a new mouse model with applications for further analyses of normal and potential cancer stem cells.

Reversible phosphorylation in haematological malignancies: potential role for protein tyrosine phosphatases in treatment?

Most aspects of leukocyte physiology are under the control of reversible tyrosine phosphorylation. It is clear that excessive phosphorylation of signal transduction elements is a pivotal element of many different pathologies including haematological malignancies and accordingly, strategies that target such phosphorylation have clinically been proven highly successful for treatment of multiple types of leukemias and lymphomas. Cellular phosphorylation status is dependent on the resultant activity of kinases and phosphatases. The cell biology of the former is now well understood; for most cellular phosphoproteins we now know the kinases responsible for their phosphorylation and we understand the principles of their aberrant activity in disease. With respect to phosphatases, however, our knowledge is much patchier. Although the sequences of whole genomes allow us to identify phosphatases using in silico methodology, whereas transcription profiling allows us to understand how phosphatase expression is regulated during disease, most functional questions as to substrate specificity, dynamic regulation of phosphatase activity and potential for therapeutic intervention are still to a large degree open. Nevertheless, recent studies have allowed us to make meaningful statements on the role of tyrosine phosphatase activity in the three major signaling pathways that are commonly affected in leukemias, i.e. the Ras-Raf-ERK1/2, the Jak-STAT and the PI3K-PKB-mTOR pathways. Lessons learned from these pathways may well be applicable elsewhere in leukocyte biology as well.

The inositol phosphatase SHIP-1 is negatively regulated by Fli-1 and its loss accelerates leukemogenesis

The activation of Fli-1, an Ets transcription factor, is the critical genetic event in Friend murine leukemia virus (F-MuLV)-induced erythroleukemia. Fli-1 overexpression leads to erythropoietin-dependent erythroblast proliferation, enhanced survival, and inhibition of terminal differentiation, through activation of the Ras pathway. However, the mechanism by which Fli-1 activates this signal transduction pathway has yet to be identified. Down-regulation of the Src homology 2 (SH2) domain-containing inositol-5-phosphatase-1 (SHIP-1) is associated with erythropoietin-stimulated erythroleukemic cells and correlates with increased proliferation of transformed cells. In this study, we have shown that F-MuLV-infected SHIP-1 knockout mice display accelerated erythroleukemia progression. In addition, RNA interference (RNAi)-mediated suppression of SHIP-1 in erythroleukemia cells activates the phosphatidylinositol 3-kinase (PI 3-K) and extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) pathways, blocks erythroid differentiation, accelerates erythropoietin-induced proliferation, and leads to PI 3-K-dependent Fli-1 up-regulation. Chromatin immunoprecipitation and luciferase assays confirmed that Fli-1 binds directly to an Ets DNA binding site within the SHIP-1 promoter and suppresses SHIP-1 transcription. These data provide evidence to suggest that SHIP-1 is a direct Fli-1 target, SHIP-1 and Fli-1 regulate each other in a negative feedback loop, and the suppression of SHIP-1 by Fli-1 plays an important role in the transformation of erythroid progenitors by F-MuLV.

SHIP1 inhibition increases immunoregulatory capacity and triggers apoptosis of hematopoietic cancer cells

Genetic studies revealed that SHIP1 limits blood cell production and immune regulatory cell numbers in vivo. We postulated that molecular targeting of SHIP1 might enhance blood cell production and increase immunoregulatory capacity. In this study, we report the identification of a chemical inhibitor of SHIP1, 3 alpha-aminocholestane (3AC). Treatment with 3AC significantly expands the myeloid immunoregulatory cell compartment and impairs the ability of peripheral lymphoid tissues to prime allogeneic T cell responses. In addition, 3AC treatment profoundly increases granulocyte production without triggering the myeloid-associated lung consolidation observed in SHIP1(-/-) mice. Moreover, 3AC also enhances RBC, neutrophil, and platelet recovery in myelosuppressed hosts. Intriguingly, we also find that chemical inhibition of SHIP1 triggers apoptosis of blood cancer cells. Thus, SHIP1 inhibitors represent a novel class of small molecules that have the potential to enhance allogeneic transplantation, boost blood cell production, and improve the treatment of hematologic malignancies.

Mammalian phosphoinositide kinases and phosphatases

Phosphoinositides are lipids that are present in the cytoplasmic leaflet of a cell's plasma and internal membranes and play pivotal roles in the regulation of a wide variety of cellular processes. Phosphoinositides are molecularly diverse due to variable phosphorylation of the hydroxyl groups of their inositol rings. The rapid and reversible configuration of the seven known phosphoinositide species is controlled by a battery of phosphoinositide kinases and phosphoinositide phosphatases, which are thus critical for phosphoinositide isomer-specific localization and functions. Significantly, a given phosphoinositide generated by different isozymes of these phosphoinositide kinases and phosphatases can have different biological effects. In mammals, close to 50 genes encode the phosphoinositide kinases and phosphoinositide phosphatases that regulate phosphoinositide metabolism and thus allow cells to respond rapidly and effectively to ever-changing environmental cues. Understanding the distinct and overlapping functions of these phosphoinositide-metabolizing enzymes is important for our knowledge of both normal human physiology and the growing list of human diseases whose etiologies involve these proteins. This review summarizes the structural and biological properties of all the known mammalian phosphoinositide kinases and phosphoinositide phosphatases, as well as their associations with human disorders.

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.

SHIP is required for a functional hematopoietic stem cell niche

SH2-domain-containing inositol 5'-phosphatase-1 (SHIP) deficiency significantly increases the number of hematopoietic stem cells (HSCs) present in the bone marrow (BM). However, the reconstitution capacity of these HSCs is severely impaired, suggesting that SHIP expression might be an intrinsic requirement for HSC function. To further examine this question, we developed a model in which SHIP expression is ablated in HSCs while they are resident in a SHIP-competent milieu. In this setting, we find that long-term repopulation by SHIP-deficient HSCs is not compromised. Moreover, SHIP-deficient HSCs from this model repopulate at levels comparable with wild-type HSCs upon serial transfer. However, when HSCs from mice with systemic ablation of SHIP are transplanted, they are functionally compromised for repopulation. These findings demonstrate that SHIP is not an intrinsic requirement for HSC function, but rather that SHIP is required for the BM milieu to support functionally competent HSCs. Consistent with these findings, cells that comprise the BM niche express SHIP and SHIP deficiency profoundly alters their function.

Regulation of myeloproliferation and M2 macrophage programming in mice by Lyn/Hck, SHIP, and Stat5

The proliferation and differentiation of hematopoietic stem cells (HSCs) is finely regulated by extrinsic and intrinsic factors via various signaling pathways. Here we have shown that, similar to mice deficient in the lipid phosphatase SHIP, loss of 2 Src family kinases, Lyn and Hck, profoundly affects HSC differentiation, producing hematopoietic progenitors with increased proliferation, reduced apoptosis, growth factor-independent survival, and skewed differentiation toward M2 macrophages. This phenotype culminates in a Stat5-dependent myeloproliferative disease that is accompanied by M2 macrophage infiltration of the lung. Expression of a membrane-bound form of SHIP in HSCs lacking both Lyn and Hck restored normal hematopoiesis and prevented myeloproliferation. In vitro and in vivo studies suggested the involvement of autocrine and/or paracrine production of IL-3 and GM-CSF in the increased proliferation and myeloid differentiation of HSCs. Thus, this study has defined a myeloproliferative transformation-sensitive signaling pathway, composed of Lyn/Hck, SHIP, autocrine/paracrine cytokines, and Stat5, that regulates HSC differentiation and M2 macrophage programming.

Phosphoinositide lipid phosphatases: natural regulators of phosphoinositide 3-kinase signaling in T lymphocytes

The phosphoinositide 3-kinase signaling pathway has been implicated in a range of T lymphocyte cellular functions, particularly growth, proliferation, cytokine secretion, and survival. Dysregulation of phosphoinositide 3-kinase-dependent signaling and function in leukocytes, including B and T lymphocytes, has been implicated in many inflammatory and autoimmune diseases. As befits a pivotal signaling cascade, several mechanisms exist to ensure that the pathway is tightly regulated. This minireview focuses on two lipid phosphatases, viz. the 3'-phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome 10) and SHIP (Src homology 2 domain-containing inositol-5-phosphatase). We discuss their role in regulating T lymphocyte signaling as well their potential as future therapeutic targets.

Induced SHIP deficiency expands myeloid regulatory cells and abrogates graft-versus-host disease

Graft-vs-host disease (GVHD) is the leading cause of treatment-related death in allogeneic bone marrow (BM) transplantation. Immunosuppressive strategies to control GVHD are only partially effective and often lead to life-threatening infections. We previously showed that engraftment of MHC-mismatched BM is enhanced and GVHD abrogated in recipients homozygous for a germline SHIP mutation. In this study, we report the development of a genetic model in which SHIP deficiency can be induced in adult mice. Using this model, we show that the induction of SHIP deficiency in adult mice leads to a rapid and significant expansion of myeloid suppressor cells in peripheral lymphoid tissues. Consistent with expansion of myeloid suppressor cells, splenocytes and lymph node cells from adult mice with induced SHIP deficiency are significantly compromised in their ability to prime allogeneic T cell responses. These results demonstrate that SHIP regulates homeostatic signals for these immunoregulatory cells in adult physiology. Consistent with these findings, induction of SHIP deficiency before receiving a T cell-replete BM graft abrogates acute GVHD. These findings indicate strategies that target SHIP could increase the efficacy and utility of allogeneic BM transplantation, and thereby provide a curative therapy for a wide spectrum of human diseases.

s-SHIP associates with receptor complexes essential for pluripotent stem cell growth and survival

Embryonic stem (ES) cells are pluripotent cells that have the ability to either self-renew or differentiate into any cell type found in the mammalian body. The signaling pathways required for self-renewal of these cells are yet to be defined. Previously we identified a stem cell-specific isoform of the protein SH2 domain-containing 5'-inositol phosphatase (SHIP) that we call s-SHIP, which is expressed in both pluripotent ES cells and adult tissue-specific multipotent cells, such as hematopoietic stem cells (HSCs). s-SHIP lacks an SH2 domain but contains a 5'-inositol phosphatase domain and several protein-protein interaction domains that potentially enable its participation in many different signaling pathways. Here we show that s-SHIP associates with gp130, which forms a heterodimeric complex with the leukemia inhibitory factor receptor (LIFR). Signaling through LIFR and other receptors that heterodimerize with gp130 is critical for growth and survival of ES cells and HSCs. Our findings provide biochemical evidence that s-SHIP participates in signaling pathways important for the maintenance of pluripotent stem cell populations.