SHIP2 multiple functions: a balance between a negative control of PtdIns(3,4,5)P₃ level, a positive control of PtdIns(3,4)P₂ production, and intrinsic docking properties

Regulation of inositol 5-phosphatase activity by the C2 domain of SHIP1 and SHIP2

SHIP1, an inositol 5-phosphatase, plays a central role in cellular signaling. As such, it has been implicated in many conditions. Exploiting SHIP1 as a drug target will require structural knowledge and the design of selective small molecules. We have determined apo, and magnesium and phosphate-bound structures of the phosphatase and C2 domains of SHIP1. The C2 domains of SHIP1 and the related SHIP2 modulate the activity of the phosphatase domain. To understand the mechanism, we performed activity assays, hydrogen-deuterium exchange mass spectrometry, and molecular dynamics on SHIP1 and SHIP2. Our findings demonstrate that the influence of the C2 domain is more pronounced for SHIP2 than SHIP1. We determined 91 structures of SHIP1 with fragments bound, with some near the interface between the two domains. We performed a mass spectrometry screen and determined four structures with covalent fragments. These structures could act as starting points for the development of potent, selective probes.

Multiorgan failure with abnormal receptor metabolism in mice mimicking Samd9/9L syndromes

Autosomal dominant sterile α motif domain containing 9 (Samd9) and Samd9L (Samd9/9L) syndromes are a large subgroup of currently established inherited bone marrow failure syndromes that includes myelodysplasia, infection, growth restriction, adrenal hypoplasia, genital phenotypes, and enteropathy (MIRAGE), ataxia pancytopenia, and familial monosomy 7 syndromes. Samd9/9L genes are located in tandem on chromosome 7 and have been known to be the genes responsible for myeloid malignancies associated with monosomy 7. Additionally, as IFN-inducible genes, Samd9/9L are crucial for protection against viruses. Samd9/9L syndromes are caused by gain-of-function mutations and develop into infantile myelodysplastic syndromes associated with monosomy 7 (MDS/-7) at extraordinarily high frequencies. We generated mice expressing Samd9LD764N, which mimic MIRAGE syndrome, presenting with growth retardation, a short life, bone marrow failure, and multiorgan degeneration. In hematopoietic cells, Samd9LD764N downregulates the endocytosis of transferrin and c-Kit, resulting in a rare cause of anemia and a low bone marrow reconstitutive potential that ultimately causes MDS/-7. In contrast, in nonhematopoietic cells we tested, Samd9LD764N upregulated the endocytosis of EGFR by Ship2 phosphatase translocation to the cytomembrane and activated lysosomes, resulting in the reduced expression of surface receptors and signaling. Thus, Samd9/9L is a downstream regulator of IFN that controls receptor metabolism, with constitutive activation leading to multiorgan dysfunction.

Scaffold dependent role of the inositol 5'-phosphatase SHIP2, in regulation of oxidative stress induced apoptosis

Cell apoptosis is an important process that occurs during development or in response to stress stimuli such as oxidative stress. The serine-threonine kinase Akt enhances survival and suppress apoptosis. SHIP2 is known as a negative regulator of Akt. In addition to its lipid 5'-phosphatase activity, SHIP2 interacts and signals as a scaffolding complex with several proteins. Several findings have pointed out a possible role of SHIP2 in apoptosis regulation. However, the molecular mechanisms behind remain unknown. Using embryonic fibroblast lacking the lipid 5'-phosphatase domain as a genetic model system and human liver cancer cells treated with SHIP2 inhibitor (AS1949490), as a pharmacological model system. We provide the first evidence that SHIP2 regulates apoptosis independently of its 5'-phosphates activity. Indeed, absence of the 5'-phosphatase domain of SHIP2 did not prevent H2O2-induced apoptosis in fibroblasts. Whereas chemical inactivation or RNAi knockdown of SHIP2 blocked H2O2-induced apoptosis in HepG2 cells. We found that suppression of apoptosis upon SHIP2 inhibition is PI3K/Akt independent but rather MAP kinase dependent. In addition, we found that AS1949490 altered both 5'-phosphatase and scaffolding function of SHIP2. Indeed, AS1949490 mediated SHIP2 inhibition promotes protein complex formation of SHIP2 together with non-receptor tyrosine kinase SRC and ABL which in turn enhances PI3K/Akt and MAP kinase pathways activation. Dual inhibition of SRC/ABL blocked activation of both pathways upon SHIP2 inhibition and H2O2 treatment. Altogether, these findings indicate that SHIP2 protein play a determinant role in H2O2-induced apoptosis.

SHIP2 inhibition alters redox-induced PI3K/AKT and MAP kinase pathways via PTEN over-activation in cervical cancer cells

Phosphatidylinositol (3,4,5)‐trisphosphate (PI(3,4,5)P3) is required for protein kinase B (AKT) activation. The level of PI(3,4,5)P3 is constantly regulated through balanced synthesis by phosphoinositide 3‐kinase (PI3K) and degradation by phosphoinositide phosphatases phosphatase and tensin homologue (PTEN) and SH2‐domain containing phosphatidylinositol‐3,4,5‐trisphosphate 5‐phosphatase 2 (SHIP2), known as negative regulators of AKT. Here, I show that SHIP2 inhibition in cervical cancer cell lines alters H₂O₂‐mediated AKT and mitogen‐activated protein kinase/extracellular signal‐regulated kinase pathway activation. In addition, SHIP2 inhibition enhances reactive oxygen species generation. Interestingly, I found that SHIP2 inhibition and H₂O₂ treatment enhance lipid and protein phosphatase activity of PTEN. Pharmacological targeting or RNA interference(RNAi) mediated knockdown of PTEN rescues extracellular signal‐regulated kinase and AKT activation. Using a series of pharmacological and biochemical approaches, I provide evidence that crosstalk between SHIP2 and PTEN occurs upon an increase in oxidative stress to modulate the activity of mitogen‐activated protein kinase and phosphoinositide 3/ATK pathways.

Gingival tissue autophagy pathway gene expression profiles in periodontitis and aging

OBJECTIVE

We hypothesized that autophagy-related genes will be differentially expressed in periodontitis, suggesting an impaired gingival autophagic response associated with disease.

BACKGROUND

Autophagy is a cellular physiologic mechanism to maintain tissue homeostasis, while deficient autophagic responses increase inflammation and susceptibility to infection.

METHODS

Rhesus monkeys [<3 years to 23 years of age (n = 34)] were examined for periodontal health and naturally occurring periodontitis. Gingival tissues samples were obtained from healthy or diseased sites, total RNA was isolated, and the Rhesus Gene Chip 1.0 ST (Affymetrix) was used for gene expression analysis of 150 autophagy-related genes.

RESULTS

Comparison of expression levels with adult healthy tissues demonstrated a rather limited number of individual genes that were significantly different across the age-groups. In contrast, with periodontitis in the adults and aged animals, about 15% of the genes were significantly increased or decreased. The differences were reflected in the mTOR complex (5/12), ULK1/ATG1 complex (5/9), PI3K complex (5/21), ATG9 complex (2/7), ATG12 conjugation/LC3 lipidation (7/22), and lysosome fusion/vesicle degradation [LF/VD (5/10)] activities within the broader autophagic pathway. The genes most greatly altered in gingival tissues of naturally occurring periodontitis were identified in the ATG12 and LF/VD pathways that approximated 50% of the genes in each of those categories. While healthy gingival aging did not appear to reflect altered autophagy gene expression, substantial differences were noted with periodontitis irrespective of the age of the animals. Future studies into the role of autophagy in periodontitis and could offer potential new therapeutic strategies to prevent and/or treat periodontal disease.

Angiosarcoma of an Arteriovenous Fistula for Hemodialysis in a Kidney Transplant Recipient Affected by Lowe’s Syndrome

Objective/Background. To describe an uncommon, life-threatening condition such as angiosarcoma of a fistula for hemodialysis occurring in a transplant recipient affected by Lowe's syndrome. Summary. We present the case of a 56-year-old male kidney transplant recipient affected by Lowe's syndrome, also known as oculocerebrorenal syndrome, a rare X-linked disorder characterized by congenital cataracts, hypotonia, intellectual disability, and Fanconi-like renal tubular dysfunction, who was diagnosed with angiosarcoma of a functioning arteriovenous fistula for hemodialysis. Conclusion. Angiosarcoma is a rare soft tissue tumor, and only 22 cases of angiosarcoma of arteriovenous fistulae were described so far; although a correlation between Lowe's syndrome and a higher risk of tumor compared to the general population has not been described so far, the mechanisms of disease causation could be an interesting starting point for future studies on a possible connection between the two events.

Helicobacter pylori CagA oncoprotein interacts with SHIP2 to increase its delivery into gastric epithelial cells

Chronic infection with Helicobacter pylori cagA‐positive strains is causally associated with the development of gastric diseases, most notably gastric cancer. The cagA‐encoded CagA protein, which is injected into gastric epithelial cells by bacterial type IV secretion, undergoes tyrosine phosphorylation at the Glu‐Pro‐Ile‐Tyr‐Ala (EPIYA) segments (EPIYA‐A, EPIYA‐B, EPIYA‐C, and EPIYA‐D), which are present in various numbers and combinations in its C‐terminal polymorphic region, thereby enabling CagA to promiscuously interact with SH2 domain‐containing host cell proteins, including the prooncogenic SH2 domain‐containing protein tyrosine phosphatase 2 (SHP2). Perturbation of host protein functions by aberrant complex formation with CagA has been considered to contribute to the development of gastric cancer. Here we show that SHIP2, an SH2 domain‐containing phosphatidylinositol 5′‐phosphatase, is a hitherto undiscovered CagA‐binding host protein. Similar to SHP2, SHIP2 binds to the Western CagA‐specific EPIYA‐C segment or East Asian CagA‐specific EPIYA‐D segment through the SH2 domain in a tyrosine phosphorylation‐dependent manner. In contrast to the case of SHP2, however, SHIP2 binds more strongly to EPIYA‐C than to EPIYA‐D. Interaction with CagA tethers SHIP2 to the plasma membrane, where it mediates production of phosphatidylinositol 3,4‐diphosphate [PI(3,4)P₂]. The CagA‐SHIP2 interaction also potentiates the morphogenetic activity of CagA, which is caused by CagA‐deregulated SHP2. This study indicates that initially delivered CagA interacts with SHIP2 and thereby strengthens H. pylori‐host cell attachment by altering membrane phosphatidylinositol compositions, which potentiates subsequent delivery of CagA that binds to and thereby deregulates the prooncogenic phosphatase SHP2.

SHIPping out diabetes-Metformin, an old friend among new SHIP2 inhibitors

SHIP2 (Src homology 2 domain‐containing inositol 5′‐phosphatase 2) belongs to the family of 5′‐phosphatases. It regulates the phosphoinositide 3‐kinase (PI3K)‐mediated insulin signalling cascade by dephosphorylating the 5′‐position of PtdIns(3,4,5)P3 to generate PtdIns(3,4)P2, suppressing the activity of the pathway. SHIP2 mouse models and genetic studies in human propose that increased expression or activity of SHIP2 contributes to the pathogenesis of the metabolic syndrome, hypertension and type 2 diabetes. This has raised great interest to identify SHIP2 inhibitors that could be used to design new treatments for metabolic diseases. This review summarizes the central mechanisms associated with the development of diabetic kidney disease, including the role of insulin resistance, and then moves on to describe the function of SHIP2 as a regulator of metabolism in mouse models. Finally, the identification of SHIP2 inhibitors and their effects on metabolic processes in vitro and in vivo are outlined. One of the newly identified SHIP2 inhibitors is metformin, the first‐line medication prescribed to patients with type 2 diabetes, further boosting the attraction of SHIP2 as a treatment target to ameliorate metabolic disorders.

Excitable networks controlling cell migration during development and disease

The directed movements of individual, groups, or sheets of cells at specific times in particular locations bring about form and complexity to developing organisms. Cells move by extending protrusions, such as macropinosomes, pseudopods, lamellipods, filopods, or blebs. Although many of the cytoskeletal components within these structures are known, less is known about the mechanisms that determine their location, number, and characteristics. Recent evidence suggests that control may be exerted by a signal transduction excitable network whose components and activities, including Ras, PI3K, TorC2, and phosphoinositides, self-organize on the plasma membrane and propagate in waves. The waves drive the various types of protrusions, which in turn, determine the modes of cell migration. Acute perturbations at specific points in the network produce abrupt shifts in protrusion type, including transitions from pseudopods to filopods or lamellipods. These observations have also contributed to a delineation of the signal transduction network, including candidate fast positive and delayed negative feedback loops. The network contains many oncogenes and tumor suppressors, and other molecules which have recently been implicated in developmental and metabolic abnormalities. Thus, the concept of signal transduction network excitability in cell migration can be used to understand disease states and morphological changes occurring in development.

Downregulation of SHIP2 by Hepatitis B Virus X Promotes the Metastasis and Chemoresistance of Hepatocellular Carcinoma through SKP2

Hepatitis B virus (HBV)-encoded X protein (HBx) plays an important role in the development of hepatocellular carcinoma (HCC). The protein SH2 domain containing inositol 5-phosphatase 2 (SHIP2) belongs to the family of enzymes that dephosphorylate the 5 position of PI(3,4,5)P3 to produce PI(3,4)P2. Expression of SHIP2 has been associated with several cancers including HCC. However, its role in the development of HBV-related HCC remains elusive. In this study, we performed tissue microarray analysis using 49 cases of HCC to explore SHIP2 expression changes and found that SHIP2 was downregulated in HBV-positive HCC. In addition, S-phase kinase-associated protein 2 (SKP2), a component of the E3 ubiquitin–ligase complex, was increased in HCC cell lines that overexpressed HBx, which also showed a notable accumulation of polyubiquitinated SHIP2. Moreover, HCC cells with silenced SHIP2 had increased expression of mesenchymal markers, which promotes cell migration, enhances glucose uptake, and leads to resistance to the chemotherapy drug (5-Fluorouracil, 5-FU). Taken together, our results demonstrate that HBx downregulates SHIP2 through SKP2 and suggest a potential role for SHIP2 in HBx-mediated HCC migration.

Generation of endogenous pH-sensitive EGF receptor and its application in high-throughput screening for proteins involved in clathrin-mediated endocytosis

Previously we used gene-editing to label endogenous EGF receptor (EGFR) with GFP and demonstrate that picomolar concentrations of EGFR ligand drive signaling and endocytosis of EGFR in tumors in vivo (Pinilla-Macua et al., 2017). We now use gene-editing to insert a fluorogen activating protein (FAP) in the EGFR extracellular domain. Binding of the tandem dye pair MG-Bis-SA to FAP-EGFR provides a ratiometric pH-sensitive model with dual fluorescence excitation and a single far-red emission. The excitation ratio of fluorescence intensities was demonstrated to faithfully report the fraction of FAP-EGFR located in acidic endosomal/lysosomal compartments. Coupling native FAP-EGFR expression with the high method sensitivity has allowed development of a high-throughput assay to measure the rates of clathrin-mediated FAP-EGFR endocytosis stimulated with physiological EGF concentrations. The assay was utilized to screen a phosphatase siRNA library. These studies highlight the utility of endogenous pH-sensitive FAP-receptor chimeras in high-throughput analysis of endocytosis.

Phosphatidylinositol 5 Phosphate (PI5P): From Behind the Scenes to the Front (Nuclear) Stage

Phosphatidylinositol (PI)-related signaling plays a pivotal role in many cellular aspects, including survival, cell proliferation, differentiation, DNA damage, and trafficking. PI is the core of a network of proteins represented by kinases, phosphatases, and lipases which are able to add, remove or hydrolyze PI, leading to different phosphoinositide products. Among the seven known phosphoinositides, phosphatidylinositol 5 phosphate (PI5P) was the last to be discovered. PI5P presence in cells is very low compared to other PIs. However, much evidence collected throughout the years has described the role of this mono-phosphoinositide in cell cycles, stress response, T-cell activation, and chromatin remodeling. Interestingly, PI5P has been found in different cellular compartments, including the nucleus. Here, we will review the nuclear role of PI5P, describing how it is synthesized and regulated, and how changes in the levels of this rare phosphoinositide can lead to different nuclear outputs.

Polyphosphoinositide-Binding Domains: Insights from Peripheral Membrane and Lipid-Transfer Proteins

Within eukaryotic cells, biochemical reactions need to be organized on the surface of membrane compartments that use distinct lipid constituents to dynamically modulate the functions of integral proteins or influence the selective recruitment of peripheral membrane effectors. As a result of these complex interactions, a variety of human pathologies can be traced back to improper communication between proteins and membrane surfaces; either due to mutations that directly alter protein structure or as a result of changes in membrane lipid composition. Among the known structural lipids found in cellular membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the membrane-anchored precursor of low-abundance regulatory lipids, the polyphosphoinositides (PPIn), which have restricted distributions within specific subcellular compartments. The ability of PPIn lipids to function as signaling platforms relies on both non-specific electrostatic interactions and the selective stereospecific recognition of PPIn headgroups by specialized protein folds. In this chapter, we will attempt to summarize the structural diversity of modular PPIn-interacting domains that facilitate the reversible recruitment and conformational regulation of peripheral membrane proteins. Outside of protein folds capable of capturing PPIn headgroups at the membrane interface, recent studies detailing the selective binding and bilayer extraction of PPIn species by unique functional domains within specific families of lipid-transfer proteins will also be highlighted. Overall, this overview will help to outline the fundamental physiochemical mechanisms that facilitate localized interactions between PPIn lipids and the wide-variety of PPIn-binding proteins that are essential for the coordinate regulation of cellular metabolism and membrane dynamics.

Metformin increases glucose uptake and acts renoprotectively by reducing SHIP2 activity

Metformin, the first-line drug to treat type 2 diabetes (T2D), inhibits mitochondrial glycerolphosphate dehydrogenase in the liver to suppress gluconeogenesis. However, the direct target and the underlying mechanisms by which metformin increases glucose uptake in peripheral tissues remain uncharacterized. Lipid phosphatase Src homology 2 domain-containing inositol-5-phosphatase 2 (SHIP2) is upregulated in diabetic rodent models and suppresses insulin signaling by reducing Akt activation, leading to insulin resistance and diminished glucose uptake. Here, we demonstrate that metformin directly binds to and reduces the catalytic activity of the recombinant SHIP2 phosphatase domain in vitro. Metformin inhibits SHIP2 in cultured cells and in skeletal muscle and kidney of db/db mice. In SHIP2-overexpressing myotubes, metformin ameliorates reduced glucose uptake by slowing down glucose transporter 4 endocytosis. SHIP2 overexpression reduces Akt activity and enhances podocyte apoptosis, and both are restored to normal levels by metformin. SHIP2 activity is elevated in glomeruli of patients with T2D receiving nonmetformin medication, but not in patients receiving metformin, compared with people without diabetes. Furthermore, podocyte loss in kidneys of metformin-treated T2D patients is reduced compared with patients receiving nonmetformin medication. Our data unravel a novel molecular mechanism by which metformin enhances glucose uptake and acts renoprotectively by reducing SHIP2 activity.-Polianskyte-Prause, Z., Tolvanen, T. A., Lindfors, S., Dumont, V., Van, M., Wang, H., Dash, S. N., Berg, M., Naams, J.-B., Hautala, L. C., Nisen, H., Mirtti, T., Groop, P.-H., Wähälä, K., Tienari, J., Lehtonen, S. Metformin increases glucose uptake and acts renoprotectively by reducing SHIP2 activity.

The impact of phosphoinositide 5-phosphatases on phosphoinositides in cell function and human disease

Phosphoinositides (PIs) are recognized as major signaling molecules in many different functions of eukaryotic cells. PIs can be dephosphorylated by multiple phosphatase activities at the 5-, 4-, and 3- positions. Human PI 5-phosphatases belong to a family of 10 members. Except for inositol polyphosphate 5-phosphatase A, they all catalyze the dephosphorylation of PI(4,5)P₂ and/or PI(3,4,5)P₃ at the 5- position. PI 5-phosphatases thus directly control the levels of PI(3,4,5)P₃ and participate in the fine-tuning regulatory mechanisms of PI(3,4)P₂ and PI(4,5)P₂ Second messenger functions have been demonstrated for PI(3,4)P₂ in invadopodium maturation and lamellipodia formation. PI 5-phosphatases can use several substrates on isolated enzymes, and it has been challenging to establish their real substrate in vivo. PI(4,5)P₂ has multiple functions in signaling, including interacting with scaffold proteins, ion channels, and cytoskeleton proteins. PI 5-phosphatase isoenzymes have been individually implicated in human diseases, such as the oculocerebrorenal syndrome of Lowe, through mechanisms that include lipid control. Oncogenic and tumor-suppressive functions of PI 5-phosphatases have also been reported in different cell contexts. The mechanisms responsible for genetic diseases and for oncogenic or tumor-suppressive functions are not fully understood. The regulation of PI 5-phosphatases is thus crucial in understanding cell functions.

Lipid phosphatase SHIP2 functions as oncogene in colorectal cancer by regulating PKB activation

Colorectal cancer (CRC) is the second most common cause of cancer-related death, encouraging the search for novel therapeutic targets affecting tumor cell proliferation and migration. These cellular processes are under tight control of two opposing groups of enzymes; kinases and phosphatases. Aberrant activity of kinases is observed in many forms of cancer and as phosphatases counteract such "oncogenic" kinases, it is generally assumed that phosphatases function as tumor suppressors. However, emerging evidence suggests that the lipid phosphatase SH2-domain-containing 5 inositol phosphatase (SHIP2), encoded by the INPPL1 gene, may act as an oncogene. Just like the well-known tumor suppressor gene Phosphatase and Tensin Homolog (PTEN) it hydrolyses phosphatidylinositol (3,4,5) triphosphate (PI(3,4,5)P3). However, unlike PTEN, the reaction product is PI(3,4)P2, which is required for full activation of the downstream protein kinase B (PKB/Akt), suggesting that SHIP2, in contrast to PTEN, could have a tumor initiating role through PKB activation. In this work, we investigated the role of SHIP2 in colorectal cancer. We found that SHIP2 and INPPL1 expression is increased in colorectal cancer tissue in comparison to adjacent normal tissue, and this is correlated with decreased patient survival. Moreover, SHIP2 is more active in colorectal cancer tissue, suggesting that SHIP2 can induce oncogenesis in colonic epithelial cells. Furthermore, in vitro experiments performed on colorectal cancer cell lines shows an oncogenic role for SHIP2, by enhancing chemoresistance, cell migration, and cell invasion. Together, these data indicate that SHIP2 expression contributes to the malignant potential of colorectal cancer, providing a possible target in the fight against this devastating disease.

Phosphoinositide 5-phosphatase activities control cell motility in glioblastoma: Two phosphoinositides PI(4,5)P2 and PI(3,4)P2 are involved

Inositol polyphosphate 5-phosphatases or phosphoinositide 5-phosphatases (PI 5-phosphatases) are enzymes that can act on soluble inositol phosphates and/or phosphoinositides (PIs). Several PI 5-phosphatases have been linked to human genetic diseases, in particular the Lowe protein or OCRL which is mutated in the Lowe syndrome. There are 10 different members of this family and 9 of them can use PIs as substrate. One of these substrates, PI(3,4,5)P3 binds to specific PH domains and recruits as effectors specific proteins to signaling complexes. Protein kinase B is one target protein and activation of the kinase will have a major impact on cell proliferation, survival and cell metabolism. Two other PIs, PI(4,5)P2 and PI(3,4)P2, are produced or used as substrates of PI 5-phosphatases (OCRL, INPP5B, SHIP1/2, SYNJ1/2, INPP5K, INPP5J, INPP5E). The inositol lipids may influence many aspects of cytoskeletal organization, lamellipodia formation and F-actin polymerization. PI 5-phosphatases have been reported to control cell migration, adhesion, polarity and cell invasion particularly in cancer cells. In glioblastoma, reducing SHIP2 expression can positively or negatively affect the speed of cell migration depending on the glioblastoma cell type. The two PI 5-phosphatases SHIP2 or SKIP could be localized at the plasma membrane and can reduce either PI(3,4,5)P3 or PI(4,5)P2 abundance. In the glioblastoma 1321 N1 cells, SHIP2 controls plasma membrane PI(4,5)P2 thereby participating in the control of cell migration.

PTEN Regulates PI(3,4)P2 Signaling Downstream of Class I PI3K

The PI3K signaling pathway regulates cell growth and movement and is heavily mutated in cancer. Class I PI3Ks synthesize the lipid messenger PI(3,4,5)P₃. PI(3,4,5)P₃ can be dephosphorylated by 3- or 5-phosphatases, the latter producing PI(3,4)P₂. The PTEN tumor suppressor is thought to function primarily as a PI(3,4,5)P₃ 3-phosphatase, limiting activation of this pathway. Here we show that PTEN also functions as a PI(3,4)P₂ 3-phosphatase, both in vitro and in vivo. PTEN is a major PI(3,4)P₂ phosphatase in Mcf10a cytosol, and loss of PTEN and INPP4B, a known PI(3,4)P₂ 4-phosphatase, leads to synergistic accumulation of PI(3,4)P₂, which correlated with increased invadopodia in epidermal growth factor (EGF)-stimulated cells. PTEN deletion increased PI(3,4)P₂ levels in a mouse model of prostate cancer, and it inversely correlated with PI(3,4)P₂ levels across several EGF-stimulated prostate and breast cancer lines. These results point to a role for PI(3,4)P₂ in the phenotype caused by loss-of-function mutations or deletions in PTEN.

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PTEN is a PI(3,4)P₂ 3-phosphatase

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PTEN and INPP4B regulate PI(3,4)P₂ accumulation downstream of class I PI3K

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PTEN regulates PI(3,4)P₂-dependent activation of Akt and formation of invadopodia

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PI(3,4)P₂ signaling may play a role in the tumor suppressor function of PTEN

Novel compound heterozygous mutations in inositol polyphosphate phosphatase-like 1 in a family with severe opsismodysplasia

This study aimed to identify the genetic basis of a severe skeletal lethal dysplasia. The main clinical features of two affected fetuses included short limbs with flared metaphyses, bowed radii, femora and tibiae, irregular ossification of hands and feet, and marked platyspondyly. Affected and nonaffected family members were subjected to whole-exome sequencing, followed by immunoblot analysis on amniocytes isolated from one of the affected individuals. Unique compound heterozygous variants in the inositol polyphosphate phosphatase-like 1 (INPPL1) gene encoding the SHIP2 protein were identified in both affected individuals. One variant was inherited from each unaffected parent. Both allelic variants, c.(2327-1G>C);(1150_1151delGA), are predicted to result in premature stop codons leading to nonsense-mediated mRNA decay of the mutant alleles and no production of SHIP2. The absence of SHIP2 was confirmed by immunoblot analysis of proband amniocytes. This skeletal disorder is caused by the complete absence of the SHIP2 protein. INPPL1 mutations have been reported in opsismodysplasia, an autosomal recessive skeletal dysplasias with significant delayed bone formation. Our finding highlights the critical role that INPPL1/SHIP2 plays in skeletal development.

Importance of Radioactive Labelling to Elucidate Inositol Polyphosphate Signalling

Inositol polyphosphates, in their water-soluble or lipid-bound forms, represent a large and multifaceted family of signalling molecules. Some inositol polyphosphates are well recognised as defining important signal transduction pathways, as in the case of the calcium release factor Ins(1,4,5)P₃, generated by receptor activation-induced hydrolysis of the lipid PtdIns(4,5)P₂ by phospholipase C. The birth of inositol polyphosphate research would not have occurred without the use of radioactive phosphate tracers that enabled the discovery of the “PI response”. Radioactive labels, mainly of phosphorus but also carbon and hydrogen (tritium), have been instrumental in the development of this research field and the establishment of the inositol polyphosphates as one of the most important networks of regulatory molecules present in eukaryotic cells. Advancements in microscopy and mass spectrometry and the development of colorimetric assays have facilitated inositol polyphosphate research, but have not eliminated the need for radioactive experimental approaches. In fact, such experiments have become easier with the cloning of the inositol polyphosphate kinases, enabling the systematic labelling of specific positions of the inositol ring with radioactive phosphate. This approach has been valuable for elucidating their metabolic pathways and identifying specific and novel functions for inositol polyphosphates. For example, the synthesis of radiolabelled inositol pyrophosphates has allowed the discovery of a new protein post-translational modification. Therefore, radioactive tracers have played and will continue to play an important role in dissecting the many complex aspects of inositol polyphosphate physiology. In this review we aim to highlight the historical importance of radioactivity in inositol polyphosphate research, as well as its modern usage.

Emerging evidence of signalling roles for PI(3,4)P2 in Class I and II PI3K-regulated pathways

There are eight members of the phosphoinositide family of phospholipids in eukaryotes; PI, PI3P, PI4P, PI5P, PI(4,5)P2, PI(3,4)P2, PI(3,5)P2 and PI(3,4,5)P3. Receptor activation of Class I PI3Ks stimulates the phosphorylation of PI(4,5)P2 to form PI(3,4,5)P3. PI(3,4,5)P3 is an important messenger molecule that is part of a complex signalling network controlling cell growth and division. PI(3,4,5)P3 can be dephosphorylated by both 3- and 5-phosphatases, producing PI(4,5)P2 and PI(3,4)P2, respectively. There is now strong evidence that PI(3,4)P2 generated by this route does not merely represent another pathway for removal of PI(3,4,5)P3, but can act as a signalling molecule in its own right, regulating macropinocytosis, fast endophilin-mediated endocytosis (FEME), membrane ruffling, lamellipodia and invadopodia. PI(3,4)P2 can also be synthesized directly from PI4P by Class II PI3Ks and this is important for the maturation of clathrin-coated pits [clathrin-mediated endocytosis (CME)] and signalling in early endosomes. Thus PI(3,4)P2 is emerging as an important signalling molecule involved in the coordination of several specific membrane and cytoskeletal responses. Further, its inappropriate accumulation contributes to pathology caused by mutations in genes encoding enzymes responsible for its degradation, e.g. Inpp4B.

INPPL1 gene mutations in opsismodysplasia

The INPPL1 (inositol polyphosphate phosphatase-like 1) gene encodes the inositol phosphatase, SHIP2 (for src homology 2 domain-containing inositol phosphatase 2). SHIP2 functions to dephosphorylate, and negatively regulate, the lipid second messenger phosphatidylinositol (3,4,5)P3. SHIP2 has been well studied in the area of insulin resistance and obesity but has roles in cancer and other disorders. Recently, it was reported that mutations in INPPL1 cause opsismodysplasia, a rare, autosomal recessive severe skeletal dysplasia. This review focuses on the mutations associated with opsismodysplasia and explores the role of INPPL1/ SHIP2 in skeletal development.

Immunoreceptors on neutrophils

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Neutrophil activities must be tightly controlled to maintain immune homeostasis.

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Activating and inhibitory receptors balance the outcome of immune cell activation.

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Immunoreceptors contain Ig-like extracellular domains and signal via ITAMs or ITIMs.

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Syk or SHP/SHIP mediate downstream signaling after immunoreceptor activation.

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Targeting immunoreceptors provides opportunities for therapeutic interventions.

Neutrophils play a critical role in the host defense against infection, and they are able to perform a variety of effector mechanisms for this purpose. However, there are also a number of pathological conditions, including autoimmunity and cancer, in which the activities of neutrophils can be harmful to the host. Thus the activities of neutrophils need to be tightly controlled. As in the case of other immune cells, many of the neutrophil effector functions are regulated by a series of immunoreceptors on the plasma membrane. Here, we review what is currently known about the functions of the various individual immunoreceptors and their signaling in neutrophils. While these immunoreceptors allow for the recognition of a diverse range of extracellular ligands, such as cell surface structures (like proteins, glycans and lipids) and extracellular matrix components, they commonly signal via conserved ITAM or ITIM motifs and their associated downstream pathways that depend on the phosphorylation of tyrosine residues in proteins and/or inositol lipids. This allows for a balanced homeostatic regulation of neutrophil effector functions. Given the number of available immunoreceptors and their fundamental importance for neutrophil behavior, it is perhaps not surprising that pathogens have evolved means to evade immune responses through some of these pathways. Inversely, some of these receptors evolved to specifically recognize these pathogens. Finally, some interactions mediated by immunoreceptors in neutrophils have been identified as promising targets for therapeutic intervention.

SHIP2 controls plasma membrane PI(4,5)P2 thereby participating in the control of cell migration in 1321 N1 glioblastoma

Phosphoinositides, particularly PI(3,4,5)P3, and PI(4,5)P2, are recognized by SHIP2 a member of the inositol polyphosphate 5-phosphatase family. SHIP2 dephosphorylates PI(3,4,5)P3 to form PI(3,4)P2; the latter interacts with specific target proteins (e.g. lamellipodin). Although the SHIP2 preferred substrate is PI(3,4,5)P3, PI(4,5)P2 could also be dephosphorylated to PI4P. Through depletion of SHIP2 in a glioblastoma cell line 1321 N1 cells, we show that SHIP2 inhibits cell migration. In different glioblastoma cell lines and primary cultures, SHIP2 staining at the plasma membrane partly overlaps with PI(4,5)P2 immunoreactivity. PI(4,5)P2 was upregulated in SHIP2-deficient N1 cells as compared to control cells; in contrast, PI4P was very much decreased in SHIP2-deficient cells. Therefore, SHIP2 controls both PI(3,4,5)P3 and PI(4,5)P2 levels in intact cells. In N1 cells, the PI(4,5)P2 binding protein myosin-1c was identified as a new interactor of SHIP2. Regulation of PI(4,5)P2 and PI4P content by SHIP2 controls N1 cell migration through the organization of focal adhesions. Thus, our results reveal a novel role of SHIP2 in the control of PI(4,5)P2, PI4P and cell migration in PTEN-deficient glioblastoma N1 cells.

PTP1B-dependent regulation of receptor tyrosine kinase signaling by the actin-binding protein Mena

The actin-binding protein Mena regulates RTK signaling after growth factor stimulation in tumor cells by a novel mechanism. The alternatively spliced MenaINV isoform disrupts this attenuation to drive sensitivity to growth factors, resistance to targeted inhibitors, and ultimately tumor invasion and metastasis.

During breast cancer progression, alternative mRNA splicing produces functionally distinct isoforms of Mena, an actin regulator with roles in cell migration and metastasis. Aggressive tumor cell subpopulations express Mena^INV, which promotes tumor cell invasion by potentiating EGF responses. However, the mechanism by which this occurs is unknown. Here we report that Mena associates constitutively with the tyrosine phosphatase PTP1B and mediates a novel negative feedback mechanism that attenuates receptor tyrosine kinase signaling. On EGF stimulation, complexes containing Mena and PTP1B are recruited to the EGFR, causing receptor dephosphorylation and leading to decreased motility responses. Mena also interacts with the 5′ inositol phosphatase SHIP2, which is important for the recruitment of the Mena-PTP1B complex to the EGFR. When Mena^INV is expressed, PTP1B recruitment to the EGFR is impaired, providing a mechanism for growth factor sensitization to EGF, as well as HGF and IGF, and increased resistance to EGFR and Met inhibitors in signaling and motility assays. In sum, we demonstrate that Mena plays an important role in regulating growth factor–induced signaling. Disruption of this attenuation by Mena^INV sensitizes tumor cells to low–growth factor concentrations, thereby increasing the migration and invasion responses that contribute to aggressive, malignant cell phenotypes.

Identification of SHIP-1 and SHIP-2 homologs in channel catfish, Ictalurus punctatus

Src homology domain 2 (SH2) domain-containing inositol 5'-phosphatases (SHIP) proteins have diverse roles in signal transduction. SHIP-1 and SHIP-2 homologs were identified in channel catfish, Ictalurus punctatus, based on sequence homology to murine and human SHIP sequences. Full-length cDNAs for catfish SHIP-1 and SHIP-2 (IpSHIP-1 and IpSHIP-2) were obtained using 5' and 3' RACE protocols. Catfish SHIP molecules share a high degree of sequence identity to their respective SHIP sequences from diverse taxa and both are encoded by single copy genes. IpSHIP-1 and IpSHIP-2 transcripts were expressed in all catfish tissues analyzed except for skin, and IpSHIP-1 message was more abundant than IpSHIP-2 message in lymphoid tissues. Catfish clonal B, cytotoxic T, and macrophage cell lines also expressed message for both molecules. IpSHIP-1 and IpSHIP-2 SH2 domains were expressed as recombinant proteins and were both found to be bound by cross-reacting rabbit anti-mouse SHIP-1 pAb. The anti-mouse SHIP-1 pAb also reacted with cell lysates from the cytotoxic T cell lines, macrophages and stimulated PBL. SHIP-1 is also phosphorylated at a conserved tyrosine residue, as shown by immunoprecipitation studies.

Mendelian disorders of PI metabolizing enzymes

More than twenty different genetic diseases have been described that are caused by mutations in phosphoinositide metabolizing enzymes, mostly in phosphoinositide phosphatases. Although generally ubiquitously expressed, mutations in these enzymes, which are mainly loss-of-function, result in tissue-restricted clinical manifestations through mechanisms that are not completely understood. Here we analyze selected disorders of phosphoinositide metabolism grouped according to the principle tissue affected: the nervous system, muscle, kidney, the osteoskeletal system, the eye, and the immune system. We will highlight what has been learnt so far from the study of these disorders about not only the cellular and molecular pathways that are involved or are governed by phosphoinositides, but also the many gaps that remain to be filled to gain a full understanding of the pathophysiological mechanisms underlying the clinical manifestations of this steadily growing class of diseases, most of which still remain orphan in terms of treatment. This article is part of a Special Issue entitled Phosphoinositides.

Structure, function and inhibition of the phosphoinositide 3-kinase p110α enzyme

The PI3K (phosphoinositide 3-kinase) p110α isoform is activated by oncogenic mutations in many cancers. This has stimulated intense interest in identifying inhibitors of the PI3K pathway as well as p110α-selective inhibitors, and understanding the mechanisms underlying activation by the oncogenic mutations. In the present article, we review recent progress in the structure and function of the p110α enzyme and two of its most common oncogenic mutations, the development of isoform-selective inhibitors, and p110α pharmacology.

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.

PI(4,5)P2 5-phosphatase A regulates PI3K/Akt signalling and has a tumour suppressive role in human melanoma

Inositol polyphosphate 5-phosphatases can terminate downstream signalling of phosphatidylinositol-3 kinase; however, their biological role in the pathogenesis of cancer is controversial. Here we report that the inositol polyphosphate 5-phosphatase, phosphatidylinositol 4,5-bisphosphate 5-phosphatase, has a tumour suppressive role in melanoma. Although it is commonly downregulated in melanoma, overexpression of phosphatidylinositol 4,5-bisphosphate 5-phosphatase blocks Akt activation, inhibits proliferation and undermines survival of melanoma cells in vitro, and retards melanoma growth in a xenograft model. In contrast, knockdown of phosphatidylinositol 4,5-bisphosphate 5-phosphatase results in increased proliferation and anchorage-independent growth of melanocytes. Although DNA copy number loss is responsible for downregulation of phosphatidylinositol 4,5-bisphosphate 5-phosphatase in a proportion of melanomas, histone hypoacetylation mediated by histone deacetylases HDAC2 and HDAC3 through binding to the transcription factor Sp1 at the PIB5PA gene promoter appears to be another commonly involved mechanism. Collectively, these results establish the tumour suppressive role of phosphatidylinositol 4,5-bisphosphate 5-phosphatase and reveal mechanisms involved in its downregulation in melanoma.

Inositol polyphosphate 5-phosphatases, such as PIB5PA, terminate signalling downstream of phosophoinositide-3 kinase; however, their biological roles remain unclear. Here the authors report that PIB5PA has a tumour suppressive role in melanoma.

SHIP2 regulates epithelial cell polarity through its lipid product, which binds to Dlg1, a pathway subverted by hepatitis C virus core protein

The main targets of hepatitis C virus (HCV) are hepatocytes, the highly polarized cells of the liver, and all the steps of its life cycle are tightly dependent on host lipid metabolism. The interplay between polarity and lipid metabolism in HCV infection has been poorly investigated. Signaling lipids, such as phosphoinositides (PIs), play a vital role in polarity, which depends on the distribution and expression of PI kinases and PI phosphatases. In this study, we report that HCV core protein, expressed in Huh7 and Madin-Darby canine kidney (MDCK) cells, disrupts apicobasal polarity. This is associated with decreased expression of the polarity protein Dlg1 and the PI phosphatase SHIP2, which converts phosphatidylinositol 3,4,5-trisphosphate into phosphatidylinositol 4,5-bisphosphate (PtdIns(3,4)P2). SHIP2 is mainly localized at the basolateral membrane of polarized MDCK cells. In addition, PtdIns(3,4)P2 is able to bind to Dlg1. SHIP2 small interfering RNA or its catalytically dead mutant disrupts apicobasal polarity, similar to HCV core. In core-expressing cells, RhoA activity is inhibited, whereas Rac1 is activated. Of interest, SHIP2 expression rescues polarity, RhoA activation, and restricted core level in MDCK cells. We conclude that SHIP2 is an important regulator of polarity, which is subverted by HCV in epithelial cells. It is suggested that SHIP2 could be a promising target for anti-HCV treatment.

How does SHIP1/2 balance PtdIns(3,4)P2 and does it signal independently of its phosphatase activity?

The number of cellular events identified as being directly or indirectly modulated by phosphoinositides dramatically increased in the recent years. Part of the complexity results from the fact that the seven phosphoinositides play second messenger functions in many different areas of growth factors and insulin signaling, cytoskeletal organization, membrane dynamics, trafficking, or nuclear signaling. PtdIns(3,4)P2 is commonly reported as a product of the SH2 domain-containing inositol 5-phosphatases 1/2 (SHIP1 and SHIP2) that dephosphorylate PtdIns(3,4,5)P3 at the 5-position. Here we discuss recent interest in PtdIns(3,4)P2 signaling highlighting its involvement in key cellular mechanisms such as cell adhesion, migration, and cytoskeletal regulation. We question and discuss the involvement of SHIP2 either as a PI 5-phosphatase or as a scaffold protein in insulin signaling, cytoskeletal dynamics, and endocytosis of growth factor receptors.

Whole-genome analysis reveals that mutations in inositol polyphosphate phosphatase-like 1 cause opsismodysplasia

Opsismodysplasia is a rare, autosomal-recessive skeletal dysplasia characterized by short stature, characteristic facial features, and in some cases severe renal phosphate wasting. We used linkage analysis and whole-genome sequencing of a consanguineous trio to discover that mutations in inositol polyphosphate phosphatase-like 1 (INPPL1) cause opsismodysplasia with or without renal phosphate wasting. Evaluation of 12 families with opsismodysplasia revealed that INPPL1 mutations explain ~60% of cases overall, including both of the families in our cohort with more than one affected child and 50% of the simplex cases.

SHIP2 signalling at the plasma membrane, in the nucleus and at focal contacts

Phosphoinositide 5-phosphatases are critical enzymes in modulating the concentrations of PI(3,4,5)P(3), PI(4,5)P(2) and PI(3,5)P(2). The SH2 domain containing inositol 5-phosphatases SHIP1 and SHIP2 belong to this family of enzymes very much involved in physiopathology and development. Therefore activity and localization of the enzymes are particularly important taking into account both catalytic and non-catalytic mechanisms of the SHIP phosphatases. Several different mechanisms have been reported for SHIP2 targeting that often result from specific protein:protein interactions. In unstimulated astrocytoma cells, SHIP2 has a perinuclear and cytoplasmic localization. In serum-stimulated cells, SHIP2 can be localized at the plasma membrane and at focal contacts in polarized cells. A phosphorylated form of SHIP2 on S132 can be found in the nucleus and nuclear speckles. When present at the plasma membrane, SHIP2 may control the intracellular level of PI(3,4,5)P(3) thereby producing PI(3,4)P(2). When present in the nucleus, SHIP2 probably associates to other nuclear proteins such as lamin A/C and could potentially control nuclear PI(4,5)P(2). Finally, its presence at focal adhesions and lamellipodia could suggest a role in cell adhesion and migration. It is proposed that the complex phenotype observed in SHIP2 mutant mice in tissue development and growth could result from the addition of plasma membrane and nuclear effects consecutive to SHIP2 alteration.

Role of phosphatidylinositol 3,4,5-trisphosphate in cell signaling

Many lipids present in cellular membranes are phosphorylated as part of signaling cascades and participate in the recruitment, localization, and activation of downstream protein effectors. Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) is one of the most important second messengers and is capable of interacting with a variety of proteins through specific PtdIns(3,4,5)P3 binding domains. Localization and activation of these effector proteins controls a myriad of cellular functions including cell survival, proliferation, cytoskeletal rearrangement, and gene expression. Aberrations in the production and metabolism of PtdIns(3,4,5)P3 have been implicated in many human diseases including cancer, diabetes, inflammation, and heart disease. This chapter provides an overview of the role of PtdIns(3,4,5)P3 in cellular regulation and the implications of PtdIns(3,4,5)P3 dysregulation in human diseases. Additionally, recent attempts at targeting PtdIns(3,4,5)P3 signaling via small molecule inhibitors are summarized.

A phosphatidylinositol lipids system, lamellipodin, and Ena/VASP regulate dynamic morphology of multipolar migrating cells in the developing cerebral cortex

In the developing mammalian cerebral cortex, excitatory neurons are generated in the ventricular zone (VZ) and subventricular zone; these neurons migrate toward the pial surface. The neurons generated in the VZ assume a multipolar morphology and remain in a narrow region called the multipolar cell accumulation zone (MAZ) for ∼24 h, in which they extend and retract multiple processes dynamically. They eventually extend an axon tangentially and begin radial migration using a migratory mode called locomotion. Despite the potential biological importance of the process movement of multipolar cells, the molecular mechanisms remain to be elucidated. Here, we observed that the processes of mouse multipolar cells were actin rich and morphologically resembled the filopodia and lamellipodia in growth cones; thus, we focused on the actin-remodeling proteins Lamellipodin (Lpd) and Ena/vasodilator-stimulated phosphoprotein (VASP). Lpd binds to phosphatidylinositol (3,4)-bisphosphate [PI(3,4)P₂] and recruits Ena/VASP, which promotes the assembly of actin filaments, to the plasma membranes. In situ hybridization and immunohistochemistry revealed that Lpd is expressed in multipolar cells in the MAZ. The functional silencing of either Lpd or Ena/VASP decreased the number of primary processes. Immunostaining and a Förster resonance energy transfer analysis revealed the subcellular localization of PI(3,4)P₂ at the tips of the processes. A knockdown experiment and treatment with an inhibitor for Src homology 2-containing inositol phosphatase-2, a 5-phosphatase that produces PI(3,4)P₂ from phosphatidylinositol (3,4,5)-triphosphate, decreased the number of primary processes. Our observations suggest that PI(3,4)P₂, Lpd, and Ena/VASP are involved in the process movement of multipolar migrating cells.

Pharmacological targeting of phosphoinositide lipid kinases and phosphatases in the immune system: success, disappointment, and new opportunities

The predominant expression of the γ and δ isoforms of PI3K in cells of hematopoietic lineage prompted speculation that inhibitors of these isoforms could offer opportunities for selective targeting of PI3K in the immune system in a range of immune-related pathologies. While there has been some success in developing PI3Kδ inhibitors, progress in developing selective inhibitors of PI3Kγ has been rather disappointing. This has prompted the search for alternative targets with which to modulate PI3K signaling specifically in the immune system. One such target is the SH2 domain-containing inositol-5-phosphatase-1 (SHIP-1) which de-phosphorylates PI(3,4,5)P₃ at the D5 position of the inositol ring to create PI(3,4)P₂. In this article, we first describe the current state of PI3K isoform-selective inhibitor development. We then focus on the structure of SHIP-1 and its function in the immune system. Finally, we consider the current state of development of small molecule compounds that potently and selectively modulate SHIP activity and which offer novel opportunities to manipulate PI3K mediated signaling in the immune system.

Developmental defects and rescue from glucose intolerance of a catalytically-inactive novel Ship2 mutant mouse

The function of the phosphoinositide 5-phosphatase Ship2 was investigated in a new mouse model expressing a germline catalytically-inactive Ship2(∆/∆) mutant protein. Ship2(∆/∆) mice were viable with defects in somatic growth and in development of muscle, adipose tissue and female genital tract. Lipid metabolism and insulin secretion were also affected in these mice, but glucose tolerance, insulin sensitivity and insulin-induced PKB phosphorylation were not. We expected that the expression of the catalytically inactive Ship2 protein in PI 3'-kinase-defective p110α(D933A/+) mice would counterbalance the phenotypes of parental mice by restoring normal PKB signaling but, for most of the parameters tested, this was not the case. Indeed, often, the Ship2(∆/∆) phenotype had a dominant effect over the p110α(D933A/+) phenotype and, sometimes, there was a surprising additive effect of both mutations. p110α(D933A/+)Ship2(∆/∆) mice still displayed a reduced PKB phosphorylation in response to insulin, compared to wild type mice yet had a normal glucose tolerance and insulin sensitivity, like the Ship2(∆/∆) mice. Together, our results suggest that the Ship2(∆/∆) phenotype is not dependent on an overstimulated class I PI 3-kinase-PKB signaling pathway and thus, indirectly, that it may be more dependent on the lack of Ship2-produced phosphatidylinositol 3,4-bisphosphate and derived phosphoinositides.

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.

Phosphoinositides in insulin action and diabetes

Phosphoinositides play an essential role in insulin signaling, serving as a localization signal for a variety of proteins that participate in the regulation of cellular growth and metabolism. This chapter will examine the regulation and localization of phosphoinositide species, and will explore the roles of these lipids in insulin action. We will also discuss the changes in phosphoinositide metabolism that occur in various pathophysiological states such as insulin resistance and diabetes.

Endosomal transport via ubiquitination

Cell survival, growth, differentiation and homeostasis rely on exquisite control of the abundance of particular cell-surface membrane proteins. Cell-surface proteins must respond appropriately to environmental and intracellular cues, often undergoing regulated internalization and lysosomal degradation. These proteins also can sustain damage and must be recognized and removed. A unifying mechanism has emerged for the trafficking of damaged and downregulated proteins to the lysosome by their attachment to ubiquitin (Ub), which serves as a sorting signal for clathrin-mediated internalization and sorting into late endosomes. Major questions remain as to how this system is governed, how it is adapted for different proteins, and whether Ub serves as more than a one-way ticket to the lysosome for degradation. Here, we highlight recent insights and the challenges that remain.