Phosphoinositide phosphatases in a network of signalling reactions

New insights into the regulation and roles of phosphatidylinositol 3,4-bisphosphate

Phosphoinositides (PIPs) are phospholipids and components of the cellular membrane. In mammals, seven phosphorylated derivatives of PIPs have been identified. Among them, phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2] is produced by lipid phosphatases (e.g., SHIP2) or by lipid kinases PI3KC2α and PI3KC2β. Although PI(3,4)P2 is undetectable in normal mouse or human tissues and common cell lines, it appears in a mouse prostate cancer model and in cells exposed to oxidative stress, indicating that PI(3,4)P2 is involved in the pathogenesis of some diseases. Here, I summarize recent findings on the cellular roles and pathophysiological significance of PI(3,4)P2.

Exploration of gut microbiome and inflammation: A review on key signalling pathways

The gut microbiome, a crucial component of the human system, is a diverse collection of microbes that belong to the gut of human beings as well as other animals. These microbial communities continue to coexist harmoniously with their host organisms and perform various functions that affect the host's general health. Each person's gut microbiota has a unique makeup. The gut microbiota is well acknowledged to have a part in the local as well as systemic inflammation that underlies a number of inflammatory disorders (e.g., atherosclerosis, diabetes mellitus, obesity, and inflammatory bowel disease).The gut microbiota's metabolic products, such as short-chain fatty acids (butyrate, propionate, and acetate) inhibit inflammation by preventing immune system cells like macrophages and neutrophils from producing pro-inflammatory factors, which are triggered by the structural elements of bacteria (like lipopolysaccharide). The review's primary goal is to provide comprehensive and compiled data regarding the contribution of gut microbiota to inflammation and the associated signalling pathways.

CgSHIP2 negatively regulates the mRNA expressions of CgIL-17s in response to Vibrio splendidus stimulation in Crassostrea gigas

SH2 domain containing inositol polyphosphate5-phosphatase-2 (SHIP2) is a member of the 5-phosphatase family, acting as a vital negative regulator of immune response in vertebrates. In the present study, a SHIP2 homologue (designed as CgSHIP2) was identified from Pacific oyster, Crassostrea gigas. There was a SH2 domain, an IPPc domain and a SAM domain in CgSHIP2. The mRNA transcripts of CgSHIP2 were widely expressed in all the tested tissues with the highest expression in haemolymph. The mRNA expressions of CgSHIP2 in haemocytes increased significantly at 6, 12, 48 and 72 h after Vibrio splendidus stimulation. The positive green signals of CgSHIP2 protein were mainly located in cytoplasm of haemocytes. After the expression of CgSHIP2 was inhibited by RNA interference, the mRNA transcripts of interleukin 17s (CgIL-17-1, CgIL-17-2, CgIL-17-3 and CgIL-17-6) in the haemocytes increased significantly at 24 h after V. splendidus stimulation, which were 8.15-fold (p < 0.001), 3.44-fold (p < 0.05), 2.15-fold (p < 0.01) and 4.63-fold (p < 0.05) compared with that in NC-RNAi group, respectively. Obvious branchial swelling and cilium shedding in gills were observed in CgSHIP2-RNAi group at 24 h after V. splendidus stimulation. The results suggested that CgSHIP2 played an important role in controlling inflammatory response induced by bacteria in oysters.

Myo-D-inositol Trisphosphate Signalling in Oomycetes

Oomycetes are pathogens of plants and animals, which cause billions of dollars of global losses to the agriculture, aquaculture and forestry sectors each year. These organisms superficially resemble fungi, with an archetype being Phytophthora infestans, the cause of late blight of tomatoes and potatoes. Comparison of the physiology of oomycetes with that of other organisms, such as plants and animals, may provide new routes to selectively combat these pathogens. In most eukaryotes, myo-inositol 1,4,5 trisphosphate is a key second messenger that links extracellular stimuli to increases in cytoplasmic Ca2+, to regulate cellular activities. In the work presented in this study, investigation of the molecular components of myo-inositol 1,4,5 trisphosphate signaling in oomycetes has unveiled similarities and differences with that in other eukaryotes. Most striking is that several oomycete species lack detectable phosphoinositide-selective phospholipase C homologues, the enzyme family that generates this second messenger, but still possess relatives of myo-inositol 1,4,5 trisphosphate-gated Ca2+-channels.

The INPP4B paradox: Like PTEN, but different

Cancer is a complex and heterogeneous disease marked by the dysregulation of cancer driver genes historically classified as oncogenes or tumour suppressors according to their ability to promote or inhibit tumour development and growth, respectively. Certain genes display both oncogenic and tumour suppressor functions depending on the biological context, and as such have been termed dual-role cancer driver genes. However, because of their context-dependent behaviour, the tumourigenic mechanism of many dual-role genes is elusive and remains a significant knowledge gap in our effort to understand and treat cancer. Inositol polyphosphate 4-phosphatase type II (INPP4B) is an emerging dual-role cancer driver gene, primarily known for its role as a negative regulator of the phosphoinositide 3-kinase (PI3K)/AKT signalling pathway. In response to growth factor stimulation, class I PI3K generates PtdIns(3,4,5)P₃ at the plasma membrane. PtdIns(3,4,5)P₃ can be hydrolysed by inositol polyphosphate 5-phosphatases to generate PtdIns(3,4)P₂, which, together with PtdIns(3,4,5)P₃, facilitates the activation of AKT to promote cell proliferation, survival, migration, and metabolism. Phosphatase and tensin homology on chromosome 10 (PTEN) and INPP4B are dual-specificity phosphatases that hydrolyse PtdIns(3,4,5)P₃ and PtdIns(3,4)P₂, respectively, and thus negatively regulate PI3K/AKT signalling. PTEN is a bona fide tumour suppressor that is frequently lost in human tumours. INPP4B was initially characterised as a tumour suppressor akin to PTEN, and has been implicated as such in a number of cancers, including prostate, thyroid, and basal-like breast cancers. However, evidence has since emerged revealing INPP4B as a paradoxical oncogene in several malignancies, with increased INPP4B expression reported in AML, melanoma and colon cancers among others. Although the tumour suppressive function of INPP4B has been mostly ascribed to its ability to negatively regulate PI3K/AKT signalling, its oncogenic function remains less clear, with proposed mechanisms including promotion of PtdIns(3)P-dependent SGK3 signalling, inhibition of PTEN-dependent AKT activation, and enhancing DNA repair mechanisms to confer chemoresistance. Nevertheless, research is ongoing to identify the factors that dictate the tumourigenic output of INPP4B in different human cancers. In this review we discuss the dualistic role that INPP4B plays in the context of cancer development, progression and treatment, drawing comparisons to PTEN to explore how their similarities and, importantly, their differences may account for their diverging roles in tumourigenesis.

The phosphoinositide 5-phosphatase INPP5K: From gene structure to in vivo functions

INPP5K (Inositol Polyphosphate 5-Phosphatase K, or SKIP (for Skeletal muscle and Kidney enriched Inositol Phosphatase) is a member of the phosphoinositide 5-phosphatases family. Its protein structure is comprised of a N-terminal catalytic domain which hydrolyses both PtdIns(4,5)P2 and PtdIns(3,4,5)P3, followed by a SKICH domain at the C-terminus which is responsible for protein-protein interactions and subcellular localization of INPP5K. Strikingly, INPP5K is mostly concentrated in the endoplasmic reticulum, although it is also detected at the plasma membrane, in the cytosol and the nucleus. Recently, mutations in INPP5K have been detected in patients with a rare form of autosomal recessive congenital muscular dystrophy with cataract, short stature and intellectual disability. INPP5K functions extend from control of insulin signaling, endoplasmic reticulum stress response and structural integrity, myoblast differentiation, cytoskeleton organization, cell adhesion and migration, renal osmoregulation, to cancer. The goal of this review is thus to summarize and comment recent and less recent data in the literature on INPP5K, in particular on the structure, expression, intracellular localization, interactions and functions of this specific member of the 5-phosphatases family.

Exploring the controversial role of PI3K signalling in CD4+ regulatory T (T-Reg) cells

The immune system is a complex network that acts to protect vertebrates from foreign microorganisms and carries out immunosurveillance to combat cancer. In order to avoid hyper-activation of the immune system leading to collateral damage tissues and organs and to prevent self-attack, the network has the intrinsic control mechanisms that negatively regulate immune responses. Central to this negative regulation are regulatory T (T-Reg) cells, which through cytokine secretion and cell interaction limit uncontrolled clonal expansion and functions of activated immune cells. Given that positive or negative manipulation of T-Regs activity could be utilised to therapeutically treat host versus graft rejection or cancer respectively, understanding how signaling pathways impact on T-Regs function should reveal potential targets with which to intervene. The phosphatidylinositol-3-kinase (PI3K) pathway controls a vast array of cellular processes and is critical in T cell activation. Here we focus on phosphoinositide 3-kinases (PI3Ks) and their ability to regulate T-Regs cell differentiation and function.

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.

Microglia Mediated Neuroinflammation: Focus on PI3K Modulation

Immune activation in the central nervous system involves mostly microglia in response to pathogen invasion or tissue damage, which react, promoting a self-limiting inflammatory response aimed to restore homeostasis. However, prolonged, uncontrolled inflammation may result in the production by microglia of neurotoxic factors that lead to the amplification of the disease state and tissue damage. In particular, specific inducers of inflammation associated with neurodegenerative diseases activate inflammatory processes that result in the production of a number of mediators and cytokines that enhance neurodegenerative processes. Phosphoinositide 3-kinases (PI3Ks) constitute a family of enzymes regulating a wide range of activity, including signal transduction. Recent studies have focused attention on the intracellular role of PI3K and its contribution to neurodegenerative processes. This review illustrates and discusses recent findings about the role of this signaling pathway in the modulation of microglia neuroinflammatory responses linked to neurodegeneration. Finally, we discuss the modulation of PI3K as a potential therapeutic approach helpful for developing innovative therapeutic strategies in neurodegenerative diseases.

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.

Legionella pneumophila effector WipA, a bacterial PPP protein phosphatase with PTP activity

The gram-negative bacterium Legionella pneumophila invades human's lung and causes Legionnaires' disease. To benefit its survival and replication in cellular milieu, L. pneumophila secrets at least 330 effector proteins into host cells. We found that the effector WipA has the protein tyrosine phosphatase (PTP) activity but does not depend on the classical CX5R motif for activity, suggesting that WipA is an unconventional PTP. Meanwhile, the presence of three other highly conserved motifs typically seen in protein serine/threonine phosphatases and the poor inhibition of WipA activity by okadaic acid led us to propose that WipA is a bacterial protein phosphatase. In addition, the determination of the 2.55-Å crystal structure of WipA revealed that WipA resembles cold-active protein tyrosine phosphatase (CAPTPase), and therefore very likely shares the same catalytic mechanism.

Modulation of host signaling in the inflammatory response by enteropathogenic Escherichia coli virulence proteins

To successfully infect host cells and evade the host immune response, a type III secretion system (T3SS) is commonly used by enteric bacterial pathogens such as enteropathogenic Escherichia coli (EPEC). Recent findings have revealed that various effectors are injected into host cells through the T3SS and exert an inhibitory effect on inflammatory signaling pathways, subverting the immune responses to these pathogens. Here we review recent studies aimed at addressing the modulation of several important inflammatory signaling pathways modulated by EPEC effector proteins, such as the nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways, which provides insight into the unfinished work in this unexplored field and helps to identify novel positions in inflammatory signaling networks for EPEC effectors.Cellular & Molecular Immunology advance online publication, 31 October 2016; doi:10.1038/cmi.2016.52.

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.

Perturbations of PIP3 signalling trigger a global remodelling of mRNA landscape and reveal a transcriptional feedback loop

PIP3 is synthesized by the Class I PI3Ks and regulates complex cell responses, such as growth and migration. Signals that drive long-term reshaping of cell phenotypes are difficult to resolve because of complex feedback networks that operate over extended times. PIP3-dependent modulation of mRNA accumulation is clearly important in this process but is poorly understood. We have quantified the genome-wide mRNA-landscape of non-transformed, breast epithelium-derived MCF10a cells and its response to acute regulation by EGF, in the presence or absence of a PI3Kα inhibitor, compare it to chronic activation of PI3K signalling by cancer-relevant mutations (isogenic cells expressing an oncomutant PI3Kα allele or lacking the PIP3-phosphatase/tumour-suppressor, PTEN). Our results show that whilst many mRNAs are changed by long-term genetic perturbation of PIP3 signalling ('butterfly effect'), a much smaller number do so in a coherent fashion with the different PIP3 perturbations. This suggests a subset of more directly regulated mRNAs. We show that mRNAs respond differently to given aspects of PIP3 regulation. Some PIP3-sensitive mRNAs encode PI3K pathway components, thus suggesting a transcriptional feedback loop. We identify the transcription factor binding motifs SRF and PRDM1 as important regulators of PIP3-sensitive mRNAs involved in cell movement.

Characterization of the protein tyrosine phosphatase PRL from Entamoeba histolytica

Protein tyrosine phosphatase of regenerating liver (PRL) is a group of phosphatases that has not been broadly studied in protozoan parasites. In humans, PRLs are involved in metastatic cancer, the promotion of cell migration and invasion. PTPs have been increasingly recognized as important effectors of host-pathogen interactions. We characterized the only putative protein tyrosine phosphatase PRL (PTP EhPRL) in the eukaryotic human intestinal parasite Entamoeba histolytica. Here, we reported that the EhPRL protein possessed the classical HCX5R catalytic motif of PTPs and the CAAX box characteristic of the PRL family and exhibited 31-32% homology with the three human PRL isoforms. In amebae, the protein was expressed at low but detectable levels. The recombinant protein (rEhPRL) had enzymatic activity with the 3-o-methyl fluorescein phosphate (OMFP) substrate; this enzymatic activity was inhibited by the PTP inhibitor o-vanadate. Using immunofluorescence we showed that native EhPRL was localized to the cytoplasm and plasma membrane. When the trophozoites interacted with collagen, EhPRL relocalized over time to vesicle-like structures. Interaction with fibronectin increased the presence of the enzyme in the cytoplasm. Using RT-PCR, we demonstrated that EhPRL mRNA expression was upregulated when the trophozoites interacted with collagen but not with fibronectin. Trophozoites recovered from amoebic liver abscesses showed higher EhPRL mRNA expression levels than normal trophozoites. These results strongly suggest that EhPRL may play an important role in the biology and adaptive response of the parasite to the host environment during amoebic liver abscess development, thereby participating in the pathogenic mechanism.

Phosphoinositide turnover in Toll-like receptor signaling and trafficking

Lipid components in biological membranes are essential for maintaining cellular function. Phosphoinositides, the phosphorylated derivatives of phosphatidylinositol (PI), regulate many critical cell processes involving membrane signaling, trafficking, and reorganization. Multiple metabolic pathways including phosphoinositide kinases and phosphatases and phospholipases tightly control spatio-temporal concentration of membrane phosphoinositides. Metabolizing enzymes responsible for PI 4,5-bisphosphate (PI(4,5)P2) production or degradation play a regulatory role in Toll-like receptor (TLR) signaling and trafficking. These enzymes include PI 4-phosphate 5-kinase, phosphatase and tensin homolog, PI 3-kinase, and phospholipase C. PI(4,5)P2 mediates the interaction with target cytosolic proteins to induce their membrane translocation, regulate vesicular trafficking, and serve as a precursor for other signaling lipids. TLR activation is important for the innate immune response and is implicated in diverse pathophysiological disorders. TLR signaling is controlled by specific interactions with distinct signaling and sorting adaptors. Importantly, TLR signaling machinery is differentially formed depending on a specific membrane compartment during signaling cascades. Although detailed mechanisms remain to be fully clarified, phosphoinositide metabolism is promising for a better understanding of such spatio-temporal regulation of TLR signaling and trafficking. [BMB Reports 2014; 47(7): 361-368]

Phosphatidylinositol-3,4,5-triphosphate and cellular signaling: implications for obesity and diabetes

Phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P₃) is one of the most important phosphoinositides and is capable of activating a wide range of proteins through its interaction with their specific binding domains. Localization and activation of these effector proteins regulate a number of cellular functions, including cell survival, proliferation, cytoskeletal rearrangement, intracellular vesicle trafficking, and cell metabolism. Phosphoinositides have been investigated as an important agonist-dependent second messenger in the regulation of diverse physiological events depending upon the phosphorylation status of their inositol group. Dysregulation in formation as well as metabolism of phosphoinositides is associated with various pathophysiological disorders such as inflammation, allergy, cardiovascular diseases, cancer, and metabolic diseases. Recent studies have demonstrated that the impaired metabolism of PtdIns(3,4,5)P₃ is a prime mediator of insulin resistance associated with various metabolic diseases including obesity and diabetes. This review examines the current status of the role of PtdIns(3,4,5)P₃ signaling in the regulation of various cellular functions and the implications of dysregulated PtdIns(3,4,5)P₃ signaling in obesity, diabetes, and their associated complications.

PTEN

The importance of PTEN in cellular function is underscored by the frequency of its deregulation in cancer. PTEN tumor-suppressor activity depends largely on its lipid phosphatase activity, which opposes PI3K/AKT activation. As such, PTEN regulates many cellular processes, including proliferation, survival, energy metabolism, cellular architecture, and motility. More than a decade of research has expanded our knowledge about how PTEN is controlled at the transcriptional level as well as by numerous posttranscriptional modifications that regulate its enzymatic activity, protein stability, and cellular location. Although the role of PTEN in cancers has long been appreciated, it is also emerging as an important factor in other diseases, such as diabetes and autism spectrum disorders. Our understanding of PTEN function and regulation will hopefully translate into improved prognosis and treatment for patients suffering from these ailments.

PI3K signalling in inflammation

PI3Ks regulate several key events in the inflammatory response to damage and infection. There are four Class I PI3K isoforms (PI3Kα,β,γ,δ), three Class II PI3K isoforms (PI3KC2α, C2β, C2γ) and a single Class III PI3K. The four Class I isoforms synthesise the phospholipid 'PIP3'. PIP3 is a 'second messenger' used by many different cell surface receptors to control cell movement, growth, survival and differentiation. These four isoforms have overlapping functions but each is adapted to receive efficient stimulation by particular receptor sub-types. PI3Kγ is highly expressed in leukocytes and plays a particularly important role in chemokine-mediated recruitment and activation of innate immune cells at sites of inflammation. PI3Kδ is also highly expressed in leukocytes and plays a key role in antigen receptor and cytokine-mediated B and T cell development, differentiation and function. Class III PI3K synthesises the phospholipid PI3P, which regulates endosome-lysosome trafficking and the induction of autophagy, pathways involved in pathogen killing, antigen processing and immune cell survival. Much less is known about the function of Class II PI3Ks, but emerging evidence indicates they can synthesise PI3P and PI34P2 and are involved in the regulation of endocytosis. The creation of genetically-modified mice with altered PI3K signalling, together with the development of isoform-selective, small-molecule PI3K inhibitors, has allowed the evaluation of the individual roles of Class I PI3K isoforms in several mouse models of chronic inflammation. Selective inhibition of PI3Kδ, γ or β has each been shown to reduce the severity of inflammation in one or more models of autoimmune disease, respiratory disease or allergic inflammation, with dual γ/δ or β/δ inhibition generally proving more effective. The inhibition of Class I PI3Ks may therefore offer a therapeutic opportunity to treat non-resolving inflammatory pathologies in humans. This article is part of a Special Issue entitled Phosphoinositides.

SHIP2 signaling in normal and pathological situations: Its impact on cell proliferation

Phosphoinositide 5-phosphatases are critical enzymes in modulating the concentrations of PI(3,4,5)P3, PI(4,5)P2 and PI(3,5)P2. The SH2 domain containing inositol 5-phosphatases SHIP1 and SHIP2 belong to this family of enzymes that dephosphorylate the 5 position of PI(3,4,5)P3 to produce PI(3,4)P2. Data obtained in zebrafish and in mice have shown that SHIP2 is critical in development and growth. Exome sequencing identifies mutations in the coding region of SHIP2 as a cause of opsismodysplasia, a severe but rare chondrodysplasia in human. SHIP2 has been reported to have both protumorigenic and tumor suppressor function in human cancer very much depending on the cell model. This could be linked to the relative importance of PI(3,4)P2 (a product of SHIP2 phosphatase activity) which is also controlled by the PI 4-phosphatase and tumor suppressor INPP4B. In the glioblastoma cell line 1321 N1, that do not express PTEN, lowering SHIP2 expression has an impact on the levels of PI(3,4,5)P3, cell morphology and cell proliferation. It positively stimulates cell proliferation by decreasing the expression of key regulatory proteins of the cell cycle such as p27. Together the data point out to a role of SHIP2 in development in normal cells and at least in cell proliferation in some cancer derived cells.

When worlds collide: inositol pyrophosphates and phosphoinositides intersect at the plasma membrane

Highly phosphorylated inositol pyrophosphates are present in the cells of many organisms such as yeast, Dictyostelium and mammals. They can act as signal molecules in growth factor and insulin signalling both in cultured cells and in intact mice. Their action involves protein pyrophosphorylation or binding to multiple protein interactors such as PH (pleckstrin homology)-domain-containing proteins. One key enzyme in their synthesis, PPIP5K (diphosphoinositol pentakisphosphate kinase) 1/2, can phosphorylate InsP6 and 5-InsP7 to 1-InsP7 and InsP8 respectively. Stephen Shears's laboratory reported in this issue of the Biochemical Journal that PPIP5K1's unexpectedly high affinity for PtdIns(3,4,5)P3, which is synthesized at the plasma membrane, provides a recruitment mechanism for this enzyme in response to growth factor receptor activation. In competition experiments, they observed that PtdIns(3,4,5)P3 binding to PPIP5K1 could be displaced by inositol pyrophosphates and that PPIP5K1 substrates were more potent inhibitors than PPIP5K1 products. Those findings reveal a mechanism for localized depletion of InsP6 and 5-InsP7 at the plasma membrane and further translocation of PtdIns(3,4,5)P3-binding PH-domain-containing proteins.

Biochemistry and structure of phosphoinositide phosphatases

Phosphoinositides are the phosphorylated derivatives of phosphatidylinositol, and play a very significant role in a diverse range of signaling processes in eukaryotic cells. A number of phosphoinositide-metabolizing enzymes, including phosphoinositide-kinases and phosphatases are involved in the synthesis and degradation of these phospholipids. Recently, the function of various phosphatases in the phosphatidylinositol signaling pathway has been of great interest. In the present review we summarize the structural insights and biochemistry of various phosphatases in regulating phosphoinositide metabolism.

Elucidating human phosphatase-substrate networks

Phosphatases are crucially involved in cellular processes by dephosphorylating cellular components. We describe a structure-based classification scheme for all active human phosphatases that reveals previously unrecognized relationships between them. By collating protein and nonprotein substrates and integrating colocalization and coexpression data, we generated a human phosphatase-substrate network. Analysis of the protein sequences surrounding sites of dephosphorylation suggested that common recognition mechanisms may apply to both kinases and a subset of phosphatases. Analysis of three-dimensional substrate recognition by protein phosphatases revealed preferred domains in the substrates. We identified phosphatases with highly specific substrates and those with less specificity by examining the relationship between phosphatases, kinases, and their shared substrates and showed how this analysis can be used to generate testable hypotheses about phosphatase biological function. DEPOD (human DEPhOsphorylation Database, version 1.0, http://www.DEPOD.org) is an online resource with information about active human phosphatases, their substrates, and the pathways in which they function. The database includes links to kinases and chemical modulators of phosphatase activity and contains a sequence similarity search function for identifying related proteins in other species.

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 novel mass assay to quantify the bioactive lipid PtdIns3P in various biological samples

PtdIns3P is recognized as an important player in the control of the endocytotic pathway and in autophagy. Recent data also suggest that PtdIns3P contributes to molecular mechanisms taking place at the plasma membrane and at the midbody during cytokinesis. This lipid is present in low amounts in mammalian cells and remains difficult to quantify either by traditional techniques based on radiolabelling followed by HPLC to separate the different phosphatidylinositol monophosphates, or by high-sensitive liquid chromatography coupled to MS, which is still under development. In the present study, we describe a mass assay to quantify this lipid from various biological samples using the recombinant PtdIns3P 5-kinase, PIKfyve. Using this assay, we show an increase in the mass level of PtdIns3P in mouse and human platelets following stimulation, loss of this lipid in Vps34-deficient yeasts and its relative enrichment in early endosomes isolated from BHK cells.

Golgi membrane dynamics and lipid metabolism

The striking morphology of the Golgi complex has fascinated cell biologists since its discovery over 100 years ago. Yet, despite intense efforts to understand how membrane flow relates to Golgi form and function, this organelle continues to baffle cell biologists and biochemists alike. Fundamental questions regarding Golgi function, while hotly debated, remain unresolved. Historically, Golgi function has been described from a protein-centric point of view, but we now appreciate that conceptual frameworks for how lipid metabolism is integrated with Golgi biogenesis and function are essential for a mechanistic understanding of this fascinating organelle. It is from a lipid-centric perspective that we discuss the larger question of Golgi dynamics and membrane trafficking. We review the growing body of evidence for how lipid metabolism is integrally written into the engineering of the Golgi system and highlight questions for future study.

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.

Quantification of Signaling Lipids by Nano-Electrospray Ionization Tandem Mass Spectrometry (Nano-ESI MS/MS)

Lipids, such as phosphoinositides (PIPs) and diacylglycerol (DAG), are important signaling intermediates involved in cellular processes such as T cell receptor (TCR)-mediated signal transduction. Here we report identification and quantification of PIP, PIP2 and DAG from crude lipid extracts. Capitalizing on the different extraction properties of PIPs and DAGs allowed us to efficiently recover both lipid classes from one sample. Rapid analysis of endogenous signaling molecules was performed by nano-electrospray ionization tandem mass spectrometry (nano-ESI MS/MS), employing lipid class-specific neutral loss and multiple precursor ion scanning for their identification and quantification. Profiling of DAG, PIP and PIP2 molecular species in primary human T cells before and after TCR stimulation resulted in a two-fold increase in DAG levels with a shift towards 1-stearoyl-2-arachidonoyl-DAG in stimulated cells. PIP2 levels were slightly reduced, while PIP levels remained unchanged.

An introduction to phosphoinositides

Phosphoinositides (PIs) are minor components of cellular membranes that play critical regulatory roles in several intracellular functions. This chapter describes the main enzymes regulating the turnover of each of the seven PIs in mammalian cells and introduces to some of their intracellular functions and to some evidences of their involvement in human diseases. Due to the complex interrelation between the distinct PIs and the plethora of functions that they can regulate inside a cell, this chapter is not meant to be a comprehensive coverage of all aspects of PI signalling but rather an introduction to this complex signalling field. For more details of their regulation/functions and extensive description of their intracellular roles, more detailed reviews are suggested on each single topic.

Role of phosphoinositides at the neuronal synapse

Synaptic transmission is amongst the most sophisticated and tightly controlled biological phenomena in higher eukaryotes. In the past few decades, tremendous progress has been made in our understanding of the molecular mechanisms underlying multiple facets of neurotransmission, both pre- and postsynaptically. Brought under the spotlight by pioneer studies in the areas of secretion and signal transduction, phosphoinositides and their metabolizing enzymes have been increasingly recognized as key protagonists in fundamental aspects of neurotransmission. Not surprisingly, dysregulation of phosphoinositide metabolism has also been implicated in synaptic malfunction associated with a variety of brain disorders. In the present chapter, we summarize current knowledge on the role of phosphoinositides at the neuronal synapse and highlight some of the outstanding questions in this research field.

A small molecule mimicking a phosphatidylinositol (4,5)-bisphosphate binding pleckstrin homology domain

Inositol phospholipids have emerged as important key players in a wide variety of cellular functions. Among the seven existing inositol phospholipids, phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P(2)) has attracted much attention in recent years due to its important role in numerous cellular signaling events and regulations, which in turn impact several human diseases. This particular lipid is recognized in the cell by specific lipid binding domains, such as the Pleckstrin-homology (PH) domain, which is also employed as a tool to monitor this important lipid. Here, we describe the synthesis and biological characterization of a small molecule that mimics the PH domain as judged by its ability to bind specifically to only PI(4,5)P(2) and effectively compete with the PH domain in vitro and in a cellular environment. The binding constant of this small molecule PH domain mimetic (PHDM) was determined to be 17.6 ± 10.1 μM, similar in potency to the PH domain. Using NIH 3T3 mouse fibroblast cells we demonstrated that this compound is cell-permeable and able to modulate PI(4,5)P(2)-dependent effects in a cellular environment such as the endocytosis of the transferrin receptor, loss of mitochondria, as well as stress fiber formation. This highly PI(4,5)P(2)-specific chemical mimetic of a PH domain not only is a powerful research tool but might also be a lead compound in future drug developments targeting PI(4,5)P(2)-dependent diseases such as Lowe syndrome.

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

The SH2 domain containing inositol 5-phosphatase 2 (SHIP2) belongs to the family of the mammalian inositol polyphosphate 5-phosphatases. The two closely related isoenzymes SHIP1 (or SHIP) and SHIP2 contain a N-terminal SH2 domain, a catalytic domain, potential PTB domain-binding sites (NPXY), and C-terminal proline-rich regions with consensus sites for SH3 domain interactions. In addition, SHIP2 contains a unique sterile alpha motif (SAM) domain that could be involved in SAM-SAM domain interactions with other proteins or receptors. SHIP2 also shows the presence of an ubiquitin interacting motif at the C-terminal end. SHIP2 is essentially a PI(3,4,5)P(3) 5-phosphatase that negatively controls PI(3,4,5)P(3) levels in intact cells and produce PI(3,4)P(2) . Depending on the cells and stimuli, PI(3,4)P(2) could accumulate at important levels and be a "second messenger" by its own. It could interact with a very large number of target proteins such as PKB or TAPP1 and 2 that control insulin sensitivity. In addition to its catalytic activity, SHIP2 is also a docking protein for a large number of proteins: Cytoskeletal, focal adhesion proteins, scaffold proteins, adaptors, protein phosphatases, and tyrosine kinase associated receptors. These interactions could play a role in the control of cell adhesion, migration, or endocytosis of some receptors. SHIP2 could be acting independently of its phosphatase activity being part of a protein network of some receptors, e.g., the EGF receptor or BCR/ABL. These non-catalytic properties associated to a PI phosphatase have also been reported for other enzymes of the metabolism of myo-inositol such as Ins(1,4,5)P(3) 3-kinases, inositol phosphate multikinase (IPMK), or PTEN.

Evidence of SHIP2 Ser132 phosphorylation, its nuclear localization and stability

PtdIns(3,4,5)P3 and PtdIns(3,4)P2 are major signalling molecules in mammalian cell biology. PtdIns(3,4)P2 can be produced by PI3Ks [PI (phosphoinositide) 3-kinases], but also by PI 5-phosphatases including SHIP2 [SH2 (Src homology 2)-domain-containing inositol phosphatase 2]. Proteomic studies in human cells revealed that SHIP2 can be phosphorylated at more than 20 sites, but their individual function is unknown. In a model of PTEN (phosphatase and tensin homologue deleted on chromosome 10)-null astrocytoma cells, lowering SHIP2 expression leads to increased PtdIns(3,4,5)P3 levels and Akt phosphorylation. MS analysis identified SHIP2 phosphosites on Ser132, Thr1254 and Ser1258; phosphotyrosine-containing sites were undetectable. By immunostaining, total SHIP2 concentrated in the perinuclear area and in the nucleus, whereas SHIP2 phosphorylated on Ser132 was in the cytoplasm, the nucleus and nuclear speckles, depending on the cell cycle stage. SHIP2 phosphorylated on Ser132 demonstrated PtdIns(4,5)P2 phosphatase activity. Endogenous phospho-SHIP2 (Ser132) showed an overlap with PtdIns(4,5)P2 staining in nuclear speckles. SHIP2 S132A was less sensitive to C-terminal degradation and more resistant to calpain as compared with wild-type enzyme. We have identified nuclear lamin A/C as a novel SHIP2 interactor. We suggest that the function of SHIP2 is different at the plasma membrane where it recognizes PtdIns(3,4,5)P3, and in the nucleus where it may interact with PtdIns(4,5)P2, particularly in speckles.

Phosphatidylinositol transfer proteins: negotiating the regulatory interface between lipid metabolism and lipid signaling in diverse cellular processes

Phosphoinositides represent only a small percentage of the total cellular lipid pool. Yet, these molecules play crucial roles in diverse intracellular processes such as signal transduction at membrane-cytosol interface, regulation of membrane trafficking, cytoskeleton organization, nuclear events, and the permeability and transport functions of the membrane. A central principle in such lipid-mediated signaling is the appropriate coordination of these events. Such an intricate coordination demands fine spatial and temporal control of lipid metabolism and organization, and consistent mechanisms for specifically coupling these parameters to dedicated physiological processes. In that regard, recent studies have identified Sec14-like phosphatidylcholine transfer protein (PITPs) as "coincidence detectors," which spatially and temporally link the diverse aspects of the cellular lipid metabolome with phosphoinositide signaling. The integral role of PITPs in eukaryotic signal transduction design is amply demonstrated by the mammalian diseases associated with the derangements in the function of these proteins, to stress response and developmental regulation in plants, to fungal dimorphism and pathogenicity, to membrane trafficking in yeast, and higher eukaryotes. This review updates the recent advances made in the understanding of how these proteins, specifically PITPs of the Sec14-protein superfamily, operate at the molecular level and further describes how this knowledge has advanced our perception on the diverse biological functions of PITPs.

Regulation and roles of PI3Kβ, a major actor in platelet signaling and functions

Phosphoinositide 3-kinases (PI3Ks) are important signaling enzymes involved in the regulation of a number of critical cell functions. Significant progress has been made during the last few years in defining the implication of individual PI3K isoforms. The role of the class IA PI3Kβ in different cell types has only been recently uncovered by the use of isoform-selective inhibitors and the development of mouse models harboring p110β catalytic subunit knock-out or germline knock-in of a kinase-dead allele of p110β. Although it is classically admitted that class IA PI3Ks are activated by receptor tyrosine kinases through recruitment of the regulatory subunits to specific tyrosine phosphorylated motifs via their SH2 domains, PI3Kβ is activated downstream of G protein-coupled receptors, and by co-operation between heterotrimeric G proteins and tyrosine kinases. PI3Kβ has been extensively studied in platelets where it appears to play an important role downstream of ITAM signaling, G protein-coupled receptors and aIIbβ3 integrin. Accordingly, mouse exhibiting p110β inactivation selectively in megakaryocyte/platelets are resistant to thromboembolism induced by carotid injury. The present review summarizes recent data concerning the mechanisms of PI3Kβ regulation and the roles of this PI3K isoform in blood platelet functions and other cell types.

Role of lipid metabolism in smoothened derepression in hedgehog signaling

The binding of Hedgehog (Hh) to its receptor Patched causes derepression of Smoothened (Smo), resulting in the activation of the Hh pathway. Here, we show that Smo activation is dependent on the levels of the phospholipid phosphatidylinositol-4 phosphate (PI4P). Loss of STT4 kinase, which is required for the generation of PI4P, exhibits hh loss-of-function phenotypes, whereas loss of Sac1 phosphatase, which is required for the degradation of PI4P, results in hh gain-of-function phenotypes in multiple settings during Drosophila development. Furthermore, loss of Ptc function, which results in the activation of Hh pathway, also causes an increase in PI4P levels. Sac1 functions downstream of STT4 and Ptc in the regulation of Smo membrane localization and Hh pathway activation. Taken together, our results suggest a model in which Ptc directly or indirectly functions to suppress the accumulation of PI4P. Binding of Hh to Ptc derepresses the levels of PI4P, which, in turn, promotes Smo activation.

A new family of phosphoinositide phosphatases in microorganisms: identification and biochemical analysis

Background

Phosphoinositide metabolism is essential to membrane dynamics and impinges on many cellular processes, including phagocytosis. Modulation of phosphoinositide metabolism is important for pathogenicity and virulence of many human pathogens, allowing them to survive and replicate in the host cells. Phosphoinositide phosphatases from bacterial pathogens are therefore key players in this modulation and constitute attractive targets for chemotherapy. MptpB, a virulence factor from Mycobacterium tuberculosis, has phosphoinositide phosphatase activity and a distinct active site P-loop signature HCXXGKDR that shares characteristics with eukaryotic lipid phosphatases and protein tyrosine phosphatases. We used this P-loop signature as a "diagnostic motif" to identify related putative phosphatases with phosphoinositide activity in other organisms.

Results

We found more than 200 uncharacterised putative phosphatase sequences with the conserved signature in bacteria, with some related examples in fungi and protozoa. Many of the sequences identified belong to recognised human pathogens. Interestingly, no homologues were found in any other organisms including Archaea, plants, or animals. Phylogenetic analysis revealed that these proteins are unrelated to classic eukaryotic lipid phosphatases. However, biochemical characterisation of those from Listeria monocytogenes and Leishmania major, demonstrated that, like MptpB, they have phosphatase activity towards phosphoinositides. Mutagenesis studies established that the conserved Asp and Lys in the P-loop signature (HCXXGKDR) are important in catalysis and substrate binding respectively. Furthermore, we provide experimental evidence that the number of basic residues in the P-loop is critical in determining activity towards poly-phosphoinositides.

Conclusion

This new family of enzymes in microorganisms shows distinct sequence and biochemical characteristics to classic eukaryotic lipid phosphatases and they have no homologues in humans. This study provides a foundation for examining the biological role of this new family of phosphatases and their potential as pharmaceutical targets against infectious diseases.

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.

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

Phosphoinositide 3-kinase-regulated adapters in lymphocyte activation

Signaling via phosphoinositide 3-kinases (PI3Ks) has emerged as a central component of lymphocyte activation via immunoreceptors, costimulatory receptors, cytokine receptors, and chemokine receptors. The discovery of phosphoinositide-binding pleckstrin homology (PH) domains has substantially increased understanding of how PI3Ks activate cellular responses. Accumulating evidence indicates that PH-domain containing adapter molecules provide important links between PI3K and lymphocyte function. Here, we review data on PI3K-regulated adapter proteins of the Grb-associated binder (GAB), Src kinase-associated phosphoprotein (SKAP), and B-lymphocyte adapter molecule of 32 kDa (Bam32)/ dual-adapter for phosphotyrosine and 3-phosphoinositides (DAPP)/TAPP families, with a focus on the latter group. Current data support the model that recruitment of these adapters to the plasma membrane of activated lymphocytes is driven by the phosphoinositides phosphatidylinositol-3,4,5-tris-phosphate and phosphatidylinositol-3,4-bisphosphate, generated through the action of PI3Ks and under the regulatory control of lipid phosphatases Src homology 2 domain-containing inositol phosphatase (SHIP), phosphatase and tensin homolog, and inositol polyphosphate 4-phosphatase. At the plasma membrane, these adapters serve to assemble distinct protein complexes. Bam32/DAPP1 and SKAPs function to promote activation of monomeric guanosine triphosphatases, including Rac and Rap, and promote integrin activation, lymphocyte adhesion to matrix proteins, and cell:cell interactions between B and T lymphocytes. GABs can provide feedforward amplification or feedback inhibition of PI3K signaling. Current work is further defining the molecular interactions driven by these molecules and identifying the functions of TAPP adapters, which also appear to be involved in lymphocyte adhesion and are specific effectors downstream of the SHIP product phosphatidylinositol-3,4-bisphosphate.

5-Stabilized phosphatidylinositol 3,4,5-trisphosphate analogues bind Grp1 PH, inhibit phosphoinositide phosphatases, and block neutrophil migration

Metabolically stabilized analogues of PtdIns(3,4,5)P3 have shown long-lived agonist activity for cellular events and selective inhibition of lipid phosphatase activity. We describe an efficient asymmetric synthesis of two 5-phosphatase-resistant analogues of PtdIns(3,4,5)P3, the 5-methylene phosphonate (MP) and 5-phosphorothioate (PT). Furthermore, we illustrate the biochemical and biological activities of five stabilized PtdIns(3,4,5)P3 analogues in four contexts. First, the relative binding affinities of the 3-MP, 3-PT, 5-MP, 5-PT, and 3,4,5-PT3 analogues to the Grp1 PH domain are shown, as determined by NMR spectroscopy. Second, the enzymology of the five analogues is explored, showing the relative efficiency of inhibition of SHIP1, SHIP2, and phosphatase and tensin homologue deleted on chromosome 10 (PTEN), as well as the greatly reduced ability of these phosphatases to process these analogues as substrates as compared to PtdIns(3,4,5)P3. Third, exogenously delivered analogues severely impair complement factor C5a-mediated polarization and migration of murine neutrophils. Finally, the new analogues show long-lived agonist activity in mimicking insulin action in sodium transport in A6 cells.

Changing phosphoinositides "on the fly": how trafficking vesicles avoid an identity crisis

Joining an antagonistic phosphoinositide (PtdInsP) kinase and phosphatase into a single protein complex may regulate rapid and local PtdInsP changes. This may be important for processes such as membrane fission that require a specific PtdInsP and that are innately local and rapid. Such a complex could couple vesicle formation, with erasing of the identity of the donor organelle from the vesicle prior to its fusion with target organelles, thus preventing organelle identity intermixing. Coordinating signals are postulated to switch the relative activities of the kinase and phosphatase in a spatio-temporal manner that matches membrane fission events. The discovery of two such complexes supports this hypothesis. One regulates the interconversion of phosphatidylinositol and PtdIns(3)P by joining the Vps34 PtdIns 3-kinase and the myotubularin 3-phosphatases. The other regulates the interconversion between PtdIns(3)P and PtdIns(3,5)P(2) through the Fab1/PIKfyve kinase and the Fig4/mFig4 phosphatase. These lipids are essential components of the endosomal identity code.

Overexpression of PIP5KL1 suppresses cell proliferation and migration in human gastric cancer cells

Phosphatidylinositol-4-phosphate 5-kinase-like 1 (PIP5KL1), the forth member of phosphatidylinositol-4-phosphate 5-kinases (PIPKs) type I, acts as a scaffold for localization and activation of PIPKs, which mediates numerous cellular processes. However, the role of PIP5KL1 in the development of human cancer is still lacking. We therefore examined the expression of PIP5KL1 in human normal and cancer tissues by tissue microarrays (TMAs). Reverse transcription-polymerase chain reaction (RT-PCR) and immunofluorescence imaging analysis were used to testify the mRNA and protein levels of PIP5KL1 in human gastric cancer cell line (BGC823). The cell proliferation was investigated with 3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide (MTT) assay. Both wound healing and transwell migration assay were performed to study the cell migration. The phosphorylation of v-akt murine thymoma viral oncogene homolog 1 (AKT1) was determined by western immunoblot analysis. Immunostaining of gastric cancer tissue microarrays revealed a negative correlation between PIP5KL1 overexpression and gastric cancer in situ. Transient transfection PIP5KL1 induced a significant increase expression at both transcriptional and translational levels and consequent robust inhibition of proliferation (P < 0.05) and migration (P < 0.05) of BGC823 cells. Overexpression of PIP5KL1 markedly inhibited (P < 0.05) serum-induced phosphorylation of AKT1. Taken together, these studies indicate a functional negative correlation between elevated levels of PIP5KL1 and the development of human gastric cancer, suggesting that PIP5KL1 overexpression may suppress gastric cancer formation.