Regulation Of Phosphoinositide Metabolism Research Articles

Regulation of phosphoinositide metabolism in Apicomplexan parasites.

Phosphoinositides are a biologically essential class of phospholipids that contribute to organelle membrane identity, modulate membrane trafficking pathways, and are central components of major signal transduction pathways that operate on the cytosolic face of intracellular membranes in eukaryotes. Apicomplexans (such as Toxoplasma gondii and Plasmodium spp.) are obligate intracellular parasites that are important causative agents of disease in animals and humans. Recent advances in molecular and cell biology of Apicomplexan parasites reveal important roles for phosphoinositide signaling in key aspects of parasitosis. These include invasion of host cells, intracellular survival and replication, egress from host cells, and extracellular motility. As Apicomplexans have adapted to the organization of essential signaling pathways to accommodate their complex parasitic lifestyle, these organisms offer experimentally tractable systems for studying the evolution, conservation, and repurposing of phosphoinositide signaling. In this review, we describe the regulatory mechanisms that control the spatial and temporal regulation of phosphoinositides in the Apicomplexan parasites Plasmodium and T. gondii. We further discuss the similarities and differences presented by Apicomplexan phosphoinositide signaling relative to how these pathways are regulated in other eukaryotic organisms.

A Targeted shRNA Knockdown Screen of Phosphoinositide Modulator Genes Reveals INPP5B As Required for Cell Cycle Progression of MLL-AF9 Acute Myeloid Leukemia Cells

Abstract 2444Phosphoinositides are pivotal regulatory and scaffolding lipids that coordinate key aspects of cellular physiology. For example, phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates endocytosis, exocytosis, membrane ruffling and the function of actin, and phosphatidylinositol (3,4,5)-trisphosphate (PIP3), downstream of phosphoinositide-3-kinase (PI3K), is a critical mediator of signal transduction. In acute myeloid leukemia (AML) mutations of FLT3 or KIT induce constitutive activation of PI3K/AKT signaling; and PTEN and SHIP1, two PI3K antagonists, have been linked to hematopoietic stem cell maintenance, with experimental deletion of these genes leading to leukemia. Little is known about the role of other phosphoinositide modulators in normal or leukemic hematopoiesis. We hypothesized that a genetic knockdown screen, utilising lentivirally delivered short hairpin RNAs (345 pLKO.1 puro vectors) targeting 92 genes with known or supposed roles in the regulation of phosphoinositide metabolism, would reveal novel regulators of proliferation and differentiation in AML, representing candidate therapeutic targets.In an initial screen, hairpins were tested for their ability to impact on proliferation or survival of human THP-1 AML cells using fold change in fluorescence of the cell viability indicator alamarBlue relative to control cells (infected with a non-targeting control hairpin) over four days as a surrogate. THP-1 cells harbor a t(9;11) chromosomal translocation which is the cytogenetic hallmark of MLL-AF9. Lentiviral supernatants, prepared individually in 96 well plates with one vector per well, were used to infect target cells, again in 96 well plate format with one lentiviral supernatant per well. 24 hours following infection puromycin was added. The alamarBlue dye assay was performed on replicate samples 48 hours and six days later. All non-infected cells were killed by puromycin exposure within 48 hours. 38 shRNAs, including two targeting PTEN, increased proliferation or enhanced survival, whereas 158, including multiple constructs targeting AKT1, AKT2 and PI3KR2, reduced proliferation or induced apoptosis (versus control cells fold change± 2SD). These results confirmed that the screening strategy was effective because it correctly identified known pro- and anti-proliferative regulators of phosphoinositide metabolism.Next, we expanded the screen to include six other AML cell lines (MonoMac1, HL60, K562, NB4, Kasumi-1 and U937). 14 genes met our criteria for a “hit”: at least two knockdown constructs per gene needed to reduce proliferation on average by at least 55% compared to non-targeting shRNA control across all cell lines screened. We focused our further investigations on type II inositol-1,4,5-trisphosphate 5-phosphatase (INPP5B), because Inpp5b−/- mice are viable into adulthood with no known hematopoietic phenotype, suggesting INPP5B might selectively regulate leukemia cells. INPP5B catalyses the conversion of inositol-1,4,5-trisphosphate (IP3) to inositol-1,4-bisphphate (IP2) and hence contributes to cellular calcium signalling mediated through phospholipase C. Knockdown of Inpp5b also reduced proliferation of murine MLL-AF9 AML cells in liquid culture using three separate shRNA constructs that reduced transcript levels by 70–80%, and largely abolished colony formation in methylcellulose; residual colonies were much smaller than control colonies. While wild-type KIT+ bone marrow stem and progenitor cells were robustly immortalized by infection with an MLL-AF9 retroviral vector, Inpp5b−/− KIT+ cells either failed to immortalize or exhibited weak immortalisation, with fewer, smaller colonies, reminiscent of the knockdown phenotype. In cell cycle analyses of cells harvested from the third round of serial replating in methylcellulose, 90% of Inpp5b−/− MLL-AF9 cells were in G1 versus 41% of WT cells. No increase in apoptosis was observed.These data implicate INPP5B as a novel and selective regulator of leukemic versus normal hematopoiesis. Disclosures:No relevant conflicts of interest to declare.

Phosphoinositides and signal transduction.

Phosphoinositides comprise a family of eight minor membrane lipids which play important roles in many signal transducing pathways in the cell. Signaling through various phosphoinositides has been shown to mediate cell growth and proliferation, apoptosis, cytoskeletal changes, insulin action and vesicle trafficking. A number of advances in signal transduction in the last decade has resulted in the discovery of a growing list of proteins which directly interact with high affinity and specificity with distinct phosphoinositides. Equally important, a number of phosphoinositide binding domains such as the pleckstrin homology domain have emerged as critical mediators of phosphoinositide signaling. Here, recent advances in phosphoinositide signaling are discussed. The aim of this review is to highlight particularly exciting advances made in the field over the last few years. The regulation of phosphoinositide metabolism by lipid kinases, phosphatases and phospholipases is reviewed, and considerable emphasis is placed on phosphoinositide-binding proteins. Finally, the role of these lipids in regulating signaling pathways and cell function is described.

Regulation of Phosphoinositide Metabolism, Akt Phosphorylation, and Glucose Transport by PTEN (Phosphatase and Tensin Homolog Deleted on Chromosome 10) in 3T3-L1 Adipocytes

To investigate the roles of PTEN (phosphatase and tensin homolog deleted on chromosome 10) in the regulation of 3-position phosphorylated phosphoinositide metabolism as well as insulin-induced Akt phosphorylation and glucose metabolism, wild-type PTEN and its phosphatase-dead mutant (C124S) with or without an N-terminal myristoylation tag were overexpressed in Sf-9 cells and 3T3-L1 adipocytes using baculovirus and adenovirus systems, respectively. When expressed in Sf-9 cells together with the p110α catalytic subunit of phosphoinositide 3-kinase, myristoylated PTEN markedly reduced the accumulations of both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate induced by p110α. In contrast, overexpression of the C124S mutants apparently increased these accumulations. In 3T3-L1 adipocytes, insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate were markedly suppressed by overexpression of wild-type PTEN with the N-terminal myristoylation tag, but not by that without the tag. On the contrary, the C124S mutants of PTEN enhanced insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. Interestingly, the phosphorylation level of Akt at Thr308 (Akt2 at Thr309), but not at Ser473 (Akt2 at Ser474), was revealed to correlate well with the accumulation of phosphatidylinositol 3,4,5-trisphosphate modified by overexpression of these PTEN proteins. Finally, insulin-induced increases in glucose transport activity were significantly inhibited by the overexpression of myristoylated wild-type PTEN, but were not enhanced by expression of the C124S mutant of PTEN. Therefore, in conclusion, 1) PTEN dephosphorylates both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in vivo, and the C124S mutants interrupt endogenous PTEN activity in a dominant-negative manner. 2) The membrane targeting process of PTEN may be important for exerting its function. 3) Phosphorylations of Thr309 and Ser474 of Akt2 are regulated differently, and the former is regulated very sensitively by the function of PTEN. 4) The phosphorylation level of Ser474, but not that of Thr309, in Akt2 correlates well with insulin-stimulated glucose transport activity in 3T3-L1 adipocytes. 5) The activity of endogenous PTEN may not play a major role in the regulation of glucose transport activity in 3T3-L1 adipocytes.

Regulation of phosphoinositide metabolism, Akt phosphorylation, and glucose transport by PTEN (phosphatase and tensin homolog deleted on chromosome 10) in 3T3-L1 adipocytes.

To investigate the roles of PTEN (phosphatase and tensin homolog deleted on chromosome 10) in the regulation of 3-position phosphorylated phosphoinositide metabolism as well as insulin-induced Akt phosphorylation and glucose metabolism, wild-type PTEN and its phosphatase-dead mutant (C124S) with or without an N-terminal myristoylation tag were overexpressed in Sf-9 cells and 3T3-L1 adipocytes using baculovirus and adenovirus systems, respectively. When expressed in Sf-9 cells together with the p110alpha catalytic subunit of phosphoinositide 3-kinase, myristoylated PTEN markedly reduced the accumulations of both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate induced by p110alpha. In contrast, overexpression of the C124S mutants apparently increased these accumulations. In 3T3-L1 adipocytes, insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate were markedly suppressed by overexpression of wild-type PTEN with the N-terminal myristoylation tag, but not by that without the tag. On the contrary, the C124S mutants of PTEN enhanced insulin-induced accumulations of phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. Interestingly, the phosphorylation level of Akt at Thr308 (Akt2 at Thr309), but not at Ser473 (Akt2 at Ser474), was revealed to correlate well with the accumulation of phosphatidylinositol 3,4,5-trisphosphate modified by overexpression of these PTEN proteins. Finally, insulin-induced increases in glucose transport activity were significantly inhibited by the overexpression of myristoylated wild-type PTEN, but were not enhanced by expression of the C124S mutant of PTEN. Therefore, in conclusion, 1) PTEN dephosphorylates both phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate in vivo, and the C124S mutants interrupt endogenous PTEN activity in a dominant-negative manner. 2) The membrane targeting process of PTEN may be important for exerting its function. 3) Phosphorylations of Thr309 and Ser474 of Akt2 are regulated differently, and the former is regulated very sensitively by the function of PTEN. 4) The phosphorylation level of Ser474, but not that of Thr309, in Akt2 correlates well with insulin-stimulated glucose transport activity in 3T3-L1 adipocytes. 5) The activity of endogenous PTEN may not play a major role in the regulation of glucose transport activity in 3T3-L1 adipocytes.

Possible involvement of a tyrosine kinase-dependent pathway in the regulation of phosphoinositide metabolism by vanadate in normal mouse islets

The potential roles of protein tyrosine kinases (TKs) and of phosphotyrosine phosphatases (PTPs) in pancreatic islet function are not known. In this study, we investigated whether vanadate, a potent PTP inhibitor, affects phosphoinositide (PI) metabolism by a TK-dependent pathway in isolated mouse islets. To avoid the confounding effects of changes in Ca2+ influx, all experiments were performed in the absence of Ca2+. In the presence of 15mM glucose, vanadate, acetylcholine (ACh) or [Arg]vasopressin (AVP) strongly stimulated InsP production. Vanadate also increased PtdInsP levels in membranes. The TK inhibitor genistein (not its inactive analogues genistin and daidzein) significantly reduced vanadate effects, but was without effect in the absence of stimulation or in the presence of ACh or AVP. Islet proteins resolved by SDS/PAGE were analysed by immunobloting with anti-phosphotyrosine antibody. Under control conditions, several phosphotyrosyl-proteins (PYPs) were present. Vanadate increased phosphotyrosine residues on several PYPs, notably two proteins of 145 and 85 kDa. This effect was prevented by genistein, p145 and p85 could correspond to phospholipate Cgamma(PLCgamma) and the regulatory subunit of PtdIns-3-kinase (PtdIns-3K) respectively. Both proteins are expressed in islets, as revealed by immunoblots with specific antibodies. Tungstate, another PTP inhibitor, reproduced vanadate effects, but inhibition of PtdIns-3K by wortmannin failed to affect vanadate-increased PtdInsP levels. Incubation of the islets in the presence of 10% (v/v) fetal calf serum instead of BSA increased InsP production and this effect was prevented by genistein. These results suggest that inhibition of PTP increases InsP production in mouse islets by a TK-dependent pathway. They also provide evidence for a potential role of TK and PTP in pancreatic B-cell function.

Two distinct pathways of platelet-activating factor-induced hydrolysis of phosphoinositides in primary cultures of rat Kupffer cells.

Stimulation of rat Kupffer cells in primary culture with platelet-activating factor (PAF) caused a rapid hydrolysis of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate with a concomitant increase in the levels of myo-inositol 1,4,5-trisphosphate and myo-inositol 1,4-bisphosphate. This phospholipase C-mediated hydrolysis of polyphosphoinositides was independent of extracellular Ca2+ but was inhibited by the intracellular Ca2+ antagonist TMB-8. A second slower response to PAF was characterized by deacylation of PI leading to the accumulation of glycerophosphoinositol (GPI). PAF-induced GPI synthesis was not inhibited by TMB-8. These effects of PAF were accompanied by initial transient mobilization of Ca2+ from intracellular stores followed by a rather slow influx of Ca2+ from the extracellular medium. PAF-stimulated deacylation and phosphodiesteric hydrolysis of inositol lipids were differentially affected by cholera toxin and pertussis toxin. Pretreatment of the Kupffer cells with either of these toxins caused inhibition of phospholipase C activity. Pertussis toxin also inhibited PAF-stimulated deacylation. However, cholera toxin itself stimulated GPI release and addition of PAF to the cholera toxin-treated cells caused a further increase in GPI release. Phorbol ester inhibited PAF-induced phosphodiesteric hydrolysis of phosphoinositides, but not deacylation. PAF-induced metabolism of phosphoinositides was inhibited by the PAF antagonist, U66985. These results suggest that PAF-induced phosphodiesteric hydrolysis and deacylation of inositol phospholipids are regulated via distinct mechanisms involving activation of separate G-proteins in rat Kupffer cells. Also the regulation of phosphoinositide metabolism by Ca2+ mobilization from two separate Ca2+ pools is indicated by this study.

The action of the protein kinase C inhibitor, staurosporine, on human platelets. Evidence against a regulatory role for protein kinase C in the formation of inositol trisphosphate by thrombin.

The ability of several putative inhibitors of protein kinase C (PKC) to block dioctanoylglycerol (DC8)-induced phosphorylation of a 47 kDa protein (a recognized substrate for PKC) in human platelets was investigated. Staurosporine (1 microM) caused complete inhibition of phosphorylation, whereas the other reagents were either inactive (polymyxin B) or gave only partial inhibition (C-1, H-7, tamoxifen). Staurosporine (1 microM) fully inhibited the phosphorylation of the 47 kDa protein in platelets challenged with thrombin, but also inhibited the phosphorylation of a 20 kDa protein which is a substrate for myosin light-chain kinase. The inhibition of both kinases by staurosporine was associated with the inhibition of thrombin-induced secretion of ATP and 5-hydroxytryptamine and a slowing of the aggregation response; staurosporine, however, had no effect on the formation of phosphatidic acid and inositol phosphates induced by thrombin. Staurosporine also reversed the inhibitory action of phorbol esters on thrombin-induced formation of phosphatidic acid. These data are consistent with a role for these two kinases in secretion and aggregation (although there must be additional control signals, since aggregation was only slowed, not inhibited), but suggest that neither kinase is involved in the regulation of phosphoinositide metabolism. This latter conclusion contradicts previous observations that the activation of PKC by phorbol esters or membrane-permeable diacylglycerols alters the apparent activity of both phospholipase C and inositol trisphosphatase. Possible explanations for this discrepancy are discussed.

Role of the aggregation factor in the regulation of phosphoinositide metabolism in sponges. Possible consequences on calcium efflux and on mitogenesis.

The aggregation factor (AF) of the marine sponge Geodia cydonium recognizes the aggregation receptor (AR) which is inserted in the plasma membrane, under formation of species-specific aggregates. The specific cell-binding fragment of the AF was used to investigate for the first time the phosphoinositide metabolism in a lower avertebrate system. We found that after binding of the cell-binding fragment to the aggregation receptor a strong and rapid stimulation of the phosphate incorporation into phosphatidylinositol occurs followed by an increased turnover of phosphoinositides in the Geodia cells. The consequences of an increased degradation of phosphatidylinositol 4,5-bisphosphate into the two second messengers inositol-1,4,5-trisphosphate and diacylglycerol are 2-fold. First, after the addition of the extracellular stimulus the cytosolic Ca2+ concentration rises, resulting in a rapid increased Ca2+ efflux rate. The functional consequence of the increase of the extracellular Ca2+ level is an initiation of the aggregate formation that is mediated by the collagen assembly factor (= primary aggregation factor). Second, some experimental evidences are presented, showing that the other second messenger formed, diacylglycerol, causes a translocation of protein kinase C within the cell. Incubation of Geodia cells with the cell-binding fragment of the AF, or with the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate, resulted within 5 min after treatment in a 70% decrease in protein kinase C activity in the cytosolic fraction and in a 700% increase in enzyme activity in the membrane fraction. It is proposed that by membrane association protein kinase C becomes activated. As a result of this event a series of cellular proteins are phosphorylated, a process which ultimately leads to an unusually strong induction of DNA polymerase alpha activity. From these data we conclude that inositol trisphosphate and protein kinase C also play a fundamental role in cellular signal transduction in lower eukaryotes.