Membrane transport and in vitro metabolism of the Ras cascade messenger, glycerophosphoinositol 4-phosphate

Alteration of the lipid profile in lymphomas induced by MYC overexpression

Overexpression of the v-myc avian myelocytomatosis viral oncogene homolog (MYC) oncogene is one of the most commonly implicated causes of human tumorigenesis. MYC is known to regulate many aspects of cellular biology including glucose and glutamine metabolism. Little is known about the relationship between MYC and the appearance and disappearance of specific lipid species. We use desorption electrospray ionization mass spectrometry imaging (DESI-MSI), statistical analysis, and conditional transgenic animal models and cell samples to investigate changes in lipid profiles in MYC-induced lymphoma. We have detected a lipid signature distinct from that observed in normal tissue and in rat sarcoma-induced lymphoma cells. We found 104 distinct molecular ions that have an altered abundance in MYC lymphoma compared with normal control tissue by statistical analysis with a false discovery rate of less than 5%. Of these, 86 molecular ions were specifically identified as complex phospholipids. To evaluate whether the lipid signature could also be observed in human tissue, we examined 15 human lymphoma samples with varying expression levels of MYC oncoprotein. Distinct lipid profiles in lymphomas with high and low MYC expression were observed, including many of the lipid species identified as significant for MYC-induced animal lymphoma tissue. Our results suggest a relationship between the appearance of specific lipid species and the overexpression of MYC in lymphomas.

Mammalian Glycerophosphodiester Phosphodiesterases

Bacterial glycerophosphodiester phosphodiesterases (GP-PDEs), GlpQ and UgpQ, are well-characterized periplasmic and cytosolic proteins that play critical roles in the hydrolysis of deacylated glycerophospholipids to glycerol phosphate and alcohol, which are utilized as major sources of carbon and phosphate. In contrast, two novel mammalian GP-PDEs, GDE1/MIR16 and GDE3, were recently identified, and were shown to be involved in several physiological functions. GDE1/MIR16 was identified as a membrane protein interacting with RGS16, a regulator of G protein signaling, and found to hydrolyze glycerophosphoinositol preferentially. We have found that expression of GDE3 is significantly up-regulated during osteoblast differentiation and is involved in morphological changes of cells. Furthermore, five mammalian GP-PDEs were virtually identified, and very recent studies indicate that retinoic acid-induced expression of GDE2 plays essential roles in neuronal differentiation and neurite outgrowth. Thus mammalian GP-PDEs are likely to be important in controlling numerous cellular events, indicating that the GP-PDE superfamily in mammals might be a pharmacological target in the future.

The glycerophosphoinositols: from lipid metabolites to modulators of T-cell signaling

Glycerophosphoinositols (GPIs) are bioactive, diffusible phosphoinositide metabolites of phospholipase A₂ that act both intracellularly and in a paracrine fashion following their uptake by specific transporters. The most representative compound, glycerophosphoinositol (GroPIns), is a ubiquitous component of eukaryotic cells that participates in central processes, including cell proliferation and survival. Moreover, glycerophosphoinositol 4-phosphate (GroPIns4P) controls actin dynamics in several cell systems by regulating Rho GTPases. Recently, immune cells have emerged as targets of the biological activities of the GPIs. We have shown that exogenous GroPIns4P enhances CXCL12-induced T-cell chemotaxis through activation of the kinase Lck in a cAMP/PKA-dependent manner. While highlighting the potential of GroPIns4P as an immunomodulator, this finding raises questions on the role of endogenously produced GroPIns4P as well as of other GPIs in the regulation of the adaptive immune responses under homeostatic and pathological settings. Here we will summarize our current understanding of the biological activities of the GPIs, with a focus on lymphocytes, highlighting open questions and potential developments in this promising new area.

The glycerophosphoinositols: cellular metabolism and biological functions

The glycerophosphoinositols are cellular products of phospholipase A(2) and lysolipase activities on the membrane phosphoinositides. Their intracellular concentrations can vary upon oncogenic transformation, cell differentiation and hormonal stimulation. Specific glycerophosphodiester phosphodiesterases are involved in their catabolism, which, as with their formation, is under hormonal regulation. With their mechanisms of action including modulation of adenylyl cyclase, intracellular calcium levels, and Rho-GTPases, the glycerophosphoinositols have diverse effects in multiple cell types: induction of cell proliferation in thyroid cells; modulation of actin cytoskeleton organisation in fibroblasts; and reduction of the invasive potential of tumour cell lines. More recent investigations include their effects in inflammatory and immune responses. Indeed, the glycerophosphoinositols enhance cytokine-dependent chemotaxis in T-lymphocytes induced by SDF-1alpha-receptor activation, indicating roles for these compounds as modulators of T-cell signalling and T-cell responses.

SRC-dependent signalling regulates actin ruffle formation induced by glycerophosphoinositol 4-phosphate

The glycerophosphoinositols are diffusible phosphoinositide metabolites reported to modulate actin dynamics and tumour cell spreading. In particular, the membrane permeant glycerophosphoinositol 4-phosphate (GroPIns4P) has been shown to act at the level of the small GTPase Rac1, to induce the rapid formation of membrane ruffles. Here, we have investigated the signalling cascade involved in this process, and show that it is initiated by the activation of Src kinase. In NIH3T3 cells, exogenous addition of GroPIns4P induces activation and translocation of Rac1 and its exchange factor TIAM1 to the plasma membrane; in addition, in in-vitro assays, GroPIns4P favours the formation of a protein complex that includes Rac1 and TIAM1. Neither of these processes involves direct actions of GroPIns4P on these proteins. Thus, through the use of specific inhibitors of tyrosine kinases and phospholipase C (and by direct evaluation of kinase activities and inositol 1,4,5-trisphosphate production), we show that GroPIns4P activates Src, and as a consequence, phospholipase Cgamma and Ca(2+)/calmodulin kinase II, the last of which directly phosphorylates TIAM1 and leads to TIAM1/Rac1-dependent ruffle formation.

Glycerophospholipid identification and quantitation by electrospray ionization mass spectrometry

Glycerophospholipids are the structural building blocks of the cellular membrane. In addition to creating a protective barrier around the cell, lipids are precursors of intracellular signaling molecules that modulate membrane trafficking and are involved in transmembrane signal transduction. Phospholipids are also increasingly recognized as important participants in the regulation and control of cellular function and disease. Analysis and characterization of lipid species by mass spectrometry (MS) have evolved and advanced with improvements in instrumentation and technology. Key advances, including the development of "soft" ionization techniques for MS such as electrospray ionization (ESI), matrix-assisted laser desorption/ionization (MALDI), and tandem mass spectrometry (MS/MS), have facilitated the analysis of complex lipid mixtures by overcoming the earlier limitations. ESI-MS has become the technique of choice for the analysis of multi-component mixtures of lipids from biological samples due to its exceptional sensitivity and capacity for high throughput. This chapter covers qualitative and quantitative MS methods used for the elucidation of glycerophospholipid identity and quantity in cell or tissue extracts. Sections are included on the extraction, MS analysis, and data analysis of glycerophospholipids and polyphosphoinositides.

Glycerophosphoinositol-4-phosphate enhances SDF-1alpha-stimulated T-cell chemotaxis through PTK-dependent activation of Vav

Glycerophosphoinositols (GPIs) are water-soluble phosphoinosite metabolites produced by all cell types, whose levels increase in response to a variety of extracellular stimuli, and are particularly high in Ras-transformed cells. GPIs are released to the extracellular space, wherefrom they can be taken up by other cells through a specific transporter. Exogenous GPIs affect a plethora of cellular functions. Among these compounds the most active is GroPIns4P, which affects cAMP levels and PKA-dependent functions through the inhibition of heterotrimeric Gs proteins. GroPIns4P has also recently been found to promote actin cytoskeleton reorganization by inducing Rho and Rac activation through an as yet unidentified mechanism. Here we have assessed the potential effects of GroPIns4P on T-cells. We found that GroPIns4P enhances CXCR4-dependent chemotaxis. This activity results from the capacity of GroPIns4P to activate the Rho GTPase exchange factor, Vav, through an Lck-dependent pathway which also results in activation of the stress kinases JNK and p38. GroPIns4P was also found to activate with a delayed kinetics the Lck-dependent activation of ZAP-70, Shc and Erk1/2. The activities of GroPIns4P were found to be dependent on its capacity to inhibit cAMP production and PKA activation. Collectively, the data provide the first evidence of a role of glycerophosphoinositols as modulators of T-cell signaling and establish a mechanistic basis for the effects of this phosphoinositide derivative on F-actin dynamics.

Involvement of membrane protein GDE2 in retinoic acid-induced neurite formation in Neuro2A cells

We show that a glycerophosphodiester phosphodiesterase homolog, GDE2, is widely expressed in brain tissues including primary neurons, and that the expression of GDE2 in neuroblastoma Neuro2A cells is significantly upregulated during neuronal differentiation by retinoic acid (RA) treatment. Stable expression of GDE2 resulted in neurite formation in the absence of RA, and GDE2 accumulated at the regions of perinuclear and growth cones in Neuro2A cells. Furthermore, a loss-of-function of GDE2 in Neuro2A cells by RNAi blocked RA-induced neurite formation. These results demonstrate that GDE2 expression during neuronal differentiation plays an important role for growing neurites.

Molecular characterization of a glycerophosphoinositol transporter in mammalian cells

The glycerophosphoinositols are ubiquitous phosphoinositide metabolites involved in the control of several cell functions. They exert their actions both intracellularly and by rapidly equilibrating across the plasma membrane when added to cells, implying the existence of a transporter for their membrane permeation. Such a transporter, GIT1, has been cloned in yeast. By PSI-BLAST analysis, we have identified the Glut2 transporter as a human-genome candidate ortholog of GIT1. This was supported directly through the use of inhibitors, siRNAs and competition studies of specific uptake of GroPIns in HeLa cells over-expressing human Glut2. These data identify Glut2 as a GroPIns transporter in mammals, and define a physiologically relevant cell-permeation mechanism.

A novel pathway of cell growth regulation mediated by a PLA2α‐derived phosphoinositide metabolite

The phosphoinositides have well-defined roles in the control of cellular functions, including cytoskeleton dynamics, membrane trafficking, and cell signaling. However, the interplay among the phosphoinositides and their diffusible derivatives that originate through phospholipase A2 action (the lysophosphoinositides and glycerophosphoinositols) remains to be fully elucidated. Here we demonstrate that in PCCl3 rat thyroid cells, the intracellular levels of glycerophosphoinositol are finely modulated by ATP and norepinephrine through the P2Y metabotropic and alpha-adrenergic receptors, respectively. The enzyme involved here is phospholipase A2 IValpha (PLA2 IValpha), which in these cells specifically hydrolyzes phosphatidylinositol, forming lysophosphatidylinositol, glycerophosphoinositol, and arachidonic acid. This receptor-mediated activation of PLA2 IValpha leads to stimulation of PCCl3 cell growth. The involvement of a PLA2 IValpha-mediated pathway is demonstrated by inhibition of the increase in intracellular glycerophosphoinositol levels and cell proliferation by specific inhibitors, RNA interference, and overexpression of the dominant-negative PLA2 IValpha(1-522). Modulation of PCCl3 cell growth is not seen with inhibitors of arachidonic acid metabolism. In conclusion, these data characterize glycerophosphoinositol as a mediator of the purinergic and adrenergic regulation of PCCl3 cell proliferation, defining a novel regulatory cascade specifically involving this soluble phosphoinositide derivative and widening the involvement of the phosphoinositides in the regulation of cell function.

Transport and metabolism of glycerophosphodiesters produced through phospholipid deacylation

Phospholipid deacylation results in the formation of glycerophosphodiesters and free fatty acids. In Saccharomyces cerevisiae, four gene products with phospholipase B (deacylating) activity have been characterized (PLB1, PLB2, PLB3, NTE1), and those activities account for most, if not all, of the glycerophosphodiester production observed to date. The glycerophosphodiesters themselves are hydrolyzed into glycerol-3-phosphate and the corresponding alcohol by glycerophosphodiester phosphodiesterases. Although only one glycerophosphodiester phosphodiesterase-encoding gene (GDE1) has been characterized in S. cerevisiae, others certainly exist. Both internal and external glycerophosphodiesters (primarily glycerophosphocholine and glycerophosphoinositol) are formed as a result of phospholipid turnover in S. cerevisiae. A permease encoded by the GIT1 gene imports extracellular glycerophosphodiesters across the plasma membrane, where their hydrolytic products can provide crucial nutrients such as inositol, choline, and phosphate to the cell. The importance of this metabolic pathway in various aspects of S. cerevisiae cell physiology is being explored.

Glycerophosphocholine-dependent growth requires Gde1p (YPL110c) and Git1p in Saccharomyces cerevisiae

Glycerophosphocholine is formed via the deacylation of the phospholipid phosphatidylcholine. The protein encoded by Saccharomyces cerevisiae open reading frame YPL110c effects glycerophosphocholine metabolism in vivo, most likely by acting as a glycerophosphocholine phosphodiesterase. Deletion of YPL110c causes an accumulation of glycerophosphocholine in cells prelabeled with [14C]choline. Correspondingly, overexpression of YPL110c results in reduced intracellular glycerophosphocholine in cells prelabeled with [14C]choline. Glycerophospho[3H]choline supplied in the growth medium accumulates to a much greater extent in the intracellular fraction of a YPL110Delta strain than in a wild type strain. Furthermore, glycerophospho[3H]choline accumulation requires the transporter encoded by GIT1, a known glycerophosphoinositol transporter. Growth on glycerophosphocholine as the sole phosphate source requires YPL110c and the Git1p permease. In contrast to glycerophosphocholine, glycerophosphoinositol metabolism is unaffected by deletion of YPL110c. The open reading frame YPL110c has been termed GDE1.

Glycerophosphoinositols inhibit the ability of tumour cells to invade the extracellular matrix

The naturally occurring phosphoinositide metabolite, glycerophosphoinositol 4-phosphate, has recently been shown to induce rearrangements in the actin cytoskeleton through modulation of the small GTPases, Rac and Rho. Since this is directly linked to cell spreading and remodelling, we have evaluated the potential role of glycerophosphoinositol 4-phosphate and related metabolites in tumour cell invasion. The biological effects of these compounds were tested in a number of cellular activities related to cell spreading, including cell migration and cell invasion. We find that unlike other inositol-containing molecules, such as the inositol phosphates, glycerophosphoinositol and glycerophosphoinositol 4-phosphate prevent the invasion of epithelium-derived MDA-MB-231 breast carcinoma and A375MM melanoma cell lines through the extracellular matrix; this is due to a decreased ability to degrade matrix components. These data identify a specific activity of the glycerophosphoinositols that can be exploited for their development as novel anti-invasive drugs.

Glycerophosphoinositol and dexamethasone improve transendothelial electrical resistance in an in vitro study of the blood–brain barrier

The blood-brain barrier (BBB) maintains the homeostasis of the brain microenvironment, which is crucial for neuronal activity and function. Under pathological conditions, the BBB may fail due to yet unknown mechanisms. BBB failure is accompanied by an increase in the transendothelial permeability to substances such as sucrose that are normally extruded. Furthermore, altered BBB function may also lead to development of abnormal drug extrusion mechanisms including expression of multiple drug resistance proteins. Therefore, it is not surprising that strategies have been developed to "repair" the BBB in order to restore normal brain homeostasis and penetration/extrusion of pharmacologically active (noxious) substances. To this end, steroidal hormones and synthetic analogues such as dexamethasone (DEX) have been used to counteract BBB failure. However, several side effects limit the usefulness of steroid treatment in humans leading to the quest for developing novel strategies for BBB repair. We here show that, in an in vitro model of the BBB based on a co-culture of endothelial cells (EC) and glia, the natural compound glycerophosphoinositol (GPI) may replicate the effects of DEX. Thus, GPI in concentrations ranging from 3 to 100 microM promoted both BBB formation and repair in a dose dependent fashion. Similar effects were obtained with an elevated dose of DEX (10 microM); at higher concentrations (100 microM), DEX was cytotoxic. We conclude that the endogenous anti-inflammatory agent GPI may ameliorate BBB function with efficacy comparable to that of steroids, but with significantly fewer side effects. Further experiments will confirm the efficacy of this treatment in vivo and elucidate the pathways that lead to BBB repair after exposure to GPI.

GDE1/MIR16 is a glycerophosphoinositol phosphodiesterase regulated by stimulation of G protein-coupled receptors

Previously we identified MIR16 (membrane interacting protein of RGS16) as an integral membrane glycoprotein that interacts with regulator of G protein signaling proteins and shares significant sequence homology with bacterial glycerophosphodiester phosphodiesterases (GDEs), suggesting that it is a putative mammalian GDE. Here we show that MIR16 belongs to a large, evolutionarily conserved family of GDEs with a characteristic putative catalytic domain that shares a common motif (amino acids 92-116) with the catalytic domains of mammalian phosphoinositide phospholipases C. Expression of wild-type MIR16 (renamed GDE1), but not two catalytic domain mutants (E97A/D99A and H112A), leads to a dramatic increase in glycerophosphoinositol phosphodiesterase (GPI-PDE) activity in HEK 293T cells. Analysis of substrate specificity shows that GDE1/MIR16 selectively hydrolyzes GPI over glycerophosphocholine. The GPI-PDE activity of GDE1/MIR16 expressed in HEK 293T cells can be regulated by stimulation of G protein-coupled, alpha/beta-adrenergic, and lysophospholipid receptors. Membrane topology studies suggest a model in which the catalytic GDE domain faces the lumenextracellular space and the C terminus faces the cytoplasm. Our results suggest that by serving as a PDE for GPI with its activity regulated by G protein signaling, GDE1/MIR16 provides a link between phosphoinositide metabolism and G protein signal transduction.

Reorganization of actin cytoskeleton by the phosphoinositide metabolite glycerophosphoinositol 4-phosphate

Glycerophosphoinositol 4-phosphate (GroPIns-4P) is a biologically active, water-soluble phospholipase A metabolite derived from phosphatidylinositol 4-phosphate, whose cellular concentrations have been reported to increase in Ras-transformed cells. It is therefore important to understand its biological activities. Herein, we have examined whether GroPIns-4P can regulate the organization of the actin cytoskeleton, because this could be a Ras-related function involved in cell motility and metastatic invasion. We find that in serum-starved Swiss 3T3 cells, exogenously added GroPIns-4P rapidly and potently induces the formation of membrane ruffles, and, later, the formation of stress fibers. These actin structures can be regulated by the small GTPases Cdc42, Rac, and Rho. To analyze the mechanism of action of GroPIns-4P, we selectively inactivated each of these GTPases. GroPIns-4P requires active Rac and Rho, but not Cdc42, for ruffle and stress fiber formation, respectively. Moreover, GroPIns-4P induces a rapid translocation of the green fluorescent protein-tagged Rac into ruffles, and increases the fraction of GTP-bound Rac, in intact cells. The activation of Rac by GroPIns-4P was near maximal and long-lasting. Interestingly, this feature seems to be critical in the induction of actin ruffles by GroPIns-4P.

Maintenance of PtdIns45P2 pools under limiting inositol conditions, as assessed by liquid chromatography-tandem mass spectrometry and PtdIns45P2 mass evaluation in Ras-transformed cells

Inositol-containing molecules are involved in important cellular functions, including signalling, membrane transport and secretion. Our interest is in lysophosphatidylinositol and the glycerophosphoinositols, which modulate cell proliferation and G-protein-dependent activities such as adenylyl cyclase and phospholipase A(2). To investigate the role of glycerophosphoinositol (GroPIns) in the modulation of Ras-dependent pathways and its correlation to Ras transformation, we employed a novel liquid chromatography-tandem mass spectrometry technique to directly measure GroPIns in cell extracts. The cellular levels of GroPIns in selected parental and Ras-transformed cells, and in some carcinoma cells, ranged from 44 to 925 microM, with no consistent correlation to Ras transformation across all cell lines. Moreover, the derived cellular inositol concentrations revealed a wide range ( approximately 150 microM to approximately 100 mM) under standard [(3)H]-inositol-loading, suggesting a complex relationship between the inositol pool and the phosphoinositides and their derivatives. We have investigated these pools under specific loading conditions, designing a further HPLC analysis for GroPIns, combined with mass determinations of cellular phosphatidylinositol 4,5-bisphosphate. The data demonstrate that limiting inositol conditions identify a preferred pathway of inositol incorporation and retention into the polyphosphoinositides pool. Thus, under conditions of increased metabolic activity, such as receptor stimulation or cellular transformation, the polyphosphoinositide levels will be maintained at the expense of phosphatidylinositol and the turnover of its aqueous derivatives.

Identification of a new polyphosphoinositide in plants, phosphatidylinositol 5-monophosphate (PtdIns5P), and its accumulation upon osmotic stress

Polyphosphoinositides play an important role in membrane trafficking and cell signalling. In plants, two PtdInsP isomers have been described, PtdIns3P and PtdIns4P. Here we report the identification of a third, PtdIns5P. Evidence is based on the conversion of the endogenous PtdInsP pool into PtdIns(4,5)P(2) by a specific PtdIns5P 4-OH kinase, and on in vivo (32)P-labelling studies coupled to HPLC head-group analysis. In Chlamydomonas, 3-8% of the PtdInsP pool was PtdIns5P, 10-15% was PtdIns3P and the rest was PtdIns4P. In seedlings of Vicia faba and suspension-cultured tomato cells, the level of PtdIns5P was about 18%, indicating that PtdIns5P is a general plant lipid that represents a significant proportion of the PtdInsP pool. Activating phospholipase C (PLC) signalling in Chlamydomonas cells with mastoparan increased the turnover of PtdIns(4,5)P(2) at the cost of PtdIns4P, but did not affect the level of PtdIns5P. This indicates that PtdIns(4,5)P(2) is synthesized from PtdIns4P rather than from PtdIns5P during PLC signalling. However, when cells were subjected to hyperosmotic stress, PtdIns5P levels rapidly increased, suggesting a role in osmotic-stress signalling. The potential pathways of PtdIns5P formation are discussed.

CD28 co-stimulates TCR/CD3-induced phosphoinositide turnover in human T lymphocytes

Upon engagement of TCR with peptide-MHC complexes displayed on the surface of antigen-presenting cells, T lymphocytes undergo a sustained elevation of intracellular Ca(2+) concentration([Ca(2+)](i)), which is required for cytokine production. In the present work, we investigate how inositol lipid metabolism can be activated for a prolonged time to ensure a sustained link between receptor triggering and downstream signaling effectors. Four lines of evidence indicate that an extensive phosphoinositide turnover induced by TCR and CD28 engagement allows this task to be accomplished: (i) continuous phosphoinositide breakdown is required for a sustained [Ca(2+)](i )increase in antigen-stimulated T cells; (ii) TCR triggering results in a continuous release of inositol phosphates from the cell membrane paralleled by a massive and sustained phosphoinositide re-synthesis due to free inositol re-incorporation; (iii) TCR-induced phosphoinositide turnover is strongly increased by CD28 ligation; and (iv) CD28 engagement augments and sustains the TCR-induced [Ca(2+)](i )increase. Our results show that the T cell pool of phosphoinositides is continuously re-formed during T cell-APC cognate interaction, thereby explaining how sustained receptor triggering can transduce an equally sustained [Ca(2+)](i) increase. Importantly, our data identify a novel step in the signaling cascade where co-stimulation converges with TCR-generated signals to sustain and amplify the activation process.