Changes in the levels of glycerophosphoinositols during differentiation of hepatic and neuronal cells

Study on plasma metabolomics profiling of depression in Chinese community-dwelling older adults based on untargeted LC/GC‒MS

Depression is a serious psychiatric illness that causes great inconvenience to the lives of elderly individuals. However, the diagnosis of depression is somewhat subjective. Nontargeted gas chromatography (GC)/liquid chromatography (LC)-mass spectrometry (MS) was used to study the plasma metabolic profile and identify objective markers for depression and metabolic pathway variation. We recruited 379 Chinese community-dwelling individuals aged ≥ 65. Plasma samples were collected and detected by GC/LC‒MS. Orthogonal partial least squares discriminant analysis and a heatmap were utilized to distinguish the metabolites. Receiver operating characteristic curves were constructed to evaluate the diagnostic value of these differential metabolites. Additionally, metabolic pathway enrichment was performed to reveal metabolic pathway variation. According to our standard, 49 people were included in the depression cohort (DC), and 49 people age- and sex-matched individuals were included in the non-depression cohort (NDC). 64 metabolites identified via GC‒MS and 73 metabolites identified via LC‒MS had significant contributions to the differentiation between the DC and NDC, with VIP values > 1 and p values < 0.05. Three substances were detected by both methods: hypoxanthine, phytosphingosine, and xanthine. Furthermore, 1-(sn-glycero-3-phospho)-1D-myo-inositol had the largest area under the curve (AUC) value (AUC = 0.842). The purine metabolic pathway is the most important change in metabolic pathways. These findings show that there were differences in plasma metabolites between the depression cohort and the non-depression cohort. These identified differential metabolites may be markers of depression and can be used to study the changes in depression metabolic pathways.

Direct LC-MS/MS Analysis of Extra- and Intracellular Glycerophosphoinositol in Model Cancer Cell Lines

Glycerophosphoinositols (GPIs) are water-soluble bioactive phospholipid derivatives of increasing interest as intracellular and paracrine mediators of eukaryotic cell functions. The most representative compound of the family is glycerophosphoinositol (GroPIns), an ubiquitous component of mammalian cells that participates in cell proliferation, cell survival and cell response to stimuli. Levels and activity of this compound vary among cell types and deciphering these functions requires accurate measurements in in vitro and in vivo models. The conventional approaches for the analysis of GroPIns pose several issues in terms of sensitivity and product resolution, especially when the product is in the extracellular milieu. Here we present an UPLC-MS study for the quantitative analysis of this lipid derivative in cells and, for the first time, culture supernatants. The method is based on a solid-phase extraction that allows for fast desalting and analyte concentration. The robustness of the procedure was tested on the simultaneous measurements of intra- and extracellular levels of GroPIns in a number of human cell lines where it has been shown that the non-transformed cells are characterized by high extracellular level of GroPIns, whereas the tumor cells tended to have higher intracellular levels.

A signalling cascade involving receptor-activated phospholipase A2, glycerophosphoinositol 4-phosphate, Shp1 and Src in the activation of cell motility

Background

Shp1, a tyrosine-phosphatase-1 containing the Src-homology 2 (SH2) domain, is involved in inflammatory and immune reactions, where it regulates diverse signalling pathways, usually by limiting cell responses through dephosphorylation of target molecules. Moreover, Shp1 regulates actin dynamics. One Shp1 target is Src, which controls many cellular functions including actin dynamics. Src has been previously shown to be activated by a signalling cascade initiated by the cytosolic-phospholipase A₂ (cPLA₂) metabolite glycerophosphoinositol 4-phosphate (GroPIns4P), which enhances actin polymerisation and motility. While the signalling cascade downstream Src has been fully defined, the mechanism by which GroPIns4P activates Src remains unknown.

Methods

Affinity chromatography, mass spectrometry and co-immunoprecipitation studies were employed to identify the GroPIns4P-interactors; among these Shp1 was selected for further analysis. The specific Shp1 residues interacting with GroPIns4P were revealed by NMR and validated by site-directed mutagenesis and biophysical methods such as circular dichroism, isothermal calorimetry, fluorescence spectroscopy, surface plasmon resonance and computational modelling. Morphological and motility assays were performed in NIH3T3 fibroblasts.

Results

We find that Shp1 is the direct cellular target of GroPIns4P. GroPIns4P directly binds to the Shp1-SH2 domain region (with the crucial residues being Ser 118, Arg 138 and Ser 140) and thereby promotes the association between Shp1 and Src, and the dephosphorylation of the Src-inhibitory phosphotyrosine in position 530, resulting in Src activation. As a consequence, fibroblast cells exposed to GroPIns4P show significantly enhanced wound healing capability, indicating that GroPIns4P has a stimulatory role to activate fibroblast migration. GroPIns4P is produced by cPLA₂ upon stimulation by diverse receptors, including the EGF receptor. Indeed, endogenously-produced GroPIns4P was shown to mediate the EGF-induced cell motility.

Conclusions

This study identifies a so-far undescribed mechanism of Shp1/Src modulation that promotes cell motility and that is dependent on the cPLA₂ metabolite GroPIns4P. We show that GroPIns4P is required for EGF-induced fibroblast migration and that it is part of a cPLA₂/GroPIns4P/Shp1/Src cascade that might have broad implications for studies of immune-inflammatory response and cancer.

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0329-3) contains supplementary material, which is available to authorized users.

An improved UPLC-MS/MS platform for quantitative analysis of glycerophosphoinositol in mammalian cells

The glycerophosphoinositols constitute a class of biologically active lipid-derived mediators whose intracellular levels are modulated during physiological and pathological cell processes. Comprehensive assessment of the role of these compounds expands beyond the cellular biology of lipids and includes rapid and unambiguous measurement in cells and tissues. Here we describe a sensitive and simple liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for quantitative analysis of the most abundant among these phosphoinositide derivatives in mammalian cells, the glycerophosphoinositol (GroPIns). The method has been developed in mouse Raw 264.7 macrophages with limits of quantitation at 3 ng/ml. Validation on the same cell line showed excellent response in terms of linear dynamic range (from 3 to 3,000 ng/ml), intra-day and inter-day precision (coefficient of variation ≤ 7.10%) and accuracy (between 98.1 and 109.0%) in the range 10-320 ng/ml. As proof of concept, a simplified analytical platform based on this method and external calibration was also tested on four stimulated and unstimulated cell lines, including Raw 264.7 macrophages, Jurkat T-cells, A375MM melanoma cells and rat basophilic leukemia RBL-2H3 cells. The results indicate a wide variation in GroPIns levels among different cell lines and stimulation conditions, although the measurements were always in line with the literature. No significant matrix effects were observed thus indicating that the here proposed method can be of general use for similar determinations in cells of different origin.

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 and their cellular functions

Interest in the glycerophosphoinositols has been increasing recently, on the basis of their biological activities. The cellular metabolism of these water-soluble bioactive phosphoinositide metabolites has been clarified, with the identification of the specific enzyme involved in their synthesis, PLA2IVα (phospholipase A2 IVα), and the definition of their phosphodiesterase-based catabolism, and thus inactivation. The functional roles and mechanisms of action of these compounds have been investigated in different cellular contexts. This has led to their definition in the control of various cell functions, such as cell proliferation in the thyroid and actin cytoskeleton organization in fibroblasts and lymphocytes. Roles for the glycerophosphoinositols in immune and inflammatory responses are also being defined. In addition to these physiological functions, the glycerophosphoinositols have potential anti-metastatic activities that should lead to their pharmacological exploitation.

L-α-lysophosphatidylinositol meets GPR55: a deadly relationship

Evidence points to a role of L-α-lysophosphatidylinositol (LPI) in cancer. First, clinical data identified LPI as a biomarker for poor prognosis in cancer patients. Second, in vitro studies demonstrated significantly elevated levels of LPI in highly proliferative cancer cells. Third, LPI displays mitogenic activity in cancer cell lines, in which the lipid significantly increased cell proliferation. However, a receptor target for LPI remained elusive until very recently. It has now been revealed that LPI activates GPR55, a G protein-coupled receptor that couples to G(12/13) and G(q) proteins, which direct oncogenic signalling. New evidence indicates that LPI and GPR55 are key partners in driving cancer cell proliferation and migration. GPR55 is expressed in human tumours and drives proliferation and its expression correlates with tumour aggressiveness. Overall patient survival is lower in patients whose glioblastomas express higher levels of GPR55. Thus, evidence suggests that interaction with GPR55 might underlie the pro-tumoural actions of LPI.

The glycerophospho metabolome and its influence on amino acid homeostasis revealed by brain metabolomics of GDE1(-/-) mice

GDE1 is a mammalian glycerophosphodiesterase (GDE) implicated by in vitro studies in the regulation of glycerophophoinositol (GroPIns) and possibly other glycerophospho (GroP) metabolites. Here, we show using untargeted metabolomics that GroPIns is profoundly (>20-fold) elevated in brain tissue from GDE1(-/-) mice. Furthermore, two additional GroP metabolites not previously identified in eukaryotic cells, glycerophosphoserine (GroPSer) and glycerophosphoglycerate (GroPGate), were also highly elevated in GDE1(-/-) brains. Enzyme assays with synthetic GroP metabolites confirmed that GroPSer and GroPGate are direct substrates of GDE1. Interestingly, our metabolomic profiles also revealed that serine (both L-and D-) levels were significantly reduced in brains of GDE1(-/-) mice. These findings designate GroPSer as a previously unappreciated reservoir for free serine in the nervous system and suggest that GDE1, through recycling serine from GroPSer, may impact D-serine-dependent neural signaling processes in vivo.

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.

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.

Isolation and characterization of two serpentine membrane proteins containing glycerophosphodiester phosphodiesterase, GDE2 and GDE6

Serpentine membrane protein with a glycerophosphodiester phosphodiesterase (GP-PDE) motif, GDE3, is involved in morphological change of cells and accelerates the program of osteoblast differentiation, suggesting that mammalian GP-PDEs play an important role in the regulation of cytoskeletal modification. Here, we isolated two cDNAs encoding serpentine membrane proteins, GDE2 and GDE6, containing GP-PDE motif from mouse cDNA libraries. The deduced sequence of GDE2 contains 607 amino acids with seven putative transmembrane regions. GDE6 was composed of 633 amino acids also with seven putative transmembrane regions. In amino acid sequences, GDE2 and GDE6 are 43.7% and 34.3% identical to GDE3, respectively. Although GDE3 mRNA is highly expressed in bone tissue and spleen, GDE2 mRNA was expressed in a variety of mouse tissues including lung and heart, while the GDE6 transcript was particularly abundant in spermatocytes of mouse testis. Immunohistochemical analyses using anti-GDE2 antibody showed that GDE2 protein is expressed in the epithelial cell layer of mouse lung. These results suggest that GP-PDEs are differentially expressed in mouse tissues, and might have distinct roles.

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.

The protein kinase C-dependent phosphorylation of serine 166 is controlled by the phospholipid species bound to the phosphatidylinositol transfer protein alpha

The charge isomers of bovine brain PI-TPalpha (i.e. PI-TPalphaI containing a phosphatidylinositol (PI) molecule and PI-TPalphaII containing a phosphatidylcholine (PC) molecule) were phosphorylated in vitro by rat brain protein kinase C (PKC) at different rates. From the double-reciprocal plot, it was estimated that the V(max) values for PI-TPalphaI and II were 2.0 and 6.0 nmol/min, respectively; the K(m) values for both charge isomers were about equal, i.e. 0.7 micrometer. Phosphorylation of charge isomers of recombinant mouse PI-TPalpha confirmed that the PC-containing isomer was the better substrate. Phosphoamino acid analysis of in vitro and in vivo (32)P-labeled PI-TPalphas showed that serine was the major site of phosphorylation. Degradation of (32)P-labeled PI-TPalpha by cyanogen bromide followed by high pressure liquid chromatography and sequence analysis yielded one (32)P-labeled peptide (amino acids 104-190). This peptide contained Ser-148, Ser-152, and the consensus PKC phosphorylation site Ser-166. Replacement of Ser-166 with an alanine residue confirmed that indeed this residue was the site of phosphorylation. This mutation completely abolished PI and PC transfer activity. This was also observed when Ser-166 was replaced with Asp, implying that this is a key amino acid residue in regulating the function of PI-TPalpha. Stimulation of NIH3T3 fibroblasts by phorbol ester or platelet-derived growth factor induced the rapid relocalization of PI-TPalpha to perinuclear Golgi structures concomitant with a 2-3-fold increase in lysophosphatidylinositol levels. This relocalization was also observed for Myc-tagged wtPI-TPalpha expressed in NIH3T3 cells. In contrast, the distribution of Myc-tagged PI-TPalpha(S166A) and Myc-tagged PI-TPalpha(S166D) were not affected by phorbol ester, suggesting that phosphorylation of Ser-166 was a prerequisite for the relocalization to the Golgi. A model is proposed in which the PKC-dependent phosphorylation of PI-TPalpha is linked to the degradation of PI.

Overexpression of phosphatidylinositol transfer protein alpha in NIH3T3 cells activates a phospholipase A

In order to investigate the cellular function of the mammalian phosphatidylinositol transfer protein alpha (PI-TPalpha), NIH3T3 fibroblast cells were transfected with the cDNA encoding mouse PI-TPalpha. Two stable cell lines, i.e. SPI6 and SPI8, were isolated, which showed a 2- and 3-fold increase, respectively, in the level of PI-TPalpha. Overexpression of PI-TPalpha resulted in a decrease in the duration of the cell cycle from 21 h for the wild type (nontransfected) NIH3T3 (wtNIH3T3) cells and mock-transfected cells to 13-14 h for SPI6 and SPI8 cells. Analysis of exponentially growing cultures by fluorescence-activated cell sorting showed that a shorter G(1) phase is mainly responsible for this decrease. The saturation density of the cells increased from 0.20 x 10(5) cells/cm(2) for wtNIH3T3 cells to 0.53 x 10(5) cells/cm(2) for SPI6 and SPI8 cells. However, anchorage-dependent growth was maintained as shown by the inability of the cells to grow in soft agar. Upon equilibrium labeling of the cells with myo-[(3)H] inositol, the relative incorporation of radioactivity in the total inositol phosphate fraction was 2-3-fold increased in SPI6 and SPI8 cells when compared with wtNIH3T3 cells. A detailed analysis of the inositol metabolites showed increased levels of glycerophosphoinositol, Ins(1)P, Ins(2)P, and lysophosphatidylinositol (lyso-PtdIns) in SPI8 cells, whereas the levels of phosphatidylinositol (PtdIns) and phosphatidylinositol 4, 5-bisphosphate were the same as those in control cells. The addition of PI-TPalpha to a total lysate of myo-[(3)H]inositol-labeled wtNIH3T3 cells stimulated the formation of lyso-PtdIns. The addition of Ca(2+) further increased this formation. Based on these observations, we propose that PI-TPalpha is involved in the production of lyso-PtdIns by activating a phospholipase A acting on PtdIns. The increased level of lyso-PtdIns that is produced in this reaction could be responsible for the increased growth rate and the partial loss of contact inhibition in SPI8 and SPI6 cells. The addition of growth factors (platelet-derived growth factor, bombesin) to these overexpressers did not activate the phospholipase C-dependent degradation of phosphatidylinositol 4,5-bisphosphate.

Fast receptor-induced formation of glycerophosphoinositol-4-phosphate, a putative novel intracellular messenger in the Ras pathway

Glycerophosphoinositols are phosphoinositide metabolites whose levels are constitutively elevated in Ras-transformed cells. Here, we show that one of these compounds, glycerophosphoinositol-4-phosphate (GroPIns-4-P) responds acutely to the stimulation of the epidermal growth factor receptor, with a fast, massive and transient increase. The mechanism leading to GroPIns-4-P formation involves the activation of phosphoinositide-3 kinase and the small GTP-binding protein Rac, since GroPIns-4-P was neither formed in cells expressing the dominant negative form of Rac nor in cells treated with the phosphoinositide-3 kinase inhibitor wortmannin. GroPIns-4-P has been previously shown to inhibit adenylyl cyclase. Accordingly, epidermal growth factor also decreased the basal, cholera toxin-stimulated, and forskolin-stimulated cyclic AMP levels with kinetics similar to those of GroPIns-4-P formation, suggesting that GroPIns-4-P mediates this inhibitory effect. The hormone-induced formation of GroPIns-4-P was detected in several cell lines of various origin, suggesting that GroPIns-4-P is a novel intracellular messenger of the Ras pathway, possibly able to convey information from tyrosine kinase receptors to the cyclic AMP cascade.