Regulation of phosphatidylcholine homeostasis by calcium-independent phospholipase A2

iPLA2-VIA is required for healthy aging of neurons, muscle, and the female germline in Drosophila melanogaster

Neurodegenerative disease (ND) is a growing health burden worldwide, but its causes and treatments remain elusive. Although most cases of ND are sporadic, rare familial cases have been attributed to single genes, which can be investigated in animal models. We have generated a new mutation in the calcium-independent phospholipase A₂ (iPLA₂) VIA gene CG6718, the Drosophila melanogaster ortholog of human PLA2G6/PARK14, mutations in which cause a suite of NDs collectively called PLA2G6-associated neurodegeneration (PLAN). Our mutants display age-related loss of climbing ability, a symptom of neurodegeneration in flies. Although phospholipase activity commonly is presumed to underlie iPLA₂-VIA function, locomotor decline in our mutants is rescued by a transgene carrying a serine-to-alanine mutation in the catalytic residue, suggesting that important functional aspects are independent of phospholipase activity. Additionally, we find that iPLA₂-VIA knockdown in either muscle or neurons phenocopies locomotor decline with age, demonstrating its necessity in both neuronal and non-neuronal tissues. Furthermore, RNA in situ hybridization shows high endogenous iPLA₂-VIA mRNA expression in adult germ cells, and transgenic HA-tagged iPLA₂-VIA colocalizes with mitochondria there. Mutant males are fertile with normal spermatogenesis, while fertility is reduced in mutant females. Mutant female germ cells display age-related mitochondrial aggregation, loss of mitochondrial potential, and elevated cell death. These results suggest that iPLA₂-VIA is critical for mitochondrial integrity in the Drosophila female germline, which may provide a novel context to investigate its functions with parallels to PLAN.

Formulative Study and Intracellular Fate Evaluation of Ethosomes and Transethosomes for Vitamin D3 Delivery

In this pilot study, ethosomes and transethosomes were investigated as potential delivery systems for cholecalciferol (vitamin D3), whose deficiency has been correlated to many disorders such as dermatological diseases, systemic infections, cancer and sarcopenia. A formulative study on the influence of pharmaceutically acceptable ionic and non-ionic surfactants allowed the preparation of different transethosomes. In vitro cytotoxicity was evaluated in different cell types representative of epithelial, connective and muscle tissue. Then, the selected nanocarriers were further investigated at light and transmission electron microscopy to evaluate their uptake and intracellular fate. Both ethosomes and transethosomes proven to have physicochemical properties optimal for transdermal penetration and efficient vitamin D3 loading; moreover, nanocarriers were easily internalized by all cell types, although they followed distinct intracellular fates: ethosomes persisted for long times inside the cytoplasm, without inducing subcellular alteration, while transethosomes underwent rapid degradation giving rise to an intracellular accumulation of lipids. These basic results provide a solid scientific background to in vivo investigations aimed at exploring the efficacy of vitamin D3 transdermal administration in different experimental and pathological conditions.

Hypothesis: Chemical activity regulates and coordinates the processes maintaining glycerophospholipid homeostasis in mammalian cells

Mammalian cells maintain the complex glycerophospholipid (GPL) class compositions of their various membranes within close limits because this is essential to their well‐being or viability. Surprisingly, however, it is still not understood how those compositions are maintained except that GPL synthesis and degradation are closely coordinated. Here, we hypothesize that abrupt changes in the chemical activity of the individual GPL classes coordinate synthesis and degradation as well other the homeostatic processes. We have previously proposed that only a limited number of “allowed” or “optimal” GPL class compositions exist in cellular membranes because those compositions are energetically more favorable than others, that is, they represent local free energy minima (Somerharju et al 2009, Biochim. Biophys. Acta 1788, 12‐23). This model, however, could not satisfactorily explain how the “optimal” compositions are sensed by the key homeostatic enzymes, that is, rate‐limiting synthetizing enzymes and homeostatic phospholipases. We now hypothesize that when the mole fraction of a GPL class exceeds an optimal value, its chemical activity abruptly increases which (a) increases its propensity to efflux from the membrane thus making it susceptible for hydrolysis by homeostatic phospholipases; (b) increases its potency to inhibit its own biosynthesis via a feedback mechanism; (c) enhances its conversion to another glycerophospholipid class via a novel process termed “head group remodeling” or (d) enhances its translocation to other subcellular membranes. In summary, abrupt change in the chemical activity of the individual GPL classes is proposed to regulate and coordinate those four processes maintaining GPL class homeostasis in mammalian cells.

Dysregulated Lipid Metabolism and Its Role in α-Synucleinopathy in Parkinson's Disease

Parkinson's disease (PD) is the second most common neurodegenerative disease, the main pathological hallmark of which is the accumulation of α-synuclein (α-syn) and the formation of filamentous aggregates called Lewy bodies in the brainstem, limbic system, and cortical areas. Lipidomics is a newly emerging field which can provide fresh insights and new answers that will enhance our capacity for early diagnosis, tracking disease progression, predicting critical endpoints, and identifying risk in pre-symptomatic persons. In recent years, lipids have been implicated in many aspects of PD pathology. Biophysical and lipidomic studies have demonstrated that α-syn binds preferentially not only to specific lipid families but also to specific molecular species and that these lipid-protein complexes enhance its interaction with synaptic membranes, influence its oligomerization and aggregation, and interfere with the catalytic activity of cytoplasmic lipid enzymes and lysosomal lipases, thereby affecting lipid metabolism. The genetic link between aberrant lipid metabolism and PD is even more direct, with mutations in GBA and SMPD1 enhancing PD risk in humans and loss of GALC function increasing α-syn aggregation and accumulation in experimental murine models. Moreover, a number of lipidomic studies have reported PD-specific lipid alterations in both patient brains and plasma, including alterations in the lipid composition of lipid rafts in the frontal cortex. A further aspect of lipid dysregulation promoting PD pathogenesis is oxidative stress and inflammation, with proinflammatory lipid mediators such as platelet activating factors (PAFs) playing key roles in arbitrating the progressive neurodegeneration seen in PD linked to α-syn intracellular trafficking. Lastly, there are a number of genetic risk factors of PD which are involved in normal lipid metabolism and function. Genes such as PLA2G6 and SCARB2, which are involved in glycerophospholipid and sphingolipid metabolism either directly or indirectly are associated with risk of PD. This review seeks to describe these facets of metabolic lipid dysregulation as they relate to PD pathology and potential pathomechanisms involved in disease progression, while highlighting incongruous findings and gaps in knowledge that necessitate further research.

Comparative Proteomic Analysis of Rana chensinensis Oviduct

As one of most important traditional Chinese medicine resources, the oviduct of female Rana chensinensis (Chinese brown frog) was widely used in the treatment of asthenia after sickness or delivery, deficiency in vigor, palpitation, and insomnia. Unlike other vertebrates, the oviduct of Rana chensinensis oviduct significantly expands during prehibernation, in contrast to the breeding period. To explain this phenomenon at the molecular level, the protein expression profiles of Rana chensinensis oviduct during the breeding period and prehibernation were observed using isobaric tags for relative and absolute quantitation (iTRAQ) technique. Then, all identified proteins were used to obtain gene ontology (GO) annotation. Ultimately, KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis was performed to predict the pathway on differentially expressed proteins (DEPs). A total of 4479 proteins were identified, and 312 of them presented different expression profiling between prehibernation and breeding period. Compared with prehibernation group, 86 proteins were upregulated, and 226 proteins were downregulated in breeding period. After KEGG enrichment analysis, 163 DEPs were involved in 6 pathways, which were lysosome, RNA transport, glycosaminoglycan degradation, extracellular matrix (ECM)⁻receptor interaction, metabolic pathways and focal adhesion. This is the first report on the protein profiling of Rana chensinensis oviduct during the breeding period and prehibernation. Results show that this distinctive physiological phenomenon of Rana chensinensis oviduct was mainly involved in ECM⁻receptor interaction, metabolic pathways, and focal adhesion.

Membrane lipid compositional sensing by the inducible amphipathic helix of CCT

The amphipathic helical (AH) membrane binding motif is recognized as a major device for lipid compositional sensing. We explore the function and mechanism of sensing by the lipid biosynthetic enzyme, CTP:phosphocholine cytidylyltransferase (CCT). As the regulatory enzyme in phosphatidylcholine (PC) synthesis, CCT contributes to membrane PC homeostasis. CCT directly binds and inserts into the surface of bilayers that are deficient in PC and therefore enriched in lipids that enhance surface charge and/or create lipid packing voids. These two membrane physical properties induce the folding of the CCT M domain into a ≥60 residue AH. Membrane binding activates catalysis by a mechanism that has been partially deciphered. We review the evidence for CCT compositional sensing, and the membrane and protein determinants for lipid selective membrane-interactions. We consider the factors that promote the binding of CCT isoforms to the membranes of the ER, nuclear envelope, or lipid droplets, but exclude CCT from other organelles and the plasma membrane. The CCT sensing mechanism is compared with several other proteins that use an AH motif for membrane compositional sensing. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

A novel calcium-independent cellular PLA2 acts in insect immunity and larval growth

Phospholipase A2 (PLA2) catalyzes the position-specific hydrolysis of fatty acids linked to the sn-2 position of phospholipids (PLs). PLA2s make up a very large superfamily, with more than known 15 groups, classified into secretory PLA2 (sPLA2), Ca(2+)-dependent cellular PLA2 (sPLA2) and Ca(2+)-independent cellular PLA2 (iPLA2). Only a few insect sPLA2s, expressed in venom glands and immune tissues, have been characterized at the molecular level. This study aimed to test our hypothesis that insects express iPLA2, using the beet armyworm, Spodoptera exigua, our model insect. Substantial PLA2 activities under calcium-free condition were recorded in several larval tissue preparations. The PLA2 activity was significantly reduced in reactions conducted in the presence of a specific iPLA2 inhibitor, bromoenol lactone (BEL). Analysis of a S. exigua hemocyte transcriptome identified a candidate iPLA2 gene (SeiPLA2-A). The open reading frame encoded 816 amino acid residues with a predicted molecular weight of 90.5 kDa and 6.15 pI value. Our phylogenetic analysis clustered SeiPLA2-A with the other vertebrate iPLA2s. SeiPLA2-A was expressed in all tissues we examined, including hemocytes, fat body, midgut, salivary glands, Malpighian tubules and epidermis. Heterologous expression in Sf9 cells indicated that SeiPLA2-A was localized in cytoplasm and exhibited significant PLA2 activity, which was independent of Ca(2+) and inhibited by BEL. RNA interference (RNAi) of SeiPLA2-A using its specific dsRNA in the fifth instar larvae significantly suppressed iPLA2 expression and enzyme activity. dsSeiPLA2-A-treated larvae exhibited significant loss of cellular immune response, measured as nodule formation in response to bacterial challenge, and extended larval-to-pupal developmental time. These results support our hypothesis, showing that SeiPLA2-A predicted from the transcriptome analysis catalyzes hydrolysis of fatty acids from cellular PLs and plays crucial physiological roles in insect immunity and larval growth.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Substrate efflux propensity is the key determinant of Ca2+-independent phospholipase A-β (iPLAβ)-mediated glycerophospholipid hydrolysis

The A-type phospholipases (PLAs) are key players in glycerophospholipid (GPL) homeostasis and in mammalian cells; Ca(2+)-independent PLA-β (iPLAβ) in particular has been implicated in this essential process. However, the regulation of this enzyme, which is necessary to avoid futile competition between synthesis and degradation, is not understood. Recently, we provided evidence that the efflux of the substrate molecules from the bilayer is the rate-limiting step in the hydrolysis of GPLs by some secretory (nonhomeostatic) PLAs. To study whether this is the case with iPLAβ as well, a mass spectrometric assay was employed to determine the rate of hydrolysis of multiple saturated and unsaturated GPL species in parallel using micelles or vesicle bilayers as the macrosubstrate. With micelles, the hydrolysis decreased with increasing acyl chain length independent of unsaturation, and modest discrimination between acyl positional isomers was observed, presumably due to the differences in the structure of the sn-1 and sn-2 acyl-binding sites of the protein. In striking contrast, no significant discrimination between positional isomers was observed with bilayers, and the rate of hydrolysis decreased with the acyl chain length logarithmically and far more than with micelles. These data provide compelling evidence that efflux of the substrate molecule from the bilayer, which also decreases monotonously with acyl chain length, is the rate-determining step in iPLAβ-mediated hydrolysis of GPLs in membranes. This finding is intriguing as it may help to understand how homeostatic PLAs are regulated and how degradation and biosynthesis are coordinated.

Tissue-specific differences in brain phosphodiesters in late-life major depression

OBJECTIVE

Late-life depression has been hypothesized to have a neurodegenerative component that leads to impaired executive function and increases in subcortical white matter hyperintensities. Phosphorus magnetic resonance spectroscopy (MRS) can quantify several important phosphorus metabolites in the brain, particularly the anabolic precursors and catabolic metabolites of the constituents of cell membranes, which could be altered by neurodegenerative activity.

METHODS

Ten patients with late-life major depression who were medication free at time of study and 11 aged normal comparison subjects were studied using (31)P MRS three-dimensional chemical shift imaging at 4 Tesla. Phosphatidylcholine and phosphatidylethanolamine comprise 90% of cell membranes in brain but cannot be quantified precisely with (31)P MRS. We measured phosphocholine and phosphoethanolamine, which are anabolic precursors, as well as glycerophosphocholine and glycerophosphoethanolamine, which are catabolic metabolites of phosphatidylcholine and phosphatidylethanolamine.

RESULTS

In accordance with our hypotheses, glycerophosphoethanolamine was elevated in white matter of depressed subjects, suggesting enhanced breakdown of cell membranes in these subjects. Glycerophosphocholine did not show any significant difference between comparison and depressed subjects but both showed an enhancement in white matter compared with gray matter. Contrary to our hypotheses, neither phosphocholine nor phosphoethanolamine showed evidence for reduction in late-life depression.

CONCLUSION

These findings support the hypothesis that neurodegenerative processes occur in white matter in patients with late-life depression more than in the normal elderly population.

Interaction of brain fatty acid-binding protein with the polyunsaturated fatty acid environment as a potential determinant of poor prognosis in malignant glioma

Malignant gliomas are the most common adult brain cancers. In spite of aggressive treatment, recurrence occurs in the great majority of patients and is invariably fatal. Polyunsaturated fatty acids are abundant in brain, particularly ω-6 arachidonic acid (AA) and ω-3 docosahexaenoic acid (DHA). Although the levels of ω-6 and ω-3 polyunsaturated fatty acids are tightly regulated in brain, the ω-6:ω-3 ratio is dramatically increased in malignant glioma, suggesting deregulation of fundamental lipid homeostasis in brain tumor tissue. The migratory properties of malignant glioma cells can be modified by altering the ratio of AA:DHA in growth medium, with increased migration observed in AA-rich medium. This fatty acid-dependent effect on cell migration is dependent on expression of the brain fatty acid binding protein (FABP7) previously shown to bind DHA and AA. Increased levels of enzymes involved in eicosanoid production in FABP7-positive malignant glioma cells suggest that FABP7 is an important modulator of AA metabolism. We provide evidence that increased production of eicosanoids in FABP7-positive malignant glioma growing in an AA-rich environment contributes to tumor infiltration in the brain. We discuss pathways and molecules that may underlie FABP7/AA-mediated promotion of cell migration and FABP7/DHA-mediated inhibition of cell migration in malignant glioma.

The role of phosphatidylcholine and choline metabolites to cell proliferation and survival

The reorganization of metabolic pathways in cancer facilitates the flux of carbon and reducing equivalents into anabolic pathways at the expense of oxidative phosphorylation. This provides rapidly dividing cells with the necessary precursors for membrane, protein and nucleic acid synthesis. A fundamental metabolic perturbation in cancer is the enhanced synthesis of fatty acids by channeling glucose and/or glutamine into cytosolic acetyl-CoA and upregulation of key biosynthetic genes. This lipogenic phenotype also extends to the production of complex lipids involved in membrane synthesis and lipid-based signaling. Cancer cells display sensitivity to ablation of fatty acid synthesis possibly as a result of diminished capacity to synthesize complex lipids involved in signaling or growth pathways. Evidence has accrued that phosphatidylcholine, the major phospholipid component of eukaryotic membranes, as well as choline metabolites derived from its synthesis and catabolism, contribute to both proliferative growth and programmed cell death. This review will detail our current understanding of how coordinated changes in substrate availability, gene expression and enzyme activity lead to altered phosphatidylcholine synthesis in cancer, and how these changes contribute directly or indirectly to malignant growth. Conversely, apoptosis targets key steps in phosphatidylcholine synthesis and degradation that are linked to disruption of cell cycle regulation, reinforcing the central role that phosphatidylcholine and its metabolites in determining cell fate.

Energetic and cell membrane metabolic products in patients with primary insomnia: a 31-phosphorus magnetic resonance spectroscopy study at 4 tesla

STUDY OBJECTIVES

Primary insomnia (PI) is a sleep disorder characterized by difficulty with sleep initiation, maintenance, and/or the experience of nonrestorative sleep combined with a subsequent impairment of daytime functioning. The hyperarousal hypothesis has emerged as the leading candidate to explain insomnia symptoms in the absence of specific mental, physical, or substance-related causes. We hypothesized that the cellular energetic metabolites, including beta nucleoside triphosphate, which in magnetic resonance spectroscopy approximates adenosine triphosphate (ATP), and phosphocreatine (PCr), would show changes in PI reflecting increased energy demand.

DESIGN AND SETTING

Matched-groups, cross-sectional study performed at two university-based hospitals.

PATIENTS

Sixteen medication-free individuals (eight males, eight females; mean ± standard deviation (SD) age = 37.2 ± 8.4 y) with PI and 16 good sleepers (nine males, seven females; mean ± SD age = 37.6 ± 4.7 y).

MEASUREMENTS

Diagnosis was established for all individuals by unstructured clinical interview, Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (SCID), sleep diary, and actigraphy. Polysomnography was collected in individuals with PI. Phosphorous magnetic resonance spectroscopy (31P MRS) data were collected on all individuals at 4 Tesla. We assessed cell membrane (anabolic precursors and catabolic metabolites) and bioenergetic (ATP, phosphocreatine) metabolites in gray matter and white matter to determine their relationship to the presence and severity of PI.

RESULTS

Individuals with PI showed lower phosphocreatine in gray matter and an unexpected decrease of phosphocholine, a precursor of the cell membrane compound phosphatidylcholine, in white matter. In addition, there was a trend toward a negative association between polysomnographically determined wake after sleep onset and gray matter beta-nucleoside triphosphate and white matter phosphocholine in the primary insomnia group.

CONCLUSIONS

These results support the hyperarousal hypothesis in PI based on lower phosphocreatine in gray matter in the PI group.

An in vivo ratio control mechanism for phospholipid homeostasis: evidence from lipidomic studies

While it is widely accepted that the lipid composition of eukaryotic membranes is under homeostatic control, the mechanisms through which cells sense lipid composition are still the subject of debate. It has been postulated that membrane curvature elastic energy is the membrane property that is regulated by cells, and that lipid composition is maintained by a ratio control function derived from the concentrations of type II and type 0 lipids, weighted appropriately. We assess this proposal by seeking a signature of ratio control in quantified lipid composition data obtained by electrospray ionization mass spectrometry from over 40 independent asynchronous cell populations. Our approach revealed the existence of a universal 'pivot' lipid, which marks the boundary between type 0 lipids and type II lipids, and which is invariant between different cell types or cells grown under different conditions. The presence of such a pivot species is a distinctive signature of the operation in vivo, in human cell lines, of a control function that is consistent with the hypothesis that membrane elastic energy is homeostatically controlled.

Induction of group IVC phospholipase A2 in allergic asthma: transcriptional regulation by TNFα in bronchoepithelial cells

Airway inflammation in allergen-induced asthma is associated with eicosanoid release. These bioactive lipids exhibit anti- and pro-inflammatory activities with relevance to pulmonary pathophysiology. We hypothesized that sensitization/challenge using an extract from the ubiquitous fungus Aspergillus fumigatus in a mouse model of allergic asthma would result in altered phospholipase gene expression, thus modulating the downstream eicosanoid pathway. We observed the most significant induction in the group IVC PLA2 (phospholipase A2) [also known as cPLA2γ (cytosolic PLA2γ) or PLA2G4C]. Our results infer that A. fumigatus extract can induce cPLA2γ levels directly in eosinophils, whereas induction in lung epithelial cells is most likely to be a consequence of TNFα (tumour necrosis factor α) secretion by A. fumigatus-activated macrophages. The mechanism of TNFα-dependent induction of cPLA2γ gene expression was elucidated through a combination of promoter deletions, ChIP (chromatin immunoprecipitation) and overexpression studies in human bronchoepithelial cells, leading to the identification of functionally relevant CRE (cAMP-response element), NF-κB (nuclear factor κB) and E-box promoter elements. ChIP analysis demonstrated that RNA polymerase II, ATF-2 (activating transcription factor 2)-c-Jun, p65-p65 and USF (upstream stimulating factor) 1-USF2 complexes are recruited to the cPLA2γ enhancer/promoter in response to TNFα, with overexpression and dominant-negative studies implying a strong level of co-operation and interplay between these factors. Overall, our results link cytokine-mediated alterations in cPLA2γ gene expression with allergic asthma and outline a complex regulatory mechanism.

Catalytic function of PLA2G6 is impaired by mutations associated with infantile neuroaxonal dystrophy but not dystonia-parkinsonism

BACKGROUND

Mutations in the PLA2G6 gene have been identified in autosomal recessive neurodegenerative diseases classified as infantile neuroaxonal dystrophy (INAD), neurodegeneration with brain iron accumulation (NBIA), and dystonia-parkinsonism. These clinical syndromes display two significantly different disease phenotypes. NBIA and INAD are very similar, involving widespread neurodegeneration that begins within the first 1-2 years of life. In contrast, patients with dystonia-parkinsonism present with a parkinsonian movement disorder beginning at 15 to 30 years of age. The PLA2G6 gene encodes the PLA2G6 enzyme, also known as group VIA calcium-independent phospholipase A(2), which has previously been shown to hydrolyze the sn-2 acyl chain of phospholipids, generating free fatty acids and lysophospholipids.

METHODOLOGY/PRINCIPAL FINDINGS

We produced purified recombinant wildtype (WT) and mutant human PLA2G6 proteins and examined their catalytic function using in vitro assays with radiolabeled lipid substrates. We find that human PLA2G6 enzyme hydrolyzes both phospholipids and lysophospholipids, releasing free fatty acids. Mutations associated with different disease phenotypes have different effects on catalytic activity. Mutations associated with INAD/NBIA cause loss of enzyme activity, with mutant proteins exhibiting less than 20% of the specific activity of WT protein in both lysophospholipase and phospholipase assays. In contrast, mutations associated with dystonia-parkinsonism do not impair catalytic activity, and two mutations produce a significant increase in specific activity for phospholipid but not lysophospholipid substrates.

CONCLUSIONS/SIGNIFICANCE

These results indicate that different alterations in PLA2G6 function produce the different disease phenotypes of NBIA/INAD and dystonia-parkinsonism. INAD/NBIA is caused by loss of the ability of PLA2G6 to catalyze fatty acid release from phospholipids, which predicts accumulation of PLA2G6 phospholipid substrates and provides a mechanistic explanation for the accumulation of membranes in neuroaxonal spheroids previously observed in histopathological studies of INAD/NBIA. In contrast, dystonia-parkinsonism mutations do not appear to directly impair catalytic function, but may modify substrate preferences or regulatory mechanisms for PLA2G6.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Depletion of phosphatidylcholine affects endoplasmic reticulum morphology and protein traffic at the Golgi complex

The mutant Chinese hamster ovary cell line MT58 contains a thermosensitive mutation in CTP:phosphocholine cytidylyltransferase, the regulatory enzyme in the CDP-choline pathway. As a result, MT58 cells have a 50% decrease in their phosphatidylcholine (PC) level within 24 h when cultured at the nonpermissive temperature (40 degrees C). This is due to a relative rapid breakdown of PC that is not compensated for by the inhibition of de novo PC synthesis. Despite this drastic decrease in cellular PC content, cells are viable and can proliferate by addition of lysophosphatidylcholine. By [(3)H]oleate labeling, we found that the FA moiety of the degraded PC is recovered in triacylglycerol. In accordance with this finding, an accumulation of lipid droplets is seen in MT58 cells. Analysis of PC-depleted MT58 cells by electron and fluorescence microscopy revealed a partial dilation of the rough endoplasmic reticulum, resulting in spherical structures on both sites of the nucleus, whereas the morphology of the plasma membrane, mitochondria, and Golgi complex was unaffected. In contrast to these morphological observations, protein transport from the ER remains intact. Surprisingly, protein transport at the level of the Golgi complex is impaired. Our data suggest that the transport processes at the Golgi complex are regulated by distal changes in lipid metabolism.

Lipid droplet biogenesis induced by stress involves triacylglycerol synthesis that depends on group VIA phospholipase A2

This work investigates the metabolic origin of triacylglycerol (TAG) formed during lipid droplet (LD) biogenesis induced by stress. Cytotoxic inhibitors of fatty acid synthase induced TAG synthesis and LD biogenesis in CHO-K1 cells, in the absence of external sources of fatty acids. TAG synthesis was required for LD biogenesis and was sensitive to inhibition and down-regulation of the expression of group VIA phospholipase A(2) (iPLA(2)-VIA). Induction of stress with acidic pH, C(2)-ceramide, tunicamycin, or deprivation of glucose also stimulated TAG synthesis and LD formation in a manner dependent on iPLA(2)-VIA. Overexpression of the enzyme enhanced TAG synthesis from endogenous fatty acids and LD occurrence. During stress, LD biogenesis but not TAG synthesis required phosphorylation and activation of group IVA PLA(2) (cPLA(2)alpha). The results demonstrate that iPLA(2)-VIA provides fatty acids for TAG synthesis while cPLA(2)alpha allows LD biogenesis. LD biogenesis during stress may be a survival strategy, recycling structural phospholipids into energy-generating substrates.

Decreased phospholipase A2 activity in cerebrospinal fluid of patients with dementia

Phospholipase A2 (PLA2) is involved in important aspects of dementia, for example neurotransmission and memory processing, membrane function, choline availability, and antioxidative defense. Reduced PLA2-activity has been reported so far in blood samples and postmortem neuronal tissue in Alzheimer disease. For the first time, we studied PLA2 in cerebrospinal fluid (CSF) in Alzheimer disease (AD), vascular (VD), and mixed Alzheimer/vascular dementia (MD). Intracellular PLA2 was assessed in CSF of 16 AD, 12 VD, 15 MD patients, and 19 healthy control subjects. A fluorometric assay was applied using the PLA2-specific substrate NBDC6-HPC. Significantly reduced PLA2 activity was not only found in AD, but also in VD and MD. This finding was independent of demographic co-variates and medication. PLA2 results in CSF corroborate previous findings of impaired PLA2 function in Alzheimer's disease and extend these to patients with VD. They are likely to reflect an involvement of PLA2 impairment in a variety of pathomechanisms crucial in different dementia subtypes, in which disruption of cholinergic neurotransmission and disturbance of intact membrane function appear to be the key mechanisms.

The increase of cell-membranous phosphatidylcholines containing polyunsaturated fatty acid residues induces phosphorylation of p53 through activation of ATR

The G1 phase of the cell cycle is marked by the rapid turnover of phospholipids. This turnover is regulated by CTP:phosphocholine-cytidylyltransferase (CCT) and group VIA Ca(2+)-independent-phospholipase A(2) (iPLA(2)). We previously reported that inhibition of iPLA(2) arrests cells in G1 phase of the cell cycle by activating the p53-p21 checkpoint. Here we further characterize the mechanism of p53 activation. We show that specific inhibition of iPLA(2) induces a time dependent phosphorylation of Ser15 in p53 in the absence of DNA damage. This phosphorylation requires the kinase ataxia-telangiectasia and Rad-3-related (ATR) but not the ataxia-telangiectasia-mutated (ATM) kinase. Moreover, we identify in cell membranes a significant increase of phosphatidylcholines (PCs) containing chains of polyunsaturated fatty acids and a decrease of PCs containing saturated fatty acids in response to inhibition of iPLA(2). The time course of phosphorylation of Ser15 in p53 correlates with increasing levels of PCs containing polyunsaturated fatty acids. We further demonstrate that the PCs with linoleic acid in their sn-2 position (18:2n6) induce phosphorylation of Ser15 in p53 in an ATR-dependent manner. Our findings establish that cells can regulate the levels of polyunsaturated fatty acids in phospholipids through iPLA(2)-mediated deacylation of PCs. Disruption of this regulation increases the proportions of PCs containing polyunsaturated fatty acids and activates the ATR-p53 signalling pathway.

Role of calcium-independent phospholipase A2 in complement-mediated glomerular epithelial cell injury

In experimental membranous nephropathy, complement C5b-9-induced glomerular epithelial cell (GEC) injury leads to morphological changes in GEC and proteinuria, in association with phospholipase A2 (PLA2) activation. The present study addresses the role of calcium-independent PLA2 (iPLA2) in GEC injury. iPLA2β short and iPLA2γ were expressed in cultured rat GEC and normal rat glomeruli. To determine whether iPLA2 is involved in complement-mediated arachidonic acid (AA) release, GEC were stably transfected with iPLA2γ or iPLA2β cDNAs (GEC-iPLA2γ; GEC-iPLA2β). Compared with control cells (GEC-Neo), GEC-iPLA2γ and GEC-iPLA2β demonstrated greater expression of iPLA2 proteins and activities. Complement-mediated release of [3H]AA was augmented significantly in GEC-iPLA2γ compared with GEC-Neo, and the augmented [3H]AA release was inhibited by the iPLA2-directed inhibitor bromoenol lactone (BEL). For comparison, overexpression of iPLA2γ also amplified [3H]AA release after incubation of GEC with H2O2, or chemical anoxia followed by reexposure to glucose (in vitro ischemia-reperfusion injury). In parallel with release of [3H]AA, complement-mediated production of prostaglandin E2 was amplified in GEC-iPLA2γ. Complement-mediated cytotoxicity was attenuated significantly in GEC-iPLA2γ compared with GEC-Neo, and the cytoprotective effect of iPLA2γ was reversed by BEL, and in part by indomethacin. Overexpression of iPLA2β did not amplify complement-dependent [3H]AA release, but nonetheless attenuated complement-mediated cytotoxicity. Thus iPLA2γ may be involved in complement-mediated release of AA. Expression of iPLA2γ or iPLA2β induces cytoprotection against complement-dependent GEC injury. Modulation of iPLA2 activity may prove to be a novel approach to reducing GEC injury.

Inhibition of calcium-independent phospholipase A2 suppresses proliferation and tumorigenicity of ovarian carcinoma cells

PLA2 (phospholipase A2) enzymes play critical roles in membrane phospholipid homoeostasis and in generation of lysophospholipid growth factors. In the present study, we show that the activity of the cytosolic iPLA2 (calcium-independent PLA2), but not that of the calcium-dependent cPLA2 (cytosolic PLA2), is required for growth-factor-independent, autonomous replication of ovarian carcinoma cells. Blocking iPLA2 activity with the pharmacological inhibitor BEL (bromoenol lactone) induces cell cycle arrest in S- and G2/M-phases independently of the status of the p53 tumour suppressor. Inhibition of iPLA2 activity also leads to modest increases in apoptosis of ovarian cancer cells. The S- and G2/M-phase accumulation is accompanied by increased levels of the cell cycle regulators cyclins B and E. Interestingly, the S-phase arrest is released by supplementing the growth factors LPA (lysophosphatidic acid) or EGF (epidermal growth factor). However, inhibition of iPLA2 activity with BEL remains effective in repressing growth-factor- or serum-stimulated proliferation of ovarian cancer cells through G2/M-phase arrest. Down-regulation of iPLA2b expression with lentivirus-mediated RNA interference inhibited cell proliferation in culture and tumorigenicity of ovarian cancer cell lines in nude mice. These results indicate an essential role for iPLA2 in cell cycle progression and tumorigenesis of ovarian carcinoma cells.

Attenuated free cholesterol loading-induced apoptosis but preserved phospholipid composition of peritoneal macrophages from mice that do not express group VIA phospholipase A2

Mouse macrophages undergo ER stress and apoptosis upon free cholesterol loading (FCL). We recently generated iPLA(2)beta-null mice, and here we demonstrate that iPLA(2)beta-null macrophages have reduced sensitivity to FCL-induced apoptosis, although they and wild-type (WT) cells exhibit similar increases in the transcriptional regulator CHOP. iPLA(2)beta-null macrophages are also less sensitive to apoptosis induced by the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin and the scavenger receptor A ligand fucoidan, and restoring iPLA(2)betaexpression with recombinant adenovirus increases apoptosis toward WT levels. WT and iPLA(2)beta-null macrophages incorporate [(3)H]arachidonic acid ([(3)H]AA]) into glycerophosphocholine lipids equally rapidly and exhibit identical zymosan-induced, cPLA(2)alpha-catalyzed [(3)H]AA release. In contrast, although WT macrophages exhibit robust [(3)H]AA release upon FCL, this is attenuated in iPLA(2)beta-null macrophages and increases toward WT levels upon restoring iPLA(2)beta expression. Recent reports indicate that iPLA(2)beta modulates mitochondrial cytochrome c release, and we find that thapsigargin and fucoidan induce mitochondrial phospholipid loss and cytochrome c release into WT macrophage cytosol and that these events are blunted in iPLA(2)beta-null cells. Immunoblotting studies indicate that iPLA(2)beta associates with mitochondria in macrophages subjected to ER stress. AA incorporation into glycerophosphocholine lipids is unimpaired in iPLA(2)beta-null macrophages upon electrospray ionization-tandem mass spectrometry analyses, and their complex lipid composition is similar to WT cells. These findings suggest that iPLA(2)beta participates in ER stress-induced macrophage apoptosis caused by FCL or thapsigargin but that deletion of iPLA(2)beta does not impair macrophage arachidonate incorporation or phospholipid composition.

The immunosuppressant drug FTY720 inhibits cytosolic phospholipase A2 independently of sphingosine-1-phosphate receptors

FTY720 is a potent immunomodulator drug that inhibits the egress of lymphocytes from secondary lymphoid tissues and thymus. FTY720 is phosphorylated in vivo by sphingosine kinase 2 to FTY720-phosphate, which acts as a potent sphingosine-1-phosphate (S1P) receptor agonist. However, in contrast to S1P, FTY720 has no effect on mast-cell degranulation, yet significantly reduces antigen-induced secretion of PGD2 and cysteinyl-leukotriene. Unexpectedly, this effect of FTY720 was independent of its phosphorylation and S1P receptor functions. The rate-limiting step in the biosynthesis of all eicosanoids is the phospholipase A2 (PLA2)-mediated release of arachidonic acid from glycerol phospholipids. Although FTY720 also reduced arachidonic acid release in response to antigen, it had no effect on translocation of cPLA2 or ERK1/2 activation, suggesting that it does not interfere with FcepsilonRI-mediated events leading to cPLA2 activation. Remarkably, however, FTY720 drastically inhibited recombinant cPLA2alpha activity, whereas FTY720-phosphate, sphingosine, or S1P had no effect. This study has uncovered a unique action of FTY720 as an inhibitor of cPLA2alpha and hence on production of all eicosanoids. Our results have important implications for the potential therapeutic mechanism of action of FTY720 in eicosanoid-driven inflammatory disorders such as asthma and multiple sclerosis.

Insulin secretory responses and phospholipid composition of pancreatic islets from mice that do not express Group VIA phospholipase A2 and effects of metabolic stress on glucose homeostasis

Studies involving pharmacologic or molecular biologic manipulation of Group VIA phospholipase A(2) (iPLA(2)beta) activity in pancreatic islets and insulinoma cells suggest that iPLA(2)beta participates in insulin secretion. It has also been suggested that iPLA(2)beta is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels and arachidonate incorporation into phosphatidylcholine (PC). We have generated iPLA(2)beta-null mice by homologous recombination and have reported that they exhibit reduced male fertility and defective motility of spermatozoa. Here we report that pancreatic islets from iPLA(2)beta-null mice have impaired insulin secretory responses to D-glucose and forskolin. Electrospray ionization mass spectrometric analyses indicate that the abundance of arachidonate-containing PC species of islets, brain, and other tissues from iPLA(2)beta-null mice is virtually identical to that of wild-type mice, and no iPLA(2)beta mRNA was observed in any tissue from iPLA(2)beta-null mice at any age. Despite the insulin secretory abnormalities of isolated islets, fasting and fed blood glucose concentrations of iPLA(2)beta-null and wild-type mice are essentially identical under normal circumstances, but iPLA(2)beta-null mice develop more severe hyperglycemia than wild-type mice after administration of multiple low doses of the beta-cell toxin streptozotocin, suggesting an impaired islet secretory reserve. A high fat diet also induces more severe glucose intolerance in iPLA(2)beta-null mice than in wild-type mice, but PLA(2)beta-null mice have greater responsiveness to exogenous insulin than do wild-type mice fed a high fat diet. These and previous findings thus indicate that iPLA(2)beta-null mice exhibit phenotypic abnormalities in pancreatic islets in addition to testes and macrophages.

Disruption of G1-phase phospholipid turnover by inhibition of Ca2+-independent phospholipase A2 induces a p53-dependent cell-cycle arrest in G1 phase

The G1 phase of the cell cycle is characterized by a high rate of membrane phospholipid turnover. Cells regulate this turnover by coordinating the opposing actions of CTP:phosphocholine cytidylyltransferase and the group VI Ca2+-independent phospholipase A2 (iPLA2). However, little is known about how such turnover affects cell-cycle progression. Here, we show that G1-phase phospholipid turnover is essential for cell proliferation. Specific inhibition of iPLA2 arrested cells in the G1 phase of the cell cycle. This G1-phase arrest was associated with marked upregulation of the tumour suppressor p53 and the expression of cyclin-dependent kinase inhibitor p21cip1. Inactivation of iPLA2 failed to arrest p53-deficient HCT cells in the G1 phase and caused massive apoptosis of p21-deficient HCT cells, suggesting that this G1-phase arrest requires activation of p53 and expression of p21cip1. Furthermore, downregulation of p53 by siRNA in p21-deficient HCT cells reduced the cell death, indicating that inhibition of iPLA2 induced p53-dependent apoptosis in the absence of p21cip1. Thus, our study reveals hitherto unrecognized cooperation between p53 and iPLA2 to monitor membrane-phospholipid turnover in G1 phase. Disrupting the G1-phase phospholipid turnover by inhibition of iPLA2 activates the p53-p21cip1 checkpoint mechanism, thereby blocking the entry of G1-phase cells into S phase.

Effects of stable suppression of Group VIA phospholipase A2 expression on phospholipid content and composition, insulin secretion, and proliferation of INS-1 insulinoma cells

Studies involving pharmacologic inhibition or transient reduction of Group VIA phospholipase A2 (iPLA2beta) expression have suggested that it is a housekeeping enzyme that regulates cell 2-lysophosphatidylcholine (LPC) levels, rates of arachidonate incorporation into phospholipids, and degradation of excess phosphatidylcholine (PC). In insulin-secreting islet beta-cells and some other cells, in contrast, iPLA2beta signaling functions have been proposed. Using retroviral vectors, we prepared clonal INS-1 beta-cell lines in which iPLA2beta expression is stably suppressed by small interfering RNA. Two such iPLA2beta knockdown (iPLA2beta-KD) cell lines express less than 20% of the iPLA2beta of control INS-1 cell lines. The iPLA2beta-KD INS-1 cells exhibit impaired insulin secretory responses and reduced proliferation rates. Electrospray ionization mass spectrometric analyses of PC and LPC species that accumulate in INS-1 cells cultured with arachidonic acid suggest that 18:0/20:4-glycerophosphocholine (GPC) synthesis involves sn-2 remodeling to yield 16:0/20:4-GPC and then sn-1 remodeling via a 1-lyso/20:4-GPC intermediate. Electrospray ionization mass spectrometric analyses also indicate that the PC and LPC content and composition of iPLA2beta-KD and control INS-1 cells are nearly identical, as are the rates of arachidonate incorporation into PC and the composition and remodeling of other phospholipid classes. These findings indicate that iPLA2beta plays signaling or effector roles in beta-cell secretion and proliferation but that stable suppression of its expression does not affect beta-cell GPC lipid content or composition even under conditions in which LPC is being actively consumed by conversion to PC. This calls into question the generality of proposed housekeeping functions for iPLA2beta in PC homeostasis and remodeling.

Neuropathy target esterase and phospholipid deacylation

Certain organophosphates react with the active site serine residue of neuropathy target esterase (NTE) and cause axonal degeneration and paralysis. Cloning of NTE revealed the presence of homologues in eukaryotes from yeast to man and that the protein has both a catalytic and a regulatory domain. The latter contains sequences similar to the regulatory subunit of protein kinase A, suggesting that NTE may bind cyclic AMP. NTE is tethered via an amino-terminal transmembrane segment to the cytoplasmic face of the endoplasmic reticulum. Unlike wild-type yeast, mutants lacking NTE activity cannot deacylate CDP-choline pathway-synthesized phosphatidylcholine (PtdCho) to glycerophosphocholine (GroPCho) and fatty acids. In cultured mammalian cells, GroPCho levels rise and fall, respectively, in response to experimental over-expression, and inhibition, of NTE. A complex of PtdCho and Sec14p, a yeast phospholipid-binding protein, both inhibits the rate-limiting step in PtdCho synthesis and enhances deacylation of PtdCho by NTE. While yeast can maintain PtdCho homeostasis in the absence of NTE, certain post-mitotic metazoan cells may not be able to, and some NTE-null animals have deleterious phenotypes. NTE is not required for cell division in the early mammalian embryo or in larval and pupal forms of Drosophila, but is essential for placenta formation and survival of neurons in the adult. In vertebrates, the relative importance of NTE and calcium-independent phospholipase A2 for homeostatic PtdCho deacylation in particular cell types, possible interactions of NTE with Sec14p homologues and cyclic AMP, and whether deranged phospholipid metabolism underlies organophosphate-induced neuropathy are areas which require further investigation.

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.

The expression and function of a group VIA calcium-independent phospholipase A2 (iPLA2beta) in beta-cells

Many cells express a Group VIA phospholipase A2, designated iPLA2beta, that does not require calcium for activation, is stimulated by ATP, and is sensitive to inhibition by a bromoenol lactone suicide substrate (BEL). Studies in various cell systems have led to the suggestion that iPLA2beta has a role in phospholipid remodeling, signal transduction, cell proliferation, and apoptosis. We have found that pancreatic islets, beta-cells, and glucose-responsive insulinoma cells express an iPLA2beta that participates in glucose-stimulated insulin secretion but is not involved in membrane phospholipid remodeling. Additionally, recent studies reveal that iPLA2beta is involved in pathways that contribute to beta-cell proliferation and apoptosis, and that various phospholipid-derived mediators are involved in these processes. Detailed characterization of the enzyme suggests that the beta-cells express multiple isoforms of iPLA2beta, and we hypothesize that these participate in different cellular functions.

Cell Cycle Dependence of Group VIA Calcium-independent Phospholipase A2 Activity

Homeostasis of phosphatidylcholine (PC) is regulated by the opposing actions between CTP:phosphocholine cytidylyltransferase (CT) and the group VIA Ca(2+)-independent phospholipase A(2) (iPLA(2)). We investigated this process during the cell cycle. PC mass doubles during late G(1) and early S phase when its rate of catabolism is lowest. We show that iPLA(2) activity is cell cycle-dependent with peak activity during G(2)/M and late S phase. iPLA(2) activity declines during G(1) and is lowest at the G(1)/S transition and early S phase. The accumulation of PC correlates with decreased iPLA(2) activity, suggesting that regulation of this enzyme contributes to phospholipid accumulation. The levels of 80 kDa iPLA(2) protein do not change and thus cannot account for changes in enzyme activity. Reverse transcriptase and real-time PCR experiments show that splice variant iPLA(2) mRNAs are preferentially expressed during G(2)/M. Immunoblot analyses with an antibody directed against the N terminus of iPLA(2) revealed a approximately 50 kDa protein that is of appropriate size to be the truncated protein encoded by the ankyrin-iPLA(2)-1 splice variant mRNA. The levels of truncated iPLA(2) protein were high in cells in late G(1) and S phase cells that had low iPLA(2) activity and low in G(2)/M cells that had high iPLA(2) activity. The truncated protein co-immunoprecipitated with full-length iPLA(2), indicating a physical interaction between the two proteins. Together, these data suggest that truncated iPLA(2) proteins associate with active iPLA(2) and down-regulate its activity during G(1). This down-regulation may contribute to phospholipid accumulation during the cell cycle.

Contribution of lipid second messengers to the regulation of phosphatidylcholine synthesis during cell cycle re-entry

During entry into the cell cycle a phosphatidylcholine (PC) metabolic cycle is activated. We have examined the hypothesis that PC synthesis during the G(0) to G(1) transition is controlled by one or more lipid products of PC turnover acting directly on the rate-limiting enzyme in the synthesis pathway, CTP: phosphocholine cytidylyltransferase (CCT). The acceleration of PC synthesis was two- to threefold during the first hour after addition of serum to quiescent IIC9 fibroblasts. The rate increased to approximately 15-fold above the basal rate during the second hour. The production of arachidonic acid, diacylglycerol (DAG), and phosphatidic acid (PA) preceded the second, rapid phase of PC synthesis. However, an increase in the cellular content of these lipid mediators was detected only for DAG. CCT activation and translocation to membranes accompanied the second phase of the PC synthesis acceleration. Bromoenol lactone (BEL), an inhibitor of calcium-independent phospholipase A(2) and PA phosphatase, blocked production of fatty acids and DAG, inhibited both phases of the PC synthesis response to serum, and reduced CCT activity and membrane affinity. The effect of BEL on PC synthesis was partially reversed by in situ generation of DAG via exogenous PC-specific phospholipase C to generate approximately 2-fold elevation in PC-derived DAG. Exogenous arachidonic acid also partially reversed the inhibition by BEL, but only at a concentration that generated a supra-physiological cellular content of free fatty acid. 1-Butanol, which blocks PA production, had no effect on DAG generation, or on PC synthesis. We conclude that fatty acids and DAG could contribute to the initial slow phase of the PC synthesis response. DAG is the most likely lipid regulator of CCT activity and the rapid phase of PC synthesis. However, processes other than direct activation of CCT by lipid mediators likely contribute to the highly accelerated phase during entry into the cell cycle.

Regulation of group VIA phospholipase A2 expression by sterol availability

Several lines of evidence suggest that glycerophospholipid mass is maintained through the coordinate regulation of CTP:phosphocholine cytidylyltransferase-alpha (CTalpha) and the group VIA calcium-independent phospholipase A2 (iPLA2). CTalpha expression is modulated by sterol and this is mediated in part through sterol regulatory element binding proteins (SREBP). In this report, we investigate the possibility that iPLA2 expression is controlled in a similar manner. When Chinese hamster ovary (CHO) cells were cultured under sterol-depleted conditions, iPLA2 catalytic activity, mRNA, and protein were induced by between two- and threefold. These inductions were suppressed when the cells were supplemented with exogenous sterols. Luciferase reporter assays indicated that sterol depletion induced transcription of iPLA2, an analysis of the 5' flanking region suggested that the iPLA2 gene contained a putative sterol regulatory element (SRE), and electrophoretic mobility shift assay (EMSA) analysis indicated that this element can bind SREBP-2. Notably, a mutant CHO cell line (SRD4) that constitutively generates mature SREBP proteins exhibited increased iPLA2 activity and expression compared to wild-type cells. These data suggest that iPLA2 expression is regulated in a manner consistent with other important genes in sterol and glycerophospholipid metabolism. Such coordinate regulation may be essential for maintaining the lipid composition of cell membranes.

p44/42ERK1/2 MAPK and PLD activation by PGD2 preserves papillary phosphatidylcholine homeostasis

Previous works from our laboratory demonstrated that PGD(2) modulates phosphatidylcholine (PC) biosynthesis in renal papillary tissue. In the present work, we have evaluated the mechanism by which PGD(2) exerts this action. PGD(2) caused two stimulatory waves in PC synthesis which were reproduced by its full-agonist BW245C. At 1min stimulation, PGD(2) increased PC synthesis by 131%; this increase was blocked by neomycin and ethanol, cheleritrine and U0126, PLD, PKC, and MEK1/2 inhibitors, respectively. A second PC synthesis increase (100%) was observed after 15min, which was blocked by PLD inhibitors. PGD(2) also increased phospho-ERK1/2 MAPK in a biphasic-fashion, which was abolished by PLC and PKC inhibitors but not by ethanol, which overincreased phospho-ERK1/2, suggesting that PGD(2)-induced ERK1/2 activation requires previous PLC-PKC activation while PLD down-regulates it. Our results indicate that PGD(2) stimulatory effect involves both PLD and ERK1/2-MAPK activation, and both pathways operate independently of PC synthesis homeostasis.

Disruption of CCTbeta2 expression leads to gonadal dysfunction

There are two mammalian genes that encode isoforms of CTP:phosphocholine cytidylyltransferase (CCT), a key rate-controlling step in membrane phospholipid biogenesis. Quantitative determination of the CCT transcripts reveals that CCTalpha is ubiquitously expressed and is found at the highest levels in the testis and lung, with lower levels in the liver and ovary. CCTbeta2 is a very minor isoform in most tissues but is significantly expressed in the brain, lung, and gonads. CCTbeta3 is the third isoform recently discovered in mice and is expressed in the same tissues as CCTbeta2, with its highest level in testes. We investigated the role(s) of CCTbeta2 by generating knockout mice. The brains and lungs of mice lacking CCTbeta2 expression did not exhibit any overt defects. On the other hand, a large percentage of the CCTbeta2(-/-) females were sterile and their ovaries exhibited defective ovarian follicle development. The proportion of female CCTbeta2(-/-) mice with defective ovaries increased as the animals aged. The rare litters born from CCTbeta2(-/-) x CCTbeta2(-/0) matings had the normal number of pups. The abnormal ovarian histopathology was characterized by disorganization of the tissue in young adult mice and absence of follicles and ova in older mice, along with interstitial stromal cell hyperplasia which culminated in the emergence of tubulostromal ovarian tumors by 16 months of age. Grossly defective CCTbeta2(-/-) ovaries were associated with high follicle-stimulating (FSH) and luteinizing (LH) hormone levels. Male CCTbeta2(-/0) mice exhibited progressive multifocal testicular degeneration and reduced fertility but had normal FSH and LH levels. Thus, the most notable phenotype of CCTbeta2 knockout mice was gonad degeneration and reproductive deficiency. The results indicate that although CCTbeta2 is expressed at very low levels compared to the alpha-isoform, loss of CCTbeta2 expression causes a breakdown in the gonadal response to hormonal stimulation.

Islet complex lipids: involvement in the actions of group VIA calcium-independent phospholipase A(2) in beta-cells

The beta-isoform of group VIA calcium-independent phospholipase A(2) (iPLA(2)beta) does not require calcium for activation, is stimulated by ATP, and is sensitive to inhibition by a bromoenol lactone suicide substrate. Several potential functions have been proposed for iPLA(2)beta. Our studies indicate that iPLA(2)beta is expressed in beta-cells and participates in glucose-stimulated insulin secretion but is not involved in membrane phospholipid remodeling. If iPLA(2)beta plays a signaling role in glucose-stimulated insulin secretion, then conditions that impair iPLA(2)beta functions might contribute to the diminished capacity of beta-cells to secrete insulin in response to glucose, which is a prominent characteristic of type 2 diabetes. Our recent studies suggest that iPLA(2)beta might also participate in beta-cell proliferation and apoptosis and that various phospholipid-derived mediators are involved in these processes. Detailed characterization of the iPLA(2)beta protein level reveals that beta-cells express multiple isoforms of the enzyme, and our studies involve the hypothesis that different isoforms have different functions.

COX-2-mediated PGD2 synthesis regulates phosphatidylcholine biosynthesis in rat renal papillary tissue

Phosphatidylcholine (PC) is the major membrane phospholipid in mammalian cells. Previous works from our laboratory demonstrated a close metabolic relationship between the maintenance of PC biosynthesis and the prostaglandins endogenously synthesized by cyclooxygenase (COX) in rat renal papilla. In the present work, we studied the COX isoform involved in papillary PC biosynthesis regulation. The incorporation of [methyl-3H]choline and [32P]orthophosphate to PC was determined in the absence and presence of SC-560 and NS-398, COX-1 and COX-2 specific inhibitors. PC synthesis was highly sensitive to COX-2 inhibition, while COX-1 inhibition only reduced PC synthesis at high SC-560 concentration. The analysis of choline-containing metabolites showed that COX-2 inhibition affected the formation of CDP-choline intermediary. The evaluation of PC biosynthetic enzymes revealed that microsomal, as well as nuclear, CTP:phosphocholine cytidylyltransferase (CCT), and nuclear-CDP-choline:1,2-diacylglycerol cholinephosphotransferase (CTP) activities were affected by COX-2 inhibition. The addition of exogenous prostaglandin D(2) (PGD(2)) restored nuclear-CCT and -CPT activities but not microsomal CCT. Papillary synthesis of PGD(2) was only detected in nuclear fraction where it was blocked by COX-2 inhibitor NS-398, but not by COX-1 inhibitor. All together, the present results demonstrated that COX-2-mediated PGD(2) synthesis is a PC biosynthesis regulator in rat renal papilla. Considering the importance of the maintenance of PC biosynthesis for the preservation of cell membrane homeostasis to ensure cell viability, and the extensive use of COX-2 inhibitors in therapeutics, the present results could have great pharmacological implications, and can constitute a biochemical explanation for the nephrotoxic effect of non-steroidal anti-inflammatory drugs.

Cellular Function of Calcium-Independent Phospholipase A2

The catalytic activity of calcium-independent phospholipase A2 (iPLA2), which is classified as a group VI PLA2, is regulated by protein kinase C, calmodulin, and others such as reactive oxygen species. Numerous findings have shown that iPLA2 is involved in stimulus-induced arachidonic acid release and lysophospholipid generation, although the participation is dependent upon the cell type and stimulus. The catalytic action of iPLA2 is known to be responsible for phospholipid remodeling as a housekeeping function. However, it has been widely accepted that arachidonic acid and lysophospholipid generated by iPLA2 act as a signaling molecule in cellular functions. Those include eicosanoid production, glucose-induced insulin secretion, Fas-induced apoptosis, cellular proliferation, membrane traffic in fusion, contribution to myocardial ischemia, and others. In this review, the functional role of iPLA2 in cellular responses upon stimulation is the focus.