Phospholipase D modulation by ceramide in senescence

The Role of Neutral Sphingomyelinase-2 (NSM2) in the Control of Neutral Lipid Storage in T Cells

The accumulation of lipid droplets (LDs) and ceramides (Cer) is linked to non-alcoholic fatty liver disease (NAFLD), regularly co-existing with type 2 diabetes and decreased immune function. Chronic inflammation and increased disease severity in viral infections are the hallmarks of the obesity-related immunopathology. The upregulation of neutral sphingomyelinase-2 (NSM2) has shown to be associated with the pathology of obesity in tissues. Nevertheless, the role of sphingolipids and specifically of NSM2 in the regulation of immune cell response to a fatty acid (FA) rich environment is poorly studied. Here, we identified the presence of the LD marker protein perilipin 3 (PLIN3) in the intracellular nano-environment of NSM2 using the ascorbate peroxidase APEX2-catalyzed proximity-dependent biotin labeling method. In line with this, super-resolution structured illumination microscopy (SIM) shows NSM2 and PLIN3 co-localization in LD organelles in the presence of increased extracellular concentrations of oleic acid (OA). Furthermore, the association of enzymatically active NSM2 with isolated LDs correlates with increased Cer levels in these lipid storage organelles. NSM2 enzymatic activity is not required for NSM2 association with LDs, but negatively affects the LD numbers and cellular accumulation of long-chain unsaturated triacylglycerol (TAG) species. Concurrently, NSM2 expression promotes mitochondrial respiration and fatty acid oxidation (FAO) in response to increased OA levels, thereby shifting cells to a high energetic state. Importantly, endogenous NSM2 activity is crucial for primary human CD4+ T cell survival and proliferation in a FA rich environment. To conclude, our study shows a novel NSM2 intracellular localization to LDs and the role of enzymatically active NSM2 in metabolic response to enhanced FA concentrations in T cells.

The wide world of non-mammalian phospholipase D enzymes

Phospholipase D (PLD) hydrolyses phosphatidylcholine (PtdCho) to produce free choline and the critically important lipid signaling molecule phosphatidic acid (PtdOH). Since the initial discovery of PLD activities in plants and bacteria, PLDs have been identified in a diverse range of organisms spanning the taxa. While widespread interest in these proteins grew following the discovery of mammalian isoforms, research into the PLDs of non-mammalian organisms has revealed a fascinating array of functions ranging from roles in microbial pathogenesis, to the stress responses of plants and the developmental patterning of flies. Furthermore, studies in non-mammalian model systems have aided our understanding of the entire PLD superfamily, with translational relevance to human biology and health. Increasingly, the promise for utilization of non-mammalian PLDs in biotechnology is also being recognized, with widespread potential applications ranging from roles in lipid synthesis, to their exploitation for agricultural and pharmaceutical applications.

Ceramide levels in blood plasma correlate with major depressive disorder severity and its neutralization abrogates depressive behavior in mice

Major depressive disorder (MDD) is a severe disease of unknown pathogenesis that will affect ∼10% of people during their lifetime. Therapy for MDD requires prolonged treatment and often fails, predicating a need for novel treatment strategies. Here, we report increased ceramide levels in the blood plasma of MDD patients and in murine stress-induced models of MDD. These blood plasma ceramide levels correlated with the severity of MDD in human patients and were independent of age, sex, or body mass index. In addition, intravenous injection of anti-ceramide antibodies or neutral ceramidase rapidly abrogated stress-induced MDD, and intravenous injection of blood plasma from mice with MDD induced depression-like behavior in untreated mice, which was abrogated by ex vivo pre-incubation of the plasma with anti-ceramide antibodies or ceramidase. Mechanistically, we demonstrate that ceramide accumulated in endothelial cells of the hippocampus of stressed mice, evidenced by the quantitative measurement of ceramide in purified hippocampus endothelial cells. We found ceramide inhibited the activity of phospholipase D (PLD) in endothelial cells in vitro and in the hippocampus in vivo and thereby decreased phosphatidic acid in the hippocampus. Finally, we show intravenous injection of PLD or phosphatidic acid abrogated MDD, indicating the significance of this pathway in MDD pathogenesis. Our data indicate that ceramide controls PLD activity and phosphatidic acid formation in hippocampal endothelial cells and thereby mediates MDD. We propose that neutralization of plasma ceramide could represent a rapid-acting targeted treatment for MDD.

Role of phospholipase D in the lifespan of Caenorhabditis elegans

We have previously shown that phospholipase D (PLD) downregulation accelerates cellular senescence, which is widely believed to play an important role in aging, by stimulating reactive oxygen species (ROS) accumulation in human cells. In this study, we examined the role of PLD in aging using the nematode Caenorhabditis elegans. The mRNA level of pld-1 was found to be inversely correlated with aging. RNAi-mediated knockdown of pld-1 expression in nematodes enhanced ROS and lipofuscin accumulation and decreased lifespan, motility, and resistance to stress compared to that in nematodes treated with control RNAi. Pld-1 knockdown repressed the long lifespan of age-1 and akt-1 mutants but did not further reduce the short lifespan of daf-16 mutants, suggesting that PLD functions between AKT-1 and DAF-16. The ROS scavenger N-acetyl-L-cysteine, a PLD effector phosphatidic acid and a possible CK2 activator spermidine attenuated the lifespan shortening and age-related biomarkers triggered by pld-1 knockdown. Pld-1 RNAi downregulated the expression of DAF-16 target genes such as sod-3, dod-11, and mtl-1 in nematodes. In human cells, furthermore, PLD2 downregulation decreased the transcription of FoxO3a target genes (Cu/ZnSOD, MnSOD, catalase, thioredoxin-2, and peroxiredoxin-5), whereas ectopic PLD2 expression elevated the mRNA levels of these antioxidant genes. Taken together, these results indicated that PLD downregulation shortens longevity and induces age-related biomarkers through ROS accumulation by inhibiting the DAF-16/FoxO3a pathway in nematodes.

Regulation of lipids is central to replicative senescence

Cellular replicative senescence, a state of permanent cell-cycle arrest, has been linked to organismal aging, tissue repair and tumorigenesis. In this study, we comparatively investigated the global lipid profiles and mRNA content of proliferating and senescent-state BJ fibroblasts. We found that both expression levels of lipid-regulating genes and the abundance of specific lipid families, are actively regulated. We further found that 19 specific polyunsaturated triacylglycerol species constituted the most prominent changes in lipid composition during replicative senescence. Based on the transcriptome analysis, we propose that the activation of CD36-mediated fatty acid uptake and diversion to glycerolipid biosynthesis could be responsible for the accumulation of triacylglycerols during replicative senescence. This, in turn, could be a cellular mechanism to prevent lipotoxicity under increased oxidative stress conditions observed in this process. Our results indicate that regulation of specific lipid species has a central role during replicative senescence.

Sphingolipids and glycerophospholipids - The "ying and yang" of lipotoxicity in metabolic diseases

Sphingolipids in general and ceramides in particular, contribute to pathophysiological mechanisms by modifying signalling and metabolic pathways. Here, we present the available evidence for a bidirectional homeostatic crosstalk between sphingolipids and glycerophospholipids, whose dysregulation contributes to lipotoxicity induced metabolic stress. The initial evidence for this crosstalk originates from simulated models designed to investigate the biophysical properties of sphingolipids in plasma membrane representations. In this review, we reinterpret some of the original findings and conceptualise them as a sort of "ying/yang" interaction model of opposed/complementary forces, which is consistent with the current knowledge of lipid homeostasis and pathophysiology. We also propose that the dysregulation of the balance between sphingolipids and glycerophospholipids results in a lipotoxic insult relevant in the pathophysiology of common metabolic diseases, typically characterised by their increased ceramide/sphingosine pools.

Rapid evaluation of 25 key sphingolipids and phosphosphingolipids in human plasma by LC-MS/MS

We report on a new, sensitive, and fast LC-MS/MS method for the simultaneous determination of 25 key sphingolipid components in human plasma, including phosphorylated sphinganine and sphingosine, in a single 9-min run. This method enables an effective and high-throughput coverage of the metabolic changes involving the sphingolipidome during physiological or pathological states. The method is based on liquid–liquid extraction followed by reversed-phase LC-MS/MS. Exogenous odd-chain lipids are used as cost-effective but reliable internal standards. The method was fully validated in surrogate matrix and naive human plasma following FDA guidelines. Sample stability and dilution integrity were also tested and verified.

Sphingolipids and lifespan regulation

Diseases including cancer, type 2 diabetes, cardiovascular and immune dysfunction and neurodegeneration become more prevalent as we age, and combined with the increase in average human lifespan, place an ever increasing burden on the health care system. In this chapter we focus on finding ways of modulating sphingolipids to prevent the development of age-associated diseases or delay their onset, both of which could improve health in elderly, fragile people. Reducing the incidence of or delaying the onset of diseases of aging has blossomed in the past decade because of advances in understanding signal transduction pathways and cellular processes, especially in model organisms, that are largely conserved in most eukaryotes and that can be modulated to reduce signs of aging and increase health span. In model organisms such interventions must also increase lifespan to be considered significant, but this is not a requirement for use in humans. The most encouraging interventions in model organisms involve lowering the concentration of one or more sphingolipids so as to reduce the activity of key signaling pathways, one of the most promising being the Target of Rapamycin Complex 1 (TORC1) protein kinase pathway. Other potential ways in which modulating sphingolipids may contribute to improving the health profile of the elderly is by reducing oxidative stresses, inflammatory responses and growth factor signaling. Lastly, perhaps the most interesting way to modulate sphingolipids and promote longevity is by lowering the activity of serine palmitoyltransferase, the first enzyme in the de novo sphingolipid biosynthesis pathway. Available data in yeasts and rodents are encouraging and as we gain insights into molecular mechanisms the strategies for improving human health by modulating sphingolipids will become more apparent. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Correlation between composition of the outer layer and phase asymmetry for vesicles ruptured by phospholipase D

Spherical phospholipid bilayers, vesicles, were prepared by using the layer-by-layer double emulsion technique, which allows individual layers to be formed asymmetrically. Phases of the layers were adjusted by selecting the lipid tail group. The head group composition of the vesicle outer layer varied 0-100 % of phosphatidylcholine (PC) by 10 % under the condition that the diameter of the vesicle was kept constant. On the outer layer of the vesicle, the phospholipase D (PLD) reacted to convert PC to phosphatidic acid. The reaction induced a curvature change of the vesicles, which eventually led them to rupture. Response time from the PLD injection to the rupture was measured against the different compositions of the outer layer at each phase (solid and liquid) using the fluorescence intensity change of pH-sensitive dye encapsulated in the vesicles. From this measurement, the rupture caused by the PLD reaction was analyzed with respect to the phase asymmetry of the layers and the composition of the outer layer. These results were interpreted with the lipid density and stability of the layers. It was observed that the solid phase of the outer layer had a variance in response time according to the phase of the inner layer, whereas the liquid phase did not. Additionally, the response of the solid phase of the outer layer at the liquid phase of the inner layer was faster than at the solid phase of the inner layer as a result of its stability.

p53 and Ceramide as Collaborators in the Stress Response

The sphingolipid ceramide mediates various cellular processes in response to several extracellular stimuli. Some genotoxic stresses are able to induce p53-dependent ceramide accumulation leading to cell death. However, in other cases, in the absence of the tumor suppressor protein p53, apoptosis proceeds partly due to the activity of this "tumor suppressor lipid", ceramide. In the current review, we describe ceramide and its roles in signaling pathways such as cell cycle arrest, hypoxia, hyperoxia, cell death, and cancer. In a specific manner, we are elaborating on the role of ceramide in mitochondrial apoptotic cell death signaling. Furthermore, after highlighting the role and mechanism of action of p53 in apoptosis, we review the association of ceramide and p53 with respect to apoptosis. Strikingly, the hypothesis for a direct interaction between ceramide and p53 is less favored. Recent data suggest that ceramide can act either upstream or downstream of p53 protein through posttranscriptional regulation or through many potential mediators, respectively.

Effect of mixed-phospholipid layer on phospholipase D reaction-induced vesicle rupture

Spherical phospholipid bilayers, or vesicles, were prepared layer by layer using a double-emulsion technique, which allows the outer layer of the vesicles to be formed with two phospholipids that have different head groups: phosphatidylcholine (PC) and phosphatidylethanolamine. At the outer layer of the vesicles, the phospholipase D (PLD) catalyzed for the conversion of PC to phosphatidic acid. The reaction caused by PLD induced the curvature change of the vesicles, which eventually led to the rupture of the vesicles. Before the investigation, the ratio of dioleoylphosphatidylethanolamine to oleoylhydroxyphosphatidylethanolamine was found as a condition such that the vesicles made with the mixed lipids were as stable as those made with pure dioleoylphosphatidylcholine. Response time from the PLD injection to vesicle rupture was monitored by the composition of the outer layer by the fluorescence intensity change of pH-sensitive dye encapsulated in the vesicles. The response time began to be slowed at approximately 30 % PC. The response times for the compositions were associated with the surface density of PC at the outer layer. These results also seem to be determined by the size of PLD, specifically the PLD active site.

3,4-Disubstituted oxazolidin-2-ones as constrained ceramide analogs with anticancer activities

Heterocyclic analogs of ceramide as 3-alkanoyl or benzoyl-4-(1-hydroxy-2-enyl)-oxazolidin-2-ones were designed by binding of primary alcohol and amide in sphinogosine backbone as a carbamate. They were synthesized by addition of acyl halide to the common ring 4-(1-t-butyldimethylsilyloxyhexadec-2-enyl)-oxazolidin-2-one which was elaborated from chiral aziridine-2-carboxylate including stereoselective reduction and ring opening reactions as key steps. Other analogs with different carbon frame at C4 position which is corresponding to the sphingoid backbone were prepared from 3-cyclopentanecarbonyl-4-(1-t-butyldimethylsilyloxybut-2-enyl)-oxazolidin-2-one and straight and cyclic alkenes by cross metathesis. All compounds were tested as antileukemic drugs against human leukemia HL-60 cells. Many of them including propionyl, cyclopentanoyl and p-nitrobenzoyl-4-(1-hydroxyhexadec-2-enyl)-oxazolidin-2-ones showed better antileukemic activities than natural C2-ceramide with good correlation between cell death and DNA fragmentation. There is a drastic change of the activities by the carbon chain lengths at C4 position. Cytotoxicity was induced by caspase activation without significant accumulation of endogenous ceramide concentration or any perturbation of ceramide metabolism.