Implication of acyl chain of diacylglycerols in activation of different isoforms of protein kinase C

The role of adhesion molecules in osteocalcin-induced effects on glucose and lipid metabolism in adipocytes

Recent findings suggest that uncarboxylated osteocalcin (GluOC) promotes glucose and lipid metabolism via its putative receptor GPRC6A; however, its direct effect on adipocytes remains elusive. In this study, we elucidated the effects of GluOC on adipocytes, with an emphasis on the role of cell adhesion molecules. We determined that GluOC promoted the expression of adipocyte adhesion molecule (ACAM) and its transcription factor Krüppel-like factor 4 and enhanced the cortical actin filament assembly, which ameliorated lipid droplet hypertrophy. Additionally, GluOC upregulated the expression of integrin αVβ3 and activation of focal adhesion kinase (FAK) and prevented insulin receptor substrate 1 (IRS1) degradation by inhibiting the ubiquitin-proteasome system via the FAK-PLC-PKC axis, which activated IRS1-Akt-mediated glucose transporter 4 (GLUT4) transport. Furthermore, we showed that GluOC elevated the expression of the insulin-independent glucose transporters GLUT1 and GLUT8, which facilitated insulin stimulation-independent glucose transport. The GluOC-induced activation of integrin αVβ3 signaling promoted microtubule assembly, which improved glucose and lipid metabolism via its involvement in intracellular vesicular transport. GluOC treatment also suppressed collagen type 1 formation, which might prevent adipose tissue fibrosis in obese individuals. Overall, our results imply that GluOC promotes glucose and lipid metabolism via ACAM, integrin αVβ3, and GLUT1 and 8 expression, directly affecting adipocytes.

Insertion Depth Modulates Protein Kinase C-δ-C1b Domain Interactions with Membrane Cholesterol as Revealed by MD Simulations

Protein kinase C delta (PKC-δ) is an important signaling molecule in human cells that has both proapoptotic as well as antiapoptotic functions. These conflicting activities can be modulated by two classes of ligands, phorbol esters and bryostatins. Phorbol esters are known tumor promoters, while bryostatins have anti-cancer properties. This is despite both ligands binding to the C1b domain of PKC-δ (δC1b) with a similar affinity. The molecular mechanism behind this discrepancy in cellular effects remains unknown. Here, we have used molecular dynamics simulations to investigate the structure and intermolecular interactions of these ligands bound to δC1b with heterogeneous membranes. We observed clear interactions between the δC1b-phorbol complex and membrane cholesterol, primarily through the backbone amide of L250 and through the K256 side-chain amine. In contrast, the δC1b-bryostatin complex did not exhibit interactions with cholesterol. Topological maps of the membrane insertion depth of the δC1b-ligand complexes suggest that insertion depth can modulate δC1b interactions with cholesterol. The lack of cholesterol interactions suggests that bryostatin-bound δC1b may not readily translocate to cholesterol-rich domains within the plasma membrane, which could significantly alter the substrate specificity of PKC-δ compared to δC1b-phorbol complexes.

Parkin coordinates mitochondrial lipid remodeling to execute mitophagy

Autophagy has emerged as the prime machinery for implementing organelle quality control. In the context of mitophagy, the ubiquitin E3 ligase Parkin tags impaired mitochondria with ubiquitin to activate autophagic degradation. Although ubiquitination is essential for mitophagy, it is unclear how ubiquitinated mitochondria activate autophagosome assembly locally to ensure efficient destruction. Here, we report that Parkin activates lipid remodeling on mitochondria targeted for autophagic destruction. Mitochondrial Parkin induces the production of phosphatidic acid (PA) and its subsequent conversion to diacylglycerol (DAG) by recruiting phospholipase D2 and activating the PA phosphatase, Lipin-1. The production of DAG requires mitochondrial ubiquitination and ubiquitin-binding autophagy receptors, NDP52 and optineurin (OPTN). Autophagic receptors, via Golgi-derived vesicles, deliver an autophagic activator, EndoB1, to ubiquitinated mitochondria. Inhibition of Lipin-1, NDP52/OPTN, or EndoB1 results in a failure to produce mitochondrial DAG, autophagosomes, and mitochondrial clearance, while exogenous cell-permeable DAG can induce autophagosome production. Thus, mitochondrial DAG production acts downstream of Parkin to enable the local assembly of autophagosomes for the efficient disposal of ubiquitinated mitochondria.

Insights into the behavior of unsaturated diacylglycerols in mixed lipid bilayers in relation to protein kinase C activation-A molecular dynamics simulation study

The lipid second messenger diacylglycerol (DAG) is known for its involvement in many types of cellular signaling, especially as an endogenous agonist for protein kinase C (PKC). Evidence has emerged that the degree of saturation of the DAG molecules can affect PKC activation. DAG molecules with different acyl chain saturation have not only been observed to induce varying extents of PKC activation, but also to express selectivity towards different PKC isozymes. Both qualities are important for precise therapeutic activation of PKC; understanding DAG behavior at the molecular level in different environments has much potential in the development of drugs to target PKC. We used molecular dynamics simulations to study the behavior of two different unsaturated DAG species in lipid environments with varying degrees of unsaturation. We focus on phosphatidylethanolamine (PE) instead of phosphatidylcholine (PC) to more accurately model the relevant biomembranes. The effect of cholesterol (CHOL) on these systems was also explored. We found that both high level of unsaturation in the acyl chains of the DAG species and presence of CHOL in the surrounding membrane increase DAG molecule availability at the lipid-water interface. This can partially explain the previously observed differences in PKC activation strength and specificity, the complete mechanism is, however, likely to be more complex. Our simulations coupled with the current understanding of lipids highlight the need for more simulations of biologically accurate lipid environments in order to determine the correct correlations between molecular mechanisms and biological behavior when studying PKC activation.

Using comprehensive lipid profiling to study effects of PFHxS during different stages of early zebrafish development

PFHxS (Perfluorohexane sulfonic acid) is one of the short-chain perfluoroalkyl substances (PFASs) which are widely used in many industrial and consumer applications. However, limited information is available on the molecular mechanism of PFHxS toxicity (e.g. lipid metabolism). This study provides in-depth information on the lipid regulation of zebrafish embryos with and without PFHxS exposure. Lipid changes throughout zebrafish development (4 to 120 h post fertilization (hpf)) were closely associated with lipid species and lipid composition (fatty acyl chains). A comprehensive lipid analysis of four different PFHxS exposures (0, 0.3, 1, 3, and 10 μM) at different zebrafish developmental stages (24, 48, 72, and 120 hpf) was performed. Data on exposure concentration, lipids, and developmental stage showed that all PFHxS concentrations dysregulated the lipid metabolism and these were developmental-dependent. The pattern of significantly changed lipids revealed that PFHxS caused effects related to oxidative stress, inflammation, and impaired fatty acid β-oxidation. Oxidative stress and inflammation caused the remodeling of glycerophospholipid (phosphatidylcholine (PC) and phosphatidylethanolamine (PE)), with increased incorporation of omega-3 PUFA and a decreased incorporation of omega-6 PUFA.

Hepatic insulin sensitivity is improved in high-fat diet-fed Park2 knockout mice in association with increased hepatic AMPK activation and reduced steatosis

Park2 is an E3 ubiquitin ligase known for its role in mitochondrial quality control via the mitophagy pathway. Park2 KO mice are protected from diet‐induced obesity and hepatic insulin sensitivity is improved in high‐fat diet (HFD)‐fed Park2 KO mice even under body weight‐matched conditions. In order to better understand the cellular mechanism by which Park2 KO mice are protected from diet‐induced hepatic insulin resistance, we determined changes in multiple pathways commonly associated with the pathogenesis of insulin resistance, namely levels of bioactive lipid species, activation of the endoplasmic reticulum (ER) stress response and changes in cytokine levels and signaling. We report for the first time that whole‐body insulin sensitivity is unchanged in regular chow (RC)‐fed Park2 KO mice, and that liver diacylglycerol levels are reduced and very‐long‐chain ceramides are increased in Park2 KO mice fed HFD for 1 week. Hepatic transcriptional markers of the ER stress response were reduced and plasma tumor necrosis factor‐α (TNFα), interleukin‐6 and −10 (IL6, IL10) were significantly increased in HFD‐fed Park2 KO mice; however, there were no detectable differences in hepatic inflammatory signaling pathways between groups. Interestingly, hepatic adenylate charge was reduced in HFD‐fed Park2 KO liver and was associated increased activation of AMPK. These data suggest that negative energy balance that contributed to protection from obesity during chronic HFD manifested at the level of the hepatocyte during short‐term HFD feeding and contributed to the improved hepatic insulin sensitivity.

Defining lipid mediators of insulin resistance: controversies and challenges

Essential elements of all cells - lipids - play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects on insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

Insulin Resistance and Vulnerability to Cardiac Ischemia

Hepatic and myocardial ectopic lipid deposition has been associated with insulin resistance (IR) and cardiovascular risk. Lipid overload promotes increased hepatic oxidative capacity, oxidative stress, and impaired mitochondrial efficiency, driving the progression of nonalcoholic fatty liver disease (NAFLD). We hypothesized that higher lipid availability promotes ischemia-induced cardiac dysfunction and decreases myocardial mitochondrial efficiency. Mice with adipose tissue-specific overexpression of sterol element-binding protein 1c as model of lipid overload with combined NAFLD-IR and controls underwent reperfused acute myocardial infarcts (AMIs). Whereas indexes of left ventricle (LV) contraction were similar in both groups at baseline, NAFLD-IR showed severe myocardial dysfunction post-AMI, with prominent LV reshaping and increased end-diastolic and end-systolic volumes. Hearts of NAFLD-IR displayed hypertrophy, steatosis, and IR due to 18:1/18:1-diacylglycerol-mediated protein kinase Cε (PKCε) activation. Myocardial fatty acid-linked respiration and oxidative stress were increased, whereas mitochondrial efficiency was decreased. In humans, decreased myocardial mitochondrial efficiency of ventricle biopsies related to IR and troponin levels, a marker of impaired myocardial integrity. Taken together, increased lipid availability and IR favor susceptibility to ischemia-induced cardiac dysfunction. The diacylglycerol-PKCε pathway and reduced mitochondrial efficiency both caused by myocardial lipotoxicity may contribute to the impaired LV compensation of the noninfarcted region of the myocardium.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Lipid Cell Biology: A Focus on Lipids in Cell Division

Cells depend on hugely diverse lipidomes for many functions. The actions and structural integrity of the plasma membrane and most organelles also critically depend on membranes and their lipid components. Despite the biological importance of lipids, our understanding of lipid engagement, especially the roles of lipid hydrophobic alkyl side chains, in key cellular processes is still developing. Emerging research has begun to dissect the importance of lipids in intricate events such as cell division. This review discusses how these structurally diverse biomolecules are spatially and temporally regulated during cell division, with a focus on cytokinesis. We analyze how lipids facilitate changes in cellular morphology during division and how they participate in key signaling events. We identify which cytokinesis proteins are associated with membranes, suggesting lipid interactions. More broadly, we highlight key unaddressed questions in lipid cell biology and techniques, including mass spectrometry, advanced imaging, and chemical biology, which will help us gain insights into the functional roles of lipids.

The impact of obesity in the cardiac lipidome and its consequences in the cardiac damage observed in obese rats

AIMS

To explore the impact of obesity on the cardiac lipid profile in rats with diet-induced obesity, as well as to evaluate whether or not the specific changes in lipid species are associated with cardiac fibrosis.

METHODS

Male Wistar rats were fed either a high-fat diet (HFD, 35% fat) or standard diet (3.5% fat) for 6 weeks. Cardiac lipids were analyzed using by liquid chromatography-tandem mass spectrometry.

RESULTS

HFD rats showed cardiac fibrosis and enhanced levels of cardiac superoxide anion (O₂), HOMA index, adiposity, and plasma leptin, as well as a reduction in those of cardiac glucose transporter (GLUT 4), compared with control animals. Cardiac lipid profile analysis showed a significant increase in triglycerides, especially those enriched with palmitic, stearic, and arachidonic acid. An increase in levels of diacylglycerol (DAG) was also observed. No changes in cardiac levels of diacyl phosphatidylcholine, or even a reduction in total levels of diacyl phosphatidylethanolamine, diacyl phosphatidylinositol, and sphingomyelins (SM) was observed in HFD, as compared with control animals. After adjustment for other variables (oxidative stress, HOMA, cardiac hypertrophy), total levels of DAG were independent predictors of cardiac fibrosis while the levels of total SM were independent predictors of the cardiac levels of GLUT 4.

CONCLUSIONS

These data suggest that obesity has a significant impact on cardiac lipid composition, although it does not modulate the different species in a similar manner. Nonetheless, these changes are likely to participate in the cardiac damage in the context of obesity, since total DAG levels can facilitate the development of cardiac fibrosis, and SM levels predict GLUT4 levels.

Decreased 16:0/20:4-phosphatidylinositol level in the post-mortem prefrontal cortex of elderly patients with schizophrenia

The etiology of schizophrenia includes phospholipid abnormalities. Phospholipids are bioactive substances essential for brain function. To analyze differences in the quantity and types of phospholipids present in the brain tissue of patients with schizophrenia, we performed a global analysis of phospholipids in multiple brain samples using liquid chromatography electrospray ionization mass/mass spectrometry (LC-ESI/MS/MS) and imaging mass spectrometry (IMS). We found significantly decreased 16:0/20:4-phosphatidylinositol (PI) levels in the prefrontal cortex (PFC) in the brains from patients with schizophrenia in the LC-ESI/MS/MS, and that the 16:0/20:4-PI in grey matter was most prominently diminished according to the IMS experiments. Previous reports investigating PI pathology of schizophrenia did not identify differences in the sn-1 and sn-2 fatty acyl chains. This study is the first to clear the fatty acid composition of PI in brains from patients with schizophrenia. Alteration in the characteristic fatty acid composition of PI may also affect neuronal function, and could play a role in the etiology of schizophrenia. Although further studies are necessary to understand the role of reduced 16:0/20:4-PI levels within the prefrontal cortex in the etiology of schizophrenia, our results provide insight into the development of a novel therapy for the clinical treatment of schizophrenia.

Ginsenoside Rg5 Inhibits Succinate-Associated Lipolysis in Adipose Tissue and Prevents Muscle Insulin Resistance

Endoplasmic reticulum (ER) stress, inflammation, and lipolysis occur simultaneously in adipose dysfunction and contribute to insulin resistance. This study was designed to investigate whether ginsenoside Rg5 could ameliorate adipose dysfunction and prevent muscle insulin resistance. Short-term high-fat diet (HFD) feeding induced hypoxia with ER stress in adipose tissue, leading to succinate accumulation due to the reversal of succinate dehydrogenase (SDH) activity. Rg5 treatment reduced cellular energy charge, suppressed ER stress and then prevented succinate accumulation in adipose tissue. Succinate promoted IL-1β production through NLRP3 inflammasome activation and then increased cAMP accumulation by impairing PDE3B expression, leading to increased lipolysis. Ginsenoside Rg5 treatment suppressed NLRP3 inflammasome activation, preserved PDE3B expression and then reduced cAMP accumulation, contributing to inhibition of lipolysis. Adipose lipolysis increased FFAs trafficking from adipose tissue to muscle. Rg5 reduced diacylglycerol (DAG) and ceramides accumulation, inhibited protein kinase Cθ translocation, and prevented insulin resistance in muscle. In conclusion, succinate accumulation in hypoxic adipose tissue acts as a metabolic signaling to link ER stress, inflammation and cAMP/PKA activation, contributing to lipolysis and insulin resistance. These findings establish a previously unrecognized role of ginsenosides in the regulation of lipid and glucose homeostasis and suggest that adipose succinate-associated NLRP3 inflammasome activation might be targeted therapeutically to prevent lipolysis and insulin resistance.

Activation of conventional and novel protein kinase C isozymes by different diacylglycerol molecular species

A variety of diacylglycerol (DG) molecular species are produced in stimulated cells. Conventional (α, βII and γ) and novel (δ, ε, η and θ) protein kinase C (PKC) isoforms are known to be activated by DG. However, a comprehensive analysis has not been performed. In this study, we analyzed activation of the PKC isozymes in the presence of 2–2000 mmol% 16:0/16:0-, 16:0/18:1-, 18:1/18:1-, 18:0/20:4- or 18:0/22:6-DG species. PKCα activity was strongly increased by DG and exhibited less of a preference for 18:0/22:6-DG at 2 mmol%. PKCβII activity was moderately increased by DG and did not have significant preference for DG species. PKCγ activity was moderately increased by DG and exhibited a moderate preference for 18:0/22:6-DG at 2 mmol%. PKCδ activity was moderately increased by DG and exhibited a preference for 18:0/22:6-DG at 20 and 200 mmol%. PKCε activity moderately increased by DG and showed a moderate preference for 18:0/22:6-DG at 2000 mmol%. PKCη was not markedly activated by DG. PKCθ activity was the most strongly increased by DG and exhibited a preference for 18:0/22:6-DG at 2 and 20 mmol% DG. These results indicate that conventional and novel PKCs have different sensitivities and dependences on DG and a distinct preference for shorter and saturated fatty acid-containing and longer and polyunsaturated fatty acid-containing DG species, respectively. This differential regulation would be important for their physiological functions.

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We comprehensively analyzed activation of c/nPKC isozymes by different DG species.

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c/nPKCs have different sensitivities and dependences on DG.

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c/nPKCs have a distinct preference for different fatty acid-containing DG species.

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This differential regulation would be important for PKCs' physiological functions.

Features of the Phosphatidylinositol Cycle and its Role in Signal Transduction

The phosphatidylinositol cycle (PI-cycle) has a central role in cell signaling. It is the major pathway for the synthesis of phosphatidylinositol and its phosphorylated forms. In addition, some lipid intermediates of the PI-cycle, including diacylglycerol and phosphatidic acid, are also important lipid signaling agents. The PI-cycle has some features that are important for the understanding of its role in the cell. As a cycle, the intermediates will be regenerated. The PI-cycle requires a large amount of metabolic energy. There are different steps of the cycle that occur in two different membranes, the plasma membrane and the endoplasmic reticulum. In order to complete the PI-cycle lipid must be transferred between the two membranes. The role of the Nir proteins in the process has recently been elucidated. The lipid intermediates of the PI-cycle are normally highly enriched with 1-stearoyl-2-arachidonoyl molecular species in mammals. This enrichment will be retained as long as the intermediates are segregated from other lipids of the cell. However, there is a significant fraction (>15 %) of lipids in the PI-cycle of normal cells that have other acyl chains. Phosphatidylinositol largely devoid of arachidonoyl chains are found in cancer cells. Phosphatidylinositol species with less unsaturation will not be as readily converted to phosphatidylinositol-3,4,5-trisphosphate, the lipid required for the activation of Akt with resulting effects on cell proliferation. Thus, the cyclical nature of the PI-cycle, its dependence on acyl chain composition and its requirement for lipid transfer between two membranes, explain many of the biological properties of this cycle.

Role of T-Cell Polarization and Inflammation and Their Modulation by n-3 Fatty Acids in Gestational Diabetes and Macrosomia

Th (T helper) cells are differentiated into either Th1 or Th2 phenotype. It is generally considered that Th1 phenotype is proinflammatory, whereas Th2 phenotype exerts anti-inflammatory or protective effects. Gestational diabetes mellitus (GDM) has been associated with a decreased Th1 phenotype, whereas macrosomia is marked with high expression of Th1 cytokines. Besides, these two pathological situations are marked with high concentrations of inflammatory mediators like tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), known to play a pivotal role in insulin resistance. Dietary n-3 polyunsaturated fatty acids (n-3 PUFAs) may exert a beneficial effect by shifting Th1/Th2 balance to a Th2 phenotype and increasing insulin sensitivity. In this paper, we shed light on the role of T-cell malfunction that leads to an inflammatory and pathophysiological state, related to insulin resistance in GDM and macrosomia. We will also discuss the nutritional management of these pathologies by dietary n-3 polyunsaturated fatty acids (PUFAs).

Analysis of diacylglycerols by ultra performance liquid chromatography-quadrupole time-of-flight mass spectrometry: Double bond location and isomers separation

Diacylglycerols (DAGs) are important lipid intermediates and have been implicated in human diseases. Isomerism complicates their mass spectrometric analysis; in particular, it is difficult to identify fatty acid substituents and locate the double bond positions in unsaturated DAGs. We have developed an analytical strategy using ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF MS) in conjunction with dimethyl disulfide (DMDS) derivatization and collision cross-section (CCS) measurement to characterize DAGs in biological samples. The method employs non-aqueous reversed-phase chromatographic separation and profile collision energy (CE) mode for MS(E) and MS/MS analyses. Three types of fragment ions were produced simultaneously. Hydrocarbon ions (m/z 50-200) obtained at high CE helped to distinguish unsaturated and saturated DAGs rapidly. Neutral loss ions and acylium ions (m/z 300-400) produced at low CE were used to identify fatty acid substituents. Informative methyl thioalkane fragment ions were used to locate the double bonds of unsaturated DAGs. Mono-methylthio derivatives were formed mainly by the reaction of DAGs with DMDS, where methyl thiol underwent addition to the first double bond farthest from the ester terminus of unsaturated fatty acid chains. The addition of CCS values maximized the separation of isomeric DAG species and improved the confidence of DAG identification. Fourteen DAGs were identified in mouse myotube cells based on accurate masses, characteristic fragment ions, DMDS derivatization, and CCS values.

Lipid metabolism and signaling in cardiac lipotoxicity

The heart balances uptake, metabolism and oxidation of fatty acids (FAs) to maintain ATP production, membrane biosynthesis and lipid signaling. Under conditions where FA uptake outpaces FA oxidation and FA sequestration as triacylglycerols in lipid droplets, toxic FA metabolites such as ceramides, diacylglycerols, long-chain acyl-CoAs, and acylcarnitines can accumulate in cardiomyocytes and cause cardiomyopathy. Moreover, studies using mutant mice have shown that dysregulation of enzymes involved in triacylglycerol, phospholipid, and sphingolipid metabolism in the heart can lead to the excess deposition of toxic lipid species that adversely affect cardiomyocyte function. This review summarizes our current understanding of lipid uptake, metabolism and signaling pathways that have been implicated in the development of lipotoxic cardiomyopathy under conditions including obesity, diabetes, aging, and myocardial ischemia-reperfusion. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.

Diacylglycerol Kinase ζ (DGKζ) Is a Critical Regulator of Bone Homeostasis Via Modulation of c-Fos Levels in Osteoclasts

Increased diacylglycerol (DAG) levels are observed in numerous pathologies, including conditions associated with bone loss. However, the effects of DAG accumulation on the skeleton have never been directly examined. Because DAG is strictly controlled by tissue-specific diacylglycerol kinases (DGKs), we sought to examine the biological consequences of DAG accumulation on bone homeostasis by genetic deletion of DGKζ, a highly expressed DGK isoform in osteoclasts (OCs). Strikingly, DGKζ(-/-) mice are osteoporotic because of a marked increase in OC numbers. In vitro, DGKζ(-/-) bone marrow macrophages (BMMs) form more numerous, larger, and highly resorptive OCs. Surprisingly, although increased DAG levels do not alter receptor activator of NF-κB (RANK)/RANK ligand (RANKL) osteoclastogenic pathway, DGKζ deficiency increases responsiveness to the proliferative and pro-survival cytokine macrophage colony-stimulating factor (M-CSF). We find that M-CSF is responsible for increased DGKζ(-/-) OC differentiation by promoting higher expression of the transcription factor c-Fos, and c-Fos knockdown in DGKζ(-/-) cultures dose-dependently reduces OC differentiation. Using a c-Fos luciferase reporter assay lacking the TRE responsive element, we also demonstrate that M-CSF induces optimal c-Fos expression through DAG production. Finally, to demonstrate the importance of the M-CSF/DGKζ/DAG axis on regulation of c-Fos during osteoclastogenesis, we turned to PLCγ2(+/-) BMMs, which have reduced DAG levels and form fewer OCs because of impaired expression of the master regulator of osteoclastogenesis NFATc1 and c-Fos. Strikingly, genetic deletion of DGKζ in PLCγ2(+/-) mice rescues OC formation and normalizes c-Fos levels without altering NFATc1 expression. To our knowledge, this is the first report implicating M-CSF/DGKζ/DAG axis as a critical regulator of bone homeostasis via its actions on OC differentiation and c-Fos expression.

Lipid-mediated muscle insulin resistance: different fat, different pathways?

Increased dietary fat intake and lipolysis result in excessive lipid availability, which relates to impaired insulin sensitivity. Over the last years, several mechanisms possibly underlying lipid-mediated insulin resistance evolved. Lipid intermediates such as diacylglycerols (DAG) associate with changes in insulin sensitivity in many models. DAG activate novel protein kinase C (PKC) isoforms followed by inhibitory serine phosphorylation of insulin receptor substrate 1 (IRS1). Activation of Toll-like receptor 4 (TLR4) raises another lipid class, ceramides (CER), which induce pro-inflammatory pathways and lead to inhibition of Akt phosphorylation. Inhibition of glucosylceramide and ganglioside synthesis results in improved insulin sensitivity and increased activatory tyrosine phosphorylation of IRS1 in the muscle. Incomplete fat oxidation can increase acylcarnitines (ACC), which in turn stimulate pro-inflammatory pathways. This review analyzed the effects of lipid metabolites on insulin action in skeletal muscle of humans and rodents. Despite the evidence for the association of both DAG and CER with insulin resistance, its causal relevance may differ depending on the subcellular localization and the tested cohorts, e.g., athletes. Nevertheless, recent data indicate that individual lipid species and their degree of fatty acid saturation, particularly membrane and cytosolic C18:2 DAG, specifically activate PKCθ and induce both acute lipid-induced and chronic insulin resistance in humans.

Targeting skeletal muscle mitochondria to prevent type 2 diabetes in youth

The prevalence of type 2 diabetes (T2D) has increased dramatically over the past two decades, not only among adults but also among adolescents. T2D is a systemic disorder affecting every organ system and is especially damaging to the cardiovascular system, predisposing individuals to severe cardiac and vascular complications. The precise mechanisms that cause T2D are an area of active research. Most current theories suggest that the process begins with peripheral insulin resistance that precedes failure of the pancreatic β-cells to secrete sufficient insulin to maintain normoglycemia. A growing body of literature has highlighted multiple aspects of mitochondrial function, including oxidative phosphorylation, lipid homeostasis, and mitochondrial quality control in the regulation of peripheral insulin sensitivity. Whether the cellular mechanisms of insulin resistance in adults are comparable to that in adolescents remains unclear. This review will summarize both clinical and basic studies that shed light on how alterations in skeletal muscle mitochondrial function contribute to whole body insulin resistance and will discuss the evidence supporting high-intensity exercise training as a therapy to circumvent skeletal muscle mitochondrial dysfunction to restore insulin sensitivity in both adults and adolescents.

Phospholipase C and D regulation of Src, calcium release and membrane fusion during Xenopus laevis development

This review emphasizes how lipids regulate membrane fusion and the proteins involved in three developmental stages: oocyte maturation to the fertilizable egg, fertilization and during first cleavage. Decades of work show that phosphatidic acid (PA) releases intracellular calcium, and recent work shows that the lipid can activate Src tyrosine kinase or phospholipase C during Xenopus fertilization. Numerous reports are summarized to show three levels of increase in lipid second messengers inositol 1,4,5-trisphosphate and sn 1,2-diacylglycerol (DAG) during the three different developmental stages. In addition, possible roles for PA, ceramide, lysophosphatidylcholine, plasmalogens, phosphatidylinositol 4-phosphate, phosphatidylinositol 5-phosphate, phosphatidylinositol 4,5-bisphosphate, membrane microdomains (rafts) and phosphatidylinositol 3,4,5-trisphosphate in regulation of membrane fusion (acrosome reaction, sperm-egg fusion, cortical granule exocytosis), inositol 1,4,5-trisphosphate receptors, and calcium release are discussed. The role of six lipases involved in generating putative lipid second messengers during fertilization is also discussed: phospholipase D, autotaxin, lipin1, sphingomyelinase, phospholipase C, and phospholipase A2. More specifically, proteins involved in developmental events and their regulation through lipid binding to SH3, SH4, PH, PX, or C2 protein domains is emphasized. New models are presented for PA activation of Src (through SH3, SH4 and a unique domain), that this may be why the SH2 domain of PLCγ is not required for Xenopus fertilization, PA activation of phospholipase C, a role for PA during the calcium wave after fertilization, and that calcium/calmodulin may be responsible for the loss of Src from rafts after fertilization. Also discussed is that the large DAG increase during fertilization derives from phospholipase D production of PA and lipin dephosphorylation to DAG.

Dictyostelium uses ether-linked inositol phospholipids for intracellular signalling

Inositol phospholipids are critical regulators of membrane biology throughout eukaryotes. The general principle by which they perform these roles is conserved across species and involves binding of differentially phosphorylated inositol head groups to specific protein domains. This interaction serves to both recruit and regulate the activity of several different classes of protein which act on membrane surfaces. In mammalian cells, these phosphorylated inositol head groups are predominantly borne by a C38:4 diacylglycerol backbone. We show here that the inositol phospholipids of Dictyostelium are different, being highly enriched in an unusual C34:1e lipid backbone, 1-hexadecyl-2-(11Z-octadecenoyl)-sn-glycero-3-phospho-(1'-myo-inositol), in which the sn-1 position contains an ether-linked C16:0 chain; they are thus plasmanylinositols. These plasmanylinositols respond acutely to stimulation of cells with chemoattractants, and their levels are regulated by PIPKs, PI3Ks and PTEN. In mammals and now in Dictyostelium, the hydrocarbon chains of inositol phospholipids are a highly selected subset of those available to other phospholipids, suggesting that different molecular selectors are at play in these organisms but serve a common, evolutionarily conserved purpose.

Choline inadequacy impairs trophoblast function and vascularization in cultured human placental trophoblasts

Maternal choline intake during gestation may influence placental function and fetal health outcomes. Specifically, we previously showed that supplemental choline reduced placental and maternal circulating concentrations of the anti-angiogenic factor, fms-like tyrosine kinase-1 (sFLT1), in pregnant women as well as sFLT1 production in cultured human trophoblasts. The current study aimed to quantify the effect of choline on a wider array of biomarkers related to trophoblast function and to elucidate possible mechanisms. Immortalized HTR-8/SVneo trophoblasts were cultured in different choline concentrations (8, 13, and 28 µM [control]) for 96-h and markers of angiogenesis, inflammation, apoptosis, and blood vessel formation were examined. Choline insufficiency altered the angiogenic profile, impaired in vitro angiogenesis, increased inflammation, induced apoptosis, increased oxidative stress, and yielded greater levels of protein kinase C (PKC) isoforms δ and ϵ possibly through increases in the PKC activators 1-stearoyl-2-arachidonoyl-sn-glycerol and 1-stearoyl-2-docosahexaenoyl-sn-glycerol. Notably, the addition of a PKC inhibitor normalized angiogenesis and apoptosis, and partially rescued the aberrant gene expression profile. Together these results suggest that choline inadequacy may contribute to placental dysfunction and the development of disorders related to placental insufficiency by activating PKC.

Role of diacylglycerol activation of PKCθ in lipid-induced muscle insulin resistance in humans

Muscle insulin resistance is a key feature of obesity and type 2 diabetes and is strongly associated with increased intramyocellular lipid content and inflammation. However, the cellular and molecular mechanisms responsible for causing muscle insulin resistance in humans are still unclear. To address this question, we performed serial muscle biopsies in healthy, lean subjects before and during a lipid infusion to induce acute muscle insulin resistance and assessed lipid and inflammatory parameters that have been previously implicated in causing muscle insulin resistance. We found that acute induction of muscle insulin resistance was associated with a transient increase in total and cytosolic diacylglycerol (DAG) content that was temporally associated with protein kinase (PKC)θ activation, increased insulin receptor substrate (IRS)-1 serine 1101 phosphorylation, and inhibition of insulin-stimulated IRS-1 tyrosine phosphorylation and AKT2 phosphorylation. In contrast, there were no associations between insulin resistance and alterations in muscle ceramide, acylcarnitine content, or adipocytokines (interleukin-6, adiponectin, retinol-binding protein 4) or soluble intercellular adhesion molecule-1. Similar associations between muscle DAG content, PKCθ activation, and muscle insulin resistance were observed in healthy insulin-resistant obese subjects and obese type 2 diabetic subjects. Taken together, these data support a key role for DAG activation of PKCθ in the pathogenesis of lipid-induced muscle insulin resistance in obese and type 2 diabetic individuals.

Activation of Src and release of intracellular calcium by phosphatidic acid during Xenopus laevis fertilization

We report a new step in the fertilization in Xenopus laevis which has been found to involve activation of Src tyrosine kinase to stimulate phospholipase C-γ (PLC-γ) which increases inositol 1,4,5-trisphosphate (IP3) to release intracellular calcium ([Ca](i)). Molecular species analysis and mass measurements suggested that sperm activate phospholipase D (PLD) to elevate phosphatidic acid (PA). We now report that PA mass increased 2.7 fold by 1 min after insemination and inhibition of PA production by two methods inhibited activation of Src and PLCγ, increased [Ca](i) and other fertilization events. As compared to 14 other lipids, PA specifically bound Xenopus Src but not PLCγ. Addition of synthetic PA activated egg Src (an action requiring intact lipid rafts) and PLCγ as well as doubling the amount of PLCγ in rafts. In the absence of elevated [Ca](i), PA addition elevated IP3 mass to levels equivalent to that induced by sperm (but twice that achieved by calcium ionophore). Finally, PA induced [Ca](i) release that was blocked by an IP3 receptor inhibitor. As only PLD1b message was detected, and Western blotting did not detect PLD2, we suggest that sperm activate PLD1b to elevate PA which then binds to and activates Src leading to PLCγ stimulation, IP3 elevation and [Ca](i) release. Due to these and other studies, PA may also play a role in membrane fusion events such as sperm-egg fusion, cortical granule exocytosis, the elevation of phosphatidylinositol 4,5-bisphosphate and the large, late increase in sn 1,2-diacylglycerol in fertilization.

Fish oil selectively improves heart function in a mouse model of lipid-induced cardiomyopathy

Fish oil (FO) supplementation may improve cardiac function in some patients with heart failure, especially those with diabetes. To determine why this occurs, we studied the effects of FO in mice with heart failure either due to transgenic expression of the lipid uptake protein acyl CoA synthetase 1 (ACS1) or overexpression of the transcription factor peroxisomal proliferator-activated receptor (PPAR) γ via the cardiac-specific myosin heavy chain (MHC) promoter. ACS1 mice and control littermates were fed 3 diets containing low-dose or high-dose FO or nonpurified diet (NPD) for 6 weeks. MHC-PPARγ mice were fed low-dose FO or NPD. Compared with control mice fed with NPD, ACS1, and MHC-PPARγ, mice fed with NPD had reduced cardiac function and survival with cardiac fibrosis. In contrast, ACS1 mice fed with high-dose FO had better cardiac function, survival, and less myocardial fibrosis. FO increased eicosapentaenoic and docosahexaenoic acids and reduced saturated fatty acids in cardiac diacylglycerols. This was associated with reduced protein kinase C alpha and beta activation. In contrast, low-dose FO reduced MHC-PPARγ mice survival with no change in protein kinase C activation or cardiac function. Thus, dietary FO reverses fibrosis and improves cardiac function and survival of ACS1 mice but does not benefit all forms of lipid-mediated cardiomyopathy.

New insights into the regulation of neutrophil NADPH oxidase activity in the phagosome: a focus on the role of lipid and Ca(2+) signaling

SIGNIFICANCE

Reactive oxygen species, produced by the phagosomal NADPH oxidase of neutrophils, play a significant physiological role during normal defense. Their role is not only to kill invading pathogens, but also to act as modulators of global physiological functions of phagosomes. Given the importance of NADPH oxidase in the immune system, its activity has to be decisively controlled by distinctive mechanisms to ensure appropriate regulation at the phagosome.

RECENT ADVANCES

Here, we describe the signal transduction pathways that regulate phagosomal NADPH oxidase in neutrophils, with an emphasis on the role of lipid metabolism and intracellular Ca(2+) mobilization.

CRITICAL ISSUES

The potential involvement of Ca(2+)-binding S100A8 and S100A9 proteins, known to interact with the plasma membrane NADPH oxidase, is also considered.

FUTURE DIRECTIONS

Recent technical progress in advanced live imaging microscopy will permit to focus more accurately on phagosomal rather than plasma membrane NADPH oxidase regulation during neutrophil phagocytosis.

Distinguishing health benefits of eicosapentaenoic and docosahexaenoic acids

Long chain omega-3 polyunsaturated fatty acids (LC n-3 PUFAs) are recommended for management of patients with wide-ranging chronic diseases, including coronary heart disease, rheumatoid arthritis, dementia, and depression. Increased consumption of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) is recommended by many health authorities to prevent (up to 0.5 g/day) or treat chronic disease (1.0 g/day for coronary heart disease; 1.2–4 g/day for elevated triglyceride levels). Recommendations for dietary intake of LC n-3 PUFAs are often provided for α-linolenic acid, and for the combination of EPA and DHA. However, many studies have also reported differential effects of EPA, DHA and their metabolites in the clinic and at the laboratory bench. The aim of this article is to review studies that have identified divergent responses to EPA and DHA, and to explore reasons for these differences. In particular, we review potential contributing factors such as differential membrane incorporation, modulation of gene expression, activation of signaling pathways and metabolite formation. We suggest that there may be future opportunity to refine recommendations for intake of individual LC n-3 PUFAs.

Vitamin E isoforms directly bind PKCα and differentially regulate activation of PKCα

Vitamin E isoforms have opposing regulatory effects on leucocyte recruitment during inflammation. Furthermore, in vitro, vitamin E isoforms have opposing effects on leucocyte migration across endothelial cells by regulating VCAM (vascular cell-adhesion molecule)-1 activation of endothelial cell PKCα (protein kinase Cα). However, it is not known whether tocopherols directly regulate cofactor-dependent or oxidative activation of PKCα. We report in the present paper that cofactor-dependent activation of recombinant PKCα was increased by γ-tocopherol and was inhibited by α-tocopherol. Oxidative activation of PKCα was inhibited by α-tocopherol at a 10-fold lower concentration than γ-tocopherol. In binding studies, NBD (7-nitrobenz-2-oxa-1,3-diazole)-tagged α-tocopherol directly bound to full-length PKCα or the PKCα-C1a domain, but not PKCζ. NBD-tagged α-tocopherol binding to PKCα or the PKCα-C1a domain was blocked by diacylglycerol, α-tocopherol, γ-tocopherol and retinol, but not by cholesterol or PS (phosphatidylserine). Tocopherols enhanced PKCα-C2 domain binding to PS-containing lipid vesicles. In contrast, the PKCα-C2 domain did not bind to lipid vesicles containing tocopherol without PS. The PKCα-C1b domain did not bind to vesicles containing tocopherol and PS. In summary, α-tocopherol and γ-tocopherol bind the diacylglycerol-binding site on PKCα-C1a and can enhance PKCα-C2 binding to PS-containing vesicles. Thus the tocopherols can function as agonists or antagonists for differential regulation of PKCα.

Catalytic activity and acyl-chain selectivity of diacylglycerol kinase ɛ are modulated by residues in and near the lipoxygenase-like motif

Diacylglycerol kinase (DGK) ɛ plays an important role in the resynthesis of phosphatidylinositol by mediating the phosphorylation of diacylglycerol to phosphatidic acid. DGKɛ is unique among mammalian DGK isoforms in that it is the only one that shows acyl-chain selectivity, preferring diacylglycerols with an sn-2 arachidonoyl group. The region responsible for this arachidonoyl specificity is the lipoxygenase (LOX)-like motif found in the accessory domain, adjacent to DGKɛ's catalytic site. Many mutations within the LOX-like motif result in a loss of enzyme activity. However, the few mutants that retain significant activity exhibit some decrease in selectivity for the arachidonoyl chain. In the present work, we have explored mutations in a region adjacent to the LOX-like motif, which is also contained within the same hydrophobic segment of the protein. This adjacent region also contains a cholesterol recognition/interaction amino acid consensus motif. Being outside of the LOX-like motif, this region likely has less direct contact with the substrate, and more activity is retained with mutations. This has allowed us to probe in more detail the relationship between this region of the protein and substrate specificity. We demonstrate that this cholesterol recognition/interaction amino acid consensus domain also plays a role in acyl-chain selectivity. Despite the high degree of conservation of the amino acid sequence in this region of the protein, certain mutations result in proteins with higher activity than the wild-type protein. These mutations also result in a selective gain of acyl-chain preferences for diacylglycerols with different acyl-chain profiles. In addition to the LOX-like motif, adjacent residues also contribute to selectivity for diacylglycerols with specific acyl-chain compositions, such as those found in the phosphatidylinositol cycle.

Analysis of cholesteryl esters and diacylglycerols using lithiated adducts and electrospray ionization-tandem mass spectrometry

Cholesteryl ester (CE) and diacylglycerol (DAG) molecular species are important lipid storage and signaling molecules. Mass spectrometric analyses of these lipids are complicated by the presence of isobaric molecular ions shared by these lipid classes and by relatively poor electrospray ionization, which is a consequence of an inherently weak dipole moment in these lipid classes. The current study demonstrates that lithiated adducts of CE and DAG molecular ions have enhanced ionization and lipid class-specific fragmentation in tandem mass spectrometry (MS/MS) scan modes, thereby allowing the implementation of strategies capable of lipid class-specific detection. Using neutral loss (NL) mode for the loss of cholestane from cholesterol esters (NL 368.5) and specific selected reaction monitoring for DAG molecular species, the response of specific molecular species to that of internal standards was determined. CE and DAG molecular species were quantified in human coronary artery endothelial cells (HCAECs) incubated with both palmitic acid and oleic acid. Furthermore, NL 368.5 spectra revealed the oxidation of the aliphatic fatty acid residues of CE molecular species. Taken together, these studies demonstrate a new analytical approach to assessing CE and DAG molecular species that exploits the utility of lithiated adducts in conjunction with MS/MS approaches.

Comparative lipidomics of azole sensitive and resistant clinical isolates of Candida albicans reveals unexpected diversity in molecular lipid imprints

Although transcriptome and proteome approaches have been applied to determine the regulatory circuitry behind multidrug resistance (MDR) in Candida, its lipidome remains poorly characterized. Lipids do acclimatize to the development of MDR in Candida, but exactly how the acclimatization is achieved is poorly understood. In the present study, we have used a high-throughput mass spectrometry-based shotgun approach and analyzed the lipidome of genetically matched clinical azole-sensitive (AS) and -resistant (AR) isolates of C. albicans. By comparing the lipid profiling of matched isolates, we have identified major classes of lipids and determined more than 200 individual molecular lipid species among these major classes. The lipidome analysis has been statistically validated by principal component analysis. Although each AR isolate was similar with regard to displaying a high MIC to drugs, they had a distinct lipid imprint. There were some significant commonalities in the lipid profiles of these pairs, including molecular lipid species ranging from monounsaturated to polyunsaturated fatty acid-containing phosphoglycerides. Consistent fluctuation in phosphatidyl serine, mannosylinositolphosphorylceramides, and sterol esters levels indicated their compensatory role in maintaining lipid homeostasis among most AR isolates. Notably, overexpression of either CaCdr1p or CaMdr1p efflux pump proteins led to a different lipidomic response among AR isolates. This study clearly establishes the versatility of lipid metabolism in handling azole stress among various matched AR isolates. This comprehensive lipidomic approach will serve as a resource for assessing strategies aimed at disrupting the functions of Candida lipids, particularly the functional interactions between lipids and MDR determinants.

TACE/ADAM-17 phosphorylation by PKC-epsilon mediates premalignant changes in tobacco smoke-exposed lung cells

BACKGROUND

Tobacco smoke predisposes humans and animals to develop lung tumors, but the molecular events responsible for this are poorly understood. We recently showed that signaling mechanisms triggered by smoke in lung cells could lead to the activation of a growth factor signaling pathway, thereby promoting hyperproliferation of lung epithelial cells. Hyperproliferation is considered a premalignant change in the lung, in that increased rates of DNA synthesis are associated with an increased number of DNA copying errors, events that are exacerbated in the presence of tobacco smoke carcinogens. Despite the existence of DNA repair mechanisms, a small percentage of these errors go unrepaired and can lead to tumorigenic mutations. The results of our previous study showed that an early event following smoke exposure was the generation of oxygen radicals through the activation of NADPH oxidase. Although it was clear that these radicals transduced signals through the epidermal growth factor receptor (EGFR), and that this was mediated by TACE-dependent cleavage of amphiregulin, it remained uncertain how oxygen radicals were able to activate TACE.

PRINCIPAL FINDINGS

In the present study, we demonstrate for the first time that phosphorylation of TACE at serine/threonine residues by tobacco smoke induces amphiregulin release and EGFR activation. TACE phosphorylation is triggered in smoke-exposed lung cells by the ROS-induced activation of PKC through the action of SRC kinase. Furthermore, we identified PKCε as the PKC isoform involved in smoke-induced TACE activation and hyperproliferation of lung cells.

CONCLUSIONS

Our data elucidate new signaling paradigms by which tobacco smoke promotes TACE activation and hyperproliferation of lung cells.

Regulatory activity of polyunsaturated fatty acids in T-cell signaling

n-3 Polyunsaturated fatty acids (PUFA) are considered to be authentic immunosuppressors and appear to exert beneficial effects with respect to certain immune-mediated diseases. In addition to promoting T-helper 1 (Th1) cell to T-helper 2 (Th2) cell effector T-cell differentiation, n-3 PUFA may also exert anti-inflammatory actions by inducing apoptosis in Th1 cells. With respect to mechanisms of action, effects range from the modulation of membrane receptors to gene transcription via perturbation of a number of second messenger cascades. In this review, the putative targets of anti-inflammatory n-3 PUFA, activated during early and late events of T-cell activation will be discussed. Studies have demonstrated that these fatty acids alter plasma membrane micro-organization (lipid rafts) at the immunological synapse, the site where T-cells and antigen-presenting cells (APC) form a physical contact for antigen initiated T-cell signaling. In addition, the production of diacylglycerol and the activation of different isoforms of protein kinase C (PKC), mitogen-activated protein kinase (MAPK), calcium signaling, and nuclear translocation/activation of transcriptional factors, can be modulated by n-3 PUFA. Advantages and limitations of diverse methodologies to study the membrane lipid raft hypothesis, as well as apparent contradictions regarding the effect of n-3 PUFA on lipid rafts will be critically presented.

Cardioprotection by omega-3 fatty acids: involvement of PKCs?

It has been known since the 1970s that an increased consumption of n-3 long chain polyunsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid has cardioprotective effects. Epidemiological studies have reported that this effect is due to the prevention of the arrhythmias responsible for sudden cardiac death. Mechanistically, different hypotheses have been put forward to give an explanation. Among them, there are a direct effect of the polyunsaturated fatty acids on ion channels and/or a modification of the regulation of ion channels by protein kinase C's.

Polyunsaturated fatty acids in the modulation of T-cell signalling

n-3 polyunsaturated fatty acids (PUFA) have been shown to modulate immune responses. These agents, being considered as adjuvant immunosuppressants, have been used in the treatment of various inflammatory and autoimmune diseases. However, the molecular mechanisms of action of n-3 PUFA-induced immunosuppressive effects are not well-understood. Since exogenous n-3 PUFA, under in vitro and in vivo conditions, are efficiently incorporated into T-cell plasma membranes, a number of recent studies have demonstrated that these agents may modulate T-cell signalling. In this review, the interactions of n-3 PUFA with the second messenger cascade initiated during early and late events of T-cell activation are discussed. We particularly focus on how these fatty acids can modulate the production of diacylglycerol and the activation of protein kinase C, mitogen activated protein kinase, calcium signalling and translocation of transcriptional factors, implicated in the regulation of gene transcription in T-cells.