Diacylglycerol and ceramide kinetics in primary cultures of activated T-lymphocytes

Mathematical modelling and analysis of cellular signalling in macrophages

Cell signalling pathways play a crucial role in proper cell development and behaviour, with implications to survival, chemotaxis, proliferation, and even programmed cell death known as apoptosis. In this article, we outline a mathematical model of the G-protein signalling pathway in a particular cell line of macrophages, focusing on activation of a particular G-protein-coupled receptor, P2Y(6). The model is based on the kinetics of P2Y(6) surface receptors, inositol trisphosphate, cytosolic calcium, and differential dynamics of multiple species of diacylglycerol. Insight into the dynamics of the system is given through recently available experimental results and incorporated into the model. Mathematical analysis of the model, including establishment of global existence, uniqueness, positivity, and boundedness of solutions, and global stability of a unique steady-state solution is discussed.

Modeling species-specific diacylglycerol dynamics in the RAW 264.7 macrophage

A mathematical model of the G protein signaling pathway in RAW 264.7 macrophages downstream of P2Y(6) receptors activated by the ubiquitous signaling nucleotide uridine 5'-diphosphate is developed. The model, which is based on time-course measurements of inositol trisphosphate, cytosolic calcium, and diacylglycerol, focuses particularly on differential dynamics of multiple chemical species of diacylglycerol. When using the canonical pathway representation, the model predicted that key interactions were missing from the current network structure. Indeed, the model suggested that accurate depiction of experimental observations required an additional branch to the signaling pathway. An intracellular pool of diacylglycerol is immediately phosphorylated upon stimulation of an extracellular receptor for uridine 5'-diphosphate and subsequently used to aid replenishment of phosphatidylinositol. As a result of sensitivity analysis of the model parameters, key predictions can be made regarding which of these parameters are the most sensitive to perturbations and are therefore most responsible for output uncertainty.

Dietary docosahexaenoic and eicosapentaenoic acid: emerging mediators of inflammation

The inflammatory response is designed to help fight and clear infection, remove harmful chemicals, and repair damaged tissue and organ systems. Although this process, in general, is protective, the failure to resolve the inflammation and return the target tissue to homeostasis can result in disease, including the promotion of cancer. A plethora of published literature supports the contention that dietary n-3 polyunsaturated fatty acids (PUFA), and eicosapentaenoic (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) in particular, are important modulators of a host's inflammatory/immune responses. The following review describes a mechanistic model that may explain, in part, the pleiotropic anti-inflammatory and immunosuppressive properties of EPA and DHA. In this review, we focus on salient studies that address three overarching mechanisms of n-3 PUFA action: (i) modulation of nuclear receptor activation, i.e., nuclear factor-kappaB (NF-kappaB) suppression; (ii) suppression of arachidonic acid-cyclooxygenase-derived eicosanoids, primarily prostaglandin E(2)-dependent signaling; and (iii) alteration of the plasma membrane micro-organization (lipid rafts), particularly as it relates to the function of Toll-like receptors (TLRs), and T-lymphocyte signaling molecule recruitment to the immunological synapse (IS). We propose that lipid rafts may be targets for the development of n-3 PUFA-containing dietary bioactive agents to down-modulate inflammatory and immune responses and for the treatment of autoimmune and chronic inflammatory diseases.

Immunomodulatory effects of (n-3) fatty acids: putative link to inflammation and colon cancer

Chronic inflammation and colorectal cancer are closely linked. Although the overall mechanisms of inflammation-associated gastrointestinal carcinogenesis are complex, it is clear that antiinflammatory therapy is efficacious against neoplastic progression and malignant conversion. From a dietary perspective, fish oil containing (n-3) polyunsaturated fatty acids (PUFAs) has antiinflammatory properties, but for years the mechanism has remained obscure. Of relevance to the immune system in the intestine, we showed that (n-3) PUFA feeding alters the balance between CD4+ T-helper (Th1 and Th2) subsets by directly suppressing Th1 cell development (i.e., clonal expansion). This is noteworthy because Th1 cells mediate inflammatory diseases and resistance to intracellular pathogens or allergic hypersensitivity, and Th2 cells mediate resistance to extracellular pathogens. Therefore, any changes induced by (n-3) PUFAs in T-cell subset balance and function are important because the outcome is expected to suppress the development of autoimmune diseases and possibly the occurrence of colon cancer. Precisely how the immunomodulatory effects of (n-3) PUFAs influence inflammation-associated colonic tumor development is the subject of an ongoing investigation.

Disruption of Lipid Rafts Stimulates Phospholipase D Activity in Human Lymphocytes: Implication in the Regulation of Immune Function

Recent evidence suggests that phospholipase D (PLD) can be regulated through its association/dissociation to lipid rafts. We show here that modifying lipid rafts either by cholesterol depletion using methyl-beta-cyclodextrin and filipin or by conversion of sphingomyelin to ceramide with exogenous bacterial sphingomyelinase (bSMase) markedly activated the PLD of human PBMC. bSMase was the most potent PLD activator, giving maximal 6- to 7-fold increase in PLD activity. Triton X-100-treated lysates prepared from control PBMC and from bSMase-treated cells were fractionated by centrifugation on sucrose density gradient. We observed that bSMase treatment of the cells induced a larger ceramide increase in raft than in nonraft membranes and displaced both the Src kinase Lck and PLD1 out of the raft fractions. In addition, the three raft-modifying agents markedly inhibited the lymphoproliferative response to mitogenic lectin. To examine further the potential role of PLD activation in the control of lymphocyte responses, we transiently overexpressed either of the PLD1 and PLD2 isoforms in Jurkat cells and analyzed the phorbol ester plus ionomycin-induced expression of IL-2 mRNA, which is one of the early responses of lymphocyte to activation. We observed a 43% decrease of IL-2 mRNA level in Jurkat cells overexpressing PLD1 as compared with mock- or PLD2-transfected cells, which indicates that elevated PLD1, but not PLD2, activity impairs lymphocyte activation. Altogether, the present results support the hypothesis that PLD1 is activated by exclusion from lipid rafts and that this activation conveys antiproliferative signals in lymphoid cells.

Dietary Docosahexaenoic Acid Suppresses T Cell Protein Kinase Cθ Lipid Raft Recruitment and IL-2 Production

To date, the proximal molecular targets through which dietary n-3 polyunsaturated fatty acids (PUFA) suppress the inflammatory process have not been elucidated. Because cholesterol and sphingolipid-enriched rafts have been proposed as platforms for compartmentalizing dynamically regulated signaling assemblies at the plasma membrane, we determined the in vivo effects of fish oil and highly purified docosahexaenoic acid (DHA; 22:6n-3) on T cell microdomain lipid composition and the membrane subdomain distribution of signal-transducing molecules (protein kinase C (PKC)theta;, linker for activation of T cells, and Fas/CD95), before and after stimulation. Mice were fed diets containing 5 g/100 g corn oil (control), 4 g/100 g fish oil (contains a mixture of n-3 PUFA) plus 1 g/100 g corn oil, or 4 g/100 g corn oil plus 1 g/100 g DHA ethyl ester for 14 days. Dietary n-3 PUFA were incorporated into splenic T cell lipid raft and soluble membrane phospholipids, resulting in a 30% reduction in raft sphingomyelin content. In addition, polyclonal activation-induced colocalization of PKCtheta; with lipid rafts was reduced by n-3 PUFA feeding. With respect to PKCtheta; effector pathway signaling, both AP-1 and NF-kappaB activation, IL-2 secretion, and lymphoproliferation were inhibited by fish oil feeding. Similar results were obtained when purified DHA was fed. These data demonstrate for the first time that dietary DHA alters T cell membrane microdomain composition and suppresses the PKCtheta; signaling axis.

Dietary n-3 PUFA affect TcR-mediated activation of purified murine T cells and accessory cell function in co-cultures

Diets enriched in n-3 polyunsaturated fatty acids (PUFA) suppress several functions of murine splenic T cells by acting directly on the T cells and/or indirectly on accessory cells. In this study, the relative contribution of highly purified populations of the two cell types to the dietary suppression of T cell function was examined. Mice were fed diets containing different levels of n-3 PUFA; safflower oil (SAF; control containing no n-3 PUFA), fish oil (FO) at 2% and 4%, or 1% purified docosahexaenoic acid (DHA) for 2 weeks. Purified (>90%) T cells were obtained from the spleen, and accessory cells (>95% adherent, esterase-positive) were obtained by peritoneal lavage. Purified T cells or accessory cells from each diet group were co-cultured with the alternative cell type from every other diet group, yielding a total of 16 different co-culture combinations. The T cells were stimulated with either concanavalin A (ConA) or antibodies to the T cell receptor (TcR)/CD3 complex and the costimulatory molecule CD28 (alphaCD3/alphaCD28), and proliferation was measured after four days. Suppression of T cell proliferation in the co-cultures was dependent upon the dose of dietary n-3 PUFA fed to mice from which the T cells were derived, irrespective of the dietary treatment of accessory cell donors. The greatest dietary effect was seen in mice consuming the DHA diet (P = 0.034 in the anova; P=0.0053 in the Trend Test), and was observed with direct stimulation of the T cell receptor and CD28 costimulatory ligand, but not with ConA. A significant dietary effect was also contributed accessory cells (P = 0.033 in the Trend Test). We conclude that dietary n-3 PUFA affect TcR-mediated by T cell activation by both direct and indirect (accessory cell) mechanisms.

Dietary n-3 polyunsaturated fatty acids modulate purified murine T-cell subset activation

Studies in humans and murine disease models have clearly shown dietary fish oil to possess anti-inflammatory properties, apparently mediated by the n-3 polyunsaturated fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). To determine the mechanisms by which dietary EPA and DHA modulate mouse T-cell activation, female C57BL/6 mice were fed diets containing either 2% safflower oil (SAF), 2% fish oil (FO), or a 2% purified EPA/DHA ethyl ester mixture for 14 days. Splenic CD4 T cells ( approximately 90% purity) or CD8 T cells ( approximately 85% purity) were incubated with agonists which act at the plasma membrane receptor level [anti(alpha)-CD3/anti(alpha)-CD28], the intracellular level (PMA/Ionomycin), or at both the receptor and intracellular levels (alphaCD3/PMA). CD4 T cells stimulated with alphaCD3/alphaCD28 or PMA/Ionomycin proliferated and produced principally IL-2 (i.e. a Th1 phenotype), whereas the proliferation of CD4 T cells stimulated with alphaCD3/PMA was apparently driven principally by IL-4 (i.e. a Th2 phenotype). The IL-4 driven proliferation of putative Th2 CD4 cells was enhanced by dietary n-3 fatty acids (P = 0.02). Conversely, IL-2 production by alphaCD3/alpha CD28-stimulated CD4 T cells was reduced in FO-fed animals (P < 0.0001). The alphaCD3/alphaCD28-stimulated CD8 cells cultured from FO-fed animals exhibited a significant decrease (P < 0.05) in proliferation. There were no dietary effects seen in alphaCD3/PMA-stimulated CD8 cells, which produced both IL-2 and IL-4, or in PMA/Ionomycin-stimulated CD8 cells, which produced principally IL-2. These data suggest that dietary n-3 fatty acids down-regulated IL-2 driven CD4 and CD8 activation, while up-regulating the activation of the Th2 CD4 T-cell subset. Thus, the anti-inflammatory effects of n-3 fatty acids may result in both the direct suppression of IL-2-induced Th1 cell activation and the indirect suppression of Th1 cells by the enhanced cross-regulatory function of Th2 cells.

Docosahexaenoic acid suppresses function of the CD28 costimulatory membrane receptor in primary murine and Jurkat T cells

(n-3) polyunsaturated fatty acids (PUFA) have been widely documented to reduce inflammation in diseases such as rheumatoid arthritis. This study sought to elucidate the mechanism whereby (n-3) PUFA downregulate T-cell proliferation. We hypothesized that membrane incorporation of dietary PUFA would alter membrane structure and consequently membrane receptor function. Female C57BL/6 mice were fed for 14 d one of three diets containing arachidonic acid (AA), fish oil or docosahexaenoic acid (DHA) that varied in lipid composition only. Spleens were harvested and T cells ( approximately 90% purity) were activated with agonists that stimulated proliferation at the receptor level [anti-CD3 (alphaCD3)/anti-CD28 (alphaCD28)], intracellularly [phorbol-12-myristate-13-acetate (PMA)/ionomycin] or with a combined receptor/intracellular agonist (alphaCD3/PMA). Although there was no significant difference (P > 0.05) in proliferative response across dietary groups within each agonist set, interleukin (IL)-2 secretion was significantly reduced (P = 0.05) in cells from DHA-fed mice stimulated with alphaCD3/alphaCD28. In parallel in vitro experiments, Jurkat T cells were incubated with 50 micromol/L linoleic acid, AA, or DHA. Similar agonists sets were employed, and cells incubated with DHA and AA had a significantly reduced (P < 0.05) IL-2 secretion in three of the agonist sets. However, only when the CD28 receptor was stimulated was there a significant difference (P < 0.05) between DHA and AA. The results of this study suggest the involvement of the CD28 receptor in reducing IL-2 secretion in DHA-fed mice and DHA-incubated Jurkat cells and that purified T cells from DHA-fed mice require accessory cells to modulate proliferative suppression.

Effects of dietary n-3 fatty acids on T cell activation and T cell receptor-mediated signaling in a murine model

A short-term feeding paradigm in mice, with diets enriched with eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), was used to study the modulation of T cell activation via the T cell receptor (TcR) and the downstream pathways of intracellular signaling. Diets enriched in EPA and DHA suppressed antigen-specific delayed hypersensitivity reactions and mitogen-induced proliferation of T cells. Cocultures of accessory cells and T cells from mice given different diets revealed that purified fatty acid ethyl esters acted directly on the T cell, rather than through the accessory cell. The loss of proliferative capacity was accompanied by reductions in interleukin (IL)-2 secretion and IL-2 receptor alpha chain mRNA transcription, suggesting that dietary EPA and DHA act, in part, by interrupting the autocrine IL-2 activation pathway. Dietary EPA and DHA blunted the production of intracellular second messengers, including diacylglycerol and ceramide, following mitogen stimulation in vitro. Dietary effects appear to vary with the agonist employed (i.e., anti-CD3 [TcR], anti-CD28, exogenous IL-2, or phorbol myristate acetate and ionomycin).

The yins and yangs of ceramide

Since their discovery over 100 years ago, sphingolipids have caught the eyes and the imagination of scientists. Modern science has made many new insights on the cell biology and day-to-day functions of many integral sphingolipids, especially those of ceramide. Ceramide is recognized as a vital second messenger in the signal transduction process mediated by receptors of many cytokines and growth factors. A great part of our current understanding of ceramide has been achieved from apoptosis-related studies, however recent data in the fields of immunology, endocrinology and neurobiology, also suggest a fundamental involvement of ceramide in the onset of diseases. Therefore, understanding the biology of ceramide could be a key to unraveling many biological mechanisms and provide information for the treatment of some common diseases.

The effects of dietary lipids on gene expression and apoptosis

The beneficial effects of dietary FO with respect to autoimmune disease, CVD and some types of cancer are well established. Studies conducted over the last 10-15 years have established the potent effects of FO on gene expression in the previously mentioned diseases. The effects of dietary FO appear to be selective in nature, with the expression of individual genes simultaneously being increased, decreased or completely unaffected. In order to elucidate the molecular mechanism(s) involved, recent studies have focused on analysing the effects of the long-chain polyunsaturated n-3 fatty acids EPA and DHA which are highly enriched in FO and thought to be the primary mediators of its biological activity. Indeed, it has been found that EPA and DHA appear to both directly and indirectly modulate gene expression in vivo, depending on the gene examined. The direct effects of EPA and DHA are most probably mediated by their ability to bind to positive and/or negative regulatory transcription factors, while the indirect effects appear to be mediated through alterations in the generation of intracellular lipid second messengers (e.g. diacylglycerol and ceramide). Future studies need to be focused on further elucidation of the inter- and intracellular signalling events mediated by dietary n-3 fatty acids. Understanding the molecular mechanism(s) modified by dietary FO will ultimately lead to improved dietary strategies to aid in the prevention of autoimmune disease, CVD and/or certain types of cancer.

n-3 polyunsaturated fatty acids and immune function

n-3 PUFA have been shown to reduce the risk of cardiovascular and inflammatory diseases. However, they have also been shown to suppress T-cell-mediated immune function, an undesirable effect, especially in immuno-suppressed individuals. Studies have thus far suggested that this immuno-suppression may be in part attributable to increased lipid peroxidation and decreased antioxidant (especially vitamin E) levels, which can be prevented by appropriate vitamin E supplementation. Further well-designed human studies are needed to determine the appropriate levels of n-3 PUFA and vitamin E supplementation to optimize the beneficial anti-inflammatory effect of n-3 PUFA and minimize their suppressive effect on T-cell function.

Dietary (n-3) polyunsaturated fatty acids suppress murine lymphoproliferation, interleukin-2 secretion, and the formation of diacylglycerol and ceramide

Elucidation of the mechanism(s) by which dietary fish oil, enriched in eicosapentaenoic acid (EPA, 20:5(n-3)] and docosahexaenoic acid [DHA, 22:6(n-3)], suppresses the inflammatory process is essential in maximizing this potentially therapeutic effect. Murine T-lymphocyte function and signal transduction were examined in response to a low fat, short term diet enriched in highly purified EPA or DHA ethyl esters. For 10 d, mice were fed comparable diets containing either 3% safflower oil ethyl esters (SAF), 2% SAF + 1% arachidonic acid triglyceride (AA), 2% SAF + 1% EPA, or 2% SAF + 1% DHA. Concanavalin A-induced T-lymphocyte proliferation in splenocyte cultures was significantly suppressed by dietary EPA and DHA while AA had no effect relative to the SAF control. The suppressed proliferative response in EPA- and DHA-fed mice was preceded temporally by a significant reduction in IL-2 secretion. Kinetics of mitogen-induced diacyl-sn-glycerol (DAG) and ceramide production did not differ significantly between SAF and AA diet groups. In contrast, DAG production was significantly suppressed in EP- and DHA-fed mice relative to the SAF and AA groups. The reduced DAG mass was paralleled by reduced ceramide mass following EPA and DHA feeding compared to the SAF and AA groups. Thus, low dose, short term dietary exposure to highly purified EPA or DHA appears to suppress mitogen-induced T-lymphocyte proliferation by inhibiting IL-2 secretion, and these events are accompanied by reductions in the production of essential lipid second messengers, DAG and ceramide.