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

Dietary long-chain n-3 fatty acids modify blood and cardiac phospholipids and reduce protein kinase-C-delta and protein kinase-C-epsilon translocation

The effects of an n-3 PUFA-enriched diet on cardiac cell membrane phospholipid fraction compositions and associated protein kinase-C (PKC) translocation modification have never been studied in higher mammals. This is of importance since membrane fatty acid composition has been shown to influence PKC signalling pathways. In the present study, we have tested whether the incorporation of n-3 PUFA in cardiac membrane phospholipids correlated with changes in the fatty acid composition of diacylglycerols (DAG) and led to a differential translocation of PKC isoforms. Two groups of five dogs were fed the standard diet supplemented with palm oil or fish oil for 8 weeks. Dogs fed a fish oil-enriched diet showed a preferential incorporation of EPA and, to a lesser extent, of DHA, at the expense of arachidonic acid, in the circulating TAG, plasma phospholipids, erythrocyte phospholipids and cardiomyocyte phospholipid fractions. Analysis of 1,2-DAG fatty acid composition also indicated a preferential enrichment of EPA compared with DHA. Associated with these results, a reduction in the expression of PKC-delta and PKC-epsilon isoforms in the particulate fractions was observed whereas no effect was seen for PKC-alpha and PKC-zeta. We conclude that a fish oil-enriched diet induces a modification in fatty acid composition of cardiac membrane phospholipids, associated with a differential translocation of PKC isoforms. These results can be explained by the production of structurally different DAG that may participate in some of the protective effects of n-3 PUFA against various chronic diseases.

Mechanisms by which fatty acids regulate leucocyte function

Fatty acids (FAs) have been shown to alter leucocyte function and thus to modulate inflammatory and immune responses. In this review, the effects of FAs on several aspects of lymphocyte, neutrophil and macrophage function are discussed. The mechanisms by which FAs modulate the production of lipid mediators, activity of intracellular signalling pathways, activity of lipid-raft-associated proteins, binding to TLRs (Toll-like receptors), control of gene expression, activation of transcription factors, induction of cell death and production of reactive oxygen and nitrogen species are described in this review. The rationale for the use of specific FAs to treat patients with impaired immune function is explained. Substantial improvement in the therapeutic usage of FAs or FA derivatives may be possible based on an improvement in the understanding of the precise molecular mechanisms of action with respect to the different leucocyte types and outcome with respect to the inflammatory responses.

Long-term n-3 polyunsaturated fatty acids administration ameliorates arteriosclerosis by modulating T-cell activity in a rat model of small intestine transplantation

BACKGROUND

Fish oil, rich in n-3 polyunsaturated fatty acids (n-3 PUFAs), has been found to reduce graft rejection and increase allografts survival. But these studies mainly focused on acute rejection. We imitated long-term fish oil administration to investigate the effects of n-3 PUFAs on graft arteriosclerosis, and T cells in a rat model of small intestine transplantation.

METHODS

From 2 weeks pre-transplantation to the 60th day post-transplantation, the Lewis rats were supplemented by gavage with phosphate buffer saline, corn oil and fish oil respectively. Total small intestine was heterotopically transplanted from F344 to Lewis rat. Graft arteriosclerosis was assessed by histological grading of intimal thickening. The expression of CD25 and CD154, IL-2 level, and NF-kappaB activation in T cells were analyzed by western blotting, ELISA, and electrophoretic mobility shift assay respectively.

RESULTS

Compared with corn oil, graft arteriosclerosis was ameliorated by fish oil significantly. The expression of CD25 and CD154, IL-2 level, and NF-kappaB activation were markedly reduced by fish oil.

CONCLUSIONS

Long-term n-3 PUFAs administration pre- and post-transplantation could inhibit T-cell activity by reducing CD154 expression and NF-kappaB activation, which might contribute to amelioration of graft arteriosclerosis.

Enzymes and receptors of prostaglandin pathways with arachidonic acid-derived versus eicosapentaenoic acid-derived substrates and products

Dietary fish oil containing omega 3 highly unsaturated fatty acids has cardioprotective and anti-inflammatory effects. Prostaglandins (PGs) and thromboxanes are produced in vivo both from the omega 6 fatty acid arachidonic acid (AA) and the omega 3 fatty acid eicosapentaenoic acid (EPA). Certain beneficial effects of fish oil may result from altered PG metabolism resulting from increases in the EPA/AA ratios of precursor phospholipids. Here we report in vitro specificities of prostanoid enzymes and receptors toward EPA-derived, 3-series versus AA-derived, 2-series prostanoid substrates and products. The largest difference was seen with PG endoperoxide H synthase (PGHS)-1. Under optimal conditions purified PGHS-1 oxygenates EPA with only 10% of the efficiency of AA, and EPA significantly inhibits AA oxygenation by PGHS-1. Two- to 3-fold higher activities or potencies with 2-series versus 3-series compounds were observed with PGHS-2, PGD synthases, microsomal PGE synthase-1 and EP1, EP2, EP3, and FP receptors. Our most surprising observation was that AA oxygenation by PGHS-2 is only modestly inhibited by EPA (i.e. PGHS-2 exhibits a marked preference for AA when EPA and AA are tested together). Also unexpectedly, TxA(3) is about equipotent to TxA(2) at the TP alpha receptor. Our biochemical data predict that increasing phospholipid EPA/AA ratios in cells would dampen prostanoid signaling with the largest effects being on PGHS-1 pathways involving PGD, PGE, and PGF. Production of 2-series prostanoids from AA by PGHS-2 would be expected to decrease in proportion to the compensatory decrease in the AA content of phospholipids that would result from increased incorporation of omega 3 fatty acids such as EPA.

Identification of genomic classifiers that distinguish induction failure in T-lineage acute lymphoblastic leukemia: a report from the Children's Oncology Group

The clinical and cytogenetic features associated with T-cell acute lymphoblastic leukemia (T-ALL) are not predictive of early treatment failure. Based on the hypothesis that microarrays might identify patients who fail therapy, we used the Affymetrix U133 Plus 2.0 chip and prediction analysis of microarrays (PAM) to profile 50 newly diagnosed patients who were treated in the Children's Oncology Group (COG) T-ALL Study 9404. We identified a 116-member genomic classifier that could accurately distinguish all 6 induction failure (IF) cases from 44 patients who achieved remission; network analyses suggest a prominent role for genes mediating cellular quiescence. Seven genes were similarly upregulated in both the genomic classifier for IF patients and T-ALL cell lines having acquired resistance to neoplastic agents, identifying potential target genes for further study in drug resistance. We tested whether our classifier could predict IF within 42 patient samples obtained from COG 8704 and, using PAM to define a smaller classifier for the U133A chip, correctly identified the single IF case and patients with persistently circulating blasts. Genetic profiling may identify T-ALL patients who are likely to fail induction and for whom alternate treatment strategies might be beneficial.

Lipid rafts in health and disease

Lipid rafts are sphingolipid- and cholesterol-rich domains of the plasma membrane which contain a variety of signalling and transport proteins. Different subtypes of lipid rafts can be distinguished according to their protein and lipid composition. Caveolae are types of rafts that are rich in proteins of the caveolin family (caveolin-1, -2 and -3) which present a distinct signalling platform. The importance of lipid raft signalling in the pathogenesis of a variety of conditions, such as Alzheimer's, Parkinson's, cardiovascular and prion diseases, systemic lupus erythematosus and HIV, has been elucidated over recent years and makes these specific membrane domains an interesting target for pharmacological approaches in the cure and prevention of these diseases. This Review analyses the importance of lipid raft proteins and lipids in health and disease, with a focus on the current state of knowledge.

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.

Modulation of lipid rafts by Omega-3 fatty acids in inflammation and cancer: implications for use of lipids during nutrition support

Current understanding of biologic membrane structure and function is largely based on the concept of lipid rafts. Lipid rafts are composed primarily of tightly packed, liquid-ordered sphingolipids/cholesterol/saturated phospholipids that float in a sea of more unsaturated and loosely packed, liquid-disordered lipids. Lipid rafts have important clinical implications because many important membrane-signaling proteins are located within the raft regions of the membrane, and alterations in raft structure can alter activity of these signaling proteins. Because rafts are lipid-based, their composition, structure, and function are susceptible to manipulation by dietary components such as omega-3 polyunsaturated fatty acids and by cholesterol depletion. We review how alteration of raft lipids affects the raft/nonraft localization and hence the function of several proteins involved in cell signaling. We focus our discussion of raft-signaling proteins on inflammation and cancer.

Dietary fish oil increases the number of splenic macrophages secreting TNF-alpha and IL-10 but decreases the secretion of these cytokines by splenic T cells from mice

Dietary fish oil has immunomodulatory effects that are partly mediated by its effects on cytokine secretion. In this paper, we examine whether dietary fish oil has different effects on cytokine secretion by T cells and macrophages. Female BalbC mice were fed diets supplemented with 18% fish oil + 2% corn oil or 20% corn oil. Concanavalin A (ConA)- and LPS-induced TNF-alpha and IL-10 secretion by splenocytes was examined using ELISA. Dietary fish oil decreased ConA induced-, but increased LPS-induced, TNF-alpha and IL-10 secretion by total murine splenocytes. Dietary fish oil increased the number of splenocytes secreting TNF-alpha and IL-10, following stimulation with LPS, by 123 and 38%, respectively, but did not affect cytokine secretion by each cell, as determined using enzyme-linked immunospot. Spleens from mice fed the fish oil diet had over 2-fold higher proportion of macrophages with high expression of CD11b than spleens from mice fed the corn oil diet. In addition, fish oil increased the proportion of total and CD11b(+) splenocytes that expressed the LPS receptor complex molecules, CD14 and toll-like receptor (TLR)4/myeloid differentiation factor-2 (MD-2), by 85 and 28%, respectively. The increased proportion of macrophages expressing the LPS receptor complex molecules, CD14 and TLR4/MD-2, in spleens from mice fed the fish oil diet may explain the increased number of cells that secreted the cytokines after LPS stimulation.

Inhibition of cytokine signaling in human retinal endothelial cells through modification of caveolae/lipid rafts by docosahexaenoic acid

PURPOSE

Docosahexaenoic acid (DHA(22:6,n3)) is the principal n3 polyunsaturated fatty acid (PUFA) in the retina. The authors previously demonstrated that DHA(22:6,n3) inhibited cytokine-induced adhesion molecule expression in primary human retinal vascular endothelial (hRVE) cells, the target tissue affected by diabetic retinopathy. Despite the importance of vascular inflammation in diabetic retinopathy, the mechanisms underlying anti-inflammatory effects of DHA(22:6,n3) in vascular endothelial cells are not understood. In this study the authors address the hypothesis that DHA(22:6,n3) acts through modifying lipid composition of caveolae/lipid rafts, thereby changing the outcome of important signaling events in these specialized plasma membrane microdomains.

METHODS

hRVE cells were cultured in the presence or absence of DHA(22:6,n3). Isolated caveolae/lipid raft-enriched detergent-resistant membrane domains were prepared using sucrose gradient ultracentrifugation. Fatty acid composition and cholesterol content of caveolae/lipid rafts before and after treatment were measured by HPLC. The expression of Src family kinases was assayed by Western blotting and immunohistochemistry.

RESULTS

Disruption of the caveolae/lipid raft structure with a cholesterol-depleting agent, methyl-cyclodextrin (MCD), diminished cytokine-induced signaling in hRVE cells. Growth of hRVE cells in media enriched in DHA(22:6,n3) resulted in significant incorporation of DHA(22:6,n3) into the major phospholipids of caveolae/lipid rafts, causing an increase in the unsaturation index in the membrane microdomain. DHA(22:6,n3) enrichment in the caveolae/raft was accompanied by a 70% depletion of cholesterol from caveolae/lipid rafts and displacement of the SFK, Fyn, and c-Yes from caveolae/lipid rafts. Adding water-soluble cholesterol to DHA(22:6,n3)-treated cells replenished cholesterol in caveolae/lipid rafts and reversed the effect of DHA(22:6,n3) on cytokine-induced signaling.

CONCLUSIONS

Incorporation of DHA(22:6,n3) into fatty acyl chains of phospholipids in caveolae/lipid rafts, followed by cholesterol depletion and displacement of important signaling molecules, provides a potential mechanism for anti-inflammatory effect of DHA(22:6,n3) in hRVE cells.

Colon cancer, fatty acids and anti-inflammatory compounds

PURPOSE OF REVIEW

To outline recent findings on the efficacy of n-3 polyunsaturated fatty acids in the prevention/treatment of inflammatory bowel disease and colorectal cancer.

RECENT FINDINGS

Compelling data indicate a functional link between chronic inflammation and colon cancer. With respect to environmental risk factors, there is growing evidence that long-chain n-3 polyunsaturated fatty acids found in fish oil suppress inflammatory bowel diseases and colon cancer risk in humans. Unfortunately, the molecular basis of the effect of n-3 polyunsaturated fatty acids on inflammation/colitis-associated colon cancer risk is still largely obscure. In this review, we focus on recent studies which address three emerging mechanisms of n-3 polyunsaturated fatty acids action: (1) metabolic interconversion into bioactive eicosanoids, (2) modulation of nuclear receptor activation, and (3) alteration of membrane phospholipid composition and functionality of lipid microdomains.

SUMMARY

The consumption of dietary fish oil may prove to be an effective adjuvant therapy in colon cancer. Therefore, it is both appropriate and timely to determine precisely how n-3 polyunsaturated fatty acids modulate cell signaling networks, and reduce the risk of developing colon cancer and inflammatory disorders of the intestine.

Immunosuppressive effects of polyunsaturated fatty acids on antigen presentation by human leukocyte antigen class I molecules

Dietary supplementation with polyunsaturated fatty acids (PUFAs) has immunosuppressive effects; however, the molecular targets of PUFAs and their mode of action remain unclear. One possible target is antigen presentation to T cells through the human leukocyte antigen (HLA) class I pathway. Here we show that incorporation of PUFAs lowers target cell susceptibility to lysis by effector T cells. Treatment of B lymphoblast targets with the omega-6 PUFA arachidonic acid (AA) or omega-3 docosahexaenoic acid lowered their susceptibility to lysis by alloreactive CD8+ T cells by approximately 20-25%. HLA class I surface levels and their rate of endoplasmic reticulum (ER)-Golgi traffic were also reduced by PUFA treatment. Calibration experiments showed that the approximately 15% reduction in surface HLA I was not sufficient to completely account for the decreased lysis. However, PUFAs significantly lowered antigen-presenting cell-T cell conjugate formation, by approximately 30-40%. Taken together, our data show for the first time that an omega-6 and an omega-3 PUFA affect the HLA class I pathway of B lymphoblasts. Our findings suggest that elimination of self- and pathogen-derived peptides by effectors may be compromised by dietary PUFA supplementation. In addition, PUFA-mediated changes in ER-Golgi trafficking point to a new area of PUFA modulation of immune responses.

Polyunsaturated fatty acids, membrane organization, T cells, and antigen presentation

Dietary supplementation with polyunsaturated fatty acids (PUFAs), especially those of the n-3 class, has immunosuppressive effects on both innate and adaptive immunity through various mechanisms. In this review, we focus on the PUFA modulation of membrane architecture and its consequent effects on both T cell responses and antigen presentation. We first use data from in vitro and in vivo experiments to make the case that the immunosuppressive effects of PUFAs begin with membrane incorporation and modulation of lipid-protein lateral organization. This in turn inhibits downstream signaling mediated by T cell receptors and suppresses T cell activation and proliferation. Next, we review evidence for PUFA-mediated alteration of major histocompatibility complex class I and II surface expression and antigen presentation. We propose that PUFAs influence the expression of major histocompatibility complex by altering its conformation, orientation, lateral organization, and trafficking, with consequences for recognition by effector T cells. Finally, we present data from model membrane studies to explain the physical principles that make PUFA acyl chains unique in modifying membrane lateral organization and protein function. An important concept to emerge from these studies is that PUFA acyl chains and cholesterol molecules are sterically incompatible. By applying this concept to the T cell activation and signaling model, mechanisms emerge by which PUFAs can modulate membrane lipid-protein lateral organization. Our data-based models show that membrane modification of both effectors and targets is an important, often overlooked, mechanism of immunomodulation by PUFAs.

Effect of docosahexaenoic acid-rich fish oil supplementation on human leukocyte function

BACKGROUND

The effect of a docosahexaenoic acid (DHA)-rich fish oil (FO) supplementation on human leukocyte function was investigated.

METHODS

Ten male volunteers were supplemented with 3g/day FO containing 26% eicosapentaenoic acid (EPA, 20:5, n-3) and 54% DHA (22:6, n-3) for 2 months.

RESULTS

FO supplementation changed the fatty acid (FA) composition of leukocytes resulting in an increase of n-3/n-6 ratio from 0.18 to 0.62 in lymphocytes and from 0.15 to 0.70 in neutrophils. DHA-rich FO stimulated an increase in phagocytic activity by 62% and 145% in neutrophils and monocytes, respectively. Neutrophil chemotactic response was increased by 128%. The rate of production of reactive oxygen species by neutrophils was also increased, as it was with lymphocyte proliferation. These changes were partially reversed after a 2-month wash out period. With respect to cytokine production by lymphocytes, interleukin (IL)-4 release was not altered, whereas secretions of IL-10, interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha were raised. These results are in contrast to those described by others using EPA-rich FO supplementation. Lymphocyte pleiotropic gene expression was analyzed by a macroarray technique. Of the analyzed genes (588 in total), 77 were modified by the supplementation. FO supplementation resulted in up-regulation of 6 genes (GATA binding protein 2, IL-6 signal transducer, transforming growth factor alpha, TNF, heat shock 90kDa protein 1-alpha and heat shock protein 70kDa 1A) and a down regulation of 71 genes (92.2% of total genes changed). The largest functional group of altered genes was that related to signaling pathways (22% of the total modified genes).

CONCLUSIONS

Therefore, although EPA and DHA are members of n-3 FA family, changes in the proportion of DHA and EPA exert different effects on neutrophil, monocyte and lymphocyte function, which may be a result of specific changes in gene expression.

Fatty acids as modulators of the immune response

Research describing fatty acids as modulators of inflammation and immune responses abounds. Many of these studies have focused on one particular group of fatty acids, omega-3. The data from animal studies have shown that these fatty acids can have powerful anti-inflammatory and immunomodulatory activities in a wide array of diseases (e.g., autoimmunity, arthritis, and infection). However, the evidence from human trials is more equivocal. In this review, a historical framework for understanding how and why fatty acids may affect the immune system is provided. Second, highlights of two recent landmark reports from the Agency for Healthcare Research and Quality are presented. These reports critically evaluate the evidence from human clinical trials of omega-3 fatty acids and rheumatoid arthritis, asthma, and a few other immune-mediated diseases. Third, the data from human clinical trials investigating the impact of various bioactive fatty acids on ex vivo and in vivo immune response are reviewed. Limitations in experimental design and immune assays commonly used are discussed. The discordance between expectation and evidence in this field has been a disappointment. Recommendations for improving both animal-based and human studies are provided.

Modulation of phosphoinositide-protein kinase C signal transduction by omega-3 fatty acids: implications for the pathophysiology and treatment of recurrent neuropsychiatric illness

The phosphoinositide (PI)-protein kinase C (PKC) signal transduction pathway is initiated by pre- and postsynaptic Galphaq-coupled receptors, and regulates several clinically relevant neurochemical events, including neurotransmitter release efficacy, monoamine receptor function and trafficking, monoamine transporter function and trafficking, axonal myelination, and gene expression. Mounting evidence for PI-PKC signaling hyperactivity in the peripheral (platelets) and central (premortem and postmortem brain) tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, coupled with evidence that PI-PKC signal transduction is down-regulated in rat brain following chronic, but not acute, treatment with antipsychotic, mood-stabilizer, and antidepressant medications, suggest that PI-PKC hyperactivity is central to an underlying pathophysiology. Evidence that membrane omega-3 fatty acids act as endogenous antagonists of the PI-PKC signal transduction pathway, coupled with evidence that omega-3 fatty acid deficiency is observed in peripheral and central tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, support the hypothesis that omega-3 fatty acid deficiency may contribute to elevated PI-PKC activity in these illnesses. The data reviewed in this paper outline a potential molecular mechanism by which omega-3 fatty acids could contribute to the pathophysiology and treatment of recurrent neuropsychiatric illness.

Dietary fish oil inhibits antigen-specific murine Th1 cell development by suppression of clonal expansion

To determine the mechanisms by which dietary fish oil (FO) affects antigen-stimulated Th1 cell development, DO11.10 Rag 2(-/-) T cell receptor transgenic mice were fed a control diet (5% corn oil (CO) or a FO diet (1% CO + 4% FO, (n-3) PUFA) for 2 wk. CD4(+) T cells were cultured under neutral or Th1 polarizing conditions. FO feeding suppressed (P < 0.05) ovalbumin peptide-induced proliferation of nonpolarized CD4(+) T cells. Differentiation in vitro to Th1 cells was not affected by dietary FO, as evidenced by similar percentages of KJ1-26(+), IFN-gamma(+), IL-4(-) Th1 cells in cultures from CO-fed (99%) and FO-fed (97%) mice. However, the absolute number of viable Th1 cells in polarized cultures from FO-fed mice was less than half that observed in CO-fed mice (P < 0.05), indicating that FO inhibits in vitro Th1 clonal expansion. The reduced number of Th1 cells in FO cultures was not a result of increased apoptosis, because similar percentages of apoptotic Th1 cells were observed in cultures from FO- and CO-fed mice. IL-2-induced cell proliferation was significantly decreased in polarized Th1 cells from the FO group; however, the suppressed proliferation was not linked to reduced CD25 surface expression on antigen-stimulated CD4(+) T cells. Adoptively transferred CFSE-labeled DO11.10 CD4(+) cells into immunized mice (Th1 polarizing agents) showed that dietary FO reduced (P < 0.05) the number of cell divisions in vivo. These studies suggest that the attenuated inflammatory response which accompanies FO feeding may be explained, at least in part, by suppression of Th1 clonal expansion.

Leukocyte lipid bodies regulation and function: contribution to allergy and host defense

Lipid bodies are lipid-rich organelles found in the cytoplasm of a variety of cells, including leukocytes. Lipid body morphology, its ability to interact with other organelles and its functions are dictated by its lipid arrangement, as well as its protein composition. Both may vary according to the cell type and with the specific lipid body biogenic stimulatory pathways. Nascent lipid bodies, which are formed in vivo in the course of a variety of immunopathological conditions, are sites of enzyme localization, eicosanoid production, as well as, sites for cytokine storage in inflammatory leukocytes, suggesting that lipid bodies function as inducible intracellular platforms for spatial segregation and organization of signaling leading to inflammatory mediator secretion during inflammation. The emerging role of lipid bodies as inflammatory organelles raises lipid body status to critical regulators of different inflammatory diseases, key markers of leukocyte activation and attractive targets for novel anti-inflammatory therapies.

Endogenous versus exogenous fatty acid availability affects lysosomal acidity and MHC class II expression

Although the immune system, inflammation, and cellular metabolism are linked to diseases associated with dyslipidemias, the mechanism(s) remain unclear. To determine whether there is a mechanistic link between lipid availability and inflammation/immune activation, we evaluated macrophage cell lines incubated under conditions of altered exogenous and endogenous lipid availability. Limiting exogenous lipids results in decreased lysosomal acidity and decreased lysosomal enzymatic activity. Both lysosomal parameters are restored with the addition of oleoyl-CoA, suggesting that fatty acids play a role in the regulation of lysosomal function. Cell surface expression of major histocompatibility complex (MHC)-encoded molecules is also decreased in the absence of exogenous lipids. Additionally, we observe decreased gamma-interferon stimulation of cell surface MHC class II. Using cerulenin to limit the endogenous synthesis of fatty acids results in decreased cell surface expression of MHC class II but does not appear to alter lysosomal acidity, suggesting that lysosomal acidity is dependent on exogenous, but not endogenous, fatty acid availability. Testing these conclusions in an in vivo mouse model, we observed statistically significant, diet-dependent differences in lysosomal acidity and MHC class II cell surface expression. Collectively, these data demonstrate a mechanistic link between lipid availability and early events in the immune response.

n-3 fatty acids and gene expression

Accumulating evidence in both humans and animal models clearly indicates that a group of very-long-chain polyunsaturated fatty acids, the n-3 fatty acids (or omega-3), have distinct and important bioactive properties compared with other groups of fatty acids. n-3 Fatty acids are known to reduce many risk factors associated with several diseases, such as cardiovascular diseases, diabetes, and cancer. The mechanisms whereby n-3 fatty acids affect gene expression are complex and involve multiple processes. As examples, n-3 fatty acids regulate 2 groups of transcription factors, such as sterol-regulatory-element binding proteins and peroxisome proliferator-activated receptors, that are critical for modulating the expression of genes controlling both systemic and tissue-specific lipid homeostasis. Modulation of specific genes by n-3 fatty acids and cross-talk between these genes are responsible for many effects of n-3 fatty acids.

Signal transduction mechanism involved in Clostridium perfringens alpha-toxin-induced superoxide anion generation in rabbit neutrophils

Clostridium perfringens alpha-toxin induces the generation of superoxide anion (O2(-)) via production of 1,2-diacylglycerol (DG) in rabbit neutrophils. The mechanism of the generation, however, remains poorly understood. Here we report a novel mechanism for the toxin-induced production of O2(-) in rabbit neutrophils. Treatment of the cells with the toxin resulted in tyrosine phosphorylation of a protein of about 140 kDa. The protein reacted with anti-TrkA (nerve growth factor high-affinity receptor) antibody and bound nerve growth factor. Anti-TrkA antibody inhibited the production of O2(-) and binding of the toxin to the protein. The toxin induced phosphorylation of 3-phosphoinositide-dependent protein kinase 1 (PDK1). K252a, an inhibitor of TrkA receptor, and LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K), reduced the toxin-induced production of O2(-) and phosphorylation of PDK1, but not the formation of DG. These inhibitors inhibited the toxin-induced phosphorylation of protein kinase C theta (PKCtheta). U73122, a phospholipase C (PLC) inhibitor, and pertussis toxin inhibited the toxin-induced generation of O2(-) and formation of DG, but not the phosphorylation of PDK1. These observations show that the toxin independently induces production of DG through activation of endogenous PLC and phosphorylation of PDK1 via the TrkA receptor signaling pathway and that these events synergistically activate PKCtheta in stimulating an increase in O2(-). In addition, we show the participation of mitogen-activated protein kinase-associated signaling events via activation of PKCtheta in the toxin-induced generation of O2(-).

Effect of docosahexaenoic acid on interleukin-2 receptor signaling pathway in lipid rafts

Recent studies have shown that polyunsaturated fatty acids (PUFA) regulated the functions of membrane receptors in T cells and suppressed T cell -mediated immune responses. But the molecular mechanisms of immune regulation are not yet elucidated. Lipid rafts are plasma membrane microdomains, in which many receptors localized. The purpose of this study was to investigate the effect of DHA on IL-2R signaling pathway in lipid rafts. We isolated lipid rafts by discontinuous sucrose density gradient ultracentrifugation, and found that DHA could change the composition of lipid rafts and alter the distribution of key molecules of IL-2R signaling pathway, which transferred from lipid rafts to detergent-soluble membrane fractions. These results revealed that DHA treatment increased the proportion of polyunsaturated fatty acids especially n-3 polyunsaturated fatty acids in lipid rafts and changed the lipid environment of membrane microdomains in T cells. Compared with controls, DHA changed the localization of IL-2R, STAT5a and STAT5b in lipid rafts and suppressed the expression of JAK1, JAK3 and tyrosine phosphotyrosine in soluble membrane fractions. Summarily, this study concluded the effects of DHA on IL-2R signaling pathway in lipid rafts and explained the regulation of PUFAs in T cell-mediated immune responses.

Dietary eicosapentaenoic acid modulates CTLA-4 expression in murine CD4+ T-cells

We have demonstrated that downregulation of proliferation by CD4(+) T-cells in mice fed n-3 PUFA diets is dependent on the involvement of CD28. Therefore, we hypothesized that the balance of co-stimulatory and downregulatory properties of CD28 and CTLA-4, respectively, would be altered by diet. Mice were fed a control corn oil (CO)-enriched diet devoid of n-3 PUFA, or diets enriched with either docosahexaenoic acid (DHA) or eicosapentaenoic acid (EPA) for 14d. The proliferation of splenic CD4(+) T-cells was suppressed by DHA and EPA following stimulation with anti-CD3 and anti-CD28. Surprisingly, the number of surface CD28 molecules was not reduced in activated CD4(+) T-cells from either group of n-3 PUFA-fed mice. However, in mice fed EPA, CTLA-4 protein levels were enhanced significantly 72 h post-activation (P<0.01). Therefore, we conclude that dietary EPA may suppress CD4(+) T-cell activation by enhancing the downregulatory co-receptor CTLA-4, while not altering the levels of CD28.

RETRACTED: Effects of Omega-3 Polyunsaturated Fatty Acids on Inflammatory Markers in COPD

BACKGROUND

COPD, the fifth-leading cause of death worldwide, is characterized by chronic inflammation. However, no available agent can effectively cure this inflammation. A dietary supplement containing omega-3 polyunsaturated fatty acids (PUFAs) has anti-inflammatory effects. In this study, we hypothesized that nutritional support with omega-3 PUFA-rich diets may be useful for treating COPD, and we compared the clinical features and inflammatory mediator levels between the COPD patients who received an omega-3 PUFA-rich supplement and those who received a nonrich supplement.

METHODS

Sixty-four COPD patients received 400 kilocalories per day of an omega-3 PUFA-rich supplement (n-3 group) or an omega-3 PUFA-nonrich supplement (n-6 group) for 2 years. We prospectively investigated the clinical features of these patients and measured the levels of inflammatory mediators.

RESULTS

In 6-min walk testing, the dyspnea Borg scale and decrease of arterial oxygen saturation measured by pulse oximetry significantly improved in the n-3 group. Leukotriene B4 levels in serum and sputum and tumor necrosis factor-alpha and interleukin-8 levels in sputum decreased significantly in the n-3 group, while there was no significant change in the n-6 group. Two patients in the n-3 group and three patients in the n-6 group had mild diarrhea, and three patients in the n-3 group and three patients in the n-6 group had nausea; however, their symptoms were controllable and they improved with treatment. With multiple regression analysis, it was proved that the omega-3 PUFA-rich diet significantly contributed to the change in cytokine levels in this study.

CONCLUSION

We suggest nutritional support with an omega-3 PUFA-rich diet as a safe and practical method for treating COPD.

Docosahexaenoic acid changes lipid composition and interleukin-2 receptor signaling in membrane rafts

Polyunsaturated fatty acids, including docosahexaenoic acid (DHA, 22:6n-3), modulate immune responses and exert beneficial immunosuppressive effects, but the molecular mechanisms inhibiting T-cell activation are not yet elucidated. Lipid rafts have been shown to play an important role in the compartmentalization and modulation of cell signaling. We investigated the role of DHA in modulating the lipid composition in lipid rafts and membrane subdomain distribution of interleukin-2 (IL-2) receptor signaling molecules. We found that DHA altered lipid components of rafts and modified the IL-2-induced Janus kinase-signal transducer and activator of transcription (STAT) signaling pathway by partially displacing IL-2 receptors from lipid rafts. We fractionated plasma membrane subcellular compartments and discovered that certain amounts of STAT5a and STAT5b existed in detergent-resistant plasma membrane fractions of T-cells. After DHA treatment, STAT5a and STAT5b were not detected in lipid raft fractions and were located in detergent-soluble fractions. These data demonstrate for the first time that DHA alters the lipid composition of membrane microdomains and suppresses IL-2 receptor signaling in T-cells. Thus, our data provide evidence for a functional modification in lipid rafts by DHA treatment and explain PUFA-mediated immunosuppressive effects.

Dietary (n-3) polyunsaturated fatty acids modulate murine Th1/Th2 balance toward the Th2 pole by suppression of Th1 development

We showed that dietary long-chain (n-3) PUFAs present in fish oil (FO) affect CD4(+) T cell proliferation and cytokine production in C57BL/6 mice. To test the hypothesis that the anti-inflammatory effect of dietary (n-3) PUFAs could be due to the indirect suppression of T helper (Th)1 cells by cross-regulation of enhanced Th2 activation, mice were fed a wash-out control diet [5% corn oil (CO), (n-6) PUFA] for 1 wk, followed by the control diet or a fish oil diet [1% CO + 4% FO, (n-3) PUFA] for 2 wk. Splenic CD4+ T cells were cultured under both neutral and Th2 polarizing conditions for 2 d. Cells were reactivated and analyzed for interleukin-4 and interferon-gamma by intracellular cytokine staining. Dietary fish oil significantly increased the percentage of Th2 polarized cells and suppressed Th1 cell frequency under neutral conditions. However, under Th2 polarizing conditions, although the suppression of Th1 cells was maintained in FO-fed mice, no effect was observed in Th2 cells. Dietary fish oil increased the Th2/Th1 ratio in the presence of homologous mouse serum under both neutral (P = 0.0009) and Th2 polarizing conditions (P = 0.0185). The FO diet did not significantly affect proliferation under Th2 polarizing conditions. Thus, the anti-inflammatory effects of FO may be explained in part by a shift in the Th1/Th2 balance, due to the direct suppression of Th1 development, and not by enhancement of the propensity of CD4+ T cells to be polarized toward a Th2 phenotype, at least in vitro.

Effects of dietary n-3 polyunsaturated fatty acids on T-cell membrane composition and function

Dietary n-3 PUFA have been shown to attenuate T-cell-mediated inflammation, in part, by suppressing T-cell activation and proliferation. n-3 PUFA have also been shown to promote apoptosis, another important mechanism for the prevention of chronic inflammation by maintaining T-cell homeostasis through the contraction of populations of activated T cells. Recent studies have specifically examined Fas death receptor-mediated activation-induced cell death (AICD), since it is the form of apoptosis associated with peripheral T-cell deletion involved in immunological tolerance and T-cell homeostasis. Data from our laboratory indicate that n-3 PUFA promote AICD in T helper 1 polarized cells, which are the mediators of chronic inflammation. Since Fas and components of the death-inducing signaling complex are recruited to plasma membrane microdomains (rafts), the effect of dietary n-3 PUFA on raft composition and resident protein localization has been the focus of recent investigations. Indeed, there is now compelling evidence that dietary n-3 PUFA are capable of modifying the composition of T-cell membrane microdomains (rafts). Because the lipids found in membrane microdomains actively participate in signal transduction pathways, these results support the hypothesis that dietary n-3 PUFA influence signaling complexes and modulate T-cell cytokinetics in vivo by altering T-cell raft composition.

Cyclooxygenases, peroxide tone and the allure of fish oil

Skepticism about the health benefits of fish oil is largely the result of our incomplete understanding of the biochemistry of omega3 essential fatty acids. Recent work has confirmed the roles of omega3 fatty acids in gene transcription and signal transduction, and has given insight into the effects of eicosapentaenoic acid (EPA) and the EPA/arachidonic acid (AA) ratio on prostanoid (PG) metabolism and function. One pronounced effect of fish-oil-induced increases in EPA/AA ratios is decreased PG formation from AA via cyclooxygenase-1, because EPA inhibits this isoform. In addition, cells lacking endogenous alkyl-peroxide-generating systems and thus having a low 'peroxide tone' cannot oxygenate EPA via cyclooxygenase-1. Platelets, however, which are equipped with a lipoxygenase that can produce an abundance of hydroperoxide from AA, can form small amounts of thromboxane A3 from EPA via cyclooxygenase-1. A second major consequence of elevated EPA/AA ratios is significantly increased production of 3-series PGs, including PGE3, via cyclooxygenase-2. There are four PGE receptor subtypes and at least one of these types--not yet identified--has a significantly different response to PGE3 than to PGE2; this difference may underlie the ability of omega3 fatty acids to mitigate inflammation and tumorigenesis.

Mechanisms of leukocyte lipid body formation and function in inflammation

An area of increasingly interest for the understanding of cell signaling are the spatio-temporal aspects of the different enzymes involved in lipid mediator generation (eicosanoid-forming enzymes, phospholipases and their regulatory kinases and phosphatases) and pools of lipid precursors. The compartmentalization of signaling components within discrete and dynamic sites in the cell is critical for specificity and efficiency of enzymatic reactions of phosphorilation, enzyme activation and function. We hypothesized that lipid bodies--inducible non-membrane bound cytoplasmic lipid domains--function as specialized intracellular sites of compartmentalization of signaling with major roles in lipid mediator formation within leukocytes engaged in inflammatory process. Over the past years substantial progresses have been made demonstrating that all enzymes involved in eicosanoid synthesis localize at lipid bodies and lipid bodies are distinct sites for eicosanoid generation. Here we will review our current knowledge on the mechanisms of formation and functions of lipid bodies pertinent to inflammation.

Polyunsaturated fatty acids block dendritic cell activation and function independently of NF-kappaB activation

Polyunsaturated fatty acids (PUFAs) modulate immune responses leading to clinically significant beneficial effects in a variety of inflammatory disorders. PUFA effects on T cells have been extensively studied, but their influence on human dendritic cells (DCs), which are the most potent antigen-presenting cells and play a key role in initiating immune responses, has not been elucidated so far. Here we show that PUFAs of the n-3 and n-6 series (arachidonic and eicosapentaenoic acid) affect human monocyte-derived DC differentiation and inhibit their activation by LPS, resulting in altered DC surface molecule expression and diminished cytokine secretion. Furthermore, the potency to stimulate T cells was markedly inhibited in PUFA-treated DCs. The PUFA-mediated block in LPS-induced DC activation is reflected by diminished TNF-alpha, IL-12p40, CD40, and COX-2 mRNA levels. Strikingly, typical LPS-induced signaling events such as degradation of IkappaB and activation of NF-kappaB were not affected by PUFAs, even though DC membrane lipid composition was markedly altered. Arachidonic and eicosapentaenoic acid both altered DC prostaglandin production, but inhibitors of cyclooxygenases and lipoxygenases did not abolish PUFA effects, indicating that the observed PUFA actions on DCs were independent of autoregulation via eicosanoids. These data demonstrate a unique interference with DC activation and function that could significantly contribute to the well known anti-inflammatory effects of PUFAs.

Immunomodulation by polyunsaturated fatty acids: Impact on T-cell signaling

In recent years the potential application of the immunomodulatory effects of polyunsaturated FA (PUFA), particularly those of the n-3 series, in a variety of inflammatory disorders has been of considerable interest. However, the mechanisms underlying inhibition of T-cell activation have so far been unclear. In this short review we summarize possible mechanisms for the modulation of immune responses by PUFA. Effects of PUFA on T-cell signal transduction pathways and underlying molecular mechanisms are described in detail. These recent results add considerably to the understanding of mechanisms of PUFA actions, but their relevance in the in vivo situation must still be elucidated.