Benjamin Drew Rockett

Docosahexaenoic and Eicosapentaenoic Acids Segregate Differently between Raft and Nonraft Domains

Omega-3 polyunsaturated fatty acids (n-3 PUFA), enriched in fish oils, are increasingly recognized to have potential benefits for treating many human afflictions. Despite the importance of PUFA, their molecular mechanism of action remains unclear. One emerging hypothesis is that phospholipids containing n-3 PUFA acyl chains modify the structure and composition of membrane rafts, thus affecting cell signaling. In this study the two major n-3 PUFA found in fish oils, eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, are compared. Using solid-state (2)H NMR spectroscopy we explored the molecular organization of 1-[(2)H(31)]palmitoyl-2-eicosapentaenoylphosphatidylcholine (PEPC-d(31)) and 1-[(2)H(31)]palmitoyl-2-docosahexaenoylphosphatidylcholine (PDPC-d(31)) in mixtures with sphingomyelin (SM) and cholesterol (chol). Our results indicate that whereas both PEPC-d(31) and PDPC-d(31) can accumulate into SM-rich/chol-rich raftlike domains, the tendency for DHA to incorporate into rafts is more than twice as great as for EPA. We propose that DHA may be the more bioactive component of fish oil that serves to disrupt lipid raft domain organization. This mechanism represents an evolution in the view of how PUFA remodel membrane architecture.

Docosahexaenoic acid modifies the clustering and size of lipid rafts and the lateral organization and surface expression of MHC class I of EL4 cells.

An emerging molecular mechanism by which docosahexaenoic acid (DHA) exerts its effects is modification of lipid raft organization. The biophysical model, based on studies with liposomes, shows that DHA avoids lipid rafts because of steric incompatibility between DHA and cholesterol. The model predicts that DHA does not directly modify rafts; rather, it incorporates into nonrafts to modify the lateral organization and/or conformation of membrane proteins, such as the major histocompatibility complex (MHC) class I. Here, we tested predictions of the model at a cellular level by incorporating oleic acid, eicosapentaenoic acid (EPA), and DHA, compared with a bovine serum albumin (BSA) control, into the membranes of EL4 cells. Quantitative microscopy showed that DHA, but not EPA, treatment, relative to the BSA control diminished lipid raft clustering and increased their size. Approximately 30% of DHA was incorporated directly into rafts without changing the distribution of cholesterol between rafts and nonrafts. Quantification of fluorescence colocalization images showed that DHA selectively altered MHC class I lateral organization by increasing the fraction of the nonraft protein into rafts compared with BSA. Both DHA and EPA treatments increased antibody binding to MHC class I compared with BSA. Antibody titration showed that DHA and EPA did not change MHC I conformation but increased total surface levels relative to BSA. Taken together, our findings are not in agreement with the biophysical model. Therefore, we propose a model that reconciles contradictory viewpoints from biophysical and cellular studies to explain how DHA modifies lipid rafts on several length scales. Our study supports the notion that rafts are an important target of DHAs mode of action.

Fish oil increases raft size and membrane order of B cells accompanied by differential effects on function

Fish oil (FO) targets lipid microdomain organization to suppress T-cell and macrophage function; however, little is known about this relationship with B cells, especially at the animal level. We previously established that a high FO dose diminished mouse B-cell lipid raft microdomain clustering induced by cross-linking GM1. To establish relevance, here we tested a FO dose modeling human intake on B-cell raft organization relative to a control. Biochemical analysis revealed more docosahexaenoic acid (DHA) incorporated into phosphatidylcholines than phosphatidylethanolamines of detergent-resistant membranes, consistent with supporting studies with model membranes. Subsequent imaging experiments demonstrated that FO increased raft size, GM1 expression, and membrane order upon cross-linking GM1 relative to no cross-linking. Comparative in vitro studies showed some biochemical differences from in vivo measurements but overall revealed that DHA, but not eicosapentaenoic acid (EPA), increased membrane order. Finally, we tested the hypothesis that disrupting rafts with FO would suppress B-cell responses ex vivo. FO enhanced LPS-induced B-cell activation but suppressed B-cell stimulation of transgenic naive CD4+ T cells. Altogether, our studies with B cells support an emerging model that FO increases raft size and membrane order accompanied by functional changes; furthermore, the results highlight differences in EPA and DHA bioactivity.

n-3 PUFA improves fatty acid composition, prevents palmitate-induced apoptosis, and differentially modifies B cell cytokine secretion in vitro and ex vivo.

n-3 polyunsaturated fatty acids (PUFAs) modify T-cell activation, in part by remodeling lipid composition; however, the relationship between n-3 PUFA and B-cell activation is unknown. Here we tested this relationship in vitro and ex vivo by measuring upregulation of B-cell surface molecules, the percentage of cells activated, and cytokine secreted in response to lipopolysaccharide (LPS) activation. In vitro, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) improved the membrane n-6/n-3 PUFA ratio, and DHA lowered interleukin (IL)-6 secretion; overall, n-3 PUFAs did not suppress B-cell activation compared with BSA, oleate, or elaidate treatment. Palmitate treatment suppressed the percentage of B cells activated through lipoapoptosis, which was differentially prevented by cosupplementing cells with MUFAs and PUFAs. Ex vivo, we tested the hypothesis with mice fed a control or high-fat saturated, hydrogenated, MUFA or n-3 PUFA diets. n-3 PUFAs had no effect on the percentage of B cells activated. Unexpectedly, the n-3 PUFA diet increased B-cell CD69 surface expression, IL-6 and IFNγ secretion, and it significantly increased body weight gain. Overall, we propose that changes in lipid composition with n-3 PUFA and suppression of lymphocyte activation is not universal. The study highlights that high-fat n-3 PUFA diets can promote pro-inflammatory responses, at least from one cell type.

Fish oil disrupts MHC class II lateral organization on the B-cell side of the immunological synapse independent of B-T cell adhesion☆

Fish oil-enriched long chain n-3 polyunsaturated fatty acids disrupt the molecular organization of T-cell proteins in the immunological synapse. The impact of fish oil derived n-3 fatty acids on antigen-presenting cells, particularly at the animal level, is unknown. We previously demonstrated B-cells isolated from mice fed with fish oil-suppressed naïve CD4(+) T-cell activation. Therefore, here we determined the mechanistic effects of fish oil on murine B-cell major histocompatibility complex (MHC) class II molecular distribution using a combination of total internal reflection fluorescence, Förster resonance energy transfer and confocal imaging. Fish oil had no impact on presynaptic B-cell MHC II clustering. Upon conjugation with transgenic T-cells, fish-oil suppressed MHC II accumulation at the immunological synapse. As a consequence, T-cell protein kinase C theta (PKCθ) recruitment to the synapse was also diminished. The effects were independent of changes in B-T cell adhesion, as measured with microscopy, flow cytometry and static cell adhesion assays with select immune ligands. Given that fish oil can reorganize the membrane by lowering membrane cholesterol levels, we then compared the results with fish oil to cholesterol depletion using methyl-B-cyclodextrin (MβCD). MβCD treatment of B-cells suppressed MHC II and T-cell PKCθ recruitment to the immunological synapse, similar to fish oil. Overall, the results reveal commonality in the mechanism by which fish oil manipulates protein lateral organization of B-cells compared to T-cells. Furthermore, the data establish MHC class II lateral organization on the B-cell side of the immunological synapse as a novel molecular target of fish oil.

Dendritic cell activation, phagocytosis and CD69 expression on cognate T cells are suppressed by n-3 long-chain polyunsaturated fatty acids.

Docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) are bioactive n‐3 long‐chain polyunsaturated fatty acids (LCPUFAs) in fish oil that exert immunosuppressive effects. A significant amount of literature shows that n‐3 LCPUFAs suppress dendritic cell (DC) function in vitro; however, few studies have determined if the effects are emulated at the animal level. In this study, we first focused on the functional consequences of 5% (weight/weight) fish oil on splenic CD11c+ DCs. Administration of n‐3 LCPUFAs, modelling human pharmacological intake (2% of total kcal from EPA,1·3% from DHA), to C57BL/6 mice for 3 weeks reduced DC surface expression of CD80 by 14% and tumour necrosis factor‐α secretion by 29% upon lipopolysaccharide stimulation relative to a control diet. The n‐3 LCPUFAs also significantly decreased CD11c+ surface expression and phagocytosis by 12% compared with the control diet. Antigen presentation studies revealed a 22% decrease in CD69 surface expression on transgenic CD4+ T lymphocytes activated by DCs from mice fed fish oil. We then determined if the functional changes were mechanistically associated with changes in lipid microdomain clustering or plasma membrane microviscosity with n‐3 LCPUFAs, as reported for B and T lymphocytes. Fish oil administration to mice did not influence cholera‐toxin induced lipid microdomain clustering or microviscosity, even though EPA and DHA levels were significantly elevated relative to the control diet. Overall, our data show that n‐3 LCPUFAs exert immunosuppressive effects on DCs, validating in vitro studies. The results also show that DC microdomain clustering and microviscosity were not changed by the n‐3 LCPUFA intervention used in this study.

High dose of an n-3 polyunsaturated fatty acid diet lowers activity of C57BL/6 mice

n-3 Polyunsaturated fatty acids (PUFA) are increasingly consumed as food additives and supplements; however, the side effects of these fatty acids, especially at high doses, remain unclear. We previously discovered a high fat n-3 PUFA diet made of fish/flaxseed oils promoted significant weight gain in C57BL/6 mice, relative to a control, without changes in food consumption. Therefore, here we tested the effects of feeding mice high fat (HF) and low fat (LF) n-3 PUFA diets, relative to a purified control diet (CD), on locomotor activity using metabolic cages. Relative to CD, the HF n-3 PUFA diet, but not the LF n-3 PUFA diet, dramatically reduced ambulatory, rearing, and running wheel activities. Furthermore, the HF n-3 PUFA diet lowered the respiratory exchange ratio. The data suggest mixed fish/flaxseed oil diets at high doses could exert some negative side effects and likely have limited therapeutic applications.

N-3 Polyunsaturated Fatty Acid Incorporates Directly into Lipid Rafts to Disrupt Domain Clustering and MHC Lateral Organization of Antigen Presenting Cells

N-3 polyunsaturated fatty acids (PUFA) are under clinical testing for the treatment of symptoms associated with inflammatory disorders such as cardiovascular disease. However, effective use n-3 PUFAs as nutraceuticals has been limited by a poor understanding of their molecular mechanisms. Here we addressed how the n-3 PUFAs eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids modified lipid raft and protein lateral organization of antigen presenting cells, whose function is suppressed by n-3 PUFAs. Quantitative fluorescence microscopy showed that DHA, but not EPA, relative to controls, diminished lipid raft clustering and increased their size. A significant amount of DHA incorporated directly into rafts without changing cholesterol distribution between rafts and non-rafts. Quantification of fluorescence co-localization images showed that DHA selectively altered the lateral organization of the major histocompatibility complex (MHC) class I protein. FRET microscopy measurements showed that DHA modified MHC class I clustering on a nanometer scale. Taken together, our findings are not in agreement with studies in model membranes on unsaturated fatty acids and lipid rafts. Therefore, we propose a new model, which reconciles contradictory viewpoints from biophysical and cellular studies, to explain how an unsaturated fatty acid modifies lipid rafts on several length scales. Our study provides mechanistic details by which DHA suppresses antigen presenting cell function, which allows us to more effectively use these fatty acids in the clinic.