LAT Displacement from Lipid Rafts as a Molecular Mechanism for the Inhibition of T Cell Signaling by Polyunsaturated Fatty Acids

Abstract

Polyunsaturated fatty acids (PUFAs) suppress immune responses and inhibit T cell activation through largely unknown mechanisms. The displacement of signaling proteins from membrane lipid rafts has recently been suggested as underlying PUFA-mediated T cell inhibition. We show here that PUFA treatment specifically interferes with T cell signal transduction by blocking tyrosine phosphorylation of LAT (linker for activation of T cells) and phospholipase C␥1. A significant fraction of LAT was displaced from rafts by PUFA treatment along with other signaling proteins. However, retaining LAT alone in lipid rafts effectively restored phospholipase C␥1/calcium signaling in PUFA-treated T cells. These data reveal LAT displacement from lipid rafts as a molecular mechanism by which PUFAs inhibit T cell signaling and underline the predominant importance of LAT localization in rafts for efficient T cell activation. Polyunsaturated fatty acids (PUFAs)1 suppress immune responses (1) and are clinically applied as adjuvant immunosuppressive agents in the treatment of inflammatory disorders (e.g. rheumatoid arthritis and inflammatory bowel disease) (2, 3) and following organ transplantation (4). PUFAs of the n-3 series have been shown to be particularly effective and are known to interfere with proinflammatory eicosanoid (prostaglandin/leukotriene) synthesis (1). However, PUFA-mediated inhibition of T lymphocyte activation and function has repeatedly been shown to be independent of eicosanoid synthesis (5-8). The molecular mechanism by which PUFAs inhibit T cell signal transduction has remained largely unknown. Because PUFA treatment changes the lipid composition of lymphocyte membranes (9) and functionally important membrane lipid rafts (10), alterations of rafts have been suggested as underlying the PUFA-mediated inhibition of T cell signaling (11).Membrane lipid rafts or microdomains are specialized regions within the plane of the plasma membrane and have been proposed as playing an essential role in T cell signal transduction (12-16). Lipid rafts are characterized by a high concentration of cholesterol and sphingolipids such as sphingomyelin and glycolipids, and their polar lipids contain predominantly saturated fatty acyl residues (17, 18). Such lipids spontaneously form liquid-ordered membrane regions that are insoluble in non-ionic detergents, facilitating raft isolation as detergentresistant membrane domains (18,19). Cytoplasmic and transmembrane proteins are targeted to lipid rafts mostly by acylation with saturated fatty acyl moieties (palmitoyl and myristoyl) which link these proteins to the cytoplasmic lipid leaflet of microdomains (20). Several proteins involved in T cell signaling such as Src family protein-tyrosine kinases (PTKs) and LAT (linker for activation of T cells) are concentrated in rafts because of post-translational palmitoylation, emphasizing the role of lipid rafts in T cell signaling (14, 21).The activation of T cells requires stimulation of the T cell antigen receptor (TCR)/CD3 complex. Triggering CD3...

Polyunsaturated Fatty Acids Block Dendritic Cell Activation and Function Independently of NF-κB Activation

“…However, a large variety of eicosanoids with the potential to influence DC function remains that has to be analyzed in detail before a possible implication of eicosanoids in mediating PUFA effects on DCs can definitely be ruled out. Lipid raft modification has been shown to underlie inhibitory effects of PUFAs on T cell signaling (21,30). PUFAs were incorporated into DC membrane lipids in our studies and probably also in raft lipids since raft lipid alterations by exogenous fatty acids generally reflect those in cellular membranes (21).…”

“…showed that PUFA treatment alters T cell membrane lipid composition in vitro and in vivo (21,27,28) causing functional defects in T cell signaling (29,30). Hence, we investigated whether also DCs incorporate exogenous PUFAs into cell membranes leading to altered membrane lipid composition.…”

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

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