PUFAs can influence inflammatory cell function, and so inflammatory processes, by a variety of mechanisms as follows:
- PUFA intake can influence complex lipid, lipoprotein, metabolite and hormone concentrations that in turn influence inflammation;
- Non-esterified PUFAs can act directly on inflammatory cells via surface or intracellular “fatty acid receptors” – the latter may include transcription factors like peroxisome proliferator activated receptors (PPARs);
- PUFAs can be oxidized (enzymatically or non-enzymatically) and the oxidized derivatives can act directly on inflammatory cells via surface or intracellular receptors – oxidation can occur to the non-esterified form of the PUFA or to PUFAs esterified into more complex lipids including circulating or cell membrane phospholipids and intact lipoproteins such as low density lipoprotein (LDL);
- PUFAs can be incorporated into the phospholipids of inflammatory cell membranes(as described above). Here they play important roles assuring the correct environment for membrane protein function, maintaining membrane order (“fluidity”) and influencing lipid raft formation . Membrane phospholipids are substrates for the generation of second messengers like diacylglycerol and it has been demonstrated that the fatty acid composition of such second messengers, which is determined by that of the precursor phospholipid, can influence their activity . In addition, membrane phospholipids are substrates for the release of (non-esterfied) PUFAs intracellularly – the released PUFAs can act as signaling molecules, ligands (or precursors of ligands) for transcription factors, or precursors for biosynthesis of lipid mediators which are involved in regulation of many cell and tissue responses, including aspects of inflammation and immunity (see below). Thus, changes in membrane phospholipid fatty acid composition, as described above, can influence the function of cells involved in inflammation via:
- ○ alterations in the physical properties of the membrane such as membrane order and raft structure;
- ○ effects on cell signaling pathways, either through modifying the expression, activity or avidity of membrane receptors or modifying intracellular signal transduction mechanisms that lead to altered transcription factor activity and changes in gene expression;
- ○ alterations in the pattern of lipid mediators produced, with the different mediators having different biological activities and potencies (see below).
The multitude of potential mechanisms involved and their complexity has made it difficult to fully understand the actions of PUFAs within inflammatory processes. This difficulty has been further compounded by the variety of experimental approaches that have been used, including the method of presentation of PUFAs of interest to inflammatory cells in order to study their effects. For example, many in vitro
studies have exposed cells to non-esterified fatty acids, often at concentrations that might not be achieved physiologically. Thus, effects of non-esterified PUFAs on responses of lymphocytes [2
], monocytes [28
], macrophages [8
], neutrophils [34
] and endothelial cells [37
] have been demonstrated. These effects may involve a direct effect of the non-esterified PUFA or of an oxidized derivative of the PUFA [40
] or they may be secondary to incorporation of the PUFA into cell membrane phospholipids. Physiologically, the concentration of non-esterified n-3 PUFAs (and also arachidonic acid) is quite low. These fatty acids are carried in the bloodstream at much higher concentrations in more complex lipids (triglycerides, phospholipds, cholesteryl esters) within lipoproteins. Many of the cell types involved in inflammatory responses express lipoprotein receptors (e.g., LDL receptor, very low density lipoprotein receptor, scavenger receptors) and so are able to take up intact lipoproteins, subsequently utilising the fatty acid components. Thus, lipoproteins may affect inflammatory cell function [43
], perhaps due to their component fatty acids. Inflammatory cells may also access fatty acids from lipoproteins by hydrolysing them extracellularly as has been demonstrated for macrophages [45
] and lymphocytes [46
]. Thus, cells involved in inflammatory processes are exposed to fatty acids, including PUFAs, in many different forms, and they may access fatty acids from their environment by a variety of mechanisms. The effect of the form of presentation of PUFAs to inflammatory cells can be examined in the cell culture setting and studies to date indicate that non-esterified fatty acids [28
], complex lipids like triglycerides [46
], intact lipoproteins [44
], and oxidized forms of fatty acids and other lipids [40
] all influence inflammatory cell responses, frequently with different effects or different potencies of n-6 and n-3 PUFAs.
Following increased dietary intake of marine n-3 PUFAs their concentrations increase in complex lipids within the bloodstream (triglycerides, phospholipids, cholesteryl esters), as well as within the membrane phospholipids of cells and tissues including those involved in inflammatory responses (see above), and there is a small increase in their concentration within the circulating non-esterfied fatty acid pool; the latter increase is small because circulating non-esterfied fatty acids derive principally from adipose tissue triglyceride breakdown and adipose tissue triglycerides contain very little EPA and DHA. Thus, following increased intake of EPA and DHA, both the cells involved in inflammation and their extracellular environment (e.g., blood plasma) are enriched in those fatty acids, so that the n-3 PUFA enriched inflammatory cells will be in contact with n-3 PUFA-rich complex lipids and lipoproteins. Many studies have examined the effect of increased intake of marine n-3 PUFAs on the function of cells typically involved in inflammation taken from the bloodstream (neutrophils, eosinophils, monocytes, lymphocytes) or, in the case of animal studies tissues and subsequently cultured. In many, probably most, cases the in vivo situation is not maintained during the ex vivo culture period, in that the n-3 PUFA enriched cells are maintained in an environment that is different from that to which they were exposed in vivo i.e., to an n-3 PUFA poor environment. Thus, the in vivo situation is not replicated in the ex vivo setting. This hampers the full interpretation of the findings of such research.