Resolvins and Protectins
The omega-3 essential fatty acids are the focus of considerable interest among nutritionists, because of the perceived beneficial effects for the health of consumers. The mechanisms by which such effects are exerted is still a matter for controversy, but it seems likely that oxygenated metabolites derived from eicosapentaenoic acid (20:5(n-3) or EPA) and docosahexaenoic acid (22:6(n-3) or DHA), the resolvins and (neuro)protectins, must play a significant part as they have potent anti-inflammatory and immunoregulatory actions at concentrations in the nanomolar and picomolar range. The term 'resolvins' or 'resolution-phase interaction products' was coined by Professor Charles N. Serhan and colleagues because these compounds were first encountered in resolving inflammatory exudates. Compounds derived from EPA are designated as resolvins of the E series, while those formed from the precursor DHA are denoted as either resolvins or protectins ('neuroprotectins') of the D series. Increasing numbers of such compounds are being identified and only the more important series are discussed here. They have been collectively termed 'specialized pro-resolving mediators' or SPMs. The lipoxins are also important in this context.
1. 18R and 18S Resolvins of the E Series
Biosynthesis of the 18R (illustrated) and 18S resolvins of the E series (name derived from EPA) has elements in common with the synthesis of the epi-lipoxins and leukotrienes.
In vascular endothelial cells derived from blood vessels during inflammation, the cyclooxygenase enzyme COX-2 that has been acetylated by aspirin introduces an 18R hydroperoxy-group into the EPA molecule (c.f. the role of aspirin in the biosynthesis of the epi-lipoxins). The product is reduced to the corresponding hydroxy compound and transferred to a neighbouring leukocyte before a 5S-hydroperoxy group is introduced into the molecule by the action of 5-lipoxygenase as in the biosynthesis of the leukotrienes. A further reduction step produces 15S,18R-dihydroxy-EPE or resolvin E2. Alternatively, the 5S-hydroperoxy,18R-hydroxy-EPE intermediate is converted to a 5,6-epoxy fatty acid in polymorphonuclear neutrophils in humans and eventually to 5S,12R,18R-trihydroxy-6Z,8E,10E,14Z,16E-eicosapentaenoic acid or resolvin E1 by a an enzyme required for the biosynthesis of leukotrienes in leukocytes. It is noteworthy that the initial oxygenation reaction is not catalysed by unacetylated COX-2 or COX-1.
The highly specific stereochemistry of resolvin E1 is required for activation of a ligand-specific receptor and thence for its biological activity down to picomolar concentrations. However, epimeric 18S-resolvins are also produced in vivo by related biosynthetic pathways with their own distinctive biological activities, and similarly the precursor 18(R/S)-hydroxy-EPE has been shown to have anti-inflammatory effects in vitro. Resolvin E1 is eventually de-activated in tissues by oxidation via at least four distinct pathways, including conversion to 18- and 12-oxo-RvE1, for example, prior to further catabolism.
2. 17R- and 17S-Resolvins of the D Series
DHA is converted to 17R-resolvins by a similar aspirin-triggered COX-2 mechanism to the previous. (In the absence of aspirin, COX-2 in human microvascular endothelial cells converts DHA to 13S-hydroxy-DHA). Thence, enzymatic epoxidation generates either 7S,(8)-epoxy or a 4S,(5)-epoxy intermediate, which are acted upon by 5-lipoxygenase to yield the resolvins. The former produces the aspirin-triggered resolvins D1 and D2 as illustrated (D nomenclature derived from DHA), while the latter produces the aspirin-triggered resolvins D3 and D4. All contain a 17R hydroxyl group. AT-RvD1 is 7S,8R,17R-trihydroxy-docosa-4Z,9E,11E,13Z,15E,19Z-hexaenoic acid. Again, there are parallels in the biosynthesis of resolvins (and protectins below) with that of epi-lipoxins (aspirin-triggered lipoxins) from arachidonate.
In an alternative reaction in the absence of aspirin, 15-lipoxygenase generates 17S-hydroxy-DHA as the initial product. This is converted to 7S-hydroperoxy,17S-hydroxy-DHA by the action of a lipoxygenase, and thence via epoxy intermediates to resolvin (RvD1 or 7S,8R,17S-trihydroxy-docosa-4Z,9E,11E,13Z,15E,19Z-hexaenoic acid) and epimeric resolvin D2 (RvD2 or 7S,16R,17S-trihydroxy-docosa-4Z,8E,10Z,12E,14E,19Z-hexaenoic acid), i.e. all contain a 17S hydroxyl group.
A further lipoxygenase-generated intermediate from 17S-hydroxy-DHA, i.e. 4S-hydroperoxy,17S-hydroxy-DHA, is transformed via an epoxide to resolvins D3 and D4. 17R- and 17S-hydroxy-DHA have anti-inflammatory properties of their own, although they have generally been viewed simply as pathway markers and have been found in blood samples. Two further resolvins RvD5 and RvD6 have now been characterized.
Interestingly, both 22:5(n-3) and 22:5(n-6) are also good substrates for 15-lipoxygenase, and the latter gives 17S-hydroxy-22:5(n-6) and 10,17S-dihydroxy-22:5(n-6) as the main products. Both of these (n-6) docosanoids are potent anti-inflammatory agents when administered either intravenously or orally.
As with RvE1, RvD1 is de-activated by oxidation to 8- or 17-oxo-RvD as the first step in catabolism.
In studies of resolvin formation in brain tissue in response to aspirin treatment, it was shown that new docosatrienes termed initially ‘neuroprotectins’ were produced. Like the leukotrienes, there are three double bonds in conjugation, hence the term ‘triene’, though there are six double bonds in total. As it is now recognized that the formation and actions of these docosanoids are not restricted to neuronal tissue, it has been suggested that the simpler term ‘protectins’ is preferable. For example, PD1 is present in murine inflammatory exudates and lung, in peripheral human blood and exhaled breath condensates, and in a wide range of cell types.
The biosynthetic pathway to neuroprotectin or protectin in brain tissue is illustrated -
The lipoxygenase product 17S-hydroperoxy-DHA is converted first to a 16(17)-epoxide and then to the 10,17-dihydroxy docosatriene (10R,17S-dihydroxy-docosa-4Z,7Z,11E,13E,15Z,19Z-hexaenoic acid), denoted as 10R,17S-DT or PD1 (or NPD1). Synthesis of NPD1 is induced as a response to oxidative stress and/or activation of neurotrophins, and again it appears that this highly stereospecific structure is essential for biological activity. Further oxygenation can occur, and 17S-hydroperoxy-DHA can be oxidized at the terminal carbon atom by cytochrome P450 enzymes or it can react with 5-lipoxygenase to form two regioisomeric dihydroxy products, for example.
An alternative single oxygenation is found in human macrophages and platelets in which a mediator termed maresin 1 (7R,14S-dihydroxy-docosa-4Z,8E,10E,12Z,16Z,19Z-hexaenoic acid - ‘macrophage mediators in resolving inflammation’ or 'MaR1') is formed via the action of human 12-lipoxygenase. Biosynthesis is initiated in macrophages by 14-lipoxygenation of DHA, producing the hydroperoxy intermediate 14S-hydroperoxydocosa-4Z,7Z,10Z,12E,16Z,19Z-hexaenoic acid, which undergoes further conversion via 13(14)-epoxidation.
MaR1 is a powerful regulator of inflammation resolution, tissue regeneration and pain resolution. An epoxy intermediate in maresin biosynthesis, 13S,14S-epoxy-maresin, affects macrophage activity and function independently. Similar oxygenated compounds with ani-inflammatory properties are formed from 22:5(n-3) and 22:5(n-6) fatty acids.
4. Biological Activity
Acute inflammation in response to infection or tissue damage is usually characterized by heat, redness, swelling and pain at a simple observational level, and by oedema, accumulation of leukocytes, and then by accumulation of monocytes and macrophages at a cellular level. The latter are the first line of defense of the innate immune system to defeat pathogens and toxins. Leukotrienes (especially LTB4) and prostaglandins (PGE2 and PGD2) derived from arachidonic acid are important in the early stages of the inflammatory process. As tissues return to health, resolvins and protectins, together with lipoxins and maresins, promote resolution of the inflammation through removal of the leukocytes together with cellular debris, ideally without leaving remnants of the host defences or of the invading microorganisms or other inflammatory initiators. Both initiation and resolution of acute inflammation are required to maintain healthy tissues.
The resolvins and neuroprotectins are distinctive and highly stereospecific lipids, which are endogenous local mediators with strong anti-inflammatory effects in addition to some immunoregulatory activities at picomolar to nanomolar concentrations. They are part of the molecular mechanisms that contribute to removal of inflammatory cells and restoration of tissue integrity once the need for the inflammatory response is over, i.e. they actively assist in the resolution of inflammation, once thought to be a passive process. It is evident that the presence of aspirin uniquely facilitates the resolution of inflammation. Thus, at local sites of inflammation, aspirin treatment enhances the conversion of the omega-3 fatty acids EPA and DHA to 18R- and 17R-oxygenated products, respectively,, i.e. resolvins of the E and D series, which carry potent anti-inflammatory signals. So far two receptors have been identified that mediate the activities of RvE1.
During inflammation, polymorphonuclear neutrophils are produced which have generally beneficial effects in countering disease by removing invading pathogens by phagocytosis, but in the longer term or if malfunctioning they may eventually cause trauma and tissue damage through infiltration into tissues. The resolvins, like the lipoxins, appear to have an important role in regulating and indeed inhibiting these harmful effects. In so doing they oppose the effects of some of the pro-inflammatory prostanoids. For example, nanomolar concentrations of resolvin E1 dramatically reduce dermal inflammation, peritonitis, dendritic cell migration and interleukin production. RvE1 blocks excessive platelet aggregation, and it also limits the effects of certain human pathogens by enhancing phagocytosis by polymorphonuclear leukocytes. Similarly, RvD2 and RvD3 have extremely potent regulatory actions on neutrophil trafficking in the picogram range in vivo by stimulating resolution and enhancing innate host defense mechanisms via a specific receptor. RvD3 appears late in resolution suggesting it that it has a specific role. Maresins appear to be especially important for tissue regeneration and wound healing.
The (neuro)protectins appear to operate in the same way as the resolvins in brain tissue. Thus, (N)PD1 has anti-inflammatory effects and protects retinal epithelial cells from apoptosis induced by oxidative stress. In addition, it has protective effects in animal models of stroke and of Alzheimer's disease. Amongst its activities in nonneuronal tissues, it promotes apoptosis of T cells and it has beneficial effects towards asthma in nanogram amounts. Protectins and maresins reduce neuropathic pain in experimental animals.
It is now well established that administration of lipoxins, resolvins and protectins in vivo and in vitro in animal models can aid the process of recovery from inflammation without compromising host defences by causing immune suppression. It is evident that such compounds and their metabolism have considerable potential for therapeutic intervention in acute inflammation or chronic inflammatory disease, and they have been tested in a wide range of experimental models, including peritonitis, periodontitis, colitis, arthritis, dry eye, inflammatory pain, cardiovascular disease, asthma and even cancer. A phase III clinical trial of a RvE1 analogue against dry eye syndrome is underway. They also appear to be beneficial towards attack by bacteria, fungi, viruses and parasites, including such debilitating diseases as influenza and malaria. From a nutritional or health standpoint, it has been suggested that dietary supplements of the precursor omega-3 fatty acids, taken together with aspirin, may ameliorate the clinical symptoms of many inflammatory disorders by regulating the time course of resolution via the production of resolvins and protectins
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Updated July 1, 2014