Resolvins are dihydroxy or trihydroxy metabolites of the polyunsaturated omega-3 fatty acids, primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) but also docosapentaenoic acid (DPA). These metabolites have been shown to be made by the cells and tissues of various animal species and humans. Resolvin (or Rv) metabolites of EPA are termed E resolvins (RvEs), those of DHA are termed D resolvins(RvDs), and those of DPA are termed resolvins D (RvDsn-3DPA) and resolvins T (RvTs) (RvDn-3DPA). The EPA-derived resolvins are nonclassic eicosanoids. Certain isomers of RvDs are termed aspirin-triggered resolvin Ds (AT-RvDs) because their synthesis is initiated by aspirin-acetylated COX2 to form 17(R)-hydroxy rather than the more classical 17(S)-hydroxy resolvins whose synthesis is initiated by lipoxygenases; however, an as yet unidentified cytochrome P450 enzyme(s) also forms this 17(R)-hydroxy intermediate and thereby contributes to the production of AT-RvDs. Resolvins are members of an expanding family of polyhydroxylated polyunsaturated fatty acid (PUFA) metabolites named "specialized pro-resolving mediators" (SPM). In addition to the resolvins, SPM include lipoxin, maresin, and protectin D1 (also termed neuroprotectin) metabolites of arachidonic acid (for lipoxins) or DHA (for maresins and protectins) as well as more recently defined and therefore less fully studied metabolites of the omega-3 PUFA isomer of DPA, 7Z,10Z,13Z,16Z,19Z-docosapentaenoic acid (clupanodonic acid) and the metabolites of the N-acetylated fatty acid amide of the DHA metabolite, docosahexaenoyl ethanolamide. SPM form during the later stages of inflammatory responses, have various, often complementary anti-inflammatory, and are thought to be key mediators in resolving these responses. The putative anti-inflammatory effects of dietary omega-3 fatty acids such as fish oil, it is suggested, are due to the conversion of EPA, DHA, and/or DPA to resolvins.
One or, acting in series, two of the following oxygenase enzymes metabolize EPA or DHA to the resolvins: 15-lipoxygenase-1 (i.e. ALOX15) (and possibly 15-lipoxygenase-2, i.e. ALOX15B); 5-lipoxygenase (i.e. ALOX5); COX-2 (especially in the presence of aspirin); and/or an as yet unidentified Cytochrome P450(s). When two oxygenases are involved in the production of a resolvin, each oxygenase often resides in different cell types; one cell type initiates the oxygenation and passes the intermediate product to a nearby cell of a different type which further metabolizes the intermediate to a resolving; this route often occurs in two different cell types each of which expresses one of the two oxygenases but lacks or is deficient in the other oxygenase needed to make resolvins. As an example of this "transcellular metabolism", aspirin triggered resolvin Ds (AT-RvDs) are 17(R)-hydroxy epimers; they are made by the initial conversion of DHA by aspirin-acetylated COX-2 in vascular endothelial cells to 17(R)-hydroxy-DHA followed by the transfer of this 17(R) intermediate to nearby neutrophils which then use ALOX5 to further oxygenate the 17(R)-hydroxyl-DHA to AT-resolvin Ds. Other examples of transcellular metabolism exist such as the production of 5(S)-HETE by neutrophils and its conversion to its more potent analog, 5-oxo-ETE, by nearby blood platelets or dendritic cells (see 5-oxo-eicosatetraenoic acid#5-Oxo-ETE production). Lipoxins which are made by the same enzymes as, and function similarly to respolvins, are also often made complementary cell types via the transcellular route (see lipoxin#Biosynthesis).
E series resolvins
For the E series of resolvins, aspirin-treated COX2 or a microbial cytochrome P450 metabolizes EPA to the 18(R)-hydroperoxy-5Z,8Z,11Z,14Z,16E-eicosapentaenoic acid intermediate and then to resolvin E3 (RvE3), i.e. 17(R),18(R)-dihydroxy-5Z,8Z,11Z,14Z,16E-eicosapentaenoic acid. Alternatively, ALOX5 metabolizes the intermediate to a 5,6-epoxide and then to Resolvin E1 (RvE1), i,e. 5S,12R,18R-trihydroxy-6Z,8E,10E,14Z,16E-eicosapentaenoic acid, or the epoxide is reduced to resolvin E2 (RvE2), i.e. 5S,18R-dihydroxy-6E,8Z,11Z,14Z,16E-eicosapentaenoic acid.
D series resolvins
For the D series of resolvins, 15-LOX (ALOX15 or possibly ALOX15B) metabolizes DHA to 17(S)-hydroperoxy-DHA which is then converted to 7(S)-hydroxy,17(S)-hydroperxy-DHA or to 5(S)-hydroxyl-17(S)-hydroperoxy-DHA. These two hydroperoxy metabolites may be rapidly reduced to their respective hydroxyl analogs viz., RvD5 (i.e. 7(S),17(S)-dihydroxy-4Z,8E,10Z,13Z,15E,19Z-docosahexaenoic acid) and RvD6 (i.e. 7(S),17(S)-dihydroxy-5E,7Z,10Z,13Z,15E,19Z-docosahexaenoic acid). Alternatively, the 15-LOX may convert the two hydroperoxy metabolites to their respective 7(S),8(S) and 4(S),5(S)-epoxy intermediates. The 7(S),8(S)-epoxy intermediate then is converted to RvD1 (i.e. 7(S),17(S)-dihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid) and RvD2 (i.e. 7(S),17(S)-dihydroxy-4Z,8E,10Z,12E,14E,19Z-docosahexaenoic acid which is shown here in the attached figure) while the (S),5(S)-epoxy intermediate is converted to RvD3 (i.e. 4,17(S)-dihydroxy-5Z,7E,9E,13Z,15E,19Z-docosahexaenoic acid) and RvD4 (i.e. 4,17(S)-dihydroxy-5Z,7E,9E,13Z,15E,19Z-docosahexaenoic acid).
Aspirin-triggered resolvin Ds
Aspirin-triggered resolvins (AT-RvDs) are 17(R)-hydroxy diastereomers of the 17(S) resolvins. AT-RvDs resolvins are formed by the initial attack of aspirin or a cytochrome P450 on DHA to form a 17(R)-hydroperoxy intermediate which is then converted by the same pathway given for the corresponding resolvins described above. Four RT-Dvs have been described: they are the 17(R)-hydroxyl diastereomers of RvD1, RvD2, RvD3, and RvD4 viz., AT-RvD1 viz., AT-RvD1 (i.e. 7(S),17(R)-dihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid), AT-RvD2 (i.e. 7(S),17(R)-dihydroxy-4Z,8E,10Z,12E,14E,19Z-docosahexaenoic acid), AT-RvD3 (i.e. 4,17(R)-dihydroxy-5Z,7E,9E,13Z,15E,19Z-docosahexaenoic acid), and AT-RvD4 (i.e. 4,17(S)-dihydroxy-5Z,7E,9E,13Z,15E,19Z-docosahexaenoic acid), respectively.
Docosapentaenoic acid resolvin Ds
The omega-3 DPA, 7Z,10Z,13Z,16Z,19Z-docosapentaenoic acid (clupanodonic acid), is metabbolized by a 15-LOX to form its 17-hydroperoxy intermediate which is further metabolized to form two omega-3 products, 10-hydroxy,17-hydroperoxy-DHA and 7-hydroxy,17-hydroperoxy-DPA. The 17-hydroperoxy intermediate and its products may by reduced to their corresponding 17-hydroxy metabolites viz., 17-hydroxy-DPA, 10,17-dihydroxy-DPA, and RvD5n-3DPA (i.e. 7,17-dihydroxy-8,10,13,15,19-DPA). Alternatively, the 7-hydroxy,17-hydroperoxy-DPA product may be further processed by a 15-LOX to form RvD1n-3DPA (i.e. 7,8,17-trihydroxy-9,11,13,15,19-DPA) and RvD2n-3DPA (i.e. 6,16,17-trihydroxy-8,10,12,14,19-DPA).
Docosapentaenoic acid resolvin Ts
The omega-3 DPA is also metabolized to RvT1 (i.e. 7,13,20-trihydroxy-E,10Z,14E,16Z,18E-DPA), RvT2 (i.e. 7,8,13-trihydroxy-9E,11E,14E,16Z,19Z-DPA), Rvt3 (i.e. 7,12,13-trihydroxy-8Z,10E,14E,16Z,19Z-DPA), and Tvt4 (i.e. 7,13-dihydroxy-8E,10Z,14E,16Z,19Z)-DPA; the R,S chirality of the hydroxyl residues in these Rvt's and the exact enzymes responsible for their production by human cells and tissues has not yet been unambiguously defined.
Mechanisms of Action
The mechanism(s) by which each of the resolvins activate cells has not been fully elucidated. However, many resolvins appear to operate at least in part by acting through the following G protein-coupled receptors: 1) RvD1 and AT-RvD1 act through the Formyl peptide receptor 2, which is also activated by certain lipoxins and is therefore often termed the ALX/FPR2 receptor; 2) RvD1, AT-RVD1, RvD3, AT-RvD3, and RvD5 act through the GPR32 receptor which is now also termed the RVD1 receptor; 3) RvD2 acts through the GPR18 receptor also now termed the RvD2 receptor, is also a receptor for N-Arachidonylglycine, Anandamide, Arachidonylcyclopropylamide, Abnormal cannabidiol, Δ9-Tetrahydrocannabinol, and other agents (see GPR18; 4) RvE1 and the 18(S) analog of RvE1 are full activators while RvE2 is a partaial actiator of the CMKLR1 receptor which is also known as the chemR23 or Chemokine-like receptor 1 and is also activated by chemerin and Adipokines); and 5) RvE1, the 18(S) analog of RvE1 along with RvE2 inhibit the Leukotriene B4 receptor 1 which is the receptor for inflammation-promoting PUFA metabolites LTB4 and the R stereoisomer of 12-HETE.
Activities and functions
A prevailing theory holds that inflammation provoking insults lead to the production of arachidonic acid metabolites (e.g. prostaglandins, leukotrienes, and 5-oxo-eicosatetraenoic acids) and various cytokines (e.g. Interleukin 8, Interleukin 2, and Granulocyte macrophage colony-stimulating factor) that orchestrate the ensuing innate immunity-based inflammatory responses. Later in these responses, production of the cited types of arachidonic acid metabolites switches to the production of SPMs, i.e. the resolvin metabolites of EPA, DHA, and DPA and the Maresin and Protectin D1 metabolites of DHA. The SPMs proceed to resolve these responses and initiate healing.
The resolvins, similar to all of the SPMs, targets cells that mediate inflammatory responses. For example, resolvins act on blood leukocytes to inhibit their migration out of the circulation into sites of inflammation and allergy; stimulate macrophage phagocytosis of apoptotic (i.e. dying) leukocytes (i.e. neutrophils and eosinophils) and of allergens at tissue sites of inflammation and allergy in a physiological process termed Efferocytosis; stimulates Natural killer T cells to clear leukocytes from inflamed tissues; stimlates leukocytes to engulf (i.e. eat) pathogenic microbes from sites of inflammation; and inhibit the activation and/or function of dendritic cells, the activation and aggregation of platelets, and the migration of vascular smooth muscle. In consequence of these activities, the resolvins prevent or reduce the severity, tissue destruction, or mortality in animal models of a truly diverse set inflammatory and allergic reactions to bacterial pathogens, mechanical injuries, burns, cigarette smoke, autoimmune diseases, and degenerative diseases. Specific diseases that are pertinent to human diseases wherein these animal models implicate the resolvins as mediators include not only standard inflammatory and allergic diseases but also Alzheimer's disease, fibromyalgia, atherosclerosis, neuralgias, obesity, Retinopathy involving neovascularization, and, by its presence in mothers' milk, protection of suckling infants. Preclinical studies finding that human cells and issues in culture respond to and show the presence and increases in the levels of the resolvins as well as findings that the resolvins are found at increased levels in the fluids and/or tissues of humans suffering the cited diseases lend further support to the notion that they contribute to these diseases in humans.
A recent translational study questions the notion that resolvins and other members of the SPM family are indeed formed in the human body from omega-3 PUFA. This study failed to detect a consistent signal of resolvin formation in urine or plasma of healthy volunteers who had taken fish oil. This study also found no alteration in the formation of resolvins during the resolution of inflammation which was induced by bacterial lipopolysaccharide in these volunteers. By contrast, formation of a series of established enzymatic and nonenzymatic oxidation products formed from omega-3 PUFA could readily be demonstrated in vivo. On this basis, the study authors concluded that their study fails to provide evidence consistent with the hypothesis that resolvins mediate an anti-inflammatory action of fish oil. Further information can be found in a commentary accompanying this translational work. This commentary indicated that the area of dietary omega-3 PUFA in the prevention and amelioration of inflammatory diseases remains somewhat clouded with contradictory results, controversial, and in need of further rigorous study. It will be important to establish the production and presence of resolvins and other SPMs given their potent protective actions in humans. In this regard, using rigorous and validated mass spectrometry based methods many human tissues and cell types have been shown to produce SPM in ror example, human white blood cells such as macrophages, serum, lymph nodes, and spleen, urine, placenta, diseased kidney, and, importantly, in milk from lactating mothers. Expert investigators in this field of resolution biology and physiology are now able to simultaneoulsy identify and measure both pro-inflammatory and anti-inflammatory-pro-resolving mediators such as the SPM in human tissues. Thus in the near future it is very likely that lipid mediator and SPM signature profiles will be useful in both precision and personalized medicine.
Regardless of their presence and function relative to omega-3 PUFA as well as to human disease, the resolvins and other SPMs, or more particularly, their pharmacological analogs that resist being metabolized and therefore exhibit longer life and greater potency in vivo may find clinical utility in suppressing a wide range of human diseases and untoward reactions. To date, however, only one clinical development study on a resolvin or resolvin analog has been reported: the RvE1 analog, RX-10045, significantly improved signs and symptoms in a phase 2 clinical trial of patients with Keratoconjunctivitis sicca (i.e. dry eye syndrome). The follow-up phase III clinical trial is in progress (Safety and Efficacy Study of RX-10045 on the Signs and Symptoms of Dry Eye, identifier NCT00799552; www.clinical trials.gov). A single clinical study on another type of SPM is relevant here: the 15(R/S)-methyl-LXA4 analog of LXA4 significantly reduced the severity of eczema in a double-blind, placebo-controlled, randomized, parallel-groups comparative study of 60 patients. Many more clinical studies will be needed to define the efficacy of such analogs in human disease.
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