E-series resolvin metabolome, biosynthesis and critical role of stereochemistry of specialized pro-resolving mediators (SPMs) in inflammation-resolution: Preparing SPMs for long COVID-19, human clinical trials, and targeted precision nutrition

Abstract

The COVID-19 pandemic has raised international awareness of the importance of rigorous scientific evidence and the havoc caused by uncontrolled excessive inflammation. Here we consider the evidence on whether the specialized pro-resolving mediators (SPMs) are ready to meet this challenge as well as targeted metabololipidomics of the resolution-inflammation metabolomes. Specific stereochemical mechanisms in the biosynthesis of SPMs from omega-3 essential fatty acids give rise to unique local-acting lipid mediators. SPMs possess stereochemically defined potent bioactive structures that are high-affinity ligands for cognate G protein-coupled surface receptors that evoke the cellular responses required for efficient resolution of acute inflammation. The SPMs biosynthesized from the major omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are coined Resolvins (resolution phase interaction products; E series and D-series), Protectins and Maresins (macrophage mediators in resolving inflammation). Their biosynthesis and stereochemical assignments are established and confirmed (>1,441 resolvin publications in PubMed.gov) as well as their functional roles on innate immune cells and adaptive immune cells (both lymphocyte T-cell subsets and B-cells). The resolution of a protective acute inflammatory response is governed mainly by phagocytes that actively clear apoptotic cells, debris, blood clots and pathogens. These resolution phase functions of the acute inflammatory response are enhanced by SPMs, which together prepare the inflammatory loci for homeostasis and stimulate tissue regeneration via activating stem cells and the biosynthesis of novel cys-SPMs (e.g. MCTRs, PCTRs and RCTRs). These cys-SPMs also activate regeneration, are organ protective and stimulate resolution of local inflammation. Herein, we review the biosynthesis and functions of the E-series resolvins, namely resolvin E1 (the first n-3 resolvin identified), resolvin E2, resolvin E3 and resolvin E4 biosynthesized from their precursor eicosapentaenoic acid (EPA), and the critical role of total organic synthesis in confirming SPM complete stereochemistry, establishing their potent functions in resolution of inflammation, and novel structures. The physical properties of each biologically derived SPM, i.e., ultra-violet (UV) absorbance, chromatographic behavior, and tandem mass spectrometry (MS²) fragmentation, were matched to SPMs biosynthesized and prepared by stereospecific total organic synthesis. We briefly review this approach, also used with the endogenous D-series resolvins, protectins and maresins confirming their potent functions in resolution of inflammation, that paves the way for their rigorous evaluation in human tissues and clinical trials. The assignment of complete stereochemistry for each of the E and D series Resolvins, Protectins and Maresins was a critical and required step that enabled human clinical studies as in SPM profiling in COVID-19 infections and experimental animal disease models that also opened the promise of resolution physiology, resolution pharmacology and targeted precision nutrition as new areas for monitoring health and disease mechanisms.

Keywords: Docosahexaenoic acid (DHA); Eicosapentaenoic acid (EPA); Human phagocytes; M2 macrophages; Neutrophils; Omega-3 polyunsaturated fatty acids.

Fig. 1

Schematic illustration of the acute inflammatory response with the strategic positions of lipid mediators in this local protective response. Panel A: Initiation of the acute inflammatory response ideally proceeds to complete resolution and homeostasis. Edema, regulated by PGE₂ in seconds to minutes, enables neutrophil tissue/exudate influx by chemotaxis to the arachidonic-acid-derived leukotriene B₄ (LTB₄). The prostaglandins PGE₂ and PGD₂ activate the lipid mediator class switch [13], triggering the increase in 15-lipoxygenase [211] that leads to production of lipoxins starting the decrease in neutrophil numbers in the inflammatory exudate and beginning the resolution phase and biosynthesis of resolvins [8,21], protectins, and maresins (the SPM), which carry functions critical to timely resolution and homeostasis [4]. These include limiting further PMN recruitment to the site and the uptake and removal of apoptotic PMN and cell debris by macrophages. The resolvins down regulate and counteract the cytokines, chemokines, and eicosanoids, e.g., prostaglandins and leukotrienes, the pro-inflammatory signals produced via inflammasomes, and NF-κB are reduced and cellular adhesion molecules C11b/CD18 are also downregulated. The production of protectins and maresins from DHA is organ protective and activates tissue regeneration as needed to repair the injured site for homeostasis (see text for further details). Adenosine blocks the biosynthesis of SPMs from omega-3 fatty acids [194], illustrating that there are many checkpoint regulators involved in the resolution of the acute inflammatory response and the resolution of this complex leukocyte trafficking event that is critical to host defense, elimination of invading bacteria, and survival. Each SPM reduces proinflammatory mediator production and expression as well as enhances PMN and macrophage-mediated bacterial phagocytosis and killing. Panel B: SPM-producing cell types. Apoptotic PMN, brain microglial cells and M2 macrophages biosynthesize SPM from the single cell type, while intact neutrophil and vascular endothelial cells biosynthesize SPM via transcellular biosynthesis (see text for details and original references; for example, see Fig. 3). Eosinophils are a source of SPM, lipoxins, and protectins and are rich in 15-lipoxygenase [209,212], giving a new role for these cells in resolution and wound healing via their production of SPM.

Fig. 2

Schematic illustration of the transcellular and single cell biosynthesis and functions of the E-series Resolvins in efferocytosis, phagocytosis and limiting further PMN infiltration. The transcellular biosynthesis of E-series resolvins involves hypoxic vascular endothelial cells and neutrophils. Physiologic hypoxia activates the conversion of EPA by COX-2 and the upregulation of this enzyme [8,21]. Each SPM limits further PMN recruitment by blocking diapedesis. RvE1 and RvE2 activate specific GPCRs such as ERV1, ChemR23 and BLT1 [72,213].

Fig. 3

Proposed Biosynthesis of the E-series Resolvins. 18R-HEPE produced from unesterified EPA via acetylated or modified COX-2 as well as p450 [21] is next converted by leukocyte 5-lipoxygenase to the intermediate 5S-hydroperoxy-18R-HEPE, which is either reduced to RvE2 or converted to the epoxide intermediate 5S,6S-epoxy-18R-HEPE to produce RvE1 [77] to carry out their specific functions listed above; see text for details. RvE1 (5S,12R,18R-trihydroxy-6Z,8E,10E,14Z,16E-eicosapentaenoic acid) [8,21,72]. EPA is also a substrate for lipoxygenation by 15-LOX to produce RvE3 (17R,18R-dihydroxy-5Z,8Z,11Z,13E,15E-eicosapentaenoic acid) [83]. Stereochemistry and proposed biosynthetic route of RvE4 are confirmed as well as RvE4’s potent actions and functions in vivo [61,87].

Fig. 4

Biosynthetic Mechanism and Intermediates in Resolvin E1 Production from EPA. The enzymes and structures of 5-LOX and LTA₄H were determined with recombinant enzymes and the 5,6-epoxide intermediate identified by methanol trapping [78]. The biosynthesis of the 18S E-series resolvins and their potent functions were described in [77,78].

Fig. 5

Further Local Metabolism of Resolvin E1 and Structures: metabolic inactivation. RvE1 is converted to less active metabolites by neutrophils and macrophages [214]. The Resolvin E1 further metabolome is cell-type specific, macrophages carry out dehydrogenation, and neutrophil p450 converts RvE1 to 20-carboxy-metabolites [214]. We prepared and designed stable analogs of Rv [60,82] that delay their rapid local inactivation. For examples, benzo RvD1 analog has been prepared [60] that retains potent bioactions of RvD1, and the 19-p-fluorophenoxy-RvE1 analog delays rapid leukocyte inactivation and is a potent anti-inflammatory and proresolving molecule [82]. Also, the benzo-Resolvin E2 has femtomolar potencies in mouse peritonitis [124].