Lipid peroxidation generates biologically active phospholipids including oxidatively N-modified phospholipids

Abstract

Peroxidation of membranes and lipoproteins converts "inert" phospholipids into a plethora of oxidatively modified phospholipids (oxPL) that can act as signaling molecules. In this review, we will discuss four major classes of oxPL: mildly oxygenated phospholipids, phospholipids with oxidatively truncated acyl chains, phospholipids with cyclized acyl chains, and phospholipids that have been oxidatively N-modified on their headgroups by reactive lipid species. For each class of oxPL we will review the chemical mechanisms of their formation, the evidence for their formation in biological samples, the biological activities and signaling pathways associated with them, and the catabolic pathways for their elimination. We will end by briefly highlighting some of the critical questions that remain about the role of oxPL in physiology and disease.

Keywords: CD36; Lipid aldehydes; Lipid peroxidation; Oxidized phospholipids; Platelet-activating factor acetyl hydrolase; Toll-like receptors.

Figure 1

Peroxidation of arachidonyl-PC forms six phospholipid hydroperoxide regioisomers. Hydrogen abstraction is the first step of lipid peroxidation and this readily occurs at bis-allylic hydrogens of 1,4-pentadiene groups because the resulting radical is in resonance across all five carbons of the pentadiene group. There are three pairs of bis-allylic hydrogens in arachidonyl-PC (labeled a, b, c) and the regioisomer of hydroperoxy-eicosatetraenoyl-PC (HPETE-PC) formed depends on the position of the initial hydrogen abstraction. Molecular oxygen, which is a diradical, reacts with the lipid radical to form a phospholipid peroxyl radical. In the absence of local environmental factors, addition of oxygen is equally favored at either the 1 or 5 positions of the pentadiene group so that yields of the two regioisomers are similar. Subsequent hydrogen abstraction (often from an adjacent PUFA) gives the PL-OOH and propagates the radical reaction to neighboring phospholipids.

Figure 2

Peroxidation of linoleoyl-PC. Linoleoyl-PC has only one pair of bis-allylic hydrogens so that hydrogen abstraction at this position (labeled a) yields two regioisomers of phospholipid hydroperoxides.

Figure 3

Peroxidation of membrane phospholipids generate four classes of oxidatively modified phospholipids (oxPL). Representative examples from peroxidation of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (PAPC) and 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphoethanolamine (PAPE) are shown. Class I oxPL include all mildly oxygenated phospholipids such as 15-HETE-PE and 14,15-EET-PC. Class II oxPL include all truncated oxPL such as OV-PC and KOdiA-PC. Class III oxPL include cyclized oxPL such as 15-F2-IsoP-PE and EI-PC. Class IV include oxidatively N-modified PL such as N-IsoLG-PE and N-C4:0CA-

Figure 4

Mildly oxygenated phospholipids can form by three general mechanisms: direct enzymatic oxygenation of the phospholipid (center mechanism), indirect enzymatic oxygenation of PUFA with subsequent re-esterification (left panel), or non-enzymatic peroxidation via free radicals (right panel). Direct enzymatic peroxidation of phospholipid (PE is shown in this example) can be performed by lipoxygenases such as 15-lipoxygenase to generate HPETE-PL which are subsequent reduced by glutathione peroxidase to HETE-PL. The HETE-PL can then be converted to KETE-PL by 15-prostaglandin dehydrogenase. Alternatively, 15-lipoxygenase can act on unesterified fatty acid to generate HPETE, which is then reduced by glutathione peroxidase to its HETE, and this is subsequently esterified into phospholipid by currently unknown oxidized fatty acid synthetase and acyltransferase. These same reactions can take place non-enzymatically, with hydroxyl radicals generated as byproducts of oxidase being particularly good at peroxidation of phospholipids.

Figure 5

Formation of epoxyeicosanoid (EET) esterified phospholipids by the action of cytochrome P450s.

Figure 6

Oxidative fragmentation of PUFA esterified phospholipids generates two complementary fragments. The first fragment is a lipid aldehyde (or its oxidized or reduced analog), that is no longer attached to the phospholipid and is therefore free to diffuse into cytoplasm or extracellular space. The remaining fragment is a phospholipid now carrying a highly truncated sn-2 chain.

Figure 7

The most abundant oxidatively truncated oxPL are predicted by position of first double bond (marked by asterisk). Major products have O-acyl chains one carbon shorter than the position of this first double bond or O-acyl chain with one of three terminal ω-oxygen moieties on the carbon of this first double bond. The major products for arachidonyl-PC and linoleoyl-PC are listed, as they represent the most abundant products.

Figure 8

Beta-scission generates volatile alkanes and oxPL with very short O-acyl chains lacking ω-oxygens. Lipid peroxyl radicals form lipid alkoxyl radicals in the presence of iron. Reaction of the alkoxyl radical with the neighboring carbon-carbon bond fragments the acyl chain, leaving an alkyl radical and a lipid aldehyde. Abstraction of hydrogen from a nearby PUFA or other H donor by the alkyl radical generates an alkane. The major beta-scission products for each of the major PL-OOH are listed.

Figure 9

Hock rearrangement of PL-OOH fragments PUFA into two complimentary lipid aldehydes. Alkyl chain migration to the beta oxygen of the peroxide and subsequent addition of water to the resulting cation forms a hemiacetal which readily hydrolyzes to two aldehydic fragments. The predicted products for each HPETE-PC and HPODE-PC are listed. It is worth noting that for the two internal double bonds of arachidonyl-PC, the hydroperoxide located on either of the two original carbons in the double bond will generate the same two products (e.g. 8-HPETE-PC and 9-HPETE-PC give the same two products). Additionally some products such as hexanal and nonenal can form from peroxidation of either arachidonyl-PC or linoleoyl-PC.

Figure 10

Predicted secondary peroxidation of Hock rearrangement products to generate γ-ketoalkenoates and γ-hydroxyalkenoates. Hock rearrangement products with double bonds at γ-position relative to aldehyde (e.g. 8-oxo-oct-6-enoyl-PC) are predicted to be susceptible to H abstraction at the beta position. Addition of molecular oxygen to this lipid radical at the γ-position generates a peroxyl radical (e.g. 5-peroxy-8-oxo-oct-6-enoate-PC) that can subsequently react to form either γ-keto or γ-hydroxy moieties. Oxidation of the aldehyde generates the final γ-keto-or γ-hydroxy-alkenoate moieties (e.g. KOdiA-PC and HOdiA-PC).

Figure 11

Alternative mechanism proposed for formation of γ-keto- or γ-hydroxy-alkenoate moieties via diepoxy carbinyl intermediate (Gu and Salomon, 2012). Formation of alkoxyl radical leads to monoepoxy carbinyl, which adds molecular oxygen to form epoxyperoxide. Subsequent scission of peroxide to form epoxy alkoxyl radical allows formation of diepoxy carbinyl radical which can undergo fragmentation at either epoxide. Fragmentation of one epoxide leads to formation of hydroxyl or keto groups by the other epoxide.

Figure 12

Another alternative mechanism for formation of γ-keto- or γ-hydroxy-alkenals and –alkenoates. Peroxidation of arachidonyl-PC can generate peroxide trimers. When these peroxides are on adjacent carbons, they readily fragment to form aldehydes, which would lead to the formation of γ-hydroxyalkenals.

Figure 13

Oxidatively truncated oxPL compete with LPS for binding to MD-2, CD14, and LBP and thereby inhibit LPS signaling. MD-2 bound to LPS promotes formation of highly active TLR4 dimer that activates TIRAP and MyD88 and therefore NFkB. Truncated oxPL like OV-PC or KOdiA-PC also bind to MD-2, CD14, or LBP but only weakly activate TLR4 signaling, so that their net effect is to inhibit LPS induced signaling via TLR4. Adapted in part from (Kawai and Akira, 2011) and (Park et al., 2009).

Figure 14

Products of lipid 1,2-dioxoanyl radical include bicyclic endoperoxides, diepoxyalcohols, isofurans, and serial cyclic endoperoxides. 5-exo-cyclization of initial peroxyl radical generates 1,2-dioxolanyl radical. If the radical reacts with double bond on opposite side of the endoperoxide, a bicyclic endoperoxide is formed that can then go on to form various prostaglandin-like moieties. Alternatively, 1,2-dioxoanyl the radical can react with the endoperoxide to form a diepoxy radical that with the addition of another molecule of oxygen leads to the generation of diepoxyalcohol. This diepoxyalcohol has been postulated to be a precursor for the isofurans. Finally, the 1,2-dioxoanyl radical can react with a molecule of oxygen and then, if another double bond is appropriately positioned, undergo another round of 5-exo-cyclization to form a serial cyclic endoperoxide.

Figure 15

Formation of a large family of prostaglandin like molecules (isoprostanes, IsoP) by peroxidation of arachidonyl-PC. Initial peroxidation forms four regioisomers of bicyclic endoperoxides (H₂-IsoP), designated by the position of their hydroxyl group. Each of these regio-isomers can then form PC esterified with F2-IsoP, A2-isothromboxane (A2-IsoTx), D2-IsoP, E2-IsoP, epoxy IsoP (potentially including epoxy E2, D2, F2, A2, and J2-IsoP), and the acyclic isolevuglandin (IsoLG). Only F2-IsoP is chemically stable, with the remaining products undergoing further chemical reactions to form secondary products. In particular, IsoLG reacts with any nearby amines to form pyrrole adducts.

Figure 16

Reaction of lipid aldehydes with amines generates imine or Michael adducts. Targeted amines include the lysyl residues of proteins, PE, PS, nucleic acids, and small molecular cytosolic amines such as spermidines. Recent studies suggest that some aldehydes like IsoLG modify PE to a greater extent than other targets.

Figure 17

Characterized products of HNE reaction with PE and PS. Reaction of PE with HNE generated primarily the N-HNE-PE Michael adduct, but also the N-HNE-PE pyrrole adducts. Reaction of HNE with PS generated only very low yield of N-HNE-PS Michael adduct (Guichardant et al., 1998).

Figure 18

Proposed products of PE reaction with acrolein. Based on the reaction of acrolein with lysine, it was expected that the reaction of acrolein with PE would generate both N-(3-methylpyridinium)-PE (N-MP-PE) and N-(3-formyl-3,4-dehydropiperidino)-PE (N-FDP-PE) (Furuhata et al., 2003; Uchida et al., 1998). However, only formation N-FDP-PE has been detected in vitro (Zemski Berry and Murphy, 2007).

Figure 19

Characterized products of IsoLG reaction with PE. Reaction of IsoLG with PE initially forms hemiaminal, which then either forms the highly irreversible imine adduct (minor product) or the highly stable N-IsoLG-PE pyrrole adduct (major product). Under oxidizing conditions, the pyrrole adduct generally rapidly evolves to form the N-IsoLG-PE hydroxylactam adduct (Bernoud-Hubac et al., 2004; Li et al., 2009; Sullivan et al., 2010).

Figure 20

Characterized products of malondialdehyde (MDA) reaction with PE. Reaction of MDA with PE initially forms imine adduct, which undergoes double bond rearrangement to form the more stable N-propenal-PE. This can then undergo further reaction to yield crosslinked products (either with PE or proteins) or undergo reaction with two additional MDA to form N-1,4-dihydropyridine-3,5-dicarbeldehyde-PE (N-DHP-PE) (Guo et al., 2012).

Figure 21

Major species of oxidatively N-modified PE with amide linkage found during in vitro peroxidation of arachidonyl-PC in presence of PE (Guo et al., 2012).

Figure 22

Proposed mechanisms for formation of N-modified PE with amide linkage. Lipid aldehydes formed by Hock rearrangement (e.g. hexanal or 8-oxo-oct-6-enoyl-PC) react with PE to give imine adducts. Neighboring lipid peroxides transfer their peroxides to the imine adducts to yield peroxyheminals that then lose water to form the highly stable amide bond (e.g. N-hexanoyl-PE and N-C7:1CA-PE).

Figure 23

Formation of long-chained aldehydes from oxidation of plasmalogen with subsequent formation of N-alkyl-PE (Felde and Spiteller, 1995; Jira and Spiteller, 1996; Morand et al., 1988; Stadelmann-Ingrand et al., 2004).

Figure 24

Characterized products of glucose reaction with PE. The open form of glucose reacts with PE to form an imine adduct which rearranges to form the stable N-Amadori-PE. A series of dehydration and rearrangement steps has been proposed to lead from this N-Amadori-PE to N-(5-hydroxmethyl-1-pyrrole-2 –carbaldehyde)-PE. N-Amadori-PE also likely decomposes to form N-carboxymethyl-PE.