Review
Selectivity of phospholipid hydrolysis by phospholipase A2 enzymes in activated cells leading to polyunsaturated fatty acid mobilization

https://doi.org/10.1016/j.bbalip.2018.07.002Get rights and content

Abstract

Phospholipase A2s are enzymes that hydrolyze the fatty acid at the sn-2 position of the glycerol backbone of membrane glycerophospholipids. Given the asymmetric distribution of fatty acids within phospholipids, where saturated fatty acids tend to be present at the sn-1 position, and polyunsaturated fatty acids such as those of the omega-3 and omega-6 series overwhelmingly localize in the sn-2 position, the phospholipase A2 reaction is of utmost importance as a regulatory checkpoint for the mobilization of these fatty acids and the subsequent synthesis of proinflammatory omega-6-derived eicosanoids on one hand, and omega-3-derived specialized pro-resolving mediators on the other. The great variety of phospholipase A2s, their differential substrate selectivity under a variety of pathophysiological conditions, as well as the different compartmentalization of each enzyme and accessibility to substrate, render this class of enzymes also key to membrane phospholipid remodeling reactions, and the generation of specific lipid mediators not related with canonical metabolites of omega-6 or omega-3 fatty acids. This review highlights novel findings regarding the selective hydrolysis of phospholipids by phospholipase A2s and the influence this may have on the ability of these enzymes to generate distinct lipid mediators with essential functions in biological processes. This brings a new understanding of the cellular roles of these enzymes depending upon activation conditions.

Introduction

It is well established that the fatty acyl chains of membrane lipids play a wide variety of biological functions including signaling; thus the enzymes regulating phospholipid fatty acid recycling constitute a key step for the fine regulation of lipid mediator production during cell activation.

A prime example of bioactive fatty acid is arachidonic acid (20:4n-6, AA), an omega-6 fatty acid that is found at relatively high levels in cells involved in innate immunity reactions, such as monocytes, macrophages and dendritic cells [[1], [2], [3]]. AA is the common precursor of the eicosanoids, a family of lipid mediators with fundamental roles in physiology and pathophysiology, particularly in inflammatory reactions [[4], [5], [6]]. The eicosanoids affect immune regulation by modulating cell activation at different points, including differentiation and migration, phagocytic capacity, and cytokine production [[7], [8], [9], [10]].

Similarly, docosahexaenoic acid (22:6n-3, DHA) and related long-chain omega-3 fatty acids eicosapentaenoic acid (20:5n-3, EPA) and docosapentaenoic acid (22:5n-3, DPA), also found in major inflammatory cells, can be oxygenated to generate biomolecules known as protectins, resolvins, and maresins (collectively called specialized pro-resolving mediators, SPM), which account for much of the biological activity of omega-3 fatty acids, and are involved in the resolution phase of inflammation, clearance of apoptotic cells, tissue repair and regeneration, and anti-nociceptive actions [11]. In addition, omega-3 fatty acids may promote anti-inflammatory reactions by themselves by acting on fatty acid-sensing receptors [12,13].

Fatty acid-derived mediators are produced during inflammation in two temporal waves with opposite effects, when cells switch the type of mediators produced from pro- to anti-inflammatory [14]. Thus, the immediate production of proinflammatory AA-derived eicosanoids after the insult is progressively followed by accumulation of anti-inflammatory lipoxins and other pro-resolving lipid mediators derived from omega-3 fatty acids, a process that initiates resolution of inflammation and the return to homeostasis [11,14]. Thus, cells appear to possess intrinsic mechanisms to dampen inflammation to avoid excessive damage that might lead to irreversible injury.

In addition to the expression of polyunsaturated fatty acid-metabolizing enzymes, availability of the fatty acid in free form is well established to constitute a limiting factor for the biosynthesis of eicosanoids and pro-resolving lipid mediators [1,15]. Such free fatty acid availability is provided by phospholipase A2s, the enzymes that cleave the sn-2 position of glycerophospholipids [16]. Multiple PLA2 enzymes co-exist in a single cell, each exhibiting potentially different headgroup and/or fatty acid preferences. Acting frequently in a co-ordinate manner, cellular PLA2s provide a tight regulation of biological processes involving membrane phospholipid fatty acid rearrangement (Fig. 1). PLA2s are found in practically all types of organisms, and in mammals they are ubiquitously expressed throughout most cells and tissues, suggesting their importance in life processes. The variety of functions of PLA2s in physiology, far from being only circumscribed to activated states of immune cells, have become more evident in the last years with the study of the phenotypes of genetically-manipulated mice [16,17].

More than thirty enzymes with PLA2 activity have been described and, based on sequence similarities, they are currently classified in 16 groups, each containing several sub-groups [16]. However, based on biochemical features these enzymes are frequently grouped into six major families: secreted phospholipase A2s (sPLA2), calcium-independent phospholipase A2s (iPLA2), cytosolic phospholipase A2s (cPLA2), platelet activating factor acetylhydrolases (PAF-AH, also known as lipoprotein-associated phospholipase A2, Lp-PLA2), lysosomal phospholipase A2 (L-PLA2) and the adipose phospholipase A (AdPLA2) [[16], [17], [18], [19], [20]]. Extensive in vitro kinetic studies have been recently carried out with most of these enzymes. Many of the studies have taken advantage of the analytical power of mass spectrometry-based lipidomics [[21], [22], [23], [24]], which provided valuable information as to the substrate preference of these enzymes. Nevertheless, factors that take part in the microenvironment of the enzymes, such as the complex membrane composition, compartmentalization of the enzyme and the different physiological and pathophysiological scenarios of the cell (including cross-talk between PLA2 forms), may produce as a result a variety of lipid molecules that orchestrate global responses and cannot be easily reproduced in in vitro assays.

In general terms, PLA2s participate in the Lands cycle of phospholipid fatty acid recycling [1,15,25], whereby the fatty acid composition at the sn-2 position of phospholipids is tightly controlled by a balance between hydrolytic reactions mediated by PLA2s versus activation of the free fatty acid by acyl-CoA synthetases and subsequent incorporation into phospholipids by lysophospholipid:acyl-CoA acyltransferases. Further remodeling reactions also occur that are catalyzed primarily by CoA-independent transacylase (CoA-IT) [15,26,27]. In resting cells the reacylation reactions dominate, but in stimulated cells the dominant reaction is the PLA2-mediated deacylation step, which results in a dramatic increase in the levels of free fatty acids, notably AA and omega-3 fatty acids, which will now be available for eicosanoid [1,14,15,28,29] or SPM [[30], [31], [32]] synthesis, depending on the temporal phase of the activation process (Fig. 1).

While our current knowledge on the mechanisms governing the expression levels of PLA2s both at gene and protein level is still scarce for the majority of members of this superfamily of enzymes, much information has accumulated on the cellular regulation of their enzymatic activities and in vitro substrate preferences. This review is aimed at relating recent findings on the ability of PLA2s to selectively hydrolyze different phospholipid substrates in cells with the generation of bioactive lipid mediators. Key current studies are discussed, focusing primarily on cPLA2α, iPLA2-VIA, sPLA2-V and sPLA2-X, as these are the PLA2 forms classically involved in the production of fatty acid-derived mediators [15,[33], [34], [35], [36]].

Section snippets

Group IVA phospholipase A2 (cPLA2α)

Group IVA PLA2, also known as cytosolic phospholipase A2α (cPLA2α), is long known to exhibit marked preference for phospholipid substrates containing AA at the sn-2 position. The aromatic residues of cPLA2α interact with the double bonds of AA, making the enzyme selective for this fatty acid. cPLA2α also displays significant activity towards EPA but, very remarkably, it shows little or no activity towards DHA [21,24,37]. This may be related to the fact that, unlike AA or EPA, DHA does not have

Ca2+- Independent group VIA phospholipase A2 (iPLA2-VIA)

iPLA2-VIA, also often abbreviated as iPLA2β, is perhaps one of the PLA2 enzymes for which more functions have recently been proposed. The enzyme was first found to participate in the regulation of lysophospholipid levels within the Lands' cycle [[78], [79], [80]]. Later work demonstrated that iPLA2-VIA is a multifaceted enzyme with multiple roles in cell physiology and pathophysiology [35,[81], [82], [83], [84]], being of special relevance in regulating intracellular signaling leading to

Group V phospholipase A2 (sPLA2-V)

An abundant body of work dating back from the 90's has documented the involvement of sPLA2-V in AA mobilization and attendant eicosanoid production [131]. In general terms, sPLA2-V acts by amplifying the action of cPLA2α, which is the key enzyme in the process, via activity-dependent or -independent mechanisms. sPLA2-V shows no clear fatty acid preference [24], and is able to release other fatty acids from cells, e.g. oleic acid or linoleic acid [[132], [133], [134]], with regulatory features

Group X phospholipase A2 (sPLA2-X)

Of all members of the sPLA2 family of enzymes, sPLA2-X is the one that shows the highest activity towards PC [155]. The enzyme is long known to release various fatty acids including AA and oleic acid, and increases prostaglandin E2 production when added exogenously to phagocytic cells, suggesting a role for this enzyme in inflammation [156]. Later, its role in inflammatory lung diseases, both mouse and human, was defined [[157], [158], [159], [160], [161]]. More recently, using an inhaled

Other phospholipase A2s

Oxidized phospholipids are formed from unsaturated acyl residues under oxidative stress in lipid membranes [169,170]. Oxidized phospholipids are frequently found in vascular tissues and lipoproteins, and usually contain an oxovaleroyl or glutaroyl residue at the sn-2 position, which result from the truncation and oxidation of an AA or EPA residue at C5. Traditionally, two PLA2 enzymes were thought to hydrolyze truncated phopholipids, namely PAF acetylhydrolase I (group VIIA PLA2), which is

CoA-independent transacylation reactions

The most common forms of membrane glycerophospholipids contain two acyl chains attached to the sn-1 and sn-2 positions of the glycerol backbone by ester bonds. However, there are glycerophospholipids that possess an sn-1 ether bond instead of an ester bond. Additionally, some of these ether-containing phospholipids also possess a cis double bond that is conjugated with the ether oxygen, i.e. forming a vinyl ether (Fig. 4). These phospholipids are called plasmalogens. In humans, plasmalogens

Concluding remarks

Much progress has been made in recent years to understand the cellular regulation of the selective hydrolysis of membrane phospholipids by PLA2s. Still, the different activation conditions and the accessibility to different pools in the cell may lead to the production of yet unidentified lipid mediators that participate in crucial pathophysiological events. It is important to emphasize that PLA2 represents the very first step of signaling pathways that involve lipid mediators which act per se

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Acknowledgments

Work in the authors' laboratory was supported by Grants SAF2016-80883-R and SAF2015-73000-EXP from the Spanish Ministry of Economy, Industry and Competitiveness, and Grant CSI073U16 from the Education Department of the Regional Government of Castile and Leon. CIBERDEM is an initiative of Instituto de Salud Carlos III.

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