3. Glycerophospholipid Hydrolysis as a Major Pathway for the Mobilization of AA
The production of lipid mediators is intrinsically linked to the availability of polyunsaturated fatty acid (PUFA) precursors necessary for their synthesis. This depends on the activities of numerous enzymes and proteins that regulate the uptake, transport, storage, hydrolysis, remodeling and trafficking of PUFA among the different cellular and extracellular lipid pools. The major metabolic route supplying free PUFA for lipid mediator synthesis is that regulated by PLA
2s, because these enzymes can directly access the major cellular reservoir of readily mobilizable fatty acid, i.e., the sn-2 position of membrane glycerophospholipids, mainly PC, PE, and PI
[35][36][37]. It is relevant to indicate, however, that despite the overwhelming quantitative importance of PLA
2s to overall PUFA release, there are other minor routes not involving PLA
2 which can play important roles under limited, tightly controlled conditions. These include monoacylglycerol lipases acting on endocannabinoids
[69][70], acid lipases acting on lysosomal lipids
[71], and triacylglycerol lipase (ATGL) acting on TG
[72][73].
The PLA
2 enzymes typically involved in cellular signaling leading to lipid mediator production have been classically categorized into three major families, namely the Ca
2+-dependent cytosolic enzymes (cPLA
2), the Ca
2+-independent enzymes (iPLA
2), and the secreted enzymes (sPLA
2). A number of excellent comprehensive reviews covering the classification, characteristics and activation properties of the more than 30 members of the PLA
2 superfamily have recently been published, and the reader is kindly referred to these for specific details
[34][74][75][76][77][78][79][80][81][82][83][84].
The group IVA PLA
2, or cPLA
2α, is widely recognized as the key enzyme effecting the AA release because of its unique preference for AA-containing phospholipid substrates and its activation properties, which place it at the center of a number of key signaling pathways involving phosphorylation cascades and/or intracellular Ca
2+ movements
[34][76][77]. In accordance, studies using cPLA
2α-deficient mice have confirmed that this enzyme is essential for stimulus-induced eicosanoid production in practically all cells and tissues
[34][74][75][76][77]. A myriad of stimuli, acting on surface receptors, are able to trigger the translocation activation of cPLA
2α from the cytosol to a number of intracellular membranes, including the LD monolayer
[19][20]. This allows positioning of the enzyme in the vicinity of cyclooxygenases and lipoxygenases for efficient supply of the free fatty acid for eicosanoid formation
[85][86].
The group VIA calcium-independent PLA
2, frequently referred to as iPLA
2β, is another important enzyme for lipid mediator production which, unlike cPLA
2α, does not manifest overt specificity for any particular fatty acid, being able to efficiently hydrolyze all kinds of phospholipid substrates
[87]. Studies in macrophages have suggested that cPLA
2α and iPLA
2β preferentially act on different membrane phospholipid subsets, the former cleaving AA-containing phospholipids, and the latter liberating other fatty acids, such as adrenic acid and palmitoleic acid
[47][48][88]. These in vivo preferences suggest that the activity of each PLA
2 in stimulated cells can also be limited by the nature of the stimulus, the subcellular localization of the enzyme, and the accessibility to a given phospholipid pool
[89][90].
The secreted PLA
2s (sPLA
2) constitute the third family of PLA
2 enzymes involved in lipid mediator production
[79][91]. The sPLA
2s are secreted by a variety of cells, particularly those of the innate immune system, and act mainly on extracellular substrates such as lipoproteins, microparticles, bacteria and viruses, and the outer plasma membrane of mammalian cells. In some cases, they can also be incorporated back to the cells that first released them or to neighboring cells, and act on different intracellular membrane locations to regulate innate immune responses
[92][93][94]. sPLA
2 enzymes often act in concert with cPLA
2α to assist and/o amplify the cPLA
2α regulated response leading to PUFA mobilization
[95][96][97]. However, under some circumstances, some sPLA
2s, especially those of groups IID, III, and X, elicit significant production of PUFAs and associated mediators by acting on their own on different extracellular targets, and showing certain degree of selectivity for hydrolysis of PUFA over other monounsaturated/saturated fatty acids
[98][99][100][101].
While less generally recognized, free AA levels can also be significantly increased in cells if the reacylation of lysophospholipids is inhibited
[37][45][77][102][103][104]. This is because AA is an intermediate of a reacylation/deacylation cycle, the so-called Lands cycle, where the fatty acid is hydrolyzed from phospholipids by PLA
2s and reincorporated back by CoA-dependent acyltrasferases
[37][45]. In resting cells, reacylation reactions dominate and, as a result, free AA levels in unstimulated cells are kept at very low levels. In stimulated cells, activation of cPLA
2α makes deacylation to dominate over reacylation, which results in the net accumulation of free AA. Nevertheless, AA reacylation under activation conditions is still significant, as demonstrated by the finding that a large amount of the AA initially liberated by cPLA
2α is returned back to phospholipids. Hence, blockade of the CoA-dependent acyltransferases involved in phospholipid AA reacylation can result in greatly elevated levels of the free AA which is then available for eicosanoid synthesis
[45]. The substrate specificity of the acyltransferases involved in AA reacylation is the reason why this fatty acid is overwhelmingly incorporated in the position sn-2 of phospholipids
[77].
A third layer of complexity in the regulation of free AA availability stems from the fact that, once incorporated into phospholipids, the AA does not remain in the phospholipid molecular species that initially incorporated it but moves between different phospholipid species by a series of CoA-independent transacylation reactions
[37][104][105]. These reactions are key for the cells to maintain the appropriate distribution of AA within the various cellular pools so that, depending on stimulation conditions, they are accessible to the relevant PLA
2 [89][90]. This is an important aspect for the regulation of eicosanoid synthesis, as the quantity and distribution of eicosanoids produced under a given condition also depends on the composition and cellular localization of the phospholipid pool where the AA-hydrolyzing PLA
2 primarily acts
[89][90].
These transacylation reactions are catalyzed by CoA-independent transacylase (CoA-IT), an enzyme that primarily transfers AA moieties from PC (diacyl species) directly to PE (both diacyl and plasmalogens species), circumventing the need for CoA or ATP
[37][104][105]. The sequence of CoA-IT is still unknown which has made it difficult to advance our knowledge on the cellular regulation of phospholipid transacylation. Recent studies have provided suggestive evidence that the CoA-IT-mediated reaction is primarily catalyzed by a well described PLA
2 enzyme, the group IVC phospholipase A
2γ (cPLA
2γ)
[89][105]. Unlike its homolog cPLA
2α, cPLA
2γ is a calcium-independent enzyme and does not manifest clear selectivity for AA residues
[106]. Since the CoA-IT reaction involving AA-containing phospholipids appears to be critically involved in the inflammatory response of macrophages to certain stimuli
[90][107], it is conceivable that other enzymes in addition to cPLA
2γ may also serve a role as CoA-IT in cells. This is currently an area of active research
[89][90][105][106][107][108].