Updating Phospholipase A₂ Biology

Abstract

The phospholipase A₂ (PLA₂) superfamily contains more than 50 enzymes in mammals that are subdivided into several distinct families on a structural and biochemical basis. In principle, PLA₂ has the capacity to hydrolyze the sn-2 position of glycerophospholipids to release fatty acids and lysophospholipids, yet several enzymes in this superfamily catalyze other reactions rather than or in addition to the PLA₂ reaction. PLA₂ enzymes play crucial roles in not only the production of lipid mediators, but also membrane remodeling, bioenergetics, and body surface barrier, thereby participating in a number of biological events. Accordingly, disturbance of PLA₂-regulated lipid metabolism is often associated with various diseases. This review updates the current state of understanding of the classification, enzymatic properties, and biological functions of various enzymes belonging to the PLA₂ superfamily, focusing particularly on the novel roles of PLA₂s in vivo.

1. Introduction

Based on their structural relationships, the PLA₂ superfamily is classified into several families, including the secreted PLA₂ (sPLA₂), cytosolic PLA₂ (cPLA₂), Ca²⁺-independent PLA₂ (iPLA₂, also called patatin-like phospholipase (PNPLA)), platelet-activating factor acetylhydrolase (PAF-AH), lysosomal PLA₂ (LPLA₂), PLA/acyltransferase (PLAAT), α/β hydrolase (ABHD), and glycosylphosphatidylinositol (GPI)-specific PLA₂ families. PLA₂s trigger the production of lipid mediators by releasing polyunsaturated fatty acids (PUFAs) and lysophospholipids from membrane phospholipids, and also participate in membrane homeostasis by altering phospholipid composition, in energy production by supplying fatty acids for β-oxidation, in generation of barrier lipids, or in fine-tuning of the microenvironmental balance between saturated and unsaturated fatty acids, among others. Many of the PLA₂ enzymes recognize the differences in the fatty acyl and/or head group moieties of their substrate phospholipids, and several enzymes catalyze even non-PLA₂ reactions such as phospholipase A₁ (PLA₁), lysophospholipase, neutral lipid lipase, and transacylase reactions. The in vivo functions of individual PLA₂s rely on their enzymatic, biochemical, and cell biological properties, their tissue and cellular distributions, lipid composition in target membranes, the spatiotemporal availability of downstream lipid-metabolizing enzymes, or the presence of cofactor(s) that can modulate the enzymatic function, in various pathophysiological settings.

During the last two decades, the functions of various PLA₂s have been clarified by studies based on not only gene-manipulated (knockout and transgenic) mice but also human diseases caused by mutations of these enzymes. Here, we provide an overview of the biological roles of various PLA₂s and their underlying lipid pathways, focusing mainly on new findings in the last five years. Readers interested in older views as a starting point for further readings should refer to our current reviews describing the classification of the PLA₂ superfamily [1], those covering PLA₂s and lipid mediators broadly [2,3], and those focusing on sPLA₂s [4,5,6,7].

2. The sPLA₂ Family

2.1. General Features

The sPLA₂ family comprises low-molecular-mass, Ca²⁺-requiring enzymes with a conserved His-Asp catalytic dyad. There are 11 mammalian sPLA₂s (IB, IIA, IIC, IID, IIE, IIF, III, V, X, XIIA, and XIIB), which are structurally subdivided into group I/II/V/X, group III, and group XII branches [8]. Individual sPLA₂s exhibit unique tissue or cellular distributions and enzymatic properties, suggesting their distinct biological roles. With regard to the substrate specificity of sPLA₂s as assessed by an assay using tissue-extracted natural membranes, sPLA₂-IB, -IIA and -IIE do not discriminate sn-2 fatty acid species, sPLA₂-V tends to prefer those with a lower degree of unsaturation such as oleic acid (OA; C18:1) and linoleic acid (LA; C18:2), and sPLA₂-IID, -IIF, -III and -X tend to prefer PUFAs such as ω6 arachidonic acid (AA; C20:4) and ω3 docosahexaenoic acid (DHA; C22:6). With regard to the polar head groups, sPLA₂-III, -V and -X efficiently hydrolyze phosphatidylcholine (PC), while sPLA₂s in the group II subfamily hydrolyze phosphatidylethanolamine (PE) much better than PC. Individual sPLA₂s exert their specific functions by producing lipid mediators, by altering membrane phospholipid composition, by degrading foreign phospholipids in microorganisms or diets, or by modifying extracellular non-cellular lipid components such as lipoproteins, pulmonary surfactant or microvesicles in response to given microenvironmental cues. In certain cases, the sPLA₂-binding protein PLA2R1 modulates the functions of sPLA₂s in either a positive or negative way. The pathophysiological roles of individual sPLA₂s, as revealed by studies using sPLA₂ knockout or transgenic mice in combination with comprehensive lipidomics, have been detailed in several recent reviews [3,6,7,9].

2.2. sPLA₂-IB in Digestion and Immunity

sPLA₂-IB (encoded by PLA2G1B in human) is synthesized as an inactive zymogen in the pancreas, and its N-terminal propeptide is cleaved by trypsin to yield an active enzyme in the duodenum [10]. The main role of sPLA₂-IB, a “pancreatic sPLA₂”, is to digest dietary and biliary phospholipids in the intestinal lumen. Perturbation of this process by gene disruption or pharmacological inhibition of sPLA₂-IB leads to resistance to diet-induced obesity, insulin resistance, and atherosclerosis due to decreased phospholipid digestion and absorption in the gastrointestinal tract [11,12,13,14]. Indeed, the human PLA2G1B gene maps to an obesity susceptibility locus [15]. These functions of sPLA₂-IB have been summarized in a recent review [16].

Beyond the well-established role of sPLA₂-IB as a “digestive sPLA₂” as outlined above, two recent studies have uncovered novel immunological functions of this sPLA₂. Entwistle et al. [17] showed that sPLA₂-IB is induced in a population of intestinal epithelial cells during helminth infection and is responsible for killing tissue-embedded larvae. Pla2g1b^−/− mice fail to expel the intestinal helminths Heligmosomoides polygyrus and Nippostrongylus brasiliensis. Treatment of the parasite with sPLA₂-IB hydrolyzes worm phospholipids (e.g., PE) and impairs development to the adult stage, suggesting that exposure to this enzyme in the intestine is an important mechanism of host-mediated defense against such parasites.

Pothlichet et al. [18] reported that sPLA₂-IB is involved in CD4⁺ T cell lymphopenia in patients infected with human immunodeficiency virus (HIV). sPLA₂-IB, in synergy with the HIV gp41 envelope protein, induces CD4⁺ T cell anergy, inhibiting the responses to IL-2, IL-4, and IL-7 as well as activation, proliferation, and survival of CD4⁺ T cells. Other sPLA₂s fail to display a similar function, implying a specific action of sPLA₂-IB. Importantly, the effects of HIV on CD4⁺ T cell anergy can be blocked by a sPLA₂-IB-specific neutralizing antibody in vivo. Thus, the sPLA₂-IB/gp41 pair constitutes a new mechanism of immune dysfunction, although the cellular source of plasma sPLA₂-IB in this context remains to be determined.

2.3. sPLA₂-IIA in Host Defense, Sterile Inflammation, and Colon Cancer

The best-known physiological function of sPLA₂-IIA (encoded by PLA2G2A) is the degradation of bacterial membranes, particularly those in Gram-positive bacteria, thereby providing the first line of antimicrobial defense as a “bactericidal sPLA₂” [19,20]. The ability of sPLA₂-IIA to hydrolyze PE and phosphatidylglycerol, which are abundant in bacterial membranes, appears to fit with its anti-bacterial function. In the lungs of patients with cystic fibrosis, sPLA₂-IIA-resistant Gram-negative Pseudomonas aeruginosa upregulates the expression of sPLA₂-IIA, which then eradicates sPLA₂-IIA-sensitive Gram-positive Staphylococcus aureus, allowing the former bacterium to become dominant within the niche [21]. Thus, sPLA₂-IIA-mediated regulation of the bacterial community in the lung microenvironment crucially affects the pathology of cystic fibrosis.

sPLA₂-IIA is often referred to as an “inflammatory sPLA₂”, as its expression is induced by pro-inflammatory cytokines and lipopolysaccharide (LPS) [22]. Besides its action on bacterial membranes as noted above, sPLA₂-IIA targets phospholipids in extracellular microvesicles (EVs), particularly those in extracellular mitochondria (organelles that have evolved from bacteria), which are released from activated platelets or leukocytes at sites of inflammation [23]. Hydrolysis of EV phospholipids by sPLA₂-IIA results in the production of lipid mediators as well as the release of mitochondrial DNA as a danger-associated molecular pattern (DAMP) that contributes to amplification of inflammation. Specifically, the AA released from platelet-derived EVs by sPLA₂-IIA is metabolized by platelet-type 12-lipoxygenase to 12-hydroxyeicosatetraenoic acid (HETE), which then acts on the BLT2 receptor on neutrophils facilitating the uptake of the EVs [24]. Thus, sPLA₂-IIA is primarily involved in host defense by killing bacteria and triggering innate immunity, while over-amplification of the response leads to exacerbation of inflammation by hydrolyzing EVs (Figure 1). These dual functions of sPLA₂-IIA were summarized in a recent review [25].

sPLA₂-IIA also appears to play a role in host defense against the malaria pathogen Plasmodium falciparum. Several sPLA₂s, including sPLA₂-IIF, -V and -X, which efficiently hydrolyze plasma lipoproteins to release free fatty acids, have the capacity to inhibit parasite growth in vitro, yet these sPLA₂s are undetectable in human plasma. sPLA₂-IIA, though hardly hydrolyzing normal lipoproteins, is increased in the plasma of malaria patients and hydrolyzes “oxidized” lipoproteins to block Plasmodium growth [26]. Injection of recombinant sPLA₂-IIA into Plasmodium-infected mice reduces the peak of parasitemia when the level of plasma peroxidation is increased during infection. Thus, malaria-induced oxidation of lipoproteins converts them into a preferential substrate for sPLA₂-IIA, thus promoting its parasite-killing effect.

PLA2G2A-transgenic mice display notable skin abnormalities with hair loss and epidermal hyperplasia [27]. Using K14-driven, skin-specific PLA2G2A-transgenic mice, Chovatiya et al. [28] recently demonstrated that sPLA₂-IIA promotes hair follicle stem cell proliferation through JNK signaling and that sPLA₂-IIA-knockdown skin cancers xenografted into NOD-SCID mice show a concomitant reduction of tumor volume and decreased JNK signaling. Kuefner et al. [29] showed that PLA2G2A-transgenic mice are protected from diet-induced obesity and become more prone to adipocyte browning with increased expression of thermogenic markers. However, since C57BL/6 mice do not express sPLA₂-IIA endogenously due to a frameshift mutation in the Pla2g2a gene [30], the physiological relevance of these results obtained from transgenic overexpression of sPLA₂-IIA in this mouse strain should be interpreted with caution. The increased energy expenditure in PLA2G2A-transgenic mice might be simply due to their lack of fur, which results in increased heat dissipation from the body surface. Indeed, transgenic mice overexpressing sPLA₂-IIF or sPLA₂-X are also furless and lean [31]. Studies using a series of sPLA₂-knockout mice have suggested that sPLA₂-IIF and sPLA₂-IID are the main endogenous regulators of skin homeostasis and adipocyte browning, respectively, as described below.

Although sPLA₂-IIA has been identified as a genetic modifier of mouse intestinal tumorigenesis [30], the underlying mechanism has long remained unclear. Schewe et al. [32] recently provided a potential mechanism by which sPLA₂-IIA, expressed by Paneth cells in the small intestine, suppresses colon cancer. In a normal state, sPLA₂-IIA inhibits Wnt signaling through intracellular activation of Yap1. Upon inflammation, sPLA₂-IIA is secreted into the intestinal lumen, where it promotes inflammation through prostaglandin (PG) E₂ synthesis via the PLA2R1-cPLA₂α pathway and Wnt signaling. Transgenic overexpression of sPLA₂-IIA delays recovery from colonic inflammation but decreases colon cancer susceptibility due to perturbation of its homeostatic Wnt-inhibitory function. Thus, this trade-off effect could provide a mechanism whereby sPLA₂-IIA acts as a genetic modifier of colonic inflammation and cancer. However, the inflammatory sPLA₂-IIA-PLA2R1-cPLA₂α-PGE₂ signaling axis proposed in this study may require further clarification, since contrary to this hypothesis, there is ample evidence that cPLA₂α and its product PGE₂ contribute to attenuation of colitis and promotion of colon cancer [33]. Other factors, such as intestinal dysbiosis due to loss of sPLA₂-IIA, might primarily underlie the increased susceptibility to gastrointestinal cancer—a possibility that awaits future study.

Apart from its potential signaling role, PLA2R1 acts as a clearance receptor for sPLA₂s [8,34]. In a model of experimental autoimmune myocarditis induced by immunizing BALB/c mice, a strain possessing a normal Pla2g2a gene, with the murine α-myosin heavy chain [30], PLA2R1 deficiency markedly increases sPLA₂-IIA and -IB proteins (but not mRNAs) in the myocardium, probably as a result of their impaired clearance [35]. In the affected myocardium, PLA2R1 and these two sPLA₂s are localized in α-SMA⁺ myofibroblasts and infiltrating neutrophils, respectively. The Pla2r1^−/− myocardium shows increased areas of inflammatory cell infiltration, accompanied by an increase in PGE₂, which promotes IL-23-induced expansion of Th17 cells. Thus, it appears that this increase of sPLA₂-IIA and -IB proteins due to their defective clearance contributes to exacerbation of autoimmune myocarditis, although it remains unclear whether these sPLA₂s act through PGE₂ synthesis or through other mechanisms, whether some other sPLA₂s that are expressed in the myocardium and have the capacity to bind to PLA2R1 are also involved in this process, or whether the effect of PLA2R1 ablation is sPLA₂-independent.

2.4. sPLA₂-IID in Immunosuppression, Host Defense, and Adaptive Thermogenesis

sPLA₂-IID (encoded by PLA2G2D), constitutively expressed in dendritic cells (DCs) in lymphoid organs, is a “resolving sPLA₂” that attenuates DC-mediated adaptive immunity by hydrolyzing PE to mobilize anti-inflammatory ω3 PUFAs and their metabolites such as DHA-derived resolvin D1 [36]. Pla2g2d^−/− mice exhibit more severe contact hypersensitivity (Th1 response) and psoriasis (Th17 response), accompanied by marked reductions of ω3 PUFAs and their metabolites in the draining lymph nodes and spleen. On the other hand, Pla2g2d^−/− mice are protected against skin cancer likely because of the enhanced anti-tumor immunity in association with increased IFN-γ⁺CD8⁺ T cells [37]. The immunosuppressive role of sPLA₂-IID in the Th1, Th2, and Th17 responses was detailed in a recent review [9].

Chronic low-grade inflammation is associated with age-related immune dysfunction in the lung, and is countered by enhanced expression of pro-resolving/anti-inflammatory factors to maintain tissue homeostasis. Vijay et al. [38] reported that sPLA₂-IID with an anti-inflammatory property contributes to worse outcomes in mice infected with severe acute respiratory syndrome coronavirus (SARS-CoV) or influenza A virus. Strikingly, Pla2g2d^−/− mice are highly protected against SARS-CoV infection, with enhanced migration of DCs to the lymph nodes, augmented anti-viral T cell responses, reduced lung damage, and increased survival. In the context of the current worldwide SARS-CoV2 pandemic, inhibition of sPLA₂-IID in the lungs of older patients with severe respiratory infections could be a potentially attractive therapeutic intervention for restoration of immune function.

ω3 PUFAs confer health benefits by preventing inflammation and obesity and by increasing thermogenesis in brown and beige adipocytes. sPLA₂-IID is constitutively expressed in M2 macrophages in white adipose tissue (WAT) and downregulated during obesity [39]. Global or macrophage-specific sPLA₂-IID deficiency decreases energy expenditure and thermogenesis by preventing adipocyte browning, thus exacerbating diet-induced obesity, insulin resistance, and WAT inflammation [39]. In WAT, PLA2G2D constitutively supplies a pool of ω3 PUFAs, which acts on the PUFA receptor GPR120 and thereby promotes the thermogenic program as a “thermogenic sPLA₂”. Importantly, dietary supplementation with ω3 PUFAs normalizes the metabolic derangement in Pla2g2d^−/− mice. These findings highlight the contribution of the macrophage-driven PLA2G2D-ω3 PUFA axis to metabolic health (Figure 2). Possibly in relation to this, a polymorphism in the human PLA2G2D gene is linked to body weight changes in patients with chronic lung disease [40].

2.5. sPLA₂-IIE and sPLA₂-IIF in the Skin

sPLA₂-IIE (encoded by PLA2G2E) is hardly detected in human tissues, whereas sPLA₂-IIE instead of sPLA₂-IIA is upregulated in several mouse tissues under inflammatory or other conditions. For instance, sPLA₂-IIE is highly upregulated in adipocytes during diet-induced obesity [41], and is expressed in hair follicles in correlation with the growth phase of the hair cycle [42]. sPLA₂-IIE hydrolyzes PE without apparent fatty acid selectivity in lipoproteins and hair follicles, and Pla2g2e^−/− mice display modest metabolic and hair follicle abnormalities.

sPLA₂-IIF (encoded by PLA2G2F) is an “epidermal sPLA₂” expressed in the suprabasal epidermis and upregulated by the Th17 cytokines IL-17A and IL-22 in psoriatic skin [31]. sPLA₂-IIF preferentially hydrolyzes PUFA-containing plasmalogen-type PE to produce lysoplasmalogen (plasmalogen-type lysophosphatidylethanolamine; P-LPE), which in turn promotes epidermal hyperplasia. Accordingly, Pla2g2f⁻⁻ mice are protected against psoriasis and skin cancer, while Pla2g2f-transgenic mice spontaneously develop psoriasis-like skin and are more susceptible to skin cancer [31]. Overall, two skin sPLA₂s, sPLA₂-IIE in the outer root sheath of hair follicles and sPLA₂-IIF in epidermal keratinocytes, play non-redundant roles in distinct compartments of mouse skin, underscoring the functional diversity of multiple sPLA₂s in the coordinated regulation of skin homeostasis and diseases. The roles of sPLA₂-IIE and -IIF were detailed in a recent review [9].

2.6. sPLA₂-III in Male Reproduction, Anaphylaxis, and Colonic Diseases

sPLA₂-III (encoded by PLA2G3) consists of three domains, in which the central sPLA₂ domain similar to bee venom group III sPLA₂ is flanked by large and unique N- and C-terminal domains [43]. The enzyme is processed to the sPLA₂ domain-only form that retains full enzymatic activity [44]. sPLA₂-III is expressed in the epididymal epithelium and acts on immature sperm cells passing through the epididymal duct in a paracrine manner to allow sperm membrane phospholipid remodeling, a process that is prerequisite for sperm motility [45]. Homozygous and even heterozygous Pla2g3-deficient sperm have impaired motility and thereby fail to fertilize oocytes, leading to hypofertility. In the context of allergy, sPLA₂-III secreted from immature mast cells is functionally coupled with lipocalin-type PGD₂ synthase (L-PGDS) in neighboring fibroblasts to supply a microenvironmental pool of PGD₂, which in turn acts on the PGD₂ receptor DP1 on mast cells to promote their proper maturation [46]. Accordingly, mice lacking sPLA₂-III, as well as those lacking L-PGDS or DP1, have immature mast cells and display reduced local and systemic anaphylactic responses.

Several lines of evidence suggest a potential link between sPLA₂-III and the development of colon cancer. For instance, sPLA₂-III-transfected colon cancer cells xenografted into nude mice show increased growth [47], higher expression of sPLA₂-III in human colorectal cancer is positively correlated with a higher rate of lymph node metastasis and shorter survival [48], and polymorphisms in the human PLA2G3 gene are significantly associated with a higher risk of colorectal cancer [49]. Importantly, Pla2g3^−/− mice are resistant to several models of colon cancer [50]. Furthermore, Pla2g3^−/− mice are less susceptible to colitis, with lower expression of pro-inflammatory and pathogenic Th17 cytokines and higher expression of epithelial barrier genes [50], implying that amelioration of colonic inflammation by sPLA₂-III ablation underlies the protective effect against colon cancer. The Pla2g3^−/− colon displays significant reduction of several lysophospholipids including lysophophatidic acid (LPA) and lysophosphatidylinositol (LPI) [50], which may promote colon inflammation or cancer through their receptors LPA₂ and GPR55, respectively [51,52]. Overall, these results establish a role for sPLA₂-III in the aggravation of colonic inflammation and cancer and point to sPLA₂-III as a novel druggable target for colorectal diseases. The biological roles of sPLA₂-III were summarized in a recent review [9].

2.7. sPLA₂-V in Obesity, Type-2 Immunity, and Aortic Protection

Although sPLA₂-V (encoded by PLA2G5) was previously thought to be a regulator of AA metabolism [53,54], it has become obvious that this sPLA₂ has a preference for phospholipids bearing sn-2 fatty acids with a lower degree of unsaturation. Transgenic overexpression of sPLA₂-V results in neonatal death due to a respiratory defect attributable to the ability of sPLA₂-V to potently hydrolyze dipalmitoyl-PC, a major component of lung surfactant [55]. Mice that are transgenic for other sPLA₂s do not exhibit such a phenotype, implying the particular ability of sPLA₂-V to hydrolyze PC with sn-2 palmitic acid (C16:0) in the lung microenvironment. sPLA₂-V is markedly induced in adipocytes during obesity as a “metabolic sPLA₂” and hydrolyzes PC in hyperlipidemic LDL to release OA and LA, which counteract adipose tissue inflammation and thereby ameliorate metabolic disorders [41]. Impairment of this process in Pla2g5^−/− mice leads to exacerbation of diet-induced obesity and insulin intolerance, accompanied by elevated phospholipid levels in plasma LDL. This phenotype is reminiscent of clinical evidence that a PLA2G5 polymorphism is associated with plasma LDL levels in patients with type 2 diabetes [56] and that the levels of PLA2G5 mRNA expression in WAT are inversely correlated with plasma LDL levels in obese subjects [41].

sPLA₂-V is a “Th2-prone sPLA₂” induced in M2 macrophages by the Th2 cytokines IL-4 and IL-13 and promotes Th2-driven pathologies such as asthma. Gene ablation of sPLA₂-V perturbs proper polarization and function of M2 macrophages in association with decreased Th2 immunity [57]. Pla2g5^−/− mice show reduced activation of type 2 innate lymphoid cells (ILC2) and infiltration of eosinophils in the lung following repetitive inhalation of the fungal allergen Alternaria Alternata [58]. Adoptive transfer experiments have revealed the contribution of sPLA₂-V expressed in both macrophages and non-hematopoietic cells (probably bronchial epithelial cells) to the pathology. Lipidomics analysis has demonstrated reduction of OA and LA in the lung and macrophages in Pla2g5^−/− mice. Exogenous administration of these unsaturated fatty acids to Pla2g5^−/− mice restores IL-33-induced inflammation and ILC2 expansion, implying that macrophage-associated sPLA₂-V contributes to type 2 immunity by promoting ILC2 activation though the release of OA and LA. The biological roles of sPLA₂-V in asthma were summarized in a recent review [59]. Probably because of the alteration in the macrophage phenotype, Pla2g5^−/− macrophages have a reduced ability to phagocytose extracellular materials, thereby being more susceptible to fungal infection and arthritis due to defective clearance of hazardous fungi and immune complexes, respectively [60,61]. Likewise, Pla2g5^−/− mice suffer from more severe lung inflammation caused by bacterial infection [62], which could also be explained by poor clearance of these microbes by alveolar macrophages. Additionally, local generation of LPE in the plasma membrane by sPLA₂-V may also contribute to macrophage phagocytosis [63].

Aortic dissection is a life-threatening aortopathy involving separation of the aortic wall. Since aortic dissection occurs suddenly without preceding clinical signs and current treatment strategies are limited mainly to antihypertensive agents and emergency surgery, biomarkers that can predict fragility and/or therapeutic targets for stabilization of the aortic wall are needed in order to improve patient outcomes. Behind its proposed role in atherosclerosis development [64], sPLA₂-V is a primary “endothelial sPLA₂” that protects against aortic dissection by endogenously mobilizing vasoprotective fatty acids [65]. Global and endothelial cell-specific deletion of sPLA₂-V leads to dissection of the thoracic ascending aorta shortly after infusion of angiotensin II (AT-II). In the AT-II-treated aorta, endothelial sPLA₂-V mobilizes OA and LA, which attenuate endoplasmic reticulum (ER) stress and increase the expression of lysyl oxidase, an enzyme that crosslinks extracellular matrix (ECM) proteins, thereby stabilizing the ECM in the aorta. Of note, dietary supplementation with OA and LA reverses the increased susceptibility of Pla2g5^−/− mice to aortic dissection. These findings reveal an unexplored functional link between sPLA₂-driven phospholipid metabolism and aortic stability (Figure 3), possibly contributing to the development of improved diagnostic and/or therapeutic strategies for preventing aortic dissection. Importantly, this work provides in vivo relevance for the actions of this sPLA₂ that had been proposed by several in vitro studies: (i) it releases OA and LA in preference to PUFAs, (ii) it preferentially acts on membranes of agonist-stimulated rather than quiescent cells, and (iii) it is retained on the cell surface through binding to heparan sulfate proteoglycan. Furthermore, this avenue of cardiovascular research has revealed a potential mechanism that could underlie the benefits of the olive oil-rich (i.e., OA-rich) Mediterranean diet in terms of cardiovascular health.

2.8. sPLA₂-X in Sperm Activation, Colitis, and Asthma

Among the mammalian sPLA₂s, sPLA₂-X (encoded by PLA2G10) has the highest activity on PC leading to release of fatty acids, particularly PUFAs, and is activated by cleavage of the N-terminal propeptide by furin-type convertases [66]. In mice, sPLA₂-X is expressed abundantly in the testis and gastrointestinal tract and to a much lesser extent in the lung, whereas its expression in other tissues is very low. In the process of reproduction, sPLA₂-X is secreted from the acrosomes of activated spermatozoa and hydrolyzes sperm membrane phospholipids to release DHA, docosapentaenoic acid (DPA, C22:5), and LPC, which facilitate in vitro fertilization with oocytes [33,67]. Pla2g10^−/− spermatozoa also show impairment of the late phase of the progesterone-induced acrosome reaction ex vivo [68]. Thus, the two particular sPLA₂s expressed in male reproductive organs, i.e., sPLA₂-III secreted from the epididymal epithelium (see above) and sPLA₂-X secreted from sperm acrosomes, act as “reproductive sPLA₂s” to coordinately regulate male reproduction.

sPLA₂-X is expressed abundantly in colorectal epithelial and goblet cells and plays a protective role against colitis by mobilizing anti-inflammatory ω3 PUFAs such as EPA and DHA [33]. Accordingly, Pla2g10^−/− mice display more severe epithelial damage and inflammation with reduction of colonic ω3 PUFAs rather than ω6 AA in a colitis model, while PLA2G10-transgenic mice exhibit global anti-inflammatory phenotypes in association with elevation of systemic levels of ω3 PUFAs and their metabolites [33]. Supplementation with exogenous EPA restores the colitis phenotype in Pla2g10^−/− mice. Furthermore, Pla2g10^−/− mice have lower fecal LA levels [69], suggesting that gastrointestinal sPLA₂-X may have a role in the digestion of dietary and biliary phospholipids (as in the case of sPLA₂-IB) or, alternatively, contribute to shaping of the gut microbiota. The latter possibility may help to explain the fact that Pla2g10^−/− mice display certain inflammatory, cardiovascular, and metabolic phenotypes that are not necessarily consistent among different laboratories [69,70,71,72].

sPLA₂-X is expressed constitutively in the airway epithelium and increased after antigen challenge in mice, and also in asthma patients. Pla2g10^−/− mice are protected from antigen-induced asthma, with marked reductions of airway hyperresponsiveness, eosinophil and T cell trafficking to the airways, airway occlusion, secretion of type-2 cytokines, generation of antigen-specific IgE, and synthesis of pulmonary eicosanoids including cysteinyl leukotrienes [73]. Further, Pla2g10^−/− mice have reduced IL-33 levels and fewer ILC2 cells in the lung, lower IL-33-induced IL-13 expression in mast cells, and a marked reduction in both the number of newly recruited macrophages and the M2 polarization of these macrophages in the lung [74]. These results indicate that sPLA₂-X serves as a key regulator of both innate and adaptive immune responses to allergens. Interestingly, as in the case of bee venom group III sPLA₂ that elicits strong type-2 immune responses, exogenous administration of sPLA₂-X serves as an adjuvant, leading to augmented type-2 immune responses with increased airway hypersensitivity and antigen-specific type-2 inflammation following peripheral sensitization and subsequent airway challenge with the antigen [75]. The biological roles of sPLA₂-X in asthma were detailed in a recent review [76].

3. The sPLA₂ Family

3.1. General Features

The cytosolic PLA₂ (cPLA₂) family comprises 6 isoforms (α–ζ), among which cPLA₂β, δ, ε, and ζ map to the same chromosomal locus [77]. There is structural similarity between the cPLA₂ and iPLA₂ families in that their catalytic domain is characterized by a three-layer α/β/α architecture employing a conserved Ser/Asp catalytic dyad [78,79]. It appears that these two families were evolved from a common ancestral gene, with the cPLA₂ family emerging from the iPLA₂ family at the branching point of vertebrates in correlation with the development of the lipid mediator signaling pathways. Enzymes belonging to the cPLA₂ family are characterized by the presence of a C2 domain at their N-terminal region, with the exception of cPLA₂γ in which this domain is absent. The C2 domain is responsible for the Ca²⁺-dependent association with membranes. Herein, we will overview several recent topics for cPLA₂α as well as potential functions of other cPLA₂ isoforms.

3.2. New Insights into cPLA₂α

cPLA₂α (group IVA PLA₂; encoded by PLA2G4A) is the best known PLA₂ that plays a central role in stimulus-coupled AA metabolism. cPLA₂α is the only PLA₂ that shows a striking substrate specificity for phospholipids containing AA (and also those containing EPA, if this ω3 PUFA is present in cell membranes at substantial levels). Upon cell activation, cPLA₂α translocates from the cytosol to the perinuclear (particularly Golgi) membranes in response to an increase in the µM range of cytosolic Ca²⁺ concentration, and is maximally activated by phosphorylation through mitogen-activated protein kinases and other kinases [80,81]. The phosphoinositide PIP₂ modulates the subcellular localization and activation of cPLA₂α [82]. The regulatory roles of cPLA₂α in eicosanoid generation in various pathophysiological events, as revealed by biochemical analyses as well as by studies using Pla2g4a^−/− mice, have been well summarized in several elegant reviews [83,84].

By means of comprehensive lipidomics, Slatter et al. [85] showed that human platelets acutely increase mitochondrial energy generation following stimulation and that the substrates for this, including multiple fatty acids and oxidized species that support energy generation via β-oxidation, are exclusively provided by cPLA₂α. This implies that cPLA₂α is a central regulator of both lipid mediator generation and energy flux in human platelets and that acute phospholipid membrane remodeling is required to support energy demands during platelet activation.

Ceramide-1-phosphate (C1P), a sphingolipid-derived bioactive lipid, directly binds to and activates cPLA₂α to stimulate the production of eicosanoids in vitro [86], but in vivo evidence for this event has been lacking. Recently, MacKnight et al. [87] addressed this issue by generating knockin mice in which endogenous cPLA₂α is replaced with a mutant form having an ablated C1P-interaction site. In a skin wound healing model, wound maturation, rather than wound closure, is enhanced in the mutant cPLA₂α-knockin mice compared to control mice. Primary dermal fibroblasts from the knockin mice show substantially increased collagen deposition and migration. The knockin mice also show an altered eicosanoid profile, with a reduction of PGE₂ and TXB₂ (a stable end-metabolite of TXA₂) as well as an increase of HETE species, which enhances the migration and collagen deposition of dermal fibroblasts. This gain-of-function role for the mutant cPLA₂α is associated with its relocalization to the cytoplasm and cytoplasmic vesicles. These findings clarify the key mechanisms by which wound healing is regulated by cPLA₂α-C1P interaction in vivo and provide insight into the roles of cPLA₂α and eicosanoids in orchestrating wound repair.

Chao et al. [88] reported that the C2 domain in cPLA₂α interacts with the CARD domain in mitochondrial antiviral signaling protein (MAVS), boosting NF-κB-driven transcriptional programs that promote experimental autoimmune encephalomyelitis, a model of multiple sclerosis. cPLA₂α recruitment to MAVS also disrupts MAVS-hexokinase 2 interactions, decreasing hexokinase activity and the production of lactate involved in the metabolic support of neurons. These findings define a novel role of cPLA₂α in driving pro-inflammatory astrocyte activities in cooperation with MAVS through protein–protein interaction in the context of neuroinflammation. It remains unknown whether some cPLA₂α-driven lipid mediators are involved in this process.

Oncogenic PIK3CA (encoding a PI3K isoform) results in an increase of AA and eicosanoids, thus promoting cell proliferation to beyond a cell-autonomous degree. Mechanistically, mutant PIK3CA drives a multimodal signaling network involving mTORC2-PKCζ-mediated activation of cPLA₂α [89]. Notably, inhibition of cPLA₂α acts synergistically with a fatty acid-free diet to restore immunogenicity and selectively reduce mutant PIK3CA-induced tumorigenicity. This reveals an important role for activated PI3K signaling in regulation of AA metabolism, highlighting a targetable metabolic vulnerability that depends largely on dietary fat restriction.

3.3. cPLA₂β Fusion Protein in Carcinoma Cell Proliferation and Survival

cPLA₂β (group IVB PLA₂; encoded by PLA2G4B) displays PLA₁, PLA₂ and more potent lysophospholipase activities in vitro. A kinetic study has demonstrated that cPLA₂β associates with a membrane surface that is rich in phosphoinositides when intracellular Ca²⁺ is low, whereas it moves to a cardiolipin-rich membrane such as the mitochondrial membrane when intracellular Ca²⁺ rises. Among three splice variants termed cPLA₂β1, β2 and β3, only the β3 form is identified as an endogenous protein and is constitutively associated with mitochondrial and early endosomal membranes [90].

Cheng et al. [91] reported that JMJD7-PLA2G4B, a read-through fusion gene formed by splicing of the neighboring JMJD7 (jumonji domain containing 7) and PLA2G4B genes, is expressed in human squamous cell carcinoma (HNSCC), as well as several other cancers. Ablation of JMJD7-PLA2G4B, but not JMJD7 or PLA2G4B alone, significantly inhibits proliferation of SCC cells by promoting G1 arrest and increases starvation-induced cell death. These findings provide a novel insight into the oncogenic control of JMJD7-PLA2G4B in HNSCC cell proliferation and survival and suggest that this fusion protein may serve as an important therapeutic target and prognostic marker for HNSCC development and progression. cPLA₂β has also been implicated in age-related changes in phospholipids and decreased energy metabolism in monocytes [92]. However, it remains unknown whether certain lipid metabolites generated by cPLA₂β would be involved in these events.

3.4. cPLA₂γ in Lipid Droplet Formation

Human cPLA₂γ (group IVC PLA₂; encoded by PLA2G4C), lacking the C2 domain characteristic of the cPLA₂ family, is C-terminally farnesylated, is tightly associated with membranes, and possesses lysophospholipase and transacylase activities in addition to PLA₂ activity [93]. Several lines of evidence suggest that cPLA₂γ acts mainly as a CoA-independent transacylase, transferring a fatty acid from one phospholipid to the other phospholipid, in cells [94,95]. cPLA₂γ is widely expressed in human tissues with a tendency for higher expression in the heart and skeletal muscle, whereas its expression in most mouse tissues is very low, making it difficult to address the in vivo roles of cPLA₂γ using a knockout strategy. Exceptionally, mouse cPLA₂γ is highly expressed in oocytes during the stage of germinal vesicle breakdown, when it dynamically relocates from the cortex to the nuclear envelope, suggesting its possible role in nuclear membrane remodeling in developing oocytes [96].

Lipid droplet (LD) accumulation in hepatocytes is a typical characteristic of steatosis. Hepatitis C virus (HCV) infection, one of the risk factors related to hepatic steatosis, induces LD accumulation in human hepatocytes. cPLA₂γ has been identified as a host factor upregulated by HCV infection and involved in HCV replication, where it promotes LD biogenesis and HCV assembly [97,98]. cPLA₂γ, through the domain around the amino acid residues 260–292, is tightly associated normally with ER membranes and relocated into LDs. Importantly, PLA2G4C knockdown hampers LD formation upon HCV stimulation, while PLA2G4C overexpression leads to LD formation in hepatocytes and enhances LD accumulation in the liver of mice fed a high-fat diet, suggesting its potential role in fatty liver disease.

3.5. cPLA₂δ in Psoriasis

cPLA₂δ (group IVD PLA₂; encoded by PLA2G4D) was first identified as a keratinocyte-specific cPLA₂ isoform that is induced during psoriasis and releases LA selectively [99]. However, subsequent studies showed that the PLA₂ activity of cPLA₂δ is much weaker than that of cPLA₂α [77] and that its PLA₁ activity is superior to its PLA₂ activity [90]. Cheung et al. [100] recently demonstrated the expression of cPLA₂δ in psoriatic mast cells, and found unexpectedly that its activity is extracellular. IFN-α-stimulated mast cells release exosomes, which transfer cytoplasmic cPLA₂δ to neighboring Langerhans cells expressing CD1a, which present lipid antigens to T cells. Thus, the exosome-mediated transfer of cPLA₂δ from mast cells to Langerhans cells leads to the generation of neolipid antigens and subsequent recognition by lipid-specific CD1a-reactive T cells, inducing the production of IL-22 and IL-17A. These data offer an alternative model of psoriasis pathogenesis in which lipid-specific CD1a-reactive T cells contribute to psoriatic inflammation, suggesting that cPLA₂δ inhibition or CD1a blockade may have potential for treatment of psoriasis. However, given that CD1a is present in humans but not in mice, and that cPLA₂δ is located mainly in epidermal keratinocytes, the regulatory roles of cPLA₂δ in psoriatic skin require further exploration.

3.6. cPLA₂ε as an N-Acyltransferase for N-Acylethanolamine Biosynthesis

N-acylethanolamines (NAEs) represent a group of endocannabinoid lipid mediators, including arachidonoylethanolamine (also known as anandamide; AEA), which acts on the endocannabinoid receptors CB1 or CB2, and palmitoyletanolamine (PEA) and oleoylethanolamine (OEA), which act on the nuclear receptor PPARα or through other mechanisms. The biosynthesis of NAEs, particularly PEA and OEA, occurs in two steps; transfer of sn-1 saturated or monounsaturated fatty acid of PC to the amino group of PE by N-acyltransferases to generate N-acyl-PE (NAPE), followed by hydrolysis mainly by NAPE-specific phospholipase D (NAPE-PLD) to give rise to NAEs [101]. There are two types of N-acyltransferase, i.e., Ca²⁺-dependent and -independent enzymes. Recently, it has been shown that cPLA₂ε (group IVE PLA₂; encoded by PLA2G4E) functions as a Ca²⁺-dependent N-acyltransferase [102]. In response to Ca²⁺ ionophore stimulation, HEK293 cells overexpressing cPLA₂ε produce various NAPE and NAE species, accompanied by concomitant decreases in PE and PC and increases in LPE and LPC, indicating that cPLA₂ε produces NAPEs by utilizing the diacyl- and plasmalogen types of PE as acyl acceptors and the diacyl types of PC and PE as acyl donors [103] (Figure 4A). The activity of cPLA₂ε is markedly enhanced by the presence of phosphatidylserine (PS) or other anionic phospholipids, and cPLA₂ε largely co-localizes with PS in the plasma membrane and organelles involved in the endocytic pathway [104,105]. This localization might be related to the observation that cPLA₂ε drives recycling through the clathrin-independent endocytic route [106].

However, it still remains to be determined whether cPLA₂ε indeed contributes to NAPE and NAE biosynthesis under certain in vivo conditions. Since genetic deletion or pharmacological inhibition of NAPE-PLD disturbs lipid metabolism in the liver, intestine, and adipose tissue [107], as well as emotional behavior [108], it is tempting to speculate that cPLA₂ε, which lies upstream of NAPE-PLD for NAE biosynthesis, may also participate in these events to improve metabolism and neuronal functions. The potential neuroprotective role of cPLA₂ε is supported by the association between reduced expression of cPLA₂ε and dementia, where adenoviral overexpression of cPLA₂ε in hippocampal neurons completely restores cognitive deficits in the elderly APP/PS1 mouse, a model of Alzheimer’s disease [109].

3.7. cPLA₂ζ in Myocardial Mitochondria

The observation that multiple fatty acids are non-selectively released from Ca²⁺-stimulated Pla2g4a^−/− lung fibroblasts, an event that is suppressed by cPLA₂α inhibitors, suggests that other cPLA₂ isoform(s) might contribute to this event. This fatty acid release and PGE₂ production by Pla2g4a^−/− fibroblasts depend on cPLA₂ζ (group IVF PLA₂, encoded by PLA2G4F) [110]. In response to ionomycin, cPLA₂ζ translocates to ruffles and dynamic vesicular structures, while cPLA₂α translocates to the Golgi and ER, suggesting distinct mechanisms of regulation for the two enzymes.

Moon et al. [111] reported that mitochondria isolated from human heart contain at least two PLA₂s—cPLA₂ζ and iPLA₂γ—of which cPLA₂ζ mediates Ca²⁺-activated release of AA from mitochondria in normal heart. The AA pool mobilized by cPLA₂ζ is preferentially channeled into cytochrome P450 epoxygenases for the synthesis of epoxyeicosatrienoic acids (EETs), which have a protective effect against heart failure. In contrast, in the failing heart, iPLA₂γ mainly mobilizes mitochondrial AA, which preferentially couples with lipoxygenases for the synthesis of toxic HETEs that open mitochondrial permeability transition pores, leading to further progression of heart failure. These results reveal an unexplored biological role of cPLA₂ζ as well as iPLA₂γ (see below) in the myocardium, although confirmation using Pla2g4f-null mice will be required.

4. The iPLA₂/PNPLA Family

4.1. General Features

The human genome encodes 9 Ca²⁺-independent PLA₂ (iPLA₂) enzymes. These enzymes are now more generally known as patatin-like phospholipase domain-containing lipases (PNPLA1-9), since all members in this family share a patatin domain, which was initially discovered in patatin (iPLA₂α), a potato protein [2,112]. Unlike the cPLA₂ family, which is present only in vertebrates, the iPLA₂/PNPLA family is widely expressed in many eukaryotes including yeast, ameba, nematode, fly and vertebrates. iPLA₂/PNPLA isoforms display lipid hydrolase or transacylase/acyltransferase activities with specificities for diverse lipids such as phospholipids, neutral lipids, sphingolipids, and retinol esters. In principle, enzymes with a large and unique N-terminal region (PNPLA6~9) act mainly on phospholipids as PLA₁, PLA₂ or lysophospholipases, whereas those lacking the N-terminal domain (PNPLA1~5) act on neutral lipids as lipases or transacylases. Analysis of gene disruption or mutation of the iPLA₂/PNPLA enzymes in mice and humans have provided valuable insights into their physiological roles in homeostatic lipid metabolism that are fundamental for life. Since there have been many excellent reviews on this enzyme family [2,3,112,113], we will herein highlight several recent topics that shed further light on the pathophysiological roles of this enzyme family.

4.2. iPLA₂β in Neurodegeneration and Hepatic Steatosis

iPLA₂β (also known as group VIA PLA₂; encoded by PLA2G6) is the only iPLA₂ isoform that acts primarily as a PLA₂ with poor fatty acid selectivity [114,115]. Different splice variants of iPLA₂β are associated with the plasma membrane, mitochondria, ER, and the nuclear envelope. Although iPLA₂β lacks a transmembrane domain, it has putative protein-interaction motifs such as ankyrin repeats, which are capable of interacting with multiple cognate receptor proteins, and a calmodulin-binding site, which interacts with the inhibitory calmodulin. The crystal structure of iPLA₂β reveals a dimer formation of the catalytic domains, which are surrounded by ankyrin repeats that adopt an outwardly flared orientation, poised to interact with membrane proteins [116]. The closely integrated active sites are positioned for cooperative activation and internal transacylation. The structure also suggests allosteric inhibition by calmodulin, where a single calmodulin molecule interacts with two catalytic domains, altering the conformation of the dimer interface and active sites.

The roles of iPLA₂β in male fertility, neuronal disorders, metabolic diseases, and inflammation, among others, have been studied in a number of Pla2g6 knockout, knockdown, and overexpression studies and are well summarized in recent reviews [113,117]. In particular, the roles of iPLA₂β in neuronal function have received extensive attention, since numerous mutations of the PLA2G6 gene have been discovered in patients with neurodegenerative disorders such as infantile neuroaxonal dystrophy (INAD) and Parkinson’s disease [118]. In fact, iPLA₂β is also referred to as the parkinsonism-associated protein PARK14, mutations of which are associated with impaired Ca²⁺ signaling in dopaminergic neurons [119].

Three recent studies using mutant flies have provided novel insights into the regulatory roles of iPLA₂β in the brain in the context of Parkinson’s disease with α-synucleinopathy. Kinghorn et al. [120] reported that knockout of the PLA2G6 gene in Drosophila results in reduced survival, locomotor deficits, and organismal hypersensitivity to oxidative stress, accompanied by mitochondrial abnormalities and increased lipid peroxidation levels. Inhibition of lipid peroxidation partially rescues the locomotor abnormalities and mitochondrial membrane potential caused by iPLA₂β deficiency. Lin et al. [121] showed that the loss of iPLA₂β causes an increase of brain ceramide, leading to lysosomal stress and neurodegeneration. iPLA₂β binds to the retromer subunits Vps35 and Vps26 and enhances their function to promote protein and lipid recycling. Loss of iPLA₂β impairs retromer function resulting in a progressive increase of ceramide, thus inducing a positive feedback loop that affects membrane fluidity and impairs retromer function and neuronal function. Mori et al. [122] showed that iPLA₂β deficiency in Drosophila results in defective neurotransmission during the early developmental stages and progressive cell loss throughout the brain, including degeneration of the dopaminergic neurons. In the brain, iPLA₂β loss results in shortening of the acyl-chain length of phospholipids, resulting in membrane lipid disequilibrium and thereby ER stress. Introduction of the mitochondria-ER contact site-resident protein C19orf12, another causal gene for Parkinson’s disease, in PLA2G6-deficient flies rescues the phenotypes associated with altered lipid composition, ER stress, and neurodegeneration. Moreover, the acceleration of α-synuclein aggregation by iPLA₂β deficiency is suppressed by administration of LA, which corrects the brain phospholipid composition. Thus, membrane remodeling by iPLA₂β is required for the survival of dopaminergic neurons and α-synuclein stability.

The roles of iPLA₂β in phospholipid remodeling in the context of hepatic lipid metabolism have recently been studied using Pla2g6^−/− mice. iPLA₂β deficiency attenuates obesity and hepatic steatosis in ob/ob mice through hepatic fatty-acyl phospholipid remodeling [123]. Aging sensitized by iPLA₂β deficiency induces liver fibrosis and intestinal atrophy involving suppression of homeostatic genes and alteration of intestinal lipids and bile acids [124]. Pla2g6^−/− mice fed a high-fat diet show attenuation of hepatic steatosis through correction of defective phospholipid remodeling [125]. Lastly, Pla2g6^−/− mice fed a methionine/choline-deficient diet do not show correction of this defect, but the hepatocellular injury is attenuated via inhibition of lipid uptake genes [126].

4.3. iPLA₂γ in Bioenergetics and Signaling

iPLA₂γ (also known as group VIB PLA₂, encoded by PNPLA8) displays PLA₂ activity, but acts as a PLA₁ toward phospholipids bearing sn-2 PUFA [127,128]. Accordingly, hydrolysis of PUFA-bearing phospholipids by iPLA₂γ typically gives rise to 2-lysophospholipids (having a PUFA at the sn-2 position) rather than 1-lysophospholipids (having a saturated or monounsaturated acid at the sn-1 position). Pnpla8^−/− mice display multiple bioenergetic and neuronal dysfunctions, including growth retardation, kyphosis, muscle weakness with atrophy of myofilaments, cold intolerance, reduced exercise endurance, increased mortality due to cardiac stress, resistance to diet-induced obesity and insulin resistance, performance deficits in spatial learning and memory, and abnormal mitochondrial function with a dramatic decrease in fatty acid β-oxidation and oxygen consumption [129]. The alterations in myocardial and hippocampal cardiolipin content and composition indicate that iPLA₂γ is involved in cardiolipin remodeling. These features of Pnpla8^−/− mice are reminiscent of those seen in patients with Barth syndrome, a disease caused by mutations in the human TAZ gene, which encodes tafazzin, a mitochondrial transacylase required for cardiolipin remodeling [130]. Moreover, loss-of-function variants of the human PNPLA8 gene recapitulate the mitochondriopathy observed in Pnpla8^−/− mice [131].

iPLA₂γ also participates in the generation of lipid mediators under certain conditions. Pnpla8^−/− mice display reduced ADP- or collagen-stimulated TXA₂ generation by platelets ex vivo and show prolonged bleeding time and reduced pulmonary thromboembolism in vivo [132]. Cardiomyocyte-specific deletion of iPLA₂γ decreases infarct size upon ischemia/reperfusion, with reduction of oxygenated metabolites of ω3 and ω6 PUFAs including PGs, HETEs, and hydroxy-DHAs [133]. iPLA₂γ releases 9-hydroxyoctadecenoic acid (9-HODE), an oxygenated metabolite of LA, from cardiolipin, integrating mitochondrial bioenergetics and signaling [134]. Heart failure-induced activation of iPLA₂γ leads to mitochondrial generation of HETEs that open the mitochondrial permeability transition pore, thus further amplifying myocardial damage [111]. Phospholipids bearing sn-2 AA are cleaved by the PLA₁ activity of iPLA₂γ to give rise to 2-arachidonoyl-lysophospholipids, which are then oxygenized directly by cyclooxygenase-2 or 12-lipoxygenase for conversion to eicosanoid-esterified lysophospholipids [134,135]. The generation of eicosanoid-esterified lysophospholipids is attenuated by the absence of iPLA₂γ, underscoring an iPLA₂γ-initiated pathway generating new classes of lipid metabolites with potential signaling functions.

4.4. PNPNA6 and PNPLA7 as Lysophospholipases

PNPLA6 (iPLA₂δ) and its closest paralog PNPLA7 (iPLA₂θ) have a lysophospholipase activity that cleaves LPC to yield fatty acid and glycerophosphocholine (GPC) [136,137]. Counterparts of these enzymes in yeast and fly act as a phospholipase B, which converts PC to GPC by liberating both sn-1 and sn-2 fatty acids. PNPLA6, also referred to as neuropathy target esterase (NTE), was originally identified as a target enzyme for the poisonous effect of organophosphates, which cause a severe neurological disorder characterized by degeneration of long axons in the spinal cord and peripheral nerves, leading to paralysis of the lower limbs [138]. In cultured renal cells, the production of GPC, an osmoprotective metabolite, is enhanced by PNPLA6 overexpression and is diminished by its siRNA knockdown or its inhibitor organophosphate [139]. Global Pnpla6^−/− mice die in utero due to placental defects, while neuron-specific Pnpla6^−/− mice exhibit progressive neuronal degeneration, leading to prominent neuronal pathology in the hippocampus and thalamus and also defects in the cerebellum [140,141]. Neuronal absence of PNPLA6 results in disruption of the ER, vacuolation of nerve cell bodies and abnormal reticular aggregates, and sustained elevation of PC over many months is accompanied by progressive degeneration and massive swelling of axons in the sensory and motor spinal tracts and worsening hindlimb dysfunction [142]. In humans, PNPLA6 mutations near the catalytic site cause a severe motor neuron disease characterized by progressive spastic paraplegia and distal muscle wasting [143] as well as childhood blindness with retinal degeneration, including Leber congenital amaurosis, Oliver McFarlane syndrome, and Boucher-Neuhäuser syndrome [144]. Although the in vivo role of PNPLA7, also known as NTE-related esterase (NRE), is still unknown, it is downregulated by insulin in WAT [136] and interacts with LDs through its catalytic domain [145], suggesting its metabolic role.

4.5. PNPLA2 and PNPLA3 in Triglyceride Metabolism

Although PNPLA2 and PNPLA3 correspond to iPLA₂ζ and iPLA₂ε, respectively, according to the PLA₂ classification, the latter names are not used here because these enzymes act essentially as neutral lipid lipases, but not as phospholipases. PNPLA2 is upregulated, while PNPLA3 is downregulated, upon starvation, and vice versa upon feeding, indicating that these two closely related lipases are nutritionally regulated in a reciprocal way [146]. PNPLA2, more generally known as adipose triglyceride lipase (ATGL), is a major lipase that hydrolyzes triglycerides in LDs to release fatty acids for β-oxidation-coupled energy production, a process known as lipolysis [147]. Genetic deletion or mutation of PNPLA2 leads to accumulation of triglycerides in multiple tissues leading to heart failure, while conferring protection from fatty liver and glucose intolerance, likely because these mice are able to utilize glucose but not free fatty acids as a fuel [148,149]. PNPLA2 deficiency also protects against cancer-associated cachexia by preventing fat loss [150]. The fatty acids released from LDs by PNPLA2 act as endogenous ligands for the nuclear receptor PPARα or PPARδ, which drives energy consumption [151,152]. In addition, the AA released from triglycerides by PNPLA2-driven lipolysis is utilized for eicosanoid generation in certain situations [153]. The activity of PNPLA2 is regulated positively by ABHD5 (also known as CGI-58) and negatively by perilipin, G0S2 and mysterin, which modulate the accessibility of PNPLA2 to LDs [152,154]. Adipocyte-specific G0S2-transgenic mice show attenuated lipolysis and adipocyte hypertrophy, accompanied by a reduced hepatic triglyceride level and increased insulin sensitivity [155], thus recapitulating the phenotypes observed in Pnpla2^−/− mice [149]. Mutations in the human PNPLA2 gene cause Chanarin–Dorfman syndrome, a condition in which triglycerides are stored abnormally in the body [156]. ABHD5 mutations also cause a similar neutral lipid storage disease but also additionally cause ichthyosis [157], likely because ABHD5 acts as a cofactor for not only PNPLA2-mediated lipolysis, but also PNPLA1-driven ω-O-acylceramide synthesis in the skin (see below). The regulatory mechanisms and metabolic roles of PNPLA2 have been detailed in other elegant reviews [158,159].

Mutations in the human PNPLA3 gene are highly associated with non-alcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH) [160]. However, Pnpla3^−/− mice do not display a fatty liver phenotype [161], whereas PNPLA3^I158M knockin mice develop hepatic steatosis [162], illustrating the complexity of the regulatory roles of PNPLA3 in hepatic lipid metabolism. PNPLA3 reportedly acts as an acyltransferase that alters the fatty acid composition of triglycerides [163], as a transacylase that promotes the transfer of PUFAs from triglycerides to phospholipids in hepatic LDs [164], or as a retinyl-palmitate lipase in hepatic stellate cells to fine-tune the plasma levels of retinoids, which might influence the differentiation of stellate cells to myofibroblasts [165]. It has now become recognized that PNPLA3 functions primarily as a triglyceride lipase and that I158M mutation leads to loss of function. Importantly, PNPLA3, and PNPLA3^I158M to an even greater extent, strongly interact with ABHD5, a cofactor for PNPLA2, thereby interfering with the lipolytic activity of PNPLA2 [166,167]. Overexpression of PNPLA3^I158M greatly suppresses PNPLA2-dependent lipolysis, leading to massive triglyceride accumulation in hepatocytes and brown adipocytes. Moreover, transgenic overexpression of PNPLA3^I158M increases hepatic triglyceride levels in WT mice, but not in Abhd5^−/− mice, confirming that the pro-steatotic effects of PNPLA3 require the presence of ABHD5. Thus, the increased abundance of PNPLA3^I148M results in sequestration of ABHD5 on LDs, thereby limiting the availability of this cofactor for activation of PNPLA2. The question remains of how PNPLA2, PNPLA3, and ABHD5 each find their way to hepatocyte LDs in the correct stoichiometric proportions and function to regulate LD assembly and turnover under conditions of fasting or nutrient excess.

4.6. PNPLA1 in Acylceramide Synthesis for Skin Barrier Function

ω-O-acylceramide, a unique sphingolipid present specifically in the stratum corneum of the epidermis, is essential for skin barrier formation, and impairment of its biosynthesis leads to ichthyosis or atopic dermatitis. Mutations in the human PNPLA1 gene cause autosomal recessive congenital ichthyosis [168]. Unlike most PNPLA isoforms that are ubiquitously expressed in many tissues, PNPLA1 is localized predominantly in the upper layer of the epidermis. PNPLA1 acts as a unique transacylase, catalyzing the transfer of LA in triglyceride to the ω-hydroxy group of ultra-long-chain fatty acid in ceramide to give rise to ω-O-acylceramide [169,170,171] (Figure 4B). Global or keratinocyte-specific deletion of PNPLA1 hampers epidermal ω-O-acylceramide formation, thereby severely impairing skin barrier function leading to neonatal death due to excessive dehydration. The enzymatic activity of PNPLA1 is enhanced by ABHD5, which may present triglycerides to PNPLA1 to facilitate substrate recognition [172,173], providing a mechanism whereby ABHD5 mutations cause Chanarin-Dorfman syndrome accompanied by ichthyosis with impaired ω-O-acylceramide formation. The role of PNPLA1 in ω-O-acylceramide biosynthesis has been described in detail in a recent review [174].

5. Other PLA₂ Family

5.1. The PAF-AH Family

The PAF-acetylhydrolase (PAF-AH) family comprises one extracellular and three intracellular enzymes that were originally found to have the capacity to deacetylate and thereby inactivate the lysophospholipid-derived lipid mediator PAF [175,176]. Plasma-type PAF-AH (group VIIA PLA₂; encoded by PLA2G7) is a secreted protein produced by macrophages, mast cells or other sources, and is now more generally referred to as lipoprotein-associated PLA₂ (Lp-PLA₂), existing as a low-density lipoprotein (LDL)-bound form in human plasma [177]. Although PAF is a potent mediator of allergic responses, Lp-PLA₂ deficiency fails to augment airway inflammation or hyperresponsiveness after PAF+LPS treatment or passive or active allergic sensitization and challenge [178]. A series of clinical studies have revealed a correlation of Lp-PLA₂ with atherosclerosis, likely because this enzyme hydrolyzes oxidized phospholipids (i.e., phospholipids having an oxidized fatty acid at the sn-2 position) in modified LDL with pro-atherogenic potential [179,180]. Although darapladib, a potent Lp-PLA₂ inhibitor, failed to meet the primary endpoints of two large phase III trials for treatment of atherosclerosis [181], recent clinical and preclinical studies have revealed that Lp-PLA₂ inhibition may have therapeutic effects in diabetic macular edema and Alzheimer’s disease [182,183]. Lp-PLA₂ deficiency in Apc^Min/+ mice leads to decreased intestinal polyposis and tumorigenesis, suggesting a role of PAF or some oxidized lipids in cancer development [184].

Type-I PAF-AH is a heterotrimer composed of two catalytic α1 and α2 subunits (group XIIIA and XIIIB PLA₂s, encoded by PAFAH1B2 and PAFAH1B3, respectively), and a regulatory β subunit that is identical to LIS-1, a causative gene for a type of Miller-Dieker syndrome [185]. Loss of both the α1 and α2 catalytic subunits leads to male infertility [186], reduction of amyloid-β generation by promoting the degradation of amyloid precursor protein C-terminal fragments [187], and an increase in the size of the ganglionic eminences resulting from increased proliferation of GABAergic neurons through perturbation of the Wnt signaling pathway [188].

Type-II PAF-AH (PAF-AH2 or group VIIB PLA₂; encoded by PAFAH2) shows significant homology with Lp-PLA₂ and preferentially hydrolyzes oxidized phospholipids in cells. In a CCl₄-induced liver injury model, Pafah2^−/− mice show a delay in hepatic injury recovery with unusual accumulation of 8-isoprostaglandin F_2α (8-iso-PGF_2α) in membrane phospholipids, indicating that PAF-AH2 removes toxic oxidized lipids from cell membranes and thereby protects the tissue from oxidative stress-induced injury [189]. Mast cells spontaneously produce ω3 PUFA-derived epoxides (ω3 epoxides), such as EPA-derived 17,18-epoxyeicosatetraenoic acid (17,18-EpETE) and DHA-derived 19,20-epoxydocosapentaenoic acid (19,20-EpDPE), whose production depends on PAF-AH2-driven cleavage of ω3 epoxide-esterified phospholipids in mast cell membranes [190]. Genetic or pharmacological inactivation of PAF-AH2 reduces the steady-state production of ω3 epoxides, leading to attenuated mast cell activation and anaphylaxis following FcεRI crosslinking (Figure 5). Mechanistically, the ω3 epoxides promote IgE-mediated activation of mast cells by down-regulating Srcin1, a Src-inhibitory protein that counteracts FcεRI signaling, through a pathway involving PPARγ. Thus, the PAF-AH2–ω3 epoxide pathway ensures optimal mast cell activation and presents new potential drug targets for allergic diseases. The properties and functions of the PAF-AH family have been summarized in a recent review [191].

5.2. Lysosomal PLA₂s

Lysosomal PLA₂ (LPLA₂ or group XV PLA_2; encoded by PLA2G15) is catalytically active under mildly acidic conditions and structurally homologous with lecithin:cholesterol acyltransferase (LCAT), an enzyme that transfers the sn-2 fatty acid of PC to cholesterol to produce cholesteryl ester in high-density lipoprotein (HDL) [192]. LPLA₂ hydrolyzes both sn-1 and sn-2 fatty acids in phospholipids and contributes to phospholipid degradation in lysosomes. Genetic deletion of LPLA₂ results in unusual accumulation of non-degraded lung surfactant phospholipids in lysosomes of alveolar macrophages leading to phospholipidosis [193], reduced presentation of lysophospholipid antigens to CD1d by invariant natural killer T (iNKT) cells [194], and impairment of adaptive T cell immunity against mycobacterium [195].

Peroxiredoxin 6 (Prdx6) is another lysosomal PLA₂ that is also called acidic Ca²⁺-independent PLA₂ (aiPLA₂). Prdx6 is a multifunctional enzyme since it also possesses glutathione peroxidase and lysophospholipid acyltransferase activities. The aiPLA₂ activity of Prdx6 has important physiological roles in the turnover (synthesis and degradation) of lung surfactant phospholipids [196], protection against LPS-induced lung injury [197], repair of peroxidized cell membranes [198], and sperm fertilizing competence [199]. The properties and functions of LPLA₂ and Prdx6/aiPLA₂ have been summarized in recent reviews [200,201].

5.3. The PLAAT Family

The PLAAT family, which comprises 3 enzymes in mice and 5 enzymes in humans, is structurally similar to lecithin:retinol acyltransferase (LRAT). Because of the ability of some members in this family to suppress H-ras signaling, it is also referred to as the HRASLS (for H-ras-like suppressor) family. Members of this family, including PLA2G16 (group XVI PLA₂, also known as PLAAT3 or HRASLS3), display PLA₁ and PLA₂ activities, as well as Ca²⁺-independent N-acyltransferase activity that synthesizes NAPE, to various degrees [202]. PLA2G16 is highly expressed in adipocytes, and Pla2g16^−/− mice are resistant to diet-induced obesity [203]. Adipocyte-derived LPC produced by PLA2G16 activates NLRP3 inflammasomes in adipocytes and adipose tissue macrophages mediating homocysteine-induced insulin resistance [204]. PLA2G16 and its paralogs in this family have also been implicated in tumor invasion and metastasis [205], vitamin A metabolism [206], peroxisome biogenesis [207], and cellular entry and clearance of Picornaviruses [208]. The properties and functions of the PLAAT/PLA2G16 family have been summarized in recent reviews [209,210].

5.4. The ABHD Family

The ABHD family is a newly recognized group of lipolytic enzymes, comprising at least 19 enzymes in humans [211]. Enzymes in this family typically possess both hydrolase and acyltransferase motifs. Although the functions of many of the ABHD isoforms still remain uncertain, some of them have been demonstrated to act on neutral lipids or phospholipids as lipid hydrolases. ABHD3 selectively hydrolyzes phospholipids with medium-chain fatty acids [212]. ABHD4 releases fatty acids from multiple classes of N-acyl-phospholipids to produce N-acyl-lysophospholipids [213]. ABHD6 acts as lysophospholipase or monoacylglycerol lipase, the latter being possibly related to signaling of 2-arachidonoyl glycerol, an endocannabinoid lipid mediator that plays a role in retrograde neurotransmission [214]. ABHD12 hydrolyzes lysophosphatidylserine (LysoPS), and is therefore referred to as LysoPS lipase [215]. Genetic or pharmacological blockade of ABHD12 stimulates immune responses in vivo, pointing to a key role for this enzyme in regulating immunostimulatory lipid pathways. Mutations in the human ABHD12 gene result in accumulation of LysoPS in the brain and cause a disease known as PHARC, which is characterized by polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract [216]. ABHD16A acts as a PS-selective PLA₂ (referred to as PS lipase), lying upstream of ABHD12 in the PS-catabolic pathway [217]. Disruption of ABHD12 and ABHD16A in macrophages respectively increases and decreases LysoPS levels and cytokine production. Although ABHD5 does not have catalytic activity because of the absence of a serine residue in the catalytic center, it interacts with and modifies the functions of several PNPLA members including PNPLA1-3, as described above.

5.5. GPI-Specific PLA₂s

GPI is a complex glycolipid covalently linked to the C terminus of proteins on the plasma membrane, particularly in the raft microdomain. The biosynthesis of GPI and its attachment to proteins occur in the ER. GPI-anchoring proteins (GPI-APs) are subjected to fatty acid remodeling, which replaces an unsaturated fatty acid at the sn-2 position of the PI moiety with a saturated fatty acid. PGAP3, which resides in the Golgi, and PGAP6, which is localized mainly on the cell surface, are GPI-specific PLA₂s (GPI-PLA₂s) involved in fatty acid remodeling of GPI-APs [218]. PGAP3-dependent fatty acid remodeling of GPI-APs has a significant role in the control of autoimmunity, possibly by the regulation of apoptotic cell clearance and Th1/Th2 balance [219]. CRIPTO, a GPI-AP that plays critical roles in early embryonic development by acting as a Nodal co-receptor, is a highly sensitive substrate of PGAP6. CRIPTO is released by PGAP6 as a glycosyllysophosphatidylinositol-bound form and acts as a co-receptor in Nodal signaling. Pgap6^−/− mice show defects in early embryonic development, particularly in the formation of the anterior-posterior axis, as do Cripto^−/− embryos, suggesting that PGAP6 plays a critical role in Nodal signaling modulation through CRIPTO shedding [220].

6. Conclusions

In this review, we have provided an overview of the biological functions of a nearly full set of PLA₂s identified to date, particularly over the past five years during which considerable advances in this research field have been made. In the interests of brevity, we have referenced previous reviews whenever possible and apologize to the authors of the numerous original papers that were not explicitly cited. By applying lipidomics approaches to knockout or transgenic mice for various PLA₂s, it has become evident that individual enzymes regulate specific forms of lipid metabolism, perturbation of which can be eventually linked to distinct pathophysiological outcomes. Knowledge of individual PLA₂s acquired from studies using animal models is now being translated to humans. Note that the designation “PLA₂” is used more broadly for enzymes that have significant homology with prototypic PLA₂ subtypes, even if they do not necessarily exert PLA₂ activity. These examples include, for instance, N-acyltransferase (cPLA₂ε), ω-O-acylceramide synthase (PNPLA1), triglyceride lipase (PNPLA2 and PNPLA3), and lysophospholipase (PNPLA6 and PNPLA7). Several PLA₂s, such as cPLA₂δ, iPLA₂γ, and PLA2G16, possess PLA₁ activity that is even superior to PLA₂ activity, although biological importance of the sn-1 cleavage by these PLA₂s is largely unclear. There are also several enzymes that possess PLA₂ activity but are not designated as PLA₂, as exemplified by the ABHD and GPI-specific PLA₂ families. Thus, the term “PLA₂” is somehow confusing and should be interpreted with caution if researchers encounter the genetic symbol “PLA2” on genome-wide transcriptome and proteome analyses.

Lipid metabolic pathways are complex, often redundant and highly interconnected, in some cases being counter regulatory. As recently reviewed by Kokotos and coworkers [221], various PLA₂ subtypes are being targeted pharmacologically to alleviate the symptoms of various disease models, but none of the PLA₂ inhibitors currently developed has reached the market yet. If the modulation of one PLA₂ pathway does not suffice therapeutically, targeting two or more pathways could be effective. On the other hand, blunting of related PLA₂ subtypes simultaneously may not be therapeutically efficient in some cases, since this strategy would block both positive and negative pathways regulated by distinct isoforms. For instance, pan-sPLA₂ inhibitors (e.g., varespladib), which inhibit group I/II/V/X sPLA₂s altogether, have been reported to exert a poor therapeutic effect on atherosclerosis, arthritis, and allergy, probably because they blunt both offensive and defensive sPLA₂s. This points to potential prophylactic or therapeutic use of an agent that could specifically inhibit a particular PLA₂ subtype, even though such a strategy will be a challenge since individual PLA₂s appear to play highly selective and often opposite roles in specific organs and disease states.

At present, the tools that are available for following the dynamics of individual PLA₂s and associated lipid metabolites and for monitoring their precise modifications and spatiotemporal localizations are still technically limited. The actions of lipids are frequently masked by the large steady-state mass of structural lipids in membranes, making it difficult to detect spatiotemporal lipid dynamics and functions. Further advances in this field and their integration for therapeutic use are likely to benefit from improved, time- and space-resolved lipidomics technology for monitoring individual PLA₂s and associated lipid metabolisms within tissue microenvironments. It seems that more work will be necessary to dissect each of the many regulated pathways of bioactive lipids and to define the mechanisms responsible for the regulatory actions of individual PLA₂s, as well as the roles of the pathways in specific responses at the cell, tissue, and organism level. Hopefully, the next decade will yield a more integrated view of the overall map of the PLA₂ network in biology, allowing the therapeutic application of inhibitors or the lipid products of some enzymes to human diseases.