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The Roles of Phospholipase A2 in Phagocytes

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The Roles of Phospholipase A2 in Phagocytes fcell-09-673502 June 5, 2021 Time: 16:58 # 1 REVIEW published: 10 June 2021 doi: 10.3389/fcell.2021.673502 The Roles of Phospholipase A2 in Phagocytes Deepti Dabral* and Geert van den Bogaart* Department of Molecular Immunology and Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Groningen, Netherlands Phagocytic cells, such as macrophages, neutrophils, and dendritic cells, ingest particles larger than about 0.5 mM and thereby clear microbial pathogens and malignant cells from the body. These phagocytic cargoes are proteolytically degraded within the lumen of phagosomes, and peptides derived from them are presented on Major Histocompatibility Complexes (MHC) for the activation of T cells. Mammalian PLA2 isozymes belong to a large family of enzymes that cleave phospholipids at the second position of the glycerol backbone, releasing a free fatty acid and a lysolipid moiety. In human macrophages, at least 15 different PLA2 forms are expressed, and expression of many of these is dependent on pathogenic stimulation. Intriguing questions are why Edited by: so many PLA2 forms are expressed in macrophages, and what are the functional James T. Murray, consequences of their altered gene expression after encountering pathogenic stimuli. In Swansea University Medical School, this review, we discuss the evidence of the differential roles of different forms of PLA in United Kingdom 2 phagocytic immune cells. These roles include: lipid signaling for immune cell activation, Reviewed by: Scott Kobayashi, initial phagocytic particle uptake, microbial action for the killing and degradation of Rocky Mountain Laboratories (NIAID), ingested microbes, and the repair of membranes induced by oxygen radicals. We also United States Anetta Härtlova, discuss the roles of PLA2 in the subsequent digestion of ingested phagocytic cargoes University of Gothenburg, Sweden for antigen presentation to T cells. *Correspondence: Keywords: phagocytosis, PLA , pathogens, macrophages, trafficking Deepti Dabral 2 [email protected] Geert van den Bogaart [email protected] INTRODUCTION Specialty section: A role for phospholipase A2 (PLA2) in phagocytic immune cells has been suggested since the 1980s This article was submitted to (Waite et al., 1979; Scott et al., 1980). These phagocytic cells, such as macrophages, neutrophils, Signaling, and dendritic cells, ingest particles larger than about 0.5 mM and thereby clear microbial a section of the journal pathogens and malignant cells from the body (Uribe-Querol and Rosales, 2020). Particularly Frontiers in Cell and Developmental macrophages and dendritic cells also present peptides derived from these phagocytosed antigens Biology on major histocompatibility complex (MHC) types I and II to activate cytolytic and helper T cells, Received: 27 February 2021 respectively, while lipids can be presented on CD1 (Burgdorf and Kurts, 2008). The process of Accepted: 11 May 2021 phagocytosis and the subsequent processing of the ingested antigen are essential processes that Published: 10 June 2021 contribute to both innate and adaptive immunity. As we will discuss in this review, evidence Citation: suggests that PLA2 is involved in all stages of pathogen encounter, from the initial signaling, to Dabral D and van den Bogaart G the uptake, degradation and presentation of the antigen. (2021) The Roles of Phospholipase A2 in Phagocytes. The nomenclature of PLA2 is very complex and disordered as it follows a chronology reflecting Front. Cell Dev. Biol. 9:673502. their time-line of discovery (Dennis et al., 2011; Murakami et al., 2015; Ramanadham et al., 2015). doi: 10.3389/fcell.2021.673502 Mammalian PLA2 isozymes belong to a large family, which have been assigned into groups I to Frontiers in Cell and Developmental Biology| www.frontiersin.org 1 June 2021| Volume 9| Article 673502 fcell-09-673502 June 5, 2021 Time: 16:58 # 2 Dabral and van den Bogaart PLA2 in Phagocytic Immune Cells XVI based on their primary amino acid sequence (Dennis et al., signaling by lipids carried out by cPLA2a, (ii) the focal exocytosis 2011; Murakami et al., 2015; Ramanadham et al., 2015). Based on to support phagosome formation by iPLA2b, (iii) the killing of 2C their subcellular localization and Ca requirement, these groups the microbe by sPLA2-V, and LPLA2 forms, and (iv) the repair can be further categorized into six types called sPLA2, iPLA2, of damaged phagosomal membranes inflicted by reactive oxygen cPLA2, LPLA2, aiPLA2, and PAF-AH, and all these types are species by LPLA2 and aiPLA2. In this review, we aim is to provide 2C further categorized into subtypes (Table 1) (i) sPLA2 are Ca - an overview of these functions of PLA2 forms. dependent secretory PLA2 forms found in secretions, such as a tears, plasma and pancreatic juice (Dennis et al., 2011; Murakami et al., 2015; Ramanadham et al., 2015). These enzymes belong LIPID SIGNALING BY cPLA2a to groups I, II, III, V, IX, X, XI, XII, XIII, and XIV (Dennis et al., 2011; Murakami et al., 2015; Ramanadham et al., 2015). Phospholipase A2 enzymes hydrolyze membrane phospholipids Within the sPLA2 subgroup, a capital English letter indicates that are composed of a glycerol backbone esterified to two the subtype of enzymes. For example, sPLA2-IIA is present in hydrophobic fatty acids tails at the sn- (stereospecifically synovial fluid, while sPLA2-IID is present in pancreas and spleen numbered) 1 and 2 positions, and a hydrophilic head group 2C (Dennis et al., 2011). (ii) cPLA2 are Ca -dependent cytosolic at the sn-3 position (Figure 2). Phospholipids with the head enzymes which belong to group IV, and its subtypes are cPLA2a, groups choline, ethanolamine, serine, and inositol form the main -b,-g,-d,-+, and -z (Dennis et al., 2011; Murakami et al., 2015; classes of phospholipids and are called phosphatidylcholine 2C Ramanadham et al., 2015). (iii) iPLA2 are Ca -independent (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), cytosolic forms which belong to group VI, and its subtypes are and phosphatidylinositol (PI), respectively. The catalytic action iPLA2-b,-g,-d,-+,-z, and -! (Dennis et al., 2011; Murakami et al., of PLA2 releases the free fatty acid from the sn-2 position, 2015; Ramanadham et al., 2015). Within the cPLA2 and iPLA2 such as arachidonic acid (ARA; 20:4), docosahexaenoic acid types, the different subtypes are mostly indicated by Greek letters (C22:6), oleic acid (C18:1), while lysophospholipids, such as lyso- (Dennis et al., 2011; Murakami et al., 2015; Ramanadham et al., phosphatidyl-choline/-ethanolamine/-inositol (LPC/LPE/LPI), 2015). However, this nomenclature is not always consistently remain esterified in the membrane. used and for example cPLA2-IVa and iPLA2-VIb are also known Both the lyso-phospholipids and the free fatty acids produced as cPLA2-IVA and iPLA2-VIA. (iv) LPLA2 is a lysosomal PLA2 by PLA2 can be bioactive molecules and/or form precursors which belongs to group XV (Shayman et al., 2011; Shayman for the generation of bioactive lipid hormones (Brash, 2001; 2C and Tesmer, 2019). (v) aiPLA2 are acidic Ca -independent Carneiro et al., 2013). PLA2 activity is especially important for PLA2 forms which belong to group XVI, and are better known the generation of a class of lipid hormones called eicosanoids. as peroxisomal PLA2 (Sorokina et al., 2009; Fisher, 2018). (vi) Eicosanoids are a broad family of oxygenated lipid compounds platelet-activating factor hydrolases (PAFAH) belong to groups that include prostaglandins, thromboxanes, and leukotrienes. VIIA (also known as lipoprotein-associated PLA2I Lp-PLA2), Eicosanoids are generated via non-enzymatic oxidation of ARA VIIB and VIII (Stafforini, 2009; Dennis et al., 2011). or by the action of enzymes, such as cyclooxygenase (COX), In human macrophages, 15 PLA2 forms are found at the lipooxygenase (LOX), and cytochrome P450 (CYP) (Dennis transcript level, including sPLA2 (sPLA2-IID, -V, and -XIIA), and Norris, 2015; Gil-de-Gómez et al., 2020). Eicosanoids exert cPLA2 (cPLA2-IV a, b, and g), iPLA2 (iPLA2-VI b, g, d, +, z, and immunomodulatory functions and, depending on the species !), LPLA2, LpPLA2, and aiPLA2 (Elstad et al., 1989; Rubio et al., of eicosanoids, can have pro- or anti-inflammatory effects 2015). Of these forms, LPLA2 and LpPLA2 carry a signal peptide (Dennis and Norris, 2015). Upon encountering a pathogen, required for cotranslational insertion into the ER and subsequent immune phagocytes produce elevated levels of eicosanoids and transport through the Golgi where they undergo N-glycosylation, an increased amount of eicosanoids in circulation is considered while others are either cytosolic or membrane bound proteins a hallmark of inflammation (Dennis and Norris, 2015). Non- (Dennis et al., 2011; Murakami et al., 2015; Ramanadham et al., steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, 2015). Analysis of published gene expression data of blood indomethacin, and asprin, target COX and PLA2 to alleviate pain, monocyte-derived dendritic cells from 38 healthy individuals redness, and swelling associated with inflammation (Singh et al., (Lee et al., 2014) revealed that the expression of some of these 2005; Dennis and Norris, 2015). PLA2 forms is dependent on pathogenic stimulation (Figure 1). Mammalian cells are rich in ARA, but this is mostly For example, the expression of cPLA2-IVa and -IVg are incorporated in phospholipids by esterification to the glycerol upregulated, while sPLA2-XIIA and PAFAHII are downregulated backbone at the sn-2 position. For example,
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