Lipoquality control by phospholipase A2 enzymes

Abstract

The phospholipase A₂ (PLA₂) family comprises a group of lipolytic enzymes that typically hydrolyze the sn-2 position of glycerophospholipids to give rise to fatty acids and lysophospholipids. The mammalian genome encodes more than 50 PLA₂s or related enzymes, which are classified into several subfamilies on the basis of their structures and functions. From a general viewpoint, the PLA₂ family has mainly been implicated in signal transduction, producing bioactive lipid mediators derived from fatty acids and lysophospholipids. Recent evidence indicates that PLA₂s also contribute to phospholipid remodeling for membrane homeostasis or energy production for fatty acid β-oxidation. Accordingly, PLA₂ enzymes can be regarded as one of the key regulators of the quality of lipids, which I herein refer to as lipoquality. Disturbance of PLA₂-regulated lipoquality hampers tissue and cellular homeostasis and can be linked to various diseases. Here I overview the current state of understanding of the classification, enzymatic properties, and physiological functions of the PLA₂ family.

Keywords: fatty acid; lipid; lipidomics; membrane; phospholipase; phospholipid.

Figure 1.

The eicosanoid-biosynthetic pathway (AA metabolism). The AA released by PLA₂ from cellular membrane is metabolized to various eicosanoids through the COX and LOX pathways. Structures and representative bioactivities of individual eicosanoids and their biosynthetic enzymes are shown. H- and L-PGDS, hematopoietic and lipocalin-type PGD₂ synthases, respectively; PGFS, PGF_2α synthase, PGIS, PGI₂ synthase; mPGES-1, microsomal PGE₂ synthase-1; TXS, TX synthase; 12-HHT, 12-hydroxyheptadecatrenoic acid; 12-HETE, 12-hydroxyeicosatetraenoic acid; FLAP, 5-LOX-activating protein; LTA₄H, LTA₄ hydrolase; LTC₄S, LTC₄ synthase.

Figure 2.

Lysophospholipid-derived lipid mediators (LPA and PAF) and PUFA-derived anti-inflammatory lipid mediators (lipoxin, resolvin and protectin). (A) Two biosynthetic pathways for LPA. LPA is produced by fatty acid deacylation of phosphatidic acid (PA) by PLA₂ (or PLA₁), or by removal of the polar head group of lysophosphatidylcholine (LPC), which is produced from PC by PLA₂ (or PLA₁), by a lysophospholipase D termed autotaxin (ATX). In most if not all in vivo situations, the ATX-dependent route is dominant for the production of LPA. DAG, diacylglycerol; DGK, diacylglycerol kinase; PLD, phospholipase D. (B) Biosynthesis and degradation of PAF. Alkyl-PC is converted by PLA₂ to alkyl-LPC (LysoPAF), which is then acetylated by LPC acyltransferase 2 (LPCAT2) to give rise to PAF. PAF is deacetylated to LysoPAF by PAFAH, a unique group of PLA₂s. LysoPAF is converted back to alkyl-PC by LPCAT3. (C) Anti-inflammatory PUFA metabolites derived from ω6 AA (lipoxin A₄; LXA₄), ω3 EPA (resolvin E1; RvE1), and ω3 DHA (RvD1 and protectin D1; PD1). The double bond characteristic of the ω3 and ω6 PUFAs is shadowed.

Figure 3.

The cPLA₂ family. (A) Structures of cPLA₂ enzymes (α-ζ). The C2 domain, which is essential for Ca²⁺-dependent membrane translocation, is conserved in cPLA₂ enzymes except for cPLA₂γ, whose C-terminal region is farnesylated. (B) A schematic diagram of stimulus-induced cPLA₂α activation. For details, see the text.

Figure 4.

The iPLA₂/PNPLA family. Structures of iPLA₂/PNPLA enzymes (PNPLA1∼9), which are subdivided into lipase and phospholipase types, are shown. The patatin domain, which is characteristic of this family, is conserved in all of these enzymes. The biological functions and enzymatic properties of the individual enzymes are indicated on the right. For details, see the text.

Figure 5.

The role of PNPLA1 in epidermal acylceramide biosynthesis. Structures of the metabolites and enzymes or transporters responsible for individual steps in the acylceramide-biosynthetic pathway are indicated. Mutations or deletions of these enzymes cause ichthyosis in both human and mouse. PNPLA1 catalyzes the transacylation of LA from triglyceride to ω-OH ceramide, leading to the formation of ω-O-acylceramide, which is an essential component of lipid lamellae and the cornified lipid envelope in the uppermost epidermis. For details, see the text. ELOVL6, fatty acid elongase 6; CYP4F22/39, cytochrome P450 family F22 (in human) and F39 (in mouse); CERS3, ceramide synthase 3; ABCA12, ABC transporter 12; UGCG, UDP-glucose ceramide glucosyltransferase; GBA, β-glucocerebrosidase; ALOXE3, epidermal-type lipoxygenase 3; ALOX12B, 12R-lipoxygenase; TGM1, transglutaminase 1.

Figure 6.

The sPLA₂ family. The phylogenetic tree of sPLA₂ isoforms, which are subdivided into classical sPLA₂s (I/II/V/X branch) and atypical sPLA₂s (III and XII branches), is shown. The pathophysiological roles and related types of lipid metabolism (target substrates or products; shown in blue) for the individual isoforms are indicated. For details, see the text.

Figure 7.

Properties of sPLA₂-IIF. (A) A schematic procedure for identification of the lipid metabolism driven by sPLA₂-IIF in differentiating keratinocytes. Phospholipids extracted from the culture supernatants of mouse keratinocytes (a representative mass spectrometric profile of phospholipids is shown; IS, internal standard; cps, count per second) were incubated with a physiologically relevant concentration of recombinant sPLA₂-IIF and then taken for the lipidomics analysis. (B) In the assay shown in (A), sPLA₂-IIF preferentially increased plasmalogen-type (P-) lysophosphatidylethanolamine (LPE) species as well as PUFAs. Values represent AUC (area under the curve; mean ± SEM, n = 4). (C) The results shown in (B), together with in vivo analyses using sPLA₂-IIF-transgenic and knockout mice,⁶⁾ indicate that sPLA₂-IIF preferentially hydrolyzes P-PE bearing DHA to liberate P-LPE and DHA under physiological conditions. For more details, please see ref. .

Figure 8.

Fatty acid selectivity of sPLA₂-V. Lipids extracted from the spleen of 1-year-old sPLA₂-V-deficient (−/−) and littermate control (+/+) mice were subjected to mass spectrometric lipidomics analysis (values are mean ± SEM, *P < 0.05 and **P < 0.01). Experiments were performed in accordance with the procedure described previously (5). Y-axis indicate relative abundance (AUC; area under the curve) of each product per mg tissue. Free fatty acid (FFA) species with a lower degree of unsaturation, including PA (16:0), palmitoleic acid (16:1), stearic acid (18:0; SA), OA (18:1), LA (18:2), eicosanoic acid (20:0) and eicosenoic acid (C20:1), but not PUFAs including AA (20:4), EPA (20:5), DPA (22:5) and DHA (22:6), were significantly reduced in sPLA₂-V-deficient mice relative to control mice. Accordingly, LA metabolites, including 9- and 13-hydroxyoctadecadienoic acids (HODEs) among others, were substantially decreased in mutant mice relative to control mice, whereas none of the AA, EPA and DHA metabolites differed significantly between the genotypes. These results are consistent with the view that sPLA₂-V has a propensity to preferentially hydrolyze phospholipids having sn-2 fatty acids with a lower degree of unsaturation, as illustrated at right bottom.