1. Introduction
Phospholipase A
1 (PLA
1) is an enzyme that hydrolyzes an ester bond at the
sn-1 position of glycerophospholipids (GPLs), usually producing a saturated or mono-unsaturated fatty acid and a 1-lyso-2-acyl-phospholipid (2-acyl-lysophospholipid, 2-acyl-LPL) (
Figure 1). PLA
1 has not attracted as much attention as other mammalian acyl hydrolases, such as phospholipase A
2 (PLA
2), which hydrolyzes fatty acids, mainly unsaturated fatty acids, and acts as a first step in producing enzymes for bioactive lipids such as eicosanoids and platelet-activating factor (PAF)
[1][2]. Some PLA
1s and PLA
2 target neutral lipids such as triacylglycerol (TAG) and diacylglycerol (DAG) in addition to GPLs (
Figure 1).
Figure 1. Structures of glycerolipids and their metabolic enzymes. (
A) Glycerophospholipids (GPL) and phospholipases. GPLs are composed of a polar head group (six major classes), a glycerol backbone and fatty acid moieties (esterified at the
sn-1 and
sn-2 positions). Phospholipase A
1 (PLA
1) hydrolyzes a fatty acid at the
sn-1 position, generating
sn-2-acyl-1-lyso-phospholipids (
sn-2-acyl LPLs), while phospholipase A
2 (PLA
2) hydrolyzes a fatty acid at the
sn-2 position generating
sn-1-acyl-2-lyso-phospholipids (
sn-1-acyl LPLs). Phospholipase B (PLB) hydrolyzes a fatty acid at both
sn-1 and
sn-2 positions. LysoPLA hydrolyzes a fatty acid of
sn-2-acyl LPLs and
sn-1-acyl LPLs, generating glycerophosphate compounds. (
B) Triacylglycerol (TAG) has three fatty acids at the
sn-1,
sn-2 and
sn-3 positions of glycerol backbone, diacylglycerol (DAG) has two fatty acids and monoacylglycerol (MAG) has one fatty acid. TAG lipase hydrolyzes a fatty acid of TAG, generating
sn-1, 2,
sn-2, 3 or
sn-1, 3-diacylglycerols (DAGs). DAG lipase hydrolyzes a fatty acid of DAG and MAG lipase hydrolyzes a fatty acid of MAG.
Much is known about the functions of PLA
2, whereas those of PLA
1 remain limited. However, because fatty acids at both the
sn-1 and
sn-2 positions of GPLs have a high turnover rate
[3], PLA
1 as well as PLA
2, appears to be involved in the rapid turnover and remodeling of cellular GPLs (
Figure 2). In addition, some PLA
1s also have a specific role in the production of 2-acyl-1-lysophospholipids, which serve as lysophospholipid mediators. For example, one type of PLA
1, membrane-associated phosphatidic acid-selective PLA
1 (mPA-PLA
1α in
Table 1,
Figure 3A), produces 2-acyl-1-lysophosphatidic acid (2-acyl-lysoPA (LPA)) with an unsaturated fatty acid residue
[4]. The 2-acyl-LPA acts as a potent ligand for LPAR3/EDG7 and LPAR6/P2Y5, with LPA receptors preferring 2-acyl-LPA over 1-acyl-LPA
[5][6]. Phosphatidylserine-specific PLA
1 (PS-PLA
1 in
Table 1,
Figure 3A) also acts as a producing enzyme of another lysophospholipid mediator, 2-acyl-lysophosphatidylserine (2-acyl-lysoPS (LysoPS)), which further supports the idea that PLA
1s function as producing enzymes for lysophospholipid mediators.
Figure 2. Fatty acid remodeling reactions of GPLs. Glycerophospholipids (GPL) in the cells are constantly subjected to two kinds of fatty acid hydrolyzing reactions mediated by phospholipase A
1 (PLA
1) and phospholipase A
2 (PLA
2), resulting in the production of 2-acyl-1-lysophospholipid (
sn-2 LPL) and 1-acyl-2-lysophospholipid (
sn-1 LPL). The LPLs thus produced are further subjected to acylation reactions to re-form the GPLs. Several kinds of lysophospholipid acyltransferases (LPLAT) are responsible for the introduction of fatty acids to lysophospholipids. By these sequential GPL remodeling reactions, the fatty acids of GPLs are constantly replaced.
Figure 3. Physiological roles of extracellular PLA
1s. (
A) PS-PLA
1 and mPA-PLA
1α serve as producing enzymes for lysophospholipid mediators. PS-PLA
1 has a strict substrate specificity in that it only acts on serine containing GPLs such as phosphatidylserine (PS) and lysophosphatidylserine (LysoPS). LysoPS then acts on GPCR-type LysoPS receptors. Three such LysoPS receptors have been identified. These include LPSR1/GPR34, LPSR2/P2Y10, and LPSR3/GPR174. mPA-PLA
1α acts on PA in a specific manner and produces
sn-2 LPA, which then acts on GPCR-type LPA receptors, LPAR1-LPAR6 evoking various biological responses. (
B) Pancreatic lipase (PL) is secreted from the pancreas into the lumen of the intestine, where it, with the aid of bile acids, hydrolyzes the fatty acids of triacylglycerol (TAG) and GPLs in the digestive juice yielding diacylglycerol (DAG), monoacylglycerol (MAG) and fatty acids. The liberated fatty acids are absorbed by intestinal cells as nutrients. (
C) Lipoprotein lipase (LPL), hepatic lipase (HL), and endothelial lipase (EL), which are mainly present in the blood, are associated with endothelial cell surfaces in adipose tissues (LPL), heart (LPL), liver (HL) and various tissues. These lipases have both TAG lipase and PLA
1 activities. They hydrolyze fatty acids of TAG and GPLs present in the circulating lipoproteins such as low-density lipoproteins (LDL) and high-density lipoproteins (HDL), yielding diacylglycerol (DAG), monoacylglycerol (MAG), lysophospholipids (LPL) and fatty acids. The free fatty acids are absorbed by corresponding cells for energy source and storage of neutral lipids.
Table 1. Mammalian PLA1s.
|
Primary Name |
Other Name |
Human Gene |
Substrate |
Reaction Mediated |
PDB ID (H: Human, R: Rat) |
Ref. |
extracellular PLA1s |
PS-PLA1 |
PLA1A |
PLA1A |
PS |
Producing enzyme for bioactive lysophospholipid, LysoPS |
- |
[7][8] |
PA-PLA1α |
LIPH, mPA-PLA1α |
LIPH |
PA |
Producing enzyme for bioactive lysophospholipid, LPA |
- |
[4][9] |
lipoprotein lipase |
LPL, LIPD |
LIPD |
TAG, PL |
TAG lipase and PLA1 activity |
H: 6E7K, 6OAU, 6OAZ, 6OB0, 6WN4 |
[10] |
hepatic lipase |
HL, LIPC |
LIPC |
TAG, PL |
TAG lipase and PLA1 activity |
- |
[10] |
endothelial cell-derived lipase |
EDL, EL, LIPG |
LIPG |
PL |
Predominant PLA1 activity |
- |
|
pacreatic lipase |
PL, PNLIP |
PNLIP |
TAG, PL |
TAG lipase and PLA1 activity |
H: 1GPL, 1LPA, 1LPB, 1N8S |
[11][12][13] |
pancreatic lipase-related protein 1 |
PLRP1 |
PLRP1 |
TAG, PL |
TAG lipase and PLA1 activity |
H: 2PPL |
|
pancreatic lipase-related protein 2 |
PLRP2 |
PLRP2 |
TAG, PL |
TAG lipase and PLA1 activity |
H: 2OXE, 2PVS; R: 1BU8 |
[11][13] |
intracellular PLA1s |
PA-PLA1 |
DDHD1, iPLA1α |
DDHD1 |
PL |
PLA1 activity |
- |
[14][15] |
KIAA0725p |
DDHD2, iPLA1γ |
DDHD2 |
PL |
PE, DAG, CL |
- |
[16][17][18] |
p125 |
iPLA1β |
P125 |
n.d. |
Enzymatic activity has not been detected |
- |
[19] |
PNPLA6 |
iPLA2δ, NTE |
PNPLA6 |
PC, LPC |
PLB, LysoPLA activity cleaving FAs at both sn-1 and sn-2 positions |
- |
[20][21][22] |
PNPLA7 |
iPLA2θ, NRE |
PNPLA7 |
PC, LPC |
PLB, LysoPLA activity cleaving FAs at both sn-1 and sn-2 positions |
- |
[23] |
PNPLA8 |
iPLA2γ, Group VIB PLA2 |
PNPLA8 |
PC |
PLB activity cleaving FAs at both sn-1 and sn-2 positions |
- |
[24][25] |
cPLA2α |
PLA2G4A, Group IVA PLA2 |
PLA2G4A |
PL |
PLB activity cleaving FAs at both sn-1 and sn-2 positions |
H: 1BCI, 1CJY, 1RLW |
[26] |
cPLA2β |
PLA2G4B, Group IVB PLA2 |
PLA2G4B |
PL |
PLB activity cleaving FAs at both sn-1 and sn-2 positions |
- |
[26][27] |
cPLA2ζ |
PLA2G4F, Group IVF PLA2 |
PLA2G4F |
PL |
PLB activity cleaving FAs at both sn-1 and sn-2 positions |
- |
|
PLA2G16 |
Group XVI PLA2, PLAAT3, HRASLS3, H-Rev107 |
PLA2G16 |
PL |
PLB activity cleaving FAs at both sn-1 and sn-2 positions |
H: 2KYT, 4DOT, 4FA0, 4Q95, 7C3Z, 7C41 |
[28] |
Although PLA
1 activity has been detected in many mammalian tissues and cells
[29][30][31][32][33][34][35][36], only a few PLA
1s have been purified and cloned. Some triacylglycerol (TAG)-hydrolyzing lipases (
Figure 1) such as hepatic lipase, endothelial cell lipase, and lipoprotein lipase also show PLA
1 activity
[11].
2. History of PLA1 Research
The following is a brief history of mammalian PLA
1 research. PLA
1 as well as PLA
2 activities have been detected in various tissues and plasma. In the 1990’s, two PLA
1 molecules were biochemically purified and identified. These two PLA
1s are phosphatidic acid-preferential PLA
1 (PA-PLA
1) and phosphatidylserine-specific PLA
1 (PS-PLA
1) (
Table 1). In 1994, Higgs and Glomset purified a novel PLA
1 from a cytosolic fraction of bovine testes that preferentially hydrolyzed PA
[14] (
Table 1). Shortly after, the same group cloned a cDNA of the PLA
1, and PA-PLA
1 was found to be an intracellular protein composed of 875-amino acids with a calculated molecular mass of approximately 100 kDa. Simultaneously, Exton’s group purified a very similar PLA
1 from the bovine brain
[37]. It was not clear whether the two intracellular PLA
1s were identical, because the two groups used different PLA
1 assay conditions and substrates. Later, Kudo’s group purified similar PLA
1s from the brain and testes of mice, rats, and cows
[38] and showed that they were identical to the PA-PLA
1 mentioned above. They demonstrated that PA-PLA
1 was predominantly hydrolyzed phosphatidic acid (PA) in the presence of Triton X-100 and phosphatidylethanolamine (PE) in its absence. The result showed clearly that the substrate specificity of PA-PLA
1 in vitro is affected by the assay conditions, which makes it difficult to identify the natural substrates of PA-PLA
1. This also implies that the name, PA-PLA
1, is not suitable for the enzyme.
Horigome et al., detected two PLA activities in the supernatant of activated rat platelets
[39][40]. One was identified as secretory PLA
2, now known as Group IIA secretory phospholipase A
2 (sPLA
2IIA). Sato et al., succeeded in purifying and cloning another PLA that showed a high preference for serine-containing phospholipids
[7]. The PLA is now known as PS-specific PLA
1 (PS-PLA
1) (
Table 1). PS-PLA
1 specifically hydrolyzed PS in vitro. In addition, it acted on the intact cell membrane, hydrolyzed PS and produced 2-acyl-LysoPS. Thus, PS-PLA
1 is believed to be a LysoPS-producing enzyme.
After discovering the two PLA
1 molecules, several researchers found similar PLA
1s (homologs) in the nucleotide databases, expressed them as recombinant proteins and characterized them biochemically. These analyses identified the homologs as novel PLA
1s. These include extracellular enzyme membrane-associated PA-selective PLA
1α (mPA-PLA
1α, a PS-PLA
1 homologue)
[4] (
Table 1) and intracellular enzyme iPLA
1γ/DDHD2/KIAA0725 (a PA-PLA
1 homologue)
[16] and iPLA
1β/SEC23IP/p125 (PLA
1 activities have not been detected)
[19] (
Table 1). In addition, similar biochemical characterization was performed for PLA
2 homologs. Interestingly, biochemical characterizations of the PLA
2 homologs showed that some of them exhibited PLA
1 activity in addition to their PLA
2 activity (PLB activity (
Figure 1)) (
Table 1). It should also be noted here that certain lipases that hydrolyze TAG have PLA
1 activity, as mentioned above (
Table 1). Lipases hydrolyze fatty acids at the
sn-1 and
sn-3 positions of triacylglycerol (TAG) and diacylglycerol (DAG). They also hydrolyze fatty acids at the
sn-1 or
sn-3 positions of monoacylglycerol (MAG). This re
svie
archw will summarize current knowledge on PLA
1 molecules reported thus far, discussing their discoveries, structures, tissue and cellular distributions, and possible biological functions.
PLA1s are roughly divided into two groups: (1) extracellular PLA1/lipase family and (2) intracellular PLA1 family (Table 1), depending on their cellular localization and primary amino acid sequences. The intracellular PLA1 family was further subdivided into several groups, consisting of an iPLA1 family, PNPLA family, cPLA2 family and PLAAT family.
3. Structural Evaluation of PLA1 Molecules
Recently, a computational machine learning method named AlphaFold was developed enabling researchers to predict protein structures with high accuracy, even when no similar experimentally solved structure is available
[41]. In this re
svie
arch, researchersw, we utilized AlphaFold to generate structural predictions of PLA
1 molecules.
Figure 4 summarizes the AlphaFold-generated structures of some PLA
1 molecules focused on in this re
svie
archw. Note that some structures have been determined, e.g., by X-ray crystallography, while others were structure-predicted. Since structures of any PNPLA and iPLA
1 family members have not been determined, AlphaFold was unable to predict their structures. Thus, predicted structures were shown only for extracellular PLA
1/lipase and for cPLA
2 family members (
Figure 4).
Figure 4. Structures of PLA
1 molecules. 3D structures of three PLA
1 family members (extracellular PLA
1/lipase, cPLA
2 and PLAAT family members) were shown. For extracellular PLA
1/lipase family members, β5 and the β9 loops and the lid domain are shown in yellow, cyan and green, respectively. The three residues forming a catalytic triad (Ser, Asp and His) are described as sticks (red). For cPLA
2 family members (cPLA
2α, cPLA
2β and cPLA
2ζ), the conserved lipase motifs (GXSGX and DXG) are shown in red, and the two residues forming a catalytic dyad (Ser and Asp) are shown as sticks. For PLAAT3/PLA2G16, the three residues forming a catalytic triad (two histidines and cysteine) are shown as sticks (red). The structures without asterisk were acquired from RCSB Protein Data Bank. (Reference PDB IDs; PL (1LPB), LPL (6OB0), PLRP1 (2PPL), cPLA
2α (1CJY), PLA2G16 (4DOT)). The predicted structures of lipases with asterisk were acquired from AlphaFold Protein Structure Database. All the structures were visualized using PyMOL software.