New developments in phospholipase A2

Pathophysiology of LPS-induced gastrointestinal injury in the rat: role of secretory phospholipase A2

A hydrophobic layer of phosphatidylcholine (PC) overlies and protects the surface of the gastrointestinal (GI) tract, contributing to barrier integrity. During critical illness such as sepsis, gut barrier integrity is compromised, which could be related to degradation of PC. The purpose of this study was to investigate a role for luminal (secretory) phospholipase A2 (sPLA(2)) in LPS-induced GI injury. Rats were treated with LPS (5 mg/kg) or saline for 0.5, 1, 3, and 5 h. The gastric and ileal luminal contents were collected for determination of sPLA(2) activity, and the luminal lipids were analyzed using thin layer chromatography for lyso-PC content. The GI permeability was assessed in vivo with fluorescein-isothiocyanate dextran 4000 and rats were tested with or without a specific sPLA(2) inhibitor. LPS induced significant increases in sPLA(2) activity and lyso-PC content in the gastric and ileal lumens at 5 h. In addition, LPS treated rats showed a significant increase in GI permeability to fluorescein-isothiocyanate dextran in both the stomach and ileum at 5 h, which was prevented by pretreatment with the sPLA(2) inhibitor. In response to LPS, sPLA(2) activity increases in the GI tract lumen where it may degrade the extracellular protective phospholipid layer and membranes, producing injurious lyso-PC and increased GI permeability. Pretreatment with an orally active sPLA(2) inhibitor blocks the LPS-induced increase in GI permeability, and may suggest a new approach to fortify the GI mucosal barrier and prevent complications from endotoxin in trauma and other patients.

Incorporation of phytosterols in human keratinocytes

We have designed experimental conditions allowing the replacement of 50% of cholesterol of human keratinocytes (SVK14 line) with sitosterol or stigmasterol without affecting cellular viability. We have investigated the influence of incorporating phytosterol on the ultraviolet-A-induced formation of lipid-peroxidation products (thiobarbituric reactive substances (TBARS)) in these cells. Our results show that ultraviolet-A-induced lipid peroxidation depends on the nature of the phytosterol. Sitosterol induces a significant decrease (-30%) of TBARS relative to the control whereas stigmasterol markedly increases lipid peroxidation (+70%). We have also studied the effect of plant sterols on prostaglandin release by using the Ca(2+) ionophore A23187 as an in vitro model of the inflammation induced by UVA radiation. We show that in the presence of 50% of phytosterol (particularly stigmasterol), the release of prostaglandin (6-ketoPG(1alpha), PGE(2)) is increased compared to untreated cells. This pro-inflammatory effect of phytosterols is correlated with a loss of the regulation of the intracellular Ca(2+) concentration.

Activity of phospholipases A and lysophospholipase in turkey semen and oviducal fluid

Changes in lipid composition of turkey semen have previously been reported to occur during in vitro storage and may be mediated by endogenous hydrolysis of phospholipids. To investigate the presence of phospholipases able to initiate such degradation, phospholipaseA2 (PLA2), phospholipase A1 (PLA1), and lysophospholipase (LPLase) activities were measured in turkey spermatozoa and seminal plasma. These enzymes were also measured in the oviductal fluid because they may be involved in the process prior to fertilization in the female. In spermatozoa and seminal plasma, the major PLA2 was a calcium-dependent and sodium deoxycholate (DOC) stimulated enzyme. However, calcium-independent PLA2 activities were also detected with different characteristics in spermatozoa (DOC inhibited enzyme) and seminal plasma (DOC stimulated enzyme). Additionally, PLA1 activity and high LPLase activity were present in spermatozoa and seminal plasma. In vitro storage of semen for 48 h did not affect PLA2 and LPLase activities. By contrast, PLA1 was the major phospholipase activity detected in oviductal fluid. A PLA2 activity stimulated by calcium or DOC and LPLase activity were also detected, but both were low relative to PLA1. These results showed that turkey semen had several enzymatic activities able to hydrolyze phospholipids. In addition, the phospholipase activities described here in the oviductal fluid could be involved in membrane destabilization prior to fertilization.

Phospolipase A2 and apoptosis

Phospolipase A(2) (PLA(2)) is the esterase activity that cleaves the sn-2 ester bond in glycerophospholipids, releasing free fatty acids and lysophospholipids. The PLA(2) activity is found in a variety of enzymes which can be divided in several types based on their Ca(2+) dependence for their activity; Ca(2+)-dependent secretory phosholipases (sPLA(2)s) and cytosolic phospholipases (cPLA(2)s), and Ca(2+)-independent phospholipase A(2)s (iPLA(2)s). These enzymes also show diverse size and substrate specificity (i.e., in the fatty acid chain length and extent of saturation). Among the fatty acids released by PLA(2), arachidonic acid (AA) is of particular biological importance, because it is subsequently converted to prostanoids and leukotrienes by cyclooxygenases (COX) and lipoxygenases (LOX), respectively. Free AA may also stimulate apoptosis through activation of sphingomyelinase. Alternatively, it is suggested that oxidized metabolites generated from AA by LOX induce apoptosis. Although the precise mechanisms remain to be elucidated, changes are observed in glycerolipid metabolism during apoptotic processes. In some cells induced to undergo apoptosis, AA is released concomitant with loss of cell viability, caspase activation and DNA fragmentation. Such AA releases appear to be mediated by activation of cPLA(2) and/or iPLA(2). For example, tumor necrosis factor-alpha (TNF-alpha)-induced cell death is mediated by cPLA(2), whereas Fas-induced apoptosis appears to be mediated by iPLA(2). Some discrepancies among early experimental results were probably caused by differences in the experimental conditions such as the serum concentration, inhibitors used that are not necessarily specific to a single-type enzyme, or differential expression of each PLA(2) in cells employed in the experiments. Recent studies eliminated such problems, by carefully defining the experimental conditions, and using multiple inhibitors that show different specificities. Accordingly, more convincing data are available that demonstrate involvement of some PLA(2)s in the apoptotic processes. In addition to cPLA(2) and iPLA(2), sPLA(2)s were recently found to play roles in apoptosis. Moreover, new proteins that appear to control PLA(2)s are being discovered. Here, the roles of PLA(2)s in apoptosis are discussed by reviewing recent reports.

Guinea pig phospholipase B, identification of the catalytic serine and the proregion involved in its processing and enzymatic activity

Guinea pig phospholipase B (GPPLB) is a glycosylated ectoenzyme of intestinal brush border membrane. It displays a broad substrate specificity and is activated by trypsin cleavage. The primary sequence contains four tandem repeat domains (I to IV) and several serines in lipase consensus sequences. We used site-directed mutagenesis to demonstrate that only the serine 399 present in repeat II is responsible for the various enzymatic activities of GPPLB. Furthermore, we characterized for the first time the retinyl esterase activity of the enzyme. We also constructed and expressed in COS-7 cells, an NH(2)-terminal repeat I deletion mutant which was detected at a very low level by immunoblot. However, confocal microscopy study showed a strong intracellular accumulation with a weak membrane expression of the mutated protein, indicating a role of the NH(2)-terminal repeat I in the processing of GPPLB. Nevertheless, the Western blot-detected protein presented a glycosylation and trypsin sensitivity patterns similar to wild type PLB. The mutant is also fully active without trypsin treatment, in contrast to native enzyme. Thus, we propose a structural model for GPPLB, in which the repeat I constitutes a lid covering the active site and impairing enzymatic activity, its removal by trypsin leading to an active protein.

Discrete role for cytosolic phospholipase A(2)alpha in platelets: studies using single and double mutant mice of cytosolic and group IIA secretory phospholipase A(2)

Among several different types of phospholipase A(2) (PLA(2)), cytosolic PLA(2) (cPLA(2))alpha and group IIA (IIA) secretory PLA(2) (sPLA(2)) have been studied intensively. To determine the discrete roles of cPLA(2)alpha in platelets, we generated two sets of genetically engineered mice (cPLA(2)alpha(-/-)/sPLA(2)-IIA(-/-) and cPLA(2)alpha(-/-)/sPLA(2)-IIA(+/+)) and compared their platelet function with their respective wild-type C57BL/6J mice (cPLA(2)alpha(+/+)/sPLA(2)-IIA(-/-)) and C3H/HeN (cPLA(2)alpha(+/+)/sPLA(2)-IIA(+/+)). We found that cPLA(2)alpha is needed for the production of the vast majority of thromboxane (TX)A(2) with collagen stimulation of platelets. In cPLA(2)alpha-deficient mice, however, platelet aggregation in vitro is only fractionally decreased because small amounts of TX produced by redundant phospholipase enzymes sufficiently preserve aggregation. In comparison, adenosine triphosphate activation of platelets appears wholly independent of cPLA(2)alpha and sPLA(2)-IIA for aggregation or the production of TX, indicating that these phospholipases are specifically linked to collagen receptors. However, the lack of high levels of TX limiting vasoconstriction explains the in vivo effects seen: increased bleeding times and protection from thromboembolism. Thus, cPLA(2)alpha plays a discrete role in the collagen-stimulated production of TX and its inhibition has a therapeutic potential against thromboembolism, with potentially limited bleeding expected.

Human epidermis is a novel site of phospholipase B expression

Phospholipase B (PLB) is an enzyme that displays both phospholipase A(2) and lysophospholipase activities. Analysis of human epidermis homogenates indicated the presence of a 97 kDa PLB protein, as well as a phospholipase A(2) activity, both being enriched in the soluble fraction. Immunolabelling and in situ hybridization experiments showed that this enzyme is expressed in the different layers of epidermis with an accumulation at the dermo-epidermis junction. RT-PCR data indicated that PLB is specifically expressed in natural and reconstructed epidermis. By 3'-RACE-PCR and screening of human genome databases, we obtained a 3600 bp cDNA coding for human PLB highly homologous to already described intestinal brush border PLBs. These data led us to conclude that the soluble PLB corresponds to a proteolytic cleavage of the membrane anchored protein. Altogether, our results provide the first characterization of human PLB which should play an important role in epidermal barrier function.

Ca2+-independent phospholipase A2 activity in apical plasma membranes from the rat parotid gland

An apical-enriched plasma membrane fraction (A-PM) was prepared from rat parotid gland by Mn2+ precipitation. In this fraction, phosphatidylcholine (PC) labelled at the sn-2 position was mainly decomposed into two labelled compounds (free fatty acid and 1,2-diacylglycerol) under Ca2+-free conditions. Studies using double-labelled PC and 2,3-diphosphoglycerate (as a phospholipase D inhibitor) showed that they were produced through different pathways: free fatty acid was released by phospholipase A2 (PLA2) while 1,2-diacylglycerol may be produced by sequential action of phospholipase D and phosphatidate phosphatase. The PLA2 in A-PM did not require Ca2+ for its activity and was highly activated by Triton X-100 and ATP. The inhibitor of the well-documented Ca2+-independent PLA2, bromoenol lactone, did not inhibit the PLA2 activity in A-PM. Although PLA2 activity was detected in other subcellular fractions, the highest specific activity was in A-PM. Its distribution among various fractions was roughly similar to that of the marker enzyme of apical plasma membranes. These findings suggested that Ca2+-independent PLA2 activity is present in apical plasma membranes from rat parotid gland. In addition, to clarify the involvement of the PLA2 in exocytosis, the fusion of exogenous PLA2-treated membranes with secretory granules was examined by fluorescence dequenching assay. This study clearly demonstrated the facilitation of fusion by PLA2 treatment, which suggests some involvement of apical PLA2 in saliva secretion.

ATTENUATION OF ISCHEMIA AND REPERFUSION INJURY OF CANINE LIVERS BY INHIBITION OF TYPE II PHOSPHOLIPASE A2 WITH LY3297221

BACKGROUND

Membrane phospholipid breakdown, caused by ischemia and reperfusion (I/R) of the liver, releases free fatty acids including arachidonic acids and lysophospholipids, which serve as precursors of various inflammatory lipid derivatives. Phospholipase A2 (PLA2) is a key enzyme that initiates this reaction. In this study, we tested our hypothesis that a type II PLA2 inhibitor, LY329722, could attenuate hepatic I/R injury caused by a 2-hr total hepatic vascular exclusion (THVE) in dogs.

METHODS

Eighteen beagle dogs, subjected to a 2-hr THVE, were divided into three groups. Group 1 (n=6) was untreated and served as a control group. LY329722 was administered to animals in group 2 (n=6) intravenously (0.2 mg x kg(-1) x hr(-1)) for 60 min before ischemia, and to animals in group 3 (n=6) for 60 min starting 15 min before reperfusion (0.2 mg x kg(-1) x hr(-1)). Animal survival, systemic and splanchnic hemodynamics, hepatic tissue blood flow, liver functions, energy metabolism, hepatic venous thromboxane B2 and endothelin-1 levels, phospholipid levels and tumor necrosis factor-a mRNA expression in liver tissue, and histopathologic findings were evaluated.

RESULTS

Two-week animal survival was 33% (two of six) in group 1, and 100% (six of six) in groups 2 and 3. LY329722 improved systemic and splanchnic hemodynamics, hepatic tissue blood flow, and energy metabolism, reduced liver enzyme, thromboxane B2, and endothelin-1 release, prevented hepatic phospholipid degradation and tumor necrosis factor-alpha mRNA expression, and lessened histopathologic damage and the number of neutrophil infiltrating into the liver tissue.

CONCLUSION

The present study demonstrated that a type II PLA2 inhibitor, LY329722, attenuated hepatic I/R injury caused by a 2-hr THVE model in dogs.

Purification, characterization, and molecular cloning of group I phospholipases A2 from the gills of the red sea bream, Pagrus major

Phospholipase A2 (PLA2) activity was investigated in various tissues of male and female red sea bream. In both male and female fishes, the specific activity of PLA2 in the gills was 70 times higher than that in other tissues, such as the adipose tissue, intestine, and hepatopancreas. Therefore, we tried to purify PLA2 from the gill filaments of red sea bream to near homogeneity by sequential chromatography on Q-Sepharose Fast Flow, Butyl-Cellulofine, and DEAE-Sepharose Fast Flow columns, and by reversed-phase high-performance liquid chromatography. Two minor and one major PLA2, tentatively named G-1, G-2 and G-3 PLA2, were purified, and all showed a single band with an apparent molecular mass of approximately 15 kDa by sodium dodecylsulfate-polyacrylamide gel electrophoresis. The exact molecular mass values of G-1, G-2, and G-3 PLA2 were 14,040, 14,040 and 14,005 Da, respectively. G-1, G-2, and G-3 PLA2 had a Cys 11 and were all identical in N-terminal amino acid sequences from Ala-1 to Glu-56. A full-length cDNA encoding G-3 PLA2 was cloned by reverse transcriptase-polymerase chain reaction and rapid amplification of cDNA ends methods, and G-3 PLA2 was found to be classified to group IB PLA2 from the deduced amino acid sequence. G-1, G-2, and G-3 PLA2 had a pH optimum in an alkaline region at around pH 9-10 and required Ca2+ essentially for enzyme activity, using a mixed-micellar phosphatidylcholine substrate with sodium cholate. These results demonstrate that three group I PLA2, G-1, G-2, and G-3 PLA2, are expressed in the gill filaments of red sea bream.

Identification of pancreatic type I secreted phospholipase A2 in human epidermis and its determination by tape stripping

Phospholipases A2 (PLA2) catalyse the release of fatty acids from the sn-2 position of phospholipids and have been suggested to play a key part in permeability barrier homeostasis. Using a sensitive and versatile fluorometric method, significant PLA2 activity has been detected in both human skin homogenates and tape strippings of stratum corneum. Based on various properties (resistance to heat and sulphuric acid treatment, neutral optimal pH, absolute requirement for millimolar calcium concentrations, inhibition by dithiothreitol and p-bromophenacyl bromide, and resistance to a trifluoromethyl ketone derivative of arachidonic acid, AACOCF3, a specific inhibitor of cytosolic PLA2), this enzyme was characterized as a secretory PLA2 (sPLA2). Immunohistochemistry revealed strong labelling of type I pancreatic sPLA2 at the stratum corneum-stratum granulosum junction, type II sPLA2 being undetectable. An increase in PLA2 activity in tape-stripped material from the deepest level of the stratum corneum was correlated with partial morphological disappearance of type I sPLA2 immunolabelling. Our data thus provide the first convincing evidence that pancreatic sPLA2 is significantly expressed in human epidermis, where it might participate in the accumulation of free fatty acids contributing to the permeability barrier. In addition, our method for determining PLA2 activity in easily available tape strippings should allow further clinical studies aimed to explore possible PLA2 abnormalities in various dermatoses.

Genetic evidence for distinct roles of COX-1 and COX-2 in the immediate and delayed phases of prostaglandin synthesis in mast cells

Activation of mast cells by aggregation of their high-affinity IgE receptors stimulates prostaglandin (PG) D(2) synthesis and secretion. An immediate phase of PGD(2) synthesis, complete within 30 min, is followed by a delayed, second phase of PGD(2) production that reaches a maximum 4 to 8 h after activation. Activation of mast cells from COX-2 (-/-) mice stimulates the release of PGD(2) during the first 30 min, whereas activation of mast cells from COX-1 (-/-) mice does not generate any PGD(2) in the first 2 h. On the other hand, COX-2 (-/-) cells do not participate in delayed phase of PGD(2) synthesis, while COX-1 (-/-) cells secrete low levels of PGD(2) between 2 and 4 h after activation. These data demonstrate that (i) the first phase of PG synthesis is COX-1 dependent and (ii) the second, delayed phase of PG synthesis is dependent on activation-induced synthesis and activity of COX-2.