Group IID heparin-binding secretory phospholipase A(2) is expressed in human colon carcinoma cells and human mast cells and up-regulated in mouse inflammatory tissues

COVID-19, Blood Lipid Changes, and Thrombosis

Although there is increasing evidence that oxidative stress and inflammation induced by COVID-19 may contribute to increased risk and severity of thromboses, the underlying mechanism(s) remain to be understood. The purpose of this review is to highlight the role of blood lipids in association with thrombosis events observed in COVID-19 patients. Among different types of phospholipases A₂ that target cell membrane phospholipids, there is increasing focus on the inflammatory secretory phospholipase A₂ IIA (sPLA₂-IIA), which is associated with the severity of COVID-19. Analysis indicates increased sPLA₂-IIA levels together with eicosanoids in the sera of COVID patients. sPLA₂ could metabolise phospholipids in platelets, erythrocytes, and endothelial cells to produce arachidonic acid (ARA) and lysophospholipids. Arachidonic acid in platelets is metabolised to prostaglandin H2 and thromboxane A₂, known for their pro-coagulation and vasoconstrictive properties. Lysophospholipids, such as lysophosphatidylcholine, could be metabolised by autotaxin (ATX) and further converted to lysophosphatidic acid (LPA). Increased ATX has been found in the serum of patients with COVID-19, and LPA has recently been found to induce NETosis, a clotting mechanism triggered by the release of extracellular fibres from neutrophils and a key feature of the COVID-19 hypercoagulable state. PLA2 could also catalyse the formation of platelet activating factor (PAF) from membrane ether phospholipids. Many of the above lipid mediators are increased in the blood of patients with COVID-19. Together, findings from analyses of blood lipids in COVID-19 patients suggest an important role for metabolites of sPLA₂-IIA in COVID-19-associated coagulopathy (CAC).

Secretory Phospholipase A2 (sPLA2) Isozymes as Potential Targets in Tobacco Condensate- induced Colon Damage

AIMS

To find out the role of secretory phospholipase A2 (sPLA2) isozymes as potential targets in tobacco condensate-induced colon damage.

BACKGROUND

The effects of cigarette smoke condensate (CSC) and the molecular mechanisms involved in the regulation of phospholipase A2 (PLA2) and its isozymes in colon cells, which are still unclear and emerging, are studied.

OBJECTIVES

The study aimed to check the effect of CSC on cell viability and reactive oxygen species (ROS) and superoxide. Also, the effect of CSC on gene expression of different secretory phospholipase A2 (sPLA2) was evaluated. Moreover, the impact of inhibition of sPLA2 on various cell properties i.e. cell viability, cell proliferation, membrane damage and free radicals' generation is also studied.

METHODS

CSC-induced changes were evaluated in cell viability by MTT assay, followed by the evaluation of membrane modulation by flow cytometry, free radical generation by fluorescent dyes, PLA2 isoforms gene expression patterns and their suppression by small interfering RNA (siRNA) studied in HCT-15 male and HT-29 female colon cells.

RESULTS

Our results demonstrate that HCT-15 and HT-29 cells treated with CSC significantly reduced the cell viability by 50% within 48 h and significantly enhanced the total reactive oxygen species (ROS) by 2 to 10-fold, and mitochondrial ROS (mtROS) and superoxide radicals (SOR) by 2-fold each. Treatment with CSC significantly unregulated secretory phospholipase A2 (sPLA2) IID group and down-regulated IB and cytosolic phospholipase (cPLA2) IVA groups in HCT-15 cells without affecting them in HT-29 cells. Silencing the sPLA2 IID group results in an increase in cell viability and a decrease in ROS. Silencing the PLA2 IVA gene in the HCT-15 cells showed a reduced expression which had no impact on the CSC-induced cell proliferation, membrane damage and free radicals (ROS, mtROS, and SOR) generation.

CONCLUSION

Therefore, identifying cell-specific sPLA2 isozymes seems to play a key role in controlling the ROSinduced damage by CSC and helps develop specific therapeutic strategies.

Structural Modeling of Wild and Mutant Forms of Human Plasma Platelet Activating Factor-Acetyl Hydrolase Enzyme

Purpose

To investigate the structural features of wild and mutant forms of the pPAF-AH enzyme that are responsible for coronary artery disease.

Methods

Mutant variants of human pPAF-AH having either V279F, Q281R, or both were modelled and evaluated for stereo chemical and structural correctness. The 3D coordinates of substrate PAF were retrieved from the PubChem database was solvated and minimized on Discovery Studio, and docked to the wild and mutant enzyme models. The top docked pose complex was refined by MD simulation.

Results

pPAF-AH model comprises of 420 amino acids in a α/β-hydrolase fold that contains a substrate-binding hydrophobic channel with an active site pocket having a catalytic triad of Ser273, Asp296 and His351. Mutations at positions 279 and 281 are opposite one another on the middle of 12 residues long H5 helix that forms the hydrophobic core of the enzyme. V279F causes a tilt on the axis of the mutation bearing helix to avoid steric clashes with the hydrophobic residues on the β-sheets adjacent to it, inducing subtle conformational changes on the H5-β8 loop, β8 sheet, and the loop bearing Asp296. A cascade of conformational changes induces a change in the orientation of His351 resulting in loss of hydrogen bonded interaction with catalytic Ser273. Q281R causes a shortening of H5 and β8, which induces conformational changes of the loops bearing Ser273 and Asp296, respectively. Simultaneous conformational changes of secondary structural elements result in the flipping of His351 causing a break in the catalytic triad. Also, there is a compromise in the substrate-binding area and volume in the mutants resulting in loss of binding to its substrate.

Conclusion

Mutant enzymes show changes at the site of the mutation, secondary motif conformations and global structural conformations that adversely affect the active site, decrease substrate channel volume and decrease stability, thereby affecting enzymatic function.

Human Secretary Phospholipase A2 Mutations and Their Clinical Implications

Phospholipases A2 (PLA₂s) belong to a superfamily of enzymes responsible for hydrolysis of the sn-2 fatty acids of membrane phospholipids to release arachidonic acid. PLA₂s are the rate limiting enzyme for the downstream synthesis of prostaglandins and leukotrienes that are the main mediators of inflammation. The extracellular forms of this enzyme are also called the secretary phospholipase A2 (sPLA₂) and are distributed extensively in most of the tissues in the human body. Their integral role in inflammatory pathways has been the primary reason for the extensive research on this molecule. The catalytic mechanism of sPLA₂ is initiated by a histidine/aspartic acid/calcium complex within the active site. Though they are known to have certain housekeeping functions, certain mutations of sPLA₂ are known to be implicated in causation of certain pathologies leading to diseases such as atherosclerosis, cardiovascular diseases, benign fleck retina, neurodegeneration, and asthma. We present an overview of human sPLA₂ and a comprehensive compilation of the mutations that result in various disease phenotypes. The study not only helps to have a holistic understanding of human sPLA₂ mutations and their clinical implications, but is also a useful platform to initiate research pertaining to structure–function relationship of the mutations to develop effective therapies for management of these diseases.

Synthesis and Evaluation of Cytosolic Phospholipase A(2) Activatable Fluorophores for Cancer Imaging

Activatable fluorophores selective to cytosolic phospholipase A2 (cPLA2) were synthesized and evaluated for their ability to image triple negative breast cancer cells. The activatable constructs were synthesized by esterification of a small molecule fluorophore with a fatty acid resulting in ablated fluorescence. Selectivity for cPLA2 was generated through the choice of fluorophore and fatty acid. Esterification with arachidonic acid was sufficient to impart specificity to cPLA2 when compared to esterification with palmitic acid. In vitro analysis of probes incorporated into phosphatidylcholine liposomes demonstrated that a nonselective phospholipase (sPLA2 group IB) was able to hydrolyze both arachidonate and palmitate coupled fluorophores resulting in the generation of fluorescence. Of the four fluorophores tested, DDAO (7-hydroxy-9H-(1,3-dichloro-9,9-dimethylacridin-2-one)) was observed to perform optimally in vitro and was analyzed further in 4175-Luc+ cells, a metastatic triple negative human breast cancer cell line expressing high levels of cPLA2. In contrast to the in vitro analysis, DDAO arachidonate was shown to activate selectively in 4175-Luc+ cells compared to the control DDAO palmitate as measured by fluorescence microscopy and quantitated with fluorescence spectroscopy. The addition of two agents known to activate cPLA2 enhanced DDAO arachidonate fluorescence without inducing any change to DDAO palmitate. Inhibition of cPLA2 resulted in reduced fluorescence of DDAO arachidonate but not DDAO palmitate. Together, we report the synthesis of a cPLA2 selective activatable fluorophore capable of detecting cPLA2 in triple negative breast cancer cells.

Secreted phospholipase A2 and mast cells

Phospholipase A2s (PLA2s) are a group of enzymes that hydrolyze the sn-2 position of phospholipids to release (typically unsaturated) fatty acids and lysophospholipids, which serve as precursors for a variety of bioactive lipid mediators. Among the PLA2 superfamily, secreted PLA2 (sPLA2) enzymes comprise the largest subfamily that includes 11 isoforms with a conserved His-Asp catalytic dyad. Individual sPLA2 enzymes exhibit unique tissue and cellular localizations and specific enzymatic properties, suggesting their distinct biological roles. Recent studies using transgenic and knockout mice for individual sPLA2 isofoms have revealed their involvement in various pathophysiological events. Here, we overview the current state of knowledge about sPLA2s, specifically their roles in mast cells (MCs) in the context of allergology. In particular, we highlight group III sPLA2 (PLA2G3) as an "anaphylactic sPLA2" that promotes MC maturation and thereby anaphylaxis through a previously unrecognized lipid-orchestrated circuit.

Acidification in the epidermis and the role of secretory phospholipases

The function of the epidermis is to form an effective barrier between the dry, external environment and the interior of the body. The barrier specifically resides in the extracellular lipid membranes of the stratum corneum (SC) and an acidic pH is necessary to maintain its competency against various insults. The purpose of this review is to explore the mechanisms which are postulated to contribute to the acidification of the stratum corneum, including both exogenous and endogenous sources. However, recent research as pointed to several endogenous mechanisms as the major source of acidification, including a sodium/proton pump (NHE1) and free fatty acid conversion from phospholipids by secretory phospholipase A₂ (sPLA₂). sPLA₂ has been shown to play a central role in the formation of the SC “acid mantle” in the early maturation of the epidermis postnatally. Many aspects of this enzyme family are complex and still being elucidated in research and the most recent findings on the localization and functions of sPL A₂-IB, -IIA, -IIC, -IID, -IIE, -IIF, -III, -V, -X and -XII in the epidermis are presented here. Given their role in inflammatory dermatoses, such as psoriasis and atopic dermatitis, understanding this complex enzyme family can lead to novel, life-changing therapies.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Group III secreted phospholipase A2 transgenic mice spontaneously develop inflammation

PLA2 (phospholipase A2) group III is an atypical sPLA2 (secretory PLA2) that is homologous with bee venom PLA2 rather than with other mammalian sPLA2s. In the present paper, we show that endogenous group III sPLA2 (PLA2G3) is expressed in mouse skin and that Tg (transgenic) mice overexpressing human PLA2G3 spontaneously develop skin inflammation. Pla2g3-Tg mice over 9 months of age frequently developed dermatitis with hyperkeratosis, acanthosis, parakeratosis, erosion, ulcer and sebaceous gland hyperplasia. The dermatitis was accompanied by infiltration of neutrophils and macrophages and by elevated levels of pro-inflammatory cytokines, chemokines and prostaglandin E2. In addition, Pla2g3-Tg mice had increased lymph aggregates and mucus in the airway, lymphocytic sialadenitis, hepatic extramedullary haemopoiesis, splenomegaly with increased populations of granulocytes and monocytes/macrophages, and increased serum IgG1. Collectively, these observations provide the first demonstration of spontaneous development of inflammation in mice with Tg overexpression of mammalian sPLA2.

Inhibition of Secreted Phospholipase A2. 4-Glycerol Derivatives of 4,5-Dihydro-3-(4-tetradecyloxybenzyl)-1,2,4-4H-oxadiazol-5-one with Broad Activities

Secreted phospholipases A2 (sPLA2s) have been reported to play an important role in various inflammatory conditions and thus represent an attractive therapeutic target. Previous SAR studies from our laboratory have revealed certain important features of our recently discovered specific hGIIA sPLA2 inhibitors, and we report here the synthesis and biological activities of glycerol-containing derivatives of our lead compound III (Figure 1). Efficient and selective synthesis methods have been developed to make glycerol trisubstituted by different groups on desired positions. In terms of biological activities, the best compounds (A3, A6, and A15) are more active than III (Figure 1), as potent as Me-Indoxam, an sPLA2s inhibitor of reference, against hGIIA, hGV, and hGX sPLA2s and at least 10 times less active toward the GIB enzymes in two in vitro assay systems. By synthesis of enantiopure (S)-A6, we demonstrated that no important improvement of the inhibitory potency could be achieved by this approach. Furthermore, the results show that the global lipophilicity is likely responsible for the anti-PLA2 activity and two oxadiazolone moieties seem too big to be accommodated by the active site of the hGIIA enzyme.

Interferon gamma-induced gene expression of the novel secretory phospholipase A2 type IID in human monocyte-derived macrophages is inhibited by lipopolysaccharide

Phospholipase A(2) (PLA(2)) is a superfamily of enzymes that may play a major role in airways inflammation. We investigated the effect of interferon-gamma (IFN-gamma) on the gene expression of 19 different PLA(2) types in human monocyte-derived macrophages and nasal epithelial cells (RPMI 2650). The cells were stimulated with IFN-gamma for different lengths of time (up to 48 h), and the mRNA levels of the different PLA(2) types were determined by reverse transcriptase-PCR (RT-PCR) and normalized to those of the house-keeping gene, GAPDH. It appeared that IFN-gamma clearly increased the expression of secretory PLA(2) IID (but not IIA) in macrophages, while both PLA(2) IID and IIA were upregulated in RPMI 2650 cells. Moreover, after 18 h, the mRNA levels of cytosolic PLA(2) IVA were 2-3 times higher in IFN-gamma-stimulated macrophages than controls, while there was no such effect of IFN-gamma in RPMI 2650 cells. Lipopolysaccharide (LPS) augmented the increased gene expression of PLA(2) IVA but decreased both the basal and the IFN-gamma-induced PLA(2) IID mRNA expression in macrophages (but not in RPMI 2650 cells). The NF-kappaB inhibitor Pyrrolidine dithiocarbamate (PDTC) and the phoshatidylinositol 3-kinase (PI3K) inhibitor wortmannin were employed to get an insight into the mechanism behind these observations. Incubation of macrophages with PDTC had no effect on the LPS impairment of PLA(2) IID gene expression, but inhibited the LPS mediated activation of PLA(2) IVA. No significant effect was noted of PDTC on IFN-gamma stimulation, while PI3K had no effect at all on any of the stimuli used. Furthermore, LPS (but not IFN-gamma) increased the mRNA levels of the nuclear factor (NF)-kappaB inhibitors alpha and xi in macrophages, but not in RPMI 2650 cells. These findings indicate that (a) the gene expression of secretory types PLA(2) IID and IIA in response to IFN-gamma is much dependent on cell type, and (b) the regulation of PLA(2) type IID in human macrophages is clearly different from that of PLA(2) type IVA. (c) PLA(2) IVA is probably under control of both NF-kappaB and IFN-gamma-responsive elements (GRE) or IFN-gamma-activating sites (GAS). The possibility that PLA(2) IID is involved in cytokine-mediated inflammation in the nasal mucosa is inferred, as is the potential role of PLA(2) IID in the host defense against LPS-containing bacteria.

A novel polymorphism in secretory phospholipase A2-IID is associated with body weight loss in chronic obstructive pulmonary disease

RATIONALE

Chronic and systemic inflammation, a potential cause of body weight loss in patients with chronic obstructive pulmonary disease (COPD), may be associated with the proinflammatory properties of secretory phospholipases A2 (sPLA2s), especially the group II subfamily sPLA2s.

OBJECTIVES

We tested our hypothesis that the individual susceptibility to body weight loss in patients with COPD is attributed to the genetic variances of this sPLA2 gene region.

METHODS

A total of 12 single nucleotide polymorphisms (SNPs) encompassing the sPLA2 gene region were determined in 276 male patients with COPD.

MEASUREMENTS AND MAIN RESULTS

We first analyzed our patients whose body mass index (BMI) was at the bottom 100 (BMI, 17.13 +/- 1.29 kg/m2) and at the top 100 (23.83 +/- 1.98) in relation to SNPs. Both the Fisher's exact test (odds ratio, 2.36; 95% confidence interval, 1.34-4.18; p = 0.004) and logistic regression analysis (odds ratio, 2.10; 95% confidence interval, 1.13-3.90; p = 0.019) showed statistical significance between one SNP (National Center for Biotechnology Information SNP reference: rs584367) and the reduction of BMI in the recessive model in patients with COPD. Using all the patients, a significant difference between the values of BMI (log transformed) of the mutant group (CT + TT) and that of the nonmutant group (CC) of this SNP (mean [SE], 1.293 [0.005] vs. 1.317 [0.006]; p = 0.003) was found after adjustment for age, smoking habit, and pulmonary function (analysis of covariance). Importantly, this SNP caused a change in amino acids in sPLA2-IID protein (Gly80Ser).

CONCLUSIONS

These results suggest that sPLA2-IID may be one of the susceptibility genes that contribute to body weight loss in patients with COPD.

Cellular Distribution, Post-translational Modification, and Tumorigenic Potential of Human Group III Secreted Phospholipase A2

Human group III secreted phospholipase A(2) (sPLA(2)-III) consists of a central group III sPLA(2) domain flanked by unique N- and C-terminal domains. We found that the sPLA(2) domain alone was sufficient for its catalytic activity and for its prostaglandin E(2) (PGE(2))-generating functions in various cell types. In several if not all cell types, the N- and C-terminal domains of sPLA(2)-III were proteolytically removed, leading to the production of the form containing only the sPLA(2) domain, which could be further N-glycosylated at two consensus sites. Immunohistochemistry demonstrated that sPLA(2)-III was preferentially expressed in the microvascular endothelium in human tissues with inflammation, ischemic injury, and cancer. In support of this, sPLA(2)-III was induced in cultured microvascular endothelial cells after stimulation with proinflammatory cytokines. Expression of sPLA(2)-III was also associated with various tumor cells, and colorectal cancer cells transfected with sPLA(2)-III exhibited enhanced PGE(2) production and cell proliferation, which required sPLA(2)-III catalytic activity. When implanted into nude mice, the sPLA(2)-III-transfected cells formed larger solid tumors with increased angiogenesis compared with control cells. Moreover, small interfering RNA for sPLA(2)-III significantly reduced PGE(2) production and proliferation of colorectal cancer cells. Taken together, these results reveal unique cell type-specific processing and N-glycosylation of sPLA(2)-III and the potential role of this enzyme in cancer development by stimulating tumor cell growth and angiogenesis.

Various secretory phospholipase A2 enzymes are expressed in rheumatoid arthritis and augment prostaglandin production in cultured synovial cells

Although group IIA secretory phospholipase A2 (sPLA2-IIA) is known to be abundantly present in the joints of patients with rheumatoid arthritis (RA), expression of other sPLA2s in this disease has remained unknown. In this study, we examined the expression and localization of six sPLA2s (groups IIA, IID, IIE, IIF, V and X) in human RA. Immunohistochemistry of RA sections revealed that sPLA2-IIA was generally located in synovial lining and sublining cells and cartilage chondrocytes, sPLA2-IID in lymph follicles and capillary endothelium, sPLA2-IIE in vascular smooth muscle cells, and sPLA2-V in interstitial fibroblasts. Expression levels of these group II subfamily sPLA2s appeared to be higher in severe RA than in inactive RA. sPLA2-X was detected in synovial lining cells and interstitial fibers in both active and inactive RA sections. Expression of sPLA2-IIF was partially positive, yet its correlation with disease states was unclear. Expression of sPLA2 transcripts was also evident in cultured normal human synoviocytes, in which sPLA2-IIA and -V were induced by interleukin-1 and sPLA2-X was expressed constitutively. Adenovirus-mediated expression of sPLA2s in cultured synoviocytes resulted in increased prostaglandin E2 production at low ng x mL(-1) concentrations. Thus, multiple sPLA2s are expressed in human RA, in which they may play a role in the augmentation of arachidonate metabolism or exhibit other cell type-specific functions.

Group VIB Ca2+-independent phospholipase A2gamma promotes cellular membrane hydrolysis and prostaglandin production in a manner distinct from other intracellular phospholipases A2

Although group VIA Ca2+-independent phospholipase A2beta (iPLA2beta) has been implicated in various cellular events, the functions of other iPLA2 isozymes remain largely elusive. In this study, we examined the cellular functions of group VIB iPLA2gamma. Lentiviral transfection of iPLA2gamma into HEK293 cells resulted in marked increases in spontaneous, stimulus-coupled, and cell death-associated release of arachidonic acid (AA), which was converted to prostaglandin E2 with preferred cyclooxygenase (COX)-1 coupling. Conversely, treatment of HEK293 cells with iPLA2gamma small interfering RNA significantly reduced AA release, indicating the participation of endogenous iPLA2gamma. iPLA2gamma protein appeared in multiple sizes according to cell types, and a 63-kDa form was localized mainly in peroxisomes. Electrospray ionization mass spectrometry of cellular phospholipids revealed that iPLA2gamma and other intracellular PLA2 enzymes acted on different phospholipid subclasses. Transfection of iPLA2gamma into HCA-7 cells also led to increased AA release and prostaglandin E2 synthesis via both COX-1 and COX-2, with a concomitant increase in cell growth. Immunohistochemistry of human colorectal cancer tissues showed elevated expression of iPLA2gamma in adenocarcinoma cells. These results collectively suggest distinct roles for iPLA2beta and iPLA2gamma in cellular homeostasis and signaling, a functional link between peroxisomal AA release and eicosanoid generation, and a potential contribution of iPLA2gamma to tumorigenesis.

Distribution of type V secretory phospholipase A2 expression in human hepatocytes damaged by liver disease

BACKGROUND AND AIM

Type V secretory phospholipase A2 (sPLA2-V) is a key enzyme in the arachidonate cascade. However, the distribution of sPLA2-V in human liver has not yet been investigated. In this study, the significance of sPLA2-V expression in human hepatocytes damaged by liver disease was investigated.

METHODS

Samples of liver tissue from patients with chronic hepatitis B and C, hepatitis virus-related liver cirrhosis, and congestive hepatocyte injury were immunostained with antibodies against sPLA2-V, cyclooxygenase (COX)-2, hepatitis viral antigens, transforming growth factor (TGF)-beta1, interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha.

RESULTS

In chronic hepatitis patients, sPLA2-V-positive hepatocytes were scattered in the liver lobules, while cyclooxygenase-2, IL-1beta, and TNF-alpha were diffusely expressed. Hepatocytes around necroinflammatory lesions were strongly positive for sPLA2-V. Some sPLA2-V-positive hepatocytes were also positive for viral antigens. TGF-beta1 was expressed only in fibrotic lesions. The pattern of distribution of these proteins in liver cirrhosis patients was similar to that in chronic hepatitis patients, but sPLA2-V expression tended to be more intense than in chronic hepatitis. In the congestive liver, sPLA2-V, COX-2, and the two cytokines were diffusely expressed in surviving hepatocytes.

CONCLUSIONS

sPLA2-V expression in hepatocytes is induced by viral infection, fibrosis, and circulatory disturbance. Immunostaining using sPLA2-V antibody is useful for the detection of injured hepatocytes in patients with liver diseases.

Novel mammalian group XII secreted phospholipase A2 lacking enzymatic activity

An increasing number of mammalian secreted phospholipases A(2) (sPLA(2)s) has been identified over the past few years. Here, we report the identification and recombinant expression of a novel sPLA(2)-like protein in mouse and human species that has been called group XIIB (GXIIB). The mature protein has a molecular mass of 19.7 kDa and structural features similar to those of the previously identified GXII sPLA(2), now called GXIIA. Strikingly, the GXIIB sPLA(2) has a mutation in the active site, replacing the canonical histidine by a leucine, suggesting that this sPLA(2) is catalytically inactive. Recombinant expression of human (hGXIIB) and mouse (mGXIIB) sPLA(2)s in Escherichia coli indicates that GXIIB sPLA(2)s display no measurable lipolytic activity on various types of phospholipid substrates. Furthermore, these sPLA(2)-like proteins display relatively weak affinity to phospholipid vesicles. Binding experiments indicate that these proteins are also unable to bind to the well-known M-type sPLA(2) receptor. The RNA tissue distribution of GXIIB sPLA(2)s is distinct from that of other sPLA(2)s including the homologous GXIIA. Strong expression was observed in liver, small intestine, and kidney in both human and mouse species. Interestingly, the expression of the novel sPLA(2) is dramatically decreased in human tumors from the same tissues. The absence of enzymatic activity suggests that the GXIIB sPLA(2)-like proteins probably exert their biological roles by acting as ligands for as yet unidentified receptors.

New phospholipase A(2) isozymes with a potential role in atherosclerosis

PURPOSE OF REVIEW

Inflammation is an integral feature of atherosclerosis, in which inflammatory processes contribute to the initiation, progression and rupture of lipid-rich atherosclerotic plaques. Recent studies have suggested the involvement of the proinflammatory secretory phospholipase A2 (sPLA2)-IIA in the development of atherosclerosis. This enzyme has been proposed to hydrolyze phosphatidylcholine (PC) in lipoproteins to liberate lyso-PC and free fatty acids in the arterial wall, thereby facilitating the accumulation of bioactive lipids and modified lipoproteins in atherosclerotic foci. However, the recent discovery of several novel sPLA2 isozymes has raised the question of which types of sPLA2 truly contribute to the atherosclerotic process.

RECENT FINDINGS

Amongst the 10 mammalian sPLA2 isozymes, sPLA2-X, -V, -IIF and -III exhibit much more potent PC-hydrolyzing activity than do the others, and can release free fatty acids and lysophospholipids from the PC-rich outer leaflet of the cellular plasma membrane. In particular, sPLA2-X and sPLA2-V hydrolyze PC in lipoproteins far more efficiently than does sPLA2-IIA. Moreover, sPLA2-X promotes foam cell formation in vitro and is expressed in the atherosclerotic arterial walls of apolipoprotein E deficient mice in vivo.

SUMMARY

PC-hydrolyzing sPLA2 isozymes, particularly sPLA2-V and sPLA2-X, are attractive candidates for proatherosclerotic factors that may act in place of sPLA2-IIA. However, their expression in human atherosclerotic lesions requires confirmation by specific methods that can distinguish between the different sPLA2 isozymes.

Differentiation-dependent regulation of secreted phospholipases A2 in murine epidermis

The action of secreted phospholipases A2 in skin is thought to be essential for epidermal barrier homeostasis. The incomplete knowledge of presence and functions of the novel secreted phospholipase A2 subtypes in skin prompted us to explore their expression in epidermis and primary keratinocytes from murine neonatal skin. We detected secreted phospholipases A2-IB, -IIA, -IIC, -IID, -IIE, -IIF, -V, -X, and -XII. To study secreted phospholipase A2 expression during epidermal differentiation, primary keratinocytes from the basal, suprabasal, and upper differentiated layers of neonatal mouse epidermis were obtained by density gradient centrifugation. mRNA for secreted phospholipases A2-IB, -IIE, -IIF, -V, and -XII-1 are mainly expressed in the upper differentiated layers, whereas the most prominent enzymes in the basal and suprabasal layers are secreted phospholipases A2-IIA, -IID, and -X. The mRNA for secreted phospholipase A2-IIC was found in all fractions. Immunohistochemical analysis in mouse skin sections reflected the mRNA distribution patterns in the different epidermal cell fractions. After in vitro induction of keratinocyte differentiation by increasing the calcium concentration of the medium, secreted phospholipases A2-IB, -IIE, -IIF, -V, and -XII-1 were upregulated, whereas secreted phospholipases A2-IIA, -IIC, -IID, and -X were mainly expressed in proliferating keratinocytes. The specific secreted phospholipase A2 expression profile in the skin suggests a distinct function for each enzyme in the epidermis.

Secretory phospholipases A2 as multivalent mediators of inflammatory and allergic disorders

Phospholipases A(2) (PLA(2)s) are enzymes responsible for mobilization of fatty acids, including arachidonic acid (AA), from phospholipids. These enzymes are classified as high-molecular-weight cytosolic PLA(2)s (cPLA(2)s) and low-molecular-weight secretory PLA(2)s (sPLA(2)s). There is increasing evidence that large quantities of sPLA(2)s are released in the plasma of patients with systemic inflammatory and autoimmune diseases. In addition, high levels of sPLA(2)s can be detected at sites of allergic inflammation including the upper airways of patients with rhinitis and the lower airways of patients with asthma. These extracellular enzymes play an important role in inflammation by releasing AA, which can be subsequently converted to proinflammatory prostaglandins and leukotrienes. Generation of AA mediated by sPLA(2)s occurs through different mechanisms, including (1) the direct hydrolysis of outer cell membrane phospholipids, (2) internalization and transfer of sPLA(2)s to intracellular pools of phospholipids enriched in AA, and (3) activation of cPLA(2)s. In addition, sPLA(2)s induce degranulation and production of cytokines and chemokines from a variety of cells involved in inflammatory and immune responses. These effects are exerted by mechanisms that are independent of the enzymatic activity and are mediated by the interaction of sPLA(2)s with specific or promiscuous membrane receptors. Therefore, sPLA(2)s may have an important role in inflammatory and allergic reactions by activating multiple mechanisms within inflammatory and immune cells, leading to the production of eicosanoids, cytokines and chemokines.

Cellular arachidonate-releasing function of novel classes of secretory phospholipase A2s (groups III and XII)

Here we report cellular arachidonate (AA) release and prostaglandin (PG) production by novel classes of secretory phospholipase A(2)s (sPLA(2)s), groups III and XII. Human group III sPLA(2) promoted spontaneous AA release, which was augmented by interleukin-1, in HEK293 transfectants. The central sPLA(2) domain alone was sufficient for its in vitro enzymatic activity and for cellular AA release at the plasma membrane, whereas either the unique N- or C-terminal domain was required for heparanoid-dependent action on cells to augment AA release, cyclooxygenase-2 induction, and PG production. Group III sPLA(2) was constitutively expressed in two human cell lines, in which other sPLA(2)s exhibited different stimulus inducibility. Human group XII sPLA(2) had a weak enzymatic activity in vitro and minimally affects cellular AA release and PG production. Cells transfected with group XII sPLA(2) exhibited abnormal morphology, suggesting a unique functional aspect of this enzyme. Based on the present results as well as our current analyses on the group I/II/V/X sPLA(2)s, general properties of cellular actions of a full set of mammalian sPLA(2)s in regulating AA metabolism are discussed.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.