Secretory phospholipases A2 induce cytokine release from blood and synovial fluid monocytes

IL-6/STAT3 axis dictates the PNPLA3-mediated susceptibility to non-alcoholic fatty liver disease

BACKGROUND & AIMS

A number of genetic polymorphisms have been associated with susceptibility to or protection against non-alcoholic fatty liver disease (NAFLD), but the underlying mechanisms remain unknown. Here, we focused on the rs738409 C>G single nucleotide polymorphism (SNP), which produces the I148M variant of patatin-like phospholipase domain-containing protein 3 (PNPLA3) and is strongly associated with NAFLD.

METHODS

To enable mechanistic dissection, we developed a human pluripotent stem cell (hPSC)-derived multicellular liver culture by incorporating hPSC-derived hepatocytes, hepatic stellate cells, and macrophages. We first applied this liver culture to model NAFLD by utilising a lipotoxic milieu reflecting the circulating levels of disease risk factors in affected individuals. We then created an isogenic pair of liver cultures differing only at rs738049 and compared NAFLD phenotype development.

RESULTS

Our hPSC-derived liver culture recapitulated many key characteristics of NAFLD development and progression including lipid accumulation and oxidative stress, inflammatory response, and stellate cell activation. Under the lipotoxic conditions, the I148M variant caused the enhanced development of NAFLD phenotypes. These differences were associated with elevated IL-6/signal transducer and activator of transcription 3 (STAT3) activity in liver cultures, consistent with transcriptomic data of liver biopsies from individuals carrying the rs738409 SNP. Dampening IL-6/STAT3 activity alleviated the I148M-mediated susceptibility to NAFLD, whereas boosting it in wild-type liver cultures enhanced NAFLD development. Finally, we attributed this elevated IL-6/STAT3 activity in liver cultures carrying the rs738409 SNP to increased NF-κB activity.

CONCLUSIONS

Our study thus reveals a potential causal link between elevated IL-6/STAT3 activity and 148M-mediated susceptibility to NAFLD.

IMPACT AND IMPLICATIONS

An increasing number of genetic variants manifest in non-alcoholic fatty liver disease (NAFLD) development and progression; however, the underlying mechanisms remain elusive. To study these variants in human-relevant systems, we developed an induced pluripotent stem cell-derived multicellular liver culture and focused on a common genetic variant (i.e. rs738409 in PNPLA3). Our findings not only provide mechanistic insight, but also a potential therapeutic strategy for NAFLD driven by this genetic variant in PNPLA3. Our liver culture is therefore a useful platform for exploring genetic variants in NAFLD development.

Citicoline and COVID-19-Related Cognitive and Other Neurologic Complications

With growing concerns about COVID-19’s hyperinflammatory condition and its potentially damaging impact on the neurovascular system, there is a need to consider potential treatment options for managing short- and long-term effects on neurological complications, especially cognitive function. While maintaining adequate structure and function of phospholipid in brain cells, citicoline, identical to the natural metabolite phospholipid phosphatidylcholine precursor, can contribute to a variety of neurological diseases and hypothetically toward post-COVID-19 cognitive effects. In this review, we comprehensively describe in detail the potential citicoline mechanisms as adjunctive therapy and prevention of COVID-19-related cognitive decline and other neurologic complications through citicoline properties of anti-inflammation, anti-viral, neuroprotection, neurorestorative, and acetylcholine neurotransmitter synthesis, and provide a recommendation for future clinical trials.

Plasma Phospholipids: A Promising Simple Biochemical Parameter to Evaluate COVID-19 Infection Severity

BACKGROUND

Coronavirus-19 (COVID-19) pandemic is a worldwide public health problem that has been known in China since December 25, 2019. Phospholipids are structural components of the mammalian cytoskeleton and cell membranes. They suppress viral attachment to the plasma membrane and subsequent replication in lung cells. In the virus-infected lung, phospholipids are highly prone to oxidation by reactive oxygen species, leading to the production of oxidized phospholipids (OxPLs).

OBJECTIVE

This study was carried out to explain the correlation between the level of plasma phospholipids in patients with COVID-19 infection and the levels of cytokine storms to assess the severity of the disease.

METHODS

Plasma samples from 34 enrolled patients with mild, moderate, and severe COVID-19 infection were collected. Complete blood count (CBC), plasma levels of D-dimer, ferritin, C-reactive protein (CRP), cholesterol, high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), phospholipids, secretory phospholipase A2 (sPLA2)α2, and cytokine storms were estimated, and lung computed tomography (CT) imaging was detected.

RESULTS

The CBC picture showed the presence of leukopenia, lymphopenia, and eosinopenia in patients with COVID-19 infection. Furthermore, a significant increase was found in plasma levels of D-dimer, CRP, ferritin, tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-13 as well as sPLA2α2 activity compared to normal persons. However, plasma levels of phospholipids decreased in patients with moderate and severe COVID-19 infection, as well as significantly decreased in levels of triacylglycerols and HDL-C in plasma from patients with severe infection only, compared to normal persons. Furthermore, a lung CT scan showed the presence of inflammation in a patient with mild, moderate, and severe COVID-19 infection.

CONCLUSIONS

This study shows that there is a correlation between plasma phospholipid depletion and elevated cytokine storm in patients with COVID-19 infection. Depletion of plasma phospholipid levels in patients with COVID-19 infection is due to oxidative stress, induction of cytokine storm, and systemic inflammatory response after endothelial cell damage promote coagulation. According to current knowledge, patients with COVID-19 infection may need to administer surfactant replacement therapy and sPLA2 inhibitors to treat respiratory distress syndrome, which helps them to maintain the interconnected surfactant structures.

Phenotypic and Functional Heterogeneity of Low-Density and High-Density Human Lung Macrophages

BACKGROUND

Pulmonary macrophages are a highly heterogeneous cell population distributed in different lung compartments.

METHODS

We separated two subpopulations of macrophages from human lung parenchyma according to flotation over density gradients.

RESULTS

Two-thirds 65.4% of the lung macrophages have a density between 1.065 and 1.078 (high-density macrophages: HDMs), and the remaining one-third (34.6) had a density between 1.039 and 1.052 (low-density macrophages: LDMs). LDMs had a larger area (691 vs. 462 μm²) and cell perimeter (94 vs. 77 μm) compared to HDMs. A significantly higher percentage of HDMs expressed CD40, CD45, and CD86 compared to LDMs. In contrast, a higher percentage of LDMs expressed the activation markers CD63 and CD64. The release of TNF-α, IL-6, IL-10 and IL-12 induced by lipopolysaccharide (LPS) was significantly higher in HDMs than in LDMs.

CONCLUSION

The human lung contains two subpopulations of macrophages that differ in buoyancy, morphometric parameters, surface marker expression and response to LPS. These subpopulations of macrophages probably play distinct roles in lung inflammation and immune responses.

Functional interaction of bacterial virulence factors of Xenorhabdus nematophila with a calcium-independent cytosolic PLA2 of Spodoptera exigua

Phospholipase A₂ (PLA₂) hydrolyzes the ester bond of phospholipids (PLs) at sn-2 and releases free fatty acids and lysophospholipids that are subsequently changed into various signal molecules to mediate various physiological processes. Numerous PLA₂s are known in various biological systems and can be divided into at least 16 groups. Although different PLA₂s recently have been annotated from several insect species, physiological roles are known for only a few genes. Two calcium-independent PLA₂s (Se-iPLA₂A and Se-iPLA₂B) are known in the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae). We generated and purified a recombinant Se-iPLA₂B (rSe-iPLA₂B) using a bacterial expression system and analyzed the enzyme kinetics. rSe-iPLA₂B exhibited catalytic activities against both arachidonyl (AA)-PL and non-AA-PL substrates. It was highly susceptible to iPLA₂-specific inhibitor, but insensitive to inhibitors specific to secretory PLA₂s or calcium-dependent cytosolic PLA₂s. Increasing calcium concentrations prevented enzyme activity, and culture medium of an entomopathogenic bacterium, Xenorhabdus nematophila, or its organic extracts significantly inhibited enzyme activity. Binding assays of rSe-iPLA₂B with known secondary metabolites identified from X. nematophila indicated that benzylideneacetone was the most potent inhibitor with a high binding affinity at 0.2 μM against rSe-iPLA₂B. Furthermore, rSe-iPLA₂B catalyzed the release of fatty acids from PLs extracted from S. exigua fat body, suggesting its physiological role in maintaining PL integrity. All these catalytic activities indicate that Se-iPLA₂B has the typical biochemical properties of other iPLA₂s. Its high binding affinity to secondary metabolites of X. nematophila suggests that it is a molecular target of X. nematophila, an entomopathogen.

A Potential Role of Phospholipase 2 Group IIA (PLA2-IIA) in P. gingivalis-Induced Oral Dysbiosis

Porphyromonas gingivalis is an oral pathogen with the ability to induce oral dysbiosis and periodontal disease. Nevertheless, the mechanisms by which P. gingivalis could abrogate the host-microbe symbiotic relationship leading to oral dysbiosis remain unclear. We have recently demonstrated that P. gingivalis specifically increased the antimicrobial properties of oral epithelial cells, through a strong induction of the expression of PLA₂-IIA in a mechanism that involves activation of the Notch-1 receptor. Moreover, gingival expression of PLA₂-IIA was significantly increased during initiation and progression of periodontal disease in non-human primates and interestingly, those PLA₂-IIA expression changes were concurrent with oral dysbiosis. In this chapter, we present an innovative hypothesis of a potential mechanism involved in P. gingivalis-induced oral dysbiosis and inflammation based on our previous observations and a robust body of literature that supports the antimicrobial and proinflammatory properties of PLA₂-IIA as well as its role in other chronic inflammatory diseases.

A prophylactic role of a secretory PLA2 of Spodoptera exigua against entomopathogens

Phospholipase A₂ (PLA₂) hydrolyses phospholipids at sn-2 position to release free fatty acids and lysophospholipids. Secretory type of PLA₂ (sPLA₂) has been found in many different animals including insects. Insect sPLA₂s have been divided into venomous and nonvenomous PLA₂s. A non-venomous sPLA₂ (Se-sPLA₂) has been identified in beet armyworm, Spodoptera exigua. Its high enzyme activity is detected in hemolymph of naïve larvae. However, the physiological role of high sPLA₂ activity in hemolymph remains unclear. To determine the physiological role of sPLA₂ in hemolymph, a recombinant Se-sPLA₂ (rSe-sPLA₂) was expressed in a bacterial expression system and purified to test antimicrobial activity against various microbes. Purified rSe-sPLA₂ exhibited typical enzyme kinetic properties, including becoming saturated at high substrate concentrations, exhibiting optimal activity at pH 7-9, and being inactivated at high temperatures. However, a reducing agent (dithiothreitol) or calcium chelator treatment inhibited the catalytic activity. A specific inhibitor to sPLA₂ also inhibited the enzyme activity of rSe-sPLA₂ while other type PLA₂ inhibitors did not. Furthermore, eight bacterial metabolites of Xenorhabdus and Photorhabdus known to be inhibitory against insect PLA₂ significantly inhibited the enzyme activity of rSe-sPLA₂. High concentrations of rSe-sPLA₂ (above 0.5 mM) showed significant cytotoxicity to hemocytes of S. exigua. At concentrations without showing cytotoxicity, rSe-sPLA₂ possessed significant antimicrobial activities against entomopathogenic bacteria (Serratia marscens and Entercoccus mondtii) and fungi (Beauveria bassiana and Metarhyzium rileyi). Hemolymph obtained from larvae treated with RNA interference specific to Se-sPLA₂ significantly lost such antimicrobial activities. However, the addition of rSe-sPLA₂ to the hemolymph significantly rescued such antimicrobial activities. These results indicate that Se-sPLA₂ possesses antimicrobial activity, suggesting that it might act as a prophylactic agent against microbial pathogens in the hemolymph of S. exigua.

Secreted phospholipase A2 is increased in meconium-stained amniotic fluid of term gestations: potential implications for the genesis of meconium aspiration syndrome

BACKGROUND

Meconium-stained amniotic fluid (MSAF) represents the passage of fetal colonic content into the amniotic cavity. Meconium aspiration syndrome (MAS) is a complication that occurs in a subset of infants with MSAF. Secreted phospholipase A2 (sPLA2) is detected in meconium and is implicated in the development of MAS. The purpose of this study was to determine if sPLA2 concentrations are increased in the amniotic fluid of women in spontaneous labor at term with MSAF.

MATERIALS AND METHODS

This was a cross-sectional study of patients in spontaneous term labor who underwent amniocentesis (n = 101). The patients were divided into two study groups: (1) MSAF (n = 61) and (2) clear fluid (n = 40). The presence of bacteria and endotoxin as well as interleukin-6 (IL-6) and sPLA2 concentrations in the amniotic fluid were determined. Statistical analyses were performed to test for normality and bivariate analysis. The Spearman correlation coefficient was used to study the relationship between sPLA2 and IL-6 concentrations in the amniotic fluid.

RESULTS

Patients with MSAF have a higher median sPLA2 concentration (ng/mL) in amniotic fluid than those with clear fluid [1.7 (0.98-2.89) versus 0.3 (0-0.6), p < 0.001]. Among patients with MSAF, those with either microbial invasion of the amniotic cavity (MIAC, defined as presence of bacteria in the amniotic cavity), or bacterial endotoxin had a significantly higher median sPLA2 concentration (ng/mL) in amniotic fluid than those without MIAC or endotoxin [2.4 (1.7-6.0) versus 1.7 (1.3-2.5), p < 0.05]. There was a positive correlation between sPLA2 and IL-6 concentrations in the amniotic fluid (Spearman Rho = 0.3, p < 0.05).

CONCLUSION

MSAF that contains bacteria or endotoxin has a higher concentration of sPLA2, and this may contribute to induce lung inflammation when meconium is aspirated before birth.

Comparative efficacy of a secretory phospholipase A2 inhibitor with conventional anti-inflammatory agents in a rat model of antigen-induced arthritis

Introduction

Previously, secretory phospholipase A₂ (sPLA₂) inhibition has been used as an adjunct to conventional rheumatoid arthritis therapy in human clinical trials without significant improvement of arthritic pathology. In this study, we compared the efficacy of a potent and orally active group IIa secretory phospholipase A₂ inhibitor (sPLA₂I) to conventional anti-arthritic agents; infliximab, leflunomide and prednisolone, in a rat model of antigen-induced arthritis.

Methods

Initially, to establish efficacy and dose-response, rats were orally dosed with the sPLA₂I (1 and 5 mg/kg) two days prior to arthritis induction, and then daily throughout the 14-day study period. In the second trial, rats were orally dosed with the sPLA₂I (5 and 10 mg/kg/day) beginning two days after the induction of arthritis, at the peak of joint swelling. Separate groups of rats were also dosed with the tumour necrosis factor-alpha (TNF-α) inhibitor infliximab (single 3 mg/kg i.v. injection), leflunomide (10 mg/kg/day, oral) or prednisolone (1 mg/kg/day, oral) at this same time point and used as comparative treatments.

Results

In the pathology prevention trial, both 1 and 5 mg/kg dose groups of sPLA₂I demonstrated a significant reduction in joint swelling and gait disturbances; however, only the higher 5 mg/kg dose resulted in significantly reduced histopathology scores. In the post-induction trial, rats dosed with sPLA₂I showed a significant improvement in joint swelling and gait scoring, whereas none of the conventional therapeutics achieved a significant decrease in both of these two disease markers. Histopathological scoring at the end-point of the study demonstrated significantly reduced median scores in rats treated with 10 mg/kg sPLA₂I and leflunomide.

Conclusions

The results from this study suggest a pathogenic role for sPLA₂ enzymes in this model of arthritis in rats, and the potential clinical utility of sPLA₂ inhibition as a safer, and more effective, alternative to conventional anti-arthritic therapeutics.

Induction of inflammatory cell accumulation by TM-N49 and promutoxin, two novel phospholipase A(2)

Local inflammation is a prominent characteristic of snakebite wound. Snake venom phospholipase A(2)s (PLA(2)s) are one of the main components which contribute to accumulation of inflammatory cells. We have isolated TM-N49 and promutoxin from Protobothrops mucrosquamatus venom and investigated their ability in induction of cell accumulation by using an in vivo mouse model. The results showed that both TM-N49 and promutoxin are potent stimuli for induction of neutrophil, lymphocyte, macrophage and eosinophil accumulation in the mouse peritoneum. The TM-N49- and promutoxin-induced inflammatory cell accumulation was inhibited by pretreatment of animals with cyproheptadine, terfenadine and Ginkgolide B, indicating that histamine and PAF is likely to contribute to the cells accumulation. Pre-injection of antibodies against adhesion molecules ICAM-1, CD18, CD11a and L-selectin showed that ICAM-1 is a key adhesion molecule of TM-N49- and promutoxin-induced lymphocyte, macrophage and eosinophil accumulation; CD18 and CD11a plays an important role in the migration of neutrophils, eosinophils and macrophages; and L-selectin is involved in the neutrophil and eosinophil accumulation. In conclusion, induction of inflammatory cell accumulation by TM-N49 and promutoxin confirms that group II PLA(2)s is pivotal stimulus for cell infiltration, through which they participate in the formation of snakebite inflammation.

Group V secreted phospholipase A2 contributes to LPS-induced leukocyte recruitment

Secreted phospholipases A(2) (sPLA(2)s) are well known for their contribution in the biosynthesis of inflammatory eicosanoids. These enzymes also participate in the inflammatory process by regulating chemokine production and protein expression of adhesion molecules. The majority of sPLA(2) isoforms are up-regulated by proinflammatory stimuli such as bacterial lipopolysaccharide (LPS), which predominantly increases the expression of group V sPLA(2) (sPLA(2)-V). Furthermore, it has recently been shown that sPLA(2)-V is a critical messenger in the regulation of cell migration during allergic airway responsiveness. Herein, we investigated the effect of sPLA(2)-V on LPS-mediated leukocyte recruitment and its capacity to modulate adhesion molecule expression. We conducted our study in the murine air pouch model, using sPLA(2)-V null mice (sPLA(2)-V(-/-)) and control wild-type (WT) littermates. We observed that LPS (1 microg/ml)-mediated leukocyte emigration in sPLA(2)-V(-/-) was attenuated by 52% and 86% upon 6 and 12 h of treatment respectively, as compared to WT mice. In WT mice, treatment with the cell-permeable sPLA(2) inhibitor (12-epi-scalaradial; SLD) reduced LPS-mediated leukocyte recruitment by 67%, but had no additional inhibitory effect in sPLA(2)-V(-/-) mice. Protein analyses from the air pouch skin were carried out upon LPS-challenge, and the expression of intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1 were both significantly reduced in sPLA(2)-V(-/-) mice as compared to control WT mice. Together, our data demonstrate the role of sPLA(2)-V in LPS-induced ICAM-1 and VCAM-1 protein overexpression and leukocyte recruitment, supporting the contribution of sPLA(2)-V in the development of inflammatory innate immune responses.

Mouse strain-dependent differences in susceptibility to Neisseria gonorrhoeae infection and induction of innate immune responses

Acute gonorrhea in women is characterized by a mucopurulent exudate that contains polymorphonuclear leukocytes (PMNs) with intracellular gonococci. Asymptomatic infections are also common. Information on the innate response to Neisseria gonorrhoeae in women is limited to studies with cultured cells, isolated immune cells, and analyses of cervicovaginal fluids. 17beta-Estradiol-treated BALB/c mice can be experimentally infected with N. gonorrhoeae, and a vaginal PMN influx occurs in 50 to 80% of mice. Here, we compared the colonization loads and proinflammatory responses of BALB/c, C57BL/6 and C3H/HeN mice to N. gonorrhoeae. BALB/c and C57BL/6 mice were colonized at similar levels following inoculation with 10(6) CFU of N. gonorrhoeae. BALB/c, but not C57BL/6, mice exhibited a marked vaginal PMN influx. Tumor necrosis factor alpha, interleukin-6, macrophage inflammatory protein 2 (MIP-2), and keratinocyte-derived chemokine were elevated in vaginal secretions from infected BALB/c mice, but not in those from C57BL/6 mice. MIP-2 levels positively correlated with a vaginal PMN influx. In contrast to BALB/c and C57BL/6 mice, C3H/HeN mice were resistant to infection, and there was no evidence of an inflammatory response. We conclude that N. gonorrhoeae causes a productive infection in BALB/c mice that is characterized by the induction of proinflammatory cytokines and chemokines and the recruitment of PMNs. Infection of C57BL/6 mice, in contrast, is more similar to asymptomatic infection. C3H/HeN mice are inherently resistant to N. gonorrhoeae infection, and this resistance is not due to an overwhelming inflammatory response to infection. Host genetic factors can therefore impact susceptibility and the immune response to N. gonorrhoeae.

Modulation of the pharmacological effects of enzymatically-active PLA2 by BTL-2, an isolectin isolated from the Bryothamnion triquetrum red alga

Background

An interaction between lectins from marine algae and PLA₂ from rattlesnake was suggested some years ago. We, herein, studied the effects elicited by a small isolectin (BTL-2), isolated from Bryothamnion triquetrum, on the pharmacological and biological activities of a PLA₂ isolated from rattlesnake venom (Crotalus durissus cascavella), to better understand the enzymatic and pharmacological mechanisms of the PLA₂ and its complex.

Results

This PLA₂ consisted of 122 amino acids (approximate molecular mass of 14 kDa), its pI was estimated to be 8.3, and its amino acid sequence shared a high degree of similarity with that of other neurotoxic and enzymatically-active PLA₂s. BTL-2 had a molecular mass estimated in approximately 9 kDa and was characterized as a basic protein. In addition, BTL-2 did not exhibit any enzymatic activity.

The PLA₂ and BTL-2 formed a stable heterodimer with a molecular mass of approximately 24–26 kDa, estimated by molecular exclusion HPLC. In the presence of BTL-2, we observed a significant increase in PLA₂ activity, 23% higher than that of PLA₂ alone. BTL-2 demonstrated an inhibition of 98% in the growth of the Gram-positive bacterial strain, Clavibacter michiganensis michiganensis (Cmm), but only 9.8% inhibition of the Gram-negative bacterial strain, Xanthomonas axonopodis pv passiflorae (Xap). PLA₂ decreased bacterial growth by 27.3% and 98.5% for Xap and Cmm, respectively, while incubating these two proteins with PLA₂-BTL-2 inhibited their growths by 36.2% for Xap and 98.5% for Cmm.

PLA₂ significantly induced platelet aggregation in washed platelets, whereas BTL-2 did not induce significant platelet aggregation in any assay. However, BTL-2 significantly inhibited platelet aggregation induced by PLA₂. In addition, PLA₂ exhibited strong oedematogenic activity, which was decreased in the presence of BTL-2. BTL-2 alone did not induce oedema and did not decrease or abolish the oedema induced by the 48/80 compound.

Conclusion

The unexpected results observed for the PLA₂-BTL-2 complex strongly suggest that the pharmacological activity of this PLA₂ is not solely dependent on the presence of enzymatic activity, and that other pharmacological regions may also be involved. In addition, we describe for the first time an interaction between two different molecules, which form a stable complex with significant changes in their original biological action. This opens new possibilities for understanding the function and action of crude venom, an extremely complex mixture of different molecules.

Purification, characterization and cytokine release function of a novel Arg-49 phospholipase A(2) from the venom of Protobothrops mucrosquamatus

Group IIA phospholipase A(2) (PLA(2)) are major components in Viperidae/Crotalidae venom. In the present study, a novel PLA(2) named promutoxin with Arg at the site 49 has been purified from the venom of Protobothrops mucrosquamatus by chromatography. It consists of 122 amino acid residues with a molecular mass of 13,656 Da assessed by MALDI-TOF. It has the structural features of snake venom group IIA PLA(2)s, but has no PLA(2) enzymatic activity. Promutoxin shows higher amino acid sequence identity to the K49 PLA(2)s (72-95%) than to D49 PLA(2)s (52-58%). Promutoxin exhibits potent myotoxicity in the animal model with as little as 1 microg of promutoxin causing myonecrosis and myoedema in the gastrocnemius muscle of mice. Promutoxin is also able to stimulate the release of IL-12, TNFalpha, IL-6 and IL-1beta from human monocytes, and induce IL-2, TNFalpha and IL-6 release from T cells, indicating that this snake venom group IIA PLA(2) is actively involved in the inflammatory process in man caused by snake venom poisoning.

Signaling events involved in cytokine and chemokine production induced by secretory phospholipase A2 in human lung macrophages

Secretory phospholipases A(2) (sPLA(2)) are enzymes released during inflammatory reactions. These molecules activate immune cells by mechanisms either related or unrelated to their enzymatic activity. We examined the signaling events activated by group IA (GIA) and group IB (GIB) sPLA(2) in human lung macrophages leading to cytokine/chemokine production. sPLA(2) induced the production of cytokines (TNF-alpha, IL-6 and IL-10) and chemokines (CCL2, CCL3, CCL4 and CXCL8), whereas no effect was observed on IL-12, CCL1, CCL5 and CCL22. sPLA(2) induced the phosphorylation of the MAPK p38 and ERK1/2, and inhibition of these kinases by SB203580 and PD98059, respectively, reduced TNF-alpha and CXCL8 release. Suppression of sPLA(2) enzymatic activity by a site-directed inhibitor influenced neither cytokine/chemokine production nor activation of MAPK, whereas alteration of sPLA(2) secondary structure suppressed both responses. GIA activated the phosphatidylinositol 3-kinase (PI3 K)/Akt system and a specific inhibitor of PI3 K (LY294002) reduced sPLA(2)-induced release of TNF-alpha and CXCL8. GIA promoted phosphorylation and degradation of IkappaB and inhibition of NF-kappaB by MG-132 and 6-amino-4-phenoxyphenylethylamino-quinazoline suppressed the production of TNF-alpha and CXCL8. These results indicate that sPLA(2) induce the production of cytokines and chemokines in human macrophages by a non-enzymatic mechanism involving the PI3 K/Akt system, the MAPK p38 and ERK1/2 and NF-kappaB.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.

Activation of human inflammatory cells by secreted phospholipases A2

Secreted phospholipases A(2) (sPLA(2)s) are enzymes detected in serum and biological fluids of patients with various inflammatory, autoimmune and allergic disorders. Different isoforms of sPLA(2)s are expressed and released by human inflammatory cells, such as neutrophils, eosinophils, T cells, monocytes, macrophages and mast cells. sPLA(2)s generate arachidonic acid and lysophospholipids thus contributing to the production of bioactive lipid mediators in inflammatory cells. However, sPLA(2)s also activate human inflammatory cells by mechanisms unrelated to their enzymatic activity. Several human and non-human sPLA(2)s induce degranulation of mast cells, neutrophils and eosinophils and activate exocytosis in macrophages. In addition some, but not all, sPLA(2) isoforms promote cytokine and chemokine production from macrophages, neutrophils, eosinophils, monocytes and endothelial cells. These effects are primarily mediated by binding of sPLA(2)s to specific membrane targets (heparan sulfate proteoglycans, M-type, N-type or mannose receptors) expressed on effector cells. Thus, sPLA(2)s may play an important role in the initiation and amplification of inflammatory reactions by at least two mechanisms: production of lipid mediators and direct activation of inflammatory cells. Selective inhibitors of sPLA(2)-enzymatic activity and specific antagonists of sPLA(2) receptors are current being tested for pharmacological treatment of inflammatory and autoimmune diseases.

Secretory phospholipase A2 induces apoptosis through TNF-α and cytochrome c-mediated caspase cascade in murine macrophage RAW 264.7 cells

Phospholipase A2 (PLA2) is an esterase that cleaves the sn-2 ester bond in glycerophospholipids, thereby releasing free fatty acids and lysophospholipids. In addition to the apoptotic activity of cytosolic PLA2 and Ca2+-independent PLA2, recent studies showed that secretory PLA2 (sPLA2) also play a role in apoptosis. However, the details of molecular mechanism have not been fully elucidated. Our data demonstrated that group IB PLA (IB PLA2)-exposed murine macrophage 264.7 cells showed characteristic features of apoptosis such as morphological changes, DNA laddering, staining positive for propidium iodide (PI) as well as Annexin V and activation of caspases and subsequent cleavage of poly (ADP-ribose) polymerase (PARP) in dose- and time-dependent manner. Moreover, IB PLA2 was found to elicit tumor necrosis factor (TNF)-alpha production and release of cytochrome c, suggesting that IB PLA2 exerts its apoptotic activity via the induction of TNF-alpha production and cytochrome c release, which results in triggering the activation of caspase cascade and PARP cleavage.

Secretory phospholipase A2 of group IIA: Is it an offensive or a defensive player during atherosclerosis and other inflammatory diseases?

Since its discovery in the serum of patients with severe inflammation and in rheumatoid arthritic fluids, the secretory phospholipase A2 of group IIA (sPLA2-IIA) has been chiefly considered as a proinflammatory enzyme, the result of which has been very intense interest in selective inhibitors of sPLA2-IIA in the hope of developing new and efficient therapies for inflammatory diseases. The recent discovery of the antibacterial properties of sPLA2-IIA, however, has raised the question of whether the upregulation of sPLA2-IIA during inflammation is to be considered uniformly negative and the hindrance of sPLA2-IIA in every instance beneficial. The aim of this review is for this reason, along with the results of various investigations which argue for the proinflammatory and proatherogenic effects of an upregulation of sPLA2-IIA, also to array data alongside which point to a protective function of sPLA2-IIA during inflammation. Thus, it could be shown that sPLA2-IIA, apart from the bactericidal effects, possesses also antithrombotic properties and indeed plays a possible role in the resolution of inflammation and the accelerated clearance of oxidatively modified lipoproteins during inflammation via the liver and adrenals. Based on these multipotent properties the knowledge of the function of sPLA2-IIA during inflammation is a fundamental prerequisite for the development and establishment of new therapeutic strategies to prevent and treat severe inflammatory diseases up to and including sepsis.

cAMP elevating agents suppress secretory phospholipase A(2)-induced matrix metalloproteinase-2 activation

Phospholipase A2 proteins are major regulators of the arachidonic acid cascade and are involved in various cellular responses. Previously, we reported that group IB PLA2 proteins stimulate MMP-2 activation and subsequent cell migration. Here, we describe a novel mechanism whereby sPLA2-induced proMMP-2 activation is regulated by intracellular cAMP in HT1080 cells, although sPLA2 itself had no effect on the regulation of cAMP levels. Exogenous dibutyryl cAMP (a cAMP analogue) strongly inhibited proMMP-2 activation, and cAMP elevating agents, namely, cholera toxin (a Gs activator) and forskolin (an adenylyl cyclase activator), abrogated basal and sPLA2-induced proMMP-2 activation. We also found that the down-regulation of TIMP-2 expression and extracellular signal-regulated kinase (ERK)1/2 activation by sPLA2 were blocked by increasing the intracellular cAMP level. Taken together, our data indicate that sPLA2-induced proMMP-2 activation is influenced by intracellular cAMP levels via the modulations of TIMP-2 expression and ERK1/2 activation.

In Vivo Protective Role of Human Group IIA Phospholipase A2against Experimental Anthrax

Anthrax is an acute disease caused by Bacillus anthracis. Some animal species are relatively resistant to anthrax infection. This trait has been correlated to the extent of the local inflammatory reaction, suggesting innate immunity to be the first line of defense against B. anthracis infection in nonimmunized hosts. Group IIA secreted phospholipase A2 (sPLA2-IIA) is produced in particular by macrophages and possesses potent antibacterial activity especially against Gram-positive bacteria. We have previously shown in vitro that sPLA2-IIA kills both germinated B. anthracis spores and encapsulated bacilli. Here we show that sPLA2-IIA plays in vivo a protective role against experimental anthrax. Transgenic mice expressing human sPLA2-IIA are resistant to B. anthracis infection. In addition, in vivo administration of recombinant human sPLA2-IIA protects mice against B. anthracis infection. The protective effect was observed both with a highly virulent encapsulated nontoxinogenic strain and a wild-type encapsulated toxinogenic strain, showing that toxemia did not hinder the sPLA2-IIA-afforded protection. sPLA2-IIA, a natural component of the immune system, may thus be considered a novel therapeutic agent to be used in adjunct with current therapy for treating anthrax. Its anthracidal activity would be effective even against strains resistant to multiple antibiotics.

Secretory phospholipases A2 in inflammatory and allergic diseases: not just enzymes

Secretory phospholipases A(2) (sPLA(2)s) are molecules released in plasma and biologic fluids of patients with systemic inflammatory, autoimmune, and allergic diseases. Several sPLA(2) isoforms are expressed and released by such human inflammatory cells as neutrophils, eosinophils, basophils, T cells, monocytes, macrophages, and mast cells. Certain sPLA(2)s release arachidonic acid, thereby providing the substrate for the biosynthesis of proinflammatory eicosanoids. However, there are other mechanisms by which sPLA(2)s might participate in the synthesis of lipid mediators. Interestingly, sPLA(2)s activate inflammatory cells through mechanisms unrelated to their enzymatic activity. Several sPLA(2)s induce degranulation of mast cells and eosinophils and activate exocytosis in macrophages. Furthermore, sPLA(2)s promote cytokine and chemokine production from macrophages, neutrophils, eosinophils, monocytes, and endothelial cells. Some of these effects are mediated by the binding of sPLA(2)s to specific receptors expressed on effector cells. Thus sPLA(2)s might play important roles in the initiation and amplification of the inflammatory reaction. Selective inhibitors of sPLA(2)s and specific antagonists of sPLA(2) receptors might prove useful in the treatment of allergic and autoimmune diseases, such as bronchial asthma and rheumatoid arthritis.

Group IB secretory phospholipase A2 stimulates leukotriene B4 production by a unique mechanism in human neutrophils

We found that group IB secretory phospholipase A(2) (sPLA(2)-IB) stimulates leukotriene B4 (LTB4) production in the absence of cytochalasin B in human neutrophils. Although LTB4 production has been reported to be associated with arachidonic acid release, the exogenous addition of sPLA(2)-IB did not induce this release from human neutrophils, suggesting that sPLA(2)-IB stimulates LTB4 production without affecting arachidonic acid. Moreover, the intracellular signaling events induced by sPLA(2)-IB included an increase in intracellular Ca(2+), which is required for LTB4 production. sPLA(2)-IB also stimulated mitogen-activated protein kinase ERK, but its activity was not required for LTB4 production. In terms of functional aspects, the supernatant of sPLA(2)-IB-stimulated human neutrophils caused chemotactic migration, which was almost completely inhibited by preincubating these cells with three different 5-lipoxygenase inhibitors (MK-886, AA-861, or NDGA). Taken together, we suggest that sPLA(2)-IB plays a role in the modulation of inflammatory and immune responses by inducing LTB4 production in human neutrophils.

A potent and selective inhibitor of group IIa secretory phospholipase A2 protects rats from TNBS-induced colitis

Secretory phospholipase A(2) (sPLA(2)) enzymes have been implicated in the pathogenesis of human inflammatory bowel disease (IBD). In this study we compared the efficacy of a potent, new and highly selective inhibitor of group IIa human sPLA(2) enzyme (5-(4-benzyloxyphenyl)-4S-(7-phenylheptanoylamino)-pentanoic acid; sPLA(2)I), with that of sulfasalazine, in a rat model of trinitrobenzene sulfonic acid (TNBS)-induced colitis. Following a single oral dose of sPLA(2)I (5 mg/kg), pharmacoactive levels of drug were detected in the serum within 15 min and for up to 24 h by liquid chromatography mass spectrometry analysis. Rats treated with sPLA(2)I (5 mg/kg/day) prior to induction of colitis were significantly healthier than TNBS-alone rats, as shown by reduced mortality, improved food intake and increased body weight, and significantly reduced colon myeloperoxidase levels, edema, tumour necrosis factor-alpha levels, and colon macroscopic pathology scores after 8 days. Rats pretreated with sulfasalazine (100 mg/kg/day) also had reduced disease expression markers similar to the sPLA(2)I, but exhibited no improvement in colon edema. This study supports a role for the group IIa sPLA(2) enzyme in pathology associated with the TNBS rat model of IBD, and suggests a possible therapeutic application for selective inhibitors of group IIa sPLA(2) inhibitors in the treatment of IBD.

The catalytically active secretory phospholipase A2 type IIA is involved in restenosis development after PTCA in human coronary arteries and generation of atherogenic LDL

Secretory phospholipase A2 type IIA (sPLA2) may actively contribute to atherogenesis, acting either within the arterial wall or in plasma. Proinflammatory eicosanoids and lysophospholipids, generated through hydrolysis of cell membrane phospho-lipids by sPLA2, initiate and prolong the inflammatory process. In the present study we examined the possible involvement of sPLA2 in development of restenosis in patients undergoing percutaneous transluminal coronary angioplasty (PTCA). We also investigated whether serum sPLA2 could catalyze accumulation of lysophosphatidylcholine (LPC) in LDL. Concentrations and catalytic activities of sPLA2 were measured in blood serum of 49 consenting patients immediately before, 1-7 and 180 days after PTCA. All patients had repeat angiograms at 180-day follow-up. Restenosis was registered in 19 patients. Accumulation of LPC in LDL was evaluated by thin-layer chromatography after incubation of blood serum with LDL. Serum sPLA2 concentrations increased in all study patients by day 1 post-PTCA, but the increase was significantly greater and more protracted in patients who developed restenosis. Catalytic activities increased significantly 6 days post-PTCA in patients who developed restenosis, whereas for patients without restenosis there was no change in serum sPLA2 activity throughout the study period in spite of the sPLA2 presence in blood. Incubation of blood serum (6 days post-PTCA) with LDL resulted in accumulation of LPC only for those patients who subsequently developed restenosis. Manoalide, a specific inhibitor of sPLA2, completely blocked the LPC accumulation. The data indicate that elevated serum sPLA2 activity after PTCA is associated with restenosis development and may be involved in atherogenic modification of LDL in blood serum.

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.

Activation of Cytokine Production by Secreted Phospholipase A2 in Human Lung Macrophages Expressing the M-Type Receptor

Secreted phospholipases A(2) (sPLA(2)) are enzymes released in plasma and extracellular fluids during inflammatory diseases. Because human group IB and X sPLA(2)s are expressed in the lung, we examined their effects on primary human lung macrophages (HLM). Both sPLA(2)s induced TNF-alpha and IL-6 release in a concentration-dependent manner by increasing their mRNA expression. This effect was independent of their enzymatic activity because 1) the capacity of sPLA(2)s to mobilize arachidonic acid from HLM was unrelated to their ability to induce cytokine production; and 2) two catalytically inactive isoforms of group IB sPLA(2) (bromophenacyl bromide-inactivated human sPLA(2) and the H48Q mutant of the porcine sPLA(2)) were as effective as the catalytically active sPLA(2)s in inducing cytokine production. HLM expressed the M-type receptor for sPLA(2)s at both mRNA and protein levels, as determined by RT-PCR, immunoblotting, immunoprecipitation, and flow cytometry. Me-indoxam, which decreases sPLA(2) activity as well as binding to the M-type receptor, suppressed sPLA(2)-induced cytokine production. Incubation of HLM with the sPLA(2)s was associated with phosphorylation of ERK1/2, and a specific inhibitor of this pathway, PD98059, significantly reduced the production of IL-6 elicited by sPLA(2)s. In conclusion, two distinct sPLA(2)s produced in the human lung stimulate cytokine production by HLM via a mechanism that is independent of their enzymatic activity and involves activation of the ERK1/2 pathway. HLM express the M-type receptor, but its involvement in eliciting cytokine production deserves further investigation.

Group IB Secretory Phospholipase A2 Stimulates CXC Chemokine Ligand 8 Production via ERK and NF-κB in Human Neutrophils

Although the level of group IB secretory phospholipase A(2) (sPLA(2)-IB) has been reported to be up-regulated during inflammatory response, the role of sPLA(2)-IB on the regulation of inflammation and immune responses has not been fully elucidated. In this study, we found that sPLA(2)-IB stimulates the expression and secretion of CXCL8 without affecting other proinflammatory cytokines, such as IL-1beta or TNF alpha in human neutrophils. The induction of CXCL8 secretion by sPLA(2)-IB occurs at both the transcription and translational levels and correlates with activation of NF-kappaB. Moreover, the NF-kappaB inhibitors pyrrolidinedithiocarbamate, dexamethasone, or sulfasalazine were found to prevent CXCL8 production by sPLA(2)-IB in human neutrophils. In addition, the signaling events induced by sPLA(2)-IB included activation of the MAPK ERK and an increase in intracellular Ca(2+), which are both required for CXCL8 production. The exogenous addition of sPLA(2)-IB did not induce arachidonic acid release from human neutrophils, and the inactivation of sPLA(2)-IB by EGTA did not affect CXCL8 production by sPLA(2)-IB in human neutrophils. Taken together, we suggest that sPLA(2)-IB plays a role in the modulation of inflammatory and immune responses via the sPLA(2) receptor, by inducing CXCL8 in human neutrophils.

Secretory phospholipase A2 induces dendritic cell maturation

High level of phospholipase A(2) (PLA(2)) activity is found in serum and biological fluids during the acute-phase response (APR). Extracellular PLA(2) in fluids of patients with inflammatory diseases such as sepsis, acute pancreatitis or rheumatoid arthritis is also associated with propagation of inflammation. PLA(2) activity is involved in the release of both pro- and anti-inflammatory lipid mediators from phospholipids of cellular membranes or circulating lipoproteins. PLA(2) may thus generate signals that influence immune responses. Here, group III secretory PLA(2) were tested for their ability to promote generation of functionally mature human dendritic cells (DC). PLA(2) treatment of differentiating monocytes in the presence of granulocyte/macrophage colony-stimulating factor and IL-4 yielded cells with phenotypical and functional characteristics of mature DC. This maturation was dependent on the dose of PLA(2), and PLA(2)-generated DC stimulated IFN-gamma secretion by allogeneic T cells. The effects of PLA(2) on DC maturation was mainly dependent on enzyme activity and correlated with the activation of NF-kappaB, AP-1 and NFAT. The data suggest that transient increase in PLA(2) activity generates signals that promote transition of innate to adaptive immunity during the APR.

Effect of phospholipase A2 inhibitory peptide on inflammatory arthritis in a TNF transgenic mouse model: a time-course ultrastructural study

We evaluated the therapeutic effect of secretory phospholipase A₂ (sPLA₂)-inhibitory peptide at a cellular level on joint erosion, cartilage destruction, and synovitis in the human tumor necrosis factor (TNF) transgenic mouse model of arthritis. Tg197 mice (N = 18) or wild-type (N = 10) mice at 4 weeks of age were given intraperitoneal doses (7.5 mg/kg) of a selective sPLA₂ inhibitory peptide, P-NT.II, or a scrambled P-NT.II (negative control), three times a week for 4 weeks. Untreated Tg197 mice (N = 10) were included as controls. Pathogenesis was monitored weekly for 4 weeks by use of an arthritis score and histologic examinations. Histopathologic analysis revealed a significant reduction after P-NT.II treatment in synovitis, bone erosion, and cartilage destruction in particular. Conspicuous ultrastructural alterations seen in articular chondrocytes (vacuolated cytoplasm and loss of nuclei) and synoviocytes (disintegrating nuclei and vacuoles, synovial adhesions) of untreated or scrambled-P-NT.II-treated Tg197 mice were absent in the P-NT.II-treated Tg197 group. Histologic scoring and ultrastructural evidence suggest that the chondrocyte appears to be the target cell mainly protected by the peptide during arthritis progression in the TNF transgenic mouse model. This is the first time ultrastructural evaluation of this model has been presented. High levels of circulating sPLA₂ detected in untreated Tg197 mice at age 8 weeks of age were reduced to basal levels by the peptide treatment. Attenuation of lipopolysaccharide- and TNF-induced release of prostaglandin E₂ from cultured macrophage cells by P-NT.II suggests that the peptide may influence the prostaglandin-mediated inflammatory response in rheumatoid arthritis by limiting the bioavailability of arachidonic acid through sPLA₂ inhibition.

Snake venom phospholipase A2s (Asp49 and Lys49) induce mechanical allodynia upon peri-sciatic administration: involvement of spinal cord glia, proinflammatory cytokines and nitric oxide

Snakebites constitute a serious public health problem in Central and South America, where species of the lancehead pit vipers (genus Bothrops) cause the majority of accidents. Bothrops envenomations are very painful, and this effect is not neutralized by antivenom treatment. Two variants of secretory phospholipases A2 (sPLA2), corresponding to Asp49 and Lys49 PLA2s, have been isolated from Bothrops asper venom. These sPLA2s induce hyperalgesia in rats following subcutaneous injection. However, venom in natural Bothrops bites is frequently delivered intramuscularly, thereby potentially reaching peripheral nerve bundles. Thus, the present series of experiments tested whether these sPLA2s could exert pain-enhancing effects following administration around healthy sciatic nerve. Both were found to produce mechanical allodynia ipsilateral to the injection site; no thermal hyperalgesia was observed. As no prior study has examined potential spinal mechanisms underlying sPLA2 actions, a series of anatomical and pharmacological studies were performed. These demonstrated that both sPLA2s produce activation of dorsal horn astrocytes and microglia that is more prominent ipsilateral to the site of injection. As proinflammatory cytokines and nitric oxide have each been previously implicated in spinally mediated pain facilitation, the effect of pharmacological blockade of these substances was tested. The results demonstrate that mechanical allodynia induced by both sPLA2s is blocked by interleukin-1 receptor antagonist, anti-rat interleukin-6 neutralizing antibody, the anti-inflammatory cytokine interleukin-10, and a nitric oxide synthesis inhibitor (L-NAME). As a variety of immune cells also produce and release sPLA2s during inflammatory states, the data may have general implications for the understanding of inflammatory pain.

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.

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.

Secreted phospholipase A2 enzymes as therapeutic targets

Homology cloning through in silico database search analysis has led to the definition of ten structurally-related mammalian secreted phospholipase A(2) (sPLA(2)) enzyme forms at present, each expressed in a species-, genotype- and cell-type-specific manner and with different enzymatic properties. These studies have shown that models based on the premise that there is only one PLA(2) drug target are now inadequate. Type IIA sPLA(2) remains the most advanced clinical target, with rationally designed inhibitors in Phase II clinical trials. However, progress in our understanding of the functional role of the ten secreted enzymes in phospholipid (PL) metabolism and in eicosanoid-mediated disorders, together with their emerging activity-independent and receptor-mediated functions, is likely to significantly impact on current and future drug development efforts.

Secretory phospholipases A2 activate selective functions in human eosinophils

Secretory phospholipases A(2) (sPLA(2)s) are released in large amounts in the blood of patients with systemic inflammatory diseases and accumulate at sites of chronic inflammation, such as the airways of patients with bronchial asthma. Blood eosinophils or eosinophils recruited in inflammatory areas therefore can be exposed in vivo to high concentrations of sPLA(2). We have examined the effects of two structurally different sPLA(2)s (group IA and group IIA) on several functions of eosinophils isolated from normal donors and patients with hypereosinophilia. Both group IA and IIA sPLA(2) induced a concentration-dependent release of beta-glucuronidase, IL-6, and IL-8. Release of the two cytokines was associated with the accumulation of their specific mRNA. In addition, sPLA(2)s induced the surface expression of CD44 and CD69, two major activation markers of eosinophils. In contrast, none of the sPLA(2)s examined induced the production of IL-5, the de novo synthesis of leukotriene C(4) and platelet-activating factor, or the generation of superoxide anion from human eosinophils. Incubation of eosinophils with the major enzymatic products of the sPLA(2)s (arachidonic acid, lysophosphatidylcholine, or lysophosphatidic acid) did not reproduce any of the enzymes' effects. In addition, inactivation of sPLA(2) enzymatic activity by bromophenacyl bromide did not influence the release of beta-glucuronidase or of cytokines. Stimulation of eosinophils by sPLA(2)s was associated with activation of extracellular signal-regulated kinases 1/2. These results indicate that sPLA(2)s selectively activate certain proinflammatory and immunoregulatory functions of human eosinophils through mechanism(s) independent from enzymatic activity and from the generation of arachidonic acid.

Immunoglobulin superantigen protein L induces IL-4 and IL-13 secretion from human Fc epsilon RI+ cells through interaction with the kappa light chains of IgE

Peptostreptococcus magnus protein L is a multidomain bacterial surface protein that correlates with virulence. It consists of up to five homologous Ig-binding domains (B1-B5) that interact with the variable domain of Ig kappa L chains. Intact protein L stimulates the synthesis and the release of IL-4 and IL-13 from human basophils in vitro. A protein L fragment covering the Ig-binding domains B1-B4 also induced IL-4 and IL-13 release from basophils. There was an excellent correlation (r(s) = 0.82; p < 0.001) between the maximal percent IL-4 release induced by protein L and that induced by anti-IgE and between intact protein L and the B1-B4 fragment (r(s) = 0.90; p < 0.01). Removal of IgE bound to basophils markedly reduced the IL-4 release induced by anti-IgE, protein L, and B1-B4. Preincubation of basophils with protein L or anti-IgE caused complete cross-desensitization to subsequent challenge with the heterologous stimulus. IgE purified from myeloma patients PS and PP (lambda chains) blocked anti-IgE-induced IL-4 release, but not the releasing activity of protein L. In contrast, IgE purified from myeloma patient ADZ (kappa chains) blocked both anti-IgE- and protein L-induced secretion. Cyclosporin A, but not cyclosporin H, inhibited protein L-induced release of IL-4 and IL-13 from basophils. Thus, protein L acts as a bacterial Ig superantigen to induce the synthesis and release of IL-4 and IL-13 from basophils by interacting with kappa L chains of the IgE isotype.

Secreted phospholipases A2 induce the expression of chemokines in microvascular endothelium

Acute respiratory distress syndrome (ARDS) is characterized by alterations in microvascular permeability. In ARDS secreted phospholipase A(2) (sPLA(2)) IB and IIA are found to be highly upregulated. In this study, we therefore investigated the influence of exogenously added sPLA(2)-IB and sPLA(2)-IIA on the production of chemokines and adhesion molecules in lung microvascular endothelial cells (LMVEC). Treatment of LMVEC with sPLA(2)s resulted in a significant increase in the production of chemokines and adhesion molecules due to an increased expression of their mRNA and in an enhanced release of oleic acid. The upregulation of chemokines and adhesion molecules by LPS was stronger in the presence of sPLA(2). Activation of NF-kappaB occurred upon stimulation with sPLA(2). Moreover the MAPkinase pERK seems to be involved since a specific pERK inhibitor, e.g., U0126, but not a p38Kinase inhibitor, e.g., SB203580 prevented sPLA(2)-induced chemokine upregulation. Our data therefore suggest that LMVEC are a highly sensitive target for the direct action of extracellular sPLA(2)s.

Biological effects of group IIA secreted phosholipase A(2)

Group IIA secreted phospholipase A(2) (sPLA(2)-IIA) is the most abundant element in human tissues of a large family of low molecular weight phospholipases A(2), which shows properties different from those displayed by the cytosolic phospholipase A(2) involved in the release of arachidonic acid. sPLA(2)-IIA behaves as a ligand for a group of receptors inside the C-type multilectin mannose receptor family and also interacts with heparan sulfate proteoglycans such as glypican, the dermatan/chondroitin sulfate-rich decorin, and the chondroitin sulfate-rich versican, thus being able to internalize to specific compartments within the cell and producing biological responses. This review provides a short summary of the biological actions of sPLA(2)-IIA on intracellular signaling pathways.

Cooperation between secretory phospholipase A2 and TNF-receptor superfamily signaling: implications for the inflammatory response in atherogenesis

Atherogenesis is the consequence of a variety of effector mechanisms rather than the result of a single functional molecule. In this connection, type IIA secretory phospholipase A2 (sPLA2) is an acute-phase reactant, which accumulates in atherosclerotic arterial walls, elicits several effects on monocytes, and has been related to the development of atherosclerosis. CD40/CD40 ligand pair is also a strong proatherogenic system. sPLA2 produced an increase of the surface expression of CD40 in THP-1 monocytes and enhanced the effect of CD40 ligation on the expression of both Fas and FasL, thus indicating the existence of a positive cooperation between sPLA2 and different elements of the TNF-receptor superfamily. Activation of the CD40/CD40L dyad with anti-CD40 antibody produced a small release of arachidonic acid and lacked any significant effect on the induction of cyclooxygenase-2, whereas the secretion of the chemokine MCP-1 and the surface display of CD11b, the alpha chain of the integrin Mac-1, were upregulated. Engagement of CD40 did not influence the survival of THP-1 monocytes, but coincubation of THP-1 monocytes pretreated with anti-CD40 antibody and Jurkat cells induced a significant increase of the number of Jurkat cells showing binding of annexin-V, and nuclear condensation and fragmentation, thus indicating that this treatment might trigger a juxtacrine/paracrine mechanism of apoptotic death in sensitive cell types. This data indicates the existence of overlapping routes for the response to CD40, TNF-alpha, and sPLA2, thus allowing the development of distinct patterns of response in monocytic cells.