Enhanced tissue expression and elevated circulating level of phospholipase A(2) receptor during murine endotoxic shock

Ectodomain shedding of PLA2R1 is mediated by the metalloproteases ADAM10 and ADAM17

Phospholipase A2 receptor 1 (PLA2R1) is a 180-kDa transmembrane protein that plays a role in inflammation and cancer and is the major autoantigen in membranous nephropathy, a rare but severe autoimmune kidney disease. A soluble form of PLA2R1 has been detected in mouse and human serum. It is likely produced by proteolytic shedding of membrane-bound PLA2R1 but the mechanism is unknown. Here, we show that human PLA2R1 is cleaved by A Disintegrin And Metalloprotease 10 (ADAM10) and ADAM17 in HEK293 cells, mouse embryonic fibroblasts, and human podocytes. By combining site-directed mutagenesis and sequencing, we determined the exact cleavage site within the extracellular juxtamembrane stalk of human PLA2R1. Orthologs and paralogs of PLA2R1 are also shed. By using pharmacological inhibitors and genetic approaches with RNA interference and knock-out cellular models, we identified a major role of ADAM10 in the constitutive shedding of PLA2R1 and a dual role of ADAM10 and ADAM17 in the stimulated shedding. We did not observe evidence for cleavage by β- or γ-secretase, suggesting that PLA2R1 may not be a substrate for regulated intramembrane proteolysis. PLA2R1 shedding occurs constitutively and can be triggered by the calcium ionophore ionomycin, the protein kinase C activator PMA, cytokines, and lipopolysaccharides, in vitro and in vivo. Altogether, our results show that PLA2R1 is a novel substrate for ADAM10 and ADAM17, producing a soluble form that is increased in inflammatory conditions and likely exerts various functions in physiological and pathophysiological conditions including inflammation, cancer, and membranous nephropathy.

sPLA2GIB Promotes PGD2 and IL-13 Production in Eosinophilic Chronic Rhinosinusitis with Nasal Polyps

OBJECTIVE

Secreted phospholipase A2 Group IB (sPLA2GIB) regulates the release of arachidonic acid, prostaglandins, and other inflammatory lipid mediators. Although it has been well involved in extensive inflammatory diseases, its specific mechanism in chronic rhinosinusitis with nasal polyps (CRSwNP) remains unclear. In this study, we investigated the role of sPLA2GIB in the pathophysiology of CRSwNP.

METHODS

Quantitative PCR, immunofluorescence staining, western blotting, and enzyme-linked immunosorbent assay (ELISA) were used to analyze the expression of sPLA2s, phospholipase A2 receptor (PLA2R), and prostaglandin D2 (PGD2) in nasal samples. Human nasal epithelial cells (HNECs) were cultured at an air-liquid interface (ALI) and stimulated with various cytokines. The human mast cell line HMC-1 was stimulated with sPLA2GIB, and the expression of PGD2 and cytokines in the culture supernatant was detected by ELISA.

RESULTS

The mRNA and protein levels of sPLA2GIB were significantly higher in eosinophilic CRSwNP than in control tissues. sPLA2GIB was predominantly expressed in the nasal epithelial cells. PLA2R mRNA and protein levels were upregulated in both eosinophilic and non-eosinophilic CRSwNP compared with the control groups. IL-4, IL-13, TNF-α, and IL-1β upregulated the expression of sPLA2GIB in ALI-cultured HNECs. sPLA2GIB induced PGD2 and IL-13 production in HMC-1 cells in a hydrolytic activity-independent manner. PGD2 protein expression was elevated in tissue homogenates of eosinophilic CRSwNP, and PGD2 upregulated the expression of IL-13 in HMC-1 cells.

CONCLUSION

Increased secretion of sPLA2GIB by epithelial cells may promote eosinophilic inflammation in CRSwNP by enhancing PGD2 and IL-13 production in mast cells via binding to PLA2R.

LEVEL OF EVIDENCE

N/A Laryngoscope, 134:1107-1117, 2024.

The characters of antibodies against PLA2R in healthy individuals and in the patient with PLA2R associated membranous nephropathy

BACKGROUND

Most primary membranous nephropathy (MN) is mediated by anti-phospholipase A2 receptor (PLA2R) antibodies. Recently, these antibodies have been revealed months to years before the disease's onset. Their production and pathogenicity need further investigation.

METHODS

Anti-PLA2R antibodies were purified from plasma of eight healthy individuals, 12 patients with PLA2R-related MN and negative circulating antibody (Ab-), and 18 patients with positive anti-PLA2R antibodies (Ab +), using affinity column coupled with recombinant human PLA2R. The antigen specificity, antibody amount, titer, IgG subclass, and affinity were assessed by Western blot, immunofluorescence, ELISA, and surface plasmon resonance.

RESULTS

The natural anti-PLA2R antibodies recognized the conformational structure of PLA2R which locates on the cell membrane of podocytes. The amount of natural IgG was 0.12 ± 0.04 g/L, which accounted for 0.80% of total IgG and was lower than that of patients (2.36%, P < 0.001). The titer of natural antibodies was lower than that of patients in Ab- and Ab + groups (1:16 vs. 1:43 vs. 1:274, P < 0.001). IgG2(45.1%) was predominant in natural antibodies, while IgG4 was predominant in Ab + group (45.7 vs. 25.0%, P < 0.001). IgG1 was increasing from natural antibodies to Ab- and Ab + groups. The affinity of natural antibodies was lower than that of patients (K_D: 641.0 vs. 269.0 vs. 99.6 nM, P = 0.002). The antibody titer, affinity, and IgG4 percentage were associated with the severity of proteinuria and the stages of membranous lesion.

CONCLUSIONS

The natural anti-PLA2R antibodies exist in healthy plasma. The antibody titer, IgG subclass, and affinity may participate in the pathogenesis of anti-PLA2R antibodies.

Deficiency of Phospholipase A2 Receptor Exacerbates Autoimmune Myocarditis in Mice

Secretory phospholipase A₂ (sPLA₂) plays a critical role in the pathogenesis of various inflammatory diseases through production of pro-inflammatory eicosanoids. PLA₂ receptor 1 (PLA₂R) acts as a clearance receptor for sPLA₂s. This study examined whether PLA₂R plays a role in the pathogenesis of experimental autoimmune myocarditis using PLA₂R-deficient (PLA₂R KO) mice on a BALB/c background. Autoimmune myocarditis was induced by immunization with murine α-myosin heavy chain. In the immunostaining of PLA₂R wild-type (WT) myocardium, PLA₂R and sPLA₂s were expressed in α-SMA⁺ cells and neutrophils, respectively. In immunoblot analyses, tissue from PLA₂R KO myocardium after immunization had five to tenfold increases in the protein level of sPLA₂-IB and sPLA₂-IIA compared with PLA₂R WT myocardium. However, the mRNA expression levels of these sPLA₂s were similar in PLA₂R KO and WT myocardium. Compared with PLA₂R WT myocardium, PLA₂R KO myocardium after immunization showed 40% increase in areas affected by infiltration of inflammatory cells, eight to tenfold increase in levels of PGE₂ and TXB₂, and a threefold increase in number of Th17 cells in heart infiltrates assessed by flow cytometric analysis. Finally, PGE₂ promoted IL-23-induced expansion of Th17 cells in vitro. In conclusion, PLA₂R-deficiency increased sPLA₂-IB and sPLA₂-IIA levels in the myocardium after immunization probably through impaired clearance, leading to increased levels of PGE₂ in the myocardium. Elevated PGE₂ induced Th17 cell expansion, exacerbating myocarditis in PLA₂R KO mice. Thus, PLA₂R plays an important role in pathogenesis of experimental autoimmune myocarditis.

Current insights into functions of phospholipase A2 receptor in normal and cancer cells: More questions than answers

Lipid signaling network was proposed as a potential target for cancer prevention and treatment. Several recent studies revealed that phospholipid metabolising enzyme, phospholipase A2 (PLA₂), is a critical regulator of cancer accelerating pathologies and apoptosis in several types of cancers. In addition to functioning as an enzyme, PLA₂ can activate a phospholipase A2 receptor (PLA2R1) in plasma membrane. While the list of PLA₂ targets extends to glucose homeostasis, intracellular energy balance, adipocyte development, and hepatic lipogenesis, the PLA2R1 downstream effectors are few and scarcely investigated. Among the most addressed PLA2R1 effects are regulation of pro-inflammatory signaling, autoimmunity, apoptosis, and senescence. Localized in glomeruli podocytes, the receptor can be identified by circulating anti-PLA2R1 autoantibodies leading to development of membranous nephropathy, a strong autoimmune inflammatory cascade. PLA2R1 was shown to induce activation of Janus-kinase 2 (JAK2) and estrogen-related receptor α (ERRα)-controlled mitochondrial proteins, as well as increasing the accumulation of reactive oxygen species, thus leading to apoptosis and senescence. These findings indicate the potential role of PLA2R1 as tumor suppressor. Epigenetic investigations addressed the role of DNA methylation, histone modifications, and specific microRNAs in the regulation of PLA2R1 expression. However, involvement of PLA2R1 in suppression of malignant growth and metastasis remains controversial. In this review, we summarize the recent findings that highlight the role of PLA2R1 in the regulation of carcinogenesis-related intracellular signaling.

PLA2R binds to the annexin A2-S100A10 complex in human podocytes

Phospholipase A₂ receptor (PLA₂R) is a member of the mannose receptor family found in podocytes in human kidney. PLA₂R is the target of the autoimmune disease, membranous nephropathy, characterised by production of anti-PLA₂R autoantibodies which bind to the podocyte. However the function of PLA₂R in health and in disease remains unclear. To gain insight into the molecular mechanisms of PLA₂R function, we searched for its endogenous binding partners. Proteomic analysis identified annexinA2 as a potential interactor with the extracellular domains of PLA₂R. We confirmed that PLA₂R binds to annexinA2-S100A10 (A2t) complex with specific high affinity to the S100A10 component. The binding occured within the PLA₂R NC3 fragment and was increased in acidic pH. Furthermore Ca²⁺ promoted the association of the PLA₂R-A2t complex with phospholipid membranes in vitro. Within the podocyte, all three proteins were enriched in the plasma membrane and organelle membrane compartments. PLA₂R co-localised with S100A10 at the cell surface and in extracellular vesicles. This novel interaction between PLA₂R and the A2t complex offers insights into the role of PLA₂R in podocytes and how autoantibodies might disrupt PLA₂R function. The ability of podocytes to secrete vesicles containing PLA₂R provides a route for engagement of PLA₂R with the immune system.

Host-targeted approaches to managing animal health: old problems and new tools

Our fellow medical and regulatory scientists question the animal producer's dependence on antibiotics and antimicrobial chemicals in the production of animal products. Retail distributors and consumers are putting even more pressure on the animal industry to find new ways to produce meat without antibiotics and chemicals. In addition, federal funding agencies are increasingly pressuring researchers to conduct science that has application. In the review that follows, we outline our approach to finding novel ways to improve animal performance and health. We use a strict set of guidelines in our applied research as follows: (1) Does the work have value to society? (2) Does our team have the skills to innovate in the field? (3) Is the product we produce commercially cost-effective? (4) Are there any reasons why the general consumer will reject the technology? (5) Is it safe for the animal, consumer, and the environment? Within this framework, we describe 4 areas of research that have produced useful products, areas that we hope other scientists will likewise explore and innovate such as (1) methods to detect infection in herds and flocks, (2) methods to control systemic and mucosal inflammation, (3) improvements to intestinal barrier function, and (4) methods to strategically potentiate immune defense. We recognize that others are working in these areas, using different strategies, but believe our examples will illustrate the vast opportunity for research and innovation in a world without antibiotics. Animal scientists have been given a new challenge that may help shape the future of both animal and human medicine.

Lack of phospholipase A2 receptor increases susceptibility to cardiac rupture after myocardial infarction

RATIONALE

Recent evidence indicates that the biological effects of secretory phospholipase A2 (sPLA2) cannot be fully explained by its catalytic activity. A cell surface receptor for sPLA2 (PLA2 receptor 1 [PLA2R]) and its high-affinity ligands (including sPLA2-IB, sPLA2-IIE, and sPLA2-X) are expressed in the infarcted myocardium.

OBJECTIVE

This study asked whether PLA2R might play a pathogenic role in myocardial infarction (MI) using mice lacking PLA2R (PLA2R(-/-)).

METHODS AND RESULTS

MI was induced by permanent ligation of the left coronary artery. PLA2R(-/-) mice exhibited higher rates of cardiac rupture after MI compared with PLA2R wild-type (PLA2R(+/+)) mice (46% versus 21%, respectively; P=0.015). PLA2R(-/-) mice had a 31% decrease in collagen content and a 45% decrease in the number of α-smooth muscle actin-positive fibroblasts in the infarcted region compared with PLA2R(+/+) mice. PLA2R was primarily found in myofibroblasts in the infarcted region. PLA2R(-/-) myofibroblasts were impaired in collagen-dependent migration, proliferation, and activation of focal adhesion kinase in response to sPLA2-IB. Binding of sPLA2-IB to PLA2R promoted migration and proliferation of myofibroblasts through functional interaction with integrin β1, independent of the catalytic activity of sPLA2-IB. In rescue experiments, the injection of PLA2R(+/+) myofibroblasts into the infarcted myocardium prevented post-MI cardiac rupture and reversed the decrease in collagen content in the infarcted region in PLA2R(-/-) mice.

CONCLUSIONS

PLA2R deficiency increased the susceptibility to post-MI cardiac rupture through impaired healing of the infarcted region. This might be partly explained by a reduction in integrin β1-mediated migratory and proliferative responses of PLA2R(-/-) myofibroblasts.

Deficiency of phospholipase A2 receptor exacerbates ovalbumin-induced lung inflammation

Secretory phospholipase A2 (sPLA2) plays a critical role in the genesis of lung inflammation through proinflammatory eicosanoids. A previous in vitro experiment showed a possible role of cell surface receptor for sPLA2 (PLA2R) in the clearance of extracellular sPLA2. PLA2R and groups IB and X sPLA2 are expressed in the lung. This study examined a pathogenic role of PLA2R in airway inflammation using PLA2R-deficient (PLA2R(-/-)) mice. Airway inflammation was induced by immunosensitization with OVA. Compared with wild-type (PLA2R(+/+)) mice, PLA2R(-/-) mice had a significantly greater infiltration of inflammatory cells around the airways, higher levels of groups IB and X sPLA2, eicosanoids, and Th2 cytokines, and higher numbers of eosinophils and neutrophils in bronchoalveolar lavage fluid after OVA treatment. In PLA2R(-/-) mice, intratracheally instilled [(125)I]-labeled sPLA2-IB was cleared much more slowly from bronchoalveolar lavage fluid compared with PLA2R(+/+) mice. The degradation of the instilled [(125)I]-labeled sPLA2-IB, as assessed by trichloroacetic acid-soluble radioactivity in bronchoalveolar lavage fluid after instillation, was lower in PLA2R(-/-) mice than in PLA2R(+/+) mice. In conclusion, PLA2R deficiency increased sPLA2-IB and -X levels in the lung through their impaired clearance from the lung, leading to exaggeration of lung inflammation induced by OVA treatment in a murine model.

Sepsis biomarkers: a review

INTRODUCTION

Biomarkers can be useful for identifying or ruling out sepsis, identifying patients who may benefit from specific therapies or assessing the response to therapy.

METHODS

We used an electronic search of the PubMed database using the key words "sepsis" and "biomarker" to identify clinical and experimental studies which evaluated a biomarker in sepsis.

RESULTS

The search retrieved 3370 references covering 178 different biomarkers.

CONCLUSIONS

Many biomarkers have been evaluated for use in sepsis. Most of the biomarkers had been tested clinically, primarily as prognostic markers in sepsis; relatively few have been used for diagnosis. None has sufficient specificity or sensitivity to be routinely employed in clinical practice. PCT and CRP have been most widely used, but even these have limited ability to distinguish sepsis from other inflammatory conditions or to predict outcome.

Upregulation of group IB secreted phospholipase A(2) and its M-type receptor in rat ANTI-THY-1 glomerulonephritis

Treatment of rat glomerular mesangial cell (GMC) cultures with pancreatic secreted phospholipase A(2) (sPLA(2)-IB) results in an enhanced expression of sPLA(2)-IIA and COX-2, possibly via binding to its specific M-type sPLA(2) receptor. In the current study, we have investigated the expression and regulation of sPLA(2)-IB and its receptor during glomerulonephritis (GN). In vivo we used the well-established rat model of anti-Thy 1.1 GN (anti-Thy 1.1-GN) to study the expression of sPLA(2)-IB and the M-type sPLA(2) receptor by immunohistochemistry. In addition, in vitro we determined the interkeukin (IL)-1beta-regulated mRNA and protein expression in primary rat glomerular mesangial and endothelial cells as well as in rat peripheral blood leukocytes (PBLs). Shortly after induction of anti-Thy 1.1-GN, sPLA(2)-IB expression was markedly upregulated in the kidney at 6-24 h. Within glomeruli, the strongest sPLA(2)-IB protein expression was detected on infiltrated granulocytes and monocytes. However, at the same time, the M-type receptor was also markedly upregulated on resident glomerular cells. In vitro, the most prominent cytokine-stimulated secretion of sPLA(2)-IB was observed in monocytes isolated from rat PBLs. Treating glomerular endothelial cells (GECs) with cytokines elicited only weak sPLA(2)-IB expression, but treatment of these cells with exogenous sPLA(2)-IB resulted in a marked expression of the endogenous sPLA(2)-IB. Mesangial cells did not express sPLA(2)-IB at all. The M-type sPLA(2) receptor protein was markedly upregulated on cytokine-stimulated mesangial and endothelial cells as well as on lymphocytes and granulocytes. During anti-Thy 1.1 rat GN, sPLA(2)-IB and the M-type sPLA(2) receptor are induced as primary downstream genes stimulated by inflammatory cytokines. Subsequently, both sPLA(2)-IB and the M-type sPLA(2) receptor are involved in the autocrine and paracrine amplification of the inflammatory process in different resident and infiltrating cells.

Group X secretory phospholipase A2 can induce arachidonic acid release and eicosanoid production without activation of cytosolic phospholipase A2 alpha

Group X secretory phospholipase A2 (sPLA2-X) and cytosolic phospholipase A2 alpha (cPLA2alpha) are involved in the release of arachidonic acid (AA) from membrane phospholipids linked to the eicosanoid production in various pathological states. Recent studies have indicated the presence of various types of cross-talk between sPLA2s and cPLA2alpha resulting in effective AA release. Here we examined the dependence of sPLA2-X-induced potent AA release on the cPLA2alpha activation by using specific cPLA2alpha or sPLA2 inhibitors as well as cPLA2alpha-deficient mice. We found that Pyrrophenone, a cPLA2alpha-specific inhibitor, did not suppress the sPLA2-X-induced potent AA release and prostaglandin E2 formation in mouse spleen cells. Furthermore, the amount of AA released by sPLA2-X from spleen cells was not significantly altered by cPLA2alpha deficiency. These results suggest that sPLA2-X induces potent AA release without activation of cPLA2a, which might be relevant to eicosanoid production in some pathological states where cPLA2a is not activated.

Comparative analysis of gene expression mechanisms between group IA and IB phospholipase A2 genes from sea snake Laticauda semifasciata

Phospholipase A(2) (PLA(2)) genes expressed in the venom glands of the sea snake, Laticauda semifasciata, were investigated. Both mRNAs, encoding group IA (without a pancreatic loop) and group IB (with pancreatic loop), were detected from venom glands by Northern blot hybridization analysis and RT-PCR. The results of quantitative PCR analysis indicated that the expression amount of group IA genes was around 100-300 times greater than that of group IB genes. Sequence analysis of 5'-upstream regions and a reporter gene assay of the genes (groups IA and IB) previously cloned showed that the functional sequence (411 bp) was inserted in the 5'-flanking region of the group IA PLA(2) genes. It seemed that the contribution of the inserted sequence to the amount of transcribed mRNAs was greater than that of number of genes present in the genome. Comparative analysis of the 5'-flanking sequences from several snake genes encoding toxic PLA(2)s revealed that this sequence was probably inserted into an ancestral gene of PLA(2) with a pancreatic loop. After the duplication of the gene, which contained the inserted sequence, the PLA(2) gene without a pancreatic loop evolved from one of the duplicate genes. This inserted sequence might determine the future of the genes expressed in the venom glands.

Group IB secretory phospholipase A(2)induces cell death in the cultured cortical neurons: a possible involvement of its binding sites

In primary cultures of rat cortical neurons, group IB secretory phospholipase A(2) (sPLA(2)-IB) induced cell death. In rat cortical membranes, there were high affinity binding sites of [125I]sPLA(2)-IB. The high-affinity binding sites were decreased by sPLA(2)-IB and anti-sPLA(2) receptor immunoglobulin G (anti-sPLA(2)R IgG). Furthermore, anti-sPLA(2)R IgG caused neuronal cell death in a concentration-dependent manner. The present study suggests that sPLA(2)-IB induces neuronal cell death via its high-affinity binding sites.

Phospholipase A2 receptor: a regulator of biological functions of secretory phospholipase A2

The phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family that includes the mannose receptor. PLA2R regulates a variety of biological responses elicited by specific types of secretory PLA2s (sPLA2s). Group IB sPLA2 (sPLA2-IB) acts as an endogenous PLA2R ligand to induce cell proliferation, cell migration, and lipid mediator production. Analysis of PLA2R-deficient mice has suggested a potential role of the sPLA2-IB/PLA2R pathway in the production of pro-inflammatory cytokines in endotoxic shock. PLA2R is also involved in the clearance of sPLA2s, including group X sPLA2 (sPLA2-X) and a particular type of snake venom sPLA2, and clearance suppresses their potent enzymatic activities. In the circulation, the soluble form of PLA2R is constitutively present as anendogenous inhibitor of sPLA2s. This review will focus on recent findings on the roles of PLA2R in regulating sPLA2 functions and summarize what is known about the otherbinding proteins for mammalian and snake venom sPLA2s.

Endotoxin induces respiratory failure and increases surfactant turnover and respiration independent of alveolocapillary injury in rats

Although endotoxin-induced acute lung injury is associated with inflammation, alveolocapillary injury, surfactant dysfunction, and altered lung mechanics, the precise sequence of these changes is polemic. We have studied the early pathogenesis of acute lung injury in spontaneously breathing anesthetized rats after intravenous infusion of Salmonella abortus equi endotoxin. The animals became hypoxic, and airway resistance, tissue resistance, lung elastance, and static compliance all deteriorated well before any change in alveolar neutrophils, macrophages, lung fluid (99mTc-labeled diethylenetriamine pentaacetic acid), or 125I-albumin flux, which were only appreciably increased at 8.5 hours. Lung elastance deteriorated before airway resistance, indicating that the compliance change was specific rather than caused by reduced lung volume. The subcellular and alveolar content of surfactant proteins A and B, cholesterol, disaturated phospholipids, and phospholipid classes remained normal in the face of a dramatic increase in the synthesis and turnover of 3H-disaturated phosphatidylcholine. Our findings indicate that the increase in surfactant disaturated phospholipid turnover reflects, at least in part, an approximately five-fold increase in "sigh frequency." We suggest that endotoxin has direct effects on tissue resistance and lung elastance independent of surfactant composition and that the initial respiratory failure results primarily from endotoxin-induced ventilation/perfusion mismatch independent of edema or alveolocapillary injury per se.

Identification of a soluble form phospholipase A2 receptor as a circulating endogenous inhibitor for secretory phospholipase A2

Venomous snakes have various types of phospholipase A(2) inhibitory proteins (PLIs) in their circulatory system to protect them from attack by their own phospholipase A(2)s (PLA(2)s). Here we show the first evidence for the existence of circulating PLI against secretory PLA(2)s (sPLA(2)s) in mammals. In mouse serum, we detected specific binding activities of group IB and X sPLA(2)s, which was in contrast with the absence of binding activities in serum prepared from mice deficient in PLA(2) receptor (PLA(2)R), a type I transmembrane glycoprotein related to the C-type animal lectin family. Western blot analysis after partial purification with group IB sPLA(2) affinity column confirmed the identity of serum sPLA(2)-binding protein as a soluble form of PLA(2)R (sPLA(2)R) that retained all of the extracellular domains of the membrane-bound receptor. Both purified sPLA(2)R and the recombinant soluble receptor having all of the extracellular portions blocked the biological functions of group X sPLA(2), including its potent enzymatic activity and its binding to the membrane-bound receptor. Protease inhibitor tests with PLA(2)R-overexpressing Chinese hamster ovary cells suggested that sPLA(2)R is produced by cleavage of the membrane-bound receptor by metalloproteinases. Thus, sPLA(2)R is the first example of circulating PLI that acts as an endogenous inhibitor for enzymatic activities and receptor-mediated functions of sPLA(2)s in mice.

Clearance of group X secretory phospholipase A(2) via mouse phospholipase A(2) receptor

Given the potent hydrolyzing activity toward phosphatidylcholine, group X secretory phospholipase A(2) (sPLA(2)-X) elicits a marked release of arachidonic acid linked to the potent production of lipid mediators in various cell types. We have recently shown that sPLA(2)-X can also act as a ligand for mouse phospholipase A(2) receptor (PLA(2)R). Here, we found that sPLA(2)-X was internalized and degraded via binding to PLA(2)R associated with the diminished prostaglandin E(2) (PGE(2)) formation in PLA(2)R-expressing Chinese hamster ovary (CHO) cells compared to CHO cells. Indirect immunocytochemical analysis revealed that internalized sPLA(2)-X was co-localized with PLA(2)R in the punctate structures in PLA(2)R-expressing CHO cells. Moreover, in mouse osteoblastic MC3T3-E(1) cells that endogenously express the PLA(2)R, the internalized sPLA(2)-X was localized in lysosomes. These findings demonstrate that PLA(2)R acts as a clearance receptor for sPLA(2)-X to suppress its strong enzymatic activity.

Endotoxin-stimulated innate immunity: A contributing factor for asthma

Exposure to airborne endotoxin in infancy may protect against asthma by promoting enhanced T(H)1 response and tolerance to allergens. On the other hand, later in life, it adversely affects patients with asthma. Endotoxin binding to receptors on macrophages and other cells generates IL-12, which inhibits IgE responses. It also generates cytokines like IL-1, TNF-alpha, and IL-8, which cause inflammation. These signal transduction pathways resemble those leading to the generation of cytokines, such as IL-4, IL-13, and IL-5, which are responsible for the inflammation of IgE-mediated allergic disease. The main difference seems to be that endotoxin recruits neutrophils, but IgE recruits eosinophils, and the details of the tissue injury from these granulocytes differ. Sources of airborne endotoxin include many agricultural dusts, aerosols from contaminated water in many industrial plants, contaminated heating and air-conditioning systems, mist-generating humidifiers, and damp or water-damaged homes. Acute inhalation of high concentrations of endotoxin can cause fever, cough, and dyspnea. Chronic inhalation of lesser amounts causes chronic bronchitis and emphysema and is associated with airway hyperresponsiveness. Airborne endotoxin adversely affects patients with asthma in 3 ways: (1) by increasing the severity of the airway inflammation; (2) by increasing the susceptibility to rhinovirus-induced colds; and (3) by causing chronic bronchitis and emphysema with development of irreversible airway obstruction after chronic exposure of adults. The most effective management is mitigating exposure. The potential of drug treatments requires further clinical investigation.

[Functional analysis of phospholipase A2 receptor by gene knockout studies]

Phospholipase A2 receptor (PLA2R) is a type I transmembrane glycoprotein related to the C-type animal lectin family and mediates a variety of biological responses elicited by group IB secretory phospholipase A2 (sPLA2-IB). In the present study, we have shown the evidence that a novel type of sPLA2, sPLA2-X, also acts as one of the high-affinity ligands for mouse PLA2R. We then generated PLA2R-deficient mice and found that the knockout mice exhibited the resistance to an endotoxic shock with reduced plasma levels of proinflammatory cytokines, such as TNF-alpha and IL-1 beta. In situ hybridization analysis revealed that the expression of PLA2R transcript was markedly enhanced in type II alveolar epithelial cells and a subset of splenic lymphocytes in accordance with the elevated expression of sPLA2-IB and TNF-alpha mRNAs during endotoxic shock. In addition, the elevated expression level of TNF-alpha transcript was significantly reduced by the deficiency of PLA2R, suggesting that PLA2R plays a role in the regulation of TNF-alpha expression in these cell types. We then synthesized a specific sPLA2 inhibitor, indoxam, which blocked the binding of sPLA2-IB and X to PLA2R. Indoxam was found to suppress the elevation of the plasma level of TNF-alpha and prolonged the survival of endotoxin-challenged mice. The inhibitory effects of indoxam were abolished by the deficiency of PLA2R, demonstrating the involvement of PLA2R in the progression of endotoxic shock. We also detected and characterized a soluble form of PLA2R protein in the plasma of mouse with anti-PLA2R antibody, and showed its potential role as an endogenous sPLA2 inhibitor. Taken together, a series of studies with PLA2R-knockout mice have elucidated a critical role of PLA2R in the regulation of the development of endotoxic shock.