Time-resolved fluoroimmunoassays of the complete set of secreted phospholipases A2 in human serum

Biology of secretory phospholipase A2

Introduction

The secretory phospholipase A₂ (sPLA₂) family provides a seemingly endless array of potential biological functions that is only beginning to be appreciated. In humans, this family comprises 9 different members that vary in their tissue distribution, hydrolytic activity, and phospholipid substrate specificity. Through their lipase activity, these enzymes trigger various cell-signaling events to regulate cellular functions, directly kill bacteria, or modulate inflammatory responses. In addition, some sPLA₂’s are high affinity ligands for cellular receptors.

Objective

This review merely scratches the surface of some of the actions of sPLA₂s in innate immunity, inflammation, and atherosclerosis. The goal is to provide an overview of recent findings involving sPLA₂s and to point to potential pathophysiologic mechanisms that may become targets for therapy.

Biochemistry and physiology of mammalian secreted phospholipases A2

Phospholipases A(2) (PLA2s) are esterases that hydrolyze the sn-2 ester of glycerophospholipids and constitute one of the largest families of lipid hydrolyzing enzymes. The mammalian genome contains 10 enzymatically active secreted PLA2s (sPLA2s) and two sPLA2-related proteins devoid of lipolytic enzymatic activity. In addition to the well-established functions of one of these enzymes in digestion of dietary phospholipids and another in host defense against bacterial infections, accumulating evidence shows that some of these sPLA2s are involved in arachidonic acid release from cellular phospholipids for the biosynthesis of eicosanoids, especially during inflammation. More speculative results suggest the involvement of one or more sPLA2s in promoting atherosclerosis and cancer. In addition, the mammalian genome encodes several types of sPLA2-binding proteins, and mounting evidence shows that sPLA2s may have functions related to binding to cellular target proteins in a manner independent of their lipolytic enzymatic activity.

Secretory phospholipase A(2) in newborn infants with sepsis

OBJECTIVE

To investigate secretory phospholipase A(2) (sPLA(2)) activity in neonatal sepsis.

STUDY DESIGN

Plasma sPLA(2) activity, C-reactive protein (CRP) concentration, leukocyte count and immature/total neutrophil (I/T) ratio were assessed in a group of 156 infants admitted for neonatal intensive care, who were classified as documented sepsis (n=24), suspected infection (n=77) and controls (n=55). Interleukin-6 (IL-6) concentrations were assessed in a subgroup (n=29).

RESULT

sPLA(2) activity, CRP concentration and I/T ratio were higher in sepsis than in suspected infection or control groups. sPLA(2) activity advanced with increasing CRP, I/T ratio and IL-6 was highest in infants with respiratory distress syndrome (RDS). Compared to CRP, sPLA(2) had equal sensitivity and lower specificity. Compared to I/T ratio, sensitivity and specificity of sPLA(2) were higher.

CONCLUSION

Plasma sPLA(2) activity is increased in neonatal sepsis and highest in infants with RDS. Further studies should assess the potential of sPLA(2) inhibition in neonatal sepsis.

Distinct expression pattern of the full set of secreted phospholipases A2 in human colorectal adenocarcinomas: sPLA2-III as a biomarker candidate

Recent studies suggest that secreted phospholipases A₂ (sPLA₂s) represent attractive potential tumour biomarkers and therapeutic targets for various cancers. As a first step to address this issue in human colorectal cancer, we examined the expression of the full set of sPLA₂s in sporadic adenocarcinomas and normal matched mucosa from 21 patients by quantitative PCR and immunohistochemistry. In normal colon, PLA2G2A and PLA2G12A were expressed at high levels, PLA2G2D, PLA2G5, PLA2G10 and PLA2G12B at moderate levels, and PLA2G1B, PLA2G2F and PLA2G3 at low levels. In adenocarcinomas from left and right colon, the expression of PLA2G3 was increased by up to 40-fold, while that of PLA2G2D and PLA2G5 was decreased by up to 23- and 14-fold. The variations of expression for sPLA₂-IID, sPLA₂-III and sPLA₂-V were confirmed at the protein level. The expression pattern of these sPLA₂s appeared to be linked respectively to the overexpression of interleukin-8, defensin α6, survivin and matrilysin, and downregulation of SFRP-1 and RLPA-1, all these genes being associated to colon cancer. This original sPLA₂ profile observed in adenocarcinomas highlights the potential role of certain sPLA₂s in colon cancer and suggests that sPLA₂-III might be a good candidate as a novel biomarker for both left and right colon cancers.

Kinetics of Group IB and IIA Phospholipase A2 During Low-Volume Continuous Hemodiafiltration in Severe Acute Pancreatitis

Continuous hemodiafiltration (CHDF) has been performed for the treatment of severe acute pancreatitis. Phospholipase A2 (PLA2) is one of the important mediators which exacerbate acute pancreatitis, but whether PLA2 can be removed by CHDF is unclear. In this study, the kinetics of group IB and group IIA PLA2 was examined at the first session of low-volume CHDF in eight patients with severe acute pancreatitis. CHDF was performed using polysulfone hemofilters (surface area: 0.7 m(2)) at a blood flow rate of 100 mL/min and a filtration and dialysate flow rate of 10 mL/min each. The plasma concentrations of group IB and IIA PLA2 before the start of CHDF were 47.4 +/- 52.0 microg/L and 352 +/- 390 microg/L, respectively, and did not change significantly. The clearances of group IB and IIA PLA2 achieved by the CHDF circuit 1 h after the start of CHDF were 20.7 +/- 11.6 mL/min and 16.7 +/- 4.4 mL/min, respectively, with both clearances decreasing significantly with time. The clearance of group IB PLA2 into the waste fluid tended to increase with time; however, the concentrations of group IIA PLA2 in the waste fluid were less than the measurable sensitivity. These results indicate that group IB PLA2 is adsorbed on the hemofilter membrane in preference to being removed into the waste fluid, while group IIA PLA2 is mainly removed by adsorption. However, low-volume CHDF is not effective at eliminating the group IB and IIA PLA2 plasma concentration.

Importance of group X-secreted phospholipase A2 in allergen-induced airway inflammation and remodeling in a mouse asthma model

Arachidonic acid metabolites, the eicosanoids, are key mediators of allergen-induced airway inflammation and remodeling in asthma. The availability of free arachidonate in cells for subsequent eicosanoid biosynthesis is controlled by phospholipase A₂s (PLA₂s), most notably cytosolic PLA₂-α. 10 secreted PLA₂s (sPLA₂s) have also been identified, but their function in eicosanoid generation is poorly understood. We investigated the role of group X sPLA₂ (sPLA₂-X), the sPLA₂ with the highest in vitro cellular phospholipolysis activity, in acute and chronic mouse asthma models in vivo. The lungs of sPLA₂-X^−/− mice, compared with those of sPLA₂-X^+/+ littermates, had significant reduction in ovalbumin-induced infiltration by CD4⁺ and CD8⁺ T cells and eosinophils, goblet cell metaplasia, smooth muscle cell layer thickening, subepithelial fibrosis, and levels of T helper type 2 cell cytokines and eicosanoids. These data direct attention to sPLA₂-X as a novel therapeutic target for asthma.

Differential behavior of sPLA2-V and sPLA2-X in human neutrophils

Neutrophils and differentiated PLB-985 cells contain various types of PLA(2)s including the 85 kDa cytosolic PLA(2) (cPLA(2)), Ca(2+)-independent PLA(2) (iPLA(2)) and secreted PLA(2)s (sPLA(2)s). The present study focuses on the behavior of sPLA(2)s in neutrophils and PLB cells and their relationship to cPLA(2)alpha. The results of the present research show that the two types of sPLA(2) present in neutrophils, sPLA(2)-V and sPLA(2)-X, which are located in the azurophil granules, are differentially affected by physiological stimuli. While sPLA(2)-V is secreted to the extacellular milieu, sPLA(2)-X is detected on the plasma membranes after stimulation. Stimulation of neutrophils with formyl-Met-Leu-Phe (fMLP), opsonized zymosan (OZ) or A23187 resulted in a different kinetics of sPLA(2) secretion as detected by its activity in the neutrophil supernatants. Neutrophil priming by inflammatory cytokines or LPS enhanced sPLA(2) activity detected in the supernatant after stimulation by fMLP. This increased activity was due to increased secretion of sPLA(2)-V to the supernatant and not to release of sPLA(2)-X. sPLA(2) in granulocyte-like PLB cells exhibit identical characteristics to neutrophil sPLA(2), with similar activity and optimal pH of 7.5. Granulocyte-like cPLA(2)alpha-deficient PLB cells serve as a good model to study whether sPLA(2) activity is regulated by cPLA(2)alpha. Secretion and activity of sPLA(2) were found to be similar in granulocyte-like PLB cells expressing or lacking cPLA(2)alpha, indicating that they are not under cPLA(2)alpha regulation.

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.

Analysis of expression of secreted phospholipases A2 in mouse tissues at protein and mRNA levels

Secreted phospholipases A(2) (sPLA(2)) form a group of low-molecular weight enzymes that catalyze the hydrolysis of phospholipids. Some sPLA(2)s are likely to play a role in inflammation, cancer, and as antibacterial enzymes in innate immunity. We developed specific and sensitive time-resolved fluroimmunoassays (TR-FIA) for mouse group (G) IB, GIIA, GIID, GIIE, GIIF, GV and GX sPLA(2)s and measured their concentrations in mouse serum and tissues obtained from both Balb/c and C57BL/6J mice. We also analyzed the mRNA expression of the sPLA(2)s by quantitative real-time reverse transcriptase PCR (qPCR). In most tissues, the concentrations of sPLA(2) proteins corresponded to the expression of sPLA(2)s at the mRNA level. With a few exceptions, the sPLA(2) proteins were found in the gastrointestinal tract. The qPCR results showed that GIB sPLA(2) is synthesized widely in the gastrointestinal tract, including esophagus and colon, in addition to stomach and pancreas. Our results also suggest that the loss of GIIA sPLA(2) in the intestine of GIIA sPLA(2)-deficient C57BL/6J mice is not compensated by other sPLA(2)s under normal conditions. Outside the gastrointestinal tract, sPLA(2)s were expressed occasionally in a number of tissues. The TR-FIAs developed in the current study may serve as useful tools to measure the levels of sPLA(2) proteins in mouse serum and tissues in various experimental settings.

Serum calcium-independent phospholipase A2 activity in bipolar affective disorder

OBJECTIVE

Phospholipases A2 (PLA2) are a family of enzymes involved in membrane phospholipid metabolism and cell signalling. The gene encoding one form, type VI calcium-independent phospholipase A2, is located in a region of DNA that may contain a gene important in the aetiology of psychosis. Moreover, the activity of calcium-independent PLA2 is reported to be elevated in the blood and brain of patients with schizophrenia. In this study we determined whether a similar change takes place in patients with bipolar disorder with and without a history of psychosis.

METHODS

Serum calcium-independent and -dependent PLA2 activities were determined in 24 patients with bipolar I disorder.

RESULTS

Serum calcium-independent and -dependent PLA2 activities in bipolar cases did not differ significantly from that in healthy volunteers (HVs). However, calcium-independent PLA2 activity was significantly (p < 0.05) higher in patients with a history of psychosis compared with those with no history of psychosis (by 55%) or to HVs (by 31%).

CONCLUSIONS

Our data suggest that a subset of bipolar I disorder patients with a history of psychosis have elevated calcium-independent PLA2 activity. Given that this enzyme activity is also increased in schizophrenia, elevated rates of phospholipid turnover mediated by the enzyme could represent a common biochemical feature of psychotic illness.

Roles of Group IIA Phospholipase A2 and Complement in Killing of Bacteria by Acute Phase Serum

The complement system is regarded as an important component of the innate defence system against invading bacteria. However, synergistic actions between the complement and the other components of innate immunity are incompletely known. Human group IIA phospholipase A(2) (hGIIA PLA(2)) is an effective antibacterial enzyme in serum of patients with severe bacterial infections. Our aim was to investigate the significance of complement and hGIIA PLA(2) in acute phase serum. Serum samples were collected from patients with acute bacterial infections and from healthy control subjects. We prepared hGIIA PLA(2)-depleted serum by immunoadsorption and inhibited the activity of complement by a specific inhibitor, compstatin. The bactericidal effects of treated and untreated serum were compared by incubating Staphylococcus aureus and Listeria monocytogenes in the presence of serum. Acute phase serum effectively killed S. aureus and L. monocytogenes, and depletion of hGIIA PLA(2) significantly reduced the antibacterial effect. Complement had a weak bactericidal effect against L. monocytogenes. We conclude that hGIIA PLA(2) is the major antibacterial factor in human acute phase serum against the gram-positive bacteria S. aureus and L. monocytogenes, exceeding complement in efficiency.