A novel enzyme immunoassay for the determination of phosphatidylserine-specific phospholipase A(1) in human serum samples

Lysophospholipids and their producing enzymes: Their pathological roles and potential as pathological biomarkers

Accumulating evidence suggests that lysophospholipids (LPL) serve as lipid mediators that exert their diverse pathophysiological functions via G protein-coupled receptors. These include lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P), lysophosphatidylserine (LysoPS) and lysophosphatidylinositol (LPI). Unlike S1P, which is produced intracellularly and secreted from various cell types, some LPLs, such as LPA, LysoPS and LPI, are produced in lesions, especially under pathological conditions, where they positively or negatively regulate disease progression through their autacoid-like actions. Although these LPLs are minor components of the cell membrane, recent developments in mass spectrometry techniques have made it possible to detect and quantify them in a variety of biological fluids, including plasma, serum, urine and cerebrospinal fluid. The synthetic enzymes of LPA and LysoPS are also present in these biological fluids, which also can be detected by antibody-based methods. Importantly, their levels have been found to dramatically increase during various pathological conditions. Thus, LPLs and their synthetic enzymes in these biological fluids are potential biomarkers. This review discusses the potential of these LPLs and LPL-related molecules as pathological biomarkers, including methods and problems in their measurement.

Current Knowledge on Mammalian Phospholipase A1, Brief History, Structures, Biochemical and Pathophysiological Roles

Phospholipase A₁ (PLA₁) is an enzyme that cleaves an ester bond at the sn-1 position of glycerophospholipids, producing a free fatty acid and a lysophospholipid. PLA₁ activities have been detected both extracellularly and intracellularly, which are well conserved in higher eukaryotes, including fish and mammals. All extracellular PLA₁s belong to the lipase family. In addition to PLA₁ activity, most mammalian extracellular PLA₁s exhibit lipase activity to hydrolyze triacylglycerol, cleaving the fatty acid and contributing to its absorption into the intestinal tract and tissues. Some extracellular PLA₁s exhibit PLA₁ activities specific to phosphatidic acid (PA) or phosphatidylserine (PS) and serve to produce lysophospholipid mediators such as lysophosphatidic acid (LPA) and lysophosphatidylserine (LysoPS). A high level of PLA₁ activity has been detected in the cytosol fractions, where PA-PLA₁/DDHD1/iPLA₁ was responsible for the activity. Many homologs of PA-PLA₁ and PLA₂ have been shown to exhibit PLA₁ activity. Although much has been learned about the pathophysiological roles of PLA₁ molecules through studies of knockout mice and human genetic diseases, many questions regarding their biochemical properties, including their genuine in vivo substrate, remain elusive.

Urine autotaxin levels reflect the disease activity of sarcoidosis

Since the clinical outcome of patients with sarcoidosis is still unpredictable, a good prognostic biomarker is necessary. Autotaxin (ATX) and phosphatidylserine-specific phospholipase A1 (PS-PLA1) function as main enzymes to produce lysophospholipids (LPLs), and these enzymes are attracting attention as useful biomarkers for several chronic inflammatory diseases. Here, we investigated the relationships between LPLs-producing enzymes and the disease activity of sarcoidosis. In total, 157 patients with sarcoidosis (active state, 51%) were consecutively enrolled. Using plasma or urine specimens, we measured the values of LPLs-producing enzymes. Urine ATX (U-ATX) levels were significantly lower in the active state compared to those in the inactive state, while the plasma ATX (P-ATX) and PS-PLA1 levels showed no significant difference between these two states. Concerning the comparison with existing clinical biomarkers for sarcoidosis, U-ATX showed a weak negative correlation to ACE, P-ATX a weak positive correlation to both ACE and sIL-2R, and PS-PLA1 a weak positive one to sIL-2R. Notably, only the U-ATX levels inversely fluctuated depending on the status of disease activity whether OCS had been used or not. These findings suggest that U-ATX is likely to be a novel and useful molecule for assessing the disease activity of sarcoidosis.

Phospholipase A1 Member A Activates Fibroblast-like Synoviocytes through the Autotaxin-Lysophosphatidic Acid Receptor Axis

Lysophosphatidylserine (lysoPS) is known to regulate immune cell functions. Phospholipase A1 member A (PLA1A) can generate this bioactive lipid through hydrolysis of sn-1 fatty acids on phosphatidylserine (PS). PLA1A has been associated with cancer metastasis, asthma, as well as acute coronary syndrome. However, the functions of PLA1A in the development of systemic autoimmune rheumatic diseases remain elusive. To investigate the possible implication of PLA1A during rheumatic diseases, we monitored PLA1A in synovial fluids from patients with rheumatoid arthritis and plasma of early-diagnosed arthritis (EA) patients and clinically stable systemic lupus erythematosus (SLE) patients. We used human primary fibroblast-like synoviocytes (FLSs) to evaluate the PLA1A-induced biological responses. Our results highlighted that the plasma concentrations of PLA1A in EA and SLE patients were elevated compared to healthy donors. High concentrations of PLA1A were also detected in synovial fluids from rheumatoid arthritis patients compared to those from osteoarthritis (OA) and gout patients. The origin of PLA1A in FLSs and the arthritic joints remained unknown, as healthy human primary FLSs does not express the PLA1A transcript. Besides, the addition of recombinant PLA1A stimulated cultured human primary FLSs to secrete IL-8. Preincubation with heparin, autotaxin (ATX) inhibitor HA130 or lysophosphatidic acid (LPA) receptor antagonist Ki16425 reduced PLA1A-induced-secretion of IL-8. Our data suggested that FLS-associated PLA1A cleaves membrane-exposed PS into lysoPS, which is subsequently converted to LPA by ATX. Since primary FLSs do not express any lysoPS receptors, the data suggested PLA1A-mediated pro-inflammatory responses through the ATX-LPA receptor signaling axis.

Current Knowledge on the Biology of Lysophosphatidylserine as an Emerging Bioactive Lipid

Lysophosphatidylserine (LysoPS) is an emerging lysophospholipid (LPL) mediator, which acts through G protein-coupled receptors, like lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P). LysoPS is detected in various tissues and cells and thought to be produced mainly by the deacylation of phosphatidylserine. LysoPS has been known to stimulate degranulation of mast cells. Recently, four LysoPS-specific G protein-coupled receptors (GPCRs) were identified. These GPCRs belong to the P2Y family which covers receptors for nucleotides and LPLs and are predominantly expressed in immune cells such as lymphocytes and macrophages. Studies on knockout mice of these GPCRs have revealed that LysoPS has immune-modulatory functions. Up-regulation of a LysoPS-producing enzyme, PS-specific phospholipase A1, was frequently observed in situations where the immune system is activated including autoimmune diseases and organ transplantations. Therefore, modulation of LysoPS signaling appears to be a promising method for providing therapies for the treatment of immune diseases. In this review, we summarize the biology of LysoPS-producing enzymes and receptors, recent developments in LysoPS signal modulators, and prospects for future therapeutic applications.

Phosphatidylserine-specific phospholipase A1: A friend or the devil in disguise

Various human tissues and cells express phospholipase A1 member A (PLA1A), including the liver, lung, prostate gland, and immune cells. The enzyme belongs to the pancreatic lipase family. PLA1A specifically hydrolyzes sn-1 fatty acid of phosphatidylserine (PS) or 1-acyl-lysophosphatidylserine (1-acyl-lysoPS). PS externalized by activated cells or apoptotic cells or extracellular vesicles is a potential source of substrate for the production of unsaturated lysoPS species by PLA1A. Maturation and functions of many immune cells, such as T cells, dendritic cells, macrophages, and mast cells, can be regulated by PLA1A and lysoPS. Several lysoPS receptors, including GPR34, GPR174 and P2Y10, have been identified. High serum levels and high PLA1A expression are associated with autoimmune disorders such as Graves' disease and systemic lupus erythematosus. Increased expression of PLA1A is associated with metastatic melanomas. PLA1A may contribute to cardiometabolic disorders through mediating cholesterol transportation and producing lysoPS. Furthermore, PLA1A is necessary for hepatitis C virus assembly and can play a role in the antivirus innate immune response. This review summarizes recent findings on PLA1A expression, lysoPS and lysoPS receptors in autoimmune disorders, cancers, cardiometabolic disorders, antivirus immune responses, as well as regulations of immune cells.

PLA1A expression as a diagnostic marker of BRAF-mutant metastasis in melanoma cancer

BRAF and NRAS are the most reported mutations associated to melanomagenesis. The lack of accurate diagnostic markers in response to therapeutic treatment in BRAF/NRAS-driven melanomagenesis is one of the main challenges in melanoma personalized therapy. In order to assess the diagnostic value of phosphatidylserine-specific phospholipase A1-alpha (PLA1A), a potent lysophospholipid mediating the production of lysophosphatidylserine, PLA1A mRNA and serum levels were compared in subjects with malignant melanoma (n = 18), primary melanoma (n = 13), and healthy subjects (n = 10). Additionally, the correlation between histopathological subtypes of BRAF/NRAS-mutated melanoma and PLA1A was analyzed. PLA1A expression was significantly increased during melanogenesis and positively correlated to disease severity and histopathological markers of metastatic melanoma. PLA1A mRNA and serum levels were significantly higher in patients with BRAF-mutated melanoma compared to the patients with NRAS-mutated melanoma. Notably, PLA1A can be used as a diagnostic marker for an efficient discrimination between naïve melanoma samples and advanced melanoma samples (sensitivity 91%, specificity 57%, and AUC 0.99), as well as BRAF-mutated melanoma samples (sensitivity 62%, specificity 61%, and AUC 0.75). Our findings suggest that PLA1A can be considered as a potential diagnostic marker for advanced and BRAF-mutated melanoma.

Higher serum levels of autotaxin and phosphatidylserine-specific phospholipase A1 in patients with lupus nephritis

BACKGROUND

Recent studies revealed that lysophospholipids (LPLs) and related molecules, such as autotaxin (ATX) and phosphatidylserine-specific phospholipase A₁ (PS-PLA₁ ), are candidates for novel biomarkers in melanoma, glaucoma and diabetic nephropathy. However, it is not clear whether serum levels of ATX/ PS-PLA₁ would be associated with pathological and clinical findings of lupus nephritis (LN).

METHODS

In this retrospective cohort study, serum samples were collected from 39 patients with LN and 37 patients with other glomerular diseases. The serum levels of ATX and PS-PLA₁ were evaluated for an association with renal pathology and clinical phenotypes of LN.

RESULTS

The serum levels of ATX and PS-PLA₁ were higher in the patients with LN as compared to those with other glomerular diseases. Among the classes of LN, the patients with class IV showed the trend of lower serum levels of ATX. Moreover, the patients with lower levels of ATX exhibited higher scores of activity index (AI) and chronicity index (CI). The level of ATX tended to be negatively correlated with AI and CI. These results might be explained by the effect of treatment, because the serum levels of ATX and PS-PLA₁ were inversely correlated with the daily amount of oral prednisolone. Moreover, they did not reflect the level of proteinuria or kidney survival in LN patients.

CONCLUSION

Although the serum levels of ATX and PS-PLA₁ were affected by the treatment, these levels were higher in the patients with LN. The potential clinical benefits of these markers need to be clarified in further studies.

Possible involvement of PS-PLA1 and lysophosphatidylserine receptor (LPS1) in hepatocellular carcinoma

Lysophosphatidylserine (LysoPS) is a lysophospholipid, its generating enzyme, phosphatidylserine-specific phospholipase A1 (PS-PLA1), reportedly plays roles in stomach and colon cancers. Here, we examined the potential roles of LysoPS in hepatocellular carcinoma (HCC). The ninety-seven HCC patients who underwent surgical treatment were enrolled in this study and approved by the institutional review board. Among LysoPS-related enzymes and receptors, increased PS-PLA1 or LysoPS receptor 1 (LPS1) mRNA was observed in HCC tissues compared to non-HCC tissues. PS-PLA1 mRNA in HCC was associated with no clinical parameters, while LPS1 mRNA in HCC was correlated inversely with tumor differentiation. Furthermore, higher serum PS-PLA1 was observed in HCC patients compared to healthy control and correlated with PS-PLA1 mRNA in non-HCC tissues and with serum AST or ALT. Additionally, serum levels of PS-PLA1 were higher in HCC patients with HCV-related liver injury than in those with HBV or non-HBV-, non-HCV-related liver diseases. In conclusion, among LysoPS-related enzymes and receptors, PS-PLA1 and LPS1 mRNA were increased in HCC. Based on the correlation between the serum PS-PLA1 and the mRNA level of PS-PLA1 in non-HCC tissues, the liver may be the main source of serum PS-PLA1, and serum PS-PLA1 levels may be a useful marker for liver injury.

Association between serum autotaxin or phosphatidylserine-specific phospholipase A1 levels and melanoma

Autotaxin (ATX), a producing enzyme for lysophosphatidic acids, was first identified from the medium of a melanoma cell line and has been considered to be one of the candidate targets to treat melanoma; however, the association between serum ATX and melanoma in human subjects has not been elucidated. Along with ATX, phosphatidylserine-specific phospholipase A1 (PS-PLA₁ ) is a producing enzyme for lysophosphatidylserine, a similar glycero-lysophospholipid mediator to lysophosphatidic acids. In the present study, we aimed to investigate the association between serum ATX or PS-PLA₁ levels and melanoma. We measured the serum levels of ATX, ATX isoforms and PS-PLA₁ in subjects with melanoma (n = 57) and healthy subjects (n = 58). We further investigated the existence of trends according to the clinical stages of melanoma. We observed that serum total ATX and classical ATX levels were significant higher and serum novel ATX levels tended to be higher in male subjects with melanoma, while no significant difference was observed between the two groups in female subjects. The trend test revealed that the serum total ATX and ATX isoforms were significantly associated with the clinical stages of female subjects with melanoma. Regarding PS-PLA₁ , serum PS-PLA₁ levels were significantly higher in the melanoma subjects and associated with the clinical stages. The present study is the first study which revealed the association between ATX or PS-PLA₁ and melanoma, suggesting the possible involvement of ATX/lysophosphatidic acids or PS-PLA₁ /lysophosphatidylserine axis in the pathogenesis of melanoma.

Lysophospholipids in laboratory medicine

Lysophospholipids (LPLs), such as lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P), and lysophosphatidylserine (LysoPS), are attracting attention as second-generation lipid mediators. In our laboratory, the functional roles of these lipid mediators and the mechanisms by which the levels of these mediators are regulated in vivo have been studied. Based on these studies, the clinical introduction of assays for LPLs and related proteins has been pursued and will be described in this review. Although assays of these lipids themselves are possible, autotaxin (ATX), apolipoprotein M (ApoM), and phosphatidylserine-specific phospholipase A₁ (PS-PLA₁) are more promising as alternate biomarkers for LPA, S1P, and LysoPS, respectively. Presently, ATX, which produces LPA through its lysophospholipase D activity, has been shown to be a useful laboratory test for the diagnosis and staging of liver fibrosis, whereas PS-PLA₁ and ApoM are considered to be promising clinical markers reflecting the in vivo actions induced by LysoPS and S1P.

Analysis of glycero-lysophospholipids in gastric cancerous ascites

Lysophosphatidic acid (LysoPA) has been proposed to be involved in the pathogenesis of various cancers. Moreover, glycero-lysophospholipids (glycero-LysoPLs) other than LysoPA are now emerging as novel lipid mediators. Therefore, we aimed to elucidate the possible involvement of glycero-LysoPLs in the pathogenesis of gastric cancer by measuring glycero-LysoPLs, autotaxin (ATX), and phosphatidylserine-specific phospholipase A1 (PS-PLA₁) in ascites obtained from patients with gastric cancer and those with cirrhosis (as a control). We observed that after adjustments according to the albumin levels, the lysophosphatidylserine (LysoPS) and lysophosphatidylglycerol (LysoPG) levels were significantly higher, while the LysoPA and ATX levels were lower, in the ascites from patients with gastric cancer. We also found that multiple regression analyses revealed that ATX was selected as a significant explanatory factor for all the detectable LysoPA species only in the cirrhosis group and that a significant positive correlation was observed between LysoPS and PS-PLA₁ only in the gastric cancer group. In conclusion, the LysoPA levels might be determined largely by LysoPC and LysoPI (possible precursors) and the PS-PLA₁-mediated pathway might be involved in the production of LysoPS in gastric cancer. Glycero-LysoPLs other than LysoPA might also be involved in the pathogenesis of cancer directly or through being converted into LysoPA.

Different origins of lysophospholipid mediators between coronary and peripheral arteries in acute coronary syndrome

Lysophosphatidic acids (LysoPAs) and lysophosphatidylserine (LysoPS) are emerging lipid mediators proposed to be involved in the pathogenesis of acute coronary syndrome (ACS). In this study, we attempted to elucidate how LysoPA and LysoPS become elevated in ACS using human blood samples collected simultaneously from culprit coronary arteries and peripheral arteries in ACS subjects. We found that: 1) the plasma LysoPA, LysoPS, and lysophosphatidylglycerol levels were not different, while the lysophosphatidylcholine (LysoPC), lysophosphatidylinositol, and lysophosphatidylethanolamine (LysoPE) levels were significantly lower in the culprit coronary arteries; 2) the serum autotaxin (ATX) level was lower and the serum phosphatidylserine-specific phospholipase A₁ (PS-PLA₁) level was higher in the culprit coronary arteries; 3) the LysoPE and ATX levels were significant explanatory factors for the mainly elevated species of LysoPA, except for 22:6 LysoPA, in the peripheral arteries, while the LysoPC and LysoPE levels, but not the ATX level, were explanatory factors in the culprit coronary arteries; and 4) 18:0 and 18:1 LysoPS were significantly correlated with PS-PLA₁ only in the culprit coronary arteries. In conclusion, the origins of LysoPA and LysoPS might differ between culprit coronary arteries and peripheral arteries, and substrates for ATX, such as LysoPC and LysoPE, might be important for the generation of LysoPA in ACS.

Blood levels of serotonin are specifically correlated with plasma lysophosphatidylserine among the glycero-lysophospholipids

Backgrounds

Glycero-lysophospholipids (glycero-LPLs), which are known to exert potent biological activities, have been demonstrated to be secreted from activated platelets in vitro; however, their association with platelet activation in vivo has not been yet elucidated. In this study, we investigated the correlations between the blood levels of each glycero-LPL and serotonin, a biomarker of platelet activation, in human subjects to elucidate the involvement of platelet activation in glycero-LPLs in vivo.

Methods and Results

We measured the plasma serotonin levels in 141 consecutive patients undergoing coronary angiography (acute coronary syndrome, n = 38; stable angina pectoris, n = 71; angiographically normal coronary arteries, n = 32) and investigated the correlations between the plasma levels of serotonin and glycero-LPLs. The results revealed the existence of a specific and significant association between the plasma serotonin and plasma lysophosphatidylserine (LysoPS) levels. On the contrary, regular aspirin intake failed to affect the plasma LysoPS levels despite the fact that the plasma lysophosphatidic acid, lysophosphatidylethanolamine, lysophosphatidylglycerol, and lysophosphatidylinositol levels were lower in those who had taken aspirin regularly.

Conclusion

We found a specific positive correlation between the blood levels of serotonin and LysoPS, a new lipid mediator. Thus, LysoPS might be specifically involved in strong platelet activation, which is associated with the release of serotonin.

General Significance

Our present results suggest the possible involvement of LysoPS in the pathogenesis of atherosclerotic diseases.

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A significant positive correlation between the plasma serotonin and lysophosphatidylserine was observed.

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Regular intake of aspirin had no influence on plasma lysophosphatidylserine.

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PS-PLA1 was correlated with lysophosphatidylserine only in acute coronary syndrome.

Surface loops of extracellular phospholipase A(1) determine both substrate specificity and preference for lysophospholipids

Members of the pancreatic lipase family exhibit both lipase activity toward triacylglycerol and/or phospholipase A(1) (PLA(1)) activity toward certain phospholipids. Some members of the pancreatic lipase family exhibit lysophospholipase activity in addition to their lipase and PLA(1) activities. Two such enzymes, phosphatidylserine (PS)-specific PLA(1) (PS-PLA(1)) and phosphatidic acid (PA)-selective PLA(1)α (PA-PLA(1)α, also known as LIPH) specifically hydrolyze PS and PA, respectively. However, little is known about the mechanisms that determine their substrate specificities. Crystal structures of lipases and mutagenesis studies have suggested that three surface loops, namely, β5, β9, and lid, have roles in determining substrate specificity. To determine roles of these loop structures in the substrate recognition of these PLA(1) enzymes, we constructed a number of PS-PLA(1) mutants in which the three surface loops are replaced with those of PA-PLA(1)α. The results indicate that the surface loops, especially the β5 loop, of PA-PLA(1)α play important roles in the recognition of PA, whereas other structure(s) in PS-PLA(1) is responsible for PS preference. In addition, β5 loop of PS-PLA(1) has a crucial role in lysophospholipase activity toward lysophosphatidylserine. The present study revealed the critical role of lipase surface loops, especially the β5 loop, in determining substrate specificities of PLA(1) enzymes.