Group X secretory phospholipase A(2) augments angiotensin II-induced inflammatory responses and abdominal aortic aneurysm formation in apoE-deficient mice

Modulation of immunity by the secreted phospholipase A2 family

Among the phospholipase A2 (PLA2 ) superfamily, which typically catalyzes the sn-2 hydrolysis of phospholipids to yield fatty acids and lysophospholipids, the secreted PLA2 (sPLA2 ) family contains 11 isoforms in mammals. Individual sPLA2 s have unique enzymatic specificity toward fatty acids and polar heads of phospholipid substrates and display distinct tissue/cellular distributions, suggesting their distinct physiological functions. Recent studies using knockout and/or transgenic mice for a full set of sPLA2 s have revealed their roles in modulation of immunity and related disorders. Application of mass spectrometric lipidomics to these mice has enabled to identify target substrates and products of individual sPLA2 s in given tissue microenvironments. sPLA2 s hydrolyze not only phospholipids in the plasma membrane of activated, damaged or dying mammalian cells, but also extracellular phospholipids such as those in extracellular vesicles, microbe membranes, lipoproteins, surfactants, and dietary phospholipids, thereby exacerbating or ameliorating various diseases. The actions of sPLA2 s are dependent on, or independent of, the generation of fatty acid- or lysophospholipid-derived lipid mediators according to the pathophysiological contexts. In this review, we make an overview of our current understanding of the roles of individual sPLA2 s in various immune responses and associated diseases.

Regulatory Roles of Phospholipase A2 Enzymes and Bioactive Lipids in Mast Cell Biology

Lipids play fundamental roles in life as an essential component of cell membranes, as a major source of energy, as a body surface barrier, and as signaling molecules that transmit intracellular and intercellular signals. Lipid mediators, a group of bioactive lipids that mediates intercellular signals, are produced via specific biosynthetic enzymes and transmit signals via specific receptors. Mast cells, a tissue-resident immune cell population, produce several lipid mediators that contribute to exacerbation or amelioration of allergic responses and also non-allergic inflammation, host defense, cancer and fibrosis by controlling the functions of microenvironmental cells as well as mast cell themselves in paracrine and autocrine fashions. Additionally, several bioactive lipids produced by stromal cells regulate the differentiation, maturation and activation of neighboring mast cells. Many of the bioactive lipids are stored in membrane phospholipids as precursor forms and released spatiotemporally by phospholipase A₂ (PLA₂) enzymes. Through a series of studies employing gene targeting and lipidomics, several enzymes belonging to the PLA₂ superfamily have been demonstrated to participate in mast cell-related diseases by mobilizing unique bioactive lipids in multiple ways. In this review, we provide an overview of our current understanding of the regulatory roles of several PLA₂-driven lipid pathways in mast cell biology.

Old but New: Group IIA Phospholipase A2 as a Modulator of Gut Microbiota

Among the phospholipase A₂ (PLA₂) superfamily, the secreted PLA₂ (sPLA₂) family contains 11 mammalian isoforms that exhibit unique tissue or cellular distributions and enzymatic properties. Current studies using sPLA₂-deficient or -overexpressed mouse strains, along with mass spectrometric lipidomics to determine sPLA₂-driven lipid pathways, have revealed the diverse pathophysiological roles of sPLA₂s in various biological events. In general, individual sPLA₂s exert their specific functions within tissue microenvironments, where they are intrinsically expressed through hydrolysis of extracellular phospholipids. Recent studies have uncovered a new aspect of group IIA sPLA₂ (sPLA₂-IIA), a prototypic sPLA₂ with the oldest research history among the mammalian PLA₂s, as a modulator of the gut microbiota. In the intestine, Paneth cell-derived sPLA₂-IIA acts as an antimicrobial protein to shape the gut microbiota, thereby secondarily affecting inflammation, allergy, and cancer in proximal and distal tissues. Knockout of intestinal sPLA₂-IIA in BALB/c mice leads to alterations in skin cancer, psoriasis, and anaphylaxis, while overexpression of sPLA₂-IIA in Pla2g2a-null C57BL/6 mice induces systemic inflammation and exacerbates arthritis. These phenotypes are associated with notable changes in gut microbiota and fecal metabolites, are variable in different animal facilities, and are abrogated after antibiotic treatment, co-housing, or fecal transfer. These studies open a new mechanistic action of this old sPLA₂ and add the sPLA₂ family to the growing list of endogenous factors capable of affecting the microbe–host interaction and thereby systemic homeostasis and diseases.

Specific inhibition of SHP2 suppressed abdominal aortic aneurysm formation in mice by augmenting the immunosuppressive function of MDSCs

AIMS

To address the roles of SHP2 in regulating angiotensin II (Ang II) induced abdominal aortic aneurysm (AAA) and the potential molecular mechanisms.

MAIN METHODS

AAA model was established in apolipoprotein E-deficient (apoE^-/-) mice infused with Ang II. Suprarenal aortic luminal diameters were ultrasonically measured to determine the presentation of AAA in mice. The inflammatory and immunosuppressive factors in serum were detected by ELISA. AAA lesion size, positive macrophages and elastic laminae degradation were examined by histological analysis. Myeloid-derived suppressor cells (MDSCs) were measured by flow cytometry after magnetic bead sorting. Bioinformatics analysis was applied to screen the crucial genes related the progression of AAA.

KEY FINDINGS

Treatment with PHPS1 (SHP2 inhibitor) significantly decreased the vascular diameter of AAA. Histological analysis showed that PHPS1 obviously reduced the Masson positive area, macrophages positive area, as well as the damage rate of elastic laminae. Moreover, PHPS1 suppressed the expression of INF-γ, TNF-α and MMPs, as well as elevated IL-10 and arginase-1 expression. Additionally, PHPS1 enhanced the expression of granulocytic MDSCs (G-MDSCs). By consulting with bioinformatics, STAT3 was selected. In G-MDSCs, PHPS1 stimulation obviously increased the phosphorylation level of STAT3, as well as elevated the protein expression of C/EBPβ and arginase-1. However, the above phenomena can be blocked after Stattic (STAT3 inhibitor) treatment.

SIGNIFICANCE

SHP2 may affect the AAA progression by interfering with expansion and function of MDSCs to regulate the body immunity, which might afford a novel direction for the treatment of patients with AAA.

Updating Phospholipase A2 Biology

The phospholipase A₂ (PLA₂) superfamily contains more than 50 enzymes in mammals that are subdivided into several distinct families on a structural and biochemical basis. In principle, PLA₂ has the capacity to hydrolyze the sn-2 position of glycerophospholipids to release fatty acids and lysophospholipids, yet several enzymes in this superfamily catalyze other reactions rather than or in addition to the PLA₂ reaction. PLA₂ enzymes play crucial roles in not only the production of lipid mediators, but also membrane remodeling, bioenergetics, and body surface barrier, thereby participating in a number of biological events. Accordingly, disturbance of PLA₂-regulated lipid metabolism is often associated with various diseases. This review updates the current state of understanding of the classification, enzymatic properties, and biological functions of various enzymes belonging to the PLA₂ superfamily, focusing particularly on the novel roles of PLA₂s in vivo.

Group V secreted phospholipase A2 plays a protective role against aortic dissection

Aortic dissection is a life-threatening aortopathy involving separation of the aortic wall, whose underlying mechanisms are still incompletely understood. Epidemiological evidence suggests that unsaturated fatty acids improve cardiovascular health. Here, using quantitative RT-PCR, histological analyses, magnetic cell sorting and flow cytometry assays, and MS-based lipidomics, we show that the activity of a lipid-metabolizing enzyme, secreted phospholipase A₂ group V (sPLA₂-V), protects against aortic dissection by endogenously mobilizing vasoprotective lipids. Global and endothelial cell-specific sPLA₂-V-deficient mice frequently developed aortic dissection shortly after infusion of angiotensin II (AT-II). We observed that in the AT-II-treated aorta, endothelial sPLA₂-V mobilized oleic and linoleic acids, which attenuated endoplasmic reticulum stress, increased the expression of lysyl oxidase, and thereby stabilized the extracellular matrix in the aorta. Of note, dietary supplementation with oleic or linoleic acid reversed the increased susceptibility of sPLA₂-V-deficient mice to aortic dissection. These findings reveal an unexplored functional link between sPLA₂-driven phospholipid metabolism and aortic stability, possibly contributing to the development of improved diagnostic and/or therapeutic strategies for preventing aortic dissection.

After 50 Years of Heart Transplants: What Does the Next 50 Years Hold for Cardiovascular Medicine? A Perspective From the International Society for Applied Cardiovascular Biology

The first successful heart transplant 50 years ago by Dr.Christiaan Barnard in Cape Town, South Africa revolutionized cardiovascular medicine and research. Following this procedure, numerous other advances have reduced many contributors to cardiovascular morbidity and mortality; yet, cardiovascular disease remains the leading cause of death globally. Various unmet needs in cardiovascular medicine affect developing and underserved communities, where access to state-of-the-art advances remain out of reach. Addressing the remaining challenges in cardiovascular medicine in both developed and developing nations will require collaborative efforts from basic science researchers, engineers, industry, and clinicians. In this perspective, we discuss the advancements made in cardiovascular medicine since Dr. Barnard's groundbreaking procedure and ongoing research efforts to address these medical issues. Particular focus is given to the mission of the International Society for Applied Cardiovascular Biology (ISACB), which was founded in Cape Town during the 20th celebration of the first heart transplant in order to promote collaborative and translational research in the field of cardiovascular medicine.

Group IID, IIE, IIF and III secreted phospholipase A2s

Among the 11 members of the secreted phospholipase A₂ (sPLA₂) family, group IID, IIE, IIF and III sPLA₂s (sPLA₂-IID, -IIE, -IIF and -III, respectively) are “new” isoforms in the history of sPLA₂ research. Relative to the better characterized sPLA₂s (sPLA₂-IB, -IIA, -V and -X), the enzymatic properties, distributions, and functions of these “new” sPLA₂s have remained obscure until recently. Our current studies using knockout and transgenic mice for a nearly full set of sPLA₂s, in combination with comprehensive lipidomics, have revealed unique and distinct roles of these “new” sPLA₂s in specific biological events. Thus, sPLA₂-IID is involved in immune suppression, sPLA₂-IIE in metabolic regulation and hair follicle homeostasis, sPLA₂-IIF in epidermal hyperplasia, and sPLA₂-III in male reproduction, anaphylaxis, colonic diseases, and possibly atherosclerosis. In this article, we overview current understanding of the properties and functions of these sPLA₂s and their underlying lipid pathways in vivo.

Deletion of resistin-like molecule-beta attenuates angiotensin II-induced abdominal aortic aneurysm

In the present study, we want to test whether deletion of resistin-like molecule-beta (RELMβ) attenuates angiotensin II (Ang II)-induced formation of abdominal aortic aneurysm (AAA). RELMβ gene expression was inhibited by siRNA both in vivo and in vitro. Apolipoprotein E-knockout (ApoE^−/−) mice were randomly divided into saline, Ang II, siRNA negative control (si-NC) and siRNA RELMβ (si-RELMβ) groups (n=15 each), and mice in the last three groups underwent Ang II infusion for 4 weeks to induce AAA. RELMβ gene deficiency significantly decreased AAA incidence and severity, which was associated with reduced macrophage accumulation and decreased expression of proinflammatory cytokines (monocyte chemoattractant protein 1 and interleukin 6), matrix metalloproteinase 2 (MMP-2) and MMP-9 in the aortic wall. In cultured macrophages, RELMβ deficiency blunted the response of macrophages to Ang II and downregulated the levels of proinflammatory cytokines, MMP-2 and MMP-9. Recombinant RELMβ promoted the secretion of proinflammatory cytokines, MMP-2 and MMP-9 in macrophages and activated extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK) signaling, which was reversed with pretreatment with inhibitors of ERK1/2 and JNK. Deletion of RELMβ attenuated Ang II-induced AAA formation in ApoE^−/− mice. The inherent mechanism may involve the reduced expression of proinflammatory cytokines, MMP-2 and MMP-9, which was mediated by ERK1/2 and JNK activation. Therefore, inhibiting RELMβ secretion may be a novel approach for anti-aneurysm treatment.

Lipoquality control by phospholipase A2 enzymes

The phospholipase A₂ (PLA₂) family comprises a group of lipolytic enzymes that typically hydrolyze the sn-2 position of glycerophospholipids to give rise to fatty acids and lysophospholipids. The mammalian genome encodes more than 50 PLA₂s or related enzymes, which are classified into several subfamilies on the basis of their structures and functions. From a general viewpoint, the PLA₂ family has mainly been implicated in signal transduction, producing bioactive lipid mediators derived from fatty acids and lysophospholipids. Recent evidence indicates that PLA₂s also contribute to phospholipid remodeling for membrane homeostasis or energy production for fatty acid β-oxidation. Accordingly, PLA₂ enzymes can be regarded as one of the key regulators of the quality of lipids, which I herein refer to as lipoquality. Disturbance of PLA₂-regulated lipoquality hampers tissue and cellular homeostasis and can be linked to various diseases. Here I overview the current state of understanding of the classification, enzymatic properties, and physiological functions of the PLA₂ family.

The Roles of the Secreted Phospholipase A2 Gene Family in Immunology

Within the phospholipase A₂ (PLA₂) family that hydrolyzes phospholipids to yield fatty acids and lysophospholipids, secreted PLA₂ (sPLA₂) enzymes comprise the largest group containing 11 isoforms in mammals. Individual sPLA₂s exhibit unique tissue or cellular distributions and enzymatic properties, suggesting their distinct biological roles. Although PLA₂ enzymes, particularly cytosolic PLA₂ (cPLA₂α), have long been implicated in inflammation by driving arachidonic acid metabolism, the precise biological roles of sPLA₂s have remained a mystery over the last few decades. Recent studies employing mice gene-manipulated for individual sPLA₂s, in combination with mass spectrometric lipidomics to identify their target substrates and products in vivo, have revealed their roles in diverse biological events, including immunity and associated disorders, through lipid mediator-dependent or -independent processes in given microenvironments. In this review, we summarize our current knowledge of the roles of sPLA₂s in various immune responses and associated diseases.

Regulation and involvement of matrix metalloproteinases in vascular diseases

Matrix metalloproteinases (MMPs) are a family of zinc dependent endopeptidases whose main function is to degrade and deposit structural proteins within the extracellular matrix (ECM). A dysregulation of MMPs is linked to vascular diseases. MMPs are classified into collagenases, gelatinases, membrane-type, metalloelastase, stromelysins, matrilysins, enamelysins, and unclassified subgroups. The production of MMPs is stimulated by factors such as oxidative stress, growth factors and inflammation which lead to its up- or down-regulation with subsequent ECM remodeling. Normally, excess activation of MMPs is controlled by their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMPs). An imbalance of MMPs and TIMPs has been implicated in hypertension, atherosclerotic plaque formation and instability, aortic aneurysms and varicose vein wall remodeling. Also, recent evidence suggests epigenetic regulation of some MMPs in angiogenesis and atherosclerosis. Over the years, pharmacological inhibitors of MMPs have been used to modify or prevent the development of the disease with some success. In this review, we discuss recent advances in MMP biology, and their involvement in the manifestation of vascular disease.

Identification of key genes associated with the human abdominal aortic aneurysm based on the gene expression profile

The present study was aimed at screening the key genes associated with abdominal aortic aneurysm (AAA) in the neck, and to investigate the molecular mechanism underlying the development of AAA. The gene expression profile, GSE47472, including 14 AAA neck samples and eight donor controls, was downloaded from the Gene Expression Omnibus database. The total AAA samples were grouped into two types to avoid bias. Differentially expressed genes (DEGs) were screened in patients with AAA and subsequently compared with donor controls using linear models for microarray data, or the Limma package in R, followed by gene ontology enrichment analysis. Furthermore, a protein-protein interaction (PPI) network based on the DEGs was constructed to detect highly connected regions using a Cytoscape plugin. In total, 388 DEGs in the AAA samples were identified. These DEGs were predominantly associated with limb development, including embryonic limb development and appendage development. Nuclear receptor co-repressor 1 (NCOR1), histone 4 (H4), E2F transcription factor 4 (E2F4) and hepatocyte nuclear factor 4α (HNF4A) were the four transcription factors associated with AAA. Furthermore, HNF4A indirectly interacted with the other three transcription factors. Additionally, six clusters were selected from the PPI network. The DEG screening process and the construction of an interaction network enabled an understanding of the mechanism of AAA to be gleaned. HNF4A may exert an important role in AAA development through its interactions with the three other transcription factors (E2F4, NCOR1 and H4), and the mechanism of this coordinated regulation of the transcription factors in AAA may provide a suitable target for the development of therapeutic intervention strategies.

A new era of secreted phospholipase A₂

Among more than 30 members of the phospholipase A2 (PLA2) superfamily, secreted PLA2 (sPLA2) enzymes represent the largest family, being Ca(2+)-dependent low-molecular-weight enzymes with a His-Asp catalytic dyad. Individual sPLA2s exhibit unique tissue and cellular distributions and enzymatic properties, suggesting their distinct biological roles. Recent studies using transgenic and knockout mice for nearly a full set of sPLA2 subtypes, in combination with sophisticated lipidomics as well as biochemical and cell biological studies, have revealed distinct contributions of individual sPLA2s to various pathophysiological events, including production of pro- and anti-inflammatory lipid mediators, regulation of membrane remodeling, degradation of foreign phospholipids in microbes or food, or modification of extracellular noncellular lipid components. In this review, we highlight the current understanding of the in vivo functions of sPLA2s and the underlying lipid pathways as revealed by a series of studies over the last decade.

Deficiency of Endogenous Acute-Phase Serum Amyloid A Protects apoE-/- Mice From Angiotensin II-Induced Abdominal Aortic Aneurysm Formation

OBJECTIVE

Rupture of abdominal aortic aneurysm (AAA), a major cause of death in the aged population, is characterized by vascular inflammation and matrix degradation. Serum amyloid A (SAA), an acute-phase reactant linked to inflammation and matrix metalloproteinase induction, correlates with aortic dimensions before aneurysm formation in humans. We investigated whether SAA deficiency in mice affects AAA formation during angiotensin II (Ang II) infusion.

APPROACH AND RESULTS

Plasma SAA increased ≈60-fold in apoE(-/-) mice 24 hours after intraperitoneal Ang II injection (100 μg/kg; n=4) and ≈15-fold after chronic 28-day Ang II infusion (1000 ng/kg per minute; n=9). AAA incidence and severity after 28-day Ang II infusion was significantly reduced in apoE(-/-) mice lacking both acute-phase SAA isoforms (SAAKO; n=20) compared with apoE(-/-) mice (SAAWT; n=20) as assessed by in vivo ultrasound and ex vivo morphometric analyses, despite a significant increase in systolic blood pressure in SAAKO mice compared with SAAWT mice after Ang II infusion. Atherosclerotic lesion area of the aortic arch was similar in SAAKO and SAAWT mice after 28-day Ang II infusion. Immunostaining detected SAA in AAA tissues of Ang II-infused SAAWT mice that colocalized with macrophages, elastin breaks, and enhanced matrix metalloproteinase activity. Matrix metalloproteinase-2 activity was significantly lower in aortas of SAAKO mice compared with SAAWT mice after 10-day Ang II infusion.

CONCLUSIONS

Lack of endogenous acute-phase SAA protects against experimental AAA through a mechanism that may involve reduced matrix metalloproteinase-2 activity.

Induction of dendritic cell-mediated T-cell activation by modified but not native low-density lipoprotein in humans and inhibition by annexin a5: involvement of heat shock proteins

OBJECTIVE

Atherosclerosis is an inflammatory disease, where activated immunocompetent cells, including dendritic cells (DCs) and T cells are abundant in plaques. Low-density lipoprotein modified either by oxidation (oxLDL) or by human group X-secreted phospholipase A2 (LDLx) and heat shock proteins (HSP), especially HSP60 and 90, have been implicated in atherosclerosis. We previously reported that Annexin A5 inhibits inflammatory effects of phospholipids, decreases vascular inflammation and improves vascular function in apolipoprotein E(-/-) mice. Here, we focus on the LDLx effects on human DCs and T cells.

APPROACH AND RESULTS

Human DCs were differentiated from peripheral blood monocytes, stimulated by oxLDL or LDLx. Naive autologous T cells were cocultured with pretreated DCs. oxLDL and LDLx, in contrast to LDL, induced DC-activation and T-cell proliferation. T cells exposed to LDLx-treated DCs produced interferon-γ, interleukin (IL)-17 but not IL-4 and IL-10. Annexin A5 abrogated LDLx effects on DCs and T cells and increased production of transforming growth factor-β and IL-10. Furthermore, IL-10 producing T cells suppressed primary T-cell activation via soluble IL-10, transforming growth factor-β, and cell-cell contact. Lentiviral-mediated shRNA knock-down HSP60 and 90 in DCs attenuated the effect of LDLx on DCs and subsequent T-cell proliferation. Experiments on DC and T cells derived from carotid atherosclerotic plaques gave similar results.

CONCLUSIONS

Our data show that modified forms of LDL such as LDLx but not native LDL activate human T cells through DCs. HSP60 and 90 contribute to such T-cell activation. Annexin A5 promotes induction of regulatory T cells and is potentially interesting as a therapeutic agent.

Emerging roles of secreted phospholipase A2 enzymes: the 3rd edition

Within the phospholipase A2 (PLA2) superfamily, secreted PLA2 (sPLA2) enzymes comprise the largest family that contains 11 to 12 mammalian isoforms with a conserved His-Asp catalytic dyad. Individual sPLA2s exhibit unique tissue and cellular localizations and specific enzymatic properties, suggesting distinct biological roles. Individual sPLA2s are involved in diverse biological events through lipid mediator-dependent or -independent processes and act redundantly or non-redundantly in a given microenvironment. In the past few years, new biological aspects of sPLA2s have been clarified using their transgenic and knockout mouse lines in combination with mass spectrometric lipidomics to unveil their target substrates and products in vivo. In the 3rd edition of this review series, we highlight the newest understanding of the in vivo functions of sPLA2s in pathophysiological conditions in the context of immunity and metabolism. We will also describe the latest knowledge on PLA2R1, the best known sPLA2 receptor, which may serve either as a clearance or signaling receptor for sPLA2 or may even act independently of sPLA2 function.

Novel mechanisms of abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are a common but asymptomatic disease that has high susceptibility to rupture. Current therapeutic options are limited to surgical procedures because no pharmacological approaches have been proven to decrease either expansion or rupture of human AAAs. The current dearth of effective medical treatment is attributed to insufficient understanding of the mechanisms underlying the initiation, propagation and rupture of AAAs. This review will emphasize recent advances in mechanistic studies that may provide insights into potential pharmacological treatments for this disease. While we primarily focus on recent salient findings, we also discuss mechanisms that continue to be controversial depending on models under study. Despite the progress on exploring mechanisms of experimental AAAs, ultimate validation of mechanisms will require completion of prospective double-blinded clinical trials. In addition, we advocate increased emphasis of collaborative studies using animal models and human tissues for determination of mechanisms that explore expansion and rupture of existing AAAs.

Group X Secreted Phospholipase A2 Limits the Development of Atherosclerosis in LDL Receptor–Null Mice

Objective—

Several secreted phospholipases A2 (sPLA2s), including group IIA, III, V, and X, have been linked to the development of atherosclerosis, which led to the clinical testing of A-002 (varespladib), a broad sPLA2 inhibitor for the treatment of coronary artery disease. Group X sPLA2 (PLA2G10) has the most potent hydrolyzing activity toward phosphatidylcholine and is believed to play a proatherogenic role.

Methods and Results—

Here, we show that Ldlr –/– mice reconstituted with bone marrow from mouse group X–deficient mice ( Pla2g10 –/– ) unexpectedly display a doubling of plaque size compared with Pla2g10 +/+ chimeric mice. Macrophages of Pla2g10 –/– mice are more susceptible to apoptosis in vitro, which is associated with a 4-fold increase of plaque necrotic core in vivo. In addition, chimeric Pla2g10 –/– mice show exaggerated T lymphocyte (Th)1 immune response, associated with enhanced T-cell infiltration in atherosclerotic plaques. Interestingly, overexpression of human PLA2G10 in murine bone marrow cells leads to significant reduction of Th1 response and to 50% reduction of lesion size.

Conclusion—

PLA2G10 expression in bone marrow cells controls a proatherogenic Th1 response and limits the development of atherosclerosis. The results may provide an explanation for the recently reported inefficacy of A-002 (varespladib) to treat patients with coronary artery disease. Indeed, A-002 is a nonselective sPLA2 inhibitor that inhibits both proatherogenic (groups IIA and V) and antiatherogenic (group X) sPLA2s. Our results suggest that selective targeting of individual sPLA2 enzymes may be a better strategy to treat cardiovascular diseases.

Interleukin 6 destabilizes atherosclerotic plaques by downregulating prolyl-4-hydroxylase α1 via a mitogen-activated protein kinase and c-Jun pathway

Interleukin 6 (IL-6) is a pivotal cytokine that regulates extracellular matrix metabolism by ameliorating the modification of collagen content, important in fibrous caps of atherosclerotic plaque. Prolyl-4-hydroxylase α1 (P4Hα1) is a key intracellular enzyme required for synthesis of collagen in animals. We investigated the relationship of IL-6 and P4Hα1 in atherosclerosis-prone mice and human aortic smooth muscle cells (HASMCs). Apolipoprotein E (ApoE)-/- mice were fed a high-fat diet and a perivascular constrictive silica collar was placed on the right common carotid artery to induce atherosclerotic lesions, then mice were divided into two groups for transfection with empty lentivirus or IL-6 lentivirus. HASMCs were transfected with small interfering RNA or treated with recombinant human IL-6. IL-6 significantly downregulated collagen, P4Hα1 and smooth muscle cell contents in atherosclerotic mouse arteries. Macrophage and lipid contents in the atherosclerotic area were significantly increased with IL-6 treatment. IL-6 significantly downregulated P4Hα1 expression in HASMCs through an RAF-MEK1/2-ERK1/2 mitogen-activated protein kinase (MAPK) pathway, and c-Jun was involved in the process. Our findings highlight IL-6 destabilize atherosclerotic plaques in mice by downregulating P4Hα1 via an RAF-MEK1/2-ERK1/2 MAPK and c-Jun pathway.

The calcium chloride-induced rodent model of abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) affects ∼5% men aged over 65 years and is an important cause of death in this population. Research into AAA pathogenesis has been fuelled by the need to identify new diagnostic biomarkers and therapeutic targets for this disease. One animal model of AAA involves peri-vascular application of calcium chloride (CaCl(2)) onto the infra-renal aorta of mice and rats to induce extracellular matrix remodelling. Twenty-three studies assessing CaCl(2)-induced AAA and six studies assessing AAA induced by a modified CaCl(2) method were identified. In the current report the preparation and pathological features of this AAA model are discussed. We also compared this animal model to human AAA. CaCl(2)-induced AAA shows the following pathological characteristics typically found in human AAA: calcification, inflammatory cell infiltration, oxidative stress, neovascularisation, elastin degradation and vascular smooth muscle cell apoptosis. A number of mechanisms involved in CaCl(2)-induced AAA have been identified which may be relevant to the pathogenesis of human AAA. Key molecules include c-Jun N-terminal kinase, peroxisome proliferator-activated receptor-γ, chemokine (C-C motif) receptor 2, group x secretory phospholipase A2 and plasminogen. CaCl(2)-induced AAA does not display aortic thrombus, atherosclerosis and rupture which are classical features of human AAA. Advantages of the CaCl(2)-induced AAA technique include (1) it can be applied to wild type mice making assessment of transgenic rodent models more straight forward and rapid; and (2) CaCl(2)-induced AAAs are usually developed in the infra-renal abdominal aorta, which is the most common location of human AAA. Currently findings obtained from the CaCl(2)-induced AAA model or other animal models of AAA have not been translated into the human situation. It is hoped that this deficiency will be corrected over the next decade with a number of clinical trials currently examining novel treatment options for AAA patients.

Involvement of the renin-angiotensin system in abdominal and thoracic aortic aneurysms

Aortic aneurysms are relatively common maladies that may lead to the devastating consequence of aortic rupture. AAAs (abdominal aortic aneurysms) and TAAs (thoracic aortic aneurysms) are two common forms of aneurysmal diseases in humans that appear to have distinct pathologies and mechanisms. Despite this divergence, there are numerous and consistent demonstrations that overactivation of the RAS (renin-angiotensin system) promotes both AAAs and TAAs in animal models. For example, in mice, both AAAs and TAAs are formed during infusion of AngII (angiotensin II), the major bioactive peptide in the RAS. There are many proposed mechanisms by which the RAS initiates and perpetuates aortic aneurysms, including effects of AngII on a diverse array of cell types and mediators. These experimental findings are complemented in humans by genetic association studies and retrospective analyses of clinical data that generally support a role of the RAS in both AAAs and TAAs. Given the lack of a validated pharmacological therapy for any form of aortic aneurysm, there is a pressing need to determine whether the consistent findings on the role of the RAS in animal models are translatable to humans afflicted with these diseases. The present review compiles the recent literature that has shown the RAS as a critical component in the pathogenesis of aortic aneurysms.

Rosiglitazone negatively regulates c-Jun N-terminal kinase and toll-like receptor 4 proinflammatory signalling during initiation of experimental aortic aneurysms

OBJECTIVE

Development and rupture of aortic aneurysms (AA) is a complex process involving inflammation, cell death, tissue and matrix remodelling. The thiazolidinediones (TZDs) including Rosiglitazone (RGZ) are a family of drugs which act as agonists of the nuclear peroxisome proliferator-activated receptors and have a broad spectrum of effects on a number of biological processes in the cardiovascular system. In our previous study we have demonstrated that RGZ has a marked effect on both aneurysm rupture and development, however, the precise mechanism of this is unknown.

METHODS AND RESULTS

In the present study, we examined possible targets of RGZ action in the early stages of Angiotensin II-induced AA in apolipoprotein E-deficient mice. For this purpose we employed immunoblotting, ELISA and antibody array approaches. We found that RGZ significantly inhibited c-Jun N-terminal kinase (JNK) phosphorylation and down-regulated toll-like receptor 4 (TLR4) expression at the site of lesion formation in response to Angiotensin II infusion in the initiation stage (6-72 h) of experimental AA development. Importantly, this effect was also associated with a decrease of CD4 antigen and reduction in production of TLR4/JNK-dependant proinflammatory chemokines MCP-1 and MIP-1α.

CONCLUSION

These data suggest that RGZ can modulate inflammatory processes by blocking TLR4/JNK signalling in initiation stages of AA development.

Group V secretory phospholipase A2 enhances the progression of angiotensin II-induced abdominal aortic aneurysms but confers protection against angiotensin II-induced cardiac fibrosis in apoE-deficient mice

Abdominal aortic aneurysms (AAAs) and heart failure are complex life-threatening diseases whose etiology is not completely understood. In this study, we investigated whether deficiency of group V secretory phospholipase A(2) (GV sPLA(2)) protects from experimental AAA. The impact of GV sPLA(2) deficiency on angiotensin (Ang) II-induced cardiac fibrosis was also investigated. Apolipoprotein E (apoE)(-/-) mice and apoE(-/-) mice lacking GV sPLA(2) (GV DKO) were infused with 1000 ng/kg per minute Ang II for up to 28 days. Increases in systolic blood pressure, plasma aldosterone level, and urinary and heart prostanoids were similar in apoE(-/-) and GV DKO mice after Ang II infusion. The incidence of aortic rupture in Ang II-infused GV DKO mice (10%) was significantly reduced compared with apoE(-/-) mice (29.4%). Although the incidence of AAA in GV DKO mice (81.3%) and apoE(-/-) mice (100%) was similar, the mean percentage increase in maximal luminal diameter of abdominal aortas was significantly smaller in GV DKO mice (68.5% ± 7.7%) compared with apoE(-/-) mice (92.6% ± 8.3%). Deficiency of GV sPLA(2) resulted in increased Ang II-induced cardiac fibrosis that was most pronounced in perivascular regions. Perivascular collagen, visualized by picrosirius red staining, was associated with increased TUNEL staining and increased immunopositivity for macrophages and myofibroblasts and nicotinamide adenine dinucleotide phosphate oxidase (NOX)-2 and NOX-4, respectively. Our findings indicate that GV sPLA(2) modulates pathological responses to Ang II, with different outcomes for AAA and cardiac fibrosis.

Group X secretory PLA2 in neutrophils plays a pathogenic role in abdominal aortic aneurysms in mice

Group X secretory PLA(2) (sPLA(2)-X) is expressed in neutrophils and plays a role in the pathogenesis of neutrophil-mediated tissue inflammation and injury. This study tested the hypothesis that sPLA(2)-X in neutrophils may contribute to the pathogenesis of abdominal aortic aneurysms (AAA) using sPLA(2)-X(-/-) mice. AAA was created by application of CaCl(2) to external surface of aorta. As a result, the aortas of sPLA(2)-X(-/-) mice had smaller diameters (percent increase from baseline; 24.8 ± 3.5% vs. 49.9 ± 9.1%, respectively; P < 0.01), a reduced grade of elastin degradation, and lower activities of elastase and gelatinase (26% and 19% lower, respectively) after CaCl(2) treatment compared with sPLA(2)-X(+/+) mice. In sPLA(2)-X(+/+) mice, immunofluorescence microscopic images showed that the immunoreactivity of sPLA(2)-X was detected only in neutrophils within aortic walls 3 days, 1, 2, and 6 wk after CaCl(2) treatment, whereas the immunoreactivity was not detected in macrophages or mast cells in aortic walls. sPLA(2)-X immunoreactivity also was colocalized in cells expressing matrix metalloproteinase (MMP)-9. Neutrophils isolated from sPLA(2)-X(-/-) mice had lower activities of elastase, gelatinase, and MMP-9 in response to stimuli compared with sPLA(2)-X(+/+) mice. The attenuated release of elastase and gelatinase from sPLA(2)-X(-/-) neutrophils was reversed by exogenous addition of mouse sPLA(2)-X protein. The adoptive transfer of sPLA(2)-X(+/+) neutrophils days 0 and 3 after CaCl(2) treatment reversed aortic diameters and elastin degradation grades in the lethally irradiated sPLA(2)-X(+/+) mice reconstituted with sPLA(2)-X(-/-) bone marrow to an extent similar to that seen in sPLA(2)-X(+/+) mice. In conclusion, sPLA(2)-X in neutrophils plays a pathogenic role in AAA in a mice model.

Chinese red yeast rice attenuates the development of angiotensin II-induced abdominal aortic aneurysm and atherosclerosis

OBJECTIVE

Abdominal aortic aneurysm (AAA) is a chronic vascular disease characterized by medial degradation and inflammation. No medical approaches have been validated for treating AAA, and therapeutic options are limited to regular surveillance leading to surgical intervention. This study aimed to investigate whether administration of Chinese red yeast rice (Monascus purpureus; RYR) suppressed angiotensin II (AngII)-induced AAA and atherosclerosis.

METHODS AND RESULTS

Apolipoprotein E-deficient male mice fed a normal diet were administered either RYR extract (200 mg/kg/day) or vehicle by gavage for 1 week before initiating AngII infusion (1000 ng/kg/min) via subcutaneous osmotic pumps for 28 days. Red yeast rice extract administration significantly suppressed AngII-induced expansion of suprarenal diameter and area (P<.05). Furthermore, RYR extract significantly reduced atherosclerotic lesion areas in both the intima of aortic arches and cross sections of aortic roots (P<.05). These effects were associated with reductions of serum total cholesterol, intercellular adhesion molecule 1, vascular cell adhesion molecule 1, matrix metalloproteinase (MMP) 2 and increases of serum macrophage migration inhibitory factor, but no changes in serum interleukin (IL) 1α, IL-6, monocyte chemoattractant protein 1, MMP-9 and expression of MMP-2 and MMP-9 in aortic walls.

CONCLUSIONS

This study demonstrated that RYR extract administration suppressed AngII-induced AAA and atherosclerosis associated with regulating inflammation responses independent of lipid-lowering effects. Red yeast rice may have preventive potential for patients with AAA.

Secreted phospholipase A2 revisited

Phospholipase A(2) (PLA(2)) catalyses the hydrolysis of the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. So far, more than 30 enzymes that possess PLA(2) or related activity have been identified in mammals. About one third of these enzymes belong to the secreted PLA(2) (sPLA(2)) family, which comprises low molecular weight, Ca(2+) requiring, secreted enzymes with a His/Asp catalytic dyad. Individual sPLA(2)s display distinct localizations and enzymatic properties, suggesting their specialized biological roles. However, in contrast to intracellular PLA(2)s, whose roles in signal transduction and membrane homoeostasis have been well documented, the biological roles of sPLA(2)s in vivo have remained obscure until recently. Over the past decade, information fuelled by studies employing knockout and transgenic mice as well as specific inhibitors, in combination with lipidomics, has clarified when and where the different sPLA(2) isoforms are expressed, which isoforms are involved in what types of pathophysiology, and how they exhibit their specific functions. In this review, we highlight recent advances in PLA(2) research, focusing mainly on the physiological functions of sPLA(2)s and their modes of action on 'extracellular' phospholipid targets versus lipid mediator production.

Group X secretory phospholipase A2 enhances TLR4 signaling in macrophages

Secretory phospholipase A(2)s (sPLA(2)) hydrolyze glycerophospholipids to liberate lysophospholipids and free fatty acids. Although group X (GX) sPLA(2) is recognized as the most potent mammalian sPLA(2) in vitro, its precise physiological function(s) remains unclear. We recently reported that GX sPLA(2) suppresses activation of the liver X receptor in macrophages, resulting in reduced expression of liver X receptor-responsive genes including ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1), and a consequent decrease in cellular cholesterol efflux and increase in cellular cholesterol content (Shridas et al. 2010. Arterioscler. Thromb. Vasc. Biol. 30: 2014-2021). In this study, we provide evidence that GX sPLA(2) modulates macrophage inflammatory responses by altering cellular cholesterol homeostasis. Transgenic expression or exogenous addition of GX sPLA(2) resulted in a significantly higher induction of TNF-α, IL-6, and cyclooxygenase-2 in J774 macrophage-like cells in response to LPS. This effect required GX sPLA(2) catalytic activity, and was abolished in macrophages that lack either TLR4 or MyD88. The hypersensitivity to LPS in cells overexpressing GX sPLA(2) was reversed when cellular free cholesterol was normalized using cyclodextrin. Consistent with results from gain-of-function studies, peritoneal macrophages from GX sPLA(2)-deficient mice exhibited a significantly dampened response to LPS. Plasma concentrations of inflammatory cytokines were significantly lower in GX sPLA(2)-deficient mice compared with wild-type mice after LPS administration. Thus, GX sPLA(2) amplifies signaling through TLR4 by a mechanism that is dependent on its catalytic activity. Our data indicate this effect is mediated through alterations in plasma membrane free cholesterol and lipid raft content.

Physiological roles of group X-secreted phospholipase A2 in reproduction, gastrointestinal phospholipid digestion, and neuronal function

Although the secreted phospholipase A(2) (sPLA(2)) family has been generally thought to participate in pathologic events such as inflammation and atherosclerosis, relatively high and constitutive expression of group X sPLA(2) (sPLA(2)-X) in restricted sites such as reproductive organs, the gastrointestinal tract, and peripheral neurons raises a question as to the roles played by this enzyme in the physiology of reproduction, digestion, and the nervous system. Herein we used mice with gene disruption or transgenic overexpression of sPLA(2)-X to clarify the homeostatic functions of this enzyme at these locations. Our results suggest that sPLA(2)-X regulates 1) the fertility of spermatozoa, not oocytes, beyond the step of flagellar motility, 2) gastrointestinal phospholipid digestion, perturbation of which is eventually linked to delayed onset of a lean phenotype with reduced adiposity, decreased plasma leptin, and improved muscle insulin tolerance, and 3) neuritogenesis of dorsal root ganglia and the duration of peripheral pain nociception. Thus, besides its inflammatory action proposed previously, sPLA(2)-X participates in physiologic processes including male fertility, gastrointestinal phospholipid digestion linked to adiposity, and neuronal outgrowth and sensing.

Secreted phospholipase A2, lipoprotein hydrolysis, and atherosclerosis: integration with lipidomics

Phospholipase A₂ (PLA₂) is a group of enzymes that hydrolyze the sn-2 position of glycerophospholipids to yield fatty acids and lysophospholipids. Of many PLA₂s or related enzymes identified to date, secreted PLA₂s (sPLA₂s) comprise the largest family that contains 10 catalytically active isozymes. Besides arachidonic acid released from cellular membranes for eicosanoid synthesis, several if not all sPLA₂s have recently been implicated in hydrolysis of phospholipids in lipoprotein particles. The sPLA₂-processed low-density lipoprotein (LDL) particles contain a large amount of lysophospholipids and exhibit the property of “small-dense” or “modified” LDL, which facilitates foam cell formation from macrophages. Transgenic overexpression of these sPLA₂s leads to development of atherosclerosis in mice. More importantly, genetic deletion or pharmacological inhibition of particular sPLA₂s significantly attenuates atherosclerosis and aneurysm. In this article, we will give an overview of current understanding of the role of sPLA₂s in atherosclerosis, with recent lipidomics data showing the action of a subset of sPLA₂s on lipoprotein phospholipids.

Integrated lipidomics in the secreted phospholipase A(2) biology

Mammalian genomes encode genes for more than 30 phospholipase A(2)s (PLA(2)s) or related enzymes, which are subdivided into several subgroups based on their structures, catalytic mechanisms, localizations and evolutionary relationships. More than one third of the PLA(2) enzymes belong to the secreted PLA(2) (sPLA(2)) family, which consists of low-molecular-weight, Ca(2+)-requiring extracellular enzymes, with a His-Asp catalytic dyad. Individual sPLA(2) isoforms exhibit unique tissue and cellular localizations and enzymatic properties, suggesting their distinct pathophysiological roles. Recent studies using transgenic and knockout mice for several sPLA(2) isoforms, in combination with lipidomics approaches, have revealed their distinct contributions to various biological events. Herein, we will describe several examples of sPLA(2)-mediated phospholipid metabolism in vivo, as revealed by integrated analysis of sPLA(2) transgenic/knockout mice and lipid mass spectrometry. Knowledge obtained from this approach greatly contributes to expanding our understanding of the sPLA(2) biology and pathophysiology.

Hair follicular expression and function of group X secreted phospholipase A2 in mouse skin

Although perturbed lipid metabolism can often lead to skin abnormality, the role of phospholipase A(2) (PLA(2)) in skin homeostasis is poorly understood. In the present study we found that group X-secreted PLA(2) (sPLA(2)-X) was expressed in the outermost epithelium of hair follicles in synchrony with the anagen phase of hair cycling. Transgenic mice overexpressing sPLA(2)-X (PLA2G10-Tg) displayed alopecia, which was accompanied by hair follicle distortion with reduced expression of genes related to hair development, during a postnatal hair cycle. Additionally, the epidermis and sebaceous glands of PLA2G10-Tg skin were hyperplasic. Proteolytic activation of sPLA(2)-X in PLA2G10-Tg skin was accompanied by preferential hydrolysis of phosphatidylethanolamine species with polyunsaturated fatty acids as well as elevated production of some if not all eicosanoids. Importantly, the skin of Pla2g10-deficient mice had abnormal hair follicles with noticeable reduction in a subset of hair genes, a hypoplasic outer root sheath, a reduced number of melanin granules, and unexpected up-regulation of prostanoid synthesis. Collectively, our study highlights the spatiotemporal expression of sPLA(2)-X in hair follicles, the presence of skin-specific machinery leading to sPLA(2)-X activation, a functional link of sPLA(2)-X with hair follicle homeostasis, and compartmentalization of the prostanoid pathway in hair follicles and epidermis.