Secreted Phospholipase A2 Group X Acts as an Adjuvant for Type 2 Inflammation, Leading to an Allergen-Specific Immune Response in the Lung

Lipid droplets control mitogenic lipid mediator production in human cancer cells

OBJECTIVES

Polyunsaturated fatty acids (PUFAs) are structural components of membrane phospholipids and precursors of oxygenated lipid mediators with diverse functions, including the control of cell growth, inflammation and tumourigenesis. However, the molecular pathways that control the availability of PUFAs for lipid mediator production are not well understood. Here, we investigated the crosstalk of three pathways in the provision of PUFAs for lipid mediator production: (i) secreted group X phospholipase A2 (GX sPLA2) and (ii) cytosolic group IVA PLA2 (cPLA2α), both mobilizing PUFAs from membrane phospholipids, and (iii) adipose triglyceride lipase (ATGL), which mediates the degradation of triacylglycerols (TAGs) stored in cytosolic lipid droplets (LDs).

METHODS

We combined lipidomic and functional analyses in cancer cell line models to dissect the trafficking of PUFAs between membrane phospholipids and LDs and determine the role of these pathways in lipid mediator production, cancer cell proliferation and tumour growth in vivo.

RESULTS

We demonstrate that lipid mediator production strongly depends on TAG turnover. GX sPLA2 directs ω-3 and ω-6 PUFAs from membrane phospholipids into TAG stores, whereas ATGL is required for their entry into lipid mediator biosynthetic pathways. ATGL controls the release of PUFAs from LD stores and their conversion into cyclooxygenase- and lipoxygenase-derived lipid mediators under conditions of nutrient sufficiency and during serum starvation. In starving cells, ATGL also promotes the incorporation of LD-derived PUFAs into phospholipids, representing substrates for cPLA2α. Furthermore, we demonstrate that the built-up of TAG stores by acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is required for the production of mitogenic lipid signals that promote cancer cell proliferation and tumour growth.

CONCLUSION

This study shifts the paradigm of PLA2-driven lipid mediator signalling and identifies LDs as central lipid mediator production hubs. Targeting DGAT1-mediated LD biogenesis is a promising strategy to restrict lipid mediator production and tumour growth.

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.

Recent insights into the mechanisms of anaphylaxis

Anaphylaxis is an acute life-threatening systemic allergic reaction that can have a wide range of clinical manifestations. The most common triggers for anaphylaxis include food, medication, and venom. What is curious regarding anaphylaxis is how so many different agents can induce a severe systemic clinical response but only in a select subgroup of patients. Over the past decade, several important advances have been made in understanding the underlying cellular and molecular mechanisms contributing to anaphylaxis, with mast cells (MCs) being an essential component. Classically, cross-linked immunoglobulin E (IgE) bound to its high- affinity receptor induces MC mediator release. However, toll-like, complement, or Mas-related G-protein-coupled receptors also activate mouse and human MCs. While anaphylaxis secondary to foods historically has been more extensively characterized clinically and mechanistically, more recent studies have shifted focus toward understanding drug-induced anaphylaxis. The focus of this review is to highlight recent basic science developments and compare what is currently known regarding anaphylaxis to food, medications, and venom.

MicroRNA-21-5p promotes mucosal type 2 inflammation via regulating GLP1R/IL-33 signaling in chronic rhinosinusitis with nasal polyps

BACKGROUND

It has been known that chronic rhinosinusitis with nasal polyps (CRSwNP) is a type 2 inflammation-dominated disease; however, the reasons causing such type of mucosal inflammation in CRSwNP are not well elucidated.

OBJECTIVE

We sought to investigate the role of microRNA-21-5p (miR-21-5p) in regulating mucosal type 2 inflammation in CRSwNP.

METHODS

miR-21-5p expression was detected in nasal mucosa of patients with CRSwNP. Correlations between miR-21-5p and indicators of type 2 inflammation were further analyzed. miR-21 knockout mice were used to explore the role of miR-21-5p in a murine model of eosinophilic (E) CRSwNP. Target gene of miR-21-5p related to type 2 inflammation in CRSwNP was identified.

RESULTS

The upregulated miR-21-5p in the nasal mucosa of patients with CRSwNP, compared with control subjects, was expressed higher in patients with ECRSwNP than in patients with nonECRSwNP. miR-21-5p expression was positively correlated with mucosal eosinophil infiltrations and the expression of type 2 inflammatory cytokines. In the CRSwNP mice, miR-21 knockout significantly attenuated type 2 inflammation, as indicated by eosinophil infiltrations and expression of cytokines/chemokines in nasal mucosa and lavage fluid; moreover, genes associated with type 2 inflammation were extensively downregulated at the transcriptome level in miR-21 knockout mice. Glucagon-like peptide-1 receptor, which was negatively correlated with miR-21-5p expression in human nasal mucosa, was identified as the target of miR-21-5p. Overexpression of miR-21-5p induced IL-33 expression, whereas glucagon-like peptide-1 receptor agonist decreased IL-33 production in airway epithelial cells.

CONCLUSIONS

miR-21-5p aggravates type 2 inflammation in the nasal mucosa of patients with CRSwNP via targeting glucagon-like peptide-1 receptor/IL-33 signaling, which may be a potential therapeutic target for CRSwNP.

Emerging Role of Phospholipase-Derived Cleavage Products in Regulating Eosinophil Activity: Focus on Lysophospholipids, Polyunsaturated Fatty Acids and Eicosanoids

Eosinophils are important effector cells involved in allergic inflammation. When stimulated, eosinophils release a variety of mediators initiating, propagating, and maintaining local inflammation. Both, the activity and concentration of secreted and cytosolic phospholipases (PLAs) are increased in allergic inflammation, promoting the cleavage of phospholipids and thus the production of reactive lipid mediators. Eosinophils express high levels of secreted phospholipase A2 compared to other leukocytes, indicating their direct involvement in the production of lipid mediators during allergic inflammation. On the other side, eosinophils have also been recognized as crucial mediators with regulatory and homeostatic roles in local immunity and repair. Thus, targeting the complex network of lipid mediators offer a unique opportunity to target the over-activation and 'pro-inflammatory' phenotype of eosinophils without compromising the survival and functions of tissue-resident and homeostatic eosinophils. Here we provide a comprehensive overview of the critical role of phospholipase-derived lipid mediators in modulating eosinophil activity in health and disease. We focus on lysophospholipids, polyunsaturated fatty acids, and eicosanoids with exciting new perspectives for future drug development.

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.