Cell-cell interactions in lipoxin generation and characterization of lipoxin A4 receptors

Promising Anti-Inflammatory Tools: Biomedical Efficacy of Lipoxins and Their Synthetic Pathways

Lipoxins (LXs) have attracted widespread attention as a class of anti-inflammatory lipid mediators that are produced endogenously by the organism. LXs are arachidonic acid (ARA) derivatives that include four different structures: lipoxin A4 (LXA4), lipoxin B4 (LXB4), and the aspirin-induced differential isomers 15-epi-LXA4 and 15-epi-LXB4. Because of their unique biological activity of reducing inflammation in the body, LXs have great potential for neuroprotection, anti-inflammatory treatment of COVID-19, and other related diseases. The synthesis of LXs in vivo is achieved through the action of lipoxygenase (LO). As a kind of important enzyme, LO plays a major role in the physiological processes of living organisms in mammals and functions in some bacteria and fungi. This suggests new options for the synthesis of LXs in vitro. Meanwhile, there are other chemical and biochemical methods to synthesize LXs. In this review, the recent progress on physiological activity and synthetic pathways of LXs is summarized, and new insights into the synthesis of LXs in vitro are provided.

Lipoxins in the Nervous System: Brighter Prospects for Neuroprotection

Lipoxins (LXs) are generated from arachidonic acid and are involved in the resolution of inflammation and confer protection in a variety of pathological processes. In the nervous system, LXs exert an array of protective effects against neurological diseases, including ischemic or hemorrhagic stroke, neonatal hypoxia-ischemia encephalopathy, brain and spinal cord injury, Alzheimer's disease, multiple sclerosis, and neuropathic pain. Lipoxin administration is a potential therapeutic strategy in neurological diseases due to its notable efficiency and unique superiority regarding safety. Here, we provide an overview of LXs in terms of their synthesis, signaling pathways and neuroprotective evidence. Overall, we believe that, along with advances in lipoxin-related drug design, LXs will bring brighter prospects for neuroprotection.

Lipoxin A4 activates ALX/FPR2 to attenuate inflammation in Aspergillus fumigatus keratitis

PURPOSE

To explore the anti-inflammatory effect of lipoxin A4 (LXA4) in Aspergillus fumigatus (A. fumigatus) keratitis and the underlying mechanisms.

METHODS

In A. fumigatus keratitis mouse models, enzyme-linked immunosorbent assay (ELISA) was used to detect the level of LXA4. Clinical scores were utilized to evaluate fungal keratitis (FK) severity. Fungal load was assessed by plate count. Immunofluorescence staining, HE staining and myeloperoxidase (MPO) assays were carried out to evaluate the neutrophil infiltration and activity. In A. fumigatus infected mouse corneas and inactivated A. fumigatus-stimulated RAW264.7 cells, quantitative real time polymerase chain reaction (qRT-PCR) and ELISA were applied to assess the expression of pro-inflammatory mediators and anti-inflammatory factors.Reactive oxygen species (ROS) was determined by 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) staining in RAW264.7 cells.

RESULTS

LXA4 level was significantly increased in mice with A. fumigatus keratitis. In an A. fumigatus keratitis mouse model, LXA4 treatment alleviated FK severity, reduced fungal load and repressed neutrophil infiltration and activity. Additionally, LXA4 inhibited the expression of pro-inflammatory mediators including IL-1β, TNF-α, IL-6, cyclooxygenase-2 (COX-2), TLR-2, TLR-4, Dectin-1 and iNOS, and promoted the expression of anti-inflammatory factors IL-10 and Arg-1. In RAW264.7 cells, LXA4 receptor/formyl peptide receptor 2 (ALX/FPR2) blockade reversed the anti-inflammatory effect of LXA4. LXA4 suppressed inactivated A. fumigatus induced elevated ROS production in RAW264.7 cells, which was abrogated by ALX/FPR2 antagonist Boc-2.

CONCLUSION

LXA4 ameliorated inflammatory response by suppressing neutrophil infiltration, downregulating the expression of pro-inflammatory mediators and ROS production through ALX/FPR2 receptor in A. fumigatus keratitis.

Resolution of inflammation in asthma

The resolution of inflammation in healthy airways is an active process, with specialized mediators and cellular mechanisms enlisted to restore tissue homeostasis. This article focuses on recent discoveries of natural mediators derived from essential fatty acids, including ω-3 fatty acids, with anti-inflammatory and pro-resolving. These pro-resolving mediators serve as agonists at specific receptors. Asthma is an incurable disease of chronic, nonresolving inflammation of the airways. While the biosynthesis of pro-resolving mediators occurs during asthma, defects in their production are associated with disease severity, suggesting that the pathobiology of asthma may result in part from impaired resolution of airway inflammation.

Paradigm shift in the pharmacological management of periodontal diseases

It is becoming clear that variations in inflammatory response are a major determinant in susceptibility to periodontitis. However, our understanding of the relationship of the causal agents in periodontitis to the pathogenesis is not as clear as we once thought, and thus therapies based on etiopathogenesis are similarly in question. We are entering a new era of therapeutic discovery that may have a major impact on our management of the periodontal diseases. Fundamentally, periodontitis is an irreversible condition and once both soft and hard tissues are lost, the healthy periodontal architecture cannot be completely or predictably rebuilt. The discovery of new families of lipid mediators of resolution of inflammation (the lipoxins) and eicosapentaenoic-acid- and docosahexaenoic-acid-derived chemical mediators (the resolvins and protectins) opens new avenues to designing resolution-targeted therapies to control the unwanted side effects of excessive inflammation. The novel protective and therapeutic actions of pro-resolution lipid mediators following microbial challenge are mediated by regulation of the local and systemic inflammatory response that has a direct impact on the organization of the biofilm (plaque) and suggests a new paradigm in clinical periodontal therapeutics.

Resolution of inflammation in periodontitis

Chronic inflammatory illnesses such as diabetes, arthritis, and heart disease are now seen as problems that might have impacts on the periodontium, and reciprocal effects of periodontal diseases are being considered as factors potentially affecting the progression of these diseases. Successful management of the inflammatory disorders in the human body depends on the identification of common pathways that would lead to a better understanding of the disease processes and development of novel treatment strategies. In this review, our objective is to identify the inflammatory basis of periodontal disease and common inflammatory mechanisms underlying several disorders elsewhere in the body, with an emphasis on how the potential extrinsic and intrinsic control methods could be used to prevent or treat the harmful effects linked to inflammation.

Lipoxin signaling in neutrophils and their role in periodontal disease

Endogenous molecules involved in counterregulation of inflammatory responses provide an opportunity to explore new therapeutic approaches based on manipulation of new pathways that may reduce the possibility of unwanted toxic side effects. Lipoxins (LX) are trihydroxytetraene-containing eicosanoids that are generated within the vascular lumen during cell-cell interactions or at mucosa through leukocyte-epithelial cell interactions. Transcellular biosynthetic pathways are the major lipoxin biosynthetic routes where LX are formed in vivo during inflammation and serve as "stop signals" that regulate key steps in leukocyte trafficking. In this review, recent findings in lipoxin generation, impact on the resolution of acute inflammation, and organ protection from neutrophil-mediated injury are presented. Periodontitis, specifically localized aggressive periodontitis, which is recognized as an example of neutrophil-mediated tissue injury, is discussed as a disease model where LX and other endogenous pro-resolution pathway mediators could have potential value.

Differential inside-out activation of beta2-integrins by leukotriene B4 and fMLP in human neutrophils

We have investigated how LTB4, an endogenous chemoattractant encountered early in the inflammatory process, and fMLP, a bacteria-derived chemotactic peptide emanating from the site of infection, mediate inside-out regulation of the beta2-integrin. The role of the two chemoattractants on beta2-integrin avidity was investigated by measuring their effect on beta2-integrin clustering and surface mobility, whereas their effect on beta2-integrin affinity was measured by the expression of a high affinity epitope, a ligand-binding domain on beta2-integrins, and by integrin binding to s-ICAM. We find that the two chemoattractants modulate the beta2-integrin differently. LTB4 induces an increase in integrin clustering and surface mobility, but only a modest increase in integrin affinity. fMLP evokes a large increase in beta2-integrin affinity as well as in clustering and mobility. Lipoxin, which acts as a stop signal for the functions mediated by pro-inflammatory agents, was used as a tool for further examining the inside-out mechanisms. While LTB4-induced integrin clustering and mobility were inhibited by lipoxin, only a minor inhibition of fMLP-induced beta2-integrin avidity and no inhibition of integrin affinity were detected. The different modes of the inside-out regulation of beta2-integrins suggest that distinct mechanisms are involved in the beta2-integrin modulation induced by various chemoattractants.

International Union of Pharmacology XXXVII. Nomenclature for leukotriene and lipoxin receptors

The leukotrienes and lipoxins are biologically active metabolites derived from arachidonic acid. Their diverse and potent actions are associated with specific receptors. Recent molecular techniques have established the nucleotide and amino acid sequences and confirmed the evidence that suggested the existence of different G-protein-coupled receptors for these lipid mediators. The nomenclature for these receptors has now been established for the leukotrienes. BLT receptors are activated by leukotriene B(4) and related hydroxyacids and this class of receptors can be subdivided into BLT(1) and BLT(2). The cysteinyl-leukotrienes (LT) activate another group called CysLT receptors, which are referred to as CysLT(1) and CysLT(2). A provisional nomenclature for the lipoxin receptor has also been proposed. LXA(4) and LXB(4) activate the ALX receptor and LXB(4) may also activate another putative receptor. However this latter receptor has not been cloned. The aim of this review is to provide the molecular evidence as well as the properties and significance of the leukotriene and lipoxin receptors, which has lead to the present nomenclature.