Lipid Mediators of Inhalation Exposure-Induced Pulmonary Toxicity and Inflammation

Abstract

Many inhalation exposures induce pulmonary inflammation contributing to disease progression. Inflammatory processes are actively regulated via mediators including bioactive lipids. Bioactive lipids are potent signaling molecules involved in both pro-inflammatory and resolution processes through receptor interactions. The formation and clearance of lipid signaling mediators are controlled by multiple metabolic enzymes. An imbalance of these lipids can result in exacerbated and sustained inflammatory processes which may result in pulmonary damage and disease. Dysregulation of pulmonary bioactive lipids contribute to inflammation and pulmonary toxicity following exposures. For example, inhalation of cigarette smoke induces activation of pro-inflammatory bioactive lipids such as sphingolipids, and ceramides contributing to chronic obstructive pulmonary disease. Additionally, exposure to silver nanoparticles causes dysregulation of inflammatory resolution lipids. As inflammation is a common consequence resulting from inhaled exposures and a component of numerous diseases it represents a broadly applicable target for therapeutic intervention. With new appreciation for bioactive lipids, technological advances to reliably identify and quantify lipids have occurred. In this review, we will summarize, integrate, and discuss findings from recent studies investigating the impact of inhaled exposures on pro-inflammatory and resolution lipids within the lung and their contribution to disease. Throughout the review current knowledge gaps in our understanding of bioactive lipids and their contribution to pulmonary effects of inhaled exposures will be presented. New methods being employed to detect and quantify disruption of pulmonary lipid levels following inhalation exposures will be highlighted. Lastly, we will describe how lipid dysregulation could potentially be addressed by therapeutic strategies to address inflammation.

1. Introduction

Inhaled exposures can contribute to the development and progression of a variety of prevalent cardiopulmonary and systemic diseases and health outcomes including asthma, heart disease, stroke, pregnancy difficulties, and others (Fauroux et al. 2000; Yhee et al. 2016). Numerous in vitro and in vivo investigations demonstrateinhaled exposures often result in acute and/or chronic inflammatory responses. Pulmonary inflammation is a primary mediator facilitating most acute and chronic lung injuriesand diseases (Jousilahti et al. 2002; Rubenfeld et al. 2005; Martínez-García et al. 2008; Girdhar et al. 2011; Solomon et al. 2013). Inappropriate resolution and/or exacerbated pulmonary inflammatory responses can result in chronic inflammation, tissue damage, and the development of diseases including fibrosis, and chronic lung disorders such as asthma, chronic obstructive pulmonary disease (COPD), and interstitial lung disease (ILD). Inhalation exposures can induce robust pro-inflammatory responses and may impair resolution processes resulting in pulmonary disease development. For example, cigarette smoke inhalation induces sustained pulmonary inflammation leading to the development of COPD by promoting pro-inflammatory signaling and decreasing resolution processes(Tashkin and Murray 2009; Snelgrove et al. 2010; Parikh et al. 2016). Although inhalation exposures consist of a variety of chemical classes, they often generate common inflammatory processes. Therefore, understanding fundamental inflammatory processes could be impactful in the generation of broadly applicable interventional therapeutic approaches to treat and prevent inflammatory-mediated diseases.

Inflammation is a complex biological mechanism initiated in response to harmful stimuli. An inflammatory response aims to eliminate potential instigators of cell injury and coordinate the healing process. The innate immune system identifies the instigator, recruits and activates immune cells, and inititates resolution of inflammation which are all critical components of the typical inflammatory response. Lung inflammation can occur in response to inhaled bacterial and viral pathogens as well as environmental pollutants (e.g. ozone, asbestos, silica, heavy metals, ambient particulate matter, etc.). Pulmonary epithelial cells and alveolar macrophages release proinflammatory mediators including cytokines, chemokines, and bioactive lipids in response to exposures.Thismediator release recruitsimmune cells including neutrophils, macrophages, and lymphocytes to the lung to phagocytize and eliminate foreign materials.When the number of inflammatory cells is at its highest during the inflammatory response, a variety of pro-resolving mediators (such as endogenous lipid-derived mediators) are produced (Sansbury and Spite 2016; Recchiuti et al. 2019).These mediators actively resolve inflammation through a complex process involving: (i) restricting and regulating neutrophilic infiltration; (ii) controlling chemokine and cytokine release; and (iii) promoting neutrophil death and removal via macrophage efferocytosis (Reville et al. 2006). The ideal result of inflammation response is resolution and the restoration of tissue equilibrium.(Ciaccia 2011).

Recently, the importance of bioactive lipids in governing inflammation has been identified and they are being actively studied for their role in toxicant-induced inflammatory responses. Improved understanding of the regulation of these lipids and their signaling has facilitated a deeper comprehension of disease pathophysiology and the discovery of novel therapeutic targets. The most prominent groups of bioactive lipids regulating inflammatory responses are eicosanoids, specialized pro-resolving mediators (SPMs), and lysophospholipids.Assessing changes in the production of these lipids and their linked signaling, induced by inhalation exposure, deepens our comprehension of disease mechanisms and can function as biomarkers for toxicity and disease.The ability to appreciate lipid-mediated mechanisms of inflammatory regulation is associated with recent progress in lipid profiling approaches. These technical advancements have led to the identification and quantification of lipid molecules through multianalyte approach associated with various globally significant diseases and disease severity.

The present review will describe and critically evaluate our current knowledge regarding the role of lipids in the pulmonary inflammatory response following inhalation exposures. This includes assessments of pro-inflammatory and resolution lipid mediators as well as alterations in lipid metabolism processes following exposures. Our assessment of the current literature will focus on the most studied inhalation exposures which include tobacco smoke, traffic related air pollution, and allergens. Advances in the techniques and methodologies for evaluation lipids have enabled our ability to perform these assessments. In this review we will highlight some of these novel approaches and their utilization for toxicology assessments. This new understanding of lipid-mediated mechanisms of toxicity can be applied to targeted interventions and therapeutic strategies to mitigate pulmonary inflammation. The potential utilization of lipid modulation as a treatment strategy will also be described within this review. Overall, this review will examine 1) exposure-induced lipid alterations and the impact on pulmonary inflammation, 2) novel methods that can be employed by the field of toxicology for the assessment of lipids, and 3) utilization of strategies to target lipid-dysregulation following exposures to address inflammation (Figure 1). Throughout the review the common lipid names are utilized, however, at first usage we have provided the systematic name as defined by LIPID MAPS(Fahy et al. 2005; Fahy et al. 2009).

Figure 1.

Representative schematic demonstrating inhalation exposure-induced lipid dysregulation and its contribution to pulmonary inflammation. Inhalation of numerous exposures can modify metabolism of lipid mediators of inflammation within the lung resulting in pulmonary inflammation facilitating toxicity and disease. Exposure-induced alterations in these lipid mediators are often assessed by mass spectrometry-based approaches. Targeting of this exposure-induced dysregulation of lipids can potentially provide therapeutic benefits (Figure created at biorender.com).

2. Lipid Metabolism and Inflammatory Regulation

Two processes regulating lipid availability include anabolism and catabolism. In general, anabolism involves creating new lipids from smaller molecules, whereas catabolism, involves the breakdown of lipids resulting in the production of energy and/or other lipid mediators. (Walther and Farese 2012) Previously, lipids such as fatty acids were primarily considered as metabolic energy reserves, however, are now recognized as crucial elements of cellular signal transduction pathways including the promotion and resolution of host inflammatory responses (Yedgar et al. 2007). Primary lipids within the lung functioning as precursors to bioactive lipids include phospholipids and free fatty acids. Phospholipids are components of biological membranes and pulmonary surfactants which function to preserve membrane fluidity and lower alveolar surface tension, respectively. In addition to these functions, phospholipids can act as a source of secondary messengers and active mediators. Mediators produced by phospholipid metabolism includearachidonic acid(AA) (5Z,8Z,11Z,14Z-eicosatetraenoic acid), lysophospholipids, endocannabinoids (N-(5Z,8Z,11Z,14Z-eicosatetraenoyl)-ethanolamine), eicosapentaenoic acid (EPA)(5Z,8Z,11Z,14Z,17Z-eicosapentaenoic acid)and docosahexaenoic acid (DHA) (4Z,7Z,10Z,13Z,16Z,19Z-docosahexaenoic acid). Fatty acids derived via phospholipid metabolism can be incorporated into cellular membranes and freed via phospholipase mediatedcleavage. These free fatty acids serve as substrates for enzymatic conversion to numerous bioactive lipid mediators involved in inflammatory regulation. Free fatty acid-derived bioactive lipids are classified as pro-inflammatory or resolution mediators. Specifically, metabolism of free fatty acids produces pro-inflammatory eicosanoids such as leukotrienes, prostaglandins, thromboxane, lipoxinsas well as resolution mediators including SPMs such asresolvin, protectin, and maresinsulfido-conjugates(Dalli et al. 2015).Distinct metabolic processes leading to production of pro-inflammatory and resolution mediators have been extensively reviewed (Serhan Charles N. et al. 2008; Serhan C. N. et al. 2014; Chiurchiù et al. 2018; Park et al. 2021; Zhang et al. 2021)elsewhere and are summarized in Figure 2.

Figure 2.

Overview of metabolic pathways of the primary endogenous bioactive lipids.Bioactive lipid mediators of inflammation are produced by PLA2 actions on membrane phospholipids, releasingArachidonic acid, DHA and EPA. These precursors then undergo a series of oxidation reactions facilitated by a variety enzymes resulting in the formation of pro-inflammatory (red) and resolution (green) mediators. Ceramide, a pro-inflammatory mediator, is produced via sphingomyelinase mediated metabolism of sphingomyelin or the de novo pathway via metabolism of serine palmitoyl CoA. CESR, Ceramide Synthase; COX, cyclooxygenase; CYP450, Cytochrome P450: DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; HETEs, hydroxyeicosatetraenoic acids; KDSR, 3-Ketodihydrosphingosine Reductase; LOX, lipoxygenase; LTs, leukotrienes; LX, lipoxin; MaR, maresin; PD, protectin; PDX, protectin DX; PGs, prostaglandins; PLA2, phospholipase A2; Rv, resolvin; SPT, serine palmitoyltransferase; TXs, thromboxanes (Figure created at biorender.com).

3. Effect of inhaled exposures on proinflammatory lipids

Although inhalation exposures are distinct due to variations in exposure scenarios and toxicant properties (i.e. chemical composition, particulate size, concentration, exposure duration, individual susceptibility, etc), activation of pro-inflammatory signaling is a commoneffectofmost exposures. Additionally, robust or sustained inflammation contributes to many diseases associated with inhalation exposures such as acute respiratory distress syndrome, hypersensitivity pneumonitis, bronchitis, bronchiolitis, and emphysema. Pro-inflammatory conditions arise due to a disruption in the balance between inflammatory and resolution processes.(Bozinovski et al. 2013; Dudek et al. 2016).Studies have begun to assess the contribution of proinflammatory lipids in mediating these inhalation exposure-induced responses (Table 1A). Below we examine specific exposures identified to distrupt pulmonary lipids and their role in pro-inflammatory signaling contributing to toxicity and disease.

Table 1:

Effect of inhaled exposures on inflammatory lipidmediators.

A. Proinflammatory Lipid Mediators
Exposure	Model/Specimen	Lipids change	References
Tobacco Smoke	Mouse	Ceramide↑	(Pilecki et al. 2018b)
	Human - sputum	168 different sphingolipids↑ 36 Phosphatidylethanolamine↑	(Telenga et al. 2014)
Traffic Source	Human -Bronchoalveolar lavage (BAL) fluid	13S-hydroxy-9Z,11E-octadecadienoic acid(13-HODE)↑ 12,13-dihydroxy-9Z-octadecenoic acid(12,13-diHOME)↑ Prostaglandin E2 (PGE2)↑	(Gouveia-Figueira et al. 2017; Loxham and Nieuwenhuijsen 2019)
Carbon Nanoparticles	Rat-lung epithelial cell line RLE-6TN	Ceramide↑	(Peuschel H. et al. 2012)
Subway Air	Human- Bronchoscopies	Prostaglandin E2 (PGE2)↓ 9,10,13-trihydroxy-11-octadecenoic acid↓ 9S,12S,13S-trihydroxy-10E-octadecenoic acid TriHOME↓ 9,- and 13-OH-9Z,11E,15Z-octadecatrienoic acid (HOTE)↓ 9-hydroxy-10E,12Z-octadecadienoic acid↓ 13- HODE↓ 9-oxo-10,12-octadecadienoic acid.↓ 13-keto-9Z,11E-octadecadienoic acid↓	(Lundström et al. 2011)
Particulate Matter 2.5 (PM2.5)	Mice- BALF	Prostaglandin D2 (PGD2)	(Yang et al. 2020)
House Dust Mite (HDM)	Human- Blood	Cysteinyl leukotrienes↑ Hydroxyeicosatetraenoic acids (HETEs) and Hydroxyoctadecadienoic acids (HODEs)↑	(Henkel et al. 2019)
B. Resolution-inflammatory Lipid Mediators
Tobacco Smoke	Human - BALF	Resolvin E1 (RvE1)↓ Resolvin D1 (RvD1)↓ Resolvin D2 (RvD2)↓	(Singh et al. 2019; Khan et al. 2020)
Carbon Nanoparticles	Mice- BALF	Leukotriene B4 (LTB4)↓ Prostaglandin E2 (PGE2)↓ Resolvin D1 (RvD1) and Resolvin E1 (RvE1)↑	(Lim et al. 2020)
Silver Nanoparticles	Mice- Lung	Eicosapentaenoic acid (EPA), Docosahexaenoic acid (DHA), Resolvin E1 (RvE1),Resolvin E2(RvE2), and Maresin	(Alqahtani S. et al. 2020)
Ozone	Mice- Lung	14(S)-hydroxy Docosahexaenoic acid (14(S)-HDHA), 17(S)-hydroxy Docosahexaenoic acid (17(S)-HDHA), SPMs Prostaglandins (PDX), andResolvin D5(RvD5)↓	(Kilburg-Basnyat et al. 2018a)

↑- Increased, ↓- Decreased.

3.1. Tobacco smoke exposure

Tobacco smoke contains thousands of distinct compounds, many of which are free radicals, that can significantly affect lung lipids (Gong et al. 2019) and lipid metabolism in a variety ofrespiratory cell types(Telenga et al. 2014; Dobri et al. 2021). Specifically, healthy smokers showed a significant and positive correlation between their lung function parameters and a higher rate of fatty acid metabolism(Agarwal et al. 2019).Additionally, smoking reduces lipid content in the plasma membranes of alveolar macrophages causing cytoplasmic lipid accumulation (Davies et al. 1978; Hannan et al. 1989). Following tobacco smoke exposure, respiratory tract epithelial cells havedemonstrated alterations in their ability to metabolize fatty acids, thus causing an imbalance in the amount of saturated to unsaturated fatty acids within their plasma membranes(van der Does et al. 2019). This finding suggests increased activity of enzymes such as phospholipase A2 responsible for cleavage of proinflammatory lipid precursors from cell membranes within the lung following tobacco smoke exposure. Further exposure to Electronic Nicotine Delivery Systems (ENDS) in the airway of the lung showed aberrant phospholipids and significant elevated surfactant-associated phospholipids in alveolar macrophages (Madison et al. 2020). The use of the lipophilic dye BODIPY in mice revealed that pulmonary macrophages exposed to smoke possessed a significantly large number of lipid droplets (Morissette et al. 2015). These lipid droplets contain enzymes for eicosanoid synthesis and serve as sites for eicosanoid generation during inflammation and are also known to regulate lipid metabolism, cell signaling, and inflammation. (Bozza and Viola 2010). The increases in intracellular lipid droplets may mediate pro-inflammatory signaling by contributing to eicosanoid synthesis within macrophages. Aerosols generated from both traditional cigarettes and electronic cigarettes (e-cigs) have alsobeen shown to compromise pulmonary endothelial barrier integrity by increasing intracellular ceramide (N-acyl-sphing-4-enine)production(Schweitzer et al. 2015) in a neutral sphingomyelinase 2-dependent manner (Levy Michal et al. 2009).The ceramide metabolic pathway significantly influences inflammation and stimulates nuclear factor-β(NF-β)(Won et al. 2004), which controls a number of proinflammatory cytokines, including tumor necrosis factor (TNF), interleukin 1 (IL-1), IL-6, and adhesion molecules to recruit neutrophils to sites of inflammation (Müller-Ladner et al. 2002; Gwinn and Vallyathan 2006). In a mouse model, cigarette smoke exposure induced ceramide synthase mRNA levelsresulting in increased ceramide production and upregulation of macrophage-mediated airway inflammation(Pilecki et al. 2018a). Overall, these findingssuggest exposure to tobacco smoke may induce lipid metabolism processes within multiple pulmonary cell types resulting in increased levels of proinflammatory lipid mediators such as ceramides. Increases in these mediators likely contribute to tobacco smoke-induced inflammation and disease development.

Cigarette smoking is the leading risk factor for developing COPD, and proinflammatory lipids are believed to contribute to disease advancement.(Bai et al. 2017).Specifically, 168 different sphingolipid(SP) and 36 phosphatidylethanolamine (PE) (1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine) lipidsspecies were determined to be elevated within the sputum of smokers with COPD compared tosmokers without COPD(Telenga et al. 2014). Lung function impairment and the number of inflammatory cells in the sputum were also determined to relate to these elevations in sphingolipids (Telenga et al. 2014). In the murine ovalbumin model of asthma, there is evidence suggesting that sphingosine-1-phosphate (Sphing-4-enine-1-phosphate) contributes to eosinophilic inflammation.(Nishiuma et al. 2008).It is important to consider PE species have side chains containing polyunsaturated fatty acids. Therefore, increases in PE levels could be connected to sphingolipid turnover via sphingosine-1-phosphate. When phospholipase A2 cleaves Pes, it creates LysoPEs along with fatty acids like AA(Murakami et al. 2020).. The conversion of AA has the potential to generate proinflammatoryeicosanoids, such as leukotrienes (FA0302) and prostaglandins (FA0301). These elevations in sphingolipids and Pessupport the involvement of sphingolipids in the pathophysiology of smokers withCOPD and potentially other diseases associated with lung inflammation.

3.2. Traffic-related air pollution exposure

Traffic-related air pollution is a significant component of environmental air pollution in urban settings, is composed of multiple components, and is associated with adverse lung health(Zheng et al. 2022). Inflammation is a primary outcome of inhalation and lipids may contribute to this process(Orach et al. 2023). Following exposure to biodiesel exhaust, proinflammatorylipid mediators were elevated including13S-hydroxy-9Z,11E-octadecadienoic acid(13-HODE), and 12,13-dihydroxy-9Z-octadecenoic acid(12,13-diHOME), and prostaglandin E2 (PGE2)(9-oxo-11R,15S-dihydroxy-5Z,13E-prostadienoic acid)in collected human bronchoalveolar lavage (BAL) fluid (Gouveia-Figueira et al. 2017; Loxham and Nieuwenhuijsen 2019). Both 13-HODE and 12,13-diHOME are produced by metabolism of consumed linoleic acid. Specifically, 12,13-DiHOME is produced viacytochrome P450-mediated metabolism, whereas 13-HODE is produced vialipoxygenase (LOX)-mediated metabolism (Warner et al. 2021). PGE2 is produced by metabolism of AA viacyclooxygenase (COX). This suggests biodiesel exhaust exposure induces linoleic acid and AA metabolism by enhancing the activity of multiple oxygenases (LOX,COX and Cytochrome P450(CYP-450)leading to the generation of proinflammatory lipid mediators. Additionally, since linoleic acid must be consumed by mammals it suggests diet may contribute to inflammatory responses due to the availability of linoleic acid-derived proinflammatory mediators.

Ceramide may contribute to airway inflammation after inhaling high doses of diesel exhaust. Specifically, pulmonary treatment of mice with the ceramide synthaseinhibitorfumonisin B1following diesel exhaust exposureinhibited the upregulation of sphingosine kinase 1 (Sphk1) mRNA(Shaheen H. M. et al. 2016). Inhibition of ceramide synthase reduces the conversion of sphingosine to ceramide thereby inhibiting ceramide mediated proinflammatory signaling and enhancing cell survival. Ceramide synthase is a class of six transmembrane proteins which has selectivity for the fatty acyl CoA moiety (Levy M. and Futerman 2010). Ceramide Synthase 2 and 5 are frequently found in high concentrations in the lung, especially within epithelial cells(Petrache et al. 2005; Xu et al. 2005). This high level of expression within the lung suggests that ceramides may be a primary regulator of pulmonary inflammatory response following exposures. Diesel exhaust is an environmental factor which can contribute to the generation of asthma. Underlying inflammation is a component of asthma and diesel exhaust particulate exposure increases ceramide synthesis in bronchial epithelial cells (Shaheen Hazem M. et al. 2016).Studies also have demonstratedhigher amounts of 12,13-DiHOME in the breath condensate of asthmatics after allergen exposure suggesting it may also contribute to inflammation(Nording et al. 2010).Lastly, 13-HODEcontributes to airway epithelium damage throughperoxisome proliferator-activated receptor-γ(PPARγ) receptorsignaling modulating core inflammation in numerous airway conditions.Exhaust generated from biodiesel and diesel vehicles are extremely complex exposures. Future research is needed to determine specific components of these exhaust exposures and their contribution to the induction of pro-inflammatory lipid within the lung. The present data suggests inhibition of the production of 13-HODE, 12, 13-diHOME, and ceramides as well as PGE2 may somewhat alleviate vehicle exhaust-induced pulmonary inflammation.

Other exposures have been demonstrated to also alter ceramidescontributing to pulmonary inflammation. Inhalation of carbon nanoparticlesinduces pulmonary inflammation and causes ceramides to accumulate in lipid raft signaling domains of epithelial cells(Peuschel Henrike et al. 2012). Specifically, the increase in ceramides within the lipid rafts initiates signaling cascades via epidermal growth factor receptor (EGFR) in the lung epithelium leading to neutrophilic lung inflammation in vivo.The signaling cascade resulting from the externally supplied ceramide serves as evidence of the significance of ceramides as molecules which may contribute to inflammation occurring after inhalation of multiple toxicants.

3.3. Subway air pollution exposure

Many individuals in urban areas utilze subway systems for transportation. Subway air is a complex mixture of inhalable components which may contribute to pulmonary inflammation. Subway air exposure affects proinflammatory eicosanoid levels of individuals based on their health condition. Specifically, in healthy individuals, exposure to subway air increasedeicosanoid levels, whereas individuals with asthma demonstrated no changes ordecreased eicosanoid levels.Responses of asthmatics to Stockholm subway air exposures decreased significantly from healthy individuals in terms of the nine eicosanoids including PGE2, 9,10,13-trihydroxy-11-octadecenoic acid- and 9S,12S,13S-trihydroxy-10E-octadecenoic acidTriHOME9,- and 13-OH-9Z,11E,15Z-octadecatrienoic acid (HOTE) (LMFA02000029), 9-hydroxy-10E,12Z-octadecadienoic acid- and 13-HODEand 9-oxo-10,12-octadecadienoic acid–and 13-keto-9Z,11E-octadecadienoic acid(Lundström et al. 2011). Overall, these nine eicosanoids included PGE2 and eight byproducts of 15-lipoxygenasebiosynthesis from linoleic orα-linolenic acids, indicating a potentially shared regulatory mechanism induced following exposure. Following exposure to subway air, healthy individuals had considerably higher expressions levels of COX-1 compared to asthmatics. (Harrington et al. 2008; Swedin et al. 2009). The levels of 15-hydroxy-5E,8Z,11Z,13Z-eicosatetraenoic acid(15-HETE) were considerably higher at baseline among asthmatics compared to healthy individuals, potentially due to enhanced production mediated by COX-1and other enzymes. After subway air exposure no differences in 15-HETE levels between asthmatic and healthy individuals were observed. This increased 15-HETE levels at baseline in asthmatics supports its role in the chronic pulmonary inflammation associated with the disease. Further, since 15-HETE levels were increased in healthy following subway air exposure but not asthmatics it suggests healthy individuals are able to induce inflammation via this mechanism while asthmatics have no further capacity. Predominance of linoleic-derived eicosanoids in comparison to AA-derived eicosanoids, both in terms of overall abundance and changes after exposure to subway air were observed.This result suggestssubway air exposure may preferentially induce metabolism of linoleic or α-linolenic acids possibly through enhanced activity of 15-LOX. Multiple underlying disease states exist with inflammation as a common component. Inflammation associated with these diseases may be driven by distinct proinflammatory lipid mediators such as was observed in asthmatics. It is possible that in these settings additional production of proinflammatory lipid mediators may be impaired due to baseline elevations. This suggests an inability to respond to exposures via upregulation of specific pathways resulting decreased clearance and response to exposures. Future investigations are needed to examine lipid dysregulation in prevalent disease states where lipids are modified which may modulate pulmonary responses to exposures.

Exposure to airborne particulate matter 2.5(PM2.5)increasesBAL inflammatory cells, multiple proinflammatory cytokines including IL-1, monocyte chemoattractant protein (MCP-1), IL-12 as well asProstaglandin D2(PGD2) (9S,15S-dihydroxy-11-oxo-5Z,13E-prostadienoic acid). Individuals with allergic asthma exposed to allergens to which they had already developed sensitivities demonstrate increased PGD2 levels in their BAL fluid(Yang et al. 2020).PGD2’s pro-inflammatory actions are mediated via DP1 and DP2/CRTH2 receptors. Both receptors have an equally strong affinity to PGD2 and depending on whether the DP1 or DP2/CRTH2 receptors are expressed, PGD2 generated by activated mast cells or T cells may activate numerous signaling pathways leading to diverse outcomes.The DP1 receptor on the bronchial epithelium regulates the production of chemokines and cytokines that attract inflammatory cells, contributing to airway inflammation and hyperreactivity in asthma.(Kabashima and Narumiya 2003).OVA-induced asthmatic mice lacking DP1 demonstrated lowered airway sensitivity and Th2-mediated lung inflammation, indicating that DP1 is crucial in mediating the effects of PGD2 produced by mast cells during an asthmatic response(Matsuoka et al. 2000).DP2/CRTH2 receptors are involved in pathogenic reactions by controlling inflammatory cell trafficking and effector activities. In humans, the DP2/CRTH2 receptor is expressed on Th2 lymphocytes, eosinophils, and basophils(Nagata K, Hirai, et al. 1999; Nagata K, Tanaka, et al. 1999; Hirai et al. 2001).The DP2/CRTH2 receptor produced from mast cells may directly mediate the recruitment of Th2 lymphocytes and eosinophils(Ricciotti and FitzGerald 2011). These findings suggest therapeutics disrupting PGD2 signaling may be useful in addressing PM-induced exacerbation of asthma while also fundamentally treating the disease.

3.4. House dust mites (HDM) exposure

House dust mites are present in most housing, represent a major instigator of allergic and asthmatic responses. Inflammation of the airways caused by house dust mites (HDM) is accompanied by elevated levels of cysteinyl leukotrienes (cysLTs) and 12/15-LOX metabolites such as HETEs and HODEs(Henkel et al. 2019). Studies show eosinophils and airway epithelial cells can produce LOX metabolites in airway inflammation induced by HDM, including cysLTs, HETEs, and HODEs(Salvi et al. 1999). Despite this, macrophages, which are prevalent in the airways and significantly express and easily upregulate LOX and COX enzymes, probably provide a substantial source of lipid mediators during the initial exposure to HDM. Interleukins linked to asthma, such as IL-4 and IL-13, have the ability to activate the human variant of the 15-LOX enzyme in monocytes and alveolar macrophages (Kühn and O’Donnell 2006).The respiratory tract contains a variety of cell types with the capacity to express 15-LOX, including bronchial epithelial cells, macrophages, eosinophils, and mast cells, with epithelial cells demonstrating the highest amounts of 15-LOX (Nadel et al. 1991; Liu et al. 2009). According to several investigations, asthmatic pro-inflammatory reactions are mostly indicated by high 15-LOX activity and levels of the AA 15-LOX product 15-HETE (Chu et al. 2002). These findings can be explained in the context of the relative abundance of the various fatty acid precursors and the selectivity of the enzyme substrate.

Inflammation processes may differ based on sex altering susceptibility to exposures. In comparison to controls, both male and female airways exposed to HDM and ozone had considerably higher levels of glycosphingolipids, which were linked to eosinophilia and airway resistance(Yaeger M. J. et al. 2021). Only 6 glycosphingolipid species were increased in males, while 15 were increased in females suggesting differential alterations in lipid metabolism following exposure(Stevens et al. 2022). Before and after ozone exposure, the amount of eicosanoids was higher in female lung tissue than in male lung tissue (Yaeger Michael J et al. 2021).Overall, these responses to HDM and ozone exposure demonstrate the potential for sex-specific differences in exposure-induced pulmonary inflammation due to differential lipid metabolism.

4. Effect of inhaled exposures on resolution lipids

Inflammation following exposure was originally assumed to be passively resolved following removal of the stimulus. Recent studies have identified the resolution process is an active process mediated by bioactive lipids such as SPMs. Following their production, SPMsmodify the immune system’s inflammatory response through direct and immunological pathways, including reducing pro-inflammatory mediator activity, boosting leukocyte phagocytosis and efferocytosis, increasing the synthesis of anti-inflammatory mediators, raising the killing and clearance of antigen, controlling transient receptor potential channels, and promoting tissue regeneration(Serhan C. N. 2017; Osthues and Sisignano 2019).Exposures may result in dysregulation of resolution signaling contributing to exacerbated and sustained inflammation (Table 1B).

Tobacco smoking is a well-known inducer of inflammation and leads to significant pulmonary diseases. Compared to nonsmokers, smokers with COPD have serum levels of monounsaturated fatty acids that are greater and levels of ω−3 fatty acids that are lower (Titz et al. 2016). These alterations in fatty acid profiles may decrease the availability of ω−3 fatty acid-derived SPMs impairing resolution and enhancing inflammation. COPD is associated with smoking and individuals with COPD have abnormally high leukotrienes and low SPMs in their BAL fluid and inducible sputum(van der Does et al. 2019). In plasma from smokers of cigarettes or waterpipe tobacco and electronic cigarettes, IL-6, IL-8, and TNF were found to be elevated, and resolvin E1 (RvE1) (5S,12R,18R-trihydroxy-6Z,8E,10E,14Z,16E-eicosapentaenoic acid) (Khan et al. 2020), RvD1(7S,8R,17S-trihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid and RvD2 (7S,16R,17S-trihydroxy-4Z,8E,10Z,12E,14E,19Z-docosahexaenoic acid)(Singh et al. 2019) were decreased in all smokers compared to non-smokers.It has been shown that SPMs play a time-dependent role in the resolution of inflammation related to damage in mice exposed to synthetic carbon nanoparticles. These mice displayed an early neutrophilic inflammation with elevated Leukotriene B4 (LTB4) (5S,12R-dihydroxy-6Z,8E,10E,14Z-eicosatetraenoic acid) and PGE2 followed by increased RvD1 and RvE1 on day 7 (Lim et al. 2020). Exposures which decrease SPMs such as resolvins may cause prolonged inflammation through inhibition of resolution processes resulting in disease progression.

Individuals with COPD have demonstrated a dysregulation of resolution mediators which may contribute to disease.For instance, in the BAL of individuals with COPD, RvD1 is decreased whereas 17S-hydroxy-4Z,7Z,10Z,13Z,15E,19Z-docosahexaenoicacid(17-HDHA), 14S,21S-dihydroxy-4Z,7Z,10Z,12E,16Z,19Z-docosahexaenoic acid (14-HDHA), 12R-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12-HETE), and 15-hydroxy-5Z,8Z,11Z,13E-eicosatetraenoic acid (15-HETE) are raised (Croasdell et al. 2015). These findings suggest metabolism of 17-HDHA and production of the SPM RvD1 is inhibited in COPD and may contribute to ongoing inflammation(Bozinovski et al. 2013). Additionally, Lipoxin A4 (LXA4) (5S,6R,15S-trihydroxy-7E,9E,11Z,13E-eicosatetraenoic acid) functions to resolve inflammation through the N-formyl peptide receptor 2 (ALX/FPR2)and its signaling capacity is reduced in COPD patients due to competitive inhibition by serum amyloid A. (Balode et al. 2012; Bozinovski et al. 2013; Duvall and Levy 2016). This countering of LXA4’s resolution signaling by the pro-inflammatory mediator serum amyloid A supports a disruption of lipid-mediated resolution contributes to acute COPD exacerbations (Titz et al. 2016).

Subpopulations of individuals may be at risk for enhanced toxicity following exposures due to disruption of lipids. Epidemiological evaluations have demonstrated that individuals with metabolic syndrome are increasingly sensitive to inhalation exposure-induced inflammation, however, the mechanisms are unknown. This susceptibility may be due to the dysregulation of lipids associated with the underlying disease enhancing theirinflammatory response following exposure. In our earlier research, isolated lung tissue showed reductions in lipid mediators of inflammatory resolution following silver nanoparticle exposure specifically in a mouse model of metabolic syndrome with fewer alterations in the healthy mouse model. Silver nanoparticles (AgNP) exposure in metabolic syndrome mice reduced levels of EPA, DHA, RvE1, RvE2, and Maresin. (Alqahtani S. et al. 2020). Interestingly, protectin D1 was decreased in both the metabolic syndrome and healthy model following silver nanoparticle exposure. This suggests exposure-induced reductions in pulmonary protectin D1 levels may contribute to inflammation in both models whereas inhibition of other SPMs may cause the enhanced inflammation observed due to metabolic syndrome. Additionally, sex differences exist in lipid regulation which may cause sex-specific differences in pulmonary inflammation following exposures. Following ozone inhalation, the levels of the DHA-derived SPM precursors 14(S)-HDHA, 17(S)-HDHA, SPMs PDX, and RvD5 were higher in female lung tissue than in male lung tissue(Yaeger Michael J et al. 2021).Lung damage from ozone Pro-resolving lipid mediators (SPMs) such 14-HDHA and 17-HDHA as well as the SPM protectin DX (PDX) with increased expression in the lung of male C57Bl/6J mice have been associated to pulmonary inflammation(Kilburg-Basnyat et al. 2018a). These findings suggest susceptibility to exposure-induced pulmonary inflammation and disease development may result from differential availability of SPMs due to underlying disease and/or sex.

5. Laboratory Methods to Isolate, Identify, and Quantify Pulmonary Lipids

To comprehensively understand mechanisms of inflammation and toxicity following inhalation exposures it is necessary to elucidate lipid contributions. Lipids exhibit a significant degree of disorder under physiological settings in contrast to nucleic acids (DNA and RNA) and proteins, making clear structure-function correlations difficult. Development of global lipidomics has advanced because of significant advances in bioinformatics and mass spectrometry technology. Lipidomic analysis involves lipid extraction, lipid separation, lipid detection, and bioinformatics analysis.

The two procedures most frequently used for lipid extraction and isolation from biological samples are the Folch method and the Bligh and Dyer approach (Folch et al. 1957; Bligh and Dyer 1959). These procedures are effective for many types of lipids, and can be modified for various low-abundance lipids and lipid mediators such as sphingolipids (Matyash et al. 2008; Löfgren et al. 2012). To separate lipids before mass measurement, high-performance liquid chromatography (HPLC) is widely employed (Hall and Murphy 1998). There are three primary methods for detecting lipids: desorption ionization MS methods, liquid-phase separations coupled to mass spectrometry (MS) methods, and infusion MS analysis (Holčapek et al. 2018). These methods have been applied to the evaluation of lipid mediated mechanisms of lung disease following inhalationexposures (Table 2).Overall, profiling approaches utilizing mass spectrometry generate large data set requiring specialized databases and software to annotate and analyze the data (Table 3). Multiple reviews have been written on the methods(Jones et al. 2017; Garikapati et al. 2019; Kadesch et al. 2019; Liang et al. 2021; Michael et al. 2022). Therefore, we will highlight two recent advancements in technology and methodology benefiting lipid assessments that can be applied to advance toxicological assessments.

Table 2:

Analytical techniques used in lipidomic studies to investigate the correlation between lipid profiles and pulmonary disease.

Condition/Exposure	Sample	Separation(method)	MS (ionization mode)	References
Chronic Obstructive Pulmonary Disease (COPD)	Human-Exhaled breath condensate (EBC), serum	LC (direct injection)	MS (+ and−ESI)	(Kilk et al. 2018)
	Human-Plasma, BALF	LC (HILIC and RP)	MS (+ and−ESI)	(Naz et al. 2017)
	Human-Urine	NMR (600 MHz)1D-1H		(Adamko et al. 2015)
Asthma	Human-Urine	LC (RP)	MS (+ and−ESI)	(Carraro et al. 2018)
	Human-Serum	LC (HILIC and RP)	MS (+ and−ESI)	(Reinke et al. 2017)
	Human-Plasma	LC (RP)	MS (+ ESI)	(McGeachie et al. 2015)
Idiopathic pulmonary fibrosis (IPF)	Human-Serum	LC (RP)	MS (+ and−ESI)	(Rindlisbacher et al. 2018)
	Human-Lung tissue	LC GC	MS/MS MS	(Zhao et al. 2017)
	Human-Lung tissue	GC	MS (EI)	(Kang et al. 2016)
	Human-Plasma	UPLC	MS (+ ESI)	(Nambiar et al. 2021)
Administration of therapeutic surfactants and isotopic tracers	Mice-Lung Tissue	MALDI-MSI	ESI	(Ellis et al. 2021)
Drug-induced lipidosis	Rat-Lung Tissue	DESI -MSI	MS and MS/MS	(Dexter et al. 2019)
Lung Cancer	Human-Lung Tissue	DESI -MSI	MS/MS (ESI)	(Bensussan et al. 2020)
Normal Condition	Human-Lung Tissue	Nano-DESI Imaging	MS/MS (ESI)	(Nambiar et al. 2021)
Naphthalene Toxicity	Mice-Lung tissue	LC-tandem mass spectrometry	MS (+ ESI)	(Lee et al. 2018)
Silver Nanoparticles (AgNP) Toxicity	Mice-Lung tissue	LC-tandem mass spectrometry	MS/MS (ESI)	(Alqahtani S. et al. 2021)
Normal Condition	Mice-Lung tissue	LC MRM- Profiling.	MS	(Kobos et al. 2021)
Volatile organic compounds (VOCs) Toxicity	Mice-Serum	LC	MS/MS (Q-TOF)	(Xia et al. 2023)

Table-3:

Commonly utilized databases for the processing and annotation of MS data for lipid analysis

Database	Applications
LIPID MAPS	Lipid database and classification
IUPAC	IUPAC lipid nomenclature
KEGG	Fatty acid, sterol, and phospholipid metabolism
METACYC	Lipid metabolism
HMDB	Metabolome database (MS and MS/MS spectra)
METLIN	Metabolome database (MS and MS/MS spectra)
Free software
LipidBlast	Tandem mass spectrometry to identify lipids.
LipidXplorer	Molecular fragmentation query language (MFQL) in shot-gunlipidomics
Skyline	Open-source application for proteomics and lipidomics data.
LIQUID	Open source software for lipids in LC-MS/MS
LipiDex	Unifies the identification of lipids based on LC-MS/MS using intelligent data filtering.”
LipidHunter	de novo identification of native phospholipids
Commercial software
LipidView (AB/Sciex)	LC-MS data processing
Marketlynx (Waters)	LC-MS, LC-MS/MS, GC/MS, and GC-MS/ MS data processing
Metabolic Profiler (Bruker)	NMR and MS data processing
Lipid (ThermoScientific)	Search LC-MS/MS data processing

5.1. Mass Spectrometry Imaging based identification of lipids.

Mass spectrometry imaging (MSI) provides useful spatial information about the distribution of molecules in biological tissues. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) and desorption electrospray ionization mass spectrometry imaging (DESI MSI) are sensitive analytical techniques providing molecular information and spatial distribution of analytes directly from biological specimens.MALDI-MSI allows for the ionization of natural sample elements at ambient environments and is frequently more time- and space-efficient(Takats et al. 2017).MALDI-MSI has gained popularity because no labeling is required, hundreds of biomolecules can be detected and their spatial distribution in biological tissues can be determined in a single measurement.(Spengler 2015). Technical developments have transformed MALDI MSI into a potent tool for the analysis and imaging specifically lipids in biological tissues (Garikapati et al. 2019; Kadesch et al. 2019).Recently, laser technological advancements have enhanced the spatial resolution and analytical speed of MALDI MSI(Prentice et al. 2015; Bowman et al. 2020; Balluff et al. 2021; RamalloGuevara et al. 2021).DESI MSI is the most well-known and least tissue-damaging ambient ionization imaging approach(Takáts et al. 2004).Without the use of external matrices or laser implementation, it is possible to quickly map, measure, and identify hundreds of lipids from an unmodified sample (tissue or cells). As a result, there is little chance that the physiological distribution of molecules in tissues will be distorted while sensitivity is also improved(Takáts et al. 2005; Takats et al. 2017).Overall, DESI-MSI is a very powerful discovery tool and its workflow is considered fairly simple(Bennet et al. 2013).The size of the spray limits the spatial resolution of DESI-MSI, therefore nanoDESI has been developed to increase the spatial resolution in the imaging mode(Nguyen et al. 2019).Nano-DESI MSI is becoming increasingly popular for lipidomicsexaminations sincethis technique gives both relative abundances and spatial localization of many different lipid species(Lanekoff et al. 2014). 265 distinct lipids were discovered using nano-DESI MSI in both positive and negative ionization modes in mouse lung tissues(Nguyen et al. 2019).The nano-DESI MSI lipidomics technique is particularly helpful for examining changes in lipid distributions occurring during lung development and those caused by illness or exposure to environmental toxicants.

Overall, these MSI approaches and specifically the spatial information provided are important for assessments of lipid mediators within the lung due to inhalation exposure depositional differences. Following inhalation, exposures do not uniformly distribute within the lung and bioactive lipids are extremely potent signaling molecules functioning at nanomolar(nM) or picomolar(pM) concentrations. Traditionally, to evaluate lipids, either whole or representative slices of tissues are homogenized and processed for mass spectrometry analysis. The homogenization of an entire lung negatively impacts the ability to detect alterations in bioactive lipids as lipid signals are diluted due to a significant amount of the tissue likely being unaffected by the exposure. Further, isolation of a lung slice introduces variability due to depositional differences in exposure. The incorporation of spatial information provided by MSI approaches can allow for more sensitive identification of lipid alterations following exposures overcoming limitations of traditional mass spectrometry approaches.

5.2. Multiple Reaction Monitoring Profiling of Lipids

MRM-Profiling is a two-stage diagnostic method for lipidomic discovery(de Lima et al. 2018; Xie et al. 2019a, 2019b). LC-MS/MS techniques use MRM for sensitive and simultaneous analysis of multiple metabolites, reducing analysis time.(Griffiths et al. 2011). The use of LC simplifies analysis and provides significant chromatographic benefit for separating mixtures of lipid mediators because no prior derivatization of the analytes is needed (Wang et al. 2014). Electron spray ionization (ESI) generates positive and negative molecular ion species ([M+H]+ and [M-H])- for MS detection of biomolecules(Rago and Fu 2013). Tandem mass spectrometry is known to improve quantitative lipid analysis sensitivity and specificity (Blanksby and Mitchell 2010). Because the acquisition mode MRM is used to monitor analytes in most lipidomics studies to transition a specific precursor ion to a specific product ion (Sandra and Sandra 2013), there are several lists that feature characteristic eicosanoids MRM transitions performed on triple-quadrupole instruments (Wang et al. 2014). For example, researchers were able to create a focused bioanalytical technique for eicosanoid quantification using high-resolution MRM(Sorgi et al. 2018)to examine inflammatory signaling in lung tissue. The MRM profiling method was previously utilized to examine pulmonary lipid modifications in a mouse model following induction of inflammation due to oropharyngeal aspiration of silver nanoparticles(Alqahtani S. et al. 2020). These MRM profiling results suggested lipid pathways affected by exposure and allowed for subsequent evaluations to utilize specific lipid treatments and targeted mass spectrometry approaches to examine lipid-mediated inflammation. Overall, the MRM profiling method assists in the discovery of alterations in bioactive lipids following exposure. This provides a quick and more comprehensive assessment which can inform future more targeted studies investigating lipid mediated inflammation following exposures.

6. Potential treatments targeting lipids to address exposure-induced inflammation

Following identification of lipid dysregulation associated with inhalation exposures or pulmonary disease this information can be applied to inform therapeutic strategies. Multiple strategies are currently targeting lipid pathways involved in inflammation for therapeutic benefit. Here we highlight some examples of how these strategies are employed clinically and some preclinical evaluations which may translate in the future to clinical application (Table 4).

Table-4:

Examples oflipid-targeting treatments to reduce exposure-induced inflammation.

Intervention	Lipid Mechanism Targeted	Outcome	Reference
Non-steroidal anti-inflammatory drugs	Reduce Prostaglandins, Prostacyclin, and Thromboxanes	NSAID- reduces the short-term effects of air pollution on pulmonary function in the elderly. Ibuprofen - decreases the late response to allergens and may therefore protect in cases of asthma caused by air pollution	(Gao et al. 2020) (Nomani et al. 2016)
Leukotriene receptor antagonists	Inhibitory effect on 5-LOX activity, which may reduce the production of pro-inflammatory leukotriene	Montelukast-Effective treatment for smokers with asthma patients. Therapy is more effective in obese asthma patients.	(Price et al. 2013)
Prostaglandin D2 (PGD2) receptor antagonists	Disrupt PGD2 signaling. Lowering the activation of Th2 lymphocytes and eosinophils.	Timapiprant and monoclonal antibodies -Effective treatment for T2 asthma and other inflammatory disorders and Reduce PM2.5-induced mast cell activation.	(Fajt et al. 2013; Straumann et al. 2013; Bhat et al. 2021)
Specialized pro-resolving mediators (SPM)	Reprogramming of immune cells to boost phagocytosis and moderate inflammation.	Resolvins- reduce exacerbated inflammation induced by inhalation exposure and decrease the inflammatory cytokines and inflammatory cells. Maresin- decreased methacholine-induced airway hyperreactivity, airway eosinophilia and serum IgE and improved airway barrier integrity as indicated by decreased lung edema. Lipoxin A4- Encouraging the production of LIM kinase facilitates the polarization of lytic granules to the immune synapse and boosts NK cell cytotoxicity.	(Bhat et al. 2021) (Godson 2020; Gilbert et al. 2021) (Duvall et al. 2020)

6.1. Non- steroidal anti-inflammatory drugs.

Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used to treat acute and chronic inflammation. These function through modulating pro-inflammatory eicosanoid production. NSAIDs are categorized based on their chemical composition and selectivity. These categories include acetylated salicylates (e.g. aspirin), non-acetylated salicylates (e.g. diflunisal, salsalate), propionic acids (e.g. naproxen, ibuprofen), acetic acids (e.g. diclofenac, indomethacin), enolic acids (e.g. meloxicam, piroxicam), and anthran (e.g. celecoxib, etoricoxib, valdecoxib).(Ghlichloo and Gerriets 2022). NSAIDs have diverse effects on inflammation and immune responses, ranging from anti-inflammatory, immunosuppressive, and anti-thrombotic to pro-resolving, because of their nonselective inhibition of prostaglandin-endoperoxide synthase 1 and 2 also known as cyclooxygenases-1 and −2 (COX-1 and COX-2). This inhibition by NSAIDS reduces the production of pro-inflammatory lipid mediators, including various PGs, prostacyclin, and TXs (Andreakos et al. 2021; Sokolowska et al. 2021). Reductions in these eicosanoids is thought to be the primary mechanism responsible for NSAID therapeutic benefits.

COX-2 is not constitutively expressed, but is induced in tissues such as the lung during the inflammatory response following inhalation exposures. NSAID usage alleviatesshort-term air pollution reductions inlung function in older individuals(Gao et al. 2020).Additionally, ibuprofen decreases the late response to allergens and may therefore provide protection in cases of asthma caused by air pollution(Nomani et al. 2016).Overall, reductions in pro-inflammatory lipid production via NSAID inhibition of COX-2 may be a useful treatment strategy that could further be clinically applied to address inhalation exposure-induced pulmonary inflammation.

6.2. Leukotriene receptor antagonists.

Several inflammatory reactions, such as bronchoconstriction, mucus secretion, and increased vascular permeability, are caused by cysteinyl leukotrienes(CysLTs) activating the cysteinyl leukotriene type 1 receptor (CysLT1). The CysLT1 is hypothesized to act as the site of competitive antagonism where all therapeutically available leukotriene receptor agntagonists(LTRAs)(montelukast, zafirlukast, and pranlu-kast) regulate airway inflammatory reactions(Capra et al. 2007; Peters-Golden and Henderson 2007; Diamant et al. 2009). LTRAs also have CysLT1-independent mechanisms which may reduce inflammation such as suppression of eosinophil protease activity (Langlois et al. 2006) and prevention of TNF- (Tahan et al. 2008) or uridine diphosphate (UDP)- mediated (Woszczek et al. 2010) cytokine production, as well as NF-B activation in human mononuclear (Ichiyama et al. 2003) or epithelial cells (Ishinaga et al. 2005). Further, it has also been discovered that LTRAs may have a strong inhibitory effect on 5-LOX activity, which may reduce the production of pro-inflammatory LTs (Ramires et al. 2004), while also disrupting transport of LTs via the multidrug resistance protein ATP-binding cassette sub-family C member 4(Rius et al. 2008).

Overall, these mechanisms by which LTRAs are known to inhibit inflammation may be translatable to the treatment of pulmonary inflammation following inhalation exposures. Smoking inhibits steroid anti-inflammatory responses, (Chalmers et al. 2002) while also increasing cysteinyl-LT synthesis(Gaki et al. 2007). This results in smokers with asthma responding more favorably to montelukast, indicating that LTRA may be more effective in treating these patients (Lazarus et al. 2007). In fact, montelukast treatment for asthmatic individuals with a smoking history greater than 11 pack-years was more effective than inhaled steroids (Price et al. 2013). Interestingly, (Giouleka et al. 2011)the therapeutic response to inhaled corticosteroids declines with increasing body mass index (BMI), but response to montelukast is unaffected, which suggests that LTRA therapy is more effective in obese patients(Giouleka et al. 2011).This may also be associated with increased pro-inflammatory LT production as BMI increases in asthmatics(Peters-Golden et al. 2006). Future, preclinical studies should be performed to determine if LTRAs could be applied to the modulation of lipid mediators in the lung to treat pulmonary inflammation induced by inhaled exposures.

6.3. PGD2 receptor antagonists

Allergen activation of mast cells releasesthe proinflammatory lipid-mediator PGD2, which is a mediator of allergic immune reactions. PGD2 interacts with the G-protein-coupled receptors (GPCR) DP1, thromboxane (TP), and Chemoattractant receptor-homologous molecules expressed on Th2 cells (CRTH2 or DP2) to produce proinflammatory effects (Pettipher et al. 2007). PGD2 interacts with the DP2-receptors on effector cells to operate as a connection between the early phase allergy-response (EAR) and late phase allergy response (LAR), in addition to its broncho- and vaso-active features in allergic airway disease. DP2 antagonists were initially developed to treat allergic airway illness (allergic rhinitis, asthma) (Rudulier et al. 2019; Brightling et al. 2020).Clinical studies have been conducted on a range of small molecule DP2inhibitors to disrupt PGD2 signaling to treat bronchial asthma, allergic rhinitis, and eosinophilic esophagitis.A novel oral DP2 antagonist known as timapiprant (previously OC000459 [Atopix Therapeutics Ltd, Sheffield, UK]) has demonstrated safety and efficacy in the treatment of eosinophilic esophagitis and asthma(Kupczyk and Kuna 2017)by substantially lowering the activation of Th2 lymphocytes and eosinophils(Straumann et al. 2013).Various selective DP2 receptor antagonists such as monoclonal antibodies are also being developed to disrupt lipid-mediated immune responses mediated by PGD2(Nagata N et al. 2017). DP2 inhibition may therefore prove effective in treating T2 asthma and other inflammatory disorders (Fajt et al. 2013).

Inhalation exposures such as PM_2.5 have been demonstrated to induce mast cell activation and release of PGD2(Yang et al. 2020). Application of DP2 antagonist to inhibit PGD2 interactions following inhalation exposures that induce inflammation via PGD2 may be beneficial as a treatment option. Studies screening for PGD2 induction following inhalation exposures could identify exposures were DP2 antagonist could be a useful treatment option.

6.4. Specialized pro-resolving mediators (SPM)

Previously discussed treatment options inhibit pro-inflammatory processes stimulated by lipid mediators, however, SPMs represent a treatment option that could be applied to engage resolution processes and facilitate a return to homeostasis. SPMs are potent endogeneous ligands that stimulate inflammatory resolution processes. Supplementation of SPMs has been demonstrated to potentially benefit numerous diseases associated with inflammation. It is likely strategies to increase SPM availability could benefit inhalation exposure-induced diseases by reducing inflammation. Here we will briefly describe some of these potential SPM supplementation strategies and how they could be applied to resolveinflammation following inhalation exposures.

Fish oil supplements are heavily utilized in our population and function by delivering high amounts of the SPM precursors EPA and DHA. These ω−3 PUFAs can reduce inflammation by being metabolized to SPMs which stimulate resolution and alsoby competing with omega-6 PUFAs, that serve as pro-inflammatory precursors, for incorporation into the cell membrane. ω−3 PUFA supplementation has been demonstrated to reduce inflammation in numerous human diseases including sepsis, cancer, cystic fibrosis, and COPD(Yu et al. 2021; Sohouli et al. 2023; Amiri Khosroshahi et al. 2024; Zhou et al. 2024). This suggests that ω−3 PUFA supplementation could be employed to treat inflammation induced via inhalation exposures. Preclinical studies have demonstrated the potential for their application to address exposure-induced inflammation(Lovins et al. 2023). For example, administration of a combination of SPM precursors (14-HDHA, 17-HDHA, PDX)was determined to reduce ozone-induced pulmonary inflammation in a mouse model(Kilburg-Basnyat et al. 2018b). Use of a specific SPM, RvD1, was also determined to reduce cigarette smoke and influenza induced inflammation. Lastly, RvD1 treatment reduced inflammation in a mouse model of metabolic syndrome following silver nanoparticle exposure, however, did not impact the inflammatory response in a healthy model(Alqahtani Saeed et al. 2022). This finding suggests that distinct subpopulations may benefit from SPM supplementation. Overall, theses studies and others suggest supplementation of ω−3 PUFAs may alleviate inflammation following inhalation exposure (Table 4). There are limitations to the utilization of SPMs as interventions. Some epidemiological studies have demonstrated variable benefits to their utilization in some diseases(Julliard et al. 2022). This means that they may not be effective in all diseases or for all exposures. Specific SPMs are also difficult to manufacture and are susceptible to metabolic inactivation limiting our ability to produce and utilize them. To overcome this limitation the production of more stable SPM receptor agonists may be needed. Overall, significant research efforts are necessary to determine if SPM supplementation can be applied to humans to treat inhalation exposures. These research efforts will need to identify which SPMs are useful, how best to deliver them, timeframes of utilization, and subpopulations that would most benefit.

Beyond supplementation another mechanism to elevate SPMs would be to modulate endogeneous production and elimination. SPMs such as epoxyeicosatrienoic acids (EETs) are eliminated by soluble epoxide hydrolase (sEH) thereby limiting their activity. Treatments to inhibit sEH would increase SPM levels and their activity. Preclinical studies investigating sEH inhibits have demonstrated their ability to treat pulmonary diseases such as pulmonary hypertension, tobacco smoke-induced COPD, COVID-19, and inflammation in animal models through enhancement of SPM-mediated resolution signaling(Bhat et al. 2020; Callan et al. 2020; Bhat et al. 2021; Manickam et al. 2022). These studies suggest that modulation of lipid production and available could be utilized as treatment options to address exposure-indued inflammation. A limitation of this approach would be the potential for immunosuppression which may limite immune responses to other challenges. Studies are needed to understand appropriate dosing regiments that would be beneficial after exposures.

7. Conclusions

Bioactivelipids are intricately involved in the lungs’ inflammatory and recovery response followinginhalation exposures. Recent research demonstrates exposure-induced alterations in bioactive lipids mediating pulmonary inflammation following inhalation of a variety of environmental toxicants. Specifically, exposures can upregulate pulmonary eicosanoids contributing to inflammatory processes or decrease SPMs inhibiting inflammatory resolution. These lipids signaling pathways may be robust and key regulators which can be targeted for potential therapeutic benefits and interventional strategies. Research and development of these strategiesare in their earlystages but offer promising potential to improve health outcomes following exposures.Additional molecular research on pulmonary bioactive lipids and exposure-induced dysregulation is needed. Knowledge is lacking regarding the exposure-specific effects on metabolism and clearance enzymes responsible for the availability of bioactive lipids. Further, additional evaluation is needed to understand specifically lipid mediators of resolution and their role in sustain inflammation often associated with disease development. Methodological and technological advancements have been made in the mass spectrometry-based identification and quantification of lipids. However, due to their high potency, it will be necessary in the future to employ advanced techniques that include spatial information to understand alterations in bioactive lipids within tissues following exposure. Lipid dysregulation is a key component of numerous diseases which may contribute to disease-associated inflammation. Identification of specific lipid mediators and pathways would inform therapeutic interventions that could be applied to subpopulations. In conclusion, bioactive lipids represent significant mediators likely contributing to inhalation exposure-induced inflammation and health effects. A thorough understanding of their modulation following exposures is necessary for protection of public health.

Data Availability Statement.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.