Ozone-induced airway epithelial cell death, the neurokinin-1 receptor pathway, and the postnatal developing lung

Alteration of glycosphingolipid metabolism by ozone is associated with exacerbation of allergic asthma characteristics in mice

Asthma is a common chronic respiratory disease exacerbated by multiple environmental factors. Acute ozone exposure has previously been implicated in airway inflammation, airway hyperreactivity, and other characteristics of asthma, which may be attributable to altered sphingolipid metabolism. This study tested the hypothesis that acute ozone exposure alters sphingolipid metabolism within the lung, which contributes to exacerbations in characteristics of asthma in allergen-sensitized mice. Adult male and female BALB/c mice were sensitized intranasally to house dust mite (HDM) allergen on days 1, 3, and 5 and challenged on days 12-14. Mice were exposed to ozone following each HDM challenge for 6 h/day. Bronchoalveolar lavage, lung lobes, and microdissected lung airways were collected for metabolomics analysis (N = 8/sex/group). Another subset of mice underwent methacholine challenge using a forced oscillation technique to measure airway resistance (N = 6/sex/group). Combined HDM and ozone exposure in male mice synergistically increased airway hyperreactivity that was not observed in females and was accompanied by increased airway inflammation and eosinophilia relative to control mice. Importantly, glycosphingolipids were significantly increased following combined HDM and ozone exposure relative to controls in both male and female airways, which was also associated with both airway resistance and eosinophilia. However, 15 glycosphingolipid species were increased in females compared with only 6 in males, which was concomitant with significant associations between glycosphingolipids and airway resistance that ranged from R2 = 0.33-0.51 for females and R2 = 0.20-0.34 in male mice. These observed sex differences demonstrate that glycosphingolipids potentially serve to mitigate exacerbations in characteristics of allergic asthma.

Integrative analysis reveals mouse strain-dependent responses to acute ozone exposure associated with airway macrophage transcriptional activity

Acute ozone (O3) exposure is associated with multiple adverse cardiorespiratory outcomes, the severity of which varies across individuals in human populations and inbred mouse strains. However, molecular determinants of response, including susceptibility biomarkers that distinguish who will develop severe injury and inflammation, are not well characterized. We and others have demonstrated that airway macrophages (AMs) are an important resident immune cell type that are functionally and transcriptionally responsive to O3 inhalation. Here, we sought to explore influences of strain, exposure, and strain-by-O3 exposure interactions on AM gene expression and identify transcriptional correlates of O3-induced inflammation and injury across six mouse strains, including five Collaborative Cross (CC) strains. We exposed adult mice of both sexes to filtered air (FA) or 2 ppm O3 for 3 h and measured inflammatory and injury parameters 21 h later. Mice exposed to O3 developed airway neutrophilia and lung injury with strain-dependent severity. In AMs, we identified a common core O3 transcriptional response signature across all strains, as well as a set of genes exhibiting strain-by-O3 exposure interactions. In particular, a prominent gene expression contrast emerged between a low- (CC017/Unc) and high-responding (CC003/Unc) strain, as reflected by cellular inflammation and injury. Further inspection indicated that differences in their baseline gene expression and chromatin accessibility profiles likely contribute to their divergent post-O3 exposure transcriptional responses. Together, these results suggest that aspects of O3-induced respiratory responses are mediated through altered AM transcriptional signatures and further confirm the importance of gene-environment interactions in mediating differential responsiveness to environmental agents.

Modulation of the EMT/MET Process by E-Cadherin in Airway Epithelia Stress Injury

Persistent injury and the following improper repair in bronchial epithelial cells are involved in the pathogenesis of airway inflammation and airway remodeling of asthma. E-cadherin (ECAD) has been shown to be involved in airway epithelium injury repair, but its underlying mechanisms to this process is poorly understood. Here, we describe a previously undetected function of ECAD in regulating the balance of EMT and MET during injury repair. Injury in mice and human bronchial epithelial cells (HBECs) was induced by successive ozone stress for 4 days at 30 min per day. ECAD overexpression in HBECs was induced by stable transfection. EMT features, transforming growth factor beta1 (TGF-β1) secretion, transcriptional repressor Snail expression, and β-catenin expression were assayed. Ozone exposure and then removal successfully induced airway epithelium injury repair during which EMT and MET occurred. The levels of TGF-β1 secretion and Snail expression increased in EMT process and decreased in MET process. While ECAD overexpression repressed EMT features; enhanced MET features; and decreased TGF-β1 secretion, Snail mRNA level, and β-catenin protein expression. Moreover, activating β-catenin blocked the effects of ECAD on EMT, MET and TGF-β1 signaling. Our results demonstrate that ECAD regulates the balance between EMT and MET, by preventing β-catenin to inhibit TGFβ1 and its target genes, and finally facilitates airway epithelia repair.

Regulatory Effects of Nur77 on Airway Remodeling and ASMC Proliferation in House Dust Mite-Induced Asthma

Airway remodeling played a vital role in the development of asthma, and airway smooth muscle (ASM) mass was its hallmark. However, few strategies targeting ASM remodeling were developed in treating asthma. Nur77 was the transcription factor nuclear receptor involved in the pathogenesis of several lung diseases. Nur77 distribution and expression were determined in an HDM-mediated allergic asthma model. Its effect on airway hyperresponsiveness (AHR), chronic inflammation, and ASM remodeling in asthmatic mice was evaluated using a lentivirus-mediated shRNA. Possible mechanisms were explored by examining Nur77 actions and its underlying pathways in primary human AMC cells (ASMCs). In this study, we reported that Nur77 expression was mainly distributed along ASM and increased in lungs of HDM-challenged mice. Nur77 depletion by lentivirus-mediated shRNA ameliorated AHR, chronic inflammation, goblet cell hyperplasia, and airway remodeling in the asthmatic mouse model. By means of primary human ASMC, we discovered that Nur77 upregulation by HDM stimulation promoted cell proliferation and ROS production, as well as reduced antioxidant gene expression. These alterations might associate with MFN2/MAPK/AKT pathways. These findings broadened our understanding of airway remodeling and ASMC proliferation, which might provide a novel therapeutic target for asthma patients.

Transcriptional Effects of Ozone and Impact on Airway Inflammation

Epidemiological and challenge studies in healthy subjects and in individuals with asthma highlight the health impact of environmental ozone even at levels considered safe. Acute ozone exposure in man results in sputum neutrophilia in 30% of subjects particularly young children, females, and those with ongoing cardiopulmonary disease. This may be associated with systemic inflammation although not in all cases. Chronic exposure amplifies these effects and can result in the formation of asthma-like symptoms and immunopathology. Asthmatic patients who respond to ozone (responders) induce a greater number of genes in bronchoalveolar (BAL) macrophages than healthy responders with up-regulation of inflammatory and immune pathways under the control of cytokines and chemokines and the enhanced expression of remodeling and repair programmes including those associated with protease imbalances and cell-cell adhesion. These pathways are under the control of several key transcription regulatory factors including nuclear factor (NF)-κB, anti-oxidant factors such as nuclear factor (erythroid-derived 2)-like 2 NRF2, the p38 mitogen activated protein kinase (MAPK), and priming of the immune system by up-regulating toll-like receptor (TLR) expression. Murine and cellular models of acute and chronic ozone exposure recapitulate the inflammatory effects seen in humans and enable the elucidation of key transcriptional pathways. These studies emphasize the importance of distinct transcriptional networks in driving the detrimental effects of ozone. Studies indicate the critical role of mediators including IL-1, IL-17, and IL-33 in driving ozone effects on airway inflammation, remodeling and hyperresponsiveness. Transcription analysis and proof of mechanisms studies will enable the development of drugs to ameliorate the effects of ozone exposure in susceptible individuals.

Key Functions and Therapeutic Prospects of Nur77 in Inflammation Related Lung Diseases

The transcription factor Nur77 belongs to the NR4A subfamily of nuclear hormone receptors. It features an atypical ligand-binding site that precludes canonical ligand binding, leading to the designation orphan nuclear receptor. However, recent studies show that small molecules can interact with the receptor and modulate its activity by inducing a conformational change in the Nur77 ligand-binding site. Nur77 expression and activation are rapidly induced by various physiological and pathologic stimuli. Once expressed, Nur77 initiates transcriptional activity and modulates expression of its target genes. Both in vitro and in vivo evidence shows that Nur77 dampens the immune response to proinflammatory stimuli, such as tumor necrosis factor-α, Toll-like receptor ligands, and oxidized lipids, primarily by suppressing NF-κB signaling. Although studies focusing on Nur77's role in lung pathophysiology are currently incomplete, available data support its involvement in the pathogenesis of lung diseases, including asthma, acute lung injury, and pulmonary fibrosis, and thus suggest a therapeutic potential for Nur77 activation in these diseases. This review addresses the mechanisms that control Nur77 as well as its known roles in inflammation-related lung diseases. Evidence regarding the therapeutic potential of Nur77-targeting molecules will also be presented. Although current knowledge is limited, additional research followed by clinical studies may firmly identify Nur77 as a pharmacologic target for inflammation-related lung diseases.

Targeting nuclear receptor NR4A1-dependent adipocyte progenitor quiescence promotes metabolic adaptation to obesity

Adipocyte turnover in adulthood is low, suggesting that the cellular source of new adipocytes, the adipocyte progenitor (AP), resides in a state of relative quiescence. Yet the core transcriptional regulatory circuitry (CRC) responsible for establishing a quiescent state and the physiological significance of AP quiescence are incompletely understood. Here, we integrate transcriptomic data with maps of accessible chromatin in primary APs, implicating the orphan nuclear receptor NR4A1 in AP cell-state regulation. NR4A1 gain and loss of function in APs ex vivo decreased and enhanced adipogenesis, respectively. Adipose tissue of Nr4a1-/- mice demonstrated higher proliferative and adipogenic capacity compared with that of WT mice. Transplantation of Nr4a1-/- APs into the subcutaneous adipose tissue of WT obese recipients improved metrics of glucose homeostasis relative to administration of WT APs. Collectively, these data identify NR4A1 as a previously unrecognized constitutive regulator of AP quiescence and suggest that augmentation of adipose tissue plasticity may attenuate negative metabolic sequelae of obesity.

Role of CD1d- and MR1-Restricted T Cells in Asthma

Innate T lymphocytes are a group of relatively recently identified T cells that are not involved in either innate or adaptive immunity. Unlike conventional T cells, most innate T lymphocytes express invariant T cell receptor to recognize exogenous non-peptide antigens presented by a family of non-polymorphic MHC class I-related molecules, such as CD1d and MHC-related molecule-1 (MR1). Invariant natural killer T (iNKT) cells and mucosal-associated invariant T (MAIT) cells quickly respond to the antigens bound to CD1d and MR1 molecules, respectively, and immediately exert effector functions by secreting various cytokines and granules. This review describes the detrimental and beneficial roles of iNKT cells in animal models of asthma and in human asthmatic patients and also addresses the mechanisms through which iNKT cells are activated by environmental or extracellular factors. We also discuss the potential for therapeutic interventions of asthma by specific antibodies against NKT cells. Furthermore, we summarize the recent reports on the role of MAIT cells in allergic diseases.

The Impact of Early-Life Exposure to Air-borne Environmental Insults on the Function of the Airway Epithelium in Asthma

The airway epithelium is both a physical barrier protecting the airways from environmental insults and a significant component of the innate immune response. There is growing evidence that exposure of the airway epithelium to environmental insults in early life may lead to permanent changes in structure and function that underlie the development of asthma. Here we review the current published evidence concerning the link between asthma and epithelial damage within the airways and identify gaps in knowledge for future studies.

Recent advances in the mechanisms of lung alveolarization and the pathogenesis of bronchopulmonary dysplasia

Alveolarization is the process by which the alveoli, the principal gas exchange units of the lung, are formed. Along with the maturation of the pulmonary vasculature, alveolarization is the objective of late lung development. The terminal airspaces that were formed during early lung development are divided by the process of secondary septation, progressively generating an increasing number of alveoli that are of smaller size, which substantially increases the surface area over which gas exchange can take place. Disturbances to alveolarization occur in bronchopulmonary dysplasia (BPD), which can be complicated by perturbations to the pulmonary vasculature that are associated with the development of pulmonary hypertension. Disturbances to lung development may also occur in persistent pulmonary hypertension of the newborn in term newborn infants, as well as in patients with congenital diaphragmatic hernia. These disturbances can lead to the formation of lungs with fewer and larger alveoli and a dysmorphic pulmonary vasculature. Consequently, affected lungs exhibit a reduced capacity for gas exchange, with important implications for morbidity and mortality in the immediate postnatal period and respiratory health consequences that may persist into adulthood. It is the objective of this Perspectives article to update the reader about recent developments in our understanding of the molecular mechanisms of alveolarization and the pathogenesis of BPD.