Activation of neurokinin-1 receptors during ozone inhalation contributes to epithelial injury and repair

Air Pollutant impacts on the brain and neuroendocrine system with implications for peripheral organs: a perspective

Air pollutants are being increasingly linked to extrapulmonary multi-organ effects. Specifically, recent studies associate air pollutants with brain disorders including psychiatric conditions, neuroinflammation and chronic diseases. Current evidence of the linkages between neuropsychiatric conditions and chronic peripheral immune and metabolic diseases provides insights on the potential role of the neuroendocrine system in mediating neural and systemic effects of inhaled pollutants (reactive particulates and gases). Autonomically-driven stress responses, involving sympathetic-adrenal-medullary and hypothalamus-pituitary-adrenal axes regulate cellular physiological processes through adrenal-derived hormones and diverse receptor systems. Recent experimental evidence demonstrates the contribution of the very stress system responding to non-chemical stressors, in mediating systemic and neural effects of reactive air pollutants. The assessment of how respiratory encounter of air pollutants induce lung and peripheral responses through brain and neuroendocrine system, and how the impairment of these stress pathways could be linked to chronic diseases will improve understanding of the causes of individual variations in susceptibility and the contribution of habituation/learning and resiliency. This review highlights effects of air pollution in the respiratory tract that impact the brain and neuroendocrine system, including the role of autonomic sensory nervous system in triggering neural stress response, the likely contribution of translocated nano particles or metal components, and biological mediators released systemically in causing effects remote to the respiratory tract. The perspective on the use of systems approaches that incorporate multiple chemical and non-chemical stressors, including environmental, physiological and psychosocial, with the assessment of interactive neural mechanisms and peripheral networks are emphasized.

The redox modulatory effects of SP/NK1R system: Implications for oxidative stress-associated disorders

Oxidative stress which refers to redox imbalance with increased generation of reactive oxygen species (ROS) has been associated with the pathophysiology of diverse disease conditions. Recently, a close, yet not fully understood, relation between oxidative stress and neuropeptides, in particular, substance P (SP), has been reported in certain conditions. SP has been shown to affect the cellular redox environment through activation of neurokinin-1receptor (NK1R). It seems that SP/NK1R system and oxidative stress can act either synergistically or antagonistically in a context-dependent manner, thereby, influencing the pathology of various clinical disorders either destructively or protectively. Importantly, the interactions between oxidative stress and SP/NK1R system can be pharmacologically targeted. Therefore, a better understanding of the redox modulatory properties of SP/NK1R signaling will pave the way for identifying new therapeutic possibilities for attenuating oxidative stress-mediated damage. Towards this end, we performed a comprehensive search through PubMed/Medline and Scopus databases and discussed all related existing literature regarding the interplay between oxidative stress and SP/NK1R system as well as their implication in various clinical disorders, to provide a clear view and hence better management of oxidative damage.

Aprepitant: an antiemetic drug, contributes to the prevention of acute lung injury with its anti-inflammatory and antioxidant properties

OBJECTIVES

We investigated, the effects of aprepitant (APRE) on the lung tissues of rats with an experimental polymicrobial sepsis model (CLP: cecal ligation and puncture) biochemically, molecularly and histopathologically.

METHODS

A total of 40 rats were divided into 5 groups with 8 animals in each group. Group 1 (SHAM), control group; Group 2 (CLP), cecal ligation and puncture; Group 3 (CLP + APRE10), rats were administered CLP + 10 mg/kg aprepitant; Group 4 (CLP + APRE20), rats were administered CLP + 20 mg/kg aprepitant; and Group 5 (CLP + APRE40), rats were administered CLP + 40 mg/kg aprepitant. A polymicrobial sepsis model was induced with CLP. After 16 h, lung tissues were taken for examination. Tumour necrosis factor α (TNF-α) and nuclear factor-kappa b (NFK-b) messenger ribonucleic acid (mRNA) expressions were analysed by real-time PCR (RT-PCR), biochemically antioxidant parameters such as superoxide dismutase (SOD) and glutathione (GSH) and oxidant parameters such as malondialdehyde (MDA) and lung damage histopathologically.

KEY FINDINGS AND CONCLUSIONS

The GSH level and SOD activity increased while the MDA level and the expressions of TNF-α and NFK-b were reduced in the groups treated with APRE, especially in the CLP + APRE40 group. The histopathology results supported the molecular and biochemical results.

Nur77 attenuates endothelin-1 expression via downregulation of NF-κB and p38 MAPK in A549 cells and in an ARDS rat model

Acute respiratory distress syndrome (ARDS) is characterized by inflammatory injury to the alveolar and capillary barriers that results in impaired gas exchange and severe acute respiratory failure. Nuclear orphan receptor Nur77 has emerged as a regulator of gene expression in inflammation, and its role in the pathogenesis of ARDS is not clear. The objective of this study is to investigate the potential role of Nur77 and its underlying mechanism in the regulation of endothelin-1 (ET-1) expression in lipopolysaccharide (LPS)-induced A549 cells and an ARDS rat model. We demonstrate that LPS induced Nur77 expression and nuclear export in A549 cells. Overexpression of Nur77 markedly decreased basal and LPS-induced ET-1 expression in A549 cells, whereas knockdown of Nur77 increased the ET-1 expression. LPS-induced phosphorylation and nuclear translocation of NF-κB and p38 MAPK were blocked by Nur77 overexpression and augmented by Nur77 knockdown in A549 cells. In vivo, LPS induced Nur77 expression in lung in ARDS rats. Pharmacological activation of Nur77 by cytosporone B (CsnB) inhibited ET-1 expression in ARDS rats, decreased LPS-induced phosphorylation of NF-κB and p38 MAPK, and relieved lung, liver, and kidney injury. Pharmacological deactivation of Nur77 by 1,1-bis-(3'-indolyl)-1-(p-hydroxyphenyl)methane (DIM-C-pPhOH, C-DIM8) had no effect on ET-1 expression and lung injury. These results indicated that Nur77 decreases ET-1 expression by suppressing NF-κB and p38 MAPK in LPS-stimulated A549 cells in vitro, and, in an LPS-induced ARDS rat model, CsnB reduced ET-1 expression and lung injury in ARDS rats.

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

Children are uniquely susceptible to ozone because airway and lung growth continue for an extensive period after birth. Early-life exposure of the rhesus monkey to repeated ozone cycles results in region-specific disrupted airway/lung growth, but the mediators and mechanisms are poorly understood. Substance P (SP), neurokinin-1 receptor (NK-1R); and nuclear receptor Nur77 (NR4A1) are signaling pathway components involved in ozone-induced cell death. We hypothesize that acute ozone (AO) exposure during postnatal airway development disrupts SP/NK-1R/Nur77 pathway expression and that these changes correlate with increased ozone-induced cell death. Our objectives were to 1) spatially define the normal development of the SP/NK-1R/Nur77 pathway in conducting airways; 2) compare how postnatal age modulates responses to AO exposure; and 3) determine how concomitant, episodic ozone exposure modifies age-specific acute responses. Male infant rhesus monkeys were assigned at age 1 mo to two age groups, 2 or 6 mo, and then to one of three exposure subgroups: filtered air (FA), FA+AO (AO: 8 h/day × 2 days), or episodic biweekly ozone exposure cycles (EAO: 8 h/day × 5 days/14-day cycle+AO). O3 = 0.5 ppm. We found that 1) ozone increases SP/NK-1R/Nur77 pathway expression in conducting airways, 2) an ozone exposure cycle (5 days/cycle) delivered early at age 2 mo resulted in an airway that was hypersensitive to AO exposure at the end of 2 mo, and 3) continued episodic exposure (11 cycles) resulted in an airway that was hyposensitive to AO exposure at 6 mo. These observations collectively associate with greater overall inflammation and epithelial cell death, particularly in early postnatal (2 mo), distal airways.

Effect of Substance P on type II alveolar epithelial cells exposed to hyperoxia and its regulation of the Sonic hedgehog signaling pathway

Oxidative stress injury and cell death in alveolar epithelial cells may lead to abnormal repair, further resulting in acute and chronic pulmonary diseases. Substance P (SP) has multiple biological activities. The Sonic hedgehog (SHH) pathway is important in lung development and decreasing epithelial injury. To investigate the effects of SP on alveolar epithelial type II cells (AEC IIs), AEC IIs were exposed to 95% oxygen and the SHH signaling pathway was examined. Primary AEC IIs were isolated and purified from premature rats. The cells were divided into four groups: The air (21% oxygen) group, hyperoxia (95% oxygen) group, hyperoxia + SP group and hyperoxia + SP + L703.606 group. The activity of AEC IIs was examined using a 3‑(4,5‑dimethylthiazol‑2‑yl)‑2,5‑diphenyltetrazolium bromide assay. The apoptotic rate of AEC IIs was analyzed by flow cytometry. The oxidative damage was evaluated by flow cytometry and reactive oxygen species (ROS) were detected using a 2',7'‑dichlorodihydrofluorescein diacetate probe. Quantitative polymerase chain reaction and western blotting were used to detect the mRNA and protein expression of the SHH signaling molecule Smoothened (SMO). The results demonstrated that exposure to 95% oxygen for 24 h significantly increased the level of ROS, contributed to apoptosis and markedly decreased the proliferation of AEC IIs. Compared with hyperoxia exposure, SP treatment decreased the level of ROS, reduced AEC II apoptosis and improved the cell survival sequentially. SMO was found to be expressed in AEC IIs and its expression increased when the cells were in hyperoxic conditions. These effects were enhanced by treatment with SP, which was able to significantly increase the expression of SMO. The aforementioned protective effect was weakened following treatment with L703.606. These findings suggested that SP was a protective regulatory factor that was able to decrease the hyperoxia‑induced cell injury and death, and improve the survival of AEC IIs exposed to hyperoxia, which may be associated with the activation of the SHH signaling pathway.

Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease

The tachykinins, exemplified by substance P, are one of the most intensively studied neuropeptide families. They comprise a series of structurally related peptides that derive from alternate processing of three Tac genes and are expressed throughout the nervous and immune systems. Tachykinins interact with three neurokinin G protein-coupled receptors. The signaling, trafficking, and regulation of neurokinin receptors have also been topics of intense study. Tachykinins participate in important physiological processes in the nervous, immune, gastrointestinal, respiratory, urogenital, and dermal systems, including inflammation, nociception, smooth muscle contractility, epithelial secretion, and proliferation. They contribute to multiple diseases processes, including acute and chronic inflammation and pain, fibrosis, affective and addictive disorders, functional disorders of the intestine and urinary bladder, infection, and cancer. Neurokinin receptor antagonists are selective, potent, and show efficacy in models of disease. In clinical trials there is a singular success: neurokinin 1 receptor antagonists to treat nausea and vomiting. New information about the involvement of tachykinins in infection, fibrosis, and pruritus justifies further trials. A deeper understanding of disease mechanisms is required for the development of more predictive experimental models, and for the design and interpretation of clinical trials. Knowledge of neurokinin receptor structure, and the development of targeting strategies to disrupt disease-relevant subcellular signaling of neurokinin receptors, may refine the next generation of neurokinin receptor antagonists.

Substance P attenuates hyperoxia‑induced lung injury in neonatal rats

The aim of the study was to investigate the effects of substance P (SP) in hyperoxia‑induced lung injury in newborn rats and to elucidate its protective mechanism of action via the sonic hedgehog (SHH) signaling pathway. Twelve‑hour‑old neonatal Sprague‑Dawley rats were randomly divided into one of four groups: air, hyperoxia, air + SP and hyperoxia + SP. In a separate set of experiments, the neonatal rat pups were exposed to 21 or 95% O2 for 14 days with or without intraperitoneal administration of rat SP. The animals were sacrificed at 3, 7 and 14 days, respectively, of hyperoxia exposure. Lung pathology and grade of lung tissue injury were examined by light microscopy. Oxidative stress was evaluated by malondialdehyde (MDA) and antioxidant activity was measured by superoxide dismutase (SOD) in tissue homogenates. The expression of SHH mRNA and protein were detected by quantitative polymerase chain reaction (qPCR) and western blot analysis, respectively. In the hyperoxia group, marked characteristics of acute lung injury (ALI) were observed. Compared with the simple hyperoxia treatment, the lung damage was significantly ameliorated following the addition of SP. Furthermore, the levels of MDA were decreased and SOD was significantly increased following the addition of SP. SP stimulation may result in activation of the SHH signaling pathway and the expression of SHH markedly increased following treatment with SP. The present study demonstrated that SP protected against the hyperoxia‑induced lung damage by attenuating oxidative stress, elevating the antioxidant activities and upregulating the signaling pathway of SHH.

Ozone exposure alters serotonin and serotonin receptor expression in the developing lung

Ozone, a pervasive environmental pollutant, adversely affects functional lung growth in children. Animal studies demonstrate that altered lung development is associated with modified signaling within the airway epithelial mesenchymal trophic unit, including mediators that can change nerve growth. We hypothesized that ozone exposure alters the normal pattern of serotonin, its transporter (5-HTT), and two key receptors (5-HT2A and 5-HT4), a pathway involved in postnatal airway neural, epithelial, and immune processes. We exposed monkeys to acute or episodic ozone during the first 2 or 6 months of life. There were three exposure groups/age: (1) filtered air, (2) acute ozone challenge, and (3) episodic ozone + acute ozone challenge. Lungs were prepared for compartment-specific qRT-PCR, immunohistochemistry, and stereology. Airway epithelial serotonin immunopositive staining increased in all exposure groups with the most prominent in 2-month midlevel and 6-month distal airways. Gene expression of 5-HTT, 5-HT2AR, and 5-HT4R increased in an age-dependent manner. Overall expression was greater in distal compared with midlevel airways. Ozone exposure disrupted both 5-HT2AR and 5-HT4R protein expression in airways and enhanced immunopositive staining for 5-HT2AR (2 months) and 5-HT4R (6 months) on smooth muscle. Ozone exposure increases serotonin in airway epithelium regardless of airway level, age, and exposure history and changes the spatial pattern of serotonin receptor protein (5-HT2A and 5-HT4) and 5-HTT gene expression depending on compartment, age, and exposure history. Understanding how serotonin modulates components of reversible airway obstruction exacerbated by ozone exposure sets the foundation for developing clinically relevant therapies for airway disease.

Postnatal exposure history and airways: oxidant stress responses in airway explants

Postnatally, the lung continues to grow and differentiate while interacting with the environment. Exposure to ozone (O(3)) and allergens during postnatal lung development alters structural elements of conducting airways, including innervation and neurokinin abundance. These changes have been linked with development of asthma in a rhesus monkey model. We hypothesized that O(3) exposure resets the ability of the airways to respond to oxidant stress and that this is mediated by changes in the neurokinin-1 receptor (NK-1R). Infant rhesus monkeys received episodic exposure to O(3) biweekly with or without house dust mite antigen (HDMA) from 6 to 12 months of age. Age-matched monkeys were exposed to filtered air (FA). Microdissected airway explants from midlevel airways (intrapulmonary generations 5-8) for four to six animals in each of four groups (FA, O(3), HDMA, and HDMA+O(3)) were tested for NK-1R gene responses to acute oxidant stress using exposure to hydrogen peroxide (1.2 mM), a lipid ozonide (10 μM), or sham treatment for 4 hours in vitro. Airway responses were measured using real-time quantitative RT-PCR of NK-1R and IL-8 gene expression. Basal NK-1R gene expression levels were not different between the exposure groups. Treatment with ozonide or hydrogen peroxide did not change NK-1R gene expression in animals exposed to FA, HDMA, or HDMA+O(3). However, treatment in vitro with lipid ozonide significantly increased NK-1R gene expression in explants from O(3)-exposed animals. We conclude that a history of prior O(3) exposure resets the steady state of the airways to increase the NK-1R response to subsequent acute oxidant stresses.

SERCA2 regulates non-CF and CF airway epithelial cell response to ozone

Calcium mobilization can regulate a wide range of essential functions of respiratory epithelium, including ion transport, ciliary beat frequency, and secretion of mucus, all of which are modified in cystic fibrosis (CF). SERCA2, an important controller of calcium signaling, is deficient in CF epithelium. We conducted this study to determine whether SERCA2 deficiency can modulate airway epithelial responses to environmental oxidants such as ozone. This could contribute to the pathogenesis of pulmonary exacerbations, which are important and frequent clinical events in CF. To address this, we used air-liquid interface (ALI) cultures of non-CF and CF cell lines, as well as differentiated cultures of cells derived from non-CF and CF patients. We found that ozone exposure caused enhanced membrane damage, mitochondrial dysfunction and apoptotic cell death in CF airway epithelial cell lines relative to non-CF. Ozone exposure caused increased proinflammatory cytokine production in CF airway epithelial cell lines. Elevated proinflammatory cytokine production also was observed in shRNA-mediated SERCA2 knockdown cells. Overexpression of SERCA2 reversed ozone-induced proinflammatory cytokine production. Ozone-induced proinflammatory cytokine production was NF-κB- dependent. In a stable NF-κB reporter cell line, SERCA2 inhibition and knockdown both upregulated cytomix-induced NF-κB activity, indicating importance of SERCA2 in modulating NF-κB activity. In this system, increased NF-κB activity was also accompanied by increased IL-8 production. Ozone also induced NF-κB activity and IL-8 release, an effect that was greater in SERCA2-silenced NF-κB-reporter cells. SERCA2 overexpression reversed cytomix-induced increased IL-8 release and total nuclear p65 in CFTR-deficient (16HBE-AS) cells. These studies suggest that SERCA2 is an important regulator of the proinflammatory response of airway epithelial cells and could be a potential therapeutic target.

Involvement of preprotachykinin A gene-encoded peptides and the neurokinin 1 receptor in endotoxin-induced murine airway inflammation

Tachykinins encoded by the preprotachykinin A (TAC1) gene such as substance P (SP) and neurokinin A (NKA) are involved in neurogenic inflammatory processes via predominantly neurokinins 1 and 2 (NK1 and NK2) receptor activation, respectively. Endokinins and hemokinins encoded by the TAC4 gene also have remarkable selectivity and potency for the NK1 receptors and might participate in inflammatory cell functions. The aim of the present study was to investigate endotoxin-induced airway inflammation and consequent bronchial hyper-reactivity in TAC1(-/-), NK1(-/-) and also in double knockout (TAC1(-/-)/NK1(-/-)) mice. Sub-acute interstitial lung inflammation was evoked by intranasal Escherichia coli lipopolysaccharide (LPS) in the knockout mice and their wildtype C57BL/6 counterparts 24 h before measurement. Respiratory parameters were measured with unrestrained whole body plethysmography. Bronchoconstriction was induced by inhalation of the muscarinic receptor agonist carbachol and Penh (enhanced pause) correlating with airway resistance was calculated. Lung interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) concentrations were measured with ELISA. Histological evaluation was performed and a composite morphological score was determined. Myeloperoxidase (MPO) activity in the lung was measured with spectrophotometry to quantify the number of infiltrating neutrophils/macrophages. Airway hyper-reactivity was significantly reduced in the TAC1(-/-) as well as the TAC1(-/-)/NK1(-/-) groups. However, LPS-induced histological inflammatory changes (perivascular/peribronchial oedema, neutrophil infiltration and goblet cell hyperplasia), MPO activity and TNF-alpha concentration were markedly diminished only in TAC1(-/-) mice. Interestingly, the concentrations of both cytokines, IL-1beta and TNF-alpha, were significantly greater in the NK1(-/-) group. These data clearly demonstrated on the basis of histology, MPO and cytokine measurements that TAC1 gene-derived tachykinins, SP and NKA, play a significant role in the development of endotoxin-induced murine airway inflammation, but not solely via NK1 receptor activation. However, in inflammatory bronchial hyper-responsiveness other tachykinins, such as hemokinin-1 acting through NK1 receptors also might be involved.

Apoptosis induced by ozone and oxysterols in human alveolar epithelial cells

The mechanism of ozone-induced lung cell injury is poorly understood. One hypothesis is that ozone induces lipid peroxidation and that these peroxidated lipids produce oxidative stress and DNA damage. Oxysterols are lipid peroxides formed by the direct effects of ozone on pulmonary surfactant and cell membranes. We studied the effects of ozone and the oxysterol 5beta,6beta-epoxycholesterol (beta-epoxide) and its metabolite cholestan-6-oxo-3,5-diol (6-oxo-3,5-diol) on human alveolar epithelial type I-like cells (ATI-like cells) and type II cells (ATII cells). Ozone and oxysterols induced apoptosis and cytotoxicity in ATI-like cells. They also generated reactive oxygen species and DNA damage. Ozone and beta-epoxide were strong inducers of nuclear factor erythroid 2-related factor 2, heat shock protein 70, and Fos-related antigen 1 protein expression. Furthermore, we found higher sensitivity of ATI-like cells compared to ATII cells exposed to ozone or treated with beta-epoxide or 6-oxo-3,5-diol. In general the response to the cholesterol epoxides was similar to the effect of ozone. Understanding the response of human ATI-like cells and ATII cells to oxysterols may be useful for further studies, because these compounds may represent useful biomarkers in other diseases.

Neural control of airway inflammation

Abnormal neural function contributes to the pathogenesis of airway disease. In addition to affecting airway physiology, the nerves produce and release inflammatory mediators, contributing to the recruitment and activation of leukocytes. Activated inflammatory cells in turn affect the function of airway nerves, changing the production and release of neurotransmitters. Cross-talk between airway nerves and leukocytes helps to maintain chronic inflammation and accentuates neural control of the airways.

Ozone activates airway nerves via the selective stimulation of TRPA1 ion channels

Inhalation of ozone is a major health risk in industrialized nations. Ozone can impair lung function and induce respiratory symptoms through sensory neural-mediated pathways, yet the specific interaction of ozone with airway sensory nerves has yet to be elucidated. Here we demonstrate, using a vagally innervated ex vivo tracheal-lung mouse preparation, that ozone selectively and directly evokes action potential discharge in a subset of nociceptive bronchopulmonary nerves, namely slow conducting C-fibres. Sensitivity to ozone correlated with the transient receptor potential (TRP) A1 agonist, cinnamaldehyde, with ozone having no effect on cinnamaldehyde-insensitive fibres. C-fibre responses to ozone were abolished by ruthenium red (TRP inhibitor). Ozone also stimulated a subset of nociceptive sensory neurones isolated from vagal ganglia of wild-type mice, but failed to activate neurones isolated from transient receptor potential ankyrin 1 (TRPA1) knockout mice. Ozone activated HEK293 cells transfected with TRPA1, but failed to activate non-transfected HEK293 or HEK293 transfected with the capsaicin-sensitive transient receptor potential vanilloid 1 (TRPV1) channel. Thus, ozone is not an indiscriminate neuronal activator, but rather it potently and selectively activates a subset of airway C-fibres by directly stimulating TRPA1.

Activation of calcitonin gene-related peptide receptor during ozone inhalation contributes to airway epithelial injury and repair

The authors investigated the importance of the neuropeptide, calcitonin gene-related peptide (CGRP), in epithelial injury, repair, and neutrophil emigration after ozone exposure. Wistar rats were administered either a CGRP-receptor antagonist (CGRP(8-37)) or saline and exposed to 8 hours of 1-ppm ozone or filtered air with an 8-hour postexposure period. Immediately after exposure, ethidium homodimer was instilled into lungs as a marker of necrotic airway epithelial cells. After fixation, airway dissected lung lobes were stained for 5'-bromo-2'-deoxyuridine, a marker of epithelial proliferation. Positive epithelial cells were quantified in specific airway generations. Rats treated with CGRP(8-37) had significantly reduced epithelial injury in terminal bronchioles and reduced epithelial proliferation in proximal airways and terminal bronchioles. Bronchoalveolar lavage and sections of terminal bronchioles showed no significant difference in the number of neutrophils emigrating into airways in CGRP(8-37)-treated rats. The airway epithelial cell line, HBE-1, showed no difference in the number of oxidant stress positive cells during exposure to hydrogen peroxide and a range of CGRP(8-37) doses, demonstrating no antioxidant effect of CGRP(8-37). We conclude that activation of CGRP receptors during ozone inhalation contributes to airway epithelial injury and subsequent epithelial proliferation, a critical component of repair, but does not influence neutrophil emigration into airways.