Metabolic shift in lung alveolar cell mitochondria following acrolein exposure

Countermeasures against Pulmonary Threat Agents

Inhaled toxicants are used for diverse purposes, ranging from industrial applications such as agriculture, sanitation, and fumigation to crowd control and chemical warfare, and acute exposure can induce lasting respiratory complications. The intentional release of chemical warfare agents (CWAs) during World War I caused life-long damage for survivors, and CWA use is outlawed by international treaties. However, in the past two decades, chemical warfare use has surged in the Middle East and Eastern Europe, with a shift toward lung toxicants. The potential use of industrial and agricultural chemicals in rogue activities is a major concern as they are often stored and transported near populated areas, where intentional or accidental release can cause severe injuries and fatalities. Despite laws and regulatory agencies that regulate use, storage, transport, emissions, and disposal, inhalational exposures continue to cause lasting lung injury. Industrial irritants (e.g., ammonia) aggravate the upper respiratory tract, causing pneumonitis, bronchoconstriction, and dyspnea. Irritant gases (e.g., acrolein, chloropicrin) affect epithelial barrier integrity and cause tissue damage through reactive intermediates or by direct adduction of cysteine-rich proteins. Symptoms of CWAs (e.g., chlorine gas, phosgene, sulfur mustard) progress from airway obstruction and pulmonary edema to acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), which results in respiratory depression days later. Emergency treatment is limited to supportive care using bronchodilators to control airway constriction and rescue with mechanical ventilation to improve gas exchange. Complications from acute exposure can promote obstructive lung disease and/or pulmonary fibrosis, which require long-term clinical care. SIGNIFICANCE STATEMENT: Inhaled chemical threats are of growing concern in both civilian and military settings, and there is an increased need to reduce acute lung injury and delayed clinical complications from exposures. This minireview highlights our current understanding of acute toxicity and pathophysiology of a select number of chemicals of concern. It discusses potential early-stage therapeutic development as well as challenges in developing countermeasures applicable for administration in mass casualty situations.

The Tobacco Smoke Component, Acrolein, as a Major Culprit in Lung Diseases and Respiratory Cancers: Molecular Mechanisms of Acrolein Cytotoxic Activity

Acrolein, a highly reactive unsaturated aldehyde, is a ubiquitous environmental pollutant that seriously threatens human health and life. Due to its high reactivity, cytotoxicity and genotoxicity, acrolein is involved in the development of several diseases, including multiple sclerosis, neurodegenerative diseases such as Alzheimer’s disease, cardiovascular and respiratory diseases, diabetes mellitus and even the development of cancer. Traditional tobacco smokers and e-cigarette users are particularly exposed to the harmful effects of acrolein. High concentrations of acrolein have been found in both mainstream and side-stream tobacco smoke. Acrolein is considered one of cigarette smoke’s most toxic and harmful components. Chronic exposure to acrolein through cigarette smoke has been linked to the development of asthma, acute lung injury, chronic obstructive pulmonary disease (COPD) and even respiratory cancers. This review addresses the current state of knowledge on the pathological molecular mechanisms of acrolein in the induction, course and development of lung diseases and cancers in smokers.

Impact of sub-acute acrolein inhalation on the molecular regulation of mitochondrial metabolism in rat lung

Nowadays, mitochondria are recognized as key players in the pathogenesis of a variety of smoking-associated lung diseases. Acrolein, a component of cigarette smoke, is a known driver of biological mechanisms underlying smoking-induced respiratory toxicity. The impact of sub-acute acrolein inhalation in vivo on key processes controlling mitochondrial homeostasis in cells of the airways however is unknown. In this study, we investigated the activity/abundance of a myriad of molecules critically involved in mitochondrial metabolic pathways and mitochondrial quality control processes (mitochondrial biogenesis and mitophagy) in the lungs of Sprague-Dawley rats that were sub-acutely exposed to filtered air or 3 ppm acrolein by whole-body inhalation (5 h/day, 5 days/week for 4 weeks). Acrolein exposure induced a general inflammatory response in the lung as gene expression analysis revealed an increased expression of Icam1 and Cinc1 (p < 0.1; p < 0.05). Acrolein significantly decreased enzyme activity of hydroxyacyl-Coenzyme A dehydrogenase (p < 0.01), and decreased Pdk4 transcript levels (p < 0.05), suggestive of acrolein-induced changes in metabolic processes. Investigation of constituents of the mitochondrial biogenesis pathways and mitophagy machinery revealed no pronounced alterations. In conclusion, sub-acute inhalation of acrolein did not affect the regulation of mitochondrial metabolism and quality control, which is in contrast to more profound changes after acute exposure in other studies.

Disruption of the Molecular Regulation of Mitochondrial Metabolism in Airway and Lung Epithelial Cells by Cigarette Smoke: Are Aldehydes the Culprit?

Chronic obstructive pulmonary disease (COPD) is a devastating lung disease for which cigarette smoking is the main risk factor. Acetaldehyde, acrolein, and formaldehyde are short-chain aldehydes known to be formed during pyrolysis and combustion of tobacco and have been linked to respiratory toxicity. Mitochondrial dysfunction is suggested to be mechanistically and causally involved in the pathogenesis of smoking-associated lung diseases such as COPD. Cigarette smoke (CS) has been shown to impair the molecular regulation of mitochondrial metabolism and content in epithelial cells of the airways and lungs. Although it is unknown which specific chemicals present in CS are responsible for this, it has been suggested that aldehydes may be involved. Therefore, it has been proposed by the World Health Organization to regulate aldehydes in commercially-available cigarettes. In this review, we comprehensively describe and discuss the impact of acetaldehyde, acrolein, and formaldehyde on mitochondrial function and content and the molecular pathways controlling this (biogenesis versus mitophagy) in epithelial cells of the airways and lungs. In addition, potential therapeutic applications targeting (aldehyde-induced) mitochondrial dysfunction, as well as regulatory implications, and the necessary required future studies to provide scientific support for this regulation, have been covered in this review.

Smoking-Associated Exposure of Human Primary Bronchial Epithelial Cells to Aldehydes: Impact on Molecular Mechanisms Controlling Mitochondrial Content and Function

Chronic obstructive pulmonary disease (COPD) is a devastating lung disease primarily caused by exposure to cigarette smoke (CS). During the pyrolysis and combustion of tobacco, reactive aldehydes such as acetaldehyde, acrolein, and formaldehyde are formed, which are known to be involved in respiratory toxicity. Although CS-induced mitochondrial dysfunction has been implicated in the pathophysiology of COPD, the role of aldehydes therein is incompletely understood. To investigate this, we used a physiologically relevant in vitro exposure model of differentiated human primary bronchial epithelial cells (PBEC) exposed to CS (one cigarette) or a mixture of acetaldehyde, acrolein, and formaldehyde (at relevant concentrations of one cigarette) or air, in a continuous flow system using a puff-like exposure protocol. Exposure of PBEC to CS resulted in elevated IL-8 cytokine and mRNA levels, increased abundance of constituents associated with autophagy, decreased protein levels of molecules associated with the mitophagy machinery, and alterations in the abundance of regulators of mitochondrial biogenesis. Furthermore, decreased transcript levels of basal epithelial cell marker KRT5 were reported after CS exposure. Only parts of these changes were replicated in PBEC upon exposure to a combination of acetaldehyde, acrolein, and formaldehyde. More specifically, aldehydes decreased MAP1LC3A mRNA (autophagy) and BNIP3 protein (mitophagy) and increased ESRRA protein (mitochondrial biogenesis). These data suggest that other compounds in addition to aldehydes in CS contribute to CS-induced dysregulation of constituents controlling mitochondrial content and function in airway epithelial cells.

Phenotypic and Metabolomic Characterization of 3D Lung Cell Cultures Exposed to Airborne Particulate Matter from Three Air Quality Network Stations in Catalonia

Air pollution constitutes an environmental problem that it is known to cause many serious adverse effects on the cardiovascular and respiratory systems. The chemical characterization of particulate matter (PM) is key for a better understanding of the associations between chemistry and toxicological effects. In this work, the chemical composition and biological effects of fifteen PM₁₀ air filter samples from three air quality stations in Catalonia with contrasting air quality backgrounds were investigated. Three-dimensional (3D) lung cancer cell cultures were exposed to these sample extracts, and cytotoxicity, reactive oxygen species (ROS) induction, metabolomics, and lipidomics were explored. The factor analysis method Multivariate Curve Resolution-Alternating Least-Squares (MCR-ALS) was employed for an integrated interpretation of the associations between chemical composition and biological effects, which could be related to urban traffic emission, biomass burning smoke, and secondary aerosols. In this pilot study, a novel strategy combining new approach methodologies and chemometrics provided new insights into the biomolecular changes in lung cells associated with different sources of air pollution. This approach can be applied in further research on air pollution toxicity to improve our understanding of the causality between chemistry and its effects.

Acrolein inhalation acutely affects the regulation of mitochondrial metabolism in rat lung

Exposure of the airways to cigarette smoke (CS) is the primary risk factor for developing several lung diseases such as Chronic Obstructive Pulmonary Disease (COPD). CS consists of a complex mixture of over 6000 chemicals including the highly reactive α,β-unsaturated aldehyde acrolein. Acrolein is thought to be responsible for a large proportion of the non-cancer disease risk associated with smoking. Emerging evidence suggest a key role for CS-induced abnormalities in mitochondrial morphology and function in airway epithelial cells in COPD pathogenesis. Although in vitro studies suggest acrolein-induced mitochondrial dysfunction in airway epithelial cells, it is unknown if in vivo inhalation of acrolein affects mitochondrial content or the pathways controlling this. In this study, rats were acutely exposed to acrolein by inhalation (nose-only; 0-4 ppm), 4 h/day for 1 or 2 consecutive days (n = 6/group). Subsequently, the activity and abundance of key constituents of mitochondrial metabolic pathways as well as expression of critical proteins and genes controlling mitochondrial biogenesis and mitophagy were investigated in lung homogenates. A transient decreasing response in protein and transcript abundance of subunits of the electron transport chain complexes was observed following acrolein inhalation. Moreover, acrolein inhalation caused a decreased abundance of key regulators associated with mitochondrial biogenesis, respectively a differential response on day 1 versus day 2. Abundance of components of the mitophagy machinery was in general unaltered in response to acrolein exposure in rat lung. Collectively, this study demonstrates that acrolein inhalation acutely and dose-dependently disrupts the molecular regulation of mitochondrial metabolism in rat lung. Hence, understanding the effect of acrolein on mitochondrial function will provide a scientifically supported reasoning to shortlist aldehydes regulation in tobacco smoke.

Global metabolome profiling of exhaled breath condensates in male smokers with asthma COPD overlap and prediction of the disease

Asthma-chronic obstructive pulmonary disease (COPD) overlap, termed as ACO, is a complex heterogeneous disease characterised by persistent airflow limitation, which manifests features of both asthma and COPD. These patients have a worse prognosis, in terms of more frequent and severe exacerbations, more frequent symptoms, worse quality of life, increased comorbidities and a faster lung function decline. In absence of clear diagnostic or therapeutic guidelines, ACO presents as a challenge to clinicians. The present study aims to investigate whether ACO patients have a distinct exhaled breath condensate (EBC) metabolic profile in comparison to asthma and COPD. A total of 132 age and BMI matched male smokers were recruited in the exploratory phase which consisted of (i) controls = 33 (ii) asthma = 34 (iii) COPD = 30 and (iv) ACO = 35. Using nuclear magnetic resonance (NMR) metabolomics, 8 metabolites (fatty acid, propionate, isopropanol, lactate, acetone, valine, methanol and formate) were identified to be significantly dysregulated in ACO subjects when compared to both, asthma and COPD. The expression of these dysregulated metabolites were further validated in a fresh patient cohort consisting of (i) asthma = 32 (ii) COPD = 32 and (iii) ACO = 40, which exhibited a similar expression pattern. Multivariate receiver operating characteristic (ROC) curves generated using these metabolites provided a robust ACO classification model. The findings were also integrated with previously identified serum metabolites and inflammatory markers to develop a robust predictive model for differentiation of ACO. Our findings suggest that NMR metabolomics of EBC holds potential as a platform to identify robust, non-invasive biomarkers for differentiating ACO from asthma and COPD.

Metabolic requirements of pulmonary fibrosis: role of fibroblast metabolism

Fibrosis is a pathologic condition characterized by excessive deposition of extracellular matrix and chronic scaring that can affect every organ system. Organ fibrosis is associated with significant morbidity and mortality, contributing to as many as 45% of all deaths in the developed world. In the lung, many chronic lung diseases may lead to fibrosis, the most devastating being idiopathic pulmonary fibrosis (IPF), which affects approximately 3 million people worldwide and has a median survival of 3.8 years. Currently approved therapies for IPF do not significantly extend lifespan, and thus, there is pressing need for novel therapeutic strategies to treat IPF and other fibrotic diseases. At the heart of pulmonary fibrosis are myofibroblasts, contractile cells with characteristics of both fibroblasts and smooth muscle cells, which are the primary cell type responsible for matrix deposition in fibrotic diseases. Much work has centered around targeting the extracellular growth factors and intracellular signaling regulators of myofibroblast differentiation. Recently, metabolic changes associated with myofibroblast differentiation have come to the fore as targetable mechanisms required for myofibroblast function. In this review, we will discuss the metabolic changes associated with myofibroblast differentiation, as well as the mechanisms by which these changes promote myofibroblast function. We will then discuss the potential for this new knowledge to lead to the development of novel therapies for IPF and other fibrotic diseases.

Cellular Specificity and Inter-cellular Coordination in the Brain Bioenergetic System: Implications for Aging and Neurodegeneration

As an organ with a highly heterogenous cellular composition, the brain has a bioenergetic system that is more complex than peripheral tissues. Such complexities are not only due to the diverse bioenergetic phenotypes of a variety of cell types that differentially contribute to the metabolic profile of the brain, but also originate from the bidirectional metabolic communications and coupling across cell types. While brain energy metabolism and mitochondrial function have been extensively investigated in aging and age-associated neurodegenerative disorders, the role of various cell types and their inter-cellular communications in regulating brain metabolic and synaptic functions remains elusive. In this review, we summarize recent advances in differentiating bioenergetic phenotypes of neurons, astrocytes, and microglia in the context of their functional specificity, and their metabolic shifts upon aging and pathological conditions. Moreover, the metabolic coordination between the two most abundant cell populations in brain, neurons and astrocytes, is discussed regarding how they jointly establish a dynamic and responsive system to maintain brain bioenergetic homeostasis and to combat against threats such as oxidative stress, lipid toxicity, and neuroinflammation. Elucidating the mechanisms by which brain cells with distinctive bioenergetic phenotypes individually and collectively shape the bioenergetic system of the brain will provide rationale for spatiotemporally precise interventions to sustain a metabolic equilibrium that is resilient against synaptic dysfunction in aging and neurodegeneration.

Systemic Immuno-metabolic alterations in chronic obstructive pulmonary disease (COPD)

BACKGROUND

Metabolic adaptation in immune cells is necessary to modulate immune cell function as it is intricately coupled with intracellular metabolism. We aimed to characterize the metabolic state of human peripheral blood mononuclear cells (PBMCs) after long-term exposure to tobacco smoke in smokers with preserved lung function and COPD subjects.

METHODS

PBMCs were isolated from healthy non-smokers (HNS), healthy smokers (HS) and COPD subjects, cultured and the mitochondrial respiration while utilizing glucose (glycolysis), fatty acids (β-oxidation) or pyruvate (direct Krebs' cycle substrate) was measured using the XFp Extracellular Flux Analyzer. Plasma levels of inflammatory cytokines IFN-γ, IL-17, TNF-α, IL-5, IL-9 and IFN-α were measured using flow cytometry. RAW264.7 cells were exposed to cigarette smoke condensate (CSC) for 1 h and its effect on cell viability, cellular metabolism and phagocytosis ability were also studied. Patient's data was analyzed using the Mann Whitney U test, whereas Student's t test was performed to analyze the in-vitro data.

RESULTS

PBMCs from COPD subjects showed a significant decrease in extracellular acidification rate (ECAR) while utilizing glucose as compared to HNS (151.9 Vs 215%). Mitochondrial oxygen consumption rate (OCR) on palmitate or pyruvate was also found to be significantly lower in COPD subjects as compared to HS and a strong positive correlation between palmitate OCR in PBMCs and FEV1 (r = 0.74, p < 0.05) and FVC (r = 0.79, p < 0.05) values in HS was observed. The metabolic shift towards fatty acid metabolism in healthy smokers promoted an inflammatory cytokine response with a greater increase in the levels of IL-5, IL-9 and IFN-α as compared to IFN-γ, IL-17 and TNF-α. In-vitro experiments with RAW 264.7 cells showed similar metabolic alterations and a reduced ability to phagocytose Streptococcus pneumonia and Haemophilus influenza after cigarette smoke exposure in the presence of glucose or palmitate.

CONCLUSIONS

These findings indicate a metabolic basis for the inflammatory response in COPD and could suggest a new therapeutic target for controlling the immune response and delaying the onset of disease.

TRIAL REGISTRATION

This observational study was retrospectively registered in the Clinical Trails Registry - India (ICMR - NIMS) on 19th January 2018 with the registration number CTRI/2018/01/011441 .

1H-NMR-based metabolic profiling of healthy individuals and high-resolution CT-classified phenotypes of COPD with treatment of tiotropium bromide

Background

Heterogeneity of COPD results in different therapeutic effects for different patients receiving the same treatment. COPD patients need to be individually treated according to their own characteristics. The purpose of this study was to explore the differences in different CT phenotypic COPD by molecular metabolites through the use of metabolomics.

Methods

According to the characteristics of CT imaging, 42 COPD patients were grouped into phenotype E (n=20) or phenotype M (n=24). Each COPD patient received tiotropium bromide powder for inhalation for a therapeutic period of 3 months. All subjects were assigned into phenotype E in pre-therapy (EB, n=20), phenotype E in post-therapy (EA, n=20), phenotype M in pre-therapy (MB, n=22), phenotype M in post-therapy (MA, n=22), or normal control (N, n=24). The method of metabolomics based on ¹H nuclear magnetic resonance (¹H-NMR) was used to compare the changes in serum metabolites between COPD patients and normal controls and between different phenotypes of COPD patients in pre- and post-therapy.

Results

Patients with COPD phenotype E responded better to tiotropium bromide than patients with COPD phenotype M in terms of pulmonary function and COPD assessment test scores. There were differences in metabolites in COPD patients vs normal control people. Differences were also observed between different COPD phenotypic patients receiving the treatment in comparison with those who did not receive treatment. The changes of metabolites involved lactate, phenylalanine, fructose, glycine, asparagine, citric acid, pyruvic acid, proline, acetone, ornithine, lipid, pyridoxine, maltose, betaine, lipoprotein, and so on. These identified metabolites covered the metabolic pathways of amino acids, carbohydrates, lipids, genetic materials, and vitamin.

Conclusion

The efficacy of tiotropium bromide on COPD phenotype E is better than that of phenotype M. Metabolites detected by ¹H-NMR metabolomics have potentialities of differentiation of COPD and healthy people, discrimination of different COPD phenotypes, and giving insight into the individualized treatment of COPD.

Exosomal transfer of mitochondria from airway myeloid-derived regulatory cells to T cells

Chronic inflammation involving both innate and adaptive immune cells is implicated in the pathogenesis of asthma. Intercellular communication is essential for driving and resolving inflammatory responses in asthma. Emerging studies suggest that extracellular vesicles (EVs) including exosomes facilitate this process. In this report, we have used a range of approaches to show that EVs contain markers of mitochondria derived from donor cells which are capable of sustaining a membrane potential. Further, we propose that these participate in intercellular communication within the airways of human subjects with asthma. Bronchoalveolar lavage fluid of both healthy volunteers and asthmatics contain EVs with encapsulated mitochondria; however, the % HLA-DR⁺ EVs containing mitochondria and the levels of mitochondrial DNA within EVs were significantly higher in asthmatics. Furthermore, mitochondria are present in exosomes derived from the pro-inflammatory HLA-DR⁺ subsets of airway myeloid-derived regulatory cells (MDRCs), which are known regulators of T cell responses in asthma. Exosomes tagged with MitoTracker Green, or derived from MDRCs transduced with CellLight Mitochondrial GFP were found in recipient peripheral T cells using a co-culture system, supporting direct exosome-mediated cell-cell transfer. Importantly, exosomally transferred mitochondria co-localize with the mitochondrial network and generate reactive oxygen species within recipient T cells. These findings support a potential novel mechanism of cell-cell communication involving exosomal transfer of mitochondria and the bioenergetic and/or redox regulation of target cells.

Reduced oxygen tension culturing conditionally alters toxicogenic response of differentiated H9c2 cardiomyoblasts to acrolein

Acrolein is a reactive electrophilic aldehyde known to cause mitochondrial dysfunction, oxidative stress, and dysregulation of signaling transduction in vitro. Most in vitro systems employ standard cell culture maintenance conditions of 95% air/5% CO₂, translating to a culture oxygen tension of approximately 20%, far above most physiological tissues. The purpose of this investigation was to examine whether low-serum, retinoic acid differentiated H9c2 cells were less sensitive to acrolein insult when cultured under reduced oxygen tension. H9c2 cells were maintained separately in 20% and 5% oxygen, differentiated for 5 d, and then exposed to acrolein for 30 min in media containing varying concentrations of tricarboxylic acid and glycolytic substrates, followed by fresh medium replacement. Cells were then assessed for MTT reduction at 2 h and 24 h after acrolein insult. We showed that pyruvate supplementation in combination with lowered oxygen culturing significantly attenuated acrolein-induced viability loss at 24 h. Poly(ADP-ribose) polymerase inhibition and EGTA preferentially provided partial rescue to low oxygen cultures, but not for standard cultures. Collectively, these results offer evidence supporting altered toxicogenic response of H9c2 during physiologically relevant oxygen tension culturing.

Phenotyping of Chronic Obstructive Pulmonary Disease Based on the Integration of Metabolomes and Clinical Characteristics

Apart from the refined management-oriented clinical stratification of chronic obstructive pulmonary disease (COPD), the molecular pathologies behind this highly prevalent disease have remained obscure. The aim of this study was the characterization of patients with COPD, based on the metabolomic profiling of peripheral blood and exhaled breath condensate (EBC) within the context of defined clinical and demographic variables. Mass-spectrometry-based targeted analysis of serum metabolites (mainly amino acids and lipid species), untargeted profiles of serum and EBC of patients with COPD of different clinical characteristics (n = 25) and control individuals (n = 21) were performed. From the combined clinical/demographic and metabolomics data, associations between clinical/demographic and metabolic parameters were searched and a de novo phenotyping for COPD was attempted. Adjoining the clinical parameters, sphingomyelins were the best to differentiate COPD patients from controls. Unsaturated fatty acid-containing lipids, ornithine metabolism and plasma protein composition-associated signals from the untargeted analysis differentiated the Global Initiative for COPD (GOLD) categories. Hierarchical clustering did not reveal a clinical-metabolomic stratification superior to the strata set by the GOLD consensus. We conclude that while metabolomics approaches are good for finding biomarkers and clarifying the mechanism of the disease, there are no distinct co-variate independent clinical-metabolic phenotypes.

Alda-1 Protects Against Acrolein-Induced Acute Lung Injury and Endothelial Barrier Dysfunction

Inhalation of acrolein, a highly reactive aldehyde, causes lung edema. The underlying mechanism is poorly understood and there is no effective treatment. In this study, we demonstrated that acrolein not only dose-dependently induced lung edema but also promoted LPS-induced acute lung injury. Importantly, acrolein-induced lung injury was prevented and rescued by Alda-1, an activator of mitochondrial aldehyde dehydrogenase 2. Acrolein also dose-dependently increased monolayer permeability, disrupted adherens junctions and focal adhesion complexes, and caused intercellular gap formation in primary cultured lung microvascular endothelial cells (LMVECs). These effects were attenuated by Alda-1 and the antioxidant N-acetylcysteine, but not by the NADPH inhibitor apocynin. Furthermore, acrolein inhibited AMP-activated protein kinase (AMPK) and increased mitochondrial reactive oxygen species levels in LMVECs-effects that were associated with impaired mitochondrial respiration. AMPK total protein levels were also reduced in lung tissue of mice and LMVECs exposed to acrolein. Activation of AMPK with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside blunted an acrolein-induced increase in endothelial monolayer permeability, but not mitochondrial oxidative stress or inhibition of mitochondrial respiration. Our results suggest that acrolein-induced mitochondrial dysfunction may not contribute to endothelial barrier dysfunction. We speculate that detoxification of acrolein by Alda-1 and activation of AMPK may be novel approaches to prevent and treat acrolein-associated acute lung injury, which may occur after smoke inhalation.

Energy metabolism and inflammation in brain aging and Alzheimer's disease

The high energy demand of the brain renders it sensitive to changes in energy fuel supply and mitochondrial function. Deficits in glucose availability and mitochondrial function are well-known hallmarks of brain aging and are particularly accentuated in neurodegenerative disorders such as Alzheimer's disease. As important cellular sources of H₂O₂, mitochondrial dysfunction is usually associated with altered redox status. Bioenergetic deficits and chronic oxidative stress are both major contributors to cognitive decline associated with brain aging and Alzheimer's disease. Neuroinflammatory changes, including microglial activation and production of inflammatory cytokines, are observed in neurodegenerative diseases and normal aging. The bioenergetic hypothesis advocates for sequential events from metabolic deficits to propagation of neuronal dysfunction, to aging, and to neurodegeneration, while the inflammatory hypothesis supports microglia activation as the driving force for neuroinflammation. Nevertheless, growing evidence suggests that these diverse mechanisms have redox dysregulation as a common denominator and connector. An independent view of the mechanisms underlying brain aging and neurodegeneration is being replaced by one that entails multiple mechanisms coordinating and interacting with each other. This review focuses on the alterations in energy metabolism and inflammatory responses and their connection via redox regulation in normal brain aging and Alzheimer's disease. Interaction of these systems is reviewed based on basic research and clinical studies.

Emerging concepts in smooth muscle contributions to airway structure and function: implications for health and disease

Airway structure and function are key aspects of normal lung development, growth, and aging, as well as of lung responses to the environment and the pathophysiology of important diseases such as asthma, chronic obstructive pulmonary disease, and fibrosis. In this regard, the contributions of airway smooth muscle (ASM) are both functional, in the context of airway contractility and relaxation, as well as synthetic, involving production and modulation of extracellular components, modulation of the local immune environment, cellular contribution to airway structure, and, finally, interactions with other airway cell types such as epithelium, fibroblasts, and nerves. These ASM contributions are now found to be critical in airway hyperresponsiveness and remodeling that occur in lung diseases. This review emphasizes established and recent discoveries that underline the central role of ASM and sets the stage for future research toward understanding how ASM plays a central role by being both upstream and downstream in the many interactive processes that determine airway structure and function in health and disease.

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Low-dose exposures to common environmental chemicals that are deemed safe individually may be combining to instigate carcinogenesis, thereby contributing to the incidence of cancer. This risk may be overlooked by current regulatory practices and needs to be vigorously investigated.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology.

Cigarette smoke extract affects mitochondrial function in alveolar epithelial cells

Cigarette smoke is the main risk factor for chronic obstructive pulmonary disease (COPD). Exposure of cells to cigarette smoke induces an initial adaptive cellular stress response involving increased oxidative stress and induction of inflammatory signaling pathways. Exposure of mitochondria to cellular stress alters their fusion/fission dynamics. Whereas mild stress induces a prosurvival response termed stress-induced mitochondrial hyperfusion, severe stress results in mitochondrial fragmentation and mitophagy. In the present study, we analyzed the mitochondrial response to mild and nontoxic doses of cigarette smoke extract (CSE) in alveolar epithelial cells. We characterized mitochondrial morphology, expression of mitochondrial fusion and fission genes, markers of mitochondrial proteostasis, as well as mitochondrial functions such as membrane potential and oxygen consumption. Murine lung epithelial (MLE)12 and primary mouse alveolar epithelial cells revealed pronounced mitochondrial hyperfusion upon treatment with CSE, accompanied by increased expression of the mitochondrial fusion protein mitofusin 2 and increased metabolic activity. We did not observe any alterations in mitochondrial proteostasis, i.e., induction of the mitochondrial unfolded protein response or mitophagy. Therefore, our data indicate an adaptive prosurvival response of mitochondria of alveolar epithelial cells to nontoxic concentrations of CSE. A hyperfused mitochondrial network, however, renders the cell more vulnerable to additional stress, such as sustained cigarette smoke exposure. As such, cigarette smoke-induced mitochondrial hyperfusion, although part of a beneficial adaptive stress response in the first place, may contribute to the pathogenesis of COPD.

Short-term cigarette smoke exposure leads to metabolic alterations in lung alveolar cells

Cigarette smoke (CS)-induced alveolar destruction and energy metabolism changes are known contributors to the pathophysiology of chronic obstructive pulmonary disease (COPD). This study examines the effect of CS exposure on metabolism in alveolar type II cells. Male A/J mice (8 wk old) were exposed to CS generated from a smoking machine for 4 or 8 weeks, and a recovery group was exposed to CS for 8 weeks and allowed to recover for 2 weeks. Alveolar type II cells were isolated from air- or CS- exposed mice. Acute CS exposure led to a reversible airspace enlargement in A/J mice as measured by the increase in mean linear intercept, indicative of alveolar destruction. The effect of CS exposure on cellular respiration was studied using the XF Extracellular Flux Analyzer. A decrease in respiration while metabolizing glucose was observed in the CS-exposed group, indicating altered glycolysis that was compensated by an increase in palmitate utilization; palmitate utilization was accompanied by an increase in the expression of CD36 and carnitine-palmitoyl transferase 1 in type II alveolar cells for the transport of palmitate into the cells and into mitochondria, respectively. The increase in palmitate use for energy production likely affects the surfactant biosynthesis pathway, as evidenced by the decrease in phosphatidylcholine levels and the increase in phospholipase A2 activity after CS exposure. These findings help our understanding of the mechanism underlying the surfactant deficiency observed in smokers and provide a target to delay the onset of COPD.