Factors modulating the epithelial response to toxicants in tracheobronchial airways

Comparison of acute respiratory epithelial toxicity for 4-Methylimidazole and naphthalene administered by oral gavage in B6C3F1 mice

4-Methylimidazole (4MEI) is a contaminant in food and consumer products. Pulmonary toxicity and carcinogenicity following chronic dietary exposures to 4MEI is a regulatory concern based on previous rodent studies. This study examined acute pulmonary toxicity in B6C3F1 mice from 6 h to 5 days after oral gavage with a single dose of 150 mg/kg 4MEI, a double dose delivered 6 h apart, or vehicle controls. Oral gavage of 150 mg/kg naphthalene, a prototypical Club cell toxicant, was used as a positive control. Intrapulmonary conducting airway cytotoxicity was assessed in fixed-pressure inflated lungs using qualitative histopathology scoring, quantitative morphometric measurement of vacuolated and exfoliating epithelial cells, and immunohistochemistry. 4MEI treatment did not change markers of cytotoxicity including the mass of vacuolated epithelium, the thickness of the epithelium, or the distributions of epithelial proteins: secretoglobin 1A1, proliferating cell nuclear antigen, calcitonin gene-related peptide, and myeloperoxidase. 4MEI and vehicle controls caused slight cytotoxicity with rare vacuolization of the epithelium relative to the severe bronchiolar epithelial cell toxicity found in the naphthalene exposed mice at terminal bronchioles, intrapulmonary airways, or airway bifurcations. In summary, 4MEI caused minimal airway epithelial toxicity without characteristic Club Cell toxicity when compared to naphthalene, a canonical Club Cell toxicant.

Onset of alveolar recirculation in the developing lungs and its consequence on nanoparticle deposition in the pulmonary acinus

The structure of the gas exchange region of the human lung (the pulmonary acinus) undergoes profound change in the first few years of life. In this paper, we investigate numerically how the change in alveolar shape with time affects the rate of nanoparticle deposition deep in the lung during postnatal development. As human infant data is unavailable, we use a rat model of lung development. The process of postnatal lung development in the rat is remarkably similar to that of the human, and the structure of the rat acinus is indistinguishable from that of the human acinus. The current numerical predictions support our group's recent in vivo findings, which were also obtained by using growing rat lung models, that nanoparticle deposition in infants is strongly affected by the change in the structure of the pulmonary acinus. In humans, this major structural change occurs over the first 2 yr of life. Our current predictions would suggest that human infants at the age of ∼ 2 yr might be most at risk to the harmful effects of air pollution. Our results also suggest that dose estimates for inhalation therapies using nanoparticles, based on fully developed adult lungs with simple body weight scaling, are likely to overestimate deposition by up to 55% for newborns and underestimate deposition by up to 17% for 2-yr-old infants.

Gender differences in pulmonary regenerative response to naphthalene-induced bronchiolar epithelial cell injury

OBJECTIVES

Accumulating evidence suggests that gender affects the incidence and severity of several pulmonary diseases. Previous studies on mice have shown gender differences in susceptibility to naphthalene-induced lung injury, where Clara cell damage was found to occur earlier and to be more extensive in females than in males. However, very little is known about whether there are any gender differences in subsequent lung repair responses. The aim of this study was to investigate whether gender plays an important role in pulmonary regenerative response to naphthalene-induced Clara cell ablation.

MATERIALS AND METHODS

Adult male and female mice were injected with a low, medium, or high dose of naphthalene, and lung tissue regeneration was examined by immunohistochemical staining for cell proliferation marker (Ki-67) and mitosis marker (phosphohistone-3).

RESULTS

Histopathological analysis showed that naphthalene-induced Clara cell necrosis was more prominent in the lungs of female mice as compared to male mice. Cell proliferation and mitosis in both the distal bronchiolar airway epithelium and peribronchiolar interstitium of female mice was significantly greater than that of male mice after treatment with the low and medium doses. However, after treatment with high dose of naphthalene, lung regeneration was delayed in female mice, while male mice mounted a timely regenerative response.

CONCLUSIONS

These findings show that there are clear gender differences in naphthalene-induced lung injury and repair.

A genetic mouse model to investigate hyperoxic acute lung injury survival

Acute lung injury (ALI) is a devastating disease that maintains a high mortality rate, despite decades of research. Hyperoxia, a universal treatment for ALI and other critically ill patients, can itself cause pulmonary damage, which drastically restricts its therapeutic potential. We stipulate that having the ability to use higher levels of supplemental O2 for longer periods would improve recovery rates. Toward this goal, a mouse model was sought to identify genes contributing to hyperoxic ALI (HALI) mortality. Eighteen inbred mouse strains were screened in continuous >95% O2. A significant survival difference was identified between sensitive C57BL/6J and resistant 129X1/SvJ strains. Although resistant, only one-fourth of 129X1/SvJ mice survived longer than any C57BL/6J mouse, demonstrating decreased penetrance of resistance. A survival time difference between reciprocal F1 mice implicated a parent-of-origin (imprinting) effect. To further evaluate imprinting and begin to delineate the genetic components of HALI survival, we generated and phenotyped offspring from all four possible intercrosses. Segregation analysis supported maternal inheritance of one or more genes but paternal inheritance of one or more contributor genes. A significant sex effect was demonstrated, with males more resistant than females for all F2 crosses. Survival time ranges and sensitive-to-resistant ratios of the different F2 crosses also supported imprinting and predicted that increased survival is due to dominant resistance alleles contributed by both the resistant and sensitive parental strains. HALI survival is multigenic with a complex mode of inheritance, which should be amenable to genetic dissection with this mouse model.

Environmental factors and developmental outcomes in the lung

The developing lung is highly susceptible to damage from exposure to environmental toxicants particularly due to the protracted maturation of the respiratory system, extending from the embryonic phase of development in utero through to adolescence. The functional organization of the lungs requires a coordinated ontogeny of critical developmental processes that include branching morphogenesis, cellular differentiation and proliferation, alveolarization, and maturation of the pulmonary immune, vasculature, and neural systems. Therefore, exposure to environmental pollutants during crucial periods of prenatal and/or postnatal development may determine the course of lung morphogenesis and maturation. Depending on the timing of exposure and pathobiological response of the affected tissue, exposure to environmental pollutants can potentially result in long-term alterations that affect the structure and function of the respiratory system. Besides an immature respiratory system at birth, children possess unique differences in their physiology and behavioral characteristics compared to adults that are believed to augment the vulnerability of their developing lungs to perturbations by environmental toxins. Furthermore, an interaction between genetic predisposition and increased opportunity for exposure to chemical and infectious disease increase the hazards and risks for infants and children. In this article, the evidence for perturbations of lung developmental processes by key ambient pollutants (environmental tobacco smoke [ETS], ozone, and particulate matter [PM]) are discussed in terms of biological factors that are intrinsic to infants and children and that influence exposure-related lung development and respiratory outcomes.

Interaction of epithelium with mesenchyme affects global features of lung architecture: a computer model of development

Lung airway morphogenesis is simulated in a simplified diffusing environment that simulates the mesenchyme to explore the role of morphogens in airway architecture development. Simple rules govern local branching morphogenesis. Morphogen gradients are modeled by four pairs of sources and their diffusion through the mesenchyme. Sensitivity to lobar architecture and mesenchymal morphogen are explored. Even if the model accurately represents observed patterns of local development, it could not produce realistic global patterns of lung architecture if interaction with its environment was not taken into account, implying that reciprocal interaction between airway growth and morphogens in the mesenchyme plays a critical role in producing realistic global features of lung architecture.

The Epithelial Cell in Lung Health and Emphysema Pathogenesis

Cigarette smoking is the primary cause of the irreversible lung disease emphysema. Historically, inflammatory cells such as macrophages and neutrophils have been studied for their role in emphysema pathology. However, recent studies indicate that the lung epithelium is an active participant in emphysema pathogenesis and plays a critical role in the lung's response to cigarette smoke. Tobacco smoke increases protease production and alters cytokine expression in isolated epithelial cells, suggesting that these cells respond potently even in the absence of a complete inflammatory program. Tobacco smoke also acts as an immunosuppressant, reducing the defense function of airway epithelial cells and enhancing colonization of the lower airways. Thus, the paradigm that emphysema is strictly an inflammatory-cell based disease is shifting to consider the involvement of resident epithelial cells. Here we review the role of epithelial cells in lung development and emphysema. To better understand tobacco-epithelial interactions we performed microarray analyses of RNA from human airway epithelial cells exposed to smoke extract for 24 hours. These studies identified differential regulation of 425 genes involved in diverse biological processes, such as apoptosis, immune function, cell cycle, signal transduction, proliferation, and antioxidants. Some of these genes, including VEGF, glutathione peroxidase, IL-13 receptor, and cytochrome P450, have been previously reported to be altered in the lungs of smokers. Others, such as pirin, cathepsin L, STAT1, and BMP2, are shown here for the first time to have a potential role in smoke-associated injury. These data broaden our understanding of the importance of epithelial cells in lung health and cigarette smoke-induced emphysema.

Estrous cycle alters naphthalene metabolism in female mouse airways

Previous studies have shown variability in naphthalene cytotoxicity, expression of CYP2F2 gene and protein, and naphthalene metabolism in random cycling female mice (NIH:Swiss). CYP2F2 metabolizes naphthalene to cytotoxic metabolites in lungs of mice. This study was designed to address the question: do hormonal changes associated with the estrous cycle alter metabolism of naphthalene in the lung? Adult virgin female mice were manipulated into defined stages of the reproductive cycle: estrus, proestrus, and noncycling. Cycling was confirmed by cytology on vaginal swabs. At specific cycle times, extrapulmonary (tracheal and bronchial) and intrapulmonary (bronchiolar) conducting airways were microdissected from the lung parenchyma and incubated with naphthalene, and the products of naphthalene metabolism were trapped and measured using high-performance liquid chromatography. Circulating estradiol levels were measured at necropsy using an enzyme-linked immunosorbent assay. CYP2F2 gene expression was determined by airway level using real-time reverse transcription-polymerase chain reaction and did not vary by estrous cycle stage in intrapulmonary airways but did in extrapulmonary airways. Metabolism of naphthalene varied significantly by estrous cycle stage with the highest level of total metabolism occurring in proestrus (when estrogen is lowest) in intrapulmonary airways. Total activity and metabolite profiles in both extrapulmonary and intrapulmonary airways were affected by cycle stage. We conclude that the hormonal patterns associated with different stages of the estrous cycle 1) alter metabolism of naphthalene in the lungs of mice and 2) alter naphthalene metabolism differentially in extrapulmonary versus intrapulmonary airways.

Smooth muscle hypertrophy in distal airways of sensitized infant rhesus monkeys exposed to house dust mite allergen

BACKGROUND

Airway smooth muscle hypertrophy is closely associated with the pathophysiology of hyper-reactive airways in allergic asthma.

OBJECTIVE

To determine whether repeated exposure to allergens during postnatal lung development promotes remodelling of airway smooth muscle.

METHODS

Infant, male rhesus monkeys (30-day-old) were sensitized to house dust mite allergen (HDMA) and then exposed to HDMA aerosol periodically over 5 months. Smooth muscle mass and bundle size and abundance in conducting airways were measured and compared with age-matched control (filtered air-exposed) monkeys.

RESULTS

Total smooth muscle mass and average bundle size were significantly greater in the conducting airways of monkeys exposed to HDMA. Smooth muscle bundle abundance was not affected by exposure to HDMA.

CONCLUSION

Repeated cycles of allergen exposure alter postnatal morphogenesis of smooth muscle, affecting both total mass and bundle size, in conducting airways of infant monkeys.

Prevention of naphthalene-induced pulmonary toxicity by glutathione prodrugs: Roles for glutathione depletion in adduct formation and cell injury

Naphthalene is metabolized in the lung and liver to reactive intermediates by cytochrome P450 enzymes. These reactive species deplete glutathione, covalently bind to proteins, and cause necrosis in Clara cells of the lung. The importance of glutathione loss in naphthalene toxicity was investigated by using the glutathione prodrugs (glutathione monoethylester or cysteine-glutathione mixed disulfide) to maintain glutathione pools during naphthalene exposure. Mice given a single intraperitoneal injection of naphthalene (1.5 mmol/kg) were treated with either prodrug (2.5 mmol/kg) 30 min later. Both compounds effectively maintained glutathione levels and decreased naphthalene-protein adducts in the lung and liver. However, cysteine-glutathione mixed disulfide was more effective at preventing Clara cell injury. To study the prodrugs in Clara cells without the influence of hepatic naphthalene metabolism and circulating glutathione, dose-response and time-course studies were conducted with intrapulmonary airway explant cultures. Only the ester of glutathione raised GSH in vitro; however, both compounds limited protein adducts and cell necrosis. In vitro protection was not associated with decreased naphthalene metabolism. We conclude that (1) glutathione prodrugs can prevent naphthalene toxicity in Clara cells, (2) the prodrugs effectively prevent glutathione loss in vivo, and (3) cysteine-glutathione mixed disulfide prevents naphthalene injury in vitro without raising glutathione levels.

Animal models of chronic obstructive pulmonary disease: some critical remarks

A variety of animal models of chronic obstructive pulmonary disease (COPD) are available, comprising elastase instillation or inhalation of noxious agents and genetic models like mouse mutants, gene-targeted and transgenic mice. The present review discusses some critical aspects which should be taken into account when evaluating these animal models of human COPD. Critical aspects related to the human disease itself: (1) diagnosis is largely symptom based, and symptoms cannot be mimicked in animals; (2) COPD is not a well-defined entity, but comprises patients with variable contribution of chronic bronchitis, mucus hypersecretion and emphysema, and (3) various factors contribute to the development of COPD, indicating that different pathways may converge into a single endpoint. Critical aspects related to the animals used: (1) species-, strain- and gender-related differences in lung structure and function preclude mimicking some features of the human disease; (2) genetic models frequently present with air space enlargement as a result of the disturbance of early postnatal alveolization (conditionally controlled transgenic animals are recommended); (3) inhalation models frequently use young animals during lung growth, which may preclude distinguishing effects owing to growth retardation or to loss of existing alveolar walls; (4) inhalation of noxious agents may result in reduction of food intake and loss of body weight, which itself may result in emphysema, and (5) the presence of emphysema cannot be concluded based on measurement of air space enlargement alone, but should include the determination of total alveolar surface area.

Isolation of rodent airway epithelial cell proteins facilitates in vivo proteomics studies of lung toxicity

Recent developments in genomics, proteomics, and metabolomics hold substantial promise for understanding cellular responses to toxicants. Gene expression profiling is now considered standard procedure, but numerous publications reporting a lack of correlation between mRNA and protein expression emphasize the importance of conducting parallel proteomics studies. The cellular complexity of the lung presents great challenges for in vivo proteomics, and improved isolation methods for proteins from specific lung cell phenotypes are required. To address this issue, we have developed a novel method for isolation of rodent airway epithelial cell proteins that facilitates in vivo proteomics studies of two target-cell pheno-types of the lung, Clara cells and ciliated cells. The airway epithelial cell proteins are reproducibly solubilized, leaving the underlying basement membrane and smooth muscle intact as shown by histopathological analyses. The method yields epithelial cell-specific proteins in fivefold higher concentrations and reduces the yield of nonepithelial cell proteins 13-fold compared with homogenates from microdissected airways. In addition, 36% more protein spots were detectable by two-dimensional gel electrophoresis.

Terminal bronchioles harbor a unique airway stem cell population that localizes to the bronchoalveolar duct junction

Cellular mechanisms contributing to renewal of terminal bronchioles remain poorly defined. Our previous studies identified pollutant-resistant Clara cell secretory protein (CCSP)-expressing stem cells that localize to the neuroepithelial body (NEB) and contribute to renewal of the proximal bronchiolar epithelium. However, activation of NEB-associated stem cells is unlikely to contribute to renewal of terminal bronchiolar epithelium because of the paucity of NEBs at this location. Goals of this study were to determine the location and properties of cells contributing to renewal of terminal bronchioles after Clara cell depletion. Pollutant-resistant CCSP-expressing cells were identified that localized to the bronchoalveolar duct junction (BADJ) and contribute to restoration of a phenotypically diverse epithelium. CCSP-expressing cells comprise the predominant proliferative population in initial terminal bronchiolar repair and include a population of label-retaining cells suggesting that they maintain characteristics of a stem cell population. Furthermore, immunohistochemical co-localization studies involving CCSP and the NEB-specific marker calcitonin gene-related peptide indicate that BADJ-associated CCSP-expressing stem cells function independently of NEB microenvironments. These studies identify a BADJ-associated, NEB-independent, CCSP-expressing stem cell population in terminal bronchioles and support the notion that regiospecific stem cell niches function to maintain epithelial diversity after injury.