Experimental suppression of tolerance to ozone and of cross-tolerance (NO2-O3) in rats by actinomycin D and colchicine

Impaired accumulation of granulocytes in the lung during ozone adaptation

Respiratory alterations induced by an acute exposure to ozone (O(3)) paradoxically resolve during multiday exposure. This adaptation is characteristically accompanied by a gradual attenuation of lung neutrophilia. As maintenance of neutrophilia at the site of inflammation is due to cytokine-mediated delayed neutrophil apoptosis, which is associated with reduced levels of Bax, a proapoptotic protein, we sought to determine whether defects in these mechanisms could account for O(3) adaptation. Lung granulocytes obtained at different time points from calves exposed to 0.75 ppm O(3) for 12 h/d for 7 consecutive days neither showed enhancement of survival nor Bax deficiency, when compared to blood granulocytes. To further investigate the effects of an exogenous oxidative stress on neutrophil survival, human granulocytes were treated with hydrogen peroxide alone, or in combination with granulocyte/macrophage colony-stimulating factor, an antiapoptotic cytokine. Both treatments led to rapid apoptosis associated with downregulation of Bcl-x(L) and Bcl-2, two antiapoptotic proteins. This study shows that O(3) adaptation is associated with a failure in the mechanisms leading to accumulation of neutrophils at the site of inflammation, and suggests that this defect is due to direct proapoptotic effects of exogenous oxidative stress on granulocytes.

Early molecular and cellular events of oxidant-induced pulmonary fibrosis in rats

To evaluate the early molecular events of oxidant-induced pulmonary fibrosis, rats were continuously exposed to 0.4 ppm ozone and 7 ppm nitrogen dioxide. The early responses to the combined gases could be divided into three phases. Acute pulmonary inflammation indicated by an increase in pulmonary edema as well as an influx of neutrophils into the airspaces first occurred on days 1 to 3 of the exposure. The pulmonary inflammation was reversed by day 8, and no biochemical or morphologic aspects of tissue responses were detected from days 15 to 45, suggesting that rats adapted to the stimuli during that period. Pulmonary fibrosis could be detected by an increase in the biomarker of lung collagen content at day 60 and by histopathologic evaluation by day 90. Enhanced expression of macrophage inflammatory protein-2 was observed only at day 1, whereas the pulmonary expression of transforming growth factor-beta was upregulated on days 60 and 90 of the exposure. Macrophage expressions of interleukin-1beta and interleukin-6 were enhanced during acute pulmonary inflammation; however, macrophage expression of tumor necrosis factor-alpha was elevated at both day 1 and days 60-90. Activation of nuclear factor-kappa B and increased expression of thioredoxin in the lungs was also observed at day 1 and days 60-90. The expression of antioxidant enzymes, such as manganeous superoxide dismutase and glutathione peroxidase, was not altered during exposure. These results indicate that macrophage activation and the expression of macrophage-derived cytokines may play an important role in the early pulmonary responses against the combined gases.

Stimulated pulmonary cell hyperplasia underlies resistance to alpha-naphthylthiourea

The rodenticide alpha-naphthylthiourea (ANTU) causes pulmonary edema and pleural effusion that leads to death via pulmonary insufficiency. Rats become resistant to the lethal effect of ANTU if they are first exposed to a small, nonlethal dose of ANTU. Young rats are also resistant to ANTU. The mechanism by which rats develop resistance by a prior, small dose exposure has yet to be determined. Growth factor induced-pulmonary hyperplasia has been demonstrated to attenuate ANTU-induced lung leak. We hypothesized that a small dose of ANTU protects against a large dose through pulmonary cell hyperplasia induced by the protective dose. Furthermore, we hypothesized that this hyperplasia is associated with altered transcription of growth factors. Male Sprague-Dawley rats (175-225 g) were treated with a low dose of ANTU (5 mg ANTU/kg; ANTU(L)) 24 h before challenge with a 100% lethal dose of ANTU (70 mg ANTU/kg; ANTU(H)) resulting in 100% protection against the lethal effect of ANTU(H). ANTU(L) protection against ANTU(H) lasted for 5 days, slowly phased out, all being lost by day 20. Injury was assessed by estimating pulmonary vascular permeability and through histopathological examination. ANTU(H) alone resulted in an increase in pulmonary edema leading to animal death. However, injury was prevented if the rats were first treated with ANTU(L). There was a stimulation of pulmonary cell hyperplasia in the lungs of ANTU(L) treated rats as measured by [3H]-thymidine and bromodeoxyuridine incorporation. Treatment with the antimitotic agent colchicine abolished ANTU(L)-induced resistance to ANTU(H). ANTU resistant rats were also resistant to the lethal effect of paraquat. Paraquat is not taken up by pneumocytes if they are undergoing hyperplasia. ANTU(L) administration resulted in an up regulation of gene transcription for keratinocyte growth factor, transforming growth factor-beta, keratinocyte growth factor receptor and epidermal growth factor receptor as determined through reverse transcription-polymerase chain reaction. A significant increase in transforming growth factor-alpha was not observed. These findings collectively suggest that ANTU(L)-induced pulmonary cell hyperplasia underlies resistance to ANTU(H). Furthermore, the stimulation of hyperplasia may be due to altered growth factor and growth factor receptor expressions.

Cytokine mediation of ozone-induced pulmonary adaptation

Previous studies have shown that a single exposure of animals to ozone (O3) can induce protection or adaptation to the acute injurious effects of a subsequent O3 challenge. Although a number of mechanisms have been proposed to account for this response, none appear to be fully explanatory. We examined the role interleukin (IL)-6 may play in the induction of adaptation to O3-induced pulmonary injury. A statistically significant 29-fold increase in bronchoalveolar lavage fluid IL-6 levels was observed in rats exposed to 0.5 ppm O3 during nighttime hours when compared with daytime hours even though similar kinetics of inflammation were induced by each exposure. Animals receiving an initial nighttime O3 exposure showed a lesser degree of inflammation following a subsequent O3 exposure when compared with animals which received an initial daytime exposure. Rats pretreated with IL-6 both intratracheally and intraperitoneally and subsequently exposed to O3 showed a lesser degree of cellular inflammation when compared with respective controls. Pretreatment of rats with anti-IL-6-receptor antibodies (ra) prior to the nighttime O3 exposure completely abrogated the O3-induced cellular adaptive response without effecting the inflammatory response induced by the initial nighttime O3 exposure. In fact, administration of anti-IL-6ra augmented the neutrophil influx following the second O3 exposure. Anti-IL-6ra treatment did not alter the pulmonary edema adaptive response, suggesting that the O3-induced cellular and edema adaptive responses are regulated by different mechanisms. Our data indicate that mobilization of pulmonary antioxidants does not play a role in the IL-6-mediated early cellular adaptive response and suggest that IL-6 is an essential mediator of the O3-induced cellular adaptive response.

Structural changes in airways of rats exposed to nitrogen dioxide intermittently for seven days. Comparison between major bronchi and terminal bronchioles

Structural changes caused by exposure to NO2 were compared between the largest intrapulmonary bronchi (major bronchi) and the terminal bronchioles. Rats were exposed to 50 ppm NO2 for 5 h a day for 7 consecutive days, and microscopic findings were assessed subjectively and quantitatively. Destruction of airway epithelium was present on Day 1, and the reparative phase was observed between Days 3 and 7. Light microscopy showed that on Day 1, loss of cilia and disintegration of epithelium occurred, and that the latter was worse in major bronchi than in terminal bronchioles; apical projections of Clara cells in terminal bronchioles were lost almost totally. Recovery of epithelium began by Day 3 and cilia by Day 5, and ciliogenesis progressed less promptly in terminal bronchioles. Clara cells did not return to normal during the experiment. Furthermore, a decrease in the airway diameter occurred in peripheral airways along with an increase in mural thickness in terminal bronchioles. Mural inflammation was also seen more conspicuously in terminal bronchioles. Electron microscopy revealed that epithelial cells lost secretory granules on Day 1 and began to repair them by Day 5 in major bronchi and by Day 7 in terminal bronchioles. It is likely that epithelium in terminal bronchioles is not particularly sensitive to NO2 compared with that in major bronchi, but that recovery occurs more promptly in major bronchi than in terminal bronchioles. Mechanisms underlying bronchoconstriction occurring preferentially in peripheral airways are also discussed.

Morphological basis of tolerance to ozone

The purpose of this research was to study Type 1 epithelial cells in the ozone (O3)-tolerant lung epithelium. Rats were made tolerant by exposure to 0.5 ppm O3 for 2 days and allowed to recover in air. Reexposure to a lethal concentration of O3 (6 ppm) at 3, 7, and 15 days of recovery revealed that tolerance was present at 3 days but almost absent at 7 and 15 days of recovery. Using Type 2 cell proliferation as a means of quantitating Type 1 cell injury, it was observed that when the preexposed rats were reexposed to 0.5 ppm at 3, 7, and 15 days, very little Type 1 cell injury occurred at 3 days. However, at 7 and 15 days the amount of Type 1 cell injury was the same as that associated with the original exposure. To determine whether there was any change in the alveolar epithelial cell populations between the periods of tolerance (3 days) and its decline (7 and 15 days), the percentage of tritiated thymidine [( 3H]TdR-labeled Type 1 and 2 cells at these times were determined. There was a significant decrease in [3H]TdR-labeled Type 1 and 2 cells between the third and fifteenth days of recovery as excess cells were sloughed off and the tissue returned to normal. Using electron microscopic morphometry, Type 1 and 2 cells were then studied during the decline of tolerance. No change was found in the morphology of Type 2 cells; however, the morphology of Type 1 cells revealed a 58% decrease in surface area and a 25% increase in the arithmetic mean thickness when tolerance was present at 3 days. As tolerance declined (7 and 15 days), Type 1 cell morphology returned to normal. It was concluded that tolerance exists when the surface area of a cell exposed to a particular concentration of ozone is small enough so that the existing antioxidant mechanism contained within that cell volume can protect it from damage.

Antioxidant system and ozone tolerance

Rats were initially exposed to 2 ppm ozone for 3 hr in order to induce ozone tolerance. The time course of the content of nonprotein SH as well as the activities of glutathione peroxidase, glutathione reductase, and glucose 6-phosphate dehydrogenase (so-called antioxidant system) in lungs of the animals was compared to the development of tolerance in the animals to a challenge exposure of 5.6 ppm ozone for 3 hr. An enhancement of a part of the antioxidant system was detected on the second day and thereafter following the initial exposure, but the clear and complete tolerance to ozone was demonstrated 1 day before the enhancement of the antioxidant system. Alternatively, the induction of tolerance to ozone was found to be experimentally suppressed by actinomycin D or colchicine, but the levels of the antioxidant system in animals thus treated were not significantly different from the matched controls, or only partly enhanced. These events suggest that the tolerance to ozone is not related to an enhancement of the antioxidant system.