Effect of high concentrations of oxygen on reparative regeneration of damaged alveolar epithelium in mice

Diet restriction modulates lung response and survivability of rats exposed to ozone

Ozone (O(3)) is a powerful oxidant component of photochemical smog polluting the air of urban cities. Exposure to low-level O(3) causes lung injury and increased morbidity of the sensitive segment of population, and exposure to high levels can be lethal to experimental animals. Injury from O(3) exposure is generally associated with free radical formation and oxidative stress. Because diet restriction is proposed to enhance antioxidant status, we examined whether it would influence the response to inhaled O(3). Twenty-four Sprague-Dawley rats, 1 month old, weighing 150 g, were divided into two dietary regimens (12 rats/regimen); one was freely-fed (FF), and the second was diet-restricted (DR) to 20% the average daily intake of the FF. After 60 days of dietary conditioning, the body weight of DR rats was reduced to 50% that of FF rats. Then, in one experiment, two groups (six rats/group), one FF and the other DR, were exposed to 0.8+/-0.1 p.p.m. (1570+/-196 microg/m(3)) O(3), continuously for 3 days. Another two similar groups of rats were exposed to filtered room air and served as matched controls. After exposure, all rats were euthanized and the lungs analyzed for biochemical markers of oxidative stress. In a second experiment, 24 rats were divided into two groups (12 rats/group), one FF and the other DR, then exposed to high-level O(3) for 8 h (4 p.p.m., 7848+/-981 microg/m(3)) and the mortality noted during exposure and for 16 h post-exposure. Following low-level O(3), inhalation, greater alterations were observed in FF rats compared with DR rats. With high-level O(3) exposure, DR rats exhibited a much greater survivability compared with FF rats (90% versus 8%, respectively). These observations suggest that diet restriction leading to significant reduction of body weight is beneficial, and may play a role in the resistance to the adverse effects of O(3).

Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance

At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na(+)-selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 microM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of < 1 microM). Both FDLE and ATII cells contain the alpha-, beta-, and gamma-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na(+)-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'-amiloride. A variety of agents, including cAMP, oxygen, glucocorticoids, and in some cases Ca2+, increased the activity and/or rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na(+)-absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn alpha-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.

Exosurf enhances adenovirus-mediated gene transfer to alveolar type II cells

We assessed the role of surfactant replacement mixtures in the enhancement of adenovirus-mediated gene transfer to pulmonary epithelial cells both in vitro and in vivo. A549 cells, a pulmonary epithelium-derived adenocarcinoma cell line, were incubated with either media alone or media containing 10 microg phospholipid/ml Exosurf or Infasurf for 50 min followed by addition of a replication-deficient adenovirus (E1-deleted) expressing the luciferase reporter gene [AdCMV-Luc; 10 plaque-forming units (PFU)/cell] for 4 h. Pretreatment with Exosurf, but not Infasurf, at 37 degrees C, but not at 4 degrees C, enhanced luciferase activity in A549 cells 24 h later by 156% (P < 0.01). Intratracheal instillation of AdCMV-Luc (2 x 10(9) PFU) into rats resulted in luciferase expression mainly in alveolar macrophages and to a smaller extent in alveolar type II (ATII) cells 24 h later. However, when the AdCMV-Luc instillation was preceded by Exosurf (250 microl; 25 mg/ml), a 10-fold increase in ATII cell luciferase activity was noted. Preincubation of cultured ATII cells with Exosurf also enhanced their transfection by AdCMV-Luc by 515% (P < 0.001). The results of these studies provide a new strategy for targeting ATII cells for gene delivery.

Augmentation of superoxide dismutase and catalase activity in alveolar type II cells

This study tested whether adducts formed by covalent linkage of superoxide dismutase (SOD) or catalase to polyethylene glycol (PEG) could augment SOD and catalase activity in alveolar type II cells and document enhanced resistance to oxidant damage. Alveolar type II cells were isolated from adult, pathogen-free rats. Antioxidant enzymes were added to the medium of cell cultures in various concentrations for periods up to 48 h. Incubation with 500 to 3,000 U of PEG-SOD or 10,000 to 40,000 U of PEG-catalase/10(6) cells produced a dose-response-related increase in intracellular enzyme activity in comparison with controls (untreated or treated with SOD or catalase, inactivated PEG-SOD or PEG-catalase, or PEG alone). Uptake was maximal during the first 4 h. Using fluorescent label (fluorescein isothiocyanate) bound to PEG-catalase, we found intracellular localization of the labeled enzyme. Exposure to H2O2 led to reduced cytotoxicity in cells pretreated with PEG-catalase than in controls. We conclude that supplementation with PEG-SOD or PEG-catalase enhanced the activity of these enzymes in alveolar type II cells and increased their resistance to oxidant stress.

Effects of oxygen and ozone on mouse lung tumorigenesis

Oxygen and ozone both have been found to enhance or to inhibit the development of tumors in mouse lung. As a general rule, preexposure to the oxidant, before administration of a carcinogen, or exposure to high levels for a comparatively short time immediately following carcinogen administration favors development of tumors. On the other hand, prolonged exposure begun after a certain time following carcinogen exposure inhibits tumor development. The paradoxical effects of the two oxidants depend on experimental design; results can be tentatively explained in terms of oxidant-induced cell proliferation or by oxidant-mediated cytotoxicity. Besides being capable of modifying chemically induced lung tumorigenesis, ozone and oxygen also appear to induce tumors in mouse lung on their own. The conclusions drawn from the study of mouse lung tumors have recently been reinforced in experiments with hamsters, where hyperoxia has clear tumor-modulating effects.

The production of alveolar macrophage-derived growth-regulating proteins in response to lung injury

Tissue injury elicits an inflammatory response, one element of which is the activation of the local macrophage population. Macrophages are recognized as the source of multiple growth-regulating proteins, and are thus thought to play an important role in wound healing. Injury to the lung by exposure to oxidant gases, particulates, chemicals or drugs is often followed by replication of the cells of the alveoli. The growth-regulating proteins released by alveolar macrophages (AM) may be one mechanism which controls the proliferation of these cells. This article describes the AM growth factors, the cell types which they affect, and the injuries known to cause their release. In view of the multiplicity and overlapping functions of the macrophage growth factors, potential mechanisms which might regulate the growth response of the surrounding cells are also considered.

Response of fetal rat lung fibroblasts to elevated oxygen concentrations after liposome-mediated augmentation of antioxidant enzymes

Cultured fetal rat lung fibroblasts are growth-inhibited and have increased lactate dehydrogenase release and prostaglandin synthesis in response to 50% and 95% oxygen exposure. Extended exposure to 50% oxygen, but not to 95% oxygen, was associated with tolerance to the cytotoxic effects of oxygen. Pretreatment of fibroblasts with liposome-encapsulated superoxide dismutase and catalase also conferred protection against the cytotoxic effects of 50% and 95% oxygen. Exogenous enhancement of intracellular superoxide dismutase and catalase specific activities did not attenuate the growth inhibition or increased prostaglandin synthesis associated with exposure to 50% or 95% oxygen. The growth inhibition could not be attributed to an autocrine prostaglandin effect, since inhibition of prostaglandin synthesis with 50 microM ibuprofen did not prevent O2-mediated inhibition of DNA synthesis.

Mitigation of pulmonary oxygen toxicity in premature lambs with intravenous antioxidants

Deficiencies of antioxidants and increased free radical generation may explain the high incidence of bronchopulmonary dysplasia in premature infants. Long-acting antioxidants such as polyethylene glycol (PEG) conjugated superoxide dismutase (SOD), and catalase might modify this process. We delivered 32 premature lambs, 16 pairs of twins, by cesarean section at 125-141 days of gestation (term 146 days) and stabilized them on ventilators in normocapnic hyperoxia for a period of 8 h. One lamb of each twin pair received an intravenous dose of 7,500-50,000 IU/kg of PEG-SOD and of 37,500-1,000,000 IU/kg of PEG-catalase at birth. Their siblings acted as controls. Mean airway pressure, arterial pressure, and heart rate were recorded continuously. Arterial blood gases and pH were obtained every 30 min. After sacrifice, standardized lung biopsies were prepared for quantitative morphometrics and electron microscopy. Administration of PEG antioxidants at birth reduced the influx of neutrophils and macrophages into the lung and damage to arterioles, bronchiolar mucosa, and type II pneumocytes without major changes in alveolar surface area or pulmonary function. These effects were dose-related and detectable even at the lowest doses of PEG antioxidants administered.

Acute sensitivity of BHT-induced alveolar toxicity to a diquat challenge in murine lungs

In this study we investigated the effects of a mild toxic challenge at selected points in time on the nonspecific cellular events that occurred in acutely damaged pulmonary alveoli. Swiss-Webster mice were treated with butylated hydroxytoluene (BHT, 400 mg/kg ip) and sacrificed at Days 1, 3, and 5 thereafter; either 24 or 48 hr prior to each sacrifice, the herbicide diquat was administered (4 mg/kg ip) as a challenge to the ongoing cellular events in the pulmonary alveoli. Standard morphometric techniques were used at both the levels of light and electron microscopy to evaluate the alveolar response to BHT and diquat treatments. Following BHT, early type I epithelial and endothelial damage triggered inflammatory changes in alveolar septa. Proliferation and differentiation of type II pneumocytes, aiming at the regeneration of the respiratory epithelium, ensued and peaked at Day 3. Treatment with diquat alone caused mild inflammatory changes and hypertrophy of type II pneumocytes, in the absence of necrosis of any alveolar cell type. The pinpoint administration of diquat in the early days following treatment with BHT significantly disorganized the temporal pattern of alveolar reaction. Diquat enhanced epithelial and endothelial damage only if administered before the onset of BHT-induced injury. Afterwards, the alveolar response to the combined effects of BHT and diquat could not be predicted from their known individual effects. Treatment with diquat modified either proliferation or differentiation of type II pneumocytes, depending upon time along the BHT schedule. Inflammatory and interstitial reactions were lowered when diquat was given at Days 1 and 3 post-BHT, but potentiated when given at Day 4. The results document time-related changes in the sensitivity of damaged and regenerating alveolar cells to a mild exogenous chemical challenge. They may further indicate that low levels of urban and industrial toxicants might influence pulmonary alveolar events in individuals made more susceptible by acute or chronic respiratory diseases.

Oxygen and resolution of lung injury

We investigated the effects of varying inspired oxygen concentrations on the resolution of oleic acid-induced lung injury in rabbits. Rabbits were injected intravenously with oleic acid and maintained in room air, or exposed to 60, 70, or 80% oxygen for periods of 7 or 10 days. Oleic acid caused hemorrhagic pulmonary edema with hypoxemia. Hypoxemia was more profound in the oxygen-treated animals, a difference that was significant after 7 days' exposure to 60 and 70% oxygen, and after 4 days to 80% oxygen. Mortality was increased in the animals maintained in 80% oxygen. The data suggest that environmental oxygen concentrations greater than 60% interfere with the return to normal lung function following oleic acid injury in rabbits. The hypoxemia may be due to either mismatching of ventilation and perfusion or to a diffusion block resulting from the increased septal width. There was no evidence of massive pulmonary edema as a cause of the hypoxemia. It was not possible to distinguish between injury primarily caused by oxygen and its interference with the healing process.

Potentiating effects of oxygen in lungs damaged by methylcyclopentadienyl manganese tricarbonyl, cadmium chloride, oleic acid, and antitumor drugs

The intraperitoneal administration of methylcyclopentadienyl manganese tricarbonyl (MMT) and cyclophosphamide, exposure to an aerosol of cadmium chloride, intravenous administration of oleic acid, and intratracheal instillation of bleomycin to young female BALB/c mice or CD/CR rats result in acute lung injury. Pulmonary morphology and lung collagen content were examined in animals treated with these chemicals alone or in combination with an elevated oxygen concentration (80%) in the inspired air. In mice, the development of fibrosis could be significantly enhanced if animals treated with MMT, cadmium chloride, cyclophosphamide, or bleomycin were exposed to 80% oxygen immediately following exposure to these agents. In rats only cyclophosphamide- and bleomycin-induced acute lung injury was potentiated by hyperoxia, resulting in significant enhancement of lung collagen content. The pathogenesis responsible for this differential species response of pulmonary injury to hyperoxia remains to be investigated.