Administration of supplemental oxygen is frequently encountered in infants suffering from pulmonary insufficiency and in adults with acute respiratory distress syndrome. However, hyperoxia causes acute lung damage in experimental animals. In the present study, we investigated the roles of the Ah receptor (AHR) in the modulation of cytochrome P4501A (CYP1A) enzymes and in the development of lung injury by hyperoxia. Adult male wild-type [AHR (ϩ/ϩ)] mice and AHR-deficient animals [AHR (Ϫ/Ϫ)] were maintained in room air or exposed to hyperoxia (Ͼ95% oxygen) for 24 to 72 h, and pulmonary and hepatic expression of CYP1A and lung injury were studied. Hyperoxia caused significant increases in pulmonary and hepatic CYP1A1 activities (ethoxyresorufin O-deethylase) and mRNA levels in wild-type (C57BL/6J) AHR (ϩ/ϩ), but not AHR (Ϫ/Ϫ) mice, suggesting that AHR-dependent mechanisms contributed to CYP1A1 induction. On the other hand, hyperoxia augmented hepatic CYP1A2 expression in both wild-type and AHR (Ϫ/Ϫ) animals, suggesting that AHR-independent mechanisms contributed to the CYP1A2 regulation by hyperoxia. AHR (Ϫ/Ϫ) mice exposed to hyperoxia were more susceptible than wildtype mice to lung injury and inflammation, as indicated by significantly higher lung weight/body weight ratios, increased pulmonary edema, and enhanced neutrophil recruitment into the lungs. In conclusion, our results support the hypothesis that the hyperoxia induces CYP1A1, but not CYP1A2, expression in vivo by AHR-dependent mechanisms, a phenomenon that may mechanistically contribute to the beneficial effects of the AHR in hyperoxic lung injury.

Supplemental oxygen therapy is frequently used in the treatment of pulmonary insufficiency, as is encountered in premature infants, and in patients with acute respiratory distress syndrome. However, hyperoxia causes lung damage in experimental animals and may do so in humans. Cytochrome P4501A enzymes have been implicated in hyperoxic lung injury. In this study, we investigated the mechanisms of CYP1A1 regulation by hyperoxia and tested the hypothesis that aryl hydrocarbon receptor (AHR)-dependent mechanisms contribute to induction of CYP1A1 and that modulation of CYP1A by hyperoxia may have implications for lung injury. Exposure of adult male Sprague-Dawley rats to hyperoxia for 24 to 48 h led to increased expression of pulmonary CYP1A1 enzyme, which was preceded by enhancement of the corresponding mRNA, followed by decline of induction at 60 h, when the animals displayed severe respiratory distress and lung inflammation. Similarly, hepatic CYP1A1/1A2 mRNAs were markedly induced between 24 and 48 h of hyperoxia, with induction declining by 60 h. Electrophoretic mobility shift assays (EMSA) and experiments with AHR (Ϫ/Ϫ) mice indicated that AHR-dependent mechanisms contributed to CYP1A induction. The AHR (Ϫ/Ϫ) mice were refractory to CYP1A1 induction by hyperoxia and were more sensitive to lung injury than wild-type mice. Lungs of hyperoxic rats showed increase in the expression of CYP1A1 in airway epithelial cells, type II pneumocytes, and endothelial cells. In conclusion, our results suggest that induction of CYP1A1 by hyperoxia is mediated by AHR-dependent mechanisms and that modulation of CYP1A enzymes by hyperoxia may have implications for hyperoxic lung injury.

Supplemental oxygen, frequently used in premature infants, has been implicated in the development of bronchopulmonary dysplasia (BPD). While the mechanisms of oxygen-induced lung injury are not known, reactive oxygen species (ROS) are most likely involved in the process. Here, we tested the hypothesis that upregulation of cytochrome P450 (CYP) 1A isoforms in lung and liver may lead to protection against hyperoxic lung injury. Adult male Sprague-Dawley rats were pretreated with the CYP1A inducer beta-naphthoflavone (beta-NF) (80 mg/kg/day), once daily for 4 days, followed by exposure to hyperoxic environment (O2 > 95%) or room air (normoxia) for 60 h. Pleural effusions were measured as estimates of lung injury. Activities of hepatic and pulmonary CYP1A1 were determined by measurement of ethoxyresorufin O-deethylation (EROD) activity. Northern hybridization and Western blot analysis of lung and liver were performed to assess mRNA and protein levels, respectively. Our results showed that beta-NF-treated animals, which displayed the highest pulmonary and hepatic induction in EROD activity (10-fold and 8-fold increase over corn oil (CO) controls, respectively), offered the most protective effect against hyperoxic lung injury, p < 0.05. Northern and Western blot analysis correlated well with enzyme activities. Our results showed an inverse correlation between pulmonary and hepatic CYP1A expression and the extent of lung injury, which supports the hypothesis that CYP1A enzyme plays a protective role against oxygen-mediated tissue damage.

Hyperoxia contributes to the development of bronchopulmonary dysplasia in premature infants. Earlier we observed that aryl hydrocarbon receptor (AhR)-deficient mice are more susceptible to hyperoxic lung injury than AhR-sufficient mice, and this phenomenon was associated with a lack of expression of cytochrome P450 1A enzymes. Omeprazole, a proton pump inhibitor, used in humans with gastric acid related disorders, activates AhR in hepatocytes in vitro. However, the effects of omeprazole on AhR activation in the lungs and its impact on hyperoxia-induced ROS generation and inflammation are unknown. In this study, we tested the hypothesis that omeprazole attenuates hyperoxia-induced cytotoxicity, ROS generation, and expression of monocyte chemoattractant protein-1 (MCP-1) in the human lung derived H441 cells via AhR activation. Experimental groups included cells transfected with AhR small interfering RNA (siRNA). Hyperoxia resulted in significant increases in cytotoxicity, ROS generation, and MCP-1 production, which were significantly attenuated with the functional activation of AhR by omeprazole. The protective effects of omeprazole on cytotoxicity, ROS production, and MCP-1 production were lost in H441 cells whose AhR gene was silenced by AhR siRNA. These findings support the hypothesis that omeprazole protects against hyperoxic injury in vitro via AhR activation that is associated with decreased ROS generation and expression of MCP-1.