[Apoptosis in neonatal rat lung exposed to hyperoxia]

Exposure to high concentrations of oxygen in the neonatal period may impair lung growth and is a major contributing factor to the development of bronchopulmonary dysplasia (BPD). Cell death from hyperoxic injury may occur through either an apoptotic or nonapoptotic pathway. The aim of the present study was to investigate the effect of hyperoxia on caspase-3 and p53 gene expression and apoptosis in the lungs of neonatal rats, so as to determine the type of cell death that occurs in the lungs of neonatal rats exposed to hyperoxia.

Hyperoxic lung injury model was established by exposing to 95% O(2) in the neonatal period of Spraque-Dawley rats. The levels of caspase-3 mRNA and p53 mRNA expression in the lungs of neonatal rats exposed to 95% hyperoxia or room air were detected by RT-PCR. To quantify PCR products, PCR products were electrophoretically separated with 1.5% agarose gels. The optical density (A) values of the DNA bands were quantified by complete gel documentation and analysis system. The A ratios of p53/beta-actin denoted the relative content of p53 mRNA, results were showed as mean +/- standard deviation. The specific positive or negative bands of caspase-3 in electrophoresis gels were counted, Fisher's exact test of propabilities was used to determined statistically significant differences between two groups. We determined the extent of apoptosis taking place in the lungs of neonatal rats exposed to 95% hyperoxia using terminal deoxyribonucleotide transferase-mediated deoxyuridine triphosphate-fluorescence nick-end labeling (TUNEL) in 7-d-old neonatal lung. Under light microscope, five areas of lung parenchyma were systematically and randomly photographed from each animal and positive cells among 500 lung cells were calculated. Results were showed as mean +/- standard deviation.

We found increased levels of p53 messenger RNA, a gene associated with apoptosis, in the lungs of neonatal rat born and raised in 95% hyperoxia. Moderate increase in the level of p53 mRNA was found in the hyperoxic-treated group at 24 h (q = 3.2305, P > 0.05). Significant increase in the level of p53 mRNA was found in the hyperoxic-treated group at 48 h (q = 7.2941, P < 0.01). The levels of p53 mRNA expression in neonatal rat lungs exposed to 95% O(2) for 72 h or 96 h returned to normal level. The levels of caspase-3 mRNA expression were very low or absent in the hyperoxic-treated groups at 12 h, 24 h, 48 h, 72 h and 96 h or in the air-breathing groups at 12 h, 24 h, 48 h, 72 h and 96 h. An increase in the number of cells undergoing apoptosis was found in the hyperoxic-treated group at 7 d (F = 56.5010, P < 0.001) which was significantly greater than the number of apoptotic cells found in the lungs of rats of the same age exposed to room air.

Our results suggested that 95% hyperoxia could temporarily up-regulate the gene expression of p53, which induced the transcription of p21(WAF/CIP1) mRNA. Furthermore, p21(WAF/CIP1) could lead to cell cycle arrest and inhibit proliferation of lung cells. Meanwhile, p53 could also promote apoptosis of lung cells. Therefore, exposure to high concentrations of oxygen in the neonatal period may impair lung growth and is a major contributing factor to the development of bronchopulmonary dysplasia (BPD), and hyperoxia may affect the future lung growth and lead to barrier of lung development. The treatments of anti-apoptosis and promoting alveoli growth hold a promising perspective in hyperoxic lung injury. The level and ratio of caspase-3 gene expression were very low or absent in the lungs of neonatal rats exposed to 95% O(2) or room air. We speculated that caspase-3 gene expression was not essential in the hyperoxia induced lung cell apoptosis in neonatal rats.