[Effect of intra-amniotic endotoxin priming plus hyperoxic exposure on the expression of vascular endothelial growth factor and its receptors in lungs of preterm newborn rats].

OBJECTIVE: Bronchopulmonary displasia (BPD) is characterized by decreased alveolar development resulting in large saccular airspaces that are thought to result from the superposition of injury on the developing lung. Inflammation and hyperoxia appear to be the common factors. Preterm delivery is frequently preceded by chorioamnionitis, resulting in exposure of the fetal lung to inflammation. Subsequently, resuscitation with positive-pressure ventilation or supplemental O2 can continue to injure the lungs. Although mechanisms that impair lung growth in BPD are poorly understood, recent studies have shown that disruption of vascular endothelial growth factor (VEGF) function plays a pivotal role in the pathogenesis of BPD. The purpose of this study was to investigate the expression of VEGF and its receptors in premature lungs of rats with intra-amniotic endotoxin priming and/or exposed to 60% O2 and to elucidate the relationship between intrauterine inflammatory/chronic high O2 exposure and the pathogenesis of BPD.

METHODS: Timed pregnant Sprague Dawley (SD) rats were randomly divided into two groups: lipopolysaccharide (LPS) group and saline solution group. LPS or saline solution was intra-amniotically injected into the sacs on gestational age day 15 (70% of term). Six days after intra-amniotic injection, the preterm rats of each group were delivered and further randomized to put in 60% O2 exposure or in room air. On days 1, 7 and 14 after birth, the lungs of the rats from both groups were removed and dissected from the main bronchi for analysis. Left lungs of each group were used to assess lung histological changes after hematoxylin and eosin (HE) staining. Total RNA and protein were extracted from the right frozen lung tissues. Lung VEGF and its receptors mRNA and protein levels were measured by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) and polypeptides analysis (Western blot).

RESULTS: (1) Lung histology revealed striking differences in lung structure in the four experimental groups. Compared with control rat lungs, endotoxin and/or hyperoxia treated groups showed a histological pattern of alveolar simplification characterized by the presence of larger and fewer distal air spaces. To quantify the apparent decreases in alveolar number, radial alveolar counts (RAC) were measured in the four experimental groups: RAC increased along with the lung development from d 1 to d 14 in both control and endotoxin alone groups. However, in endotoxin alone group, RAC was significantly lower than that of controls from d 1 to d 14. In hyeroxia alone group, RAC began to decrease on day 7, and became significantly lower than that of the control group on day 14 (P < 0.05). In endotoxin plus hyperoxia groups, RAC significantly decreased as compared to controls on days 1 and 7, and became significantly lower than those of the other three groups on day 14. (2) Western blotting showed changes in protein content of VEGF164 and its receptor Flk-1 increased from day 1 to day 14 in both control group and endotoxin alone group. However, In endotoxin alone group, they were significantly lower than those of controls from d 1 to d 7 (P < 0.05). Conversely, they decreased from day 1 to day 14 in the other two groups, and were significantly lower than those of the control group on days 7 and 14 in both hyperoxia alone group and endotoxin plus hyperoxia groups. Importantly, the expression of VEGF and its receptor in endotoxin plus hyperoxia group was lower than those of both the hyperoxia alone group and the endotoxin alone group with significant difference. The levels of Flt-1 had obviously an age-dependent increase from day 1 to day 14 in the four experimental groups. (3) The changes of mRNA expression of VEGF and its receptors were basically consistent with the changes on protein expression.

CONCLUSIONS: These results suggest that the intra-amniotic endotoxin and/or hyperoxia can down-regulate expression of VEGF and Flk-1, which may contribute to the pathogenesis of BPD.