Apoptosis of airway epithelial cells in response to meconium

Abstract

Meconium aspiration syndrome (MAS) is common among newborn children but its mechanism is unclear. The syndrome is known to produce a strong inflammatory reaction in the lungs resulting in massive cell death. In this work we studied lung cell death by apoptosis after meconium aspiration in forty two-week-old rabbit pups. Analyzing lung samples by ISEL-DNA end labeling demonstrated the specific spread of apoptotic bodies throughout the lungs. These bodies were shrunken and smaller in size compared to normal cells and many of them were lacking cell membranes. About 70% of all apoptotic bodies were found among the airway epithelium cell eight hours after meconium instillation. In comparison, among lung alveolar cells, only about 20% cells were apoptotic in the same animals. In meconium-treated lungs and A549 cells, a significant increase of angiotensinogen mRNA and Caspase-3 expression were observed. The pretreatment of cells with Caspase-3 inhibitor ZVAD-fmk significantly inhibited meconium-induced lung cell death by apoptosis. These findings demonstrate the apoptotic process in meconium-instilled lungs or A549 cells in culture. Our results show lung airway epithelial and A549 cell apoptosis after meconium instillation. We suggest that studies of lung airway epithelial cell death are essential to understanding the pathophysiology of MAS and may present a key point in future therapeutic applications.

Introduction

MAS is a perinatal aspiration of meconium followed by acute lung inflammation and respiratory failure, in some cases leading to death. Previously we demonstrated meconium-induced damage of airways and alveoli (Zagariya et al., 2000). Increased pulmonary vascular resistance, severe inflammatory response, meconium toxicity and deactivation of pulmonary surfactant were observed by others (Tyler et al., 1978, Davey et al., 1993). Recent studies have suggested that apoptosis of airways and alveolar epithelial cells is an important factor in the pathogenesis of MAS. The apoptotic process in alveolar epithelium has been well studied. Hagimoto found apoptosis and expression of FAS in the alveolar epithelium of mice exposed to bleomycin (Hagimoto et al., 1995a, Hagimoto et al., 1995b). The angiotensinogen-converting enzyme (ACE) inhibitor captopril was able to inhibit experimentally induced lung fibrosis induced by the plant alkaloid monocrotaline (Molteni et al., 1985) or by gamma irradiation (Wang et al., 1999a). Recently, Uhal et al. found that captopril is also a potent inhibitor of FAS-induced apoptosis in human or rat lungs in vitro (Uhal et al., 1998).

Apoptosis and loss of alveolar epithelium cells were also seen in acute lung injury (Hagimoto et al., 1995a). In present study we investigated what type of cells in the lung tissue is damaged first after meconium instillation.

We hypothesized that meconium can induce lung cell apoptosis, which is preferentially localized in lung airway epithelial cells. We studied a meconium-induced injury in two-week-old rabbits.