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Injury to the alveolar region is a hallmark of the adult respiratory distress syndrome (ARDS) whereas injury to the epithelium of the conducting airways is a characteristic of asthma. Reactive oxygen species have been implicated as mediators of lung injury in both of these conditions. We have investigated the relationship between intracellular nonprotein thiols (NPSH), and the release of the cytosolic enzyme lactate dehydrogenase (LDH), as an index of cell injury, following treatment of the human alveolar type II-like epithelial cell line (A549 cells) or the human bronchial epithelial cell line (16HBE140-) with hydrogen peroxide (H2O2). We have also assessed the protective effects of pre-incubation of both of these cells lines with H2O2 or enhancement of intracellular NPSH against H2O2-induced cell injury. Exposure of A549 and 16HBE140- cells to H2O2 (0.1 mM and 1 mM respectively for 16 h) produced the release of 40% of the total cellular LDH. H2O2 exposure produced an initial dose-dependent decrease in NPSH in A549 cells, with a subsequent increase to above control values. 16HBE140- cells also showed a dose-dependent decrease in NPSH following exposure to H2O2. Pretreatment of A549 cells with 0.1 mM H2O2 followed by subsequent exposure to H2O2 did not protect against H2O2-induced LDH release in this epithelial cell line. Pre-incubation with 2 mM N-acetylcysteine (NAC) increased NPSH but not intracellular reduced glutathione and resulted in total inhibition of H2O2-induced LDH release in both cell types. Pretreatment with reduced glutathione protected both cell types against the injurious effects of H2O2, whereas glutathione monethyl ester (GSHMEE) only partially protected A549 cells and had no effect in 16HBE140- cells. Intracellular cysteine levels were increased in both cell lines following NAC exposure but not sufficiently to account for the increase in NPSH levels. These observations raise the possibility that a critical concentration of nonprotein thiols may be necessary to protect pulmonary epithelial cells against hydrogen peroxide-induced injury.

We studied the regulation of GSH and the enzymes involved in GSH regulation, γ-glutamylcysteine synthetase (γ-GCS) and γ-glutamyl transpeptidase (γ-GT), in response to the oxidants menadione, xanthine/xanthine oxidase, hyperoxia, and cigarette smoke condensate in human alveolar epithelial cells (A549). Menadione (100 μM), xanthine/xanthine oxidase (50 μM/10 mU), and cigarette smoke condensate (10%) exposure produced increased GSH levels (240 ± 6, 202 ± 12, and 191 ± 2 nmol/mg protein, respectively; P < 0.001) compared with the control level (132 ± 8 nmol/mg protein), which were associated with a significant increase in γ-GCS activity (0.18 ± 0.006, 0.16 ± 0.01, and 0.17 ± 0.008 U/mg protein, respectively; P < 0.01) compared with the control level (0.08 ± 0.001 U/mg protein) at 24 h. Exposure to hyperoxia (95% O2) resulted in a time-dependent increase in GSH levels. γ-GCS activity increased significantly at 4 h ( P < 0.001), returning to control values after 12 h of exposure. Dexamethasone (3 μM) exposure produced a significant time-dependent decrease in the levels of GSH and γ-GCS activity at 24–96 h. The activity of γ-GT did not change after oxidant treatment; however, it was decreased significantly by dexamethasone at 24–96 h. Thus oxidants and dexamethasone modulate GSH levels and activities of γ-GT and γ-GCS by different mechanisms. We suggest that the increase in γ-GCS activity but not in γ-GT activity may be required for the increase in intracellular GSH under oxidative stress in alveolar epithelial cells.

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