DIFFERENTIAL REGULATION OF GLUTATHIONE BY OXIDANTS AND DEXAMETHASONE IN ALVEOLAR EPITHELIAL-CELLS

We studied the regulation of GSH and the enzymes involved in GSH regul ation, gamma-glutamylcysteine synthetase (gamma-GCS) and gamma-glutamy l transpeptidase (gamma-GT), in response to the oxidants menadione, xa nthine/xanthine oxidase, hyperoxia, and cigarette smoke condensate in human alveolar epithelial cells (A549). Menadione (100 mu M), xanthine /xanthine oxidase (50 mu M/10 mU), and cigarette smoke condensate (10% ) exposure produced increased GSH levels (240 +/- 6, 202 +/- 12, and 1 91 +/- 2 nmol/mg protein, respectively; P < 0.001) compared with the c ontrol level (132 +/- 8 nmol/mg protein), which were associated with a significant increase in gamma-GCS activity (0.18 +/- 0.006, 0.16 +/- 0.01, and 0.17 +/- 0.008 U/mg protein, respectively; P < 0.01) compare d with the control level (0.08 +/- 0.001 U/mg protein) at 24 h. Exposu re to hyperoxia (95% O-2) resulted in a time-dependent increase in GSH levels, gamma-GCS activity increased significantly at 4 h (P < 0.001) , returning to control values after 12 h of exposure. Dexamethasone (3 mu M) exposure produced a significant time-dependent decrease in the levels of GSH and gamma-GCS activity at 24-96 h. The activity of gamma -GT did not change after oxidant treatment; however, it was decreased significantly by dexamethasone at 24-96 h. Thus oxidants and dexametha sone modulate GSH levels and activities of gamma-GT and gamma-GCS by d ifferent mechanisms. We suggest that the increase in gamma-GCS activit y but not in gamma-GT activity may be required for the increase in int racellular GSH under oxidative stress in alveolar epithelial cells.