Ozone inactivates cyclooxygenase in human tracheal epithelial cells without altering PGHS-2 mRNA or protein

Exposure of human tracheal epithelial (TE) cells to ozone (0.1-0.5 ppm) leads to a transient increase followed by decreased production of prostaglandin (PG) E2 concomitant with dose-dependent loss and delayed recovery of cyclooxygenase (CO) activity [S.E. Alpert and R.W. Walenga. Am. J. Physiol. 269 (Lung Cell. Mol. Physiol. 13): L734-L743, 1995]. Formation of reactive oxygen species (ROS) in cultured tracheobronchial epithelial cells during ozone exposure was recently demonstrated (L.C. Chen and Q.Qu. Toxicol. Appl. Pharmacol. 143: 96-101, 1997). In the present study, we investigated if ROS generated by ozone-exposed human TE cells contribute to PGE2 production and/or CO inactivation and whether the delay in recovery of CO activity after ozone reflects impaired gene transcription and/or protein synthesis. Rapid, dose-dependent ROS generation, assessed by fluorescence of dihydrorhodamine 123, was detected in human TE monolayers exposed to 0.21-0.63 ppm ozone. In a different system, TE cells were exposed to air or 0.5 ppm ozone for 1 h by serial renewal/collection of an adherent film of media. Ozone-induced ROS formation, the transient increase and decline in PGE2, and CO inactivation were attenuated by an intracellular hydroxyl radical scavenger, 1,3-dimethyl-2-thiourea. Ibuprofen, a reversible CO inhibitor, prevented PGE2 release during ozone exposure (and hence autocatalytic CO inactivation) but not loss of CO activity. Although CO activity remained depressed for hours after ozone exposure, compared with air-exposed cultures, no differences were detected in mRNA and protein levels of prostaglandin endoperoxide G/H synthase 2 (PGHS-2), the only CO isoform present in human TE cells, or in the rate of de novo PGHS-2 synthesis. Our findings suggest that ozone-induced PGE2 production and CO inactivation are primarily the result of formation of intracellular oxidant molecules and that delayed recovery of CO activity in human TE cells after short-term ozone exposure is due to persistent inactivation of PGHS-2, rather than to interference with its synthesis.