Ozone inhibits endothelial cell cyclooxygenase activity through formation of hydrogen peroxide

We have previously demonstrated that a 2H exposure of cultured pulmonary endothelial cells to ozone (0.0–1.0 ppm) resulted in a concentration-dependent reduction of endothelial prostacyclin production (90% decrease at the 1.0 ppm level). Ozone-exposed endothelial cells, incubated with 20 uM arachidonate, also demonstrated a significant inhibition of prostacyclin synthesis. To further examine the mechanisms of the inhibition of prostacyclin synthesis, bovine pulmonary endothelial cells were exposedto 1.0 ppm ozone for 2H. A significant decease in protacyclin synthesis was found within 5 min of exposure (77 ± 36% of air-exposed control values, p < 0.05). Endothelial prostacyclin synthesis returned to baseline levels by 12H after ozone exposure, a time point which was similar to the recovery time of unexposed endothelium treated with 0.5 uM acetylsalicylic acid. Incubation of endothelial cells, previously exposed to 1.0 ppm ozone for 2 hours, with 4 uM PGH₂ resulted in restoration of essentially normal prostacyclin synthesis. When endothelial cells were co-incubated with catalase (5U/ml) during ozone exposure, no inhibition of prostacycline synthesis was observed. Co-incubation with either heat-inactivated catalase or superoxide dismutase (10U/ml) did not affect the ozone-induced inhibition of prostacycline synthesis. These data suggest that H₂O₂ is a major toxic species produced in endothelial cells during ozone exposure and responsible for the inhibiton of endothelial cyclooxygenase activity.