Activation of eicosanoid metabolism in human airway epithelial cells by ozonolysis products of membrane fatty acids.

Inhaled ozone can react with a variety of cellular macromolecules within the lung. Recent analyses of the chemistry of ozone reactions with unsaturated fatty acids, which are present in all membranes and in mucus in the airways, indicate that ozonolysis yields one aldehyde and one hydroxyhydroperoxide molecule for each molecule of ozone. The hydroxyhydroperoxide molecule is unstable in aqueous environments, and subsequently yields a second aldehyde and hydrogen peroxide. The structure of common unsaturated fatty acids is such that attack by ozone at the carbon-carbon double bonds will yield 3-, 6-, and 9-carbon saturated and unsaturated aldehydes and hydroxyhydroperoxide. This study examines the effects of ozonolysis products on eicosanoid metabolism in human airway epithelial cells. Eicosanoid biosynthesis is important in a wide array of pathophysiological responses in the airway, and the release of eicosanoids by the epithelial barrier is likely to be significant in diseases induced by environmental factors. Previously, we demonstrated that ozone can increase eicosanoid synthesis from airway epithelial cells exposed in vitro. Human exposures to concentrations of ozone below the current National Ambient Air Quality Standard (0.12 ppm, not to be exceeded for more than one hour once per year) also resulted in increased eicosanoids in bronchoalveolar lavage fluid. To determine whether ozonolysis products could activate eicosanoid release, we exposed human airway epithelial cells to 3-, 6-, and 9-carbon aldehydes, hydroxyhydroperoxides, and hydrogen peroxide. We measured (1) eicosanoid metabolism using high-performance liquid chromatography and radioimmunoassays, and (2) the effects of the aldehydes, hydroxyhydroperoxides, and hydrogen peroxide on cell lysis. Eicosanoid release increased after exposure to aldehyde; release induced by 9-carbon (nonanal) aldehyde was greater than that induced by the 6-carbon (hexanal) or 3-carbon (propanal) aldehydes. Hydroxyhydroperoxides induced greater eicosanoid release than the corresponding aldehydes of equivalent chain length. Again, the longer the aliphatic chain length of the hydroxyhydroperoxide the greater the effect. These effects were noted at concentrations of hydroxyhydroperoxide below those that produce cell lysis, and the time course of the two responses was dissimilar. Because hydroxyhydroperoxides can degrade into an aldehyde and hydrogen peroxide, it is conceivable that the effects observed were attributable to the formation of either hydrogen peroxide or hydrogen peroxide and aldehyde. This mechanism is unlikely, however, because the effects of hydroxyhydroperoxides on eicosanoid release were dependent on chain length, whereas each hydroxyhydroperoxide can produce only one hydrogen peroxide molecule. Although hydrogen peroxide alone also stimulated eicosanoid metabolism, this effect was not augmented when aldehyde and hydrogen peroxide were added together. In addition, the dose of hydroxyhydroperoxide needed to produce an effect (10 to 100 microM) was lower than that of hydrogen peroxide (300 microM). We could not fully evaluate the effects of the unsaturated aldehydes and hydroxyhydroperoxides. Although the 6-carbon and 9-carbon cis-3-aldehydes could be synthesized from the cis-3-alcohols, the resulting aldehydes were not chemically stable. The cis-3-aldehydes were useful for producing the corresponding 1-hydroxy-alkenyl-hydroperoxides of high purity. These results support the method selected for chemical synthesis, but further studies are required to establish proper storage and handling methods before these compounds can be tested in assays of eicosanoid metabolism.