Prostaglandin H synthase-dependent co-oxygenation of (±)-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene in hamster trachea and human bronchus explants

Abstract

The role of prostaglandin H synthase (PHS) in the metabolism of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) has been examined in short-term explant cultures of hamster and human tracheobronchial tissues. Labeled BP-7,8-diol was incubated with the explants in the presence and absence of the PHS substrate arachidonic acid (20:4) and the PHS inhibitor indomethadn. The addition of 10 μM to 200 μM 20:4 to incubations of hamster trachea with 5 μM BP-7,8-diol caused significant increases in the formation of 7 r ,8 t -dihydroxy-9 t ,10 t -epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE). These increases were not seen when 1 μM or 20 μM BP-7,8-diol was employed. The stimulation of anti-BPDE formation was observed after incubations of from 1 to 48 h. This stimulation was inhibited to the basal level by 20 μM indomethacin, supporting the role of PHS in the response. No effect of 20:4 was seen on the uptake of BP-7,8-diol by the tracheas or on the formation of water-soluble metabolites. Significant increases in covalent binding of BP-7,8-diol metabolites to DNA of the tracheal epithelium were also elicited by the addition of 20:4, however these increases were not well correlated quantitatively with the increases in anti-BPDE formation. H.p.l.c. profiles of deoxynucleoside adducts from basal and 20:4-stimulated incubations were qualitatively identical. Far greater variability of metabolism was seen in human bronchus explants, but 20:4-dependent increases in anti-BPDE formation could be demonstrated in those tissues as well. Inhibition of this stimulation by indomethacin was either absent or incomplete. This variation in the effect of indomethacin was explained by the examination of the products of 20:4 metabolism by the two tissues. Hamster trachea produced almost exclusively PHS metabolites whereas human bronchus yielded predominantly products of lipoxygenases, enzymes insensitive to indomethacin. In conclusion, this study indicates that co-oxygenation of chemical carcinogens can occur in hamster and human tracheobronchial tissues. The concentration-dependence observed with BP-7,8-diol, however, suggests that this pathway is of minor importance in the activation of BP in these tissues.