CELLULAR AND METABOLIC BASIS OF CLARA CELL TOLERANCE TO MULTIPLE DOSES OF CYTOCHROME P450-ACTIVATED CYTOTOXICANTS .1. BRONCHIOLAR EPITHELIAL REORGANIZATION AND EXPRESSION OF CYTOCHROME-P450 MONOOXYGENASES IN MICE EXPOSED TO MULTIPLE DOSES OF NAPHTHALENE

The objectives of this study were to quantitatively define alterations in the epithelial populations and expression of cytochrome P450 monoo xygenases in distal airways which are associated with the tolerance re sulting from repeated exposure of mice to the acute Clara cell toxican t, naphthalene. Bronchiolar epithelium of mice treated daily for 7 day s with an acutely cytotoxic dose of naphthalene (200 mg/kg/day) quanti tatively resembles the bronchiolar epithelium of control animals in te rms of proportions of ciliated and nonciliated cells and nuclear and c ytoplasmic volumes. Subsequent treatment of tolerant animals with high er doses (300 mg/kg) does not produce the same Clara cell injury obser ved in previously untreated mice after a single treatment with 300 mg/ kg. After repeated exposures to naphthalene, cellular expression of im munoreactive cytochrome P450 monooxygenases 2B and 2F, P450 reductase and Clara cell secretory protein significantly decreased in bronchiola r epithelium. Although metabolism of naphthalene to the 1R,2S-naphthal ene oxide is depressed in microsomes derived from whole-lung homogenat es of tolerant animals, metabolism of naphthalene in distal airways is olated by microdissection is unchanged from the controls at saturating substrate concentrations. When substrate concentrations are less than the apparent K-m for naphthalene (<0.05 mM), the rate of naphthalene metabolite production is markedly lower in distal airways of tolerant mice compared with controls. Covalent binding of reactive naphthalene metabolites in lungs of tolerant mice is also unchanged from control. In conclusion, after repeated exposure to short-term cytotoxic doses o f naphthalene, distal bronchioles of tolerant mice 1) are lined by epi thelium which resembles that of controls; 2) express lower levels of P 450 proteins; 3) have reduced levels of naphthalene monooxygenase acti vity, but only at less than saturating concentrations and 4) have no d ecrease in covalent binding of reactive metabolites to proteins.