title: Development of tolerance to Clara cell necrosis with repeat administration of coumarin.
abstract: Coumarin was identified as a mouse-lung carcinogen following oral gavage administration in a chronic bioassay, and was shown to cause the selective necrosis of terminal bronchiolar Clara (non-ciliated bronchiolar epithelial) cells in the mouse lung after acute administration. After oral gavage, a similar effect was not observed in the terminal bronchioles of rats, suggesting that coumarin-mediated Clara cell toxicity is a species-specific effect. Using coumarin dosages (50 and 200 mg/kg) and a dosing schedule modeled after the chronic bioassay, the current study examined the effects of repeated coumarin administration in mouse lung. A single dosage of coumarin (200 mg/kg) caused swelling of Clara cells and necrosis in mouse-lung terminal bronchioles. However, after 5 consecutive oral doses of coumarin (200 mg/kg), the mouse lung became tolerant to coumarin, and although areas of bronchiolar epithelial flattening and hyperplasia were noted, Clara cell necrosis was not observed. After 10 doses of coumarin, mouse lungs appeared nearly normal. Coumarin-mediated Clara cell injury is thought to result from the cytochrome P450-catalyzed formation of coumarin 3,4-epoxide and Western analysis of whole mouse lung microsomal P450 content indicated that, commensurate with Clara cell necrosis, many P450s were decreased. However, P450 levels appeared qualitatively normal in lung microsomes from tolerant mice. Similarly, coumarin epoxidation and 7-hydroxylation rates in whole lung microsomes from tolerant animals were similar to controls. To determine if animals tolerant to coumarin were tolerant to other Clara cell toxicants, a single toxic dose of naphthalene (200 mg/kg) was administered to coumarin-tolerant mice. Coumarin pretreatment reduced naphthalene-mediated Clara cell toxicity, supporting the hypothesis that tolerance may result from general biochemical and molecular changes and not exclusively from alterations in chemical metabolism.
authors.affiliation: Procter and Gamble Company, Miami Valley Laboratories, Cincinnati, Ohio 45253, USA. born.sl@pg.com
authors.forename: S L
authors.initials: SL
authors.lastname: Born
authors.forename: A S
authors.initials: AS
authors.lastname: Fix
authors.forename: D
authors.initials: D
authors.lastname: Caudill
authors.forename: L D
authors.initials: LD
authors.lastname: Lehman-McKeeman
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000493:pharmacokinetics / Q000633:toxicity*; D001711:Biotransformation; D002273:Carcinogens / Q000493:pharmacokinetics / Q000633:toxicity*; D003374:Coumarins / Q000493:pharmacokinetics / Q000633:toxicity*; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000378:metabolism; D004361:Drug Tolerance; D004847:Epithelial Cells / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism / Q000473:pathology; D005260:Female; D008168:Lung / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism / Q000473:pathology*; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D009336:Necrosis
chemical_list: D000970:Antineoplastic Agents; D002273:Carcinogens; D003374:Coumarins; D003577:Cytochrome P-450 Enzyme System; C030123:coumarin