Cytotoxicity of naphthalene toward cells from target and non-target organs in vitro

Chronic inhalation exposure to high concentrations of naphthalene produced nasal tumors in rats and lung tumors in female mice. Naphthalene bioactivation is required for target organ toxicity and cytotoxicity in target organs may be involved in tumor development. The present studies characterized the dose-response relationships for naphthalene-induced glutathione (GSH) depletion, effects on cellular ATP, and cytotoxicity in cells from both target (lung, nasal epithelium) and non-target (liver) organs in vitro using cells from F-344 rats, B6C3F1 mice and humans. The cells were incubated with various concentrations of naphthalene in sealed glass flasks for 3h, then placed in monolayer culture in fresh media for 24h to examine the repair or progression of damage. Naphthalene was a low potency cytotoxicant in vitro, with 500 μM frequently observed as a no-observed adverse effect concentration or lowest observed adverse effect concentration. Naphthalene exposure produced dose-dependent decreases in cellular GSH, ATP and viability in rat, mouse and human hepatocytes at concentrations >500 μM. Human nasal respiratory epithelial cells exhibited greater naphthalene cytotoxicity than rat or mouse nasal respiratory epithelial cell preparations. Rat nasal respiratory epithelial cell preparations metabolized naphthalene through pathways leading to the preferential formation of 1,2-naphthoquinone GSH conjugates rather than 1,4-naphthoquinone GSH conjugates observed in rat hepatocytes or mouse nasal respiratory epithelial cells, consistent with the suggestion that this bioactivation pathway may be involved in rat nasal tumor development. Naphthalene exposures of ≥500 μM decreased cellular GSH and ATP in rat, mouse and human lung cell preparations. The variability of the responses of the human lung cell preparations was consistent with the known variability of CYP activities in human lung tissue. The results of these studies can be used as the basis for future studies of the mechanisms involved in naphthalene-induced cytotoxicity and the relevance of the bioactivation pathways for human exposure to naphthalene.