EXAMINATION OF POTENTIAL MECHANISMS OF CARCINOGENICITY OF 1,4-DIOXANE IN RAT NASAL EPITHELIAL-CELLS AND HEPATOCYTES

Several long-term studies with 1,4-dioxane (dioxane) have shown it to induce liver tumors in mice, and nasal and liver tumors in rats when administered in amounts from 0.5 to 1.8% in the drinking water (Argus et al. 1965; Kociba et al. 1974; National Cancer Institute, 1978). In order to examine potential mechanisms of action, chemically-induced DNA repair (as an indicator of DNA reactivity) and cell proliferation (as an indicator of promotional activity) were examined in nasal turbinate epithelial cells and hepatocytes of male Fischer-344 rats treated with dioxane. Neither dioxane nor 1,4-dioxane-2-one, one of the proposed metabolites, exhibited activity in the in vitro primary rat hepatocyte DNA repair assay, even from cells that had been isolated from animals given either 1 or 2% dioxane in the drinking water for 1 week to induce enzymes that might be responsible for producing genotoxic metabolites. No activity was seen in the in vivo hepatocyte DNA repair assay in animals given a single dose of up to 1000 mg/kg dioxane or up to 2% dioxane in the drinking water for 1 week. Treatment of rats with 1.0% dioxane in the drinking water for 5 days yielded no increase in liver/body weight nor induction of palmitoyl CoA oxidase, indicating that dioxane does not fit into the class of peroxisomal proliferating carcinogens. The percentage of cells in DNA synthesis phase (S-phase) was determined by administration of H-3-thymidine and subsequent quantitative histoautoradiography. The hepatic labeling index (LI) did not increase at either 24 or 48 h following a single dose of 1000 mg/kg dioxane. The LI did increase approximately two-fold in animals given dioxane in the drinking water for 2 weeks. No DNA repair was seen in either nasoturbinate or maxilloturbinate nasal epithelial cells isolated from animals treated with 1% dioxane in the drinking water for 8 days followed by a single dose of up to 1000 mg/kg dioxane by gavage 12h before sacrifice. Reexamination of the nasal passages of male rats in archived material from the NTP bioassay (National Cancer Institute 1978), revealed that the primary site of tumor formation was the anterior third of the dorsal meatus. The location of these tumors supports the proposal that inhalation of dioxane-containing drinking water may account for the site specificity of these nasal lesions. In vivo studies showed no increase relative to controls in cell proliferation at the site of highest tumor formation in the nose in response to 1.0% dioxane in the drinking water for 2 weeks. Thus, repair-inducing DNA adduct formation, peroxisomal proliferation in the liver, and short-term induction of cell proliferation in the nose do not appear to be involved in tumor formation by dioxane. There may be a role of dioxane-induced cell proliferation in the formation of the liver tumors. However, the quantitative relationships between induced cell proliferation and tumorigenic potential have yet to be established.