Promotion by 17 β-estradiol and β-hexachlorocyclohexane of hepatocellular tumors in medaka, Oryzias latipes

Abstract

A feature common to many laboratory and field studies with various fish species is a higher prevalence of hepatocellular neoplasia in females than in males. During female sexual maturation, endogenous estrogens stimulate substantial increases in synthetic activity, including production of vitellogenin and choriogenin and proliferation of hepatocytes. We tested the hypothesis that estrogens, either natural or xenobiotic, promote growth of hepatic preneoplastic lesions and tumors. Medaka ( Oryzias latipes) were exposed to a low dose of the carcinogen diethylnitrosamine (DEN; 200 μg l −1 aqueous bath for 24 h) at 3 weeks of age, then fed purified casein-based diet or the same diet containing 17 β-estradiol (E2; 0.01–10.0 μg g −1 dry diet) or a xenoestrogen, β-hexachlorocyclohexane ( βHCH, 0.01–100.0 μg g −1 dry diet) daily from 1 to 7 months of age. Livers were removed, embedded in glycol methacrylate, step-sectioned, stained with hematoxylin and eosin, and examined for foci of cellular alteration (FCA) and hepatocellular tumors. E2 increased prevalences of hepatocellular adenoma or carcinoma (26% in DEN plus 10 ppm E2 group versus 4.6% in DEN only group, P<0.01). With increasing level of E2, average numbers of basophilic FCA (BF) rose and numbers of eosinophilic FCA (EF) sharply declined. There were greater numbers of tumors in most and greater numbers of BF in all of the DEN plus βHCH treatment groups, but statistical analyses indicated no significant elevation in tumor or BF prevalence relative to treatment with DEN only. In all DEN-treated groups, BF were more common in female medaka and EF more common in males. No tumors were found in fish fed E2 or βHCH without DEN exposure. Among control medaka, liver weights were significantly larger in females, but treatment with 0.1, 1.0 or 10.0 ppm E2 elevated liver weights in males similar to that in females. βHCH had no effect on liver weights. This data shows E2 is a tumor promoter in medaka. Because tumor increases were not statistically significant, βHCH was considered a weakly positive modulator. E2 particularly promoted tumor development in male medaka, indicating xenobiotics with mechanism of action like that of E2 may escalate growth in wild fish of previously initiated cells into tumors.