Abstract 5049: Multiple hepatic lesion sequences in dichloroacetic acid (DCA) stop and promotion studies in the B6C3F1 male mouse

Dichloroacetic acid (DCA) is a hepatocellular carcinogen in the male B6C3F1 mouse. Histopathologic analysis of a timed DCA dose response study has demonstrated three hepatic lesion sequences in this model. The purpose of the present study of 465 B6C3F1 male mice was to determine: (1) the persistence of early lesions in a “stop” study and (2) the efficacy of phenobarbital (PB) to promote these early lesions. The persistence of early lesions in mice exposed to drinking water containing 3.5 g/l DCA for 10 week intervals to 50 weeks was determined. One group remained exposed until the study was terminated at 100 weeks. Control animals received distilled water. In the promotion study, groups of animals exposed to drinking water containing 3.5 g/l DCA for 30 weeks were transferred to 0.6% phenobarbital (PB) or water alone and terminated at 60 or 90 weeks. Histopathologic analysis of liver lesions was performed on 1,481 formalin fixed, paraffin embedded tissue sections stained with hematoxylin and eosin. Lesions were identified and classified as altered foci (AF), hyperplastic nodules (HN), adenomas (AD) or carcinomas (CA). 10-50 weeks of DCA exposure doubled the prevalence of atypical (irregular misshapen and hyperchromatic) nuclei (82-95% vs 32% in untreated animals). Lifetime exposure to DCA was not required for induction of neoplastic lesions. Numerous neoplasms arose in animals exposed to 3.5 g/L DCA for 10, 20, 30, 40, or 50 weeks and then placed on water until 100 weeks. The prevalence of AD and CA increased with time to 50 weeks but decreased after 100 weeks, possibly reflecting the dose dependent suppression of proliferation in premalignant hepatic lesions such as hyperplastic nodules and adenomas by DCA. DCA exposure for 10-30 weeks doubled the prevalence of enlarged nuclei which were highly correlated with zonal changes. With increasing DCA exposure there was a shift in frequency of lesion types from hyperplastic nodues (20 weeks) to adenomas (40 weeks) to carcinomas (50 weeks). The major effect of PB promotion was to increase the number of eosinophilic AD at 60 and 90 weeks. These data demonstrate the continued development of preneoplastic hepatocellular lesions following termination of DCA treatment and are in agreement with studies which demonstrated that early-life exposure was as carcinogenic as life-long exposures for 98 weeks. Histopathologic analysis of these (DCA) stop and promotion studies in the B6C3F1 male mouse confirms that carcinomas can arise directly from single initiated cells in liver as well as within hyperplastic nodules and adenomas. Potentially, epigenetic-mediated effects may be involved since earlier reports indicate that RNA and micro-RNA profiles associated with prior DCA treatment were not sustainable. This work as supported by USEPA Cooperative Agreement #CR-814803-01-0. Citation Format: Julia H. Carter, Harry W. Carter, Anthony B. DeAngelo. Multiple hepatic lesion sequences in dichloroacetic acid (DCA) stop and promotion studies in the B6C3F1 male mouse [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5049.