Sex differences in the activation of tamoxifen to DNA binding species in rat liver in vivo and in rat hepatocytes in vitro: Role of sulfotransferase induction

Previous work has indicated that metabolic activation of tamoxifen in fat l iver cells involves cytochrome P450-mediated alpha-hydroxylation, followed by sulfate ester formation, mediated by hydroxysteroid sulfotransferase a ( rHSTa), a member of the SULT2A subfamily, which efficiently metabolizes deh ydroepiandrosterone. Because it is known that the expression of rHSTa and o ther SULT2A forms is substantially higher in female rats than in males, it might be predicted that tamoxifen would be a more potent liver carcinogen i n females than in males. Yet tamoxifen has been shown to be equipotent in b oth sexes, To investigate this paradox, primary cultures of hepatocytes wer e prepared from Fischer F-344 rats and treated with tamoxifen (10 mu M) or alpha-hydroxytamoxifen (1 mu M). Rats were also treated dth tamoxifen daily by gavage (0.12 mmol/kg/day) for up to 14 days. DNA was isolated from hepa tocytes and liver and analyzed by P-32-postlabeling. Liver cytosol fraction s were prepared and analyzed for dehydroepiandrosterone sulfotransferase ac tivity and SULT2A protein levels. In tamoxifen-treated hepatocytes and afte r a single dose of tamoxifen in vivo, DNA adduct formation in male cells wa s significantly lower than in female cells, 11- and 6-fold, respectively. H owever,,vith increasing daily doses of rats with tamoxifen, the adduct leve l in males increased to a level 89% of that in females by 14 days. Dehydroe piandrosterone sulfotransferase activity In male rat liver cytosols was onl y 17% of the activity of female cytosols after one dose of tamoxifen but 64 % after 14 days of exposure to the compound. This increase in activity corr elated with increases in the levels of SULT2A protein, detected by Western blotting. Western blotting did not allow the unambiguous identification of the induced SULT2A form(s). However, by using a specific reverse transcript ase/PCR technique, it was found that It was primarily rHSTa that was induce d. Thus, after prolonged exposure to tamoxifen, DNA adduct formation and rH STa expression in males are significantly closer to the levels in females t han they are after initial exposure. These changes explain the similar susc eptibility of male and female rats to tamoxifen carcinogenesis.