Delayed effects of tamoxifen in hepatocarcinogenesis-resistant Fischer 344 rats as compared with susceptible strains

The anti-oestrogenic drug tamoxifen has been under investigation as a breast cancer chemopreventive agent for at least a decade. However, its use for this purpose is still debatable since it is able to induce liver tumours in rats via a mechanism involving metabolic activation to a DNA adduct-forming electrophilic intermediate. The metabolic activation and adduct-forming properties of tamoxifen are now well characterized but less is known about its ability to induce hepatic cell proliferation, which is also essential for the carcinogenic process. The effects of tamoxifen on liver weight and cell proliferation were compared in female Fischer 344 (F344), Wistar and Lewis rats given the drug in the diet for up to 26 weeks. The onset and duration of hepatic cell proliferation varied between the strains of rat. In Wistar and Lewis but not F344 rats there was a marked increase in hepatocellular proliferation during the first 4 weeks of tamoxifen administration. In the Wistar strain this was associated with an increase in DNA adduct levels; no such increase was observed in the F344 strain. The onset of the proliferative response was delayed until the 13 week time point in the F344 strain. By the 13 and 26 week time points, cell proliferation in tamoxifen-treated Wistar and Lewis rat liver had returned to normal, but the amount of apoptotic activity in these livers was elevated. This suggests that excess cells generated during the proliferative phase of tamoxifen treatment were being eliminated by apoptosis. In the F344 strain, however, increased proliferative activity was associated with relatively low apoptotic activity at the 26 week time point, suggesting that the delayed proliferative response had yet to be balanced by apoptotic deletion. This is consistent with the fact that tamoxifen-induced hepatocellular tumours develop very late, towards the end of the lifespan, in this strain. The cell proliferative activity of tamoxifen in the Wistar rat liver was compared with that of a non-mutagenic analogue, toremifene. Tamoxifen induced increased cell cycle activity in the livers of rats following gavage dosing at all sampling times (1-12 weeks), whereas toremifene had no effect on the incidence of cycling in hepatic cells, demonstrating that the hepatic cell proliferation is not a general response to anti-oestrogen treatment. These observations suggest that the rate of promotion of liver tumours by tamoxifen is a function of the rate, time of onset and duration of increased cell replication. The susceptibility of rat strains to the hepatocarcinogenic effects of tamoxifen appears to depend upon the balance between initiation via DNA adduct formation, promotion via increased cell proliferation and cell deletion via apoptosis. Our findings suggest that an early proliferative response to tamoxifen is important in this process.