Localization of tamoxifen-induced DNA adducts in rat liver

Abstract

Tamoxifen is a non-steroidal anti-estrogen and is one of the most widely used and successful agents in the treatment of breast cancer. Tamoxifen competes with endogenous estrogens for estrogen receptors in mammary tissue, and thereby inhibits transcription of estrogen-responsive genes. The success of tamoxifen in the adjuvant therapy of breast cancer has resulted in its current widespread use as a prophylactic in healthy women who are at high risk of developing breast cancer. However, the prophylactic use of tamoxifen is controversial because of mixed results from clinical trials and, in particular, the increase in the incidence of endometrial cancer associated with tamoxifen therapy.The reason for this increase in endometrial cancer in humans is a matter of considerable debate. One theory is that tamoxifen is a genotoxic carcinogen – that is, it is metabolized to electrophilic, DNA-reactive intermediates that damage the genetic material and form DNA ‘adducts’. In support of this hypothesis, tamoxifen is a potent hepatocarcinogen in rats, and adducts are indeed found in the liver DNA of rats treated with tamoxifen. In humans, however, there are inconsistencies in the literature concerning the presence of DNA adducts in endometrial tissue from women undergoing tamoxifen therapy. It is therefore unclear whether tamoxifen is a genotoxic carcinogen in humans, or whether it exerts its carcinogenic effects by another mechanism.A recent report by Divi et al. 1xImmunohistochemical localization and semi-quantitation of hepatic tamoxifen-DNA adducts in rats exposed orally to tamoxifen. Divi, R.L. et al. Carcinogenesis. 2001; 10: 1693–1699CrossrefSee all References1 might help shed light on this contentious issue. The authors have developed a sensitive immunohistochemical assay to detect tamoxifen–DNA adducts in rat liver tissue sections. An advantage of this technique over conventional assays for adduct detection, in which DNA must first be isolated from tissue or cells, is that adducts can be localized to particular regions of the tissue in situ. In a beautiful series of immunohistochemical staining images, Divi and colleagues demonstrate nuclear localization of DNA adducts in liver sections from rats treated orally with tamoxifen at various doses. In subsequent experiments, the authors located regions within the liver tissue that expressed an altered form of glutathione-S-transferase, a phase II detoxification enzyme that reduces adduct formation. It was found that, in these regions, levels of tamoxifen–DNA adducts were much lower. Most striking of all is the presence of discrete foci of cells, positive for tamoxifen–DNA adducts, surrounded by areas of tissue that are relatively adduct free; this demonstrates localization of adducts at single-cell resolution within the tissue.The authors raise the possibility that this technique might be suitable for the analysis of human endometrial samples from women undergoing tamoxifen therapy, and might reveal specific regions or cells within the endometrial tissue that contain significant levels of tamoxifen–DNA adducts. This technique might well be a more suitable and sensitive method of analysis than other adduct-detection methods in which DNA must be isolated, and in which the presence of adducts in certain regions of the tissue are masked by the vast majority of surrounding tissue that contains no DNA adducts. If this is indeed the case, analysis of the localization of tamoxifen–DNA adducts in human tissues might help us understand the mechanism of tamoxifen-induced carcinogenesis in humans.