Tamoxifen and raloxifene are widely used in clinical practice. It has been found that tamoxifen treatment increases the risk of development of endometrial cancer. The effects of tamoxifen and raloxifene on endometrium might be caused by different estrogen receptor expression. The aim of the present study was immunohistochemical evaluation of the effects of tamoxifen and raloxifene on estrogen receptors, and Ki-67 antigen expression in the human endometrial adenocarcinoma Ishikawa cell line. Tamoxifen in concentrations of 10 µM and 20 µM increased ERα expression without any effect on ERβ. All used concentrations of tamoxifen and raloxifene (0.1 nM, 1 nM, 10 nM, 1 µM, 10 µM and 20 µM) had no effect on expression of ERβ. Tamoxifen, but not raloxifene, increased Ki-67 antigen expression in the Ishikawa cell line. Tamoxifen, in contrast to raloxifene, increased proliferation of endometrial adenocarcinoma cells as well as exerted the shift of ERα/ERβ ratio. Thus, it could be responsible for increased carcinogenic effect during tamoxifen treatment.

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