Maturitas 49 (2004) 235–240 Breast cancer risk during HRT: influence of estradiol metabolites on breast cancer and endothelial cell proliferation  Harald Seeger, Fritz-Ulrich Deuringer, Diethelm Wallwiener, Alfred O. Mueck ∗ Section of Endocrinology and Menopause, University Women’s Hospital, Calwerstrasse 7, 72 076 Tuebingen, Germany Received 23 July 2003; received in revised form 3 February 2004; accepted 17 February 2004 Abstract Objectives: Long-term hormone replacement therapy is associated with an increased breast cancer risk. Evidence is accumu- lating that estradiol metabolites are involved in carcinogenesis. These metabolites may have proliferating and anti-proliferative properties. We have investigated the effect of 14 metabolites on the proliferation of human breast cancer cells and on the pro- liferation of human vascular endothelial cells. Methods: As cell model, human umbilical vein endothelial cells (HUVEC) and the human breast cancer cell line MCF-7 were used. The relationship between dosage and effect was tested over the pharmaco- logical concentration range of 10 -8 to 10 -5 M. Results: In HUVECs, all of 10 A-ring metabolites tested stimulated the growth of the endothelial cells at the lower concentrations. At the highest concentration, some A-ring metabolites caused significant inhibitions. The D-ring metabolites showed no marked effects compared to the A-ring metabolites. In MCF-7 cells also, nearly all A-ring metabolites demonstrated a biphasic reaction behaviour on cell proliferation. For the D-ring metabolites, this biphasic pattern was only found for 16-hydroxyestrone, but the inhibitory effect of this metabolite was weak. Conclusion: These results indicate that certain endogenous estradiol metabolites are able to stimulate breast cancer cell proliferation, and others may be suitable for breast cancer treatment when used in high dosages, since they inhibit cancer cell growth as well as neoangiogen- sis. This may be of special importance for therapy, since some of these metabolites are virtually devoid of any oestrogenic activity. © 2004 Elsevier Ireland Ltd. All rights reserved. Keywords: Estradiol metabolites; Human vascular endothelial cells; Human breast cancer cells; Proliferation  The data presented have been awarded with the poster prize at the 6th European Congress on Menopause, 24–28 May 2003, Bucharest, Romania. Maturitas 2003; 44 (Suppl. 2): S126 (Poster P01.02). ∗ Corresponding author. Fax: +49-7071-29-4801. E-mail address: endo.meno@med.uni-tuebingen.de (A.O. Mueck). 1. Introduction Long-term hormone replacement therapy is as- sociated with an increased breast cancer risk [1,2]. Discussion is ongoing whether estradiol is only a promoter of carcinogenesis or is carcinogenic per se. Evidence is accumulating that estradiol metabolites are involved in cancer development [3]. Estradiol metabolism occurs primarily via two mutually ex- clusive pathways yielding the A-ring and the D-ring 0378-5122/$ – see front matter © 2004 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.maturitas.2004.02.004