Tibolone actions on normal and breast cancer cells

Tibolone and its main derivatives were studied in an original model of cultures of normal human epithelial breast (HBE) cells on proliferation, differentiation and apoptosis, the three mechanisms responsible for breast homeostasis. Tibolone and its Delta4 isomer were antiproliferative, both in the absence and presence of oestradiol (E2). The oestrogenic 3alpha and 3beta hydroxy derivatives did not display any mitogenic activities in HBE cells. Moreover, at 1 microM, they were antiproliferative. Tibolone and its Delta isomer increased the 17beta hydroxysteroid dehydrogenase activity similarly to that observed with progestins [1]. Apoptosis was increased in HBE cells to a similar range as with the pure pregnane progestin, Org2058. We also studied the extent of apoptosis in hormone-dependent breast cancer cell lines. Tibolone and its Delta4 isomer also increased apoptosis, especially in ZR75-1 cells containing progesterone and androgen receptors [2]. We could demonstrate that these pro-apoptotic actions of tibolone and its Delta4 isomer were mediated at least partially through the bcl-2-family of proteins. Moreover, the antiproliferative and pro-apoptotic activities of tibolone, as well as Org2058, were mediated by increasing catalase activities in breast cancer cells. Thus, in breast cells, tibolone slows down the proliferation rate, increases differentiation and apoptosis. These actions seem to be optimal on breast tissue.