Tibolone is a tissue specific steroid hormone newly recognized as an estrogenic agent for hormone replacement therapy (HRT). The aim of the study is to characterize the basic mechanism of tibolone in the PLD signal transduction pathway of breast cancer cell lines.

The levels of phospholipase D (PLD), caspase 3 mRNA and protein, and the cell counts were measured in estrogen receptor positive MCF-7 and negative MDA-MB-231 cell lines treated with estradiol, tamoxifen and tibolone and three metabolite forms of tibolone (3β-OH-tibolone, ▵4 isomer, 3α-OH-tibolone). Multimodality methods such as RT-PCR, immunoblot analysis and in vivo enzyme activity assay were used.

The addition of estradiol to MCF-7 cell line resulted in cell proliferation in a time-dependent manner while that of tamoxifen and tibolone showed antiproliferative effects. The addition of tamoxifen or tibolone to MCF-7 and MDA-MB-231 cell lines resulted in the elevation of caspase 3 mRNA levels, indicating the induction of apoptosis. PLD mRNA level was elevated in both cell lines treated with tamoxifen, but decreased in those treated with the various tibolones, except for 3β-OH-tibolone. In immunoblot analysis, while MCF-7 cell line treated with tamoxifen showed an increased level of PLD expression, in MDA-MB-231 cell line the expression was decreased. Similar results were observed in the addition of tibolones, which resulted in an increase of PLD expression level in MCF-7 cell line and a decrease in MDA-MB-231 cell line. In vitro PLD activity assay showed decreased activity after estradiol treatment and increased activity after tamoxifen and tibolone treatment in MDA-MB231 cell line. In MCF-7 cell line,among the tibolones only ▵4 isomer increased PLD activity. Tiboloneshowed antiproliferative and apoptosis-inducing effects on MCF-7 and MDA-MB-231 cell lines. But its influence on the signal transduction pathway varied slightly between the two cell lines.

we were able to find the antiestrogenic properties of the estrogenic agent tibolone.