Antiestrogens are pro-apoptotic in normal human breast epithelial cells.

Estrogens promote cell proliferation in normal and transformed mammary epithelial cells by inducing expression of hormone-responsive genes involved in the cell cycle. The action of antiestrogens is therefore central in regard to their potent inhibitory effects on estrogen-induced cell growth. We used normal human epithelial breast cells from primary cultures (HBE cells) to study hormonal (estrogen and antiestrogen) regulation on 3 key proteins involved in the apoptotic process: Bcl-2, p53 and caspase-3. The mammary adenocarcinoma cell line, MCF-7, was also used to study the molecular regulation of Bcl-2. In both HBE and MCF-7 cells, we found that estradiol (E2) induced an increase in Bcl-2 mRNA levels. This effect was counteracted in the presence of a pure antiestrogen, ICI 182780 (ICI). Alone, ICI did not modify either the Bcl-2 protein or mRNA levels in HBE cells, whereas in MCF-7, a strong downregulation of Bcl-2 mRNA was observed. In parallel, in HBE cells, we observed that E2 caused a decrease in p53 and caspase-3 protein levels, whereas ICI alone increased p53 and caspase-3 protein levels. The ICI effects on p53 and caspase-3 were partially counteracted by E2. Under the same experimental conditions, ICI exerts a potent pro-apoptotic effect, which was not counteracted by E2. In contrast, 4-hydroxytamoxifen was slightly weaker as a pro-apoptotic agent in HBE cells and its effects were reversed by E2. We demonstrate that in HBE cells, ICI reverses the anti-apoptotic action of E2 and alone acts as a highly potent pro-apoptotic molecule. These results provide new insight into treatment for breast cancer prevention.