Tamoxifen still remains the most frequently used antioestrogen for the treatment of breast cancer. However, its efficacy is often limited by the emergence of acquired resistance and it has been suggested that, in some instances, this may involve oestrogen receptor (ER) loss. This study addresses this issue by examining long-term tamoxifen treatment of breast cancer cells and identifies that progressive ER loss does occur, leading to greatly increased aggressive tumour cell behaviour. Encouragingly, even after 30 months treatment, ER loss is reversible by a c-src inhibitor. Our data therefore provide a new model to study the cellular mechanisms associated with antihormone promoted ER loss and its possible prevention/reversal by signal transduction inhibitors.

Using quantitative PCR based on SYBR Green fluorescence, the expression of total ERα mRNA and its constituent mRNA variants were quantified in MCF7 cells and in our in vitro developed tamoxifen-resistant breast cancer cells (TamR), which have been cultured in the presence of tamoxifen for 30 months. Specific PCR amplification of all ERα mRNA variants was possible using forward primers designed to bind specifically to the 5' untranslated regions of ERα mRNA and used separately with a common reverse primer that anneals to the 5' end of the protein encoding region of exon 1 of ERα cDNA. Expression of ERα protein was assessed by western blot and immunohistochemistry.

In MCF7 cells, the ERα mRNA isoforms A, B and C were detected as the most predominant variants, with C ERα mRNA showing the highest expression level. In TamR cells, about a 40% fall in total ERα mRNA was observed in comparison with MCF7 cells and was most apparent for the C variant. Extension of the tamoxifen treatment period to 30 months produced a further dramatic decrease in ERα mRNA (all variants) and protein levels, resulting in ER negativity being recorded in >90% of the cells by immunohistochemistry. These cells show increased levels of phosphorylated Erk 1&2, AKT, PKCα and src, and are highly aggressive in their growth behaviour, with increased cell motility and invasiveness. Treatment of the cells with the demethylating agent 5-azacytidine did not restore ERα expression, suggesting that epigenetic alterations are unlikely to be responsible for the reduced ER levels. However, Affymetrix data in the TamR cells showed that some positive regulators of ER expression, such as p53 and Foxo3A, are downregulated during the development of the resistant phenotype and their continued absence may contribute to the progressive ER loss. Significantly, pathway inhibitor studies revealed c-src to be an important regulator of ER loss, since its inhibition rapidly restored ER levels.

Our data indicate that considerable ER loss can occur during antihormonal treatment of breast cancer cells and that this can lead to a more aggressive phenotype. Encouragingly, however, even after 30 months exposure to tamoxifen, the process is reversible by inhibition of c-src. These data suggest that combinations of antihormones with signal transduction inhibitors could retain ER functions in treated cells and prevent a drift towards more aggressive cancer cell behaviour.