Effects of long-term tamoxifen treatment on ERα expression in breast cancer cells

Tamoxifen has long been the drug of choice in endocrine therapy of oestrogen receptor a ERα-positive breast cancer and still remains widely used due to its well-documented beneficial effect. However, its efficacy is often limited by the onset of acquired resistance and several clinical studies have suggested that a proportion of tumours, which are initially ERα-positive, lack the receptor at the time of tamoxifen relapse in the adjuvant or metastatic setting. The mechanisms underlying acquired ERα-negativity remain largely unknown and their elucidation is of therapeutic importance since breast cancer lacking ERα is associated with endocrine resistance, aggressive tumour biology and poor prognosis. This study addressed the issue of acquired ERα-negativity during tamoxifen therapy by assessing ERα expression in tamoxifen-resistant MCF-7-derived cells, which were cultured in the presence of the anti-hormone over a 30-month period. Results showed a progressive and significant reduction of total ERα mRNA and protein expression in response to tamoxifen therapy and this was accompanied with greatly increased aggressive tumour cell behaviour. The tamoxifen-treatment regimen also resulted in reduced expression of all ERα mRNA variants, which are generated through alternative promoter usage of the ERα gene. Such reduction was most apparent for ERα-mRNA variants C. Importantly, pharmacological modulation of cell signalling pathways identified the epidermal growth factor receptor EGFR signalling maintaining ERα levels, whilst c-Src kinase activity appeared to be the key underlying cause of ERα loss during tamoxifen therapy. Encouragingly, even after a 30-month treatment regime with tamoxifen, ERα loss was reversible with a c-Src inhibitor. The data presented in this thesis suggest that combinations of anti-hormones with c-Src inhibitors could retain ERα functions during tamoxifen therapy and prevent a drift towards more aggressive cancer cell behaviour