Activation of the Her2/neu receptor leads to antiestrogen resistance, but acquired resistance is not caused by altered ErbB expression and activation

Full text

Development of acquired resistance against antiestrogen treatment is a serious problem in human breast cancer, and knowledge of alterations resulting in resistance is important for selection of further treatment. To mimic the clinical situation we have established a series of MCF-7 human breast cancer cell lines by long term treatment with the antiestrogens tamoxifen, ICI 164,384, and ICI 182,780 (Faslodex^™). Common for these cell lines is a decreased expression of the estrogen receptor α (ERα). In human breast cancer, lack of response to endocrine therapy is often associated with decreased expression of the estrogen receptor and increased expression of epidermal growth factor receptor (EGFR) and/or HER-2/neu (ErbB-2). Our antiestrogen-resistant cell lines did not express altered levels of EGFR, HER-2/neu, ErbB-3 and ErbB-4. Estrogen and antiestrogen regulation of HER-2/neu expression was essentially similar in parent and resistant MCF-7 cells. Treatment with antibodies to HER-2/neu (Herceptin^™) did not affect growth of MCF-7 cells or resistant cells, indicating that in this in vitro model system, acquired antiestrogen resistance does not emerge from activation of the HER-2/neu signalling pathway. However, addition of heregulin1-β 1 abolished the inhibitory activity of ICI 182,780 on MCF-7 cells, demonstrating that activation of the HER-2/neu receptor signalling pathway can override the growth inhibitory effect of ICI 182,780. The effect of heregulin1-β 1 could be abrogated by Herceptin^™.