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Contains fulltext : 200105.pdf (Publisher’s version ) (Open Access) Estrogen receptor-alpha (ERalpha)-positive breast cancer is often treated with antihormonal regimens. However, resistance to treatment is common, leading to metastatic disease. ERalpha activity requires the functional involvement of pioneer factors FOXA1 and GATA3, which enable ERalpha-chromatin binding and are crucial for ERalpha-driven cell proliferation. FOXA1 was found increased in metastatic breast cancers in relation to the primary tumor, but a comprehensive clinical assessment thereof, in relation to different metastatic sites and endocrine therapy usage, is currently lacking. Prior cell line-based reports, however, have revealed that FOXA1 is required for tamoxifen-resistant tumor cell proliferation. We studied expression levels of ERalpha, GATA3, and FOXA1 by immunohistochemistry in samples from both primary tumors and various metastatic sites. For all factors, expression levels varied between the metastatic sites. For pleural metastases, strong variation was found in FOXA1 and GATA3 levels. Although GATA3 levels remained unaltered between primary breast cancer and pleural metastases, FOXA1 levels were reduced exclusively in metastases of patients who received endocrine therapies in the adjuvant setting, even though ERalpha was still expressed. Importantly, decreased FOXA1 levels in pleural metastases correlated with hormone irresponsiveness in the palliative setting, while no such correlation was found for GATA3. With this, we show divergent clinical correlations of the two ERalpha pioneer factors FOXA1 and GATA3 in metastatic breast cancer, where endocrine therapy resistance was associated with decreased FOXA1 levels in pleural metastases. 01 november 2018