Tamoxifen Treatment in Correlation with Increased ET-1 Levels Is Associated with the Development of Breast Cancer Metastases

Background: In breast cancer patients, a correlation between endothelin-1 (ET-1) and lymph node metastasis was found. While breast cancer with a positive ER status can be treated with Tamoxifen, several studies describe increasing Tamoxifen resistance in patients. We analyzed the relationship between Tamoxifen, ET-1 overexpression, and ER leading to Tamoxifen resistance. Methods: Breast cancer cell lines were treated with Tamoxifen, ET-1, estrogen and combinations. Using qRT-PCR, immune-precipitation, Western blot, EMSA and immunohistology target gene expression and ER complex partners were investigated. Human biopsies and mastectomy specimens were immunohistologically studied for Vimentin 3, and ERβ. Results: Breast cancer cells stimulated with a combination of Tamoxifen and ET-1 downregulate ERα, while upregulating intracellular ET-1, and ERβ. Immunoprecipation of nuclear extracts with ET-1, ERα or ERβ agarose conjugated antibodies reveals a complex formation change replacing ERα by ERβ once Tamoxifen forms a complex with ET-1. ERβand ET-1 migrate into the nucleus. ET-1 stimulation upregulates metastases promoting target genes (IL-6, Wnt11), including a novel one, Vimentin 3. Tissue analyses show Vim3 and ERβ expression in metastases of ERα positive breast cancer, and in ERα negative biopsies/mastectomy specimens. Conclusion: We are the first to describe a complex consisting of Tamoxifen, ERβ and ET-1, whose nuclear transmigration causes an overexpression of target genes. This mechanism may explain Tamoxifen resistance. Future pathologic analyses should include estrogen beta receptor status as well as the ET-1 expression. This concept presents a new treatment approach for individualized medicine in breast cancer patients with increased ET-1 levels.