Tamoxifen-resistant, ER-positive MAC 51 cell line with a high metastatic potential developed from a spontaneous breast cancer mouse model

We developed and characterized an estrogen-responsive and ER-positive murine breast cancer cell line (MAC51) from a spontaneous breast cancer animal model. These cells are overexpressed with K8, K18 and K19 proteins in an immunofluoresence assay. Upregulation of ER alpha was observed in the immunofluoresence assay, real-time PCR analysis and western blot assay. A colocalization experiment in MAC 51 showed cytoplasmic colocalization of K18 and K19 proteins with ER α. Real-time analysis of tumor samples from engrafted animals, MAC 51, metastatic liver and metastatic ovary revealed overexpression of K8 and K18 compared to the respective controls. A hormone responsive experiment in immunodeficient mice showed highly significant decreases in estrogen and tumor volume after 14 days ovariectomization. The tumorogenicity assay showed higher (3 × 10 5) and lower (3 × 104) concentrations of MAC 51 cells that developed tumors within 2 weeks post-transplantation. Tumor morphology and histology resembled a sarcoma pattern but our spontaneous model appeared in an adenocarcinoma pattern. Metastasis to different organs occurred through hematogenous and lymphatic routes. We assessed the potency of the anticancer effect in MAC 51 cells by treating various anticancer drugs with E2, followed by studying apoptotic gene expression profiles. E2 and E2+ tamoxifen-treated cells showed upregulation of apoptotic genes caspase 1, 3, 9, P53 and Bcl-xl but the tamoxifen- and paclitaxel-treated cells did not upregulate the apoptotic genes. Tamoxifen-resistant, ER-positive and high metastatic potential cell lines from murine origin are very rare. Also, estrogen greatly induced apoptosis in this cell line, hence MAC 51 has a greater application potential to evaluate low doses of estrogen with other targeted therapeutic drugs to treat breast cancer.