Drug resistance is frequently found in estrogen receptor–positive (ER⁺) breast cancer patients during and after prolonged tamoxifen treatment. Although tamoxifen rechallenge has been proposed for treating recurrent breast tumors, the clinical benefit of this treatment is still controversial. The aims of this study are to identify the possible tamoxifen cytotoxicity-resistant subpopulation of MCF7 cells and to determine the effects of tamoxifen rechallenge on these cells.

Western blot analysis was used to determine the expression levels of various epithelial–mesenchymal transition– and cell survival/proliferation–related proteins in MCF7 and MCF7-derived, tamoxifen-mediated cytotoxicity-resistant MCF7-TAM12.5 breast cancer cells. Wound healing, Transwell migration, and invasion assays were used to examine the metastatic potential of cells. Clonogenic assays, trypan blue exclusion assays, and bromodeoxyuridine assays were used to examine clonogenicity and to determine the proliferation rate of cells.

We found that MCF7-TAM12.5 cells exhibited higher tolerance to tamoxifen-mediated cytotoxicity, higher metastatic potential, higher expression levels of XIAP, and lower expression levels of ERα/ERβ/HER2/Smac than MCF7 cells. In addition, MCF7 cells endogenously expressed Bcl-2α, whereas MCF7-TAM12.5 cells only expressed Bcl-2β. Interestingly, tamoxifen rechallenge decreased the metastatic potential but increased the proliferation and clonogenicity of MCF7-TAM12.5 cells. At the molecular level, tamoxifen rechallenge upregulated the expression of phosphorylated Aurora A and Aurora B kinase in MCF7-TAM12.5 cells.

Our findings further support the existence of highly heterogenetic cancer cell populations in ER⁺ breast tumors. It will be of clinical importance to determine the protein expression and the genetic profiles of tamoxifen-resistant/recurrent ER⁺ breast tumors to predict the potential effects of tamoxifen readministration in the future.

^{长期他莫昔芬治疗期间和之后，雌激素受体阳性 (ER +} ) 乳腺癌患者经常出现耐药性。尽管已经提出他莫昔芬再治疗用于治疗复发性乳腺肿瘤，但这种治疗的临床益处仍然存在争议。本研究的目的是确定 MCF7 细胞可能的三苯氧胺抗细胞毒性亚群，并确定三苯氧胺再激发对这些细胞的影响。

Western印迹分析用于确定MCF7和MCF7衍生的、他莫昔芬介导的细胞毒性抗性MCF7-TAM12.5乳腺癌细胞中各种上皮-间质转化和细胞存活/增殖相关蛋白的表达水平。伤口愈合、Transwell 迁移和侵袭试验用于检查细胞的转移潜力。克隆形成测定、台盼蓝排除测定和溴脱氧尿苷测定用于检查克隆形成性并确定细胞的增殖率。

我们发现，与 MCF7 细胞相比，MCF7-TAM12.5 细胞对他莫昔芬介导的细胞毒性具有更高的耐受性、更高的转移潜能、更高的 XIAP 表达水平和更低的 ERα/ERβ/HER2/Smac 表达水平。此外，MCF7 细胞内源性表达 Bcl-2α，而 MCF7-TAM12.5 细胞仅表达 Bcl-2β。有趣的是，他莫昔芬再激发降低了转移潜能，但增加了 MCF7-TAM12.5 细胞的增殖和克隆形成。在分子水平上，他莫昔芬再激发上调了 MCF7-TAM12.5 细胞中磷酸化 Aurora A 和 Aurora B 激酶的表达。

^{我们的研究结果进一步支持 ER +}乳腺肿瘤中存在高度异质的癌细胞群。^{确定他莫昔芬耐药/复发性 ER +}乳腺肿瘤的蛋白质表达和遗传谱对于预测他莫昔芬再给药在未来的潜在影响具有临床意义。