Estrogen receptor alpha is cell cycle-regulated and regulates the cell cycle in a ligand-dependent fashion

Estrogen receptor alpha (ERα) has been implicated in several cell cycle regulatory events and is an important predictive marker of disease outcome in breast cancer patients. Here, we aimed to elucidate the mechanism through which ERα influences proliferation in breast cancer cells. Our results show that ERα protein is cell cycle-regulated in human breast cancer cells and that the presence of 17-β-estradiol (E2) in the culture medium shortened the cell cycle significantly (by 4.5 hours, <i>P</i> &lt; 0.05) compared with unliganded conditions. The alterations in cell cycle duration were observed in the S and G₂/M phases, whereas the G₁ phase was indistinguishable under liganded and unliganded conditions. In addition, ERα knockdown in MCF-7 cells accelerated mitotic exit, whereas transfection of ERα-negative MDA-MB-231 cells with exogenous ERα significantly shortened the S and G₂/M phases (by 9.1 hours, <i>P</i> &lt; 0.05) compared with parental cells. Finally, treatment of MCF-7 cells with antiestrogens revealed that tamoxifen yields a slower cell cycle progression through the S and G₂/M phases than fulvestrant does, presumably because of the destabilizing effect of fulvestrant on ERα protein. Together, these results show that ERα modulates breast cancer cell proliferation by regulating events during the S and G₂/M phases of the cell cycle in a ligand-dependent fashion. These results provide the rationale for an effective treatment strategy that includes a cell cycle inhibitor in combination with a drug that lowers estrogen levels, such as an aromatase inhibitor, and an antiestrogen that does not result in the degradation of ERα, such as tamoxifen.

雌激素受体α（Estrogen receptor alpha, ERα）参与多种细胞周期调控事件，同时也是乳腺癌患者疾病预后的重要预测标志物。本研究旨在阐明雌激素受体α调控乳腺癌细胞增殖的具体分子机制。研究结果显示，人乳腺癌细胞中的雌激素受体α蛋白呈现细胞周期依赖性表达；与未配体结合状态相比，培养基中添加17β-雌二醇（17-β-estradiol, E2）可显著缩短细胞周期（缩短时长4.5小时，P<0.05）。细胞周期时长的改变主要发生在S期和G₂/M期，而配体结合与未结合状态下的G₁期无显著差异。此外，在MCF-7细胞中敲低雌激素受体α可加速有丝分裂退出；而在雌激素受体α阴性的MDA-MB-231细胞中转染外源性雌激素受体α后，与亲本细胞相比，其S期和G₂/M期显著缩短（缩短时长9.1小时，P<0.05）。最后，采用抗雌激素药物处理MCF-7细胞的实验结果表明，他莫昔芬相较于氟维司群，会更显著延缓S期和G₂/M期的细胞周期进程，推测这一现象与氟维司群对雌激素受体α蛋白的去稳定化作用有关。综上，本研究结果证实，雌激素受体α可通过配体依赖性方式调控细胞周期S期与G₂/M期的相关事件，从而调节乳腺癌细胞的增殖能力。本研究结果为有效的临床治疗策略提供了理论依据：该策略可联合使用细胞周期抑制剂、降低雌激素水平的药物（如芳香化酶抑制剂）以及不会引发雌激素受体α降解的抗雌激素药物（如他莫昔芬）。