SMAD signaling and redox imbalance cooperate to induce prostate cancer cell dormancy

Metastasis involves the dissemination of single or small clumps of cancer cells through blood or lymphatic vessels and their extravasation into distant organs. Despite the strong regulation of metastases development by a cell dormancy phenomenon, the dormant state of cancer cells remains poorly characterized due to the difficulty of <i>in vivo</i> studies. We have recently shown <i>in vitro</i> that clonogenicity of prostate cancer cells is regulated by a dormancy phenomenon that is strongly induced when cells are cultured both at low cell density and in a slightly hypertonic medium. Here, we characterized by RT-qPCR a genetic expression signature of this dormant state which combines the presence of both stemness and differentiation markers. We showed that both TFGβ/BMP signaling and redox imbalance are required for the full induction of this dormancy signature and cell quiescence. Moreover, reconstruction experiments showed that TFGβ/BMP signaling and redox imbalance are sufficient to generate a pattern of genetic expression displaying all characteristic features of the dormancy signature. Finally, we observed that low cell density was sufficient to activate TGFβ/BMP signaling and to generate a slight redox imbalance thus priming cells for dormancy that can be attained with a co-stimulus like hypertonicity, most likely through an increased redox imbalance. The identification of a dual regulation of dormancy provides a framework for the interpretation of previous reports showing a restricted ability of BMP signaling to regulate cancer cell dormancy <i>in vivo</i> and draws attention on the role of oxidative stress in the metastatic process.

癌转移（Metastasis）指单个癌细胞或小簇癌细胞经血管或淋巴管播散，并渗出至远处器官的病理过程。尽管细胞休眠现象可对转移发生起到较强的调控作用，但由于体内（in vivo）研究存在诸多技术难点，癌细胞的休眠状态至今仍未得到充分阐释。我们近期的体外（in vitro）研究证实，前列腺癌细胞的克隆形成能力受一种休眠现象调控：当细胞以低细胞密度培养且处于轻度高渗培养基中时，该休眠现象会被显著诱导。本研究通过实时定量聚合酶链反应（RT-qPCR），对该休眠状态的基因表达特征进行了系统表征，发现其同时兼具干细胞性（stemness）与分化标志物的表达特征。我们证实，转化生长因子β/骨形态发生蛋白（TGFβ/BMP）信号通路与氧化还原失衡均是完整诱导该休眠特征及细胞静息状态的必要条件。进一步的重构实验表明，仅激活TGFβ/BMP信号通路并造成氧化还原失衡，即可诱导出完全匹配该休眠特征的基因表达模式。此外我们观察到，低细胞密度本身即可激活TGFβ/BMP信号通路并引发轻度氧化还原失衡，从而使细胞具备进入休眠状态的潜能；而诸如高渗刺激这类协同刺激因子，可通过进一步加剧氧化还原失衡，最终促使细胞完全进入休眠。本研究揭示的休眠双重调控机制，既为既往报道中BMP信号通路在体内调控癌细胞休眠能力受限的研究结果提供了合理的解释框架，也提示氧化应激在肿瘤转移进程中发挥着关键作用。