Tumor-initiating cells (TICs) are considered the culprits of cancer development and progression. Dysregulation of metastasis suppressor protein 1 (MTSS1) has been widely observed in tumor metastasis, but its functional contribution and mechanism in cancer is poorly understood. Here we report a role of MTSS1 in suppressing TICs in breast cancer. Mtss1 knockout (KO) enhances the mammary epithelial TIC subpopulation in both luminal and basal-like breast cancer mouse models. MTSS1 also suppresses tumorsphere formation in breast cancer cells. Mechanistically, MTSS1 interacts with the E3 ligase RanBP2-type and C3HC4-type zinc finger containing 1 (RBCK1) to facilitate RBCK1-mediated p65 ubiquitination and degradation, thus suppressing the NF-κB signaling pathway and tumorigenesis. In addition, actin beta-like 2 (ACTBL2) competes with RBCK1 for MTSS1 binding, leading to p65 stabilization. Importantly, MTSS1 silencing promotes patient-derived organoid formation and xenograft growth. MTSS1 downregulation in clinical tumors is also linked to worse prognosis. Overall our data reveal a new paradigm of NF-κB regulation and may have important implications in therapeutics targeting TICs.

肿瘤起始细胞 (TIC) 被认为是癌症发展和进展的罪魁祸首。转移抑制蛋白 1 (MTSS1) 的失调已在肿瘤转移中广泛观察到，但其在癌症中的功能贡献和机制却知之甚少。在这里，我们报告了 MTSS1 在抑制乳腺癌中 TIC 中的作用。Mtss1敲除 (KO) 增强了管腔和基底样乳腺癌小鼠模型中的乳腺上皮 TIC 亚群。MTSS1 还抑制乳腺癌细胞中肿瘤球的形成。机制上，MTSS1 与 E3 连接酶 RanBP2 型和 C3HC4 型含 1 (RBCK1) 的锌指相互作用，促进 RBCK1 介导的 p65 泛素化和降解，从而抑制 NF-κB 信号通路和肿瘤发生。此外，肌动蛋白 β 样 2 (ACTBL2) 与 RBCK1 竞争 MTSS1 结合，导致 p65 稳定。重要的是，MTSS1沉默促进患者衍生的类器官形成和异种移植物生长。临床肿瘤中的 MTSS1 下调也与较差的预后有关。总体而言，我们的数据揭示了一种新的 NF-κB 调节范式，并且可能对针对 TIC 的治疗具有重要意义。