Department of Molecular Oncology, Moffitt Cancer Center and Research Institute, Tampa, Florida.
Mol Cancer Res. 2019 Jan;17(1):277-288. doi: 10.1158/1541-7786.MCR-18-0558. Epub 2018 Sep 26.
Transforming growth factor β1 (TGFβ1) is a potent inhibitor of cell growth that targets gene-regulatory events, but also inhibits the function of CDC45-MCM-GINS helicases (CMG; MCM, Mini-Chromosome Maintenance; GINS, Go-Ichi-Ni-San) through multiple mechanisms to achieve cell-cycle arrest. Early in G, TGFβ1 blocks MCM subunit expression and suppresses Myc and Cyclin E/Cdk2 activity required for CMG assembly, should MCMs be expressed. Once CMGs are assembled in late-G, TGFβ1 blocks CMG activation using a direct mechanism involving the retinoblastoma (Rb) tumor suppressor. Here, in cells lacking Rb, TGFβ1 does not suppress Myc, Cyclin E/Cdk2 activity, or MCM expression, yet growth arrest remains intact and Smad2/3/4-dependent. Such arrest occurs due to inhibition of MCM hexamer assembly by TGFβ1, which is not seen when Rb is present and MCM subunit expression is normally blocked by TGFβ1. Loss of Smad expression prevents TGFβ1 suppression of MCM assembly. Mechanistically, TGFβ1 blocks a Cyclin E-Mcm7 molecular interaction required for MCM hexamer assembly upstream of CDC10-dependent transcript-1 (CDT1) function. Accordingly, overexpression of CDT1 with an intact MCM-binding domain abrogates TGFβ1 arrest and rescues MCM assembly. The ability of CDT1 to restore MCM assembly and allow S-phase entry indicates that, in the absence of Rb and other canonical mediators, TGFβ1 relies on inhibition of Cyclin E-MCM7 and MCM assembly to achieve cell cycle arrest. IMPLICATIONS: These results demonstrate that the MCM assembly process is a pivotal target of TGFβ1 in eliciting cell cycle arrest, and provide evidence for a novel oncogenic role for CDT1 in abrogating TGFβ1 inhibition of MCM assembly.
转化生长因子β1（TGFβ1）是一种有效的细胞生长抑制剂，其靶向基因调控事件，但也通过多种机制抑制 CDC45-MCM-GINS 解旋酶（CMG；MCM，微染色体维持；GINS，Go-Ichi-Ni-San）的功能，以实现细胞周期停滞。在 G1 早期，TGFβ1 阻断 MCM 亚基表达，并抑制 Myc 和细胞周期蛋白 E/Cdk2 活性，这些是 CMG 组装所必需的，除非 MCM 被表达。一旦在 G1 晚期组装了 CMG，TGFβ1 就会通过一种直接机制阻断 CMG 的激活，该机制涉及视网膜母细胞瘤（Rb）肿瘤抑制因子。在这里，在缺乏 Rb 的细胞中，TGFβ1 不会抑制 Myc、细胞周期蛋白 E/Cdk2 活性或 MCM 表达，但生长停滞仍然完整，并且依赖于 Smad2/3/4。这种停滞是由于 TGFβ1 抑制 MCM 六聚体组装所致，而在 Rb 存在时，MCM 亚基表达通常被 TGFβ1 阻断，因此不会出现这种情况。Smad 表达的缺失阻止了 TGFβ1 对 MCM 组装的抑制。从机制上讲，TGFβ1 阻断了细胞周期蛋白 E-Mcm7 分子相互作用，该相互作用是在 CDC10 依赖的转录物-1（CDT1）功能之前进行的，对于 MCM 六聚体组装是必需的。相应地，具有完整 MCM 结合结构域的 CDT1 的过表达可消除 TGFβ1 抑制并挽救 MCM 组装。CDT1 恢复 MCM 组装并允许 S 期进入的能力表明，在缺乏 Rb 和其他经典介质的情况下，TGFβ1 依赖于抑制细胞周期蛋白 E-MCM7 和 MCM 组装来实现细胞周期停滞。意义：这些结果表明，MCM 组装过程是 TGFβ1 引发细胞周期停滞的关键靶点，并为 CDT1 在消除 TGFβ1 抑制 MCM 组装中的新型致癌作用提供了证据。
