PDF(Pubmed)
Abstract：
Regulated cell cycle progression ensures homeostasis and prevents cancer. In proliferating cells, premature S phase entry is avoided by the E3 ubiquitin ligase anaphasepromoting complex/cyclosome (APC/C), although the APC/C substrates whose degradation restrains G1-S progression are not fully known. The APC/C is also active in arrested cells that exited the cell cycle, but it is not clear whether APC/C maintains all types of arrest. Here, by expressing the APC/C inhibitor, EMI1, we show that APC/C activity is essential to prevent S phase entry in cells arrested by pharmacological cyclin-dependent kinases 4 and 6 (CDK4/6) inhibition (Palbociclib). Thus, active protein degradation is required for arrest alongside repressed cell cycle gene expression. The mechanism of rapid and robust arrest bypass from inhibiting APC/C involves CDKs acting in an atypical order to inactivate retinoblastoma-mediated E2F repression. Inactivating APC/C first causes mitotic cyclin B accumulation which then promotes cyclin A expression. We propose that cyclin A is the key substrate for maintaining arrest because APC/C-resistant cyclin A, but not cyclin B, is sufficient to induce S phase entry. Cells bypassing arrest from CDK4/6 inhibition initiate DNA replication with severely reduced origin licensing. The simultaneous accumulation of S phase licensing inhibitors, such as cyclin A and geminin, with G1 licensing activators disrupts the normal order of G1-S progression. As a result, DNA synthesis and cell proliferation are profoundly impaired. Our findings predict that cancers with elevated EMI1 expression will tend to escape CDK4/6 inhibition into a premature, underlicensed S phase and suffer enhanced genome instability.
摘要：
受调控的细胞周期进程确保稳态并预防癌症。在增殖细胞中，E3泛素连接酶后酶促进复合物/环体（APC/C）避免了过早的S期进入，尽管降解抑制G1-S进展的APC/C底物尚不完全清楚。APC/C在退出细胞周期的停滞细胞中也很活跃，但目前尚不清楚APC/C是否维持所有类型的逮捕。这里,通过表达APC/C抑制剂，EMI1，我们表明APC/C活性对于防止由药理学细胞周期蛋白依赖性激酶4和6（CDK4/6）抑制（Palbociclib）阻止的细胞中的S期进入至关重要。因此，抑制细胞周期基因表达需要活性蛋白降解。抑制APC/C的快速而强大的阻止旁路的机制涉及CDK以非典型的顺序作用，以失活视网膜母细胞瘤介导的E2F抑制。使APC/C失活首先引起有丝分裂细胞周期蛋白B的积累，然后促进细胞周期蛋白A的表达。我们认为细胞周期蛋白A是维持阻滞的关键底物，因为抗APC/C细胞周期蛋白A，但不是细胞周期蛋白B，足以诱导S相进入。绕过CDK4/6抑制阻滞的细胞启动DNA复制，来源许可严重减少。同时积累S期许可抑制剂，如细胞周期蛋白A和geminin，使用G1许可激活器破坏了G1-S进展的正常顺序。因此,DNA合成和细胞增殖严重受损。我们的研究结果预测，EMI1表达升高的癌症将倾向于逃避CDK4/6的抑制，未获得许可的S期，并遭受增强的基因组不稳定性。
