美国威尔康奈尔医学Tobias Meyer等研究人员合作发现，Rb-E2F的中间活性状态保障增殖承诺。相关论文于2024年6月26日在线发表在《自然》杂志上。

研究人员表示，组织修复、免疫防御和癌症进展都依赖于细胞在静止和增殖之间做出的重要决定。哺乳动物细胞通过触发正反馈机制来增殖。转录因子E2F会激活细胞周期蛋白依赖性激酶2（CDK2），CDK2又会使E2F抑制蛋白视网膜母细胞瘤（Rb）磷酸化并失活。这一作用进一步提高了E2F的活性，从而表达增殖所需的基因。鉴于正反馈会无意中放大微小的信号，了解细胞如何控制这种正反馈仍是一个难题。

研究人员测量了单细胞中E2F和CDK2信号的变化，发现正反馈机制仅在G1期晚期才发挥作用。细胞在进入增殖之前，会在可逆的中间E2F活性状态下度过不同的时间，而且时间往往会延长。这种中间E2F活性与CDK2或CDK4/CDK6介导的Rb中保守的T373残基磷酸化量成正比。这种T373磷酸化的Rb仍结合在染色质上，但一旦Rb在许多位点上被过度磷酸化，就会与染色质分离，从而完全激活E2F。T373去磷酸化的速度相对较慢，这就解释了T373偏好初始磷酸化的原因。总之，该研究确定了E2F激活的中间激活状态，在这种状态下，细胞会感知外部和内部信号，并决定是逆转并退出静止状态，还是触发正反馈机制以启动细胞增殖。

附：英文原文

Title: An intermediate Rb–E2F activity state safeguards proliferation commitment

Author: Konagaya, Yumi, Rosenthal, David, Ratnayeke, Nalin, Fan, Yilin, Meyer, Tobias

Issue&Volume: 2024-06-26

Abstract: Tissue repair, immune defence and cancer progression rely on a vital cellular decision between quiescence and proliferation1,2. Mammalian cells proliferate by triggering a positive feedback mechanism3,4. The transcription factor E2F activates cyclin-dependent kinase2 (CDK2), which in turn phosphorylates and inactivates the E2F inhibitor protein retinoblastoma (Rb). This action further increases E2F activity to express genes needed for proliferation. Given that positive feedback can inadvertently amplify small signals, understanding how cells keep this positive feedback in check remains a puzzle. Here we measured E2F and CDK2 signal changes in single cells and found that the positive feedback mechanism engages only late in G1 phase. Cells spend variable and often extended times in a reversible state of intermediate E2F activity before committing to proliferate. This intermediate E2F activity is proportional to the amount of phosphorylation of a conserved T373 residue in Rb that is mediated by CDK2 or CDK4/CDK6. Such T373-phosphorylated Rb remains bound on chromatin but dissociates from it once Rb is hyperphosphorylated at many sites, which fully activates E2F. The preferential initial phosphorylation of T373 can be explained by its relatively slower rate of dephosphorylation. Together, our study identifies a primed state of intermediate E2F activation whereby cells sense external and internal signals and decide whether to reverse and exit to quiescence or trigger the positive feedback mechanism that initiates cell proliferation.

DOI: 10.1038/s41586-024-07554-2

Source: https://www.nature.com/articles/s41586-024-07554-2