The p53 tumor suppressor protein is a potent activator of proliferative arrest and cell death. In normal cells, this pathway is restrained by p53 protein degradation mediated by the E3-ubiquitin ligase activity of MDM2. Oncogenic stress releases p53 from MDM2 control, so activating the p53 response. However, many tumors that retain wild-type p53 inappropriately maintain the MDM2-p53 regulatory loop in order to continuously suppress p53 activity. We have shown previously that single point mutations in the human MDM2 RING finger domain prevent the interaction of MDM2 with the E2/ubiquitin complex, resulting in the loss of MDM2's E3 activity without preventing p53 binding. Here, we show that an analogous mouse MDM2 mutant (MDM2 I438K) restrains p53 sufficiently for normal growth but exhibits an enhanced stress response in vitro. In vivo, constitutive expression of MDM2 I438K leads to embryonic lethality that is rescued by p53 deletion, suggesting MDM2 I438K is not able to adequately control p53 function through development. However, the switch to I438K expression is tolerated in adult mice, sparing normal cells but allowing for an enhanced p53 response to DNA damage. Viewed as a proof of principle model for therapeutic development, our findings support an approach that would inhibit MDM2 E3 activity without preventing MDM2/p53 binding as a promising avenue for development of compounds to activate p53 in tumors with reduced on-target toxicities.

中文翻译：

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胚胎发生过程和成年小鼠对 MDM2 E3 活性的不同要求

p53 肿瘤抑制蛋白是增殖停滞和细胞死亡的有效激活剂。在正常细胞中，该途径受到由 MDM2 的 E3-泛素连接酶活性介导的 p53 蛋白降解的限制。致癌应激从 MDM2 控制中释放 p53，从而激活 p53 反应。然而，许多保留野生型 p53 的肿瘤不恰当地维持 MDM2-p53 调节环以持续抑制 p53 活性。我们之前已经表明，人类 MDM2 环指结构域中的单点突变阻止了 MDM2 与 E2/泛素复合物的相互作用，导致 MDM2 的 E3 活性丧失而不阻止 p53 结合。在这里，我们显示类似的小鼠 MDM2 突变体 (MDM2 I438K) 足以抑制 p53 正常生长，但在体外表现出增强的应激反应。体内, MDM2 I438K 的组成型表达导致胚胎致死率被 p53 缺失挽救，表明 MDM2 I438K 不能通过发育充分控制 p53 功能。然而，成年小鼠对 I438K 表达的转换是可以耐受的，不会影响正常细胞，但会增强 p53 对 DNA 损伤的反应。作为治疗开发原理模型的证明，我们的研究结果支持一种在不阻止 MDM2/p53 结合的情况下抑制 MDM2 E3 活性的方法，作为开发在肿瘤中激活 p53 并降低靶向毒性的化合物的有希望的途径。