p53 Tumor Suppressor Pathway

P53在体内肿瘤抑制、干细胞功能和衰老等方面发挥着重要作用。我们的实验室继续研究癌症相关p53突变的临床意义和特定突变的生物学活性（如密码子249处Arg-to-Ser突变）。我们最近的研究重点是p53和端粒封盖蛋白复合物（称为庇护蛋白）之间的功能相互作用，保护端粒免受不必要的DNA损伤反应。在细胞复制寿命结束时，无帽、功能失调的端粒失去了这种保护机制，触发端粒启动的DNA损伤信号，激活p53，从而诱导复制衰老。我们已经确定了一个涉及p53、Siah-1（一种p53诱导的E3泛素连接酶）和TRF2（一种庇护蛋白复合物的成分）的信号通路。内源性TRF2和Siah-1分别在生理性激活p53的复制性衰老中下调和上调。等位基因缺失、shRNA敲低、天然p53亚型（delta133p53）的显性阴性抑制、nutlin-3a的激活和p53的过表达均表明p53诱导Siah-1并抑制TRF2。TRF2的p53依赖性蛋白酶体降解归因于Siah-1的E3连接酶活性，使TRF2泛素化。siRNA敲低Siah-1可稳定TRF2蛋白，延长细胞复制寿命。因此，本研究表明，p53、Siah-1和TRF2通过与先前发现的下游miR-34a和p21WAF1通路合作，构成了一种新的信号通路，调节p53介导的细胞衰老。最重要的是，p53，一个来自未封顶端粒的损伤信号的下游效应体，已经被证明也在端粒封顶蛋白复合体的上游起作用，这表明存在一个正反馈回路，协调端粒启动的DNA损伤信号到细胞衰老。我们正在进行的项目也为合作研究做出了重大贡献，包括：鉴定miR-22作为控制p21WAF1表达和细胞命运决定（即细胞凋亡与细胞衰老）的p53新靶点；在肝细胞癌中发现与p53突变相关的干细胞样基因表达特征，进一步研究p53在癌症和正常干细胞中的作用。

p53 plays critical roles in tumor suppression, stem cell functions and aging in vivo. Our laboratory continues to study clinical significance of cancer-associated p53 mutations and biological activities of specific mutations (e.g., Arg-to-Ser mutation at codon 249). Our recent focus has been on functional interactions between p53 and the telomere-capping protein complex (called shelterin), which protects telomeres from initiating unwanted DNA damage response. Uncapped, dysfunctional telomeres at the end of cellular replicative lifespan lose this protective mechanism and trigger telomere-initiated DNA damage signaling to activate p53 and thereby induce replicative senescence. We have identified a signaling pathway involving p53, Siah-1 (a p53-inducible E3 ubiquitin ligase) and TRF2 (a component of the shelterin complex). Endogenous TRF2 and Siah-1 were down- and up-regulated, respectively, at replicative senescence with physiologically activated p53. Allelic loss, shRNA knockdown, dominant-negative inhibition by a natural p53 isoform (delta133p53), nutlin-3a activation and overexpression of p53 all showed that p53 induced Siah-1 and repressed TRF2. The p53-dependent proteasomal degradation of TRF2 was attributed to the E3 ligase activity of Siah-1 to ubiquitinate TRF2. siRNA knockdown of Siah-1 stabilized TRF2 protein and extended the cellular replicative lifespan. This study thus suggests that the p53, Siah-1 and TRF2 constitute a novel signaling pathway that regulates p53-mediated cellular senescence by co-operating with the previously identified downstream pathways involving miR-34a and p21WAF1. Most importantly, p53, a downstream effector of the damage signaling from uncapped telomeres, has been shown to also function upstream of the telomere-capping protein complex, suggesting the presence of a positive feedback loop that orchestrates the telomere-initiated DNA damage signaling to cellular senescence. Our ongoing project has also made significant contribution to collaborative studies including: identification of miR-22 as a novel p53 target that controls p21WAF1 expression and cell fate decision (i.e., apoptosis vs. cellular senescence); and discovery of stem cell-like gene expression traits associated with p53 mutations in hepatocellular carcinoma, which prompts further investigation of the roles of p53 in cancer and normal stem cells.