ARF MDM2 P53 TUMOR SUPPRESSION PATHWAY

p53和Rb介导两种主要的肿瘤抑制途径，这两种途径被认为在大多数（如果不是全部）人类癌症中都是功能性失活的。了解这两种途径是如何被调节的已经成为当代癌细胞生物学的一个主要目标。与p53一样，ARF-INK4a基因座是人类癌症中最常改变的两个基因座之一。功能上，p16INK4a抑制细胞周期蛋白d依赖性激酶（CDK4和CDK6）的活性，从而维持视网膜母细胞瘤蛋白（Rb）的生长抑制状态。另一方面，ARF通过结合和拮抗MDM2的核输出，介导癌基因激活的超增殖检查点通路，从而阻止p53的细胞质降解。通过关注ARF和p53之间的联系，我试图为我们对这两种主要途径的理解以及癌症的发展做出贡献。我之前发现ARF通过结合并拮抗p53的负调节因子mdm2的活性来稳定p53，从而揭示了ARF- mdm2 -p53的抑瘤途径。随后，我进一步阐明了ARF稳定p53的机制：ARF与MDM2和p53在核质中形成核小体，从而阻断p53的核输出，阻止其胞质降解。我还证明了人类ARF蛋白中频繁发生的肿瘤衍生突变损害了其阻断p53核输出的功能。最近，我获得了新的证据表明：(a) MDM2与核糖体蛋白L5的关联对于MDM2核输出是必要的，而这种调节被MDM2中频繁的癌症衍生突变所破坏。(b)核糖体蛋白L5结合缺陷的MDM2不能促进p53降解，但保留了抑制p53反活化活性的能力。(c) ARF参与多肽复合物，(d) MDM2通过蛋白水解有效降解。关于ARF-MDM2-p53通路的调控，我目前和未来的研究主要针对以下几个问题：(a)阐明核糖体蛋白L5和/或ARF控制p53和MDM2核胞质穿梭的机制。(b)通过纯化ARF复合物来确定ARF-MDM2-p53核小体的功能；(c)作为长期目标，确定MDM2泛素化和降解的机制和调控。总之，这些实验应该促进对ARF-MDM2-p53通路的调控及其在人类癌症中改变的功能后果的理解。

p53 and Rb mediate two major tumor suppression pathways that are believed to be functionally inactivated inmost, if not all, human cancers. Understanding how these two pathways are regulated has become a major goal of contemporary cancer cell biology. Along withp53, the ARF-INK4a locus is one of the two most frequently altered loci in human cancer. Functionally, p16INK4a inhibits the activity of cyclin D-dependent kinases (CDK4 and CDK6), thereby maintaining the retinoblastoma protein (Rb) in its growth suppressive state. ARF, on the other hand, mediates an oncogene- activated hyperproliferative checkpoint pathway through binding to and antagonizing the nuclear export of MDM2, thereby preventing cytoplasmic degradation of p53. With a focus on the connection between ARF and p53, I have tried to contribute to our understanding of these two major pathways and thereby cancer development. I had previously discovered that ARF stabilizes p53 through binding to and antagonizing the activity of MDM2-a negative regulator of p53, and thus revealed an ARF-MDM2-p53 tumor suppression pathway. Subsequently, I further elucidated the mechanism of ARFs p53 stabilization: ARF forms nuclear bodies in the nucleoplasm with MDM2 and p53, thereby blocking nuclear export of p53 and preventing its cytoplasmic degradation. I also demonstrated that frequently occurring tumor-derived mutations in the human ARF protein impair its function in blocking p53 nuclear export. More recently, I obtained new evidence showing that: (a) Association of MDM2 with ribosomal protein L5 is necessary for MDM2 nuclear export and this regulation is disrupted by frequent cancer-derived mutations in MDM2. (b) Ribosomal protein L5- binding deficient MDM2 failed to promote p53 degradation but retains its ability to suppress p53's transactivation activity. (c) ARF participates in a multipeptide complex, and (d) MDM2 is efficiently degraded by proteolysis. My current and future studies are aimed at several issues concerning the regulation of ARF-MDM2-p53 pathway: (a) Elucidate the mechanism of p53 and MDM2 nucleo-cytoplasmic shuttling controlled by ribosomal protein L5 and/or ARF. (b) Define the function of ARF-MDM2-p53 nuclear bodies by purifying the ARF complex, and (c) as a long-term goal to identify the mechanism and regulation of MDM2 ubiquitination and degradation. Together these experiments should advance understanding of the regulation of the ARF-MDM2-p53 pathway and the functional consequences of its alterations in human cancer.