ARF MDM2 P53 TUMOR SUPPRESSION PATHWAY

P53和Rb介导了两个主要的肿瘤抑制途径，据信在大多数(如果不是全部)人类癌症中是功能失活的。了解这两条通路是如何调控的，已成为当代癌细胞生物学的主要目标。与P53一样，ARF-INK4a基因座是人类癌症中最常见的两个改变基因座之一。在功能上，p16INK4a抑制细胞周期蛋白D依赖的蛋白(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通过与MDM2的核输出结合并拮抗MDM2的核输出，介导癌基因激活的高增殖检查点通路，从而阻止P53的细胞质降解。我把重点放在ARF和P53之间的联系上，试图帮助我们理解这两条主要途径，从而促进癌症的发展。我以前发现ARF通过结合和拮抗p53的负调控因子MDM2的活性来稳定P53，从而揭示了ARF-MDM2-P53的肿瘤抑制途径。随后，我进一步阐明了ARF稳定P53的机制：ARF与MDM2和P53在核浆中形成核小体，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会削弱其阻止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的结合是MDM2核出口所必需的，这一调控被MDM2频繁的癌症起源突变所扰乱。核糖体蛋白L5结合缺陷的MDM2不能促进P53的降解，但仍能抑制P53的S反式激活活性。(C)ARF参与多肽复合体，(D)MDM2通过蛋白降解有效降解。我目前和未来的研究主要针对ARF-MDM2-P53途径调控的几个问题：(A)阐明核糖体蛋白L5和/或ARF控制P53和MDM2核质穿梭的机制。(B)通过纯化ARF复合体来确定ARF-MDM2-P53核体的功能，以及(C)作为确定MDM2泛素化和降解的机制和调控的长期目标。总之，这些实验应该会促进对ARF-MDM2-P53通路的调节及其在人类癌症中变化的功能后果的理解。

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.