Cellular Mechanisms Controlling Myc Protein Stability

描述(申请人提供)：Myc癌蛋白在多种人类肿瘤中过度表达。为了在静止的细胞中维持正常的Myc蛋白水平，新合成的Myc通过泛素/蛋白酶体途径迅速降解。泛素介导的Myc降解受细胞生长调节，依赖于两条RAS效应通路的作用。具体地说，RAS激活Raf-MEK-ERK介导的丝氨酸62位Myc的磷酸化稳定Myc，而GSK-3β对苏氨酸58位Myc的磷酸化依赖于丝氨酸62的先前磷酸化，并受RAS激活的PI3K-Akt调节，刺激Myc降解。此外，丝氨酸62磷酸化的丧失先于Myc的泛素化，而蛋白磷酸酶2A(PP2A)负责这种去磷酸化。PP2A是一种构象特异的磷酸酶，其活性被磷酸特异的Pro异构酶Pin1增强。Pin1促进PP2A去除Myc中的丝氨酸62磷酸，从而促进Myc的降解。这些观察确定了一系列程序性事件，确保了在细胞生长反应的初始阶段Myc的瞬时积累。这项拨款提案旨在进一步阐明调节Myc蛋白稳定性的细胞机制，目的是更好地了解Pin1和PP2A对Myc的作用，并发现其他专门促进Myc降解的细胞蛋白。这些Myc降解蛋白将被用来抑制Myc诱导的细胞转化。1.研究Myc蛋白稳定性的细胞调控机制；2.分析Pin1和PP2A对人肿瘤Myc降解的调控及其在Myc过表达中的作用；3.鉴定参与Myc降解的E3泛素连接酶，研究强迫Myc降解对Myc介导的肿瘤发生的影响。此前的研究表明，抑制癌症发生过程中的一种细胞事件可以逆转肿瘤的形成。因此，分析控制Myc积累的分子机制，并识别专门针对Myc降解的蛋白质，可能有助于开发Myc的特定抑制剂，最终目标是设计一种治疗依赖于高水平Myc蛋白的人类癌症的方法。

DESCRIPTION (provided by applicant): The Myc oncoprotein is overexpressed in a wide variety of human tumors. In order to maintain normal levels of Myc protein in quiescent cells, newly synthesized Myc is rapidly degraded via the ubiquitin/proteasome pathway. Ubiquitin-mediated degradation of Myc is cell growth regulated, dependent upon the action of two Ras effector pathways. Specifically, Ras activated Raf-MEK-ERK-mediated phosphorylation of Myc at Serine 62 stabilizes Myc, whereas phosphorylation of Myc at Threonine 58 by GSK-3Beta, which is dependent on prior phosphorylation of Serine 62 and is regulated by Ras activated PI3K-Akt, stimulates Myc degradation. In addition, loss of Serine 62 phosphorylation precedes ubiquitination of Myc and the Protein Phosphatase 2A (PP2A) is responsible for this dephosphorylation. PP2A is a conformation-specific phosphatase whose activity is enhanced by the phospho-specific prolyl isomerase Pin1. Pin1 facilitates the removal of the Serine 62 phosphate in Myc by PP2A, thus promoting Myc degradation. These observations have defined a cascade of programmed events that insure the transient accumulation of Myc during the initial phase of a cellular growth response. This grant proposal is intended to further elucidate cellular mechanisms that regulate Myc protein stability, with the goal of better understanding Pin1 and PP2A actions on Myc and discovering other cellular proteins that specifically promote Myc degradation. These Myc-degrading proteins will then be used to inhibit Myc-induced cellular transformation. The following specific aims will be pursued: 1. Investigate cellular mechanisms regulating Myc protein stability, 2. Analyze Pin1 and PP2A regulation of Myc degradation and their contribution to Myc overexpression in human tumors, 3. Identify the E3 ubiquitin ligase involved in Myc degradation and study the effects of forced Myc degradation on Myc-mediated oncogenesis. Research has previously shown that inhibition of one of the cellular events in the genesis of cancer can revert tumor formation. Thus, analysis of molecular mechanisms that control Myc accumulation and identification of proteins that specifically target Myc for degradation could be instrumental in developing specific inhibitors of Myc, with the ultimate goal of designing a therapeutic approach for human cancers that are dependent upon high levels of Myc protein.