Mechanisms of MEK/ERK growth arrest signaling

描述（由申请人提供）：本项目的目标是确定MEK/ERK信号传导的机制差异，该信号传导介导响应异常Ras和/或Raf信号的生长停滞与细胞增殖。Ras/Raf/MEK/ERK通路的异常激活是许多癌症的中心特征。然而，矛盾的是，持续激活的途径诱导细胞周期停滞和衰老在正常细胞和某些癌症类型。已经提出，生长停滞作为抵抗Ras/Raf-mediated致癌作用的防御机制，并且克服这种“生长停滞屏障”是肿瘤进展的必要步骤。我们对这一在MEK/ERK水平控制的致癌关键事件的理解目前相当有限。我们已经开始使用正常和Ras/Raf-responsive肿瘤细胞作为模型来解决MEK/ERK信号传导的潜在机制。第一个有趣的发现是，ERK不仅可以利用其“典型”激酶活性，而且还可以利用尚未鉴定的非催化功能来介导生长停滞。我们最近报道，致癌Raf信号诱导的生长停滞被废除的ERK 1/2耗尽，但引入催化失活的突变ERK到ERK 1/2耗尽的细胞可以选择性地恢复生长停滞现象。基于MEK/ERK与特定蛋白相互作用以介导生长停滞信号的假设，我们进行了串联亲和纯化，并将mortalin鉴定为MEK/ERK-生长停滞信号的潜在负调节剂。我们的初步研究表明，mortalin结合无活性，但不活跃，MEK和mortalin消耗增加基础以及Raf-induced MEK/ERK活性。此外，死亡蛋白耗竭促进生长抑制信号，而死亡蛋白过表达发挥相反的作用。这些初步研究表明，MEK/ERK利用一种独特的信号传导机制来介导生长停滞，其中死亡蛋白可能通过其与该通路的物理相互作用而具有负调节作用，而ERK调节一种需要其非催化功能的机制。为了检验这些假设，我们建议（i）通过在正常和K-Rasor B-Raf突变的肿瘤细胞中研究功能的获得或丧失，并通过比较肿瘤组织标本中死亡蛋白表达水平与改变的MEK/ERK活性，来确定死亡蛋白在Raf/MEK/ERK介导的生长停滞信号传导的调节中的作用;（ii）确定死亡蛋白是否通过差异螯合MEK 1或MEK 2调节Raf/MEK/ERK途径;和（iii）确定非催化ERK功能的分子机制。这项研究将提高我们的知识MEK/ERK信号的具体机制，中断Ras/Raf-driven致癌。 公共卫生相关性：MEK/ERK途径的活化诱导源自小细胞肺癌、甲状腺髓样癌、前列腺癌、嗜铬细胞瘤、神经胶质瘤和胃肠道类癌的某些类型的癌系中的终末生长停滞。探索这些癌症类型可以通过调节途径经由死亡蛋白到生长停滞的方向来有效抑制的可能性，可以允许开发新的治疗策略来控制它们。

DESCRIPTION (provided by applicant): The goal of this project is to determine the mechanistic differences in MEK/ERK signaling that mediates growth arrest versus cell proliferation in response to aberrant Ras and/or Raf signals. Aberrant activation of the Ras/Raf/MEK/ERK pathway is a central feature in many cancers. However, paradoxically, sustained activation of the pathway induces cell cycle arrest and senescence in normal cells and certain cancer types. It has been proposed that growth arrest acts as a defense mechanism against Ras/Raf-mediated carcinogenesis and that overcoming this "growth arrest barrier" is a necessary step in tumor progression. Our understanding of this key event in carcinogenesis, controlled at the level of MEK/ERK, is currently quite limited. We have begun to address the underlying mechanisms of MEK/ERK signaling using normal and Ras/Raf-responsive tumor cells as models. The first intriguing finding is that ERK can mediate growth arrest by utilizing not only its "canonical" kinase activity but also, as yet unidentified, non-catalytic functions. We recently reported that oncogenic Raf signal-induced growth arrest is abrogated by ERK1/2 depletion but introduction of catalytically disabled mutant ERK into ERK1/2- depleted cells can selectively restore the growth arrest phenomenon. Based upon a hypothesis that MEK/ERK would interact with specific proteins to mediate growth arrest signaling, we conducted tandem affinity purification and identified mortalin as a potential negative regulator of MEK/ERK-growth arrest signaling. Our preliminary studies show that mortalin binds to inactive, but not active, MEK and that mortalin depletion increases basal as well as Raf-induced MEK/ERK activity. In addition, mortalin depletion promotes growth inhibitory signaling whereas mortalin overexpression exerts the opposite effects. These preliminary studies suggest that MEK/ERK utilizes a unique signaling mechanism to mediate growth arrest, for which mortalin has a negative-regulatory role possibly via its physical interaction with the pathway and ERK modulates a mechanism that requires its non-catalytic function. To test these hypotheses, we propose to (i) determine the role of mortalin in the regulation of Raf/MEK/ERK-mediated growth arrest signaling by gain or loss of function studies in normal and K-Rasor B-Raf mutated tumor cells and by comparing mortalin expression levels with altered MEK/ERK activity in tumor tissue specimens; (ii) determine whether mortalin regulates the Raf/MEK/ERK pathway by differentially sequestering MEK1 or MEK2; and (iii) determine molecular mechanisms underlying the non-catalytic ERK functions. This study will enhance our knowledge of the specific mechanisms of MEK/ERK signaling that interrupts Ras/Raf-driven carcinogenesis. PUBLIC HEALTH RELEVANCE: Activation of the MEK/ERK pathway induces terminal growth arrest in certain types of cancer lines derived from small cell lung carcinoma, medullary thyroid carcinoma, prostate carcinoma, pheochromocytoma, glioma, and gastrointestinal carcinoid. Exploration of the possibility that these cancer types can be effectively suppressed by modulating the pathway via mortalin into the direction of growth arrest may allow development of new therapeutic strategies to control them.