PR55-alpha controlled PP2A in the regulation of the Hippo/YAP pathway

摘要 胰腺癌（PC）是美国最致命的癌症之一，5年生存率为1.7%， 在过去的40年里基本上没有变化。尽管进行了广泛的研究，但没有有效的针对性 这种疾病的治疗，表明对PC中致癌信号网络的理解不足。而 几乎所有与癌症有关的信号网络都依赖于激酶和磷酸酶的合作， 尽管蛋白激酶是促进癌症的信号，但大多数研究都集中在癌症中的蛋白激酶上 而留下蛋白磷酸酶作为盲点。PP 2A是一个异源三聚体家族，其作为主要的 人类细胞中Ser/Thr磷酸酶活性的来源，每个亚基均由支架亚基（Aα/β）、A β亚基组成 催化亚基（Cα/β）和控制底物的许多非保守调节（B）亚基之一（>27 PP 2A复合物的特异性。我们的初步研究确定了PP 2A的调节亚基PR 55 α，作为一种蛋白， 促进恶性PC的重要新参与者。特别是，我们的研究表明，PR 55 β抑制了 Hippo肿瘤抑制途径的MOB 1/LATS自激活环，其功能是诱导肿瘤细胞凋亡。 雅普磷酸化导致其细胞质保留和蛋白酶体降解。根据基本的 雅普信号在Kras驱动的PC发育和进展中的作用，PR 55 α的敲低阻碍了 PC细胞的非贴壁依赖性生长、致瘤性和转移。我们的初步数据还显示 PR 55 α蛋白的稳定性受抑癌基因p53及其靶基因的负调控 FBXL 20，SCF（SKP 1-CUL 1-F-box蛋白）E3连接酶的底物衔接子。我们一致观察到， PR 55 α在PC标本中升高，其表达与患者生存率低相关。基于这些 研究结果，我们假设p53功能的丧失导致PR 55 α蛋白的稳定，反过来， 激活Kras驱动PC启动和进展所需的雅普致癌信号传导。我们将测试 目的1：明确PR 55 α/PP 2A促进雅普的机制 激活和该级联在PC的肿瘤发生和恶性潜能中的意义，目的2阐明 p53如何抑制PR 55 α水平，目的3阐明PR 55 α/雅普轴在胰腺癌中的作用。 KPC自发小鼠模型中的癌症发展和进展。这项工作是创新的，因为它将 阐明通过PR 55 β/PP 2A激活的雅普促进PC起始和进展的关键途径 信令，这对于PP 2A领域和PC领域都是新颖的。成功完成拟议的研究 将产生一个深入的机制的理解PR 55 β/PP 2A复合物的关键贡献， 观察到在PC发展过程中Kras激活和p53肿瘤抑制因子的丢失之间的合作 和进步。获得的知识不仅将提高对PP 2A在癌症中作用的理解， 而且将有助于确定新的治疗靶点以改善PC的治疗。

Abstract Pancreatic cancer (PC) is one of the most lethal cancers in the U.S. with a 5-year survival rate of 7%, which has essentially unchanged for the past 40 years. Despite extensive research, there are no effective targeted therapies for this disease, suggesting an insufficient understanding of oncogenic signaling networks in PC. While almost all of the signaling networks implicated in cancer rely on the cooperation of kinases and phosphatases to transduce signals promoting cancer, most efforts have only been put on the study of protein kinases in cancer while leaving protein phosphatases as a blind spot. PP2A is a family of heterotrimers that serve as the main source of Ser/Thr phosphatase activities in human cells, with each consisting of a scaffold subunit (Aα/β), a catalytic subunit (Cα/β), and one of many non-conserved regulatory (B) subunits (>27) that control the substrate specificities of PP2A complexes. Our preliminary studies identified PR55α, a regulatory subunit of PP2A, as an important new player in promoting malignant PC. In particular, our studies revealed that PR55 inhibits the MOB1/LATS auto-activation loop of the Hippo tumor suppression pathway, whose function is to induce the phosphorylation of YAP causing its cytoplasmic retention and proteasomal degradation. In line with the essential role of YAP signaling in Kras-driven PC development and progression, knockdown of PR55α impeded the anchorage-independent growth, tumorigenicity, and metastasis of PC cells. Our preliminary data also revealed that the stability of PR55α protein is negatively regulated by the tumor suppressor p53 and its target gene FBXL20, a substrate adaptor for the SCF (SKP1-CUL1-F-box protein) E3 ligase. Consistently, we observed that PR55α is elevated in PC specimens, where its expression correlates with poor patient survival. Based on these findings, we hypothesize that loss of p53 function results in the stabilization of PR55α protein, which, in turn, activates the YAP oncogenic signaling required for Kras-driving PC initiation and progression. We will test the hypothesis by three specific aims: Aim 1 to define the mechanism by which PR55α/PP2A promotes YAP activation and the significance of this cascade in oncogenesis and malignant potential of PC, Aim 2 to elucidate how p53 suppresses the level of PR55α, and Aim 3 to delineate the role of the PR55/YAP axis in pancreatic cancer development and progression in the KPC spontaneous mouse model. The work is innovative, as it will illuminate a critical pathway that promotes PC initiation and progression through PR55/PP2A activated YAP signaling, which is novel to both the PP2A field and the PC field. Successful completion of the proposed studies will yield an in-depth mechanistic understanding of the critical contribution of PR55/PP2A complex to the observed cooperation between Kras activation and the loss of the p53 tumor suppressor during PC development and progression. The acquired knowledge not only will improve the understanding of the role of PP2A in cancer but also will contribute to identifying novel therapeutic targets for improving the treatment of PC.