The role of MDM2-MTBP axis in cancer metastasis

描述（由申请人提供）：小鼠双分钟（MDM2）癌基因通过肿瘤抑制因子p53以外的途径促进癌症转移。然而，p53非依赖性mdm2介导的癌症进展机制尚不清楚。通过了解MDM2不依赖p53的功能，确定了MDM2结合蛋白（MDM2 Binding Protein， MTBP）。我们之前通过以下发现证明了MTBP独立于p53抑制癌症的迁移和转移：1)小鼠MTBP单倍性不足增加肝细胞癌（HCC）、肉瘤和其他类型癌症的转移；2) MTBP过表达抑制缺乏野生型p53活性的骨肉瘤细胞的迁移和转移；3) MTBP内源性结合诱导迁移的肌动蛋白交联蛋白α -肌动蛋白4 (ACTN4)，抑制ACTN4介导的迁移和丝状足形成。临床上，MTBP在头颈部鳞状细胞癌组织中的表达降低与患者生存率降低相关。本提案的目的是确定MTBP的作用及其与MDM2的功能关联，如果有的话，在癌症转移中。HCC是美国癌症相关死亡的上升原因，其5年生存率低于12%。这种不良预后的主要原因是转移性扩散。由于这些原因，我们将使用HCC作为模型来检查MDM2和MTBP在癌症迁移和转移中的作用。我们的初步结果表明：1)MTBP在人HCC组织中的表达降低与血管/囊浸润和淋巴结转移有关；2)MTBP抑制HCC细胞迁移；3)MTBP抑制ACTN4介导的丝状足形成和迁移；4)MTBP抑制Ets癌基因家族转录因子Elk-1的活性，Elk-1是Erk1/2 MAP激酶的靶点。基于这些观察，我们假设MTBP通过抑制ACTN4和Elk-1活性来抑制HCC转移；而MDM2通过抑制与MTBP的结合促进肿瘤转移。我们将通过实现以下两个特定目标来验证这一假设：目的1是通过探索影响转移的MDM2-MTBP相互作用的下游介质来确定MDM2对MTBP在HCC中抑制转移的抑制作用；目的2是通过使用我们最近生成的MTBP表达水平仅为野生型20%的半胚MTBPH小鼠，确定MDM2和MTBP在小鼠模型中对HCC进展的体内贡献。本项目的完成将填补MTBP抑制肿瘤转移和MDM2通过抑制MTBP以不依赖p53的方式促进肿瘤进展机制方面的知识空白。鉴于大约70%的HCC组织中发现MTBP表达降低，30%的HCC中检测到MDM2过表达，并且转移是癌症死亡的主要原因，我们的研究有可能显著影响HCC以及其他类型的MDM2和/或MTBP表达改变的癌症的诊断、治疗和预后。

DESCRIPTION (provided by applicant): The mouse double minute (MDM2) oncogene enhances cancer metastasis via pathways other than the tumor suppressor p53. However, the mechanisms underlying p53-independent MDM2-mediated cancer progression remain unclear. Through the efforts to understand the p53-independent function of MDM2, MDM2 Binding Protein (MTBP) was identified. We previously demonstrated that MTBP suppresses cancer migration and metastasis independently of p53 through the following findings: 1) MTBP haploinsufficiency in mice increases metastasis of hepatocellular carcinoma (HCC), sarcoma, and other types of cancer; 2) MTBP overexpression inhibits the migration and metastasis of osteosarcoma cells lacking wild-type p53 activity; 3) MTBP endogenously binds to a migration-inducing actin-crosslinking protein alpha-actinin-4 (ACTN4) and inhibits the migration and filopodia formation mediated by ACTN4. Clinically, reduced MTBP expression in head and neck squamous cell carcinoma tissues is shown to be associated with reduced survival of patients. The goal of this proposal is to determine the roles of MTBP and its functional association with MDM2, if any, in cancer metastasis. HCC is a rising cause of cancer-related death in the United States with 5-year survival rate below 12%. The leading cause of this poor prognosis is metastatic spread. For these reasons, we will use HCC as a model for examining the contributions of MDM2 and MTBP in cancer migration and metastasis. Our preliminary results demonstrated the following: 1) reduced MTBP expression in human HCC tissues was associated with the presence of vascular/capsular invasion and lymph node metastasis, 2) MTBP inhibited HCC cell migration, 3) MTBP inhibited the filopodia formation and migration mediated by ACTN4, and 4) MTBP inhibited the activity of Elk-1, an Ets oncogene family transcription factor and a target of Erk1/2 MAP kinase. Based on these observations, we hypothesize that MTBP inhibits HCC metastasis by suppressing ACTN4 and Elk-1 activities; whereas, MDM2 promotes cancer metastasis by its inhibitory binding to MTBP. We will test this hypothesis by achieving the following two Specific Aims: Aim 1 is to determine the inhibitory effects of MDM2 on the metastasis suppression by MTBP in HCC by exploring downstream mediators of MDM2-MTBP interactions that affect metastasis; Aim 2 is to determine the in vivo contributions of MDM2 and MTBP to HCC progression in mouse models by using our recently generated hypomorphic MTBPH mice that express MTBP at only 20% of wild-type levels. Completion of this project will fill the knowledge gaps pertaining to the mechanisms by which MTBP inhibits cancer metastasis and MDM2 promotes tumor progression in a p53- independent manner through inhibition of MTBP. Given that reduced MTBP expression is found in about 70% of HCC tissues, MDM2 overexpression is detected in 30% of HCC, and metastasis is the major cause of cancer mortality, our study has the potential to significantly impact the diagnosis, treatment, and prognosis for HCC and likely other types of cancer having altered expression of MDM2 and/or MTBP.