Decoding the MST1 and MST2 kinases in cellular physiology and tumor suppression

描述（由申请人提供）：我们最近发现Mst1/2丝氨酸/苏氨酸激酶是重要的肝细胞癌（HCC）肿瘤抑制因子。Mst1/2是果蝇肿瘤抑制因子Hippo的同源基因，在发育过程中促进细胞凋亡和抑制细胞增殖。Hippo的作用是由一组在人类中保守的关键底物介导的，这些底物反过来抑制转录辅激活因子Yorkie，其人类同源物Yap是一种已建立的癌基因和器官大小的调节因子。通过生成Mst1/2 KO小鼠，我们发现Mst1和Mst2是维持肝细胞静止所必需的，它们的肝脏特异性消融导致Yap激活、肝脏过度生长、细胞凋亡抵抗和HCC的快速发展。值得注意的是，我们已经确定约30%的人类hcc解除了Mst1/2-Yap信号轴的调控。这些结果证实Mst1/2是与人类HCC发病机制相关的重要抑癌因子，并指出Yap是一个关键的下游靶点。尽管Mst1/Mst2位于一个重要的生长控制网络的中心，但关于Mst1/2的生理调节和功能及其作为肿瘤抑制因子的作用仍有待确定。我们发现，哺乳动物细胞中Mst1和Mst2的上游调控和输出明显不同于果蝇Hippo通路和体外过表达研究预测的结果。例如，在小鼠初始T细胞中，Mst1/2独立于Yap负性调节增殖。此外，Mst1/2是肿瘤抑制多肽Rassf家族的组成伙伴，其中一些（如Nore1/Rassf5）受ras样GTPases调控。这多个上游元素中哪一个控制肝脏中的Mst1/Mst2是一个悬而未决的问题。该提案描述了一种系统的方法，从分子角度定义Mst1/2的上游调控输入和关键输出，这些输入和关键输出决定了Mst1/2在体内调节肝脏生长和抑制肿瘤的能力。Yap的调控及其在癌变中的作用将被详细研究。我们还发现了对Mst1/2缺乏的Yap非依赖性反应，其激活机制和对肝细胞恶性转化的贡献将被确定。最后，我们将描述表现出Mst1/Mst2通路失调的人类HCC，并确定这种失调如何影响人类HCC的组织病理学和临床特征。这些研究可能会揭示肝癌以及其他具有Mst1/2激酶调控缺陷的人类癌症的治疗新策略。

DESCRIPTION (provided by applicant): We have recently identified the Mst1/2 ser/thr kinases as important hepatocellular carcinoma (HCC) tumor suppressors. Mst1/2 are the orthologs of the Drosophila tumor suppressor Hippo, which promotes apoptosis and inhibits cell proliferation during development. Hippo action is mediated by a set of critical substrates which are conserved in humans, and which in turn suppress the transcriptional coactivator, Yorkie, whose human ortholog, Yap, is an established oncogene and regulator of organ size. By generating Mst1/2 KO mice we found that Mst1 and Mst2 are required to maintain quiescence of liver cells and that their liver-specific ablation results in Yap activation, liver overgrowth, resistance to apoptosis, and the rapid development of HCC. Significantly, we have determined that ~30% of human HCCs have deregulation of the Mst1/2-Yap signaling axis. These results establish Mst1/2 as important tumor suppressors relevant to the pathogenesis of HCC in humans and point to Yap as one critical downstream target. Although Mst1/Mst2 lie at the center of an important growth control network, much about the physiologic regulation and functions of Mst1/2 and their role as tumor suppressors remains to be determined. We find that the upstream regulation and outputs of Mst1 and Mst2 in mammalian cells differ markedly from those predicted by the Drosophila Hippo pathway and by in vitro overexpression studies. In murine naive T cells, e.g., Mst1/2 negatively regulate proliferation independently of Yap. Moreover Mst1/2 are constitutive partners of the Rassf family of tumor suppressor polypeptides, some of which (e.g., Nore1/Rassf5) are regulated by Ras-like GTPases. Which of these multiple upstream elements control Mst1/Mst2 in liver is an open question. This proposal describes a systematic approach to define in molecular terms the upstream regulatory inputs and critical outputs of Mst1/2 that determine the capacity of Mst1/2 to regulate liver growth and exert tumor suppression in vivo. The regulation of Yap and its contribution to carcinogenesis will be investigated in detail. We have also identified Yap- independent responses to Mst1/2 deficiency, whose mechanism of activation and contribution to the malignant transformation of hepatocytes will be determined. Finally we will characterize the human HCCs that exhibit deregulation of the Mst1/Mst2 pathway, and determine how such deregulation influences the histopathologic and clinical features of human HCC. These studies are likely to reveal new strategies for the therapy of HCC and as well as of other human cancers that have defective regulation of the Mst1/2 kinases. PUBLIC HEALTH RELEVANCE: MST1 and MST2 are two very similar protein kinases that are central parts of a network of tumor suppressors and oncogenes. Deletion of both together in mouse liver produces liver cancer. We want to find out how they are regulated, the identity of their key targets in suppressing tumor development, how their removal enables liver cancer to develop and their role in human liver cancer.