Novel posttranslational modification in LKB1 activation and cancer development

描述(申请人提供)：最近出现的癌症新陈代谢在癌症的发展和耐药性中扮演着重要的角色。参与癌症代谢的一个关键调节因子是丝氨酸/苏氨酸激酶LKB1。它在抑制和促进癌症的过程中发挥着“双刃剑”的作用，并与另外两种蛋白质--假激酶STRAD和支架蛋白MO25--形成异源三聚体复合体。LKB1与这两种蛋白的结合促进并稳定了LKB1的活化构象。尽管LKB1已被证明在调节癌症的发展中起着关键作用，但LKB1的激活和活性是如何被调节的还不是很清楚。我们的初步结果揭示了一种维持LKB1活性的新机制。我们发现LKB1的活性受致癌基因Skp2的正调控，Skp2与Skp1、Cul-1和Rbx1形成SCF复合体，构成E3连接酶。有趣的是，我们发现Skp2是LKB1的直接E3连接酶，而Skp2介导的LKB1泛素化是LKB1活性所必需的。我们推测，K63连接的LKB1泛素化对于维持LKB1-Strad-MO25复合体的完整性是必不可少的，从而调节LKB1的激活。这项研究的目标是测试这一假设模型，并确定这种泛素化在癌症发展中的作用。我们将通过追求三个具体目标来检验我们的中心假设。目的1.研究Skp2介导的K63连接的LKB1泛素化对LKB1活性的影响。基本原理。目的2.确定LKB1在调节Akt信号激活中的作用。目的3.探讨Skp2和LKB1在致癌功能中的相互作用。总而言之，我们的研究有几个具有重要临床意义的新发现。首先，我们首次证明了K63与LKB1 I的泛素化有关，这对LKB1的活性至关重要。其次，我们发现LKB1是EGF激活致癌Akt所必需的，从而为致癌Akt激活提供了一个洞察力和范式。第三，我们发现Skp2 E3连接酶是LKB1的E3连接酶，它是LKB1激活的关键。第四，我们的研究可能通过激活LKB1-AMPK-ACC通路为癌基因Skp2在细胞转化和肿瘤发生中的作用提供新的视角和机制。最后，我们的研究不仅可能揭示LKB1在肿瘤维持中的新的致癌功能和机制，而且有助于确定LKB1-AMPK作为癌症预防和干预的潜在治疗靶点。

DESCRIPTION (provided by applicant): Cancer metabolism has recently emerged to play an important role in cancer development and drug resistance. A key regulator involved in cancer metabolism is serine/threonine kinase LKB1. It displays a "double-edged sword" role in cancer suppression and promotion and forms a heterotrimeric complex with two other proteins, the pseudokinase STRAD and the scaffolding protein MO25. Binding of LKB1 to these two protein promotes and stabilizes the activated conformation of LKB1. Although LKB1 has been shown to play a critical role in regulating cancer development, how LKB1 activation and activity are regulated is poorly defined. Our preliminary results revealed a novel mechanism by which LKB1 kinase activity is maintained. We showed LKB1 kinase activity is positively regulated by oncogenic Skp2, which forms a SCF complex with Skp1, Cul-1 and Rbx1 to constitute an E3 ligase. Interestingly, we found that Skp2 is a direct E3 ligase for LKB1, and Skp2-mediated ubiquitination of LKB1 is required for LKB1 activity. We hypothesized that K63-linked ubiquitination of LKB1 is essential for maintaining the integrity of LKB1-STRAD-MO25 complex, thereby regulating LKB1 activation. The goal of this study is to test this hypothetical model and to determine the role of this ubiquitination in cancer development. We will test our central hypothesis by pursuing three specific aims. Aim 1. To characterize the role of Skp2-mediated K63-linked ubiquitination of LKB1 in LKB1 activity. Rationale. Aim 2. To identify the role of LKB1 in regulating Akt signaling activation. Aim 3. To explore the interplays between Skp2 and LKB1 in oncogenic functions. In summary, our study has several novel discoveries with important clinical implications. First, we showed for the first time that K63-linked ubiquitination of LKB1 i critical for LKB1 kinase activity. Second, we found that LKB1 is required for oncogenic Akt activation in response to EGF, thereby providing an insight and paradigm for oncogenic Akt activation. Third, we showed that Skp2 E3 ligase is an E3 ligase for LKB1, which is critical for LKB1 activation. Fourth, our study may offer a novel insight and mechanism for oncogenic Skp2 in cell transformation and tumorigenesis by activating the LKB1-AMPK-ACC pathway. Finally, our study may not only reveal novel oncogenic function and mechanism for LKB1 in tumor maintenance, but also help identify LKB1-AMPK as potential therapeutic targets for cancer prevention and intervention.