Histone methyltransferase EZH2 in liver pathophysiology and carcinogenesis

描述（由申请人提供）：长期目标是了解肝癌发生的分子机制，以便开发有效预防和治疗肝癌的新策略。表观遗传异常是肝细胞癌发生的主要标志。组蛋白甲基转移酶增强子zeste同源物2（EZH 2）通过催化组蛋白H3在赖氨酸-27处的三甲基化并促进DNA甲基化在胚胎发育中起关键作用。EZH 2在人肝癌中过表达，敲低EZH 2可导致肝癌细胞凋亡和裸鼠肿瘤消退。然而，关于EZH 2在肝脏病理生理学和致癌作用中的体内重要性知之甚少，这主要是由于EZH 2缺失小鼠的胚胎致死性。此外，癌基因c-Myc的激活导致小鼠肝癌，c-Myc过表达与人类肝细胞的恶性转化有关;然而，c-Myc诱导肝癌发生的潜在机制仍不清楚。本提案的目的是利用肝脏特异性敲除（LKO）EZH 2和/或过表达c-Myc的小鼠新模型阐明EZH 2在肝脏中的病理生理学重要性。我们发现EZH 2-LKO小鼠表现正常，表明肝脏中EZH 2的损失是耐受良好的。EZH 2对于胰腺β细胞的再生至关重要。我们发现EZH 2在肝再生过程中被显著诱导，表明其在肝再生中的重要性。核心假设是EZH 2对肝再生很重要，并且通过抑制c-Myc诱导的细胞衰老/凋亡和促进c-Myc诱导的细胞增殖而为c-Myc诱导的肝癌发生所需。EZH 2基因敲除将导致肝再生的显著延迟和c-Myc诱导的肝癌发生的阻断。这一核心假设将在两个具体目标中得到检验。目的1将使用EZH 2-LKO小鼠中的2/3部分肝切除模型来确定EZH 2在肝再生中的重要性。工作假设是EZH 2-LKO小鼠在肝再生方面将具有相当大的延迟，但由于代偿机制最终将再生肝。目的2将使用肝脏特异性敲除EZH 2但过表达c-Myc的小鼠来确定EZH 2在c-Myc诱导的肝癌发生中的重要性。工作假设是EZH 2缺陷将通过抑制细胞增殖但增强细胞凋亡/衰老来预防c-Myc诱导的肝癌。这项研究的结果将提供关于EZH 2在肝再生和肝癌发生中的重要性的关键新知识，这可能最终有助于将EZH 2确定为预防和治疗肝癌的新靶点，肝癌是一种在美国发病率不断增加的致命疾病。此外，了解EZH 2在肝再生中的作用将有助于确定EZH 2抑制剂如何影响肝损伤修复以及EZH 2抑制剂与其他抗癌药物联合治疗各种恶性肿瘤时化疗的安全性。 公共卫生相关性：美国肝癌的发病率越来越高，这是一种缺乏有效治疗的致命疾病。提出的研究是重要的，在调查领域的表观遗传控制的肝再生和肝癌的一种新的组蛋白甲基转移酶EZH 2。因此，这些发现有望提高我们对肝脏疾病和肝癌的认识，这些知识可用于开发预防和治疗肝癌的新药物靶点。

DESCRIPTION (provided by applicant): The long term goal is to understand the molecular mechanism of hepatocarcinogenesis so that novel strategies for the effective prevention and treatment of liver cancer can be developed. Epigenetic abnormity is a major hallmark in hepatocellular carcinogenesis. The histone methyltransferase enhancer of zeste homolog 2 (EZH2) plays a key role in embryonic development via catalyzing trimethylation of histone H3 at lysine-27 and facilitating DNA methylation. EZH2 is overexpressed in human liver cancer, and knockdown of EZH2 causes apoptosis of liver cancer cells and tumor regression in nude mice. However, little is known about the in vivo importance of EZH2 in liver pathophysiology and carcinogenesis, largely due to the embryonic lethality of EZH2-null mice. Additionally, activation of the oncogene c-Myc causes liver cancer in mice, and c-Myc overexpression is associated with malignant transformation of hepatocytes in humans; however, the underlying mechanism of c-Myc-induced hepatocarcinogenesis remains unclear. The objective of this proposal is to elucidate the pathophysiological importance of EZH2 in liver utilizing novel models of mice with liver-specific knockout (LKO) of EZH2 and/or overexpression of c-Myc. We found that EZH2-LKO mice appeared normal, suggesting that loss of EZH2 in liver is well-tolerated. EZH2 is essential for the regeneration of pancreatic beta cells. We found that EZH2 was markedly induced during liver regeneration, suggesting its importance for the regeneration of liver. The central hypothesis is that EZH2 is important for liver regeneration and is required for c-Myc-induced hepatocarcinogenesis through inhibiting c-Myc-induced cell senescence/apoptosis and promoting c-Myc-induced cell proliferation. Knockout of EZH2 will cause a significant delay of liver regeneration and blockage of c-Myc-induced hepatocarcinogenesis. This central hypothesis will be tested in two specific aims. Aim 1 will determine the importance of EZH2 in liver regeneration using models of 2/3 partial hepatectomy in EZH2-LKO mice. The working hypothesis is that EZH2-LKO mice will have considerable delay in liver regeneration but will eventually regenerate the liver due to compensatory mechanism(s). Aim 2 will determine the importance of EZH2 in c-Myc-induced liver carcinogenesis using mice with liver-specific knockout of EZH2 but overexpression of c-Myc. The working hypothesis is that EZH2 deficiency will prevent c-Myc-induced liver cancer via inhibiting cell proliferation but enhancing cell apoptosis/senescence. Results from this study will provide crucial novel knowledge about the importance of EZH2 in liver regeneration and liver carcinogenesis, which may ultimately help to identify EZH2 as a novel target for the prevention and treatment of liver cancer, a deadly disease with increasing incidence in the USA. Moreover, understanding the role of EZH2 in liver regeneration will help to determine how EZH2 inhibitors may influence liver injury-repair and the safety of chemotherapy when EZH2 inhibitors are combined with other anticancer drugs to treat diverse malignancies. PUBLIC HEALTH RELEVANCE: The United States has increasing incidence of liver cancer, a deadly disease that lacks effective treatment. The proposed studies are of importance in the under-investigated area of epigenetic control of liver regeneration and liver carcinogenesis by a novel histone methyltransferase EZH2. Thus, the findings are expected to enhance our understanding of liver diseases and liver cancer, and this knowledge can be used to develop new drug target to prevent and treat liver cancer.