Histone methyltransferase EZH2 in liver pathophysiology and carcinogenesis

描述（申请人提供）：长期目标是了解肝癌发生的分子机制，从而制定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。zeste同源物2的组蛋白甲基转移酶增强子（EZH2）通过催化组蛋白H3在赖氨酸-27位点的三甲基化和促进DNA甲基化，在胚胎发育中起关键作用。EZH2在人肝癌中过表达，敲低EZH2可引起裸鼠肝癌细胞凋亡和肿瘤消退。然而，对于EZH2在肝脏病理生理和癌变中的体内重要性知之甚少，这主要是由于EZH2缺失小鼠的胚胎致死性。此外，致癌基因c-Myc的激活会导致小鼠肝癌，而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尚不清楚。本提案的目的是利用EZH2肝脏特异性敲除（LKO）和/或c-Myc过表达的新型小鼠模型阐明EZH2在肝脏中的病理生理重要性。我们发现EZH2- lko小鼠表现正常，表明肝脏中EZH2的丢失是耐受良好的。EZH2对于胰腺细胞的再生至关重要。我们发现EZH2在肝脏再生过程中被显著诱导，提示其在肝脏再生中的重要性。核心假设是EZH2对肝脏再生很重要，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，是c- myc诱导的肝癌发生所必需的。敲除EZH2会导致肝脏再生的显著延迟和c- myc诱导的肝癌发生的阻断。这一中心假设将在两个具体目标中得到检验。目的1将通过EZH2- lko小鼠2/3部分肝切除模型确定EZH2在肝再生中的重要性。工作假设是EZH2-LKO小鼠的肝脏再生会有相当大的延迟，但最终会由于代偿机制而再生肝脏。Aim 2将通过肝脏特异性敲除EZH2但c-Myc过表达的小鼠，确定EZH2在c-Myc诱导的肝癌发生中的重要性。工作假设是EZH2缺乏通过抑制细胞增殖而增强细胞凋亡/衰老来预防c- myc诱导的肝癌。这项研究的结果将提供关于EZH2在肝脏再生和肝癌发生中的重要性的重要新知识，这可能最终有助于确定EZH2作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上升的致命疾病。此外，了解EZH2在肝脏再生中的作用将有助于确定当EZH2抑制剂与其他抗癌药物联合治疗各种恶性肿瘤时，EZH2抑制剂如何影响肝损伤修复和化疗的安全性。

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.