Role of Epigenetic Repression of IRF8 in Tumor Progression/Metastasis

描述(申请人提供)：结直肠癌是美国男性和女性中第二常见的癌症。绝大多数结直肠癌患者的死亡与肝转移的形成有关。根据美国癌症协会的数据，如果癌症没有转移，结直肠癌患者的5年生存率为92%。然而，一旦癌症转移到肝脏，5年存活率就会下降到7%。转移性结直肠癌目前还没有有效的治疗方法，治愈的前景仍然很渺茫。对细胞凋亡的抵抗是转移性人类结直肠癌的标志。在人类，Fas在正常结肠组织中有结构性表达，而在结直肠癌中Fas表达减弱，在转移性结直肠癌中经常出现Fas表达和功能的完全丧失。诱导肿瘤细胞凋亡是许多癌症治疗的基础，包括结直肠癌治疗。因此，获得对细胞凋亡的抵抗是结直肠癌治疗中最重要的挑战之一。我们已经确定干扰素调节因子8(IRF8)是人类结直肠癌细胞凋亡的重要调节因子。此外，我们观察到，在绝大多数转移性人类结直肠癌标本中，通过IRF8启动子DNA的超甲基化，IRF8的表达被沉默。此外，异位表达IRF8在体外恢复了转移性人结肠癌细胞对Fas介导的凋亡的敏感性，并在体内抑制了原发人结肠癌在异种小鼠模型中的生长。基于这些新的观察结果，我们假设IRF8是一种自发的结直肠癌转移抑制因子，它通过调节肿瘤细胞对凋亡的敏感性而发挥作用，转移性结直肠癌细胞利用IRF8启动子的超甲基化来沉默IRF8的表达，从而获得转移表型。为了验证我们的假设，我们将追求以下两个特定的目标：1)阐明IRF8在人结肠癌细胞中表达的表观遗传学调控的分子机制；2)确定IRF8在体内自发性结肠癌转移中的作用，并探索基于IRF8机制的治疗在抑制结直肠癌转移中的潜力。该研究项目的成功完成不仅验证了我们的假设，即IRF8是一种新型的自发转移抑制因子，而且将为以IRF8机制为基础的针对致命转移性人类结直肠癌的治疗提供分子基础。公共卫生相关性：这项拟议的研究项目将阐明IRF8在转移性结肠癌中表达的表观遗传调控的分子机制，并探索靶向IRF8表达与TRAIL联合治疗在体内干预人结肠癌转移的可能性。这项拟议研究的成功完成将为针对致命的转移性人类结直肠癌提供一种新的策略。

DESCRIPTION (provided by applicant): Colorectal cancer is the second most commonly diagnosed cancer found in men and women in the United States. The vast majority of mortality of colorectal cancer patients is associated with formation of liver metastasis. According to American Cancer Society, the 5-year survival rate of colorectal cancer patients is 92% if the cancers have not metastasized. However, the 5-year survival rate drops to 7% once the cancer has metastasized to the liver. There is no effective therapy for metastatic colorectal cancer and prospects for cure remains poor. Resistance to apoptosis is a hallmark of metastatic human colorectal cancer. In humans, Fas is constitutively expressed in normal colon tissues, However, Fas expression is diminished in colorectal carcinoma, and complete loss of Fas expression and function is frequently observed in metastatic colorectal cancers. Induction of tumor cell apoptosis is the basis of many cancer therapies, including colorectal cancer therapies. Therefore, acquisition of resistance to apoptosis is one of the most significant challenges in colorectal cancer therapy. We have identified Interferon Regulatory Factor 8 (IRF8) as an essential regulator of apoptosis in human colorectal cancer cells. Furthermore, we observed that IRF8 expression is silenced in the vast majority of metastatic human colorectal cancer specimens through the IRF8 promoter DNA hypermethylation. In addition, ectopic expression of IRF8 restored the sensitivity of metastatic human colon carcinoma cells to Fas-mediated apoptosis in vitro and inhibited the growth of primary human colon carcinoma in a xenograft mouse model in vivo. Based on these novel observations, we hypothesize that IRF8 is a spontaneous colorectal cancer metastasis suppressor that functions through regulating tumor cell sensitivity to apoptosis, and metastatic colorectal cancer cells use the IRF8 promoter hypermethylation to silence IRF8 expression to acquire a metastatic phenotype. To test our hypothesis, we will pursue the following two specific aims: 1) to elucidate the molecular mechanisms underlying epigenetic regulation of IRF8 expression in human colon carcinoma cells; and 2) to determine the roles of IRF8 in spontaneous colon carcinoma metastasis in vivo, and to explore the potential of IRF8 mechanism-based therapies in suppression of colorectal carcinoma metastasis. Successful completion of this proposed research project will not only validate our hypothesis that IRF8 is a novel spontaneous metastasis suppressor but will also provide the molecular basis for the development of IRF8 mechanism-based therapies to target the deadly metastatic human colorectal cancer. PUBLIC HEALTH RELEVANCE: This proposed research project will elucidate the molecular mechanisms underlying epigenetic regulation of IRF8 expression in metastatic human colon cancer, and explore the potential of targeting IRF8 expression in combination with TRAIL therapy for the intervention of human colon carcinoma metastasis in vivo. Successful completion of the proposed research will provide a novel strategy for targeting the deadly metastatic human colorectal cancer.