Nrf2在生命早期砷暴露影响肝癌易感性效应中的作用与机制

Arsenic is a common environmental carcinogen with the liver as one of the major targets. Accumulating studies found that arsenic exposure in early life was associated with enhanced susceptibility to cancers in later life. The underlying mechanism is elusive. Nrf2 is a well-known master regulator of anti-oxidative system. However, recent evidence suggests a cancer promoting role of Nrf2 when it is constitutively activated, which is more relevant to chronic exposure to environmental carcinogens. We have also observed a constitutively elevated expression of Nrf2 and its downstream genes during arsenic-induced malignant transformation of human liver cells. Hence we hypothesize that exposure to an environment-related dose of arsenic in early life induces constitutive Nrf2 activation, and thus leads to enhanced susceptibility to liver cancers in later life. In the proposed project, global and liver-specific Nrf2-knock out mice will be used to analyze the role of Nrf2 in long-term effect of early-life arsenic exposure on susceptibility to liver cancers, and to explore important epigenetic alterations regulating Nrf2 in this long-term effect. The effect of Nrf2 in abnormal autophagy, proliferation and differentiation of liver-related stem cells will also be investigated to elucidate possible mechanisms by which Nrf2 regulates susceptibility to liver cancers affected by arsenic. A comprehensive understanding of oxidative stress in environmental carcinogenesis will be achieved based on aforementioned studies. In addition, the present project will provide new insights into carcinogenic mechanisms, as well as prevention and treatment strategies, for cancer resulting from arsenic and similar kind of chemicals.

砷是常见的环境致癌物，以肝脏为主要靶器官之一。生命早期砷暴露显著增强生命晚期的癌症易感性，但机制不清。Nrf2是抗氧化防御体系的重要调控因子，而最新的证据提示，Nrf2持续活化对癌症发生起促进作用。后者与环境致癌物的长期慢性低水平暴露特点更具相关性。我们在砷恶性转化肝细胞的过程中也观察到Nrf2及其下游基因持续高表达。由此我们假设：生命早期环境水平砷暴露诱发Nrf2持续活化，进而造成生命晚期癌症易感性升高。本项目以生命早期为砷暴露窗口，应用全身和肝脏特异性Nrf2基因敲除小鼠模型分析Nrf2在砷暴露增加肝癌易感性远期效应中的作用；探讨从生命早期砷暴露到生命晚期肝癌发生过程中，对Nrf2起关键调控作用的表观遗传学改变；从干细胞自噬缺陷和增殖分化异常两个角度解析Nrf2参与砷影响肝癌易感性的机制。上述研究将加深我们对氧化应激在砷及同类化学毒物致癌过程中作用的认识，为相关癌症防治工作提供新思路。

砷是常见的环境致癌物，以肝脏为致癌效应的靶器官之一。生命早期砷暴露显增强生命晚期癌症易感性，但机制不清。Nrf2是机体抗氧化防御体系主要上游调控因子，其在癌症发生发展中的作用具有两面性。本研究建立生命早期无机砷暴露小鼠模型，证实生命早期环境相关水平（0.5 ppm、5 ppm）无机砷暴露诱导肝脏Nrf2的表达与活化，导致适应性抗氧化反应，增强小鼠对于二乙基亚硝胺（DEN）所致的肝癌易感性。Nrf2敲除对无机砷砷甲基化代谢无显著影响，但抑制DEN和生命早期砷暴露引起的肝癌效应。Nrf2参与生命早期砷暴露增强DEN所致肝癌易感性效应的原因可能与其在维持CYP2E1蛋白稳定性中的作用有关。Nrf2敲除时，肝细胞CYP2E1蛋白降解速度加快，此时，外源化学物（如硫代乙酰胺）的代谢活化抑制，肝毒性减弱。对断乳期（4周龄）子代小鼠的肝脏分析提示，生命早期砷暴露对肝脏Nrf2表达与活化的诱导作用与microRNA 144-3p表达降低有关，与Nrf2基因DNA甲基化无关。生命早期砷暴露对肝脏内源性和外源性干细胞数量无显著影响，但无细胞毒性水平的无机砷暴露可显著抑制小鼠骨髓间充质干细胞成脂、成骨分化能力。对氧化还原基础值迥异的两种小鼠（C57BL/6和129X1/SvJ）进行的比较研究发现，与129X1/SvJ小鼠相比，C57BL/6小鼠对生命早期砷暴露增强肝癌易感性效应和急性砷暴露所致肝损伤均更为敏感。C57BL/6小鼠砷甲基化与外排能力弱于129X1/SvJ小鼠，这种砷代谢差异一定程度上解释了两种鼠系在急性砷暴露所致肝毒性上的差别，但不足以解释生命早期砷暴露所致的远期肝癌易感性差别。上述结果证实了本研究提出的核心假说，即“生命早期环境水平砷暴露诱发Nrf2持续活化，进而造成生命晚期癌症易感性升高”。本研究丰富了对Nrf2功能与意义的认识，揭示了Nrf2在环境无机砷暴露增强癌症易感性中扮演的“癌基因”角色，为相关癌症防治工作提供了新思路。