The epigenetic mechanism of hexavalent chromium carcinogenesis

 描述（由申请人提供）：六价铬[Cr（VI）]是导致肺癌和其他癌症的最常见环境污染物之一。Cr（VI）致癌的机制尚未阐明。本研究的长期目标是确定Cr（VI）致癌性的机制，并确定更好地治疗和预防Cr（VI）暴露导致的肺癌的靶点。表观遗传学是指基因表达模式的可遗传变化，这些变化不是由DNA序列的变化引起的，而是由DNA甲基化、组蛋白翻译后修饰、microRNA和长非编码RNA介导的。癌症干细胞（CSCs）是具有正常干细胞特征的癌细胞。CSC或CSC样细胞被认为是癌症起始细胞。基因组印记是指某些基因以亲本来源特异性方式表达的表观遗传机制，限制它们仅从两个亲本染色体中的一个表达。许多研究表明，表观遗传学和基因组印记的失调在肿瘤发生中起着关键作用。越来越多的证据表明，Cr（VI）也会引起表观遗传效应，如改变DNA甲基化和组蛋白翻译后修饰。然而，Cr（VI）触发这些表观遗传修饰的机制在很大程度上仍然是未知的，这些表观遗传修饰是否在Cr（VI）致癌中起因果作用尚不清楚。我们的初步研究发现：（i）慢性Cr（VI）暴露诱导CSC样性质和细胞恶性转化;（ii）慢性Cr（VI）暴露增加几种HMT酶的水平，其在Cr（VI）诱导的CSC样性质和细胞转化中起因果作用;（iii）在暴露于铬的小鼠和人的肺癌组织中也检测到更高水平的H3抑制性甲基化标记及其相关HMT酶;（iv）HMT酶的上调在慢性Cr（VI）暴露引起的Dlk 1-Dio 3基因组印迹簇的失调中起关键作用;（v）miR-494下调在Cr（VI）诱导的细胞转化中起因果作用;和（vi）氧化应激增加HMT酶的水平。基于文献和我们的初步数据，我们假设：（i）Cr（VI）暴露产生氧化应激，这增加了HMT酶的水平;（ii）HMT酶的上调使Dlk 1-Dio 3基因组印迹簇失调，这降低了miR-494的水平;和（iii）miR-494的下调增加了其靶标和关键CSC因子c-Myc和BMI 1的水平，其产生CSC样性质，促进Cr（VI）诱导的细胞转化和肿瘤发生。这项研究不仅填补了表观遗传学和Cr（VI）致癌作用的知识空白，而且为氧化应激在Cr（VI）致癌作用中的关键作用提供了新的机制见解。提出了三个目标：目标1：下调Dlk 1-Dio 3印迹簇的miR-494诱导CSC样特性及其在Cr（VI）诱导的细胞转化和肿瘤发生中的作用目的2：慢性Cr（VI）暴露诱导的HMTases通过下调Dlk 1-Dio 3基因组印迹簇下调miR-494。目的3：慢性Cr（VI）暴露通过氧化应激上调HMT酶。

 DESCRIPTION (provided by applicant): Hexavalent chromium [Cr(VI)] is one of the most common environmental pollutants causing lung and other cancer. The mechanism of Cr(VI) carcinogenesis has not been elucidated. The long-term goal of this study is to determine the mechanism of Cr(VI) carcinogenicity and identify targets for better treatment and prevention of lung cancer resulting from Cr(VI) exposure. Epigenetics refers to heritable changes in the pattern of gene expression that are not caused by changes in DNA sequence, but are mediated by DNA methylation, histone posttranslational modifications, microRNAs and long noncoding RNAs. Cancer stem cells (CSCs) are cancer cells possessing characteristics of normal stem cells. CSCs or CSC-like cells are considered as cancer initiating cells. Genomic imprinting refers to an epigenetic mechanism by which certain genes are expressed in a parent- of-origin-specific manner, restricting their expression from only one of the two parental chromosomes. Many studies demonstrated that deregulation of epigenetics and genomic imprinting plays key roles in carcinogenesis. Accumulating evidence shows that Cr(VI) also causes epigenetic effects such as changing DNA methylation and histone posttranslational modifications. However, the mechanisms by which Cr(VI) triggers these epigenetic modifications remain largely unknown and whether these epigenetic modifications play a causal role in Cr(VI) carcinogenesis are not clear. Our preliminary studies found: (i) Chronic Cr(VI) exposure induces CSC-like property and cell malignant transformation; (ii) Chronic Cr(VI) exposure increases the levels of several HMTases, which play a causal role in Cr(VI)-induced CSC-like property and cell transformation; (iii) Higher levels of H3 repressive methylation marks and their related HMTases are also detected in lung cancer tissues of mice and humans exposed to chromium; (iv) Up-regulation of HMTases plays a crucial role in chronic Cr(VI) exposure- caused deregulation of Dlk1-Dio3 genomic imprinting cluster; (v) miR-494 down-regulation plays a causal role in Cr(VI)-induced cell transformation; and (vi) Oxidative stress increases the levels of HMTases. Based on literature and our preliminary data, we hypothesize: (i) Cr(VI) exposure generates oxidative stress, which increases the levels of HMTases; (ii) Up-regulation of HMTases deregulates the Dlk1-Dio3 genomic imprinting cluster, which decreases the level of miR-494; and (iii) Down-regulation of miR-494 increases the levels of its targets and key CSC factors c-Myc and BMI1, which produce CSC-like property promoting Cr(VI)-induced cell transformation and tumorigenesis. This proposed study not only fills the knowledge gap of epigenetics and Cr(VI) carcinogenesis, but also provides novel mechanistic insights for the crucial role of oxidative stress in Cr(VI) carcinogenesis. Three aims are proposed: Aim 1: Induction of CSC-like property by down-regulating miR-494 of Dlk1-Dio3 imprinting cluster and its role in Cr(VI)-induced cell transformation and tumorigenesis. Aim 2: Down-regulation of miR-494 by chronic Cr(VI) exposure-induced HMTases through deregulating Dlk1-Dio3 genomic imprinting cluster. Aim 3: Up-regulation of HMTases by chronic Cr(VI) exposure through oxidative stress.