Department of Toxicology, School of Public Health, Guangzhou Medical University, Guangzhou 510182, People's Republic of China.
Toxicol Sci. 2012 Feb;125(2):412-7. doi: 10.1093/toxsci/kfr320. Epub 2011 Nov 23.
Cadmium (Cd) and its compounds are well-known human carcinogens, but the mechanisms underlying the carcinogenesis are not entirely understood yet. Aberrant methylation was investigated in order to obtain insight into the DNA repair-related epigenetic mechanisms underlying CdCl(2)-induced malignant transformation of human bronchial epithelial cells (16HBE). Gene expression and DNA methylation were assessed in untreated control cells; 5th, 15th, and 35th passage of CdCl2-treated cells and tumorigenic cells (TCs) from nude mice by using high-performance liquid chromatography, real-time PCR, Western blot analysis, and methylation-specific PCR assay. During Cd-induced malignant transformation, global DNA methylation progressively increased and was associated with the overexpression of the DNA methyltransferase genes DNMT1 and DNMT3a but not DNMT3b. Expression of both the messenger RNA and proteins of the DNA repair genes (hMSH2, ERCC1, XRCC1, and hOGG1) progressively reduced and DNA damage increased with Cd-induced transformation. The promoter regions of hMSH2, ERCC1, XRCC1, and hOGG1 were heavily methylated in the 35th passage transformed cells and the TCs. The DNA demethylating agent 5-aza-2'-deoxycytidine could reverse the Cd-induced global DNA hypermethylation, DNMT hyperactivity, and the silencing of hMSH2, ERCC1, XRCC1, and hOGG1 in a time-dependent manner. The results indicate that DNMT1 and DNMT3a overexpression can result in global DNA hypermethylation and silencing of the hMSH2, ERCC1, XRCC1, and hOGG1 genes. They may partly explain the epigenetic mechanisms underlying the carcinogenesis due to Cd.
镉（Cd）及其化合物是众所周知的人类致癌物，但致癌的机制尚未完全了解。为了深入了解 CdCl2 诱导人支气管上皮细胞（16HBE）恶性转化相关的 DNA 修复的表观遗传机制，研究了异常甲基化。通过使用高效液相色谱、实时 PCR、Western blot 分析和甲基化特异性 PCR 分析，在未经处理的对照细胞中；CdCl2 处理的细胞的第 5、15 和 35 代以及裸鼠中的肿瘤细胞（TCs）中评估基因表达和 DNA 甲基化。在 Cd 诱导的恶性转化过程中，全基因组 DNA 甲基化逐渐增加，并与 DNA 甲基转移酶基因 DNMT1 和 DNMT3a 的过度表达相关，但与 DNMT3b 无关。DNA 修复基因（hMSH2、ERCC1、XRCC1 和 hOGG1）的信使 RNA 和蛋白表达逐渐减少，随着 Cd 诱导的转化，DNA 损伤增加。在 35 代转化细胞和 TCs 中，hMSH2、ERCC1、XRCC1 和 hOGG1 的启动子区域高度甲基化。DNA 去甲基化剂 5-氮杂-2'-脱氧胞苷可以在时间依赖性方式逆转 Cd 诱导的全基因组 DNA 过度甲基化、DNMT 活性增强以及 hMSH2、ERCC1、XRCC1 和 hOGG1 的沉默。结果表明，DNMT1 和 DNMT3a 的过度表达可导致全基因组 DNA 过度甲基化和 hMSH2、ERCC1、XRCC1 和 hOGG1 基因的沉默。它们可能部分解释了 Cd 致癌的表观遗传机制。
