Reactive oxygen species-induced global DNA hypomethylation in bladder cancer is mediated via increased formation of 8-hydroxy-2'-deoxyguanosine

Abstract

Oxidative stress, as a consequence of the elevated reactive oxygen species (ROS), has been implicated in various chronic diseases including cancers by, for examples, causing oxidative DNA damage and altering epigenetic regulation. Long interspersed element-1 (LINE-1) is an only active retrotransposable elements in human DNA, comprising up to 17% of the whole genome. LINE-1 hypomethylation is reported in bladder cancer and evidently it is a consequence of oxidative stress. In this study, we investigated the expression of 5-methylcytosine (5-mC), 8-hydroxydeoxyguanosine (8-OHdG), LINE-1-endoced protein (ORF1p), histone H3K9me3 and HP1 alpha in bladder cancer tissues. Mechanistic insight into how oxidative stress affected DNA methylation and histone modifications that further caused aberrant expression of LINE-1 elements in bladder cancer cells was also explored. Elevated expression of 8-OHdG, decreased 5-mC expression, upregulation of ORF1p expression, and increase in histone H3K9me3 expression were observed in bladder cancer tissues, indicating increases in oxidative stress and epigenetic alterations. The in vitro study in bladder cancer cell lines revealed that LINE-1 hypomethylation was occurred through the oxidative DNA damage on LINE-1 promoter. Based on ChIP experiments, oxidative stress disturbed histone modification patterns, especially on LINE-1 promoter. Enrichments of histone H3K4me3 and H3K18Ac at LINE-1 promoter were increased in ROS-treated cells compared to the untreated control. In addition, ROS-treated bladder cancer cells had migration and invasion capability greater than the untreated control cells. In conclusion, elevated formation of oxidative DNA lesions, overexpression of ORF1p, and alteration of chromatin modification were shown in human bladder cancer tissues. Experimentally, oxidative stress induced by ROS caused hypomethylation of LINE-1 and enrichment of active chromatin marks at LINE-1 promoter. These, at least in part, led to increased expression of LINE-1 ORF1p protein. The current findings suggest that LINE-1 reactivation by oxidative stress promotes bladder cancer progression. Therefore, downregulation of LINE-1 expression and attenuation of oxidative stress could be potential targets for bladder cancer therapy in the future.