Nickel induced transcriptional changes persist post exposure through epigenetic reprograming

Nickel is an occupational and environmental toxicant associated with a number of diseases in humans including pulmonary fibrosis, bronchitis and lung and nasal cancers. Our earlier studies showed that the nickel-exposure-induced genome-wide transcriptional changes, which persist even after the termination of exposure may underlie nickel pathogenesis. However, the mechanisms that drive nickel-induced persistent changes to the transcriptome remain elusive. To elucidate the mechanisms that underlie nickel induced long-term transcriptional changes, in this study, we examined the transcriptome and the epigenome of human lung epithelial cells during nickel exposure and after the termination of exposure. We identified two categories of persistently differentially expressed genes based on the timing of expression changes: i) the genes that were differentially expressed during nickel exposure; and ii) the genes that were differentially expressed only after the termination of nickel exposure. Interestingly, the majority of nickel-induced transcriptional changes occurred only after the termination of exposure. We found robust genome-wide alterations to the activating histone modification, H3K4me3, after the termination of nickel exposure, which coincided with the post-exposure gene expression changes. In addition, we found significant post-exposure alterations to the repressive histone modification, H3K27me3. By uncovering a new category of transcriptional and epigenetic changes, which occur only after the termination of exposure, this study sheds new light on the post-exposure effects of nickel and provides a novel understanding of the long-term deleterious consequences of nickel exposure on human health.