Alterations of histone modifications and transgene silencing by nickel chloride

Abstract

Although it has been well established that insoluble nickel compounds are potent carcinogens and soluble nickel compounds are less potent, the mechanisms remain unclear. Nickel compounds are weakly mutagenic, but cause epigenetic effects in cells. Previous studies have shown that insoluble nickel compounds enter cells by phagocytosis and silence gene expression, but the entry of soluble nickel compounds and their effects on gene silencing have not been well studied. Here, we have demonstrated, using a dye that fluoresces when nickel ions bind, that soluble nickel compounds were taken up by cells. Nickel ions localized initially in the cytoplasm, but later entered the nucleus and eventually silenced a transgene. In addition, we described three major changes in histone modification of cells exposed to soluble nickel compounds: (i) loss of acetylation of H2A, H2B, H3 and H4; (ii) increases of H3K9 dimethylation; and (iii) substantial increases of the ubiquitination of H2A and H2B. These effects were observed at nickel exposure conditions that had minimum effects on cell cytotoxicity. Moreover, we demonstrated that nickel-induced transgene silencing was associated with similar changes of histone modifications in their nuclesomes. This study is the first to show that nickel compounds increase histone ubiquitination in cells. These new findings will further our understanding of the epigenetic mechanisms of nickel-mediated carcinogenesis.

Nickel compounds induce histone ubiquitination by inhibiting histone deubiquitinating enzyme activity

“…Previous studies have shown that nickel exposure results in alteration of histone modifications in cells, and thereby disrupts epigenetic homeostasis. A loss of histone acetylation and an increased histone methylation on H3K9 were previously observed in nickel-exposed cells Ke et al, 2006;Ke, 2007). In this study, it has been demonstrated that nickel compounds, both soluble NiCl 2 and insoluble Ni 3 S 2 , increase histone ubiquitination of H2A and H2B in A549 cells in a dose-and time-dependent manner.…”

“…This nickel-induced transgene silencing did not involve mutation or deletion of any portion of the transgene (Lee et al, 1995). Instead, increased DNA methylation and chromatin condensation (Lee et al, 1995), as well as a broad loss of histone acetylation of all four core histones and an increase in histone H3K9 methylation were observed at the silenced transgene locus (Yan et al, 2003;Ke et al, 2006). This indicated that an epigenetic, rather than a mutagenic mechanism was involved in the nickel-induced transgene silencing.…”

“…Previous studies have shown that exposure of cells with nickel compounds [soluble nickel chloride (NiCl 2 ) and insoluble nickel subsulfide (Ni 3 S 2 )] results in intracellular accumulation of nickel ion, a loss of acetylation at all four core histones, and increased methylation at histone H3 lysine 9 (H3K9) at the global level Ke et al, 2006;Ke, 2007). Research on the mechanisms by which nickel compounds alter histone modifications has revealed that nickel exposure decreases histone acetylation by inhibiting histone acetyltransferase (HAT) activity, but has no effects on histone deacetylases (HDAC) (Kang et al, 2003).…”

Epigenetic dysregulation by nickel through repressive chromatin domain disruption

“…Ni-induced spreading of H3K9me2 is reminiscent of the local discrete changes, suggesting that misregulation of this regulatory switch results in altered gene expression. Furthermore, several earlier studies have suggested ectopic spreading of H3K9me2 to be an important signal for transcriptional silencing (27)(28)(29)(30)39). However, the alternate possibility of gene silencing resulting in spreading of H3K9me2 in a subset of the spreading domains cannot be ruled out.…”

“…that showed association of H3K9me2 spreading with decreased gene expression (27)(28)(29)(30). To examine this on a genome-wide scale, we stratified all of the genes into those in H3K9me2-spreading domains (3,930) and genes that are not in H3K9me2 in either control or Ni-exposed cells (8,096) ( Fig.…”