Heterochromatinization as a Potential Mechanism of Nickel-Induced Carcinogenesis

Abstract

Epigenetics refers to heritable patterns of gene expression that do not depend on alterations of the genomic DNA sequence. Nickel compounds have demonstrated carcinogenicity without any associated mutagenesis, suggesting that its mechanism of carcinogenesis is epigenetic in nature. One such potential mechanism is the heterochromatinization of chromatin within a region of the genome containing a gene sequence, inhibiting any further molecular interactions with that underlying gene sequence and effectively inactivating that gene. We report here the observation, by atomic force microscopy and circular dichroism spectropolarimetry, that nickel ion (Ni 2+ ) condenses chromatin to a greater extent than the natural divalent cation of the cell, magnesium ion (Mg 2+ ). In addition, we use a model experimental system that incorporates a transgene, the bacterial xanthine guanine phosphoribosyl transferase gene (gpt) differentially near to, and far away from, a heterochromatic region of the genome, in two cell lines, the Chinese hamster V79-derived G12 and G10 cells, respectively, to demonstrate by DNase I protection assay that nickel treatement protects the gpt gene sequence from DNase I exonuclease digestion in the G12 cells, but not in the G10 cells. We conclude that condensation of chromatin by nickel is a potential mechanism of nickel-mediated gene regulation. Keywords chromatin; heterochromatin; nickel; divalent cation; carcinogenesisChromatin is the natural state of protein-associated DNA in all eukaryotes. It is a dynamic polymer of subunits called nucleosomes. The nucleosome subunit consists of approximately 146 bps of DNA wrapped around an octamer of histone proteins. The histone octamer is made up of two each of histone H2A, H2B, H3, and H4. The DNA makes one and a half turns around the octamer, with the entry and exit points of the DNA at opposite sides of the disk-shaped histone octameric complex. This complex of 146 bps and the octamer of histones is known as the nucleosome core particle. In chromatin, the exiting and entering DNA duplex strands of this nucleosome core particle extend to the next nucleosome core particles on either side of it. The intervening DNA connecting each pair of nucleosomes is termed the linker DNA. The lengths of these intervening DNA duplex strands vary from species to species and from tissue types to tissues types, as well as from one stretch between two nucleosomes to another. Linker DNA length ranges from a minimum of 25 bps to upwards of 100 bps, averaging approximately 50 to 60 bps (1). A fifth histone type, the linker histone (H1), binds to the linker DNA in a stoichiometry of 1 H1 to 1 nucleosome. The complex of the core particle (the histone octamer

Epigenetic dysregulation by nickel through repressive chromatin domain disruption

“…These results suggest a loss of CTCF-mediated insulation function as a potential reason for domain disruption and spreading. Because epigenetic alterations are believed to be the basis of Niinduced cancers (14,16), our finding of H3K9me2 domain disruption by Ni and a potential role for CTCF in maintaining H3K9me2 domains will have profound implications in understanding the process of carcinogenesis.…”

“…Intriguingly, Ni-induced silencing of gpt and increased H3K9me2 levels occurred only when the transgene was integrated near a heterochromatic region (13,14,21). This suggests that Ni could induce H3K9me2 spreading.…”

“…However, how H3K9me2 domains are maintained and the consequences of aberrant domain disruption are poorly understood. An important clue toward understanding the H3K9me2 domains comes from studies that investigated silencing of a transgene following the exposure of cells to nickel (13,14). Nickel compounds are environmental carcinogens that cause a multitude of human health risks including allergic dermatitis, bronchitis, pulmonary fibrosis, pulmonary edema, diseases of the kidney and cardiovascular system, and lung and nasal cancers (15,16).…”

“…In the transgenic gpt+ Chinese hamster V79-derived G12 cells, silencing of the bacterial xanthine-guanine phosphoribosyltransferase (gpt) transgene following Ni exposure is associated with an increase in H3K9me2 and a decrease in H3 and H4 acetylation at the promoter and coding regions (13,14,21). Intriguingly, Ni-induced silencing of gpt and increased H3K9me2 levels occurred only when the transgene was integrated near a heterochromatic region (13,14,21).…”

In vitro profiling of epigenetic modifications underlying heavy metal toxicity of tungsten-alloy and its components

“…Recent studies have reported histone hypoacetylation, increased methylation/ubiquitination of histones Ke et al, 2006;Li et al, 2009) and changes in the chromatin stationary structure (Ellen et al, 2009) in response to nickel and cobalt exposure. These studies also pointed to hypoxia and ROSs generation as causes of decreased acetyl-CoA, a substrate for histone acetylation due to heavy metal exposure .…”