The histone deacetylase inhibitor trichostatin A reduces nickel-induced gene silencing in yeast and mammalian cells

We have previously reported that nickel (Ni)-silenced expression of the URA 3 gene in yeast (Saccharomyces cerevisiae) and gpt transgene in G12 Chinese hamster cells. In both cases, close proximity to a heterochromatic region was required for gene silencing. Yeast exposed to Ni exhibited reduced acet ylation of the lysine residues in the N-terminal tail of histone H4. Ni-ind uced silencing of the gpt gene in mammalian cells involved hypermethylation of promoter region DNA. Yeast do not employ DNA methylation to silence gen e expression. To determine if histone deacetylation participates in Ni-indu ced silencing of the URA3 and gpt genes, we exposed yeast and G12 hamster c ells to the histone deacetylase inhibitor trichostatin A (TSA) prior to and concurrently with Ni. Treatment of yeast cells with 0.2-0.6 mM NiCl2 resul ted in reduced expression of the URA3 gene as assessed by increased resista nce to I g/l 5-fluorotic acid (5-FOA). This effect was lessened when yeast were pre-treated with 50 mug TSA/ml. Similarly, treatment of G12 cells with 5 ng/ml TSA during and after exposure to 0.3 mug Ni3S2/cm(2) reduced silen cing of the gpt gene as gauged by resistance to 10 mug/ml 6-thioguanine (6- TG). The ability of TSA alone and in combination with the DNA-demethylating agent (5-AzaC) to reactivate the gpt gene in Ni-silenced variants was also assessed. Although treatment with 100 ng/ml TSA for 48 h was partially eff ective in reactivating the gpt gene, treatment with 5 muM 5-AzaC was more e fficacious. The greatest gpt gene reversion frequencies were observed follo wing a sequential 5-AzaC/TSA treatment. Taken all together, our data from m ammalian cells suggests that both DNA methylation and histone deacetylation participate in Ni-induced silencing of the gpt gene with DNA hypermethylat ion playing the more dominant role in maintaining the silenced state. (C) 2 001 Elsevier Science B.V. All rights reserved.