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To determine a possible function of histone modifications in stomach carcinogenesis, we analyzed global and MGMT-promoter levels of di-methyl-H3-K9, di-methyl-H3-K4 and acetyl-H3-K9, as well as MGMT DNA methylation and mRNA expression following treatment with 5-aza-2′ -deoxycytidine and/or Trichostatin A. We found that histone H3-K9 di-methylation, H3-K4 di-methylation, H3-K9 acetylation and DNA methylation work in combination to silence MGMT. The results indicate that histone modifications as well as DNA methylation may be involved in stomach carcinogenesis. In addition to its effect on DNA methylation, 5-aza-2′ -deoxycytidine can act at histone modification level to reactivate MGMT expression in a region-specific and DNA methylation-dependent manner. [ABSTRACT FROM AUTHOR]

In the course of gastric cancer development, gene silencing by DNA hypermethylation is an important mechanism. While DNA methylation often co-exists with histone modifications to regulate gene expression, the function of histone modifications in gene silencing in gastric cancer has not been evaluated in detail. p16, a well-known tumor suppressor gene, is frequently silenced in DNA hypermethylation manner in gastric cancer. Accordingly, we chose p16 to clarify whether there is a correlation among histone H3 lysine 9 (H3-K9) di-methylation, H3-K9 acetylation, DNA methylation and p16 expression in human gastric cancer. Three gastric cancer cells, MKN-45, SGC-7901 and BGC-823, were treated with 5-aza-2'-deoxycytidine (5-Aza-dC) and/or trichostatin A (TSA). We investigated p16 promoter DNA methylation status, p16 mRNA levels, regional and global levels of di-methyl-H3-K9 and acetyl-H3-K9 in four groups: i) 5-Aza-dC, ii) TSA, iii) the combination of 5-Aza-dC and TSA and iv) control group with no treatments. p16 silencing is characterized by DNA hypermethylation, H3-K9 hypoacetylation and H3-K9 hypermethylation at the promoter region. Treatment with TSA, increased H3-K9 acetylation at the hypermethylated promoter, but did not affect H3-K9 di-methylation or p16 expression. By contrast, treatment with 5-Aza-dC, reduced H3-K9 di-methylation, increased H3-K9 acetylation at the hypermethylated promoter and reactivated the expression of p16. Combined treatment restored the expression of p16 synergistically. In addition, 5-Aza-dC and the combined treatment did not result in global alteration of H3-K9 di-methylation. These results suggest that H3-K9 di-methylation, H3-K9 acetylation and DNA methylation work in combination to silence p16 in gastric cancer. The decreased H3-K9 di-methylation correlates with DNA demethylation and reactivation of p16. H3-K9 di-methylation as well as DNA methylation related to p16 silencing is limited to the promoter region. In addition to its effect on DNA methylation, 5-Aza-dC can act at histone modification levels to reactivate p16 expression in region-specific and DNA methylation-dependent manner.

Objective: To investigate the effects of 5-Aza-2'-deoxycytidine (5-Aza-dC) and trichostatin A (TSA) on DNA methylation and expression of P16, hMLH1 and MGMT genes in the human gastric cancer cell line MGC-803, and to explore the mechanism of P16, hMLH1 and MGMT gene silencing in human gastric cancer cells., Methods: MGC-803 cells were cultured in RPMI-1640 medium and were treated with 5-Aza-dC or TSA. Methylation-specific polymerase chain reaction (MS-PCR) was used to detect the promoter methylation status of P16, hMLH1 and MGMT genes. RT-PCR was used to detect the mRNA expressions of P16, hMLH1 and MGMT., Results: Promoter hypermethylation of P16, hMLH1 and MGMT genes were detected in MGC-803 cells, and mRNA expressions of P16, hMLH1 and MGMT were absent before treatment. After treatment with 5-Aza-dC, the promoter region of the P16, hMLH1 and MGMT gene exhibited a demethylation status, and their mRNA expressions were increased. The treatment with TSA had no effects on DNA demethylation or restoration of P16 or hMLH1 expression. P16, hMLH1 and MGMT mRNA relative expression levels after treatment with a combination of 5-Aza-dC and TSA were 0.412+/-0.030, 0.397+/-0.024 and 0.553+/-0.043 respectively, which were higher than those after 5-Aza-dC treatment alone (0.221+/-0.022, 0.214+/-0.018 and 0.156+/-0.017, all P<0.05)., Conclusions: Promoter hypermethylation is a major mechanism of P16, hMLH1 and MGMT gene silencing in human gastric cancer cells. Treatment with 5-Aza-dC alone or the combination of 5-Aza-dC and TSA can reactivate the expressions of these genes.

Objective: To identify the association of his tone H3 lysine 9 (H3-K9) methylation with DNA methylation and the expression of the mismatch repair gene hMLH1 in human gastric cancer cells., Methods: Gastric cancer cells of the lines BGC-823 and MGC-803 were cultured and treated with 5-Aza-2'-deoxycytidine (5-Aza-dC), a demethylation agent, for 72 hour. Chromatin immunoprecipitation (ChIP) assay was used to assess the status of histone H3 lysine 9 methylation in the promoter regions of hMLH1 gene. Methylation-specific PCR (MSP) was used to evaluate the effect of 5-Aza-dC on DNA methylation status. RT-PCR was used to examine the hMLH1 gene expression., Results: In the MGC-803 cells, silenced hMLH1 gene was characterized by DNA methylation and histone H3-K9 hypermethylation; 5-Aza-dC demethylated the DNA and reduced the histone H3-K9 methylation at silenced loci and resulted in reactivation of hMLH1 gene therein. Contrary to the MGC-803 cells, BGC-823 cells expressed hMLH1 gene with DNA demethylation and histone H3-K9 hypomethylation; and 5-Aza-dC had no effects on the gene expression, DNA methylation, and histone H3-K9 methylation therein., Conclusion: Hypermethylation of DNA in the promoter region is related to transcriptional silencing of hMLH1 gene. Histone H3-K9 methylation in different regions of the promoter studied correlates with DNA methylation status of hMLH1 gene in gastric cancer cells. Alteration of DNA methylation affects histone H3-K9 methylation. 5-Aza-dC can control hMLH1 expression, DNA methylation, and histone H3-K9 methylation in the promoter.