Inactivation of retinoic acid receptor β by promoter CpG hypermethylation in gastric cancer

Inactivation of nuclear retinoic acid receptor beta (RAR beta) expression is implicated in tumorigenesis. We hypothesized that loss of RAR beta in gastric cancer cells may occur as a result of multiple factors, including epigenetic modifications which alter RAR beta promoter chromatin structure. We examined hypermethylation of CpG islands present in the RAR beta promoter by methylation-specific PCR and the expression of RAR beta in gastric cancer cell lines and tissues. Three (MKN-28, -45 and -74) out of eight gastric cancer cell lines had a loss of RAR expression associated with promoter methylation. RAR beta expression was retrieved in these cell lines by treatment with 5-azacytidine or by the histone deacetylase inhibitor trichostatin A. Promoter hypermethylation was detected in 64% (7/11) of gastric carcinoma tissues with reduced expression of RAR beta, whereas it was detected in 22% (2/9) of tumors with retained RAR beta expression. To investigate the functions of exogenous RAR beta in gastric cancer cells, we transfected a retroviral RAR beta expression vector (LNS beta) into MKN-28 cells that have hypermethylation of the RAR beta promoter. Overexpression of RAR in MKN-28 cells appeared to regulate the expression of DNA methyltransferase and DNA demethylase and the acetylation of hitone H4. These results suggest that the transcriptional inactivation of the RAR beta by promoter CpG hypermethylation is frequently associated with gastric carcinoma. Our data also suggests that DNA methylation plays a pivotal role in establishing and maintaining an inactive state of RAR beta by rendering the chromatin structure inaccessible to the transcription machinery.