5-Aza-2′-deoxycytidine induces retinoic acid receptor-β2 demethylation and growth inhibition in esophageal squamous carcinoma cells

Esophageal squamous cell carcinoma (ESCC) is one of the most common tumors in human. Previous studies showed that multiple genetic and epigenetic alterations involved in carcinogenesis of esophagus, whereas the molecular mechanisms are poorly understood. So far, more and more ESCC-related genes have been found and retinoic acid receptor beta(2) (RAR beta(2)) is such a gene which was recognized as a putative tumor suppressor gene since reduced RAR beta(2) mRNA expression has been observed in several solid tumors, including ESCC. A growing evidence indicated that RAR beta(2) was required for the growth inhibitory effect of retinoic acid (RA). However, the molecular mechanism of its inactivation remained obscure in ESCC. The RAR beta 2 methylation status was assessed by methylation-specific PCR (MSP) in 12 ESCC cell lines and compared with their mRNA and protein expression level. Bisulfite sequencing of RAR beta(2) promoter region was performed to confirm the MSP results. After 5-aza-2'-deoxycytidine (5-aza-dc) treatment the expression of RAR beta(2) was reversed in two RAR beta(2)-downregulated cell lines. Therefore, hypermethylation of the promoter regions of RAR beta 2 gene is a major mechanism of transcriptional inactivation and might be involved in tumor development of esophagus in some ESCC cell lines suggesting that multiple mechanisms contribute to the loss of RAR beta(2) expression in ESCC cell lines. Furthermore, the methylation status of RAR beta(2) promoter region and its expression was analyzed in 51 ESCC tissue samples with their adjacent normal epithelia and two normal esophageal epithelia. The results showed that there was a statistically significant correlation between methylation status of RAR beta(2) and tumor grade; Moreover, a relationship between methylation status and decreased RAR beta(2) expression was found only in G(2) stage tumors. After 5-aza-dc treatment, RAR beta(2) restoration was accompanied by growth inhibition and this might be one of the mechanisms but not the only mechanism for the tumor cell growth inhibition by 5-aza-dc. This study may have clinical applications for ESCC therapy and prevention. (c) 2005 Elsevier Ireland Ltd. All rights reserved.