MicroRNA-148a can regulate runt-related transcription factor 3 gene expression via modulation of DNA methyltransferase 1 in gastric cancer

Underexpression of the gene runt-related transcription factor 3 (RUNX3), an important tumor suppressor, is known to contribute to gastric cancer progression. However, the mechanism underlying aberrant RUNX3 expression has not been fully elucidated. We investigated the role of microRNA-148a (miR-148a) and DNA methyltransferases (DNMTs) in RUNX3 promoter methylation and gene expression. RUNX3 mRNA, RUNX3 protein, and methylation levels were assayed in human gastric cancer tissues and matched normal tissues, and AGS and BGC-823 cells by real-time reverse transcription PCR, Western blot, and methylation-specific PCR, respectively. A correlation between RUNX3 mRNA levels and that of miR-148a was also investigated in gastric cancer tissues. We found that RUNX3 mRNA levels were significantly downregulated in gastric cancer tissues compared with their matched normal tissues, and were closely associated with miR-148a expression. After treatment of human gastric cancer AGS and BGC-823 cells with the DNA methylation inhibitor 5-aza-2'-deoxycytidine, a significant increase in RUNX3 mRNA, RUNX3 protein, and the non-methylated form of the RUNX3 promoter were observed relative to untreated cells. Enforced expression of miR-148a, which can modulate DNMT1 and DNMT3B, also increased the expression of RUNX3 in gastric cancer cells. Knockdown of DNMT1 was associated with increased levels of RUNX3 mRNA and RUNX3 protein, while knockdown of DNMT3B did not have any effect on these in BGC-823 cells. Our results show that miR-148a may regulate RUNX3 expression through modulation of DNMT1-dependent DNA methylation in gastric cancer and highlight a miRNA-epigenetics regulation mechanism of gene expression.