Dysregulation of microRNA-34a expression causes drug-resistance to 5-FU in human colon cancer DLD-1 cells

MiR-34a was identified as one of the down-regulated micro-RNAs (miRs) in human colorectal cancer 5-fluorouracil (5-FU)-resistant DLD-1 cells compared with those in the parental DLD-1 cells. Exposure to 5-FU at 30 μM activated phosphoinositide 3-kinase (PI3K)/Akt signaling markedly from 12 h up to 48 h in the 5-FU-resistant cells compared with that in the parental cells and resulted in an overt difference in growth at those times. Furthermore, the expression of miR-34a in the 5-FU-resistant cells was sustained at a low-level, whereas it was up-regulated in the parental cells after the 5-FU treatment. Sirt1, which is one of the target genes for miR-34a and related to drug-resistance, was strikingly up-regulated in the 5-FU-resistant cells. The ectopic expression of miR-34a in the 5-FU-resistant cells inhibited growth, as in the parental cells, and attenuated the resistance to 5-FU through the down-regulation of Sirt1 and E2F3. Moreover, the silencing of Sirt1 significantly canceled the resistance to 5-FU in the 5-FU-resistant cells. These findings suggest that miR-34a targeting the Sirt1 and E2F3 genes could negatively regulate, at least in part, the resistance to 5-FU in human colorectal cancer DLD-1 cells.