Epithelial-mesenchymal transition (EMT) and changes in the expression of the microRNA-200 (miR-200) family were examined in the human colorectal cancer (CRC) cell line SW620 with acquired oxaliplatin (L-OHP) resistance. Two CRC cell lines, SW480, derived from primary CRC, and SW620, derived from lymph node metastasis, which were obtained from the same patient, were used in the present study. L-OHP-resistant SW620 cells were obtained by exposure to L-OHP for 155 d. The concentration of L-OHP was increased to 80 µM in a stepwise manner. The IC50 value of L-OHP was increased 16-fold in L-OHP-resistant SW620 cells, which also displayed mesenchymal cell-like characteristics, such as the down-regulation of E-cadherin and up-regulation of vimentin. However, L-OHP-resistant SW480 cells were not obtained when the concentration of L-OHP was increased in a similar stepwise manner. The expression levels of members of the miR-200 family (miR-200a, miR-200b, miR-429, miR-200c, and miR-141) were significantly higher in SW480 cells than in SW620 cells. The expression levels of miR-200c and miR-141 were significantly lower in L-OHP-resistant SW620 cells than in control SW620 cells. L-OHP-resistant SW620 cells did not exhibit cross-resistance to other anti-cancer drugs used to treat CRC, such as 5-fluorouracil, irinotecan, and the active metabolite of irinotecan (SN-38). These results suggest that the down-regulated expression of miR-200c and miR-141 plays a role in selective resistance to L-OHP and EMT in CRC cells during repeated treatments with L-OHP.

The effects of decitabine (DAC), a DNA methyltransferase (DNMT) inhibitor, on metastasis and exosomal expression of microRNAs were examined in SW620/OxR cells, a human colorectal cancer (CRC) cell line (SW620) with acquired resistance to oxaliplatin. This cell line shows an invasive phenotype by epithelial-mesenchymal transition. Two CRC cell lines, SW480, derived from primary CRC, and SW620, derived from lymph node metastasis, which were obtained from the same patient, as well as SW620/OxR, were also used in the present study. Cytarabine (Ara-C), a non-DNMT-inhibiting cytidine analog, was used as negative control of DAC. No significant difference was observed in the invasion abilities of SW480 cells treated with DAC or Ara-C. On the other hand, invasion ability was suppressed by treatment with DAC in SW620 and SW620/OxR cells. Up-regulated expression of E-cadherin, microRNA-200c (miR-200c), and miR-141 following DAC treatment indicated the acquisition of epithelial cell-like characteristics in SW620 and SW620/OxR cells. Exosomal expression levels of miR-200c and miR-141 were also up-regulated by DAC treatment in SW620 and SW620/OxR but not in SW480 cells. This increase in exosomal miRNA expression negatively correlated with invasion ability. These results suggest that DNA demethylation treatment caused acquisition of epithelial cell-like characteristics in SW620 and SW620/OxR cells. Furthermore, the observed increased exosomal expression of miR-200c and miR-141 may be an indicator or biomarker candidate for mesenchymal-epithelial transition of CRC cells.

DNA hypermethylation, an epigenetic change that silences gene expression without altering nucleotide sequences, plays a critical role in the formation and progression of colorectal cancers as well as in the acquisition of drug resistance. Decitabine (DAC), a DNA methyltransferase 1 inhibitor of nucleoside analogues, has been shown to restore gene expression silenced by hypermethylation. In the present study, the mechanisms underlying both uridine and DAC uptake were examined in the human colon cancer cell line HCT116. Real-time polymerase chain reaction analysis revealed that ENT1 mRNA was the most abundant among the nucleoside transporters examined in HCT116 cells. The ENT1 protein was detected in the membrane fraction, as determined by Western blotting. The uptake of uridine or DAC was time-and concentration-dependent, but also Na -independent. The uptake of these agents was inhibited by S-(4-nitrobenzyl)-6-thioinosine (NBMPR), an inhibitor of equilibrative nucleoside transporters (ENTs), and was also decreased in cells treated with ENT1 small interfering RNA. The uptake of both uridine and DAC was inhibited by uridine, cytidine, adenosine, or inosine, while that of DAC was also inhibited by thymidine. The expression of MAGEA1 mRNA, the DNA of which was methylated in HCT116 cells, was increased by DAC treatment, and this increment was attenuated by concomitant treatment with NBMPR. The IC 50 value of DAC was also increased in the presence of NBMPR. These results suggest that DAC is mainly taken up by ENT1 and that this uptake is one of the key determinants of the activity of DAC in HCT116 cells. Key words equilibrative nucleoside transporter; decitabine; uridineColorectal cancer is the second most common cancer in females and third most common cancer in males worldwide including Japan.1,2) Approximately 600000 people die from this cancer in the world each year. The majority of colon cancers arise sporadically, and the deficient expression of DNA repair genes has frequently been observed in these sporadic colorectal cancers. Previous studies reported that this deficient expression was often due to epigenetic alterations that did not include changes in the DNA sequence.3,4) DNA hypermethylation is one of the epigenetic alterations in colorectal cancers that occur in the CG-rich regions of the promoter region of protein-coding genes. These changes cause the transcriptional silencing of cancer-related genes such as tumor suppressor genes.3) DNA hypermethylation also plays a critical role in the acquisition of drug resistance by colorectal cancers. For example, promoter methylation of the DNA mismatch repair gene, human MLH1, has been associated with the loss of DNA mismatch repair and resistance to 5-FU in colon cancer cell lines or to cisplatin, carboplatin, temozolomide, or epirubicin in human tumor xenografts. 5,6) Decitabine (5-aza-2′-deoxycytidine; DAC) is a DNA methyltransferase 1 inhibitor that restores gene expression silenced as a result of DNA hypermethylation. 7,8) Several studies have demonstrated that...

The aim of the present study was to determine the effects of long-term exposure of decitabine (DAC) to HCT116 colorectal cancer (CRC) cells on the acquisition of resistance to DAC as well as cross-resistance to anticancer drugs used for CRC or other epigenetic modifiers. In the present study, DAC-resistant HCT116 CRC cells were established through long-term treatment with increasing concentrations of DAC (10 to 540 nM); and the cross-resistance to other drugs was subsequently examined. DAC-resistant HCT116 cells were obtained following a 104-day treatment with DAC, including DAC-free intervals. The results demonstrated that the IC50 value of DAC was increased ~100-fold in DAC-resistant HCT116 cells. Messenger (m)RNA expression of secreted frizzed-related protein 1 (SFRP1), which is regulated by DNA methylation, was not detected in DAC-resistant cells; however, SFRP1 mRNA was present in HCT116 cells treated with DAC for 52 days. DNA methyltransferase 1 (DNMT1) protein levels were slightly decreased until day 81 and then returned to control levels in DAC-resistant cells. Further experiments using DAC-resistant HCT116 cells revealed that these cells exhibited cross-resistance to gemcitabine (Gem); however, cross-resistance was not observed for other DNMT inhibitors (azacitidine and zebularine), histone deacetylase inhibitors (trichostatin A, vorinostat and valproic acid) or anticancer drugs for CRC (5-fluorouracil, irinotecan and oxaliplatin). Furthermore, the protein expression levels of cytidine deaminase (CDA) were increased, while those of deoxycytidine kinase (dCK) were decreased in DAC-resistant HCT116 cells; by contrast, the mRNA expression levels for these proteins were not significantly altered. In conclusion, the results of the present study indicated that the long-term treatment of HCT116 cells with DAC led to the acquisition of resistance to both DAC and Gem. In addition, these results may be partly attributed to changes in CDA and/or dCK, which are involved in metabolic pathways common to these two drugs.

The effects of zebularine, a DNA methyltransferase inhibitor, on the invasion activity as well as intracellular expression level of let-7b, tumor suppressor microRNA, were examined in three human colorectal cancer (CRC) cell lines: SW480, SW620, and oxaliplatin-resistant SW620 (SW620/OxR). Zebularine suppressed the invasion activity of SW620 and SW620/OxR cells. The intracellular expression level of let-7b was upregulated by zebularine in SW620 and SW620/OxR cells. The overexpression of let-7b by the transfection of let-7b mimic suppressed invasion activity in SW620 and SW620/OxR cells. These results suggest that zebularine may inhibit invasion activity by up-regulating the intracellular expression level of let-7b in high-invasive CRC cells.Key words invasion; zebularine; let-7b; oxaliplatin-resistant; colorectal cancer DNA methylation is a major epigenetic mechanism of gene expression level. Zebularine (Zeb) has been shown to inhibit DNA methyltransferase (DNMT). Furthermore, Zeb and other DNMT inhibitors, such as decitabine (DAC) and azacitidine (AC), exert differential effects on gene expression levels.1) Flotho et al. reported that Zeb did not regulate the expression levels of a large number of mRNAs or markers for genome-wide methylation.1) We previously showed that DAC suppressed invasion activity with the up-regulation of epithelial cell marker expression levels not only in parental SW620 cells, high-invasive colorectal cancer (CRC) cells, but also in oxaliplatin (L-OHP)-resistant SW620 (SW620/OxR) cells. 2)Pakneshan et al. demonstrated that AC enhanced invasion activity by up-regulating the urokinase plasminogen activator in prostate cancer.3) However, information on the effects of Zeb on invasion activity is limited.MicroRNAs (miRNAs), small non-coding RNAs that suppress target mRNA, play important roles in cancer metastasis. Members of the let-7 family (let-7a, let-7b, let-7c, let-7d, let-7e, let-7f, let-7g, and let-7i in addition to miR-98 and miR-202) are tumor suppressor miRNAs. 4) Kang et al. previously reported that let-7b suppressed invasion activity in gastric cancer.5) In addition, Kahlert et al. showed that let-7b expression level in CRC tumor invasion front was lower than that in CRC tumor center.6) The expression levels of let-7 family members are also regulated by DNA methylation. The location of let-7b is in the 3′-untranslated region of MIRLET7BHG as a long intergenic noncoding RNA. 7) Nishi et al. found that intracellular expression level of let-7b may be regulated by the DNA methylation status of CpG islands near transcription start site (TSS) of MIRLET7BHG.7) Kobayashi et al. showed that let-7 family members were more abundant in exosomes, small membrane vesicles of approximately 100 nm in diameter, from high-invasive ovarian cancer cells than in those from low-invasive ovarian cancer cells.8) In addition, Wong et al. reported that the expression levels of let-7 family members were upregulated by Zeb in nasopharyngeal carcinoma cells. We previously demonstrated that the invasion activ...