Extracellular Disposal of Tumor-Suppressor miRs-145 and -34a via Microvesicles and 5-FU Resistance of Human Colon Cancer Cells

Abstract

The dysregulation of microRNA (miRNA) expression causes various kinds of diseases. Especially, alterations in miRNA expression levels are frequently observed in human tumor cells and are associated with cancer pathogenesis. Earlier we established Fluorouracil (5-FU)-resistant human colon cancer DLD-1 cells (DLD-1/5FU) from parental 5-FU- sensitive DLD-1 cells. In the present study, we examined the expression of miRNA in each cell line and in its extracellular microvesicles (MVs) before and after treatment with 5-FU. The nascent RNAs of anti-oncogenic miR-34a and -145 labeled with EU in both cells were proved to be transferred into MVs in both cell lines. The levels of miR-34a and -145 in the cells and in their MVs were not largely different in the two cell lines, and a substantial amount of both miRNAs was secreted by both cell lines even in the steady-state condition. The exposure of both cell lines to 5-FU significantly increased the intracellular levels of miR-145 and miR-34a in the 5-FU-sensitive DLD-1 cells, whereas the level of neither miR was elevated in the DLD-1/5FU cells. Interestingly, the amount of miR-145 detected in the small MVs shed into the medium of the parental cells was reduced after the treatment with 5-FU. On the other hand, the intracellular expression of miR-34a in the DLD-1/5FU cells was down-regulated compared with that in the parental DLD-1 cells even in the steady-state condition. As to the miR-34a secreted into MVs, the increase in the level in DLD-1/5FU cells was greater than that in the parental DLD-1 cells after the treatment with 5-FU. Thus, the intra- and extracellular miR-145 and -34a were closely associated with 5-FU resistance, and the resistance was in part due to the enhanced secretion of miR-145 and -34a via MVs, resulting in low intracellular levels of both miRNAs.

The tumor-suppressive and potential therapeutic functions of miR-34a in epithelial carcinomas

“…Interestingly, CDF has previously been shown to reduce colonosphere formation and colon cancer cell growth when combined with 5-FU and oxaliplatin[58], suggesting that demethylation of miR-34a may be part of the mechanism underlying this effect. Independent of this study, the intracellular expression of miR-34a has been reported to be downregulated in 5-FU-resistant human colon cancer cells as compared to 5-FU-sensitive cells, with miR-34a secreted at higher rates into extracellular microvesicles in response to 5-FU treatment in 5-FU resistant versus sensitive cells[59]. Therefore, further studies investigating the sensitizing effects of miR-34a to 5-FU are warranted to determine if the combination of these agents can be used in treating colorectal cancer patients.…”

Exosomes play an important role in the process of psoralen reverse multidrug resistance of breast cancer

“…Such phenomenon was observed in several tumour models, including ovarian cancer [7], prostate cancer [8, 9] and osteosarcoma [10]. In colon cancer cells, enhanced secretion of miR-145 and miR-34a via exosomes increased the cells’ resistance to 5-fluorouracil [11]. Docetaxel resistance is related to the enhancement of exosome secretion in a prostate cancer model, probably due to docetaxel efflux through exosomes [12].…”

Microvesicle removal of anticancer drugs contributes to drug resistance in human pancreatic cancer cells

“…Release of exosomes was shown to be a mechanism in acidosis-mediated cisplatin resistance in human melanoma cells [18, 19]. In colon cancer cells, enhanced secretion of miR-145 and miR-34a via MVs increased the cells' resistance to 5-fluorouracil [20]. In an attempt to make vehicles to deliver anticancer agents, exosomes released by mesenchymal stem cells treated with paclitaxel were found to contain paclitaxel [21].…”