miR-106b modulates cancer stem cell characteristics through TGF-β/Smad signaling in CD44-positive gastric cancer cells

Cancer stem cells have the capacity to form new tumors and are thus considered to be a cause of metastasis and tumor recurrence. However, many of the mechanisms determining cancer stem cell characteristics are still unknown. MicroRNAs (miRNAs) are possible modulators of cancer stem cell generation and may be involved in the retention of cancer stem cell characteristics. The aim of this study was to examine the miRNA expression profiles regulating the cancer stem-like cell characteristics in gastric cancer. We sorted gastric cancer stem-like cells using the stem cell marker CD44 by fluorescence-activated cell sorting. CD44(+) cells formed more and larger spheres compared with CD44(-) cells. Cancer stem cell markers were overexpressed in CD44(+) cells. CD44(+) cells showed increased expression of mesenchynnal cell markers, whereas epithelial markers were downregulated. In miRNA microarray, the miR-106b family comprising miR-106b, miR-93, and miR-25 was significantly upregulated in CD44(+) cells than in CD44(-) cells. Smad7, which inhibits transforming growth factor-beta (TGF-beta)/Smad signaling as a target of the miR-106b family, was downregulated in CD44(+) cells. Furthermore, expression of TGF-fl/Smad signal molecules was activated in CD44(+) cells, in accordance with the action of the miR-106b family. Inhibition of miR-106b showed suppression of the TGF-beta/Smad signaling pathway and decreased self-renewal capacity and cell invasiveness. Our study suggests that CD44(+) gastric cancer cells show cancer stem cell properties with epithelial nnesenchynnal transition (EMT). Increased miR-106b family expression regulated cancer stem-like cell properties, particularly EMT characteristics, through the TGF-beta/Smad signaling pathway in CD44(+) stem-like cells. Taken together, these results indicate that targeting miR-106b may be an effective form of cancer therapy in gastric cancer through the modulation of cancer stem cell characteristics.