Although microRNA-21 (miR-21) is emerging as an oncogene and has been shown to 
target several tumor suppressor genes, including programmed cell death 4 
(PDCD4), its precise mechanism of action on cancer stem cells (CSCs) is unclear. 
Herein, we report that FOLFOX-resistant HCT-116 and HT-29 cells that are 
enriched in CSCs show a 3- to 7-fold upregulation of pre- and mature miR-21 and 
downregulation of PDCD4. Likewise, overexpression of miR-21 in HCT-116 cells, 
achieved through stable transfection, led to the downregulation of PDCD4 and 
transforming growth factor beta receptor 2 (TGFβR2). In contrast, the levels of 
β-catenin, TCF/LEF activity and the expression of c-Myc, Cyclin-D, which are 
increased in CSCs, are also augmented in miR-21 overexpressing colon cancer 
cells, accompanied by an increased sphere forming ability in vitro and tumor 
formation in SCID mice. Downregulation of TGFβR2 could be attributed to 
decreased expression of the receptor as evidenced by reduction in the activity 
of the luciferase gene construct comprising TGFβR2-3' untranslated region (UTR) 
sequence that binds to miR-21. Moreover, we observed that downregulation of 
miR-21 enhances luciferase-TGFβR2-3' UTR activity suggesting TGFβR2 as being one 
of the direct targets of miR-21. Further support is provided by the observation 
that transfection of TGFβR2 in HCT-116 cells attenuates TCF/LEF luciferase 
activity, accompanied by decreased expression of β-catenin, c-Myc and Cyclin-D1. 
Our current data suggest that miR-21 plays an important role in regulating 
stemness by modulating TGFβR2 signaling in colon cancer cells.