MICRORNA INVOLVEMENT IN COLON CANCER PROGRESSION

Loss of response to TGF-β occurs in many cancers and disruption of its regulatory circuitry appears as a central event in the genesis of colorectal cancer (CRC) malignancy. Lack of an inhibitory response to TGF-β is common to most colon cancer cell lines. Recently, abrogation of the TGF-β growth inhibitory response mediated by different miRNAs has been reported. By searching for miRNAs in regions showing copy number changes and concordant gene expression in 36 sporadic CRCs compared to their normal counterpart, we identified the miR-17-92 cluster localized on the 13q31 locus, gained and highly expressed at early stages of CRC. We selected the TGF-β sensitive FET colon carcinoma and investigated the relationship between enhanced expression of miR-20a and TGF-β mediated growth inhibition. We found that miR-20a affects p21 levels and has a negative and significant effect on the cytostatic response mediated by TGF-β. We confirmed that p21 down-modulation is addressed by direct binding of its 3’-UTR by miR-20a. Moreover, we observed also that miR-20a is able to block the transactivation of the 2.3-kb CDKN1A promoter upon TGF-β stimulation, but not the activation of the Smad3/4-reponsive reporter. We also found that two of the c-MYC repressor genes, E2F5 and KLF11, are directly targeted by miR-20a, thus resulting in abrogation of the TGF-β mediated repression of c-MYC. Our experiments suggest for miR-20a an interference with the TGF-β homeostasis in colon cancer addressing the up-regulation of p21 expression, through mechanisms involving more effectors of the TGF-β cascade.