p53/MDM2 Feed-back Loop Sustains miR-221 Expression and Dictates the Response to Anticancer Treatments in Hepatocellular Carcinoma

The overexpression of miR-221 is reported in several human cancers including hepatocellular carcinoma (HCC) and its targeting by tailored treatments has been proposed. The evidence supporting the role of miR-221 in cancer is growing and has been mainly focused on the discovery of miR-221 targets as well as on its possible therapeutic exploitations. However, the mechanism sustaining miR-221 aberrant expression remains to be elucidated. In this study, MDM2, a known p53 (TP53) modulator, is identified as a direct target of miR-221 and a feed-forward loop is described that sustains miR-221 aberrant expression. Interestingly, miR-221 can activate the p53/MDM2 axis by inhibiting MDM2 and, in turn, p53 activation contributes to miR-221 enhanced expression. Moreover, by modulating p53 axis, miR-221 impacts cell cycle progression and apoptotic response to Doxorubicin in HCC-derived cell lines. Finally, CpG island methylation status was assessed as a causative event associated with miR-221 up-regulation in HCC cells and primary tumor specimens. In HCC-derived cell lines, pharmacologically-induced DNA hypomethylation potentiated a significant increase in miR-221 expression. These data were confirmed in clinical specimens of HCC in which elevated miR-221 expression was associated with the simultaneous presence of wild-type p53 and DNA hypomethylation.