Abnormal activation of the NF-kappaB pathway is closely related to tumorigenesis and chemoresistance. Therefore, microRNAs that possess the NF-kappaB inhibitory activity may provide novel targets for anti-cancer therapy. miR-26 family members have been shown to be frequently downregulated in hepatocellular carcinoma (HCC) and correlated with the poor survival of HCC patients. To date, there is no report disclosing the regulatory role of miR-26 on the NF-kappaB pathway and its biological significance.

The effects of miR-26b on the NF-kappaB signaling pathway and the chemosensitivity of cancer cells were examined in two HCC cell lines, QGY-7703 and MHCC-97H, using both gain- and loss-of-function studies. The correlation between miR-26b level and apoptosis rate was further investigated in clinical HCC specimens.

Both TNFalpha and doxorubicin treatment activated the NF-kappaB signaling pathway in HCC cells. However, the restoration of miR-26b expression significantly inhibited the phosphorylation of IkappaBalpha and p65, blocked the nuclear translocation of NF-kappaB, reduced the NF-kappaB reporter activity, and consequently abrogated the expression of NF-kappaB target genes in TNFalpha or doxorubicin-treated HCC cells. Furthermore, the ectopic expression of miR-26b dramatically sensitized HCC cells to the doxorubicin-induced apoptosis, whereas the antagonism of miR-26b attenuated cell apoptosis. Consistently, the miR-26b level was positively correlated with the apoptosis rate in HCC tissues. Subsequent investigations revealed that miR-26b inhibited the expression of TAK1 and TAB3, two positive regulators of NF-kappaB pathway, by binding to their 3'-untranslated region. Moreover, knockdown of TAK1 or TAB3 phenocopied the effects of miR-26b overexpression.

These data suggest that miR-26b suppresses NF-kappaB signaling and thereby sensitized HCC cells to the doxorubicin-induced apoptosis by inhibiting the expression of TAK1 and TAB3. Our findings highlight miR-26b as a potent inhibitor of the NF-kappaB pathway and an attractive target for cancer treatment.