Inhibition of the Canonical IKK/NFκB Pathway Sensitizes Human Cancer Cells to Doxorubicin

The NFκB family is composed by five subunits (p65/RelA, c-Rel, RelB, p105-p50/NFκB1, p100-p52/NF-κB2) and controls the expression of many genes that participate in cell cycle, apoptosis, and other key cellular processes. In a canonical pathway, NF-κB activation depends on the IKK complex activity, which is formed by three subunits (IKKα and IKKβ and IKKγ/NEMO). There is an alternative NFκB activation pathway that does not require IKKβ or IKKγ/NEMO, in which RelB is a major player. We report in a panel of human breast cancer cells that the IKK/NFκB system is generally overexpressed in breast cancer cells and there is heterogeneity in expression levels of individual members between different cell lines. Doxorubicin, an anticancer agent used in patients with breast cancer, activated NFκB and appeared to be less effective in cells expressing predominantly members of the canonical IKK/NFκB. Two NFκB inhibitors, bortezomib and NEMO-Binding Domain Inhibitory Peptide, prevented doxorubicin-induced NFκB activation and increased doxorubicin antitumor effects in BT-474 cells. Transient downregulation of members of the canonical pathway (p65, p52, c-Rel and IKKγ/NEMO) by siRNA in HeLa cells increased doxorubicin cytotoxicity. In contrast, silencing of RelB, a key subunit of the alternative pathway, had no evident effects on doxorubicin cytotoxicity. To conclude, NFκB inhibition sensitized cells to doxorubicin, implying directly p65, p52, c-Rel and IKKγ/NEMO subunits in chemoresistance, but not RelB. These findings suggest that selective inhibition of the canonical NFκB pathway is sufficient to improve doxorubicin antitumor effects.