Inhibition of the canonical IKK/NFκB pathway sensitizes human cancer cells to doxorubicin

The NF kappa B family is composed by five subunits (p65/RelA, c-Rel, RelB, p105-p50/NF kappa B-1, p100-p52/NF kappa B2) and controls the expression of many genes that participate in cell cycle, apoptosis, and other key cellular processes. In a canonical pathway, NF kappa B activation depends on the IKK complex activity, which is formed by three subunits (IKK alpha and IKK beta and IKK gamma/NEMO). There is an alternative NF kappa B activation pathway that does not require IKKb or IKKg/ NEMO, in which RelB is a major player. We report in a panel of human breast cancer cells that the IKK/NF kappa B 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 NF kappa B and appeared to be less effective in cells expressing predominantly members of the canonical IKK/NF kappa B. Two NF kappa B inhibitors, bortezomib and NEMO-Binding Domain Inhibitory Peptide, prevented doxorubicin-induced NF kappa B activation and increased doxorubicin antitumor effects in BT-474 cells. Transient down-regulation of members of the canonical pathway (p65, p52, c-Rel and IKKg/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, NF kappa B inhibition sensitized cells to doxorubicin, implying directly p65, p52, c-Rel and IKKg/NEMO subunits in chemoresistance, but not RelB. These findings suggest that selective inhibition of the canonical NF kappa B pathway is sufficient to improve doxorubicin antitumor effects.