Doxorubicin is one of the most effective agents used in the treatment of various tumors. Its use is restricted by the development of resistance to apoptosis, the mechanism of which is not fully understood. Nuclear transcription factor kappaB (NF-kappaB) has been shown both to block apoptosis and to promote cell proliferation, and hence has been considered as an important target for anticancer drug development. We found that in wild type and Dox-revertant MCF-7 cells, Doxorubicin induced NF-kappaB was transient and Dox-resistant cells showed high basal activity of NF-kappaB and expression of genes dependent on it. Moreover, in resistant cells Doxorubicin was unable to induce apoptosis as detected by assays for reactive oxygen intermediates generation, lipid peroxidation, cytotoxicity, PARP degradation and Bcl-2 expression. High basal expressions of multi-drug resistant protein and transglutaminase were found in Dox-resistant cells and inhibition of NF-kappaB decreased those amounts and also sensitized these cells by Doxorubicin. These observations collectively suggest that high NF-kappaB activity confers resistance to Doxorubicin and its inhibition potentiates apoptosis. This study indicates that NF-kappaB plays an important role in chemoresistance and establishes the fact that inhibition of NF-kappaB will be a novel approach in chemotherapy. 2009 Wiley-Liss, Inc.