Inhibition of Nuclear Factor-κB Activity by Temozolomide Involves O6-Methylguanine–Induced Inhibition of p65 DNA Binding

The alkylating agent temozolomide, commonly used in the treatment of malignant glioma, causes cellular cytotoxicity by forming O 6 -methylguanine adducts. In this report, we investigated whether temozolomide alters the activity of the transcription factor nuclear factor-κB (NF-κB). Temozolomide inhibits basal and tumor necrosis factor α (TNFα)–induced NF-κB transcriptional activity without altering phosphorylation or degradation of inhibitor of κB-α. Inhibition of NF-κB is secondary to attenuation of p65 DNA binding, not nuclear translocation. Inhibition of DNA binding is shown both in vitro , with gel shift studies and DNA binding assays, and in vivo at κB sites. Consistent with inhibition of NF-κB activity, temozolomide reduces basal and TNFα-induced κB-dependent gene expression. Temozolomide also inhibits NF-κB activated by inducers other than TNFα, including lipopolysaccharide, doxorubicin, and phorbol 12-myristate 13-acetate. The inhibitory action of temozolomide on NF-κB is observed to be maximal following pretreatment of cells with temozolomide for 16 h and is also seen with the S N 1-type methylating agent methylnitrosourea. The ability of temozolomide to form O 6 -methylguanine adducts is important for inhibition of NF-κB as is the presence of a functioning mismatch repair system. Activation of NF-κB with TNFα before administration of temozolomide reduces the cytotoxicity of temozolomide, whereas 16-h pretreatment with temozolomide resensitizes cells to killing. This work shows a mechanism whereby O 6 -methylguanine adducts formed by temozolomide lead to inhibition of NF-κB activity and illustrates a link between mismatch repair processing of alkylator-induced DNA damage and cell death. [Cancer Res 2007;67(14):6889–98]