Constitutive nuclear factor-kappa B mRNA, protein overexpression and enhanced DNA-binding activity in thymidylate synthase inhibitor-resistant tumour cells

In this study, the gene copy number, mRNA and protein expression levels and nuclear DNA-binding activity of nuclear factor kappa B (NF-κB) were compared in a panel of five pairs of thymidylate synthase (TS) inhibitor-resistant and wild-type parent cancer cell lines. High constitutive NF-κB DNA-binding activity was detected in all chemoresistant cell lines. The upregulated NF-κB activity was composed of NF-κB subunits p50 and p65. Four out of five resistant cell lines constitutively overexpressed NF-κB p50 and p65 mRNA and protein. One resistant cell line with the highest NF-κB DNA-binding activity showed normal p50 and p65 protein expression. No NF-κB gene amplification was detected in resistant cell lines. Transient exposure of wild-type RKO(WT) and H630(WT) cells to 5-FU induced NF-κB DNA-binding activity but had no effect on NF-κB protein expression in these cells. Our results indicate that high constitutive NF-κB activity caused by its gene overexpression is an intrinsic character of TS inhibitor-resistant cells. NF-κB can antagonise anticancer drug-induced apoptosis. High NF-κB expression and nuclear activity in TS inhibitor-resistant cancer cells may play an important role in the chemoresistance.