Tumor necrosis factor α-dependent drug resistance to purine and pyrimidine analogues in human colon tumor cells mediated through IKK

Development of drug resistance in cancer is one of the main challenges in chemotherapy, and many mechanisms are still unknown. In this study, we show that tumor necrosis factor alpha (TNFalpha) increases postdrug survival from 5-fluoro-2'-deoxyuridine (FdUrd) in two human colon tumor cell lines. This resulted in the development of drug-resistant cells in a TNFalpha-dependent manner. Interestingly, although the drug-resistant cells were selected using FdUrd, they are also resistant to a number of other antimetabolites in the DNA synthesis pathway in a TNFalpha-dependent manner. Only in the drug-resistant cells (p35- colo201) TNFalpha treatment resulted in G(0)-G(1) arrest but not in the parental colo201 and other cell types. Blocking TNFalpha-induced cell cycle arrest sensitized drug- resistant cells to FdUrd. TNFalpha-induced cell cycle arrest required IKK. IKK inhibition by a small molecule inhibitor or by the knockdown of IKKalpha, IKKbeta, or RelA/p65 using siRNA, but not the inhibition of JNK, MEK, p38, or caspase-8 pathways, blocked TNFalpha-induced G(0)-G(1) arrest and restored sensitivity to FdUrd of drug-resistant cells. TNFalpha reduced the transcripts and protein levels of phosphorylated retinoblastoma protein (Rb), Rb, E2F1, and Cdk4 only in drugresistant p35-colo201 cells. This effect of TNFalpha was reversed by IKK inhibitor, suggesting that TNFalpha-induced cell cycle arrest is probably due to the reduction of Rb, E2F1, and Cdk4. Taken together, this study shows that, in vitro, TNFalpha-induced cell cycle arrest through IKK can provide a mechanism for the development of drug resistance to anti-cancer drugs, purine and pyrimidine analogues.