Inhibition of NF-κB Signaling by Quinacrine Is Cytotoxic to Human Colon Carcinoma Cell Lines and Is Synergistic in Combination with Tumor Necrosis Factor-related Apoptosis-inducing Ligand (TRAIL) or Oxaliplatin

Colorectal cancer is the third most common malignancy in the United States. Modest advances with therapeutic approaches that include oxaliplatin (l-OHP) have brought the median survival rate to 22 months, with drug resistance remaining a significant barrier. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is undergoing clinical evaluation. Although human colon carcinomas express TRAIL receptors, they can also demonstrate TRAIL resistance. Constitutive NF-κB activation has been implicated in resistance to TRAIL and to cytotoxic agents. We have demonstrated constitutive NF-κB activation in five of six human colon carcinoma cell lines; this activation is inhibited by quinacrine. Quinacrine induced apoptosis in colon carcinomas and potentiated the cytotoxic activity of TRAIL in RKO and HT29 cells and that of l-OHP in HT29 cells. Similarly, overexpression of IκBα mutant (IκBαM) or treatment with the IKK inhibitor, BMS-345541, also sensitized these cells to TRAIL and l-OHP. Importantly, 2 h of quinacrine pretreatment resulted in decreased expression of c-FLIP and Mcl-1, which were determined to be transcriptional targets of NF-κB. Extended exposure for 24 h to quinacrine did not further sensitize these cells to TRAIL- or l-OHP-induced cell death; however, exposure caused the down-regulation of additional NF-κB-dependent survival factors. Short hairpin RNA-mediated knockdown of c-FLIP or Mcl-1 significantly sensitized these cells to TRAIL and l-OHP. Taken together, data demonstrate that NF-κB is constitutively active in colon cancer cell lines and NF-κB, and its downstream targets may constitute an important target for the development of therapeutic approaches against this disease.