Chemosensitization by Diverging Modulation by Short-term and Long-term TNF-α Action on ABCB1 Expression and NF-κB Signaling in Colon Cancer

Multidrug resistance (MDR) is a major cause for cancer chemotherapy failure. Among the numerous strategies to overcome persistent action of proinflammatory cytokines, such as tumor necrosis factor α (TNF-α) permits downregulation of MDR-associated genes, including ATP-binding cassette, subfamily B 1 gene (ABCB1). A key regulator of ABCB1 expression is the transcription factor nuclear factor κ light chain enhancer (NF-κB)/p65. We analyzed diverging short- and long-term effects of TNF-α regarding modulation of NF-κB/p65 signaling and ABCB1 expression in colon cancer cells. Highly resistant ABCB1 overexpressing human HCT15 colorectal carcinoma cells were subjected to short- (30-120 min) or long-term (24-96 h) TNF-α treatment. TNF-α mediated modulation of ABCB1 expression was analyzed by real-time RT-PCR and western blot analysis. The TNF-mediated chemosensitization was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cytotoxicity assay. The involvement of TNF receptors and of NF-κB/p65 signaling was analyzed by western blot analysis, ABCB1 promoter analysis and electrophoretic mobility shift assay (EMSA). The study revealed, that long-term, but not short-term TNF-α treatment leads to TNF-receptor 1 (TNFR1) mediated downregulation of ABCB1 resulting in sensitization towards drug treatment. It dampens NF-κB/p65 activation and nuclear factor of κ light polypeptide gene enhancer in B-cells inhibitor α (IκBα) resynthesis, associated with reduced nuclear accumulation of NF-κB/p65 and reduced binding to its consensus sequence in the ABCB1 promoter. The study reveals the diverging effects of short- or long-term TNF-α action and provides novel insights on downregulation of ABCB1 expression by TNF-mediated repression of NF-κB signaling.