Inhibition of transcription factor nuclear factor-κB by a mutant inhibitor-κBα attenuates resistance of human head and neck squamous cell carcinoma to TNF-α caspase-mediated cell death

Tumour necrosis factor-α (TNF-α) is a cytokine that can induce cell death of different cancers via a cellular cascade of proteases, the caspases. However, TNF-α has been detected in tumour and serum of patients with head and neck squamous cell carcinoma (HNSCC), and tumour cell lines derived from this environment often exhibit resistance to TNF-α-induced cell death. Cell death mediated by TNF-α and caspases may be inhibited by cytoprotective genes regulated by transcription factor nuclear factor-κB (NF-κB). We recently showed that NF-κB is constitutively activated in HNSCC, and that inhibition of NF-κB by expression of a nondegradable mutant inhibitor of NF-κB, IκBαM, markedly decreased survival and growth of HNSCC cells in vivo. In the present study, we examined the TNF-α sensitivity and response of HNSCC with constitutively active NF-κB, and of HNSCC cells in which NF-κB is inhibited by stable expression of a dominant negative mutant inhibitor, IκBαM. Human lines UM-SCC-9, 11B and 38, previously shown to exhibit constitutive activation of NF-κB, were found to be highly resistant to growth inhibition by up to 10(4)U/ml of TNF-α in 5 day MTT assay. These TNF-α resistant HNSCC lines expressed TNF receptor I, and exhibited constitutive and TNF-α-inducible activation of NF-κB as demonstrated by nuclear localization of NF-κB p65 by immunohistochemistry. UM-SCC-9 I11 cells which stably expressed an inhibitor of NF-κB, IκBαm, were susceptible to TNF-α-induced growth inhibition. DNA cell cycle analysis revealed that TNF-α induced growth inhibition was associated with accumulation of cells with sub-G0/G1 DNA content. Cell death was demonstrated by trypan blue staining, and was blocked by caspase inhibitor. We conclude that HNSCC that exhibit constitutive and TNF-α-inducible activation of transcription factor NF-κB are resistant to TNF-α, and that inhibition of NF-κB sensitizes HNSCC to TNF-α caspase-mediated cytotoxicity. The demonstration of the role of activation of NF-κB in resistance of HNSCC to TNF-α may be helpful in the identification of potential targets for pharmacological, molecular and immune therapy of HNSCC. © 2000 Cancer Research Campaign