Evidence that TNF-TNFR1-TRADD-TRAF2-RIP-TAK1-IKK pathway mediates constitutive NF-κB activation and proliferation in human head and neck squamous cell carcinoma

Constitutively activated nuclear factor-κB (NF-κB) has been associated with a variety of aggressive tumor types, including head and neck squamous cell carcinoma (HNSCC); however, the mechanism of its activation is not fully understood. Therefore, we investigated the molecular pathway that mediates constitutive activation of NF-κB in a series of HNSCC cell lines. We confirmed that NF-κB was constitutively active in all HNSCC cell lines (FaDu, LICR-LON-HN5 and SCC4) examined as indicated by DNA binding, immunocytochemical localization of p65, by NF-κB-dependent reporter gene expression and its inhibition by dominant-negative (DN)-inhibitory subunit of NF-κB (IκBα), the natural inhibitor of NF-κB. Constitutive NF-κB activation in HNSCC was found to be due to constitutive activation of IκBα kinase (IKK); and this correlated with constitutive expression of phosphorylated forms of IκBα and p65 proteins. All HNSCC showed the expression of p50, p52, p100 and receptor-interacting protein; all linked with NF-κB activation. The expression of constitutively active NF-κB in HNSCC is mediated through the tumor necrosis factor (TNF) signaling pathway, as NF-κB reporter activity was inhibited by DN-TNF receptor-associated death domain (TRADD), DN-TNF receptor-associated factor (TRAF)2, DN-receptor-interacting protein (RIP), DN-transforming growth factor-β-activated kinase 1 (TAK1), DN-κ-Ras, DN-AKT and DN-IKK but not by DN-TRAF5 or DN-TRAF6. Constitutive NF-κB activation was also associated with the autocrine expression of TNF, TNF receptors and receptor-activator of NF-κB and its ligand in HNSCC cells but not interleukin (IL)-1β. All HNSCC cell lines expressed IL-6, a NF-κB-regulated gene product. Furthermore, treatment of HNSCC cells with anti-TNF antibody downregulated constitutively active NF-κB, and this was associated with inhibition of IL-6 expression and cell proliferation. Our results clearly demonstrate that constitutive activation of NF-κB is mediated through the TRADD-TRAF2-RIP-TAK1-IKK pathway, making TNF a novel target in the treatment of head and neck cancer.