Protein Kinase Casein Kinase 2 Mediates Inhibitor-κB Kinase and Aberrant Nuclear Factor-κB Activation by Serum Factor(s) in Head and Neck Squamous Carcinoma Cells

Abstract

We showed previously that the signal transcription factor nuclear factor-κB (NF-κB) is aberrantly activated and that inhibition of NF-κB induces cell death and inhibits tumorigenesis in head and neck squamous cell carcinomas (HNSCC). Thus, identification of specific kinases underlying the activation of NF-κB could provide targets for selective therapy. Inhibitor-κB (IκB) kinase (IKK) is known to activate NF-κB by inducing NH₂-terminal phosphorylation and degradation of its endogenous inhibitor, IκB. Casein kinase 2 (CK2) was previously reported to be overexpressed in HNSCC cells and to be a COOH-terminal IKK, but its relationship to NF-κB activation in HNSCC cells is unknown. In this study, we examined the contribution of IKK and CK2 in the regulation of NF-κB in HNSCC in vitro. NF-κB activation was specifically inhibited by kinase-dead mutants of the IKK1 and IKK2 subunits or small interfering RNA targeting the β subunit of CK2. CK2 and IKK kinase activity, as well as NF-κB transcriptional activity, was shown to be serum responsive, indicating that these kinases mediate aberrant activation of NF-κB in response to serum factor(s) in vitro. Recombinant CK2α was shown to phosphorylate recombinant IKK2 as well as to promote immunoprecipitated IKK complex from HNSCC to phosphorylate the NH₂-terminal S32/S36 of IκBα. We conclude that the aberrant NF-κB activity in HNSCC cells in response to serum is partially through a novel mechanism involving CK2-mediated activation of IKK2, making these kinases candidates for selective therapy to target the NF-κB pathway in HNSCC. (Cancer Res 2006; 66(13): 6722-31)