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

We showed previously that the signal transcription factor nuclear factor-kappa B (NF-kappa B) is aberrantly activated and that inhibition of NF-kappa B induces cell death and inhibits tumorigenesis in head and neck squamous cell carcinomas (HNSCC). Thus, identification of specific kinases underlying the activation of NF-kappa B could provide targets for selective therapy. Inhibitor-kappa B (I kappa B) kinase (IKK) is known to activate NF-kappa B by inducing NH2-terminal phosphorylation and degradation of its endogenous inhibitor, I kappa B. 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-kappa B activation in HNSCC cells is unknown. In this study, we examined the contribution of IKK and CK2 in the regulation of NF-kappa B in HNSCC in vitro. NF-kappa B activation was specifically inhibited by kinase-dead mutants of the IKK1 and IKK2 subunits or small interfering RNA targeting the 0 subunit of CK2. CK2 and IKK kinase activity, as well as NF-kappa B transcriptional activity, was shown to be serum responsive, indicating that these kinases mediate aberrant activation of NF-kappa B in response to serum' factor(s) in vitro. Recombinant CK2 alpha was shown to phosphorylate recombinant IKK2 as well as to promote immunoprecipitated IKK complex from HNSCC to phosphorylate the NH2-terminal S32/S36 of I kappa B alpha. We conclude that the aberrant NF-kappa B 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-kappa B pathway in HNSCC.