Okadaic acid induces tyrosine phosphorylation of IκBα that mediated by PKR pathway in human osteoblastic MG63 cells

Treatment of human osteosarcoma cell line MG63 cells with okadaic acid stimulated phosphorylation of I kappa B alpha, as judged from the results of Western blot analysis and a lambda protein phosphatase dephosphorylation assay. The stimulated phosphorylation of I kappa B alpha was both time- and dose-dependent. The phosphorylation sites of I kappa B alpha were taken to be tyrosine residues because the anti-phospho-tyrosine antibody bound to the samples immunoprecipitated with the anti-I kappa B alpha antibody. In the cells treated with 100 nM okadaic acid consequential translocation of NF-kappa B p65 from the cytosol to the nucleus occurred. Double-stranded RNA-dependent protein kinase (PKR) is a player in the cellular antiviral response and is involved in transcriptional stimulation through activation of NF-kappa B. We investigated the functional relationship between PKR and I kappa B alpha phosphorylation by constructing MG63 PKR K/R cells that produced a catalytically inactive mutant PKR. NF-kappa B p65 was detected in the nucleus of these cells, even in the unstimulated cells. Although I kappa B alpha was degraded phosphorylation of eIF-2 alpha, a substrate of PKR, did not occur in the mutant cells treated with okadaic acid. Our results suggest that okadaic acid-induced tyrosine phosphorylation of I kappa B alpha was mediated by PKR kinase activity, thus indicating the involvement of this kinase in the control mechanism governing the activation of NF-kappa B.