Cyclin-Dependent Kinase 6 Phosphorylates NF-κB P65 at Serine 536 and Contributes to the Regulation of Inflammatory Gene Expression

Nuclear factor kappa-B (NF-kB) activates multiple genes with overlapping roles in cell proliferation, inflammation and cancer. Using an unbiased approach we identified human CDK6 as a novel kinase phosphorylating NF-kB p65 at serine 536. Purified and reconstituted CDK6/cyclin complexes phosphorylated p65 in vitro and in transfected cells. The physiological role of CDK6 for basal as well as cytokine-induced p65 phosphorylation or NF-kB activation was revealed upon RNAimediated suppression of CDK6. Inhibition of CDK6 catalytic activity by PD332991 suppressed activation of NF-kB and TNFinduced gene expression. In complex with a constitutively active viral cyclin CDK6 stimulated NF-kB p65-mediated transcription in a target gene specific manner and this effect was partially dependent on its ability to phosphorylate p65 at serine 536. Tumor formation in thymi and spleens of v-cyclin transgenic mice correlated with increased levels of p65 Ser536 phosphorylation, increased expression of CDK6 and upregulaton of the NF-kB target cyclin D3. These results suggest that aberrant CDK6 expression or activation that is frequently observed in human tumors can contribute through NF-kB to chronic inflammation and neoplasia.