IκB kinase α regulates subcellular distribution and turnover of cyclin D1 by phosphorylation

I kappa B kinases (IKKs), IKK alpha and IKK beta, with a regulatory subunit IKK gamma/NEMO constitute a high molecular weight IKK complex that regulates NF-kappa B activity. Although IKK alpha and IKK beta share structural and biochemical similarities, IKK alpha has been shown to have distinct biological roles. Here we show that IKK alpha plays a critical role in regulating cyclin D1 during the cell cycle. Analysis of IKK alpha(-/-) mouse embryo fibroblast cells showed that cyclin D1 is overexpressed and localized in the nucleus compared with parental mouse embryo fibroblasts. IKK alpha associates with and phosphorylates cyclin D1. Analysis on cyclin D1 mutants demonstrated that IKK alpha phosphorylates cyclin D1 at Thr(286). Reconstitution of IKK alpha in knockout cells leads to nuclear export and increased degradation of cyclin D1. Further, RNAi-mediated knockdown of IKK alpha results in similar changes as observed in IKK alpha(-/-) cells. These results suggest a novel role of IKK alpha in regulating subcellular localization and proteolysis of cyclin D1 by phosphorylation of cyclin D1 at Thr286, the same residue earlier found to be a target for glycogen synthase kinase-3 beta-induced phosphorylation.