Tyrosine Phosphorylation Is Required for IκB Kinase-β (IKKβ) Activation and Function in Osteoclastogenesis

The transcription factor NF-kappa B is crucial for numerous cellular functions such as survival, differentiation, immunity, and inflammation. A key function of this family of transcription factors is regulation of osteoclast differentiation and function, which in turn controls skeletal homeostasis. The I kappa B kinase (IKK) complex, which contains IKK alpha, IKK beta, and IKK gamma, is required for activation of NF-kappa B, and deletion of either IKK alpha or IKK beta resulted with defective osteoclast differentiation and survival. We have recently investigated the details of the mechanisms governing the role of IKK beta in osteoclastogenesis and found that constitutively active IKK beta in which serine residues 177/181 were mutated into negatively charged glutamic acids instigates spontaneous bona fide receptor activator of NF-kappa B ligand (RANKL)-independent osteoclastogenesis. To better understand and define the functional role of IKK beta domains capable of regulating the osteoclastogenic activity of IKK, we investigated key motifs in the activation T loop of IKK beta, which are potentially capable of modulating its osteoclastogenic activity. We discovered that dual serine (traditional serine residues 177/181) and tyrosine (188/199) phosphorylation events are crucial for IKK beta activation. Mutation of the latter tyrosine residues blunted the NF-kappa B activity of wild type and constitutively active IKK beta, and tyrosine 188/199-deficient IKK beta inhibited osteoclastogenesis. Thus, tyrosines 188/199 are a novel target for regulating IKK beta activity, at least in osteoclasts.