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

Abstract

The transcription factor NF-kappaB 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 IkappaB kinase (IKK) complex, which contains IKKalpha, IKKbeta, and IKKgamma, is required for activation of NF-kappaB, and deletion of either IKKalpha or IKKbeta resulted with defective osteoclast differentiation and survival. We have recently investigated the details of the mechanisms governing the role of IKKbeta in osteoclastogenesis and found that constitutively active IKKbeta in which serine residues 177/181 were mutated into negatively charged glutamic acids instigates spontaneous bona fide receptor activator of NF-kappaB ligand (RANKL)-independent osteoclastogenesis. To better understand and define the functional role of IKKbeta domains capable of regulating the osteoclastogenic activity of IKK, we investigated key motifs in the activation T loop of IKKbeta, 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 IKKbeta activation. Mutation of the latter tyrosine residues blunted the NF-kappaB activity of wild type and constitutively active IKKbeta, and tyrosine 188/199-deficient IKKbeta inhibited osteoclastogenesis. Thus, tyrosines 188/199 are a novel target for regulating IKKbeta activity, at least in osteoclasts.