Constitutively active canonical NF-κB pathway induces severe bone loss in mice.
Physiologic osteoclastogenesis entails activation of multiple signal transduction pathways distal to the cell membrane receptor RANK. However, atypical osteoclastogenesis driven by pro-inflammatory stimuli has been described. We have reported recently a novel mechanism whereby endogenous mutational activation of the classical NF-κB pathway is sufficient to induce RANKL/RANK-independent osteoclastogenesis. Here we investigate the physiologic relevance of this phenomenon in vivo. Using a knock-in approach, the active form of IKK2, namely IKK2SSEE, was introduced into the myeloid lineage with the aid of CD11b-cre mice. Phenotypic assessment revealed that expression of IKK2SSEE in the myeloid compartment induced significant bone loss in vivo. This observation was supported by a dramatic increase in the number and size of osteoclasts in trabecular regions, elevated levels of circulating TRACP-5b, and reduced bone volume. Mechanistically, we observed that IKK2SSEE induced high expression of not only p65 but also p52 and RelB; the latter two molecules are considered exclusive members of the alternative NF-κB pathway. Intriguingly, RelB and P52 were both required to mediate the osteoclastogenic effect of IKK2SSEE and co-expression of these two proteins was sufficient to recapitulate osteoclastogenesis in the absence of RANKL or IKK2SSEE. Furthermore, we found that NF-κB2/p100 is a potent inhibitor of IKK2SSEE-induced osteoclastogenesis. Deletion of p52 enabled more robust osteoclast formation by the active kinase. In summary, molecular activation of IKK2 may play a role in conditions of pathologic bone destruction, which may be refractory to therapeutic interventions targeting the proximal RANKL/RANK signal.
Department of Orthopedic Surgery, University of Iowa Hospitals and Clinics, Iowa City, Iowa, United States of America.
Jesse E
JE
Otero
Tim
T
Chen
Kaihua
K
Zhang
Yousef
Y
Abu-Amer
D000135:Acid Phosphatase / Q000097:blood; D000818:Animals; D015153:Blotting, Western; D015519:Bone Density; D001854:Bone Marrow Cells / Q000378:metabolism; D001862:Bone Resorption / Q000097:blood / Q000235:genetics / Q000378:metabolism*; D001842:Bone and Bones / Q000000981:diagnostic imaging / Q000378:metabolism / Q000473:pathology; D002454:Cell Differentiation / Q000235:genetics; D002478:Cells, Cultured; D051550:I-kappa B Kinase / Q000235:genetics / Q000378:metabolism*; D053583:Interleukin-1beta / Q000097:blood; D007527:Isoenzymes / Q000097:blood; D008264:Macrophages / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism*; D052003:NF-kappa B p52 Subunit / Q000235:genetics / Q000378:metabolism; D010010:Osteoclasts / Q000378:metabolism; D010012:Osteogenesis / Q000235:genetics; D053245:RANK Ligand / Q000097:blood; D034622:RNA Interference; D015398:Signal Transduction; D000071681:Tartrate-Resistant Acid Phosphatase; D051841:Transcription Factor RelB / Q000235:genetics / Q000378:metabolism; D055114:X-Ray Microtomography
D053583:Interleukin-1beta; D007527:Isoenzymes; D016328:NF-kappa B; D052003:NF-kappa B p52 Subunit; D053245:RANK Ligand; C505689:Tnfsf11 protein, mouse; D051841:Transcription Factor RelB; D051550:I-kappa B Kinase; C496562:Ikbkb protein, mouse; D000135:Acid Phosphatase; D000071681:Tartrate-Resistant Acid Phosphatase