Abstract
Paget's disease of bone (PDB) is characterized by abnormal osteoclasts with unique characteristics that include increased sensitivity of osteoclast progenitors to 1,25(OH)2 D3 , receptor activator of NF-κB ligand (RANKL), and TNF-α; increased osteoclast numbers; and increased expression of IL-6 and several transcription factors. We recently reported that measles virus nucleocapsid protein (MVNP) plays a key role in the development of these abnormal osteoclasts. MVNP can induce the pagetic osteoclast phenotype in vitro and in vivo in TRAP-MVNP transgenic mice. However, the molecular mechanisms by which MVNP generates pagetic osteoclasts have not been determined. TANK-binding kinase 1 (TBK1) and IκB kinase-ϵ (IKKϵ) are IKK family members that complex with MVNP and activate both IRF3 and NF-κB pathways. MVNP increases the amount of TBK1 protein in bone marrow monocytes (BMM). Interestingly, we found that RANKL increased TBK1 and IKKϵ early in osteoclast differentiation, suggesting a possible role in normal osteoclastogenesis. However, only TBK1 is further increased in osteoclasts formed by TRAP-MVNP BMM owing to increased TBK1 protein stability. TBK1 overexpression induced IL6 promoter reporter activity, and elevated endogenous IL6 mRNA and p65 NF-κB, TAF12, and ATF7 proteins in several cell lines. Overexpression of TBK1 was insufficient to induce pagetic osteoclasts from WT BMM but synergized with MVNP to increase pagetic osteoclast formation from TRAP-MVNP BMM. BX795 inhibition of TBK1 impaired MVNP-induced IL-6 expression in both NIH3T3 cells and BMM, and shRNA knockdown of Tbk1 in NIH3T3 cells impaired IL-6 secretion induced by MVNP and decreased TAF12 and ATF7, factors involved in 1,25(OH)2 D3 hypersensitivity of pagetic osteoclasts. Similarly, Tbk1 knockdown in BMM from TRAP-MVNP and WT mice specifically impaired development of the MVNP-induced osteoclast pagetic phenotype. These results demonstrate that TBK1 plays a critical role in mediating the effects of MVNP on osteoclast differentiation and on the expression of IL-6, a key contributor to the pagetic osteoclast phenotype.