Chronic inflammatory bone diseases, such as rheumatoid arthritis, are characterized by local bone loss that is largely due to the effects of increased production of pro-inflammatory cytokines, IL-1, IL-6, and TNF. These cytokines cause enhanced osteoclastogenesis, and reduced osteoblast activity and survival. Although much is known about how cytokines stimulate focal osteoclastic bone erosion, the mechanisms by which cytokines inhibit osteoblast function remain unclear. In the past several years, the E3 ubiquitin ligase, Smad ubiquitin regulatory factor (Smurf1), has been reported to promote degradation of the osteoblast- specific transcription factor, Runx2, thereby inhibiting osteoblast differentiation. However, the regulation of Smurf and its role in the pathogenesis of inflammation-induced local bone loss have not been investigated. Recently, we have demonstrated that TNF stimulates Smurf1 expression in osteoblasts, which result in Runx2 degradation. TNF-mediated Runx2 degradation is associated with increased ubiquitination, which can be blocked by inhibitory RNA to Smurfl and proteasomal inhibitors. Consistent with this, tissues from joints of rheumatoid arthritic patients and TNF transgenic arthritic mice have high levels of Smurf1 and TNF expression. Based on these preliminary findings, we hypothesize that under inflammatory conditions, TNF or/and other cytokines inhibit osteoblast function through the up-regulation of Smurf-proteasomal degradation pathway. Here we propose to determine the effect of TNF on the Smurf pathway in osteoblasts in vitro, and the effect of Smurf1 deficiency on TNF-mediated osteoblast inhibition in vivo using TNF transgenic and Smurf1-/- mice. The overall purpose of this application is to validate the significance of the Smurf-proteasomal pathway in cytokine-mediated local bone loss. Results from this study should open up a new field of investigation into the involvement of Smurf E3 ligases in inflammation-mediated osteoblast inhibition in erosive bone diseases. ? ? ?
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