## 6 The UPS and Bone Metabolism  
Over the last decade, numerous efforts have been made to understand the role of UPS in bone remodeling. The early evidence arose from the observations that several bone proteins, such as those belonging to BMP and Wnt pathway, as well as the transcription factors Runx2 and Osterix were modulated by ubiquitin-mediated proteasome degradation [43; 44; 45]. A preliminary study from [PERSON] et al. showed that the proteolytic activity is essential for OB proliferation and that OBs have high levels of the main three catalytic activities of proteasome [46]. Moreover, Runx2 accumulation in OBs is regulated by ubiquitination, and different E3 ubiquitin ligases have been involved in the proteasome-dependent Runx2 degradation [46]. Specifically, [PERSON] and colleagues showed that the E3 ubiquitin ligase Smad ubiquitin regulatory factor 1 (Smurf1) interacts with Runx2 favoring its degradation by proteasome [47]. Consistently, overexpression of Smurf1 in OBs inhibits bone formation without inducing OB apoptosis, whereas deletion of Smurf1 in mice markedly increases OB activity and bone mass [47; 48]. In OBs, Smurf1 also regulates the degradation of Smad1, the downstream molecule of the BMP pathway, thus impairing OB formation. Importantly, the interplay between Runx2 and Smurf1 explains the mechanisms by which PTH acts on OBs. It is widely known that PTH induces bone resorption when administered continuously, whereas intermittently, it stimulates bone formation [49; 50]. The anabolic effect of PTH is mainly due to the reduced OB apoptosis through the transcription of Runx2-dependent survival genes. At the same time, PTH decreases the level of Runx2 by inducing its Smurf1-dependent proteasomal degradation, thus explaining why repeated exposure of PTH is needed to exert its anabolic effect [51]. PTH also stimulates all proteolytic activities in OBs as well as the accumulation of polyubiquitinated proteins [46].  
Many other E3 ubiquitin ligases are involved in Runx2 degradation as evidenced by reports that Schnurri-3 (Shn3), a zinc finger adaptor protein, recruiting the E3 ubiquitin ligase WWP1, inhibits Runx2 activity through polyubiquitination and proteasome degradation [52]. Wnt signaling, one of the main pathways involved in OB formation, is tightly regulated at proteasome level. Indeed, Wnt inhibitors, by blocking the interplay between Wnts and their own receptors, lead to \(\beta\)-catenin ubiquitination by \(\beta\)-TrCP1 and proteasomal-somal destruction, thus preventing its translocation into the nucleus [53]. Importantly, the transcription factor Osterix has been proven to be tightly regulated by ubiquitination and degradation. Specifically, the E3 ubiquitin ligases Cbl-b and c-Cbl inhibit OB differentiation of MSCs through the proteasome-dependent degradation of Osterix [44]. More recently, other mechanisms potentially involved in the regulation of proteasome-dependent bone metabolism have been investigated. In particular, the ubiquitin-specific peptidase 53 (USP53), a member of the DUBs family, has been identified as a positive regulator of OB differentiation both in vitro and in vivo [54]. The authors showed that USP53 interacts with the ubiquitin ligase FBXO31 during OB differentiation and regulates \(\beta\)-catenin degradation. Interestingly, USP53 expression decreases in osteoporoptic patients and in a mouse model of osteoporosis [54]. These preliminary findings provide novel insights into the involvement of DUBs in bone disease and potential therapeutic strategies for bone regeneration.