Phosphorylation‐dependent osterix degradation negatively regulates osteoblast differentiation

Abstract

Proteasome inhibitors exert an anabolic effect on bone formation with elevated levels of osteoblast markers. These findings suggest the important role of the proteasomal degradation of osteogenic regulators, while the underlying molecular mechanisms are not fully understood. Here, we report that the proteasome inhibitors bortezomib and ixazomib markedly increased protein levels of the osteoblastic key transcription factor osterix/Sp7 (Osx). Furthermore, we revealed that Osx was targeted by p38 and Fbw7 for proteasomal degradation. Mechanistically, p38-mediated Osx phosphorylation at S73/77 facilitated Fbw7 interaction to trigger subsequent Osx ubiquitination. Consistent with these findings, p38 knockdown or pharmacological p38 inhibition resulted in Osx protein stabilization. Treatment with p38 inhibitors following osteogenic stimulation efficiently induced osteoblast differentiation through Osx stabilization. Conversely, pretreatment of p38 inhibitor followed by osteogenic challenge impaired osteoblastogenesis via suppressing Osx expression, suggesting that p38 exerts dual but opposite effects in the regulation of Osx level to fine-tune its activity during osteoblast differentiation. Furthermore, Fbw7-depleted human mesenchymal stem cells and primary mouse calvarial cells resulted in increased osteogenic capacity. Together, our findings unveil the molecular mechanisms underlying the Osx protein stability control and suggest that targeting the Osx degradation pathway could help enhance efficient osteogenesis and bone matrix regeneration.

Mild Chronic Kidney Disease Associated with Low Bone Formation and Decrease in Phosphate Transporters and Signaling Pathways Gene Expression

“…To capture the early molecular events in the progression of CKD-MBD we examined pathways associated with bone remodeling at the gene expression level. In parallel with the bone histomorphometric alterations in Nx2 and Nx6, we found the down-regulation of gene expression profile related to osteoblastogenesis (Sp7 [28], Ctnnb1 [29], Bmp4 [30]), and Pi transport and sensing (Slc20a1, Slc20a2, Xpr1, Ankh, Fgfr2, Mapk1, Mapk3 [31,32]). Contrariwise, there were differences neither in osteoclast number nor in osteoclast regulatory genes expression (Tnfrsf11B, Tnfsf11, Lgr4 [9]) between experimental groups and controls.…”

Sp7 Action in the Skeleton: Its Mode of Action, Functions, and Relevance to Skeletal Diseases

“…Another report showed that phosphorylation of Sp7 at Ser-73 and Ser-77 is necessary for its interaction with Fbw7 (F-box/WD repeat-containing protein 7), which works as a receptor subunit of the Skp1-Cullin1-F-box-protein (SCF) Fbw7 E3 ligase complex; Fbw7-targeted Sp7 undergoes ubiquitination and subsequent degradation [ 60 ]. Thus, p38-mediated phosphorylation directs Sp7 to degradation in this context.…”

Ubiquitin modification in osteogenic differentiation and bone formation: From mechanisms to clinical significance

“…S-phase kinase associated protein 2 of the Skp-Cullin-F-box family negatively regulates osteogenic differentiation by targeting RUNX2 degradation (Thacker et al, 2016). F-box/WD repeatcontaining protein 7, another component of the Skp-Cullin-Fbox ubiquitin ligase complex, has been shown to negatively regulate osterix protein stability and osteoblast differentiation (Hoshikawa et al, 2020).…”