Akt enhances Runx2 protein stability by regulating Smurf2 function during osteoblast differentiation

Abstract

Runx2 plays essential roles in bone formation and chondrocyte maturation. Akt promotes osteoblast differentiation induced by the bone morphogenetic proteins BMP2 and enhances the function and transcriptional activity of Runx2. However, the precise molecular mechanism underlying the relationship between Runx2 and Akt is not well understood. In this study, we examined the role of Akt in regulating Runx2 function. We found that Akt increases the stability of Runx2 protein. However, the level of Runx2 mRNA was not affected by Akt, and we did not find any evidence for direct modification of Runx2 by Akt. Instead, we found evidence that Akt induces the phosphorylation of the Smad ubiquitination regulatory factor Smurf2 and decreases the level of Smurf2 protein through ubiquitin/proteasome-mediated degradation of Smurf2. Akt also alleviates Smurf2-mediated suppression of Runx2 transcriptional activity. Taken together, our results suggest that Akt regulates osteoblast differentiation, at least in part, by enhancing the protein stability and transcriptional activity of Runx2 through regulation of ubiquitin/proteasome-mediated degradation of Smurf2.

E3 Ubiquitin Ligase Fbw7 Negatively Regulates Osteoblast Differentiation by Targeting Runx2 for Degradation

“…4). In addition, our data also provide a possible explanation for enhanced Runx2 activity via osteoanabolic agents including insulin and insulinlike growth factor-1, leading to activation of PI3K/Akt pathway that stabilizes Runx2 apparently by inactivating GSK3␤ by phosphorylating it at Ser 9 (26,42,43,47). Here, it is noteworthy to mention that although Wnt signaling is known to a play key role in maintaining bone mass where GSK3␤ and ␤-catenin are crucial signaling molecules, regulation of Runx2 by GSK3␤ in osteoblasts might be independent of Wnt signaling.…”

“…5c, lower panel), suggesting that Fbw7 inhibition may restore Runx2 expression and its function. Increased Akt expression upon Fbw7 knockdown may apparently be attributed to stabilized Runx2, which is known to activate Akt (25,27,42,43). Like MC3T3-E1, Fbw7 RNAi in mouse primary calvarial osteoblasts showed similar results (Fig.…”

“…We hypothesized that inactivated GSK3␤, in turn, fails to phosphorylate Runx2, thereby preventing its recognition and subsequent Fbw7-mediated proteasomal degradation. Moreover, increased Runx2 levels are known to up-regulate Akt protein levels, thereby regulating Runx2-dependent osteoblast differentiation (42,43). We, therefore, determined the effect of Fbw7 depletion on GSK3␤ and Akt expression in MC3T3-E1 cells cultured in both growth medium and differentiation induction medium upon Fbw7 RNAi.…”

Ablation of Adenosine Monophosphate-Activated Protein Kinase α1 in Vascular Smooth Muscle Cells Promotes Diet-Induced Atherosclerotic Calcification In Vivo

“…Indeed, it has been reported that several E3 ubiquitin ligases promote Runx2 ubiquitination and degradation in osteoblasts, such as Smurf1, Smurf2, and WWP1. [40][41][42] We also checked whether AMPKα1 may regulate Runx2 via these E3 ubiquitin ligase. However, we did not find evidence of AMPKα1 regulating these E3 ubiquitin ligases or these E3 ubiquitin ligases regulating Runx2 in VSMC (data not shown).…”

MicroRNA-29a mitigates glucocorticoid induction of bone loss and fatty marrow by rescuing Runx2 acetylation

“…These phenomena were evidently mitigated in the miR-29a transgenic mice. Post-translational modification of Runx2, a master osteogenic transcription factor, reportedly changes osteogenic reactions of osteoblast cultures [25]. We tested whether miR-29a modulation of HDAC4 affected the acetylation status of Runx2 in glucocorticoid-treated bone tissue.…”