Smurf1 inhibits mesenchymal stem cell proliferation and differentiation into osteoblasts through JunB degradation

Ubiquitin ligase Smurf1-deficient mice develop an increased-bone-mass phenotype in an age-dependent manner. It was reported that such a bone-mass increase is related to enhanced activities of differentiated osteoblasts. Although osteoblasts are of mesenchymal stem cell (MSC) origin and MSC proliferation and differentiation can have significant impacts on bone formation, it remains largely unknown whether regulation of MSCs plays a role in the bone-mass increase of Smurf1-deficient mice. In this study we found that bone marrow mesenchymal progenitor cells from Smurf1(-/-) mice form significantly increased alkaline phosphatase-positive colonies, indicating roles of MSC proliferation and differentiation in bone-mass accrual of Smurf1(-/-) mice. Interestingly, Smurf1(-/-) cells have an elevated protein level of AP-1 transcription factor JunB. Biochemical experiments demonstrate that Smurf1 interacts with JunB through the PY motif and targets JunB protein for ubiquitination and proteasomal degradation. Indeed, Smurf1-deficient MSCs have higher proliferation rates, consistent with the facts that cyclin D1 mRNA and protein both are increased in Smurf1(-/-) cells and JunB can induce cyclinD1 promoter. Moreover, JunB overexpression induces osteoblast differentiation, shown by higher expression of osteoblast markers, and JunB knock-down not only decreases osteoblast differentiation but also restores the osteogenic potential to wild-type level in Smurf1(-/-) cells. In conclusion, our results suggest that Smurf1 negatively regulates MSC proliferation and differentiation by controlling JunB turnover through an ubiquitin-proteasome pathway.