Regulation of osteoblastic differentiation by the proteasome inhibitor bortezomib.

In eukaryotic cells, degradation of most intracellular proteins is carried out by the ubiquitin-proteasome pathway. Recent investigations suggest that bone metabolism is also regulated by this pathway. The clinical efficacy of bortezomib, a 26S proteasome inhibitor used as an anticancer drug, has been linked to an increase in bone formation. In this study, we show that proteasome inhibitors induce expression of osteoblastic differentiation-related genes such as osteocalcin and alkaline phosphatase in C2C12 cells. In contrast, myogenic differentiation is inhibited. Among the proteasome inhibitors tested, bortezomib induced the greatest increase in osteocalcin expression. Although these effects were similar to that of bone morphogenetic protein (BMP) 2, proteasome inhibitors did not induce transcriptional activity of Smad1/4-dependent reporter or BMP2 signaling target gene expression. Transient transfection of osteocalcin promoter-luciferase constructs with bortezomib resulted in an increase in luciferase activity. Mutation of OSE2, but not OSE1, sites of the osteocalcin promoter diminished the bortezomib-induced activity. Also, Runx2 binding activity and protein levels were induced by bortezomib treatment. These results suggest that the bortezomib induces osteoblastic differentiation by modifying the activity of Runx2 and that the function of the proteasome in controlling degradation of differentiation-related transcription factors plays an important role in osteoblast differentiation.