Cyclooxygenase-2 negatively regulates osteogenic differentiation in murine bone marrow mesenchymal stem cells via the FOXO3a/p27kip1 pathway.

Aims: Cyclooxygenase-2 (COX-2) is an enzyme that synthesizes prostaglandins from arachidonic acid. Previous reports have indicated that COX-2 is constitutively expressed in osteogenic cells instead of being expressed only after pathogenic induction, and that it facilitates osteoblast proliferation via PTEN/Akt/p27^kip1 signalling. However, the role of COX-2 in osteogenic differentiation of murine bone marrow mesenchymal stromal cells (BMSCs) remains controversial. In this study, we investigated the function of COX-2 in the osteogenic differentiation of BMSCs.

Methods: COX-2 inhibitor, COX-2 overexpression vector, and p27^kip1 small interfering RNA (siRNA) were used to evaluate the role of COX-2 in osteogenic differentiation and related signalling pathways in BMSCs.

Results: We found that the messenger RNA (mRNA) and protein levels of COX-2 decreased gradually during osteogenic differentiation. Inhibition of COX-2 activity promoted FOXO3a and p27^kip1 expression and simultaneously enhanced osteogenesis, as indicated by increased osteogenic gene expression and mineralization in BMSCs. Furthermore, when p27^kip1 was silenced, the suppressive effects of COX-2 on osteogenesis were reversed. It demonstrated that the negative regulatory effect of COX-2 on osteogenesis was mediated by p27^kip1. In addition, our results showed that overexpression of COX-2 reduced the mRNA and protein levels of FOXO3a and p27^kip1, and thus attenuated osteogenic gene expression. These results indicate that COX-2 negatively regulates osteogenic differentiation by reducing the expression of osteogenic genes via the FOXO3a/p27^kip1 signalling pathway.

Conclusion: Together with the findings from previous and current studies, these results indicate that COX-2 has a different role in proliferation versus differentiation during osteogenesis via FOXO3a/p27^kip1 signalling in osteoblasts or BMSCs.