c-Jun N-terminal kinase 1 negatively regulates osteoblastic differentiation induced by BMP2 via phosphorylation of Runx2 at Ser104.

Runx2 plays a crucial role in osteoblastic differentiation, which can be upregulated by bone morphogenetic proteins 2 (BMP2). Mitogen-activated protein kinase (MAPK) cascades, such as extracellular signal-regulated kinase (ERK) and p38, have been reported to be activated by BMP2 to increase Runx2 activity. The role of cjun-N-terminal kinase (JNK), the other kinase of MAPK, in osteoblastic differentiation has not been well elucidated. In this study, we first showed that JNK1 is activated by BMP2 in multipotent C2C12 and preosteoblastic MC3T3-E1 cell lines. We then showed that early and late osteoblastic differentiation, represented by ALP expression and mineralization, respectively, are significantly enhanced by JNK1 loss-of-function, such as treatment of JNK inhibitor, knockdown of JNK1 and ectopic expression of a dominant negative JNK1 (DN-JNK1). Consistently, BMP2-induced osteoblastic differentiation is reduced by JNK1 gain-of-function, such as enforced expression of a constitutively active JNK1 (CA-JNK1). Most importantly, we showed that Runx2 is required for JNK1-mediated inhibition of osteoblastic differentiation, and identified Ser104 of Runx2 is the site phosphorylated by JNK1 upon BMP2 stimulation. Finally, we found that overexpression of the mutant Runx2 (Ser104Ala) stimulates osteoblastic differentiation of C2C12 and MC3T3-E1 cells to the extent similar to that achieved by overexpression of wild-type (WT) Runx2 plus JNK inhibitor treatment. Taken together, these data indicate that JNK1 negatively regulates BMP2-induced osteoblastic differentiation through phosphorylation of Runx2 at Ser104. In addition, unraveling these mechanisms may help to develop new strategies in enhancing osteoblastic differentiation and bone formation.