Opposing TNF-α/IL-1β- and BMP-2-activated MAPK signaling pathways converge on Runx2 to regulate BMP-2-induced osteoblastic differentiation

In patients who were treated with exogenous BMP-2 to repair bone fractures or defects, the levels of the inflammatory cytokines such as TNF-α and IL-1β in sera are significantly elevated, which may affect the outcome of bone regeneration. Mitogen-activated protein kinase (MAPK) cascades such as extracellular signal-regulated kinase 1/2 (ERK1/2), p38, and c-Jun NH2-terminal kinase 1/2 (JNK1/2) have a crucial role in osteogenic differentiation and are activated by both BMP-2 and TNF-α/IL-1β. However, previous studies suggested that the effects of BMP-2 and TNF-α/IL-1β in osteoblastic differentiation are opposite. Here, we investigated the exact role of MAPKs in a BMP-2 and TNF-α/IL-1β co-existed condition. Treatment with TNF-α/IL-1β inhibited BMP-2-induced alkaline phosphatase activity, calcium deposition, osteogenic transcriptional factor Runx2, and the expression of osteogenic markers in C2C12 and MC3T3-E1 cells. This inhibitory effect was independent of the canonical BMP/Smad pathway, suggesting the presence of an alternate regulatory pathway for BMP-2-induced Runx2 activity and subsequent osteoblastic differentiation. We then confirmed that BMP-2, TNF-α, and IL-1β alone can activate p38, ERK1/2, and JNK1/2, respectively. However, only inhibition of p38 and ERK1/2 signaling were required to modulate BMP-2-induced Runx2 expression. Finally, we determined that TNF-α/IL-1β decreased BMP-2-induced Runx2 expression through the activation of p38 and ERK1/2 signaling. Furthermore, strong activation of p38 and ERK1/2 signaling by transfection with CA-MKK3 or CA-MEK1 inhibited BMP-2-induced Runx2 expression and osteoblastic differentiation in C2C12 and MC3T3-E1 cells. Based on these results, we conclude that TNF-α/IL-1β- and BMP-2-activated p38 and ERK1/2 signaling have opposing roles that converge on Runx2 to regulate osteoblastic differentiation. The elucidation of these mechanisms may hasten the development of new strategies and improve the osteoinductive efficacy of BMP-2 in the clinic to enhance osteoblastic differentiation and bone formation.