LIM Mineralization Protein-1 Enhances Bone Morphogenetic Protein-2-Mediated Osteogenesis Through Activation of ERK1/2 MAPK Pathway and Upregulation of Runx2 Transactivity

LIM mineralization protein-1 (LMP-1) is an intracellular regulator of bone formation. Upregulation of bone morphogenetic proteins (BMPs) and stabilization of BMP/Smad signaling have been proven to be the key mechanisms through which LMP-1 enhances osteogenesis. However, how LMP-1 regulates BMPs expression and related bone formation remains unclear. In this study, a LMP-1-induced osteogenesis cell model was used to study the molecular action of LMP-1 on BMP-2 expression and bone formation. The results show that overexpression of LMP-1 significantly increases, whereas downregulation of endogenous LMP-1 decreases BMP-2 expression and bone formation. Antagonism of BMP-2 with noggin or short hairpin BMP-2 significantly attenuates the osteoinductive effect of LMP-1, suggesting that the osteoinductive effect of LMP-1 is mediated by BMP-2. LMP-1 regulation of BMP-2 is found to occur at the transcription level using a luciferase reporter assay with a reporter construct containing a BMP-2 promoter. A promoter deletion assay reveals that -1000/-500 bp is the key regulated region by LMP-1. A Runx2-binding site is then located at -934/-920 bp and confirmed by luciferase assay using a reporter construct containing repeats of this Runx2-binding site and the site-directed mutagenesis analysis. Overexpression of LMP-1 significantly increases Runx2 expression. Downregulation of Runx2 expression significantly decreases BMP-2 promoter activity and BMP-2 expression. A ChIP assay demonstrates that LMP-1 increases the interaction between Runx2 and BMP-2 promoter. A luciferase reporter assay using the OSE2 promoter containing a Runx2-binding site confirms that Runx2 transactivity can be upregulated by LMP-1. Moreover, inhibiting the activation of different pathways with specific pathway inhibitors reveals that ERK1/2 MAPK activation is essential for LMP-1-induced upregulation of Runx2 transactivity and subsequent BMP-2 expression. In conclusion, our novel findings describe a positive regulatory effect of LMP-1 on BMP-2 expression and BMP-2-mediated osteogenesis. This effect occurs through activation of ERK1/2 pathway and subsequent upregulation of Runx2 transactivity.