Abstract
Runx2 is a critical transcription factor for osteoblast differentiation. Regulation of Runx2 expression levels and transcriptional activity is important for bone morphogenetic protein (BMP)-induced osteoblast differentiation. Previous studies have shown that extracellular signal-regulated kinase (Erk) activation enhances the transcriptional activity of Runx2 and that BMP-induced Runx2 acetylation increases Runx2 stability and transcriptional activity. Because BMP signaling induces Erk activation in osteoblasts, we sought to investigate whether BMP-induced Erk signaling regulates Runx2 acetylation and stability. Erk activation by overexpression of constitutively active MEK1 increased Runx2 transcriptional activity, whereas U0126, an inhibitor of MEK1/2, suppressed basal Runx2 transcriptional activity and BMP-induced Runx2 acetylation and stabilization. Overexpression of constitutively active MEK1 stabilized Runx2 protein via up-regulation of acetylation and down-regulation of ubiquitination. Erk activation increased p300 protein levels and histone acetyltransferase activity. Knockdown of p300 using siRNA diminished Erk-induced Runx2 stabilization. Overexpression of Smad5 increased Runx2 acetylation and stabilization. Erk activation further increased Smad-induced Runx2 acetylation and stabilization, whereas U0126 suppressed these functions. On the other hand, knockdown of Smad1 and Smad5 by siRNA suppressed both basal and Erk-induced Runx2 protein levels. Erk activation enhanced the association of Runx2 with p300 and Smad1. Taken together these results indicate that Erk signaling increases Runx2 stability and transcriptional activity, partly via increasing p300 protein levels and histone acetyltransferase activity and subsequently increasing Runx2 acetylation by p300. In addition to the canonical Smad pathway, a BMP-induced non-Smad Erk signaling pathway cooperatively regulates osteoblast differentiation partly via increasing the stability and transcriptional activity of Runx2.
Highlights
The bone morphogenetic proteins (BMPs)3 are members of the transforming growth factor- superfamily and are primary growth factors that induce formation of both cartilage and bone
extracellular signal-regulated protein kinase (Erk) Activation Is Involved in BMP-2-induced Osteoblast Differentiation—To investigate the role of activated Erk in BMP-2-induced osteoblast differentiation, we used pluripotent mesenchymal C2C12 cells that can be differentiated into osteoblasts by BMP-2 treatment [35]
BMP2-induced Erk phosphorylation was observed within 5 min and persisted for at least 24 h, there was a fluctuation in Erk phosphorylation level
Summary
The bone morphogenetic proteins (BMPs) are members of the transforming growth factor- superfamily and are primary growth factors that induce formation of both cartilage and bone. In addition to the Smad pathway, diverse intracellular signaling molecules participate in BMP-induced osteoblast differentiation These are collectively called the non-Smad pathway of BMP signaling and include extracellular signal-regulated protein kinase (Erk), p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, phosphatidylinositol 3-kinase, and protein kinases C and D [3,4,5,6,7]. A recent in vivo study has elucidated that transgenic mice overexpressing a constitutively active MEK (Mek-sp) in osteoblasts show enhanced bone formation and that crossing Mek-sp transgenic mice to Runx heterozygote mice partially rescues the phenotype of cleidocranial dysplasia [24] These reports suggest that the MEK/Erk pathway plays a pivotal role in osteogenesis, at least through stimulation of Runx phosphorylation and transcriptional activity. Histone deacetylase 4 (HDAC4) and HDAC5 have been shown to deacetylate Runx, allowing the protein to undergo Smurf-mediated ubiquitination and subsequent degradation [28]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
