Osx transcriptional regulation is mediated by additional pathways to BMP2/Smad signaling

Abstract

Bone morphogenetic protein (BMP)-2 induces Osterix (Osx) in mouse C2C12 cells and chondrocytes. Genetic studies place Osx downstream to the BMP-2/Smad/Runx2 signaling pathway; however, limited information is available on the mediators of Osx expression in osteoblast lineage commitment. Several lines of research implicate the presence of Runx2-independent ossification. Therefore, the purpose of this study was to identify possible mediators of Osx expression beyond the BMP-2/Smad pathway. Using real-time RT-PCR, we showed upregulation of Osx in response to BMP-2 in human mesenchymal stem cells (hMSC). Insulin-like growth factor (IGF)-I upregulated Osx, but not Runx2. Further, IGF-I in combination with BMP-2 was synergistic for Osx, suggesting a pathway beyond Smad signaling. MAPK was tested as a common mediator across BMP-2 and IGF-I signaling pathways. Inhibition of MAPK component ERK1/2 did not affect Runx2 gene expression, but inhibited Osx expression and matrix mineralization. BMP-2-mediated Osx expression was downregulated in response to p38 inhibition. We therefore conclude that during osteogenic lineage progression, in addition to the BMP-2/Smad pathway, IGF-I and MAPK signaling may mediate Osx.

Focal adhesion kinase signaling pathways regulate the osteogenic differentiation of human mesenchymal stem cells

“…Blocking Runx2/ Cbfa-1 activity using a dominant negative Runx2/Cbfa-1 construct and an ubiquitination mediator for Runx2 degradation decreases BMP-2 induction of osterix [29]. We may therefore be observing the assembly of this important pathway in our uncommitted cells, and suggest that FAK activation is an essential early step in this process.…”

“…This is of course not the first report that FAK activates ERK family members, but it is the first demonstration that specific ECM proteins induce hMSC differentiation via the FAK-ERK pathway. Significant cross-talk also occurs between MAPK, bone morphogenic proteins (BMP)/Smads (which are also regulators of bone formation) [7], and protein kinase D [29] signaling cascades in osteoblasts. Together these observations suggest that FAK phosphorylation of ERK 1/2 via MEK may be only one of many mechanisms for regulating Runx2/Cbfa-1 in our cells.…”

“…This pathway also functions to maintain osteogenic differentiation in committed cells: inhibiting with ERK activity with dominant negative mutants of ERK 1/2 or the MEK1 inhibitor U0126 blocks osterix expression and matrix mineralization [29]. Blocking Runx2/ Cbfa-1 activity using a dominant negative Runx2/Cbfa-1 construct and an ubiquitination mediator for Runx2 degradation decreases BMP-2 induction of osterix [29].…”

“…Chondrogenic differentiation of mesenchymal progenitor cells requires Runx2/Cbfa-1 activity [28] but not osterix, suggesting that Runx2/Cbfa-1 may be the key regulator of hMSC commitment along the osteogenic or chondrogenic lineages. MAPK signaling pathways are linked to osterix expression in hMSC [29], but the role of FAK signaling in regulation of osterix is entirely unknown.…”

Cationic Nanogel-mediated Runx2 and Osterix siRNA Delivery Decreases Mineralization in MC3T3 Cells

“…2B). This suggests Osx expression was upregulated through an alternate pathway [12], independent of SMAD protein signaling and Runx2 activation [31].…”