P38 Regulates Expression of Osteoblast-specific Genes by Phosphorylation of Osterix

María José Ortuño,Silvia Ruiz-Gaspà,Edgardo Rodríguez-Carballo,Antonio R.G Susperregui,Ramon Bartrons,José Luis Rosa,Francesc Ventura

doi:10.1074/jbc.m110.123612

Abstract

Osterix, a zinc finger transcription factor, is specifically expressed in osteoblasts and osteocytes of all developing bones. Because no bone formation occurs in Osx-null mice, Osterix is thought to be an essential regulator of osteoblast differentiation. We report that, in several mesenchymal and osteoblastic cell types, BMP-2 induces an increase in expression of the two isoforms of Osterix arising from two alternative promoters. We identified a consensus Sp1 sequence (GGGCGG) as Osterix binding regions in the fibromodulin and the bone sialoprotein promoters in vitro and in vivo. Furthermore, we show that Osterix is a novel substrate for p38 MAPK in vitro and in vivo and that Ser-73 and Ser-77 are the regulatory sites phosphorylated by p38. Our data also demonstrate that Osterix is able to increase recruitment of p300 and Brg1 to the promoters of its target genes fibromodulin and bone sialoprotein in vivo and that it directly associates with these cofactors through protein-protein interactions. Phosphorylation of Osterix at Ser-73/77 increased its ability to recruit p300 and SWI/SNF to either fibromodulin or bone sialoprotein promoters. We therefore propose that Osterix binds to Sp1 sequences on target gene promoters and that its phosphorylation by p38 enhances recruitment of coactivators to form transcriptionally active complexes.

Highlights

OCTOBER 15, 2010 VOLUME 285 NUMBER 42 entiation to the osteoblast lineage; they promote osteoblast differentiation in vitro and in vivo, bone regeneration, and ectopic bone formation in vivo [1,2,3]
Activation of p38 MAPK signaling by BMP-2, insulin-like growth factor I, or mechanical stress has been shown to be relevant in the induction of their osteogenic effects (20 –25)
Expression of Two Isoforms of Osterix Is Stimulated by BMP2 in Mesenchymal and Osteogenic Cells—Previous studies showed that Osx mRNA levels increase after BMP-2 addition in mesenchymal cell types, including primary osteoblasts, mesenchymal stem cells, mesenchymal C2C12 cells, and osteoblast MC3T3-E1 cells [11, 17, 20, 27]

Summary

Introduction

OCTOBER 15, 2010 VOLUME 285 NUMBER 42 entiation to the osteoblast lineage; they promote osteoblast differentiation in vitro and in vivo, bone regeneration, and ectopic bone formation in vivo [1,2,3]. The reporter assays of Fmod and Ibsp indicate that, in addition to the induction of Osx expression, BMP-2 activates alternative pathways that result in further Fmod and Ibsp transcriptional activation by Osx. Osx Interacts with Sp1 Consensus Regions in Regulatory Sites of Fmod and Ibsp—It has been shown that Osx interacts with the promoters of osteogenic genes such as Col1a1, Dkk1, or Bglap in vivo [14, 15, 31].

Results

Conclusion