Glucocorticoid-induced osteogenesis is negatively regulated by Runx2/Cbfa1 serine phosphorylation

Abstract

Glucocorticoid hormones have complex stimulatory and inhibitory effects on skeletal metabolism. Endogenous glucocorticoid signaling is required for normal bone formation in vivo, and synthetic glucocorticoids, such as dexamethasone, promote osteoblastic differentiation in several in vitro model systems. The mechanism by which these hormones induce osteogenesis remains poorly understood. We demonstrate here that the coordinate action of dexamethasone and the osteogenic transcription factor Runx2/Cbfa1 synergistically induces osteocalcin and bone sialoprotein gene expression, alkaline phosphatase activity, and biological mineral deposition in primary dermal fibroblasts. Dexamethasone decreased Runx2 phosphoserine levels, particularly on Ser125, in parallel with the upregulation of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1) through a glucocorticoid-receptor-mediated mechanism. Inhibition of MKP-1 abrogated the dexamethasone-induced decrease in Runx2 serine phosphorylation, suggesting that glucocorticoids modulate Runx2 phosphorylation via MKP-1. Mutation of Ser125 to glutamic acid, mimicking constitutive phosphorylation, inhibited Runx2-mediated osteoblastic differentiation, which was not rescued by dexamethasone treatment. Conversely, mutation of Ser125 to glycine, mimicking constitutive dephosphorylation, markedly increased osteoblastic differentiation, which was enhanced by, but did not require, additional dexamethasone supplementation. Collectively, these results demonstrate that dexamethasone induces osteogenesis, at least in part, by modulating the phosphorylation state of a negative-regulatory serine residue (Ser125) on Runx2. This work identifies a novel mechanism for glucocorticoid-induced osteogenic differentiation and provides insights into the role of Runx2 phosphorylation during skeletal development.

Anti-inflammatory functions of glucocorticoid-induced genes

“…It is expressed in response to GCs A Clark Anti-inflammatory functions of glucocorticoid-induced genes Page 18 of 53 in a wide variety of cell types including mast cells (Kassel et al, 2001), monocytes or macrophages (Abraham et al, 2006;Aeberli et al, 2006;Bhattacharyya et al, 2007;Chen et al, 2002;Zhao et al, 2005), microglia (Zhou et al, 2007), T lymphocytes , dermal, lung and synovial fibroblasts (Phillips et al, 2006;Toh et al, 2004;Yang et al, 2006), endothelial cells (Furst et al, 2007), osteoblasts (Engelbrecht et al, 2003;Leclerc et al, 2004), keratinocytes (Onda et al, 2006;Stojadinovic et al, 2006), adipocytes (Bazuine et al, 2004), lung epithelial cells (Chivers et al, 2006;Hermoso et al, 2004), airway smooth muscle cells (Issa et al, 2007) and HeLa cells (Imasato et al, 2002;Lasa et al, 2002). Typically expression is quite rapid (within one hour), sustained (up to 24 hours), requires relatively low concentrations of GC, and is blocked by the GR antagonist RU486.…”

Engineering graded tissue interfaces

“…Furthermore, recent studies have reported promising results for this factor as a therapeutic transgene in orthopedic tissue engineering applications (16,17). Importantly, we have previously shown that retroviral overexpression of Runx2 reprograms nonosteoblastic dermal fibroblasts into a mineralizing, osteoblast-like phenotype in vitro and in vivo (18)(19)(20). Here we demonstrate that a continuous bone-soft tissuemimetic interface can be engineered by a simple, one-step seeding of primary dermal fibroblasts onto polymeric scaffolds containing a graded distribution of immobilized Runx2 retrovirus.…”

Effects of the Dichloromethane Fraction of Dipsaci Radix on the Osteoblastic Differentiation of Human Alveolar Bone Marrow-Derived Mesenchymal Stem Cells

“…37) Dipsaci Radix, a regimen to treat bone fractures or diseases over long periods in traditional Korean medicine can be used in regenerative medicine without serious side effects like those of DEX. DR DM and hederagenin 3-O-(2-O-acetyl)--L-arabinopyranoside increased important bone markers to a degree comparable to DEX, the representative osteogenic factor, but the precise mechanisms of osteoblast differentiation by DR DM and hederagenin 3-O-(2-O-acetyl)--L-arabinopyranoside are unknown.…”