title: Opposing effects of glucocorticoids and Wnt signaling on Krox20 and mineral deposition in osteoblast cultures.
abstract: Krox20 is expressed in osteoblasts and chondrocytes, and is required for trabecular bone formation during embryogenesis. Here we show by RT-qPCR and Western blot analysis that Krox20 is up-regulated during late stages of osteoblast differentiation in culture. Glucocorticoids (GCs) rapidly inhibit the expression of Krox20 as well its co-activator, HCF-1, resulting in inhibition of the Osteocalcin Krox20-binding Enhancer (OKE). GCs also inhibit expression of EGR1, EGR3, and EGR4. OKE activity, which is dependent on the presence of Runx2, was independent of the osteocalcin promoter Runx2 binding site. In contrast to GCs, activation of the Wnt, but not the BMP or the PTH signaling pathways, stimulated Krox20 expression as well as activity of the OKE. GC-mediated suppression of Krox20 expression was compromised, albeit not completely, in the presence of DKK1, suggesting that the inhibition occurs in both Wnt-dependent and Wnt-independent manners. Furthermore, Wnt3A partially rescued Krox20 expression in GC-arrested osteoblast cultures and this was accompanied by rescue of mineralization. These findings are consistent with a role for Krox20 in osteoblast function and suggest that this transcription factor may contribute to the opposing effects of GCs and Wnt signaling on bone formation.
authors.affiliation: Department of Biochemistry & Molecular Biology, Institute for Genetic Medicine, Keck School of Medicine at the University of Southern California, Los Angeles, California 90033, USA.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Leclerc
authors.forename: Tommy
authors.initials: T
authors.lastname: Noh
authors.forename: Jon
authors.initials: J
authors.lastname: Cogan
authors.forename: Dilan B
authors.initials: DB
authors.lastname: Samarawickrama
authors.forename: Elisheva
authors.initials: E
authors.lastname: Smith
authors.forename: Baruch
authors.initials: B
authors.lastname: Frenkel
mesh_terms: D000818:Animals; D002113:Calcification, Physiologic / Q000187:drug effects / Q000502:physiology*; D002454:Cell Differentiation; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000378:metabolism; D051767:Early Growth Response Protein 2 / Q000378:metabolism*; D005786:Gene Expression Regulation; D005938:Glucocorticoids / Q000494:pharmacology*; D000076249:Heat Shock Transcription Factors; D051379:Mice; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D015398:Signal Transduction; D014157:Transcription Factors / Q000378:metabolism; D051153:Wnt Proteins / Q000502:physiology*; D060508:Wnt3 Protein; D060509:Wnt3A Protein
chemical_list: D004268:DNA-Binding Proteins; D051767:Early Growth Response Protein 2; C497327:Egr2 protein, mouse; D005938:Glucocorticoids; D000076249:Heat Shock Transcription Factors; D014157:Transcription Factors; D051153:Wnt Proteins; D060508:Wnt3 Protein; D060509:Wnt3A Protein; C555511:Wnt3a protein, mouse