title: T-box 3 negatively regulates osteoblast differentiation by inhibiting expression of osterix and runx2.
abstract: T-box (Tbx)3, a known transcriptional repressor, is a member of a family of transcription factors, which contain a highly homologous DNA binding domain known as the Tbx domain. Based on the knowledge that mutation of the Tbx3 gene results in limb malformation, Tbx3 regulates osteoblast proliferation and its expression increases during osteoblast differentiation, we predicted that Tbx3 is an important regulator of osteoblast cell functions. In this study, we evaluated the consequence of transgenic overexpression of Tbx3 on osteoblast differentiation. Retroviral overexpression increased Tbx3 expression >100-fold at the mRNA and protein level. Overexpression of Tbx3 blocked mineralized nodule formation (28 +/- 8 vs. 7 +/- 1%) in MC3T3-E1 cells. In support of these data, alkaline phosphatase (ALP) activity was reduced 33-70% (P < 0.05) in both MC3T3-E1 cells and primary calvaria osteoblasts overexpressing Tbx3. In contrast, Tbx3 overexpression did not alter ALP activity in bone marrow stromal cells. Tbx3 overexpression blocked the increase in expression of key osteoblast marker genes, ALP, bone sialoprotein, and osteocalcin that occurs during normal osteoblast differentiation, but had little or no effect on expression of proliferation genes p53 and Myc. In addition, Tbx3 overexpression abolished increased osterix and runx2 expression observed during normal osteoblast differentiation, but the change in Msx1 and Msx2 expression over time was similar between control and Tbx3 overexpressing cells. Interestingly, osterix and runx2, but not Msx1 and Msx2, contain Tbx binding site in the regulatory region. Based on these data and our previous findings, we conclude that Tbx3 promotes proliferation and suppresses differentiation of osteoblasts and may be involved in regulating expression of key transcription factors involved in osteoblast differentiation.
authors.affiliation: Musculoskeletal Disease Center, Jerry L. Pettis VA Medical Center, Loma Linda, CA 92357, USA.
authors.forename: K E
authors.initials: KE
authors.lastname: Govoni
authors.forename: G R
authors.initials: GR
authors.lastname: Linares
authors.forename: S-T
authors.initials: ST
authors.lastname: Chen
authors.forename: S
authors.initials: S
authors.lastname: Pourteymoor
authors.forename: S
authors.initials: S
authors.lastname: Mohan
mesh_terms: D000818:Animals; D015415:Biomarkers; D002454:Cell Differentiation; D002478:Cells, Cultured; D050659:Core Binding Factor Alpha 1 Subunit / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000235:genetics*; D006801:Humans; D051379:Mice; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism*; D000076182:Sp7 Transcription Factor; D020825:T-Box Domain Proteins / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: D015415:Biomarkers; D050659:Core Binding Factor Alpha 1 Subunit; C493716:Runx2 protein, mouse; D000076182:Sp7 Transcription Factor; C479466:Sp7 protein, mouse; D020825:T-Box Domain Proteins; C491460:Tbx3 protein, mouse; D014157:Transcription Factors