Early B cell factor 1 (Ebf1) is a transcription factor that regulates B cell, neuronal and adipocyte differentiation. We and others have shown that Ebf1 is expressed in osteoblasts and that deletion of Ebf1 results in increased bone formation. Conversely, overexpression of Ebf1 in osteoblasts leads to impaired bone formation. The role of Ebf1 during early osteoblast differentiation remains unclear. We aimed to determine which of the effects are autonomous for bone cells and whether Ebf1 could regulate adipocyte and osteoblast lineages. Overexpression of Ebf1 in mesenchymal cell line C3H10T1/2 cells led to enhanced osteoblast differentiation with increased expression of osterix. Conversely, in Ebf1-/- osteoblast cultures differentiation was impaired. To reconcile these data with previous in vivo findings, we hypothesised that Ebf1 could have a dual role in osteoblast differentiation promoting early but inhibiting late stages. To allow for some Ebf1 activity to overcome the early defect in differentiation we used haploinsufficient Ebf1+/- calvarial cells, which showed reduced expression of Osx and ALP. We also identified an Ebf binding site in Osx promoter by ChIP assay suggesting regulation by Ebf1. Interestingly, adipogenesis and adipocyte markers PPARG and aP2 were increased by more than 50% already on day 7 of Ebf1+/- culture. To confirm these findings in vivo, we generated conditional Ebf1 knockout mice, in which Ebf1 deletion was targeted to early or late OBs by crossing with Osx- or hOC-Cre mouse lines, respectively. Deletion of Ebf1 in early osteoblasts resulted in significantly increased bone volume and trabecular number in the tibia at the age of 12 weeks by mCT analysis, while Ebf1hOC-/- mice did not have a bone phenotype. Our data suggest that Ebf1 promotes early osteoblast differentiation at the crossroads of osteoblasts and adipocytes, possibly via inducing Osx expression. However, Ebf1 inhibits osteoblast function in committed Osx-expressing osteoblasts.
Disclosure: The authors declared no competing interests. This study was supported by grants from the Academy of Finland, Sigrid Juselius Foundation, Emil Aaltonen Foundation and Turku University Hospital Research Funds.