Nuclear factor-E2-related factor-1 mediates ascorbic acid induction of osterix expression via interaction with antioxidant-responsive element in bone cells

We recently found that deletion of the gulonolactone oxidase gene, which is involved in the synthesis of ascorbic acid (AA), was responsible for the fracture phenotype in spontaneous fracture mice. To explore the molecular mechanisms by which AA regulates osteoblast differentiation, we examined the effect of AA on osterix expression via Nrf1 (NF-E2-related factor-1) binding to antioxidant-responsive element (ARE) in bone marrow stromal (BMS) cells. AA treatment caused a 6-fold increase in osterix expression in mutant BMS cells at 24 h, which was unaffected by pretreatment with protein synthesis inhibitor. Sequence analyses of mouse osterix promoter revealed a putative ARE located at -1762 to -1733 upstream of the transcription start site to which Nrf potentially binds. A gel mobility shift assay revealed that nuclear proteins from AA-treated BMS cells bound to radiolabeled ARE much more strongly than nuclear extracts from AA-untreated cells. A chromatin immunoprecipitation assay with Nrf1 antibody confirmed the interaction of Nrf1 with the mouse osterix promoter. A reporter assay demonstrated that the promoter activity of mouse osterix containing an ARE was stimulated 4-fold by a 48-h treatment with AA in spontaneous fracture BMS cells. Treatment of mutant BMS cells with AA resulted in a 3.9-fold increase in the nuclear accumulation of Nrf1. Transfection of mutant BMS cells with Nrf1 small interfering RNA decreased Nrf1 protein by 4.5-fold, blocked AA induction of osterix expression, and impaired BMS cell differentiation. Our data provided the first experimental evidence that AA modulated osterix expression via a novel mechanism involving Nrf1 nuclear translocation and Nrf1 binding to ARE to activate genes critical for cell differentiation.