1α,25-dehydroxyvitamin D3 synergism toward transforming growth factor-β1-induced AP-1 transcriptional activity in mouse osteoblastic cells via its nuclear receptor

The present study demonstrates 1 alpha,25-dehydroxyvitamin D-3 (1 alpha-25-(OH)(2)D-3) synergism toward transforming growth factor (TGF)-beta 1-induced activation protein-1 (AP-1) activity in mouse osteoblastic MC3T3-E1 cells via the nuclear receptor of the vitamin. 1 alpha-25-(OH)(2)D-3 synergistically stimulated TGF-beta 1-induced expression of the c-jun gene in the cells but not that of the c-fos gene. We actually showed by a gel mobility shift assay 1 alpha-25(OH)(2)D-3 synergism of TGF-beta 1-induced AP-1 binding to the 12-(O-tetradecanoylphorbol-13-acetate response element (TRE), 1 alpha-25-(OH)(2)D-3 markedly stimulated the transient activity of TGF-beta 1-induced AP-1 in the cells transfected with a TRE-chloramphenicol acetyltransferase (CAT) reporter gene. Also, a synergistic increase in TGF-beta 1-induced CAT activity was observed in the cells cotransfected with an expression vector encoding vitamin D-3 receptor (VDR) and the reporter gene. However, the synergistic CAT activity was inhibited by pretreatment with VDR antisense oligonucleotides. In addition, in a Northern blot assay, we observed 1 alpha-25(OH)(2)D-3 synergism of TGF-beta 1-induced expression of the c-jun gene in the cells transfected with the VDR expression vector and also found that the synergistic action was clearly blocked by VDR antisense oligonucleotide pretreatment. The present study strongly suggests a novel positive regulation by 1 alpha-25-(OH)(2)D-3 of TGF-beta 1-induced AP-1 activity in osteoblasts via "genommc action.".