Functional Cooperation between CCAAT/Enhancer-Binding Proteins and the Vitamin D Receptor in Regulation of 25-Hydroxyvitamin D3 24-Hydroxylase

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Abstract
1,25-Dihydroxyvitamin D3 [1,25(OH)2D3] induces the synthesis of 25-hydroxyvitamin D3 24-hydroxylase [24(OH)ase], an enzyme involved in its catabolism, thereby regulating its own metabolism. Here we demonstrate that CCAAT enhancer binding protein β (C/EBPβ) is induced by 1,25(OH)2D3 in kidney and in osteoblastic cells and is a potent enhancer of vitamin D receptor (VDR)-mediated 24(OH)ase transcription. Transfection studies indicate that 1,25(OH)2D3 induction of 24(OH)ase transcription is enhanced a maximum of 10-fold by C/EBPβ. Suppression of 1,25(OH)2D3-induced 24(OH)ase transcription was observed with dominant negative C/EBP or osteoblastic cells from C/EBPβ−/− mice. A C/EBP site was identified at positions −395 to −388 (−395/−388) in the rat 24(OH)ase promoter. Mutation of this site inhibited C/EBPβ binding and markedly attenuated the transcriptional response to C/EBPβ. We also report the cooperation of CBP/p300 with C/EBPβ in regulating VDR-mediated 24(OH)ase transcription. We found that not only 1,25(OH)2D3 but also parathyroid hormone (PTH) can induce C/EBPβ expression in osteoblastic cells. PTH potentiated the induction of C/EBPβ and 24(OH)ase expression in response to 1,25(OH)2D3 in osteoblastic cells. Data with the human VDR promoter (which contains two putative C/EBP sites) indicate a role for C/EBPβ in the protein kinase A-mediated induction of VDR transcription. From this study a fundamental role has been established for the first time for cooperative effects and cross talk between the C/EBP family of transcription factors and VDR in 1,25(OH)2D3-induced transcription. These findings also indicate a novel role for C/EBPβ in the cross talk between PTH and 1,25(OH)2D3 that involves the regulation of VDR transcription.

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