Novel regulation of 25-hydroxyvitamin D3 24-hydroxylase (24(OH)ase) transcription by glucocorticoids: Cooperative effects of the glucocorticoid receptor, C/EBPβ, and the Vitamin D receptor in 24(OH)ase transcription

Glucocorticoid‐induced bone loss has been proposed to involve direct effects on bone cells as well as alterations in calcium absorption and excretion. Since vitamin D is important for the maintenance of calcium homeostasis, in the present study the effects of glucocorticoids on vitamin D metabolism through the expression of 24(OH)ase, an enzyme involved in the catabolism of 1,25(OH)2D3, were examined. Injection of vitamin D replete mice with dexamethasone (dex) resulted in a significant induction in 24(OH)ase mRNA in kidney, indicating a regulatory effect of glucocorticoids on vitamin D metabolism. Whether glucocorticoids can affect 24(OH)ase transcription is not known. Here we demonstrate for the first time a glucocorticoid receptor (GR) dependent enhancement of 1,25(OH)2D3‐induced 24(OH)ase transcription. Dex treatment of GR and vitamin D receptor (VDR) transfected COS‐7 cells and dex treatment of osteoblastic cells (in which VDR and GR are present endogenously) potentiated 1,25(OH)2D3‐induced 24(OH)ase transcription. In addition, GR was found to cooperate with C/EBPβ to enhance VDR‐mediated 24(OH)ase transcription. Using the rat 24(OH)ase promoter with the C/EBP site mutated, GR‐mediated potentiation of 1,25(OH)2D3‐induced 24(OH)ase transcription was inhibited. Immunoprecipitation indicated that that GR can interact with C/EBPβ and ChIP/re‐ChIP analysis showed that C/EBPβ and GR bind simultaneously to the 24(OH)ase promoter. These findings indicate a novel mechanism whereby glucocorticoids can alter VDR‐mediated 24(OH)ase transcription through functional cooperation between C/EBPβ and GR that results in an enhanced ability of C/EBPβ to cooperate with VDR in the regulation of 24(OH)ase. J. Cell. Biochem. 110: 1314–1323, 2010. © 2010 Wiley‐Liss, Inc.