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Enhanced Coactivator Binding and Transcriptional Activation of Mutant Vitamin D Receptors From Patients With Hereditary 1,25‐Dihydroxyvitamin D‐Resistant Rickets by Phosphorylation and Vitamin D Analogs
Author(s) -
Liu Yan,
Shen Qi,
Malloy Peter J,
Soliman Emad,
Peng Xiaorong,
Kim Sungtae,
Pike J Wesley,
Feldman David,
Christakos Sylvia
Publication year - 2005
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1359/jbmr.050410
Subject(s) - calcitriol receptor , mutant , coactivator , chromatin immunoprecipitation , biology , microbiology and biotechnology , phosphatase , transcription factor , phosphorylation , biochemistry , receptor , gene expression , promoter , gene
In this study, we report that the function of certain mutant VDRs from patients with hereditary HVDRR can at least be partially restored by phosphorylation and hexafluoro 1,25(OH) 2 D 3 analogs. Our study provides new insights into mechanisms involved in enhancement of mutant VDR function. Introduction : 1,25‐Dihydroxyvitamin D‐resistant rickets (HVDRR) is a rare genetic disorder caused by inactivating mutations in the vitamin D receptor (VDR). In this study, we examined VDR from patients with HVDRR having mutations in the ligand‐binding domain (F251C, I268T, H305Q, E420K). We examined methods of restoring transcriptional activity of these mutants and the mechanisms involved. Materials and Methods : Reporter gene transcriptional assays were used to examine the activation of mutant VDRs. Western‐blot analysis, glutathione S ‐transferase (GST) pull‐down assays, and chromatin immunoprecipitation (ChIP) assays were also used in this study. Results : Using mutant VDRs, H305Q, F251C, I268T, and 10 −8 M 1,25(OH) 2 D 3 , only 10–30% of the activity of wildtype (WT) VDR in activating 24(OH)ase transcription was observed. The transcriptional response of mutant VDR mutants was significantly enhanced 2‐ to 3‐fold by co‐treatment of VDR mutant transfected COS‐7 cells with 1,25(OH) 2 D 3 and okadaic acid (OA; inhibitor of phosphatase; 50 nM). The H305Q mutant was the most responsive (90% of the response exhibited by WT VDR was restored). The E420K mutant was unresponsive to 1,25(OH) 2 D 3 in the presence or absence of OA. The increased transcriptional response correlated with an increase in the interaction between DRIP205 and the mutant VDR. We further provide evidence that OA induces the phosphorylation of CREB‐binding protein (CBP), indicating for the first time a correlation between phosphorylation of CBP and enhanced VDR function. Hexafluoro 1,25(OH) 2 D 3 analogs (RO‐26‐2198 and RO‐4383561) also resulted in at least a partial restoration of the transcriptional responsiveness of mutant VDRs I268T, F251C, and H305Q. Our data indicate that the enhanced potency of the hexafluoro analogs may be caused by increased DRIP205 and glucocorticoid receptor interacting protein 1 (GRIP‐1) binding to VDRs and enhanced association of VDRs with DNA, as suggested by results of ChIP assays. Conclusion : Our study provides new insights into the mechanisms involved in the enhancement of VDR function by both phosphorylation and hexafluoro analogs and forms a basis for future study of vitamin D analogs or specifically designed kinase activity mediators as potential therapy for the treatment of selected patients with HVDRR.