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Division of Endocrinology, Diabetes and Metabolism, Burns and Allen Research Institute, Cedars-Sinai Medical Center, Room D-3088, 8700 Beverly Boulevard, Los Angeles, California 90048, USA

(Requests for offprints should be addressed to M Hewison; Email: martin.hewison{at}cshs.org)

The constitutively expressed member of the heat shock protein-70 family (hsc70) is a chaperone with multiple functions in cellular homeostasis. Previously, we demonstrated the ability of hsc70 to bind 25-hydroxyvitamin D₃ (25-OHD₃) and 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃). Hsc70 also recruits and interacts with the co-chaperone Bcl2-associated athanogene (BAG)-1 via the ATP-binding domain that resides on hsc70. Competitive ligand-binding assays showed that, like hsc70, recombinant BAG-1 is able to bind 25-OHD₃ (K_d=0.71±0.25 nM, B_max=69.9±16.1 fmoles/µg protein) and 1,25(OH)₂D₃ (K_d=0.16±0.07 nM, B_max=38.1±3.5 fmoles/µg protein; both n=3 separate binding assays, P<0.001 for K_d and B_max). To investigate the functional significance of this, we transiently overexpressed the S, M, and L variants of BAG-1 into human kidney HKC-8 cells stably transfected with a 1,25(OH)₂D₃-responsive 24-hydroxylase (CYP24) promoter–reporter construct. As HKC-8 cells also express the enzyme 1-hydroxylase, both 25-OHD₃ (200 nM) and 1,25(OH)₂D₃ (5 nM) were able to induce CYP24 promoter activity. This was further enhanced following overexpression of all the three BAG-1 isoforms. By contrast, BAG-1 isoforms had no effect on metabolism of 25-OHD₃ by HKC-8 cells (either via 1- or 24-hydroxylase activities). Further studies showed that a mutant form of BAG-1S exhibited decreased binding of 1,25(OH)₂D₃ and this resulted in a concomitant loss of potentiation of CYP24 promoter transactivation. Similar effects were not observed for 25-OHD₃. These data highlight a novel role for BAG-1 as an intracellular-binding protein for 1,25(OH)₂D₃ and further suggest that BAG-1 is able to potentiate vitamin D receptor-mediated transactivation by acting as a nuclear chaperone for 1,25(OH)₂D₃.

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				R. F Chun, J. S Adams, and M. Hewison Back to the future: a new look at 'old' vitamin D J. Endocrinol., August 1, 2008; 198(2): 261 - 269. [Abstract] [Full Text] [PDF]