Intracellular vitamin D binding proteins: Novel facilitators of vitamin D-directed transactivation

Previously recognized intracellular proteins with an affinity for vitamin D metabolites include the vitamin D receptor and the cytochrome P-450-based vitamin D metabolizing mixed-function oxidases. We recently characterized a third set of high-capacity, intracellular vitamin D binding proteins (IDBP s) in the inducible heat shock protein-70 (hsp-70) family. Here we report t he cloning and expression of cDNAs coding for two IDBPs. The full-length cD NAs for IDBP-1 and IDBP-P demonstrated 95% and 94% nucleotide homology, res pectively, with the cDNAs for human constitutively expressed heat shock pro tein 70 (hsc-70) and hsp-70. Transient expression of the IDBP cDNAs in a vi tamin D-responsive primate cell line increased extractable 25-hydroxylated vitamin D metaboliteIDBP-binding 25-fold. Transfection experiments also dem onstrated that the majority of the constitutively expressed 25-hydroxylated vitamin D metabolite binding activity was attributable to expression of th e hsc-70-related IDBP-1 and that metabolite binding activity sublocalized t o the highly conserved ATP-binding/ATPase domain of hsp708. Stable overexpr ession of IDBP-1 in wild-type cells enhanced vitamin D-directed responsiven ess of endogenous vitamin D-24-hydroxylase, osteopontin, and osteocalcin ge nes by several-fold over that observed in cells transfected with an empty v ector. These results suggest that IDBP-1 facilitates the intracellular loca lization of active vitamin D metabolites and vitamin D receptor-mediated tr ansactivation.