Inhibition of vitamin D receptor-retinoid X receptor-vitamin D response element complex formation by nuclear extracts of vitamin D-resistant New World primate cells.

Abstract

Most New World primate (NWP) genera evolved to require high circulating levels of steroid hormones and vitamin D. We hypothesized that an intracellular vitamin D binding protein @BP), present in both nuclear and cytoplasmic fractions of NWP cells, or another protein(s) may cause or contribute to the steroid hormone-resistant state in NWP by disruption of the receptor dime&&ion process and/or by inmrtbrence of receptor complex binding to the consensus response elements present in the enhancer regions of steroid-responsive genes. We employed electromobility shift assay (EMSA) to screen for the presence of proteins capable of binding to the vitamin D response element (VDRB). Nuclear and post-nuclear extracta were prepared from two B-lymphoblastoid cell lines known to be. qresentative of the vitamin Dresistant and wild type phenotypes, respectively. The extracts were compared for their ability to retard the migration of radiolabeled double stranded oligomers representative of the VDREs of the human osteocalcin and the mouse osteopontin gene promoters. A specific, mtarded band containing VDR-RXR was identified when wild type cell but not when vitamin D-resistant cell nuclear extract was used in the binding reaction with either probe. In addition, vitamin D-resistant cell nuclear extract contained a protein(s) which was bound specifically to the VDRE and was capable of completely inhibiting VDR-RXR-VDRE complex formation, these effects were not demonstrated with nuclear extract t?om the wild type cell line or with the post-nuclear extract of the vitamin D resistant cell line. We conclude that a VDRE-binding protein(s), distinct from JDBP and present in nuclear extract of cells from a prototypical vitamin D-resistant NWP, is capable of inhibiting normal VDR-RXR heterodimer binding to the VDRE

Back to the future: a new look at ‘old’ vitamin D

“…However, as already described, subsequent studies showed that the opposite was true in that IDBPs such as hsc70 were potent enhancers of VDRmediated signaling (Wu et al 2000). An alternative mechanism for vitamin D resistance in NWPs was proposed based on the experiments showing nuclear extracts from NWP cells contained a soluble factor that was able to compete with the VDR-retinoid X receptor (RXR) complex for binding to VDRE during 1,25(OH) 2 D-VDR-directed gene regulation (Arbelle et al 1996). This factor was subsequently referred to as the VDRE-BP.…”

Functional Characterization and Purification of an Intracellular Vitamin D-binding Protein in Vitamin D-resistant New World Primate Cells

“…Levels of 25-OHD may be up to 10-fold and 1,25-(OH) 2 D up to 100-fold greater than those observed in Old World primates (OWP), including man. Unlike the majority of resistant states described for other steroid hormones and vitamin D in Homo sapiens (5,6), resistance in NWP does not appear to be related to a mutation in the vitamin D receptor (VDR) protein (1,7). Rather, the vitamin D-resistant state in NWP is associated with the apparent overexpression of an intracellular vitamin D-binding protein (IDBP) (1), which is distinct from members of the serum vitamin D-binding protein DBP/albumin families of proteins (8,9).…”

Functional Characterization of Heterogeneous Nuclear Ribonuclear Protein C1/C2 in Vitamin D Resistance

“…Fifth are receptor-associated coactivators and co-repressors that influence recruitment of other elements of the transcriptional machinery to the promoter (11,12). And sixth are the "co-modulator" Cis-acting proteins of the heterogeneous nuclear ribonucleoprotein (hnRNP) family that compete with the VDR-RXR for binding to the vitamin D response element (VDRE), thus altering hormone receptor-directed transactivation (13)(14)(15).…”