Endogenous blockade of 1,25-dihydroxyvitamin D-receptor binding in New World primate cells

Abstract

When assessed by 1,25-dihydroxyvitamin D3 (1,25(OH)2-D3)-receptor (VDR) binding analysis or 1,25(OH)2-D3-VDR-directed bioresponsiveness, cultured cells from some New World primates (platyrrhines) demonstrate a variable decrement in VDR when compared with Old World primate (catarrhine) cells. To study this difference in VDR expression among primates, we performed immunoblot analysis of the VDR in cultured dermal fibroblasts from platyrrhines in the genera Pithecia and Aotus and from catarrhines in the genus Presbytis; although a platyrrhine, the owl monkey (Aotus) expresses a VDR of the catarrhine (wild type) phenotype. Despite a 10-fold difference in the content of VDR by ligand binding analysis among cells from the three prototypic primate genera, there was a less than or equal to 10% difference in the steady-state level of 50-kD VDR detected by immunoblot analysis of cellular extracts. We investigated this apparent discrepancy in the content of VDR in immunoblots and ligand binding analyses by mixing VDR-containing nuclear extracts of equivalent protein concentration from the various primates. Coincubation of Pithecia and Aotus fibroblast extracts with Presbytis extract diminished specific 1,25(OH)2-D3 binding in the mix by 90% and 95% respectively. Similar results were obtained by mixing nuclear extracts of the owl monkey cell line, OMK, and the vitamin D resistant marmoset B-lymphoblast cell line B95-8. A wild type 1,25(OH)2-D3-binding profile was restored in mixtures after trypsin or heat treatment of the B95-8 extract. These data indicate that some New World primate cells contain a soluble protein that prevents intracellular 1,25(OH)2-D3-VDR binding. It is possible that the quantitative differences in the expression of this protein are responsible for 1,25(OH)2-D3 and other steroid hormone resistant states of variable severity in New World primates.