Monoclonal antibodies to the porcine intestinal receptor for 1,25-dihydroxyvitamin D3: interaction with distinct receptor domains.

Monoclonal antibodies to different domains of the porcine intestinal 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor have been produced. A nuclear extract enriched in the 1,25-(OH)2D3 receptor was prepared from small intestinal mucosa of young pigs. The receptor was purified an additional 6600-fold by chromatography on DNA-cellulose, ammonium sulfate precipitation, gel filtration high-performance liquid chromatography, and DEAE-Sepharose chromatography, with an overall yield of 23% and an average purity of 24%. A BALB/c mouse immunized with this material developed serum polyclonal antibodies to the 1,25-(OH)2D3 receptor, as demonstrated by a change in sedimentation of the porcine receptor on sucrose gradients. Spleen cells from this animal were fused with mouse myeloma cells (P3-NSI/1-Ag4-1, SP2/0-Ag14), and 24 hybridomas secreting antibodies to the 1,25-(OH)2D3 receptor were identified by both a radiometric immunosorbent assay and an immunoprecipitation assay. Twenty-one hybridoma lines were cloned by limiting dilution and further characterized as subclass IgG1 antibodies with the exception of one which is an IgA. All but two of the antibodies cross-react with the 1,25-(OH)2D3 receptor from both mammalian (human, monkey, and rat) and avian (chicken) intestine; two antibodies recognize only porcine intestinal receptor. All antibodies are unreactive to the vitamin D serum transport protein. Eight of the antibodies bind denatured receptor on an immunoblot. A solid-phase competition assay was used to identify four groups of antibodies that bind to distinct epitopes on the 1,25-(OH)2D3 receptor. One antibody from each of the four groups was used to examine the effect of antibody binding on the DNA-binding activity of the receptor-hormone complex. One antibody completely inhibited the binding of the 1,25-(OH)2D3 receptor complex to DNA-cellulose, suggesting that the epitope for this antibody may be located in the polynucleotide binding domain of the protein. Antibodies from two additional groups only slightly perturbed DNA binding, while one had no effect, suggesting that these antibodies bind to receptor epitopes distant from the region of the polypeptide directly involved in polynucleotide binding. These antibodies that are directed to several different binding sites on the 1,25-(OH)2D3 receptor provide important new tools to probe the biochemistry and topology of the 1,25-(OH)2D3 receptor and to investigate its role in mediating target tissue response to hormone.