Interaction between 1,25-dihydroxyvitamin D3 receptors and intestinal nuclei. Binding to nuclear constituents in vitro

The molecular action of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is thought to involve its localization within the nucleus of target cells, a process mediated by intracellular receptors. This report probes both the association between chick intestinal 1,25(OH)2D3 receptors and purified homologous nuclei and the interaction between this receptor and nucleic acids. 1,25(OH)2D3 receptors bound to purified nuclei in a apparently saturable manner (Kd = 2.2-4.8 X 10(-10) M) under conditions of intermediate ionic strength and constant protein concentration. Nuclear binding was hormone-dependent; whereas receptor-hormone complex (Rs) binds to nuclei under the ionic conditions employed here (greater than 70%), hormone-free (R0) receptors do not bind (less than 10%). Binding was localized to the nuclear chromatin fraction and was extremely sensitive to KCl concentration both in the incubation medium and during postincubation treatment of nuclei. The interaction appeared to be temperature-independent, suggesting the lack of a classic activation event characteristic of most steroid receptors. Partial digestion of intestinal nuclei with DNase I eliminated subsequent receptor binding by greater than 95%, pointing to the involvement of DNA in the binding interaction. In turn, receptors were found to bind to both DNA and RNA, a characteristic independent of receptor aggregation, but sensitive to disruption with increasing ionic strength buffers. Elution of both Rs and R0 from DNA appeared identical (0.28 M KCl), whereas the strength of interaction with RNA was much less (0.12 M KCl). Thus, while there appeared to be a fundamental difference between R0 and Rs, such that only the binding of receptor-hormone complex to nuclei was allowed under the conditions employed here, this characteristic was not observed during DNA binding. Nevertheless, the possibility exists that the in vivo interaction between 1,25(OH)2D3 receptor and nuclei involves DNA and that this nuclear constituent may be the ultimate site of action of this unique sterol hormone.