Regulation of 1,25-Dihydroxyvitamin D3 Receptors by Vitamin D Analogs in Cultured Mammalian Cells*

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Abstract
The pig kidney cell line (LLC-PK1) has been shown to possess 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors and to exhibit functional responses to vitamin D metabolites. Here we report that these receptors appear to undergo homologous up-regulation by 1,25-(OH)2D3 and other vitamin D analogs. This phenomenon was also observed in other cell lines, including human skin fibroblasts and human mammary cancer cells (MCF-7). Treatment with active hormone or vitamin D analogs results in a substantial increase (200-400%) in the number of 1,25-(OH)2D3 receptors without altering the affinity of receptor for hormone. The up-regulated receptor, like the basal receptor, has an apparent Kd of about 0.04 nM and sediments at 3.3S on hypertonic sucrose gradients. In addition, approximately 50% of the total receptors from both control and treated cells bind to DNA-cellulose and elute at 0.18 M KCl. These results indicate that the up-regulated receptor is similar to the classical 1,25-(OH)2D3 receptor. While the time necessary to achieve the maximal receptor increment is 16-20 h, there is a rapid component in the rise observed within 5 min. The maximal effect persists for 4-6 h after hormone removal. The increased binding is not a result of differential receptor localization of extractability. 1,25-(OH)2D3, 1,24,25-trihydroxyvitamin D3,24,25-(OH)2D3 and 25-hydroxyvitamin D3 all increase receptor binding to similar levels, and the dose required closely reflects the affinities of the various metabolites for the receptor. Treatment of cells with the RNA synthesis inhibitor actinomycin D indicates that the increase in receptors is partially dependent on RNA synthesis. Mutant skin fibroblasts from patients with vitamin D-dependent rickets type II, containing nonresponsive 1,25-(OH)2D3 receptors, failed to exhibit the characteristic up-regulation observed in normal cells. Taken together, these results indicate that vitamin D metabolites regulate the number of 1,25-(OH)2D3 receptors in part by receptor occupancy and, more importantly, by a receptor-mediated induction mechanism.