Measurement of 1,25-Dihydroxyvitamin D3 Receptor Turnover by Dense Amino Acid Labeling: Changes during Receptor Up-Regulation by Vitamin D Metabolites*

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Abstract
We have previously demonstrated that 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors are up-regulated by exposure to 24,25-(OH)2D3 and other vitamin D metabolites in several mammalian cell culture systems as well as in vivo in rats. The goal of the current study was to determine how changes in receptor turnover could give rise to increased receptor levels after treatment with 24,25-(OH)2D3, whether by an increased rate of synthesis or a decreased rate of degradation. Receptor turnover was studied by the dense amino acid shift technique in cultured pig kidney cells (LLC-PK1). In control cells, the half-life of the 1,25-(OH)2D3 receptor was estimated to be 4.3 .+-. 0.4 h, with a degradation rate constant (kD) of 0.16 .+-. 0.01 h-1. In parallel experiments, cells were treated with 24,25(OH)2D3 for about 48 h to a reach a new up-regulated state before the measurement of receptor turnover. In up-regulated cells, the receptor half-life was prolonged to 8.9 .+-. 1.0 h and the kD was 0.07 .+-. 0.01 h-1. Assuming a steady state, the values obtained for total receptor levels and the degradation rate allow calculation of the synthesis rate (kS) at equilibrium. Control cells synthesized receptors at the rate of 3.4 .+-. 0.4 vs. up-regulated cells at 4.9 .+-. 0.5 fmol mg protein-1 h-1. In conclusion, the data indicate that up-regulation of receptors results predominantly from a prolongation of receptor half-life in addition to a relatively smaller increase in the rate of receptor synthesis.