Abnormal Expression and Regulation of Vitamin D Receptor in Experimental Uremia

Abstract

The low concentration of the biologically active metabolite of vitamin D, namely lα,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃), is critical to the pathogenesis of secondary hyperparathyroidism in chronic renal failure. The actions of 1,25(OH)₂D₃ are mediated through binding to a cellular receptor protein, the vitamin D receptor (VDR). In order to further investigate expression and regulation of VDR in uremia, we measured specific [³H]-1,25(OH)₂D₃ binding capacity and VDR mRNA concentration in intestinal mucosa and in parathyroid glands of subtotally nephrectomized rats (Nx) and compared Nx to sham-operated rats with normal kidney function (Intact). Intestinal [³H]-1,25(OH)₂D₃ binding capacity in short-term Nx (6-10 days after nephrectomy) was 663 ± 114 fmol/mg protein; it was 517 ± 34 in Intact (p = 0.06, n = 6 experiments). Intestinal VDR mRNA concentration was comparable between Nx and Intact. Specific 1,25(OH)₂D₃ binding capacity in parathyroid glands was higher in Nx (195 ± 9 fmol/mg protein) than in Intact (116 ± 14 fmol/mg protein, n = 5, p < 0.05). The affinity of the VDR for l,25(OH)₂D₃ (KD) did not change in Nx. The 1,25(OH)₂D₃ binding capacity in intestinal mucosa of more long-term uremic animals (14-16 weeks after subtotal Nx) was 519 ± 32 fmol/mg protein versus 349 ± 31 in Intact (n = 3, p < 0.01). Parathyroid VDR was 171 ± 9 fmol/mg protein in long-term Nx and 125 ± 3 in Intact (p < 0.01). These results were confirmed when 1,25(OH)₂D₃ binding capacity in uremic rats with hereditary polycystic kidney disease was compared to control rats with normal kidney function (757 ± 54 fmol/mg protein versus 495 ± 59 in intestinal mucosa, p < 0.05; 273 ± 48 versus 104 ± 27 in parathyroid glands, p < 0.05). In parallel to changes in intestinal 1,25(OH)₂D₃ binding capacity, 1,25(OH)₂D₃-mediated stimulation of intestinal 25(OH)D₃-24-hydroxylase activity was significantly higher in long-term subtotally Nx (1.43 ± 0.06 pmol 24,25-dihydroxyvitamin D₃/mg protein) than in sham-operated normal rats (1.04 ± 0.10, p < 0.05). Administration of 1,25(OH)₂D₃ to sham-operated normal rats resulted in an increase of 1,25(OH)₂D₃ binding capacity by 20-40% in intestinal mucosa and by 40-50% in parathyroid glands. In contrast, 1,25(OH)₂D₃ caused down-regulation of mean 1,25(OH)₂D₃ binding capacity in short-term Nx by 38% in intestinal mucosa (p < 0.01) and by 43% in parathyroid glands (p < 0.01). In long-term Nx, mean 1,25(OH)₂D₃ binding capacity was reduced by 20% in intestinal mucosa (p < 0.05) and by 22% in parathyroid glands (p < 0.01). After prolonged exposure to 1,25(OH)₂D₃ for 6 weeks, intestinal 1,25(OH)₂D₃ bin...