Normal Vitamin D Receptor Concentration and Responsiveness to 1, 25-dihydroxyvitamin D3 in Skin Fibroblasts from Patients with Absorptive Hypercalciuria

To evaluate whether there is an increase in vitamin D receptor (VDR) concentration which could raise intestinal calcium absorption in absorptive hypercalciuric (AH) patients and promote hypercalciuria, we measured VDR concentration and VDR mRNA levels in skin fibroblasts from 16 patients with AH and 17 age-matched normal subjects before and following a 16-hour incubation in the presence of 10(-8) M 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. There were no significant differences in VDR concentration between normal subjects and AH patients in the basal state (30 +/- 11 vs. 30 +/- 15 ng/mg protein, respectively) or following 1,25(OH)2D3-mediated upregulation (43 +/- 18 vs. 42 +/- 16 ng/mg protein) as measured by immunoblot methodology. Analysis of VDR mRNA/beta-actin mRNA ratios demonstrated no significant differences between normal subjects and AH patients prior to (2.1 +/- 1.7 vs. 1.8 +/- 2.4) or following (2.7 +/- 2.8 vs. 1.9 +/- 1.8) 1,25(OH)2D3 exposure. As a measure of VDR bioactivity, we quantitated 1,25(OH)2D3-mediated induction of 25-hydroxyvitamin D3-24-hydroxylase. Again, no significant differences were observed between normal subjects and all patients (2.1 +/- 1.6 vs. 1.9 +/- 1.6 pmol/mg/30 min, respectively). These findings indicate that there is neither an increase in VDR concentration in skin fibroblasts, a recognized vitamin D responsive cell, nor increased sensitivity to upregulation of VDR numbers by 1, 25(OH)2D3 in patients with AH. This suggests an alternative cause of intestinal hyperabsorption of calcium in AH other than alteration of the VDR number.