Specific Inter realization and Action of 1,25-Dihydroxyvit; min D₃in Cultured Dermal Fibroblasts from Patients with X-Linked Hypophosphatemia*

An abnormality in the interaction of 1,25-dihydroxyvitamin D [1,25-(OH)2D] with its specific intracellular receptor has been postulated to explain the resistance of patients with X-linked hypophosphatemia (XLH) to treatment with vitamin D. To investigate this possibility, the specific internalization of [3H]1,25-(OH)2D3 by suspensions of cultured dermal fibroblasts from six patients with XLH was evaluated. Scatchard analysis of the specific uptake of [3H]1,25-(OH)2D3 by intact XLH fibroblasts revealed a mean internalization constant (Kin) of 0.72 +/- 0.10 nM (mean +/- SE) and a cellular capacity for hormone (Nmax) of 61.5 +/- 5.4 fmol/mg protein. The Kin and Nmax for XLH cells were similar to the mean values obtained for normal fibroblasts (0.59 nM and 72 fmol/mg protein, respectively) and cells from a patient with acquired hypophosphatemia (0.67 nM and 73 fmol/mg protein, respectively). Conversely, cultured fibroblasts from a patient with vitamin D-dependent rickets type II demonstrated a marked reduction of sterol uptake when compared to that in cells from either a hypophosphatemic or normal host. Neither the affinity nor the capacity of XLH fibroblasts for [3H]1,25-(OH)2D3 was influenced by exposure of proliferating or confluent monolayer cultures to a low phosphate concentration (2.0 mg/dl) in the extracellular medium before or during assay. In addition, the normal responsiveness of cultured fibroblasts from three patients with XLH to 1,25-(OH)2D3 was confirmed by the provocative induction of the cellular 25-hydroxyvitamin D3-24-hydroxylase. These data suggest that 1,25-(OH)2-D3-receptor binding as well as postreceptor events are normal in patients with XLH.