Variable phosphate-mediated regulation of vitamin D metabolism in the murine hypophosphatemic rachitic/osteomalacic disorders

The familial hypophosphatemic (vitamin D-resistant) disorders are a variety of genetic and acquired syndromes that exhibit unexpected biochemical heterogeneity, manifest as variably abnormal or apparently normal regulation of 1,25-dihydroxyvitamin D [1,25-(OH)2D] production. Recently, we observed that Hyp and Gy mice, murine homologs of X-linked hypophosphatemic rickets, exhibit similarly disparate regulation of vitamin D metabolism. While Gy mice under basal conditions maintain an appropriate elevation (relative to hypophosphatemia) of renal 25-hydroxyvitamin D (25OHD)-1 alpha-hydroxylase, Hyp mice manifest only normal, not increased, enzyme activity. Whether such diversity results from maintenance of phosphate (P)-regulated 1,25-(OH)2D production in Gy mice and loss of this function in Hyp mice or from other variations remains unknown. Therefore, we examined the integrity of P-regulated enzyme activity in the Gy and Hyp mice by testing the effects of enzyme inhibition and alteration of the serum phosphorus concentration on 1,25-(OH)2D production. Initially, we discovered that inhibition of renal 25OHD-1 alpha-hydroxylase suppressed enzyme function in Hyp mice, but did not prevent expression of P-mediated activity in Gy mice. In this regard, while administration of a high calcium diet or 1,25-(OH)2D (0.4 ng/h, sc, for 48 h) resulted in a comparable inhibition of enzyme activity in Hyp (5.9 +/- 0.5 vs. 2.8 +/- 0.7 fmol/mg.min) and normal mice (4.4 +/- 0.6 vs. 2.0 +/- 0.2 fmol/mg.min), similar treatment did not effect complete inhibition of 25OHD-1 alpha-hydroxylase in Gy (10.3 +/- 0.6 vs. 4.9 +/- 0.3 fmol/mg.min) or P-depleted mice (10.2 +/- 0.5 vs. 5.1 +/- 0.4 fmol/mg.min). In accord with the apparent persistence of P-mediated stimulation of enzyme function in Gy mice, dietary P repletion in this mutant resulted in a serum phosphorus concentration similar to that of normal mice and decreased enzyme activity (4.0 +/- 0.8 fmol/mg.min) to a level no different from that expressed in controls (3.4 +/- 0.3 fmol/mg.min). However, in the absence of apparent P-mediated stimulation of enzyme activity, identical treatment of Hyp mice increased the serum phosphorus level comparably, but paradoxically enhanced 25OHD-1 alpha-hydroxylase (3.1 +/- 0.4 vs. 11.7 +/- 2.0 fmol/mg.min). Collectively, these data indicate that enhanced renal 25OHD-1 alpha-hydroxylase expressed in Gy mice and probably in related human diseases results from normally maintained P regulation of enzyme activity, an action absent or mutated in the genetically distinct Hyp mouse.