TRANSCRIPTIONAL REGULATION AND RENAL LOCALIZATION OF 1,25-DIHYDROXYVITAMIN D-3-24-HYDROXYLASE GENE-EXPRESSION - EFFECTS OF THE HYP MUTATIONAND 1,25-DIHYDROXYVITAMIN D-3

X-Linked hypophosphatemic (Hyp) mice respond to low phosphate (Pi) int ake with a fall in the serum concentration of 1,25-dihydroxyvitamin D- 3 [1,25-(OH)(2)D-3] and an increase in the renal activity of 1,25-(OH) (2)D-3-24-hydroxylase (24-hydroxylase), the first enzyme in the C-24 o xidation pathway that degrades 1,25-(OH)(2)D-3 to its final inactivati on product. In contrast, normal mice respond to a low Pi diet with an adaptive increase in serum 1,25-(OH)(2)D-3 levels and no change in ren al 24-hydroxylase. The low Pi response in Hyp mice involves a 3-fold i ncrease in renal 24-hydroxylase maximum velocity and a corresponding i ncrease in 24-hydroxylase immunoreactive protein and messenger RNA (mR NA). To determine the mechanism for the increase in 24-hydroxylase mRN A in the mutant strain, we examined the effect of actinomycin D on ren al 24-hydroxylase mRNA abundance and measured renal 24-hydroxylase gen e transcription by nuclear run-off assay in Hyp mice fed control and l ow Pi diets for 4 h. Vehicle and 1,25-(OH)(2)D-3-treated normal mice w ere also studied 2-4 h posttreatment. Actinomycin D abrogated the incr ease in renal 24-hydroxylase mRNA elicited by a low Pi diet in Hyp mic e and by 1,25-(OH)(2)D-3 in both normal and Hyp mice. 24-Hydroxylase g ene transcription, relative to that of glyceraldehyde-3-phosphate dehy drogenase, was increased 2-fold by feeding the low Pi diet to Hyp mice (n = 4; P < 0.05) and 5.4-fold by 1,25-(OH)(2)D-3 administration to n ormal mice (n = 3; P < 0.01). In situ hybridization localized 24-hydro xylase transcripts to the proximal tubule of normal and mutant mice fe d control and low Pi diets and showed that the 1,25-(OH)(2)D-3-induced increase in 24-hydroxylase mRNA occurred in the same nephron segment. The present study demonstrates that 1) transcriptional activation can account for the increase in renal 24-hydroxylase mRNA in Pi-deprived Hyp mice and for 24-hydroxylase mRNA induction by 1,25-(OH)(2)D-3; and 2) the renal proximal tubule is the primary site of increased express ion of 24-hydroxylase mRNA.