Dietary and Serum Phosphorus Regulate 25-hydroxyvitamin D-1α- and 24-hydroxylase Gene Expression in Mouse Kidney

Abstract 1979 Nephrology Platform, Sunday, 5/2

In healthy humans and experimental animals, restriction of dietary phosphorous (Pi) predictably will induce a decrease in serum concentration of Pi and an increase in the renal production and serum concentration of 1,25(OH)₂D. By contrast, in X-linked hypophosphatemic rickets, a disease attributable to loss of Phex function and in which serum Pi is reduced, renal production and serum 1,25(OH)₂D are inappropriately low and do not increase in response to Pi restriction. To elucidate the mechanisms by which dietary and serum Pi regulate serum 1,25(OH)₂D, we examined the renal expression of the enzymes involved in the synthesis (1α-hydroxylase) and catabolism (24-hydroxylase) of 1,25(OH)₂D in normal mice, in mice homozygous for the disrupted renal Na⁺-Pi cotransporter gene Npt2 (Npt2-/-), and in X-linked hypophosphatemic (Hyp) mice, the murine homologue of the human disease. Riboprobes were prepared for each hydroxylase and mRNA abundance, relative to that of β-actin, was determined by ribonuclease production assay. In normal mice in which dietary Pi was restricted for 4 days, 1α-hydroxylase enzyme activity, measured in renal mitochondria, was 5-fold higher due to an increase in its V_max, compared to that in normal mice fed a control diet. Renal abundance of 1α-hydroxylase mRNA was 7-fold higher and 24-hydroxylase mRNA was 70% lower, compared to abundance in normal mice fed a control diet. Renal 1α-hydroxylase enzyme activity and mRNA expression were highly correlated. Similarly, in Npt2-/- mice in which hypophosphatemia was secondary to impaired renal reabsorption of Pi, abundance of 1α-hydroxylase mRNA was 3-fold higher and 24-hydroxylase mRNA 3-fold lower compared to abundance in wildtype (Npt2+/+) mice. By contrast, when dietary Pi was restricted in Hyp mice, 1α-hydroxylase mRNA decreased by 50% and 24-hydroxylase mRNA increased by ¼ relative to abundance in Hyp mice fed a control diet. These data demonstrate that: a) dietary and serum Pi are important determinants of renal 1α- and 24-hydroxylase mRNA expression, and that such expression is regulated in a reciprocal manner; b) functional Npt2 protein is neither necessary nor sufficient to mediate changes in 1α- and 24-hydroxylase mRNA expression induced by Pi restriction; and, c) due to loss of Phex function in Hyp mice, 1α- and 24-hydroxylase genes are regulated by Pi in a paradoxical manner, opposite to that in normal mice.