Growth Hormone Normalizes Renal 1,25‐Dihydroxyvitamin D3‐24‐Hydroxylase Gene Expression but Not Na+‐Phosphate Cotransporter (Npt2) mRNA in Phosphate‐Deprived Hyp Mice

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Abstract
The murine X-linked Hyp mutation is characterized by decreased renal expression of type II Na+-phosphate (Pi) cotransporter (Npt2) mRNA and an abnormal vitamin D response to Pi deprivation. The latter is manifest by an aberrant fall in serum 1,25-dihydroxyvitamin D3 (1,25(OH)2D) levels that is associated with an increase in renal 1,25(OH)2D-24-hydroxylase (24-hydroxylase), the first enzyme in the C-24 oxidation pathway. Because growth hormone (GH) enhances renal Na+-Pi cotransport and permits the adaptive 1,25(OH)2D response in Pi-deprived hypophysectomized rats, we examined the effects of GH on vitamin D metabolism and renal Npt2 mRNA abundance in Hyp mice fed control and low Pi diets. GH significantly decreased renal 24-hydroxylase activity (0.202+/-0.020 to 0.098+/-0.008 pmol/mg of protein/minute, p < 0.05) and mRNA abundance, relative to beta-actin mRNA (299+/-13 to 78+/-14, p < 0.05), in Hyp mice fed the low Pi diet but had no effect on either parameter in mutants fed the control diet. Moreover, after GH treatment, renal 24-hydroxylase gene expression was no longer elevated in Pi-deprived Hyp mice relative to mutants fed control diet. In contrast, GH did not correct the serum concentration of 1,25(OH)2D in Pi-deprived Hyp mice. We also demonstrate that GH did not normalize renal Npt2 mRNA expression, relative to beta-actin mRNA, in Hyp mice fed either control or low Pi diets. The present data demonstrate that normalization of renal 24-hydroxylase gene expression in Pi-deprived Hyp mice by GH is not sufficient to correct the serum concentration of 1,25(OH)2D and is not associated with an alteration in renal Npt2 mRNA expression.

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