Roy S, Tenenhouse H S
Medical Research Council Genetics Group, McGill University-Montreal Children's Hospital Research Institute, Montreal, Quebec, Canada.
Endocrinology. 1996 Jul;137(7):2938-46. doi: 10.1210/endo.137.7.8770917.
X-Linked hypophosphatemic (HYP) mice respond to low phosphate (Pi) intake with a fall in the serum concentration of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and an increase in the renal activity of 1,25-(OH)2D3-24-hydroxylase (24-hydroxylase), the first enzyme in the C-24 oxidation pathway that degrades 1,25-(OH)2D3 to its final inactivation product. In contrast, normal mice respond to a low Pi diet with an adaptive increase in serum 1,25-(OH)2D3 levels and no change in renal 24-hydroxylase. The low Pi response in Hyp mice involves a 3-fold increase in renal 24-hydroxylase maximum velocity and a corresponding increase in 24-hydroxylase immunoreactive protein and messenger RNA (mRNA). To determine the mechanism for the increase in 24-hydroxylase mRNA in the mutant strain, we examined the effect of actinomycin D on renal 24-hydroxylase mRNA abundance and measured renal 24-hydroxylase gene transcription by nuclear run-off assay in Hyp mice fed control and low Pi diets for 4 h. Vehicle and 1,25-(OH)2D3-treated normal mice were also studied 2-4 h post-treatment. Actinomycin D abrogated the increase in renal 24-hydroxylase mRNA elicited by a low Pi diet in Hyp mice and by 1,25-(OH)2D3 in both normal and Hyp mice. 24-Hydroxylase gene transcription, relative to that of glyceraldehyde-3-phosphate dehydrogenase, 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)2D3 administration to normal mice (n = 3; P < 0.01). In situ hybridization localized 24-hydroxylase transcripts to the proximal tubule of normal and mutant mice fed control and low Pi diets and showed that the 1,25(OH)2D3-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)2D3; and 2) the renal proximal tubule is the primary site of increased expression of 24-hydroxylase mRNA.
X连锁低磷血症（HYP）小鼠对低磷（Pi）摄入的反应是血清1,25 - 二羟基维生素D3 [1,25-(OH)2D3]浓度下降，以及1,25-(OH)2D3 - 24 - 羟化酶（24 - 羟化酶）的肾脏活性增加，24 - 羟化酶是C - 24氧化途径中的首个酶，可将1,25-(OH)2D3降解为其最终失活产物。相比之下，正常小鼠对低Pi饮食的反应是血清1,25-(OH)2D3水平适应性增加，而肾脏24 - 羟化酶无变化。HYP小鼠的低Pi反应涉及肾脏24 - 羟化酶最大速度增加3倍，以及24 - 羟化酶免疫反应性蛋白和信使核糖核酸（mRNA）相应增加。为了确定突变株中24 - 羟化酶mRNA增加的机制，我们研究了放线菌素D对肾脏24 - 羟化酶mRNA丰度的影响，并通过核转录分析测量了喂食对照和低Pi饮食4小时的HYP小鼠的肾脏24 - 羟化酶基因转录。还在治疗后2 - 4小时研究了给予载体和1,25-(OH)2D3的正常小鼠。放线菌素D消除了低Pi饮食在HYP小鼠中以及1,25-(OH)2D3在正常和HYP小鼠中引起的肾脏24 - 羟化酶mRNA增加。相对于甘油醛 - 3 - 磷酸脱氢酶，给HYP小鼠喂食低Pi饮食（n = 4；P < 0.05）使24 - 羟化酶基因转录增加2倍，给正常小鼠给予1,25-(OH)2D3（n = 3；P < 0.01）使转录增加5.4倍。原位杂交将24 - 羟化酶转录本定位到喂食对照和低Pi饮食的正常和突变小鼠的近端小管，并表明1,25(OH)2D3诱导的24 - 羟化酶mRNA增加发生在同一肾单位节段。本研究表明：1）转录激活可解释Pi缺乏的HYP小鼠中肾脏24 - 羟化酶mRNA的增加以及1,25-(OH)2D3对24 - 羟化酶mRNA的诱导；2）肾脏近端小管是24 - 羟化酶mRNA表达增加的主要部位。
