Physiological regulation of phosphate by vitamin D, parathyroid hormone (PTH) and phosphate (Pi)

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Abstract

Inorganic phosphate (Pi) is an abundant element in the body and is essential for a wide variety of key biological processes. It plays an essential role in cellular energy metabolism and cell signalling, e.g. adenosine and guanosine triphosphates (ATP, GTP), and in the composition of phospholipid membranes and bone, and is an integral part of DNA and RNA. It is an important buffer in blood and urine and contributes to normal acid-base balance. Given its widespread role in almost every molecular and cellular function, changes in serum Pi levels and balance can have important and untoward effects. Pi homoeostasis is maintained by a counterbalance between dietary Pi absorption by the gut, mobilisation from bone and renal excretion. Approximately 85% of total body Pi is present in bone and only 1% is present as free Pi in extracellular fluids. In humans, extracellular concentrations of inorganic Pi vary between 0.8 and 1.2 mM, and in plasma or serum Pi exists in both its monovalent and divalent forms (H2PO4 − and HPO4 2−). In the intestine, approximately 30% of Pi absorption is vitamin D regulated and dependent. To help maintain Pi balance, reabsorption of filtered Pi along the renal proximal tubule (PT) is via the NaPi-IIa and NaPi-IIc Na+-coupled Pi cotransporters, with a smaller contribution from the PiT-2 transporters. Endocrine factors, including, vitamin D and parathyroid hormone (PTH), as well as newer factors such as fibroblast growth factor (FGF)-23 and its coreceptor α-klotho, are intimately involved in the control of Pi homeostasis. A tight regulation of Pi is critical, since hyperphosphataemia is associated with increased cardiovascular morbidity in chronic kidney disease (CKD) and hypophosphataemia with rickets and growth retardation. This short review considers the control of Pi balance by vitamin D, PTH and Pi itself, with an emphasis on the insights gained from human genetic disorders and genetically modified mouse models.

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Mutation of the mouse klotho gene leads to a syndrome resembling ageing.

M Kuro-O, Y Matsumura, H Aizawa … (1997)

A new gene, termed klotho, has been identified that is involved in the suppression of several ageing phenotypes. A defect in klotho gene expression in the mouse results in a syndrome that resembles human ageing, including a short lifespan, infertility, arteriosclerosis, skin atrophy, osteoporosis and emphysema. The gene encodes a membrane protein that shares sequence similarity with the beta-glucosidase enzymes. The klotho gene product may function as part of a signalling pathway that regulates ageing in vivo and morbidity in age-related diseases.

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Fibroblast growth factor 23 is elevated before parathyroid hormone and phosphate in chronic kidney disease.

Tamara Isakova, Patricia Wahl, Gabriela S. Vargas … (2011)

Fibroblast growth factor 23 (FGF23) regulates phosphorus metabolism and is a strong predictor of mortality in dialysis patients. FGF23 is thought to be an early biomarker of disordered phosphorus metabolism in the initial stages of chronic kidney disease (CKD). We measured FGF23 in baseline samples from 3879 patients in the Chronic Renal Insufficiency Cohort study, which is a diverse cohort of patients with CKD stage 2-4. Mean serum phosphate and median parathyroid hormone (PTH) levels were in the normal range, but median FGF23 was markedly greater than in healthy populations, and increased significantly with decreasing estimated glomerular filtration rate (eGFR). High levels of FGF23, defined as being above 100 RU/ml, were more common than secondary hyperparathyroidism and hyperphosphatemia in all strata of eGFR. The threshold of eGFR at which the slope of FGF23 increased was significantly higher than the corresponding threshold for PTH based on non-overlapping 95% confidence intervals. Thus, increased FGF23 is a common manifestation of CKD that develops earlier than increased phosphate or PTH. Hence, FGF23 measurements may be a sensitive early biomarker of disordered phosphorus metabolism in patients with CKD and normal serum phosphate levels.

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Vitamin D metabolism, mechanism of action, and clinical applications.

Daniel Bikle (2014)

Vitamin D3 is made in the skin from 7-dehydrocholesterol under the influence of UV light. Vitamin D2 (ergocalciferol) is derived from the plant sterol ergosterol. Vitamin D is metabolized first to 25 hydroxyvitamin D (25OHD), then to the hormonal form 1,25-dihydroxyvitamin D (1,25(OH)2D). CYP2R1 is the most important 25-hydroxylase; CYP27B1 is the key 1-hydroxylase. Both 25OHD and 1,25(OH)2D are catabolized by CYP24A1. 1,25(OH)2D is the ligand for the vitamin D receptor (VDR), a transcription factor, binding to sites in the DNA called vitamin D response elements (VDREs). There are thousands of these binding sites regulating hundreds of genes in a cell-specific fashion. VDR-regulated transcription is dependent on comodulators, the profile of which is also cell specific. Analogs of 1,25(OH)2D are being developed to target specific diseases with minimal side effects. This review will examine these different aspects of vitamin D metabolism, mechanism of action, and clinical application. Copyright © 2014 Elsevier Ltd. All rights reserved.