Latest findings in phosphate homeostasis

The bone and the kidney

Renal tubular diseases may present with osteopenia, osteoporosis or osteomalacia, as a result of significant derangements in body electrolytes. In case of insufficient synthesis of calcitriol, as in renal failure, the more complex picture of renal osteodystrophy may develop. Hypothetically, also disturbed renal production of BMP-7 and Klotho could cause bone disease. However, the acknowledgment that osteocytes are capable of producing FGF23, a phosphaturic hormone at the same time modulating renal synthesis of calcitriol, indicates that it is also bone that can influence renal function. Importantly, a feed-back mechanism exists between FGF23 and calcitriol synthesis, while Klotho, produced by the kidney, determines activity and selectivity of FGF23. Identification of human diseases linked to disturbed production of FGF23 and Klotho underlines the importance of this new bone-kidney axis. Kidney and bone communicate reciprocally to regulate the sophisticated machinery responsible for divalent ions homeostasis and for osseous or extraosseous mineralisation processes.

Phosphate homeostasis and the renal-gastrointestinal axis

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Mutation analysis and serum FGF23 level in a patient with pulmonary alveolar microlithiasis

Pulmonary alveolar microlithiasis (PAM) is a rare, hereditary disorder characterized by ectopic formation of calcium-phosphate microliths in the alveolar space. PAM has been reported to arise from inactivating mutations in SLC34A2, encoding a sodium-dependent phosphate co-transporter essential for phosphate transport in the lungs and small intestine. Serum levels of the phosphaturic hormone fibroblast growth factor-23 (FGF23) in PAM have not been determined. Our objectives were to investigate the genetic etiology and circulating level of FGF23 in a 50-year-old male with clinical characteristics of PAM and extra-pulmonary calcifications. The SLC34A2 and FGF23 genes were sequenced for mutations and serum FGF23 analyzed by ELISA. We found no disease-causing mutations or single nucleotide polymorphisms in the genes investigated. Importantly, repeated measurements revealed undetectable or markedly low serum FGF23 (<3-11 RU/ml). Surprisingly, in the face of low serum FGF23, 1,25-dihydroxy vitamin D₃ level was low-normal and parathyroid hormone mildly elevated. Total 24-h urinary excretion of phosphate and calcium were low, as was fractional urinary excretion of calcium. In contrast, fractional excretion of phosphate was above normal, likely due to elevated PTH. Collectively, PAM may be a polygenic disorder that arises from mutations other than in SLC34A2. The low FGF23 level in our PAM patient supports an intestinal-bone axis, leading to decreased FGF23 expression when intestinal phosphate absorption is compromised.

The influence of glomerular filtration rate and age on fibroblast growth factor 23 serum levels in pediatric chronic kidney disease

BACKGROUND

Fibroblast growth factor 23 (FGF23) is a phosphaturic factor and a suppressor of 1alpha-hydroxylase activity in the kidney. Although its importance in chronic kidney disease (CKD) has been demonstrated in adults, there is little information in pediatric patients.

OBJECTIVES

The aims of this study were: 1) to determine reference values for FGF23 serum levels according to glomerular filtration rate (GFR) (measured by the reference standard, inulin clearance), gender, and age; and 2) to evaluate the effects of different etiologies and treatments on FGF23 serum levels in a prospective single-center cohort of 227 CKD children (119 boys).

RESULTS

Age, body weight, height, and GFR (mean +/- sd) values were: 11.3 +/- 4.1 yr, 37 +/- 16 kg, 140 +/- 20 cm, and 98 +/- 34 ml/min per 1.73 m(2), respectively. Calcium, phosphate, PTH, 25 hydroxyvitamin D, 1,25 dihydroxyvitamin D, C-terminal FGF23, and intact FGF23 (mean +/- sd) levels were: 2.43 +/- 0.11 mmol/liter, 1.41 +/- 0.22 mmol/liter, 41 +/- 23 pg/ml, 24 +/- 10 ng/ml, 152 +/- 72 pmol/liter, 76 +/- 134 relative units/ml, and 44 +/- 37 pg/ml, respectively. There was a wide range of FGF23 serum levels, but FGF23 levels increased when GFR decreased. FGF23 serum levels were not modified by gender, but they increased with age. In univariate analysis, corticosteroid therapy seemed to be associated with increased FGF23 serum levels. A multivariate linear regression analysis found a significant impact of GFR, body mass index, and solid organ transplantation on FGF23 serum levels.

CONCLUSION

Age, GFR, body mass index, and solid organ transplantation seem to influence FGF23 serum levels in a pediatric population. The impact of corticosteroids on FGF23 metabolism should be further investigated; further longitudinal studies will also help to better define the prognostic impact of FGF23 serum levels in pediatric CKD in terms of disease progression, cardiovascular morbidities, and bone disabilities.

Phosphate binders in CKD: chalking out the differences

Plasma phosphate levels are important in the evolution of hyperparathyroidism and ectopic calcification in chronic kidney disease (CKD). Although dietary management may be adequate to control plasma phosphate in its early stages, most patients develop hyperphosphataemia by CKD stages 3-4 and require the addition of a phosphate binder. Calcium-containing phosphate binders are the most used and cheapest binders but have fallen out of favour because of the potential for positive calcium balance and calcium toxicity. This problem may be attenuated by newer phosphate binders such as sevelamer hydrochloride and lanthanum carbonate. In this review, the role of phosphate as a uraemic toxin and the advantages and disadvantages of the currently available phosphate binders are discussed.

Compensatory regulation of the sodium/phosphate cotransporters NaPi-IIc (SCL34A3) and Pit-2 (SLC20A2) during Pi deprivation and acidosis

The role of four Pi transporters in the renal handling of Pi was analyzed using functional and molecular methods. The abundance of NaPi-IIa, NaPi-IIc, and Pit-2 was increased by 100% in kidney from rats on a 0.1% Pi diet, compared to a 0.6% Pi diet. Pit-1 was not modified. Type II-mediated Pi uptake in Xenopus oocytes increased as the pH of the uptake medium increased, and the opposite occurred with Pit-1 and Pit-2. At pH 6.0, Pi uptake mediated through type II was approximately 10% of the uptake at pH 7.5, but the uptake through Pit-2 was 250% of the activity at pH 7.5. Real brush-border membrane vesicles (BBMV) responded to pH changes following the same pattern as type II transporters. Adaptation to a 0.1% Pi diet was accompanied by a 65% increase in the V (max) of BBMV Pi transport at pH 7.5, compared to a 0.6% Pi diet. The increase was only 11% at pH 6.0. Metabolic acidosis increased the expression of NaPi-IIc and Pit-2 in animals adapted to a low Pi diet, and phosphaturia was only observed in control diet animals. The combination of the pH effect, Pi adaptation, and metabolic acidosis suggests very modest involvement of Pit-2 in renal Pi handling. Real-time PCR and mathematical analyses of transport findings suggest that NaPi-IIa RNA accounts for 95% of all Pi transporters and that type II handles 97% of Pi transport at pH 7.5 and 60% of Pi transport at pH 6.0, depending on the pH and the physiological conditions.

Identification of a novel function of PiT1 critical for cell proliferation and independent of its phosphate transport activity

PiT1 is a Na(+)-phosphate (P(i)) cotransporter located at the plasma membrane that enables P(i) entry into the cell. Its broad tissue expression pattern has led to the idea that together with the closely related family member PiT2, PiT1 is the ubiquitous supplier of P(i) to the cell. Moreover, the role of P(i) in phosphorylation reactions, ATP production, DNA structure, and synthesis has led to the view that P(i) availability could be an important determinant of cell growth. However, these issues have not been clearly addressed to date, and the role of either P(i) or PiT proteins in cell proliferation is unknown. Using RNA interference in HeLa and HepG2 cells, we show that transient or stable PiT1 depletion markedly reduces cell proliferation, delays cell cycle, and impairs mitosis and cytokinesis. In vivo, PiT1 depletion greatly reduced tumor growth when engineered HeLa cells were injected into nude mice. We provide evidence that this effect on cell proliferation is specific to PiT1 and not shared by PiT2 and is not the consequence of impaired membrane Na(+)-P(i) transport. Moreover, we show that modulation of cell proliferation by PiT1 is independent from its transport function because the proliferation of PiT1-depleted cells can be rescued by non-transporting PiT1 mutants. PiT1 depletion leads to the phosphorylation of p38 mitogen-activated protein (MAP) kinase, whereas other MAP kinases and downstream targets of mammalian target of rapamycin (mTOR) remain unaffected. This study is the first to describe the effects of a P(i) transporter in cell proliferation, tumor growth, and cell signaling.

A blueprint for randomized trials targeting phosphorus metabolism in chronic kidney disease

The diagnosis of chronic kidney disease (CKD) confers dismal clinical outcomes regardless of whether patients are initiating dialysis and face a median survival of only 2-3 years or they have earlier-stage CKD and face a risk of death that is greater than the risk of progression to dialysis. These poor outcomes are driven by extraordinarily high rates of cardiovascular disease that historically have not responded to risk-factor modification strategies proven to attenuate risk in the general population. Nor have measures aimed at increasing the dose or quality of dialysis made an appreciable dent in mortality. Still worse, interventions that were expected to be beneficial resulted in increased mortality in recent trials. Although this apparent lack of progress in advancing the care of CKD is discouraging, resignation is not an option. On the contrary, with the rising rates of CKD worldwide, there is an urgent need to rigorously test novel therapeutic strategies in randomized trials. The breadth of accumulating evidence linking disordered phosphorus metabolism to adverse outcomes spans in vitro, animal, and human studies, and positions phosphorus management as an attractive target for intervention. Although opinion-based practice guidelines promote phosphorus management strategies that are widely accepted in dialysis patients, there is a clear need to perform randomized controlled trials to prove or disprove the benefits of therapy. Perhaps even more important, the discovery of fibroblast growth factor 23 (FGF23) and its potential as a novel diagnostic to identify disordered phosphorus metabolism at an early, subclinical state has presented the opportunity to develop placebo-controlled randomized trials in pre-dialysis CKD patients with normal serum phosphate levels. This commentary considers the justification and challenges for such trials and presents a 'first-draft' blueprint of distinct trial approaches to initiate a dialog that will ultimately culminate in studies aimed at improving survival across the spectrum of CKD.