Hypophosphatemia: an update on its etiology and treatment

Dietary and genetic evidence for phosphate toxicity accelerating mammalian aging

Identifying factors that accelerate the aging process can provide important therapeutic targets for slowing down this process. Misregulation of phosphate homeostasis has been noted in various skeletal, cardiac, and renal diseases, but the exact role of phosphate toxicity in mammalian aging is not clearly defined. Phosphate is widely distributed in the body and is involved in cell signaling, energy metabolism, nucleic acid synthesis, and the maintenance of acid-base balance by urinary buffering. In this study, we used an in vivo genetic approach to determine the role of phosphate toxicity in mammalian aging. Klotho-knockout mice (klotho(-/-)) have a short life span and show numerous physical, biochemical, and morphological features consistent with premature aging, including kyphosis, uncoordinated movement, hypogonadism, infertility, severe skeletal muscle wasting, emphysema, and osteopenia, as well as generalized atrophy of the skin, intestine, thymus, and spleen. Molecular and biochemical analyses suggest that increased renal activity of sodium-phosphate cotransporters (NaPi2a) leads to severe hyperphosphatemia in klotho(-/-) mice. Genetically reducing serum phosphate levels in klotho(-/-) mice by generating a NaPi2a and klotho double-knockout (NaPi2a(-/-)/klotho(-/-)) strain resulted in amelioration of premature aging-like features. The NaPi2a(-/-)/klotho(-/-) double-knockout mice regained reproductive ability, recovered their body weight, reduced their organ atrophy, and suppressed ectopic calcifications, with the resulting effect being prolonged survival. More important, when hyperphosphatemia was induced in NaPi2a(-/-)/klotho(-/-) mice by feeding with a high-phosphate diet, premature aging-like features reappeared, clearly suggesting that phosphate toxicity is the main cause of premature aging in klotho(-/-) mice. The results of our dietary and genetic manipulation studies provide in vivo evidence for phosphate toxicity accelerating the aging process and suggest a novel role for phosphate in mammalian aging.

Serum phosphate level in burn patients

INTRODUCTION

Despite plasma phosphate imbalance being rare, it is a relatively common finding in certain subsets of burn patients. It may occur due to the burn itself or as a result of the treatment. Severe hypophosphataemia (<1.0 mg dl(-1)) is associated with a significant morbidity and a fourfold increase in mortality. In this study, the relation between serum phosphate level and the total body surface area (TBSA) of the burn was compared.

METHODS

According to the percentage of TBSA of the burn, the patients (n=155) were divided into three groups: group A with 20-29% TBSA burns, group B with 30-39% and group C with more than 40% TBSA burns (62, 48 and 45 patients, respectively). Analysis of variance (ANOVA)-repeated measure was used to detect any statistically significant difference in the three post-burn time-points of 3rd, 6th and 9th days and the mean score of the serum phosphate level between the three groups.

RESULTS

The incidence of hypophosphataemia at 9th post-burn day in the three groups was 6.1%, 32.4% and 73.5%, respectively. There were significant differences (p<0.05) between mean serum phosphate levels of groups A and C, B and C and A and B as well. We found significant differences between the three post-burn follow-up time stages.

DISCUSSION

We have shown that hypophosphataemia, defined as mean serum phosphate levels below 3.0 mg dl(-1), was very common following burn, based on 75.6% of patients with more than 40% burn at the 3rd post-burn day. As the percentage of TBSA of burn increases, the incidence of hypophosphataemia significantly increases. We suggest that phosphate level be routinely measured after a major burn, especially in patients with a complicated course, so that appropriate replacement therapy may be started in a timely manner.

Isolated C-terminal tail of FGF23 alleviates hypophosphatemia by inhibiting FGF23-FGFR-Klotho complex formation

Fibroblast growth factor (FGF) 23 inhibits renal phosphate reabsorption by activating FGF receptor (FGFR) 1c in a Klotho-dependent fashion. The phosphaturic activity of FGF23 is abrogated by proteolytic cleavage at the RXXR motif that lies at the boundary between the FGF core homology domain and the 72-residue-long C-terminal tail of FGF23. Here, we show that the soluble ectodomains of FGFR1c and Klotho are sufficient to form a ternary complex with FGF23 in vitro. The C-terminal tail of FGF23 mediates binding of FGF23 to a de novo site generated at the composite FGFR1c-Klotho interface. Consistent with this finding, the isolated 72-residue-long C-terminal tail of FGF23 impairs FGF23 signaling by competing with full-length ligand for binding to the binary FGFR-Klotho complex. Injection of the FGF23 C-terminal tail peptide into healthy rats inhibits renal phosphate excretion and induces hyperphosphatemia. In a mouse model of renal phosphate wasting attributable to high FGF23, the FGF23 C-terminal peptide reduces phosphate excretion, leading to an increase in serum phosphate concentration. Our data indicate that proteolytic cleavage at the RXXR motif abrogates FGF23 activity by a dual mechanism: by removing the binding site for the binary FGFR-Klotho complex that resides in the C-terminal region of FGF23, and by generating an endogenous inhibitor of FGF23. We propose that peptides derived from the C-terminal tail of FGF23 or peptidomimetics and small-molecule organomimetics of the C-terminal tail can be used as therapeutics to treat renal phosphate wasting.

The FGF23-Klotho axis: endocrine regulation of phosphate homeostasis

Appropriate levels of phosphate in the body are maintained by the coordinated regulation of the bone-derived growth factor FGF23 and the membrane-bound protein Klotho. The endocrine actions of FGF23, in association with parathyroid hormone and vitamin D, mobilize sodium-phosphate cotransporters that control renal phosphate transport in proximal tubular epithelial cells. The availability of an adequate amount of Klotho is essential for FGF23 to exert its phosphaturic effects in the kidney. In the presence of Klotho, FGF23 activates downstream signaling components that influence the homeostasis of phosphate, whereas in the absence of this membrane protein, it is unable to exert such regulatory effects, as demonstrated convincingly in animal models. Several factors, including phosphate and vitamin D, can regulate the production of both FGF23 and Klotho and influence their functions. In various acquired and genetic human diseases, dysregulation of FGF23 and Klotho is associated with vascular and skeletal anomalies owing to altered phosphate turnover. In this Review, I summarize how the endocrine effects of bone-derived FGF23, in coordination with Klotho, can regulate systemic phosphate homeostasis, and how an inadequate balance of these molecules can lead to complications that are caused by abnormal mineral ion metabolism.

In vivo genetic evidence for suppressing vascular and soft-tissue calcification through the reduction of serum phosphate levels, even in the presence of high serum calcium and 1,25-dihydroxyvitamin d levels

BACKGROUND

Klotho-knockout mice (klotho(-/-)) have increased renal expression of sodium/phosphate cotransporters (NaPi2a), associated with severe hyperphosphatemia. Such serum biochemical changes in klotho(-/-) mice lead to extensive soft-tissue anomalies and vascular calcification. To determine the significance of increased renal expression of the NaPi2a protein and concomitant hyperphosphatemia and vascular calcification in klotho(-/-) mice, we generated klotho and NaPi2a double-knockout (klotho(-/-)/NaPi2a(-/-)) mice.

METHODS AND RESULTS

Genetic inactivation of NaPi2a activity from klotho(-/-) mice reversed the severe hyperphosphatemia to mild hypophosphatemia or normophosphatemia. Importantly, despite significantly higher serum calcium and 1,25-dihydroxyvitamin D levels in klotho(-/-)/NaPi2a(-/-) mice, the vascular and soft-tissue calcifications were reduced. Extensive soft-tissue anomalies and cardiovascular calcification were consistently noted in klotho(-/-) mice by 6 weeks of age; however, these vascular and soft-tissue abnormalities were absent even in 12-week-old double-knockout mice. Klotho(-/-)/NaPi2a(-/-) mice also regained body weight and did not develop the generalized tissue atrophy often noted in klotho(-/-) single-knockout mice.

CONCLUSIONS

Our in vivo genetic manipulation studies have provided compelling evidence for a pathological role of increased NaPi2a activities in regulating abnormal mineral ion metabolism and soft-tissue anomalies in klotho(-/-) mice. Notably, our results suggest that serum phosphate levels are the important in vivo determinant of calcification and that lowering serum phosphate levels can reduce or eliminate soft-tissue and vascular calcification, even in presence of extremely high serum calcium and 1,25-dihydroxyvitamin D levels. These in vivo observations have significant clinical importance and therapeutic implications for patients with chronic kidney disease with cardiovascular calcification.

Inactivation of klotho function induces hyperphosphatemia even in presence of high serum fibroblast growth factor 23 levels in a genetically engineered hypophosphatemic (Hyp) mouse model

Hyp mice possess a mutation that inactivates the phosphate-regulating gene, which is homologous to the endopeptidases of the X-chromosome (PHEX). The mutation is associated with severe hypophosphatemia due to excessive urinary phosphate wasting. Such urinary phosphate wasting in Hyp mice is associated with an increased serum accumulation of fibroblast growth factor (FGF) 23. We wanted to determine the biological significance of increased serum FGF23 levels and concomitant hypophosphatemia in Hyp mice and to evaluate whether FGF23 activity could be modified by manipulating klotho (a cofactor of FGF23 signaling). We generated Hyp and klotho double-mutant mice (Hyp/klotho(-/-)). Severe hypophosphatemia of Hyp mice was reversed to hyperphosphatemia in Hyp/klotho(-/-) double mutants, despite the fact that the double mutants showed significantly increased serum levels of FGF23. Hyperphosphatemia in Hyp/klotho(-/-) mice was associated with increased renal expression of sodium/phosphate cotransporter 2a (NaPi2a) protein. Exogenous injection of bioactive parathyroid hormone 1-34 down-regulated renal expression of NaPi2a and consequently reduced serum levels of phosphate in Hyp/klotho(-/-) mice. Moreover, in contrast to the Hyp mice, the Hyp/klotho(-/-) mice showed significantly higher serum levels of 1,25-dihydroxyvitamin D and developed extensive calcification in soft tissues and vascular walls. Furthermore, compared with the Hyp mice, Hyp/klotho(-/-) mice were smaller in size, showed features of generalized tissue atrophy, and generally died by 15-20 wk of age. Our in vivo studies provide genetic evidence for a pathological role of increased FGF23 activities in regulating abnormal phosphate homeostasis in Hyp mice. Moreover, these results suggest that even when serum levels of FGF23 are significantly high, in the absence of klotho, FGF23 is unable to regulate systemic phosphate homeostasis. Our in vivo observations have significant clinical implications in diseases associated with increased FGF23 activity and suggest that the functions of FGF23 can be therapeutically modulated by manipulating the effects of klotho.

FGF23 elevation and hypophosphatemia after intravenous iron polymaltose: a prospective study

CONTEXT

Parenteral iron administration has been associated with hypophosphatemia. Fibroblast growth factor 23 (FGF23) has a physiological role in phosphate homeostasis via suppression of 25-hydroxyvitamin D [25(OH)D] activation and promotion of phosphaturia. We recently reported a case of iron-induced hypophosphatemic osteomalacia associated with marked FGF23 elevation.

OBJECTIVE

Our objective was to prospectively investigate the effect of parenteral iron polymaltose on phosphate homeostasis and to determine whether any observed change was related to alterations in circulating FGF23.

DESIGN, SETTING, AND PARTICIPANTS

Eight medical outpatients prescribed iv iron polymaltose were recruited. Plasma phosphate, 25(OH)D, 1,25-dihydroxyvitamin D [1,25(OH)(2)D], PTH, FGF23, and urinary tubular reabsorption of phosphate were measured prior to iron administration and then weekly for a minimum of 3 wk.

RESULTS

Plasma phosphate fell from 3.4 +/- 0.6 mg/dl at baseline to 1.8 +/- 0.6 mg/dl at wk 1 (P < 0.0001) associated with a fall in percentage tubular reabsorption of phosphate (90 +/- 4.8 to 68 +/- 13; P < 0.001) and 1,25(OH)(2)D (54 +/- 25 to 9 +/- 8 pg/ml; P < 0.001). These indices remained significantly suppressed at wk 2 and 3. 25(OH)D levels were unchanged. FGF23 increased significantly from 43.5 pg/ml at baseline to 177 pg/ml at wk 1 (P < 0.001) with levels correlating with both serum phosphate (R = -0.74; P <0.05) and 1,25(OH)(2)D (R = -0.71; P < 0.05).

CONCLUSION

Parenteral iron suppresses renal tubular phosphate reabsorption and 1alpha-hydroxylation of vitamin D resulting in hypophosphatemia. Our data suggest that this is mediated by an increase in FGF23.

Hypophosphataemia: An Easy Strategy for Diagnosis and Treatment in HIV Patients

Because HIV infection has become a chronic disease, it is crucial that metabolic complications secondary to HIV infection or prolonged therapy be diagnosed and managed appropriately over time. Therefore the optimal follow-up becomes complex and time consuming. Our review aimed to provide physicians in charge of HIV-infected patients with key data helping them to diagnose and understand hypophosphataemia in HIV patients. Hypophosphataemia is frequent and sometimes secondary to renal phosphate wasting. It is very rarely a component of a complex proximal tubular disorder, such as Fanconi syndrome. When isolated, hypophosphataemia is easy to rule out and treat. In rare cases, prolonged hypophosphataemia, when related to renal phosphate wasting and tubular dysfunction, might have potential consequences on bone outcome, however, more studies are needed. HIV infection by itself might be a risk factor for bone metabolism abnormalities; antiretroviral drugs might also be involved. Therefore, it seems valuable for patients that the minimal screening should be performed routinely, in order to prevent long-term disabilities.

Electrolyte disorders related to EGFR-targeting drugs

It is now clearly established that anti-vascular endothelial growth factor (VEGF) drug class induces hypertension and proteinuria sometimes related to thrombotic microangiopathy and/or various glomerulopathies, according to capillary and glomerular VEGF and VEGF-receptor expressions. As reported in the literature, anti-epidermal growth factor receptor (EGFR) therapies seem to be less nephrotoxic. Indeed, many reports of anti-EGFR nephrotoxicity are tubular dependent such as acute tubular necrosis, electrolyte disorders (hypophosphatemia, hypomagnesemia, etc.) or both. This is explained by elective tubular expression of renal EGF/EGFR. In this paper, we focus on electrolyte disorders related to anti-EGFR treatment and discuss the tubular involvement of these drugs based on their renal expression.

Hypophosphatemia in critically ill children: prevalence and associated risk factors

BACKGROUND

Hypophosphatemia is a disorder with potential complications and is often unrecognized in critically ill patients.

AIMS

To identify the prevalence of hypophosphatemia and risk factors associated to this disorder in critically ill children.

METHODS

In a prospective cohort study, 82 children admitted consecutively to a pediatric intensive care unit (ICU) were monitored regarding phosphorus serum levels during the first 10 days of admission. The following variables were analyzed as independent for hypophosphatemia: age, gender, diagnosis at admission, malnutrition, phosphorus intake, clinical severity score at admission (pediatric index of mortality 2) and daily scores (Pediatric Logistic Organ Dysfunction), sepsis, use of dopamine, furosemide and steroids, starvation period, and refeeding. Children with a z score of less than -2 of expected weight for age or body mass index (National Center for Health Statistics, 2000) were considered malnourished. Variables significantly associated with hypophosphatemia by bivariate analysis (p < 0.1) were included in a multiple logistic regression model.

RESULTS

The rate of hypophosphatemia was 61% during the first 10 days of pediatric ICU stay, and 12 patients developed hypophosphatemia during the study period. Malnutrition was present in 39.1% of patients, and the sera phosphorus concentration was significantly lower in malnourished than in well-nourished children (2.6 +/- 0.7 mg/dL vs. 3.5 +/- 0.8 mg/dL, p = 0.01). The multiple logistic regression model indicated the diagnosis of acute respiratory disease (odds ratio: 3.22; confidence interval: 1.03-10.1; p = 0.04), use of dopamine (odds ratio: 8.65; confidence interval: 1.58-47.3; p = 0.01), and malnutrition (odds ratio: 3.96; confidence interval: 1.19-13.3; p = 0.02) as independent risk factors for hypophosphatemia. None of the other potential risk factors discriminated for hypophosphatemia.

CONCLUSIONS

Hypophosphatemia was common in the first 10 days of ICU hospitalization and was associated with the diagnosis of respiratory disease, use of dopamine, and malnutrition. These factors should be taken into account during clinical follow up of critically ill children, especially when these conditions are found together.

FGF23-mediated regulation of systemic phosphate homeostasis: is Klotho an essential player?

Understanding the physiological regulation of mineral ion metabolism is essential for determining the pathomechanisms of skeletal, vascular, and renal diseases associated with an abnormal regulation of calcium and phosphate homeostasis. Normal calcium and phosphate balance is delicately maintained by endocrine factors that coordinate to influence the functions of the intestine, bone, parathyroid gland, and kidney. Under physiological conditions, the kidneys play an important role in maintaining normal mineral ion balance by fine-tuning the amount of urinary excretion of calcium and phosphate according to the body's needs. Fibroblast growth factor (FGF)23 regulates urinary phosphate excretion to maintain systemic phosphate homeostasis. The exact mode of action of the phosphaturic effects of FGF23 is not fully understood and is an intense area of research. Studies suggest, however, that FGF23, by interacting with FGF receptors, can initiate downstream signaling events and that Klotho, a transmembrane protein, facilitates the interaction of FGF23 with its receptor. FGF23 can inhibit the activities of 1-alpha-hydroxylase and sodium-phosphate cotransporter in the kidney to influence the overall systemic phosphate balance. This article briefly summarizes how FGF23 might coordinately regulate systemic phosphate homeostasis and how Klotho is involved in such regulation.

Severe hypophosphataemia mimicking Guillain-Barré syndrome

A 42 year-old woman presented with neurological symptoms suggestive of Guillain-Barré syndrome and required ventilatory support. She was found to have severe hypophosphataemia that was corrected promptly after which her trachea was extubated. She then revealed that she had undergone a prolonged religious fast and had just started eating again when her symptoms started. Over the next 2 weeks she made a complete recovery.

Phosphate is a uremic toxin

Hyperphosphatemia is one of the more prevalent metabolic disturbances in kidney failure. Phosphate can be considered a uremic toxin based on the accumulation of phosphate during chronic kidney disease, the effects of phosphate on biological systems, and the adverse effects of hyperphosphatemia. The renal clearance of phosphate is maintained until later stages of chronic kidney disease, when the remaining nephrons are no longer able to excrete sufficient phosphate to offset dietary phosphate absorption. Clearance of phosphate by conventional forms of dialysis is insufficient to prevent hyperphosphatemia in most endstage kidney-disease patients. Phosphate contributes to metabolic disturbances such as hyperparathyroidism, vitamin D resistance, and hypocalemia. In combination with these and other factors, hyperphosphatemia damages many organs, including the parathyroid glands, bones, and most importantly the cardiovascular system. Elevated phosphorus is associated with arterial and valvular calcification, arteriosclerosis, and an increased risk of cardiovascular death. Importantly, the adverse effects of hyperphosphatemia are partially preventable with the effective treatments available today.

Fever as a cause of hypophosphatemia in patients with malaria

Hypophosphatemia occurs in 40 to 60% of patients with acute malaria, and in many other conditions associated with elevations of body temperature. To determine the prevalence and causes of hypophosphatemia in patients with malaria, we retrospectively studied all adults diagnosed with acute malaria during a 12-year period. To validate our findings, we analyzed a second sample of malaria patients during a subsequent 10-year period. Serum phosphorus correlated inversely with temperature (n = 59, r = −0.62; P<0.0001), such that each 1°C increase in body temperature was associated with a reduction of 0.18 mmol/L (0.56 mg/dL) in the serum phosphorus level (95% confidence interval: −0.12 to −0.24 mmol/L [−0.37 to −0.74 mg/dL] per 1°C). A similar effect was observed among 19 patients who had repeat measurements of serum phosphorus and temperature. In a multiple linear regression analysis, the relation between temperature and serum phosphorus level was independent of blood pH, PCO2, and serum levels of potassium, bicarbonate, calcium, albumin, and glucose. Our study demonstrates a strong inverse linear relation between body temperature and serum phosphorus level that was not explained by other factors known to cause hypophosphatemia. If causal, this association can account for the high prevalence of hypophosphatemia, observed in our patients and in previous studies of patients with malaria. Because hypophosphatemia has been observed in other clinical conditions characterized by fever or hyperthermia, this relation may not be unique to malaria. Elevation of body temperature should be added to the list of causes of hypophosphatemia.

The emerging role of the fibroblast growth factor-23-klotho axis in renal regulation of phosphate homeostasis

Normal mineral ion homeostasis is tightly controlled by numerous endocrine factors that coordinately exert effects on intestine, kidney, and bone to maintain physiological balance. The importance of the fibroblast growth factor (FGF)-23-klotho axis in regulating mineral ion homeostasis has been proposed from recent research observations. Experimental studies suggest that 1) FGF23 is an important in vivo regulator of phosphate homeostasis, 2) FGF23 acts as a counter regulatory hormone to modulate the renal 1alpha-hydroxylase and sodium-phosphate cotransporter activities, 3) there is a trend of interrelationship between FGF23 and parathyroid hormone activities, 4) most of the FGF23 functions are conducted through the activation of FGF receptors, and 5) such receptor activation needs klotho, as a cofactor to generate downstream signaling events. These observations clearly suggest the emerging roles of the FGF23-klotho axis in maintaining mineral ion homeostasis. In this brief article, we will summarize how the FGF23-klotho axis might coordinately regulate normal mineral ion homeostasis, and how their abnormal regulation could severely disrupt such homeostasis to induce disease pathology.

Hypophosphatemia: clinical consequences and management

Current evidence regarding the clinical consequences of hypophosphatemia is not straightforward. Given the potentially different implications of hypophosphatemia among various patient groups, this commentary touches on patients with low serum phosphate after acute hospitalization, those with chronic ambulatory hypophosphatemia, and those with hypophosphatemia in the setting of advanced renal disease. Finally, this commentary examines the evidence regarding how best to replete phosphorous in the hypophosphatemic patient.

Premature aging in klotho mutant mice: cause or consequence?

Suitable mammalian models for aging with a wide range of age-associated pathology are desirable to study molecular mechanisms of human aging. Recent studies have identified that fibroblast growth factor 23 (Fgf-23) null mice and klotho hypomorphs could generate multiple premature aging-like features, including shortened lifespan, infertility, kyphosis, atherosclerosis, extensive soft tissue calcifications, skin atrophy, muscle wasting, T cell dysregulation, pulmonary emphysema, osteoporosis/osteopenia, abnormal mineral ion metabolism, and impaired vitamin-D homeostasis. The strikingly similar in vivo phenotypes of two separate genetically altered mouse lines implicate that the premature aging-like features may be partly regulated through a common signaling pathway involving both Fgf-23 and klotho; such speculation is experimentally supported by the observation that Fgf-23 requires klotho as a cofactor to exert its functions. Despite about 2000-fold higher serum levels of Fgf-23 in klotho mutants (compared to wild-type animals), these mice show physical, biochemical and morphological features similar to Fgf-23 null mice, but not as Fgf-23 transgenic mice; these observations suggest that widely encountered premature aging-like features in klotho mutant mice are due to the inability of Fgf-23 to exert its bioactivities in absence of klotho. The results of recent studies showing klotho as a cofactor in Fgf-23 signaling consequently explains that the premature aging-like features in klotho-deficient mice is not a primary cause, rather a consequence of lacking Fgf-23 activity. These understandings will help us to redefine the role of klotho as an aging factor.

Hepatic surgery-related hypophosphatemia

This review describes pathophysiology of post-surgical hypophosphatemia (HP), which has particularly high incidence following liver transplantation. HP remains poorly understood; and there is a lack of universally accepted guidelines for its investigation and management. The pathogenesis of HP following major liver surgery has been hypothesized as being due either to excessive utilization by regenerating liver or increased urinary losses of phosphate. This review provides evidence that excessive urinary loss rather than increased Pi uptake by the liver is the most likely mechanism, and this may be mediated by recently described phosphaturic factors, known as phosphatonins. Until recently blood Pi homeostasis had been explained solely in terms of classical hormones, i.e., vitamin D and PTH. It is however increasingly recognized that phosphatonins may play a critical role in the post-operative HP, but the exact mechanism and candidate phosphaturic factor has not yet been identified. In this review, we have described likely mechanisms and suggest candidate phosphatonins that may mediate urinary Pi loss following liver transplantation. We also discuss the biochemical consequences of cellular Pi depletion, which exposes some gaps in the utilization of established knowledge and therefore in the management of HP. The main aspects of pathophysiology of HP and cellular Pi depletion are presented to provide rational for novel biochemical investigations, which are likely to improve monitoring of HP associated metabolic stress as well as extent of severity of HP, and thereby enhance management of these patients.