FGF23 induces left ventricular hypertrophy

Calcium regulation and bone mineral metabolism in elderly patients with chronic kidney disease

The elderly chronic kidney disease (CKD) population is growing. Both aging and CKD can disrupt calcium (Ca2+) homeostasis and cause alterations of multiple Ca2+-regulatory mechanisms, including parathyroid hormone, vitamin D, fibroblast growth factor-23/Klotho, calcium-sensing receptor and Ca2+-phosphate product. These alterations can be deleterious to bone mineral metabolism and soft tissue health, leading to metabolic bone disease and vascular calcification and aging, termed CKD-mineral and bone disorder (MBD). CKD-MBD is associated with morbid clinical outcomes, including fracture, cardiovascular events and all-cause mortality. In this paper, we comprehensively review Ca2+ regulation and bone mineral metabolism, with a special emphasis on elderly CKD patients. We also present the current treatment-guidelines and management options for CKD-MBD.

FGF23: a mature renal and cardiovascular risk factor?

High FGF23 predicts renal function loss in chronic kidney disease (CKD) patients and graft failure in transplant patients. FGF23, parathyroid hormone and serum phosphate are all interrelated but among these CKD-MBD biomarkers only FGF23 is independently related with CKD progression. High FGF23 associates with endothelial dysfunction in CKD patients and in elderly individuals in the general population. Furthermore, independently of serum phosphate, high FGF23 associates with mortality and left ventricular hypertrophy in dialysis patients and with atherosclerosis in elderly individuals in the general population. FGF23 also predicts a high risk for death and cardiovascular events in predialysis CKD patients and in subjects with coronary artery disease. A recent trial in CKD patients showed that low phosphate intake associated with a phosphate binder produces a 35% decrease in plasma FGF23. Yet in this and in another trial testing several phosphate binders, FGF23 levels remained well beyond the upper limit of the normal range. Of note, in this latter study, calcification of the coronary arteries and abdominal aorta actually increased, rather than decreased, during treatment with these drugs in the face of evidence of negative phosphate balance and amelioration of hyperparathyroidism. Mechanistic studies are still needed before testing the hypothesis that FGF23 is implicated in a causal manner in cardiovascular and renal diseases. Given the modest effects of phosphate binders on serum FGF23 in CKD patients, pharmacologic interventions antagonizing the effects of this growth factor rather than phosphate-lowering interventions should be put in place to properly test this hypothesis in the clinical scenario in CKD.

Blood pressure, proteinuria, and phosphate as risk factors for progressive kidney disease: a hypothesis

Chronic kidney disease (CKD) affects approximately 500 million people worldwide and is increasingly common in both industrialized and emerging countries. Although the mechanisms underlying the inexorable progression of CKD are incompletely defined, recent discoveries may pave the way to a more comprehensive understanding of the pathophysiology of CKD progression and the development of new therapeutic strategies. In particular, there is accumulating evidence indicating a key role for the complex and yet incompletely understood system of divalent cation regulation, which includes phosphate metabolism and the recently discovered fibroblast growth factor 23 (FGF-23)/klotho system, which seems inextricably associated with vitamin D deficiency. The aim of this review is to discuss the links between high blood pressure, proteinuria, phosphate levels, and CKD progression and explore new therapeutic strategies to win the fight against CKD.

Klotho and chronic kidney disease

Through alternative splicing, Klotho protein exists both as a secreted and a membrane form whose extracellular domain could be shed from the cell surface by secretases and released into the circulation to act as endocrine factor. Unlike membrane Klotho which functions as a coreceptor for fibroblast growth factor-23 (FGF23) to modulate FGF23 signal transduction, soluble Klotho is a multifunction protein present in the biological fluids including blood, urine and cerebrospinal fluid and plays important roles in antiaging, energy metabolism, inhibition of Wnt signaling, antioxidation, modulation of ion transport, control of parathyroid hormone and 1,25(OH)2VD3 production, and antagonism of renin-angiotensin-aldosterone system. Emerging evidence from clinical and basic studies reveal that chronic kidney disease is a state of endocrine and renal Klotho deficiency, which may serve as an early biomarker and a pathogenic contributor to chronic progression and complications in chronic kidney disease including vascular calcification, cardiac hypertrophy, and secondary hyperparathyroidism. Supplementation of exogenous Klotho and/or upregulation of endogenous Klotho production by using rennin angiotensin system inhibitors, HMG CoA reductase inhibitors, vitamin D analogues, peroxisome proliferator-activated receptors-gamma agonists, or anti-oxidants may confer renoprotection from oxidation and suppression of renal fibrosis, and also on prevention or alleviation of complications in chronic kidney disease. Therefore, Klotho is a highly promising candidate on the horizon as an early biomarker, and as a novel therapeutic agent for chronic kidney disease.

Fibroblast growth factor-23: what we know, what we don't know, and what we need to know

Traditional risk factors of cardiovascular morbidity and mortality such as hypertension, hypercholesterolemia and obesity are paradoxically associated with better outcomes in dialysis patients, and the few trials of interventions targeting modifiable traditional risk factors have yielded disappointing results in this patient population. Non-traditional risk factors such as inflammation, anemia and abnormalities in bone and mineral metabolism have been proposed as potential explanations for the excess mortality seen in patients with chronic kidney disease (CKD) and end-stage renal disease (ESRD), but without clear understanding of what the most important pathophysiologic mechanisms of these risk factors are, which ones might be ideal treatment targets and which therapeutic interventions may be effective and safe in targeting them. Among the novel risk factors, fibroblast growth factor-23 (FGF23) has recently emerged as one of the most powerful predictors of adverse outcomes in patients with CKD and ESRD. FGF23 is a hormone produced by osteoblasts/osteocytes in bone that acts on the kidney to regulate phosphate and vitamin D metabolism through activation of FGF receptor/α-Klotho co-receptor complexes. It is possible that elevated FGF23 may exert its negative impact through distinct mechanisms of action independent from its role as a regulator of phosphorus homeostasis. Elevated circulating FGF23 concentrations have been associated with left ventricular hypertrophy (LVH), and it has been suggested that FGF23 exerts a direct effect on the myocardium. While it is possible that 'off target' effects of FGF23 present in very high concentrations could induce LVH, this possibility is controversial, since α-klotho is not expressed in the myocardium. Another possibility is that FGF23's effect on the heart is mediated indirectly, via 'on target' activation of other humoral pathways. We will review the physiology and pathophysiology of FGF23, the outcomes associated with elevated FGF23 levels, and describe putative mechanisms of action responsible for its negative effects and potential therapeutic strategies to treat these.

The effect of vitamin D status on risk factors for cardiovascular disease

Vitamin-D-related pathways are implicated in various endocrine, inflammatory and endothelial functions. An estimated 1 billion people in the world have vitamin D deficiency or insufficiency, and undiagnosed vitamin D deficiency is common. Vitamin D deficiency is associated with substantial increases in the incidence of hypertension, hyperlipidaemia, myocardial infarction and stroke, as well as in diseases such as chronic kidney disease and type 2 diabetes. Low vitamin D levels also upregulate the renin-angiotensin-aldosterone system, increase inflammation and cause endothelial dysfunction. However, the role of vitamin D deficiency in cardiovascular morbidity and mortality is an emerging and hotly debated topic. Epidemiological studies suggest an association between low vitamin D levels and risk factors for cardiovascular disease, but a causal relationship has not been established, and clinical trials and meta-analyses have not demonstrated convincing evidence that vitamin D therapy improves cardiovascular outcomes. Some evidence suggests that vitamin D status is a biomarker of lifestyle, since unhealthy and sedentary lifestyles are associated with vitamin D insufficiency or deficiency and are also risk factors for cardiovascular complications.

Fibroblast growth factor 23 is a predictor of aortic artery calcification in maintenance hemodialysis patients

BACKGROUND

Our study aimed to investigate the factors associated with elevated plasma FGF23 (cFGF23) levels in maintenance hemodialysis (MHD) patients and to determine whether plasma FGF23 level is related to aortic artery calcification (AAC).

METHODS

This study included 120 MHD patients and 20 controls. The FGF23 level was measured using a C-terminal assay and AAC was detected by a lateral lumbar X-ray plain.

RESULTS

Plasma FGF23 levels were significantly higher among dialysis patients compared to controls: FGF23 level of 27691.42 ± 55646.41 RU/mL in MHD patients versus 49.89 ± 23.94 RU/mL in health people. Significant correlations were observed between FGF23 levels and vintage, intact parathyroid hormone (iPTH), serum phosphate, total calcium, 25(OH)D, urea nitrogen (BUN), and serum creatinine (SCR). Stepwise multiple regression analysis showed that the independent parameters associated with FGF23 level were serum phosphate, total calcium, parathyroid hormone (PTH), SCR, and prealbumin. There were 73 patients (60.83%) with visible calcification in the abdominal aorta. Bivariate analysis showed that AAC score correlated with FGF23, phosphate, total calcium, vintage, age, and diastolic blood pressure. Forward logistic analysis showed that the independent parameters associated with AAC were age, total protein, and Lg FGF23.

CONCLUSION

Plasma FGF23 level is significant increased in hemodialysis patients and is independently associated with AAC.

FGF23 is a novel regulator of intracellular calcium and cardiac contractility in addition to cardiac hypertrophy

Fibroblast growth factor 23 (FGF23) is a hormone released primarily by osteocytes that regulates phosphate and vitamin D metabolism. Recent observational studies in humans suggest that circulating FGF23 is independently associated with cardiac hypertrophy and increased mortality, but it is unknown whether FGF23 can directly alter cardiac function. We found that FGF23 significantly increased cardiomyocyte cell size in vitro, the expression of gene markers of cardiac hypertrophy, and total protein content of cardiac muscle. In addition, FGFR1 and FGFR3 mRNA were the most abundantly expressed FGF receptors in cardiomyocytes, and the coreceptor α-klotho was expressed at very low levels. We tested an animal model of chronic kidney disease (Col4a3(-/-) mice) that has elevated serum FGF23. We found elevations in common hypertrophy gene markers in Col4a3(-/-) hearts compared with wild type but did not observe changes in wall thickness or cell size by week 10. However, the Col4a3(-/-) hearts did show reduced fractional shortening (-17%) and ejection fraction (-11%). Acute exposure of primary cardiomyocytes to FGF23 resulted in elevated intracellular Ca(2+) ([Ca(2+)](i); F/F(o) + 86%) which was blocked by verapamil pretreatment. FGF23 also increased ventricular muscle strip contractility (67%), which was inhibited by FGF receptor antagonism. We hypothesize that although FGF23 can acutely increase [Ca(2+)](i), chronically this may lead to decreases in contractile function or stimulate cardiac hypertrophy, as observed with other stress hormones. In conclusion, FGF23 is a novel bone/heart endocrine factor and may be an important mediator of cardiac Ca(2+) regulation and contractile function during chronic kidney disease.

Arterial klotho expression and FGF23 effects on vascular calcification and function

Recent studies support a role for FGF23 and its co-receptor Klotho in cardiovascular pathology, yet the underlying mechanisms remain largely elusive. Herein, we analyzed the expression of Klotho in mouse arteries and generated a novel mouse model harboring a vascular smooth muscle cell specific deletion of Klotho (Sm22-KL(-/-) ). Arterial Klotho expression was detected at very low levels with quantitative real-time PCR; Klotho protein levels were undetectable by immunohistochemistry and Western blot. There was no difference in arterial Klotho between Sm22-KL(-/-) and wild-type mice, as well as no changes in serum markers of mineral metabolism. Intravenous delivery of FGF23 elicited a rise in renal (0.005; p<0.01) but not arterial Egr-1 expression, a marker of Klotho-dependent FGF23 signaling. Further, the impact of FGF23 on vascular calcification and endothelial response was evaluated in bovine vascular smooth muscle cells (bVSMC) and in a murine ex vivo model of endothelial function, respectively. FGF23 treatment (0.125-2 ng/mL) did not modify calcification in bVSMCs or dilatory, contractile and structural properties in mice arterial specimen ex vivo. Collectively, these results demonstrate that FGF23-Klotho signaling is absent in mouse arteries and that the vascular response was unaffected by FGF23 treatment. Thus, our data do not support Klotho-mediated FGF23 effects in the vasculature although confirmative studies in humans are warranted.

Phosphate binders, vitamin D and calcimimetics in the management of chronic kidney disease-mineral bone disorders (CKD-MBD) in children

In order to minimize complications on the skeleton and to prevent extraskeletal calcifications, the specific aims of the management of chronic kidney disease mineral and bone disorder (CKD-MBD) are to maintain blood levels of serum calcium and phosphorus as close to the normal range as possible, thereby maintaining serum parathyroid hormone (PTH) at levels appropriate for CKD stage, preventing hyperplasia of the parathyroid glands, avoiding the development of extra-skeletal calcifications, and preventing or reversing the accumulation of toxic substances such as aluminum and β2-microglobulin. In order to limit cardiovascular calcification, daily intake of elemental calcium, including from dietary sources and from phosphate binders, should not exceed twice the daily recommended intake for age and should not exceed 2.5 g/day. Calcium-free phosphate binders, such as sevelamer hydrochloride and sevelamer carbonate, are safe and effective alternatives to calcium-based binders, and their use widens the margin of safety for active vitamin D sterol therapy. Vitamin D deficiency is highly prevalent across the spectrum of CKD, and replacement therapy is recommended in vitamin D-deficient and insufficient individuals. Therapy with active vitamin D sterols is recommended after correction of the vitamin D deficiency state and should be titrated based on target PTH levels across the spectrum of CKD. Although the use of calcimimetic drugs has been proven to effectively control the biochemical features of secondary hyperparathyroidism, there is very limited experience with the use of such agents in pediatric patients and especially during the first years of life. Studies are needed to further define the role of such agents in the treatment of pediatric CKD-MBD.

Phosphate is a vascular toxin

Elevated phosphate (P) levels are seen in advanced renal failure and, together with dysregulated calcium, parathyroid hormone and vitamin D levels, contribute to the complex of chronic kidney disease-mineral and bone disease (CKD-MBD). Converging evidence from in vitro, clinical and epidemiological studies suggest that increased P is associated with vascular calcification and mortality. When vessels are exposed to high P conditions in vitro, they develop apoptosis, convert to bone-like cells and develop extensive calcification. Clinical studies in children on dialysis show that high P is associated with increased vessel wall thickness, arterial stiffness and coronary calcification. Epidemiological studies in adult dialysis patients demonstrate a significant and independent association between raised P and mortality. Importantly, raised P is associated with cardiovascular changes even in pre-dialysis CKD, and also in subjects with normal renal function but high P. All P binders can effectively reduce serum P, and this decrease is linked to improved survival. Raised serum P triggers the release of fibroblast growth factor 23 (FGF-23), which has the beneficial effect of increasing P excretion in early CKD, but is increased several 1,000-fold in dialysis, and may be an independent cardiovascular risk factor. Both FGF-23 and its co-receptor Klotho may have direct effects on the vasculature leading to calcification. Fascinatingly, disturbances in FGF-23-Klotho and raised P have also been associated with premature aging. These data suggest that high P levels have adverse vascular effects and that maintaining the serum P levels in the normal range reduces cardiovascular risk and mortality.

Hypophosphatemia and growth

Over the last decade the discovery of fibroblast growth factor 23 (FGF23) and the progressive and ongoing clarification of its role in phosphate and mineral metabolism have led to expansion of the diagnostic spectrum of primary hypophosphatemic syndromes. This article focuses on the impairment of growth in these syndromes. Growth retardation is a common, but not constant, feature and it presents with large variability. As a result of the very low prevalence of other forms of primary hypophosphatemic syndromes, the description of longitudinal growth and the pathogenesis of its impairment have been mostly studied in X-linked hypophosphatemia (XLH) patients and in Hyp mice, the animal model of this disease. In general, children with XLH have short stature with greater shortness of lower limbs than trunk. Treatment with phosphate supplements and 1α vitamin D derivatives heals active lesions of rickets, but does not normalize growth of XLH patients. Patients might benefit from recombinant human growth hormone (rhGH) therapy, which may accelerate the growth rate without increasing body disproportion or correcting hypophosphatemia. These clinical data as well as research findings obtained in Hyp mice suggest that the pathogenesis of defective growth in XLH and other hypophosphatemic syndromes is not entirely dependent on the mineralization disorder and point to other effects of hypophosphatemia itself or FGF23 on the metabolism of bone and growth plate.

Cardiovascular effects of sevelamer in stage 3 CKD

Serum phosphate independently predicts cardiovascular mortality in the general population and CKD, even when levels are in the normal range. Associations between serum phosphate, arterial stiffness, and left ventricular (LV) mass suggest a possible pathophysiological mechanism, potentially mediated by the phosphaturic hormone fibroblast growth factor-23 (FGF-23). To what extent the phosphate binder sevelamer modulates these effects is not well understood. In this single-center, randomized, double-blind, placebo-controlled trial, we enrolled 120 patients with stage 3 nondiabetic CKD. After a 4-week open-label run-in period, during which time all patients received sevelamer carbonate, we randomly assigned 109 patients to sevelamer (n=55) or placebo (n=54) for an additional 36 weeks. We assessed LV mass and systolic and diastolic function with cardiovascular magnetic resonance imaging and echocardiography, and we assessed arterial stiffness by carotid-femoral pulse wave velocity. The mean age was 55 years, and the mean eGFR was 50 ml/min per 1.73 m(2). After 40 weeks, we found no statistically significant differences between sevelamer and placebo with regard to LV mass, systolic and diastolic function, or pulse wave velocity. Only 56% of subjects took ≥ 80% of prescribed therapy; in this compliant subgroup, treatment with sevelamer associated with lower urinary phosphate excretion and serum FGF-23 but not serum phosphate, klotho, vitamin D, or cardiovascular-related outcomes of interest. In conclusion, this study does not provide evidence that sevelamer carbonate improves LV mass, LV function, or arterial stiffness in stage 3 nondiabetic CKD.

Beyond mineral metabolism, is there an interplay between FGF23 and vitamin D in innate immunity?

Fibroblast growth factor 23 (FGF23) is an "endocrine" FGF acting in the kidney as a phosphaturic hormone and a suppressor of active vitamin D, through an inhibition of the 1α hydroxylase and a stimulation of the 24 hydroxylase. Beyond its well-known effects on the bone/kidney/parathyroid axis and its deregulation during chronic kidney disease (CKD), recent evidence has revealed its direct systemic effects on cardiovascular health. In the meantime, studies have highlighted the health implications for vitamin D inside and outside CKD that also extend beyond its classical actions on mineral homeostasis and bone metabolism: vitamin D has indeed been shown to exert pluripotent non-classical effects as a modulator of immune function in monocytes, mainly through the stimulation of the antimicrobial cathelicidin. The aim of this review is to provide new insights on the interplay between FGF23 and vitamin D in innate immunity in the context of CKD.

Phosphorus and risk of renal failure in subjects with normal renal function

PURPOSE

Whether higher serum phosphorus levels increase risk for kidney disease onset and progression to end-stage renal disease in those with normal renal function is largely unknown. We sought to determine whether higher serum phosphorus levels increase risk for end-stage renal disease within a large ethnically diverse population with normal kidney function.

METHODS

A retrospective longitudinal cohort study was performed in the period January 1, 1998 through December 31, 2008 of adults within a vertically integrated health plan (3.4 million members). The primary objective was to determine risk of incident end-stage renal disease. Baseline and time-averaged phosphorus were used for Cox regressions analyses to calculate hazard ratios (HR) adjusting for age, sex, race, pre-existing hypertension, and diabetes.

RESULTS

A total of 94,989 subjects were identified in the 11-year observation period. Mean age of the cohort was 50 years, with 61% female, 38% white, 14% black, and 25% Hispanic. Population-based phosphorus quartile ranges were 1.9-3.0 mg/dL, 3.1-3.4 mg/dL, 3.5-3.8 mg/dL, and 3.9-5.7 mg/dL. End-stage renal disease occurred in 130 (0.1%) subjects. Every 0.5-mg/dL phosphorus increase demonstrated an adjusted HR of 1.40 (95% confidence interval [CI], 1.06-1.84) and HR for mortality of 1.09 (95% CI, 1.06-1.13). Adjusted HRs were 0.64 (95% CI, 0.37-1.11), 0.83 (95% CI, 0.50-1.39), and 1.48 (95% CI, 0.96-2.28) in the 2nd, 3rd, and 4th quartile, respectively, compared with the first phosphorus quartile. Time-averaged serum phosphorus demonstrated a similar relationship across quartiles and as a continuous variable.

CONCLUSION

In our large, ethnically diverse cohort of non kidney disease subjects, higher serum phosphorus levels were associated with greater risk for end-stage renal disease and mortality.

Pharmacological inhibition of fibroblast growth factor (FGF) receptor signaling ameliorates FGF23-mediated hypophosphatemic rickets

Fibroblast growth factor 23 (FGF23) is a circulating factor secreted by osteocytes that is essential for phosphate homeostasis. In kidney proximal tubular cells FGF23 inhibits phosphate reabsorption and leads to decreased synthesis and enhanced catabolism of 1,25-dihydroxyvitamin D3 (1,25[OH]2 D3 ). Excess levels of FGF23 cause renal phosphate wasting and suppression of circulating 1,25(OH)2 D3 levels and are associated with several hereditary hypophosphatemic disorders with skeletal abnormalities, including X-linked hypophosphatemic rickets (XLH) and autosomal recessive hypophosphatemic rickets (ARHR). Currently, therapeutic approaches to these diseases are limited to treatment with activated vitamin D analogues and phosphate supplementation, often merely resulting in partial correction of the skeletal aberrations. In this study, we evaluate the use of FGFR inhibitors for the treatment of FGF23-mediated hypophosphatemic disorders using NVP-BGJ398, a novel selective, pan-specific FGFR inhibitor currently in Phase I clinical trials for cancer therapy. In two different hypophosphatemic mouse models, Hyp and Dmp1-null mice, resembling the human diseases XLH and ARHR, we find that pharmacological inhibition of FGFRs efficiently abrogates aberrant FGF23 signaling and normalizes the hypophosphatemic and hypocalcemic conditions of these mice. Correspondingly, long-term FGFR inhibition in Hyp mice leads to enhanced bone growth, increased mineralization, and reorganization of the disturbed growth plate structure. We therefore propose NVP-BGJ398 treatment as a novel approach for the therapy of FGF23-mediated hypophosphatemic diseases.

FGF23 production by osteocytes

Fibroblast growth factor 23 (FGF23), a known regulator of phosphate homeostasis, is produced by cells residing in bone, namely, osteocytes, to target a distant organ, the kidney. Elevated FGF23 levels have recently been found systemically and in osteocytes in patients and animal models of chronic kidney disease. Associations between serum FGF23 level and vascular dysfunction, vascular calcification, and increased risk of cardiovascular disease have also been observed. In this review we discuss FGF23 expression in osteocytes and the potential means to regulate expression and function of this protein at the osteocyte level.

Fractional excretion of phosphorus modifies the association between fibroblast growth factor-23 and outcomes

Fibroblast growth factor-23 (FGF23) induces phosphaturia through its effects on renal tubules. Higher levels of FGF23 associate with cardiovascular disease (CVD) events and all-cause mortality, but it is unknown whether these associations differ by the degree of phosphaturia. Here, we measured serum FGF23 and 24-hour urine fractional excretion of phosphorus (FePi) in 872 outpatients with stable CVD and a mean estimated GFR of 71 ml/min per 1.73 m(2). During an average 7.5 years of follow-up, there were 337 deaths and 199 CVD events. Urinary FePi significantly modified the association of FGF23 with each outcome (P interaction<0.001 for all-cause mortality and P interaction<0.05 for CVD events). In models adjusted for CVD risk factors, kidney function, and PTH, those patients who had FGF23 above the median (≥ 42.3 relative units [RU]/ml) but FePi below the median (<15.7%) had the highest risks of both all-cause mortality (HR=1.98, 95% CI=1.42-2.77) and CVD events (HR=1.92, 95% CI=1.25-2.94) compared with those patients who had low concentrations of FGF23 and low FePi. In summary, associations of FGF23 with mortality and CVD events are stronger in persons with lower FePi independent of PTH and kidney function. In such individuals, the renal tubular response to FGF23 may be suboptimal.

Effects of dietary phosphate restriction and phosphate binders on FGF23 levels in CKD

BACKGROUND

Elevated levels of fibroblast growth factor 23 (FGF23) are associated with increased risk of adverse outcomes in patients with CKD. Reducing dietary phosphate intake or absorption may decrease FGF23 levels, but data on the combined effects of dietary phosphate restriction and phosphate binders in CKD are limited.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this 2×2 factorial, single-blinded, placebo-controlled, 3-month study, conducted between July 2009 and March 2012, 39 patients with CKD stages 3 or 4 and normal serum phosphate levels were randomly assigned to one of four groups: ad libitum diet plus lanthanum carbonate (LC) placebo (n=10), 900-mg phosphate diet plus LC placebo (n=10), ad libitum diet plus LC (n=11), or 900-mg phosphate diet plus LC (n=8). The dose of LC was 1000 mg three times daily with meals. Dietary restriction was accomplished with outpatient counseling. The primary end point was change in FGF23 levels from baseline.

RESULTS

Compared with ad libitum diet, the 900-mg phosphate diet did not significantly reduce FGF23 levels (diet × time interaction, P=0.05). Compared with placebo, LC alone also did not significantly reduce FGF23 levels (LC × time interaction, P=0.21). However, the dual intervention significantly decreased FGF23 levels throughout the study period (diet × LC × time interaction, P=0.02), resulting in a 35% (95% confidence interval, 8%-62%) reduction by study end.

CONCLUSION

The combination of LC plus counseling for a phosphate-restricted diet decreased FGF23 levels in patients with CKD stages 3-4 and normal serum phosphate levels.

Emerging effects of sevelamer in chronic kidney disease

Sevelamer, a non-absorbable anion exchange resin, is used to control hyperphosphatemia in chronic kidney disease (CKD) by binding to dietary phosphate in the gastrointestinal tract. Lipid-lowering effect is a widely recognized pleiotropic effect of sevelamer. In addition, many studies have reported that sevelamer leads to reduced vascular calcification compared with calcium-containing phosphate binders, which is attributed to the improved lipid profiles and decreased calcium load. In addition, recent studies have suggested novel pleiotropic effects on bone structure, inflammation, oxidative stress, anemia, fetuin-A, and trace element metabolism in CKD patients. All of these effects have the potential to suppress the development/progression of cardiovascular lesions and reduce mortality. This review summarizes novel findings from recent studies and discusses the potential pleiotropic effects of sevelamer on non-traditional cardiovascular risk factors in CKD patients.

Sodium- and phosphorus-based food additives: persistent but surmountable hurdles in the management of nutrition in chronic kidney disease

Sodium- and phosphorus-based food additives are among the most commonly consumed nutrients in the world. This is because both have diverse applications in processed food manufacturing, leading to their widespread use by the food industry. Since most foods are naturally low in salt, sodium additives almost completely account for the excessive consumption of sodium throughout the world. Similarly, phosphorus additives represent a major and "hidden" phosphorus load in modern diets. These factors pose a major barrier to successfully lowering sodium or phosphorus intake in patients with CKD. As such, any serious effort to reduce sodium or phosphorus consumption will require reductions in the use of these additives by the food industry. The current regulatory environment governing the use of food additives does not favor this goal, however, in large part because these additives have historically been classified as generally safe for public consumption. To overcome these barriers, coordinated efforts will be needed to demonstrate that high intake of these additives is not safe for public consumption and as such should be subject to greater regulatory scrutiny.

Renal and extrarenal actions of Klotho

Klotho is a single-pass transmembrane protein highly expressed in the kidney. Membrane Klotho protein acts as a co-receptor for fibroblast growth factor-23. Its extracellular domain is shed from the cell surface and functions as an endocrine substance that exerts multiple renal and extrarenal functions. An exhaustive review is beyond the scope and length of this article; thus, only effects with pertinence to mineral metabolism and renoprotection are highlighted here. Klotho participates in mineral homeostasis via interplay with other calciophosphoregulatory hormones (parathyroid hormone, fibroblast growth factor-23, and 1,25-[OH]2 vitamin D3) in kidney, bone, intestine, and parathyroid gland. Klotho also may be involved in acute and chronic kidney disease development and progression. Acute kidney injury is a temporary and reversible state of Klotho deficiency and chronic kidney disease is a sustained state of systemic Klotho deficiency. Klotho deficiency renders the kidney more susceptible to acute insults, delays kidney regeneration, and promotes renal fibrosis. In addition to direct renal effects, Klotho deficiency also triggers and aggravates deranged mineral metabolism, secondary hyperparathyroidism, vascular calcification, and cardiac hypertrophy and fibrosis. Although studies examining the therapeutic effect of Klotho replacement were performed in animal models, it is quite conceivable that supplementation of exogenous Klotho and/or up-regulation of endogenous Klotho production may be a viable therapeutic strategy for patients with acute or chronic kidney diseases.

Long term calcium intake and rates of all cause and cardiovascular mortality: community based prospective longitudinal cohort study

OBJECTIVE

To investigate the association between long term intake of dietary and supplemental calcium and death from all causes and cardiovascular disease.

DESIGN

Prospective longitudinal cohort study.

SETTING

Swedish mammography cohort, a population based cohort established in 1987-90.

PARTICIPANTS

61 433 women (born between 1914 and 1948) followed-up for a median of 19 years.

MAIN OUTCOME MEASURES

Primary outcome measures, identified from registry data, were time to death from all causes (n=11 944) and cause specific cardiovascular disease (n=3862), ischaemic heart disease (n=1932), and stroke (n=1100). Diet was assessed by food frequency questionnaires at baseline and in 1997 for 38 984 women, and intakes of calcium were estimated. Total calcium intake was the sum of dietary and supplemental calcium.

RESULTS

The risk patterns with dietary calcium intake were non-linear, with higher rates concentrated around the highest intakes (≥1400 mg/day). Compared with intakes between 600 and 1000 mg/day, intakes above 1400 mg/day were associated with higher death rates from all causes (hazard ratio 1.40, 95% confidence interval 1.17 to 1.67), cardiovascular disease (1 49, 1.09 to 2.02), and ischaemic heart disease (2.14, 1.48 to 3.09) but not from stroke (0.73, 0.33 to 1.65). After sensitivity analysis including marginal structural models, the higher death rate with low dietary calcium intake (<600 mg/day) or with low and high total calcium intake was no longer apparent. Use of calcium tablets (6% users; 500 mg calcium per tablet) was not on average associated with all cause or cause specific mortality but among calcium tablet users with a dietary calcium intake above 1400 mg/day the hazard ratio for all cause mortality was 2.57 (95% confidence interval 1.19 to 5.55).

CONCLUSION

High intakes of calcium in women are associated with higher death rates from all causes and cardiovascular disease but not from stroke.

Cardiorenal syndrome: the emerging role of protein-bound uremic toxins

Cardiorenal syndrome is a condition in which a complex interrelationship between cardiac dysfunction and renal dysfunction exists. Despite advances in treatment of both cardiovascular and kidney disease, cardiorenal syndrome remains a major global health problem. Characteristic of the pathophysiology of cardiorenal syndrome is bidirectional cross-talk; mediators/substances activated by the disease state of 1 organ can play a role in worsening dysfunction of the other by exerting their biologically harmful effects, leading to the progression of the syndrome. Accumulation of uremic toxins is a hallmark of renal excretory dysfunction. Removal of some toxins by conventional dialysis is particularly problematic because of their high protein binding. In this review, we demonstrate that protein-bound uremic toxins may play an important role in progression of cardiovascular disease in the setting of chronic kidney disease. The highly protein-bound uremic toxin indoxyl sulfate has emerged as a potent toxin adversely affecting both the kidney and heart. Direct cardiac effects of this toxin have been recently demonstrated both in vitro and in vivo. Specifically, potent fibrogenic and prohypertrophic effects, as well as oxidative stress-inducing effects, appear to play a central role in both renal and cardiac pathology. Many of these adverse effects can be suppressed by use of a gut adsorbent, AST-120. Potential mechanisms underlying indoxyl sulfate-induced cardiorenal fibrosis are discussed. Future research and clinical implications conclude this review.

Chronic kidney disease: a clinical model of premature aging

Premature aging is a process associated with a progressive accumulation of deleterious changes over time, an impairment of physiologic functions, and an increase in the risk of disease and death. Regardless of genetic background, aging can be accelerated by the lifestyle choices and environmental conditions to which our genes are exposed. Chronic kidney disease is a common condition that promotes cellular senescence and premature aging through toxic alterations in the internal milieu. This occurs through several mechanisms, including DNA and mitochondria damage, increased reactive oxygen species generation, persistent inflammation, stem cell exhaustion, phosphate toxicity, decreased klotho expression, and telomere attrition. Because recent evidence suggests that both increased local signaling of growth factors (through the nutrient-sensing mammalian target of rapamycin) and decreased klotho expression are important modulators of aging, interventions that target these should be tested in this prematurely aged population.

FGF23 suppresses chondrocyte proliferation in the presence of soluble α-Klotho both in vitro and in vivo

Fibroblast growth factor-23 (FGF23) is well established to play crucial roles in the regulation of phosphate homeostasis. X-linked hypophosphatemic rickets (XLH) is characterized by impaired mineralization and growth retardation associated with elevated circulating FGF23 levels. Administration of phosphate and calcitriol is effective in improving growth retardation, but is not sufficient to fully reverse impaired growth, suggesting the existence of a disease-specific mechanism in the development of growth retardation in addition to dysregulated phosphate metabolism. However, the precise mechanisms of growth retardation in XLH remain elusive. Here, we postulated that FGF23 suppressed chondrocyte proliferation in the presence of soluble α-Klotho (sKL). In vitro and ex vivo studies revealed that FGF23 formed a protein complex with sKL through KL1 internal repeat and suppressed the linear growth of metatarsals in the presence of sKL, which was antagonized by co-incubation with neutralizing antibodies against FGF23 or by knocking-down FGFR3 expression. Additionally, FGF23 binding to FGFR3 was enhanced in the presence of sKL. Histologically, the length of the proliferating zone was diminished and was associated with decreased chondrocyte proliferation. FGF23/sKL suppressed Indian hedgehog (Ihh) expression and administration of Ihh protein partially rescued the suppressive effect of FGF23/sKL on metatarsal growth. Intraperitoneal administration of sKL in Hyp mice, a murine model for XLH, caused a decrease in the length of the proliferating zone associated with decreased chondrocyte proliferation without altering circulating phosphate levels. These findings suggest that suppression of chondrocyte proliferation by FGF23 could have a causative role in the development of growth retardation in XLH.

Serum FGF23 and risk of cardiovascular events in relation to mineral metabolism and cardiovascular pathology

BACKGROUND AND OBJECTIVES

Circulating fibroblast growth factor-23 is associated with adverse cardiovascular outcomes in CKD and non-CKD individuals, but the underlying mechanism remains unclear. This study tested whether this association is independent of mineral metabolism and indices of subclinical cardiovascular pathology.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The prospective association between fibroblast growth factor-23 and major cardiovascular events (a composite of hospital-treated myocardial infarction, hospital-treated stroke, or all-cause mortality) was investigated in the community-based Prospective Investigation of the Vasculature in Uppsala Seniors (n=973; mean age=70 years, 50% women) using multivariate logistic regression. Subjects were recruited between January of 2001 and June of 2004.

RESULTS

During follow-up (median=5.1 years), 112 participants suffered a major cardiovascular event. In logistic regression models adjusted for age, sex, and estimated GFR, higher fibroblast growth factor-23 was associated with increased risk for major cardiovascular events (odds ratio for tertiles 2 and 3 versus tertile 1=1.92, 95% confidence interval=1.19-3.09, P<0.01). After additional adjustments in the model, adding established cardiovascular risk factors, confounders of mineral metabolism (calcium, phosphate, parathyroid hormone, and 25(OH)-vitamin D), and indices of subclinical pathology (flow-mediated vasodilation, endothelial-dependent and -independent vasodilation, arterial stiffness, and atherosclerosis and left ventricular mass) attenuated this relationship, but it remained significant (odds ratio for tertiles 2 and 3 versus tertile 1=1.69, 95% confidence interval=1.01-2.82, P<0.05).

CONCLUSIONS

Fibroblast growth factor-23 is an independent predictor of cardiovascular events in the community, even after accounting for mineral metabolism abnormalities and subclinical cardiovascular damage. Circulating fibroblast growth factor-23 may reflect novel and important aspects of cardiovascular risk yet to be unraveled.

Management of hyperphosphataemia in chronic kidney disease-challenges and solutions

Hyperphosphataemia is a clinical consequence of the advanced stages of chronic kidney disease (CKD). Considerable evidence points to a role of hyperphosphataemia in the pathogenesis of CKD-associated cardiovascular (CV) complications, including vascular calcification, and with increased all-cause and CV mortality. These observations place management of hyperphosphataemia at the centre of CKD treatment. Although our increased understanding of the physiological role of FGF-23 may provide a long-term alternative biomarker of phosphate load and underlying disease progression, regular determination of serum phosphate is currently the most frequently used parameter to evaluate phosphate load in clinical practice. This review considers the challenges physicians and patients face in trying to control hyperphosphataemia. Amongst these are the limitations of dietary phosphate restriction, giving rise to the need for phosphate binder therapy to maintain serum phosphate control. Once the decision to use phosphate binders has been made, considerations include the relative efficacy, different potential side effects and pill burden associated with various phosphate binders. Although a number of phosphate binders are available, adherence poses a major obstacle to effective treatment. This emphasizes that further improvements to phosphate binder therapy can be made. Evaluation of novel agents and their potential role in the clinic should continue.

FGF-23: the rise of a novel cardiovascular risk marker in CKD

Elevated plasma levels of the phosphaturic hormone fibroblast growth factor 23 (FGF-23) are a hallmark of chronic kidney disease (CKD)-mineral and bone disorder. FGF-23 allows serum phosphate levels within physiological limits to be maintained in progressive CKD until end-stage renal disease is reached. Despite its seemingly beneficial role in phosphate homeostasis, several prospective studies in dialysis patients and in patients with less advanced CKD associated elevated FGF-23 with poor cardiovascular and renal outcome. Moreover, very recent evidence suggests an adverse prognostic impact of elevated FGF-23 even in subjects without manifest CKD. These epidemiological data are supplemented by laboratory findings that reveal a pathophysiological role of FGF-23 in the pathogenesis of myocardial injury. In aggregate, these clinical and experimental data identify FGF-23 as a promising target of novel therapeutic interventions in CKD and beyond, which should be tested in future clinical trials.

Reducing cardiovascular mortality in chronic kidney disease: something borrowed, something new

CLINICAL VIGNETTE: A 48-year-old man with chronic kidney disease stage five due to type II diabetes mellitus and hypertension was referred for hemodialysis initiation. His physical exam showed a blood pressure of 150/80, normal fundi, a positive fourth heart sound (S4), and trace pedal edema. Moderate aortic calcification was present on prior chest X-ray. The ECG showed left ventricle hypertrophy by voltage and slight prolongation of the QT interval. Medications included chlorthalidone, amlodipine, carvedilol, cholecalciferol, erythropoietin, and a phosphate binder. What additional therapy should be initiated to reduce vascular calcifications and cardiovascular mortality?

Fibroblast growth factor 23: associations with cardiovascular disease and mortality in chronic kidney disease

Fibroblast growth factor-23 (FGF-23) has emerged as an important hormone involved in phosphorus and vitamin D homeostasis. Chronic kidney disease (CKD) is the most common clinical condition in which FGF-23 levels are persistently and markedly elevated. Abnormal phosphate homeostasis and high circulating levels of FGF-23 are early complications of CKD. Although increases in FGF-23 levels may help maintain serum phosphate levels in the normal range in CKD, the long-term effects of its sustained elevated levels are unclear. Patients with CKD have high risks of developing end-stage renal disease (ESRD), cardiovascular disease, and premature death. Recent prospective studies in populations with predialysis CKD, ESRD on hemodialysis, and kidney transplant recipients demonstrate that elevated FGF-23 levels are independently associated with cardiovascular events and mortality. It was originally thought that FGF-23 was only a biomarker of disturbed phosphate balance; however, recent studies have shown that FGF-23 can have a direct effect on the heart, inducing left ventricular hypertrophy. This suggests that elevated FGF-23 levels may be a novel mechanism that explains the poor cardiovascular outcomes in CKD patients. Interventional studies are required in order to clarify the relation of causality between FGF-23 and cardiovascular mortality in this population.

Ergocalciferol treatment and aspects of mineral homeostasis in patients with chronic kidney disease stage 4-5

BACKGROUND

Focus on non-classical effects and possible less side effects of treatment with nutritional vitamin D, raises the expectation of possible benefits from treating chronic kidney disease (CKD) patients with ergocalciferol (vitamin D2). Treatment with 1,25(OH)2 vitamin D (calcitriol) induces elevated fibroblast growth factor 23 (FGF23), while epidemiological studies have found positive effects of nutritional and 25(OH)vitamin D on mortality in CKD. Disturbed mineral homeostasis in CKD is correlated to adverse outcome and cardiovascular mortality. The objective was to examine the possible effects of treatment with high doses of ergocalciferol on parameters of mineral homeostasis in predialysis CKD patients.

METHODS

A total of 43 adult patients with CKD stage 4-5, not receiving vitamin D supplementation, were studied, and allocated by simple randomization to either an intervention (n = 26) or a control group (n = 17). The intervention group received ergocalciferol, 50.000 IU/week for 6 weeks. Plasma FGF23, creatinine, parathyroid hormone (PTH), phosphate and ionized calcium were obtained at baseline and after the 6 weeks.

RESULTS

The intervention group had a significant increase in 25(OH)D2 concentration from < 10 to 90 ± 4 nmol/L, while 1,25(OH)2D (62 ± 6 at baseline and 67 ± 6 pmol/L at 6 weeks) remained stable. No changes were seen in the circulating vitamin D concentrations in the control group. After the 6 weeks of treatment no significant changes were seen in concentration of creatinine, phosphate, ionized calcium, PTH and FGF23 remained stable.

CONCLUSION

No harmful effects of short-term treatment with high-dose ergocalciferol were observed on markers of mineral homeostasis and FGF23 in CKD patients stage 4-5.

Dietary phosphorus is associated with greater left ventricular mass

Dietary phosphorus consumption has risen steadily in the United States. Oral phosphorus loading alters key regulatory hormones and impairs vascular endothelial function which may lead to an increase in left ventricular mass (LVM). We investigated the association of dietary phosphorus with LVM in 4,494 participants from the Multi-Ethnic Study of Atherosclerosis, a community-based study of individuals free of known cardiovascular disease. The intake of dietary phosphorus was estimated using a 120-item food frequency questionnaire and the LVM was measured using magnetic resonance imaging. Regression models were used to determine associations of estimated dietary phosphorus with LVM and left ventricular hypertrophy (LVH). Mean estimated dietary phosphorus intake was 1,167 mg/day in men and 1,017 mg/day in women. After adjustment for demographics, dietary sodium, total calories, lifestyle factors, comorbidities, and established LVH risk factors, each quintile increase in the estimated dietary phosphate intake was associated with an estimated 1.1 gram greater LVM. The highest gender-specific dietary phosphorus quintile was associated with an estimated 6.1 gram greater LVM compared to the lowest quintile. Higher dietary phosphorus intake was associated with greater odds of LVH among women, but not men. These associations require confirmation in other studies.

Lanthanum Carbonate Modulates Inflammatory Profile in Hemodialysis Patients: Relationship with Fibroblast Growth Factor-23

Elevated fibroblast growth factor 23 (FGF-23) is independently associated with increased inflammatory markers in chronic kidney disease. Lanthanum carbonate (LaCa) reduces FGF-23. We studied the effects of LaCa on inflammatory profile of hemodialysis patients, and the relationship with changes in FGF-23. This prospective study was performed under habitual clinical practice conditions. Twenty-six hemodialysis patients with serum phosphate > 5 mg/dl receiving calcium-based phosphate binders were switched to LaCa. Ten patients with phosphate ≤ 5 mg/dl under calcium-based phosphate binders were enrolled as a control group for comparison. Serum calcium, phosphorus, calcium-phosphate product (CaxP), intact parathyroid hormone and the inflammatory profile [including serum high-sensitivity C-reactive protein (hs-CRP), tumor necrosis factor-α (TNF-α), interleukin (IL)-6 and IL-10, as well as mRNA expression levels of TNF-α and IL-6 in peripheral blood mononuclear cells] were analyzed. Serum FGF-23 significantly decreased in patients switched to LaCa (P< 0.01), with a concomitant reduction in serum hsCRP (-10.9%, P < 0.01), TNF-α (-6.7%, P < 0.05) and IL-6 (-8.1%, P < 0.01). mRNA expression levels of TNF-α and IL-6 in PBMC also decreased by 7.8% (P< 0.05) and 10.3% (P< 0.01), respectively. Multivariate regression analysis demonstrated that variations in FGF-23 were the only independent determinant of the changes in serum and mRNA expression levels of inflammatory parameters. In conclusion, LaCa posses anti-inflammatory actions, which are significant and independently associated with the reduction of FGF-23. FGF-23 may regulate inflammatory cytokine gene expression at the transcriptional level. Whether these effects have influence on clinical outcomes warrants consideration.

Fibroblast growth factor 23 inhibits extrarenal synthesis of 1,25-dihydroxyvitamin D in human monocytes

Vitamin D is a potent stimulator of monocyte innate immunity, and this effect is mediated via intracrine conversion of 25-hydroxyvitamin D (25OHD) to 1,25-dihydroxyvitamin D (1,25(OH)(2) D). In the kidney, synthesis of 1,25(OH)(2) D is suppressed by fibroblast growth factor 23 (FGF23), via transcriptional suppression of the vitamin D-activating enzyme 1α-hydroxylase (CYP27B1). We hypothesized that FGF23 also suppresses CYP27B1 in monocytes, with concomitant effects on intracrine responses to 1,25(OH)(2) D. Healthy donor peripheral blood mononuclear cell monocytes (PBMCm) and peritoneal dialysate monocyte (PDm) effluent from kidney disease patients were assessed at baseline to confirm the presence of mRNA for FGF23 receptors (FGFRs), with Klotho and FGFR1 being more strongly expressed than FGFR2/3/4 in both cell types. Immunohistochemistry showed coexpression of Klotho and FGFR1 in PBMCm and PDm, with this effect being enhanced following treatment with FGF23 in PBMCm but not PDm. Treatment with FGF23 activated mitogen-activated protein kinase (MAPK) and protein kinase B (Akt) pathways in PBMCm, demonstrating functional FGFR signaling in these cells. FGF23 treatment of PBMCm and PDm decreased expression of mRNA for CYP27B1. In PBMCm this was associated with downregulation of 25OHD to 1,25(OH)(2) D metabolism, and concomitant suppression of intracrine induced 24-hydroxylase (CYP24A1) and antibacterial cathelicidin (LL37). FGF23 suppression of CYP27B1 was particularly pronounced in PBMCm treated with interleukin-15 to stimulate synthesis of 1,25(OH)(2) D. These data indicate that FGF23 can inhibit extra-renal expression of CYP27B1 and subsequent intracrine responses to 1,25(OH)(2) D in two different human monocyte models. Elevated expression of FGF23 may therefore play a crucial role in defining immune responses to vitamin D and this, in turn, may be a key determinant of infection in patients with chronic kidney disease (CKD).

Fibroblast growth factor 23 and Klotho: physiology and pathophysiology of an endocrine network of mineral metabolism

The metabolically active and perpetually remodeling calcium phosphate-based endoskeleton in terrestrial vertebrates sets the demands on whole-organism calcium and phosphate homeostasis that involves multiple organs in terms of mineral flux and endocrine cross talk. The fibroblast growth factor (FGF)-Klotho endocrine networks epitomize the complexity of systems biology, and specifically, the FGF23-αKlotho axis highlights the concept of the skeleton holding the master switch of homeostasis rather than a passive target organ as hitherto conceived. Other than serving as a coreceptor for FGF23, αKlotho circulates as an endocrine substance with a multitude of effects. This review covers recent data on the physiological regulation and function of the complex FGF23-αKlotho network. Chronic kidney disease is a common pathophysiological state in which FGF23-αKlotho, a multiorgan endocrine network, is deranged in a self-amplifying vortex resulting in organ dysfunction of the utmost severity that contributes to its morbidity and mortality.

Cardiovascular disease in chronic kidney disease: untying the Gordian knot

Chronic kidney disease (CKD) affects around 10-13% of the general population, with only a small proportion in end stage renal disease (ESRD), either on dialysis or awaiting renal transplantation. It is well documented that CKD patients have an extremely high risk of developing cardiovascular disease (CVD) compared with the general population, so much so that in the early stages of CKD patients are more likely to develop CVD than they are to progress to ESRD. Various pathophysiological pathways and explanations have been advanced and suggested to account for this, including endothelial dysfunction, dyslipidaemia, inflammation, left ventricular hypertrophy and cardiac autonomic dysfunction. In this review, we try to understand and further explore the link between CKD and CVD, as well as offering interventional advice where available, while exposing the current lack of RCT-based research and trial evidence in this area. We also suggest pragmatic Interim measures we could take while we wait for definitive RCTs.

Cardiorenal syndrome: pathophysiology and potential targets for clinical management

Combined dysfunction of the heart and the kidneys, which can be associated with haemodynamic impairment, is classically referred to as cardiorenal syndrome (CRS). Cardiac pump failure with resulting volume retention by the kidneys, once thought to be the major pathophysiologic mechanism of CRS, is now considered to be only a part of a much more complicated phenomenon. Multiple body systems may contribute to the development of this pathologic constellation in an interconnected network of events. These events include heart failure (systolic or diastolic), atherosclerosis and endothelial cell dysfunction, uraemia and kidney failure, neurohormonal dysregulation, anaemia and iron disorders, mineral metabolic derangements including fibroblast growth factor 23, phosphorus and vitamin D disorders, and inflammatory pathways that may lead to malnutrition-inflammation-cachexia complex and protein-energy wasting. Hence, a pathophysiologically and clinically relevant classification of CRS based on the above components would be prudent. With the existing medical knowledge, it is almost impossible to identify where the process has started in any given patient. Rather, the events involved are closely interrelated, so that once the process starts at a particular point, other pathways of the network are potentially activated. Current therapies for CRS as well as ongoing studies are mostly focused on haemodynamic adjustments. The timely targeting of different components of this complex network, which may eventually lead to haemodynamic and vascular compromise and cause refractoriness to conventional treatments, seems necessary. Future studies should focus on interventions targeting these components.

The emerging role of Klotho in clinical nephrology

Klotho is highly expressed in the kidney and a soluble form of Klotho functions as an endocrine substance that exerts multiple actions including the modulation of renal solute transport and the protection of the kidney from a variety of insults in experimental models. At present, the Klotho database is still largely preclinical, but the anticipated forthcoming impact on clinical nephrology can be immense. This manuscript puts these potentials into perspective for the clinician. There is renal and systemic Klotho deficiency in both acute kidney injury (AKI) and chronic kidney disease (CKD). Klotho plummets very early and severely in AKI and represents a pathogenic factor that exacerbates acute kidney damage. In CKD, Klotho deficiency exerts a significant impact on progression of renal disease and extra renal complications. In AKI, soluble Klotho levels in plasma and/or urine may serve as an early biomarker for kidney parenchymal injury. Restoration by exogenous supplementation or stimulation of endogenous Klotho may prevent and/or ameliorate kidney injury and mitigate CKD development. In CKD, Klotho levels may be an indicator of early disease and predict the rate of progression, and presence and severity of soft tissue calcification. The correction of Klotho deficiency may delay progression and forestall development of extra renal complications in CKD. Rarely does one find a molecule with such broad potential applications in nephrology. Klotho can possibly emerge on the horizon as a candidate for an unprecedented sole biomarker and intervention. Nephrologists should monitor the progress of the preclinical studies and the imminently emerging human database.

Mineral metabolites and CKD progression in African Americans

CKD progresses more rapidly to ESRD among African Americans compared with Caucasians. Disordered mineral metabolism is more severe among African Americans with CKD, which might partially explain the accelerated progression of their kidney disease. Here, using data from the African American Study of Kidney Disease and Hypertension, we evaluated longitudinal changes in serum levels of fibroblast growth factor-23 (FGF23), parathyroid hormone (PTH), phosphate, and 25-hydroxyvitamin D in a subset of 420 participants followed for a median of 4 years. We also examined the association of baseline levels of mineral metabolites with risk for ESRD or death in 809 participants. FGF23, PTH, and phosphate levels rose over time; participants with faster rates of decline in measured GFR had the greatest increases in these parameters (P<0.01 for each). Higher baseline levels of FGF23, PTH, and phosphate each associated with increased risk for ESRD or death independent of GFR. FGF23 exhibited a dose-response relationship with outcomes (HR=1.30 per doubling, 95% CI=1.15-1.47; HR=2.24 for highest compared with lowest quartile, 95% CI=1.39-3.60), whereas PTH and phosphate showed nonlinear relationships. Vitamin D insufficiency (<30 ng/ml) was present in 95% of participants, but lower levels did not independently associate with outcomes. Using death-censored ESRD as the outcome produced qualitatively similar results. In conclusion, abnormalities of mineral metabolism worsen with progressive CKD and associate with higher risk for ESRD among African Americans with hypertensive nephrosclerosis.