FGF23 fails to inhibit uremic parathyroid glands

Current therapeutic approach of chronic kidney disease-mineral and bone disorder

Chronic kidney disease (CKD) has emerged as one of the leading noncommunicable diseases affecting >10% of the population worldwide. Bone and mineral disorders are a common complication among patients with CKD resulting in a poor life quality, high fracture risk, increased morbidity and cardiovascular mortality. According to Kidney Disease: Improving Global Outcomes, renal osteodystrophy refers to changes in bone morphology found in bone biopsy, whereas CKD-mineral and bone disorder (CKD-MBD) defines a complex of disturbances including biochemical and hormonal alterations, disorders of bone and mineral metabolism and extraskeletal calcification. As a result, the management of CKD-MBD should focus on the aforementioned parameters, including the treatment of hyperphosphatemia, hypocalcemia, abnormal PTH and vitamin D levels. Regarding the bone fragility fractures, osteoporosis and renal osteodystrophy, which constitute the bone component of CKD-MBD, anti-osteoporotic agents constitute the mainstay of treatment. However, a thorough elucidation of the CKD-MBD pathogenesis is crucial for the ideal personalized treatment approach. In this paper, we review the pathology and management of CKD-MBD based on the current literature with special attention to recent advances.

Secondary hyperparathyroidism in chronic kidney disease: pathophysiology, current treatments and investigational drugs

INTRODUCTION

Secondary hyperparathyroidism (SHPT) is a common complication of chronic kidney disease (CKD). It begins as an adaptive increase in parathyroid hormone levels to prevent calcium and phosphate derangements. Over time, this condition becomes maladaptive and is associated with increased morbidity and mortality. Current therapies encompass phosphate-lowering strategies, vitamin D analogues, calcimimetics and parathyroidectomy. These approaches harbor inherent limitations, stimulating interest in the development of new drugs for SHPT to overcome these limitations and improve survival and quality of life among CKD patients.

AREAS COVERED

This review delves into the main pathophysiological mechanisms involved in SHPT, alongside the treatment options that are currently available and under active investigation. Data presented herein stem from a comprehensive search conducted across PubMed, Web of Science, ClinicalTrials.gov and International Clinical Trials Registry Platform (ICTRP) spanning from 2000 onwards.

EXPERT OPINION

The advancements in investigational drugs for SHPT hold significant promise for enhancing treatment efficacy while minimizing side effects associated with conventional therapies. Although several challenges still hinder their adoption in clinical practice, ongoing research will likely continue to expand the available therapeutic options, refine treatment strategies, and tailor them to individual patient profiles.

Roles of Parathyroid Hormone and Fibroblast Growth Factor 23 in Advanced Chronic Kidney Disease

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) each play a central role in the pathogenesis of chronic kidney disease (CKD)-mineral and bone disorder. Levels of both hormones increase progressively in advanced CKD and can lead to damage in multiple organs. Secondary hyperparathyroidism (SHPT), characterized by parathyroid hyperplasia with increased PTH secretion, is associated with fractures and mortality. Emerging evidence suggests that these associations may be partially explained by PTH-induced browning of adipose tissue and increased energy expenditure. Observational studies suggest a survival benefit of PTHlowering therapy, and a recent study comparing parathyroidectomy and calcimimetics further suggests the importance of intensive PTH control. The mechanisms underlying the regulation of FGF23 secretion by osteocytes in response to phosphate load have been unclear, but recent experimental studies have identified glycerol-3-phosphate, a byproduct of glycolysis released by the kidney, as a key regulator of FGF23 production. Elevated FGF23 levels have been shown to be associated with mortality, and experimental data suggest off-target adverse effects of FGF23. However, the causal role of FGF23 in adverse outcomes in CKD patients remains to be established. Further studies are needed to determine whether intensive SHPT control improves clinical outcomes and whether treatment targeting FGF23 can improve patient outcomes.

The Molecular Mechanisms Underlying the Systemic Effects Mediated by Parathormone in the Context of Chronic Kidney Disease

Chronic kidney disease (CKD) stands as a prominent non-communicable ailment, significantly impacting life expectancy. Physiopathology stands mainly upon the triangle represented by parathormone-Vitamin D-Fibroblast Growth Factor-23. Parathormone (PTH), the key hormone in mineral homeostasis, is one of the less easily modifiable parameters in CKD; however, it stands as a significant marker for assessing the risk of complications. The updated "trade-off hypothesis" reveals that levels of PTH spike out of the normal range as early as stage G2 CKD, advancing it as a possible determinant of systemic damage. The present review aims to review the effects exhibited by PTH on several organs while linking the molecular mechanisms to the observed actions in the context of CKD. From a diagnostic perspective, PTH is the most reliable and accessible biochemical marker in CKD, but its trend bears a higher significance on a patient's prognosis rather than the absolute value. Classically, PTH acts in a dichotomous manner on bone tissue, maintaining a balance between formation and resorption. Under the uremic conditions of advanced CKD, the altered intestinal microbiota majorly tips the balance towards bone lysis. Probiotic treatment has proven reliable in animal models, but in humans, data are limited. Regarding bone status, persistently high levels of PTH determine a reduction in mineral density and a concurrent increase in fracture risk. Pharmacological manipulation of serum PTH requires appropriate patient selection and monitoring since dangerously low levels of PTH may completely inhibit bone turnover. Moreover, the altered mineral balance extends to the cardiovascular system, promoting vascular calcifications. Lastly, the involvement of PTH in the Renin-Angiotensin-Aldosterone axis highlights the importance of opting for the appropriate pharmacological agent should hypertension develop.

Roles of PTH and FGF23 in kidney failure: a focus on nonclassical effects

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) each play a central role in the pathogenesis of chronic kidney disease-mineral and bone disorder (CKD-MBD). Both hormones increase as kidney function declines, presumably as a response to maintain normal phosphate balance, but when patients reach kidney failure, PTH and FGF23 fail to exert their phosphaturic effects, leading to hyperphosphatemia and further elevations in PTH and FGF23. In patients with kidney failure, the major target organ for PTH is the bone, but elevated PTH is also associated with mortality presumably through skeletal and nonskeletal mechanisms. Indeed, accumulated evidence suggests improved survival with PTH-lowering therapies, and a more recent study comparing parathyroidectomy and calcimimetic treatment further suggests a notion of "the lower, the better" for PTH control. Emerging data suggest that the link between SHPT and mortality could in part be explained by the action of PTH to induce adipose tissue browning and wasting. In the absence of a functioning kidney, the classical target organ for FGF23 is the parathyroid gland, but FGF23 loses its hormonal effect to suppress PTH secretion owing to the depressed expression of parathyroid Klotho. In this setting, experimental data suggest that FGF23 exerts adverse nontarget effects, but it remains to be confirmed whether FGF23 directly contributes to multiple organ injury in patients with kidney failure and whether targeting FGF23 can improve patient outcomes. Further efforts should be made to determine whether intensive control of SHPT improves clinical outcomes and whether nephrologists should aim at controlling FGF23 levels just as with PTH levels.

Silver jubilee: 25 years of the first demonstration of the direct effect of phosphate on the parathyroid cell

Although phosphorus is an essential element for life, it is not found in nature in its native state but rather combined in the form of inorganic phosphates (PO₄^3-), with tightly regulated plasma levels that are associated with deleterious effects and mortality when these are out of bounds. The growing interest in the accumulation of PO₄^3- in human pathophysiology originated in its attributed role in the pathogenesis of secondary hyperparathyroidism (SHPT) in chronic kidney disease. In this article, we review the mechanisms by which this effect was justified and we commemorate the important contribution of a Spanish group led by Dr. M. Rodríguez, just 25 years ago, when they first demonstrated the direct effect of PO₄^3- on the regulation of the synthesis and secretion of parathyroid hormone by maintaining the structural integrity of the parathyroid glands in their original experimental model. In addition to demonstrating the importance of arachidonic acid (AA) and the phospholipase A2-AA pathway as a mediator of parathyroid gland response, these findings were predecessors of the recent description of the important role of PO₄^3- on the activity of the calcium sensor-receptor, and also fueled various lines of research on the importance of PO₄^3- overload not only for the pathophysiology of SHPT but also in its systemic pathogenic role.

Mineral Bone Disorders in Kidney Disease Patients: The Ever-Current Topic

Chronic kidney disease (CKD) is a complex and multifactorial disease, and one of the most prevalent worldwide. Chronic kidney disease-mineral bone disorders (CKD-MBD) with biochemical and hormonal alterations are part of the complications associated with the progression of CKD. Pathophysiology of CKD-MBD focused on abnormalities in serum levels of several biomarkers (such as FGF-23, klotho, phosphate, calcium, vitamin D, and PTH) which are discussed in this review. We therefore examine the prognostic association between CKD-MBD and the increased risk for cardiovascular events, mortality, and CKD progression to end-stage kidney disease (ESKD). Lastly, we present specific treatments acting on CKD to prevent and treat the complications associated with secondary hyperparathyroidism (SHPT): control of hyperphosphatemia (with dietary restriction, intestinal phosphate binders, and adequate dialysis), the use of calcimimetic agents, vitamin D, and analogues, and the use of bisphosphonates or denosumab in patients with osteoporosis.

The direct effect of fibroblast growth factor 23 on vascular smooth muscle cell phenotype and function

BACKGROUND

In chronic kidney disease (CKD) patients, increased levels of fibroblast growth factor 23 (FGF23) are associated with cardiovascular mortality. The relationship between FGF23 and heart hypertrophy has been documented, however, it is not known whether FGF23 has an effect on vasculature. Vascular smooth muscle cells VSMCs may exhibit different phenotypes; our hypothesis is that FGF23 favours a switch from a contractile to synthetic phenotype that may cause vascular dysfunction. Our objective was to determine whether FGF23 may directly control a change in VSMC phenotype.

METHODS

This study includes in vitro, in vivo and ex vivo experiments and evaluation of patients with CKD stages 2-3 studying a relationship between FGF23 and vascular dysfunction.

RESULTS

In vitro studies show that high levels of FGF23, by acting on its specific receptor FGFR1 and Erk1/2, causes a change in the phenotype of VSMCs from contractile to synthetic. This change is mediated by a downregulation of miR-221/222, which augments the expression of MAP3K2 and PAK1. miR-221/222 transfections recovered the contractile phenotype of VSMCs. Infusion of recombinant FGF23 to rats increased vascular wall thickness, with VSMCs showing a synthetic phenotype with a reduction of miR-221 expression. Ex-vivo studies on aortic rings demonstrate also that high FGF23 increases arterial stiffening. In CKD 2-3 patients, elevation of FGF23 was associated with increased pulse wave velocity and reduced plasma levels of miR-221/222.

CONCLUSION

In VSMCs, high levels of FGF23, through the downregulation of miR-221/222, causes a change to a synthetic phenotype. This change in VSMCs increases arterial stiffening and impairs vascular function, which might ultimately worsen cardiovascular disease.

Pediatric Mineral and Bone Disorder of Chronic Kidney Disease and Cardiovascular Disease

Chronic kidney disease is common and causes significant morbidity including shortened lifespans and decrease in quality of life for patients. The major cause of mortality in chronic kidney disease is cardiovascular disease. Cardiovascular disease within the chronic kidney disease population is closely tied with disordered calcium and phosphorus metabolism and driven in part by renal bone disease. The complex nature of renal, bone, and cardiovascular diseases was renamed as mineral and bone disorder of chronic kidney disease to encompass how bone disease drives vascular calcification and contributes to the development of long-term cardiovascular disease, and recent data suggest that managing bone disease well can augment and improve cardiovascular disease status. Pediatric nephrologists have additional obstacles in optimal mineral and bone disorder of chronic kidney disease management such as linear growth and skeletal maturation. In this article, we will discuss cardiovascular and bone diseases in chronic kidney disease and end-stage kidney disease patients with a focus on pediatric issues and concerns.

Chronic Kidney Disease-Mineral Bone Disease biomarkers in kidney transplant patients

Background:

Chronic Kidney Disease associated with Mineral Bone Disease (CKD-MBD) is frequent in kidney transplant patients. Post-transplantation bone disease is complex, especially in patients with pre-existing metabolic bone disorders that are further affected by immunosuppressive medications and changes in renal allograft function. Main biochemical abnormalities of mineral metabolism in kidney transplantation (KTx) include hypophosphatemia, hyperparathyroidism (HPTH), insufficiency or deficiency of vitamin D, and hypercalcemia.

Objective:

This review aimed to summarize the pathophysiology and main biomarkers of CKD-MBD in KTx.

Methods:

A comprehensive and non-systematic search in PubMed was independently made with an emphasis on biomarkers in mineral bone disease in KTx.

Results:

CKD-MBD can be associated with numerous factors including secondary HPTH, metabolic dysregulations before KTx, and glucocorticoids therapy in post-transplant subjects. Fibroblast growth factor 23 (FGF23) reaches normal levels after KTx with good allograft function, while calcium, vitamin D and phosphorus, ultimately, result in hypercalcemia, persistent vitamin D insufficiency, and hypophosphatemia respectively. As for PTH levels, there is an initial tendency of a significant decrease, followed by a raise due to secondary or tertiary HPTH. In regard to sclerostin levels, there is no consensus in the literature.

Conclusion:

KTx patients should be continuously evaluated for mineral homeostasis and bone status, both cases with successful kidney transplantation and those with reduced functionality. Additional research on CKD-MBD pathophysiology, diagnosis, and management is essential to guarantee long-term graft function, better prognosis, good quality of life, and reduced mortality for KTx patients.

Molecular Mechanisms of Parathyroid Disorders in Chronic Kidney Disease

Secondary hyperparathyroidism (SHP) is a common complication of chronic kidney disease (CKD) that induces morbidity and mortality in patients. How CKD stimulates the parathyroid to increase parathyroid hormone (PTH) secretion, gene expression and cell proliferation remains an open question. In experimental SHP, the increased PTH gene expression is post-transcriptional and mediated by PTH mRNA–protein interactions that promote PTH mRNA stability. These interactions are orchestrated by the isomerase Pin1. Pin1 participates in conformational change-based regulation of target proteins, including mRNA-binding proteins. In SHP, Pin1 isomerase activity is decreased, and thus, the Pin1 target and PTH mRNA destabilizing protein KSRP fails to bind PTH mRNA, increasing PTH mRNA stability and levels. An additional level of post-transcriptional regulation is mediated by microRNA (miRNA). Mice with parathyroid-specific knockout of Dicer, which facilitates the final step in miRNA maturation, lack parathyroid miRNAs but have normal PTH and calcium levels. Surprisingly, these mice fail to increase serum PTH in response to hypocalcemia or uremia, indicating a role for miRNAs in parathyroid stimulation. SHP often leads to parathyroid hyperplasia. Reduced expressions of parathyroid regulating receptors, activation of transforming growth factor α-epidermal growth factor receptor, cyclooxygenase 2-prostaglandin E2 and mTOR signaling all contribute to the enhanced parathyroid cell proliferation. Inhibition of mTOR by rapamycin prevents and corrects the increased parathyroid cell proliferation of SHP. This review summarizes the current knowledge on the mechanisms that stimulate the parathyroid cell at multiple levels in SHP.

The association of fibroblast growth factor 23 at mid-life and late-life with subsequent risk of cardiovascular disease: The Atherosclerosis Risk in Communities (ARIC) Study

Background

Fibroblast growth factor 23 (FGF-23) regulates phosphorus and is associated with cardiovascular disease (CVD), particularly in patients with chronic kidney disease. However, data are limited regarding its contribution to different CVD subtypes across wide age ranges in the general population.

Methods

Using data from ARIC, we evaluated the associations of FGF-23 with heart failure (HF), coronary heart disease (CHD), stroke, and composite CVD (any CVD event) in 12,039 participants at mid-life (visit 3 [1993-1995], mean age 60.0 [SD 5.7] years) and 5608 of the same participants at late-life (visit 5 [2011-2013], 75.5 [5.1] years).

Results

During a median of 9.0 years from visit 3 and 6.9 years from visit 5, we observed 1636 and 1137 composite CVD events, respectively. Higher FGF-23 at visit 5, but not necessarily at visit 3, was significantly associated with the risk of CVD independently of potential confounders including kidney function (adjusted HRs for top vs. bottom quartile, 1.56 [95% CI, 1.30-1.87] at visit 5 and 1.10 [95% CI, 0.95-1.27] at visit 3, p-for-difference < 0.001). We observed similar patterns in key demographic and clinical subgroups without interactions. Among CVD subtypes, HF was the only subtype robustly associated with higher FGF-23 at both visits.

Conclusion

Higher FGF-23 concentrations at late-life but not necessarily at mid-life were independently associated with the risk of CVD. Among CVD subtypes tested, only HF showed robust associations with FGF-23 at both mid-life and late-life.

Klotho and Mesenchymal Stem Cells: A Review on Cell and Gene Therapy for Chronic Kidney Disease and Acute Kidney Disease

Chronic kidney disease (CKD) and acute kidney injury (AKI) are public health problems, and their prevalence rates have increased with the aging of the population. They are associated with the presence of comorbidities, in particular diabetes mellitus and hypertension, resulting in a high financial burden for the health system. Studies have indicated Klotho as a promising therapeutic approach for these conditions. Klotho reduces inflammation, oxidative stress and fibrosis and counter-regulates the renin-angiotensin-aldosterone system. In CKD and AKI, Klotho expression is downregulated from early stages and correlates with disease progression. Therefore, the restoration of its levels, through exogenous or endogenous pathways, has renoprotective effects. An important strategy for administering Klotho is through mesenchymal stem cells (MSCs). In summary, this review comprises in vitro and in vivo studies on the therapeutic potential of Klotho for the treatment of CKD and AKI through the administration of MSCs.

Clinical factors associated with severe hypophosphataemia after kidney transplant

BACKGROUND

The mechanism by which hypophosphataemia develops following kidney transplantation remains debated, and limited research is available regarding risk factors. This study aimed to assess the association between recipient and donor variables, and the severity of post-transplantation hypophosphataemia.

METHODS

We performed a single-centre retrospective observational study. We assessed the association between demographic, clinical and biochemical variables and the development of hypophosphataemia. We used linear regression analysis to assess association between these variables and phosphate nadir.

RESULTS

87.6% of patients developed hypophosphataemia. Patients developing hypophosphataemia were younger, had a shorter time on renal replacement therapy, were less likely to have had a parathyroidectomy or to experience delayed graft function, were more likely to have received a living donor transplant, from a younger donor. They had higher pre-transplantation calcium levels, and lower alkaline phosphatase levels. Receipt of a living donor transplant, lower donor age, not having had a parathyroidectomy, receiving a transplant during the era of tacrolimus-based immunosuppression, not having delayed graft function, higher pre-transplantation calcium, and higher pre-transplantation phosphate were associated with lower phosphate nadir by multiple linear regression.

CONCLUSIONS

This analysis demonstrates an association between variables relating to better graft function and hypophosphataemia. The links with biochemical measures of mineral-bone disease remain less clear.

Non-classical Vitamin D Actions for Renal Protection

Chronic Kidney Disease (CKD), a disorder that affects 11% of the world's population, is characterized by an acceleration in skeletal, immune, renal, and cardiovascular aging that increases the risk of cardiovascular mortality by 10- to 20-fold, compared to that in individuals with normal renal function. For more than two decades, the progressive impairment in renal capacity to maintain normal circulating levels of the hormonal form of vitamin D (1,25-dihydroxyvitamin D or calcitriol) was considered the main contributor to the reduced survival of CKD patients. Accordingly, calcitriol administration was the treatment of choice to attenuate the progression of secondary hyperparathyroidism (SHPT) and its adverse impact on bone health and vascular calcification. The development of calcitriol analogs, designed to mitigate the resistance to calcitriol suppression of PTH associated with CKD progression, demonstrated survival benefits unrelated to the control of SHPT or skeletal health. The exhaustive search for the pathophysiology behind survival benefits associated with active vitamin D analogs has identified novel anti-inflammatory, anti-hypertensive, anti-aging actions of the vitamin D endocrine system. A major paradigm shift regarding the use of calcitriol or active vitamin D analogs to improve survival in CKD patients emerged upon demonstration of a high prevalence of vitamin D (not calcitriol) deficiency at all stages of CKD and, more significantly, that maintaining serum levels of the calcitriol precursor, 25(OH)vitamin D, above 23 ng/ml delayed CKD progression. The cause of vitamin D deficiency in CKD, however, is unclear since vitamin D bioactivation to 25(OH)D occurs mostly at the liver. Importantly, neither calcitriol nor its analogs can correct vitamin D deficiency. The goals of this chapter are to present our current understanding of the pathogenesis of vitamin D deficiency in CKD and of the causal link between defective vitamin D bioactivation to calcitriol and the onset of molecular pathways that promote CKD progression independently of the degree of SHPT. An understanding of these mechanisms will highlight the need for identification of novel sensitive biomarkers to assess the efficacy of interventions with vitamin D and/or calcitriol(analogs) to ameliorate CKD progression in a PTH-independent manner.

A feline-focused review of chronic kidney disease-mineral and bone disorders - Part 2: Pathophysiology of calcium disorder and extraosseous calcification

Derangements in mineral metabolism are one of the main entities in chronic kidney disease-mineral and bone disorder (CKD-MBD). This is the second of a two-part review of the physiology and pathophysiology of calcium homeostasis in feline CKD-MBD. While dysregulation in calcium homeostasis is known to contribute to the development of vascular calcification in CKD, evidence characterising the relationship between serum calcium concentration and nephrocalcinosis and nephrolithiasis is limited. Recently, fibroblast growth factor 23 (FGF23) and α-Klotho have gained increased research interest and been shown to be important biomarkers for the prediction of CKD progression in human patients. However, conflicting evidence exists on their role in calcium homeostasis and vascular and soft tissue calcification. This review details the pathophysiology of calcium disorders associated with CKD-MBD and its implications on vascular and soft tissue mineralisation in human and feline patients. Further prospective studies investigating the clinical consequences of calcium disturbances in cats with CKD are warranted and this may provide additional insight into the pathophysiology of feline CKD-MBD.

Bone Disease in CKD in Children

This manuscript discusses mineral and bone disorders of chronic kidney disease (MBD-CKD) in pediatric patients with special emphasis on the underlying pathophysiology, the causes and clinical profile of growth retardation, the alterations in the growth plate, the strategies to optimize growth and the medical recommendations for prevention and treatment.

Factors Affecting Bone Health in Kidney Transplant Recipients: Klotho Gene Single-Nucleotide Polymorphisms and Other Clinical Features

OBJECTIVES

Posttransplant bone diseases are a major cause of morbidity in kidney transplant recipients. We investigated the relationship between klotho gene single-nucleotide polymorphisms and bone diseases after kidney transplant. We also aimed to identify possible risk factors for development of bone disease.

MATERIALS AND METHODS

The study consisted of 251 kidney transplant recipients (164 men and 87 women) with minimum follow-up of 3 years after kidney transplant. Patients with prolonged immobilization, malignancy, parathyroidectomy, glomerular filtration rates less than 30 mL/min/1.73 m², hypo- or hyperthyroidism, and treatment with drugs that affect bone metabolism were excluded. We investigated the relationship between 6 single-nucleotide polymorphisms of the klotho gene (rs480780, rs211234, rs576404, rs211235, rs9536314, and rs1207568) and development of osteoporosis, avascular bone necrosis, and persistent hyperparathyroidism.

RESULTS

Longer dialysis treatment (odds ratio, 1.13; P = .002) and rs211235 single-nucleotide polymorphism in the klotho gene (odds ratio, 9.87; P = .001 for GG genotype) were significantly associated with persistent hyperparathyroidism. A higher magnesium level was detected as a protective factor from development of persistent hyperparathyroidism (odds ratio, 0.19; P = .009). Persistent hyperparathyroidism was defined as a risk factor for development of osteopenia/osteoporosis (odds ratio, 2.76; P = .003) and avascular bone necrosis (odds ratio, 2.52; P= .03). Although the rs480780 (odds ratio, 8.73; P = .04) single-nucleotide polymorphism in the klotho gene was defined as a risk factor for development of osteopenia/osteoporosis, none of the klotho single-nucleotide polymorphisms was found to be associated with development of avascular bone necrosis.

CONCLUSIONS

Persistent hyperparathyroidism could be an important indicator for development of bone disease in kidney transplant recipients. Also, some of the klotho gene single-nucleotide polymorphisms are associated with higher risk for bone disease after kidney transplant.

New Aspects of the Kidney in the Regulation of Fibroblast Growth Factor 23 (FGF23) and Mineral Homeostasis

The bone-derived hormone fibroblast growth factor 23 (FGF23) acts in concert with parathyroid hormone (PTH) and the active vitamin D metabolite calcitriol in the regulation of calcium (Ca) and phosphate (P) homeostasis. More factors are being identified to regulate FGF23 levels and the endocrine loops between the three hormones. The present review summarizes the complex regulation of FGF23 and the disturbed FGF23/Klotho system in chronic kidney disease (CKD). In addition to the reduced ability of the injured kidney to regulate plasma levels of FGF23, several CKD-related factors have been shown to stimulate FGF23 production. The high circulating FGF23 levels have detrimental effects on erythropoiesis, the cardio-vascular system and the immune system, all contributing to the disturbed system biology in CKD. Moreover, new factors secreted by the injured kidney and the uremic calcified vasculature play a role in the mineral and bone disorder in CKD and create a vicious pathological crosstalk.

Osteoporosis in Patients with Chronic Kidney Diseases: A Systemic Review

Chronic kidney disease (CKD) is associated with the development of mineral bone disorder (MBD), osteoporosis, and fragility fractures. Among CKD patients, adynamic bone disease or low bone turnover is the most common type of renal osteodystrophy. The consequences of CKD-MBD include increased fracture risk, greater morbidity, and mortality. Thus, the goal is to prevent the occurrences of fractures by means of alleviating CKD-induced MBD and treating subsequent osteoporosis. Changes in mineral and humoral metabolism as well as bone structure develop early in the course of CKD. CKD-MBD includes abnormalities of calcium, phosphorus, PTH, and/or vitamin D; abnormalities in bone turnover, mineralization, volume, linear growth, or strength; and/or vascular or other soft tissue calcification. In patients with CKD-MBD, using either DXA or FRAX to screen fracture risk should be considered. Biomarkers such as bALP and iPTH may assist to assess bone turnover. Before initiating an antiresorptive or anabolic agent to treat osteoporosis in CKD patients, lifestyle modifications, such as exercise, calcium, and vitamin D supplementation, smoking cessation, and avoidance of excessive alcohol intake are important. Managing hyperphosphatemia and SHPT are also crucial. Understanding the complex pathogenesis of CKD-MBD is crucial in improving one's short- and long-term outcomes. Treatment strategies for CKD-associated osteoporosis should be patient-centered to determine the type of renal osteodystrophy. This review focuses on the mechanism, evaluation and management of patients with CKD-MBD. However, further studies are needed to explore more details regarding the underlying pathophysiology and to assess the safety and efficacy of agents for treating CKD-MBD.

miR-129 Blocks Secondary Hyperparathyroidism-Inducing Fgf23/αKlotho Signaling in Mice with Chronic Kidney Disease

BACKGROUND

Secondary hyperparathyroidism, a condition of excess parathyroid hormone (PTH, Pth) production, is often seen in chronic kidney disease (CKD) patients with elevated fibroblast growth factor 23 (FGF23, Fgf23). Elevated FGF23 levels stimulate secondary hyperparathyroidism-associated parathyroid αKlotho signaling. As overexpression of rationally selected microRNAs can suppress target gene activation, we hypothesized that microRNA-based suppression of parathyroid FGF23/αKlotho axis activity may be a potential strategy to combat secondary hyperparathyroidism.

METHODS

In vitro luciferase assays and human parathyroid adenoma cell experiments were used to determine miR-129-1-3p's effects on αKlotho expression in vitro. We also studied the effects of parathyroid-specific miR-129-1 overexpression (miR-129Ox) in CKD and non-CKD mice and parathyroid tissue cultures derived therefrom.

RESULTS

miR-129-1-3p directly targets the αKlotho mRNA strand in human parathyroid cells. miR-129Ox CKD mice and control CKD mice displayed comparable serum levels of calcium, phosphate, Fgf23, and 1,25-dihydroxyvitamin D (1,25(OH)₂D). However, miR-129Ox CKD mice displayed reduced parathyroid αKlotho expression and lower circulating Pth levels. In vitro culture of miR-129Ox CKD murine parathyroid tissue showed suppressed responses to Fgf23, with decreased Pth secretion and diminished cell proliferation after four days.

CONCLUSIONS

miR-129 negatively regulates pro-proliferative, Pth-inducing Fgf23/α​Klotho signaling in the parathyroid glands of CKD mice.

Recent advances in understanding and managing secondary hyperparathyroidism in chronic kidney disease

Secondary hyperparathyroidism is a complex pathology that develops as chronic kidney disease progresses. The retention of phosphorus and the reductions in calcium and vitamin D levels stimulate the synthesis and secretion of parathyroid hormone as well as the proliferation rate of parathyroid cells. Parathyroid growth is initially diffuse but it becomes nodular as the disease progresses, making the gland less susceptible to be inhibited. Although the mechanisms underlying the pathophysiology of secondary hyperparathyroidism are well known, new evidence has shed light on unknown aspects of the deregulation of parathyroid function. Secondary hyperparathyroidism is an important feature of chronic kidney disease-mineral and bone disorder and plays an important role in the development of bone disease and vascular calcification. Thus, part of the management of chronic kidney disease relies on maintaining acceptable levels of mineral metabolism parameters in an attempt to slow down or prevent the development of secondary hyperparathyroidism. Here, we will also review the latest evidence regarding several aspects of the clinical and surgical management of secondary hyperparathyroidism.

Parathyroid Cell Proliferation in Secondary Hyperparathyroidism of Chronic Kidney Disease

Secondary hyperparathyroidism (SHP) is a common complication of chronic kidney disease (CKD) that correlates with morbidity and mortality in uremic patients. It is characterized by high serum parathyroid hormone (PTH) levels and impaired bone and mineral metabolism. The main mechanisms underlying SHP are increased PTH biosynthesis and secretion as well as increased glandular mass. The mechanisms leading to parathyroid cell proliferation in SHP are not fully understood. Reduced expressions of the receptors for calcium and vitamin D contribute to the disinhibition of parathyroid cell proliferation. Activation of transforming growth factor-α-epidermal growth factor receptor (TGF-α-EGFR), nuclear factor kappa B (NF-kB), and cyclooxygenase 2- prostaglandin E2 (Cox2-PGE2) signaling all correlate with parathyroid cell proliferation, underlining their roles in the development of SHP. In addition, the mammalian target of rapamycin (mTOR) pathway is activated in parathyroid glands of experimental SHP rats. Inhibition of mTOR by rapamycin prevents and corrects the increased parathyroid cell proliferation of SHP. Mice with parathyroid-specific deletion of all miRNAs have a muted increase in serum PTH and fail to increase parathyroid cell proliferation when challenged by CKD, suggesting that miRNA is also necessary for the development of SHP. This review summarizes the current knowledge on the mechanisms of parathyroid cell proliferation in SHP.

Endocrine fibroblast growth factors as potential biomarkers for chronic kidney disease

INTRODUCTION

Among the family of fibroblast growth factors (FGFs), FGF19, FGF21, and FGF23 act as circulating hormones and are called endocrine FGFs. FGF19 and FGF21 regulate bile acid and energy homeostasis, respectively, whereas FGF23 regulates vitamin D and phosphate homeostasis. Accumulating evidence suggests that FGF23 plays a critical role in disturbed mineral metabolisms, left ventricular hypertrophy, immunosuppression, inflammation, among others in patients with chronic kidney disease (CKD), highlighting the potential both as a biomarker and a therapeutic target. Several studies have also examined the potential role of FGF19 and FGF21 in CKD patients.

AREAS COVERED

In this review, we present a brief overview of the biology of FGF19, FGF21, and FGF23, and summarize recent clinical and experimental studies on the pathophysiological roles of endocrine FGFs, mainly FGF23, in CKD patients.

EXPERT OPINION

Among the endocrine FGFs, FGF23 represents the most promising biomarker in CKD patients. If future studies confirm that FGF23 is directly toxic in CKD patients, FGF23 could be regarded as a therapeutic target and its measurement would be valuable if applied in clinical practice. Despite their potentially important roles, the clinical relevance of FGF19 and FGF21 in CKD patients is unclear, and much more studies are required.

Severe chronic kidney disease environment reduced calcium-sensing receptor expression in parathyroid glands of adenine-induced rats even without high phosphorus diet

Background

Chronic kidney disease (CKD) disrupts mineral homeostasis and its main underlying cause is secondary hyperparathyroidism (SHPT). We previously reported that calcium-sensing receptor (CaSR) mRNA and protein expression in parathyroid glands (PTGs) significantly decreased in a CKD rat model induced by a 5/6 nephrectomy that were fed a high phosphorus diet. However, there was a significant difference in the severity of CKD between high phosphorus and adequate phosphorus diet groups. Thus, it was unclear whether CKD environment or the high phosphorus diet influenced CaSR expression, and the underlying mechanism remains largely unknown.

Methods

CKD was induced in rats with 0.75% adenine-containing diet. CKD and control rats were maintained for 5 days and 2 weeks on diets with 0.7% or 1.3% phosphorus. For gene expression analysis, quantitative real-time polymerase chain reaction was performed with TaqMan probes. Protein expression was analyzed by immunohistochemistry.

Results

PTG CaSR expression significantly decreased in the presence of a severe CKD environment, even without the high phosphate load. Ki67 expressing cells in PTGs were significantly higher only in the CKD rats fed a high phosphorus diet. Furthermore, among the many genes that could affect CaSR expression, only vitamin D receptor (VDR) and glial cells missing 2 (Gcm2) showed significant changes. Moreover, Gcm2 was significantly reduced at an early stage without significant changes in serum calcium, phosphorus and 1,25(OH)₂ vitamin D, and there was no significant reduction in CaSR and VDR expressions. Then, significantly elevated Ki67-positive cell numbers were also only observed in the early CKD PTGs with high-phosphorus diets.

Conclusions

Our data suggest that the cause of the decreased PTG CaSR expression is the reduction in VDR and Gcm2 expression; Gcm2 may play a role in the onset and progression of SHPT.

Interaction between serum FGF-23 and PTH in renal phosphate excretion, a case-control study in hypoparathyroid patients

BACKGROUND

phosphate homeostasis is mediated through complex counter regulatory feed-back balance between parathyroid hormone, FGF-23 and 1,25(OH)2D. Both parathyroid hormone and FGF-23 regulate proximal tubular phosphate excretion through signaling on sodium- phosphate cotransporters II_a and II_c. However, the interaction between these hormones on phosphate excretion is not clearly understood. We performed the present study to evaluate whether the existence of sufficient parathyroid hormone is necessary for full phosphaturic function of FGF-23 or not.

METHODS

In this case-control study, 19 patients with hypoparathyroidism and their age- and gender-matched normal population were enrolled. Serum calcium, phosphate, alkaline phosphatase,parathyroid hormone, FGF-23, 25(OH)D, 1,25(OH)2D and Fractional excretion of phosphorous were assessed and compared between the two groups, using SPSS software.

RESULTS

The mean serum calcium and parathyroid hormone level was significantly lower in hypoparathyroid patients in comparison with the control group (P < 0.001 and P < 0.001, respectively). We found high serum level of phosphate and FGF-23 in hypoparathyroid patients compared to the control group (P < 0.001 and P < 0.001, respectively). However, there was no significant difference in Fractional excretion of phosphorous or 1,25OH2D level between the two groups. There was a positive correlation between serum FGF-23 and Fractional excretion of phosphorous just in the normal individuals (P < 0.001, r = 0.79).

CONCLUSIONS

Although the FGF-23 is a main regulator of urinary phosphate excretion but the existence of sufficient parathyroid hormone is necessary for the full phosphaturic effect of FGF-23.

FGF23: Is It Another Biomarker for Phosphate-Calcium Metabolism?

Fibroblast growth factor 23 (FGF23) is a protein produced by mature osteoblasts involved in mineral homeostasis by binding to its receptor complex FGFR/Klotho located mainly in the kidneys. Although this protein participates in numerous biological processes, increase in the levels of FGF23 is responsible for many pathologies, such as X-linked hypophosphataemia (XLH), chronic kidney disease, cardiovascular disease or even mortality. For this reason, both FGF23 and its receptors have become elements of interest for the development of treatments. However, FGF23 can be altered for many other reasons, such as inflammatory processes, iron, hypoxia, heart failure or erythropoietin, that negatively affect mortality. This article will review the role of FGF23 in phosphate homeostasis, its relationship to mortality, fractures and chronic renal failure, and how the levels of this factor can be reduced.

Early response of the parathyroid gland to withdrawal of a calcimimetic compound in uremic rats

Little is known about changes in parathyroid cells when calcimimetics are withdrawn. We examined the response of parathyroid glands to cinacalcet (Cina) withdrawal in uremic Sprague-Dawley rats fed a high-phosphate diet to develop secondary hyperparathyroidism and divided into groups treated with vehicle (UC), Cina, and Cina and maxacalcitol (Maxa), a vitamin D receptor activator (CiNa + Maxa). After 2 wk of treatment, vehicle and Cina were withdrawn and Maxa was continued. Rats were analyzed immediately (day 0) and 7 days (day 7) after withdrawal. The Cina and CiNa + Maxa groups had significantly lower parathyroid hormone (PTH) than the UC group on day 0, although PTH in the Cina group reached UC levels on day 7. On day 0, there were significantly more proliferating cell nuclear antigen-positive cells in the UC group compared with normal controls, and this increase was significantly suppressed in the Cina and CiNa + Maxa groups. On day 7, the Cina group, but not the CiNa + Maxa group, showed a significant increase in proliferating cell nuclear antigen-positive cells compared with the UC group. This increase was related to parathyroid cell diameter regression to UC levels, whereas combination treatment maintained diameter suppression. These results indicate that parathyroid growth activity is stimulated by Cina withdrawal, although the PTH level was not further increased. Continuous administration of Cina may be required for optimal control of secondary hyperparathyroidism, and simultaneous use of a vitamin D receptor activator may be advisable during Cina withdrawal.

Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD): Current Perspectives

Despite the availability of global and regional guidelines to curtail the adverse clinical outcomes associated with chronic kidney disease–mineral and bone disorder (CKD-MBD), most CKD patients are still affected by the consequences of abnormalities of CKD-MBD. This important clinical complication of CKD continues to be studied, in order to improve the understanding and management of CKD-MBD. Some notable discoveries include the role of fibroblast growth factor 23 (FGF23) in the pathogenesis of CKD-MBD, leading to a shift from the previous well-established classic trade-off hypothesis to the updated trade-off hypothesis. More recently, there has been a shift from the treatment of CKD-MBD based on a single level of biomarkers to serial measurements of calcium, phosphate and parathyroid hormone (PTH). Furthermore, some clinical trials have emerged after the 2009 Kidney Disease-Improving Global Outcomes (KDIGO) Guidelines, leading to the 2017 KDIGO updated recommendations. Hence, this review gives an overview of the rapidly evolving trends in CKD-MBD, linking the past and current concepts of CKD-MBD.

Disputable Issues of Etiology and Pathogenesis of Tertiary Hyperparathyroidism

Hyperparathyroidism is a clinical and laboratory syndrome characterized by high production of the chief cells of the parathyroid hormone, a calcium-phosphorus metabolism disorder and the organ failure (kidneys, bone tissue).There are primary, secondary and tertiary hyperparathyroidism.This literature review is focused on tertiary hyperparathyroidism and includes the following sections: definition of different forms of hyperparathyroidism, the role of vitamin D in the tertiary hyperparathyroidism development, the development of tertiary hyperparathyroidism in chronic kidney disease patients, the development of tertiary hyperparathyroidism in patients after kidney transplantation, differential diagnosis various forms of hyperparathyroidism, indications for surgical tertiary hyperparathyroidism treatment in patients with kidney disease, in patients with normal kidney function.Objective. The objective of this literature review is to study the current information about this definition, pathogenesis, diagnosis and treatment of tertiary hyperparathyroidism.Methodology. The literature review was taken in English data bases MEDLINE (Pubmed), Scopus, Cochlear library, using following keywords: “secondary hyperparathyroidism pathogenesis diagnosis treatment”, “tertiary hyperparathyroidism pathogenesis diagnosis treatment”, “development of tertiary hyperparathyroidism from secondary hyperparathyroidism”, “chronic vitamin D deficiency, hyperparathyroidism”, “early stages of chronic renal failure, hyperparathyroidism”. Also, search for the same keywords was completed in Russian data base Elibrary.Discussion. Both the lack of a common understanding of this problem, and the presence of diverse and contradictory data of the etiology and pathogenesis indicate the need for further study of tertiary hyperparathyroidism.

FGF23 and Phosphate-Cardiovascular Toxins in CKD

Elevated levels of fibroblast growth factor 23 (FGF23) and phosphate are highly associated with increased cardiovascular disease and mortality in patients suffering from chronic kidney disease (CKD). As the kidney function declines, serum phosphate levels rise and subsequently induce the secretion of the phosphaturic hormone FGF23. In early stages of CKD, FGF23 prevents the increase of serum phosphate levels and thereby attenuates phosphate-induced vascular calcification, whereas in end-stage kidney disease, FGF23 fails to maintain phosphate homeostasis. Both hyperphosphatemia and elevated FGF23 levels promote the development of hypertension, vascular calcification, and left ventricular hypertrophy by distinct mechanisms. Therefore, FGF23 and phosphate are considered promising therapeutic targets to improve the cardiovascular outcome in CKD patients. Previous therapeutic strategies are based on dietary and pharmacological reduction of serum phosphate, and consequently FGF23 levels. However, clinical trials proving the effects on the cardiovascular outcome are lacking. Recent publications provide evidence for new promising therapeutic interventions, such as magnesium supplementation and direct targeting of phosphate and FGF receptors to prevent toxicity of FGF23 and hyperphosphatemia in CKD patients.

Mineral Bone Abnormalities and Vascular Calcifications

Vascular calcification (VC) is common in chronic kidney disease, increases in prevalence as patients progress to end-stage renal disease, and is significantly associated with mortality. VC is a complex and highly regulated process similar to bone formation whereby hydroxyapatite crystals deposit in the intimal or medial layer of arteries. Mineral bone abnormalities are common in chronic kidney disease; reduction in glomerular filtration rate and changes in vitamin D, parathyroid hormone, and fibroblast growth factor 23 result in the dysregulation of phosphorus and calcium metabolism. Cell culture studies, animal models, and observational and clinical studies all suggest this abnormal mineral metabolism plays a role in the initiation and progression of VC in kidney disease. This review will focus on these mineral bone abnormalities and how they may contribute to mechanisms that induce VC in kidney disease.

FGF23, Biomarker or Target?

Fibroblast growth factor 23 (FGF23) plays a key role in the complex network between the bones and other organs. Initially, it was thought that FGF23 exclusively regulated phosphate and vitamin D metabolism; however, recent research has demonstrated that an excess of FGF23 has other effects that may be detrimental in some cases. The understanding of the signaling pathways through which FGF23 acts in different organs is crucial to develop strategies aiming to prevent the negative effects associated with high FGF23 levels. FGF23 has been described to have effects on the heart, promoting left ventricular hypertrophy (LVH); the liver, leading to production of inflammatory cytokines; the bones, inhibiting mineralization; and the bone marrow, by reducing the production of erythropoietin (EPO). The identification of FGF23 receptors will play a remarkable role in future research since its selective blockade might reduce the adverse effects of FGF23. Patients with chronic kidney disease (CKD) have very high levels of FGF23 and may be the population suffering from the most adverse FGF23-related effects. The general population, as well as kidney transplant recipients, may also be affected by high FGF23. Whether the association between FGF23 and clinical events is causal or casual remains controversial. The hypothesis that FGF23 could be considered a therapeutic target is gaining relevance and may become a promising field of investigation in the future.

Not all forms of dietary phosphorus are equal: an evaluation of postprandial phosphorus concentrations in the plasma of the cat

Phosphorus is present in diets as naturally occurring P from raw materials or added as an inorganic salt. However, little is known about postprandial kinetics of P absorption in cats. Here, we describe several studies quantifying postprandial kinetics following the ingestion of diets of varying composition. Briefly, cats were fed a meal consisting of 50 % of their metabolic energy requirement in a randomised crossover design. A pre-meal baseline blood sample was taken via cephalic catheter and repeated measurements taken regularly up to 6 h post-meal to assess the whole blood ionised Ca, plasma P and parathyroid hormone concentrations. A diet containing 4·8 g total P/4184 kJ (1000 kcal), 3·5 g P from sodium dihydrogen phosphate (NaH2PO4)/4184 kJ (1000 kcal) and Ca:P 0·6 caused a marked increase in plasma P from baseline to a peak of 1·976 (95% CI 1·724, 2·266) mmol/l (P <0·001), whereas a diet containing 3·38 g total P/4184 kJ (1000 kcal), no added inorganic P and Ca:P 1·55 resulted in a postprandial decrease in plasma P (P = 0·008). Subsequent data indicate that added inorganic P salts in the diet above 0·5 g P/4184 kJ (1000 kcal) cause an increase in plasma P in cats, while diets below this do not. The data presented here demonstrate that sources of added inorganic P salts cause a temporary postprandial increase in plasma P in a dose-dependent manner, prolonged in diets with Ca:P <1·0. Dietary P derived from natural food ingredients (e.g. meat or vegetable matter) does not appear to have any effect on postprandial plasma P.

Parathyroidectomy in dialysis patients: Indications, methods, and consequences

Secondary hyperparathyroidism, characterized by increased PTH synthesis and secretion, is often seen in advanced stages of chronic kidney disease. Excessive proliferation of parathyroid cells leads to the development of diffuse hyperplasia that subsequently progresses to nodular histology. Refractory hyperparathyroidism occurs when parathyroid glands fail to respond to medical therapy. Parathyroidectomy (PTX), surgical resection of parathyroid glands, is usually performed in cases of persistent serum levels of PTH above 1000 pg/mL associated with hypercalcemia or when hyperparathyroidism is refractory to conservative therapy. Parathyroidectomy can be carried out using different procedures: subtotal PTX or total PTX with or without parathyroid autotransplantation. Parathyroid surgery may have undesirable consequences due to PTH oversuppression, such as the development of adynamic bone disease; hungry bone syndrome is quite common after this surgery. However, PTX improves survival and parameters of mineral metabolism. Parathyroidectomy needs to be considered in those patients with severe hyperparathyroidism with a poor response to pharmacological treatment and with distinct undesirable effects of PTH on bone and mineral metabolism parameters.

Secondary Hyperparathyroidism: Pathogenesis and Latest Treatment

The classic pathogenesis of secondary hyperparathyroidism (SHPT) began with the trade-off hypothesis based on parathyroid hormone hypersecretion brought about by renal failure resulting from a physiological response to correct metabolic disorder of calcium, phosphorus, and vitamin D. In dialysis patients with failed renal function, physiological mineral balance control by parathyroid hormone through the kidney fails and hyperparathyroidism progresses. In this process, many significant genetic findings have been established. Abnormalities of Ca-sensing receptor and vitamin D receptor are associated with the pathogenesis of SHPT, and fibroblast growth factor 23 has also been shown to be involved in the pathogenesis. Vitamin D receptor activators (VDRAs) are widely used for treatment of SHPT. However, VDRAs have calcemic and phosphatemic effects that limit their use to a subset of patients, and calcimimetics have been developed as alternative drugs for SHPT. Hyperphosphatemia also affects progression of SHPT, and control of hyperphosphatemia is, therefore, thought to be fundamental for control of SHPT. Currently, a combination of a VDRA and a calcimimetic is recognized as the optimal strategy for SHPT, and for other outcomes such as reduced cardiovascular disease and improved survival. The latest findings on the pathogenesis and treatment of SHPT are summarized in this review.

The Emerging Role of Nutritional Vitamin D in Secondary Hyperparathyroidism in CKD

In chronic kidney disease (CKD), hyperphosphatemia induces fibroblast growth factor-23 (FGF-23) expression that disturbs renal 1,25-dihydroxy vitamin D (1,25D) synthesis; thereby increasing parathyroid hormone (PTH) production. FGF-23 acts on the parathyroid gland (PTG) to increase 1α-hydroxylase activity and results in increase intra-gland 1,25D production that attenuates PTH secretion efficiently if sufficient 25D are available. Interesting, calcimimetics can further increase PTG 1α-hydroxylase activity that emphasizes the demand for nutritional vitamin D (NVD) under high PTH status. In addition, the changes in hydroxylase enzyme activity highlight the greater parathyroid 25-hydroxyvitmain D (25D) requirement in secondary hyperparathyroidism (SHPT); the higher proportion of oxyphil cells as hyperplastic parathyroid progression; lower cytosolic vitamin D binding protein (DBP) content in the oxyphil cell; and calcitriol promote vitamin D degradation are all possible reasons supports nutritional vitamin D (NVD; e.g., Cholecalciferol) supplement is crucial in SHPT. Clinically, NVD can effectively restore serum 25D concentration and prevent the further increase in PTH level. Therefore, NVD might have the benefit of alleviating the development of SHPT in early CKD and further lowering PTH in moderate to severe SHPT in dialysis patients.

Correlation between Soluble α-Klotho and Renal Function in Patients with Chronic Kidney Disease: A Review and Meta-Analysis

Objective

Over decades, numerous inconsistent studies are reported on the relationship between soluble α-Klotho and renal function in patients with chronic kidney disease (CKD). This study aims to perform a meta-analysis to figure out the correlations between soluble α-Klotho and renal function in patients with CKD.

Materials and Methods

We searched medical and scientific literature databases, PubMed and EMBASE (from the inception to October 2017), for publications that reported studies on associations between soluble α-Klotho and renal function in patients with CKD. Only publications in English were extracted. Summary correlation coefficient (r) values were extracted from each study, and 95% confidence intervals (CIs) were calculated. Publication bias was tested, and sensitivity and subgroup analyses were performed to investigate potential heterogeneity.

Results

Of 611 studies, 9 publications with 1457 patients were included into the analysis. The following data were extracted from the literature: first author, year of publication, research region, research index, sample size, average age and Pearson or Spearman correlation coefficient, study design, the αKlotho/FGF23 assays utilized, full length, or the C-terminal fragment of FGF23. The pooled r between α-Klotho and estimated glomerular filtration rate (eGFR), FGF-23 were 0.35 (95%CI, 0.23~0.46, and P<0.05), -0.10 (95%CI, -0.19~-0.01, and P<0.05) with remarkable significance, indicating moderate heterogeneity. There was no significant heterogeneity between subgroups in analyses of α-Klotho and eGFR stratified by research region, mean age, and eGFR, but heterogeneity exists in analyses of α-Klotho and FGF-23 stratified by research region. There was no significant correlation between a-klotho and Ca and PTH and PHOS. There was no evidence of publication bias with Egger's test (p=0.360) or with Begg's test (p=0.902) and the distribution of funnel plots was symmetrical in all of our analysis.

Conclusions

There exists a significant positive correlation between soluble α-Klotho and eGFR in patients with CKD. Also, a significant negative correlation between α-Klotho and FGF23 levels is proven. This raises hope to employ αKlotho and FGF23 as early biomarkers of CKD. However, further large prospective follow-up researches are needed to validate this hypothesis and to explore whether maintaining or elevating the Klotho level could improve renal function and complications in CKD patients.

TGF-Beta Signaling in Bone with Chronic Kidney Disease

Transforming growth factor (TGF)-β signaling is not only important in skeletal development, but also essential in bone remodeling in adult bone. The bone remodeling process involves integrated cell activities induced by multiple stimuli to balance bone resorption and bone formation. TGF-β plays a role in bone remodeling by coordinating cell activities to maintain bone homeostasis. However, mineral metabolism disturbance in chronic kidney disease (CKD) results in abnormal bone remodeling, which leads to ectopic calcification in CKD. High circulating levels of humoral factors such as parathyroid hormone, fibroblast growth factor 23, and Wnt inhibitors modulate bone remodeling in CKD. Several reports have revealed that TGF-β is involved in the production and functions of these factors in bone. TGF-β may act as a factor that mediates abnormal bone remodeling in CKD.

Phosphate control in reducing FGF23 levels in hemodialysis patients

Background

In hemodialysis patients, high levels of Fibroblast Growth Factor 23 (FGF23) predict mortality. Our study was designed to test whether the control of serum phosphate is associated with a reduction in serum FGF23 levels. Additionally other variables with a potential effect on FGF23 levels were evaluated.

Material and methods

The effect of sustained (40-weeks) control of serum phosphate on FGF23 levels (intact and c-terminal) was evaluated in 21 stable hemodialysis patients that were not receiving calcimimetics or active vitamin D. Patients received non-calcium phosphate binders to maintain serum phosphate below 4.5 mg/dl. In an additional analysis, values of intact-FGF23 (iFGF23) and c-terminal FGF23 (cFGF23) from 150 hemodialysis patients were correlated with parameters of mineral metabolism and inflammation. Linear mixed models and linear regression were performed to evaluate longitudinal trajectories of variables and the association between FGF23 and the other variables examined.

Results

During the 40-week treatment, 12 of 21 patients achieved the target of serum phosphate <4.5 mg/dl. In these 12 patients, iFGF23 decreased to less than half whereas cFGF23 did not reduce significantly. In patients with serum phosphate >4.5 mg, iFGF23 and cFGF23 increased two and four-fold respectively as compared with baseline. Furthermore, changes in serum phosphate correlated with changes in C-reactive protein (hs-CRP). In our 150 hemodialysis patients, those in the higher tertile of serum phosphate also showed increased hs-CRP, iPTH, iFGF23 and cFGF23. Multiple regression analysis revealed that iFGF23 levels directly correlated with both serum phosphate and calcium, whereas cFGF23 correlated with serum phosphate and hs-CRP but not with calcium.

Conclusions

The control of serum phosphate reduced iFGF23. This reduction was also associated with a decreased in inflammatory parameters. Considering the entire cohort of hemodialysis patients, iFGF23 levels correlated directly with serum phosphate levels and also correlated inversely with serum calcium concentration. The levels of cFGF23 were closely related to serum phosphate and parameters of inflammation.

Mineral and Bone Disorders After Kidney Transplantation

The risk of mineral and bone disorders among patients with chronic kidney disease is substantially elevated, owing largely to alterations in calcium, phosphorus, vitamin D, parathyroid hormone, and fibroblast growth factor 23. The interwoven relationship among these minerals and hormones results in maladaptive responses that are differentially affected by the process of kidney transplantation. Interpretation of conventional labs, imaging, and other fracture risk assessment tools are not standardized in the post-transplant setting. Post-transplant bone disease is not uniformly improved and considerable variation exists in monitoring and treatment practices. A spectrum of abnormalities such as hypophosphatemia, hypercalcemia, hyperparathyroidism, osteomalacia, osteopenia, and osteoporosis are commonly encountered in the post-transplant period. Thus, reducing fracture risk and other bone-related complications requires recognition of these abnormalities along with the risk incurred by concomitant immunosuppression use. As kidney transplant recipients continue to age, the drivers of bone disease vary throughout the post-transplant period among persistent hyperparathyroidism, de novo hyperparathyroidism, and osteoporosis. The use of anti-resorptive therapies require understanding of different options and the clinical scenarios that warrant their use. With limited studies underscoring clinical events such as fractures, expert understanding of MBD physiology, and surrogate marker interpretation is needed to determine ideal and individualized therapy.

Role of etelcalcetide in the management of secondary hyperparathyroidism in hemodialysis patients: a review on current data and place in therapy

Secondary hyperparathyroidism (sHPT) is a frequently occurring severe complication of advanced kidney disease. Its clinical consequences include extraskeletal vascular and valvular calcifications, changes in bone metabolism resulting in renal osteodystrophy, and an increased risk of cardiovascular morbidity and mortality. Calcimimetics are a cornerstone of parathyroid hormone (PTH)-lowering therapy, as confirmed by the recently updated 2017 Kidney Disease: Improving Global Outcomes chronic kidney disease – mineral and bone disorder clinical practice guidelines. Contrary to calcitriol or other vitamin D-receptor activators, calcimimetics reduce PTH without increasing serum-calcium, phosphorus, or FGF23 levels. Etelcalcetide is a new second-generation calcimimetic that has been approved for the treatment of sHPT in adult hemodialysis patients. Whereas the first-generation calcimimetic cinacalcet is taken orally once daily, etelcalcetide is given intravenously thrice weekly at the end of the hemodialysis session. Apart from improving drug adherence, etelcalcetide has proven to be more effective in lowering PTH when compared to cinacalcet, with an acceptable and comparable safety profile. The hope for better gastrointestinal tolerance with intravenous administration did not come true, as etelcalcetide did not significantly mitigate the adverse gastrointestinal effects associated with cinacalcet. Enhanced adherence and strong reductions in PTH, phosphorus, and FGF23 could set the stage for a future large randomized controlled trial to demonstrate that improved biochemical control of mineral metabolism with etelcalcetide in hemodialysis patients translates into cardiovascular and survival benefits and better health-related quality of life.

Interleukin-6 contributes to the increase in fibroblast growth factor 23 expression in acute and chronic kidney disease

The high serum fibroblast growth factor 23 (FGF23) levels in patients with acute kidney injury (AKI) and chronic kidney disease (CKD) are associated with increased morbidity and mortality. Mice with folic acid-induced AKI had an increase in bone FGF23 mRNA expression together with an increase in serum FGF23 and several circulating cytokines including interleukin-6 (IL-6). Dexamethasone partially prevented the increase in IL-6 and FGF23 in the AKI mice. IL-6 knock-out mice fed an adenine diet to induce CKD failed to increase bone FGF23 mRNA and had a muted increase in serum FGF23 levels, compared with the increases in wild-type mice with CKD. Therefore, IL-6 contributes to the increase in FGF23 observed in CKD. Hydrodynamic tail injection of IL-6/soluble IL-6 receptor (sIL-6R) fusion protein hyper IL-6 (HIL-6) plasmid increased serum FGF23 levels. Circulating sIL-6R levels were increased in both CKD and AKI mice, suggesting that IL-6 increases FGF23 through sIL-6R-mediated trans-signaling. Renal IL-6 mRNA expression was increased in mice with either AKI or CKD, suggesting the kidney is the source for the increased serum IL-6 levels in the uremic state. HIL-6 also increased FGF23 mRNA in calvaria organ cultures and osteoblast-like UMR106 cells in culture, demonstrating a direct effect of IL-6 on FGF23 expression. HIL-6 increased FGF23 promoter activity through STAT3 phosphorylation and its evolutionarily conserved element in the FGF23 promoter. Thus, IL-6 increases FGF23 transcription and contributes to the high levels of serum FGF23 in both acute and chronic kidney disease.

Biology of Fibroblast Growth Factor 23: From Physiology to Pathology

Fibroblast growth factor (FGF)23 is a phosphaturic hormone produced by osteocytes and osteoblasts that binds to FGF receptors in the presence of the transmembrane protein αKlotho. FGF23 mainly targets the renal proximal tubule to inhibit calcitriol production and the expression of the sodium/phosphate cotransporters NaPi2a and NaPi2c, thus inhibiting renal phosphate reabsorption. FGF23 also acts on the parathyroid glands to inhibit parathyroid hormone synthesis and secretion. FGF23 regulation involves many systemic and local factors, among them calcitriol, phosphate, and parathyroid hormone. Increased FGF23 is primarily observed in rare acquired or genetic disorders, but chronic kidney disease is associated with a reactional increase in FGF23 to combat hyperphosphatemia. However, high FGF23 levels induce left ventricular hypertrophy (LVH) and are associated with an increased risk of mortality. In this review, we describe FGF23 physiology and the pathological consequences of high or low FGF23 levels.

Fibroblast Growth Factor (FGF) 23 Regulates the Plasma Levels of Parathyroid Hormone In Vivo Through the FGF Receptor in Normocalcemia, But Not in Hypocalcemia

The calcium and phosphate homeostasis is regulated by a complex interplay between parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and calcitriol. Experimental studies have demonstrated an inhibitory effect of FG23 on PTH production and secretion; the physiological role of this regulation is however not well understood. Surprisingly, in uremia, concomitantly elevated FGF23 and PTH levels are observed. The parathyroid gland rapidly loses its responsiveness to extracellular calcium in vitro and a functional parathyroid cell line has currently not been established. Therefore, the aim of the present investigation was to study the impact of FGF23 on the Ca²⁺/PTH relationship in vivo under conditions of normocalcemia and hypocalcemia. Wistar rats were allocated to treatment with intravenous recombinant FGF23 and inhibition of the FGF receptor in the setting of normocalcemia and acute hypocalcemia. We demonstrated that FGF23 rapidly inhibited PTH secretion and that this effect was completely blocked by inhibition of the FGF receptor. Furthermore, inhibition of the FGF receptor by itself significantly increased PTH levels, indicating that FGF23 has a suppressive tonus on the parathyroid gland's PTH secretion. In acute hypocalcemia, there was no effect of either recombinant FGF23 or FGF receptor inhibition on the physiological response to the low ionized calcium levels. In conclusion, FGF23 has an inhibitory tonus on PTH secretion in normocalcemia and signals through the FGF receptor. In acute hypocalcemia, when increased PTH secretion is needed to restore the calcium homeostasis, this inhibitory effect of FGF23 is abolished.

Coupling between phosphate and calcium homeostasis: a mathematical model

We developed a mathematical model of calcium (Ca) and phosphate (PO₄) homeostasis in the rat to elucidate the hormonal mechanisms that underlie the regulation of Ca and PO₄ balance. The model represents the exchanges of Ca and PO₄ between the intestine, plasma, kidneys, bone, and the intracellular compartment, and the formation of Ca-PO₄-fetuin-A complexes. It accounts for the regulation of these fluxes by parathyroid hormone (PTH), vitamin D₃, fibroblast growth factor 23, and Ca²⁺-sensing receptors. Our results suggest that the Ca and PO₄ homeostatic systems are robust enough to handle small perturbations in the production rate of either PTH or vitamin D₃ The model predicts that large perturbations in PTH or vitamin D₃ synthesis have a greater impact on the plasma concentration of Ca²⁺ ([Ca²⁺]_p) than on that of PO₄ ([PO₄]_p); due to negative feedback loops, [PO₄]_p does not consistently increase when the production rate of PTH or vitamin D₃ is decreased. Our results also suggest that, following a large PO₄ infusion, the rapidly exchangeable pool in bone acts as a fast, transient storage PO₄ compartment (on the order of minutes), whereas the intracellular pool is able to store greater amounts of PO₄ over several hours. Moreover, a large PO₄ infusion rapidly lowers [Ca²⁺]_p owing to the formation of CaPO₄ complexes. A large Ca infusion, however, has a small impact on [PO₄]_p, since a significant fraction of Ca binds to albumin. This mathematical model is the first to include all major regulatory factors of Ca and PO₄ homeostasis.

Tissue expression and source of circulating αKlotho

αKlotho (Klotho), a type I transmembrane protein and a coreceptor for Fibroblast Growth Factor-23, was initially thought to be expressed only in a limited number of tissues, most importantly the kidney, parathyroid gland and choroid plexus. Emerging data may suggest a more ubiquitous Klotho expression pattern which has prompted reevaluation of the restricted Klotho paradigm. Herein we systematically review the evidence for Klotho expression in various tissues and cell types in humans and other mammals, and discuss potential reasons behind existing conflicting data. Based on current literature and tissue expression atlases, we propose a classification of tissues into high, intermediate and low/absent Klotho expression. The functional relevance of Klotho in organs with low expression levels remain uncertain and there is currently limited data on a role for membrane-bound Klotho outside the kidney. Finally, we review the evidence for the tissue source of soluble Klotho, and conclude that the kidney is likely to be the principal source of circulating Klotho in physiology.

Differential regulation of renal Klotho and FGFR1 in normal and uremic rats

In renal failure, hyperphosphatemia occurs despite a marked elevation in serum fibroblast growth factor (FGF)-23. Abnormal regulation of the FGFR1-Klotho receptor complex may cause a resistance to the phosphaturic action of FGF23. The purpose of the present study was to investigate the regulation of renal Klotho and FGF receptor (FEFR)-1 in healthy and uremic rats induced by 5/6 nephrectomy. In normal rats, the infusion of rat recombinant FGF23 enhanced phosphaturia and increased renal FGFR1 expression; however, Klotho expression was reduced. Uremic rats on a high-phosphate (HP) diet presented hyperphosphatemia with marked elevation of FGF23 and an increased fractional excretion of phosphate (P) that was associated with a marked reduction of Klotho expression and an increase in FGFR1. After neutralization of FGF23 by anti-FGF23 administration, phosphaturia was still abundant, Klotho expression remained low, and the FGFR1 level was reduced. These results suggest that the expression of renal Klotho is modulated by phosphaturia, whereas the FGFR1 expression is regulated by FGF23. Calcitriol (CTR) administration prevented a decrease in renal Klotho expression. In HEK293 cells HP produced nuclear translocation of β-catenin, together with a reduction in Klotho. Wnt/β-catenin inhibition with Dkk-1 prevented the P-induced down-regulation of Klotho. The addition of CTR to HP medium was able to recover Klotho expression. In summary, high FGF23 levels increase FGFR1, whereas phosphaturia decreases Klotho expression through the activation of Wnt/β-catenin pathway.-Muñoz-Castañeda, J. R., Herencia, C., Pendón-Ruiz de Mier, M. V., Rodriguez-Ortiz, M. E., Diaz-Tocados, J. M., Vergara, N., Martínez-Moreno, J. M., Salmerón, M. D., Richards, W. G., Felsenfeld, A., Kuro-O, M., Almadén, Y., Rodríguez, M. Differential regulation of renal Klotho and FGFR1 in normal and uremic rats.

Vitamin D prohormone in the treatment of secondary hyperparathyroidism in patients with chronic kidney disease

Secondary hyperparathyroidism (sHPT) represents the adaptive and very often, finally, maladaptive response of the organism to control the disturbed homeostasis of calcium, phosphorus, and vitamin D metabolism caused by declining renal function in chronic kidney disease (CKD). sHPT leads to cardiovascular and extravascular calcifications and is directly linked to an increased risk of cardiovascular morbidity and mortality as well as excess all-cause mortality. Vitamin D plays an important role in the development of sHPT. CKD patients are characterized by a high prevalence of hypovitaminosis D. Supplementation with both vitamin D prohormones cholecalciferol and ergocalciferol enables the achievement and maintenance of a normal vitamin D status when given in adequate doses over an appropriate treatment period. In patients with earlier stages of CKD, sHPT is influenced by and can be successfully treated with vitamin D prohormone supplementation, whereas in patients with very late stages of CKD and those requiring dialysis, treatment with prohormones seems to be of limited efficacy. This review gives an overview of the pathogenesis of sHPT, summarizes vitamin D metabolism, and discusses the existing literature regarding the role of vitamin D prohormone in the treatment of sHPT in patients with CKD.