Differential expression and regulation of Klotho by paricalcitol in the kidney, parathyroid, and aorta of uremic rats. Kidney Int. 2015;87:1141-1152. [PMID: 25692955 PMCID: PMC4449811 DOI: 10.1038/ki.2015.22]

The role of Klotho and sirtuins in sleep-related cardiovascular diseases: a review study

The prevalence of sleep disorders has been reported from 1.6% to 56.0%, worldwide. Sleep deprivation causes cardiovascular diseases (CVDs) including atherosclerosis, vascular aging, hypertension, heart dysfunction, reduced heart rate variability, and cardiac arrhythmia. Reduced tissue oxygen causes various CVDs by activating pro-inflammatory factors and increasing oxidative stress. Sleep disorders are more important and prevalent in older people and cause more severe cardiovascular complications. On the other hand, the reduction of Klotho level, an age-dependent protein whose expression decreases with age, is associated with age-related diseases. Sirtuins, class III histone deacetylases, also are among the essential factors in postponing cellular aging and increasing the lifespan of organisms, and they do this by regulating different pathways in the cell. Sirtuins and Klotho play an important role in the pathophysiology of CVDS and both have anti-oxidative stress and anti-inflammatory activity. Studies have shown that the levels of Klotho and sirtuins are altered in sleep disorders. In this article, alterations of Klotho and sirtuins in sleep disorders and in the development of sleep-related CVDs were reviewed and the possible signaling pathways were discussed. The inclusion criteria were studies with keywords of different types of sleep disorders and CVDs, klotho, SIRT1-7, and sirtuins in PubMed, Scopus, Embase، Science Direct، Web of Sciences and Google Scholar by the end of 2023. The studies revealed there is a bidirectional relationship between sleep disorders and the serum and tissue levels of Klotho and sirtuins and sleep related-CVDs.

Emerging concepts on the FGF23 regulation and activity

Fibroblast growth factor 23 (FGF23) discovery has provided new insights into the regulation of Pi and Ca homeostasis. It is secreted by osteoblasts and osteocytes, and acts mainly in the kidney, parathyroid, heart, and bone. The aim of this review is to highlight the current knowledge on the factors modulating the synthesis of FGF23, the canonical and non-canonical signaling pathways of the hormone, the role of FGF23 in different pathophysiological conditions, and the anti-FGF23 therapy. This is a narrative review based on the search of PubMed database in the range of years 2000-2023 using the keywords local and systemic regulators of FGF23 synthesis, FGF23 receptors, canonical and non-canonical pathways, pathophysiological conditions and FGF23, and anti-FGF23 therapy, focusing the data on the molecular mechanisms. The regulation of FGF23 synthesis is complex and multifactorial. It is regulated by local factors and systemic regulators mainly involved in bone mineralization. The excessive FGF23 production is associated with different congenital diseases and with diseases occurring with a secondary high FGF23 production such as in chronic disease kidney and tumor-induced osteomalacia (TIO). The anti-FGF23 therapy appears to be useful to treat chromosome X-linked hypophosphatemia and TIO, but there are doubts about the handle of excessive FGF23 production in CKD. FGF23 biochemistry and pathophysiology are generating a plethora of knowledge to reduce FGF23 bioactivity at many levels that might be useful for future therapeutics of diseases associated with high-serum FGF23 levels.

Soluble Klotho, a Potential Biomarker of Chronic Kidney Disease-Mineral Bone Disorders Involved in Healthy Ageing: Lights and Shadows

Shortly after the discovery of Klotho, interest grew in its potential role in chronic kidney disease (CKD). There are three isoforms of the Klotho protein: αKlotho, βKlotho and γKlotho. This review will focus on αKlotho due to its relevance as a biomarker in CKD. αKlotho is synthesized mainly in the kidneys, but it can be released into the bloodstream and urine as soluble Klotho (sKlotho), which undertakes systemic actions, independently or in combination with FGF23. It is usually accepted that sKlotho levels are reduced early in CKD and that lower levels of sKlotho might be associated with the main chronic kidney disease-mineral bone disorders (CKD-MBDs): cardiovascular and bone disease. However, as results are inconsistent, the applicability of sKlotho as a CKD-MBD biomarker is still a matter of controversy. Much of the inconsistency can be explained due to low sample numbers, the low quality of clinical studies, the lack of standardized assays to assess sKlotho and a lack of consensus on sample processing, especially in urine. In recent decades, because of our longer life expectancies, the prevalence of accelerated-ageing diseases, such as CKD, has increased. Exercise, social interaction and caloric restriction are considered key factors for healthy ageing. While exercise and social interaction seem to be related to higher serum sKlotho levels, it is not clear whether serum sKlotho might be influenced by caloric restriction. This review focuses on the possible role of sKlotho as a biomarker in CKD-MBD, highlighting the difference between solid knowledge and areas requiring further research, including the role of sKlotho in healthy ageing.

Klotho inversely relates with carotid intima- media thickness in atherosclerotic patients with normal renal function (eGFR ≥60 mL/min/1.73m2): a proof-of-concept study

Introduction

Klotho protein is predominantly expressed in the kidneys and has also been detected in vascular tissue and peripheral blood circulating cells to a lesser extent. Carotid artery intima-media thickness (CIMT) burden, a marker of subclinical atherosclerosis, has been associated with reductions in circulating Klotho levels in chronic kidney disease patients, who show reduced levels of this protein at all stages of the disease. However, the contribution of serum Klotho and its expression levels in peripheral blood circulating cells and in the carotid artery wall on the CIMT in the absence of kidney impairment has not yet been evaluated.

Methods

We conducted a single-center study in 35 atherosclerotic patients with preserved kidney function (eGFR≥60 mL/min/1.73m2) subjected to elective carotid surgery. Serum levels of Klotho and cytokines TNFa, IL6 and IL10 were determined by ELISA and transcripts encoding for Klotho (KL), TNF, IL6 and IL10 from vascular segments were measured by qRT-PCR. Klotho protein expression in the intima-media and adventitia areas was analyzed using immunohistochemistry.

Results

APatients with higher values of CIMT showed reduced Klotho levels in serum (430.8 [357.7-592.9] vs. 667.8 [632.5-712.9] pg/mL; p<0.001), mRNA expression in blood circulating cells and carotid artery wall (2.92 [2.06-4.8] vs. 3.69 [2.42-7.13] log.a.u., p=0.015; 0.41 [0.16-0.59] vs. 0.79 [0.37-1.4] log.a.u., p=0.013, respectively) and immunoreactivity in the intimal-medial area of the carotids (4.23 [4.15-4.27] vs. 4.49 [4.28-4.63] log µm2 p=0.008). CIMT was inversely related with Klotho levels in serum (r= -0.717, p<0.001), blood mRNA expression (r=-0.426, p=0.011), and with carotid artery mRNA and immunoreactivity levels (r= -0.45, p=0.07; r= -0.455, p= 0.006, respectively). Multivariate analysis showed that serum Klotho, together with the gene expression levels of tumor necrosis factor TNFa in blood circulating cells, were independent determinants of CIMT values (adjusted R2 = 0.593, p<0.001).

Discussion

The results of this study in subjects with eGFR≥60mL/min/1.73m2 show that patients with carotid artery atherosclerosis and higher values of CIMT present reduced soluble Klotho levels, as well as decreased KL mRNA expression in peripheral blood circulating cells and Klotho protein levels in the intima-media of the carotid artery wall.

Phosphatonins: From Discovery to Therapeutics

OBJECTIVE

Phosphate is crucial for cell signaling, energy metabolism, nucleotide synthesis, and bone mineralization. The gut-bone-parathyroid-kidney axis is influenced by parathyroid hormone, 1,25-dihydroxyvitamin D, and phosphatonins, especially fibroblast growth factor 23 (FGF23). These hormones facilitate maintenance of phosphate homeostasis. This review summarizes current knowledge regarding the phosphate homeostasis, phosphatonin pathophysiology, and clinical implications of FGF23-related hypophosphatemic disorders, with specific focus on burosumab treatment.

METHOD

A focused literature search of PubMed was conducted.

RESULTS

Phosphatonins including FGF23, secreted frizzled-related protein 4, matrix extracellular phosphoglycoprotein, and fibroblast growth factor 7 play a pathogenic role in several hypophosphatemic disorders. Excess FGF23 inhibits sodium-dependent phosphate cotransporters (NaPi-2a and NaPi-2c), resulting in hyperphosphaturia and hypophosphatemia. Additionally, FGF23 suppresses 1,25-dihydroxyvitamin D synthesis in the proximal renal tubule, and thus, it indirectly inhibits intestinal phosphate absorption. Disorders of FGF23-related hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia/McCune-Albright syndrome, and tumor-induced osteomalacia (TIO). Complications of conventional therapy with oral phosphate and vitamin D analogs comprise gastrointestinal distress, hypercalcemia, nephrocalcinosis, and secondary/tertiary hyperparathyroidism. In both children and adults with XLH and TIO, the anti-FGF23 antibody burosumab exhibits a favorable safety profile and is associated with healing of rickets in affected children and improvement of osteomalacia in both children and adults.

CONCLUSION

The treatment paradigm for XLH and TIO is changing based on data from recent clinical trials. Research suggest that burosumab is effective and safe for pediatric and adult patients with XLH or TIO.

Pathophysiology and genetics of salt-sensitive hypertension

Most hypertensive cases are primary and heavily associated with modifiable risk factors like salt intake. Evidence suggests that even small reductions in salt consumption reduce blood pressure in all age groups. In that regard, the ACC/AHA described a distinct set of individuals who exhibit salt-sensitivity, regardless of their hypertensive status. Data has shown that salt-sensitivity is an independent risk factor for cardiovascular events and mortality. However, despite extensive research, the pathogenesis of salt-sensitive hypertension is still unclear and tremendously challenged by its multifactorial etiology, complicated genetic influences, and the unavailability of a diagnostic tool. So far, the important roles of the renin-angiotensin-aldosterone system, sympathetic nervous system, and immune system in the pathogenesis of salt-sensitive hypertension have been studied. In the first part of this review, we focus on how the systems mentioned above are aberrantly regulated in salt-sensitive hypertension. We follow this with an emphasis on genetic variants in those systems that are associated with and/or increase predisposition to salt-sensitivity in humans.

High sodium reduced the expression of PTH1R and Klotho by inhibiting 1,25(OH)2D3 synthesis in cultured proximal tubule epithelial cells

Background

The proximal tubule is the sensing site of sodium and phosphate and the main place for the synthesis and metabolism of 1,25(OH)2D3. We aimed to investigate the effects of high sodium on the synthesis and function of active vitamin D and local phosphate regulation in proximal tubular epithelial cells.

Methods

Human proximal tubule epithelial (HK-2) cells were treated with different concentrations of sodium/phosphate. The expression of 1α-OHase and 24-OHase was determined. Liquid chromatography/mass spectrometry (LC/MS) and enzyme-linked immunosorbent assay (ELISA) were used to detect the levels of 1,25(OH)2D3. RNA sequencing and bioinformatics analysis was used to probe into the possible pathways. Chromatin samples were immunoprecipitated with antibodies against parathyroid receptor 1 (PTH1R) and Klotho.

Results

We found that high sodium decreased the expression of 1,25(OH)2D3 by reducing 1α-OHase and 24-OHase, reduced the expression of PTH1R and Klotho, and increased the intracellular calcium concentration. These effects were reversed by sodium phosphate transporter inhibitor, sodium hydrogen transporter inhibitor, and a chelator of the extracellular calcium, whereas enhanced by ouabain. Vitamin D receptor (VDR) agonists significantly increased the recruitment of VDR to the vitamin D response element (VDRE) of PTH1R and Klotho promoter, thus increasing the expression of PTH1R and Klotho.

Conclusions

High sodium can decrease the synthesis of active vitamin D in the proximal tubules, affect the gene regulation of 1,25(OH)2D3/VDR, and significantly reduce the expression of PTH1R and Klotho. It revealed the influence of a high-sodium diet on mineral metabolism and the core role of vitamin D in kidney mineral metabolism.

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.

Inflammation and Oxidative Stress in Chronic Kidney Disease and Dialysis Patients

Significance: Chronic kidney disease (CKD) can be regarded as a burden of lifestyle disease that shares common underpinning features and risk factors with the aging process; it is a complex constituted by several adverse components, including chronic inflammation, oxidative stress, early vascular aging, and cellular senescence. Recent Advances: A systemic approach to tackle CKD, based on mitigating the associated inflammatory, cell stress, and damage processes, has the potential to attenuate the effects of CKD, but it also preempts the development and progression of associated morbidities. In effect, this will enhance health span and compress the period of morbidity. Pharmacological, nutritional, and potentially lifestyle-based interventions are promising therapeutic avenues to achieve such a goal. Critical Issues: In the present review, currents concepts of inflammation and oxidative damage as key patho-mechanisms in CKD are addressed. In particular, potential beneficial but also adverse effects of different systemic interventions in patients with CKD are discussed. Future Directions: Senotherapeutics, the nuclear factor erythroid 2-related factor 2-kelch-like ECH-associated protein 1 (NRF2-KEAP1) signaling pathway, the endocrine klotho axis, inhibitors of the sodium-glucose cotransporter 2 (SGLT2), and live bio-therapeutics have the potential to reduce the burden of CKD and improve quality of life, as well as morbidity and mortality, in this fragile high-risk patient group. Antioxid. Redox Signal. 35, 1426-1448.

Klotho in kidney diseases: A crosstalk between the renin-angiotensin system and endoplasmic reticulum stress

Klotho is a transmembrane anti-ageing protein that exists in three forms, i.e., α-Klotho, β-Klotho, and γ-Klotho with distinct organ-specific expression and functions in the body. Here we focus on α-Klotho (mentioned as 'Klotho' only), abundantly expressed by the distal and proximal convoluted tubules of the kidney. Significant decline in systemic and renal Klotho level is a new hallmark for kidney disease progression. Emerging research portrays Klotho as a promising diagnostic as well as a therapeutic target for diabetic and non-diabetic kidney disease. Even so, the underlying mechanisms of Klotho regulation and the strategies to restore its systemic as well as the renal level are still lacking. Angiotensin-converting enzyme inhibitors (ACEi) and/or angiotensin receptor blockers (ARBs) are the current standard of care for kidney diseases where the molecular mechanisms for their nephroprotective action are still ambiguous. Moreover, endoplasmic reticulum stress (ER stress) also plays a crucial role in kidney disease progression. Few studies have claimed that RAAS has a direct relation with ER stress generation and vice versa in kidney disease. Interestingly, RAAS and ER stress modulation is associated with Klotho regulation in kidney disease. Here we focus on how the RAAS and ER stress connects with Klotho regulation in kidney disease. We also discuss Klotho and ER stress in an alliance with the concept of hemodynamic and metabolic overload in kidney disease. In addition, we highlight novel approaches to implement Klotho as a therapeutic target via RAAS and ER stress modulation for the treatment of diabetic and non-diabetic kidney disease.

Fibroblast growth factor 23-Klotho and hypertension: experimental and clinical mechanisms

Hypertension (HTN) and chronic kidney disease (CKD) are increasingly recognized in pediatric patients and represent risk factors for cardiovascular morbidity and mortality later in life. In CKD, enhanced tubular sodium reabsorption is a leading cause of HTN due to augmented extracellular fluid volume expansion. The renin-angiotensin-aldosterone system (RAAS) upregulates various tubular sodium cotransporters that are also targets of the hormone fibroblast growth factor 23 (FGF23) and its co-receptor Klotho. FGF23 inhibits the activation of 1,25-dihydroxyvitamin D that is a potent suppressor of renin biosynthesis. Here we review the complex interactions and disturbances of the FGF23-Klotho axis, vitamin D, and the RAAS relevant to blood pressure regulation and discuss the therapeutic strategies aimed at mitigating their pathophysiologic contributions to HTN.

Correlation Between Soluble Klotho and Vascular Calcification in Chronic Kidney Disease: A Meta-Analysis and Systematic Review

Background: The correlation between soluble Klotho (sKlotho) level and vascular calcification (VC) in patients with chronic kidney disease (CKD) remains controversial. Using meta-analysis, we aimed to address this controversy and assess the feasibility of applying sKlotho as a biomarker for VC. Methods: Medical electronic databases were thoroughly searched for eligible publications on the association between sKlotho level and VC in CKD patients. Effectors, including correlation coefficients (r), odds ratios (ORs), hazard ratio (HR) or β-values, and 95% confidence intervals (CIs) were extracted and combined according to study design or effector calculation method. Pooled effectors were generated using both random-effects models and fixed-effects models according to I 2-value. Origin of heterogeneity was explored by sensitivity analysis and subgroup analysis. Results: Ten studies with 1,204 participants from a total of 1,199 publications were eligible and included in this meta-analysis. The combined correlation coefficient (r) was [-0.33 (-0.62, -0.04)] with significant heterogeneity (I 2 = 89%, p < 0.001) based on Spearman correlation analysis, and this significant association was also demonstrated in subgroups. There was no evidence of publication bias. The combined OR was [3.27 (1.70, 6.30)] with no evidence of heterogeneity (I 2 = 0%, p = 0.48) when sKlotho was treated as a categorical variable or [1.05 (1.01, 1.09)] with moderate heterogeneity (I 2 = 63%, p = 0.10) when sKlotho was treated as a continuous variable based on multivariate logistic regression. No significant association was observed and the pooled OR was [0.29 (0.01, 11.15)] with high heterogeneity (I 2 = 96%, p < 0.001) according to multivariate linear regression analysis. There was an inverse association between sKlotho and parathyroid hormone levels. The combined coefficient (r) was [-0.20 (-0.40, -0.01)] with significant heterogeneity (I 2 = 86%, p < 0.001), and without obvious publication bias. No significant association was found between sKlotho and calcium or phosphate levels. Conclusion: There exists a significant association between decreased sKlotho level and increased risk of VC in CKD patients. This raises the possibility of applying sKlotho as a biomarker for VC in CKD populations. Large, prospective, well-designed studies or interventional clinical trials are required to validate our findings.

The Effects of Cholecalciferol Supplementation on FGF23 and α-Klotho in Hemodialysis Patients With Hypovitaminosis D: A Randomized, Double-Blind, Placebo-Controlled Trial

OBJECTIVE

Vitamin D-fibroblast growth factor-23 (FGF-23)-klotho forms an axis that takes part at least in cardiovascular complications in patients with chronic kidney disease. This study aimed to assess the effects of cholecalciferol supplementation on FGF23 and α-klotho in patients with hypovitaminosis D requiring hemodialysis.

METHODS

In a single-center, parallel-arm, randomized, double-blind, placebo-controlled trial, 86 patients with hypovitaminosis D requiring hemodialysis were enrolled. The patients were randomized into 2 groups (n = 43 each) to receive either 50,000 IU of cholecalciferol or placebo every week for 12 weeks. Accordingly, the serum levels of FGF23 and klotho were measured by ELISA and compared between both groups.

RESULTS

Serum 25OH(D) levels increased in participants who received cholecalciferol supplementation compared with participants who received placebo (P = .006). In addition, serum FGF23 decreased and α-klotho levels increased in the supplemented group compared with placebo. However, the before-after differences between cholecalciferol supplement and placebo were significant only for α-klotho (P = .035). These effects were not accompanied by changes in the levels of phosphate, total and ionized calcium, and intact parathyroid hormone.

CONCLUSION

Cholecalciferol supplementation of 50,000 IU for 12 weeks increases α-klotho levels in the serum of kidney failure patients undergoing hemodialysis. This may suggest that patients receiving maintenance hemodialysis can benefit from using cholecalciferol supplementation and increase in serum α-klotho levels.

Myostatin in the Arterial Wall of Patients with End-Stage Renal Disease

AIM

Myostatin (Mstn) has been described as a trigger for the progression of atherosclerosis. In this study, we evaluated the role of Mstn in arterial remodeling in patients with end-stage renal disease (ESRD).

METHODS

Vascular specimens were collected from 16 ESRD patients (56.4±7.9 years) undergoing renal transplant (recipients) and 15 deceased kidney non-uremic donors (55.4±12.1 years). We studied gene and protein expression of Mstn, ubiquitin ligases, Atrogin-1, and muscle ring finger protein-1 (MuRF-1), inflammatory marker CCL2, cytoskeleton components, and Klotho by reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemistry. Moreover, we assessed vascular calcification and collagen deposition. Finally, we studied the effects of recombinant Mstn on rat vascular smooth muscle cells (VSMCs, A7r5) and evaluated the effects of uremic serum (US) on primary human VSMCs.

RESULTS

Myostatin mRNA was upregulated in the arterial vascular wall of recipients compared with donors (~15- folds, p＜0.05). This response was accompanied by the upregulation of gene expression of Atrogin-1 and MuRF-1 (＋2.5- and ＋10-fold) and CCL2 (＋3-fold). Conversely, we found downregulation of protein expression of Smoothelin, α-smooth muscle actin (α-SMA), vimentin, and Klotho (-85%, -50%, -70%, and -80%, respectively; p＜0.05) and gene expression of vimentin and Klotho. Exposition of A7r5 to Mstn induced a time-dependent SMAD 2/SMAD 3 phosphorylation and expression of collagen-1 and transforming growth factor β (TGFβ) mRNA, while US induced overexpression of Mstn and Atrogin-1 and downregulation of Smoothelin and Klotho.

CONCLUSIONS

Our data suggest that uremia might induce vascular Mstn gene expression together with a complex pathway of molecular and structural changes in the vascular wall. Myostatin, in turn, can translate the metabolic alterations of uremia into profibrotic and stiffness inducing signals.

Klotho, Aging, and the Failing Kidney

Klotho has been recognized as a gene involved in the aging process in mammals for over 30 years, where it regulates phosphate homeostasis and the activity of members of the fibroblast growth factor (FGF) family. The α-Klotho protein is the receptor for Fibroblast Growth Factor-23 (FGF23), regulating phosphate homeostasis and vitamin D metabolism. Phosphate toxicity is a hallmark of mammalian aging and correlates with diminution of Klotho levels with increasing age. As such, modulation of Klotho activity is an attractive target for therapeutic intervention in the diseasome of aging; in particular for chronic kidney disease (CKD), where Klotho has been implicated directly in the pathophysiology. A range of senotherapeutic strategies have been developed to directly or indirectly influence Klotho expression, with varying degrees of success. These include administration of exogenous Klotho, synthetic and natural Klotho agonists and indirect approaches, via modulation of the foodome and the gut microbiota. All these approaches have significant potential to mitigate loss of physiological function and resilience accompanying old age and to improve outcomes within the diseasome of aging.

A Land of Controversy: Fibroblast Growth Factor-23 and Uremic Cardiac Hypertrophy

Cardiac hypertrophy is a common feature in patients with CKD. Recent studies revealed that two phosphate regulators, fibroblast growth factor-23 and α-Klotho, are highly involved in the pathophysiologic process of CKD-induced cardiac hypertrophy. With decreasing renal function, elevated fibroblast growth factor-23 and decreased α-Klotho may contribute to cardiac hypertrophy by targeting the heart directly or by inducing systemic changes, such as vascular injury, hemodynamic disorders, and inflammation. However, several studies have demonstrated that disturbances in the fibroblast growth factor-23/α-Klotho axis do not lead to cardiac hypertrophy. In this review, we describe the cardiac effects of the fibroblast growth factor-23/α-Klotho axis and summarize recent progress in this field. In addition, we present not only the main controversies in this field but also provide possible directions to resolve these disputes.

FGF23 measurement in chronic kidney disease: What is it really reflecting?

Fibroblast growth factor can be measured in clinical practice using ELISA, with acceptable validity. Different from many metabolites and minerals, its value can differ by a thousand-fold between individuals, largely because of differences in kidney function and dietary habits. This wide range complicates the proper interpretation of the concentration of FGF23, both in terms of the appropriateness of a given value for a given estimated GFR, and in terms of estimating the magnitude of risk for clinical events, with which FGF23 is clearly associated. In this narrative review, the impact of kidney function, exposure to phosphate from diet, and novel emerging factors that influence FGF23 concentrations are discussed. These and yet to define determinants of FGF23 question the causality of the association of FGF23 with hard (cardiovascular) endpoints, as observed in several epidemiological studies.

Clinical Potential of Targeting Fibroblast Growth Factor-23 and αKlotho in the Treatment of Uremic Cardiomyopathy

Chronic kidney disease is highly prevalent, affecting 10% to 15% of the adult population worldwide and is associated with increased cardiovascular morbidity and mortality. As chronic kidney disease worsens, a unique cardiovascular phenotype develops characterized by heart muscle disease, increased arterial stiffness, atherosclerosis, and hypertension. Cardiovascular risk is multifaceted, but most cardiovascular deaths in patients with advanced chronic kidney disease are caused by heart failure and sudden cardiac death. While the exact drivers of these deaths are unknown, they are believed to be caused by uremic cardiomyopathy: a specific pattern of myocardial hypertrophy, fibrosis, with both diastolic and systolic dysfunction. Although the pathogenesis of uremic cardiomyopathy is likely to be multifactorial, accumulating evidence suggests increased production of fibroblast growth factor-23 and αKlotho deficiency as potential major drivers of cardiac remodeling in patients with uremic cardiomyopathy. In this article we review the increasing understanding of the physiology and clinical aspects of uremic cardiomyopathy and the rapidly increasing knowledge of the biology of both fibroblast growth factor-23 and αKlotho. Finally, we discuss how dissection of these pathological processes is aiding the development of therapeutic options, including small molecules and antibodies, directly aimed at improving the cardiovascular outcomes of patients with chronic kidney disease and end-stage renal disease.

Klotho Gene in Human Salt-Sensitive Hypertension

BACKGROUND AND OBJECTIVES

Hypertension is a common aging-related disorder. Salt intake is one of the main environmental factors contributing to the development of hypertension. Transgenic mice with one-half Klotho deficiency displayed a spontaneous BP increase and salt-sensitive hypertension in response to high sodium intake. Usually circulating levels of α-Klotho decrease with age, and this reduction may be stronger in patients with several aging-related diseases. This study aimed at exploring the association of Klotho with salt sensitivity in humans.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The role of Klotho polymorphisms and α-Klotho serum levels was evaluated in patients with hypertension who were treatment naive and underwent an acute salt-sensitivity test (discovery n=673, intravenous 2 L of 0.9% saline in 2 hours). Salt sensitivity was defined as a mean BP increase of >4 mm Hg at the end of the infusion. A total of 32 single nucleotide polymorphisms in the Klotho gene (KL), previously identified with a genome-wide association study, were used in the genetic analysis and studied for a pressure-natriuresis relationship.

RESULTS

Of the patients with hypertension, 35% were classified as salt sensitive. The most relevant polymorphism associated with pressure natriuresis was the common missense single nucleotide polymorphism rs9536314, and the GG and GT genotypes were more represented among patients who were salt sensitive (P=0.001). Those carrying the G allele showed a less steep pressure-natriuresis relationship, meaning that a significant increase in mean BP was needed to excrete the same quantity of salt compared with patients who were salt resistant. KL rs9536314 also replicated the pressure-natriuresis association in an independent replication cohort (n=193) and in the combined analysis (n=866). There was an inverse relationship between circulating Klotho and mean BP changes after the saline infusion (r=-0.14, P=0.03). Moreover, circulating α-Klotho was directly related to kidney function at baseline eGFR (r=0.22, P<0.001).

CONCLUSIONS

KL rs9536314 is associated with salt-sensitive hypertension in patients with hypertension who are treatment naive. Moreover, circulating α-Klotho levels were mainly related to diastolic BP changes at the end of a salt load and to eGFR as an expression of kidney aging.

Evolving concepts in the pathogenesis of uraemic cardiomyopathy

The term uraemic cardiomyopathy refers to the cardiac abnormalities that are seen in patients with chronic kidney disease (CKD). Historically, this term was used to describe a severe cardiomyopathy that was associated with end-stage renal disease and characterized by severe functional abnormalities that could be reversed following renal transplantation. In a modern context, uraemic cardiomyopathy describes the clinical phenotype of cardiac disease that accompanies CKD and is perhaps best characterized as diastolic dysfunction seen in conjunction with left ventricular hypertrophy and fibrosis. A multitude of factors may contribute to the pathogenesis of uraemic cardiomyopathy, and current treatments only modestly improve outcomes. In this Review, we focus on evolving concepts regarding the roles of fibroblast growth factor 23 (FGF23), inflammation and systemic oxidant stress and their interactions with more established mechanisms such as pressure and volume overload resulting from hypertension and anaemia, respectively, activation of the renin-angiotensin and sympathetic nervous systems, activation of the transforming growth factor-β (TGFβ) pathway, abnormal mineral metabolism and increased levels of endogenous cardiotonic steroids.

Aldosterone induces renal fibrosis by promoting HDAC1 expression, deacetylating H3K9 and inhibiting klotho transcription

Aldosterone has an important role in the progression of renal fibrosis. In the present study, the concentration of aldosterone and klotho (KL) in the serum of patients with chronic kidney disease (CKD) were analyzed. A negative correlation was observed between aldosterone and KL, suggesting that KL may serve a protective role in CKD. Subsequently, an aldosterone‑induced CKD mouse model was established using a single nephrectomy and subcutaneous osmotic pump with aldosterone and 1% high‑salt drinking water. It was demonstrated that fibronectin 1 (Fn1) expression levels were higher in high aldosterone mice, whereas KL expression levels were low. In addition, the results demonstrated that histone deacetylase 1 (HDAC1) protein expression levels were upregulated in the renal distal convoluted tubules of high aldosterone mice, whereas acetylated H3K9 (H3K9Ac) was significantly downregulated. To determine the transcriptional activation status, chromatin immunoprecipitation polymerase chain reaction (PCR) was used to validate binding of H3K9Ac to the KL gene promoter site. It was revealed that the binding product of the KL promoter could be PCR‑amplified at the H3K9Ac site from wild‑type and low aldosterone mice; however, amplification of the binding product was not observed in high aldosterone mice. In conclusion, aldosterone significantly inhibited H3K9 acetylation by upregulating HDAC1 protein expression levels in the renal distal convoluted tubule cells, resulting in its inability to bind to the KL promoter, loss of transcription of the KL gene and increased expression of the renal fibrosis gene, Fn1.

The role of klotho in chronic kidney disease

Chronic kidney disease (CKD) is an inherently systemic disease that refers to a long-term loss of kidney function. The progression of CKD has repercussions for other organs, leading to many kinds of extrarenal complications. Intensive studies are now being undertaken to reveal the risk factors and pathophysiological mechanism of this disease. During the past 20 years, increasing evidence from clinical and basic studies has indicated that klotho, which was initially known as an anti-aging gene and is mainly expressed in the kidney, is significantly correlated with the development and progression of CKD and its complications. Here, we discuss in detail the role and pathophysiological implications of klotho in ion disorders, the inflammation response, vascular calcification, mineral bone disorders, and renal fibrosis in CKD. Based on the pathogenic mechanism of klotho deficiency and klotho decline in urine early in CKD stage 2 and even earlier in CKD stage 1, it is not difficult to understand that soluble klotho can serve as an early and sensitive marker of CKD. Moreover, the prevention of klotho decline by several mechanisms can attenuate renal injuries, retard CKD progression, ameliorate extrarenal complications, and improve renal function. In this review, we focus on the functions and pathophysiological implications of klotho in CKD and its extrarenal complications as well as its potential applications as a diagnostic and/or prognostic biomarker for CKD and as a novel treatment strategy to improve and decrease the burden of comorbidity in CKD.

Vitamin D Attenuates Endothelial Dysfunction in Uremic Rats and Maintains Human Endothelial Stability

Background Dysfunctional endothelium may contribute to the development of cardiovascular complications in chronic kidney disease ( CKD ). Supplementation with active vitamin D has been proposed to have vasoprotective potential in CKD , not only by direct effects on the endothelium but also by an increment of α-Klotho. Here, we explored the capacity of the active vitamin D analogue paricalcitol to protect against uremia-induced endothelial damage and the extent to which this was dependent on increased α-Klotho concentrations. Methods and Results In a combined rat model of CKD with vitamin D deficiency, renal failure induced vascular permeability and endothelial-gap formation in thoracic aorta irrespective of baseline vitamin D, and this was attenuated by paricalcitol. Downregulation of renal and serum α-Klotho was found in the CKD model, which was not restored by paricalcitol. By measuring the real-time changes of the human endothelial barrier function, we found that paricalcitol effectively improved the recovery of endothelial integrity following the addition of the pro-permeability factor thrombin and the induction of a wound. Furthermore, immunofluorescence staining revealed that paricalcitol promoted vascular endothelial-cadherin-based cell-cell junctions and diminished F-actin stress fiber organization, preventing the formation of endothelial intracellular gaps. Conclusions Our results demonstrate that paricalcitol attenuates the CKD -induced endothelial damage in the thoracic aorta and directly mediates endothelial stability in vitro by enforcing cell-cell interactions.

Upregulation of Klotho potentially inhibits pulmonary vascular remodeling by blocking the activation of the Wnt signaling pathway in rats with PM2.5-induced pulmonary arterial hypertension

We evaluated the effects of Klotho on pulmonary vascular remodeling and cell proliferation and apoptosis in rat models with PM2.5-induced pulmonary arterial hypertension (PAH) via the Wnt signaling pathway. After establishing rat models of PM2.5-induced PAH, these Sprague-Dawley male rats were randomized into control and model groups. Cells extracted from the model rats were sub-categorized into different groups. Activation of Wnt/β-catenin signaling transcription factor was detected by a TOPFlash/FOPFlash assay. A serial of experiment was conducted to identify the mechanism of Klotho on PHA via the Wnt signaling pathway. VEGF levels and PaCO₂ content were higher in the model group, while PaO_2, NO₂^- /NO₃^- content and Klotho level was lower compared to the control group. In comparison to the control group, the model group had decreased Klotho and Bax levels, and elevated Wnt-1, β-catenin, bcl-2, survivin, and PCNA expression, VEGF, IL-6, TNF-α, TNF-β1, and bFGF levels, as well as the percentage of pulmonary artery ring contraction. The Klotho vector, DKK-1 and DKK-1 + Klotho vector groups exhibited reduced cell proliferation, luciferase activity, and the expression of Wnt-1, β-catenin, bcl-2, survivin, and PCNA, as well as shortened S phase compared with the blank and NC groups. Compared with the Klotho vector and DKK-1 groups, the DKK-1 + Klotho vector groups had reduced cell proliferation, luciferase activity, and the expression of Wnt-1, β-catenin, bcl-2, survivin, and PCNA, as well as a shortened S phase. Conclusively, Klotho inhibits pulmonary vascular remodeling by inactivation of Wnt signaling pathway.

The role of fibroblast growth factor 23 and Klotho in uremic cardiomyopathy

PURPOSE OF REVIEW

In chronic kidney disease (CKD), multiple factors contribute to the development of cardiac hypertrophy by directly targeting the heart or indirectly by inducing systemic changes such as hypertension, anemia, and inflammation. Furthermore, disturbances in phosphate metabolism have been identified as nonclassical risk factors for cardiovascular mortality in these patients. With declining kidney function, the physiologic regulators of phosphate homeostasis undergo changes in their activity as well as their circulating levels, thus potentially contributing to cardiac hypertrophy once they are out of balance. Recently, two of these phosphate regulators, fibroblast growth factor 23 (FGF23) and Klotho, have been shown to affect cardiac remodeling, thereby unveiling a novel pathomechanism of cardiac hypertrophy in CKD. Here we discuss the potential direct versus indirect effects of FGF23 and the soluble form of Klotho on the heart, and their crosstalk in the regulation of cardiac hypertrophy.

RECENT FINDINGS

In models of CKD, FGF23 can directly target cardiac myocytes via FGF receptor 4 and induce cardiac hypertrophy in a blood pressure-independent manner. Soluble Klotho may directly target the heart via an unknown receptor thereby protecting the myocardium from pathologic stress stimuli that are associated with CKD, such as uremic toxins or FGF23.

SUMMARY

Elevated serum levels of FGF23 and reduced serum levels of soluble Klotho contribute to uremic cardiomyopathy in a synergistic manner.

Pathophysiology and therapeutics of premature ageing in chronic kidney disease, with a focus on glycative stress

Chronic kidney disease (CKD) is a major concern in public health. The pathology of CKD includes premature ageing in the kidney and vessels, which results in a high risk of cardiovascular events and end-stage renal disease. Many factors are involved in premature ageing in CKD, including hormonal imbalance, glycative stress, nitrogenous metabolites, and oxidative stress. Of these, the most important role in premature ageing in CKD is played by glycative stress, namely a massive and unfavourable glycation state, since the kidney is responsible for the clearance of advanced glycation endproducts (AGEs). In an animal model, overexpression of glyoxalase I (GLO-1), a detoxifier of AGEs, has been found to alleviate premature ageing in the kidney and vessels. Both lifestyle changes and drug therapy have shown promise in overcoming premature ageing. Promising drug therapies include a GLO-1 activator and an absorbent against glycotoxin and nitrogenous metabolites.

Suppression of Wnt Signaling and Osteogenic Changes in Vascular Smooth Muscle Cells by Eicosapentaenoic Acid

Vascular medial calcification is often observed in patients with arteriosclerosis. It is also associated with systolic hypertension, wide pulse pressure, and fluctuation of blood pressure, which results in cardiovascular events. Eicosapentaenoic acid (EPA) has been shown to suppress vascular calcification in previous animal experiments. We investigated the inhibitory effects of EPA on Wnt signaling, which is one of the important signaling pathways involved in vascular calcification. Intake of food containing 5% EPA resulted in upregulation of the mRNA expression of Klotho, an intrinsic inhibitor of Wnt signaling, in the kidneys of wild-type mice. Expression levels of β-catenin, an intracellular signal transducer in the Wnt signaling pathway, were increased in the aortas of Klotho mutant (kl/kl) mice compared to the levels in the aortas of wild-type mice. Wnt3a or BIO, a GSK-3 inhibitor that activates β-catenin signaling, upregulated mRNA levels of AXIN2 and LEF1, Wnt signaling marker genes, and RUNX2 and BMP4, early osteogenic genes, in human aorta smooth muscle cells. EPA suppressed the upregulation of AXIN2 and BMP4. The effect of EPA was cancelled by T0070907, a PPARγ inhibitor. The results suggested that EPA could suppress vascular calcification via the inhibition of Wnt signaling in osteogenic vascular smooth muscle cells via PPARγ activation.

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.

Potential application of klotho in human chronic kidney disease

The extracellular domain of transmembrane alpha-Klotho (αKlotho, hereinafter simply called Klotho) is cleaved by secretases and released into the circulation as soluble Klotho. Soluble Klotho in the circulation starts to decline early in chronic kidney disease (CKD) stage 2 and urinary Klotho possibly even earlier in CKD stage 1. Therefore soluble Klotho could serve as an early and sensitive marker of kidney function decline. Moreover, preclinical animal data support Klotho deficiency is not just merely a biomarker, but a pathogenic factor for CKD progression and extrarenal CKD complications including cardiovascular disease and disturbed mineral metabolism. Prevention of Klotho decline, re-activation of endogenous Klotho production or supplementation of exogenous Klotho are all associated with attenuation of renal fibrosis, retardation of CKD progression, improvement of mineral metabolism, amelioration of cardiomyopathy, and alleviation of vascular calcification in CKD. Therefore Klotho is not only a diagnostic and/or prognostic marker for CKD, but the treatment of Klotho deficiency may be a promising strategy to prevent, retard, and decrease the burden of comorbidity in CKD.

Vascular calcification in CKD-MBD: Roles for phosphate, FGF23, and Klotho

Vascular calcification (VC) is highly prevalent in aging, diabetes mellitus, and chronic kidney disease (CKD). VC is a strong predictor of cardiovascular morbidity and mortality in the CKD population. Complex pathological mechanisms are involved in the development of VC, including osteochondrogenic differentiation and apoptosis of vascular smooth muscle cells, instability and release of extracellular vesicles loaded calcium and phosphate, and elastin degradation. Elevated serum phosphate is a late manifestation of CKD, and has been shown to accelerate mineral deposition in both the vessel wall and heart valves. α-Klotho and fibroblast growth factor 23 (FGF23) are emerging factors in CKD-mineral and bone disorder (CKD-MBD) and are thought to be involved in the pathogenesis of uremic VC. There are discordant reports regarding the biomedical effects of FGF23 on VC. In contrast, mounting evidence supports a well-supported protective role for α-Klotho on VC. Further studies are warranted to elucidate potential roles of FGF23 and α-Klotho in VC and to determine where and how they are synthesized in normal and disease conditions. A thorough systemic evaluation of the biomedical interplay of phosphate, FGF23, and α-Klotho may potentially lead to new therapeutic options for patients with CKD-MBD.

The role of the anti-ageing protein Klotho in vascular physiology and pathophysiology

Klotho is an anti-ageing protein that functions in many pathways that govern ageing, like regulation of phosphate homeostasis, insulin signaling, and Wnt signaling. Klotho expression levels and levels in blood decline during ageing. The vascular phenotype of Klotho deficiency features medial calcification, intima hyperplasia, endothelial dysfunction, arterial stiffening, hypertension, and impaired angiogenesis and vasculogenesis, with characteristics similar to aged human arteries. Klotho-deficient phenotypes can be prevented and rescued by Klotho gene expression or protein supplementation. High phosphate levels are likely to be directly pathogenic and are a prerequisite for medial calcification, but more important determinants are pathways that regulate cellular senescence, suggesting that deficiency of Klotho renders cells susceptible to phosphate toxicity. Overexpression of Klotho is shown to ameliorate medial calcification, endothelial dysfunction, and hypertension. Endogenous vascular Klotho expression is a controversial subject and, currently, no compelling evidence exists that supports the existence of vascular membrane-bound Klotho expression, as expressed in kidney. In vitro, Klotho has been shown to decrease oxidative stress and apoptosis in both SMCs and ECs, to reduce SMC calcification, to maintain the contractile SMC phenotype, and to prevent μ-calpain overactivation in ECs. Klotho has many protective effects with regard to the vasculature and constitutes a very promising therapeutic target. The purpose of this review is to explore the etiology of the vascular phenotype of Klotho deficiency and the therapeutic potential of Klotho in vascular disease.

Comparison of the effects of novel vitamin D receptor analog VS-105 and paricalcitol on chronic kidney disease-mineral bone disorder in an experimental model of chronic kidney disease

When using vitamin D, the most important clinical problems are hypercalcemia, hyperphosphatemia, and vascular calcification. VS-105 is a novel vitamin D receptor (VDR) analog. In the present study, we compared the effects of VS-105 and paricalcitol on chronic kidney disease-mineral bone disorder (CKD-MBD) in a CKD rat model. We used male Sprague-Dawley (SD) rats and performed 5/6 nephrectomy at 8-9 weeks. At 10 weeks, the rats were classified into five groups and administered vehicle, low-dose paricalcitol (LP, 0.1μg/kg), high-dose paricalcitol (HP, 0.3μg/kg), low-dose VS-105 (LV, 0.2μg/kg), and high-dose VS-105 (HV, 0.6 μg/kg) three times a week for 10 weeks. There were no significant differences in blood pressure or renal function among the five groups. Alhough serum calcium levels were comparable between the LP and LV groups, they were higher in the HP group than in the HV group. Serum phosphate levels were higher in the paricalcitol-treated groups than in the VS-105-treated groups and paticularly higher in the HP group than in the other groups. The urinary excretion of phosphate was greater in the VS-105-treated groups than in the paricalcitol-treated groups. Serum parathyroid hormone (PTH) levels decreased and serum fibroblast growth factor-23 (FGF23) levels were elevated after administering paricalcitol and VS-105; however, serum FGF23 levels were remarkably elevated in the paricalcitol-treated groups. Further biochemical analyses revealed that the calcium content of the aorta was higher in the paricalcitol-treated groups than in the VS-105-treated group. VDR and Klotho expression in the kidney was significantly higher in the VS-105-treated groups than in the paricalcitol-treated groups although both agents increased these expressions. Our data suggest that VS-105 had a lesser effect on CKD-MBD than paricalcitol except in the case of serum PTH levels. The mechanism appears to be associated with the difference in VDR and Klotho expression.

The impact of oat (Avena sativa) consumption on biomarkers of renal function in patients with chronic kidney disease: A parallel randomized clinical trial

BACKGROUND & OBJECTIVE

Animal studies report that oat (Avena sativa L) intake has favorable effects on kidney function. However, the effects of oat consumption have not been assessed in humans. The aim of this study was to examine the impact of oat intake on biomarkers of renal function in patients with chronic kidney disease (CKD).

METHODS

Fifty-two patients with CKD were randomly assigned to a control group (recommended to reduce intake of dietary protein, phosphorus, sodium and potassium) or an oat consumption group (given nutritional recommendations for controls +50 g/day oats). Blood urea nitrogen (BUN), serum creatinine (SCr), urine creatinine, serum albumin, serum potassium, parathyroid hormone (PTH), serum klotho and urine protein concentration were measured at baseline and after an eight-week intervention. Creatinine clearance was calculated using urine creatinine concentration.

RESULTS

Within group analysis showed a significant increase in BUN (P = 0.02) and serum potassium (P = 0.01) and a marginally significant increment in SCr (P = 0.08) among controls. However, changes in the oat group were not significant. In a multivariate adjusted model, we observed a significant difference in change of serum potassium (-0.03 mEq/L for oat group and 0.13 mEq/L for control group; P = 0.01) and a marginally significant difference in change of serum albumin (0.01 g/dl for oat group and -0.08 for control group; P = 0.08) between the two groups. There was no change in PTH concentration.

CONCLUSION

Intake of oats may have a beneficial effect on serum albumin and serum potassium in patients with CKD.

REGISTRATION CODE

Present study registered under IRCT.ir identifier no. IRCT2015050414551N2.

Klotho/FGF23 Axis in Chronic Kidney Disease and Cardiovascular Disease

BACKGROUND

Membrane αKlotho (hereinafter called Klotho) is highly expressed in the kidney and functions as a coreceptor of FGF receptors (FGFRs) to activate specific fibroblast growth factor 23 (FGF23) signal pathway. FGF23 is produced in bones and participates in the maintenance of mineral homeostasis. The extracellular domain of transmembrane Klotho can be cleaved by secretases and released into the circulation as soluble Klotho. Soluble Klotho does not only weakly activate FGFRs to transduce the FGF23 signaling pathway, but also functions as an enzyme and hormonal substance to play a variety of biological functions. FGF23 exerts its biological effects through activation of FGFRs in a Klotho-dependent manner. However, extremely high FGF23 can exert its pathological action in a Klotho-independent manner.

SUMMARY

The decline in serum and urinary Klotho followed by a rise in serum FGF23 at an early stage of chronic kidney disease (CKD) functions as an early biomarker for kidney dysfunction and can also serve as a predictor for risk of cardiovascular disease (CVD) and mortality in both CKD patients and the general population. Moreover, Klotho deficiency is a pathogenic factor for CKD progression and CVD. FGF23 may also contribute to CVD. Prevention of Klotho decline, reactivation of endogenous Klotho production, or supplementation of exogenous Klotho attenuate renal fibrosis, retard CKD progression, improve mineral metabolism, ameliorate cardiomyopathy, and alleviate vascular calcification in CKD. However, the poor CVD outcome after depletion of FGF23 with FGF23 antibody stimulates the generation of a more specific inhibitor of FGF23 for CKD treatment.

KEY MESSAGE

Klotho/FGF23 may not only be diagnostic and/or prognostic biomarkers for CKD and CVD, but are also pathogenic contributors to CKD progression and CVD development. The Klotho/FGF23 axis should be a novel target for renal clinics.

Indoxyl Sulfate Enhance the Hypermethylation of Klotho and Promote the Process of Vascular Calcification in Chronic Kidney Disease

Chronic kidney disease (CKD) is a state of Klotho deficiency. The Klotho expression may be suppressed due to DNA hypermethylation in cancer cells so we have investigated the effects and possible mechanisms by which Klotho expression is regulated in human aortic smooth muscle cells (HASMCs). The vascular Klotho hypermethylation in radial arteries of patients with end-stage renal disease was described. Cultured HASMCs and 5/6-nephrectomized Sprague Dawley (SD) rats treated with indoxyl sulfate (IS) were used as in vitro and in vivo models, respectively. IS increased CpG hypermethylation of the Klotho gene and decreased Klotho expression in HASMCs, and potentiated HASMCs calcification. The expression of DNA methyltransferase (DNMT) 1 and 3a in HASMCs treated with IS was significantly increased and specific inhibition of DNA methyltransferase 1 by 5-aza-2'-deoxycytidine(5Aza-2dc) caused demethylation of the Klotho gene and increased Klotho expression. In rats, injection of IS potentiated vascular calcification, increased CpG hypermethylation of the Klotho gene and decreased Klotho expression in the aortic medial layer and all of these changes could be reverted by 5Aza-2dc treatment. Transcriptional suppression of vascular Klotho gene expression by IS and epigenetic modification of Klotho by IS may be an important pathological mechanism of vascular calcification in CKD.

Phosphate Toxicity in CKD: The Killer among Us

Maintenance of a normal serum phosphate level depends on absorption in the gut, reabsorption and excretion by the kidney, and the flux between the extracellular and skeletal pools. Phosphate homeostasis is a coordinated, complex system of crosstalk between the bone, intestine, kidney, and parathyroid gland. Dysfunction of this system has serious clinical consequences in healthy individuals and those with conditions, such as CKD, in which hyperphosphatemia is associated with increased risks of cardiovascular morbidity and mortality. The last half-century of renal research has helped define the contribution of the parathyroid hormone, calcitriol, fibroblast growth factor 23, and Klotho in the regulation of phosphate. However, despite new discoveries and insights gained during this time, what remains unchanged is the recognition that phosphate retention is the initiating factor for the development of many of the complications observed in CKD, namely secondary hyperparathyroidism and bone and cardiovascular diseases. Controlling phosphate load remains the primary goal in the treatment of CKD. This review discusses the clinical effects of dysregulated phosphate metabolism, particularly in CKD, and its association with cardiovascular disease. The importance of early control of phosphate load in the treatment of CKD is emphasized, and the latest research in the treatment of phosphate retention is discussed.

Stop chronic kidney disease progression: Time is approaching

Progression of chronic kidney disease (CKD) is inevitable. However, the last decade has witnessed tremendous achievements in this field. Today we are optimistic; the dream of withholding this progression is about to be realistic. The recent discoveries in the field of CKD management involved most of the individual diseases leading the patients to end-stage renal disease. Most of these advances involved patients suffering diabetic kidney disease, chronic glomerulonephritis, polycystic kidney disease, renal amyloidosis and chronic tubulointerstitial disease. The chronic systemic inflammatory status and increased oxidative stress were also investigated. This inflammatory status influences the anti-senescence Klotho gene expression. The role of Klotho in CKD progression together with its therapeutic value are explored. The role of gut as a major source of inflammation, the pathogenesis of intestinal mucosal barrier damage, the role of intestinal alkaline phosphatase and the dietary and therapeutic implications add a novel therapeutic tool to delay CKD progression.

Sclerostin, TNF-alpha and Interleukin-18 Correlate and are Together with Klotho Related to Other Growth Factors and Cytokines in Haemodialysis Patients

Patients with chronic renal failure are known to have renal osteodystrophy (bone disease) and increased calcification of vessels. A new marker of bone disease, sclerostin, the two pro-inflammatory cytokines tumour necrosis factor-alpha (TNF-alpha) and interleukin-18 (IL-18), and the fibroblast growth factor-23 (FGF-23) receptor-associated marker Klotho were tested in 84 haemodialysis (HD) patients and in healthy controls. The patients had significantly higher levels of the three former markers than of the controls while Klotho was significantly higher in the controls. Low level, but significant, correlations were observed in the patient group when the levels of these four markers were compared to each other and to those of 5 cytokines and growth factors tested earlier; high-sensitive CRP (hsCRP), interleukin-6 (IL-6), hepatocyte growth factor (HGF), fibroblast growth factor-23 (FGF-23) and soluble urokinase plasminogen activator (suPAR). Ln sclerostin correlated positively to Ln hsTNF-alpha, Ln HGF and Ln suPAR. Ln hsTNF-alpha correlated positively to Ln sclerostin, Ln hsCRP, Ln IL-6, Ln FGF-23, Ln suPAR and Ln IL-18. Ln IL-18 correlated positively to Ln suPAR and Ln TNF-alpha. Ln Klotho correlated negatively to Ln hsCRP but did not correlate to Ln FGF-23. The markers studied here may be involved in the calcification of vessels seen in HD patients due to a combination of inflammation and bone disease. The mechanisms are still not fully known but may be of importance for future therapeutic possibilities in this group of patients.

Bone mineral disorder in chronic kidney disease: Klotho and FGF23; cardiovascular implications

Cardiovascular factors are one of the main causes of morbidity and mortality in patients with chronic kidney disease. Bone mineral metabolism disorders and inflammation are pathological conditions that involve increased cardiovascular risk in chronic kidney disease. The cardiovascular risk involvement of bone mineral metabolism classical biochemical parameters such as phosphorus, calcium, vitamin D and PTH is well known. The newest markers, FGF23 and klotho, could also be implicated in cardiovascular disease.

αKlotho and Chronic Kidney Disease

Alpha-Klotho (αKlotho) protein is encoded by the gene, Klotho, and functions as a coreceptor for endocrine fibroblast growth factor-23. The extracellular domain of αKlotho is cleaved by secretases and released into the circulation where it is called soluble αKlotho. Soluble αKlotho in the circulation starts to decline in chronic kidney disease (CKD) stage 2 and urinary αKlotho in even earlier CKD stage 1. Therefore soluble αKlotho is an early and sensitive marker of decline in kidney function. Preclinical data from numerous animal experiments support αKlotho deficiency as a pathogenic factor for CKD progression and extrarenal CKD complications including cardiac and vascular disease, hyperparathyroidism, and disturbed mineral metabolism. αKlotho deficiency induces cell senescence and renders cells susceptible to apoptosis induced by a variety of cellular insults including oxidative stress. αKlotho deficiency also leads to defective autophagy and angiogenesis and promotes fibrosis in the kidney and heart. Most importantly, prevention of αKlotho decline, upregulation of endogenous αKlotho production, or direct supplementation of soluble αKlotho are all associated with attenuation of renal fibrosis, retardation of CKD progression, improvement of mineral metabolism, amelioration of cardiac function and morphometry, and alleviation of vascular calcification in CKD. Therefore in rodents, αKlotho is not only a diagnostic and prognostic marker for CKD but the enhancement of endogenous or supplement of exogenous αKlotho are promising therapeutic strategies to prevent, retard, and decrease the comorbidity burden of CKD.

The vascular secret of Klotho

Klotho is an evolutionarily highly conserved protein related to longevity. Increasing evidence of a vascular protecting effect of the Klotho protein has emerged and might be important for future treatments of uremic vascular calcification. It is still disputed whether Klotho is locally expressed in the vasculature or whether its vascular effects arise uniquely from its presence in the circulation.

Effect of chronic uremia on the transcriptional profile of the calcified aorta analyzed by RNA sequencing

The development of vascular calcification (VC) in chronic uremia (CU) is a tightly regulated process controlled by factors promoting and inhibiting mineralization. Next-generation high-throughput RNA sequencing (RNA-seq) is a powerful and sensitive tool for quantitative gene expression profiling and the detection of differentially expressed genes. In the present study, we, for the first time, used RNA-seq to examine rat aorta transcriptomes from CU rats compared with control rats. Severe VC was induced in CU rats, which lead to extensive changes in the transcriptional profile. Among the 10,153 genes with an expression level of >1 reads/kilobase transcript/million mapped reads, 2,663 genes were differentially expressed with 47% upregulated genes and 53% downregulated genes in uremic rats. Significantly deregulated genes were enriched for ontologies related to the extracellular matrix, response to wounding, organic substance, and ossification. The individually affected genes were of relevance to osteogenic transformation, tissue calcification, and Wnt modulation. Downregulation of the Klotho gene in uremia is believed to be involved in the development of VC, but it is debated whether the effect is caused by circulating Klotho only or if Klotho is produced locally in the vasculature. We found that Klotho was neither expressed in the normal aorta nor calcified aorta by RNA-seq. In conclusion, we demonstrated extensive changes in the transcriptional profile of the uremic calcified aorta, which were consistent with a shift in phenotype from vascular tissue toward an osteochondrocytic transcriptome profile. Moreover, neither the normal vasculature nor calcified vasculature in CU expresses Klotho.

α-Klotho Expression in Human Tissues

CONTEXT

α-Klotho has emerged as a powerful regulator of the aging process. To date, the expression profile of α-Klotho in human tissues is unknown, and its existence in some human tissue types is subject to much controversy.

OBJECTIVE

This is the first study to characterize systemwide tissue expression of transmembrane α-Klotho in humans. We have employed next-generation targeted proteomic analysis using parallel reaction monitoring in parallel with conventional antibody-based methods to determine the expression and spatial distribution of human α-Klotho expression in health.

RESULTS

The distribution of α-Klotho in human tissues from various organ systems, including arterial, epithelial, endocrine, reproductive, and neuronal tissues, was first identified by immunohistochemistry. Kidney tissues showed strong α-Klotho expression, whereas liver did not reveal a detectable signal. These results were next confirmed by Western blotting of both whole tissues and primary cells. To validate our antibody-based results, α-Klotho-expressing tissues were subjected to parallel reaction monitoring mass spectrometry (data deposited at ProteomeXchange, PXD002775) identifying peptides specific for the full-length, transmembrane α-Klotho isoform.

CONCLUSIONS

The data presented confirm α-Klotho expression in the kidney tubule and in the artery and provide evidence of α-Klotho expression across organ systems and cell types that has not previously been described in humans.

Membrane-bound Klotho is not expressed endogenously in healthy or uraemic human vascular tissue

AIMS

Cardiovascular disease (CVD) is the leading cause of death in patients with chronic kidney disease (CKD), a disease state that is strongly associated with loss of renal and systemic (alpha-)Klotho. Reversely, murine Klotho deficiency causes marked medial calcification. It is therefore thought that Klotho conveys a vasculoprotective effect. Klotho expression in the vessel wall, however, is disputed.

METHODS AND RESULTS

We assessed Klotho expression in healthy human renal donor arteries (n = 9), CKD (renal graft recipient) arteries (n = 10), carotid endarterectomy specimens (n = 8), other elastic arteries (three groups of n = 3), and cultured human aortic smooth muscle cells (HASMCs) (three primary cell lines), using immunohistochemistry (IHC), immunofluorescence, quantitative reverse transcriptase-polymerase chain reaction, and western blotting (WB). We have extensively validated anti-Klotho antibody KM2076 by comparing staining patterns with other anti-Klotho antibodies (SC-22220, SC-22218, and AF1819), competition assays with recombinant Klotho, IHC on Klotho-deficient kl/kl mouse kidney, and WB with recombinant Klotho. Using KM2076, we could not detect full-length Klotho in vascular tissues or HASMCs. On the mRNA level, using primers against all four exon junctions, klotho expression could not be detected either. Fibroblast growth factor 23 (FGF23) injections in mice induced FGF23 signalling in kidneys but not in the aorta, indicating the absence of Klotho-dependent FGF23 signalling in the aorta.

CONCLUSION

Using several independent and validated methods, we conclude that full-length, membrane-bound Klotho is not expressed in healthy or uraemic human vascular tissue.

Biological Role of Anti-aging Protein Klotho

Klotho-deficient mice have accelerated aging phenotypes, whereas overexpression of Klotho in mice extends lifespan. Klotho is an anti-aging single-pass membrane protein predominantly produced in the kidney, with shedding of the amino-terminal extracellular domain into the systemic circulation. Circulating levels of soluble Klotho decrease with age, and the klotho gene is associated with increased risk of age-related diseases. The three forms of Klotho protein have distinct functions. Membrane Klotho forms a complex with fibroblast growth factor (FGF) receptors, functions as an obligatory co-receptor for FGF23, which is involved in aging and the development of chronic diseases via regulation of P_i and vitamin D metabolism. Secreted Klotho functions as a humoral factor with pleiotropic activities including regulation of oxidative stress, growth factor signaling, and ion homeostasis. Secreted Klotho is also involved in organ protection. The intracellular form of Klotho suppresses inflammation-mediated cellular senescence and mineral metabolism. Herein we provide a brief overview of the structure and function and recent research about Klotho.