FGF23 and Klotho in chronic kidney disease

Muscle plasticity is influenced by renal function and caloric intake through the FGF23-vitamin D axis

Skeletal muscle, the main metabolic engine in the body of vertebrates, is endowed with great plasticity. The association between skeletal muscle plasticity and two highly prevalent health problems: renal dysfunction and obesity, which share etiologic links as well as many comorbidities, is a subject of great relevance. It is important to know how these alterations impact on the structure and function of skeletal muscle because the changes in muscle phenotype have a major influence on the quality of life of the patients. This literature review aims to discuss the influence of a nontraditional axis involving kidney, bone, and muscle on skeletal muscle plasticity. In this axis, the kidneys play a role as the main site for vitamin D activation. Renal disease leads to a direct decrease in 1,25(OH)₂-vitamin D, secondary to reduction in renal functional mass, and has an indirect effect, through phosphate retention, that contributes to stimulate fibroblast growth factor 23 (FGF23) secretion by bone cells. FGF23 downregulates the renal synthesis of 1,25(OH)₂-vitamin D and upregulates its metabolism. Skeletal production of FGF23 is also regulated by caloric intake: it is increased in obesity and decreased by caloric restriction, and these changes impact on 1,25(OH)₂-vitamin D concentrations, which are decreased in obesity and increased after caloric restriction. Thus, both phosphate retention, that develops secondary to renal failure, and caloric intake influence 1,25(OH)₂-vitamin D that in turn plays a key role in muscle anabolism.

Klotho's impact on diabetic nephropathy and its emerging connection to diabetic retinopathy

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease worldwide and is a significant burden on healthcare systems. α-klotho (klotho) is a protein known for its anti-aging properties and has been shown to delay the onset of age-related diseases. Soluble klotho is produced by cleavage of the full-length transmembrane protein by a disintegrin and metalloproteases, and it exerts various physiological effects by circulating throughout the body. In type 2 diabetes and its complications DN, a significant decrease in klotho expression has been observed. This reduction in klotho levels may indicate the progression of DN and suggest that klotho may be involved in multiple pathological mechanisms that contribute to the onset and development of DN. This article examines the potential of soluble klotho as a therapeutic agent for DN, with a focus on its ability to impact multiple pathways. These pathways include anti-inflammatory and oxidative stress, anti-fibrotic, endothelial protection, prevention of vascular calcification, regulation of metabolism, maintenance of calcium and phosphate homeostasis, and regulation of cell fate through modulation of autophagy, apoptosis, and pyroptosis pathways. Diabetic retinopathy shares similar pathological mechanisms with DN, and targeting klotho may offer new insights into the prevention and treatment of both conditions. Finally, this review assesses the potential of various drugs used in clinical practice to modulate klotho levels through different mechanisms and their potential to improve DN by impacting klotho levels.

Correlation of Serum FGF23 and Chronic Kidney Disease-Mineral and Bone Abnormality Markers With Cardiac Structure Changes in Maintenance Hemodialysis Patients

Background

CKD-MBD is a mineral and bone metabolism syndrome caused by chronic kidney disease. FGF23 is an important factor regulating phosphorus and is the main influencer in the CKD-MBD process. In this study, we observed the correlation among serum FGF23 and calcium, phosphorus and parathyroid hormone, and the correlation between FGF23 levels and cardiac structural changes in MHD patients.

Methods

We examined serum FGF23 concentrations in 107 cases of MHD patients using the ELISA method, recorded demographic information and biochemical data, and analyzed the correlation between serum FGF23 levels and blood calcium and blood phosphorus and PTH levels. All patients were evaluated by cardiac color ultrasound, and we finally analyzed the association between the FGF23 level and cardiac structural changes.

Results

In 107 cases of MHD patients, serum FGF23 levels were linearly associated with serum calcium (r = 0.27 P < 0.01) and parathyroid hormone levels (r = 0.25, P < 0.05). FGF 23 was negatively correlated with age (r = -0.44, P < 0.01).Serum FGF23 levels were correlated with right atrial hypertrophy in HD patients (P < 0.05). No correlation was found among FGF23, left ventricular hypertrophy/enlargement, and valve calcification stenosis (P > 0.05).

Conclusion

Serum FGF23 showed a positive correlation among blood calcium levels and PTH levels in hemodialysis patients, and FGF23 levels can affect the incidence of right atrial hypertrophy in MHD patients.

Hypertension in chronic kidney disease: What lies behind the scene

Hypertension is a frequent condition encountered during kidney disease development and a leading cause in its progression. Hallmark factors contributing to hypertension constitute a complexity of events that progress chronic kidney disease (CKD) into end-stage renal disease (ESRD). Multiple crosstalk mechanisms are involved in sustaining the inevitable high blood pressure (BP) state in CKD, and these play an important role in the pathogenesis of increased cardiovascular (CV) events associated with CKD. The present review discusses relevant contributory mechanisms underpinning the promotion of hypertension and their consequent eventuation to renal damage and CV disease. In particular, salt and volume expansion, sympathetic nervous system (SNS) hyperactivity, upregulated renin–angiotensin–aldosterone system (RAAS), oxidative stress, vascular remodeling, endothelial dysfunction, and a range of mediators and signaling molecules which are thought to play a role in this concert of events are emphasized. As the control of high BP via therapeutic interventions can represent the key strategy to not only reduce BP but also the CV burden in kidney disease, evidence for major strategic pathways that can alleviate the progression of hypertensive kidney disease are highlighted. This review provides a particular focus on the impact of RAAS antagonists, renal nerve denervation, baroreflex stimulation, and other modalities affecting BP in the context of CKD, to provide interesting perspectives on the management of hypertensive nephropathy and associated CV comorbidities.

Vascular Calcification in Chronic Kidney Disease: An Update and Perspective

Chronic kidney disease is a devastating condition resulting from irreversible loss of nephron numbers and function and leading to end-stage renal disease and mineral disorders. Vascular calcification, an ectopic deposition of calcium-phosphate salts in blood vessel walls and heart valves, is an independent risk factor of cardiovascular morbidity and mortality in chronic kidney disease. Moreover, aging and related metabolic disorders are essential risk factors for chronic kidney disease and vascular calcification. Marked progress has been recently made in understanding and treating vascular calcification in chronic kidney disease. However, there is a paucity of systematic reviews summarizing this progress, and investigating unresolved issues is warranted. In this systematic review, we aimed to overview the underlying mechanisms of vascular calcification in chronic kidney diseases and discuss the impact of chronic kidney disease on the pathophysiology of vascular calcification. Additionally, we summarized potential clinical diagnostic biomarkers and therapeutic applications for vascular calcification with chronic kidney disease. This review may offer new insights into the pathogenesis, diagnosis, and therapeutic intervention of vascular calcification.

Administration of α-Klotho Does Not Rescue Renal Anemia in Mice

Renal anemia is a common complication in chronic kidney disease (CKD), associated with decreased production of erythropoietin (EPO) due to loss of kidney function, and subsequent decreased red blood cell (RBC) production. However, many other factors play a critical role in the development of renal anemia, such as iron deficiency, inflammation, and elevated fibroblast growth factor 23 (FGF23) levels. We previously reported that inhibition of FGF23 signaling rescues anemia in mice with CKD. In the present study we sought to investigate whether α-Klotho deficiency present in CKD also contributes to the development of renal anemia. To address this, we administered α-Klotho to mice with CKD induced by an adenine-rich diet. Mice were sacrificed 24 h after α-Klotho injection, and blood and organs were collected immediately post-mortem. Our data show that α-Klotho administration had no beneficial effect in mice with CKD-associated anemia as it did not increase RBC numbers and hemoglobin levels, and it did not stimulate EPO secretion. Moreover, α-Klotho did not improve iron deficiency and inflammation in CKD as it had no effect on iron levels or inflammatory markers. Interestingly, Klotho supplementation significantly reduced the number of erythroid progenitors in the bone marrow and downregulated renal Epo and Hif2α mRNA in mice fed control diet resulting in reduced circulating EPO levels in these mice. In addition, Klotho significantly decreased intestinal absorption of iron in control mice leading to reduced serum iron and transferrin saturation levels. Our findings demonstrate that α-Klotho does not have a direct role in renal anemia and that FGF23 suppresses erythropoiesis in CKD via a Klotho-independent mechanism. However, in physiological conditions α-Klotho appears to have an inhibitory effect on erythropoiesis and iron regulation.

A phosphate and calcium-enriched diet promotes progression of 5/6-nephrectomy-induced chronic kidney disease in C57BL/6 mice

C57BL/6 mice are known to be rather resistant to the induction of experimental chronic kidney disease (CKD) by 5/6-nephrectomy (5/6-Nx). Here, we sought to characterize the development of CKD and its cardiac and skeletal sequelae during the first three months after 5/6-Nx in C57BL/6 mice fed a calcium- and phosphate enriched diet (CPD) with a balanced calcium/phosphate ratio. 5/6-NX mice on CPD showed increased renal fibrosis and a more pronounced decrease in glomerular filtration rate when compared to 5/6-Nx mice on normal diet (ND). Interestingly, despite comparable levels of serum calcium, phosphate, and parathyroid hormone (PTH), circulating intact fibroblast growth factor-23 (FGF23) was 5 times higher in 5/6-Nx mice on CPD, relative to 5/6-Nx mice on ND. A time course experiment revealed that 5/6-Nx mice on CPD developed progressive renal functional decline, renal fibrosis, cortical bone loss, impaired bone mineralization as well as hypertension, but not left ventricular hypertrophy. Collectively, our data show that the resistance of C57BL/6 mice to 5/6-Nx can be partially overcome by feeding the CPD, and that the CPD induces a profound, PTH-independent increase in FGF23 in 5/6-Nx mice, making it an interesting tool to assess the pathophysiological significance of FGF23 in CKD.

Renal Replacement Modality Affects Uremic Toxins and Oxidative Stress

Nowadays, the high prevalence of kidney diseases and their related complications, including endothelial dysfunction and cardiovascular disease, represents one of the leading causes of death in patients with chronic kidney diseases. Renal failure leads to accumulation of uremic toxins, which are the main cause of oxidative stress development. The renal replacement therapy appears to be the best way to lower uremic toxin levels in patients with end-stage renal disease and reduce oxidative stress. At this moment, despite the increasing number of recognized toxins and their mechanisms of action, it is impossible to determine which of them are the most important and which cause the greatest complications. There are many different types of renal replacement therapy, but the best treatment has not been identified yet. Patients treated with diffusion methods have satisfactory clearance of small molecules, but the clearance of medium molecules appears to be insufficient, but treatment with convection methods cleans medium molecules better than small molecules. Hence, there is an urgent need of new more validated, appropriate, and reliable information not only on toxins and their role in metabolic disorders, including oxidative stress, but also on the best artificial renal replacement therapy to reduce complications and prolong the life of patients with chronic kidney disease.

Excessive fibroblast growth factor 23 promotes renal fibrosis in mice with type 2 cardiorenal syndrome

Cardiorenal syndrome (CRS) has a high mortality, but its pathogenesis remains elusive. Fibroblast growth factor 23 (FGF23) is increased in both renal dysfunction and cardiac dysfunction, and FGF receptor 4 (FGFR4) has been identified as a receptor for FGF23. Deficiency of FGF23 causes growth retardation and shortens the lifespan, but it is unclear whether excess FGF23 is detrimental in CRS. This study sought to investigate whether FGF23 plays an important role in CRS-induced renal fibrosis. A mouse model of CRS was created by surgical myocardial infarction for 12 weeks. CRS mice showed a significant increase of circulatory and renal FGF23 protein levels, as well as an upregulation of p-GSK, active-β-catenin, TGF-β, collagen I and vimentin, a downregulation of renal Klotho expression and induction of cardiorenal dysfunction and cardiorenal fibrosis. These changes were enhanced by cardiac overexpression of FGF23 and attenuated by FGF receptor blocker PD173074 or β-catenin blocker IGC001. In fibroblasts (NRK-49F), expression of FGFR4 rather than Klotho was detected. Recombinant FGF23 upregulated the expression of p-GSK, active-β-catenin, TGF-β, collagen I and vimentin proteins. These changes were attenuated by FGFR4 blockade with BLU9931 or β-catenin blockade with IGC001. We concluded that FGF23 promotes CRS-induced renal fibrosis mediated by partly activating FGFR4/β-catenin signaling pathway.

Fibrosis in Chronic Kidney Disease: Pathogenesis and Consequences

Fibrosis is a process characterized by an excessive accumulation of the extracellular matrix as a response to different types of tissue injuries, which leads to organ dysfunction. The process can be initiated by multiple and different stimuli and pathogenic factors which trigger the cascade of reparation converging in molecular signals responsible of initiating and driving fibrosis. Though fibrosis can play a defensive role, in several circumstances at a certain stage, it can progressively become an uncontrolled irreversible and self-maintained process, named pathological fibrosis. Several systems, molecules and responses involved in the pathogenesis of the pathological fibrosis of chronic kidney disease (CKD) will be discussed in this review, putting special attention on inflammation, renin-angiotensin system (RAS), parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), Klotho, microRNAs (miRs), and the vitamin D hormonal system. All of them are key factors of the core and regulatory pathways which drive fibrosis, having a great negative kidney and cardiac impact in CKD.

Uremic Toxins and Vascular Dysfunction

Vascular dysfunction is an essential element found in many cardiovascular pathologies and in pathologies that have a cardiovascular impact such as chronic kidney disease (CKD). Alteration of vasomotricity is due to an imbalance between the production of relaxing and contracting factors. In addition to becoming a determining factor in pathophysiological alterations, vascular dysfunction constitutes the first step in the development of atherosclerosis plaques or vascular calcifications. In patients with CKD, alteration of vasomotricity tends to emerge as being a new, less conventional, risk factor. CKD is characterized by the accumulation of uremic toxins (UTs) such as phosphate, para-cresyl sulfate, indoxyl sulfate, and FGF23 and, consequently, the deleterious role of UTs on vascular dysfunction has been explored. This accumulation of UTs is associated with systemic alterations including inflammation, oxidative stress, and the decrease of nitric oxide production. The present review proposes to summarize our current knowledge of the mechanisms by which UTs induce vascular dysfunction.

Vascular Calcification-New Insights Into Its Mechanism

Vascular calcification (VC), which is categorized by intimal and medial calcification, depending on the site(s) involved within the vessel, is closely related to cardiovascular disease. Specifically, medial calcification is prevalent in certain medical situations, including chronic kidney disease and diabetes. The past few decades have seen extensive research into VC, revealing that the mechanism of VC is not merely a consequence of a high-phosphorous and -calcium milieu, but also occurs via delicate and well-organized biologic processes, including an imbalance between osteochondrogenic signaling and anticalcific events. In addition to traditionally established osteogenic signaling, dysfunctional calcium homeostasis is prerequisite in the development of VC. Moreover, loss of defensive mechanisms, by microorganelle dysfunction, including hyper-fragmented mitochondria, mitochondrial oxidative stress, defective autophagy or mitophagy, and endoplasmic reticulum (ER) stress, may all contribute to VC. To facilitate the understanding of vascular calcification, across any number of bioscientific disciplines, we provide this review of a detailed updated molecular mechanism of VC. This encompasses a vascular smooth muscle phenotypic of osteogenic differentiation, and multiple signaling pathways of VC induction, including the roles of inflammation and cellular microorganelle genesis.

Targeting Vascular Calcification in Chronic Kidney Disease

Cardiovascular (CV) disease remains an important cause of morbidity and mortality for patients with chronic kidney disease (CKD). Although clustering of traditional risk factors with CKD is well recognized, kidney-specific mechanisms are believed to drive the disproportionate burden of CV disease. One perturbation that is frequently observed at high rates in patients with CKD is vascular calcification, which may be a central mediator for an array of CV sequelae. This review summarizes the pathophysiological bases of intimal and medial vascular calcification in CKD, current strategies for diagnosis and management, and posits vascular calcification as a risk marker and therapeutic target.

The relationship between estimated glomerular filtration rate and urine m/creatinine ratio and parathyroid hormone in elderly Koreans

This study aimed to assess the relationship between estimated glomerular filtration rate (eGFR), urine microalbumin/creatinine ratio (uACR), and parathyroid hormone (PTH) levels in elderly Korean men and women. Data were analyzed from 2880 adults (1262 men and 1618 women) aged ≥ 50 years from the Fifth Korean National Health and Nutrition Examination Survey V-2 (KNHANES V-2). In men, after adjusting for related variables, the odds ratios (ORs) of the high PTH levels (PTH > 78.0 ng/L) were significantly higher in the decreased eGFR group [eGFR < 60 ml/min/1.73 m²; 1.353 (95% confidence interval [CI], 0.989-1.851)], elevated uACR group [uACR ≥ 30 mg/g; 1.804 (95% CI, 1.004-3.239)], and decreased eGFR plus elevated uACR group [eGFR < 60 ml/min/1.73 m² and uACR ≥ 30 mg/g; 2.158 (95% CI, 1.208-3.858)] relative to the normal group (eGFR ≥ 60 ml/min/1.73 m² and uACR < 30 mg/g). In women, after adjusting for related variables, the ORs of the high PTH levels were significantly higher in the decreased eGFR group [1.585 (95% CI, 0.911-2.760)], elevated uACR group [1.645 (95% CI, 1.127-2.400)], and decreased eGFR plus elevated uACR group [2.934 (95% CI, 1.267-6.795)] relative to the normal group. In conclusion, PTH was positively associated with an increase in uACR in elderly Korean men and women but was not associated with a decrease in eGFR. In addition, PTH increased greatly when both a decrease in eGFR and increase in uACR appeared simultaneously.

The Signaling of Cellular Senescence in Diabetic Nephropathy

Diabetic nephropathy is the leading cause of chronic kidney disease (CKD) in western countries. Notably, it has a rapidly rising prevalence in China. The patients, commonly complicated with cardiovascular diseases and neurologic disorders, are at high risk to progress into end-stage renal disease (ESRD) and death. However, the pathogenic mechanisms of diabetic nephropathy have not been determined. Cellular senescence, which recently has gained broad attention, is thought to be an important player in the onset and development of diabetic nephropathy. In this issue, we generally review the mechanisms of cellular senescence in diabetic nephropathy, which involve telomere attrition, DNA damage, epigenetic alterations, mitochondrial dysfunction, loss of Klotho, Wnt/β-catenin signaling activation, persistent inflammation, and accumulation of uremic toxins. Moreover, we highlight the potential therapeutic targets of cellular senescence in diabetic nephropathy and provide important clues for clinical strategies.

FGF23, Hypophosphatemia, and Emerging Treatments

FGF23 is an important hormonal regulator of phosphate homeostasis. Together with its co-receptor Klotho, it modulates phosphate reabsorption and both 1α-hydroxylation and 24-hydroxylation in the renal proximal tubules. The most common FGF23-mediated hypophosphatemia is X-linked hypophosphatemia (XLH), caused by mutations in the PHEX gene. FGF23-mediated forms of hypophosphatemia are characterized by phosphaturia and low or low-normal calcitriol concentrations, and unlike nutritional rickets, these cannot be cured with nutritional vitamin D supplementation. Autosomal dominant and autosomal recessive forms of FGF23-mediated hypophosphatemias show a similar pathophysiology, despite a variety of different underlying genetic causes. An excess of FGF23 activity has also been associated with a number of other conditions causing hypophosphatemia, including tumor-induced osteomalacia, fibrous dysplasia of the bone, and cutaneous skeletal hypophosphatemia syndrome. Historically phosphate supplementation and therapy using analogs of highly active vitamin D (eg, calcitriol, alfacalcidol, paricalcitol, eldecalcitol) have been used to manage conditions involving hypophosphatemia; however, recently a neutralizing antibody for FGF23 (burosumab) has emerged as a promising treatment agent for FGF23-mediated disorders. This review discusses the progression of clinical trials for burosumab for the treatment of XLH and its recent availability for clinical use. Burosumab may have potential for treating other conditions associated with FGF23 overactivity, but these are not yet supported by trial data. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Effect of osthole on advanced glycation end products-induced renal tubular hypertrophy and role of klotho in its mechanism of action

BACKGROUND

Osthole has been widely reported to have pharmacological activities such as anti-cancer, anti-inflammation and anti-hyperlipidemic effects. Klotho was identified as an anti-senescence protein in a variety of tissues. Loss of klotho has been associated with chronic kidney disease. However, potential roles and molecular events for osthole and klotho in diabetic nephropathy remain unclear.

PURPOSE

In the current study, we undertook to study the effect of osthole on klotho expression in advanced glycation end products (AGE)-cultured human renal proximal tubular cells, and to investigate the molecular mechanisms of osthole and exogenous klotho against AGE-induced renal tubular hypertrophy.

METHODS

Cell viability was elucidated by MTT assay. Protein expression was measured by Western blotting. mRNA level was analyzed by real-time PCR. Cellular hypertrophy growth was evaluated by hypertrophy index. Relative cell size was detected by flow cytometry.

RESULTS

We found that raising the ambient AGE concentration causes a dose-dependent decrease in klotho synthesis. Osthole significantly increased AGE-inhibited klotho mRNA and protein expression. Osthole and exogenous klotho treatments significantly attenuated AGE-induced Janus kinase 2 (JAK2)-signal transducers and activators of transcription 1 (STAT1) and STAT3 activation. Moreover, protein levels of suppressor of cytokine signaling 1 (SOCS1) and SOCS3 were augmented by osthole and exogenous klotho. The abilities of osthole and exogenous klotho to reverse AGE-induced cellular hypertrophy were verified by the observation that osthole and exogenous klotho inhibited p21^Waf1/Cip1/collagen IV/RAGE expression, total protein content, and cell size.

CONCLUSION

Consequently, we found that osthole attenuated AGE-induced renal tubular hypertrophy via induction of klotho expression and suppression of the JAK2-STAT1/STAT3 signaling. These results also showed that klotho might be used as a unique molecular target for the treatment of diabetic nephropathy.

Serum phosphate and phosphate-regulatory hormones in COPD patients

Background

Fibroblast growth factor 23 (FGF23) regulates phosphate metabolism by increasing renal phosphate excretion and decreasing 1.25-dihydroxyvitamin D synthesis. Reports about hypophosphatemia in patients with chronic obstructive pulmonary disease (COPD) suggest altered phosphate metabolism. Therefore, we hypothesized that disturbances in phosphate-regulatory hormones such as FGF23 and parathyroid hormone (PTH) are present in COPD patients.

Methods

We investigated 40 COPD patients (63.5 ± 9.9 years, 27 male), each matched with two age- and sex-matched controls without any primary lung disease. COPD patients underwent lung function testing in advance. All patients had a glomerular filtration rate (GFR) > 60 mL/min/1.73m². We measured concentrations of intact FGF23 (iFGF23) and c-terminal FGF23 (c-term FGF23), phosphate, parathyroid hormone (PTH) and C-reactive protein (CRP) levels in COPD patients and controls.

Results

Phosphate (1.0 ± 02 vs. 1.1 ± 0.2 mmol/L; p = 0.027), PTH (54.2 ± 29.4 vs. 68.7 ± 31.8 pg/mL; p = 0.002) and iFGF23 (46.3 ± 29.0 vs. 57.5 ± 33.5 pg/mL; p = 0.026 ) levels were significantly lower in COPD patients compared with controls. There was a significant negative correlation between c-term FGF23 and total lung capacity (r = − 0.4; p = 0.01), and between c-term FGF23 and CRP in COPD patients (r = 0.48; p = 0.002). iFGF23 and c-term FGF23 were positively correlated with phosphate and PTH in the control group.

Conclusion

We confirmed lower average serum phosphate levels in COPD patients compared with controls. However, our data do not suggest a causative role for FGF23 or PTH in COPD because levels of both phosphate-lowering hormones appear to be adaptively decreased as well. Therefore, further investigations are needed to identify the pathogenesis of low phosphate levels in patients with COPD and the relationship between phosphate-regulatory hormones and disease progression.

Menaquinone-4 modulates the expression levels of calcification-associated factors to inhibit calcification of rat aortic vascular smooth muscle cells in a dose-dependent manner

Vascular calcification (VC) caused by chronic kidney disease (CKD)-mineral and bone disorder is a common complication of CKD. Recent studies have demonstrated that menaquinone-4 (MK-4) is negativly associated with VC in patients with CKD. Furthermore, we have previously shown that runt-related transcription factor 2 (Runx2) is important in the phenotypic transformation process of rat vascular smooth muscle cells (VSMCs), which is the key step for the development of VC. The present study investigated the influence of MK-4 on the phenotypic transformation process of rat VSMCs in order to illustrate its role in the process of VC. Calcification assays were perfomed to access the calcified degree of rat VSMCs. Additionally, the genes and proteins related to phenotypic transformation were measured by reverse transcription-polymerase chain reaction and western blotting methods. It was revealed that calcium deposition in the cells was evidently increased with an addition of β-glycerophosphate (β-GP) and could be completely prevented by co-incubation with MK-4 in a dose-dependent manner. Furthermore, the expression of Runx2 in the β-GP-induced VSMCs was inhibited by MK-4. It was also revealed that the expression of SMAD1 and bone morphogenetic protein (BMP)-2 were decreased in the β-GP-induced VSMCs treated with MK-4 in a dose-dependent manner; however, the expression of SMAD7 was increased in the β-GP-induced VSMCs treated with MK-4 in a dose-dependent manner. These observations suggest that MK-4 reduces mineralization by regulating the BMP-2 signaling pathway in order to attenuate the expression of Runx2.

Uremic Toxins and Clinical Outcomes: The Impact of Kidney Transplantation

Non-transplanted and transplanted patients with chronic kidney disease (CKD) differ in terms of mortality and the risk of clinical events. This difference is probably due to the difference of both traditional and non-traditional risk factors. Uremic retention solutes may constitute important non-traditional risk factors in this population. In the present review, we selected a set of uremic toxins that have been associated with harmful effects, and are an appealing target for adjuvant therapy in CKD. For each toxin reviewed here, relevant studies were selected and the relationship with hard clinical outcomes of uremic toxins were compared between non-transplanted CKD patients and transplanted patients taking into account the level of glomerular filtration rate in these two situations.

The FGF-23/klotho axis and its relationship with phosphorus, calcium, vitamin D, PTH, aldosterone, severity of disease, and outcome in hospitalised foals

BACKGROUND

Fibroblast growth factor-23 (FGF-23) and klotho are key regulators of vitamin D and parathyroid hormone (PTH) synthesis as well as phosphorus and calcium homeostasis; however, information on the FGF-23/klotho axis in healthy and hospitalised foals is lacking.

OBJECTIVES

The aims of this study were to measure serum FGF-23 and klotho concentrations and determine their association with serum phosphorus, total calcium (TCa), vitamin D metabolite [25(OH)D, 1,25(OH)₂ D], PTH, and aldosterone concentrations, disease severity, and mortality in hospitalised foals.

STUDY DESIGN

Prospective, multicentre, cross-sectional study.

METHODS

A total of 91 foals ≤72 h old were classified as hospitalised (n = 81; 58 septic; 23 sick non-septic [SNS]) and healthy (n = 10). Blood samples were collected on admission. Hormone concentrations were determined by immunoassays.

RESULTS

Serum FGF-23, PTH, phosphorus, and aldosterone concentrations were higher while klotho, 25(OH)D, 1,25(OH)₂ D, and TCa concentrations were lower in septic and SNS compared to healthy foals (P<0.05). In hospitalised and septic foals, increased FGF-23 and aldosterone concentrations were associated with high phosphorus and PTH but not with TCa and vitamin D metabolite concentrations. Hospitalised foals with the highest FGF-23 and lowest klotho concentrations were more likely to die (odds ratio (OR): 3.3; 95% confidence interval (CI): 1.1-10.3 and OR: 3.1; CI: 1.1-8.0, respectively).

MAIN LIMITATIONS

Blood gas, ionised calcium, blood culture information not being available for many foals, and use of the sepsis score to classify hospitalised foals.

CONCLUSIONS

Imbalances in the FGF-23/klotho axis may contribute to mineral dyshomeostasis and disease progression in critically ill foals. Elevated FGF-23 and reduced klotho, together with high phosphorus and PTH concentrations suggests FGF-23 resistance. FGF-23 and klotho are good markers of disease severity and likelihood of mortality in hospitalised foals. Aldosterone may influence phosphorus and PTH dynamics in hospitalised foals. Routine measurement of phosphorus concentrations in sick foals is recommended.

FGF23 promotes myocardial fibrosis in mice through activation of β-catenin

Fibroblast growth factor 23 (FGF23) has been reported to induce left ventricular hypertrophy, but it remains unclear whether FGF23 plays a role in cardiac fibrosis. This study is attempted to investigate the role of FGF23 in post-infarct myocardial fibrosis in mice. We noted that myocardial and plasma FGF23 and FGF receptor 4 were increased in mice with heart failure as well as in cultured adult mouse cardiac fibroblasts (AMCFs) exposed to angiotensin II, phenylephrine, soluble fractalkine. Recombinant FGF23 protein increased active β-catenin , procollagen I and procollagen III expression in cultured AMCFs. Furthermore, intra-myocardial injection of adeno-associated virus-FGF23 in mice significantly increased left ventricular end-diastolic pressure and myocardial fibrosis, and markedly upregulated active β-catenin, transforming growth factor β (TGF-β), procollagen I and procollagen III in both myocardial infarction (MI) and ischemia/reperfusion (IR) mice, while β-catenin inhibitor or silencing of β-catenin antagonized the FGF23-promoted myocardial fibrosis in vitro and in vivo. These findings indicate that FGF23 promotes myocardial fibrosis and exacerbates diastolic dysfunction induced by MI or IR, which is associated with the upregulation of active β-catenin and TGF-β.

Cellular and Molecular Mediators of Intestinal Fibrosis

Intestinal fibrosis is a major complication of the inflammatory bowel diseases (IBD) and although inflammation is necessary for its development, it would appear that it plays a minor role in its progression as anti-inflammatory treatments in IBD do not prevent fibrosis once it has started. The processes that regulate fibrosis would thus appear to be distinct from those regulating inflammation and, therefore, a detailed understanding of these pathways is vital to the development of anti-fibrogenic strategies. There have been several recent reviews exploring what is known, and what remains unknown, about the development of intestinal fibrosis. This review is designed to add to this literature but with a focus on the cellular components that are involved in the development of fibrogenesis and the major molecular mediators that impact on these cells. The aim is to heighten the understanding of the factors involved in intestinal fibrogenesis so that detailed research can be encouraged in order to advance the processes that could lead to effective treatments.

Cellular and Molecular Mediators of Intestinal Fibrosis

Intestinal fibrosis is a major complication of the inflammatory bowel diseases (IBD) and although inflammation is necessary for its development, it would appear that it plays a minor role in its progression as anti-inflammatory treatments in IBD do not prevent fibrosis once it has started. The processes that regulate fibrosis would thus appear to be distinct from those regulating inflammation and, therefore, a detailed understanding of these pathways is vital to the development of anti-fibrogenic strategies. There have been several recent reviews exploring what is known, and what remains unknown, about the development of intestinal fibrosis. This review is designed to add to this literature but with a focus on the cellular components that are involved in the development of fibrogenesis and the major molecular mediators that impact on these cells. The aim is to heighten the understanding of the factors involved in intestinal fibrogenesis so that detailed research can be encouraged in order to advance the processes that could lead to effective treatments.

The mechanism of attenuation of epithelial-mesenchymal transition by a phosphodiesterase 5 inhibitor via renal klotho expression

Phosphodiesterase-5 (PDE-5) inhibitors induces vasodilation in several organs by blocking cyclic GMP (guanosine monophosphate) degradation. However, the existence of alternative mechanism of action in case of an impaired nitric oxide (NO) system remains controversial. Previous studies suggested that decreased NO bioavailability may result in the downregulation of klotho expression, but the relationship between klotho and NO remains obscure. Therefore, we investigated whether a PDE-5 inhibitor could preserve epithelial-mesenchymal transition (EMT) and relationship exists between the NO and renal klotho expression. Ten-week-old SD rats (N = 24, 200 g, male) were divided (N = 6) into four groups, which received: A LSD, L-NAME 1 mg/mL in drinking water, Udenafil 5 mg/kg subcutaneously and both for 4 weeks. Urine nitrate/nitrite, NGAL (Neutrophil gelatinase-associated lipocalin), and cGMP were measured using ELISA. Kidney was subjected to evaluate PCNA (proliferative cell nuclear antigen), α-SMA (smooth muscle cell antigen), E-cadherin, and klotho expression. Urine cGMP decreased after treatment of PDE-5 inhibitor compared with control due to blocking degradation of cGMP (P < .05, control vs Udenafil and L-NAME with Udenafil groups). Urine NGAL increased after treating of L-NAME and attenuated after using PDE-5 inhibitor (P < .05, control vs L-NAME and L-NAME with Udenafil). PCNA, α-SMA, and E-cadherin (EMT markers) increased after L-NAME treatment and normalized after using PDE-5 inhibitor. Klotho expression showed trend to increase in the L-NAME with PDE-5 inhibitor group compared with the L-NAME group, however, eNOS expression did not change after treatment of L-NAME or PDE-5 inhibitor compared with control. PDE-5 inhibitor alleviates EMT in the kidney via klotho modulation independent of the NO system.

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.

Extrarenal effects of FGF23

Chronic kidney disease (CKD) is associated with an increased risk of cardiovascular mortality, infections, and impaired cognitive function. It is characterized by excessively increased levels of the phosphaturic hormone fibroblast growth factor 23 (FGF23) and a deficiency of its co-receptor Klotho. Despite the important physiological effect of FGF23 in maintaining phosphate homeostasis, there is increasing evidence that higher FGF23 levels are a risk factor for mortality and cardiovascular disease. FGF23 directly induces left ventricular hypertrophy via activation of the FGF receptor 4/calcineurin/nuclear factor of activated T cells signaling pathway. By contrast, the impact of FGF23 on endothelial function and the development of atherosclerosis are poorly understood. The results of recent experimental studies indicate that FGF23 directly impacts on hippocampal neurons and may thereby impair learning and memory function in CKD patients. Finally, it has been shown that FGF23 interferes with the immune system by directly acting on polymorphonuclear leukocytes and macrophages. In this review, we discuss recent data from clinical and experimental studies on the extrarenal effects of FGF23 with respect to the cardiovascular, central nervous, and immune systems.

Klotho preservation via histone deacetylase inhibition attenuates chronic kidney disease-associated bone injury in mice

Bone loss and increased fracture are the devastating outcomes of chronic kidney disease-mineral and bone disorder (CKD-MBD) resulting from Klotho deficit-related mineral disturbance and hyperparathyroidism. Because Klotho down-regulation after renal injury is presumably affected by aberrant histone deacetylase (HDAC) activities, here we assess whether HDAC inhibition prevents Klotho loss and attenuates the CKD-associated bone complication in a mouse model of CKD-MBD. Mice fed adenine-containing diet developed the expected renal damage, a substantial Klotho loss and the deregulated key factors causally affecting bone remodeling, which were accompanied by a marked reduction of bone mineral density. Intriguingly, administration of a potent HDAC inhibitor trichostatin A (TSA) impressively alleviated the Klotho deficit and the observed alterations of serum, kidney and bone. TSA prevented Klotho loss by increasing the promoter-associated histone acetylation, therefore increasing Klotho transcription. More importantly the mice lacking Klotho by siRNA interference largely abolished the TSA protections against the serum and renal abnormalities, and the deranged bone micro-architectures. Thus, our study identified Klotho loss as a key event linking HDAC deregulation to the renal and bone injuries in CKD-MBD mice and demonstrated the therapeutic potentials of endogenous Klotho restoration by HDAC inhibition in treating CKD and the associated extrarenal complications.

Effect of Osteocyte-Ablation on Inorganic Phosphate Metabolism: Analysis of Bone-Kidney-Gut Axis

In response to kidney damage, osteocytes increase the production of several hormones critically involved in mineral metabolism. Recent studies suggest that osteocyte function is altered very early in the course of chronic kidney disease. In the present study, to clarify the role of osteocytes and the canalicular network in mineral homeostasis, we performed four experiments. In Experiment 1, we investigated renal and intestinal Pi handling in osteocyte-less (OCL) model mice [transgenic mice with the dentin matrix protein-1 promoter-driven diphtheria toxin (DT)-receptor that were injected with DT]. In Experiment 2, we administered granulocyte colony-stimulating factor to mice to disrupt the osteocyte canalicular network. In Experiment 3, we investigated the role of osteocytes in dietary Pi signaling. In Experiment 4, we analyzed gene expression level fluctuations in the intestine and liver by comparing mice fed a high Pi diet and OCL mice. Together, the findings of these experiments indicate that osteocyte ablation caused rapid renal Pi excretion (P < 0.01) before the plasma fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) levels increased. At the same time, we observed a rapid suppression of renal Klotho (P < 0.01), type II sodium phosphate transporters Npt2a (P < 0.01) and Npt2c (P < 0.05), and an increase in intestinal Npt2b (P < 0.01) protein. In OCL mice, Pi excretion in feces was markedly reduced (P < 0.01). Together, these effects of osteocyte ablation are predicted to markedly increase intestinal Pi absorption (P < 0.01), thus suggesting that increased intestinal Pi absorption stimulates renal Pi excretion in OCL mice. In addition, the ablation of osteocytes and feeding of a high Pi diet affected FGF15/bile acid metabolism and controlled Npt2b expression. In conclusion, OCL mice exhibited increased renal Pi excretion due to enhanced intestinal Pi absorption. We discuss the role of FGF23-Klotho on renal and intestinal Pi metabolism in OCL mice.

Fibroblast Growth Factor 21 (FGF-21) in Peritoneal Dialysis Patients: Natural History and Metabolic Implications

Background

Human fibroblast growth factor 21 (FGF-21) is an endocrine liver hormone that stimulates adipocyte glucose uptake independently of insulin, suppresses hepatic glucose production and is involved in the regulation of body fat. Peritoneal dialysis (PD) patients suffer potential interference with FGF-21 status with as yet unknown repercussions.

Objectives

The aim of this study was to define the natural history of FGF-21 in PD patients, to analyze its relationship with glucose homeostasis parameters and to study the influence of residual renal function and peritoneal functional parameters on FGF-21 levels and their variation over time.

Methods

We studied 48 patients with uremia undergoing PD. Plasma samples were routinely obtained from each patient at baseline and at 1, 2 and 3 years after starting PD therapy.

Results

Plasma FGF-21 levels substantially increased over the first year and were maintained at high levels during the remainder of the study period (253 pg/ml (59; 685) at baseline; 582 pg/ml (60.5–949) at first year and 647 pg/ml (120.5–1116.6) at third year) (p<0.01). We found a positive correlation between time on dialysis and FGF-21 levels (p<0.001), and also, those patients with residual renal function (RRF) had significantly lower levels of FGF-21 than those without RRF (ρ -0.484, p<0.05). Lastly, there was also a significant association between FGF-21 levels and peritoneal protein losses (PPL), independent of the time on dialysis (ρ 0.410, p<0.05).

Conclusion

Our study shows that FGF-21 plasma levels in incident PD patients significantly increase during the first 3 years. This increment is dependent on or is associated with RRF and PPL (higher levels in patients with lower RRF and higher PPL). FGF-21 might be an important endocrine agent in PD patients and could act as hormonal signaling to maintain glucose homeostasis and prevent potential insulin resistance. These preliminary results suggest that FGF-21 might play a protective role as against the development of insulin resistance over time in patients undergoing a continuous glucose load.

Klotho Prevents Translocation of NFκB

Klotho protein is a β-glucuronidase capable of hydrolyzing steroid β-glucuronides. Two molecules are produced by the Klotho gene, a membrane bound form and a circulating form. This protein is recognized as an antiaging gene with pleiotropic functions. The activation of cellular systems is associated with the pathogenesis of several chronic and degenerative diseases associated with an inflammatory state. Inflammation is characterized by an activation of NFκB. Klotho suppresses nuclear factor NFκB activation and the subsequent transcription of proinflammatory genes. This review focuses on the current understanding of Klotho protein function and its relationship with NFκB regulation, emphasizing its potential involvement in the pathophysiologic process.

Hypercalcemia. Pathophysiological Aspects

The metabolic pathways that contribute to maintain serum calcium concentration in narrow physiological range include the bone remodeling process, intestinal absorption and renal tubule resorption. Dysbalance in these regulations may lead to hyper- or hypocalcemia. Hypercalcemia is a potentionally life-threatening and relatively common clinical problem, which is mostly associated with hyperparathyroidism and/or malignant diseases (90 %). Scarce causes of hypercalcemia involve renal failure, kidney transplantation, endocrinopathies, granulomatous diseases, and the long-term treatment with some pharmaceuticals (vitamin D, retinoic acid, lithium). Genetic causes of hypercalcemia involve familial hypocalciuric hypercalcemia associated with an inactivation mutation in the calcium sensing receptor gene and/or a mutation in the CYP24A1 gene. Furthermore, hypercalcemia accompanying primary hyperparathyroidism, which develops as part of multiple endocrine neoplasia (MEN1 and MEN2), is also genetically determined. In this review mechanisms of hypercalcemia are discussed. The objective of this article is a review of hypercalcemia obtained from a Medline bibliographic search.

Fibroblast growth factor 23 and Klotho as cardiovascular risk factors in heart transplant recipients

BACKGROUND

Fibroblast growth factor (FGF) 23 is one of the most recently discovered FGFs. This phosphaturic hormone produced in bones is a risk factor for cardiovascular diseases and thus mortality. Klotho is an essential coreceptor for FGF23 and at the same time it is known as a "longevity" hormone. There are no data considering FGF23 and Klotho roles in heart transplant (HT) recipients. The aim of this study was to assess Klotho and FGF23 serum concentration in heart transplant recipients depending on immunosuppressive therapy regimen and comorbidities.

METHODS

Eighty-four stable heart transplant recipients were enrolled in the study; 22 healthy volunteers served as control subjects. FGF23 and Klotho protein concentration, markers of renal function, such as cystatin C and neutrophil gelatinase-associated lipocalin (NGAL), and heart failure markers, such as copeptine and N-termiinal pro-B-type natriuretic peptide (NT-proBNP), were evaluated.

RESULTS

FGF23 concentration was significantly higher in the HT group whereas Klotho protein was significantly lower. FGF23 correlated with creatinine level (r = 0.72; P < .001), estimated glomerular filtration rate (eGFR; r = -0.32; P < .01), cystatin C (r = 0.36; P < .01), NGAL (r = 0.51; P < .001), hemoglobin (r = -0.39; P < .001), NT-proBNP (r = 0.51; P < .001), high-density lipoprotein (HDL; r = 0.27; P < .05), intraventricular septum thickness (r = 0.42; P < .01) and right ventricular systolic pressure (r = 0.34; P < .05). Klotho protein correlated only with age (r = -0.21; P < .05), creatinine (r = -0.21; P < .05), and eGFR (r = -0.31; P < .01). FGF23 concentration was significantly higher in patients with eGFR <60 mL/min whereas Klotho protein was significantly lower. FGF23 predictors were renal function (creatinine concentration; β = 0.45; P = .0001), HDL (β = 0.33; P = .003), intraventricular septum thickness (β = 0.38; P = .0003), and right ventricular systolic pressure (β = 0.34; P = .003), explaining 70% of FGF23 variability.

CONCLUSIONS

FGF23/Klotho system disorders in HT recipients are related to cardiovascular system function and kidney failure and could cause increased risk of cardiovascular disease.

Phosphate and FGF-23 homeostasis after kidney transplantation

Dysregulated phosphate metabolism is a common consequence of chronic kidney disease, and is characterized by a high circulating level of fibroblast growth factor (FGF)-23, hyperparathyroidism, and hyperphosphataemia. Kidney transplantation can elicit specific alterations to phosphate metabolism that evolve over time, ranging from severe hypophosphataemia (<0.5 mmol/l) to hyperphosphataemia (>1.50 mmol/l) and high FGF-23 levels. The majority of renal transplant recipients develop hypophosphataemia during the first 3 months after transplantation as a consequence of relatively slow adaptation of FGF-23 and parathyroid hormone levels to restored renal function, and the influence of immunosuppressive drugs. By 3-12 months after transplantation, phosphate homeostasis is at least partially restored in the majority of recipients, which is paralleled by a substantially reduced risk of cardiovascular-associated morbidity and mortality compared with the pre-transplantation setting. Many renal transplant recipients, however, exhibit persistent abnormalities in phosphate homeostasis, which is often due to multifactorial causes, and may contribute to adverse outcomes on the cardiovascular system, kidney, and bone. Dietary and pharmacologic interventions might improve phosphate homeostasis in renal transplant recipients, but additional insight into the pathophysiology of transplantation-associated abnormalities in phosphate homeostasis is needed to further optimize disease management and improve prognosis for renal transplant recipients.

The ABCC6 Transporter as a Paradigm for Networking from an Orphan Disease to Complex Disorders

The knowledge on the genetic etiology of complex disorders largely results from the study of rare monogenic disorders. Often these common and rare diseases show phenotypic overlap, though monogenic diseases generally have a more extreme symptomatology. ABCC6, the gene responsible for pseudoxanthoma elasticum, an autosomal recessive ectopic mineralization disorder, can be considered a paradigm gene with relevance that reaches far beyond this enigmatic orphan disease. Indeed, common traits such as chronic kidney disease or cardiovascular disorders have been linked to the ABCC6 gene. While during the last decade the awareness of the wide ramifications of ABCC6 has increased significantly, the gene itself and the transmembrane transporter it encodes have not unveiled all of the mysteries that surround them. To gain more insights, multiple approaches are being used including next-generation sequencing, computational methods, and various "omics" technologies. Much effort is made to place the vast amount of data that is gathered in an integrated system-biological network; the involvement of ABCC6 in common disorders provides a good view on the wide implications and potential of such a network. In this review, we summarize the network approaches used to study ABCC6 and the role of this gene in several complex diseases.

Associations between Thyroid Hormones, Calcification Inhibitor Levels and Vascular Calcification in End-Stage Renal Disease

Introduction

Vascular calcification is a common, serious and elusive complication of end-stage renal disease (ESRD). As a pro-calcifying risk factor, non-thyroidal illness may promote vascular calcification through a systemic lowering of vascular calcification inhibitors such as matrix-gla protein (MGP) and Klotho.

Methods and Material

In 97 ESRD patients eligible for living donor kidney transplantation, blood levels of thyroid hormones (fT3, fT4 and TSH), total uncarboxylated MGP (t-ucMGP), desphospho-uncarboxylated MGP (dp-ucMGP), descarboxyprothrombin (PIVKA-II), and soluble Klotho (sKlotho) were measured. The degree of coronary calcification and arterial stiffness were assessed by means of cardiac CT-scans and applanation tonometry, respectively.

Results

fT3 levels were inversely associated with coronary artery calcification (CAC) scores and measures of arterial stiffness, and positively with dp-ucMGP and sKlotho concentrations. Subfractions of MGP, PIVKA-II and sKlotho did not associate with CAC scores and arterial stiffness. fT4 and TSH levels were both inversely associated with CAC scores, but not with arterial stiffness.

Discussion

The positive associations between fT3 and dp-ucMGP and sKlotho suggest that synthesis of MGP and Klotho is influenced by thyroid hormones, and supports a link between non-thyroidal illness and alterations in calcification inhibitor levels. However, the absence of an association between serum calcification inhibitor levels and coronary calcification/arterial stiffness and the fact that MGP and Klotho undergo post-translational modifications underscore the complexity of this association. Further studies, measuring total levels of MGP and membrane bound Klotho, should examine this proposed pathway in further detail.

Longevity factor klotho and chronic psychological stress

Chronic psychological stress is associated with accelerated aging and premature morbidity and mortality; however, the biology linking chronic psychological stress and its maladaptive effects remains largely unknown. Klotho is a pleiotropic hormone that regulates the aging process and promotes better brain and body health. Whether klotho is linked to psychosocial stress or its negative impact in humans has not been investigated. To address this gap, we recruited 178 healthy women who were either chronically high-stress maternal caregivers for a child with autism spectrum disorder (n = 90) or low-stress control mothers of a typically developing child (n = 88). We found that women under high chronic stress displayed significantly lower levels of the longevity hormone klotho compared with low-stress controls (t(176) = 2.92, P = 0.004; d = 0.44), and the decrease among those under high stress was age-dependent. In addition, high-stress caregivers who reported more depressive symptoms displayed even lower klotho levels compared with low-stress participants. These findings provide the first evidence that klotho levels are sensitive to psychosocial stressors and raise the possibility that klotho may serve as a novel biological link connecting stress, depression and risk for accelerated disease development. Furthermore, these findings have important implications for understanding the plasticity of the aging process and may represent a therapeutic target for mitigating the deleterious effects of chronic psychological stress on health and well-being.

Alpha Klotho and phosphate homeostasis

The Klotho family consists of three single-pass transmembrane proteins—αKlotho, βKlotho and γKlotho. Each of them combines with fibroblast growth factor (FGF) receptors (FGFRs) to form receptor complexes for various FGF’s. αKlotho is a co-receptor for physiological FGF23 signaling and appears essential for FGF23-mediated regulation of mineral metabolism. αKlotho protein also plays a FGF23-independent role in phosphate homeostasis. Animal experimental studies and clinical observations have demonstrated that αKlotho deficiency leads to severe hyperphosphatemia; moderate elevation of αKlotho reduces serum phosphate and extremely high αKlotho induces hypophosphatemia and high-FGF23. αKlotho maintains circulating phosphate in a narrow range by modulating intestinal phosphate absorption, urinary phosphate excretion by the kidney, and phosphate distribution into bone rather than soft tissue in concerted interaction with other calciophosphotropic hormones such as PTH, FGF23, and 1,25-(OH)₂ vitamin D. The role of αKlotho in maintenance of phosphate homeostasis is mediated by direct suppression of Na-dependent phosphate cotransporters in target organs. Therefore, αKlotho manipulation may be a novel strategy for genetic and acquired phosphate disorders and for medical conditions with αKlotho deficiency such as chronic kidney disease in future.

The future of phosphate binders: a perspective on novel therapeutics

Chronic kidney disease-mineral bone disorder (CKD-MBD) is a common complication of CKD. The therapeutic strategies for the treatment of CKD-MBD include phosphate binders, active vitamin D analogs and calcimimetics. The first class of drugs provided nephrologists with a range of phosphate binders that are able to decrease circulating phosphate and parathyroid hormone but involve some tolerability and safety issues. In the past 2 years, new phosphate binders have been launched and others are still under development. Serum phosphate increases only in the late stages of CKD but clinical abnormalities begin to occur earlier when multiple mechanisms try to compensate for the progressive reduced ability of the kidney to eliminate phosphorus with urine. Accordingly, starting phosphate binders when phosphatemia reaches values higher than normal may represent a late therapeutic approach. Serum phosphorus is not the ideal biomarker for the diagnosis and treatment of phosphate imbalance. This role could be better played by fibroblast growth factor 23, whose serum concentrations rise earlier in CKD. A more detailed knowledge of the mechanisms underlying CKD-MBD development will provide new therapeutic targets and then new perspectives for the treatment of phosphate imbalance in the future.

Chronic kidney disease results in deficiency of ABCC6, the novel inhibitor of vascular calcification

BACKGROUND

Chronic kidney disease (CKD) is associated with arterial medial calcification which plays a major role in the pathogenesis of cardiovascular disease in this population. Several factors are known to promote soft tissue and accelerated arterial calcification in CKD including systemic inflammation, altered calcium and phosphate homeostasis, hypertension, and deficiency of endogenous calcification inhibitors. The ABCC6 transporter (ATP-binding cassette subfamily C number 6), also known as multidrug resistance-associated protein 6 (MRP6), is highly expressed in the liver and kidney. Mutation of ABCC6 results in pseudoxanthoma elasticum, an inherited disorder characterized by arterial and soft tissue calcification. Given the prevalence of arterial medial calcification in CKD, the present study was undertaken to test the hypothesis that CKD may lead to acquired ABCC6 deficiency.

METHODS

CKD was induced via 5/6 nephrectomy in male Sprague-Dawley rats and by adenine-containing diet to cause chronic interstitial nephropathy in female DBA/2J mice. Sham-operated rats and mice fed regular diet served as controls. Liver and kidney tissues were harvested and processed for ABCC6 protein and mRNA analysis.

RESULTS

ABCC6 protein levels were significantly reduced in the liver and kidney tissues from CKD rats and mice. However, ABCC6 mRNA levels were unchanged, pointing to post-transcriptional or post-translational mechanisms for the observed ABCC6 deficiency. Additionally, plasma levels of the calcification inhibitor fetuin-A were significantly decreased in CKD animals compared to controls.

CONCLUSIONS

CKD results in acquired ABCC6 transporter deficiency. To our knowledge this abnormality has not been previously reported and may contribute to CKD-associated vascular and soft tissue calcification.

Klotho attenuates high glucose-induced fibronectin and cell hypertrophy via the ERK1/2-p38 kinase signaling pathway in renal interstitial fibroblasts

Although exogenous klotho attenuates renal fibrosis, it is not known if exogenous klotho attenuates diabetic nephropathy (DN). Thus, we studied the anti-fibrotic mechanisms of klotho in terms of transforming growth factor-β (TGF-β) and signaling pathways in high glucose (HG, 30 mM)-cultured renal interstitial fibroblast (NRK-49F) cells. We found that HG increased klotho mRNA and protein expression. HG also activated TGF-β Smad2/3 signaling and activated extracellular signal-regulated kinase (ERK1/2) and p38 kinase signaling. Exogenous klotho (400 pM) attenuated HG-induced TGF-β bioactivity, type II TGF-β receptor (TGF-βRII) protein expression and TGF-β Smad2/3 signaling. Klotho also attenuated HG-activated ERK1/2 and p38 kinase. Additionally, klotho and inhibitors of ERK1/2 or p38 kinase attenuated HG-induced fibronectin and cell hypertrophy. Finally, renal tubular expression of klotho decreased in the streptozotin-diabetic rats at 8 weeks. Thus, exogenous klotho attenuates HG-induced profibrotic genes, TGF-β signaling and cell hypertrophy in NRK-49F cells. Moreover, klotho attenuates HG-induced fibronectin expression and cell hypertrophy via the ERK1/2 and p38 kinase-dependent pathways.

Soluble α-klotho and its relation to kidney function and fibroblast growth factor-23

CONTEXT

Relations between fibroblast growth factor-23 (FGF-23), soluble α-klotho (s-α-klotho), and kidney function in chronic kidney disease (CKD) are still unclear. Especially the role of s-α-klotho requires further study.

OBJECTIVES

Our objectives were to analyze the relation of s-α-klotho to estimated glomerular filtration rate (eGFR), FGF-23, and other parameters of calcium-phosphate metabolism and to investigate the response of s-α-klotho to cholecalciferol.

PATIENTS, DESIGN, AND SETTING

Twenty-four CKD (stage 1-5) patients participated in this 8-week randomized controlled trial (vitamin D and chronic renal insufficiency).

INTERVENTIONS

Interventions included 40 000 IU cholecalciferol or placebo weekly.

MAIN OUTCOME MEASURE

S-α-klotho was determined by ELISA with antihuman klotho antibodies 67G3 and 91F1.

RESULTS

For all patients, s-α-klotho concentrations did not differ between CKD stages. When patients were subdivided based on FGF-23 concentrations, a positive association of s-α-klotho with eGFR became apparent in patients with lower than median FGF-23 concentrations but not in those above median value. Patients with s-α-klotho below 204 pg/mL showed higher age, lower phosphate clearance, and lower bone-specific alkaline phosphatase compared with patients with higher s-α-klotho. Treatment with cholecalciferol significantly increased 1,25-dihydroxyvitamin D. The increase of FGF-23 had only borderline significance. There was no significant effect of high-dose cholecalciferol administration for 8 weeks on plasma s-α-klotho.

CONCLUSIONS

CKD patients with s-α-klotho below 204 pg/mL had higher age, lower phosphate clearance, and lower bone-specific alkaline phosphatase. An association of s-α-klotho with eGFR was observed only in the presence of close to normal, but not high, FGF-23 concentrations. Cholecalciferol treatment did not change s-α-klotho concentrations.

Biochemical and functional characterization of the klotho-VS polymorphism implicated in aging and disease risk

Klotho (KL) is an age-regulating protein named after the Greek goddess who spins the thread of life. Mice deficient in KL are normal throughout development, but rapidly degenerate and display a variety of aging-associated abnormalities that eventually lead to decreased life expectancy. While multiple genetic association studies have identified KL polymorphisms linked with changes in disease risk, there is a paucity of concrete mechanistic data to explain how these amino acid substitutions alter KL protein function. The KLVS polymorphism is suggested to lead to changes in protein trafficking although the mechanism is unclear. Our studies have sought to further investigate the functional differences in the KLVS variant that result in increased risk of many age-related diseases. Our findings suggest that the F352V and C370S substitutions lead to alterations in processing as seen by differences in shedding and half-life. Their co-expression in KLVS results in a phenotype resembling wild-type, but despite this intragenic complementation there are still changes in homodimerization and interactions with FGFR1c. Taken together, these studies suggest that KLVS leads to altered homodimerization that indirectly leads to changes in processing and FGFR1c interactions. These findings help elucidate the functional differences that result from the VS polymorphism, which will help clarify how alterations in KL function can lead to human disease and affect cognition and lifespan.