Endothelial dysfunction in the klotho mouse and downregulation of klotho gene expression in various animal models of vascular and metabolic diseases

Intact FGF23 and α-Klotho during acute inflammation/sepsis in CKD patients

BACKGROUND

High FGF23 and low α-Klotho levels associate with systemic inflammation and reduced nitric oxide (NO) bioavailability, but the dynamics of this relationship in patients with CKD has not been investigated.

METHODS

We sequentially measured serum intact FGF23 and carboxyl-terminal (iFGF23, cFGF23), the iFGF23/cFGF23 ratio, αKlotho, biomarkers of inflammation (hs-CRP, IL-6 and TNF-α) and sepsis (procalcitonin), nitrotyrosine (reflecting NO synthesis and oxidative stress), serum iron and ferritin and CKD-MBD biomarkers, PTH, 25(OH)VD, 1,25(OH)2 VD at peak of intercurrent sepsis and after complete resolution in a series of 17 patients with CKD.

RESULTS

At peak infection, biomarkers of inflammation/sepsis, ferritin and nitrotyrosine were all very high (all P < 0·01) and declined towards the normal range thereafter (P < 0·01). iFGF23 was 191 ± 10 pg/ml (geometric mean, SD) and doubled to 371 ± 8 pg/ml (P = 0·003) after the resolution of infection, while cFGF23 did not change (246 ± 5 pg/mL vs. 248 ± 5 pg/mL, P = 0·50). As a consequence, the iFGF23/cFGF23 ratio, an indicator of the proteolytic cleavage of the FGF23 molecule, was 0·78 ± 3·87 at peak infection and increased to 1·49 ± 3·00 after resolution of infection (P < 0·001). In contrast, serum α-Klotho levels were upregulated at peak infection (peak infection: 526 ± 4 pg/ml, postinfection: 447 ± 4 pg/ml, P = 0·001). The eGFR, PTH and vitamin D did not change significantly throughout.

CONCLUSIONS

Acute inflammation/sepsis suppresses the active form of FGF23 and activates α-Klotho, the latter effect being likely attributable to enhance proteolysis of FGF23 molecule. iFGF23 downregulation and α-Klotho upregulation during acute sepsis may participate into the counter-regulatory response to severe inflammation in CKD patients with sepsis.

Is Low Serum Klotho Level Associated with Alterations in Coronary Flow Reserve?

BACKGROUND

The Klotho gene, described as an "aging suppressor" gene, encodes a single-pass transmembrane protein. The extracellular part of Klotho is cleaved and released into the circulation where it may function as a vasculoprotective hormone. Coronary flow reserve (CFR) is accepted as a marker of coronary microvascular dysfunction when epicardial coronary stenosis is absent. There are no data regarding the relationship between serum Klotho levels and disorders in coronary microcirculation in healthy adults. We aimed to investigate the association between serum Klotho levels and alterations in coronary microcirculation in healthy adults using echocardiographic measurements of CFR.

METHODS

Thirty-four healthy volunteers (median age: 34 [27-39], 14 males) were enrolled in this study. The study population was divided into two subgroups according to the median value of serum Klotho levels: a high Klotho (HK) group (n = 17, median age: 34 [30-38]; 6 males) and a low Klotho (LK) group (n = 17, median age: 32 [26-39]; 8 males). The analysis of coronary flow velocities was performed by transthoracic Doppler echocardiography.

RESULTS

Hyperemic diastolic peak flow velocities and CFR were significantly higher in the HK group than in the LK group (70 [66-92] versus 61 [47-66], P = 0.003 and 3.0 [2.6-3.8] versus 2.2 [1.7-2.8], respectively, P = 0.001). Serum Klotho levels were positively correlated with CFR (P < 0.001).

CONCLUSION

Serum Klotho levels correlate with CFR in a healthy population. Low serum Klotho levels may potentially identify patients with impaired CFR.

Influence of Klotho gene polymorphisms on vascular gene expression and its relationship to cardiovascular disease

Klotho protein has been associated with beneficial effects that contribute to the maintenance of cardiovascular health. Diverse studies suggest that alterations in the levels of this molecule may be associated with pathophysiological abnormalities that result in increased cardiovascular risk. The primary aim of this proof-of-concept study was to analyse the existence of a potential link between Klotho gene polymorphisms and the expression level of this gene in the vascular wall, and additionally with the incidence of cardiovascular disease and cardiovascular risk factors. Our results indicate that the variant G-395A, located in the promoter region, influences Klotho gene vascular expression and is associated with the incidence of diabetes. Similarly, the exonic variant KL-VS was associated with the incidence of atherosclerotic vascular disease and coronary artery disease. Moreover, vascular expression levels of Klotho were related with the incidence of diabetes mellitus and coronary artery disease. These findings, which need to be confirmed in larger studies, suggest a potential role of Klotho in the pathogenesis of vascular damage.

Klotho in cardiovascular disease: Current and future perspectives

Protein Klotho, beyond its role as a regulator of the phosphatemia, is also involved in the maintaining of the cardiovascular health, being associated its alterations with the development of cardiovascular damage and increased morbi-mortality. For all this, nowadays Klotho is the subject of a thorough research which is focused on uncover its intimate mechanisms of action, and in analyzing the utility of its modulation as a potential strategy with clinical applicability. Molecular mechanisms of Klotho are not well understood but an emerging research area links Klotho deficiency with vascular pathology. Changes in this protein have been associated with cardiovascular-related complications like inflammation, vascular calcification, and endothelial dysfunction. All this is particularly relevant if considering the recent discovery of Klotho expression in vascular tissue.

Klotho inhibits human follicular thyroid cancer cell growth and promotes apoptosis through regulation of the expression of stanniocalcin-1

The new anti-aging gene Klotho has been identified as a multi-functional humoral factor which influences multiple biological processes, including tumor progression. Although ample evidence indicates that Klotho plays important roles in cervical, lung and breast cancer, the role and mechanism of Klotho in thyroid cancer are still unclear. The present study aimed to investigate the effects and mechanisms of Klotho in human thyroid cancer cell lines FTC133 and FTC238. Klotho overexpression markedly reduced thyroid cancer FTC133 and FTC238 cell proliferation and enhanced apoptosis, whereas, Klotho silencing in the FTC133 and FTC238 cells increased cell growth. Moreover, soluble human KL1 (sKL) and Klotho overexpression had a similar effect on FTC133 and FTC238 cell growth. A high level of Klotho was also found to be associated with a low level of stanniocalcin 1 (STC1) in both the FTC133 and FTC238 cell lines. STC1 silencing significantly inhibited thyroid cancer cell proliferation, whereas recombinant human STC1 (hSTC1) markedly enhanced cell proliferation. In addition, our study demonstrated that hSTC1 treatment attenuated Klotho-induced inhibition of cell proliferation and promotion of apoptosis. Our data revealed the existence of a moderating effect between Klotho and STC1, where Klotho may inhibit thyroid tumor progression by inhibiting the tumor marker level of STC1.

The role of klotho on vascular calcification and endothelial function in chronic kidney disease

Recent insights into novel roles of klotho in vascular biology make this primarily kidney-derived protein a possible candidate to form a link between chronic kidney disease and cardiovascular morbidity and mortality. Typical features of vascular dysfunction or structural abnormalities in the arterial wall are exacerbated in klotho-deficient states. Reported klotho functions include inhibition of local phosphate transport in vascular cells, phenotypic switches of vascular cellular elements into bone-forming cells, attenuation of matrix mineralization and calcification, and also preservation of endothelial functional properties and viability. To a large extent these insights rely on animal models of kidney or cardiovascular diseases. In this review the current state of knowledge on these issues is summarized, and we aim to provide a possible new perspective on cardiovascular disease in chronic kidney disease.

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.

Molecular basis of Klotho: from gene to function in aging

The discovery of the Klotho (KL) gene, which was originally identified as a putative aging-suppressor gene, has generated tremendous interest and has advanced understanding of the aging process. In mice, the overexpression of the KL gene extends the life span, whereas mutations to the KL gene shorten the life span. The human KL gene encodes the α-Klotho protein, which is a multifunctional protein that regulates the metabolism of phosphate, calcium, and vitamin D. α-Klotho also may function as a hormone, although the α-Klotho receptor(s) has not been found. Point mutations of the KL gene in humans are associated with hypertension and kidney disease, which suggests that α-Klotho may be essential to the maintenance of normal renal function. Three α-Klotho protein types with potentially different functions have been identified: a full-length transmembrane α-Klotho, a truncated soluble α-Klotho, and a secreted α-Klotho. Recent evidence suggests that α-Klotho suppresses the insulin and Wnt signaling pathways, inhibits oxidative stress, and regulates phosphatase and calcium absorption. In this review, we provide an update on recent advances in the understanding of the molecular, genetic, biochemical, and physiological properties of the KL gene. Specifically, this review focuses on the structure of the KL gene and the factors that regulate KL gene transcription, the key sites in the regulation of α-Klotho enzyme activity, the α-Klotho signaling pathways, and the molecular mechanisms that underlie α-Klotho function. This current understanding of the molecular biology of the α-Klotho protein may offer new insights into its function and role in aging.

αKlotho and vascular calcification: an evolving paradigm

PURPOSE OF REVIEW

Cardiovascular disease remains the single most serious contributor to mortality in chronic kidney disease (CKD). Although conventional risk factors are prevalent in CKD, both cardiomyopathy and vasculopathy can be caused by pathophysiologic mechanisms specific to the uremic state. CKD is a state of systemic αKlotho deficiency. Although the molecular mechanism of action of αKlotho is not well understood, the downstream targets and biologic functions of αKlotho are astonishingly pleiotropic. An emerging body of literature links αKlotho to uremic vasculopathy.

RECENT FINDINGS

The expression of αKlotho in the vasculature is controversial because of conflicting data. Regardless of whether αKlotho acts as a circulating or resident protein, there are good data associating changes in αKlotho levels with vascular pathology including vascular calcification and in-vitro data of the direct action of αKlotho on both the endothelium and vascular smooth muscle cells in terms of cytoprotection and prevention of mineralization.

SUMMARY

It is critical to understand the pathogenic role of αKlotho on the integral endothelium-vascular smooth muscle network rather than each cell type in isolation in uremic vasculopathy, as αKlotho can serve as a potential prognostic biomarker and a biological therapeutic agent.

Implications of Klotho in vascular health and disease

Cardiovascular disease (CVD) is a prevalent condition in general population and the first cause of death overall. Klotho, a pleiotropic protein related to longevity that acts as a co-receptor of the fibroblast growth factor 23, has been proposed as a key regulator of the development of CVD. In the few clinical studies made, it has been observed a relationship between low levels of soluble Klotho and the occurrence and severity of CVD, as well as a reduction of cardiovascular risk when they are high. Also, different polymorphisms of human Klotho gene have been related to the incidence of cardiovascular events. Moreover, several experimental studies indicate that this protein acts in the maintenance of vascular homeostasis. Klotho improves endothelial dysfunction through promotion of NO production and mediates anti-inflammatory and anti-aging effects such as suppression of adhesion molecules expression, attenuation of nuclear factor-kappa B or inhibition of Wnt signaling. Furthermore, this protein is related to the attenuation of vascular calcification as well as prevention of cardiac hypertrophy. The expression of this protein in the vascular wall implies a new scenario for the treatment of vascular disorders. The purpose of this review is to provide an overview of the relationship between the Klotho protein and CVD, in addition to its role in the maintenance of functional vascular integrity.

Klotho: its various functions and association with sickle cell disease subphenotypes

The Klotho protein, whose gene has predominant renal expression, acts in the control of serum phosphorus and 1,25-dihydroxyvitamin D3 and regulates the function of ion channels. It also participates in the mechanism of protection against oxidative stress and acts on the vascular endothelium by inducing the production of nitric oxide. Mutations that reflect defects in the Klotho gene expression may be implicated in the onset of osteonecrosis, priapism, and leg ulcers in patients with sickle cell disease, as a result of oxidative stress and endothelial impairment, important factors in the development and severity of this disease. Previous reports regarding the association of Klotho single nucleotide polymorphisms with sickle cell disease subphenotypes have found that these polymorphisms are important to identify genetic markers of risk in these individuals and allow early and more effective therapeutic intervention.

The aging kidney revisited: a systematic review

As for the whole human body, the kidney undergoes age-related changes which translate in an inexorable and progressive decline in renal function. Renal aging is a multifactorial process where gender, race and genetic background and several key-mediators such as chronic inflammation, oxidative stress, the renin-angiotensin-aldosterone (RAAS) system, impairment in kidney repair capacities and background cardiovascular disease play a significant role. Features of the aging kidney include macroscopic and microscopic changes and important functional adaptations, none of which is pathognomonic of aging. The assessment of renal function in the framework of aging is problematic and the question whether renal aging should be considered as a physiological or pathological process remains a much debated issue. Although promising dietary and pharmacological approaches have been tested to retard aging processes or renal function decline in the elderly, proper lifestyle modifications, as those applicable to the general population, currently represent the most plausible approach to maintain kidney health.

Fibroblast growth factor 23, Klotho, and disordered mineral metabolism in chronic kidney disease: unraveling the intricate tapestry of events and implications for therapy

Fibroblast growth factor 23 (FGF23) and Klotho play key roles, along with serum phosphate and parathyroid hormone (PTH), in disordered mineral metabolism in chronic kidney disease (CKD). Through independent and combined effects on renal phosphorus and vitamin D metabolism, alterations in FGF23 and Klotho expression are essential for maintaining mineral homeostasis in kidney disease. However, increasing evidence showing that disturbances in FGF23 and Klotho expression produce direct cardiovascular and renal toxicity has accelerated interest in therapies that directly target these hormones in the interest of improving outcomes in CKD. Importantly, recent studies showing that reversing CKD-induced elevations in FGF23 levels may have adverse cardiovascular effects have shed new light on the potential unintended consequences linked to developing therapeutic targets in this arena. These findings ultimately support the notion that addressing root causes of disturbances of FGF23 and Klotho, such as phosphorus excess, should remain the main focus in managing mineral metabolism in CKD.

Ramipril lowers plasma FGF-23 in patients with diabetic nephropathy

BACKGROUND/AIMS

Ramipril attenuates renal Fibroblast growth factor-23 (FGF-23) expression, ameliorates proteinuria and normalizes serum phosphate in the diabetic Zucker rat with progressive renal disease suggesting that the renoprotective effect by this drug may be in part due to a FGF-23-lowering effect of angiotensin-converting enzyme (ACE) inhibition.

METHODS

In this nonrandomized study, we tested whether ACE-inhibition reduces circulating FGF-23 in type-2 diabetics with stage-1 chronic kidney disease (CKD) and proteinuria. Intact FGF-23, the eGFR, proteinuria and the endothelium-dependent flow-mediated (FMD) response to ischemia and other parameters were measured at baseline and after 12-weeks of treatment with ramipril (n = 68) or amlodipine (n = 32).

RESULTS

Blood Pressure (BP) fell to a similar extent (p < 0.001) in the two groups. However, 24 h proteinuria and the FMD improved more (both p < 0.01) in ramipril-treated patients than in amlodipine-treated patients. Changes in proteinuria (r = 0.47) and in FMD (r = -0.49) by ramipril were closely associated (p < 0.001) with simultaneous changes in FGF-23 and this link was confirmed in multiple regression analyses. In these analyses, the relationship between FMD and proteinuria changes attained statistical significance (p < 0.01) only in a model excluding FGF-23 suggesting that endothelial dysfunction and FGF-23 share a common pathway conducive to renal damage.

CONCLUSION

Findings in this study contribute to generate the hypothesis that FGF-23 may be implicated in proteinuria and in endothelial dysfunction in diabetic nephropathy (clinicaltrials.gov (NCT01738945)).

Hyperlipidemia-associated renal damage decreases Klotho expression in kidneys from ApoE knockout mice

Background

Klotho is a renal protein with anti-aging properties that is downregulated in conditions related to kidney injury. Hyperlipidemia accelerates the progression of renal damage, but the mechanisms of the deleterious effects of hyperlipidemia remain unclear.

Methods

We evaluated whether hyperlipidemia modulates Klotho expression in kidneys from C57BL/6 and hyperlipidemic apolipoprotein E knockout (ApoE KO) mice fed with a normal chow diet (ND) or a Western-type high cholesterol-fat diet (HC) for 5 to 10 weeks, respectively.

Results

In ApoE KO mice, the HC diet increased serum and renal cholesterol levels, kidney injury severity, kidney macrophage infiltration and inflammatory chemokine expression. A significant reduction in Klotho mRNA and protein expression was observed in kidneys from hypercholesteromic ApoE KO mice fed a HC diet as compared with controls, both at 5 and 10 weeks. In order to study the mechanism involved in Klotho down-regulation, murine tubular epithelial cells were treated with ox-LDL. Oxidized-LDL were effectively uptaken by tubular cells and decreased both Klotho mRNA and protein expression in a time- and dose-dependent manner in these cells. Finally, NF-κB and ERK inhibitors prevented ox-LDL-induced Klotho downregulation.

Conclusion

Our results suggest that hyperlipidemia-associated kidney injury decreases renal expression of Klotho. Therefore, Klotho could be a key element explaining the relationship between hyperlipidemia and aging with renal disease.

The connection between dietary phosphorus, cardiovascular disease, and mortality: where we stand and what we need to know

Disorders of phosphorus metabolism are independent risk factors for cardiovascular disease. Because excess dietary phosphorus intake is common in the general population and plays a central role in disturbances in phosphorus homeostasis, these findings have fueled interest in restricting phosphorus intake as a potential therapy for improving cardiovascular outcomes. Although experimental and observational data support this possibility, current limitations in the assessment of dietary phosphorus consumption in free-living populations and the lack of reliable biomarkers of the effects of dietary phosphorus on cardiovascular health pose major barriers to the design and conduct of trials assessing the efficacy of phosphorus restriction in improving cardiovascular health. Fibroblast growth factor 23 and Klotho are novel mediators of phosphorus metabolism that are tightly linked to dietary phosphorus intake and show promise as integrated biomarkers of phosphorus excess and its long-term health consequences. Advances in the understanding of how these hormones are associated with diet and phosphorus metabolism will likely bolster future efforts to assess the true health consequences of excess phosphorus intake and whether restricting phosphorus intake has salutary effects on cardiovascular health.

Reduced Klotho is associated with the presence and severity of coronary artery disease

OBJECTIVE

Klotho is involved in vascular health. We aimed to analyse in a cross-sectional study the relationship between Klotho and human coronary artery disease (CAD).

METHODS

The study included 371 subjects who underwent coronary angiography and 70 patients who underwent elective cardiac surgery recruited between May 2008 and June 2009. The presence and severity (stenosis index) of CAD, cardiovascular risk factors, Klotho gene expression in the thoracic aorta, and serum soluble Klotho concentrations were evaluated.

RESULTS

The soluble Klotho concentration was lower (p<0.001) in patients with significant CAD (n=233). The maximal stenosis observed in every epicardial artery and the stenosis severity index was significantly lower in patients within the higher soluble Klotho concentrations (p<0.0001). Multiple regression analysis showed that serum Klotho concentrations were inverse and significantly associated with CAD (adjusted R(2)=0.67, p<0.001). Multivariate logistic regression analysis showed that risk factors for significant CAD included age, diabetes, smoking and inflammation, whereas high serum Klotho values were associated with a lower risk for CAD. Lower mRNA expression level of Klotho was observed in 46 patients with significant CAD, as compared with subjects without CAD (p=0.01). Logistic regression analysis showed that high Klotho gene expression was independently associated with lower risk for CAD.

CONCLUSIONS

Patients with significant CAD present lower soluble concentrations of Klotho, as well as reduced levels of Klotho gene expression in the vascular wall. Reduced serum Klotho concentrations and decreased vascular Klotho gene expression were associated with the presence and severity of CAD independently of established cardiovascular risk factors.

Significance of the anti-aging protein Klotho

The Klotho gene was identified as an 'aging suppressor' in mice. Overexpression of the Klotho gene extends lifespan and defective Klotho results in rapid aging and early death. Both the membrane and secreted forms of Klotho have biological activity that include regulatory effects on general metabolism and a more specific effect on mineral metabolism that correlates with its effect on aging. Klotho serves as a co-receptor for fibroblast growth factor (FGF), but it also functions as a humoral factor that regulates cell survival and proliferation, vitamin D metabolism, and calcium and phosphate homeostasis and may serve as a potential tumor suppressor. Moreover, Klotho protects against several pathogenic processes in a FGF23-independent manner. These processes include cancer metastasis, vascular calcification, and renal fibrosis. This review covers the recent advances in Klotho research and discusses novel Klotho-dependent mechanisms that are clinically relevant in aging and age-related diseases.

The anti-aging factor α-klotho during human pregnancy and its expression in pregnancies complicated by small-for-gestational-age neonates and/or preeclampsia

OBJECTIVE

α-klotho, a protein with anti-aging properties, has been involved in important biological processes, such as calcium/phosphate metabolism, resistance to oxidative stress, and nitric oxide production in the endothelium. Recent studies have suggested a role of α-klotho in endocrine regulation of mineral metabolism and postnatal growth in infants. Yet, the role of α-klotho during pregnancy remains largely unknown. The aim of this study was to determine whether maternal plasma concentration of α-klotho changes during pregnancy and evaluate its expression in pregnancies complicated by small for gestational age (SGA) and/or preeclampsia (PE).

STUDY DESIGN

This cross-sectional study included patients in the following groups: (1) non pregnant women (n = 37); (2) uncomplicated pregnancy (n = 130); (3) PE without an SGA neonate (PE; n = 58); (4) PE with an SGA neonate (PE and SGA; n = 52); and (5) SGA neonate without PE (SGA; n = 52). Plasma concentrations of α-klotho were determined by ELISA.

RESULTS

The median plasma α-klotho concentration was higher in pregnant than in non-pregnant women. Among women with an uncomplicated pregnancy, the median plasma concentration of α-klotho increased as a function of gestational age (Spearman Rho = 0.2; p = 0.006). The median (interquartile range) plasma concentration of α-klotho in women with PE and SGA [947.6 (762-2013) pg/mL] and SGA without PE [1000 (585-1567) pg/mL] were 21% and 17% lower than that observed in women with an uncomplicated pregnancy [1206.6 (894-2012) pg/mL], (p = 0.005 and p = 0.02), respectively. Additionally, there were no significant differences in the median plasma concentration of α-klotho between uncomplicated pregnancies and women with PE without an SGA neonate (p = 0.5).

CONCLUSION

Maternal plasma concentration of α-klotho was higher during pregnancy than in a non-pregnant state. Moreover, the median maternal plasma concentration of α-klotho was lower in mothers who delivered an SGA neonate than in those with an uncomplicated pregnancy regardless of the presence or absence of PE.

The antiaging protein Klotho enhances oligodendrocyte maturation and myelination of the CNS

We have previously shown that myelin abnormalities characterize the normal aging process of the brain and that an age-associated reduction in Klotho is conserved across species. Predominantly generated in brain and kidney, Klotho overexpression extends life span, whereas loss of Klotho accelerates the development of aging-like phenotypes. Although the function of Klotho in brain is unknown, loss of Klotho expression leads to cognitive deficits. We found significant effects of Klotho on oligodendrocyte functions, including induced maturation of rat primary oligodendrocytic progenitor cells (OPCs) in vitro and myelination. Phosphoprotein analysis indicated that Klotho's downstream effects involve Akt and ERK signal pathways. Klotho increased OPC maturation, and inhibition of Akt or ERK function blocked this effect on OPCs. In vivo studies of Klotho knock-out mice and control littermates revealed that knock-out mice have a significant reduction in major myelin protein and gene expression. By immunohistochemistry, the number of total and mature oligodendrocytes was significantly lower in Klotho knock-out mice. Strikingly, at the ultrastructural level, Klotho knock-out mice exhibited significantly impaired myelination of the optic nerve and corpus callosum. These mice also displayed severe abnormalities at the nodes of Ranvier. To decipher the mechanisms by which Klotho affects oligodendrocytes, we used luciferase pathway reporters to identify the transcription factors involved. Together, these studies provide novel evidence for Klotho as a key player in myelin biology, which may thus be a useful therapeutic target in efforts to protect brain myelin against age-dependent changes and promote repair in multiple sclerosis.

Circulating α-klotho levels in CKD and relationship to progression

BACKGROUND

α-Klotho is reported to have protective effects against kidney injury, and its renal expression is decreased in many experimental models of kidney disease. However, circulating α-klotho levels in human chronic kidney disease (CKD) and the relationship to progression are unknown.

STUDY DESIGN

Post hoc analysis of a prospective cohort study.

SETTING & PARTICIPANTS

243 of 301 participants from a CKD cohort at our institution between January 2006 and December 2011 were eligible for the study.

PREDICTOR

Baseline α-klotho levels.

OUTCOMES

Primary outcome was the composite of doubling of baseline serum creatinine concentration, end-stage renal disease, or death. End-stage renal disease was defined as onset of treatment by renal replacement therapy.

MEASUREMENTS

Serum α-klotho and fibroblast growth factor 23 (FGF-23) were measured using enzyme-linked immunosorbent assay.

RESULTS

Lower serum α-klotho levels were associated with more severe CKD stage in the cross-sectional analysis of the baseline data (P for trend < 0.001). In the adjusted multivariable linear regression model, log(α-klotho) was associated independently with estimated glomerular filtration rate (β = 0.154; P = 0.001). Cox regression analysis showed that baseline α-klotho level independently predicted the composite outcome after adjustment for age, diabetes, blood pressure, estimated glomerular filtration rate, proteinuria, parathyroid hormone level, and FGF-23 level (HR per 10-pg/mL increase, 0.96; 95% CI, 0.94-0.98; P < 0.001). When patients were categorized into 2 groups according to baseline median α-klotho value, 43 (35.2%) patients with α-klotho levels ≤396.3 pg/mL reached the primary composite outcome compared with 19 (15.7%) with α-klotho levels >396.3 pg/mL (HR, 2.03; 95% CI, 1.07-3.85; P = 0.03).

LIMITATIONS

Uncontrolled dietary phosphorus intake and use of frozen samples.

CONCLUSIONS

This observational study showed that low circulating α-klotho levels were associated with adverse kidney disease outcome, suggesting that α-klotho is a novel biomarker for CKD progression. More data from larger prospective longitudinal studies are required to validate our findings.

Klotho endows hepatoma cells with resistance to anoikis via VEGFR2/PAK1 activation in hepatocellular carcinoma

Klotho was originally characterized as an aging suppressor gene that predisposed Klotho-deficient mice to premature aging-like syndrome. Although Klotho was recently reported to exhibit tumor suppressive properties during various malignant transformations, the functional role and molecular mechanism of Klotho in hepatocarcinogenesis remains poorly understood. In our present study, immunohistochemical Klotho staining levels in a clinical follow-up of 52 hepatoma patients were significantly associated with liver cirrhosis, tumor multiplicity and venous invasion. The overall survival rate of hepatoma patients with high Klotho expression was significantly lower than those patients with low Klotho expression. Moreover, Klotho overexpression increased cellular migration, anchorage-independent growth, and anoikis resistance in hepatoma cells. Klotho overexpression elevated p21-activated kinase 1 (PAK1) expression and shRNA-mediated PAK1 knockdown and kinase activity inhibition with kinase dead mutant PAK1 K299R coexpression or allosteric inhibitor IPA3 treatment reversed anoikis resistance in Klotho-overexpressed hepatoma cells. More importantly, the pivotal significance of upregulated VEGFR2 protein levels mediated by Klotho expression was confirmed by VEGFR2 inhibitor Axitinib and blocking antibody treatment in hepatoma cells. Axitinib treatment sensitized anoikis was reversed by constitutive active mutant PAK1 T423E coexpression in Klotho-overexpressed hepatoma cells. Conversely, knockdown of Klotho reduced VEGFR2/PAK1 dependent anoikis resistance, which could be reversed by PAK1 T423E. These results revealed a novel oncogenic function of Klotho in promoting anoikis resistance via activating VEGFR2/PAK1 signaling, thus facilitating tumor migration and invasion during hepatoma progression, which could provide a putative molecular mechanism for tumor metastasis.

FGF23 associated bone diseases

Recently, fibroblast growth factor 23 (FGF23) has sparked widespread interest because of its potential role in regulating phosphate and vitamin D metabolism. In this review, we summarized the FGF superfamily, the mechanism of FGF23 on phosphate and vitamin D metabolism, and the FGF23 related bone disease.

Renal and extrarenal actions of Klotho

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

A decreased level of serum soluble Klotho is an independent biomarker associated with arterial stiffness in patients with chronic kidney disease

Background

Klotho was originally identified in a mutant mouse strain unable to express the gene that consequently showed shortened life spans. In humans, low serum Klotho levels are related to the prevalence of cardiovascular diseases in community-dwelling adults. However, it is unclear whether the serum Klotho levels are associated with signs of vascular dysfunction such as arterial stiffness, a major determinant of prognosis, in human subjects with chronic kidney disease (CKD).

Methods

We determined the levels of serum soluble Klotho in 114 patients with CKD using ELISA and investigated the relationship between the level of Klotho and markers of CKD-mineral and bone disorder (CKD-MBD) and various types of vascular dysfunction, including flow-mediated dilatation, a marker of endothelial dysfunction, ankle-brachial pulse wave velocity (baPWV), a marker of arterial stiffness, intima-media thickness (IMT), a marker of atherosclerosis, and the aortic calcification index (ACI), a marker of vascular calcification.

Results

The serum Klotho level significantly correlated with the 1,25-dihydroxyvitamin D level and inversely correlated with the parathyroid hormone level and the fractional excretion of phosphate. There were significant decreases in serum Klotho in patients with arterial stiffness defined as baPWV≥1400 cm/sec, atherosclerosis defined as maximum IMT≥1.1 mm and vascular calcification scores of ACI>0%. The serum Klotho level was a significant determinant of arterial stiffness, but not endothelial dysfunction, atherosclerosis or vascular calcification, in the multivariate analysis in either metabolic model, the CKD model or the CKD-MBD model. The adjusted odds ratio of serum Klotho for the baPWV was 0.60 (p = 0.0075).

Conclusions

Decreases in the serum soluble Klotho levels are independently associated with signs of vascular dysfunction such as arterial stiffness in patients with CKD. Further research exploring whether therapeutic approaches to maintain or elevate the Klotho level could improve arterial stiffness in CKD patients is warranted.

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

BACKGROUND AND OBJECTIVES

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

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

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

RESULTS

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

CONCLUSIONS

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

Klotho modulates the stress response in human senescent endothelial cells

Lack of Klotho expression in mice leads to premature aging and age-related diseases, including vascular diseases. The aim of this study was to determine how endothelial cell line senescence affects Klotho expression and whether intra- or extracellular Klotho has any effect on the response of senescent cells to oxidative stress. The study was performed using human endothelial cells (HUVEC); cell aging was obtained by prolongation of cell division to 42 population doublings (PD). Senescence was also obtained by exposure to TNFα, which causes cell changes resembling cellular senescence. The decline in Klotho preceded the manifestations of cell ageing: telomere shortening and β-galactosidase expression. Klotho was also reduced in cells exposed to the proinflammatory cytokine TNFα. The addition of exogenous Klotho to aging cells did not modify the proportion of cells with short telomeres or any other feature of cell aging; however, exogenous Klotho prevented the changes resembling premature cellular senescence associated with TNFα, such as the decrease in telomere length and the increase in β-galactosidase-positive cells. Likewise exogenous Klotho prevented the increases in reactive oxygen species (ROS) activity, mitochondrial potential and cell apoptosis induced by TNFα.

Soluble serum Klotho in diabetic nephropathy: relationship to VEGF-A

OBJECTIVES

Both kidney expression and soluble serum Klotho are influenced by chronic kidney disease (CKD) and diabetes. Serum Klotho is a yet poorly explored biomarker. We describe, for the first time to our knowledge, serum Klotho in diabetic patients with CKD and its relationship to vascular endothelial growth factor A (VEGF-A).

DESIGN AND METHODS

We included 43 controls and 146 diabetic patients with different stages of CKD. Laboratory evaluation, urinary albumin/creatinine ratio (UACR), Klotho (ELISA), VEGF-A (ELISA) were performed.

RESULTS

Klotho was 0.40(0.10-1.30)ng/mL in diabetic patients without CKD and 0.80(0.30-1.30)ng/mL in controls, p=0.20; VEGF-A was higher in diabetic patients 73.85(57.32-119.00)pg/mL than in controls 43.20(30.1-65.9)pg/mL, p<0.0001. Klotho increased with CKD stage: 0.2(0.10-0.40)ng/mL in CKD 1/2, 0.60(0.20-1.1)ng/mL in CKD 3/4 and 1.45(0.425-2.90)ng/mL in dialysis patients, p<0.0001; it also increased with decreasing glomerular filtration rate (GFR). Klotho was lower in albuminuric (UACR>30 mg/g) patients 0.20(0.10-0.70)ng/mL than in normoalbuminuric (UACR<30 mg/g) ones 0.50(0.20-1.30)ng/mL, p=0.03; lowest Klotho was found in microalbuminuric (UACR 30-300 mg/g) patients, p=0.07. VEGF was lower in microalbuminuric patients but was not influenced by GFR. In diabetic patients but not in controls, Klotho correlated to VEGF-A (r=0.29, p=0.0003); in multiple regression VEGF-A was the only significant predictor of Klotho: b=0.27, 95%CI (0.01-0.04), p=0.001.

CONCLUSIONS

In diabetic patients, Klotho is decreased in early CKD and increases thereafter, paralleling reduced GFR. VEGF-A is higher in diabetic patients than in controls. Both Klotho and VEGF-A are decreased in the presence of microalbuminuria. In diabetes, Klotho strongly correlates to VEGF-A.

The role of vitamin D in the FGF23, klotho, and phosphate bone-kidney endocrine axis

1,25-dihydroxyvitamin D (1,25D), through association with the nuclear vitamin D receptor (VDR), exerts control over a novel endocrine axis consisting of the bone-derived hormone FGF23, and the kidney-expressed klotho, CYP27B1, and CYP24A1 genes, which together prevent hyperphosphatemia/ectopic calcification and govern the levels of 1,25D to maintain bone mineral integrity while promoting optimal function of other vital tissues. When occupied by 1,25D, VDR interacts with RXR to form a heterodimer that binds to VDREs in the region of genes directly controlled by 1,25D (e.g., FGF23, klotho, Npt2c, CYP27B1 and CYP24A1). By recruiting complexes of comodulators, activated VDR initiates a series of events that induces or represses the transcription of genes encoding proteins such as: the osteocyte-derived hormone, FGF23; the renal anti-senescence factor and protein co-receptor for FGF23, klotho; other mediators of phosphate transport including Npt2a/c; and vitamin D hormone metabolic enzymes, CYP27B1 and CYP24A1. The mechanism whereby osteocytes are triggered to release FGF23 is yet to be fully defined, but 1,25D, phosphate, and leptin appear to play major roles. The kidney responds to FGF23 to elicit CYP24A1-catalyzed detoxification of the 1,25D hormone while also repressing both Npt2a/c to mediate phosphate elimination and CYP27B1 to limit de novo 1,25D synthesis. Comprehension of these skeletal and renal actions of 1,25D should facilitate the development of novel mimetics to prevent ectopic calcification, chronic renal and vascular disease, and promote healthful aging.

Identification of novel small molecules that elevate Klotho expression

The absence of Klotho (KL) from mice causes the development of disorders associated with human aging and decreased longevity, whereas increased expression prolongs lifespan. With age, KL protein levels decrease, and keeping levels consistent may promote healthier aging and be disease-modifying. Using the KL promoter to drive expression of luciferase, we conducted a high-throughput screen to identify compounds that activate KL transcription. Hits were identified as compounds that elevated luciferase expression at least 30%. Following validation for dose-dependent activation and lack of cytotoxicity, hit compounds were evaluated further in vitro by incubation with opossum kidney and Z310 rat choroid plexus cells, which express KL endogenously. All compounds elevated KL protein compared with control. To determine whether increased protein resulted in an in vitro functional change, we assayed FGF23 (fibroblast growth factor 23) signalling. Compounds G-I augmented ERK (extracellular-signal-regulated kinase) phosphorylation in FGFR (fibroblast growth factor receptor)-transfected cells, whereas co-transfection with KL siRNA (small interfering RNA) blocked the effect. These compounds will be useful tools to allow insight into the mechanisms of KL regulation. Further optimization will provide pharmacological tools for in vivo studies of KL.

Nuclear localization of Klotho in brain: an anti-aging protein

Klotho is a putative age-suppressing gene whose overexpression in mice results in extension of life span. The Klotho gene encodes a single-pass transmembrane protein whose extracellular domain is shed and released into blood, urine, and cerebrospinal fluid, potentially functioning as a humoral factor. The extracellular domain of Klotho has an activity that increases the expression of antioxidant enzymes and confers resistance to oxidative stress in cultured cells and in whole animals. The transmembrane form of the Klotho protein directly binds to multiple fibroblast growth factor receptors and modifies their ligand affinity and specificity. The purpose of the present study was to determine the precise cellular localization of Klotho in the mouse brain. Using light microscopic immunohistochemical methods, we found the highest levels of Klotho immunoreactivity in 2 brain regions: the choroid plexus, and cerebellar Purkinje cells. In the choroid plexus cells, Klotho was found not only on the plasma membrane but also in large amounts near the nuclear membrane. Likewise, in the Purkinje cell Klotho was found throughout the cell including dendrites, axon and soma with large amounts near the nuclear membrane. Using immunoelectron microscopy, we found Klotho in the cell membrane, but the highest concentration was localized in the peripheral portion of the nucleus and the nucleolus in both cell types. This new finding suggests that in addition to Klotho being secreted from cells in brain, it also has a nuclear function.

Comparison of calcium acetate and sevelamer on vascular function and fibroblast growth factor 23 in CKD patients: a randomized clinical trial

BACKGROUND

Fibroblast growth factor 23 (FGF-23) is a marker of endothelial dysfunction and atherosclerotic complications in patients with chronic kidney disease (CKD). Because previous studies suggested that sevelamer may exert effects on FGF-23 level and endothelial function independently of its phosphate-lowering action, we tested the effect of sevelamer versus calcium acetate on vascular function and FGF-23 levels.

STUDY DESIGN

Randomized prospective open-label trial.

SETTING & PARTICIPANTS

Patients with stage 4 CKD with hyperphosphatemia (n = 100).

INTERVENTION

An 8-week intervention with sevelamer (n = 47) and calcium acetate (n = 53).

OUTCOMES

The primary study outcome was change in flow-mediated vasodilatation in the forearm. The secondary outcome was change in FGF-23 levels.

RESULTS

Serum phosphate levels decreased in both treatment arms (P < 0.001), but more markedly in the sevelamer group (P < 0.001). Flow-mediated vasodilatation increased from 6.1% to 7.1% (P < 0.001) in sevelamer-treated patients, whereas it was unchanged in the calcium-acetate group (6.0% vs 6.0%). In a combined analysis, treatment-induced changes in flow-mediated vasodilatation were (P < 0.001) associated with simultaneous changes in FGF-23 levels (-27.1% [-33.2% to -8.8%] for the sevelamer group; 3.5% [-8.4% to 12.1%] for the calcium acetate group), as well as with C-reactive protein and fetuin A levels. These relationships were confirmed in multiple regression analysis adjusting for changes in serum phosphate levels and other factors.

LIMITATIONS

Unblinded randomized controlled study that cannot establish mechanisms of effect.

CONCLUSIONS

In hyperphosphatemic patients with stage 4 CKD, treatment with phosphate lowering induces measurable improvements in flow-mediated vasodilatation. Furthermore, independently of serum phosphate level, FGF-23 level changes induced by phosphate binders are associated with simultaneous changes in flow-mediated vasodilatation. These observations are compatible with the hypothesis that FGF-23 may contribute to vascular dysfunction in this population.

The putative role of the antiageing protein klotho in cardiovascular and renal disease

Ageing is a multifactorial process often characterized by a progressive decline in physiological function(s). Ageing can and is often associated with an increased incidence of cardiovascular and renal disease. Klotho is a novel antiageing gene that encodes a protein with multiple pleiotropic functions including an emerging role in cardiorenal disease. Mice deficient for this gene display a phenotype of premature human ageing characterized by diffuse vascular calcification, altered calcium/phosphate metabolism, and shortened lifespan. Klotho is mainly expressed in the renal tubules but it also exists as circulating soluble form detectable in the blood, with systemic effects. Reduction in soluble Klotho has been associated with renal disease, hyperphosphataemia, increased oxidative stress, endothelial dysfunction, and diffuse vascular calcification. Conversely, overexpression of Klotho promotes cardiovascular-renal protection. The majority of the research on Klotho has been conducted in vitro and in animal studies but there is emerging data from human studies which suggest that Klotho may be a modifiable factor involved in the pathogenesis of cardiovascular and renal disease in at-risk populations. Further data is required to confirm if this novel protein can emerge as therapeutic tool that may be used to prevent or slow progression of cardiorenal disease.

Genetic causation of neointimal hyperplasia in hemodialysis vascular access dysfunction

The major cause of hemodialysis vascular access failure is venous stenosis resulting from neointimal hyperplasia. Genetic factors have been shown to be associated with cardiovascular disease and peripheral vascular disease (PVD) in the general population. Genetic factors may also play an important role in vascular access stenosis and development of neointimal hyperplasia by affecting pathways that lead to inflammation, endothelial function, oxidative stress, and vascular smooth muscle proliferation. This review will discuss the role of genetics in understanding neointimal hyperplasia development in hemodialysis vascular access dysfunction and other disease processes with similar neointimal hyperplasia development such as coronary artery disease and PVD.

Dietary and genetic evidence for enhancing glucose metabolism and reducing obesity by inhibiting klotho functions

Klotho is a multifunctional protein involved in numerous biological functions, ranging from mineral ion metabolism to signaling activities. Recent studies have identified klotho as a target gene for peroxisome proliferator-activated receptor-γ (PPAR-γ), a master regulator of adipocyte differentiation, and an adipogenesis-promoting factor. In a similar line of observation, eliminating klotho function from mice resulted in the generation of lean mice with almost no detectable fat tissue. In contrast to the klotho-knockout mice (11.7±0.3 g at 9 wk), leptin-deficient (ob/ob) mice are severely obese (49.3±0.6 g at 9 wk), due to excessive fat accumulation. To study the in vivo role of klotho in obesity, we have generated and characterized ob/ob mice lacking klotho activity [ob/ob-klotho double-knockout (DKO) mice]. The ob/ob mice started to get bigger from 3 wk onward and gained almost 2 times more weight than their wild-type (WT) counterparts (WT vs. ob/ob: 34.8±1.3 vs. 65.5±1.2 g at 21 wk). The generated ob/ob-klotho DKO mice were not only viable throughout their adulthood but also showed markedly reduced fat tissue accumulation compared to their ob/ob littermates. The ob/ob-klotho DKO mice had significantly (P<0.01) less retroperitoneal, mesenteric, and epididymal fat accumulation, compared to their ob/ob counterparts. Similarly, the fatty liver that was consistently observed in the ob/ob mice was eliminated in the ob/ob-klotho DKO mice. Such structural improvement in the liver was also evident from markedly reduced fasting blood glucose levels in ob/ob-klotho DKO mice, compared to their ob/ob counterparts (ob/ob vs. ob/ob-klotho DKO: 266 ± 36 vs. 65±2 mg/dl). Finally, to study whether the absence of klotho can induce resistance to high-fat-diet-induced obesity, we provided a high-fat (60%) diet to klotho-knockout mice and compared them with normal-fat (20%) diet-fed klotho-knockout mice. No significant difference in body weight was detected in klotho-knockout mice fed either the normal-fat diet or high-fat diet, while WT mice fed the high-fat diet gradually gained body weight, compared to the normal-fat-diet-fed counterparts. The results of our dietary and genetic manipulation studies provide in vivo evidence for a role of klotho in obesity and offer a novel target to manipulate obesity and associated complications.

Klotho suppresses RIG-I-mediated senescence-associated inflammation

It is well known that aged or senescent cells develop a complex senescence-associated secretory phenotype (SASP), which is observed both in culture and in vivo. However, the mechanisms underlying the induction of the SASP are largely unknown. We demonstrate that retinoic-acid-inducible gene-I (RIG-I) is induced through the ataxia telangiectasia mutated-interferon regulatory factor 1 (ATM-IRF1) axis in senescent cells and that RIG-I signalling mediates the expression of two important mediators of inflammation, interleukin-6 (IL-6) and IL-8. Klotho has been associated with ageing. We show here that the intracellular, but not the secreted, form of klotho interacts with RIG-I and that this interaction inhibits RIG-I-induced expression of IL-6 and IL-8 both in vitro and in vivo. Our study uncovers a mechanism in which klotho functions as an anti-ageing factor through the suppression of RIG-I-mediated inflammation.

The phosphate regulating hormone fibroblast growth factor-23

Over the last decade, the regulation of phosphate (Pi) homeostasis has been under intense investigation. By utilizing modern biochemical and genetic tools, the pathophysiological mechanisms behind several known hereditary and acquired hypo- and hyperphosphatemic diseases have been clarified. The results of these efforts have opened new insights into the causes of Pi dysregulation and hereby also the physiological mechanisms determining Pi homeostasis. Although several potential Pi-regulating proteins have been discovered and investigated, current data strongly argues for fibroblast growth factor-23 (FGF23), a hormonal factor produced in bone, as a particularly important regulator of Pi homeostasis. In this article, we review the discovery of the FGF23 protein, as well as its biochemistry, localization of production, receptor specificity and mechanisms of action.

Angiotensin II blockade upregulates the expression of Klotho, the anti-ageing gene, in an experimental model of chronic cyclosporine nephropathy

BACKGROUND

The Klotho gene plays a role in suppressing ageing-related disorders. It is suggested that activation of renin-angiotensin system (RAS) or oxidative stress suppresses Klotho in the kidney. This study evaluated the association between Klotho expression and RAS in cyclosporine (CsA)-induced renal injury.

METHODS

Chronic CsA nephropathy was induced by administering CsA (30 mg/kg) to mice on a low-salt diet (LSD) for 4 weeks. A normal-salt diet (NSD) was used as the control. Reverse transcription-polymerase chain reaction, western blot and immunohistochemistry were performed for Klotho and intrarenal RAS activity was measured using immunohistochemistry for angiotensinogen and renin. Oxidative stress was measured with urinary excretion of 8-hydroxy-2'-deoxyguanosine (8-OHdG).

RESULTS

CsA treatment decreased Klotho mRNA and protein in mouse kidney in a dose-dependent and time-dependent manner, but a concurrent treatment with losartan, an angiotensin II type 1 (AT1) receptor blocker, reversed the decrease in Klotho expression with histological improvement. This finding was more marked in the LSD than the NSD. Klotho expression was correlated with angiotensinogen and renin expression, tubulointerstitial fibrosis score and urinary 8-OHdG excretion.

CONCLUSIONS

Angiotensin II may play a pivotal role in regulating Klotho expression in CsA-induced renal injury. AT1 receptor blocker may inhibit the ageing process by decreasing oxidative stress caused by CsA.

FGF-23 and vascular dysfunction in patients with stage 3 and 4 chronic kidney disease

Studies in animals show that fibroblast growth factor (FGF)-23 interferes with vascular reactivity induced by the nitric oxide (NO) system. To investigate the relationship between circulating FGF-23 levels and the response of forearm blood flow to ischemia (flow-mediated vasodilatation, FMD) and nitroglycerin, we tested 183 patients with stage 3-4 chronic kidney disease (CKD). None of them had cardiovascular complications or were taking drugs interfering with vascular function. Patients with FGF-23 levels above the median had significantly lower glomerular filtration rate, FMD, and fetuin-A levels (an anti-inflammatory molecule and potent inhibitor of calcification). They also had higher proteinuria and phosphate levels when compared to patients whose FGF-23 levels were below the median. The response to nitroglycerin was not different between the two groups. Multiple regression analysis showed that the relationship between FGF-23 and FMD was only modestly sensitive to adjustment for classical risk factors, biomarkers of bone mineral metabolism, high-sensitivity C-reactive protein, and homeostatic model assessment index. Adjustment for asymmetrical dimethyl arginine (ADMA) weakened the strength of this link; however, it remained highly significant. There was no independent association between FGF-23 and nitroglycerin. Thus, attenuation of FMD by ADMA suggests that this endogenous inhibitor of NO synthase may, in part, mediate the vascular effects of FGF-23 in patients with CKD.

Decreased expression of klotho gene in uremic atherosclerosis in apolipoprotein E-deficient mice

Chronic renal failure (CRF) markedly accelerates the development of atherosclerosis, but the pathogenesis of uremic atherosclerosis remains to be elucidated. The klotho gene, predominantly expressed in the kidney, plays a key role in regulating aging and the development of age-related diseases in mammals. A loss of klotho results in multiple aging-like phenotypes including atherosclerosis. This study examines the relationship between the klotho expression and the development of accelerated atherosclerosis in uremic state. Eight-week-old apolipoprotein E-deficient (apo-E(-/-)) male mice underwent 5/6 partial kidney ablation to induce CRF or sham-operation. At 6 wk after nephrectomy, CRF mice showed significantly increased aortic plaque area fraction, aortic root plaque area and aortic cholesterol content as compared with non-CRF mice. Serum urea, total cholesterol and triglyceride concentrations were significantly higher in CRF apo-E(-/-) mice compared with non-CRF controls. Moreover, the expression of renal klotho gene and the serum levels of klotho protein were markedly decreased in CRF mice compared with controls. These results suggested that CRF favored atherosclerosis in apo-E(-/-) mice and uremic atherosclerosis was accompanied by down-regulation of klotho expression.

Klotho suppresses TNF-alpha-induced expression of adhesion molecules in the endothelium and attenuates NF-kappaB activation

Klotho is a senescence suppressor protein that, when overexpressed, extends the lifespan of mice. Klotho-disrupted mice exhibit atherosclerosis and endothelial dysfunction, which led us to investigate the effect of the Klotho protein on vascular inflammation, particularly adhesion molecule expression. In this study, human umbilical vein endothelial cells (HUVECs) were preincubated with Klotho protein and then exposed to tumor necrosis factor-alpha (TNF-alpha) or vehicle. Reverse transcription-PCR and Western blot analyses revealed that Klotho suppressed TNF-alpha-induced expression of intracellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1). NF-kappaB activation, IkappaB phosphorylation induced by TNF-alpha were also attenuated by Klotho protein administration. The inhibition of eNOS phosphorylation by TNF-alpha was reversed by Klotho. Furthermore, Klotho inhibited TNF-alpha-induced monocyte adhesion to HUVECs and suppressed adhesion molecule expression in an organ culture of the rat aorta. These results suggest that Klotho suppresses TNF-alpha-induced expression of adhesion molecules and NF-kappaB activation. Klotho may have a role in the modulation of endothelial inflammation.