Hyperphosphatemia-induced nanocrystals upregulate the expression of bone morphogenetic protein-2 and osteopontin genes in mouse smooth muscle cells in vitro

Osteogenic gene markers are epigenetically reprogrammed during contractile-to-calcifying vascular smooth muscle cell phenotype transition

The understanding of vascular calcification-based mechanism is an urgent pending task in vascular biology and this prompted us to better address this issue by investigating whether DNA methylation mechanism might drive osteogenic marker genes modulation in primary human vascular smooth muscle cells (VSMCs) responding to calcium and phosphate levels overload up to 72 h. Firstly, our data shows this calcifying process recapitulates the molecular repertory of osteogenic biomarkers and specifically requiring RUNX2, Osterix and ALP, BSP genes activations along 72 h in vitro, and this behavior was validated here using other lineages. Conversely, both BMPs 4 and 7 were significantly overexpressed, maybe already as a mechanism in response to RUNX2 and Osterix genes activities identified earlier in response to the calcifying condition, and taken into maintain the calcifying phenotype of VSMCs. Additionally, survival signaling was maintained active and accompanied by a dynamic cytoskeleton rearrangement signaling requiring MAPK and AKT phosphorylations. Moreover, during the contractile-to-calcifying transition phenotype of VSMCs, epigenetic machinery was finely modulated, requiring the translocation of DNMT3B and TET2 into nucleus and this prompted us evaluating whether the profile of osteogenic-related gene promoters' methylation might contribute with this process. By firstly estimating 5meC/5 hmeC ratio changes, we further specifically show the significance of the epigenetic modulation of Osterix and Bone sialoprotein related gene promoters, presenting a positive correlation between the epigenetic signature of their gene promoters and transcriptional patterns. Altogether, our results show for the first time the importance of epigenetic mechanism on modulating osteogenic gene markers reprogramming during calcifying VSMCs phenotype acquisition, which might drive the genesis of vascular ectopic calcification.

The effect of lanthanum carbonate on calciprotein particles in hemodialysis patients

Background

Aggregation of solid-phase calcium–phosphate and fetuin-A form nanoparticles called calciprotein particles (CPP). Serum CPP levels are increased in CKD patients and correlated with vascular stiffness and calcification. In this study, we evaluated effects of lanthanum carbonate (LC) and calcium carbonate (CC) on serum CPP levels in hemodialysis (HD) patients.

Methods

Twenty-four (24) HD patients (50% men, age; 68 ± 12 years, dialysis period; 6.2 ± 4.8 years, Kt/v; 1.74 ± 0.34) were treated with CC during 0–8 weeks and then switched to LC during 9–16 weeks. Blood samples were obtained at 0, 8, 16 weeks. Serum CPP levels (TCPP) were measured by the gel-filtration method. Low-density CPP (LCPP) levels were determined by centrifuging the serum samples at 16,000 g for 2 h and measuring CPP levels in the supernatant. The difference between TCPP and LCPP was defined as the high-density CPP (HCPP) level. We evaluated association of TCPP, LCPP, and HCPP with serum calcium (Ca), phosphorus (P), intact PTH, FGF23, Klotho, fetuin-A, aortic calcification index (ACI), LDL cholesterol, and hs-CRP.

Results

TCPP and LCPP levels were significantly decreased after switching CC to LC, whereas Ca and P levels were not changed. HCPP levels were below the lower limit quantification in all patients. The changes in P, Ca × P, LDL cholesterol, but not ACI and the changes in hs-CRP, were correlated with the change in TCPP levels.

Conclusion

The TCPP levels were significantly decreased after switching CC to LC. Non-calcium-containing phosphate binders may be preferable for lowering CPP levels.

Shape-dependent toxicity and mineralization of hydroxyapatite nanoparticles in A7R5 aortic smooth muscle cells

Vascular smooth muscle cell damage is a key step in inducing vascular calcification that yields hydroxyapatite (HAP) as a major product. The effect of the shape of HAP on the damage to vascular smooth muscle cells has yet to be investigated. In this study, we compared the differences in toxicity of four various morphological nano-HAP crystals, namely, H-Rod, H-Needle, H-Sphere, and H-Plate, in rat aortic smooth muscle cells (A7R5). The sizes of these crystals were 39 nm × 115 nm, 41 nm ×189 nm, 56 nm × 56 nm, and 91 nm × 192 nm, respectively. Results showed that all HAPs decreased cell viability, disorganized cell morphology, disrupted cell membranes, increased intracellular reactive oxygen species concentration, decreased mitochondrial membrane potential, decreased lysosome integrity, increased alkaline phosphatase activity, and increased intracellular calcium concentration, resulting in cell necrosis. The cytotoxicity of the four kinds of HAP was ranked as follows: H-Plate > H-Sphere > H-Needle > H-Rod. The cytotoxicity of each crystal was positively correlated with the following factors: large specific surface area, high electrical conductivity and low surface charge. HAP accelerated calcium deposits on the A7R5 cell surface and induced the expression of osteogenic proteins, such as BMP-2, Runx2, OCN, and ALP. The crystals with high cytotoxicity caused more calcium deposits on the cell surface, higher expression levels of osteogenic protein, and stronger osteogenic transformation abilities. These findings elucidated the relationship between crystal shape and cytotoxicity and provided theoretical references for decreasing the risks of vascular calcification.

The Klotho proteins in health and disease

The Klotho proteins, αKlotho and βKlotho, are essential components of endocrine fibroblast growth factor (FGF) receptor complexes, as they are required for the high-affinity binding of FGF19, FGF21 and FGF23 to their cognate FGF receptors (FGFRs). Collectively, these proteins form a unique endocrine system that governs multiple metabolic processes in mammals. FGF19 is a satiety hormone that is secreted from the intestine on ingestion of food and binds the βKlotho-FGFR4 complex in hepatocytes to promote metabolic responses to feeding. By contrast, under fasting conditions, the liver secretes the starvation hormone FGF21, which induces metabolic responses to fasting and stress responses through the activation of the hypothalamus-pituitary-adrenal axis and the sympathetic nervous system following binding to the βKlotho-FGFR1c complex in adipocytes and the suprachiasmatic nucleus, respectively. Finally, FGF23 is secreted by osteocytes in response to phosphate intake and binds to αKlotho-FGFR complexes, which are expressed most abundantly in renal tubules, to regulate mineral metabolism. Growing evidence suggests that the FGF-Klotho endocrine system also has a crucial role in the pathophysiology of ageing-related disorders, including diabetes, cancer, arteriosclerosis and chronic kidney disease. Therefore, targeting the FGF-Klotho endocrine axes might have therapeutic benefit in multiple systems; investigation of the crystal structures of FGF-Klotho-FGFR complexes is paving the way for the development of drugs that can regulate these axes.

High-phosphorus environment promotes calcification of A7R5 cells induced by hydroxyapatite nanoparticles

This study simulated the high-phosphorus (Pi) environment in patients with chronic kidney disease. Nano-hydroxyapatite (HAP) crystals were used to damage rat aortic smooth muscle cells (A7R5) pre-damaged with different concentrations of Pi solution to compare the differences in HAP-induced calcification in A7R5 cells before and after injury by high-Pi condition. After the A7R5 cells were damaged by high-Pi environment, the following were observed. HAP resulted in declined cell viability and lysosomal integrity, release of lactate dehydrogenase, and increased reactive oxygen species production. The ability of high-Pi damaged cells to internalize HAP crystals declined; crystal adhesion and calcium deposition on the cell surface and alkaline phosphatase activities increased. Osteopontin expression and level of Runt-related transcription factor 2 were increased, and HAP-induced osteogenic transformation was enhanced. High-Pi condition promoted the adhesion of A7R5 cells to nano-HAP crystals and inhibited HAP endocytosis, increasing the risk of calcification.

New Insights into the Roles of Monocytes/Macrophages in Cardiovascular Calcification Associated with Chronic Kidney Disease

Cardiovascular disease (CVD) is an important cause of death in patients with chronic kidney disease (CKD), and cardiovascular calcification (CVC) is one of the strongest predictors of CVD in this population. Cardiovascular calcification results from complex cellular interactions involving the endothelium, vascular/valvular cells (i.e., vascular smooth muscle cells, valvular interstitial cells and resident fibroblasts), and monocyte-derived macrophages. Indeed, the production of pro-inflammatory cytokines and oxidative stress by monocyte-derived macrophages is responsible for the osteogenic transformation and mineralization of vascular/valvular cells. However, monocytes/macrophages show the ability to modify their phenotype, and consequently their functions, when facing environmental modifications. This plasticity complicates efforts to understand the pathogenesis of CVC-particularly in a CKD setting, where both uraemic toxins and CKD treatment may affect monocyte/macrophage functions and thereby influence CVC. Here, we review (i) the mechanisms by which each monocyte/macrophage subset either promotes or prevents CVC, and (ii) how both uraemic toxins and CKD therapies might affect these monocyte/macrophage functions.

Synthesis of Extracellular Pyrophosphate Increases in Vascular Smooth Muscle Cells During Phosphate-Induced Calcification

Objective- Hydroxyapatite deposition on the medial layer of the aortic walls is the hallmark of vascular calcification and the most common complication in aging individuals and in patients with diabetes mellitus and those undergoing hemodialysis. Extracellular pyrophosphate is a potent physicochemical inhibitor of hydroxyapatite crystal formation. This study analyzed changes in extracellular pyrophosphate metabolism during the phosphate-induced calcification process. Approach and Results- Phosphate-induced calcification of ex vivo-cultured aortic rings resulted in calcium accumulation after 7 days. This accumulation was enhanced when aortic walls were devitalized. BMP2 (bone morphogenic protein 2) expression was associated with calcium accumulation in cultured aortic rings, as well as in cultured vascular smooth muscle cells (VSMCs) and in calcitriol-induced calcification in rats. Hydroxyapatite dose dependently induced BMP2 overexpression in VSMCs. Moreover, TNAP (tissue nonspecific alkaline phosphatase) mRNA levels and activity were found to be downregulated in early phases and upregulated in later phases of calcification in all 3 models studied. eNPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) increased from early to later phases of calcification, whereas eNTPD1 (ectonucleoside triphosphate diphosphohydrolase 1) was downregulated during later phases. Synthesis of pyrophosphate in VSMCs increased significantly over time, in all 3 models studied. Because the rate of pyrophosphate hydrolysis was 10× slower than the rate of pyrophosphate synthesis, pyrophosphate synthesis is determined mainly by the ratio of eNPP1 to eNTPD1 activity. Hydroxyapatite also induces increments both in TNAP and eNPP1/eNTPD1 ratio in VSMCs. Conclusions- Pyrophosphate synthesis increases in VSMCs during phosphate-induced calcification because of compensatory regulation of extracellular pyrophosphate metabolism.

The interplay between mineral metabolism, vascular calcification and inflammation in Chronic Kidney Disease (CKD): challenging old concepts with new facts

Chronic kidney disease (CKD) is one of the most powerful predictors of premature cardiovascular disease (CVD), with heightened susceptibility to vascular intimal and medial calcification associated with a high cardiovascular mortality. Abnormal mineral metabolism of calcium (Ca) and phosphate (P) and underlying (dys)regulated hormonal control in CKD-mineral and bone disorder (MBD) is often accompanied by bone loss and increased vascular calcification (VC). While VC is known to be a multifactorial process and a major risk factor for CVD, the view of primary triggers and molecular mechanisms complexity has been shifting with novel scientific knowledge over the last years. In this review we highlight the importance of calcium-phosphate (CaP) mineral crystals in VC with an integrated view over the complexity of CKD, while discuss past and recent literature aiming to highlight novel horizons on this major health burden. Exacerbated VC in CKD patients might result from several interconnected mechanisms involving abnormal mineral metabolism, dysregulation of endogenous calcification inhibitors and inflammatory pathways, which function in a feedback loop driving disease progression and cardiovascular outcomes. We propose that novel approaches targeting simultaneously VC and inflammation might represent valuable new prognostic tools and targets for therapeutics and management of cardiovascular risk in the CKD population.

Shape-dependent adhesion and endocytosis of hydroxyapatite nanoparticles on A7R5 aortic smooth muscle cells

The interaction between nanohydroxyapatite (HAP) and smooth muscle cells is an important step in vascular calcification. However, the effect of the shape of HAP on adhesion and endocytosis to aortic smooth muscle cells has been rarely reported. Four different morphological HAP crystals (H-Rod, H-Needle, H-Sphere, and H-Plate) were selected to interact with rat aortic smooth muscle cells (A7R5). Fluorescence-labeled HAP was used to detect crystal adhesion and endocytosis and then pretreated with different endocytic inhibitors to explore the pathway of endocytotic crystals. The distribution of crystals inside and outside the cells and the crystal localization in lysosomes was observed through laser confocal microscopy. The effect of crystal on the cell cycle and the changes in the expression of phosphatidylserine, osteopontin, α-actin, core binding factor alpha 1, and osterix on the surface of A7R5 cells were detected. The adhesion and endocytosis of HAP on A7R5 cells were closely related to crystal shapes and ranked as follows: H-Plate > H-Sphere > H-Needle > H-Rod. H-Sphere and H-Needle were internalized into the cells mainly via the clathrin-mediated pathway, whereas H-Plate and H-Rod were internalized into the cells mainly via macropinocytosis. The endocytosed nano-HAP was mainly distributed in the cell lysosome. The adhesion and endocytosis of HAP to A7R5 cells were positively correlated with the specific surface area, and contact area of HAP and negatively correlated with the absolute value of Zeta and contact angle of HAP. This study provided insights into the effect of crystal morphology on vascular calcification and its mechanism.

Signaling pathways involved in vascular smooth muscle cell calcification during hyperphosphatemia

Medial vascular calcification has emerged as a putative key factor contributing to the excessive cardiovascular mortality of patients with chronic kidney disease (CKD). Hyperphosphatemia is considered a decisive determinant of vascular calcification in CKD. A critical role in initiation and progression of vascular calcification during elevated phosphate conditions is attributed to vascular smooth muscle cells (VSMCs), which are able to change their phenotype into osteo-/chondroblasts-like cells. These transdifferentiated VSMCs actively promote calcification in the medial layer of the arteries by producing a local pro-calcifying environment as well as nidus sites for precipitation of calcium and phosphate and growth of calcium phosphate crystals. Elevated extracellular phosphate induces osteo-/chondrogenic transdifferentiation of VSMCs through complex intracellular signaling pathways, which are still incompletely understood. The present review addresses critical intracellular pathways controlling osteo-/chondrogenic transdifferentiation of VSMCs and, thus, vascular calcification during hyperphosphatemia. Elucidating these pathways holds a significant promise to open novel therapeutic opportunities counteracting the progression of vascular calcification in CKD.

Tissue engineered bone mimetics to study bone disorders ex vivo: Role of bioinspired materials

Recent advances in materials development and tissue engineering has resulted in a substantial number of bioinspired materials that recapitulate cardinal features of bone extracellular matrix (ECM) such as dynamic inorganic and organic environment(s), hierarchical organization, and topographical features. Bone mimicking materials, as defined by its self-explanatory term, are developed based on the current understandings of the natural bone ECM during development, remodeling, and fracture repair. Compared to conventional plastic cultures, biomaterials that resemble some aspects of the native environment could elicit a more natural molecular and cellular response relevant to the bone tissue. Although current bioinspired materials are mainly developed to assist tissue repair or engineer bone tissues, such materials could nevertheless be applied to model various skeletal diseases in vitro. This review summarizes the use of bioinspired materials for bone tissue engineering, and their potential to model diseases of bone development and remodeling ex vivo. We largely focus on biomaterials, designed to re-create different aspects of the chemical and physical cues of native bone ECM. Employing these bone-inspired materials and tissue engineered bone surrogates to study bone diseases has tremendous potential and will provide a closer portrayal of disease progression and maintenance, both at the cellular and tissue level. We also briefly touch upon the application of patient-derived stem cells and introduce emerging technologies such as organ-on-chip in disease modeling. Faithful recapitulation of disease pathologies will not only offer novel insights into diseases, but also lead to enabling technologies for drug discovery and new approaches for cell-based therapies.

Magnesium: A Magic Bullet for Cardiovascular Disease in Chronic Kidney Disease?

Magnesium is essential for many physiological functions in the human body. Its homeostasis involves dietary intake, absorption, uptake and release from bone, swifts between the intra- and extracellular compartment, and renal excretion. Renal excretion is mainly responsible for regulation of magnesium balance. In chronic kidney disease (CKD), for a long time the general policy has been limiting magnesium intake. However, this may not be appropriate for many patients. The reference ranges for magnesium are not necessarily optimal concentrations, and risks for insufficient magnesium intake exist in patients with CKD. In recent years, many observational studies have shown that higher (in the high range of “normal” or slightly above) magnesium concentrations are associated with better survival in CKD cohorts. This review gives an overview of epidemiological associations between magnesium and overall and cardiovascular survival in patients with CKD. In addition, potential mechanisms explaining the protective role of magnesium in clinical cardiovascular outcomes are described by reviewing evidence from in vitro studies, animal studies, and human intervention studies with non-clinical endpoints. This includes the role of magnesium in cardiac arrhythmia, heart failure, arterial calcification, and endothelial dysfunction. Possible future implications will be addressed, which will need prospective clinical trials with relevant clinical endpoints before these can be adopted in clinical practice.

Clinical implications of fetuin-A

Fetuin-A, also termed alpha2-Heremans-Schmid glycoprotein, is a 46kDa hepatocyte derived protein (hepatokine) and serves multifaceted functions.

Recent Advances on Relationship Between Inorganic Phosphate and Pathologic Calcification: Is Calcification After Breast Augmentation with Fat Grafting Correlated with Locally Increased Concentration of Inorganic Phosphate?

BACKGROUND

Pathologic calcification has frequently occurred after breast augmentation with fat grafting as well as other conditions such as breast cancer, trauma, myocardial infarction, arteriosclerosis and even after reduction mammoplasty. Inorganic phosphate, correlated with fat metabolism, is an important factor that induces pathologic calcification such as vascular calcification.

METHODS

A literature search was conducted using PubMed with the keywords: calcification, inorganic phosphate, fat. Studies related to the process of pathologic calcification, correlation between inorganic phosphate and pathologic calcification, between inorganic phosphate and fat metabolism in pathologic calcification were collected.

RESULTS

Various mechanisms were referred to in pathologic calcification among which inorganic phosphate played an important role. Inorganic phosphate could be liberated, under the effect of various enzymes, in the process of fat metabolism. The authors hypothesized that a large-scale necrotizing zone, which could occur in fat grafting with large amounts per cannula, might provide a high-phosphate environment which might contribute to differentiation of surrounding cells such as stem cells or regenerated vessel cells into osteoblast-like cells that induce pathologic calcification.

CONCLUSION

Inorganic phosphate, which was correlated with fat metabolism, played a significant role in pathologic calcification. We firstly hypothesize that calcification after fat grafting may be related to locally increasing concentrations of phosphate in a necrotizing zone. Further research should be conducted to verify this hypothesis.

LEVEL OF EVIDENCE V

This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Efficacy of linalool to ameliorate uremia induced vascular calcification in wistar rats

BACKGROUND

Uremia is the condition generally associated with the last stage of chronic kidney disease (CKD) due to highly reduced glomerular filtration rate. Mortality of the patients diagnosed with Uremia generally occurs due to cardiovascular involvement. This occurs due to the transdifferentiation of vascular smooth muscle cells (VSMCs) into osteogenic cells in hyperphosphatemic condition that is associated with kidney failure promoting extra-osseous calcification.

PURPOSE

Linalool is an essential oil that has been recently studied for its procardiovascular effects, thus the aim of the study involved to identify its potential role on vascular calcification (VC).

METHODS

Uremia was induced in male wistar rats, weighing 250-350 gm by giving adenine diet for 4 weeks followed by phosphate diet for next 4 weeks. Linalool was given orally at two different doses (100 mg/kg bodyweight and 150 mg/kg bodyweight) daily for 4 weeks with phosphate diet.

RESULTS

Linalool being a moderate antioxidant probably scavenged superoxide radicals (in vitro analysis). Deposition of calcium was observed by alizarin and von-kossa stains in aorta of uremic rats whereas linalool co-administration prevents calcium deposition in aorta of uremic rats. Elevated mRNA expression of calcification markers, increased lipid peroxidation levels and increased levels of catalase and superoxide dismutase (SOD) was found in aorta of uremic animals. However, with supplementation of linalool reduction in the mRNA expression of calcification markers, lipid peroxidation and antioxidant enzymes were observed.

CONCLUSION

Therefore it can be concluded that linalool could be a promising therapeutic candidate for exploring its clinical application in VC.

Control of metabolic predisposition to cardiovascular complications of chronic kidney disease by effervescent calcium magnesium citrate: a feasibility study

Aims

Cardiovascular (CV) complications are common in chronic kidney disease (CKD). Numerous metabolic disturbances including hyperphosphatemia, high circulating calciprotein particles (CPP), hyperparathyroidism, metabolic acidosis, and magnesium deficiency are associated with, and likely pathogenic for CV complications in CKD. The goal of this feasibility study was to determine whether effervescent calcium magnesium citrate (EffCaMgCit) ameliorates the aforementioned pathogenic intermediates.

Methods

Nine patients with Stage 3 and nine patients with Stage 5D CKD underwent a randomized crossover study, where they took EffCaMgCit three times daily for 7 days in one phase, and a conventional phosphorus binder calcium acetate (CaAc) three times daily for 7 days in the other phase. Two-hour postprandial blood samples were obtained on the day before and on the 7th day of treatment.

Results

In Stage 5D CKD, EffCaMgCit significantly increased T50 (half time for conversion of primary to secondary CPP) from baseline by 63% (P = 0.013), coincident with statistically non-significant declines in serum phosphorus by 25% and in saturation of octacalcium phosphate by 35%; CaAc did not change T50. In Stage 3 CKD, neither EffCaMgCit nor CaAc altered T50. With EffCaMgCit, a significant increase in plasma citrate was accompanied by statistically non-significant increase in serum Mg and phosphate. CaAc was without effect in any of these parameters in Stage 3 CKD. In both Stages 3 and 5D, both drugs significantly reduced serum parathyroid hormone. Only EffCaMgCit significantly increased serum bicarbonate by 3 mM (P = 0.015) in Stage 5D.

Conclusions

In Stage 5D, EffCaMgCit inhibited formation of CPP, suppressed PTH, and conferred magnesium and alkali loads. These effects were unique, since they were not observed with CaAc. In Stage 3 CKD, neither of the regimens have any effect. These metabolic changes suggest that EffCaMgCit might be useful in protecting against cardiovascular complications of CKD by ameliorating pathobiologic intermediates.

Molecular Mechanisms Underlying Accelerated Aging by Defects in the FGF23-Klotho System

The basic research of aging has been primarily focused on elucidating mechanisms of aging and longevity that are evolutionarily conserved from yeasts to primates. Such efforts have culminated in the notion that (1) senescence at the cellular level is associated with aging at the organismal level and that (2) calorie restriction and growth suppression decelerate aging. However, these important findings in the basic research have not necessarily been linked to improvement of daily medical practice in the aging society. It has become increasingly important to investigate mechanisms of aging unique to mammals or humans and apply the research fruits for the treatment of major age-related disorders to extend the health span. Seminal studies on the klotho mouse, a mutant exhibiting a premature aging syndrome, have identified phosphate as a proaging factor in mammals. In this review, mechanisms of phosphate-induced premature aging and potential therapeutic targets will be discussed, which may be directly applicable for developing novel strategies for the treatment of chronic kidney disease and its complications.

Biochemical transformation of calciprotein particles in uraemia

Calciprotein particles (CPP) have emerged as nanoscale mediators of phosphate-induced toxicity in Chronic Kidney Disease (CKD). Uraemia favors ripening of the particle mineral content from the amorphous (CPP-I) to the crystalline state (CPP-II) but the pathophysiological significance of this transformation is uncertain. Clinical studies suggest an association between CPP ripening and inflammation, vascular dysfunction and mortality. Although ripening has been modelled in vitro, it is unknown whether particles synthesised in serum resemble their in vivo counterparts. Here we show that in vitro formation and ripening of CPP in uraemic serum is characterised by extensive physiochemical rearrangements involving the accretion of mineral, loss of surface charge and transformation of the mineral phase from a spherical arrangement of diffuse domains of amorphous calcium phosphate to densely-packed lamellar aggregates of crystalline hydroxyapatite. These physiochemical changes were paralleled by enrichment with small soluble apolipoproteins, complement factors and the binding of fatty acids. In comparison, endogenous CPP represent a highly heterogeneous mixture of particles with characteristics mostly intermediate to synthetic CPP-I and CPP-II, but are also uniquely enriched for carbonate-substituted apatite, DNA fragments, small RNA and microbe-derived components. Pathway analysis of protein enrichment predicted the activation of cell death and pro-inflammatory processes by endogenous CPP and synthetic CPP-II alike. This comprehensive characterisation validates the use of CPP-II generated in uraemic serum as in vitro equivalents of their endogenous counterparts and provides insight into the nature and pathological significance of CPP in CKD, which may act as vehicles for various bioactive ligands.

Magnesium as a Calcification Inhibitor

Vascular calcification (VC) is associated with elevated cardiovascular mortality rates in patients with CKD. Recent clinical studies of patients with advanced CKD have observed an association between low serum magnesium (Mg) levels on one hand and elevated VC and cardiovascular mortality on the other. These findings have stimulated interest in understanding Mg's impact on CKD in general and the associated VC in particular. In vitro and preclinical in vivo data indicate that Mg has the potential to protect vascular smooth muscle cells against calcification via several different molecular mechanisms. Accordingly, data from pilot interventional studies in the clinic suggest that oral Mg supplementation reduces VC in patients with CKD. The present review provides an overview of our current understanding of the impact of Mg on the development of VC in patients with CKD.

Prevention and treatment of hyperphosphatemia in chronic kidney disease

Hyperphosphatemia has consistently been shown to be associated with dismal outcome in a wide variety of populations, particularly in chronic kidney disease (CKD). Compelling evidence from basic and animal studies elucidated a range of mechanisms by which phosphate may exert its pathological effects and motivated interventions to treat hyperphosphatemia. These interventions consisted of dietary modifications and phosphate binders. However, the beneficial effects of these treatment methods on hard clinical outcomes have not been convincingly demonstrated in prospective clinical trials. In addition, exposure to high amounts of dietary phosphate may exert untoward actions even in the absence of overt hyperphosphatemia. Based on this concept, it has been proposed that the same interventions used in CKD patients with normal phosphate concentrations be used in the presence of hyperphosphatemia to prevent rise of phosphate concentration and as an early intervention for cardiovascular risk. This review describes conceptual models of phosphate toxicity, summarizes the evidence base for treatment and prevention of hyperphosphatemia, and identifies important knowledge gaps in the field.

Phosphate stimulates myotube atrophy through autophagy activation: evidence of hyperphosphatemia contributing to skeletal muscle wasting in chronic kidney disease

Background

Accelerated muscle atrophy is associated with a three-fold increase in mortality in chronic kidney disease (CKD) patients. It is suggested that hyperphosphatemia might contribute to muscle wasting, but the underlying mechanisms remain unclear. Although evidence indicates that autophagy is involved in the maintenance of muscle homeostasis, it is not known if high phosphate levels can result in activation of autophagy, leading to muscle protein loss.

Methods

Immortalized rat L6 myotubes were exposed to a high concentration of phosphate, with or without autophagy inhibition. Myotube atrophy was examined by phase contrast microscopy. Autophagic activity was assessed by measuring the expression of microtubule-associated protein 1 light chain 3 (LC3) and p62 using quantitative real-time polymerase chain reaction and western blot.

Results

Phosphate induced cell atrophy in L6 myotubes in a dose- and time-dependent manner, and these responses were not associated with calcification or osteogenesis. Phosphate also dose- and time-dependently increased the LC3-II/LC3-I ratio. Inhibition of autophagy with wortmannin or knockdown of Atg5 significantly suppressed myotube atrophy caused by high phosphate concentration.

Conclusions

High phosphate concentration induces muscle cell atrophy through the activation of autophagy. Targeting autophagy could be a therapeutic strategy for preventing muscle wasting caused by hyperphosphatemia.

Magnesium prevents vascular calcification in vitro by inhibition of hydroxyapatite crystal formation

Magnesium has been shown to effectively prevent vascular calcification associated with chronic kidney disease. Magnesium has been hypothesized to prevent the upregulation of osteoblastic genes that potentially drives calcification. However, extracellular effects of magnesium on hydroxyapatite formation are largely neglected. This study investigated the effects of magnesium on intracellular changes associated with transdifferentiation and extracellular crystal formation. Bovine vascular smooth muscle cells were calcified using β-glycerophosphate. Transcriptional analysis, alkaline phosphatase activity and detection of apoptosis were used to identify transdifferentiation. Using X-ray diffraction and energy dispersive spectroscopy extracellular crystal composition was investigated. Magnesium prevented calcification in vascular smooth muscle cells. β-glycerophosphate increased expression of osteopontin but no other genes related to calcification. Alkaline phosphatase activity was stable and apoptosis was only detected after calcification independent of magnesium. Blocking of the magnesium channel TRPM7 using 2-APB did not abrogate the protective effects of magnesium. Magnesium prevented the formation of hydroxyapatite, which formed extensively during β-glycerophosphate treatment. Magnesium reduced calcium and phosphate fractions of 68% and 41% extracellular crystals, respectively, without affecting the fraction of magnesium. This study demonstrates that magnesium inhibits hydroxyapatite formation in the extracellular space, thereby preventing calcification of vascular smooth muscle cells.

Novel treatment strategies for chronic kidney disease: insights from the animal kingdom

Many of the >2 million animal species that inhabit Earth have developed survival mechanisms that aid in the prevention of obesity, kidney disease, starvation, dehydration and vascular ageing; however, some animals remain susceptible to these complications. Domestic and captive wild felids, for example, show susceptibility to chronic kidney disease (CKD), potentially linked to the high protein intake of these animals. By contrast, naked mole rats are a model of longevity and are protected from extreme environmental conditions through mechanisms that provide resistance to oxidative stress. Biomimetic studies suggest that the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2) offers protection in extreme environmental conditions and promotes longevity in the animal kingdom. Similarly, during months of fasting, immobilization and anuria, hibernating bears are protected from muscle wasting, azotaemia, thrombotic complications, organ damage and osteoporosis - features that are often associated with CKD. Improved understanding of the susceptibility and protective mechanisms of these animals and others could provide insights into novel strategies to prevent and treat several human diseases, such as CKD and ageing-associated complications. An integrated collaboration between nephrologists and experts from other fields, such as veterinarians, zoologists, biologists, anthropologists and ecologists, could introduce a novel approach for improving human health and help nephrologists to find novel treatment strategies for CKD.

Microcalcifications, Their Genesis, Growth, and Biomechanical Stability in Fibrous Cap Rupture

For many decades, cardiovascular calcification has been considered as a passive process, accompanying atheroma progression, correlated with plaque burden, and apparently without a major role on plaque vulnerability. Clinical and pathological analyses have previously focused on the total amount of calcification (calcified area in a whole atheroma cross section) and whether more calcification means higher risk of plaque rupture or not. However, this paradigm has been changing in the last decade or so. Recent research has focused on the presence of microcalcifications (μCalcs) in the atheroma and more importantly on whether clusters of μCalcs are located in the cap of the atheroma. While the vast majority of μCalcs are found in the lipid pool or necrotic core, they are inconsequential to vulnerable plaque. Nevertheless, it has been shown that μCalcs located within the fibrous cap could be numerous and that they behave as an intensifier of the background circumferential stress in the cap. It is now known that such intensifying effect depends on the size and shape of the μCalc as well as the proximity between two or more μCalcs. If μCalcs are located in caps with very low background stress, the increase in stress concentration may not be sufficient to reach the rupture threshold. However, the presence of μCalc(s) in the cap with a background stress of about one fifth to one half the rupture threshold (a stable plaque) will produce a significant increase in local stress, which may exceed the cap rupture threshold and thus transform a non-vulnerable plaque into a vulnerable one. Also, the classic view that treats cardiovascular calcification as a passive process has been challenged, and emerging data suggest that cardiovascular calcification may encompass both passive and active processes. The passive calcification process comprises biochemical factors, specifically circulating nucleating complexes, which would lead to calcification of the atheroma. The active mechanism of atherosclerotic calcification is a cell-mediated process via cell death of macrophages and smooth muscle cells (SMCs) and/or the release of matrix vesicles by SMCs.

The Role of Secondary Calciprotein Particles in the Mineralisation Paradox of Chronic Kidney Disease

Mineralisation paradox is prevalent in chronic kidney disease and ageing where increased vascular calcification is accompanied by reduced bone mineralisation and osteopenia. Secondary calciprotein particles (CPP2), colloidal nanoparticles containing hydroxyapatite crystal stabilised by a protein shell, have been implicated in vascular calcification in chronic kidney disease. Here, we describe the effect of CPP2 on osteoblasts and vascular smooth muscle cells (VSMC) mineralisation in an in vitro model system. The mineralisation paradox can be simulated in vitro by the addition of phosphate ions (Pi, 3 mM) and CPP2 (10 µg/ml of Ca equivalent). Pi alone induced osteoblast mineralisation but had no effect on VSMC mineralisation. CPP2 alone had no effect on mineralisation in either cell line, but when combined with elevated Pi, reduced osteoblast-like mineralisation (P < 0.001) whilst induced VSMC mineralisation (P < 0.001). These results suggest that in an in vitro system the synergistic interaction between Pi and CPP2 could mimic the mineralisation paradox, and may provide a potential mechanistic link to explain these clinical observations.

High phosphate-induced downregulation of PPARγ contributes to CKD-associated vascular calcification

Medial arterial calcification associated with hyperphosphatemia is a main cause of cardiovascular mortality in patients with chronic kidney disease (CKD), but the mechanisms underlying high phosphate-induced vascular calcification remain largely unknown. Here, we observed a significant decrease in the expression of peroxisome proliferator-activated receptor-gamma (PPARγ) in calcified arteries both in CKD patients and in a mouse model of CKD with hyperphosphatemia. In vitro, high phosphate treatment led to a decreased expression of PPARγ in mouse vascular smooth muscle cells (VMSCs), accompanied by apparent osteogenic differentiation and calcification. Pretreatment with PPARγ agonist rosiglitazone significantly reversed high phosphate-induced VSMCs calcification. Further investigation showed that methyl-CpG binding protein 2 (Mecp2)-mediated epigenetic repression was involved in high phosphate-induced PPARγ downregulation. Moreover, the expression of Klotho that has the ability to inhibit vascular calcification by regulating phosphate uptake decreased with the PPARγ reduction in VSMCs after high phosphate treatment, and rosiglitazone failed to inhibit high phosphate-induced calcification in VSMCs with knockdown of Klotho or in aortic rings from Klotho-deficient (kl/kl) mice. Finally, an in vivo study demonstrated that oral administration of rosiglitazone could increase Klotho expression and protect against high phosphate-induced vascular calcification in CKD mice. These findings suggest that the inhibition of PPARγ expression may contribute to the pathogenesis of high phosphate-induced vascular calcification, which may provide a new therapeutic target for vascular calcification in CKD patients.

Diethyl citrate and sodium citrate reduce the cytotoxic effects of nanosized hydroxyapatite crystals on mouse vascular smooth muscle cells

OBJECTIVE

This study aimed to investigate the damage mechanism of nanosized hydroxyapatite (nano-HAp) on mouse aortic smooth muscle cells (MOVASs) and the injury-inhibiting effects of diethyl citrate (Et2Cit) and sodium citrate (Na3Cit) to develop new drugs that can simultaneously induce anticoagulation and inhibit vascular calcification.

METHODS

The change in cell viability was evaluated using a cell proliferation assay kit, and the amount of lactate dehydrogenase (LDH) released was measured using an LDH kit. Intracellular reactive oxygen species (ROS) and mitochondrial damage were detected by DCFH-DA staining and JC-1 staining. Cell apoptosis and necrosis were detected by Annexin V staining. Intracellular calcium concentration and lysosomal integrity were measured using Fluo-4/AM and acridine orange, respectively.

RESULTS

Nano-HAp decreased cell viability and damaged the cell membrane, resulting in the release of a large amount of LDH. Nano-HAp entered the cells and damaged the mitochondria, and then induced cell apoptosis by producing a large amount of ROS. In addition, nano-HAp increased the intracellular Ca2+ concentration, leading to lysosomal rupture and cell necrosis. On addition of the anticoagulant Et2Cit or Na3Cit, cell viability and mitochondrial membrane potential increased, whereas the amount of LDH released, ROS, and apoptosis rate decreased. Et2 Cit and Na3Cit could also chelate with Ca+ to inhibit the intracellular Ca2+ elevations induced by nano-HAp, prevent lysosomal rupture, and reduce cell necrosis. High concentrations of Et2Cit and Na3Cit exhibited strong inhibitory effects. The inhibitory capacity of Na3Cit was stronger than that of Et2Cit at similar concentrations.

CONCLUSION

Both Et2Cit and Na3Cit significantly reduced the cytotoxicity of nano-HAp on MOVASs and inhibited the apoptosis and necrosis induced by nano-HAp crystals. The chelating function of citrate resulted in both anticoagulation and binding to HAp. Et2Cit and Na3Cit may play a role as anticoagulants in reducing injury to the vascular wall caused by nano-HAp.

Nano-Hydroxyapatite Stimulation of Gene Expression Requires Fgf Receptor, Phosphate Transporter, and Erk1/2 Signaling

Hydroxyapatite (HAp) is critical to health both as the main structural material of the skeleton and storage material of calcium and phosphate. Nanosized HAp (nHAp) is naturally produced by mineralizing cells during bone formation and remodeling and is the main constituent of the skeleton. As such, HAp is currently being investigated as a therapeutic biomaterial for orthopedic and dental purposes. Recent studies have suggested that extracellular nHAp can influence osteoblast lineage commitment and cell function through changes in gene expression; however, the mechanisms remain to be elucidated. Here, the cellular and molecular mechanism by which rod-shaped nHAp (10 × 100 nm) stimulates gene expression in preosteoblast bone marrow stromal cells was investigated. Electron microscopy detected a rapid and stable interaction of nHAp with the cell membrane, which correlated with a strong stimulation of the Erk1/2 signaling pathway. Results also identified the requirement of the Fgf receptor signaling and phosphate-transporters for nHAp regulated gene expression whereas a calcium-sensing receptor inhibitor had no effect. Collectively, the study uncovers novel signaling pathways and cellular events specifically stimulated by and required for the cellular response to free extracellular HAp. The results provide insight into the osteoblastic response to HAp relevant to functional mineralization and pathological calcification and could be used in the development of biomaterials for orthopedic purposes.

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

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

Hydrogen sulfide attenuates calcification of vascular smooth muscle cells via KEAP1/NRF2/NQO1 activation

BACKGROUND AND AIMS

Vascular calcification is a common health problem related to oxidative stress, inflammation, and circulating calciprotein particles (CPP). Hydrogen sulfide is an endogenous signaling molecule with antioxidant properties and potential for drug development targeting redox signaling. Yet, its molecular mechanisms of action in vascular smooth muscle cell (VSMC) calcification have not been delineated. We therefore sought to identify key pathways involved in the calcification-inhibitory properties of sulfide employing our recently developed CPP-induced VSMC calcification model.

METHODS

Using next-generation sequencing, we investigated the transcriptomic changes of sodium hydrosulfide-treated versus non-treated calcifying VSMCs. The potential role of candidate genes and/or regulatory pathways in prevention of calcification was investigated by small interfering RNA (siRNA).

RESULTS

CPP led to a pronounced accumulation of cell-associated calcium, which was decreased by sulfide in a concentration-dependent manner. Both, CPP-induced hydrogen peroxide production and enhanced pro-inflammatory/oxidative stress-related gene expression signatures were attenuated by sulfide-treatment. Gene ontology enrichment and in silico pathway analysis of our transcriptome data suggested NAD(P)H dehydrogenase [quinone] 1 (NQO1) as potential mediator. Corroborating these findings, silencing of Kelch-like ECH-associated protein 1 (KEAP1), an inhibitor of nuclear factor (erythroid-derived 2)-like 2 (NRF2) nuclear activity, enhanced NQO1 expression, whereas NRF2 silencing reduced the expression of NQO1 and abrogated the calcification-suppressing activity of sulfide. Moreover, immunofluorescence microscopy and Western blot analysis confirmed nuclear translocation of NRF2 by sulfide in VSMC.

CONCLUSIONS

Sulfide attenuates CPP-induced VSMC calcification in vitro via the KEAP1-NRF2 redox sensing/stress response system by enhancing NQO1 expression.

A novel fluorescent probe-based flow cytometric assay for mineral-containing nanoparticles in serum

Calciprotein particles, nanoscale aggregates of insoluble mineral and binding proteins, have emerged as potential mediators of phosphate toxicity in patients with Chronic Kidney Disease. Although existing immunochemical methods for their detection have provided compelling data, these approaches are indirect, lack specificity and are subject to a number of other technical and theoretical shortcomings. Here we have developed a rapid homogeneous fluorescent probe-based flow cytometric method for the detection and quantitation of individual mineral-containing nanoparticles in human and animal serum. This method allows the discrimination of membrane-bound from membrane-free particles and different mineral phases (amorphous vs. crystalline). Critically, the method has been optimised for use on a conventional instrument, without the need for manual hardware adjustments. Using this method, we demonstrate a consistency in findings across studies of Chronic Kidney Disease patients and commonly used uraemic animal models. These studies demonstrate that renal dysfunction is associated with the ripening of calciprotein particles to the crystalline state and reveal bone metabolism and dietary mineral as important modulators of circulating levels. Flow cytometric analysis of calciprotein particles may enhance our understanding of mineral handling in kidney disease and provide a novel indicator of therapeutic efficacy for interventions targeting Chronic Kidney Disease-Mineral Bone Disorder.

Hydroxyapatite nanocrystals stimulate osteogenic differentiation in primary human aortic smooth muscle cells by activation of oxidative stress and the ERK pathway

BACKGROUND

Hydroxyapatite nanocrystal (HN) deposition underlies the development of vascular calcification, which is an actively regulated process resembling bone formation. This study investigated the role of HNs in inducing osteogenic differentiation of primary human aortic smooth muscle cells (HASMCs).

METHODS

Primary HASMCs were incubated with HNs, cell osteogenic differentiation was evaluated by von kossa staining and calcium content. The expressions of SM-α-actin and bone markers, including runt-related transcription factor 2 (Runx2), osteopontin (OPN), osterix, and collagen 1 (COL1) were also determined. Antioxidants, ERK-specific inhibitor were used to examine whether oxidative stress and the ERK pathway were required for this transition.

RESULTS

Stimulation of HASMCs with HNs increased calcium deposition, expression of bone markers and decreased SM-α-actin expression. HNs produced reactive oxygen species (ROS) in HASMCs, as evaluated by fluorescent probe. Antioxidants inhibited HN-induced osteogenic differentiation. Furthermore, the inhibitor of the ERK pathway, PD98059, suppressed the effect of HNs on bone marker expression.

CONCLUSIONS

These findings suggest that HNs stimulated osteogenic differentiation of vascular smooth muscle cells that build biomineralized deposits partly by activating oxidative stress and the ERK pathway.

TNAP stimulates vascular smooth muscle cell trans-differentiation into chondrocytes through calcium deposition and BMP-2 activation: Possible implication in atherosclerotic plaque stability

Atherosclerotic plaque calcification varies from early, diffuse microcalcifications to a bone-like tissue formed by endochondral ossification. Recently, a paradigm has emerged suggesting that if the bone metaplasia stabilizes the plaques, microcalcifications are harmful. Tissue-nonspecific alkaline phosphatase (TNAP), an ectoenzyme necessary for mineralization by its ability to hydrolyze inorganic pyrophosphate (PP_i), is stimulated by inflammation in vascular smooth muscle cells (VSMCs). Our objective was to determine the role of TNAP in trans-differentiation of VSMCs and calcification. In rodent MOVAS and A7R5 VSMCs, addition of exogenous alkaline phosphatase (AP) or TNAP overexpression was sufficient to stimulate the expression of several chondrocyte markers and induce mineralization. Addition of exogenous AP to human mesenchymal stem cells cultured in pellets also stimulated chondrogenesis. Moreover, TNAP inhibition with levamisole in mouse primary chondrocytes dropped mineralization as well as the expression of chondrocyte markers. VSMCs trans-differentiated into chondrocyte-like cells, as well as primary chondrocytes, used TNAP to hydrolyze PP_i, and PP_i provoked the same effects as TNAP inhibition in primary chondrocytes. Interestingly, apatite crystals, associated or not to collagen, mimicked the effects of TNAP on VSMC trans-differentiation. AP and apatite crystals increased the expression of BMP-2 in VSMCs, and TNAP inhibition reduced BMP-2 levels in chondrocytes. Finally, the BMP-2 inhibitor noggin blocked the rise in aggrecan induced by AP in VSMCs, suggesting that TNAP induction in VSMCs triggers calcification, which stimulates chondrogenesis through BMP-2. Endochondral ossification in atherosclerotic plaques may therefore be induced by crystals, probably to confer stability to plaques with microcalcifications.

The FGF23 and Klotho system beyond mineral metabolism

FGF23 is a bone-derived hormone that acts primarily on the kidney to induce phosphaturia and suppress synthesis of 1,25-dihydroxyvitamin D₃. The unique feature of FGF23 is that it requires Klotho as an obligate co-receptor. The FGF23-Klotho system has emerged as an endocrine axis indispensable for maintaining phosphate homeostasis. Mineral and bone disorders associated with chronic kidney disease (CKD-MBD) can be viewed as a series of events triggered by a compensatory response of the FGF23-Klotho system to excess phosphate intake relative to the residual nephron number. Furthermore, the fact that disruption of the FGF23-Klotho system causes phosphate retention and a syndrome resembling aging in mammals has led to the notion that phosphate accelerates aging. The aging-like pathology caused by phosphate, or phosphatopathy, may be unique to the higher organisms having the Klotho gene and provides new insights into the molecular mechanism of aging in humans.

Calcium Phosphate Crystals from Uremic Serum Promote Osteogenic Differentiation in Human Aortic Smooth Muscle Cells

Recent study demonstrated that calcium phosphate (CaP) crystals isolated from high phosphate medium were a key contributor to arterial calcification. The present study further investigated the effects of CaP crystals induced by uremic serum on calcification of human aortic smooth muscle cells. This may provide a new insight for the development of uremic cardiovascular calcification. We tested the effects of uremic serum or normal serum on cell calcification. Calcification was visualized by staining and calcium deposition quantified. Expression of various bone-calcifying genes was detected by real-time PCR, and protein levels were quantified by western blotting or enzyme-linked immunosorbent assays. Pyrophosphate was used to investigate the effects of CaP crystals' inhibition. Finally, CaP crystals were separated from uremic serum to determine its specific pro-calcification effects. Uremic serum incubation resulted in progressively increased calcification staining and increased calcium deposition in HASMCs after 4, 8 and 12 days (P vs 0 day <0.001 for all). Compared to cells incubated in control serum, uremic serum significantly induced the mRNA expression of bone morphogenetic factor-2, osteopontin and RUNX2, and increased their protein levels as well (P < 0.05 for all). Inhibition of CaP crystals with pyrophosphate incubation prevented calcium deposition and bone-calcifying gene over-expression increased by uremic serum. CaP crystals, rather than the rest of uremic serum, were responsible for these effects. Uremic serum accelerates arterial calcification by mediating osteogenic differentiation. This effect might be mainly attributed to the CaP crystal content.

WNT/β-catenin signaling promotes VSMCs to osteogenic transdifferentiation and calcification through directly modulating Runx2 gene expression

Arterial medial calcification (AMC) is prevalent in patients with chronic kidney disease (CKD) and contributes to elevated risk of cardiovascular events and mortality. Vascular smooth muscle cells (VSMCs) to osteogenic transdifferentiation (VOT) in a high-phosphate environment is involved in the pathogenesis of AMC in CKD. WNT/β-catenin signaling is indicated to play a crucial role in osteogenesis via promoting Runx2 expression in osteoprogenitor cells, however, its role in Runx2 regulation and VOT remains incompletely clarified. In this study, Runx2 was induced and β-catenin was activated by high-phosphate in VSMCs. Two forms of active β-catenin, dephosphorylated on Ser37/Thr41 and phosphorylated on Ser675 sites, were upregulated by high-phosphate. Activation of β-catenin, through ectopic expression of stabilized β-catenin, inhibition of GSK-3β, or WNT-3A protein, induced Runx2 expression, whereas blockade of WNT/β-catenin signaling with Porcupine (PORCN) inhibitor or Dickkopf-1 (DKK1) protein inhibited Runx2 induction by high-phosphate. WNT-3A promoted osteocalcin expression and calcium deposition in VSMCs, whereas DKK1 ameliorated calcification of VSMCs induced by high-phosphate. Two functional T cell factor (TCF)/lymphoid enhancer-binding factor binding sites were identified in the promoter region of Runx2 gene in VSMCs, which interacted with TCF upon β-catenin activation. Site-directed mutation of each of them attenuated Runx2 response to β-catenin, and deletion or destruction of both of them completely abolished this responsiveness. In the aortic tunica media of rats with chronic renal failure, followed by AMC, Runx2 and β-catenin was induced, and the Runx2 mRNA level was positively associated with the abundance of phosphorylated β-catenin (Ser675). Collectively, our study suggested that high-phosphate may activate WNT/β-catenin signaling through different pathways, and the activated WNT/β-catenin signaling, through direct downstream target Runx2, could play an important role in promoting VOT and AMC.

Phosphate-a poison for humans?

Maintenance of phosphate balance is essential for life, and mammals have developed a sophisticated system to regulate phosphate homeostasis over the course of evolution. However, due to the dependence of phosphate elimination on the kidney, humans with decreased kidney function are likely to be in a positive phosphate balance. Phosphate excess has been well recognized as a critical factor in the pathogenesis of mineral and bone disorders associated with chronic kidney disease, but recent investigations have also uncovered toxic effects of phosphate on the cardiovascular system and the aging process. Compelling evidence also suggests that increased fibroblastic growth factor 23 and parathyroid hormone levels in response to a positive phosphate balance contribute to adverse clinical outcomes. These insights support the current practice of managing serum phosphate in patients with advanced chronic kidney disease, although definitive evidence of these effects is lacking. Given the potential toxicity of excess phosphate, the general population may also be viewed as a target for phosphate management. However, the widespread implementation of dietary phosphate intervention in the general population may not be warranted due to the limited impact of increased phosphate intake on mineral metabolism and clinical outcomes. Nonetheless, the increasing incidence of kidney disease or injury in our aging society emphasizes the potential importance of this issue. Further work is needed to more completely characterize phosphate toxicity and to establish the optimal therapeutic strategy for managing phosphate in patients with chronic kidney disease and in the general population.

Where do we stand on vascular calcification?

Vascular disease, such as atherosclerosis and diabetic vasculopathy, is frequently complicated by vascular calcification. Previously believed to be an end-stage process of unregulated mineral precipitation, it is now well established to be a multi-faceted disease influenced by the characteristics of its vascular location, the origins of calcifying cells and numerous regulatory pathways. It reflects the fundamental plasticity of the vasculature that is gradually being revealed by progress in vascular and stem cell biology. This review provides a brief overview of where we stand in our understanding of vascular calcification, facing the challenge of translating this knowledge into viable preventive and therapeutic strategies.

Calcification of vascular smooth muscle cells is induced by secondary calciprotein particles and enhanced by tumor necrosis factor-α

BACKGROUND AND AIMS

Vascular calcification is prevalent in clinical states characterized by low-grade chronic inflammation, such as chronic kidney disease (CKD). Calciprotein particles (CPP) are calcium phosphate-containing nano-aggregates, which have been found in the blood of CKD patients and appear pro-inflammatory in vitro. The interplay of CPPs and inflammatory cytokines with regard to the calcification of vascular smooth muscle cells (VSMC), in vitro, has not been investigated yet.

METHODS

Primary or secondary CPP were generated using phosphate-enriched culture medium (DMEM/10% FBS) incubated at 37 °C. Human VSMC were cultured with these media and mineralization was measured. Expression of TNF-α was detected by qPCR, ELISA and Western blot in calcified VSMC. To further characterize the significance of TNF-α and its receptors for the calcification of VSMC, RNA interference experiments using siTNF-α, siTNFR1 and siTNFR2 were performed.

RESULTS

The addition of phosphate to cell culture medium containing DMEM/10% FBS led to the rapid formation of primary CPP, which underwent spontaneous transformation to secondary CPP. Exposure of VSMC towards secondary CPP led to pronounced and concentration-dependent calcification, whereas exposure towards primary CPP did not. Importantly, secondary CPP induced oxidative stress, and led to the up-regulation and release of TNF-α. Addition of TNF-α to the cell culture medium enhanced, whereas the suppression of endogenous TNF-α or TNF receptor type 1 (TNFR1) expression by siRNA, ameliorated calcification.

CONCLUSIONS

Secondary, but not primary CPP, induce VSMC calcification. Secondary CPP induce the expression and release of TNF-α, which enhances calcification via its receptor TNFR1.

Fetuin-A-containing calciprotein particles in mineral trafficking and vascular disease

Calcium and phosphate combine to form insoluble precipitates in both inorganic and organic materials. This property is useful biologically and has been used by numerous organisms to create hard tissues, a process referred to as biomineralisation [1]. In humans, calcium and phosphate combine to form useful crystal structures largely composed of calcium hydroxyapatite [Ca10(PO4)6(OH)2] and these are essential in the growth, maintenance and strength of parts of the skeleton and other structures like teeth. However, it remains unclear how the body achieves the exquisite specificity involved in biomineralisation. In ageing and disease, these pathways are perturbed, resulting in ectopic calcium crystal deposition impairing tissue function and, interestingly, frequently accompanied by simultaneous loss of mineral from sites where it is useful (e.g. bone). One paradigm for this maladaptive situation is renal failure; a situation that we know is associated with vascular stiffening and calcification, along with mineral loss from the skeleton. Mineral trafficking is a loose term used to describe the movements of calcium salts around the body, and new insights into these pathways may explain some of the problems of previous models of bone mineral disease in renal failure and point to potential future therapeutic strategies.

Emerging roles for vascular smooth muscle cell exosomes in calcification and coagulation

Vascular smooth muscle cell (VSMC) phenotypic conversion from a contractile to 'synthetic' state contributes to vascular pathologies including restenosis, atherosclerosis and vascular calcification. We have recently found that the secretion of exosomes is a feature of 'synthetic' VSMCs and that exosomes are novel players in vascular repair processes as well as pathological vascular thrombosis and calcification. Pro-inflammatory cytokines and growth factors as well as mineral imbalance stimulate exosome secretion by VSMCs, most likely by the activation of sphingomyelin phosphodiesterase 3 (SMPD3) and cytoskeletal remodelling. Calcium stress induces dramatic changes in VSMC exosome composition and accumulation of phosphatidylserine (PS), annexin A6 and matrix metalloproteinase-2, which converts exosomes into a nidus for calcification. In addition, by presenting PS, VSMC exosomes can also provide the catalytic surface for the activation of coagulation factors. Recent data showing that VSMC exosomes are loaded with proteins and miRNA regulating cell adhesion and migration highlight VSMC exosomes as potentially important communication messengers in vascular repair. Thus, the identification of signalling pathways regulating VSMC exosome secretion, including activation of SMPD3 and cytoskeletal rearrangements, opens up novel avenues for a deeper understanding of vascular remodelling processes.