Mechanism of phosphate-induced calcification in rat aortic tissue culture: possible involvement of Pit-1 and apoptosis

Early diagnosis of aortic calcification through dental X-ray examination for dental pulp stones

Vascular calcification, an ectopic calcification exacerbated by aging and renal dysfunction, is closely associated with cardiovascular disease. However, early detection indicators are limited. This study focused on dental pulp stones, ectopic calcifications found in oral tissues that are easily identifiable on dental radiographs. Our investigation explored the frequency and timing of these calcifications in different locations and their relationship to aortic calcification. In cadavers, we examined the association between the frequency of dental pulp stones and aortic calcification, revealing a significant association. Notably, dental pulp stones appeared prior to aortic calcification. Using a rat model of hyperphosphatemia, we confirmed that dental pulp stones formed earlier than calcification in the aortic arch. Interestingly, there were very few instances of aortic calcification without dental pulp stones. Additionally, we conducted cell culture experiments with vascular smooth muscle cells (SMCs) and dental pulp cells (DPCs) to explore the regulatory mechanism underlying high phosphate-mediated calcification. We found that DPCs produced calcification deposits more rapidly and exhibited a stronger augmentation of osteoblast differentiation markers compared with SMCs. In conclusion, the observation of dental pulp stones through X-ray examination during dental checkups could be a valuable method for early diagnosis of aortic calcification risk.

Impact of Transferrin Saturation and Anemia on Radial Artery Calcification in Patients with End-Stage Kidney Disease

Background: Arterial calcification is an important factor in determining the prognosis of patients with chronic kidney disease (CKD). Few studies on aortic calcification have involved radial artery calcification (RAC). This study aimed to analyze risk factors for RAC in patients with end-stage kidney disease (ESKD) and investigate the relationship between subsequent cardiovascular events (CVE) and vascular access trouble (VAT). Methods: This cohort study included 64 consecutive patients with ESKD who initiated hemodialysis and underwent a procedure for the creation of a primary radiocephalic arteriovenous fistula (RCAVF). Small arterial specimens were obtained from patients during RCAVF surgery. Tissue samples were stained with von Kossa, and arterial microcalcification was evaluated. We analyzed the association between preexisting arterial microcalcifications, clinical characteristics, CVE, and VAT. Results: In the univariate analysis, RAC patients demonstrated high systolic blood pressure (sBP), low hemoglobin (Hb), and low transferrin saturation (TSAT) (<0.05, <0.05, and <0.05, respectively). In the multivariate analysis, Hb (HR−0.516 (0.278−0.959), p < 0.05), TSAT (HR−0.0012 (0.00000248−0.597), p < 0.05), and sBP (HR−1.037 (1.001−1.073), p < 0.05) were independent risk factors for RAC. The cumulative incidence rate of CVE/VAT was not associated with RAC for one year. Conclusion: RAC was associated with sBP, TSAT, and anemia; however, no association with CVE/VAT was observed.

Chronic Kidney Disease-Mineral and Bone Disorders: Pathogenesis and Management

The key players of the chronic kidney disease-mineral and bone disorders (CKD-MBD) are calcium, phosphate, PTH, FGF23, and the vitamin D hormonal system. The progressive reduction of kidney function greatly modifies the tightly interrelated mechanisms that control these parameters. As a result, important changes occur in the bone and mineral hormonal axis, leading to changes in bone turnover with relevant consequences in clinical outcomes, such as decrease in bone mass with increased bone fragility and bone fractures and increased vascular and valvular calcification, also with great impact in the cardiovascular outcomes. So far, the knowledge of the mineral and bone disorders in CKD and the increased variety of efficacious therapies should lead to a better prevention and management of CKD-MBD.

The Novel gem-Dihydroperoxide 12AC3O Suppresses High Phosphate-Induced Calcification via Antioxidant Effects in p53LMAco1 Smooth Muscle Cells

The excessive intake of phosphate (Pi), or chronic kidney disease (CKD), can cause hyperphosphatemia and eventually lead to ectopic calcification, resulting in cerebrovascular diseases. It has been reported that reactive oxygen species (ROS), induced by high concentrations of Pi loading, play a key role in vascular calcification. Therefore, ROS suppression may be a useful treatment strategy for vascular calcification. 12AC3O is a newly synthesized gem-dihydroperoxide (DHP) that has potent antioxidant effects. In the present study, we investigated whether 12AC3O inhibited vascular calcification via its antioxidative capacity. To examine whether 12AC3O prevents vascular calcification under high Pi conditions, we performed Alizarin red and von Kossa staining, using the mouse aortic smooth muscle cell line p53LMAco1. Additionally, the effect of 12AC3O against oxidative stress, induced by high concentrations of Pi loading, was investigated using redox- sensitive dyes. Further, the direct trapping effect of 12AC3O on reactive oxygen species (ROS) was investigated by ESR analysis. Although high concentrations of Pi loading exacerbated vascular smooth muscle calcification, calcium deposition was suppressed by the treatment of both antioxidants and 12AC3O, suggesting that the suppression of ROS may be a candidate therapeutic approach for treating vascular calcification induced by high concentrations of Pi loading. Importantly, 12AC3O also attenuated oxidative stress. Furthermore, 12AC3O directly trapped superoxide anion and hydroxyl radical. These results suggest that ROS are closely involved in high concentrations of Pi-induced vascular calcification and that 12AC3O inhibits vascular calcification by directly trapping ROS.

Effects of repetitive diet-induced fluctuations in plasma phosphorus on vascular calcification and inflammation in rats with early-stage chronic kidney disease

Cardiovascular disease is a major cause of death among hemodialysis patients. Hyperphosphatemia induces cardiovascular disease through vascular endothelial dysfunction and calcification. Repetition of a short-term excessive-phosphorus (P) diet causes transient elevations in plasma P and subsequent vascular endothelial dysfunction in normal rats. The purpose of this study was to investigate the effects of the P fluctuation on vascular calcification and inflammation in rats after unilateral nephrectomy as an early-stage chronic kidney disease (CKD) model. Rats were bred for 36 days; CP group, fed a control P (0.6%) diet; HP group, fed a high-P (1.2%) diet; and P fluctuation group, fed low-P (0.02%) and high-P diets alternately every 2 days. Influences on vascular calcification were analyzed using Von Kossa staining and measurement of vessel Ca content. The influence on inflammation was measured as urinary levels of 8-hydroxy-2'-deoxyguanosine. We demonstrated that the P fluctuation group showed similar vascular calcification and inflammation to the HP group, despite having the same total P intake as the CP group. A diet avoiding P fluctuations may be important for patients with early-stage CKD.

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 .

Effect of cross-linked chitosan iron (III) on vascular calcification in uremic rats

Cross-linked chitosan iron (III) is a chitin-derived polymer with a chelating effect on phosphorus, but it is untested in vascular calcification. We evaluated this compound's ability to reduce hyperphosphatemia and its effect on vascular calcification in uremic rats using an adenine-based, phosphorus-rich diet for seven weeks. We used a control group to characterize the uremia. Uremic rats were divided according the treatment into chronic kidney disease, CKD-Ch-Fe(III)CL (CKD-Ch), CKD-calcium carbonate, or CKD-sevelamer groups. We measured creatinine, phosphorus, calcium, alkaline phosphatase, phosphorus excretion fraction, parathyroid hormone, and fibroblast growth factor 23. Vascular calcification was assessed using the aortic calcium content, and a semi-quantitative analysis was performed using Von Kossa and hematoxylin-eosin staining. At week seven, rats in the chronic kidney disease group had higher creatinine, phosphorus, phosphorus excretion fraction, calcium, alkaline phosphatase, fibroblast growth factor 23, and aortic calcium content than those in the Control group. Treatments with cross-linked chitosan iron (III) and calcium carbonate prevented phosphorus increase (20%-30% reduction). The aortic calcium content was lowered by 88% and 85% in the CKD-Ch and CKD-sevelamer groups, respectively. The prevalence of vascular changes was higher in the chronic kidney disease and CKD-calcium carbonate (62.5%) groups than in the CKD-Ch group (37.5%). In conclusion, cross-linked chitosan iron (III) had a phosphorus chelating effect similar to calcium carbonate already available for clinical use, and prevented calcium accumulation in the aorta. Impact statement Vascular calcification (VC) is a common complication due to CKD-related bone and mineral disorder (BMD) and is characterized by deposition of calcium in vessels. Effective therapies are not yet available but new phosphorus chelators can prevent complications from CV. We tested the effect of chitosan, a new phosphorus chelator, on the VC of uremic animals. It has recently been proposed that chitosan treatment may be effective in the treatment of hyperphosphataemia. However, its action on vascular calcification has not been investigated yet. In this study, we demonstrated that chitosan reduced the calcium content in the aorta, suggesting that this may be a therapeutic approach in the treatment of hyperphosphatemia by preventing CV.

Caspase-independent programmed cell death triggers Ca2PO4 deposition in an in vitro model of nephrocalcinosis

Nephrocalcinosis involves the deposition of microscopic crystals in the tubular lumen or interstitium. While the clinical, biochemical, and genetic aspects of the diseases causing nephrocalcinosis have been elucidated, little is known about the cellular events in this calcification process. We previously reported a phenomenon involving the spontaneous formation of Ca2PO4 nodules in primary papillary renal cells from a patient with medullary nephrocalcinosis harboring a rare glial cell-derived neurotrophic factor (GDNF) gene variant. We also demonstrated that cultivating GDNF-silenced human kidney-2 (HK-2) cells in osteogenic conditions for 15 days triggered Ca2PO4 deposits. Given the reportedly close relationship between cell death and pathological calcification, aim of the present study was to investigate whether apoptosis is involved in the calcification of GDNF-silenced HK-2 cells under osteogenic conditions. Silenced and control cells were cultured in standard and osteogenic medium for 1, 5, and 15 days, and any Ca2PO4 deposition was identified by means of von Kossa staining and environmental SEM (ESEM) analyses. Based on the results of annexin V and propidium iodide (PI) analysis, and terminal deoxynucleotidyl transferase dUTP-biotin nick end labeling (TUNEL) assay, the silenced cells in the osteogenic medium showed a significant increase in the percentage of cells in the late phase of apoptosis and an increased Ca2PO4 deposition at 15 days. The results of quantitative real-time PCR (qRT-PCR) of BAX and BCL2, and in-cell Western analysis of caspases indicated that the cell death process was independent of caspase-3, -6, -7, and -9 activation, however. Using this model, we provide evidence of caspase-independent cell death triggering the calcification process in GDNF-silenced HK-2 cells.

Coronary Artery Bypass Surgery in End-Stage Renal Disease Patients

The number of patients requiring hemodialysis is continuously increasing around the world. Hemodialysis affects patient quality of life and it is also associated with a higher risk for cardiovascular events. In addition to traditional risk factors for cardiovascular events such as hypertension, hyperlipidemia, and diabetes, hemodialysis is associated with hyperphosphatemia, chronic inflammation, vascular calcification, and anemia which accelerate atherosclerosis, vascular stiffness, and cardiac ischemia. Treatment strategy for coronary revascularization in this progressive disease remains controversial. However, a systematic treatment including medical therapy and complete revascularization through a less invasive strategy should be considered in addressing this problem. This review discusses the epidemiology, vascular pathology and current treatment options in patients with end-stage renal disease requiring coronary revascularization.

Calcium Overload Accelerates Phosphate-Induced Vascular Calcification Via Pit-1, but not the Calcium-Sensing Receptor

Aim: Vascular calcification (VC) is a risk factor of cardiovascular and all-cause mortality in patients with chronic kidney disease (CKD). CKD–mineral and bone metabolism disorder is an important problem in patients with renal failure. Abnormal levels of serum phosphate and calcium affect CKD–mineral and bone metabolism disorder and contribute to bone disease, VC, and cardiovascular disease. Hypercalcemia is a contributing factor in progression of VC in patients with CKD. However, the mechanisms of how calcium promotes intracellular calcification are still unclear. This study aimed to examine the mechanisms underlying calcium-induced calcification in a rat aortic tissue culture model.

Methods: Aortic segments from 7-week-old male Sprague–Dawley rats were cultured in serum-supplemented medium for 10 days. We added high calcium (HiCa; calcium 3.0 mM) to high phosphate (HPi; phosphate 3.8 mM) medium to accelerate phosphate and calcium-induced VC. We used phosphonoformic acid and the calcimimetic R-568 to determine whether the mechanism of calcification involves Pit-1 or the calcium-sensing receptor.

Results: Medial VC was significantly augmented by HPi+HiCa medium compared with HPi alone (300%, p < 0.05), and was associated with upregulation of Pit-1 protein. Pit-1 protein concentrations in HPi+HiCa medium were greater than those in HPi medium. Phosphonoformic acid completely negated the augmentation of medial VC induced by HPi+HiCa. R-568 had no additive direct effect on medial VC.

Conclusion: These results indicated that exposure to HPi+HiCa accelerates medial VC, and this is mediated through Pit-1, not the calcium-sensing receptor.

7-Ketocholesterol-induced lysosomal dysfunction exacerbates vascular smooth muscle cell calcification via oxidative stress

Vascular calcification is known to reduce the elasticity of aorta. Several studies have suggested that autophagy-lysosomal pathway (ALP) in vascular smooth muscle cells (VSMCs) is associated with vascular calcification. A major component of oxidized low-density lipoproteins, 7-ketocholesterol (7-KC), has been reported to promote inorganic phosphorus (Pi)-induced vascular calcification and induce ALP. The aim of this study was to unravel the relationship between ALP and the progression of calcification by 7-KC. Calcification of human VSMCs was induced by Pi stimulation in the presence or absence of 7-KC. FACS analysis showed that 7-KC-induced apoptosis at a high concentration (30 μM), but not at a low concentration (15 μM). Interestingly, 7-KC promoted calcification in VSMCs regardless of apoptosis. Immunoblotting and immunostaining showed that 7-KC inhibits not only the fusion of autophagosomes and lysosomes but also causes a swell of lysosomes with the reduction of cathepsin B and D. Moreover, lysosomal protease inhibitors exacerbated the apoptosis-independent calcification by 7-KC although inhibition of autophagosome formation by Atg5 siRNA did not. Finally, the 7-KC-induced progression of calcification was alleviated by the treatment with antioxidant. Taken together, our data showed that 7-KC promotes VSMC calcification through lysosomal-dysfunction-dependent oxidative stress.

Mineral Composition of Phosphate-Induced Calcification in a Rat Aortic Tissue Culture Model

AIM

High phosphorus conditions promote vascular calcification (VC) in both chronic kidney disease (CKD) patients and experimental models. However, the composition of medial calcification has not been accurately determined, so the objective of this study was to evaluate the mineral composition of calcification in a tissue culture model, not a cell culture system.

METHODS

Aortic rings obtained from male Sprague-Dawley rats were incubated in serum-supplemented medium for 10 days. The inorganic phosphate (Pi) concentration of the medium was increased to induce VC, which was assessed by histology, imaging, and spectroscopy. The mineral composition of the calcification was analyzed using Fourier transform infrared (FTIR) spectroscopic imaging, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX) mapping.

RESULTS

The calcium content significantly increased only in aortic rings cultured for 10 days in the high-Pi medium (HiP: 3.8 mmol/L). The concentration of the phosphate transporter Pit-1 in the aortic tissue exposed to HiP was higher than that in the control incubated sections. The FTIR images and spectra indicated that PO4(3-) was mostly distributed as hydroxyapatite in the medial calcification of aortic rings cultured in HiP. A small quantity of carbonate was identified. The SEM-EDX overlay map demonstrated that phosphorus and calcium simultaneously accumulated and localized in the area of medial calcification induced by exposure to HiP.

CONCLUSION

This is the first report of accurate determination of the chemical composition of aortic medial calcification. Exposure to high Pi concentration augments aortic calcification via an increase in Pit-1, which mainly contains calcium phosphate.

High phosphorus level leads to aortic calcification via β-catenin in chronic kidney disease

AIMS

Vascular calcification is a risk factor for causing cardiovascular events and has a high prevalence among chronic kidney disease (CKD) patients. However, the molecular mechanism underlying this pathogenic process is still obscure.

METHODS

Vascular smooth muscle cells (VSMCs) were induced by a concentration of phosphorus (Pi) of 2.5 mM, and were subjected to cell calcification analyses. The effect of high Pi on the Wnt/β-catenin pathway was measured using a TOP/FOP-Flash reporter assay. The transcriptional regulation of β-catenin on PIT1 (a type III sodium-dependent phosphate cotransporter) was confirmed by promoter reporter and chromatin immunoprecipitation assays. The 5/6 nephrectomized rat was used as an in vivo model and was fed a high Pi diet to induce aortic calcification. Serum levels of phosphate, calcium, creatine, and blood urea nitrogen were measured, and abdominal aortic calcification was examined.

RESULTS

High Pi induced VSMC calcification, downregulated expression levels of VSMC markers, and upregulated levels of osteogenic markers. High Pi activated the Wnt/β-catenin pathway and β-catenin activity. β-Catenin was involved in the process of high Pi-induced VSMC calcification. Further investigation revealed that β-catenin transcriptionally regulated Pit1, a necessary player in VSMC osteogenic phenotype change and calcification. The in vivo study showed that β-catenin was involved in rat abdominal aortic calcification induced by high Pi. When knockdown expression of β-catenin in the rat model was investigated, we found that aortic calcification was reduced.

CONCLUSION

These results suggest that β-catenin is an important player in high phosphorus level-induced aortic calcification in CKD.

5-aza-2'-Deoxycytidine, a DNA methyltransferase inhibitor, facilitates the inorganic phosphorus-induced mineralization of vascular smooth muscle cells

AIM

Vascular calcification, an independent risk factor for cardiovascular disease in patients with chronic kidney disease(CKD), refers to the mineralization of vascular smooth muscle cells(VSMCs) caused by phenotypic changes toward osteoblast-like cells. DNA methylation, mediated by DNA methyltransferases(DNMTs), plays an important role in the differentiation of osteoblasts. We herein assessed the effects of a DNMT inhibitor on phenotypic changes in VSMCs and the development of vascular calcification.

METHODS

The effects of 5-aza-2'-deoxycytidine(5-aza-dC), a DNMT inhibitor, on human aortic smooth muscle cells(HASMCs) were evaluated. The expression and DNA methylation status of osteogenic genes were determined using RT-qPCR and bisulfite sequencing, respectively. Mineralization of HASMCs was induced by high concentrations of inorganic phosphate(Pi), as confirmed by quantitation of the calcium levels and von Kossa staining. Moreover, we examined the effects of the suppression of DNMT1 and/or alkaline phosphatase(ALP) on the mineralization of HASMCs.

RESULTS

5-aza-dC increased the expression and activity of ALP and reduced the DNA methylation levels of the ALP promoter region in the HASMCs. In addition, both treatment with 5-aza-dC and downregulation of the DNMT1 expression promoted the Pi-induced mineralization of HASMCs. Moreover, both treatment with phosphonoformic acid(PFA), a sodium-dependent phosphate transporter inhibitor, and suppression of the ALP expression inhibited the 5-aza-dC-promoted mineralization of HASMCs.

CONCLUSIONS

The present study showed that DNMT inhibitors facilitate the Pi-induced development of vascular calcification via the upregulation of the ALP expression along with a reduction in the DNA methylation level of the ALP promoter region.

Phosphate: an old bone molecule but new cardiovascular risk factor

Phosphate handling in the body is complex and involves hormones produced by the bone, the parathyroid gland and the kidneys. Phosphate is mostly found in hydroxyapatite. however recent evidence suggests that phosphate is also a signalling molecule associated with bone formation. Phosphate balance requires careful regulation of gut and kidney phosphate transporters, SLC34 transporter family, but phosphate signalling in osteoblasts and vascular smooth muscle cells is likely mediated by the SLC20 transporter family (PiT1 and PiT2). If not properly regulated, phosphate imblanace could lead to mineral disorders as well as vascular calcification. In chronic kidney disease-mineral bone disorder, hyperphosphataemia has been consistently associated with extra-osseous calcification and cardiovascular disease. This review focuses on the physiological mechanisms involved in phosphate balance and cell signalling (i.e. osteoblasts and vascular smooth muscle cells) as well as pathological consequences of hyperphosphataemia. Finally, conventional as well as new and experimental therapeutics in the treatment of hyperphosphataemia are explored.

Fibroblast growth factor 23 accelerates phosphate-induced vascular calcification in the absence of Klotho deficiency

Fibroblast growth factor 23 (FGF23) is a phosphate-regulating hormone that acts primarily on the kidney and parathyroid. With declining kidney function there is an increase in circulating FGF23 levels, which is associated with vascular calcification and mortality in chronic kidney disease. Whether FGF23 exerts direct effects on vasculature is unclear. We evaluated the expression of Klotho and FGF receptors in rat aortic rings and rat aorta vascular smooth muscle cells maintained in culture by reverse transcription-PCR, western blotting, and immunostaining. Signaling pathways underlying FGF23 effects were assessed by western blotting, and effects of FGF23 on osteogenic markers and phosphate transporters were assessed by real-time reverse transcription-PCR. We detected Klotho and FGFR1 in total aorta but not in vascular smooth muscle cells. FGF23 augmented phosphate-induced vascular calcification in the aortic rings from uremic rats and dose dependently increased ERK1/2 phosphorylation in Klotho-overexpressing but not naive vascular smooth muscle cells. FGF23-induced ERK1/2 phosphorylation was inhibited by SU5402 (FGFR1 inhibitor) and U0126 (MEK inhibitor). FGF23 enhanced phosphate-induced calcification in Klotho-overexpressing vascular smooth muscle cells and increased osteoblastic marker expression, which was inhibited by U0126. In contrast, phosphate transporter expression was not affected by phosphate or FGF23. Thus, FGF23 enhances phosphate-induced vascular calcification by promoting osteoblastic differentiation involving the ERK1/2 pathway.

Cardiovascular disease in an adenine-induced model of chronic kidney disease: the temporal link between vascular calcification and haemodynamic consequences

OBJECTIVES

Medial vascular calcification is highly prevalent in chronic kidney disease (CKD), and it is a risk factor for mortality. This study characterizes the time course and the link between calcification of major arteries, changes in blood pressure (BP) and cardiac growth in experimental CKD.

METHODS

CKD (elevated serum creatinine and urea) was induced with a 0.25% adenine diet (5, 8 and 11 weeks). BP was measured by radiotelemetry in conscious rats or indwelling catheter under anaesthesia. At each time point, serum biochemistry and tissue calcification was quantified.

RESULTS

CKD was present in all animals by 5 weeks and the ensuing 6 weeks (11 weeks total). CKD animals developed elevated serum phosphate (5-8 weeks) and fibroblast growth factor-23 (FGF-23; 5-11 weeks) levels. There was a 100% incidence of calcification at 11 weeks, 71% at 8 weeks and 33% at 5 weeks, and distal arteries appeared more susceptible than proximal arteries. Calcification was associated with widening of pulse pressure (PP), and a higher pulse wave. Continuous radiotelemetry revealed a significant increase in SBP variability and an accelerated (<24 h) elevation in PP of at least 10 mmHg following 8 weeks of CKD. This precipitous change was driven by a drop in mean DBP rather than elevated mean SBP. PP, duration of CKD and FGF-23 levels correlated with left ventricular hypertrophy.

CONCLUSION

The unique haemodynamic consequences of medial calcification, combined with the hormonal consequences of hyperphosphatemia (i.e. FGF-23), seem to have an exacerbated risk for left ventricular hypertrophy.

Microcalcifications in breast cancer: Lessons from physiological mineralization

Mammographic mammary microcalcifications are routinely used for the early detection of breast cancer, however the mechanisms by which they form remain unclear. Two species of mammary microcalcifications have been identified; calcium oxalate and hydroxyapatite. Calcium oxalate is mostly associated with benign lesions of the breast, whereas hydroxyapatite is associated with both benign and malignant tumors. The way in which hydroxyapatite forms within mammary tissue remains largely unexplored, however lessons can be learned from the process of physiological mineralization. Normal physiological mineralization by osteoblasts results in hydroxyapatite deposition in bone. This review brings together existing knowledge from the field of physiological mineralization and juxtaposes it with our current understanding of the genesis of mammary microcalcifications. As an increasing number of breast cancers are being detected in their non-palpable stage through mammographic microcalcifications, it is important that future studies investigate the underlying mechanisms of their formation in order to fully understand the significance of this unique early marker of breast cancer.

Trichostatin A, an HDAC class I/II inhibitor, promotes Pi-induced vascular calcification via up-regulation of the expression of alkaline phosphatase

AIM

Vascular calcification, a major complication of chronic kidney disease (CKD), refers to the mineralization of vascular smooth muscle cells (VSMCs), resulting from a phenotypic change towards osteoblast-like cells. Histone deacetylase inhibitors (HDIs), potential therapeutic agents for CKD, are known to promote the differentiation and mineralization of osteoblasts. In this study, we aimed to determine the effects of an HDI on the phenotypic change of VSMCs and the development of vascular calcification.

METHODS

The effect of trichostatin A (TSA), an HDI, on human aortic smooth muscle cells (HASMCs) was determined. The mineralization of HASMCs was induced by inorganic phosphorus (Pi), and was confirmed by quantitation of Ca levels and by von Kossa staining. Furthermore, we examined the effect of alkaline phosphatase (ALP) suppression using siRNA on Pi-induced vascular calcification in the presence or absence of TSA.

RESULTS

TSA increased the expression and activity of ALP in HASMCs at a concentration which showed an inhibitory effect of histone deacetylase (HDAC) activity but not on cell viability. Moreover, TSA promoted the Pi-induced mineralization of HASMCs. In addition, both phosphonoformic acid (PFA), which is a sodium-dependent phosphate transporter inhibitor, and suppression of ALP expression by siRNA markedly inhibited the TSA-promoted mineralization of HASMCs.

CONCLUSION

These data show that inhibition of HDAC activity promotes Pi-induced vascular calcification via the up-regulation of ALP expression. Taken together, HDIs may increase the risk of vascular calcification in CKD patients.

High expression of the Pi-transporter SLC20A1/Pit1 in calcific aortic valve disease promotes mineralization through regulation of Akt-1

The regulation of phosphate (Pi) handling is crucial during calcification of the aortic valve. Gene profiling of Pi transporters revealed that VIC culture expresses SLC201A1/Pit1 and SLC20A2/Pit2. On exposure to a mineralizing medium (2 mM Pi), the expression of Pi transporters in VIC culture is increased several folds, with the highest magnitude for SLC20A1. By using siRNAs, we established that silencing SLC20A1 significantly reduced Pi-induced mineralization of VICs. In human pathological specimens, we found that the expression of SCL20A1 was increased in CAVD tissues compared to control non-mineralized aortic valves. Treatment of VIC culture with Pi promoted the loss of mitochondrial membrane potential (ΔΨm) and cytochrome c release within the cytosol, leading to apoptosis. Inhibition of Pi transporters with phosphonoformic acid (PFA) prevented Pi-mediated apoptosis of VICs. Moreover, we discovered that the level of the Akt-1 transcript is diminished in CAVD tissues compared with control valves. Accordingly, treatment with Pi caused a reduction of the Akt-1 transcript in VIC culture, and treatment with PFA or siRNA against SLC20A1 restored the level of Akt-1. Overexpression of Akt-1 (pCMVAkt-1) prevented both Pi-induced apoptosis and mineralization of VIC culture. These results strongly suggest that overexpression of SLC20A1 promotes apoptosis and mineralization by altering the level of Akt-1.

Relationship between aortic mineral elements and osteodystrophy in mice with chronic kidney disease

In chronic kidney disease (CKD), osteodystrophy and arterial calcification often coexist. However, arterial alterations have not been addressed in CKD unaccompanied by evidence of calcification. We investigated the association of phosphate (P) and calcium (Ca) accumulation in calcification-free aortas with CKD-induced osteodystrophy. Aortic accumulation of magnesium (Mg), an inhibitor of calcification, was also examined. Male mice aged 26 weeks with CKD characterized by hyperparathyroidism and hyperphosphatemia (Nx, n = 8) and age-matched healthy male mice (shams, n = 8) were sampled for blood, and thoracic vertebrae and aortas were harvested. Bone structure and chemicals were analyzed by microcomputed tomography and infrared microspectroscopy, respectively, and aortic accumulation of P, Ca, and Mg was evaluated by plasma-atomic emission spectrometry. Volume fractions of cortical and trabecular bones were smaller in Nx than in sham animals (P < 0.05), attributed to cortical thinning and reduction in trabecular number, respectively. Bone chemicals were not different between the groups. No calcification was found in either group, but P, Ca, and Mg contents were higher in Nx than in shams (P < 0.05). The mass ratio of Ca/P was lower in Nx than in shams (P < 0.05), but that of Mg/Ca and Mg/P was not different between the groups. Aortic P and Ca contents were inversely correlated with the volume fraction of cortical bone (P < 0.05). In conclusion, the relationship of osteodystrophy with aortic P and Ca accumulation suggests the existence of a bone-vascular axis, even in calcification-free arteries in CKD. The preservation of ratios of Mg/Ca and Mg/P despite CKD development might contribute to calcification resistance.

Inorganic phosphate accelerates the migration of vascular smooth muscle cells: evidence for the involvement of miR-223

Backgound

An elevated serum inorganic phosphate (Pi) level is a major risk factor for kidney disease and downstream vascular complications. We focused on the effect of Pi levels on human aortic vascular smooth muscle cells (VSMCs), with an emphasis on the role of microRNAs (miRNAs).

Methodology/Principal Findings

Exposure of human primary VSMCs in vitro to pathological levels of Pi increased calcification, migration rate and concomitantly reduced cell proliferation and the amount of the actin cytoskeleton. These changes were evidenced by significant downregulation of miRNA-143 (miR-143) and miR-145 and concomitant upregulation of their targets and key markers in synthetic VSMCs, such as Krüppel-like factors−4 and −5 and versican. Interestingly, we also found that miR-223 (a marker of muscle damage and a key factor in osteoclast differentiation) is expressed in VSMCs and is significantly upregulated in Pi-treated cells. Over-expressing miR-223 in VSMCs increased proliferation and markedly enhanced VSMC migration. Additionally, we found that the expression of two of the known miR-223 targets, Mef2c and RhoB, was highly reduced in Pi treated as well as miR-223 over-expressing VSMCs. To complement these in vitro findings, we also observed significant downregulation of miR-143 and miR-145 and upregulation of miR-223 in aorta samples collected from ApoE knock-out mice, which display vascular calcification.

Conclusions/Significance

Our results suggest that (i) high levels of Pi increase VSMC migration and calcification, (ii) altered expression levels of miR-223 could play a part in this process and (iii) miR-223 is a potential new biomarker of VSMC damage.

The management of hyperphosphatemia by lanthanum carbonate in chronic kidney disease patients

Hyperphosphatemia has been shown to be involved not only in the onset and progression of secondary hyperparathyroidism but also in vascular calcification. In addition, it influences the clinical course of patients with chronic kidney disease. Phosphate (Pi) binder is required in the management of hyperparaphosphatemia, because dietary Pi restriction and Pi removal by hemodialysis alone are insufficient. Lanthanum carbonate, a powerful Pi binder, has a similar effect to aluminum hydroxide in reducing serum Pi levels. As it is excreted via the liver, lanthanum carbonate has an advantage in patients with renal failure. The effect of lanthanum carbonate on serum Pi levels is almost two times higher than that of calcium (Ca) carbonate, which is commonly used. Lanthanum carbonate and Ca carbonate have an additive effect. Worldwide, there is 6 years worth of clinical treatment data on lanthanum carbonate; however, we have 3 years of clinical use in Japanese patients with hyperphosphatemia. No serious side effects have been reported. However, the most important concern is bone toxicity, which has been observed with use of aluminum hydroxide. For this study, clinical research involved analysis of bone biopsies. Although osteomalacia is the most noticeable side effect, this was not observed. Both the high- and the low-turnover bone disease concentrated into a normal bone turnover state. However, as the authors have less than 10 years’ clinical experience with lanthanum carbonate, patients should be monitored carefully. In addition, it is necessary to demonstrate whether potent treatment effects on hyperphosphatemia improve the long-term outcome.

Magnesium reduces calcification in bovine vascular smooth muscle cells in a dose-dependent manner

Background.

Vascular calcification (VC), mainly due to elevated phosphate levels, is one major problem in patients suffering from chronic kidney disease. In clinical studies, an inverse relationship between serum magnesium and VC has been reported. However, there is only few information about the influence of magnesium on calcification on a cellular level available. Therefore, we investigated the effect of magnesium on calcification induced by β-glycerophosphate (BGP) in bovine vascular smooth muscle cells (BVSMCs).

Methods.

BVSMCs were incubated with calcification media for 14 days while simultaneously increasing the magnesium concentration. Calcium deposition, transdifferentiation of cells and apoptosis were measured applying quantification of calcium, von Kossa and Alizarin red staining, real-time reverse transcription–polymerase chain reaction and annexin V staining, respectively.

Results.

Calcium deposition in the cells dramatically increased with addition of BGP and could be mostly prevented by co-incubation with magnesium. Higher magnesium levels led to inhibition of BGP-induced alkaline phosphatase activity as well as to a decreased expression of genes associated with the process of transdifferentiation of BVSMCs into osteoblast-like cells. Furthermore, estimated calcium entry into the cells decreased with increasing magnesium concentrations in the media. In addition, higher magnesium concentrations prevented cell damage (apoptosis) induced by BGP as well as progression of already established calcification.

Conclusions.

Higher magnesium levels prevented BVSMC calcification, inhibited expression of osteogenic proteins, apoptosis and further progression of already established calcification. Thus, magnesium is influencing molecular processes associated with VC and may have the potential to play a role for VC also in clinical situations.

Endothelin but Not Angiotensin II May Mediate Hypertension-Induced Coronary Vascular Calcification in Chronic Kidney Disease

To understand the relationship between putative neurohormonal factors operative in hypertension and coronary artery calcification (CAC), the relevant cellular actions of angiotensin (Ang II) and endothelin-1 (ET-1) are reviewed. There is compelling evidence to implicate ET-1 in CAC. ET-1 increases phosphate transport with a 42 to 73% increase in V_max. Increased cellular phosphate may induce CAC through increased Ca x phosphate product, transformation of vascular smooth muscle cells into a bone-producing phenotype or cell apoptosis that releases procalcific substances. ET-1 is increased in several models of vascular calcification. ET-1 inhibits inhibitors of calcification, matrix Gla and osteoprotegerin, while enhancing pro-calcific factors such as BMP-2 and osteopontin. In contrast, Ang II inhibits phosphate transport decreasing V_max by 38% and increases matrix Gla. Ang II also stimulates bone resorption. Vascular calcification is reduced by ET-1 A receptor antagonists and to a greater extent than angiotensin receptor blockade although both agents reduce blood pressure.

Cartilage Oligomeric Matrix Protein Inhibits Vascular Smooth Muscle Calcification by Interacting With Bone Morphogenetic Protein-2

Rationale:

Vascular calcification is a significant contributor to cardiovascular morbidity and mortality. We recently reported that cartilage oligomeric matrix protein (COMP) is pivotal for maintaining the homeostasis of vascular smooth muscle cells (VSMCs). Whether COMP affects the process of vascular calcification is unknown.

Objective:

We aimed to test whether COMP modulates vascular calcification.

Methods and Results:

VSMC calcification in vitro was induced by calcifying media containing high inorganic phosphate or calcium. In vivo medial vessel calcification was induced in rats by 5/6 nephrectomy with a high-phosphate diet or by periadventitial application of CaCl 2 to the abdominal aorta. COMP protein level was markedly reduced in both calcified VSMCs and arteries. COMP deficiency remarkably exacerbated VSMC calcification, whereas ectopic expression of COMP greatly reduced calcification. Furthermore, COMP knockdown facilitated osteogenic markers expression by VSMCs even in the absence of calcifying media. By contrast, COMP overexpression significantly inhibited high phosphate– or high calcium–induced VSMC osteochondrogenic transition. Induction of osteogenic marker expression by COMP silencing was reversed by a soluble form of bone morphogenetic protein (BMP)-2 receptor IA, which suggests a BMP-2–dependent mechanism. Our data revealed that COMP bound directly to BMP-2 through the C terminus, inhibited BMP-2 receptor binding, and blocked BMP-2 osteogenic signaling, indicating COMP inhibits osteochondrogenic transition of VSMCs at least partially through inhibiting BMP-2.

Conclusions:

Our data strongly suggest that COMP is a novel inhibitor of vascular calcification. The imbalance between the effects of COMP and BMP-2 may provide new insights into the pathophysiology of vascular calcification.

Phosphate-induced apoptosis in human peritoneal mesothelial cells in vitro

BACKGROUND

It has been demonstrated that phosphate uptake through the type III sodium-dependent phosphate co-transporter, Pit-1, induced apoptosis of aortic vascular smooth muscle cells and endothelial cells in vitro. However, the apoptotic effects of high phosphate (HP) level in human peritoneal mesothelial cells (HPMCs) are not known.

METHODS

To examine whether Pit-1 is expressed in HPMCs, we checked the Western blot assay of immunoreactive Pit-1 and the transcription of Pit-1 by reverse transcriptase PCR. We treated several different phosphate concentrations (1-4 mM) and calcium concentrations (1.8 and 2.8 mM) on HPMCs to assess the effects of concentration. MTT, TUNEL assays, and flow cytometry analysis using Annexin V and propidium iodide were performed to identify cell death and apoptosis. Bax and Bcl-2 by Western blot and caspase-3 activity were evaluated by colorimetric assay. In addition, phosphonoformic acid (PFA) and pan-caspase inhibitor, Z-VAD-FMK, were given to prevent phosphate-induced apoptosis.

RESULTS

Pit-1 expression on HPMCs was demonstrated. Apoptosis in HPMCs significantly increased with a high concentration of phosphate in a dose- and time-dependent manner, and was enhanced in the presence of 2.8 mM calcium. HP concentrations significantly decreased the anti-apoptotic Bcl-2/Bax ratio and increased caspase-3 activity. The treatment with PFA and Z-VAD-FMK prevented cell death by HP.

CONCLUSION

Phosphate uptake through Pit-1 induces apoptosis in HPMCs by a caspase-related mechanism.

Vascular smooth muscle cell differentiation to an osteogenic phenotype involves TRPM7 modulation by magnesium

Arterial calcification, common in vascular diseases, involves vascular smooth muscle cell (VSMC) transformation to an osteoblast phenotype. Clinical studies suggest that magnesium may prevent this, but mechanisms are unclear. We assessed whether increasing magnesium levels reduce VSMC calcification and differentiation and questioned the role of the Mg(2+) transporter, transient receptor potential melastatin (TRPM)7 cation channels in this process. Rat VSMCs were exposed to calcification medium in the absence and presence of magnesium (2.0 to 3.0 mmol/L) or 2-aminoethoxy-diphenylborate (2-APB) (TRPM7 inhibitor). VSMCs from mice with genetically low (MgL) or high-normal (MgH) [Mg(2+)](i) were also studied. Calcification was assessed by von Kossa staining. Expression of osteocalcin, osteopontin, bone morphogenetic protein (BMP)-2, BMP-4, BMP-7, and matrix Gla protein and activity of TRPM7 (cytosol:membrane translocation) were determined by immunoblotting. Calcification medium induced osteogenic differentiation, reduced matrix Gla protein content, and increased expression of the sodium-dependent cotransporter Pit-1. Magnesium prevented calcification and decreased osteocalcin expression and BMP-2 activity and increased expression of calcification inhibitors, osteopontin and matrix Gla protein. TRPM 7 activation was decreased by calcification medium, an effect reversed by magnesium. 2-APB recapitulated the VSMC osteoblastic phenotype in VSMCs. Osteocalcin was increased by calcification medium in VSMCs and intact vessels from MgL but not MgH, whereas osteopontin was increased in MgH, but not in MgL mice. Magnesium negatively regulates vascular calcification and osteogenic differentiation through increased/restored TRPM7 activity and increased expression of anticalcification proteins, including osteopontin, BMP-7, and matrix Gla protein. New molecular insights are provided whereby magnesium could protect against VSMC calcification.

Regulation of vascular smooth muscle cell calcification by extracellular pyrophosphate homeostasis: synergistic modulation by cyclic AMP and hyperphosphatemia

Vascular calcification is a multifaceted process involving gain of calcification inducers and loss of calcification inhibitors. One such inhibitor is inorganic pyrophosphate (PP(i)), and regulated generation and homeostasis of extracellular PP(i) is a critical determinant of soft-tissue mineralization. We recently described an autocrine mechanism of extracellular PP(i) generation in cultured rat aortic vascular smooth muscle cells (VSMC) that involves both ATP release coupled to the ectophosphodiesterase/pyrophosphatase ENPP1 and efflux of intracellular PP(i) mediated or regulated by the plasma membrane protein ANK. We now report that increased cAMP signaling and elevated extracellular inorganic phosphate (P(i)) act synergistically to induce calcification of these VSMC that is correlated with progressive reduction in ability to accumulate extracellular PP(i). Attenuated PP(i) accumulation was mediated in part by cAMP-dependent decrease in ANK expression coordinated with cAMP-dependent increase in expression of TNAP, the tissue nonselective alkaline phosphatase that degrades PP(i). Stimulation of cAMP signaling did not alter ATP release or ENPP1 expression, and the cAMP-induced changes in ANK and TNAP expression were not sufficient to induce calcification. Elevated extracellular P(i) alone elicited only minor calcification and no significant changes in ANK, TNAP, or ENPP1. In contrast, combined with a cAMP stimulus, elevated P(i) induced decreases in the ATP release pathway(s) that supports ENPP1 activity; this resulted in markedly reduced rates of PP(i) accumulation that facilitated robust calcification. Calcified VSMC were characterized by maintained expression of multiple SMC differentiation marker proteins including smooth muscle (SM) alpha-actin, SM22alpha, and calponin. Notably, addition of exogenous ATP (or PP(i) per se) rescued cAMP + phosphate-treated VSMC cultures from progression to the calcified state. These observations support a model in which extracellular PP(i) generation mediated by both ANK- and ATP release-dependent mechanisms serves as a critical regulator of VSMC calcification.