Osteopontin is associated with bioprosthetic heart valve calcification in humans

Calcium-Dependent Cytosolic Phospholipase A2α as Key Factor in Calcification of Subdermally Implanted Aortic Valve Leaflets

Calcium-dependent cytosolic phospholipase A2α (cPLA2α) had been previously found to be overexpressed by aortic valve interstitial cells (AVICs) subjected to in vitro calcific induction. Here, cPLA2α expression was immunohistochemically assayed in porcine aortic valve leaflets (iAVLs) that had undergone accelerated calcification subsequent to 2- to 28-day-long implantation in rat subcutis. A time-dependent increase in cPLA2α-positive AVICs paralleled mineralization progression depending on dramatic cell membrane degeneration with the release of hydroxyapatite-nucleating acidic lipid material, as revealed by immunogold particles decorating organelle membranes in 2d-iAVLs, as well as membrane-derived lipid byproducts in 7d- to 28d-iAVLs. Additional positivity was detected for (i) pro-inflammatory IL-6, mostly exhibited by rat peri-implant cells surrounding 14d- and 28d-iAVLs; (ii) calcium-binding osteopontin, with time-dependent increase and no ossification occurrence; (iii) anti-calcific fetuin-A, mostly restricted to blood plasma within vessels irrorating the connective envelopes of 28d-iAVLs; (iv) early apoptosis marker annexin-V, limited to sporadic AVICs in all iAVLs. No positivity was found for either apoptosis executioner cleaved caspase-3 or autophagy marker MAP1. In conclusion, cPLA2α appears to be a factor characterizing AVL calcification concurrently with a distinct still uncoded cell death form also in an animal model, as well as a putative target for the prevention and treatment of calcific valve diseases.

Noncalcific Mechanisms of Bioprosthetic Structural Valve Degeneration

Bioprosthetic heart valves (BHVs) largely circumvent the need for long‐term anticoagulation compared with mechanical valves but are increasingly susceptible to deterioration and reduced durability with reoperation rates of ≈10% and 30% at 10 and 15 years, respectively. Structural valve degeneration is a common, unpreventable, and untreatable consequence of BHV implantation and is frequently characterized by leaflet calcification. However, 25% of BHV reoperations attributed to structural valve degeneration occur with minimal leaflet mineralization. This review discusses the noncalcific mechanisms of BHV structural valve degeneration, highlighting the putative roles and pathophysiological relationships between protein infiltration, glycation, oxidative and mechanical stress, and inflammation and the structural consequences for surgical and transcatheter BHVs.

In Vitro Calcification of Bioprosthetic Heart Valves: Test Fluid Validation on Prosthetic Material Samples

Calcification is a major failure mode of bioprosthetic heart valves. So far, cost and time saving in vitro analyses of calcification potentials are unreliable, mostly due to superficial or spontaneous precipitation of the applied fluids. In this study, we developed a near-physiological non-spontaneously precipitating fluid for an accelerated in vitro calcification assessment, and validated it by analyzing the calcification potential of two prosthetic materials within two reference-tests. The first test focused on the comparison of four calcification fluids under dynamic contact with n=12 commercial bovine pericardium patches. The second one focused on the validation of the most appropriate fluid by analyzing the calcification potential of pericardium vs. polyurethane. The patches were mounted in separate test compartments and treated simultaneously with the respective fluids at an accelerated test frequency. Calcification propensity and progression were detected macroscopically and microscopically. Structural analyses of all deposits indicated hydroxyapatite by X-ray powder diffraction, which is also most commonly observed in vivo. Histological examination by von Kossa staining showed matrix internal and superficial calcifications, depending on the fluid composition. The present study reveals promising results towards the development of a meaningful, cost and time saving in vitro analysis of the calcification potential of bioprosthetic heart valves.

Degeneration of Bioprosthetic Heart Valves: Update 2020

The implantation of bioprosthetic heart valves (BHVs) is increasingly becoming the treatment of choice in patients requiring heart valve replacement surgery. Unlike mechanical heart valves, BHVs are less thrombogenic and exhibit superior hemodynamic properties. However, BHVs are prone to structural valve degeneration (SVD), an unavoidable condition limiting graft durability. Mechanisms underlying SVD are incompletely understood, and early concepts suggesting the purely degenerative nature of this process are now considered oversimplified. Recent studies implicate the host immune response as a major modality of SVD pathogenesis, manifested by a combination of processes phenocopying the long‐term transplant rejection, atherosclerosis, and calcification of native aortic valves. In this review, we summarize and critically analyze relevant studies on (1) SVD triggers and pathogenesis, (2) current approaches to protect BHVs from calcification, (3) obtaining low immunogenic BHV tissue from genetically modified animals, and (4) potential strategies for SVD prevention in the clinical setting.

Bioprosthetic valve thrombosis and degeneration following transcatheter aortic valve implantation (TAVI)

Bioprosthetic valve thrombosis (BPVT) is a recognised complication of prosthetic aortic valves and can be found in up to 13% of patients after transcatheter implantation. The mechanism of BPVT is not well known, abnormal flow conditions in the new and native sinuses and lack of functional endothelialisation are suspected causes. BPVT may result in valve dysfunction, possibly related to degeneration, and recurrence of patient symptoms, or remain subclinical. BPVT is best diagnosed at multiphase gated computed tomography (CT) angiography as the presence of reduced leaflet motion (RELM) and hypoattenuating aortic leaflet thickening (HALT). Although CT is used to exclude BPVT in symptomatic patients and those with increased valve gradients, the value of screening and prophylactic anticoagulation is debatable.

Glycation and Serum Albumin Infiltration Contribute to the Structural Degeneration of Bioprosthetic Heart Valves

•

Two novel and interacting mechanisms contributing to BHV SVD are reported: glycation and serum albumin infiltration.

•

Glycation product formation and serum albumin deposition were observed in 45 clinical BHV explanted due to SVD as well as BHV tissue subcutaneously implanted in rats.

•

In vitro exposure to glycation and serum albumin elicited collagen network misalignment similar to that seen in clinical and rat explant BHV tissue.

•

Glycation was sufficient to impair BHV hydrodynamic function in ISO-5840-compliant pulse duplication testing and concomitant serum albumin infiltration exacerbated these effects.

Valvular heart diseases are associated with significant cardiovascular morbidity and mortality, and often require surgical and/or percutaneous repair or replacement. Valve replacement is limited to mechanical and biological prostheses, the latter of which circumvent the need for lifelong anticoagulation but are subject to structural valve degeneration (SVD) and failure. Although calcification is heavily studied, noncalcific SVD, which represent roughly 30% of BHV failures, is relatively underinvestigated. This original work establishes 2 novel and interacting mechanisms—glycation and serum albumin incorporation—that occur in clinical valves and are sufficient to induce hallmarks of structural degeneration as well as functional deterioration.

Osteopontin is elevated in patients with mitral annulus calcification independent from classic cardiovascular risk factors

Background

Osteopontin (OPN) regulates the Ca⁺⁺-deposition in bone and coronary arteries. Elevated OPN were also associated with (aortic) valve calcification in healthy individuals. This study aimed to investigate the association between OPN levels and mitral annulus calcification (MAC) in patients with coronary artery disease (CAD).

Methods

In this cross-sectional study OPN-levels were measured in 223 non-or ex-smoking patients (160 male, mean age: 61,09 ± 11,02 years; 63 female: mean age: 67,49 ± 7,87 years) with CAD. Plasma OPN levels were measured by ELISA and MAC was evaluated by echocardiography.

Results

Forward stepwise logistic regression analysis (likelihood quotient) showed significantly higher OPN-levels in patients with MAC compared to patient without calcified mitral annulus independent from the classic risk factors age and severity of coronary artery disease (CAD). In addition to age and the severity of CAD, the circulating OPN amount was a significant predictor for MAC.

Conclusions

This is the first clinical trial which observed increased circulating OPN levels in MAC, suggesting a distinct role of OPN in the process of MAC. Considering the current knowledge about OPN it is more likely that OPN does not promote but counteracts valve calcification and therefore is elevated in course of a calcification processes.

Osteopontin-CD44v6 interaction mediates calcium deposition via phospho-Akt in valve interstitial cells from patients with noncalcified aortic valve sclerosis

OBJECTIVE

The activation of valve interstitial cells (VICs) toward an osteogenic phenotype characterizes aortic valve sclerosis, the early asymptomatic phase of calcific aortic valve disease. Osteopontin is a phosphorylated acidic glycoprotein that accumulates within the aortic leaflets and labels VIC activation even in noncalcified asymptomatic patients. Despite this, osteopontin protects VICs against in vitro calcification. Here, we hypothesize that the specific interaction of osteopontin with CD44v6, and the related intracellular pathway, prevents calcium deposition in human-derived VICs from patients with aortic valve sclerosis.

APPROACH AND RESULTS

On informed consent, 23 patients and 4 controls were enrolled through the cardiac surgery and heart transplant programs. Human aortic valves and VICs were tested for osteogenic transdifferentiation, ex vivo and in vitro. Osteopontin-CD44 interaction was analyzed using proximity ligation assay and the signaling pathways investigated. A murine model based on angiotensin II infusion was used to mimic early pathological remodeling of the aortic valves. We report osteopontin-CD44 functional interaction as a hallmark of early stages of calcific aortic valve disease. We demonstrated that osteopontin-CD44 interaction mediates calcium deposition via phospho-Akt in VICs from patients with noncalcified aortic valve sclerosis. Finally, microdissection analysis of murine valves shows increased cusp thickness in angiotensin II-treated mice versus saline infused along with colocalization of osteopontin and CD44 as seen in human lesions.

CONCLUSIONS

Here, we unveil a specific protein-protein association and intracellular signaling mechanisms of osteopontin. Understanding the molecular mechanisms of early VIC activation and calcium deposition in asymptomatic stage of calcific aortic valve disease could open new prospective for diagnosis and therapeutic intervention.

Aortic stenosis: a general overview of clinical, pathophysiological and therapeutic aspects

Aortic stenosis is the most prevalent valve pathology and calcific aortic valve disease (CAVD) is its most frequent etiology in developed countries. There is extensive evidence that CAVD represents an active disease process similar to that of atherosclerosis with similar classical cardiovascular risk factors and pathological mechanisms. Given that in the vast majority of situations the only treatment available is valve replacement there is a need to develop pharmacological therapies that retard the disease progression. Lipid-lowering therapies have been the focus of research, however, so far with negative results. Future studies, including animal models, shall provide an opportunity to further evaluate the disease mechanisms of CAVD and to discover potential disease biomarkers and pharmacological interventions that can reduce the need for valve replacement.

Renal papillary calcification and the development of calcium oxalate monohydrate papillary renal calculi: a case series study

BACKGROUND

The objective of this study is to determine in a case series (four patients) how calcified deposits in renal papillae are associated with the development of calcium oxalate monohydrate (COM) papillary calculi.

METHODS

From the recently collected papillary calculi, we evaluated retrospectively patients, subjected to retrograde ureteroscopy, with COM papillary lithiasis.

RESULTS

The COM papillary calculi were found to result from subepithelial injury. Many of these lesions underwent calcification by hydroxyapatite (HAP), with calculus morphology and the amount of HAP in the concave zone dependent on the location of the calcified injury. Most of these HAP deposits grew, eroding the epithelium covering the renal papillae, coming into contact with urine and starting the development of COM calculi. Subepithelial HAP plaques may alter the epithelium covering the papillae, resulting in the deposit of COM crystals directly onto the epithelium. Tissue calcification depends on a pre-existing injury, the continuation of this process is due to modulators and/or crystallization inhibitors deficiency.

CONCLUSIONS

Since calculus morphology and the amount of detected HAP are dependent on the location and widespread of calcified injury, all types of papillary COM calculi can be found in the same patient. All patients had subepithelial calcifications, with fewer papillary calculi, demonstrating that some subepithelial calcifications did not further evolve and were reabsorbed. A high number of subepithelial calcifications increases the likelihood that some will be transformed into COM papillary calculi.

Selection of artificial valve for the patients on hemodialysis

The selection of artificial valve for the hemodialysis patients is still controversial. Initially ACC/AHA guideline recommended using mechanical valve because of concern on the durability of bioprosthesis after replacement on the dialysis patients; however, revised guideline deleted that recommendation. Although many reports recognized rapid deterioration of bioprosthesis mainly due to calcification after valve replacement, there is no difference on survival between both types of valve. The recently conducted meta-analysis reported the same conclusions. Actually the long-term survival of the dialysis patients is poorer than that of non-dialysis people; however, it differs according to the etiology of renal failure. For example, the long-term survival of the non-diabetic patients seems longer than that of diabetic patients requiring longer durability of artificial valve. According to ACC/AHA guideline and the meta-analysis, surgeon should not hesitate to use bioprosthetic valve; however, surgeon should consider stratification of the dialysis patients by prediction for the long survival of each patient.

Dephosphorylation of circulating human osteopontin correlates with severe valvular calcification in patients with calcific aortic valve disease

CONTEXT

Calcific Aortic Valve Disease (CAVD) is an active pathological process leading to biomineralization of the aortic cusps. We characterized circulating and tissue Osteopontin (OPN) as a biomarker for CAVD.

OBJECTIVES

Here we investigate the post-translational modifications of circulating OPN and correlate the phosphorylation status with the ability to prevent calcification.

METHODS

Circulating OPN levels were estimated in CAVD patients (n = 51) and controls (n = 56). In a subgroup of 27 subjects, OPN was purified and the phosphorylation status analyzed.

RESULTS

Plasma OPN levels were significantly elevated in CAVD patients as compared to the controls and correlates with the aortic valve calcium score. Our study demonstrates that phospho-threonine levels of OPN purified from controls were higher when compared to CAVD subjects, whereas phospho-serine and phospho-tyrosine levels were comparable between the two groups.

CONCLUSION

The dephosphorylation of circulating OPN correlates with severe valvular calcification in patients with CAVD.

Analysis of osteopontin levels for the identification of asymptomatic patients with calcific aortic valve disease

BACKGROUND

Calcific aortic valve disease (CAVD) is the most common cause of acquired valve disease. Initial phases of CAVD include thickening of the cusps, whereas advanced stages are associated with biomineralization and reduction of the aortic valve area. These conditions are known as aortic valve sclerosis (AVSc) and aortic valve stenosis (AVS), respectively. Because of its asymptomatic presentation, little is known about the molecular determinants of AVSc. The aim of this study was to correlate plasma and tissue osteopontin (OPN) levels with echocardiographic evaluation for the identification of asymptomatic patients at risk for CAVD. In addition, our aim was to analyze the differential expression and biological function of OPN splicing variants as biomarkers of early and late stages of CAVD.

METHODS

From January 2010 to February 2011, 310 patients were enrolled in the study. Patients were divided into 3 groups based on transesophageal echocardiographic (TEE) evaluation: controls (56 patients), AVSc (90 patients), and AVS (164 patients). Plasma and tissue OPN levels were measured by immunohistochemical evaluation, enzyme-linked immunosorbent assay (ELISA), and real-time quantitative polymerase chain reaction (qPCR).

RESULTS

Patients with AVSc and AVS have higher OPN levels compared with controls. OPN levels are elevated in asymptomatic patients with AVSc with no appearance of calcification during TEE evaluation. OPN splicing variants OPN-a, OPN-b, and OPN-c are differentially expressed during CAVD progression and are able to inhibit biomineralization in a cell-based biomineralization assay.

CONCLUSIONS

The analysis of the differential expression of OPN splicing variants during CAVD may help in developing diagnostic and risk stratification tools to follow the progression of asymptomatic aortic valve degeneration.

In vivo calcification of glutaraldehyde-fixed cardiac valve and pericardium of Phoca groenlandica

Calcification remains the main reason for failure of bioprosthetic valves. The aim of this study was to evaluate the in vivo calcification response of a new bioprosthetic valve, derived from cardiac tissue of Phoca groenlandica. Aortic and pulmonary leaflets, bovine, and Phoca groenlandica pericardia were fixed in buffered glutaraldehyde solution. Tissues were divided into four groups: group 1, bovine pericardium (BP); group 2, pulmonary leaflets; group 3, seal pericardium; and group 4, aortic leaflets. All samples were implanted subdermally into four sets of eight female 12-day-old Wistar rats for 21 days. The tissues were divided into two parts for calcium measurement, and histology with hematoxylin-eosin, von Kossa, and Weigert Van Gieson staining. All groups experienced significant calcification. Group 1 with 1.39 mg/g (0.34) before and 125.78 mg/g (21.48) after implantation (p < 0.001), group 2 with 1.50 mg/g (0.43) before and 151.85 mg/g (19.1) after (p < 0.001), group 3 with 3.15 mg/g (0.62) before and 116.38 mg/g (33.74) after (p < 0.001), and group 4 with 1.84 mg/g (0.52) before and 126.95 mg/g (13.37) after (p < 0.001). Explant samples showed foreign body response, disorganized collagen, and obvious calcification. The cardiac valve and pericardium of Phoca groenlandica calcify to the same extent as the BP.

Role of carbonic anhydrase II in ectopic calcification

INTRODUCTION

Osteopontin (OPN) is a potent inhibitor of ectopic calcification. Previous studies suggested that, in addition to blocking apatite crystal growth, OPN promoted regression of ectopic calcification by inducing the expression of acid-generating carbonic anhydrase II (CAR2) in monocyte-derived cells.

METHODS

To test this hypothesis, OPN and CAR2 expression and calcification of subcutaneously implanted glutaraldehyde-fixed bovine pericardium (GFBP) were studied in CAR2 mutant mice.

RESULTS

Consistent with previous studies in Black Swiss mice, GFBP calcified to a greater extent in OPN-deficient mice compared to wild types on the C57Bl/6 background. GFBP implanted in CAR2-deficient mice (CAR2(-/-)) were significantly more calcified than those implanted into wild-type mice (CAR2(+/+)) [37+/-5 vs. 20+/-6.5 microg Ca/mg tissue, respectively, at 30 days (P<.001), and 42+/-5 versus 20+/-4 microg Ca/mg tissue at 60 days, respectively (P<.001)]. On the other hand, OPN levels within and surrounding the implants were similar in CAR2(+/+) and CAR2(-/-) mice, suggesting that OPN expression in the absence of CAR2 was not sufficient to mitigate ectopic calcification.

CONCLUSIONS

These results indicate that CAR2 expression is an important regulator of ectopic calcification, potentially by facilitating OPN mediated mineral regression.

Phytate inhibits bovine pericardium calcification in vitro

OBJECTIVE

The present study examined the inhibitory effects of pyrophosphate, etidronate, and phytate on bovine pericardium calcification in vitro.

METHODS

Bovine pericardium was glutaraldehyde fixed and then placed in a flow chamber in the presence of a synthetic physiological fluid alone (control) or the fluid plus various concentrations of pyrophosphate, etidronate, or phytate. Following a 96-h incubation, fragments were removed and assayed for calcification by measuring calcium and phosphorus levels.

RESULTS

The data indicated that both pyrophosphate and etidronate at 1 mg/l (5.75 and 4.95 microM, respectively) inhibited bovine pericardium calcification, whereas neither agent had an effect at 0.5 mg/l (2.87 and 2.47 microM, respectively). Phytate was the most potent inhibitor of calcification, and the effects of this agent were apparent at levels as low as 0.25 mg/l (0.39 microM).

CONCLUSIONS

While pyrophosphate, etidronate, and phytate were all able to inhibit bovine pericardium calcification in vitro, phytate was found to be the most effective.

Bioprosthetic valve degeneration due to cholesterol deposition in a patient with normal lipid profile

Hypercholesterolemia has been identified as a risk factor for bioprosthetic valvular degeneration, and it has been suggested that statin therapy reduces this risk. The case of a 77-year-old man with low levels of low-density lipoprotein cholesterol (LDL-C) and apolipoprotein B who developed marked LDL-C crystal deposition and a severe foreign body giant cell reaction 21.5 years after aortic bioprosthetic replacement is reported. This observation confirms that cholesterol deposition contributes to bioprosthetic valve degeneration, but that this can occur even in patients with low levels of LDL-C. It suggests that the characteristics of the valve are more critical than the patient's level of LDL-C.

Effect of Crystallization Inhibitors on Vascular Calcifications Induced by Vitamin D A Pilot Study in Sprague-Dawley Rats

BACKGROUND

Pathological calcification in soft tissues (ie, ectopic calcification) can have severe consequences. Hydroxyapatite is the common mineral phase present in all tissue calcifications. In general, the development of tissue calcifications requires a pre-existing injury as an inducer (heterogeneous nucleant), whereas further progression requires the presence of other promoter factors (such as hypercalcemia and/or hyperphosphatemia) and/or a deficiency in calcification repressor factors (crystallization inhibitors and cellular defense mechanisms). The present study investigated the capacity of etidronate (a bisphosphonate used in osteoporosis treatment) and phytate (a natural product) to inhibit vascular calcification in rats.

METHODS AND RESULTS

Six male Sprague-Dawley rats in each of the 3 treatment groups were subcutaneously injected with either a placebo (physiological serum solution), etidronate (0.825 micromol x kg(-1) x day (-1)) or phytate (0.825 micromol x kg (-1) x day(-1)) for 8 days. Four days into this regimen, calcinosis was induced by subcutaneous injections of 500,000 IU/kg vitamin D at 0 h, 24 h and 48 h. Ninety-six hours after the final vitamin D injection, the rats were killed and aortas and their hearts were removed for histological and calcium analyses. The data showed that phytate-treated rats had lower levels of aortic calcium than placebo-treated rats. All groups had similar heart calcium levels.

CONCLUSIONS

The present study found that phytate acted as a vascular calcification inhibitor. Thus, the action of polyphosphates could be important in protecting against vascular calcification.

Mammary Gland Secretory Concretions Contain Non-Collagenous Bone Matrix Proteins

Secretory concretions in mammary gland alveoli are commonly of microscopical size. However, some concretions reach clinically palpable dimensions and may occlude teat canals and obstruct milk flow. We studied secretory concretions in sheep, goat and cow mammary glands, using routine histological staining methods, conventional histochemistry and electron microscopy. As concretions frequently mineralize, immunostaining for keratan sulphate and calcium-binding non-collagenous bone matrix proteins (bone sialoprotein, osteocalcin, osteonectin and osteopontin) was performed. Concretions consisted of organic matrix (condensed secretions) with calcium precipitates. Mineralized deposits mostly show concentric organization, bound haematoxylin, and were readily identified in H&E-stained sections. Mineral components of concretions reacted for calcium carbonate and phosphate, organic matrix was found to contain sialoglycan material. Immunohistochemistry revealed bone sialoprotein, osteonectin and keratan sulphate in cow and goat concretions. Osteocalcin was detected in sheep, cow and goat concretions, whilst osteopontin was not identified in any of the specimens studied. Our results indicate the presence of non-collagenous bone matrix proteins (except osteopontin) in mammary gland concretions. These glycoproteins are commonly thought to govern mineralization of organic matrix and are assumed also to promote mineral deposition in mammary gland secretory concretions. Besides caseins, these particular glycoproteins have to be considered as calcium-binding milk proteins.

Degeneration of native and tissue prosthetic valve in aortic position: do statins play an effective role in prevention?

Degenerative aortic valve stenosis is a common disease in western countries. When it becomes severe, it confers significant morbidity and mortality. Aortic stenosis has been recognized as a complex inflammatory and highly regulated process with histological and immunochemical similarities with the process of atherosclerosis. Hypertension, smoking and diabetes mellitus have consistently been linked to the development of aortic stenosis. Endothelial injury or other processes that contribute to coronary disease may play a role in calcific aortic stenosis. Several observational studies suggests that the key factors of aortic stenosis are lipoproteins and that medical therapies with cholesterol lowering drugs may retard its progression. Similarly, it has been suggested that the process of degeneration of the tissue heart valve has been associated with the same risk factors of atherosclerosis and shares many histological and molecular characteristics. Assuming all this concept, and evaluating the results of a retrospective study it has been suggested to use statin also as medical therapy able to prevent tissue valve degeneration. Randomized controlled clinical trials will be needed to demonstrate the role of lipid intervention to prevent the progression of aortic stenosis and the degeneration of tissue heart valves.

Clinical manifestations and pathogenesis of hydroxyapatite crystal deposition in juvenile dermatomyositis

Pathologic deposition of mineral in the form of bone-like hydroxyapatite is a frequent occurrence in juvenile dermatomyositis (JDM) and other connective tissue diseases. Although the sizes of the mineral crystals in JDM are similar to those in bone, there is much more mineral in the deposits than there is in bone. Bone matrix proteins also accumulate associated with the deposits. The reasons for the formation of these deposits are not known. It is our hypothesis that persistent inflammation is a component of JDM and other hydroxyapatite deposition diseases. Other contributing factors are genetic, environmental, and physical chemical. This paper discusses the influence of inflammation on the deposition of hydroxyapatite, with emphasis on the clinical and environmental factors that may facilitate the formation of calcific deposits in JDM.

Regulation of vascular calcification: roles of phosphate and osteopontin

Vascular calcification is prevalent in aging as well as a number of pathological conditions, and it is now recognized as a strong predictor of cardiovascular events in the general population as well as diabetic and end-stage renal disease patients. Vascular calcification is a highly regulated process involving inductive and inhibitory mechanisms. This article focuses on two molecules, phosphate and osteopontin, that have been implicated in the induction or inhibition of vascular calcification, respectively. Elevated phosphate is of interest because hyperphosphatemia is recognized as a major nonconventional risk factor for cardiovascular disease mortality in end-stage renal disease patients. Studies to date suggest that elevated phosphate stimulates smooth muscle cell phenotypic transition and mineralization via the activity of a sodium-dependent phosphate cotransporter. Osteopontin, however, appears to block vascular calcification most likely by preventing calcium phosphate crystal growth and inducing cellular mineral resorption.

Regulation of cardiovascular calcification

Vascular calcification is highly correlated with cardiovascular disease (CVD) and is a significant predictor of cardiovascular events, especially in high risk patients such as the end stage renal disease (ESRD) population. Vascular calcification can lead to serious problems including valve stenosis, decreased vascular compliance, calciphylaxis, and even sudden death. However, the contribution of vascular calcification to progression of atherosclerosis is unknown and needs more study. Biochemical, histological, and genetic studies indicate that vascular calcification is actively regulated and involves both positive and negative modulators. Several nonmutually exclusive theories to account for vascular calcification based on current studies are discussed.

Osteopontin inhibits mineral deposition and promotes regression of ectopic calcification

Ectopic calcification, the abnormal calcification of soft tissues, can have severe clinical consequences especially when localized to vital organs such as heart valves, arteries, and kidneys. Recent observations suggest that ectopic calcification, like bone biomineralization, is an actively regulated process. These observations have led a search for molecular determinants of ectopic calcification. A candidate molecule is osteopontin (OPN), a secreted phosphoprotein invariantly associated with both normal and pathological mineral deposits. In the present study, OPN was found to be a natural inhibitor of ectopic calcification in vivo. Glutaraldehyde-fixed aortic valve leaflets showed accelerated and fourfold to fivefold greater calcification after subcutaneous implantation into OPN-null mice compared to wild-type mice. In vitro and in vivo studies suggest that OPN not only inhibits mineral deposition but also actively promotes its dissolution by physically blocking hydroxyapatite crystal growth and inducing expression of carbonic anhydrase II in monocytic cells and promoting acidification of the extracellular milieu. These findings suggest a novel mechanism of OPN action and potential therapeutic approach to the treatment of ectopic calcification.

Inactivation of the osteopontin gene enhances vascular calcification of matrix Gla protein-deficient mice: evidence for osteopontin as an inducible inhibitor of vascular calcification in vivo

Osteopontin (OPN) is abundantly expressed in human calcified arteries. To examine the role of OPN in vascular calcification, OPN mutant mice were crossed with matrix Gla protein (MGP) mutant mice. Mice deficient in MGP alone (MGP(-/-) OPN(+/+)) showed calcification of their arteries as early as 2 weeks (wk) after birth (0.33 +/- 0.01 mmol/g dry weight), and the expression of OPN in the calcified arteries was greatly up-regulated compared with MGP wild-types. OPN accumulated adjacent to the mineral and colocalized to surrounding cells in the calcified media. Cells synthesizing OPN lacked smooth muscle (SM) lineage markers, SM alpha-actin and SM22alpha. However, most of them were not macrophages. Importantly, mice deficient in both MGP and OPN had twice as much arterial calcification as MGP(-/-) OPN(+/+) at 2 wk, and over 3 times as much at 4 wk, suggesting an inhibitory effect of OPN in vascular calcification. Moreover, these mice died significantly earlier (4.4 +/- 0.2 wk) than MGP(-/-) OPN(+/+) counterparts (6.6 +/- 1.0 wk). The cause of death in these animals was found to be vascular rupture followed by hemorrhage, most likely due to enhanced calcification. These studies are the first to demonstrate a role for OPN as an inducible inhibitor of ectopic calcification in vivo.

Protein adsorption of calcified and noncalcified valvular bioprostheses after human implantation

BACKGROUND

The incidence of calcification of porcine valve bioprostheses shows important, and as yet unexplained, variations. Previous studies by others showed that osteopontin and osteocalcin are expressed in calcified porcine valve bioprostheses. However, no study has yet explored other proteins that could also be involved.

METHODS

Twelve porcine valve bioprostheses were retrieved from 12 patients and were separated into two groups. Group 1 (n = 6) had early calcification after 4 to 9 years (mean, 6+/-2.3 years). The mean age of the patients at the time of implantation was 46+/-9 years. Group 2 (n = 6) had no calcification after 8 to 14 years (mean, 12+/-2.8 years). The mean age was 47+/-13.4 years. These valves were analyzed by electrophoresis, and the bands were quantified by densitometry.

RESULTS

A 14-kd protein showed a 50% increase in the calcified group. A 31-kd protein found in the calcified group was not detected in the noncalcified group. Three other proteins (45, 39, and 28 kd) showed reduced adsorption in the calcified group.

CONCLUSIONS

Important differences were found in the proteins adsorbed in calcified and noncalcified bioprostheses after implantation in patients. Besides osteopontin and osteocalcin, several other proteins may play a role in the process of calcification of valvular bioprostheses.

Effect of ethanol and ether in the prevention of calcification of bioprostheses

BACKGROUND

Lipids play a significant role in the process of calcification of bioprostheses. We assessed whether lipid extraction by ethanol, ether, or a surfactant could mitigate calcification of glutaraldehyde-treated bioprostheses.

METHODS

On 200 bovine pericardium samples pretreated with 0.6% glutaraldehyde, lipid extraction was carried out by ethanol, ether, or the tween 80 surfactant, and combinations thereof. The treated tissues were implanted subcutaneously in 50 juvenile rats for 4 and 6 months. Lipids were analyzed by Fourier transform infrared spectrophotometer and chromatography before implantation. Calcium content of implanted tissues was assessed by atomic absorption spectrometer.

RESULTS

Ethanol, ether, or surfactant did mitigate calcification. The most efficient pretreatments were the combination of ethanol and surfactant (calcium content: 15.5+/-6.8 microg/mg dry tissue after 6 months implantation) or the combination of ethanol, ether, and surfactant (13.1+/-6.2 microg/mg dry tissue) when compared with surfactant alone (42.9+/-12.7 microg/mg dry tissue).

CONCLUSIONS

Ethanol or the combination of ethanol and ether added to the currently used glutaraldehyde-surfactant treatment further mitigates calcification.

Osteopontin

Osteopontin (OPN) is a highly phosphorylated sialoprotein that is a prominent component of the mineralized extracellular matrices of bones and teeth. OPN is characterized by the presence of a polyaspartic acid sequence and sites of Ser/Thr phosphorylation that mediate hydroxyapatite binding, and a highly conserved RGD motif that mediates cell attachment/signaling. Expression of OPN in a variety of tissues indicates a multiplicity of functions that involve one or more of these conserved motifs. While the lack of a clear phenotype in OPN "knockout" mice has not established a definitive role for OPN in any tissue, recent studies have provided some novel and intriguing insights into the versatility of this enigmatic protein in diverse biological events, including developmental processes, wound healing, immunological responses, tumorigenesis, bone resorption, and calcification. The ability of OPN to stimulate cell activity through multiple receptors linked to several interactive signaling pathways can account for much of the functional diversity. In this review, we discuss the structural features of OPN that relate to its function in the formation, remodeling, and maintenance of bones and teeth.

Calcification of vascular smooth muscle cell cultures: inhibition by osteopontin

Calcification of vascular tissue is a common complication in aging, atherosclerosis, diabetes, renal failure, aortic stenosis, and prosthetic valve replacement. Osteopontin is a noncollagenous adhesive protein routinely found at sites of dystrophic calcification and synthesized at high levels by macrophages in calcified aortic valves and atherosclerotic plaques. In the present study, we have characterized the calcification of bovine aortic smooth muscle cell (BASMC) cultures in vitro and have studied the effects of exogenous osteopontin on mineral deposition. Induction of calcification in BASMC cultures was alkaline phosphatase-dependent and was characterized by a multilayer cell morphology. Mineral deposition occurred in the basal matrix of multilayered areas as indicated by von Kossa staining, and transmission electron microscopy and electron diffraction identified the mineral as apatite. Ultrastructural analysis of the cultures showed the presence of extracellular matrix vesicles, calcifying collagen fibrils, and nodular-type calcifications similar to those found in calcified heart valves and atherosclerotic plaques. Purified osteopontin (0.05 to 5 microgram/mL) dose dependently inhibited calcification of BASMC cultures, whereas vitronectin and fibronectin had no effect. In contrast to the inhibitory mechanism of levamisole on mineral deposition, osteopontin did not inhibit alkaline phosphatase activity or reduce phosphorus levels in the culture medium. Addition of calcium to the cultures overcame the inhibitory effect of osteopontin on BASMC culture calcification and resulted in decreased levels of calcium in the culture medium and increased levels in the cell layer. Moreover, using high-resolution, colloidal-gold immunocytochemistry, osteopontin was found intimately associated with growing apatite crystals. These data indicate that the effect of osteopontin, although calcium-dependent, was not mediated by simple calcium chelation but most likely by direct interaction of osteopontin with crystal surfaces. These studies suggest that BASMCs can be used to model vascular calcification in vitro and that soluble osteopontin released near sites of vascular calcification may represent an adaptive mechanism aimed at preventing vascular calcification.

Infrared spectroscopy in the evaluation of the process of calcification of valvular bioprostheses

BACKGROUND

The methods currently used to analyze the process of calcification of bioprostheses give only global information on calcium deposition. We investigated the potential advantage of infrared spectroscopy, which makes it possible to analyze the various components of the calcification process, ie, lipids, proteins, and calcium deposits.

METHODS

Sixty porcine aortic leaflets were fixed in 0.6% glutaraldehyde and then subsequently implanted in 10-day-old Wistar rats. The valve leaflets were removed 2, 7, 14, 21, 35, and 56 days after implantation.

RESULTS

Before implantation infrared spectroscopic analysis revealed the presence of proteins only. On day 2 after implantation, all valves showed minor lipid deposits. On day 7, amorphous calcium phosphate was detected. Between days 7 and 14, crystalline forms of calcium phosphate appeared and amorphous calcium phosphate progressively changed into carbapatite over the 56-day period.

CONCLUSIONS

Infrared spectroscopy yields valuable additional information on the nature and kinetics of the various components of glutaraldehyde-treated tissues after implantation. It may prove to be important in the evaluation of new techniques of calcium mitigation.