Colocalization of cholesterol and hydroxyapatite in human atherosclerotic lesions

Ex-vivo atherosclerotic plaque characterization using spectral photon-counting CT: Comparing material quantification to histology

BACKGROUND AND AIMS

Atherosclerotic plaques are characterized as being vulnerable to rupture based on a series of histologically defined features, including a lipid-rich necrotic core, spotty calcification and ulceration. Existing imaging modalities have limitations in their ability to distinguish between different materials and structural features. We examined whether X-ray spectral photon-counting computer tomography (SPCCT) images were able to distinguish key plaque features in a surgically excised specimen from the carotid artery with comparison to histological images.

METHODS

An excised carotid plaque was imaged in the diagnostic X-ray energy range of 30-120 keV using a small-bore SPCCT scanner equipped with a Medipix3RX photon-counting spectral X-ray detector with a cadmium telluride (CdTe) sensor. Material identification and quantification (MIQ) images of the carotid plaque were generated using proprietary MIQ software at 0.09 mm volumetric pixels (voxels). The plaque was sectioned, stained and photographed at high resolution for comparison.

RESULTS

A lipid-rich core with spotty calcification was identified in the MIQ images and confirmed by histology. MIQ showed a core region containing lipid, with a mean concentration of 260 mg lipid/ml corresponding to a mean value of -22HU. MIQ showed calcified regions with mean concentration of 41 mg Ca/ml corresponded to a mean value of 123HU. An ulceration of the carotid wall at the bifurcation was identified to be lipid-lined, with a small calcification identified near the breach of the artery wall.

CONCLUSIONS

SPCCT derived material identification and quantification images showed hallmarks of vulnerable plaque including a lipid-rich necrotic core, spotty calcifications and ulcerations.

Characteristics of non-culprit plaques in acute coronary syndrome patients with calcified plaque at the culprit lesion

OBJECTIVES

To investigate the non-culprit plaques (NCPs) characteristics in acute coronary syndrome (ACS) patients with calcified plaques (CP).

BACKGROUND

Recently, a new in vivo classification of calcified culprit plaques in patients with ACS was proposed. Characteristics of NCPs in this group of patients are unknown.

METHODS

A total of 692 NCPs from 492 ACS patients were retrospectively compared based on the culprit plaque phenotype: 71 from CP patients, 383 from plaque rupture (PR) patients, 238 from plaque erosion (PE) patients.

RESULTS

NCPs of CP patients had greater maximal calcium thickness, wider calcium arc, longer calcium length, and greater calcium index, compared to PR or PE patients (CP vs. PR: all p < .001, CP vs. PE: all p < .001). Thin-cap fibroatheroma was less prevalent (p = .023), fibrous cap was thicker (p = .035), and mean lipid arc was narrower in CP than in PR (p < .001).

CONCLUSIONS

In conclusion, NCPs of CP patients had greater calcium burden and less vulnerability. This information may help to better understand the underlying mechanisms of ACS and to develop strategy for tailored management.

Interactions between hydroxyapatite and cholesterol associated with calcification in age-related macular degeneration

Hydroxyapatite deposition and calcification occurs over cholesterol-containing lipid droplets between Bruch's membrane and sub-retinal pigment epithelium (sub - RPE) in the eyes of patients affected by age-related macular degeneration (AMD) as spherules, nodules, and Bruch's membrane plaques. In the present study, an attempt has been made to prepare a composite containing hydroxyapatite and cholesterol to elucidate interactions involved in the formation of such organic-inorganic interphase. To understand the mechanism of hydroxyapatite deposition on cholesterol, we have applied various biophysical techniques such as dynamic light scattering (DLS) measurements, transmission electron microscopy (TEM) imaging, Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance (ssNMR) spectroscopy on the prepared composite. Our results give molecular level insight into the mechanism of biocalcification in the disease system.

Emerging techniques in atherosclerosis imaging

Atherosclerosis is a chronic immunomodulated disease that affects multiple vascular beds and results in a significant worldwide disease burden. Conventional imaging modalities focus on the morphological features of atherosclerotic disease such as the degree of stenosis caused by a lesion. Modern CT, MR and positron emission tomography scanners have seen significant improvements in the rapidity of image acquisition and spatial resolution. This has increased the scope for the clinical application of these modalities. Multimodality imaging can improve cardiovascular risk prediction by informing on the constituency and metabolic processes within the vessel wall. Specific disease processes can be targeted using novel biological tracers and "smart" contrast agents. These approaches have the potential to inform clinicians of the metabolic state of atherosclerotic plaque. This review will provide an overview of current imaging techniques for the imaging of atherosclerosis and how various modalities can provide information that enhances the depiction of basic morphology.

Role of lipoprotein (a) and LPA KIV2 repeat polymorphism in bicuspid aortic valve stenosis and calcification: a proof of concept study

Hemodynamic valvular impairment is a frequent determinant of the natural history of bicuspid aortic valve (BAV). The role of elevated Lp(a) levels and LPA Kringle IV type 2 (KIV-2) size polymorphism in influencing aortic valve calcification and stenosis development in patients with tricuspid aortic valve was recognized. In this study, we investigate the association between Lp(a) and LPA KIV-2 repeat number, and the presence of calcification and stenosis in BAV patients. Sixty-nine patients [79.7% males; median age 45(30-53) yrs], consecutively referred to Center for Cardiovascular Diagnosis or Referral Center for Marfan syndrome or related disorders, AOU Careggi, from June to November 2014, were investigated. For each patient, clinical (ECG and echocardiography) and laboratory [Lp(a) (Immunoturbidimetric assay) and LPA KIV-2 repeat number (real-time PCR)] evaluation were performed. Patients were compared with 69 control subjects. No significant association between Lp(a) circulating levels and LPA KIV-2 repeat number and BAV was evidenced. Among BAV patients, significantly higher Lp(a) levels according to calcification degree were found [no calcifications:78(42-159) mg/L, mild/moderate: 134(69-189) mg/L; severe: 560(286-1511) mg/L, p = 0.008]. Conversely, lower LPA KIV-2 repeat numbers in subjects with more severe calcification degree were observed. Furthermore, higher Lp(a) levels in patients with aortic stenosis [214(67-501) mg/L vs 104(56-169) mg/L, p = 0.043] were also found. In conclusion, present data suggest the potential role for Lp(a) as a possible risk marker useful to stratify, among BAV patients, those with a higher chance to develop valvular calcifications and aortic stenosis.

Cells and extracellular matrix interplay in cardiac valve disease: because age matters

Cardiovascular aging is a physiological process affecting all components of the heart. Despite the interest and experimental effort lavished on aging of cardiac cells, increasing evidence is pointing at the pivotal role of extracellular matrix (ECM) in cardiac aging. Structural and molecular changes in ECM composition during aging are at the root of significant functional modifications at the level of cardiac valve apparatus. Indeed, calcification or myxomatous degeneration of cardiac valves and their functional impairment can all be explained in light of age-related ECM alterations and the reciprocal interplay between altered ECM and cellular elements populating the leaflet, namely valvular interstitial cells and valvular endothelial cells, is additionally affecting valve function with striking reflexes on the clinical scenario. The initial experimental findings on this argument are underlining the need for a more comprehensive understanding on the biological mechanisms underlying ECM aging and remodeling as potentially constituting a pharmacological therapeutic target or a basis to improve existing prosthetic devices and treatment options. Given the lack of systematic knowledge on this topic, this review will focus on the ECM changes that occur during aging and on their clinical translational relevance and implications in the bedside scenario.

The incidence and morphology of Monckeberg's medial calcification in banked vascular segments from a monocentric donor population

Little is known about the real incidence and the clinical relevance of the enigmatic Monckeberg's medial calcification in the patency of the femoral artery allograft. Here we present a retrospective study on 143 multiorgan donors (mean age 38 years, range 14-59 years), to describe the incidence and the morphological features of vascular calcifications in banked femoral arteries suitable for clinical use. In the present series, focal vascular calcifications were present in 36 (25 %) cases, 23 cases localized in the intima, 7 in the media, and 6 were mixed. No correlation was found between the incidence of calcifications and the classical cardiovascular clinical risk factors (n = 9); only hypertension correlated with the medial localization, but not with the incidence, of the calcification (P = 0.017). While the macroscopic exclusion criteria of vascular grafts include atheromatous and not-atheromatous lesions, we ignore the actual impact of Monckeberg's medial calcification on vessel transplantation and allograft life. In our opinion this is a very important topic, since when the histological criteria for Monckeberg's calcification diagnosis are used, 25 % of our young donors population was affected. Whether Monckeberg's medial calcification is a stable arterial condition, apparently underestimated in the general population, or a dynamic process evolving with age and atherosclerosis, or a banking-related vascular alteration, still remain an open issue deserving further studies with subjects of different ages.

Lipoprotein (a) in calcific aortic valve disease: from genomics to novel drug target for aortic stenosis

Calcific aortic stenosis (AS) is the most common form of valve disease in the Western world and affects over 2.5 million individuals in North America. Despite the large burden of disease, there are no medical treatments to slow the development of AS, due at least in part to our incomplete understanding of its causes. The Cohorts for Heart and Aging Research in Genetic Epidemiology extra-coronary calcium consortium reported a genome-wide association study demonstrating that genetic variants in LPA are strongly associated with aortic valve (AV) calcium and clinical AS. Using a Mendelian randomization study design, it was demonstrated that the effect of this genetic variant is mediated by plasma lipoprotein (a) [Lp(a)], directly implicating elevations in Lp(a) as a cause of AV calcium and progression to AS. This discovery has sparked intense interest in Lp(a) as a modifiable cause for AV disease. Herein, we will review the mounting epidemiological and genetic findings in support of Lp(a)-mediated valve disease, discuss potential mechanisms underlying this observation, and outline the steps to translate this discovery to a much needed novel preventive and/or therapeutic strategy for AV disease.

Carotenoids co-localize with hydroxyapatite, cholesterol, and other lipids in calcified stenotic aortic valves. Ex vivo Raman maps compared to histological patterns

Unlike its application for atherosclerotic plaque analysis, Raman microspectroscopy was sporadically used to check the sole nature of bioapatite deposits in stenotic aortic valves, neglecting the involvement of accumulated lipids/lipoproteins in the calcific process. Here, Raman microspectroscopy was employed for examination of stenotic aortic valve leaflets to add information on nature and distribution of accumulated lipids and their correlation with mineralization in the light of its potential precocious diagnostic use. Cryosections from surgically explanted stenotic aortic valves (n=4) were studied matching Raman maps against specific histological patterns. Raman maps revealed the presence of phospholipids/triglycerides and cholesterol, which showed spatial overlapping with one another and Raman-identified hydroxyapatite. Moreover, the Raman patterns correlated with those displayed by both von-Kossa-calcium- and Nile-blue-stained serial cryosections. Raman analysis also provided the first identification of carotenoids, which co-localized with the identified lipid moieties. Additional fit concerned the distribution of collagen and elastin. The good correlation of Raman maps with high-affinity staining patterns proved that Raman microspectroscopy is a reliable tool in evaluating calcification degree, alteration/displacement of extracellular matrix components, and accumulation rate of different lipid forms in calcified heart valves. In addition, the novel identification of carotenoids supports the concept that valve stenosis is an atherosclerosis-like valve lesion, consistently with their previous Raman microspectroscopical identification inside atherosclerotic plaques.

Association of low-density lipoprotein cholesterol-related genetic variants with aortic valve calcium and incident aortic stenosis

IMPORTANCE

Plasma low-density lipoprotein cholesterol (LDL-C) has been associated with aortic stenosis in observational studies; however, randomized trials with cholesterol-lowering therapies in individuals with established valve disease have failed to demonstrate reduced disease progression.

OBJECTIVE

To evaluate whether genetic data are consistent with an association between LDL-C, high-density lipoprotein cholesterol (HDL-C), or triglycerides (TG) and aortic valve disease.

DESIGN, SETTING, AND PARTICIPANTS

Using a Mendelian randomization study design, we evaluated whether weighted genetic risk scores (GRSs), a measure of the genetic predisposition to elevations in plasma lipids, constructed using single-nucleotide polymorphisms identified in genome-wide association studies for plasma lipids, were associated with aortic valve disease. We included community-based cohorts participating in the CHARGE consortium (n = 6942), including the Framingham Heart Study (cohort inception to last follow-up: 1971-2013; n = 1295), Multi-Ethnic Study of Atherosclerosis (2000-2012; n = 2527), Age Gene/Environment Study-Reykjavik (2000-2012; n = 3120), and the Malmö Diet and Cancer Study (MDCS, 1991-2010; n = 28,461).

MAIN OUTCOMES AND MEASURES

Aortic valve calcium quantified by computed tomography in CHARGE and incident aortic stenosis in the MDCS.

RESULTS

The prevalence of aortic valve calcium across the 3 CHARGE cohorts was 32% (n = 2245). In the MDCS, over a median follow-up time of 16.1 years, aortic stenosis developed in 17 per 1000 participants (n = 473) and aortic valve replacement for aortic stenosis occurred in 7 per 1000 (n = 205). Plasma LDL-C, but not HDL-C or TG, was significantly associated with incident aortic stenosis (hazard ratio [HR] per mmol/L, 1.28; 95% CI, 1.04-1.57; P = .02; aortic stenosis incidence: 1.3% and 2.4% in lowest and highest LDL-C quartiles, respectively). The LDL-C GRS, but not HDL-C or TG GRS, was significantly associated with presence of aortic valve calcium in CHARGE (odds ratio [OR] per GRS increment, 1.38; 95% CI, 1.09-1.74; P = .007) and with incident aortic stenosis in MDCS (HR per GRS increment, 2.78; 95% CI, 1.22-6.37; P = .02; aortic stenosis incidence: 1.9% and 2.6% in lowest and highest GRS quartiles, respectively). In sensitivity analyses excluding variants weakly associated with HDL-C or TG, the LDL-C GRS remained associated with aortic valve calcium (P = .03) and aortic stenosis (P = .009). In instrumental variable analysis, LDL-C was associated with an increase in the risk of incident aortic stenosis (HR per mmol/L, 1.51; 95% CI, 1.07-2.14; P = .02).

CONCLUSIONS AND RELEVANCE

Genetic predisposition to elevated LDL-C was associated with presence of aortic valve calcium and incidence of aortic stenosis, providing evidence supportive of a causal association between LDL-C and aortic valve disease. Whether earlier intervention to reduce LDL-C could prevent aortic valve disease merits further investigation.

Label-free quantitative imaging of cholesterol in intact tissues by hyperspectral stimulated Raman scattering microscopy

A finger on the pulse: Current molecular analysis of cells and tissues routinely relies on separation, enrichment, and subsequent measurements by various assays. Now, a platform of hyperspectral stimulated Raman scattering microscopy has been developed for the fast, quantitative, and label-free imaging of biomolecules in intact tissues using spectroscopic fingerprints as the contrast mechanism.

Lipids in biocalcification: contrasts and similarities between intimal and medial vascular calcification and bone by NMR

Pathomechanisms underlying vascular calcification biogenesis are still incompletely understood. Biomineral from human atherosclerotic intimal plaques; human, equine, and bovine medial vascular calcifications; and human and equine bone was released from collagenous organic matrix by sodium hydroxide/sodium hypochlorite digestion. Solid-state (13)C NMR of intimal plaque mineral shows signals from cholesterol/cholesteryl esters and fatty acids. In contrast, in mineral from pure medial calcifications and bone mineral, fatty acid signals predominate. Refluxing (chloroform/methanol) intimal plaque calcifications removes the cholesterylic but not the fatty acyl signals. The lipid composition of this refluxed mineral now closely resembles that of the medial and bone mineral, which is unchanged by reflux. Thus, intimal and medial vascular calcifications and bone mineral have in common a pool of occluded mineral-entrained fatty acyl-rich lipids. This population of fatty acid may contain methyl-branched fatty acids, possibly representing lipoprotein particle remnants. Cell signaling and mechanistic parallels between physiological (orthotopic) and pathological (ectopic) calcification are also reflected thus in the NMR spectroscopic fingerprints of mineral-associated and mineral-entrained lipids. Additionally the atherosclerotic plaque mineral alone shows a significant independent pool of cholesterylic lipids. Colocalization of mineral and lipid may be coincidental, but it could also reflect an essential mechanistic component of biomineralization.

Atherosclerosis: Viewing the Problem from a Different Perspective Including Possible Treatment Options

This paper proposes that atherosclerosis is initiated by a signaling event that deposits calcium hydroxyapatite (Ca-HAP). This event is preceded by a loss of mechanical structure in the arterial wall. After Ca-HAP has been deposited, it is unlikely that it will be reabsorbed because the solubility product constant (Ksp) is very small, and the large stores of Ca+2 and PO4−3 in the bones oppose any attempts to dissolve Ca-HAP by decreasing the common ions. The hydroxide ion (OH–) of Ca-HAP can be displaced in nature by fluoride (F–) and carbonate (CO3−2) ions, and it is proposed that anions associated with cholesterol ester hydrolysis and, in very small quantities, the enolate of 7-ketocholesterol could also displace the OH– of Ca-HAP, forming an ionic bond. The free energy of hydration of Ca-HAP at 310 K is most likely negative, and the ionic radii of the anions associated with the hydrolysis of cholesterol ester are compatible with the substitution. Furthermore, examination of the pathology of atherosclerotic lesions by Raman and NMR spectroscopy and confocal microscopy supports deposition of Ca-HAP associated with cholesterol. Investigating the affinity of intermediates of cholesterol hydrolysis for Ca-HAP compared to lipoproteins such as HDL, LDL, and VLDL using isothermic titration calorimetry could add proof of this concept and may lead to the development of a new class of medications targeted at the deposition of cholesterol within Ca-HAP. Treatment of acute ischemic events as a consequence of atherosclerosis with denitrogenation and oxygenation is discussed.

Vascular smooth muscle cell phenotypic plasticity and the regulation of vascular calcification

Vascular smooth muscle cells (VSMCs) exhibit an extraordinary capacity to undergo phenotypic change during development, in vitro and in association with disease. Unlike other muscle cells they do not terminally differentiate. Development and maintenance of the mature contractile phenotype is regulated by a number of interacting transcription factors. In response to injury contractile VSMCs can be induced to change phenotype, proliferate and migrate to effect repair. On completion of the repair process VSMCs return to a nonproliferating contractile phenotype. In this way, in the context of atherosclerosis, a protective fibrous cap is formed and maintained at sites of injury. However in disease, when modulatory signals are perturbed, this phenotypic transition is dysregulated and VSMCs are induced to undergo inappropriate differentiation into cells with features of other mesenchymal lineages such as osteoblasts, chondrocytes and adipocytes. Moreover, evidence is accumulating that these aberrant phenotypic transitions contribute to the pathogenesis of vascular diseases such as atherosclerosis and Monckeberg's Sclerosis. Indeed, the osteo/chondrocytic conversion of VSMCs and the association of this phenotype with vascular calcification is a paradigm for how inappropriate differentiation can influence disease processes. Understanding of the mechanisms and signalling pathways involved in this particular phenotype change is well advanced offering the possibility for the design of successful therapeutic interventions in the future.

Arterial calcification in mice after freeze-thaw injury

Vascular calcification is highly correlated with atherosclerosis and cardiovascular disease and is a significant predictor of cardiovascular morbidity and mortality. Studies in mice indicate a genetic contribution to this dystrophic extra osseous calcification. We sought to elaborate a method to induce dystrophic arterial calcification in mice and further examine the pathogenetical mechanisms involved in the phenotype. We established a method of freeze-thaw injury of the infrarenal aorta producing a limited tissue necrosis and histologically investigated the occurrence of dystrophic calcification within the aortic wall 1, 3 and 7 days after injury in C57BL/6 (a mouse strain shown to be resistant to dystrophic cardiac calcification after injury) and C3H/He (susceptible to dystrophic cardiac calcification). C57BL/6 mice exhibited no dystrophic calcification at all within the vessel wall upon injury of the infrarenal aorta (0/5 mice 1 day after injury and 0/10 animals 7 days after injury). By contrast C3H/He mice displayed a remarkable extent of calcification mainly present within the media of the infrarenal aorta which was evident as early as 24 h (three out of five animals 1 day after injury) and reached its maximum extent 7 days after injury (10 out of 10 animals at the seventh postoperative day, p<0.001 compared to C57BL/6 mice). Upon immuno-histological analysis calcification was accompanied by the occurrence of certain bone-matrix associated proteins. Osteopontin and Bone Morphogenetic Protein 2/4 expression was detected co-localized with the calcified lesions. Our results demonstrate that freeze-thaw injury of the infrarenal aorta is a sufficient method to induce dystrophic arterial calcification in mice. We present evidence that the occurrence of arterial calcification in C3H/He mice seems to be actively regulated by certain bone-matrix associated proteins.

Growth of calcium hydroxyapatite (Ca-HAp) on cholesterol and cholestanol crystals from a simulated body fluid: A possible insight into the pathological calcifications associated with atherosclerosis

An experimental study into calcium phosphate (CP) nucleation and growth on cholesterol and cholestanol surfaces from a supersaturated simulated body fluid (SBF) is presented with the overall aim of gaining some fundamental insights into the pathological calcifications associated with atherosclerosis. Soaking of pressed cholesterol disks at physiological temperature in SBF solutions was found to lead to CP nucleation and growth if the disks were surface roughened and if an SBF with concentrations of the calcium and hydrogen phosphate ions at 2.25x physiological concentrations was used. The CP phase deposited was shown via SEM micrographs to possess a florette type morphology akin to that observed in earlier reported studies. The use of recrystallised cholesterol and cholestanol microcrystals as substrates for soaking in SBF facilitated the observation of CP deposition. In general, cholesterol recrystallised from polar solvents like 95% ethanol as a cholesterol monohydrate phase which was a better substrate for CP growth than cholesterol recrystallised from more non-polar solvents (e.g., benzene) which produced anhydrous cholesterol phases. CP was also observed to form on recrystallised cholestanol microcrystals, a molecule closely related to cholesterol. Inductively coupled plasma optical emission spectrometry (ICP-OES) data gave confirmation that Ca:P mole ratios of the grown CP were 1.3-1.5 suggesting a mixed phase of octacalcium phosphate (OCP) and Ca-deficient HAp and that the CP coating grows (with time of soaking) on the substrates after nucleation in the SBF growth medium. Infrared (IR) spectra of the extracted coatings from the cholesterol substrates confirmed that the CP phase deposited is a semi crystalline HAp with either carbonate substituted into its structure or else co-deposited as calcium carbonate. Soaking experiments involving modified cholesterol substrates in which the OH group in the molecule was replaced with the oleiyl or phosphonate group showed no CP nucleation and growth. This observation illustrates the importance of the known epitaxial relationship between cholesterol and HAp (which theoretically predicts favourable deposition of one phase upon the other) and the consequences of its destruction (by chemical modification of the cholesterol). In the case of the phosphorylated cholesterol, failure of this substrate to nucleate CP phases may have also been caused by the reduction in concentration of free solution Ca2+ in the SBF medium by complexation with the phosphonate groups on the phosphorylated cholesterol. This would have reduced the ion product of Ca2+ and inorganic phosphate and lowered the degree of supersaturation in the SBF medium.

Effect of lipid modification on progression of coronary calcification

Coronary artery calcification (CAC) reflects the anatomic presence of coronary atherosclerosis and the relative burden of coronary artery disease (CAD). Higher levels of CAC are seen in the presence of CAD risk factors, older age, and chronic kidney disease. The lipid profile (primarily low HDL cholesterol, elevated triglycerides, elevated LDL cholesterol, and elevated total cholesterol) are important factors in the calcification process. The annual progression of CAC can be reduced from 25 to 30% to 0 to 6% with LDL cholesterol reduction caused by statins and possibly sevelamer. At treated LDL cholesterol levels somewhere below 100 mg/dl, several sources of data suggest the anatomic burden of CAD, including CAC, regresses. Additional supportive studies indicate that carotid intimal medial thickness and the volume of coronary atheroma also can be reduced by LDL cholesterol reduction in concert with elevation of HDL cholesterol. This article reviews the data in support of altering the natural history of CAC with lipid modification.

Dyslipidemia and progression of cardiovascular calcification (CVC) in patients with end-stage renal disease (ESRD)

Dyslipidemia and progression of cardiovascular calcification (CVC) in patients with end-stage renal disease (ESRD). Cardiovascular calcification (CVC) is commonly encountered both in the general population as well as in patients with end-stage renal disease (ESRD). The etiology of CVC in patients with ESRD is multifactorial. Despite that, current debate remains narrowly focused on the role of calcium loading from calcium-based phosphate binders (CBPB) in the pathogenesis and progression of CVC. Yet, the alleged link between these binders and CVC has not been substantiated in well-designed controlled trials. In contrast, the purported role of sevelamer, a non-calcium-based phosphate binder, in slowing the progression of CVC in dialysis patients has attracted widespread attention. The beneficial effect of sevelamer on progression of calcification was thought to be due to lower calcium loading during its use. However, an alternative and possibly more likely mechanism involves sevelamer-induced lowering of LDL cholesterol. In this context, previous studies in individuals with normal renal function have documented amelioration of coronary artery calcification (CAC) with reduction of LDL-cholesterol by treatment with HMG-CoA reductase inhibitors (statins). Given that CAC is a well-accepted marker of atherosclerosis, and that high plasma cholesterol concentration is one of the main risk factors for atherosclerosis, then it is not unreasonable to suspect that CAC may be halted or even reversed by lowering of LDL cholesterol level with statin therapy. Unfortunately, the effect of lowering the LDL-cholesterol level on CAC has not been studied in patients with ESRD. Therefore, conclusions about this important topic should await the results of well-designed clinical studies that control for all factors potentially implicated in the CVC burden of patients with ESRD. In this review, I will discuss the role of various potential mechanisms involved in the pathogenesis of CVC in patients with ESRD, and emphasize the role of dyslipidemia and its treatment in this important clinical entity.

Vitamin D, Shedding Light on the Development of Disease in Peripheral Arteries

Vitamin D is generally associated with calcium metabolism, especially in the context of uptake in the intestine and the formation and maintenance of bone. However, vitamin D influences a wide range of metabolic systems through both genomic and nongenomic pathways that have an impact on the properties of peripheral arteries. The genomic effects have wide importance for angiogenesis, elastogenesis, and immunomodulation; the nongenomic effects have mainly been observed in the presence of hypertension. Although some vitamin D is essential for cardiovascular health, excess may have detrimental effects, particularly on elastogenesis and inflammation of the arterial wall. Vitamin D is likely to have a role in the paradoxical association between arterial calcification and osteoporosis. This review explores the relationship between vitamin D and a range of physiological and pathological processes relevant to peripheral arteries.

Role of bicarbonate/CO2 buffer in the initiation of vesicle-mediated calcification: mechanisms of aortic calcification related to atherosclerosis

Calcifying vesicles play an important role in the mechanism of aortic calcification induced by dietary cholesterol interventions. This study was initiated to test the hypothesis that alterations in the ratio of bicarbonate/CO2, which is a main physiological buffer, could affect vesicle-mediated calcification. Using rabbits as a model, in vitro calcification of vesicles isolated from aortas was performed to study the effect of the bicarbonate buffer on the mineralization process. When Tris buffer was initially used to maintain pH of the media, ATP-dependent vesicle calcification increased with pH of calcifying media. By replacing Tris with physiological bicarbonate/CO2 buffer, ATP-dependent vesicle calcification increased rapidly with increased ratios of bicarbonate/CO2. The increase appears to be a result of elevated levels of pH through the alteration in the ratios of bicarbonate/CO2. The effect of the physiological concentration of bicarbonate (30 mM) on pH of calcifying media was remarkable since 50 mM of Tris buffer at pH 7.6 failed to prevent a rapid rise in pH under atmospheric CO2. The effect of bicarbonate and CO2 on vesicle calcification was dependent on the ratio of the surface area to the volume of calcifying media, since the ratio profoundly affects the exchange rate between the gas and liquid phases of CO2. Although the pathological conditions that alter the pH remain unknown, it is conceivable that blockage in the supply of blood CO2 to the media by intimal thickening in the lesions could contribute to focal calcification. We conclude that bicarbonate buffer could provide a dynamic and rapid transitional increase in pH of extracellular fluids, thereby creating a favorable condition for the initiation of vesicle-mediated calcification under pathological conditions.

Left main coronary artery aneurysm associated with extensive coronary arterial calcification: case report and review

We present a 68-year-old male with left main coronary artery aneurysm and extensive coronary calcification involving the entire coronary arterial tree detected by coronary angiography and electron beam computerized tomography. With this article we also discussed the relationships between the pathogenesis of coronary atherosclerosis, coronary calcification, and coronary aneurysm formation.

Cardiovascular calcification in patients with chronic renal failure: Are we on target with this risk factor?

End-stage renal disease (ESRD) is comprised of conditions associated with metabolic disorders associated with soft tissue and coronary artery calcification (CAC). The most consistent determinants of CAC in these patients are extent and duration of renal dysfunction and older age. The majority of published studies have not found a causal relationship between measures of calcium-phosphorus balance and CAC. When taken into consideration, the lipid profile [primarily low high-density lipoprotein cholesterol, elevated triglycerides, elevated low-density lipoprotein (LDL-C), and elevated total cholesterol] are important factors in the calcification process. Recent data seems to indicate that CAC is regulated both positively and negatively by a wide variety of mechanisms affecting patients with renal disease. The progression of CAC can be reduced from a 25% to 30% to 0% to 6% annual increase with LDL-C reduction caused by statins or possibly sevelamer. It is currently unclear whether the calcium-phosphorus balance and its related treatments are involved in CAC progression in ESRD patients. Further research into the determinants and potential treatments for CAC in association with ESRD is warranted.

Effects of LDL, cholesterol, and their oxidized forms on the precipitation kinetics of calcium phosphates

BACKGROUND

LDL, cholesterol, and their oxidized forms are known cardiovascular risk factors and are often found in atherosclerotic lesions of various stages. Little is known, however, about whether they are directly involved in the formation of calcium phosphate compounds.

METHODS

We used the pH-stat technique to follow the kinetics of calcium phosphate precipitation at pH 7.4, 37 degrees C, and ionic strength 0.150 mol/L, in the presence or absence of LDL, oxidized LDL, cholesterol, cholestane-3beta,5alpha,6beta-triol, and cholesteryl linoleate. The precipitates were characterized by x-ray diffraction, scanning and transmission electronic microscopy coupled with energy-dispersion x-ray analysis, and inductively coupled plasma atomic emission spectroscopy.

RESULTS

Under the experimental conditions, LDL (14.8 and 43.1 mg/L protein) had no significant effect on the precipitation kinetics. Oxidized LDL (14.8 and 43.1 mg/L protein) prolonged the nucleation phase and diminished the amount of total precipitate, and both the extent of oxidation and the concentration of the protein affected the kinetics. Cholesterol microcrystals (71.4 and 143 mg/L) made the nucleation phase shorter (300 min vs 390 min for the control), and the precipitated particles had an organic core and a shell composed of calcium phosphates. L-alpha-Phosphatidylcholine vesicles (143 mg/L), cholesterol (71.4 mg/L)/phospholipid (143 mg/L) mixed vesicles, cholesteryl linoleate (143 mg/L), and cholestane-3beta,5alpha,6beta-triol (71.4 mg/L) prolonged the nucleation phase.

CONCLUSIONS

LDL is not involved directly in the precipitation of calcium phosphates. Oxidized LDL inhibits both nucleation and crystal growth, possibly by attracting calcium ions in the solution and thus reducing supersaturation. Cholesterol microcrystals serve as seeds for the precipitation of hydroxyapatite, whereas L-alpha-phosphatidylcholine, cholesteryl linoleate, and cholestane-3beta,5alpha,6beta-triol exhibit inhibitive effects on the nucleation of calcium phosphates.

Coronary artery calcium and its relationship to coronary artery disease

Electron-beam computed tomography (EBCT) and the recent generation of multi-slice computed tomography scanners (MSCT) permit high-resolution imaging of the beating heart and the coronary arteries. The visualization of coronary calcium offers the opportunity to non-invasively obtain direct information on coronary anatomy and plaque burden. For clinical purposes, coronary calcium represents the presence of arteriosclerotic plaques. Coronary calcium is deposited in an actively regulated process related to lipid content of and apoptosis within coronary plaques. The amount of coronary calcium is related to the extent of coronary plaque disease, which has substantial diagnostic and prognostic implications. Visualization of coronary calcium by cardiac CT allows to non-invasively detect and localize coronary plaques and describe their distribution in the coronary tree. Approximately 50% to 70% of all plaques are calcified. Calcium cannot be used to reliably identify plaques at risk for developing complications such as rupture or erosion with ensuing thrombus formation. However, data are accumulating that indicate that calcium is an indicator of coronary arteriosclerotic disease activity. A scan negative for coronary calcium has a high negative predictive value indicating absence of stenotic coronary artery disease and an excellent short- to mid-term prognosis. Studies using serial CT scans indicate that the annual progression of coronary calcium varies between 30% to 50% in symptomatic or high-risk individuals and 0% to 20% in patients treated effectively with lipid-lowering medication. An increased rate of progression of coronary calcium seems to indicate a substantially increased risk for adverse cardiac events.

In vitro effect of cholesterol on calcifying activity of vesicles isolated from rabbit aortas

It has been shown that vesicles play a key role in the onset mechanism of aortic calcification related to cholesterol-induced atherosclerosis. This study using a rabbit model was conducted to determine whether cholesterol exerts a direct effect on vesicle's calcifiability. Inclusion of cholesterol in calcifying media stimulated ATP-initiated deposition of calcium in a dose-dependent manner by vesicles isolated from normal aortas using crude collagenase digestion. By contrast, cholesterol did not significantly affect ATP-promoted calcification if vesicles were isolated from atherosclerotic aortas. To determine whether high cholesterol levels in atherosclerotic vesicle preparations may have already maximized calcifying activity and therefore account for lack of the vesicle's response to the sterol, Fourier transform infrared spectroscopy (FT-IR) was used to compare the cholesterol contents in control and atherosclerotic vesicles. The spectral patterns revealed higher levels of cholesterol in vesicle preparations from atherosclerotic aortas than those from normal aortas. Removal of extra-vesicular cholesterol micelles from atherosclerotic vesicles by a relatively low centrifugal force sensitized the vesicles to cholesterol stimulation causing a 2-fold increase in calcifying activity. Of various oxidized forms of cholesterol tested, 7-keto and 6-keto cholesterol enhanced the activity by 2-fold. Altogether, these observations suggest that cholesterol and especially its oxidized forms may induce aortic calcification by directly enhancing the vesicle's ability to calcify.

High resolution three-dimensional visualization and characterization of coronary atherosclerosis in vitro by synchrotron radiation x-ray microtomography and highly localized x-ray diffraction

Human atherosclerotic plaques in both native and bypass arteries have been visualized using microtomography to provide additional information on the nature of coronary artery disease. Plaques contained within arteries removed from three white males aged 51, 55 and 70 are imaged in three-dimensions with monochromatic synchrotron x-ray radiation. Fields of view are 658 x 658 x 517 voxels. with cubic voxels ranging from 12 to 13 microm on a side. X-ray energies range from 11 to 15 keV (bandpass approximately 10 eV). At lower energies, high local absorption tends to generate reconstruction artefacts, while at higher energies the arterial wall is scarcely visible. At all energies, calcifications are clearly visible and differences are observed between plaques in native arteries (lifetime accumulations) versus bypass arteries (plaques developing in the interval between the heart bypass operation and the autopsy). In order to characterize coronary calcification, a microfocused, 50 microm2, 25 keV x-ray beam was used to acquire powder diffraction data from selected calcifications. Also, large calcifications were removed from the native arteries and imaged with 25 keV x-ray energy. Calcifications are composed of hydroxyapatite crystallites and an amorphous phase. In summary, native calcifications are larger and have a higher fraction of hydroxyapatite than calcifications from the bypass arteries.

Induction of calcification in rabbit aortas by high cholesterol diets: roles of calcifiable vesicles in dystrophic calcification

Atherosclerotic calcification may weaken the aorta wall and thereby lead to rupture of the vessel. The mechanism whereby aortas undergo calcification remains unclear. Previous reports in this laboratory showed that, after 2 months of cholesterol-supplemental feeding, an increase in calcifiability of membrane vesicles isolated from rabbit aortas precedes substantial arterial calcification. Further, the mineral was deposited by isolated calcifiable vesicles as an amorphous phase similar to minerals in human aortas at an early stage of atherosclerosis. In the current study, atherosclerotic calcification was induced by exposing rabbits to a 1% cholesterol-rich diet for 3 or 6 months. After 3 months of dietary interventions, atherosclerotic lesions were fully developed. Fatty streaks were evident in areas proximal to the heart and became less frequent in the distal areas. However, calcification was not yet identifiable histologically or by using Fourier transform spectroscopy (FT-IR). After 6 months of high cholesterol treatment, aortas were partially calcified. Histochemical staining for mineral revealed that calcification appeared to occur predominantly in the intimal areas immediately adjacent to the media. Fourier Transform Imaging analysis demonstrated that the mineral deposited in atherosclerotic rabbit aortas was a hydroxyapatite-like phase. To determine whether aorta vesicles play a role in mineral formation in aortas, vesicles were isolated from calcified aortas and then their calcifiability was compared to that in normal vesicles. Interestingly, during the course of vesicle isolation, we found that calcifiable vesicles with much higher calcifiability than normal vesicles could be readily isolated from atherosclerotic aortas simply by suspending minced tissues in PBS. The characteristics of the calcification process and the enzymatic contents of isolated vesicles were similar to those obtained using collagenase digestion. Correlatively, mineral deposited by calcifiable vesicles isolated from the calcified aortas was also of hydroxyapatite-like phases. Altogether, these observations indicate that (1) aortic calcification is a later event during atherogenesis, (2) calcifiable vesicles are loosely bound to the matrices of the lesions as the result of the disease process and (3) similarities in the mineral phases between those in aortas and by vesicles during atherogenesis further support the role of calcifiable vesicles in dystrophic calcification.

Monocyte/macrophage regulation of vascular calcification in vitro

BACKGROUND

Calcification is a common complication of atherosclerosis and other chronic inflammatory processes that involves infiltration of monocytes and accumulation of macrophages.

METHODS AND RESULTS

To determine whether these cells modulate vascular calcification in vitro, calcifying vascular cells (CVCs), a subpopulation of osteoblast-like cells derived from the artery wall, were cocultured with human peripheral blood monocytes for 5 days. Results showed that alkaline phosphatase (ALP) activity, a marker of osteoblastic differentiation, was significantly greater in cocultures than in cultures of CVCs or monocytes alone. Both ALP activity and matrix mineralization increased in proportion to the number of monocytes added. Activation of monocyte/macrophages (M/Ms) by oxidized LDL further increased ALP activity in cocultures. However, neither conditioned medium from oxidized-LDL-activated M/Ms or transwell coculture had this effect on CVCs, which suggests a need for cell-to-cell contact. In contrast, conditioned medium from lipopolysaccharide-activated M/Ms increased ALP activity of CVCs. ELISA showed that lipopolysaccharide-activated M/Ms secreted tumor necrosis factor-alpha, and neutralizing antibody to tumor necrosis factor-alpha attenuated the induction of ALP activity by the conditioned media.

CONCLUSIONS

These results suggest that M/Ms enhance in vitro vascular calcification via 2 independent mechanisms: cell-cell interaction and production of soluble factors such as tumor necrosis factor-alpha.

Raman microspectroscopy of human coronary atherosclerosis: biochemical assessment of cellular and extracellular morphologic structures in situ

BACKGROUND

We have previously shown that Raman spectroscopy can be used for chemical analysis of intact human coronary artery atherosclerotic lesions ex vivo without tissue homogenization or extraction. Here, we report the chemical analysis of individual cellular and extracellular components of atherosclerotic lesions in different stages of disease progression in situ using Raman microspectroscopy.

METHODS

Thirty-five coronary artery samples were taken from 16 explanted transplant recipient hearts, and thin sections were prepared. Using a high-resolution confocal Raman microspectrometer system with an 830-nm laser light, high signal-to-noise Raman spectra were obtained from the following morphologic structures: internal and external elastic lamina, collagen fibers, fat, foam cells, smooth muscle cells, necrotic core, beta-carotene, cholesterol crystals, and calcium mineralizations. Their Raman spectra were modeled by using a linear combination of basis Raman spectra from the major biochemicals present in arterial tissue, including collagen, elastin, actin, myosin, tropomyosin, cholesterol monohydrate, cholesterol linoleate, phosphatidyl choline, triolein, calcium hydroxyapatite, calcium carbonate, and beta-carotene.

RESULTS

The results show that the various morphologic structures have characteristic Raman spectra, which vary little from structure to structure and from artery to artery. The biochemical model described the spectrum of each morphologic structure quite well, indicating that the most essential biochemical components were included in the model. Furthermore, the biochemical composition of each structure, indicated by the fit contributions of the biochemical basis spectra of the morphologic structure spectrum, was very consistent.

CONCLUSIONS

The Raman spectra of various morphologic structures in normal and atherosclerotic coronary artery may be used as basis spectra in a linear combination model to analyze the morphologic composition of atherosclerotic coronary artery lesions.

Medial localization of mineralization-regulating proteins in association with Mönckeberg's sclerosis: evidence for smooth muscle cell-mediated vascular calcification

BACKGROUND

Calcification of the media of peripheral arteries is referred to as Mönckeberg's sclerosis (MS) and occurs commonly in aged and diabetic individuals. Its pathogenesis is unknown, but its presence predicts risk of cardiovascular events and leg amputation in diabetic patients. Several studies have documented expression of bone-associated genes in association with intimal atherosclerotic calcification, leading to the suggestion that vascular calcification may be a regulated process with similarities to developmental osteogenesis. Therefore, we examined gene expression in vessels with MS to determine whether there was evidence for a regulated calcification process in the vessel media.

METHODS AND RESULTS

In situ hybridization, immunohistochemistry, and semiquantitative reverse-transcription polymerase chain reaction were used to examine the expression of mineralization-regulating proteins in human peripheral arteries with and without MS. MS occurred in direct apposition to medial vascular smooth muscle cells (VSMCs) in the absence of macrophages or lipid. These VSMCs expressed the smooth muscle-specific gene SM22alpha and high levels of matrix Gla protein but little osteopontin mRNA. Compared with normal vessels, vessels with MS globally expressed lower levels of matrix Gla protein and osteonectin, whereas alkaline phosphatase, bone sialoprotein, bone Gla protein, and collagen II, all indicators of osteogenesis/chondrogenesis, were upregulated. Furthermore, VSMCs derived from MS lesions exhibited osteoblastic properties and mineralized in vitro.

CONCLUSIONS

These data indicate that medial calcification in MS lesions is an active process potentially orchestrated by phenotypically modified VSMCs.

Interdependence of serum concentrations of vitamin K1, vitamin E, lipids, apolipoprotein A1, and apolipoprotein B: importance in assessing vitamin status

Vitamin E (alpha-tocopherol) and vitamin K1 (phylloquinone) are fat-soluble vitamins and are important nutrients in health and disease. In this study serum concentrations of vitamin E and vitamin K1, lipids and apolipoproteins A1 and B were measured in neonates, normal and hyperlipidaemic individuals in an attempt to establish their interrelationships. A high degree of correlation was observed between the concentrations of the vitamins and those of lipids and apolipoproteins (r ranged from 0.42 to 0.92; p<0.001). Stepwise linear regression methods determined that serum concentrations of both vitamin E and vitamin K1 could best be predicted by using equations excluding lipids but containing only apolipoprotein A1 and B concentrations. Correlation coefficients between predicted and measured values were 0.89 for serum vitamin E, and 0.83 for serum vitamin K1 concentrations. To test the validity of the derived formulae, measured and estimated vitamin K1 and vitamin E concentrations in serum were determined in another group of neonates, normal adults and hypercholesterolemic adults and the comparisons were shown to be very good. These results indicate that the serum levels of both vitamins depend critically on the concentration of the lipoprotein carriers, apolipoproteins A1 and B. Hence, in order to identify variations in serum vitamin K1 and vitamin E concentrations, which are independent of variations in carrier concentration, it will be necessary to express these serum vitamins as ratios of vitamins to apolipoprotein A1 and B carriers.

Noncollagenous bone matrix proteins, calcification, and thrombosis in carotid artery atherosclerosis

Advanced atherosclerosis is often associated with dystrophic calcification, which may contribute to plaque rupture and thrombosis. In this work, the localization and association of the noncollagenous bone matrix proteins osteonectin, osteopontin, and osteocalcin with calcification, lipoproteins, thrombus/hemorrhage (T/H), and matrix metalloproteinases (MMPs) in human carotid arteries from endarterectomy samples have been determined. According to the recent American Heart Association classification, 6 of the advanced lesions studied were type V (fibroatheroma) and 16 type VI (complicated). Osteonectin, osteocalcin, and osteopontin were identified by monoclonal antibodies IIIA(3)A(8), G12, and MPIIIB10(1) and antiserum LF-123. Apolipoprotein (apo) AI, B, and E; lipoprotein(a); fibrinogen; fibrin; fragment D/D-dimer; MMP-2 (gelatinase A); and MMP-3 (stromelysin-1) were identified with previously characterized antibodies. Calcium phosphate deposits (von Kossa's stain) were present in 82% of samples (3 type V and 15 type VI). Osteonectin was localized in endothelial cells, SMCs, and macrophages and was associated with calcium deposits in 33% of type V and 88% of type VI lesions. Osteopontin was distributed similarly to osteonectin and was associated with calcium deposits in 50% of type V and 94% of type VI lesions. Osteocalcin was localized in large calcified areas only (in 17% of type V and 38% of type VI lesions). ApoB colocalized with cholesterol crystals and calcium deposits. Lipoprotein(a) was localized in the intima, subintima, and plaque shoulder. Fibrin (T/H) colocalized with bone matrix proteins in 33% of type V and 69% of type VI lesions. MMP-3 was cytoplasmic in most cells and colocalized with calcium and fibrin deposits. MMP-2 was less often associated with calcification. The results of this study show that osteonectin, osteopontin, and osteocalcin colocalized with calcium deposits with apoB, fibrin, and MMP-3 in advanced, symptomatic carotid lesions. These data suggest that the occurrence of T/H might contribute to dystrophic arterial calcification in the progression and complications of atherosclerosis.

Imaging of coronary artery calcification. Its importance in assessing atherosclerotic disease

Coronary artery calcification is a marker for atherosclerotic disease. The calcifications frequently occur early in the disease process and often before the development of luminal narrowing or cardiac events. Electron beam CT has a high accuracy in detecting calcifications, and thus has prognostic value in predicting luminal narrowing and future cardiac events.

Direct nucleation of calcium oxalate dihydrate crystals onto the surface of living renal epithelial cells in culture

BACKGROUND

The interaction of the most common crystal in human urine, calcium oxalate dihydrate (COD), with the surface of monkey renal epithelial cells (BSC-1 line) was studied to identify initiating events in kidney stone formation.

METHODS

To determine if COD crystals could nucleate directly onto the apical cell surface, a novel technique utilizing vapor diffusion of oxalic acid was employed. Cells were grown to confluence in the inner four wells of 24-well plates. At the start of each experiment, diethyloxalate in water was placed into eight adjacent wells, and the plates were sealed tightly with tape so that oxalic acid vapor diffused into a calcium-containing buffer overlying the cells.

RESULTS

Small crystals were visualized on the cell surface after two hours, and by six hours the unambiguous habitus of COD was confirmed. Nucleation onto cells occurred almost exclusively via the (001) face, one that is only rarely observed when COD crystals nucleate onto inanimate surfaces. Similar results were obtained when canine renal epithelial cells (MDCK line) were used as a substrate for nucleation. Initially, COD crystals were internalized almost as quickly as they formed on the apical cell surface.

CONCLUSIONS

Face-specific COD crystal nucleation onto the apical surface of living renal epithelial cells followed by internalization is a heretofore unrecognized physiological event, suggesting a new mechanism to explain crystal retention within the nephron, and perhaps kidney stone formation when this process is dysregulated or overwhelmed.

Calcification in atherosclerotic plaque of human carotid arteries: associations with mast cells and macrophages

Calcification has been examined in 250 samples of atherosclerotic lesions (types II to VI) of human carotid arteries using von Kossa and haematoxylin staining. Early calcification described as 'stippling' was first noted in stage III specimens, with intermediate and solid calcifications becoming increasingly prominent within advanced plaques, especially stages Vb and VI. Although the relative frequencies of stippling, intermediate and large calcified deposits varied between plaques of the same stage, the prevalent sites of calcification were recognized as the deeper regions of the intima and the atheroma. Immunolocalization and histochemical techniques were used to identify the associations of mast cells (MCs), macrophages, smooth muscle cells (SMCs), and elastin with the different stages of calcification. Early, dispersed stippling was commonly associated with local accumulations of macrophages (HAM56 and CD68-positive), MCs and extracellular MC tryptase, the presence of immunoreactive elastin, but the relative absence of SMCs. Intermediate stages of calcification described as 'morula' deposits were also associated with local increases in the numbers of macrophages and MCs. Larger calcified deposits, even within the same plaque specimen, showed no regular pattern of cellular or elastin associations. However, in the vast majority of specimens, macrophages represented the predominant cell type associated with different phases of calcification. By contrast, the calcification less frequently observed in the media beneath advanced plaques was commonly associated with SMCs and elastin; only rarely were macrophages or MCs present. These studies are the first to demonstrate that macrophages, MCs, and extracellular tryptase frequently occupy micro-environmental loci showing the first stages of calcification within the atherosclerotic plaque; similar associations with more advanced mineral deposits are discussed in relation to plaque rupture.

Calcifying subpopulation of bovine aortic smooth muscle cells is responsive to 17 beta-estradiol

BACKGROUND

Arterial calcification, common in atherosclerosis, is associated with an increased risk of clinical events such as myocardial infarction. We previously identified a subpopulation of bovine aortic medial cells, calcifying vascular cells (CVCs), that have osteoblastic characteristics and form bone mineral in vitro in the form of calcified nodules. To assess whether estrogen modulates arterial calcification as well as bone calcification, we tested CVCs for estrogen receptors and for the effect of 17 beta-estradiol on formation of calcified nodules, calcium content, alkaline phosphatase activity, and osteocalcin concentration in the culture medium.

METHODS AND RESULTS

Estrogen receptor immunoreactivity was identified in the cytoplasm and the perinuclear region of CVCs by immunocytochemistry. CVCs were treated with 17 beta-estradiol at concentrations of 0, 5, and 10 nmol/L. Twenty-one days of 17 beta-estradiol treatment resulted in a significantly increased number of calcified nodules, visualized by von Kossa staining, as well as increased calcium content of the cultures. Increases in alkaline phosphatase activity, a marker for early osteoblastic differentiation, and secreted osteocalcin, a marker for late osteoblastic differentiation, were enhanced in cells treated with 17 beta-estradiol compared with control cells.

CONCLUSIONS

These results suggest that 17 beta-estradiol promotes osteoblastic differentiation and calcification in vascular cells and that estrogen may play a regulatory role in arterial calcification.

Lipid oxidation products have opposite effects on calcifying vascular cell and bone cell differentiation. A possible explanation for the paradox of arterial calcification in osteoporotic patients

Atherosclerotic calcification and osteoporosis often coexist in patients, yielding formation of bone mineral in vascular walls and its simultaneous loss from bone. To assess the potential role of lipoproteins in both processes, we examined the effects of minimally oxidized low-density lipoprotein (MM-LDL) and several other lipid oxidation products on calcifying vascular cells (CVCs) and bone-derived preosteoblasts MC3T3-E1. In CVCs, MM-LDL but not native LDL inhibited proliferation, caused a dose-dependent increase in alkaline phosphatase activity, which is a marker of osteoblastic differentiation, and induced the formation of extensive areas of calcification. Similar to MM-LDL, oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (ox-PAPC) and the isoprostane 8-iso prostaglandin E2 but not PAPC or isoprostane 8-iso prostaglandin F2 alpha induced alkaline phosphatase activity and differentiation of CVCs. In contrast, MM-LDL and the above oxidized lipids inhibited differentiation of the MC3T3-E1 bone cells, as evidenced by their stimulatory effect on proliferation and their inhibitory effect on the induction of alkaline phosphatase and calcium uptake. These results suggest that specific oxidized lipids may be the common factors underlying the pathogenesis of both atherosclerotic calcification and osteoporosis.

Role of molecular regulation in vascular calcification

Calcium deposits account for most of the dry weight of atherosclerotic lesions. Previously considered uncommon, vascular calcification is now known to be present in 80% of significant lesions and in at least 90% of patients with coronary artery disease. Previously considered a passive process, it is increasingly recognized as an active, regulated process. Previously considered benign, it is now becoming recognized as a major risk factor for cardiovascular events, and a major contributor to systolic hypertension, heart failure, plaque rupture and stenosis. To confirm the similarity of vascular calcification with embryonic osteogenesis, we demonstrated the expression of bone morphogenetic protein in calcified human lesions, and we developed an in vitro model of vascular calcification that provides a useful experimental system for elucidating the molecular regulation of this process, which we have shown to include alkaline phosphatase induction and expression of bone matrix proteins and differentiation factors. Understanding the regulatory mechanisms of vascular calcification will allow future therapeutic approaches to prevent and possibly reverse this disease and its clinical consequences.

High expression of genes for calcification-regulating proteins in human atherosclerotic plaques

Calcification is common in atheromatous plaques and may contribute to plaque rupture and subsequent thrombosis. However, little is known about the mechanisms which regulate the calcification process. Using in situ hybridization and immunohistochemistry we show that two bone-associated proteins, osteopontin (OP) and matrix Gla protein (MGP), are highly expressed in human atheromatous plaques. High levels of OP mRNA and protein were found in association with necrotic lipid cores and areas of calcification. The predominant cell type in these areas was the macrophage-derived foam cell, although some smooth muscle cells could also be identified. MGP was expressed uniformly by smooth muscle cells in the normal media and at high levels in parts of the atheromatous intima. Highest levels of this matrix-associated protein were found in lipid-rich areas of the plaque. The pattern of expression of these two genes contrasted markedly with that of calponin and SM22 alpha, genes expressed predominantly by differentiated smooth muscle cells and whose expression was generally confined to the media of the vessel. The postulated function of OP and MGP as regulators of calcification in bone and the high levels and colocalization of both in atheromatous plaques suggest they have an important role in plaque pathogenesis and stability.