Microcalcifications in early intimal lesions of atherosclerotic human coronary arteries

Effect of vitamin K in bone metabolism and vascular calcification: A review of mechanisms of action and evidences

Osteoporosis is a public health concern associated with an increased risk of bone fractures and vascular calcification. Vitamin K presents unique benefits on these issues, although understudied. The two main forms of vitamin K are phylloquinone (vitamin K₁) and menaquinone (vitamin K₂). In this study, it was especially investigated the action of vitamin K₂ in bones and vessels. Vitamin K₂ has shown to stimulate bone formation by promoting osteoblast differentiation and carboxylation of osteocalcin, and increasing alkaline phosphatase, insulin-like growth factor-1, growth differentiation factor-15, and stanniocalcin 2 levels. Furthermore, vitamin K₂ reduces the pro-apoptotic proteins Fas and Bax in osteoblasts, and decreases osteoclast differentiation by increasing osteoprotegerin and reducing the receptor activator of nuclear factor kappa-B ligand. In blood vessels, vitamin K₂ reduces the formation of hydroxyapatite, through the carboxylation of matrix Gla protein and Gla rich protein, inhibits the apoptosis of vascular smooth muscle cells, by increasing growth arrest-specific gene 6, and reduces the transdifferentiation of vascular smooth muscle cells to osteoblasts. The commonly used dosage of vitamin K₂ in human studies is 45 mg/day and its application can be an interesting strategy in benefitting bone and vascular health, especially to osteoporotic post-menopausal women.

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

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

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

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

Identifying Small Coronary Calcification in Non-Contrast 0.5-mm Slice Reconstruction to Diagnose Coronary Artery Disease in Patients with a Conventional Zero Coronary Artery Calcium Score

Aims: In a new-generation computed tomography (CT) scanner, coronary artery calcium (CAC) scores were measured using 3.0-mm slice reconstruction images originally acquired with 0.5 mm thickness scans in a single beat. This study investigated the usefulness of thin-slice (0.5 mm) reconstruction for identifying small calcifications in coronary arteries and evaluated the association with coronary plaques and stenosis compared to conventional 3.0-mm reconstruction images.

Methods: We evaluated 132 patients with zero CAC scores in conventional 3.0-mm Agatston method using a 320-slice CT. Then, 0.5-mm slice reconstruction was performed to identify small calcifications. The presence of stenosis and coronary plaques was assessed using coronary CT angiography.

Results: In total, 22 small calcifications were identified in 18 patients. There were 28 (21%) patients with any (≥ 25%) stenosis (34 lesions). Forty-seven coronary plaques were found in 33 patients (25%), including 7 calcified plaques in 7 patients (5%), 34 noncalcified plaques in 27 patients (20%), and 6 partially calcified plaques in 5 patients (4%). Patients with small calcifications had a significantly higher prevalence of noncalcified or partially calcified plaques (83% vs 14%; p < 0.001) and obstructive stenosis (33% vs 5.2%; p < 0.001) compared to those without small calcifications. The addition of small calcifications to the coronary risk factors when diagnosing stenosis significantly improved the diagnostic value.

Conclusion: Small calcifications detected by thin-slice 0.5-mm reconstruction are useful for distinguishing coronary atherosclerotic lesions in patients with zero CAC scores from conventional CT reconstruction.

Correlation between osteocalcin-positive endothelial progenitor cells and spotty calcification in patients with coronary artery disease

Immature endothelial progenitor cells (EPC) carrying osteocalcin (OCN) might mediate vascUlar calcification in coronary artery disease (CAD). Spotty calcification within atherosclerotic plaque is associated with cardiovascular events. The aim of the present study was to assess the correlation between immature EPC levels and spotty calcification in CAD patients. In the 224 CAD patients studied, 76 had acute myocardial infarction (AMI), 102 had unstable angina pectoris (UAP), and 46 had stable angina pectoris (SAP). The levels of OCN-positive (OCN+) EPC were analysed by flow cytometry. The status of spotty calcification was determined by cardiac computed tomography angiography. OCN+ EPC and calcium deposits were significantly increased in acute coronary artery syndrome (ACS) when compared with those in SAP patients. Positive correlation was also revealed between the number of OCN+ EPC and the frequency of spotty calcification and levels of serum high-sensitivity C-reactive protein (hs-CRP) and serum alkaline phosphatase in AMI and UAP patients. In summary, the number of OCN+ EPC is positively related to the frequency of spotty calcification in ACS patients. Serum hs-CRP and serum alkaline levels are thought to contribute to the elevation of OCN+ EPC.

Cardiovascular PET-CT imaging: a new frontier?

Cardiovascular positron-emission tomography combined with computed tomography (PET-CT) has recently emerged as an imaging technology with the potential to simultaneously describe both anatomical structures and physiological processes in vivo. The scope for clinical application of this technique is vast, but to date this promise has not been realised. Nonetheless, significant research activity is underway to explore these possibilities and it is likely that the knowledge gained will have important diagnostic and therapeutic implications in due course. This review provides a brief overview of the current state of cardiovascular PET-CT and the likely direction of future developments.

Osteopontin protects against high phosphate-induced nephrocalcinosis and vascular calcification

Pathologic calcification is a significant cause of increased morbidity and mortality in patients with chronic kidney disease. The precise mechanisms of ectopic calcification are not fully elucidated, but it is known to be caused by an imbalance of procalcific and anticalcific factors. In the chronic kidney disease population, an elevated phosphate burden is both highly prevalent and a known risk factor for ectopic calcification. Here we tested whether osteopontin, an inhibitor of calcification, protects against high phosphate load-induced nephrocalcinosis and vascular calcification. Osteopontin knockout mice were placed on a high phosphate diet for 11 weeks. Osteopontin deficiency together with phosphate overload caused uremia, nephrocalcinosis characterized by substantial renal tubular and interstitial calcium deposition, and marked vascular calcification when compared with control mice. Although the osteopontin-deficient mice did not exhibit hypercalcemia or hyperphosphatemia, they did show abnormalities in the mineral metabolism hormone fibroblast growth factor-23. Thus, endogenous osteopontin plays a critical role in the prevention of phosphate-induced nephrocalcinosis and vascular calcification in response to high phosphate load. A better understanding of osteopontin's role in phosphate-induced calcification will hopefully lead to better biomarkers and therapies for this disease, especially in patients with chronic kidney disease and other at-risk populations.

New Insights into the Pros and Cons of the Clinical Use of Vitamin K Antagonists (VKAs) Versus Direct Oral Anticoagulants (DOACs)

Vitamin K-antagonists (VKA) are the most widely used anticoagulant drugs to treat patients at risk of arterial and venous thrombosis for the past 50 years. Due to unfavorable pharmacokinetics VKA have a small therapeutic window, require frequent monitoring, and are susceptible to drug and nutritional interactions. Additionally, the effect of VKA is not limited to coagulation, but affects all vitamin K-dependent proteins. As a consequence, VKA have detrimental side effects by enhancing medial and intimal calcification. These limitations stimulated the development of alternative anticoagulant drugs, resulting in direct oral anticoagulant (DOAC) drugs, which specifically target coagulation factor Xa and thrombin. DOACs also display non-hemostatic vascular effects via protease-activated receptors (PARs). As atherosclerosis is characterized by a hypercoagulable state indicating the involvement of activated coagulation factors in the genesis of atherosclerosis, anticoagulation could have beneficial effects on atherosclerosis. Additionally, accumulating evidence demonstrates vascular benefit from high vitamin K intake. This review gives an update on oral anticoagulant treatment on the vasculature with a special focus on calcification and vitamin K interaction.

Intra-Section Analysis of Human Coronary Arteries Reveals a Potential Role for Micro-Calcifications in Macrophage Recruitment in the Early Stage of Atherosclerosis

BACKGROUND

Vascular calcification is associated with poor cardiovascular outcome. Histochemical analysis of calcification and the expression of proteins involved in mineralization are usually based on whole section analysis, thereby often ignoring regional differences in atherosclerotic lesions. At present, limited information is available about factors involved in the initiation and progression of atherosclerosis.

AIM OF THIS STUDY

This study investigates the intra-section association of micro-calcifications with markers for atherosclerosis in randomly chosen section areas of human coronary arteries. Moreover, the possible causal relationship between calcifying vascular smooth muscle cells and inflammation was explored in vitro.

TECHNICAL APPROACH

To gain insights into the pathogenesis of atherosclerosis, we performed analysis of the distribution of micro-calcifications using a 3-MeV proton microbeam. Additionally, we performed systematic analyses of 30 to 40 regions of 12 coronary sections obtained from 6 patients including histology and immuno-histochemistry. Section areas were classified according to CD68 positivity. In vitro experiments using human vascular smooth muscle cells (hVSMCs) were performed to evaluate causal relationships between calcification and inflammation.

RESULTS

From each section multiple areas were randomly chosen and subsequently analyzed. Depositions of calcium crystals at the micrometer scale were already observed in areas with early pre-atheroma type I lesions. Micro-calcifications were initiated at the elastica interna concomitantly with upregulation of the uncarboxylated form of matrix Gla-protein (ucMGP). Both the amount of calcium crystals and ucMGP staining increased from type I to IV atherosclerotic lesions. Osteochondrogenic markers BMP-2 and osteocalcin were only significantly increased in type IV atheroma lesions, and at this stage correlated with the degree of calcification. From atheroma area type III onwards a considerable number of CD68 positive cells were observed in combination with calcification, suggesting a pro-inflammatory effect of micro-calcifications. In vitro, invasion assays revealed chemoattractant properties of cell-culture medium of calcifying vascular smooth muscle cells towards THP-1 cells, which implies pro-inflammatory effect of calcium deposits. Additionally, calcifying hVSMCs revealed a pro-inflammatory profile as compared to non-calcifying hVSMCs.

CONCLUSION

Our data indicate that calcification of VSMCs is one of the earliest events in the genesis of atherosclerosis, which strongly correlates with ucMGP staining. Our findings suggest that loss of calcification inhibitors and/or failure of inhibitory capacity is causative for the early precipitation of calcium, with concomitant increased inflammation followed by osteochondrogenic transdifferentiation of VSMCs.

Translational Coronary Atherosclerosis Imaging with PET

Although still in its infancy, coronary atherosclerosis imaging with PET holds promise in improving understanding of the pathophysiologic processes that underlie plaque progression and adverse cardiovascular events. Fludeoxyglucose F 18 offers the potential to measure inflammatory activity within the plaque itself whereas fluoride F 18 allows detection of microcalcification, both of which are key characteristics of plaques at risk of rupture. Further work is required to improve these imaging techniques and to assess their ability to predict cardiac events prospectively.

Effect of Oxidized Low Density Lipoprotein on the Expression of Runx2 and SPARC Genes in Vascular Smooth Muscle Cells

Background:

Vascular calcification is an important stage in atherosclerosis. During this stage, vascular smooth muscle cells (VSMC) synthesize many osteogenic factors such as osteonectin (encoded by SPARC). Oxidative stress plays a critical role in atherosclerosis progression, and its accumulation in the vascular wall stimulates the development of atherosclerosis and vascular calcification. The osteonectin overexpression has been observed in the arterial wall during the course of atherosclerosis. However, the regulatory mechanism of oxidized low density lipoprotein (oxLDL)-mediated vascular calcification remains to be clarified. The aim of this study was to investigate the effect of oxLDL on the osteonectin gene expression through the Runx2 transcription factor.

Methods:

In this experimental study, VSMC were cultured in F-12K media and then treated with oxLDL. The expression of Runx2 and osteonectin genes was determined by real-time PCR method. Protein levels were investigated by the western blotting technique. The Runx2 gene was knocked down using siRNA in order to determine whether Runx2 regulates the osteonectin expression in VSMC induced by oxLDL. Then transfected cells were treated with oxLDL, and the expression levels of Runx2 and osteonectin were determined again.

Results:

oxLDL was found to increase Runx2 and osteonectin gene expression (4.8 ± 0.47- and 9.2 ± 1.96-fold, respectively) after 48 h. Western blotting analysis confirmed the induced levels of Runx2 and osteonectin proteins. However, oxLDL-induced osteonectin expression was not observed to be blocked by Runx2 knockdown.

Conclusion:

The up-regulation of osteonectin by oxLDL is independent of Runx2, and it may be mediated by other transcription factors.

A Drosophila model identifies a critical role for zinc in mineralization for kidney stone disease

Ectopic calcification is a driving force for a variety of diseases, including kidney stones and atherosclerosis, but initiating factors remain largely unknown. Given its importance in seemingly divergent disease processes, identifying fundamental principal actors for ectopic calcification may have broad translational significance. Here we establish a Drosophila melanogaster model for ectopic calcification by inhibiting xanthine dehydrogenase whose deficiency leads to kidney stones in humans and dogs. Micro X-ray absorption near edge spectroscopy (μXANES) synchrotron analyses revealed high enrichment of zinc in the Drosophila equivalent of kidney stones, which was also observed in human kidney stones and Randall's plaques (early calcifications seen in human kidneys thought to be the precursor for renal stones). To further test the role of zinc in driving mineralization, we inhibited zinc transporter genes in the ZnT family and observed suppression of Drosophila stone formation. Taken together, genetic, dietary, and pharmacologic interventions to lower zinc confirm a critical role for zinc in driving the process of heterogeneous nucleation that eventually leads to stone formation. Our findings open a novel perspective on the etiology of urinary stones and related diseases, which may lead to the identification of new preventive and therapeutic approaches.

Imaging and analysis of microcalcifications and lipid/necrotic core calcification in fibrous cap atheroma

The presence of microcalcifications (µCalcs) >5 µm within the cap of human fibroatheroma has been shown to produce a 200-700% increase in peak circumferential stress, which can transform a stable plaque into a vulnerable one, whereas µCalcs < 5 µm do not appear to increase risk. We quantitatively examine the possibility to distinguish caps with µCalcs > 5 µm based on the gross morphological features of fibroatheromas, and the correlation between the size and distribution of µCalcs in the cap and the calcification in the lipid/necrotic core beneath it. Atherosclerotic lesions (N = 72) were imaged using HR-μCT at 2.1-μm resolution for detailed analysis of atheroma morphology and composition, and validated using non-decalcified histology. At 2.1-μm resolution one observes four different patterns of calcification within the lipid/necrotic core, and is able to elucidate the 3D spatial progression of the calcification process using these four patterns. Of the gross morphological features identified, only minimum cap thickness positively correlated with the existence of µCalcs > 5 µm in the cap. We also show that µCalcs in the cap accumulate in the vicinity of the lipid/necrotic core boundary with few on the lumen side of the cap. HR-μCT enables three-dimensional assessment of soft tissue composition, lipid content, calcification patterns within lipid/necrotic cores and analysis of the axial progression of calcification within individual atheroma. The distribution of µCalcs within the cap is highly non-uniform and decreases sharply as one proceeds from the lipid pool/necrotic core boundary to the lumen.

The effect of adiponectin on osteonectin gene expression by oxidized low density lipoprotein-treated vascular smooth muscle cells

Osteonectin is a bone- associated protein involved in vascular calcification. Adiponectin may protect against cardiovascular disease but possible effects on vascular calcification have been poorly studied. The aim of this study was to investigate the modulatory effect of adiponectin on oxidized low density lipoprotein (oxLDL)- induced expression of osteonectin in human aorta vascular smooth muscle cells (HA/VSMCs). HA/VSMCs were cultured in F12K media and then treated with oxLDL (100 µg/mL) in the presence or absence of adoponectin (5 µg/mL) for 24 and 48 hours. mRNA expression and protein level of osteonectin were determined by quantitative real-time PCR and western blot analysis, respectively. After exposure to oxLDL, osteonectin expression increased 1.62 ± 0.23- and 6.62 ± 0.48-fold after 24 and 48 hours respectively compared to the control. Adiponectin increased oxLDL- induced osteonectin expression in a time-dependent manner after 24 and 48 hours (3.24 ± 0.39- and 24.93 ± 2.15-fold, respectively). Western blotting confirmed that osteonectin protein was upregulated by adiponectin.Our data suggest that OxLDL might cause the increase of osteonectin expression both at mRNA and protein level. This upregulation is intensified by adiponectin.

Matrix Gla protein and osteocalcin: from gene duplication to neofunctionalization

Osteocalcin (OC or bone Gla protein, BGP) and matrix Gla protein (MGP) are two members of the growing family of vitamin K-dependent (VKD) proteins. They were the first VKD proteins found not to be involved in coagulation and synthesized outside the liver. Both proteins were isolated from bone although it is now known that only OC is synthesized by bone cells under normal physiological conditions, but since both proteins can bind calcium and hydroxyapatite, they can also accumulate in bone. Both OC and MGP share similar structural features, both in terms of protein domains and gene organization. OC gene is likely to have appeared from MGP through a tandem gene duplication that occurred concomitantly with the appearance of the bony vertebrates. Despite their relatively close relationship and the fact that both can bind calcium and affect mineralization, their functions are not redundant and they also have other unrelated functions. Interestingly, these two proteins appear to have followed quite different evolutionary strategies in order to acquire novel functionalities, with OC following a gene duplication strategy while MGP variability was obtained mostly by the use of multiple promoters and alternative splicing, leading to proteins with additional functional characteristics and alternative gene regulatory pathways.

Calcium micro-depositions in jugular truncular venous malformations revealed by Synchrotron-based XRF imaging

It has been recently demonstrated that the internal jugular vein may exhibit abnormalities classified as truncular venous malformations (TVMs). The investigation of possible morphological and biochemical anomalies at jugular tissue level could help to better understand the link between brain venous drainage and neurodegenerative disorders, recently found associated with jugular TVMs. To this end we performed sequential X-ray Fluorescence (XRF) analyses on jugular tissue samples from two TVM patients and two control subjects, using complementary energies at three different synchrotrons. This investigation, coupled with conventional histological analyses, revealed anomalous micro-formations in the pathological tissues and allowed the determination of their elemental composition. Rapid XRF analyses on large tissue areas at 12.74 keV showed an increased Ca presence in the pathological samples, mainly localized in tunica adventitia microvessels. Investigations at lower energy demonstrated that the high Ca level corresponded to micro-calcifications, also containing P and Mg. We suggest that advanced synchrotron XRF micro-spectroscopy is an important analytical tool in revealing biochemical changes, which cannot be accessed by conventional investigations. Further research on a larger number of samples is needed to understand the pathogenic significance of Ca micro-depositions detected on the intramural vessels of vein walls affected by TVMs.

Small entities with large impact: microcalcifications and atherosclerotic plaque vulnerability

PURPOSE OF REVIEW

Atherosclerotic plaque rupture and subsequent acute events, such as myocardial infarction and stroke, contribute to the majority of cardiovascular-related deaths. Calcification has emerged as a significant predictor of cardiovascular morbidity and mortality, challenging previously held notions that calcifications stabilize atherosclerotic plaques. In this review, we address this discrepancy through recent findings that not all calcifications are equivalent in determining plaque stability.

RECENT FINDINGS

The risk associated with calcification is inversely associated with calcification density. As opposed to large calcifications that potentially stabilize the plaque, biomechanical modeling indicates that small microcalcifications within the plaque fibrous cap can lead to sufficient stress accumulation to cause plaque rupture. Microcalcifications appear to derive from matrix vesicles enriched in calcium-binding proteins that are released by cells within the plaque. Clinical detection of microcalcifications has been hampered by the lack of imaging resolution required for in-vivo visualization; however, recent studies have demonstrated promising new techniques to predict the presence of microcalcifications.

SUMMARY

Microcalcifications play a major role in destabilizing atherosclerotic plaques. The identification of critical characteristics that lead to instability along with new imaging modalities to detect their presence in vivo may allow early identification and prevention of acute cardiovascular events.

Has our understanding of calcification in human coronary atherosclerosis progressed?

Coronary artery calcification is a well-established predictor of future cardiac events; however, it is not a predictor of unstable plaque. The intimal calcification of the atherosclerotic plaques may begin with smooth muscle cell apoptosis and release of matrix vesicles and is almost always seen microscopically in pathological intimal thickening, which appears as microcalcification (≥0.5 μm, typically <15 μm in diameter). Calcification increases with macrophage infiltration into the lipid pool in early fibroatheroma where they undergo apoptosis and release matrix vesicles. The confluence of calcified areas involves extracellular matrix and the necrotic core, which can be identified by radiography as speckled (≤2 mm) or fragmented (>2, <5 mm) calcification. The calcification in thin-cap fibroatheromas and plaque rupture is generally less than what is observed in stable plaques and is usually speckled or fragmented. Fragmented calcification spreads into the surrounding collagen-rich matrix forming calcified sheets, the hallmarks of fibrocalcific plaques. The calcified sheets may break into nodules with fibrin deposition, and when accompanied by luminal protrusion, it is associated with thrombosis. Calcification is highest in fibrocalcific plaques followed by healed plaque rupture and is the least in erosion and pathological intimal thickening. The extent of calcification is greater in men than in women especially in the premenopausal period and is also greater in whites compared with blacks. The mechanisms of intimal calcification remain poorly understood in humans. Calcification often occurs in the presence of apoptosis of smooth muscle cells and macrophages with matrix vesicles accompanied by expression of osteogenic markers within the vessel wall.

The realm of vitamin K dependent proteins: shifting from coagulation toward calcification

In the past few decades vitamin K has emerged from a single-function "haemostasis vitamin" to a "multi-function vitamin." The use of vitamin K antagonists (VKA) inevitably showed that the inhibition was not restricted to vitamin K dependent coagulation factors but also synthesis of functional extrahepatic vitamin K dependent proteins (VKDPs), thereby eliciting undesired side effects. Vascular calcification is one of the recently revealed detrimental effects of VKA. The discovery that VKDPs are involved in vascular calcification has propelled our mechanistic understanding of this process and has opened novel avenues for diagnosis and treatment. This review addresses mechanisms of VKDPs and their significance for physiological and pathological calcification.

Bone metabolism regulators and arterial stiffness in postmenopausal women

BACKGROUND

Osteoprotegerin (OPG), osteopontin (OPN) and matrix Gla protein (MGP) are markers of bone metabolism but they are also involved in vascular calcification. However, their precise role is not completely understood. Arterial stiffness is considered an independent predictor of cardiovascular events and it may be one of the causes of the increased cardiovascular risk associated with postmenopausal status. Medial and intimal calcification may increase arterial stiffness. The aim of our study was to assess the relationship of OPG, OPN and MGP with aortic pulse wave velocity (aPWV) as a marker of arterial stiffness in postmenopausal women.

MATERIALS AND METHODS

Circulating OPG, OPN and serum total MGP were measured in 144 postmenopausal women using the enzyme-linked immunosorbent assay method. Aortic PWV was determined by an oscillometric method.

RESULTS

Osteoprotegerin correlated with age (p<0.001, r=0.27), aPWV (p<0.001, r=0.32) and hypersensitive C reactive protein (hsCRP) (p<0.001, r=0.37), OPN correlated directly with hsCRP (p<0.001, r=0.39) and inversely with high density lipoprotein cholesterol (p=0.02, r=-0.02). No significant association was found between total MGP and clinical, biochemical and vascular parameters. The correlation between OPG and aPWV persisted even after the adjustment for various potential confounders (p=0.02, r=0.19). In multiple regression analysis in the whole study population the most important predictors of aPWV were OPG (β=0.230, p=0.006), hsCRP (β=0.212, p=0.01) and systolic blood pressure (β=0.163, p=0.04). After exclusion of patients treated with statins the independent predictors were hsCRP (β=0.275, p=0.005) and OPG (β=0.199, p=0.04).

CONCLUSION

Circulating OPG, but not OPN and total MGP, is associated with aPWV and may be a marker of the increased arterial stiffness and cardiovascular risk in postmenopausal women.

Revised microcalcification hypothesis for fibrous cap rupture in human coronary arteries

Using 2.1-µm high-resolution microcomputed tomography, we have examined the spatial distribution, clustering, and shape of nearly 35,000 microcalcifications (µCalcs) ≥ 5 µm in the fibrous caps of 22 nonruptured human atherosclerotic plaques. The vast majority of these µCalcs were <15 µm and invisible at the previously used 6.7-µm resolution. A greatly simplified 3D finite element analysis has made it possible to quickly analyze which of these thousands of minute inclusions are potentially dangerous. We show that the enhancement of the local tissue stress caused by particle clustering increases rapidly for gap between particle pairs (h)/particle diameter (D) < 0.4 if particles are oriented along the tensile axis of the cap. Of the thousands of µCalcs observed, there were 193 particle pairs with h/D ≤ 2 (tissue stress factor > 2), but only 3 of these pairs had h/D ≤ 0.4, where the local tissue stress could increase a factor > 5. Using nondecalcified histology, we also show that nearly all caps have µCalcs between 0.5 and 5 µm and that the µCalcs ≥ 5 µm observed in high-resolution microcomputed tomography are agglomerations of smaller calcified matrix vesicles. µCalcs < 5 µm are predicted to be not harmful, because the tiny voids associated with these very small particles will not explosively grow under tensile forces because of their large surface energy. These observations strongly support the hypothesis that nearly all fibrous caps have µCalcs, but only a small subset has the potential for rupture.

Vitamin K-dependent carboxylation of matrix Gla-protein: a crucial switch to control ectopic mineralization

Vascular mineralization has recently emerged as a risk factor for cardiovascular morbidity and mortality. Previously regarded as a passive end-stage process, vascular mineralization is currently recognized as an actively regulated process with cellular and humoral contributions. The discovery that the vitamin K-dependent matrix Gla-protein (MGP) is a strong inhibitor of vascular calcification has propelled our mechanistic understanding of this process and opened novel avenues for diagnosis and treatment. This review focuses on molecular mechanisms of vascular mineralization involving MGP and discusses the potential for treatments and biomarkers to monitor patients at risk for vascular mineralization.

Composition and distribution of elements and ultrastructural topography of a human cardiac calculus

Trace elements (TEs) may contribute to the formation of calculi or stones or be involved in the aetiopathogenesis of stone diseases. The compositions and spatial distribution of elements from the inner nucleus to outer crust of the cardiac calculus were investigated by energy-dispersive X-ray fluorescence (EDXRF) spectrometer. The surface topograph, distribution map of elements, elemental and chemical compositions were also determined by environmental scanning electron microscope (ESEM)-energy-dispersive X-ray (EDX) analysis. Twenty-five elements were identifiable from 18 positions on the cardiac calculus by EDXRF spectrometer, in which the highest concentrations of toxic TEs (Ni, Pt, Hg, Sn, Pb, W, Au, Al, Si) and higher levels of essential TEs (Ca, Sr, Cr, P) were detected. A moderate positive Pearson's correlation between TEs concentrations of Mg, Ca or P and location differences from centre to periphery in the cardiac calculus was observed. A positive correlation was also found for Ca/Zn and Ca/Cu, indicating the gradual increase of calcium concentration from inner nucleus to outer crust of cardiac calculus. The drop-like nodules/crystals on the surface of petrous part of cardiac calculus were observed from ESEM analysis. ESEM-EDX analysis determined the calculus to be predominantly composed of calcium hydroxyapatite and cholesterol, as indicated by the petrous surface and drop-like nodules/crystals, respectively. This composition was confirmed using a portable Raman analyser. The spatial distribution analysis indicated a gradual increase in Mg, P and Ca concentrations from the inner nucleus to the outer crust of the cardiac calculus. The major chemical compositions of calcium hydroxyapatite and cholesterol were detected on this cardiac calculus.

Matrix gla protein gene polymorphism is associated with increased coronary artery calcification progression

OBJECTIVE

Matrix gla protein (MGP) inhibits arterial and cartilaginous calcification. A threonine to alanine (Thr83Ala) polymorphism (codon 83) in MGP is associated with myocardial infarction and femoral artery calcification. We examined the association of the MGP Thr83Ala polymorphism with quantity and progression of coronary artery calcification (CAC), a noninvasive measure of subclinical coronary atherosclerosis.

METHODS AND RESULTS

In 605 participants of the Epidemiology of Coronary Artery Calcification Study, generalized linear mixed models were fit to determine the association of MGP Thr83Ala with CAC quantity and progression. There was a significant additive relation between MGP Thr83Ala and CAC progression (P=0.001). In the fully adjusted model, every 1 Ala83 allele increase was associated with an estimated 1.9% (95% confidence interval, 0.7%-3.0%) per year since baseline larger increase in CAC quantity. A proxy single nucleotide polymorphism for MGP Thr83Ala (rs6488724) was similarly associated with CAC progression in an independent cohort from the Genetic Epidemiology Network of Arteriopathy (GENOA) study.

CONCLUSIONS

Increased risk of myocardial infarction associated with MGP ThrAla83 genotype observed elsewhere may be related to faster progression of subclinical coronary atherosclerosis. MGP genotype could be a potential candidate for identifying individuals at increased risk of atherosclerotic disease who would benefit from aggressive primary prevention strategies.

The explosive growth of small voids in vulnerable cap rupture; cavitation and interfacial debonding

While it is generally accepted that ruptures in fibrous cap atheromas cause most acute coronary deaths, and that plaque rupture occurs in the fibrous cap at the location where the tissue stress exceeds a certain critical peak circumferential stress, the exact mechanism of rupture initiation remains unclear. We recently reported the presence of multiple microcalcifications (μCalcs) <50 μm diameter embedded within the fibrous cap, μCalcs that could greatly increase cap instability by introducing up to a 5-fold increase in local tissue stress. Here, we explore the hypothesis that, aside from cap thickness, μCalc size and interparticle spacing are principal determinants of cap rupture risk. Also, we propose that cap rupture is initiated near the poles of the μCalcs due to the presence of tiny voids that explosively grow at a critical tissue stress and then propagate across the fibrous cap. We develop a theoretical model based on classic studies in polymeric materials by Gent (1980), which indicates that cavitation as opposed to interfacial debonding is the more likely mechanism for cap rupture produced by μCalcs <65 μm diameter. This analysis suggests that there is a critical μCalc size range, from 5 μm to 65 μm, in which cavitation should be prevalent. This hypothesis for cap rupture is strongly supported by our latest high resolution μCT studies in which we have observed trapped voids in the vicinity of μCalcs within fibrous caps in human coronaries.

Vitamin K-antagonists accelerate atherosclerotic calcification and induce a vulnerable plaque phenotype

BACKGROUND

Vitamin K-antagonists (VKA) are treatment of choice and standard care for patients with venous thrombosis and thromboembolic risk. In experimental animal models as well as humans, VKA have been shown to promote medial elastocalcinosis. As vascular calcification is considered an independent risk factor for plaque instability, we here investigated the effect of VKA on coronary calcification in patients and on calcification of atherosclerotic plaques in the ApoE(-/-) model of atherosclerosis.

METHODOLOGY/PRINCIPAL FINDINGS

A total of 266 patients (133 VKA users and 133 gender and Framingham Risk Score matched non-VKA users) underwent 64-slice MDCT to assess the degree of coronary artery disease (CAD). VKA-users developed significantly more calcified coronary plaques as compared to non-VKA users. ApoE(-/-) mice (10 weeks) received a Western type diet (WTD) for 12 weeks, after which mice were fed a WTD supplemented with vitamin K(1) (VK(1), 1.5 mg/g) or vitamin K(1) and warfarin (VK(1)&W; 1.5 mg/g & 3.0 mg/g) for 1 or 4 weeks, after which mice were sacrificed. Warfarin significantly increased frequency and extent of vascular calcification. Also, plaque calcification comprised microcalcification of the intimal layer. Furthermore, warfarin treatment decreased plaque expression of calcification regulatory protein carboxylated matrix Gla-protein, increased apoptosis and, surprisingly outward plaque remodeling, without affecting overall plaque burden.

CONCLUSIONS/SIGNIFICANCE

VKA use is associated with coronary artery plaque calcification in patients with suspected CAD and causes changes in plaque morphology with features of plaque vulnerability in ApoE(-/-) mice. Our findings underscore the need for alternative anticoagulants that do not interfere with the vitamin K cycle.

A mechanistic analysis of the role of microcalcifications in atherosclerotic plaque stability: potential implications for plaque rupture

The role of microcalcifications (μCalcs) in the biomechanics of vulnerable plaque rupture is examined. Our laboratory previously proposed (Ref. 44), using a very limited tissue sample, that μCalcs embedded in the fibrous cap proper could significantly increase cap instability. This study has been greatly expanded. Ninety-two human coronary arteries containing 62 fibroatheroma were examined using high-resolution microcomputed tomography at 6.7-μm resolution and undecalcified histology with special emphasis on calcified particles <50 μm in diameter. Our results reveal the presence of thousands of μCalcs, the vast majority in lipid pools where they are not dangerous. However, 81 μCalcs were also observed in the fibrous caps of nine of the fibroatheroma. All 81 of these μCalcs were analyzed using three-dimensional finite-element analysis, and the results were used to develop important new clinical criteria for cap stability. These criteria include variation of the Young's modulus of the μCalc and surrounding tissue, μCalc size, and clustering. We found that local tissue stress could be increased fivefold when μCalcs were closely spaced, and the peak circumferential stress in the thinnest nonruptured cap (66 μm) if no μCalcs were present was only 107 kPa, far less than the proposed minimum rupture threshold of 300 kPa. These results and histology suggest that there are numerous μCalcs < 15 μm in the caps, not visible at 6.7-μm resolution, and that our failure to find any nonruptured caps between 30 and 66 μm is a strong indication that many of these caps contained μCalcs.

Vascular calcification: the price to pay for anticoagulation therapy with vitamin K-antagonists

Vitamin K-antagonists (VKA) are the most widely used anti-thrombotic drugs with substantial efficacy in reducing risk of arterial and venous thrombosis. Several lines of evidence indicate, however, that VKA inhibit not only post-translational activation of vitamin K-dependent coagulation factors but also synthesis of functional extra-hepatic vitamin K-dependent proteins thereby eliciting undesired side-effects. Vascular calcification is one of the recently revealed side-effects of VKA. Vascular calcification is an actively regulated process involving vascular cells and a number of vitamin K-dependent proteins. Mechanistic understanding of vascular calcification is essential to improve VKA-based treatments of both thrombotic disorders and atherosclerosis. This review addresses vitamin K-cycle and vitamin K-dependent processes of vascular calcification that are affected by VKA. We conclude that there is a growing need for better understanding of the effects of anticoagulants on vascular calcification and atherosclerosis.

Phosphate: a stealthier killer than previously thought?

The kidneys excrete excess dietary phosphate, and patients with chronic kidney disease may suffer from phosphate overload and hyperphosphatemia. In chronic kidney disease, hyperphosphatemia has emerged as a risk factor for vascular calcification, cardiovascular mortality, left ventricular hypertrophy, and progression of chronic kidney disease. Serum phosphate at the upper limits of normal has also been associated with adverse outcomes in patients with relatively preserved kidney function. Of note, hyperphosphatemia is not a sensitive indicator of phosphate overload. In this regard, increased circulating fibroblast growth factor-23, a phosphatonin that is released in response to phosphate overload, is independently associated with adverse outcomes in patients with and without chronic kidney disease. Direct effects of extracellular phosphate on vascular calcification or cardiovascular cell biology; adverse consequences of adaptive mechanisms activated to limit phosphate overload, such as left ventricular hypertrophy induced by fibroblast growth factor-23; or epidemiological associations of additional cardiovascular risk factors with chronic kidney disease may underlie these observations. We now review the pathophysiology of phosphate, its relationship with cardiovascular outcomes, the potential consequences for patient care related to dietary phosphate and phosphate binders, and the clinical relevance for patients without overt chronic kidney disease.

Impaired iron status in aging research

Aging is associated with disturbances in iron metabolism and storage. During the last decade, remarkable progress has been made toward understanding their cellular and molecular mechanisms in aging and age-associated diseases using both cultured cells and animal models. The field has moved beyond descriptive studies to potential intervention studies focusing on iron chelation and removal. However, some findings remain controversial and inconsistent. This review summarizes important features of iron dyshomeostasis in aging research with a particular emphasis on current knowledge of the mechanisms underlying age-associated disorders in rodent models.

Acute myocardial infarction and atherosclerosis of the coronary arteries in patients treated with drugs against osteoporosis: calcium in the vessels and not the bones?

We studied the association between bisphosphonate use and risk of acute myocardial infarction (AMI) or atherosclerosis of the coronary vessels using a nationwide retrospective cohort from Denmark. All users of bisphosphonates and other drugs against osteoporosis between 1996 and 2006 (n = 103,562) comprised the exposed group and three age- and gender-matched controls from the general population (n = 310,683), the unexposed group. The main outcomes were occurrence of AMI or atherosclerosis of the coronary vessels. An excess risk of AMI was seen in users of alendronate compared to the unexposed. However, an inverse dose-response relationship was seen, with an increased risk in those with low adherence (≤0.66 DDD, HR = 1.50, 95% CI 1.24-1.82) and a nonsignificantly decreased risk in those who were adherent to the drug (≥1 DDD, HR = 0.80, 95% CI 0.59-1.09; P for trend <0.01). For etidronate and raloxifene, no excess risk was present and no dose-response relationship was seen. For atherosclerosis of the coronary vessels, a similar trend as for AMI was seen for alendronate but a protective effect was present at high doses (≥1 DDD, HR = 0.58, 95% CI 0.49-0.70). For etidronate, an increased risk of atherosclerosis was seen at all doses, with no dose-response relationship. For raloxifene, no excess of atherosclerosis was observed. At high doses of alendronate a decreased risk of atherosclerosis of the coronary vessels was seemingly present, whereas at low doses an increased risk was present. The finding may be spurious due to the "healthy user" effect, but a causal relationship cannot be excluded.

Role of vitamin K-dependent proteins in the arterial vessel wall

Vitamin K was discovered early last century at the same time as the vitamin K-antagonists. For many years the role of vitamin K was solely ascribed to coagulation and coagulation was thought to be involved only at the venous blood side. This view has dramatically changed with the discovery of vitamin K-dependent proteins outside the coagulation cascade and the role of coagulation factors at the arterial side. Vitamin K-dependent proteins are involved in the regulation of vascular smooth muscle cell migration, apoptosis, and calcification. Vascular calcification has become an important independent predictor of cardiovascular disease. Vitamin K-antagonists induce inactivity of inhibitors of vascular calcification, leading to accelerated calcification. The involvement of vitamin K-dependent proteins such as MGP in vascular calcification make that calcification is amendable for intervention with high intake of vitamin K. This review focuses on the effect of vitamin K-dependent proteins in vascular disease.