Current Pharmaceutical Design - Volume 20, Issue 37, 2014

Our society faces a growing burden of atherosclerosis and arteriosclerosis, diseases that result from dysregulated lipid and mineral metabolisms. Atherosclerosis and arteriosclerosis lead to increased acute cardiovascular events. A number of risk factors, including hypercholesterolemia, metabolic syndrome, diabetes mellitus and endstage renal diseases, have been found to contribute to acceleration of vascular calcification [1-5]. Vascular calcification occurs as a result of the deposition of calcium, predominantly in the form of hydroxyapatite, in both the tunica intima and the tunica media of the arterial wall [2-5]. Vascular calcification is known to occur in two distinct forms: intimal calcification, which always occurs in the context of atherosclerosis, and medial calcification, which can occur in its absence [12, 13, 19]. These two processes differ not only in morphology, but also in the pathological mechanisms involved [6, 12, 13, 19]. Inflammatory cells such as macrophages and mast cells infiltrating plaque lipid-rich regions play an important role in atherosclerotic calcification [5, 10-20]. In contrast to intimal calcification, medial calcification is known to occur in the absence of inflammatory cell infiltration and lipid deposition [5,10-20]. Apart from arteriosclerotic calcification of the media in large elastic-type arteries of nondiabetic individuals, the calcification process, known as Monckeberg's sclerosis, commonly affects the media of peripheral medium-sized arteries in aged and diabetic individuals [19]. Over the last decades, there has been growing interest in the identification of molecular and cellular mechanisms involved in vascular calcification [1-24]. Accumulating evidence suggests that the pathological processes that are instrumental for cardiovascular calcification utilize the mechanisms which are the same or similar to those of bone development, such as osteoblastic differentiation and biomineralization [1-24]. A significant body of evidence has accumulated to support a view that arterial calcification is not a passive but an active cell-regulated process similar to osteogenesis [1-24]. This view is supported by the findings of the expression of a variety of bone-associated proteins in atherosclerotic plaque, particularly at sites prone to or undergoing calcification as well as by identification of cases of the formation of bone in arterial tissue. Several cell types have been suggested to be responsible for arterial calcification. Vascular smooth muscle cells (SMCs), microvascular smooth muscle-like cells, and pericytes can differentiate in culture to form osteoblast-like cells producing a calcified matrix and the concept of multipotent mesenchymal calcifying vascular cells, which are capable of forming calcifying nodules, has been developed [2, 23]. The association of vascular calcification with SMC phenotypic transition, in which several osteogenic proteins including osteopontin, osteocalcin, the bone-determining factor Cbfa, S100 proteins were gained [1-24]. It has been reported that both vascular SMCs and macrophages express a variety of chondrocytic, osteoblastic, and osteoclast-associated proteins that may govern the calcification process in the arterial wall. The invasion of pluripotent bone marrow-derived cells into atherosclerotic plaques has been suggested as well, adding a new dimension to speculations about the origin of vascular calcifying cells [5, 10, 22]. Currently, no therapies are available to prevent vascular calcification. Despite the clinical importance of the elucidation of the mechanisms and the identification of biomarkers of vascular calcification, the mechanisms of vascular calcification and biomarkers are insufficiently studied and insufficiently understood. In 2010, the National Heart, Lung and Blood Institute (NHLBI) Working Group on Calcific Aortic Stenosis emphasized the importance of understanding the mechanisms of vascular calcification markers and the need to develop new imaging modalities for detection of subclinical calcification [15]. A special issue entitled” Targeting vascular calcification: Up-date” highlights the recent advances in our understanding of the mechanisms of vascular calcification and discuss challenges for treatment or suppression of the involved pathological processes. Vascular calcification is common in aging as well as a number of genetic and metabolic disorders. Ectopic mineralization in the arteries complicates the prognosis and increases the morbidity in diseases such as atherosclerosis, diabetes and chronic kidney disease (CKD). In this special issue, several reviews provide up-date information about the mechanism of vascular calcification associated with several human diseases [25-35], including atherosclerosis [25, 26], diabetes [27], and CKD [28]. Traditionally abdominal aortic calcification (AAC) has received less intensive study than artery calcification in atherosclerosis and diabetes, but the widespread use of abdominal imaging has however encouraged recent investigation of this problem. In this issue, Jonathan Golledge (Australia) provides the current information about AAC [29]. In a review article entitled "Modulators of networks: Molecular targets of arterial calcification identified in man and mice" Yvonne Nitschke, Frank Rutsch (Germany) show that genetic studies of rare monogenic human disorders and studies of naturally occurring or mutant mouse models have identified specific inductors and inhibitors of arterial calcification, which can be classified according to the networks they participate in [30]. These networks include ATP and pyrophosphate metabolism, Phosphate homeostasis and Vitamin D Receptor Signalling [30]. Furthermore, intracellular signalling molecules, including SMAD6 and a number of systemic circulatory inhibitors of arterial calcification, including fetuin, tumor necrosis factor receptor superfamily member 11b, matrix GLA protein, adiponectin and Family with sequence similarity 20 member A have been identified by human and mouse genetics [30]. Based on the in vivo evidence of their functional relevance, the above listed proteins will serve as excellent targets for the prevention and treatment of arterial calcification [30]. The involvement of the osteoprotegerin (OPG)/ receptor activator of nuclear factor-ΚB (RANK)/ Receptor activator of nuclear factor-ΚB ligand (RANKL) triad and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) in cardiovascular pathology is well recognized nowadays and attracts increasing interest. In this special issue, two review articles highlight the most current information on the relationship between RANKL, OPG and TRAIL, and unambiguously demonstrate that a better understanding of RANKL-mediated signalling may help develop more sophisticated cell-based therapies to inhibit calcification of the vessel wall [31,32]. Another review article, entitled "Role of Bone-Type Tissue- Nonspecific Alkaline Phosphatase and PHOSPO1 in Vascular Calcification" shows that pathologic roles of bone-type TNAP and PHOSPHO1 make them to be attractive targets for cardiovascular anti-calcification therapy [33]. In a paper entitled “Blood Serum Atherogenicity and Coronary Artery Calcification’ a collaborative research group (Russia and Germany) reports an interesting finding indicating that seruminduced intracellular cholesterol accumulation is not related to the processes of calcium deposition in arterial wall [34]. Despite of a remarkable progress in our understanding of cell elements involved in arterial calcification, there is still controversy about the contribution of mesenchymal cell progenitors and blood origin circulating cell precursors to vascular calcification. In this issue of the journal, Mattia Albiero and colleagues (Italy) discuss potential roles of different populations of circulating calcifying cells in vascular calcification [35]. Thus, in this special issue, a team of international experts discuss the most novel topics relating to the problem of vascular calcification. I would like to thank the contributors to this issue for their participation. We hope that this special issue will be helpful for the development of novel therapeutic drugs against vascular calcification and novel approaches for prevention, diagnosis and treatment of this pathological condition.

Mineralization of bone and tooth extracellular matrix (ECM) is a physiologic process, while soft tissue mineralization, also known as ectopic mineralization (calcification), is a pathologic condition. Vascular calcification is common in aging and also in a number of genetic and metabolic disorders. The calcific deposits in arteries complicate the prognosis and increase the morbidity in diseases such as atherosclerosis, diabetes and chronic kidney disease (CKD). To completely understand the pathophysiology of these lifethreatening diseases, it is critical to elucidate the molecular mechanisms underlying vascular calcification. Unveiling these mechanisms will eventually identify new therapeutic targets and also improve the management of the associated complications. In the current review, we discussed the common determinants of ECM mineralization, the mechanism of vascular calcification associated with several human diseases and outlined the most common therapeutic approaches to prevent its progression.

Arterial calcification (AC) is a hallmark of many serious diseases, including atherosclerosis, chronic kidney disease, and diabetes. AC may also develop as a side-effect of therapy with anticoagulants, such as warfarin which is widely used for prophylaxis of thrombosis. In our studies, we established the relation between warfarin-induced AC and activation of enzyme transglutaminase 2 (TG2) and β-catenin signaling. We showed that TG2-specific inhibitor KCC-009 significantly attenuated the damaging effects of warfarin on arterial tissue. A similar protective effect was also achieved with a dietary bioflavonoid quercetin that inhibits TG2 and β-catenin signaling. We have shown that quercetin intercepts the chondrogenic transformation of vascular smooth muscle and also drastically attenuates calcifying cartilaginous metaplasia in another model of AC caused by genetic loss of matrix gla protein (MGP). These findings suggest that quercetin may be considered as a promising anti-AC therapeutic in the clinical settings of warfarin supplementation and MGP dysfunction. Further studies are required to test the efficacy of quercetin on other types of AC.

Matrix vesicle (MV)-mediated mineralization is important for bone ossification. However, under certain circumstances such as atherosclerosis, mineralization may occur in the arterial wall. Bone-type tissue-nonspecific alkaline phosphatase (TNAP) hydrolyzes inorganic pyrophosphate (PPi) and generates inorganic phosphate (Pi), which is essential for MV-mediated hydroxyapatite formation. MVs contain another phosphatase, PHOSPHO1, that serves as an additional supplier of Pi. Activation of bone-type tissue-nonspecific alkaline phosphatase (TNAP) in vascular smooth muscle cells precedes vascular calcification. By degrading PPi, TNAP plays a procalcific role changing the Pi/PPi ratio toward mineralization. A pathologic role of bone-type TNAP and PHOSPHO1 make them to be attractive targets for cardiovascular therapy.

In patients with chronic kidney disease (CKD), vascular calcification is associated with significant morbidity and mortality. The prevalence of vascular calcification increases as glomerular filtration rate (GFR) declines and calcification occurs years earlier in CKD patients than in the general population. The mechanisms of vascular calcification in CKD patients are complex and not completely understood but likely involve non-traditional risk factors, which may be unique to patients with CKD. These unique risk factors may predispose patients to early and more accelerated calcification. Experimental and clinical studies show that disorders in mineral metabolisms including calcium and phosphorus homeostasis initiate and promote vascular calcification in patients with CKD. It is currently unknown if vascular calcification can be prevented or reversed with therapies aimed at maintaining calcium and phosphorus homeostasis. This review focuses on the potential mechanisms by which disordered mineral metabolism may promote vascular calcification in patients with CKD.

Traditionally abdominal aortic calcification (AAC) has received less intensive study than coronary artery calcification. The widespread use of abdominal imaging has however encouraged recent investigation of this problem. Human association studies suggest that older age, chronic kidney disease and osteoporosis are the most important risk factors for AAC. AAC severity has been consistently associated with death and cardiovascular events and therefore there is growing interest in identifying potential therapies to limit AAC. At present there have only been a small number of well controlled trials designed to assess effective interventions for AAC. Further studies are expected over the coming years. Whether an intervention which effectively limits AAC will also reduce the incidence of cardiovascular events remains to be established.

In recent years, mechanisms of arterial calcifications are beginning to be elucidated. Arterial calcification is now considered as an actively regulated process resembling osteogenesis within the arterial wall orchestrated by a number of systemic or constitutively expressed mediators. Genetic studies of rare monogenic human disorders and studies of naturally occurring or mutant mouse models have identified specific inductors and inhibitors of arterial calcification, which can be classified according to the networks they participate in. These networks include ATP and pyrophosphate metabolism, phosphate homeostasis and vitamin D receptor signaling. Furthermore, intracellular signaling molecules, including SMAD6 and a number of systemic circulatory inhibitors of arterial calcification, including fetuin, tumor necrosis factor receptor superfamily member 11b, matrix GLA protein, adiponectin and family with sequence similarity 20 member A have been identified by human and mouse genetics. Based on the in vivo evidence of their functional relevance, these proteins will serve as excellent targets for the prevention and treatment of arterial calcification. In this review we discuss the functional role of the identified modulators of arterial calcification and describe the networks they belong to.

Receptor activator of nuclear factor-ΚB ligand (RANKL) is a member of the tumour necrosis factor family important in bone remodelling. Recent evidence suggest that calcification in the vessel wall is equivalent to mechanisms mediating bone formation. This review highlights the role of RANKL in vascular arterial calcification. Here, the relationship between RANKL, osteoprotegerin (OPG) and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) is discussed. Furthermore, we focus on the regulatory mechanisms mediating RANKL gene expression and transcription in cells of the vessel wall. A better understanding of RANKL-mediated signalling may help develop more sophisticated cell-based therapies to inhibit calcification of the vessel wall.

The OPG/RANK/RANKL axis is now recognized as a master regulator of bone remodeling, controlling osteoclastapos;s maturation and extracellular matrix calcification. Nevertheless, a number of clinical and basic science studies conducted in the last few years demonstrated that the triad could be also involved in several physiological and pathological processes outside the bone tissue. In particular, evidences have been collected showing an active participation of OPG and RANKL in vascular pathology, including atherogenesis and arterial calcification. A series of epidemiological studies also showed that increased circulating levels of OPG are associated with significant, independent predictive value for future cardiovascular mortality/morbidity. However, the human studies did not unravel whether OPG should be considered as a promoter, a protective mechanism or is instead neutral with regard of vascular disease progression. Main objective of the present review is to summarize findings from both in vivo and in vitro investigations on the role played by OPG in vascular disease progression and to delineate a plausible scenario on the actual involvement of the OPG/RANK/RANKL triad and TRAIL in cardiovascular pathology.

Medial artery calcification (MAC) is a characteristic feature of diabetes. MAC represents a concentric calcification that proceeds via matrix vesicle-nucleated mineralization accompanied with apatitic calcium phosphate deposits in the arterial tunica media in the absence of atheroma and neointima. Multiple factors contribute to the induction and progression of diabetic MAC including inflammation, oxidative stress, adiposity, insulin resistance, advanced glycation end-products, and hyperphosphatemia. Osteoblast-like cells form in the vessel wall from vascular smooth muscle cells and multipotent vascular mesenchymal progenitors. These mineralizing cells as well as the recruitment of undifferentiated progenitors to the osteochondrocyte lineage play a critical role in the calcification process. Important transcription factors such as Msx 2, Osterix, and RUNX2 are crucial in the programming of osteogenesis. Currently, no therapy is available to reverse vascular calcification. Available therapies can only reduce and slow the progression of vascular calcification. Targeting regulatory proteins and enzymes directly involved in osteochondrogenesis and hydroxyapatite accumulation in the vascular wall may be beneficial for generating new efficient anti-calcific drugs.

The phenomenon of blood serum atherogenicity was described as the ability of human serum to induce lipid accumulation in cultured cells. The results of recent two-year prospective study in asymptomatic men provided the evidence for association between the changes in serum atherogenicity and dynamics of carotid intima-media thickness progression. The present study was undertaken to test the hypothesis that blood serum atherogenicity and its changes in dynamics may be associated with accumulation of coronary calcium in subclinical atherosclerosis. It was performed in 782 CHD-free participants of The Heinz Nixdorf RECALL (Risk Factors, Evaluation of Coronary Calcium and Lifestyle) Study, in whom blood samples have been taken at the baseline and at the end of 5-year follow-up. Opposite to the previous findings, the changes in serum atherogenicity did not correlate neither with the extent of coronary artery calcification, nor with the changes in Agatston CAC score. There was a moderate but significant rise in serum atherogenicity after 5-year followup period, and the same dynamics was observed for Agatston CAC score, but not for convenient lipid-related risk factors. The absence of association of the changes in serum atherogenicity with the changes in Agatston CAC score, along with previous findings, provides a point of view that serum-induced intracellular cholesterol accumulation is not related to the processes of calcium deposition in arterial wall, since the last one reflects the progression of already existing subclinical atherosclerotic lesions.

Vascular calcification is the deposition of calcium-phosphate salts in the form of hydroxyapatite within the arterial wall. This is a finely regulated process to such an extent that it shares some mechanisms with endochondral and membranous embryonic ossification. Current theories describe vascular calcification as the imbalance between mechanisms, which promote calcification and those that inhibit it. Canonical cellular players in this scenario include endothelial cells, resident vascular smooth muscle cells and immune cells. Nevertheless, the last decade has seen the rise of extraparietal cells as important players in vascular biology and also in the setting of ectopic calcification. After an overview of the mechanism involved in vascular calcification, we herein discuss the potential role of different populations of circulating calcifying cells.

Neuroinflammation is a well-orchestrated, dynamic, multicellular process playing a major role in neurodegenerative disorders. The microglia which make up the innate immune system of the central nervous system are key cellular mediators of neuroinflammatory processes. In normal condition they exert a protective function, providing tissue repair by releasing anti-inflammatory cytokines and neurotrophic factors. Upon neuronal injury or infection, they become overactivated, thereby releasing neurotoxic substances, amplifying neuroinflammation leading to neurodegeneration. Positron emission tomography (PET) provides a sensitive non-invasive imaging technique to study and quantify receptor and enzyme expression. A radiolabeled tracer for a protein (over)expressed in neuroinflammation and more specifically for the overactivated microglia would be useful as a diagnostic tool in the follow-up of neuroinflammation progression and to study the efficacy of anti-inflammatory therapy over time. In this manuscript, an overview of potential PET tracer targets upregulated during neuroinflammation is provided together with the current radiotracers used to image these targets. In addition, lead structures to develop radiotracers for new targets are suggested.

Objectives: There are two major forms of vitamin D, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). We studied the effect of the different vitamin D fractions (D3/D2) on arterial wall properties in coronary artery disease (CAD) patients. Methods: We included 252 subjects with CAD. Endothelial function was evaluated by flow mediated dilation (FMD). Carotid femoral pulse wave velocity (PWV) was measured as an index of arterial stiffness and augmentation index (AI) as a measure of reflected waves. Measures for 25(OH)D2 and 25(OH)D3 were performed using Liquid Chromatography Mass Spectrometry technology. Results: From the study population, 155(62%), 66(26%) and 31(12%) were categorized as having vitamin D deficiency, insufficiency and sufficiency respectively. There was no difference between subjects with vitamin D deficiency, insufficiency and sufficiency in FMD, AI and PWV (p=NS for all). Subjects with vitamin D insufficiency/deficiency had significantly higher D2 to D ratio compared to subjects with vitamin D sufficiency. Interestingly, FMD was positively associated with D2 to D ratio (rho=0.13, p=0.02) and subjects with D2 levels<0.3ng/ml had impaired FMD compared to those with increased D2 levels (p=0.048). Conclusion: Vitamin D insufficiency/deficiency is highly prevalent in CAD subjects. Vitamin D2 concentrations are positively associated with endothelial function. These findings may suggest a beneficial role of vitamin D2 levels in vascular health.