Osteogenesis associates with inflammation in early-stage atherosclerosis evaluated by molecular imaging in vivo

Optical and multimodality molecular imaging: insights into atherosclerosis

Imaging approaches that visualize molecular targets rather than anatomic structures aim to illuminate vital molecular and cellular aspects of atherosclerosis biology in vivo. Several such molecular imaging strategies stand ready for rapid clinical application. This review describes the growing role of in vivo optical molecular imaging in atherosclerosis and highlights its ability to visualize atheroma inflammation, calcification, and angiogenesis. In addition, we discuss advances in multimodality probes, both in the context of multimodal imaging as well as multifunctional, or "theranostic," nanoparticles. This review highlights particular molecular imaging strategies that possess strong potential for clinical translation.

Matrix Metalloproteinases in Medial Arterial Calcification: Potential Mechanisms and Actions

Arterial calcification is now understood to be an actively regulated process with promoters and inhibitors similar to those seen remodeling bone. It occurs in two distinct forms involving either the atherosclerotic intimal or the media. The amount of calcification found in the tibial arteries of the lower extremity is a better predictor of amputation than atherosclerosis risk factors and the ankle brachial index. We and others have recently demonstrated that matrix metalloproteinases (MMPs) play a critical role in the development of experimental arterial calcification in rodent models. The mechanisms by which MMPs may regulate arterial calcification, however, are not completely understood. While MMPs have traditionally been thought to function primarily in the degradation of extracellular matrix molecules, recent data suggest that MMPs may also function as important regulators of matrix biology, inflammation, and osteogenesis. In this review, we will examine recent data on the potential mechanisms by which MMPs may function in the control of arterial calcification.

Notch signaling induces osteogenic differentiation and mineralization of vascular smooth muscle cells: role of Msx2 gene induction via Notch-RBP-Jk signaling

OBJECTIVE

Vascular calcification is closely correlated with cardiovascular morbidity and mortality. Here, we demonstrate the role of Notch signaling in osteogenic differentiation and mineralization of vascular smooth muscle cells (SMCs).

METHODS AND RESULTS

The Msx2 gene, a key regulator of osteogenesis, was highly induced by coculture with Notch ligand-expressing cells or overexpression of Notch intracellular domains (NICDs) in human aortic SMCs (HASMCs). Furthermore, the Notch1 intracellular domain (N1-ICD) overexpression markedly upregulated alkaline phosphatase (ALP) activity and matrix mineralization of HASMCs. A knockdown experiment with a small interfering RNA confirmed that Msx2 mediated N1-ICD-induced osteogenic conversion of HASMCs. Interestingly, Msx2 induction by N1-ICD was independent of bone morphogenetic protein-2 (BMP-2), an osteogenic morphogen upstream of Msx2. The transcriptional activity of the Msx2 promoter was significantly enhanced by N1-ICD overexpression. The RBP-Jk binding element within the Msx2 promoter was critical to Notch-induced Msx2 gene expression. Correspondingly, N1-ICD overexpression did not induce the Msx2 expression in RBP-Jk-deficient fibroblasts. Immunohistochemistry of human carotid artery specimens revealed localization of Notch1, Jagged1 and Msx2 to fibrocalcific atherosclerotic plaques.

CONCLUSIONS

These results imply a new mechanism for osteogenic differentiation of vascular SMCs in which Notch/RBP-Jk signaling directly induces Msx2 gene expression and suggest its crucial role in mediating vascular calcification.

Fetuin-A protects against atherosclerotic calcification in CKD

Reduced serum levels of the calcification inhibitor fetuin-A associate with increased cardiovascular mortality in dialysis patients. Fetuin-A-deficient mice display calcification of various tissues but notably not of the vasculature. This absence of vascular calcification may result from the protection of an intact endothelium, which becomes severely compromised in the setting of atherosclerosis. To test this hypothesis, we generated fetuin-A/apolipoprotein E (ApoE)-deficient mice and compared them with ApoE-deficient and wild-type mice with regard to atheroma formation and extraosseous calcification. We assigned mice to three treatment groups for 9 wk: (1) Standard diet, (2) high-phosphate diet, or (3) unilateral nephrectomy (causing chronic kidney disease [CKD]) plus high-phosphate diet. Serum urea, phosphate, and parathyroid hormone levels were similar in all genotypes after the interventions. Fetuin-A deficiency did not affect the extent of aortic lipid deposition, neointima formation, and coronary sclerosis observed with ApoE deficiency, but the combination of fetuin-A deficiency, hyperphosphatemia, and CKD led to a 15-fold increase in vascular calcification in this model of atherosclerosis. Fetuin-A deficiency almost exclusively promoted intimal rather than medial calcification of atheromatous lesions. High-phosphate diet and CKD also led to an increase in valvular calcification and aorta-associated apoptosis, with wild-type mice having the least, ApoE-deficient mice intermediate, and fetuin-A/ApoE-deficient mice the most. In addition, the combination of fetuin-A deficiency, high-phosphate diet, and CKD in ApoE-deficient mice greatly enhanced myocardial calcification, whereas the absence of fetuin-A did not affect the incidence of renal calcification. In conclusion, fetuin-A inhibits pathologic calcification in both the soft tissue and vasculature, even in the setting of atherosclerosis.

Arterial and aortic valve calcification abolished by elastolytic cathepsin S deficiency in chronic renal disease

BACKGROUND

Clinical studies have demonstrated that 50% of individuals with chronic renal disease (CRD) die of cardiovascular causes, including advanced calcific arterial and valvular disease; however, the mechanisms of accelerated calcification in CRD remain obscure, and no therapies can prevent disease progression. We recently demonstrated in vivo that inflammation triggers cardiovascular calcification. In vitro evidence also indicates that elastin degradation products may promote osteogenesis. Here, we used genetically modified mice and molecular imaging to test the hypothesis in vivo that cathepsin S (catS), a potent elastolytic proteinase, accelerates calcification in atherosclerotic mice with CRD induced by 5/6 nephrectomy.

METHODS AND RESULTS

Apolipoprotein-deficient (apoE(-/-))/catS(+/+) (n=24) and apoE(-/-)/catS(-/-) (n=24) mice were assigned to CRD and control groups. CRD mice had significantly higher serum phosphate, creatinine, and cystatin C levels than those without CRD. To visualize catS activity and osteogenesis in vivo, we coadministered catS-activatable and calcification-targeted molecular imaging agents 10 weeks after nephrectomy. Imaging coregistered increased catS and osteogenic activities in the CRD apoE(-/-)/catS(+/+) cohort, whereas CRD apoE(-/-)/catS(-/-) mice exhibited less calcification. Quantitative histology demonstrated greater catS-associated elastin fragmentation and calcification in CRD apoE(-/-)/catS(+/+) than CRD apoE(-/-)/catS(-/-) aortas and aortic valves. Notably, catS deletion did not cause compensatory increases in RNA levels of other elastolytic cathepsins or matrix metalloproteinases. Elastin peptide and recombinant catS significantly increased calcification in smooth muscle cells in vitro, a process further amplified in phosphate-enriched culture medium.

CONCLUSIONS

The present study provides direct in vivo evidence that catS-induced elastolysis accelerates arterial and aortic valve calcification in CRD, providing new insight into the pathophysiology of cardiovascular calcification.

Advances in molecular imaging of atherosclerotic vascular disease

PURPOSE OF REVIEW

Molecular imaging aims to illuminate vital molecular and cellular aspects of disease in vivo, and is rapidly translating into the clinical arena. Advantages of this field include enabling serial biological studies in living subjects, assessment of pharmaceutical efficacy, and in-vivo characterization of clinical diseases. Here we present recent exciting advances in molecular imaging of atherosclerotic vascular disease.

RECENT FINDINGS

Atherosclerosis molecular imaging approaches are now available for magnetic resonance, nuclear, computed tomography, ultrasound, and near-infrared fluorescence imaging. Advances in agent synthesis and detection technology are now enabling in-vivo imaging of endothelial cell activation, macrophages, cellular metabolism, protease activity, apoptosis, and osteogenic activity. Several agents show clinical utility for the detection of high-risk plaques.

SUMMARY

Molecular imaging is actively unraveling the biological basis of atherosclerosis in living subjects. In the near-term, molecular imaging will play an important role in assessing novel atherosclerosis pharmacotherapies in clinical trials. Longer term, molecular imaging should enable accurate identification of high-risk plaques responsible for myocardial infarction, stroke, and ischemic limbs.

TNF-alpha and IL-1beta inhibit RUNX2 and collagen expression but increase alkaline phosphatase activity and mineralization in human mesenchymal stem cells

AIMS

Joint inflammation leads to bone erosion in rheumatoid arthritis (RA), whereas it induces new bone formation in spondyloarthropathies (SpAs). Our aims were to clarify the effects of tumour necrosis factor alpha (TNF-alpha) and interleukin 1beta (IL-1beta) on osteoblast differentiation and mineralization in human mesenchymal stem cells (MSCs).

MAIN METHODS

In MSCs, expression of osteoblast markers was assessed by real-time PCR and ELISA. Activity of tissue-nonspecific alkaline phosphatase (TNAP) and mineralization were determined by the method of Lowry and alizarin red staining respectively. Involvement of RUNX2 in cytokine effects was investigated in osteoblast-like cells transfected with a dominant negative construct.

KEY FINDINGS

TNF-alpha (from 0.1 to 10 ng/ml) and IL-1beta (from 0.1 to 1 ng/ml) stimulated TNAP activity and mineralization in MSCs. Addition of 50 ng/ml of IL-1 receptor antagonist in TNF-alpha-treated cultures did not reverse TNF-alpha effects, indicating that IL-1 was not involved in TNF-alpha-stimulated TNAP activity. Both TNF-alpha and IL-1beta decreased RUNX2 expression and osteocalcin secretion, suggesting that RUNX2 was not involved in mineralization. This hypothesis was confirmed in osteoblast-like cells expressing a dominant negative RUNX2, in which TNAP expression and activity were not reduced. Finally, since mineralization may merely rely on increased TNAP activity in a collagen-rich tissue, we investigated cytokine effects on collagen expression, and observed that cytokines decreased collagen expression in osteoblasts from MSC cultures.

SIGNIFICANCE

The different effects of cytokines on TNAP activity and collagen expression may therefore help explain why inflammation decreases bone formation in RA whereas it induces ectopic ossification from collagen-rich entheses during SpAs.

Efficacy of simvastatin treatment of valvular interstitial cells varies with the extracellular environment

OBJECTIVE

The lack of therapies that inhibit valvular calcification and the conflicting outcomes of clinical studies regarding the impact of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors on valve disease highlight the need for controlled investigations to characterize the interactions between HMG-CoA reductase inhibitors and valve tissue. Thus, we applied multiple in vitro disease stimuli to valvular interstitial cell (VIC) cultures and examined the impact of simvastatin treatment on VIC function.

METHODS AND RESULTS

VICs were cultured on 3 different substrates that supported various levels of nodule formation. Transforming growth factor (TGF)-beta1 was also applied as a disease stimulus to VICs on 2-D surfaces or encapsulated in 3-D collagen gels and combined with different temporal applications of simvastatin. Simvastatin inhibited calcific nodule formation in a dose-dependent manner on all materials, although the level of statin efficacy was highly substrate-dependent. Simvastatin treatment significantly altered nodule morphology, resulting in dramatic nodule dissipation over time, also in a substrate-dependent manner. These effects were mimicked in 3-D cultures, wherein simvastatin reversed TGF-beta1-induced contraction. Decreases in nodule formation were not achieved via the HMG-CoA reductase pathway, but were correlated with decreases in ROCK activity.

CONCLUSIONS

These studies represent a significant contribution to understanding how simvastatin may impact heart valve calcification.

Evolving concepts of cardiac valve dynamics: the continuum of development, functional structure, pathobiology, and tissue engineering

Considerable progress has been made in recent years toward elucidating a conceptual framework that integrates the dynamic functional structure, mechanical properties, and pathobiological behavior of the cardiac valves. This communication reviews the evolving paradigm of a continuum of heart valve structure, function, and pathobiology and explores its implications. Specifically, we discuss (1) the interactions of valve biology and biomechanics (eg, correlations of function with structure at the cell, tissue, and organ levels and mechanical considerations, development, endothelial cell and interstitial cell biology, extracellular matrix biology, homeostasis, and adaptation to environmental change); (2) mechanisms of disease (eg, valve cell and matrix pathobiology in congenital anomalies, aortic valve calcification, and mitral valve prolapse); (3) considerations in replacement and repair (eg, cell/matrix biology of tissue valve substitutes and their degeneration and durability of repairs); and (4) the potential for tissue engineering approaches to therapeutic regeneration of the cardiac valves. Opportunities for research and clinical translation are highlighted.

Mineral surface in calcified plaque is like that of bone: further evidence for regulated mineralization

OBJECTIVE

Cell biological studies demonstrate remarkable similarities between mineralization processes in bone and vasculature, but knowledge of the components acting to initiate mineralization in atherosclerosis is limited. The molecular level microenvironment at the organic-inorganic interface holds a record of the mechanisms controlling mineral nucleation. This study was undertaken to compare the poorly understood interface in mineralized plaque with that of bone, which is considerably better characterized.

METHODS AND RESULTS

Solid state nuclear magnetic resonance (SSNMR) spectroscopy provides powerful tools for studying the organic-inorganic interface in calcium phosphate biominerals. The rotational echo double resonance (REDOR) technique, applied to calcified human plaque, shows that this interface predominantly comprises sugars, most likely glycosaminoglycans (GAGs). In this respect, and in the pattern of secondary effects seen to protein (mainly collagen), calcified plaque strongly resembles bone.

CONCLUSIONS

The similarity between biomineral formed under highly controlled (bone) and pathological (plaque) conditions suggests that the control mechanisms are more similar than previously thought, and may be adaptive. It is strong further evidence for regulation of plaque mineralization by osteo/chondrocytic vascular smooth muscle cells.