Identification and characterization of vascular calcification-associated factor, a novel gene upregulated during vascular calcification in vitro and in vivo

Adipose tissue-derived omentin-1 attenuates arterial calcification via AMPK/Akt signaling pathway

Adipose tissue-derived adipokines mediate various kind of crosstalk between adipose tissue and other organs and thus regulate metabolism balance, inflammation state as well as disease progression. In particular, omentin-1, a newly found adipokine, has been reported to exhibit anti-calcification effects in vitro and in vivo. However, little is known about the function of endogenous adipose tissue-derived omentin-1 in arterial calcification and the detailed mechanism involved. Here, we demonstrated that global omentin-1 knockout (omentin-1^-/-) resulted in more obvious arterial calcification in 5/6-nephrectomy plus high phosphate diet treated (5/6 NTP) mice while overexpression of omentin-1 attenuated attenuates osteoblastic differentiation and mineralisation of VSMCs in vitro and 5/6 NTP-induced mice arterial calcification in vivo. Moreover, we found that omentin-1 induced AMPK and Akt activation while inhibition of AMP-activated protein kinase (AMPK) and Akt signaling reversed the anti-calcification effect induced by omentin-1 both in vitro and in vivo. Our results suggest that adipose tissue-derived omentin-1 serves as a potential therapeutic target for arterial calcification and cardiovascular disease.

Pericytes in the Heart

Mural cells known as pericytes envelop the endothelial layer of microvessels throughout the body and have been described to have tissue-specific functions. Cardiac pericytes are abundantly found in the heart, but they are relatively understudied. Currently, their importance is emerging in cardiovascular homeostasis and dysfunction due to their pleiotropism. They are known to play key roles in vascular tone and vascular integrity as well as angiogenesis. However, their dysfunctional presence and/or absence is critical in the mechanisms that lead to cardiac pathologies such as myocardial infarction, fibrosis, and thrombosis. Moreover, they are targeted as a therapeutic potential due to their mesenchymal properties that could allow them to repair and regenerate a damaged heart. They are also sought after as a cell-based therapy based on their healing potential in preclinical studies of animal models of myocardial infarction. Therefore, recognizing the importance of cardiac pericytes and understanding their biology will lead to new therapeutic concepts.

Vascular Calcification: Is it rather a Stem/Progenitor Cells Driven Phenomenon?

Vascular calcification (VC) has witnessed a surge of interest. Vasculature is virtually an omnipresent organ and has a notably high capacity for repair throughout embryonic and adult life. Of the vascular diseases, atherosclerosis is a leading cause of morbidity and mortality on account of ectopic cartilage and bone formation. Despite the identification of a number of risk factors, all the current theories explaining pathogenesis of VC in atherosclerosis are far from complete. The most widely accepted response to injury theory and smooth muscle transdifferentiation to explain the VC observed in atherosclerosis is being challenged. Recent focus on circulating and resident progenitor cells in the vasculature and their role in atherogenesis and VC has been the driving force behind this review. This review discusses intrinsic cellular players contributing to fate determination of cells and tissues to form ectopic cartilage and bone formation.

Discovering cardiac pericyte biology: From physiopathological mechanisms to potential therapeutic applications in ischemic heart disease

Microvascular pericytes and the more recently discovered adventitial pericyte-like progenitor cells are a subpopulation of vascular stem cells closely associated with small and large blood vessels respectively. These populations of perivascular cells are remarkably abundant in the heart. Pericytes control important physiological processes such as angiogenesis, blood flow and vascular permeability. In the heart, this pleiotropic activity makes pericytes extremely interesting for applications in regenerative medicine. On the other hand, dysfunction of pericytes could participate in the pathogenesis of cardiovascular disease, such as arterial hypertension, fibro-calcific cardiovascular remodeling, myocardial edema and post-ischemic coronary no-reflow. On a therapeutic standpoint, preclinical studies in small animal models of myocardial infarction have demonstrated the healing potential of pericytes transplantation, which has been ascribed to direct vascular incorporation and paracrine pro-angiogenic and anti-apoptotic activities. These promising findings open the door to the clinical use of pericytes for the treatment of cardiovascular diseases.

Elevated levels of endothelial-derived microparticles, and serum CXCL9 and SCGF-β are associated with unstable asymptomatic carotid plaques

Endothelial microparticles (EMPs) are released from dysfunctional endothelial cells. We hypothesised that patients with unstable carotid plaque have higher levels of circulating microparticles compared to patients with stable plaques, and may correlate with serum markers of plaque instability and inflammation. Circulating EMPs, platelet MPs (PMPs) and inflammatory markers were measured in healthy controls and patients undergoing carotid endarterectomy. EMP/PMPs were quantified using flow cytometry. Bioplex assays profiled systemic inflammatory and bone-related proteins. Immunohistological analysis detailed the contribution of differentially-regulated systemic markers to plaque pathology. Alizarin red staining showed calcification. EMPs and PMPs were significantly higher in patients with carotid stenosis (≥70%) compared to controls, with no differences between asymptomatic vs symptomatic patients. Asymptomatic patients with unstable plaques exhibited higher levels of EMPs, CXCL9 and SCGF-β compared to those with stable plaques. CXCL9, and SCGF-β were detected within all plaques, suggesting a contribution to both localised and systemic inflammation. Osteopontin and osteoprotegerin were significantly elevated in the symptomatic vs asymptomatic group, while osteocalcin was higher in asymptomatic patients with stable plaque. All plaques exhibited calcification, which was significantly greater in asymptomatic patients. This may impact on plaque stability. These data could be important in identifying patients at most benefit from intervention.

RANKL-OPG and RAGE modulation in vascular calcification and diabetes: novel targets for therapy

Type 2 diabetes is associated with increased cardiovascular morbidity and mortality and early vascular ageing. This takes the form of atherosclerosis, with progressive vascular calcification being a major complication in the pathogenesis of this disease. Current research and drug targets in diabetes have hitherto focused on atherosclerosis, but vascular calcification is now recognised as an independent predictor of cardiovascular morbidity and mortality. An emerging regulatory pathway for vascular calcification in diabetes involves the receptor activator for nuclear factor κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG). Important novel biomarkers of calcification are related to levels of glycation and inflammation in diabetes. Several therapeutic strategies could have advantageous effects on the vasculature in patients with diabetes, including targeting the RANKL and receptor for AGE (RAGE) signalling pathways, since there has been little success-at least in macrovascular outcomes-with conventional glucose-lowering therapy. There is substantial and relevant clinical and basic science evidence to suggest that modulating RANKL-RANK-OPG signalling, RAGE signalling and the associated proinflammatory milieu alters the natural course of cardiovascular complications and outcomes in people with diabetes. However, further research is critically needed to understand the precise mechanisms underpinning these pathways, in order to translate the anti-calcification strategies into patient benefit.

Connective tissue growth factor induces osteogenic differentiation of vascular smooth muscle cells through ERK signaling

Connective tissue growth factor (CTGF) plays an important role in the pathogenesis of atherosclerosis by promoting vascular smooth muscle cell (VSMC) growth, migration, apoptosis, adhesion and the secretion of matrix components. The osteogenic differentiation of VSMCs is essential in the development of vascular calcification. However, the role of CTGF in the transdifferentiation and calcification of VSMCs is unclear. In the present study, we examined whether CTGF stimulates VSMC transdifferentiation. Primary VSMCs were obtained from mouse thoracic aortas by enzymatic digestion and identified by immunostaining for smooth muscle specific α-actin antibody (α-SMA). VSMC calcification was induced by the addition of CTGF to the osteogenic mediaum containing 5-10% FBS in the presence of 0.25 mM ascorbic acid and 10 mM β-glycerophosphate for 14 days. Calcified cells were determined by Alizarin Red S staining. Our results revealed that CTGF induced the expression of several bone markers, including alkaline phosphatase (ALP), osteocalcin (OC), osteoprotegerin (OPG) and core-binding factor subunit α1 (Cbfα1)/runt-related transcription factor 2 (Runx2), as well as calcification. However, the inhibition of extracellular signal-regulated kinase (ERK) activity using the ERK-specific inhibitor, PD98059, blocked the induction of these proteins and VSMC calcification. Based on these data, we conclude that CTGF stimulates the transdifferentiation of VSMCs into osteoblasts and that the ERK signaling pathway appears to play a critical role in this process.

Ghrelin attenuates the osteoblastic differentiation of vascular smooth muscle cells through the ERK pathway

Vascular calcification results from osteoblastic differentiation of vascular smooth muscle cells (VSMCs) and is a major risk factor for cardiovascular events. Ghrelin is a newly discovered bioactive peptide that acts as a natural endogenous ligand of the growth hormone secretagog receptor (GHSR). Several studies have identified the protective effects of ghrelin on the cardiovascular system, however research on the effects and mechanisms of ghrelin on vascular calcification is still quite rare. In this study, we determined the effect of ghrelin on osteoblastic differentiation of VSMCs and investigated the mechanism involved using the two universally accepted calcifying models of calcifying vascular smooth muscle cells (CVSMCs) and beta-glycerophosphate (beta-GP)-induced VSMCs. Our data demonstrated that ghrelin inhibits osteoblastic differentiation and mineralization of VSMCs due to decreased alkaline phosphatase (ALP) activity, Runx2 expression, bone morphogenetic protein-2 (BMP-2) expression and calcium content. Further study demonstrated that ghrelin exerted this suppression effect via an extracellular signal-related kinase (ERK)-dependent pathway and that the suppression effect of ghrelin was time dependent and dose dependent. Furthermore, inhibition of the growth hormone secretagog receptor (GHSR), the ghrelin receptor, by siRNA significantly reversed the activation of ERK by ghrelin. In conclusion, our study suggests that ghrelin may inhibit osteoblastic differentiation of VSMCs through the GHSR/ERK pathway.

Apelin attenuates the osteoblastic differentiation of vascular smooth muscle cells

Vascular calcification, which results from a process osteoblastic differentiation of vascular smooth muscle cells (VSMCs), is a major risk factor for cardiovascular morbidity and mortality. Apelin is a recently discovered peptide that is the endogenous ligand for the orphan G-protein-coupled receptor, APJ. Several studies have identified the protective effects of apelin on the cardiovascular system. However, the effects and mechanisms of apelin on the osteoblastic differentiation of VSMCs have not been elucidated. Using a culture of calcifying vascular smooth muscle cells (CVMSCs) as a model for the study of vascular calcification, the relationship between apelin and the osteoblastic differentiation of VSMCs and the signal pathway involved were investigated. Alkaline phosphatase (ALP) activity and osteocalcin secretion were examined in CVSMCs. The involved signal pathway was studied using the extracellular signal-regulated kinase (ERK) inhibitor, PD98059, the phosphatidylinositol 3-kinase (PI3-K) inhibitor, LY294002, and APJ siRNA. The results showed that apelin inhibited ALP activity, osteocalcin secretion, and the formation of mineralized nodules. APJ protein was detected in CVSMCs, and apelin activated ERK and AKT (a downstream effector of PI3-K). Suppression of APJ with siRNA abolished the apelin-induced activation of ERK and Akt. Furthermore, inhibition of APJ expression, and the activation of ERK or PI3-K, reversed the effects of apelin on ALP activity. These results showed that apelin inhibited the osteoblastic differentiation of CVSMCs through the APJ/ERK and APJ/PI3-K/AKT signaling pathway. Apelin appears to play a protective role against arterial calcification.

Transglutaminase 2 is central to induction of the arterial calcification program by smooth muscle cells

Arterial calcification is a phenotype of vascular repair in atherosclerosis, diabetes, hyperphosphatemic renal failure, and aging. Arterial calcification is modulated by transition of arterial smooth muscle cells (SMCs) from contractile to chondro-osseous differentiation programmed in response to increases in P(i), bone morphogenetic protein-2, and certain other stimuli. Transglutaminase (TG)2 release modulates tissue repair, partly by transamidation-catalyzed covalent crosslinking of extracellular matrix substrates. TG2 regulates cultured SMC differentiation, resistance artery remodeling to vasoconstriction, and atherosclerotic lesion size. Here, TG2 expression was required for the majority of TG activity in mouse and human aortic SMCs. TG2(-/-) SMCs lost the capacity for P(i) donor-induced formation of multicellular bone-like nodules and for increased expression of the type III sodium-dependent P(i) cotransporter Pit-1 and certain osteoblast and chondrocyte genes (tissue-nonspecific alkaline phosphatase, the osteoblast master transcription factor runx2, and chondrocyte-restricted aggrecan), and for P(i) donor- and bone morphogenetic protein-2-induced calcification. Uniquely in TG2(-/-) SMCs, P(i) donor treatment increased expression of the physiological SMC chondro-osseous differentiation and calcification inhibitors osteoprotegerin, matrix Gla protein, and osteopontin. Conversely, TG2(-/-) SMCs, unlike wild-type SMCs, failed to maintain contractile differentiation on laminin. Exogenous catalytically active TG2 augmented calcification by TG2(-/-) SMC in response to P(i) donor treatment. TG2 expression also drove P(i)-stimulated calcification of mouse aortic ring organ cultures, which was suppressed by the TG2 catalytic site-specific inhibitor Boc-DON-Gln-Ile-Val-OMe (10 micromol/L). Our results suggest that TG2 release in injured arteries is critical for programming chondro-osseous SMC differentiation and calcification in response to increased P(i) and bone morphogenetic protein-2.

Hepatocyte growth factor and c-Met expression in pericytes: implications for atherosclerotic plaque development

Intraplaque neovascularization contributes to the progression of atherosclerosis. Our aim is to understand the mobilization of cells and factors involved in this process. We investigated the localization of hepatocyte growth factor (HGF) and its receptor, c-Met, in human atherosclerotic plaques, together with the effects of HGF on pericyte migration in vitro. Atherosclerotic femoral arterial segments were collected and analysed from 13 subjects who were undergoing lower limb amputation. Pericytes were identified in human lesions using a 3G5 antibody. Immunohistochemical analysis localized HGF mainly around microvessels, in association with some, but not all, CD31-positive endothelial cells. c-Met expression was mainly associated with smooth muscle cells and pericytes, around some, but not all, microvessels within the atherosclerotic lesions; no detection was apparent in normal internal mammary arteries. Using RT-PCR, we demonstrated expression of HGF and c-Met in a rat pericyte cell-line, TR-PCT1, and in primary pericytes. HGF treatment of TR-PCT1 cells induced their migration, but not their proliferation, in a dose-dependent manner (10-100 ng/ml, p<0.01), an effect mediated by activation of the serine/threonine kinase Akt, shown by western blot analysis. Treating the cells with the PI3K inhibitors Wortmannin (0.1 microM) or LY294002 (10 microM) abolished these effects. This work demonstrates the expression of c-Met and HGF in human atherosclerotic arteries, in association with SM-actin-positive cells and CD-31-positive cells, respectively. HGF induces pericyte migration via PI3-kinase and Akt activation in vitro. HGF and c-Met may be involved in neovascularization during plaque development, and may recruit pericytes to neovessels. Since pericytes are thought to mechanically stabilize new blood vessels, these factors may function to protect against haemorrhage.

Contribution of VCAF-positive cells to neovascularization and calcification in atherosclerotic plaque development

Calcification of the vessel wall is a regulated process with many similarities to osteogenesis. Progenitor cells may play a role in this process. Previously, we identified a novel gene, Vascular Calcification Associated Factor (VCAF), which was shown to be important in pericyte osteogenic differentiation. The aim of this study was to determine the localization and expression pattern of VCAF in human cells and tissues. Immunohistochemical analysis of seven atherosclerotic arteries confirmed VCAF protein expression within calcified lesions. In addition, individual VCAF-positive cells were detected within the intima and adventitia in areas where sporadic 3G5-positive pericytes were localized. Furthermore, VCAF-positive cells were identified in newly formed microvessels in association with CD34-positive/CD146-positive/c-kit-positive cells as well as in intact CD31-positive endothelium in internal mammary arteries. Western blot analysis confirmed the presence of VCAF (18 kD) in protein lysates extracted from human smooth muscle cells, endothelial cells, macrophages, and osteoblasts. In fracture callus samples from three patients, VCAF was detected in osteoblasts and microvessels. This study demonstrates the presence of VCAF in neovessels and raises the possibility that VCAF could be a new marker for vascular progenitor cells involved in a number of differentiation pathways. These data may have implications for the prevention or treatment of vascular disease.

Axl/Phosphatidylinositol 3-Kinase Signaling Inhibits Mineral Deposition by Vascular Smooth Muscle Cells

The calcification of blood vessels correlates with increased morbidity and mortality in patients with atherosclerosis, diabetes, and end-stage kidney disease. The receptor tyrosine kinase Axl is emerging as an important regulator of adult mammalian physiology and pathology. This study tests the hypothesis that Axl prevents the deposition of a calcified matrix by vascular smooth muscle cells (VSMCs) and that this occurs via the phosphatidylinositol 3-kinase (PI3K) signaling pathway. First, we demonstrate that Axl is expressed and phosphorylated in confluent VSMCs and that its expression is markedly downregulated as these cells calcify their matrix. Second, we demonstrate that overexpression of wild-type Axl, using recombinant adenoviruses, enhances Axl phosphorylation and downstream signaling via PI3K and Akt. Furthermore, overexpression of Axl significantly inhibits mineral deposition by VSMCs, as assessed by alizarin red staining and 45 Ca accumulation. Third, the addition of a PI3K inhibitor, wortmannin, negates the inhibition of mineralization by overexpression of wild-type Axl, suggesting that activation of downstream signaling via PI3K is crucial for its inhibitory activity. In contrast, Axl-mediated signaling is not enhanced by overexpression of kinase-dead Axl and mineralization is accelerated, although β-glycerophosphate is still required for this effect. Finally, the caspase inhibitor zVAD.fmk attenuates the increased mineralization induced by kinase-dead Axl, suggesting that kinase-dead Axl stimulates mineralization by inhibiting the antiapoptotic effect of endogenous Axl. Together, these results demonstrate that signaling through Axl inhibits vascular calcification in vitro and suggest that therapeutics targeting this receptor may open up new avenues for the prevention of vascular calcification in vivo.

Dexamethasone downregulates calcification-inhibitor molecules and accelerates osteogenic differentiation of vascular pericytes: implications for vascular calcification

Vascular calcification is present in many pathological conditions and is recognized as a strong predictor of future cardiovascular events. Current evidence suggests that it is a regulated process involving inducing and inhibitory molecules. Glucocorticoids have great clinical importance as antiinflammatory drugs and can act as potent inducers of osteogenic differentiation in vitro. The effect of glucocorticoids on vascular cells in vivo remains obscure. Pericytes are pluripotent cells that can differentiate into osteoblasts, and recent evidence suggests that they could participate in vascular calcification. We hypothesized that the synthetic glucocorticoid dexamethasone would enhance the rate of pericyte differentiation and mineralization in vitro with a concomitant suppression of calcification-inhibitory molecules. Three weeks of dexamethasone treatment induced a 2-fold increase in (1) alkaline phosphatase activity, (2) calcium deposition, and (3) the number of nodules formed in vitro; and a reduction in the expression of matrix Gla protein (MGP), osteopontin (OPN), and vascular calcification-associated factor (VCAF) mRNAs. The glucocorticoid receptor antagonist Org 34116 abolished dexamethasone-accelerated pericyte differentiation, nodule formation, and mineralization. Data obtained using Org 34116, the transcription inhibitor actinomycin D, and the protein synthesis inhibitor cyclohexamide suggest that MGP, OPN, and VCAF mRNA abundance are controlled at different and multiple levels by dexamethasone. This is the first report showing that dexamethasone enhances the osteogenic differentiation of pericytes and downregulates genes associated with inhibition of mineralization. Our study highlights the need for further investigation into the long-term consequences of prolonged glucocorticoid therapy on vascular calcification.