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

Regulatory mechanisms in vascular calcification

In the past decade, the prevalence, significance, and regulatory mechanisms of vascular calcification have gained increasing recognition. Over a century ago, pathologists recognized atherosclerotic calcification as a form of extraskeletal ossification. Studies are now identifying the mechanism of this remarkable process as a recapitulation of embryonic endochondral and membranous ossification through phenotypic plasticity of vascular cells that function as adult mesenchymal stem cells. These embryonic developmental programs, involving bone morphogenetic proteins and potent osteochondrogenic transcription factors, are triggered and modulated by a variety of inflammatory, metabolic, and genetic disorders, particularly hyperlipidemia, chronic kidney disease, diabetes, hyperparathyroidism, and osteoporosis. They are also triggered by loss of powerful inhibitors, such as fetuin A, matrix Gla protein, and pyrophosphate, which ordinarily restrict biomineralization to skeletal bone. Teleologically, soft-tissue calcification might serve to create a wall of bone to sequester noxious foci such as chronic infections, parasites, and foreign bodies. This Review focuses on atherosclerotic and medial calcification. The capacity of the vasculature to produce mineral in culture and to produce de novo, vascularized, trabecular bone and cartilage tissue, even in patients with osteoporosis, should intrigue investigators in tissue engineering and regenerative biology.

Calcium, phosphorus, PTH and death rates in a large sample of dialysis patients from Latin America. The CORES Study

BACKGROUND

Mineral metabolism parameters may play a role in the survival of patients with chronic kidney disease (CKD).

METHODS

In the CORES Study, we analysed the association between calcium, phosphorus and PTH and mortality (all-cause and cardiovascular) in 16 173 haemodialysis (HD) patients over 18 years from six Latin American countries, who underwent haemodialysis up to 54 months. Unadjusted, case-mix-adjusted and time-dependent multivariable-adjusted hazard ratio (HR) of death were calculated for categories of serum albumin-corrected calcium (Ca(Alb)), phosphorus and PTH using as 'reference values' the range in which the lowest death rate was observed. Age, gender, vitamin D treatment, diabetes, vintage, vascular access, weight, blood pressure and laboratory variables (serum albumin, haemoglobin, creatinine, ferritin and Kt/V) were used as confounding variables.

RESULTS

Low (<9.5 mg/dL) and high (>10.5 mg/dL) Ca(Alb) increased the HR for all-cause mortality. Low (<9.0 mg/dL) Ca(Alb) increased the HR for cardiovascular mortality. High phosphorus (>5.5 mg/dL) increased the HR for both all-cause and cardiovascular mortality. Low phosphorus (<4.0 and <3.0 mg/dL) increased the HR for both all-cause and cardiovascular mortality. Furthermore, low (<150 pg/mL) and high (>500 and >300 pg/mL) PTH increased the HR for both all-cause and cardiovascular mortality. In addition, only phosphorus >6.0 mg/dL increased the HR for cardiovascular hospitalizations. No effect was observed with Ca(Alb) or PTH.

CONCLUSIONS

In summary, in 16,173 HD patients, elevated and reduced serum levels of albumin-corrected calcium, phosphorus and PTH levels were associated with increments in all-cause mortality. Similar results were obtained when only cardiovascular mortality was analysed.

Myeloid CD34+CD13+ precursor cells transdifferentiate into chondrocyte-like cells in atherosclerotic intimal calcification

Chondrogenic differentiation is pivotal in the active regulation of artery calcification. We investigated the cellular origin of chondrocyte-like cells in atherosclerotic intimal calcification of C57BL/6 LDLr(-/-) mice using bone marrow transplantation to trace ROSA26-LacZ-labeled cells. Immunohistochemical costaining of collagen type II with LacZ and leukocyte defining surface antigens was performed and analyzed by high-resolution confocal microscopy. Chondrocyte-like cells were detected in medium and advanced atherosclerotic plaques accounting for 7.1 +/- 1.6% and 14.1 +/- 1.7% of the total plaque cellularity, respectively. Chimera analysis exhibited a mean of 89.8% LacZ(+) cells in peripheral blood and collagen type II costaining with LcZ revealed an average 88.8 +/- 7.6% cytoplasmatic LacZ(+) evidence within the chondrocyte-like cells. To examine whether hematopoietic stem cells contribute to the phenotype, stem cell marker CD34 and myeloid progenitor-associated antigen CD13 were analyzed. CD34(+) was detectable in 86.9 +/- 8.1% and CD13(+) evidence in 54.2 +/- 7.6% of chondrocyte-like cells, attributable most likely because of loss of surface markers during transdifferentiation. Chondrocyte differentiation factor Sox-9 was detected in association with chondrocyte-like cells, whereas Sm22alpha, a marker for smooth muscle cells, could not be demonstrated. The results show that the majority of chondrocyte-like cells were of bone marrow origin, whereas CD34(+)/CD13(+) myeloid precursors appeared to infiltrate the plaque actively and transdifferentiated into chondrocytes-like cells in the progression of atherosclerosis.

Regulation of vascular smooth muscle cell calcification by extracellular pyrophosphate homeostasis: synergistic modulation by cyclic AMP and hyperphosphatemia

Vascular calcification is a multifaceted process involving gain of calcification inducers and loss of calcification inhibitors. One such inhibitor is inorganic pyrophosphate (PP(i)), and regulated generation and homeostasis of extracellular PP(i) is a critical determinant of soft-tissue mineralization. We recently described an autocrine mechanism of extracellular PP(i) generation in cultured rat aortic vascular smooth muscle cells (VSMC) that involves both ATP release coupled to the ectophosphodiesterase/pyrophosphatase ENPP1 and efflux of intracellular PP(i) mediated or regulated by the plasma membrane protein ANK. We now report that increased cAMP signaling and elevated extracellular inorganic phosphate (P(i)) act synergistically to induce calcification of these VSMC that is correlated with progressive reduction in ability to accumulate extracellular PP(i). Attenuated PP(i) accumulation was mediated in part by cAMP-dependent decrease in ANK expression coordinated with cAMP-dependent increase in expression of TNAP, the tissue nonselective alkaline phosphatase that degrades PP(i). Stimulation of cAMP signaling did not alter ATP release or ENPP1 expression, and the cAMP-induced changes in ANK and TNAP expression were not sufficient to induce calcification. Elevated extracellular P(i) alone elicited only minor calcification and no significant changes in ANK, TNAP, or ENPP1. In contrast, combined with a cAMP stimulus, elevated P(i) induced decreases in the ATP release pathway(s) that supports ENPP1 activity; this resulted in markedly reduced rates of PP(i) accumulation that facilitated robust calcification. Calcified VSMC were characterized by maintained expression of multiple SMC differentiation marker proteins including smooth muscle (SM) alpha-actin, SM22alpha, and calponin. Notably, addition of exogenous ATP (or PP(i) per se) rescued cAMP + phosphate-treated VSMC cultures from progression to the calcified state. These observations support a model in which extracellular PP(i) generation mediated by both ANK- and ATP release-dependent mechanisms serves as a critical regulator of VSMC calcification.

Discoidin domain receptor-1 deficiency attenuates atherosclerotic calcification and smooth muscle cell-mediated mineralization

Intimal calcification is a feature of advanced atherosclerotic disease that predicts a two- to eightfold increase in the risk of coronary events. Type I collagen promotes vascular smooth muscle cell-mediated calcification, although the mechanism by which this occurs is unknown. The discoidin domain receptor 1 (DDR1) is a collagen receptor that is emerging as a critical mediator of atherosclerosis. To determine whether DDR1 is involved in intimal calcification, we fed male Ddr1(-/-);Ldlr(-/-) and Ddr1(+/+);Ldlr(-/-) mice an atherogenic diet for 6, 12, or 24 weeks. DDR1 deficiency significantly reduced the calcium content of the aortic arch, and microcomputed tomography demonstrated a significant decrease in hydroxyapatite deposition after 24 weeks of atherogenic diet. Reduced calcification was correlated with decreases in macrophage accumulation and tumor necrosis factor alpha staining, suggesting that the reduction in calcification was in part due to decreased inflammation. The chondrogenic markers type II collagen, type X collagen, and Sox-9 were expressed within the mineralized foci. An in vitro assay performed with vascular smooth muscle cells revealed that DDR1 was required for cell-mediated calcification of the matrix, and Ddr1(+/+) smooth muscle cells expressed more alkaline phosphatase activity, whereas Ddr1(-/-) smooth muscle cells expressed elevated levels of mRNA for nucleotide pyrophosphatase phosphodiesterase 1, an inhibitor of tissue mineralization. Taken together, our results demonstrate that DDR1 mediates an important mechanism for atherosclerotic calcification.

Regulation of valvular interstitial cell calcification by components of the extracellular matrix

Understanding the interactions between extracellular matrix (ECM) components and valvular interstitial cells (VICs) is relevant to both treating heart valve disease and designing heart valve tissue engineering scaffolds, yet the VIC-ECM relationship has not been well characterized. Thus, the aim of this study was to characterize VIC-ECM interactions, paying specific attention to whether ECM composition affected the in vitro calcification of VICs. Our results show that the number and size of calcific nodules formed in VIC cultures, as well as the expression of the mineralization markers alkaline phosphatase (ALP) and CBFa1, were highly dependent upon the composition of the culture surface. VICs cultured on certain ECM components, that is, collagen and fibronectin, were resistant to calcification, even upon treatment with mineralization-inducing growth factors. Meanwhile, cultures of VICs on fibrin, laminin, and heparin coatings had a high number of calcified nodules, although only VICs on fibrin expressed significantly elevated levels of ALP and CBFa1. Nodule composition analysis revealed the presence of multiple types of mineralization. Although apoptotic and necrotic cells were more concentrated in nodules, these nodules did contain a strong majority population of viable cells. Characterizing this ECM-dependence of VIC calcification will help us to identify appropriate biomaterial environments for heart valve tissue engineering as well as elucidate mechanisms of valvular disease.

Lanthanum acetate inhibits vascular calcification induced by vitamin D3 plus nicotine in rats

Lanthanum, a rare earth element, has been used to decrease serum phosphorus level in patients with chronic renal disease and hyperphosphatemia. We aimed to observe the effect and mechanism of two doses of lanthanum acetate (375 and 750 mg/kg/day) on vascular calcification induced by vitamin D3 plus nicotine treatment in rats for 4 weeks. As compared with control rats, rats with calcification showed widespread calcified nodules and irregular elastic fibers in calcified aorta on von Kossa calcium staining and increased aortic calcium and phosphorus contents, alkaline phosphatase (ALP) activity and bone-related protein expressions for osteopontin (OPN) and type III sodium dependent phosphate cotransporter Pit-1 (Pit-1). After treatment with either dose of lanthanum acetate, the calcified nodules and degree of irregular elastic fibers decreased in aortas. Lanthanum acetate at 750 mg/kg/day was more effective than 375 mg/kg/day in lessening vascular calcification by significantly reducing plasma phosphorus level, calcium x phosphorus product and ALP activity, by 30.3%, 28.6%, and 68.6%, respectively; reducing aortic phosphorus and calcium contents and ALP activity, by 48%, 53.1%, and 63.5% (all P < 0.01), respectively; reducing aortic mRNA level of OPN and Pit-1, by 55.8% (P < 0.01) and 38.8% (P < 0.05) and protein level of OPN and Pit-1, by 37.2% and 27.2% (both P < 0.01), respectively; and increasing carboxylated matrix Gla-protein (MGP) protein expression by 33.7% (P < 0.05), as compared with rats treated with vitamin D3 and nicotine alone. Lanthanum acetate could effectively inhibit the pathogenesis of vascular calcification.

Differentiation patterning of vascular smooth muscle cells (VSMC) in atherosclerosis

To investigate the involvement of transdifferentiation and dedifferentiation phenomena inside atherosclerotic plaques, we analyzed the differentiation status of vascular smooth muscle cells (VSMC) in vitro and in vivo. Forty normal autoptic and 20 atherosclerotic carotid endarterectomy specimens as well as 20 specimens of infrarenal and suprarenal aortae were analyzed for the expression of cytokeratins 7 and 18 and beta-catenin as markers (epithelial transdifferentiation) as well as CD31 and CD34 (embryonic dedifferentiation) by conventional and double fluorescence immunohistochemistry and reverse transcription polymerase chain reaction. Looking at these markers, additional cell culture experiments with human aortic (HA)-VSMC were done under stimulation with IL-1beta, IL-6, and TNF-alpha. Cytokeratins and beta-catenin were expressed significantly higher in atherosclerotic than in normal carotids primarily localized in VSMC of the shoulder/cap region of atherosclerotic lesions. Additionally, heterogeneous cellular coexpression of CD31 and/or CD34 was observed in subregions of progressive atherosclerotic lesions by VSMC. The expression of those differentiation markers by stimulated HA-VSMC showed a time and cytokine dependency in vitro. Our findings show that (1) VSMC of progressive atheromas have the ability of differentiation, (2) that transdifferentiation and dedifferentiation phenomena are topographically diverse localized in the subregions of advanced atherosclerotic lesions, and (3) are influenced by inflammatory cytokines like IL-1beta, IL-6, and TNF-alpha.

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.

Collagens in the progression and complications of atherosclerosis

Collagens constitute a major portion of the extracellular matrix in the atherosclerotic plaque, where they contribute to the strength and integrity of the fibrous cap, and also modulate cellular responses via specific receptors and signaling pathways. This review focuses on the diverse roles that collagens play in atherosclerosis; regulating the infiltration and differentiation of smooth muscle cells and macrophages; controlling matrix remodeling through feedback signaling to proteinases; and influencing the development of atherosclerotic complications such as plaque rupture, aneurysm formation and calcification. Expanding our understanding of the pathways involved in cell-matrix interactions will provide new therapeutic targets and strategies for the diagnosis and treatment of atherosclerosis.

Autocrine ATP release coupled to extracellular pyrophosphate accumulation in vascular smooth muscle cells

Extracellular inorganic pyrophosphate (PP(i)) is a potent suppressor of physiological calcification in bone and pathological calcification in blood vessels. Ectonucleotide pyrophosphatase/phosphodiesterases (eNPPs) generate PP(i) via the hydrolysis of ATP released into extracellular compartments by poorly understood mechanisms. Here we report that cultured vascular smooth muscle cells (VSMC) from rat aorta generate extracellular PP(i) via an autocrine mechanism that involves ATP release tightly coupled to eNPP activity. The nucleotide analog beta,gamma-methylene ATP (MeATP or AMPPCP) was used to selectively suppress ATP metabolism by eNPPs but not the CD39-type ecto-ATPases. In the absence of MeATP, VSMC generated extracellular PP(i) to accumulate >or=600 nM within 2 h while steadily maintaining extracellular ATP at 1 nM. Conversely, the presence of MeATP completely suppressed PP(i) accumulation while increasing ATP accumulation. Probenecid, which inhibits PP(i) efflux dependent on ANK, a putative PP(i) transporter or transport regulator, reduced extracellular PP(i) accumulation by approximately twofold. This indicates that autocrine ATP release coupled to eNPP activity comprises >or=50% of the extracellular PP(i)-generating capacity of VSMC. The accumulation of extracellular PP(i) and ATP was markedly attenuated by reduced temperature but was insensitive to brefeldin A, which suppresses constitutive exocytosis of Golgi-derived secretory vesicles. The magnitude of extracellular PP(i) accumulation in VSMC cultures increased with time postplating, suggesting that ATP release coupled to PP(i) generation is upregulated as cultured VSMC undergo contact-inhibition of proliferation or deposit extracellular matrix.

Degradation and healing characteristics of small-diameter poly(epsilon-caprolactone) vascular grafts in the rat systemic arterial circulation

BACKGROUND

Long-term patency of conventional synthetic grafts is unsatisfactory below a 6-mm internal diameter. Poly(epsilon-caprolactone) (PCL) is a promising biodegradable polymer with a longer degradation time. We aimed to evaluate in vivo healing and degradation characteristics of small-diameter vascular grafts made of PCL nanofibers compared with expanded polytetrafluoroethylene (ePTFE) grafts.

METHODS AND RESULTS

We prepared 2-mm-internal diameter grafts by electrospinning using PCL (M(n)=80, 000 g/mol). Either PCL (n=15) or ePTFE (n=15) grafts were implanted into 30 rats. Rats were followed up for 24 weeks. At the conclusion of the follow-up period, patency and structural integrity were evaluated by digital subtraction angiography. The abdominal aorta, including the graft, was harvested and investigated under light microscopy. Endothelial coverage, neointima formation, and transmural cellular ingrowth were measured by computed histomorphometry. All animals survived until the end of follow-up, and all grafts were patent in both groups. Digital subtraction angiography revealed no stenosis in the PCL group but stenotic lesions in 1 graft at 18 weeks (40%) and in another graft at 24 weeks (50%) in the ePTFE group. None of the grafts showed aneurysmal dilatation. Endothelial coverage was significantly better in the PCL group. Neointimal formation was comparable between the 2 groups. Macrophage and fibroblast ingrowth with extracellular matrix formation and neoangiogenesis were better in the PCL group. After 12 weeks, foci of chondroid metaplasia located in the neointima of PCL grafts were observed in all samples.

CONCLUSIONS

Small-diameter PCL grafts represent a promising alternative for the future because of their better healing characteristics compared with ePTFE grafts. Faster endothelialization and extracellular matrix formation, accompanied by degradation of graft fibers, seem to be the major advantages. Further evaluation of degradation and graft healing characteristics may potentially lead to the clinical use of such grafts for revascularization procedures.

Arterial calcification in race horses

Calcification of large arteries has been sporadically reported in horses. The pathogenesis is still unknown, but recent studies in humans suggest that this is a regulated biomineralizing process. This study surveyed the prevalence, distribution, and severity of vascular calcification in Thoroughbred and Standardbred racehorses. Histopathologic, ultrastructural imaging, and energy dispersive X-ray elemental analyses were used to examine the lesions. Calcification of the tunica media, predominantly the pulmonary artery, was found in 82% of horses (83/101). Young adult horses (mean [SD] age in years, 4.44 +/- 2.17) of both breeds and sexes were similarly affected. Lesions appeared as white-to-yellowish, hard, and gritty plaques of variable size. On microscopic examination, elastic fibers within the tunica media were thinned, fragmented, and calcified, and surrounded by dense collagen matrix. Elemental analysis showed distinct peaks for calcium and phosphorus, consistent with hydroxyapatite mineral. The frequent occurrence of calcification in the tunica media of large pulmonary arteries of young racing horses indicates the need to investigate its pathogenesis and potential clinical implications.

Osteoprotegerin, vascular calcification and atherosclerosis

The association of bone pathologies with atherosclerosis has stimulated the search for common mediators linking the skeletal and the vascular system. Since its initial discovery as a key regulator in bone metabolism, osteoprotegerin (OPG) has become the subject of intense interest for its role in vascular disease and calcification. Studies in vitro and in animal models suggest that OPG inhibits vascular calcification. Paradoxically however, clinical studies suggest that serum OPG levels increase in association with vascular calcification, coronary artery disease, stroke and future cardiovascular events. This has led to an extensive debate on the potential of OPG as a biomarker of vascular disease. However the exact significance and mechanisms by which this bone-regulatory protein influences cardiovascular pathophysiology is still unclear. The need for a more complete picture is being addressed in increasing valuable research indicating OPG as not only a marker but also a mediator of vascular pathology modulating osteogenic, inflammatory and apoptotic responses. By integrating the results of recent experimental research, animal models and clinical studies, this review summarises the present understanding of the role of OPG in vascular disease and calcification.

Calcium phosphate crystals induce cell death in human vascular smooth muscle cells: a potential mechanism in atherosclerotic plaque destabilization

Vascular calcification is associated with an increased risk of myocardial infarction; however, the mechanisms linking these 2 processes are unknown. Studies in macrophages have suggested that calcium phosphate crystals induce the release of proinflammatory cytokines; however, no studies have been performed on the effects of calcium phosphate crystals on vascular smooth muscle cell function. In the present study, we found that calcium phosphate crystals induced cell death in human aortic vascular smooth muscle cells with their potency depending on their size and composition. Calcium phosphate crystals of approximately 1 microm or less in diameter caused rapid rises in intracellular calcium concentration, an effect that was inhibited by the lysosomal proton pump inhibitor, bafilomycin A1. Bafilomycin A1 also blocked vascular smooth muscle cell death suggesting that crystal dissolution in lysosomes leads to an increase in intracellular calcium levels and subsequent cell death. These studies give novel insights into the bioactivity of calcified deposits and suggest that small calcium phosphate crystals could destabilize atherosclerotic plaques by initiating inflammation and by causing vascular smooth muscle cell death.

In search of the in vivo identity of mesenchymal stem cells

In spite of the advances in the knowledge of adult stem cells (ASCs) during the past few years, their natural activities in vivo are still poorly understood. Mesenchymal stem cells (MSCs), one of the most promising types of ASCs for cell-based therapies, are defined mainly by functional assays using cultured cells. Defining MSCs in vitro adds complexity to their study because the artificial conditions may introduce experimental artifacts. Inserting these results in the context of the organism is difficult because the exact location and functions of MSCs in vivo remain elusive; the identification of the MSC niche is necessary to validate results obtained in vitro and to further the knowledge of the physiological functions of this ASC. Here we show an analysis of the evidence suggesting a perivascular location for MSCs, correlating these cells with pericytes, and present a model in which the perivascular zone is the MSC niche in vivo, where local cues coordinate the transition to progenitor and mature cell phenotypes. This model proposes that MSCs stabilize blood vessels and contribute to tissue and immune system homeostasis under physiological conditions and assume a more active role in the repair of focal tissue injury. The establishment of the perivascular compartment as the MSC niche provides a basis for the rational design of additional in vivo therapeutic approaches. This view connects the MSC to the immune and vascular systems, emphasizing its role as a physiological integrator and its importance in tissue repair/regeneration.

Role of homocysteine in aortic calcification and osteogenic cell differentiation

BACKGROUND

The role of homocysteine in atherosclerosis is unclear. We examined the relationship between plasma homocysteine and infrarenal aortic calcification, the presence of homocysteine in human atheroma and the influence of homocysteine on osteogenic differentiation in vitro.

METHODS AND RESULTS

In 194 patients with symptomatic peripheral artery disease or abdominal aortic aneurysm, fasting plasma total homocysteine was independently associated with the severity of infrarenal aortic calcification measured by Computer Tomography Angiography (odds ratio 1.91, 95% confidence interval 1.17-3.21 for calcification >or=median). Homocysteine was identified in all 60 atheroma biopsies from 16 patients undergoing endarterectomy, and concentrations were significantly greater in the calcified biopsies (p=0.003). In vitro studies demonstrated that 100 micromol/L homocysteine doubled the calcium deposition by mesenchymal stem cells during 16 days incubation in osteogenic medium (74+/-4 compared to 42+/-5 microg calcium/well without homocysteine, p<0.001). Homocysteine also stimulated monocytic THP1 cells to promote aortic smooth muscle cell calcification as evidenced by significant higher calcium deposition and alkaline phosphatase activity compared to incubation without homocysteine (p<or=0.05).

CONCLUSIONS

Homocysteine plays an important role in vascular calcification via multiple mechanisms. The presence of homocysteine in atheroma and its ability to enhance osteogenic cell differentiation may partly explain the association of homocysteine with atherosclerotic events.

Extracellular transglutaminase 2 activates beta-catenin signaling in calcifying vascular smooth muscle cells

Accumulation of transglutaminase 2 (TG2) is often associated with mineral deposits in vasculature. Here, we demonstrate that purified TG2 stimulated a 3-fold increase in matrix mineralization and up-regulation of osteoblastic markers in cultured primary vascular smooth muscle cells (VSMCs). Extracellular TG2 interacts with the low density lipoprotein related-protein 5 receptor and activates beta-catenin signaling in VSMCs. These results suggest that TG2 may promote vascular calcification by activating the beta-catenin signaling pathway.

Oxidative stress induces vascular calcification through modulation of the osteogenic transcription factor Runx2 by AKT signaling

Oxidative stress plays a critical role in the pathogenesis of atherosclerosis including the formation of lipid laden macrophages and the development of inflammation. However, oxidative stress-induced molecular signaling that regulates the development of vascular calcification has not been investigated in depth. Osteogenic differentiation of vascular smooth muscle cells (VSMC) is critical in the development of calcification in atherosclerotic lesions. An important contributor to oxidative stress in atherosclerotic lesions is the formation of hydrogen peroxide from diverse sources in vascular cells. In this study we defined molecular signaling that is operative in the H2O2-induced VSMC calcification. We found that H2O2 promotes a phenotypic switch of VSMC from contractile to osteogenic phenotype. This response was associated with an increased expression and transactivity of Runx2, a key transcription factor for osteogenic differentiation. The essential role of Runx2 in oxidative stress-induced VSMC calcification was further confirmed by Runx2 depletion and overexpression. Inhibition of Runx2 using short hairpin RNA blocked VSMC calcification, and adenovirus-mediated overexpression of Runx2 alone induced VSMC calcification. Inhibition of H2O2-activated AKT signaling blocked VSMC calcification and Runx2 induction concurrently. This blockage did not cause VSMC apoptosis. Taken together, our data demonstrate a critical role for AKT-mediated induction of Runx2 in oxidative stress-induced VSMC calcification.

Ion channel switching and activation in smooth-muscle cells of occlusive vascular diseases

Blood vessels are essential for animal life, allowing flow of oxygen and nutrients to tissues and removal of waste products. Consequently, inappropriate remodelling of blood vessels, resulting in occlusion, can lead to disabling or catastrophic events: heart attacks, strokes and claudication. An important cell type of remodelling is the VSMC (vascular smooth-muscle cell), a fascinating cell that contributes significantly to occlusive vascular diseases by virtue of its ability to 'modulate' to a cell that no longer contracts and arranges radially in the medial layer of the vessel wall but migrates, invades, proliferates and adopts phenotypes of other cells. An intriguing aspect of modulation is switching to different ion transport systems. Initial events include loss of the Ca(V)1.2 (L-type voltage-gated calcium) channel and gain of the K(Ca)3.1 (IKCa) potassium channel, which putatively occur to enable membrane hyperpolarization that increases rather than decreases a type of calcium entry coupled with cell cycle activity, cell proliferation and cell migration. This type of calcium entry is related to store- and receptor-operated calcium entry phenomena, which, in VSMCs, are contributed to by TRPC [TRP (transient receptor potential) canonical] channel subunits. Instead of being voltage-gated, these channels are chemically gated - importantly, by key phospholipid factors of vascular development and disease. This brief review focuses on the hypothesis that the transition to a modulated cell may require a switch from predominantly voltage- to predominantly lipid-sensing ion channels.

Metabolic acidosis inhibits soft tissue calcification in uremic rats

Metabolic acidosis is common in patients with chronic kidney disease, which is known to affect bone metabolism. We examined the effect of metabolic acidosis on the development of vascular and other soft-tissue calcifications in uremic rats treated with calcitriol. Extraskeletal calcification was measured in vivo, in control rats and rats with a remnant kidney model of uremia with or without ammonium chloride-induced acidosis. Soft-tissue calcification was assessed histologically, by measurement of the expression of the sodium-dependent phosphate cotransporter Pit-1 and by quantification of tissue calcium and phosphorus. Calcitriol administration to uremic rats resulted in significant deposition of material positive for von Kossa stain in the aorta, stomach, and kidney, elevated aortic calcium and phosphorus, increased aortic Pit-1 expression, and high mortality. Calcitriol-treated uremic rats with acidosis did not develop aortic or soft-tissue calcification, did not increase aortic Pit-1 expression, and had significantly lower mortality. Additionally, an acidotic environment prevented calcification of vascular smooth muscle cells in vitro. Our study shows that metabolic acidosis inhibits extraskeletal calcification.

Regulation of in vitro vascular calcification by BMP4, VEGF and Wnt3a

Vascular calcification is a common clinical complication of cardiovascular disease, diabetes and end-stage renal failure, associated with significant morbidity and mortality. In this study we demonstrate that factors secreted by the hypertrophic chondrocytes induce matrix mineralization and osteoblastic transformation in cultured mouse vascular smooth muscle cells (VSMCs). In addition, these factors render VSMCs responsive to BMP4 and Wnt3a ligands. Neither BMP-4 nor Wnt3a could induce mineralization in short-term (up to 8 days) cultures of primary mouse VSMCs. However, both ligands act synergistically with the chondrocyte-conditioned medium causing a further increase in VSMC calcification. Finally, we show that commitment of VSMCs towards the BMP-regulated mineralization can be induced by the chondrocyte-secreted bone anabolic factor VEGF. In addition, expression profiling suggests a novel role in vascular calcification for the matrix proteins previously known to regulate bone formation and mineralization (including MMP3, fibulin, 11betahydroxysteroid dehydrogenase 1 and retinoic acid receptor responder 2). The results of this study may contribute to further understanding of the cellular mechanisms responsible for vascular calcification and provide important information for the treatment of this pathology.

Control of phenotypic plasticity of smooth muscle cells by bone morphogenetic protein signaling through the myocardin-related transcription factors

Vascular smooth muscle cells (VSMCs), unlike other muscle cells, do not terminally differentiate. In response to injury, VSMCs change phenotype, proliferate, and migrate as part of the repair process. Dysregulation of this plasticity program contributes to the pathogenesis of several vascular disorders, such as atherosclerosis, restenosis, and hypertension. The discovery of mutations in the gene encoding BMPRII, the type II subunit of the receptor for bone morphogenetic proteins (BMPs), in patients with pulmonary arterial hypertension (PAH) provided an indication that BMP signaling may affect the homeostasis of VSMCs and their phenotype modulation. Here we report that BMP signaling potently induces SMC-specific genes in pluripotent cells and prevents dedifferentiation of arterial SMCs. The BMP-induced phenotype switch requires intact RhoA/ROCK signaling but is not blocked by inhibitors of the TGFbeta and PI3K/Akt pathways. Furthermore, nuclear localization and recruitment of the myocardin-related transcription factors (MRTF-A and MRTF-B) to a smooth muscle alpha-actin promoter is observed in response to BMP treatment. Thus, BMP signaling modulates VSMC phenotype via cross-talk with the RhoA/MRTFs pathway, and may contribute to the development of the pathological characteristics observed in patients with PAH and other obliterative vascular diseases.

Bone marrow-derived cells and arterial disease

This article reviews the association between bone and artery disease, with particular relevance to progenitor cells. The review was based on insight gained by analysis of previous publications and on-going work by the authors. A large number of studies have demonstrated a correlation between bone pathology, particularly osteoporosis, and atherosclerosis. In this review we highlight the particular aspect of bone marrow progenitor cells in the bone-artery link. Progenitor cells, primarily those believed to give rise to endothelial cells, have been inversely correlated with atherosclerosis severity and risk factors. Therapeutic approaches aimed at manipulating progenitor cells in revascularization and vascular repair have demonstrated some promising results. Subtypes of progenitor cells have also been linked with vascular pathology, however, and further studies are required to assess relative beneficial and pathologic effects of bone marrow-derived progenitors. Further understanding of the link between bone and artery pathophysiology is likely to be of significant value in developing new therapies for vascular disease.

Clinical, cellular, and molecular aspects of arterial calcification

Arterial calcification is a complex and independently regulated process with risk factors similar to those for atherosclerotic occlusive disease. It may develop either within the atherosclerotic intima or in the media. When calcification is found in coronary or lower extremity arteries, it is an independent predictor of cardiovascular events and lower extremity amputation. Recent evidence suggests a role for several endogenous stimulators and inhibitors in the pathogenesis of arterial calcification. Inflammatory mediators and matrix-degrading enzymes are also thought to control the progression of calcification in humans. Current research involves efforts to define the complex interactions between cellular and molecular mediators of arterial calcification.