Inactivation of the osteopontin gene enhances vascular calcification of matrix Gla protein-deficient mice: evidence for osteopontin as an inducible inhibitor of vascular calcification in vivo

Regulation of vascular calcification: roles of phosphate and osteopontin

Vascular calcification is prevalent in aging as well as a number of pathological conditions, and it is now recognized as a strong predictor of cardiovascular events in the general population as well as diabetic and end-stage renal disease patients. Vascular calcification is a highly regulated process involving inductive and inhibitory mechanisms. This article focuses on two molecules, phosphate and osteopontin, that have been implicated in the induction or inhibition of vascular calcification, respectively. Elevated phosphate is of interest because hyperphosphatemia is recognized as a major nonconventional risk factor for cardiovascular disease mortality in end-stage renal disease patients. Studies to date suggest that elevated phosphate stimulates smooth muscle cell phenotypic transition and mineralization via the activity of a sodium-dependent phosphate cotransporter. Osteopontin, however, appears to block vascular calcification most likely by preventing calcium phosphate crystal growth and inducing cellular mineral resorption.

Acidosis inhibits bone formation by osteoblasts in vitro by preventing mineralization

The negative effect of acidosis on the skeleton has been known for almost a century. Bone mineral serves an important pathophysiologic role as a reserve of hydroxyl ions to buffer systemic protons if the kidneys and lungs are unable to maintain acid-base balance within narrow physiologic limits. Extracellular hydrogen ions are now thought to be the primary activation signal for osteoclastic bone resorption, and osteoclasts are very sensitive to small changes in pH within the pathophysiologic range. Herein, we investigated the effects of acidosis on osteoblast function by using mineralized bone nodule-forming primary osteoblast cultures. Osteoblasts harvested from neonatal rat calvariae were cultured up to 21 days in serum-containing medium, with ascorbate, beta-glycerophosphate and dexamethasone. pH was manipulated by addition of 5 to 30 mmol/L HCl and monitored by blood gas analyzer. Abundant, matrix-containing mineralized nodules formed in osteoblast cultures at pH 7.4, but acidification progressively reduced mineralization of bone nodules, with complete abolition at pH 6.9. Osteoblast proliferation and collagen synthesis, assessed by 3H-thymidine and 3H-proline incorporation, respectively, were unaffected by pH in the range 7.4 to 6.9; no effect of acidification on collagen ultrastructure and organization was evident. The apoptosis rate of osteoblasts, assessed by the enrichment of nucleosomes in cell lysates, was also unaffected by pH within this range. However, osteoblast alkaline phosphatase activity, which peaked strongly near pH 7.4, was reduced eight-fold at pH 6.9. Reducing pH to 6.9 also downregulated messenger ribonucleic acid (mRNA) for alkaline phosphatase, but upregulated mRNA for matrix Gla protein, an inhibitor of mineralization. The same pH reduction is associated with two-and four-fold increases in Ca2+ and PO4(3-) solubility for hydroxyapatite, respectively. Our results show that acidosis exerts a selective, inhibitory action on matrix mineralization that is reciprocal with the osteoclast activation response. Thus, in uncorrected acidosis, the deposition of alkaline mineral in bone by osteoblasts is reduced, and osteoclast resorptive activity is increased in order to maximize the availability of hydroxyl ions in solution to buffer protons.

Osteoporosis and atherosclerosis: biological linkages and the emergence of dual-purpose therapies

Osteoporosis and atherosclerosis are both widely prevalent in an ageing population, and induce serious morbidities and death. There is growing evidence that in addition to their relationship to ageing, osteoporosis and atherosclerosis are also linked by biological associations. This article reviews their clinical interrelations, discusses the basic biology of bone and the arterial wall, and presents five examples that illustrate their biological linkages. Current therapeutic approaches emerging from these linkages, including statins, bisphosphonates, and the thiazolidinediones, have dual effects on bone and the vasculature. Additional therapies derived from experimental studies that enhance bone density and reduce atherogenesis hold further promise to diminish the morbidity and mortality of osteoporosis and atherosclerosis, with attendant benefits to society.

Reduced plasma pyrophosphate levels in hemodialysis patients

Pyrophosphate (PPi) is a known inhibitor of hydroxyapatite formation and has been shown to inhibit medial vascular calcification in vitamin D-toxic rats. It was demonstrated recently that endogenous production of PPi prevents calcification of rat aorta that are cultured in high concentrations of calcium and phosphate. For determining whether PPi metabolism is altered in hemodialysis patients, plasma levels and dialytic clearance of PPi were measured in stable hemodialysis patients. Predialysis plasma [PPi] was 2.26 +/- 0.19 microM in 38 clinically stable hemodialysis patients compared with 3.26 +/- 0.17 in 36 normal subjects (P < 0.01). Approximately 30% of plasma PPi was protein bound, and this was not altered in dialysis patients. There was a weak inverse correlation with age in normal individuals but not in dialysis patients. Plasma [PPi] in dialysis patients was correlated with plasma [PO4(3-)] (r = 0.56) but not with [Ca2+], parathyroid hormone, or the dose of dialysis, and levels did not vary between interdialytic periods of 2 and 3 d. Plasma [PPi] decreased 32 +/- 5% after standard hemodialysis in 17 patients. In vitro clearance of PPi by a 2.1-m2 cellulose acetate dialyzer was 36%, and the mean PPi removal in five patients was 43 +/- 5 micromol, consistent with a similar in vivo clearance. Cleared PPi was greater than the plasma pool but less than the estimated extracellular fluid pool. Erythrocyte PPi content decreased 24 +/- 4%, indicating that intracellular PPi is removed as well. It is concluded that plasma [PPi] is reduced in hemodialysis patients and that PPi is cleared by dialysis. Plasma levels in some patients were below those that have previously been shown to prevent calcification of vessels in culture, suggesting that altered PPi metabolism could contribute to vascular calcification in hemodialysis patients.

Osteopontin expression and adventitial angiogenesis induced by local vascular endothelial growth factor 165 reduces experimental aortic calcification

BACKGROUND

Vascular calcification is a common pathologic and precisely regulated process involving bone-associated proteins such as osteopontin. In this study, we investigated mechanisms by which recombinant human vascular endothelial growth factor 165 protects the arterial wall from severe vascular remodeling, including calcification, a newly discovered biologic action of vascular endothelial growth factor.

METHODS

In a rabbit model of thoracic aortic end-to-end anastomosis that simulates cardiovascular intervention, recombinant human vascular endothelial growth factor 165 at a dose of 0.75 mug (n = 19) or albumin (control; n = 19) was delivered intraluminally and on the serosal surface. Animals were killed, and aortic tissue was evaluated by Western blotting, immunohistochemistry, and immunofluorescence at 4, 8, and 24 hours; 1 week; and 1 month after surgery.

RESULTS

All controls revealed extensive aortic medial calcification at 1 month, whereas calcification was significantly reduced or absent with vascular endothelial growth factor treatment. Compared with controls, vascular endothelial growth factor treatment resulted in an earlier infiltration of macrophages in the vessel media (at 8 hours: 5.7 +/- 2.3 macrophages per high-power field in control vs 32.1 +/- 7.5 in vascular endothelial growth factor-treated aortas; P < .001), whereas controls showed an increase in macrophages starting at 1 week (24.1 +/- 6.9 vs 4.3 +/- 1.8; P < .001). Osteopontin expression was transiently increased and detected in macrophages and endothelial cells in vascular endothelial growth factor-treated vessels, and adventitial microvascular density was significantly increased by 1 week (9.5 +/- 0.43 vs 25.0 +/- 1.3; P < .001).

CONCLUSIONS

Our data suggest that exogenous vascular endothelial growth factor is capable of increasing adventitial angiogenesis and shifting macrophage infiltration and osteopontin expression in the media to an earlier time point, thereby promoting prompt repair and diminishing vascular remodeling and calcification after acute vascular injury.

Matrix Gla protein and osteopontin genetic associations with coronary artery calcification and bone density: the CARDIA study

A role for matrix proteins has previously been proposed in the pathogenesis of arterial calcification in the setting of atherosclerosis, and a link has been suggested between osteoporosis and arterial calcification. Our aim has been to investigate whether matrix Gla protein (MGP) T-138C, osteopontin (SPP1) T-443C, and Asp94Asp single nucleotide polymorphisms are associated with the development of arterial calcification and bone density. The individual effects of the MGP and SPP1 polymorphisms with coronary calcification are weak and not statistically significant. Bone mineral density differences at both the hip and spine do not vary statistically by genotype for any of the polymorphisms studied. Given the significant role of both MGP and SPP1 in arteriosclerosis, further research in higher risk, older populations are needed to determine fully the way in which MGP and SPP1 polymorphisms are associated with disease.

Calcification of advanced atherosclerotic lesions in the innominate arteries of ApoE-deficient mice: potential role of chondrocyte-like cells

OBJECTIVE

Advanced atherosclerotic lesions in the innominate arteries of chow-fed apolipoprotein E-deficient mice become highly calcified with 100% frequency by 75 weeks of age. The time course, cell types, and mechanism(s) associated with calcification were investigated.

METHODS AND RESULTS

The deposition of hydroxyapatite is preceded by the formation of fibro-fatty nodules that are populated by cells that morphologically resemble chondrocytes. These cells are spatially associated with small deposits of hydroxyapatite in animals between 45 and 60 weeks of age. Immunocytochemical analyses with antibodies recognizing known chondrocyte proteins show that these cells express the same proteins as chondrocytes within developing bone. Histological and electron microscopic analyses of lesions from animals between 45 and 60 weeks of age show that the chondrocyte-like cells are surrounded by dense connective tissue that stains positive for type II collagen. Nanocrystals of hydroxyapatite can be seen within matrix vesicles derived from the chondrocyte-like cells. In mice between 75 and 104 weeks of age, the lesions have significantly reduced cellularity and contain large calcium deposits. The few remaining chondrocyte-like cells are located adjacent to or within the large areas of calcification.

CONCLUSIONS

Calcification of advanced lesions in chow-fed apolipoprotein E-deficient mice occurs reproducibly in mice between 45 and 75 weeks of age. The deposition of hydroxyapatite is mediated by chondrocytes, which suggests that the mechanism of calcification may in part recapitulate the process of endochondral bone formation.

Msx2 promotes cardiovascular calcification by activating paracrine Wnt signals

In diabetic LDLR-/- mice, an ectopic BMP2-Msx2 gene regulatory program is upregulated in association with vascular calcification. We verified the procalcific actions of aortic Msx2 expression in vivo. CMV-Msx2 transgenic (CMV-Msx2Tg(+)) mice expressed 3-fold higher levels of aortic Msx2 than nontransgenic littermates. On high-fat diets, CMV-Msx2Tg(+) mice exhibited marked cardiovascular calcification involving aortic and coronary tunica media. This corresponded to regions of Msx2 immunoreactivity in adjacent adventitial myofibroblasts, suggesting a potential paracrine osteogenic signal. To better understand Msx2-regulated calcification, we studied actions in 10T1/2 cells. We found that conditioned media from Msx2-transduced 10T1/2 cells (Msx2-CM) is both pro-osteogenic and adipostatic; these features are characteristic of Wnt signaling. Msx2-CM stimulated Wnt-dependent TCF/LEF transcription, and Msx2-transduced cells exhibited increased nuclear beta-catenin localization with concomitant alkaline phosphatase induction. Msx2 upregulated Wnt3a and Wnt7a but downregulated expression of the canonical inhibitor Dkk1. Dkk1 treatment reversed osteogenic and adipostatic actions of Msx2. Teriparatide, a PTH1R agonist that inhibits murine vascular calcification, suppressed vascular BMP2-Msx2-Wnt signaling. Analyses of CMV-Msx2Tg(+) mice confirmed that Msx2 suppresses aortic Dkk1 and upregulates vascular Wnts; moreover, TOPGAL(+) (Wnt reporter); CMV-Msx2Tg(+) mice exhibited augmented aortic LacZ expression. Thus, Msx2-expressing cells elaborated an osteogenic milieu that promotes vascular calcification in part via paracrine Wnt signals.

Calcification of Tissue Heart Valve Substitutes: Progress Toward Understanding and Prevention

Calcification plays a major role in the failure of bioprosthetic and other tissue heart valve substitutes. Tissue valve calcification is initiated primarily within residual cells that have been devitalized, usually by glutaraldehyde pretreatment. The mechanism involves reaction of calcium-containing extracellular fluid with membrane-associated phosphorus to yield calcium phosphate mineral deposits. Calcification is accelerated by young recipient age, valve factors such as glutaraldehyde fixation, and increased mechanical stress. Recent studies have suggested that pathologic calcification is regulated by inductive and inhibitory factors, similar to the physiologic mineralization of bone. The most promising preventive strategies have included binding of calcification inhibitors to glutaraldehyde fixed tissue, removal or modification of calcifiable components, modification of glutaraldehyde fixation, and use of tissue cross linking agents other than glutaraldehyde. This review summarizes current concepts in the pathophysiology of tissue valve calcification, including emerging concepts of endogenous regulation, progress toward prevention of calcification, and issues related to calcification of the aortic wall of stentless bioprosthetic valves.

Arterial osteoprotegerin: increased amounts in diabetes and modifiable synthesis from vascular smooth muscle cells by insulin and TNF-alpha

AIMS/HYPOTHESIS

Extracellular matrix modifications and linear medial calcifications are elements of diabetic macroangiopathy. We hypothesised that the bone-related protein osteoprotegerin (OPG) may occur in altered amounts in the arterial wall in diabetes, putatively associated with altered synthesis from vascular cells.

METHODS

The amount of OPG in the thoracic aorta, obtained at autopsy from 21 diabetic and 42 sex- and age-matched controls, was measured in tissue extracts by an ELISA. The production of OPG was estimated in conditioned media by an ELISA, and OPG mRNA was estimated by RT-PCR in vascular cells grown in vitro.

RESULTS

The content of OPG was increased in tunica media samples from diabetic individuals. No differences between diabetic and non-diabetic subjects were observed in tunica intima. Human vascular smooth muscle cells (HVSMCs) produced approximately 30 times more OPG than human umbilical vein endothelial cells. The OPG production into the medium decreased dose- and time-dependently after insulin treatment (maximal effect approximately 60% of control) in HVSMCs, whereas TNF-alpha supplement gave rise to increased OPG synthesis in a time- and dose-dependent manner (maximal effect approximately 200% of control). Similar effects on OPG mRNA expression were observed. Addition of growth hormone (10 ng/ml) or extra glucose (25 mmol/l) to the growth medium had no effect.

CONCLUSIONS/INTERPRETATION

Increased OPG concentrations in the arterial wall in diabetes may be part of generalised matrix alterations, putatively related to the development of vascular calcifications. Altered arterial OPG content may be a consequence of the effects of hormones and cytokines, like insulin and TNF-alpha.

Role of matrix Gla protein in parathyroid hormone inhibition of osteoblast mineralization

Parathyroid hormone (PTH) exerts biphasic effects on bone, dependent on the frequency and dose of administration. The catabolic actions of PTH on bone have been associated with continuous treatment, an increase in osteoblast-mediated resorption of bone via osteoclast activation, and inhibition of osteoblast activity and mineralization. Downregulation of differentiation markers and inhibition of mineralization by PTH have been reported in primary calvarial explants and osteoblast cell lines. Using MC3T3-E1 osteoblast-like cells, we have shown that matrix Gla protein (MGP) can be induced by PTH, and that this induction may explain the PTH-mediated inhibition of osteoblast biomineralization. MGP is a known inhibitor of mineralization, and mice deficient in Mgp show severe vascular calcification and premature bone mineralization. This review discusses the role of MGP in mineralization, comparing bone and vascular mineralization. In addition to MGP, the regulation and possible role of osteopontin, another known regulator of osteoblast mineralization, in PTH-mediated regulation of bone and vascular mineralization is discussed.

Chondrogenesis mediated by PPi depletion promotes spontaneous aortic calcification in NPP1-/- mice

OBJECTIVE

We recently linked human arterial media calcification of infancy to heritable PC-1/nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) deficiency. NPP1 hydrolyzes ATP to generate PP(i), a physicochemical inhibitor of hydroxyapatite crystal growth. But pathologic calcification in NPP1 deficiency states is tissue-restricted and in perispinal ligaments is endochondral differentiation-mediated rather than simply a dystrophic process. Because ectopic chondro-osseous differentiation promotes artery calcification in atherosclerosis and other disorders, we tested the hypothesis that NPP1 and PP(i) deficiencies regulate cell phenotype plasticity to promote artery calcification.

METHODS AND RESULTS

Using cultured multipotential NPP1-/- mouse bone marrow stromal cells, we demonstrated spontaneous chondrogenesis inhibitable by treatment with exogenous PP(i). We also demonstrated cartilage-specific gene expression, upregulated alkaline phosphatase, decreased expression of the physiological calcification inhibitor osteopontin, and increased calcification in NPP1-/- aortic smooth muscle cells (SMCs). Similar changes were demonstrated in aortic SMCs from ank/ank mice, which are extracellular PP(i)-depleted because of defective ANK transmembrane PP(i) transport activity. Moreover, NPP1-/- and ank/ank mice demonstrated aortic media calcification by von Kossa staining, and intra-aortic cartilage-specific collagen gene expression was demonstrated in situ in NPP1-/- mice.

CONCLUSIONS

NPP1 and PP(i) deficiencies modulate phenotype plasticity in artery SMCs and chondrogenesis in mesenchymal precursors, thereby stimulating artery calcification by modulating cell differentiation.

Polymorphisms in the osteopontin promoter affect its transcriptional activity

Understanding the molecular mechanisms that underlie regulation of transcription of the human osteopontin encoding gene (OPN) may help to clarify several processes, such as fibrotic evolution of organ damage, tumorigenesis and metastasis, and immune response, in which OPN overexpression is observed. With the aim to evaluate variants with functional effect on transcription, we have analyzed the promoter region and focused our investigation on three common variants present in the first 500 bp upstream of the transcription start site. Transfection of constructs carrying the four most frequent haplotypes relative to variants at -66, -156, and -443 fused to the luciferase reporter gene in a panel of different cell lines showed that one haplotype conferred a significantly reduced level of reporter gene expression in all tested cell lines. We describe that the -66 polymorphism modifies the binding affinity for the SP1/SP3 transcription factors, the -156 polymorphism is included in a yet uncharacterized RUNX2 binding site, and the -443 polymorphism causes differential binding of an unknown factor. The finding of differential effects of various combination of variants in haplotypes may contribute to explain data of association studies reported in several already published articles. Future association studies using haplotypes instead of single OPN variants will allow to achieve more accurate results referable to differential expression of OPN in several common diseases, in which OPN is considered a candidate susceptibility gene.

Progression of diabetic nephropathy enhances the plasma osteopontin level in type 2 diabetic patients

Osteopontin (OPN) is thought to play multiple roles in the progression of atherosclerotic plaque including diabetic vascular complications. However, it still remains unclear whether the level of OPN in vivo is indeed clinically associated with the progression of diabetic complications. This study evaluated whether the levels of OPN in plasma and urine are correlated with the progression of diabetic complications, such as retinopathy, neuropathy, and nephropathy in patients with type 2 diabetes. In 229 patients with type 2 diabetes, OPN level in plasma and urine was evaluated by both the severity of diabetic complications, such as retinopathy, neuropathy, and nephropathy, and the clinical characteristics and the substantial laboratory findings. Plasma OPN level increased significantly with aging and the progression of diabetic nephropathy, especially at the stage of renal failure (p<0.05). However, the level was not related to the progression of retinopathy or neuropathy, or to laboratory findings, such as HbA1c or serum lipids. In contrast, urinary OPN level was not associated with diabetic complications in any of the subjects. There was no correlation between the plasma and urinary values of OPN. The results established that the plasma OPN was elevated in proportion to the progression of diabetic nephropathy, indicating that the plasma concentration may be a potential diagnostic predictor of diabetic end-stage renal disease.

Tamm-Horsfall protein is a critical renal defense factor protecting against calcium oxalate crystal formation

BACKGROUND

The tubular fluid of the mammalian kidney is often supersaturated with mineral salts, but crystallization rarely occurs under normal conditions. The unique ability of the kidney to avoid harmful crystal formation has long been attributed to the inhibitory activity of the urinary macromolecules, although few in vivo studies have been carried out to examine this hypothesis. Here we examined the role of Tamm-Horsfall protein (THP), the principal urinary protein, in urinary defense against renal calcium crystal formation, using a THP knockout model that we recently developed.

METHODS

Wild-type and THP knockout mice were examined for the spontaneous formation of renal calcium crystals using von Kossa staining. The susceptibility of these mice to experimentally induced renal crystal formation was evaluated by administering mice with ethylene glycol, a precursor of oxalate, and vitamin D(3), which increases calcium absorption. Renal calcium crystals were visualized by von Kossa stain, dark field microscopy with polarized light and scanning electron microscopy.

RESULTS

Inactivating the THP gene in mouse embryonic stem cells results in spontaneous formation of calcium crystals in adult kidneys. Excessive intake of calcium and oxalate, precursors of the most common type of human renal stones, dramatically increases both the frequency and the severity of renal calcium crystal formation in THP-deficient, but not in wild-type mice. Under high calcium/oxalate conditions, the absence of THP triggers a marked, adaptive induction in renal epithelial cells of osteopontin (OPN), a potent inhibitor of bone mineralization and vascular calcification. Thus, OPN may serve as an inducible inhibitor of calcium crystallization, whereas THP can serve as a constitutive and apparently more effective inhibitor.

CONCLUSION

These results provide the first in vivo evidence that THP is a critical urinary defense factor and suggest that its deficiency could be an important contributing factor in human nephrolithiasis, a condition afflicting tens of millions of people in the world annually.

Vascular calcification: mechanisms and clinical ramifications

Vascular calcification, long thought to result from passive degeneration, involves a complex, regulated process of biomineralization resembling osteogenesis. Evidence indicates that proteins controlling bone mineralization are also involved in the regulation of vascular calcification. Artery wall cells grown in culture are induced to become osteogenic by inflammatory and atherogenic stimuli. Furthermore, osteoclast-like cells are found in calcified atherosclerotic plaques, and active resorption of ectopic vascular calcification has been demonstrated. In general, soft tissue calcification arises in areas of chronic inflammation, possibly functioning as a barrier limiting the spread of the inflammatory stimulus. Atherosclerotic calcification may be one example of this process, in which oxidized lipids are the inflammatory stimulus. Calcification is widely used as a clinical indicator of atherosclerosis. It progresses nonlinearly with time, following a sigmoid-shaped curve. The relationship between calcification and clinical events likely relates to mechanical instability introduced by calcified plaque at its interface with softer, noncalcified plaque. In general, as calcification proceeds, interface surface area increases initially, but eventually decreases as plaques coalesce. This phenomenon may account for reports of less calcification in unstable plaque. Vascular calcification is exacerbated in certain clinical entities, including diabetes, menopause, and osteoporosis. Mechanisms linking them must be considered in clinical decisions. For example, treatments for osteoporosis may have unanticipated effects on vascular calcification; the converse also applies. Further understanding of processes governing vascular calcification may yield new therapeutic options for vascular disease.

Vascular calcification in chronic kidney disease

Dialysis patients have increased cardiovascular morbidity, mortality, and vascular calcification, and the latter appears to impact the former. Recent evidence indicates that vascular calcification is an active, cell-mediated process. Osteoblast differentiation factor Cbfa1 and several bone-associated proteins (osteopontin, bone sialoprotein, alkaline phosphatase, type I collagen) are present in histologic sections of arteries obtained from patients with end-stage renal disease (chronic kidney disease stage V [CKD-V]). This supports the theory that vascular smooth muscle cells can dedifferentiate or transform to osteoblast-like cells, possibly by up-regulation of Cbfa1. In in vitro experiments, addition of pooled serum from dialysis patients (versus normal healthy controls) accelerated mineralization and increased expression of Cbfa1, osteopontin, and alkaline phosphatase in cultured vascular smooth muscle cells. Clinically, the pathogenesis of vascular calcification is not completely understood, although increased levels of phosphorus and/or other potential uremic toxins may play an important role by transforming vascular smooth muscle cells into osteoblast-like cells. Presumably, once this process begins, increased serum calcium X phosphorus product, or calcium load from binders, accelerates this process. In addition, it is likely that circulating inhibitors of calcification are also important. Further understanding of the pathophysiology of vascular calcification is needed to intervene appropriately.

Inhibition of hydroxyapatite formation by osteopontin phosphopeptides

Osteopontin (OPN) is an acidic phosphoglycoprotein that is believed to function in the prevention of soft tissue calcification. In vitro studies have shown that OPN can inhibit the formation of hydroxyapatite (HA) and other biologically relevant crystal phases, and that this inhibitory activity requires phosphorylation of the protein; however, it is not known which phosphorylated residues are involved. We have synthesized peptides corresponding to four phosphoserine-containing sequences in rat OPN: OPN7-17, containing phosphoserines 10 and 11; OPN41-52, containing phosphoserines 46 and 47; OPN248-264, containing phosphoserines 250, 257 and 262; and OPN290-301, containing phosphoserines 295-297. The abilities of these peptides to inhibit de novo HA formation were determined using a constant-composition autotitration assay. All four OPN phosphopeptides caused a dose-dependent increase in nucleation lag time, but did not significantly affect subsequent formation of the crystals. However, OPN41-52 (inhibitory constant 73.5 min/microM) and OPN290-301 (72.2 min/microM) were approx. 4 times more potent inhibitors than OPN7-17 (19.7 min/microM) and OPN247-264 (16.3 min/microM). 'Scrambling' the amino acid sequence of OPN290-301 resulted in decreased potency (45.6 min/microM), whereas omission of the phosphate groups from this peptide caused a greater decrease (5.20 min/microM). These findings have identified phosphorylated sequences that are important for the ability of rat bone OPN to inhibit HA crystal formation, and suggest that negative-charge density is an important factor in this activity.

Osteogenic regulation of vascular calcification: an early perspective

Cardiovascular calcification is a common consequence of aging, diabetes, hypercholesterolemia, mechanically abnormal valve function, and chronic renal insufficiency. Although vascular calcification may appear to be a uniform response to vascular insult, it is a heterogenous disorder, with overlapping yet distinct mechanisms of initiation and progression. A minimum of four histoanatomic variants-atherosclerotic (fibrotic) calcification, cardiac valve calcification, medial artery calcification, and vascular calciphylaxis-arise in response to metabolic, mechanical, infectious, and inflammatory injuries. Common to the first three variants is a variable degree of vascular infiltration by T cells and macrophages. Once thought benign, the deleterious clinical consequences of calcific vasculopathy are now becoming clear; stroke, amputation, ischemic heart disease, and increased mortality are portended by the anatomy and extent of calcific vasculopathy. Along with dystrophic calcium deposition in dying cells and lipoprotein deposits, active endochondral and intramembranous (nonendochondral) ossification processes contribute to vascular calcium load. Thus vascular calcification is subject to regulation by osteotropic hormones and skeletal morphogens in addition to key inhibitors of passive tissue mineralization. In response to oxidized lipids, inflammation, and mechanical injury, the microvascular smooth muscle cell becomes activated. Orthotopically, proliferating stromal myofibroblasts provide osteoprogenitors for skeletal growth and fracture repair; however, in valves and arteries, vascular myofibroblasts contribute to cardiovascular ossification. Current data suggest that paracrine signals are provided by bone morphogenetic protein-2, Wnts, parathyroid hormone-related polypeptide, osteopontin, osteoprotegerin, and matrix Gla protein, all entrained to endocrine, metabolic, inflammatory, and mechanical cues. In end-stage renal disease, a "perfect storm" of vascular calcification often occurs, with hyperglycemia, hyperphosphatemia, hypercholesterolemia, hypertension, parathyroid hormone resistance, and iatrogenic calcitriol excess contributing to severe calcific vasculopathy. This brief review recounts emerging themes in the pathobiology of vascular calcification and highlights some fundamental deficiencies in our understanding of vascular endocrinology and metabolism that are immediately relevant to human health and health care.

A locus on chromosome 7 determines dramatic up-regulation of osteopontin in dystrophic cardiac calcification in mice

Calcification of necrotic tissue is frequently observed in chronic inflammation and atherosclerosis. A similar response of myocardium to injury, referred to as dystrophic cardiac calcinosis (DCC), occurs in certain inbred strains of mice. We now examined a putative inhibitor of calcification, osteopontin, in DCC after transdiaphragmal myocardial freeze-thaw injury. Strong osteopontin expression was found co-localizing with calcification in DCC-susceptible strain C3H/HeNCrlBr, which exhibited low osteopontin plasma concentrations otherwise. Osteopontin mRNA induction was 20-fold higher than in resistant strain C57BL/6NCrlBr, which exhibited fibrous lesions without calcification and little osteopontin expression. Sequence analysis identified several polymorphisms in calcium-binding and phosphorylation sites in osteopontin cDNA. Their potential relevance for DCC was tested in congenic mice, which shared the osteopontin locus with C57BL/6NCrlBr, but retained a chromosomal segment from C3H/HeNCrlBr on proximal chromosome 7. These mice exhibited strong osteopontin expression and DCC comparable to C3H/HeNCrlBr suggesting that a trans-activator of osteopontin transcription residing on chromosome 7 and not the osteopontin gene on chromosome 5 was responsible for the genetic differences in osteopontin expression. A known osteopontin activator encoded by a gene on chromosome 7 is the transforming growth factor-beta1, which was more induced (3.5x) in C3H/HeNCrlBr than in C57BL/6NCrlBr mice.

Regulation of cardiovascular calcification

Vascular calcification is highly correlated with cardiovascular disease (CVD) and is a significant predictor of cardiovascular events, especially in high risk patients such as the end stage renal disease (ESRD) population. Vascular calcification can lead to serious problems including valve stenosis, decreased vascular compliance, calciphylaxis, and even sudden death. However, the contribution of vascular calcification to progression of atherosclerosis is unknown and needs more study. Biochemical, histological, and genetic studies indicate that vascular calcification is actively regulated and involves both positive and negative modulators. Several nonmutually exclusive theories to account for vascular calcification based on current studies are discussed.

Plasma osteopontin levels are associated with the presence and extent of coronary artery disease

Recently, osteopontin (OPN) mRNA was reported to be highly expressed in atherosclerotic plaques, most strikingly in calcified plaques. We examined if plasma OPN levels are associated with coronary stenosis and calcification in patients with coronary artery disease (CAD). We measured plasma OPN levels in 178 patients undergoing coronary angiography. Compared with 71 patients without CAD, 107 with CAD had higher OPN levels (616+/-308 ng/ml versus 443+/-237 ng/ml, P<0.001). A stepwise increase in OPN levels was found depending on the number of >50% stenotic coronary vessels: 540+/-293 ng/ml in 1-vessel, 615+/-230 ng/ml in 2-vessel, and 758+/-416 ng/ml in 3-vessel disease. OPN levels also correlated with the numbers of >50% and >25% stenotic segments (r=0.35 and 0.43, respectively, P<0.001). In multivariate analysis, OPN levels were significantly associated with CAD (odds ratio=1.21, 95% CI=1.05-1.39 for a 100 ng/ml increase) independent of traditional risk factors. Coronary calcification was found in 86 patients. OPN levels were higher in patients with calcification than in those without calcification (608+/-328 ng/ml versus 490+/-246 ng/ml, P<0.01) and correlated with the number of calcified segment (r=0.26, P<0.001). However, OPN levels were not independently associated with coronary calcification. Thus, plasma OPN levels were found to be associated with the presence and extent of CAD.

Calciphylaxis in chronic, non-dialysis-dependent renal disease

Background

Calciphylaxis cutis is characterized by media calcification of arteries and, most prominently, of cutaneous and subcutaneous arterioles occurring in renal insufficiency patients.

Case Report

A 53-year-old woman with chronic cardiac and renal failure complained of painful crural, non-varicosis ulcers. She was hospitalized in an immobilized condition due to both the crural ulcerations and the existing heart-failure state (NYHA III-IV) having pleural and pericardial effusions, atrial fibrillation and weight loss of 30 kg over the past year. Despite normalization of calcium-phosphorus balance and improvement of renal function, the clinical course of crural ulcerations deteriorated during the following 3 months. After failure of surgical debridements, multiple courses of sterile-maggot therapy were introduced at a late stage to stabilize the wounds. The patient died of recurrent wound infections and sepsis paralleled by exacerbations of renal malfunction.

Conclusions

The role of renal disease in vascular complications is discussed. Sterile-maggot debridement may constitute a therapy for the ulcerated calciphylaxis at an earlier stage, i.e. when first ulcerations appear.

Teriparatide (human parathyroid hormone (1-34)) inhibits osteogenic vascular calcification in diabetic low density lipoprotein receptor-deficient mice

Cardiovascular calcification is a common consequence of diabetes. High fat diets induce diabetes and arterial calcification in male low density lipoprotein receptor (LDLR) -/- mice; calcification occurs via Msx2 signaling that promotes the osteogenic differentiation of arterial myofibroblasts. We studied regulation of arterial osteogenesis by human parathyroid hormone (PTH) (1-34) (also called teriparatide) in LDLR -/- mice fed diabetogenic diets for 4 weeks. LDLR -/- mice were treated with vehicle or 0.4 mg/kg of PTH(1-34) subcutaneously five times/week. Gene expression was determined from single aortas and hind limb RNA by fluorescence reverse transcription-PCR. Valve calcification was determined by histological staining of cardiac sections using image analysis to quantify valve leaflet mineralization. PTH(1-34) increased bone mineral content (by dual energy x-ray absorptiometry) in LDLR -/- mice, with induction of osseous osteopontin (OPN) expression and serum OPN levels (>150 nM); PTH(1-34) did not significantly change serum glucose, lipids, body weight, or fat mass. PTH(1-34) suppressed aortic OPN and Msx2 expression >50% and decreased cardiac valve calcification 80% (8.3 +/- 1.5% versus 1.4 +/- 0.5%; p < 0.001). Of the known circulating regulators of vascular calcification (OPN, osteoprotegerin, and leptin), PTH(1-34) regulated only serum OPN. We therefore studied actions of PTH(1-34) and OPN in vitro on cells induced to mineralize with Msx2. OPN (5-50 nM) reversed Msx2-induced mineralization. PTH(1-34) inhibited mineralization by 40% and down-regulated Msx2 in aortic myofibroblasts. PTH(1-34) inhibits vascular calcification and aortic osteogenic differentiation via direct actions and potentially via circulating OPN. PTH(1-34) exerts beneficial actions at early stages of macrovascular disease responses to diabetes and dyslipidemia.

The serum protein alpha 2-Heremans-Schmid glycoprotein/fetuin-A is a systemically acting inhibitor of ectopic calcification

Ectopic calcification is a frequent complication of many degenerative diseases. Here we identify the serum protein alpha2-Heremans-Schmid glycoprotein (Ahsg, also known as fetuin-A) as an important inhibitor of ectopic calcification acting on the systemic level. Ahsg-deficient mice are phenotypically normal, but develop severe calcification of various organs on a mineral and vitamin D-rich diet and on a normal diet when the deficiency is combined with a DBA/2 genetic background. This phenotype is not associated with apparent changes in calcium and phosphate homeostasis, but with a decreased inhibitory activity of the Ahsg-deficient extracellular fluid on mineral formation. The same underlying principle may contribute to many calcifying disorders including calciphylaxis, a syndrome of severe systemic calcification in patients with chronic renal failure. Taken together, our data demonstrate a critical role of Ahsg as an inhibitor of unwanted mineralization and provide a novel therapeutic concept to prevent ectopic calcification accompanying various diseases.

Alterations of arterial physiology in osteopontin-null mice

OBJECTIVE

In this study, we characterized the effects of an osteopontin (OPN)-null mutation in normal arterial function and remodeling in a murine model.

METHODS AND RESULTS

OPN-null mutant mice were compared with wild-type mice before and after carotid artery ligation. Before ligation, OPN-null mice had increased heart rate, lower blood pressure, and increased circulating lymphocytes compared with wild-type mice. OPN-null vessels also demonstrated greater compliance accompanied by a loosely organized collagen network. After carotid artery ligation, significant differences were also found in the remodeling response of OPN-null animals. At 4 days after ligation, leukocyte adhesion/invasion was diminished by 10-fold in OPN-null mice compared with wild-type mice. At 14 days after ligation, the ligated arteries of OPN-null mice had smaller neointimal lesions but greater constrictive remodeling compared with wild-type mice, resulting in similar lumen areas. Continued remodeling resulted in a similar morphological phenotype in both groups at 28 days.

CONCLUSIONS

These data show that endogenous OPN regulates normal vascular physiology and contributes to the vascular remodeling response by regulating vascular compliance and the inflammatory response.

Osteopontin deficiency attenuates atherosclerosis in female apolipoprotein E-deficient mice

OBJECTIVE

Osteopontin (OPN), a noncollagenous adhesive protein, is implicated in atherosclerosis, in which macrophages within atherosclerotic plaques express OPN. However, it is not known whether the elevated OPN expression is a cause or result of atherosclerosis.

METHODS AND RESULTS

We generated mice that lacked OPN and crossed them with apolipoprotein (apo) E-deficient mice and analyzed these mice with a mixed C57BL/6x129 background after 36 weeks on a normal chow diet. In female mice, OP+/-E-/- and OP-/-E-/- mice had significantly smaller atherosclerotic and inflammatory lesions compared with OP+/+E-/- mice, and that was reflected by smaller area of MOMA-2-positive staining. In male mice, however, there was no significant difference in the atherosclerosis lesion areas among 3 genotypes. In both OP-/-E-/- and OP+/+E-/- mice, typical atherosclerotic lesions were detected, which include necrotic core, foamy cell collections, and cholesterol clefts. However, we found that vascular mineral-deposited areas in 60-week-old male OP-/-E-/- mice were significantly increased compared with those in OP+/+E-/- male mice.

CONCLUSIONS

These results suggest that OPN plays a promoting effect in atherosclerosis and inhibitory effect in vascular calcification. The suppression of OPN expression in females should be considered a therapeutic possibility in atherosclerosis.

Targeted disruption of the osteoblast/osteocyte factor 45 gene (OF45) results in increased bone formation and bone mass

We have previously described osteoblast/osteocyte factor 45 (OF45), a novel bone-specific extracellular matrix protein, and demonstrated that its expression is tightly linked to mineralization and bone formation. In this report, we have cloned and characterized the mouse OF45 cDNA and genomic region. Mouse OF45 (also called MEPE) was similar to its rat orthologue in that its expression was increased during mineralization in osteoblast cultures and the protein was highly expressed within the osteocytes that are imbedded within bone. To further determine the role of OF45 in bone metabolism, we generated a targeted mouse line deficient in this protein. Ablation of OF45 resulted in increased bone mass. In fact, disruption of only a single allele of OF45 caused significantly increased bone mass. In addition, knockout mice were resistant to aging-associated trabecular bone loss. Cancellous bone histomorphometry revealed that the increased bone mass was the result of increased osteoblast number and osteoblast activity with unaltered osteoclast number and osteoclast surface in knockout animals. Consistent with the bone histomorphometric results, we also determined that OF45 knockout osteoblasts produced significantly more mineralized nodules in ex vivo cell cultures than did wild type osteoblasts. Osteoclastogenesis and bone resorption in ex vivo cultures was unaffected by OF45 mutation. We conclude that OF45 plays an inhibitory role in bone formation in mouse.

Aortic valve sclerosis as a marker of active atherosclerosis

Aortic sclerosis is a calcific disease of the aortic valvular leaflets defined as focal leaflet thickening without significant obstruction to left ventricular outflow. Several clinical factors are associated with calcific aortic valve disease, including male sex, smoking, hypertension, age, hypercholesterolemia, and diabetes. Histologic and biochemical studies suggest similarities between the mechanisms involved in the development of aortic sclerosis and atherosclerosis, suggesting these two diseases may share common pathophysiologic mechanisms. In a recent prospective trial, the presence of aortic sclerosis was associated with an approximately 50% increase in cardiovascular mortality and myocardial infarction, even after correction for age, gender, known coronary artery disease, and clinical factors associated with a aortic sclerosis.