Clinical Science (2010) 119, 111–121 (Printed in Great Britain) doi:10.1042/CS20090631 111

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Vascular calcification in chronic kidney disease

Adrian COVIC∗, Mehmet KANBAY†, Luminita VORONEANU∗, Faruk TURGUT†,Dragomir N. SERBAN‡§, Ionela Lacramioara SERBAN‡§ and David J. GOLDSMITH‖∗Clinic of Nephrology, “C. I. Parhon” University Hospital, “Gr. T. Popa” University of Medicine and Pharmacy, Bd. Carol I,Nr. 50, Iasi 700503, Romania, †Section of Nephrology, Department of Internal Medicine, Fatih University School of Medicine,Bestepe/Cankaya, Ankara, Turkey, ‡Department of Physiology, “Gr. T. Popa” University of Medicine and Pharmacy, Str.Universitatii, Nr. 16, Iasi 700115, Romania, and §Unit of Cell Physiology and Pharmacology at Center for Study and Therapyof Pain, “Gr. T. Popa” University of Medicine and Pharmacy, Str. Mihail Kogalniceanu, Nr. 9, Iasi 700454, Romania, and‖Renal Unit, Guy’s Hospital, Great Maze Pond, London SE1 9RT, U.K.

A B S T R A C T

VC (vascular calcification) is highly prevalent in patients with CKD (chronic kidney disease), butits mechanism is multifactorial and incompletely understood. In addition to increased traditionalrisk factors, CKD patients also have a number of non-traditional cardiovascular risk factors, whichmay play a prominent role in the pathogenesis of arterial calcification, such as duration of dialysisand disorders of mineral metabolism. The transformation of vascular smooth muscle cells intochondrocytes or osteoblast-like cells seems to be a key element in VC pathogenesis, in the contextof passive calcium and phosphate deposition due to abnormal bone metabolism and impaired renalexcretion. The process may be favoured by the low levels of circulating and locally produced VCinhibitors. VC determines increased arterial stiffness, left ventricular hypertrophy, a decreasein coronary artery perfusion, myocardial ischaemia and increased cardiovascular morbidity andmortality. Although current therapeutic strategies focus on the correction of phosphate, calcium,parathyroid hormone or vitamin D, a better understanding of the mechanisms of abnormal tissuecalcification may lead to development of new therapeutic agents, which could reduce VC andimprove cardiovascular outcome in CKD patients. The present review summarizes the followingaspects: (i) the pathophysiological mechanism responsible for VC and its promoters and inhibitors,(ii) the methods for detection of VC in patients with CKD, including evaluation of arterial stiffness,and (iii) the management of VC in CKD patients.

INTRODUCTION

VC (vascular calcification) is very common in CKD(chronic kidney disease), becoming more prevalentwith the worsening of kidney function and withCKD duration. VC is associated with many adverse

clinical outcomes, including ischaemic cardiac eventsand subsequent vascular mortality [1]. Pathogenesis ofVC is complex, and it does not consist of a simpleprecipitation of calcium and phosphate, but is insteadan active process in which VSMCs (vascular smoothmuscle cells) undergo apoptosis and vesicle formation

Key words: arterial stiffness, calcimimetic, haemodialysis, hyperphosphataemia, non-calcium phosphate binder, vitamin D.Abbreviations: CAC, coronary arterial calcification; Ca×P, calcium-phosphate product; CBPB, calcium-based phosphate binder;CKD, chronic kidney disease; CT, computer tomography; CYP27B1, cytochrome P450, family 27, subfamily B, polypeptide 1;EBCT, electron beam CT; FGF-23, fibroblast growth factor-23; HD, haemodialysis; HDL, high-density lipoprotein; IS, indoxylsulfate; LDL, low-density lipoprotein; MGP, matrix GIa protein; MSCT, multi-slice spiral CT; OPG, osteoprotegerin; PPi,pyrophosphate; PTH, parathyroid hormone; PWV, pulse wave velocity; Runx2, Runt-related transcription factor 2; SHPT, secondaryhyperparathyroidism; TNAP, tissue non-specific alkaline phosphatase; VC, vascular calcification; VDR, vitamin D receptor; VSMC,vascular smooth muscle cell;.Correspondence: Professor Adrian Covic (email adrianccovic@gmail.com).

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Figure 1 Concentric VC in the coronary artery territories of an 83-year-old Type 2 diabetic women on long-term HDThe images were obtained using EBCT. This Figure was reproduced from Nephrology Dialysis and Transplantation, vol. 19 (9), pp. 2307–2312, 2004, Coronary arterycalcification is related to coronary atherosclerosis in chronic renal disease patients: a study comparing EBCT-generated coronary artery calcium scores and coronaryangiography, with permission from Oxford University Press. c© (2004) European Renal Association-European Dialysis and Transplant Association.

and are transformed into osteoblast-like cells, inducingmatrix formation and also attracting local factors thatare involved in the mineralization process [2]. VC isusually seen in aging, after vascular injury and in variousclinical conditions, such as diabetes, atherosclerosis andMonckeburg’s medial sclerosis. However, there is nodoubt that CKD patients have a high risk for and a highprevalence of VC because of multiple risk factors thatinduce the phenotypic transformation of VSMCs intoosteoblast-like cells capable of the tissue mineralizationprocess [3]. VC has been associated with numerous‘traditional’ risk factors such as aging, hypertension,diabetes or dyslipidaemia, as well as with ‘non-traditional’ risk factors, such as hyperphosphataemia,hyperparathyroidism, hypervitaminosis D or excessadministration of calcium salts [4]. The haemodynamicconsequences of VC include a loss of arterial elasticity, anincrease in PWV (pulse wave velocity) [5], developmentof left ventricular hypertrophy [6], decrease in coronaryartery perfusion and myocardial ischaemia. In addition,VC in iliac and femoral vessels can prejudice orcompromise renal transplantation. At the site of vessel

injury, such as needle sites in arteriovenous fistulae, VCcan compromise patency and function. An example ofdense concentric VC in all coronary artery territoriesof an 83-year-old Type 2 diabetic woman on long-term HD (haemodialysis) is shown in Figure 1. Currentstrategies employed towards delaying VC are focused onthe correction of mineral metabolism markers of bonedisease, such as phosphate, calcium, PTH (parathyroidhormone) and vitamin D. The use of agents such asbisphosphonates and cinacalcet show much initial prom-ise, but further clinical data are urgently required ([7]and http://clinicaltrials.gov/ct2/show/NCT00379899).Cutting-edge scientific research on the mechanismsunderlying VC is increasingly being undertaken andfurther insight into these mechanisms may lead to thedevelopment of several types of therapeutic agents, whichcould improve cardiovascular outcome in CKD patients.The present review will summarize current knowledgeregarding the pathogenic determinants and inhibitors ofVC, methods for assessing VC, and management of VC inCKD patients, all aimed at evaluating the potential thera-peutical impact of recent progress in understanding VC.

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PATHOPHYSIOLOGICAL MECHANISMS OFVC, ITS PROMOTERS AND INHIBITORS

The pathophysiology of vascular disease in CKD isincreasingly recognized to be distinct from that relatedto atherosclerosis in the general population [8]. Initially,VC was viewed as a passive phenomenon; however,it has been subsequently recognized as an active cell-mediated process [9,10]. VC may occur in either theintimal layer or the medial layer of the vessel wall,e.g. Monckeberg’s sclerosis, which is very common inCKD patients [11]. It is not yet clear to what extentthese two patterns of VC might overlap in termsof pathogenetic mechanism, but both are associatedwith increased mortality in CKD patients [12]. Intimalcalcification is focal, associated with inflammation andthe development of plaques and occlusive lesions, whileadjacent regions of the vessel wall may remain remarkablynormal. This form of calcification represents an advancedstage of atherosclerosis and is seen in the aorta, coronaryarteries and other large vessels [9]. Medial calcification,characterized by diffuse mineral deposition throughoutthe vascular tree, can occur completely independent ofatherosclerosis or alongside, and is commonly observedin muscle-type conduit arteries, such as femoral, tibialand uterine arteries [6,10]. The two forms of calcificationmay well co-exist in the same vessel, which could be evenmore detrimental.

The mechanisms underlying accelerated VC in CKDare not completely understood. It is well recognizedthat in patients with CKD changes in the arterialwall, fibro-elastic intimal thickening, calcification ofelastic lamellae, increased extracellular matrix and morecollagen deposition with relatively less elastic fibrecontent all cause arterial remodelling [13]. In this process,many bone-associated proteins [including osteocalcin,osteopontin and OPG (osteoprotegerin)] and manybone morphogenetic proteins are involved; they areexpressed in calcified arterial lesions and are associatedwith VC [14]. VSMCs are the major component ofthe medial arterial layer and, in the setting of CKD,can differentiate into chondrocyte- or osteoblast-likecells, by up-regulation of transcription factors such asRunx2 (Runt-related transcription factor 2) and Msx2(Msh homeobox 2), which are critical factors for normalbone development [15]. This phenotypic switch maylead to a calcified smooth muscle, in a process similarto bone formation. In other words, this pattern of VCis actually an ectopic ossification. Moreover, uraemiainduces differentiation of VSMCs into an osteoblast-like phenotype, and also inhibits the differentiation ofmonocyte macrophages into osteoclasts [15]. Dialysisvessels also show increased alkaline phosphatase activityand Runx2 and Osx (Osterix) expression, indicativeof VSMC osteogenic transformation [16]. Furthermore,chronic vascular injury secondary to chronic volume

overload, high blood pressure and an overactiverenin–angiotensin system may cause smooth muscleproliferation, medial hyperplasia and, eventually, therecruitment of osteoblast-like cells into the vessel wallsin CKD patients.

Determinants of VCNumerous risk factors have been reported for VC.Several are ‘traditional’, such as increasing age,hypertension, diabetes or dyslipidaemia; a numberof other, ‘non-traditional’ factors, such as mineralmetabolism abnormalities, extreme PTH serum levels,excess administration of calcium salts, inflammation,malnutrition and oxidative stress have been described inCKD patients [1].

Phosphate and calcium metabolismAdvanced CKD patients develop hyperphosphataemiadue to impaired renal phosphate excretion [17]. Thereis strong evidence that VC is closely associated withserum levels of calcium, phosphate and Ca×P (calcium-phosphate product) [6,18,19]. High serum phosphatelevels might be considered as a ‘vascular toxin’ [20].Clinical studies have shown that patients with the worstphosphate control have the most rapid progression ofVC [21]. Two different mechanisms are proposed toexplain the relationship between calcium and phosphatedisorders and VC: (i) a passive one, the directcalcium-phosphate precipitation in the vasculature, and(ii) an active one, that induces the expression ofbone-associated genes in VSMCs, which acquire thephenotype of bone-forming (osteoblast-like) cells (seeabove) [22]. Experimental studies have demonstratedthat calcium plays a role in the development of VCby stimulating mineralization of VSMCs under normalphosphate conditions [22]. Furthermore, with elevatedphosphate levels, this calcium-driven mineralization isaccelerated synergistically [22,23]. Hyperphosphataemiamay directly induce vascular injury and indirectlystimulates osteoblastic differentiation through a type IIIsodium-dependent phosphate co-transporter (PiT-1)[24]. Jono et al. [25] suggested that elevated intracellularphosphate may directly stimulate VSMCs to transforminto calcifying cells by activating genes associatedwith osteoblastic functions [24]. Additionally, in animportant recent paper [26], Giachelli’s group fromSeattle reported a good animal model of CKD-relatedVC (calcification-prone DBA/2 mouse); using varyinglevels of renoprivation, this model then allowed thedevelopment of mild-to-severe VC as a result of differentdiets. The extensive arterial calcification only developsonce the animals are on a high-phosphate diet, suggestingthat hyperphosphataemia is a marked accelerator of thisprocess. These findings provide strong evidence thathyperphosphataemia and calcium load are probably themost important pathogenetic factors in VC. Nevertheless,

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a recent study [27] showed a high prevalence (83 %) ofradiological VC in HD patients, even in patients withgood control of hyperphosphataemia, indicating thatphosphate is not the only factor in the pathogenesis ofVC.

PTHSHPT (secondary hyperparathyroidism), occurring evenat early stages of the disease, is a common finding in CKDpatients [19]. The progression of CKD is associated withdisorders of mineral metabolism (hyperphosphataemiaand hypocalcaemia), leading to the development of SHPT,characterized by increased PTH and parathyroid glandhyperplasia [17]. High PTH levels are responsible forthe enhanced number and activity of osteoclasts, being amajor contributor to increased bone resorption in CKD[17]. As this process increases in severity, mesenchymalcells are activated and differentiate into fibroblast-likecells which form fibrous tissue, and fibrosis develops inthe marrow space.

Vitamin DA significant decrease in serum vitamin D is observedfrom the early stages of CKD [28] as result of renaland non-renal factors, including reduced sun exposure,impaired production of the 25-hydroxy vitamin Dprecursor molecule and reduced dietary intake [17,29].As vitamin D deficiency progresses in CKD, theparathyroid glands are maximally stimulated, causingSHPT [17]. Recently, an association between vitaminD deficiency and increased cardiovascular morbidityand mortality, including increased VC and stiffness, wasdescribed [30]. Vitamin D increases the gastrointestinalabsorption of calcium and phosphate, and also induces theproliferation and osteoblastic differentiation of VSMCs.Moreover, 1,25-dihydroxy vitamin D has been shown tooperate as a negative hormonal regulator of the renin–angiotensin system, which plays an important role inthe cardiovascular system by modulating volume andelectrolyte homoeostasis [31].Vitamin D deficiency mightalso be associated with blood levels of inflammatoryfactors, including TNF-α (tumour necrosis factor-α) andIL-10 (interleukin-10) [32]. Although it has been reportedthat administration of supra-physiological doses of 1,25-dihydroxy vitamin D induces VC [33], physiologicaldoses are protective against aortic calcification in animalmodels of CKD [34]. Schroff et al. [35] showed thatthe vitamin D level is the most important predictor ofincreased arterial thickness, stiffness and calcification inchildren on dialysis, a population where atheroscleroticlesions are minimal. VSMC drop out by apoptosis wasalso shown in the arteries of uraemic children [35].

FGF-23 (fibroblast growth factor-23)FGF-23, a novel hormone produced by osteoblasts, isinvolved in the regulation of phosphate and vitamin

D metabolism [36]. FGF-23 level rises in CKD fromearly stages and causes renal phosphate loss by inhibitingNPT2a (sodium/phosphate co-transporter type IIa) inthe renal proximal tubule. It also suppresses the renalexpression of CYP27B1 (cytochrome P450, family 27,subfamily B, polypeptide 1), resulting in the impairmentof 1,25-dihydroxy vitamin D synthesis [37,38]. FGF-23 binds to its receptor via α-Klotho, a pleiotropictransmembrane protein expressed in the kidney [39].There is also evidence that FGF-23 also controls bonemineralization independently of phosphate homoeostasis[40]. Reduced FGF-23 activity is associated with vascularand soft tissue calcification in both experimental animalsand human studies [41]. Extensive vascular and soft tissuecalcification is observed in FGF-23-knockout mice by6 weeks of age; small-and medium-sized arteries and theproximal tubules in the kidneys are the most extensivelyaffected sites, in addition to the aorta [42]. In accordancewith the animal studies, human diseases associated withinactivating mutations in either FGF-23 or Klotho geneexpress severe ectopic calcification. FGF-23 can reducecalcification by inhibiting vitamin D activity [43]. On theother hand, Cozzolino et al. [44] reported that, in CKDpatients undergoing dialysis, extensive VC is present,despite their significantly high serum FGF-23 levels.Gutierrez et al. [37] reported that increased FGF-23 levelsare independently associated with mortality in newlystarted HD patients. Finally, in a recent study, Roos et al.[38] found no correlation between serum intact FGF-23and coronary artery score in subjects with normal kidneyfunction. These findings suggest that, under normal renalfunction, where the kidneys are effectively maintaininga normal phosphate balance, FGF-23 is not a suitablemarker for coronary artery calcification. Further studiesare needed to better understand the role of FGF-23in vascular and soft tissue calcification under variouspathological conditions.

IS (indoxyl sulfate)IS a protein-bound uraemic toxin, resulting fromdietary tryptophan, which is metabolized into indoleby intestinal bacteria and, after intestinal absorption, isconverted further into IS in the liver [45]. Normallythe kidneys excrete IS via proximal tubular secretionbut, because of impaired renal function, IS accumulatesin CKD patients [45,46]. Additionally, IS cannot beremoved efficiently by conventional HD because ofits high affinity for albumin [45]. Several studies haveproposed that IS may play a role in VC. Yamamoto etal. [47] showed for the first time that IS can stimulatethe proliferation of rat VSMCs. It was subsequentlydemonstrated that IS induces aortic calcification, withaortic wall thickening and the expression of osteoblast-specific proteins, in a rat model of hypertension [48].It was also shown that IS induces oxidative stress bymodifying the balance between pro- and anti-oxidant

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mechanisms in endothelial cells [49]. Finally, it has beenshown that serum IS levels increase as renal failureprogresses, and they are closely associated with aorticcalcification in CKD patients [49].

To summarize, there are many factors that play a rolein the development of VC. Some of them are unique todialysis patients, whereas some are also present in thegeneral population. Current knowledge does not offerthe answers to important questions: which is the mostimportant contributing factor, how does each risk factorexactly work, how do they interact with each other, andat what stage can we stop the process?

Inhibitors of VCAlthough VC is very common in CKD patients, notall patients will develop VC, despite similar exposureto the uraemic environment, suggesting that protectivemechanisms also exist.

Extracellular calcium-regulatory proteins: fetuin-A and MGP(matrix GIa protein)Fetuin-A is mostly synthesized in the liver, andcirculating concentrations fall during the cellularimmunity phase of inflammation [50]. Fetuin A isan extracellular calcium-regulatory protein acting asa potent inhibitor of calcium-phosphate precipitation[51], inhibits calcification by binding hydroxyapatitestructures [52] and it protects VSMCs from thedetrimental effects of calcium overload and subsequentcalcification [53]. Thus fetuin-A inhibits VSMC apoptosisby perturbing death-signalling pathways: (i) it isinternalized by VSMCs and concentrated in intracellularvesicles and it is secreted via vesicle release fromapoptotic and viable VSMCs; (ii) the presence offetuin-A in vesicles abrogates their ability to nucleatebasic calcium phosphate; and (iii) in addition, fetuin-A enhances phagocytosis of vesicles by VSMCs. Theseobservations provide evidence that the uptake of theserum protein fetuin-A by VSMCs is a key event inthe inhibition of vesicle-mediated VSMC calcification[53]. In vitro, fetuin-A antagonizes the antiproliferativeaction of TGF-β1 (transforming growth factor-β1) andblocks osteogenesis and deposition of calcium-containingmatrix in dexamethosone-treated rat bone marrow cells[51]. Fetuin-A-knockout mice develop extensive ectopiccalcifications in the myocardium, kidney, lung, tongueand skin [51]. Ketteler et al. [51] showed that CKDpatients with lower serum fetuin-A levels have increasedmortality due to cardiovascular events, suggesting thatfetuin-A is involved in preventing the accelerated extra-skeletal calcification.

MGP, a small ubiquitous matrix protein, initiallyisolated from bone [54], is a key regulator of VC.To achieve full biological activity, MPG needs to beactivated and this depends on the availability of vitamin

K [55]. Studies have demonstrated that MGP inhibitscalcification of cartilage and blood vessels [56]. MGPexerts its effects on VC directly, via inhibition ofcalcium crystal formation, and indirectly, by influencingtranscription factors that inhibit VSMC differentiation tothe osteoblast-like phenotype [57]. MGP also appearsto be an important factor in ensuring correctdifferentiation of VSMCs [56]. It has been shown thatdeclining GFR (glomerular filtration rate) results in adecreased uncarboxylated MGP level which is associatedwith VC and atherosclerosis [58].

OPGOPG forms a system with the receptor activator of NF-κB (nuclear factor κ-light-chain-enhancer of activatedB-cells), called RANK, and the ligand of this receptor(e.g. RANKL). This system may play a role in bone–VC imbalance and could be a marker of VC extentand progression. In a recent study, Morena et al. [59]found that, in CKD patients, CAC (coronary arterialcalcification) is strongly associated with plasma OPG:values of OPG >757.7 pg/ml were predictive of thepresence of CAC in these patients. The mechanism bywhich OPG levels might be related to CAC is unknown.OPG is recognized as a protective factor for vascularcalcium deposition in animal models [60]. Surprisingly,higher levels of OPG have been reported in patientswith vascular damage [61], suggesting that an increase inOPG level may represent a compensatory self-defensivemechanism against factors promoting VC, atherosclerosisand other forms of vascular damage [61].

Vitamin KVitamin K is an essential nutrient and a cofactor inthe production of coagulation factors, osteocalcin andMPG. Vitamin K contributes to bone health, probablythrough its role as a cofactor in the carboxylation ofosteocalcin. The efficiency of vitamin K in preventingfractures is controversial, but it has proven to be effectivein preventing fractures in postmenopausal women [62].In a meta-analytical review, Iwamoto et al. [63] showedthat vitamin K supplementation has inconsistent effectson serum total osteocalcin levels, with a modest increasein bone mineral density, but it improves indices of bonestrength in the femoral neck and reduces the incidence ofclinical fractures. In the Rotterdam trial, low vitamin Kintake was associated with a higher incidence of severeaortic calcification [64]. In rodent studies, the therapeuticadministration of vitamin K2 resulted in reduced VC[65]. Intervention studies in humans showed increasedosteocalcin and MPG after vitamin K administration. Ina pilot study in 50 dialysis patients, Ketteler et al. [51]demonstrated that the daily administration of vitaminK causes a significant increase in MPG and osteocalcin.Although these studies are encouraging, further largerclinical trials are needed.

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PPi (pyrophosphate)PPi is a potent inhibitor of medial VC, directly blockinghydroxyapatite formation [66]. PPi is produced byarterial smooth muscle and its level is controlledby hydrolysis via a TNAP (tissue non-specific alkalinephosphatase), which hydrolyses PPi and generatesphosphate [67]. A deficiency of the ectoenzyme thatsynthesizes extracellular PPi results in massive arterialcalcification in mice and humans [68]. In HD patients,plasma PPi is deficient; these lower circulating levels ofPPi in HD patients are not understood, but could resultfrom higher hydrolysis (an increase in TNAP activity andPPi hydrolysis was demonstrated in uraemic rats) [69]and from dialytic clearance (PPi is removed during theHD session). In a recent study, O’Neill et al. [70] foundthat plasma PPi is negatively associated with VC in end-stage renal disease. Taken together, these studies show aninhibitory effect of PPi on VC, but further studies areneeded to establish a causal role.

METHODS FOR ASSESSING VC

A number of non-invasive imaging techniques areavailable to screen for the presence of VC: plain X-raysto identify macroscopic calcifications of aorta andperipheral arteries; two-dimensional ultrasound forcalcification of carotid arteries, femoral arteries and aorta;echocardiography for assessment of valvular calcification;and, of course, CT (computer tomography) technologiesthat constitute the gold standard for quantification ofcoronary artery and aorta calcification.

EBCT (electron beam CT) and MSCT(multi-slice spiral CT)EBCT and the newer MSCT are highly sensitivemethods, assessing accurately and quantitatively, es-pecially coronary artery calcification, by using anelectrocardiographic trigger for heart imaging only indiastole, thus avoiding motion artifacts [71]. Thesemethods could be successfully used to study prevalentcalcifications, progressive VC and the impact of therapyon VC [72]. EBCT is not readily available in manyhospitals. In contrast, almost every hospital has a multi-purpose spiral CT and, with software adjustments toallow gated imaging, the newer faster spiral CTs canassess VC. However, there are conflicting results aboutthe correlation between the severity of CAC measured byEBCT and subsequent clinical cardiac events in dialysispatients [17,73,74]. This can be explained by the fact thatthe arterial calcification score generated by CT scanningis a composite of both medial and intimal calcification.This is a limitation of these CT-based imaging techniques,as they are unable to distinguish between the twopredominant arterial calcification sites [17]. EBCT and

MSCT could be also used for assessment of VC in theaorta [75].

Conventional CTConventional CT may be used to evaluate non-coronaryVC, especially aortic calcifications. Measuring theproportion of aortic circumference showing calcificationcan generate an ACI (aortic calcification index). Thismethod seems to be simple, relatively inexpensive anduseful for an initial diagnosis of VC. Taniwaki et al. [76]used this method for the quantification of VC in HDpatients with diabetes mellitus. Again, this method couldnot quantify the medial/intimal distribution of VC.

Latero-abdominal plain radiographyLatero-abdominal plain radiography is a valuable andinexpensive tool for the detection of VC in CKD patients,but the method is semi-quantitative, possibly missingsubtle changes in the evolution of VC. It is the onlytechnique for the detection of VC included in theKDIGO (Kidney Disease: Improving Global Outcomes)guidelines for cardiovascular disease in dialysis patients[77]. The pattern of VC seen on plain radiographsmay yield some information about the localization ofcalcification within the arterial wall (intima comparedwith media). Kauppila et al. [78] used lateral lumbar filmsto detect the presence of calcification in the abdominalaortic wall, in the region corresponding to the first to thefourth lumbar vertebrae. This semi-quantitative methodis more widely available and less expensive for studyingcalcification and could be used for cardiovascular riskstratification.

Ultrasound-based methodsUltrasound-based methods indirectly evaluate calciumcontent and are typically used to study carotid arterycalcification. Ultrasound uses a universally availabletechnology, is relatively inexpensive and does not requireexposure to ionizing radiation. This method has twosignificant limitations: first, it is unable to differentiatemedial from intimal calcification; and secondly, dataderived from ultrasound are qualitative and are unlikelyto detect small changes, at least over the shorterterm. Using transthoracic echocardiography, the valvularcalcification could be quantified. In a very recent study,Leskinen et al. [79] found that valvular calcification inCKD patients is associated with increased carotid intima-media thickness, carotid plaque, coronary artery diseaseand peripheral arterial disease.

Arterial stiffnessArterial stiffness and increased PWV are induced by VC[80]. PWV measured in large elastic arteries could beanother indirect method for the quantification of VC.A positive correlation between VC and arterial stiffnessmeasured by PWV has been demonstrated in both

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early CKD and HD patients [80–82]. Several possiblemechanisms for the association between arterial stiffnessand VC can be hypothesized. First, arterial calcificationmay induce arterial wall stiffness and increased PWV[81,83]. A previous study in adult HD patients reportedan association between 25-hydroxy vitamin D deficiencyand arterial stiffness [84]. Secondly, increased arterialstiffness may cause vessel wall damage and atherosclerosis[85]. Thirdly, changes in the intrinsic properties of arterialwall by arterial remodelling may contribute to bothprocesses in CKD patients [14].

MANAGEMENT OF VC

Non-calcium phosphate bindersHyperphosphataemia is involved in SHPT, vascularevents, cardiovascular mortality and all-cause mor-tality. Phosphate binders currently used to managehyperphosphataemia include sevelamer, lanthanum andthe CBPBs (calcium-based phosphate binders) CaCO3

and calcium acetate. Sevelamer is an aluminum- andcalcium-free phosphate binder, which does not promotehypercalcaemia, allows a better serum phosphoruscontrol compared with CBPBs, suppresses the pro-gression of aortic calcification in HD patients and hasfavourable effects on the lipid profile, with an associatedreduction in LDL (low-density lipoprotein)-cholesteroland an increase in HDL (high-density lipoprotein)-cholesterol [86]. In a comparative study including 200HD patients, Chertow et al. [87] demonstrated thatsevelamer attenuates the progression of coronary andaortic calcification better than CBPBs after 1 year. Thesefindings were confirmed by Cozzolino et al. [88], whoshowed that treatment with sevelamer, when comparedwith CaCO3, is associated with less VC within themyocardium, aorta and kidney. The probable mechanismconsists of a strong phosphate-binding capacity in theintestine, without excessive calcium loading. In contrast,in the RIND (Renagel in New Dialysis) study, in patientswith baseline CAC scores of 30 or higher, there wasno significant difference in the rates of calcificationprogression between the patients treated with sevelamerand those treated with CBPBs, at any point up to18 months of follow-up [89]. The improvement inlipid metabolism could be another factor contributingto decreased VC. In vitro studies have shown thatacetylated LDL stimulates, whereas HDL inhibits,VSMC calcification [90]. In human studies, sevelamer hasbeen shown to consistently reduce LDL and frequentlyincrease HDL levels. Such an improved lipid profilecould potentially play a role in the lower degree of VCseen after sevelamer treatment; however, in the CARE-2(Calcium Acetate Renagel Evaluation-2) study, intensiveLDL-cholesterol-lowering therapy with atorvastatin isassociated with a similar progression of CAC in HD

patients treated with either calcium acetate or sevelamer[91].

VDR (vitamin D receptor) activatorsVDR activators are an essential part of treatment amongstage 5 CKD patients. Different VDR activators exertdifferential effects on VC. Cardus et al. [92] showed inuraemic rats that paricalcitol and calcitriol had differentialeffects on VC. Although both drugs raised serumcalcium and Ca×P, only calcitriol caused an increasein the calcification of the abdominal aorta [92]. Hirataet al. [93] found similar results in 5/6 nephrectomizedrats, and demonstrated that 1,25-dihydroxy vitamin D3,but not 22-oxacalcitriol, induced the calcification ofthe aorta. Noonan et al. [94] found that paricalcitoland doxercalciferol display differential effects on aorticcalcification in vivo, which is independent of serumcalcium, phosphate and Ca×P. The mechanism by whichVDR activation inhibits aortic calcification seems to bethe inhibition of osteoblastic gene expression in thevessels. In addition, VDR activators stimulated osteoblastfunction instead of suppressing it and increased osteoblastsurfaces and bone formation rates, contributing to theirprotection against aortic calcification [95,96].

CalcimimeticsBecause calcimimetics reduce PTH levels without theinduction of hypercalcaemia, it is likely that patients whoare treated with a calcimimetic may show less risk for VCthan patients who are treated with vitamin D sterols. Ina rat model of SHPT (5/6 nephrectomy), Henley et al.[33] showed that calcitriol-treated rats have moderate-to-marked aortic calcification, whereas no significantcalcification was observed in cinacalcet-treated groups.Lopez et al. [97] studied the effect of the calcimimeticR-568 alone or in combination with calcitriol on thedevelopment of VC and other soft tissue calcificationsin a rat model of uraemia-associated SHPT. They showedthat the calcimimetic R-568 reduces PTH levels withoutinducing VC and can also attenuate the calcitriol-inducedcalcifying effects on vascular tissue and decrease mortalityassociated with calcitriol. They concluded that, in uraemicrats, R-568 reduces elevated PTH levels without inducingVC and prevents calcitriol-induced VC [97]. The mostvalid explanation is related to the control of PTH levelwithout increasing Ca×P.

BisphosphonatesBisphosphonates may have a future role in themanagement of VC, as they have been shown to reduceVC in experimental models. Tamura et al. [98] showed in5/6 nephrectomized rats that aortic calcification inducedby calcitriol could be reduced by etidronate. Initially,they demonstrated that low-dose etidronate (2 mg/kg ofbody weight) was ineffective, but a dose of 5–10 mg/kgof body weight inhibited calcification. In another study

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using BASMCs (bovine aortic smooth muscle cells),pamidronate inhibited arterial calcification [99]. In HDpatients, etidronate reduced and even reversed theprogression of CAC in some, but not all, of the patientsafter 6 months [100] and 12 months [101] of treatment.However, the mechanism is not clear. Bisphosphonatescould inhibit bone resorption, with reduced efflux ofcalcium and phosphate, limiting their availability fordeposition in the vasculature [102], or could influencethe activity of the sodium/phosphate co-transporter inVSMCs. Alternatively, bisphosphonates may have directeffects on the vessel wall and, like pyrophosphate, oncrystal formation [102].

CONCLUSIONS

Vascular disease is the most important cause of morbidityand mortality among patients with CKD, while VC iscommon and is nearly ubiquitous in these patients andprogresses rapidly. It can severely prejudice therapeuticoptions for patients with CKD and is therefore somethingwe cannot view with equanimity. VC is an active process,involving multiple risk factors with additive effects,which has a profound influence on arterial stiffnessand cardiovascular function. Recent cell biology studieshave provided novel findings about the important roleof the VSMCs in the complex process of ectopic bonedevelopment (calcified matrix) in CKD patients. Plasmaphosphate remains in our view the key clinical driverof the process. If this is valid, urgent efforts shouldbe made to achieve rigorous phosphate control withoutcompromising nutrition. New strategies may improve themanagement of vascular diseases, possibly with a specificpositive impact on the high prevalence of VC in CKD.These are encouraging, but we need solid epidemiologicaland clinical data to underpin a better management ofthe bone–cardiovascular axis in CKD patients. The besttherapeutical strategy available remains the control ofaltered bone metabolism and inflammation.

FUNDING

This work was supported by Romanian Ministry ofEducation and Research [grant number ID-1156; 2007-2010], via Consiliul National al Cercetarii Stiintifice dinInvatamantul Superior (CNCSIS) and Unitatea Executivapentru Finantarea Invatamantului Superior si a CercetariiStiintifice Universitare (UEFISCSU) [plan PN2, programIDEI, section PCE].

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Received 3 December 2009/4 February 2010; accepted 2 March 2010Published on the Internet 28 April 2010, doi:10.1042/CS20090631

C© The Authors Journal compilation C© 2010 Biochemical Society