Bioengineering Department, University of Washington, Seattle, Washington.
Correspondence to Dr. Cecilia M. Giachelli, Bioengineering Department, Box 351720, University of Washington, Okanogan Lane, Bagley Hall, Seattle, WA 98195. Phone: 206-543-0205; Fax: 206-616-9763; E-mail: ceci{at}u.washington.edu
Vascular calcification is highly correlated with cardiovasculardisease mortality, especially in patients with ESRD or diabetes.In addition to the devastating effects of inappropriate biomineralizationseen in cardiac valvulopathies, calciphylaxis, and idiopathicarterial calcification, vascular calcification is now recognizedas a marker of atherosclerotic plaque burden as well as a majorcontributor to loss of arterial compliance and increased pulsepressure seen with age, diabetes, and renal insufficiency. Inrecent years, several mechanisms to explain vascular calcificationhave been identified including (1) loss of inhibition, (2) inductionof bone formation, (3) circulating nucleational complexes, and(4) cell death. Alterations in calcium (Ca) and phosphorus (P)balance as seen in patients with ESRD promotes vascular calcificationvia multiple mechanisms and may explain the alarmingly highlevels of cardiovascular disease deaths in these patients. Strategiesto control Ca and P levels in patients with ESRD have met withearly success in preventing progression of vascular calcification.Whether or not vascular calcification can be reversed is notyet known, but exciting new studies suggest that this may bepossible in the future.
Pathologic calcification of cardiovascular structures, or vascularcalcification, is associated with a number of diseases includingESRD and cardiovascular disease. Calcium phosphate deposition,in the form of bioapatite, is the hallmark of vascular calcificationand can occur in the blood vessels, myocardium, and cardiacvalves. In blood vessels, calcified deposits are found in distinctlayers of the blood vessel and are related to underlying pathology.Intimal calcification occurs in atherosclerotic lesions (1,2),whereas medial calcification (also known as Monckebergsmedial sclerosis) is associated with vascular stiffening andarteriosclerosis observed with age, diabetes, and ESRD (3,4).Intimal calcification may occur independently of medial calcificationand vice versa. In patients with ESRD, a mixture of intimaland medial calcification has been observed in affected vessels(5,6).
Clinical Consequences of Vascular Calcification
Vascular calcification can lead to devastating organ dysfunctiondepending on its extent and the organ affected. In the heart,calcification of cardiac valve leaflets is recognized as a majormode of failure of native as well as bioprosthetic valves (7,8).In dialysis patients, vascular medial calcification is responsiblefor calcific uremic arteriolopathy, a necrotizing skin conditionassociated with extremely high mortality rates (9). Finally,a genetic deficiency in pyrophosphate levels causes idiopathicinfantile arterial calcification, a disease characterized byarterial calcification, fibrosis, and stenosis that leads topremature death in affected neonates (10).
In contrast, calcification of blood vessels commonly seen withaging, ESRD, diabetes, and atherosclerosis has historicallybeen considered a benign finding. However, the introductionof new techniques to measure vascular calcification noninvasively,such as electron beam computed tomography, have revolutionizedour current thinking about the risks of vascular calcification.In coronary arteries, calcification is positively correlatedwith atherosclerotic plaque burden (11,12), increased risk ofmyocardial infarction (1315), and plaque instability(2,16). Although some of these findings may relate to the correlationof coronary calcification with extent of underlying atheroscleroticdisease, it is also possible that vascular calcification itselfmay contribute to initiation or progression of cardiovasculardisease (CVD). This possibility seems particularly plausiblein the case of coronary calcification associated with ESRD (seebelow). Finally, vascular calcification, especially that foundin the media of large arteries, leads to increased stiffeningand therefore decreased compliance of these vessels. The consequentloss of the important cushioning function of these arteriesis associated with increased arterial pulse wave velocity andpulse pressure, and leads to impaired arterial distensibility,increased afterload favoring left ventricular hypertrophy, andcompromised coronary perfusion (17,18). Indeed, medial arterialcalcification is strongly correlated with coronary artery diseaseand future cardiovascular events in patients with type 1 (19,20)and is a strong prognostic marker of CVD mortality in patientswith ESRD (21). Thus, vascular calcification has a profoundinfluence on cardiovascular function and health.
Cardiovascular Calcification and CVD Mortality in ESRD
More than half the deaths in patients with ESRD are due to CVD.In fact, the risk of CVD mortality in adult patients with ESRDis 20 to 30 times higher than that of the general population(22). Growing evidence suggests that this increased risk ofCVD mortality may be partly explained by the predispositionof this population to vascular calcification. Hyperphosphatemiaand elevated Ca x P ion product (Ca x P; prevalent in patientswith ESRD) promote vascular calcification, and are significantlylinked to all cause and CVD mortality in patients with ESRD(23). In a landmark study, Goodman et al. (24) found that coronaryartery calcification occurred in young patients with ESRD decadesbefore this pathology was observed in the normal population.Furthermore, progression of vascular calcification in this groupwas positively correlated with serum P levels, Ca x P, and dailyintake of Ca (24). Similar findings were observed by Eifingeret al. (25) as well as Oh et al. (26) in young adults with childhood-onsetESRD. In addition, Raggi et al. (27) found that coronary arterycalcification was very common and severe in adult hemodialysispatients, and significantly correlated with ischemic CVD. Again,calcification was highly correlated with elevated serum Ca andP levels. Finally, arterial medial calcification, a strong prognosticmarker for CVD mortality in patients with ESRD, was also associatedwith elevated serum Ca, P, Ca x P, and prescribed Ca intake(18,21). Thus derangements in Ca and P balance are now consideredmajor nontraditional risk factors for CVD in ESRD.
Mechanisms of Vascular Calcification
Vascular calcification is currently considered an actively regulatedprocess that may arise by several different, nonmutuallyexclusive mechanisms (28). As shown in Figure 1, four differentmechanisms for initiating vascular calcification have been proposed.First, human and mouse genetic findings have determined thatblood vessels normally express inhibitors of mineralization,such as pyrophosphate and matrix gla protein, respectively,and that lack of these molecules ("loss of inhibition") leadsto spontaneous vascular calcification and increased mortality(10,29). Likewise, fetuin/2-HS-glycoprotein is a major inhibitorof apatite found in the circulation, and decreased fetuin levelshave recently been correlated with elevated CVD mortality inhemodialysis patients (30). Second, the presence of bone proteinssuch as osteopontin (31), osteocalcin (32), and BMP2 (33), matrixvesicles (34), and outright bone and cartilage formation incalcified vascular lesions (1,35) has suggested that osteogenicmechanisms may also play a role in vascular calcification. Indeed,cells derived from the vascular media undergo bone- and cartilage-likephenotypic change and calcification in vitro under various conditions,and is discussed in further detail below (3640). Third,bone turnover leading to release of circulating nucleationalcomplexes has been proposed to explain the link between vascularcalcification and osteoporosis in postmenopausal women (4143).Fourth, cell death can provide phospholipid-rich membranousdebris and apoptotic bodies that may serve to nucleate apatite,especially in diseases where necrosis and apoptosis are prevalent,such as atherosclerosis (34,44,45). Finally, via thermodynamicmechanisms (sometimes referred to as "passive" mechanisms),elevated Ca, P, and Ca x P promote apatite nucleation and crystalgrowth and would be expected to exacerbate vascular calcificationinitiated by any of the other mechanisms described above. Furthermore,new evidence suggests that Ca and P may additionally have directeffects on vascular cells that predispose to mineralization,as described below.
Figure 1. Schematic illustrating four, nonmutually exclusive theories for vascular calcification: (1) loss of inhibition as a result of deficiency of constitutively expressed tissue-derived and circulating mineralization inhibitors leads to default apatite deposition, (2) induction of bone formation resulting from altered differentiation of vascular smooth muscle or stem cells (3), circulating nucleational complexes released from actively remodeling bone, and (4) cell death leading to release of apoptotic bodies and/or necrotic debris that may serve to nucleate apatite at sites of injury. Figure reprinted from (28) with permission from Elsevier.
Roles of Ca and P in Vascular Smooth Muscle Cell Mineralization
As indicated above, elevated serum P, Ca, and increased Ca burdenare correlated with vascular calcification and cardiovascularmortality in patients with ESRD. To determine whether elevatedCa or P directly affect cell-mediated regulation of vascularcalcification, we and others have made use of vascular smoothmuscle cell culture systems. Increasing inorganic P in the culturemedia to those seen in hyperphosphatemia (>2.4 mM) leadsto deposition of apatite into the extracellular matrix surroundingsthe cells (37,4648). Concomitant with mineralization,the cells undergo a phenotypic change characterized by lossof smooth muscle specific gene expression and upregulation ofgenes commonly associated with bone differentiation includingosteocalcin, osteopontin and Runx2. Similar phenotypic changeshave also been observed in vivo in human as well as animal modelsof vascular calcification (46,49,50). Elevated P-induced phenotypictransition and mineralization were shown to be dependent ona sodium-dependent phosphate cotransporter, Pit-1, on the basisof their ability to be inhibited by phosphonoformic acid (37)and Pit-1 specific small interfering RNA (Giachelli and Li,unpublished data). These data confirm the importance of inorganicP as a signaling molecule with the ability to initiate bothphenotypic change and mineralization in vascular smooth musclecells.
Likewise, elevating Ca levels in the culture media to levelsconsidered hypercalcemic (>2.6 mM) leads to enhanced mineralizationand phenotypic transition of vascular smooth muscle cells (51).Elevated calcium-induced mineralization was also dependent onthe function of a sodium-dependent phosphate cotransporter.Although elevated Ca did not appear to increase P uptake acutely,prolonged exposure of smooth muscle cell cultures to elevatedCa induced Pit-1 mRNA levels, suggesting that elevated Ca regulatesP sensitivity of vascular smooth muscle cells. These findingswere recently confirmed and extended by Proudfoot et al. (52).In those studies, elevated Ca (1.8 to 5.0 mM) stimulated humansmooth muscle cell calcification in vitro. Furthermore, elevatedCa levels stimulated release of mineralization-competent matrixvesicles from human smooth muscle cells, and diltiazem and BAPTA,both inhibitors of intracellular Ca influx, blocked smooth musclecell mineralization. A rise in intracellular Ca was also associatedwith altered alkaline phosphatase, decreased matrix Gla protein(MGP), and increased fetuin levels. Together with our own, thesefindings confirm that elevated Ca has pro-mineralizing effectsbeyond simply raising the Ca x P, and regulates multiple systemsin smooth muscle cells that promote susceptibility to matrixmineralization. A diagram summarizing the possible roles ofelevated Ca and P on vascular smooth muscle calcification isshown in Figure 2.
Figure 2. Proposed model for the effects of elevated Ca and P on vascular smooth muscle cell (SMC) matrix mineralization. Elevated Ca and P are proposed to stimulate vascular matrix mineralization in two ways. First, both Ca and P increase the activity of Pit-1: elevated P stimulates P uptake via Pit-1, and elevated Ca induces expression of Pit-1 mRNA. Both mechanisms are proposed to enhance P uptake into SMC as well as matrix vesicles. Elevated intracellular P than leads to SMC phenotypic modulation, which includes upregulation of osteogenic genes (Runx2, osteocalcin, and alkaline phosphatase), and generation of a mineralization-competent extracellular matrix. In addition, increased Pit-1 in matrix vesicles promotes P loading of matrix vesicles, promoting nucleation of mineral within the extracellular matrix. Second, elevated Ca and/or P lead to increased Ca x P ion product, thereby promoting growth of apatite crystals in the matrix via thermodynamic mechanisms.
Can Vascular Calcification Be Controlled?
Several recent studies suggest that vascular calcification maybe slowed, and potentially even reversed, in humans as wellas experimental animal models. In light of the findings thatelevated serum P and Ca are strongly correlated with vascularcalcification and CVD mortality in ESRD, an emphasis has beenplaced on the use of nonCa-containing P binders, suchas sevelamer, to treat hyperphosphatemia in these patients (53).Indeed, when sevelamer was compared to commonly used Ca-basedP binders in a large hemodialysis patient group it was foundthat patients receiving sevelamer had unchanged median coronaryartery and aorta calcification scores after 1 yr as opposedto Ca-treated patients whose arterial calcification scores increased28% over baseline (12). Significantly, although both treatmentscontrolled P levels equivalently, treatment with Ca containingbinders led to an increased frequency of hypercalcemic episodesand greater suppression of serum parathyroid hormone (PTH) levelsin hemodialysis patients (54,55). Similar effects of sevelamerwere also noted in a rat uremia model, where renal calcificationwas greatly reduced compared to Ca carbonate treatment (56).Thus, decreasing Ca and P burdens appear to be beneficial inblocking vascular calcification.
Antihypertensive agents have also been implicated in controlof vascular calcification. In a clinical study, the Ca channelblocker nifedipine slowed progression of coronary calcificationin hypertensive patients compared to diuretics (57). Moreover,Moreaus group has developed a novel animal model of isolatedsystolic hypertension that is caused by arterial calcification(58,59). In this model, rats are treated with warfarin and vitaminK and this leads to calcification of the aortic wall and isolatedsystolic hypertension. Treatment with an endothelin (ET-1) receptorantagonist or angiotensin II blocker prevents increases in pulsepressure as well as calcification of the vessels. Remarkably,treatment of rats with ET-1 antagonist after calcification wasestablished caused regression of vascular calcification andnormalization of pulse pressure. These data support the ideathat calcification of compliance vessels leads to hypertensiveeffects, especially increased pulse pressure as a result ofincreased stiffness, and identify ET-1 antagonists as majorregulators of vascular calcification. Furthermore, these datasuggest that regression of vascular calcification can occurand is actively regulated. Indeed, evidence for regression ofcoronary calcification in humans has also been obtained. Ina study of 102 symptomatic patients with coronary calcificationcurrently being treated with standard medications, 15% of patientsshowed evidence of regression on the basis of electron beamcomputed tomography calcium scores measured at baseline andapproximately 6 mo later (60).
Vascular calcification is highly correlated with CVD morbidityand mortality, especially in high-risk populations such patientswith ESRD or diabetes. Four nonmutually exclusive mechanismsfor vascular calcification are emerging, including (1) lossof inhibition, (2) induction of bone formation, (3) circulatingnucleational complexes, and (4) cell death. Derangements inCa and P metabolism effect all of these mechanisms by elevatingthe Ca x P, and also by direct effects on smooth muscle cellsthat promote bonelike differentiation. The challenge remainsto understand which mechanisms are active and/or predominateunder various disease states, and to develop effective therapeuticstrategies that may prevent and potentially reverse vascularcalcification.
Acknowledgments
Dr. Giachellis research is supported by NIH grants HL62329,AR48798, NSF grant EEC9529161, and a grant from Genzyme.
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