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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

Update on the pathophysiology of aortic stenosis

Nalini M. Rajamannan*

Division of Cardiology, Department of Pathology, Bluhm Cardiovascular Institute, Feinberg School of Medicine, Northwestern University, 300 East Chicago Avenue, Tarry 12-717, Chicago, IL 60611, USA

* Corresponding author. Tel: +1 312 695 4965; fax: +1 312 695 5774. E-mail address: n-rajamannan{at}northwestern.edu


   Abstract
Top
 Abstract
Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
The objective of this review is to explain the basis for the current appreciation of the role of active atherosclerotic and bone-forming processes in the pathophysiology of calcific aortic stenosis (AS). Relevant epidemiological, histopathological, and experimental studies are reviewed. Calcific AS and atherosclerosis share similar risk factors, including elevated LDL cholesterol levels, hypertension, male gender, smoking, and diabetes mellitus. Both atherosclerosis and calcific AS often occur in individuals with familial hypercholesterolaemia, and elderly patients with AS are also at an increased risk of myocardial infarction and death from cardiovascular causes. Lipoprotein deposition is prominent in both conditions. Aortic valve mineralisation occurs as the result of mechanisms similar to those active in bone formation, including upregulation of the Lrp5 pathway and the presence of an osteoblast phenotype in calcified valves. An active atherosclerosis-type pathophysiology involving oxidative stress, inflammation, and endothelial dysfunction in aortic valves has been induced by hypercholesterolaemia, and inhibited by administering HMG-CoA reductase inhibitors (statins), in animal models. Evidence of endothelial dysfunction with neovascularisation and an increase in endothelial macrophages and metalloproteinases has been found in surgically excised human valves with calcific AS. The initiating factors and pathophysiology of calcific AS are broadly similar to those of atherosclerosis of coronary and other arteries. The sclerosis and calcification of aortic valves may therefore be addressed with pharmacological therapies that are effective in slowing or reversing atherosclerosis.

Key Words: Aortic valve stenosis • Pathophysiology • Atherosclerosis • Calcification • Endothelial dysfunction


   Introduction
Top
 Abstract
 Introduction
Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
Calcific aortic stenosis (AS) is the most common valve disorder in the western world.1 Calcification of an aortic valve was once thought to be a largely passive process, with serum calcium attaching to the surface of a valve leaflet, forming a nodule. Consistent with this understanding, surgical aortic valve replacement (AVR) has been a mainstay of treatment for calcific AS with outflow obstruction2 and remains the primary treatment modality recommended for symptomatic severe calcific AS.3,4 Calcific AS is the chief indication for surgical valve replacement3 and, in the USA, the number of such procedures has been increasing in recent years, with 16 330 isolated AVRs and 14 976 AVRs in conjunction with coronary artery bypass procedures reported by the Society of Thoracic Surgeons in 2006.5

However, emerging epidemiological, histopathological, and experimental evidence indicates that calcification of an aortic valve is an active rather than a passive biological process within the valve leaflet, causing regulated bone formation. Recent advances have underscored the parallels of this active process to coronary atherosclerosis. The present article reviews the evidence for an atherosclerotic basis to aortic valve calcification and stenosis.


   Epidemiological evidence
Top
 Abstract
 Introduction
 Epidemiological evidence
Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
Similarities in risk factors strongly suggest that a similar process underlies both the development and progression of calcific AS as well as atherosclerosis. Risk factors shared by calcific AS and atherosclerosis include hypertension, elevated LDL cholesterol, male gender, smoking, and diabetes mellitus.6–11 Epidemiological evidence consistent with these similarities includes the occurrence of valvular heart disease in patients with homozygous familial hypercholesterolaemia, a condition characterised by profoundly elevated LDL cholesterol and premature development of atherosclerosis,12,13 as well as the connection between aortic valve sclerosis and cardiovascular mortality and morbidity in elderly patients; there is an ~50% increased risk of myocardial infarction and death from cardiovascular causes in patients with AS, even among individuals without left-ventricular outflow obstruction.14

Palta et al.8 investigated the clinical, echocardiographic, and biochemical characteristics that might bear on the rate of progression of AS in a group of 170 consecutive patients who had paired echocardiograms ≥3 months (23 ± 11) apart. Baseline values were left-ventricular outflow tract (LVOT) velocity, 0.9 ± 0.2 m/s; peak aortic velocity, 2.7 ± 0.07 m/s; and aortic valve area (AVA) 1.17 ± 0.38 cm2. The annual reduction in AVA was significantly related to the initial AVA (r = 0.46, P < 0.0001), mean aortic valve gradient (r = –0.27, P = 0.04), LVOT velocity (r = 0.26, P = 0.001), and LV end-diastolic diameter (r = 0.20, P = 0.04) and marginally related to the baseline serum creatinine level (r = 0.15, P = 0.08). Patients with a serum cholesterol level >5.18 mmol/L (200 mg/dL) had an AVA reduction rate that was about two times as high as that of patients with lower cholesterol levels (P = 0.04). The investigators concluded that, in patients with AS, increased levels of serum cholesterol, creatinine, and calcium, as well as current smoking, accelerate the reduction in AVA.


   Histopathological evidence
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
Important histopathological evidence that calcific AS unfolds in ways similar to those of atherosclerosis was reported by O’Brien et al.,15 who found that apolipoproteins B and E are present in early as well as advanced aortic valve lesions but not in normal valve regions; a similar pattern of lipoprotein deposition also occurs in coronary atherosclerosis.

Evidence that the aortic valve is associated with an active cellular biology was shown in our case report of the appearance of lipid-rich plaque in an aortic valve leaflet, coronary artery, and aorta of a 7-year-old boy, who was born in 1949 with congenital hyperlipidaemia and multiple prominent skin xanthomas.13 His serum lipid levels included a total cholesterol of 24.6 mmol/L (951 mg/dL); cholesterol esters 17.2 mmol/L (666 mg/dL); and fatty acids 36.6 mmol/L (1,032 mg/dL). At this time, he was treated with a low-animal-fat diet, because the use of tight lipid-lowering treatment regimens was not a common practice. The child had recurrent asthma and, in 1956, had an acute onset of cyanosis and cardiac arrhythmia. He was hospitalised but suddenly became apnoeic and died.

Our post-mortem study of the heart in 2003, and examination of tissues by light microscopy after staining, showed lipid-rich plaque with foam cells (similar to findings observed in coronary atherosclerosis) and collagen deposition along the aortic valve leaflet surface (Figure 1).13 The left circumflex artery was severely narrowed, with evidence of atherosclerosis and acute thrombosis, which was judged to be the likely cause of death. The thoracic aorta also showed evidence of atherosclerosis. The coexistence of these findings in a patient with familial hypercholesterolaemia suggested the possibility of a common underlying pathophysiological disease mechanism for vascular and aortic valve disease.


Figure 1
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  		Figure 1 Aortic valve, circumflex coronary artery, and thoracic aorta of a 7-year-old boy. The aortic valve cusp contains atheromatous plaque (arrow) along its aortic aspect (x20 magnification; top panels). The circumflex coronary artery is severely stenotic (x12 magnification; middle panels). The thoracic aorta contains a shallow intimal plaque (arrowhead; x12 magnification). Reprinted with permission from N Engl J Med.13 Copyright © 2003 Massachusetts Medical Society. All rights reserved.

 

   Experimental evidence
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
Several studies have helped to explain the process by which aortic valves become calcified. Mohler et al.16 demonstrated that osteopontin, a protein in bone matrix that regulates the deposition of calcium, is found in minimally and severely calcified aortic valves and located mainly in the calcified areas (Figure 2).


Figure 2
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  		Figure 2 Association of osteopontin and minimal valve calcification in a congenital bicuspid aortic valve that was replaced. Radiograph (A) of calcific deposits (bright white regions at the tip and base of the left leaflet); stained (B; alizarin red S) section of the calcified tip; low-power (C) and high-power (D) photomicrographs of the stained (antiosteopontin antibody) calcified tip. Reprinted with permission from Mohler et al.16 Copyright © 1997 Lippincott Williams & Wilkins.

 
Additional evidence that aortic valve mineralisation is an active biological process, similar to that occurring in bone, derives from our study of surgically removed valves, which demonstrated an osteoblast phenotype. As shown by reverse transcription–polymerase chain reaction (RT–PCR), increased transcription of genes for osteopontin, bone sialoprotein, osteocalcin, bone-specific transcriptional factor Cbfa1, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (i.e. increased mRNA levels) was evident in calcified (vs. normal) aortic valves (Figure 3).17


Figure 3
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  		Figure 3 Phenotype of calcified aortic valve obtained with reverse transcription–polymerase chain reaction. There was an increase in mRNA expression for all markers shown, except alkaline phosphatase. Norm, normal aortic valve; Calc, calcified aortic valve; GAPDH, glyceraldehyde-3-phosphate dehydrogenase. Reprinted with permission from Circulation.17 Copyright © 2003 Lippincott Williams & Wilkins.

 
In addition to increased gene transcription, certain signal transduction mechanisms that are upregulated in bone formation also seem to be activated in mineralising aortic valve leaflets. In particular, LDL receptor-related protein 5 (Lrp5), osteocalcin, and other bone markers are upregulated in calcified tricuspid and bicuspid aortic valves.18 In the presence of hypercholesterolaemia, the Lrp5 pathway seems to be activated in mesenchymal myofibroblast cells within the valve, switching them to a bone-producing phenotype. This ‘phenotypic switch’ occurs through the binding of Lrp5 to the glycoprotein Wnt, which in turn activates β-catenin to induce the formation of bone. Upregulation of osteogenic bone signalling markers may be expressed as cartilage formation in human myxomatous (degenerative) mitral valves and calcification in bicuspid and tricuspid valves (Figure 4).18


Figure 4
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  		Figure 4 Calcification and cartilage in human degenerative (myxomatous) mitral valves and calcified tricuspid and bicuspid aortic valves (x25 magnification). (A) Alizarin red stain; (B) alcian blue stain; (C) microCT scan; white area is calcification. Reprinted from J Am Coll Cardiol.18 Copyright © 2006 with permission from the American College of Cardiology as published by Elsevier.

 
We conducted a study to determine whether Lrp5 plays a role in cellular proliferation and osteoblastogenesis in aortic valves of rabbits rendered hypercholesterolaemic, and whether these processes might be affected by administration of a statin.19 Three groups of 18 Watanabe rabbits each were studied: Group I received a normal diet, Group II a 0.25% cholesterol diet, and Group III a 0.25% cholesterol diet with atorvastatin. The rabbits’ aortic valves were examined for cellular proliferation, Lrp5/β-catenin as an indicator of Wnt pathway activation, and bone matrix markers. The cholesterol-enriched diet resulted in bone formation in the calcified valves, with increased expression of Lrp5 receptors, osteopontin, and p42/44 (an indicator of cellular proliferation). In this study, administration of atorvastatin reduced bone formation, cellular proliferation, and Lrp5/β-catenin protein levels in the valves, which suggested that hypercholesterolaemic aortic valve calcification in rabbits may be mediated by activation of the Lrp5/β-catenin signal-transduction mechanism.

We also investigated whether apoptosis (programmed cell death) contribution is relevant as a mechanism of aortic valve degeneration.20 New Zealand white rabbits were fed either a 1% cholesterol diet (n = 8) or a normal diet (n = 8) for 12 weeks, after which their aortic valves were dissected. Apoptosis was identified in valvular lesions by the TUNEL (TdT-mediated dUTP-biotin nick end-labelling) technique, which was verified with electron microscopy. Computed morphometry was used to determine the number of apoptotic cells. Valves from hypercholesterolaemic rabbits showed increased apoptosis in their atherosclerotic layer compared with valves of control rabbits (P < 0.0001). Because apoptosis is increased in rabbit aortic valves during experimental hypercholesterolaemia, this process may contribute to the pathogenesis of valvular disease.

Weiss et al.21 published the first report of a predisposition towards spontaneous development of the full aortic valvular heart disease syndrome in a laboratory animal. This group showed that senescent mice with moderate hypercholesterolaemia were prone to develop AS and valvular heart disease, and that oxidative stress played a role in the development of the disease. The animal model used was the genetic knockout mouse, which lacks the gene for the LDL receptor (LDLr) and expresses only the receptor for the human apolipoprotein B-100 (LDLr–/–ApoB100/100). This same genotype has been associated with the development of large human aortic intimal atheromata. Von Kossa staining of the aortic valve showed mineralisation in LDLr–/–ApoB100/100 mice but not in control mice. Superoxide (shown as oxyethidium fluorescence), evidence of oxidative stress, was present in greater concentration in the LDLr–/–ApoB100/100 mice with AS than in those without.

Further evidence of endothelial dysfunction with oxidative stress and inflammation in the pathogenesis of cholesterol-initiated AS was provided by another of our studies of hypercholesterolaemic rabbits.22 In this study, we assessed the effects of atorvastatin on endothelial nitric oxide synthase (eNOS) expression, serum nitrite concentration, and aortic valve calcification. Three groups of 16 rabbits each were fed a normal diet, a high-cholesterol (0.5%) diet, or a high-cholesterol diet plus atorvastatin for 3 months, after which valves were examined using eNOS immunostains and western blotting. Results showed lower serum concentrations of nitrite and higher serum concentrations of the inflammation marker high-sensitivity C-reactive protein in hypercholesterolaemic compared with normocholesterolaemic rabbits. Bone mineralisation was present in the aortic valves of the hypercholesterolaemic animals. Atorvastatin administration was associated with a reduction in oxidative stress, as indicated by higher serum nitrite concentrations in atorvastatin-treated rabbits (vs. those fed a high-cholesterol diet without statin administration).

Charest et al.23 reported further evidence of endothelial dysfunction associated with degenerative AS in human valves that had been surgically removed. In their study of 30 valves with AS and 20 normal aortic valves, investigators found normal valves to be avascular, whereas valves with degenerative AS contained blood vessels in specific areas. The secreted protein, acidic and rich in cysteine/osteonectin (SPARC), which is a protein involved in ossification, angiogenesis, and metalloproteinase production, was present in AS valves in mature blood vessels and in macrophages.

To assess whether aortic valve atherosclerosis and early bone matrix expression occur secondary to experimental hypercholesterolaemia and that treatment with a statin can modify this transformation, we examined 16 male New Zealand white rabbits fed a normal diet (control), 16 fed a diet supplemented with cholesterol (cholesterol-fed animals), and 16 cholesterol-fed animals treated with atorvastatin for 8 weeks.24 Osteoblast bone markers, including one for osteopontin, were determined by RT–PCR. Rabbits rendered hypercholesterolaemic developed a proliferative atherosclerosis-like process in the aortic valve, with increased staining for macrophages as well as increased expression of osteopontin bone matrix expression; these were reduced in animals receiving atorvastatin (Figure 5).24


Figure 5
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  		Figure 5 Macrophage (upper row left) and osteopontin (lower row left) production shown by light microscopy (x12.5 magnification). In each panel, the aortic valve leaflet is to the left of the aorta. Upper right row shows RT–PCR with the total RNA from aortic valves for osteopontin, and lower row right shows these results normalised to GAPDH. Atorv, atorvastatin; Chol, cholesterol-fed; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; OPN, osteopontin; RT–PCR, reverse transcription–polymerase chain reaction. Reprinted with permission from Circulation.24 Copyright © 2002 Lippincott Williams & Wilkins.

 
Currently, there are a number of ongoing clinical trials testing the hypothesis that medical therapy can slow the progression of aortic valve disease. The most recent published study is RAAVE (Rosuvastatin Affecting Aortic Valve Endothelium), from Porto Portugal, which determined that earlier treatment with a statin is more efficacious in the prevention of progression of aortic valve stenosis than late treatment.25


   First clinical evidence
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
In the RAAVE trial, we studied the prospective treatment of AS with rosuvastatin targeting serum LDL cholesterol. This treatment slowed progression of AS according to echocardiographic haemodynamic measurements and improved inflammatory biomarkers, providing initial clinical evidence for targeted therapy in patients with asymptomatic AS. The study’s aim was to assess the effects of rosuvastatin on the haemodynamic progression and inflammatory markers of AS by treating LDL cholesterol in patients with AS according to the National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III) guidelines26 for 1 year. The RAAVE investigators performed an open-label, prospective study evaluating 118 consecutive patients with asymptomatic moderate AS (age 73.5 ± 8.8 years; 56 men and 62 women) with and without rosuvastatin according to the NCEP ATP III guidelines.26 Echocardiographic parameters and serum markers for high-sensitivity C-reactive protein, interleukin-6 (IL-6), and CD40 were measured at baseline and every 6 months for 1 year. A total of 65 patients (55.1%) with an elevated LDL cholesterol (162 ± 35.7 mg/dL), aortic valve velocity (AVV) of 3.62 ± 0.63 m/s, and AVA of 1.11 ± 0.38 cm2 received rosuvastatin 20 mg/day, compared with 53 (44.9%) patients having a normal LDL cholesterol (108.4 ± 20.9 mg/dL), an AVV of 3.64 ± 0.61 m/s, and an AVA of 1.10 ± 0.37 cm2 who did not receive a statin. During a mean follow-up of 53 ± 3.2 weeks, the decrease in AVA was 0.11 ± 0.18 cm2 in the non-statin group compared with 0.04 ± 0.13 cm2 in the statin treatment group (P = 0.059). The change in peak AVV was 0.18 ± 0.26 m/s in the statin vs. 0.04 ± 0.29 m/s in the non-statin treatment group (P = 0.016). There were statistically significant improvements in all serum markers in the statin group. Prospective treatment of moderate AS with rosuvastatin targeting serum LDL cholesterol slowed progression of peak AVV and improved inflammatory biomarkers, providing the first clinical evidence for targeted therapy in asymptomatic moderate-to-severe AS.25 The first prospective randomised study of statins for AS published (SALTIRE: Scottish Aortic Stenosis and Lipid Lowering Trial) found that high-dose atorvastatin did not halt or significantly slow disease progression (or induce disease regression) compared with placebo; however, patients had evidence of advanced AS at baseline.27 Other studies, such as the Simvastatin and Ezetimibe in Aortic Stenosis (SEAS),28 ASTRONOMER (Aortic Stenosis Progression Observation Measuring Effects of Rosuvastatin),29 and STOP-AS30 trials, are further evaluating the effects of statins and other lipid-modifying therapies in patients with aortic valve disease.


   Summary
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 
Calcific AS remains the leading indication for surgical valve replacement. The pathophysiology of calcific AS is no longer considered to be a largely passive process but is rather understood to be an active one, with initiating factors and mechanisms of progression that are broadly similar to those of atherosclerosis in coronary and other arteries. Risk factors shared by calcific AS and atherosclerosis include hypertension, elevated LDL cholesterol, male gender, smoking, and diabetes mellitus. Calcific AS often develops in persons with atherosclerosis, as indicated (especially in the elderly) by an increased risk of deaths from cardiovascular disease in patients with calcific AS. In addition, calcific AS and atherosclerosis may coexist in persons with homozygous familial hypercholesterolaemia.

Animal studies have shown that, as in atherosclerosis, experimental hypercholesterolaemia initiates endothelial dysfunction in aortic valves, with upregulation of oxidative stress and inflammatory processes leading to plaque formation, as well as a switch to an osteogenic phenotype with valve mineralisation. Evidence for the presence of endothelial dysfunction in the development of calcific AS has been demonstrated in human valves that have been removed surgically. In animal studies of AS, administration of a statin blunted hypercholesterolaemia-induced valve mineralisation and cellular proliferation. This background on AS pathophysiology helps to set the stage for potential benefits of LDL cholesterol-lowering therapy in humans, with the hope that medical therapy may be able to prevent or delay the need for valve replacement.


   Clinical implications
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
Conclusions
 Funding
 Acknowledgements
 References
 
Hypercholesterolaemia may be an important factor in the initiation and progression of AS, and statins may inhibit the induction of this process in animals. If the human aortic valve undergoes a pathophysiological process initiated by hypercholesterolaemia with oxidative modification of cholesterol, as suggested by the studies described earlier, medical therapy might help to decelerate AS progression.


   Conclusions
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
Funding
 Acknowledgements
 References
 
The leading indication for surgical valve replacement is calcific AS. Once considered a passive process, the pathophysiology of calcific AS is now understood to be active, with initiating factors and mechanisms of progression that are broadly similar to those of atherosclerosis in coronary and other arteries. Animal studies have shown that experimental hypercholesterolaemia initiates endothelial dysfunction in valves, with upregulation of oxidative stress and inflammatory processes leading to plaque formation and bone-like valve mineralisation. Evidence of endothelial dysfunction in the development of calcific AS in humans has been obtained through the study of surgically excised aortic valves. Appreciation that calcific AS reflects the results of active biological processes has led to the consideration that these processes may be amenable to elimination or slowing by pharmacotherapeutic intervention, potentially avoiding or at least postponing the need for valve replacement.


   Funding
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
Acknowledgements
 References
 
This work was completed with the support of an American Heart Association Grant-in-Aid (0555714Z) and a grant from the National Institute of Health (5K08HL073927-04, 1R01HL085591-01A1). The author is an inventor on a patent for the use of statins in the degeneration of aortic valve disease. This patent is owned by the Mayo Clinic, and the author does not receive any royalties from this patent. This work was received from the Merck/Schering-Plough Joint Venture.


   Acknowledgements
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
References
 
Assistance in the preparation of the manuscript was provided by Rete Biomedical Communications Corp. (Ridgewood, NJ, USA).

Conflict of interest: none declared.


   References
Top
 Abstract
 Introduction
 Epidemiological evidence
 Histopathological evidence
 Experimental evidence
 First clinical evidence
 Summary
 Clinical implications
 Conclusions
 Funding
 Acknowledgements
 References
 

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