Lowering Plasma Cholesterol Levels Halts Progression of Aortic Valve Disease in Mice

Abstract

Background-Treatment of hyperlipidemia produces functional and structural improvements in atherosclerotic vessels.However, the effects of treating hyperlipidemia on the structure and function of the aortic valve have been controversial, and any effects could be confounded by pleiotropic effects of hypolipidemic treatment. The goal of this study was to determine whether reducing elevated plasma lipid levels with a "genetic switch" in Reversa mice (LdlrϪ/Ϫ/Apob ) reduces oxidative stress, reduces pro-osteogenic signaling, and retards the progression of aortic valve disease. Methods and Results-After 6 months of hypercholesterolemia, Reversa mice exhibited increases in superoxide, lipid deposition, myofibroblast activation, calcium deposition, and pro-osteogenic protein expression in the aortic valve. Maximum aortic valve cusp separation, as judged by echocardiography, was not altered. During an additional 6 months of hypercholesterolemia, superoxide levels, valvular lipid deposition, and myofibroblast activation remained elevated. Furthermore, calcium deposition and pro-osteogenic gene expression became more pronounced, and the aortic cusp separation decreased from 0.85Ϯ0.04 to 0.70Ϯ0.04 mm (meanϮSE; PϽ0.05). Rapid normalization of cholesterol levels at 6 months of age (by inducing expression of Cre recombinase) normalized aortic valve superoxide levels, decreased myofibroblast activation, reduced valvular calcium burden, suppressed pro-osteogenic signaling cascades, and prevented reductions in aortic valve cusp separation. Conclusions-Collectively, these data indicate that reducing plasma lipid levels by genetic inactivation of the mttp gene in hypercholesterolemic mice with early aortic valve disease normalizes oxidative stress, reduces pro-osteogenic signaling, and halts the progression of aortic valve stenosis. Key Words: aortic valve stenosis Ⅲ calcification Ⅲ free radicals Ⅲ hypercholesterolemia Ⅲ valves R eplacement of the aortic valve is the primary treatment for patients with symptomatic calcific aortic valve stenosis and is the second-most-common thoracic surgery procedure in the United States. 1 Risk factors for the development of aortic valve stenosis are similar to those of atherosclerosis and include older age, 2 male sex, hypertension, smoking, diabetes mellitus, 1,2 and hypercholesterolemia. 2 Stenotic aortic valves resemble atherosclerotic lesions pathologically and contain calcium, 3 high levels of matrix-remodeling enzymes, 4 -6 reduced endothelial nitric oxide synthase levels, 7 and increased oxidative stress. 8 -10 Both stenotic valves and atherosclerotic lesions contain a subpopulation of cells with osteoblast-like activity, [11][12][13][14] suggesting that deposition of calcium in these lesions is an active process. Editorial see p 2653 Clinical Perspective on p 2701Reducing plasma cholesterol levels in humans and experimental animals slows the progression and/or reduces the size of atherosclerotic lesions, 15,16 reduces oxidative stress, 17 and improves nitric oxide bioavailability, 15,18 ...

Discovery of an Experimental Model of Unicuspid Aortic Valve

“…Immunostaining for the procalcific transcription factor Runx2 was minimal or absent in Velvet valves and control valves. α‐SMA, which is a marker for valve interstitial cell transdifferentiation to myofibroblasts,16 was minimal or absent in Velvet aortic valves and control valves (Figure 6E through 6H).…”

“…Runx2 staining is minimal or absent. H, Immunostains from a Reversa valve, which is known to undergo valve interstitial cell transdifferentiation to myofibroblasts (α‐ SMA ) and osteogenic transdifferentiation (Runx2),16 serve as “positive control.” Black bar=500 μm. White bar=100 μm.…”

“…But, at this preliminary stage, we are able to discount some mechanisms that are present in other forms of aortic valve disease. In an established mouse model of acquired AS, valve interstitial cells undergo transdifferentiation into myofibroblasts, which express α‐SMA and produce interstitial collagen 16. Myofibroblasts may then transdifferentiate further into osteoblast‐like cells, which produce matrix calcification 24.…”

Calcific Aortic Valve Disease

“…They also show increases in valve thickness, macrophage accumulation, activated myofibroblasts and osteoblasts, and ectopic mineralization (Matsumoto et al, 2010). An expansion of this model, the Reversa mouse, is achieved by inhibiting apolipoprotein B (ApoB) 100 and incorporating a conditional knockout of the microsomal triglyceride transfer www.intechopen.com protein (Mttp) under the control of an inducible Mx1-Cre+/+ gene (LDLr -/-, ApoB100/100, Mttp fl/fl, Mx1-Cre +/+ ) (Miller et al, 2009). Fed a high-cholesterol diet, these mice develop calcific aortic stenosis in 6 months; profibrotic signalling, myofibroblast activation, and procalcific signalling are also observed (Miller et al, 2009, Miller et al, 2010.…”

“…An expansion of this model, the Reversa mouse, is achieved by inhibiting apolipoprotein B (ApoB) 100 and incorporating a conditional knockout of the microsomal triglyceride transfer www.intechopen.com protein (Mttp) under the control of an inducible Mx1-Cre+/+ gene (LDLr -/-, ApoB100/100, Mttp fl/fl, Mx1-Cre +/+ ) (Miller et al, 2009). Fed a high-cholesterol diet, these mice develop calcific aortic stenosis in 6 months; profibrotic signalling, myofibroblast activation, and procalcific signalling are also observed (Miller et al, 2009, Miller et al, 2010. Our recent studies have induced chronic renal disease (CRD) in ApoE -/-mice, to cause accelerated ectopic calcification (Aikawa et al, 2009.…”