Novel Mechanisms Contributing to Fibrosis and Matrix Remodeling in Myxomatous Mitral Valve Disease

Abstract

Myxomatous mitral valve disease is characterized by fibrosis and matrix degradation and increases in transforming growth factor beta‐1 (TGFbeta‐1). Little is known, however, about changes in molecules that regulate TGFbeta‐1 signaling in myxomatous valves. Thus, we tested the hypothesis that TGFbeta‐1 promotes valvular fibrosis and matrix remodeling in myxomatous valves through canonical Smad2 signaling and permissive reductions in Smad inhibitors. Expression of TGFbeta‐1, TGFbeta‐1 target genes (connective tissue growth factor, matrix metalloproteinases 2/9), E3 ubiquitin‐protein ligases (Smurf 1/2, which degrade Smad2), and NAD(P)H oxidase catalytic subunits (Nox 2, 4, and 5) was measured using quantitative real‐time RT‐PCR. Phospho‐Smad2 levels were measured using western blotting. TGFbeta‐1 expression was higher in myxomatous valves (138.3 ± 12.5% of control, n = 24 per group; p < 0.05), while CTGF and MMP2 were increased by 5‐ and 2‐fold, respectively. Surprisingly, phospho‐Smad2 levels were unchanged between myxomatous and control tissue due to selective increases in Smurf2 expression in myxomatous valve tissue (165.2 ± 17.7% of control, p < 0.05), as Smurf1 expression was unchanged. Expression of Nox2 and Nox4—which can drive pro‐fibrotic and matrix remodeling gene expression in other tissues—were significantly elevated in myxomatous valves (211 ± 44% and 224 ± 35 % of control, respectively; p < 0.05). In conclusion, our data suggest that canonical TGFbeta‐1/Smad2 signaling is not required for progression of myxomatous valve disease, and that reducing Nox2‐and Nox4‐derived reactive oxygen species may be a novel intervention to slow progression of fibrosis and matrix remodeling in myxomatous valves.