Background: The ubiquitin-proteasome system (UPS) breaks down misfolded and normal proteins, including cell cycle regulatory proteins involved in cardiac hypertrophy. Because congestive heart failure (CHF) increases cardiomyocyte cellular mass, indicative of increased protein synthesis and/or impaired breakdown, and ventricular unloading decreases cardiac hypertrophy and changes regulation of multiple molecular systems ("reverse cardiac remodeling"), we tested the hypothesis that ventricular unloading alters myocardial UPS.
Methods: In 23 paired myocardial specimens (before and after unloading) ubiquitin, 20S proteasome, and cyclin D1 were investigated immunohistochemically and morphometrically quantified in relation to cardiomyocyte hypertrophy, DNA content, nuclear profile area and perimeter, and cyclin D1 protein expression. Moreover, 20S proteasome plasma concentrations were measured by enzyme-linked immunoassay (ELISA).
Results: In CHF, sarcoplasmic 20S proteasome protein expression was significantly decreased compared with controls, but significantly increased after unloading. In contrast, sarcoplasmic ubiquitin protein was increased in CHF but significantly decreased after unloading, and both variables were inversely correlated. Cardiomyocyte 20S proteasome expression correlated inversely with cell size, mean DNA content, and cyclin D1, whereas ubiquitin protein expression was positively correlated with these parameters. The 20S proteasome plasma concentration was significantly increased after unloading.
Conclusions: Our data indicate that: (1) the UPS is depressed in CHF; and (2) this is reversed by ventricular unloading and associated with decreased cardiomyocyte hypertrophy, mean DNA content, and cell cycle regulatory proteins. The findings support the view that the UPS is involved in both the pathogenesis of cardiac hypertrophy and "reverse cardiac remodeling" after ventricular unloading.
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