Balancing protein turnover in the heart
Alterations in the ubiquitin proteasome system (UPS), which tags proteins for degradation, underlies some cardiomyopathies and age-related cardiac dysfunction.
In the heart, the UPS is essential for the precise balance between cardiomyocyte atrophy and hypertrophy. In skeletal muscle, the E3 ubiquitin ligase atrogin-1 promotes atrophy by targeting hypertrophy-associated proteins for degradation; however, a role for atrogin-1 in cardiac proteostasis is not clear.
In this issue of the Journal of Clinical Investigation, Marco Sandri, Marco Mongillo and colleagues at the Venetian Institute of Molecular Medicine investigated cardiac homeostasis in atrogin-1-deficient mice.
Aged animals lacking atrogin-1 exhibited enlarged and abnormally shaped hearts, thickened left ventricular walls, increased fibrosis and apoptosis, and reduced function.
Loss of atrogin-1 led to increased ER stress and upregulation of genes involved in the unfolded protein response (UPR). Moreover, autophagy, which is inhibited by the UPR in skeletal muscle, was also repressed in cardiac tissue of atrogin-1 null animals, suggesting that atrogin-1 may target autophagy-related proteins for degradation.
Evaluation of cardiac protein turnover in atrogin-1-deficient animals revealed reduced turnover of lysosomal targets, including the endosomal sorting complex III protein CHMP2B.
Silencing CHMP2B in atrogin-1 null mice restored markers of autophagy and reduced apoptosis in the heart. These results indicated that UPS-regulated autophagy in the protein turnover necessary for cardiomyocyte health.