Abstract 1770: Proteasome Switching is Associated with Adverse Cardiac Remodeling in a Transgenic Mouse Model of Sustained Inflammatory Signaling

Abstract

The 26S proteasome possess proteolytic activity and deubiquitinating (DUB) activity of ubiquitin tagged proteins. Whereas the proteolytic activity of the 26S proteasome facilitates protein degradation, proteasome DUB activity spares proteins from degradation by shortening the length of the ubiquitin chains, thereby preventing proteins from being degraded by the 26S proteasome. In yeast, increased DUB activity is beneficial by preventing depletion of ubiquitin pools critical for cell signaling. However, the role of DUB activity in mammalian systems is not known. We have shown that mice with cardiac restricted overexpression of tumor necrosis factor (sTNF mice) develop a heart failure phenotype characterized by progressive left ventricular (LV) remodeling and accumulation of pro-apoptotic proteins, including Smac/Diablo. To determine whether the adverse LV remodeling in sTNF mice was related to alterations in DUB activity we measured the cleavage of ubiquitin-AMC, an in vitro fluorescent substrate for DUBs, in purified preparations of the 26S proteasome obtained from hearts of 4 week old sTNF and littermate (LM) control mice. Compared to LM controls we observed a significant (p < 0.001) 60.8% decrease in activity of the 26S proteasome and a significant (p < 0.01) 24.2% increase in DUB activity in sTNF mouse hearts. There was also a significant (p < 0.01) 11-fold increase myocardial protein levels of USP14, a critical DUB associated with the 26S proteasome in sTNF mouse hearts. The decrease in 26S proteasome activity and increased DUB activity in sTNF mouse hearts was accompanied by an increase in myocardial levels of ubiquitinated SMAC/Diablo. Taken together these results show for the first time that sustained myocardial inflammation leads to switch in the function of the proteasome from a proteolytic function to a protein sparing function. Although this “proteasome switching” may provide a short-term adaptive benefit by preventing depletion of critical ubiquitin pools, it may lead to long-term maladaptive consequences by allowing the progressive accumulation of potentially harmful pro-apoptotic proteins in the cytosol, which may in turn promote programmed cell death and adverse cardiac remodeling.