Abstract 214: Muscle Ring Finger-1 (MuRF1) Expression Modulates Proteins of the Cardiac Myofibrillar Complex and Metabolism in vivo
Muscle Ring Finger-1 (MuRF1) has recently been identified as having anti-hypertrophic activity in cardiomyocytes. Specifically, cardiac over-expression of MuRF1 in transgenic mice inhibits the development of cardiac hypertrophy. We have identified that MuRF1 has ubiquitin ligase activity, which can tag specific substrates for subsequent ubiquitin-dependent degradation by the proteasome. Specifically, MuRF1 recognizes and ubiquitinates cardiac troponin I (cTnI) in vitro, resulting in its subsequent proteasome-dependent degradation. Recently, other potential MuRF1 substrates were identified by yeast two hybrid screening including additional sarcomeric proteins (i.e. cTnT, cTnC, telethonin, MLC2) and proteins regulating glycolysis (i.e. pyruvate dehydrogenase kinase 4 (PDK4))(J Mol Biol 350:713, 2005). To determine the physiological relevance of these findings, we investigated a MuRF1 Tg mouse line with cardiac specific over-expression of MuRF1 (~65 fold wild type levels). Steady state protein levels of cTnI, cTnT, cTnC, telethonin, and MLC2 did not differ between MuRF1 Tg from wild type littermate controls. Similarly, cTnI mRNA did not differ between the MuRF1 Tg and wild type mice either, suggesting that transcriptional compensation was not occurring at baseline. These findings suggest that the baseline cardiac dysfunction in MuRF1 Tg mice (FS% 35 36.9±1.7 vs. 53.2±1.5% in wild type controls) is not due to quantitative changes in sarcomeric proteins. When MuRF1 Tg and wild type mice underwent trans-aortic constriction (TAC) to induce hypertrophy, we identified a significant decrease in telethonin, MLC2, and PDK4 protein expression in the MuRF1 Tg mice compared to wild type mice at 2 weeks post TAC. cTnI, cTnT, cTnC protein levels were not affected despite the significantly blunted hypertrophic response post TAC. An increased afterload in MuRF1 Tg mice therefore results in a decrease of specific sarcomeric proteins and the glycolysis regulatory PDK4 (protein pyruvate dehydrogenase kinase 4). Taken together, these results demonstrate for the first time that the muscle specific MuRF1 ubiquitin ligase may inhibit cardiac hypertrophy by multiple mechanisms including the modulation of the myofibrillar apparatus and glycolytic pathway.