Abstract 3429: SIRT3 Blocks Agonist Mediated Cardiomyocyte Hypertrophy by Suppressing the ROS Mediated Ras Activation and the Downstream ERK and Akt/mTOR Pathways
SIRT3, a class III HDAC has been genetically linked to extended life span in animals as well as in humans. The cell specific effects of SIRT3 have not been examined yet. This study was designed to examine the effect of SIRT3 in the development of cardiomyocyte hypertrophy. Primary cultures of neonatal rat heart myocytes grown in low serum conditions were treated with 10 μM phenylephrine (PE) for 48 hours to induce hypertrophy. The effect of SIRT3 in cardiomyocytes was examined by over- expressing cells with wild-type SIRT3 or the mutant, lacking deacetylase activity, using the adenoviral expression vectors. Over-expression of SIRT3 wild-type, but not the mutant, in myocytes inhibited the PE-induced [3H]leucine incorporation into cellular proteins, sarcomere organization, ANF release from nuclei as well as the induction of fetal genes expression (e.g. CARP, β-MHC and ANF), thus suggesting that SIRT3 has the potential to block the hypertrophy response of cardiomyocytes. To explore the mechanisms by which SIRT3 impairs the hypertrophy response, we examined the activation of signaling pathways leading to hypertrophy of cardiomyocytes. We found that SIRT3 blocks the PE-mediated nuclear localization of GATA4 and NFAT3, which are known to control the cardiac fetal gene program. SIRT3 over-expression also inhibited the phosphorylation of ERK1/2, Akt and mTOR, which are known to be involved in the development of cardiac hypertrophy. Since SIRT3 is a deacetylase, we hypothesized that SIRT3 might control an upstream signaling factor which regulates the activity of both the ERK and AKT/mTOR signaling pathways. We found that SIRT3 was capable of blocking the activation of Ras, an oncogenic protein upstream of ERK and AKT pathways. To get further insights for how SIRT3 controls Ras activation, we quantified mitochondrial reactive oxygen species (ROS), a common activator of Ras during agonist induced cardiac hypertrophy. As expected, we found that over-expression of SIRT3 wild-type significantly inhibited the PE-mediated mitochondrial ROS production, but not the mutant-SIRT3. From these studies we conclude that SIRT3 protects cardiomyocytes from hypertrophy through suppression of ROS-mediated Ras activation and downstream ERK and AKT/mTOR signaling pathways.