Sirtuin 3 (SIRT3) is a member of the sirtuin family of proteins that promote longevity in many organisms. Increased expression of SIRT3 has been linked to an extended life span in humans. Here, we have shown that Sirt3 protects the mouse heart by blocking the cardiac hypertrophic response. Although Sirt3-deficient mice appeared to have normal activity, they showed signs of cardiac hypertrophy and interstitial fibrosis at 8 weeks of age. Application of hypertrophic stimuli to these mice produced a severe cardiac hypertrophic response, whereas Sirt3-expressing Tg mice were protected from similar stimuli. In primary cultures of cardiomyocytes, Sirt3 blocked cardiac hypertrophy by activating the forkhead box O3a-dependent (Foxo3a-dependent), antioxidant-encoding genes manganese superoxide dismutase (MnSOD) and catalase (Cat), thereby decreasing cellular levels of ROS. Reduced ROS levels suppressed Ras activation and downstream signaling through the MAPK/ERK and PI3K/Akt pathways. This resulted in repressed activity of transcription factors, specifically GATA4 and NFAT, and translation factors, specifically eukaryotic initiation factor 4E (elf4E) and S6 ribosomal protein (S6P), which are involved in the development of cardiac hypertrophy. These results demonstrate that SIRT3 is an endogenous negative regulator of cardiac hypertrophy, which protects hearts by suppressing cellular levels of ROS.

Abnormal activation of insulin-like growth factor (IGF)-Akt signaling is implicated in the development of various diseases, including heart failure. However, the molecular mechanisms that regulate activation of this signaling pathway are not completely understood. Here we show that sirtuin 6 (SIRT6), a nuclear histone deacetylase, functions at the level of chromatin to directly attenuate IGF-Akt signaling. SIRT6-deficient mice developed cardiac hypertrophy and heart failure, whereas SIRT6 transgenic mice were protected from hypertrophic stimuli, indicating that SIRT6 acts as a negative regulator of cardiac hypertrophy. SIRT6-deficient mouse hearts showed hyperactivation of IGF signaling–related genes and their downstream targets. Mechanistically, SIRT6 binds to and suppresses the promoter of IGF signaling–related genes by interacting with c-Jun and deacetylating histone 3 at Lys9 (H3K9). We also found reduced SIRT6 expression in human failing hearts. These findings disclose a new link between SIRT6 and IGF-Akt signaling and implicate SIRT6 in the development of cardiac hypertrophy and failure.

Since the discovery of NAD-dependent deacetylases, sirtuins, it has been recognized that maintaining intracellular levels of NAD is crucial for the management of stress response of cells. Here we show that agonist-induced cardiac hypertrophy is associated with loss of intracellular levels of NAD, but not exerciseinduced physiologic hypertrophy. Exogenous addition of NAD was capable of maintaining intracellular levels of NAD and blocking the agonist-induced cardiac hypertrophic response in vitro as well as in vivo. NAD treatment blocked the activation of pro-hypertrophic Akt1 signaling, and augmented the activity of anti-hypertrophic LKB1-AMPK signaling in the heart, which prevented subsequent induction of mTOR-mediated protein synthesis. By using gene knock-out and transgenic mouse models of SIRT3 and SIRT1, we showed that the anti-hypertrophic effects of exogenous NAD are mediated through activation of SIRT3, but not SIRT1. SIRT3 deacetylates and activates LKB1, thus augmenting the activity of the LKB1-AMPK pathway. These results reveal a novel role of NAD as an inhibitor of cardiac hypertrophic signaling, and suggest that prevention of NAD depletion may be critical in the treatment of cardiac hypertrophy and heart failure.Cardiac hypertrophy is a complex growth response of the heart, whereby terminally differentiated cardiac myocytes structurally, genetically, and functionally remodel in response to a variety of physiologic and pathologic stimuli. In settings of pathologic stimuli, such as hypertension, ischemic disease, or valvular insufficiency, cardiac hypertrophy develops with enlarged cardiomyocytes, which are associated with formation of new sarcomeres and induction of a group of genes (fetal genes), which are usually expressed during development of the fetal heart. These changes provide a short term mechanism for decreasing ventricular wall stress and improving heart function. However, during prolonged intervals of pathologic hypertrophy, this program becomes maladaptive, resulting in myocyte cell death, fibrosis, and ventricular dilation and the transition to heart failure (1). Recent evidence suggests that reduction of cardiac hypertrophy could block the onset of heart failure and improve patient survival (1-3). One novel approach that is gaining increasing attention in this direction is the activation of endogenous cell signaling pathways that negatively regulate cardiac hypertrophy (4). Exogenous agents that can facilitate the activity of these pathways are of particular interest as new therapeutic tools for the management of cardiac hypertrophy and heart failure.At the cellular level various signaling mechanisms have been described that lead to development of cardiac hypertrophy. Among them, oxidative stress is recognized as a critical common signal to various stimuli, which directs to evolution of pathologic hypertrophy (5). Severe oxidative stress can result in increased NAD turnover due to increased activity of NAD-consuming enzymes such as poly(ADP-ribose) polymerase-1 and/or decreased activity...

Deacetylation of Akt and its activating kinase PDK1 promotes cell growth in physiological and pathological settings.

Poly(ADP-ribose) polymerase 1 (PARP1) and SIRT1 deacetylase are two NAD-dependent enzymes which play major roles in the decision of a cell to live or to die in a stress situation. Because of the dependence of both enzymes on NAD, cross talk between them has been suggested. Here, we show that PARP1 is acetylated after stress of cardiomyocytes, resulting in the activation of PARP1, which is independent of DNA damage. SIRT1 physically binds to and deacetylates PARP1. Increased acetylation of PARP1 was also detected in hearts of SIRT1 ؊/؊ mice, compared to that detected in the hearts of SIRT1 ؉/؉ mice, confirming a role of SIRT1 in regulating the PARP1 acetylation in vivo. SIRT1-dependent deacetylation blocks PARP1 activity, and it protects cells from PARP1-mediated cell death. We also show that SIRT1 negatively regulates the activity of the PARP1 gene promoter, thus suggesting that the deacetylase controls the PARP1 activity at the transcriptional level as well. These data demonstrate that the activity of PARP1 is under the control of SIRT1, which is necessary for survival of cells under stress conditions. During cellular stress, proteins undergo a variety of posttranslational modifications that result in their increased or decreased activity. One such modification is poly(ADP-ribosyl) ation, which is catalyzed by a family of enzymes called poly(ADP-ribose) polymerases (PARPs). This is initiated by transfer of an ADP-ribose unit from NAD to glutamate or aspartate residues of the target protein, and it proceeds with successive additions of many ADP-ribose units to the substrate, resulting in the synthesis of a large chain of branched ADP-ribose polymers, which are subsequently degraded by poly(ADP-ribose) glycohydrolase (30).PARP1 (116 kDa) is a prototype member of the PARP family of enzymes. It is ubiquitously expressed and accounts for most of the poly(ADP-ribosyl)ation of proteins in vivo (30). PARP1 is located in the nucleus as well as in the mitochondria, and it plays an important role in the DNA repair process and in the maintenance of genome stability. The enzyme consists of a characteristic three-domain structure: a DNA binding domain at the amino terminus, a catalytic domain at the carboxy terminus, and an automodification domain in the middle, which is poly(ADP-ribosyl)ated by itself (30). PARP1 is activated in response to DNA damage, such as single-strand breaks, which could develop as a response to various pathological conditions, such as inflammatory diseases, diabetes, reperfusion injury, or oxidative stress. PARP1 is also known to be activated by processes independent of DNA damage, including phosphorylation, and high levels of Mg 2ϩ , Ca 2ϩ , and polyamines (31). While basal activation of PARP1 is needed for maintenance of normal homeostasis of the cell, overactivation of PARP1 consumes NAD and results in cell death due to depletion of intracellular NAD stores (10). This characteristic makes it important for PARP1 activity to be tightly regulated for survival of the cell. One group of factors which a...