The long-term goal of this study is to understand the role of PARP-Sir2a axis of signaling in the onset and progression of cardiac hypertrophy. Both PARP (poly-ADP-ribose polymerase) and Sir2a (Class-Ill histone deacetylase, HDACs) are NAD-dependent, redox-sensitive, chromatin remodeling enzymes. They play a central role in cell survival and gene regulation, and are considered to be the nuclear integrators of oxidative-stress signaling. During increased work load on the heart, PARP is activated in a linear fashion proportional to the intensity of cardiac hypertrophy, and massive PARP expression occurs in failing hearts. PARP (-/-) mice are protected from ischemic insults, and produce an attenuated hypertrophic response to pressure overload. However, the mechanism behind the deleterious effect of PARP in hearts is virtually unknown. PARP catalyzes ADP-ribosylation of proteins by transferring multiple ADP-ribose units from NAD to the target protein. Since NAD is essential for the activity of class III HDACs, it is believed that the depletion of NAD by PARP over-activation represses the activity of class-Ill HDACs (e.g., Sir2a). Sir2a is considered to be a longevity factor, and is implicated in calorie restriction-mediated increases in mammalian cell-survival. Recent data obtained in our laboratory indicate that a reciprocal expression of these two enzymes (PARP &Sir2a) plays a central role in myocyte cell-survival/death and the progression of cardiac hypertrophy to failure. For example, in failing hearts of both animals and humans, PARP activation was found to be associated with the loss of Sir2a activity. In cultured cardiac myocytes, PARP over-expression resulted in massive repression of gene transcription and myocyte cell-death;over expression of Sir2a protected myocytes from Ang-ll and oxidative-stress mediated cell-death, and enhanced the expression of key contractile genes, e.g., the cardiac a-MHC gene. Based on these results, we believe that during myocardial stress, induction of PARP plays a central role in the translation of oxidative-stress signals to hypertrophy and, subsequently, to the heart's progression from hypertrophy to failure. This proposal is designed to test the hypothesis that PARP over activation during hypertrophy attenuates Sir2a deacetylase activity due to cellular NAD depletion. These changes shift the balance from cell-survival towards cell death during oxidative stress, resulting in gene repression and myocyte cell death, which eventually lead to chamber dilation and muscle decompensation associated with the failing heart. A successful outcome of this project will allow us to understand the role of Sir2a, which is implicated as a survival factor for many cell-types (including neurons) during the development of cell hypertrophy. As such, these studies are likely to provide new strategies for the management of heart failure.
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