| dc.description.abstract | It is believed that the health benefits of exercise are mediated in part by exercise-induced adaptations of skeletal muscle tissue. The mechanisms whereby alterations in cellular energy state are sensed and how these signals are then transduced into physiologic adaptations of skeletal muscle are subjects of intense investigation. Sirtuin 1 (Sirt1) is an NAD+-dependent deacetylase that regulates cellular energy output. Based on in vitro studies performed using C2C12 myoblasts, it is widely assumed that Sirt1 in skeletal myofibers is required for activating PGC1α, a transcriptional coactivator that potently stimulates mitochondrial biogenesis and the release of angiogenic factors in response to exercise. We propose that Sirt1 within skeletal myofibers is not required for the activity of PGC1α in vivo, and we hypothesize instead that Sirt1 within endothelial cells is required for PGC1α-induced angiogenesis in skeletal muscle tissue.
To investigate this hypothesis, overexpression of PGC1α in skeletal myofibers (MCK-PGC1α) was used to simulate exercise training in mice lacking Sirt1 activity in skeletal myofibers (Sirt1flox Myog-Cre) or endothelial cells (Sirt1flox Tie2-Cre). The activity of PGC1α was not suppressed by knockout of Sirt1 in skeletal myofibers, as evaluated by qPCR analysis of oxidative metabolism genes, Western blotting of proteins of oxidative phosphorylation, quantification of cross-sectional vascular density in skeletal muscle tissue, and treadmill endurance testing. In contrast, knockout of Sirt1 in endothelial cells resulted in impaired angiogenesis in response to PGC1α overexpression, as evaluated by quantification of vascular density within the quadriceps muscle group. Thus, we conclude that Sirt1 within skeletal myofibers is not required for PGC1α activity when PGC1α is overexpressed in vivo; however, endothelial Sirt1 is required for angiogenesis within skeletal muscle tissue in response to PGC1α, an essential mediator of exercise-induced angiogenesis. Furthermore, we demonstrate that, in MS1 endothelial cells, Sirt1 appears to regulate chemotaxis through Notch-independent pathways.
Ultimately, these findings identify endothelial Sirt1 as a key regulator of angiogenesis in skeletal muscle tissue and as a new therapeutic target for diseases of vascular insufficiency. In addition to further characterizing the mechanism by which endothelial Sirt1 modulates angiogenic behavior, our current studies now seek to characterize the effects of administering nicotinamide mononucleotide (NMN), a pharmacologic activator of Sirt1, in mice with age-associated endothelial dysfunction and in mouse models of hindlimb ischemia. This has therapeutic potential for peripheral artery disease, myocardial ischemia, and vascular diseases of the central nervous system. | en_US |
