Background: Myocardial infarction (MI) produces severe hypoxia within regions of the myocardium, resulting in the formation of scar tissue and significant cardiac cell death. Adult hearts contain endogenous cardiac stem cells (CSCs) that have regenerative capacity and participate in myocardial tissue homeostasis and repair post-MI, but are insufficient to promote complete regeneration. The histone deacetylase Sirtuin 1 (SIRT1) likely mediates this hypoxia-induced decrease in regeneration via roles in cell cycle progression and conferring protection from senescence and oxidative damage.
Hypothesis: Hypoxia decreases CSC proliferation through reduced SIRT1-mediated deacetylation.
Methods and Results: Murine CSCs were grown at room air (21% O2), physiologic (5% O2), and ischemic hypoxic (0.5% O2) conditions for 72 hours. Ischemic hypoxia, but not physiologic, reduced CSC proliferation and DNA synthesis to 25±2.0% (N=3; p<0.05) and 54±7.0% (N=7; p<0.05), respectively, relative to cells grown in room air. SIRT1 protein expression was decreased by 58±10.0% (N=4; p<0.05) and acetylation of Histone H3 Lys9 (2.4-fold; N=5; p<0.05) and p53 Lys379 (1.64-fold; N=4; p<0.05) were increased after 72 hours of growth in 0.5% compared to 21% O2. However, SIRT1 mRNA transcripts remained unchanged. Furthermore, SIRT1 protein was 59±6.0% less stable (N=4; p<0.05) following 8 hours of cyclohexamide treatment in CSCs exposed to 0.5% O2 for 72 hours compared to CSCs exposed to 21% O2. SIRT1 knockdown by RNA interference significantly reduced proliferation of CSCs grown in room air (N=3 and, p<0.05), similar to that observed in un-transfected cells grown under ischemic hypoxia conditions.
Conclusion: The decrease in CSC proliferation with hypoxia is in part due to a reduction in SIRT1 protein stability. These results suggest that SIRT1 expression is regulated post-translationally and support a role for SIRT1 in preserving CSC self-renewal under hypoxic conditions.