Stem cells and Antioxidants

Endogenous c-kit-positive cardiac progenitor cells (CPCs) are a promising cell type for myocardial regenerative therapies. They have robust cardiovascular differentiation abilities and on direct intramyocardial delivery, these cells structurally integrate and improve the performance of the myocardium. Oxidative stress is increased in the ischemic myocardium and indirect evidence suggests the vulnerability of CPCs to oxidative stress. Survival of CPCs and mesenchymal stem cells (MSCs) are less than 10% within 4 days of transplantation within the ischemic myocardium. The survival of cells, such as skeletal myoblasts and cardiomyoblasts, are improved if antioxidants, such as superoxide dismutase (SOD) and Tempol (SOD mimetic), are delivered to the myocardium during cell transplantation. Further, enhanced endogenous expression of antioxidants, including SOD2, protects endothelial progenitors during oxidative stress. In addition to the effect of reactive oxygen species (ROS) and antioxidants on survival, they also regulate other important properties of stem cells, such as their self-renewal and senescence.

SODs are one of the major endogenous antioxidant enzymes providing first line protection against oxidative stress. For example, higher SOD2 and catalase levels in circulating endothelial progenitors protect them from oxidative stress induced apoptosis compared to related cell types. In addition, enhanced expression of antioxidant genes is considered a stemness trait in many progenitor cells. Therefore, the basal SOD protein and activity levels of CPCs were compared with other adult cardiac cell types. Interestingly, CPCs had significantly higher protein levels of both SOD1 and SOD2 enzymes compared to FBs and myocytes, suggesting this as a potential mechanism by which CPCs could be protected from oxidative stress unlike cardiomyocytes. SOD activity assays confirmed Western analysis, though SOD2 levels were not significantly lower in cardiomyocytes compared to CPCs, despite the large difference in protein levels. SOD activities measured in CPCs show that the cells lose about 25% of total SOD activity within 6 h of oxidative stress. However, after 48 h of oxidative stress, CPCs had not only regained activity, but had significantly higher total SOD activity levels compared to control cells. Additionally, 2-way ANOVA conducted on CPCs with time-matched controls demonstrate significantly higher activity of SOD1 and SOD2 following 48 h of XXO treatment, suggesting that the increased SOD mRNA expression led to changes in protein levels. While there appears to be some lag between the new mRNA expression (3–6 h) and new protein expression (24–48 h) and activity levels (48 h), this may be due to both the highly regulated destruction and generation of the protein. Observations exist in the literature that shows that SOD2, but not SOD1, is critical to protect the cells against oxidative stress-induced apoptosis. SOD2 is known to maintain mitochondrial membrane integrity, which when disrupted leads to cytochrome release and begins the chain of events leading to apoptosis [11].