Caspases are a family of cysteine proteases that include at least 14 members, and play a critical role in mammalian apoptosis or proteolytic activation of cytokines. Caspase-1, the best characterized inflammatory caspase was originally identified on the basis of its proteolytic activity for cleaving the inactive interleukin-1b (IL-12) precursor in the mature cytokine. Manganese superoxide dismutase (MnSOD) is an antioxidant enzyme located in the mitochondria that provides critical protection against reactive oxygen species (ROS) -induced tissue injury. We recently made the novel observation that downregulation of basal level of MnSOD (also known as Sod2) by RNA interference upregulated caspase-1 expression in microvascular endothelial cells (EC). Additionally, MnSOD downregulation increased LPS or TNFa stimulated increase in caspase-1 expression. In contrast, downregulation of CuZnSOD (also known as Sod1) by RNA interference did not increase the level of caspase-1 expression alone or in response to LPS stimulation in endothelial cells. This observation suggests that MnSOD plays a specific negative regulatory role in the expression of caspase-1. Our central hypothesis is that increased caspase-1 expression in Sod2-/- mice would result in severe inflammation and apoptosis, whereas Sod1-/- mice would not show increased expression of caspase-1 and inflammation or apoptosis. In the first specific aim we will evaluate whether Sod2 -/- mice would show increased caspase-1 expression and increased cytokine expression. In addition, we will also evaluate the level of IL-12, IL-18 in the heart tissue and sera of Sod2-/- mice. We will also evaluate the expression of caspase-1, IL-12 and IL-18 in Sod1 -/- mice. In addition, we will inject Sod2-/- mice with caspase-1 inhibitor YVAD-CHO, and will evaluate whether we could ameliorate inflammation markers in these mice. In this aim, we will evaluate whether Sod2-/- mice would show increased apoptosis of cardiac cells and endothelial cells. We will also evaluate apoptosis using caspase-3, caspase-7, PARP cleavage, and detection of Tunel nuclei as markers. In addition, we will inject Sod2 -/- mice with broad spectrum caspase inhibitor YVAD-CHO, and evaluate the expression of apoptosis markers. In addition, we will determine whether MnSOD would regulate caspase-1 expression in a pathophysiological setting such as ischemia-reperfusion of heart. These studies would demonstrate whether constitutive MnSOD expression is necessary to maintain caspase-1 at a low level in vivo. The results obtained in this investigation would also show for the first time a novel role of MnSOD in the negative control of caspase-1 expression in vivo.
Apoptosis of cardiomyocytes is increased in human heart failure, and several studies have subsequently suggested that cardiomyocyte apoptosis is a central mechanism in heart failure. The exploratory studies proposed in this application will demonstrate a novel function of a well known antioxidant enzyme in the control of inflammatory caspase-1 expression. The outcome of these studies may help in the therapeutic use of MnSOD or substances that induce MnSOD in the protection of myocardial injury in heart failure, endothelial dysfunction or other cardiovascular diseases.
