Molecular Cardiology Mitochondrial Thioredoxin Reductase Is Essential for Early Postischemic Myocardial Protection

Background—Excessive formation of reactive oxygen species contributes to tissue injury and functional deterioration after myocardial ischemia/reperfusion. Especially, mitochondrial reactive oxygen species are capable of opening the mitochondrial permeability transition pore, a harmful event in cardiac ischemia/reperfusion. Thioredoxins are key players in the cardiac defense against oxidative stress. Mutations in the mitochondrial thioredoxin reductase (thioredoxin reductase-2, Txnrd2) gene have been recently identified to cause dilated cardiomyopathy in patients. Here, we investigated whether mitochondrial thioredoxin reductase is protective against myocardial ischemia/reperfusion injury. Methods and Results—In mice, ␣-MHC-restricted Cre-mediated Txnrd2 deficiency, induced by tamoxifen (Txnrd2-/-ic), aggravated systolic dysfunction and cardiomyocyte cell death after ischemia (90 minutes) and reperfusion (24 hours). Txnrd2-/-ic was accompanied by a loss of mitochondrial integrity and function, which was resolved on pretreatment with the reactive oxygen species scavenger N-acetylcysteine and the mitochondrial permeability transition pore blocker cyclosporin A. Likewise, Txnrd2 deletion in embryonic endothelial precursor cells and embryonic stem cell-derived cardiomyocytes, as well as introduction of Txnrd2-shRNA into adult HL-1 cardiomyocytes, increased cell death on hypoxia and reoxygenation, unless N-acetylcysteine was coadministered. Conclusions—We report that Txnrd2 exerts a crucial function during postischemic reperfusion via thiol regeneration. The efficacy of cyclosporin A in cardiac Txnrd2 deficiency may indicate a role for Txnrd2 in reducing mitochondrial reactive oxygen species, thereby preventing opening of the mitochondrial permeability transition pore. E xcessive formation of reactive oxygen species (ROS) during postischemic reperfusion significantly contributes to tissue injury and functional deterioration of the heart. Cardio-myocytes contain a high amount of mitochondria to provide energy equivalents for generating force. However, mitochondria are the major site of generation of an oxygen radical burst on reperfusion in vitro 1 and in vivo. 2 Under normoxic conditions, in vitro ROS formation occurs at a rate of approximately 1% to 2% of oxygen consumption. 3 In contrast, posthypoxic reoxygenation results in massive mitochondrial superoxide formation, triggering subsequent formation of hydrogen peroxide (H 2 O 2) and hydroxyl radical (OH .). 4,5 Myocardial ischemia and reperfusion (I/R) injury has been shown to be attenuated by endogenous ROS scavenging systems, such as mitochondrial manganese superoxide dismutase, detoxifying O 2 Ϫ , 6,7 and catalase or gluta-thione peroxidase, which inactivate H 2 O 2. Less well studied is the thioredoxin / thioredoxin reductase system. Thioredoxins work through a –Cys-Gly-Pro-Cys–active site and supply electrons for the reduction of a wide variety of substrates, including peroxiredoxins, which are critically involved in ROS scavenging. 10 Thioredoxin-1 …