Die Rolle von SIRT4 in der myokardialen Ischämie-Reperfusion
Zusammenfassung der Projektergebnisse
Mitochondrial derangements causally contribute to myocardial ischemia reperfusion (IR) injury, including defects in oxidative phosphorylation, increased reactive oxygen species (ROS) generation and increased opening of the mitochondrial permeability transition pore (mPTP). Sirtuin 4 (SIRT4) is a mitochondrial NAD+-dependent deacylase which inhibits glucose and fatty acid oxidation by deacetylation of pyruvate dehydrogenase and malonyl- CoA decarboxylase, respectively, and may regulate mitochondrial ROS generation and mPTP opening. During IR, mitochondrial NAD+ levels become depleted, which may result in impaired SIRT4 activity and modulation of the extent of myocardial IR injury. In the current project, we proposed (1) to investigate the effect of modulating SIRT4 levels on myocardial IR injury, and (2) to elucidate the mechanism of cardioprotection in mice lacking SIRT4. Deficiency or overexpression of SIRT4 did not affect cardiac function under physiological conditions in vivo using echocardiography. In contrast, SIRT4 deficiency resulted in a marked decrease in cardiac infarct size following ligation of the left anterior descending coronary artery in vivo. Similarly, recovery of contractile function following global no-flow ischemia in the Langendorff model was improved in SIRT4-/- mice. Overexpression of SIRT4 did not change infarct size or functional recovery following IR. While mitochondrial respiratory function was unaffected and mPTP opening only mildly affected in SIRT4-/- mice, the IR-induced increase in mitochondrial ROS was completely blunted in SIRT4-/- mice following IR. Analysis of gene expression and metabolomics revealed that increased expression of several antioxidative enzymes as well as a blunting of ischemia-induced accumulation of succinate may be major mechanisms contributing to decreased oxidative stress in SIRT4-/- mice following IR. Based on the results of this project, we believe that impairment of SIRT4 activity during pathological NAD+ depletion may represent an endogenous and conserved mechanism to protect the heart from damage during conditions of impaired bioenergetics, e.g. ischemia reperfusion. Pharmacological inhibition of SIRT4 may thus be a useful therapeutic strategy to use the cardioprotective effect of SIRT4 suppression. Of note, SIRT4 is the only sirtuin which shows protective effects in its absence. Thus, supplementation of NAD+ as currently investigated both in animals and human trials, may not result in a beneficial effect by all sirtuins. Therefore, a therapeutic strategy combining NAD+ supplementation and SIRT4 inhibition may result in the most effective disease protection, including cardiac IR injury.
