Sirtuin 1 suppresses mitochondrial dysfunction of ischemic mouse livers in a mitofusin 2-dependent manner

Ischemia/reperfusion (I/R) injury is a major cause of morbidity and mortality after liver surgery. The role of Sirtuin 1 (SIRT1) in hepatic I/R injury remains elusive. Using human and mouse livers, we investigated the effects of I/R on hepatocellular SIRT1. SIRT1 expression was significantly decreased after I/R. Genetic overexpression or pharmacological activation of SIRT1 markedly suppressed defective autophagy, onset of the mitochondrial permeability transition, and hepatocyte death after I/R, whereas SIRT1-null hepatocytes exhibited increased sensitivity to I/R injury. Biochemical approaches revealed that SIRT1 interacts with mitofusin-2 (MFN2). Furthermore, MFN2, but not MFN1, was deacetylated by SIRT1. Moreover, SIRT1 overexpression substantially increased autophagy in wild-type cells, but not in MFN2-deficient cells. Thus, our results demonstrate that the loss of SIRT1 causes a sequential chain of defective autophagy, mitochondrial dysfunction, and hepatocyte death after I/R.During hepatic resection and liver transplantation operations, inflow occlusion is employed to temporarily limit blood flow to minimize intraoperative blood loss. Although prolonged ischemia eventually causes tissue injury, severe damage paradoxically does not occur until recovery of blood flow and restitutions of normal physiological pH.¹ Ischemia/reperfusion (I/R) injury is a key cause of postoperative liver failure during hemorrhagic shock, hepatectomy, and liver transplantation. Despite continuous efforts, substantial benefits from current strategies have not been realized, mainly because of the multifactorial nature of I/R injury.I/R initiates opening of high-conductance permeability transition pores in the mitochondrial inner membranes, leading to mitochondrial permeability transition (MPT).² Onset of the MPT uncouples oxidative phosphorylation and depolarizes mitochondrial membrane potential (ΔΨ_m) that in turn causes ATP depletion and cell death.Autophagy is an evolutionarily conserved catabolic process. Among the three forms of autophagy, macroautophagy is of particular importance in the liver, as it not only degrades unneeded intracellular proteins but also digests injured or dysfunctional organelles such as abnormal mitochondria.³ We have shown that impaired autophagy contributes to liver I/R injury.^{4, 5, 6}Sirtuin1 (SIRT1) deacetylates Lys residues of both histone and nonhistone targets, and is activated in response to fasting and calorie restriction in the liver, a condition inducing autophagy.^{7, 8} Despite its extramitochondrial localization, SIRT1 appears to affect mitochondrial biogenesis⁹ and bioenergetics,¹⁰ but its mechanisms remain elusive.Using isolated hepatocytes, mouse livers, SIRT1-null mice, and human livers, we here demonstrate that I/R depletes livers of SIRT1 and that specific overexpression of SIRT1 mitigates defective autophagy, onset of the MPT, and subsequent hepatocyte death after both in vitro and in vivo I/R. Furthermore, we show that mitofusin-2 (MFN2) is a new substrate for SIRT1.