The role of mitochondrial regulation in graft injury after liver transplantation

Abstract

The application of steatotic liver graft expanded the donor pool for liver transplantation. However, it remains to be controversial due to the potential high morbidity and mortality, which were resulted from severe ischemia/reperfusion injury. It was broadly investigated that mitochondrial dysfunction was involved in ischemia/reperfusion injury during liver resection and transplantation. However, the role of mitochondria in steatotic graft injury after adult living donor liver transplantation was not fully understood. Thus elucidating the mechanism of mitochondrial functional regulation in steatotic graft after liver transplantation will be crucial to develop therapeutic strategies targeting at graft injury and thereafter to further expand the donor pool. Among 1279 liver transplant patients over the last two decades with 100% patient follow-up in our center, 530 patients receiving living donor liver transplantation (LDLT) were included in current study for risk factor analyses and outcomes comparison. The long term graft survival for the patients implanted with moderate to severe steatotic graft (10% - 80% steatosis) was significant short compared to that with mild and non-steatotic grafts (HR=2.652, p<0.01). They also had higher incidences of post-transplantation complications such as primary non-function (6.1%, p<0.001), portal vein thrombosis (9.1%, p<0.01) and hepatic vein thrombosis (3.0%, p<0.01). Further comparisons for liver function showed both ALT (Day0, 541U/L vs 347U/L vs 332U/L, p<0.05) and AST (Day1, 468U/L vs 243U/L vs 229U/L, p<0.01) increased significantly in the patients with moderate to severe steatotic graft within post-operation 7 days. Consistently, serum total bilirubin level recovery was also delayed compared to control groups. In addition, significant down-regulation of mitochondrial metabolism regulating genes, especially for AMP-activated protein kinase (AMPK) phosphorylation, was mainly found in steatotic graft after LDLT by PCR array screening. We further established animal models to investigate the role of mitochondria in steatotic graft injury. First of all, AMPK down-regulation was identified to be associated with mitochondrial dysfunction and biogenesis impairment during steatotic graft injury. Secondly, AMPK down-regulation was responsible for steatotic graft injury through decreasing ATP production, dysregulating oxidative phosphorylation, increasing membrane permeability transition (MPT) as well as impairing biogenesis in a series of in vivo and in vitro experiments. In particular, mitochondrial biogenesis impairment was indicated by down-regulated PGC-1α signaling, decreased electron transition chain subunits expressions, and reduced mitochondrial phospholipids content. Further attempt of metformin treatment was made to explore the therapeutic value of restoring mitochondrial function against post-transplantation graft injury. The beneficial effect of metformin was observed in attenuating hepatic architecture destruction, ameliorating serum aminotransferases increase and promoting liver regeneration. It was also indicated that metformin treatment could increase ATP production, reduce MPT and promote mitochondrial biogenesis. In conclusion, we first identified graft steatosis (>10%) as an independent risk factor for long term graft failure and short term graft function deterioration during LDLT. The mechanistic findings suggested that AMPK mediated mitochondrial functional dysregulation was responsible for severe steatotic graft injury. By combining clinical, mechanistic and interventional findings, we could elicit a new approach of steatotic graft injury treatment by restoring mitochondrial biogenesis and energy homeostasis through drugs such as metformin.