Mitochondrial Uncoupling Protein-2 Mediates Steatotic Liver Injury following Ischemia/Reperfusion

Abstract

Steatotic livers are not used for transplantation because they have a reduced tolerance for ischemic events with reduced ATP levels and greater levels of cellular necrosis, which ultimately result in total organ failure. Mitochondrial uncoupling protein-2 (UCP2) is highly expressed in steatotic livers and may be responsible for liver sensitivity to ischemia through mitochondrial and ATP regulation. To test this hypothesis, experiments were conducted in lean and steatotic (ob/ob), wild-type, and UCP2 knock-out mice subjected to total warm hepatic ischemia/reperfusion. Although ob/ob UCP2 knock-out mice and ob/ob mice have a similar initial phenotype, ob/ob UCP2 knock-out animal survival was 83% when compared with 30% in ob/ob mice 24 h after reperfusion. Serum alanine aminotransferase concentrations and hepatocellular necrosis were decreased in the ob/ob UCP2 knock-out mice when compared with ob/ob mice subjected to ischemia. Liver ATP levels were increased in the ob/ob UCP2 knock-out animals after reperfusion when compared with the ob/ob mice but remained below the concentrations from lean livers. Lipid peroxidation (thiobarbituric acid-reactive substances) increased after reperfusion most significantly in the steatotic groups, but the increase was not affected by UCP2 deficiency. These results reveal that UCP2 expression is a critical factor, which sensitizes steatotic livers to ischemic injury, regulating liver ATP levels after ischemia and reperfusion.Complications from liver steatosis represent a significant clinical concern, especially for liver surgeries including resection and transplantation. This is of escalating importance as nonalcoholic fatty liver disease is independently correlated to obesity and insulin resistance, which are both epidemic in the Unites States (1, 2). Steatotic livers are considerably more sensitive to acute stressors including ischemia/reperfusion (I/R) 2 as experienced in transplantation, and organs meeting this criterion are routinely turned down for donation (3-6). Under I/R conditions, steatotic livers are ATP-depleted, and the predominant hepatocellular fate is shifted from apoptosis to oncotic necrosis, strongly implicating inappropriate energy homeostasis as the primary cause of liver sensitivity (7-10).In a state of energy substrate abundance and forward shift in cellular redox potential, hepatocytes are thought to combat mitochondrial electron transport chain-derived reactive oxygen species (ROS) production through mitochondrial uncoupling. In normal lean livers, mitochondrial uncoupling protein-2 (UCP2) is confined to Kupffer cells; however, hepatocellular concentrations of UCP2 greatly increase with steatosis (7,11,12). Although the mechanism is not known, UCP2 facilitates passive proton conductance across the mitochondrial inner membrane into the matrix during respiration (13,14). Maximum employment of electrochemical potential at ATP synthase is sacrificed by UCP2, and heat, rather than ATP, is produced. Proton conductance is thought to require posttranslational activ...

Targeted Expression of Uncoupling Protein 2 to Mouse Liver Increases the Susceptibility to Lipopolysaccharide/Galactosamine-Induced Acute Liver Injury†

“…Previous studies suggested that UCP2 promotes mitochondrial proton leak and increases susceptibility of liver to ischemia/reperfusion injury. 27,39 However, the underlying mechanisms of how UCP2 sensitizes hepatocytes to death are not fully understood. Our data for the first time suggest the possible mechanism of how the compromised energy metabolism signals the core apoptotic machinery (Fig.…”

“…23 An interesting note is that UCP2 expression becomes significantly abundant in hepatocytes of fatty liver, steatotic liver, and chronic liver disease conditions. [24][25][26] Under these conditions, hepatocytes are prone to ischemia/reperfusion-induced liver injury, 27 implying that UCP2 has pathogenic roles in the development of liver diseases. It was found that UCP2 expression induced by obesity in hepatocytes promotes liver ATP depletion.…”

The Protective Role of CD59 and Pathogenic Role of Complement in Hepatic Ischemia and Reperfusion Injury

“…28 After 45 minutes of hepatic ischemia, surviving mice were sacrificed at 6, 12, or 24 hours after reperfusion.…”

A Role for Complement in the Enhanced Susceptibility of Steatotic Livers to Ischemia and Reperfusion Injury

“…At 8 wk of age, following dietary treatment, wt lean, wt fat feed, C3 Ϫ/Ϫ lean, and C3 Ϫ/Ϫ fat feed mice were subjected to total warm hepatic ischemia as previously described (21). Ischemia was performed for 45 min and surviving mice were sacrificed 24 h after reperfusion.…”

“…H&E staining of liver sections was performed as previously described (21). Sections stained with H&E were graded in a blinded fashion for centrilobular necrosis on a 0 -3 scale as described by Neil and Hubscher.…”