Objective: Liver injury and cell death are prominent features in the pathogenesis of acute liver failure. Mitochondrial uncoupling protein 2 plays a controversial role in liver cell death through its involvement in the production of reactive oxygen species and adenosine triphosphate.
Design: This randomized controlled animal study was designed to investigate the exact role of uncoupling protein 2 in the pathogenesis of endotoxemic acute liver failure.
Setting: Research laboratory of an academic institution. SUBJECTS, INTERVENTIONS, AND MEASUREMENTS: Uncoupling protein 2+/+ and uncoupling protein 2-/- mice were challenged with D-galactosamine (Gal, 720 mg/kg intraperitoneally) and Escherichia coli lipopolysaccharide (10 microg/kg intraperitoneally) and studied 6 hrs thereafter (n = 5 per group). Control mice received physiologic saline (n = 5 per group). Analysis included in vivo fluorescence microscopy of hepatic microcirculation and hepatocellular apoptosis as well as plasma malondialdehyde concentrations as reactive oxygen species-dependent lipid peroxidation product and hepatic adenosine triphosphate levels.
Main results: Administration of Gal-lipopolysaccharide in uncoupling protein 2+/+ mice caused systemic cytokine release and malondialdehyde production. Further, it provoked marked hepatic damage, characterized by intrahepatic leukocyte recruitment (10.5 +/- 1.3 n/mm2 vs. 3.3 +/- 0.5 n/mm2), microvascular perfusion failure (33.1% +/- 1.6% vs. 2.3% +/- 0.4%), and adenosine triphosphate depletion (3.4 +/- 0.9 micromol/g vs. 6.4 +/- 0.9 micromol/g). Furthermore, uncoupling protein +/+ mice revealed a huge rise in cell apoptosis, given by high numbers of hepatocytes exhibiting nuclear chromatin fragmentation (44.9 +/- 11.5 n/mm2 vs. 0.0 +/- 0.0 n/mm2) and cleaved caspase-3 expression (1.24 +/- 0.24 vs. 0.06 +/- 0.04). Liver injury was coexistent with enzyme release (alanine aminotransferase 442 +/- 126 U/L vs. 57 +/- 12 U/L) and necrotic cell death. Of interest, Gal-lipopolysaccharide-exposed uncoupling protein 2-/- mice exhibited higher rates of hepatocellular apoptosis (135.6 +/- 46.0 n/mm2) as well as cleaved caspase-3 expression (1.75 +/- 0.25), however, preserved hepatic adenosine triphosphate (6.4 +/- 1.7), milder perfusion failure (24.5 +/- 2.4) and decreased leukocyte recruitment (2.7 +/- 0.2), less necrotic injury, lower transaminase levels (340 +/- 91), and finally better survival rates.
Conclusion: The higher adenosine triphosphate availability in uncoupling protein 2-deficient mice might allow hepatocytes to undergo apoptosis as an energy-consuming mode of cell death, while at the same time cellular adenosine triphosphate levels seem to increase hepatic resistance against harmful effects upon Gal-lipopolysaccharide exposure. As net result, uncoupling protein 2 deficiency provided protection under endotoxemic stress conditions, underlining the significant role of the bioenergetic status in critical illness.