(33) MITOCHONDRIAL UNCOUPLING ASSOCIATED WITH EXPRESSION OF UNCOUPLING PROTEIN-2 IN HEPATOCYTES FROM GENETICALLY OBESE OB/OB MICE

Jan B. Hoek (1), Elizabeth Walajtys-Rode (2) and Anna-Mae Diehl (3)
(1) Department of Pathology, Anatomy and Cell Biology, Thomas Jefferson University, Philadelphia, PA
(2) Institute of Biotechnology and Antibiotics, Warsaw, Poland
(3) Department of Medicine, John Hopkins University, Baltimore, MD

Genetically obese ob/ob mice are deficient in leptin and exhibit marked hyperphagia in combination with hyperinsulinemia and hyperglycemia. Although ob/ob mice develop fatty liver (both macrovesicular and microvesicular), the factors controlling energy disposition in the liver and the metabolic adaptations in the control of energy metabolism associated with this condition are not well understood. In vivo 32P-NMR studies demonstrated that liver ATP levels in ob/ob mice are highly susceptible to hypoxic or metabolic stress and are deficient in the recovery of normal ATP levels following a challenge, suggesting a defect in mitochondrial function. We have investigated energy conservation characteristics in liver mitochondria isolated from ob/ob mice and their lean littermates. Mitochondria from ob/ob mice exhibited a significant 65% increase in the rate of succinate oxidation compared to mitochondria from lean littermates, and maintained a normal respiratory control index, in agreement with previous studies. There was no significant difference in the membrane potential under State 4 conditions as detected by the accumulation of tetraphenylphosphonium (TPP+), although the State 3 membrane potential was elevated in the mitochondria from ob/ob mice, in accordance with the increased rate of respiration. The mitochondrial proton leak characteristics as a function of membrane potential were studied by titration of succinate oxidation with malonate in the presence of oligomycin. Although the membrane potential maintained at maximal rates of substrate oxidation was not significantly different between the two preparations, the slope of proton flux vs. membrane potential was markedly increased (3-5 fold) in mitochondria from ob/ob mice compared to lean littermate preparations. These characteristics are compatible with an increased proton leak rate across the liver mitochondrial membrane in ob/ob mice that is compensated by an increased respiratory capacity. Consistent with this functional evidence of obesity-related mitochondrial uncoupling, ob/ob hepatocytes, but not hepatocytes from lean littermates, showed constitutive expression of uncoupling protein-2 (UCP-2) mRNA (but not UCP-1 or UCP-3 mRNA). By contrast, in normal liver UCP-2 is exclusively expressed in non-parenchymal cells of the liver. However, UCP-2 could be induced in parenchymal liver cells of normal mice by treatment with endotoxin or other inflammatory stimuli. These data suggest that expression of UCP-2 occurs in mitochondria of hepatocytes of ob/ob mice as an adaptive response to cope with an increased supply of substrates to the liver or as a protective mechanism against formation of reactive oxygen species under conditions of increased redox pressure.