Uncoupling protein-2 expression and effects on mitochondrial membrane potential and oxidant stress in heart tissue

Myocardial uncoupling protein (UCP)-2 is increased with chronic peroxisome proliferator-activated receptor γ (PPARγ) stimulation, but the effect on membrane potential and superoxide is unclear. Wild-type (WT) and UCP-2 knockout (KO) mice were given a 3-week diet of control (C) or the PPARγ agonist pioglitazone (PIO; 50 μg/g-chow per day). In isolated mitochondria, UCP-2 content by Western blots, membrane potential (ΔΨ_m) by tetraphenylphosphonium (TPP), and relative superoxide levels by dihydroethidium (DHE) were measured. Oxygen respiration was determined at baseline and after 10 min anoxia-reoxygenation. PIO induced a 2-fold increase in UCP-2 and nuclear-bound PGC1α in WT mice with no UCP-2 expression in KO mice. Mitochondrial ΔΨ_m from WT mice on C and PIO diets was -166 ± 4 mV and -147 ± 6 mV, respectively (P < 0.05). These values were lower than in UCP-2 KO mice on C and PIO (-180 ± 4 mV and -180 ± 4 mV, respectively; P < 0.05). Maximal complex III inhibitable superoxide from WT mice on C and PIO diets was 22.5 ± 1.3 and 17.8 ± 1.1 AU, respectively (P < 0.05), and were lower than UCP-2 KO on C and PIO (32.9 ± 2.3 and 29.2 ± 1.9 AU, respectively; P < 0.05). Postanoxia, the respiratory control index (RCI) in mitochondria from WT mice with and without PIO was 2.5 ± 0.3 and 2.4 ± 0.2, respectively, and exceeded that of UCP-2 KO mice on C and PIO (1.2 ± 0.1 and 1.4 ± 0.1, respectively; P < 0.05). In summary, chronic PPARγ stimulation leads to depolarization of the inner membrane and reduced superoxide of isolated heart mitochondria, which was critically dependent on increased expression of UCP-2. Thus, UCP-2 expression affords resistance to brief anoxia-reoxygenation.