3,5,3′-Triiodothyronine Induces Mitochondrial Permeability Transition Mediated by Reactive Oxygen Species and Membrane Protein Thiol Oxidation

Ca2+-loaded rat liver mitochondria treated with 3,5,3′-triiodothyronine (T3) undergo nonspecific inner membrane permeabilization, as evidenced by mitochondrial swelling, a decrease in membrane potential (ΔΨ), and an increase in the rate of oxygen uptake. T3analogues thyroxine (T4), 3′,5′-diiodothyronine (T2), and 3,5′,3′-triiodothyronine (reverse T3), in decreasing order of potency, resulted in a similar but less extensive effect. Permeabilization induced by T3is dependent on Ca2+(1 μM) and T3(0.5–25 μM) concentrations and is inhibited by cyclosporin A, a known inhibitor of mitochondrial permeability transition. Catalase or dithiothreitol also prevents membrane permeabilization, suggesting the participation of membrane protein thiol group oxidationinduced by reactive oxygen species. The determination of the mitochondrial membrane protein thiol group content after treatment with Ca2+and T3shows a significant decrease, due to thiol oxidation. When mitochondria are incubated in the presence of inorganic phosphate and the protonophore carbonyl cyanidep-trifluoromethoxyphenylhydrazone, mitochondrial swelling still occurs after treatment with T3and high Ca2+concentrations, suggesting that mitochondrial permeabilization is not dependent on T3-induced ΔΨ or matrix pH alterations. Under these experimental conditions, when no oxygen is present in the incubation medium, no permeabilization occurs, suggesting that the permeabilization is dependent on mitochondrial-generated reactive oxygen species. Confirming this hypothesis, superoxide generation in a suspension of submitochondrial particles is increased when T3is present. Our results lead to the conclusion that T3induces a situation of oxidative stress in isolated liver mitochondria, with Ca2+-mediated membrane protein thiol oxidation and nonspecific inner membrane permeabilization.