Thyroid hormone action: the mitochondrial pathway

Binding of thyroid hormone by human erythrocyte cytosol proteins

Gel filtration (G-100, 0.01 M Tris, pH 7.4) of post-100,000 x g supernatant from lysate of washed human erythrocytes (RBC) revealed 3 fractions (R-1, R-2, R-3) which bound labeled T3 and T4. Major peak R-2 emerged with the mehoglobin fraction (A560 nm) and binding by this fraction was partially dissociable; the dissociable site bound D-T4, but not tetraidothyroacetic acid or reverse T3. Non-dissociable binding characterized peaks R-1 and R-3. R-1, R-2, and R-3 were pronase-digestible and R-1 binding was acid-unstable (pH 6.8 vs. 7.4). Evidence developed herein and elsewhere indicates that hemoglobin, itself, accounts for the binding within fraction R-2. Intact RBCs maintained for 72 hr at 4C in buffer enriched with T3 or T4 showed progressive incorporation with time of iodothyronines into the hemoglobin fraction.

The use of antithyroid therapy in the treatment of neoplastic conditions with cytotoxic drugs

Taking into account the known actions of thyroid hormone, we suggest that standard antithyroid therapy may be used to create a situation of greater sensitivity to cytotoxic drugs in tumour tissues as compared with normal tissues.

Early effects of thyroidectomy and triiodothyronine administration on rat-liver mitochondria

This work was undertaken to study the action exerted by thyroid hormones on mitochondria. By day 6 after thyroidectomy, the respective activities of two inner-membrane enzymes--succinate and beta-hydroxybutyrate cytochrome c reductases--had already dropped by 32 and 50%, whereas, in the outer membrane, the activity of rotenone-insensitive NADH-cytochrome c reductase did not change significantly. The decrease in the activity of the inner-membrane enzymes closely followed the disappearance of T3 and T4 from serum. 10 h after administration of 25 micrograms/100 g T3 to thyroidectomized rats, the activity of succinate and beta-hydroxybutyrate cytochrome c reductases and the oxygen consumption rate with succinate or beta-hydroxybutyrate were significantly increased, while, in the outer membrane, the activity of monoamine oxidase and rotenone-insensitive NADH-cytochrome c reductase remained unchanged. In the thyroidectomized rat, L-[3H]leucine incorporation in vivo is diminished in all the liver mitochondrial proteins, and especially in two constituents of MW 19 000 and 28 000. The radioactivity of these two components is also decreased in the normal rat treated with chloramphenicol, a specific inhibitor of mitochondrial protein synthesis. L-[14C]leucine incorporation in isolated liver mitochondria was significantly increased in the thyroidectomized rat, 10 h after T3 treatment. Thus, thyroid hormones have an early and preferential action on the mitochondrial protein synthesizing system and on the inner-membrane enzyme activities.

Mitochondrial thyroid hormone receptor: localization and physiological significance

Binding studies of thyroid hormone to submitochondrial fractions from rat liver suggest that the component responsible for high-affinity, low-capacity (saturable) binding of hormones arises from the inner mitochondrial membrane. The partially purified component, approximately 150,000 daltons, appears to be half protein and half lipid, largely phospholipids, tentatively identified as lecithin, phosphatidyl ethanolamine, and cardiolipin. A similar hormone-binding macromolecule was found in mitochondria from rabbit kidney, from human liver and kidney, and from rat kidney, myocardium, skeletal muscle, intestinal mucosa, whole small intestine, adipose tissue, and lung. It was absent from mitochondria of adult rat brain, spleen, and testis, organs calorigenically unresponsive to thyroid hormones injected in vivo, but was present in mitochondria from brains of rats 12 days old and younger. The organ distribution of the hormone-binding protein and its presence in neonatal brain mitochondria supports the biological relevance of the mitochondrial component as a thyroid hormone receptor.

Biochemical and stereological analysis of rat liver mitochondria in different thyroid states

The concentrations of the inner mitochondrial membrane markers cardiolipin and cytochrome alpha have been measured in liver homogenates and in purified mitochondria after thyroxine administration to thyroidectomized and normal rats. The biochemical results have been correlated with stereological electron micrographic analyses of hepatocytes in liver sections, and of isolated mitochondrial pellets. There were progressive and parallel increases in homogenate and mitochondrial cardiolipin concentration, and in mitochondrial cytochrome alpha concentration, after administration of 20 microgram of thyroxine on alternate days to thyroidectomized rats, and of 300 microgram on alternate days to normal rats. Electron microscope measurements showed marked differences in the shape of the mitochondria and in the number of cristae in different thyroid states. Hypothyroid mitochondria were shorter and wider than controls, and hyperthyroid mitochondria longer but of similar width. Mitochondrial volume per unit cell volume was virtually unchanged in hypo- and hyperthyroid animals. The most striking changes were a decrease in the area of the inner membrane plus cristae in thyroidectomized rats, and a substantial increase in membrane area after thyroxine administration. The biochemical and electron micrographic results indicate that, in rat liver, thyroid hormone administration leads to a selective increase in the relative amount of mitochondrial inner membranes, with little or no change in the mitochondrial volume per unit cell volume, or in total mitochondrial protein per unit total cell protein.

Fasting decreases triiodothyronine receptor capacity

Fasting decreases the ratio of hepatic nuclear to serum triiodothyronine (T3) by diminishing the binding capacity of nuclear T3 receptors. In combination with the lower serum T3 concentration caused by fasting, the decrease in receptor content results in a marked decrease in nuclear T3-receptor complexes. The changes in T3 receptor content and circulating T3 in fasted animals appear to be independent synergistic adaptations for caloric conservation in the fasted state. Unlike changes in hormonal level, the modification of nuclear receptor content provides a mechanism that may protect cells with a low caloric reserve independently of the metabolic status of the whole animal.