Energy metabolism and oxidative status of rat liver mitochondria in conditions of experimentally induced hyperthyroidism

Highlights

• Oxygen consumption was found to be higher in rats with induced hyperthyroidism.

• An increase in the quantity of subunits of CI, CII, CIV in HLM was found.

• The activity of СI, СII, and CS was increased in HLM.

• The activity of CII+III, CIII, CIV was inhibited in HLM.

• The induced hyperthyroidism led to a disorder in the exchange of ROS

Abstract

The aim of the present work was to investigate the energy metabolism and antioxidant status of rat liver mitochondria using a model of hyperthyroidism. In experimental animals, the level of triiodothyronine and thyroxine was increased 3- and 4-fold, respectively, in comparison with that in the control group, indicating the development of hyperthyroidism in these animals. Oxygen consumption was found to be higher in rats with experimentally induced hyperthyroidism (from 20 to 60% depending on the experimental scheme used), with a slight decrease in the efficiency of oxidative phosphorylation and respiratory state ratio. It was shown for the first time that the level the respiratory complexes of the electron transport chain in hyperthyroid rats increased; however, the quantity of complexes III and V changed unreliably. The assay of respiratory chain enzymes revealed that the activities of complexes I, II, and citrate synthase increased, whereas the activities complexes II + III, III, IV decreased in liver mitochondria of the experimental animals. Alterations in the oxidative state in liver mitochondria were found: a 60% increase in the hydrogen peroxide production rate and a 45% increase in lipid peroxidation. The activities of superoxide dismutase and catalase in the liver of experimental rats were higher than in the control. At the same time, the activity of glutathione peroxidase did not change. The data obtained indicate that the known activation of metabolism and changes in the oxidative status in thyrotoxicosis are associated with variations in the respiratory chain functioning and the antioxidant enzymes of mitochondria.

Introduction

Thyroid hormones (TH) are involved in the regulation of numerous functions of the organism. Various mechanisms of action of triiodothyronine (T₃) and thyroxin (T₄) affecting the body, tissues, and cells have been widely studied. At the same time, a number of questions related to the regulatory effects of these hormones on the energy metabolism parameters in mitochondria have not been completely investigated. The regulatory effects of TH are mediated by two different mechanisms: through the influence on nuclear receptors (Cioffi et al., 2013, Cheng, 2000) and the through receptors localized in various cell compartments, such as the plasma membrane, cytoplasm, cytoskeleton and mitochondria (Pessemesse et al., 2014).

The most widely recognized effect of thyroid hormones in mammals is their effect on energy metabolism, also known as the hypermetabolic effect (calorigenesis) (Harper and Seifert, 2008). In some studies, the efficiency of phosphorylation in rat liver mitochondria does not significantly change after administration of thyroxine, while oxygen consumption markedly increases (Shears and Bronk, 1981, Horrum et al., 1990, Soboll, 1993). Whether this is due to an increase in the activity/quantity of mitochondrial respiratory complexes (including autoxidizable electron carriers) or an increase in the electron transport chain (ETC) activity remains unclear.

Previous research results on the activity of respiratory chain enzymes in mitochondria of different tissues in hyperthyroidism remain inconclusive. For example, the maximal activity of cytochrome oxidase was significantly increased (by around 30%) in heart mitochondrial preparations from hyperthyroid rats (Paradies et al., 1994). On the other hand, the Vmax of complex I of rat heart mitochondria did not increase in hyperthyroidism (Veitch et al., 1993). Another study shows, that the activities of complexes I, II, III, IV in hyperthyroid rat livers was increased (Venditti et al., 2006).

The mitochondrial respiration chain is an important site for ROS generation. Changes in the respiratory chain and the hypermetabolic state, characteristic of hyperthyroidism, can lead to increased reactive oxygen species (ROS) production. Disorder in the balance between ROS formation and utilization leads to oxidative stress development. Mitochondria possess a powerful antioxidant system of enzymes, which prevents oxidative stress under normal conditions. This system includes superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT). It has been reported that hyperthyroidism enhances ROS generation and induces changes in the antioxidant defense in different tissues, including liver (Huh et al., 1998), heart (Venditti et al., 2007, Shinohara et al., 2000), brain (Das and Chainy, 2004) and muscles (Shinohara et al., 2000, Asayama et al., 1987). The unambiguous estimation of changes in the activities of antioxidant enzymes when excess thyroid hormone levels are present is lacking.

It has been observed recently that interactions between individual respiratory complexes result in the formation of structures termed supercomplexes. These structures have been hypothesized to provide stabilization of individual complexes, kinetic advantage and control of ROS generation by the electron transport chain (Letts and Sazanov, 2017). The effects of thyroxine on individual electron transport chain complexes and supercomplexes and their relationship with energy and oxidative metabolism in hyperthyroid liver has been so far not examined.

This study presents a more comprehensive examination of parameters related to the energy metabolism and oxidative status of rat liver mitochondria using a model of hyperthyroidism. The data obtained may provide insight into molecular mechanisms underlying the disturbance in energy and oxidative metabolism during thyrotoxicosis, as well as promote a search for new targets to correct this pathology.