Nature of respiratory stimulation in hyperthyroidism: the redox behaviour of cytochrome c

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 2016.

Thyroid hormones as molecular determinants of thermogenesis

Thyroid hormones (TH) are major modulators of energy metabolism and thermogenesis. It is generally believed that 3,5,3'-triiodo-l-thyronine (T3) is the only active form of TH, and that most of its effects are mediated by nuclear T3 receptors, which chiefly affect the transcription of target genes. Some of these genes encode for the proteins involved in energy metabolism. However, a growing volume of evidence now indicates that other iodothyronines may be biologically active. Several mechanisms have been proposed to explain the calorigenic effect of TH, but none has received universal acceptance. Cold acclimation/exposure and altered nutritional status are physiological conditions in which a modulation of energy expenditure is particularly important. TH seem to be deeply involved in this modulation, and this article will review some aspects of their possible influence in these conditions.

Divergence of the mitochondrial electron transport chains from the green alga Chlamydomonas reinhardtii and its colorless close relative Polytomella sp

Compelling evidence exists that the colorless algae of the genus Polytomella arose from a green Chlamydomonas-like ancestor by losing its functional photosynthetic apparatus. Due to the close relationship between the colorless and the green chlorophyte, Polytomella sp. appeared as a useful indicative framework for structural studies of Chlamydomonas reinhardtii mitochondria. However, comparative studies reported here unexpectedly revealed significant differences between the mitochondrial respiratory systems of the two algae. Two-dimensional blue native/SDS-PAGE of isolated mitochondria indicated that cytochrome-containing respiratory complexes III and IV in the two chlorophytes contrast in size, subunit composition and relative abundance. Complex IV in Polytomella is smaller than its counterpart in C. reinhardtii and occurs in two forms that differ presumably in the presence of subunit COXIII. The cytochrome c and the iron-sulfur Rieske protein of both chlorophytes revealed structural differences on the amino acid sequence level. Under comparable culture conditions, the colorless alga exhibits lower levels of cytochrome c and complex IV but a higher respiratory activity than the green alga. Cytochrome c levels were also found to be differently regulated by the growth conditions in both algae. The divergence between the respiratory systems in the two related chlorophytes can be viewed as a consequence of the loss of photosynthetic activity and/or of the adaptation to the environment via the acquisition of a more flexible, heterotrophic metabolism. Our understanding of mitochondrial function and evolution is expected to be greatly enhanced via further parallel studies of photosynthetic/non-photosynthetic algae, for which this study forms an incentive.

Polyunsaturated fatty acid deficiency reverses effects of alcohol on mitochondrial energy metabolism

BACKGROUND/AIMS

Polyunsaturated fatty acids (PUFA) deficiency is common in patients with alcoholic liver disease. The suitability of reversing such deficiency remains controversial. The aim was to investigate the role played by PUFA deficiency in the occurrence of alcohol-related mitochondrial dysfunction.

METHODS

Wistar rats were fed either a control diet with or without alcohol (control and ethanol groups) or a PUFA deficient diet with or without alcohol (PUFA deficient and PUFA deficient+ethanol groups). After 6 weeks, liver mitochondria were isolated for energetic studies and fatty acid analysis.

RESULTS

Mitochondria from ethanol fed rats showed a dramatic decrease in oxygen consumption rates and in cytochrome oxidase activity. PUFA deficiency showed an opposite picture. PUFA deficient+ethanol group roughly reach control values, regarding cytochrome oxidase activity and respiratory rates. The relationship between ATP synthesis and respiratory rate was shifted to the left in ethanol group and to the right in PUFA-deficient group. The plots of control and PUFA deficient+ethanol groups were overlapping. Phospholipid arachidonic over linoleic ratio closely correlated to cytochrome oxidase and oxygen uptake.

CONCLUSIONS

PUFA deficiency reverses alcohol-related mitochondrial dysfunction via an increase in phospholipid arachidonic over linoleic ratio, which raises cytochrome oxidase activity. Such deficiency may be an adaptive mechanism.

Mitochondrial respiratory chain adjustment to cellular energy demand

Because adaptation to physiological changes in cellular energy demand is a crucial imperative for life, mitochondrial oxidative phosphorylation is tightly controlled by ATP consumption. Nevertheless, the mechanisms permitting such large variations in ATP synthesis capacity, as well as the consequence on the overall efficiency of oxidative phosphorylation, are not known. By investigating several physiological models in vivo in rats (hyper- and hypothyroidism, polyunsaturated fatty acid deficiency, and chronic ethanol intoxication) we found that the increase in hepatocyte respiration (from 9.8 to 22.7 nmol of O(2)/min/mg dry cells) was tightly correlated with total mitochondrial cytochrome content, expressed both per mg dry cells or per mg mitochondrial protein. Moreover, this increase in total cytochrome content was accompanied by an increase in the respective proportion of cytochrome oxidase; while total cytochrome content increased 2-fold (from 0.341 +/- 0.021 to 0.821 +/- 0.024 nmol/mg protein), cytochrome oxidase increased 10-fold (from 0.020 +/- 0.002 to 0.224 +/- 0.006 nmol/mg protein). This modification was associated with a decrease in the overall efficiency of the respiratory chain. Since cytochrome oxidase is well recognized for slippage between redox reactions and proton pumping, we suggest that this dramatic increase in cytochrome oxidase is responsible for the decrease in the overall efficiency of respiratory chain and, in turn, of ATP synthesis yield, linked to the adaptive increase in oxidative phosphorylation capacity.

Control of energy metabolism by iodothyronines

One of the most widely recognized effects of thyroid hormones (TH) in adult mammals is their influence over energy metabolism. In the past, this has received much attention but, possibly because of the complex mode of action of thyroid hormones, no universally accepted mechanism to explain this effect has been put forward so far. Significant advances in our understanding of the biochemical processes involved in the actions of TH have been made in the last three decades and now it seems clear that TH can act through both nuclear-mediated and extranuclear-mediated pathways. TH increase energy expenditure, partly by reducing metabolic efficiency, with control of specific genes at the transcriptional level, being is thought to be the major molecular mechanism. However, both the number and the identity of the thyroid-hormone-controlled genes remain unknown, as do their relative contributions. The recent discovery of uncoupling proteins (UCPs) (in addition to UCP1 in brown adipose tissue) in almost all tissues in animals, including humans, has opened new perspectives on the understanding of the mechanisms involved in the regulation of energy metabolism by thyroid hormones. Other approaches have included the various attempts made to attribute changes in respiratory activity to a direct influence of thyroid hormones over the mitochondrial energy-transduction apparatus. In addition, an increasing number of studies has revealed that TH active in the regulation of energy metabolism include not only T3, but also other iodothyronines present in the biological fluids, such as 3,5-diiodothyronine (3,5-T2). This, in turn, may make it possible to explain some of the effects exerted by TH on energy metabolism that cannot easily be attributed to T3.

Thyroid hormone-induced oxidative damage on lipids, glutathione and DNA in the mouse heart

Oxygen radicals of mitochondrial origin are involved in oxidative damage. In order to analyze the possible relationship between metabolic rate, oxidative stress and oxidative damage, OF1 female mice were rendered hyper- and hypothyroid by chronic administration of 0.0012% L-thyroxine (T4) and 0.05% 6-n-propyl-2-thiouracil (PTU), respectively, in their drinking water for 5 weeks. Hyperthyroidism significantly increased the sensitivity to lipid peroxidation in the heart, although the endogenous levels of lipid peroxidation were not altered. Thyroid hormone-induced oxidative stress also resulted in higher levels of GSSG and GSSG/GSH ratio. Oxidative damage to mitochondrial DNA was greater than that to genomic DNA. Hyperthyroidism decreased oxidative damage to genomic DNA. Hypothyroidism did not modify oxidative damage in the lipid fraction but significantly decreased GSSG and GSSG/GSH ratio and oxidative damage to mitochondrial DNA. These results indicate that thyroid hormones modulate oxidative damage to lipids and DNA, and cellular redox potential in the mouse heart. A higher oxidative stress in the hyperthyroid group is presumably neutralized in the case of nuclear DNA by an increase in repair activity, thus protecting this key molecule. Treatment with PTU, a thyroid hormone inhibitor, reduced oxidative damage in the different cell compartments.

High-throughput profiling of the mitochondrial proteome using affinity fractionation and automation

Recent studies have demonstrated the need for complementing cellular genomic information with specific information on expressed proteins, or proteomics, since the correlation between the two is poor. Typically, proteomic information is gathered by analyzing samples on two-dimensional gels with the subsequent identification of specific proteins of interest by using trypsin digestion and mass spectrometry in a process termed peptide mass fingerprinting. These procedures have, as a rule, been labor-intensive and manual, and therefore of low throughput. The development of automated proteomic technology for processing large numbers of samples simultaneously has made the concept of profiling entire proteomes feasible at last. In this study, we report the initiation of the (eventual) complete profile of the rat mitochondrial proteome by using high-throughput automated equipment in combination with a novel fractionation technique using minispin affinity columns. Using these technologies, approximately one hundred proteins could be identified in several days. In addition, separate profiles of calcium binding proteins, glycoproteins, and hydrophobic or membrane proteins could be generated. Because mitochondrial dysfunction has been implicated in numerous diseases, such as cancer, Alzheimer's disease and diabetes, it is probable that the identification of the majority of mitochondrial proteins will be a beneficial tool for developing drug and diagnostic targets for associated diseases.

Thyroid hormone status and membrane n-3 fatty acid content influence mitochondrial proton leak

Proton leak, as determined by the relationship between respiration rate and membrane potential, was lower in mitochondria from hypothyroid rats compared to euthyroid controls. Moreover, proton leak rates diminished even more when hypothyroid rats were fed a diet containing 5% of the lipid content as n-3 fatty acids. Similarly, proton leak was lower in euthyroid rats fed the 5% n-3 diet compared to one containing only 1% n-3 fatty acids. Lower proton leaks rates were associated with increased inner mitochondrial membrane levels of n-3 fatty acids and a decrease in the ratio of n-6/n-3 fatty acids. This trend was evident in the phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol and cardiolipin phospholipid fractions. These results suggest that a significant portion of the effect of thyroid hormone status on proton leak is due to alterations in membrane fatty acid composition, primarily changes in n-3 content. Both the hypothyroid state and dietary effects appear to be mediated in part by inhibition of the Delta6- and Delta5-desaturase pathways.