Thyroid hormone administration to hypothyroid rats restores the mitochondrial membrane permeability properties

Regulatory role of acetylation on enzyme activity and fluxes of energy metabolism pathways

BACKGROUND

Most of the enzymes involved in the central carbon metabolism are acetylated in Lys residues. It has been claimed that this covalent modification represents a novel regulatory mechanism by which both enzyme/transporter activities and pathway fluxes can be modulated.

METHODS

To establish which enzymes are regulated by acetylation, a systematic experimental analysis of activities and acetylation profile for several energy metabolism enzymes and pathway fluxes was undertaken in cells and mitochondria.

RESULTS

The majority of the glycolytic and neighbor enzymes as well as mitochondrial enzymes indeed showed Lys-acetylation, with GLUT1, HPI, CS, ATP synthase displaying comparatively lower acetylation patterns. The incubation of cytosolic and mitochondrial fractions with recombinant Sirt-3 produced lower acetylation signals, whereas incubation with acetyl-CoA promoted protein acetylation. Significant changes in acetylation levels of MDH and IDH-2 from rat liver mitochondria revealed no change in their activities. Similar observations were attained for the cytosolic enzymes from AS-30D and HeLa cells. A minor but significant (23%) increase in the AAT-MDH complex activity induced by acetylation was observed. To examine this question further, AS-30D and HeLa cells were treated with nicotinamide and valproic acid. These compounds promoted changes in the acetylation patterns of glycolytic proteins, although their activities and the glycolytic flux (as well as the OxPhos flux) revealed no clear correlation with acetylation.

CONCLUSION

Acetylation seems to play no predominant role in the control of energy metabolism enzyme activities and pathway fluxes.

GENERAL SIGNIFICANCE

The physiological function of protein acetylation on energy metabolism pathways remains to be elucidated.

Pathogenesis of hypothyroidism-induced NAFLD: Evidence for a distinct disease entity?

Nonalcoholic fatty liver disease (NAFLD), the most common liver disease worldwide, may be associated with primary hypothyroidism. However, the pathogenesis underlying such an association is complex and not completely understood. Here, we specifically discuss the pathogenic mechanisms potentially involved in hypothyroidism-induced NAFLD. To this end, we summarize the general pathophysiology of thyroid hormones (TH). Next, we analyze the published data from rodent studies by discussing whether hypothyroid rats may develop NAFLD via hyperphagia; whether mitochondria become energetically more efficient; what the overall energy balance is and if diversion of fatty substrates occurs; and the latest advancements in molecular pathogenesis brought about by metabolomics, cell imaging, lipophagy, autophagy and genetically engineered mouse models. Moreover, we discuss the data published regarding humans on the pathogenic role of TH, metabolic syndrome and other risk factors in hypothyroidism-related NAFLD as well as the putative mechanisms underlying the development of NAFLD-related hepatocellular carcinoma in hypothyroidism. In conclusion, although many research questions still remain unanswered, the pathophysiology of hypothyroidism-induced NAFLD makes this a potentially curable and distinct disease entity. However, further studies are needed to better elucidate the underlying mechanisms, and to ascertain whether treatment with either TH or thyromimetic agents improves NAFLD.

Supplementation of T3 recovers hypothyroid rat liver cells from oxidatively damaged inner mitochondrial membrane leading to apoptosis

Hypothyroidism is a growing medical concern. There are conflicting reports regarding the mechanism of oxidative stress in hypothyroidism. Mitochondrial oxidative stress is pivotal to thyroid dysfunction. The present study aimed to delineate the effects of hepatic inner mitochondrial membrane dysfunction as a consequence of 6-n-propyl-2-thiouracil-induced hypothyroidism in rats. Increased oxidative stress predominance in the submitochondrial particles (SMP) and altered antioxidant defenses in the mitochondrial matrix fraction correlated with hepatocyte apoptosis. In order to check whether the effects caused by hypothyroidism are reversed by T₃, the above parameters were evaluated in a subset of T₃-treated hypothyroid rats. Complex I activity was inhibited in hypothyroid SMP, whereas T₃ supplementation upregulated electron transport chain complexes. Higher mitochondrial H₂O₂ levels in hypothyroidism due to reduced matrix GPx activity culminated in severe oxidative damage to membrane lipids. SMP and matrix proteins were stabilised in hypothyroidism but exhibited increased carbonylation after T₃ administration. Glutathione content was higher in both. Hepatocyte apoptosis was evident in hypothyroid liver sections; T₃ administration, on the other hand, exerted antiapoptotic and proproliferative effects. Hence, thyroid hormone level critically regulates functional integrity of hepatic mitochondria; hypothyroidism injures mitochondrial membrane lipids leading to hepatocyte apoptosis, which is substantially recovered upon T₃ supplementation.

Thyroid hormone, thyromimetics, and metabolic efficiency

Thyroid hormone (TH) has long been recognized as a major modulator of metabolic efficiency, energy expenditure, and thermogenesis. TH effects in regulating metabolic efficiency are transduced by controlling the coupling of mitochondrial oxidative phosphorylation and the cycling of extramitochondrial substrate/futile cycles. However, despite our present understanding of the genomic and nongenomic modes of action of TH, its control of mitochondrial coupling still remains elusive. This review summarizes historical and up-to-date findings concerned with TH regulation of metabolic energetics, while integrating its genomic and mitochondrial activities. It underscores the role played by TH-induced gating of the mitochondrial permeability transition pore (PTP) in controlling metabolic efficiency. PTP gating may offer a unified target for some TH pleiotropic activities and may serve as a novel target for synthetic functional thyromimetics designed to modulate metabolic efficiency. PTP gating by long-chain fatty acid analogs may serve as a model for such strategy.

The higher susceptibility of congenital analbuminemic rats to Ca2+-induced mitochondrial permeability transition is associated with the increased expression of cyclophilin D and nitrosothiol depletion

Congenital analbuminemia is a rare autosomal recessive disorder characterized by a trace level of albumin in blood plasma and mild clinical symptoms. Analbuminemic patients generally present associated abnormalities, among which dyslipidemia is a hallmark. In this study, we show that mitochondria isolated from different tissues (liver, heart and brain) from 3-month-old analbuminemic rats (NAR) present a higher susceptibility to Ca(2+)-induced mitochondrial permeability transition (MPT), as assessed by either Ca(2+)-induced mitochondrial swelling, dissipation of membrane potential or mitochondrial Ca(2+) release. The Ca(2+) retention capacity of the liver mitochondria isolated from 3-month-old NAR was about 50% that of the control. Interestingly, the assessment of this variable in 21-day-old NAR indicated that the mitochondrial Ca(2+) retention capacity was preserved at this age, as compared to age-matched controls, which indicates that a reduced capacity for mitochondrial Ca(2+) retention is not a constitutive feature. The search for putative mediators of MPT sensitization in NAR revealed a 20% decrease in mitochondrial nitrosothiol content and a 30% increase in cyclophilin D expression. However, the evaluation of other variables related to mitochondrial redox status showed similar results between the controls and NAR, i.e., namely the contents of reduced mitochondrial membrane protein thiol groups and total glutathione, H(2)O(2) release rate, and NAD(P)H reduced state. We conclude that the higher expression of cyclophilin D, a major component of the MPT pore, and decreased nitrosothiol content in NAR mitochondria may underlie MPT sensitization in these animals.

Aromatase deficiency inhibits the permeability transition in mouse liver mitochondria

Lack of estrogens affects male physiology in a number of ways, including severe changes in liver metabolism that result in lipid accumulation and massive hepatic steatosis. Here we investigated whether estrogen deficiency may alter the functionality and permeability properties of liver mitochondria using, as an experimental model, aromatase knockout (ArKO) male mice, which cannot synthesize endogenous estrogens due to a disruption of the Cyp19 gene. Liver mitochondria isolated from ArKO mice displayed increased activity of the mitochondrial respiratory complex IV compared with wild-type mice and were less prone to undergo cyclosporin A-sensitive mitochondrial permeability transition (MPT) induced by calcium loading. The altered permeability properties of the mitochondrial membranes were not due to changes in reactive oxygen species, ATP levels, or mitochondrial membrane potential but were associated with increased content of the phospholipid cardiolipin, structural component of the mitochondrial membranes and regulator of the MPT pore, and with increased mitochondrial protein levels of Bcl-2 and the adenine nucleotide translocator (ANT), regulator and component of the MPT pore, respectively. Real-time RT-PCR demonstrated increased mRNA levels for Bcl-2 and ANT2 but not for the ANT1 isoform in ArKO livers. Supplementation of 17beta-estradiol retrieved ArKO mice from massive hepatic steatosis and restored mitochondrial permeability properties, cardiolipin, Bcl-2, and ANT2 levels. Overall, our findings demonstrate an important role of estrogens in the modulation of hepatic mitochondrial function and permeability properties in males and suggest that estrogen deficiency may represent a novel positive regulator of Bcl-2 and ANT2 proteins, two inhibitors of MPT occurrence and powerful antiapoptotic molecules.

Gating of the mitochondrial permeability transition pore by thyroid hormone

The calorigenic-thermogenic activity of thyroid hormone (T3) has long been ascribed to uncoupling of mitochondrial oxidative phosphorylation. However, the mode of action of T3 in promoting mitochondrial proton leak is still unresolved. Mitochondrial uncoupling by T3 is reported here to be transduced in vivo in rats and in cultured Jurkat cells by gating of the mitochondrial permeability transition pore (PTP). T3-induced PTP gating is shown here to be abrogated in inositol 1,4,5-trisphosphate (IP(3)) receptor 1 (IP(3)R1)(-/-) cells, indicating that the endoplasmic reticulum IP(3)R1 may serve as upstream target for the mitochondrial activity of T3. IP(3)R1 gating by T3 is due to its increased expression and truncation into channel-only peptides, resulting in IP(3)-independent Ca(2+) efflux. Increased cytosolic Ca(2+) results in activation of protein phosphatase 2B, dephosphorylation and depletion of mitochondrial Bcl2 (S70), and increase in mitochondrial free Bax leading to low-conductance PTP gating. The T3 transduction pathway integrates genomic and nongenomic activities of T3 in regulating mitochondrial energetics and may offer novel targets for thyromimetics designed to modulate energy expenditure.

Hypothyroidism provides resistance to kidney mitochondria against the injury induced by renal ischemia-reperfusion

Massive Ca(2+) accumulation in mitochondria, plus the stimulating effect of an inducing agent, i.e., oxidative stress, induces the so-called permeability transition, which is characterized by the opening of a nonspecific pore. This work was aimed at studying the influence of thyroid hormone on the opening of such a nonspecific pore in kidney mitochondria, as induced by an oxidative stress. To meet this objective, membrane permeability transition was examined in mitochondria isolated from kidney of euthyroid and hypothyroid rats, after a period of ischemia/reperfusion. It was found that mitochondria from hypothyroid rats were able to retain accumulated Ca(2+) to sustain a transmembrane potential after Ca(2+) addition, as well as to maintain matrix NAD(+) and membrane cytochrome c content. The protective effect of hypothyroidism was clearly opposed to that occurring in ischemic reperfused mitochondria from euthyroid rats. Our findings demonstrate that these mitochondria were unable to preserve selective membrane permeability, except when cyclosporin A was added. It is proposed that the protection is conferred by the low content of cardiolipin found in the inner membrane. This phospholipid is required to switch adenine nucleotide translocase from specific carrier to a non-specific pore.

Thyroid hormone regulates the hypotriglyceridemic gene APOA5

The apolipoprotein AV gene (APOA5) is a key determinant of plasma triglyceride levels, a major risk factor for coronary artery disease and a biomarker for the metabolic syndrome. Since thyroid hormones influence very low density lipoprotein triglyceride metabolism and clinical studies have demonstrated an inverse correlation between thyroid status and plasma triglyceride levels, we examined whether APOA5 is regulated by thyroid hormone. Here we report that 3,5,3'-triiodo-L-thyronine (T3) and a synthetic thyroid receptor beta (TRbeta) ligand increase APOA5 mRNA and protein levels in hepatocytes. Our data revealed that T3-activated TR directly regulates APOA5 promoter through a functional direct repeat separated by four nucleotides (DR4). Interestingly, we show that upstream stimulatory factor 1, a transcription factor associated with familial combined hyperlipidemia and elevated triglyceride levels in humans, and upstream stimulatory factor 2 cooperate with TR, resulting in a synergistic activation of APOA5 promoter in a ligand-dependent manner via an adjacent E-box motif. In rats, we observed that apoAV levels declines with thyroid hormone depletion but returned to normal levels upon T3 administration. In addition, treatments with a TRbeta-selective agonist increased apoAV and diminished triglyceride levels. The identification of APOA5 as a T3 target gene provides a new potential mechanism whereby thyroid hormones can influence triglyceride homeostasis. Additionally, these data suggest that TRbeta may be a potential pharmacological target for the treatment of hypertriglyceridemia.

Ultrastructural zonal heterogeneity of hepatocytes and mitochondria within the hepatic acinus during liver regeneration after partial hepatectomy

BACKGROUND INFORMATION

Partial hepatectomy (70%) induces cell proliferation until the original mass of the liver is restored. In the first 24 h after partial hepatectomy, drastic changes in the metabolism of the remaining liver have been shown to occur. To evaluate changes in hepatocyte ultrastructure within the hepatic acinus during the liver regenerative process, we investigated, by light and electron microscopy observations on specimens taken 0 h, 24 h and 96 h after partial hepatectomy, the hepatocyte structure and ultrastructure in the periportal and pericentral area of the hepatic acinus, with a particular emphasis on mitochondria ultrastructure. Moreover, some biochemical events that could affect the mitochondria ultrastructure and function were investigated.

RESULTS

We found that, 24 h after partial hepatectomy, mitochondria with altered ultrastructure were preferentially localized in the periportal area. Periportal hepatocytes showed also an increase in the number of peroxisomes, free ribosomes, lysosomes and autophagosomes. Altered mitochondria showed swelling, an ultrastructural index of increased membrane permeability, a reduction in the number of cristae, and a rarefied, often vacuoled, matrix. Consistently, an increase in the mitochondrial oxidized/reduced glutathione ratio was found as well as calcium release from mitochondria in a manner inhibited by cyclosporin A. Interestingly, light and electron microscopy analysis showed that the hepatocytes in the periportal area were the cells with the major structural attributes to proliferate. At 96 h after partial hepatectomy, the preferential zonation of altered mitochondria was lost and the normal mitochondrial membrane permeability properties were restored.

CONCLUSIONS

We suggest that 24 h after partial hepatectomy, a preferential zonation of altered mitochondria in the periportal hepatocytes could be involved in the changes of metabolic and functional heterogeneity of the hepatocytes within the hepatic acinus during the regenerative process.

Modulation of mitochondrial transition pore components by thyroid hormone

Thyroid hormone (TH) modulates metabolic efficiency by controlling the coupling of mitochondrial oxidative phosphorylation. However, its uncoupling mode of action is still enigmatic. Treatment of Jurkat or GH3 cells with T3 is reported here to result in limited, Cyclosporin A-sensitive mitochondrial depolarization, conforming to low conductance gating of the mitochondrial transition pore (MTP). MTP protein components induced by T3 treatment were verified in T3-treated and hypothyroid rat liver as well as in Jurkat cells. T3 treatment resulted in increase in mitochondrial Bax and Bak together with decreased mitochondrial Bcl2. T3-induced mitochondrial depolarization was aborted by overexpression of Bcl2. In contrast to Bax-Bcl2 family proteins, some other MTP components were either not induced by T3 (e.g. voltage-dependent anion channel) or were induced, but were not involved in Cyclosporin A-sensitive MTP gating (e.g. Cyclophilin D and adenine nucleotide translocase-2) Hence, TH-induced mitochondrial uncoupling may be ascribed to low conductance MTP gating mediated by TH-induced increase in mitochondrial proapoptotic combined with a decrease in mitochondrial antiapoptotic proteins of the Bax-Bcl2 family.

Advances in assessment, diagnosis, and treatment of hyperthyroidism in children

The thyroid gland is responsible for regulating multiple complex metabolic processes that affect most organs. Physical growth and cognitive development are dependent on proper levels of thyroid hormone. This article will review common challenges in the diagnosis of hyperthyroidism in children, the approaches to treatment, and the nursing interventions guided toward child and family responses to thyroid disease. A comparison of signs and symptoms of hypothyroidism and hyperthyroidism is also included. The nursing interventions addressed in the article integrate the biological, psychological, social, and environmental stresses and adaptations necessary to cope with hyperthyroid disease.

Species-specific modulation of the mitochondrial permeability transition by norbormide

In the present study, we show that norbormide stimulates the opening of the permeability transition pore (PTP) in mitochondria from various organs of the rat but not of guinea pig and mouse. Norbormide does not affect the basic parameters that modulate the PTP activity since the proton electrochemical gradient, respiration, phosphorylation and Ca(2+) influx processes are only partially affected. On the other hand, norbormide induces rat-specific changes in the fluidity of the lipid interior of mitochondrial membranes, as revealed by fluorescence anisotropy of various reporter molecules. Such changes increase the PTP open probability through the internal Me(2+) regulatory site. The lack of PTP opening by norbormide is matched by a negligible perturbation of internal lipid domains in guinea pig and mouse, suggesting that the drug does not gain access to the matrix in the mitochondria from these species. Consistent with this interpretation, we demonstrate a preferential interaction of norbormide with the mitochondrial surface leading to alterations of the Me(2+) binding affinity for the external PTP regulatory site. Our findings indicate that norbormide affects Me(2+) binding to the regulatory sites of the PTP, and suggest that the drug could be taken up by a mitochondrial transport system unique to the rat. The characterization of the norbormide target may lead to a better understanding of the mechanisms underlying the mitochondrial PTP as well as to the identification of species-specific drugs that affect mitochondrial function.

Thyroid hormone treatment of hypothyroid rats restores the regenerative capacity and the mitochondrial membrane permeability properties of the liver after partial hepatectomy

We have investigated the effect of thyroid hormone on recovery of liver mass and on the mitochondrial membrane permeability properties during rat liver regeneration after 70% partial hepatectomy (PH). In the euthyroid state, liver weight starts to recover 24 h after PH and is completely restored 96 h after PH. Cyclosporin A (CsA)-sensitive mitochondrial permeability transition (MPT) occurs 24 h after PH, and it has been suggested to act in the signaling mechanism for hepatocyte proliferation. In this study we show that hypothyroidism delays recovery of the liver mass, being only 50% of the initial weight 96 h after PH, and alters the duration and mode of MPT occurrence, first inducing a CsA-insensitive swelling 24 h after PH, followed by a CsA-sensitive swelling 96 h after PH. The occurrence of both CsA-sensitive and -insensitive swelling is shown to be associated with an increase in mitochondrial calcium content. Concurrent with mitochondrial swelling, external release of matrix proteins from mitochondria, such as aspartate aminotransferase and malate dehydrogenase, is shown to be CsA insensitive 24 h after PH and CsA sensitive 96 h after PH. After thyroid hormone administration to hypothyroid rats, the liver regenerative capacity is restored, and the duration and mode of MPT occurrence as well as changes in mitochondrial calcium content become similar to those observed in the euthyroid condition. The results of the present study suggest the involvement of a mitochondria-mediated pathway in regulation of the liver regenerative process by thyroid hormone.