Effect of hypothyroidism on the expression of cytochrome c and cytochrome c oxidase in heart and muscle during development

The assessment of fetal cardiac output in maternal hypothyroidism under levothyroxine treatment

OBJECTIVE

In this study, we investigated whether maternal hypothyroidism has a role in the cardiac output (CO) of the fetus or not.

METHODS

Pregnant women between 33 and 37 gestational weeks known to have hypothyroidism and using levothyroxine were accepted as the case group. Gestational age-matched healthy euthyroid pregnant women constituted the control group. Fetal echocardiography was performed. Diameters and the velocity waveform of the pulmonary artery (PA) and aortic valves were measured. Velocity time integral (VTI) was also measured from the ventricular outflow tract. CO was calculated using VTI × π (Aortic Valve or Pulmonary Valve diameter/2) 2 × heart rate formula.

RESULTS

The aortic and PA annulus were measured larger in the control group. (p = .003, p = .005, respectively). Furthermore, the right and left CO of the case group were lower than the control group. Whereas the mean combined CO (ml/min) of the case group was 674.8 ± 146.2, it was 827.8 ± 167.9 in the control group (p < .001). Additionally, a negative correlation was observed between thyroid-stimulating hormone and aortic VTI (r:-.480; p:.006).

CONCLUSION

The findings of our study suggest that the CO of the fetus may be affected by maternal hypothyroidism.

Cardiac Cytochrome-c Oxidase Deficiency Occurs During Late Postnatal Development in Progeny of Copper-Deficient Rats

Although cytochrome-c oxidase (CCO) is a copper-dependent enzyme, the effect of maternal copper deficiency on the expression of CCO activity during postnatal development of the neonatal rat heart has not been investigated extensively. Here, we show that CCO activity in heart mitochondria isolated from neonates of copper-deficient dams did not exhibit significant reductions until postnatal days (PND) 15 and 21. In addition, Immunoblot analysis indicated that the CCO subunit (Cox-1) was reduced on postnatal Days 10 and 21, and that Cox-4 was reduced on PND 21 in heart mitochondria of the neonates from copper-deficient dams. These findings indicate that the impairment of CCO activity in neonatal heart by maternal copper deficiency occurs late in the postnatal heart development. Furthermore, the concurrent reductions in Cox-1 and Cox-4 suggest that the impaired CCO activity reflects a CCO deficiency in heart mitochondria. CCO activity and Cox-1 in heart mitochondria were not fully restored by 6 weeks of postweaning copper repletion in the pups of copper-deficient dams. This indicates that prolonged maternal intake of moderately low dietary copper produces CCO deficiency in cardiac mitochondria of neonates during late postnatal heart development, after terminal differentiation of cardiomyocytes occurs. The resistance of CCO deficiency to repair by dietary copper supplementation may be related to the relatively slow turnover of the affected mitochondria in the terminally differentiated heart.

Quantitative Assessment of Cardiac Function in Fetuses of Women with Maternal Gestational Thyroid Dysfunction Using VVI Echocardiography

Background

The study aimed to investigate the clinical value of velocity vector imaging (VVI) in assessing heart function in fetuses of pregnant women with thyroid dysfunction. The inter-observer and intra-observer variability was assessed for all VVI parameters observed.

Material/Methods

The participants were enrolled from singleton pregnant women with gestational ages ranging 24⁺⁰ to 40⁺¹ weeks who visited the Department of Obstetrics and Gynecology at the Affiliated Hospital of Qingdao University, China, for prenatal care from July 2011 to February 2014. Digital 2-dimensional (2D) dynamic 4-chamber images of the heart were collected. A total of qualified 226 images from 125 fetuses of pregnant women with normal thyroid (control group), 64 fetuses of pregnant women with hypothyroidism (hypothyroidism group), and 37 fetuses of pregnant women with hyperthyroidism (hyperthyroidism group) were interrogated offline using VVI software. The echocardiographic parameters including the myocardium peak systolic velocity (Vs), peak diastolic velocity (Vd), peak systolic strain (S), peak systolic strain rate (SRs), peak diastolic strain rate (SRd) of RV and LV, were obtained from the velocity curves of 2D myocardial motion. The heart rate was measured using a virtual M-mode algorithm built into the software.

Results

The study found that the longitudinal Vs and Vd of both ventricles in the control group gradually decreased from basal segments to apical segments and significantly increased over the gestation. S, SRs, and SRd of both ventricles remained stable after middle gestation. Compared with the control group, the hypothyroidism and hyperthyroidism groups exhibited significantly reduced S, SRs, and SRd, even for fetuses at 24-weeks gestation. There were no significant differences in global Vs and global Vd between the control group and the hyperthyroidism or hypothyroidism groups.

Conclusions

The thyroid dysfunction of pregnant women may damage fetal heart function, and VVI could be a sensitive technique to measure the variation of fetal heart function.

Serum total antioxidant status and lipid peroxidation marker malondialdehyde levels in overt and subclinical hypothyroidism

BACKGROUND

Mitochondria are the main production site of free oxygen radicals, which can cause organ dysfunction by oxidation of cellular macromolecules such as carbohydrates, lipids and proteins. Oxidative stress may result from either overproduction of these species or from failure of the antioxidant defence systems. Thyroid hormones have well-known effects on mitochondrial oxygen consumption, but data about how hypothyroidism affects oxidative stress are controversial, and little is known about oxidative stress in subclinical hypothyroidism. Total antioxidant status (TAS) gives information about all of the antioxidants in the organism, while malondialdehyde (MDA) is a lipid peroxidation marker used to assess lipid peroxidation due to increased oxidative stress. We aimed to determine how hypothyroidism and subclinical hypothyroidism affect serum MDA and TAS.

SUBJECTS AND METHODS

Serum TAS, MDA, C-reactive protein levels and lipid compositions were studied in 20 hypothyroid, 40 subclinical hypothyroid and 40 healthy subjects.

RESULTS

MDA was elevated in both hypothyroid and subclinical hypothyroid patients compared with controls, while TAS levels show no significant differences between groups. Low-density lipoprotein (LDL) cholesterol levels were significantly high in both hypothyroid and subclinical hypothyroid patients. Triglyceride levels were high only in hypothyroid patients when compared with the controls. MDA showed a correlation with LDL cholesterol, total cholesterol and triglyceride.

CONCLUSIONS

These results suggest an increased oxidative stress in both hypothyroid and subclinical hypothyroidism states, which can be explained by both the insufficient increase in the antioxidant status and the altered lipid metabolism in these cases.

Diabetic myopathy differs between Ins2Akita+/- and streptozotocin-induced Type 1 diabetic models

Mechanistic studies examining the effects of Type 1 diabetes mellitus (T1DM) on skeletal muscle have largely relied on streptozotocin-induced diabetic (STZ) rodents. Unfortunately, characterization of diabetic myopathy in this model is confounded by the effects of streptozotocin on skeletal muscle independent of the diabetic phenotype. Here we define adolescent diabetic myopathy in a novel, genetic model of T1DM, Ins2(Akita+/-) mice, and contrast these findings with STZ mice. Eight weeks of diabetes resulted in significantly reduced gastrocnemius-plantaris-soleus mass (control: 0.16 +/- 0.005 g; Ins2(Akita+/-): 0.12 +/- 0.003 g; STZ: 0.12 +/- 0.01g) and IIB/D fiber area in Ins2(Akita+/-) (1,294 +/- 94 microm(2)) and STZ (1,768 +/- 163 microm(2)) compared with control (2,241 +/- 144 microm(2)). Conversely, STZ type I fibers (1,535 +/- 165 microm(2)) were significantly larger than Ins2(Akita+/-) (915 +/- 76 microm(2)) but not control (1,152 +/- 86 microm(2)). Intramyocellular lipid increased in STZ (122.9 +/- 3.6% of control) but not Ins2(Akita+/-) likely resultant from depressed citrate synthase (control: 6.2 +/- 1.2 micromol.s(-1).mg(-1); Ins2(Akita+/-): 5.2 +/- 0.8 micromol.s(-1).mg(-1); STZ: 2.8 +/- 0.5 micromol.s(-1).mg(-1)) and 3-beta-hydroxyacyl coenzyme-A dehydrogenase (control: 4.2 +/- 0.6 nmol.s(-1).mg(-1); Ins2(Akita+/-): 5.0 +/- 0.6 nmol.s(-1).mg(-1); STZ: 2.7 +/- 0.6 nmol.s(-1).mg(-1)) enzyme activity in STZ muscle. In situ muscle stimulation revealed lower absolute peak tetanic force in Ins2(Akita+/-) (70.2 +/- 8.2% of control) while STZ exhibited an insignificant decrease (87.6 +/- 7.9% of control). Corrected for muscle mass, no force loss was observed in Ins2(Akita+/-), while STZ was significantly elevated vs. control and Ins2(Akita+/-). These results demonstrate that atrophy and specific fiber-type loss in Ins2(Akita+/-) muscle did not affect contractile properties (relative to muscle mass). Furthermore, we demonstrate distinctive contractile, metabolic, and phenotypic properties in STZ vs. Ins2(Akita+/-) diabetic muscle despite similarity in hyperglycemia/hypoinsulinemia, raising concerns of our current state of knowledge regarding the effects of T1DM on skeletal muscle.

Mitochondrial gene expression is regulated at multiple levels and differentially in the heart and liver by thyroid hormones

Biogenesis of the oxidative phosphorylation system (OXPHOS) requires the coordinated expression of the nuclear and the mitochondrial genomes. Thyroid hormones play an important role in cell growth and differentiation and are one of the main effectors in mitochondrial biogenesis. To determine how mtDNA expression is regulated, we have investigated the response of two different tissues, the heart and liver, to the thyroid hormone status in vivo and in vitro. We show here that mtDNA expression is a tightly regulated process and that several levels of control can take place simultaneously. In addition, we show that the mechanisms operating in the control of mtDNA expression and their relevance differ between the two tissues, being gene dosage important only in heart while transcription rate and translation efficiency have more weight in liver cells. Another interesting difference is the lack of a direct effect of thyroid hormones on heart mitochondrial transcription.

Effect of chronic contractile activity on SS and IMF mitochondrial apoptotic susceptibility in skeletal muscle

Chronic contractile activity of skeletal muscle induces an increase in mitochondria located in proximity to the sarcolemma [subsarcolemmal (SS)] and in mitochondria interspersed between the myofibrils [intermyofibrillar (IMF)]. These are energetically favorable metabolic adaptations, but because mitochondria are also involved in apoptosis, we investigated the effect of chronic contractile activity on mitochondrially mediated apoptotic signaling in muscle. We hypothesized that chronic contractile activity would provide protection against mitochondrially mediated apoptosis despite an elevation in the expression of proapoptotic proteins. To induce mitochondrial biogenesis, we chronically stimulated (10 Hz; 3 h/day) rat muscle for 7 days. Chronic contractile activity did not alter the Bax/Bcl-2 ratio, an index of apoptotic susceptibility, and did not affect manganese superoxide dismutase levels. However, contractile activity increased antiapoptotic 70-kDa heat shock protein and apoptosis repressor with a caspase recruitment domain by 1.3- and 1.4-fold (P<0.05), respectively. Contractile activity elevated SS mitochondrial reactive oxygen species (ROS) production 1.4- and 1.9-fold (P<0.05) during states IV and III respiration, respectively, whereas IMF mitochondrial state IV ROS production was suppressed by 28% (P<0.05) and was unaffected during state III respiration. Following stimulation, exogenous ROS treatment produced less cytochrome c release (25-40%) from SS and IMF mitochondria, and also reduced apoptosis-inducing factor release (approximately 30%) from IMF mitochondria, despite higher inherent cytochrome c and apoptosis-inducing factor expression. Chronic contractile activity did not alter mitochondrial permeability transition pore (mtPTP) components in either subfraction. However, SS mitochondria exhibited a significant increase in the time to Vmax of mtPTP opening. Thus, chronic contractile activity induces predominantly antiapoptotic adaptations in both mitochondrial subfractions. Our data suggest the possibility that chronic contractile activity can exert a protective effect on mitochondrially mediated apoptosis in muscle.

Effect of denervation on mitochondrially mediated apoptosis in skeletal muscle

Chronic muscle disuse induced by denervation reduces mitochondrial content and produces muscle atrophy. To investigate the molecular mechanisms responsible for these adaptations, we assessed 1) mitochondrial biogenesis- and apoptosis-related proteins and 2) apoptotic susceptibility and cell death following denervation. Rats were subjected to 5, 7, 14, 21, or 42 days of unilateral denervation of the sciatic or peroneal nerve. Muscle mass and mitochondrial content were reduced by 40–65% after 21 and 42 days of denervation. Denervation-induced decrements in mitochondrial content occurred along with 60% and 70% reductions in transcription factor A (Tfam) and peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α, respectively. After 42 days of denervation, Bax was elevated by 115% and Bcl-2 was decreased by 89%, producing a 16-fold increase in the Bax-to-Bcl-2 ratio. Mitochondrial reactive oxygen species production was markedly elevated by 5- to 7.5-fold in subsarcolemmal mitochondria after 7, 14, and 21 days of denervation, whereas reactive oxygen species production in intermyofibrillar (IMF) mitochondria was reduced by 40–50%. Subsarcolemmal and IMF mitochondrial levels of MnSOD were also reduced by 40–50% after 14–21 days of denervation. The maximal rate of IMF mitochondrial pore opening ( Vmax) was elevated by 25–35%, and time to Vmaxwas reduced by 20–25% after 14 and 21 days, indicating increased apoptotic susceptibility. Myonuclear decay, assessed by DNA fragmentation, was elevated at 7–21 days of denervation. Our data indicate that PGC-1α and Tfam are important factors that likely contribute to the reduced mitochondrial content after chronic disuse. In addition, our results illustrate that, despite the reduced mitochondrial content, denervated muscle has greater mitochondrial apoptotic susceptibility, which coincided with elevated apoptosis, and these processes may contribute to denervation-induced muscle atrophy.

Elevated urinary 8-isoprostaglandin F(2alpha) in females with Graves' hyperthyroidism

In this study, we investigate how oxidative stress alters the urinary F2-isoprostanes' level and we examine the correlation between 8-isoPGF(2alpha) and thyroid hormones in female patients with Graves' hyperthyroidism. We quantitatively determined the concentrations of urinary F2-isoprostanes using gas chromatography-mass spectrometry in the selected ion-monitoring mode. We recruited individuals in the following three groups of subjects for this study: (1) female hyperthyroidism patients (n = 14, 21-71 years), (2) female hypothyroidism patients (n = 16, 16-66 years), and (3) female age-matched normal controls (n = 10, 20-61 years). The average concentration of 8-iso prostaglandin F(2alpha) (8-isoPGF(2alpha)) in hyperthyroidism patients was significantly higher than that in the healthy controls (P < 0.05). The concentration of 8-isoPGF(2alpha) in hypothyroidism patients was similar to that in normal controls. Although the level of 8-isoPGF(2alpha) in two of the patients was slightly elevated, the P value was not significant (NS). Our data suggest that the increased level of urinary 8-isoPGF(2alpha) may reflect possible oxidation injuries in hyperthyroidism patients due to the high level of thyroid hormones. Therefore, the elevated 8-isoPGF(2alpha) in patients could be used as an important biomarker for hyperthyroidism disease.

Tissue-specific regulation of cytochrome c oxidase subunit expression by thyroid hormone

The influence of thyroid hormone (T(3)) on respiration is partly mediated via its effect on the cytochrome c oxidase (COX) enzyme, a multi-subunit complex within the mitochondrial respiratory chain. We compared the expression of COX subunits I, III, Vb, and VIc and thyroid receptors (TR)alpha1 and TRbeta1 with functional changes in COX activity in tissues that possess high oxidative capacities. In response to 5 days of T(3) treatment, TRbeta1 increased 1.6-fold in liver, whereas TRalpha1 remained unchanged. T(3) also induced concomitant increases in the protein and mRNA expression of nuclear-encoded subunit COX Vb in liver, matched by a 1.3-fold increase in binding to a putative thyroid response element (TRE) within the COX Vb promoter in liver, suggesting transcriptional regulation. In contrast, T(3) had no effect on COX Vb expression in heart. T(3) produced a significant increase in COX III mRNA in liver but decreased COX III mRNA in heart. These changes were matched by parallel alterations in mitochondrial transcription factor A expression in both tissues. In contrast, COX I protein increased in both liver and heart 1.7- and 1.5-fold (P < 0.05), respectively. These changes in COX I closely paralleled the T(3)-induced increases in COX activity observed in both of these tissues. In liver, T(3) induced a coordinated increase in the expression of the nuclear (COX Vb) and mitochondrial (COX I) genomes at the protein level. However, in heart, the main effect of T(3) was restricted to the expression of mitochondrial DNA subunits. Thus our data suggest that T(3) regulates the expression of COX subunits by both transcriptional and posttranscriptional mechanisms. The nature of this regulation differs between tissues possessing a high mitochondrial content, like liver and heart.

Calcium-regulated changes in mitochondrial phenotype in skeletal muscle cells

Cytochrome c expression and mitochondrial biogenesis can be invoked by elevated intracellular Ca2+in muscle cells. To characterize the potential role of Ca2+as a messenger involved in mitochondrial biogenesis in muscle, we determined the effects of the Ca2+ionophore A-23187 on the expression of nuclear- and mitochondrially encoded genes. Treatment of myotubes with 1 μM A-23187 for 48–96 h increased nuclear-encoded β-subunit F1ATPase and malate dehydrogenase (MDH) mRNA levels by 50–100% ( P < 0.05) but decreased mRNA levels of glutamate dehydrogenase (GDH) by 19% ( P < 0.05). mRNA levels of the cytochrome c oxidase (COX) nuclear-encoded subunits IV, Vb, and VIc were unchanged, whereas the mitochondrially encoded subunits COX II and COX III were decreased by 30 and 70%, respectively ( P < 0.05). This was paralleled by a 20% decrease ( P < 0.05) in COX activity. These data suggest that cytoplasmic Ca2+differentially regulates the mRNA level of nuclear and mitochondrial genes. The decline in COX II and III mRNA may be mediated by Tfam, because A-23187 modestly reduced Tfam levels by 48 h. A-23187 induced time-dependent increases in Egr-1 mRNA, along with the activation of ERK1/2 and AMP-activated protein kinase. MEK inhibition with PD-98059 attenuated the increase in Egr-1 mRNA. A-23187 also increased Egr-1, serum response factor, and Sp1 protein expression, transcription factors implicated in mitochondrial biogenesis. Egr-1 overexpression increased nuclear-encoded cytochrome c transcriptional activation by 1.5-fold ( P < 0.05) and reduced GDH mRNA by 37% ( P < 0.05) but had no effect on MDH or β-subunit F1ATPase mRNA. These results indicate that changes in intracellular Ca2+can modify mitochondrial phenotype, in part via the involvement of Egr-1.

The cardiolipin–cytochrome c interaction and the mitochondrial regulation of apoptosis

While many studies have focused on cytochrome c release from mitochondria, little attention has been given to the specific interaction between cardiolipin (CL) and cytochrome c, the breaching of which likely represents a critical event in the initiation of mitochondrially mediated apoptosis. Mounting evidence suggests that a decrease in the level of CL affects cytochrome c binding to the inner membrane, thus leading to higher levels of soluble cytochrome c in the mitochondrial intermembrane space. Among the factors known to affect CL levels are thyroid status, plasma concentrations of free fatty acids, Ca2+ dysregulation, and reactive oxygen species (ROS). These factors, especially Ca2+ and ROS, have long been recognized as triggers of cell death and, more recently, as modulators of mitochondrially mediated apoptosis. In this review, we discuss the significance of the disruption of the CL-cytochrome c interaction for cytochrome c release and apoptosis.

Effect of hypo- and hyperthyroid states on phospholipid composition in developing rat heart

The aim of this study was to examine the effect of hypo- and hyperthyroidism on the phospholipid composition in developing rat heart. The hypothyroid state (PTU) was induced by 0.05% 6-n-propyl-2-thiouracil in drinking water given to nursing mothers from the postnatal day 2-21. The hyperthyroidism (T3) was made by daily injection of 3,3',5-triiodo-L-thyronine (10 microg/100 g body wt) to newborns in the same time period. Age matched intact littermates were taken as euthyroid controls. PTU decreased the concentration of total phospholipids (PL), choline phosphoglycerides (PC), ethanolamine phosphoglycerides (PE) and diphosphatidylglycerol (DPG) and increased the proportion of plasmalogen component of PE (PLPE). T3 increased the concentration of PL, PC, PE, DPG and decreased PLPE in comparison with euthyroid controls. The ratio of saturated/unsaturated fatty acids (FA) in PE was decreased in PTU and increased in T3 group. The ratio of n-6/n-3 polyunsaturated FA in PC, PE and phosphatidylinositol (PI) was increased in PTU due to increase of 18:2n-6 and decrease of 22:6n-3 proportion. T3 decreased this ratio because of decline in 20:4n-6 and rise in 22:6n-3 proportion. Both hypo- and hyperthyroidism decreased the ratio of 20:4n-6/18:2n-6 in the majority of phospholipids. PTU decreased the unsaturation index in PC, PI and phosphatidylserine. It is concluded that thyroid state plays an essential role in the development of membrane phospholipid components in cardiac membranes during the early postnatal period.

T3 increases lactate transport and the expression of MCT4, but not MCT1, in rat skeletal muscle

Triiodothyronine (T3) regulates the expression of genes involved in muscle metabolism. Therefore, we examined the effects of a 7-day T3 treatment on the monocarboxylate transporters (MCT)1 and MCT4 in heart and in red (RG) and white gastrocnemius muscle (WG). We also examined rates of lactate transport into giant sarcolemmal vesicles and the plasmalemmal MCT1 and MCT4 in these vesicles. Ingestion of T3 markedly increased circulating serum T3 (P < 0.05) and reduced weight gain (P < 0.05). T3 upregulated MCT1 mRNA (RG +77, WG +49, heart +114%, P < 0.05) and MCT4 mRNA (RG +300, WG +40%). However, only MCT4 protein expression was increased (RG +43, WG +49%), not MCT1 protein expression. No changes in MCT1 protein were observed in any tissue. T3 treatment doubled the rate of lactate transport when vesicles were exposed to 1 mM lactate (P < 0.05). However, plasmalemmal MCT4 was only modestly increased (+13%, P < 0.05). We conclude that T3 1) regulates MCT4, but not MCT1, protein expression and 2) increases lactate transport rates. This latter effect is difficult to explain by the modest changes in plasmalemmal MCT4. We speculate that either the activity of sarcolemmal MCTs has been altered or else other MCTs in muscle may have been upregulated.

PPARgamma coactivator-1alpha expression during thyroid hormone- and contractile activity-induced mitochondrial adaptations

The transcriptional coactivator the peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) has been identified as an important mediator of mitochondrial biogenesis based on its ability to interact with transcription factors that activate nuclear genes encoding mitochondrial proteins. The induction of PGC-1alpha protein expression under conditions that provoke mitochondrial biogenesis, such as contractile activity or thyroid hormone (T(3)) treatment, is not fully characterized. Thus we related PGC-1alpha protein expression to cytochrome c oxidase (COX) activity in 1) tissues of varying oxidative capacities, 2) tissues from animals treated with T(3), and 3) skeletal muscle subject to contractile activity both in cell culture and in vivo. Our results demonstrate a strong positive correlation (r = 0.74; P < 0.05) between changes in PGC-1alpha and COX activity, used as an index of mitochondrial adaptations. The highest constitutive levels of PGC-1alpha were found in the heart, whereas the lowest were measured in fast-twitch white muscle and liver. T(3) increased PGC-1alpha content similarly in both fast- and slow-twitch muscle, as well as in the liver, but not in heart. T(3) also induced early (6 h) increases in AMP-activated protein kinase (AMPKalpha) activity, as well as later (5 day) increases in p38 MAP kinase activity in slow-twitch, but not in fast-twitch, muscle. Contractile activity provoked early increases in PGC-1alpha, coincident with increases in mitochondrial transcription factor A (Tfam), and nuclear respiratory factor-1 (NRF-1) protein expression, suggesting that PGC-1alpha is physiologically important in coordinating the expression of the nuclear and mitochondrial genomes. Ca(2+) ionophore treatment of muscle cells led to an approximately threefold increase in PGC-1alpha protein, and contractile activity induced rapid and marked increases in both p38 MAP kinase and AMPKalpha activities. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) treatment of muscle cells also led to parallel increases in AMPKalpha activity and PGC-1alpha protein levels. These data are consistent with observations that indicate that increases in PGC-1alpha protein are affected by Ca(2+) signaling mechanisms, AMPKalpha activity, as well as posttranslational phosphorylation events that increase PGC-1alpha protein stability. Our data support a role for PGC-1alpha in the physiological regulation of mitochondrial content in a variety of tissues and suggest that increases in PGC-1alpha expression form part of a unifying pathway that promotes both T(3)- and contractile activity-induced mitochondrial adaptations.

Functional consequences of thyroid hormone-induced changes in the mitochondrial protein import pathway

Thyroid hormone [3,5,3'-triiodo-l-thyronine (T(3))] induces phenotypic alterations in cardiac mitochondria, in part by influencing protein import and the expression of the import motor mitochondrial heat shock protein (mtHsp70). Here we examined the adaptability of translocases of the inner membrane (Tim) proteins, as well as the outer membrane receptor Tom34, to T(3). Administration of T(3) to rats for 5 days increased cardiac Tim23 and Tim44 mRNA levels by 55 and 50%, respectively, but had no effect on Tim17. T(3) treatment also induced a 45% increase in Tom34 mRNA, with no accompanying changes at the protein level, suggesting regulation at the posttranscriptional level. In H9c2 cardiac cells, Tim17 mRNA was elevated by 114% by 9 days of differentiation, whereas Tim23 and Tim44 declined by 25 and 29%, respectively. To determine the functional consequences of these T(3)-induced changes, malate dehydrogenase (MDH) import rates were measured in H9c2 cells stably overexpressing Tim44 and mtHsp70, either alone or in combination. MDH import remained unaltered in cells overexpressing Tim44 or in cells overexpressing both Tim44 and mtHsp70. However, when mtHsp70 was overexpressed alone, a 13% (P < 0.05) increase in MDH import rate was observed. These findings indicate that import machinery components are differentially regulated in response to stimuli that induce mitochondrial biogenesis, like T(3) and differentiation. In addition, the induction of an import machinery component in response to T(3) may not necessarily result in functional changes in protein import during mitochondrial biogenesis. Finally, mtHsp70 may play a regulatory role in the import process that is independent of its interaction with Tim44.

Reduced activity of mtTFA decreases the transcription in mitochondria isolated from diabetic rat heart

To evaluate abnormalities in the mitochondrial transcription factor A (mtTFA) function as a cause of mitochondrial dysfunction in diabetes, we measured the mRNA contents of the proteins consisting of the mitochondrial respiratory chain as well as transcriptional and translational activities in the mitochondria isolated from controls and streptozotocin-induced diabetic rat hearts. Using Northern blot analysis, we found 40% reduced mRNA contents of mitochondrial-encoded cytochrome b and ATP synthase subunit 6 in diabetic rat hearts compared with control rats (P < 0.05). These abnormalities were completely recovered by insulin treatment. Furthermore, the mitochondrial activities of transcription and translation were decreased significantly in mitochondria isolated from diabetic rats by 60% (P < 0.01) and 71% (P < 0.01), respectively, compared with control rats. The insulin treatment also completely normalized these abnormalities in diabetic rats. Consistently, gel retardation assay showed a reduced binding of mtTFA to the D-loop of mitochondrial DNA in diabetic rats, although there was no difference in the mtTFA mRNA and protein content between the two groups. On the basis of these findings, a reduced binding activity of mtTFA to the D-loop region in the hearts of diabetic rats may contribute to the decreased mitochondrial protein synthesis.

In vivo regulation of human skeletal muscle gene expression by thyroid hormone

Thyroid hormones are key regulators of metabolism that modulate transcription via nuclear receptors. Hyperthyroidism is associated with increased metabolic rate, protein breakdown, and weight loss. Although the molecular actions of thyroid hormones have been studied thoroughly, their pleiotropic effects are mediated by complex changes in expression of an unknown number of target genes. Here, we measured patterns of skeletal muscle gene expression in five healthy men treated for 14 days with 75 microg of triiodothyronine, using 24,000 cDNA element microarrays. To analyze the data, we used a new statistical method that identifies significant changes in expression and estimates the false discovery rate. The 381 up-regulated genes were involved in a wide range of cellular functions including transcriptional control, mRNA maturation, protein turnover, signal transduction, cellular trafficking, and energy metabolism. Only two genes were down-regulated. Most of the genes are novel targets of thyroid hormone. Cluster analysis of triiodothyronine-regulated gene expression among 19 different human tissues or cell lines revealed sets of coregulated genes that serve similar biologic functions. These results define molecular signatures that help to understand the physiology and pathophysiology of thyroid hormone action.

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.

Mitochondrial DNA in aging and degenerative disease

The mitochondrial DNA encodes only a few gene products compared to the nuclear DNA. These products, however, play a decisive role in determining cell function. Should this DNA mutate spontaneously or be damaged by free radicals the functionality of the gene products will be compromised. A number of mitochondrial genetic diseases have been identified. Some of these are quite serious and involve the central nervous system as well as muscle, heart, liver and kidney. Aging has been characterized by a gradual increase in base deletions in this DNA. This increase in deletion mutation has been suggested to be the cumulative result of exposure to free radicals.

Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle

Chronic contractile activity produces mitochondrial biogenesis in muscle. This adaptation results in a significant shift in adenine nucleotide metabolism, with attendant improvements in fatigue resistance. The vast majority of mitochondrial proteins are derived from the nuclear genome, necessitating the transcription of genes, the translation of mRNA into protein, the targeting of the protein to a mitochondrial compartment via the import machinery, and the assembly of multisubunit enzyme complexes in the respiratory chain or matrix. Putative signals involved in initiating this pathway of gene expression in response to contractile activity likely arise from combinations of accelerations in ATP turnover or imbalances between mitochondrial ATP synthesis and cellular ATP demand, and Ca(2+) fluxes. These rapid events are followed by the activation of exercise-responsive kinases, which phosphorylate proteins such as transcription factors, which subsequently bind to upstream regulatory regions in DNA, to alter transcription rates. Contractile activity increases the mRNA levels of nuclear-encoded proteins such as cytochrome c and mitochondrial transcription factor A (Tfam) and mRNA levels of upstream transcription factors like c-jun and nuclear respiratory factor-1 (NRF-1). mRNA level changes are often most evident during the postexercise recovery period, and they can occur as a result of contractile activity-induced increases in transcription or mRNA stability. Tfam is imported into mitochondria and controls the expression of mitochondrial DNA (mtDNA). mtDNA contributes only 13 protein products to the respiratory chain, but they are vital for electron transport and ATP synthesis. Contractile activity increases Tfam expression and accelerates its import into mitochondria, resulting in increased mtDNA transcription and replication. The result of this coordinated expression of the nuclear and the mitochondrial genomes, along with poorly understood changes in phospholipid synthesis, is an expansion of the muscle mitochondrial reticulum. Further understanding of 1) regulation of mtDNA expression, 2) upstream activators of NRF-1 and other transcription factors, 3) the identity of mRNA stabilizing proteins, and 4) potential of contractile activity-induced changes in apoptotic signals are warranted.

Tom20-mediated mitochondrial protein import in muscle cells during differentiation

Mitochondrial biogenesis is accompanied by an increased expression of components of the protein import machinery, as well as increased import of proteins destined for the matrix. We evaluated the role of the outer membrane receptor Tom20 by varying its expression and measuring changes in the import of malate dehydrogenase (MDH) in differentiating C2C12 muscle cells. Cells transfected with Tom20 had levels that were twofold higher than in control cells. Labeling of cells followed by immunoprecipitation of MDH revealed equivalent increases in MDH import. This parallelism between import rate and Tom20 levels was also evident as a result of thyroid hormone treatment. Using antisense oligodeoxynucleotides, we inhibited Tom20 expression by 40%, resulting in 40-60% reductions in MDH import. In vitro assays also revealed that import into the matrix was more sensitive to Tom20 inhibition than import into the outer membrane. These data indicate a close relationship between induced changes in Tom20 and the import of a matrix protein, suggesting that Tom20 is involved in determining the kinetics of import. However, this relationship was dissociated during normal differentiation, since the expression of Tom20 remained relatively constant, whereas imported MDH increased 12-fold. Thus Tom20 is important in determining import during organelle biogenesis, but other mechanisms (e.g., intramitochondrial protein degradation or nuclear transcription) likely also play a role in establishing the final mitochondrial phenotype during normal muscle differentiation.

Effect of thyroid hormones on mitochondrial oxygen free radical production and DNA oxidative damage in the rat heart

Mitochondria seem to be involved in oxygen radical damage and aging. However, the possible relationships between oxygen consumption and oxygen radical production by functional mitochondria, and oxidative DNA damage, have not been studied previously. In order to analyze these relationships, male Wistar rats of 12 weeks of age were rendered hyper- and hypothyroid by chronic T(3) and 6-n-propyl-2-thiouracil treatments, respectively. Hypothyroidism decreased heart mitochondrial H(2)O(2) production in States 4 (to 51% of controls; P<0.05) and 3 (to 21% of controls; P<0.05). In agreement with this, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) decreased in the heart genomic DNA of hypothyroid animals to 40% of controls (P<0.001). Studies with respiratory inhibitors showed that the decrease in oxygen radical generation observed in hypothyroidism occurred at Complex III (mainly) and at Complex I; that decrease was due to the presence of a lower free radical leak in the respiratory chain (P<0.05). Hyperthyroidism did not significantly change heart mitochondrial H(2)O(2) production since the increase in State 4 oxygen consumption in comparison with control and hypothyroid animals (P<0.05) was compensated by a decrease in the free radical leak in relation to control animals (P<0.05). In agreement with this, heart 8-oxodG was not changed in hyperthyroid animals. The lack of increase in H(2)O(2) production per unit of mitochondrial protein will protect mitochondria themselves against self-inflicted damage during hyperthyroidism.

Thyroid hormone coordinates respiratory control maturation and adenine nucleotide translocator expression in heart in vivo

BACKGROUND

The signal transduction mechanism linking mitochondrial ATP synthesis with cytosolic ATP utilization in heart changes during postnatal development in vivo. This maturational process occurs in parallel with accumulation of mitochondrial adenine nucleotide translocator (ANT), which provides a possible site for respiratory control. We postulated that thyroid hormone regulates these maturational processes.

METHODS AND RESULTS

We used (31)P MR spectroscopy to determine the relationship between myocardial high-energy phosphates, phosphocreatine, and ADP and oxygen consumption (MVO(2)) during epinephrine stimulation in 32- to 40-day-old lambs thyroidectomized after birth (THY) and age-matched controls. Steady-state protein and mRNA levels for ANT isoforms and beta-F(1)-ATPase were assessed from left ventricular tissues by Western and Northern blotting. With greater doses of epinephrine, THY attained lower peak MVO(2) than controls (P:<0.05). Controls maintained high-energy phosphate levels, unlike THY, which demonstrated significantly decreased phosphocreatine/ATP and increased cytosolic ADP despite lower peak MVO(2). No significant differences in beta-F(1)-ATPase protein or mRNA occurred between groups. However, ANT isoform mRNA levels were 2-fold greater and protein levels 4-fold greater in control hearts.

CONCLUSIONS

These data imply that the maturational shift away from ADP-mediated respiratory control is regulated by thyroid hormone in vivo. Specific thyroid-modulated increases in ANT mRNA and protein imply that this regulation occurs in part at a pretranslational level.

Depolarizing stimulation upregulates GA-binding protein in neurons: a transcription factor involved in the bigenomic expression of cytochrome oxidase subunits

Neurons are unique in having dendrites that extend far away from their cell bodies. Mitochondria located in the dendrites can be separated from the nucleus for long distances. The mechanism of bigenomic coordination is of particular importance to cytochrome oxidase (CO), which has subunits that are encoded in both the nuclear and mitochondrial DNA. GA-binding protein (GABP) is a transcription factor that is required for the promoter activity of mitochondrial transcription factor A as well as several nuclear-encoded CO subunits. Thus, GABP may play a key role in coordinating the transcription of both mitochondrial and nuclear-encoded subunits of CO. The goal of the present study was to determine if GABP was expressed in neurons and whether and how it responded to increased neuronal activity. Using primary neuronal cultures, the beta-subunit of GABP was localized immunocytochemically to both the cytoplasm and the nucleus, whereas the alpha-subunit was expressed mainly in the nucleus. In KCl-treated cultures, immunoreactivity for both alpha- and beta-subunits was significantly increased in the nucleus compared with untreated sister cultures. The induction of GABP preceded that of CO gene expression from the two genomes, which, in turn, preceded that of CO activity. Thus, our data suggest that neuronal activity regulates subunit concentrations of GABP in the nucleus, and GABP may be a critical sensor of changes in neuronal activity. Our data are also consistent with the postulated role of GABP as a coordinator of both mitochondrial and nuclear transcription for subunits of CO in neurons.

Thyroid hormones differentially modulate enolase isozymes during rat skeletal and cardiac muscle development

During muscle development, an isozymic transition of the glycolytic enzyme enolase occurs from the embryonic and ubiquitous alphaalpha-isoform to the muscle-specific betabeta-isoform. Here, we demonstrate a stimulatory role of thyroid hormones on these two enolase genes during rat development in hindlimb muscles and an inhibitory effect on the muscle-specific enolase gene in cardiac muscle. In hindlimb muscles the ubiquitous alpha-transcript level is diminished by hypothyroidism, starting at birth. On the contrary, the more abundant muscle-specific beta-transcript is insensitive to hypothyroidism before establishment of the functional diversification of fibers and is greatly decreased thereafter. Our data support the hypothesis of a role of thyroid hormones in coordinating the expressions of contractile proteins and metabolic enzymes during muscle development. The subcellular localization of isoenolases, established here, is not modified by hypothyroidism. Our results underline the specificity of action of thyroid hormones, which modulate differentially two isozymes in the same muscle and regulate, in opposite directions, the expression of the same gene in two different muscles.

Cytochrome c transcriptional activation and mRNA stability during contractile activity in skeletal muscle

We evaluated contractile activity-induced alterations in cytochrome c transcriptional activation and mRNA stability with unilateral chronic stimulation (10 Hz, 3 h/day) of the rat tibialis anterior (TA) muscle for 1, 2, 3, 4, 5, and 7 days (n = 3-11/group). Transcriptional activation was assessed by direct plasmid DNA injection into the TA with a chloramphenicol acetyltransferase (CAT) reporter gene linked to 326 bp of the cytochrome c promoter. Cytochrome c mRNA in stimulated muscles increased by 1.3- to 1. 7-fold above control between 1 and 7 days. Cytochrome c protein was increased after 5 days of stimulation to reach levels that were 1. 9-fold higher than control by 7 days. Cytochrome c mRNA stability, determined with an in vitro decay assay, was greater in stimulated TA than in control between 2 and 4 days, likely mediated by the induction of a cytosolic factor. In contrast, cytochrome c transcriptional activation was elevated only after 5 days of stimulation when mRNA stability had returned to control levels. Thus the contractile activity-induced increase in cytochrome c mRNA was due to an early increase in mRNA stability, followed by an elevation in transcriptional activation, leading to an eventual increase in cytochrome c protein levels.

Calcium-dependent regulation of cytochrome c gene expression in skeletal muscle cells. Identification of a protein kinase c-dependent pathway

Mitochondrial biogenesis can occur rapidly in mammalian skeletal muscle subjected to a variety of physiological conditions. However, the intracellular signal(s) involved in regulating this process remain unknown. Using nuclearly encoded cytochrome c, we show that its expression in muscle cells is increased by changes in cytosolic Ca2+ using the ionophore A23187. Treatment of myotubes with A23187 increased cytochrome c mRNA expression up to 1.7-fold. Transfection experiments using promoter-chloramphenicol acetyltransferase constructs revealed that this increase could be transcriptionally mediated since A23187 increased chloramphenicol acetyltransferase activity by 2.5-fold. This increase was not changed by KN62, an inhibitor of Ca2+/calmodulin-dependent kinases II and IV, and it was not modified by overexpression of protein kinase A and cAMP response element-binding protein, demonstrating that the A23187 effect was not mediated through Ca2+/calmodulin-dependent kinase- or protein kinase A-dependent pathways. However, treatment of myotubes with staurosporine or 12-O-tetradecanoylphorbol-13-acetate reduced the effect of A23187 on cytochrome c transactivation by 40-50%. Coexpression of the Ca2+-sensitive protein kinase C isoforms alpha and betaII, but not the Ca2+-insensitive delta isoform, exaggerated the A23187-mediated response. The short-term effect of A23187 was mediated in part by mitogen-activated protein kinase (extracellular signal-regulated kinases 1 and 2) since its activation peaked 2 h after A23187 treatment, and cytochrome c transactivation was reduced by PD98089, a mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor. These results demonstrate the existence of a Ca2+-sensitive, protein kinase C-dependent pathway involved in cytochrome c expression and implicate Ca2+ as a signal in the up-regulation of nuclear genes encoding mitochondrial proteins.

Effect of thyroid status on lipid composition and peroxidation in the mouse liver

In order to analyze the possible relationship between metabolic rate and oxidative stress, OF1 female mice were rendered hyper- or hypothyroid for 4-5 weeks by administration of 0.0012% L-thyroxine (T4) or 0.05% 6-n-propyl-2-thiouracil (PTU), respectively, in their drinking water. Treatment with T4 resulted in increased basal metabolic rate measured by oxygen consumption and liver cytochrome oxidase activity without altering the glutathione redox system. Endogenous lipid peroxidation, sensitivity to lipid peroxidation and fatty acid unsaturation were decreased in the hyperthyroid group. Hypothyroidism also decreased phosphatidylcholine and cardiolipin fatty acid unsaturation but not in total lipids, and thus lipid peroxidation was not altered. Cardiolipin, a mainly mitochondrial lipid, was the most profoundly altered fraction by both hyper- and hypothyroidism. It is suggested that the lipid changes observed in hyperthyroid animals can protect them against an increased oxidative attack to tissue lipids due to their increased mitochondrial activities.

Effect of microgravity on the expression of mitochondrial enzymes in rat cardiac and skeletal muscles

The purpose of this study was to examine the expression of nuclear and mitochondrial genes in cardiac and skeletal muscle (triceps brachii) in response to short-duration microgravity exposure. Six adult male rats were exposed to microgravity for 6 days and were compared with six ground-based control animals. We observed a significant 32% increase in heart malate dehydrogenase (MDH) enzyme activity, which was accompanied by a 62% elevation in heart MDH mRNA levels after microgravity exposure. Despite modest elevations in the mRNAs encoding subunits III, IV, and VIc as well as a 2.2-fold higher subunit IV protein content after exposure to microgravity, heart cytochrome c oxidase (CytOx) enzyme activity remained unchanged. In skeletal muscle, MDH expression was unaffected by microgravity, but CytOx activity was significantly reduced 41% by microgravity, whereas subunit III, IV, and VIc mRNA levels and subunit IV protein levels were unaltered. Thus tissue-specific (i.e., heart vs. skeletal muscle) differences exist in the regulation of nuclear-encoded mitochondrial proteins in response to microgravity. In addition, the expression of nuclear-encoded proteins such as CytOx subunit IV and expression of MDH are differentially regulated within a tissue. Our data also illustrate that the heart undergoes previously unidentified mitochondrial adaptations in response to short-term microgravity conditions more dramatic than those evident in skeletal muscle. Further studies evaluating the functional consequences of these adaptations in the heart, as well as those designed to measure protein turnover, are warranted in response to microgravity.

The effects of thyroid hormone level and action in developing brain: are these targets for the actions of polychlorinated biphenyls and dioxins?

Alterations in thyroid hormone level or responsivity to thyroid hormone have significant neurologic sequelae throughout the life cycle. During fetal and early neonatal periods, disorders of thyroid hormone may lead to the development of motor and cognitive disorders. During childhood and adult life, thyroid hormone is required for neuronal maintenance as well as normal metabolic function. Those with an underlying disorder of thyroid hormone homeostasis or mitochondrial function may be at greater risk for developing cognitive, motor, or metabolic dysfunction upon exposure to substances which alter thyroid hormone economy. Polychlorinated biphenyls (PCBs) and dioxins have been argued to interfere with thyroid hormone action and thus may affect the developing and mature brain. Animal models provide useful tools for studying the effects of thyroid hormone disorders and the effects of environmental endocrine disruptors. The congenitally hypothyroid, hyt/hyt, mouse exhibits abnormalities in both the cognitive and motor systems. In this mouse and other animal models of thyroid hormone disorders, delayed somatic and reflexive development are noted, as are permanent deficits in hearing and locomotor and adaptive motor behavior. This animal's behavioral abnormalities are predicated on anatomic abnormalities in the nervous system. In turn, these abnormalities are correlated with differences in neuronal structural proteins. In normal mice, the expression of mRNAs coding for these proteins occurs temporally with the onset of autonomous thyroid hormone production. The hyt/hyt mouse has a mutation in the thyroid stimulating hormone receptor (TSHr) gene which renders it incapable of transducing the TSH signal in the thyrocyte to produce thyroid hormone. Some behavioral and possibly some biochemical abnormalities in mice exposed to PCBs are similar to those seen in the hyt/hyt mouse. In addition to direct effects on brain development and neuronal maintenance, thyroid hormone is necessary for maintaining metabolic functioning through its influence on mitochondria. Because the brain is particularly sensitive to inadequate energy generation, disorders of thyroid hormone economy also indirectly impair brain functioning. Alterations in thyroid hormone level result in differing expression of mitochondrial genes. Mutations in these mitochondrial genes lead to well-recognized syndromes of encephalomyopathy, myopathy, and multisystem disorder. Hence, PCBs and dioxins, by possibly altering the thyroid hormone milieu, may alter the functioning of mitochondria in the generation of adenosine triphosphate (ATP). The use of animal models of thyroid hormone deficiency for behavioral, anatomic, histologic, and molecular comparison will help elucidate the mechanisms of action of these putative endocrine-disrupting compounds. The study of thyroid hormone disorders provides a template for relating thyroid hormone mediated effects on the brain to these compounds.

Influence of thyroid hormone on the tissue-specific expression of cytochrome c oxidase isoforms during cardiac development

In mammals, cytochrome c oxidase (COX) is composed of 13 different protein subunits. In the rat, two nuclear-encoded subunits, COX VIa and VIII, exist as tissue-specific isoforms: heart and liver. Using Northern-blot analysis, the levels of transcripts for the heart and liver isoforms of VIa and VIII were examined in developing rat hearts. The liver isoform was found to be the predominant form of subunit VIa and the exclusive form of VIII in the 18-day fetal hearts. The mRNA levels of the heart isoform of both subunits increased dramatically to reach adult levels by 14 days. Although the levels of the VIa- and VIII-liver isoform mRNAs remained stable throughout early development, their levels decreased by 40 and 36% respectively between the 18-day fetal stage and 18-day neonatal stage. Therefore the up-regulation of the heart isoforms and down-regulation of the liver isoforms appear to be regulated in a co-ordinated manner during development. To determine if thyroid hormone influences the expression of these developmentally regulated isoforms, the RNA was also extracted from the hearts of 2-week-old hypothyroid rats. The results showed that the levels of VIII-heart and VIa-liver COX mRNAs were approx. 40% lower in the hypothyroid hearts, while VIII-liver and VIa-heart COX isoform expression remained unchanged. These data demonstrate that the isoforms of COX subunits VIa and VIII are not co-ordinately regulated by changes in thyroid hormone levels. Therefore we conclude that, although thyroid hormone influences the expression of isoforms, it appears to do so via a different mechanism from that which regulates the developmental transition.