Regulation of biosynthesis of the rat liver inner mitochondrial membrane by thyroid hormone

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.

Direct regulation of mitochondrial RNA synthesis by thyroid hormone

We have analyzed the influence of in vivo treatment and in vitro addition of thyroid hormone on in organello mitochondrial DNA (mtDNA) transcription and, in parallel, on the in organello footprinting patterns at the mtDNA regions involved in the regulation of transcription. We found that thyroid hormone modulates mitochondrial RNA levels and the mRNA/rRNA ratio by influencing the transcriptional rate. In addition, we found conspicuous differences between the mtDNA dimethyl sulfate footprinting patterns of mitochondria derived from euthyroid and hypothyroid rats at the transcription initiation sites but not at the mitochondrial transcription termination factor (mTERF) binding region. Furthermore, direct addition of thyroid hormone to the incubation medium of mitochondria isolated from hypothyroid rats restored the mRNA/rRNA ratio found in euthyroid rats as well as the mtDNA footprinting patterns at the transcription initiation area. Therefore, we conclude that the regulatory effect of thyroid hormone on mitochondrial transcription is partially exerted by a direct influence of the hormone on the mitochondrial transcription machinery. Particularly, the influence on the mRNA/rRNA ratio is achieved by selective modulation of the alternative H-strand transcription initiation sites and does not require the previous activation of nuclear genes. These results provide the first functional demonstration that regulatory signals, such as thyroid hormone, that modify the expression of nuclear genes can also act as primary signals for the transcriptional apparatus of mitochondria.

Thyroid hormone activates transcription from the promoter regions of some human nuclear-encoded genes of the oxidative phosphorylation system

Thyroid hormone (T3) modulates the mRNA levels for cytochrome c and the adenine nucleotide translocator-2 (ANT2) in adult rat liver. Here we show that T3 activates expression of a reporter gene driven from the human cytochrome c1 and ANT2 promoters transfected into human choriocarcinoma JEG3 cells. By contrast, the human F1-ATPase beta-subunit promoter responded marginally, thus providing a pattern of differential expression similar to that earlier observed in rats in vivo. T3-activation is dependent on co-expression of the thyroid hormone receptor (TR alpha1). Co-expression of both the TR and RXR receptors had no additional effect. Transient transfection of deletion constructs showed that T3 activation is retained by the proximal regions of the cytochrome c1 and ANT2 promoters, and, in the case of cytochrome c1, is lost upon removal of a fragment containing the transcription initiator ((nucleotides) (nt) + 1 to + 100). The promoter regions supporting T3-activation of the reporter genes appear to lack strong DNA binding sites for TR and retinoid X receptor (RXR).

Thyroid hormones regulate the onset of osmotic activity of rat liver mitochondria after birth

The effect of thyroid hormone deprivation on the osmotic activity of liver mitochondria from early newborn rats was studied. Experimentally induced hypothyroidism prevented the increase in the osmotic activity of mitochondria observed immediately after birth. Osmotic activity was restored by T4 and T3 treatment to hypothyroid newborns but not when this treatment was supplemented with cycloheximide. Under the same circumstances, streptomycin had no effect. Hypothyroidism abolished the change in the slope of the osmotic curve (plot of inverse absorbance of mitochondrial suspensions incubated in sucrose solutions vs. inverse sucrose concentration) observed in mitochondria from euthyroid newborns at 110-120 mOsm sucrose, suggesting that hypothyroidism prevents the formation of tight physical connections between mitochondrial outer and inner membranes. Thyroid hormone deprivation increased the passive permeability of the mitochondrial inner membrane to protons, resulting in a decreased respiratory control ratio. Hypothyroidism prevented the sharp decrease in the affinity of mitochondria for ATP observed in euthyroid newborns immediately after birth. These results corroborate our previous suggestion (Endocrinology, 1995, 136:4448) that, during the early neonatal period, thyroid hormones control the synthesis of some nucleus-coded protein(s) involved in the assembly of F0,F1-ATPase.

Biogenesis of thermogenic mitochondria in brown adipose tissue of Djungarian hamsters during cold adaptation

After cold exposure, cytochrome c oxidase (COX) activity increased about 2.5-fold within 2 weeks in the brown adipose tissue (BAT) of Djungarian hamsters. The mRNAs for COX subunits I and III and the 12 S rRNA, encoded on mitochondrial DNA (mtDNA), as well as mRNAs for COX subunits IV, Va and mitochondrial transcription factor A, encoded in the nucleus, were unchanged when expressed per unit of total tissue RNA. However, since total tissue RNA doubled per BAT depot, while total DNA remained unchanged, the actual levels of these transcripts were increased within BAT cells. In contrast, the abundance of mRNA for uncoupling protein was increased 10-fold, indicating specific activation of this gene. In addition, the maximal rate of protein synthesis analysed in a faithful in organello system was increased 2.5-fold in mitochondria isolated from BAT after 7 days of cold exposure. We conclude from these data that the biogenesis of thermogenic mitochondria in BAT following cold adaptation is achieved by increasing the overall capacity for synthesis of mitochondrial proteins in both compartments, by increasing their mRNAs as well as the ribosomes needed for their translation. In addition, the translational rate for COX subunits as well as all other proteins encoded on mtDNA is increased. Thus the pool of subunits encoded on mtDNA required for assembly of respiratory chain complexes is provided. By comparison with other models of increased mitochondrial biogenesis, we propose that thyroid hormone (generated within BAT cells by 5'-deiodinase, and induced upon sympathetic stimulation), which is a well known regulator of the biogenesis of mitochondria in many tissues, is also the major effector of these adaptive changes in BAT.

Hypothyroidism affects the expression of the beta-F1-ATPase gene and limits mitochondrial proliferation in rat liver at all stages of development

In order to analyze the role of thyroid hormones in mitochondrial biogenesis, we have studied the expression pattern of the beta subunit of the mitochondrial ATP-synthase complex in liver and in isolated mitochondria during postnatal development of hypothyroid rats. Chemically induced hypothyroidism promoted a significant reduction in body and liver masses at all stages of development. Furthermore, plasma 3,5,3'-triiodo-L-thyronine (T3) and 3,5,3',5'-tetraiodo-L-thyronine (T4) concentrations were significantly reduced in hypothyroid animals when compared to euthyroid animals. Remarkably, steady-state beta-F1-ATPase mRNA levels in livers of hypothyroid animals showed an approximately 50% reduction when compared to age-matched euthyroid rats at all stages of development. The relative amounts of beta-F1-ATPase protein determined in isolated mitochondria of 1-day-old and adult hypothyroid animals were similar to those determined in mitochondria of age-matched euthyroids, indicating that hypothyroidism does not affect organelle differentiation in the liver of suckling and adult rats. In contrast, the relative amount of beta-F1-ATPase protein in liver homogenates varied (0-30% reduction) due to the hypothyroid condition during development. These findings suggest the existence of compensatory mechanisms operating at the translational and/or post-translational levels which promote proliferation of mitochondria in the hypothyroid liver. However, when the liver mass was considered, hypothyroidism significantly reduced overall mitochondrial proliferation in rat liver. Interestingly, the effects of thyroid hormones on the biogenesis of the ATP synthase complex at latter stages of development provide an example in which the hypothyroid condition limits the expression of the nuclear-encoded gene with no apparent effect on the expression of the mitochondrial-encoded genes (ATP synthase subunits 6-8).

The effect of L-thyroxine on the anaemia response in Trypanosoma congolense infected rabbits

The development of anaemia is a major pathological manifestation in chronic trypanosomosis. The anaemia in African trypanosomosis coincides with a marked decrease in plasma concentration of both thyroxine (T4) and 3,5,3' triiodothyronine (T3). To evaluate the effect of trypanosome-induced hypothyroidism on the development of anaemia, sexually mature white New Zealand rabbits were used. Three groups were set up, each of ten rabbits: one group was infected with Trypanosoma congolense; the second group was infected but given replacement doses of thyroxine (treated); the third group was not infected. Small volumes of blood were collected for the determination of parasitaemia and packed cell volume (PCV). The concentrations of T3 and T4 were measured in plasma by radioimmunoassay. The decrease in PCV correlated closely (y = -0.38x + 15.2; r = 0.82, P = 0.001) with the intensity and duration of parasitaemia. The critical PCV value was 0.15 11-1 with a peak parasitaemia of approximately 5 x 10(6) trypanosomes ml-1 of blood. There was a significant correlation between the plasma T3 and PCV (y = 0.049x + 0.57; r = 0.66, P = 0.020). There was also a good positive correlation between T4 and PCV (y = 14.5 + 3.03; r = 0.95, P < 0.001) in the infected untreated group. The PCV levels were significantly different among the three groups of animals (P < 0.05). The infected-treated animals sustained longer periods of infection than the infected and untreated ones. The sustained physiological level of bioactive thyroid hormones T3 and T4 significantly arrested the decline in PCV as the disease progressed.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of thyroid hormone and promoter diversity in the regulation of nuclear encoded mitochondrial proteins

Thyroid hormone regulates the in vivo expression of a selected set of rat nuclear genes encoding mitochondrial inner membrane proteins. Certain mRNAs, such as that for cytochrome c1, are increased as much as 20-50-fold, while others, such as core protein 1 of Complex III and the F1-ATPase beta-subunit do not respond. The promoter region of human cytochrome c1 also supports thyroid hormone induction of a reporter gene in transient transfection experiments. Thus, thyroid hormone regulates only selected genes, even for subunits within the same complex and in widely varying species. By contrast, growth activation of quiescent NIH3T3 cells, a second paradigm used for stimulating mitochondrial biogenesis, does not increase cytochrome c1 mRNA but does increase F1-ATPase beta-subunit mRNA. These findings suggest that nuclear OXPHOS genes are not necessarily expressed in a coordinated manner, and that multiple regulatory circuits might exist which are linked to different physiological stimuli. Analysis of the promoters of several OXPHOS genes reveals a great diversity and heterogeneity of transfactor binding elements. No single regulatory feature exists which could account for a coordinated expression of all OXPHOS genes. The potential diversity for regulating expression of nuclear OXPHOS genes raises the possibility for the existence of disease states linked to regulatory defects.

Increased expression of mitochondrial-encoded genes in skeletal muscle of humans with diabetes mellitus

Screening subtraction libraries from normal and type II diabetic human skeletal muscle, we identified four different mitochondrially encoded genes which were increased in expression in diabetes. The genes were cytochrome oxidase I, cytochrome oxidase III, NADH dehydrogenase IV, and 12s rRNA, all of which are located on the heavy strand of the mitochondrial genome. There was a 1.5- to 2.2-fold increase in the expression of these mRNA molecules relative to total RNA in both type I and type II diabetes as assessed by Northern blot analyses. Since there was approximately 50% decrease in mitochondrial DNA copy number as estimated by Southern blot analyses, mitochondrial gene expression increased approximately 2.5-fold when expressed relative to mitochondrial DNA copy number. For cytochrome oxidase I similar changes in mitochondrial gene expression were observed in muscle of nonobese diabetic and ob/ob mice, models of type I and type II diabetes, respectively. By contrast there was no change or a slight decrease in expression of cytochrome oxidase 7a, a nuclear-encoded subunit of cytochrome oxidase, and the expression of mitochondrial transcription factor 1 in human skeletal muscle did not change with type I or type II diabetes. The increased mitochondrial gene expression may contribute to the increase in mitochondrial respiration observed in uncontrolled diabetes.

Thyroid hormones promote transcriptional activation of the nuclear gene coding for mitochondrial beta-F1-ATPase in rat liver

Thyroid hormones acutely regulate gene expression of the beta-catalytic subunit of the mitochondrial F1-ATPase complex in the liver of hypothyroid rat neonates at either a transcriptional and/or post-transcriptional level [(1990) J. Biol. Chem. 265, 9090-9097]. Administration at birth of various thyroid hormone doses to hypothyroid newborn rats promote a rapid (1 h) increase in liver steady-state amounts of both beta-F1-ATPase protein and mRNA. Induction of the beta-F1-ATPase mRNA is coincident with an elevation in gene transcription detected using nascent RNA chains synthesized by isolated nuclei. These results suggest that thyroid hormones induction of postnatal mitochondrial differentiation in the liver of hypothyroid rat neonates is mostly triggered by transcriptional regulation of beta-F1-ATPase gene.

Respiratory activity of isolated liver Mitochondria following Trypanosoma congolense infection in rabbits: the role of thyroxine

1. The effect of trypanosome infection on rabbit liver mitochondrial oxidative phosphorylation was investigated, with and without thyroxine replacement. 2. State 3 respiration, respiratory control ratio (RCR) and ADP/O ratio were significantly reduced in mitochondria from trypanosome-infected animals whereas there was no change in state 4 respiration. 3. State 3 respiration, RCR and ADP/O ratio were not significantly altered in trypanosome-infected animals given thyroxine replacement therapy. 4. Trypanosome infection leads to impairment of mitochondrial integrity, apparently through lowered thyroxine levels. Replacement of thyroxine therefore sustains optimal mitochondrial respiratory activity.

Transcript levels for nuclear-encoded mammalian mitochondrial respiratory-chain components are regulated by thyroid hormone in an uncoordinated fashion

Thyroid hormone is one of the few known physiological regulators of mammalian mitochondrial biogenesis. Although it exerts a global effect on biogenesis, it does so by regulating the expression of a limited number of unidentified mitochondrial proteins. We have investigated these hormone-regulated proteins in rat liver. Hormone injection induced a 30-fold increase in the levels of cytochrome-c1 mRNA after 3 d. In addition, the mRNA for the growth-activated adenine-nucleotide translocator, ANT2, was increased 13-fold and that for the ATPase N,N'-dicyclohexylcarbodiimide-binding protein increased 4-5-fold. Mitochondrial transcripts of cytochrome-oxidase subunit I also increased. No changes were found in the mRNA levels for the F1-ATPase beta-subunit or cytochrome oxidase IV. A single low dose of triiodothyronine induces rapid increases in cytochrome-c1 and ANT2 mRNA species which parallel changes in the activity of the hormone-responsive malic enzyme, but are earlier than other mitochondrial biogenetic events. These data strengthen the view that thyroid hormone regulates synthesis of specific components within each respiratory-chain complex and that these products apparently play key roles in inner-membrane biogenesis and assembly. The significance of ANT2 induction is also discussed with respect to the rapid respiratory response induced by thyroid hormone.

Differential action of thyroid hormones on the activity of certain enzymes in rat kidney and brain

In rat kidney several mitochondrial and soluble enzyme activities are stimulated by thyroid hormones and the mitochondrial membrane fluidity is also increased. However, the ketone metabolism enzyme activities of D-3-hydroxybutyrate dehydrogenase and of 3-oxoacid CoA-transferase are not significantly affected by the hyperthyroid state and the ketone body concentration is not greatly changed. Therefore, in hyperthyroid rats the response of the kidney, as far as the ketone bodies and their metabolizing enzymes are concerned, is at variance with that of the liver and the heart. In the brain of young rats, age 8-9 weeks, the activities of the enzymes of ketone body metabolism and those responsible for other metabolic pathways are not influenced by the hyperthyroid state. In these animals, however, the activities of two enzymes, NAD-isocitrate dehydrogenase and pyruvate kinase, are still stimulated by 28 and 41%, respectively. This can be probably related to the higher energy requirement for definitive brain maturation in young hyperthyroid rats.

The effects of thyroid hormones and starvation on hepatic mitochondrial nucleic acids of rainbow trout (Oncorhynchus mykiss)

Twenty four hours after an intraperitoneal injection of thyroxine (T4; 4.4, 44 ng/g body wt) or triiodothyronine (T3; 3.3, 33 ng/g body wt), DNA and RNA were significantly reduced in isolated liver mitochondria of rainbow trout. Total liver DNA was increased with the higher doses in both T4- and T3-injected specimens while total RNA was significantly reduced with the same doses. Total circulating plasma T4 was reduced with the injections of T3 or starvation, and plasma T3 was increased severalfold with the T3 injections. Plasma T4 also increased fivefold after a higher dose of T4 injection while starvation significantly reduced the concentration, when measured by radioimmunoassay.

Acetyl-L-carnitine increases cytochrome oxidase subunit I mRNA content in hypothyroid rat liver

The effect of acetyl-L-carnitine on the quantity of the messenger RNA for the subunit I of cytochrome oxidase in the liver mitochondria of hypothyroid rat was measured by Northern blot and solution hybridization. Three hours after pre-treatment of hypothyroid rat with acetyl-L-carnitine, the level of the transcript increased strongly. This effect was also obtained when acetyl-L-carnitine was administered to T3 pre-treated hypothyroid rats. These results add further evidence to the suggestion that acetyl-L-carnitine is able to stimulate mitochondrial transcription under altered metabolic conditions.

Thyroid hormone regulation of mitochondrial function. Comments on the mechanism of signal transduction

Thyroid hormone exerts two types of effect on mitochondria. The first of these is a rapid activation of respiration which takes place within minutes after hormone injection, and is preserved in isolated mitochondria. The second response occurs after 1 to several days of injection and leads to mitochondrial biogenesis and increases in mitochondria mass. The hormone signal for these two responses involves either triiodothyronine (T3)-responsive nuclear genes or a direct action of T3 at mitochondria binding sites.

Mitochondrial protein synthesis during thyroxine-induced cardiac hypertrophy

The goal of this paper was to determine the effects of 3,5,3'-triiodothyronine (T3)-thyroxine-induced cardiac hypertrophy on the rates of synthesis of mitochondrial proteins by both the cytoplasmic and mitochondrial protein synthesis systems and to compare the results with total protein synthesis and cardiac enlargement. Daily injections of T3-thyroxine in the rat resulted in a 25% increase in the growth of the ventricle compared with controls. The cytoplasmic synthesis of both mitochondrial and total proteins as measured in the isolated perfused heart was stimulated by T3-thyroxine injection to a peak of 155 and 146%, respectively, of vehicle-injected controls after 3 days of hormone treatment. This peak was followed by a gradual decline in stimulation in total protein synthesis to 132% of control by 9 days of injection, whereas the decline in stimulation of cytoplasmic synthesis of mitochondrial proteins was significantly steeper, falling to 119% of vehicle control. The rate of protein synthesis within the mitochondrial compartment was also measured during the time course of T3-thyroxine-induced hypertrophy. These rates were measured in an isolated intact heart mitochondrial protein synthesis system described and characterized in the companion papers [E. E. McKee, B. L. Grier, G. S. Thompson, and J. D. McCourt. Am. J. Physiol. 258 (Endocrinol. Metab. 21): E492-E502, 1990; and E. E. McKee, B. L. Grier, G. S. Thompson, A. C. F. Leung, and J. D. McCourt. Am. J. Physiol. 258 (Endocrinol. Metab. 21): E503-E510, 1990]. Rates of mitochondrial protein synthesis were dramatically stimulated by T3-thyroxine injection.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone regulation of nuclear-encoded mitochondrial inner membrane polypeptides of the liver

The effects of thyroid hormone on nuclear-encoded mitochondrial inner membrane proteins were investigated by in vitro translation of the endogenous mRNA present in a postmitochondrial fraction from the livers of rats treated in vivo with hormone. The levels of the mRNAs were estimated by quantitative immunoabsorption of the translation mixture. Total protein synthesis was increased 2.6-fold after 4 days of in vivo hormone treatment, but only 10-15% of the polypeptides were dramatically altered (greater than 5-fold). Among the most highly elevated were cytochrome c1 (greater than 10-fold increase) and the Rieske iron-sulfur protein of the cytochrome bc1 complex. Other inner membrane proteins (core protein 1, beta subunit of F1 ATPase, subunit IV of cytochrome oxidase, 3-hydroxybutyrate dehydrogenase) and non-mitochondrial proteins (rat serum albumin, beta 2-microglobulin) were not altered significantly by hormone treatment. Cytochrome c1 and the Rieske protein increased after 12 h of hormone treatment, a relatively early response in mammalian mitochondrial biogenesis. The possible significance of this response for the regulation of mitochondrial synthesis and assembly is discussed.

Rapid stimulation of hepatic oxygen consumption by 3,5-di-iodo-L-thyronine

Tri-iodothyronine (T3) and thyroxine (T4) as well as 3,5-di-iodothyronine (T2) stimulated O2 consumption by isolated perfused livers from hypothyroid rats at a concentration as low as 1 pM by about 30% within 90 min. Application of T2 resulted in a faster stimulation than with application of T3 or T4. Inhibition of iodothyronine monodeiodinase by propylthiouracil, thereby blocking the degradation of T4 to T3 and of T3 to T2, demonstrated that only T2 is the active hormone for the rapid stimulation of hepatic O2 consumption: T3 and T4 lost all of their stimulative activity, whereas T2 was as potent as in the absence of propylthiouracil. Perfusion experiments with thyroid-hormone analogues confirmed the specificity of the T2 effect. The nucleus is unlikely to contribute to the rapid T2 effect, as can be deduced from perfusion experiments with cycloheximide and lack of induction of malic enzyme by T2. In conclusion, a new scheme of regulation of mitochondrial activity is proposed: T2 acts rapidly and directly via a mitochondrial pathway, whereas T3 exerts its long-term action indirectly by induction of specific enzymes.

Control of mitochondrial transcription by thyroid hormone

Thyroid hormone regulation of rat liver mitochondrial transcription was investigated. Steady-state levels of mitochondrial transcripts were measured by Northern blot analysis using cloned fragments of rat mtDNA. Thyroid hormone increased the steady-state concentrations of all mitochondrial mRNAs by 2-8 fold after 1-3 days of hormone treatment, whereas no significant change in the mitochondrial rRNA was observed. Analysis of transcript synthesis in isolated mitochondria shows that part or all of this increase is accounted for by elevated synthesis. Mechanisms by which thyroid hormone regulates transcription of the mitochondrial genome are discussed.

Oxidative metabolism in a teleost, Anabas testudineus Bloch: effect of thyroid hormones on hepatic enzyme activities

In vivo administration of L-thyroxine (L-T4) in Anabas testudineus, while significantly stimulated the activities of cytochrome c oxidase and alpha-glycerophosphate dehydrogenase (alpha-GPDH), inhibited glucose-6-phosphate dehydrogenase (G-6-PDH), cytosolic and mitochondrial malate dehydrogenase (cyt. MDH; mit. MDH), and Mg2+ DNP-dependent adenosine triphosphatase (Mg2+ ATPase) activities. The activities of lactate dehydrogenase (LDH), succinate dehydrogenase (SDH), and catalase remained unaltered after L-T4 treatment. Administration of protein synthesis inhibitors such as actinomycin D, while significantly inhibited cytochrome oxidase, alpha-GPDH, catalase, SDH, and Mg2+ ATPase activities, did not change LDH, cyt. MDH, and mit. MDH activities. Chloramphenicol injection significantly stimulated cytochrome oxidase, alpha-GPDH, and G-6-PDH activities. Simultaneous injections of actinomycin D or chloramphenicol with 3,5,3'-triiodo-L-thyronine (L-T3) or L-T4 prevented the effects of thyroid hormones on enzyme activities, when compared to the respective controls.

The response of individual polypeptides of the mammalian respiratory chain to thyroid hormone

The effects of thyroid hormone on the accumulation of inner membrane polypeptides in rat liver mitochondria have been investigated using Western blot analysis. Respiration and mitochondrial protein synthesis were also measured. Levels of the subunits of cytochrome oxidase, the cytochrome bc1 complex, and the beta-subunit of F1-ATPase increase relatively late, requiring 3-6 days of treatment and high doses of hormone. In contrast, respiration increases under conditions in which no significant accumulation of individual subunits is observed. Our results indicate that increased oxidative capacity of mitochondria can be divided into an early response which probably involves metabolic regulation of mitochondrial respiration by hormone and a later response which is due to elevated mitochondrial protein synthesis and the accumulation of polypeptides of the respiratory chain.

The induction of mitochondrial L-3-glycerophosphate dehydrogenase by thyroid hormone

L-3-Glycerophosphate dehydrogenase was purified from porcine brain mitochondria by a shorter and simpler procedure than previously reported. Immunoblotting with antiserum to the porcine enzyme established that rat liver L-3-glycerophosphate dehydrogenase has the same Mr (76 000) by SDS-polyacrylamide gel electrophoresis. In liver mitochondria from normal and hyperthyroid rats, changes in L-3-glycerophosphate dehydrogenase activity were parallelled by changes in enzyme content assayed by immunoblotting. Similar changes were found in the amount of enzyme synthesised in vitro by reticulocyte lysate programmed with rat liver mRNA, suggesting that thyroid hormone causes specific induction of L-3-glycerophosphate dehydrogenase mRNA.

Mitochondrial DNA, RNA and protein synthesis in normal and hypothyroid developing rat liver

Mitochondrial DNA, RNA and protein synthesis in normal and hypothyroid rat liver between the ages of -3 and 21 days were followed. In normal rats DNA polymerase activity and protein synthesis behaved similarly, showing two peaks of activity, one at -3 and the other at 21 days of age. RNA polymerase activity did not change between days -3 and 14, whereas it increased by 21 days of age. Hypothyroidism delayed the developmental pattern of DNA polymerase activity, affected RNA polymerase activity only at 21 days, whereas it inhibited protein synthesis at birth and in the third week of life. The cytochrome aa3 content appeared to be affected by hypothyroidism at birth and at 21 days of age.

The translation system of rat heart muscle mitochondria is stimulated following treatment with L-triiodothyronine

In vitro translation of mitochondrial translation products by mitochondria isolated from the myocardia of rats injected with L-triiodothyronine daily for 5 days was compared with those of euthyroid animals. The incorporation of 35S-methionine into proteins was greater by heart mitochondria isolated from T3-treated rats than by those isolated from euthyroid animals. This increase is due to a stimulation of mitochondrial protein synthesis rather than being the result of a reduction in the rate of protein degradation or an increase in the specific radioactivity of the amino acid pool. The results also established that the polypeptide profile of mitochondrial translation products is the same in heart mitochondria isolated from euthyroid and T3-treated animals. The relative increase in the rates of synthesis of mitochondrial translation products is non-uniform. These results suggest that the hormone acts by inducing a general but non-uniform increase in the activity of the mitochondrial transcription and/or translation system(s).

Thyroid hormone regulation of flavocoenzyme biosynthesis

The means by which thyroid hormone regulates flavocoenzyme biosynthesis was studied in hyper-, eu-, and hypothyroid rats by determining the activities of flavocoenzyme-forming enzymes, viz., flavokinase and FAD synthetase, as well as those of flavocoenzyme-degrading enzymes, viz., FMN phosphatase and FAD pyrophosphatase. Flavokinase activity was increased in hyperthyroid animal and decreased in hypothyroid animals. Correspondence of flavokinase activity with the amount of a high-affinity flavin-binding protein quantitated immunologically in hypo-, eu-, and hyperthyroid rats indicated that the thyroid response is caused by an increased amount of enzyme; moreover, the concomitant decrease in a low-affinity flavin-binding protein suggests an inactive precursor form of flavokinase. FAD synthetase activity showed a similar but less pronounced trend than flavokinase. Activities of FMN phosphatase and FAD pyrophosphatase were not influenced by thyroid hormone. Overall results indicate that the mechanism of thyroid hormone regulation of flavocoenzyme level is in the steps of biosynthesis, especially at flavokinase, rather than in degradation steps.