Molecular cloning of a cDNA sequence for rat malic enzyme. Direct evidence for induction in vivo of rat liver malic enzyme mRNA by thyroid hormone

Modulation of gene expression by 3-iodothyronamine: genetic evidence for a lipolytic pattern

3-Iodothyronamine (T1AM) is an endogenous biogenic amine, structurally related to thyroid hormone, which is regarded as a novel chemical messenger. The molecular mechanisms underlying T1AM effects are not known, but it is possible to envisage changes in gene expression, since delayed and long-lasting phenotypic effects have been reported, particularly with regard to the modulation of lipid metabolism and body weight. To test this hypothesis we analysed gene expression profiles in adipose tissue and liver of eight rats chronically treated with T1AM (10 mg/Kg twice a day for five days) as compared with eight untreated rats. In vivo T1AM administration produced significant transcriptional effects, since 378 genes were differentially expressed in adipose tissue, and 114 in liver. The reported changes in gene expression are expected to stimulate lipolysis and beta-oxidation, while inhibiting adipogenesis. T1AM also influenced the expression of several genes linked to lipoprotein metabolism suggesting that it may play an important role in the regulation of cholesterol homeostasis. No effect on the expression of genes linked to toxicity was observed. The assay of tissue T1AM showed that in treated animals its endogenous concentration increased by about one order of magnitude, without significant changes in tissue thyroid hormone concentration. Therefore, the effects that we observed might have physiological or pathophysiological importance. Our results provide the basis for the reported effectiveness of T1AM as a lipolytic agent and gain importance in view of a possible clinical use of T1AM in obesity and/or dyslipidaemia.

Protein and enery relationships in the broiler chicken 12. Dietary protein and triidothyronine (T3) effects on the response of broilers to isoproterenol and cyclic adenosine monophosphate in vitro

Indian River male broiler chickens (7-d-old) were fed on diets containing 120, 210 or 300 g crude protein/kg + 0 or 1 mg triiodothyronine (T3)/kg diet (Expt 1) and 120, 150, 180 or 210 g crude protein/kg +0 or 1 mg T3/kg diet (Expt 2) to determine the effects of crude protein level and T3on growth and metabolism. Body composition of chickens was determined by a combination of dissection of muscle and abdominal fat pads, and chemical extraction (Expt 1).In vitrolipogenesis (IVL) was determined in both experiments by incubating liver explants for 2 h at 37° in the presence of 10-4M-dibutyryl cyclic AMP (cAMP) or 10-5M isoproterenol (ISO) and 10-2M-[2-14C]acetate. Acetate incorporation into total lipid was an indication of IVL. Activity ratios for each of these additions relative to control (?cAMP?ISO) were calculated to ascertain basal ν. inhibited rates of IVL. The relative muscle mass was increased by increasing crude protein from 120 to 210 g/kg diet but not from 210 to 300 g/kg diet. Dietary T3decreased total body lipid regardless of the dietary crude-protein level. Increasing dietary crude protein decreased (P<0.05) basal IVL (?cAMP? ISO) but not IVL (+ cAMP). Dietary T3decreased basal IVL in birds fed on the diets containing 120 and 210 g crude protein/kg but had little effect on the two inhibited states of lipogenesis (+ cAMP or +ISO). The component of lipogenesis sensitive toin vitroinhibition is also the component under dietary control.

A proteomics approach to identify protein expression changes in rat liver following administration of 3,5,3'-triiodo-L-thyronine

We analyzed whole cell protein content of rat liver following T3 administration. Fourteen differentially expressed proteins were unambiguously identified and were involved in substrates and lipid metabolism, energy metabolism, detoxification of cytotoxic products, calcium homeostasis, amino acid catabolism, and the urea cycle. This study represents the first systematic identification of T3-induced changes in liver protein expression profile and provides novel information at the molecular, cellular, and tissue level of T3 action.

Studies on doses of methimazole (MMI) and its administration regimen on broiler metabolism

We designed three experiments to determine both the optimal dose of and time on experiment for methimazole (MMI; 1-methyl-2-mercaptimidazole). Our goals were to determine if chicken growth was related to thyroid hormone levels and if intermediary metabolism changed along with changes in thyroid hormone levels. Initiating MMI at one week of age decreased (P<0.01) plasma thyroid levels and growth in four-week old birds. In contrast, initiating MMI at two and three weeks of age decreased (P<0.05) hormone levels without affecting growth as severely. Although initiating MMI at two weeks of age depressed (P<0.05) plasma thyroid hormones at four weeks, there was little change in vitro lipogenesis at four weeks. Again, initiating MMI at one week of age decreased body weight, plasma thyroid hormones and in vitro lipogenesis at four weeks of age. In addition, this treatment also decreased (P<0.05) malic enzyme activity at this same age period. The second experiment showed that MMI, initiated at 14 days, had no significant effect on 28-day body weight and again decreased both plasma T(3) and T(4) but T(3) replacement increased plasma T(3) in both 14-28-day treatment groups. All body weights were similar at 30 days, however. Lastly, diets containing graded levels of MMI decreased thyroid hormones and body weight (0>0.25>0.5>1 g MMI/kg). In contrast, only the two higher levels (0.5 and 1 g MMI/kg) decreased in vitro lipogenesis. Growth depression, caused by MMI feeding, can occur without changes in lipid metabolism. The length of MMI administration may be as important as dose level in obtaining effects (growth, thyroid hormone depression and inhibition of lipogenesis).

Methimazole and thyroid hormone replacement in broilers

Seven-day-old chickens were fed diets containing 18% crude protein + 0 or 1g methimazole/kg to produce either euthyroid or hypothyroid groups of birds at 28 days of age. These two groups were then offered diets containing either 0 or 1mg triiodothyronine (T(3))/kg diet. Birds were sampled at 0, 2, 5, and 8 days following the onset of the T(3) treatment. Measurements taken at these intervals included in vitro hepatic lipogenesis (IVL), growth and feed consumption, hepatic enzyme activities (malic enzyme, ME; isocitrate dehydrogenase, ICD; and aspartate amino transferase, AAT), plasma hormones (T(3); thyroxine, T(4); insulin like growth factors I, IGF-I; and insulin like growth factors II, IGF-II) and metabolites (glucose; fatty acids, NEFA; triglyerides; uric acid). Hypothyroidism decreased IVL and ME at 28 days of age; however, T(3) supplementation for 2 days restored both IVL and ME. Paradoxically, continuing T(3) replenishment for an additional 3-6 days decreased IVL without affecting ME activity. In contrast, supplemental T(3) decreased IVL in euthyroid birds, regardless of the dosing interval, but had no effect on ME activity. Methimazole decreased plasma T(3), T(4), uric acid, and IGF-I, but did not affect IGF-II at 28 days. Giving T(3) to birds previously on methimazole increased plasma IGF-I as did feeding a control diet. Supplemental T(3) increased NEFA in both euthyroid and hypothyroid birds, but only for a short period following the initiation of supplementation (2 days post-supplementation). These data may help to explain some of the apparent reported dichotomies in lipid metabolism elicited by changes in the thyroid state of animals. In addition, most metabolic changes in response to feeding T(3) occurred within 2-5 days, suggesting that changes in intermediary metabolism preceded morphological changes. In conclusion, the thyroid state of the animal will determine responses to exogenous T(3).

Differential role of PPAR gamma in the regulation of UCP-1 and adipogenesis by TNF-alpha in brown adipocytes

Extracellular regulated kinases (ERKs) mediate the inhibitory effect of tumor necrosis factor alpha (TNF-alpha) on uncoupling protein-1 (UCP-1), but not on lipid accumulation. TNF-alpha-induced ERK-dependent peroxisome proliferator activator receptor gamma (PPAR gamma) phosphorylation could be responsible for UCP-1 downregulation. Thus, the negative effect of TNF-alpha on UCP-1 mRNA expression at 4-5 h, under basal conditions or in cells treated with the PPAR gamma agonist, rosiglitazone, was reversed by the MEK1 inhibitor PD98059. In contrast, fatty acid synthase and malic enzyme mRNA downregulation was not prevented. Moreover, rosiglitazone has no positive effect on adipogenic gene expression or lipid accumulation. Therefore, there is a differential regulation of thermogenic and adipogenic differentiation by PPAR gamma, which might account for the differences in the TNF-alpha regulation through ERKs.

TNF-alpha inhibits UCP-1 expression in brown adipocytes via ERKs. Opposite effect of p38MAPK

Tumor necrosis factor-alpha (TNF-alpha) activates extracellular-regulated kinases (ERKs) and p38 mitogen-activated protein kinase (p38MAPK), and inhibits the expression of uncoupling protein-1 (UCP-1) and adipocyte-specific genes in rat fetal brown adipocytes. MEK inhibition with PD98059 abolished the inhibitory effect of TNF-alpha on UCP-1, but not on adipogenic genes. In contrast, inhibition of p38MAPK with SB203580 potentiated the negative effect of TNF-alpha on UCP-1 and adipogenic genes. The inhibitory action of TNF-alpha was partially correlated with changes in C/EBPalpha and beta protein levels and in their DNA binding activity, suggesting a role for these transcription factors. However, other transcription factors might explain the different regulation of UCP-1 and adipogenic genes by ERKs.

Phosphatidylinositol 3-kinase is a requirement for insulin-like growth factor I-induced differentiation, but not for mitogenesis, in fetal brown adipocytes

In the present study we have examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the insulin-like growth factor I (IGF-I)-signaling pathways involved in differentiation and in mitogenesis in fetal rat brown adipocytes. Activation of PI 3-kinase in response to IGF-I was markedly inhibited by two PI 3-kinase inhibitors (wortmannin and LY294002) in a dose-dependent manner. IGF-I-stimulated glucose uptake was also inhibited by both compounds. The expression of adipogenic-related genes such as fatty acid synthase, malic enzyme, glycerol 3-phosphate dehydrogenase, and acetylcoenzyme A carboxylase induced by IGF-I was totally prevented in the presence of IGF-I and any of those inhibitors, resulting in a marked decrease of the cytoplasmic lipid content. Moreover, the expression of the thermogenic marker uncoupling protein induced by IGF-I was also down-regulated in the presence of wortmannin/LY294002. IGF-I-induced adipogenic- and thermogenic-related gene expression was only partly inhibited by the p70S6k inhibitor rapamycin. In addition, pretreatment of brown adipocytes with either wortmannin or LY294002, but not with rapamycin, blocked protein kinase C zeta activation by IGF-I. In contrast, IGF-I-induced fetal brown adipocyte proliferation was PI 3-kinase-independent. Our results show for the first time an essential requirement of PI 3-kinase in the IGF-I-signaling pathways leading to fetal brown adipocyte differentiation, but not leading to mitogenesis. In addition, protein kinase C zeta seems to be a signaling molecule also involved in the IGF-I differentiation pathways downstream from PI 3-kinase.

Regulation of gene expression for lipogenic enzymes in the liver and adipose tissue of hereditary hypertriglyceridemic, insulin-resistant rats: effect of dietary sucrose and marine fish oil

Hypertriglyceridemia is closely linked to insulin resistance. Increased dietary intake of n-3 polyunsaturated fatty acids reverses both hypertriglyceridemia and insulin resistance. To evaluate molecular mechanisms responsible for the hypotriglyceridemic effects of n-3 polyunsaturated fatty acids, the expression of genes for lipogenic enzymes in liver and white and brown adipose tissue was estimated in hereditary hypertriglyceridemic rats which underwent an euglycemic hyperinsulinemic clamp. Before the clamp, animals were fed a basal or a high (63%) sucrose diet with or without fish oil for two weeks. Results were compared to data obtained from control animals subjected to the identical protocol. In hereditary hypertriglyceridemic rats, gene expression for malic enzyme was increased in liver and in brown adipose tissue but not in white adipose tissue. The high sucrose diet raised malic enzyme mRNA levels in liver of both hereditary hypertriglyceridemic and control rats, and this effect was more pronounced in brown adipose tissue. Supplementing the high sucrose diet with fish oil led to a suppression of malic enzyme gene expression in liver and brown adipose tissue of control rats. However, this inhibitory effect was not as pronounced in the hereditary hypertriglyceridemic rats. Raised levels of fatty acid synthase mRNA in liver and brown adipose tissue of control rats fed high sucrose diet were suppressed by consumption of diet high in n-3 fatty acids. On the other hand, in hereditary hypertriglyceridemic rats fed high sucrose diet, fish oil supplementation failed to suppress increased levels of fatty acid synthase mRNA in liver and in brown adipose tissue. It appears that hereditary hypertriglyceridemic rats have elevated levels of mRNA for lipogenic enzymes in liver and brown adipose tissue and dietary control leading to an alteration of hypertriglyceridemia influences gene expression of lipogenic enzymes only under special dietary circumstances.

Nonidentity of the cDNA sequence of human breast cancer cell malic enzyme to that from the normal human cell

A cDNA coding for human breast cancer cell cytosolic NADP(+)-dependent malic enzyme was obtained. This cDNA is composed of a length of 2084 base pairs, with 1698 base pairs coding for 565 amino acid residues and a length of 386 base pairs representing a 3'-noncoding region. Comparing this nucleotide sequence with that from the normal human tissue [Loeber, G., Dworkin, M. B., Infante, A., and Ahorn, H. (1994), FEBS Lett. 344, 181-186] reveals that three nucleotides in the open reading frame and the length of 3'-noncoding region of the cDNA are different. One of the changes results in a substitution of serine at position 438 for proline, which, however, may not cause significant changes in the predicted secondary structure. A partial cDNA lacking the first 84 nucleotides in the open reading frame was successfully cloned and expressed functionally in Escherichia coli cells. Its Km value for L-malate (1.21 +/- 0.11 mM) is four times higher than that for the natural human breast cancer cell malic enzyme (0.29 +/- 0.04 mM) but similar to that for the full-length recombinant enzyme (1.06 +/- 0.07 mM). The Km values for Mn2+ and NADP+ (0.26 +/- 0.03 and 0.97 +/- 0.4 microM, respectively) are similar to those for the natural enzyme (0.12 +/- 0.02 and 1.9 +/- 0.3 microM, respectively) or the recombinant wild-type enzyme (0.56 +/- 0.04 and 0.44 +/- 0.02 microM, respectively). A recombinant pigeon liver malic enzyme without the first 13 amino acid residues was used for comparison. The Km values for L-malate and Mn2+ of the truncated enzyme (11.2 +/- 0.9 mM and 61.2 +/- 4.6 microM, respectively) are over 40 times larger than those for the natural pigeon liver malic enzyme (0.21 +/- 0.02 mM and 1.06 +/- 0.08 microM, respectively) or the recombinant wild-type enzyme (0.25 +/- 0.01 mM and 1.48 +/- 0.05 microM, respectively). We suggest that the N-terminus of malic enzyme may be required for the substrate binding during the catalytic cycle.

Abnormal regulation of hepatic glucocorticoid receptor mRNA and receptor protein distribution in the obese Zucker rat

This study examines the cellular distribution of glucocorticoid receptor (GR) protein and transcriptional activity of the GR gene in the liver of Zucker obese (fa/fa) rats. Immunoabsorption and Western blotting showed an increase in nuclear GR protein level but a decrease in cytosolic GR levels in the liver of 5-week old male obese rats (fa/fa) compared to their lean littermates (Fa/-). These changes were confirmed by receptor-ligand binding assays with [3H]-dexamethasone which showed a sixfold increase in average obese nuclear GR binding and a twofold reduction in cytosolic GR binding. HSP90, but not HSP70, levels were reduced in hepatic cytosol and increased in hepatic nuclei prepared from obese rats. Using Northern blot analysis of hepatic RNA, we demonstrated a twofold increase in hepatic mRNAs for GR, malic enzyme (ME), tyrosine aminotransferase (TAT), and glyceraldehyde 3-PO4-dehydrogenase in the obese rat. Increased transcription of GR and ME mRNAs in obese nuclei was indicated in nuclear run-on assays. These data suggest that there is increased nuclear localization of GR in the liver of obese rats and suggests that increased transcription of GR gene may contribute to this effect. The described changes may contribute to the abnormal regulation by glucocorticoids of some hepatic genes in the Zucker fa/fa rat.

Nutritional and hormonal regulation of expression of the gene for malic enzyme

We have provided a historical and personal description of the analysis of physiological and molecular mechanisms by which diet and hormones regulate the activity of hepatic malic enzyme. For the most part, our analyses have been reductionist in approach, striving for increasingly simpler systems in which we can ask more direct questions about the molecular nature of the signaling pathways that regulate the activity of malic enzyme. The reductionist approaches that were so successful at analyzing molecular mechanisms in cells in culture may now provide the means to analyze more definitively questions about the physiological mechanisms involved in nutritional regulation of gene expression. In addition to physiological questions, however, there are still many aspects of the molecular mechanisms that have not been elucidated. Despite considerable effort from many laboratories, the molecular mechanisms by which T3 regulates transcription are not clear. Similarly, the molecular details for the mechanisms by which glucagon, insulin, glucocorticoids, and fatty acids regulate gene expression remain to be determined. The role of fatty acids is particularly interesting because it may provide a model for mechanisms by which genes are regulated by metabolic intermediates; this is a form of transcriptional regulation widely used by prokaryotic organisms and extensively analyzed in prokaryotic systems, but poorly understood in higher eukaryotes. At any specific time, there is, of course, only one rate of transcription for each copy of the malic-enzyme gene in a cell. Our long-term objective is to understand how signals from all of the relevant regulatory pathways are integrated to bring about that rate.

Direct evidence for a role of the "spot 14" protein in the regulation of lipid synthesis

"Spot 14" is a nuclear protein that is rapidly induced by thyroid hormone (T3) and dietary carbohydrate in liver. We used an antisense oligonucleotide to inhibit induction of spot 14 protein by T3 and glucose in primary cultures of rat hepatocytes to test the hypothesis that the protein could function in the regulation of lipid synthesis. Spot 14 protein was undetectable in hepatocytes maintained in 5.5 mM glucose without T3, and was induced within 4 h after addition of 27.5 mM glucose and 50 nM T3 to the culture medium, reaching a maximal level within 24 h. Accumulation of spot 14 protein was markedly inhibited in hepatocytes transfected with a spot 14 antisense oligonucleotide, but not in those treated with a control oligonucleotide. Transfection of the antisense, but not control, oligonucleotide also abrogated the increase in lipogenesis induced by T3 and glucose. Reduced triglyceride formation accounted for the diminished net lipid synthesis. In contrast to lipogenesis, glucose uptake was not significantly affected by the transfections. Antisense transfection inhibited the induction of both ATP-citrate lyase and fatty acid synthase immunoreactivities, as well as malic enzyme activity, indicating that the observed reduction in lipogenesis could be explained by diminished cellular content of lipogenic enzymes. Reduced malic enzyme activity in antisense-transfected hepatocytes was accompanied by lowered relative abundance of malic enzyme mRNA, suggesting that the antisense effects on lipogenic enzymes were mediated at the pretranslational level. The oligonucleotides did not significantly affect lipogenesis in a rat hepatoma cell line that does not express detectable spot 14 mRNA or protein. These data directly implicate the spot 14 protein in the transduction of hormonal and dietary signals for increased lipid metabolism in hepatocytes.

Transforming growth factor beta 1 induces differentiation-specific gene expression in fetal rat brown adipocytes

Fetal rat brown adipocytes show a low number of transforming growth factor beta 1 (TGF-beta 1) binding sites of high affinity, revealing the presence of type I, II and III TGF-beta 1 receptors and a minor-labeled species of approximately 140 kDa. The culture of cells in the presence of TGF-beta 1 induced the expression of the tissue-specific gene uncoupling protein in a dose- and time-dependent manner. In addition, TGF-beta 1 up-regulates the expression of genes involved in adipogenesis such as fatty acid synthase, glycerol 3-phosphate dehydrogenase, malic enzyme and glucose 6-phosphate dehydrogenase, as well as induces the expression of fibronectin (specific target gene for TGF-beta 1). Our results suggest that TGF-beta 1 is a major signal involved in initiating and/or maintaining the thermogenic and adipogenic differentiation of rat fetal brown adipocytes.

Pyruvate stimulates hormonal induction of lipogenic enzymes in primary cultured rat hepatocytes

Hormonal inductions of lipogenic enzyme activities (fatty acid synthetase, malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PD) and ATP-citrate lyase) were studied in primary cultured rat hepatocytes. Insulin, triiodothyronine and dexamethasone markedly stimulated the inductions of the enzymes (particularly G6PD and ME) in the presence of pyruvate. Lactate also induced their activities. The activities of these enzymes in the presence of appropriate hormone combinations and a substrate amount of pyruvate were as high as, or higher than those in the liver of rats on high-carbohydrate, low-fat diet. The aldolase and glucokinase activities induced by these hormones were not enhanced by the addition of pyruvate. The induction by pyruvate was inhibited by actinomycin D or cycloheximide. The ATP content of rat hepatocytes was maintained without increase during culture with pyruvate for 6 days. These results indicate that the additions of pyruvate, or its metabolites to cultures of isolated hepatocytes have specific effects on the inductions of certain hepatic enzymes, possibly acting at the level of transcription. Their effects are similar to those of feeding a high-carbohydrate, low-fat diet to intact animals.

Regulation of malic-enzyme-gene expression by cAMP and retinoic acid in differentiating brown adipocytes

Brown adipose tissue (BAT) is composed of highly specialized cells, whose main function is to produce heat under adrenergic stimulation, uncoupling oxidative phosphorylation. For this function, lipogenesis must be accurately regulated. Malic enzyme has a central role in lipogenesis and is strongly expressed in brown adipocytes. In this work, we study the modulation by adrenergic stimuli, cAMP effectors and retinoic acid on the induction produced by insulin and 3,5,3'-triiodothyronine on malic-enzyme-gene expression. Primary cultures of differentiating brown adipocytes have been used. The results obtained demonstrate that physiological doses of norepinephrine do not modify malic-enzyme mRNA levels when acting alone, but considerably reduce the induction produced by insulin, 3,5,3'-triiodothyronine or both together. Other cAMP inducers such as glucagon, forskolin or 8-bromo-cAMP, greatly inhibit both, basal and 3,5,3'-triiodothyronine-induced malic-enzyme-gene gene expression. Retinoic acid abolishes basal and also inhibits 3,5,3'-triiodothyronine-induced malic-enzyme-gene expression.

Hormonal regulation of malic enzyme and glucose-6-phosphate-dehydrogenase expression in fetal brown-adipocyte primary cultures under non-proliferative conditions

The expression of malic enzyme and glucose-6-phosphate (Glc6P) dehydrogenase was investigated in primary cultures of fetal brown adipocytes after the prolonged presence (6 d or 10 d) of various hormones under non-proliferative conditions. The presence of triiodothyronine for 6 d and 10 d resulted in maturation of the triiodothyronine regulatory mechanism of malic-enzyme expression at the mRNA level. However, triiodothyronine had no effect on Glc6P dehydrogenase expression. Insulin increased malic-enzyme and Glc6P dehydrogenase expression at the mRNA and protein level after 6 d and 10 d of culture. The joint presence of triiodothyronine and insulin produced an additive effect on malic-enzyme expression at the mRNA and protein level after 6 d and 10 d of culture, by two independent mechanisms. Noradrenaline prevented the effect at the protein level after 6 d, but not after 10 d, probably due to loss of the beta-adrenergic response of brown adipocytes after prolonged culture. Triiodothyronine overexpressed the Glc6P dehydrogenase mRNA induced by the presence of insulin at 6 d and 10 d of culture. There was no adrenergic regulation of Glc6P dehydrogenase expression in cultured fetal brown adipocytes, regardless of the time of culture.

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.

Insulin-mediated regulation of epididymal fat pad malic enzyme

The complex nature of insulin-mediated biological responses has made it difficult to interpret such data. Prior studies in our laboratory had characterized the insulin-mediated increases in hepatic malic enzyme activity in normal and diabetic rats (Drake et al., 1983; Drake and Mucenski, 1985). However, since insulin-mediated regulatory processes have been shown to be tissue specific, we decided to examine malic enzyme activity in the epididymal fat pads of normal, diabetic, and insulin-treated normal and diabetic rats. This data revealed that in direct contrast to the hepatic studies, the normal epididymal fat pad contained the low specific activity malic enzyme molecule. Insulin treatment of both normal and diabetic rats resulted in an increase in epididymal fat pad malic enzyme activity due to increases in both enzyme quantity and specific activity. In order to quantitate epididymal fat pad malic enzyme mRNA levels, we isolated a 2.4 kb malic enzyme specific cDNA which was designated pR ME 1. In both normal and diabetic rats, the observed increases in malic enzyme quantity were directly paralleled by increases in malic enzyme mRNA content. The Northern blot data revealed an apparent differential expression of the malic enzyme mRNA doublet between insulin-treated normal and diabetic epididymal fat pads. This study demonstrates that insulin modulates epididymal fat pad and hepatic malic enzyme activity in a tissue-specific manner utilizing a defined subset of insulin-sensitive parameters involving alterations in enzyme specific activity and/or quantity.

Coding nucleotide sequence of rat malic enzyme mRNA and tissue specific regulation by thyroid hormone

A nearly full length ME cDNA has been obtained and sequenced. The identity has been established by comparison of the translated nucleotide sequence with the amino acid sequence of 7 tryptic peptides from purified ME. Northern analysis with this cDNA shows that ME mRNA consists of two different messages of about 27S and 21S. The size difference between two ME mRNAs (approximately equal to 27S and 21S) is attributed to the differences in the 3' noncoding regions. The relative ratios of the two ME mRNAs differ in various tissues examined (liver, heart, kidney, brain, lung, spleen, and testis). Their regulation by T3 is tissue specific with coordinate stimulation of both mRNAs in liver, heart and kidney, suggesting a single promoter for both mRNAs and no stimulation of either in the other tissues. T3 regulates ME mRNA synthesis via a dual-tissue specific mechanism by increasing the rate of transcription in liver and heart and stabilizing nuclear ME RNA sequences only in liver.

Insulin-mediated post-transcriptional regulation of hepatic malic enzyme and albumin mRNAs

Livers of insulin-treated diabetic rats accumulate albumin and malic enzyme mRNAs at very different rates. We now report that in normal rats insulin directs a specific increase in malic enzyme mRNA, while albumin mRNA levels remain unaltered. These studies support the contention that insulin regulates the accumulation of hepatic mRNAs in a highly specific manner. To evaluate whether or not albumin and malic enzyme mRNA levels are determined by altered rates of transcription, in vitro transcription assays were performed. The results of these studies demonstrate that increased malic enzyme mRNA levels in insulin-treated normal rats and increased malic enzyme and albumin mRNA levels in insulin-treated diabetic rats do not involve altered rates of transcription of the genetic sequences encoding these proteins. For these two specific proteins, insulin mediates changes in mRNA levels by a post-transcriptional mechanism.

Cellular mechanism of action of thyroid hormones

It has emerged in the last decade that the molecular mechanism of action of thyroid hormones resembles that of steroids; thyroid hormones indeed exert their effects mainly by directly regulating gene expression, on association with specific chromatin-bound receptors. Of the two thyroid hormones, thyroxine (T4) appears to be a sort of prohormone, whereas triiodothyronine (T3) seems to be the active form; in this respect, T4-deiodination, which occurs at the level of the target tissues, may be crucial in the local homeostasis of T3. Moreover, many cellular compartments, other than the nucleus, can bind thyroid hormone, and at least some of these further sites might play some role in modulating T3 supply to the nucleus. The binding of the T3-receptor complex to chromatin is likely to regulate the structural organization of specific genes and, in some instances, of the chromatin as a whole.

Photoaffinity labeling of thyroid hormone receptors

Photoaffinity label probes of iodothyronines can interact with nuclear receptors in intact cells and in solubilized receptor preparations. These probes have certain advantages over a chemical affinity label in analyzing receptor structure. First, a photoaffinity label probe covalently cross-links only after photoactivation. Therefore, it is possible to demonstrate with appropriate competitive inhibition studies that the photoaffinity label probe associates with the receptor in question. Secondly, since cross-linking only occurs after photolysis, it is possible to adjust the concentration of the photoaffinity label to maximize association with "specific" binding sites relative to "non-specific" associations prior to covalent linkage by photoactivation. The different [125I]iodothyronine-PAL analogues may be useful as probes of the thyroid hormone receptor binding domain since PAL compounds with different affinities for receptor may photocouple to different receptor residues within or proximate to the hormone binding region. These probes may also be useful as an adjunct to receptor purification and in probing the organization of the receptor in chromatin. Lastly, they may provide insights into possible alterations of receptor structure in patients with partial end organ resistance to thyroid hormone (Refetoff et al., 1967; Eil et al., 1982).

A study of the messenger RNA encoding pyruvate kinase of Neurospora crassa

In Neurospora crassa, there is a single pyruvate kinase (PK) consisting of four identical subunits of approximately 60k daltons. Northern and dot blot hybridization studies, using most of the yeast pyruvate kinase gene as a probe, suggest the presence of two distinct mRNA species for pyruvate kinase, separable on the basis of the length of their polyadenylated tails, by oligo(dT)cellulose chromatography. These messages are present in polysomes, immuno-precipitated by anti-PK antibodies, indicating probable translation in vivo. Fractions containing both messages were translated in vitro in the heterologous systems as well as in a homologous N. crassa lysate, the newly-synthesized PK being detected by immunoadsorption. Protection studies using S1-nuclease suggest no major structural differences in the 5'-untranslated and most of the coding regions of the two messages.

Interaction of thyroid hormone and nutritional signals on thyroid hormone action

The interaction between thyroid hormone (T3) and nutritional signals has been of interest for nearly a century. Thus, enhanced glucose production, absorption and utilization are associated with hyperthyroidism, whereas diminished glucose utilization and lipogenesis characterize hypothyroidism. Recent studies have uncovered what appears to be yet another area of interaction at the molecular level. On the one hand, a marked overlap exists between the changes in rat hepatic mRNA activity profile induced by hyperthyroidism and high carbohydrate administration. On the other hand, the patterns produced by hypothyroidism, starvation and diabetes are characterized by oppositely directed shifts. These findings may be due, in part, to a synergistic relationship between carbohydrate feeding and T3 administration in the induction of many hepatic lipogenic enzymes and their respective mRNAs. Studies both in the intact rat as well as in isolated hepatocyte cultures indicate that this synergism arises from the ability of T3 to multiply an intracellular signal derived from the metabolism of glucose. The development of recombinant DNA techniques can now be applied to the study of the interaction of T3 with nutritional signals. Initial efforts have demonstrated a hepatic mRNA (mRNAS14) rapidly responsive to both T3 and carbohydrates. With this probe, studies are under way to define the precise molecular mechanisms by which T3 and carbohydrates interact to influence gene expression.

Mechanism of action of thyroid hormones

Thyroid hormones have ubiquitous effects and influence the function of most organs. The influences that thyroid hormones have on these diverse functions are primarily mediated through binding of T3 and T4 to specific nuclear receptor sites. The nuclear action of T3 results in organ-specific increases and decreases of specific mRNAs, leading to alteration in the level of the corresponding proteins. In addition to the well established nuclear action of T3, effects of thyroid hormone on other sites including cell membranes and mitochondria have been documented.

Insulin mediates the asynchronous accumulation of hepatic albumin and malic enzyme messenger RNAs

We have compared the rate of accumulation of hepatic albumin and malic enzyme mRNAs following insulin treatment of diabetic rats to determine whether insulin coordinately increases mRNA levels or specifically induces the accumulation of individuals mRNAs. Initially, the quantities of both albumin and malic enzyme mRNAs are reduced in diabetic rats compared to normal rats as determined by RNA blot analysis using complementary DNA probes. Following insulin administration for 12 h, albumin and malic enzyme mRNA levels increase at similar rates. However, after 12 h the rate of malic enzyme mRNA accumulation increases dramatically while albumin mRNA continues to increase at its initial rate. This accelerated rate of accumulation of malic enzyme mRNA continued through 60 h of hormone treatment and was associated with the onset of hepatic lipogenesis. Thus, our results suggest that insulin regulates the accumulation of mRNAs encoding these two inducible proteins in an asynchronous manner directly related to the metabolic requirements of the animal.

Triiodothyronine nuclear receptor and the role of non-histone protein factors in in vitro triiodothyronine binding

The rat liver triiodothyronine (T3) nuclear receptor rapidly looses, after a partial purification from the nuclear extract, its ability to bind T3. We previously reported that histones, in the presence of DNA, could protect against inactivation enhancing the T3 binding site concentration and maintaining the high affinity for T3. A nuclear fraction discarded during the receptor purification (fraction A) was also found able to restore T3 binding and was analyzed. As histones + DNA, fraction A stabilized the T3 binding site from irreversible inactivation during incubation with T3, increasing its concentration while keeping the same high affinity for T3. It was active even at relatively high receptor concentration, appeared slightly more active than histones (+ DNA) in the same protein concentration range (up to 50-fold increment of T3 binding at the optimal concentration of 25 micrograms/ml) and was unaffected or slightly inhibited by DNA. Other proteins (ovalbumin, soybean trypsin inhibitor, RNAase) and rat liver cytosol were several times less effective, suggesting a major role of some nuclear constituents. The active factors in fraction A essentially belong to non-histone nuclear proteins. Fraction A was found heterogeneous regarding the molecular size and pHi of the active factors, the existence of subfractions more active on a protein concentration basis being suggested but not yet clearly evidenced. Efficient in vitro T3 binding to the isolated T3 nuclear receptor thus depends on the presence of several different nuclear constituents, histones + DNA or some non-histone proteins. Whether interactions with these constituents could modulate T3 binding within the nucleus remains to be elucidated.