Regulation of glucose-6-phosphate dehydrogenase by diet and thyroid hormone

Changes of the salivary and serum proteome in canine hypothyroidism

In this study, changes in salivary and serum proteome of dogs with hypothyroidism were studied using tandem mass tags (TMT) labelling and liquid chromatography-mass spectrometry (LC-MS/MS). Saliva and serum proteome from 10 dogs with hypothyroidism were compared with 10 healthy dogs. In saliva, a total of seven proteins showed significant changes between the two groups, being six downregulated and one upregulated, meanwhile, in serum, a total of six proteins showed significant changes, being five downregulated and one upregulated. The altered proteins reflected metabolic and immunologic changes, as well as, skin and coagulation alterations, and these proteins were not affected by gender. One of the proteins that were downregulated in saliva, lactate dehydrognease (LDH), was measured by a spectrophotometric assay in saliva samples from 42 dogs with hypothyroidism, 42 dogs with non-thyroid diseases and 46 healthy dogs. The activity of LDH was lower in the saliva of hypothyroid dogs when compared to non-thyroid diseased dogs and healthy controls. This study indicates that canine hypothyroidism can produce changes in the proteome of saliva and serum. These two sample types showed different variations in their proteins reflecting physiopathological changes that occur in this disease, mainly related to the immune system, metabolism, skin and coagulation. In addition, some of the proteins identified in this study, specially LDH in saliva, should be further explored as potential biomarkers of canine hypothyroidism.

Acquired Glucose-6-Phosphate Dehydrogenase Deficiency

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is a hereditary condition caused by mutations on chromosome X and is transmitted by a sex-linked inheritance. However, impairment of G6PD activity may result from biochemical mechanisms that are able to inhibit the enzyme in specific clinical conditions in the absence of a structural gene-level defect. In this narrative review, a number of clinical settings associated with an "acquired" G6PD deficiency, phenotypically undistinguishable from the primary deficiency, as well as the mechanisms involved, were examined. Hyperaldosteronism and diabetes are the most common culprits of acquired G6PD deficiency. Additional endocrine and metabolic conditions may cause G6PD deficiency in both hospitalized and outpatients. Contrary to the inherited defect, acquired G6PD deficiency is a condition that is potentially curable by removing the factor responsible for enzyme inhibition. Awareness regarding acquired G6PD deficiency by physicians might result in improved recognition and treatment.

Action of Thyroid Hormones, T3 and T2, on Hepatic Fatty Acids: Differences in Metabolic Effects and Molecular Mechanisms

The thyroid hormones (THs) 3,3′,5,5′-tetraiodo-l-thyronine (T4) and 3,5,3′-triiodo-l-thyronine (T3) influence many metabolic pathways. The major physiological function of THs is to sustain basal energy expenditure, by acting primarily on carbohydrate and lipid catabolism. Beyond the mobilization and degradation of lipids, at the hepatic level THs stimulate the de novo fatty acid synthesis (de novo lipogenesis, DNL), through both the modulation of gene expression and the rapid activation of cell signalling pathways. 3,5-Diiodo-l-thyronine (T2), previously considered only a T3 catabolite, has been shown to mimic some of T3 effects on lipid catabolism. However, T2 action is more rapid than that of T3, and seems to be independent of protein synthesis. An inhibitory effect on DNL has been documented for T2. Here, we give an overview of the mechanisms of THs action on liver fatty acid metabolism, focusing on the different effects exerted by T2 and T3 on the regulation of the DNL. The inhibitory action on DNL exerted by T2 makes this compound a potential and attractive drug for the treatment of some metabolic diseases and cancer.

The redox imbalance and the reduction of contractile protein content in rat hearts administered with L-thyroxine and Doxorubicin

Oxidative stress and disorders in calcium balance play a crucial role in the doxorubicin-induced cardiotoxicity. Moreover, many cardiotoxic targets of doxorubicin are regulated by iodothyronine hormones. The aim of the study was to evaluate effects of tetraiodothyronine (0.2, 2 mg/L) on oxidative stress in the cardiac muscle as well as contractility and cardiomyocyte damage markers in rats receiving doxorubicin (1.5 mg/kg) once a week for ten weeks. Doxorubicin was administered alone (DOX) or together with a lower (0.2T₄ + DOX) and higher dose of tetraiodothyronine (2T₄ + DOX). Two groups received only tetraiodothyronine (0.2T₄, 2T₄). Coadministration of tetraiodothyronine and doxorubicin increased the level of lipid peroxidation products and reduced RyR2 level when compared to untreated control and group exposed exclusively to doxorubicin. Insignificant differences in SERCA2 and occasional histological changes were observed. In conclusion, an increase of tetraiodothyronine level may be an additional risk factor of redox imbalance and RyR2 reduction in anthracycline cardiotoxicity.

Oxidative damage and antioxidant enzyme activities in experimental hypothyroidism

Free radicals are now well known to damage cellular components. To investigate whether age and thyroid level affect peroxidation speed, we examined the levels of malondialdehyde and antioxidant enzyme activities in different age groups of hypothyroid rats. Hypothyroidism was induced in 30- and 60-day-old Wistar Albino rats by the i.p. administration of propylthiouracil (10 mg kg(-1) body weight) for 15 days. While malondialdehyde levels of 30- or 60-day-old hypothyroid rats were increased in liver, they were decreased in the tissues of the heart and thyroid. While glucose-6-phosphate dehydrogenase activity levels did not change in heart, brain and liver tissues of 30-day-old rats, they increased in brain and heart tissues of 60-day-old experimental groups, but decreased in the liver. Catalase activities decreased in the liver and heart of rats with hypothyroidism, but increased in erythrocytes. In control groups while malondialdehyde levels increased in brain, heart and thymus with regard to age, they decreased in plasma. Glucose-6-phosphate dehydrogenase and catalase activities were not affected by age in tissues of the thymus, thyroid and brain, but they were decreased in the heart tissue. The changes in the levels of lipid peroxidation and antioxidant enzyme activities which were determined in different tissues of hypothyroid rats indicate a cause for functional disorder of these tissues. Moreover, there may be changes depending on age at lipid peroxidation and antioxidant enzyme activity levels.

Dietary regulation of expression of glucose-6-phosphate dehydrogenase

The family of enzymes involved in lipogenesis is a model system for understanding how a cell adapts to dietary energy in the form of carbohydrate versus energy in the form of triacylglycerol. Glucose-6-phosphate dehydrogenase (G6PD) is unique in this group of enzymes in that it participates in multiple metabolic pathways: reductive biosynthesis, including lipogenesis; protection from oxidative stress; and cellular growth. G6PD activity is enhanced by dietary carbohydrates and is inhibited by dietary polyunsaturated fats. These changes in G6PD activity are a consequence of changes in the expression of the G6PD gene. Nutrients can regulate the expression of genes at both transcriptional and posttranscriptional steps. Most lipogenic enzymes undergo large changes in the rate of gene transcription in response to dietary changes; however, G6PD is regulated at a step subsequent to transcription. This step is involved in the rate of synthesis of the mature mRNA in the nucleus, specifically regulation of the efficiency of splicing of the nascent G6PD transcript. Understanding the mechanisms by which nutrients alter nuclear posttranscriptional events will help uncover new information on the breadth of mechanisms involved in gene regulation.

3,5-diiodo-L-thyronine regulates glucose-6-phosphate dehydrogenase activity in the rat

Thyroid hormones influence the activity of lipogenic enzymes such as malic enzyme (ME) and glucose-6-phosphate dehydrogenase (G6PD). The effect of T3 on ME is exerted at the transcriptional level, but it is unclear if its effect on G6PD is also nuclear mediated. Furthermore, other iodothyronines that have been shown to possess biological activity (such as diiodothyronines) could contribute to this enzyme's regulation. In this study the effects of 3,5-diiodothyronine (T2) on the aforementioned enzymes were examined and compared with those of T3. Rats made hypothyroid by propylthiouracil and iopanoic acid treatment were used throughout. Enzyme activities were determined spectrophotometrically, and G6PD messenger RNA (mRNA) expression was analyzed by Northern blotting using a human G6PD complementary DNA probe. Injections of T2 to hypothyroid animals significantly enhanced the activity of both enzymes. The effect of T2 on ME was nuclear mediated and mimicked the effect of T3. The effects of T2 and T3 on G6PD differed. Injection of T3 into hypothyroid rats induced an increase in both enzyme activity and G6PD mRNA expression, indicating a nuclear-mediated effect. The effect of T2 on G6PD activity, on the other hand, was not nuclear mediated. The injection of T2 into hypothyroid animals did not change G6PD mRNA expression, and the strong increase in the enzyme's activity (from +70% to +300%) was unaffected by simultaneous injection of protein synthesis inhibitors. As the lowest dose of 1 microg T2/100 g BW affects G6PD activity 3-5 times more than the same dose of T3, these data provide the first evidence that T2 is a factor capable of regulating G6PD activity.

Tumor necrosis factor alpha augments the expression of glucose-6-phosphate dehydrogenase in rat hepatic endothelial and Kupffer cells

Cellular activity of glucose-6-phosphate dehydrogenase (G6PD), the key enzyme of the hexose monophosphate shunt, supports several pathways involved in the nonspecific immune response. In the present study, we investigated the in vivo effects of selected pro-inflammatory cytokines on the expression of G6PD in Kupffer and hepatic endothelial cells. Murine recombinant TNF alpha, IL-1 beta, or IL-6 (1.5 x 10(5) U/kg) was injected and cellular G6PD mRNA level determined using a quantitative reverse transcription and polymerase chain reaction method. G6PD mRNA was elevated two- to threefold seven hours after the injection of TNF alpha in Kupffer and endothelial cells as compared to cells from saline-injected animals. The elevated G6PD mRNA was accompanied by increased cellular enzyme activity in both cells. The cellular activity of 6-phosphogluconate dehydrogenase (6PGD) was also increased seven hours after TNF alpha treatment in these cells. G6PD mRNA and enzyme activity returned to control levels 22h after TNF alpha administration. In contrast to the marked effects of TNF alpha, no significant alterations were found on G6PD expression following IL-1 beta or IL-6 injections in these cells. None of these cytokines caused changes in G6PD or 6PGD expression in parenchymal cells. These data indicate that the proinflammatory cytokine TNF alpha plays an important role in the regulation of cellular G6PD expression in hepatic immune competent cells.

Regulation of glucose-6-phosphate dehydrogenase synthesis and mRNA abundance in cultured rat hepatocytes

Conditions were identified which, for the first time, demonstrate that primary hepatocytes can express the same range of glucose-6-phosphate dehydrogenase (G6PD) synthesis and mRNA as in live rats. Primary hepatocytes were cultured without prior exposure to serum, hormones or carbohydrates. Five modulators implicated in G6PD induction in vivo were examined: insulin, dexamethasone, tri-iodothyronine (T3), glucose and fructose, T3 did not affect G6PD activity, and did not interact with carbohydrate to affect the activity of G6PD. Neither glucose nor fructose alone affected G6PD activity, and they did not interact with insulin to increase G6PD activity. Hepatocytes isolated from fasted rats and cultured in serum-free media with amino acids ad the only energy source how a 12-fold increase in G6PD synthesis and mRNA (measured by a solution-hybridization assay). This induction does not require added hormones or carbohydrate. The addition of insulin alone caused another increase in G6PD synthesis and mRNA. There are at least three distinct phases to G6PD induction under these conditions. The largest increase in G6PD synthesis (12-fold) occurs in the absence of any hormones and with amino acids as the only energy source. This phase is due to increased G6PD mRNA. Insulin causes an additional 2-3-fold increase in G6PD synthesis and mRNA. However, dexamethasone and insulin are both required before G6PD synthesis is equal to that in rats which are fasted and refed on a high-carbohydrate diet.

The influence of lipogenic and lipolytic conditions on the pentose phosphate pathway dehydrogenases in rat-kidney-cortex

The effects of various lipogenic and antilipogenic states on the activities of rat-kidney cortex glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase have been studied. These conditions are related to the long-term administration of different diets, such as high-carbohydrate (80%) and high-fat (23%), and also to a state of fast. Contrary to what happens in liver cells and kidney cortex during a high protein diet administration, none of these nutritional conditions produced significant changes in the kinetics of either kidney hexose monophosphate dehydrogenases.

Solution hybridization quantitation of G6PD mRNA in rat epididymal fat pads

A solution hybridization assay is systematically characterized and used to quantitate glucose-6-phosphate dehydrogenase (G6PD) mRNA from epididymal fat pads in fasted and glucose-induced rats. G6PD mRNA and specific activity increase 9-fold and 2-fold, respectively. The 9-fold increase in G6PD synthesis reported previously (Wolfe et al. (1979) Biochem. Biophys. Res. Commun. 89, 108-115) can, therefore, be accounted for by the increase in G6PD mRNA. This solution hybridization assay is sensitive enough to quantitative levels of G6PD mRNA in total liver RNA from a fasted rat, one of the least abundant sources of this mRNA. It can, therefore, be used to answer several questions about the regulation of G6PD synthesis in rat tissues. Preliminary results suggest that the dietary regulation of G6PD mRNA in rat liver is much larger than previously reported.

Quantitation of glucose-6-phosphate dehydrogenase mRNA by solution hybridization: correlation with rates of synthesis

Rat liver glucose-6-phosphate dehydrogenase (G6PD) is one of several proteins involved in lipid metabolism whose synthesis is regulated by diet. In experiments reported here, rats were fasted or fed diets until a new steady state level of G6PD was produced. Livers were used to measure G6PD activity, synthesis and mRNA simultaneously. Since accurate quantitation of G6PD mRNA by Northern blots was found to be difficult in noninduced animals a new solution hybridization assay was also used. Noninduced rats have approx. One molecule of G6PD mRNA per liver cell. Changes in G6PD mRNA are larger than previously reported and, at the steady state, can completely account for the 33-fold change in G6PD activity and synthesis when fasted rats are refed a high carbohydrate diet. In contrast, a high fat carbohydrate-free diet does not increase G6PD mRNA and dibutyryl cAMP lowers G6PD mRNA. Since changes in G6PD synthesis and activity are closely correlated, degradation of G6PD is not significantly regulated.

Long-term adaptive response to dietary protein of hexose monophosphate shunt dehydrogenases in rat kidney tubules

We have studied the effects of several different macronutrients on the kinetic behaviour of rat renal glucose 6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). Rats were meal-fed with high-carbohydrate/low-protein, high-protein/low-carbohydrate and high-fat diets. High-protein increased renal G6PDH and 6PDGH activities by 66 per cent and 70 per cent respectively, without significantly changing the Km values of either and each Hexose monophosphate dehydrogenase activity increased steadily, reaching a significant difference on day 4. A rise in carbohydrate or fat in the diets, produced no significant change in either the activity or the kinetic parameters, Vmax and Km of the two dehydrogenases. In addition, the administration of a high-protein diet for 8 days significantly increased both the pentose phosphate pathway flux (92.6 per cent) and the kidney weigth (35 per cent), whereas no significant changes in these parameters were found when the animals were treated with the other diets. Our results suggest that an increase in the levels of dietary protein induces a rise in the intracellular levels of these enzymes. The possible role of this metabolic pathway in the kidneys under these nutritional conditions is also discussed.

Nutritional regulation of hepatic glucose-6-phosphate dehydrogenase. Transient activation of transcription

Hepatic glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) is subject to nutritional and hormonal regulation. Previous work has shown that increased amounts of mRNA encoding G6PDH can account for the increase in enzyme activity. The results of this study demonstrate that transcription of the G6PDH gene is transiently elevated after ingestion of a high-carbohydrate diet. However, the increased rate of transcription cannot totally account for the increased G6PDH mRNA. The half-life of the G6PDH mRNA appears to be about 4-5-fold higher during ingestion of a high-carbohydrate diet. Thus increased transcription as well as mRNA stability are each partially responsible for the nutritional regulation of G6PDH mRNA.