Effect of different thyroid states on rat liver glucokinase synthesis and degradation in vivo

Thyroid Hormone and Diabetes Mellitus Interplay: Making Management of Comorbid Disorders Complicated

Insulin and thyroid hormones play important roles in our body. Insulin helps regulate the glucose level while the thyroid hormones affect various cells and tissues, metabolizing protein, lipids, and glucose. Hyperthyroidism and thyrotoxicosis are potential hazards for type 2 diabetes mellitus. There is a high prevalence of hypothyroidism being more common compared to hyperthyroidism coexisting with diabetes mellitus. Thyroid hormones affect glucose metabolism through its action on peripheral tissues (gastrointestinal tract, liver, skeletal muscles, adipose tissue, and pancreas). High-level thyroid hormone causes hyperglycemia, upregulation of glucose transport, and reduction in glycogen storage. The reverse is observed during low levels of thyroid hormone along with insulin clearance. The net result of thyroid disorder is insulin resistance. Type 2 diabetes mellitus can downsize the regulation of thyroid stimulating hormones and impair the conversion of thyroxine to triiodothyronine in peripheral tissues. Furthermore, poorly managed type 2 diabetes mellitus may result in insulin resistance and hyperinsulinemia, contributing to the proliferation of thyroid tissue and an increase in nodule formation and goiter size. Although metformin proves advantageous for both type 2 diabetes mellitus and thyroid disorder patients, other antidiabetics like sulfonylureas, pioglitazone, and thiazolidinediones may have adverse effects on thyroid disorders. Moreover, antithyroid drugs such as methimazole can weaken glycemic control in individuals with diabetes. Thus, an interplay between both endocrinopathies is observed and individualized care and management of the disorder needs to be facilitated.

Dynamic Metabolic Zonation of the Hepatic Glucose Metabolism Is Accomplished by Sinusoidal Plasma Gradients of Nutrients and Hormones

Being the central metabolic organ of vertebrates, the liver possesses the largest repertoire of metabolic enzymes among all tissues and organs. Almost all metabolic pathways are resident in the parenchymal cell, hepatocyte, but the pathway capacities may largely differ depending on the localization of hepatocytes within the liver acinus-a phenomenon that is commonly referred to as metabolic zonation. Metabolic zonation is rather dynamic since gene expression patterns of metabolic enzymes may change in response to nutrition, drugs, hormones and pathological states of the liver (e.g., fibrosis and inflammation). This fact has to be ultimately taken into account in mathematical models aiming at the prediction of metabolic liver functions in different physiological and pathological settings. Here we present a spatially resolved kinetic tissue model of hepatic glucose metabolism which includes zone-specific temporal changes of enzyme abundances which are driven by concentration gradients of nutrients, hormones and oxygen along the hepatic sinusoids. As key modulators of enzyme expression we included oxygen, glucose and the hormones insulin and glucagon which also control enzyme activities by cAMP-dependent reversible phosphorylation. Starting with an initially non-zonated model using plasma profiles under fed, fasted and diabetic conditions, zonal patterns of glycolytic and gluconeogenetic enzymes as well as glucose uptake and release rates are created as an emergent property. We show that mechanisms controlling the adaptation of enzyme abundances to varying external conditions necessarily lead to the zonation of hepatic carbohydrate metabolism. To the best of our knowledge, this is the first kinetic tissue model which takes into account in a semi-mechanistic way all relevant levels of enzyme regulation.

Effects of growth hormone, insulin-like growth factor I, triiodothyronine, thyroxine, and cortisol on gene expression of carbohydrate metabolic enzymes in sea bream hepatocytes

The present study investigated the regulatory effects of growth hormone (GH), human insulin-like growth factor I (hIGF-I), thyroxine (T(4)), triiodothyronine (T(3)) and cortisol, on mRNA expression of key enzymes involved in carbohydrate metabolism, including glucokinase (GK), glucose-6-phosphatase (G6Pase), glycogen synthase (GS), glycogen phosphorylase (GP) and glucose-6-phosphate dehydrogenase (G6PDH) in hepatocytes isolated from silver sea bream. Genes encoding GK, G6Pase, GS and GP were partially cloned and characterized from silver sea bream liver and real-time PCR assays were developed for the quantification of the mRNA expression profiles of these genes in order to evaluate the potential of these carbohydrate metabolic pathways. GK mRNA level was elevated by GH and hIGF-I, implying that GH-induced stimulation of GK expression may be mediated via IGF-I. GH was found to elevate GS and G6Pase expression, but reduce G6PDH mRNA expression. However, hIGF-I did not affect mRNA levels of GS, G6Pase and G6PDH, suggesting that GH-induced modulation of GS, G6Pase and G6PDH expression levels is direct, and occurs independently of the action of IGF-I. T(3) and T(4) directly upregulated transcript abundance of GK, G6Pase, GS and GP. Cortisol significantly increased transcript amounts of G6Pase and GS but markedly decreased transcript abundance of GK and G6PDH. These changes in transcript abundance indicate that (1) the potential of glycolysis is stimulated by GH and thyroid hormones, but attenuated by cortisol, (2) gluconeogenic and glycogenic potential are augmented by GH, thyroid hormones and cortisol, (3) glycogenolytic potential is upregulated by thyroid hormones but not affected by GH or cortisol, and (4) the potential of the pentose phosphate pathway is attenuated by GH and cortisol but unaffected by thyroid hormones.

Cyclic adenosine 3',5'-monophosphate increases pancreatic glucokinase activity and gene expression

Comparison of the pancreatic and hepatic glucokinase gene transcripts reveals tissue-specific control of expression and the existence of two distinct promoters in a single glucokinase gene. The existence of alternate promoters suggests that separate factors regulate glucokinase transcription in the two tissues. Hepatic glucokinase expression has been shown to be repressed by cAMP; however, in the pancreatic beta-cell it is unlikely that cAMP represses glucokinase activity, as cAMP is known to positively affect glucose-induced insulin secretion, a process that in mature islets requires pancreatic glucokinase activity. In this work we demonstrate that cAMP indeed has a stimulatory effect on pancreatic glucokinase. The cyclic nucleotide stimulates pancreatic glucokinase activity after 3-h incubation, and maximal effects are observed after 6 and 12 h of treatment. Using the bDNA assay, a sensitive signal amplification technique, we detected relative increases in glucokinase messenger RNA levels of 40.5 +/- 7.5% after 3-h incubation with cAMP. This stimulatory effect was increased to 106.3 +/- 22% after 6-h incubation and sustained up to 12 h of incubation. Inhibition of gene transcription by actinomycin D abolishes cAMP-induced glucokinase activity. In transfected fetal islets, cAMP increased the activity of the -1000 bp rat glucokinase promoter by 60 +/- 6%. These data demonstrate that cAMP has a stimulatory effect on pancreatic glucokinase gene expression and that the nucleotide has opposite effects on pancreatic and hepatic glucokinase, supporting the concept that glucokinase transcription in the liver and that in the beta-cell differ.

Effects of triiodothyronine and retinoic acid on glucokinase gene expression in neonatal rat hepatocytes

Glucokinase (EC 2.7.1.2) first appears in rat liver two weeks after birth and increases rapidly after weaning on to a high-carbohydrate diet. We investigated the role of triiodothyronine and retinoic acid in the absence of insulin on the first expression of the glucokinase gene in primary cultures of hepatocytes from 10 day-old rats. These two hormones were able to induce a rapid accumulation of liver glucokinase mRNA, secondarily to a stimulation of gene transcription during the first 24 h of culture. Moreover, the effects of individual hormones were not additive. Finally, glucokinase mRNA stability was not modified by these hormones. This suggests that triiodothyronine and retinoic acid act on glucokinase gene at the transcriptional.

The influence of thyroid state on hepatic glycolysis

The effects of thyroid status on glycolysis using 10, 20, and 40 mM glucose have been examined in hepatocytes derived from hypothyroid, euthyroid, and hyperthyroid rats. For any given concentration of added glucose, total glycolytic rates, as measured by the release of tritium from [6-3H]glucose, were similar in all thyroid states. The aerobic component of glycolysis, where cytoplasmically generated reducing equivalents are transferred to the mitochondria for oxidation, was the major component in the hyperthyroid state, at all concentrations of glucose. In contrast, the aerobic proportion of glycolysis in the hypothyroid and euthyroid states decreased with increasing concentration of added glucose and the anaerobic component became dominant above 20 mM glucose. Cytoplasmic reducing equivalents generated during aerobic glycolysis were transferred to the mitochondria via both the glycerol 1-phosphate and malate/aspartate shuttles in each thyroid state, even though the former shuttle was considerably depressed in the livers of hypothyroid rats. Both asparagine and aminooxyacetate had only minor effects on the rate of glycolysis, but aminooxyacetate depressed the contribution of aerobic glycolysis whereas asparagine had relatively little influence. The respiration rate in the presence of 40 mM glucose was twice as high in hepatocytes from hyperthyroid rats as in cells from hypothyroid animals, and 1.4 times as high as in hepatocytes from euthyroid rats. Smaller stimulations were observed with lower concentrations of added glucose. Furthermore, the increase in respiratory rate over the endogenous value, induced by 10 mM glucose, was six times higher in cells from hyperthyroid rats than in hepatocytes from hypothyroid animals and 2.7 times higher than that observed with cells from euthyroid rats. The insensitivity of glycolysis to thyroid status in contrast to the marked response of respiration provides additional support for the view that the stimulation of metabolism by thyroid hormone is mediated primarily by its action on mitochondrial processes.

Mechanisms underlying enhanced glycogenolysis in livers of 3,5,3'-triiodothyronine-treated rats

Rats trained on a diurnal controlled meal-feeding schedule and injected with a single dose of 3,5,3'-triiodothyronine (T3) failed to accumulate liver glycogen and incorporated less D-[6-3H]glucose into glycogen than normally observed during the feeding period. In the experimental group, the concentration of liver adenosine 3',5'-cyclic monophosphate (cAMP) did not fall during feeding and the pattern of activities of glycogen phosphorylase, glycogen synthase, and phosphorylase kinase remained conductive to glycogenolysis. Liver lysosomal alpha-glucosidase activity normally fell during feeding periods. After T3 treatment the activities of alpha-glucosidase and two lysosomal cathepsins (B1 and D) were elevated. The evidence suggests that T3 may induce both liver phosphorylase kinase and lysosomal alpha-glucosidase. This outcome of T3 excess, in concert with previously described T3-inducible systems, provides a plausible explanation for the failure of glycogen accumulation in this experimental model.

Regulation of hepatic glucokinase gene expression. Role of carbohydrates, and glucocorticoid and thyroid hormones

The present study investigates the effect of thyroid and glucocorticoid hormones on the induction of hepatic glucokinase mRNA activity, enzyme synthesis and activity in starved/refed adrenalectomized, thyroidectomized and intact rats. In intact rats glucose refeeding resulted within 2 h in a more than tenfold increase in the functional messenger, followed by a corresponding increase in glucokinase synthesis and, a little later, in enzyme activity. Glucokinase mRNA and synthesis remained elevated at this level for about further 6 h. Then the mRNA activity and enzyme synthesis declined considerably to a new steady state (a factor of about 4 above the starvation level) within a further 8 h, while enzyme activity remained constantly elevated. The half-life of glucokinase mRNA, as determined after administration of cordycepin, was identical during the different refeeding periods. Thus the overshoot phenomenon, provoked by carbohydrate refeeding, in glucokinase mRNA is not explained by alteration of the glucokinase mRNA decay rates. In thyroidectomized or adrenalectomized rats, glucose refeeding resulted in only a small increase in glucokinase mRNA, synthesis and activity. Application of thyroid hormones in thyroidectomized rats, refed a carbohydrate-rich diet, enhanced the specific mRNA considerably within 8-10 h, while it took 20-24 h to enhance glucokinase mRNA by glucocorticoids in adrenalectomized rats refed a carbohydrate-rich diet. The decay in translatable glucokinase mRNA, as determined after administration of cordycepin, was identical in the hypothyroid and euthyroid fed state, while adrenalectomy resulted in a significant decrease in the specific mRNA half-life. We conclude that refeeding a carbohydrate-rich diet rapidly stimulates glucokinase mRNA regeneration showing overshoot kinetics. 3,3',5-Triiodothyronine in its physiological concentration significantly enhances the response in glucokinase mRNA at the nuclear level, while glucocorticoids in their physiological concentration predominantly stabilize the translatable glucokinase mRNA.

Cooperative effect of thyroid and glucocorticoid hormones on the induction of hepatic phosphoenolpyruvate carboxykinase in vivo and in cultured hepatocytes

The present study investigates the effect and interaction of glucocorticoid and thyroid hormones on the induction of phosphoenolpyruvate carboxykinase (PEPck) mRNA and enzyme protein under in vivo conditions and in serum-free cultured hepatocytes from hypothyroid rats. In hypothyroid/adrenalectomized rats T3 significantly enhanced the cAMP induced PEPck mRNA activity within 3-6 h. This effect was further enhanced by the presence of glucocorticoids. The half-life of PEPck mRNA, as determined after administration of cordycepin, was not affected by hypothyroidism or hyperthyroidism (t 1/2 approximately equal to 45 min), but considerably prolonged by the absence of glucocorticoid hormones (t 1/2 less than 80 min). In hepatocytes in culture Bt2cAMP (0.2 mM) provoked an increase in translatable PEPck mRNA within 2 h incubation time. Preincubation with either T3 (0.1 microM) or dexamethasone (0.1 microM) for 4 h significantly enhanced the cAMP response on PEPck mRNA. Addition of both, T3 plus dexamethasone further enhanced this Bt2cAMP-mediated effect. By measurement of PEPck synthesis corresponding findings were observed. It is concluded that glucocorticoid and thyroid hormones predominantly enhance the cAMP-provoked induction of hepatic PEPck mRNA and, consequently, of PEPck synthesis. Their effect is rapid, significant and additive, indicating an independent action. While glucocorticoids, in addition, accelerate PEPck mRNA degradation, the PEPck mRNA decay rate is similar in the presence and absence of thyroid hormones.

Purification of rat hepatic glucokinase

We have developed a rapid, reliable procedure for the purification of rat hepatic glucokinase. The purification utilizes DEAE-cellulose, two affinity chromatography steps, and high-performance liquid chromatography. Glucokinase with a specific activity of 240 units/mg, a 42 K-fold purification, and a yield of 60% is obtained. The enzyme appears as a homogeneous band, with over 99% purity as assessed by polyacrylamide gel electrophoresis. The purification procedure can be completed in 5 days.

Evidence that transcription of the hexokinase gene is increased in a rapidly growing rat hepatoma

The level of hexokinase-specific mRNA in rat liver and in rat ascites hepatoma (Zajdela) was assessed by translating free polysomes from both cell types in a reticulocyte lysate. Translation of hexokinase occurred with approximately a 10 fold higher frequency in tumor cells than in liver, suggesting that transcriptional activation is in part responsible for increased levels of hexokinase in tumor.

Permissive action of thyroid hormones in the cAMP-mediated induction of phosphoenolpyruvate carboxykinase in hepatocytes in culture

Thyroid hormones are known to stimulate in vivo the synthesis of several liver proteins. In order to determine their role in the regulation of important hepatic proteins at the cell level, the effect of T3 on the induction of the glucoregulatory enzyme phosphoenolpyruvate carboxykinase (PEPck) was investigated in cultured hepatocytes. The cells were isolated from adult hypothyroid rats and exposed to a chemically defined medium, devoid of serum supplement and hormone-free over a period of 48 h. Addition of T3 to the medium had only a minor effect on PEPck induction. Dibutyryladenosine 3',5'-monophosphate (Bt2cAMP) or epinephrine provoked significant increase in PEPck synthesis within 4-6 h. Simultaneous addition of T3 was found significantly to promote the Bt2cAMP-mediated enzyme induction. Physiological concentrations of T3 (10 pM to 1 nM) were found to be effective in enhancing this cAMP-mediated signal. Exposure of the cells to T3 for only 90 min, 16 h prior to addition of Bt2cAMP was nearly as effective as continuous exposure to T3 in enhancing the cAMP-effect on PEPck synthesis. In contrast, no effect of T3 on the Bt2cAMP-induced tyrosine-aminotransferase activity could be detected. It is concluded that the role of T3 on the cAMP-elicited PEPck synthesis is selective, rapid, primarily permissive and significant within the physiological range of the hormone. In addition, these data indicate that such hepatocyte cultures prepared from hypothyroid rats are ideally suited to the analysis of the regulatory role of thyroid hormones on specific gene expression.

Thyroid hormone action on intermediary metabolism. Part III. Protein metabolism in hyper- and hypothyroidism

In their physiological concentrations, thyroid hormones stimulate the synthesis as well as the degradation of proteins, whereas in supraphysiological doses protein catabolism predominates. In hyperthyroidism skeletal muscle protein stores suffer depletion which is reflected by an increased urinary N- and methylhistidine -excretion. Due to the enhanced skeletal muscle amino acid release, the plasma concentration of glucoplastic amino acids are often enhanced, contributing by means of an elevated substrate supply to the increased hepatic gluconeogenesis. Thyroid hormone excess induces cardiac hypertrophy which is in direct contrast to the hypotroph skeletal muscle in hyperthyroid patients. Thyroid hormones stimulate a series of intracellular and secretory proteins in the liver, although in hyperthyroid liver alcohol dehydrogenase and the enzymes of histidine and tryptophan metabolism show reduced activities. The stimulatory effect is due to thyroid hormone-induced increase in the protein synthesis at a pretranslational level and is supported experimentally for malic enzyme, alpha 2u-globulin and albumin by the measurement of their specific messenger RNA activities. Thyroid hormone action at the cellular level is reflected by a generalized increase in total cellular RNA with a selective increase or decrease in a small population of specific mRNA. The activities of protein catabolizing lysosomal enzymes are stimulated by thyroid hormones; up to now effects of T3 on the degradation of specific enzymes have not been reported. Serum total protein concentration is slightly reduced or even unchanged in hyperthyroidism. The thyroid hormone-induced increase in the turnover of total body protein is part of the hypermetabolism observed in hyperthyroidism.

Thyroid hormone action on intermediary metabolism. Part I: respiration, thermogenesis and carbohydrate metabolism

The effect of thyroid hormones on mitochondrial respiration are summarized: T3 directly stimulates mitochondrial respiration and the synthesis of adenosine 5'-triphosphate (ATP). Cytosolic ATP availability is increased by a thyroid hormone-induced increase in adenine nucleotide translocation across the mitochondrial membrane; the steady state ATP concentration and the cytosolic ATP/adenosine 5'-diphosphate (ADP) ratio is even decreased in hyperthyroid tissues because of the simultaneous stimulation of the synthesis and consumption of ATP. With regard to the thyroid hormone-induced energy wasting processes, heart work, intra- and interorgan futile cycling and Na+/K+-ATPase are involved to varying degrees. As a consequence of the thyroid hormone-induced hydrolysis of ATP, thermogenesis is increased in hyper- and decreased in hypothyroidism. Despite an increased rate of glucose utilization, clinical and experimental hyperthyroidism is often characterized by an abnormal oral glucose tolerance test. This finding is due to the thyroid hormone-induced increase in intestinal glucose absorption as well as the still enhanced endogenous glucose production in the liver. Hypothyroid patients show a reduced glucose tolerance test because of a decrease in intestinal glucose absorption and a sometimes reduced glucose turnover. The thyroid hormone-induced alterations in glucose metabolism are most probably not due to alterations in serum insulin levels and/or to a peripheral insulin resistance at the receptor level.

New perspectives on pancreatic islet glucokinase

Control of blood sugar involves the complex interaction of the pancreatic glucose-sensing beta-cells with the liver, which serves as the primary site of glucose disposal after a meal. Glucokinase occupies an important role in controlling glucose phosphorylation and metabolism both in the liver and in pancreatic islets. In the beta-cells, glucokinase functions as pacemaker of glycolysis at physiological glucose levels. It determines the unique characteristics of islet hexose usage, that is, the rate, affinity, cooperativity, and anomeric discrimination of glucose metabolism. Because glycolysis controls hexose-induced insulin release, glucokinase is considered the best-qualified candidate for the elusive glucose sensor of beta-cells. A deficiency of glucokinase would disturb glucose homeostasis. Decreased islet glucokinase would diminish islet glycolysis and would result in a higher set point of beta-cells for glucose-induced insulin release. Decreased liver glucokinase would cause less efficient hepatic glucose disposal. Human maturity-onset diabetes (type II diabetes) has these characteristics. It is thus conceivable that certain forms of type II diabetes are due to a glucokinase deficiency.

Accelerated turnover of hepatic glucokinase in starved and streptozotocin-diabetic rat

Rat liver glucokinase synthesis and degradation was estimated in fasted/fed and diabetic/diabetic-insulin-treated rats by the radioimmunological technique. Starvation and Streptozotocin-diabetes led to basal rates of synthesis and, consequently, to low levels in enzyme activity. In addition, a decrease in the apparent half-life from about 19 h in the fed or diabetic-insulin substituted to about 11 h in the starved or diabetic rat, respectively, was observed. Injection of Bt2cAMP into glucose-fed animals reduced glucokinase synthesis to basal levels within 90 min, without affecting enzyme activity. It is concluded that in metabolic states associated with elevated levels in tissue cAMP glucokinase synthesis is reduced to basal values and, in addition, its rate of degradation is significantly enhanced.

Renal phosphoenolpyruvate carboxykinase turnover in hypo- and hyperthyroid rat in vivo

The effect of hypo- and hyperthyroidism on activity, synthesis and degradation of renal cytosolic phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) was studied in the rat by radioimmunological techniques. In hypo- and euthyroid rats, starvation induced similar alterations in enzyme activities and relative rates of synthesis, whereas in hyperthyroid rats the increase in both was significantly reduced. Substitution of L-thyroxine in hypothyroid rats resulted in a decrease in activity and synthesis within 18 h as observed in hyperthyroid animals. The apparent half-life of the enzyme measured by double-pulse labeling experiments was approx. 13 h in euthyroid animals. The rate of degradation was unaffected by the different thyroid states.