Influence of thyroid hormones on gluconeogenesis from glycerol in rat hepatocytes: a dose-response study

Thyroid hormone Beta receptor agonists for treatment of kidney disease: A promising agent?

BACKGROUND

Chronic kidney disease is a common disorder affecting a significant portion of the adult population with high mortality and morbidity. Obesity and hyperlipidemia are prevalent in chronic kidney disease, and they may trigger fat accumulation in renal parenchyma and eventually fatty kidney. Chronic kidney disease and fatty kidney are also strongly associated with nonalcoholic fatty liver disease. Because they both lead to detrimental effects on organ function, they both need to be treated effectively to improve the outcome.

AIM

In this narrative review, we have hypothesized that thyroid hormone beta receptor agonists, a novel drug group, may potentially be beneficial in the management of chronic kidney disease due to its promising outcomes among patients with nonalcoholic fatty liver disease, a condition sharing multiple common underlying pathophysiological mechanisms.

RESULTS AND CONCLUSION

Thyroid hormone beta receptors are abundantly expressed in liver and kidney tissues, while both nonalcoholic fatty liver disease and chronic kidney disease share various similar pathophysiological mechanisms and triggers. Therefore, thyroid hormone beta receptor agonists may become a promising tool in the management of patients with chronic kidney disease.

Obstructive Sleep Apnea, Circadian Clock Disruption, and Metabolic Consequences

Obstructive sleep apnea (OSA) is a chronic disorder characterized by recurrent episodes of apnea and hypopnea during sleep. It is associated with various cardiovascular and metabolic complications, including type 2 diabetes mellitus (T2DM) and obesity. Many pathways can be responsible for T2DM development in OSA patients, e.g., those related to HIF-1 and SIRT1 expression. Moreover, epigenetic mechanisms, such as miRNA181a or miRNA199, are postulated to play a pivotal role in this link. It has been proven that OSA increases the occurrence of circadian clock disruption, which is also a risk factor for metabolic disease development. Circadian clock disruption impairs the metabolism of glucose, lipids, and the secretion of bile acids. Therefore, OSA-induced circadian clock disruption may be a potential, complex, underlying pathway involved in developing and exacerbating metabolic diseases among OSA patients. The current paper summarizes the available information pertaining to the relationship between OSA and circadian clock disruption in the context of potential mechanisms leading to metabolic disorders.

GLIS3 mediated by the Rap1/PI3K/AKT signal pathway facilitates real-ambient PM2.5 exposure disturbed thyroid hormone homeostasis regulation

Exposure to fine particulate matter (PM_2.5) could damage multiple organs and systems. Recent epidemiological studies have shown that PM_2.5 can disrupt dynamic balance of thyroid hormone (TH). However, the underlying mechanism by which PM_2.5 interferes with TH remains unclear. This study evaluated the role of Gli-similar3 (GLIS3) in the effect of PM_2.5 on TH synthesis in mice using a real-ambient exposure system, in Shijiazhuang City, Hebei Province. The PM_2.5exposure group (PM) and filtered air group (FA) were placed in the exposure device for four and eight weeks. The results showed that the PM_2.5 exposure altered the structure of the thyroid gland. Moreover, after PM_2.5 exposure for eight weeks, the exposure level of free thyroxine (FT4) increased and the expression level of thyroid stimulating hormone (TSH) decreased in serum of mice. In addition, PM_2.5 exposure significantly increased the expression of proteins related to thyroid hormone synthesis, such as sodium iodide transporter (NIS), thyroid peroxidase (TPO) and thyroglobulin (TG). Next, we found that GLIS3 and thyroid transcription factor Paired box 8 (PAX8) also increased after PM_2.5 exposure. In order to further explore the potential molecular mechanism, we carried out transcriptome sequencing. KEGG analysis of the top 10 pathways revealed that the Ras-associated protein 1 (Rap1) signaling pathway could activate transcription factors and is related to thyroid cell survival. Additionally, PM_2.5 exposure significantly increased the protein levels of Rap1 and its active form (Rap1 +GTP). We speculate that the active state of Rap1 is believed to be involved in activating the expression of transcription factor GLIS3. In conclusion, PM_2.5 exposure induces histological changes in the thyroid gland and thyroid dysfunction in mice. The exposure activates GLIS3 through the Rap1/PI3K/AKT pathway to promote the expression of proteins related to thyroid hormone synthesis, leading to increased dysregulating TH homeostasis.

Physiochemical and nutritional evaluation of whole kumquat fruits powder and its protective effect on thyroid hormones and blood sugar levels in diabetic rats

The present study was conducted to evaluate the chemical composition, antioxidant activity and hypoglycemic effects of whole kumquat (Ku) powder in diabetic rats fed a high-fat-high-cholesterol (HFHC) diet. The antioxidant activities were evaluated using stable 1,1-diphenyl 2-picrylhydrazyl (DPPH) free radical scavenging method, 2,2´-azinobis (3-ethyl benzo thiazoline-6-sulphonic acid) radical cation (ABTS) and Ferric reducing antioxidant power (FRAP). Total phenolic content was (51.85 mg GAE/g) and total flavonoid content was (0.24 mg Cateachin Equivalent, CE/g). DPPH and ABTS values were 3.32 and 3.98 mg Trolox equivalent (TE)/g where FRAP value was 3.00 mM Fe2+/kg dry material. A total of 90 albino rats were used in the present study. Rats group were as follows: normal diet; normal treated (2, 4, and 6% Ku.), diabetic rats (non-treated), diabetic + HFHC diet (non-treated), HFHC (non-treated), Diabetic (treated), HFHC (treated) and Diabetic + HFHC (treated). The diets were followed for 8 weeks. Blood samples were collected at the end of the experiment. Serum glucose was recorded and thyroid hormones (T4, Thyroxine and T3, Triiodothyronine) were conducted. Diet supplemented with Kumquat at different concentrations have a hypoglycemic effect and improve the thyroid hormones of both diabetic rats and HFHC diabetic rats.

Analysis of metabolomics profile in hypothyroid patients before and after thyroid hormone replacement

PURPOSE

The serum metabolic changes occurring during the transition from hypothyroidism to euthyroidism are not known. This study aimed to determine the metabolomic profile in hypothyroid patients before (HypoT₀) and after (HypoT₁) euthyroidism achieved through levothyroxine (L-T4) treatment.

METHODS

Eighteen patients with overt primary hypothyroidism were recruited for the study. All patients were treated with L-T4 to achieve euthyroidism. Thyrotropin (TSH), free thyroxine (FT4), free triiodothyronine (FT3) and metabolomics profiles were measured before and after 3 months of treatment. The euthyroid control group consisted of 28 healthy volunteers. Metabolomics analysis was performed using Nuclear Magnetic Resonance (NMR) spectroscopy.

RESULTS

¹H NMR-based metabolomics profiling of patients with newly diagnosed hypothyroidism (HypoT₀) showed significantly higher levels of citrate, creatinine, glycerol, myo-inositol and serine, and lower levels of proline and taurine compared to controls. Interestingly, some metabolic changes were persistent three months after pharmacological treatments, despite normal serum TSH and thyroid hormone concentrations (HypoT₁). When an Orthogonal Partial Least Squares Discriminant Analysis (OPLS-DA) model was built to evaluate possible differences in the metabolic profile between HypoT₀ and HypoT₁, the data obtained were not significantly different.

CONCLUSION

These results suggest that metabolic changes in the patients with hypothyroidism may persist after normalization of serum levels of FT3, FT4, and TSH, which currently represent the gold standard in laboratory testing for diagnosis and evaluation of thyroid pathology. So, the metabolomics approach may contribute to integrate classical hormone assays and to determine the euthyroid status achievement with greater efficacy.

Application of Proteomics and Metabonomics to Reveal the Molecular Basis of Atractylodis Macrocephalae Rhizome for Ameliorating Hypothyroidism Instead of Hyperthyroidism

As the treatments of diseases with Chinese herbs are holistic and characterized by multiple components, pathways, and targets, elucidating the efficacy of Chinese herbs in treating diseases, and their molecular basis, requires a comprehensive, network-based approach. In this study, we used a network pharmacology strategy, as well as in vivo proteomics and metabonomics, to reveal the molecular basis by which Atractylodis macrocephalae rhizome (AMR) ameliorates hypothyroidism. Eighteen main compounds from AMR and its fractions (volatile oil fraction, crude polysaccharides fraction, lactones fraction, oligosaccharide fraction, and atractyloside fraction) were identified by HPLC, and their targets were screened using the TCMSP database and Swiss Target Prediction. Disease targets were gathered from the TTD, CTD and TCMSP databases. Hub targets were screened by different plug-ins, such as Bisogene, Merge, and CytoNCA, in Cytoscape 3.7.1 software and analyzed for pathways by the DAVID database. Hypothyroidism and hyperthyroidism pharmacological models were established through systems pharmacology based on proteomic and metabolomic techniques. Finally, AMR and its fractions were able to ameliorate the hypothyroidism model to different degrees, whereas no significant improvements were noted in the hyperthyroidism model. The lactones fraction and the crude polysaccharides fraction were considered the most important components of AMR for ameliorating hypothyroidism. These amelioration effects were achieved through promoting substance and energy metabolism. In sum, the integrative approach used in this study demonstrates how network pharmacology, proteomics, and metabolomics can be used effectively to elucidate the efficacy, molecular basis, and mechanism of action of medicines used in TCM.

Free Thyroxine Levels are Associated with Cold Induced Thermogenesis in Healthy Euthyroid Individuals

Thyroid hormone (TH) is an important regulator of mammalian metabolism and facilitates cold induced thermogenesis (CIT) in brown adipose tissue (BAT). Profound hypothyroidism or hyperthyroidism lead to alterations in BAT function and CIT. In euthyroid humans the inter-individual variation of thyroid hormones is relatively large. Therefore, we investigated whether levels of free thyroxine (T4) or free triiodothyronine (T3) are positively associated with CIT in euthyroid individuals. We performed an observational study in 79 healthy, euthyroid volunteers (mean age 25.6 years, mean BMI 23.0 kg · m^-2). Resting energy expenditure (REE) was measured by indirect calorimetry during warm conditions (EE_warm) and after a mild cold stimulus of two hours (EE_cold). CIT was calculated as the difference between EE_cold and EE_warm. BAT activity was assessed by ¹⁸F-FDG-PET after a mild cold stimulus in a subset of 26 participants. EE_cold and CIT were significantly related to levels of free T4 (R² = 0.11, p=0.0025 and R² = 0.13, p=0.0011, respectively) but not to free T3 and TSH. Cold induced BAT activity was also associated with levels of free T4 (R² = 0.21, p=0.018). CIT was approximately fourfold higher in participants in the highest tertile of free T4 as compared to the lowest tertile. Additionally, free T4 was weakly, albeit significantly associated with outdoor temperature seven days prior to the respective study visit (R² = 0.06, p=0.037). These finding suggests that variations in thyroid hormone levels within the euthyroid range are related to the capability to adapt to cool temperatures and affect energy balance.

Night-restricted feeding improves locomotor activity rhythm and modulates nutrient utilization to accelerate growth in rabbits

An unfavorable lifestyle disrupts the circadian rhythm, leading to metabolic dysfunction in adult humans and animals. Increasing evidence suggests that night-restricted feeding (NRF) can effectively prevent ectopic fat deposition caused by circadian rhythm disruption, and reduce the risk of metabolic diseases. However, previous studies have mainly focused on the prevention of obesity in adults by regulating dietary patterns, whereas limited attention has been paid to the effect of NRF on metabolism during growth and development. Here, we used weaning rabbits as models and found that NRF increased body weight gain without increasing feed intake, and promoted insulin-mediated protein synthesis through the mTOR/S6K pathway and muscle formation by upregulating MYOG. NRF improved the circadian clock, promoted PDH-regulated glycolysis and CPT1B-regulated fatty-acid β-oxidation, and reduced fat content in the serum and muscles. In addition, NRF-induced body temperature oscillation might be partly responsible for the improvement in the circadian clock and insulin sensitivity. Time-restricted feeding could be used as a nondrug intervention to prevent obesity and accelerate growth in adolescents.

Elevated serum neuregulin 4 levels in patients with hyperthyroidism

OBJECTIVE

Recent studies have shown that neuregulin 4 (Nrg4), a member of the epidermal growth factor (EGF) family of extracellular ligands, plays an important role in the prevention of obesity, insulin resistance and nonalcoholic fatty liver disease (NAFLD). Considering that thyroid hormone (TH) has profound effects on whole-body energy metabolism, we speculate that circulating Nrg4 levels might be altered in patients with hyperthyroidism.

DESIGN AND METHODS

A total of 129 hyperthyroid patients and 100 healthy subjects were recruited. Of them, 39 hyperthyroid patients received thionamide treatment for 3 months until euthyroidism. Serum Nrg4 levels were determined using the ELISA method. To further confirm the relationship between TH and Nrg4, C57BL/6 mice were treated with T3 and quantitative real-time PCR was performed to detect Nrg4 gene expression.

RESULTS

Serum Nrg4 levels were significantly elevated in hyperthyroid patients as compared with normal controls (3.84 ± 1.63 vs 2.21 ± 1.04 ng/mL, P < 0.001). After achieving euthyroidism by thionamide treatment, serum Nrg4 levels dropped markedly from 3.57 ± 1.26 to 1.94 ± 0.72 ng/ml (P < 0.001). After adjustment for potential confounders, serum Nrg4 levels were independently associated with hyperthyroidism. The upregulation of Nrg4 expression in the livers and white adipose tissues by T3 was further confirmed by animal and cell culture experiments.

CONCLUSIONS

Serum Nrg4 levels were increased in patients with hyperthyroidism. The liver and white adipose tissue might be primary sources contributing to elevated serum Nrg4 concentrations.

Metformin inhibits gluconeogenesis via a redox-dependent mechanism in vivo

Metformin, the universal first-line treatment for type 2 diabetes, exerts its therapeutic glucose-lowering effects by inhibiting hepatic gluconeogenesis. However, the primary molecular mechanism of this biguanide remains unclear, though it has been suggested to act, at least partially, by mitochondrial complex I inhibition. Here we show that clinically relevant concentrations of plasma metformin achieved by acute intravenous, acute intraportal or chronic oral administration in awake normal and diabetic rats inhibit gluconeogenesis from lactate and glycerol but not from pyruvate and alanine, implicating an increased cytosolic redox state in mediating metformin's antihyperglycemic effect. All of these effects occurred independently of complex I inhibition, evidenced by unaltered hepatic energy charge and citrate synthase flux. Normalizing the cytosolic redox state by infusion of methylene blue or substrates that contribute to gluconeogenesis independently of the cytosolic redox state abrogated metformin-mediated inhibition of gluconeogenesis in vivo. Additionally, in mice expressing constitutively active acetyl-CoA carboxylase, metformin acutely decreased hepatic glucose production and increased the hepatic cytosolic redox state without altering hepatic triglyceride content or gluconeogenic enzyme expression. These studies demonstrate that metformin, at clinically relevant plasma concentrations, inhibits hepatic gluconeogenesis in a redox-dependent manner independently of reductions in citrate synthase flux, hepatic nucleotide concentrations, acetyl-CoA carboxylase activity, or gluconeogenic enzyme protein expression.

Metabolomic profile in hyperthyroid patients before and after antithyroid drug treatment: Correlation with thyroid hormone and TSH concentration

Hyperthyroidism (HT) is characterized by an intense metabolic impact which affects the lipid, carbohydrate and amino acids metabolism, with increased resting energy expenditure and thermogenesis. Metabolomics is a new comprehensive technique that allows to capture an instant metabolic picture of an organism, reflecting peculiar molecular and pathophysiological states. The aim of the present prospective study was to identify a distinct metabolomic profile in HT patients using ¹H NMR spectroscopy before and after antithyroid drug treatment. This prospective study included 15 patients (10 female, 5 male) who were newly diagnosed hyperthyroidism. A nuclear magnetic resonance (¹H NMR) based analysis was performed on plasma samples from the same patients at diagnosis (HypT₀) and when they achieved euthyroidism (HypT₁). The case groups were compared with a control group of 26 healthy volunteers (C). Multivariate statistical analysis was performed with Partial Least Squares-Discriminant Analysis (PLS-DA). PLS-DA identified a distinct metabolic profile between C and untreated hyperthyroid patients (R²X 0.638, R²Y 0.932, Q² 0.783). Interestingly, a significant difference was also found between C and euthyroid patients after treatment (R²X 0.510, R²Y 0.838, Q² 0.607), while similar cluster emerged comparing HypT₀vs HypT₁ patients. This study shows that metabolomic profile is deeply influenced by hyperthyroidism and this alteration persists after normalization of thyrotropin (TSH) and free thyroid hormone (FT3, FT4) concentration. This suggests that TSH, FT3 and FT4 assays may not be insufficient to detect long lasting peripheral effects of the thyroid hormones action. Further studies are needed to clarify whether and to what extent the evaluation of metabolomics profile may provide relevant information in the clinical management of hyperthyroidism.

Thyroid disorders and gastrointestinal and liver dysfunction: A state of the art review

Thyroid disorders commonly impact on the gastrointestinal system and may even present with gastrointestinal symptoms in isolation; for example, metastatic medullary thyroid carcinoma typically presents with diarrhoea. Delays in identifying and treating the underlying thyroid dysfunction may lead to unnecessary investigations and treatment, with ongoing morbidity, and can potentially be life-threatening. Similarly, gastrointestinal diseases can impact on thyroid function tests, and an awareness of the concept and management of non-thyroidal illness is necessary to avoid giving unnecessary thyroid therapies that could potentially exacerbate the underlying gastrointestinal disease. Dual thyroid and gastrointestinal pathologies are also common, with presentations occurring concurrently or sequentially, the latter after a variable time lag that can even extend over decades. Such an association aetiologically relates to the autoimmune background of many thyroid disorders (e.g. Graves' disease and Hashimoto's thyroiditis) and gastrointestinal disorders (e.g. coeliac disease and inflammatory bowel disease); such autoimmune conditions can sometimes occur in the context of autoimmune polyglandular syndrome. Emphasis should also be given to the gastrointestinal side effects of some of the medications used for thyroid disease (e.g. anti-thyroid drugs causing hepatotoxicity) and vice versa (e.g. interferon therapy causing autoimmune thyroid dysfunction). In this review, we discuss disorders of the thyroid-gut axis and identify the evidence base behind the management of such disorders.

Thyroid hormone regulation of metabolism

Thyroid hormone (TH) is required for normal development as well as regulating metabolism in the adult. The thyroid hormone receptor (TR) isoforms, α and β, are differentially expressed in tissues and have distinct roles in TH signaling. Local activation of thyroxine (T4), to the active form, triiodothyronine (T3), by 5'-deiodinase type 2 (D2) is a key mechanism of TH regulation of metabolism. D2 is expressed in the hypothalamus, white fat, brown adipose tissue (BAT), and skeletal muscle and is required for adaptive thermogenesis. The thyroid gland is regulated by thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH). In addition to TRH/TSH regulation by TH feedback, there is central modulation by nutritional signals, such as leptin, as well as peptides regulating appetite. The nutrient status of the cell provides feedback on TH signaling pathways through epigentic modification of histones. Integration of TH signaling with the adrenergic nervous system occurs peripherally, in liver, white fat, and BAT, but also centrally, in the hypothalamus. TR regulates cholesterol and carbohydrate metabolism through direct actions on gene expression as well as cross-talk with other nuclear receptors, including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR), and bile acid signaling pathways. TH modulates hepatic insulin sensitivity, especially important for the suppression of hepatic gluconeogenesis. The role of TH in regulating metabolic pathways has led to several new therapeutic targets for metabolic disorders. Understanding the mechanisms and interactions of the various TH signaling pathways in metabolism will improve our likelihood of identifying effective and selective targets.

Nandrolone decanoate inhibits gluconeogenesis and decreases fasting glucose in Wistar male rats

The use of anabolic-androgenic steroids to improve physical performance or appearance has increased notably. The doses used are 10- to 100- fold higher than the therapeutic dose (TD), and this abuse can cause several side effects. Glucose metabolism is significantly affected by anabolic-androgenic steroid abuse, but studies about glycemic regulation during fasting are scarce. There are some evidences showing that testosterone can antagonize glucocorticoids action, which are crucial to glucose production during fasting. Thus, the aim of this study was to determine the impact of supraphysiological doses (SDs) of nandrolone decanoate (DECA) on rat glucose metabolism during fasting. Male Wistar rats were treated with i.m. injections of vehicle, a low TD (0.016 mg/100 g b.w.-TD group) or a high SD (1 mg/100 g b.w.-SD group) of DECA, once a week for 8 weeks. After 12 h fasting, we evaluated glucose and pyruvate tolerance tests, liver glycogen content, serum levels of gluconeogenic substrates, insulin and corticosterone, glucose uptake and hexokinase (HK) activity in skeletal muscle, and the adrenal catecholamine content. SD group had increased serum insulin levels and a blunted response to insulin regarding glucose uptake in skeletal muscle. Fasting serum glucose decreased significantly in SD group, as well as the pyruvate tolerance test and liver glycogen content. Moreover, serum levels of glycerol were increased in SD group. Our data indicate that SDs of DECA exert effects on different regulatory points of glucose metabolism, resulting in defective gluconeogenesis and decreased skeletal muscle glucose uptake in response to insulin.

Decreased fasting blood glucose is associated with impaired hepatic glucose production in thyroid-stimulating hormone receptor knockout mice

Our previous study reported that thyroid-stimulating hormone (TSH) promotes cholesterol synthesis via the cyclic adenosine monophosphate/protein kinase A/cAMP regulatory element-binding protein (cAMP/PKA/CREB) pathway after binding to TSH receptors (TSHR) in the liver. The hepatic cAMP/PKA/CREB pathway also plays an important role in maintaining fasting glucose homeostasis. These findings implied a possible role for TSH in hepatic glucose metabolism. In this study, we used TSH receptor knockout mice (Tshr-ko mice) to clarify the effect of Tshr deletion on hepatic glucose metabolism, and investigated whether the effects of TSH directly regulate hepatic gluconeogenesis in HepG2 cells. Tshr-ko mice exhibited decreased fasting blood glucose levels, increased insulin sensitivity but normal level of fasting plasma insulin. Tshr deletion impaired hepatic glucose production by down-regulating the expression of glucose-6-phosphatase (G6P) and phosphoenolpyruvate pyruvate carboxylase (PEPCK) mRNA, two rate-limiting enzymes in hepatic gluconeogenesis, and enhancing the abundance of hepatic glucokinase (GK), the first enzyme regulating glycogen synthesis. Moreover, Tshr deletion inhibited the protein expression of hepatic phospho-CREB and increased the protein expression of hepatic phospho-AMP-activated protein kinase (p-AMPK), two up-stream regulators of PEPCK and G6P mRNA. In HepG2 cells, TSH increased the expression of G6P and PEPCK at mRNA level. These results indicated the simulative effects of TSH on hepatic glucose production in vivo and in vitro, suggesting a novel role for TSH in hepatic glucose metabolism.

Consequences of dysthyroidism on the digestive tract and viscera

Thyroid hormones define basal metabolism throughout the body, particularly in the intestine and viscera. Gastrointestinal manifestations of dysthyroidism are numerous and involve all portions of the tract. Thyroid hormone action on motility has been widely studied, but more complex pathophysiologic mechanisms have been indicated by some studies although these are not fully understood. Both thyroid hormone excess and deficiency can have similar digestive manifestations, such as diarrhea, although the mechanism is different in each situation. The liver is the most affected organ in both hypo- and hyperthyroidism. Specific digestive diseases may be associated with autoimmune thyroid processes, such as Hashimoto’s thyroiditis and Grave’s disease. Among them, celiac sprue and primary biliary cirrhosis are the most frequent although a clear common mechanism has never been proven. Overall, thyroid-related digestive manifestations were described decades ago but studies are still needed in order to confirm old concepts or elucidate undiscovered mechanisms. All practitioners must be aware of digestive symptoms due to dysthyroidism in order to avoid misdiagnosis of rare but potentially lethal situations.

Hypothyroidism Reduces Tricarboxylate Carrier Activity and Expression in Rat Liver Mitochondria by Reducing Nuclear Transcription Rate and Splicing Efficiency

The tricarboxylate carrier (TCC), also known as citrate carrier, is an integral protein of the mitochondrial inner membrane. It is an essential component of the shuttle system by which mitochondrial acetyl-CoA, primer for both fatty acid and cholesterol synthesis, is transported into the cytosol, where lipogenesis occurs. The effect of hypothyroidism on the activity and expression of the hepatic mitochondrial TCC was investigated in this study. TCC activity was significantly decreased in hypothyroid rats as compared with euthyroid animals. This hormone deficiency effect was due to a reduction in the amount of carrier protein, which resulted from a proportionate decrease of the specific mRNA. Hypothyroidism did not influence TCC mRNA stability. On the other hand, nuclear run-on assay revealed that the transcriptional rate of TCC mRNA decreased by approximately 40% in the nuclei from hypothyroid versus euthyroid rats. In addition, the ribonuclease protection assay showed that, in the nuclei of hypothyroid rats, the ratio of mature to precursor RNA decreased, indicating that the splicing of TCC RNA is affected. Furthermore, we found that the ratio of polyadenylated/unpolyadenylated TCC RNA as well as the length of the TCC RNA poly(A) tail were similar in both euthyroid and hypothyroid rats. Thus, the rate of formation of the TCC 3'-end is not altered in hypothyroidism. These results suggest that hypothyroidism affects TCC expression at both the transcriptional and post-transcriptional levels.

Peripheral metabolism of thyroid hormone and glucose homeostasis

Thyroid hormone action has long been recognized as an important determinant of glucose homeostasis. Recent advances in the knowledge of the physiology of the deiodinases indicate that through tissue-specific regulation of thyroid hormone metabolism, leading to local specificity of thyroid hormone action and target gene transcription patterns, they may have an important function in the modulation of carbohydrate metabolism. This review briefly addresses the role of thyroid hormone action on glucose homeostasis with a specific focus on the significance of the peripheral metabolism of thyroid hormone in the regulation of glucose homeostasis and insulin sensitivity.

Thyroid hormone and dehydroepiandrosterone permit gluconeogenic hormone responses in hepatocytes

The importance of the sn-glycerol- 3-phosphate (G-3-P) electron transfer shuttle in hormonal regulation of gluconeogenesis was examined in hepatocytes from rats with decreased mitochondrial G-3-P dehydrogenase activity (thyroidectomized) or increased G-3-P dehydrogenase activity [triiodothyronine (T(3)) or dehydroepiandrosterone (DHEA) treated]. Rates of glucose formation from 10 mM lactate, 10 mM pyruvate, or 2.5 mM dihydroxyacetone were somewhat less in hypothyroid cells than in cells from normal rats but gluconeogenic responses to calcium addition and to norepinephrine (NE), glucagon (G), or vasopressin (VP) were similar to the responses observed in cells from normal rats. However, with 2. 5 mM glycerol or 2.5 mM sorbitol, substrates that must be oxidized in the cytosol before conversion to glucose, basal gluconeogenesis was not appreciably altered by hypothyroidism but responses to calcium and to the calcium-mobilizing hormones were abolished. Injecting thyroidectomized rats with T(3) 2 days before preparing the hepatocytes greatly enhanced gluconeogenesis from glyc erol and restored the response to Ca(2+) and gluconeogenic hormones. Feeding dehydroepiandrosterone for 6 days depressed gluconeogenesis from lactate or pyruvate but substantially increased glucose production from glycerol in euthyroid cells and restored responses to Ca(2+) in hypothyroid cells metabolizing glycerol. Euthyroid cells metabolizing glycerol or sorbitol use the G-3-P and malate/aspartate shuttles to oxidize excess NADH generated in the cytosol. The transaminase inhibitor aminooxyacetate (AOA) decreased gluconeogenesis from glycerol 40%, but had little effect on responses to Ca(2+) and NE. However, in hypothyroid cells, with minimal G-3-P dehydrogenase, AOA decreased gluconeogenesis from glycerol more than 90%. Thus, the basal rate of gluconeogenesis from glycerol in the euthyroid cells is only partly dependent on electron transport from cytosol to mitochondria via the malate/aspartate shuttle and almost completely dependent in the hypothyroid state, and the hormone enhancement of the rate in euthyroid cells involves primarily the G-3-P cycle. These data are consistent with Ca(2+) being mobilized by gluconeogenic hormones and G-3-P dehydrogenase being activated by Ca(2+) so as to permit it to transfer reducing equivalents from the cytosol to the mitochondria.

Increased glucose synthesis in renal tubule fragments from hyperthyroid rats

Rates of glucose synthesis from several substrates were examined in renal tubule fragments from hyperthyroid rats. A hyperthyroid state was induced by daily intraperitoneal injections of thyroxine (T4) (100 µg/100 g body weight) for 14 days. At the end of the experimental period, plasma triiodothyronine and T4levels were six and eight times higher, respectively, than initial values. Hyperthyroid rats gained less weight and had lower blood glucose despite an increased food intake. In both control and hyperthyroid rats, rates of glucose production by renal tubule fragments were higher with glutamine and glycerol than with lactate, alanine, or glutamate. T4treatment induced a significant increase in the de novo glucose synthesis from all substrates, except glutamine. The highest percent increase was obtained with alanine (64%), compared with 31-40% for glutamate, lactate, and glycerol. The T4treatment induced increase in glucose synthesis by renal tubule fragments suggests that renal gluconeogenesis contributes to enhance glucose production in hyperthyroidism.Key words: hyperthyroidism, rat, renal gluconeogenesis, renal tubule fragments, glutamate, glutamine, lactate, alanine, glycerol.

Effect of growth hormone deficiency on hormonal control of hepatic glycogenolysis in hypophysectomized rat

The present study was designed to investigate the hormonal regulation of rat liver glycogenolysis in growth hormone (GH) deficiency. To this end, hepatocytes were isolated from control, GH-deprived (hypophysectomized and treated with triiodothyronine [T3] and corticotropin), and 7-day GH-supplemented fed rats and incubated with glucagon and alpha 1-adrenergic agonist (phenylephrine) to measure the hormonal activation of both glycogen phosphorylase and glucose production from glycogen stores. GH deficiency induces a combined decrease of 50% of the glycogen content, the activity of glucose-6-phosphatase, and the maximal hormone-induced glycogen phosphorylase activity. Daily GH injections restore the levels of both glycogen phosphorylase and glucose-6-phosphatase. These enzymatic inductions occur without normalization of insulinemia. Despite the reduced levels of key enzymes of glycogenolysis, the stimulation of glucose production from glycogen in response to glucagon and phenylephrine is not modified in GH-deprived rats. An increase in the intrinsic activity of one or both of the enzymatic steps is postulated to compensate for the lower levels of enzymes, as indicated by the slopes of the correlation between glucose production and phosphorylase a activity (107 and 216 nmol glucose produced/min/U phosphorylase a [P < .001] in control and GH-deprived rats, respectively). GH replacement enhances maximal phosphorylase activity and brings the correlation toward the control value (slope, 128 nmol glucose produced/min/U phosphorylase a). Our findings demonstrate that glycogenolysis in hepatocytes isolated from GH-deprived rats is normal, despite a reduction of glycogen phosphorylase and glucose-6-phosphatase activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of hyperthyroidism on glucose, glutamine and ketone-body metabolism in the gut of the rat

1. The metabolism of glucose, glutamine and ketone-bodies was studied in the small intestine of rats after 5 days of hyperthyroidism. 2. Portal-drained visceral bloodflow increased by 20.1% (P < 0.05) in hyperthyroid rats and was accompanied by a decrease in the arteriovenous concentration difference of glutamine (25.7%, P < 0.05), glutamate (22.0%, P < 0.05), alanine (20.9%, P < 0.05) and ammonia (20.6%, P < 0.05) and an increase in that of glucose (27.2%, P < 0.05), lactate (28.9%, P < 0.05) and ketone-bodies (163.2%, P < 0.001). 3. The gut of hyperthyroid rats showed increased rates of extraction of glucose, lactate and ketone-bodies. 4. Enterocytes isolated from hyperthyroid rats showed increased rates of utilization of glucose and ketone-bodies but that of glutamine were decreased. 5. The maximal activities of hexokinase, 6-phosphofructokinase, pyruvate kinase, citrate synthase and oxoglutarate dehydrogenase were increased (by 13.7-36.2%) in intestinal mucosal scrapings of hyperthyroid rats, whereas the activity of glutaminase was decreased (22.1-31.4%). 6. It is concluded that hyperthyroidism increases the rates of utilization of glucose and ketone-bodies but decreases that of glutamine (both in vivo and in vitro) by the epithelial cells of the small intestine.