Fasting-induced changes in hepatic thyroid hormone metabolism in male rats are independent of autonomic nervous input to the liver

Thyroid Hormones Regulate the Thermoregulatory Mechanisms of the Body: Review

Thyroid hormones (TH) play a critical role in metabolism, energy balance and thermogenesis. The mechanisms whereby thyroid hormone increases heat production have been analyzed with emphasis in more recent developments. Thyroid hormone increases obligatory thermogenesis as a result of the stimulation of numerous metabolic pathways involved in the development, remodeling and delivery of energy to the tissues. In this section, alterations in primary hyperthyroidism and hypothyroidism will be contrasted with the physiological characteristics of TH-dependent regulation in response to fasting and exposure to cold. The current review will discuss the situation with regard to regional thyroid hormones in the Central Nervous System (CNS) and more specifically, in peripheral cells. When caused by exposure to cold or fasting, local anomalies in the CNS are distinct from peripheral compartments, in contrast to hyperthyroidism and hypothyroidism, which differ when similar changes are observed. Lower hypothalamic TH concentrations are associated with cold exposure, although higher peripheral TH levels. The TH tendency is reversed by fasting. Primary hypothyroidism and hyperthyroidism impair them. The current study aims to trace the various mechanisms used by the thyroid gland to regulate the body's energy production process.

Hepatic Energy Metabolism under the Local Control of the Thyroid Hormone System

The energy homeostasis of the organism is orchestrated by a complex interplay of energy substrate shuttling, breakdown, storage, and distribution. Many of these processes are interconnected via the liver. Thyroid hormones (TH) are well known to provide signals for the regulation of energy homeostasis through direct gene regulation via their nuclear receptors acting as transcription factors. In this comprehensive review, we summarize the effects of nutritional intervention like fasting and diets on the TH system. In parallel, we detail direct effects of TH in liver metabolic pathways with regards to glucose, lipid, and cholesterol metabolism. This overview on hepatic effects of TH provides the basis for understanding the complex regulatory network and its translational potential with regards to currently discussed treatment options of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) involving TH mimetics.

Temperature modulates systemic and central actions of thyroid hormones on BAT thermogenesis

Thyroid hormones (THs) play a major role regulating energy balance and brown adipose tissue (BAT) thermogenesis, as well as body temperature, as shown in hyperthyroid patients. However, the current landscape of preclinical thyroid hormone models is complex. For example, while rats become catabolic after TH administration, mice gain weight; so, these differences in species need to be analyzed in detail and specially whether temperature could be a factor. Here, we aimed to investigate the effect of environmental temperature on those actions. Rats were subcutaneously treated with L-thyroxine (T4) or stereotaxically within the ventromedial nucleus of the hypothalamus (VMH) with triiodothyronine (T3) and housed at 23°C, 4°C or 30°C; energy balance, BAT thermogenesis and AMP-activated protein kinase (AMPK) in the VMH were analyzed. Our data showed that the effect of both systemic T4 of central T3 on energy balance and BAT thermogenesis was dependent upon environmental temperature. This evidence is of interest in the design of experimental settings highlighting the species-specific metabolic actions of THs, and in understanding its physiological role in the adaptation to temperature.

Criminal of Adverse Pregnant Outcomes: A Perspective From Thyroid Hormone Disturbance Caused by SARS-CoV-2

Nowadays, emerging evidence has shown adverse pregnancy outcomes, including preterm birth, preeclampsia, cesarean, and perinatal death, occurring in pregnant women after getting infected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), but the underlying mechanisms remain elusive. Thyroid hormone disturbance has been unveiled consistently in various studies. As commonly known, thyroid hormone is vital for promoting pregnancy and optimal fetal growth and development. Even mild thyroid dysfunction can cause adverse pregnancy outcomes. We explored and summarized possible mechanisms of thyroid hormone abnormality in pregnant women after coronavirus disease 2019 (COVID-19) infection and made a scientific thypothesis that adverse pregnancy outcomes can be the result of thyroid hormone disorder during COVID-19. In which case, we accentuate the importance of thyroid hormone surveillance for COVID-19-infected pregnant women.

Thyroid hormone receptor phosphorylation regulates acute fasting-induced suppression of the hypothalamic-pituitary-thyroid axis

Fasting induces profound changes in the hypothalamic-pituitary-thyroid (HPT) axis. After binding thyroid hormone (TH), the TH receptor beta 2 isoform (THRB2) represses Trh and Tsh subunit genes and is the principle negative regulator of the HPT axis. Using mass spectrometry, we identified a major phosphorylation site in the AF-1 domain of THRB2 (serine 101, S101), which is conserved among many members of the nuclear hormone receptor superfamily. More than 50% of THRB2 is phosphorylated at S101 in cultured thyrotrophs (TαT1.1) and in the mouse pituitary. All other THR isoforms lack this site and exhibit limited overall levels of phosphorylation. To determine the importance of THRB2 S101 phosphorylation, we used the TαT1.1 cell line and S101A mutant knock-in mice (Thrb2 ^S101A ). We found that TH promoted S101 THRB2 phosphorylation and was essential for repression of the axis at physiologic TH concentrations. In mice, THRB2 phosphorylation was also increased by fasting and mimicked Trh and Tshb repression by TH. In vitro studies demonstrated that a master metabolic sensor, AMP-activated kinase (AMPK) induced phosphorylation at the same site and caused Tshb repression independent of TH. Furthermore, we identified cyclin-dependent kinase 2 (CDK2) as a direct kinase phosphorylating THRB2 S101 and propose that AMPK or TH increase S101 phosphorylation through the activity of CDK2. This study provides a physiologically relevant function for THR phosphorylation, which permits nutritional deprivation and TH to use a common mechanism for acute suppression of the HPT axis.

Fasting Increases Iron Export by Modulating Ferroportin 1 Expression Through the Ghrelin/GHSR1α/MAPK Pathway in the Liver

The liver is contributed to maintaining body iron homeostasis and controlling of body adaptation to fasting. Although previous studies implied a negative relationship between iron and ghrelin in both mice and humans, it remains to be explored whether fasting or ghrelin has a functional effect on iron homeostasis in the liver. In this study, we examined the roles of fasting and ghrelin in modulating the protein expression of Fpn1, transferrin receptor 1 (TfR1), and ferritin light chain (Ft-L), as well as the mRNA expression of ghrelin, hepcidin, ghrelin O-acyltransferase (GOAT), and growth hormone secretagogue receptor 1 alpha (GHSR1α) in mouse liver and cultured hepatocytes. Our in vivo results suggested that fasting significantly upregulated the mRNA expression of ghrelin, GOAT, and GHSR1α, as well as the protein levels of ghrelin, Fpn1, and Ft-L, but not TfR1, in mouse liver. Interestingly, mRNA expression of hepcidin did not change significantly after fasting. Meanwhile, in cultured hepatocytes, ghrelin significantly increased the protein expression of Fpn1 but not Ft-L and TfR1 and significantly enhanced ERK phosphorylation. Furthermore, the pretreatment of cultured hepatocytes with either a pERK inhibitor or a GHSR1α antagonist abolished the effects of ghrelin on Fpn1 expression and ERK phosphorylation. Our findings confirmed that fasting increases iron export in the liver by upregulating Fpn1 expression through the ghrelin/GHSR1α/MAPK signaling pathway.

Regulation of type 3 deiodinase in rodent liver and adipose tissue during fasting

Fasting induces profound changes in the hypothalamus-pituitary-thyroid axis and peripheral thyroid hormone (TH) metabolism, ultimately leading to lower serum thyroid hormone (TH) concentrations. In the present study, we aimed to investigate the regulation of type 3 deiodinase (D3) during fasting in two metabolic tissues: liver and white adipose tissue (WAT). To this end, we studied the effect of modulation of the mammalian target of rapamycin (mTOR) and hypoxia inducible factor 1α (HIF1α) on D3 expression in primary rat hepatocytes and in 3T3-L1 adipocytes. In addition, we studied the role of the constitutive androstane receptor (CAR) on liver TH metabolism using primary hepatocytes and CAR-/- mice. Twenty-four-hour fasting increased liver Dio3 expression in mice. Inhibition of mTOR using mTOR inhibitors markedly induced Dio3 mRNA expression in primary hepatocytes; this increase was accompanied by a small increase in D3 activity. Stimulation of these cells with a CAR agonist induced both Dio3 mRNA expression and activity. Fasting increased hepatic D3 expression in WT but not in CAR-/- mice. In WAT, Dio3 mRNA expression increased five-fold after 48-h fasting. Treatment of 3T3-L1 adipocytes with mTOR inhibitors induced Dio3 mRNA expression, whereas stimulation of these cells with cobalt chloride, a compound that mimics hypoxia and stabilizes HIF1α, did not induce Dio3 mRNA expression. In conclusion, our results indicate an important role of mTOR in the upregulation of D3 in WAT and liver during fasting. Furthermore, CAR plays a role in the fasting induced D3 increase in the liver.

Downregulation of Type 3 Deiodinase in the Hypothalamus During Inflammation

Background: Inflammation is associated with marked changes in cellular thyroid hormone (TH) metabolism in triiodothyronine (T3) target organs. In the hypothalamus, type 2 deiodinase (D2), the main T3 producing enzyme, increases upon inflammation, leading to an increase in local T3 availability, which in turn decreases thyrotropin releasing hormone expression in the paraventricular nucleus. Type 3 deiodinase (D3), the T3 inactivating enzyme, decreases during inflammation, which might also contribute to the increased T3 availability in the hypothalamus. While it is known that D2 is regulated by nuclear factor κB (NF-κB) during inflammation, the underlying mechanisms of D3 regulation are unknown. Therefore, the aim of the present study was to investigate inflammation-induced D3 regulation using in vivo and in vitro models. Methods: Mice were injected with a sublethal dose of bacterial endotoxin (lipopolysaccharide [LPS]) to induce a systemic acute-phase response. A human neuroblastoma (SK-N-AS) cell line was used to test the involvement of the thyroid hormone receptor alpha 1 (TRα1) as well as the activator protein-1 (AP-1) and NF-κB inflammatory pathways in the inflammation-induced decrease of D3. Results: D3 expression in the hypothalamus was decreased 24 hours after LPS injection in mice. This decrease was similar in mice lacking the TRα. Incubation of SK-N-AS cells with LPS robustly decreased both D3 mRNA expression and activity. This led to increased intracellular T3 concentrations. The D3 decrease was prevented when NF-κB or AP-1 was inhibited. TRα1 mRNA expression decreased in SK-N-AS cells incubated with LPS, but knockdown of the TRα in SK-N-AS cells did not prevent the LPS-induced D3 decrease. Conclusions: We conclude that the inflammation-induced D3 decrease in the hypothalamus is mediated by the inflammatory pathways NF-κB and AP-1, but not TRα1. Furthermore, the observed decrease modulates intracellular T3 concentrations. Our results suggest a concerted action of inflammatory modulators to regulate both hypothalamic D2 and D3 activities to increase the local TH concentrations.

The Thyroid Hormone Inactivating Type 3 Deiodinase Is Essential for Optimal Neutrophil Function: Observations From Three Species

Neutrophils are essential effector cells of the innate immune system that have recently been recognized as thyroid hormone (TH) target cells. Cellular TH bioavailability is regulated by the deiodinase enzymes, which can activate or inactivate TH. We have previously shown that the TH inactivating enzyme type 3 deiodinase (D3) is present in neutrophils. Furthermore, D3 knockout (D3KO) mice show impaired bacterial killing upon infection. We hypothesized that D3 plays a role in neutrophil function during infection by actively regulating local TH availability. We measured TH concentrations in cerebrospinal fluid (CSF) from patients with bacterial meningitis and controls. Bacterial meningitis resulted in marked changes in CSF TH levels, characterized by a strong increase of thyroxine and reverse-triiodothyronine concentrations. This altered TH profile was consistent with elevated D3 activity in infiltrating neutrophils at the site of infection. D3 knockdown in zebrafish embryos with pneumococcal meningitis resulted in increased mortality and reduced neutrophil infiltration during infection. Finally, stimulated neutrophils from female D3KO mice exhibited impaired NADPH-oxidase activity, an important component of the neutrophil bacterial killing machinery. These consistent findings across experimental models strongly support a critical role for reduced intracellular TH concentrations in neutrophil function during infection, for which the TH inactivating enzyme D3 appears essential.

The classic pathways of thyroid hormone metabolism

Thyroid hormones (TH) are crucial for growth and development and play an important role in energy homeostasis. Although serum TH levels are relatively constant in the physiological state, TH bioavailability at the tissue and cellular level is dependent on local TH metabolism. Circulating TH produced by the thyroid can be metabolized by a number of different pathways resulting in 1) activation of TH 2) deactivation of TH or 3) excretion of TH and subsequent metabolites. These pathways play an essential role in determining local TH levels and action. The major classical pathways of TH metabolism are deiodination, sulfation, glucuronidation, and ether-link cleavage. This review provides an overview of these pathways, their relative contributions to TH levels in the serum and in various organs and the changes in these pathways elicited by fasting and illness.

Tissue thyroid hormone metabolism is differentially regulated during illness in mice

Illness induces major modifications in central and peripheral thyroid hormone (TH) metabolism, so-called nonthyroidal illness syndrome (NTIS). As a result, organ-specific changes in local TH availability occur depending on the type and severity of illness. Local TH availability is of importance for the regulation of the tissue-specific TH target genes and determined by the interplay between deiodinating enzymes, TH transport and TH receptor (TR) expression. In the present study, we evaluated changes in TH transport, deiodination and TR expression, the resulting tissue TH concentrations and the expression of TH target genes in liver and muscle in three animal models of illness. We induced (1) acute systemic inflammation by intraperitoneal injection of bacterial endotoxin (LPS), (2) chronic local inflammation by a turpentine injection in the hind limb and (3) severe pneumonia and sepsis by intranasal inoculation with Streptococcus pneumoniae We found that all aspects of peripheral TH metabolism are differentially regulated during illness, depending on the organ studied and severity of illness. In addition, tissue TH concentrations are not equally affected by the decrease in serum TH concentrations. For example, the decrease in muscle TH concentrations is less severe than the decrease observed in liver. In addition, despite lower TH concentrations in muscle in all three models, muscle T₃ action is differentially affected. These observations help to understand the complex nature of the nonthyroidal illness syndrome.

Thyroid hormones in extreme longevity

The aim of the present review was to summarize knowledge about thyroid hormones (THs) and longevity. Longevity is a complex multifactorial phenomenon on which specific biological pathways, including hormonal networks involved in the regulation of homeostasis and survival, exert a strong impact. THs are the key responsible for growth, metabolism rate and energy expenditure, and help in maintaining cognition, bone and cardiovascular health. THs production and metabolism are fine tuned, and may help the organism to cope with a variety of environmental challenges. Experimental evidence suggests that hypothyroid state may favor longevity by reducing metabolism rate, oxidative stress and cell senescence. Data from human studies involving healthy subjects and centenarians seem to confirm this view, but THs changes observed in older patients affected by chronic diseases cannot be always interpreted as a protective adaptive mechanism aimed at reducing catabolism and prolonging survival. Medications, selected chronic diseases and multi-morbidity can interfere with thyroid function, and their impact is still to be elucidated.

Increased plasma leptin attenuates adaptive metabolism in early lactating dairy cows

Mammals meet the increased nutritional demands of lactation through a combination of increased feed intake and a collection of adaptations known as adaptive metabolism (e.g., glucose sparing via insulin resistance, mobilization of endogenous reserves, and increased metabolic efficiency via reduced thyroid hormones). In the modern dairy cow, adaptive metabolism predominates over increased feed intake at the onset of lactation and develops concurrently with a reduction in plasma leptin. To address the role of leptin in the adaptive metabolism of early lactation, we asked which adaptations could be countered by a constant 96-h intravenous infusion of human leptin (hLeptin) starting on day 8 of lactation. Compared to saline infusion (Control), hLeptin did not alter energy intake or milk energy output but caused a modest increase in body weight loss. hLeptin reduced plasma glucose by 9% and hepatic glycogen content by 73%, and these effects were associated with a 17% increase in glucose disposal during an insulin tolerance test. hLeptin attenuated the accumulation of triglyceride in the liver by 28% in the absence of effects on plasma levels of the anti-lipolytic hormone insulin or plasma levels of free fatty acids, a marker of lipid mobilization from adipose tissue. Finally, hLeptin increased the plasma concentrations of T4 and T3 by nearly 50% without affecting other neurally regulated hormones (i.e., cortisol and luteinizing hormone (LH)). Overall these data implicate the periparturient reduction in plasma leptin as one of the signals promoting conservation of glucose and energy at the onset of lactation in the energy-deficient dairy cow.

The effect of diet interventions on hypothalamic nutrient sensing pathways in rodents

The hypothalamus plays a fundamental role in regulating homeostatic processes including regulation of food intake. Food intake is driven in part by energy balance, which is sensed by specific brain structures through signaling molecules such as nutrients and hormones. Both circulating glucose and fatty acids decrease food intake via a central mechanism involving the hypothalamus and brain stem. Besides playing a role in signaling energy status, glucose and fatty acids serve as fuel for neurons. This review focuses on the effects of glucose and fatty acids on hypothalamic pathways involved in regulation of energy metabolism as well as on the role of the family of peroxisome proliferator activated receptors (PPARs) which are implicated in regulation of central energy homeostasis. We further discuss the effects of different hypercaloric diets on these pathways.

Abnormalities of Thyroid Hormone Metabolism during Systemic Illness: The Low T3 Syndrome in Different Clinical Settings

Thyroid hormone abnormalities are common in critically ill patients. For over three decades, a mild form of these abnormalities has been described in patients with several diseases under outpatient care. These alterations in thyroid hormone economy are a part of the nonthyroidal illness and keep an important relationship with prognosis in most cases. The main feature of this syndrome is a fall in free triiodothyronine (T3) levels with normal thyrotropin (TSH). Free thyroxin (T4) and reverse T3 levels vary according to the underlying disease. The importance of recognizing this condition in such patients is evident to physicians practicing in a variety of specialties, especially general medicine, to avoid misdiagnosing the much more common primary thyroid dysfunctions and indicating treatments that are often not beneficial. This review focuses on the most common chronic diseases already known to present with alterations in serum thyroid hormone levels. A short review of the common pathophysiology of the nonthyroidal illness is followed by the clinical and laboratorial presentation in each condition. Finally, a clinical case vignette and a brief summary on the evidence about treatment of the nonthyroidal illness and on the future research topics to be addressed are presented.

Chronic central leptin infusion modulates the glycemia response to insulin administration in male rats through regulation of hepatic glucose metabolism

Leptin and insulin use overlapping signaling mechanisms to modify hepatic glucose metabolism, which is critical in maintaining normal glycemia. We examined the effect of an increase in central leptin and insulin on hepatic glucose metabolism and its influence on serum glucose levels. Chronic leptin infusion increased serum leptin and reduced hepatic SH-phosphotyrosine phosphatase 1, the association of suppressor of cytokine signaling 3 to the insulin receptor in liver and the rise in glycemia induced by central insulin. Leptin also decreased hepatic phosphoenolpyruvate carboxykinase levels and increased insulin's ability to phosphorylate insulin receptor substrate-1, Akt and glycogen synthase kinase on Ser9 and to stimulate glucose transporter 2 and glycogen levels. Peripheral leptin treatment reproduced some of these changes, but to a lesser extent. Our data indicate that leptin increases the hepatic response to a rise in insulin, suggesting that pharmacological manipulation of leptin targets may be of interest for controlling glycemia.

The molecular basis of the non-thyroidal illness syndrome

The 'sick euthyroid syndrome' or 'non-thyroidal illness syndrome' (NTIS) occurs in a large proportion of hospitalized patients and comprises a variety of alterations in the hypothalamus-pituitary-thyroid (HPT) axis that are observed during illness. One of the hallmarks of NTIS is decreased thyroid hormone (TH) serum concentrations, often viewed as an adaptive mechanism to save energy. Downregulation of hypophysiotropic TRH neurons in the paraventricular nucleus of the hypothalamus and of TSH production in the pituitary gland points to disturbed negative feedback regulation during illness. In addition to these alterations in the central component of the HPT axis, changes in TH metabolism occur in a variety of TH target tissues during NTIS, dependent on the timing, nature and severity of the illness. Cytokines, released during illness, are known to affect a variety of genes involved in TH metabolism and are therefore considered a major determinant of NTIS. The availability of in vivo and in vitro models for NTIS has elucidated part of the mechanisms involved in the sometimes paradoxical changes in the HPT axis and TH responsive tissues. However, the pathogenesis of NTIS is still incompletely understood. This review focusses on the molecular mechanisms involved in the tissue changes in TH metabolism and discusses the gaps that still require further research.