Transport of 3,5,3'-triiodothyronine into the perfused rat liver and subsequent metabolism are inhibited by fasting

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.

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.

The 5'-deiodinases are not essential for the fasting-induced decrease in circulating thyroid hormone levels in male mice: possible roles for the type 3 deiodinase and tissue sequestration of hormone

Fasting in rodents is characterized by decreases in serum T4 and T3 levels but no compensatory increase in serum TSH level. The types 1 and 2 deiodinases (D1 and D2) are postulated to play key roles in mediating these changes. However, serum T4 and T3 levels in fasted 5'-deiodinase-deficient mice decreased by at least the same percentage as that observed in wild-type mice, whereas serum TSH level was unaffected. D3 activity was increased in kidney, muscle, and liver up to 4-fold during fasting, and the mean serum rT3 level was increased 3-fold in fasted D1-deficient mice, compared with fed animals. In wild-type mice, the tissue contents of T4 and T3 in liver, kidney, and muscle were unchanged or increased in fasted animals, and after the administration of [(125)I]T4 or [(125)I]T3, the radioactive content in the majority of tissues from fasted mice was increased 2- or 4-fold, respectively. These findings suggest that the observed fasting-induced reductions in the circulating T3 and T4 levels are mediated in part by increased D3 activity and by the sequestration of thyroid hormone and their metabolites in tissues. Studies performed in D3-deficient mice demonstrating a blunting of the fasting-induced decrease in serum T4 and T3 levels were consistent with this thesis. Thus, the systemic changes in thyroid hormone economy as a result of acute food deprivation are not dependent on the D1 or D2 but are mediated in part by sequestration of T4 and T3 in tissues and their enhanced metabolism by the D3.

Thyroid hormone (T3)-modification of polyethyleneglycol (PEG)-polyethyleneimine (PEI) graft copolymers for improved gene delivery to hepatocytes

Targeting of gene vectors to liver hepatocytes could offer the opportunity to cure various acquired and inherited diseases. Efficient gene delivery to the liver parenchyma has been obscured from efficient targeting of hepatocytes. Here we show that the thyroid hormone, triiodothyronine (T3), can be used to improve the gene transfer efficiency of nonviral gene vectors to hepatocytes in vitro and to the liver of mice in vivo. T3 conjugated to the distal ends of fluorescent labeled PEG-g-dextran resulted in T3-specific cellular endosomal uptake into the hepatocellular cell line HepG2. PEG-g-PEI graft copolymers with increasing molar PEG-ratios were synthesized, complexed with plasmid DNA, and transfected into HepG2 or HeLa cells. Gene transfer efficiency decreased as the number of PEG blocks increased. T3 conjugation to PEI and the distal ends of PEG blocks resulted in T3 specific gene transfer in HepG2 cells as evidenced by reduction of gene transfer efficiency after pre-incubation of cells with excess of T3. In vivo application of T3-PEG-g-PEI based gene vectors in mice after tail vein injection resulted in a significantly 7-fold increase of gene expression in the liver compared with PEG-g-PEI based gene vectors.

Effects of thyroid state on the expression of hepatic thyroid hormone transporters in rats

Liver uptake of thyroxine (T4) is mediated by transporters and is rate limiting for hepatic 3,3',5-triiodothyronine (T3) production. We investigated whether hepatic mRNA for T4 transporters is regulated by thyroid state using Xenopus laevis oocytes as an expression system. Because X. laevis oocytes show high endogenous uptake of T4, T4 sulfamate (T4NS) was used as an alternative ligand for the hepatic T4 transporters. Oocytes were injected with 23 ng liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats, and after 3-4 days uptake was determined by incubation of injected and uninjected oocytes for 1 h at 25 degrees C or for 4 h at 18 degrees C with 10 nM [125I]T4NS. Expression of type I deiodinase (D1), which is regulated by thyroid state, was studied in the oocytes as an internal control. Uptake of T4NS showed similar approximately fourfold increases after injection of liver mRNA from euthyroid, hypothyroid, or hyperthyroid rats. A similar lack of effect of thyroid state was observed using reverse T3 as ligand. In contrast, D1 activity induced by liver mRNA from hyperthyroid and hypothyroid rats in the oocytes was 2.4-fold higher and 2.7-fold lower, respectively, compared with euthyroid rats. Studies have shown that uptake of iodothyronines in rat liver is mediated in part by several organic anion transporters, such as the Na+/taurocholate-cotransporting polypeptide (rNTCP) and the Na-independent organic anion-transporting polypeptide (rOATP1). Therefore, the effects of thyroid state on rNTCP, rOATP1, and D1 mRNA levels in rat liver were also determined. Northern analysis showed no differences in rNTCP or rOATP1 mRNA levels between hyperthyroid and hypothyroid rats, whereas D1 mRNA levels varied widely as expected. These results suggest little effect of thyroid state on the levels of mRNA coding for T4 transporters in rat liver, including rNTCP and rOATP1. However, they do not exclude regulation of hepatic T4 transporters by thyroid hormone at the translational and posttranslational level.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Thyroid hormones homeostasis in rats refed after short-term and prolonged fasting

Effects of starvation on thyroid hormone homeostasis were usually determined after 2 or more days of fasting, however, both in man and in rodents, natural feeding cycles comprise far shorter fasting periods. Therefore serum levels of T4, FT4, T3, FT3 and rT3 were measured in rats refed chow diet for 1, 4, 8 or 24 hours after 14 or 48 hours of starvation. Both short-term (14 h) and long-term fasting (48 h) decreased body weight and serum glucose level. Short-term fast decreased serum FT3 and did not change serum levels of T4, FT4, T3 and rT3. Total T3 and reverse T3 increased after one and 4 hours, free T3 after 4 hours and total T4 after 4 and 8 hours of refeeding. Percent of FT3 did not change after short-term fast, declined after 1 and 4 hours of refeeding, and normalised thereafter. Prolonged starvation (48 h) decreased serum T4, T3, FT3 and % FT3 with no changes in FT4 and rT3. After 24 hours of refeeding only FT3 and % FT3 returned to control levels while total T4 and total T3 were still diminished, and reverse T3 levels did not change. The results suggest that the length of preceding fasting period may strongly influence thyroid hormone homeostasis during fasted-to-fed transition.

T4 uptake into the perfused rat liver and liver T4 uptake in humans are inhibited by fructose

Recently, we described a two-pool model for 3,5,3'-triiodothyronine uptake and metabolism in the isolated perfused rat liver. Here, we applied this model to investigate transmembrane thyroxine (T4) transport and its possible ATP dependence in vivo. These studies are performed in perfused rat livers during perfusion with or without fructose in the medium, as it has been shown that intracellular ATP is decreased after fructose loading. Furthermore, we studied serum T4 tracer disappearance curves in four human subjects before and after intravenous fructose loading. In the perfused rat liver, we found a decrease in liver ATP concentration and a decrease in medium T4 disappearance and T4 uptake in the liver pool after fructose. Furthermore, it was shown that, when corrected for differences in the medium free hormone concentration, only transport to the metabolizing liver pool was decreased after fructose perfusion, whereas uptake in the nonmetabolizing pool was unaffected. Disposal, corrected for differences in transport into the metabolizing pool, was also not affected after fructose. In the human studies, intravenous fructose administration induced a rise in serum lactic acid and uric acid, indicating a decrease in liver ATP. This was observed concomitant with a decrease in serum tracer T4 disappearance during the first 3 h after fructose administration. These results suggest ATP dependence of transport of iodothyronines into the liver in vivo and show that, in the rat liver and in humans, uptake of T4 may be regulated by intracellular energy stores; in this way the tissue uptake process may affect intracellular metabolism and bioavailability of thyroid hormone.

Different effects of amiodarone on transport of T4 and T3 into the perfused rat liver

Uptake and metabolism of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) were studied in isolated perfused livers of control and amiodarone-treated rats (40 mg.kg body wt-1.day-1, 22 days). With the use of this perfusion system and a two-pool model describing thyroid hormone kinetics, total uptake was evaluated by the half-time (t1/2) of the fast component of the biphasic thyroid hormone disappearance from the medium and by the fractional influx rate constant (k21). Metabolism was assessed by the t1/2 of the slow component, by determination of breakdown products in medium and bile, and by thyroid hormone disposal according to the two-pool model. Disposal was corrected for differences in mass transfer into the metabolizing pool. In amiodarone-treated rats, both uptake and metabolism of T4 were decreased. Furthermore, it was shown that only transport into the metabolizing liver compartment and not uptake into the nonmetabolizing liver compartment was decreased. Both uptake and total metabolism of T3 were unaffected by amiodarone. The results showed that the different transport systems for T4 and T3 described in isolated rat hepatocytes may also be operative in the intact rat liver. Furthermore, it can be concluded that the low-T3 syndrome, caused by treatment with amiodarone, may be due to both impaired transport and impaired 5'-deiodination.

Transport of thyroxine into cultured hepatocytes: effects of mild non-thyroidal illness and calorie restriction in obese subjects

OBJECTIVE

Inhibitors of cellular T4 transport leading to diminished plasma T3 production have been identified as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and indoxyl sulphate in uraemia and bilirubin and non-esterified fatty acids (NEFA) in critically ill patients with hyperbilirubinaemia. We question whether other factors are responsible for the altered thyroid hormone parameters observed in mild illness and during calorie restriction.

PATIENTS

We studied (i) 18 non-uraemic patients with non-thyroidal illness (NTI) (T4 > or = 60, T3 < or = 1.1 and rT3 > or = 0.45 nmol/l) with serum molar ratios of bilirubin:albumin < or = 0.17 and NEFA:albumin < or = 2.6. These molar ratios have been shown to be the minimum ratios which inhibited T4 transport into rat hepatocytes; (ii) four obese euthyroid subjects on 600 kcal/day for 10-14 days. This diet is known to inhibit the unidirectional T4 transport into human liver in vivo.

MEASUREMENTS

We measured iodide production from 125I-T4 by incubating rat hepatocytes with 10% human serum. The deiodination of T4 was used as an index of cellular transport of T4 in vivo.

RESULTS

The mean iodide production from 125I-T4 by rat hepatocytes in the presence of 10% serum from NTI patients (98 +/- 17%, mean +/- SD) was not significantly different from the normals (100 +/- 9%). Calorie restriction in euthyroid obese subjects resulted in a small but significant reduction (-12%) of iodide production. Calorie restriction increased the total serum NEFA by 91%.

CONCLUSIONS

Our study demonstrates that CMPF, indoxyl sulphate, bilirubin and NEFA are not responsible for the inhibition of T4 tissue uptake in patients with mild illness. In addition, studies with calorie restricted obese subjects indicate that high concentration of NEFA during calorie restriction inhibits T4 tissue uptake. This inhibition may partly explain the lower plasma T3 during calorie restriction.