Biological Effects Of Thyroid Hormone Research Articles

Thyroid Hormone in Hepatocellular Carcinoma: Cancer Risk, Growth Regulation, and Anticancer Drug Resistance.

Thyroid hormone (TH) and its receptor (TR) are involved in differentiation, metabolic process, and growth regulation in hepatocellular carcinoma (HCC). The TH/TR complexes are ligand-dependent transcriptional factors, functioning through binding to thyroid hormone response elements (TREs) upstream of the target genes. To date, deciphering the biological effects of TH in cancer progression remains challenging. Several lines of evidence suggest a growth inhibitory effect of TH in liver cancer. Mutation and aberrant expression of TRs are highly correlated with several types of cancers including HCC. Several reports show that TH inhibits cell growth in liver cancer through regulation of cell-cycle-related genes and non-coding RNAs. A case–control study indicates that hypothyroidism is associated with an increased risk of HCC. Moreover, TH/TR suppresses hepatocarcinogenesis via selective autophagy. Conversely, other groups have indicated that TH promotes cancer cell proliferation. In vitro and in vivo experiments show that TH/TR enhances cancer cell migration and invasion, anticancer drug resistance, angiogenesis, and cancer stem cell self-renewal. Adding to the complexity of this issue, non-genomic effects of TH mediated by integrin receptor on cell surface can also modulate several biological functions. Accumulating evidence indicate that regulations by genomic and non-genomic effects of TH overlap. Taken together, these observations suggest that the functions of TH depend largely on cell context, and TH/TR plays a duel role in cancer progression. Therefore, understanding the maze of biological effects of TH has become a necessity when attempting to develop effective therapeutic and preventive strategies in liver cancer.

Genetic evidence that thyroid hormone is indispensable for prepubertal insulin-like growth factor–I expression and bone acquisition in mice

Understanding how bone growth is regulated by hormonal and mechanical factors during early growth periods is important for optimizing the attainment of peak bone mass to prevent or postpone the occurrence of fragility fractures later in life. Using genetic mouse models that are deficient in thyroid hormone (TH) (Tshr(-/-) and Duox2(-/-)), growth hormone (GH) (Ghrhr(lit/lit)), or both (Tshr(-/-); Ghrhr(lit/lit)), we demonstrate that there is an important period prior to puberty when the effects of GH are surprisingly small and TH plays a critical role in the regulation of skeletal growth. Daily administration of T3/T4 during days 5 to 14, the time when serum levels of T3 increase rapidly in mice, rescued the skeletal deficit in TH-deficient mice but not in mice lacking both TH and GH. However, treatment of double-mutant mice with both GH and T3/T4 rescued the bone density deficit. Increased body fat in the TH-deficient as well as TH/GH double-mutant mice was rescued by T3/T4 treatment during days 5 to 14. In vitro studies in osteoblasts revealed that T3 in the presence of TH receptor (TR) α1 bound to a TH response element in intron 1 of the IGF-I gene to stimulate transcription. In vivo studies using TRα and TRβ knockout mice revealed evidence for differential regulation of insulin-like growth factor (IGF)-I expression by the two receptors. Furthermore, blockade of IGF-I action partially inhibited the biological effects of TH, thus suggesting that both IGF-I-dependent and IGF-I-independent mechanisms contribute to TH effects on prepubertal bone acquisition.

Rapid stimulatory effect of thyroxine on plasma membrane transport systems: Calcium uptake and neutral amino acid accumulation in immature rat testis

Although the biological effects of thyroid hormones are mediated by nuclear receptors (genomic mechanisms), interactions with receptors associated with the plasma membrane (non-genomic mechanisms) of target cells are not clear. In this study we investigated the rapid stimulatory effect of thyroxine (T 4) on 45Ca 2+ uptake as well as ionic currents and intracellular messengers involved in the stimulatory action of T 4 in amino acid accumulation in immature rat testes. Results indicated that 10 −9 M or 10 −6 M T 4 was able to increase immediately 45Ca 2+ uptake after 60 s of hormone exposure. These results indicate for the first time that voltage-dependent Ca 2+ channels and ATP-dependent K + channels can be seen as a set-point in the stimulatory effect of T 4 on amino acid accumulation. Apamin-sensitive small-conductance Ca 2+-activated K + channels (SK Ca) and chloride channels were shown to be partially involved in this mechanism. The amino acid accumulation triggered by the PKC pathway suggests a functional link between different ion channel activities and the stimulatory effect of T 4 on amino acid accumulation. In conclusion, we show in this study a rapid and stimulatory effect of T 4 on calcium uptake and on amino acid accumulation, both events initiated at the plasma membrane, which strongly characterizes a non-genomic effect of T 4 in immature rat testes.

Leptin Acute Modulation of the 5′-deiodinase Activities in Hypothalamus, Pituitary and Brown Adipose Tissue of Fed Rats

Leptin and thyroid hormones (TH) have the ability to increase energy expenditure. Biological effects of TH are dependent on thyroxine (T4) to triiodothyronine (T3) conversion by deiodinase type 1 (D1) and type 2 (D2). Leptin has been shown to stimulate the hypothalamus-pituitary-thyroid axis and, also, to modulate 5'-deiodinases in different tissues, depending on energetic status of animals. Here, we examined the acute effects of leptin on hypothalamic, pituitary and BAT D2 and pituitary D1 activities. Male fed rats received a single subcutaneous injection of saline or leptin (8 microg/100 g BW) and sacrificed 2 hours later. Leptin promoted an important decrease in hypothalamic D2 (55% reduction, p <0.001) with no changes in pituitary D2, in concomitance with a 2-fold rise in serum TSH, suggesting that leptin acted at hypothalamus in order to stimulate TRH-TSH axis. In addition, BAT D2 was decreased by 25% (p<0.05). In contrast, pituitary D1 showed a 2-fold increase (p<0.001), indicating that, as demonstrated before for liver and thyroid D1, the pituitary enzyme is also acutely up-regulated by leptin. Serum concentrations of insulin and TH of leptin-injected animals remained unchanged. Regulation of 5'-deiodinases directing the local T3 production, is a mechanism by which leptin may alter hypothalamic, pituitary and BAT functions.

Peroxisome proliferator activated receptor-γ (PPARγ) represses thyroid hormone signaling in growth plate chondrocytes

Peroxisome proliferator activated receptors (PPARs) are DNA-binding nuclear hormone receptors that are upregulated in response to high fat diets. PPARs are structurally related to the type II nuclear receptors, including the thyroid hormone receptors (TRs). To investigate if PPARs modulate TR-mediated terminal differentiation of growth plate chondrocytes, primary cultures of epiphyseal chondrocytes transiently transfected with TRα and PPARγ expression vectors were treated with the PPAR ligands ciglitazone or troglitazone. Forced overexpression of PPARγ decreased TRα1-mediated transcriptional activity and suppressed T3-induced increases in alkaline phosphatase activity and type X collagen expression. Similar effects were observed when the cells were treated with the PPARγ activator ciglitazone or troglitazone. Overexpression of retinoid X receptor-α (RXRα) partially restored not only the inhibition of transcriptional activation by PPARγ but also T3-induced hypertrophic differentiation. These data demonstrate that activation of PPARγ signaling by either addition of PPARγ ligands or overexpression of PPARγ in growth plate chondrocytes inhibits TR-mediated gene transcription and inhibits the biological effects of thyroid hormone on terminal differentiation. The molecular mechanism involved in this inhibition appears to be competition between PPARγ and TRα for limiting amounts of the heterodimeric partner RXR.

A method for efficient production of recombinant thyroid hormone receptors reveals that receptor homodimer–DNA binding is enhanced by the coactivator TIF2

Thyroid hormone receptors (TRs) are ligand-activated transcription factors that mediate the biological effects of thyroid hormone (T3) by binding to thyroid hormone response elements (TREs), typically located in the promoter regions of T3-responsive genes. It is generally held that T3-induced gene activation is mediated by retinoid X receptor (RXR)–TR heterodimers. Although TR homodimers can bind to some TREs, T3 destabilizes this interaction, calling into question the ability of TR to activate transcription in the absence of RXR. TR–DNA binding has been difficult to study in vitro because mammalian TRs are notoriously difficult to produce in Escherichia coli. We considered that this may be due to codon bias. Therefore, we produced TRβ1 in E. coli Rosetta 2(DE3) which contains a plasmid that overexpresses the tRNAs corresponding to the seven rarest E. coli codons. This resulted in an improved yield of full-length TRβ1, which we then used in electrophoretic mobility shift assays. We found the coactivator TIF2 greatly enhances binding of T3-occupied TRs to a subset of TREs, suggesting TRs may activate transcription from these TREs in an RXR-independent manner.

Hormônios tiroideanos: mecanismo de ação e importância biológica

Os hormônios tiroideanos desempenham função importante no crescimento, desenvolvimento e metabolismo de todos os vertebrados. A concentração sérica dos mesmos é controlada pelo TRH, somatostatina e TSH, os quais determinam a taxa de biossíntese e secreção hormonal, bem como por desiodases (principalmente a do tipo I), enzimas que geram, nos tecidos periféricos, aproximadamente 75% do T3 presente no soro, a partir do T4 circulante; as desiodases do tipo II, por outro lado, geram T3 principalmente para os tecidos nos quais são expressas. Os efeitos biológicos dos hormônios tiroideanos são desencadeados por meio da sua interação com receptores nucleares que se apresentam em regiões específicas do DNA, o que determina a ativação ou inibição de seus genes-alvo e o controle da síntese de proteínas específicas. Outras ações dos hormônios tiroideanos são rapidamente desencadeadas (segundos/minutos), o que sugere o envolvimento de mecanismos não genômicos nos efeitos observados.

Both Thyroid Hormone and 9-cis Retinoic Acid Receptors Are Required To Efficiently Mediate the Effects of Thyroid Hormone on Embryonic Development and Specific Gene Regulation in Xenopus laevis

Tissue culture transfection and in vitro biochemical studies have suggested that heterodimers of thyroid hormone receptors (TRs) and 9-cis retinoic acid receptors (RXRs) are the likely in vivo complexes that mediate the biological effects of thyroid hormone, 3,5,3'-triiodothyronine (T3). However, direct in vivo evidence for such a hypothesis has been lacking. We have previously reported a close correlation between the coordinated expression of TR and RXR genes and tissue-dependent temporal regulation of organ transformations during Xenopus laevis metamorphosis. By introducing TRs and RXRs either individually or together into developing Xenopus embryos, we demonstrate here that RXRs are critical for the developmental function of TRs. Precocious expression of TRs and RXRs together but not individually leads to drastic, distinct embryonic abnormalities, depending upon the presence or absence of T3, and these developmental effects require the same receptor domains as those required for transcriptional regulation by TR-RXR heterodimers. More importantly, the overexpressed TR-RXR heterodimers faithfully regulate endogenous T3 response genes that are normally regulated by T3 only during metamorphosis. That is, they repress the genes in the absence of T3 and activate them in the presence of the hormone. On the other hand, the receptors have no effect on a retinoic acid (RA) response gene. Thus, RA- and T3 receptor-mediated teratogenic effects in Xenopus embryos occur through distinct molecular pathways, even though the resulting phenotypes have similarities.

Influence of thyroid hormones on the human ATP synthase beta-subunit gene promoter.

The action of thyroid hormones on the expression of the mitochondrial ATP synthase beta-subunit gene (ATPsyn beta) is controversial. We detected a binding site for the thyroid hormone receptor between -366 and -380 in the human ATPsyn beta gene by DNase I footprint analysis and band-shift assays. However, expression vectors in which the chloramphenicol acetyl transferase (CAT) reporter gene is driven by the 5' upstream region of ATPsyn beta gene were unresponsive to T3 when transiently transfected to HepG2 or GH4C1 cells. CAT constructs driven by the rat phosphoenolpyruvate carboxykinase (PEPCK) or the growth hormone (GH) promoters were stimulated several fold by T3 in parallel experiments. It is proposed that the biological effects of thyroid hormones on the ATPsyn beta expression occur through indirect mechanisms.

Evidence of specific nuclear binding sites for T3 in the mouse cultured fibroblast

Mouse L 929 cultured fibroblasts revealed saturable and high affinity nuclear receptors for triiodothyronine. 125I-T3 bound rapidly to intact cells, with a steady state achieved between 1-2 h at 37 C. By Scatchard estimation, the maximal binding capacity averaged 18 fentomoles of T3 bound per 100 microgram of DNA, which approximated to 2,000 sites per cell nucleus; the apparent equilibrium association constant Ka averaged 4.90 x 109 LM-1. The relative affinity of T3 analogs tested correlated with their respective thyromimetic potency. Limited capacity, high affinity and specificity of T3 nuclear receptors in these fibroblasts were found to have properties similar to those previously observed in other cell lines or tissues. Such cultured fibroblasts may provide a useful experimental model to investigate the intracellular biological effects of thyroid hormones.

Effect of Decreased Hepatic Nuclear L-Triiodothyronine Receptors on the Response of Hepatic Enzymes to LTriiodothyronine in Tumor-Bearing Rats*

The activity of hepatic α-glycerophosphate dehydrogenase (α-GPD) and malic enzyme (ME) was measured in rats bearing the Walker 256 carcinoma to assess the relationship between the reduction in the nuclear L-T3 (T3) receptor concentration which occurs in tumor-bearing (T) rats and the biological effects of thyroid hormones. T3 receptor concentration was uniformly decreased when tumors exceeded 9.5% of body weight. No change in basal α-GPD activity was observed, but ME activity was decreased to between 29–61% of control. The decrease in ME activity did not appear to be related to decreased nuclear T;) receptor concentration since ME was also decreased in T rats with tumors that were too small to effect a decrease in the concentration of nuclear T3 receptors or plasma T4 and T3. The response of both ME and α-GPD to T3 administration in T rats was comparable to that of control despite reduced nuclear T3 receptor and plasma hormone concentrations. When non-T rats were subjected to food restriction to simulate ...