triiodothyronine--reverse and 3-3--5-triiodothyroacetic-acid

Thyroid hormones (THs) are produced by the thyroid gland and converted in peripheral organs by deiodinases. THs regulate cell functions through two distinct mechanisms: genomic (nuclear) and nongenomic (non-nuclear). Many TH effects are mediated by the genomic pathway--a mechanism that requires TH activation of nuclear thyroid hormone receptors. The overall nongenomic processes, emerging as important accessory mechanisms in TH actions, have been observed at the plasma membrane, in the cytoplasm and cytoskeleton, and in organelles. Some products of peripheral TH metabolism (besides triiodo-L-thyronine), now termed 'nonclassical THs', were previously considered as inactive breakdown products. However, several reports have recently shown that they may have relevant biological effects. The recent accumulation of knowledge on how classical and nonclassical THs modulate the activity of membrane receptors, components of the mitochondrial respiratory chain, kinases and deacetylases, opened the door to the discovery of new pathways through which they act. We reviewed the current state-of-the-art on the actions of the nonclassical THs, discussing the role that these endogenous TH metabolites may have in the modulation of thyroid-related effects in organisms with differing complexity, ranging from nonmammals to humans.

Topics: Animals; Diiodothyronines; Humans; Signal Transduction; Thyroid Gland; Thyroid Hormones; Thyronines; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

Topics: Chemical Phenomena; Chemistry; Diiodothyronines; Diiodotyrosine; Glucuronates; Humans; Hyperthyroidism; Iodine; Kinetics; Phenyl Ethers; Radioimmunoassay; Sulfates; Thyronines; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

To test the hypothesis that 3,5,3'-triiodothyroacetic acid (Triac) is more active as a TSH suppressor than on peripheral parameters of thyroid hormone action, the following parameters were studied: basal metabolic rate, sleeping energy expenditure (SEE), sex hormone-binding globulin, and cholesterol. In a double blind trial, 14 subjects received during 3 weeks (phase 1) 180 micrograms T4 or 1700 micrograms Triac daily, divided into 3 doses, to suppress thyroidal secretion. The dosage was doubled for the next 3 weeks (phase 2). Under T4 treatment, TSH reached 0.11 mU/L during phase 1 and less than 0.03 mU/L during phase 2. With Triac, a marked TSH inhibition occurred after 1 week (0.17 mU/L), followed by an escape during the following 2 weeks (0.63 mU/L). During phase 2, an almost complete TSH suppression was obtained (0.03 mU/L). Both Triac doses suppressed endogenous thyroid hormone secretion, as evidenced by T4 and rT3 levels. Both substances induced a 2-fold stimulation of sex hormone-binding globulin during phase 2. Serum cholesterol decreased similarly, without affecting the high/low density lipoprotein ratio. T4 increased SEE by 4.1% and 8.5% during phases 1 and 2. Triac failed to induce the expected peripheral metabolic responses of the thyroid hormones, as demonstrated by an unchanged SEE and basal metabolic rate. These results clearly show a preferential action of Triac on TSH suppression.

Topics: Basal Metabolism; Cholesterol; Energy Metabolism; Half-Life; Humans; Kinetics; Male; Respiration; Sex Hormone-Binding Globulin; Sleep; Thyrotropin; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

Steered molecular dynamics simulations of ligand dissociation from Thyroid hormone receptors indicate that dissociation is favored via rearrangements in a mobile part of the LBD comprising H3, the loop between H1 and H2, and nearby beta-sheets, contrary to current models in which the H12 is mostly involved. Dissociation is facilitated in this path by the interaction of the hydrophilic part of the ligand with external water molecules, suggesting strategies to enhance ligand binding affinity.

Topics: Computer Simulation; Ligands; Models, Chemical; Molecular Structure; Receptors, Thyroid Hormone; Structure-Activity Relationship; Time Factors; Triiodothyronine; Triiodothyronine, Reverse

We have investigated the mechanism by which thyroid hormone potentiates IFN-gamma-induced HLA-DR expression. IFN-gamma-induced HLA-DR expression requires activation of STAT1alpha and induction of the Class II trans-activator, CIITA. HeLa and CV-1 cells treated only with L-thyroxine (T4) demonstrated increased tyrosine phosphorylation and nuclear translocation (= activation) of STAT1alpha; this hormone effect on signal transduction, and T4 potentiation of IFN-gamma-induced HLA-DR expression, were blocked by the inhibitors CGP 41251 (PKC) and genistein (tyrosine kinase). Treatment of cells with T4-agarose also caused activation of STAT1alpha. In the presence of IFN-gamma, T4 enhanced cytokine-induced STAT1alpha activation. Potentiation by T4 of IFN-gamma action was associated with increased mRNA for both CIITA and HLA-DR, with peak enhancement at 16 h (CIITA), and 2 d (HLA-DR). T4 increased IFN-gamma-induced HLA-DR protein 2.2-fold and HLA-DR mRNA fourfold after 2 d. Treatment with actinomycin D after induction of HLA-DR mRNA with IFN-gamma, with or without T4, showed that thyroid hormone decreased the t(1/2) of mRNA from 2.4 to 1.1 h. HeLa and CV-1 cells lack functional nuclear thyroid hormone receptor. Tetraiodothyroacetic acid (tetrac) and 3,5,3'-triiodo-thyroacetic acid (triac) blocked T4 potentiation of IFN-gamma-induced HLA-DR expression and T4 activation of STAT1alpha. These studies define an early hormone recognition step at the cell surface that is novel, distinct from nuclear thyroid hormone receptor, and blocked by tetrac and triac. Thus, thyroid hormone potentiation of IFN-gamma-induced HLA-DR transcription is mediated by a cell membrane hormone binding site, enhanced activation of STAT1alpha, and increased CIITA induction.

Topics: Biological Transport; Cell Nucleus; Dextrothyroxine; Diiodothyronines; Drug Synergism; Genistein; HeLa Cells; HLA-DR Antigens; Humans; Interferon-gamma; Interferon-Stimulated Gene Factor 3; Nuclear Proteins; Phosphorylation; Protein Kinase C; Protein-Tyrosine Kinases; RNA, Messenger; Thyroxine; Time Factors; Trans-Activators; Transcription Factors; Triiodothyronine; Triiodothyronine, Reverse; Tyrosine

In a previous investigation, we found that murine MoAb 42C3, raised against human Tg, recognized Tg differently depending upon its level of iodination of Tg. A possible explanation for this finding is that iodine is directly involved with the specific epitope recognized by MoAb 42C3. In the present study, we report that the binding of MoAb 42C3 to iodinated Tg is inhibited by T4, T3, reverse T3 (rT3), triiodothyroacetic acid (triac), diiodothyronine (T2), diiodotyrosine (DIT), but not by thyronine (TO) or tyrosine. The order of inhibition of these iodinated compounds is T4 > T3 > rT3 > triac > T2 > DIT. The MoAb 42C3 does not have the same specificity as the T3, T4-receptor since the order of binding of these iodinated compounds on the receptor differed from the order of their inhibition of this MoAb. Monoclonal antibody 42C3 also recognized non-iodinated Tg that was subsequently iodinated in vitro. It failed to recognize another protein, bovine serum albumin, that was iodinated in vitro by the same method. These results suggest that iodinated tyrosines and thyronines determine the binding specificity of MoAb 42C3. The inhibitory effects of these compounds on MoAb 42C3 depend on their iodine content as well as location of iodine in the aromatic ring.

Topics: Antibodies, Monoclonal; Antibody Specificity; Autoantigens; Diiodothyronines; Humans; Iodine; Receptors, Thyroid Hormone; Serum Albumin, Bovine; Thyroglobulin; Thyronines; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse; Tyrosine

The mechanism of action of thyroid hormones on bone is still not clear. At low concentrations, they stimulate bone formation; at high concentrations, they elicit bone resorption in vitro and in vivo. In the present study we investigated the effect of T3 and T4 as well as their active and inactive analogs (TRIAC, SKF L-94901, rT3, and DIT) on the IGF-I and TNF-alpha content in the medium of UMR-106 rat osteoblastic cells and fetal rat limb bones. In the dose-response studies, a biphasic increase in medium IGF-I was observed in both cells and limb bones, with peak stimulatory concentrations of 10(-8) M for T3 and 10(-7) M for T4 in both systems. At higher concentrations, at which thyroid hormones elicit bone resorption, the stimulatory effect diminished and finally was no longer detectable. The active analogs TRIAC and SKF L-94901 also enhanced IGF-I release in UMR-106 cells. The inactive compounds rT3 and DIT failed to increase IGF-I in these cultures. The protein content of the cell culture wells exposed to high concentrations of thyroid hormones was similar to those containing low concentrations, indicating that the decrease in IGF-I content at high doses was not due to toxic effects. This was also confirmed by trypan blue exclusion. Time course studies with UMR-106 cells revealed a significant increase in medium IGF-I after 2 days of incubation. No significant further increase was observed after this up to 5 days of culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Bone and Bones; Bone Resorption; Culture Media; Culture Techniques; Dose-Response Relationship, Drug; Extremities; Insulin-Like Growth Factor I; Osteoblasts; Osteosarcoma; Rats; Rats, Sprague-Dawley; Thyroid Hormones; Thyronines; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse; Tumor Cells, Cultured; Tumor Necrosis Factor-alpha

125I-Triac was employed to measure hepatic thyroid hormone nuclear receptor (RT) in the rat. The binding properties of 125I-Triac and 125I-T3 were compared in a 0.4 M KCl extract of a liver nuclear preparation. The order in which the stable compounds, Triac, T3, T4 and rT3, competed for 125I-Triac and 125I-T3 binding in liver nuclear extract was similar (Triac greater than T3 greater than T4 greater than rT3), suggesting association of both radioligands with RT. Scatchard plot analysis of specific 125I-Triac and 125I-T3 binding in nuclear extract gave approximately equal estimates of the maximum binding capacity (MBC). However, the binding affinity, as represented by the equilibrium association constant (KA), was higher for 125I-Triac than for 125I-T3 (7-10 X 10(9)M-1 vs 1-3 X 10(9)M-1). To determine the effect of contaminating serum proteins on estimates of MBC and KA, a small amount of dilute rat serum was added to the same nuclear extract preparation. Addition of serum decreased the KA value and markedly increased the MBC values estimated by analysis of 125I-T3 binding data. In contrast, KA and MBC values derived from 125I-Triac binding data were not influenced appreciably by the addition of serum. These data indicate that: 1) both 125I-Triac and 125I-T3 bound to RT in rat liver nuclear extract, 2) the affinity of RT for 125I-Triac is appreciably greater than for 125I-T3, and 3) estimates of RT concentration (MBC) made with 125I-Triac are less sensitive to serum protein contamination than those made with 125I-T3. These properties of 125I-Triac may be useful in efforts to demonstrate RT in tissues that have low RT levels and/or when serum contamination is present.

Topics: Animals; Binding, Competitive; Cell Nucleus; Liver; Rats; Receptors, Cell Surface; Receptors, Thyroid Hormone; Triiodothyronine; Triiodothyronine, Reverse

The metabolism of 3, 5-[3'-125I]triiodothyronin (T3) and 3-[3', 5'-125I]triiodothyronine (rT3) was studied in cultured monkey hepatocarcinoma cells (NCLP-6E), and the deiodinations of these iodothyronines were also investigated in cultured cell homogenates and in rat liver homogenates. The metabolites were analyzed by ion exchange column chromatography. For nonphenolic ring deiodination of 3, 5-[3'-125I]triiodothyronine, the order of the inhibitory effect of excess unlabeled iodothyronine or its analog was as follows: 3,3',5-triiodothyroinine greater than triiodothyroacetic acid greater than tetraiodothyroacetic acid greater than thyroxine. This order did not differ between in the intact cells (NCLP-6E) and their homogenates. The order of effectiveness of the excess unlabeled compounds on phenolic ring deiodination of 3-[3', 5'-125I]triiodothyronine in the intact cells was as follows: tetraiodothyroacetic acid greater than triiodothyroacetic acid, 3, 3', 5-triiodothyronine greater than thyroxine. This order was the same among monkey hepatocarcinoma cell homogenates, rat hepatoma cell homogenates and rat liver homogenates, and triiodothyroacetic acid was obviously more effective than 3, 3', 5-triiodothyronine. It was concluded that 3, 3', 5-triiodothyronine had the highest affinity for nonphenolic ring deiodinase among iodothyronines and their analogs used in the present study and that tetraiodothyroacetic acid and the highest affinity for phenolic ring deiodinase. It seems, therefore, that the metabolites derived from the thyroid hormones might contribute to deiodinations which involve activation and inactivation of the hormones.

Topics: Animals; Cell Line; Chromatography, Ion Exchange; Haplorhini; Iodide Peroxidase; Liver; Liver Neoplasms; Peroxidases; Rats; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

The metabolic role of a number of the metabolites of T4 is unknown. Hence, these iodothyronines, now known to be present in human serum, were tested for their ability to displace [125I]T3 from specific binding sites in isolated pig liver nuclei. Compared with T3 (1.0), the molar inhibition ratios of the analogs tested were: triiodothyroacetic acid, 4.4; T4 6.2; 3.3'-diiodothyronine, 56; 3,5-diiodothyronine, 245; rT3, 264; and 3',5'-diiodothyronine, 60,000. In isolated pig liver nuclei, the Ka for T3 was 1.73 +/- 0.21 X 10(9) M-1 and that for T4 was 0.17 +/- 0.06 X 10(9) M-1. Nuclei stored in liquid nitrogen for up to 8 weeks leaked bound [125I]T3 into the supernatant during the incubation period. No loss of bound [125I]T3 was observed with freshly prepared nuclei. The data indicate that, with the exception of T3 and T4, iodothyronines derived from T4 are unlikely to modulate the interaction of T3 with its receptor unless their perireceptor concentration is significantly greater than their serum concentration.

Topics: Animals; Binding, Competitive; Cell Nucleus; Diiodothyronines; Kinetics; Liver; Receptors, Cell Surface; Structure-Activity Relationship; Swine; Thyronines; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse

The effects of a daily oral dose (1.4 mg) of 3,5,3'-Triiodothyroacetic acid (Triac) on thyroid hormone levels (T4, T3 and rT3) and on the TSH and PRL responses to TRH were studied in 15 normal subjects and 5 hypothyroid patients. There were no significant changes in weight, heart rate, reflex time, or serum concentration of either cholesterol or triglycerides after 6 weeks of Triac administration. However, T4 was significantly reduced to a lower mean level (mean +/- SEM, 7.3 +/- 0.7 to 4.3 +/- 0.6 microgram/dl) in the control group. T3 and rT3 concentrations increased, possibly due to a cross-reaction with Triac in their respective RIAs. The peak TSH response to TRH in the normal subjects was 17.6 +/- 3.4 muU/ml and fell significantly to 2.0 +/- 0.8 muU/ml after Triac administration. In the hypothyroid subjects the mean serum TSH level was significantly reduced from 136 +/- 66 to 12.6, 10.5, and 11.6 muU/ml in the weeks after Triac administration. The mean peak response of both TSH and PRL after TRH (206 muU and 44.8 ng/ml, respectively) declined significantly to 63.4 muU/ml and 24 ng/ml. It was concluded that this dose of Triac partially inhibits the synthesis and secretion of TSH and PRL without any major peripheral metabolic effects.

Topics: Adult; Female; Humans; Hypothyroidism; Male; Prolactin; Reference Values; Thyroidectomy; Thyrotropin; Thyrotropin-Releasing Hormone; Thyroxine; Triiodothyronine; Triiodothyronine, Reverse