Binding of thyroid hormones to isolated hepatic nuclei from Rana catesbeiana tadpoles

Low-temperature arrest of the triiodothyronine-dependent transcription in Rana catesbeiana red blood cells

We examined possible molecular mechanisms for the low-temperature arrest of T3-induced Rana catesbeiana metamorphosis. Scatchard plots revealed that the ratios of maximum binding capacity/dissociation constant for high-affinity sites of tadpole serum proteins for T3 at 20 and 28 C was 3.3-4.6 times less than that at 4 C, due to the decrease in maximum binding capacity values. Kinetic studies of T3 uptake into tadpole red blood cells demonstrated that the ratio of maximum uptake rate/Michaelis constant at 23 C was approximately 13 times greater than that at 4 C. The process of intracellular transport of T3 into the nucleus was not arrested at 4 C. The ratio of T3 incorporated into nuclei to that taken up into red blood cells was not significantly different at 4, 20, and 28 C, indicating the absence of temperature-sensitive sites in this process. T3 binding to the T3 receptors alpha and beta were not temperature sensitive at least at 4 and 20 C. Transcription of the tr genes, early primary T3 response genes, was activated by 10 nM T3 at 20 and 28 C but was barely detected at 4 C. These results indicate that the major molecular event causing the low-temperature arrest of amphibian metamorphosis occurs after T3 entry into the nucleus but before or during the transcriptional activation of the tr genes. Plasma proteins binding T3 and the cellular thyroid hormone uptake system on the plasma membrane may contribute to the slowing of the incorporation of T3 into nucleus at 4 C by decreasing the uptake velocity of T3.

The type 2 and type 3 iodothyronine deiodinases play important roles in coordinating development in Rana catesbeiana tadpoles

In developing Rana catesbeiana tadpoles, the timing of the thyroid hormone (TH)-dependent metamorphic responses varies markedly among tissues. Yet at any one time these tissues are exposed to the same plasma concentration of TH, suggesting that TH action is regulated in part at the level of the peripheral tissues. A major factor in TH action is the intracellular level of the active TH, T3. This level is dependent not only on the plasma concentration of TH (mostly T4) but also on the intracellular activities of the type 2 5'-deiodinase (D2) and the type 3 5-deiodinase (D3), which are responsible, respectively, for generating and degrading T3. (D1 is not present in this species.) To determine whether differential expression of D2 and D3 among tissues could be a significant factor in the coordination of metamorphic events, the ontogenic profiles of the two enzyme activities and corresponding messenger RNA levels in most tissues of R. catesbeiana tadpoles have been documented. The profiles of D2 expression in tail, hindlimb, forelimb, intestine, skin, and eye differed markedly at both activity and messenger RNA levels, but it was notable that expression was invariably highest in a given tissue at the time of its major metamorphic change. D2 expression was very low in brain and heart and did not vary during development. D2 was not expressed in liver, kidney, or red blood cells. With the exception of red blood cells, D3 expression was detected in all tissues studied. Furthermore, it was evident that in tissues that expressed both deiodinase genes, the two expression profiles were comparable, indicating a potential for tight control of intracellular T3 levels. Direct evidence of the importance of the intracellular conversion of T4 to T3 for TH-dependent metamorphic events was obtained in tadpoles in which endogenous TH synthesis was blocked with methimazole, and the activities of D2 and D3 were inhibited by iopanoic acid. This treatment inhibited metamorphosis. The inhibition could be overcome by the concomitant administration of replacement levels of T3, but not T4. These results strongly support the view that coordinated development in amphibia depends in part on the tissue-specific expression patterns of the D2 and D3 genes, which ensure that the requisite level of intracellular T3 is attained in a given tissue, regardless of the current level of circulating TH, at the appropriate stage of metamorphosis.

Characterization of gamma-glutamyltranspeptidase in the liver of the frog: 3. Response to freezing and thawing in the freeze-tolerant wood frog Rana sylvatica

The freeze tolerant wood frog Rana sylvatica was studied to determine the impact of the freezing and thawing of this frog on the activity of gamma-glutamyltranspeptidase in the liver. On exposure to -2.5 degrees C, for 1, 12 and 24 h, frogs were found to be cool, covered with ice crystals and frozen, respectively. Thawing for 24 h at 4 degrees C recovered the frogs completely. A 45 per cent decrease in the liver weight: body weight ratio was notable after 1 h at -2.5 degrees C, suggestive of an early hepatic capacitance response. A glycemic response to freezing was observed: blood glucose levels exhibited a 55 per cent decrease after 1 h at -2.5 degrees C on cooling; a 10.5-fold increase after 12 h at -2.5 degrees C on the initiation of freezing; and a 22-fold increase after 24 h at -2.5 degrees C in the fully frozen state. Blood glucose levels remained elevated four-fold in the thawed state. Plasma insulin levels were increased twofold in the frozen state and 1.8-fold in the thawed state, while plasma ketone levels were increased 1.8-fold in the frozen state and 1.5-fold in the thawed state. Plasma total T3 levels were decreased by 22 per cent in the frozen state and normalized on thawing. In homogenates and plasma membranes isolated from the livers of Rana sylvatica, the activity of gamma-glutamyltrans-peptidase was found to be elevated at all stages of the freeze-thaw process. After 1, 12 and 24 h at -2.5 degrees C, activities were increased 2.5-, 2.3-, 2.4-fold respectively in the homogenates and 2.5-, 2.2-, 2.4-fold respectively in the plasma membranes. After thawing, activities were still increased 1.9-fold in both homogenates and plasma membranes. In homogenates prepared from the kidneys of Rana sylvatica, the activity of gamma-glutamyltranspeptidase was increased 1.4-fold after 1 h at -2.5 degrees C after which it returned to normal. The role of thyroid hormone in producing the increase in gamma-glutamyltranspeptidase in the liver of Rana sylvatica in response to freezing is discussed as is the significance of the enzyme increase in terms of hepatic cytoprotection and freeze tolerance.

Characterization of gamma-glutamyltranspeptidase in the liver of the frog: 2. Response to season, temperature and thyroid hormone in Rana pipiens

The impact of season and temperature on frog liver gamma-glutamyltranspeptidase was assessed by measuring the activity of this enzyme in plasma membranes isolated from the livers of Rana pipiens obtained as summer and winter frogs; subjected to short-term (3 weeks) temperature acclimation; and subjected to multiple-temperature shifts. Plasma levels of T3 were determined. gamma-Glutamyltranspeptidase was found to be 2.2-fold higher in the summer frog relative to the winter frog; decreased by 44 percent in the summer frog by cold acclimation and increased by 1.7-fold in the winter frog by warm acclimation; and increased by 1.9-fold in the summer frog and 2.8-fold in the winter frog subjected to multiple-temperature shifts. Plasma T3 levels were found to be 42-fold higher in the summer frog relative to the winter frog; decreased by 42 percent by cold acclimation and increased by 2.9-fold by warm acclimation; and decreased by 39 percent and 38 percent in the summer and winter frogs subjected to multiple temperature shifts. T3 replacement during the last phase of the multiple-temperature shift protocol, restored the plasma T3 levels to 75 percent of the control levels and prevented the increase evoked by the multiple-temperature shifts in gamma-glutamyltranspeptidase activity. Indeed, enzyme activity in the T3 replaced state was 19 percent lower than in the control state. The involvement of thyroid hormone as a negative regulator of enzyme activity is discussed.

Thyroid hormone receptors and iodothyronine deiodinases in the developing Mexican axolotl, Ambystoma mexicanum

The Mexican axolotl, Ambystoma mexicanum, is a neotenous salamander that rarely undergoes anatomical metamorphosis, but can be induced to do so by administration of thyroxine (T4). The neoteny appears to be due primarily to low levels of plasma T4 secondary to a low rate of secretion of thyroid-stimulating hormone. However, other factors may also be involved. In anuran amphibia, metamorphosis is accompanied by alterations in thyroid hormone receptor concentration and marked changes in the activities of the iodothyronine deiodinase systems, all of which contribute to enhancing peripheral sensitivity to circulating T4. The present study was designed to assess these functions in the tissues of the axolotl. Putative 3,5,3'-triiodothyronine (T3) receptors were readily detected in axolotl red blood cells, and the receptor number (sites/nucleus) showed a developmental decline, comparable to that seen in anuran amphibia, as the larval cells with a high receptor number were replaced with adult cells with a low receptor number. Saturable T3 nuclear binding sites were also detected in liver, but the receptor number was too low to quantitate. Using a sensitive RIA, T4 and T3 were below detectable levels in juvenile axolotls and T4 was just detectable in some of the adults. Following injection of [125I]T4, [125I]T3 was detected in plasma and liver of adult, but not of juvenile, axolotls. Some 5'-deiodinase activity was consistently detected in preparations of larval skin, and in adult skin and gut, but it was too low to permit kinetic analyses. Activity was not increased following treatment with sufficient T4 to induce anatomical metamorphosis. 5-Deiodinase activity was not detected in any tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of putative thyroid hormone receptor in the brain nuclei of Singi fish, Heteropneustes fossilis (Bloch)

A single injection of [125I]triiodothyronine (T3) with or without stable T3 in Singi fish, Heteropneustes fossilis (Bloch), showed that the fish brain has saturable binding sites and that the specific uptake is 60-70% higher than the nonspecific uptake. The distribution kinetics of [125I]T3 in the serum, whole brain, and brain nuclei after a single injection of the labeled hormone showed that the removal of [125I]T3 from the serum was very rapid with the t1/2 of about 3.3 hr and the incorporation of the hormone into the brain and brain nuclei were very slow and achieve a maximal value after 4-6 hr of postinjection. The binding of [125I]T3 to the isolated brain nuclei of Singi fish was further studied in vitro. Binding was linearly increased with the increasing concentration of the DNA (nuclei). The binding achieved equilibrium between 15 and 20 min at 27 degrees and was stable at least for 1 hr. The binding was reversible in the presence of excess unlabeled T3. Scatchard analysis showed only a single class of binding sites. The mean dissociation constant (Kd) is 2.15 +/- 0.45 x 10(-10) M and maximum binding capacity (MBC) is 0.044 +/- 0.024 pmol/mg DNA. The relative binding affinities of thyroid hormone analogs for T3 sites were as follows: TRIAC greater than T3 greater than TETRAC greater than T4 greater than reverse T3 greater than T2. These findings were similar to those for other animals. Therefore, the nuclear binding sites in Singi fish brain, as demonstrated, may be regarded as thyroid hormone receptors.

Gonadal hormones inhibit the induction of metamorphosis by thyroid hormones in Xenopus laevis tadpoles in vivo, but not in vitro

Although the major hormones controlling amphibian metamorphosis are those of the thyroid, other hormones, notably prolactin and the adrenal steroids, modulate the effects of thyroid hormones (TH). Some authors report that the gonadal steroids stimulate the metamorphic actions of TH whereas others report inhibition. The aims of the present study were to determine the effects of gonadal steroids on TH-induced metamorphosis in Xenopus laevis and to determine the site of action of these steroids. In all cases, hormones were added to the water in which the tadpoles were swimming. The gonadal steroids, testosterone and 17 beta-estradiol, inhibited triiodothyronine (T3)-induced metamorphosis in living, premetamorphic tadpoles of X. laevis. Both steroids, at 3.4 microM, prevented the reduction in body weight and the shrinkage of head and alimentary canal brought about by 1 nM T3. In contrast, 3.4 microM corticosterone stimulated T3-induced metamorphosis. Addition of 100 nM T3 to the medium induced a large reduction in size of X. laevis tails cultured in vitro. The antagonistic effects of testosterone were not reproduced in such cultures, whereas the synergistic action of corticosterone was maintained. Testosterone had no effect upon the specific binding of T3 to X. laevis tail tissue, whereas corticosterone increased such binding. These findings indicate that, while corticosterone stimulates the metamorphic actions of T3 by acting directly in the peripheral tissues, the gonadal steroids, particularly testosterone, inhibit T3 by acting at a more central site. Prolactin is known to antagonize the metamorphic actions of T3 and one such central action could be the stimulation of prolactin synthesis. However, testosterone inhibited the prometamorphic actions of bromocriptine, which stimulates metamorphosis by inhibiting production of prolactin. Thus the central action of testosterone is unlikely to be a stimulation of prolactin production.

Corticosteroid receptors in liver cytosol of the clawed toad, Xenopus laevis: influence of thyroid and ovarian hormones

The glucocorticoid receptor capacity Ro and the dissociation constant Kd were determined in the liver of Xenopus laevis by Scatchard analysis. In 5-year-old female toads Ro was about three times higher than that in males (153.9, 54.3 fmol/mg protein) and Kd was similar in both sexes (4.0, 4.1 nM). Some of the animals used had abnormal enlarged thyroid glands, atrophic ovaries, or both defects in connection with different levels of Ro, but not of Kd, compared to those of normal animals. Females with ovarian atrophy showed significantly lower Ro values, in the same range as in normal males, and a high liver weight. In male and female toads with enlarged thyroid glands and in animals with both defects a significantly higher Ro occurred compared to that of the corresponding group without this abnormality. To study the influence of thyroid hormones on glucocorticoid receptors, young toads (2-3 years old) received injections of 4-phenyl-2-thiouracil, T3, or T4 on 7 consecutive days. Ro and Kd were determined on the following day. Doses of 50 and 500 ng T3 and of 500 and 5000 ng T4 per gram of body weight and day resulted in an increase of Ro up to 250% of the controls. Injections of T3 were more efficient in males than in females. The effect of thyroxine was about the same in both sexes. These observations suggest that thyroid and ovarian hormones exert an influence on glucocorticoid receptor capacity and may belong to the factors which regulate glucocorticoid receptors.

Physicochemical properties of the triiodothyronine nuclear receptor from tadpole liver, intestine and tail fin

The binding of triiodothyronine (T3) to tadpole liver, intestine, and tail fin nuclei was consistent with a single class of binding sites. The KD for the binding of T3 to isolated nuclei from the liver (1.02 nM), intestine (1.40 nM), and tail fin (0.831 nM) nuclei were not significantly different. The number of binding sites for T3 in the nuclei isolated from each tissue were also not significantly different. Sucrose gradient centrifugation of the salt-extracted nuclear T3-receptor complex revealed that the S20,w was not significantly different for the complex from the liver (3.7 S), intestine (3.9 S), or tail fin (3.9 S). Similarly, the Stokes radii of the complex from the liver (3.65 nm), intestine (3.84 nm), and tail fin (3.99 nm) were not significantly different. Molecular weights of 57,000, 64,000, and 67,000 were calculated from the sedimentation coefficients and Stokes radii for the T3-receptor complex from the liver, intestine, and tail fin, respectively. The similarity of the physicochemical properties of the T3-receptor complex from each of the tissues studied is consistent with the hypothesis that the same receptor is capable of inducing tissue-specific biochemical changes.

Nuclear receptors for L-triiodothyronine in trout erythrocytes

We have developed an in vitro assay to evaluate saturable specific binding of triiodothyronine (T3) by erythrocyte (RBC) nuclei isolated from rainbow trout. Our results indicate that the nuclei contain a T3-saturable protein which binds T3 with temperature, and pH dependency, high T3 affinity (Ka = 1.6 X 10(9) M-1), and relative thyroid hormone (TH) analog affinities (TRIAC greater than T3 greater than T4 greater than rT3 greater than T2) which are characteristic of TH receptors in other vertebrates. Our estimate of the maximal T3 binding capacity (MBC) of nuclei isolated from heterogeneous populations of RBCs at different maturational stages (MBC = 3.6 fM/mg DNA; 13 sites/nucleus) was 10-100 times lower than would be expected of a TH-responsive tissue. Differential cell counts revealed that 1% of the RBCs in our trout were immature (pro-RBCs). Pro-RBCs, in contrast to mature RBCs, contain abundant heterochromatin, mitochondria, and polyribosomes, and synthesize hemoglobin. Evaluation of binding data for RBC nuclei taken from trout in which erythropoiesis was stimulated by prior bleeding indicated that MBC was directly proportional to the absolute number of pro-RBC nuclei in the incubation. Our maximum estimate of MBC for pro-RBC nuclei (458 fmol/mg DNA; 1781 sites/nucleus) falls within the range of MBC values reported for other vertebrate TH-responsive tissues. These data indicate that RBCs of rainbow trout contain a nuclear protein (putative receptor) which binds T3 with characteristics similar to the TH receptor of higher vertebrates, and that nuclear T3 binding may be diminished during RBC maturation.

Thyroid hormone binding to isolated trout (Salmo gairdneri) liver nuclei in vitro: binding affinity, capacity, and chemical specificity

Isolated nuclei from trout hepatocytes demonstrated in vitro high-affinity and low-capacity binding for 3,5,3'-tri-iodo-L-thyronine (T3). Binding was reversible; the dissociation rate was 5.7 X 10(-3) min-1 at 15 degrees C. Linear Scatchard plots suggested a single class of noncooperative sites (Kd = 1.4 +/- 0.10 X 10(-10) M; maximum binding capacity (MBC) = 62 +/- 10 fmol/mg DNA). After correction for site degradation, site occupancy by endogenous T3, and the dissociation rate of endogenous bound T3, the MBC was 106 +/- 16 fmol/mg DNA. The T3 affinity exceeded slightly that of the hepatocyte nucleus of the rat; the MBC was lower than for most other vertebrates. The relative binding affinities of thyroid hormone (TH) analogs for the T3 site were: TRIPROP greater than TRIAC greater than methyl-bridged T3 greater than TETRAPROP greater than T3 greater than TETRAC greater than TRIFORM greater than 3,5-dibromo, 3'-isopropyl thyronine greater than L-thyroxine (T4) greater than DL-T4 greater than 3'-isopropyl 3',5'-dimethyl thyronine greater than reverse T3 greater than 3,5-T2. MIT and DIT did not bind at all. This structure-affinity profile was similar but not identical to that of rat liver, indicating considerable but not complete evolutionary conservation of site structure. Parallel studies of T4 binding also indicate a single class of noncooperative sites (Kd = 7.2 +/- 2.4 X 10(-10) M). Both the MBC and the structure-affinity profile for T4 corresponded to those for T3. These observations, combined with the ability of excess T3 or T4 to completely displace both labeled T3 or T4, support a previous suggestion that in teleosts T3 and T4 bind to the same class of nuclear sites. These sites probably represent TH receptors.

Immunofluorescent detection and localization of thyroxine in blood of Rana catesbeiana from early larval through metamorphic stages

Indirect immunofluorescent staining was used to detect and localize thyroxine (T4) in blood smears from individual Rana catesbeiana tadpoles in almost every stage of larval development. Earlier radioimmunoassays revealed a surge of T4 in the blood plasma during metamorphic stages, but plasma T4 concentrations in earlier stages were either very low or below the minimal detectable limits of the assays. With the present immunofluorescent method, T4 was found in plasma of tadpoles throughout the entire larval period from early limb bud stages to the end of metamorphosis. Moreover, T4 was also found in association with cytoplasm and nuclei of red blood cells, particularly nuclei of the new population of adult red cells differentiating during metamorphic stages. In conclusion, thyroid hormone is present in both blood plasma and erythrocytes of R. catesbeiana from early through late stages of larval development.

Binding of thyroid hormones by nuclei of target tissues during the chick embryo development

In the present studies we have compared the ontogeny of the binding of thyroxine (T4) and triiodothyronine (T3) to isolated nuclei from various target tissues of chick embryo. We observed a marked difference between the patterns of Satchard plots, maximal binding capacities (MBC) and association constants (Ka) of T4 and those of T3. Scatchard plots revealed that T4 and T3 had different binding sites. In liver, brain and lung MBCs and Kas of T3 and T4 were rather similar at day 9, but during the following days (12-19) T3 MBCs and Kas showed small changes, whereas T4 MBC markedly increased (4-5-fold) and T4 Ka significantly declined. In liver, for instance, T3 MBC = 395 +/- 19 (day 9) and 489 +/- 66 fmol/mg protein (day 19); T4 MBC = 631 +/- 6.5 (day 9) and 2201 +/- 516 fmol/mg protein (day 19); T4 Ka = 1.92 +/- 0.01 (day 9) and 0.56 +/- 0.21 X 10(8) M-1 (day 19). These data indicate that, during chick embryogenesis, nuclei of target tissues contain multiple T4 binding proteins, but only a single T3 binding site.

Antibodies to nuclear thyroid hormone-binding proteins. Antibody characterization and immunofluorescent localization

To further investigate the mechanism by which thyroid hormones regulate target cell function, we have prepared and partially characterized antibodies to highly purified nuclear thyroid hormone-binding proteins (NTBP). NTBPs were prepared from bovine liver nuclear extracts by bio-specific elution from an affinity gel containing immobilized 3,5,3'-triiodo-L-thyronine (T3). Antibodies (Ab) raised to NTBP in BALB/c mice were assayed for Ab-NTBP complex formation on HPLC TSK SW3000 molecular exclusion gels and found to be species-specific and non-cross-reactive with serum thyroid hormone-binding proteins. Most of the antibody activity was directed against two fractions of molecular weight (MW) 89 000 and 53 000, which were associated with thyroxine (T4)-binding activity. The 89 000 D T4-binding activity was shifted to a higher MW complex when incubated with specific antibody. Indirect immunofluorescence showed antibody activity against discrete, clumped chromatin sites, nuclear envelope and plasma membrane in hepatocytes. Intense fluorescence was also observed in the cells lining the hepatic sinusoids and in the cytoskeleton of bovine aortic endothelial cells in culture. The data suggest that thyroid hormone target cells contain extranuclear loci that share antigenic sites with NTBP and may also represent specific NTBP-like sites of thyroid hormone binding.

3,5,3'-Triiodothyronine receptors and thyroxine 5'-monodeiodinating activity in thyroid hormone-insensitive amphibia

Adult anuran Amphibia and neotenous urodeles, such as Necturus maculosus, rarely respond to thyroid hormone (TH). In this study, the possibility was examined that this lack of response is due either to an inability to convert thyroxine (T4) to 3,5,3'-triiodothyronine (T3) or to the absence of nuclear TH receptors. Following injection of [125I]T4, significant amounts of [125I]T3, analyzed by chromatography, were detected in the serum and liver of Necturus and Rana catesbeiana frog indicating that both possesses a T4 5'-monodeiodinating system. Nuclear binding of T3 was studied in suspensions of purified hepatic nuclei and intact red blood cells (RBC). Analysis of binding data revealed that frog liver nuclei contained two sets of saturable T3 binding sets with affinities comparable to those of the two sets of sites previously demonstrated in tadpole liver nuclei (Galton and Schaafsma, 1983). However, the number of sites per nuclei was small compared to tadpole; expressed as pmol/mg DNA, the maximum binding capacity of the high affinity set of sites (MBC1) was 2.2 +/- 0.31 versus 12.9 +/- 1.80 and MBC2 was 24.9 +/- 4.5 versus 42.2 +/- 54. Receptor number in RBC nuclei was also smaller in frog than tadpole: 90 +/- 26 versus 882 +/- 56 sites/nucleus (Kd less than 10(-11) M in both groups). No comparable high affinity binding sites were detected in Necturus liver, but some sites were found in Necturus RBC. These cells contained 2111 +/- 120 sites/nucleus, more than twice the number found in tadpole.(ABSTRACT TRUNCATED AT 250 WORDS)