Relationship of receptor affinity to the modulation of thyroid hormone nuclear receptor levels and growth hormone synthesis by L-triiodothyronine and iodothyronine analogues in cultured GH1 cells

The c-erb-A protein is a high-affinity receptor for thyroid hormone

Hormone binding and localization of the c-erb-A protein suggest that it is a receptor for thyroid hormone, a nuclear protein that binds to DNA and activates transcription. In contrast, the product of the viral oncogene v-erb-A is defective in binding the hormone but is still located in the nucleus.

The c-erb-A gene encodes a thyroid hormone receptor

The cDNA sequence of human c-erb-A, the cellular counterpart of the viral oncogene v-erb-A, indicates that the protein encoded by the gene is related to the steroid hormone receptors. Binding studies with the protein show it to be a receptor for thyroid hormones.

Does short-term fasting desensitize human somatotrophs to oral L-dopa stimulation?

To investigate whether short-term fasting influences hormone release from human somatotrophs and lactotrophs, two oral L-dopa tests, using 0.5 g L-dopa, were carried out in seven normal subjects. The first test was performed after an overnight fast (8 h), the second after an additional 48 h of total food withdrawal. Basal blood glucose declined by 21 +/- 2% (mean +/- SEM) between the two tests (P less than 0.001). Free T3 in serum (FT3) fell by 32 +/- 4% (P less than 0.001). GH responsiveness to L-dopa--as reflected by the GH incremental area (ng/ml X min)--decreased from 588 +/- 162 to 24 +/- 240 (P less than 0.02). The latter response area did not even differ significantly from zero. Food deprivation failed to affect the lactotrophs, inasmuch as similar PRL decremental areas (ng/ml X min) were obtained in response to L-dopa before (704 +/- 165) and after (686 +/- 128) the prolonged fast. In order to prevent blood glucose from falling significantly during the period of food deprivation eight additional normal subjects were given 9 small oral doses of glucose (each dose 0.5 g/kg) at regular intervals between the two L-dopa tests. This resulted in similar FT3 concentrations before and after the prolonged fast. Furthermore, glucose supplementation restored normal GH responsiveness to L-dopa, as evidenced by similar GH incremental areas before and after the glucose fast. These results imply that short-term fasting desensitizes human somatotrophs to exogenous L-dopa. The finding that the decreased GH responsiveness returns to normal if a decline in FT3 is prevented by oral glucose supplementation, suggests that changes in FT3 might contribute to the abolished GH responsiveness to L-dopa in fasting.

Nuclear thyroid hormone receptors in cultured human fibroblasts: improved method of isolation, partial characterization, and interaction with chromatin

In order to characterize the nuclear thyroid hormone receptors in human tissue, an improved method for isolation of nuclei from cultured human fibroblasts was developed. This method provided nuclei with a protein/DNA ratio of 2.8 and recovery of 42%. The purity of nuclei was verified by phase contrast and electron microscopy, which showed normal appearance of chromatin structure. Nuclear binding assay was performed by incubation of whole cells at 37 degrees C or isolated nuclei at 22 degrees C with L-triiodothyronine (T3). In both cases, an affinity constant (Ka) of 2.0-3.0 X 10(10) M-1 and an average binding capacity of 41 femtomoles of T3/100 micrograms DNA (3,100 binding sites/nucleus) were obtained. During incubation of the nuclei, 13% to 16% of receptors that had an identical Ka was released into the medium. Salt extraction recovered 85% to 90% of the receptors, which had a Ka of 4.5 X 10(10) M-1 and the capacity of 0.13 pmol of T3/mg protein. The Ka fo. L-thyroxine (T4) was seven to 18 times lower than that for T3, but the capacity was the same in isolated nuclei, receptors released during incubation of nuclei, and in salt-extracted receptors. Of the iodothyronines examined, affinity for triiodothyroacetic acid was the highest, followed by L-T3, D-T3, L-T4. Isokinetic glycerol gradient analysis revealed that salt-extracted receptors had a sedimentation coefficient of 3.4 S, whereas micrococcal nuclease digested receptors showed two major (6.0 to 6.5 and 12.5 S), and two minor (17 and 19 S) peaks. These results were virtually identical to those obtained with rat liver nuclei analyzed in parallel studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone responses to GRF 1-29 in patients with primary hypothyroidism before and during replacement therapy with thyroxine

It is well known that hypothyroidism is frequently associated with impaired GH responses to different stimuli. In the present study we have evaluated GH responses to GH-releasing factor (GRF) in patients with primary hypothyroidism before and during T4 replacement therapy. Fourteen patients (age range 26-60 years) underwent two GRF tests (1 microgram/kg) before and during replacement therapy (150 micrograms/d). Administration of T4 increased peak GH responses to GRF in 9 patients and in the group as a whole (mean +/- SEM, 17.0 +/- 2.8 vs 32.6 +/- 5.7 mU/l, P less than 0.02). When the data are analysed by means of area under the curve (AUC), the GH response to GRF was increased by T4 in 10 patients and in the group as a whole (mean +/- SEM, 51.7 +/- 14.3 vs 101.5 +/- 28.1, P less than 0.02). These data indicate that thyroid hormone replacement therapy enhances the responsiveness of the somatotroph to GRF 1-29 in patients with primary hypothyroidism.

Stereospecific transport of triiodothyronine to cytoplasm and nucleus in GH1 cells

We have recently demonstrated substantial stereospecific nuclear/cytosolic free triiodothyronine (T3) gradients within T3 responsive rat tissues in situ. These studies have now been extended to examine T3 transport in a rat pituitary tumor cell line, GH1. L-T3 had a 7.6-fold higher affinity for the nuclear receptor when assayed in whole cell incubations in comparison to isolated nuclei, though D-T3 affinity was not altered under these conditions. An apparently higher number of receptors for D-T3 was explained by racemic contamination of the isotopes used. Measurement of free hormone concentration ratios for both enantiomers revealed a small step up from medium to cytosol for L-T3 (1.65) but a reverse ratio for D-T3 (0.46). The nuclei were able to concentrate both enantiomers, though stereospecificity was maintained (nucleus/cytosol, L-T3, 4.5, D-T3 1.7). Transport of L-T3 at both boundaries could be inhibited by monodansylcadaverine. Thus, stereospecific transport functions are found within GH1 cells, though the magnitude of the free nucleus/cytosol gradient is reduced from those seen in rat tissues in situ.

Metabolic effect of 3,3',5'-triiodothyronine in cultured growth hormone-producing rat pituitary tumor cells. Evidence for a unique mechanism of thyroid hormone action

Physiologic levels of 3,3',5'-triiodothyronine (rT3) are generally believed to have minimal metabolic effects in the pituitary gland and other tissues. In the present studies, the regulatory role of rT3 and other thyroid hormones on iodothyronine 5'-deiodinase (I5'D) activity was studied in a growth hormone-producing rat pituitary tumor cell line (GH3 cells). I5'D activity was thiol-dependent and displayed nonlinear reaction kinetics suggesting the presence of two enzymatic processes, one having a low Michaelis constant (Km for thyroxine [T4] of 2 nM) and a second with a high Km value (0.9 microM). Growth of cells in hormone-depleted medium resulted in a two- to 3.5-fold increase in low Km I5'D activity (P less than 0.001). The addition of thyroid hormones to the culture medium resulted in a rapid, dose-dependent inhibition of low Km I5'D activity with the following order of analogue potency: rT3 greater than or equal to T4 greater than 3,5,3'-triiodothyronine (T3). Using serum-free culture conditions, rT3 was approximately 50 times more active than T3. These inhibitory effects were noted within 15 min of hormone addition and could not be attributed to substrate competition with T4. These findings suggest that the control of T4 to T3 conversion by thyroid hormones in the anterior pituitary gland is mediated by a unique cellular mechanism that is independent of the nuclear T3 receptor; and under some circumstances, rT3 may play a regulatory role in controlling this enzymatic process.

Thyroid hormone. Aldosterone antagonism in cultured epithelial cells

Thyroid hormone (T3) has been demonstrated to inhibit the action of aldosterone on sodium transport in toad urinary bladder and rat kidney. We have examined the effect of T3 on aldosterone action and specific nuclear binding in cultured epithelial cells derived from toad urinary bladder. In cell line TB6-C, addition of 5 X 10(-8) M T3 to culture media for up to 3 days results in no change in short-circuit current or transepithelial resistance. This concentration of T3 completely inhibits the maximal increase in short-circuit current in response to 1 X 10(-7) M aldosterone. The inhibition can be demonstrated with 18 h preincubation or with simultaneous addition of T3 and aldosterone. The half-maximal concentration for the inhibition of the aldosterone effect is approx. 5 X 10(-9) M T3. T3 has no effect on cyclic AMP-stimulated short-circuit current in these cells. The effect of T3 on nuclear binding of [3H]aldosterone was examined using a filtration assay with data analysis by at least-squares curve-fitting program. Best fit was obtained with a model for two binding sites. The dissociation constants for the binding were K'd1 = (0.82 +/- 0.36) X 10(-10) M and K'd2 = (3.2 +/- 0.60) X 10(-8) M. The half-maximal concentration for aldosterone-stimulated sodium transport in these cells is approx. 1 X 10(-8) M. Analysis of nuclear aldosterone binding in cells preincubated for 18 h with 5 X 10(-8) M T3 showed a K'd1 = (0.15 +/- 0.10) X 10(-10) M and K'd2 = (3.5 +/- 0.10) X 10(-8) M. We conclude that T3 inhibits the action of aldosterone on sodium transport at a site after receptor binding in the nucleus.

The binding of thyroid hormone receptors to DNA

The behaviour of tri-iodothyronine (T3)- and thyroxine (T4)-receptor complexes when bound to native DNA-cellulose is reported. Equal and large proportions of both T3- and T4-receptor complexes bind to DNA but although T3-receptor complexes are 99% recoverable by 0.5 M NaCl buffer elution, only 60-70% of the T4-receptor complexes are regained. The balance appears as free T4, apparently released as the T4-receptor complexes bind to the DNA whilst the corresponding receptor remains bound. This effect is independent of T4-receptor complex/DNA ratio up to ca. 4 fmol/micrograms DNA, of the presence of an equal amount of unoccupied receptor and of an eight-fold concentration range of both T4-receptor complex and DNA at a fixed ratio, in the cellulose matrix. Pre-formed receptor-DNA material, likewise, only accepts some 60% of the expected quantity of T4 whereas the capacity for T3 appears to be similar to that of free receptors.

Molecular dynamics of 3,3',5-triiodo-L-thyronine in model membranes: a spin label study

A spin-labeled derivative of 3,3',5-triiodo-L-thyronine, 3-[( alpha-carboxy-4-(4-hydroxy-3-iodophenoxy)-3,5-diiodophenethyl++ +] carbamoyl)-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (SL-T3) has been synthesized. Evaluation of its binding to nuclei after incubation with rat pituitary tumor GH3 cells at 37 degrees C showed that it bound to nuclei with a 18% potency of that of T3. The dynamic interaction of SL-T3 with multilamellar vesicles prepared from dimyristoylphosphatidylcholine (DMPC) was investigated using electron spin resonance techniques. At 31 degrees C, the lateral diffusion constant of SL-T3 in DMPC membranes was found to be 3.0 X 10(-8) cm2/s as determined by the ESR line-broadening method. The temperature dependency of the ESR spectrum of SL-T3 in DMPC multilamellar vesicles showed a break at 23.5 degrees C, which is close to the main phase-transition temperature, 23.7 degrees C, of DMPC membranes. This suggests that the motion of the probe reflects the motion of phospholipids in DMPC membranes, and that the probe itself does not perturb the membrane structure. SL-T3 appears to be a useful probe for studying the motion of thyroid hormone in the plasma membrane of responsive cells.

Pituitary nuclear triiodothyronine receptors during development in the rat

Studies were undertaken to measure the pituitary nuclear triiodothyronine (T3) binding capacity (BC) during development in the rat. BC was measured in 0.4 M NaCl-solubilized receptors in 5-, 14-, 20-, 27-, 30-, 40-, 50-day-old rats and in adult animals. Results indicate that BC is lower in 5-day-old rats than in adult animals (0.397 +/- 0.02 vs. 0.797 +/- 0.06 pmol T3/mg protein) (mean +/- SE) (P less than 0.01). Hypothyroidism, induced during the neonatal period or in the adult animal, results in a significant decrease in BC (P less than 0.01) when compared with control rats. However, treatment of hypothyroid animals with T3 (0.4 micrograms/100 g body wt) for a period of 7-14 days significantly increased BC as compared with hypothyroid rats without apparent changes in Ka. The relative affinities of various thyroid hormone analogues for the nuclear receptor were also measured in the adult animal. The affinities of these analogues are in the following order: TRIAC greater than L-T3 greater than D-T3 greater than L-T4 greater than D-T4 greater than DIMIT greater than L-T2 greater than rT3. The present findings demonstrate the presence of high affinity nuclear T3 binding sites in the pituitary of neonatal rat and could thus account for the effects of thyroid hormones on GH synthesis and TSH secretion observed in these animals.

Thyroxine action on the rat liver nuclear thyroid-hormone receptors. Binding of thyroxine to the nuclear non-histone protein and induction of mitochondrial alpha-glycerophosphate dehydrogenase activity

The possibility that thyroxine (T4) itself exerts the hormonal effect in vivo on the rat liver nuclear receptor was studied with the aid of iopanoic acid (IOP), an inhibitor of the conversion of T4 into tri-iodothyronine (T3). After administration of 2.4 micrograms of T4/100 g body weight to hypothyroid rats for 7 days, T4 and T3 concentrations in serum and in the liver nuclear non-histone protein (NHP) were all increased to the hyperthyroid range. Hepatic mitochondrial alpha-glycerophosphate dehydrogenase (alpha-GPD) activity and DNA content increased significantly. The equilibrium association constant (Ka) of the nuclear T3 receptor was unchanged and the maximal binding capacity (Cmax.) increased 1.4-fold. Simultaneous administration of IOP (5 mg/100 g body weight) to the rats given 2.4 micrograms of T4/100 g body weight completely blocked the conversion into T3. The serum T4 was even more increased, whereas the serum T3 decreased to the hypothyroid range. Although the NHP-bound T4 was at a concentration comparable with the rats given T4 alone, no NHP-bound T3 was detected. Yet the alpha-GPD activity was elevated 2.8-fold and the DNA content increased to the same extent as observed in the rats given T4 alone. The Ka and Cmax. of the nuclear receptor were significantly decreased. After administration of 48 or 480 micrograms of T4/100 g body weight for 3 days, serum T4 and T3 were markedly increased. The NHP-bound T3 was also increased, but no NHP-bound T4 was detected. The alpha-GPD activity was markedly elevated, but the DNA content was unchanged. The Cmax. per g of liver was increased, whereas the Ka remained unchanged. Simultaneous administration of IOP to these animals could not completely block the T4 conversion. The observed hormonal effects in the absence of nuclear T3 indicate that T4 possesses the intrinsic hormonal activities on the rat liver. T4 is less potent in induction of alpha-GPD activity but as potent in increment of hepatic DNA as T3. Although the binding site for T4 is not fully characterized, it appears to be acidic NHP. T4 is an active hormone, yet is also a prohormone of T3, offering the closest analogy with testosterone.

Association of thyroid hormone receptors with chromatin

A large body of circumstantial evidence indicates that receptors located in nuclei of T3 responsive tissues represent a site of initiation of thyroid hormone action at the cellular level. Partial characterization of T3 receptors indicates that these proteins are monomeric structures in nuclei and are chromatin-associated non-histone proteins. Treatment of rat liver nuclei with either pancreatic DNase I or micrococcal nuclease releases T3 receptors from nuclei in two forms: a predominant (95 400 Mr; 5.5-6.0S) and a minor (265 000-365 000 Mr; 12.5S) nucleoprotein complex. Similar structures are excised from rat kidney, brain, and heart nuclei and from GH1 pituitary cell nuclei by micrococcal nuclease digestion. These endonuclease-excised receptor-containing complexes are significantly larger than the salt-extracted receptor (50 000 Mr; 3.5S). The presence of DNA and other non-receptor proteins in these structures indicates that T3 receptors probably function within multimeric complexes in vivo. Although T3 receptors appear to be associated with DNA between nucleosomes, i.e. linker DNA, it is not entirely clear whether all or only a fraction of T3 receptors interact with nucleosomal components. The 12.5S receptor-containing nucleoprotein complex may represent T3 receptors in association with linker DNA and nucleosomal components. T3 receptors do not appear to be uniformly distributed to all chromatin fractions, but are associated with structures having characteristics of transcriptionally active chromatin. They are found in a region of chromatin which is enriched in RNA polymerase activity, rapidly labeled RNA and non-histone proteins, and depleted of histone Hl. This region is also highly sensitive to both micrococcal nuclease and pancreatic DNase I digestion. The association of receptors with transcriptionally active chromatin, however, must be considered provisional until additional details of the precise receptor-chromatin interaction have been established. The recent demonstration of a 20-fold increase in a specific hepatic mRNA four hours following administration of T3 to hypothyroid rats indicates that thyroid hormone potentially has very rapid effects on hepatic gene expression. However, significant changes in nuclear protein phosphorylation, nuclear protein composition, and chromatin structure have not been detected within this four-hour period. Thus, effects of T3 on hepatic gene expression are brought about by local and presumably subtle changes in nuclear function.

Multihormonal induction of alpha 2u-globulin in an established rat hepatoma cell line

A subclone of the FU5-5 rat hepatoma cell line has been isolated which is inducible more than several hundred fold for the 20,000 dalton form of the major rat urinary protein alpha 2u-globulin. The basal relative synthetic rate (RSR) in growth medium containing 10% fetal calf serum was less than 2 X 10(-6) of total protein synthesis. Both dexamethasone and insulin were necessary for induction, and yielded a maximum induced RSR of 4-8 X 10(-3). Triiodothyronine (T3), dihydrotestosterone (DHT), rat growth hormone (GH), and estrogen, all of which have been shown to influence the induction of alpha 2u-globulin in the intact rat, were without effect on the cell line. A factor present in fetal calf serum was also necessary for maximum induction, since dexamethasone plus insulin in serum-free medium raised the RSR to only 3 X 10(-5); exogenous T3, GH, and DHT could not substitute for this serum factor. The kinetics of induction by dexamethasone were slow, with a lag of approximately 48 hr followed by a period of increasing RSR for 6-20 days. Removal of dexamethasone from induced cells led to an exponential decline in the RSR (t 1/2 15 hr). The concentrations of dexamethasone and insulin that could yield half maximum induction were 5 X 10(-8)M and 3 X 10(-11)M, respectively. Higher concentrations of insulin, although still in physiological range (10(-9)M), inhibited induction. At yet higher insulin levels, beyond the physiological range, alpha 2u-globulin synthesis returned to maximum values. The lack of DHT, T3, and GH requirement for alpha 2u-globulin induction in this cell line may mean that a regulatory aberrancy has occurred in this transformed cell line, or, alternatively, that these hormones act indirectly in the intact animal. This cell line should prove useful for the study of the molecular events associated with alpha 2u-globulin induction and for genetic approaches to the problem of multihormonal regulation of gene expression.

Triiodothyronine and adipose conversion of OB17 preadipocytes : binding to high affinity sites and effects on fatty acid synthetizing and esterifying enzymes

Cells of a preadipocyte clonal line (ob 17) isolated from epididymal fat pad of ob/ob mouse possess high-affinity binding sites for triiodothyronine. A single class of sites was found on growing and early confluent cells (KD 0.14 +/- 0.025 nM ; 5,000 +/- 600 sites per cell). A two-fold increase in the number if T3 binding sites occurs during adipose conversion, with no significant change in KD values. The order of potency of structural analogs to compete with 125I-T3 is in favor of nuclear binding sites. A correlation was obtained 3 between this order of potency and the ability of the analogs, included on a long-term basis to confluent cells, to increase 14 C-acetate incorporation into lipids, suggesting an enhancement of de novo fatty acid synthesis, This hypothesis was supported by increased activity levels of fatty acid synthetase after chronic exposure to 1.5 nM triiodothyronine. Under these conditions activity levels of acid:CoA ligase and glycerol-3-phosphate dehydrogenase were also increased significantly. Inclusion of bromodeoxyuridine as a differentiation-blocking agent in the culture medium of growing cells decreases drastically the T3 effects, favoring the role of the latter hormone as amplifier of specific phenotypes expressed during adipose conversion. These results show that ob17 cell line should be an useful tool to study the role of thyroid hormones in the regulation of fatty acid synthesis and esterification in adipose cells.

Therapy of primary hypothyroidism with L-triiodothyronine: discordant cardiac and pituitary responses

Cardiac systolic time intervals were studied in ten patients with primary hypothyroidism before and during therapy with increasing doses of oral L-triiodothyronine (L-T3). Therapy was increased sequentially from 10, 20, 25 to 50 microgram of L-T3 daily on a monthly basis. On L-T3, 20 to 25 microgram/day, cardiac systolic time intervals and other peripheral responses to thyroid hormone including serum cholesterol concentration, serum creatine phosphokinase (CPK) activity, and basal metabolic rate had normalized. However, serum thyrotrophin (TSH) levels and peak TSH responses to thyrotrophin-releasing hormone (TRH) remained elevated on these doses of L-T3. As the dose of L-T3 was increased from 20 to 50 microgram/day, mean basal serum TSH levels decreased from 55 to 16 microunits/ml, and the peak TSH response to TRH decreased from 243 to 58 microunits/ml (P less than 0.001) while systolic time intervals did not decrease further. Changing to L-thyroxine (L-T4) therapy at this point resulted in further suppression of TSH secretion, without significantly altering systolic time intervals or the other peripheral responses to thyroid hormone. These data suggest (a) that some biological responses to thyroid hormone were normalized on lower doses of L-T3 than were required to normalize TSH secretion, and (b) that higher doses of L-T3 or substituting L-T4 therapy could suppress TSH secretion further without altering the other peripheral responses to thyroid hormone.

Thyroid hormone receptor-containing fragment released from chromatin by deoxyribonuclease I and micrococcal nuclease

Limited deoxyribonuclease I and micrococcal nuclease digestion of hepatic nuclei from euthyroid rats injected with 125I-labeled triiodothyronine ([125I]T3) releases a discrete [125I]T3-labeled chromatin fragment (5.8S) which is larger than the T3 receptor (3.5S). These results suggest the T3 receptor is associated with a restricted fraction of hepatic chromatin that has a nuclease sensitivity characteristic of transcriptionally active chromatin.