Interaction of thyroid and glucocorticoid hormones in rat pituitary tumor cells. Specificity and diversity of the responses analyzed by two-dimensional gel electrophoresis

Prolactin-deficient GH3B3 cells are defective in the utilization of the endogenous prolactin promoter yet are fully competent to initiate transcription from a transfected prolactin promoter

Transcription of the prolactin (PRL) gene has been analyzed in wild-type D6, PRL-deficient B3, and revertant r16 GH3 cells. Levels of processed nuclear transcripts from the PRL gene were substantially reduced in the deficient line compared to wild-type cells and returned to greater than wild-type levels in the revertant line. Rare PRL transcripts in the deficient line contained the same 5' end found on transcripts in wild-type and revertant cells as judged by primer extension and S1 nuclease protection assays, implying that the cells are deficient in utilization of the normal wild-type promoter. Deficient cells also contained wild-type levels of the PRL- and growth hormone-specific transcription factor pit-1/GHF-1, and no difference was found in the ability of extracts from wild-type and deficient cells to retard various restriction fragments from both the proximal and the distal PRL promoter regions. The deficient and wild-type cells were equally competent in initiating transcription from a transfected rat PRL promoter containing both the distal and proximal promoter elements. These observations imply that PRL-deficient cells are not defective in a trans-activating factor functioning on these PRL promoter fragments (trans model). Rather, inefficient use of the PRL promoter in the variant cells may reflect an increased methylation state of the PRL gene itself (cis model).

Thyroid hormone-inducible monoamine oxidase inhibitor in rat liver cytosol

An endogenous inhibitor of monoamine oxidase (MAO) was separated by gel-filtration from 105,000 g supernate of T4-treated rat liver cytosol. The inhibition by this inhibitor was concentration-dependent and more potent for A-form MAO than for B-form MAO. The mode of inhibition was competitive either with 5-hydroxytryptamine or beta-phenylethylamine. The molecular weight of this inhibitor was estimated to be 600-700 by gel filtration. The pI value was determined to be 3.0 by isoelectric focusing. This inhibitor was proved to be heat-stable and resistant to protease treatment. MAO inhibition activity was much lower in the cytosol of thyroidectomized, non-T4-treated rats than T4-treated rats, suggesting that this inhibitor is induced by thyroid hormone T4. MAO activity in rat liver might be regulated by the level of this inhibitor.

Dexamethasone control of growth hormone mRNA levels in GH3 pituitary cells is cycloheximide-sensitive and primarily posttranscriptional

To clarify the mechanism of growth hormone (GH) gene activation by glucocorticoids in GH3 pituitary cells, GH mRNA accumulation in nuclear and cytoplasmic compartments was measured in the presence and absence of cycloheximide. In dexamethasone-treated cells, levels of GH mRNA were increased in the nucleus by 6 h and in the cytoplasm by 12 h. Dexamethasone treatment caused a 5- to 24-fold rise in total GH mRNA levels by 48-72 h. The differential elevation of nuclear levels of GH mRNA relative to the amount of cytoplasmic GH mRNA persisted for 48 h. A transient accumulation of GH mRNA in the nucleus was followed by a brief rise in cytoplasmic GH mRNA levels in GH3 cells treated simultaneously with dexamethasone and cycloheximide. In GH3 cells pretreated for 2 h with cycloheximide, the rise in nuclear and cytoplasmic GH mRNA levels mediated by dexamethasone was blocked completely. Levels of glucocorticoid receptor were unaffected by cycloheximide. These data suggest that the stimulation of GH mRNA levels by glucocorticoids is initiated within the nucleus and that cycloheximide-sensitive events are essential for this stimulation to occur. To assess the importance of GH gene transcriptional activation by glucocorticoids, nuclear transcription run-on reactions and assays of GH promoter activity in an aminoglycoside 3'-phosphotransferase (Neo) fusion gene within stably transformed GH3 cells were performed. Evidence for a weak, transient transcriptional activation of the GH gene by dexamethasone in nuclear run-on assays was obtained. Consistent with this idea, a 30-72 h exposure to dexamethasone raised levels of Neo mRNA in GH-Neo GH3 cell transformants by less than or equal to 2-fold. We conclude that glucocorticoid stimulation of GH mRNA in GH3 cells requires ongoing protein synthesis and can occur largely independently of GH gene transcriptional activation.

Thyroid hormone regulation of nuclear-encoded mitochondrial inner membrane polypeptides of the liver

The effects of thyroid hormone on nuclear-encoded mitochondrial inner membrane proteins were investigated by in vitro translation of the endogenous mRNA present in a postmitochondrial fraction from the livers of rats treated in vivo with hormone. The levels of the mRNAs were estimated by quantitative immunoabsorption of the translation mixture. Total protein synthesis was increased 2.6-fold after 4 days of in vivo hormone treatment, but only 10-15% of the polypeptides were dramatically altered (greater than 5-fold). Among the most highly elevated were cytochrome c1 (greater than 10-fold increase) and the Rieske iron-sulfur protein of the cytochrome bc1 complex. Other inner membrane proteins (core protein 1, beta subunit of F1 ATPase, subunit IV of cytochrome oxidase, 3-hydroxybutyrate dehydrogenase) and non-mitochondrial proteins (rat serum albumin, beta 2-microglobulin) were not altered significantly by hormone treatment. Cytochrome c1 and the Rieske protein increased after 12 h of hormone treatment, a relatively early response in mammalian mitochondrial biogenesis. The possible significance of this response for the regulation of mitochondrial synthesis and assembly is discussed.

Dexamethasone blocks adaptive increase of Na+-Pi cotransport in renal brush border membrane elicited by thyroid hormone

Dexamethasone administered to rats blocks and/or reverses adaptive increases in the rate of Na+-Pi cotransport, and also in the Na+-dependent binding of [14C]-phosphonoformic acid (PFA) by renal brush border membrane (BBM) vesicles elicited by thyroid hormone (T3). In contrast, dexamethasone had no effect on Na+-independent binding of [14C]-phosphonoformic acid, on Na+-dependent transport of D-glucose or on Na+-dependent binding of phlorizin by BBMV which indicates that its inhibitory effect is specific for Na+-Pi cotransport system of BBM. These findings suggest that glucocorticoids antagonize T3-elicited adaptive enhancement of Na+-Pi cotransport in renal proximal tubules by blocking the T3-stimulated de novo synthesis of Na+-Pi symporters and/or their insertion into BBM.

Thyroid hormone transcriptional regulatory region of the growth hormone gene

By deletion-transfection analysis, a region of the rat growth hormone gene has been identified which directs accurate, thyroid hormone responsive transcriptional initiation in vivo. In addition, a thyroid hormone-responsive DNase I hypersensitive domain containing three discrete hypersensitive sites has been identified near the GH promoter. One site is coincident with the TATA homology, and the others lie approximately 150 nucleotides 5' and 3' of this sequence. The TATA and 5' flanking DNA hypersensitive sites are located in the region of the gene which promotes hormone-responsive gene transcription. Based on these results, it is possible that the molecular basis for thyroid hormone induction of GH gene transcription includes binding of the occupied receptor to chromatin sites flanking the TATA homology, promoting binding of the TATA activating protein to this sequence. Together, these events may enhance the rate of RNA polymerase II initiation at the promoter.

Expression of the 31-kD stress protein in rat myoblasts and hepatocytes

Rat tissue cultures cells respond to stress by inducing the synthesis of about 20 proteins, including two low-molecular-weight species of about 31 kD and 27 kD. We have cloned a cDNA for the 31-kD protein. This protein is induced in myoblasts and hepatoma cells in response to a 43 degrees C heat shock, or exposure to sodium arsenite or cadmium chloride salts. Furthermore, this protein is superinduced in hepatoma cells conditioned to grow in cadmium and zinc salts when they are exposed to a standard sodium arsenite stress. Induction of the gene encoding the 31-kD protein has been characterized as follows: (i) Transcripts accumulate maximally with similar kinetics when myoblasts are induced with either heat shock or sodium arsenite; (ii) accumulation of transcripts decays to preinduction levels within 4 hr of a heat shock, but requires more than 8 days after sodium arsenite stress; (iii) basal levels of transcript are reduced when myoblasts are cultured in the presence of steroid hormones; and (iv) stress induction is virtually abolished once myoblasts have differentiated.

Effects of glucocorticoid and thyroid hormones on regulatory enzymes of fatty acid synthesis and glycogen metabolism in developing fetal rat lung

Although glucocorticoid and thyroid hormones are known to act synergistically to stimulate surfactant production, they have opposite effects on other parameters of fetal lung maturation. We recently reported that the developmental increases in de novo fatty acid synthesis and glycogen accumulation in fetal rat lung were accelerated by dexamethasone but prevented by triiodothyronine and that the dexamethasone-induced increases were diminished when the two hormones were administered together. We have now examined the effects of maternal administration of these hormones on activities of enzymes of lung fatty acid synthesis and glycogen metabolism in the rat. There was a developmental increase in fatty-acid synthase activity between 19 and 21 days gestation. This activity was increased by dexamethasone but decreased by triiodothyronine. When the two hormones were administered together the stimulatory effect of dexamethasone was decreased from 56% to 29%. The stimulatory effect on fatty-acid synthase was also observed in fetal lung explants cultured in the presence of dexamethasone. This shows that the effect of the hormone was directly on the fetal lung. Dexamethasone had no effect on liver fatty-acid synthase. There was a developmental decrease in acetyl-CoA carboxylase activity but it was not affected by the hormones. These data show that the developmental and hormone-induced changes in fetal lung de novo fatty acid synthesis are mediated by fatty-acid synthase. Although there were developmental changes in fetal lung 6-phosphofructokinase, glycogen synthase and glycogen phosphorylase activities, these enzymes were not affected by the hormones.

Thyroid and glucocorticoid hormone regulation of specific protein abundance in cultured human skin fibroblasts

Effects of 3,3',5-triiodo-L-thyronine (T3) and dexamethasone (Dex) on the abundance of proteins resolvable by two-dimensional gel electrophoresis were examined in cultured human skin fibroblasts. Confluent cultures were incubated in medium supplemented with thyroidectomized calf serum and were treated with either T3 (100 nM), Dex (100 nM) or both hormones for 2 days and then labelled with [3H]leucine for 1 day. Most of the greater than 100 resolvable radiolabelled proteins were unaffected by hormone treatment. However, both hormones could increase or decrease the abundance of a limited number (approximately 10) of specific proteins. Some of these affected proteins appear to be coregulated. This result is consistent with effects demonstrated previously in pituitary adenoma cultures and suggests that both hormones regulate protein metabolism in human fibroblasts. This may be an ideal model for studying the nature of resistance to thyroid and glucocorticoid hormones.

Thyroid hormone antagonizes an aldosterone-induced protein: a candidate mediator for the late mineralocorticoid response

In the urinary bladder of the toad Bufo marinus, the basal rate of synthesis of a number of proteins was modulated in a bidirectional way (i.e., induced or repressed) by aldosterone and by triiodothyronine (T3). Each hormone was therefore characterized by a distinct domain of response. When both hormones were added simultaneously, the two domains consistently overlapped at least for one protein, termed AIP-1, or aldosterone-induced protein 1 (Mr approximately 65 kilodaltons, pi = 6.7, as analyzed by two-dimension gel electrophoresis). The physiological role of AIP-1 is unknown, but could be related to the late mineralocorticoid response. In five experiments, T3 (60 nM, 18-hr incubation) consistently repressed AIP-1, while aldosterone-dependent sodium transport (late response) was significantly inhibited, as previously described. The repression of AIP-1 was also observed as early as 6 hr after aldosterone addition. In addition, sodium butyrate (3 mM), which was previously shown to also selectively inhibit the late mineralocorticoid response, was also able to repress AIP-1. Our results suggest that AIP-1 is one of the proteins involved in the mediation of the late mineralocorticoid response.

Diabetes-induced alterations in the translational activity of specific messenger ribonucleic acids isolated from rat hearts

During diabetes mellitus, total proteins and ribonucleic acids are significantly decreased in the rat heart, and these parameters can be increased by insulin administration. To determine whether all ribonucleic acids are equally sensitive to insulin, we examined the influence of this hormone on individual translatable ribonucleic acids. Cardiac ribonucleic acid prepared from control, untreated, and insulin-treated diabetic animals was translated in vitro in the presence of [35S]methionine. The radiolabeled peptides were separated by two-dimensional gel electrophoresis and were analyzed by fluorometry. We found that diabetes induces both qualitative and quantitative changes in the predominance of a few specific translatable messenger ribonucleic acid species. The translation of 11 messenger ribonucleic acid species was significantly decreased and that of eight messenger ribonucleic acid species was significantly increased in diabetic preparation. Twelve of the 19 translation products were quantified by digital matrix photometry: three labeled peptides were observed only when cardiac ribonucleic acid from diabetic animals was added to the cell-free translation system, four new peptides appeared when cardiac ribonucleic acid from control animals was added, and although the remaining five peptides were translated in vitro after either control or diabetic ribonucleic acid was added, their relative predominance was altered 2- to 200-fold. When translation products coded for by messenger ribonucleic acids prepared from either diabetic or hypothyroid hearts were compared, we found that most of the alterations induced by diabetes were also induced by hypothyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)

A common 43 K protein induced by glucocorticoids in a variety of cells and tissues

We have investigated the domain of adrenal steroid action in a variety of mammalian cells and tissue by high resolution two-dimensional gel electrophoresis and autoradiography. Following in vivo or in vitro treatment with steroids, minced tissue or cells were pulsed with [35S]methionine, and they newly synthesized polypeptides compared with controls by visual inspection of partial protein maps. We have found a similar protein (Mr approximately 41-44K, pI approximately 6.3-6.5) to be consistently increased by dexamethasone in 3 rat tissues and 9 human, bovine and rat cell lines. The protein was constitutively synthesized in all targets; quantitative measurements on the magnitude of response show a 2-fold induction by dexamethasone in all systems, in vivo and in vitro. Glucocorticoid-specific hormones but not sex steroids increase the rate of synthesis; deoxycorticosterone has agonist or antagonist effects on 43K synthesis in different systems studied. Co-electrophoresis experiments indicated that 43K proteins co-migrate in at least 3 rat cell lines from tissues with diverse functions. The functional significance of this common glucocorticoid-induced protein is unknown; its ubiquity and electrophoretic properties suggest a highly conserved, common indicator of glucocorticoid action in diverse target cells and tissues, rather than a tissue-specific response.

Regulation of biosynthesis of the rat liver inner mitochondrial membrane by thyroid hormone

Regulation of mitochondrial protein synthesis by thyroid hormone has been studied in isolated rat hepatocytes and liver mitochondria. Small doses (5 micrograms/100 g body wt) of triiodothyronine (T3) injected into hypothyroid rats increased both state 3 and 4 respiration by approximately 100%, while the ADP:O ratio remained constant. This suggests that T3 increases the numbers of functional respiratory chain units. T3 also induces mitochondrial protein synthesis by 50-100%. Analysis of the mitochondrial translation products show that all of the products were induced. No differential translation of the peptides involved in the respiratory chain was found. Regulation of the cytoplasmically made inner membrane peptides was also investigated in isolated hepatocytes. The majority of these peptides were not influenced by T3, in contrast to the finding with mitochondrial translation products. Those found to be regulated by T3 belong to two subsets, which were either induced or repressed by hormone. Thus, T3 stimulated a general increase in the synthesis of mitochondrially translated inner membrane peptides, but regulates selectively those inner membrane peptides translated on cytoplasmic ribosomes. The findings suggest that hormone regulation of the respiratory chain is exerted through a few selective proteins, perhaps those which require subunits made from both nuclear and mitochondrial genes.

Mechanisms of glucocorticoid hormone action

This work summarizes some of our studies of the mechanisms of glucocorticoid action, including aspects of steroid binding to receptors, the activation of glucocorticoid-receptor complexes and the regulation of expression of endogenous and transferred glucocorticoid-responsive genes. Studies of the receptor-steroid interaction support the notion that steroid entry is passive. A comparative analysis of binding in isolated cytosol and intact cells suggests that the initial receptor-steroid binding reaction and not subsequent steps such as activation and nuclear binding, is predominantly responsible for the high-affinity state that is generated. The binding is driven by entropy and enthalpy changes at low temperature; at higher temperatures it is driven by entropy changes, with enthalpy working against it. Studies of the activation of the receptor-glucocorticoid complex with the use of highly purified receptors suggest that this step is associated with a change in charge of the receptor-glucocorticoid complex (such as would occur with a dephosphorylation reaction), whereas the data do not support the notion that dissociation of a bound RNA or of receptor oligomers is responsible for generating the nuclear- and DNA-binding activity of the complex. Studies of the regulation by glucocorticoids of expression of the endogenous rat growth hormone (rGH) gene in cultured rat pituitary tumor (GC, GH3D6) cells suggest that glucocorticoids increase the expression of this gene by multiple mechanisms. First, there is a modest direct stimulation of transcription by a mechanism(s) that does not depend on protein synthesis; however, if the cells have been exposed to thyroid hormone for several hours, the steroid exerts a much greater increase in rGH pre-mRNA levels. Secondly, the steroid appears to stimulate some relatively stable function or functions that increase the ability of thyroid hormone to increase rGH levels. Thirdly, the steroid probably increases rGH mRNA stability, since the fold-increases in rGH mRNA exceed those of transcription. Finally, the steroid may, by unknown mechanisms, affect rGH mRNA polyadenylation. The gene transfer experiments utilized the rat and human (h) GH genes and hybrid genes containing either rGH and Herpes Simplex virus thymidine kinase (TK) gene sequences or the human metallothionein-IIA (hMT-IIA) and TK gene sequences. The steroid was found to regulate hMT-IIA gene expression in all glucocorticoid-responsive cell types tested by actions on its 5'-flanking DNA. By contrast, the glucocorticoid regulated GH gene expression in some but not all glucocorticoid-responsive cell types.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding and degradation of 125I-labeled insulin by a clonal line of rat pituitary tumor cells

Receptor sites for insulin on GH3 cells were characterized. Uptake of 125I-labeled insulin by the cells was dependent upon time and temperature, with apparent steady-states reached by 120, 20 and 10 min at 4, 23 and 37 degrees C, respectively. The binding sites were sensitive to trypsin, suggesting that the receptors contain protein. Insulin competed with 125I-labeled insulin for binding sites, with half-maximal competition observed at 5 nM insulin. Neither adrenocorticotropic hormone nor growth hormone competed for 125I-labeled insulin binding sites. 125I-labeled insulin binding was reversible, and saturable with respect to hormone concentration. 125I-labeled insulin was degraded at both 4 and 37 degrees C by GH3 cells, but not by medium conditioned by these cells. After a 5 min incubation at 37 degrees C, products of 125I-labeled insulin degradation could be recovered from the cells but were not detected extracellularly. Extending the time of incubation resulted in the recovery of fragments of 125I-labeled insulin from both cells and the medium. Native insulin inhibited most of the degradation of 125I-labeled insulin suggesting that degradation resulted, in part, from a saturable process. At steady-state, degradation products of 125I-labeled insulin, as well as intact hormone, were recovered from GH3 cells. After 30 min incubation at 37 degrees C, 80% of the cell-bound radioactivity was not extractable from GH3, cells with acetic acid.

Phenotypic switching in GH3 rat pituitary tumor cells: linked expression of growth hormone and another hormonally responsive protein

From a phenotypically unstable, prolactin-deficient variant of GH3 rat pituitary tumor cells, variants deficient in growth hormone synthesis were isolated at high frequency (about 90%). The rate of synthesis of growth hormone (rGH) and one other hormonally responsive protein, termed p16, was substantially decreased in variant cells. Reduced synthesis of rGH was accompanied by a decrease in cytoplasmic levels of pre-rGH mRNA compared to levels found in wild-type GH3 cells. Although synthesis was decreased, inducibility of rGH and p16 synthesis by dexamethasone was unimpaired. The spontaneous reversion frequency was estimated to be near 4%. The expression of p16 was coordinately regained in an rGH-producing revertant, indicating that the expression of the two proteins is tightly coupled in GH3 cells. It was also found that p16 and p21 (another hormonally responsive protein in GH3 cells whose expression is tightly linked to that of prolactin) were detected at high levels in rat anterior pituitary. These observations suggest that, if growth hormone and p16 are encoded by distinct genes, then during the course of their evolution the two genes have developed similar genetic determinants governing basal expression and hormonal responsiveness in pituitary cells.