Regulation of growth hormone messenger RNA synthesis by dexamethasone and triiodothyronine. Transcriptional rate and mRNA stability changes in pituitary tumor cells

O-GlcNAcylation Is Essential for Rapid Pomc Expression and Cell Proliferation in Corticotropic Tumor Cells

Pituitary adenomas have a staggering 16.7% lifetime prevalence and can be devastating in many patients because of profound endocrine and neurologic dysfunction. To date, no clear genomic or epigenomic markers correlate with their onset or severity. Herein, we investigate the impact of the O-GlcNAc posttranslational modification in their etiology. Found in more than 7000 human proteins to date, O-GlcNAcylation dynamically regulates proteins in critical signaling pathways, and its deregulation is involved in cancer progression and endocrine diseases such as diabetes. In this study, we demonstrated that O-GlcNAc enzymes were upregulated, particularly in aggressive adrenocorticotropin (ACTH)-secreting tumors, suggesting a role for O-GlcNAcylation in pituitary adenoma etiology. In addition to the demonstration that O-GlcNAcylation was essential for their proliferation, we showed that the endocrine function of pituitary adenoma is also dependent on O-GlcNAcylation. In corticotropic tumors, hypersecretion of the proopiomelanocortin (POMC)-derived hormone ACTH leads to Cushing disease, materialized by severe endocrine disruption and increased mortality. We demonstrated that Pomc messenger RNA is stabilized in an O-GlcNAc-dependent manner in response to corticotrophin-releasing hormone (CRH). By affecting Pomc mRNA splicing and stability, O-GlcNAcylation contributes to this new mechanism of fast hormonal response in corticotropes. Thus, this study stresses the essential role of O-GlcNAcylation in ACTH-secreting adenomas' pathophysiology, including cellular proliferation and hypersecretion.

Mechanisms involved in glucocorticoid induction of pituitary GH expression during embryonic development

Glucocorticoid hormones are involved in functional differentiation of GH-producing somatotrophs. Glucocorticoid treatment prematurely induces GH expression in mammals and birds in a process requiring protein synthesis and Rat sarcoma (Ras) signaling. The objective of this study was to investigate mechanisms through which glucocorticoids initiate GH expression during embryogenesis, taking advantage of the unique properties of chicken embryos as a developmental model. We determined that stimulation of GH expression occurred through transcriptional activation of GH, rather than enhancement of mRNA stability, and this process requires histone deacetylase activity. Through pharmacological inhibition, we identified the ERK1/2 pathway as a likely downstream Ras effector necessary for glucocorticoid stimulation of GH. However, we also found that chronic activation of ERK1/2 activity with a constitutively active mutant or stimulatory ligand reduced initiation of GH expression by glucocorticoid treatment. Corticosterone treatment of cultured embryonic pituitary cells increased ERK1/2 activity in an apparent cyclical manner, with a rapid increase within 5 minutes, followed by a reduction to near-basal levels at 3 hours, and a subsequent increase again at 6 hours. Therefore, we conclude that ERK1/2 signaling must be strictly controlled for maximal glucocorticoid induction of GH to occur. These results are the first in any species to demonstrate that Ras- and ERK1/2-mediated transcriptional events requiring histone deacetylase activity are involved in glucocorticoid induction of pituitary GH during embryonic development. This report increases our understanding of the molecular mechanisms underlying glucocorticoid recruitment of somatotrophs during embryogenesis and should provide insight into glucocorticoid-induced developmental changes in other tissues and cell types.

Thyroid hormone regulation of gene expression in primary cerebrocortical cells: role of thyroid hormone receptor subtypes and interactions with retinoic acid and glucocorticoids

The effects of thyroid hormone on brain development and function are largely mediated by the binding of 3,5,3'-triiodo-L-thyronine (T3) to its nuclear receptors (TR) to regulate positively or negatively gene expression. We have analyzed by quantitative polymerase chain reaction the effect of T3 on primary cultured cells from the embryonic mouse cerebral cortex, on the expression of Hr, Klf9, Shh, Dio3, Aldh1a1, and Aldh1a3. In particular we focused on T3 receptor specificity, and on the crosstalk between T3, retinoic acid and dexamethasone. To check for receptor subtype specificity we used cerebrocortical cells derived from wild type mice and from mice deficient in thyroid hormone receptor subtypes. Receptor subtype specificity was found for Dio3 and Aldh1a1, which were induced by T3 only in cells expressing the T3 receptor alpha 1 subtype. Interactions of T3 with retinoic acid signaling through the control of retinoic acid metabolism are likely to be important during development. T3 had opposing influences on retinoic acid synthesizing enzymes, increasing the expression of Aldh1a1, and decreasing Aldh1a3, while increasing the retinoic acid degrading enzyme Cyp26b1. Dexamethasone increased Klf9 and Aldh1a1 expression. The effects of T3 and dexamethasone on Aldh1a1 were highly synergistic, with mRNA increments of up to 20 fold. The results provide new data on thyroid hormone regulation of gene expression and underscore the importance of thyroid hormone interactions with retinoic acid and glucocorticoids during neural development.

Rapid induction of the growth hormone gene transcription by glucocorticoids in vitro: possible involvement of membrane glucocorticoid receptors and phosphatidylinositol 3-kinase activation

The regulation of transcription of the growth hormone (GH) gene by glucocorticoids was studied in MtT/S cells, a cell line derived from an oestrogen-induced mammotrophic tumour in the rat, and in the primary culture of the anterior pituitary gland of adult mice. The levels of the GH heteronuclear RNA (GH hnRNA), which are mainly determined by the transcription rate, increased by 25-fold during 24 h in response to dexamethasone (DEX; 1 μM) in MtT/S cells that were cultured in the medium containing charcoal-stripped serum for 7 days. The stimulatory effect of DEX on the GH hnRNA levels was detectable as early as 30 min. This rapid effect of DEX did not require on-going protein synthesis, whereas it was considered that DEX requires the presence of unknown cellular proteins produced independently of DEX stimulation. By contrast, on-going protein synthesis was required for DEX action when incubated for 6 h, as has been observed in the previous studies. The specific inhibitor of glucocorticoid receptor, RU486, inhibited both rapid (30 min) and delayed (6 h) the effects of glucocorticoids on GH hnRNA levels. Membrane impermeable glucocorticoid, corticosterone-bovine serum albumin conjugate (CSBSA), was found to have effects similar to those of DEX and free corticosterone (CS), suggesting that glucocorticoids regulate GH gene transcription at least in part through the membrane bound receptors. From pharmacological studies, it was suggested that phosphatidylinositol 3-kinase (PI3K) activation is involved in the rapid effects but not in the delayed effects of glucocorticoids. This also suggests that the delayed effects of glucocorticoids depend on mechanisms other than the activation of PI3-kinase. Finally, both rapid and delayed effects of CS and CSBSA were observed not only in MtT/S cells, but also in the mouse pituitary cells in primary culture. Therefore, it is possible that the membrane initiated action of glucocorticoids is involved in the regulation of GH transcription in normal pituitary cells, as well as in pituitary tumour cells.

Transgenic mice expressing the human growth hormone gene provide a model system to study human growth hormone synthesis and secretion in non-tumor-derived pituitary cells: differential effects of dexamethasone and thyroid hormone

Growth hormone (GH) is regulated by pituitary and hypothalamic factors as well as peripheral endocrine factors including glucocorticoids and thyroid hormone. Studies on human GH are limited largely to the assessment of plasma levels in endocrine disorders. Thus, insight into the regulation of synthesis versus secretion has come mainly from studies done on non-human GH and/or pituitary tumor cells. However, primate and non-primate GH gene loci have differences in their structure and, by extension, regulation. We generated transgenic (171hGH/CS-TG) mice containing the intact hGH1 gene and locus control region, including sequences required for integration-independent and preferential pituitary expression. Here, we show hGH co-localizes with mouse (m) GH in somatotrophs in situ and in primary pituitary cells. Dexamethasone treatment increased hGH and mGH, as well as GH releasing hormone (GHRH) receptor RNA levels, and hGH release was stimulated by GHRH treatment. By contrast, triiodothyronine decreased or had no effect on hGH and mGH production, respectively, and the negative effect on hGH was also seen in the presence of dexamethasone. Thus, 171hGH/CS-TG mouse pituitary cultures represent a model system to investigate hormonal control of hGH synthesis and secretion.

T3 rapidly modulates TSHβ mRNA stability and translational rate in the pituitary of hypothyroid rats

Whereas it is well known that T3 inhibits TSHβ gene transcription, its effects on TSHβ mRNA stability and translation have been poorly investigated. This study examined these possibilities, by evaluating the TSHβ transcripts poly(A) tail length, translational rate and binding to cytoskeleton, in pituitaries of thyroidectomized and sham-operated rats treated with T3 or saline, and killed 30 min thereafter. The hypothyroidism induced an increase of TSHβ transcript poly(A) tail, as well as of its content in ribosomes and attachment to cytoskeleton. The hypothyroid rats acutely treated with T3 exhibited a reduction of TSHβ mRNA poly(A) tail length and recruitment to ribosomes, indicating that this treatment decreased the stability and translation rate of TSHβ mRNA. Nevertheless, acute T3 administration to sham-operated rats provoked an increase of TSHβ transcripts binding to ribosomes. These data add new insight to an important role of T3 in rapidly regulating TSH gene expression at posttranscriptional level.

Low doses of estradiol partly inhibit release of GH in sheep without affecting basal levels

Estradiol increases basal growth hormone (GH) concentrations in sheep and cattle. This study sought to determine the effects of estradiol on GH-releasing hormone (GRH)-stimulated GH release in sheep. Growth hormone secretory characteristics, the GH response to GRH, and steady-state GH mRNA concentrations were determined in castrated male lambs treated with 2 different doses of estradiol 17-beta for a 28-d experimental period. Although no differences between treatments in mean GH, basal GH, or GH pulse number were observed after 28 d of estradiol treatment, GH pulse amplitude was greater (P < 0.05) in the 2.00-cm implant-treated animals than in the control and 0.75-cm implant group. The effect of estradiol treatment on GRH-stimulated GH release revealed differences between the control and estradiol-treated animals (P < 0.05). The 15-min GH responses to 0.075 microg/kg hGRH in the control, 0.75-cm, and 2.00-cm implant groups, respectively, were 76 +/- 10, 22.6 +/- 2.1, and 43.6 +/- 15.0 ng/mL. Growth hormone mRNA content was determined for pituitary glands from the different treatment groups, and no differences in steady-state GH mRNA levels were observed. There were no differences in the mean plasma concentrations of IGF-I, cortisol, T(3), or T(4) from weekly samples. Growth hormone release from cultured ovine pituitary cells from control sheep was not affected by estradiol after 72 h or in a subsequent 3-h incubation with estradiol combined with GRH. These data suggest that estradiol has differing actions on basal and GRH-stimulated GH concentrations in plasma, but the increase in pulse amplitude does not represent an increased pituitary sensitivity to GRH.

Functional maturation of growth hormone cells in the anterior pituitary gland of the fetus

Recent studies have disclosed the molecular mechanisms responsible for the phenotype determination of the anterior pituitary cell types. However, as far as growth hormone (GH) cells are concerned, particular extra-cellular cues are required for the initiation of GH and GH-releasing hormone (GHRH)-receptor gene production in addition to the expression of the cell type specific transcription factor, pit-1. The glucocorticoids play a principal role in the functional maturation of nascent GH cells in the fetal pituitary glands in rodents, inducing GH and GHRH-receptor gene expression, and establish the GH secretory system regulated by the brain in late gestation. Research supporting this role for glucocorticoid in the development of GH cells is discussed.

Regulation of 5'-promoter activity of the rat growth hormone and growth hormone-releasing hormone receptor genes in the MtT/S and MtT/E cells

The MtT/E and MtT/S cells have been established from a mammotrophic pituitary tumor, and postulated to be progenitor and premature growth hormone (GH) cells, respectively. The difference in the regulation of GH and GH-releasing hormone (GHRH) receptor gene transcription in relation to the developmental stage of GH cells were examined in these two cell lines. In MtT/S cells, triiodothyronine (T3), all-trans retinoic acid (RA) and 9-cis retinoic acid (9cRA) stimulated GH promoter activity but dexamethasone (DEX) did not. On the other hand, DEX stimulated GHRH-receptor promoter alone. T3, RA and 9cRA showed little effect alone but each of them augmented the effect of DEX when used together with DEX. In MtT/E cells, DEX, RA and 9cRA showed similar effect as observed in MtT/S cells on both GH and GHRH-receptor promoter activity. However, T3 neither stimulated GH promoter activity nor augmented the DEX-induced GHRH-receptor gene transcription in MtT/E cells. RT-PCR analyses revealed that both cell types expressed TRalpha1, TRbeta1 and TRalpha2, but expression of TRbeta2, a pituitary specific isoform of TR, was only detected in MtT/S cells. However, the deficiency of TRbeta2 for its own sake does not appear to be a reason why T3 action was not observed in MtT/E cells, because co-transfection of expression plasmids for TRbeta2 and RXRalpha failed in conferring on the cells an ability to respond to T3 by increased GH or GHRH-receptor promoter activity. These results suggest that the acquisition of mechanisms responsible for the regulation of GH or GHRH-receptor transcription by T3 may be involved in the process of functional development of GH cells.

Effects of exogenous cortisol on the GH/IGF-I/IGFBP network in channel catfish

Glucocorticoids are known to hinder somatic growth in a number of vertebrate species. In order to better understand the mechanisms through which they may act in channel catfish, we examined the effects of feeding cortisol on the growth hormone (GH)/insulin-like growth factor-I (IGF-I)/IGF-binding protein (IGFBP) network. Fish (30.6+/-3.0 g) were fed once daily for 4 weeks and treatments included: (1) High-cortisol (dietary cortisol provided at 400 mg/kg feed), (2) Low-cortisol (dietary cortisol provided at 200 mg/kg feed), and (3) Control (commercial catfish feed). Fish fed diets with cortisol weighed approximately 50% less than Controls. Feed intake was reduced by approximately 30% in both treatments of cortisol fed fish compared to Controls. A approximately 20-kDa IGFBP was observed in plasma from High- and Low-treated fish while it was not detected in Control fish plasma. High-cortisol treatment increased pituitary GH mRNA expression approximately 10-fold while liver IGF-I mRNA expression was not different between cortisol-treated fish and Controls. Cortisol treatments decreased plasma levels of IGF-I. These data indicate that feeding cortisol for 4 weeks reduces weight gain, feed intake, and plasma levels of IGF-I and induces a approximately 20-kDa IGFBP. One mechanism through which cortisol may impede growth of catfish is through an increase in a low molecular weight IGFBP which may lead to inhibitory effects on the action of IGF-I.

Effects of food deprivation on expression of growth hormone receptor and proximate composition in liver of black seabream Acanthopagrus schlegeli

The effects of food deprivation on the hepatic level growth hormone receptor (GHR) were investigated in black seabream (Acanthopagrus schlegeli) both at the protein level (by radioreceptor assay) and at the mRNA level (by ribonuclease protection assay). Serum levels of growth hormone (GH) and triiodothyronine (T(3)) were also measured. Condition factor and hepatic proximate composition of the fish were also assessed. Significant decrease in hepatic GHR binding was recorded as early as on day 2 of starvation. On day 30 this decrease was even more pronounced, with the level in the starved fish reaching less than 20% the fed control level. A concomitant decrease in the hepatic GHR mRNA content was also noted during this period, with a progressive decrease from day 2 to day 30 of starvation. The extent of decrease in the mRNA content was less pronounced than the decrease in receptor binding, with the hepatic GHR mRNA content in the day 30 starved fish representing approximately 30% of the level in the fed control. In large contrast, serum GH level increased progressively during starvation. After 30 days of starvation, serum GH levels in the starved fish were more than three times the concentration found in the fed control. Serum T(3) levels, on the other hand, decreased during starvation, with the difference reaching significance on day 15 and day 30. After 30 days of starvation, serum T(3) levels in the starved fish were only approximately 40% the concentration found in the fed control. The hepatic lipid content exhibited an increasing trend during starvation. On day 30 the hepatic lipid content of the starved fish had doubled the level found in the fed control. However, the hepatic protein content did not exhibit much change during starvation. There was also a minor decrease in the moisture content of the liver during starvation, but the condition factor of the fish as a whole registered a gradual decrease during the course of food deprivation.

Thyroid hormone stimulates myoglobin gene expression in rat cardiac muscle

T3 increases the heart activity, O2 consumption and the reactive O2 species production. Myoglobin (Mb) is highly expressed in the heart, where it facilitates O2 diffusion, mitochondrial respiration, and scavenges reactive O2 species. Here we investigate, by dose-response (0.3-100 microg/100 g BW, i.p., 5 days) and time-course studies (100 microg/100 g BW, i.v., from 0.5 to 24h), whether T3 affects the Mb mRNA and protein expression in atrium (A) and ventricle (V), by Northern and Western blot. We show that the Mb gene is controlled by T3 in A and V, as indicated by Mb mRNA and protein content decrease in thyroidectomized (Tx) rats, and restoration by T3 treatment. In the A, the different doses of T3 induced the Mb mRNA and protein recovery to the euthyroid levels; in the time-course study, this occurred only with the protein levels. In the V, T3 progressively increased the Mb mRNA above the euthyroid levels at a dose of 25 microg/100g BW; higher doses decreased it to the euthyroid levels. Mb protein increased only to the euthyroid levels at all T3 doses injected. The time-course study showed a progressive increase in the ventricular Mb mRNA and protein, which exceeded the euthyroid levels from 6 to 24h, and at 2 and 6 h of the T3 treatment, respectively. We conclude that heart Mb gene expression is influenced by thyroid status.

Differential developmental regulation of rat liver canalicular membrane transporters Bsep and Mrp2

Bile formation depends on the active secretion of bile salts and other biliary constituents by specific transporters. Recently two major transporters that contribute to bile formation, the bile salt export pump (Bsep) and multidrug resistance protein-2 (Mrp2), have been cloned. The goal of the present study was to define the expression of Bsep and Mrp2 during rat liver development. mRNA expression as assessed by Northern blot and RT-PCR was higher for Mrp2 (40% of adult) at 21 d fetal age relative to Bsep (<20% of adult). The levels of Mrp2 mRNA increased to approximately 50% of adult at 1 d of life and then rapidly increased to adult levels by 1-3 wk. Nuclear run-on assays for Bsep and Mrp2 showed minimal transcription during fetal life with an increase in transcription in the postnatal period. A different pattern of expression was observed for both Mrp2 and Bsep proteins. During fetal life, there was low expression of Mrp2 and Bsep proteins (<20% of adult) with a gradual increase neonatally reaching adult levels at 4 wk. Thus, we noted a temporal delay between the maximal expression of the mRNA (1-3 wk) and protein (4 wk) for Bsep and Mrp2. These results show that 1) expression (of mRNA and protein) of canalicular transporters is developmentally regulated by both transcriptional and posttranscriptional mechanisms and 2) Mrp2 and Bsep gene expression (mRNA) are differentially regulated.

Acute decrease in circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis

In thyrotoxicosis there is an impaired GH response to GHRH, normal GH responsiveness to GHRP-6 and lack of synergistic GH response after simultaneous administration of both peptides. We have previously shown that the GHRH-induced GH release in these patients increases after an acute reduction of circulating T3 values with administration of iopanoic acid, a compound that inhibits peripheral conversion of T4 to T3. We have now studied the effect of a decrease in serum T3 levels on the GH response to GHRP-6 (1 microg/kg) plus GHRH (100 microg) in 9 hyperthyroid patients before and after 15 days of treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (600 mg/day). Nine normal subjects were also studied. In all hyperthyroid patients iopanoic acid induced a rapid decrease and normalisation of serum T3 levels. In these subjects peak GH (microg/l; mean +/- SE) and AUC (microg/l x 120 min) values after GHRP-6 plus GHRH were significantly higher on day 15 compared to pretreatment values (peak, 18.3 +/- 3.0 vs 13.4 +/- 1.9; AUC, 1227.9 +/- 212.9 vs 968.5 +/- 160.4; p<0.05). Despite the significant enhancement of the GH responsiveness to GHRP-6 plus GHRH after treatment with iopanoic acid, this response remained significantly blunted when compared to controls both in terms of peak GH (18.3 +/- 3.0 vs 83.7 +/- 15.2; p<0.05) and AUC values (1227.9 +/- 212.9 vs 4956.5 +/- 889.3; p<0.05). In conclusion, our results show that an acute decrease of circulating T3 levels enhances, but does not normalise, the GH response to GHRP-6 plus GHRH in thyrotoxicosis. This could suggest that circulating T3 does not have a major role in the mechanisms involved in the synergistic effect of these peptides.

Alkaline phosphatase expression in tissues from glucocorticoid-treated dogs

OBJECTIVE

To determine the effect of glucocorticoids on the induction of alkaline phosphatase (ALP) isoenzymes in the liver, kidneys, and intestinal mucosa, 3 tissues that are principally responsible for ALP synthesis in dogs.

SAMPLE POPULATION

Tissues from the liver, kidneys, and intestinal mucosa of 6 dogs treated with 1 mg of prednisone/kg/d for 32 days and 6 untreated control dogs.

PROCEDURES

Using canine-specific primers for the ALP isoenzymes, a reverse transcription-polymerase chain reaction assay was designed to measure liver ALP (LALP) and intestinal ALP (IALP) mRNA and heterogeneous nuclear RNA (hnRNA) expression in tissues from the liver and kidneys and intestinal mucosa of glucocorticoid-treated and control dogs. Tissue ALP isoenzyme activities were compared between the groups.

RESULTS

The LALP activity and mRNA concentrations increased in tissues of the liver and kidneys in dogs treated with prednisone, whereas LALP hnRNA increased only in liver tissues. The IALP activity and mRNA expression increased in intestinal mucosa and liver tissues in prednisone-treated dogs. We did not detect an increase in IALP hnRNA expression in these tissues.

CONCLUSIONS AND CLINICAL RELEVANCE

Synthesis of ALP is increased in the liver, kidneys, and intestinal mucosa of dogs in response to prednisone treatment. This response appears to be regulated at the transcriptional level, but mechanisms may differ between LALP and IALP.

Thyroid hormone stimulates myoglobin expression in soleus and extensorum digitalis longus muscles of rats: concomitant alterations in the activities of Krebs cycle oxidative enzymes

Myoglobin (Mb) gene expression, Citrate Synthase (CS) and Succinate Dehydrogenase (SDH) activities of Soleus (S) and Extensorum Digitalis Longus (EDL) muscles were studied in intact, thyroidectomized and T3-treated (25 microg/100g, BW, ip, 15 days) rats. The fiber type composition of S muscle was also evaluated and used as control of the T3-induced effects. In the S muscle, the T3 treatment increased the Mb mRNA and protein expression, as well as the CS and SDH activity. These changes occurred parallel to the expected increase in type II (fast) and decrease in type I (slow)-fibers in S muscle. In the hypothyroid state, the Mb mRNA was decreased, while the Mb expression and CS activity tended to decrease. In contrast the SDH activity was increased, probably due to the enhanced motor activity that occurs as a short-term response to the hypothermia induced by hypothyroidism. In the EDL, the alterations were milder than those in S muscle in both thyroid states. These findings show that Mb gene expression is induced by T3. This is concomitant with the enhancement of Krebs Cycle enzyme activities and provides additional evidence that thyroid hormone increases the aerobic potential of skeletal muscles, as well as the speed of muscle contraction.

Thyroid hormones regulate pituitary growth hormone secretagogue receptor gene expression

Thyroid hormones regulate growth hormone (GH) secretion by actions both at the hypothalamus and at the pituitary gland. At the level of the pituitary, thyroid hormones increase GH and GH-releasing hormone receptor (GHRH-R) mRNA expression. To test if thyroid hormones might also regulate the pituitary expression of mRNA for the recently identified GH secretagogue (GHS) receptor, GHS-R, primary pituitary cell cultures from adult male rats were treated with triiodothyronine (T3) and GHS-R mRNA levels were assessed by reverse transcriptase-polymerase chain reaction. T3 increased pituitary GHS-R mRNA levels in a dose- and time-dependent manner. The stimulatory action of T3 on GHS-R mRNA levels was also observed in the presence of the RNA synthesis inhibitor, actinomycin D, indicating that gene transcription is not required. Closer examination of the decay rates of GHS-R mRNA in the presence of actinomycin D revealed T3 extended the half-life of the GHS-R mRNA from 8 h (basal) to15 h, demonstrating that T3 increases GHS-R mRNA levels in vitro by increasing message stability.

Continuous measurement of targeted promoter activity by a secreted bioluminescence reporter, Vargula hilgendorfii luciferase

The promoter activity of growth hormone (GH) was continuously monitored in rat pituitary adenoma cells (GH3) by a secreted bioluminescence reporter, Vargula hilgendorfii (Vh) luciferase. The sensitivity of the reporter was approximately 60-times higher than that of the firefly luciferase. GH3 cells were transfected with a plasmid containing a DNA sequence of the GH promoter (1.8 kb) and a full length of the Vh luciferase cDNA. Using the stable transformants, the Vh luciferase activity was monitored in the plate culture through the bioluminescence of Vh luciferase secreted into the culture medium. The reporter activity was well correlated with GH mRNA as well as GH when the GH promoter activity was activated by thyroid hormone. To develop a continuous monitoring system of the promoter, the reporter activity was sequentially measured in the perfusion system. When the promoter activity of the stable transformants was suppressed by a transcription inhibitor, the reporter activity and GH in the perfusate were simultaneously decreased. The Vh luciferase reporter is a sensitive and convenient tool for continuous and prolonged measurement of promoter activity in living cell culture systems.

Dexamethasone stimulates ribosomal protein L32 gene transcription in rat myoblasts

Incubation of rat L6 myoblasts for 24 h with 10(-7) M dexamethasone, a glucocorticoid analogue, resulted in a 2.5-fold increase in the rate of ribosomal protein L32 (rpL32) gene transcription with a corresponding increase in the level of rpL32 mRNA. The increased rate of transcription was accompanied by a dramatic enhancement in binding of the delta, but not beta and gamma, factors to the rpL32 gene promoter as measured by gel mobility shift assays. This increased binding reflects a change in the activity of the delta factor since its level is unchanged by dexamethasone treatment. The presence of the glucocorticoid analogue RU38486 reversed the stimulating effect of dexamethasone on rpL32 gene transcription and binding of the delta factor to the delta element. These results suggest that the mechanism which enhances rpL32 gene transcription in dexamethasone-treated rat L6 myoblasts involves glucocorticoid-receptor mediated changes in the activity of the delta factor.

Iopanoic acid-induced decrease of circulating T3 causes a significant increase in GH responsiveness to GH releasing hormone in thyrotoxic patients

OBJECTIVE

Thyroid hormones participate in GH synthesis and secretion, and an impaired GH response to many pharmacological stimuli, including GH releasing hormone (GHRH), has been found in thyrotoxicosis. Although the mechanisms involved in this process have not been fully elucidated, there is evidence that thyroid hormones could act at both hypothalamic and pituitary levels. There are no data in the literature about the effect of an acute reduction of circulating T3 levels on GH secretion in hyperthyroidism. The GH responsiveness to GHRH was therefore evaluated in a group of hyperthyroid patients during short-term treatment with iopanoic acid. Iopanoic acid is a compound that induces a rapid decrease in serum T3 levels, mainly by inhibition of peripheral conversion of T4 to T3. To the authors' knowledge, there is no evidence of a direct effect of iopanoic acid on GH secretion.

DESIGN

Hyperthyroid patients were submitted to a GHRH test (100 microg, i.v.) before (day 0), and on days 4, 7 and 15 after oral treatment with iopanoic acid (3 g every 3 days) and propylthiouracil (200 mg every 8 h). A group of normal control subjects was also submitted to a single GHRH test (100 microg, i.v.).

PATIENTS

Nine patients with thyrotoxicosis (eight women, one man), with a mean age of 34 years, were studied. All patients had high serum levels of total T3 and total T4, and suppressed TSH levels. None of them had taken any medication for at least 3 months before the study. The patients were compared with a group of nine control subjects (five women, four men) with a mean age of 31 years.

MEASUREMENTS

GH and TSH were measured by immunofluorometric assays. Total T3, total T4 and IGF-I were determined by radioimmunoassay. Albumin levels were measured by a colorimetric method.

RESULTS

Iopanoic acid induced a rapid and maintained decrease in serum T3 concentrations, with a significant reduction on days 4, 7 and 15 compared with pre-treatment values. In hyperthyroidism, peak GH levels (mean +/- SE mU/l) after GHRH were significantly higher on day 15 (24.4 +/- 3.8) than those observed on days 0 (14.2 +/- 1.6), 4 (15.2 +/- 3.0) and 7 (19.6 +/- 5.0). There was a 79% increase in this response on day 15 compared with the pre-treatment period. Hyperthyroid patients had a blunted GH response to GHRH on days 0, 4 and 7 in comparison with control subjects. However, on day 15, no differences were observed between the area under the curve (mean +/- SE mU/l.120 min) in thyrotoxic patients (1770 +/- 306) and in the control group (3300 +/- 816). IGF-I and albumin levels did not change during iopanoic acid administration.

CONCLUSIONS

The results show that an acute reduction in serum T3 levels elicits an increase in GH responsiveness to GHRH in hyperthyroidism. Although the mechanisms involved in this process are still unknown, it is possible that T3 influences GH responsiveness to GHRH via hypothalamic somatostatin release. Alternatively, T3 could have a direct effect at the pituitary somatotroph, modulating GHRH intracellular pathways.

Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human

During the last decade, the GH axis has become the compelling focus of remarkably active and broad-ranging basic and clinical research. Molecular and genetic models, the discovery of human GHRH and its receptor, the cloning of the GHRP receptor, and the clinical availability of recombinant GH and IGF-I have allowed surprisingly rapid advances in our knowledge of the neuroregulation of the GH-IGF-I axis in many pathophysiological contexts. The complexity of the GHRH/somatostatin-GH-IGF-I axis thus commends itself to more formalized modeling (154, 155), since the multivalent feedback-control activities are difficult to assimilate fully on an intuitive scale. Understanding the dynamic neuroendocrine mechanisms that direct the pulsatile secretion of this fundamental growth-promoting and metabolic hormone remains a critical goal, the realization of which is challenged by the exponentially accumulating matrix of experimental and clinical data in this arena. To the above end, we review here the pathophysiology of the GHRH somatostatin-GH-IGF-I feedback axis consisting of corresponding key neurotransmitters, neuromodulators, and metabolic effectors, and their cloned receptors and signaling pathways. We propose that this system is best viewed as a multivalent feedback network that is exquisitely sensitive to an array of neuroregulators and environmental stressors and genetic restraints. Feedback and feedforward mechanisms acting within the intact somatotropic axis mediate homeostatic control throughout the human lifetime and are disrupted in disease. Novel effectors of the GH axis, such as GHRPs, also offer promise as investigative probes and possible therapeutic agents. Further understanding of the mechanisms of GH neuroregulation will likely allow development of progressively more specific molecular and clinical tools for the diagnosis and treatment of various conditions in which GH secretion is regulated abnormally. Thus, we predict that unexpected and enriching insights in the domain of the neuroendocrine pathophysiology of the GH axis are likely be achieved in the succeeding decades of basic and clinical research.

Protein kinase A-regulated instability site in the 3'-untranslated region of lactate dehydrogenase-A subunit mRNA

Expression of the lactate dehydrogenase A subunit (LDH-A) gene can be controlled by transcriptional as well as posttranscriptional mechanisms. In rat C6 glioma cells, LDH-A mRNA is stabilized by activation and synergistic interaction of protein kinases A and C. In the present study, we aimed to identify the sequence domain which determines and regulates mRNA stability/instability by protein kinase A and focused our attention on the 3'-untranslated region (3'-UTR) of LDH-A mRNA. We have constructed various chimeric globin/lactate dehydrogenase (ldh) genes linked to the c-fos promoter and stably transfected them into rat C6 glioma cells. After their transfection, we determined the half-life of transcribed chimeric globin/ldh mRNAs. The results showed that at least three sequence domains within the LDH-A 3'-UTR consisting of nucleotides 1286-1351, 1453-1471, and 1471-1502 are responsible for the relatively rapid rate of LDH-A mRNA turnover in the cytoplasm. Whereas chimeric globin/ldh mRNAs containing the base sequences 1286-1351 and 1453-1471 were not stabilized by (Sp)-cAMPS, an activator of protein kinase A, instability caused by the 1471-1502 domain was significantly reversed. Additional deletion and mutational analyses demonstrated that the 3'-UTR fragment consisting of the 22 bases 1478-1499 is a critical determinant for the (Sp)-cAMPS-mediated LDH-A mRNA stabilizing activity. Because of its functional characteristics, we named the 22-base region "cAMP-stabilizing region."

Estradiol regulates estrogen receptor mRNA stability

Previous studies suggest that post-transcriptional events play an important role in estrogen-induced loss of estrogen receptor expression. The present study shows that treatment of MCF-7 cells with estradiol resulted in a six-fold decrease in estrogen receptor mRNA half-life from 4 h in control cells to 40 min in estradiol treated cells. To determine the role of protein synthesis in the regulation of estrogen receptor mRNA stability, several translational inhibitors were utilized. Pactamycin and puromycin, which prevent ribosome association with mRNA, inhibited the effect of estradiol on receptor mRNA stability, whereas cycloheximide, which has no effect on ribosome association with mRNA, had no effect on estradiol regulation of estrogen receptor mRNA stability. In control cells, the total cellular content of estrogen receptor mRNA was associated with high molecular weight polyribosomes. Treatment with estradiol resulted in a 70% decrease in estrogen receptor mRNA associated with polyribosomes but had no effect on the polyribosome distribution of estrogen receptor mRNA. In an in vitro degradation assay, polyribosomes isolated from estradiol-treated cells degraded ER mRNA faster than polyribosomes isolated from control cells. The nuclease activity associated with the polysome fraction appeared to be Mg2+ independent and inhibited by RNasin. Freeze-thawing and heating at 90 degrees C for 10 min resulted in the loss of nuclease activity. These studies suggest that an estrogen-regulated nuclease activity associated with ribosomes alters the stability of estrogen receptor mRNA.

A urokinase receptor mRNA binding protein from rabbit lung fibroblasts and mesothelial cells

The urokinase receptor (uPAR) influences several biological functions relevant to lung injury and repair, including proteolysis, cell migration, and adhesion. In malignant mesothelioma cells, we recently found that a posttranscriptional mechanism involving a cis-trans interaction between a uPAR mRNA sequence and a cytoplasmic uPAR mRNA binding protein (mRNABP) regulates uPAR gene expression (S. Shetty, A. Kumar, and S. Idell. Mol. Cell Biol. 17: 1075-1083, 1997). In this study, we sought to determine if uPAR expression in lung and pleural cells involves a similar posttranscriptional pathway. We first identified and characterized the uPAR mRNABP in rabbit tissues using gel mobility shift, ultraviolet (UV) cross-linking, and RNase protection assays and detected it in liver, heart, brain, spleen, colon, and lung. Phorbol 12-myristate 13-acetate, lipopolysaccharide, transforming growth factor-beta, tumor necrosis factor-alpha, or cycloheximide induced uPAR and uPAR mRNA expression in cultured rabbit pleural mesothelial cells and lung fibroblasts and concurrently reduced the uPAR mRNA-uPAR mRNABP interaction. Using conventional and affinity chromatography, we purified a 50-kDa uPAR mRNABP that selectively binds to a 51-nucleotide fragment of the uPAR coding region. This protein migrates as a monomer when analyzed by SDS-PAGE and UV cross-linking and does not possess intrinsic RNase activity in vitro. A uPAR mRNABP physicochemically and functionally similar to that of human malignant mesothelioma is constitutively expressed in the rabbit lung and other nonneoplastic tissues. In rabbit lung fibroblasts and mesothelial cells, expression of uPAR involves posttranscriptional regulation whereby the uPAR mRNABP appears to interact with a specific coding region cis-element to decrease the stability of uPAR mRNA.

Involvement of glucocorticoid-induced factor(s) in the stimulation of growth hormone expression in the fetal rat pituitary gland in vitro

The mechanism by which glucocorticoids induce GH expression between embryonic days 18 and 19 (E18-19) in the fetal rat pituitary gland was examined with an in vitro organ culture system. Twenty-four hour incubation of E18 pituitary glands in serum-free medium containing either dexamethasone (DEX, 5-50 nM) or corticosterone (0.55 microM) resulted in a conspicuous accumulation of GH messenger RNA (mRNA), whereas no spontaneous expression of GH mRNA was noted without glucocorticoid. Triiodothyronine (1 nM) alone weakly induced GH mRNA but increased the effect of DEX 2-fold. The GH mRNA accumulation was not observed after 5 or 10 h incubation with DEX. However, a 10-h incubation with DEX followed by 14 h chase incubation without DEX resulted in apparent induction of GH mRNA. The induction of GH mRNA by DEX was completely inhibited by puromycin. These data, taken as a whole, suggest that the induction of GH mRNA by DEX in the fetal pituitary gland is not a direct effect of DEX on the GH gene but is mediated by a factor that is synthesized in the pituitary gland in response to DEX. Both immunoblot and RNase protection assays suggested that this factor is not pit-1, which is known to be required for GH mRNA expression.

Biology of the congenitally hypothyroid hyt/hyt mouse

The hyt/hyt mouse has an autosomal recessive, fetal onset, characterized by severe hypothyroidism that persists throughout life and is a reliable model of human sporadic congenital hypothyroidism. The hypothyroidism in the hyt/hyt mouse reflects the hyporesponsiveness of the thyroid gland to thyrotropin (TSH). This is attributable to a point mutation of C to T at nucleotide position 1666, resulting in the replacement of a Pro with Leu at position 556 in transmembrane domain IV of the G protein-linked TSH receptor. This mutation leads to a reduction in all cAMP-regulated events, including thyroid hormone synthesis. The diminution in T3/T4 in serum and other organs, including the brain, also leads to alterations in the level and timing of expression of critical brain molecules, i.e. selected tubulin isoforms (M beta 5, M beta 2, and M alpha 1), microtubule associated proteins (MAPs), and myelin basic protein, as well as to changes in important neuronal cytoskeletal events, i.e. microtubule assembly and SCa and SCb axonal transport. In the hyt/hyt mouse, fetal hypothyroidism leads to reductions in M beta 5, M beta 2, and M alpha 1 mRNAs, important tubulin isoforms, and M beta 5 and M beta 2 proteins, which comprise the microtubules. These molecules are localized to layer V pyramidal neurons in the sensorimotor cortex, a site of differentiating neurons, as well as a site for localization of specific thyroid hormone receptors. These molecular abnormalities in specific cells and at specific times of development or maturation may contribute to the observed neuroanatomical abnormalities, i.e. altered neuronal process growth and maintenance, synaptogenesis, and myelination, in hypothyroid brain. Abnormal neuroanatomical development in selected brain regions may be the factor underlying the abnormalities in reflexive, locomotor, and adaptive behavior seen in the hyt/hyt mouse and other hypothyroid animals.

Control of growth hormone synthesis

A large body of research, primarily in the rodent and human species, has elucidated many of the details regarding the control of GH synthesis and release. Cell type-specific transcriptional control has been identified as the main mechanism of the somatotroph-specific expression of GH. The recent detailed analysis in rodents and humans of a highly specific transcriptional activator protein, PIT-1, has opened several new areas of study. This is especially true for research in the farm animal species, where PIT-1 has been cloned and its binding elements on the GH gene are being investigated in a number of economically important species. Genetic and biochemical analyses of PIT-1 and other GH regulators have shown the central role of PIT-1 not only in the cell-autonomous stimulation of GH gene transcription, but also in the participation of PIT-1 in the response at the GH gene to exogenous hormones such as RA and TH. PIT-1 has been implicated in the proliferative development of the pituitary itself, in the maintenance of anterior pituitary cell types once cell types are defined, and in the mechanism by which the hypothalamic signal for GH release is transduced. However, PIT-1 by itself does not activate the GH gene, so that additional unknown factors exist that need to be identified to fully understand the cell type-specific activation of the GH gene. In addition, GH gene regulatory elements acting through well-characterized systems such as TH have seemingly different effects; the specific context of the regulatory elements relative to the promoter elements appear to be crucial. These contextual details of GH gene regulation are not well understood for any species and need to be further studied to be able to make predictions for particular elements and regulatory mechanisms across species. The regulation of the pulsatile secretion of GH by GHRH and SRIH is reasonably well understood after the cloning and analysis of the two releasing factors and their receptors. Modification or manipulation of the pathways involved in the regulation of GH secretion is a potential means of enhancing the lean tissue growth of meat animals. However, further understanding of the systems controlling the in vivo release of GH is needed before such manipulations are likely to be productive. Several other research questions regarding the control of GH expression and release remain to be answered. What is the biochemical connection between exogenous signal transduction (i.e., GRH/GHRH-R, TR, ER, RAR) and PIT-1 at the GH gene? Are there additional coactivators or repressors of GH that respond to cAMP levels? Do ubiquitous regulatory factors such as GHF-3 and Zn-15, identified thus far only in the rat, exist in humans or livestock? Zn-15 is expected to be found in many mammalian species, because its recognition sequence between the PIT-1 binding sites is highly conserved across mammals (Figure 2). What is the mechanism causing GH levels to drop during aging? Does PIT-1 expression decrease during the lifespan of animals? Is it possible to increase GH gene expression within target tissues by directing the expression of PIT-1 to these tissues via transgenesis, or are other factors limiting in peripheral tissues so that the lack of PIT-1 expression is not the deciding factor? Finally, is there genetic variation in the expression of GHRH and/or SRIH or in their respective receptors? These questions are relevant to and could be investigated in several of the livestock species.

Differential ontogenic regulation of basolateral and canalicular bile acid transport proteins in rat liver

The hepatic transport systems mediating bile acid uptake and excretion undergo independent, stage-specific expression during development in the rat. In this study, the mechanisms underlying ontogenic regulation of both the Na(+)-dependent basolateral bile acid transporter and canalicular bile acid transporter/ecto-ATPase were examined. Steady state mRNA levels for the basolateral transporter were less than 20% of adult values prior to birth, increased to 35% on the first postnatal day, and reached adult levels by 1 week of age. This was paralleled by transcription rates, which were low prior to birth, reached 47% by day 1, and were maximal by 1 week of age. Steady state mRNA levels for ecto-ATPase were 12% of adult values prior to birth and showed a 2-fold increase by the first day of life. Thereafter, there was a gradual increase in mRNA for this transporter, with adult levels being reached at 4 weeks of age. Transcription rates paralleled this increment, although adult levels were reached earlier. Surprisingly, for both transporters, the full complement of protein was present well before adult levels of mRNA were reached. The basolateral protein was expressed at 82% of adult levels on the first day of life but was of lower apparent molecular mass (39 kDa), a difference that persisted until 4 weeks of age. N-Glycanase digestion suggested that this difference could be fully accounted for by N-linked glycosylation. The ecto-ATPase protein was present at 33% of adult levels prior to birth, 77% by 1 day, and 84% of adult levels by 1 week of age. Unlike the basolateral transporter, the apparent molecular weight of this protein did not change during development. In summary, the ontogeny of bile acid transporters on the plasma membrane of the hepatocyte is complex and appears to be regulated at transcriptional, translational, and post-translational levels.

Third-generation model for corticosteroid pharmacodynamics: roles of glucocorticoid receptor mRNA and tyrosine aminotransferase mRNA in rat liver

A third-generation pharmacokinetic/pharmacodynamic model was proposed for receptor/gene-mediated corticosteroid effects. The roles of the messenger RNA (mRNA) for the glucocorticoid receptor (GR) in hepatic GR down-regulation and the mRNA for hepatic tyrosine aminotransferase (TAT) induction by methylprednisolone (MPL) were examined. Male adrenalectomized Wistar rats received 50 mg/kg MPL iv. Blood and liver samples were collected at various time points for a period of 18 hr. Plasma concentrations of MPL, free hepatic cytosolic GR densities, GR mRNA, TAT mRNA, and TAT activities in liver were determined. Plasma MPL profile was biexponential with a terminal t1/2 of 0.57 hr. Free hepatic GR density rapidly disappeared from cytoplasm after the MPL dose and then slowly returned to about 60% of starting level after 16 hr. Meanwhile, GR mRNA level fell to 45% of baseline within 2 hr postdosing, and remained at that level for at least 18 hr. The GR down-regulation of GR mRNA and protein turnover rate were modeled. The TAT mRNA began to increase at about 2 hr, reached a maximum at about 5 hr, and declined to baseline by 14 hr. TAT induction followed a similar pattern, except the induction was delayed about 0.5 hr. Pharmacodynamic parameters were obtained by fitting seven differential equations in a piecewise fashion. The cascade of corticosteroid steps were modeled by a series of inductions for steroid-receptor-DNA complex, two intermediate transit compartments, TAT mRNA, and TAT activity. Results indicate that GR mRNA and TAT mRNA are major controlling factors for the receptor/gene-mediated effects of corticosteroids.

Cloning and molecular characterization of the ontogeny of a rat ileal sodium-dependent bile acid transporter

Sodium-dependent bile acid transport in the rat ileum is abruptly expressed at weaning. Degenerate oligonucleotides, based on amino acid sequence identities between the rat liver and hamster ileal transporters, were used to amplify a rat ileal probe. A 1.2-kb cDNA clone, which contains the full coding region (348 amino acids, 38 kD), was isolated by hybridization screening. In vitro translation yielded a 38-kD protein which glycosylated to 48 kD. Sodium-dependent uptake of taurocholate was observed in oocytes injected with cRNA. Northern blot analysis revealed a 5.0-kb mRNA in ileum, kidney, and cecum. A 48-kD protein was detected in ileal brush border membranes and localized to the apical border of villus ileal enterocytes. mRNA and protein expression, which were negligible before weaning, increased dramatically at weaning. Nuclear transcription rates for the transporter increased 15-fold between postnatal days 7 and 28. The apparent molecular weight of the transporter also increased between days 19 and 28. In summary, the developmental regulation of the rat ileal sodium-dependent bile acid cotransporter is characterized by transcriptionally regulated increases in mRNA and protein levels at the time of weaning with changes in apparent molecular weight of the protein after weaning.

Differential regulation of the expression of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase by thyroid hormone and insulin-like growth factor-I in the L6 muscle cell line

The aim of this study was to investigate the mechanism(s) underlying the thyroid-hormone (L-tri-iodothyronine, T3)-induced elevation of fast-type sarcoplasmic-reticulum Ca(2+)-ATPase (SERCA1) levels in L6 myotubes and the potentiating effect of insulin-like growth factor-I (IGF-I) [Muller, van Hardeveld, Simonides and van Rijn (1991) Biochem. J. 275, 35-40]. T3 increased the SERCA1 protein level (per microgram of DNA) by 160%. The concomitant increase in the SERCA1 mRNA level was somewhat higher (240%). IGF-I also increased SERCA1 protein (110%) and mRNA levels (50%), whereas IGF-I + T3 increased SERCA1 protein and mRNA levels by 410% and 380% respectively. These SERCA1 mRNA analyses show that the more-than-additive action of T3 and IGF-I on SERCA1 expression is, at least in part, pre-translational in nature. Further studies showed that the half-life of SERCA1 protein in L6 cells (17.5 h) was not altered by T3. In contrast, IGF-I prolonged the half-life of SERCA1 protein 1.5-1.9-fold, which may contribute to the disproportional increase in SERCA1 protein content compared with mRNA by IGF-I. Measurements of SERCA1 mRNA half-life (as determined by actinomycin D chase) showed no difference from the control values (15.5 h) in the presence of T3 or IGF-I alone. When T3 and IGF-I were both present, the SERCA1 mRNA half-life was prolonged 2-fold. No significant effects of T3 and IGF-I were observed on the half-life of total protein (37.4 h) and total RNA (37.0 h). The absence of an effect of T3 on SERCA1 protein and mRNA stability, when it was present alone, suggested transcriptional regulation, which was confirmed by nuclear run-on experiments, showing a 3-fold increase in transcription frequency of the SERCA1 gene by T3. We conclude that the synergistic stimulating effects of T3 and IGF-I on SERCA1 expression are the result of both transcriptional and post-transcriptional regulation. T3 acts primarily at the transcriptional level by increasing the transcription frequency of the SERCA1 gene, whereas IGF-I seems to act predominantly at post-transcriptional levels by enhancing SERCA1 protein and mRNA stability, the latter, however, only in the presence of T3.

Regulation of the expression of alkaline phosphatase in a human breast-cancer cell line

Treatment of the cultured human breast-cancer cells BC-M1 with dexamethasone induced a placental-type alkaline phosphatase (ALP). Both the ALP activity and the mRNA level in the cells were increased. The induction of ALP activity by dexamethasone was time- and dose-dependent. The accumulation of ALP mRNA was inhibited by both actinomycin D and cycloheximide, indicating that its induction is a complex event and may involve other regulatory proteins. Retinoic acid showed opposing effects with dexamethasone on the expression of alkaline phosphatase. Retinoic acid (RA) and phorbol 12-myristate 13-acetate also substantially reduced the dexamethasone-induced expression of ALP. Studies on thermostability and sensitivity to various amino acid inhibitors indicated that the BC-M1 ALP is most similar to the placental form. Northern hybridization analysis also revealed that ALP mRNA transcripts in BC-M1 and term placenta are similar in size and are distinct from that of the placental-like mRNA transcript in choriocarcinoma cells. Analysis of the degradation of BC-M1 ALP mRNA showed a similar half-life of 27 h in the untreated and in dexamethasone- or RA-treated cells. These findings demonstrated that the induction of ALP in BC-M1 cells by dexamethasone is mainly due to the increase in the transcription of the ALP gene.

Transcriptional and posttranscriptional regulation of hepatic beta 2-adrenergic receptor gene expression during development

Hepatic responsiveness to beta 2-adrenergic stimulation is dynamically regulated during early development as well as following hepatic injury and disease. In the present study, the molecular mechanisms that underlie the decline in the steady-state levels of hepatic beta 2-adrenergic receptor mRNA that occurs during development in the male rat were investigated. As determined by nuclear run-on assays, an age-associated reduction in beta 2-adrenergic receptor gene transcription was observed. The transcription rate of the beta 2-adrenergic receptor gene in postnatal day 18 liver was approximately 50% lower than that of fetal liver. Stability of beta 2-adrenergic receptor gene transcripts was highest (t1/2 approximately 6h) in hepatocytes isolated from fetal rats and was lowest (t1/2 approximately 6h) in hepatocytes from postnatal day 14 rats. In fetal hepatocytes, but not postnatal day 2 hepatocytes, cycloheximide appeared to stabilize beta 2-adrenergic receptor gene transcripts in the presence of actinomycin D. These findings establish the molecular basis of reduced steady-state levels of beta 2-adrenergic receptor mRNA in liver during early postnatal development and suggest multilevel regulatory control of hepatic beta 2-adrenergic receptor gene expression.

Growth retardation in rats whose growth hormone gene expression was suppressed by antisense RNA transgene

We produced four transgenic founder rats (F0) by introducing into rat embryos a fusion gene, which consisted of rat growth hormone (GH) promoter containing with four copies of thyroid hormone response element (TRE) and antisense cDNA sequences for rat GH. This transgene promoter directed 2.8-fold stimulation of CAT gene expression in transfected GH3 rat pituitary tumor cells compared with the rat GH promoter alone. Two of four transgenic rats expressed antisense RNA in the pituitary. Transgenic offspring (F1) from each founder rat exhibited dwarfism at as early as 3-4 weeks of age, and they exhibited approximately 70-85% reduced growth rate compared with their nontransgenic littermates over 56 weeks of observation. Plasma rat GH concentration was approximately 40-50% lower in transgenic F1 rats compared to their nontransgenic littermates. In these experiments, the pituitary hormone expression controlled in a complex manner was shown to be repressed by the antisense RNA transgene. Furthermore, the suppression of gene expression could be achieved by antisense RNA transgene in the rat as well.

Retinoid-dependent transcriptional suppression of cytokeratin gene expression in human epidermal squamous cell carcinoma cells

We have previously demonstrated that cytokeratin levels are coordinately regulated in normal cultured human keratinocytes. In the present study we examine the mechanism of this regulation using human squamous cell carcinoma (SCC) cells. Treatment of SCC-13 cells with 20 or 200 nM trans-retinoic acid results in nearly complete suppression of cytokeratin K5 and K6 expression. This change is accompanied by a simultaneous reduction (> 20-fold) in the level of the mRNAs encoding K5 and K6. Transcriptional analysis indicates that the transcription rate of the K5 and K6 genes drops by approximately four to fivefold in retinoid treated nuclei. Retinol (2000 nM) also promotes this change. In contrast, cytokeratin K19 does not increase in the presence of retinoic acid, thus the normal coordinate regulation of keratin gene expression by retinoids appears to be uncoupled in SCC-13 cells. However, this does not represent a general defect in positive regulation of gene expression by retinoids, since in a transient transfection assay trans-retinoic acid positively regulates a reporter plasmid containing the retinoid response element from the retinoic acid receptor-beta gene. The synthetic retinoids Ro 13-6298 (ethyl ester) and its metabolic derivative Ro 13-7410 (free acid) are both active in modulating the differentiation of normal keratinocytes. In contrast, only Ro 13-7410 is active in SCC-13 cells. As Ro 13-6298 binds poorly to the retinoic acid receptors, this suggests that SCC-13 cells, unlike normal keratinocytes, lack the ability to convert Ro 13-6298 to the active Ro 13-7410.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids stimulate transcription of the rat phenylethanolamine N-methyltransferase (PNMT) gene in vivo and in vitro

1. Phenylethanolamine N-methyltransferase (PNMT) is regulated by glucocorticoid hormones. This study investigates the ability of glucocorticoids to modulate transcription of the rat PNMT gene in vivo and in vitro. 2. In the adrenal glands of hypophysectomized (HPX'd) rats, the synthetic glucocorticoid dexamethasone (DEX) stimulates production of PNMT mRNA. Quantitative hybridization reveals that the levels of PNMT mRNA increase approximately threefold in total and poly(A)+RNA after 4 days of DEX treatment of HPX'd rats, a level which is maximal for this treatment. 3. ACTH, the hormonal stimulus of glucocorticoid biosynthesis in the adrenal cortex, enhances PNMT mRNA production to levels comparable to that achieved with DEX in this system. The steroid responsiveness of PNMT message production is specific for glucocorticoids. DEX also increases PNMT mRNA in the brain stem, although the magnitude and speed of response are lower than observed in the adrenal gland. 4. Additional confirmation of the inductive ability of glucocorticoids is demonstrated by the increase in PNMT immunoprecipitated following translation in vitro of adrenal RNAs from DEX-treated rats. Furthermore, the PNMT mRNA signal obtained by in situ hybridization histochemistry in adrenal sections and in primary cultures of dispersed rat adrenal medullae reveals that DEX effects on PNMT mRNA can be elicited both in vivo and in vitro. 5. Specifically, glucocorticoids exert their effects on expression of PNMT mRNA by elevating the rate of PNMT gene transcription: a 2.3-fold increase in PNMT transcription persists for 18 hr following DEX treatment of HPX'd rats. In summary, this study establishes that glucocorticoids directly and rapidly stimulate transcription of the rat PNMT gene.

Dexamethasone increases apolipoprotein A-I concentrations in medium and apolipoprotein A-I mRNA abundance from Hep G2 cells

Glucocorticoid hormones increase high-density lipoprotein (HDL) levels in vivo. However, there is little known about the mechanism by which glucocorticoids alter HDL metabolism. Hep G2 cells were incubated with dexamethasone to determine the effect of glucocorticoids on apolipoprotein (apo) A-I secretion. Dexamethasone increased apo A-I concentration in a dose-dependent fashion. After 24 hours, 5.5 x 10(-5) mol/L dexamethasone increased apo A-I accumulation in culture medium by 54%. Detectable increases in apo A-I concentration were noted in medium by 5 hours of incubation and persisted up to 48 hours. Cellular apo A-I mRNA concentration increased by 28% after incubation with dexamethasone for 24 hours. The increase in apo A-I mRNA concentration was detectable within 3 hours after incubation with dexamethasone. In contrast, incubation with dexamethasone decreased apo B concentration by 43% in culture medium, but it had no effect on cellular apo B mRNA concentrations. Dexamethasone had little effect on cholesterol and triglyceride accumulation in the medium. Incubation with albumin alone did not affect apo A-I concentration, but it decreased apo B concentration by 30% in the medium. Incubation with albumin and dexamethasone had no effect on apo A-I concentration in medium and had no additive effect on apo B concentration. These data suggest dexamethasone increases secretion of apo A-I by Hep G2 cells by increasing mRNA levels.

Hormonal regulation of the nuclear localization signals of the human glucocorticosteroid receptor

Nuclear localization of the rat glucocorticosteroid receptor (rGR) transiently expressed in COS-7 cells appears to be mediated by two nuclear localization signals, NL1 and NL2, in a hormone-dependent mechanism. We investigated the intracellular distribution of the human GR (hGR) expressed in COS-7 cells, by a different immunohistochemical technique involving immunostaining of cell pellet sections, thus avoiding the use of cell permeabilizing agents and allowing rigorous comparison between successive experiments. With a large set of hGR mutants, we could define determinants of the hGR nuclear localization and compare them with those previously reported for rGR. Our study demonstrated two hormone-dependent nuclear localization signals. NL1 activity, overlapping the DNA-binding domain (DBD)-hinge boundary, was repressed by the unliganded ligand-binding domain (LBD), even if the repressed NL1 retained a residual potency to target hGR in the nucleus. Structure/function analysis suggested a bipartite structure of NL1, analogous to that of other nuclear targeting signals (the carboxy-terminal part of DBD between amino acids 478 and 487 and the beginning of the hinge region which includes a basic amino acid stretch between 491 and 498). Upon hormone binding, NL2, located in the LBD, was activated, but was unable by itself to sustain full nuclear localization, which required the derepressed NL1 activity. Only two sequences in the LBD, localized between amino acids 600 and 626 and from amino acid 696 up to the carboxyl-terminal amino acid 777, respectively, were found to inhibit NL1 activity. As previously reported, efficient nuclear retention, mandatory for gene expression, did not required DNA-binding activity. The controversial intracellular localization of the unliganded form of hGR and the role of hsp90 in cytoplasmic localization are further discussed.

Glucocorticoids modulate the induction of BLTE/granzyme A activity in the murine T cell hybridoma PC60

The cytolytic granule-associated serine esterase granzyme A cleaves the synthetic substrate benzyloxycarbonyl-L-lysinate-thiobenzylester (BLT) and has been described as a marker for cytotoxic T lymphocyte (CTL) activation. We recently showed that BLT-esterase activity (BLTE activity) can be induced in the murine CTL-hybridoma PC60 by exogenous interleukin-1 (IL-1) and/or a rise of the intracellular cAMP level, although cAMP does not act as a second messenger for IL-1 in this system. The present study demonstrates that glucocorticoids (GC) such as dexamethasone and hydrocortisone efficiently inhibit the induction of BLTE activity by IL-1 and/or cAMP and downregulate the basal BLTE levels in PC60 cells; these results could be reproduced in part with progesterone and were steroid class-specific, since estrogen did not affect the induction of BLTE activity. The GC-induced effects on the production of BLTE activity required the activation of specific GC receptors, since induction of the activity could be restored upon addition of the contragestative drug RU 38486; they further could not be related to any alteration of the cellular metabolism of arachidonic acid and did not appear to be mediated by secreted macromolecules such as lipocortins.

DNA-binding by the glucocorticoid receptor: a structural and functional analysis

The glucocorticoid receptor belongs to a family of ligand activated nuclear receptors. This family includes, in addition to the receptors for steroid hormones, receptors for thyroid hormone, retinoic acid and 1,25-dihydroxy vitamin D3 as well as some receptors with as yet unknown ligands. The glucocorticoid receptor DNA-binding domain has been expressed in E. coli. The purified protein binds to the same DNA sequences as the native receptor and is therefore suitable for biochemical and structural studies of the DNA-binding function of the receptor protein. This protein has been shown to bind as a dimer to its DNA-binding site. Protein-protein interactions facilitate DNA-binding and a segment responsible for these interactions has been identified close to the C-terminal zinc-binding site. The family of nuclear receptors, with their related DNA-binding sites, provides an opportunity to study determinants for DNA sequence recognition. A segment close to the N-terminal zinc ion has been shown to be responsible for the target specificity of glucocorticoid and estrogen receptors. DNA-binding domains of nuclear receptors include nine conserved cysteine residues which have been shown to coordinate two zinc ions and zinc has been shown to be required for the structural integrity and DNA-binding ability of the glucocorticoid receptor DNA-binding domain. A motif for DNA recognition, based around zinc ions, was first described for transcription factor IIIA and nuclear receptors were believed to recognize DNA via a similar motif. However, the three-dimensional structure determination of the glucocorticoid receptor DNA-binding domain shows that its structure is clearly different from that of the TFIIIA type zinc-binding domains.

Biological effects of glucocorticoid hormones on two rat colon adenocarcinoma cell lines

Glucocorticoid hormones are thought to play a role in carcinogenesis as they regulate cell differentiation and proliferation. We have investigated the effect of dexamethasone on two cell lines derived from a colon carcinoma, which differ by their tumorigenicity. Dexamethasone was found to inhibit growth of both the progressive (PROb) and the regressive clone (REGb). Upon glucocorticoid treatment, PROb cells were found to secrete an additional Mr approximately 40,000 protein. The synthesis and the release in the culture medium of this protein is stimulated specifically by glucocorticoid agonists, and not by other steroid hormones. The anti-glucocorticoid RU 38486 is inefficient and suppresses the induction of this protein by dexamethasone. Induction is sensitive to actinomycin D, suggesting that regulation may be related to an alteration of the rate of mRNA synthesis. The cellular effect of glucocorticoid hormones being mediated through a specific soluble receptor, we have characterized this protein. The PROb cells contained more specific glucocorticoid-binding sites (approximately 170,000 sites per cell) than the regressive ones (REGb cells; approximately 100,000 sites per cell). In both clones, the receptor was associated with the Mr approximately 90,000 heat shock protein to yield large complexes (Stokes radius Rs approximately 7.5 nm), which were dissociated to the same extent upon heat- and salt-treatment. The steroid- and DNA-binding unit of the receptor, characterized under denaturing conditions using an anti-receptor monoclonal antibody, was found to be more degraded in the PROb cell line.

Modulation of complement gene expression by glucocorticoids

The addition of dexamethasone, prednisolone or cortisol (in order of efficacy) to human monocytes in culture produced dose-related increases in the synthesis rates of the complement components C1 inhibitor (C1-inh), factor B (B) and C2. In contrast, concentrations of C3 and lysozyme in the culture supernatants were decreased. Indomethacin stimulated synthesis of C1-inh, C2 and B, but had little effect on synthesis of C3 or lysozyme. The simultaneous addition of cycloheximide (2.5 micrograms/ml) abrogated the effects of dexamethasone on synthesis of C2, B and C1-inh, but the effect of indomethacin on the synthesis of these components was unchanged. These data suggest that protein synthesis is required for the effects of glucocorticoids on the synthesis of C2, B and C1-inh to occur. Dexamethasone and indomethacin increased the abundances of C1-inh mRNA, B mRNA and C2 mRNA in parallel with changes in the synthesis rates of these proteins. The changes in mRNA abundance were not transcriptional, but were shown to be due to increased mRNA stability. In contrast, dexamethasone decreased the expression of C3 and lysozyme by decreasing the rate of transcription of these genes. Indomethacin had no effect on transcription of the C3 and lysozyme genes. The half-lives of C3 mRNA, lysozyme mRNA and actin mRNA were not altered by dexamethasone or indomethacin. It is concluded that the effects of glucocorticoids on monocyte synthesis of C2, B and C1-inh are due to increased mRNA stability and may be related to inhibition of prostaglandin synthesis, as these effects are similar to those produced by indomethacin. The effects of dexamethasone on the synthesis of C3 and lysozyme differ from those on C2, B and C1-inh as they depend upon a decrease in gene transcription, which is not affected by indomethacin.

Thyroid hormone and glucocorticoid regulation of receptor and target gene mRNAs in pituitary GH3 cells

We have examined the effects of triiodothyronine (T3), in dose-response and time-course studies, on T3 receptor (T3R) alpha and beta and glucocorticoid receptor (GR) mRNAs in rate pituitary GH3 cells, in parallel with T3 actions on expression of the growth hormone (GH) target gene. Modulatory influences of dexamethasone (dex) on T3 action were studied by treatment with dex before and during T3 treatment. T3 treatment (1-100 nM) for 24 h reduced T3R alpha mRNA, while the presence of dex (1 microM) enhanced the T3 effect on T3R alpha mRNA and induced T3 inhibition of T3R beta mRNA. Stimulatory effects of T3 treatment on GH mRNA and release were seen in the face of inhibition of T3R mRNAs; these effects on GH were also enhanced by the presence of dex. T3 treatment for 24 h increased GR mRNA; this effect was inhibited by the presence of dex. We next examined the influence of dex on GR and T3R alpha and beta mRNAs, in parallel with effects of dex on the prolactin (PRL) target gene. Modulatory influences of T3 on dex action were studied by treatment of cells with T3 before and during dex treatment. Treatment with dex (0.1-10 microM) for 24 h reduced GR mRNA, an action enhanced by the presence of T3 (100 nM). Dex treatment resulted in inhibition of PRL mRNA and release despite parallel inhibition of GR mRNA by dex; these effects were enhanced by the presence of T3. In contrast to actions on GR, dex has no effect on T3R mRNAs. These effects of T3 and dex on receptor mRNAs suggest that glucocorticoid modulation of T3 action is not related to direct actions on T3R synthesis. In contrast, the mechanism of T3 modulation of glucocorticoid action may be due in part to alteration of GR mRNA expression. Effects of T3 and dex on target gene expression were observed in the presence of parallel reduction of their respective receptor mRNAs. This provides new evidence that interactions between these hormones are likely to be mediated by mechanisms other than regulation of receptor gene expression.

Thyroid hormone increases glyceraldehyde 3-phosphate dehydrogenase gene expression in rat liver

Livers from hypophysectomized rats had low levels of glyceraldehyde 3-phosphate dehydrogenase mRNA. Administration of L-triiodothyronine increased these levels over 20-fold. The peak response was seen 72 h after hormone administration. A half-maximal response was obtained with 5 micrograms of T3 per 100 g of body weight. Thus the expression of hepatic glyceraldehyde 3-phosphate dehydrogenase appears to be regulated by thyroid hormone.