Negative regulation of expression of the pituitary-specific transcription factor GHF-1/Pit-1 by thyroid hormones through interference with promoter enhancer elements

Liganded T3 receptor β2 inhibits the positive feedback autoregulation of the gene for GATA2, a transcription factor critical for thyrotropin production

The serum concentration of thyrotropin (thyroid stimulating hormone, TSH) is drastically reduced by small increase in the levels of thyroid hormones (T3 and its prohormone, T4); however, the mechanism underlying this relationship is unknown. TSH consists of the chorionic gonadotropin α (CGA) and the β chain (TSHβ). The expression of both peptides is induced by the transcription factor GATA2, a determinant of the thyrotroph and gonadotroph differentiation in the pituitary. We previously reported that the liganded T3 receptor (TR) inhibits transactivation activity of GATA2 via a tethering mechanism and proposed that this mechanism, but not binding of TR with a negative T3-responsive element, is the basis for the T3-dependent inhibition of the TSHβ and CGA genes. Multiple GATA-responsive elements (GATA-REs) also exist within the GATA2 gene itself and mediate the positive feedback autoregulation of this gene. To elucidate the effect of T3 on this non-linear regulation, we fused the GATA-REs at -3.9 kb or +9.5 kb of the GATA2 gene with the chloramphenicol acetyltransferase reporter gene harbored in its 1S-promoter. These constructs were co-transfected with the expression plasmids for GATA2 and the pituitary specific TR, TRβ2, into kidney-derived CV1 cells. We found that liganded TRβ2 represses the GATA2-induced transactivation of these reporter genes. Multi-dimensional input function theory revealed that liganded TRβ2 functions as a classical transcriptional repressor. Then, we investigated the effect of T3 on the endogenous expression of GATA2 protein and mRNA in the gonadotroph-derived LβT2 cells. In this cell line, T3 reduced GATA2 protein independently of the ubiquitin proteasome system. GATA2 mRNA was drastically suppressed by T3, the concentration of which corresponds to moderate hypothyroidism and euthyroidism. These results suggest that liganded TRβ2 inhibits the positive feedback autoregulation of the GATA2 gene; moreover this mechanism plays an important role in the potent reduction of TSH production by T3.

The Mechanism of Negative Transcriptional Regulation by Thyroid Hormone: Lessons From the Thyrotropin β Subunit Gene

Thyroid hormone (T3) activates (positive regulation) or represses (negative regulation) target genes at the transcriptional level. The molecular mechanism of the former has been elucidated in detail; however, the mechanism for negative regulation has not been established. The best example of the gene that is negatively regulated by T3 is the thyrotropin (thyroid-stimulating hormone) β subunit (TSHβ) gene. Analogous to the T3-responsive element (TRE) in positive regulation, a negative TRE (nTRE) has been postulated in the TSHβ gene. However, TSHβ promoter analysis, performed in the presence of transcription factors Pit1 and GATA2, which are determinants of thyrotroph differentiation in the pituitary, revealed that the nTRE is dispensable for inhibition by T3. We propose a tethering model in which the T3 receptor is tethered to GATA2 via protein-protein interaction and inhibits GATA2-dependent transactivation of the TSHβ gene in a T3-dependent manner.

The thyroid hormone receptors as tumor suppressors

In addition to the well-known role of the thyroid hormone receptors (TRs) in growth, development and metabolism, there is increasing evidence that they have profound effects on cell proliferation and malignant transformation. TRs repress transcriptional induction of cyclin D1 by the ras oncogene and block transformation and tumor formation by Ras-transformed fibroblasts in nude mice. Mutant receptors that do not bind coactivators are able to display these actions, whereas receptors defective in corepressors binding are unable to antagonize the responses to the ras oncogene. Furthermore, expression of TRβ1 in hepatocarcinoma and breast cancer cells abolishes anchorage-independent growth and migration, blocks responses to growth factors and represses expression of prometastatic genes, reducing tumor growth and strongly inhibiting invasiveness, extravasation and metastasis formation in euthyroid mice. By contrast, when cells are inoculated into hypothyroid host, tumor growth is retarded, but tumors are more invasive and metastatic growth is enhanced. Increased aggressiveness and tumor growth retardation was also observed with parental cells that do not express TRs, showing that changes secondary to hypothyroidism can modulate tumor progression and metastatic growth independently of the presence of TRs on the tumor cells. Finally, increased malignancy of skin tumors is found in mice lacking TRs, further demonstrating the role of these receptors as inhibitors of tumor progression and suggesting that they represent a potential therapeutic target in cancer.

β-Hydroxybutyric sodium salt inhibition of growth hormone and prolactin secretion via the cAMP/PKA/CREB and AMPK signaling pathways in dairy cow anterior pituitary cells

β-hydroxybutyric acid (BHBA) regulates the synthesis and secretion of growth hormone (GH) and prolactin (PRL), but its mechanism is unknown. In this study, we detected the effects of BHBA on the activities of G protein signaling pathways, AMPK-α activity, GH, and PRL gene transcription, and GH and PRL secretion in dairy cow anterior pituitary cells (DCAPCs). The results showed that BHBA decreased intracellular cAMP levels and a subsequent reduction in protein kinase A (PKA) activity. Inhibition of PKA activity reduced cAMP response element-binding protein (CREB) phosphorylation, thereby inhibiting GH and PRL transcription and secretion. The effects of BHBA were attenuated by a specific G_αi inhibitor, pertussis toxin (PTX). In addition, intracellular BHBA uptake mediated by monocarboxylate transporter 1 (MCT1) could trigger AMPK signaling and result in the decrease in GH and PRL mRNA translation in DCAPCs cultured under low-glucose and non-glucose condition when compared with the high-glucose group. This study identifies a biochemical mechanism for the regulatory action of BHBA on GH and PRL gene transcription, translation, and secretion in DCAPCs, which may be one of the factors that regulate pituitary function during the transition period in dairy cows.

Short-chain fatty acids inhibit growth hormone and prolactin gene transcription via cAMP/PKA/CREB signaling pathway in dairy cow anterior pituitary cells

Short-chain fatty acids (SCFAs) play a key role in altering carbohydrate and lipid metabolism, influence endocrine pancreas activity, and as a precursor of ruminant milk fat. However, the effect and detailed mechanisms by which SCFAs mediate bovine growth hormone (GH) and prolactin (PRL) gene transcription remain unclear. In this study, we detected the effects of SCFAs (acetate, propionate, and butyrate) on the activity of the cAMP/PKA/CREB signaling pathway, GH, PRL, and Pit-1 gene transcription in dairy cow anterior pituitary cells (DCAPCs). The results showed that SCFAs decreased intracellular cAMP levels and a subsequent reduction in PKA activity. Inhibition of PKA activity decreased CREB phosphorylation, thereby inhibiting GH and PRL gene transcription. Furthermore, PTX blocked SCFAs- inhibited cAMP/PKA/CREB signaling pathway. These data showed that the inhibition of GH and PRL gene transcription induced by SCFAs is mediated by Gi activation and that propionate is more potent than acetate and butyrate in inhibiting GH and PRL gene transcription. In conclusion, this study identifies a biochemical mechanism for the regulation of SCFAs on bovine GH and PRL gene transcription in DCAPCs, which may serve as one of the factors that regulate pituitary function in accordance with dietary intake.

Alternative splicing of iodothyronine deiodinases in pituitary adenomas. Regulation by oncoprotein SF2/ASF

Pituitary tumors belong to the group of most common neoplasms of the sellar region. Iodothyronine deiodinase types 1 (DIO1) and 2 (DIO2) are enzymes contributing to the levels of locally synthesized T3, a hormone regulating key physiological processes in the pituitary, including its development, cellular proliferation, and hormone secretion. Previous studies revealed that the expression of deiodinases in pituitary tumors is variable and, moreover, there is no correlation between mRNA and protein products of the particular gene, suggesting the potential role of posttranscriptional regulatory mechanisms. In this work we hypothesized that one of such mechanisms could be the alternative splicing. Therefore, we analyzed expression and sequences of DIO1 and DIO2 splicing variants in 30 pituitary adenomas and 9 non-tumorous pituitary samples. DIO2 mRNA was expressed as only two mRNA isoforms. In contrast, nine splice variants of DIO1 were identified. Among them, five were devoid of exon 3. In silico sequence analysis of DIO1 revealed multiple putative binding sites for splicing factor SF2/ASF, of which the top-ranked sites were located in exon 3. Silencing of SF2/ASF in pituitary tumor GH3 cells resulted in change of ratio between DIO1 isoforms with or without exon 3, favoring the expression of variants without exon 3. The expression of SF2/ASF mRNA in pituitary tumors was increased when compared with non-neoplastic control samples. In conclusion, we provide a new mechanism of posttranscriptional regulation of DIO1 and show deregulation of DIO1 expression in pituitary adenoma, possibly resulting from disturbed expression of SF2/ASF.

Thyroid hormone receptors, cell growth and differentiation

BACKGROUND

Tissue homeostasis depends on the balance between cell proliferation and differentiation. Thyroid hormones (THs), through binding to their nuclear receptors, can regulate the expression of many genes involved in cell cycle control and cellular differentiation. This can occur by direct transcriptional regulation or by modulation of the activity of different signaling pathways.

SCOPE OF REVIEW

In this review we will summarize the role of the different receptor isoforms in growth and maturation of selected tissues and organs. We will focus on mammalian tissues, and therefore we will not address the fundamental role of the THs during amphibian metamorphosis.

MAJOR CONCLUSIONS

The actions of THs are highly pleiotropic, affecting many tissues at different developmental stages. As a consequence, their effects on proliferation and differentiation are highly heterogeneous depending on the cell type, the cellular context, and the developmental or transformation status. Both during development and in the adult, stem cells are essential for proper organ formation, maintenance and regeneration. Recent evidence suggests that some of the actions of the thyroid hormone receptors could be secondary to regulation of stem/progenitor cell function. Here we will also include the latest knowledge on the role of these receptors in proliferation and differentiation of embryonic and adult stem cells.

GENERAL SIGNIFICANCE

The thyroid hormone receptors are potent regulators of proliferation and differentiation of many cell types. This can explain the important role of the thyroid hormones and their receptors in key processes such as growth, development, tissue homeostasis or cancer. This article is part of a Special Issue entitled Thyroid hormone signalling.

Activin inhibits the human Pit-1 gene promoter through the p38 kinase pathway in a Smad-independent manner

The pituitary transcription factor Pit-1 regulates hormonal production from the anterior pituitary gland. However, the mechanisms by which Pit-1 gene expression is regulated in humans are poorly understood. Activin, a member of the TGFbeta superfamily, acts as a negative regulator of cell growth and prolactin gene expression in lactotrope cells. In this study, we show that activin negatively regulates the human Pit-1 gene promoter. We defined a 117-bp element within the Pit-1 promoter that is sufficient to relay these inhibitory effects. We further investigated the signaling pathways that mediate activin-induced inhibition of Pit-1 gene promoter in pituitary lactotrope cells. We found that the activin effects on Pit-1 gene regulation are Smad independent and require the p38 MAPK pathway. Specifically, blocking p38 kinase activity reverses activin-mediated inhibition of the Pit-1 gene promoter. Together, our results highlight the p38 MAPK pathway as a key regulator of activin function in pituitary lactotrope cells and further emphasizes the critical role played by activin in regulating hormonal production in the pituitary gland.

The vitamin D receptor represses transcription of the pituitary transcription factor Pit-1 gene without involvement of the retinoid X receptor

Pituitary transcription factor-1 (Pit-1) plays a key role in cell differentiation during organogenesis of the anterior pituitary, and as a transcriptional activator for the pituitary GH and prolactin genes. However, Pit-1 is also expressed in nonpituitary cell types and tissues. In breast tumors, Pit-1 mRNA and protein levels are increased with respect to normal breast, and in MCF-7 human breast adenocarcinoma cells, Pit-1 increases GH secretion and cell proliferation. We report here that 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] administration to MCF-7 cells induces a significant decrease in Pit-1 mRNA and protein levels. By deletion analyses, we mapped a region (located between -147 and -171 bp from the transcription start site of the Pit-1 gene) that is sufficient for the repressive response to 1,25-(OH)2D3. Gel mobility shift and chromatin immunoprecipitation assays confirmed the direct interaction between the vitamin D receptor (VDR) as homodimer (without the retinoid X receptor), and the Pit-1 promoter, supporting the view that Pit-1 is a direct transcriptional target of VDR. Our data also indicate that recruitment of histone deacetylase 1 is involved in this repressive effect. This ligand-dependent Pit-1 gene inhibition by VDR in the absence of the retinoid X receptor seems to indicate a new mechanism of transcriptional repression by 1,25-(OH)2D3.

An unliganded thyroid hormone beta receptor activates the cyclin D1/cyclin-dependent kinase/retinoblastoma/E2F pathway and induces pituitary tumorigenesis

Thyroid-stimulating hormone (TSH)-secreting tumors (TSH-omas) are pituitary tumors that constitutively secrete TSH. The molecular genetics underlying this abnormality are not known. We discovered that a knock-in mouse harboring a mutated thyroid hormone receptor (TR) beta (PV; TRbeta(PV/PV) mouse) spontaneously developed TSH-omas. TRbeta(PV/PV) mice lost the negative feedback regulation with highly elevated TSH levels associated with increased thyroid hormone levels (3,3',5-triiodo-l-thyronine [T3]). Remarkably, we found that mice deficient in all TRs (TRalpha1(-/-) TRbeta(-/-)) had similarly increased T3 and TSH levels, but no discernible TSH-omas, indicating that the dysregulation of the pituitary-thyroid axis alone is not sufficient to induce TSH-omas. Comparison of gene expression profiles by cDNA microarrays identified overexpression of cyclin D1 mRNA in TRbeta(PV/PV) but not in TRalpha1(-/-) TRbeta(-/-) mice. Overexpression of cyclin D1 protein led to activation of the cyclin D1/cyclin-dependent kinase/retinoblastoma protein/E2F pathway only in TRbeta(PV/PV) mice. The liganded TRbeta repressed cyclin D1 expression via tethering to the cyclin D1 promoter through binding to the cyclic AMP response element-binding protein. That repression effect was lost in mutant PV, thereby resulting in constitutive activation of cyclin D1 in TRbeta(PV/PV) mice. The present study revealed a novel molecular mechanism by which an unliganded TRbeta mutant acts to contribute to pituitary tumorigenesis in vivo and provided mechanistic insights into the understanding of pathogenesis of TSH-omas in patients.

The thyroid hormone receptor is a suppressor of ras-mediated transcription, proliferation, and transformation

The thyroid hormone triiodothyronine (T3) has a profound effect on growth, differentiation, and metabolism in higher organisms. Here we demonstrate that T3 inhibits ras-induced proliferation in neuroblastoma cells and blocks induction of cyclin D1 expression by the oncogene. The hormone, at physiological concentrations, strongly antagonizes the transcriptional response mediated by the Ras/mitogen-activated protein kinase/ribosomal-S6 subunit kinase (Rsk) signaling pathway in cells expressing thyroid hormone receptors (TRs). T3 blocks the response to the oncogenic forms of the three ras isoforms (H-, K-, and N-ras) and both TRalpha and TRbeta can mediate this action. The main target for induction of cyclin D1 transcription by oncogenic ras in neuroblastoma cells is a cyclic AMP response element (CRE) located in proximal promoter sequences, and T3 represses the transcriptional activity of b-Zip transcription factors such as CREB (CRE-binding protein) or ATF-2 (activation transcription factor 2) that are direct targets of Rsk2 and bind to this sequence. The hormone also blocks fibroblast transformation by oncogenic ras when TR is expressed. Furthermore, TRs act as suppressors of tumor formation by the oncogene in vivo in nude mice. The TRbeta isoform has stronger antitransforming properties than the alpha isoform and can inhibit tumorigenesis even in hypothyroid mice. These results show the existence of a previously unrecognized transcriptional cross talk between the TRs and the ras oncogene which influences relevant processes such as cell proliferation, transformation, or tumorigenesis.

Binding of the thyroid hormone receptor to a negative element in the basal growth hormone promoter is associated with histone acetylation

Nuclear thyroid hormone receptors (TRs) act as ligand-dependent activators, but paradoxically unliganded TRs can increase transcription of promoters containing negative response elements (nTRE), and hormone binding represses this activation. The rat growth hormone (GH) promoter contains a positive TRE and a nTRE. Ligand-dependent negative regulation mediated by the nTRE could play an important physiological role in restricting GH gene expression in non-pituitary cells that express TRs. With chromatin immunoprecipitation assays, we show here that the nTRE is responsible for binding of TR to the promoter in non-pituitary HeLa cells and that this element also governs transactivation by the unoccupied receptor and repression by triiodothyronine. Occupancy of the promoter by TR is concomitant with appearance of acetylated histone H3, and triiodothyronine causes release of the receptor as well as disappearance of the acetylated histone from the promoter. Although the nTRE overlaps the TATA box, the receptor does not exclude binding of TATA-binding protein, but could rather facilitate formation of the preinitiation complex. Furthermore, the proximal GH promoter is synergistically stimulated by unliganded TR and TATA-binding protein, whereas the ligand represses this cooperation. Constitutive receptor activity and synergism with TATA-binding protein require binding of corepressors. Furthermore, inhibitors of histone deacetylases enhance promoter activation by the unliganded receptor and reduce triiodothyronine-dependent repression, whereas expression of HDAC1 reverses promoter stimulation. This suggests that partitioning of histone acetylases and deacetylases between the receptors and basal transcription factors could be involved in regulation of the basal GH promoter by TRs.

The thyroid hormone receptor antagonizes CREB-mediated transcription

Combinatorial regulation of transcription involves binding of transcription factors to DNA as well as protein-protein interactions between them. In this paper, we demonstrate the existence of a mutual transcriptional antagonism between the thyroid hormone receptor (TR) and the cyclic AMP response element binding protein (CREB), which involves a direct association of both transcription factors. TR inhibits transcriptional activity of CREB and represses activation of cAMP response element (CRE)-containing promoters. TR does not bind to the CRE in vitro, but in vivo the liganded receptor is tethered to the promoter through protein-protein interactions. In turn, expression of CREB reduces TR-dependent transcriptional responses. The association of TR with CREB inhibits the ability of protein kinase A to phosphorylate CREB at Ser133, and leads to a reduction in the ligand-dependent recruitment of the p160 coactivators by TR. These results indicate the existence of a transcriptional cross-talk between CREB and TR signalling pathways, which can have important functional consequences.

Paradoxical triiodothyronine suppression of S14 transcription in permanent hepatic cell lines

Both in vivo and in primary rat hepatocyte culture, carbohydrate and triiodothyronine (T(3)) rapidly induce transcription of the rat S14 gene. To determine if regulation of this gene by T(3) is similar in human liver cells, we transfected the S14 upstream region into HepG2 cells. We chose this cell line because many others have used this cell line to study the effect of thyroid hormone on hepatic gene expression. We found that changing media glucose concentration did not affect S14 transcription. Furthermore, addition of T(3) to HepG2 cells caused a marked reduction of rat S14 transcription. This paradoxical reduction was dependent on cotransfection of the T(3) receptor. We obtained similar results in the other human hepatoma cell lines, HuH-7 and Hep3B. The paradoxical response was not limited to human cells. We found a similar response in the nonmalignant permanent mouse liver cell line, AML-12. This paradoxical response was specific to the S14 gene because transfection of all the cell lines with a CAT or luciferase reporter driven by a mouse mammary tumor virus promoter containing 1 or 4 copies of a palindromic thyroid hormone response element (TRE) showed marked induction by T(3). Our results show that T(3) abnormally regulates the S14 gene in proliferating liver cell lines of diverse origins. This paradoxical regulation by T(3) is caused by an interaction between T(3) and the thyroid hormone receptor. The factors that lead to this paradoxical response are not active in primary hepatocytes and normal intact liver.

Cross-repression, a functional consequence of the physical interaction of non-liganded nuclear receptors and POU domain transcription factors

Nuclear receptors (NRs) and POU domain factors form two important transcription factor families for which several levels of functional interference have been described. In this study, the adopted orphan receptors constitutive androstane receptor (CAR) and pregnane X receptor (PXR) were found to perform direct protein-protein interactions with Pit-1, a representative POU domain factor. The ligand-dependent interaction profile of Pit-1 with CAR, PXR, and the vitamin D receptor in solution was shown to be that of a corepressor. In the absence of receptor agonist Pit-1 inhibited the complex formation of NRs with the retinoid X receptor on DNA. Also in living cells, Pit-1 and Oct-1, another POU domain factor, behaved like corepressors of NR signaling, and Pit-1-mediated repression was found to involve histone deacetylases. Conversely vitamin D receptor, CAR, and PXR were shown to act as repressors of Pit-1 signaling in different cell lines (MCF-7, HaCaT, and GH4C1). This repression was found to be independent of histone deacetylases and seems to be based on a competition of NRs with coactivator and corepressor proteins for overlaying interaction interfaces on the surface of Pit-1. Taken together this study suggests that cross-repression should occur in all tissues in which POU domain factors and non-liganded NRs meet each other.

Regulation of Pit-1 expression by ghrelin and GHRP-6 through the GH secretagogue receptor

GH secretagogues are an expanding class of synthetic peptide and nonpeptide molecules that stimulate the pituitary gland to secrete GH through their own specific receptor, the GH-secretagogue receptor. The cloning of the receptor for these nonclassical GH releasing molecules, together with the more recent characterization of an endogenous ligand, named ghrelin, have unambiguously demonstrated the existence of a physiological system that regulates GH secretion. Somatotroph cell-specific expression of the GH gene is dependent on a pituitary-specific transcription factor (Pit-1). This factor is transcribed in a highly restricted manner in the anterior pituitary gland. The present experiments sought to determine whether the synthetic hexapeptide GHRP-6, a reference GH secretagogue compound, as well as an endogenous ligand, ghrelin, regulate pit-1 expression. By a combination of Northern and Western blot analysis we found that GHRP-6 elicits a time- and dose-dependent activation of pit-1 expression in monolayer cultures of infant rat anterior pituitary cells. This effect was blocked by pretreatment with actinomycin D, but not by cycloheximide, suggesting that this action was due to direct transcriptional activation of pit-1. Using an established cell line (HEK293-GHS-R) that overexpresses the GH secretagogue receptor, we showed a marked stimulatory effect of GHRP-6 on the pit-1 -2,500 bp 5'-region driving luciferase expression. We truncated the responsive region to -231 bp, a sequence that contains two CREs, and found that both CREs are needed for GHRP-6-induced transcriptional activation in both HEK293-GHS-R cells and infant rat anterior pituitary primary cultures. The effect was dependent on PKC, MAPK kinase, and PKA activation. Increasing Pit-1 by coexpression of pCMV-pit-1 potentiated the GHRP-6 effect on the pit-1 promoter. Similarly, we showed that the endogenous GH secretagogue receptor ligand ghrelin exerts a similar effect on the pit-1 promoter. These data provide the first evidence that ghrelin, in addition to its previously reported GH-releasing activities, is also capable of regulating pit-1 transcription through the GH secretagogue receptor in the pituitary, thus giving new insights into the physiological role of the GH secretagogue receptor on somatotroph cell differentiation and function.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Negative transcriptional regulation mediated by thyroid hormone response element 144 requires binding of the multivalent factor CTCF to a novel target DNA sequence

DNA target sites for a "multivalent" 11-zinc-finger CCTC-binding factor (CTCF) are unusually long ( approximately 50 base pairs) and remarkably different. In conjunction with the thyroid receptor (TR), CTCF binding to the lysozyme gene transcriptional silencer mediates the thyroid hormone response element (TRE)-dependent transcriptional repression. We tested whether other TREs, which in addition to the presence of a TR binding site require neighboring sequences for transcriptional function, might also contain a previously unrecognized binding site(s) for CTCF. One such candidate DNA region, previously isolated by Bigler and Eisenman (Bigler, J., and Eisenman, R. N. (1995) EMBO J. 14, 5710-5723), is the TRE-containing genomic element 144. We have identified a new CTCF target sequence that is adjacent to the TR binding site within the 144 fragment. Comparison of CTCF recognition nucleotides in the lysozyme silencer and in the 144 sequences revealed both similarities and differences. Several C-terminal CTCF zinc fingers contribute differently to binding each of these sequences. Mutations that eliminate CTCF binding impair 144-mediated negative transcriptional regulation. Thus, the 144 element provides an additional example of a functionally significant composite "TRE plus CTCF binding site" regulatory element suggesting an important role for CTCF in cooperation with the steroid/thyroid superfamily of nuclear receptors to mediate TRE-dependent transcriptional repression.

Mechanisms that mediate negative regulation of the thyroid-stimulating hormone alpha gene by the thyroid hormone receptor

A group of transcriptional cofactors for nuclear hormone receptors, referred to as corepressors (CoRs) and coactivators (CoAs), has been shown to induce transcriptional silencing and hormone-induced activation, respectively, of genes that contain positive hormone response elements. Transcriptional silencing by CoRs involves the recruitment of histone deacetylases (HDACs), whereas ligand-dependent activation is associated with the recruitment of CoAs, which possess or recruit histone acetyltransferases (HATs). In a reciprocal manner, negatively regulated genes are stimulated by nuclear receptors in the absence of ligand and are repressed in response to ligand binding to receptors. We show here that negative regulation of the thyroid-stimulating hormone alpha (TSHalpha) promoter by the thyroid hormone receptor (TR) involves a novel mechanism in which the recruitment of CoRs by TR is associated with transcriptional stimulation and histone acetylation. Expression of excess HDAC reverses the stimulation mediated by the TR.CoR complex, consistent with a pivotal role for acetylation in this event. Addition of the ligand, 3,5,3'-triiodothyronine (T3), induces transcriptional repression of the TSHalpha promoter and is associated with the loss of histone acetylation. T3-dependent repression is blocked by phosphorylation of cAMP response element binding protein, or by inhibition of HDAC, indicating that receptor action is subverted by maneuvers that stimulate histone acetylation of the target gene. We propose that negative regulation of a subset of genes by TR involves the active exchange of CoRs and CoAs with intrinsic promoter regulatory elements that normally strongly induce histone acetylation and transcriptional activation.

A direct protein-protein interaction is involved in the cooperation between thyroid hormone and retinoic acid receptors and the transcription factor GHF-1

The nuclear receptors for thyroid hormone (TRs) and retinoic acid (RARs and RXRs) cooperate with the pituitary-specific transcription factor GHF-1 to activate the rat growth hormone (GH) gene. The GH promoter contains a hormone response element (HRE), which binds TR/RXR and RAR/RXR heterodimers, located close to two binding sites for GHF-1. GHF-1 inhibits binding of TR/RXR and RAR/RXR heterodimers to an isolated HRE. Similarly, the receptors inhibit binding of GHF-1 to its cognate site. These results suggest the existence of direct protein to protein interactions between the receptors and the pituitary transcription factor. This was confirmed by in vitro binding studies with GST fusion proteins, which demonstrated a strong association of GHF-1 with RXR and a weaker interaction with RAR and TR. GHF-1 and the receptor heterodimers form a ternary complex with a fragment of the rat GH promoter, which contains binding sites for both, and GHF-1 increases receptor binding to the promoter when present in limiting conditions. These results suggest that the synergistic activation of the rat GH gene involves protein-DNA interactions as well as a physical association between the nuclear receptors and the pituitary-specific transcription factor GHF-1.

Sexually dimorphic interaction of insulin-like growth factor (IGF)-I and sex steroids in lactotrophs

Anterior pituitary hormone secretion is sexually dimorphic due partially to gender differences in the postpubertal hormone environment; however, differences in the pituitary's responsiveness to these signals may also play a role. We have used simple and double in situ hybridization to determine whether lactotrophs and somatotrophs from male and female rats respond differently in vitro to growth hormone-releasing hormone (GHRH), somatostatin (SS) or insulin-like growth factor (IGF)-I and whether sex steroids modulate these responses. Cultures were treated with either 17 beta-estradiol (E; 10(-9)M), testosterone (T; 10(-7)M), dihydrotestosterone (DHT; 10(-7) M) or vehicle in combination with either GHRH (10(-7)M), SS (10(-7)M), IGF-I (10(-7)M) or vehicle. Basal mRNA levels of GH, prolactin (PRL) and pituitary transcription factor-1 (Pit-1) did not differ between the sexes. The responses to peptide hormones alone were similar between the sexes, but not in the presence of gonadal steroids. In females, DHT reduced and E increased the stimulatory effect of GHRH and inhibitory effect of SS on GH mRNA levels (two-way ANOVA: P < 0.05), while having no effect in males. An additive effect of E and GHRH on PRL mRNA levels was seen only in males. The E induced rise in PRL mRNA levels was completely inhibited by SS in females, but only partially so in males (two-way ANOVA: P < 0.001). IGF-I inhibited the E induced rise in PRL and lactotroph Pit-1 mRNA levels only in females. These results suggest that sex steroids modulate the pituitary's response to hypothalamic and circulating factors differently in males and females and that this may play a role in generating the sexually dimorphic patterns of pituitary hormone secretion.

Autoregulation of eukaryotic transcription factors

The structures of several promoters regulating the expression of eukaryotic transcription factors have in recent years been examined. In many cases there is good evidence for autoregulation, in which a given factor binds to its own promoter and either activates or represses transcription. Autoregulation occurs in all eukaryotes and is an important component in controlling expression of basal, cell cycle specific, inducible response and cell type-specific factors. The basal factors are autoregulatory, being strictly necessary for their own expression, and as such must be epigenetically inherited. Autoregulation of stimulus response factors typically serves to amplify cellular signals transiently and also to attenuate the response whether or not a given inducer remains. Cell cycle-specific transcription factors are positively and negatively autoregulatory, but this frequently depends on interlocking circuits among family members. Autoregulation of cell type-specific factors results in a form of cellular memory that can contribute, or define, a determined state. Autoregulation of transcription factors provides a simple circuitry, useful in many cellular circumstances, that does not require the involvement of additional factors, which, in turn, would need to be subject to another hierarchy of regulation. Autoregulation additionally can provide a direct means to sense and control the cellular conce]ntration of a given factor. However, autoregulatory loops are often dependent on cellular pathways that create the circumstances under which autoregulation occurs.

Differential regulation of pituitary-specific gene expression by insulin-like growth factor 1 in rat pituitary GH4C1 and GH3 cells

We have compared the influence of insulin-like growth factor 1 (IGF-1) on pituitary gene expression in the rat cell lines GH4C1 and GH3. Incubation with IGF-1 increased PRL messenger RNA (mRNA) levels in GH4C1 cells by 4- to 5-fold but decreased the levels of PRL transcripts in GH3 cells. In addition, the levels of GH-mRNA that were not affected by IGF-1 in GH4C1 cells were significantly inhibited by the growth factor in GH3 cells. IGF-1 also decreased PRL and GH-mRNA response to T3, retinoic acid, and Fk in GH3 cells. Stability of PRL or GH transcripts was not altered by IGF-1 in GH3 cells, suggesting that the inhibitory effect is exerted at a transcriptional level. The pituitary-specific transcription factor GHF-1/Pit-1 activates both the GH and PRL promoters. As analyzed by Western blot, IGF-1 did not alter GHF-1/Pit-1 protein levels in GH4C1 cells but reduced the levels of the transcription factor in GH3 cells. This decrease is secondary to a reduction of GHF-1/Pit-1 transcripts in IGF-1-treated GH3 cells. Thus, a different effect of IGF-1 on the expression of GHF-1/Pit-1 in GH3 and GH4C1 cells is likely involved in the different regulation of GH and PRL gene in both cell types. IGF-1 increases the activity of the PRL promoter in transient transfection assays in GH4C1 cells by a Ras-dependent mechanism. Expression of oncogenic Ras(Val12) mimics the effect of IGF-1, and the dominant negative Ras(Asn17) blocks IGF-1-mediated stimulation of the PRL promoter in GH4C1 cells. Although IGF-1 did not stimulate the PRL promoter in GH3 cells, Ras(Val12) strongly activated the promoter in these cells. Hence, the machinery to activate Ras-dependent signaling is intact in GH3 cells. Moreover, IGF-1 stimulates the mitogen-activated protein kinase in GH3 cells, showing that the components linking the IGF-1 receptor to Ras are also active. These results suggest that, in addition to the Ras/mitogen-activated protein kinase pathway, IGF-1 could activate a different pathway and that the combination of both is required to elicit PRL gene expression by the growth factor. This second pathway may be defective in GH3 cells that respond to Ras but not to IGF-1.

Thyroid hormone inhibits the human prolactin gene promoter by interfering with activating protein-1 and estrogen stimulations

Transcription of the human PRL (hPRL) gene in the pituitary is subject to tissue-specific and multihormonal regulation involving two main regulatory regions, a proximal promoter and a distal enhancer. In this report we show that thyroid hormone inhibits the expression of the hPRL gene in rat pituitary cells. Transient expression experiments show that thyroid hormone regulation involves a strong inhibitory element, located in the proximal (-164/-35) promoter, which is modulated by a more distal stimulatory response control region. Gel retardation experiments reveal that the thyroid hormone receptor does not bind to the proximal negative element. We show the existence of an activating protein-1 (AP-1) response element located at positions -61 to -54 of the proximal promoter, conferring AP-1 stimulation to the hPRL promoter. This AP-1 induction is abolished when hormone-bound thyroid hormone receptor is present, indicating that there is an interference between the thyroid hormone receptor and AP-1 regulatory pathways. Furthermore, using the complete hPRL upstream region, we show that estrogen induction is abolished by simultaneous thyroid hormone treatment.

AP-1 and Oct-1 transcription factors down-regulate the expression of the human PIT1/GHF1 gene

The pituitary-specific transcription factor Pit-1/GHF-1 is a member of the POU domain family of regulatory proteins. It is involved in the commitment and expansion of the somatotropic cell lineage and activates the transcription of a set of anterior pituitary genes. We have cloned the human PIT1/GHF1 gene and characterized the regulatory mechanisms controlling its promoter activation and regulation. A minimal promoter region (-102 to +15) contains the cis-acting elements that confer to the human PIT1/GHF1 gene a high basal transcriptional activity, the tissue-specific expression, and the autoregulation by Pit-1/GHF-1 protein. The upstream promoter region contains a multiplicity of Pit-1/GHF-1 binding sites that do not show any synergistic interaction with the minimal promoter. The transcriptional activity is negatively regulated by Oct-1 and mediated by an octamer-binding site (OTF). In addition, we have also identified a 12-O-tetradecanoylphorbol-13-acetate-responsive element, which overlaps with a Pit-1/GHF-1 binding site. A mutually exclusive binding of the activator protein-1 (AP-1) and Pit-1/GHF-1 has been observed on this composite site, and AP-1 was shown to down-regulate PIT1/GHF1 transcription.