3,5,3'-L-triiodothyronine (thyroid hormone)-induced protein-DNA interactions in the thyroid hormone response elements and cell type-specific elements of the rat growth hormone gene revealed by in vivo dimethyl sulfate footprinting

Transcriptional activation by the thyroid hormone receptor through ligand-dependent receptor recruitment and chromatin remodelling

A bimodal switch model is widely used to describe transcriptional regulation by the thyroid hormone receptor (TR). In this model, the unliganded TR forms stable, chromatin-bound complexes with transcriptional co-repressors to repress transcription. Binding of hormone dissociates co-repressors and facilitates recruitment of co-activators to activate transcription. Here we show that in addition to hormone-independent TR occupancy, ChIP-seq against endogenous TR in mouse liver tissue demonstrates considerable hormone-induced TR recruitment to chromatin associated with chromatin remodelling and activated gene transcription. Genome-wide footprinting analysis using DNase-seq provides little evidence for TR footprints both in the absence and presence of hormone, suggesting that unliganded TR engagement with repressive complexes on chromatin is, similar to activating receptor complexes, a highly dynamic process. This dynamic and ligand-dependent interaction with chromatin is likely shared by all steroid hormone receptors regardless of their capacity to repress transcription in the absence of ligand.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Chelation of zinc amplifies induction of growth hormone mRNA levels in cultured rat pituitary tumor cells

Zinc is thought to be an integral part of nuclear receptor proteins, stabilizing them in a conformation required for binding to target genes. However, we have recently shown that restriction of zinc availability with a chelator (diethylenetriaminepenta-acetic acid, DTPA) enhances, rather than inhibits, the ability of thyroid hormone to induce growth hormone mRNA expression in GH3 rat pituitary tumor cells. In this report, we have extended these observations by showing that a prolonged (48 h) exposure to DTPA is required to see these effects. The induction by DTPA can be reversed by subsequent addition of zinc, but again, this reversal is slow. A second chelator, EDTA, can also induce growth hormone gene expression in the presence of thyroid hormone, though it is less potent than DTPA. Other agents which act via the nuclear receptor pathway, all-trans and 9-cis retinoic acid, also induce expression of growth hormone mRNA. Addition of DTPA amplifies these effects in a zinc-dependent manner. Thus chelation of zinc potentiates the action of ligands acting via nuclear receptors on growth hormone gene expression. The delayed nature of the response suggests an indirect effect.

CREB-independent regulation by CBP is a novel mechanism of human growth hormone gene expression

Hypothalamic growth hormone-releasing hormone (GHRH) stimulates growth hormone (GH) gene expression in anterior pituitary somatotrophs by binding to the GHRH receptor, a G-protein-coupled transmembrane receptor, and by mediating a cAMP-mediated protein kinase A (PKA) signal-transduction pathway. Two nonclassical cAMP-response element motifs (CGTCA) are located at nucleotides -187/-183 (distal cAMP-response element; dCRE) and -99/-95 (proximal cAMP-response element; pCRE) of the human GH promoter and are required for cAMP responsiveness, along with the pituitary-specific transcription factor Pit-1 (official nomenclature, POU1F1). Although a role for cAMP-response element binding protein (CREB) in GH stimulation by PKA has been suggested, it is unclear how the effect may be mediated. CREB binding protein (CBP) is a nuclear cofactor named for its ability to bind CREB. However, CBP also binds other nuclear proteins. We determined that CBP interacts with Pit-1 and is a cofactor for Pit-1-dependent activation of the human GH promoter. This pathway appears to be independent of CREB, with CPB being the likely target of phosphorylation by PKA.

Ligand-inducible retinoid X receptor-mediated protein: DNA interactions in the retinoic acid receptor beta2 gene promoter in vivo

Retinoid X receptors (RXRs) are recently characterized transcription factors that are members of the nuclear hormone receptor superfamily. However, it is not known whether the endogenous RXR complex requires its ligand for access to its hormone response element (HRE) of a target gene in vivo. Hence, dimethyl sulfate-based genomic footprinting was carried out to examine occupancy of HREs in the retinoic acid (RA) receptor beta2 (RARbeta2) gene promoter in the murine melanoma cell line S91 cultured in the absence or presence of T3, all-trans-RA (atRA), or CD2624, an RXR-selective retinoid. No footprint was observed at the RA-response element (betaRARE) in the absence of ligands. However, a footprint was detected at the betaRARE and other cis-acting elements after a 6 h incubation with CD2624 and atRA. Interestingly, only the betaRARE was footprinted after 60 min incubation with CD2624. These results suggest that the endogenous RXR complex can interact with an HRE of a target gene in the presence of ligand, and subsequently may initiate additional interactions between DNA and other transcription factors.

Zinc chelation enhances thyroid hormone induction of growth hormone mRNA in GH3 cells

The effects of restriction and addition of zinc on thyroid hormone responsiveness of the growth hormone gene were investigated in GH3, rat pituitary tumor cells. Addition of diethylenetriaminepenta-acetic acid (DTPA), a membrane-impermeable chelator, resulted in up to 10-fold increases in GH mRNA in the presence of 10 nM T3, with half-maximal induction at 50 microM DTPA. Only minor effects were seen in the absence of T3. Addition of zinc inhibited the stimulatory effect of DTPA in a dose-dependent manner. Equimolar concentrations of other divalent cations could not substitute for zinc, though inhibitions of the DTPA effect were observed at higher concentrations. In the absence of DTPA, exogenous zinc (100 microM) inhibited T3-induced GH mRNA by approximately 33%. Addition of DTPA or zinc did not affect T3 binding to its nuclear receptor. DTPA also enhanced the stimulatory effect of dexamethasone on GH mRNA. The results demonstrate that restricted zinc availability positively affects T3 induction of the GH gene in GH3 cells.

Role of Pit-1 in the gene expression of growth hormone, prolactin, and thyrotropin

To date, nine different mutations in the Pit-1 gene resulting in CPHD have been described in mammals. Four of these mutations alter residues important for DNA binding or alter the predicted alpha helical nature of the Pit-1 protein (A158P, R172X, E250X, and W261C). The A158P mutation, however, has minimal effects on DNA binding. Four mutations lie outside alpha helical regions (P24L, R143Q, K216E, and R271W) and do not significantly alter DNA binding either experimentally or by prediction. One mutation is a large deletion of the Pit-1 gene locus in the Jackson dwarf mouse. Mutant Pit-1 proteins that do not interfere with binding cause CPHD through interference with target gene activation and regulation. The R271W mutant acts as a dominant inhibitor of transcription of the GH and Prl genes. The A158P mutant is incapable of activating transcription from the GH-I site and has low activation of transcription of the distal enhancer and proximal promoter sites of Prl and of 320 bp of the 5' GH promoter sequence. Some mutant proteins interfere with nuclear receptors. For example, the K216E mutant has defective retinoic acid signaling on the Pit-1 gene enhancer. There is phenotypic variability in the degree of CPHD and in pituitary size in patients with Pit-1 gene mutations. Since Pit-1 has different functions in the somatotroph, lactotroph, and thyrotroph, it is not surprising that point mutations in different regions of the gene interfere in different ways with Pit-1 function. A mutant Pit-1 may be able to carry out its developmental role, but may be aberrant in GH and Prl gene activation or Pit-1 autoregulation. Study of Pit-1 mutations and their diverse pathophysiologic mechanisms should increase the understanding of anterior pituitary gland development and gene regulation in normal and disease states.

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.

Transcriptional mechanisms in anterior pituitary cell differentiation

Development of the anterior pituitary gland involves the establishment of five distinct cell lineages which are each characterized by the expression of specific trophic hormone genes. Recent studies of the thyrotrope, somatotrope, and lactotrope cell types have investigated the molecular decisions responsible for the commitment and differentiation of these cell types and have characterized the regulatory mechanisms that govern cell-specific expression of individual hormone genes. In particular, elucidation of the molecular basis of heritable dwarf phenotypes lacking particular pituitary cell lineages, such as the Snell, Jackson, and little dwarf mice, and studies of the regulation of trans-acting factors, including Pit-1, involved in pituitary cell restricted gene activation have begun to delineate the pathways responsible for development of this organ.