Thyroid hormone receptor, v-ErbA, and chromatin

Thyroid Hormone Enhances Angiogenesis and the Warburg Effect in Squamous Cell Carcinomas

Simple Summary

Cancer cells rewire their metabolism to promote growth, survival, proliferation, and long-term maintenance. Aerobic glycolysis is a prominent trait of many cancers; contextually, glutamine addiction, enhanced glucose uptake and aerobic glycolysis sustain the metabolic needs of rapidly proliferating cancer cells. Thyroid hormone (TH) is a positive regulator of tumor progression and metastatic conversion of squamous cell carcinoma (SCC). Accordingly, overexpression of the TH activating enzyme, D2, is associated with metastatic SCC. The aim of our study was to assess the ability of TH and its activating enzyme in promoting key tracts of cancer progression such as angiogenesis, response to hypoxia and metabolic adaptation. By performing in vivo and in vitro studies, we demonstrate that TH induces VEGF-A in cancer cells and fosters aerobic glycolysis inducing pro-glycolytic mediators, thus implying that TH signal attenuation represents a therapeutic tool to contrast tumor angiogenesis and tumor progression.

Abstract

Cancer angiogenesis is required to support energetic demand and metabolic stress, particularly during conditions of hypoxia. Coupled to neo-vasculogenesis, cancer cells rewire metabolic programs to sustain growth, survival and long-term maintenance. Thyroid hormone (TH) signaling regulates growth and differentiation in a variety of cell types and tissues, thus modulating hyper proliferative processes such as cancer. Herein, we report that TH coordinates a global program of metabolic reprogramming and induces angiogenesis through up-regulation of the VEGF-A gene, which results in the enhanced proliferation of tumor endothelial cells. In vivo conditional depletion of the TH activating enzyme in a mouse model of cutaneous squamous cell carcinoma (SCC) reduces the concentration of TH in the tumoral cells and results in impaired VEGF-A production and attenuated angiogenesis. In addition, we found that TH induces the expression of the glycolytic genes and fosters lactate production, which are key traits of the Warburg effect. Taken together, our results reveal a TH–VEGF-A–HIF1α regulatory axis leading to enhanced angiogenesis and glycolytic flux, which may represent a target for SCC therapy.

Nuclear Import and Export of the Thyroid Hormone Receptor

The thyroid hormone receptors, TRα1 and TRβ1, are members of the nuclear receptor superfamily that forms one of the most abundant classes of transcription factors in multicellular organisms. Although primarily localized to the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. The fine balance between nuclear import and export of TRs has emerged as a critical control point for modulating thyroid hormone-responsive gene expression. Mutagenesis studies have defined two nuclear localization signal (NLS) motifs that direct nuclear import of TRα1: NLS-1 in the hinge domain and NLS-2 in the N-terminal A/B domain. Three nuclear export signal (NES) motifs reside in the ligand-binding domain. A combined approach of shRNA-mediated knockdown and coimmunoprecipitation assays revealed that nuclear entry of TRα1 is facilitated by importin 7, likely through interactions with NLS-2, and importin β1 and the adapter importin α1 interacting with both NLS-1 and NLS-2. Interestingly, TRβ1 lacks NLS-2 and nuclear import depends solely on the importin α1/β1 heterodimer. Heterokaryon and fluorescence recovery after photobleaching shuttling assays identified multiple exportins that play a role in nuclear export of TRα1, including CRM1 (exportin 1), and exportins 4, 5, and 7. Even single amino acid changes in TRs dramatically alter their intracellular distribution patterns. We conclude that mutations within NLS and NES motifs affect nuclear shuttling activity, and propose that TR mislocalization contributes to the development of some types of cancer and Resistance to Thyroid Hormone syndrome.

Thyroid receptor: roles in cancer

The thyroid hormone receptors, encoded by the TRalpha and TRbeta genes, are ligand-dependent transcription factors that belong to the nuclear receptor superfamily. In addition to the role of these receptors in growth, development and metabolism, there is increasing evidence that they also inhibit transformation and act as tumor suppressors. Aberrant TR action, as well as receptor silencing, are common events in human cancer, and TRs also have an important role in tumor progression in experimental animal models, suggesting that these receptors constitute a novel therapeutic target in cancer. This review highlights recent studies on mechanisms by which loss of expression and/or function of these receptors results in a selective advantage for cellular transformation, tumor development and metastatic growth.

Deleted in breast cancer 1, a novel androgen receptor (AR) coactivator that promotes AR DNA-binding activity

Androgen receptor (AR) plays a critical role in development and maintenance of male reproductive functions and the etiology of prostate cancer. As a ligand-regulated transcription factor, identification and characterization of AR coregulators are essential for understanding the molecular mechanisms underlying its diverse biological functions. Here we reported the identification of a novel AR coactivator, deleted in breast cancer 1 (DBC1), through a biochemical approach. DBC1 interacts with AR in a ligand-stimulated manner and facilitates AR transcriptional activation in transfected cells as well as in Xenopus oocytes. In in vitro gel shift experiments, recombinant DBC1 drastically enhanced AR DNA-binding activity. Expression of DBC1 also enhanced the binding of AR to chromatinized template in vivo, whereas knockdown of DBC1 impaired the binding of AR to endogenous prostate-specific antigen (PSA) gene in the prostate cancer cell line LNCaP. Thus, our data identify DBC1 as a novel AR coactivator.

Molecular mechanisms for corticotropin-releasing hormone gene repression by glucocorticoid in BE(2)C neuronal cell line

The molecular mechanisms for the suppression of corticotropin-releasing hormone (CRH) gene expression by glucocorticoid remain to be clarified albeit the well-known physiological role of the glucocorticoid-induced negative feedback regulation of the gene. In this study, we examined the effect of glucocorticoid on CRH gene transcription using the human BE(2)C neuronal cell line, which expresses the CRH gene and produces CRH peptide intrinsically. Dexamethasone, a specific ligand for the glucocorticoid receptor (GR), potently suppressed human CRH 5'-promoter activity. The effect was GR-dependent, and was completely antagonized by antiglucocorticoid RU38486. Treatment with neither sodium butyrate nor trichostatin A abolished the suppression, thus making the possible involvement of histone deacetylase (HDACs) unlikely. The suppression was not influenced by the deletion or mutation of the proposed negative glucocorticoid-response element (nGRE) but was completely eliminated by that of cAMP-response element. Finally, overexpression of protein kinase A catalytic subunit antagonized the glucocorticoid suppression, whereas overexpression of GR enhanced it. Taken together, our data suggest that: (1) glucocorticoid exerts its negative effect on CRH gene transcription in a GR-dependent manner, but the GR-mediated inhibition appears to be independent of the nGRE; (2) HDACs do not play a significant role in the glucocorticoid repression; (3) some of the inhibitory events may take place through transrepression of protein kinase A by GR.

Mixed lineage kinase 2 enhances trans-repression of Alien and nuclear receptors

Alien was previously identified as a corepressor for the thyroid hormone receptor (TR) and DAX-1 which belong both to the superfamily of nuclear receptors. Here, we isolated the mixed lineage kinase 2 (MLK2) as an interacting partner for the corepressor Alien using a yeast two hybrid screen. MLK2 is an upstream activator of JNKs and activation of MLK2-mediated signaling cascades play roles in neurodegenerative and apoptotic mechanisms in the central nervous system. MLK2 has been shown to be localized both in the cytoplasm and cell nucleus. We confirmed the Alien-MLK2 interaction using GST pull-down experiments and also show that MLK2 is able to phosphorylate Alien in immune-kinase assays. Functional analyses revealed that Alien, DAX-1 and thyroid hormone receptor mediated transcriptional silencing is strongly enhanced in the presence of active MLK2. Since MAP kinase signaling pathways are important mediators of cellular responses to a wide variety of stimuli, our data suggest that signaling pathways not only regulate transactivation but also enhancement of transcriptional silencing. This novel cross-talk may represent a link between MLK2-mediated signaling and transcriptional repression of target genes during neuronal differentiation processes.

Chromatin and Epigenetics

An EMBO workshop held June 19-22, 2003, at the European Molecular Biology Laboratory in Heidelberg, Germany, was dedicated to the memory of Alan Wolffe. It provided a venue to highlight new concepts in the establishment and maintenance of functional chromatin organization.

Compartment-specific phosphorylation of rat thyroid hormone receptor alpha1 regulates nuclear localization and retention

The thyroid hormone receptor alpha1 (TRalpha1) is a transcription factor, which can activate or repress gene expression in response to thyroid hormone. In addition, some of its actions, including DNA binding and transcriptional activation, are thought to be regulated by phosphorylation. Results presented here, using Xenopus oocyte microinjection assays, demonstrate that a phosphorylated form of rat TRalpha1 is present in the nucleus, whereas unphosphorylated TRalpha1 remains cytoplasmic. Changes in the phosphorylation state of TRalpha1 occur rapidly and point to the possibility that phosphorylation occurs in the nucleus. Furthermore, increasing the overall phosphorylation state of the cell leads to enhanced nuclear retention of TRalpha1, suggesting that compartment-specific phosphorylation regulates nuclear localization of TRalpha1. Enhanced nuclear retention of TRalpha1 is not dependent on phosphorylation of serine 12, a well-characterized casein kinase II site, nor is phosphorylation of this site necessary for import of TRalpha1 into the Xenopus oocyte nucleus. Similarly, mutational analysis in mammalian cells shows that nuclear localization and partitioning of TRalpha1 to the nuclear matrix are independent of serine 12 phosphorylation. Taken together, these studies suggest that phosphorylation of one or more sites in TRalpha1, excluding serine 12, enhances nuclear retention and/or inhibits nuclear export but is not directly involved in nuclear import.

Alien/CSN2 gene expression is regulated by thyroid hormone in rat brain

Alien has been described as a corepressor for the thyroid hormone receptor (TR). Corepressors are coregulators that mediate gene silencing of DNA-bound transcriptional repressors. We describe here that Alien gene expression in vivo is regulated by thyroid hormone both in the rat brain and in cultured cells. In situ hybridization revealed that Alien is widely expressed in the mouse embryo and also throughout the rat brain. Hypothyroid animals exhibit lower expression of both Alien mRNAs and protein levels as compared with normal animals. Accordingly, we show that Alien gene is inducible after thyroid hormone treatment both in vivo and in cell culture. In cultured cells, the hormonal induction is mediated by either TRalpha or TRbeta, while cells lacking detectable amounts of functional TR lack hormonal induction of Alien. We have detected two Alien-specific mRNAs by Northern experiments and two Alien-specific proteins in vivo and in cell lines by Western analysis, one of the two forms representing the CSN2 subunit of the COP9 signalosome. Interestingly, both Alien mRNAs and both detected proteins are regulated by thyroid hormone in vivo and in cell lines. Furthermore, we provide evidence for the existence of at least two Alien genes in rodents. Taken together, we conclude that Alien gene expression is under control of TR and thyroid hormone. This suggests a negative feedback mechanism between TR and its own corepressor. Thus, the reduction of corepressor levels may represent a control mechanism of TR-mediated gene silencing.

Attenuation of glucocorticoid signaling through targeted degradation of p300 via the 26S proteasome pathway

The effects of acetylation on gene expression are complex, with changes in chromatin accessibility intermingled with direct effects on transcriptional regulators. For the nuclear receptors, both positive and negative effects of acetylation on specific gene transcription have been observed. We report that p300 and steroid receptor coactivator 1 interact transiently with the glucocorticoid receptor and that the acetyltransferase activity of p300 makes an important contribution to glucocorticoid receptor-mediated transcription. Treatment of cells with the deacetylase inhibitor, sodium butyrate, inhibited steroid-induced transcription and altered the transient association of glucocorticoid receptor with p300 and steroid receptor coactivator 1. Additionally, sustained sodium butyrate treatment induced the degradation of p300 through the 26S proteasome pathway. Treatment with the proteasome inhibitor MG132 restored both the level of p300 protein and the transcriptional response to steroid over 20 h of treatment. These results reveal new levels for the regulatory control of gene expression by acetylation and suggest feedback control on p300 activity.

Mice with a mutation in the thyroid hormone receptor beta gene spontaneously develop thyroid carcinoma: a mouse model of thyroid carcinogenesis

The molecular genetic basis of thyroid carcinogenesis is not well understood. Most of the existing models of thyroid cancer only rarely show metastases, and this has limited progress in the understanding of the molecular events in thyroid cancer invasion and metastasis. We have recently generated a mutant mouse by introducing a dominant negative mutant thyroid hormone nuclear receptor gene, TRbetaPV, into the TRbeta gene locus. In this TRbetaPV mouse, the regulation of the thyroid-pituitary axis is disrupted, leading to a mouse with high levels of circulating thyroid-stimulating hormone and extensive hyperplasia of follicular epithelium within the thyroid. As TRbeta(PV/PV) mice, but not TRbeta(PV/+) mice, aged, metastatic thyroid carcinoma developed. Histologic evaluation of thyroids of 5-14-month-old mice showed capsular invasion (91%), vascular invasion (74%), anaplasia (35%), and metastasis to the lung and heart (30%). Previous models of thyroid cancer have focused on genes that control initial carcinogenesis, but this model provides an unusual opportunity to study the alterations in gene regulation that occur with clinically relevant changes during progression and metastasis in a predictable fashion.

AR suppresses transcription of the alpha glycoprotein hormone subunit gene through protein-protein interactions with cJun and activation transcription factor 2

Previously, we reported that the AR directly suppressed transcription of the alpha glycoprotein hormone subunit (alphaGSU) gene in a ligand-dependent fashion while ER had no effect. Mutagenesis studies of the alphaGSU promoter indicated that two elements were required for AR-mediated suppression: the alpha basal element and tandem cAMP response elements (CREs). Because several members of the bZip family of transcriptional proteins can bind the CREs, we used several functional assays to determine whether AR interacts selectively with cJun, activation transcription factor 2 (ATF2), or CRE binding protein (CREB). When tested by cotransfection with AR, cJun and ATF2 specifically rescued androgen-mediated suppression of the alphaGSU-reporter construct in a gonadotrope-derived cell line. In contrast, cotransfected CREB displayed no activity in this rescue assay. In fact, overexpression of CREB alone diminished activity of the alphaGSU promoter, suggesting that the transcriptional activity normally conferred by the tandem CREs in gonadotropes requires their occupancy by cJun/ATF2 heterodimers. Binding assays carried out with a glutathione-S-transferase-AR fusion protein indicated that the receptor itself also displayed a clear preference for binding cJun and ATF2. Furthermore, we ruled out the possibility that AR suppressed activity of the alphaGSU promoter by reducing synthesis of these bZip proteins. Additional experiments suggested that phosphorylation of AR or histone acetylation are unlikely requirements for AR suppression of alphaGSU promoter activity. Thus, our data suggest that AR suppresses activity of the alphaGSU promoter through direct protein-protein interactions with cJun and ATF2.

Nucleocytoplasmic shuttling of the thyroid hormone receptor alpha

The thyroid hormone receptor alpha (TR alpha) exhibits a dual role as an activator or repressor of gene transcription in response to thyroid hormone (T(3)). Our studies show that TR alpha, formerly thought to reside solely in the nucleus tightly bound to DNA, actually shuttles rapidly between the nucleus and cytoplasm. The finding that TR alpha shuttles reveals an additional checkpoint in receptor control of gene expression. Using Xenopus oocyte microinjection assays, we show that there are two coexisting mechanisms for nuclear entry of TR alpha. First, nuclear import of TR alpha (molecular mass 46 kDa) was not sensitive to general inhibitors of signal-mediated transport, indicating that TR alpha can enter the oocyte nucleus by passive diffusion. Second, when TR alpha was tagged with glutathione-S:-transferase, import of the fusion protein (molecular mass 73 kDa) was completely blocked by these inhibitors, demonstrating that an alternative, signal-mediated import pathway exists for TR alpha. Nuclear retention of TR alpha in oocytes is enhanced in the presence of T(3), suggesting that more intranuclear binding sites are available for the ligand-bound receptor. Using mammalian cells, we show that shuttling of green fluorescent protein (GFP)-tagged and untagged TR alpha is inhibited in both chilled and energy-depleted cells, suggesting that there is an energy-requiring step in the nuclear retention/export process. Nuclear export of TR alpha is not blocked by leptomycin B, a specific inhibitor of the export receptor CRM1, indicating that TR alpha does not require the CRM1 pathway to exit the nucleus. Dominant negative mutants of TR with defects in DNA binding and transactivation accumulate in the cytoplasm at steady state, illustrating that even single amino acid changes in functional domains may alter the subcellular distribution of TR. In contrast to TR alpha, nuclear export of its oncogenic homolog v-ErbA is sensitive to leptomycin B, suggesting that the oncoprotein follows a CRM1-mediated export pathway. Acquisition of altered nuclear export capabilities may contribute to the oncogenic properties of v-ErbA.