Thyroid hormone receptor is a negative regulator in p53-mediated signaling pathways

Transcriptome and Methylome Analysis Reveal Complex Cross-Talks between Thyroid Hormone and Glucocorticoid Signaling at Xenopus Metamorphosis

Background: Most work in endocrinology focus on the action of a single hormone, and very little on the cross-talks between two hormones. Here we characterize the nature of interactions between thyroid hormone and glucocorticoid signaling during Xenopus tropicalis metamorphosis. Methods: We used functional genomics to derive genome wide profiles of methylated DNA and measured changes of gene expression after hormonal treatments of a highly responsive tissue, tailfin. Clustering classified the data into four types of biological responses, and biological networks were modeled by system biology. Results: We found that gene expression is mostly regulated by either T₃ or CORT, or their additive effect when they both regulate the same genes. A small but non-negligible fraction of genes (12%) displayed non-trivial regulations indicative of complex interactions between the signaling pathways. Strikingly, DNA methylation changes display the opposite and are dominated by cross-talks. Conclusion: Cross-talks between thyroid hormones and glucocorticoids are more complex than initially envisioned and are not limited to the simple addition of their individual effects, a statement that can be summarized with the pseudo-equation: TH ∙ GC > TH + GC. DNA methylation changes are highly dynamic and buffered from genome expression.

Novel Transcriptional Mechanisms for Regulating Metabolism by Thyroid Hormone

The thyroid hormone plays a key role in energy and nutrient metabolisms in many tissues and regulates the transcription of key genes in metabolic pathways. It has long been believed that thyroid hormones (THs) exerted their effects primarily by binding to nuclear TH receptors (THRs) that are associated with conserved thyroid hormone response elements (TREs) located on the promoters of target genes. However, recent transcriptome and ChIP-Seq studies have challenged this conventional view as discordance was observed between TH-responsive genes and THR binding to DNA. While THR association with other transcription factors bound to DNA, TH activation of THRs to mediate effects that do not involve DNA-binding, or TH binding to proteins other than THRs have been invoked as potential mechanisms to explain this discrepancy, it appears that additional novel mechanisms may enable TH to regulate the mRNA expression. These include activation of transcription factors by SIRT1 via metabolic actions by TH, the post-translational modification of THR, the THR co-regulation of transcription with other nuclear receptors and transcription factors, and the microRNA (miR) control of RNA transcript expression to encode proteins involved in the cellular metabolism. Together, these novel mechanisms enlarge and diversify the panoply of metabolic genes that can be regulated by TH.

Thyroid hormone actions in liver cancer

The thyroid hormone 3,3',5-triiodo-L-thyronine (T3) mediates several physiological processes, including embryonic development, cellular differentiation, metabolism, and the regulation of cell proliferation. Thyroid hormone receptors (TRs) generally act as heterodimers with the retinoid X receptor (RXR) to regulate target genes. In addition to their developmental and metabolic functions, TRs have been shown to play a tumor suppressor role, suggesting that their aberrant expression can lead to tumor transformation. Conversely, recent reports have shown an association between overexpression of wild-type TRs and tumor metastasis. Signaling crosstalk between T3/TR and other pathways or specific TR coregulators appear to affect tumor development. Since TR actions are complex as well as cell context-, tissue- and time-specific, aberrant expression of the various TR isoforms has different effects during diverse tumorigenesis. Therefore, elucidation of the T3/TR signaling mechanisms in cancers should facilitate the identification of novel therapeutic targets. This review provides a summary of recent studies focusing on the role of TRs in hepatocellular carcinomas (HCCs).

Small-molecule hormones: molecular mechanisms of action

Small-molecule hormones play crucial roles in the development and in the maintenance of an adult mammalian organism. On the molecular level, they regulate a plethora of biological pathways. Part of their actions depends on their transcription-regulating properties, exerted by highly specific nuclear receptors which are hormone-dependent transcription factors. Nuclear hormone receptors interact with coactivators, corepressors, basal transcription factors, and other transcription factors in order to modulate the activity of target genes in a manner that is dependent on tissue, age and developmental and pathophysiological states. The biological effect of this mechanism becomes apparent not earlier than 30-60 minutes after hormonal stimulus. In addition, small-molecule hormones modify the function of the cell by a number of nongenomic mechanisms, involving interaction with proteins localized in the plasma membrane, in the cytoplasm, as well as with proteins localized in other cellular membranes and in nonnuclear cellular compartments. The identity of such proteins is still under investigation; however, it seems that extranuclear fractions of nuclear hormone receptors commonly serve this function. A direct interaction of small-molecule hormones with membrane phospholipids and with mRNA is also postulated. In these mechanisms, the reaction to hormonal stimulus appears within seconds or minutes.

Severe pulmonary hypertension: The role of metabolic and endocrine disorders

Pulmonary arterial hypertension (PAH) is a multi-factorial condition and the underlying pulmonary vascular disease is shaped by the combined action of genetic, epigenetic and immune-related factors. Whether and how gender, obesity and the metabolic syndrome modify PAH and associated right heart failure is under intense investigation. Estrogens may enhance the process of pulmonary angioproliferation, but may also protect the right ventricle under pressure. Obesity may affect the pulmonary circulation via interactions with inflammatory cells and mediators, or via alterations in endocrine signaling. Obesity is a major risk factor for pulmonary hypertension in patients with elevated pulmonary venous pressure and preserved LV ejection fraction. Given the overlap between PAH and autoimmune diseases, hypothyroidism in patients with PAH is commonly considered a consequence of an autoimmune thyroiditis. In the clinical setting of hyperthyroidism, severe pulmonary hypertension may develop due to a hyperdynamic circulation, but a more complex situation presents itself when hyperthyroidism is associated with PAH. We recently showed in a relevant animal model of severe PAH that thyroid hormone, via its endothelial cell-proliferative action, can be permissive and drive angioproliferation. Signaling via the integrin αvβ3 and FGF receptors may participate in the formation of the lung vascular lesions in this model of PAH. Whether thyroid hormones in euthyroid PAH patients play a pathobiologically important role is unknown- as we also do not know whether the commonly diagnosed hypothyroidism in patients with severe PAH is cardioprotective. This brief review highlights some recent insights into the role of metabolic and endocrine disorders in PAH.

Molecular aspects of thyroid hormone actions

Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

Thyroid hormone receptor-beta (TR beta 1) impairs cell proliferation by the transcriptional inhibition of cyclins D1, E and A2

Thyroid hormone receptor-beta1 (TRbeta1) belongs to the ligand-inducible transcription factor superfamily. We have previously described that stable TRbeta1 expression impairs fibroblast proliferation diminishing levels and activity of the main regulators of the G(1)/S transition. To unmask the underlying molecular mechanism of this action, we have investigated the expression of cyclin D1, E and A2 upon serum stimulation in TRbeta1 expressing cells, finding a strong downregulation of their mRNAs, concomitant with low protein levels. The inhibition of the transcriptional activation in response to serum of these cyclins is differently exerted. For cyclin D1, we demonstrate that TRbeta1 represses its promoter as a consequence of the downregulation of c-jun levels, diminished AP-1 activation and loss of c-jun recruitment to its binding sites on cyclin D1 promoter. For cyclin E and A2, it is the impairment of the cyclinD/Rb/E2F pathway by TRbeta1 that prevents the activation of these two E2F target genes. Indeed, recruitment of E2F-1 to cyclin A2 promoter could not be detected. In summary, we propose that apo-TRbeta1 exerts its antiproliferative action through a mechanism that could constitute a model by which other nuclear receptors may control cell division.

Immunohistochemical Status of p53, MDM2, bcl2, bax, and ER in Invasive Ductal Breast Carcinoma in Tunisian Patients

TP53 gene alterations have been associated with sporadic breast cancer. To assess the role of p53 in invasive ductal carcinoma (IDC) of the breast among Tunisian patients, p53 protein status was studied by immuno-histochemical analysis. The p53 protein was expressed in 41 of 70 (58%) tumors. Study of the status of its target gene expression showed that MDM2 was overexpressed in 43 tumors (61%), bcl2 in 29 (41%), and bax in only 9 (12%). Estrogen receptor (ER) was detected in 38 tumor tissues (54%). The accumulated p53 was significantly associated with MDM2-positive, bcl2-negative, and ER-negative tumors (P = 0.024, P = 0.000027, and P = 0.000008, respectively), whereas with bax the correlaton was not significant. Bcl2 immunostaining displayed a positive correlation with ER (P = 0.001). A significantly higher fraction of p53-positive cells was observed in ER-negative SBRII-SBRIII tumors than in ER-positive SBRI-SBRII tumors (P = 0.000066). bcl2-positive tumors were significantly correlated with ER-positive/SBRI-SBRII tumors (P = 0.007), but negatively correlated with p53/bax (P = 0000004). MDM2 immunostaining displayed the same phenotype as p53 in the correlation with bcl2 and ER (P = 0.003), strengthened by significant associations between MDM2-positive/p53-positive and bcl2-negative or ER-negative, respectively (P = 0.00005 and P = 0.000001, respectively). MDM2-positive cells were significantly correlated with the p53-positive/bax-negative phenotype (P = 0.04). These results suggest that p53 accumulated in these tumor tissues is associated with bad prognostic markers (ER-negative, SBRIII) of IDC. MDM2 overexpression might be responsible for the accumulated p53 value in IDC. Regulation of the apoptotic process is involved in IDC; bcl2 is associated with a good prognostic marker (ER-positive and SBRI-II), whereas the regulation of bax is complex and does not necessarily correlate with the overexpression of p53.

Unliganded thyroid hormone receptor beta1 inhibits proliferation of murine fibroblasts by delaying the onset of the G1 cell-cycle signals

Thyroid hormone receptors (TRs) are members of the ligand-inducible transcription factor superfamily. The two major functional TRs (alpha1 and beta1) have different spatial and temporal expression patterns and specific physiological functions for these isoforms are now starting to emerge. By expressing these TR isoforms individually in Swiss 3T3 fibroblasts, we found that TRbeta1 expression, in the absence of hormone, provokes a proliferation arrest in G0/G1, lengthening the cycling time. Upon serum stimulation TRbeta1-expressing cells showed a marked delay in the induction of cyclins D and E, in the phosphorylation of retinoblastoma protein, and in the activation of cyclin-dependent kinase 2, accompanied by increased levels of cyclin-dependent kinase inhibitor p27Kip1. Accordingly, serum-stimulated E2F-1 transcriptional activity was repressed by TRbeta1 in transient transfection experiments. Analysis of the receptor domains required for this effect confirmed that there is no need for a functional ligand-binding domain while the DNA-binding domain is essential. In this work, we demonstrate for the first time that TRbeta1 participates in the molecular mechanisms that control cell proliferation. The unliganded TRbeta1 impairs the normal induction of the G1/S cycle regulators preventing progression into the S phase.

Cell cycle-dependent expression of thyroid hormone receptor-beta is a mechanism for variable hormone sensitivity

Thyroid hormone receptors (TRs) are ligand-regulatable transcription factors. Currently, little is known about the expression of TRs or other nuclear hormone receptors during the cell cycle. We thus developed a stable expression system to express green fluorescent protein-TRbeta in HeLa cells under tetracycline regulation, and studied TR expression during the cell cycle by laser scanning cytometry. Only approximately 9-15% of the nonsynchronized cell population expressed TR because the majority of cells were in G(1) phase and did not express detectable amounts of TR. However, when cells were synchronized in early S phase with hydroxyurea and then released, TR expression levels increased in a cell cycle-dependent manner and peaked to 30-40% cells expressing TR at late G(2)/M phase before declining to nonsynchronized levels. Moreover, we observed a direct correlation between transcriptional activity and TR expression during the cell cycle. Similar cell cycle-dependent findings also were observed for endogenous TR in rat pituitary GH(3) cells. Last, cycloheximide studies demonstrated that the increase in TR expression was primarily due to increased translation. These novel observations of cell cycle-dependent expression of TR suggest that differential hormone sensitivity can occur during the cell cycle and may contribute to cell cycle progression during normal development and oncogenesis.

Thyroid hormone receptor mutations in cancer

The thyroid hormone receptors (TRs) mediate the pleiotropic activities of the thyroid hormone (T3) in growth, development, and differentiation and in maintaining metabolic homeostasis. They are ligand-dependent transcription factors and are members of the steroid hormone/retionic acid receptor superfamily. Two TR genes, alpha and beta, located on human chromosomes 17 and 3, respectively, have been identified. That they are cellular homologs of the retroviral v-erbA oncogene suggests their possible involvement in carcinogenesis. Recent studies showed altered expression of TRs at both the mRNA and protein levels and identified somatic mutations of TRs in several human cancers. Furthermore, male transgenic mice overexpressing v-erbA oncogene develop hepatocellular carcinoma. Importantly, a targeted germline mutation of the TRbeta gene leads to the occurrence of metastatic thyroid carcinoma in homozygous mutant mice. These findings provide evidence to support the critical role of TRs in human cancer.

Thyroid hormone receptors: lessons from knockout and knock-in mutant mice

The genes encoding thyroid hormone receptor alpha and beta (TRalpha and TRbeta) encode four thyroid hormone receptors and four variant isoforms with antagonistic properties. Because of this complexity, numerous models of TR mutation have been developed to understand the functions of specific receptors. In total, 13 mutant strains are now available. Phenotype analysis has shown that the two genes serve distinct functions: TRalpha is crucial for postnatal development and cardiac function, whereas TRbeta mainly controls inner ear and retina development, liver metabolism and thyroid hormone levels. These mouse mutant strains also provide us with the unique opportunity to address the respective contribution of each receptor isoform and isotype in vivo and highlight the in vivo importance of the ligand-independent function of the TR gene products.

Expression of novel ING variants is regulated by thyroid hormone in the Xenopus laevis tadpole

The candidate tumor suppressor gene, ING1, encodes several protein isoforms as a result of alternative splicing that may possess agonistic and antagonistic roles in the control of cell proliferation and apoptosis. Recently a related gene, ING2, was isolated in human whose expression is increased in adenocarcinomas. Little is known about the cellular function and regulation of these ING family members, but the fact that ING proteins contain a plant homeodomain finger suggests that these proteins may modulate transcription factor-mediated pathways. To elucidate how ING may interact in different tissues to modulate function, we used amphibian metamorphosis as a model system in which a single stimulus, thyroid hormone (TH), initiates tissue-specific proliferation, differentiation, and apoptosis. We have isolated the first Xenopus laevis ING2 and demonstrate that transcript levels increase in response to TH treatment. We provide evidence for the existence of splice variants that are differentially expressed in tissues with different TH-induced fates. Western blots using an antibody directed against the highly conserved C-terminal end of ING proteins reveal a tissue-specific pattern of ING isoform expression in adult Xenopus tissues. Analyses of premetamorphic tadpole tissues show a TH-induced accumulation of ING proteins in tail, whereas the levels in the leg are not affected. This TH-induced accumulation is also observed in serum-free tail organ cultures and is prevented by inhibitors of tail apoptosis. Therefore, this work presents the first link between ING expression and a hormonally regulated nuclear transcription factor-mediated apoptotic response opening the possibility that ING family members may be involved in transducing the signal initiated by TH that determines cell fate.

Silencing of Wnt signaling and activation of multiple metabolic pathways in response to thyroid hormone-stimulated cell proliferation

To investigate the transcriptional program underlying thyroid hormone (T3)-induced cell proliferation, cDNA microarrays were used to survey the temporal expression profiles of 4,400 genes. Of 358 responsive genes identified, 88% had not previously been reported to be transcriptionally or functionally modulated by T3. Partitioning the genes into functional classes revealed the activation of multiple pathways, including glucose metabolism, biosynthesis, transcriptional regulation, protein degradation, and detoxification in T3-induced cell proliferation. Clustering the genes by temporal expression patterns provided further insight into the dynamics of T3 response pathways. Of particular significance was the finding that T3 rapidly repressed the expression of key regulators of the Wnt signaling pathway and suppressed the transcriptional downstream elements of the beta-catenin-T-cell factor complex. This was confirmed biochemically, as beta-catenin protein levels also decreased, leading to a decrease in the transcriptional activity of a beta-catenin-responsive promoter. These results indicate that T3-induced cell proliferation is accompanied by a complex coordinated transcriptional reprogramming of many genes in different pathways and that early silencing of the Wnt pathway may be critical to this event.

Expression of thyroid hormone receptors is disturbed in human renal clear cell carcinoma

Human renal clear cell carcinoma (RCCC) accounts for up to 2% of human cancers. To find out if thyroid hormone (T3) and its receptors (TRs) play a role in tumorigenesis of RCCC, the expression of TRs was evaluated on mRNA and protein level. It was found that TRalpha (both alpha1 and alpha2) mRNA amount was significantly decreased in tumors while compared with healthy kidney tissue, and this decrease was deepest in G1 (well differentiated) RCCCs. In contrast, TRalpha1 protein was 1.6x overexpressed in tumors. TRbeta1 mRNA amount was overexpressed in 30% and significantly decreased in 70% of examined tumors. On the protein level, TRbeta1 amount was 1.7x lower in tumors than in healthy controls.

Negative regulation of the antimetastatic gene Nm23-H1 by thyroid hormone receptors

Metastasis of various malignant cells is inversely related to the abundance of the Nm23-H1 protein. The possible role of thyroid hormones in tumor metastasis has now been investigated by examining the effect of T3 on the expression of the Nm23-H1 gene. Human hepatoma HepG2 cells, in which endogenous thyroid hormone receptor subtype alpha1 (TRalpha1) is expressed at a low level, were stably transfected, either with expression plasmids encoding wild-type TRalpha1 or a dominant negative mutant of TRalpha1, or with the empty vector (yielding HepG2-Wt, HepG2-Mt, and HepG2-Neo cells, respectively). Immunoblot analysis revealed that exposure of HepG2-Wt and HepG2-Neo cells, but not HepG2-Mt cells, to T3-induced time-dependent decreases in the abundance of Nm23-H1 messenger RNA and protein, with the extent of these effects correlating with the level of expression of TRalpha1. An in vitro assay also revealed that T3 induced a marked increase in the invasive activity of HepG2-Wt cells; it induced a smaller increase in that of HepG2-Neo cells but had no effect on that of HepG2-Mt cells. Finally, the promoter region of Nm23-H1 spanning nucleotides -471 to -437 (relative to the transcriptional initiation site) inhibited the expression of a downstream reporter gene, in a T3-dependent manner, in COS-1 cells also transfected with an expression plasmid encoding TRalpha1 or TRbeta1. The DNA binding domain of TRbeta1 was required for this inhibitory effect. These results indicate that T3, acting through TRs, inhibits transcription of Nm23-H1, and that this effect is mediated by a negative regulatory element in the promoter region of the gene. Thus, it is possible that T3 promotes tumor metastasis by inducing down-regulation of Nm23-H1 expression.

The cellular response to p53: the decision between life and death

The p53 tumor suppressor protein plays a crucial role in regulating cell growth following exposure to various stress stimuli. p53 induces either growth arrest, which prevents the replication of damaged DNA, or programmed cell death (apoptosis), which is important for eliminating defective cells. Whether the cell enters growth arrest or undergoes apoptosis, depends on the final integration of incoming signals with antagonistic effects on cell growth. Many factors affect the cellular response to activated p53. These include the cell type, the oncogenic status of the cell with emphasis on the Rb/E2F balance, the extracellular growth and survival stimuli, the intensity of the stress signals, the level of p53 expression and the interaction of p53 with specific proteins. p53 is regulated both at the levels of protein stability and biochemical activities. This complex regulation is mediated by a range of viral and cellular proteins. This review discusses this intriguing complexity which affects the cell response to p53 activation.