Germline and somatic thyroid hormone receptor mutations in man

Thyroid hormone promotes differentiation of colon cancer stem cells

Tumor formation and maintenance depend on a small fraction of cancer stem cells (CSCs) that can self-renew and generate a wide variety of differentiated cells. CSCs are resistant to chemotherapy and radiation, and can represent a reservoir of cancer cells that often cause relapse after treatment. Evidence suggests that CSCs also give rise to metastases. Thyroid hormone (TH) controls a variety of biological processes including the development and functioning of most adult tissues. Recent years has seen the emergence of an intimate link between TH and multiple steps of tumorigenesis. Thyroid hormone controls the balance between the proliferation and differentiation of CSCs, and may thus be a druggable anti-cancer agent. Here, we review current understanding of the effects of TH on colorectal CSCs, including the cross regulatory loops between TH and regulators of CSC stemness. Targeting TH in the tumor microenvironment may improve treatment strategies.

Classical nuclear hormone receptor activity as a mediator of complex biological responses: a look at health and disease

Nuclear hormone receptors are a large family of receptors that bind a wide range of lipolic hormones and intracellular ligands. They act as ligand-inducible transcription factors to regulate the expression of target genes and play important roles in normal development, reproduction, and metabolism. NRs bind to hormones steroids, thyroid hormone, and vitamin D as well as metabolites of fatty acids, cholesterol, and bild acids. Orphan receptors are another group of NRs for which no known ligands have been identified yet but appear to have major roles in regulating intracellular metabolism. Targeting NRs has been a major source for the development of new drugs, particularly selective agonists and antagonists for cancer and metabolic diseases. Additionally, hormone resistance syndromes in man have enlarged our understanding of the functions of specific NRs and their isoforms as well as genetic mechanisms for phenotype expression.

Thyroid hormone receptor mutants implicated in human hepatocellular carcinoma display an altered target gene repertoire

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that control multiple aspects of normal physiology and development. Mutations in TRs have been identified at high frequency in certain cancers, including human hepatocellular carcinomas (HCCs). The majority of HCC-TR mutants bear lesions within their DNA recognition domains, and we have hypothesized that these lesions change the mutant receptors' target gene repertoire in a way crucial to their function as oncoproteins. Using stable cell transformants and expression array analysis, we determined that mutant TRs isolated from two different HCCs do, as hypothesized, display a target gene repertoire distinct from that of their normal TR progenitors. Only a subset of genes regulated by wild-type TRs was regulated by the corresponding HCC-TR mutants. More surprisingly, the HCC-TR mutants also gained the ability to regulate additional target genes not recognized by the wild-type receptors, and were not simply restricted to repression, but could also activate a subset of their target genes. We conclude that the TR mutants isolated from HCC have sustained multiple alterations from their normal progenitors that include not only changes in their transcriptional outputs, but also changes in the genes they target; both are likely to contribute to neoplasia.

Thyroid hormone receptor mutations found in renal clear cell carcinomas alter corepressor release and reveal helix 12 as key determinant of corepressor specificity

Thyroid hormone receptors (TRs) regulate multiple normal physiological and developmental pathways, whereas mutations in TRs can result in endocrine and neoplastic disease. A particularly high rate of TR mutations has been found in human renal clear cell carcinomas (RCCCs). We report here that the majority of these RCCC TR mutants tested are defective for transcriptional activation and behave as dominant-negative inhibitors of wild-type receptor function. Although several of the dominant-negative RCCC TR mutants are impaired for hormone binding, all fail to release from corepressors appropriately in response to T(3), a trait that closely correlates with their defective transcriptional properties. Notably, many of these mutants exhibit additional changes in their specificity for different corepressor splice forms that may further contribute to the disease phenotype. Mapping of the relevant mutations reveals that the C-terminal receptor helix 12 is not simply a hormone-operated switch that either permits or prevents all corepressor binding, but is instead a selective gatekeeper that actively discriminates between different forms of corepressor even in the absence of T(3).

Distinct dysregulation of lipid metabolism by unliganded thyroid hormone receptor isoforms

Thyroid hormone receptors (TRs) play critical roles in energy homeostasis. To understand the role of TRs in lipid homeostasis in vivo, we adopted the loss-of-function approach by creating knock-in mutant mice with targeted mutation in the TRalpha gene (TRalpha1PV mouse) or TRbeta gene (TRbetaPV mouse). The PV mutation, identified in a patient with resistance to thyroid hormone, exhibits potent dominant-negative activity. Here we show that in contrast to TRalpha1PV mouse, TRbetaPV mice exhibited no significant reduction in WAT but had significant increases in serum free fatty acids and total triglycerides. Moreover, the liver of TRbetaPV mice was markedly increased (33%) with excess lipid accumulation, but the liver mass of TRalpha1PV mouse was decreased (23%) with paucity of lipids. These results indicate that apo-TRbeta and apo-TRalpha1 exerted distinct abnormalities in lipid metabolism. Further biochemical analyses indicate that increased lipogenic enzyme expression, activated peroxisome proliferator-activated receptor gamma (Ppargamma) signaling, and decreased fatty acid beta-oxidation activity contributed to the adipogenic steatosis and lipid accumulation in the liver of TRbetaPV mice. In contrast, the expression of lipogenic enzymes and Ppargamma was decreased in the liver of TRalpha1PV mice. These results suggest that the regulation of genes critical for lipid metabolism by TRs in the liver is isoform dependent. These results indicate that apo-TRbeta and apo-TRalpha1 had different effects on lipid metabolism and that both TR isoforms contribute to the pathogenesis of lipid metabolism in hypothyroidism.

Development of a thyroid hormone receptor targeting conjugate

Molecular conjugates of hormone receptor-ligands with molecular probes or functional domains are finding diverse applications in chemical biology. Whereas many examples of hormone conjugates that target steroid hormone receptors have been reported, practical ligand conjugates that target the nuclear thyroid hormone receptor (TRbeta) are lacking. TR-targeting conjugate scaffolds based on the ligands GC-1 and NH-2 and the natural ligand triiodothyronine (T3) were synthesized and evaluated in vitro and in cellular assays. Whereas the T3 or GC-1 based conjugates did not bind TRbeta with high affinity, the NH-2 inspired fluorescein-conjugate JZ01 showed low nanomolar affinity for TRbeta and could be used as a nonradiometric probe for ligand binding. A related analogue JZ07 was a potent TR antagonist that is 13-fold selective for TRbeta over TRalpha. JZ01 localizes in the nuclei of TRbeta expressing cells and may serve as a prototype for other TR-targeting conjugates.

Thyroid color flow Doppler sonography: an adjunctive tool for differentiating patients with inappropriate thyrotropin (TSH) secretion due to TSH-secreting pituitary adenoma or resistance to thyroid hormone

BACKGROUND

Thyrotropin (TSH)-secreting pituitary adenoma (TSHoma) and resistance to thyroid hormone (RTH) are two forms of inappropriate TSH secretion. Thyroid blood flow is largely TSH dependent.

OBJECTIVE

To assess whether thyroid blood flow may help to differentiate TSHoma and RTH.

DESIGN

Intrathyroidal color flow Doppler sonography (CFDS) pattern and peak systolic velocity (PSV) were assessed at baseline and during T(3) suppression test on eight consecutive patients with TSHoma and 10 with RTH.

MAIN OUTCOME

All controls had CFDS pattern 0. Three RTH patients had pattern I and seven had pattern II. Two TSHoma patients had pattern I, five had pattern II, and one had pattern III. PSV at baseline was 3.8 +/- 1.3 cm/s in controls, 8.8 +/- 2.5 cm/s in RTH, 11.1 +/- 2.7 cm/s in TSHoma (p < 0.0003 vs. controls, p = 0.087 RTH vs. TSHoma). After T3 suppression test, PSV values were lower in RTH than in TSHoma (4.6 +/- 1.8 vs. 7.7 +/- 2.6 cm/s, p = 0.008). PSV values and CFDS pattern normalized in nine and eight RTH patients, respectively, after T(3) suppression test; conversely, only one TSHoma patient had a normalization of PSV values, and none had a normalization of CFDS pattern (p < 0.003 vs. RTH).

CONCLUSIONS

Both RTH and TSHoma have increased CFDS pattern and PSV values; however, after T(3) both parameters normalized in most patients with RTH but not in those with TSHoma. Accordingly, CFDS pattern and PSV are adjunctive tools to differentiate these two forms of inappropriate TSH secretion.

Thyroid hormone receptors mutated in liver cancer function as distorted antimorphs

Aberrant thyroid hormone receptors (TRs) are found in over 70% of the human hepatocellular carcinomas (HCCs) analysed. To better understand the role(s) of these TR mutants in this neoplasia, we analysed a panel of HCC mutant receptors for their molecular properties. Virtually all HCC-associated TR mutants tested retained the ability to repress target genes in the absence of T3, yet were impaired in T3-driven gene activation and functioned as dominant-negative inhibitors of wild-type TR activity. Intriguingly, the HCC TRalpha1 mutants exerted dominant-negative interference at all T3 concentrations tested, whereas the HCC TRbeta1 mutants were dominant-negatives only at low and intermediate T3 concentrations, reverting to transcriptional activators at higher hormone levels. The relative affinity for the SMRT versus N-CoR corepressors was detectably altered for several of the HCC mutant TRs, suggesting changes in corepressor preference and recruitment compared to wild type. Several of the TRalpha HCC mutations also altered the DNA recognition properties of the encoded receptors, indicating that these HCC TR mutants may regulate a distinct set of target genes from those regulated by wild-type TRs. Finally, whereas wild-type TRs interfere with c-Jun/AP-1 function in a T3-dependent fashion and suppress anchorage-independent growth when ectopically expressed in HepG2 cells, at least certain of the HCC mutants did not exert these inhibitory properties. These alterations in transcriptional regulation and DNA recognition appear likely to contribute to oncogenesis by reprogramming the differentiation and proliferative properties of the hepatocytes in which the mutant TRs are expressed.

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.