New Insights into the Structure and Function of the Thyroid Hormone Receptor

Protocol for a feasibility study of a cohort embedded randomised controlled trial comparing phron paring reatment (NEST) for small renal masses

INTRODUCTION

Small renal masses (SRMs; ≤4 cm) account for two-thirds of new diagnoses of kidney cancer, the majority of which are incidental findings. The natural history of the SRM seems largely indolent. There is an increasing concern regarding surgical overtreatment and the associated health burden in terms of morbidity and economy. Observational data support the safety and efficacy of percutaneous cryoablation but there is an unmet need for high-quality evidence on non-surgical management options and a head-to-head comparison with standard of care is lacking. Historical interventional trial recruitment difficulties demand novel study conduct approaches. We aim to assess if a novel trial design, the cohort embedded randomised controlled trial (RCT), will enable carrying out such a comparison.

METHODS AND ANALYSIS

Single-centre prospective cohort study of adults diagnosed with SRM (n=200) with an open label embedded interventional RCT comparing nephron sparing interventions. Cohort participants will be managed at patient and clinicians' discretion and agree with longitudinal clinical data and biological sample collection, with invitation for trial interventions and participation in comparator control groups. Cohort participants with biopsy-proven renal cell carcinoma eligible for both percutaneous cryoablation and partial nephrectomy will be randomly selected (1:1) and invited to consider percutaneous cryoablation (n=25). The comparator group will be robotic partial nephrectomy (n=25). The primary outcome of this feasibility study is participant recruitment. Qualitative research techniques will assess barriers and recruitment improvement opportunities. Secondary outcomes are participant trial retention, health-related quality of life, treatment complications, blood transfusion rate, intensive care unit admission and renal replacement requirement rates, length of hospital stay, time to return to pre-treatment activities, number of work days lost, and health technologies costs.

ETHICS AND DISSEMINATION

Ethical approval has been granted (UK HRA REC 19/EM/0004). Study outputs will be presented and published.

TRIAL REGISTRATION

ISRCTN18156881; Pre-results.

Mutation analysis underlying the downregulation of the thyroid hormone receptor β1 gene in the Chinese breast cancer population

PURPOSE

There are a growing number of reports suggesting that the aberrant expression and mutation of the thyroid hormone receptor β1 (TRβ1) gene is associated with the development of human neoplasms. However, its exact role in the pathogenesis of breast cancer remains elusive. In the present study, we analyzed the mRNA expression and mutations of the TRβ1 gene in the Chinese breast cancer population.

METHODS

The expression of TRβ1 mRNA was examined by real-time quantitative reverse transcription polymerase chain reaction, and mutations in the TRβ1 gene in the hotspot region that spans exons 7-10 were analyzed by polymerase chain reaction single-strand conformation polymorphism and automated DNA sequencing.

RESULTS

TRβ1 mRNA expression was significantly reduced in all 105 breast cancer specimens examined. A total of 20 samples showed truncating mutations within the exons 7-10 of the TRβ1 gene, where eight cases harbored a frame shift mutation (five cases of c.850insA in exon 7 and three cases c.1028delA in exon 8), whereas missense mutations were observed in 12 breast cancer cases. The 20 cases with mutation in the TRβ1 gene showed a reduction in TRβ1 mRNA expression compared with that observed in matched normal tissues. The mutation was also correlated with menopausal stage and estrogen receptor status.

CONCLUSION

The findings of the present study suggest that the aberrant expression and mutations of the TRβ1 gene are associated with the development of breast cancer and that the mutations in the TRβ1 gene partly serve as the underlying mechanism for TRβ1 inactivation in the Chinese breast cancer population.

Down-regulation of thyroid hormone receptor β1 gene expression in gastric cancer involves promoter methylation

Hypermethylation has been shown in the promoter region of the thyroid hormone receptor β1 (TRβ1) gene in several human tumors. However, its role in gastric cancer formation is still unclear. In the study, we analyzed mRNA expression of TRβ1 gene using real-time quantitative PCR (qPCR). A quantitative methylation-specific PCR (Q-MSP) assay was used to determine the methylation status of the TRβ1 gene promoter region in 46 pair-matched gastric neoplastic and adjacent non-neoplastic tissues. The results showed that TRβ1 mRNA expression was significantly reduced in gastric cancer specimens. The methylation of promoter of TRβ1 gene in gastric cancer tissues was significantly higher than in adjacent normal tissues. Promoter hypermethylation of the TRβ1 gene correlated with tumor infiltration, lymph node metastasis, and distant metastasis, but it was not associated with other clinicopathological characteristics. We treated gastric cancer cell lines MKN-45, MKN-28, SGC-7901, NCI-N87, and SNU-1 with 5-Aza-2-deoxycytidine (5-Aza-dC). The results showed the expression of TRβ1 mRNA was increased in MKN-45, MKN-28, SGC-7901, but not increased in NCI-N87 and SNU-1. These results suggest that the TRβ1 gene plays important roles in the development of gastric cancer partially through epigenetic mechanisms.

Long-term results after computed-tomography-guided percutaneous radiofrequency ablation for small renal tumors

PURPOSE

To present the long-term results and to identify possible risk factors for recurrence after radiofrequency ablation (RFA) for renal tumors.

PATIENTS AND METHODS

Thirty-one patients with a total of 39 renal tumors ranging from 1.3 to 7.5 cm (mean size 3.1 cm) were treated with RFA using a Rita Medical System model 1500 RF generator attached to a 15-gauge Starburst XL probe under percutaneous CT scan guidance. The average patient age was 61.4 years (range 37-86 y). Indications for RFA were bilateral tumors, presence of serious comorbidities, a high risk of development of additional renal-cell carcinomas, marginal renal function, and patient preference. No tumor was biopsied before treatment; therefore, only local control success rates could be provided. Recurrence was defined as enhancement or lesion enlargement on follow-up CT scan. Statistical analysis was performed to identify possible risk factors for recurrence. Parameters tested were age, sex, and number of ablation sessions, tumor size, location, multiplicity, body mass index, and American Society of Anesthesiologists score.

RESULTS

Initial ablation success rate was 90% and with repeated treatment, a success rate of complete ablation reached 97%. Average follow-up was 61.2 months (range 36-84 mos). Recurrence was seen in four tumors. The risk factor associated with recurrence was tumor size exceeding 4 cm (P < 0.01, relative risk [RR] = 3.31). Overall 3- and 5-year tumor control rate was 92% and 89%, respectively. Tumor size was also predictive for recurrence in the subgroup of 17 patients followed for more than 5 years (P = 0.02, RR = 3.15). Tumor control rate for this subgroup was 90%.

CONCLUSIONS

According to our results, larger tumor size was prognostic for recurrence after RFA for renal tumors. This treatment seems to demonstrate excellent tumor control long-term results, comparable to those achieved by nephron-sparing surgery in a selected group of patients.

Human testicular orphan receptor 4 enhances thyroid hormone receptor signaling

The thyroid hormone receptor (TR) and human testicular orphan receptor 4 (TR4) belong to the nuclear hormone receptor superfamily. They are ligand-dependent transcription factors. TR and TR4 bind to a similar thyroid response element (TRE), known as a direct repeat with four nucleotide spacing (DR4). This study examined the possible interaction or cross-talking between those two receptors. We hypothesized that protein-protein interaction between TR4 and TR may promote TR-mediated transcriptional activity. Glutathione S-transferase pull-down and immunoprecipitation assays showed direct interaction between TR and TR4. Electrophoretic mobility-shift assay demonstrated that TR and TR4 could co-occupy the same TRE. The interaction between TR4 and TR may enhance regulation of genes targeted by TR, such as furin, fibrinogen, cdk2 and p21 expression. We found that TR4 function is similar with TR as TR4 alone could regulate expression of some TR target genes, and could increase cell migration or inhibit cell proliferation. Importantly, the TR-dependent inhibition of cell proliferation and stimulation of cell migration are more enhanced in the HepG2-TR cells stably over-expressing TR4. Overall, TR4 not only has modulation abilities similar to TR but also can cross-talk with TR and promote the TR signaling pathway.

Renal tumors

The treatment of RCC is rapidly changing. The introduction of minimally invasive thermal ablation techniques offers a safe and accurate alternative to open surgery in the treatment of renal tumors. Because of its technical benefits, percutaneous radiofrequency ablation took the lead among these minimally invasive techniques. Supported by convincing results from experimental studies, patient data prove this procedure to be safe and efficient. As a minimally invasive and nephron-sparing technique, it is well suited for patients with a single kidney, multiple tumors, or contraindications for open surgery.

Thyroid hormone receptor β1 in normal colon and colorectal cancer–association with differentiation, polypoid growth type and K-ras mutations

The precursors for colorectal cancer include polypoid (conventional), flat and serrated adenomas. Polypoid growth in polypoid adenomas and serrated adenomas is associated with K-ras mutations. The regulation of polypoid or nonpolypoid growth is not well known, but could be related to trophic stimuli, such as thyroid hormones. Hence, we investigated the expression pattern of thyroid hormone receptor TRbeta1 in colorectal mucosa and in colorectal tumours and its relationship to tumour growth type. One hundred fourteen colorectal carcinoma specimens were evaluated for TRbeta1. Normal mucosa, adjacent adenomatous component (N = 46) and lymph node metastases (N = 28) were analysed when present, and the results were confirmed by Western blot analysis in selected cases. Nuclear TRbeta1 was almost always present in normal epithelium (96%), but less frequent in adenomas (83%) and in cancer (68%; p < 0.001 and p < 0.001, respectively). TRbeta1 was associated with polypoid growth, presence of K-ras mutations and also with a higher WHO histological grade and advanced Dukes' stage. Cytoplasmic expression of TRbeta1 was observed in nonneoplastic and neoplastic epithelium. In Western blot analysis, a 58 kDa band corresponding to TRbeta1 was expressed in normal mucosa and in colorectal cancer specimens with positive immunohistochemistry. Association of TRbeta1 expression with growth pattern and the presence of K-ras mutations suggest that abnormalities in thyroid hormone signalling involving TRbeta1 play a role in the development of some types of colorectal adenocarcinomas.

Needle-based ablation of renal parenchyma using microwave, cryoablation, impedance- and temperature-based monopolar and bipolar radiofrequency, and liquid and gel chemoablation: laboratory studies and review of the literature

BACKGROUND AND PURPOSE

Small renal tumors are often serendipitously detected during the screening of patients for renal or other disease entities. Rather than perform a radical or partial nephrectomy for these diminutive lesions, several centers have begun to explore a variety of ablative energy sources that could be applied directly via a percutaneously placed needle-like probe. To evaluate the utility of such treatment for small renal tumors/masses, we compared the feasibility, regularity (consistency in size and shape), and reproducibility of necrosis produced in normal porcine kidneys by different modes of tissue ablation: microwaves, cold impedance-based and temperature-based radiofrequency (RF) energy (monopolar and bipolar), and chemical. Chemoablation was accomplished using ethanol gel, hypertonic saline gel, and acetic acid gel either alone or with simultaneous application of monopolar or bipolar RF energy.

MATERIALS AND METHODS

A total of 107 renal lesions were created laparoscopically in 33 domestic pigs. Microwave thermoablation (N=12) was done using a Targis T3 (Urologix) 10F antenna. Cryoablation (N=16) was done using a single 1.5-mm probe or three 17F microprobes (17F SeedNet system; Galil Medical) (N=10 single probe and N=6 three probes); a double freeze cycle with a passive thaw was employed under ultrasound guidance. Dry RF lesions were created using custom-made 18-gauge single-needle monopolar probe with two or three exposed metal tips (GelTx) (N=12) or a single-needle bipolar probe (N=6) at 50 W of 510 kHz RF energy for 5 minutes. In addition, a multitine RF probe (RITA Medical Systems) was used in one set of studies (N=6). Both impedance- and temperature-based RF were evaluated. Chemoablation was performed with 95% ethanol (4 mL), 24% hypertonic saline (4 mL), and 50% acetic acid (4 mL) as single injections. In addition, chemoablation was tested with monopolar and bipolar RF (wet RF). Tissues were harvested 1 week after ablation for light microscopy.

RESULTS

In 11 of the 15 ablation techniques, there was complete necrosis in all lesions; however, three ethanol gel lesions had skip areas, three hypertonic saline gel lesions showed no necrosis or injury, and one monopolar RF and one bipolar RF lesion showed skip areas. In contrast to impedance-based RF, heat-based RF (RITA) caused complete necrosis without skip areas. All cryolesions resulted in complete tissue necrosis, and cryotherapy was the only modality for which lesion size could be effectively monitored using ultrasound imaging.

CONCLUSIONS

Cryoablation and thermotherapy produce well-delineated, completely necrotic renal lesions. The single-probe monopolar and bipolar RF produce limited areas of tissue necrosis; however, both are enhanced by using hypertonic saline, acetic acid, or ethanol gel. Hypertonic saline gel with RF consistently provided the largest lesions. Ethanol and hypertonic saline gels tested alone failed to produce consistent cellular necrosis at 1 week. In contrast, RITA using the Starburst XL probe produced consistent necrosis, while impedance-based RF left skip areas of viable tissue. Renal cryotherapy under ultrasound surveillance produced hypoechoic lesions, which could be reasonably monitored, while all other modalities yielded hyperechoic lesions the margins of which could not be properly monitored with ultrasound imaging.

Thyroid hormones regulate DNA-synthesis and cell-cycle proteins by activation of PKC? and p42/44 MAPK in chick embryo hepatocytes

The molecular mechanism by which thyroid hormones exert their effects on cell growth is still unknown. In this study, we used chick embryo hepatocytes at different stages of development as a model to investigate the effect of the two thyroid hormones, T3 and T4, and of their metabolite T2, on the control of cell proliferation. We observed that T2 provokes increase of DNA-synthesis as well as T3 and T4, independently of developmental stage. We found that this stimulatory effect on the S phase is reverted by specific inhibitors of protein kinase C (PKC) and p42/44 mitogen-activated protein kinase (p42/44 MAPK), Ro 31-8220 or PD 98059. Furthermore, the treatment with thyroid hormones induces the activation of PKCalpha and p42/44 MAPK, suggesting their role as possible downstream mediators of cell response mediated by thyroid hormones. The increase of DNA-synthesis is well correlated with the increased levels of cyclin D1 and cdk4 that control the G1 phase, and also with the activities of cell-cycle proteins involved in the G1 to S phase progression, such as cyclin E/A-cdk2 complexes. Interestingly, the activity of cyclin-cdk2 complexes is strongly repressed in the presence of PKC and p42/44 MAPK inhibitors. In conclusion, we demonstrated that the thyroid hormones could modulate different signaling pathways that are able to control cell-cycle progression, mainly during G1/S transition.

Thyroid hormone receptor alpha 1 (c-erb A alpha 1) suppressed transforming phenotype of nasopharyngeal carcinoma cell line

Only three thyroid hormone receptor (TR) isoforms, alpha 1, beta 1, and beta 2, bind thyroid hormone (TH) and are considered to be true TRs. TR alpha 2, unable to bind TH, binds to TH response element on DNA and has been shown to exert dominant negative action on TR alpha1. TR alphas regulate many important processes such as proliferation, differentiation and apoptosis. To find out if TR alphas played roles in growth control of nasopharyngeal carcinoma cells, transfectant with inducible expression of TR alpha 1 was generated from NPC-TW 04 cell lines. Induced expression of TR alpha 1 in nasopharyngeal carcinoma cell reduced proliferation and colony-formation ability in agar. Tumor formation ability in nude mice was reduced in NPC cells with TR alpha 1 expression than those without expression or vector-transfected cells. Our results supported the hypothesis that TR alpha 1 functions as a tumor suppressor gene in nasopharyngeal carcinoma tumorigenesis.

Alterations of thyroid hormone receptor alpha gene: frequency and association with Nm23 protein expression and metastasis in gastric cancer

Thyroid hormone receptor genes (c-erbA, or TR) have been implicated as tumor suppressor genes. We investigated whether two subtypes of TR genes (TRalpha and TRbeta) are altered in human gastric cancer, as well as the role of such alterations in tumor metastasis. As a preliminary survey (n=18), Southern blot analysis was conducted with full-length TRalpha or TRbeta cDNA probes, after digestion with restriction enzyme (EcoRI, BamHI, or HindIII). The survey showed the most frequent alteration was in TRalpha gene by a loss of two HindIII fragments (7.1 and 10.1 kb). Subsequent study focused at this TRalpha alteration by increasing patient number up to a total of 42. TRalpha alteration was noted in 19 (49%) of the 39 patients with informative results. The TRalpha alteration was associated with the development of distant (hematogenous) metastasis (P=0.0084) and high expression level of Nm23 protein (P=0.020). These findings suggest that TRalpha might be involved in tumorigenesis, and that interactions between the TRalpha and nm23 genes might take part in hematogenous metastasis of gastric cancer.

Impaired interaction of mutant thyroid hormone receptors associated with human hepatocellular carcinoma with transcriptional coregulators

Thyroid hormone (T(3)) exerts its many biological activities through interaction with specific nuclear receptors (TRs) that function as ligand-dependent transcription factors at genes that contain a thyroid hormone response element (TRE). Mutant TRs have been detected in human hepatocellular carcinoma cell lines and tissue, but their contribution to carcinogenesis has remained unclear. The interaction of four such mutant TRs (J7-TRalpha1, J7-TRbeta1, H-TRalpha1, and L-TRalpha1) with transcriptional coregulators has now been investigated. With the exception of J7-TRalpha1, which in the absence of T(3) exhibited transcriptional silencing activity with a TRE-reporter gene construct in transfected cells, the mutant TRs had little effect (compared with that of wild-type receptors) on transcriptional activity of the reporter gene in the absence or presence of T(3), of the transcriptional corepressors SMRT, NCoR or of the transcriptional coactivator SRC. Electrophoretic mobility-shift assays revealed that, in the presence of T(3), the J7-TRss1 mutant did not interact with SRC, whereas J7-TRalpha1 and H-TRalpha1 exhibited reduced abilities to associate with this coactivator and L-TRalpha1 showed an ability to interact with SRC similar to that of wild-type TRalpha1. The dominant negative activity of the mutant TRs in transfected cells appeared inversely related to the ability of the receptors to interact with SRC. Whereas J7-TRss1, H-TRalpha1, and L-TRalpha1 did not interact with SMRT, and NCoR. J7-TRalpha1 bind to corepressors but failed to dissociate from them in the presence of T(3). These aberrant interactions between the mutant TRs and transcriptional coregulators may contribute to the highly variable clinical characteristics of human hepatocellular carcinoma.

Mice with a targeted mutation in the thyroid hormone beta receptor gene exhibit impaired growth and resistance to thyroid hormone

Patients with mutations in the thyroid hormone receptor beta (TRbeta) gene manifest resistance to thyroid hormone (RTH), resulting in a constellation of variable phenotypic abnormalities. To understand the molecular basis underlying the action of mutant TRbeta in vivo, we generated mice with a targeted mutation in the TRbeta gene (TRbetaPV; PV, mutant thyroid hormone receptor kindred PV) by using homologous recombination and the Cre/loxP system. Mice expressing a single PV allele showed the typical abnormalities of thyroid function found in heterozygous humans with RTH. Homozygous PV mice exhibit severe dysfunction of the pituitary-thyroid axis, impaired weight gains, and abnormal bone development. This phenotype is distinct from that seen in mice with a null mutation in the TRbeta gene. Importantly, we identified abnormal expression patterns of several genes in tissues of TRbetaPV mice, demonstrating the interference of the mutant TR with the gene regulatory functions of the wild-type TR in vivo. These results show that the actions of mutant and wild-type TRbeta in vivo are distinct. This model allows further study of the molecular action of mutant TR in vivo, which could lead to better treatment for RTH patients.

Hormone binding induces rapid proteasome-mediated degradation of thyroid hormone receptors

The thyroid hormone 3,3',5-triiodo-l-thyronine (T3) is essential for growth, differentiation, and development. Its biological activities are mediated by T3 nuclear receptors (TRs). At present, how T3 regulates TR proteins and the resulting functional consequences are still unknown. Immunofluorescence analyses of endogenous TR in the growth hormone-producing GC cells showed that the T3-induced rapid degradation of TR was specifically blocked by lactacystin, a selective inhibitor of the ubiquitin-proteasome degradation pathway. Immunoblots demonstrated that the transfected TRbeta1 was ubiquitinated and that the ubiquitination was T3 independent. Studies with a series of truncated TRbeta1 showed that the hormone-binding domain was sufficient for the T3-induced rapid degradation of TRbeta1 by the proteasome degradation pathway. T3 also induced rapid degradation of TRbeta2 and TRalpha1. In contrast, the stability of the non-T3-binding TRalpha2 and naturally occurring TRbeta1 mutants that do not bind T3 was not affected by T3 treatment, indicating that hormone binding to receptor was essential for the degradation of the wild-type receptors. In the presence of proteasome protease inhibitors, the levels of both total and ubiquitinated TRbeta1 protein increased, yet T3-dependent transcriptional activation and the expression of the growth hormone gene were diminished, suggesting that proteasome-mediated degradation played a novel role in modulating transcriptional activation by TR. The present study reveals a role of T3 in modulating the functions of TR by regulating its receptor level via the ubiquitin-proteasome degradation pathway.

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 orphan nuclear receptor Ear-2 is a negative coregulator for thyroid hormone nuclear receptor function

Thyroid hormone (T3) nuclear receptors (TR) are ligand-dependent transcription factors which regulate growth, differentiation, and development. One emerging hypothesis suggests that TR mediate these diverse effects via a large network of coregulators. Recently, we found that TR-mediated transcriptional responses varied in six cell lines derived from different tissues. We therefore used human TR subtype beta1 (TRbeta1) as bait to search for coregulators in human colon carcinoma RKO cells with a yeast two-hybrid system. RKO cells exhibited T3-dependent and -independent transcriptional activation. One of the three positive clones was identified as Ear-2, which is a distant member of the chick ovalbumin upstream promoter-transcription factors of the orphan nuclear receptor family. The physical interaction between Ear-2 and TRbeta1 was further confirmed by specific binding of Ear-2 to glutathione S-transferase-TRbeta1. In addition, Ear-2 was found to associate with TRbeta1 in cells. As a result of this physical interaction, binding of TRbeta1 to the T3 response elements was inhibited. Using reporter systems, we found that both the basal activation and the T3-dependent activation mediated by TRbeta1 were repressed by Ear-2 in CV1 cells. In RKO cells, however, the T3-independent transcriptional activity was more sensitive to the repression effect of Ear-2 than the T3-dependent transcriptional activity. The repression effect of Ear-2 was reversed by steroid hormone receptor coactivator 1. These results suggest that TR-mediated responses reflect a balance of corepressors and coactivators in cells. These findings further strengthen the hypothesis that the diverse activities of TR are achieved via a large network of coregulators that includes Ear-2.

Thyroid hormone-induced cell proliferation in GC cells is mediated by changes in G1 cyclin/cyclin-dependent kinase levels and activity

The thyroid hormone, 3,3', 5-triiodo-L-thyronine (T3), is essential for growth and regulation of metabolic functions. The biological activities of T3 are mediated by its interaction with the thyroid hormone nuclear receptors (TRs). The mechanism by which TRs mediate cell growth is unknown. We found that T3 stimulated cell growth in GC cells by shortening the doubling time approximately 3-fold. Flow cytometric analysis indicated that the growth stimulatory effect was mainly due to shortening of G1 phase accompanied by increases in S and G2/M phases of the cell cycle. These changes correlated with T3-induced increases in messenger RNA and protein levels of two key regulators of G1 progression, cyclins D1 and E, as well as cdk2. Furthermore, the kinase activities associated with cyclin D1 and E were activated up to 4-fold by T3, which led to increased phosphorylation of the retinoblastoma protein (Rb), the driving force in G1 to S cell cycle progression. These results show for the first time that the growth promoting effect of T3 in GC cells is mediated, at least in part, by increases in cyclin/cdk activities and the phosphorylation state of Rb. The functional link of T3 to Rb has important implications for the understanding of the biology of normal and cancer cells.

Expression of the mutant thyroid hormone receptor PV in the pituitary of transgenic mice leads to weight reduction

Resistance to thyroid hormone (RTH) is a genetic disease caused by mutations of the thyroid hormone receptor beta gene (TRbeta). One of the symptoms in some affected individuals is growth retardation. To understand the molecular basis of growth retardation in these patients with RTH, a transgenic mouse was prepared in which the expression of the TRbeta1 mutant PV was targeted to the pituitary using the promoter of the glycoprotein hormone alpha-subunit. The PV mutant was originally identified in a patient with severe growth impairment. The PV mutation is a C-insertion at codon 448 of the TRbeta gene and leads to a frame-shift of the carboxyl-terminal 14 amino acids of TRbeta1, resulting in total loss of triiodothyronine (T3) binding and transcriptional activation. PV was selectively expressed in the pituitary of the transgenic mouse and not in other tissues examined. The transgenic mice showed a significant impairment in weight gain. However, no changes in the serum level of thyroid-stimulating hormone were seen, and no elevation of thyroid hormones was detected in the transgenic mice. The circulating levels of growth hormone and insulin-like growth factor I were not affected in the transgenic mice, suggesting that the growth impairment in RTH is complex and is mediated by pathways that are yet to be elucidated.

Expression of mutant thyroid hormone nuclear receptors in human hepatocellular carcinoma cells

To understand the expression and role of thyroid hormone nuclear receptors (TRs) in hepatocarcinogenesis, we characterized the TRs in 16 human hepatocellular carcinoma (HCC) specimens. The full-length cDNAs for the two TR subtypes, alpha1 and beta1, were cloned from several tumors by reverse transcription-polymerase chain reaction. Southern blot analysis indicated that, in addition to the full-length cDNA, truncated TRalpha1 and TRbeta1 cDNAs were present in nine tumors (53%). In addition, point mutations detected by the mismatch RNase cleavage assay in TRalpha1 and TRbeta1 were found in 65% and 76% of the tumors, respectively. The mutations were confirmed by DNA sequencing. Interestingly, most of the TRalpha1 mutations were in amino acid codons 209-228 and 245-256, two hot-spots in HCC patients. However, no hot-spot was detected in TRbeta1. The expression of TRalpha1 and TRbeta1 proteins was determined in the tissue extracts by western blotting. TRbeta1 protein was expressed or elevated in 10 tumors but not in normal livers, whereas the expression of TRalpha1 was variable among tumors. The mutant TR proteins were translated in vitro, and their hormone- and DNA-binding activities were evaluated. Abnormal binding to the thyroid hormone response elements was observed. The proteins' DNA binding activity was either partially impaired or completely lost. The high prevalence of TR mutations found in the tumors of patients with hepatocellular carcinoma suggests that mutant TRs could play an important role in liver carcinogenesis.

Tissue-specific differential repression of gene expression by a dominant negative mutant of thyroid hormone beta1 receptor

Resistance to thyroid hormone (RTH) is a genetic disease caused by the mutations of the thyroid hormone beta receptor (TRbeta) gene, producing receptors with a dominant negative action. The present study addressed the question as to whether tissue-specific factors modulate the dominant negative function in different tissues. We prepared stably transfected pituitary GH3 (GH3-PV) and liver SK-Hep-1 (SK-Hep-1-PV) cell lines with a potent dominant negative mutant, PV. The growth hormone (GH) and the malic enzyme genes (ME) in GH3 and SK-Hep-1, respectively, are directly regulated by the thyroid hormone, 3,3,'5-triiodo-L-thyronine (T3). The ratio of the expressed PV/endogenous TRbeta1 proteins was approximately 20 and 5 for GH3-PV and SK-Hep-1-PV cells, respectively. However, the T3-activated expression of the GH gene in GH3-PV and ME gene in SK-Hep-1-PV was repressed by approximately 30% and 90%, respectively, indicating the lack of correlation of PV/TRpbeta1 protein ratio with the dominant negative potency of mutant PV. Furthermore, the synergistic effect of the pituitary-specific factor 1 on the TR-mediated GH promoter activity was not repressed by mutant PV. Taken together, these results suggest that the dominant negative effect of mutant TR is variable in the tissues studied.

Hormone-induced translocation of thyroid hormone receptors in living cells visualized using a receptor green fluorescent protein chimera

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors that regulate growth, differentiation, and development. To understand the role of the hormone, 3,3', 5-triiodo-L-thyronine (T3), in the nuclear translocation and targeting of TRs to the regulatory sites in chromatin, we appended green fluorescent protein (GFP) to the human TR subtype beta1 (TRbeta1). The fusion of GFP to the amino terminus of TRbeta1 protein did not alter T3 binding or transcriptional activities of the receptor. The subcellular localization of GFP-TRbeta1 in living cells was visualized by laser-scanning confocal microscopy. In the presence of T3, the expressed GFP-TRbeta1 was predominately localized in the nucleus, exhibiting a nuclear/cytoplasmic ratio of approximately 5.5. No GFP-TRbeta1 was detected in the nucleolus. In the absence of T3, more GFP-TRbeta1 was present in the cytoplasm, exhibiting a nuclear/cytoplasmic ratio of approximately 1.5. In these cells, cytoplasmic GFP-TRbeta1 could be induced to enter the nucleus by T3. The T3-induced translocation was blocked when Lys184-Arg185 in domain D of TRbeta1 was mutated to Ala184-Ala185. Furthermore, the inability of the mutant TR to translocate to the nucleus correlated with the loss of most of its transcriptional activity. These results suggest that TR functions may, in part, be regulated by T3-induced nuclear entry.

Hyperactivity and Learning Deficits in Transgenic Mice Bearing a Human Mutant Thyroid Hormone β1 Receptor Gene

Resistance to thyroid hormone (RTH) is a human syndrome mapped to the thyroid receptor β(TRβ) gene on chromosome 3, representing a mutation of the ligandbinding domain of the TRβ gene. The syndrome is characterized by reduced tissue responsiveness to thyroid hormone and elevated serum levels of thyroid hormones. A common behavioral phenotype associated with RTH is attention deficit hyperactivity disorder (ADHD). To test the hypothesis that RTH produces attention deficits and/or hyperactivity, transgenic mice expressing a mutant TRβ gene were generated. The present experiment tested RTH transgenic mice from the PV kindred on behavioral tasks relevant to the primary features of ADHD: hyperactivity, sustained attention (vigilance), learning, and impulsivity. Male transgenic mice showed elevated locomotor activity in an open field compared to male wild-type littermate controls. Both male and female transgenic mice exhibited impaired learning of an autoshaping task, compared to wild-type controls. On a vigilance task in an operant chamber, there were no differences between transgenics and controls on the proportion of hits, response latency, or duration of stimulus tolerated. On an operant go/no-go task measuring sustained attention and impulsivity, there were no differences between controls and transgenics. These results indicate that transgenic mice bearing a mutant human TRβ gene demonstrate several behavioral characteristics of ADHD and may serve a valuable heuristic role in elucidating possible candidate genes in converging pathways for other causes of ADHD.

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

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors which regulate growth, differentiation, and development. The molecular mechanism by which TRs mediated these effects remains unclear. A prevailing hypothesis is that TRs exert their biological effects by cooperating with other transcription factors. We have recently shown that the human TR subtype beta1 (hTRbeta1) interacts with the tumor suppressor p53, which plays a critical role in cell-cycle regulation and tumorigenesis. This interaction of hTRbeta1 with p53 leads to an impairment of TR function. The present study examined whether hTRbeta1 could modulate the function of p53. Mapping of the domains of p53 responsible for the interaction with hTRbeta1 indicated that the regions involved resided in the DNA-binding domain and carboxy terminus of p53. In agreement with this finding, hTRbeta1 increased the binding of p53 to p53 DNA-binding elements. This increase in DNA binding, however, resulted in repression of p53-dependent transcription activation in transfected cells. Furthermore, hTRbeta1 led to an inhibition of the p53-mediated induction of bax and gadd45 expression. In contrast, the p53-induced expression of p21 was not affected by hTRbeta1, suggesting that the expression of p53-regulated genes is differentially modulated by hTRbeta1. Because the expressions of bax, gadd45, and p21 are directly regulated by p53, these results indicate that hTRbeta1 can modulate p53-regulated gene expression and support the hypothesis that there is cross-talk between these two regulatory pathways. The cross-talk between these two transcription factors could play an important role in the biology of normal and cancer cells.

Dominant negative activity of mutant thyroid hormone alpha1 receptors from patients with hepatocellular carcinoma

Complementary DNAs for two mutant thyroid hormone alpha1 receptors (TR alpha1) were isolated from hepatocellular carcinomas of two patients. Sequence analyses of the complementary DNAs showed a single Val390Ala and double Pro398Ser/Glu350Lys mutations in mutants H and L, respectively. We characterized their hormone-binding, DNA-binding, and dominant negative activities. Mutants H and L did not bind the hormone T3. Their DNA-binding activities were analyzed using three types of thyroid hormone response elements (TREs) in which the half-site binding motifs are arranged in an everted repeat (Lys), an inverted repeat (Pal), or a direct repeat separated by four nucleotides (DR4). Compared with wild-type TR alpha1 (w-TR alpha1), which bound these TREs with different homodimer/monomer ratios, binding of mutant L to the three TREs as homodimers was reduced by approximately 90%. However, binding of mutant H to these TREs was more complex. Although it bound normally to DR4 as homodimers, its binding to Lys as homodimers was reduced by approximately 80%. Surprisingly, its binding to Pal was markedly enhanced compared with w-TR alpha1. The binding of these two mutants to the three TREs as heterodimers with retinoid X receptors (RXR alpha and -beta) was not significantly affected. Consistent with the lack of T3-binding activity, both mutants had lost their trans-activation capacity. Mutants H and L exhibited dominant negative activity, but differed in their TRE dependency. The dominant negative potency of mutant H was in the rank order of Pal > DR4 > Lys, whereas no TRE dependency was observed for mutant L. The present study indicates that mutations of the TR alpha gene do occur in patients and that these novel TR alpha1 mutants provide a valuable tool to further understand the molecular basis of the dominant negative action of mutant TRs.

Tumor suppressor p53 is a negative regulator in thyroid hormone receptor signaling pathways

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors which regulate growth, differentiation, and development. The molecular mechanisms by which TRs mediate these diverse effects are unclear. One emerging hypothesis suggests that TRs could mediate these diverse effects via cooperation with different transcription factors/receptors. Indeed, we have recently shown that the human TR subtype beta1 (h-TRbeta1) interacts with the tumor suppressor p53. p53 is a transcription factor that plays a critical role in cell cycle regulation and tumor development. To assess the physiological relevance of the interaction of h-TRbeta1 with p53, the present study addressed the question as to whether the functions of h-TRbeta1 could be modulated by p53. We first compared the h-TRbeta1-mediated transcriptional activity in two pairs of isogenic cell lines, RKO/RKO E6 and MCF-7/MCF-7 E6. RKO and MCF-7 cells are colon and breast carcinoma cell lines, respectively, that contain p53 but lack TRbeta1. The isogenic RKO E6 and MCF-7 E6 cells are stable clones expressing high levels of papillomavirus type 16 E6 protein. In these cells, the level of p53 protein was lower than the parental cells. The impairment of p53 functions in these E6-containing cells led to an activation of TRbeta1-mediated transcriptional activity. Furthermore, in a growth hormone-producing cell line in which the expression of the growth hormone gene is positively regulated by TRs, overexpression of the wild-type p53 led to repression in the expression of the growth hormone gene. Thus, TRs could cross-talk with p53 in its signaling pathways to regulate gene regulatory functions. The present findings further strengthen the hypothesis that mediation of the pleiotropic effects of T3 requires the cooperation of TRs with a large network of transcription factors.

The gene regulating activity of thyroid hormone nuclear receptors is modulated by cell-type specific factors

To understand whether the transcriptional activity of thyroid hormone nuclear receptors (TRs) is modulated by cell-type specific factors, full length TR subtype alpha1 (TRalpha1) and beta1 (TRbeta1) cDNAs were cloned from human hepatoma cell lines: HA22T, SK-Hep-1 and HepG2. The cloned receptor bound to the thyroid hormone 3,3',5-triiodo-L-thyronine (T3) and the thyroid hormone response elements (TREs) similarly to those cloned from other tissues. They exhibited T3- and TRE-dependent transactivation activities, indicating these TRs were transcriptionally active. The lipogenic malic enzyme (ME), a T3-target gene in liver, was stimulated approximately 3- and 1.5-fold by T3 in HA22T and SK-Hep-1, respectively. The T3-stimulated ME gene expression was inhibited in HA22T, but stimulated in SK-Hep-1 cells by insulin. These results suggest that the gene regulating activity of TRs was modulated by cell-type specific factors. Furthermore, these cell-type specific factors could modulate the cross talk between TR- and insulin receptor-mediated pathways.

Hormone-activated phosphorylation of human beta1 thyroid hormone nuclear receptor

To understand the role of phosphorylation in the hormone-dependent transcriptional activation of thyroid hormone receptors (TRs), the present study evaluated the effect of the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3) on the phosphorylation of TR, human subtype beta1 (h-TRbeta1). The extent of phosphorylation was compared in cells cultured in T3-depleted (Td) or T3-supplemented medium (Td + T3). T3 was found to activate phosphorylation of h-TRbeta1 approximately threefold. Taking into account the T3-induced fourfold downregulation in the expression of h-TRbeta1 in the same period, the specific T3-activated phosphorylation was increased approximately twelvefold. Phosphoamino acid analysis indicates that the phosphorylation of serine and threonine in a ratio of approximately 10:1 was increased approximately threefold by T3. Comparison of the [32P]-labeled tryptic maps of h-TRbeta1 phosphorylated in cells cultured in Td medium or Td + T3 medium indicates that the latter had fewer fragments and changes of intensities in several common fragments, indicating that the phosphorylation sites activated by T3-treatment differed from those of basal phosphorylation. Partial V8 and chymotrypic proteolysis indicates that h-TRbeta1 phosphorylated in cells cultured in Td + T3 medium was more resistant to proteolysis. These results indicate that T3-activated phosphorylation altered the protease susceptibility of h-TRbeta1 that could reflect structural changes in h-TRbeta1. These results raise the possibility that T3-activated phosphorylation may play an important role in transcriptional activation of h-TRbeta1.

The differential hormone-dependent transcriptional activation of thyroid hormone receptor isoforms is mediated by interplay of their domains

Human thyroid hormone nuclear receptor isoforms (TRalpha1 and TRbeta1) express differentially in a tissue-specific and development-dependent manner. It is unclear whether these two isoforms have differential functions. We analyzed their interaction with a thyroid hormone response element with half-site binding motifs arranged in an everted repeat separated by six nucleotides (F2). Despite extensive sequence homologies, the two isoforms bound to F2 with different affinities and ratios of homodimer/monomer. Using F2-containing reporter gene, we found that the transcriptional activity of TRbeta1 was approximately 6-fold higher than that of TRalpha1. The lower activity of TRalpha1 was not due to differences in expression of the two isoforms because similar nuclear localization patterns were observed. To understand the structural determinants responsible for these differences, we constructed chimeric receptors in which hinge regions (domain D), hormone binding domains (domain E), and domains (D + E) were sequentially interchanged and their activities were compared. Chimeric TRs containing the domains D, E or (D + E) of TRbeta1 showed increased propensities to form homodimers and mediated higher transactivation activities than TRalpha1. Thus, differential transactivation activities of TR isoforms are mediated by interplay of their domains and could serve as an important regulatory mechanism to achieve diversity and specificity of pleiotropic T3 effect.

Differential sensitivity of thyroid hormone receptor isoform homodimers and mutant heterodimers to hormone-induced dissociation from deoxyribonucleic acid: its role in dominant negative action

General resistance to thyroid hormone is an inheritable disease with resistance of peripheral tissues to elevated levels of thyroid hormone. Genetic studies have shown that it is due to interference in the functions of wild-type thyroid hormone nuclear receptors (wTRs) via the dominant negative effect of mutant TRs (mTRs). The present study compared the heterodimerization of the two TR isoforms, TR beta1 and TR alpha1, with mutant TRs to understand if mTRs had isoform-dependent dominant negative action. Using electrophoresis gel mobility shift assay, we have demonstrated that mutant PV, S, ED, and OK form heterodimers with wTR alpha1 and deltaTR beta1 (in which the A/B domain of wTR beta1 has been deleted), on the F2-thyroid hormone response element (TRE). In the presence of T3, both homo- and heterodimer complexes are dissociated in a T3 concentration dependent manner. The ED50 for deltaTR beta1 homodimers was 3-fold higher than that of wTR alpha1 homodimers. ED50s for deltaTR beta1/mTR heterodimers were 10- to 40-fold higher than the corresponding wTR alpha1/mTR heterodimers. Mutant ED and OK homodimers were only partially dissociated at the highest T3 concentrations used (100 nM), whereas no dissociation could be detected for PV and S homodimers, indicating differential sensitivity of the F2-bound TR dimers to the T3-induced dissociation. We presented a model that indicates the dissociation of any particular TR dimer from F2 is determined by competition of T3 for both of its constituent TRs. By transfection assays, we showed that the potency of the dominant negative action of PV on TR alpha1 and TR beta1 inversely correlated with the sensitivity of the appropriate mTR/wTR heterodimer to T3-induced dissociation from F2. The differential dominant negative action of mutants on the two TR isoforms could play an important role in the heterogeneity of tissue-specific manifestations in patients with resistance to thyroid hormone.

Tissue-specific stabilization of the thyroid hormone beta1 nuclear receptor by phosphorylation

The present study evaluated the expression and regulation of endogenous thyroid hormone receptors (TRs) in cultured cells. In COS-1 cells, the endogenous TR, subtype beta1 (TRbeta1), but not subtype beta2 or alpha1, was induced to express by okadaic acid (OA) in a concentration-dependent manner. The induced TRbeta1 had immunoreactivity and partial V8 proteolytic maps similar to those of the transfected and in vitro translated human TRbeta1 (h-TRbeta1). The OA-induced expression of endogenous TRbeta1 was, however, not observed in a variety of other cultured cell lines tested, indicating that the induction was cell type-dependent. TRbeta1 induced by OA was a multisite phosphorylated protein, in which serine and threonine in a ratio of 10:1 were phosphorylated. The induced TRbeta1 was functional as it could mediate the thyroid hormone-dependent transcriptional activity via several thyroid hormone response elements. The induction of endogenous TRbeta1 expression by OA was not accompanied by an increase in mRNA levels but was the result of an increase in the stability of the TRbeta1 protein. This is the first report to indicate that one of the mechanisms by which the TR isoforms are differentially expressed is via the tissue-specific stabilization of the TR isoform proteins. Furthermore, this selective stability of TRbeta1 could be conferred by phosphorylation.

Modulation of the transcriptional activity of thyroid hormone receptors by the tumor suppressor p53

Thyroid hormone nuclear receptors (TRs) are ligand-dependent transcriptional factors that regulate growth, differentiation, and development. The molecular mechanisms by which TRs mediate these effects are unclear. One prevailing hypothesis suggests that TRs may cooperate with other transcriptional factors to mediate their biological effects. In this study, we tested this hypothesis by examining whether the activity of TRs is modulated by the tumor suppressor p53. p53 is a nuclear protein that regulates gene expression via sequence-specific DNA binding and/or direct protein-protein interaction. We found that the human TR subtype beta 1 (h-TR beta 1) physically interacted with p53 via its DNA binding domain. As a result of this physical interaction, binding of h-TR beta 1 to its hormone response elements either as homodimer or as a heterodimer with the retinoic X receptor was inhibited by p53 in a concentration-dependent manner. In transfected cells, wild-type p53 repressed the hormone-dependent transcriptional activation of h-TR beta 1. In contrast, mutant p53 either had no effect or activated the transcriptional activity of h-TR beta 1 depending on the type of hormone response elements. These results indicate the gene regulating activity of TRs was modulated by p53, suggesting that the cross talk between these two transcriptional factors may play an important role in the biology of normal and cancer cells.

Structure of the carboxy-terminal region of thyroid hormone nuclear receptors and its possible role in hormone-dependent intermolecular interactions

The thyroid hormone nuclear receptors (TRs) are ligand-dependent transcription factors. To understand the molecular basis of ligand-dependent transactivation, we studied the structure of their carboxy-terminal activation domain. We analyzed the structures of the peptides derived from the C-terminal sequences of human TR subtypes beta 1 (h-TR beta 1) and alpha 1 (h-TR alpha 1) and a human TR mutant, PV, by circular dichroism (CD). Mutant PV has a C-terminal frameshift mutation and does not bind to the thyroid hormone, 3,3',5-triiodo-L-thyronine (T3). Analyses of the secondary structures of the peptides by CD indicate that five amino acids, EVFED, are part of an amphipathic alpha-helix which is required to maintain the structural integrity of the hormone binding domain. A monoclonal antibody, C4 (mAb C4), which recognizes both h-TR beta 1 and h-TR alpha 1 was developed. Using a series of truncated mutants and synthetic peptides, we mapped the epitope of mAb C4 to the conserved C-terminal amino acids, EVFED. Analysis of the binding data indicates that binding of T3 to either h-TR beta 1 or h-TR alpha 1 was competitively inhibited by mAb C4. Deletion of C-terminal amino acids including EVFED led to a total loss of T3 binding activity. Thus, part of the T3 binding site is located in this five amino acid segment. T3 may transduce its hormonal signal to the transcriptional machinery via interaction with EVFED at the C-terminus of TRs.