Insight Into Molecular Determinants of T3 vs T4 Recognition From Mutations in Thyroid Hormone Receptor α and β

CONTEXT

The two major forms of circulating thyroid hormones (THs) are T3 and T4. T3 is regarded as the biologically active hormone because it binds to TH receptors (TRs) with greater affinity than T4. However, it is currently unclear what structural mechanisms underlie this difference in affinity.

OBJECTIVE

Prompted by the identification of a novel M256T mutation in a resistance to TH (RTH)α patient, we investigated Met256 in TRα1 and the corresponding residue (Met310) in TRβ1, residues previously predicted by crystallographic studies in discrimination of T3 vs T4.

METHODS

Clinical characterization of the RTHα patient and molecular studies (in silico protein modeling, radioligand binding, transactivation, and receptor-cofactor studies) were performed.

RESULTS

Structural modeling of the TRα1-M256T mutant showed that distortion of the hydrophobic niche to accommodate the outer ring of ligand was more pronounced for T3 than T4, suggesting that this substitution has little impact on the affinity for T4. In agreement with the model, TRα1-M256T selectively reduced the affinity for T3. Also, unlike other naturally occurring TRα mutations, TRα1-M256T had a differential impact on T3- vs T4-dependent transcriptional activation. TRα1-M256A and TRβ1-M310T mutants exhibited similar discordance for T3 vs T4.

CONCLUSIONS

Met256-TRα1/Met310-TRβ1 strongly potentiates the affinity of TRs for T3, thereby largely determining that T3 is the bioactive hormone rather than T4. These observations provide insight into the molecular basis for underlying the different affinity of TRs for T3 vs T4, delineating a fundamental principle of TH signaling.

SUN-546 Coexistent Resistance to Thyroid Hormone Beta and Viral Thyroiditis

Background Resistance to thyroid hormone beta (RTHbeta) is a rare genetic disorder caused by a defective beta form of the thyroid hormone receptor and is characterized by elevated T3 and T4, non-suppressed TSH and variable tissue resistance to thyroid hormone (TH). In most cases a heterozygous mutation in THRB gene is identified. Clinical case A 32 year old adopted Caucasian woman presented with chest pain in the primary care setting in December 2016. Associated symptoms included palpitations, insomnia, poor concentration, restlessness, tremor, mood swings, headaches, irritability, increased appetite, dry scalp, cold extremities, weight gain and diarrhoea. She had no goitre. Based on thyroid function test (TFT) results, she was suspected to have hyperthyroidism and was commenced on antithyroid drugs for 13 months, but treatment was later stopped due to mouth ulcers. At this stage, further tests in our hospital showed elevated fT3 (8.4 pmol/L, RR 3.5-6.5 pmol/L) and fT4 (33.5 pmol/L, RR 10.0-19.8 pmol/L) and non-suppressed TSH (1.35 mU/L, RR 0.35-5.50 mU/L), without evidence of assay interference. SHBG and MRI pituitary were normal. THRB sequencing identified a heterozygous pathogenic mutation (R438H). She further reported that she struggled in school which prevented her from completing university education and remembered that her biological mother had learning disability. She was further assessed in our clinical research facility. Surprisingly, TFT results were very different: fT3 23.6 pmol/L, fT4 97.3 pmol/L and fully suppressed TSH (<0.03mU/L). Resting energy expenditure (REE) measured by indirect calorimetry was markedly raised [0.214 MJ/kg of lean body mass (LBM)]; compared to that seen in healthy controls (mean (SD): 0.148 (0.013) MJ/kg LBM). Given a recent coryzal illness and negative TRAB, thyrotoxicosis was deemed to be in keeping with thyroiditis. The TFT pattern evolved, without treatment, to mild hypothyroidism (fT3 6.8 pmol/L, fT4 16.2 pmol/L and TSH 21.54 mU/L), then returned to the pattern at presentation: elevated fT3 and fT4 and non-suppressed TSH (fT3 10.6 pmol/L, fT4 31.6 pmol/L, TSH 2.09 mU/L). Given ongoing thyrotoxic symptoms, identical to those present at time of referral, Tri-iodothyroacetic Acid (TRIAC) was commenced. Conclusion Coincident thyroid dysfunction can further complicate interpretation of TFTs in patients with RTHbeta. Notably, individuals with RTHbeta and coexistent thyrotoxicosis can exhibit a suppressed TSH, with underlying pituitary resistance to TH action only becoming evident once TH levels fall. Although thyroiditis rarely complicates RTHbeta, it has been reported that autoimmune thyroid disease occurs more commonly, so interpretation of such unusual TFT patterns may be required during the course of management of these individuals.

Role of Leucine 341 in Thyroid Hormone Receptor Beta Revealed by a Novel Mutation Causing Thyroid Hormone Resistance

Leucine 341 has been predicted from crystal structure as an important residue for thyroid hormone receptor beta (TRβ) function, but this has never been confirmed in functional studies. Here, a novel p.L341V mutation as a cause of resistance to TRβ is described, suggesting an important role for L341 in TRβ function. In silico and in vitro studies confirmed that substituting L341 with valine and other non-polar amino acids impairs sensitivity of TRβ for triiodothyronine to various degrees, depending on their side-chain size and orientation.

Homozygous loss-of-function mutations in SLC26A7 cause goitrous congenital hypothyroidism

Defects in genes mediating thyroid hormone biosynthesis result in dyshormonogenic congenital hypothyroidism (CH). Here, we report homozygous truncating mutations in SLC26A7 in 6 unrelated families with goitrous CH and show that goitrous hypothyroidism also occurs in Slc26a7-null mice. In both species, the gene is expressed predominantly in the thyroid gland, and loss of function is associated with impaired availability of iodine for thyroid hormone synthesis, partially corrected in mice by iodine supplementation. SLC26A7 is a member of the same transporter family as SLC26A4 (pendrin), an anion exchanger with affinity for iodide and chloride (among others), whose gene mutations cause congenital deafness and dyshormonogenic goiter. However, in contrast to pendrin, SLC26A7 does not mediate cellular iodide efflux and hearing in affected individuals is normal. We delineate a hitherto unrecognized role for SLC26A7 in thyroid hormone biosynthesis, for which the mechanism remains unclear.

Rapid disease progression in a patient with mismatch repair-deficient and cortisol secreting adrenocortical carcinoma treated with pembrolizumab

CONTEXT

Metastatic adrenocortical carcinoma (ACC) is an aggressive malignancy with a poor prognosis and limited therapeutic options. A subset of ACC is due to Lynch syndrome, an inherited tumor syndrome resulting from germline mutations in mismatch repair (MMR) genes. It has been demonstrated that several cancers characterized by MMR deficiency are sensitive to immune checkpoint inhibitors that target PD-1. Here, we provide the first report of PD-1 blockade with pembrolizumab in a patient with Lynch syndrome and progressive cortisol-secreting metastatic ACC.

CASE REPORT

A 58-year-old female with known Lynch syndrome presented with severe Cushing's syndrome and was diagnosed with a cortisol-secreting ACC. Three months following surgical resection and adjuvant mitotane therapy the patient developed metastatic disease and persistent hypercortisolemia. She commenced pembrolizumab, but her second cycle was delayed due to a transient transaminitis. Computed tomography performed after 12 weeks and 2 cycles of pembrolizumab administration revealed significant disease progression and treatment was discontinued. After 7 weeks, the patient became jaundiced and soon died due to fulminant liver failure.

CONCLUSION

Treatment of MMR-deficient cortisol-secreting ACC with pembrolizumab may be ineffective due to supraphysiological levels of circulating corticosteroids, which may in turn mask severe drug-induced organ damage.

Adult-onset hyperinsulinaemic hypoglycaemia in clinical practice: diagnosis, aetiology and management

OBJECTIVE

In adults with hyperinsulinaemic hypoglycaemia (HH), in particular those with insulinoma, the optimal diagnostic and management strategies remain uncertain. Here, we sought to characterise the biochemical and radiological assessment, and clinical management of adults with HH at a tertiary centre over a thirteen-year period.

DESIGN

Clinical, biochemical, radiological and histological data were reviewed from all confirmed cases of adult-onset hyperinsulinaemic hypoglycaemia at our centre between 2003 and 2016. In a subset of patients with stage I insulinoma, whole-exome sequencing of tumour DNA was performed.

RESULTS

Twenty-nine patients were identified (27 insulinoma, including 6 subjects with metastatic disease; 1 pro-insulin/GLP-1 co-secreting tumour; 1 activating glucokinase mutation). In all cases, hypoglycaemia (glucose ≤2.2 mmol/L) was achieved within 48 h of a supervised fast. At fast termination, subjects with stage IV insulinoma had significantly higher insulin, C-peptide and pro-insulin compared to those with insulinoma staged I-IIIB. Preoperative localisation of insulinoma was most successfully achieved with EUS. In two patients with inoperable, metastatic insulinoma, peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-DOTATATE rapidly restored euglycaemia and lowered fasting insulin. Finally, in a subset of stage I insulinoma, whole-exome sequencing of tumour DNA identified the pathogenic Ying Yang-1 (YY1) somatic mutation (c.C1115G/p.T372R) in one tumour, with all tumours exhibiting a low somatic mutation burden.

CONCLUSION

Our study highlights, in particular, the utility of the 48-h fast in the diagnosis of insulinoma, EUS for tumour localisation and the value of PRRT therapy in the treatment of metastatic disease.

Digenic DUOX1 and DUOX2 Mutations in Cases With Congenital Hypothyroidism

CONTEXT

The DUOX2 enzyme generates hydrogen peroxide (H2O2), a crucial electron acceptor for the thyroid peroxidase-catalyzed iodination and coupling reactions mediating thyroid hormone biosynthesis. DUOX2 mutations result in dyshormonogenetic congenital hypothyroidism (CH) that may be phenotypically heterogeneous, leading to the hypothesis that CH severity may be influenced by environmental factors (e.g., dietary iodine) and oligogenic modifiers (e.g., variants in the homologous reduced form of NAD phosphate-oxidase DUOX1). However, loss-of-function mutations in DUOX1 have not hitherto been described, and its role in thyroid biology remains undefined.

CASE DESCRIPTION

We previously described a Proband and her brother (P1, P2) with unusually severe CH associated with a DUOX2 homozygous nonsense mutation (p.R434*); P1, P2: thyrotropin >100 µU/mL [reference range (RR) 0.5 to 6.3]; and P1: free T4 (FT4) <0.09 ng/dL (RR 0.9 to 2.3). Subsequent studies have revealed a homozygous DUOX1 mutation (c.1823-1G>C) resulting in aberrant splicing and a protein truncation (p.Val607Aspfs*43), which segregates with CH in this kindred.

CONCLUSION

This is a report of digenic mutations in DUOX1 and DUOX2 in association with CH, and we hypothesize that the inability of DUOX1 to compensate for DUOX2 deficiency in this kindred may underlie the severe CH phenotype. Our studies provide evidence for a digenic basis for CH and support the notion that oligogenicity as well as environmental modulators may underlie phenotypic variability in genetically ascertained CH.

The Hypercoagulable state in Hyperthyroidism is mediated via the Thyroid Hormone β Receptor pathway

OBJECTIVE

Hyperthyroidism is associated with a hypercoagulable state, but the underlying mechanism is unknown. Patients with resistance to thyroid hormone (RTH) due to defective thyroid hormone receptor β (TRβ) exhibit elevated circulating thyroid hormones (TH) with refractoriness to TH action in TRβ-expressing tissues. We tested the hypothesis that the hypercoagulable state in hyperthyroidism is mediated via the TRβ.

DESIGN

We conducted a cross-sectional study from November 2013 to January 2015 in 3 hospitals in the Netherlands and the United Kingdom.

METHODS

Patients with RTH due to defective TRβ (n=18), patients with hyperthyroidism (n=16) and euthyroid subjects (n=18) were included. TH concentrations and markers of coagulation and fibrinolysis were measured. Data are expressed as median [interquartile range].

RESULTS

Free thyroxine (FT4) levels were slightly higher in hyperthyroid patients than in RTH patients (53.9 [30.5-70.0] and 34.9 [28.4-42.2]pmol/l, respectively, P=0.042). Both groups had raised FT4 levels compared to euthyroid subjects (14.0 [13.0-15.8] pmol/l, P≤0.001). Levels of von Willebrand factor (VWF), factor (F) VIII, fibrinogen, and D-dimer were significantly higher in hyperthyroid patients than in RTH patients (VWF 231 [195-296] vs. 111 [82-140]%, FVIII 215 [192-228] vs. 145 [97-158]%, fibrinogen 3.6 [3.0-4.4] vs. 2.8 [2.5-3.2]g/L, D-dimer 0.41 [0.31-0.88] vs. 0.20 [0.17-0.26]mg/L, respectively, P≤0.001), while there were no differences between RTH patients and euthyroid controls.

CONCLUSIONS

Parameters of coagulation and fibrinolysis were elevated in hyperthyroid patients compared to patients with RTH due to defective TRβ, whereas these parameters were not different between euthyroid controls and RTH patients, despite elevated FT4 concentrations in RTH patients. This indicates that the procoagulant effects observed in hyperthyroidism are mediated via the TRβ.

A Novel Thyrotropin-Releasing Hormone Receptor Missense Mutation (P81R) in Central Congenital Hypothyroidism

CONTEXT

Isolated central congenital hypothyroidism (CCH) is rare and evades diagnosis on TSH-based congenital hypothyroidism (CH) screening programs in the United Kingdom. Accordingly, genetic ascertainment facilitates diagnosis and treatment of familial cases. Recognized causes include TSH β subunit (TSHB) and Ig superfamily member 1 (IGSF1) mutations, with only two previous reports of biallelic, highly disruptive mutations in the TRH receptor (TRHR) gene.

CASE DESCRIPTION

A female infant presenting with prolonged neonatal jaundice was found to have isolated CCH, with TSH of 2.2 mU/L (Reference range, 0.4-3.5) and free T4 of 7.9 pmol/L (0.61 ng/dL) (Reference range, 10.7-21.8 pmol/L). Because TSHB or IGSF1 mutations are usually associated with profound or X-linked CCH, TRHR was sequenced, and a homozygous mutation (p.P81R) was identified, substituting arginine for a highly conserved proline residue in transmembrane helix 2. Functional studies demonstrated normal cell membrane expression and localization of the mutant TRHR; however, its ability to bind radio-labelled TRH and signal via Gqα was markedly impaired, likely due to structural distortion of transmembrane helix 2.

CONCLUSIONS

Two previously reported biallelic, highly disruptive (nonsense; R17*, in-frame deletion and single amino acid substitution; p.[S115-T117del; A118T]) TRHR mutations have been associated with CCH; however, we describe the first deleterious, missense TRHR defect associated with this phenotype. Importantly, the location of the mutated amino acid (proline 81) highlights the functional importance of the second transmembrane helix in mediating hormone binding and receptor activation. Future identification of other naturally occurring TRHR mutations will likely offer important insights into the molecular basis of ligand binding and activation of TRHR, which are still poorly understood.

Mild cold effects on hunger, food intake, satiety and skin temperature in humans

BACKGROUND

Mild cold exposure increases energy expenditure and can influence energy balance, but at the same time it does not increase appetite and energy intake.

OBJECTIVE

To quantify dermal insulative cold response, we assessed thermal comfort and skin temperatures changes by infrared thermography.

METHODS

We exposed healthy volunteers to either a single episode of environmental mild cold or thermoneutrality. We measured hunger sensation and actual free food intake. After a thermoneutral overnight stay, five males and five females were exposed to either 18°C (mild cold) or 24°C (thermoneutrality) for 2.5 h. Metabolic rate, vital signs, skin temperature, blood biochemistry, cold and hunger scores were measured at baseline and for every 30 min during the temperature intervention. This was followed by an ad libitum meal to obtain the actual desired energy intake after cold exposure.

RESULTS

We could replicate the cold-induced increase in REE. But no differences were detected in hunger, food intake, or satiety after mild cold exposure compared with thermoneutrality. After long-term cold exposure, high cold sensation scores were reported, which were negatively correlated with thermogenesis. Skin temperature in the sternal area was tightly correlated with the increase in energy expenditure.

CONCLUSIONS

It is concluded that short-term mild cold exposure increases energy expenditure without changes in food intake. Mild cold exposure resulted in significant thermal discomfort, which was negatively correlated with the increase in energy expenditure. Moreover, there is a great between-subject variability in cold response. These data provide further insights on cold exposure as an anti-obesity measure.

Recent advances in central congenital hypothyroidism

Central congenital hypothyroidism (CCH) may occur in isolation, or more frequently in combination with additional pituitary hormone deficits with or without associated extrapituitary abnormalities. Although uncommon, it may be more prevalent than previously thought, affecting up to 1:16 000 neonates in the Netherlands. Since TSH is not elevated, CCH will evade diagnosis in primary, TSH-based, CH screening programs and delayed detection may result in neurodevelopmental delay due to untreated neonatal hypothyroidism. Alternatively, coexisting growth hormones or ACTH deficiency may pose additional risks, such as life threatening hypoglycaemia. Genetic ascertainment is possible in a minority of cases and reveals mutations in genes controlling the TSH biosynthetic pathway (TSHB, TRHR, IGSF1) in isolated TSH deficiency, or early (HESX1, LHX3, LHX4, SOX3, OTX2) or late (PROP1, POU1F1) pituitary transcription factors in combined hormone deficits. Since TSH cannot be used as an indicator of euthyroidism, adequacy of treatment can be difficult to monitor due to a paucity of alternative biomarkers. This review will summarize the normal physiology of pituitary development and the hypothalamic-pituitary-thyroid axis, then describe known genetic causes of isolated central hypothyroidism and combined pituitary hormone deficits associated with TSH deficiency. Difficulties in diagnosis and management of these conditions will then be discussed.

An approach to quantifying abnormalities in energy expenditure and lean mass in metabolic disease

Background/objectives:

The objective of this study was to develop approaches to expressing resting energy expenditure (REE) and lean body mass (LM) phenotypes of metabolic disorders in terms of Z-scores relative to their predicted healthy values.

Subjects/methods:

Body composition and REE were measured in 135 healthy participants. Prediction equations for LM and REE were obtained from linear regression and the range of normality by the standard deviation of residuals. Application is demonstrated in patients from three metabolic disorder groups (lipodystrophy, n=7; thyrotoxicosis, n=16; and resistance to thyroid hormone (RTH), n=46) in which altered REE and/or LM were characterised by departure from the predicted healthy values, expressed as a Z-score.

Results:

REE (kJ/min)=−0.010 × age (years)+0.016 × FM (kg)+0.054 × fat-free mass (kg)+1.736 (R²=0.732, RSD=0.36 kJ/min).

LM (kg)=5.30 × bone mineral content (kg)+10.66 × height² (m)+6.40 (male).

LM (kg)=0.20 × fat (kg)+14.08 × height² (m)−2.93 (female).

(male R²=0.55, RSD=3.90 kg; female R²=0.59, RSD=3.85 kg).

We found average Z-scores for REE and LM of 1.77 kJ/min and −0.17 kg in the RTH group, 5.82 kJ/min and −1.23 kg in the thyrotoxic group and 2.97 kJ/min and 4.20 kg in the LD group.

Conclusion:

This approach enables comparison of data from individuals with metabolic disorders with those of healthy individuals, describing their departure from the healthy mean by a Z-score.

What should be done when thyroid function tests do not make sense?

Interpretation of thyroid function tests (TFTs) is generally straightforward. However, in a minority of contexts the results of thyroid hormone and thyrotropin measurements either conflict with the clinical picture or form an unusual pattern. In many such cases, reassessment of the clinical context provides an explanation for the discrepant TFTs; in other instances, interference in one or other laboratory assays can be shown to account for divergent results; uncommonly, genetic defects in the hypothalamic-pituitary-thyroid axis are associated with anomalous TFTs. Failure to recognize these potential 'pitfalls' can lead to misdiagnosis and inappropriate management. Here, focusing particularly on the combination of hyperthyroxinaemia with nonsuppressed thyrotropin, we show how a structured approach to investigation can help make sense of atypical TFTs.

Maternal isodisomy for chromosome 9 causing homozygosity for a novel FOXE1 mutation in syndromic congenital hypothyroidism

CONTEXT

Homozygous loss-of-function mutations in forkhead box E1/thyroid transcription factor 2 (FOXE1/TTF-2) cause syndromic congenital hypothyroidism, with thyroid dysgenesis, cleft palate, spiky hair, and variable choanal atresia and bifid epiglottis in three cases reported hitherto. We have elucidated the molecular basis of the disorder in a female with a similar clinical phenotype, born to nonconsanguineous parents.

OBJECTIVE AND DESIGN

The FOXE1 gene, located on chromosome 9q22, was sequenced in the proband and family members. Microsatellite marker and multiplex ligation probe amplification analyses determined chromosomal inheritance patterns and FOXE1 copy number. Mutant FOXE1 function was predicted by structural modeling and tested in transfection assays.

RESULTS

The proband was homozygous for a novel missense (c.412T-->C; F137S) FOXE1 mutation, but her mother showed heterozygous and father wild-type alleles for this gene sequence. However, the proband was also homozygous for 10 microsatellite markers spanning chromosome 9 with exclusively maternal inheritance. Multiplex ligation probe amplification assays showed two copies of FOXE1 in the proband, indicating maternal isodisomy for chromosome 9. Consistent with structural modeling, the F137S mutant FOXE1 protein failed to bind DNA and showed negligible transcriptional activity.

CONCLUSION

We have described the first case of uniparental disomy causing homozygosity for a novel, loss-of-function FOXE1/TTF-2 mutation in dysgenetic congenital hypothyroidism.

Immunoglobulin interference in serum follicle-stimulating hormone assays: autoimmune and heterophilic antibody interference

Interference in immunoassay caused by endogenous immunoglobulin is a cause of incorrect laboratory results that can drastically affect patient management. Two cases of immunoglobulin interference in serum follicle-stimulating hormone (FSH) assays are presented. These cases illustrate two common mechanisms for false-positive interference in two-site (sandwich) immunoassays. The first case describes a circulating autoimmune FSH immunoglobulin complex ('macro'-FSH), which has not been previously described for FSH, and the second a cross-linking antibody directed against the assay reagents. Immunoglobulin interference was detected and characterized using a combination of method comparison, immunosubtraction and size exclusion chromatography.

Partial lipodystrophy and insulin resistant diabetes in a patient with a homozygous nonsense mutation in CIDEC

Lipodystrophic syndromes are characterized by adipose tissue deficiency. Although rare, they are of considerable interest as they, like obesity, typically lead to ectopic lipid accumulation, dyslipidaemia and insulin resistant diabetes. In this paper we describe a female patient with partial lipodystrophy (affecting limb, femorogluteal and subcutaneous abdominal fat), white adipocytes with multiloculated lipid droplets and insulin-resistant diabetes, who was found to be homozygous for a premature truncation mutation in the lipid droplet protein cell death-inducing Dffa-like effector C (CIDEC) (E186X). The truncation disrupts the highly conserved CIDE-C domain and the mutant protein is mistargeted and fails to increase the lipid droplet size in transfected cells. In mice, Cidec deficiency also reduces fat mass and induces the formation of white adipocytes with multilocular lipid droplets, but in contrast to our patient, Cidec null mice are protected against diet-induced obesity and insulin resistance. In addition to describing a novel autosomal recessive form of familial partial lipodystrophy, these observations also suggest that CIDEC is required for unilocular lipid droplet formation and optimal energy storage in human fat.

Augmentation index in resistance to thyroid hormone (RTH)

OBJECTIVE

Resistance to thyroid hormone (RTH) is associated with a varied clinical presentation. The cardiac effects of RTH have been described but vascular function has yet to be fully evaluated in this condition. We have measured the arterial function of those with RTH to assess any vascular changes.

DESIGN

An observational study.

PATIENTS

Twelve RTH patients were recruited from the thyroid clinic (mean value +/- SD), age 40.8 +/- 18.7 years; BMI 27.2 +/- 4.2 kg/m(2) and compared with 12 healthy, euthyroid, age-matched controls (age 41.4 +/- 19.3; BMI 24.8 +/- 4.4 kg/m(2)) with no history of cardiovascular disease. No interventional measures were instituted.

MEASUREMENTS

Arterial stiffness was measured using pulse wave analysis at the radial artery. Thyroid function, fasting lipids and glucose were also measured on the same occasion in both patients and controls. Results The corrected augmentation index, a surrogate marker of arterial stiffness was significantly higher in patients compared with controls (21.0% +/- 14.1%vs. 5.4% +/- 18.2%, P < 0.03). Low density lipoprotein cholesterol (LDL-cholesterol) levels were also significantly elevated in patients compared with controls (3.0 +/- 0.6 vs. 2.1 +/- 0.5 mmol/l; P < 0.002).

CONCLUSION

RTH patients show evidence in this study of increased augmentation index consistent with an increase in arterial stiffness compared with euthyroid controls. They also demonstrate elevated LDL-cholesterol levels. Both these measures may lead to increased cardiovascular risk.

International Union of Pharmacology. LXI. Peroxisome proliferator-activated receptors

The three peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors of the nuclear hormone receptor superfamily. They share a high degree of structural homology with all members of the superfamily, particularly in the DNA-binding domain and ligand- and cofactor-binding domain. Many cellular and systemic roles have been attributed to these receptors, reaching far beyond the stimulation of peroxisome proliferation in rodents after which they were initially named. PPARs exhibit broad, isotype-specific tissue expression patterns. PPARalpha is expressed at high levels in organs with significant catabolism of fatty acids. PPARbeta/delta has the broadest expression pattern, and the levels of expression in certain tissues depend on the extent of cell proliferation and differentiation. PPARgamma is expressed as two isoforms, of which PPARgamma2 is found at high levels in the adipose tissues, whereas PPARgamma1 has a broader expression pattern. Transcriptional regulation by PPARs requires heterodimerization with the retinoid X receptor (RXR). When activated by a ligand, the dimer modulates transcription via binding to a specific DNA sequence element called a peroxisome proliferator response element (PPRE) in the promoter region of target genes. A wide variety of natural or synthetic compounds was identified as PPAR ligands. Among the synthetic ligands, the lipid-lowering drugs, fibrates, and the insulin sensitizers, thiazolidinediones, are PPARalpha and PPARgamma agonists, respectively, which underscores the important role of PPARs as therapeutic targets. Transcriptional control by PPAR/RXR heterodimers also requires interaction with coregulator complexes. Thus, selective action of PPARs in vivo results from the interplay at a given time point between expression levels of each of the three PPAR and RXR isotypes, affinity for a specific promoter PPRE, and ligand and cofactor availabilities.

Influences of age, gender, smoking, and family history on autoimmune thyroid disease phenotype

CONTEXT

Both genetic and environmental factors contribute to susceptibility to Graves' disease (GD) and Hashimoto's thyroiditis (HT), as well as disease manifestations.

OBJECTIVE

The objective of the study was to define how endogenous/environmental factors contribute to variation in phenotype.

DESIGN/SETTING

This was a multicenter cohort study. PATIENTS/OUTCOME MEASURES: We prospectively collected clinical/biochemical data as part of the protocol for a United Kingdom DNA collection for GD and HT. We investigated, in 2805 Caucasian subjects, whether age at diagnosis, gender, family history (FH), smoking history, and presence of goiter influenced disease manifestations.

RESULTS

For 2405 subjects with GD, the presence of goiter was independently associated with disease severity (serum free T4 at diagnosis) (P < 0.001). Free T4 (P < 0.05) and current smoking (P < 0.001) were both independent predictors of the presence of ophthalmopathy. Approximately half of those with GD (47.4% of females, 40.0% of males) and HT (n = 400) (56.4% of females, 51.7% of males) reported a FH of thyroid dysfunction. In GD, a FH of hyperthyroidism in any relative was more frequent than hypothyroidism (30.1 vs. 24.4% in affected females, P < 0.001). In HT, a FH of hypothyroidism was more common than hyperthyroidism (42.1 vs. 22.8% in affected females, P < 0.001). For GD (P < 0.001) and HT (P < 0.05), a FH was more common in maternal than paternal relatives. The reporting of a parent with thyroid dysfunction (hyper or hypo) was associated with lower median age at diagnosis of both GD (mother with hyperthyroidism, P < 0.001) and HT (father with hypothyroidism, P < 0.05). In GD and HT, there was an inverse relationship between the number of relatives with thyroid dysfunction and age at diagnosis (P < 0.01).

CONCLUSIONS

Marked associations among age at diagnosis, disease severity, goiter, ophthalmopathy, smoking, and FH provide evidence for interactions between genetic and environmental/endogenous factors; understanding these may allow preventive measures or better tailoring of therapies.

Leptin deficiency unmasks the deleterious effects of impaired peroxisome proliferator-activated receptor gamma function (P465L PPARgamma) in mice

Peroxisome proliferator-activated receptor (PPAR)gamma is a key transcription factor facilitating fat deposition in adipose tissue through its proadipogenic and lipogenic actions. Human patients with dominant-negative mutations in PPARgamma display lipodystrophy and extreme insulin resistance. For this reason it was completely unexpected that mice harboring an equivalent mutation (P465L) in PPARgamma developed normal amounts of adipose tissue and were insulin sensitive. This finding raised important doubts about the interspecies translatability of PPARgamma-related findings, bringing into question the relevance of other PPARgamma murine models. Here, we demonstrate that when expressed on a hyperphagic ob/ob background, the P465L PPARgamma mutant grossly exacerbates the insulin resistance and metabolic disturbances associated with leptin deficiency, yet reduces whole-body adiposity and adipocyte size. In mouse, coexistence of the P465L PPARgamma mutation and the leptin-deficient state creates a mismatch between insufficient adipose tissue expandability and excessive energy availability, unmasking the deleterious effects of PPARgamma mutations on carbohydrate metabolism and replicating the characteristic clinical symptoms observed in human patients with dominant-negative PPARgamma mutations. Thus, adipose tissue expandability is identified as an important factor for the development of insulin resistance in the context of positive energy balance.

A novel missense mutation in human TTF-2 (FKHL15) gene associated with congenital hypothyroidism but not athyreosis

BACKGROUND

Thyroid dysgenesis is the most frequent cause of congenital hypothyroidism (CH), and its genetic basis is largely unknown. Hitherto, two mutations in the human thyroid transcription factor 2 (TTF-2) gene have been described in unrelated cases of CH with cleft palate, spiky hair, variable choanal atresia, and complete thyroid agenesis. Here, we describe a novel TTF-2 mutation in a female child resulting in syndromic CH in the absence of thyroid agenesis.

RESULTS

The index case is homozygous for an arginine to cysteine mutation (R102C) of a highly conserved residue within the forkhead, DNA binding domain of TTF-2. Her consanguineous, heterozygous parents are unaffected, and the mutation was not detected in 100 control chromosomes. Consonant with its location, the R102C mutant TTF-2 protein showed loss of DNA binding and was transcriptionally inactive. CH in the proposita was associated with cleft palate, spiky hair, and bilateral choanal atresia. However, radiological studies showed the presence of thyroid tissue in a eutopic location.

CONCLUSION

Our findings indicate that human thyroid development can occur despite loss of TTF-2 function and suggest that TTF-2 gene defects should also be considered in cases of syndromic CH without total athyreosis.

Increased PTEN expression due to transcriptional activation of PPARgamma by Lovastatin and Rosiglitazone

Germline mutations in the tumor suppressor gene PTEN (protein phosphatase and tensin homolog located on chromosome ten) predispose to heritable breast cancer. The transcription factor PPARgamma has also been implicated as a tumor suppressor pertinent to a range of neoplasias, including breast cancer. A putative PPARgamma binding site in the PTEN promoter indicates that PPARgamma may regulate PTEN expression. We show here that the PPARgamma agonist Rosiglitazone, along with Lovastatin, induce PTEN in a dose- and time-dependent manner. Lovastatin- or Rosiglitazone-induced PTEN expression was accompanied by a decrease in phosphorylated-AKT and phosphorylated-MAPK and an increase in G1 arrest. We demonstrate that the mechanism of Lovastatin- and Rosiglitazone-associated PTEN expression was a result of an increase in PTEN mRNA, suggesting that this increase was transcriptionally-mediated. Compound-66, an inactive form of Rosiglitazone, which is incapable of activating PPARgamma, was unable to elicit the same response as Rosiglitazone, signifying that the Rosiglitazone response is PPARgamma-mediated. To support this, we show, using reporter assays including dominant-negative constructs of PPARgamma, that both Lovastatin and Rosiglitazone specifically mediate PPARgamma activation. Additionally, we demonstrated that cells lacking PTEN or PPARgamma were unable to induce PTEN mediated cellular events in the presence of Lovastatin or Rosiglitazone. These data are the first to demonstrate that Lovastatin can signal through PPARgamma and directly demonstrate that PPARgamma can upregulate PTEN at the transcriptional level. Since PTEN is constitutively active, our data indicates it may be worthwhile to examine Rosiglitazone and Lovastatin stimulation as mechanisms to increase PTEN expression for therapeutic and preventative strategies including cancer, diabetes mellitus and cardiovascular disease.

Dehydroepiandrosterone replacement in patients with Addison's disease has a bimodal effect on regulatory (CD4+CD25hi and CD4+FoxP3+) T cells

Oral replacement of the near-total deficiency of dehydroepiandrosterone (DHEA) in patients with Addison's disease (adrenal insufficiency) enhances mood and well-being and reduces fatigue. We studied the immunological effects of 12 wk of oral DHEA treatment in ten patients with Addison's disease receiving their normal mineralo- and glucocorticoid hormone replacement. We found that baseline circulating regulatory T cells were reduced in Addison's disease patients compared to controls, a hitherto unrecognised defect in this disorder. Oral DHEA treatment had a bimodal effect on naturally occurring regulatory (CD4+CD25hiFoxP3+) T cells and lymphocyte FoxP3 expression. Oral DHEA replacement restored normal levels of regulatory T cells and led to increased FoxP3 expression. These effects were probably responsible for a suppression of constitutive cytokine expression following DHEA withdrawal. In contrast, oral DHEA treatment led to reduced FoxP3 expression induced by TCR engagement and so augmented the cytokine response, but without a bias towards the Th1 or Th2 phenotype. NK and NKT cell numbers fell during DHEA treatment, and homeostatic lymphocyte proliferation was increased. We conclude that DHEA replacement in Addison's disease has significant immunomodulatory properties and propose that it has a greater impact on the human immune system than would be expected from its classification as a dietary supplement.

Rosiglitazone increases indexes of stearoyl-CoA desaturase activity in humans: link to insulin sensitization and the role of dominant-negative mutation in peroxisome proliferator-activated receptor-gamma

Fatty acid desaturases such as steaoryl-CoA desaturase (SCD) convert saturated to unsaturated fatty acids and are involved in lipogenesis. Observational and animal data suggest that SCD-1 activity is related to insulin sensitivity. However, the effects of insulin-sensitizing drugs on SCD gene expression and desaturase activities are unknown in humans. In a randomized, placebo-controlled, double-blind, crossover study, 24 subjects with type 2 diabetes and one subject with partial lipodystrophy and diabetes due to dominant-negative mutation in the peroxisome proliferator-activated receptor-gamma (PPARgamma) gene (P467L) received placebo and rosiglitazone for 3 months. SCD gene expression in adipose tissue was determined in 23 subjects, and in a representative subgroup (n = 10) we assessed fatty acid composition in fasting plasma triglycerides to estimate SCD and delta6- and delta5-desaturase activity, using product-to-precursor indexes. SCD mRNA expression increased by 48% after rosiglitazone (P < 0.01). SCD and delta5-desaturase but not delta6-desaturase activity indexes were increased after rosiglitazone versus placebo (P < 0.01 and P < 0.05, respectively). The change in activity index but not the expression of SCD was associated with improved insulin sensitivity (r = 0.73, P < 0.05). In the P467L PPARgamma carrier, SCD and delta5-desaturase activity indexes were exceptionally low but were restored (52- and 15-fold increases, respectively) after rosiglitazone treatment. This study shows for the first time that rosiglitazone increases SCD activity indexes and gene expression in humans. An increased SCD activity index may reflect increased lipogenesis and might contribute to insulin sensitization by rosiglitazone. The restored SCD activity index after rosiglitazone in PPARgamma mutation supports a pivotal role of PPARgamma function in SCD regulation.

Energy expenditure and adaptive responses to an acute hypercaloric fat load in humans with lipodystrophy

Humans respond to an acute excess of ingested energy by storing the surplus energy as triglyceride in white adipose tissue. To study the energetic response to acute overfeeding in human subjects with limited adipose tissue capacity, we recruited seven subjects with lipodystrophy and seven lean healthy controls. Total fat mass was approximately 70% lower in lipodystrophic subjects (mean, 6.1 kg) than in body mass index-matched lean controls (mean, 22.0 kg). Energy expenditure and macronutrient oxidation rates were assessed in chamber calorimeters on two separate occasions for 40 h, during which time subjects consumed either an energy-balanced diet or a diet incorporating 30% excess energy as fat. On the energy-balanced diet, total daily energy expenditure and basal metabolic rate were linearly associated with lean mass in both groups (r(2) = 0.83) and were not significantly different between groups when corrected for lean mass. In response to the fat challenge, total energy expenditure did not increase significantly in healthy controls (9,472 +/- 1,069 to 9,724 +/- 1,114 kJ/d; P = 0.189). Substrate oxidation results confirm that excess fat was predominantly stored. In contrast, lipodystrophic subjects significantly increased total daily energy expenditure (11,081 +/- 1,226 to 11,730 +/- 1,374 kJ/d; P < 0.005). This was largely attributable to a 29% increase in fat oxidation. Thus, subjects with lipodystrophy uniquely respond to an acute hypercaloric load with a higher energy expenditure increment and by increasing fat oxidation. Insight into the molecular mechanisms responsible for this phenomenon may yield novel therapeutic approaches for obesity.

Tyrosine agonists reverse the molecular defects associated with dominant-negative mutations in human peroxisome proliferator-activated receptor gamma

Loss-of-function mutations in the ligand-binding domain of human peroxisome proliferator-activated receptor gamma (PPARgamma) are associated with a novel syndrome characterized by partial lipodystrophy and severe insulin resistance. Here we have further characterized the properties of natural dominant-negative PPARgamma mutants (P467L, V290M) and evaluated the efficacy of putative natural ligands and synthetic thiazolidinedione (TZD) or tyrosine-based (TA) receptor agonists in rescuing mutant receptor function. A range of natural ligands failed to activate the PPARgamma mutants and their transcriptional responses to TZDs (e.g. pioglitazone, rosiglitazone) were markedly attenuated, whereas TAs (e.g. farglitazar) corrected defects in ligand binding and coactivator recruitment by the PPARgamma mutants, restoring transcriptional function comparable with wild-type receptor. Transcriptional silencing via recruitment of corepressor contributes to dominant-negative inhibition of wild type by the P467L and V290M mutants and the introduction of an artificial mutation (L318A) disrupting corepressor interaction abrogated their dominant-negative activity. More complete ligand-dependent corepressor release and reversal of dominant-negative inhibition was achieved with TA than TZD agonists. Modeling suggests a structural basis for these observations: both mutations destabilize helix 12 to favor receptor-corepressor interaction; conversely, farglitazar makes more extensive contacts than rosiglitazone within the ligand-binding pocket, to stabilize helix 12, facilitating corepressor release and transcriptional activation. Farglitazar was a more potent inducer of PPARgamma target gene (aP2) expression in peripheral blood mononuclear cells with the P467L mutation. Having shown that rosiglitazone is of variable and limited efficacy in these subjects, we suggest that TAs may represent a more rational therapeutic approach.

Potentiation of glucose uptake in 3T3-L1 adipocytes by PPAR gamma agonists is maintained in cells expressing a PPAR gamma dominant-negative mutant: evidence for selectivity in the downstream responses to PPAR gamma activation

Pharmacological agonists for the nuclear receptor PPAR gamma enhance glucose disposal in a variety of insulin-resistant states in humans and animals. The precise mechanisms whereby activation of PPAR gamma leads to increased glucose uptake in metabolically active cells remain to be determined. Notably, certain novel, synthetic PPAR gamma ligands appear to antagonize thiazolidinedione-induced adipogenesis yet stimulate cellular glucose uptake. We have explored the molecular mechanisms underlying the enhancement of glucose uptake produced by PPAR gamma agonists in 3T3-L1 adipocytes. Rosiglitazone treatment for 48 h significantly increased basal and insulin-stimulated glucose uptake and markedly increased the cellular expression of GLUT1 but not GLUT4. Rosiglitazone increased plasma membrane levels of GLUT1, but not GLUT4, both basally and after insulin stimulation. Surprisingly, adenoviral expression of a dominant-negative mutant PPAR gamma, which was demonstrated to strongly inhibit adipogenesis, completely failed to inhibit rosiglitazone-stimulated glucose uptake. Similar findings were obtained with the non-thiazolidinedione PPAR gamma agonists, GW1929 and GW7845. The insensitivity of PPAR gamma agonist-stimulated glucose uptake to expression of a dominant-negative mutant, compared with the latter's marked inhibitory effects on preadipocyte differentiation, suggests that, as is the case for other nuclear receptors, the precise molecular mechanisms linking PPAR gamma activation to downstream events may differ depending on the nature of the biological response. The growing evidence that the effects of PPAR gamma on adipogenesis and glucose uptake can be dissociated may have important implications for the development of improved antidiabetic drug treatments.

Chromatin remodeling by the thyroid hormone receptor in regulation of the thyroid-stimulating hormone alpha-subunit promoter

The chromatin architecture of a promoter is an important determinant of its transcriptional response. For most target genes, the thyroid hormone receptor (TR) activates gene expression in response to thyroid hormone (T(3)). In contrast, the thyroid-stimulating hormone alpha-subunit (TSH alpha) gene promoter is down-regulated by TR in the presence of T(3). Here we utilize the capacity for the Xenopus oocyte to chromatinize exogenous nuclear- injected DNA to analyze the chromatin architecture of the TSH alpha promoter and how this changes upon TR-mediated regulation. Interestingly, in the oocyte, the TSH alpha promoter was positively regulated by T(3). In the inactive state, the promoter contained six loosely positioned nucleosomes. The addition of TR/retinoid X receptor together had no effect on the chromatin structure, but the inclusion of T(3) induced strong positioning of a dinucleosome in the TSH alpha proximal promoter that was bordered by regions that were hypersensitive to cleavage by methidiumpropyl EDTA. We identified a novel thyroid response element that coincided with the proximal hypersensitive region. Furthermore, we examined the consequences of mutations in TR that impaired coactivator recruitment. In a comparison with the Xenopus TR beta A promoter, we found that the effects of these mutations on transactivation and chromatin remodeling were significantly more severe on the TSH alpha promoter.

Inhibition of cellular proliferation through IkappaB kinase-independent and peroxisome proliferator-activated receptor gamma-dependent repression of cyclin D1

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-regulated nuclear receptor superfamily member. Liganded PPARgamma exerts diverse biological effects, promoting adipocyte differentiation, inhibiting tumor cellular proliferation, and regulating monocyte/macrophage and anti-inflammatory activities in vitro. In vivo studies with PPARgamma ligands showed enhancement of tumor growth, raising the possibility that reduced immune function and tumor surveillance may outweigh the direct inhibitory effects of PPARgamma ligands on cellular proliferation. Recent findings that PPARgamma ligands convey PPARgamma-independent activities through IkappaB kinase (IKK) raises important questions about the specific mechanisms through which PPARgamma ligands inhibit cellular proliferation. We investigated the mechanisms regulating the antiproliferative effect of PPARgamma. Herein PPARgamma, liganded by either natural (15d-PGJ(2) and PGD(2)) or synthetic ligands (BRL49653 and troglitazone), selectively inhibited expression of the cyclin D1 gene. The inhibition of S-phase entry and activity of the cyclin D1-dependent serine-threonine kinase (Cdk) by 15d-PGJ(2) was not observed in PPARgamma-deficient cells. Cyclin D1 overexpression reversed the S-phase inhibition by 15d-PGJ(2). Cyclin D1 repression was independent of IKK, as prostaglandins (PGs) which bound PPARgamma but lacked the IKK interactive cyclopentone ring carbonyl group repressed cyclin D1. Cyclin D1 repression by PPARgamma involved competition for limiting abundance of p300, directed through a c-Fos binding site of the cyclin D1 promoter. 15d-PGJ(2) enhanced recruitment of p300 to PPARgamma but reduced binding to c-Fos. The identification of distinct pathways through which eicosanoids regulate anti-inflammatory and antiproliferative effects may improve the utility of COX2 inhibitors.

Resistance to thyroid hormone, and peroxisome-proliferator-activated receptor γ resistance

Resistance to thyroid hormone (RTH) is usually inherited in a dominant fashion, and is characterized by elevated serum thyroid hormone levels and failure to suppress pituitary secretion of thyroid-stimulating hormone, with variable refractoriness to hormone action in peripheral tissues. Two major forms of the disorder are recognized: asymptomatic individuals with generalized resistance (GRTH) and patients with thyrotoxic features suggesting predominant pituitary resistance (PRTH). In over 100 families with GRTH or PRTH, we have identified heterozygous mutations in the thyroid hormone receptor β isoform (TRβ), which localize to three regions (amino acids 234–282, 310–353 and 429–461) of the hormone-binding domain of the receptor. The mutant receptors are transcriptionally impaired, due either to reduced ligand binding or to attenuated interaction with co-activators, and inhibit wild-type TR action in a dominant-negative manner. In the TRβ crystal structure, most RTH mutations cluster around the hormone-binding pocket, with receptor regions that mediate functions (DNA binding, dimerization, corepressor recruitment) required for dominant-negative activity being devoid of natural mutations. The pathogenesis of variable tissue resistance is not fully understood, but may be related to the differing tissue distributions of TRα and TRβ, and to variable dominant-negative activity of mutant receptors on different target genes. The nuclear receptor peroxisome-proliferator-activated receptor γ (PPARγ) regulates adipogenesis and mediates the action of thiazolidinediones - novel antidiabetic agents which enhance tissue insulin sensitivity. The PPARγ gene was screened in 85 subjects with severe insulin resistance, and two different heterozygous receptor mutations (P467L and V290M) were identified in three affected individuals. The PPARγ mutants are markedly transcriptionally impaired due to altered ligand binding and co-activator recruitment. Analogous to RTH, they inhibit the function of wild-type PPARγ when co-expressed, and such dominant-negative inhibition is linked to their ability to silence basal gene transcription via aberrant interaction with co-repressors. In addition to insulin resistance, all three affected subjects developed Type II diabetes mellitus and hypertension at an unusually early age. Our findings provide compelling evidence that PPARγ is important in the control of insulin sensitivity, glucose homoeostasis and blood pressure in humans. Future studies aim to elucidate the mechanism by which this receptor regulates insulin action and vascular tone.

Arachidonic acid stimulates glucose uptake in 3T3-L1 adipocytes by increasing GLUT1 and GLUT4 levels at the plasma membrane. Evidence for involvement of lipoxygenase metabolites and peroxisome proliferator-activated receptor gamma

Exposure of insulin-sensitive tissues to free fatty acids can impair glucose disposal through inhibition of carbohydrate oxidation and glucose transport. However, certain fatty acids and their derivatives can also act as endogenous ligands for peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor that positively modulates insulin sensitivity. To clarify the effects of externally delivered fatty acids on glucose uptake in an insulin-responsive cell type, we systematically examined the effects of a range of fatty acids on glucose uptake in 3T3-L1 adipocytes. Of the fatty acids examined, arachidonic acid (AA) had the greatest positive effects, significantly increasing basal and insulin-stimulated glucose uptake by 1.8- and 2-fold, respectively, with effects being maximal at 4 h at which time membrane phospholipid content of AA was markedly increased. The effects of AA were sensitive to the inhibition of protein synthesis but were unrelated to changes in membrane fluidity. AA had no effect on total cellular levels of glucose transporters, but significantly increased levels of GLUT1 and GLUT4 at the plasma membrane. While the effects of AA were insensitive to cyclooxygenase inhibition, the lipoxygenase inhibitor, nordihydroguaiaretic acid, substantially blocked the AA effect on basal glucose uptake. Furthermore, adenoviral expression of a dominant-negative PPARgamma mutant attenuated the AA potentiation of basal glucose uptake. Thus, AA potentiates basal and insulin-stimulated glucose uptake in 3T3-L1 adipocytes by a cyclooxygenase-independent mechanism that increases the levels of both GLUT1 and GLUT4 at the plasma membrane. These effects are at least partly dependent on de novo protein synthesis, an intact lipoxygenase pathway and the activation of PPARgamma with these pathways having a greater role in the absence than in the presence of insulin.

Transcriptional repression by nuclear receptors: mechanisms and role in disease

Co-repressor proteins mediate transcriptional repression by nuclear receptors in the absence of ligand. The identification of a co-repressor-receptor interaction motif, and the finding that co-repressors and co-activators compete for the same site on the receptor, suggests a simple mechanism for the switch from repression to activation upon ligand binding. Defects in this mechanism result in dominant-negative receptors that repress transcription. Such receptors have been implicated in several clinically important diseases, including thyroid hormone resistance and diabetes mellitus.

Evidence for gene-nutrient interaction at the PPARgamma locus

The importance of the nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) in regulating insulin resistance and blood pressure has been demonstrated in families with loss of function mutations. Gain of function mutations has been associated with severe obesity. However, previous population studies of the common variant Pro12Ala have produced conflicting results. As it is likely that the natural ligands for this receptor may include fatty acids, we hypothesized that the effect of this common variant may be altered by the character of the diet, particularly the ratio of dietary polyunsaturated fat to saturated fat (P:S ratio). We studied 592 nondiabetic participants in an ongoing population-based cohort study who were genotyped for the Pro12Ala polymorphism in the PPAR gamma2 isoform. As the Ala homozygotes were uncommon (2.0%), all analyses were conducted comparing Pro homozygotes (79.1%) to Ala allele carriers. There was no difference in fasting insulin concentration or BMI between Ala allele carriers and Pro homozygotes. The fasting insulin concentration was negatively associated with the P:S ratio (P = 0.0119) after adjustment for age and sex, and a strong interaction was evident between the P:S ratio and the Pro12Ala polymorphism for both BMI (P = 0.0038) and fasting insulin (P = 0.0097). The data suggest that when the dietary P:S ratio is low, the BMI in Ala carriers is greater than that in Pro homozygotes, but when the dietary ratio is high, the opposite is seen. This gene-nutrient interaction emphasizes the difficulty of examining the effect of common polymorphisms in the absence of data on nongenetic exposures, and may explain the heterogeneity of findings in previous studies.

Improvement in mood and fatigue after dehydroepiandrosterone replacement in Addison's disease in a randomized, double blind trial

Dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS) are adrenal precursors of steroid biosynthesis and centrally acting neurosteroids. Glucocorticoid and mineralocorticoid deficiencies in Addison's disease require life-long hormone replacement, but the associated failure of DHEA synthesis is not corrected. We conducted a randomized, double blind study in which 39 patients with Addison's disease received either 50 mg oral DHEA daily for 12 weeks, followed by a 4-week washout period, then 12 weeks of placebo, or vice versa. After DHEA treatment, levels of DHEAS and Delta(4)-androstenedione rose from subnormal to within the adult physiological range. Total testosterone increased from subnormal to low normal with a fall in serum sex hormone-binding globulin in females, but with no change in either parameter in males. In both sexes, psychological assessment showed significant enhancement of self-esteem with a tendency for improved overall well-being. Mood and fatigue also improved significantly, with benefit being evident in the evenings. No effects on cognitive or sexual function, body composition, lipids, or bone mineral density were observed. Our results indicate that DHEA replacement corrects this steroid deficiency effectively and improves some aspects of psychological function. Beneficial effects in males, independent of circulating testosterone levels, suggest that it may act directly on the central nervous system rather than by augmenting peripheral androgen biosynthesis. These positive effects, in the absence of significant adverse events, suggest a role for DHEA replacement therapy in the treatment of Addison's disease.

St John's wort, a herbal antidepressant, activates the steroid X receptor

St John's wort (SJW), an extract of the medicinal plant Hypericum perforatum, is widely used as a herbal antidepressant. Recently, this agent has been found to adversely affect the metabolism of various coadministered drugs. Steroid X receptor (SXR), an orphan nuclear receptor, induces hepatic cytochrome P450 gene expression in response to diverse endogenous steroids, xenobiotics and drugs. Here, we report that, when coexpressed with SXR, a reporter construct derived from the cytochrome P450 3A promoter is activated by St John's wort. A GAL4-SXR ligand binding domain (LBD) fusion mediates concentration-dependent transactivation by SJW, whereas a mutant GAL4-SXR fusion, containing substitutions in key residues in a transactivation domain, is inactive. SJW recruits steroid receptor coactivator-1 to SXR in a two-hybrid assay and competes with radiolabelled ligand in binding studies, suggesting it interacts directly with the receptor LBD. Of two constituents of SJW, we find that hyperforin, but not hypericin, mediates both transactivation and coactivator recruitment by SXR. Our observations suggest that SXR activation by St John's wort mediates its adverse interaction with drugs metabolised via the CYP 3A pathway. Future development of SJW derivatives lacking SXR activation, may enable its antidepressant and drug-metabolising properties to be dissociated.

The localization of thyroid hormone receptor mRNAs in human bone

Thyroid hormones have well-documented effects on the skeleton although the mechanism of their action on bone is poorly understood. We have recently reported the presence of different thyroid hormone receptor isoforms in human bone. However, there is evidence to suggest that the expression of thyroid hormone receptor (TR) protein may not necessarily correlate with its mRNA. In this study, we used specific digoxigenin-labeled ribo probes to investigate the expression of TRalpha1, variant TRalpha2, TRbeta1, and in particular TRbeta2 mRNA in human osteophytic bone and osteoclastoma tissue in situ. The number of positive cells was expressed as the percentage of the total number of cells of the same phenotype. In osteophytes, at sites of endochondral ossification, TRalpha1, variant TRalpha2, TRbeta1, and TRbeta2 mRNA were widely distributed in undifferentiated, proliferating, mature and hypertrophic chondrocytes. At sites of bone remodeling, TRalpha1 mRNA was expressed in the majority (> 90%) of osteoblasts. TRbeta1 and the variant TR-alpha2 mRNA were moderately expressed in approximately 75% of cells with only a few osteoblasts (< 25%) expressing TRbeta2 mRNA. All the TR transcripts were highly expressed in multinucleated osteoclasts in osteoclastoma tissue. The distribution of TR mRNAs was similar to TR receptor protein expression (as we have previously reported) in both osteophytic bone and osteoclastoma tissue except TRalpha1 mRNA that was highly expressed in osteoclasts and in undifferentiated, proliferating, mature, and hypertrophic chondrocytes in contrast to its receptor protein expression. This study highlights the importance of studying both TR mRNA and receptor proteins in triiodothyronine (T3) responsive tissues. This is also the first demonstration of the presence of TRbeta2 mRNA in bone. The role of TRbeta2 in mediating the actions of thyroid hormones in bone is not known and requires further investigation.

A dominant-negative peroxisome proliferator-activated receptor gamma (PPARgamma) mutant is a constitutive repressor and inhibits PPARgamma-mediated adipogenesis

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) promotes adipocyte differentiation, exerts atherogenic and anti-inflammatory effects in monocyte/macrophages, and is believed to mediate the insulin-sensitizing action of antidiabetic thiazolidinedione ligands. As no complete PPARgamma antagonists have been described hitherto, we have constructed a dominant-negative mutant receptor to inhibit wild-type PPARgamma action. Highly conserved hydrophobic and charged residues (Leu(468) and Glu(471)) in helix 12 of the ligand-binding domain were mutated to alanine. This compound PPARgamma mutant retains ligand and DNA binding, but exhibits markedly reduced transactivation due to impaired coactivator (cAMP-response element-binding protein-binding protein and steroid receptor coactivator-1) recruitment. Unexpectedly, the mutant receptor silences basal gene transcription, recruits corepressors (the silencing mediator of retinoid and thyroid receptors and the nuclear corepressor) more avidly than wild-type PPARgamma, and exhibits delayed ligand-dependent corepressor release. It is a powerful dominant-negative inhibitor of cotransfected wild-type receptor action. Furthermore, when expressed in primary human preadipocytes using a recombinant adenovirus, this PPARgamma mutant blocks thiazolidinedione-induced differentiation, providing direct evidence that PPARgamma mediates adipogenesis. Our observations suggest that, as in other mutant nuclear receptor contexts (acute promyelocytic leukemia, resistance to thyroid hormone), dominant-negative inhibition by PPARgamma is linked to aberrant corepressor interaction. Adenoviral expression of this mutant receptor is a valuable means to antagonize PPARgamma signaling.

HIV protease inhibitors block human preadipocyte differentiation, but not via the PPARgamma/RXR heterodimer

A recent prospective clinical study has shown that antiviral therapy with HIV protease inhibitors (PIs) is associated with a syndrome of peripheral fat wasting (lipodystrophy) and disordered glucose and lipid metabolism (Carr et al. 1999). We have studied the effects of indinavir and saquinavir, two HIV protease inhibitors, on cultured primary human preadipocytes and report that these compounds inhibit their differentiation. However, we find that these agents do not inhibit either transcriptional activation or adipocyte P2 gene induction by the PPARgamma/RXR nuclear receptor heterodimer. Together, our findings suggest that impaired adipogenesis is the basis of PI-associated lipodystrophy, but that this occurs via a PPARgamma/RXR-independent mechanism.

Dominant negative mutations in human PPARgamma associated with severe insulin resistance, diabetes mellitus and hypertension

Thiazolidinediones are a new class of antidiabetic agent that improve insulin sensitivity and reduce plasma glucose and blood pressure in subjects with type 2 diabetes. Although these agents can bind and activate an orphan nuclear receptor, peroxisome proliferator-activated receptor gamma (PPARgamma), there is no direct evidence to conclusively implicate this receptor in the regulation of mammalian glucose homeostasis. Here we report two different heterozygous mutations in the ligand-binding domain of PPARgamma in three subjects with severe insulin resistance. In the PPARgamma crystal structure, the mutations destabilize helix 12 which mediates transactivation. Consistent with this, both receptor mutants are markedly transcriptionally impaired and, moreover, are able to inhibit the action of coexpressed wild-type PPARgamma in a dominant negative manner. In addition to insulin resistance, all three subjects developed type 2 diabetes mellitus and hypertension at an unusually early age. Our findings represent the first germline loss-of-function mutations in PPARgamma and provide compelling genetic evidence that this receptor is important in the control of insulin sensitivity, glucose homeostasis and blood pressure in man.

Three novel mutations at serine 314 in the thyroid hormone beta receptor differentially impair ligand binding in the syndrome of resistance to thyroid hormone

The syndrome of resistance to thyroid hormone is associated with diverse mutations in the ligand-binding domain of the thyroid hormone beta receptor, localizing to three clusters around the hormone binding cavity. Here, we report three novel resistance to thyroid hormone mutations (S314C, S314F, and S314Y), due to different nucleotide substitutions in the same codon, occurring in six separate families. Functional characterization of these mutant receptors showed marked differences in their properties. S314F and S314Y receptor mutants exhibited significant transcriptional impairment in keeping with negligible ligand binding and were potent dominant negative inhibitors of wild-type receptor action. In contrast, the S314C mutant bound ligand with reduced affinity, such that its functional impairment and dominant negative activity manifest at low concentrations of thyroid hormone, but are more reversible at higher T3 concentrations. The degree of functional impairment of mutant receptors in vitro may correlate with the magnitude of thyroid dysfunction in vivo. Modelling these mutations using the crystal structure of thyroid hormone receptor beta shows why ligand binding is perturbed and why the phenylalanine/tyrosine mutations are more deleterious than cysteine.

Pulsed monoclonal antibody treatment and autoimmune thyroid disease in multiple sclerosis

BACKGROUND

Multiple sclerosis results from T-cell-dependent inflammatory demyelination of the central nervous system. Our objective was long-term suppression of inflammation with short-term monoclonal antibody treatment.

METHODS

We depleted 95% of circulating lymphocytes in 27 patients with multiple sclerosis by means of a 5-day pulse of the humanised anti-CD52 monoclonal antibody, Campath-1H. Clinical and haematological consequences of T-cell depletion, and in-vitro responses of patients' peripheral-blood mononuclear cells were analysed serially for 18 months after treatment.

FINDINGS

Radiological and clinical markers of disease activity were significantly decreased for at least 18 months after treatment. However, a third of patients developed antibodies against the thyrotropin receptor and carbimazole-responsive autoimmune hyperthyroidism. The depleted peripheral lymphocyte pool was reconstituted with cells that had decreased mitogen-induced proliferation and interferon gamma secretion in vitro.

INTERPRETATION

Campath-1H causes the immune response to change from the Th1 phenotype, suppressing multiple sclerosis disease activity, but permitting the generation of antibody-mediated thyroid autoimmunity.

Isolation and characterisation of the retinoic acid receptor-alpha gene in the Japanese pufferfish, F. rubripes

Nuclear hormone receptors (NRs) are ligand-inducible transcription factors that mediate critical functions in many species. The majority of novel NRs have hitherto been cloned from cDNA libraries by virtue of their homology to previously identified receptors. In this study, we validate a genomic DNA-based approach to isolating NRs by cloning the retinoic acid receptor-alpha (RARalpha) gene from the genome of the Japanese pufferfish, Fugu rubripes. The fRARalpha gene is more compact than its human and murine counterparts and demonstrates a highly conserved genomic organisation and amino acid sequence, generating two isoforms (fRARalpha1 and fRARalpha2) with divergent aminoterminal domains. In addition, a conserved regulatory element containing a retinoic acid response element was identified upstream of the fRARalpha2-specific exon, implying that retinoid induction of this isoform is evolutionarily conserved and critical to its function in vivo. We propose two uses for the Fugu genome in the study of NRs: the isolation of novel NRs that exhibit restricted spatio-temporal expression from genomic DNA and the identification of evolutionarily conserved promoter or intragenic regulatory DNA elements.

Effect of mutations in the beta1-thyroid hormone receptor on the inhibition of T3 binding by desethylamiodarone

Desethylamiodarone (DEA) acts as a competitive inhibitor of triiodothyronine (T3) binding to the alpha1-thyroid hormone receptor (TR alpha1) but as a non-competitive inhibitor with respect to TR beta1. To gain insight into the position of the binding site of desethylamiodarone on TR beta1 we investigated the naturally occurring mutants Y321C, R429Q, P453A, P453T and the artificial mutants L421R and E457A in the ligand binding domain of human TR beta1. The IC50 values (in microM) of DEA for P453A (50 +/- 11) and P453T (55 +/- 16) mutant TR beta1 are not different from that for the wild type TR beta1 (56 +/- 15), but the IC50 values of R429Q (32 +/- 7; P<0.001) and E457A (17 +/- 3; P<0.001) are significantly lower than of the wild type. Scatchard plots and Langmuir analyses indicate a non-competitive nature of the inhibition by DEA of T3 binding to all four mutant TR beta1s tested. Mutants P453A and P453T do not influence overall electrostatic potential, and also do not influence the affinity for DEA compared to wild type. Mutant E457A causes a change from a negatively charged amino acid to a hydrophobic amino acid, enhancing the affinity for DEA. Mutant R429Q, located in helix 11, causes an electrostatic potential change from positive to uncharged, also resulting in greater affinity for DEA. We therefore postulate that amino acids R429 and E457 are at or close to the binding site for DEA, and that DEA does not bind in the T3 binding pocket itself, in line with the non-competitive nature of the inhibition of T3 binding to TR beta1 by DEA.

Prenatal diagnosis of thyroid hormone resistance

A 29-yr-old woman with pituitary resistance to thyroid hormones (PRTH) was found to harbor a novel point mutation (T337A) on exon 9 of the thyroid hormone receptor beta (TRbeta) gene. She presented with symptoms and signs of hyperthyroidism and was successfully treated with 3,5,3'-triiodothyroacetic acid (TRIAC) until the onset of pregnancy. This therapy was then discontinued in order to prevent TRIAC, a compound that crosses the placental barrier, from exerting adverse effects on normal fetal development. However, as the patient showed a recurrence of thyrotoxic features after TRIAC withdrawal, we sought to verify, by means of genetic analysis and hormone measurements, whether the fetus was also affected by RTH, in order to rapidly reinstitute TRIAC therapy, which could potentially be beneficial to both the mother and fetus. At 17 weeks gestation, fetal DNA was extracted from chorionic villi and was used as a template for PCR and restriction analysis together with direct sequencing of the TRbeta gene. The results indicated that the fetus was also heterozygous for the T337A mutation. Accordingly, TRIAC treatment at a dose of 2.1 mg/day was restarted at 20 weeks gestation. The mother rapidly became euthyroid, and the fetus grew normally up to 24 weeks gestation. At 29 weeks gestation mild growth retardation and fetal goiter were observed, prompting cordocentesis. Circulating fetal TSH was very high (287 mU/L) with a markedly reduced TSH bioactivity (B/I: 1.1 +/- 0.4 vs 12.7 +/- 1.2), while fetal FT4 concentrations were normal (8.7 pmol/L; normal values in age-matched fetuses: 5-22 pmol/L). Fetal FT3 levels were raised (7.1 pmol/L; normal values in age-matched fetuses: <4 pmol/L), as a consequence of 100% cross-reactivity of TRIAC in the FT3 assay method. To reduce the extremely high circulating TSH levels and fetal goiter, the dose of TRIAC was increased to 3.5 mg/day. To monitor the possible intrauterine hypothyroidism, another cordocentesis was performed at 33 weeks gestation, showing that TSH levels were reduced by 50% (from 287 to 144 mU/L). Furthermore, a simultaneous ultrasound examination revealed a clear reduction in fetal goiter. After this latter cordocentesis, acute complications occured, prompting delivery by cesarean section. The female neonate was critically ill, with multiple-organ failure and respiratory distress syndrome. In addition, a small goiter and biochemical features ofhypothyroidism were noted transiently and probably related to the prematurity of the infant. At present, the baby is clinically euthyroid, without goiter, and only exhibits biochemical features of RTH. In summary, although further fetal studies in cases of RTH are necessary to determine whether elevated TSH levels with a markedly reduced bioactivity are a common finding, our data suggest transient biochemical hypothyroidism in RTH during fetal development. Furthermore, we advocate prenatal diagnosis of RTH and adequate treatment of the disease in case of maternal hyperthyroidism, to avoid fetal thyrotrope hyperplasia, reduce fetal goiter, and maintain maternal euthyroidism during pregnancy.

Development of a luminescent bioassay for thyroid stimulating antibodies

The hyperthyroidism of Graves Disease (GD) is due to thyroid stimulating antibodies (TSAb) which are thyrotropin (TSH) agonists. They are detected routinely by measuring their ability to inhibit TSH binding to the receptor (TBII), which does not reflect their true biological activity. Current bioassays which measure cAMP by RIA, are not suitable for routine use. We have developed a luminescent bioassay for TSAb, by introducing a cAMP responsive luciferase construct into CHO cells stably expressing the human TSH receptor (TSHR). Clone lulul displays dose dependent TSH response detectable from 10 microU/ml and maximal at 10 mU/ml when a >25 fold increase in light output is obtained. 34 euthyroid sera were tested to determine a reference range, with values >1.5 relative light units (R.L.U.) being considered positive. An international TSAb standard responded in a dose dependent manner with 10 mIU/ml giving an R.L.U. of >10. The assay was adapted to a 96 well format for automatic readout and 100 treated GD samples (50 TBII negative and 50 TBII positive) were tested, 73% being positive. In contrast only 4% of 79 control sera from individuals with Hashimoto's, non-thyroid autoimmunity or multinodular goitre produced R.L.U. >1.5. When 44 of the GD sera were compared in a traditional salt-free bioassay, 61% were positive compared with 75% in the new luminescent assay. In conclusion, we have developed a luminescent bioassay for TSAb, using unfractionated serum which is capable of high throughput suitable for routine use.