Characterization of Quinoline Yellow Dyes As Transient Aryl Hydrocarbon Receptor Agonists.

The aryl hydrocarbon receptor (AHR) and estrogen receptor alpha (ERα) are two ligand activated transcription factors that are targeted by a wide range of anthropogenic compounds. Crosstalk between both receptors is well established but little understood. We previously developed a dual color luciferase assay (i.e., XEER) which allows time dissolved monitoring of the activation of both receptors in situ. The system was now used in conjunction with HPLC-qTOF to identify several quinophthalone dyes as transient receptor agonists of the AHR. Altogether the approach identified three widely used dyes, that is the plastic colorant latyl yellow 3G (LY), the structurally related textile dye disperse yellow 64 (DY), and the cosmetic dye quinoline yellow (QY). The latter was the most potent agonist followed by LY and DY as confirmed by the XEER assay and CYP1A1 gene induction in MCF7 cells. In addition QY, LY, and DY also inhibited ER signaling in an AHR-dependent manner. This establishes some evidence for quinoline yellow dyes as potential disruptors of AHR/ER signaling, raising potential toxicological concern. Although none of the dyes featured any signs of genotoxicity in vitro, our data point to the need for a systematic approach when screening for substances of potential toxicological and endocrine relevance.

A Novel Dual-Color Luciferase Reporter Assay for Simultaneous Detection of Estrogen and Aryl Hydrocarbon Receptor Activation.

Consumers are exposed to a plethora of anthropogenic and natural substances that can act as agonists or antagonists for various transcription factors. Depending on the exposure and potency, such interactions can potentially lead to adverse health effects, particularly for substances with multiple molecular targets. The early detection of such interactions is thus of high toxicological interest. Here, we report on the development of a new cellular dual-color reporter assay that allows for time-resolved and quantitative recording of estrogen receptor (ER) and aryl hydrocarbon receptor (AHR) activation in living cells. Both receptors are known for their ligand promiscuity. Moreover, both receptor signaling pathways are interconnected by direct protein-protein interactions as well as by shared protein factors and the competition for ligands. The assay is based on two rare beetle luciferases that emit light in the red (SLR) and green (ELuc) spectrum and that have been stably inserted into human T-47D mammary carcinoma cells. The corresponding cell line is termed "XEER" and has been successfully subjected to proof-of-principle studies using prototypical ER and AHR ligands as well as various phytochemicals, xenobiotics, and extracts from various plastic products.

Combination of Metabolomics with Cellular Assays Reveals New Biomarkers and Mechanistic Insights on Xenoestrogenic Exposures in MCF-7 Cells.

The disruptive potential of xenoestrogens like bisphenol A (BPA) lies in their 17ß-estradiol (E2)-like binding to estrogen receptors (ERs) followed by concomitant modulation of ER target gene expression. Unsurprisingly, most endocrine testing systems focus on the quantification of canonical transcripts or ER-sensitive reporters. However, only little information is available about the corresponding metabolomic changes in vitro. This knowledge gap becomes particularly relevant in the context of potential mixture effects, for example, as a consequence of coexposure to potentially estrogenically active pollutants (e.g., Cd2+). Such effects are often difficult to dissect with molecular tools, especially with regard to potential physiological relevance. Metabolomic biomarkers are well-suited to address this latter aspect as they provide a comprehensive readout of whole-cell physiology. Applying a targeted metabolomics approach (FIA-MS/MS), this study looked for biomarkers indicative of xenoestrogenic exposure in MCF-7 cells. Cells were treated with E2 and BPA in the presence or absence of Cd2+. Statistical analysis revealed a total of 11 amino acids and phospholipids to be related to the compound's estrogenic potency. Co-exposure to Cd2+ modulated the estrogenic profile. However, the corresponding changes were found to be moderate with cellular assays such as the E-screen failing to record any Cd2+-specific estrogenic effects. Overall, metabolomics analysis identified proline as the most prominent estrogenic biomarker. Its increase could clearly be related to estrogenic exposure and concomitant ERα-mediated induction of proliferation. Involvement of the latter was confirmed by siRNA-mediated knockdown studies as well as by receptor inhibition. Further, the underlying signaling was also found to involve the oncoprotein MYC. Taken together, this study provides insights into the underlying mechanisms of xenoestrogenic effects and exemplify the strength of the complementary use of metabolomics and cellular and molecular assays.

G protein-coupled receptor 30 ligand G-1 increases aryl hydrocarbon receptor signalling by inhibition of tubulin assembly and cell cycle arrest in human MCF-7 cells.

Regulatory crosstalk between the aryl hydrocarbon receptor (AHR) and oestrogen receptor α (ERα) is well established. Apart from the nuclear receptors ERα and ERß, oestrogen signalling further involves an unrelated G protein-coupled receptor termed GPR30. In order to investigate potential regulatory crosstalk, this study investigated the influence of G-1 as one of the few GPR30-specific ligands on the AHR regulon in MCF-7 cells. As a well-characterised model system, these human mammary carcinoma cells co-express all three receptors (AHR, ERα and GPR30) and are thus ideally suited to study corresponding regulatory pathway interactions on transcript level. Indeed, treatment with micromolar concentrations of the GPR30-specific agonist G-1 resulted in up-regulation of AHR as well as the transcripts for cytochromes P450 1A1 and 1B1, two well-known targets of the AHR regulon. While this was partly attributable to G-1-mediated inhibition of tubulin assembly and subsequent cell cycle arrest in the G2/M phase, the effects nevertheless required functional AHR. However, G-1-induced up-regulation of CYP 1A1 was not mediated by GPR30, as G15 antagonist treatment as well as a knockdown of GPR30 and AHR failed to inhibit this effect.

Identification of lipidomic biomarkers for coexposure to subtoxic doses of benzo[a]pyrene and cadmium: the toxicological cascade biomarker approach.

The search for model bioassay systems indicating activation of different toxicological signaling pathways is one of the paramount goals of modern toxicology. Especially coexposure scenarios need to be investigated with respect to synergistic and interdependent effects for the activation of toxicological signaling pathways. The present study introduces an experimental in vitro model system for nontoxic and low-dose coexposures of human mammary carcinoma MCF-7 cells against polycyclic aromatic hydrocarbons (PAHs) such as benzo[a]pyrene (BP) and heavy metals such as cadmium. For the first time, a multivariate model that identifies 18 metabolic biomarkers has been shown to be sufficient to separate BP-treated cells from coexposed or control cells. A "toxicological pathway color code model" is introduced to visualize the results. Different biomarker subsets can be associated with specific HER2 signaling steps. A tiered cascade biomarker approach is proposed that could be used to identify profiles associated with tumorigenic potency of environmental toxicants in coexposure scenarios, including possible synergistic or additive effects.

Application of systematic evidence mapping to identify available data on the potential human health hazards of selected market-relevant azo dyes.

BACKGROUND: Azo dyes are used in textiles and leather clothing. Human exposure can occur from wearing textiles containing azo dyes. Since the body's enzymes and microbiome can cleave azo dyes, potentially resulting in mutagenic or carcinogenic metabolites, there is also an indirect health concern on the parent compounds. While several hazardous azo dyes are banned, many more are still in use that have not been evaluated systematically for potential health concerns. This systematic evidence map (SEM) aims to compile and categorize the available toxicological evidence on the potential human health risks of a set of 30 market-relevant azo dyes. METHODS: Peer-reviewed and gray literature was searched and over 20,000 studies were identified. These were filtered using Sciome Workbench for Interactive computer-Facilitated Text-mining (SWIFT) Review software with evidence stream tags (human, animal, in vitro) yielding 12,800 unique records. SWIFT Active (a machine-learning software) further facilitated title/abstract screening. DistillerSR software was used for additional title/abstract, full-text screening, and data extraction. RESULTS: 187 studies were identified that met populations, exposures, comparators, and outcomes (PECO) criteria. From this pool, 54 human, 78 animal, and 61 genotoxicity studies were extracted into a literature inventory. Toxicological evidence was abundant for three azo dyes (also used as food additives) and sparse for five of the remaining 27 compounds. Complementary search in ECHA's REACH database for summaries of unpublished study reports revealed evidence for all 30 dyes. The question arose of how this information can be fed into an SEM process. Proper identification of prioritized dyes from various databases (including U.S. EPA's CompTox Chemicals Dashboard) turned out to be a challenge. Evidence compiled by this SEM project can be evaluated for subsequent use in problem formulation efforts to inform potential regulatory needs and prepare for a more efficient and targeted evaluation in the future for human health assessments.

Mutually opposite signal modulation by hypothalamic heterodimerization of ghrelin and melanocortin-3 receptors.

Interaction and cross-talk of G-protein-coupled receptors (GPCRs) are of considerable interest because an increasing number of examples implicate a profound functional and physiological relevance of homo- or hetero-oligomeric GPCRs. The ghrelin (growth hormone secretagogue receptor (GHSR)) and melanocortin-3 (MC3R) receptors are both known to have orexigenic effects on the hypothalamic control of body weight. Because in vitro studies indicate heterodimerization of GHSR and MC3R, we investigated their functional interplay. Combined in situ hybridization and immunohistochemistry indicated that the vast majority of GHSR-expressing neurons in the arcuate nucleus also express MC3R. In vitro coexpression of MC3R and GHSR promoted enhanced melanocortin-induced intracellular cAMP accumulation compared with activation of MC3R in the absence of GHSR. In contrast, agonist-independent basal signaling activity and ghrelin-induced signaling of GHSR were impaired, most likely due to interaction with MC3R. By taking advantage of naturally occurring GHSR mutations and an inverse agonist for GHSR, we demonstrate that the observed enhanced MC3R signaling capability depends directly on the basal activity of GHSR. In conclusion, we demonstrate a paradigm-shifting example of GPCR heterodimerization allowing for mutually opposite functional influence of two hypothalamic receptors controlling body weight. We found that the agonist-independent active conformation of one GPCR can determine the signaling modalities of another receptor in a heterodimer. Our discovery also implies that mutations within one of two interacting receptors might affect both receptors and different pathways simultaneously. These findings uncover mechanisms of important relevance for pharmacological targeting of GPCR in general and hypothalamic body weight regulation in particular.

Chemical activation of estrogen and aryl hydrocarbon receptor signaling pathways and their interaction in toxicology and metabolism.

INTRODUCTION: Estrogen receptors (ERs) and the arylhydrocarbon receptor (AHR) are ligand-activated transcription factors that regulate the expression of genes involved in many physiological processes. With both receptors binding a broad range of natural and anthropogenic ligands, they are molecular targets for many substances, raising concerns for possible health effects. Areas covered: This review shall give a brief overview on the physiological functions of both receptors including their underlying molecular mechanisms. It summarizes the interaction of the respective signaling pathways including impacts on metabolism of endogenous estrogens, transcriptional interference, inhibitory crosstalk, and proteasomal degradation. Also addressed are the AHR dependent formation of estrogenic metabolites from polycyclic aromatic hydrocarbons and the possible impact of the ER/AHR crosstalk in the context of drug metabolism. Expert opinion: Despite decade-long research, the physiological role of the AHR and ER as well as the implications of their complex mutual crosstalk remain to be determined as do resulting potential impacts on human health. With more and more endogenous AHR ligands being discovered, future research should hence systematically address the potential impact of such substances on estrogen signaling. The intimate link between these two pathways and the genes regulated therein bears the potential for impacts on drug metabolism and human health.

Evolutionary aspects in evaluating mutations in the melanocortin 4 receptor.

More than 70 missense mutations have been identified in the human melanocortin 4 receptor (MC4R), and many of them have been associated with obesity. In a number of cases, the causal link between mutations in MC4R and obesity is controversially discussed. Here, we mined evolution as an additional source of structural information that may help to evaluate the functional relevance of naturally occurring variations in MC4R. The sequence information of more than 60 MC4R orthologs enabled us to identify residues that are important for maintaining receptor function. More than 90% of all inactivating mutations found in obese patients were located at amino acid positions that are highly conserved during 450 million years of MC4R evolution in vertebrates. However, for a reasonable number of MC4R variants, we found no correlation between structural conservation of the mutated position and the reported functional consequence. By re-evaluating selected mutations in the MC4R, we demonstrate the usefulness of combining functional and evolutionary approaches.

N-ethyl-N-nitrosourea-based generation of mouse models for mutant G protein-coupled receptors.

Chemical random mutagenesis techniques with the germ line supermutagen N-ethyl-N-nitrosourea (ENU) have been established to provide comprehensive collections of mouse models, which were then mined and analyzed in phenotype-driven studies. Here, we applied ENU mutagenesis in a high-throughput fashion for a gene-driven identification of new mutations. Selected members of the large superfamily of G protein-coupled receptors (GPCR), melanocortin type 3 (Mc3r) and type 4 (Mc4r) receptors, and the orphan chemoattractant receptor GPR33, were used as model targets to prove the feasibility of this approach. Parallel archives of DNA and sperm from mice mutagenized with ENU were screened for mutations in these GPCR, and in vitro assays served as a preselection step before in vitro fertilization was performed to generate the appropriate mouse model. For example, mouse models for inherited obesity were established by selecting fully or partially inactivating mutations in Mc4r. Our technology described herein has the potential to provide mouse models for a GPCR dysfunction of choice within <4 mo and can be extended to other gene classes of interest.

Prevalence, spectrum, and functional characterization of melanocortin-4 receptor gene mutations in a representative population-based sample and obese adults from Germany.

CONTEXT: Autosomal dominant inheritance of mutations in the melanocortin-4 receptor gene (MC4R) is currently regarded as the most relevant genetic cause for extreme obesity and affects 2-4% of extremely obese individuals. OBJECTIVE: Our objective was to assess the relevance of MC4R mutations in a German population-based sample. DESIGN AND SETTING: We conducted a mutation screen of the MC4R gene by capillary electrophoresis-based single-strand conformation polymorphism analysis and denaturing HPLC. PARTICIPANTS: Subjects included 4068 individuals of a German population-based study group [Kooperative Gesundheitsforschung im Raum Augsburg, Survey 4 (KORA-S4); i.e. Cooperative Health Research in the Region of Augsburg] and 1003 German obese adults (body mass index >or= 30 kg/m(2)). MAIN OUTCOME MEASURES: Samples with aberrant capillary electrophoresis-based single-strand conformation polymorphism analysis/denaturing HPLC patterns were resequenced. Functional studies including agonistic receptor stimulation (Nle-D-Phe-alpha-, alpha-, and beta-MSH) and cell surface expression assays were performed. RESULTS: Sixteen (six novel) coding nonsynonymous mutations were detected in 27 heterozygous individuals of KORA-S4. Four of the mutation alleles led to impaired receptor function in vitro; however, none of these six heterozygous mutation carriers was obese (body mass index >or= 30 kg/m(2)). In the obese adults, six coding nonsynonymous and a nonsense mutation were detected in 13 individuals. Only the nonsense mutation allele entailed impaired receptor function. CONCLUSIONS: Our study depicts prevalence, spectrum, and functional characterization of MC4R mutations in the German population-based sample KORA-S4. In this epidemiological study group, individuals heterozygous for nonsynonymous MC4R mutation alleles entailing impaired function were not obese. Furthermore, nonsynonymous MC4R mutations causing impaired receptor function were rare in German obese adults (two in 1003 = 0.2%).

MC4R oligomerizes independently of extracellular cysteine residues.

The melanocortin 4 receptor (MC4R) plays an essential role in weight regulation. Recently we could show that the MC4R is able to form receptor dimers. In the present study we investigated the role of extracellular cysteine residues and the structure of the third extracellular loop for receptor dimerization. None of the four extracellular cysteine residues nor the structure of the third extracellular loop play a role for MC4R-MC4R interaction as all investigated mutants display the same dimerization pattern as the wild-type receptor. Therefore for MC4R dimerization structures of the transmembrane-spanning helices are more likely to be involved.

Estrogenic Activity of Mineral Oil Aromatic Hydrocarbons Used in Printing Inks.

The majority of printing inks are based on mineral oils (MOs) which contain complex mixtures of saturated and aromatic hydrocarbons. Consumer exposure to these oils occurs either through direct skin contacts or, more frequently, as a result of MO migration into the contents of food packaging that was made from recycled newspaper. Despite this ubiquitous and frequent exposure little is known about the potential toxicological effects, particularly with regard to the aromatic MO fractions. From a toxicological point of view the huge amount of alkylated and unsubstituted compounds therein is reason for concern as they can harbor genotoxicants as well as potential endocrine disruptors. The aim of this study was to assess both the genotoxic and estrogenic potential of MOs used in printing inks. Mineral oils with various aromatic hydrocarbon contents were tested using a battery of in vitro assays selected to address various endpoints such as estrogen-dependent cell proliferation, activation of estrogen receptor α or transcriptional induction of estrogenic target genes. In addition, the comet assay has been applied to test for genotoxicity. Out of 15 MOs tested, 10 were found to potentially act as xenoestrogens. For most of the oils the effects were clearly triggered by constituents of the aromatic hydrocarbon fraction. From 5 oils tested in the comet assay, 2 showed slight genotoxicity. Altogether it appears that MOs used in printing inks are potential endocrine disruptors and should thus be assessed carefully to what extent they might contribute to the total estrogenic burden in humans.

Differential cellular metabolite alterations in HaCaT cells caused by exposure to the aryl hydrocarbon receptor-binding polycyclic aromatic hydrocarbons chrysene, benzo[a]pyrene and dibenzo[a,l]pyrene.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the human environment. Since they are present in crude oilfractions used for the production of rubber and plastics, consumers may come into direct dermal contacts with these compounds (e.g., via tool handles) on a daily basis. Some individual PAHs are identified as genotoxic mutagens thereby prompting particular toxicological and environmental concern. Among this group, benzo[a]pyrene (BAP) constitutes a model carcinogen which is also used as reference compound for risk assessment purposes. It acts as a strong agonist of the aryl hydrocarbon receptor (AHR) and becomes metabolically activated toward mutagenic and carcinogenic intermediates by cytochrome P450-dependent monooxygenases (CYPs). While BAP has been exhaustively characterized with regard to its toxicological properties, there is much less information available for other PAHs. We treated an AHR-proficient immortal human keratinocyte cell line (i.e., HaCaT) with three selected PAHs: BAP, chrysene (CRY) and dibenzo[a,l]pyrene (DALP). Compound-mediated alterations of endogenous metabolites were investigated by an LC-MS/MS-based targeted approach. To examine AHR-dependent changes of the measured metabolites, AHR-deficient HaCaT knockdown cells (AHR-KD) were used for comparison. Our results reveal that 24 metabolites are sufficient to separate the PAH-exposed cells from untreated controls by application of a multivariate model. Alterations in the metabolomics profiles caused by each PAH show influences on the energy and lipid metabolism of the cells indicating reduced tricarboxylic acid (TCA) cycle activity and ß-oxidation. Up-regulation of sphingomyelin levels after exposure to BAP and DALP point to pro-apoptotic processes caused by these two potent PAHs. Our results suggest that in vitro metabolomics can serve as tool to develop bioassays for application in hazard assessment.

Extended clinical phenotype, endocrine investigations and functional studies of a loss-of-function mutation A150V in the thyroid hormone specific transporter MCT8.

OBJECTIVE: Thyroid hormones, besides having other functions, are known to be essential for the development of the human brain. Recently the monocarboxylate transporter 8 (MCT8) was identified as a thyroid hormone transporter which is expressed in different regions of the human brain. Here we describe in detail the clinical and biochemical features in response to thyroid hormone administration of a boy carrying an MCT8 mutation (A150V) in the second transmembrane domain. METHODS: To study the functional impact of the mutation we performed triiodothyronine (T3) uptake, immunofluorescence and dimerization studies. RESULTS: Thyroid hormone (l-thyroxine (LT4) and LT3) administration did not result in any significant clinical changes; however, with high doses of LT4, alone or in combination with T3, TSH suppression was achieved. We could show a robust uptake of (125)I-T3 for wild type (WT) MCT8, whereas no specific uptake could be detected for the mutant A150V. Subcellular localization of WT and mutant MCT8 revealed a strong cell surface expression for the WT MCT8, in contrast to A150V, which is mostly retained intracellularly with only weak cell surface expression. We could also demonstrate for the first time that WT MCT8 as well as the mutant are able to form multimers. CONCLUSION: Our findings open a wide field of possible interaction within the central nervous system and will help to understand the crucial role of MCT8 in early fetal brain development.

Type 2 diabetes and impaired glucose tolerance in European children and adolescents with obesity -- a problem that is no longer restricted to minority groups.

BACKGROUND: The incidence of childhood obesity and type 2 diabetes is an increasing problem in Europe. We determined the prevalence of impaired glucose regulation in a predominantly Caucasian cohort of 491 children and adolescents with obesity. METHODS: Fasting glucose and insulin levels were determined in all 491 subjects. Patients with an abnormal fasting glucose or with additional risk factors (positive family history of type 2 diabetes, acanthosis nigricans, hyperlipidemia; n=102) underwent an oral glucose tolerance test (OGTT; 1.75 g glucose/kg body weight). Homeostasis model assessment was used to estimate insulin resistance in all subjects. The insulin sensitivity index was determined in those subjects who underwent an OGTT. Screening for mutations in the melanocortin 4 receptor (MC4R) gene and the coding region of the brain-derived neutrophic factor (BDNF) in 37 patients with an impaired glucose tolerance was performed by WAVE analysis. RESULTS: Out of the total of 491 patients, 12 had an abnormal fasting glucose level. Of the 102 patients who underwent an OGTT, 37 had impaired glucose tolerance; 6 out of the 102 patients were diagnosed with type 2 diabetes. Eighty-eight per cent of patients with abnormal glucose tolerance and 66% of patients with type 2 diabetes were Caucasian. Insulin resistance indices correlated well with the degree of abnormal glucose tolerance. Using the screening algorithm for type 2 diabetes as advocated by the American Diabetes Association, 68% of patients with impaired glucose tolerance and 66% of patients with type 2 diabetes would have been missed. No abnormalities in the MC4R and BDNF genes were detected. CONCLUSIONS: Impaired glucose tolerance and type 2 diabetes are far more common in obese European children of Caucasian origin than previously thought. Using fasting glucose levels as the main screening tool appears to be insufficient in detecting these children.

Effects of triclocarban on the transcription of estrogen, androgen and aryl hydrocarbon receptor responsive genes in human breast cancer cells.

Triclocarban (TCC) is an antimicrobial agent that is used in detergents, soaps and other personal hygiene products. Similarly to triclosan the widespread use of TCC has raised concerns about its endocrine potential. In luciferase-based reporter assays TCC has been shown to enhance estrogenic and androgenic activities following cellular coexposure with estrogen or dihydrotestosterone, respectively. The present study demonstrates that although coexposure with TCC enhances the estrogenic and androgenic readout of luciferase-based reporter cell lines such as HeLa9908 and MDA-kb2, it fails to act as a xenoandrogen on transcriptional level, nor does it induce cell proliferation in the estrogen sensitive E-screen. In addition TCC did not alter the expression of estrogen responsive genes in human mammary carcinoma MCF-7 cells exposed to 17ß-estradiol, bisphenol A, butylparaben or genistein. However, TCC was shown to interfere with the regulon of the aryl hydrocarbon receptor (AhR) as TCC showed a costimulatory effect on transcription of CYP1A1 and CYP1B1, effectively lowering the transcriptional threshold for both genes in the presence of estrogens. It thus seems, that while the induction of the respective luciferase reporter assays by TCC is an unspecific false positive signal caused by luciferase stabilisation, TCC has the potential to interfere with the regulatory crosstalk of the estrogen receptor (ER) and the AhR regulon.

Inhibition of melanocortin-4 receptor dimerization by substitutions in intracellular loop 2.

Obesity is one of the most challenging global health problems. One key player in energy homeostasis is the melanocortin-4 receptor (MC4R), which is a family A G-protein-coupled receptor (GPCR). It has recently been shown that MC4R has the capacity to form homo- or heterodimers. Dimerization of GPCRs is of great importance for signaling regulation, with major pharmacological implications. Unfortunately, not enough is yet known about the detailed structural properties of MC4R dimers or the functional consequences of receptor dimerization. Our goal, therefore, was to explore specific properties related to MC4R dimerization. First, we aimed to induce the dissociation of dimers to monomers and to compare the functional parameters of wild-type and MC4R variants. To inhibit homodimerization, we designed MC4R chimeras with the cannabinoid-1 receptor, a receptor that does not interact with MC4R. Indeed, we identified several substitutions in the intracellular loop 2 (ICL2) and adjacent regions of transmembrane helix 3 (TMH3) and TMH4 that lead to partial dimer dissociation. Interestingly, the capacity for signaling activity was generally increased in these MC4R variants, although receptor expression remained unchanged. This increase in activity for dissociated receptors might indicate a link between receptor dimerization and signaling capacity. Moreover, dimer dissociation was also observed in a naturally occurring activating MC4R mutation in ICL2. Taken together, this study provides new information on the structural prerequisites for MC4R dimerization and identifies an approach to induce the dissociation of MC4R dimers. This might be useful for further investigation of pharmacological properties.

Identification of the translation start site of the human melanocortin 3 receptor.

BACKGROUND: The melanocortin-3-receptor (MC3R) is a G-protein coupled receptor participating in hypothalamic energy metabolism. So far, it was assumed that the translation of the human MC3R starts at the non-conserved first ATG, however, a second evolutionary conserved ATG is located 37 amino acids downstream. One frequent polymorphism, T6K, is located between these two ATGs. METHODS: For characterization of the two potential start ATGs, COS-7 cells were transfected with plasmids encoding the longer and the shorter form of the human MC3R. For signal transduction properties, cAMP was measured. Cell surface expression was determined by using an ELISA method. The translational start point of the MC3R was investigated by a GFP-based method. RESULTS: Signal transduction was comparable for the long and the short receptor form. Cell surface expression via aminoterminal hemagglutinin tag could only be detected in the shorter form, but not in the longer one. In our study we show that the translation of the human MC3R protein starts at the evolutionary conserved ATG codon which results in a shorter protein than previously assumed. CONCLUSION: The polymorphism T6K is not located in the coding region of the human MC3R and has no influence on translation initiation which makes an impact on body weight unlikely.

A new phenotype of nongoitrous and nonautoimmune hyperthyroidism caused by a heterozygous thyrotropin receptor mutation in transmembrane helix 6.

CONTEXT: Activating mutations in the TSHR gene were found in patients suffering from nonautoimmune hyperthyroidism. In the past, it was assumed that thyroid hyperplasia is due to constitutive activation of the Gs/adenylyl cyclase signaling pathway; however, the physiological role of the Gq/11 pathway in this context remains unclear. OBJECTIVE: In this study, we investigated molecular details of the TSHR in a patient with nonautoimmune and nongoitrous hyperthyroidism. RESULTS: We detected a heterozygous mutation in exon 10 of the TSHR gene leading to an exchange of a cysteine residue for tryptophan at amino acid position 636 in transmembrane helix 6. Functional characterization of the mutant receptor revealed a slight reduction of the cell surface expression and TSH induced cAMP accumulation compared to the wild type. Additional observations included a constitutive activation of the Gs-mediated signaling pathway and a simultaneous nearly complete loss-of-function for the Gq/11 pathway after bovine TSH stimulation. Studies on TSHR models suggest significant changes of important amino acid interactions and the overall helix arrangement caused by mutation C636W. CONCLUSION: We report a patient in whom a TSHR mutation leads to nonautoimmune hyperthyroidism due to a mutation that constitutively activates the Gs signaling pathway but additionally completely inhibits the Gq/11 pathway. The absence of goiter in the patient suggests that the Gq/11 pathway is related to thyroid growth and that different signaling pathways are mediated and regulated by TSH. These functional data could be confirmed by reproducible findings of two siblings with a constitutive activation for both pathways.