Expression cloning of a cDNA encoding the mouse pituitary thyrotropin-releasing hormone receptor

Discoveries Around the Hypothalamic-Pituitary-Thyroid Axis

Many members of the American Thyroid Association played prominent roles in discovering the various aspects of the hypothalamic-pituitary-thyroid axis. This axis is fundamental for maintaining the normal serum levels of circulating thyroid hormones (THs) and thus the euthyroid state. The pituitary glycoprotein hormone, thyrotropin (TSH), controls the activity of the thyroid gland. Thyrotropin-releasing hormone and the negative feedback mechanism of circulating TH regulate the synthesis and the secretion of TSH. The dynamic interplay of these two dominant mechanisms has essential effects on TSH release. Therefore, the finding of abnormal serum levels of TSH often indicates the presence of a disorder of thyroid gland function. A summary of key historical discoveries in the understanding of the hypothalamic-pituitary axis is presented.

Biochemical and physiological insights into TRH receptor-mediated signaling

Thyrotropin-releasing hormone (TRH) is an important endocrine agent that regulates the function of cells in the anterior pituitary and the central and peripheral nervous systems. By controlling the synthesis and release of thyroid hormones, TRH affects many physiological functions, including energy homeostasis. This hormone exerts its effects through G protein-coupled TRH receptors, which signal primarily through G_q/11 but may also utilize other G protein classes under certain conditions. Because of the potential therapeutic benefit, considerable attention has been devoted to the synthesis of new TRH analogs that may have some advantageous properties compared with TRH. In this context, it may be interesting to consider the phenomenon of biased agonism and signaling at the TRH receptor. This possibility is supported by some recent findings. Although knowledge about the mechanisms of TRH receptor-mediated signaling has increased steadily over the past decades, there are still many unanswered questions, particularly about the molecular details of post-receptor signaling. In this review, we summarize what has been learned to date about TRH receptor-mediated signaling, including some previously undiscussed information, and point to future directions in TRH research that may offer new insights into the molecular mechanisms of TRH receptor-triggered actions and possible ways to modulate TRH receptor-mediated signaling.

Synthesis and Evaluation of in Vivo Anti-hypothermic Effect of the N- and C-Terminus Modified Thyrotropin-Releasing Hormone Mimetic: [(4S,5S)-(5-Methyl-2-oxooxazolidine-4-yl)carbonyl]-[3-(thiazol-4-yl)-L-alanyl]-L-prolinamide

We explored orally effective thyrotropin-releasing hormone (TRH) mimetics, which show high central nervous system effects in structure-activity relationship studies based on in vivo antagonistic activity on reserpine-induced hypothermia (anti-hypothermic effect) in mice starting from TRH. This led us to the TRH mimetic: [(4S,5S)-(5-methyl-2-oxooxazolidine-4-yl)carbonyl]-[3-(thiazol-4-yl)-L-alanyl]-L-prolinamide 1, which shows a higher anti-hypothermic effect compared with that of TRH after oral administration. We next attempted further chemical modification of the N- and C-terminus of 1 to find more orally effective TRH mimetics. As a result, we obtained several N- and C-terminus modified TRH mimetics which showed high anti-hypothermic effects.

Characterization of a novel thyrotropin-releasing hormone receptor, TRHR3, in chickens

The physiological roles of thyrotropin-releasing hormone (TRH) are proposed to be mediated by TRH receptors (TRHR), which have been divided into 3 subtypes, namely, TRHR1, TRHR2, and TRHR3, in vertebrates. Although 2 TRH receptors (TRHR1 and TRHR3) have been predicted to exist in birds, it remains unclear whether TRHR3 is a functional TRH receptor similar to TRHR1. Here, we reported the functionality and tissue expression of TRHR3 in chickens. The cloned chicken TRHR3 (cTRHR3) encodes a receptor of 387 amino acids, which shares high-amino-acid identities (63–80%) to TRHR3 of parrots, lizards, Xenopus tropicalis, and tilapia and comparatively lower sequence identities to chicken TRHR1 or mouse TRHR2. Using cell-based luciferase reporter assays and Western blot, we demonstrated that similar to chicken TRHR1 (cTRHR1), cTRHR3 expressed in HEK 293 cells can be potently activated by TRH and that its activation stimulates multiple signaling pathways, indicating both TRH receptors are functional. Quantitative real-time PCR revealed that cTRHR1 and cTRHR3 are widely, but differentially, expressed in chicken tissues, and their expression is likely controlled by promoters located upstream of exon 1, which display strong promoter activities in cultured DF-1 cells. cTRHR1 is highly expressed in the anterior pituitary and testes, while cTRHR3 is highly expressed in the muscle, testes, fat, pituitary, spinal cord, and many brain regions (including hypothalamus). These findings indicate that TRH actions are likely mediated by 2 TRH receptors in chickens. In conclusion, our data provide the first piece of evidence that both cTRHR3 and cTRHR1 are functional TRH receptors, which helps to elucidate the physiological roles of TRH in birds.

Synthesis and evaluation of in vivo anti-hypothermic effect of all stereoisomers of the thyrotropin-releasing hormone mimetic: Rovatirelin Hydrate

We discovered the orally active thyrotropin-releasing hormone (TRH) mimetic: (4S,5S)-5-methyl-N-{(2S)-1-[(2R)-2-methylpyrrolidin-1-yl]-1-oxo-3-(1,3-thiazol-4-yl)propan-2-yl}-2-oxo-1,3-oxazolidine-4-carboxamide 1 (rovatirelin). The central nervous system (CNS) effect of rovatirelin after intravenous (iv) administration is 100-fold higher than that of TRH. As 1 has four asymmetric carbons in its molecule, there are 16 stereoisomers. We synthesized and evaluated the anti-hypothermic effect of all stereoisomers of 1, which has the (4S),(5S),(2S),(2R) configuration from the N-terminus to the C-terminus, in order to clarify the structure-activity relationship (SAR) of stereoisomers. The (4R),(5R),(2R),(2S)-isomer 16 did not show any anti-hypothermic effect. Only the (4S),(5S),(2S),(2S)-isomer 10, which has the (2S)-2-methylpyrrolidine moiety at the C-terminus showed the anti-hypothermic effect similar to 1. Stereoisomers, which have the (5R) configuration of the oxazolidinone at the N-terminus and the (2R) configuration at the middle-part, showed a much lower anti-hypothermic effect than that of 1. On the other hand, stereoisomers, which have the (4R) configuration of the oxazolidinone at the N-terminus or the (2S) configuration of the C-terminus, have little influence on the anti-hypothermic effect.

Discovery of the Orally Effective Thyrotropin-Releasing Hormone Mimetic: 1-{N-[(4S,5S)-(5-Methyl-2-oxooxazolidine-4-yl)carbonyl]-3-(thiazol-4-yl)-l-alanyl}-(2R)-2-methylpyrrolidine Trihydrate (Rovatirelin Hydrate)

We have explored orally effective thyrotropin-releasing hormone (TRH) mimetics, showing oral bioavailability and brain penetration by structure-activity relationship (SAR) study on the basis of in vivo antagonistic activity on reserpine-induced hypothermia in mice. By primary screening of the synthesized TRH mimetics, we found a novel TRH mimetic: l-pyroglutamyl-[3-(thiazol-4-yl)-l-alanyl]-l-prolinamide with a high central nervous system effect compared with TRH as a lead compound. Further SAR optimization studies of this lead compound led to discovery of a novel orally effective TRH mimetic: 1-{N-[(4S,5S)-(5-methyl-2-oxooxazolidine-4-yl)carbonyl]-3-(thiazol-4-yl)-l-alanyl}-(2R)-2-methylpyrrolidine trihydrate (rovatirelin hydrate), which was selected as a candidate for clinical trials.

Possible involvement of thyrotropin-releasing hormone receptor 3 in the release of prolactin in the metamorphosing bullfrog larvae

In amphibians, thyrotropin (TSH), corticotropin (ACTH) and prolactin (PRL) are regarded as the major pituitary hormones involved in metamorphosis, their releasing factors being corticotropin-releasing factor (CRF), arginine vasotocin (AVT), and thyrotropin-releasing hormone (TRH), respectively. It is also known that thyrotropes and corticotropes are equipped with CRF type-2 receptor and AVT V1b receptor, respectively. As for PRL cells, information about the type of receptor for TRH (TRHR) through which the action of TRH is mediated to induce the release of PRL is lacking. In order to fill this gap, an attempt was made to characterize the TRHR subtype existing in the PRL cells of the anterior pituitary gland of the bullfrog, Rana catesbeiana. We cloned cDNAs for three types of bullfrog TRHRs, namely TRHR1, TRHR2 and TRHR3, and confirmed that all of them are functional receptors for TRH by means of reporter gene assay. Analyses with semi-quantitative reverse transcription-PCR and in situ hybridization revealed that TRHR3 mRNA is expressed in the anterior lobe and that the signals reside mostly in the PRL cells. It was also noted that the expression levels of TRHR3 mRNA in the anterior pituitary as well as in the PRL cells of metamorphosing tadpoles elevate as metamorphosis progresses. Since the pattern of changes in TRHR3 mRNA levels in the larval pituitary is almost similar to that previously observed in the pituitary PRL mRNA and plasma PRL levels, we provide a view that TRHR3 mediates the action of TRH on the PRL cells to induce the release of PRL that is prerequisite for growth and metamorphosis in amphibians.

Intravenous and Intratracheal Thyrotropin Releasing Hormone and Its Analog Taltirelin Reverse Opioid-Induced Respiratory Depression in Isoflurane Anesthetized Rats

Thyrotropin releasing hormone (TRH) is a tripeptide hormone and a neurotransmitter widely expressed in the central nervous system that regulates thyroid function and maintains physiologic homeostasis. Following injection in rodents, TRH has multiple effects including increased blood pressure and breathing. We tested the hypothesis that TRH and its long-acting analog, taltirelin, will reverse morphine-induced respiratory depression in anesthetized rats following intravenous or intratracheal (IT) administration. TRH (1 mg/kg plus 5 mg/kg/h, i.v.) and talitrelin (1 mg/kg, i.v.), when administered to rats pretreated with morphine (5 mg/kg, i.v.), increased ventilation from 50% ± 6% to 131% ± 7% and 45% ± 6% to 168% ± 13%, respectively (percent baseline; n = 4 ± S.E.M.), primarily through increased breathing rates (from 76% ± 9% to 260% ± 14% and 66% ± 8% to 318% ± 37%, respectively). By arterial blood gas analysis, morphine caused a hypoxemic respiratory acidosis with decreased oxygen and increased carbon dioxide pressures. TRH decreased morphine effects on arterial carbon dioxide pressure, but failed to impact oxygenation; taltirelin reversed morphine effects on both arterial carbon dioxide and oxygen. Both TRH and talirelin increased mean arterial blood pressure in morphine-treated rats (from 68% ± 5% to 126% ± 12% and 64% ± 7% to 116% ± 8%, respectively; n = 3 to 4). TRH, when initiated prior to morphine (15 mg/kg, i.v.), prevented morphine-induced changes in ventilation; and TRH (2 mg/kg, i.v.) rescued all four rats treated with a lethal dose of morphine (5 mg/kg/min, until apnea). Similar to intravenous administration, both TRH (5 mg/kg, IT) and taltirelin (2 mg/kg, IT) reversed morphine effects on ventilation. TRH or taltirelin may have clinical utility as an intravenous or inhaled agent to antagonize opioid-induced cardiorespiratory depression.

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary-thyroid axis

This review presents the findings that led to the discovery of TRH and the understanding of the central mechanisms which control hypothalamus-pituitary-thyroid axis (HPT) activity. The earliest studies on thyroid physiology are now dated a century ago when basal metabolic rate was associated with thyroid status. It took over 50 years to identify the key elements involved in the HPT axis. Thyroid hormones (TH: T4 and T3) were characterized first, followed by the semi-purification of TSH whose later characterization paralleled that of TRH. Studies on the effects of TH became possible with the availability of synthetic hormones. DNA recombinant techniques facilitated the identification of all the elements involved in the HPT axis, including their mode of regulation. Hypophysiotropic TRH neurons, which control the pituitary-thyroid axis, were identified among other hypothalamic neurons which express TRH. Three different deiodinases were recognized in various tissues, as well as their involvement in cell-specific modulation of T3 concentration. The role of tanycytes in setting TRH levels due to the activity of deiodinase type 2 and the TRH-degrading ectoenzyme was unraveled. TH-feedback effects occur at different levels, including TRH and TSH synthesis and release, deiodinase activity, pituitary TRH-receptor and TRH degradation. The activity of TRH neurons is regulated by nutritional status through neurons of the arcuate nucleus, which sense metabolic signals such as circulating leptin levels. Trh expression and the HPT axis are activated by energy demanding situations, such as cold and exercise, whereas it is inhibited by negative energy balance situations such as fasting, inflammation or chronic stress. New approaches are being used to understand the activity of TRHergic neurons within metabolic circuits.

Absence of TRH receptor 1 in male mice affects gastric ghrelin production

TRH not only functions as a thyrotropin releasing hormone but also acts as a neuropeptide in central circuits regulating food intake and energy expenditure. As one suggested mode of action, TRH expressed in the caudal brainstem influences vagal activity by activating TRH receptor 1 (TRH-R1). In order to evaluate the impact of a diminished medullary TRH signaling on ghrelin metabolism, we analyzed metabolic changes of TRH-R1 knockout (R1ko) mice in response to 24 hours of food deprivation. Because R1ko mice are hypothyroid, we also studied eu- and hypothyroid wild-type (wt) animals and R1ko mice rendered euthyroid by thyroid hormone treatment. Independent of their thyroidal state, R1ko mice displayed a higher body weight loss than wt animals and a delayed reduction in locomotor activity upon fasting. Ghrelin transcript levels in the stomach as well as total ghrelin levels in the circulation were equally high in fasted wt and R1ko mice. In contrast, only wt mice responded to fasting with a rise in ghrelin-O-acyltransferase mRNA expression and consequently an increase in serum levels of acylated ghrelin. Together, our data suggest that an up-regulation of medullary TRH expression and subsequently enhanced activation of TRH-R1 in the vagal system represents a critical step in the stimulation of ghrelin-O-acyltransferase expression upon starvation that in turn is important for adjusting the circulating levels of acylated ghrelin to the fasting condition.

Gsp mutation in acromegaly and its influence on TRH-induced paradoxical GH response

OBJECTIVE

We recently reported that paradoxical GH response to TRH administration reflects biological characteristics in patients with acromegaly. The aim of this study is to elucidate the relationship between gsp mutations and the paradoxical GH response to TRH.

PATIENTS

Sixty-seven patients with acromegaly were included for analysis. Paradoxical increase in serum GH level to TRH, GH suppression by octreotide and bromocriptine, radiological profiles and histopathological findings were analysed with respect to tumour gsp-mutation status.

RESULTS

Twenty-six (38·8%) gsp mutations were detected, and the number of paradoxical GH responders to TRH, defined as an increase of 100% or more in GH after TRH, was 49 (73·1%). Among the paradoxical GH responders to TRH, 21 patients (42·9%) had a gsp mutation and 28 patients (57·1%) did not. The percentage of paradoxical GH responders to TRH in gsp-positive and gsp-negative patients was not significantly different (80·8% and 68·3%, respectively). The gsp-positive group showed a significantly higher paradoxical increase in serum GH level by TRH administration (1830% vs 650% GH increase, P = 0·045) and greater GH suppression by octreotide (88·7% vs 75·4% GH decrease, P = 0·003) than the gsp-negative group.

CONCLUSION

Paradoxical GH response to TRH was observed regardless of gsp mutation, although the rate of increase was significantly higher in gsp-positive patients. These results suggest that gsp mutation is not sufficient to cause the paradoxical GH response to TRH, while other unidentified factors have a strong influence on paradoxical GH response to TRH in patients with acromegaly.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Minireview: The neural regulation of the hypothalamic-pituitary-thyroid axis

Thyroid hormone (TH) signaling plays an important role in development and adult life. Many organisms may have evolved under selective pressure of exogenous TH, suggesting that thyroid hormone signaling is phylogenetically older than the systems that regulate their synthesis. Therefore, the negative feedback system by TH itself was probably the first mechanism of regulation of circulating TH levels. In humans and other vertebrates, it is well known that TH negatively regulates its own production through central actions that modulate the hypothalamic-pituitary-thyroid (HPT) axis. Indeed, primary hypothyroidism leads to the up-regulation of the genes encoding many key players in the HPT axis, such as TRH, type 2 deiodinase (dio2), pyroglutamyl peptidase II (PPII), TRH receptor 1 (TRHR1), and the TSH α- and β-subunits. However, in many physiological circumstances, the activity of the HPT axis is not always a function of circulating TH concentrations. Indeed, circadian changes in the HPT axis activity are not a consequence of oscillation in circulating TH levels. Similarly, during reduced food availability, several components of the HPT axis are down-regulated even in the presence of lower circulating TH levels, suggesting the presence of a regulatory pathway hierarchically higher than the feedback system. This minireview discusses the neural regulation of the HPT axis, focusing on both TH-dependent and -independent pathways and their potential integration.

Alterations of protein profile in zebrafish liver cells exposed to methyl parathion: a membrane proteomics approach

Methyl parathion (MP) is an extensively used organophosphorus pesticide, which has been associated with a wide spectrum of toxic effects on environmental organisms. The aim of this study is to investigate the alterations of membrane protein profiles in zebrafish liver (ZFL) cell line exposed to MP for 24 h using proteomic approaches. Two-dimensional gel electrophoresis revealed a total of 13 protein spots, whose expression levels were significantly altered by MP. These differential proteins were subjected to matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry analysis, and nine proteins were identified to be membrane proteins, among which seven were up-regulated, while two were down-regulated. In addition, the mRNA levels corresponding to these differential membrane proteins were further analyzed by quantitative real-time PCR. And the differential expression of arginase-2 was specially validated via Western blotting. Regarding the physiological functions, these proteins are involved in molecular chaperon, cytoskeleton system, cell metabolism, signal transduction, transport and hormone receptor respectively, suggesting the complexity of MP-mediated toxicity to ZFL cell. These data could provide useful insights for better understanding the hepatotoxic mechanisms of MP and develop novel protein biomarkers for effectively monitoring MP contamination level in aquatic environment.

Molecular cloning, molecular evolution and gene expression of cDNAs encoding thyrotropin-releasing hormone receptor subtypes in a teleost, the sockeye salmon (Oncorhynchus nerka)

Molecular cloning of thyrotropin-releasing hormone receptors (TRHR) was performed in a teleost, the sockeye salmon (Oncorhynchus nerka). Four different TRHR cDNAs were cloned and named TRHR1, TRHR2a, TRHR2b and TRHR3 based on their similarity to known TRHR subtypes in vertebrates. Important residues for TRH binding were conserved in deduced amino acid sequences of the three TRHR subtypes except for the TRHR2b. Seven transmembrane domains were predicted for TRHR1, TRHR2a and TRHR3 proteins but only five for TRHR2b which appears to be truncated. In silico database analysis identified putative TRHR sequences including invertebrate TRHR and reptilian, avian and mammalian TRHR3. Phylogenetic analyses predicted the molecular evolution of TRHR in vertebrates: from the common ancestral TRHR (i.e. invertebrate TRHR), the TRHR2 subtype diverged first and then TRHR1 and TRHR3 diverged. Reverse transcription-polymerase chain reaction analyses revealed TRHR1 transcripts in the brain (hypothalamus), retina, pituitary gland and large intestine; TRHR2a in the brain (telencephalon and hypothalamus); and TRHR3 in the brain (olfactory bulbs) and retina.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Thyrotropin Releasing Hormone (TRH) in goldfish (Carassius auratus): role in the regulation of feeding and locomotor behaviors and interactions with the orexin system and cocaine- and amphetamine regulated transcript (CART)

TRH is a peptide produced by the hypothalamus which major function in mammals is the regulation of TSH secretion by the pituitary. In fish, TRH does not appear to affect TSH secretion, suggesting that it might regulate other functions. In this study, we assessed the effects of central (intracerebroventricular, icv) injections of TRH on feeding and locomotor behavior in goldfish. TRH at 10 and 100 ng/g, but not 1 ng/g, significantly increased feeding and locomotor behaviors, as indicated by an increase in food intake and in the number of total feeding acts as compared to saline-injected fish. In order to assess possible interactions between TRH and other appetite regulators, we examined the effects of icv injections of TRH on the hypothalamic expression of orexin, orexin receptor and CART. The mRNA expression levels of all three peptides were significantly increased in fish injected with TRH at 100 ng/g as compared to saline-injected fish. Fasting increased TRH, orexin, and orexin receptor hypothalamic mRNA levels and decreased CART hypothalamic mRNA levels. Our results suggest that TRH is involved in the regulation of feeding/locomotor activity in goldfish and that this action is associated with a stimulation of both the orexin and CART systems.

Housing and husbandry of Xenopus laevis affect the quality of oocytes for heterologous expression studies

To assess the effect of Xenopus husbandry on oocyte quality for membrane transport physiology experiments, we compared a recirculating-water housing system with a static-water system in a 23-mo study. Two groups of frogs (n = 8) were maintained separately for the entire study: one group was housed in a multiinvestigator centrally managed Xenopus facility, which consists of 33 tanks placed on a shared and recirculating water system; the other group was housed in a satellite facility used by a single investigator and consisting of static tanks placed in a dedicated cold-room. The activity of a heterologously expressed membrane transporter was assessed every 4 to 5 wk for a total of 23 mo. Activity of the mouse cotransporter NKCC1 was assessed through isotopic (86) Rb influx measurements under 2 experimental conditions: stimulation of cotransporter by coinjection of regulatory kinases and by exposure to a hypertonic solution. The results showed a significant difference in the level of ion fluxes under these 2 experimental conditions between the 2 groups of oocytes. During the entire period, oocytes isolated from frogs maintained in the static facility demonstrated consistently robust NKCC1 function, whereas oocytes isolated from frogs maintained in the recirculating facility showed inconsistent and weaker cotransporter function. Furthermore, the oocytes isolated from frogs maintained in the recirculating facility showed significant deterioration during the summer months (April to August), a seasonal variation that was muted in frog oocytes maintained in the static facility.

Protein alterations induced by long-term agonist treatment of HEK293 cells expressing thyrotropin-releasing hormone receptor and G11alpha protein

This study aimed to determine whether sustained stimulation with thyrotropin-releasing hormone (TRH), a peptide with important physiological functions, can possibly affect expression of plasma membrane proteins in HEK293 cells expressing high levels of TRH receptor and G(11)alpha protein. Our previous experiments using silver-stained two-dimensional polyacrylamide gel electrophoretograms did not reveal any significant changes in an overall composition of membrane microdomain proteins after long-term treatment with TRH of these cells (Matousek et al. 2005 Cell Biochem Biophys 42: 21-40). Here we used a purified plasma membrane fraction prepared by Percoll gradient centrifugation and proteins resolved by 2D electrophoresis were stained with SYPRO Ruby gel stain. The high enrichment in plasma membrane proteins of this preparation was confirmed by a multifold increase in the number of TRH receptors and agonist stimulated G-protein activity, compared to postnuclear supernatant. By a combination of these approaches we were able to determine a number of clearly discernible protein changes in the plasma membrane-enriched fraction isolated from cells treated with TRH (1 x 10(-5) M, 16 h): 4 proteins disappeared, the level of 18 proteins decreased and the level of 39 proteins increased. Our concomitant immunochemical determinations also indicated a clear down-regulation of G(q/11)alpha proteins in preparations from hormone-treated cells. In parallel, we observed decrease in caspase 3 and alterations in some other apoptotic marker proteins, which were in line with the presumed antiapoptotic effect of TRH.

Distribution and axonal projections of neurons coexpressing thyrotropin-releasing hormone and urocortin 3 in the rat brain

Thyrotropin-releasing hormone (TRH) decreases food intake when administered intracerebroventricularly or into the ventromedial hypothalamus. However, it is unknown which population of TRH neurons exerts this anorexigenic function. In the rostral perifornical area, the pattern of TRH-expressing neurons is reminiscent of the distribution of neurons expressing urocortin3 (Ucn3) that also inhibits feeding when injected into the hypothalamic ventromedial nucleus (VMN). Since colocalization of TRH and Ucn3 may help to identify feeding-related TRH neurons, the putative coexpression of the two peptides was examined using fluorescent in situ hybridization combined with immunofluorescence. Almost all (95.5 +/- 0.2%) Ucn3-immunoreactive neurons in the perifornical area expressed pro-TRH mRNA, while 50.2 +/- 1.6% Ucn3 neurons were double-labeled in the bed nucleus of the stria terminalis (BNST). Only a few Ucn3/pro-TRH neurons were found outside these two areas. The distribution of axons containing both Ucn3 and TRH was examined by dual immunofluorescence. Ucn3/TRH fibers heavily innervated the VMN. In addition, high densities of double-labeled axons were observed in the lateral septal nucleus, posterior division of the BNST, medial amygdaloid nucleus, amygdalohippocampal area, and ventral hippocampus, forebrain areas associated with psychological stress and anxiety. We conclude that Ucn3 and TRH are coexpressed in a discrete, continuous population of neurons in the perifornical area and BNST, making Ucn3 a neurochemical marker to define a distinct subset of TRH neurons. The distribution of their axons suggests that Ucn3/TRH neurons may coordinate feeding and behavioral responses to stressful stimuli.

TRH acts as a multifunctional hypophysiotropic factor in vertebrates

Thyrotropin-releasing hormone (TRH) is the first hypothalamic hypophysiotropic neuropeptide whose sequence has been chemically characterized. The primary structure of TRH (pGlu-His-Pro-NH(2)) has been fully conserved across the vertebrate phylum. TRH is generated from a large precursor protein that contains multiple repeats of the TRH progenitor tetrapeptide Gln-His-Pro-Gly. In all tetrapods, TRH-expressing neurons located in the hypothalamus project towards the external zone of the median eminence while in teleosts they directly innervate the pars distalis of the pituitary. In addition, in frogs and teleosts, a bundle of TRH-containing fibers terminate in the neurointermediate lobe of the pituitary gland. Although TRH was originally named for its ability to trigger the release of thyroid-stimulating hormone (TSH) in mammals, it later became apparent that it exerts multiple, species-dependent hypophysiotropic activities. Thus, in fish TRH stimulates growth hormone (GH) and prolactin (PRL) release but does not affect TSH secretion. In amphibians, TRH is a marginal stimulator of TSH release in adult frogs, not in tadpoles, and a major releasing factor for GH and PRL. In birds, TRH triggers TSH and GH secretion. In mammals, TRH stimulates TSH, GH and PRL release. In fish and amphibians, TRH is also a very potent stimulator of alpha-melanocyte-stimulating hormone release. Because the intermediate lobe of the pituitary of amphibians is composed by a single type of hormone-producing cells, the melanotrope cells, it is a suitable model in which to investigate the mechanism of action of TRH at the cellular and molecular level. The occurrence of large amounts of TRH in the frog skin and high concentrations of TRH in frog plasma suggests that, in amphibians, skin-derived TRH may exert hypophysiotropic functions.

TRH-receptor-type-2-deficient mice are euthyroid and exhibit increased depression and reduced anxiety phenotypes

Thyrotropin-releasing hormone (TRH) is a neuropeptide that initiates its effects in mice by interacting with two G-protein-coupled receptors, TRH receptor type 1 (TRH-R1) and TRH receptor type 2 (TRH-R2). Two previous reports described the effects of deleting TRH-R1 in mice. TRH-R1 knockout mice exhibit hypothyroidism, hyperglycemia, and increased depression and anxiety-like behavior. Here we report the generation of TRH-R2 knockout mice. The phenotype of these mice was characterized using gross and histological analyses along with blood hematological assays and chemistries. Standard metabolic tests to assess glucose and insulin tolerance were performed. Behavioral testing included elevated plus maze, open field, tail suspension, forced swim, and novelty-induced hypophagia tests. TRH-R2 knockout mice are euthyroid with normal basal and TRH-stimulated serum levels of thyroid-stimulating hormone (thyrotropin), are normoglycemic, and exhibit normal development and growth. Female, but not male, TRH-R2 knockout mice exhibit moderately increased depression-like and reduced anxiety-like phenotypes. Because the behavioral changes in TRH-R1 knockout mice may have been caused secondarily by their hypothyroidism whereas TRH-R2 knockout mice are euthyroid, these data provide the first evidence for the involvement of the TRH/TRH-R system, specifically extrahypothalamic TRH/TRH-R2, in regulating mood and affect.

Selectivity-based QSAR approach for screening and evaluation of TRH analogs for TRH-R1 and TRH-R2 receptors subtypes

Design and development of therapeutically useful CNS selective thyrotropin-releasing hormone (TRH) analogs acting on TRH-R2 receptor subtype, exerting weak or no TRH-R1-mediated TSH-releasing side effects has gained imagination of researchers in the recent past. The present study reports the development and implementation of a selectivity-based QSAR approach for screening selective agonists of TRH-R2 receptor subtype. The statistically significant predictive models were thoroughly validated using an external validation set whose activity was previously unknown. The model was able to predict preference for either of the receptor subtypes successfully.

Corymbones A and B, phloroglucinols with thyrotropin releasing hormone receptor 2 binding affinity from the flowers of Corymbia peltata

High-throughput screeing of a plant and marine invertebrate extract library to find natural products with rat thytotropin releasing hormone receptor 2 binding affinity led to the isolation of two new active acylphloroglucinols, corymbones A and B (1 and 2) from flowers of the Queensland tree Corymbia peltata. Their structures were assigned from interpretation of 2D NMR and high-resolution ESIMS data. Compounds 1 and 2 showed rat TRH receptor 2 binding affinity with IC 50 values of 23 and 19 microM, respectively.

Spongian diterpenes with thyrotropin releasing hormone receptor 2 binding affinity from Spongia sp

High-throughput screening of a plant and marine invertebrate extract library to find natural products with rat thyrotropin releasing hormorne (TRH) receptor 2 binding affinity led to the isolation of four new (1-4) and one known (5) spongian diterpene from the sponge Spongia sp. The structures were assigned from interpretation of 2D NMR and high-resolution ESIMS data. The absolute configurations of 1-4 were proposed on the basis of analysis of their CD spectra. Diterpenes 1-5 showed rat TRH receptor 2 binding affinity with IC(50) values of 23 microM, 70 microM, 400 microM, 600 microM, and 1 mM, respectively.

A novel TRH analog, Glp-Asn-Pro-D-Tyr-D-TrpNH2, binds to [3H][3-Me-His2]TRH-labelled sites in rat hippocampus and cortex but not pituitary or heterologous cells expressing TRHR1 or TRHR2

Glp-Asn-Pro-D-Tyr-D-TrpNH(2) is a novel synthetic peptide that mimics and amplifies central actions of thyrotropin-releasing hormone (TRH) in rat without releasing TSH. The aim of this study was to compare the binding properties of this pentapeptide and its all-L counterpart (Glp-Asn-Pro-Tyr-TrpNH(2)) to TRH receptors in native rat brain tissue and cells expressing the two TRH receptor subtypes identified in rat to date, namely TRHR1 and TRHR2. Radioligand binding studies were carried out using [(3)H][3-Me-His(2)]TRH to label receptors in hippocampal, cortical and pituitary tissue, GH4 pituitary cells, as well as CHO cells expressing TRHR1 and/or TRHR2. In situ hybridization studies suggest that cortex expresses primarily TRHR2 mRNA, hippocampus primarily TRHR1 mRNA and pituitary exclusively TRHR1 mRNA. Competition experiments showed [3-Me-His(2)]TRH potently displaced [(3)H][3-Me-His(2)]TRH binding from all tissues/cells investigated. Glp-Asn-Pro-D-Tyr-D-TrpNH(2) in concentrations up to 10(-5)M did not displace [(3)H][3-Me-His(2)]TRH binding to membranes derived from GH4 cells or CHO-TRHR1 cells, consistent with its lack of binding to pituitary membranes and TSH-releasing activity. Similar results were obtained for the corresponding all-L peptide. In contrast, both pentapeptides displaced binding from rat hippocampal membranes (pIC(50) Glp-Asn-Pro-D-Tyr-D-TrpNH(2): 7.7+/-0.2; pIC(50) Glp-Asn-Pro-Tyr-TrpNH(2): 6.6+/-0.2), analogous to cortical membranes (pIC(50) Glp-Asn-Pro-D-Tyr-D-TrpNH(2): 7.8+/-0.2; pIC(50) Glp-Asn-Pro-Tyr-TrpNH(2): 6.6+/-0.2). Neither peptide, however, displaced [(3)H][3-Me-His(2)]TRH binding to CHO-TRHR2. Thus, this study reveals for the first time significant differences in the binding properties of native and heterologously expressed TRH receptors. Also, the results raise the possibility that Glp-Asn-Pro-D-Tyr-D-TrpNH(2) is not displacing [(3)H][3-Me-His(2)]TRH from a known TRH receptor in rat cortex, but rather a hitherto unidentified TRH receptor.

Suppression of thyrotropin receptor constitutive activity by a monoclonal antibody with inverse agonist activity

TSH binding to the TSH receptor (TSHR) induces thyrocyte growth and proliferation primarily by activating the adenylyl cyclase signaling pathway. Relative to the other glycoprotein hormone receptors, the TSHR has considerable ligand-independent (constitutive) activity. We describe a TSHR monoclonal antibody (CS-17) with the previously unrecognized property of being an inverse agonist for TSHR constitutive activity. This property is retained, even when constitutive activity is extremely high consequent to diverse TSHR extracellular region mutations. A similar effect on an activating mutation at the base of the sixth transmembrane helix (not accessible to direct CS-17 contact) indicates that CS-17 is acting allosterically. Administered to mice in vivo, CS-17 reduces serum T(4) levels. The CS-17 epitope is conformational and a significant portion lies in the C-terminal region of the TSHR leucine-rich domain (residues 260-289). By interacting with the large TSHR extracellular domain, CS-17 is, to our knowledge, the first antibody reported to be an inverse agonist for a member of the G protein receptor superfamily. After humanization of its murine constant region, CS-17 has the potential to be an adjunctive therapeutic agent in athyreotic patients with residual well-differentiated thyroid carcinoma as well as pending definitive treatment in some selected hyperthyroidism states.

Thyrotrophin-releasing hormone decreases feeding and increases body temperature, activity and oxygen consumption in Siberian hamsters

Thyrotrophin-releasing hormone (TRH) is known to play an important role in the control of food intake and energy metabolism in addition to its actions on the pituitary-thyroid axis. We have previously shown that central administration of TRH decreases food intake in Siberian hamsters. This species is being increasingly used as a physiological rodent model in which to understand hypothalamic control of long-term changes in energy balance because it accumulates fat reserves in long summer photoperiods, and decreases food intake and body weight when exposed to short winter photoperiods. The objectives of our study in Siberian hamsters were: (i) to investigate whether peripheral administration of TRH would mimic the effects of central administration of TRH on food intake and whether these effects would differ dependent upon the ambient photoperiod; (ii) to determine whether TRH would have an effect on energy expenditure; and (iii) to investigate the potential sites of action of TRH. Both peripheral (5-50 mg/kg body weight; i.p.) and central (0.5 microg/ml; i.c.v.) administration of TRH decreased food intake, and increased locomotor activity, body temperature and oxygen consumption in the Siberian hamster, with a rapid onset and short duration of action. Systemic treatment with TRH was equally effective in suppressing feeding regardless of ambient photoperiod. The acute effects of TRH are likely to be centrally mediated and independent of its role in the control of the production of thyroid hormones. We conclude that TRH functions to promote a catabolic energetic state by co-ordinating acute central and chronic peripheral (thyroid-mediated) function.

Synthesis, receptor binding, and activation studies of N(1)-alkyl-l-histidine containing thyrotropin-releasing hormone (TRH) analogues

Thyrotropin-releasing hormone (TRH) analogues in which the N(1)-position of the imidazole ring of the centrally placed histidine residue is substituted with various alkyl groups were synthesized and studied as agonists for TRH receptor subtype 1 (TRH-R1) and subtype 2 (TRH-R2). Analogue 3 (R=C2H5) exhibited binding affinity (Ki) of 0.012 microM to TRH-R1 that is about 1.1-fold higher than that of TRH. Several analogues were found to selectively activate TRH-R2 with greater potency than TRH-R1. The most selective agonist of the series 5 [R=CH(CH3)2] was found to activate TRH-R2 with a potency (EC50) of 0.018 microM but could only activate TRH-R1 at EC50 value of 1.6 microM; that is, exhibited 88-fold greater potency for TRH-R2 versus TRH-R1. The results of this study indicate that modulation of central histidine residue is important for designing analogues which were selective agonist at TRH receptor subtypes.

TRH-like antidepressant peptide, pyroglutamyltyroslyprolineamide, occurs in rat brain

We have previously reported the occurrence of pGlu-Glu-Pro-NH(2)(Glu-TRH, EEP), Val-TRH, Tyr-TRH, Leu-TRH, Phe-TRH, and Trp-TRH in rat brain using a combination of HPLC and radioimmunoassays with antibodies that cross-react with the general structure pGlu-X-Pro-NH(2) where 'X' maybe any amino acid residue (Peptides 2004; 25 : 647). This new family of TRH-like peptides, along with TRH (pGlu-His-Pro-NH(2)), has neuroprotective, anticonvulsant, antidepressant, euphoric, anti-amnesic, and analeptic effects. We now report that a combination of affinity chromatography using a rabbit antibody specific for Tyr-TRH and Phe-TRH, along with HPLC and tandem mass spectrometry operating in the multiple reaction monitoring (MRM) mode, provide conclusive evidence for the presence of Tyr-TRH in rat brain. Furthermore, synthetic Tyr-TRH is active in the Porsolt Swim Test suggesting that it is a fourth member of this family of in vivo neuroregulatory agents that have psychopharmacotherapeutic properties.

Thyrotropin-Releasing Hormone (TRH) Analogues That Exhibit Selectivity to TRH Receptor Subtype 2

Thyrotropin-releasing hormone (TRH) analogues in which the C-2 position of the imidazole ring of the centrally placed histidine residue is substituted with various alkyl groups were synthesized and studied as agonists for TRH receptor subtype 1 (TRH-R1) and subtype 2 (TRH-R2). Several analogues were found to be selective agonists for TRH-R2 exhibiting no activation of TRH-R1. For example, analogue 4 (R= c-C3H5) was found to activate TRH-R2 with a potency (EC50) of 0.41 microM but did not activate TRH-R1 (potency > 100 microM). This study describes the first discovery of TRH-R2-specific agonists and provides impetus to design predominately CNS-effective TRH peptides.

Thyrotropin-releasing hormone in cardiovascular pathophysiology

Thyrotropin (TSH)-releasing hormone (TRH) also known as thyroliberin was the first of a number of peptides exerting several roles as a hormone and as a neuropeptide. Its ubiquitous distribution in the hypothalamus and in the extrahypothalamic regions and its diverse pharmacological and physiological effects are all features of its dual functions. For this reason, TRH has been the subject of much research throughout the past 20 years, work that has examined the structure, function, distribution, and regulation of the tripeptide and it has been extensively reviewed elsewhere [O'Leary R., O'Connor B. Thyrotropin-releasing hormone. J Neurochem. 1995;65:953-963.; Nillni E., Sevarino K. The biology of pro-thyrotropin-releasing hormone-derived peptides. Endocrine Reviews, 1999;20:599-664.]. After a brief overview of its distribution, hypothalamic and extrahypothalamic functions, and receptors involved, this review discusses efforts devoted to support TRH role in cardiovascular regulation with a main focus on hypertension pathophysiology in experimental models and humans.

Thyrotropin-releasing hormone induced thermogenesis in Syrian hamsters: Site of action and receptor subtype

Early work in our laboratory has revealed the important role played by thyrotropin-releasing hormone (TRH) in the arousal from hibernation in Syrian hamsters. In the present study, we investigated the thermogenic mechanism of TRH in Syrian hamsters. Six to 10 female Syrian hamsters were used in the respective experiments. Intracerebroventricular (icv) injection of TRH elevated the intrascapular brown adipose tissue (IBAT) temperature (T(IBAT)) and rectal temperature (T rec) in Syrian hamsters. Thermogenic response of icv TRH was suppressed by bilateral denervation of the sympathetic nerve. Icv injection of TRH increased the norepinephrin (NE) turnover rate in IBAT without affecting the total serum triiodothyronine (T3) level. Moreover, TRH microinjections into the dorsomedial hypothalamus (DMH), preoptic area (PO), anterior hypothalamus (AH) and ventromedial hypothalamus (VMH) induced T(IBAT) and T(rec) increases. However, neither T(IBAT) nor T rec was affected by similar TRH administrations into the lateral hypothalamus and posterior hypothalamus. Interestingly, although TRH-induced hyperthermia was suppressed by pretreatment of anti-TRH-R1 antibodies, no changes were induced by anti-TRH-R2 antibodies. These results suggest that the sites of action of TRH associated with thermogenesis are probably localized in the DMH, PO, AH and VMH. In addition, TRH-induced thermogenesis is probably elicited by facilitation of the sympathetic nerve system via the central TRH-R1 irrelevant of T3.

Generation of Thyrotropin-Releasing Hormone Receptor 1-Deficient Mice as an Animal Model of Central Hypothyroidism

To provide an animal model of central hypothyroidism, mice deficient in the TRH-receptor 1 (TRH-R1) gene were generated by homologous recombination. The pituitaries of TRH-R1−/− mice are devoid of any TRH-binding capacity, demonstrating that TRH-R1 is the only receptor localized on TRH target cells of the pituitary. With the exception of some retardation in growth rate, TRH-R1−/− mice appear normal, but compared with control animals they exhibit a considerable decrease in serum T3, T4, and prolactin (PRL) levels but not in serum TSH levels. In situ hybridization histochemistry and real-time RT-PCR analysis revealed that in adult TRH-R1−/− animals TSHβ-mRNA expression is not impaired whereas PRL mRNA and GH mRNA levels are considerably reduced compared with control mice. The numbers of thyrotropes, somatotropes, and lactotropes, however, are not affected by the deletion of the TRH-R1 gene. The mutant mice are fertile, and the dams nourish their pups well, indicating that TRH is not a decisive factor for suckling-induced PRL release. In situ hybridization and quantitative RT-PCR analysis, furthermore, revealed that, as in control animals, pituitary PRL-mRNA expression in TRH-R1−/− is considerably increased during lactation, albeit strongly reduced as compared with lactating control animals.

A pituitary gene encodes a protein that produces differentiation of breast and prostate cancer cells

A cDNA clone of 1.1 kb encoding a 108-aa polypeptide was isolated from a human pituitary cDNA library by expression cloning. This protein was named tumor differentiation factor (TDF). The recombinant TDF protein and a 20-aa peptide, P1, selected from the ORF of the gene, induced morphological and biochemical changes consistent with differentiation of human breast and prostate cancer cells. Fibroblast, kidney, hepatoma, and leukemic lymphocytic cell lines were unaffected. Breast and prostate cancer cells aggregated in spheroid-like structures within 24 h of exposure to TDF. This effect was abrogated by a specific affinity-purified rabbit polyclonal anti-P1 Ab. E-cadherin expression was increased in a dose-dependent manner by TDF. Treatment of MCF7 cells with TDF led to production of a lactalbumin-related protein. Peptide P1 significantly decreased the growth of androgen-independent DU145 prostate cancer in severe combined immunodeficient mice. The presence of TDF protein in human sera was detected by the anti-P1 Ab, suggesting a role of TDF in endocrine metabolism. The fact that all activities of TDF can be mimicked by a peptide derived from the encoding TDF sequence opens the possibility of therapeutic applications.

The human neuroendocrine thyrotropin-releasing hormone receptor promoter is activated by the haematopoietic transcription factor c-Myb

Thyrotropin-releasing hormone (TRH) receptor (TRHR) is a G-protein-coupled receptor playing a crucial role in the anterior pituitary where it controls the synthesis and secretion of thyroid-stimulating hormone and prolactin. Its widespread presence not only in the central nervous system, but also in peripheral tissues, including thymus, indicates other important, but unknown, functions. One hypothesis is that the neuropeptide TRH could play a role in the immune system. We report here that the human TRHR promoter contains 11 putative response elements for the haematopoietic transcription factor c-Myb and is highly Myb-responsive in transfection assays. Analysis of Myb binding to putative response elements revealed one preferred binding site in intron 1 of the receptor gene. Transfection studies of promoter deletions confirmed that this high-affinity element is necessary for efficient Myb-dependent transactivation of reporter plasmids in CV-1 cells. The Myb-dependent activation of the TRHR promoter was strongly suppressed by expression of a dominant negative Myb-Engrailed fusion. In line with these observations, reverse transcriptase PCR analysis of rat tissues showed that the TRHR gene is expressed both in thymocytes and bone marrow. Furthermore, specific, high-affinity TRH agonist binding to cell-surface receptors was demonstrated in thymocytes and a haematopoietic cell line. Our findings imply a novel functional link between the neuroendocrine and the immune systems at the level of promoter regulation.

MRNA stability and the control of gene expression: implications for human disease

Regulation of gene expression is essential for the homeostasis of an organism, playing a pivotal role in cellular proliferation, differentiation, and response to specific stimuli. Multiple studies over the last two decades have demonstrated that the modulation of mRNA stability plays an important role in regulating gene expression. The stability of a given mRNA transcript is determined by the presence of sequences within an mRNA known as cis-elements, which can be bound by trans-acting RNA-binding proteins to inhibit or enhance mRNA decay. These cis-trans interactions are subject to a control by a wide variety of factors including hypoxia, hormones, and cytokines. In this review, we describe mRNA biosynthesis and degradation, and detail the cis-elements and RNA-binding proteins known to affect mRNA turnover. We present recent examples in which dysregulation of mRNA stability has been associated with human diseases including cancer, inflammatory disease, and Alzheimer's disease.

Characterization and functional expression of cDNAs encoding thyrotropin-releasing hormone receptor from Xenopus laevis

Thyrotropin-releasing hormone receptor (TRHR) has already been cloned in mammals wherethyrotropin-releasing hormone (TRH) is known to act as a powerful stimulator of thyroid-stimulating hormone (TSH) secretion. The TRH receptor of amphibians has not yet been characterized, although TRH is specifically important in the adaptation of skin color to environmental changes via the secretion of alpha-melanocyte-stimulating hormone (alpha-MSH). Using a dege-nerate PCR strategy, we report on the isolation of three distinct cDNA species encoding TRHR from the brain of Xenopus laevis. We have designated these as xTRHR1, xTRHR2 and xTRHR3. Analysis of the predicted amino acid sequences revealed that the three Xenopus TRHRs are only 54-62% identical and contain all the highly conserved residues constituting the TRH binding pocket. Amino acid sequences and phylogenetic analysis revealed that xTRHR1 is a member of TRHR subfamily 1 and xTRHR2 belongs to subfamily 2, while xTRHR3 is a new TRHR subtype awaiting discovery in other animal species. The three Xeno-pus TRHRs have distinct patterns of expression. xTRHR3 was abundant in the brain and much scarcer in the peripheral tissues, whereas xTRHR1 was found mainly in the stomach and xTRHR2 in the heart. The Xenopus TRHR subtype 1 was found specifically in the intestine, lung and urinary bladder. These observations suggest that the three xTRHRs each have specific functions that remain to be elucidated. Expression in Xenopus oocytes and HEK-293 cells indicates that the three Xenopus TRHRs are fully functional and are coupled to the inositol phosphate/calcium pathway. Interestingly, activation of xTRHR3 required larger concentrations of TRH compared with the other two receptors, suggesting marked differences in receptor binding, coupling or regulation.

Dimerization and phosphorylation of thyrotropin-releasing hormone receptors are modulated by agonist stimulation

Dimerization and phosphorylation of thyrotropin-releasing hormone (TRH) receptors was characterized using HEK293 and pituitary GHFT cells expressing epitope-tagged receptors. TRH receptors tagged with FLAG and hemagglutinin epitopes were co-precipitated only if they were co-expressed, and 10-30% of receptors were isolated as hemagglutinin/FLAG-receptor dimers under basal conditions. The abundance of receptor dimers was increased when cells had been stimulated by TRH, indicating that TRH either stabilizes pre-existing dimers or increases dimer formation. TRH increased receptor dimerization and phosphorylation within 1 min in a dose-dependent manner. TRH increased phosphorylation of both receptor monomers and dimers, documented by incorporation of (32)P and an upshift in receptor mobility reversed by phosphatase treatment. The ability of TRH to increase receptor phosphorylation and dimerization did not depend on signal transduction, because it was not inhibited by the phospholipase C inhibitor. Receptor phosphorylation required an agonist but was not blocked by the casein kinase II inhibitor apigenin, the protein kinase C inhibitor GF109203X, or expression of a dominant negative form of G protein-coupled receptor kinase 2. TRH receptors lacking most of the cytoplasmic carboxyl terminus formed dimers constitutively but failed to undergo agonist-induced dimerization and phosphorylation. TRH also increased phosphorylation and dimerization of TRH receptors expressed in GHFT pre-lactotroph cells.

Correlation between basal signaling and internalization of thyrotropin-releasing hormone receptors: evidence for involvement of similar receptor conformations

Previous studies have shown that rat thyrotropin-releasing hormone (TRH) receptor type 2 exhibits higher basal signaling activity and internalizes more rapidly upon agonist binding than rat TRH receptor type 1. The mouse TRH receptor type 2 (mR2) was recently cloned and, similar to its rat homolog, shows a higher basal signaling activity than mR1. Taking advantage of the high degree of sequence homology between mR1 and mR2, we used chimeras/mutants of these receptors to gain insight into the properties of the receptors that influence internalization and basal signaling. Chimeric receptors that have the mR1 extracellular and transmembrane domains with the carboxyl terminus and intracellular loops of mR2 (R1/R2-tail; R1/R2-I3,tail; R1/R2-I2,3,tail; R1/R2-I1,2,3,tail) exhibited internalization rates and basal activities that were similar to that of mR1. In contrast, a chimeric receptor with the extracellular and transmembrane domains of mR2 and the carboxyl terminus of mR1 exhibited the more rapid internalization rate and higher basal signaling activity characteristic of mR2. We showed previously that mutation of a highly conserved tryptophan to alanine caused mR1 to exhibit a high basal signaling activity and rapid internalization rate. In contrast, mutation of this tryptophan to alanine in mR2 decreased the rate of internalization and inhibited basal signaling activity. The rates of receptor internalization did not correlate with the binding affinities, coupling efficiencies, or potencies of the receptors. Thus, we observed that receptors with more rapid internalization rates showed relatively higher basal signaling activities, whereas receptors with lower basal signaling activities showed slower internalization rates. These data suggest that similar receptor conformations are required for productive coupling to signaling G proteins and to proteins involved in internalization.

Conformationally restricted TRH analogues: constraining the pyroglutamate region

A modified synthetic route has been developed so that the steric size of constraints added to the pyroglutamate region of TRH (pGluHisProNH(2)) can be varied. Both an analogue with a smaller ethylene bridge and a larger, more flexible propane bridge in this region have been synthesized. These analogues were synthesized in order to probe why the initial incorporation of an ethane bridge into this region of the molecule had led to an analogue with a binding constant and potency three times lower than that of an directly analogous unconstrained analogue. The data for both analogues indicated that the fall off in activity caused by the ethane bridge in the initial analogue was not caused by the size of the bridge.

Early gene expression changes preceding thyroid hormone-induced involution of a thyrotrope tumor

Treatment with thyroid hormone (TH) results in shrinkage of a thyrotropic tumor grown in a hypothyroid host. We used microarray and Northern analysis to assess the changes in gene expression that preceded tumor involution. Of the 1,176 genes on the microarray, 7 were up-regulated, whereas 40 were decreased by TH. Many of these were neuroendocrine in nature and related to growth or apoptosis. When we examined transcripts for cell cycle regulators only cyclin-dependent kinase 2, cyclin A and p57 were down-regulated, whereas p15 was induced by TH. Retinoblastoma protein, c-myc, and mdm2 were unchanged, but E2F1 was down-regulated. TH also decreased expression of brain-derived neurotrophic factor, its receptor trkB, and the receptor for TRH. These, in addition to two other genes, neuronatin and PB cadherin, which were up- and down-regulated, respectively, showed a more rapid response to TH than the cell cycle regulators and may represent direct targets of TH. Finally, p19ARF was dramatically induced by TH, and although this protein can stabilize p53 by sequestering mdm2, we found no increase in p53 protein up to 48 h of treatment. In summary, we have described early changes in the expression of genes that may play a role in TH-induced growth arrest of a thyrotropic tumor. These include repression of specific growth factor and receptors and cell cycle genes as well as induction of other factors associated with growth arrest and apoptosis.

Synthesis and biology of new thyrotropin-releasing hormone (TRH) analogues

We report synthesis and biological activities of several thyrotropin-releasing hormone (TRH) analogues in which the N-terminal pyroglutamic acid residue has been replaced with various carboxylic acids and the central histidine is modified with substituted-imidazole derivatives.

Cloning of two thyrotropin-releasing hormone receptor subtypes from a lower vertebrate (Catostomus commersoni): functional expression, gene structure, and evolution

A PCR approach was used to clone thyrotropin-releasing hormone receptors (TRH-R) from the brain and anterior pituitary of the teleost Catostomus commersoni (cc), the white sucker. Two distinct TRH-R, designated ccTRH-R1 and ccTRH-R2, were identified. ccTRH-R1 was similar to mammalian TRH-R of the subtype 1, whereas ccTRH-R2 exhibited the highest identity (61% at the amino acid level) with the recently discovered rat TRH-R2. It is postulated that ccTRH-R2 and rat TRH-R2 are members of the same TRH-R subfamily 2. Functional expression of ccTRH receptors in human embryonic kidney cells and in Xenopus laevis oocytes demonstrated that both ccTRH receptors were fully functional in both systems. Oocytes expressing either receptor responded to the application of TRH by an induction of membrane chloride currents, indicating that ccTRH-R of both subtypes are coupled to the inositol phosphate/calcium pathway. The analysis of genomic clones revealed, for the first time, both similarities and differences in the structure of TRH-R subtype genes. Both ccTRH-R genes contained an intron within the coding region at the beginning of transmembrane domain (TM) 6. The position of this intron is highly conserved, as it was found at an identical position in the human TRH-R1 gene. The ccTRH-R2 gene contained an additional intron at the end of TM 3 that was not found in any of the TRH-R1 genes identified so far. The analysis of the gene structure of ccTRH-R and the amino acid sequence comparisons of mammalian and teleost TRH-R of both subtypes suggest that TRH receptors have been highly conserved during the course of vertebrate evolution. A common ancestral TRH receptor gene that could be found much earlier in evolution, possibly in invertebrates, might be the origin of ccTRH-R genes.

Production and characterization of an antiserum which recognizes the native receptor for thyrotropin-releasing hormone

Despite attempts in several laboratories, it has been difficult to prepare antiserum to the thyrotropin-releasing hormone receptor (TRHR). We have prepared a polyclonal anti-rat TRHR antiserum by immunization of rabbits with a synthetic peptide corresponding to the C-terminus of the TRHR. The specificity of the antiserum was assessed by enzyme-linked immunosorbent assay. The affinity-purified antibody recognized a major broad band at 50-60 kDa and a minor broad band at 100-120 kDa in Western blot analysis of membrane proteins from TRHR-transfected, but not control, HEK293t cells. Binding to both bands was abolished by preincubation with the immunizing peptide but not control peptide. The approach was repeated with rat pituitary F4C1 cells, which lack endogenous TRHRs; membranes from F4C1 cells transfected with TRHR cDNA, but not control cells, showed specific binding by Western blot. Using laser confocal microscopy, the TRHR was visualized on the plasma membrane of transfected, but not control, F4C1 cells. Similar confocal findings were observed in TRHR-transfected HEK293t cells. Within 5 min after TRH addition, the TRHR signal translocated from the plasma membrane to the cytoplasm of F4C1 cells transfected with TRHR cDNA. Ten minutes after TRH addition, the TRHR signal formed aggregates in the cytoplasm. Thirty minutes after TRH treatment, both cytoplasmic and plasma membrane localizations were observed, suggesting recycling of some TRHRs back to the plasma membrane. These observations are consistent with our previous findings using an epitope-tagged TRHR. In conclusion, we have prepared an antiserum that recognizes the native TRHR by Western blot analysis and confocal microscopy.

Expression of peptides and other neurochemical markers in hypothalamus and olfactory bulb of mice devoid of all known thyroid hormone receptors

We have investigated with histochemical techniques the expression of peptides and other neurochemical markers in the hypothalamus and olfactory bulb of male mice, in which the genes encoding the alpha and beta thyroid hormone receptors (TRalpha1, TRbeta1 and TRbeta2) have been deleted. Thyrotropin-releasing hormone messenger RNA levels were increased in the hypothalamic paraventricular nucleus and in the medullary raphe nuclei of mutant mice lacking the thyroid hormone receptors alpha1 and beta (alpha1(-/-)beta(-/-)), as compared to wild-type mice. In contrast, galanin messenger RNA levels were lower in the hypothalamic paraventricular nucleus of mutant animals, as was galanin-like immunoreactivity in the internal layer of the median eminence. Substance P messenger RNA levels were unchanged in the medullary raphe nuclei. Thyrotropin-releasing hormone receptor messenger RNA levels were increased in motoneurons, unchanged in the subiculum, and lower in the amygdala of mutant animals. Galanin messenger RNA levels were unchanged in the hypothalamic dorsomedial and arcuate nuclei of the thyroid hormone receptor alpha1(-/-)beta(-/-) mice, as was the immunocytochemistry for oxytocin and for vasopressin in the hypothalamic paraventricular nucleus. A reduction in tyrosine hydroxylase messenger RNA levels was found in the arcuate nucleus of mutant mice. In the olfactory bulb, immunohistochemistry for calbindin and for tyrosine hydroxylase revealed a reduction in the intensity of labeling of nerve processes in the glomerular layer of thyroid hormone receptor alpha1(-/-)beta(-/-) mice. The tyrosine hydroxylase messenger RNA levels were also slightly reduced. In contrast, the levels of galanin and neuropeptide Y messenger RNA in this region were unchanged in thyroid hormone receptor alpha1(-/-)beta(-/-) mice as compared to wild-type mice. Together these studies reveal many regional and neurochemically selective alterations in neuronal phenotype of mice devoid of all known thyroid hormone receptors.

The expression of thyrotrophin-releasing hormone receptor 1 messenger ribonucleic acid in human pituitary adenomas

OBJECTIVE

Thyrotrophin-releasing hormone (TRH) paradoxically induces the release of growth hormone (GH) when injected intravenously into acromegalic patients, although the mechanism of this action is unknown at present. Several research groups have reported that the level of TRH receptor-1 (TRHR-1) mRNA expression is variable in pituitary adenomas, and does not correlate with the degree of paradoxical GH response to TRH administration in a limited number of acromegalic patients. We aimed to compare the expression levels of TRHR-1 mRNA among various types of pituitary adenoma and to clarify whether these levels correlate with the degree of pituitary hormone response to TRH.

PATIENTS

Pituitary adenoma tissue was obtained by surgery from 14 patients with acromegaly, four with prolactinomas, nine with nonfunctioning adenomas and one with a TSH-producing adenoma.

METHODS

The level of human TRHR-1 mRNA expression in each adenoma was quantified using the competitive reverse transcription polymerase chain reaction (RT-PCR) METHOD: For amplification of a TRHR-1 cDNA fragment, a sense primer was designed according to the sequence in exon 2 and an antisense primer designed according to the sequence located at the region in exon 3 that does not encode for the alternative splicing-generated short form of TRHR-1 mRNA.

RESULTS

TRHR-1 mRNA was detected in all pituitary adenomas examined and did not correlate with their size. The mean level of TRHR-1 mRNA expression was significantly lower in GH-producing adenomas than in prolactinomas and nonfunctioning adenomas (1.4 +/- 0.4 x 10(-2) attomol/microg total RNA, 10.7 3.4 x 10(-2) attomol/microg total RNA, and 7.2 +/- 3.3 x 10(-2) attomol/g total RNA, respectively). The ratio of plasma peak GH induced by TRH administration to the basal level of plasma GH in the patients with acromegaly correlated positively with the level of TRHR-1 mRNA expression in their GH-producing adenomas (r = 0.620, P = 0.0179). The responsiveness of plasma PRL and gonadotrophin to TRH in the patients with prolactinoma and nonfunctioning pituitary adenoma did not significantly correlate with the levels of TRHR-1 mRNA expression in their pituitary adenomas, respectively.

CONCLUSIONS

The findings of the present study suggest that the level of TRHR-1 mRNA expression varies among different types of pituitary adenoma. Furthermore, in acromegaly, the responsiveness of plasma GH to TRH administration appears to at least partially depend on the level of TRHR-1 mRNA expression in the GH-producing pituitary adenoma.