Recombinant human thyrotropins of the twenty-first century

Biologically active recombinant carp LH as a spawning-inducing agent for carp

Currently, spawning is induced in carp species by carp pituitary extract (CPE) and a combination of synthetic agonist of GnRH combined with a dopamine antagonist. The main goal of this study was the production of recombinant gonadotropins (GtHs) on a large scale to serve as an alternative to currently used agents. We produced carp (c) recombinant (r) Lh as a single chain in the methylotrophic yeast Pichia pastoris Lha subunit was joined with Lhb subunit with a flexible linker of three glycine-serine repeats and six Histidines to form a mature protein, the β-subunit formed the N-terminal part and the α-subunit formed the C-terminal part. The ability of the rcLh to elicit biological response was tested by in vivo stimulation of estradiol (E2) and 17α,20β-dihydroxy-4-pregnen-3-one (DHP) and by its in vivo potency to induce ovulation and spawning induction. rcLh tested in this work significantly enhanced both E2 and DHP secretion in a dose-dependent manner similar to the results obtained with CPE. E2 levels showed a moderate rise following the priming injection and a subsequent decrease during the rest of the trial. DHP levels were only increased after the resolving injection, approximately 5 h before spawning. At the highest dose of rcLh (350 µg/kg BW), the recombinant protein was more efficient than CPE in terms of both spawning success and fertilization rate. It is shown here that rcLh can elicit the secretion of DHP in vivo and actually trigger spawning. These novel findings introduce the potential of utilizing recombinant gonadotropins in aquaculture.

New Frontier in Glycoprotein Hormones and Their Receptors Structure-Function

Last two decades of structure-function studies performed in numerous laboratories provided substantial progress in understanding basic science, physiological, pathophysiological, pharmacological, and comparative aspects of glycoprotein hormones (GPHs) and their cognate receptors. Multiple concepts and models developed based on experimental data in the past stood the test of time and have been, at least in part, confirmed and/or remained compatible with the new structures resolved at the atomic level. Major advances in understanding of the ligand-receptor relationships are heralding the dawn of a new era for GPHs and their receptors, although many basic questions still remain unanswered. This article examines retrospectively several basic science aspects of GPH super-agonists and related "biosuperiors" in a broader context of the advances in the ligand-receptor structure-function relationships and new mechanistic models generated based on the structure elucidation. Due to selective focus of my comments and perspectives in certain parts, the reader is directed to the most relevant publications and reviews in the field for more comprehensive analyses.

Effects of recombinant human thyroid-stimulating hormone superagonists on thyroidal uptake of 18F-fluorodeoxyglucose and radioiodide

BACKGROUND

Superagonist analogs of human thyroid-stimulating hormone (hTSH) may stimulate the uptake of (131)I-iodide and (18)F-fluorodeoxyglucose ((18)F-FDG) in thyroid carcinomas to a greater degree than hTSH. We herein report the potency and efficacy of two hTSH analogs, TR1401 and TR1402, to stimulate radioiodide and (18)F-FDG uptake in FRTL-5 cells and compared the effects of hTSH and TR1401 on radioiodide uptake in the thyroid in vivo in mice.

METHODS

The effects of hTSH analogs on intracellular levels of cAMP, uptake of (131)I-iodide, and (18)F-FDG were studied in FRTL-5 cells to determine the stimulatory potency and efficacy of the compounds by calculating half-maximum effective concentration (EC(50)) values and maximal stimulatory effects (E(max)). Biodistribution studies (n = 96) and positron emission tomography/computed tomography imaging studies (single animals) on thyroid (125)I/(124)I-iodide uptake were performed with T3-suppressed CD-1 mice in a dose-dependent manner (3, 10, and 30 μg/animal).

RESULTS

The EC(50) values of TR1401 and TR1402 demonstrated a 90-fold or 800-fold higher potency for their capacity to increase intracellular cAMP levels in comparison with hTSH (p < 0.05). Similar results were demonstrated for the stimulation of (18)F-FDG uptake. Bovine TSH, TR1401, and TR1402 were 85%-490% more potent to increase iodide uptake than hTSH (p < 0.05). TR1402 was 30% more efficacious to stimulate iodide uptake than hTSH. The agonist-induced increase in radiotracer uptake was paralleled by increases in NIS and GLUT-1 expression. Ex vivo biodistribution studies showed an increased iodide uptake in the thyroid of TR1401-treated mice at the low dose of 3 μg/animal in comparison with hTSH-treated mice (n = 16, p < 0.05). Positron emission tomography/computed tomography imaging studies confirmed the increased thyroidal iodide uptake in TR1401-treated mice in vivo.

CONCLUSIONS

TR1401 and TR1402 have considerably higher potency than hTSH to stimulate thyroidal iodide and (18)F-FDG uptake in vitro. Moreover, in vivo studies indicated that at low but not higher doses, TR1401 induced an enhanced ability for the thyroid to concentrate iodide compared with hTSH. These properties makes TR1401 and TR1402 interesting candidates for use in humans to enhance uptake of radioiodine and (18)F-FDG by metastases and recurrences of thyroid carcinoma.

Human TSH receptor ligands as pharmacological probes with potential clinical application

The biologic role of thyroid-stimulating hormone (TSH; thyrotropin) as an activator (agonist) of the TSH receptor (TSHR) in the hypothalamic-pituitary-thyroid axis is well known and activation of TSHR by recombinant human TSH is used clinically in patients with thyroid cancer. TSHR ligands other than TSH could be used to probe TSHR biology in thyroidal and extrathyroidal tissues, and potentially be employed in patients. A number of different TSHR ligands have been reported, including TSH analogs, antibodies and small-molecule, drug-like compounds. In this review, we will provide an update on all these classes of TSHR agonists and antagonists but place emphasis on small-molecule ligands.

Small-molecule agonists for the thyrotropin receptor stimulate thyroid function in human thyrocytes and mice

Seven-transmembrane-spanning receptors (7TMRs) are prominent drug targets. However, small-molecule ligands for 7-transmembrane-spanning receptors for which the natural ligands are large, heterodimeric glycoprotein hormones, like thyroid-stimulating hormone (TSH; thyrotropin), have only recently been reported, and none are approved for human use. We have used quantitative high-throughput screening to identify a small-molecule TSH receptor (TSHR) agonist that was modified to produce a second agonist with increased potency. We show that these agonists are highly selective for human TSHR versus other glycoprotein hormone receptors and interact with the receptor's serpentine domain. A binding pocket within the transmembrane domain was defined by docking into a TSHR homology model and was supported by site-directed mutagenesis. In primary cultures of human thyrocytes, both TSH and the agonists increase mRNA levels for thyroglobulin, thyroperoxidase, sodium iodide symporter, and deiodinase type 2, and deiodinase type 2 enzyme activity. Moreover, oral administration of the agonist stimulated thyroid function in mice, resulting in increased serum thyroxine and thyroidal radioiodide uptake. Thus, we discovered a small molecule that activates human TSHR in vitro, is orally active in mice, and could be a lead for development of drugs to use in place of recombinant human TSH in patients with thyroid cancer.

Challenges and opportunities of trapping ligands

Because gonadotropin-releasing hormone (GnRH) analogs constitute an important class of therapeutics for various reproductive and hormone-dependent disorders, many novel compounds have been discovered and studied. Several orally active nonpeptide GnRH antagonists have recently gained increased attention. In the study published in this issue of Molecular Pharmacology, Kohout et al. (p. 238) used small-molecule TAK-013 (sufugolix; developed previously by Takeda Chemical Industries) as a tool to elucidate the mechanism of its insurmountable antagonism. On the basis of receptor mutagenesis combined with molecular modeling, the authors hypothesized that certain amino acid sequences uniquely present in the human GnRH receptor amino terminus and extracellular loop 2 may form a "trap door" retarding dissociation of TAK-013. Such a trapping mechanism could be both ligand- and receptor species-specific. Although analogous models were previously proposed for other G protein-coupled receptors, the study by Kohout et al. (2007) provides an important advance in the GnRH antagonists field and an illustration of the fact that preclinical studies using animal models with nonhuman receptors may have very limited value in predicting drug efficacy in human disease. There are many examples showing that high-affinity protein, peptide, or nonpeptide agonists or antagonists have also enhanced clinical efficacy. However, there are also numerous studies indicating that very high receptor binding affinity is not a guarantee of drug efficacy and that other factors, including pharmacokinetic profile, ligand-induced receptor desensitization, and "trafficking," are critical in design and development of optimal drugs.