Inhibition of thyrotropin binding to receptor by synthetic human thyrotropin beta peptides

The effect of posttranslational modifications on the in vitro activity of recombinant human thyroid-stimulating hormone

Posttranslational modification can influence the biologic activity of recombinant proteins. The effects of beta-subunit C-terminal truncation, oligosaccharide heterogeneity, and chemical oxidation on the in vitro activity of recombinant human thyroid-stimulating hormone (rhTSH) were investigated. beta-Subunit C-terminal truncation up to residue 113 did not effect the in vitro activity of the hormone. The relationship between the heterogeneity of oligosaccharide structures on rhTSH and specific activity of the glycoprotein hormone was also examined. Oligosaccharide profiles were generated for preparations of rhTSH containing similar sialic acid levels. A weak correlation was observed between relative levels of monosialylated biantennary, bisialylated biantennary, and trisialylated triantennary oligosaccharide species and in vitro activity of the recombinant hormone (p < 0.05). To examine the effect of chemically induced methionine oxidation on the activity of rhTSH, the hormone was treated with tert-butyl hydroperoxide and then characterized. Using peptide mapping and mass spectrometry, the degree of oxidation of the five methionine residues within rhTSH was measured. Met-71 in the alpha-subunit was the most susceptible to oxidation whereas Met-9 in the beta-subunit was the most resistant. Also, after tert-butyl hydroperoxide treatment, levels of oxidation of Met-32 in the beta-subunit, and Met-29 and Met-47 in the alpha-subunit were less than half of that observed for Met-71. The in vitro activity of rhTSH initially declined with increasing oxidation; however, the loss in activity plateaued at approximately 50% of the control sample activity. In summary, despite the possible effects that posttranslational modifications may have on the bioactivity of a protein, a limited degree of variation in bioactivity was observed for the rhTSH preparations described in this study.

Comparison of two in vitro methods for the measurement of recombinant human TSH bioactivity

Thyroid stimulating hormone (TSH), a pituitary glycoprotein hormone, is a potent inducer of intracellular cAMP production. Two methods for measuring TSH bioactivity were evaluated and compared. One assay is based on using a radioimmunoassay (RIA) to measure the recombinant human TSH-induced increase in cAMP using a bovine thyroid membrane isolate. The other is based on a Chinese hamster ovary (CHO) cell line that has been transfected with the TSH receptor and a cAMP-responsive luciferase reporter. The within-assay coefficient of variation for the membrane-based assay was determined to be approximately 35% compared with approximately 25% for the cell-based assay. Twenty-one preparations of recombinant human TSH (rhTSH) were tested using both methods. No significant difference was detected between the data sets and no assay bias was present. Both assay systems provide a suitable means for measuring the activity of rhTSH. The advantage of the membrane-based assay is the relatively small quantity of TSH needed for analysis. However, the average time required to analyse a sample using the membrane-based method was more than twice as long as that needed to test a sample in the cell-based assay. Other advantages of the cell-based method include the use of a 96-well format, which facilitates the analysis of several concentrations of rhTSH within one assay plate, and the use of a non-radioactive endpoint.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Substitution of the seat-belt region of the thyroid-stimulating hormone (TSH) beta-subunit with the corresponding regions of choriogonadotropin or follitropin confers luteotropic but not follitropic activity to chimeric TSH

The region between the 10th and 12th cysteine (Cys88-Cys105 in human thyroid-stimulating hormone beta-subunit (hTSHbeta)) of the glycoprotein hormone beta-subunits corresponds to the disulfide-linked seat-belt region. It wraps around the common alpha-subunit and has been implicated in regulating specificity between human choriogonadotropin (hCG) and human follicle-stimulating hormone (hFSH), but determinants of hTSH specificity are unknown. To characterize the role of this region for hTSH, we constructed hTSH chimeras in which the entire seat-belt region Cys88-Cys105 or individual intercysteine segments Cys88-Cys95 and Cys95-Cys105 were replaced with the corresponding sequences of hCG and hFSH or alanine cassettes. Alanine cassette mutagenesis of hTSH showed that the Cys95-Cys105 segment of the seat-belt was more important for TSH receptor binding and signal transduction than the Cys88-Cys95 determinant loop region. Replacing the entire seat-belt of hTSHbeta with the hCG sequence conferred full hCG receptor binding and activation to the hTSH chimera, whereas TSH receptor binding and activation were abolished. Conversely, introduction of the hTSHbeta seat-belt sequence into hCGbeta generated an hCG chimera that bound to and activated the TSH receptor but not the CG/lutropin (LH) receptor. In contrast, an hTSH chimera bearing hFSH seat-belt residues did not possess any follitropic activity, and its thyrotropic activity was only slightly reduced. This may in part be due to the fact that the net charge of the seat-belt is similar in hTSH and hFSH but different from hCG. However, exchanging other regions of charge heterogeneity between hTSHbeta and hFSHbeta did not confer follitropic activity to hTSH. Thus, exchanging the seat-belt region between hTSH and hCG switches hormonal specificity in a mutually exclusive fashion. In contrast, the seat-belt appears not to discriminate between the TSH and the FSH receptors, indicating for the first time that domains outside the seat-belt region contribute to glycoprotein hormone specificity.

Chimeric proteins can exceed the sum of their parts: implications for evolution and protein design

Chimeric analogs derived from pairs of homologous proteins routinely exhibit activities found in one or both parents. We describe chimeras of two glycoprotein hormones, human chorionic gonadotropin (hCG) and human follitropin (hFSH), that exhibit activity unique to a third family member, human thyrotropin (hTSH). The results show that biological activity can be separated from hormone-specific amino acid residues. This is consistent with a model for the evolution of homologous ligand-receptor pairs involving gene duplication and the creation of inhibitory determinants that restrict binding. Disruption of these determinants can unmask activities characteristic of other members of a protein family. Combining portions of two ligands to create analogs with properties of a third family member can facilitate identifying key determinants of protein-protein interaction and may be a useful strategy for creating novel therapeutics. In the case of the glycoprotein hormones, this showed that two different hormone regions (i.e., the seat-belt and the intersubunit groove) appear to limit inappropriate contacts with receptors for other members of this family. These observations also have important caveats for chimera-based protein design because an unexpected gain of function may limit the therapeutic usefulness of some chimeras.

Structure and function of the TSH receptor: its suitability as a target for radiotherapy

Systemic radiotherapy, such as radioimmunotherapy, is an exciting and rapidly growing field of medical therapeutics for a variety of solid and diffuse human malignancies. This therapy involves the systemic administration of a radionuclide, attached to a carrier ligand (such as hormone analogue or monoclonal antibody), which becomes directed at the tumor through a target receptor or antigen that resides within the malignant tissue. The thyrotropin receptor (TSHr) is a membrane-bound glycoprotein through which the pituitary communicates with thyroid follicular cells. Because it is a thyroid-specific protein and is expressed frequently in differentiated thyroid cancers, it is a potential candidate target for systemic radiotherapy of these malignancies. I will examine the general structure of TSHr and its potential utility such as a target. Several obstacles regarding the concentration and distribution of TSHr as well as the availability of a suitable carrier ligand must be overcome before radioimmunotherapy of thyroid cancers using TSHr as target becomes a reality.

Human follitropin heterodimerization and receptor binding structural motifs: identification and analysis by a combination of synthetic peptide and mutagenesis approaches

The family of human glycoprotein hormones, including follitropin (FSH), are heterodimeric proteins, each composed of single alpha- and beta-subunits that are tightly associated but non-covalently linked. To study structure and function relationships of FSH, synthetic peptides were used to inhibit subunit association, to map epitopes of FSH antibodies and as antigens to generate site specific antipeptide antibodies which could be used for topographic analysis. Interpretation of such results are generally more straightforward than when peptides are used with radioreceptor assays or in cell cultures which are complex systems. The data we collected using the synthetic peptide approach suggested that FSH residues homologous to human chorionic gonadotropin (hCG) loops L3 beta and L2 alpha are involved in subunit contact. FSH residues homologous to hCG loops L2 beta and L3 alpha seemed involved in receptor binding. Loop L2 beta also seemed involved in subunit contact. Those data provided a rationale for extensive mutagenesis of the four regions of hFSH. Mutagenesis data provided additional information and higher resolution of function when combined with the three dimensional structure of hCG. In the aggregate, this information has provided a reasonable model of the receptor binding site of hFSH. Our current model of the FSH receptor site is that of a discontinuous functional epitope including L3 beta, L2 alpha and L3 alpha. The juxtaposition of residues beta D93, alpha K5 1, alpha Y88 and of alpha Y89 in the 'binding-facet' of hFSH suggest the feasibility of designing a synthetic peptide mimetic of FSH. Additional residues of the alpha-subunit are involved, along this facet of the molecule. The data collected studying hFSH therefore demonstrates that the alpha-subunit features prominently in the mechanism of FSH binding to and stabilizing the interaction with its receptor. In contrast, the beta-subunit determinant loop serves as discriminator in addition to stabilizing the binding interaction whereas mutagenesis data indicates that L2 beta does neither. Instead, L2 beta appears to stabilize FSH conformation, possibly, the alpha-subunit, required for competent binding. In this regard, synthetic peptides provided data which were a useful guide to plan mutagenesis studies and which contributed to the process of understanding the structure and function of the gonadotropins.

Follitropin conformational stability mediated by loop 2 beta effects follitropin-receptor interaction

Follicle-stimulating hormone (FSH) is in the family of pituitary/placental glycoprotein hormones which also includes luteinizing hormone (LH), chorionic gonadotropin (hCG), and thyroid-stimulating hormone. These hormones are heterodimers composed of common alpha- and similar but unique beta-subunits. The 21 amino acid loop between Y33 and F53 of the FSH beta-subunit (L2 beta) can be switched into L2 beta of hCG beta without a loss of receptor binding, yet mutation of hFSH beta 37LVY39 to 37AAA39 was antecendent to a 20-fold reduction in receptor binding (based on ID50). A mutation in the LH beta gene, which causes Q54 to be R, causes hypogonadism. This residue is conserved in the glycoprotein hormones and corresponds to Q48 in hFSH beta. Mutation of hFSH beta 48QKTCT52 to 48AAACA52 resulted in a failure of heterodimer formation. In the current study single mutations were made to pinpoint which of the seven hFSH beta residues in the 37LVY39 to 37AAA39 and the 48QKTCT52 to 48AAACA52 mutants were responsible for the observed phenotypes. A single mutation of T52 to alanine was sufficient to cause a reduction in expression of heterodimeric hormone. Single mutants Q48A, T50A, V38A, Y39A, and, to a lesser extent, T52A formed heterodimer. However, these hFSH mutants were markedly unstable at pH 2.0. Thus, acid dissociation can be used to reveal metastable forms of this protein. Mutant hFSH beta Q48A was also 8-fold less active than wild-type hFSH when assayed for binding to hFSH receptors. hFSH beta V38A and Y39A mutants affected receptor binding; however, neither mutation alone caused greater than a 2-fold decrease in receptor binding activity. In summary, these results identify single important residues in the long loop (between Y33 and F53) of the hFSH beta-subunit which are required for proper subunit interactions that provide conformational stability which in turn is necessary for FSH-receptor interaction.

Scanning-alanine mutagenesis of long loop residues 33-53 in follicle stimulating hormone beta subunit

Follicle stimulating hormone (FSH) is a gonadotropin and member of the pituitary/placental glycoprotein hormone family which bind to G-protein-coupled receptors. These hormones are heterodimers composed of a common alpha and distinct beta -subunits. Previous experimental evidence suggested that the FSH beta -subunit long loop comprised of amino acids Tyr33 to Phe53 is involved in receptor binding and activation and in subunit interaction. According to recently reported crystal structures of human chorionic gonadotropin (hCG), the homologous long loop of the beta -subunit of hCG associates with the alpha -subunit and is partially exposed to solvent. This report describes the results of scanning alanine mutagenesis used to determine if amino acid side chains in this region of the molecule are required for receptor binding and/or subunit contact. Five mutations were made which spanned this loop and the mutant FSH beta-subunits were co-expressed with alpha-subunit in a Baculovirus-infected insect-cell expression system. Mutation of 48QKTCT52 to 48AAACA52 produced a FSH beta-subunit that failed to form heterodimer, consistent with the crystal structure of hCG which shows these amino acids are buried at the subunit interface. The four remaining mutants produced heterodimer and were assayed for binding to and activation of human FSH receptors. Mutation of 37LVY39 to 37AAA39 caused a 20-fold reduction binding (ID50 of 7.0 nM compared with 0.3 nM for wildtype). Mutation of 34TRDL37 to 34AAAA37 or 44RPKI47 to 44APAA47 caused lesser but measurable effects with ID50 values of 1.1 nM and 1.9 nM, respectively. The (40)KDPA(43) to 40KDPA43 to 40AAPA43 mutation had little effect on receptor binding (ID50 = 0.5 nM).

Secondary structure prediction of beta-subunits of the gonadotropin-thyrotropin family from its aligned sequences using environment-dependent amino-acid substitution tables and conformational propensities

The secondary structures of beta-subunits of the glycoprotein hormone family, LH (luteinizing hormone), CG (chorionic gonadotropin), FSH (follicle stimulating hormone), TSH (thyroid stimulating hormone), and GTH I/GTH II (two types of fish gonadotropins), are predicted by comparing an amino-acid substitution pattern at equivalent sites in their aligned sequences with environment-dependent amino-acid substitution tables and conformational propensities calculated from other protein families whose three-dimensional structures are known. According to the prediction results, together with other structural information obtained from experiments, the following points come up as important structural features of the beta-subunits of this family; The regions assigned to regular secondary structures (one alpha-helix and three beta-strands) are considered to constitute a core of the beta-subunits. They involve interaction sites with carbohydrate and alpha-subunit. Out of the six disulfide bonds formed in the beta-subunit, four are located together on one side of the core, and the other two on the opposite side. The two regions assumed to be a receptor binding region from experiments (therefore, species-specific regions) are predicted as loops located on the same side of the beta-subunit in this study. Some of the predicted loops are rich in proline residues. While the positions of proline residues are conserved in the family generally, there are hormone- or species-specific ones in the loop that is assumed to take part in receptor binding. The possible importance of proline residues in hormone or species specificity is discussed. (After submitting the manuscript the X-ray crystal structure of human CG was published. In order to evaluate the prediction, the original manuscript is kept intact and a comparison has been made between the prediction results and the crystal structure in an appendix).

A receptor binding site identified in the region 81-95 of the beta-subunit of human luteinizing hormone (LH) and chorionic gonadotropin (hCG)

Two series of overlapping peptides comprising the entire sequences of the beta-subunits of human lutropin (LH) and choriogonadotropin (hCG) were prepared by a comprehensive synthetic strategy in order to identify all linear regions of the subunit that may participate in binding of the hormone to its receptor. Each series of peptides (15 residues in length) spanned the entire amino acid sequences of the two beta-subunits. The peptides were tested for their ability to inhibit the binding of 125I-labeled hCG or LH to rat ovarian membranes and for their ability to inhibit hCG-stimulated progesterone production in a Leydig cell bioassay. The most potent inhibitor of LH/hCG binding was a peptide containing the sequence beta 81-95, a receptor binding site of the LH/hCG beta subunit not previously described. The concentration at which LH/hCG binding was inhibited at 50% (IC50) was 20 microM and 30 microM for hCGbeta 81-95 and LH beta 81-95, respectively. These peptides also inhibited the stimulation of progesterone production by hCG in Leydig cell bioassays. In order to determine important residues that inhibit binding within this region, a third set of peptides was synthesized in which each residue of hCG beta 81-95 was sequentially replaced with the residue L-alanine. Five residues (Leu-86, Cys-88, Cys-90, Arg-94, and Arg-95) were critical for maximal inhibition of hCG binding by CG beta 81-95. In addition to site beta 81-95, other sites that inhibited hCG/LH binding but with significantly lower potencies included hCG beta 1-15, LH beta 41-55, and LH beta 91-105.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of TSH bio-activity by synthetic beta TSH peptides

The in vitro bioactivity of the human beta TSH subunit was investigated utilizing eleven overlapping synthetic peptides representing the entire 112 residue sequence. The peptides were tested for both stimulatory and inhibitory activity in two sensitive bioassay systems: the first based on cAMP production in FRTL-5 rat thyroid cells, and the second based on stimulation of iodine trapping by the same continuous cell line. Peptides from three distinct regions of the beta-subunit showed concentration dependent inhibition of TSH bio-activity, including beta 1-15, beta 11-25, beta 31-45, beta 81-95, and beta 91-105 with IC50 values ranging from 150 to 304 microM. An additional peptide representing the entire sequence of the "intercysteine loop" region of beta TSH, beta 31-52, also inhibited TSH activity with somewhat higher potency than its fragment peptide beta 31-45 (IC50 of 87.5 +/- 14.7 microM for beta 31-52 versus 207 +/- 92.4 microM for beta 31-45). Three of these, beta 1-15, beta 31-45, and beta 31-52, also inhibited binding of TSH to the receptor in a radio-receptor assay, as previously reported (1), supporting their importance in receptor interaction. None of the synthetic peptides stimulated either cAMP production or iodine trapping. Two other overlapping peptides, beta 81-95 and beta 91-105, possessed bio-inhibitory activity but did not inhibit binding of labeled TSH. Computer analysis of this sequence predicted an extended turn structure for this region. This region has been referred to as the "determinant loop" as it is bounded by cysteine residues at positions 88 and 95 that many believe form a disulfide bond in the native subunit. The current data suggests the beta 88-95 region may play a role in receptor activation after initial binding of hormone to receptor.

Progress and approaches in mapping the surfaces of human follicle-stimulating hormone: comparison with the other human pituitary glycoprotein hormones

The pituitary glycoprotein hormones, including human follicle-stimulating hormone (hFSH), are involved in the physiologic functions of receptor binding, in subunit association during assembly and processing, and in untoward effects such as antibody formation during clinical intervention. These activities derive from specific macromolecular associations; a major research trend has been to map the surfaces of these glycoprotein hormones (hFSH, hLH, hCG, and hTSH) into discrete areas responsible for each activity by using a variety of biochemical approaches. Each surface domain or determinant of the hormone is likely to comprise discontinuous amino acid sequences, from one or both subunits, assembled into a precise, unique, macromolecular surface. The paradigm of antigen-antibody interaction may help to explain how the surfaces are assembled, how the common alpha-subunit combines with the unique beta-subunit of each, and how the receptor interacts with heterodimeric hormone.

Structure--function relationships of the glycoprotein hormones and their receptors

The primary structures of the glycoprotein hormones follitropin (FSH), lutropin (LH), human choriogonadotropin (hCG) and thyrotropin (TSH) have been determined, hCG has been crystallized and initial diffraction data obtained. Studies with synthetic peptides have provided information on regions involved in receptor interaction and signal transduction. The receptors for the glycoprotein hormones have been prepared by gene cloning methods and their primary structures deduced. As Leo Reichert and colleagues discuss here, although cAMP is involved in glycoprotein hormone signal transduction, recent evidence also implicates other second messengers, especially Ca2+ and may include both the phosphatidylinositol pathway and activation of Ca2+ channels.