Native hGH Research Articles

Human growth hormone inclusion bodies present native-like secondary and tertiary structures which can be preserved by mild solubilization for refolding

BackgroundNative-like secondary structures and biological activity have been described for proteins in inclusion bodies (IBs). Tertiary structure analysis, however, is hampered due to the necessity of mild solubilization conditions. Denaturing reagents used for IBs solubilization generally lead to the loss of these structures and to consequent reaggregation due to intermolecular interactions among exposed hydrophobic domains after removal of the solubilization reagent. The use of mild, non-denaturing solubilization processes that maintain existing structures could allow tertiary structure analysis and increase the efficiency of refolding.ResultsIn this study we use a variety of biophysical methods to analyze protein structure in human growth hormone IBs (hGH-IBs). hGH-IBs present native-like secondary and tertiary structures, as shown by far and near-UV CD analysis. hGH-IBs present similar λmax intrinsic Trp fluorescence to the native protein (334 nm), indicative of a native-like tertiary structure. Similar fluorescence behavior was also obtained for hGH solubilized from IBs and native hGH at pH 10.0 and 2.5 kbar and after decompression. hGH-IBs expressed in E. coli were extracted to high yield and purity (95%) and solubilized using non-denaturing conditions [2.4 kbar, 0.25 M arginine (pH 10), 10 mM DTT]. After decompression, the protein was incubated at pH 7.4 in the presence of the glutathione-oxidized glutathione (GSH-GSSG) pair which led to intramolecular disulfide bond formation and refolded hGH (81% yield).ConclusionsWe have shown that hGH-IBs present native-like secondary and tertiary structures and that non-denaturing methods that aim to preserve them can lead to high yields of refolded protein. It is likely that the refolding process described can be extended to different proteins and may be particularly useful to reduce the pH required for alkaline solubilization.

Soluble Expression, One-Step Purification and Characterization of Recombinant Human Growth Hormone Fused with ompA3 in Escherichia coli.

Human growth hormone (hGH) is the first recombinant protein approved for the treatment of human growth hormone deficiency. However, expression in inclusion bodies and low expression levels are enormous challenges for heterologous expression of hGH in Escherichia coli. To increase the soluble expression of recombinant hGH with correct folding in E. coli. We constructed a new recombinant expression plasmid containing the coding sequence of the outer membrane protein A (ompA3) which was used for the expression in Transetta (DE3) E. coli. In order to simplify the purification process and cleavage of recombinant proteins, the fusion sequence should contain hexahistidine-tag (His6) and enterokinase recognition sites (D4K). The effect of different expression conditions on recombinant hGH expression was optimized in flask cultivations. Furthermore, the periplasmic solution containing soluble hGH was purified by Ni-NTA affinity chromatography. Circular dichroism (CD), western blot and mass spectrometry analyses were used to characterize the protein. Moreover, the growth-promoting effect of the purified hGH was also evaluated by cell proliferation assay. High-level expression (800 μg/mL) was achieved by induction with 0.5 mM IPTG at 30°C for 10 hours. The purity of hGH was over 90%. The immunological activity, secondary structure and molecular weight of the purified hGH were consistent with native hGH. The purified hGH was found to promote the growth of MC3T3-E1 cells, and was found to show the highest activity at a concentration of 100 ng/mL. Our research provides a feasible and convenient method for the soluble expression of recombinant hGH in E. coli, and may lay a foundation for the production and application of hGH in the industry.

Surfactants as stabilizers for biopharmaceuticals: An insight into the molecular mechanisms for inhibition of protein aggregation

PurposeSurfactants are common stabilizers, often added to biopharmaceuticals formulations, but the mechanisms at the basis of their activity are unclear. The aim of this work is to provide insight into the molecular factors underlying surfactant effectiveness as protectants. MethodsMolecular Dynamics simulations of human growth hormone (hGH) in the presence of Tween 20 were performed. The effect of Tween 20 was compared with the activity of commonly used protectants, such as the disaccharides. ResultsWe found that Tween 20 could prevent the self-association of hGH, leading to the formation of a protein-surfactant complex. In the case of unfolded hGH, surfactant molecules were oriented with their hydrophilic head in the direction of the protein hydrophobic patches. This created a highly unstable situation, fostering refolding. In the case of native hGH, Tween 20 molecules oriented with their lipophilic groups in the direction of the protein surface. This thermodynamically stabilized the native conformation, preventing unfolding. ConclusionsWe found that the ability of surfactants to foster protein refolding is related to their amphiphilic nature, and, more specifically, to their specific orientation with respect to the protein surface. A molecular mechanism explaining surfactant activity is proposed, which could provide direction for improvements in biopreservation.

Selective Determination of Human Growth Hormone (Somatropin) in the Presence of Its Chemical Degradation Products.

A rapid and sensitive HPLC method was developed and validated for selective determination of the human growth hormone (hGH) somatropin in the presence of its deamidated and oxidized degradation products. Reversed-phase chromatography with an acetonitrile and ammonium bicarbonate mobile phase in gradient elution mode was used. A short run time of 15 min allowed rapid and cost-effective analysis, with an average retention time of 7.4 min for native hGH, 6.2 min for its deamidated form, and 4.3 and 6 min for its oxidized variants. The method was validated for selectivity, linearity, and intra- and interday variations according to International Conference on Harmonization guidelines. The proposed method was successfully applied for rapid evaluation of the quantity of hGH during downstream processing, formulation, and storage.

Selective Determination of Human Growth Hormone (Somatropin) in the Presence of Its Chemical Degradation Products.

A rapid and sensitive HPLC method was developedand validated for selective determination of thehuman growth hormone (hGH) somatropin in thepresence of its deamidated and oxidized degradationproducts. Reversed-phase chromatographywith an acetonitrile and ammonium bicarbonatemobile phase in gradient elution mode was used.A short run time of 15 min allowed rapid andcost-effective analysis, with an average retentiontime of 7.4 min for native hGH, 6.2 min for itsdeamidated form, and 4.3 and 6 min for its oxidizedvariants. The method was validated for selectivity,linearity, and intra- and interday variations accordingto International Conference on Harmonizationguidelines. The proposed method was successfullyapplied for rapid evaluation of the quantity ofhGH during downstream processing, formulation,and storage.

Development and Characterization of Sodium Hyaluronate Microparticle-Based Sustained Release Formulation of Recombinant Human Growth Hormone Prepared by Spray-Drying

The purpose of this study was to develop and characterize a sodium hyaluronate microparticle-based sustained release formulation of recombinant human growth hormone (SR-rhGH) prepared by spray-drying. Compared to freeze-drying, spray-dried SR-rhGH showed not only prolonged release profiles but also better particle property and injectability. The results of size-exclusion high-performance liquid chromatography showed that no aggregate was detected, and dimer was just about 2% and also did not increase with increase of inlet temperature up to 150°C. Meanwhile, the results of reversed-phase high-performance liquid chromatography revealed that related proteins increased slightly from 4.6% at 100°C to 6.3% at 150°C. Thermal mapping test proved that product temperature did not become high to cause protein degradation during spray-drying because thermal energy was used for the evaporation of surface moisture of droplets. The structural characterization by peptide mapping, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and circular dichroism revealed that the primary, secondary, and tertiary structures of rhGH in SR-rhGH were highly comparable to those of reference somatropin materials. The biological characterization by rat weight gain and cell proliferation assays provided that bioactivity of SR-rhGH was equivalent to that of native hGH. These data establish that spray-dried SR-rhGH is highly stable by preserving intact rhGH and hyaluronate microparticle-based formulation by spray-drying can be an alternative delivery system for proteins.

Characterization of recombinant human growth hormone variants from sodium hyaluronate-based sustained release formulation of rhGH under heat stress

This study provides the findings of investigations of potential product-related variants on recombinant human growth hormone (rhGH) in a once-weekly sustained release formulation (SR–rhGH) of sodium hyaluronate microparticles and on the rhGH bulk solution used as the active ingredient for SR–rhGH under extreme stress conditions of 24h at 60°C. The extent of rhGH degradation was much higher in solution (33%) than in SR–rhGH (10%). The degradation products, especiallyMet14 sulfoxide and deamidated rhGH variants, were separated and quantified by a modified reversed-phase high-performance liquid chromatography (RP–HPLC) method at reduced flow rate. The primary degradation product of rhGH was found to be deamidated rhGH, although an unknown peak was also detected. In contrast, the primary degradation product of SR–rhGH was Met14 sulfoxide rhGH, with no unknown peaks. Using a cell proliferation assay, the biological activities of the isolated products of SR–rhGH degradation were found to be equivalent to those of native hGH, as determined by comparison with a National Institute for Biological Standards and Control standard. In conclusion, SR–rhGH is structurally and functionally stable and maintains the intactness of rhGH.

High-level expression and purification of recombinant human growth hormone produced in soluble form in Escherichia coli

Human growth hormone (hGH) was one of the first recombinant proteins approved for the treatment of human growth disorders. Its small size (191 amino acids), possession of only 2 disulphide bonds and absence of posttranslational modifications make Escherichia coli the host of choice for its production on any scale. In this work, we have utilized an efficient T7 based expression system to produce high levels of soluble thioredoxin-hGH (Trx-hGH) fusion protein. We outline a relatively simple three step purification process employing two immobilized metal-affinity chromatography and one anion-exchange steps and removal of fusion partner by enterokinase cleavage yielding native hGH. The ability of cell populations to produce quantities of up to 1g/L of the soluble Trx-hGH fusion protein has been tested in flask cultivations as well as in batch and fed-batch bioreactor runs. The sequence and structure of derived hGH were confirmed by mass spectrometry and circular dichroism and its native function, to induce cell proliferation, was confirmed by employing a Nb2 cell line proliferation assay.

A Novel Long-Acting Human Growth Hormone Fusion Protein (VRS-317): Enhanced In Vivo Potency and Half-Life

A novel recombinant human growth hormone (rhGH) fusion protein (VRS-317) was designed to minimize receptor-mediated clearance through a reduction in receptor binding without mutations to rhGH by genetically fusing with XTEN amino acid sequences to the N-terminus and the C-terminus of the native hGH sequence. Although in vitro potency of VRS-317 was reduced approximately 12-fold compared with rhGH, in vivo potency was increased because of the greatly prolonged exposure to the target tissues and organs. VRS-317 was threefold more potent than daily rhGH in hypophysectomized rats and fivefold more potent than daily rhGH in juvenile monkeys. In juvenile monkeys, a monthly dose of 1.4 mg/kg VRS-317 (equivalent to 0.26 mg/kg rhGH) caused a sustained pharmacodynamic response for 1 month equivalent to 0.05 mg/kg/day rhGH (1.4 mg/kg rhGH total over 28 days). In monkeys, VRS-317, having a terminal elimination half-life of approximately 110 h, was rapidly and near-completely absorbed, and was well tolerated with no observed adverse effects after every alternate week subcutaneous dosing for 14 weeks. VRS-317 also did not cause lipoatrophy in pig and monkey studies. VRS-317 is currently being studied in GH-deficient patients to confirm the observations in these animal studies. © 2012 Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 101:2744–2754, 2012

Oxidation of Human Growth Hormone by Oxygen-Centered Radicals: Formation of Leu-101 Hydroperoxide and Tyr-103 Oxidation Products

Human growth hormone (hGH) was exposed to oxygen-centered radicals generated through the thermolysis of AAPH in the presence of dioxygen. Such conditions mimic oxidative processes which protein pharmaceuticals can encounter during formulation in the presence of polysorbates. We detected the oxidation of Met to Met sulfoxide, the formation of protein carbonyls, the oxidation of Tyr to dityrosine and several additional Tyr oxidation products, the conformation-dependent oxidation of Trp, and the site-specific formation of protein hydroperoxides. The sensitivity of Met oxidation correlates with their solvent accessible surface, i.e. the yields of MetSO decreased in the order Met-14 > Met-125 > Met-170. Trp oxidation in native hGH was negligible, but was enhanced through denaturation. Dityrosine formed predominantly intramolecularly but did not contribute significantly to protein cross-linking. Hydroperoxides formed selectively on Leu-101 and were generated specifically by alkoxyl radicals, generated through the decomposition of peroxyl radicals. Tyr-103 was converted into a series of oxidation products characterized by mass shifts of Tyr + 14 Da and Tyr + 16 Da.

History of Growth Hormone Therapy

The first human to receive GH therapy was in 1956; it was of bovine origin and was given for 3 wk for metabolic balance studies revealing no effects. By 1958, three separate laboratories utilizing different extraction methods retrieved hGH from human pituitaries, purified it and used for clinical investigation. By 1959 presumed GHD patients were being given native hGH collected and extracted by various methods. Since 1 mg of hGH was needed to treat one patient per day, >360 human pituitaries were needed per patient per year. Thus, the availability of hGH was limited and was awarded on the basis of clinical research protocols approved by the National Pituitary Agency (NPA) established in 1961. hGH was dispensed and injected on a milligram weight basis with varied concentrations between batches from 0.5 units/mg to 2.0 units/mg of hGH. By 1977 a centralized laboratory was established to extract all human pituitaries in the US, this markedly improved the yield of hGH obtained and most remarkably, hGH of this laboratory was never associated with Creutzfeld-Jacob disease (CJD) resulting from the injection of apparently prior- contaminated hGH produced years earlier. However, widespread rhGH use was not possible even if a pituitary from each autopsy performed in the US was collected, this would only permit therapy for about 4,000 patients. Thus, the mass production of rhGH required the identification of the gene structure of the hormone, methodology that began in 1976 to make insulin by recombinant technology. Serendipity was manifest in 1985 when patients who had received hGH years previously were reported to have died of CJD. This led to the discontinuation of the distribution and use of hGH, at a time when a synthetic rhGH became available for clinical use. The creation of a synthetic rhGH was accompanied by unlimited supplies of hGH for investigation and therapy. However, the appropriate use and the potential abuse of this hormone are to be dealt with. The illegitimate use of rhGH, unequivocally the abuse by athletes is, and should be, of primary concern to society and should be halted. The abuse of prescribing rhGH in an attempt to retard the aging process also should receive attention.

Pharmacokinetic/pharmacodynamic (PK/PD) differentiation of native and PEGylated recombinant human growth hormone (rhGH and PEG-rhGH) in the rat model of osteoarthritis

Osteoarthritis (OA) is a degenerative joint disease that has no FDA-approved treatment. The current standard of care does not address the regeneration of the damaged cartilage.Human growth hormone (hGH) is part of the insulin-like growth factor (IGF)-1 axis. There has been preclinical data that suggest its potential regenerative property in the joint.However, unformulated recombinant hGH (rhGH) is short-lived in the joint, and does not provide a desirable pharmacokinetic (PK) profile to support a clinical treatment paradigm.Polyethylene glycol (PEG)ylation is a potential method to extend the half-life of rhGH in the joint.The purpose of this study was to delineate the PK/PD profile of PEG-rhGH in the knee joint in a rat preclinical model of OA.After intra-articular (IA) injection of 100 μg into a rat knee joint that underwent medial meniscectomy, PEG-rhGH exhibits 2-fold longer half-lives in joint than native hGH. However, PEG-rhGH has a much longer systemic exposure. IA injections of PEG-rhGH also resulted in higher levels of IGF-1 in the joint and serum when compared with native rhGH.In order to develop PEG-rhGH as an IA therapeutic treatment for OA, careful dose selection is necessary to avoid systemic effects while retaining its anabolic efficacy in the joint.

Human growth hormone–transferrin fusion protein for oral delivery in hypophysectomized rats

Transferrin (Tf)-based recombinant fusion protein approach was investigated to achieve oral delivery for human growth hormone (hGH). Plasmid constructs expressing the fusion proteins were established by fusing coding sequences of both hGH and Tf in frame. Fusion proteins were produced in serum free media by transient transfection of human embryonic kidney HEK293 cells. The SDS-PAGE analysis of conditioned media showed that fusion proteins expressed at high purity with a 100 kDa molecular weight; the Western blot analysis with anti-hGH and anti-Tf antibodies verified the identity of fusion proteins. The Nb2 cell proliferation and Caco-2 cell Tf receptor (TfR) binding assays demonstrated that fusion proteins retained bioactivity of both hGH and Tf, respectively. A helical linker was inserted as spacer between hGH- and Tf-domain to enhance the bioactivity and the yield of the fusion protein. Two fusion proteins, hGH-Tf (GT) and hGH-(H4) 2-Tf (GHT) were obtained and assessed in hGH-deficient hypophysectomized rats for in vivo biological activity. Results from seven-day subcutaneous dosing (1.25 mg/kg/day) demonstrated that both GT and GHT fusion proteins were bioactive in vivo, comparable to native hGH. However, only the GHT, but not GT, fusion protein promoted a modest but statistically significant weight gain after oral dosing with 12.5 mg/kg/day.

Growth Hormone in Sport: Beyond Beijing 2008

Human growth hormone (hGH) is a protein endogenously produced predominantly by the anterior pituitary gland. Native hGH and, especially, its recombinant analogue (rhGH), used to treat patients with hormone deficiency, are supposed to be abused by athletes searching its anabolic and lipolytic effects. Hence, hGH use has been prohibited for a long time by the sport authorities, but until recently, hGH abuse could not be detected. Two approaches have been followed when trying to develop methods for GH abuse detection. The direct method identifies an abnormal ratio between GH isoforms--a result of hGH exogenous administration. The time window to find a cheating athlete by this approach is limited by the excretion time of the hormone. The indirect approach measures serum biomarkers directly affected by GH intake (eg, markers of released liver growth factors and of bone and collagen turnover). In this approach, the retrospective power extends further. Alternative possibilities for cheating related to hGH could be the administration of recombinant growth factors themselves, the administration of hGH metabolic precursors such as ghrelin-like GH secretagogues, or the genetic manipulation of muscle growth-related genes (gene doping). In parallel with the new types of abuse, which will surely emerge in the near future, the research and development for the improvement of the analytical detection of GH itself will continue.

A New PEG−β-Alanine Active Derivative for Releasable Protein Conjugation

A new PEGylating agent, PEG-betaAla-NHCO-OSu, has been studied for protein amino conjugation using human growth hormone (hGH) and granulocyte colony stimulating factor (G-CSF) as model therapeutic proteins. This new activated PEG possesses a convenient property for protein modification when compared to other activated carboxylate PEGs, namely, lower reactivity. When this polymer reacts with a protein, its features lead to fewer PEG-protein conjugate isomers because it preferentially binds the most nucleophilic and exposed amines. Furthermore, the conjugates obtained with PEG-betaAla-NHCO-OSu showed an interesting slow release of polymer chains upon incubation under physiological conditions. Further investigations determined that the PEG chains released are those coupled to histidine residues, and this finally yields less PEGylated species as well as free protein. This release allows a partial recovery of protein activity that is often remarkably and permanently reduced after stable PEGylation, and it occurs in water or blood without the involvement of enzymes. On the other hand, the rate of PEG release, tuned by the chemical structure of this new PEGylating agent, is not too high, and therefore, the achievement of a desired prolongation of protein half-life in vivo is still feasible. The pharmacokinetics of hGH-PEG6k-betaAla conjugate was compared to that of native hGH in rats and monkeys, and the blood residence times were increased by 10- and 7-fold, respectively. The conjugate potency was evaluated in hypophysectomized rats demonstrating a superior pharmacodynamic profile with respect to native hGH.

The conformational and biological analysis of a cyclic anti-obesity peptide from the C-terminal domain of human growth hormone.

The three-dimensional solution structure of antiobesity drug (AOD), a 15-residue, disulfide-bonded, cyclic peptide, cyclo(6,13)-H2N-Leu-Arg-Ile-Val-Gln-Cys-Arg-Ser-Val-Glu-Gly-Ser-Cys-Gly-Phe-OH, derived from the C-terminal domain of the human growth hormone (hGH) (residues 177-191) was determined using two-dimensional 1H NMR spectroscopy. AOD stimulates lipolysis and inhibits lipogenesis, in vitro, in rodent, porcine and human adipose tissues. These biological effects suggest that AOD is a potential therapeutic candidate for the treatment of obesity. Conformational studies of AOD were conducted in aqueous solution and in water/dimethylsulfoxide mixtures. In general, spectral quality was superior in the water/ dimethylsulfoxide mixtures. The cyclic region of AOD in water/dimethylsulfoxide adopts type I beta-turns at residues Ser8-Val9-Glu10-Gly11 and Ser12-Cys13-Gly14-Phe15, each preceded by loop-like structures. Comparison of the conformation of this peptide with residues 177-191 in the native hGH protein X-ray crystal structure indicates that the synthetic peptide retains some structural similarity to the intact protein. This study provides evidence that the C-terminal region of hGH is a specific functional domain of the multifunctional hGH protein.

Changes in the specificity of antibodies in mice infected with lactate dehydrogenase-elevating virus.

Infection with lactate dehydrogenase-elevating virus (LDV) modifies the isotypic distribution of antibodies (Ab) directed to several antigenic proteins with a preferential production of IgG2a. Because it was not known whether the virus could also affect the Ab specificity, the authors addressed this point using human growth hormone (hGH) as a model antigen. Anti-hGH monoclonal antibodies (MoAb) were used as probes to study the occurrence of Ab to three native hGH epitopes (3C11, F11 and 10D6) in sera from LDV-infected CBA/Ht and BALB/c mice immunized with hGH. Competition ELISA was used to determine the extent of Ab directed to cryptic hGH epitopes, i.e. antigenic determinants hidden in the native hormone. Results indicated that in LDV-infected CBA/Ht mice the titres of anti-hGH Ab were lower than in controls, although a consistent isotypic shift to IgG2a subclass was observed. Concurrently, the presence of Ab to epitopes 3C11, F11 and/or 10D6 were markedly reduced in infected animals and most anti-hGH Ab were directed to hGH cryptic epitopes. By contrast, LDV infection increased the amount of anti-KLH Ab elicited by CBA/Ht mice and did not affect Ab specificity, whilst control and LDV-infected BALB/c mice showed similar concentrations of anti-hGH Ab. Furthermore, the proportion of Ab to cryptic hGH epitopes did not change in infected animals even though an important shift to IgG2a was detected. Thus, data presented herein suggest that LDV infection modifies Ab specificity depending on the mice genetic background and on the antigenic characteristics of the immunogen.

Metal-catalyzed Oxidation of Histidine in Human Growth Hormone: MECHANISM, ISOTOPE EFFECTS, AND INHIBITION BY A MILD DENATURING ALCOHOL

Metal-catalyzed oxidation of proteins represents an important pathway of post-translational modification. We utilized human growth hormone (hGH), a protein with a well defined metal-binding site, to study the detailed mechanism of metal-catalyzed oxidation by ascorbate/Cu(II)/O2. Particularly His18 and His21 within the metal-binding site were oxidized, predominantly to 2-oxo-His with the incorporated oxygen originating from molecular oxygen, based on amino acid analysis, tryptic mapping, mass spectrometry, isotopic labeling, and 1H NMR. The anaerobic reduction of a hGH/Cu(II) mixture by ascorbate generated a hGH-Cu(I) complex with NMR spectral features different from those of native hGH and hGH/Cu(II). The anaerobic reaction of this hGH-Cu(I) complex with hydrogen peroxide resulted in the oxidation of His18 and His21, suggesting that a fraction of Cu(I) was bound at the metal-binding site of hGH. Site-specific oxidation of hGH required an intact metal-binding site and could largely (about 80%) be inhibited by the presence of >/=28% (v/v) 1-propanol which appears (i) to perturb the metal-binding site and (ii) to interact with a reactive oxygen species formed at the perturbed metal-binding site. The inhibition by 1-propanol-d7 (CD3CD2CD2OH) was significantly lower than that by 1-propanol-h7 with [residual hGH]1-propanol-h7/[residual hGH]1-propanol-d7 = 1.95 at 30% (v/v) 1-propanol, reflecting a kinetic isotope effect close to that for the reaction of a hydroxyl radical with Calpha-H/D bonds of methanol, suggesting the involvement of a hydroxyl radical-like species in the oxidation of His.

Growth Hormone (GH) and a GH Antagonist Promote GH Receptor Dimerization and Internalization

It has previously been shown that a human growth hormone (hGH) analog, hGH-G120R, acts as a GH antagonist (Chen, W. Y., Wight, D. C. , Wagner, T. E., and Kopchick, J. J. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 5061-5065; Chen, W. Y., White, M. E., Wagner, T. E., and Kopchick, J. J. (1991) Endocrinology 129, 1402-1408; Chen, W. Y., Chen, N-Y., Yun, J., Wang, X. Z., Wagner, T. E., and Kopchick, J. J. (1994) J. Biol. Chem. 269, 15892-15897). In this study, we report the ability of hGH and hGH-G120R to be internalized by GH receptor expressing cells. Additionally, results of chemical cross-linking experiments revealed that both native hGH and hGH-G120R form complexes similar in size to that expected for hGH when bound to recombinant hGH-binding protein (bp). The molecular mass of the complex was determined to be approximately 280 kDa which is consistent with multiple receptors interacting with the ligand. The predominant radiolabeled band detected was a complex of approximately 140 kDa which probably represents one GH molecule bound to one GH receptor. The cross-linked complexes were not detected in the presence of excess unlabeled hGH or hGH-G120R and were not observed in cells which do not express detectable levels of GH receptors. Also, GH induced tyrosine phosphorylation of a complex of proteins of approximately 95 kDa in these cells whereas hGH-G120R did not. Thus, we have separated the hGH or hGH-G120R/GHR binding and internalization capabilities from the ability to stimulate tyrosine phosphorylation of intracellular proteins.

G120R, a Human Growth Hormone Antagonist, Shows Zinc-dependent Agonist and Antagonist Activity on Nb2 Cells

Substitution of arginine for glycine at position 120 in native 22-kDa human growth hormone (hGH) results in an analogue, G120R, which is unable to dimerize the GH receptor and is widely used to probe the molecular mechanism of action of hGH. When acting on human GH receptors, G120R antagonizes several biological effects of hGH, but is itself inactive as an agonist. It has been reported that this mutant also antagonizes hGH activation of the rat or human prolactin (PRL) receptor in cell-based assays, with no agonist activity. We have now tested this mutant in a sensitive MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide)-ESTA (eluted stain assay) bioassay using rat PRL receptors in the Nb2 cell line. We confirm that G120R acts as an efficient antagonist of native hGH, but show that it can also act as an agonist to generate intracellular signals leading to metabolic activation and proliferation of Nb2 cells. We have demonstrated an unusual sensitivity to the presence of zinc (Zn2+). In the absence of added Zn2+, G120R shows weak but full agonist activity in the bioassay, and this can be blocked by co-incubation with recombinant hGH-binding protein. G120R can therefore be utilized to discriminate between the molecular mechanisms of hGH interactions with its somatogenic and lactogenic receptors. Future studies with G120R in the rat may need to take account of its significant agonist effects on PRL receptors.