A novel bioassay for human somatogenic activity in serum samples supports the clinical reliability of immunoassays

Short stature explained by dimerization of human growth hormone induced by a p.C53S point mutation

A homozygous mutation in growth hormone 1 (GH1) was recently identified in an individual with growth failure. This mutation, c.705G>C, causes replacement of cysteine at position 53 of the 191-amino-acid sequence of 22 kDa human GH (hGH) with serine (p.C53S). This hGH molecule (hereafter referred to as GH-C53S) lacks the disulfide bond between p.Cys-53 and p.Cys-165, which is highly conserved among species. It has been reported previously that monomeric GH-C53S has reduced bioactivity compared with WT GH (GH-WT) because of its decreased ability to bind and activate the GH receptor in vitro In this study, we discovered that substitution of p.Cys-53 in hGH significantly increased formation of hGH dimers in pituitary cells. We expressed His-tagged hGH variants in the cytoplasm of genetically modified Rosetta-gami B DE3 Escherichia coli cells, facilitating high-yield production. We observed that the bioactivity of monomeric GH-C53S is 25.2% of that of GH-WT and that dimeric GH-C53S-His has no significant bioactivity in cell proliferation assays. We also found that the expression of GH-C53S in pituitary cells deviates from that of GH-WT. GH-C53S was exclusively stained in the Golgi apparatus, and no secretory granules formed for this variant, impairing its stimulated release. In summary, the unpaired Cys-165 in GH-C53S forms a disulfide bond linking two hGH molecules in pituitary cells. We conclude that the GH-C53S dimer is inactive and responsible for the growth failure in the affected individual.

The role of human growth hormone's C-terminal disulfide bridge

OBJECTIVE

Human growth hormone (hGH), as well as the other members of the same polypeptide hormone family, have a four-helix bundle structure linked by two disulfide bridges, C53-C165 and C182-C189 in hGH. The C-terminal disulfide bridge of growth hormone is evolutionally conserved but its role is unknown. Our aim was to determine its importance for GH structure and/or function.

DESIGN

We disrupted the highly conserved C-terminal disulfide bridge of hGH by substituting one or both of its cysteines by alanines. Mutant and wild type hGH genes were expressed in human embryonic kidney (HEK)-293 cells and the hGH analogs were characterized in vitro regarding biological activity, stability and binding to GH receptor (GHR) as well as GH binding protein (GHBP).

RESULTS

Disrupting the hGH C-terminal disulfide bridge significantly reduces binding affinity to GHR and GHBP. If one of the cysteines is removed, the stability of the molecule is reduced but this feature is reversed when both cysteines are absent. However, despite decreased binding affinity and stability, biological activity is only modestly decreased when the disulfide bridge is removed.

CONCLUSIONS

Our study reveals the importance of the C-terminal disulfide bridge of GH for receptor binding and the detrimental effect of its unpaired cysteines on stability as well as, to a lesser extent, biological activity. This improved knowledge of structure-function relationships helps better understand the biology of GH and related molecules. This could have an impact on diagnosis and treatment of patients with growth disorders.

Development of a bioassay system for human growth hormone determination with close correlation to immunoassay

Serum growth hormone (GH) level is measured largely through immunoassays in clinical practice. However, a few cases with bioinactive and immunoreactive GH have also been reported. We describe here a new bioassay system for GH determination using the BaF/GM cell line, which proliferates in a dose-dependent manner on hGH addition; cell proliferation was blocked by anti-hGH antibody. This bioassay had the lowest detection limit (∼0.02 ng/ml) reported thus far and the highest specificity for GH. The bioassay results were compared with those of an immunoradiometric assay across 163 patient samples in various endocrine states. A close correlation (the ratio of bioactivity/immunoreactivity was 1.04 ± 0.33, mean ± SD) was observed between bioactivity and immunoreactivity in these samples. The newly developed system is a specific, sensitive, easy, and fast bioassay system for GH determination; we consider it useful for evaluating GH bioactivity in various endocrine states.

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.

Expression of functional recombinant human growth hormone in transgenic soybean seeds

We produced human growth hormone (hGH), a protein that stimulates growth and cell reproduction, in genetically engineered soybean [Glycine max (L.) Merrill] seeds. Utilising the alpha prime (α') subunit of β-conglycinin tissue-specific promoter from soybean and the α-Coixin signal peptide from Coix lacryma-jobi, we obtained transgenic soybean lines that expressed the mature form of hGH in their seeds. Expression levels of bioactive hGH up to 2.9% of the total soluble seed protein content (corresponding to approximately 9 g kg(-1)) were measured in mature dry soybean seeds. The results of ultrastructural immunocytochemistry assays indicated that the recombinant hGH in seed cotyledonary cells was efficiently directed to protein storage vacuoles. Specific bioassays demonstrated that the hGH expressed in the soybean seeds was fully active. The recombinant hGH protein sequence was confirmed by mass spectrometry characterisation. These results demonstrate that the utilisation of tissue-specific regulatory sequences is an attractive and viable option for achieving high-yield production of recombinant proteins in stable transgenic soybean seeds.

20-kDa placental hGH-V has diminished diabetogenic and lactogenic activities compared with 22-kDa hGH-N while retaining antilipogenic activity

Placental human growth hormone-variant (hGH-V) and pituitary human growth hormone-N (hGH-N) are of identical size (22 kDa) but differ in 13 residues scattered throughout the protein. Several isoforms of GH are produced by the hGH-N and hGH-V genes including a 20-kDa hGH-V resulting from a 45-bp deletion caused by the use of an alternative acceptor site within exon 3. To date, the biological properties of the 20-kDa GH-V have not been characterized in vivo. Using young male Wistar rats fed either chow or a high-fat (HF) diet for 4 wk postweaning, we investigated the effect of 7 days treatment with either 22-kDa hGH-N, 20-kDa hGH-V (5 ug x g(-1) x day(-1) sc), or vehicle on body composition and endocrine and metabolic profiles. Total body growth (absolute weight gain and linear growth trajectory) in the 20-kDa hGH-V-treated animals was intermediary between that of control and hGH-N-treated animals. Both 22-kDa hGH-N and 20-kDa hGH-V significantly reduced total body fat mass compared with control animals, and there were no differences between the GH isoforms in anti-lipogenic activity in animals fed the HF diet. Fasting plasma insulin and C peptide were significantly increased in animals on the HF diet and further increased by hGH-N but were unchanged in 20-kDa hGH-V-treated animals compared with saline-treated controls. Plasma volume as assessed by hematocrit was increased in hGH-N-treated animals but was unchanged in 20-kDa hGH-V-treated animals compared with controls. Furthermore, 20-kDa hGH-V had reduced lactogenic (prolactin receptor mediated) activity characteristic of hGH-N as tested in vitro compared with the 20-kDa hGH-N and 22-kDa hGH-N variants. In summary, placental 20-kDa hGH-V retains some of the growth-promoting and all antilipogenic activities of pituitary 22-kDa hGH-N but has diminished diabetogenic and lactogenic properties compared with the native 22-kDa hGH-N.

Human growth hormone measurement by means of a sensitive ELISA of whole blood spots on filter paper

BACKGROUND

Measurements of human growth hormone (hGH) are a prerequisite for identifying a deficiency or excess. Our study is the first to investigate the reliability of a very sensitive assay for the quantification of GH in dried blood spots on filter paper.

OBJECTIVE

Validation of a commercially-available enzyme-linked immunoassay (ELISA) for measuring hGH from filter paper samples of dried blood.

METHODS

We used an assay system (ELISA, E022, Mediagnost) based on polyclonal rabbit antibodies. Its suitability is ascribable to its very high sensitivity (1.6 ng/L) and virtual absence of interfering factors, excepting for a cross-reactivity with high pegvisomant concentrations.

RESULTS

hGH was found to be stable in dried blood spots on filter paper (No. 903, Whatman) over eight days at 37 degrees C. Extraction of hGH from filter paper, in comparison to EDTA plasma, was 107% (SD 8.1%; n=6) over a range from 2.4 to 34.5 microg/L. Linear regression analysis (n=119) showed a correlation of R(2)=0.97 for the hGH concentration in serum and on filter paper samples.

CONCLUSION

Our findings demonstrate the reliability of measurements of hGH in dried blood spots on filter paper. The advantages of this method are the low sample volume and the easy transport, storage, and handling of samples. This method contributes to the standardisation of diagnostics pertaining to abnormal hGH secretion as it facilitates the comparison of decisive measurements.

Determination of free growth hormone

CONTEXT

Approximately 50% of circulating GH is bound to the high-affinity GH-binding protein (GHBP), which is known to affect the pharmacokinetics, bioactivity, and quantitative determination of GH. Nevertheless, the presence of GHBP is rarely taken into account in the clinical use of GH measurements.

OBJECTIVE

Our objective was to develop an assay for free GH in serum.

METHODS

We used ultrafiltration by centrifugation. Due to the small molecular difference between GH and GHBP, the size of GHBP and GHBP-GH complexes was increased by preincubation of serum with a monoclonal GHBP antibody (MAb 263).

RESULTS

The ultrafiltration membrane almost completely retained all GHBP (>98.5%) and allowed free passage of unbound GH (>98.4%). Addition of increasing concentrations of GHBP reduced free GH dose dependently, and measured and calculated levels of free GH changed in parallel. During an insulin-tolerance test, free and total GH changed in parallel in all individuals (n = 11) and their peak values as well as area under the curve values were positively correlated (r = 0.89; P < 0.001 and r = 0.92; P < 0.001, respectively). Of note, the relative levels of free GH (calculated as the area under the curve of free to total GH) was inversely correlated with GHBP (r = -0.94; P < 0.001).

CONCLUSION

It is possible to measure free GH in human serum. Free GH correlated positively with total GH and inversely with GHBP. Measurement of free GH may be a helpful future tool in the management of GH disorders and in studies of GH-GHBP interrelationships.

Deterioration of visual acuity associated with growth hormone therapy in a child with extreme short stature and high hypermetropia

BACKGROUND

Growth hormone (GH) has long been implicated in the pathogenesis of diabetic retinopathy, although its precise role remains ill-defined. In 1998, an association between exogenous human GH and retinal pathology in non-diabetic subjects was described.

CASE REPORT

A female child with extreme short stature of unknown aetiology (height -7.38 SD at 11.3 years) and severe hypermetropia developed retinopathy with visual deterioration during two separate empiric trials of GH therapy. On the first occasion, a relatively high dose of GH (10.5 mg/m2/week) administered from age 4.4 to age 4.7 years was associated with the development of central serous retinopathy, resulting in marked reduction in visual acuity. On cessation of GH, the macular oedema resolved, and visual acuity improved. At age 5.6 years, GH therapy was re-introduced at a lower dose (3.9 mg/m2/week) and her vision monitored closely. Bilateral retinal oedema recurred after 3 months, and GH therapy was stopped. Once again, the macular oedema regressed, and visual acuity improved following withdrawal of GH. These ophthalmic changes contra-indicated further GH therapy.

CONCLUSION

We suggest that GH may be a risk factor in the development of retinopathy in certain non-diabetic patients, especially in the presence of a severe refractive error.

Activation of the growth hormone receptor

Growth hormone (GH) is a major regulator of postnatal growth and metabolism. There are extensive clinical applications for GH or its antagonists, including treatments for dwarfism, cancer and metabolic wasting. Owing to this, there is considerable interest in the mechanisms of GH receptor (GHR) activation. It is conventionally thought that GH induces dimerization of two GHR monomers, which initiates intracellular signaling cascades. However, recent studies have provided evidence for a ligand-induced conformational change within constitutively dimerized GHRs being responsible for activating signaling pathways. This review will relate the new model of GHR activation to the activation of related cytokine receptors and discuss the implication of this new model for the design of small GH mimetics and antagonists for therapeutic use.

Growth hormone isoforms and segments/fragments: Molecular structure and laboratory measurement

Endocrine diagnostic tests are dependent on the molecular characteristics of protein hormones, a property that is also intimately tied to function. The structure-function relationship is of particular importance for multifunctional protein hormones such as growth hormone (GH). For clinical diagnosis, it is imperative to understand the relationship between GH structure (and its molecular fragments) and function and their potential interaction with single or multiple receptors. The existence of a single or aggregated intact GH 22 kDa form such as the 20 kDa GH isoform has been described, but GH fragments cannot be excluded a priori. Recent advances and probable similarity of GH with other protein hormones such as natriuretic peptides (ANP, BNP and their proANP and proBNP fragments) and POMC (ACTH, beta-endorphin, etc.) lend support to the hypothesis that GH fragments should also be present. This brief review focuses on GH heterogeneity and feasible post-synthesis events. The aim of the review is to describe which GH forms/fragments have already been recognized and/or are potentially present in the circulation. The impacts of GH isoforms (namely the intact 20 and 22 kDa isoforms) and fragments thereof on quantitative measurement are discussed with reference to traditional immunoassay technology and innovative immunofunctional laboratory assays.

Physiology and pathophysiology of growth hormone-binding protein: methodological and clinical aspects

Circulating GH is partly bound to a high-affinity binding protein (GHBP), which in humans is derived from cleavage of the extracellular domain of the GH receptor. The precise biological function GHBP is unknown, although a regulation of GH bioactivity appears plausible. GHBP levels are determined by GH secretory status, body composition, age, and sex hormones, but the cause-effect relationships remain unclarified. In addition to the possible in vivo significance of GHBP, the interaction between GH and GHBP has methodological implications for both GH and GHBP assays. The present review concentrates on methodological aspects of GHBP measurements, GHBP levels in certain clinical conditions with a special emphasis on disturbances in the GH-IGF axis, and discusses the possible relationship between plasma GHBP and GH receptor status in peripheral tissues.

ELISA for Determination of Human Growth Hormone: Recognition of Helix 4 Epitopes

Human growth hormone (hGH) signal transduction initiates with a receptor dimerization in which one molecule binds to the receptor through sites 1 and 2. A sandwich enzyme-linked immunosorbent assay was developed for quantifying hGH molecules that present helix 4 from binding site 1. For this, horse anti-rhGH antibodies were eluted by an immunoaffinity column constituted by sepharose-rhGH. These antibodies were purified through a second column with synthetic peptide correspondent to hGH helix 4, immobilized to sepharose, and used as capture antibodies. Those that did not recognize synthetic peptide were used as a marker antibody. The working range was of 1.95 to 31.25 ng/mL of hGH. The intra-assay coefficient of variation (CV) was between 4.53% and 6.33%, while the interassay CV was between 6.00% and 8.27%. The recovery range was between 96.0% to 103.8%. There was no cross-reactivity with human prolactin. These features show that our assay is an efficient method for the determination of hGH.