Biological activities of synthesized 20K and 22K hGH in Nb2 bioassay and IM-9 radioreceptor assay

Human growth hormone: 45-kDa isoform with extraordinarily stable interchain disulfide links has attenuated receptor-binding and cell-proliferative activities

BACKGROUND

Human growth hormone (hGH) is a complex mixture of molecular isoforms. Gaps in our knowledge exist regarding the structures and biological significances of the uncharacterized hGH molecular variants. Mercaptoethanol-resistant 45-kDa human growth hormone (MER-45 kDa hGH) is an extraordinarily stable disulfide-linked hGH homodimer whose biological significance is unknown.

OBJECTIVES

To elucidate the pharmacokinetic abilities of dimeric MER-45-kDa hGH to bind to GH and prolactin (PRL) receptors and to elucidate its abilities to stimulate cell proliferation in lactogen-induced and somatogen-induced in vitro cell proliferation bioassays.

DESIGN

The binding of MER-45-kDa hGH to GH and PRL receptors was tested in radioreceptor assays (RRAs). Competitive displacements of [(125)I]-bovine GH from bovine liver membranes, [(125)I]-ovine PRL from lactating rabbit mammary gland membranes and [(125)I]-hGH from human IM-9 lymphocytes by unlabelled GHs, PRLs or dimeric MER-45-kDa hGH were evaluated. The abilities of dimeric MER-45-kDa hGH to stimulate proliferation of lactogen-responsive Nb2 lymphoma cells and to stimulate proliferation of somatogen-responsive T47-D human breast cancer cells were assessed by incubation of cells with GHs or PRLs and subsequently measuring growth using the MTS cell proliferation assay.

RESULTS

Dimeric MER-45-kDa hGH, compared to monomeric hGH, had reduced binding affinities to both GH and prolactin receptors. In a bovine liver GH radioreceptor assay its ED(50) (197.5 pM) was 40.8% that of monomeric hGH. In a human IM-9 lymphocyte hGH RRA its ED(50) (2.96 nM) was 26.2% that of monomeric hGH. In a lactating rabbit mammary gland prolactin RRA its ED(50) (3.56 nM) was 16.8% that of a monomeric hGH. Dimeric MER-45-kDa hGH, compared to monomeric hGH, had a diminished capacity to stimulate proliferation of cells in vitro. In a dose-response relationship assessing proliferation of Nb2 lymphoma cells its ED(50) (191 pM) was 18.0% that of monomeric hGH. While monomeric hGH stimulated a 2.2-fold proliferation of T47-D human breast cancer cells above vehicle control, dimeric MER-45-kDa hGH was unable to stimulate the cells to proliferate and slightly inhibited their proliferation to 77.6% that of control.

CONCLUSIONS

The topological arrangement of monomeric hGHs to form an unusually stable disulfide-linked dimer markedly diminishes hGH's binding affinities to both GH and PRL receptors and also drastically attenuates its ability to stimulate proliferation of cells in vitro.

22- and 20 kDa-human growth hormones bind to different sites within certain cellular receptors

Human growth hormone 20 kDa (20 kDa-hGH) is a natural variant of the main hGH isoform (22 kDa-hGH). Since some 20- and 22 kDa-hGH biological activities are not identical, we decided to map the prolactin (PRLR) and growth hormone receptor (GHR) binding sites for both isoforms. Monoclonal antibody (MAb) R7B4, directed to both receptors, was employed to estimate the relative proximity between 20- and 22 kDa-hGH receptors binding sites. Results indicated that although both hGH isoforms share the same PRLR present in Nb2-cells and rat liver, MAb R7B4 differently affected hormone binding, suggesting that their receptor binding sites would be close in Nb2-cells and separate in rat liver membranes. Since labelled 20 kDa-hGH did not bind significantly to hGHR, we added to the incubating medium an allosteric MAb anti-hGH that improved 20 kDa-hGH affinity for receptors. Under these experimental conditions MAb R7B4 inhibited 20 kDa-hGH binding to human liver but not to placenta, whereas the Ab impaired 22 kDa-hGH binding to both receptors. Data thus suggested that both hGH isoforms share the same hGHR binding site in liver tissue but bind to different overlapped regions in placenta. Consequently, results presented in this paper indicate that PRLR and GHR binding sites for 22- and 20 kDa-hGH should not be always identical, a fact that could explain some of the isoforms different activities.

A novel bioassay based on human growth hormone (hGH) receptor mediated cell proliferation: measurement of 20K-hGH and its modified forms

Previously we introduced the full-length hGH receptor (hGHR) into the mouse pro-B cell line, Ba/F3, and obtained stable transfectant (Ba/F3-hGHR), which could grow in response to 20K- and 22K-hGH in a dose-dependent manner(1). In the present study, we established a new bioassay system based on the proliferation of the Ba/F3-hGHR in combination with the eluted stain assay (ESTA). The Ba/F3-hGHR assay is completed in 18 h and requires only 10(-6)-fold amount of GH sample (1.8 ng) as compared with the rat weight gain assay. The validation study shows that the Ba/F3-hGHR assay is specific for hGH, precise (RSD = 1.1-19.7%) and ultrasensitive (lower limit of working range = 18.7 pg/mL). Four modified forms of recombinant 20K-hGH (oxidized, deamidated, des-Phe(1)and cleaved form) all of which are newly identified were measured by the Ba/F3-hGHR assay and the rat weight gain assay with our in-house recombinant 20K-hGH as standard. The oxidized and deamidated 20K-hGH were fully active, however the des-Phe(1)and cleaved 20K-hGH had significantly reduced activities in both assays. These findings suggest that the Ba/F3-hGHR assay is useful as an alternative to the rat weight gain assay.

Comparison of the somatogenic action of 20 kDa- and 22 kDa-human growth hormones in spontaneous dwarf rats

The somatogenic action of the 20 kilodalton human growth hormone (20 K) was studied using the spontaneous dwarf rat (SDR), which has an isolated GH deficiency. Saline or 2.5 microg, 10 microg, or 100 microg/rat/day of recombinant 20 K or 22 K was administered to prepubertal male and female SDRs for 10 days. Their body weights, serum IGF-I, glucose and insulin were measured, and their body composition was determined. Body weights and serum IGF-I increased dose-dependently in both the 20 K- and 22 K-treated groups. There was no significant difference in body weights and serum IGF-I between the 20 K- and 22 K-treated groups except at the dose of 100 microg/rat, in which the IGF-I concentrations were higher in the 22 K-treated male SDRs (P< 0.05: 20 K vs 22 K). Blood glucose was not significantly different between the Spague-Dawley (SD) normal rats and the SDR control groups; however, serum insulin levels of the SDR were higher than those of the SD control group (P< 0.05). Additionally, there was a tendency for serum insulin and glucose levels to increase following 22 K treatment, but the differences were not significant. The percentage of body fat decreased with hGH treatment in both groups (P< 0.01: GH 10, 100 microg/rat group vs SDR control group), however, no significant differences were observed in body composition between the 20 K and 22 K treatment groups. In summary, the 20 K-hGH showed almost the same somatogenic activity as the 22 K-hGH in prepubertal male and female SDRs.

The 20-kilodalton (kDa) human growth hormone (hGH) differs from the 22-kDa hGH in the effect on the human prolactin receptor

Previously we have demonstrated that 20-kDa human GH (20K-hGH) is a full agonist for hGH receptor (hGHR) even though its complex formation with hGHR and hGH-binding protein differs from that of 22-kDa human GH (22K-hGH). In this study, we focused on the effect of 20K-hGH on human PRL receptor (hPRLR). To elucidate the effects of 20K-hGH on hPRLR and compare them with those of 22K-hGH, we prepared two cells stably expressing full-length hPRLR, Ba/F3-hPRLR and CHO-hPRLR. In the proliferation of Ba/F3-hPRLR cells, which can grow in a dose-response to lactogenic hormones, both 20K- and 22K-hGH exhibited bell-shaped curves in the absence of exogenous zinc ion (Zn2+); however, the curve of 20K-hGH was shifted to a 10-fold higher concentration than that of 22K-hGH in view of EC50 value (the EC50 of 20K- and 22K-hGH were 15 nM and 1.5 nM, respectively). Addition of Zn2+ up to 25 microM increased the activities of both 20K- and 22K-hGH; however, the enhancement by Zn2+ was greater in 20K-hGH than in 22K-hGH, thereby the activities of both hGH isoforms reached the same level at 25 microM Zn2+. Nevertheless, in the presence of 0.25-1 microM free Zn2+, which is equal in human serum, the activity of 20K-hGH was still lower than that of 22K-hGH. The modest effect of 20K-hGH on activating hPRLR in the absence of Zn2+ was confirmed in the rat serine protease inhibitor 2.1 (Spi2.1) gene promoter activation and JAK2/Stat5 tyrosine phosphorylation in CHO-hPRLR. In addition, in human breast cancer cell T-47D, 20K-hGH was proved to stimulate Stat5 tyrosine phosphorylation to much lower degree than 22K-hGH via not hGHR but hPRLR. Taken together, our data suggest that 20K-hGH may be a weaker agonist for hPRLR than 22K-hGH in the human body; therefore 20K-hGH may alleviate the hPRLR-mediated side-effects such as breast cancer when administered to human body.

New growth hormone assays: potential benefits

Three recently published new assays are described for the measurement of growth hormone (GH). Two of these--the eluted stain assay (ESTA) and the immunofunctional assay (IFA)--have been developed to measure the bioactivity of GH, rather than the immunoactivity as measured by conventional radioimmunoassays (RIAs). The third assay--the 22 kDa exclusion assay (22 k GHEA)--is designed to measure the concentrations of the different isoforms of GH present in the circulation. The ESTA is a variant of the Nb2 bioassay for lactogenic hormones, but has been adapted for specific GH measurement in serum samples. It has a lower detection limit than previous bioassays and permits the quantification of GH in large series of samples. The IFA uses a binding-site-specific antibody in combination with GH-binding protein (GHBP) in order to quantify only those GH molecules that are able to dimerize the extracellular domain of the GH receptor (GHBP), which is a prerequisite for GH signal transduction in target cells. The IFA is as convenient to use as immunoassays and can be employed routinely for GH determinations. The clinical usefulness of the 22 k GHEA has not been established, but it should provide a means of augmenting our understanding of the regulation of GH and its various isoforms. Once the ESTA bioassay or the IFA become commercially and widely available, either could replace the RIA as the standard reference method for measuring GH, as both more closely reflect the biologically active proportion of GH in serum samples than that measured by RIA.

The effect of insulin-like growth factor-I and human growth hormone on periodontal ligament fibroblast morphology, growth pattern, DNA synthesis, and receptor binding

Repopulation of the detached root surface by cells from the periodontal ligament (PDL) is a prerequisite for new attachment formation. Stimulation of PDL-cell growth may therefore serve as an essential method to enhance formation of new attachment. Studies have demonstrated that insulin-like growth factor-I (IGF-I) has a mitogenic effect on fibroblasts originating from various connective tissues and cell-lines. Further, human growth hormone (hGH) is known to regulate the plasma concentration of IGF-I and to mediate cellular biological effects. In the present study we examined the effect of IGF-I and hGH on morphology, growth pattern, and DNA synthesis. The expression of IGF-I and hGH receptors on the surface of cultured PDL fibroblasts is also described. A primary fibroblastic cell line was established from rat PDL tissue, and blind, photographic recordings of morphology and growth pattern, as well as incorporation of [3H]thymidine in cellular DNA, was carried out in the presence and absence of IGF-I and hGH. The presence of specific membrane receptors was investigated by binding of [125I]IGF-I and [125I]hGH. The analysis of photographs showed that IGF-I and hGH had no effect on morphology and growth pattern. Incorporation of 3H-thymidine, however, was increased in a dose-dependent manner by IGF-I, whereas hGH alone or in combination with IGF-I produced no dose-dependent response. Maximum effect (% of control) on DNA synthesis was 176% for IGF-I and 91% for hGH.(ABSTRACT TRUNCATED AT 250 WORDS)

Pseudohypopituitary syndromes

In a child with short stature, the finding of normal or elevated GH levels in the presence of low concentrations of IGF-I raises the following possibilities. (1) A modification of the GH molecule, which is still detected by RIA, but inactive biologically. Therefore, an RRA or bioassay for hGH should result in considerably lower GH measurements compared with RIA determinations in the same sample. As both bioassays as well as RRAs are not widely available and are hampered by several difficulties, few children with this presumptive diagnosis have been described. So far, it has not been possible to define a specific molecular defect in one of these patients. (2) Abnormalities of the GH receptor or postreceptor mechanisms lead to a GH insensitivity syndrome. Laron-type dwarfism is usually due to a deletion in the gene for hepatic GH receptors: the serum binding protein for GH is absent. In three additional populations, the Pygmies of Zaire, the little women of Loja in Ecuador and the Mountain Ok people in Papua New Guinea, alterations of GH receptor function have been described. Finally, some reports describe patients with normal or elevated serum levels of both growth hormone and IGF-I in whom resistance to IGF has been implied in the pathogenesis of small stature.