Subdomain 2, Not the Transmembrane Domain, Determines the Dimerization Partner of Growth Hormone Receptor and Prolactin Receptor.

Growth hormone receptor (GHR) and prolactin (PRL) receptor (PRLR) are homologous transmembrane class I cytokine receptors. In humans, GH interacts with GHR homodimers or PRLR homodimers and PRL interacts with only PRLR homodimers to promote signaling. In human breast cancer cells endogenously expressing both receptors, GHR and PRLR specifically coimmunoprecipitate. We previously devised a split luciferase complementation assay to study GHR and PRLR assemblages. In this technique, firefly luciferase is split into two fragments (N- and C-terminal fragments of the luciferase), each without enzyme activity and tethered to the tails of two receptors. The fragments restore luciferase activity when brought close to each other by the receptors. Real-time ligand-induced complementation changes reflect the arrangement of receptors and indicate that GHR/PRLR is arranged as a heteromultimer comprised of GHR-GHR homodimers and PRLR-PRLR homodimers. We now dissect determinants for GHR and PRLR homodimerization versus heteroassociation. GHR and PRLR have extracellular domains comprised of the ligand-binding N-terminal subdomain 1 and a membrane-proximal subdomain 2 (S2), which fosters receptor-receptor contact. Based on previous studies of S2 versus the transmembrane domain (TMD) in GHR dimerization, we constructed GHR(PRLRS2), GHR(PRLRS2-TMD), and GHR(PRLRTMD), replacing GHR's S2 alone, S2 plus TMD, and TMD alone with PRLR's counterpart. We tested by complementation the ability of these chimeras and GHR or PRLR to homodimerize or heteroassociate. Comparing various combinations, we found GHR(PRLRS2) and GHR(PRLRS2-TMD) behaved as PRLR, whereas GHR(PRLRTMD) behaved as GHR regarding their dimerization partners. We conclude that S2 of GHR and PRLR, rather than their TMDs, determines their dimerization partner.

Growth Hormone Receptor Deficiency Protects against Age-Related NLRP3 Inflammasome Activation and Immune Senescence.

The hallmarks of age-related immune senescence are chronic inflammation, aberrant expansion of effector memory, and loss of naive T lymphocytes due in part to systemic activation of innate immune sensor NLRP3 inflammasome in myeloid lineage cells. The endogenous mechanisms that regulate inflammasome activation during aging are unknown. Here, we present evidence that growth hormone receptor (GH-R)-dependent downregulation of NLRP3 inflammasome in macrophages is linked to pro-longevity effects that maintain immune system homeostasis in aging. Deletion of GH-R prevented the macrophage-driven age-related activation of inflammasome in response to NLRP3 ligands and also increased the preservation of naive T cells, even in advanced age and with higher IFNγ secretion from effector cells. The mechanism of inflammasome inhibition is linked to autocrine somatotropic axis as ablation of IGF1R in macrophages lowered the NLRP3 inflammasome activation. Together, our findings show that functional somatotropic axis in macrophages controls inflammation, thus linking NLRP3-mediated innate immune signaling to health span and longevity.

Anti-idiotypic antibody: A new strategy for the development of a growth hormone receptor antagonist.

In general, traditional growth hormone receptor antagonist can be divided into two major classes: growth hormone (GH) analogues and anti-growth hormone receptor (GHR) antibodies. Herein, we tried to explore a new class of growth hormone receptor (GHR) antagonist that may have potential advantages over the traditional antagonists. For this, we developed a monoclonal anti-idiotypic antibody growth hormone, termed CG-86. A series of experiments were conducted to characterize and evaluate this antibody, and the results from a competitive receptor-binding assay, Enzyme Linked Immunosorbent Assays (ELISA) and epitope mapping demonstrate that CG-86 behaved as a typical Ab2ß. Next, we examined its antagonistic activity using in vitro cell models, and the results showed that CG-86 could effectively inhibit growth hormone receptor-mediated signalling and effectively inhibit growth hormone-induced Ba/F3-GHR638 proliferation. In summary, these studies show that an anti-idiotypic antibody (CG-86) has promise as a novel growth hormone receptor antagonist. Furthermore, the current findings also suggest that anti-idiotypic antibody may represent a novel strategy to produce a new class of growth hormone receptor antagonist, and this strategy may be applied with other cytokines or growth factors.

Effects of somatotrophic axis (GH/GHR) double transgenesis on structural and molecular aspects of the zebrafish immune system.

The development of growth hormone (GH) transgenic fish has been shown to be a promising method to improve growth rates. However, the role of GH is not restricted only to processes involved in growth. Several others physiological processes, including immune function, are impaired due to GH imbalances. Given the importance of generating GH transgenic organisms for aquaculture purposes, it is necessary to develop strategies to reduce or compensate for the collateral effects of GH. We hypothesized that the generation of double transgenic fish that overexpress GH and growth hormone receptor (GHR) in the skeletal muscle could be a possible alternative to compensate for the deleterious effects of GH on the immune system. Specifically, we hypothesized that increased GHR amounts in the skeletal muscle would be able to reduce the level of circulating GH, attenuating the GH signaling on the immune cells while still increasing the growth rate. To test this hypothesis, we evaluated the size of the immune organs, T cell content in the thymus and head kidney, and expression of immune-related genes in double-transgenic fish. Contrary to our expectations, we found that the overexpression of GHR does not decrease the deleterious effect of GH excess on the size of the thymus and head kidney, and in the content of CD3(+) and CD4(+) cells in the thymus and head kidney. Unexpectedly, the control GHR transgenic group showed similar impairments in immune system parameters. These results indicate that GHR overexpression does not reverse the impairments caused by GH and, in addition, could reinforce the damage to the immune functions in GH transgenic zebrafish.

Intracellular signaling transduction pathways triggered by a well-known anti-GHR monoclonal antibody, Mab263, in vitro and in vivo.

A series of studies have reported that monoclonal antibody 263 (Mab263), a monoclonal antibody against the growth hormone receptor (GHR), acts as an agonist in vitro and in vivo. However, the intracellular signaling pathways triggered by Mab263 have not yet been delineated. Therefore, we examined the intracellular signaling pathways induced by Mab263 in vivo and in vitro in the present study. The results show that this antibody activated janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3), STAT1 and extracellular signal-regulated kinase 1/2 (ERK1/2), but not STAT5. The phosphorylation kinetics of JAK2, STAT3/1 and ERK1/2 induced by Mab263 were subsequently analyzed in dose-response and time course experiments. Our observations indicate that Mab263 induced different intracellular signaling pathways than GH, which indicates that Mab263 is a signal-specific molecule and that Mab263 may be a valuable biological reagent to study the mechanism(s) of GHR-mediated intracellular signaling pathways.

Differential intracellular signalling properties of the growth hormone receptor induced by the activation of an anti-GHR antibody.

A series of studies have reported that anti-GHR antibody can function as a GHR agonist and may serve as an attractive tool for studying the mechanisms of GHR activation. However, to date, there is relatively little information about intracellular signalling triggered by anti-GHR antibody. Therefore, in this work, we have developed a panel of monoclonal antibodies to GHBP, among which one Mab, termed CG-172, was selected for further characterisation because of its signalling properties. The results from FACS assays, receptor binding and immunoprecipitation assays and western blotting demonstrated that CG-172 specifically binds to GHR expressed on target cells. Subsequently, epitope mapping studies that used receptor binding analysis showed that CG-172 specifically binds subdomain 1 of GHR ECD. We next examined the resulting signal transduction pathways triggered by this antibody in CHO-GHR638 cells and rat hepatocytes. We found that CG-172 can activate JAK2, AKT, ERK1/2 and STAT1/3 but not STAT5. The phosphorylation kinetics of STAT1/3, AKT and ERK1/2 induced by either GH or CG-172 were analysed in dose-response and time course experiments. Our observations demonstrated that an anti-GHR monoclonal antibody (CG-172) can serve as an attractive tool to study the mechanism(s) of GHR-mediated intracellular signalling pathways and may lead to the production of signal-specific molecules that are capable of inducing different biochemical responses.

The role of prolactin receptor in GH signaling in breast cancer cells.

GH and prolactin (PRL) are structurally related hormones that exert important effects in disparate target tissues. Their receptors (GHR and PRLR) reside in the cytokine receptor superfamily and share signaling pathways. In humans, GH binds both GHR and PRLR, whereas PRL binds only PRLR. Both hormones and their receptors may be relevant in certain human and rodent cancers, including breast cancer. GH and PRL promote signaling in human T47D breast cancer cells that express both GHR and PRLR. Furthermore, GHR and PRLR associate in a fashion augmented acutely by GH, even though GH primarily activates PRLR, rather than GHR, in these cells. To better understand PRLR's impact, we examined the effects of PRLR knockdown on GHR availability and GH sensitivity in T47D cells. T47D-ShPRLR cells, in which PRLR expression was reduced by stable short hairpin RNA (shRNA) expression, were compared with T47D-SCR control cells. PRLR knockdown decreased the rate of GHR proteolytic turnover, yielding GHR protein increase and ensuing sensitization of these cells to GHR signaling events including phosphorylation of GHR, Janus kinase 2, and signal transducer and activator of transcription 5 (STAT5). Unlike in T47D-SCR cells, acute GH signaling in T47D-ShPRLR cells was not blocked by the PRLR antagonist G129R but was inhibited by the GHR-specific antagonist, anti-GHR(ext-mAb). Thus, GH's use of GHR rather than PRLR was manifested when PRLR was reduced. In contrast to acute effects, GH incubation for 2 h or longer yielded diminished STAT5 phosphorylation in T47D-ShPRLR cells compared with T47D-SCR, a finding perhaps explained by markedly greater GH-induced GHR down-regulation in cells with diminished PRLR. However, when stimulated with repeated 1-h pulses of GH separated by 3-h washout periods to more faithfully mimic physiological GH pulsatility, T47D-ShPRLR cells exhibited greater transactivation of a STAT5-responsive luciferase reporter than did T47D-SCR cells. Our data suggest that PRLR's presence meaningfully affects GHR use in breast cancer cells.

Expression of the growth hormone/insulin-like growth factor axis during Balb/c thymus ontogeny and effects of growth hormone upon ex vivo T cell differentiation.

AIMS: We address the question of the expression and the role of the growth hormone/insulin-like growth factor (GH/IGF) axis in the thymus. METHODS: Using RT-qPCR, the expression profile of various components of the somatotrope GH/IGF axis was measured in different thymic cell types and during thymus embryogenesis in Balb/c mice. The effect of GH on T cell differentiation was explored via thymic organotypic culture. RESULTS: Transcription of Gh, Igf1, Igf2 and their related receptors predominantly occurred in thymic epithelial cells (TEC), while a low level of Gh and Igf1r transcription was also evidenced in thymic T cells (thymocytes). Gh, Ghr, Ins2, Igf1, Igf2, and Igfr1 displayed distinct expression profiles depending on the developmental stage. The protein concentrations of IGF-1 and IGF-2 were in accordance with the profile of their gene expression. In fetal thymus organ cultures (FTOC) derived from Balb/c mice, treatment with exogenous GH resulted in a significant increase of double negative CD4-CD8- T cells and CD4+ T cells, together with a decrease in double positive CD4+CD8+ T cells. These changes were inhibited by concomitant treatment with GH and the GH receptor (GHR) antagonist pegvisomant. However, GH treatment also induced a significant decrease in FTOC Gh, Ghr and Igf1 expression. CONCLUSION: These data show that the thymotropic properties of the somatotrope GH/IGF-1 axis involve an interaction between exogenous GH and GHR expressed by TEC. Since thymic IGF-1 is not increased by GH treatment, the effects of GH upon T cell differentiation could implicate a different local growth factor or cytokine.

Inhibitory GH receptor extracellular domain monoclonal antibodies: three-dimensional epitope mapping.

GH receptor (GHR) mediates the anabolic and metabolic effects of GH. We previously characterized a monoclonal antibody (anti-GHR(ext-mAb)) that reacts with subdomain 2 of the rabbit GHR extracellular domain (ECD) and is a conformation-specific inhibitor of GH signaling in cells bearing rabbit or human GHR. Notably, this antibody has little effect on GH binding and also inhibits inducible metalloproteolysis of the GHR that occurs in the perimembranous ECD stem region. In the current study, we demonstrate that anti-GHR(ext-mAb) inhibits GH-dependent cellular proliferation and also inhibits hepatic GH signaling in vivo in mice that adenovirally express rabbit GHR, as assessed with our noninvasive bioluminescence hepatic signaling assay. A separate monoclonal antibody (anti-GHR(mAb 18.24)) is a sister clone of anti-GHR(ext-mAb). Here, we demonstrate that anti-GHR(mAb 18.24) also inhibits rabbit and human GHR signaling and inducible receptor proteolysis. Further, we use a random PCR-generated mutagenic expression system to map the three-dimensional epitopes in the rabbit GHR ECD for both anti-GHR(ext-mAb) and anti-GHR(mAb 18.24). We find that each of the two antibodies has similar, but nonidentical, discontinuous epitopes that include regions of subdomain 2 encompassing the dimerization interface. These results have fundamental implications for understanding the role of the dimerization interface and subdomain 2 in GHR activation and regulated GHR metalloproteolysis and may inform development of therapeutics that target GHR.

Immunogenicity, toxicology, pharmacokinetics and pharmacodynamics of growth hormone ligand-receptor fusions.

A fundamental concern for all new biological therapeutics is the possibility of inducing an immune response. We have recently demonstrated that an LR-fusion (ligand-receptor fusion) of growth hormone generates a potent long-acting agonist; however, the immunogenicity and toxicity of these molecules have not been tested. To address these issues, we have designed molecules with low potential as immunogens and undertaken immunogenicity and toxicology studies in Macaca fascicularis and pharmacokinetic and pharmacodynamic studies in rats. Two variants of the LR-fusion, one with a flexible linker (GH-LRv2) and the other without (GH-LRv3), were tested. Comparison was made with native human GH (growth hormone). GH-LRv2 and GH-LRv3 demonstrated similar pharmacokinetics in rats, showing reduced clearance compared with native GH and potent agonist activity with respect to body weight gain in a hypophysectomized rat model. In M. fascicularis, a low level of antibodies to GH-LRv2 was found in one sample, but there was no other evidence of any immunogenic response to the other fusion protein. There were no toxic effects and specifically no changes in histology at injection sites after two repeated administrations. The pharmacokinetic profiles in monkeys confirmed long half-lives for both GH-LRv2 and GH-LRv3 representing exceptionally delayed clearance over rhGH (recombinant human GH). The results suggest that repeated administration of a GH LR-fusion is safe, non-toxic, and the pharmacokinetic profile suggests that two to three weekly administrations is a potential therapeutic regimen for humans.

Growth hormone in vascular pathology: neovascularization and expression of receptors is associated with cellular proliferation.

Vascular tumours are common lesions of the skin and subcutaneous tissue, but also occur in many other tissues and internal organs. The well-differentiated tumours consist of irregular anastomosing, blood-filled vascular channels that are lined by variably atypical endothelial cells. The less differentiated tumours may show solid strands and sheets, resembling carcinoma or lymphoma. Several growth factors, including basic fibroblast growth factor, transforming growth factors and vascular endothelial growth factor, play a role in tumour angiogenesis. Growth hormone (GH) is mitogenic for a variety of vascular tissue cells, including smooth muscle cells, fibroblasts and endothelial cells and exerts its regulatory functions in controlling metabolism, balanced growth and differentiated cell expression by acting on specific membrane-bound receptors, which trigger a phosphorylation cascade resulting in the modulation of numerous signalling pathways and of gene expression. Essential to the initiation of a cellular response to GH, the presence of receptors for this hormone may predict the adaptation of tumour cells resulting from GH exposure. To address the site/mode of action through which GH exerts its effects, a well characterized monoclonal antibody, obtained by hybridoma technology from Balb/c mice immunized with purified rabbit and rat liver GH-receptor (GHR) and directed against the hormone binding site of the receptor, was applied, using the ABC technique to determine GHR expression in a panel of vascular tumours. The GHR was cloned from a rabbit liver cDNA library with the aid of an oligonucleotide probe based on a 19 residue tryptic peptide sequence derived from 5900 fold purified rabbit liver receptor. A total of 64 benign and malignant vascular tumours were obtained from different human organ sites, including the chest wall, skin, axillary contents, duodenum, female breast, abdomen, stomach, colon, lymph node, bladder, body flank and neck regions. The tumours were of the following pathological entities: Haemangioma (n = 12); haemangioendothelioma (n = 10); Castleman's disease (n = 3), haemangiopericytoma (n = 4); angiosarcoma, (n = 11), Kaposi's sarcoma with focal infiltration by lymphoma, HIV +ve (n = 7), Kaposi's sarcoma (n = 17). The endothelial cell marker CD-31 was used to establish endothelial cell characteristics and microvascular density. To delineate tumour cell growth, immunohistochemical analysis of cycling nuclear protein and of proliferating cell nuclear antigen, using Ki-67 and PCNA polyclonal antibodies respectively, was used to demonstrate proliferative indexes. Results show that, compared to their normal tissue counterparts, nuclear and cytoplasmic expression of GHR consistently result in strong receptor immunoreactivity in the highly malignant angiosarcomas and Kaposi's sarcomas and was localized in the cell membranes and cytoplasm, but strong nuclear immunoreactivity was also identified. The presence of intracellular GHR is the result of endoplasmic reticulum and Golgi localization. Nuclear localization is due to identical nuclear GHR-binding protein. Furthermore, there was a positive correlation of GHR immunoreactivity with neoplastic cellular proliferation and cycling, as measured by Ki-67 and PCNA. In conclusion, this study shows that GHR expression in vascular tumours is a function of malignancy and cancer progression. Malignant cells, which are highly expressive of the receptor, have a greater proliferation rate and thereby also higher survival rate compared to tumours expressing lower or minimal receptor level. The presence of GHR in endothelial cells of vascular neoplasm indicates that they are target cells and GH is of importance in the proliferation of vascular tumour angiogenesis. GH is necessary not only for differentiation of progenitor cells, but also for their subsequent clonal expansion and maintenance. The results support the hypothesis that GH is involved in the paracrine-autocrine mechanism, acting locally in regulating vascular tumour growth and will be useful for site-specific studies of the evolution of vascular cancers. The use of anti-GHR antibodies to block tumour progression is an intriguing possibility.

A conformationally sensitive GHR [growth hormone (GH) receptor] antibody: impact on GH signaling and GHR proteolysis.

The GH receptor (GHR) mediates metabolic and somatogenic actions of GH. Its extracellular domain (ECD; residues 1-246) has two subdomains, each with seven beta strands organized into two antiparallel beta sheets, connected by a short hinge region. Most of the ECD residues involved in GH binding reside in subdomain 1, whereas subdomain 2 harbors a dimerization interface between GHR dimers that alters conformation in response to GH. A regulated GHR metalloprotease cleavage site is in the membrane-proximal stem region of subdomain 2. We have identified a monoclonal anti-ECD antibody, anti-GHR(ext-mAb), which recognizes the rabbit and human GHRs by immunoprecipitation, but less so after GH treatment. By immunoblotting and immunoprecipitation, anti-GHR(ext-mAb) recognized a glutathione-S-transferase (GST) fusion incorporating subdomain 2, but not one including subdomain 1. In transient transfection experiments, anti-GHR(ext-mAb) failed to recognize by immunoprecipitation a previously characterized dimerization interface mutant GHR that is incompetent for signaling. In signaling experiments, brief pretreatment of GH-responsive human fibrosarcoma cells with anti-GHR(ext-mAb) dramatically inhibited GH-induced Janus kinase 2 and signal transducer and activator of transcription 5 tyrosine phosphorylation and prevented GH-induced GHR disulfide linkage (a reflection of GH-induced conformational changes). In contrast, anti-GHR(ext-mAb) only partially inhibited radiolabeled GH binding, suggesting its effects on signaling were not simply via inhibition of binding. Furthermore, anti-GHR(ext-mAb) prevented phorbol ester-stimulated GHR proteolysis, but GHR cleavage site mutants were normally recognized by the antibody, indicating that the stem region cleavage site is not a direct epitope. A Fab fragment of anti-GHR(ext-mAb) inhibited GH-induced GHR disulfide linkage and signaling, as well as phorbol ester-induced GHR proteolysis, in a fashion similar to the intact antibody. Thus, our findings suggest that anti-GHR(ext-mAb) has promise as a GH antagonist and as a tool in studies of conformational changes required for GHR activation.

Epitope map for a growth hormone receptor agonist monoclonal antibody, MAb 263.

Monoclonal antibody (MAb) 263 is a widely used monoclonal antibody that recognizes the extracellular domain (ECD) of the GH receptor. It has been shown to act as a GH agonist both in vitro and in vivo, and we report here that it must be divalent to exert its effect on the full-length receptor. To understand the mechanism of its agonist action, we have determined the precise epitope for this antibody using a novel random PCR mutagenesis approach together with expression screening in yeast. A library of 5200 clones of rabbit GH receptor ECD mutants were screened both with MAb 263 and with an anticarboxy-tag antibody to verify complete ECD expression. Sequencing for clones that expressed complete ECD but were not MAb 263 positive identified 20 epitope residues distributed in a discontinuous manner throughout the ECD. The major part of the epitope, as revealed after mapping onto the crystal structure model of the ECD molecule, was located on the side and upper portion of domain 1, particularly within the D-E strand disulfide loop 79-96. Molecular dynamics docking of an antibody of the same isotype as MAb 263 was used to dock the bivalent antibody to the 1528-A2 epitope and to visualize the likely consequences of MAb binding. The minimized model enables the antibody to grasp two receptors in a pincer-like movement from opposite sides, facilitating alignment of the receptor dimerization domains in a manner similar to, but not identical with, GH.

The thymus gland: a target organ for growth hormone.

Increasing evidence has placed hormones and neuropeptides among potent immunomodulators, in both health and disease. Herein, we focus on the effects of growth hormone (GH) upon the thymus. Exogenous GH enhances thymic microenvironmental cell-derived secretory products such as cytokines and thymic hormones. Moreover, GH increases thymic epithelial cell (TEC) proliferation in vitro, and exhibits a synergistic effect with anti-CD3 in stimulating thymocyte proliferation, which is in keeping with the data showing that transgenic mice overexpressing GH or GH-releasing hormone exhibit overgrowth of the thymus. GH also influences thymocyte traffic: it increases human T-cell progenitor engraftment into the thymus; augments TEC/thymocyte adhesion and the traffic of thymocytes in the lymphoepithelial complexes, the thymic nurse cells; modulates in vivo the homing of recent thymic emigrants, enhancing the numbers of fluroscein isothiocyanate (FITC)+ cells in the lymph nodes and diminishing them in the spleen. In keeping with the effects of GH upon thymic cells is the detection of GH receptors in both TEC and thymocytes. Additionally, data indicate that insulin-like growth factor (IGF)-1 is involved in several effects of GH in the thymus, including the modulation of thymulin secretion, TEC proliferation as well as thymocyte/TEC adhesion. This is in keeping with the demonstration of IGF-1 production and expression of IGF-1 by TEC and thymocytes. Also, it should be envisioned as an intrathymic circuitry, involving not only IGF-1, but also GH itself, as intrathymic GH expression is seen both in TEC and in thymocytes, and that thymocyte-derived GH could enhance thymocyte proliferation. Finally, the possibility that GH improve thymic functions, including thymocyte proliferation and migration, places this molecule as a potential therapeutic adjuvant in immunodeficiency conditions associated with thymocyte decrease and loss of peripheral T cells.

Growth hormone prevents Fas-induced apoptosis in lymphocytes through modulation of Bcl-2 and caspase-3.

OBJECTIVE: Growth hormone (GH) has been reported to have a potent effect on the immune system. However, the detailed mechanism of the effect of GH on the immune system has not yet been clarified. This study was designed to investigate the nature of this mechanism. METHODS: In the present study, we investigated the effects of GH on the susceptibility of both human CEM/C7 lymphocytes and human IM-9 lymphocytes to Fas-induced apoptosis. RESULTS: Both cell lines expressed GH receptor mRNA. GH rescued Fas-induced suppression of [(3)H]-thymidine incorporation into each cell line. GH prevented Fas-induced apoptosis in each cell line without changing Fas antigen expression. We next investigated the mechanisms of the prevention of Fas-induced apoptosis, by focusing on intracellular molecules related to the apoptotic signal. Bcl-2 expression was increased by GH treatment in both CEM/C7 and IM-9 lymphocytes. GH also downregulated caspase-3 expression and inhibited activation of caspase-3 in both cell lines. CONCLUSION: These findings suggest that GH regulates the human immune system through inhibition of Fas-induced apoptosis in activated T and B lymphocytes.

Monoclonal antibody against human growth hormone receptor.

GHR shows a high degree of homology with the prolactin receptor and with the other receptors that belong to the hemopoietic receptor superfamily. This paper describes a monoclonal antibody (MAb) (2B4B6) specific for both the extracellular domain of human GHR and human growth hormone (GH) binding protein. Mice were immunized against a seven-aminoacid peptide sequence screened by FASTA (sequence similarity search served by Genome-Net) from the European Bioinformatics Institute to exclude the existence of human membrane proteins with significant sequence homology. MAbs were screened against the peptide sequence and 2B4B6 was selected for its capability to recognize the full-length hGHBP. As evaluated by both enzyme-linked immunoadsorbent assay (ELISA) and FACS analysis, this MAb seems to recognize and bind to a hGHR positive cell line, IM-9, as well as a murine cell line, BaF3 (8/6), transfected with a chimeric construct, hGHR/hG-CSFR and expressing hGHR on the cell membrane. Studies investigating the biological effects of this MAb showed that anti-hGHR mediated inhibition of cell proliferation was not due to competition with GH binding but rather to prevention of receptor dimerization. Because of its specificity, this MAb may be usefully applied in situations in which GHR and receptors with a high degree of homology, such as PRLR (prolactin receptor), are expressed simultaneously, as occurs in the immune system.

Growth hormone and colorectal carcinoma: localization of receptors.

Growth hormone (GH) plays a crucial role in stimulating and controlling the growth, metabolism and differentiation of many mammalian cell types by modulating the synthesis of multiple mRNA species and a paracrine or autocrine mechanism of action has been proposed. These effects are mediated by the binding of GH to its membrane-bound receptor and involve a phosphorylation cascade that results in the modulation of numerous signaling pathways. To address the side/mode of action through which GH exerts its effects, a panel of well characterized monoclonal antibodies, directed against the hormone binding side of the receptor, was applied to immunohistochemically determine growth hormone receptor (GH-receptor) expression in poorly- moderate- to well differentiated col.orectal adenocarcinomas (n = 40) from the rectum, transverse-, ascending-, descending and sigmoid colons. Of five anti-growth hormone receptor monoclonal antibodies used, human GH- receptor specific Mab 263 consistently resulted in strong receptor expression in colorectal carcinoma tumour cells. Heterogeneity of immunoreactivity was found in primary and secondary tumour lesions with a variable range of positive cells. Staining was mainly intracellular, showing either a monotonous or granular pattern, with some nuclei also reactive. The presence of intracellular GH-receptors has been previously documented and is a result of endoplasmic reticulum and Golgi localization. Immunoreactivity in surface columnar cells, independent from pathological tissue, was weak to moderate. Epithelial cells from normal tissue, adjacent to tumour lesions, were of variable intensity. Goblet and mucous cells located at the crypt base immunostained faintly or were negative for the GH-receptors. Crypt base columnar cells strongly expressed the GH-receptor, but oligomucous cells were less reactive. In conclusion, this study indicates that receptor expression may be associated with malignancy of colorectal carcinoma and supports the hypothesis that GH may act locally in colorectal tissue. The demonstration of the presence of receptors for GH will be useful for site-specific studies of the evolution of gastrointestinal tract tumours, providing valuable information concerning cellular growth kinetics and tumour prognosis. It also raises questions regarding the administration of GH to cancer-induced cachexia patients and the possible oncogenic potential of the GH-receptor.

Structural features and expression of an alternatively spliced growth factor type I receptor for follitropin signaling in the developing ovary.

The pleiotropic actions of pituitary follitropin (FSH), regulate the expression of many cell cycle genes controlling ovarian follicular development and differentiation. In this study we asked the question whether different receptor motifs are created by the alternative splicing of the single large 80-100 Kb receptor gene. A 1.2 Kb transcript identified from a cDNA library of hormone primed (immature) sheep ovaries, codes for a putative protein lacking the seven transmembrane segment. The receptor of 259 amino acids designated FSH-R3 is derived from a transcript comprising the first eight exons of the Gs coupled larger FSH receptor (R1) spliced to another DNA segment. This event produces a different carboxyl terminus at the junction creating a novel receptor motif with a single membrane spanning domain, assigning it to the growth factor type I receptor family. In transfected cells the expressed receptor localizes on the cell surface and specific antibodies directed against the unique C-terminal portion (residues 242-259) of FSH-R3 demonstrate the presence of the receptor protein in solubilized ovarian and testicular membrane preparations. FSH binding to the transfected cells induced [Ca2+]i identifying coupling of the R3 receptor to calcium signaling pathways. Thus, a growth factor type I receptor for FSH may be implicated in the growth promoting actions of FSH in the ovary. This is the first documentation of alternative splicing of a G protein coupled receptor gene creating a different signaling motif for cellular signaling.

Up-regulation of growth hormone receptor immunoreactivity in human melanoma.

Growth hormone (GH) exerts its regulatory functions in controlling metabolism, balanced growth and differentiated cell expression by acting on specific receptors, which trigger a phosphorylation cascade resulting in the modulation of numerous signalling pathways, and dictate gene expression. Immunohistochemical techniques were used to demonstrate the presence of growth hormone receptors in 126 formalin-fixed, paraffin-embedded melanocytic tumours comprising melanocytic naevi, superficial spreading melanoma, nodular melanoma, lentigo maligna melanoma and metastatic melanomas. The relative proportion of positive cells and intensity of staining was higher in neoplastic cells, compared to normal cutaneous cells. Of the 76 cases of common melanocytic naevi (CMN) studies, 46 were weakly reactive with MAb 263. Heterogeneity of immunoreactivity was found in primary melanoma lesions with a variable range of positive cells. Of 37 cases studied, 34 were moderately to strongly positive. Immunoreactivity showed subcellular localization of the GH-receptor in cell membranes, was predominantly cytoplasmic, but strong nuclear immunoreaction was also apparent in many instances. The nuclear localization of immunoreactivity is the result of nuclear GH-receptor/binding protein, identically to the cytosolic and plasma growth hormone binding protein. Intense immuno-reactivity was also observed in the cellular Golgi area of established cell lines and cultured tissue-derived cells in exponential growth phase, indicating cells are capable of GH-receptor synthesis. In the primary lesions, dermal tumour cells tended to be more immunoreactive relative to those seen in the dermal region. Metastatic lesions in various organs also expressed growth hormone receptors in secondary tumour cells and all of the metastatic cases were positive. The expression of GH-receptors in human melanoma cells means that these cells are directly responsive to GH action and that GH may stimulate local production of IGF-I, which then acts in an autocrine mechanism.

Characterization of an antagonist monoclonal antibody, GHBP116, specific for human growth hormone receptors.

To obtain an antagonist antibody against human growth hormone receptors (hGHRs), we prepared monoclonal antibodies against the recombinant hGHR extracellular domain. One of the clones, GHBP116, exhibited binding activity to intact human IM-9 cells and effectively immunoprecipitated the receptors in cell lysate. GHBP116 competitively inhibited 125I-human growth hormone (hGH) binding to the cells. The antagonist activity of GHBP116 was assessed in terms of ligand-induced receptor internalization, degradation, and phosphorylation of signal transducer and activator of transcription (STAT) 5. The antibody alone did not cause internalization or degradation of hGHRs, but a 1:25000 dilution of ascitic fluid almost completely inhibited ligand (1 nM hGH)-induced internalization and degradation of surface hGHRs. Moreover, GHBP116 alone did not stimulate the phosphorylation of STAT5, used as an indicator of Janus kinase (JAK)-STAT signaling, but almost completely inhibited hGH-induced phosphorylation of STAT5. These results suggest that GHBP116 acts as a specific antagonist of hGH.