Functional characterization of monoclonal antibodies specific to growth hormone receptor

Development and Characterization of a Novel Anti-idiotypic Monoclonal Antibody to Growth Hormone, Which Can Mimic Physiological Functions of Growth Hormone in Primary Porcine Hepatocytes

B-32 is one of a panel of monoclonal anti-idiotypic antibodies to growth hormone (GH) that we developed. To characterize and identify its potential role as a novel growth hormone receptor (GHR) agonist, we determined that B-32 behaved as a typical Ab2β based on a series of enzyme-linked immunosorbent assay assays. The results of fluorescence-activated cell sorting, indirect immunofluorescence and competitive receptor binding assays demonstrated that B-32 specifically binds to the GHR expressed on target cells. Next, we examined the resulting signal transduction pathways triggered by this antibody in primary porcine hepatocytes. We found that B-32 can activate the GHR and Janus kinase (2)/signal transducers and activators of transcription (JAK2/STAT5) signalling pathways. The phosphorylation kinetics of JAK2/STAT5 induced by either GH or B-32 were analysed in dose-response and time course experiments. In addition, B32 could also stimulate porcine hepatocytes to secrete insulin-like growth factors-1. Our work indicates that a monoclonal anti-idiotypic antibody to GH (B-32) can serve as a GHR agonist or GH mimic and has application potential in domestic animal (pig) production.

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 activation and differential signalling of the growth hormone receptor induced by pGH or anti-idiotypic monoclonal antibodies in primary rat hepatocytes

In this report, we have developed a panel of monoclonal anti-idiotypic antibodies to pGH by immunising BALB/c mice with a purified monoclonal anti-pGH antibody (1A3), among which one mAb, termed CG-8F, was selected for further characterisation. We found that CG-8F behaved as a typical Ab2β, not only conformationally competing with pGH for 1A3 but also exhibiting recognition for GHR in a rat hepatocyte model. We next examined the resulting signal transduction pathways triggered by this antibody in rat hepatocytes and found that both pGH and CG-8F could trigger the JAK2-STAT1/3/5-mediated signal transduction pathway. Furthermore, the phosphorylation kinetics of pSTAT1/3/5 induced by either pGH or CG-8F were remarkably similar in the dose-response and time course rat hepatocyte experiments. In contrast, only pGH, but not CG-8F, was capable of inducing ERK phosphorylation. Further experimental studies indicated that the two functional binding sites on CG-8F are required for GHR activation. This study partially reveals the mechanism of action of GH anti-idiotypic antibodies and also indicates that monoclonal anti-idiotypic antibodies represent an effective way to produce GH mimics, suggesting that it is possible to produce signal-specific cytokine agonists using an anti-idiotypic antibody approach.

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.

Cytosolic malate dehydrogenase confers selectivity of the nucleic acid-conducting channel

We have described previously a cell surface channel that is highly selective for nucleic acids. Nucleic acid conductance is 10 pS and the channel is at least 10,000-fold more selective for oligodeoxynucleotides than any anion tested (1). Herein we provide evidence that the nucleic acid-conducting channel (NACh) is a heteromultimeric complex of at least two proteins; a 45-kDa pore-forming subunit (p45) and a 36-kDa regulatory subunit (p36). Reconstitution of p45 in planar lipid bilayers resulted in formation of a channel which gated in the absence of nucleic acid and which was more selective for anions (including oligonucleotide) than cations. This channel exhibited transitions from one level of current to another (or to the closed state); however the incidence of transitions was rare. Channel activity was not observed when p36 was reconstituted alone. Reconstitution of p36 with p45 restored nucleic acid dependence and selectivity to the channel. Protein sequence analysis identified p36 as cytosolic malate dehydrogenase (cMDH). Experiments were performed to prove that cMDH is a regulatory subunit of NACh. Selective activity was observed when p45 was reconstituted with pig heart cMDH but not with mitochondrial MDH. Both the enzyme substrate l-malate and antiserum raised against cMDH block NACh activity. These data demonstrate that a nucleic acid conducting channel is a complex of at least two proteins, p45 and cMDH. Furthermore, these data establish that cMDH confers nucleic acid selectivity of the channel.

Augmentation of hormonal activities with antibodies from cattle immunized with a combination of synthetic and recombinant growth hormone peptide

Antibodies generated against a synthetic growth hormone (GH) peptide in a number of animal species were shown to enhance the efficacy of GH. However, the ability to produce the effective antibodies diminished over time and repeated boosters failed to overcome the hurdle. Therefore, this study was designed to address the issue on the failed antibody responses by employing different GH peptide antigen preparations in cattle. Holstein steers were repeatedly immunized with a synthetic peptide corresponding to an amino acid sequence 54-95 of porcine GH (pGH). The peptide was conjugated to ovalbumin (OVA) as a carrier. Animals initially responded to the antigen well and elicited antibodies specific to the peptide. However, the 4th challenge with the same OVA-peptide antigen rendered animals unresponsive, resulting in a decline in antibody production. This unresponsiveness was overcome by switching the antigen at the 5th immunization from OVA-peptide to a recombinant peptide preparation which was composed of maltose binding protein (MBP) as a carrier. Antibodies generated in cattle after the 5th immunization recognized not only the pGH(54-95) peptide, but also bovine GH (bGH) and pGH. These antibodies were not immunoreactive with an unrelated control peptide. Hypophysectomized (hypox) rats were used for functional analysis and bGH was active in promoting the growth of these GH-deficient rats. The growth-promoting effect of bGH was significantly enhanced by mixing it with bovine anti-peptide antibodies prior to administration. Therefore, the present findings suggest that peptide 54-95 induces cattle to elicit antibodies capable of not only recognizing bGH but also augmenting the somatogenic effectiveness of bGH in hypox rats.

Augmentation of hormonal activities with antibodies from cattle immunized with a combination of synthetic and recombinant growth hormone peptide

Antibodies generated against a synthetic growth hormone (GH) peptide in a number of animal species were shown to enhance the efficacy of GH. However, the ability to produce the effective antibodies diminished over the time and repeated boosters failed to overcome the hurdle. Therefore, this study was designed to address the issue on the fallen antibody responses by employing different GH peptide antigen preparations in cattle. Holstein steers were repeatedly immunized with a synthetic peptide corresponding to an amino acid sequence 54-95 of porcine GH (pGH). The peptide was conjugated to ovalbumin (OVA) as a carrier. Animals initially responded to the antigen well and elicited antibodies specific to the peptide. However, the 4th challenge with the same OVA-peptide antigen rendered animals unresponsive, resulting in a decline in antibody production. This unresponsiveness was overcome by switching the antigen at the 5th immunization from OVA-peptide to a recombinant peptide preparation which was composed of maltose binding protein (MBP) as a carrier. Antibodies generated in cattle after the 5th immunization recognized not only the pGH(54-95) peptide, but also bovine GH (bGH) and pGH. These antibodies were not immunoreactive with an unrelated control peptide. Hypophysectomized (hypox) rats were used for functional analysis and bGH was active in promoting the growth of these GH-deficient rats. The growth-promoting effect of bGH was significantly enhanced by mixing with bovine anti-peptide antibodies prior to administration. Therefore, the present findings suggest that peptide 54-95 induces cattle to elicit antibodies capable of not only recognizing bGH but also augmenting the somatogenic effectiveness of bGH in hypox rats.