Heterogeneity of growth-hormone receptors detected with monoclonal antibodies to human growth hormone

Immunomodulatory approaches for regulation of growth and body composition

Hormonal growth promoters (growth hormone (GH), β-adrenergic agonists, steroids) which improve growth rate and/or lean: fat ratios in the carcass have received considerable adverse publicity and are either banned or have no licence for their use in countries of the European Community. This has led to the development of a number of techniques, involving the use of antibodies, aimed at regulating metabolic processes involved in determining growth and body composition.

A number of these approaches have focused upon the GH axis, for example immunoneutralization of somatostatin (which normally inhibits GH secretion) to improve growth, the use of antibodies to GH which can enhance its effects in vivo and the development of antibodies which mimic the actions of GH. Although immunization against somatostatin has led to increased growth rates in a number of studies other studies have failed to demonstrate such an effect. A precise understanding of the mechanism of action of this approach is required before we can begin to understand why success is not assured. Antibodies which enhance GH action clearly do work reproducibly but the major problem in developing this approach is to produce an inexpensive peptide immunogen (its sequence derived from GH) which can be used to actively immunize animals so that their own antibodies enhance endogenous GH activity. Anti-idiotypic mimics of GH have also been produced which have GH actions in vivo but again this approach is of limited value until appropriate vaccines can be developed.

A different approach to the problem of excess fat deposition involves the use of antibodies directed against the plasma membranes of adipocytes in order to elicit their destruction and thereby limit the storage capacity for fat. This technique has been demonstrated in rats, sheep and pigs in both passive and active immunization techniques. Once again, however, this promising approach is limited by the lack of a commercially suitable vaccine. The identification of individual membrane proteins which are antigenic has been achieved and this provides the prospect of producing recombinant DNA-derived vaccines.

Whether these new approaches will be perceived as acceptable to the general public remains a serious concern and a potential limitation to their development as many would-be sponsors cut back their support for research in these areas.

Actions of monoclonal antibodies on the activity of human growth hormone (GH) in an in vitro bioassay

An in vitro bioassay for GH was established, based on the response of the 3T3-F442A mouse preadipocyte cell line, together with a parallel receptor-binding assay using the same cells. The effects of monoclonal antibodies on the biological activity of human GH in vitro were then explored. Antibodies that did not bind GH had no effect on the bioassay or on receptor binding. Antibodies EB1 and EB2, which strongly enhance growth-promoting actions in vivo, inhibited the actions of human GH in the in vitro bioassay, and blocked binding of human GH to receptors. Antibody NA71, which weakly enhances growth promotion by human GH in vivo, enhanced biological activity in vitro but did not affect receptor binding. Thus, enhancement of the biological activity of human GH has been shown in this in vitro system, but the effect does not correlate completely with the established enhancement effects in vivo. Of the various mechanisms that have been proposed to explain the enhancement effect these results support the 'restriction hypothesis'--the idea that monoclonal antibodies may enhance GH action in vivo by preventing binding of GH to receptors/binding sites that are not involved in growth promotion.

Veterinary vaccines

Vaccination of animals for the prevention of infectious diseases has been practised for a number of years with little change in product composition. Recent advances in molecular biology, pathogenesis and immunology have laid the groundwork for the development of a new generation of veterinary vaccines based on pure subunits as well as live vectored bacteria and viruses. Along with novel methods of antigen preparation, the use of new adjuvants and delivery systems will permit targeting of the appropriate immune response as well as offering flexibility in terms of vaccination protocols. These new technologies are also being applied to the development of vaccines to enhance animal productivity and to control reproduction.

Growth hormone signal transduction

Growth hormone (GH) promotes animal growth by stimulating bone and cartilage cell proliferation, and influences carbohydrate and lipid metabolism. Some of these effects are brought about indirectly via somatomedin induction in hepatocytes, others by acting directly on the target cells. In either case, GH first binds to specific receptors on cells to trigger a sequence of biochemical events culminating in a biological response. Recently much has been learnt about the molecular structure of GH receptor, its binding to ligand, and the ensuing signal transduction events.

Immunological manipulation of adiposity

Although hormonal regulators of adiposity are available they as yet have not been licensed for use. Withdrawal periods and delivery systems are still potential problems in maximizing their effectiveness. Immunization techniques, on the other hand, suffer none of the problems of withdrawal periods or requirement for frequent injection/implantation. As such they are clearly perceived as safe, economic and should have a positive animal welfare image. They are, however, not without their problems. Active immunization in particular involves an autoimmune response and this is typically difficult to evoke and virtually impossible to regulate. In addition, the fact that antibodies may have immunoneutralizing and immunoenhancing properties may explain the apparently contradictory results obtained in various studies as, for example, in the case of immunization against somatostatin. As our knowledge of immune responsiveness and its control increases, however, the possibilities for immune intervention should increase considerably. We may then be faced with ethical rather than practical limitations as to how far we should manipulate growth and body composition.

Modulation of human growth hormone binding to somatogenic and lactogenic receptors by monoclonal antibodies to human growth hormone

The relationship between the structure of human growth hormone (hGH) and the hormone-receptor interaction was investigated by studying the effects of specific monoclonal antibodies (MAbs) to hGH on the binding of [125I]hGH to rabbit liver and mouse liver microsomes. Receptor binding assays were carried out using a constant dose (1 ng) of [125I]hGH and varying concentrations of MAbs. The assay was carried out in the presence of either excess ovine prolactin for the measurement of somatogenic (SOM) binding sites, or excess bovine growth hormone for the determination of lactogenic (LAC) binding sites. Anti-hGH MAbs were found to have a whole spectrum of effects on hGH binding, including inhibitory, non-effect and enhancing activities. Enhancement of the binding of [125I]hGH to both SOM and LAC receptors was observed in liver membranes of rabbit or mouse. The observed amplified signal of [125I]hGH binding to various receptors in the presence of MAb no. 8 may be due to conformational changes which occur following MAb binding to hGH. On the other hand, most of the other MAbs caused inhibition of [125I]hGH binding. A negative correlation exists between the cross-reaction of various MAbs with the N-terminus truncated forms of hGH (Met14-hGH or Met8Leu-hGH) and their respective KD/IC50 values enabled the evaluation of the crucial role of the N-terminus region in hGH binding to both LAC and SOM receptors. MAb nos 1 and 19, which are directed towards acid residues 95-134 and the C-terminus, inhibited SOM binding more potently than LAC binding. Thus, it seems that these mid-molecule and C-terminus regions are also important in hGH binding, and that they play a role in the partial overlap of SOM and LAC binding.

The growth hormone-insulin-like growth factor I axis: studies in man during growth

Several major differences are noted between males and females in their patterns of growth at puberty. Accelerated pubertal growth in both males and females depends upon the integrity of the GH-receptor system. In males, acceleration of growth results primarily from enhanced sensitivity of the GH-receptor-IGF I system to GH brought about by testosterone. Whether testosterone itself is responsible for this observation is still unclear. Perhaps the initial GH, IGF I peak present in males and absent in females occurs at the time when sleep-related rises of gonadotropins and testosterone begin just prior to puberty. Though the pygmy data certainly supports a relationship between testosterone and the GH-receptor-IGF I axis, the undisputed tall stature of eunuchs remains a puzzle. It is possible that the maturing male gonad secretes another growth factor and/or growth inhibitor in conjunction with testosterone and that it is this unidentified factor which modulates growth. At any rate, acceleration of growth in males results from sensitization or the GH-receptor-IGF I system while growth acceleration in females results almost solely from increased secretion of GH and not sensitization of the system.

Soluble forms of the rabbit adipose tissue and liver growth hormone receptors are antigenically identical, but the integral membrane forms differ

Cytosolic, detergent-solubilized and membrane-bound growth hormone (GH) receptors from rabbit adipose tissue and liver were tested for reactivity with a panel of monoclonal antibodies (MAbs). The cytosolic and detergent-solubilized forms of adipose tissue and liver GH receptors were identically reactive with four precipitating and two hormone-binding-site-directed MAbs. However, the membrane-bound form of the adipose receptor was 1000-fold less reactive with one binding-site-directed MAb (MAb 7) than the membrane-bound liver GH receptor. Reactivity with another inhibitory MAb (MAb 263) was identical for adipose tissue and liver membrane GH receptors. The relative potency of 22,000-Mr and 20,000-Mr forms of human GH was identical in assays with liver and adipose tissue membrane receptors. Thus, contrary to earlier suggestions, the discrepancy between the growth-promoting and insulin-like activities of 20,000-Mr human GH cannot be rationalized by a difference in the affinity of this hormone for 'somatogenic' and 'metabolic' receptors when the comparison is made in the same species. Cross-linking studies showed that the major GH-binding subunit of liver and adipose tissue GH receptors had the same Mr (54,000 +/- 5000, reduced). The ligand-binding subunits of liver and adipose tissue receptors are identical by several criteria, but one epitope on the adipose tissue receptor appears to be masked upon membrane insertion, possibly by close association with a tissue-specific component. Tissue specificity may be determined by association of a ubiquitous GH-binding subunit with tissue-specific membrane components, rather than by differences in amino acid sequence.

Monoclonal antibodies to the pituitary growth-hormone receptor by the anti-idiotypic approach. Production and initial characterization

We obtained 10/192 and 3/384 antibody-secreting hybrids after immunization of Balb/c mice with either human growth hormone or affinity-purified rabbit anti-(human growth hormone) respectively. Radiolabelled rabbit anti-(human growth hormone) antibodies, but not human growth hormone, were specifically bound by supernatants from the 13 hybrids. The binding was completely inhibited by human-growth-hormone serum binding protein. However, anti-(human growth hormone antibodies) were detected in the sera of all the mice immunized with human growth hormone. In an independent fusion, which was carried out after immunization with fewer doses of human growth hormone, anti-(human growth hormone) antibodies were also obtained. Five hybrids, where the starting antigen was human growth hormone, were selected for ascites production, and the corresponding monoclonal antibodies were partially purified and characterized with respect to their immunoglobulin isotype and their interaction with human-growth-hormone receptors. These antibodies were found to enhance the binding of radioiodinated human growth hormone to human-growth-hormone serum binding protein from human and rabbit plasma by 40%. Scatchard analysis of the effect of one of the monoclonal antibodies showed that this enhancement was due to an increased number of binding sites. All of the partially purified antibodies but one (F12) inhibited the binding of human growth hormone to rat but not rabbit, liver microsomes to various extents, as well as to H-4-II-E rat hepatoma cells. Monoclonal antibody F12 enhanced the binding of radiolabelled human growth hormone to rat liver microsomes and H-4-II-E hepatoma cells. This enhancement was found to be due to an increase in the number of binding sites.

Indication of different lactogen and somatogen binding sites in the human growth hormone molecule as probed with monoclonal antibodies

The relationship between the structure of human growth hormone (hGH) and the hormone-receptor interaction has been investigated using as probes monoclonal antibodies (Mabs) to hGH of defined epitope specificity profile. Seven high affinity Mabs were studied for their ability (i) to inhibit the binding of 125I-hGH to Nb2-SP rat lymphoma cells and to IM-9 human lymphocytes possessing lactogen and somatogen type receptors, respectively; and (ii) to interfere with the hormone (hGH or Met8Leu hGH)-induced proliferation in Nb2-11C lymphoma cells. The ability of these Mabs to inhibit the 125I-hGH binding and the hormone-induced proliferation in Nb2-11C cells was negatively correlated with the ability of these Mabs to cross-react with met14 hGH. Furthermore, Mabs Nos. 3 and 7, which cross-reacted minimally (0.2-0.4%) with Met8Leu hGH, were unable to interfere with the mitogenic activity of Met8Leu hGH in Nb2-11C cells. These results indicate that the first 13 amino acids of the N-terminal region of hGH are necessary for its lactogen activity. The inhibition of 125I-hGH binding to IM-9 cells by these Mabs was similar to those observed in Nb2-SP cells, except for Mabs Nos. 19 and 1. These Mabs inhibited more strongly the binding of 125I-hGH to IM-9 than to Nb2 cells and recognized antigenic epitopes close to the C-terminal part of the molecule. These results suggest that the somatogen receptor binding site of hGH may be located on two sites, one at the N-terminal and the other one close to the C-terminal, while the lactogen receptor is mainly confined to the N-terminal part.

Antibody-mediated enhancement of hormone activity

The enhancement of hormone activity by antibodies has been known for many years; however, investigation into the molecular basis of the phenomenon has only recently begun. A number of mechanisms for this enhancement, including "buffering" or slow release, bivalency and Fc region, and conformational and receptor "restriction" effects, have been documented or proposed. The availability of panels of monoclonal antibodies of distinct combining site specificity have aided in these studies and contributed substantially to our understanding of hormone-receptor interactions.

Growth hormone-albumin conjugates. Reduced renal toxicity and altered plasma clearance

The effective therapeutic use of many small peptides such as growth hormone has been limited by their small molecular masses and rapid clearance by the kidneys. Moreover, various degrees of nephrotoxicity have been reported for small proteins which are readily filtered at the level of the glomerulus. We have attempted to circumvent this drawback by conjugating growth hormone (somatotropin) to serum albumin in an effort to alter the peptide's pharmacokinetics while retaining its biological activity.

Use of calcium dependence as a means to study the interaction between growth hormones and their binding proteins in rabbit liver

The affinity of 22,000-Mr human growth hormone (22 K-hGH) for GH binding proteins in rabbit liver is increased approx. 19-fold by 25 mM-Ca2+. In contrast, ovine growth hormone (oGH) binding is Ca2+-independent up to 10 mM, and decreased by greater Ca2+ concentrations. The 20,000-Mr hGH variant (20K-hGH), lacking residues 32-46, exhibits intermediate behaviour. Without Ca2+ there is a residual 40% of maximum specific binding to liver microsomes, and this increases to 65% with liver cytosolic GH binding proteins. In contrast with 22K-hGH, Scatchard analysis of 20K-hGH binding to liver microsomes produces curvilinear plots in the presence of 25 mM-Ca2+. From these results and inhibition studies with monoclonal antibodies to the GH binding proteins, it is concluded that deletion of the region 32-46 from 22K-hGH has eliminated one component of high-affinity Ca2+-potentiable binding. The Ca2+-mediated increase in Ka for the 22K-hGH-binding protein interaction is consistent with convergence of unit negative charges on the hormone and binding protein towards an intercalated Ca2+ ion. A positive charge in the critical region of nonprimate GHs would render their interactions Ca2+-independent and of lower Ka compared with 22K-hGH. A likely candidate for the negatively charged interactive residue is glutamate-33, since it is unique to human GH and is replaced by a positively charged arginine in non-primate GHs. Its absence in 20K-hGH could explain the altered calcium-dependence of 20K-hGH binding to what is probably the type 2 binding protein [Barnard & Waters (1986) Biochem. J. 237, 885-892]. The Ca2+-dependence of 20K-hGH binding to a subset of GH binding proteins provides both a verification and a mechanistic basis for the proposal [Hughes, Tokuhiro, Simpson & Friesen (1983) Endocrinology (Baltimore) 113, 1904-1906] that 20K-hGH binds with high affinity to only a subset of binding proteins in rabbit liver membranes.

Growth hormone receptors

The receptor for rabbit liver growth hormone (GH) has recently been purified and cloned. Sequencing data demonstrates that this site represents a new class of transmembrane receptor proteins containing covalently linked ubiquitin and N-linked oligosaccharides. Homology with the human GH liver receptor is estimated to be 84%. GH receptors are also present on non-liver cells, however, their function is poorly understood, and it is not yet known if there is homology with the liver site. Direct stimulation of differentiation of preadipocytes to adipocytes by GH has been demonstrated. In bone, binding of GH to the resting cell zone, but not the proliferative layers has been reported. Taken together, these data support the hypothesis of Green and colleagues that GH has a dual mechanism of action which includes direct stimulation of differentiation of precursor cells and a secondary effect of enhancing the clonal proliferative response.

The molecular basis of growth hormone deficiency

A well-known law states that 'if a thing can go wrong it will go wrong'. This clearly applies to the hypothalamic-pituitary-somatic axis as to many other physiological and biochemical systems. Defects of this axis, giving rise to stunted growth, can occur at several different points, as has been discussed in detail in this review. Defects at the level of the brain can lead to inadequate production or secretion of the factors that control growth hormone secretion. Defects at the level of the pituitary can lead to failure to produce or secrete adequate quantities of growth hormone, or to production of inactive hormone. Defects at the level of target organs can lead to inability to respond to growth hormone or somatomedins. The axis involved in the production and effects of growth hormone is a complex one, and defects have been identified at most of the points that 'could go wrong', although in many cases the molecular details are far from fully understood. Increased understanding of the biochemistry and physiology of the hormonal control of growth, and of the impairments to which it is subject, should provide an improved basis for treatment of growth defects. Nevertheless, there remain many points at which our knowledge is very incomplete. The field is a rapidly moving one and further developments in both basic understanding and clinical treatment are to be expected during the next few years.