Serum growth hormone-binding protein (GHBP) in domestic animals as measured by ELISA

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.

Inability of heterologous growth hormone (GH) to regulate GH binding protein in GH-transgenic swine

Transgenic pigs expressing bovine, ovine, or human growth hormone (GH) structural genes fused to mouse metallothionein-I (mMT-bGH), ovine MT (oMT-oGH), or mouse transferrin (mTf-hGH) promoters were used to study the effects of GH on the regulation of serum GH-binding protein (GHBP). In the 14 transgenic pigs studied, circulating concentrations of heterologous GH ranged from 15 to 2,750 ng/mL. Using chromatographic methods, specific binding of GH was detected in serum from normal pigs but was undetectable in serum from all the transgenic pigs used, probably as a result of the high serum concentrations of heterologous GH present in these animals. Thus, to avoid interference of binding by high GH concentrations, serum samples were subjected to immunoblotting using a specific anti-GHBP antibody. A specific 54-kDa band was detected in normal pig serum as well as in sera from mMT-bGH, oMT-oGH, and mTf-hGH pigs. Additionally, sera from transgenic mMT-bGH pigs and their sibling controls were subjected to immunoprecipitation with an anti-GHBP antibody followed by immunoblotting with the same antibody. With this technique, we detected two specific bands of 53 and 45 kDa that could represent different degrees of glycosylation of GHBP. As determined by densitometric analysis the amount of GHBP in transgenic pig sera was similar to that detected in sera of the respective control animals. The amount of circulating GHBP remained unchanged even in oMT-oGH and mTf-hGH pigs that were exposed from birth to circulating concentrations of GH as high as 2,750 ng/mL. Thus, we conclude that heterologous GH do not act as modulators ofthe serum GHBP in pigs.

Growth hormone binding protein 2001

The present state of knowledge about growth hormone binding proteins (GHBP) is reviewed, with particular emphasis on the high affinity GHBP, which represents the circulating ectodomain of the growth hormone receptor (GHR). GHBP is conserved through vertebrate evolution, is produced in many tissues (especially liver) by either alternative GHR mRNA splicing (rodents) or by proteolytic cleavage from the GHR (humans, rabbits and several other species). The metalloprotease TACE (tumor necrosis factor-alpha converting enzyme) is the likely enzyme responsible for cleavage, but the structural requirements for TACE recognition or catalysis, and hence the precise cleavage point in the GHR, are unknown. GHBP is widely distributed in biological fluids, with marked concentration differences amongst them. GHBP binds about half of the circulating GH under basal conditions but is easily saturated at high GH levels; it subserves complex functions, including a circulating buffer/reservoir function for GH, prolongation of plasma GH half-life, competition with GHRs for GH, and probably unproductive heterodimer formation with the GHR. The net effect of these partly enhancing and partly inhibitory functions on GH action in vivo is complex and difficult to ascertain. Serum GHBP levels roughly parallel GHR expression (particularly in liver) through the life span, with very low levels in fetal life, upregulation to adult levels during childhood, and decline in senescence. Rodent pregnancy is associated with a massive increase in GHBP expression. Although the regulation of GHBP expression/production is not necessarily tightly linked to GHR expression, in general, low GHBP levels occur in conditions associated with GH resistance (e.g., malnutrition, uncontrolled diabetes, catabolic states, renal failure, hypothyroidism). Conversely, obesity, a condition with enhanced GH responsivity, is associated with elevated GHBP levels. This suggests that in many (but not all) instances of abnormal GH action, the GHBP level reflects GHR status. Laron syndrome (genetic GHR deficiency/dysfunction due to mutations in the GHR gene) is associated with low or undetectable GHBP in 75-80% of patients; GHBP measurement can therefore be used diagnostically. Depending on the design of assays for serum GH, endogenous GHBP may interfere to varying degrees, and GH assays should be individually validated and optimized in this regard. The ultimate biological role and physiological significance of the GHBP remain to be established.

Hormones, mammary growth, and lactation: a 41-year perspective

When I was a beginning graduate student 41 yr ago it had been established that estrogen caused mammary duct growth; a combination of estrogen and progesterone was required for lobule-alveolar development of the mammary glands; and prolactin and growth hormone were essential for mammary growth. In laboratory species exogenous prolactin, glucocorticoids, and estrogen would initiate secretion of milk provided the mammary glands had a well-developed lobule-alveolar system. It was not known with certainty that progesterone inhibited the process. For some species, prolactin and thyroxine had been shown to stimulate lactation, while glucocorticoids suppressed lactation. Definitive roles for growth hormone and insulin during lactation had not been established. Studies of hormonal control of mammary growth and function in cattle were few. In vitro methods to study hormonal regulation of the mammary glands were in their infancy. Quantitative measures of changes in mammary cell numbers and specific components of milk in response to hormones were rare. The concepts for quantification of hormone concentrations, hormone receptors, growth factors, and binding proteins in blood; hormonal regulation of nutrient partitioning; and hormonally induced mechanisms of action within mammary cells were waiting to be discovered. And eventually they were. However, lest we become too enamored with our current understanding of the hormones that control mammary growth and lactation, it remains a fact that the greatest physiological stimulus for milk yield is pregnancy, not some cocktail of exogenous hormones, growth factors, receptor agonists/antagonists, or gene therapies. Viva la mom!

Anti-idiotypic responses of lactating cows immunized with monoclonal antibodies against bovine somatotropin

The objective of this study was to produce anti-idiotypic antibodies with bovine somatotropin (bST)-like activity by active immunization of lactating cows and to determine their effects on milk yield. Several monoclonal antibodies against bST were evaluated for their interaction with bST in a rat growth bioassay. Two bST-agonist monoclonal antibodies (1 and 2), and two bST-antagonist monoclonal antibodies (3 and 4) were selected. Cows were immunized with immunoglobulin G as a control (n = 12) or with one of the four anti-bST monoclonal antibodies (1, 2, 3, 4; n = 12) on d 3, 24, 45, 66, 87, 108, 129, and 150 of lactation. From wk 3 of lactation, all cows immunized with each of the four anti-bST monoclonal antibodies developed anti-idiotypes until wk 30 of lactation. Total lactation yields were not different among monoclonal antibodies 2, 3, and 4 and control cows (9299, 9321, 9733, and 9415 kg, respectively). However, cows immunized with anti-bST monoclonal antibody 1 had reduced lactation yield compared with cows on other treatments (8136 kg). Daily milk yield of cows immunized with monoclonal antibody 1 was decreased from wk 9 of lactation [36.2 vs. 40.9 kg/d (control)] until the end of lactation, concomitantly with decreased bST concentration from wk 9 of lactation. Cows immunized with anti-bST monoclonal antibody 4 had increased milk yield compared with that of controls during wk 3 to 6 and wk 18 to 21 of lactation. Therefore, anti-idiotypes directed against anti-bST 1 had bST-antagonistic effects on lactation performance; anti-idiotypes against anti-bST 4 transiently increased milk yield.

Generation, characterization and utilization of anti-human growth hormone 1-43, (hGH1-43), monoclonal antibodies in an ELISA

The major isoform of hGH is a polypeptide of 191 amino acids. Human GH1-43 is an amino terminal segment of hGH1-191 which comprises the first 43 amino acids. This peptide is a potent regulator of glucose homeostasis. To facilitate our understanding of the physiological regulation of hGH1-43 an assay to measure its levels in biological fluids and extracts is needed. This communication describes the development of anti-hGH1-43 monoclonal antibodies and their use in the development of an indirect competitive ELISA for the quantification of hGH1-43. Hybridomas were produced by the fusion of FOX-NY myeloma cells with spleen cells taken from a mouse immunized with hGH1-43. The hybridomas were screened for production of antibodies to hGH1-43 by antibody capture ELISA. Hybridomas which produced antibodies reactive to hGH1-43 were cloned by limiting dilution. Three monoclonal hybridomas, CCL-1, CCL-2, and CCL-3 were subsequently obtained. These hybridomas secreted antibodies that were highly reactive towards hGH1-43 but minimally reactive towards hGH1-191. The isotypes of the mAbs secreted by CCL-1, CCL-2 and CCL-3 were all IgG1 kappa as shown by isotype specific antibody capture analysis. An indirect competitive ELISA with a detection limit that ranged from 1 to 10 ng/ml was developed using mAbs from monoclonal hybridoma CCL-3. Dose-response curves for competing hGH1-191, hPRL, and hPL indicated minimal cross-reactivity of mAbs with these hormones and conversely, a high degree of specificity for hGH1-43. Dose-response curves for dilutions of human serum and pituitary extract were parallel to the standard. The availability of a sensitive assay for the measurement of hGH1-43 will help us answer questions regarding the biosynthesis, regulation of secretion, and role of hGH1-43 in the control of glucose homeostasis.

Hormonal regulation of peripheral blood mononuclear cells in sheep

The presence of growth hormone receptors (GHR) on sheep peripheral blood mononuclear (PBMN) cells was studied in two ways. The first was to directly measure specific GH-binding sites on PBMN cells drawn from lambs from birth to 5 months of age. The second was to measure the effect of GH on resting and interleukin-2 (IL-2)-activated PBMN cells in vitro and also the effect of other hormones such as prolactin (PRL), insulin-like growth factor-1 (IGF-1), and glucocorticoid. The specific binding of [125I]human GH (hGH) was low on PBMN cells but increased (P < 0.05) with age up to 5 months. Interestingly, serum concentrations of GH-binding protein also increased (P < 0.01) with age and were highly correlated (r = 0.89; P < 0.05) with the specific binding of GH to PBMN cells. The addition of ovine GH (oGH) to PBMN cells in vitro resulted in increased (P < 0.01) proliferation (at a dose of 100 ng/ml). Higher doses of oGH (2 micrograms/ml) also increased PBMN cell proliferation. When PBMN cells were previously stimulated with IL-2, the dose of 100 ng/ml) oGH was no longer able to stimulate proliferation. IGF-1 inhibited (P < 0.04) resting PBMN cell proliferation at 100 ng/ml but had no effect on IL-2-activated PBMN cells. Other hormones such as PRL caused a stimulation of PBMN cell proliferation (P < 0.01) at 100 ng/ml, whereas dexamethasone (dex) inhibited PBMN cell proliferation at doses as low as 63 ng/ml. These studies show that GH, PRL, IGF-1, and glucocorticoids are effective in modulating the proliferation of PBMN cells from sheep and therefore suggest that a modulation of immune system function in vitro by these hormones is consistent with a purpose in vivo.