Human growth hormone fragments 1-43 and 44-191: in vitro somatogenic activity and receptor binding characteristics in human and nonprimate systems

Blood-brain barrier transport kinetics of NOTA-modified proteins: the somatropin case

BACKGROUND

Chemical modifications such as PEG, polyamine and radiolabeling on proteins can alter their pharmacokinetic behavior and their blood-brain barrier (BBB) transport characteristics. NOTA, i.e. 1,4,7-triazacyclononane-1,4,7-triacetic acid, is a bifunctional chelating agent that has attracted the interest of the scientific community for its high complexation constant with metals like gallium. Until now, the comparative BBB transport characteristics of NOTA-modified proteins versus unmodified proteins are not yet described.

METHODS

Somatropin (i.e. recombinant human growth hormone), NOTA-conjugated somatropin and gallium-labelled NOTA-conjugated somatropin were investigated for their brain penetration characteristics (multiple time regression and capillary depletion [CD]) in an in vivo mice model to determine the blood-brain transfer properties.

RESULTS

The three compounds showed comparable initial brain influx, with Kin=0.38±0.14 µL/(g×min), 0.36±0.16 µL/(g×min) and 0.28±0.18 µL/(g×min), respectively. CD indicated that more than 80% of the influxed compounds reached the brain parenchyma. All three compounds were in vivo stable in serum and brain during the time frame of the experiments.

CONCLUSIONS

Our results show that modification of NOTA as well as gallium chelation onto proteins, in casu somatropin, does not lead to a significantly changed pharmacokinetic profile at the blood-brain barrier.

Bioactive growth hormone in humans: Controversies, complexities and concepts

OBJECTIVE

To revisit a finding, first described in 1978, which documented existence of a pituitary growth factor that escaped detection by immunoassay, but which was active in the established rat tibia GH bioassay.

METHODS

We present a narrative review of the evolution of growth hormone complexity, and its bio-detectability, from a historical perspective.

RESULTS

In humans under the age of 60, physical training (i.e. aerobic endurance and resistance training) are stressors which preferentially stimulate release of bioactive GH (bGH) into the blood. Neuroanatomical studies indicate a) that nerve fibers directly innervate the human anterior pituitary and b) that hind limb muscle afferents, in both humans and rats, also modulate plasma bGH. In the pituitary gland itself, molecular variants of GH, somatotroph heterogeneity and cell plasticity all appear to play a role in regulation of this growth factor.

CONCLUSION

This review considers more recent findings on this often forgotten/neglected subject. Comparison testing of a) human plasma samples, b) sub-populations of separated rat pituitary somatotrophs or c) purified human pituitary peptides by GH bioassay vs immunoassay consistently yield conflicting results.

Growth Hormone(s), Testosterone, Insulin-Like Growth Factors, and Cortisol: Roles and Integration for Cellular Development and Growth With Exercise

Hormones are largely responsible for the integrated communication of several physiological systems responsible for modulating cellular growth and development. Although the specific hormonal influence must be considered within the context of the entire endocrine system and its relationship with other physiological systems, three key hormones are considered the "anabolic giants" in cellular growth and repair: testosterone, the growth hormone superfamily, and the insulin-like growth factor (IGF) superfamily. In addition to these anabolic hormones, glucocorticoids, mainly cortisol must also be considered because of their profound opposing influence on human skeletal muscle anabolism in many instances. This review presents emerging research on: (1) Testosterone signaling pathways, responses, and adaptations to resistance training; (2) Growth hormone: presents new complexity with exercise stress; (3) Current perspectives on IGF-I and physiological adaptations and complexity these hormones as related to training; and (4) Glucocorticoid roles in integrated communication for anabolic/catabolic signaling. Specifically, the review describes (1) Testosterone as the primary anabolic hormone, with an anabolic influence largely dictated primarily by genomic and possible non-genomic signaling, satellite cell activation, interaction with other anabolic signaling pathways, upregulation or downregulation of the androgen receptor, and potential roles in co-activators and transcriptional activity; (2) Differential influences of growth hormones depending on the "type" of the hormone being assayed and the magnitude of the physiological stress; (3) The exquisite regulation of IGF-1 by a family of binding proteins (IGFBPs 1-6), which can either stimulate or inhibit biological action depending on binding; and (4) Circadian patterning and newly discovered variants of glucocorticoid isoforms largely dictating glucocorticoid sensitivity and catabolic, muscle sparing, or pathological influence. The downstream integrated anabolic and catabolic mechanisms of these hormones not only affect the ability of skeletal muscle to generate force; they also have implications for pharmaceutical treatments, aging, and prevalent chronic conditions such as metabolic syndrome, insulin resistance, and hypertension. Thus, advances in our understanding of hormones that impact anabolic: catabolic processes have relevance for athletes and the general population, alike.

Multiple Effects of Growth Hormone in the Body: Is it Really the Hormone for Growth?

In this review, we analyze the effects of growth hormone on a number of tissues and organs and its putative role in the longitudinal growth of an organism. We conclude that the hormone plays a very important role in maintaining the homogeneity of tissues and organs during the normal development of the human body or after an injury. Its effects on growth do not seem to take place during the fetal period or during the early infancy and are mediated by insulin-like growth factor I (IGF-I) during childhood and puberty. In turn, IGF-I transcription is dependent on an adequate GH secretion, and in many tissues, it occurs independent of GH. We propose that GH may be a prohormone, rather than a hormone, since in many tissues and organs, it is proteolytically cleaved in a tissue-specific manner giving origin to shorter GH forms whose activity is still unknown.

Generation of 5 and 17 kDa human growth hormone fragments through limited proteolysis

INTRODUCTION

The reported presence of two fragments of 5 and 17 kDa originating from the 22 kDa human growth hormone (hGH) in blood and tissues, postulated as the sequences AA 1-43 and AA 44-191, has led to the hypothesis of a post-translational proteolytic origin with respect to the abundant 22 kDa variant (AA 1-191). To evaluate this hypothesis, the activity of several endo-proteases on the 22 kDa hGH protein has been evaluated.

METHODS

Proteolysis using pepsin, trypsin, V8-protease, proteinase K and thermolysin were explored under several conditions, including incubation time and pH. Results were monitored by MALDI-TOF and HPLC-ESI mass spectrometry. Proteolytic 5 and 17 kDa fragments were purified through reversed phase HPLC-UV, and their immuno-affinity properties evaluated by surface plasmon resonance.

RESULTS

Thermolysin was shown to target mainly the AA 43-44 bond of the 22 kDa sequence at physiological pH. Interaction studies of the purified fragments with anti-GH antibodies showed some reactivity for the 17 kDa fragment.

CONCLUSIONS

Thermolysin processes hGH generating 5 and 17 kDa fragments, demonstrating the feasibility of this reaction, although the enzyme responsible for this process in humans is still unknown. Specific antibodies should be used to detect these fragments in human specimens, and, at the same time, the 17 kDa fragment could constitute an interference in some hGH immunoassays.

Characterisation of the 5 kDa growth hormone isoform

OBJECTIVES

The 5 kDa N-terminal fragment of 43 amino acids of human growth hormone (GH) shows a specific and significant in-vivo insulin-like activity. This isoform can be easily obtained by solid phase synthesis methods. Our objective in this study is to describe this procedure in detail and to provide structural information of the protein.

METHODS

Solid phase synthesis was employed for the synthesis of the 5 kDa GH isoform. Circular dichroism and limited proteolysis have been carried out to provide structural information about the folded state of the protein in solution. Surface plasmon resonance was used to compare the structural equivalence between the synthetic protein and a proteolytic homologue at an antibody binding level. For this purpose, a murine monoclonal antibody specific for the 5 kDa isoform was generated and characterised employing this and several other GH isoforms.

RESULTS

Circular dichroism and proteolysis results suggested that the C-terminal segment of the 5 kDa protein folds in an alpha-helix. The comparison of the synthetic product to its proteolytic homologue at an antibody binding level suggested structural equivalency. A highly specific antibody against the 5 kDa GH isoform was generated with null cross-reactivity for 17, 20 and 22 kDa isoforms. Kinetic data on the interaction with the synthetic 5 kDa GH was obtained.

CONCLUSIONS

The structure of the protein appears to be different in comparison to when it is included within the 22 kDa GH isoform. Finally, a highly specific antibody has been generated. The possible significance of the 5 kDa protein as a potential agent for obesity-related diseases is discussed.

The effect of exercise type on immunofunctional and traditional growth hormone

The purpose of this study was to compare the growth hormone (GH) response, including the immunfunctional (IF) GH response, between an acute bout of aerobic and resistance exercise in the same subjects. Ten cross-trained males (24.3 +/- 1.2 years) performed both 30 min of continuous cycling at 70% of VO(2max), and intermittent free weight squatting at 70% of 1-RM, in a randomly assigned crossover design, separated by at least 1 week. Blood samples were collected at 10-min intervals for 2 h (30 min rest, 30 min exercise, 60 min recovery) and analyzed for total human and IF GH. After adjusting for the amount of work performed per minute of exercise, integrated GH AUC was significantly greater during the resistance session than the aerobic session as measured by both the total and IF GH assays (P = 0.008 and P = 0.014, respectively). Peak GH concentrations were significantly greater during the resistance session than the aerobic session (P = 0.05). A similar overall GH pattern was observed in response to both types of exercise, with peak values occurring at the end of exercise, regardless of the GH assay used. These data demonstrate that in young, cross-trained males, intermittent resistance exercise elicits a greater response of GH, including IF GH, compared to a continuous aerobic session, when controlling for the work performed per minute, intersubject variability, relative exercise intensity and session duration.

Chronic resistance training in women potentiates growth hormone in vivo bioactivity: characterization of molecular mass variants

This investigation determined the influence of acute and chronic resistance exercise on responses of growth hormone (GH) molecular variants in women. Seventy-four healthy young women (23 +/- 3 yr, 167 +/- 7 cm, 63.8 +/- 9.3 kg, 26.3 +/- 4.0% body fat) performed an acute bout of resistance exercise (6 sets of 10 repetition maximum squat). Blood samples were obtained pre- and postexercise. Resulting plasma was fractionated by molecular mass (fraction A, >60 kDa; fraction B, 30-60 kDa; and fraction C, <30 kDa) using chromatography. Fractionated and unfractionated (UF) plasma was then assayed for GH using three different detection systems (monoclonal immunoassay, polyclonal immunoassay, and rat tibial line in vivo bioassay). Subjects were then matched and randomly placed into one of four resistance exercise training groups or a control group for 24 wk. All experimental procedures were repeated on completion of the 24-wk resistance training programs. After acute exercise, immunoassays showed consistent increases in UF GH samples and fractions B and C; increases in fraction A using immunoassay were seen only in the monoclonal assay. No consistent changes in bioactive GH were found following acute exercise. Conversely, chronic exercise induced no consistent changes in immunoassayable GH of various molecular masses, whereas, in general, bioassayable GH increased. In summary, although acute exercise increased only immunoactive GH, chronic physical training increased the biological activity of circulating GH molecular variants. Increased bioactive GH was observed across all fractions and training regimens, suggesting that chronic resistance exercise increased a spectrum of GH molecules that may be necessary for the multitude of somatogenic and metabolic actions of GH.

Activation of the growth hormone receptor

Growth hormone (GH) is a major regulator of postnatal growth and metabolism. There are extensive clinical applications for GH or its antagonists, including treatments for dwarfism, cancer and metabolic wasting. Owing to this, there is considerable interest in the mechanisms of GH receptor (GHR) activation. It is conventionally thought that GH induces dimerization of two GHR monomers, which initiates intracellular signaling cascades. However, recent studies have provided evidence for a ligand-induced conformational change within constitutively dimerized GHRs being responsible for activating signaling pathways. This review will relate the new model of GHR activation to the activation of related cytokine receptors and discuss the implication of this new model for the design of small GH mimetics and antagonists for therapeutic use.

Limited proteolysis of human growth hormone at low pH: isolation, characterization, and complementation of the two biologically relevant fragments 1-44 and 45-191

The limited proteolysis approach was used to analyze the conformational features of human growth hormone (hGH) under acidic solvent conditions (A-state). Pepsin was used as the proteolytic probe because of its poor substrate specificity and its activity at low pH. Limited proteolysis of hGH in its A-state results in a selective cleavage of the Phe44-Leu45 peptide bond, leading to the production of fragments 1-44 and 45-191. The two fragments were isolated in homogeneous form for studying their conformational properties by means of spectroscopic methods. Fragment 1-44 was shown to retain little secondary and tertiary structure at neutral pH, while fragment 45-191 independently folds into a highly helical secondary structure. In particular, we have shown that the two peptic fragments are able to associate into a stable and native-like hGH complex 1-44/45-191. Our proteolysis data indicate that in acid solution hGH adopts a partly folded state characterized by a local unfolding of the first minihelix (residues 38-47) encompassing the Phe44-Leu45 peptide bond. Of interest, hGH has both insulin-like and diabetogenic effects. Two fragments of hGH occur in vivo and exert these two opposite activities, namely, fragment 1-43 showing an insulin-potentiating effect and fragment 44-191 showing a diabetogenic activity. The results of this study suggest that the conformational changes of hGH induced by an acidic pH promote the generation of the two physiologically relevant fragments by proteolytic processing of the hormone. Although pepsin cannot be the enzyme responsible for the in vivo processing of the hormone, we propose that limited proteolysis of hGH at low pH is physiologically relevant, since the hormone is exposed to an acidic environment in the cell. This study reports for the first time the analysis of the conformational features of the two individual functional domains of hGH and of their complex.

Growth hormone molecular heterogeneity and exercise

There are more than 100 molecular isoforms of circulating growth hormone (GH), but the traditional measurement approach in the exercise literature has only focused on the main isoform (i.e., 22 kDa). New assay methodologies now can assess various GH isoforms. The current data suggest that exercise results in the preferential release of GH isoforms with extended half-lives, thereby sustaining biological actions.

Computational stabilization of human growth hormone

Recombinant human growth hormone (hGH) is used worldwide for the treatment of pediatric hypopituitary dwarfism and in children suffering from low levels of hGH. It has limited stability in solution, and because of poor oral absorption, is administered by injection, typically several times a week. Development has therefore focused on more stable or sustained-release formulations and alternatives to injectable delivery that would increase bioavailability and make it easier for patients to use. We redesigned hGH computationally to improve its thermostability. A more stable variant of hGH could have improved pharmacokinetics or enhanced shelf-life, or be more amenable to use in alternate delivery systems and formulations. The computational design was performed using a previously developed combinatorial optimization algorithm based on the dead-end elimination theorem. The algorithm uses an empirical free energy function for scoring designed sequences. This function was augmented with a term that accounts for the loss of backbone and side-chain conformational entropy. The weighting factors for this term, the electrostatic interaction term, and the polar hydrogen burial term were optimized by minimizing the number of mutations designed by the algorithm relative to wild-type. Forty-five residues in the core of the protein were selected for optimization with the modified potential function. The proteins designed using the developed scoring function contained six to 10 mutations, showed enhancement in the melting temperature of up to 16 degrees C, and were biologically active in cell proliferation studies. These results show the utility of our free energy function in automated protein design.

Increase of fat oxidation and weight loss in obese mice caused by chronic treatment with human growth hormone or a modified C-terminal fragment

OBJECTIVE

To observe the chronic effects of human growth hormone (hGH) and AOD9604 (a C-terminal fragment of hGH) on body weight, energy balance, and substrate oxidation rates in obese (ob/ob) and lean C57BL/6Jmice. In vitro assays were used to confirm whether the effects of AOD9604 are mediated through the hGH receptor, and if this peptide is capable of cell proliferation via the hGH receptor.

METHOD

Obese and lean mice were treated with hGH, AOD or saline for 14 days using mini-osmotic pumps. Body weight, caloric intake, resting energy expenditure, fat oxidation, glucose oxidation, and plasma glucose, insulin and glycerol were measured before and after treatment. BaF-BO3 cells transfected with the hGH receptor were used to measure in vitro 125I-hGH receptor binding and cell proliferation.

RESULTS

Both hGH and AOD significantly reduced body weight gain in obese mice. This was associated with increased in vivo fat oxidation and increased plasma glycerol levels (an index of lipolysis). Unlike hGH, however, AOD9604 did not induce hyperglycaemia or reduce insulin secretion. AOD9604 does not compete for the hGH receptor and nor does it induce cell proliferation, unlike hGH.

CONCLUSIONS

Both hGH and its C-terminal fragment reduce body weight gain, increase fat oxidation, and stimulate lipolysis in obese mice, yet AOD9604 does not interact with the hGH receptor. Thus, the concept of hGH behaving as a pro-hormone is further confirmed. This data shows that fragments of hGH can act in a manner novel to traditional hGH-stimulated pathways.

Characteristics of circulating growth hormone in women after acute heavy resistance exercise

The effects of exercise on the molecular nature of secreted human growth hormone (GH) or its biological activity are not well understood. Plasma from women (average age 23.6 yr, n = 35), drawn before and after an acute heavy resistance exercise test, was fractionated by size exclusion chromatography into three size classes, namely, > 60 kDa (fraction A), 30-60 kDa (fraction B), and < 30 kDa (fraction C), before GH assay. Concentrations of GH in these fractions, as well as in unfractioned plasma, were measured by the Nichols immunoradiometric assay, National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) polyclonal competitive RIA, Diagnostic Systems Laboratory's immunofunctional assay (measures dimerization-capable species), and the rat tibial bioassay. Significantly increased circulating GH concentrations of two- to fourfold were observed when immunoassays in unfractionated plasma samples were used, but they showed no significant change with use of the rat tibial bioassay. Significant exercise-induced increases in GH were found in fractions B and C but not in fraction A. Because chemical reduction of the samples before GH immunoassay significantly increased GH concentrations in fractions B and C (Nichols and NIDDK kits) after exercise, it is concluded that exercise may specifically increase release of disulfide-linked hormone molecules and/or fragments. Finally, because most of the GH released after exercise was able to dimerize the GH receptor in vitro, it is also concluded that these forms have the two intact binding sites required to initiate signal transduction in target cells.

Growth hormone pulsatility profile characteristics following acute heavy resistance exercise

This investigation examined the hypothesis that acute heavy resistance exercise (AHRE) would increase overnight concentrations of circulating human growth hormone (hGH). Ten men (22 +/- 1 yr, 177 +/- 2 cm, 79 +/- 3 kg, 11 +/- 1% body fat) underwent two overnight blood draws sampled every 10 min from 1700 to 0600: a control and an AHRE condition. The AHRE was conducted from 1500 to 1700 and was a high-volume, multiset exercise bout. Three different immunoassays measured hGH concentrations: the Nichols immunoradiometric assay (Nichols IRMA), National Institute of Diabetes and Digestive and Kidney Diseases radioimmunoassay (NIDDK RIA), and the Diagnostic Systems Laboratory immunofunctional assay (DSL IFA). The Pulsar peak detection system was used to evaluate the pulsatility profile characteristics of hGH. Maximum hGH was lower in the exercise (10.7 microg/l) vs. the control (15.4 microg/l) condition. Mean pulse amplitude was lower in the exercise vs. control condition when measured by the Nichols IRMA and the DSL IFA. A differential pattern of release was also observed after exercise in which hGH was lower in the first half of sleep but higher in the second half. We conclude that AHRE does influence the temporal pattern of overnight hGH pulsatility. Additionally, because of the unique molecular basis of the DSL IFA, this influence does have biological relevance because functionally intact molecules are affected.

Characterization of a bioactive 15 kDa fragment produced by proteolytic cleavage of chicken growth hormone

There is evidence for a cleaved form of GH in the chicken pituitary gland. A 25 kDa band of immunoreactive-(ir-)GH, as well as the 22 kDa monomeric form and some oligomeric forms were observed when purified GH or fresh pituitary extract were subjected to SDS-PAGE under nonreducing conditions. Under reducing conditions, the 25 kDa ir-GH was no longer observed, being replaced by a 15 kDa band, consistent with reduction of the disulfide bridges of the cleaved form. The type of protease involved was investigated using exogenous proteases and monomeric cGH. Cleaved forms of chicken GH were generated by thrombin or collagenase. The site of cleavage was found in position Arg133-Gly134 as revealed by sequencing the fragments produced. The NH2-terminal sequence of 40 amino acid residues in the 15 kDa form was identical to that of the rcGH and analysis of the remaining 7 kDa fragment showed an exact identity with positions 134-140 of cGH structure. The thrombin cleaved GH and the 15 kDa form showed reduced activity (0.8% and 0.5% of GH, respectively) in a radioreceptor assay employing a chicken liver membrane preparation. However, this fragment had a clear bioactivity in an angiogenic bioassay and was capable to inhibit the activity of deiodinase type III in the chicken liver.

Proteolytic processing of human growth hormone (GH) by rat tissues in vitro: influence of sex and age

Although a wealth of evidence exists indicating that proteolytic cleavage can enhance the biological activity of the growth hormone (GH) molecule, the mechanisms responsible for the generation of GH fragments are not completely understood. In the present work we investigated the ability of different rat tissues to cleave 22 kDa GH, as well as the influence of sex and age, the two major physiological regulators of GH secretion on this process. Our results show that tissue homogenates obtained from rat liver, skeletal muscle or adipose tissue (three well-documented target organs for the hormone) are able to cleave 22K-GH, while the hormone is resistant to cleavage by rat brain homogenates. This process is rather selective for 22K-GH, since the 20 kDa GH variant exhibits stability to degradation by all tissue homogenates investigated. Moreover, only a minor fraction of 22 kDa GH is cleaved under our experimental conditions, suggesting that GH microheterogeneity within the 22 kDa range may also determine hormone susceptibility. Finally, we also found that 22K-GH processing shows important age-related changes (the greatest intensity observed in 4-day-old pups), while no gender-related differences exist in any of the tissues investigated.

Active immunization of ewes against ovine placental lactogen increases birth weight of lambs and milk production with no adverse effect on conception rate

In two experiments, 16 Booroola-Assaf and 35 Assaf ewe-lambs were actively immunized at 5 months of age against recombinant ovine placental lactogen (oPL). At 9 months of age, the ewe-lambs were mated for the first time and then introduced into a frequent mating-system. Anti-oPL antibody titers, reproductive performance, maternal serum levels of oPL during pregnancy, lamb birth weight and milk production of the ewes were followed in the immunized ewes and in their non-immunized control counterparts. All the immunized ewes developed anti-oPL antibodies, which interfered with oPL bioactivity in an in vitro cell proliferation assay. Conception rates did not differ (P>0.05) between immunized and non-immunized ewes. Abundant antibody-bound non-active oPL detected in sera of immunized ewes by western blotting indicated enhanced oPL production by the placenta following immunization. An increase (P<0.02) in serum oPL bioactivity, but not immunoreactivity, was observed in the immunized ewes in late gestation relative to control ewes. The average litter size was 1.83 and 1.32 lambs born per ewe lambing in the first and second experiments, respectively. Average birth weights of lambs born to the immunized ewes were higher (P<0.01) than for lambs born to control ewes by 10, 17 and 39% for those born as singles, twins and triplets, respectively. Immunized ewes produced 19 and 33% more milk (P<0.02) than the control ewes in the first 3.5 months of the first and second lactations, respectively. These findings do not suggest a role for oPL in maternal recognition of pregnancy, but they strongly suggest important roles for oPL in fetal growth and mammogenesis. Immunization of ewes against oPL may thus represent a novel practical technique for enhancing birth weights of lambs born to prolific sheep, as well as milk production by both dairy and mutton ewes.

Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism

A small synthetic peptide sequence of human growth hormone (hGH), AOD-9401, has lipolytic and antilipogenic activity similar to that of the intact hormone. Here we report its effect on lipid metabolism in rodent models of obesity and in human adipose tissue to assess its potential as a pharmacological agent for the treatment of human obesity. C57BL/6J (ob/ob) mice were orally treated with either saline (n = 8) or AOD-9401 (n = 10) for 30 days. From day 16 onward, body weight gain in AOD-9401-treated animals was significantly lower than that of saline-treated controls. Food consumption did not differ between the two groups. Analyses of adipose tissue ex vivo revealed that AOD-9401 significantly reduced lipogenic activity and increased lipolytic activity in this tissue. Increased catabolism was also reflected in an acute increase in energy expenditure and glucose and fat oxidation in ob/ob mice treated with AOD-9401. In addition, AOD-9401 increased in vitro lipolytic activity and decreased lipogenic activity in isolated adipose tissue from obese rodents and humans. Together, these findings indicate that oral administration of AOD-9401 alters lipid metabolism in adipose tissue, resulting in a reduction of weight gain in obese animals. The marked lipolytic and antilipogenic actions of AOD-9401 in human adipose tissues suggest that this small synthetic hGH peptide has potential in the treatment of human obesity.

Immunofunctional vs immunoreactive growth hormone responses after resistance exercise in men and women

Immunoassays for growth hormone (GH) may yield variable concentrations for the same sample due to the molecular heterogeneity of growth hormone and epitope specificity of their antibodies. Strasburger et al. developed an "immunofunctional" assay that only detects those GH molecules possessing intact sites 1 and 2, which are necessary for inducing receptor dimerization and subsequent signal transduction. This study compared the immunoreactive (IR) vs immunofunctional (IF) GH concentrations before and after acute resistance exercise (i.e. six sets of 10 repetition maximum squats separated by 2 min rest periods) in 8 men and 6 women. IF concentrations were determined by an ELISA(DSL)and IR GH by a monoclonal IRMA(Nichols). Both men (M) and women (W) demonstrated similar increases for IR (M: 1.47 vs 25.0 ng/ml; W: 4.0 vs 25.4 ng/ml) and IF (M: 0.55 vs 11.66 ng/ml; W: 1.94 vs 10.41 ng/ml) GH following exercise. Post-exercise IF GH was significantly less than IR GH for both M and W. The ratio of IR/IF after exercise was approximately 2 and similar for both M and W. In summary, dynamic exercise elicited a similar rise in M and W for immunofunctionally active GH molecules, but the magnitude is lower than when detected with another conventional assay.

Ruminant placental lactogens act as antagonists to homologous growth hormone receptors and as agonists to human or rabbit growth hormone receptors

Growth hormone receptor (GHR)-mediated activity of ruminant placental lactogens (PLs) and ovine (o) GH was compared, using cells transfected with full size human (h), rabbit (rb), and oGHRs. All three PLs acted as agonists in heterologous bioassays, whereas in homologous bioassays in cells transfected with oGHRs they antagonized the oGH activity. Despite these differences, oGH and PLs bound with similar affinity to the oGHR extracellular domain (oGHR-ECD), indicating that the binding occurs through hormone site I. Gel filtration of complexes between oPL and oGHR-ECD showed a 1:1 stoichiometry, confirming this conclusion. The oPL T185D and bPL T188D, which exhibited weak biological activity mediated through GHRs, behaved as site I antagonists, whereas oPL G130R and bPL G133R formed a 1:1 complex with GHR-ECDs and bound to h/rb/oGHR-ECDs with affinity similar to that of wild-type oPL. They had no agonistic activity in all models transfected with h/rb and oGHRs, but were antagonistic to all of them. In conclusion, ruminant PLs antagonize the activity of oGH in homologous systems, because they cannot homodimerize oGHRs, whereas in heterologous systems they act as agonists. The structural analysis hints that minor differences in the sequence of the GHR-ECDs may account for this difference. Since the initial step in the activity transduced through cytokine/hemapoietic receptors family is receptor homodimerization or heterodimerization, we suggest that the question of homologous versus heterologous interactions should be reexamined.

Novel recombinant analogues of bovine placental lactogen. G133K and G133R provide a tool to understand the difference between the action of prolactin and growth hormone receptors

Two new analogues of bovine placental lactogen (bPL), bPL(G133K) and bPL(G133R), were expressed in Escherichia coli, refolded, and purified to a native form. Binding experiments, which are likely to represent the binding to site 1 only, to intact FDC-P1 cells transfected with rabbit (rb) growth hormone receptor (GHR) or with human (h) GHR, to Nb2 rat lymphoma cells, or to rabbit mammary gland membranes prolactin receptor (PRLR), revealed only small or no reduction in binding capacity. The complex formation between these analogues and receptor extracellular domains (R-ECD) of various hormones was determined by gel filtration. Wild type bPL yielded 1:2 complex with hGHR-ECD, rat PRLR-ECD, and rbPRLR-ECD, whereas both analogues formed only 1:1 complexes with all R-ECDs tested. Real time kinetics experiments demonstrated that the ability of the analogues to form homodimeric complexes was compromised in both PRLR- and GHR-ECDs. The biological activity transduced through lactogenic receptors in in vitro bioassays in rabbit mammary gland acini culture and in Nb2 cells was almost fully retained, whereas the activity transduced through somatogenic receptors in FDC-P1 cells transfected with rbGHRs or with hGHRs was abolished. Both analogues exhibited antagonistic activity in the latter cells. To explain the discrepancy between the effect of the mutation on the signal transduced by PLR versus GHRs we suggest that: 1) the mutation impairs the ability of site 2 of bPL to form a stable homodimeric complex with both lactogenic and somatogenic receptors by a drastic shortening of the half-life of 2:1 complex; 2) the transient existence of the homodimeric complex is still sufficient to initiate the signal transduced through lactogenic receptors but not through somatogenic receptors; and 3) one possible reason for this difference is that JAK2, which serves as a mediator of both receptors, is already associated with lactogenic receptors prior to hormone binding-induced receptor dimerization, whereas in somatogenic receptors the JAK2 receptor association occurs subsequently to receptor dimerization.

Activation of chimeric and full-length growth hormone receptors by growth hormone receptor monoclonal antibodies. A specific conformational change may be required for full-length receptor signaling

Signal transduction by the growth hormone receptor (GHR) occurs through growth hormone (GH)-induced dimerization of two GHRs to form a trimeric complex. It is thought that dimerization alone is sufficient for signaling, since monoclonal antibodies (mAbs) against the extracellular domain of the GHR elicit proliferation of FDC-P1 cells transfected with a chimeric receptor comprising the extracellular domain of the GHR and the fibronectin and cytoplasmic domains of the murine granulocyte colony-stimulating factor receptor. We have screened 14 GHR mAbs for proliferative activity against characterized FDC-P1 and BaF-B03 cell lines stably expressing the full-length human, rabbit, or rat GHR, or the chimeric human GHR/granulocyte colony-stimulating factor receptor, and for transactivation of the c-fos promoter and STAT activation. With the chimeric receptor, eight mAbs were able to elicit proliferation, although there was no correlation between inhibition of hormone binding and agonist activity. In contrast, no mAbs were able to act as agonists with the full-length GHR FDC-P1 cell lines, although nine competed with GH for binding. A weak proliferative response was observed in the BaF-B03 cell lines with two of the mAbs (263 and 1C9), and the addition of anti-mouse F(ab)2 resulted in increased signaling in the hGHR BaF-B03 cell line to a plateau of 28 +/- 4% of the GH maximum for mAb 263. These data could indicate considerable stringency in the ability of mAbs to correctly dimerize the full-length GHR. However, the ability of mAb 263 to stimulate a mutant hGHR altered in the F'-G' loop of domain 2 was nearly abolished, concurrent with an increased affinity of this mAb for the receptor. Since the F'-G' loop undergoes a conformational change on GH binding and is necessary for full proliferative signaling, we propose that in addition to promoting receptor dimerization, mAb 263 may induce specific changes in receptor conformation similar to GH, which are required for the biological response.

Site-directed mutagenesis of recombinant bovine placental lactogen at lysine-73 leads to selective attenuation of its somatogenic activity

Bovine placental lactogen (bPL) is capable of binding and transducing biological activity via somatogenic and lactogenic receptors. To modify this capability, three analogs, bPL(K73D), bPL(K73F) and bPL(K73A), mutated at position 73, and corresponding to R64 in human GH (hGH), were produced in Escherichia coli. Circular dichroic spectrum analyses indicated proper refolding in all cases. Biological activity of these analogs was tested in vitro. In a lactogenic-receptor-mediated Nb2 rat lymphoma cell bioassay, bPL and its analogs acted similarly. In another lactogenic bioassay that measures beta-casein synthesis by HC-11 mouse mammary-gland cells, the analogs were 30-40% as potent as bPL. In contrast, somatogenic receptor-mediated bioactivity in FDC-P1 cells transfected with either rabbit (rb) or hGH receptor (R) was almost completely abolished in these analogs. In receptor binding assays, the effect was more conspicuous and the mutations affected not only somatogenic but also lactogenic binding. Binding to rat (r) and rabbit PRL receptor extracellular domains (ECDs) or membrane-embedded receptors was only slightly changed, except for bPL (K73D), which displayed very low affinity. In somatogenic binding assays to intact IM-9 human lymphocytes, hGHR-ECD or bovine liver membranes, bPL (K73D) did not bind at all, and bPL(K73F) or bPL(K73A) binding was drastically reduced. Binding experiments performed in real time using a BIAcore apparatus revealed that the decreased binding could be mainly attributed to increased k(off) rather than decreased k(on) values. The complex with hGHR-ECD revealed a 2:1 stoichiometry with bPL, bPL(K73F) and bPL(K73A), although the complex with these analogs was less stable than with bPL, whereas bPL(K73D) scarcely assembled a 1:1 complex. In contrast, bPL and the three analogs formed stable 1:2 complexes with rPRL-ECD. These results suggest that position 73 in bPL is more important for somatogenic than lactogenic properties and concurs with results from other groups, which have shown that R64, the analogous amino acid in hGH holds the same differential importance with respect to somatogenic binding.

Human growth hormone fragment (hGH44-91) produces insulin resistance and hyperinsulinemia but is less potent than 22 kDa hGH in the rat

A 17 kDa fragment of human growth hormone (22 kDa hGH), identified as hGH44-191, has lower binding affinity for growth hormone receptors (GHRs), but has been reported to be more potent in producing glucose intolerance in yellow obese mice. Out aim was to investigate this anomaly by comparing acute development of hyperinsulinemia and insulin resistance ("diabetogenic activity") during hGH44-191 or 22 kDa hGH infusion in normal rats. Fasted awake make rats (350-370 g) were infused via a carotid cannula with saline (CON), 22 kDa hGH (at 0.125 micrograms/min), or hGH44-191 (at 0.64 or 0.32 micrograms/min) for 5.75 h. Over the last 2 h, a euglycemic hyperinsulinemic clamp (insulin infusion rate 0.25 U/kg/h) was performed. After 3.75 h infusion, 22 kDa hGH at 0.125 and hGH44-191 at 0.64 micrograms/min produced basal (preclamp) hyperinsulinemia compared to CON. During the clamp, insulin resistance was consistently produced by 22 kDa hGH at 0.125 and hGH44-191, at 0.64 micrograms/min compared to CON. Using specific radioimmunoassays for 22 kDa hGH and hGH44-191, we determined that under conditions of equivalent diabetogenic activity, molar circulating levels of hGH44-191 were 50-60-fold higher than 22 kDa hGH. It was concluded that whereas 22 kDA hGH and hGH44-191 are both capable of generating acute hyperinsulinemia and insulin resistance in the normal rat, the diabetogenic potency of hGH44-191 is not enhanced compared to 22 kDa hGH, and that diabetogenic potency is in accord with the reported lower binding affinity of hGH44-191 to the GHR.

Recombinant analogues of prolactin, growth hormone, and placental lactogen: correlations between physical structure, binding characteristics, and activity

The availability of recombinant growth hormones, prolactins, placental lactogens and a few soluble extracellular domains of their receptors have extended our ability to study the interaction of somatogenic and lactogenic hormones with their receptors. Modifications of their respective cDNAs have enabled the preparation of sufficient amounts of the corresponding proteins. The present review summarizes two aspects of these interactions: (a) the relationship between binding, the apparent ability to dimerize the receptors and biological activities in vitro and in vivo; and (b) the effect of mutations on selective changes in the ability of human growth hormone and bovine placental lactogen to interact with somatogenic and lactogenic receptors. In view of this summary, strategies for preparing a second generation of biologically relevant recombinant hormones are discussed.