Biosynthetic 20-kilodalton methionyl-human growth hormone has diabetogenic and insulin-like activities

The Effects of 20-kDa Human Placental GH in Male and Female GH-deficient Mice: An Improved Human GH?

A rare 20K isoform of GH-V (here abbreviated as GHv) was discovered in 1998. To date, only 1 research article has characterized this isoform in vivo, observing that GHv treatment in male high-fat fed rats had several GH-like activities, but unlike GH lacked diabetogenic and lactogenic activities and failed to increase IGF-1 or body length. Therefore, the current study was conducted to further characterize the in vivo activities of GHv in a separate species and in a GH-deficient model (GH-/- mice) and with both sexes represented. GHv-treated GH-/- mice had significant increases to serum IGF-1, femur length, body length, body weight, and lean body mass and reduced body fat mass similar to mice receiving GH treatment. GH treatment increased circulating insulin levels and impaired insulin sensitivity; in contrast, both measures were unchanged in GHv-treated mice. Since GHv lacks prolactin receptor (PRLR) binding activity, we tested the ability of GH and GHv to stimulate the proliferation of human cancer cell lines and found that GHv has a decreased proliferative response in cancers with high PRLR. Our findings demonstrate that GHv can stimulate insulin-like growth factor-1 and subsequent longitudinal body growth in GH-deficient mice similar to GH, but unlike GH, GHv promoted growth without inhibiting insulin action and without promoting the growth of PRLR-positive cancers in vitro. Thus, GHv may represent improvements to current GH therapies especially for individuals at risk for metabolic syndrome or PRLR-positive cancers.

Characterization of growth hormone disulfide-linked molecular isoforms during post-exercise release vs nocturnal pulsatile release reveals similar milieu composition

OBJECTIVE

To characterize the influence of mode (aerobic/resistance) and volume of exercise (moderate/high) on circulating GH immediately post-exercise as well as following the onset of sleep.

DESIGN

This study used repeated measures in which subjects randomly completed 5 separate conditions: control (no exercise), moderate volume resistance exercise (MR), high-volume resistance exercise (HR), moderate volume aerobic exercise (MA), and high volume aerobic exercise (HA).

METHODS

Subjects had two overnight stays on each of the 5 iterations. Serial blood draws began as soon as possible after the completion of the exercise session. Blood was obtained every 20 min for 24-h. GH was measured using a chemiluminescent immunoassay. Pooled samples representing post exercise (PE) and first nocturnal pulse (NP) were divided into two aliquots. One of these aliquots was chemically reduced by adding 10 mM glutathione (GSH) to break down disulfide-linked aggregates.

RESULTS

No differences were observed when pooling GH response at post-exercise (2.02 ± 0.21) and nocturnal pulse (2.63 ± 0.51; p = .32). Pairwise comparisons revealed main effect differences between controls (1.19 ± 0.29) and both MA (2.86 ± 0.31; p = .009) and HA (3.73 ± 0.71; p = .001). Both MA (p = .049) and HA (p = .035) responses were significantly larger than the MR stimulus (1.96 ± 0.28). With GSH reduction, controls significantly differed from MA (p = .018) and HA (p = .003) during PE, but only differed from HA (p = .003) during NP.

CONCLUSIONS

This study demonstrated similar GH responses to exercise and nocturnal pulse, indicating that mode and intensity of exercise does not proportionately affect GH dimeric isoform concentration.

Ghrelin stimulation of growth hormone isoforms: parallel secretion of total and 20-kDa growth hormone and relation to insulin sensitivity in healthy humans

CONTEXT

The 20-kDa human GH (hGH) is produced in the pituitary by alternative splicing of the hGH-N gene. The 20-kDa hGH promotes growth similarly to 22-kDa or total hGH, the predominant form in circulation, but the relative effects of these isoforms on glucose metabolism have been debated.

OBJECTIVE

To investigate the effect of ghrelin on 20-kDa and total hGH secretion in healthy, nonobese subjects. We also studied associations between basal GH concentration and fasting glucose and insulin as well as between dynamic GH secretion and insulin sensitivity.

DESIGN AND SETTING

Synthetic human acyl ghrelin (0.2 or 0.6 nmol/kg · h) or saline was infused in random order in 14 healthy subjects (six males, eight females; age 27.7 ± 6.3 yr; body mass index 22.0 ± 2.7 kg/m(2), mean ± SEM) on 3 separate days. Ghrelin was infused for 45 min to achieve steady-state levels and continued through a 3-h frequently sampled i.v. glucose tolerance test. Insulin sensitivity index was quantified using the minimal model of glucose kinetics.

RESULTS

Basal 20-kDa and total GH concentrations were 0.4 ± 0.1 and 2.2 ± 0.4 ng/ml, respectively, with a 20-kDa to total GH ratio of 0.13 ± 0.02. Females had significantly higher baseline GH levels. Ghrelin administration increased 20-kDa and total GH levels in a parallel and dose-dependent fashion, with no significant change in the ratio of the isoforms. Basal 20-kDa and total GH levels were negatively correlated with fasting glucose, insulin, and homeostasis model assessment of insulin resistance. During the frequently sampled iv glucose tolerance test, GH secretion was positively correlated with insulin sensitivity index with saline infusion.

CONCLUSION

Ghrelin dose-dependently increases endogenous 20-kDa and total GH secretion in a parallel fashion in healthy subjects. Both basal and stimulated levels of the different GH isoforms were positively associated with insulin sensitivity in this cohort of healthy men and women.

20-kDa placental hGH-V has diminished diabetogenic and lactogenic activities compared with 22-kDa hGH-N while retaining antilipogenic activity

Placental human growth hormone-variant (hGH-V) and pituitary human growth hormone-N (hGH-N) are of identical size (22 kDa) but differ in 13 residues scattered throughout the protein. Several isoforms of GH are produced by the hGH-N and hGH-V genes including a 20-kDa hGH-V resulting from a 45-bp deletion caused by the use of an alternative acceptor site within exon 3. To date, the biological properties of the 20-kDa GH-V have not been characterized in vivo. Using young male Wistar rats fed either chow or a high-fat (HF) diet for 4 wk postweaning, we investigated the effect of 7 days treatment with either 22-kDa hGH-N, 20-kDa hGH-V (5 ug x g(-1) x day(-1) sc), or vehicle on body composition and endocrine and metabolic profiles. Total body growth (absolute weight gain and linear growth trajectory) in the 20-kDa hGH-V-treated animals was intermediary between that of control and hGH-N-treated animals. Both 22-kDa hGH-N and 20-kDa hGH-V significantly reduced total body fat mass compared with control animals, and there were no differences between the GH isoforms in anti-lipogenic activity in animals fed the HF diet. Fasting plasma insulin and C peptide were significantly increased in animals on the HF diet and further increased by hGH-N but were unchanged in 20-kDa hGH-V-treated animals compared with saline-treated controls. Plasma volume as assessed by hematocrit was increased in hGH-N-treated animals but was unchanged in 20-kDa hGH-V-treated animals compared with controls. Furthermore, 20-kDa hGH-V had reduced lactogenic (prolactin receptor mediated) activity characteristic of hGH-N as tested in vitro compared with the 20-kDa hGH-N and 22-kDa hGH-N variants. In summary, placental 20-kDa hGH-V retains some of the growth-promoting and all antilipogenic activities of pituitary 22-kDa hGH-N but has diminished diabetogenic and lactogenic properties compared with the native 22-kDa hGH-N.

Growth hormone isoforms

Human growth hormone (GH) is a heterogeneous protein hormone consisting of several isoforms. The sources of this heterogeneity reside at the level of the genome, mRNA splicing, post-translational modification and metabolism. The GH gene cluster on chromosome 17q contains 2 GH genes (GH1 or GH-N and GH2 or GH-V) in addition to 2(-3) genes encoding the related chorionic somatomammotropin. Alternative mRNA splicing of the GH1 transcript yields two products: 22K-GH (the principal pituitary GH form) and 20K-GH. Post-translationally modified GH forms include N(alpha)-acylated, deamidated and glycosylated monomeric GH forms, as well as both non-covalent and disulfide-linked oligomers up to at least pentameric GH. GH fragments generated in the course of peripheral metabolism may be measured in immunoassays for GH. The GH-N gene is expressed in the pituitary, the GH-V gene in the placenta. Secretion of pituitary GH forms is pulsatile under control from the hypothalamus, whereas secretion of placental GH-V is tonic and rises progressively in maternal blood during the 2nd and 3rd trimester. Pituitary GH forms are co-secreted during a secretory pulse; no isoform-specific stimuli have been identified. There are minor differences in somatogenic and metabolic bioactivity among the GH isoforms, depending on species and assay system used. Both 20K-GH and GH-V have poor lactogenic activity. Oligomeric GH forms have variably diminished bioactivity compared to monomeric forms. GH isoforms cross-react in most immunoassays, but assays specific for 22K-GH, 20K-GH and GH-V have been developed. The metabolic clearance of 20K-GH and GH oligomers is delayed compared to that of 22K-GH. The heterogeneous mixture of GH isoforms in blood is further complicated by the presence of two GH-binding proteins, which form complexes with GH; isoform proportions also vary depending on the lag time from a secretory pulse because of different half-lives. GH forms excreted in the urine reflect monomeric GH isoforms in blood, but constitute only a minute fraction of the GH production rate. The heterogeneity of GH is one important reason for the notorious disparity among assay results. It also presents an opportunity for distinguishing endogenous from exogenous GH.

22- and 20 kDa-human growth hormones bind to different sites within certain cellular receptors

Human growth hormone 20 kDa (20 kDa-hGH) is a natural variant of the main hGH isoform (22 kDa-hGH). Since some 20- and 22 kDa-hGH biological activities are not identical, we decided to map the prolactin (PRLR) and growth hormone receptor (GHR) binding sites for both isoforms. Monoclonal antibody (MAb) R7B4, directed to both receptors, was employed to estimate the relative proximity between 20- and 22 kDa-hGH receptors binding sites. Results indicated that although both hGH isoforms share the same PRLR present in Nb2-cells and rat liver, MAb R7B4 differently affected hormone binding, suggesting that their receptor binding sites would be close in Nb2-cells and separate in rat liver membranes. Since labelled 20 kDa-hGH did not bind significantly to hGHR, we added to the incubating medium an allosteric MAb anti-hGH that improved 20 kDa-hGH affinity for receptors. Under these experimental conditions MAb R7B4 inhibited 20 kDa-hGH binding to human liver but not to placenta, whereas the Ab impaired 22 kDa-hGH binding to both receptors. Data thus suggested that both hGH isoforms share the same hGHR binding site in liver tissue but bind to different overlapped regions in placenta. Consequently, results presented in this paper indicate that PRLR and GHR binding sites for 22- and 20 kDa-hGH should not be always identical, a fact that could explain some of the isoforms different activities.

Diabetogenic activity of 20 kDa human growth hormone (20K-hGH) and 22K-hGH in rats

To compare the diabetogenic activity of 20 kDa human growth hormone (20K-hGH) with that of 22K-hGH, we evaluated insulin sensitivity with a euglycaemic clamp in rats. The glucose infusion rate (GIR) in euglycaemic clamp studies was measured as an indicator of insulin sensitivity. [(14)C]glucose and 2-[(3)H] deoxy- D -glucose injection were used to calculate the rate of glucose utilization (R(d)), the hepatic glucose output (HGO), and the glucose metabolic index (R(g)'). Both 20K- and 22K-hGH were infused at equivalent rates (1.0 (mg/kg)/day). A 24 h infusion of 20K-hGH had no significant effects on the GIR, R(d), HGO and R(g)(')except for in gastrocnemius muscle. In contrast, 22K-hGH significantly lowered the GIR compared with the control (P< 0.001) and 20K-hGH groups (P< 0.01). The infusion of 22K-hGH also reduced R(d)compared with the controls and the 20K-hGH rats by 46.6% (P< 0.001) and 39.6% (P< 0.05) respectively, while no differences were observed in the HGO. Moreover, 22K-hGH inhibited glucose uptake, which was estimated from the insulin-stimulated R(g)' in some tissues. These results suggest that 22K-hGH inhibits the uptake and use of glucose in various tissues, which then leads to insulin resistance. In conclusion, the diabetogenicity of 20K-hGH is much weaker than that of 22K-hGH, and the reduced insulin-antagonizing action of 20K-hGH could have important clinical benefits.

Differences in the effects of 20 K- and 22 K-hGH on water retention in rats

Antidiuretic actions induced by two growth hormone (GH) isoforms (20 K- and 22 K-hGH; 0.2 and 2.0 mg/kg) were evaluated in rats, as fluid retention may cause oedema, one of the adverse effects of GH. Both GH isoforms (2.0 mg/kg) suppressed urine excretion in hypophysectomized rats (P< 0.01), but only the 22 K-hGH isoform (2.0 mg/kg) suppressed urine excretion in intact rats (P< 0.01). In addition, prolactin (PRL) suppressed urine excretion in intact rats (P< 0.05). In conclusion, 20 K-hGH has less potency in causing urine retention than 22 K-hGH and since 20 K-hGH is missing 15 amino acids found in 22 K-hGH, these amino acids may be important for the antidiuretic action of GH. Since prolactin suppressed urine excretion, a part of the antidiuretic action of GH may be related to PRL-R activation.

Reversible dimerization of 20 kilodalton human growth hormone (hGH)

A noncovalent dimer of the 22 kilodalton human growth hormone (22 K-hGH) is known to have diminished somatogenic activity compared with monomeric 22 K-hGH. In the present study, we examined the biological activity and physicochemical behaviour of a noncovalent dimer of the 20 kilodalton human growth hormone (20 K-hGH), an isoform of 22 K-hGH. Analysis of the equilibrium between monomeric and associated forms revealed that the associated 20 K-hGH was present in the dimeric form in aqueous solution. The kinetics of dimerization in rat plasma followed the theory of dissociation-association equilibrium, and more than 99% of 20 K-hGH molecules existed as a monomer in the equilibrium state at the physiological hGH concentration. Analysis of the pharmacokinetics showed that the ratio of the administrated dimer in rat circulation decreased from 43% to less than 4% in 2 h. A preparation of noncovalent dimeric 20 K-hGH had essentially the same degree of biological potency as that of a monomer in both in vitro and in vivo bioassays. In conclusion, dimerization of 20 K-hGH is reversible both in vitro and in vivo and a noncovalent dimer can function as a pharmaceutically active component of a 20 K-hGH preparation, in contrast to a 22 K-hGH preparation.

Secretion of authentic 20-kDa human growth hormone (20K hGH) in Escherichia coli and properties of the purified product

Using Bacillus amyloliquefaciens neutral protease gene (npr), we have constructed a secretion system of 20-kDA human growth hormone (20K hGH) in E. coli. The secretion-signal region from npr was modified inserting a fragment coding a 2Lys-5Leu cluster. In this system we found that co-expression of glutathione reductase remarkably increased accumulation level of 20K hGH in periplasm and confirmed that secreted 20K hGH was correctly processed. The recombinant 20K hGH was highly purified and subjected to analyses of physicochemical properties and biological activities which are still unclear and controversial due to difficulty in preparing the sample with authentic structure. The secreted recombinant product had authentic disulfide linkages and showed molecular weight of 20,270.5 +/- 3.7 (theoretical value, 20,269.9). The results suggest that the recombinant 20K hGH is a full agonist on rat growth promotion and lipolysis stimulation in isolated rat adipose tissues. In particular, the lipolysis-stimulating activity of 20K hGH was distinct as compared with that of 22K hGH under physiological concentration. Cell proliferation activity via prolactin-receptor in Nb-2 lymphoma was obviously low as compared with that of 22K hGH.

Nutrition support in critical illness

Sepsis, shock, multiple trauma, and burns are often associated with altered metabolism characterized by severe catabolism, wasting of the lean body mass, immune dysfunction, and compromised wound healing. Nutrition support is one of the mainstays in the management of these critically ill patients and is aimed at minimizing these complications. The purpose of this article is to compare stress hypermetabolism and starvation metabolism, to review current recommendations for the provision of energy and substrate to the critically ill patient, and to review pertinent literature regarding enteral vs parenteral nutrition. Finally, this article will provide a brief overview of new and future therapies with emphasis on specific substrates and growth factors and the potential for their use in the critically ill patient.

Contribution of hGH20K variant to blood hGH response in sauna and exercise

Exercise-induced increases in blood somatotropin (hGH) have always been considered in terms of quantity of the circulating molecules. Knowing that the hypophysis can release several GH species, we investigated the differential release in blood of total hGH (hGHT) and the main hGH variant (hGH20K) molecules in six trained male swimmers exposed to three different conditions known to favor GH release in blood: 45 min--70% maximum oxygen uptake (VO2max) bicycling and swimming, and 20 min of sauna bathing. Based on the binding specificity of hGH antibodies, hGH20K was isolated then assayed using the Nichols immunoradiometric assay system. All three experimental conditions produced significant (P less than 0.001) elevations in blood hGHT and hGH20K. In all three cases, mean blood hGH20K contribution to blood hGHT was relatively constant (11.9, SE 0.7%). Rises in rectal temperature were not statistically related to the changes in blood hGHT. This demonstration of a relatively constant elevation in hGH20K during bicycling, swimming, and sauna bathing can hardly explain the large differences in blood hGHT responses reported in literature under similar conditions.

Growth hormone binding proteins and various forms of growth hormone: implications for measurements

GH measurements are complicated by numerous physiologically occurring GH forms, by the lack of availability of completely specific reagents for the various GH isoforms, and by interference of the circulating BP in some assays. The discrepancies between assay results are partly due to these factors, with monoclonal immunoassays or RRAs being more affected than polyclonal immunoassays.

A new natural hGH variant--17.5 kd--produced by alternative splicing. An additional consensus sequence which might play a role in branchpoint selection

From a human pituitary cDNA library, we have cloned 3 distinct human growth hormone (hGH) cDNAs, coding respectively for the 22 K hGH, the 20 K variant, and a yet unknown 17.5 K variant. S1 mapping analysis using human pituitary RNA confirms the existence of at least four distinct hGH mRNAs originating from alternative acceptor sites at the second intron of the primary transcript. We have analysed the hGH gene sequence to explain the high frequency of alternative splicings which occur only at this location. In this study we propose CTTGNNPyPyPy as an additional consensus sequence guiding the selection of the branched nucleotide.

The acute GH action in rat adipocytes is associated with enhanced phosphorylation of a 46 kDa plasma membrane protein enriched by GH-Sepharose

The acute anti-lipolytic effect of human growth hormone (hGH) in maximally noradrenaline-stimulated intact rat adipocytes was selectively associated with increased phosphorylation of a 46 kDa plasma membrane protein which was highly enriched by hGH-Sepharose chromatography. The same protein was also phosphorylated by an endogenous protein kinase in isolated plasma membranes, although then no hGH effect could be demonstrated. About 14% of the phosphate incorporated into the protein in isolated plasma membranes was found in tyrosine residues and the remainder in serine and threonine. The possible relation of the 46 kDa protein with the hGH plasma membrane receptor is discussed.