Comparison of the somatogenic action of 20 kDa- and 22 kDa-human growth hormones in spontaneous dwarf rats

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.

Growth hormone: isoforms, clinical aspects and assays interference

The measurement of circulating concentrations of growth hormone (GH) is an indispensable tool in the diagnosis of both GH deficiency and GH excess. GH is a heterogeneous protein composed of several molecular isoforms, but the physiological role of these different isoforms has not yet been fully understood. The 22KD GH (22 K-GH) is the main isoform in circulation, followed by 20KD GH (20 K-GH) and other rare isoforms. Studies have been performed to better understand the biological actions of the different isoforms as well as their importance in pathological conditions. Generally, the non-22 K- and 20 K-GH isoforms are secreted in parallel to 22 K-GH, and only very moderate changes in the ratio between isoforms have been described in some pituitary tumors or during exercise. Therefore, in a diagnostic approach, concentrations of 22 K-GH accurately reflect total GH secretion. On the other hand, the differential recognition of GH isoforms by different GH immunoassays used in clinical routine contributes to the known discrepancy in results from different GH assays. This makes the application of uniform decision limits problematic. Therefore, the worldwide efforts to standardize GH assays include the recommendation to use 22 K-GH specific GH assays calibrated against the pure 22 K-GH reference preparation 98/574. Adoption of this recommendation might lead to improvement in diagnosis and follow-up of pathological conditions, and facilitate the comparison of results from different laboratories.

Specific monoclonal antibodies and ultrasensitive immunoassays for 20K and 22K human growth hormone

OBJECTIVE

Generation of specific monoclonal antibodies (mAbs) against 20K and 22K human growth hormone (hGH) and development of ultrasensitive immunoassays to quantify 20K and 22K hGH.

DESIGN

Mice were immunized with recombinant 20K or 22K hGH. Hybridoma cells were screened with biotinylated 20K and 22K hGH simultaneously. The specific mAbs were further characterized and used for construction of isoform specific assays. The ultrasensitive chemiluminescent assays were developed with AMDEX streptavidin-HRP and a sensitive substrate.

RESULTS

The 20K hGH specific mAb 1G12 and the 22K hGH specific mAb 5E1 showed less than 0.1% cross-reactivity to 22K or 20K hGH by competitive binding assay, respectively. Western blot analysis also confirmed the specificity of mAb 1G12 and mAb 5E1. Using mAb 1G12 and mAb 5E1, 20K and 22K specific assays with working range of 2-2000 pg/mL were constructed. The 22K hGH concentrations in 103 serum samples from different healthy subjects in the basal GH state were 343.7+/-421.5 pg/mL (18.6-1820 pg/mL). The 20K hGH concentrations were 30.7+/-37.5 pg/mL (2.4-205pg/mL). The ratios of 20K to 20K plus 22K hGH were 9.8+/-4.4% (3.3-28.3%). Both 22K hGH and 20K hGH concentrations in women (465.9+/-476.3 pg/mL and 43.7+/-46.1 pg/mL, n=47) were significantly higher than those (241.1+/-337.0 pg/mL and 20K hGH 19.8+/-23.0 pg/mL, n=56, P<0.01) in men. However, there was no difference in the proportion of 20K to 20K plus 22K between men and women (P>0.05). The strong correlation between 20K and 22K hGH (R=0.914, P<0.01) indicated the constant proportion between 20K and 22K hGH in the basal GH state of healthy subjects.

CONCLUSIONS

Specific monoclonal antibodies and ultrasensitive chemiluminescent immunoassays for 20K and 22K hGH were generated. The ultrasensitive immunoassays are essential for the determination of 20K and 22K hGH in the basal GH state. This universal ultrasensitive immunoassay form can be adapted to other immunoassays for broad application.

Growth hormone isoforms in acromegalic patients before and after treatment with octreotide LAR

BACKGROUND

Human growth hormone (hGH) circulates as a mixture of different isoforms. It has been previously reported that the ratio of 20kDa to 20kDa plus 22kDa (%20kDa-hGH) is increased in patients with active acromegaly.

OBJECTIVES

To evaluate the GH isoforms (20kDa- and 22kDa-hGH) in acromegalic patients before and after six months of treatment with octreotide LAR, and to compare the results with those in healthy controls. In addition, the relationships between the %20kDa-hGH, tumor size and biochemical measurements were also investigated.

DESIGN

Random serum samples from 23 acromegalic patients evaluated before and after six months of treatment with octreotide LAR and from 23 matched healthy controls were studied. Growth hormone, IGF-I and prolactin (PRL) were measured by chemiluminescence immunometric assay and the 20kDa- and 22kDa-hGH isoforms were measured by specific time-resolved fluorescence immunoassays.

RESULTS

In acromegalic patients before treatment, there was a significantly higher median %20Da-hGH in comparison to healthy controls (14.31% vs. 9.59%, p<0.001). After six months of treatment, the median %20kDa-hGH was similar to the baseline values. Patients with GH<2.5ng/mL after six months of treatment had already lower GH and %20kDa-hGH at baseline (p<0.01). The IGF-I (SD-scores) was positively correlated to total GH levels in acromegalic patients after treatment. There was no correlation between the %20kDa-hGH and PRL levels or tumor size.

CONCLUSIONS

Our study confirmed that acromegalic patients have an increased proportion of circulating 20kDa-hGH isoform. Consequently, the use of a 22kDa-hGH specific assay may underestimate the tumor production of total GH. Although octreotide LAR promoted a significant decrease in the GH and IGF-I levels, it did not normalize the GH isoforms composition and suggests that the secretion of GH isoforms is equally inhibited by somatostatin analogues and that it is the disease control that normalizes the GH isoforms composition in acromegaly.

Repetitive Stimulation of the Pituitary with Growth-Hormone-Releasing Hormone Alters the Proportion of 22 and 20 Kilodalton Human-Growth Hormone Released

Background/Aims. 20 Kilodalton-hGH (20 K-hGH) is the second most abundant pituitary GH variant after 22 K-hGH. In the steady state the proportion of 20 : 22 K-hGH appears constant; does this proportion change with repetitive somatotroph stimulation? Methods. Forty adult males were randomised to receive a GHRH(1–29)NH₂ bolus (0.5 μg/kg (n = 20) or 1.0 μg/kg (n = 20)), preceded or followed by a saline bolus, 1 week apart. Four to six weeks later, 10 subjects received 0.5 μg/kg GHRH(1–29)NH₂ at 0, 60, 120, and 180 minutes. Clearance rate of 22 and 20 K-hGH was measured in 10 subjects. Results. Total amount/proportion of 22 K-hGH/20 K-hGH secreted was similar for both GHRH(1–29)NH₂ doses. Repetitive stimulation reduced the amount of 22 K-hGH released whereas the amount of 20 K-hGH did not change significantly leading to an increase in the proportion of 20 K-hGH (P = .05). Half-life of 20 and 22 K-hGH were not significantly different (P = .55). Conclusions. Repetitive stimulation of the somatotroph may alter the proportion of GH variant released.

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.

20K-GH and its use in detecting GH abuse

Following the successful production of recombinant 20K-GH, several studies investigating the physiology of this GH isoform have been undertaken. In this report, we review studies of its biological effect, measurement and secretion. To use serum 20K-GH level in detecting GH abuse, new method has been established and serum 22K-GH, 20K-GH were measured in normal subjects and athletes, and no abnormal results were found among athletes. Another study confirmed that serum 22K-GH increased remarkably and 20K-GH decreased following the exogenous administration of 22K-GH. The duration was relatively short, approximately 24-36h in our and other studies. The increase of the ratio, 22K-GH/20K-GH was the most suitable indicator of GH abuse. Studies supported by the WADA were undertaken in collaboration with an Australian Group. A new approach for the GH isoform assay by beads assay platform is being developed. It is concluded that the direct measurement of 20K-GH is a valid scientific approach, for detecting GH abuse, although the duration of the positive results is short. Our method will be useful in combination with the marker method, an out-of-competition test or test for target cases. Furthermore, its application for the doping test passport is considered to be a possible future strategy.

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.

Growth hormone isoforms release in response to physiological and pharmacological stimuli

Ten healthy subjects used to performing regular physical activity and eight subjects affected by idiopathic isolated GH deficiency (GHD) were enrolled; 22- and 20-kDa GH secretion and its biological activity were evaluated in response to pharmacological stimuli such as arginine, L-dopa or glucagon in GHD children, while the hormonal response to exercise was studied according to Bruce protocol in healthy subjects. We found a significant increase in 22- and 20-kDa GH level in healthy subjects after monitored physical exercise (MPE; basal 0.28+/-0.12 vs 7.37+/-2.08 ng/ml and basal 0.076+/-0.04 vs 0.18+/-0.05 ng/ml, respectively). Furthermore, the 22-kDa/20-kDa ratio significantly increased in children who had undergone MPE and the GH bioactivity basal mean value also increased significantly after exercise (basal 2.86+/-0.76 vs 7.64+/-1.9 ng/ml). The mean value of 22-kDa GH in GHD patients increased significantly following GH pharmacological stimulation (2.78+/-0.63 ng/ml) when compared with mean basal (0.20+/-0.11 ng/ml) value. In the GHD group the basal concentration of 20-kDa GH significantly increased following GH pharmacological stimulation (0.34+/-0.11 vs 0.72+/-0.2 ng/ml); the 22-kDa/20-kDa ratio significantly increased too. Likewise, GH bioactivity in children with GHD increased significantly after pharmacological stimulation test (basal 2.53+/-0.56 vs 7.33+/-1.26 ng/ml). Both GH isoform concentrations and their biological activity are significantly increased in healthy subjects after submaximal exercise protocol and in GHD children after pharmacological stimuli.

Large-scale preparation and in vitro characterization of biologically active human placental (20 and 22K) and pituitary (20K) growth hormones: placental growth hormones have no lactogenic activity in humans

Expression plasmids containing DNA sequences optimized for expression in Escherichia coli were prepared encoding human pituitary (hGH-N 20K) and placental (hGH-V 20 and 22K) growth hormones. The proteins were expressed in bacteria, refolded and purified to homogeneity by anion-exchange chromatography on Q-Sepharose according to a unique protocol developed for each protein. The yields from 5l of fermentation culture varied between 400 and 700mg of electrophoretically pure, over 95% monomeric protein. Circular dichroism (CD) analysis revealed similarity of the purified hGHs' secondary structure to that of the pituitary hGH-N 22K, except for hGH-V 20K, in which the alpha-helix content was lower. The purified proteins were stable as a 0.1% sterile solution held at pH 10-11 at 4 degrees C for at least one month. All three purified hGH molecules formed a 1:2 complex with hGH receptor extracellular domain (hGHR-ECD), similar to hGH-N 22K. Binding experiments using hGHR-ECD revealed that the differences between the two 22K variants or between the two 20K variants were not significant, except that hGH-V 20K exhibited slightly lower affinity. Somatogenic activity was tested in vitro using FDC-P1 cell lines. Whereas the bioactivity of 22K hGHs and hGH-N 20K in FDC-P1-9D11 cells stably transfected with hGHR was almost equal and two to threefold higher than that of hGH-V 20K, in FDC-P1 3B9 cells stably transfected with rabbit (rb) GHR, the bioactivity of both 20K analogues was significantly (five to ninefold) lower than that of the 22K hormones. The lactogenic activity measured in heterologous assays (Nb2-11C cells and Baf/3 cells stably transfected with the long form of rabbit prolactin receptor) revealed that the activity of hGH-N 20K was close to that of hGH-N 22K in the Baf/3 cells, but 4.5-fold lower in the Nb2 cells. The activity of hGH-V 22K was ninefold less in Nb2 cells and 55-fold less in Baf/3 cells, whereas hGH-V 20K had no lactogenic activity in either bioassay. In contrast, in a homologous lactogenic assay using Baf/3 LP cells stably transfected with hPRLR, the activity of both placental hGHs was nil and the activity of hGH-N 20K was 4.3-fold lower than that of hGH-N 22K. The latter finding raises the question of whether the lack of intrinsic lactogenic activity in the placental hGHs that dominate during pregnancy has any physiological relevance.

The 20-kD human growth hormone reduces body fat by increasing lipolysis and decreasing lipoprotein lipase activity

AIM

The aim of this study was to estimate the lipolytic activity of the human growth hormone variant, 20-kD human growth hormone (20K-hGH).

METHODS

Obese KV-A(y) mice were given daily subcutaneous injections of 20K-hGH (0.25, 0.5, 1.0 mg/kg), 22K-hGH (0.25 mg/kg) or saline as a control for 2 weeks. Body composition (fat, water and protein), lipolysis and lipoprotein lipase (LPL) activity were measured 24 h after the final injection.

RESULTS

Both growth hormone isoforms significantly reduced relative fat pad and whole body lipids. In addition, 20K-hGH produced an inhibition of LPL activity in adipose tissue and stimulated lipolysis in adipocytes.

CONCLUSION

These data strongly suggest that inhibition of LPL activity in adipose tissue and stimulation of lipolysis in adipocytes by 20K-hGH treatment reduce adipose tissue mass, resulting in body fat reduction.

Physiological and pharmacological regulation of 20-kDa growth hormone

The 20-kDa growth hormone (GH) is generated from alternative splicing of the primary transcript of full-length 22-kDa GH. We have studied the regulation of 20-kDa GH over a range of pathophysiological conditions and in response to pharmacological stimulation using isoform-specific enzyme-linked immunosorbent assays (ELISAs). Mean 24-h levels of 20- and 22-kDa GH were higher in acromegaly and lower in GH deficiency than in normal subjects, with the 20-to-22-kDa ratio not different between the three groups. In normal subjects, 20-kDa GH was secreted in a pulsatile manner throughout the day, with peaks coinciding with those of 22-kDa GH. However, the half-life of 20-kDa GH (18.7 +/- 0.8 min) was significantly longer than that of 22-kDa GH (14.7 +/- 0.8 min; P < 0.02). Insulin-induced hypoglycemia, androgen, and oral estrogen caused a parallel and proportionate increase in both isoforms. Octreotide suppressed 20-kDa less rapidly than 22-kDa GH in blood. Administration of recombinant 22-kDa GH in normal subjects rapidly reduced the 20-kDa GH levels. In conclusion, 20-kDa GH is cosecreted with and circulates at a constant proportion of 22-kDa GH. The 20-kDa GH level is reduced by administration of exogenous 22-kDa GH, suggesting rapid negative feedback regulation on pituitary release.

Differential secretion of chicken growth hormone variants after growth hormone-releasing hormone stimulation in vitro

Variants of growth hormone (GH) are present in most vertebrates. Chicken GH (cGH) undergoes posttranslational modifications that contribute to its structural diversity. Although the 22-kDa form of GH is the most abundant, some other variants have discrete bioactivities that may not be shared by others. The proportion of cGH variants changes during ontogeny, suggesting that they are regulated differentially. The effect of growth hormone-releasing hormone (GHRH) on the release of cGH variants was studied in both pituitary gland and primary cell cultures, employing sodium dodecyl sulfate polyacrylamide gel electrophoresis, Western blotting, and densitometry. GHRH (2 nM, 2 h) stimulated the secretion of most of the size variants of cGH although the amplitude of increase was not equal for each one. A differential effect on the secretion of GH size variants, particularly on the 22- (monomer) and 26-kDa (putatively glycosylated) cGH isoforms was found in both systems. In the whole pituitary culture, the proportion of the 26-kDa immunoreactive cGH increased 35% while the 22 kDa decreased 31% after GHRH treatment in comparison with the controls. In the primary cell culture system, the proportion of the glycosylated variant increased 43% whereas the monomer and the dimer decreased 22.26 and 29%, respectively, after GHRH stimulation. Activators of intracellular signals such as 1 mM 8-bromo-cAMP and 1 microM phorbol myristate acetate had a similar effect to that obtained with GHRH. The data support the hypothesis that GH variants may be under differential control and that GHRH promotes the release of a glycosylated cGH variant that has an extended half-life in circulation.