Insulin and insulin-like growth factor-I acutely inhibit surface translocation of growth hormone receptors in osteoblasts: a novel mechanism of growth hormone receptor regulation

Can Human Growth Hormone Accelerate Tendon and Ligament Injury Recovery?

BACKGROUND

Studies involving human fibroblasts and use of human growth hormone (HGH) administration for injury recovery are limited. It is plausible that if the administration of HGH to human cells increased cellular proliferation and differentiation, then HGH might be able to assist in accelerating recovery from injury.

HYPOTHESIS

HGH will increase proliferation and differentiation of human tendon and ligament fibroblasts in vitro based on both a single-dose and a sustained-dose model of HGH administration.

STUDY DESIGN

Basic science cellular study.

METHODS

Human tendon and ligament tissue were harvested from 24 patients. Tissue samples were digested with type I collagenase to isolate the target cell types. HGH was administered directly to isolated cells at doses ranging from 100 pg/mL to 10 µg/mL, either in a single-dose or a sustained-dose model. Proliferation was analyzed at days 4 and 7. Differentiation of ligament and tendon fibroblasts was assessed at day 14.

RESULTS

Administration of a single-dose of HGH to both cell types demonstrated similar or inferior cellular proliferation compared with controls after 7 days. For the sustained-dosing model of ligament fibroblasts, only the 100 ng/mL concentration demonstrated at least statistically similar or improved proliferation compared with controls. When examining the 100 ng/mL HGH concentration with larger sample sizes, cellular proliferation was not improved over controls for any cell type for the single- or sustained-dosing models. Proliferation for tendon fibroblasts was either similar or inferior to the control group at all concentrations of HGH. There was no clear dose-response relationship demonstrating enhanced collagen production with administration of HGH to suggest it enhances injury recovery.

CONCLUSION

HGH administered to human tendon and ligament fibroblasts does not appear to positively affect cellular proliferation and differentiation.

CLINICAL RELEVANCE

This study does not support the use of HGH for accelerating recovery from injury.

Growth Hormone Receptor Regulation in Cancer and Chronic Diseases

The GHR signaling pathway plays important roles in growth, metabolism, cell cycle control, immunity, homeostatic processes, and chemoresistance via both the JAK/STAT and the SRC pathways. Dysregulation of GHR signaling is associated with various diseases and chronic conditions such as acromegaly, cancer, aging, metabolic disease, fibroses, inflammation and autoimmunity. Numerous studies entailing the GHR signaling pathway have been conducted for various cancers. Diverse factors mediate the up- or down-regulation of GHR signaling through post-translational modifications. Of the numerous modifications, ubiquitination and deubiquitination are prominent events. Ubiquitination by E3 ligase attaches ubiquitins to target proteins and induces proteasomal degradation or starts the sequence of events that leads to endocytosis and lysosomal degradation. In this review, we discuss the role of first line effectors that act directly on the GHR at the cell surface including ADAM17, JAK2, SRC family member Lyn, Ubc13/CHIP, proteasome, βTrCP, CK2, STAT5b, and SOCS2. Activity of all, except JAK2, Lyn and STAT5b, counteract GHR signaling. Loss of their function increases the GH-induced signaling in favor of aging and certain chronic diseases, exemplified by increased lung cancer risk in case of a mutation in the SOCS2-GHR interaction site. Insight in their roles in GHR signaling can be applied for cancer and other therapeutic strategies.

Mechanisms Underlying the Synergistic Action of Insulin and Growth Hormone on IGF-I and -II Expression in Grass Carp Hepatocytes

In mammals, insulin is known to modify growth hormone (GH)-induced IGF-I expression at the hepatic level, which also contributes to the functional crosstalk between energy homeostasis and somatotropic axis. However, the studies on the comparative aspects of this phenomenon are limited and the mechanisms involved have not been fully characterized. Using a serum-free culture of grass carp hepatoctyes, the functional interaction between GH and insulin on hepatic expression of IGF-I and -II was examined in a fish model. In carp hepatocytes, GH could up-regulate IGF-I and -II mRNA expression via the JAK₂/STAT₅, MEK/ERK and PI3K/Akt pathways. These stimulatory effects were mimicked by insulin via activation of the PI3K/Akt but not MEK/ERK and P38 MAPK cascades. Although insulin did not activate JAK₂ and STAT₅ at hepatocyte level, insulin-induced IGF-I and -II mRNA expression were highly dependent on the normal functioning of JAK₂/STAT₅ pathway. In parallel experiments, insulin co-treatment was found to markedly enhance IGF-I and -II responses induced by GH and these potentiating effects were mediated by insulin receptor (InsR) but not IGF-I receptor. Interestingly, co-treatment with GH also enhanced insulin-induced InsR phosphorylation with a current elevation in protein:protein interaction between GH receptor and phosphorylated InsR and these stimulatory effects were noted with further enhancement in STAT₅, ERK_1/2 and Akt phosphorylation at hepatocyte level. Consistent with these findings, the potentiating effects of GH and insulin co-treatment on IGF-I and -II mRNA expression were found to be suppressed/abolished by inhibiting JAK₂/STAT₅, MEK/ERK and PI3K/Akt but not P38 MAPK pathways. These results, as a whole, suggest that insulin and GH can act in a synergistic manner in the carp liver to up-regulate IGF-I and -II expression through protein:protein interaction at the receptor level followed by potentiation in post-receptor signaling.

Comparative inhibition of the GH/IGF-I axis obtained with either the targeted secretion inhibitor SXN101959 or the somatostatin analog octreotide in growing male rats

Abnormally high GH/IGF-I levels, most often caused by adenomas arising from pituitary somatotrophs, generate deleterious effects. We recently described a targeted secretion inhibitor (SXN101742) comprising a GHRH domain and the endopeptidase domain of botulinum toxin serotype D (GHRH-light chain endopeptidase type D domain [LC/D] associated to a heavy chain translocation domain [HN]) able to down-regulate the GH/IGF-I axis. In the present study, we compared the effect of a single iv bolus of a related molecule developed for clinical studies (SXN101959, 1 mg/kg) with a sc infusion of the somatostatin analog octreotide (SMS201-995, 10 μg/kg · h) to lower GH/IGF-I activity in growing male rats. Ten days after administration of SXN101959 or initiation of the octreotide infusion, body and pituitary weights, body length, GH peaks, and IGF-I production were reduced by both treatments but to a greater extent with SXN101959. In contrast to unaltered GH gene expression and increased GH storage in pituitaries from octreotide-treated rats, the inhibition of GH secretion was associated with a collapse of both GH mRNA and protein level in pituitaries from SXN101959-treated rats, in line with a specific decrease in hypothalamic GHRH production, not observed with octreotide. SXN101959 did not induce major apoptotic events in anterior pituitary and exhibited a reversible mode of action with full recovery of somatotroph cell functionality 30 days after treatment. Octreotide infusion permanently decreased ghrelin levels, whereas SXN101959 only transiently attenuated ghrelinemia. Both treatments limited bone mass acquisition and altered specifically tissues development. In conclusion, SXN101959 exerts a powerful and reversible inhibitory action on the somatotropic axis. Specific features of SXN101959, including long duration of action coupled to a strong inhibition of pituitary GH synthesis, represent advantages when treating overproduction of GH.

Estrogens regulate the hepatic effects of growth hormone, a hormonal interplay with multiple fates

The liver responds to estrogens and growth hormone (GH) which are critical regulators of body growth, gender-related hepatic functions, and intermediate metabolism. The effects of estrogens on liver can be direct, through the direct actions of hepatic ER, or indirect, which include the crosstalk with endocrine, metabolic, and sex-differentiated functions of GH. Most previous studies have been focused on the influence of estrogens on pituitary GH secretion, which has a great impact on hepatic transcriptional regulation. However, there is strong evidence that estrogens can influence the GH-regulated endocrine and metabolic functions in the human liver by acting at the level of GHR-STAT5 signaling pathway. This crosstalk is relevant because the widespread exposition of estrogen or estrogen-related compounds in human. Therefore, GH or estrogen signaling deficiency as well as the influence of estrogens on GH biology can cause a dramatic impact in liver physiology during mammalian development and in adulthood. In this review, we will summarize the current status of the influence of estrogen on GH actions in liver. A better understanding of estrogen-GH interplay in liver will lead to improved therapy of children with growth disorders and of adults with GH deficiency.

A botulinum toxin-derived targeted secretion inhibitor downregulates the GH/IGF1 axis

Botulinum neurotoxins (BoNTs) are zinc endopeptidases that block release of the neurotransmitter acetylcholine in neuromuscular synapses through cleavage of soluble N-ethylmaleimide-sensitive fusion (NSF) attachment protein receptor (SNARE) proteins, which promote fusion of synaptic vesicles to the plasma membrane. We designed and tested a BoNT-derived targeted secretion inhibitor (TSI) targeting pituitary somatotroph cells to suppress growth hormone (GH) secretion and treat acromegaly. This recombinant protein, called SXN101742, contains a modified GH-releasing hormone (GHRH) domain and the endopeptidase domain of botulinum toxin serotype D (GHRH-LHN/D, where HN/D indicates endopeptidase and translocation domain type D). In vitro, SXN101742 targeted the GHRH receptor and depleted a SNARE protein involved in GH exocytosis, vesicle-associated membrane protein 2 (VAMP2). In vivo, administering SXN101742 to growing rats produced a dose-dependent inhibition of GH synthesis, storage, and secretion. Consequently, hepatic IGF1 production and resultant circulating IGF1 levels were reduced. Accordingly, body weight, body length, organ weight, and bone mass acquisition were all decreased, reflecting the biological impact of SXN101742 on the GH/IGF1 axis. An inactivating 2-amino acid substitution within the zinc coordination site of the endopeptidase domain completely abolished SXN101742 inhibitory actions on GH and IGF1. Thus, genetically reengineered BoNTs can be targeted to nonneural cells to selectively inhibit hormone secretion, representing a new approach to treating hormonal excess.

Acromegaly pathogenesis and treatment

Dysregulated growth hormone (GH) hypersecretion is usually caused by a GH-secreting pituitary adenoma and leads to acromegaly - a disorder of disproportionate skeletal, tissue, and organ growth. High GH and IGF1 levels lead to comorbidities including arthritis, facial changes, prognathism, and glucose intolerance. If the condition is untreated, enhanced mortality due to cardiovascular, cerebrovascular, and pulmonary dysfunction is associated with a 30% decrease in life span. This Review discusses acromegaly pathogenesis and management options. The latter include surgery, radiation, and use of novel medications. Somatostatin receptor (SSTR) ligands inhibit GH release, control tumor growth, and attenuate peripheral GH action, while GH receptor antagonists block GH action and effectively lower IGF1 levels. Novel peptides, including SSTR ligands, exhibiting polyreceptor subtype affinities and chimeric dopaminergic-somatostatinergic properties are currently in clinical trials. Effective control of GH and IGF1 hypersecretion and ablation or stabilization of the pituitary tumor mass lead to improved comorbidities and lowering of mortality rates for this hormonal disorder.

Insulin regulation of growth hormone receptor gene expression. Evidence for a transcriptional mechanism of down-regulation in rat hepatoma cells

The role of insulin in regulating responsiveness to growth hormone (GH) remains unclear. Continuous insulin treatment reduces GH binding, which suggests that insulin may effect growth hormone receptor (GHR) levels. The present study used rat hepatoma cells to examine the effects of insulin and GH on GHR gene expression. Prolonged insulin treatment (greater than 3h) significantly reduced GHR mRNA, and removal of insulin led to a gradual recovery. This effect of insulin occurred at physiologic concentrations, occurred many hours before the insulin-regulated decrease in GHR protein, and was mediated by reduction of GHR transcription. GH treatment dramatically reduced GHR protein, but caused only a modest reduction in GHR mRNA. These findings indicate that the heterologous reduction of GHR by insulin occurs via transcriptional downregulation, and the homologous reduction of GHR by GH occurs via a different mechanism. Furthermore, with insulin, extended time of exposure may be necessary for appreciable reduction of GHR.

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.

Reduced exercise-associated response of the GH-IGF-I axis and catecholamines in obese children and adolescents

Obesity blunts catecholamine and growth hormone (GH) responses to exercise in adults, but the effect of obesity on these exercise-associated hormonal responses in children is unclear. Therefore, the aim of the present study was to asses the effect of childhood obesity on the counterregulatory hormonal response to acute exercise. Twenty-five obese children (Ob; body mass index > 95%), and 25 age, gender, and maturity-matched normal-weight controls (NW) participated in the study. Exercise consisted of ten 2-min bouts of constant-cycle ergometry above the anaerobic threshold, with 1-min rest intervals between each bout. Pre-, post-, and 120-min postexercise blood samples were collected for circulating components of the GH-IGF-I axis and catecholamines. There were no differences in peak exercise heart rate, serum lactate, and peak O2 uptake normalized to lean body mass between the groups. Obesity attenuated the GH response to exercise (8.9 +/- 1.1 vs. 3.4 +/- 0.7 ng/ml in NW and Ob participants, respectively; P < 0.02). No significant differences in the response to exercise were found for other components of the GH-IGF-I axis. Obesity attenuated the catecholamine response to exercise (epinephrine: 52.5 +/- 12.7 vs. 18.7 +/- 3.7 pg/ml, P < 0.02; norepinephrine: 479.5 +/- 109.9 vs. 218.0 +/- 26.0 pg/ml, P < 0.04; dopamine: 17.2 +/- 2.9 vs. 3.5 +/- 1.9 pg/ml, P < 0.006 in NW and Ob, respectively). Insulin levels were significantly higher in the obese children and dropped significantly after exercise in both groups. Despite the elevated insulin levels and the blunted counterregulatory response, none of the participants developed hypoglycemia. Childhood obesity was associated with attenuated GH and catecholamine response to acute exercise. These abnormalities were compensated for, so that exercise was not associated with hypoglycemia, despite increased insulin levels in obese children.

Glucose homeostasis in abdominal obesity: hepatic hyperresponsiveness to growth hormone action

It has been suggested that (abdominally) obese individuals are hypersensitive to growth hormone (GH) action. Because GH affects glucose metabolism, this may impact glucose homeostasis in abdominal obesity. Therefore, we studied the effect of GH on glucose metabolism in abdominally obese (OB) and normal-weight (NW) premenopausal women. A 1-h intravenous infusion of GH or placebo was randomly administered to six NW [body mass index (BMI) 21.1 +/- 1.9 kg/m(2)] and six OB (BMI 35.5 +/- 1.5 kg/m(2)) women in a crossover design. Insulin, glucagon, and GH secretion were suppressed by concomitant infusion of somatostatin. Glucose kinetics were measured using a 10-h infusion of [6,6-(2)H(2)]glucose. In both groups, similar physiological GH peaks were reached by infusion of GH. GH strongly stimulated endogenous glucose production (EGP) in both groups. The percent increase was significantly greater in OB than in NW women (29.8 +/- 11.3 vs. 13.3 +/- 7.4%, P = 0.014). Accordingly, GH responsiveness, defined as the maximum response of EGP per unit GH, was increased in OB vs. NW subjects (6.0 +/- 2.1 vs. 2.2 +/- 1.5 micromol.min(-1).mU(-1).l(-1), P = 0.006). These results suggest that the liver is hyperresponsive to GH action in abdominally obese women. The role of the somatotropic ensemble in the control of glucose homeostasis in abdominal obesity is discussed.

Inducible heat shock protein 70 promotes myelin autoantigen presentation by the HLA class II

In this study, we investigated the role of the inducible form of heat shock protein 70 (hsp70) in the presentation of the major putative autoantigen in multiple sclerosis, myelin basic protein (MBP), in the context of appropriate MHC class II. By coimmunoprecipitation, we found that MBP is associated with hsp70 in APC in an ATP/ADP-dependent manner. Additionally, using confocal microscopy, hsp70 was detected in the endocytic pathway of APC, where it colocalized with MBP and HLA-DR. The immunodominant epitopes of MBP 85-99 and 80-99 were shown to bind selectively and specifically to hsp70 by surface plasmon resonance. The functional significance of MBP interaction with hsp70 was demonstrated by the detection of enhanced responses of an MBP-specific T cell hybridoma to MBP and MBP 80-99 with increasing levels of hsp70 and reduced responses when hsp70 expression was diminished within APC-expressing DRA*0101, DRB1*1501 (DR1501). However, when MBP 85-99 was used as the stimulus, T cell hybridoma responses were not enhanced by hsp70 overexpression within APC, suggesting that hsp70 contributes to Ag processing rather than Ag presentation. The importance of a direct association between MBP and hsp70 in the presentation pathways was demonstrated by enhanced efficacy of MBP presentation by APC transfected with a plasmid vector encoding a fusion hsp70-MBP protein. This is the first report on the involvement of self-inducible hsp70 in MHC class II-dependent autoantigen processing by APC. It implicates that aberrant self hsp expression may lead to the enhancement/modulation of autoimmune responses.

Regulation of bone mass by growth hormone

Growth hormone (GH) is a peptide hormone secreted from the pituitary gland under the control of the hypothalamus. It has a many actions in the body, including regulating a number of metabolic pathways. Some, but not all, of its effects are mediated through insulin-like growth factor-I (IGF-I). Both GH and IGF-I play significant roles in the regulation of growth and bone metabolism and hence are regulators of bone mass. Bone mass increases steadily through childhood, peaking in the mid 20s. Subsequently, there is a slow decline that accelerates in late life. During childhood, the accumulation in bone mass is a combination of bone growth and bone remodeling. Bone remodeling is the process of new bone formation by osteoblasts and bone resorption by osteoclasts. GH directly and through IGF-I stimulates osteoblast proliferation and activity, promoting bone formation. It also stimulates osteoclast differentiation and activity, promoting bone resorption. The result is an increase in the overall rate of bone remodeling, with a net effect of bone accumulation. The absence of GH results in a reduced rate of bone remodeling and a gradual loss of bone mineral density. Bone growth primarily occurs at the epiphyseal growth plates and is the result of the proliferation and differentiation of chondrocytes. GH has direct effects on these chondrocytes, but primarily regulates this function through IGF-I, which stimulates the proliferation of and matrix production by these cells. GH deficiency severely limits bone growth and hence the accumulation of bone mass. GH deficiency is not an uncommon complication in oncology and has long-term effects on bone health.

Pharmacokinetics and acute lipolytic actions of growth hormone. Impact of age, body composition, binding proteins, and other hormones

The biologic actions of endogeneous growth hormone (GH) depend on its secretion and clearance rates as well as sensitivity at the receptor level. Aberrations in GH pharmacokinetics and pharmacodynamics may occur with increasing age, and have been implicated in diseases such as obesity, diabetes mellitus, and critical illness. In this review, recent insights into the association between GH metabolism and age, body composition, binding proteins and other hormones are discussed.

Impairment of liver GH receptor signaling by fasting

Fasting causes a state of GH resistance responsible for low circulating IGF-I levels. To investigate whether this resistance may result from alterations in the GH signaling pathway, we determined the effects of fasting on the GH transduction pathway in rat liver. Forty-eight-hour fasted or fed male rats were injected with recombinant rat GH via the portal vein. Liver was removed 0 and 15 min after injection. Although GH stimulated Janus kinase 2 (JAK2) phosphorylation in all animals, this was severely blunted in fasted animals. Similarly, the phosphorylation of the GH receptor, although observed in both fasted and fed rats after GH injection, was markedly reduced in fasted rats. A rapid signal transducer and activator of transcription 5 (STAT5) tyrosine phosphorylation was also induced in the liver of fed animals in response to GH. In contrast, in fasted rats only a slight phosphorylated STAT5 signal was observed. The inhibitory effect of fasting on these GH signaling molecules occurred without changes in their protein content. Furthermore, the impairment of the JAK-STAT pathway in fasted animals was associated with increased liver suppressor of cytokine signaling 3 mRNA levels. Although glucocorticoids, which are increased by fasting, may cause GH resistance, adrenalectomy failed to prevent alterations in the JAK-STAT pathway caused by fasting. In conclusion, the GH resistance induced by fasting is associated with impairment of the JAK-STAT signaling pathway. This might contribute to the decrease in liver IGF-I production observed in fasting.

Growth hormone signalling and its regulation: preventing too much of a good thing

Growth hormone (GH) is a major growth-promoting and metabolic regulatory hormone. Interaction of GH with its cell surface GH receptor (GHR), by virtue of receptor dimerization, causes activation of the GHR-associated cytoplasmic tyrosine kinase, JAK2. Several signalling pathways, including the STAT5, PI3 kinase and MAP kinase pathways, are thereby accessed, resulting in various biochemical and biological cellular signalling outcomes which are rapidly becoming deciphered. Various mechanisms probably exist to terminate, modulate and prevent GH signalling. Some of these mechanisms regulate receptor abundance and/or availability while others may alter the responsiveness of downstream signalling molecules to receptor engagement. In this review, recent insights into modulation of GH signalling are discussed. Special emphasis is placed on mechanisms of homologous and heterologous desensitization and on the likelihood that inducible GHR proteolysis, in addition to causing GH binding protein generation, may also serve as an important mechanism of heterologous GHR downregulation.

Binding and functional studies with the growth hormone receptor antagonist, B2036-PEG (pegvisomant), reveal effects of pegylation and evidence that it binds to a receptor dimer

GH actions are dependent on receptor dimerization. The GH receptor antagonist, B2036-PEG, has been developed for treating acromegaly. B2036 has mutations in site 1 to enhance receptor binding and in site 2 to block receptor dimerization. Pegylation (B2036-PEG) increases half-life and lowers immunogenicity, but high concentrations are required to control insulin-like growth factor-I levels. We examined antagonist structure and function and the impact of pegylation on biological efficacy. Unpegylated B2036 had a 4.5-fold greater affinity for GH binding protein (GHBP) than GH but similar affinity for membrane receptor. Pegylation substantially reduced membrane binding affinity and receptor antagonism, as assessed by a transcription assay, by 39- and 20-fold, respectively. GHBP reduced antagonist activity of unpegylated B2036 but did not effect antagonism by B2036-PEG. B2036 down-regulated receptors, and membrane binding sites doubled in the presence of dimerization-blocking antibodies, suggesting that B2036 binds to a receptor dimer. It is concluded that the high concentration requirement of B2036-PEG for clinical efficacy relates to pegylation, which decreases binding to membrane receptor but has the advantages of reduced clearance, immunogenicity, and interactions with GHBP. Our studies suggest that B2036 binds to a receptor dimer and induces internalization but not signaling.

Insulin regulation of human hepatic growth hormone receptors: divergent effects on biosynthesis and surface translocation

Insulin modulates the biological actions of GH, but little is known about its effect on human hepatic GH receptors (GHRs). Using the human hepatoma cell line HuH7 as a model, we investigated insulin regulation of total, intracellular, and cell surface GHRs and receptor biosynthesis and turnover. Insulin up-regulated total and intracellular GHRs in a concentration-dependent manner. It increased surface GHRs in a biphasic manner, with a peak response at 10 nmol/L, and modulated GH-induced Janus kinase-2 phosphorylation in parallel with expression of surface GHRs. The abundance of GHR messenger ribonucleic acid and protein, as assessed by RT-PCR and Western analysis, respectively, markedly increased with insulin treatment. To examine whether insulin regulates GHRs at the posttranslational level, its effects on receptor surface translocation and internalization were investigated. Insulin suppressed surface translocation in a concentration-dependent manner, whereas internalization was unaffected. Moreover, insulin actions on total GHRs and surface translocation were inhibited by PD98059 and wortmannin, respectively. In conclusion, insulin regulates hepatic GHR biosynthesis and surface translocation in a reciprocal manner, with surface receptor availability the net result of the divergent effects. The divergent actions of insulin appear to be mediated by the mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathways, respectively.

Distribution and abundance of messenger ribonucleic acid for growth hormone receptor isoforms in human tissues

Two alternatively spliced exon 9 variants of human GH receptor (GHR) messenger ribonucleic acid (mRNA), GHR-(1-279) and GHR(1-277), were recently identified in liver. They encode receptor proteins lacking most of the intracellular domain and inhibit GH action in a dominant negative manner. Little is known about tissue distribution and abundance of these GHR isoforms. We have developed quantitative RT-PCR assays specific for the full-length and truncated GHRs and investigated their expression in various human tissues and cell lines. The mRNA of full-length GHR and GHR-(1-279) were readily detectable in all tissues investigated, with liver, fat, muscle, and kidney showing high levels of expression. These two receptor isoforms were also detected in a range of human cell lines, with strongest expression in IM9, a lymphoblastoid cell line. In contrast, GHR-(1277) message was expressed at low levels in liver, fat, muscle, kidney, and prostate and in trace amount in IM9 cells. Full-length GHR was the most abundant isoform, accounting for over 90% of total receptor transcripts in liver, fat, and muscle for quantitative RT-PCR. However, liver had 2- to 4-fold more full-length receptor mRNA and 16- to 40-fold more GHR-(1-277) mRNA than fat and muscle, whereas the mRNA levels of GHR-(1-279) were similar in the three tissues. GHR-(1-279) constituted less than 4% in liver and 7-10% in fat and muscle. GHR-(1-277) accounted for 0.5% of total GHR transcripts in liver and less than 0.1% in the other two tissues. These data suggest that the absolute and relative abundance of mRNA of the three GHR isoforms may be tissue specific. The regulation of expression of exon 9 alternatively spliced GHR variants may provide a potential mechanism for modulation of GH sensitivity at the tissue level.

Studies with a growth hormone antagonist and dual-fluorescent confocal microscopy demonstrate that the full-length human growth hormone receptor, but not the truncated isoform, is very rapidly internalized independent of Jak2-Stat5 signaling

We have investigated trafficking of two negative regulators of growth hormone receptor (GHR) signaling: a human, truncated receptor, GHR1-279, and a GH antagonist, B2036. Fluorescent-labeled growth hormone (GH) was rapidly internalized by the full-length GHR, with >80% of the hormone internalized within 5 min of exposure to GH. In contrast, <5% of labeled GH was internalized by cells expressing truncated GHR1-279. Using another truncated receptor, GHR1-317 fused to enhanced green fluorescent protein (EGFP), we have exploited fluorescence energy transfer to monitor the trafficking of ligand-receptor complexes. The data confirmed that internalization of this truncated receptor is very inefficient. It was possible to visualize the truncated GHR1-317-EGFP packaged in the endoplasmic reticulum, its rapid movement in membrane bound vesicles to the Golgi apparatus, and subsequent transport to the cell membrane. The GH antagonist, B2036, blocked Jak2-Stat5-mediated GHR signaling but was internalized with a similar time course to native GH.

THE RESULTS

1) demonstrate the rapid internalization of GH when studied under physiological conditions; 2) confirm the hypothesis that internalization of cytoplasmic domain truncated human GHRs is very inefficient, which explains their dominant negative action; and 3) show that the antagonist action of B2036 is independent of receptor internalization.

Insulin inhibits growth hormone signaling via the growth hormone receptor/JAK2/STAT5B pathway

Insulin is important for maintaining the responsiveness of the liver to growth hormone (GH). Insulin deficiency results in a decrease in liver GH receptor (GHR) expression, which can be reversed by insulin administration. In osteoblasts, continuous insulin treatment decreases the fraction of cellular GHR localized to the plasma membrane. Thus, it is not clear whether hyperinsulinemia results in an enhancement or inhibition of GH action. We asked whether continuous insulin stimulation, similar to what occurs in hyperinsulinemic states, results in GH resistance. Our present studies suggest that insulin treatment of hepatoma cells results in a time-dependent inhibition of acute GH-induced phosphorylation of STAT5B. Whereas total protein levels of JAK2 were not reduced after insulin pretreatment for 16 h, GH-induced JAK2 phosphorylation was inhibited. There was a concomitant decrease in GH binding and a reduction in immunoreactive GHR levels following pretreatment with insulin for 8-24 h. In summary, continuous insulin treatment in rat H4 hepatoma cells reduces GH binding, immunoreactive GHR, GH-induced phosphorylation of JAK2, and GH-induced tyrosine phosphorylation of STAT5B. These findings suggest that hepatic GH resistance may develop when a patient exhibits chronic hyperinsulinemia, a condition often observed in patients with obesity and in the early stage of Type 2 diabetes.