Biomedical subjects

R J Ross

Publications and source records attributed to R J Ross.

At least 19 recordsLinked to original sources

PubMed2001

High-dose leptin activates human leukocytes via receptor expression on monocytes.

Leptin is capable of modulating the immune response. Proinflammatory cytokines induce leptin production, and we now demonstrate that leptin can directly activate the inflammatory response. RNA expression for the leptin receptor (Ob-R) was detectable in human PBMCs. Ob-R expression was examined at the protein level by whole blood flow cytometry using an anti-human Ob-R mAb 9F8. The percentage of cells expressing leptin receptor was 25 +/- 5% for monocytes, 12 +/- 4% for neutrophils, and 5 +/- 1% for lymphocytes (only B lymphocytes). Incubation of resting PBMCs with leptin induced rapid expression of TNF-alpha and IL-6 mRNA and a dose-dependent production of TNF-alpha and IL-6 by monocytes. Incubation of resting PBMCs with high-dose leptin (250 ng/ml, 3-5 days) induced proliferation of resting cultured PBMCs and their secretion of TNF-alpha (5-fold), IL-6 (19-fold), and IFN-gamma (2.5-fold), but had no effect on IL-4 secretion. The effect of leptin was distinct from, and additive to, that seen after exposure to endotoxin or activation by the mixed lymphocyte reaction. In conclusion, Ob-R is expressed on human circulating leukocytes, predominantly on monocytes. At high doses, leptin induces proinflammatory cytokine production by resting human PBMCs and augments the release of these cytokines from activated PBMCs in a pattern compatible with the induction of Th1 cytokines. These results demonstrate that leptin has a direct effect on the generation of an inflammatory response. This is of relevance when considering leptin therapy and may partly explain the relationship among leptin, proinflammatory cytokines, insulin resistance, and obesity.

Adult↗

PubMed2001

A single nucleotide polymorphism (SNP) in the leptin receptor is associated with BMI, fat mass and leptin levels in postmenopausal Caucasian women.

The human leptin (obese) receptor gene contains a number of single nucleotide polymorphisms, including GLN223ARG, which changes an amino acid on the extracellular region common to all isoforms of the receptor. Here, we demonstrate that, in postmenopausal Caucasian women, genotypes at that locus are associated with differences in body mass index (BMI), fat mass and serum leptin levels. Measurement of serum leptin-binding activity indicates that this may reflect changed receptor function associated with genotype. These observations indicate that functional variations in the leptin receptor gene are important factors in the regulation of adiposity and BMI.

Adipose Tissue↗

PubMed2001

Is there a medical need to explore the clinical use of insulin-like growth factor I?

Cloning of the insulin-like growth factor I (IGF-I) gene led to the development in 1987 of recombinant IGF-I available for clinical use. Trials were started targeting endocrine, metabolic and neurological disorders, and beneficial results have been demonstrated in IGF-I deficiency states caused by IGF-I gene deletion and growth hormone (GH) receptor deficiency, type 1 and type 2 diabetes mellitus, and severe insulin resistance syndromes. Results of equivocal benefit have also been reported in osteoporosis and amyotrophic lateral sclerosis. Recent encouraging data using the IGF-I-IGF-binding protein 3 (IGFBP-3) complex in diabetes mellitus suggest that this preparation may eventually replace recombinant free IGF-I. The lack of an established therapeutic indication for IGF-I has resulted in its supplies being severely limited. It will probably be decided during the next decade whether use of IGF-I or the IGF-I-IGFBP-3 complex becomes firmly established as an accepted endocrine therapy.

Clinical Trials as Topic↗

PubMed2001

Generation of human soluble leptin receptor by proteolytic cleavage of membrane-anchored receptors.

The leptin receptor (ObR) exists in multiple isoforms. In rodents, a soluble isoform is generated by alternative splicing; but in humans, there is no mRNA encoding soluble receptor (leptin binding protein). We investigated the hypothesis that human leptin binding protein can be generated by proteolytic cleavage of membrane-anchored leptin receptors (ObRb and ObRa). Leptin binding protein of similar size to that previously detected in human serum was detected by HPLC in medium of cells transfected with ObRa. ObRa exhibited higher expression at the cell surface than ObRb and generated greater levels of leptin binding protein. Ligand-mediated immunofunctional and immunofluorometric assays revealed that the leptin binding protein in medium bound both leptin and an ObR-specific antibody and that the level of leptin binding protein correlated with receptor expression at the cell surface. Phorbol 12-myristate-13-acetate and N-ethylmaleimide increased the accumulation of leptin binding protein, an indication that the production of leptin binding protein was up-regulated by PKC and sulfhydryl group activation. The protease inhibitors, TNFalpha protease inhibitor 1 and Immunex compound 2, could inhibit the production of leptin binding protein, indicating that the enzyme responsible for leptin binding protein cleavage belongs to the metalloprotease family. In conclusion, human leptin binding protein is generated by proteolytic cleavage of membrane-anchored leptin receptor by a metalloprotease.

Carrier Proteins↗

PubMed2001

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.

Carrier Proteins↗

PubMed2001

Differential expression of suppressors of cytokine signalling genes in response to nutrition and growth hormone in the septic rat.

GH treatment during critical illness and sepsis may increase mortality. A family of negative regulators of cytokine signalling, the suppressors of cytokine signalling (SOCS), have been characterised. SOCS provide a mechanism for cross-talk between the cytokine receptors, including GH. Here, we have investigated the impact of nutrition and GH treatment on GH receptor, SOCS1, SOCS-2, SOCS-3 and cytokine-inducible SH2-containing protein (CIS) hepatic mRNA expression in a rat model of sepsis, caecal ligation and puncture (CLP). Four groups of rats were studied: control (food given ad libitum, n=7), CLP only (n=8), CLP and total parenteral nutrition (TPN) (n=9), and CLP, TPN and GH (n=10). CLP rats underwent surgery and 18 h later received saline or TPN or TPN+GH for 6 h before they were killed. Serum IGF-I levels were lower in all CLP groups (P<0.001). The combination of TPN and GH treatment increased IGF-I levels compared with the saline-treated CLP rats (P<0.01), but IGF-I levels remained lower than control animals (P<0.001). GH receptor and GH-binding protein expression in liver was reduced in animals subjected to CLP and was unaffected by nutrition or GH treatment. Hepatic SOCS-1 was detectable in normal rats, induced in all CLP animals but was unaffected by nutrition and GH. Hepatic SOCS-2 expression was difficult to detect in normal and CLP rats but was greatly induced in CLP rats treated with GH. Hepatic SOCS-3 expression was only just detectable in the control group but was elevated in all CLP groups and unaffected by nutrition and GH. Hepatic CIS expression was difficult to detect in normal rats, was not induced by CLP but was induced by both nutrition and GH. In conclusion, CLP induced low IGF-I levels associated with increased expression of SOCS-1 and SOCS-3, both of which are known to inhibit GH receptor signalling. GH induced SOCS-2 and CIS in the CLP rat despite resistance with respect to IGF-I generation, and parenteral feeding induced CIS in the CLP rat. Thus, there is potential for a complex interaction between GH and cytokine signalling at the level of SOCS expression whereby the inflammatory response may alter GH signalling and GH may influence the inflammatory response.

Adaptor Proteins, Signal Transducing↗

PubMed2001

Influence of fear conditioning on elicited ponto-geniculo-occipital waves and rapid eye movement sleep.

The amygdala plays a central role in fear conditioning, a model of anticipatory anxiety. It has massive projections to brainstem regions involved in rapid eye movement sleep (REM) and ponto-geniculo-occipital (PGO) wave generation. PGO waves occur spontaneously in REM or in response to stimuli. Electrical stimulation of the central nucleus of the amygdala enhances spontaneous PGO wave activity during REM and the amplitude of both the acoustic startle response and the elicited PGO wave (PGOE), a neural marker of alerting. This study examined the effects of fear conditioning on REM and on PGOE. On conditioning days, the number of REM episodes, the average REM duration and the REM percentage were decreased while REM latency was increased. The presentation of auditory stimuli in the presence of a light conditioned stimulus produced PGOE of greater amplitudes. The results suggest that fear, most likely involving the amygdala, can influence REM and brainstem alerting mechanisms.

Amygdala↗

PubMed2000

In rats with sepsis, the acute fall in IGF-I is associated with an increase in circulating growth hormone-binding protein levels.

OBJECTIVE: Growth hormone (GH) given to reverse muscle catabolism in critical illness increased mortality, illustrating the need for better understanding of the pathophysiology of the GH axis. We describe the relationship between changes in plasma insulin-like growth factor-I (IGF-I) and growth hormone-binding protein (GHBP) levels and hepatic growth hormone-binding in rats with sepsis. DESIGN: Randomised, controlled study. SETTING: University research laboratory. SUBJECTS: One hundred and eleven male Wistar rats. INTERVENTION: Three groups of rats underwent caecal ligation and puncture (CLP) and three groups laparotomy only (LAP). Survivors were killed at 24, 72, and 96 h. All animals were starved during the study. Twelve rats were killed at the start of the experiment (baseline) and twelve (allowed food) at 96 h. MEASUREMENTS AND RESULTS: Plasma levels of IGF-I and GHBP and binding of 125I-labelled human GH in liver homogenates were measured. IGF-I fell significantly following both CLP and LAP; at 24 h, IGF-I levels were lower after CLP than LAP (950 +/- 74 vs 1,522 +/- 60 microg/l, P = < 0.001). GHBP increased at 24 h following both CLP and LAP (45.6 +/- 1.87 and 47.7 +/- 3.01 vs 38.7 +/- 1.98 ng/ml at baseline, P = < 0.05). In LAP animals GHBP fell to below baseline by 72 h, and significantly so by 96 h (33.5 +/- 1.43, P = < 0.05), whereas GHBP remained elevated 72 h following CLP, returning to baseline by 96 h. The density of GH-binding sites in liver tended to increase, following both CLP and LAP at both 24 and 96 h, but these changes failed to achieve statistical significance. CONCLUSION: Reduced IGF-I levels in sepsis in the rat are associated with elevations in GHBP and a trend to increased hepatic GH binding. This suggests that in sepsis 'GH resistance' is not associated with reduced GH receptor numbers.

Animals↗

PubMed2000

Inflammatory cytokines and acquired growth hormone resistance.

Resistance to growth hormone (GH)-mediated induction of insulin-like growth factor I (IGF-I) is a common complication of catabolic diseases, including critical illness and post-surgical conditions. This resistance to GH is believed to be permissive to the development of protein catabolism, cachexia and wasting, which are associated with an increased mortality rate. Data from in vitro studies and animal models suggest that increased levels of inflammatory cytokines can induce cachexia and might inhibit the effects of GH on target tissues. The molecular mechanisms involved are unclear, although an effect of cytokines on GH receptor signalling has been suggested. The GH-activated pathways that mediate the increase in IGF-I levels are not well understood, thereby impeding the elucidation of the effect of inflammatory cytokines. Several signalling cascades, like the JAK-STAT and MAP kinase pathways, have been shown to be activated by GH and some inflammatory cytokines, hence raising the possibility of crosstalk on this level. Our data, however, indicate that inflammatory cytokines have little or no effect on GH-mediated JAK-STAT signalling. In this review, we discuss these results and the possibility that secondary changes in the structure of chromatin are likely to be involved in the induction of IGF-I gene transcription by GH.

Animals↗

PubMed2000

GH, IGF-I and binding proteins in altered nutritional states.

Nutritional state has profound actions at all levels of the GH/IGF-I axis and growth hormone has potent effects on nutritional state. The growth promoting effects of GH on linear height in children has long been recognised and, more recently, the important action of GH in maintaining adult body composition have been appreciated. The strong anabolic actions of GH have made it a drug of abuse among athletes and, of interest, clinicians, as a potent anti-catabolic therapy. In this review we consider the effects of nutrition on the GH/IGF-I axis and then discuss its potential use as an anti-catabolic agent. We also note the recent study in critically ill patients which has been associated with a poor outcome using high dose GH therapy.

Chronic Disease↗

PubMed2000

Association of elevated insulin-like growth factor binding protein-1 with insulin resistance in hyperthyroidism.

OBJECTIVE: Insulin-like growth factor binding-protein-1 (IGFBP-1) has a role in glucose homeostasis and is present at high concentrations in hyperthyroidism. We have investigated the relationship between IGFBP-1 concentration and glucose homeostasis in hyperthyroidism. DESIGN: Patients and controls had intravenous glucose tolerance tests (IVGTT) and/or oral glucose tolerance tests (OGTT). Patients were tested when hyperthyroid and when euthyroid whilst the controls were tested once. The IVGTT was used to assess insulin sensitivity and the OGTT to establish that the study group had abnormal glucose tolerance. The hyperthyroid patients were treated with methimazole to restore euthyroidism. PATIENTS: Ten patients (9 females) and 13 healthy controls (9 females) consented to the study. Ten patients and nine controls (7 females) had IVGTT. Six patients (5 females) and six controls (4 females) had OGTT. MEASUREMENTS: Glucose, insulin, glucagon, GH and IGFBP-1 were measured during GTT. IGF-I, free thyroid hormones, and TSH concentrations were measured basally. RESULTS: Hyperthyroid subjects were insulin resistant and 67% had impaired glucose tolerance. Fasting IGFBP-1 levels were doubled in hyperthyroid subjects compared to healthy controls and correlated positively with free T4 (r = 0.84, P < 0.0001), with peak glucose during the OGTT (r = 0.68, P < 0.005) with peak insulin during the IVGTT (r = 0.51, P < 0.005) and negatively with glucose disappearance constant (r = - 0.52, P < 0.005). IGFBP-1 was highly phosphorylated in hyperthyroid and control subjects. Fasting insulin and IGFBP-1 levels were unrelated but IGFBP-1 suppressed acutely during GTT in all groups. GH levels fell less in patients with hyperthyroidism than in normals during GTTs. CONCLUSIONS: We conclude that in hyperthyroidism thyroid hormones directly increase fasting IGFBP-1 concentration but acute regulation of IGFBP-1 by insulin is normal and that elevated fasting phosphorylated IGFBP-1 concentration is associated with insulin resistance.

Adult↗

PubMed2000

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.

Adipose Tissue↗

PubMed2000

High-dose growth hormone does not affect proinflammatory cytokine (tumor necrosis factor-alpha, interleukin-6, and interferon-gamma) release from activated peripheral blood mononuclear cells or after minimal to moderate surgical stress.

High-dose GH therapy, with GH doses 10-20 times the normal replacement dose for GH-deficient adults, has been used as an anti-catabolic agent in a number of different patient groups. A recent study, however, has shown an increase in mortality in critically ill patients treated with high-dose GH. The increased mortality was associated with multiorgan failure, septic shock, and uncontrolled infection, suggesting that GH may have altered the immune response. The GH receptor and GH are both expressed in peripheral blood mononuclear cells (PBMCs); thus, GH could act as either an endocrine or an autocrine modulator of the immune response. We have examined the hypothesis that high-dose GH therapy may induce proinflammatory cytokines, which are implicated in septic shock. To do this we measured cytokine production by PBMCs incubated in conditions that simulated high-dose GH therapy, and we measured cytokine levels in patients undergoing laparoscopic cholecystectomy who were randomized to receive either high-dose GH therapy (13 IU/m2 x day) or placebo. To confirm the biological activity of GH in our cell culture system we used a Stat5 functional assay. In this assay GH induced a bell-shaped curve, with a maximal response at GH levels between 100-1,000 ng/mL. PBMCs from healthy volunteers were incubated with GH in doses from 1-1,000 ng/mL for 6-72 h under resting conditions and after activation with endotoxin and the mixed lymphocyte reaction. Studies were repeated with PBMCs from six individuals using a GH dose of 100 ng/mL (the level of GH found after high-dose GH therapy) and an endotoxin dose that gave a submaximal response (0.01 ng/mL). GH had no effect on cell proliferation or the production of tumor necrosis factor-alpha (TNFalpha), interleukin-6 (IL-6), or interferon-gamma (IFNgamma). In patients undergoing laparoscopic cholecystectomy there was a time-related effect of surgery on cytokine levels. There was a rise in IL-6 and a fall in TNFalpha at 24 h after surgery; however, high-dose GH therapy had no effect on the cytokine response. We considered the possibility that endogenous GH production by PBMCs could influence the cytokine response in activated PBMCs; however, incubation of PBMCs in the presence of the GH receptor antagonist, B2036, had no effect on TNFalpha, IL-6, or IFNgamma production by PBMCs in either the mixed lymphocyte reaction or when activated by endotoxin. These results suggest that high-dose GH therapy does not alter the proinflammatory cytokine response to surgery or endotoxin. The results do not exclude an effect of GH on the immune response, but they suggest that the mortality seen in critically ill patients may be due to factors other than immune modulation.

Adult↗

PubMed2000

Stimulation of endogenous GH and interleukin-6 receptors selectively activates different Jaks and Stats, with a Stat5 specific synergistic effect of dexamethasone.

The interaction of GH, interleukin (IL)-6 and glucocorticoids is likely to be important in regulating the GH-insulin-like growth factor (IGF)-I axis. The signalling cascades activated by GH and IL-6 appear to be very similar, as demonstrated by studies using overexpression of the receptor and other components of the Jak-Stat and mitogen-activated protein (MAP) kinase pathways. Here we show that the human embryonic kidney cell line 293 (HEK293) expresses GH and IL-6 receptors endogenously. To determine which specific pathways might be activated by the two cytokines, at physiological levels of all components, we studied GH and IL-6 mediated signal transduction both under basal conditions and in the presence of overexpressed receptors and Stat proteins. Our results suggest a receptor specificity of Jak2 for GH receptors, and Jak1 for IL-6 receptors. Stat activation in response to GH and IL-6 was determined by reporter gene induction. Both GH and IL-6 were able to induce the reporter gene containing the Stat5 responsive element (LHRE) but the IL-6 response appeared to be mediated mainly through Stat3 activation. In contrast, the reporter gene containing the Stat3 responsive element (SIE) was IL-6 specific. The levels of gene induction by GH and IL-6 were not altered by the co-stimulation with GH and IL-6, suggesting that there is little cross-talk at the Jak-Stat activation level between the two cytokines. Neither GH nor IL-6 activated the MAP-kinase responsive serum response element (SRE), unless GH receptors or gp130 were overexpressed. Transfection of Stat3 or Stat5 expression vectors enhanced the response to GH and IL-6. Stimulation with dexamethasone synergistically enhanced GH activation of the LHRE reporter gene but had no effect on the IL-6 activation of the same reporter or on the SIE reporter gene. Thus, our studies suggest that while each cytokine, GH and IL-6, may activate various members of the Jak-Stat pathway in overexpression studies, specific activation of Stat3 by IL-6 and of Jak2 and Stat5 by GH can be observed in HEK293 cells and that in this system the synergistic effect of dexamethasone appears specific for Stat5.

Cell Line↗

PubMed1999

Rapid eye movement sleep changes during the adaptation night in combat veterans with posttraumatic stress disorder.

BACKGROUND: Hyperarousal in posttraumatic stress disorder (PTSD) is manifested during sleep as well as waking. Elevated rapid eye movement sleep (REMS) phasic activity, likely signifying central nervous system alerting, has been identified in PTSD. The authors reasoned that PTSD compared to control subjects would show particularly increased REMS phasic activity on the first night of polysomnography, with adaptation to a novel environment. METHODS: First-night polysomnograms of 17 veterans with PTSD were compared with those of 11 control subjects. Sleep was also studied in subsets of both groups over two nights. RESULTS: On the first night, the PTSD subjects had a higher density of rapid eye movements in the first REMS period. This measure was increased on the first compared to the second night, but there was no interaction effect between night and group. CONCLUSIONS: REMS changes are again demonstrated in veterans with PTSD. Introduction to a novel environment activated a REMS phasic process, but not differentially in PTSD compared to control subjects.

Adaptation, Physiological↗

PubMed1999

Lack of association between thyroid and adrenal nodules: a histological study.

The prevalence of thyroid nodules is increased in patients with Cushing's disease, but the possibility of an association between thyroid and adrenal nodules in other patient groups has not been formally tested. We have evaluated the co-existence of thyroid and adrenal nodules in retrospective and prospective autopsy studies. Retrospective (83 autopsies) and prospective (29 autopsies) blinded studies of thyroid and adrenal gland histopathology were performed by two experienced histopathologists in unselected autopsies. The presence of nodules, defined as areas of tissue having discrete edges within the gland parenchyma seen as a step difference between the cells or architecture adjacent to the nodule, was determined for each gland. No association was found between the presence of adrenal and thyroid nodules in either the retrospective or prospective studies (p>0.2 for both). In the retrospective study, 23% of specimens had thyroid nodules and 28% adrenal nodules. In the prospective study, 24% of specimens had thyroid nodules and 7% adrenal nodules. The proportion of patients with adrenal nodules in the prospective study was significantly less than that in the retrospective study. In conclusion, thyroid and adrenal nodules are frequent autopsy findings in the general population but we have found no evidence of a relationship between the occurrence of nodules in these glands.

Adrenal Gland Diseases↗

PubMed1999

Electrical stimulation of the amygdala increases the amplitude of elicited ponto-geniculo-occipital waves.

The amygdala projects massively via its central nucleus (CNA) into brain stem regions involved in alerting and ponto-geniculo-occipital (PGO) wave generation. Electrical stimulation of CNA is known to enhance the acoustic startle response (ASR) and influence spontaneous PGO waves. The role of the amygdala in the modulation of ASR and elicited PGO waves (PGOE) was investigated in albino rats. Electrically stimulating CNA within 25 ms prior to an auditory stimulus enhanced ASR and PGOE amplitude in a similar way, with the largest response occurring when the electrical and auditory stimuli were given simultaneously. The data suggest that CNA modulates alerting mechanisms.

Acoustic Stimulation↗

PubMed1999

The GH receptor and GH insensitivity.

Growth hormone (GH) resistance/insensitivity (GHIS) is the finding of elevated GH levels associated with a reduction in the biologic actions of GH. It may be congenital or acquired. In congenital GHIS, over 30 mutations in the GH receptor (GHR) have been described. Dimerization of the GHR activates phosphorylation cascades involving MAP kinases and the Janus kinase, JAK2. This in turn activates the STAT (Signal transducers and activators of transcription) proteins, which translocate to the nucleus. A soluble form of the GHR circulates as a GH binding protein (GHBP), and is derived from the proteolytic cleavage of the extracellular domain of the GHR. The majority of GHR mutations resulting in GHIS are though to affect GH binding, hence the finding of low levels of GHBP in many patients. However, a small group of patients with GHIS have been identified in whom there are normal or high levels of GHBP. Mutations in these patients include, among others, a splice-site mutation that results in the skipping of exon 9. The resultant protein is a truncated GHR that lacks 97% of the intracellular domain of the normal receptor. Patients heterozygous for this mutation have GHIS. In vitro studies have shown the truncated receptor acts as a dominant-negative inhibitor of receptor signalling. The truncated receptor lacks the domains essential for internalization, and is therefore highly expressed on the cell surface. It heterodimerizes with the full-length receptor and blocks signaling. Analysis of GHIS has increased our understanding of the molecular basis for GHIS and helped elucidate the factors that regulate GHR trafficking and signalling.

Alternative Splicing↗