Growth hormone (GH) and a GH antagonist promote GH receptor dimerization and internalization

Preparation of a Growth Hormone Receptor/Prolactin Receptor Bispecific Antibody Antagonist Which Exhibited Anti-Cancer Activity

Prolactin receptor (PRLR) and growth hormone receptor (GHR) are closely related to the occurrence and development of breast cancer, and breast cancer cell endogenously express GHR, PRLR and GHR-PRLR heterodimer. In this case, the combined use of PRLR or GHR inhibitors may produce better anti-breast cancer potential than PRLR or GHR inhibitors alone. In this case, it is necessary to develop the dual-function GHR/PRLR antagonists with anti-breast cancer potential. For this, we used hybridoma technology to generate an anti-idiotypic antibody (termed H53). We then used various techniques, including competitive ELISA, competitive receptor binding analysis, and indirect immunofluorescence assay to identify H53, and the results show that H53 behaves as a typical internal image anti-idiotypic antibody (Ab2β). Further experiments indicate that H53 is a dual-function inhibitor, which not only inhibited PRLR-mediated intracellular signaling, but also blocked GHR-mediated intracellular signaling in a dose-dependent manner. Furthermore, H53 could inhibit PRL/GH-driven cancer cell proliferation in vivo and in vitro. This study indicates that H53 exhibits potential biological activity against breast tumors, which implies that internal image anti-idiotypic antibodies may be a useful strategy for the development of PRLR/GHR dual-function antagonists for breast cancer therapy.

Growth hormone-specific induction of the nuclear localization of porcine growth hormone receptor in porcine hepatocytes

The phenomenon of nuclear translocation of growth hormone receptor (GHR) in human, rat, and fish has been reported. To date, this phenomenon has not been described in a domestic animal (such as pig). In addition, the molecular mechanisms of GHR nuclear translocation have not been thoroughly elucidated. To this end, porcine hepatocytes were isolated and used as a cell model. We observed that porcine growth hormone (pGH) can induce porcine GHR's nuclear localization in porcine hepatocytes. Subsequently, the dynamics of pGH-induced pGHR's nuclear localization were analyzed and demonstrated that pGHR's nuclear localization occurs in a time-dependent manner. Next, we explored the mechanism of pGHR nuclear localization using different pGHR ligands, and we demonstrated that pGHR's nuclear translocation is GH(s)-dependent. We also observed that pGHR translocates into cell nuclei in a pGH dimerization-dependent fashion, whereas further experiments indicated that IMPα/β is involved in the nuclear translocation of the pGH-pGHR dimer. The pGH-pGHR dimer may form a pGH-GHR-JAK2 multiple complex in cell nuclei, which would suggest that similar to its function in the cell membrane, the nuclear-localized pGH-pGHR dimer might still have the ability to signal.

Targeting growth hormone receptor in human melanoma cells attenuates tumor progression and epithelial mesenchymal transition via suppression of multiple oncogenic pathways

Recent reports have confirmed highest levels of growth hormone (GH) receptor (GHR) transcripts in melanoma, one of the most aggressive forms of human cancer. Yet the mechanism of GH action in melanoma remains mostly unknown. Here, using human malignant melanoma cells, we examined the effects of GH excess or siRNA mediated GHR knock-down (GHRKD) on tumor proliferation, migration and invasion. GH promoted melanoma progression while GHRKD attenuated the same. Western blot analysis revealed drastic modulation of multiple oncogenic signaling pathways (JAK2, STAT1, STAT3, STAT5, AKT, mTOR, SRC and ERK1/2) following addition of GH or GHRKD. Further, we show that GH excess upregulates expression of markers of epithelial mesenchymal transition in human melanoma, while the effects were reversed by GHRKD. Interestingly, we observed consistent expression of GH transcript in the melanoma cells as well as marked modulation of the IGF receptors and binding proteins (IGF1R, IGF2R, IR, IGFBP2, IGFBP3) and the oncogenic HGF-MET mRNA, in response to excess GH or GHRKD. Our study thus identifies the mechanistic model of GH-GHR action in human melanoma and validates it as an important pharmacological target of intervention.

Lessons learned from studies with the growth hormone receptor

Findings related to GH's biological activities have continued to be a fascinating topic over the past decade. Below, I will review several items related to the actions of GH including the GH/GHR interaction, pegvisomant (a GH receptor antagonist), GHR gene disruptions in mice, and clinical consequences of human GHR gene mutations.

Genetic Predictors of Response to Different Medical Therapies in Acromegaly

In the era of predictive medicine, management of diseases is evolving into a more personal and individualized approach, as more data are available regarding clinical, biochemical, radiological, molecular, histopathological, and genetic aspects. In the particular setting of acromegaly, which is a rare, chronic, debilitating, and disfiguring disease, an optimized approach deems even more necessary, especially because of an associated increased morbidity and mortality, the impact on patients' quality of life, and the increased cost of frequently necessary life-long treatments. In this paper, we review the available studies that address potential genetic influences on acromegaly, their role in the outcome, and response to treatments, as well as their contribution to the risk of developing side effects. We focus mainly on pharmacogenetic factors involved during treatment with dopamine agonists, somatostatin analogs, and pegvisomant. Specifically, mutations in dopamine receptors, somatostatin receptors, growth hormone receptors, and metabolic pathways involved in growth hormone action; polymorphisms in the insulin-like growth factor and the insulin-like growth factor binding proteins; and polymorphisms in other genes that may determine differences in the frequency of developing adverse events.

Autocrine/Paracrine Human Growth Hormone-stimulated MicroRNA 96-182-183 Cluster Promotes Epithelial-Mesenchymal Transition and Invasion in Breast Cancer

Human growth hormone (hGH) plays critical roles in pubertal mammary gland growth, development, and sexual maturation. Accumulated studies have reported that autocrine/paracrine hGH is an orthotopically expressed oncoprotein that promotes normal mammary epithelial cell oncogenic transformation. Autocrine/paracrine hGH has also been reported to promote mammary epithelial cell epithelial-mesenchymal transition (EMT) and invasion. However, the underlying mechanism remains largely obscure. MicroRNAs (miRNAs) are reported to be involved in regulation of multiple cellular functions of cancer. To determine whether autocrine/paracrine hGH promotes EMT and invasion through modulation of miRNA expression, we performed microarray profiling using MCF-7 cells stably expressing wild type or a translation-deficient hGH gene and identified miR-96-182-183 as an autocrine/paracrine hGH-regulated miRNA cluster. Forced expression of miR-96-182-183 conferred on epithelioid MCF-7 cells a mesenchymal phenotype and promoted invasive behavior in vitro and dissemination in vivo. Moreover, we observed that miR-96-182-183 promoted EMT and invasion by directly and simultaneously suppressing BRMS1L (breast cancer metastasis suppressor 1-like) gene expression. miR-96 and miR-182 also targeted GHR, providing a potential negative feedback loop in the hGH-GHR signaling pathway. We further demonstrated that autocrine/paracrine hGH stimulated miR-96-182-183 expression and facilitated EMT and invasion via STAT3 and STAT5 signaling. Consistent with elevated expression of autocrine/paracrine hGH in metastatic breast cancer tissue, miR-96-182-183 expression was also remarkably enhanced. Hence, we delineate the roles of the miRNA-96-182-183 cluster and elucidate a novel hGH-GHR-STAT3/STAT5-miR-96-182-183-BRMS1L-ZEB1/E47-EMT/invasion axis, which provides further understanding of the mechanism of autocrine/paracrine hGH-stimulated EMT and invasion in breast cancer.

Chlamydia pneumoniae effector chlamydial outer protein N sequesters fructose bisphosphate aldolase A, providing a benefit to bacterial growth

Background

Pathogenic chlamydiae are obligate intracellular pathogens and have adapted successfully to human cells, causing sexually transmitted diseases or pneumonia. Chlamydial outer protein N (CopN) is likely a critical effector protein secreted by the type III secretion system in chlamydiae, which manipulates host cells. However, the mechanisms of its action remain to be clarified. In this work, we aimed to identify previously unidentified CopN effector target in host cells.

Results

We first performed a pull-down assay with recombinant glutathione S-transferase (GST) fusion CopN proteins (GST–CpCopN: Chlamydia pneumoniae TW183, GST–CtCopN: Chlamydia trachomatis D/UW-3/CX) as “bait” and soluble lysates obtained from human immortal epithelial HEp-2 cells as “prey”, followed by SDS-PAGE with mass spectroscopy (MS). We found that a host cell protein specifically bound to GST–CpCopN, but not GST–CtCopN. MS revealed the host protein to be fructose bisphosphate aldolase A (aldolase A), which plays a key role in glycolytic metabolism. We also confirmed the role of aldolase A in chlamydia-infected HEp-2 cells by using two distinct experiments for gene knockdown with an siRNA specific to aldolase A transcripts, and for assessment of glycolytic enzyme gene expression levels. As a result, both the numbers of chlamydial inclusion-forming units and RpoD transcripts were increased in the chlamydia-infected aldolase A knockdown cells, as compared with the wild-type HEp-2 cells. Meanwhile, chlamydial infection tended to enhance expression of aldolase A.

Conclusions

We discovered that one of the C. pneumoniae CopN targets is the glycolytic enzyme aldolase A. Sequestering aldolase A may be beneficial to bacterial growth in infected host cells.

Role of the tumor suppressor IQGAP2 in metabolic homeostasis: Possible link between diabetes and cancer

Deficiency of IQGAP2, a scaffolding protein expressed primarily in liver leads to rearrangements of hepatic protein compartmentalization and altered regulation of enzyme functions predisposing development of hepatocellular carcinoma and diabetes. Employing a systems approach with proteomics, metabolomics and fluxes characterizations, we examined the effects of IQGAP2 deficient proteomic changes on cellular metabolism and the overall metabolic phenotype. Iqgap2^-/- mice demonstrated metabolic inflexibility, fasting hyperglycemia and obesity. Such phenotypic characteristics were associated with aberrant hepatic regulations of glycolysis/gluconeogenesis, glycogenolysis, lipid homeostasis and futile cycling corroborated with corresponding proteomic changes in cytosolic and mitochondrial compartments. IQGAP2 deficiency also led to truncated TCA-cycle, increased anaplerosis, increased supply of acetyl-CoA for de novo lipogenesis, and increased mitochondrial methyl-donor metabolism necessary for nucleotides synthesis. Our results suggest that changes in metabolic networks in IQGAP2 deficiency create a hepatic environment of a 'pre-diabetic' phenotype and a predisposition to non-alcoholic fatty liver disease (NAFLD) which has been linked to the development of hepatocellular carcinoma.

Growth hormone mediates pubertal skeletal development independent of hepatic IGF-1 production

Deficiencies in either growth hormone (GH) or insulin-like growth factor 1 (IGF-1) are associated with reductions in bone size during growth in humans and animal models. Liver-specific IGF-1-deficient (LID) mice, which have 75% reductions in serum IGF-1, were created previously to separate the effects of endocrine (serum) IGF-1 from autocrine/paracrine IGF-1. However, LID mice also have two- to threefold increases in GH, and this may contribute to the observed pubertal skeletal phenotype. To clarify the role of GH in skeletal development under conditions of significantly reduced serum IGF-1 levels (but normal tissue IGF-1 levels), we studied the skeletal response of male LID and control mice to GH inhibition by pegvisomant from 4 to 8 weeks of age. Treatment of LID mice with pegvisomant resulted in significant reductions in body weight, femur length (Le), and femur total area (Tt.Ar), as well as further reductions in serum IGF-1 levels by 8 weeks of age, compared with the mean values of vehicle-treated LID mice. Reductions in both Tt.Ar and Le were proportional after treatment with pegvisomant. On the other hand, the relative amount of cortical tissue formed (RCA) in LID mice treated with pegvisomant was significantly less than that in both vehicle-treated LID and control mice, indicating that antagonizing GH action, either directly (through GH receptor signaling inhibition) or indirectly (through further reductions in serum/tissue IGF-1 levels), results in disproportionate reductions in the amount of cortical bone formed. This resulted in bones with significantly reduced mechanical properties (femoral whole-bone stiffness and work to failure were markedly decreased), suggesting that compensatory increases of GH in states of IGF-1 deficiency (LID mice) act to protect against a severe inhibition of bone modeling during growth, which otherwise would result in bones that are too weak for normal and/or extreme loading conditions.

Growth hormone excess and the development of growth hormone receptor antagonists

In 1990, a single amino acid substitution in the growth hormone (GH) gene at position 119 was found to transform the consequent protein from an agonist to an antagonist at the growth hormone receptor (GHR). Further amino acid substitutions plus prolongation of the half-life of the protein by pegylation resulted in the first clinically effective GHR antagonist, pegvisomant. Following extensive clinical trials, this medication has emerged as the most efficacious therapy for treatment-resistant acromegaly. Subsequent advances in our understanding of GH-GHR interactions and downstream GH signalling pathways suggest that pegvisomant binds to preformed GHR dimers and prevents rotational changes within the receptor-GH complex necessary for intracellular signalling to occur. This article reviews the discovery of pegvisomant, from initial experimental data to successful licensing of the drug for treatment-resistant acromegaly, and discusses its other potential therapeutic uses in diseases with abnormalities in the GH-IGF-I axis.

Rational design of competitive prolactin/growth hormone receptor antagonists

There is increasing evidence that prolactin (PRL) and growth hormone (GH) act as growth-promoters of breast tumors. Recent arguments have accumulated to suggest that when they are locally-produced within the mammary tissue, these hormones, acting by an autocrine-paracrine mechanism may have enhanced, or even specific functions compared to endocrine PRL and GH. Classical drugs blocking pituitary hormone production (dopamine and somatostatin analogs) are ineffective on extrapituitary expression of PRL/GH genes, therefore the undesirable effects of these locally-produced hormones remain a target of interest for alternative strategies. This has encouraged the development of competitive PRL and/or GH receptor antagonists, which involve engineered variants of natural receptor ligands (PRL or GH) aimed at blocking receptor activation rather than hormone production in peripheral tissues. This article overviews the rational design of this new class of molecules, their specific molecular features (receptor specificity, biological properties, etc.) and whenever available, the data that have been obtained in cell or animal models of breast cancer.

Formulation, lyophilization and solid-state properties of a pegylated protein

In this paper the importance of formulation and process parameters on the solid-state properties of a lyophilized, pegylated growth hormone antagonist (pegvisomant) was studied. The degree of solid-state disorder (amorphicity), protein/polyethylene glycol (PEG)/sucrose interactions, and dissolution characteristics of the resultant cakes were examined. Using isothermal microcalorimetry (IMC) and differential scanning calorimetry (DSC), it was shown that in co-lyophilized pegylated protein/sucrose systems there was an interaction between sucrose and pegylated protein molecules. This interaction was evidenced by a decrease in the melting temperature (Tm) and melting enthalpy of PEG as a function of sucrose concentration. It was also shown that the sum of the heat of interaction with water for the individual constituents, lyophilized pegylated protein and lyophilized sucrose, was higher than the heat of interaction for the co-lyophilized system. As the concentration of sucrose was increased, the degree of solid-state disorder increased and the solid dissolved faster. A correlation was found among heat of interaction with water, degree of solid-state disorder, and dissolution time. Pegylation caused a shorter dissolution time, lower moisture content, increased amorphicity, and a more rapid moisture-induced crystallization of sucrose.

Role of the growth hormone (GH) receptor transmembrane domain in receptor predimerization and GH-induced activation

The GH receptor (GHR) mediates GH effects by activating the GHR-associated cytoplasmic tyrosine kinase, Janus kinase 2. Recent studies indicate that GHRs exist as dimers independently of GH binding. Some authors suggest that receptor predimerization is mediated by the transmembrane domain (TMD) and that GH binding initiates signaling by triggering changes in the orientation of the two GHRs within the dimer. In this study, we investigate the role of GHR TMD in GH-independent receptor dimerization and ligand-induced activation. We prepared a GHR mutant, GHR(LDLR), in which the TMD is replaced with the TMD of the human low-density lipoprotein receptor (LDLR). The resultant chimera has a TMD two residues shorter than the native GHR TMD; thus, in addition to possessing a different TMD, the altered GHR(LDLR) TMD helical register may change positions of the GHR extracellular domain (ECD) and intracellular domain relative to the TMD when compared with the wild-type (WT) receptor. When each was coexpressed with an intracellular domain-truncated GHR mutant, GHR(1-274-Myc), both WT GHR and GHR(LDLR) were specifically coprecipitated with GHR(1-274-Myc), indicating that the GHR TMD was not required for GHR heterodimerization with GHR(1-274-Myc). We further examined the contribution of the so-called "dimerization interface," a GHR ECD region that is critical for GH-induced signaling, to receptor predimerization. Coimmunoprecipitation experiments with either WT GHR, a dimerization interface mutant (GHR-H150D), or a control mutant (GHR-T147D) with GHR(1-274-Myc) showed dramatically reduced coprecipitation of GHR-H150D with GHR(1-274-Myc) when compared with WT GHR or GHR-T147K. This result suggests that, in contrast to some recent models, the dimerization interface contributes to GHR predimerization. We also compared WT GHR with GHR(LDLR) and GHR(LDLRDelta4) (a chimera in which the LDLR TMD has an internal deletion of four residues) with regard to response to GH stimulation. Although the chimeras had similar GH dose responses and time courses for signaling as WT GHR, they were markedly less sensitive to inhibition of signaling by a conformation-sensitive GHR ECD monoclonal antibody. Further, the chimeras were much less sensitive to inducible metalloprotease cleavage than was WT GHR, implying that the ECD conformations of the chimera receptors differ from WT GHR. Collectively, our data indicate that the composition and/or length of the TMD affect some aspects of GHR function, but do not affect receptor predimerization or GH-induced GHR activation. Further, they suggest that the GHR ECD-TMD is more flexible than previously thought in terms of the ability to achieve the active conformation in response to GH.

A liver specific gene that is expressed in growth hormone transgenic mice and in normal female mice as a function of age

Growth hormone (GH) acts on various organs to exert its growth and metabolic effects. GH induces transcription of a number of genes in different organs including liver. By performing subtractive hybridization analysis on liver cDNAs of GH transgenic and non-transgenic mice, differentially expressed cDNAs were obtained. This paper describes the isolation and characterization of a liver cDNA, termed cDNA #5, that contains 1897 bp and is predicted to encode a protein (P5) of 512 aa residues. P5 has five immunoglobulin related domains thus allowing it to be classified as a member of the immunoglobulin super family (IGSF). Also, P5 shows significant similarity to both rat and human alpha-1-B glycoprotein which is an acidic serum protein of unknown function. mRNA #5 was detected in the liver hepatocytes of male and female GH transgenic mice and in the liver of female, but not of male, non-transgenic mice. mRNA #5 was not present in dwarf mice including the Ames dwarf, those that express a GH antagonist and those with the GH receptor and binding protein gene disrupted. These findings suggest that induction of mRNA #5 in the liver requires a continuous pattern of GH secretion and an intact GH-GH receptor-signaling complex. mRNA #5 levels in female non-transgenic mice were observed to vary with age implying that gender-specific age-dependent factor(s) may be involved in the induction of mRNA #5. The appearance of mRNA #5 in post-hepatectomized liver that coincides with the proliferative phase of liver regeneration suggests that it may be involved in hepatocyte proliferation. Together these data suggest that expression of cDNA #5 is liver-specific, sexually dimorphic, age-dependent, and may be involved in hepatocyte hyperplasia and liver enlargement.

Growth hormone receptor antagonists

The currently available long-acting somatostatin analogs normalize serum growth hormone (GH) levels and insulin-like growth factor-I levels in approximately 60% of patients with acromegaly. The recently introduced GH receptor antagonist, pegvisomant, is able to normalize insulin-like growth factor-I levels in virtually all acromegalic subjects. Although no correlation between increased GH concentrations and tumor size has been found, long-term safety studies are still in progress. Also, pegvisomant monotherapy is administered once daily and is very costly. Combined treatment of a somatostatin analog with pegvisomant appears to be an effective and rational approach.

Nanomedicines in the treatment of acromegaly: focus on pegvisomant

This article examines the role of pegvisomant in the treatment of acromegaly. This syndrome, caused by excessive growth hormone (GH) secretion by a pituitary adenoma, is associated with a doubled mortality rate and poor quality of life. Pituitary microsurgery has long been the first choice of treatment since it cures many patients, especially those with localized tumors. Adjuvant irradiation was given if insulin-like growth factor-I (IGF-I) or GH did not normalize. The introduction of long-acting slow- release somatostatin analogs was a breakthrough for adjuvant treatment, although not always effective. Rather, targeting excessive GH production, muting the GH signal at its receptor, was a totally different approach. The development of GH antagonists (by mutation ofglycine at position 120) and other modifications to enhance receptor binding, and subsequent pegylation of the molecule led to the development of B2036. After pegylation of B2036 at 5 positions the distribution volume is restricted and its serum half-life considerably increased. In short-term clinical studies performed in selected, mostly pretreated, acromegalic patients, IGF-I normalized in the majority of cases. Combination therapy with long-acting somatostatin analogs and weekly rather than daily pegvisomant injections appears to be successful in one clinical study and might limit the high cost of pegvisomant. Long-term efficacy and safety has to be demonstrated. The drug does not cross the blood-brain barrier, and whether it distributes freely into the extracellular space of other organs than the liver has not been investigated, which might have implications for persistent local IGF-I production under unrestrained GH concentrations.

Model for growth hormone receptor activation based on subunit rotation within a receptor dimer

Growth hormone is believed to activate the growth hormone receptor (GHR) by dimerizing two identical receptor subunits, leading to activation of JAK2 kinase associated with the cytoplasmic domain. However, we have reported previously that dimerization alone is insufficient to activate full-length GHR. By comparing the crystal structure of the liganded and unliganded human GHR extracellular domain, we show here that there is no substantial change in its conformation on ligand binding. However, the receptor can be activated by rotation without ligand by inserting a defined number of alanine residues within the transmembrane domain. Fluorescence resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET) and coimmunoprecipitation studies suggest that receptor subunits undergo specific transmembrane interactions independent of hormone binding. We propose an activation mechanism involving a relative rotation of subunits within a dimeric receptor as a result of asymmetric placement of the receptor-binding sites on the ligand.

Gene Expression Profiling to Identify Oncogenic Determinants of Autocrine Human Growth Hormone in Human Mammary Carcinoma

We have exploited a discrepancy in the oncogenic potential of autocrine and exogenous human growth hormone (hGH) in an attempt to identify molecules that could potentially be involved in oncogenic transformation of the human mammary epithelial cell. Microarray analysis of 19,000 human genes identified a subset of 305 genes in a human mammary carcinoma cell line that were remarkably different in their response to autocrine and exogenous hGH. Autocrine and exogenous hGH also regulated 167 common genes. Semiquantitative reverse transcription-PCR confirmed differential regulation of genes by either autocrine or exogenous hGH. Functional analysis of one of the identified autocrine hGH-regulated genes, TFF3, determined that its expression is sufficient to support anchorage-independent growth of human mammary carcinoma cells. Small interfering RNA-mediated knockdown of TFF3 concordantly abrogated anchorage-independent growth of mammary carcinoma cells and abrogated the ability of autocrine hGH to stimulate oncogenic transformation of immortalized human mammary epithelial cells. Further functional characterization of the identified subset of specifically autocrine hGH regulated genes will delineate additional novel oncogenes for the human mammary epithelial cell.

The role of growth hormone-receptor antagonism in relation to acromegaly

Acromegaly is a rare but disabling condition associated with reduced life expectancy. It is caused almost invariably by a growth hormone-secreting pituitary adenoma. Transsphenoidal surgery and/or radiotherapy are still considered to be the treatment of choice, but despite recent advances in both these forms of treatment, the overall surgical cure rate remains approximately 60%, and radiotherapy is characterised by delayed effect and a high incidence of hypopituitarism. Medical therapy in the form of dopamine agonists and somatostatin analogues has traditionally been used as an adjunct to surgery and/or radiotherapy, but is increasingly being used as first line therapy in the treatment of acromegaly. Recently, a third form of medical therapy, the growth hormone receptor antagonist, pegvisomant, has been licensed for use in acromegaly. This article examines the design, properties, clinical efficacy and safety of pegvisomant.

History and future of growth hormone research

The understanding of the mechanisms of growth hormone (GH) action has seen great accomplishments over the last two decades. These achievements include the cloning of a variety of GH and GH receptor (GHR) genes and cDNAs; solving of the three-dimensional structure of GH and the GH/GHR complex, and the discovery of GH antagonists. These GH antagonists have resulted in a new class of drugs with important clinical implications. Animal models in which the GH/insulin-like growth factor (IGF)-I axis has been perturbed also have resulted in many novel findings. We have now entered the era of genomics and proteomics. Genes and proteins that are up- or downregulated as a function of GH action (or lack thereof) will add to the repertoire of knowledge that will lead to a better understanding of the molecular basis of GH action.

Ubiquitin system-dependent regulation of growth hormone receptor signal transduction

The growth hormone (GH) receptor is a key regulator of cellular metabolism. Unlike most growth factor receptors, its downregulation is not initiated by its ligand. Like many growth factor receptors, specific molecular mechanisms guarantee that a receptor can signal only once in its lifetime. Three features render the GH receptor unique: (a) an active ubiquitination system is required for both uptake (endocytosis) and degradation in the lysosomes; (b) uptake of the receptor is a continuous process, independent of both GH binding and Jak2 signal transduction; (c) only the cell surface expression of dimerised GH receptors is controlled by the ubiquitin system. This system enables two independent regulatory mechanisms for the endocrinology of the GH/GHR axis: the pulsatile secretion of GH by the pituitary and the GH sensitivity of individual cells of the body by the effects of the ubiquitin system on GH receptor availability.

The inhibition of apoptosis in EL4 lymphoma cells overexpressing growth hormone

The antiapoptotic action of exogenous growth hormone (GH) has been reported for several lymphoid cell lines; however, the potential role of endogenous GH in apoptosis has not been thoroughly investigated. This study was designed to investigate the effects of endogenous GH on apoptosis induced by methyl methanesulfonate (MMS) in a T cell lymphoma overexpressing GH (GHo). The results of these experiments have shown that in EL4 lymphoma cells, overexpression of GH sustained viability after exposure to MMS compared to control cells. The extent of DNA fragmentation measured by ladder formation on agarose gels was reduced in GHo cells following treatment with MMS, when compared to control cells. Adding exogenous GH to control cells and treatment of GHo cells with antibodies to GH had no effect on MMS-induced DNA ladder formation. In further studies, DNA microarray analysis suggested a marked decrease in the constitutive expression of bax, BAD, and caspases 3, 8, and 9 in GHo cells compared to controls. In addition, after treatment with MMS, the activities of caspases 2, 3, 6, 8, and 9 were all lower than control in GHo cells. Western blot analysis detected an increase in Bcl-2 while the levels of nuclear factor kappa B (NFkappaB) remained unchanged in GHo cells. Treatment of EL4 cells with antisense deoxyoligonucleotides to GH and specific inhibitors of NFkappaB (SN-50) increased DNA fragmentation. GHo cells show increased levels of phosphorylated Akt and GSK-3, suggesting inactivation of this proapoptotic protein. The results, taken together with our previous data which showed increased nitric oxide formation in GHo cells, suggest a possible mechanism for the antiapoptotic effects of endogenous GH through the production of nitric oxide and support the idea that endogenous GH may play an important role in the survival of lymphocytes exposed to stressful stimuli.

Function of a conserved residue in the amino terminal alpha-helix of four helical bundle cytokines

The Glu residue in the helix A is conserved among many cytokines. Mutation in this residue converts some cytokines to an antagonist. Such an artificial cytokine with an antagonist activity may be useful in a clinical area. In this study we generated a mutant granulocyte colony-stimulating factor (G-CSF) termed G-CSF.E20K in which this residue is substituted to Lys. It is known that G-CSF binds to a homodimeric receptor, while other cytokines which can be converted to antagonists bind to heterodimeric receptors. We showed that G-CSF.E20K does not bind to the receptor at all, and that it fails to stimulate proliferation. Thus, the mutant did not act as an antagonist. We propose that the nature of the receptor, namely whether it is a homodimer or heterodimer, determines the antagonist activity of the mutant.

Effects of rGH and G118RrGH on the induction of CYP2C12 and IGF-I in primary rat hepatocytes

We have investigated the induction of the CYP2C12 and IGF-I genes by rGH and a "binding site 2 mutant", G118RrGH, in primary hepatocytes derived from male and female rats. Both the basal and the induced levels of CYP2C12, but not of IGF-I, were markedly lower in male derived than in female derived hepatocytes. A lower degree of receptor occupancy appears needed to elicit the CYP2C12 than the IGF-I response in cells obtained from both gender. At 7h of GH exposure high doses of rGH caused a bell-shaped induction of IGF-1 and a plateau was reached for CYP2C12 in female derived cells. G118RrGH did not antagonize the rGH effect; in contrast G118RrGH had an agonistic effect on CYP2C12 induction in female derived cells that in dose-response experiments was bell-shaped. The difficulty in achieving bell-shaped dose-response curves with rGH, together with the lack of an antagonistic effect of G118RrGH, suggests that both binding sites of rGH have similar affinity for the rGHR. The agonistic and bell-shaped dose-related effect of G118RrGH on CYP2C12 expression indicates that G118RrGH interacts with two receptors. The results could also imply that rGH, via its site 2, can interact with other receptor molecules than rGHR.

Impact of experimental blockade of peripheral growth hormone (GH) receptors on the kinetics of endogenous and exogenous GH removal in healthy women and men

Organs that respond to and metabolize GH are enriched in cognate high-affinity receptors. However, whether isologous receptors mediate the de facto access of ligand to cellular degradative pathways is not known. To address this query, we assessed the distribution and whole-body elimination kinetics of (endogenous and exogenous) GH before and after administration of a novel, potent, and selective recombinant human (rh) GH receptor antagonist peptide, pegvisomant. Sixteen healthy young adults (nine men and seven women) participated in a double-blind, prospectively randomized, within-subject cross-over study. The intervention comprised a single sc injection of placebo vs. a high dose of pegvisomant (1 mg/kg sc) timed 62 and 74 h before the overnight sampling and daytime infusion sessions, respectively. The half-life, metabolic clearance rate (MCR), and distribution volume of GH were quantitated by way of: 1) deconvolution analysis of serum GH concentration time series collected every 10 min for 10 h; 2) exponential regression analysis of the decay of GH concentrations after a 6-min iv pulse of rhGH (1 and 10 micro g/kg); 3) calculation of the MCR during constant iv infusion of rhGH (0.5 and 5.0 micro g/kg every 2 h); and 4) exponential fitting of the elimination time-course of GH concentrations following cessation of each constant infusion. Concentrations of GH and pegvisomant were measured in separate, noncross-reactive, two-site monoclonal, immunofluorometric assays. Pegvisomant concentrations averaged 4860 +/- 480 micro g/liter (+/-SEM) across the infusion interval, thus exceeding low steady state GH concentrations by 3000-fold. Inhibitory efficacy of the GH receptor antagonist peptide was affirmed by way of a 34% reduction in the serum total IGF-I concentration, i.e., from 257 +/- 37 (placebo) to 170 +/- 24 (drug) micro g/liter (P < 0.001); and a reciprocal 77% elevation of the (10-h) mean GH concentration, i.e., from 1.3 +/- 0.23 (placebo) to 2.3 +/- 0.42 (drug) micro g/liter (P = 0.003). ANOVA disclosed that prior administration of pegvisomant (compared with placebo) did not alter: 1) the calculated half-life (minutes) of secreted GH, which averaged 15 +/- 1.3 (placebo) and 14 +/- 0.69 (drug); 2) the half-time of disappearance (minutes) of an iv pulse of rhGH, 15 +/- 1.0 (placebo) and 13 +/- 0.5 (drug) (for the 10 micro g/kg dose); 3) the distribution volume (milliliters per kilogram) of rhGH, 59 +/- 6.2 (placebo) and 58 +/- 3.5 (drug); 4) the steady state GH concentration (micrograms per liter) attained during constant iv infusion of rhGH (at a rate of 5 micro g/kg every 2 h), 18.2 +/- 2.4 (placebo) and 18.3 +/- 2.3 (drug); 5) the half-life (minutes) of elimination of GH from equilibrium, 16 +/- 0.98 (placebo) and 16 +/- 1.8 (drug); and 6) the steady state MCR (liters per kilogram per day) of rhGH, 3.8 +/- 0.32 (placebo) and 3.5 +/- 0.31 (drug). In ensemble, the present data refute the a priori postulate that vascular-accessible GH receptors determine the in vivo pseudoequilibrium kinetics of GH disappearance in the human.

The ubiquitin-proteasome pathway and the regulation of growth hormone receptor availability

The number of growth hormone receptors (GHR) per cell are regulated and this feature plays a major role in the hormone responsiveness of the body. This article deals with the regulatory mechanisms underlying the availability of GHR for serum growth hormone. The availability of membrane proteins at the cell surface can be regulated at different locations within the cell: (1) The amount of protein synthesized in the endoplasmic reticulum (ER) is largely controlled by gene transcription. In addition, the ER quality control system regulates the exiting of properly folded proteins from the ER. (2) In the trans-Golgi network, proteins can either be diverted directly to the lysosomes or be transported to the cell surface. (3) At the plasma membrane, the endocytic machinery can select proteins for endocytosis via clathrin-coated pits or proteins may be subject to proteolysis, resulting in shedding of the extracellular domain. (4) In endosomes, internalized proteins are either recycled back to the plasma membrane or targeted to the lysosome for degradation. At each of these cellular locations the ubiquitin-proteasome pathway can specifically regulate protein levels via different mechanisms. In transfected Chinese hamster lung cells, GHR availability is determined by three factors: endocytosis (75%), shedding (10%), and other undetermined mechanisms (15%). As outlined in this article the level of GHR at the cell surface, defined as GHR availability, is mainly regulated by the ubiquitin-proteasome pathway.

Growth hormone receptor antagonists: discovery, development, and use in patients with acromegaly

An understanding of the events that occur during GH receptor (GHR) signaling has facilitated the development of a GHR antagonist (pegvisomant) for use in humans. This molecule has been designed to compete with native GH for the GHR and to prevent its proper or functional dimerization-a process that is critical for GH signal transduction and IGF-I synthesis and secretion. Clinical trials in patients with acromegaly show GHR blockade to be an exciting new mode of therapy for this condition, and pegvisomant may have a therapeutic role in diseases, such as diabetes and malignancy, in which abnormalities of the GH/IGF-I axis have been observed. This review charts the discovery and development of GHR antagonists and details the experience gained in patients with acromegaly.

Ligand-independent growth hormone receptor dimerization occurs in the endoplasmic reticulum and is required for ubiquitin system-dependent endocytosis

The regulatory effect of growth hormone (GH) on its target cells is mediated via the GH receptor (GHR). GH binding to the GHR results in the formation of a GH-(GHR)(2) complex and the initiation of signal transduction cascades via the activation of the tyrosine kinase JAK2. Subsequent endocytosis and transport to the lysosome of the ligand-receptor complex is regulated via the ubiquitin system and requires the presence of an intact ubiquitin-dependent endocytosis (UbE) motif in the cytosolic tail of the GHR. Recently, the model of ligand-induced receptor dimerization has been challenged. In this study, ligand-independent GHR dimerization is demonstrated in the endoplasmic reticulum and at the cell surface by coimmunoprecipitation of an epitope-tagged truncated GHR with wild-type GHR. In addition, evidence is provided that the extracellular domain of the GHR is not required to maintain this interaction. Internalization of a chimeric receptor, which fails to dimerize, is independent of an intact UbE-motif. Therefore, we postulate that dimerization of GHR molecules is required for ubiquitin system-dependent endocytosis.

A novel bioassay for human somatogenic activity in serum samples supports the clinical reliability of immunoassays

OBJECTIVE

Because there is discordance between different immunoassay values for serum hGH, and because clinical state may not correlate with immunoreactive hGH, we have developed an assay to accurately measure serum hGH somatogenic bioactivity. The results of this assay were compared with the Elegance two-site ELISA assay across 135 patient samples in a variety of clinical states.

DESIGN

The somatogenic assay was based on stable expression of hGH receptor in the murine BaF line, allowing these cells to proliferate in response to hGH. To eliminate interference by other growth factors in serum, we created a specific antagonist of the hGH receptor (similar to Trovert or Pegvisomant) which allowed us to obtain a true measure of hGH somatogenic activity by subtraction of the activity in the presence of the antagonist. The assay was carried out in microtiter plates over 24 h, with oxidation of a chromogenic tetrazolium salt (MTT) as the endpoint.

PATIENTS

These encompassed a number of different clinical conditions related to short stature, including idiopathic short stature, neurosecretory dysfunction and renal failure, as well as obese patients on dietary restriction and normal volunteers.

MEASUREMENTS

In addition to the colourimetric (MTT) response to hGH, we measured free hGH by stripping out GHBP-bound hGH using beads coupled to a monoclonal antibody to the GHBP (GH binding protein). All samples were measured in both bioassay and ELISA assay.

RESULTS

This bioassay was sensitive (5 mU/l or 2 microg/l) and precise, and not subject to interference by the GHBP. There was a good correlation (r = 0.95) between bioactivity and immunoactivity across clinical states. There was, however, an increased bioactivity during secretory peaks (over 25 mU/l), which has been reported previously for the Nb2 bioassay. Free hGH did not correlate with clinical state.

CONCLUSIONS

Because the results of the Elegance ELISA and the bioassay correlate well, even though there is greater bioactivity at higher hormone concentrations, it is evident that an appropriate immunoassay is able to act as a reliable indicator for clinical assessment. In those rare cases where bio-inactive GH exists, our bioassay should provide an appropriate means to demonstrate this.

Receptor dimerization in GH and erythropoietin action--it takes two to tango, but how?

The receptors for GH and erythropoietin are members of the cytokine receptor superfamily. They are single membrane-spanning proteins that bind ligand in the extracellular domain and couple to cytosolic JAK tyrosine kinases to initiate signaling. The ligand-engaged GH receptor (GHR) and erythropoietin receptor (EpoR) extracellular domains are believed to exist in a dimerized configuration in which a single ligand molecule engages two receptor extracellular domains. The last several years have witnessed a rapid expansion in our knowledge of the structural and functional details of this dimerization process and have forced a reexamination of how the ligand-containing complexes achieve their conformation. For EpoR, there is good evidence that the unliganded receptor is already a preformed dimer that is activated by a ligand-induced change in the receptor conformation. Owing in some measure to the unavailability of the analogous crystal structure of the unliganded GHR extracellular domain, it is still unknown whether GHR adopts a similar preformed dimer/conformational change in response to GH as is found for EpoR. This review critically examines the state of our knowledge pertaining to GHR and EpoR dimerization, noting differences and similarities between the two.

Identification, isolation, and cloning of growth hormone (GH)-inducible interscapular brown adipose complementary deoxyribonucleic acid from GH antagonist mice

In a dwarf mouse line that expresses a GH antagonist, we found that the interscapular brown adipose tissue (iBAT) mass is significantly greater than that in nontransgenic littermates. We proposed that gene expression in iBAT may be up- or down-regulated by GH. To identify these genes, we employed the PCR-select subtraction approach to construct subtractive libraries from iBAT total RNAs. We have generated forward and reverse subtractive libraries. Clones were screened by differential hybridization and identified by BLAST similarity to expressed sequence tags and complementary DNA sequences. Four novel expressed sequence tags were isolated from the reverse subtractive library. Of them, clone 42, was further analyzed. It encodes a 2475-bp messenger RNA with an open reading frame of 346 amino acids. Northern blot analysis demonstrated two RNA isoforms (2.5 and 1.3 kb) in various tissues. Differential expression of both isoforms was verified in GH antagonist and nontransgenic mouse iBAT. BLAST searches suggested that clone 42 is highly homologous to a gene found in a human female fetal brain and a related gene found in a human pituitary tumor.

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.

Regulators of growth hormone signaling

Growth hormone acts through binding to membrane receptors that belong to the cytokine receptor superfamily. Ligand binding induces receptor dimerization and activation of the receptor-associated kinase: JAK2; this results in phosphorylation of the kinase itself, of the receptor, and of many cellular proteins. Among these are the Stat proteins as well as adaptors leading to the activation of the Ras/MAP kinase pathway and of the PI-3 kinase pathway. Activation by growth hormone is very transient and several mechanisms are involved in this downregulation: internalization and degradation of the receptor and recruitment of phosphatases or of specific inhibitors of the JAK/Stat pathway, the SOCS proteins.

Functional domains of human growth hormone necessary for the adipogenic activity of hGH/hPL chimeric molecules

Genetic analysis through construction of chimeric genes and their transfection in mammalian cells could provide a better understanding of biological functions of native or modified proteins, and would allow the design of new gene constructs encoding peptides that mimic or block ligand interaction with target tissues. To identify the hGH domains responsible for induction of adipose differentiation we constructed hGH/hPL chimeric molecules using homologous DNA mutagenesis, since hGH, but not human placental lactogen (hPL), promotes adipose differentiation in mouse 3T3-F442A cells. We assayed their adipogenic activity in an autocrine/paracrine biological model consisting of transiently transfected 3T3-F442A cells with the chimeric constructs. Plasmid DNAs carrying these constructs were transfected into growing 3T3-F442A cells, and cultures were further maintained for 7 days to differentiate into adipocytes. Secretion of transfected hGH/hPL chimeric proteins into the medium was in the range of 5–25 ng/ml. Adipogenic activity was a property only of those chimeric proteins that contained hGH exon III together with either hGH exon II or hGH IV. Our results also suggest that hGH binding site-2 is composed of two structural subdomains: subsite 2A encoded by exon II of hGH and subsite-2B encoded by exon IV. We also suggest that full adipogenic activity requires the presence of binding site-1 and any of the subsites of binding site-2. This simple autocrine/paracrine biological model of gene transfection allows the analysis of specific biological activity of products encoded by modified genes.

Transgenic models of growth hormone action

A growth-promoting principle of the pituitary gland was discovered in 1921, and bovine growth hormone (GH) was isolated in 1944. Since then, the structure of GH as it relates to its biological activities has been an exciting research topic. Equally fascinating is the relationship between GH structure and its metabolic activities. In attempts to define some of these activities, several investigators have used GH transgenic mice as models. In this review we summarize what is known about the molecular mechanisms of GH action. We then describe some of the GH transgenic models and point out potential targets for nutrition research.

Subcellular localization and internalization of the four human leptin receptor isoforms

There are four known isoforms of the human leptin receptor (HLR) with different C-terminal cytoplasmic domains (designated by the number of unique C-terminal amino acids). In cells expressing HLR-5, -15, or -274, 15-25% of the leptin binding sites were located at the plasma membrane. In contrast, in cells expressing HLR-67, only 5% of the total binding sites were at the plasma membrane. Immunofluorescent microscopy showed that all four isoforms partially co-localized with calnexin and beta-COP, markers of the endoplasmic reticulum and the Golgi, respectively. All isoforms were also detected in an unidentified punctate compartment. All isoforms were internalized via clathrin-mediated endocytosis, but at different rates. After 20 min at 37 degrees C, 45% of a bound cohort of labeled ligand had been internalized by HLR-15, 30% by HLR-67, 25% by HLR-274, and 15% by HLR-5. Degradation of internalized leptin occurred in lysosomes. Overnight exposure to leptin down-regulated all isoforms, but to a variable extent. HLR-274 displayed the greatest down-regulation and also appeared to reach lysosomes more quickly than the other isoforms. The faster degradation of HLR-274 may help to terminate leptin signaling.

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.

Heteromeric and homomeric transforming growth factor-beta receptors show distinct signaling and endocytic responses in epithelial cells

Transforming growth factor-beta (TGF-beta) induces distinct responses dependent upon the cellular context. It is unclear whether the initial receptor interactions identified in one cell type will be operative in another. Utilizing a chimeric receptor strategy we have examined the signaling and endocytic activity of both heteromeric (type I/type II) and homomeric (type I/type I or type II/type II) TGF-betaR interactions in Mv1Lu epithelial cells. In agreement with that observed in mesenchymal cells, all TGF-betaR signaling in Mv1Lu cells required the formation of a heteromeric type I-type II receptor complex. However, the initial endocytic response to TGF-betaR oligomerization was distinctly regulated in the two cell types. While heteromeric TGF-beta receptors were internalized and down-regulated, homomeric TGF-betaR interactions showed diminished endocytic activity in Mv1Lu cells. This contrasts to that observed in mesenchymal cultures where ligand bound to TGF-betaR homomers was internalized, yet the receptors were not down-regulated. Moreover, while previous reports have suggested that mutations at serine 172 or threonine 176 in the type I TGF-betaR separated transcriptional from proliferative responses, we found no separation of pathways or effect on initial endocytic activity when the analogous mutations were made in the chimeric receptors.

Homodimerization restores biological activity to an inactive erythropoietin mutant

Erythropoietin (Epo) is believed to transduce a signal by bringing two Epo receptors into close proximity, enabling cross-phosphorylation. We compared monomeric Epos with homodimers in which two Epo monomers are linked by polyglycine. Monomeric Epo mutant R103A is unable to support Epo-dependent cell growth or trigger Janus kinase 2 and STAT5 activation, even at concentrations greater than 7,000 times that sufficient for wild-type Epo activity. In contrast, R103A homodimer induces proliferation and transduces signal at concentrations similar to that of wild-type Epo monomer and homodimer. These experiments show that two discrete domains on Epo are required for receptor binding and activation. Our results also suggest that the EpoR can be dimerized by different forms and sizes of molecules, as long as two recognition motifs are provided in the same molecule. Design of other dimeric molecules may enhance our understanding of cytokine specificity and signal transduction.

The molecular basis of growth hormone action

Recent studies have begun to elucidate the molecular actions of growth hormone, a major regulator of somatic growth and metabolic functions. The cell surface growth hormone receptor, a member of the cytokine receptor superfamily, binds as a dimer to a single growth hormone molecule. Receptor dimerization precedes signal transduction, which is predominantly mediated by the non-receptor tyrosine kinase, Jak2. Activation of Jak2 leads to mitogenic proliferation, phosphorylation of intracellular proteins, MAP kinase activation, activation of Stats 1, 3, and 5, and induction of target gene expression. Specific cytoplasmic domains of the growth hormone receptor mediate Jak2 activation, metabolic actions of growth hormone, Stat activation, and calcium influx.

Distinct endocytic responses of heteromeric and homomeric transforming growth factor beta receptors

Transforming growth factor beta (TGF beta) family ligands initiate a cascade of events capable of modulating cellular growth and differentiation. The receptors responsible for transducing these cellular signals are referred to as the type I and type II TGF beta receptors. Ligand binding to the type II receptor results in the transphosphorylation and activation of the type I receptor. This heteromeric complex then propagates the signal(s) to downstream effectors. There is presently little data concerning the fate of TGF beta receptors after ligand binding, with conflicting reports indicating no change or decreasing cell surface receptor numbers. To address the fate of ligand-activated receptors, we have used our previously characterized chimeric receptors consisting of the ligand binding domain from the granulocyte/macrophage colony-stimulating factor alpha or beta receptor fused to the transmembrane and cytoplasmic domain of the type I or type II TGF beta receptor. This system not only provides the necessary sensitivity and specificity to address these types of questions but also permits the differentiation of endocytic responses to either homomeric or heteromeric intracellular TGF beta receptor oligomerization. Data are presented that show, within minutes of ligand binding, chimeric TGF beta receptors are internalized. However, although all the chimeric receptor combinations show similar internalization rates, receptor down-regulation occurs only after activation of heteromeric TGF beta receptors. These results indicate that effective receptor down-regulation requires cross-talk between the type I and type II TGF beta receptors and that TGF beta receptor heteromers and homomers show distinct trafficking behavior.

Conformational changes required in the human growth hormone receptor for growth hormone signaling

Growth hormone (GH) plays a significant role in normal growth and development. Signaling to the cell is believed to require growth hormone receptor (GHR) dimerization, which occurs following binding of a single growth hormone molecule to each of two receptors. We have developed human growth hormone receptor-specific monoclonal antibodies, one of which was used here to characterize hormone/receptor interactions. This antibody, GHR05, is directed against the hinge spanning subdomains I and II of the receptor's extracellular region. Antibody binding to the cell surface receptor increases upon receptor binding to growth hormone, but not when it binds a mutant form, hGHG120R, which does not trigger receptor activation. Growth hormone binding thus appears to lead to a conformational change in the receptor epitope recognized by GHR05, giving rise to the active dimer configuration, necessary for signal transduction. Using a chimeric receptor-expressing, growth hormone-dependent murine cell line, we find that GHR05 binds to the receptor in the absence of human GH and delivers a signal leading to cell proliferation. Finally, GHR05 treatment of IM-9 cells, a human cell line expressing a functional human GHR, leads to cell proliferation mediated by the generation of GH-specific signals, including phosphorylation of the JAK2 tyrosine kinase and activation of STAT5.

Crystal structure of an antagonist mutant of human growth hormone, G120R, in complex with its receptor at 2.9 A resolution

Human growth hormone binds two receptor molecules and thereby induces signal transduction through receptor dimerization. At high concentrations, growth hormone acts as an antagonist because of a large difference in affinities at the respective binding sites. This antagonist action can be enhanced further by reducing binding in the low affinity binding site. A growth hormone antagonist mutant Gly-120 --> Arg, has been crystallized with its receptor as a 1:1 complex and the crystal structure determined at 2.9 A resolution. The 1:1 complex is remarkably similar to the native growth hormone-receptor 1:2 complex. A comparison between the two structures reveals only minimal differences in the conformations of the hormone or its receptor in the two complexes, including the angle between the two immunoglobulin-like domains of the receptor. Further, two symmetry-related 1:1 complexes in the crystal form a 2:2 complex with a large solvent inaccessible area between two receptor molecules. In addition, we present here a native human growth hormone-human growth hormone-binding protein 1:2 complex structure at 2.5 A resolution. One important difference between our structure and the previously published crystal structure at 2.8 A is revealed. Trp-104 in the receptor, a key residue in the hormone-receptor interaction, has an altered conformation in the low affinity site enabling a favorable hydrogen bond to be formed with Asp-116 of the hormone.