Disulfide linkage of growth hormone (GH) receptors (GHR) reflects GH-induced GHR dimerization. Association of JAK2 with the GHR is enhanced by receptor dimerization

The Mammary Gland: Basic Structure and Molecular Signaling during Development

The mammary gland is a compound, branched tubuloalveolar structure and a major characteristic of mammals. The mammary gland has evolved from epidermal apocrine glands, the skin glands as an accessory reproductive organ to support postnatal survival of offspring by producing milk as a source of nutrition. The mammary gland development begins during embryogenesis as a rudimentary structure that grows into an elementary branched ductal tree and is embedded in one end of a larger mammary fat pad at birth. At the onset of ovarian function at puberty, the rudimentary ductal system undergoes dramatic morphogenetic change with ductal elongation and branching. During pregnancy, the alveolar differentiation and tertiary branching are completed, and during lactation, the mature milk-producing glands eventually develop. The early stages of mammary development are hormonal independent, whereas during puberty and pregnancy, mammary gland development is hormonal dependent. We highlight the current understanding of molecular regulators involved during different stages of mammary gland development.

Diphenylbutylpiperidine Antipsychotic Drugs Inhibit Prolactin Receptor Signaling to Reduce Growth of Pancreatic Ductal Adenocarcinoma in Mice

BACKGROUND & AIMS

Prolactin (PRL) signaling is up-regulated in hormone-responsive cancers. The PRL receptor (PRLR) is a class I cytokine receptor that signals via the Janus kinase (JAK)-signal transducer and activator of transcription and mitogen-activated protein kinase pathways to regulate cell proliferation, migration, stem cell features, and apoptosis. Patients with pancreatic ductal adenocarcinoma (PDAC) have high plasma levels of PRL. We investigated whether PRLR signaling contributes to the growth of pancreatic tumors in mice.

METHODS

We used immunohistochemical analyses to compare levels of PRL and PRLR in multitumor tissue microarrays. We used structure-based virtual screening and fragment-based drug discovery to identify compounds likely to bind PRLR and interfere with its signaling. Human pancreatic cell lines (AsPC-1, BxPC-3, Panc-1, and MiaPaCa-2), with or without knockdown of PRLR (clustered regularly interspaced short palindromic repeats or small hairpin RNA), were incubated with PRL or penfluridol and analyzed in proliferation and spheroid formation. C57BL/6 mice were given injections of UNKC-6141 cells, with or without knockdown of PRLR, into pancreas, and tumor development was monitored for 4 weeks, with some mice receiving penfluridol treatment for 21 days. Human pancreatic tumor tissues were implanted into interscapular fat pads of NSG mice, and mice were given injections of penfluridol daily for 28 days. Nude mice were given injections of Panc-1 cells, xenograft tumors were grown for 2 weeks, and mice were then given intraperitoneal penfluridol for 35 days. Tumors were collected from mice and analyzed by histology, immunohistochemistry, and immunoblots.

RESULTS

Levels of PRLR were increased in PDAC compared with nontumor pancreatic tissues. Incubation of pancreatic cell lines with PRL activated signaling via JAK2-signal transducer and activator of transcription 3 and extracellular signal-regulated kinase, as well as formation of pancospheres and cell migration; these activities were not observed in cells with PRLR knockdown. Pancreatic cancer cells with PRLR knockdown formed significantly smaller tumors in mice. We identified several diphenylbutylpiperidine-class antipsychotic drugs as agents that decreased PRL-induced JAK2 signaling; incubation of pancreatic cancer cells with these compounds reduced their proliferation and formation of panco spheres. Injections of 1 of these compounds, penfluridol, slowed the growth of xenograft tumors in the different mouse models, reducing proliferation and inducing autophagy of the tumor cells.

CONCLUSIONS

Levels of PRLR are increased in PDAC, and exposure to PRL increases proliferation and migration of pancreatic cancer cells. Antipsychotic drugs, such as penfluridol, block PRL signaling in pancreatic cancer cells to reduce their proliferation, induce autophagy, and slow the growth of xenograft tumors in mice. These drugs might be tested in patients with PDAC.

Autocrine/paracrine actions of growth hormone in human melanoma cell lines

Melanoma is the most aggressive skin cancer. Its aggressiveness is most commonly attributed to ERK pathway mutations leading to constitutive signaling. Though initial tumor regression results from targeting this pathway, resistance often emerges. Interestingly, interrogation of the NCI-60 database indicates high growth hormone receptor (GHR) expression in melanoma cell lines. To further characterize melanoma, we tested responsiveness to human growth hormone (GH). GH treatment resulted in GHR signaling and increased invasion and migration, which was inhibited by a GHR monoclonal antibody (mAb) antagonist in WM35, SK-MEL 5, SK-MEL 28 and SK-MEL 119 cell lines. We also detected GH in the conditioned medium (CM) of human melanoma cell lines. GHR, JAK2 and STAT5 were basally phosphorylated in these cell lines, consistent with autocrine/paracrine GH production. Together, our results suggest that melanomas are enriched in GHR and produce GH that acts in an autocrine/paracrine manner. We suggest that GHR may constitute a therapeutic target in melanoma.

Growth hormone (GH) receptor (GHR)-specific inhibition of GH-Induced signaling by soluble IGF-1 receptor (sol IGF-1R)

Human growth hormone (GH) binds and activates GH receptor (GHR) and prolactin (PRL) receptor (PRLR). LNCaP human prostate cancer cells express only GHR. A soluble fragment of IGF-1 receptor (IGF-1R) extracellular domain (sol IGF-1R) interacts with GHR and blocks GH signaling. We now explore sol IGF-1R's specificity for inhibiting GH signaling via GHR vs. PRLR and test GHR and PRLR extracellular domain inhibition determinants. Although T47D human breast cancer cells express GHR and PRLR, GH signaling is largely PRLR-mediated. In T47D, sol IGF-1R inhibited neither GH- nor PRL-induced STAT5 activation. However, sol IGF-1R inhibited GH-induced STAT5 activation in T47D-shPRLR cells, which harbor reduced PRLR. In MIN6 mouse β-cells, bovine GH (bGH) activates mouse GHR, not PRLR, while human GH activates mouse GHR and PRLR. In MIN6, sol IGF-1R inhibited bGH-induced STAT5 activation, but partially inhibited human GH-induced STAT5 activation. These findings suggest sol IGF-1R's inhibition is GHR-specific. Using a cellular reconstitution system, we compared effects of sol IGF-1R on signaling through GHR, PRLR, or chimeras in which extracellular subdomains 2 (S2) of the receptors were swapped. Sol IGF-1R inhibited GH-induced STAT5 activation in GHR-expressing, not PRLR-expressing cells, consistent with GHR specificity of sol IGF-1R. Interestingly, we found that GHR S2 (which harbors the GHR-GHR dimer interface) was required, but not sufficient for sol IGF-1R inhibition of GHR signaling. These results suggest sol IGF-1R specifically inhibits GH-induced GHR-mediated signaling, possibly through interaction with GHR S1 and S2 domains. Our findings have implications for GH antagonist development.

Chicken GHR natural antisense transcript regulates GHR mRNA in LMH cells

Growth hormone receptor (GHR) played key roles in human and animal growth. Both human laron type dwarfism and sex linked dwarf chicken were caused by the mutation of GHR gene. In this study, we identified an endogenously expressed long non-coding natural antisense transcript, GHR-AS, which overlapped with the GHR mRNA (GHR-S) in a tail to tail manner. Spatial and temporal expression analyses indicated that GHR-AS were highly expressed in chicken liver and displayed ascending with the development of chicken from E10 to 3 w of age. Interfering GHR-AS caused GHR-S decreasing, accompanied with increasing of the inactive gene indicator, H3K9me2, in the GHR-S promoter regions in LMH cells. RNase A experiment exhibited that GHR-AS and GHR-S can form double strand RNAs at the last exon of GHR gene in vivo and in vitro, which hinted they could act on each other via the region. In addition, the levels of GHR-S and GHR-AS can be affected by DNA methylation. Compared the normal chicken with the dwarfs, the negative correlation trends were showed between the GHR-S promoter methylation status and the GHR-AS levels. This is the first report of that GHR gene possessed natural antisense transcript and the results presented here further highlight the fine and complicated regulating mechanism of GHR gene in chicken development.

Subdomain 2, Not the Transmembrane Domain, Determines the Dimerization Partner of Growth Hormone Receptor and Prolactin Receptor

Growth hormone receptor (GHR) and prolactin (PRL) receptor (PRLR) are homologous transmembrane class I cytokine receptors. In humans, GH interacts with GHR homodimers or PRLR homodimers and PRL interacts with only PRLR homodimers to promote signaling. In human breast cancer cells endogenously expressing both receptors, GHR and PRLR specifically coimmunoprecipitate. We previously devised a split luciferase complementation assay to study GHR and PRLR assemblages. In this technique, firefly luciferase is split into two fragments (N- and C-terminal fragments of the luciferase), each without enzyme activity and tethered to the tails of two receptors. The fragments restore luciferase activity when brought close to each other by the receptors. Real-time ligand-induced complementation changes reflect the arrangement of receptors and indicate that GHR/PRLR is arranged as a heteromultimer comprised of GHR-GHR homodimers and PRLR-PRLR homodimers. We now dissect determinants for GHR and PRLR homodimerization versus heteroassociation. GHR and PRLR have extracellular domains comprised of the ligand-binding N-terminal subdomain 1 and a membrane-proximal subdomain 2 (S2), which fosters receptor-receptor contact. Based on previous studies of S2 versus the transmembrane domain (TMD) in GHR dimerization, we constructed GHR(PRLRS2), GHR(PRLRS2-TMD), and GHR(PRLRTMD), replacing GHR's S2 alone, S2 plus TMD, and TMD alone with PRLR's counterpart. We tested by complementation the ability of these chimeras and GHR or PRLR to homodimerize or heteroassociate. Comparing various combinations, we found GHR(PRLRS2) and GHR(PRLRS2-TMD) behaved as PRLR, whereas GHR(PRLRTMD) behaved as GHR regarding their dimerization partners. We conclude that S2 of GHR and PRLR, rather than their TMDs, determines their dimerization partner.

TIMP3 Modulates GHR Abundance and GH Sensitivity

GH receptor (GHR) binds GH at the cell surface via its extracellular domain and initiates intracellular signal transduction, resulting in important anabolic and metabolic actions. GH signaling is subject to dynamic regulation, which in part is exerted by modulation of cell surface GHR levels. Constitutive and inducible metalloprotease-mediated cleavage of GHR regulate GHR abundance and thereby modulate GH action. We previously demonstrated that GHR proteolysis is catalyzed by the TNF-α converting enzyme (TACE; ADAM17). Tissue inhibitors of metalloproteases-3 (TIMP3) is a natural specific inhibitor of TACE, although mechanisms underlying this inhibition are not yet fully understood. In the current study, we use two model cell lines to examine the relationships between cellular TACE, TIMP3 expression, GHR metalloproteolysis, and GH sensitivity. These two cell lines exhibited markedly different sensitivity to inducible GHR proteolysis, which correlated directly to their relative levels of mature TACE vs unprocessed TACE precursor and indirectly to their levels of cellular TIMP3. Our results implicate TIMP3 as a modulator of cell surface GHR abundance and the ability of GH to promote cellular signaling; these modulatory effects may be conferred by endogenous TIMP3 expression as well as exogenous TIMP3 exposure. Furthermore, our analysis suggests that TIMP3, in addition to regulating the activity of TACE, may also modulate the maturation of TACE, thereby affecting the abundance of the active form of the enzyme.

Development and characterization of a novel GHR antibody antagonist, GF185

Here, we describe the development of a panel of monoclonal antibodies targeting the growth hormone receptor (GHR). Of these monoclonal antibodies (Mabs), GF185 was selected for further characterization due to its activities. Competitive receptor-binding assays and Western blotting analyses were used to demonstrate that GF185's epitopes are localized within subdomain 1 of the growth hormone receptor extracellular domain (GHR-ECD). Subsequently, we evaluated GF185's antagonistic activities in vivo and in vitro and showed that GF185 was able to neutralize growth hormone (GH) signalling and inhibit GH-induced Ba/F3-GHR proliferation. Our findings suggest that GF185 may serve as an attractive tool for GHR-related research and has a potential future application for the treatment of GH-dependent disease.

JAK2 activation by growth hormone and other cytokines

Growth hormone (GH) and structurally related cytokines regulate a great number of physiological and pathological processes. They do this by coupling their single transmembrane domain (TMD) receptors to cytoplasmic tyrosine kinases, either as homodimers or heterodimers. Recent studies have revealed that many of these receptors exist as constitutive dimers rather than being dimerized as a consequence of ligand binding, which has necessitated a new paradigm for describing their activation process. In the present study, we describe a model for activation of the tyrosine kinase Janus kinase 2 (JAK2) by the GH receptor homodimer based on biochemical data and molecular dynamics simulations. Binding of the bivalent ligand reorientates and rotates the receptor subunits, resulting in a transition from a form with parallel TMDs to one where the TMDs separate at the point of entry into the cytoplasm. This movement slides the pseudokinase inhibitory domain of one JAK kinase away from the kinase domain of the other JAK within the receptor dimer-JAK complex, allowing the two kinase domains to interact and trans-activate. This results in phosphorylation and activation of STATs and other signalling pathways linked to this receptor which then regulate postnatal growth, metabolism and stem cell activation. We believe that this model will apply to most if not all members of the class I cytokine receptor family, and will be useful in the design of small antagonists and agonists of therapeutic value.

Human GH receptor-IGF-1 receptor interaction: implications for GH signaling

GH signaling yields multiple anabolic and metabolic effects. GH binds the transmembrane GH receptor (GHR) to activate the intracellular GHR-associated tyrosine kinase, Janus kinase 2 (JAK2), and downstream signals, including signal transducer and activator of transcription 5 (STAT5) activation and IGF-1 gene expression. Some GH effects are partly mediated by GH-induced IGF-1 via IGF-1 receptor (IGF-1R), a tyrosine kinase receptor. We previously demonstrated in non-human cells that GH causes formation of a GHR-JAK2-IGF-1R complex and that presence of IGF-1R (even without IGF-1 binding) augments proximal GH signaling. In this study, we use human LNCaP prostate cancer cells as a model system to further study the IGF-1R's role in GH signaling. GH promoted JAK2 and GHR tyrosine phosphorylation and STAT5 activation in LNCaP cells. By coimmunoprecipitation and a new split luciferase complementation assay, we find that GH augments GHR/IGF-1R complex formation, which is inhibited by a Fab of an antagonistic anti-GHR monoclonal antibody. Short hairpin RNA-mediated IGF-1R silencing in LNCaP cells reduced GH-induced GHR, JAK2, and STAT5 phosphorylation. Similarly, a soluble IGF-1R extracellular domain fragment (sol IGF-1R) interacts with GHR in response to GH and blunts GH signaling. Sol IGF-1R also markedly inhibits GH-induced IGF-1 gene expression in both LNCaP cells and mouse primary osteoblast cells. On the basis of these and other findings, we propose a model in which IGF-1R augments GH signaling by allowing a putative IGF-1R-associated molecule that regulates GH signaling to access the activated GHR/JAK2 complex and envision sol IGF-1R as a dominant-negative inhibitor of this IGF-1R-mediated augmentation. Physiological implications of this new model are discussed.

Dynamic analysis of GH receptor conformational changes by split luciferase complementation

The transmembrane GH receptor (GHR) exists at least in part as a preformed homodimer on the cell surface. Structural and biochemical studies suggest that GH binds GHR in a 1:2 stoichiometry to effect acute GHR conformational changes that trigger the activation of the receptor-associated tyrosine kinase, Janus kinase 2 (JAK2), and downstream signaling. Despite information about GHR-GHR association derived from elegant fluorescence resonance energy transfer/bioluminescence resonance energy transfer studies, an assessment of the dynamics of GH-induced GHR conformational changes has been lacking. To this end, we used a split luciferase complementation assay that allowed detection in living cells of specific ligand-independent GHR-GHR interaction. Furthermore, GH treatment acutely augmented complementation of enzyme activity between GHRs fused, respectively, to N- and C-terminal fragments of firefly luciferase. Analysis of the temporal pattern of GH-induced complementation changes, pharmacological manipulation, genetic alteration of JAK2 levels, and truncation of the GHR intracellular domain (ICD) tail suggested that GH acutely enhances proximity of the GHR homodimer partners independent of the presence of JAK2, phosphorylation of GHR-luciferase chimeras, or an intact ICD. However, subsequent reduction of complementation requires JAK2 kinase activity and the ICD tail. This conclusion is in contrast to existing models of the GHR activation process.

Mechanism of activation of protein kinase JAK2 by the growth hormone receptor

Signaling from JAK (Janus kinase) protein kinases to STAT (signal transducers and activators of transcription) transcription factors is key to many aspects of biology and medicine, yet the mechanism by which cytokine receptors initiate signaling is enigmatic. We present a complete mechanistic model for activation of receptor-bound JAK2, based on an archetypal cytokine receptor, the growth hormone receptor. For this, we used fluorescence resonance energy transfer to monitor positioning of the JAK2 binding motif in the receptor dimer, substitution of the receptor extracellular domains with Jun zippers to control the position of its transmembrane (TM) helices, atomistic modeling of TM helix movements, and docking of the crystal structures of the JAK2 kinase and its inhibitory pseudokinase domain with an opposing kinase-pseudokinase domain pair. Activation of the receptor dimer induced a separation of its JAK2 binding motifs, driven by a ligand-induced transition from a parallel TM helix pair to a left-handed crossover arrangement. This separation leads to removal of the pseudokinase domain from the kinase domain of the partner JAK2 and pairing of the two kinase domains, facilitating trans-activation. This model may well generalize to other class I cytokine receptors.

Regulation of pancreatic islet beta-cell mass by growth factor and hormone signaling

Dysfunction and destruction of pancreatic islet beta cells is a hallmark of diabetes. Better understanding of cellular signals in beta cells will allow development of therapeutic strategies for diabetes, such as preservation and expansion of beta-cell mass and improvement of beta-cell function. During the past several decades, the number of studies analyzing the molecular mechanisms, including growth factor/hormone signaling pathways that impact islet beta-cell mass and function, has increased exponentially. Notably, somatolactogenic hormones including growth hormone (GH), prolactin (PRL), and insulin-like growth factor-1 (IGF-1) and their receptors (GHR, PRLR, and IGF-1R) are critically involved in beta-cell growth, survival, differentiation, and insulin secretion. In this chapter, we focus more narrowly on GH, PRL, and IGF-1 signaling, and GH-IGF-1 cross talk. We also discuss how these signaling aspects contribute to the regulation of beta-cell proliferation and apoptosis. In particular, our novel findings of GH-induced formation of GHR-JAK2-IGF-1R protein complex and synergistic effects of GH and IGF-1 on beta-cell signaling, proliferation, and antiapoptosis lead to a new concept that IGF-1R may serve as a proximal component of GH/GHR signaling.

Emerging drugs for acromegaly

INTRODUCTION

Acromegaly is a rare disease that severely impacts patients' health all the while, being a slowly progressing illness. In the past decades, advancements in treatment modalities, especially development of new drugs, as well as focused guidelines has improved management of acromegaly. Still, many patients are considered not sufficiently treated and there remains an ongoing need for further development.

AREAS COVERED

This article reviews new medical treatments currently under clinical investigation (such as pasireotide, oral octreotide and somatoprim) and under experimental development (such as octreotide implants, CAM2029 and ATL-1103).

EXPERT OPINION

As it seems unlikely that one single agent may achieve cure in 100% of cases, there is an urgent need for new agents that help patients where current medication fails. Imperatively, this means we have to improve our understanding of the underlying pathogenetic and molecular mechanisms.

The co-existence of two growth hormone receptors and their differential expression profiles between female and male tongue sole (Cynoglossus semilaevis)

Growth hormone receptor (Ghr) is a single-transmembrane pass protein which is important in initiating the ability of growth hormone (Gh) to regulate development and somatic growth in vertebrates. In this study, molecular cloning, expression analysis of two different ghr genes (ghr1 and ghr2) in the tongue sole (Cynoglossus semilaevis) was conducted. As a result, the ghr1 and ghr2 cDNA sequences are 2364 bp and 3125 bp, each of which encodes a transmembrane protein of 633 and 561 amino acids (aa), respectively. Besides, the ghr1 gene includes nine exons and eight introns. The sex-specific tissue expression was analyzed by using 14 tissues from females, normal males and extra-large male adults. Both the ghr1 and ghr2 were predominantly expressed in the liver, and the ghr1 expression level in normal males was 1.6 and 1.4 times as much as those in females and extra-large males, while the ghr2 mRNA expression level in normal males was 1.1 and 1.2 times as much as those in females and extra-large males, respectively. Ontogenetic expression analysis at early life stages indicated that the ghr1 and ghr2 mRNAs were detected at all of the 35 sampling points (from oosphere to 410days-old). Furthermore, the sex differences in ghr mRNA expressions were also examined by using a full-sib family of C. semilaevis. Significantly higher levels of ghr1 mRNA were observed in males than in females at most stages of the sampling period (P<0.01). The ghr2 mRNA expression at most stages exhibited a significant sexual difference at each sampling point (P<0.01) without any variation trend related with the sexes during the whole sampling period.

Molecular mechanisms of prolactin and its receptor

Prolactin and the prolactin receptors are members of a family of hormone/receptor pairs which include GH, erythropoietin, and other ligand/receptor pairs. The mechanisms of these ligand/receptor pairs have broad similarities, including general structures, ligand/receptor stoichiometries, and activation of several common signaling pathways. But significant variations in the structural and mechanistic details are present among these hormones and their type 1 receptors. The prolactin receptor is particularly interesting because it can be activated by three sequence-diverse human hormones: prolactin, GH, and placental lactogen. This system offers a unique opportunity to compare the detailed molecular mechanisms of these related hormone/receptor pairs. This review critically evaluates selected literature that informs these mechanisms, compares the mechanisms of the three lactogenic hormones, compares the mechanism with those of other class 1 ligand/receptor pairs, and identifies information that will be required to resolve mechanistic ambiguities. The literature describes distinct mechanistic differences between the three lactogenic hormones and their interaction with the prolactin receptor and describes more significant differences between the mechanisms by which other related ligands interact with and activate their receptors.

Growth hormone-induced JAK2 signaling and GH receptor down-regulation: role of GH receptor intracellular domain tyrosine residues

GH receptor (GHR) mediates important somatogenic and metabolic effects of GH. A thorough understanding of GH action requires intimate knowledge of GHR activation mechanisms, as well as determinants of GH-induced receptor down-regulation. We previously demonstrated that a GHR mutant in which all intracellular tyrosine residues were changed to phenylalanine was defective in its ability to activate signal transducer and activator of transcription (STAT)5 and deficient in GH-induced down-regulation, but able to allow GH-induced Janus family of tyrosine kinase 2 (JAK2) activation. We now further characterize the signaling and trafficking characteristics of this receptor mutant. We find that the mutant receptor's extracellular domain conformation and its interaction with GH are indistinguishable from the wild-type receptor. Yet the mutant differs greatly from the wild-type in that GH-induced JAK2 activation is augmented and far more persistent in cells bearing the mutant receptor. Notably, unlike STAT5 tyrosine phosphorylation, GH-induced STAT1 tyrosine phosphorylation is retained and augmented in mutant GHR-expressing cells. The defective receptor down-regulation and persistent JAK2 activation of the mutant receptor do not depend on the sustained presence of GH or on the cell's ability to carry out new protein synthesis. Mutant receptors that exhibit resistance to GH-induced down-regulation are enriched in the disulfide-linked form of the receptor, which reflects the receptor's activated conformation. Furthermore, acute GH-induced internalization, a proximal step in down-regulation, is markedly impaired in the mutant receptor compared to the wild-type receptor. These findings are discussed in the context of determinants and mechanisms of regulation of GHR down-regulation.

A common model for cytokine receptor activation: combined scissor-like rotation and self-rotation of receptor dimer induced by class I cytokine

The precise mechanism by which the binding of a class I cytokine to the extracellular domain of its corresponding receptor transmits a signal through the cell membrane remains unclear. Receptor activation involves a cytokine-receptor complex with a 1∶2 stoichiometry. Previously we used our transient-complex theory to calculate the rate constant of the initial cytokine-receptor binding to form a 1∶1 complex. Here we computed the binding pathway leading to the 1∶2 activation complex. Three cytokine systems (growth hormone, erythropoietin, and prolactin) were studied, and the focus was on the binding of the extracellular domain of the second receptor molecule after forming the 1∶1 complex. According to the transient-complex theory, translational and rotation diffusion of the binding entities bring them together to form a transient complex, which has near-native relative separation and orientation but not the short-range specific native interactions. Subsequently conformational rearrangement leads to the formation of the native complex. We found that the changes in relative orientations between the two receptor molecules from the transient complex to the 1∶2 native complex are similar for the three cytokine-receptor systems. We thus propose a common model for receptor activation by class I cytokines, involving combined scissor-like rotation and self-rotation of the two receptor molecules. Both types of rotations seem essential: the scissor-like rotation separates the intracellular domains of the two receptor molecules to make room for the associated Janus kinase molecules, while the self-rotation allows them to orient properly for transphosphorylation. This activation model explains a host of experimental observations. The transient-complex based approach presented here may provide a strategy for designing antagonists and prove useful for elucidating activation mechanisms of other receptors.

Inhibitory GH receptor extracellular domain monoclonal antibodies: three-dimensional epitope mapping

GH receptor (GHR) mediates the anabolic and metabolic effects of GH. We previously characterized a monoclonal antibody (anti-GHR(ext-mAb)) that reacts with subdomain 2 of the rabbit GHR extracellular domain (ECD) and is a conformation-specific inhibitor of GH signaling in cells bearing rabbit or human GHR. Notably, this antibody has little effect on GH binding and also inhibits inducible metalloproteolysis of the GHR that occurs in the perimembranous ECD stem region. In the current study, we demonstrate that anti-GHR(ext-mAb) inhibits GH-dependent cellular proliferation and also inhibits hepatic GH signaling in vivo in mice that adenovirally express rabbit GHR, as assessed with our noninvasive bioluminescence hepatic signaling assay. A separate monoclonal antibody (anti-GHR(mAb 18.24)) is a sister clone of anti-GHR(ext-mAb). Here, we demonstrate that anti-GHR(mAb 18.24) also inhibits rabbit and human GHR signaling and inducible receptor proteolysis. Further, we use a random PCR-generated mutagenic expression system to map the three-dimensional epitopes in the rabbit GHR ECD for both anti-GHR(ext-mAb) and anti-GHR(mAb 18.24). We find that each of the two antibodies has similar, but nonidentical, discontinuous epitopes that include regions of subdomain 2 encompassing the dimerization interface. These results have fundamental implications for understanding the role of the dimerization interface and subdomain 2 in GHR activation and regulated GHR metalloproteolysis and may inform development of therapeutics that target GHR.

Signaling cross talk between growth hormone (GH) and insulin-like growth factor-I (IGF-I) in pancreatic islet β-cells

Dysfunction and destruction of pancreatic islet β-cells is a hallmark of diabetes. Better understanding of cell signals regulating β-cell growth and antiapoptosis will allow development of therapeutic strategies for diabetes by preservation and expansion of β-cell mass. GH and IGF-I share a complicated physiological relationship and have both been implicated in β-cell function. GH and IGF-I exert their biological effects through binding to respective receptors (GHR and IGF-IR) and subsequently engaging downstream signaling pathways. However, their collaborative roles in modulation of β-cell mass and the underlying molecular mechanisms remain poorly understood. In this study, we demonstrate that cultured β-cells are appealing systems for investigating potential GH-IGF-I signaling cross talk. We uncover that GH specifically promotes formation of a protein complex containing GHR, Janus kinase 2 (a nonreceptor kinase coupled to GH/GHR signaling), and IGF-IR. More importantly, GH and IGF-I synergistically activate both signal transducer and activator of transcription 5 and Akt pathways. Concomitantly, β-cells proliferate more robustly and are better protected from serum deprivation-induced apoptosis when exposed to GH and IGF-I in combination vs. GH or IGF-I alone. The augmented proliferative effects by GH and IGF-I are confirmed in isolated islets. Taken together, our findings strongly suggest that there exists a novel signaling relationship between GH/GHR and IGF-I/IGF-IR systems in β-cells, i.e. IGF-IR may serve as a proximal component of GH/GHR signaling, contributing to enhancement of β-cell mass and function. In support of this, IGF-IR knockdown in β-cells resulted in the desensitization of acute GH-induced signal transducer and activator of transcription 5 activation.

Non-cysteine linked MUC1 cytoplasmic dimers are required for Src recruitment and ICAM-1 binding induced cell invasion

Background

The mucin MUC1, a type I transmembrane glycoprotein, is overexpressed in breast cancer and has been correlated with increased metastasis. We were the first to report binding between MUC1 and Intercellular adhesion molecule-1 (ICAM-1), which is expressed on stromal and endothelial cells throughout the migratory tract of a metastasizing breast cancer cell. Subsequently, we found that MUC1/ICAM-1 binding results in pro-migratory calcium oscillations, cytoskeletal reorganization, and simulated transendothelial migration. These events were found to involve Src kinase, a non-receptor tyrosine kinase also implicated in breast cancer initiation and progression. Here, we further investigated the mechanism of MUC1/ICAM-1 signalling, focusing on the role of MUC1 dimerization in Src recruitment and pro-metastatic signalling.

Methods

To assay MUC1 dimerization, we used a chemical crosslinker which allowed for the detection of dimers on SDS-PAGE. We then generated MUC1 constructs containing an engineered domain which allowed for manipulation of dimerization status through the addition of ligands to the engineered domain. Following manipulation of dimerization, we immunoprecipitated MUC1 to investigate recruitment of Src, or assayed for our previously observed ICAM-1 binding induced events. To investigate the nature of MUC1 dimers, we used both non-reducing SDS-PAGE and generated a mutant construct lacking cysteine residues.

Results

We first demonstrate that the previously observed MUC1/ICAM-1signalling events are dependent on the activity of Src kinase. We then report that MUC1 forms constitutive cytoplasmic domain dimers which are necessary for Src recruitment, ICAM-1 induced calcium oscillations and simulated transendothelial migration. The dimers are not covalently linked constitutively or following ICAM-1 binding. In contrast to previously published reports, we found that membrane proximal cysteine residues were not involved in dimerization or ICAM-1 induced signalling.

Conclusions

Our data implicates non-cysteine linked MUC1 dimerization in cell signalling pathways required for cancer cell migration.

Growth hormone signaling in human T47D breast cancer cells: potential role for a growth hormone receptor-prolactin receptor complex

GH receptor (GHR) and prolactin (PRL) receptor (PRLR) are structurally similar cytokine receptor superfamily members that are highly conserved among species. GH has growth-promoting and metabolic effects in various tissues in vertebrates, including humans. PRL is essential for regulation of lactation in mammals. Recent studies indicate that breast tissue bears GHR and PRLR and that both GH and PRL may impact development or behavior of breast cancer cells. An important facet of human GH (hGH) and human PRL (hPRL) biology is that although hPRL interacts only with hPRLR, hGH binds well to both hGHR and hPRLR. Presently, we investigated potential signaling effects of both hormones in the estrogen receptor- and progesterone receptor-positive human T47D breast cancer cell line. We found that this cell type expresses ample GHR and PRLR and responds well to both hGH and hPRL, as evidenced by activation of the Janus kinase 2/signal transducer and activator of transcription 5 pathway. Immunoprecipitation studies revealed specific GHR-PRLR association in these cells that was acutely enhanced by GH treatment. Although GH caused formation of disulfide-linked and chemically cross-linked GHR dimers in T47D cells, GH preferentially induced tyrosine phosphorylation of PRLR rather than GHR. Notably, both a GHR-specific ligand antagonist (B2036) and a GHR-specific antagonist monoclonal antibody (anti-GHR(ext-mAb)) failed to inhibit GH-induced signal transducer and activator of transcription 5 activation. In contrast, although the non-GHR-specific GH antagonist (G120R) and the PRL antagonist (G129R) individually only partially inhibited GH-induced activation, combined treatment with these two antagonists conferred greater inhibition than either alone. These data indicate that endogenous GHR and PRLR associate (possibly as a GHR-PRLR heterodimer) in human breast cancer cells and that GH signaling in these cells is largely mediated by the PRLR in the context of both PRLR-PRLR homodimers and GHR-PRLR heterodimers, broadening our understanding of how these related hormones and their related receptors may function in physiology and pathophysiology.

Distinct growth hormone receptor signaling modes regulate skeletal muscle development and insulin sensitivity in mice

Skeletal muscle development, nutrient uptake, and nutrient utilization is largely coordinated by growth hormone (GH) and its downstream effectors, in particular, IGF-1. However, it is not clear which effects of GH on skeletal muscle are direct and which are secondary to GH-induced IGF-1 expression. Thus, we generated mice lacking either GH receptor (GHR) or IGF-1 receptor (IGF-1R) specifically in skeletal muscle. Both exhibited impaired skeletal muscle development characterized by reductions in myofiber number and area as well as accompanying deficiencies in functional performance. Defective skeletal muscle development, in both GHR and IGF-1R mutants, was attributable to diminished myoblast fusion and associated with compromised nuclear factor of activated T cells import and activity. Strikingly, mice lacking GHR developed metabolic features that were not observed in the IGF-1R mutants, including marked peripheral adiposity, insulin resistance, and glucose intolerance. Insulin resistance in GHR-deficient myotubes derived from reduced IR protein abundance and increased inhibitory phosphorylation of IRS-1 on Ser 1101. These results identify distinct signaling pathways through which GHR regulates skeletal muscle development and modulates nutrient metabolism.

A CSF-1 receptor phosphotyrosine 559 signaling pathway regulates receptor ubiquitination and tyrosine phosphorylation

Receptor tyrosine kinase (RTK) activation involves ligand-induced receptor dimerization and transphosphorylation on tyrosine residues. Colony-stimulating factor-1 (CSF-1)-induced CSF-1 receptor (CSF-1R) tyrosine phosphorylation and ubiquitination were studied in mouse macrophages. Phosphorylation of CSF-1R Tyr-559, required for the binding of Src family kinases (SFKs), was both necessary and sufficient for these responses and for c-Cbl tyrosine phosphorylation and all three responses were inhibited by SFK inhibitors. In c-Cbl-deficient macrophages, CSF-1R ubiquitination and tyrosine phosphorylation were substantially inhibited. Reconstitution with wild-type, but not ubiquitin ligase-defective C381A c-Cbl rescued these responses, while expression of C381A c-Cbl in wild-type macrophages suppressed them. Analysis of site-directed mutations in the CSF-1R further suggests that activated c-Cbl-mediated CSF-1R ubiquitination is required for a conformational change in the major kinase domain that allows amplification of receptor tyrosine phosphorylation and full receptor activation. Thus the results indicate that CSF-1-mediated receptor dimerization leads to a Tyr-559/SFK/c-Cbl pathway resulting in receptor ubiquitination that permits full receptor tyrosine phosphorylation of this class III RTK in macrophages.

Deletion of IGF-I receptor (IGF-IR) in primary osteoblasts reduces GH-induced STAT5 signaling

GH promotes longitudinal growth and regulates multiple cellular functions in humans and animals. GH signals by binding to GH receptor (GHR) to activate the tyrosine kinase, Janus kinase 2 (JAK2), and downstream pathways including signal transducer and activator of transcription 5 (STAT5), thereby regulating expression of genes including IGF-I. GH exerts effects both directly and via IGF-I, which signals by activating the IGF-I receptor (IGF-IR). IGF-IR is a cell surface receptor that contains intrinsic tyrosine kinase activity within its intracellular domain. In this study, we examined the potential role of IGF-IR in facilitating GH-induced signal transduction, using mouse primary calvarial osteoblasts with Lox-P sites flanking both IGF-IR alleles. These cells respond to both GH and IGF-I and in vitro infection with an adenovirus that drives expression of Cre recombinase (Ad-Cre) dramatically reduces IGF-IR abundance without affecting the abundance of GHR, JAK2, STAT5, or ERK. Notably, infection with Ad-Cre, but not a control adenovirus, markedly inhibited acute GH-induced STAT5 activity (more than doubling the ED(50) and reducing the maximum activity by nearly 50%), while sparing GH-induced ERK activity, and markedly inhibited GH-induced transactivation of a STAT5-dependent luciferase reporter. The effect of Ad-Cre on GH signaling was specific, as platelet-derived growth factor-induced signaling was unaffected by Ad-Cre-mediated reduction of IGF-IR. Ad-Cre-mediated inhibition of GH signaling was reversed by adenoviral reexpression of IGF-IR, but not by infection with an adenovirus that drives expression of a hemagglutination-tagged somatostatin receptor, which drives expression of the unrelated somatostatin receptor, and Ad-Cre infection of nonfloxed osteoblasts did not affect GH signaling. Notably, infection with an adenovirus encoding a C-terminally truncated IGF-IR that lacks the tyrosine kinase domain partially rescued both acute GH-induced STAT5 activity and GH-induced IGF-I gene expression in cells in which endogenous IGF-IR was reduced. These data, in concert with our earlier findings that GH induces a GHR-JAK2-IGF-IR complex, suggest a novel function for IGF-IR. In addition to its role as a key IGF-I signal transducer, this receptor may directly facilitate acute GH signaling. The implications of these findings are discussed.

Growth hormone receptor modulators

Growth hormone (GH) regulates somatic growth, substrate metabolism and body composition. Its actions are elaborated through the GH receptor (GHR). GHR signalling involves the role of at least three major pathways, STATs, MAPK, and PI3-kinase/Akt. GH receptor function can be modulated by changes to the ligand, to the receptor or by factors regulating signal transduction. Insights on the physico-chemical basis of the binding of GH to its receptor and the stoichiometry required for activation of the GH receptor-dimer has led to the development of novel GH agonists and antagonists. Owing to the fact that GH has short half-life, several approaches have been taken to create long-acting GHR agonists. This includes the pegylation, sustained release formulations, and ligand-receptor fusion proteins. Pegylation of a GH analogue (pegvisomant) which binds but not activate signal transduction forms the basis of a new successful approach to the treatment of acromegaly. GH receptors can be regulated at a number of levels, by modifying receptor expression, surface availability and signalling. Insulin, thyroid hormones and sex hormones are among hormones that modulate GHR through some of these mechanisms. Estrogens inhibit GH signalling by stimulating the expression of SOCS proteins which are negative regulators of cytokine receptor signalling. This review of GHR modulators will cover the effects of ligand modification, and of factors regulating receptor expression and signalling.

Interruption of growth hormone signaling via SHC and ERK in 3T3-F442A preadipocytes upon knockdown of insulin receptor substrate-1

Insulin receptor substrate-1 (IRS-1) is a docking protein tyrosine phosphorylated in response to insulin, IGF-1, GH, and other cytokines. IRS-1 has an N-terminal plekstrin homology domain (which facilitates membrane localization), a phosphotyrosine-binding domain [which associates with tyrosine-phosphorylated insulin receptor or IGF-1 receptor (IGF-1R)], and tyrosine residues that, when phosphorylated, bind signaling molecules. The role of IRS-1 in GH signaling is uncertain. We previously reported that IRS-1 and Janus kinase 2 associate independently of tyrosine phosphorylation via IRS-1's N terminus and that IRS-1 reconstitution greatly enhances GH-induced ERK, but not STAT5, activation. We now use GH-responsive 3T3-F442A preadipocytes to study the influence of IRS-1 on GH action. We stably transfected cells with vector only (Control) or a vector encoding IRS-1 short hairpin RNA [knockdown (KD)] and compared representative clones. Immunoblotting confirmed more than 80% knockdown of IRS-1 in KD cells. GH caused characteristic Janus kinase 2 and STAT5 activation in both Control and KD cells, but ERK activation was dramatically reduced in KD cells in GH time course and dose-response experiments. Notably, GH-induced Src homology collagen (SHC) activation and SHC-Grb2 association in KD cells were also markedly diminished compared with Control cells. Subcellular fractionation revealed that IRS-1 in Control cells was largely cytosolic, but the component isolated with plasma membranes was highly enriched in lipid raft membranes (LR). In KD cells, GH-induced ERK activation in the LR fraction was particularly diminished compared with Control cells. These data suggest that LR-enriched IRS-1 contributes substantially to GH-induced ERK activation in LR in 3T3-F442A fibroblasts. Furthermore, our results are consistent with IRS-1 residing upstream of SHC in the GH-induced ERK-signaling pathway.

ERK-dependent threonine phosphorylation of EGF receptor modulates receptor downregulation and signaling

Epidermal growth factor (EGF) signaling is critical in normal and aberrant cellular behavior. Extracellular signal-regulated kinase (ERK) mediates important downstream aspects of EGF signaling. Additionally, EGFR undergoes MEK1-dependent ERK consensus site phosphorylation in response to EGF or cytokines such as growth hormone (GH) and prolactin (PRL). GH- or PRL-induced EGFR phosphorylation alters subsequent EGF-induced EGFR downregulation and signal characteristics in an ERK-dependent fashion. We now use reconstitution to study mutation of the sole EGFR ERK phosphorylation consensus residue, (669)T. CHO-GHR cells, which lack EGFR and express GHR, were stably transfected to express human wild-type or T669A ((669)T changed to alanine) EGFRs at similar abundance. Treatment of cells with GH or EGF caused phosphorylation of WT, but not T669A EGFR, in an ERK activity-dependent fashion that was detected with an antibody that recognizes phosphorylation of ERK consensus sites, indicating that (669)T is required for this phosphorylation. Notably, EGF-induced downregulation of EGFR abundance was much more rapid in cells expressing EGFR T669A vs. WT EGFR. Further, pretreatment with the MEK1/ERK inhibitor PD98059 enhanced EGF-induced EGFR loss in cells expressing WT EGFR, but not EGFR T669A, suggesting that the ERK-dependent effects on EGFR downregulation required phosphorylation of (669)T. In signaling experiments, EGFR T669A displayed enhanced acute (15 min) EGFR tyrosine phosphorylation (reflecting EGFR kinase activity) compared to WT EGFR. Further, acute EGF-induced ubiquitination of WT EGFR was markedly enhanced by PD98059 pretreatment and was increased in EGFR T669A-expressing cells independent of PD98059. These signaling data suggest that ERK-mediated (669)T phosphorylation negatively modulates EGF-induced EGFR kinase activity. We furthered these investigations using a human fibrosarcoma cell line that endogenously expresses EGFR and ErbB-2 and also harbors an activating Ras mutation. In these cells, EGFR was constitutively detected with the ERK consensus site phosphorylation-specific antibody and EGF-induced EGFR downregulation was modest, but was substantially enhanced by pretreatment with MEK1/ERK inhibitor. Collectively, these data indicate that ERK activity, by phosphorylation of a threonine residue in the EGFR juxtamembrane cytoplasmic domain, modulates EGFR trafficking and signaling.

Endotoxin-induced proteolytic reduction in hepatic growth hormone (GH) receptor: a novel mechanism for GH insensitivity

GH is an important anabolic hormone. We previously demonstrated in cell culture that the cell surface GH receptor (GHR) is susceptible to inducible metalloproteolytic cleavage that yields the shed receptor extracellular domain (called GH binding protein) and renders the cells desensitized to subsequent GH stimulation. Sepsis and inflammatory states are associated with hepatic desensitization to GH, although disparate mechanisms have been postulated in various animal models. Using C3H/HeJ mice, we now demonstrate that administration of lipopolysaccharide (LPS) causes marked hepatic desensitization to GH, assessed by monitoring signal transducer and activator of transcription 5 tyrosine phosphorylation and nuclear accumulation and with a novel noninvasive bioluminescence imaging system to track in vivo hepatic GH signaling serially in individual mice. This endotoxin-induced desensitization was accompanied by marked loss of hepatic GHR, which was not explained by changes in GHR mRNA abundance. Furthermore, we observe that LPS causes GH-binding protein shedding of a hepatically expressed wild-type GHR but not a GHR with a mutation in the metalloprotease cleavage site. These data suggest that in this model system, LPS-induced desensitization to GH is associated with proteolytic GHR cleavage. These data are the first to demonstrate inducible in vivo GHR proteolysis and suggest this is a mechanism to regulate GH sensitivity and its anabolic effects during sepsis or inflammation.

Mechanistic aspects of crosstalk between GH and PRL and ErbB receptor family signaling

Growth hormone (GH) and prolactin (PRL) are anterior pituitary hormones that have multiple roles in growth and metabolism. Both hormones are important in mammary development and breast cancer. The epidermal growth factor (EGF) family of peptides and the receptors that they activate (the ErbB family) are also major players in mammary biology and pathophysiology. Recent studies in signal transduction have highlighted the interplay between signaling pathways referred to as crosstalk. In this review, cell biological and signaling studies related to crosstalk between GH and PRL and the ErbB family are discussed. In particular, the role of GH- and PRL-induced phosphorylation of ErbB receptors in regulating EGF responsiveness is highlighted with attention to potential pathophysiological relevance.

Activation of growth hormone receptors by growth hormone and growth hormone antagonist dimers: insights into receptor triggering

GH binds dimerized GH receptors (GHRs) to form a trimolecular complex and induces downstream signaling events. The mechanism by which GH binding converts the inactive predimerized GHR to its active signaling conformation is uncertain. GH has no axis of symmetry. Its interaction with GHR is mediated by two asymmetric binding sites on GH, each with distinct affinity. Site 1 is of high affinity and is thought to mediate the first binding step. Mutation of binding site 2 (as in the human GH mutant, G120R) disrupts the second binding but leaves site 1 binding intact. G120R is a GH antagonist; it binds only one GHR and thus fails to signal, and it prevents productive GHR binding by normal GH. We previously demonstrated that prolactin receptor signaling was achieved by a dimeric version of a prolactin antagonist. We now employ assays of cellular signaling and receptor conformational changes to examine whether GH molecules harboring two site 1 regions can trigger GHR activation. We used recombinantly produced GH-GH and G120R-G120R dimers in which monomers in tandem are connected by a short linker peptide. Rabbit GHR-expressing human fibrosarcoma cells (C14) were treated with GH, G120R, GH-GH, or G120R-G120R. As expected, GH and GH-GH, but not G120R, induced GHR disulfide linkage, as assessed by anti-GHR blotting of cell extracts resolved by SDS-PAGE under nonreducing conditions. Disulfide linkage of GHRs reflects attainment of the active signaling conformation. Likewise, GH and GH-GH, but not G120R, caused Janus kinase 2 (JAK2) and signal transducer and activator of transcription 5 (STAT5) activation. Notably, G120R-G120R, despite its lack of an intact site 2 in either dimer partner, also promoted GHR disulfide linkage and JAK2 and STAT5 activation, albeit less potently than either GH or GH-GH. Time-course responses of the three agonists were similar in terms of JAK2 and STAT5 activation. Pretreatment of cells with our conformation-sensitive inhibitory monoclonal antibody, anti-GHR ext-mAb, prevented ligand-induced receptor activation for all three agonists. GHR was also rendered less immunoprecipitable by anti-GHR ext-mAb after treatment with these agonists. These results are important in that they indicate that a ligand with two intact binding sites 1 causes GHR to adopt similar conformational changes as does GH and thus triggers activation of JAK2 and downstream signaling. Furthermore, we infer that there is substantial flexibility in the GHR extracellular domain, such that it productively accommodates GH dimers that are much larger than GH.

Evolution of receptors for growth hormone and somatolactin in fish and land vertebrates: lessons from the lungfish and sturgeon orthologues

Two cognate hormones, growth hormone (GH) and somatolactin (SL), control several important physiological processes in vertebrates. Knowledge about GH and its receptor (GHR) has accumulated over the last decades. However, much less is known about SL and its receptor (SLR). SL is found only in fish (including lungfish), suggesting that it was present in the common ancestor of vertebrates, but was lost secondarily in the lineage leading to land vertebrates after the lungfish branched off. SLR was suggested to be a duplicated copy of GHR acquired only in teleosts via the fish-specific genome duplication (FSGD). This scenario (i.e., the existence of SL but not SLR in the vertebrate ancestors) is intriguing but contested. In this study, we first evaluated the plausibility of this scenario through synteny analyses and found that the loci for GHR and SLR are located in syntenic genomic positions, whereas the loci for GH and SL are not. Next, we cloned GHRs of lungfish and sturgeon, which possess SL but did not undergo the FSGD (i.e., they should not possess SLR). Their phylogenetic positions in the GHR/SLR gene tree further support the fish-specific scenario for the GHR-SLR duplication. Interestingly, their sequences share greater similarity with teleost SLRs and reptilian/amphibian GHRs than with the GHRs of mammals, birds, and teleosts. On the basis of these results, we discuss the validity of the nomenclature of the teleost-specific copy of GHR as SLR and an ancestral receptor(s) for SL before the evolution of SLR during the FSGD.

Insulin regulation of growth hormone receptor gene expression: involvement of both the PI-3 kinase and MEK/ERK signaling pathways

The mechanism(s) of insulin's effects on growth hormone receptor (GHR) gene expression are poorly understood. Using rat hepatoma cells, we have previously shown that insulin treatment reduces GHR mRNA and protein in a time- and concentration-dependent manner, at least in part via down-regulation of GHR transcription. The present study determines whether the phosphatidylinositol-3 kinase (PI-3 kinase) and mitogen activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathways are involved in mediating these effects of insulin. Inhibition of the PI-3 kinase pathway partially blocked insulin's reduction of GHR mRNA, as did inhibition of the MEK/ERK pathway, resulting in higher GHR mRNA levels. Inhibition of both pathways was necessary to completely block insulin effects. Similar results were obtained for GHR protein. Collectively, these data suggest that insulin signaling via either the PI-3 kinase or MEK/ERK pathway may result in partial reduction of GHR gene expression, whereas signaling via both pathways may be required to achieve the full insulin effect.

A growth hormone receptor mutation impairs growth hormone autofeedback signaling in pituitary tumors

Pituitary tumors are a diverse group of neoplasms that are classified based on clinical manifestations, hormone excess, and histomorphologic features. Those that cause growth hormone (GH) excess and acromegaly are subdivided into morphologic variants that have not yet been shown to have pathogenetic significance or predictive value for therapy and outcome. Here, we identify a selective somatic histidine-to-leucine substitution in codon 49 of the extracellular domain of the GH receptor (GHR) in a morphologic subtype of human GH-producing pituitary tumors that is characterized by the presence of cytoskeletal aggresomes. This GHR mutation significantly impairs glycosylation-mediated receptor processing, maturation, ligand binding, and signaling. Pharmacologic GH antagonism recapitulates the morphologic phenotype of pituitary tumors from which this mutation was identified, inducing the formation of cytoskeletal keratin aggresomes. This novel GHR mutation provides evidence for impaired hormone autofeedback in the pathogenesis of these pituitary tumors. It explains the lack of responsiveness to somatostatin analogue therapy of this tumor type, in contrast to the exquisite sensitivity of tumors that lack aggresomes, and has therapeutic implications for the safety of GH antagonism as a therapeutic modality in acromegaly.

Short stature and decreased insulin-like growth factor I (IGF-I)/growth hormone (GH)-ratio in an adult GH-deficient patient pointing to additional partial GH insensitivity due to a R179C mutation of the growth hormone receptor

OBJECTIVE

Genetic factors play an expanding role in understanding growth hormone (GH) disorders, therefore the German KIMS Pharmacogenetics Study was initiated with the aim of genotyping various GH-/IGF-I-axis-related genes of GH-deficient adult patients to investigate genotype:phenotype relationships and response to GH therapy.

PATIENTS AND METHODS

129 consecutively enrolled GH-deficient adult patients were genotyped for variant 1 (V1) of the alternatively spliced noncoding exons in the 5'-untranslated region and for the nine coding exons of the GH receptor (GHR) gene, which obviously play a striking role in the function of the GH-IGF-I-axis. After detection of a heterozygous, non-synonymous mutation R179C in exon 6 in one single patient with acquired GH-deficiency (GHD) in late adulthood, analysis of her clinical data followed, leading to the diagnosis of mild short stature (-1.5SD). For further endocrine evaluation, five pituitary stimulation tests (arginine) of this patient were statistically compared to stimulation tests (arginine) of ten GH-deficient control patients, retrospectively.

RESULTS

The formerly in patients with Laron syndrome and idiopathic short stature reported mutation R179C leads to an amino acid change from an arginine residue (codon CGC) to a cysteine residue (codon TGC) in position 179 of the extracellular domain of the GHR. Statistical analysis revealed significant decreased IGF-I/GH(0) ratio (p=0.004) and IGF-I/GH(max) ratio (p=0.001) of the index patient compared to the control patients, implying growth hormone resistance of the index patient at the level of the GHR, according to the detected R179C mutation.

CONCLUSIONS

This study reports on the unusual case of a patient with mild short stature, who acquired GHD in late adulthood due to a non-secreting pituitary adenoma and get additionally diagnosed for pre-existing growth hormone insensitivity due to a formerly in two short statured patients described, single, heterozygous, non-synonymous mutation in the GHR. Our findings support the theory that heterozygous mutations in the GHR gene can have mild phenotypical consequences.

Primary growth hormone (GH) insensitivity and insulin-like growth factor deficiency caused by novel compound heterozygous mutations of the GH receptor gene: genetic and functional studies of simple and compound heterozygous states

CONTEXT

Primary GH insensitivity (GHI) or Laron syndrome, caused by mutations of the GH receptor (GHR) gene, has a clinical phenotype of postnatal growth failure associated with normal elevated serum concentrations of GH and low serum levels of IGF-I.

OBJECTIVE

We investigated the clinical and biochemical implications of molecular defects in the GHR gene in an Austrian family with two daughters who were GHI.

PATIENTS

Patient 1 [height, -4.8 sd score (SDS)] and patient 2 (height, -5.0 SDS) had elevated circulating levels of GH, low-normal levels of GH-binding protein, and abnormally low IGF-I (-5.0 SDS and -2.6 SDS, respectively) and IGF-binding protein-3 (-2.6 SDS and -2.0 SDS, respectively).

RESULTS

Both patients carry novel compound, missense, heterozygous GHR mutations, C94S and H150Q. In vitro reconstitution experiments demonstrated that whereas each of the mutants could be stably expressed, GHR(C94S) lost its affinity for GH and could neither activate signal transducer and activator of transcription (STAT)-5b nor drive STAT5b-dependent gene transcription in response to GH (1-100 ng/ml). GHR(H150Q) showed normal affinity for GH but impaired capacity for signal transduction. The compound heterozygote and C94S heterozygote, but not the H150Q heterozygote, showed significant deficiency in activating GH-induced gene expression, corroborating diminished GH-induced STAT5b activation in fibroblasts carrying GHR(C94S) as either a compound heterozygote (in the patients) or a simple heterozygote (in one parent).

CONCLUSIONS

Each of the compound heterozygous mutations contributed additively to the pathological condition seen in the patients, and the more detrimental of the two mutations, C94S, may cause (partial) primary GHI, even in a heterozygous state.

Molecular cloning, characterization and distribution of two types of growth hormone receptor in orange-spotted grouper (Epinephelus coioides)

The cDNAs encoding two distinct growth hormone receptors (GHRs) were firstly cloned and sequenced from the liver of orange-spotted grouper (Epinephelus coioides). The cDNA of grouper GHR1 consisted of 2673-bp and encoded 658 amino acids, while the cDNA of grouper GHR2 consisted of 2989-bp and encoded 577 amino acids. The two cDNAs shared 78.6% identity in nucleotide sequence and 37.8% identity in deduced amino acid sequence. Northern blot analysis demonstrated a single GHR1 transcript of approximately 4.3 kb in liver and a single GHR2 transcript of approximately 3.9 kb in the liver and muscle. In the Real-time PCR assay, grouper GHR1 and GHR2 were expressed in all tissues tested. The expression of GHR2 was significantly higher than that of GHR1 in telencephalon, cerebellum, pituitary, heart and white muscle, whereas the expression of GHR1 was significantly higher in liver. These results indicated that there were two types of GHRs existing in orange-spotted grouper, and they had different structural features and tissues expression patterns. However, the functional differences between GHR1 and GHR2 in orange-spotted grouper remained further research.

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.

Two growth hormone receptors in Nile tilapia (Oreochromis niloticus): molecular characterization, tissue distribution and expression profiles in the gonad during the reproductive cycle

Growth hormone (GH) has many important physiological roles in the control of growth, metabolism and reproduction, which is mediated by growth hormone receptor (GHR). In this study, two cDNAs encoding GHR were isolated from the liver of Nile tilapia (Oreochromis niloticus). The two cDNAs, one consisting of 1908 bp and the other of 1728 bp, encoding for putative 635- and 575-amino acid GHR (designated ntGHR1 and ntGHR2, respectively), shared 34.4% identity in nucleotide sequence and 29.6% in deduced amino acid sequence. Northern blot analysis indicated a single 6.0-kb transcript of ntGHR1 and a single 4.0-kb transcript of ntGHR2 in the liver. Real-time RT-PCR analysis showed that both ntGHR1 and ntGHR2 mRNAs were presented in all tissues tested and expressed extremely highly in the liver. In most tissues, ntGHR2 expressed significantly higher than ntGHR1. Analysis of the ntGHRs expression profiles in the gonad during reproductive cycle indicated that the mRNA levels of ntGHRs in ovary were significantly higher at sexual matured stage while those in testis were significantly higher at sexual recrudescent stage, suggesting that GH/IGF-I axis might be involved in reproduction under a regulatory mechanism of GHR gene expression.

Membrane reinsertion of a myristoyl-peptidyl anchored extracellular domain growth hormone receptor

The actions of GH are mediated through a cell surface cytokine receptor. We previously demonstrated that naturally occurring truncated membrane bound GH receptors (GHRs) can block GH receptor signaling. We have now investigated whether recombinant extracellular GHR can be conjugated to a myristoylated-peptide (mp) tail and inserted into cell membranes to modulate GHR signaling. Recombinant human extracellular domain (1-241) GHR was expressed in Escherichia coli, purified, and refolded from cell lysate. The free C-terminal cysteine was then reduced and conjugated to an activated preformed mp tail. The properties of the purified tailed GHR (GHR-mp) were then compared with those of the untailed purified GHR 1-241. Fluorescence-activated cell sorter analysis and cell surface binding assays demonstrated that GHR-mp inserted into the cell surface membranes of CHO cells, whereas untailed GHR 1-241 showed no insertion. In a cell-based bioassay GHR-mp partially inhibited wild-type GHR signaling, whereas GHR 1-241 had no effect. Truncated extracellular domain GHR can, when specifically modified with a membrane-localizing mp unit, insert into cell surface membranes and modulate GHR signaling.

Disulfide bonds determine growth hormone receptor folding, dimerisation and ligand binding

The growth hormone receptor contains seven cysteine residues in its extracellular domain. The six in the growth hormone binding domain form disulfide bonds, and help the receptor to gain its correct three-dimensional structure. In this study we replaced the cysteine for serine and alanine residues and investigated their role in growth hormone receptor folding, dimerisation and signal transduction. Folding and growth hormone binding capacity of the wild-type growth hormone receptor require less than two minutes for completion. Although less efficient, all mutant receptors arrive at the cell surface as pre-formed dimers. Disulfide bond C38-C48 is important for efficient maturation. The middle disulfide-bond, C83-C94, is important for ligand binding. Removing disulfide bond C108-C122 has little effect without affecting signalling. When two or all disulfide bonds are changed, ligand binding and activation are blocked. Dimerisation is delayed when all disulfide bonds are destroyed.

Activation of the growth hormone receptor

Growth hormone (GH) is a major regulator of postnatal growth and metabolism. There are extensive clinical applications for GH or its antagonists, including treatments for dwarfism, cancer and metabolic wasting. Owing to this, there is considerable interest in the mechanisms of GH receptor (GHR) activation. It is conventionally thought that GH induces dimerization of two GHR monomers, which initiates intracellular signaling cascades. However, recent studies have provided evidence for a ligand-induced conformational change within constitutively dimerized GHRs being responsible for activating signaling pathways. This review will relate the new model of GHR activation to the activation of related cytokine receptors and discuss the implication of this new model for the design of small GH mimetics and antagonists for therapeutic use.

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.

Janus kinase 2 enhances the stability of the mature growth hormone receptor

The abundance of surface GH receptor (GHR) is an important determinant of cellular GH sensitivity and is regulated at both transcriptional and posttranscriptional levels. In previous studies of GHR-expressing Janus kinase 2 (JAK2)-deficient human fibrosarcoma cells (gamma2A-GHR), we demonstrated that stable transfection with JAK2 resulted in increased steady-state levels of mature GHR (endoH-resistant; relative molecular mass, 115-140 kDa) relative to precursor GHR (endoH-sensitive; relative molecular mass, 100 kDa). We now examine further the effects of JAK2 on GHR trafficking by comparing gamma2A-GHR to gamma2A-GHR cells stably reconstituted with JAK2 (C14 cells). In the presence of JAK2, GHR surface expression was increased, as assessed by surface biotinylation, 125I-labeled human GH cell surface binding, and immunofluorescence microscopy assays. Although the absence of JAK2 precluded GH-stimulated signaling, GH-induced GHR disulfide linkage (a proxy for the GH-induced conformational changes in the GHR dimer) proceeded independent of JAK2 expression, indicating that the earliest steps in GH-induced GHR triggering are not prevented by the absence of JAK2. RNA interference-mediated knockdown of JAK2 in C14 cells resulted in a decreased mature to precursor ratio, supporting a primary role for JAK2 either in enhancing GHR biogenesis or dampening mature GHR degradation. To address these potential mechanisms, metabolic pulse-chase labeling experiments and experiments in which the fate of previously synthesized GHR was followed by anti-GHR immunoblotting after cycloheximide treatment (cycloheximide chase experiments) were performed. These indicated that the presence of JAK2 conferred modest enhancement (1.3- to 1.5-fold) in GHR maturation but substantially prolonged the t1/2 of the mature GHR, suggesting a predominant effect on mature GHR stability. Cycloheximide chase experiments with metalloprotease, proteasome, and lysosome inhibitors indicated that the enhanced stability of mature GHR conferred by JAK2 is not related to effects on constitutive receptor metalloproteolysis but rather is a result of reduced constitutive endosomal/lysosomal degradation of the mature GHR. These results are discussed in the context of emerging information on how JAK-family members modulate surface expression of other cytokine receptors.

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.