Physical and functional interaction of growth hormone and insulin-like growth factor-I signaling elements

Starvation-induced transcription factor CREBH negatively governs body growth by controlling GH signaling

cAMP responsive element-binding protein H (CREBH) is a hepatic transcription factor to be activated during fasting. We generated CREBH knock-in flox mice, and then generated liver-specific CREBH transgenic (CREBH L-Tg) mice in an active form. CREBH L-Tg mice showed a delay in growth in the postnatal stage. Plasma growth hormone (GH) levels were significantly increased in CREBH L-Tg mice, but plasma insulin-like growth factor 1 (IGF1) levels were significantly decreased, indicating GH resistance. In addition, CREBH overexpression significantly increased hepatic mRNA and plasma levels of FGF21, which is thought to be as one of the causes of growth delay. However, the additional ablation of FGF21 in CREBH L-Tg mice could not correct GH resistance at all. CREBH L-Tg mice sustained GH receptor (GHR) reduction and the increase of IGF binding protein 1 (IGFBP1) in the liver regardless of FGF21. As GHR is a first step in GH signaling, the reduction of GHR leads to impairment of GH signaling. These data suggest that CREBH negatively regulates growth in the postnatal growth stage via various pathways as an abundant energy response by antagonizing GH signaling.

Classical and novel GH receptor signaling pathways

In this review, I summarize historical and recent features of the classical pathways activated by growth hormone (GH) through the cell surface GH receptor (GHR). GHR is a cytokine receptor superfamily member that signals by activating the non-receptor tyrosine kinase, JAK2, and members of the Src family kinases. Activation of the GHR engages STATs, PI3K, and ERK pathways, among others, and details of these now-classical pathways are presented. Modulating elements, including the SOCS proteins, phosphatases, and regulated GHR metalloproteolysis, are discussed. In addition, a novel physical and functional interaction of GHR with IGF-1R is summarized and discussed in terms of its mechanisms, consequences, and physiological and therapeutic implications.

Insulin-Like Growth Factors Are Key Regulators of T Helper 17 Regulatory T Cell Balance in Autoimmunity

Appropriate balance of T helper 17 (Th17) and regulatory T (Treg) cells maintains immune tolerance and host defense. Disruption of Th17-Treg cell balance is implicated in a number of immune-mediated diseases, many of which display dysregulation of the insulin-like growth factor (IGF) system. Here, we show that, among effector T cell subsets, Th17 and Treg cells selectively expressed multiple components of the IGF system. Signaling through IGF receptor (IGF1R) activated the protein kinase B-mammalian target of rapamycin (AKT-mTOR) pathway, increased aerobic glycolysis, favored Th17 cell differentiation over that of Treg cells, and promoted a heightened pro-inflammatory gene expression signature. Group 3 innate lymphoid cells (ILC3s), but not ILC1s or ILC2s, were similarly responsive to IGF signaling. Mice with deficiency of IGF1R targeted to T cells failed to fully develop disease in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. Thus, the IGF system represents a previously unappreciated pathway by which type 3 immunity is modulated and immune-mediated pathogenesis controlled.

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.

Targeting growth hormone function: strategies and therapeutic applications

Human growth hormone (GH) is a classical pituitary endocrine hormone that is essential for normal postnatal growth and has pleiotropic effects across multiple physiological systems. GH is also expressed in extrapituitary tissues and has localized autocrine/paracrine effects at these sites. In adults, hypersecretion of GH causes acromegaly, and strategies that block the release of GH or that inhibit GH receptor (GHR) activation are the primary forms of medical therapy for this disease. Overproduction of GH has also been linked to cancer and the microvascular complications that are associated with diabetes. However, studies to investigate the therapeutic potential of GHR antagonism in these diseases have been limited, most likely due to difficulty in accessing therapeutic tools to study the pharmacology of the receptor in vivo. This review will discuss current and emerging strategies for antagonizing GH function and the potential disease indications.

Emerging therapies are offering an expanded toolkit for combatting the effects of human growth hormone overproduction. Human growth hormone (GH) is a major driver of postnatal growth; however, systemic or localized overproduction is implicated in the aberrant growth disease acromegaly, cancer, and diabetes. In this review, researchers led by Jo Perry, from the University of Auckland, New Zealand, discuss strategies that either inhibit GH production, block its systemic receptor, or interrupt its downstream signaling pathways. The only licensed GH receptor blocker is pegvisomant, but therapies are in development that include long-acting protein and antibody-based blockers, and nucleotide complexes that degrade GHR production have also shown promise. Studies investigating GHR antagonism are limited, partly due to difficulty in accessing therapeutic tools which block GHR function, but overcoming these obstacles may yield advances in alleviating chronic disease.

Imatinib Inhibits GH Secretion From Somatotropinomas

Background: Imatinib, a tyrosine kinase inhibitor, causes growth failure in children with chronic myeloid leukemia probably by targeting the growth hormone (GH)/insulin like growth factor-1 (IGF-1) axis. We aim to explore the imatinib targets expression in pituitary adenomas and study the effect of imatinib on GH secretion in somatotropinoma cells and GH3 cell line. Materials and Methods: The expression pattern of imatinib's targets (c-kit, VEGF, and PDGFR-α/β) was studied using immunohistochemistry and immunoblotting 157 giant (≥4 cm) pituitary adenomas (121 non-functioning pituitary adenomas, 32 somatotropinomas, and four prolactinomas) and compared to normal pituitary (n = 4) obtained at autopsy. The effect imatinib on GH secretion, cell viability, immunohistochemistry, electron microscopy, and apoptosis was studied in primary culture of human somatotropinomas (n = 20) and in rat somato-mammotroph GH3 cell-line. A receptor tyrosine kinase array was applied to human samples to identify altered pathways. Results: Somatotropinomas showed significantly higher immunopositivity for c-kit and platelet-derived growth factor receptor-β (PDGFR-β; P < 0.009 and P < 0.001, respectively), while staining for platelet-derived growth factor receptor-α (PDGFR-α) and vascular endothelial growth factor (VEGF) revealed a weaker expression (P < 0.001) compared to normal pituitary. Imatinib inhibited GH secretion from both primary culture (P < 0.01) and GH3 cells (P < 0.001), while it did not affect cell viability and apoptosis. The receptor tyrosine kinase array showed that imatinib inhibits GH signaling via PDGFR-β pathway. Conclusion: Imatinib inhibits GH secretion in somatotropinoma cells without affecting cell viability and may be used as an adjunct therapy for treating GH secreting pituitary adenomas.

Different intracellular signalling properties induced by human and porcine growth hormone

Growth hormone (GH) is reportedly species-specific. Primate growth hormone can trigger non-primate growth hormone receptor (GHR), but primates GHR cannot be activated by non-primate GH. However, it is also unclear that why primate GH and non-primate GH have different biological activities. Thus, we analysed primate growth hormone (human growth hormone (hGH)) or non-primate GH (porcine growth hormone (pGH))-induced intracellular signalling in 3T3-F442A cells and rat hepatocytes in a dose- and time-dependent manner to explore the different biological activities between them. The results revealed that both hGH and pGH can activate Janus kinase 2 (JAK2), Signal transducers and activators of transcription 1, 3 and 5 (STATs 1, 3 and 5) and extracellular signal-regulated kinase 1/2 (ERK1/2). There were no significant differences in JAK2 or ERK1/2 tyrosine phosphorylation after hGH and pGH treatment, but there were different between hGH and pGH in STAT/1/3/5 tyrosine phosphorylation, and JAK2, STAT/1/3/5 tyrosine phosphorylation was time-dependent and dose-dependent, whereas ERK1/2 was not. Both hGH and pGH demonstrated similar kinetics for STATs 1, 3 and 5 phosphorylation, but the pGH-mediated tyrosine phosphorylation was weaker than that mediated by hGH. Our observations indicated that the levels of main signalling proteins phosphorylation triggered by hGH or pGH were not exactly the same, which may explain the different biological activities showed by primate GH and non-primate GH.

DMP-1-mediated Ghr gene recombination compromises skeletal development and impairs skeletal response to intermittent PTH

Bone minerals are acquired during growth and are key determinants of adult skeletal health. During puberty, the serum levels of growth hormone (GH) and its downstream effector IGF-1 increase and play critical roles in bone acquisition. The goal of the current study was to determine how bone cells integrate signals from the GH/IGF-1 to enhance skeletal mineralization and strength during pubertal growth. Osteocytes, the most abundant bone cells, were shown to orchestrate bone modeling during growth. We used dentin matrix protein (Dmp)-1-mediated Ghr knockout (DMP-GHRKO) mice to address the role of the GH/IGF axis in osteocytes. We found that DMP-GHRKO did not affect linear growth but compromised overall bone accrual. DMP-GHRKO mice exhibited reduced serum inorganic phosphate and parathyroid hormone (PTH) levels and decreased bone formation indices and were associated with an impaired response to intermittent PTH treatment. Using an osteocyte-like cell line along with in vivo studies, we found that PTH sensitized the response of bone to GH by increasing Janus kinase-2 and IGF-1R protein levels. We concluded that endogenously secreted PTH and GHR signaling in bone are necessary to establish radial bone growth and optimize mineral acquisition during growth.

Novel Therapy to Treat Corneal Epithelial Defects: A Hypothesis with Growth Hormone

Impaired corneal wound healing that occurs with ocular surface disease, trauma, systemic disease, or surgical intervention can lead to persistent corneal epithelial defects (PCED), which result in corneal scarring, ulceration, opacification, corneal neovascularization, and, ultimately, visual compromise and vision loss. The current standard of care can include lubricants, ointments, bandage lenses, amniotic membranes, autologous serum eye drops, and corneal transplants. Various inherent problems exist with application and administration of these treatments, which often may not result in a completely healed surface. A topically applicable compound capable of promoting corneal epithelial cell proliferation and/or migration would be ideal to accelerate healing. We hypothesize that human growth hormone (HGH) is such a compound. In a recent study, HGH was shown to activate signal transducer and activators of transcription-5 (STAT5) signaling and promote corneal wound healing by enhancing corneal epithelial migration in a co-culture system of corneal epithelial cells and fibroblasts. These effects require an intact communication between corneal epithelia and fibroblasts. Further, HGH promotes corneal wound healing in a rabbit debridement model, thus demonstrating the effectiveness of HGH in vivo as well. In conclusion, HGH may represent an exciting and effective topical therapeutic to promote corneal wound healing.

Intracellular signaling transduction pathways triggered by a well-known anti-GHR monoclonal antibody, Mab263, in vitro and in vivo

A series of studies have reported that monoclonal antibody 263 (Mab263), a monoclonal antibody against the growth hormone receptor (GHR), acts as an agonist in vitro and in vivo. However, the intracellular signaling pathways triggered by Mab263 have not yet been delineated. Therefore, we examined the intracellular signaling pathways induced by Mab263 in vivo and in vitro in the present study. The results show that this antibody activated janus kinase 2 (JAK2), signal transducer and activator of transcription 3 (STAT3), STAT1 and extracellular signal-regulated kinase 1/2 (ERK1/2), but not STAT5. The phosphorylation kinetics of JAK2, STAT3/1 and ERK1/2 induced by Mab263 were subsequently analyzed in dose-response and time course experiments. Our observations indicate that Mab263 induced different intracellular signaling pathways than GH, which indicates that Mab263 is a signal-specific molecule and that Mab263 may be a valuable biological reagent to study the mechanism(s) of GHR-mediated intracellular signaling pathways.

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.

Minireview: mechanisms of growth hormone-mediated gene regulation

GH exerts a diverse array of physiological actions that include prominent roles in growth and metabolism, with a major contribution via stimulating IGF-1 synthesis. GH achieves its effects by influencing gene expression profiles, and Igf1 is a key transcriptional target of GH signaling in liver and other tissues. This review examines the mechanisms of GH-mediated gene regulation that begin with signal transduction pathways activated downstream of the GH receptor and continue with chromatin events at target genes and additionally encompasses the topics of negative regulation and cross talk with other cellular inputs. The transcription factor, signal transducer and activator of transcription 5b, is regarded as the major signaling pathway by which GH achieves its physiological effects, including in stimulating Igf1 gene transcription in liver. Recent studies exploring the mechanisms of how activated signal transducer and activator of transcription 5b accomplishes this are highlighted, which begin to characterize epigenetic features at regulatory domains of the Igf1 locus. Further research in this field offers promise to better understand the GH-IGF-1 axis in normal physiology and disease and to identify strategies to manipulate the axis to improve human health.

Functional collaboration of insulin-like growth factor-1 receptor (IGF-1R), but not insulin receptor (IR), with acute GH signaling in mouse calvarial cells

GH signals through the GH receptor (GHR), a cytokine receptor linked to Janus kinase 2 (JAK2). GH activates signal transducer and activator of transcription 5 (STAT5), causing expression of genes including IGF-I. IGF-I binds IGF-I receptor (IGF-IR), a heterotetrameric (α2-β2) tyrosine kinase growth factor receptor similar to insulin receptor (IR). In addition to this GH -> GHR -> IGF-I -> IGF-IR pathway, GH induces a complex including GHR, JAK2, and IGF-IR and deletion of floxed IGF-1R in primary murine calvarial cells with Cre-recombinase-expressing adenovirus (Ad-Cre) desensitizes cells to GH for STAT5 activation and IGF-I mRNA accumulation. Diminished GH-induced STAT5 phosphorylation in Ad-Cre-treated cells is rescued by adenoviruses encoding either IGF-IR or IGF-IR lacking the β-chain intracellular domain. Reasoning that IGF-IR's extracellular portion (α or extracellular β) mediates functional interaction with GH signaling, we pursued reconstitution studies. Although structurally related to IGF-IR, IR expressed adenovirally did not rescue GH-induced STAT5 phosphorylation in Ad-Cre-treated cells. We thus created chimeras, swapping homologous IR extracellular regions into IGF-IR. IR and IGF-IR possess N-terminal L1, cysteine-rich (CR), and L2 α-chain domains. We created Ad-IGF-IR/IR-L1 and Ad-IGF-IR/IR-L1-CR-L2, in which L1 alone or L1, CR, and L2 of IR replace corresponding IGF-IR regions, respectively. Ad-IGF-IR/IR-L1, but not Ad-IGF-IR/IR-L1-CR-L2, rescued GH-induced STAT5 phosphorylation in Ad-Cre-treated cells. Additionally, medium containing a soluble IGF-IR (including only L1-CR-L2) dampened GH-induced STAT5 phosphorylation in calvarial cells and two other GH-responsive cell lines. Thus, an extracellular determinant(s), likely in CR-L2, specifically allows IGF-IR to collaborate with GHR and JAK2 for robust GH-induced acute STAT5 phosphorylation.

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.

Lymphocyte GH-axis hormones in immunity

The production and utilization of common ligands and their receptors by cells of the immune and neuroendocrine systems constitutes a biochemical information circuit between and within the immune and neuroendocrine systems. The sharing of ligands and receptors allows the immune system to serve as the sixth sense notifying the nervous system of the presence of foreign entities. Within this framework, it is also clear that immune cell functions can be altered by neuroendocrine hormones and that cells of the immune system have the ability to produce neuroendocrine hormones. This review summarizes a part of this knowledge with particular emphasis on growth hormone (GH). The past two decades have uncovered a lot of detail about the actions of GH, acting through its receptor, at the molecular and cellular level and its influence on the immune system. The production and action of immune cell-derived GH is less well developed although its important role in immunity is also slowly emerging. Here we discuss the production of GH, GH-releasing hormone (GHRH) and insulin-like growth factor-1 (IGF-1) and their cognate receptors on cells of the immune system and their influence via endocrine/autocrine/paracrine and intracrine pathways on immune function. The intracellular mechanisms of action of immune cell-derived GH are still largely unexplored, and it is anticipated that further work in this particular area will establish an important role for this source of GH in normal physiology and in pathologic situations.

IGF-1R modulation of acute GH-induced STAT5 signaling: role of protein tyrosine phosphatase activity

GH is a potent anabolic and metabolic factor that binds its cell surface receptor (GHR), activating the GHR-associated tyrosine kinase, Janus kinase 2, which phosphorylates and activates the latent transcription factor, signal transducer and activator of transcription 5 (STAT5). Some GH actions are mediated by the elaboration of IGF-1, which exerts effects by binding and activating the heterotetrameric tyrosine kinase growth factor receptor, IGF-1R. In addition to this GH-GHR-IGF-1-IGF-1R scheme, we have demonstrated in primary osteoblasts and in islet β-cells that then deletion or silencing of IGF-1R results in diminished GH-induced STAT5 phosphorylation, suggesting that the presence of IGF-1R may facilitate GH signaling. In this study, we explore potential roles for protein tyrosine phosphatase activity in modulating GH-induced signaling, comparing conditions in which IGF-1R is present or diminished. We confirm that in mouse primary osteoblasts harboring loxP sites flanking the IGF-1R gene, infection with an adenovirus that expresses the Cre recombinase results in IGF-1R deletion and diminished acute GH-induced STAT5 phosphorylation. Furthermore, we present a new model of IGF-1R silencing, in which expression of short hairpin RNA directed at IGF-1R greatly reduces IGF-1R abundance in LNCaP human prostate cancer cells. In both models, treatment with a chemical inhibitor of protein tyrosine phosphatase-1B (PTP-1B), but not one of src homology region 2 domain-containing phosphotase-1 (SHP-1) and SHP-2, reverses the loss of GH-induced STAT5 phosphorylation in cells lacking IGF-1R but has no effect in cells with intact IGF-1R. Furthermore, expression of either a dominant-negative PTP-1B or the PTP-1B-interacting inhibitory protein, constitutive photomorphogenesis 1, also rescues acute GH-induced STAT5 signaling in IGF-1R-deficient cells but has no effect in IGF-1R replete cells. By expressing a substrate-trapping mutant PTP-1B, we demonstrate that tyrosine phosphorylated Janus kinase-2 is a PTP-1B substrate only in cells lacking IGF-1R. Collectively, our data suggest that IGF-1R positively regulates acute GH signaling by preventing access of PTP-1B activity to Janus kinase 2 and thereby preventing PTP-1B-mediated suppression of GH-induced STAT5 activation.

Growth hormone potentiates 17β-estradiol-dependent breast cancer cell proliferation independently of IGF-I receptor signaling

Estrogen action in mammary gland development and breast cancer progression is tightly linked to the GH/IGF-I axis. Although many of the effects of GH on mammary gland growth and development require IGF-I, the extent to which GH action in breast cancer depends on IGF-I is not known. We examined GH action in a panel of estrogen receptor-positive breast cancer cell lines and found that T47D cells express significant levels of GH receptor and that GH significantly enhances 17β-estradiol (E2)-stimulated proliferation in these cells. GH action in the T47D cells was independent of changes in IGF-I and IGF-I receptor (IGF-IR) expression and IGF-IR signaling, suggesting that GH can exert direct effects on breast cancer cells. Although E2-dependent proliferation required IGF-IR signaling, the combination of GH+E2 overcame inhibition of IGF-IR activity to restore proliferation. In contrast, GH required both Janus kinase 2 and epidermal growth factor receptor signaling for subsequent ERK activation and potentiation of E2-dependent proliferation. Downstream of these pathways, we identified a number of immediate early-response genes associated with proliferation that are rapidly and robustly up-regulated by GH. These findings demonstrate that GH can have important effects in breast cancer cells that are distinct from IGF-IR activity, suggesting that novel drugs or improved combination therapies targeting estrogen receptor and the GH/IGF axis may be beneficial for breast cancer patients.

Growth hormone and cancer: an update on progress

PURPOSE OF REVIEW

Animals born with a deficiency in the cell surface receptor for growth hormone (GH) have a significantly reduced risk of developing cancer. Conversely, increased expression levels of GH and the GH receptor (GHR) are detectable in a variety of different human cancers. Here we discuss recent literature contributing to our understanding of the field.

RECENT FINDINGS

In addition to animal evidence, studies of individuals with Laron syndrome suggest that congenital GHR deficiency may also protect humans against cancer. GH expression in certain malignancies is correlated with clinicohistopathological parameters and may contribute the therapeutic resistance. Other recent studies have identified novel aspects of the GH signal transduction pathway, including receptor crosstalk and the involvement of microRNA in endocrine regulation of GH.

SUMMARY

Substantial evidence suggests the GH/insulin-like growth factor-1 axis initiates and promotes progression of cancer. However, important questions remain unanswered regarding the therapeutic utility of GH or GHR antagonism in cancer. Further clinical studies regarding the clinical association of GH expression with human malignancies and translational studies investigating GHR antagonism in animal models of human cancer are critical.

Effects of insulin and IGF-I on growth hormone- induced STAT5 activation in 3T3-F442A adipocytes

BACKGROUND

Growth hormone (GH) and insulin signaling pathways are known important regulators of adipose homeostasis. The cross-talk between GH and insulin signaling pathways in mature adipocytes is poorly understood.

METHODS

In the present study, the impact of insulin on GH-mediated signaling in differentiated 3T3-F442A adipocytes and primary mice adipocytes was examined.

RESULTS

Insulin alone did not induce STAT5 tyrosine phosphorylation, but enhanced GH-induced STAT5 activation. This effect was more pronounced when insulin was added 20 min prior to GH treatment. The above results were further confirmed by in vivo study, showing that insulin pretreatment potentiated GH- induced STAT5 tyrosine phosphorylation in visceral adipose tissues of C57/BL6 mice. In addition, our in vitro results showed that IGF-I had similar potentiating effect as insulin on GH-induced STAT5 activation. In vitro, insulin and IGF-I had an additive effect on GH- induced MAPK activation.

CONCLUSION

These results indicate that both insulin and IGF-I specifically potentiated GH mediated STAT5 activation in mature adipose cells. These findings suggest that insulin and GH, usually with antagonistic functions, might act synergistically to regulate some specific functions in mature adipocytes.

Osteoblast-restricted Disruption of the Growth Hormone Receptor in Mice Results in Sexually Dimorphic Skeletal Phenotypes

Growth hormone (GH) exerts profound anabolic actions during postnatal skeletal development, in part, through stimulating the production of insulin-like growth factor-1 (IGF-1) in liver and skeletal tissues. To examine the requirement for the GH receptor (GHR) in osteoblast function in bone, we used Cre-LoxP methods to disrupt the GHR from osteoblasts, both in vitro and in vivo. Disruption of GHR from primary calvarial osteoblasts in vitro abolished GH-induced signaling, as assessed by JAK2/STAT5 phosphorylation, and abrogated GH-induced proliferative and anti-apoptotic actions. Osteoblasts lacking GHR exhibited reduced IGF-1-induced Erk and Akt phosphorylation and attenuated IGF-1-induced proliferation and anti-apoptotic action. In addition, differentiation was modestly impaired in osteoblasts lacking GHR, as demonstrated by reduced alkaline phosphatase staining and calcium deposition. In order to determine the requirement for the GHR in bone in vivo, we generated mice lacking the GHR specifically in osteoblasts (ΔGHR), which were born at the expected Mendelian frequency, had a normal life span and were of normal size. Three week-old, female ΔGHR mice had significantly reduced osteoblast numbers, consistent with the in vitro data. By six weeks of age however, female ΔGHR mice demonstrated a marked increase in osteoblasts, although mineralization was impaired; a phenotype similar to that observed previously in mice lacking IGF-1R specifically in osteoblasts. The most striking phenotype occurred in male mice however, where disruption of the GHR from osteoblasts resulted in a "feminization" of bone geometry in 16 week-old mice, as observed by μCT. These results demonstrate that the GHR is required for normal postnatal bone development in both sexes. GH appears to serve a primary function in modulating local IGF-1 action. However, the changes in bone geometry observed in male ΔGHR mice suggest that, in addition to facilitating IGF-1 action, GH may function to a greater extent than previously appreciated in establishing the sexual dimorphism of the skeleton.

MSM enhances GH signaling via the Jak2/STAT5b pathway in osteoblast-like cells and osteoblast differentiation through the activation of STAT5b in MSCs

Methylsulfonylmethane (MSM) is a naturally occurring sulfur compound with well-known anti-oxidant properties and anti-inflammatory activities. But, its effects on bone are unknown. Growth hormone (GH) is regulator of bone growth and bone metabolism. GH activates several signaling pathways such as the Janus kinase (Jak)/signal transducers and activators of transcription (STAT) pathway, thereby regulating expression of genes including insulin-like growth factor (IGF)-1. GH exerts effects both directly and via IGF-1, which signals by activating the IGF-1 receptor (IGF-1R). In this study, we investigated the effects of MSM on the GH signaling via the Jak/STAT pathway in osteoblasts and the differentiation of primary bone marrow mesenchymal stem cells (MSCs). MSM was not toxic to osteoblastic cells and MSCs. MSM increased the expression of GH-related proteins including IGF-1R, p-IGF-1R, STAT5b, p-STAT5b, and Jak2 in osteoblastic cells and MSCs. MSM increased IGF-1R and GHR mRNA expression in osteoblastic cells. The expression of MSM-induced IGF-1R and GHR was inhibited by AG490, a Jak2 kinase inhibitor. MSM induced binding of STAT5 to the IGF-1R and increased IGF-1 and IGF-1R promoter activities. Analysis of cell extracts by immunoprecipitation and Western blot showed that MSM enhanced GH-induced activation of Jak2/STAT5b. We found that MSM and GH, separately or in combination, activated GH signaling via the Jak2/STAT5b pathway in UMR-106 cells. Using siRNA analysis, we found that STAT5b plays an essential role in GH signaling activation in C3H10T1/2 cells. Osteogenic marker genes (ALP, ON, OCN, BSP, OSX, and Runx2) were activated by MSM, and siRNA-mediated STAT5b knockdown inhibited MSM-induced expression of osteogenic markers. Furthermore, MSM increased ALP activity and the mineralization of MSCs. Taken together, these results indicated that MSM can promote osteogenic differentiation of MSCs through activation of STAT5b.

The insulin-like growth factors in adipogenesis and obesity

Adipose tissue has been recognized as a major target of growth hormone (GH) action. GH was shown to inhibit adipocyte differentiation but stimulated preadipocyte proliferation in vitro. GH acts directly via its receptor or via upregulating insulin-like growth factor (IGF)-I, which is a critical mediator of preadipocyte proliferation, differentiation, and survival. Results from clinical studies on GH treatment in patients with GH deficiency or GH insensitivity syndrome can be used to dissect GH and IGF as well as IGF-binding protein (IGFBP) actions in vivo. In this article, changes of the GH/IGF system during adipocyte differentiation in vitro as well as related signaling pathways and their impact on adipose tissue growth and function are discussed. Clinical considerations include the effects of GH and IGF-I on adipose tissue during treatment of GH deficiency, differences in the IGF system between visceral and subcutaneous adipose tissue depots as well as the recently emerging role for adipose tissue in the regulation of glucose homeostasis.

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.

Endothelial cell-specific molecule 2 (ECSM2) localizes to cell-cell junctions and modulates bFGF-directed cell migration via the ERK-FAK pathway

Background

Despite its first discovery by in silico cloning of novel endothelial cell-specific genes a decade ago, the biological functions of endothelial cell-specific molecule 2 (ECSM2) have only recently begun to be understood. Limited data suggest its involvement in cell migration and apoptosis. However, the underlying signaling mechanisms and novel functions of ECSM2 remain to be explored.

Methodology/Principal Findings

A rabbit anti-ECSM2 monoclonal antibody (RabMAb) was generated and used to characterize the endogenous ECSM2 protein. Immunoblotting, immunoprecipitation, deglycosylation, immunostaining and confocal microscopy validated that endogenous ECSM2 is a plasma membrane glycoprotein preferentially expressed in vascular endothelial cells (ECs). Expression patterns of heterologously expressed and endogenous ECSM2 identified that ECSM2 was particularly concentrated at cell-cell contacts. Cell aggregation and transwell assays showed that ECSM2 promoted cell-cell adhesion and attenuated basic fibroblast growth factor (bFGF)-driven EC migration. Gain or loss of function assays by overexpression or knockdown of ECSM2 in ECs demonstrated that ECSM2 modulated bFGF-directed EC motility via the FGF receptor (FGFR)-extracellular regulated kinase (ERK)-focal adhesion kinase (FAK) pathway. The counterbalance between FAK tyrosine phosphorylation (activation) and ERK-dependent serine phosphorylation of FAK was critically involved. A model of how ECSM2 signals to impact bFGF/FGFR-driven EC migration was proposed.

Conclusions/Significance

ECSM2 is likely a novel EC junctional protein. It can promote cell-cell adhesion and inhibit bFGF-mediated cell migration. Mechanistically, ECSM2 attenuates EC motility through the FGFR-ERK-FAK pathway. The findings suggest that ECSM2 could be a key player in coordinating receptor tyrosine kinase (RTK)-, integrin-, and EC junctional component-mediated signaling and may have important implications in disorders related to endothelial dysfunction and impaired EC junction signaling.

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.

Acute regulation of IGF-I by alterations in post-exercise macronutrients

This investigation sought to examine the contributions of exercise and nutrient replenishment on in vivo regulation of the insulin-like growth factor-I (IGF-I) axis components. Eight college-aged males completed three high-intensity interval training (HIIT) protocols followed by three post-exercise nutritional protocols: (1) placebo (EX); (2) carbohydrate only (CHO); and (3) essential amino acid/carbohydrate (EAA/CHO). Samples were analyzed for growth hormone (GH), free IGF-I, IGFBP-1, IGFBP-2, insulin, hematocrit, hemoglobin, serum leucine, matrix metalloproteinase-9 (MMP-9) proteolytic activity, and presence of IGFBP-3 protease activity. No evidence for IGFBP-3 proteolysis was observed. Significant increases in [free IGF-I] and [leucine] were observed in the EAA/CHO group only. Significant differences were noted in [IGFBP-1] and [IGFBP-2] across conditions. Significant increases in [GH] and MMP-9 activity were observed in all groups. These results indicate that post-exercise macronutrient ratio is a determinant of [free IGF-I], [IGFBP-1 and -2] and may play a role in modulating the IGF-I axis in vivo.

Synergy in ERK activation by cytokine receptors and tyrosine kinase growth factor receptors

Epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) signal through EGF and PDGF receptors, which are important receptor tyrosine kinases (RTKs). Growth hormone (GH) and prolactin (PRL) are four helical bundle peptide hormones that signal via GHR and PRLR, members of the cytokine receptor superfamily. In this study, we examine crosstalk between signaling pathways emanating from these disparate receptor groups (RTKs and cytokine receptors). We find that GH and EGF specifically synergize for activation of ERK in murine preadipocytes. The locus of this synergy resides at the level of MEK activation, but not above this level (i.e., not at the level of EGFR, SHC, or Raf activation). Furthermore, dephosphorylation of the scaffold protein, KSR, at a critical serine residue is also synergistically promoted by GH and EGF, suggesting that GH sensitizes these cells to EGF-induced ERK activation by augmenting the actions of KSR in facilitating MEK-ERK activation. Similarly specific synergy in ERK activation is also detected in human T47D breast cancer cells by cotreatment with PRL and PDGF. This synergy also resides at the level of MEK activation. Consistent with this synergy, PRL and PDGF also synergized for c-fos-dependent transactivation of a luciferase reporter gene in T47D cells, indicating that events downstream of ERK activation reflect this signaling synergy. Important conceptual and physiological implications of these findings are discussed.

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.

Biological effects of growth hormone on carbohydrate and lipid metabolism

This review will summarize the metabolic effects of growth hormone (GH) on the adipose tissue, liver, and skeletal muscle with focus on lipid and carbohydrate metabolism. The metabolic effects of GH predominantly involve the stimulation of lipolysis in the adipose tissue resulting in an increased flux of free fatty acids (FFAs) into the circulation. In the muscle and liver, GH stimulates triglyceride (TG) uptake, by enhancing lipoprotein lipase (LPL) expression, and its subsequent storage. The effects of GH on carbohydrate metabolism are more complicated and may be mediated indirectly via the antagonism of insulin action. Furthermore, GH has a net anabolic effect on protein metabolism although the molecular mechanisms of its actions are not completely understood. The major questions that still remain to be answered are (i) What are the molecular mechanisms by which GH regulates substrate metabolism? (ii) Does GH affect substrate metabolism directly or indirectly via IGF-1 or antagonism of insulin action?

Substance P promotes expansion of human mesenteric preadipocytes through proliferative and antiapoptotic pathways

White adipose tissue is intimately involved in the regulation of immunity and inflammation. We reported that human mesenteric preadipocytes express the substance P (SP)-mediated neurokinin-1 receptor (NK-1R), which signals proinflammatory responses. Here we tested the hypothesis that SP promotes proliferation and survival of human mesenteric preadipocytes and investigated responsible mechanism(s). Preadipocytes were isolated from mesenteric fat biopsies during gastric bypass surgery. Proliferative and antiapoptotic responses were delineated in 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS), bromodeoxyuridine (BrdU), caspase-3, and TUNEL assays, as well as Western immunoanalysis. SP (10(-7) M) increased MTS and proliferation (BrdU) and time dependently (15-30 min) induced Akt, EGF receptor, IGF receptor, integrin alphaVbeta3, phosphatidylinositol 3-kinase, and PKC-theta phosphorylation. Furthermore, pharmacological antagonism of Akt and PKC-theta activation significantly attenuated SP-induced preadipocyte proliferation. Exposure of preadipocytes to the proapoptotic Fas ligand (FasL, 100 microM) resulted in nuclear DNA fragmentation (TUNEL assay), as well as increased cleaved poly (ADP-ribose) polymerase, cleaved caspase-7, and caspase-3 expression. Cotreatment with SP almost completely abolished these responses in a NK-1R-dependent fashion. SP (10(-7) M) also time dependently stimulated expression 4E binding protein 1 and phosphorylation of p70 S6 kinase, which increased protein translation efficiency. SP increases preadipocyte viability, reduces apoptosis, and stimulates proliferation, possibly via cell cycle upregulation and increased protein translation efficiency. SP-induced proliferative and antiapoptotic pathways in fat depots may contribute to development of the creeping fat and inflammation characteristic of Crohn's disease.

AKT/eNOS signaling module functions as a potential feedback loop in the growth hormone signaling pathway

Background

While evidence suggested that the activity states of Protein kinase B (AKT/PKB) and endothelial nitric oxide synthase (eNOS) play an important role in the progression of the Growth Hormone (GH) signal cascade, the implication of the activation of AKT/PKB and eNOS in terms of their function in the signaling pathway was not clear.

Results

Using a specific AKT/PKB inhibitor and a functional proteomic approach, we were able to detect the activities of multiple signal transduction pathway elements, the downstream targets of the AKT/PKB pathway and the modification of those responses by treatment with GH. Inhibiting the AKT/PKB activity reduced or eliminated the activation (phosphorylation) of eNOS. We demonstrated that the progression of the GH signal cascade is influenced by the activity status of AKT and eNOS, wherein the suppression of AKT activity appears to augment the activity of extracellular signal-regulated kinases 1 and 2 (Erk1/2) and to antagonize the deactivation (phosphorylation) of cyclin-dependent kinase 2 (CDC2/Cdk1) induced by GH. Phosphorylation of GSK3a/b (glycogen synthase kinase 3), the downstream target of AKT/PKB, was inhibited by the AKT/PKB inhibitor. GH did not increase phosphorylation of ribosomal S6 kinase 1 (RSK1) in normal cells but increases phosphorylation of RSK1 in cells pre-treated with the AKT and eNOS inhibitors.

Conclusion

The MAP kinase and CDC2 kinase-dependent intracellular mechanisms are involved in or are the targets of the GH's action processes, and these activities are probably directly or indirectly modulated by AKT/PKB pathways. We propose that the AKT/PKB-eNOS module likely functions as a negative feedback mediator of GH actions.

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.

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.

Mode of Growth Hormone Action in Osteoblasts

Growth hormone (GH) affects bone size and mass in part through stimulating insulin-like growth factor type 1 (IGF-1) production in liver and bone. Whether GH acts independent of IGF-1 in bone remains unclear. To define the mode of GH action in bone, we have used a Cre/loxP system in which the type 1 IGF-1 receptor (Igf1r) has been disrupted specifically in osteoblasts in vitro and in vivo. Calvarial osteoblasts from mice homozygous for the floxed IGF-1R allele (IGF-1R(flox/flox)) were infected with adenoviral vectors expressing Cre. Disruption of IGF-1R mRNA (>90%) was accompanied by near elimination of IGF-1R protein but retention of GHR protein. GH-induced STAT5 activation was consistently greater in osteoblasts with an intact IGF-1R. Osteoblasts lacking IGF-1R retained GH-induced ERK and Akt phosphorylation and GH-stimulated IGF-1 and IGFBP-3 mRNA expression. GH-induced osteoblast proliferation was abolished by Cre-mediated disruption of the IGF-1R or co-incubation of cells with an IGF-1-neutralizing antibody. By contrast, GH inhibited apoptosis in osteoblasts lacking the IGF-1R. To examine the effects of GH on osteoblasts in vivo, mice wild type for the IGF-1R treated with GH subcutaneously for 7 days showed a doubling in the number of osteoblasts lining trabecular bone, whereas osteoblast numbers in similarly treated mice lacking the IGF-1R in osteoblasts were not significantly affected. These results indicate that although direct IGF-1R-independent actions of GH on osteoblast apoptosis can be demonstrated in vitro, IGF-1R is required for anabolic effects of GH in osteoblasts in vivo.

Chronobiology in the endocrine system

Biological signaling occurs in a complex web with participation and interaction of the central nervous system, the autonomous nervous system, the endocrine glands, peripheral endocrine tissues including the intestinal tract and adipose tissue, and the immune system. All of these show an intricate time structure with rhythms and pulsatile variations in multiple frequencies. Circadian (about 24-hour) and circannual (about 1-year) rhythms are kept in step with the cyclic environmental surrounding by the timing and length of the daily light span. Rhythmicity of many endocrine variables is essential for their efficacy and, even in some instances, for the qualitative nature of their effects. Indeed, the continuous administration of certain hormones and their synthetic analogues may show substantially different effects than expected. In the design of drug-delivery systems and treatment schedules involving directly or indirectly the endocrine system, consideration of the human time organization is essential. A large amount of information on the endocrine time structure has accumulated, some of which is discussed in this review.

Growth hormone (GH)-associated nitration of Janus kinase-2 at the 1007Y-1008Y epitope impedes phosphorylation at this site: mechanism for and impact of a GH, AKT, and nitric oxide synthase axis on GH signal transduction

A generalized increase in liver protein tyrosine nitration (3'-nitrotyrosine, 3'-NT) occurs after GH injection in a time frame consistent with observed acute GH hyporesponsiveness. Here we investigated whether the GH-associated nitration process might be targeted to the (1007)Y-(1008)Y-phosphorylation epitope of Janus kinase (JAK)-2 because of its homology to a defined peptide nitration motif. Using antibodies we developed to the 3'NT-substituted peptide analog of the (1007)Y-(1008)Y-JAK2 site (nitro-JAK2), we demonstrated a rapid increase in membrane-associated nitro-JAK2 after GH. In vivo (bovine liver) and in vitro (porcine hepatocytes), GH-induced cellular levels of nitro-(1007)Y-(1008)Y-JAK2 persisted significantly longer after a stimulatory GH pulse than did levels of phospho-JAK2. Treatment of cultured cells with inhibitors of AKT or endothelial nitric oxide synthase prior to GH challenge attenuated the increases in nitro-JAK2 predominantly in the membrane subcellular fraction. In instances in which GH effected orthophosphorylation of (694)Y-signal transducer and activator of transcription (STAT)-5b, the addition of AKT and endothelial nitric oxide synthase inhibitors prior to GH significantly increased the levels of phospho-(694)Y-STAT5b and phospho-(1007)Y-JAK2 over those arising from GH alone. Nuclear magnetic resonance molecular modeling of natural and 3'-NT- and orthophosphate-substituted peptide analogs of the (1007)Y-(1008)Y site demonstrated significant effects of 3'-nitration on the planar orientation and intramolecular stabilizing points of the affected tyrosines. When these peptides were used as substrates for in vitro tyrosine kinase phosphorylation reactions, 3'-NT in the (1007)Y and/or (1008)Y positions blocked the generation of (1007)Y-phosphotyrosine. The data suggest that the nitration of JAK2 may act as an inhibitory counterpart to phosphorylation activation, reflecting a very localized break on the progression of GH signal transduction processes spanning JAK-STAT-AKT interactions.

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.

Mechanisms of disease: metabolic effects of growth hormone and insulin-like growth factor 1

Insulin-like growth factor (IGF) 1 is a member of a family that is involved in growth, development, cell differentiation, and metabolism. IGF1, IGF2 and insulin act primarily through tyrosine-kinase-linked receptors--the IGF1 receptor (IGF1R) and insulin receptor (IR). The IGF1R binds IGF1 and IGF2 with high affinity and the IR binds insulin with high affinity; however, since both receptors share a high degree of structural and functional homology, the IGF1R can bind insulin and the IR can bind the IGFs with reduced affinity. These two receptors can, moreover, form heterodimers, which bind both ligands. Upon binding to the receptors, cascades of tyrosine and serine kinases are stimulated to facilitate growth or metabolism. The IGF2 receptor is a scavenger receptor, and is, therefore, not involved in mediation of growth or metabolic effects of the IGF family and will not be discussed in the current article. IGF1 is a major gene target of growth hormone and its product mediates many of the actions of growth hormone on growth and development; however, IGF1 has actions distinct from those of growth hormone in carbohydrate, lipid, and protein metabolism. For example, excess growth hormone causes insulin resistance and hyperglycemia, whereas IGF1 has insulin-like effects that reduce blood glucose levels and has been used experimentally to treat both type 1 and type 2 diabetes.

Diagnostik und Therapie posttraumatischer Pseudolipome

BACKGROUND

Both trauma and lipomas are frequently encountered in day-to-day clinical practice. Although lipomas are defined both clinically and pathologically as benign fatty tissue tumours, their aetiology is still not clear.

METHODS

In this study 19 patients with 23 posttraumatic lipomas were analysed retrospectively with reference to ultrasound and MRI diagnosis, history, laboratory results and histopathological investigations.

RESULTS

The mean age of the patients was 50.5 years (+/-15.5). The causative soft tissue trauma dated back an average of 2.6 years. When the histories were taken, 16 of the 19 patients reported vast, slow-resorbing posttraumatic haematomas. Nine of the 23 lipomas were sited in the upper extremity, 3 in the lower extremity, 9 in the trunk and 2 in the face. All were located epifascially. In 22 cases the lipomas were excised, and in 1 case the lipoma was removed by liposuction. Histological examination demonstrated capsulated benign fatty tissue tumours in 19 cases and uncapsulated benign fatty tissue tumours in 4. The average body mass index (BMI) was 29 kg/m(2). Removal of the tumour resulted in a good aesthetic result in all patients.

CONCLUSIONS

The link between a blunt soft tissue injury and the development of a posttraumatic lipoma is still the subject of controversy; there are two mechanisms that seem more likely than any others proposed: (1) the "pseudolipoma" as the result of a prolapse of fatty tissue as an immediate result of trauma and (2) the development of a lipoma by way of differentiation of pre-adipocytes mediated by cytokines released by a posttraumatic haematoma. There appears to be a correlation between an increased partial thromboplastin time (PTT) and the development of posttraumatic lipomas. The generalised increase in the volume of body fat documented by the elevated BMI supports the idea that lipomas arise from the prolapse of adipose tissue. However, there is no single mechanistic explanation for the development of posttraumatic lipomas. They are probably caused by multiple factors and not by isolated pathological mechanisms.

Involvement of JAK2 and Src kinase tyrosine phosphorylation in human growth hormone-stimulated increases in cytosolic free Ca2+and insulin secretion

We previously reported that human growth hormone (hGH) increases cytoplasmic Ca2+concentration ([Ca2+]i) and proliferation in pancreatic β-cells (Sjöholm Å, Zhang Q, Welsh N, Hansson A, Larsson O, Tally M, and Berggren PO. J Biol Chem 275: 21033–21040, 2000) and that the hGH-induced rise in [Ca2+]iinvolves Ca2+-induced Ca2+release facilitated by tyrosine phosphorylation of ryanodine receptors (Zhang Q, Kohler M, Yang SN, Zhang F, Larsson O, and Berggren PO. Mol Endocrinol 18: 1658–1669, 2004). Here we investigated the tyrosine kinases that convey the hGH-induced rise in [Ca2+]iand insulin release in BRIN-BD11 β-cells. hGH caused tyrosine phosphorylation of Janus kinase (JAK)2 and c-Src, events inhibited by the JAK2 inhibitor AG490 or the Src kinase inhibitor PP2. Although hGH-stimulated rises in [Ca2+]iand insulin secretion were completely abolished by AG490 and JAK2 inhibitor II, the inhibitors had no effect on insulin secretion stimulated by a high K+concentration. Similarly, Src kinase inhibitor-1 and PP2, but not its inactive analog PP3, suppressed [Ca2+]ielevation and completely abolished insulin secretion stimulated by hGH but did not affect responses to K+. Ovine prolactin increased [Ca2+]iand insulin secretion to a similar extent as hGH, effects prevented by the JAK2 and Src kinase inhibitors. In contrast, bovine GH evoked a rise in [Ca2+]ibut did not stimulate insulin secretion. Neither JAK2 nor Src kinase inhibitors influenced the effect of bovine GH on [Ca2+]i. Our study indicates that hGH stimulates rise in [Ca2+]iand insulin secretion mainly through activation of the prolactin receptor and JAK2 and Src kinases in rat insulin-secreting cells.

Prolactin modulates phosphorylation, signaling and trafficking of epidermal growth factor receptor in human T47D breast cancer cells

Prolactin (PRL) is a polypeptide hormone produced by the anterior pituitary gland and other sites that acts both systemically and locally to cause lactation and other biological effects by interacting with the PRL receptor, a Janus kinase (JAK)2-coupled cytokine receptor family member, and activating downstream signal pathways. Recent evidence suggests PRL is a player in the pathogenesis and progression of breast cancer. Epidermal growth factor (EGF) also has effects on breast tissue, working through its receptors, epidermal growth factor receptor (EGFR) and ErbB-2 (c-neu, HER2), both intrinsic tyrosine kinase growth factor receptors. EGFR promotes pubertal breast ductal morphogenesis in mice, and both EGFR and ErbB-2 are relevant in pathogenesis and behavior of breast and other human cancers. Previous studies showed that PRL and EGF synergize to enhance motility in the human breast cancer cell line, T47D. In this study, we explored crosstalk between the PRL and EGF signaling pathways in T47D cells, with an ultimate aim of understanding how these two important factors might work together in vivo to affect breast cancer behavior. Both PRL and EGF caused robust signaling in T47D cells; PRL acutely activated JAK2, signal transducer and activator of transcription-5 (STAT5), and extracellular signal-regulated kinase-1 and -2 (ERK1 and ERK2), whereas EGF caused EGFR activation and consequent src homology collagen (SHC) activation and ERK activation. Notably, PRL also caused phosphorylation of the EGFR and ErbB-2 at sites detected by PTP101, an antibody that recognizes threonine phosphorylation at consensus motifs for ERK-induced phosphorylation. PRL-induced PTP101-reactive phosphorylation was prevented by pretreatment with PD98059, an ERK pathway inhibitor. Furthermore, PRL synergized with EGF in activating SHC and ERK and transactivating a luciferase reporter driven by c-fos gene enhancer elements, suggesting that PRL allowed markedly enhanced EGF signaling. This was accompanied by substantial inhibition of EGF-induced EGFR downregulation when PRL and EGF cotreatment was compared to EGF treatment alone. This effect of PRL was abrogated by ERK pathway inhibitor pretreatment. Our data suggest that PRL synergistically augments EGF signaling in T47D breast cancer cells at least in part by lessening EGF-induced EGFR downregulation and that this effect requires PRL-induced ERK activity and threonine phosphorylation of EGFR.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

Insulin enhances growth hormone induction of the MEK/ERK signaling pathway

Growth hormone (GH) plays an important role in growth and metabolism by signaling via at least three major pathways, including STATs, ERK1/2, and phosphatidylinositol 3-kinase/Akt. Physiological concentrations of insulin promote growth probably by modulating liver GH receptor (GHR) levels in vivo, but the possible effects of insulin on GH-induced post-GHR signaling have yet to be studied. We hypothesized that short-term insulin, similar to the fluctuations that occur following feeding, affects GH-induced post-GHR signaling. Our present studies suggest that, in rat H4IIE hepatoma cells, insulin (4 h or less) selectively enhanced GH-induced phosphorylation of MEK1/2 and ERK1/2, but not GH-induced activation of STAT5 and Akt. Although insulin pretreatment altered GH-induced formation of Shc.Grb2.SOS complex, it did not significantly affect GH-induced activation of other signaling intermediates upstream of MEK/ERK, including JAK2, Ras, and Raf-1. Immunofluorescent staining indicated that insulin pretreatment facilitated GH-induced cell membrane translocation of MEK1/2. Insulin pretreatment also increased the amount of MEK association with its scaffolding protein, KSR. In summary, short-term insulin treatment of cultured, liver-derived cells selectively sensitized GH-induced MEK/ERK phosphorylation independent of JAK2, Ras, and Raf-1, but likely resulted from increased cell membrane translocation of MEK1/2. These findings suggest that insulin may be necessary for sensitization of cells to GH-induced ERK1/2 activation and provides a potential cellular mechanism by which insulin promotes growth.