Stimulation by growth hormone of MAP kinase activity in 3T3-F442A fibroblasts

2022 Cannon lecture: an ode to signal transduction: how the growth hormone pathway revealed insight into height, malignancy, and obesity

Walter Cannon was a highly regarded American neurologist and physiologist with extremely broad interests. In the tradition of Cannon and his broad interests, we discuss our laboratory's multifaceted work in signal transduction over the past 40+ years. We show how our questioning of how growth hormone (GH) in the blood communicates with cells throughout the body to promote body growth and regulate body metabolism led to insight into not only body height but also important regulators of malignancy and body weight. Highlights include finding that 1) A critical initiating step in GH signal transduction is GH activating the GH receptor-associated tyrosine kinase JAK2; 2) GH activation of JAK2 leads to activation of a number of signaling proteins, including STAT transcription factors; 3) JAK2 is autophosphorylated on multiple tyrosines that regulate the activity of JAK2 and recruit signaling proteins to GH/GH receptor/JAK2 complexes; 4) Constitutively activated STAT proteins are associated with cancer; 5) GH activation of JAK2 recruits the adapter protein SH2B1 to GH/GH receptor/JAK2 complexes where it facilitates GH regulation of the actin cytoskeleton and motility; and 6) SH2B1 is recruited to other receptors in the brain, where it enhances satiety, most likely in part by regulating leptin action and neuronal connections of appetite-regulating neurons. These findings have led to increased understanding of how GH functions, as well as therapeutic interventions for certain cancer and obese individuals, thereby reinforcing the great importance of supporting basic research since one never knows ahead of time what important insight it can provide.

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

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.

Skeletal effects of growth hormone and insulin-like growth factor-I therapy

The growth hormone/insulin-like growth factor (GH/IGF) axis is critically important for the regulation of bone formation, and deficiencies in this system have been shown to contribute to the development of osteoporosis and other diseases of low bone mass. The GH/IGF axis is regulated by a complex set of hormonal and local factors which can act to regulate this system at the level of the ligands, receptors, IGF binding proteins (IGFBPs), or IGFBP proteases. A combination of in vitro studies, transgenic animal models, and clinical human investigations has provided ample evidence of the importance of the endocrine and local actions of both GH and IGF-I, the two major components of the GH/IGF axis, in skeletal growth and maintenance. GH- and IGF-based therapies provide a useful avenue of approach for the prevention and treatment of diseases such as osteoporosis.

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.

Nuclear factor-kappaB (NF-kappaB) p65 interacts with Stat5b in growth plate chondrocytes and mediates the effects of growth hormone on chondrogenesis and on the expression of insulin-like growth factor-1 and bone morphogenetic protein-2

Growth hormone (GH) stimulates growth plate chondrogenesis and longitudinal bone growth with its stimulatory effects primarily mediated by insulin-like growth factor-1 (IGF-1) both systemically and locally in the growth plate. It has been shown that the transcription factor Stat5b mediates the GH promoting effect on IGF-1 expression and on chondrogenesis, yet it is not known whether other signaling molecules are activated by GH in growth plate chondrocytes. We have previously demonstrated that nuclear factor-κB p65 is a transcription factor expressed in growth plate chondrocytes where it facilitates chondrogenesis. We have also shown that fibroblasts isolated from a patient with growth failure and a heterozygous mutation of inhibitor-κBα (IκB; component of the nuclear factor-κB (NF-κB) signaling pathway) exhibit GH insensitivity. In this study, we cultured rat metatarsal bones in the presence of GH and/or pyrrolidine dithiocarbamate (PDTC), a known NF-κB inhibitor. The GH-mediated stimulation of metatarsal longitudinal growth and growth plate chondrogenesis was neutralized by PDTC. In cultured chondrocytes isolated from rat metatarsal growth plates, GH induced NF-κB-DNA binding and chondrocyte proliferation and differentiation and prevented chondrocyte apoptosis. The inhibition of NF-κB p65 expression and activity (by NF-κB p65 siRNA and PDTC, respectively) in chondrocytes reversed the GH-mediated effects on chondrocyte proliferation, differentiation, and apoptosis. Lastly, the inhibition of Stat5b expression in chondrocytes prevented the GH promoting effects on NF-κB-DNA binding, whereas the inhibition of NF-κB p65 expression or activity prevented the GH-dependent activation of IGF-1 and bone morphogenetic protein-2 expression.

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.

STAT5 activity in pancreatic β-cells

Signal transducer and activator of transcription (STAT)5A and -5B are latent transcription factors activated by cytokines and hormones of the cytokine family. In pancreatic insulin-secreting β-cells, STAT5A and -5B are activated primarily by prolactin and growth hormone stimulation and are important mediators of the potent stimulation of proliferation and insulin production caused by these hormones. STAT5A and -5B are both expressed in β-cells and control the expression of a number of mRNAs implicated in cell replication control, insulin biosynthesis and secretion. In addition to STAT5A and -5B being transcriptional activators, they may also repress gene transcription. By these means, STAT5 proteins increase the levels of anti-apoptotic transcripts in β-cells and repress expression of pro-apoptotic genes. This review focuses on the anti-apoptotic role of STAT5 signaling, providing a mechanism for β-cell resistance to pro-apoptotic cytokines, Type 1 diabetes mellitus and obesity-associated β-cell stress. It is clear from studies of STAT5 signaling in pancreatic β-cells that STAT5 is important for postnatal β-cell compensatory growth (as in pregnancy or obesity) and in the defense against β-cell stress factors.

JAK2, but not Src family kinases, is required for STAT, ERK, and Akt signaling in response to growth hormone in preadipocytes and hepatoma cells

Janus kinase 2 (JAK2), a tyrosine kinase that associates with the GH receptor and is activated by GH, has been implicated as a key mediator of GH signaling. Several published reports suggest that members of the Src family of tyrosine kinases may also participate in GH signaling. We therefore investigated the extent to which JAK2 and Src family kinases mediate GH activation of signal transducers and activators of transcription (STATs) 1, 3, and 5a/b, ERKs 1 and 2, and Akt, in the highly GH-responsive cell lines 3T3-F442A preadipocytes and H4IIE hepatoma cells. GH activation of Src family kinases was not detected in either cell line. Further, blocking basal activity of Src kinases with the Src inhibitors PP1 and PP2 did not inhibit GH activation of STATs 1, 3, or 5a/b, or ERKs 1 and 2. When levels of JAK2 were depressed by short hairpin RNA in 3T3-F442A and H4IIE cells, GH-stimulated activation of STATs 1, 3, and 5a/b, ERKs 1 and 2, and Akt were significantly reduced; however, basal activity of Src family kinases was unaffected. These results were supported genetically by experiments showing that GH robustly activates JAK2, STATs 3 and 5a/b, ERKs 1 and 2, and Akt in murine embryonic fibroblasts derived from Src/Yes/ Fyn triple-knockout embryos that lack known Src kinases. These results strongly suggest that JAK2, but not Src family kinases, is critical for transducing these GH signals in 3T3-F442A and H4IIE cells.

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.

Dose-specific or dose-dependent effect of growth hormone treatment on the proliferation and differentiation of cultured neuronal cells

OBJECTIVE

GH controls the proliferation of cartilage, fibroblasts or the differentiation of adipose and muscle tissue. However, the effect of GH on neuronal cells remains unknown. The present study was conducted to determine the proliferative or differentiating effect of GH on the nervous system in vitro.

DESIGN

Neuronal hybrid cells (VSC4.1) were cultured with GH. The concentration ranged from 0.134 microg/ml up to 1.34 mg/ml. A cell confluency and MTT assay, cell cycle phase analysis with flow cytometry, extracellular receptor kinase (ERK) phosphorylation and mitogen activated protein kinase (MAPK) inhibitor (PD98050) assays were all performed to determine the effect on proliferation. Differentiation was evaluated by neurite outgrowth and neurofilament expression. Terminally differentiated neurons were stained by Hoechst 33342 for apoptotic nuclear fragmentation by degeneration. Poly-adenosyl ribose polymerase (PARP) expression and its cleavage products were evaluated.

RESULTS

Cells at concentrations between 0.134 microg/ml and 1.34 microg/ml of GH proliferated with ERK phosphorylation, which was attenuated by MAPK inhibitors. Proliferation decreased at concentrations higher than 13.4 microg/ml; however, neurite outgrowth was observed at these concentrations. Terminally differentiated cells underwent apoptosis and showed nuclear fragmentation by Hoechst 33342 staining. PARP expression was increased with caspase-3 dependent-cleaved fragment.

CONCLUSIONS

Our in vitro data demonstrate that GH exerts dual effects; proliferation with a specific GH dose window, or differentiation in a dose-dependent manner in cultured neuronal hybrid cells.

Pituitary-Specific Expression and Pit-1 Regulation of the Rat Growth Hormone-Releasing Hormone Receptor Gene

The GHRH receptor is expressed in the somatotroph cell of the anterior pituitary, where it functions to mediate GHRH-stimulated GH release. To study pituitary and somatotroph cell-specific expression of this gene, a transgenic mouse model and complementary cell culture experiments were developed. The activity of the 1.6-kb proximal rat GHRH receptor promoter was examined in vivo by generating transgenic mice with the promoter directing expression of a luciferase reporter. The promoter directs tissue-specific expression; luciferase is highly expressed in the pituitary but absent from 14 other tissues. Immunocytochemistry experiments show that transgene expression is targeted to GH-expressing somatotroph cells. The transgene is 5-fold more highly expressed in males than females, and there is an increase in transgene expression leading up to the onset of puberty. The 1.6-kb promoter was further examined in cell culture experiments, which revealed that the promoter is selectively activated in pituitary cells and that promoter-reporter expression in nonpituitary cells can be enhanced by the pituitary-specific transcription factor Pit-1. EMSAs identified 10 short regions that specifically bind Pit-1 with highly variable relative affinities. The highest affinity site was previously identified and is required for Pit-1 activation of the promoter. Four additional sites contribute to Pit-1 regulation of the promoter and are important to achieving full activation of the gene. The results show that the 1.6-kb promoter is sufficient to direct tissue- and cell-specific expression in vivo and is regulated by Pit-1.

Phosphorylation of JAK2 at serine 523: a negative regulator of JAK2 that is stimulated by growth hormone and epidermal growth factor

The tyrosine kinase JAK2 is a key signaling protein for at least 20 receptors in the cytokine/hematopoietin receptor superfamily and is a component of signaling for multiple receptor tyrosine kinases and several G-protein-coupled receptors. In this study, phosphopeptide affinity enrichment and mass spectrometry identified serine 523 (Ser523) in JAK2 as a site of phosphorylation. A phosphoserine 523 antibody revealed that Ser523 is rapidly but transiently phosphorylated in response to growth hormone (GH). MEK1 inhibitor UO126 suppresses GH-dependent phosphorylation of Ser523, suggesting that extracellular signal-regulated kinases (ERKs) 1 and/or 2 or another kinase downstream of MEK1 phosphorylate Ser523 in response to GH. Other ERK activators, phorbol 12-myristate 13-acetate and epidermal growth factor, also stimulate phosphorylation of Ser523. When Ser523 in JAK2 was mutated, JAK2 kinase activity as well as GH-dependent tyrosyl phosphorylation of JAK2 and Stat5 was enhanced, suggesting that phosphorylation of Ser523 inhibits JAK2 kinase activity. We hypothesize that phosphorylation of Ser523 in JAK2 by ERKs 1 and/or 2 or other as-yet-unidentified kinases acts in a negative feedback manner to dampen activation of JAK2 in response to GH and provides a mechanism by which prior exposure to environmental factors that regulate Ser523 phosphorylation might modulate the cell's response to GH.

Merlin inhibits growth hormone-regulated Raf–ERKs pathways by binding to Grb2 protein

Numerous studies have suggested that the NF2 protein merlin is involved in the regulation of abnormal cell growth and proliferation. In this study, to better understand the merlin's mechanisms that contribute to the inhibition of tumorigenesis, we examined the potential action of merlin on the cell proliferative signaling pathways in response to growth hormone (GH). Merlin effectively attenuated the GH-induced serum response element (SRE) and Elk-1-mediated transcriptional activation, as well as the endogenous SRE-regulated gene c-fos expression in NIH3T3 cells. In addition, merlin prevented the Raf-1 complex activation process, which resulted in the suppression of MAP kinase/ERK, extracellular signal-regulated kinase (ERKs), and Elk-1 phosphorylation, which are the downstream signals of Raf-1. Moreover, it was shown that merlin interacted with endogenous growth factor receptor bound 2 (Grb2) protein and inhibited its expression. These results suggest that merlin contributes, via its protein-to-protein interaction with Grb2 and consequent inhibition of the MAPK pathways, to the regulation of the abnormal cell proliferation, and this provides a further mechanism underlying the tumor suppressor function of merlin.

p44/42 MAP kinase-dependent regulation of catalase by autocrine human growth hormone protects human mammary carcinoma cells from oxidative stress-induced apoptosis

Previous microarray expression analyses have indicated autocrine human growth hormone (hGH) regulation of genes involved in the oxidative stress response. Expression analysis of antioxidant enzymes revealed that autocrine hGH increased both the mRNA and protein levels of catalase, superoxide dismutase 1 (SOD1), glutathione peroxidase and glutamylcysteine synthetase but not that of SOD2. As a consequence, autocrine hGH increased the antioxidant capacity of mammary carcinoma cells and protected against oxidative stress-induced apoptosis. Catalase activity was increased by autocrine production of hGH in mammary carcinoma cells and a catalase inhibitor abrogated protection from oxidative stress afforded by autocrine hGH. Autocrine hGH transcriptionally regulated catalase gene expression in a p44/42 MAP kinase-dependent manner and inhibition of MEK concordantly abrogated the protective effect of autocrine hGH against oxidative stress-induced apoptosis. Given that increased cellular oxidative stress is a key effector mechanism of specific chemotherapeutic agents, we propose that antagonism of autocrine hGH will improve the efficacy of chemotherapeutic regimes utilized for human mammary carcinoma.

Growth hormone attenuates the transcriptional activity of Runx2 by facilitating its physical association with Stat3beta

We document that GH controls osteoblast function by modulating the biological activity of the osteospecific transcription factor Runx2. Evidence is provided for a physical interaction between Runx2 and Stat3beta, which is enhanced by GH and downregulates the transcriptional properties of this key osteogenic regulator.

INTRODUCTION

Growth hormone (GH) signals to bone either through insulin-like growth factor-1 or directly by influencing the function of osteoblasts, the bone-forming cells. This study aimed at exploring the molecular events that underlie the direct biological action of GH on osteoblastic cells, and specifically, the effects that it might exert on the function of the bone-specific transcriptional regulator Runx2.

MATERIALS AND METHODS

The GH-responsive human osteoblastic cell line Saos-2 was used as our experimental system. Western blot analyses were used to monitor the presence of several parameters known to be affected by GH in these cells (i.e., downregulation of GH receptor, induction of STATs, and extracellular signal-regulated kinase [ERK] mitogen-activated protein kinase [MAPK] pathways). Electrophoretic mobility shift assays were used to assess Runx2 and Stat3 binding activity on an osteoblast-specific element (OSE2) after GH treatment. A combination of yeast two-hybrid and co-immunoprecipitation assays were performed to test for the existence of a physical Runx2.Stat3beta association. Finally, co-transfection experiments were used to investigate the interplay of the two transcription factors on the activity of a p6OSE2-Luc promoter after GH stimulation.

RESULTS

We show that GH signaling through Stat3/ERK MAPK potentiates the DNA binding activity of Runx2 but, at the same time, restrains its transcriptional potential. Moreover, a novel physical interaction of Runx2 with transcription factor Stat3beta, which is enhanced by GH stimulation, was documented both in vitro and in vivo. Importantly, this interaction impairs the transcriptional activity of Runx2 without affecting its DNA binding capacity.

CONCLUSION

Our data provide the first evidence that GH modulates the transcriptional function of Runx2 in osteoblastic cells by promoting its inhibitory interaction with Stat3beta. Shedding light on such mechanisms will contribute to a better understanding of GH effects on skeletal homeostasis that may impact on decisions at the clinical level, especially in diseases affecting bone quantity and quality (e.g., osteoporosis).

Growth hormone alters epidermal growth factor receptor binding affinity via activation of extracellular signal-regulated kinases in 3T3-F442A cells

Epidermal growth factor receptor (EGFR) is a transmembrane protein that binds EGF in its extracellular domain and initiates signaling via intrinsic tyrosine kinase activity in its cytoplasmic domain. EGFR is important in development, cellular proliferation, and cancer. GH is a critical growthpromoting and metabolic regulatory hormone that binds the GH receptor, thereby engaging various signaling pathways, including ERKs. Prior studies suggest cross-talk between the GH receptor and EGFR signaling systems. Using the GH- and EGF-responsive 3T3-F442A preadipocyte, we previously observed that GH, in addition to causing EGFR tyrosine phosphorylation, also induced EGFR phosphorylation that was detected by PTP101, an antibody reactive with ERK consensus phosphorylation sites. This latter phosphorylation was prevented by pretreatment with MAPK kinase (MEK)1 inhibitors, suggesting ERK pathway dependence. Furthermore, GH cotreatment with EGF markedly slowed EGF-induced EGFR degradation and down-regulation, thereby potentiating EGF-induced EGFR signaling. These effects were also MEK1 dependent and suggested ERK pathway-dependent influence of GH on EGF-induced EGFR postendocytic trafficking and signaling. We now explore the impact of GH on cell surface binding of EGF in 3T3-F442A cells. We found that GH pretreatment caused transient, but substantial, lessening of (125)I-EGF binding. Competitive binding experiments revealed that the decreased binding was primarily due to decreased affinity, rather than a change in the number of EGF binding sites. The effect of GH on EGF binding was concentration dependent and temporally correlated with GH-induced ERK activation and EGFR PTP101-reactive phosphorylation. Blockade of the MEK1/ERK but not the protein kinase C pathway, prevented GH's effects on EGF binding, and our results indicate that the mechanisms of GH- and phorbol-12-myristate-13-acetateinduced inhibition of EGF binding differ substantially. Overall, our findings suggest that GH can modulate both EGF binding kinetics and the EGFR's postbinding signaling itinerary in a MEK1/ERK pathway-dependent fashion.

Growth hormone (GH) receptors in prostate cancer: gene expression in human tissues and cell lines and characterization, GH signaling and androgen receptor regulation in LNCaP cells

Various hormones and growth factors have been implicated in progression of prostate cancer, but their role and the underlying molecular mechanism(s) involved remain poorly understood. In this study, we investigated the role of human growth hormone (GH) and its receptor (GHR) in human prostate cancer. We first demonstrated mRNA expression of GHR and of its exon 9-truncated isoform (GHR(tr)) in benign prostate hyperplasia (BPH) and prostate adenocarcinoma patient tissues, as well as in LNCaP, PC3 and DU145 human prostate cancer cell lines. GHR mRNA levels were 80% higher and GHR(tr) only 25% higher, in the carcinoma tissues than in BPH. Both isoforms were also expressed in LNCaP and PC3 cell lines and somewhat less so in DU145 cells. The LNCaP cell GHR protein was further characterized, on the basis of its M(r) of 120kDa, its binding to two different GHR monoclonal antibodies, its high affinity and purely somatogenic binding to (125)I-hGH and its ability to secrete GH binding protein, all characteristic of a functional GHR. Furthermore, GH induced rapid, time- and dose-dependent signaling events in LNCaP cells, including phosphorylation of JAK2 tyrosine kinase, of GHR itself and of STAT5A (JAK2-STAT5A pathway), of p42/p44 MAPK and of Akt/PKB. No effect of GH (72h) could be shown on basal or androgen-induced LNCaP cell proliferation nor on PSA secretion. Interestingly, however, GH caused a rapid (2-12h) though transient striking increase in immunoreactive androgen receptor (AR) levels (< or =5-fold), followed by a slower (24-48h) reduction (< or = 80%), with only modest parallel changes in serine-phosphorylated AR. In conclusion, the GH-induced activation of signaling pathways, its effects on AR protein in LNCaP cells and the isoform-specific regulation of GHR in prostate cancer patient tissues, suggest that GH, most likely in concert with other hormones and growth factors, may play an important role in progression of human prostate cancer.

Src-CrkII-C3G-dependent activation of Rap1 switches growth hormone-stimulated p44/42 MAP kinase and JNK/SAPK activities

We demonstrate here that growth hormone (GH) stimulates the activation of Rap1 and Rap2 in NIH-3T3 cells. Full activation of Rap1 and Rap2 by GH necessitated the combined activity of both JAK2 and c-Src kinases, although c-Src was predominantly required. GH-stimulated Rap1 and Rap2 activity was also demonstrated to be CrkII-C3G-dependent. GH stimulated the tyrosine phosphorylation of C3G, which again required the combined activity of JAK2 and c-Src. C3G tyrosine residue 504 was required for GH-stimulated Rap activation. Activated Rap1 inhibited GH-stimulated activation of RalA and subsequent GH-stimulated p44/42 MAP kinase activity and Elk-1-mediated transcription. In addition, we demonstrated that C3G-Rap1 mediated CrkII enhancement of GH-stimulated JNK/SAPK activity. We have therefore identified a linear JAK2-independent pathway switching GH-stimulated p44/42 MAP kinase and JNK/SAPK activities.

Growth hormone-induced phosphorylation of epidermal growth factor (EGF) receptor in 3T3-F442A cells. Modulation of EGF-induced trafficking and signaling

Growth hormone (GH) promotes signaling by causing activation of the non-receptor tyrosine kinase, JAK2, which associates with the GH receptor. GH causes phosphorylation of epidermal growth factor receptor (EGFR; ErbB-1) and its family member, ErbB-2. For EGFR, JAK2-mediated GH-induced tyrosine phosphorylation may allow EGFR to serve as a scaffold for GH signaling. For ErbB-2, GH induces serine/threonine phosphorylation that dampens basal and EGF-induced ErbB-2 kinase activation. We now further explore GH-induced EGFR phosphorylation in 3T3-F442A, a preadipocytic fibroblast cell line that expresses endogenous GH receptor, EGFR, and ErbB-2. Using a monoclonal antibody that recognizes ERK consensus site phosphorylation (PTP101), we found that GH caused PTP101-reactive phosphorylation of EGFR. This GH-induced EGFR phosphorylation was prevented by MEK1 inhibitors but not by a protein kinase C inhibitor. Although GH did not discernibly affect EGF-induced EGFR tyrosine phosphorylation, we observed by immunoblotting a substantial decrease of EGF-induced EGFR degradation in the presence of GH. Fluorescence microscopy studies indicated that EGF-induced intracellular redistribution of an EGFR-cyan fluorescent protein chimera was markedly reduced by GH cotreatment, in support of the immunoblotting results. Notably, protection from EGF-induced degradation and inhibition of EGF-induced intracellular redistribution afforded by GH were both prevented by a MEK1 inhibitor, suggesting a role for GH-induced ERK activation in regulating the trafficking itinerary of the EGF-stimulated EGFR. Finally, we observed augmentation of early aspects of EGF signaling (EGF-induced ERK2 activation and EGF-induced Cbl tyrosine phosphorylation) by GH cotreatment; the GH effect on EGF-induced Cbl tyrosine phosphorylation was also prevented by MEK1 inhibition. These data indicate that GH, by activating ERKs, can modulate EGF-induced EGFR trafficking and signaling and expand our understanding of mechanisms of cross-talk between the GH and EGF signaling systems.

Commitment of 3T3-F442A cells to adipocyte differentiation takes place during the first 24-36 h after adipogenic stimulation: TNF-alpha inhibits commitment

We studied the commitment of 3T3-F442A cells during stimulation with adipogenic serum or growth hormone. Confluent 3T3-F442A preadipocytes were incubated with adipogenic medium for increasing times; the number of adipose clusters, GPDH activity, and lipid accumulation were evaluated. Results show that cell commitment took place during the first 24-36 h after stimulation under adipogenic conditions. Then, cultures underwent a 2-fold increase in total cell number through selective multiplication of committed cells, followed by a dramatic decrease in colony-forming ability and 300- to 1000-fold raise in GPDH activity. Cell commitment was not modulated by insulin, but this hormone stimulated clonal expansion of committed cells and lipogenesis. Commitment was inhibited by TNF-alpha at concentrations as low as 5 ng/ml, and by retinoic acid. The results show that TNF-alpha inhibits adipose conversion at two different levels: at concentrations as low as 5 ng/ml, it blocks commitment, and at concentrations of 100 ng/ml or higher the cytokine seems to block mitotic expansion and other steps of differentiation after cell commitment. The identification of a specific time for cell commitment would allow the study of the early genes that might regulate cell reprogramming into adipocytes.

A role for Grb2-associated binder-1 in growth hormone signaling

GH signaling begins with activation of the GH receptor (GHR)-associated cytoplasmic tyrosine kinase, Janus kinase-2. GH-induced Janus kinase-2 activation leads to engagement of several signaling pathways, including the extracellular signal-regulated kinase (ERK), mitogen-activated protein kinase, phosphoinositol 3-kinase, and signal transducer and activator of transcription-5 (STAT5) pathways. Previous work suggests that ERK activation in response to GH may be modulated by several proteins acting as docking molecules, including the epidermal growth factor receptor (EGFR) and insulin receptor substrate-1. In this study we investigate potential roles for the pleckstrin homology (PH) domain-containing insulin receptor substrate-like protein, Grb-2-associated binder-1 (Gab1), in GH signaling. We find in 3T3-F442A preadipocytes that GH promotes tyrosine phosphorylation of Gab1 and its association with SHP2, an Src homology 2-containing cytoplasmic tyrosine phosphatase. The Grb2 adapter protein, in contrast, is specifically coimmunoprecipitated with Gab1, even in the absence of GH exposure. Using a COS-7 cell transient reconstitution system, we observed that GH-induced Gab1 tyrosine phosphorylation is dependent on the Gab1 PH domain, whereas GH-induced coimmunoprecipitation of SHP2 requires tyrosine 627 of Gab1, as previously reported for EGF-induced Gab1-SHP2 association. Deletion of the Gab1 PH domain significantly attenuates GH-induced ERK activation and trans-activation of a c-fos enhancer-driven reporter construct compared with wild-type Gab1 in this system. In contrast, GH-induced STAT5 tyrosine phosphorylation and STAT5-dependent trans-activation are similar in cells expressing wild-type or PH domain-deleted Gab1. Notably, neither the ERK nor the STAT5 GH-dependent signaling outcome is affected by expression of the Gab1 mutant with tyrosine 627 changed to phenylalanine. Finally, we observed GH-dependent translocation of a wild-type, but not a PH domain-deleted, Gab1-green fluorescent protein chimera from the cytoplasm to the plasma membrane. Our results suggest selective involvement of Gab1 in GH-induced ERK activation and implicate the Gab1 PH domain as critical in this involvement.

Identification of a JAK2-independent pathway regulating growth hormone (GH)-stimulated p44/42 mitogen-activated protein kinase activity. GH activation of Ral and phospholipase D is Src-dependent

We have demonstrated here that growth hormone (GH) stimulates the formation of the active GTP-bound form of both RalA and RalB in NIH-3T3 cells. Full activation of RalA and RalB by GH required the combined activity of c-Src and JAK2, both kinases activated by GH independent of the other. Activation of RalA and RalB by growth hormone did not require the activity of JAK2 per se. Ras was also activated by GH and was required for the GH-stimulated formation of GTP-bound RalA and RalB. Activation of RalA by GH subsequently resulted in increased phospholipase D activity and the formation of its metabolite, phosphatidic acid. GH-stimulated RalA-phospholipase D-dependent formation of phosphatidic acid was required for activation of p44/42 MAPK and subsequent Elk-1-mediated transcription stimulated by GH. Thus we report the identification of a JAK2-independent pathway regulating GH-stimulated p44/42 MAPK activity.

Dual regulation of phosphorylation and dephosphorylation of C/EBPbeta modulate its transcriptional activation and DNA binding in response to growth hormone

The phosphorylation state of transcription factors is a critical determinant of their function. C/EBPbeta occurs in cells as the transcriptional activator liver-enriched activating protein (LAP) and in the truncated form liver-enriched inhibitory protein (LIP) that inhibits transcription. Analysis of C/EBPbeta phosphorylation by isoelectric focusing (IEF) shows that LAP is present in multiple forms, each with a different degree of phosphorylation in 3T3-F442A fibroblasts. Growth hormone (GH) treatment induces a new band near the negative pole, consistent with GH-promoted dephosphorylation of LAP. In addition, bands near the positive pole are rapidly and transiently induced, suggesting that GH also stimulates phosphorylation at some site(s) on LAP. C/EBPbeta contains a highly conserved MAPK consensus site that corresponds to Thr(188) in murine (m) LAP and Thr(37) in mLIP. Immunoblotting with antiphosphopeptide antibodies specific for Thr(188/37) of C/EBPbeta (anti-P-C/EBPbeta) shows that GH rapidly and transiently promotes phosphorylation of mLAP and mLIP on the MAPK site. MEK inhibitors prevent this GH-promoted phosphorylation of LAP and LIP, suggesting that such phosphorylation depends on GH-activated MAPK signaling. Mutation of Thr(235) to Ala in the homologous MAPK site of human (h) LAP (hLAPT235A) inhibits transcription mediated by the c-fos promoter in response to GH, indicating that phosphorylation at the MAPK site is required for LAP to be transcriptionally active in the context of GH-stimulated activation of the c-fos promoter. Complexes bound to the c-fos C/EBP site transiently contain C/EBPbeta phosphorylated at the MAPK site. As phosphorylation subsides, the binding of less transcriptionally active forms of LAP increases, consistent with the transient nature of c-fos stimulation by GH and other growth factors. Thus, both phosphorylation and dephosphorylation of C/EBPbeta, in response to a single physiological stimulus such as GH, coordinately modulate the ability of C/EBPbeta to activate transcription by modulating its DNA binding activity and its transactivation capacity.

Intracellular signaling by growth hormone variant (GH-V)

Placental growth hormone, or growth hormone variant (GH-V), is a member of the growth hormone gene family. Its physiologic role during pregnancy remains undefined. Although extensive work has been done characterizing the signaling pathways activated by hGH, the signaling pathways which are activated by GH-V have not been characterized. We investigated cellular signaling by GH-V in model systems in which signaling by hGH has been well characterized. GH-V activates the STAT5b transcription factor in the IM-9 human lymphocyte and 3T3-F442A preadipocyte cell lines, and in primary porcine smooth muscle cells, which all homologously express GH receptors. In contrast, hCS does not activate signaling in these GH receptor-expressing cells. Like hGH, signaling by GH-V is inhibited by the GH antagonist (G120K). These results provide preliminary evidence that GH-V plays a major role in affecting target cells expressing the GH receptor, thus potentially exerting significant GH-like effects on maternal physiology during pregnancy.

c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription

We have previously demonstrated that cellular stimulation with GH results in the formation of a multiprotein signaling complex. One component of this multiprotein signaling complex is the adapter molecule c-Cbl. Here we have examined the role of c-Cbl in the mechanism of GH signal transduction. Forced expression of c-Cbl in NIH3T3 cells did not alter GH-stimulated Janus kinase 2 tyrosine phosphorylation nor GH-stimulated p44/42 MAPK activation and consequent Elk-1- mediated transcription. c-Cbl overexpression did, however, result in enhanced and prolonged GH-stimulated activation of phosphatidylinositol 3-kinase. Forced expression of c-Cbl did not affect GH-stimulated STAT5 tyrosine phosphorylation, nuclear translocation, nor binding to DNA but markedly abrogated GH-stimulated STAT5-mediated transactivation. c-Cbl overexpression resulted in increased ubiquitination and proteosomal degradation of STAT5 and increased degradation of GH-stimulated tyrosine phosphorylated STAT5. Cellular pretreatment with the proteosomal inhibitor MG132 reversed the effect of c-Cbl overexpression with prolonged duration of GH-stimulated STAT5 tyrosine phosphorylation and restoration of STAT5-mediated transcription. Thus, c-Cbl is a negative regulator of GH-stimulated STAT5-mediated transcription by direction of STAT5 for proteosomal degradation.

Growth hormone signal transduction

Multiple signaling pathways mediate the diverse effects of growth hormone (GH) on growth and metabolism. The interaction of GH with GH receptors (GHR) on target cells promotes the association of the cellular tyrosine kinase JAK2 with the GHR, initiating tyrosine phosphorylation of GHR and JAK2, and activation of multiple signaling cascades. GH-stimulated activation of signal transducers and activators of transcription (STATs), mitogen activated protein kinase (MAPK) and phosphatidylinositol 3' kinase (PI3K) cascades have been shown to regulate the transcription of GH-responsive genes. Cross-talk among these signaling cascades in regulating specific genes suggests that GH signaling to the nucleus involves a GH-regulated signaling network.

Differential phosphorylation of Janus kinase 2, Stat5A and Stat5B in response to growth hormone in primary rat adipocytes

In vitro growth hormone (GH) stimulation of Janus kinase 2 (Jak2) tyrosine phosphorylation and activation has been detected in rat adipocytes where GH exerts both chronic diabetogenic and acute insulin-like effects but not in adipocytes where only chronic diabetogenic effects are exerted. The 95 kDa transcription factor Stat5, which is tyrosine phosphorylated in response to GH in both cases, is here identified as the 5A-isoform. Stat5B was not tyrosine phosphorylated in response to GH in adipocytes but subject to a gel supershift indicating regulation by serine and/or threonine phosphorylation. The differential tyrosine phosphorylation of these proteins suggests involvement of a kinase other than Jak2 in Stat5A activation. However, in adipocytes where GH exerts both diabetogenic and insulin-like effects, and both Jak2 and Stat5A were activated, their phosphorylation kinetics and downregulation of tyrosine phosphorylation were almost identical. We conclude that Stat5A is important for the diabetogenic actions of GH and that Jak2 still is the most probable candidate kinase for Stat5A in primary adipocytes.

Signal transduction via the growth hormone receptor

Rapid progress has been made recently in the definition of growth hormone (GH) receptor signal transduction pathways. It is now apparent that many cytokines, including GH, share identical or similar signalling components to exert their cellular effects. This review provides a brief discourse on the signal transduction pathways, which have been demonstrated to be utilized by GH. The identification of such pathways provides a basis for understanding the pleiotropic actions of GH. The mechanisms by which the specific cellular effects of GH are achieved remain to be elucidated.

Growth hormone regulates phosphorylation and function of CCAAT/enhancer-binding protein beta by modulating Akt and glycogen synthase kinase-3

Growth hormone (GH) regulates transcription factors associated with c-fos, including C/EBPbeta. Two forms of C/EBPbeta, liver-activating protein (LAP) and liver inhibitory protein (LIP), are dephosphorylated in GH-treated 3T3-F442A fibroblasts. GH-induced dephosphorylation of LAP and LIP is reduced when cells are preincubated with phosphatidylinositol 3'-kinase (PI3K) inhibitors. GH activates Akt and inhibits glycogen synthase kinase-3 (GSK-3). Lithium, a GSK-3 inhibitor, increases GH-dependent dephosphorylation of LAP and LIP. Both are in vitro substrates of GSK-3, suggesting that GSK-3 inactivation contributes to GH-promoted dephosphorylation of C/EBPbeta. Alkaline phosphatase increases binding of LAP homodimers and decreases binding of LIP homodimers to c-fos, suggesting that dephosphorylation of C/EBPbeta modifies their ability to bind DNA. Both alkaline phosphatase- and GH-mediated dephosphorylation comparably increase binding of endogenous LAP in 3T3-F442A cells. In cells overexpressing LAP and GSK-3, LAP binding decreases, suggesting that GSK-3-mediated phosphorylation interferes with LAP binding. Expression of constitutively active GSK-3 reduced GH-stimulated c-fos promoter activity. These studies indicate that PI3K/Akt/GSK-3 mediates signaling between GH receptor and the nucleus, promoting dephosphorylation of C/EBPbeta. Dephosphorylation increases binding of LAP complexes to the c-fos promoter and may contribute to the participation of C/EBPbeta in GH-stimulated c-fos expression.

The somatomedin hypothesis: 2001

Since the original somatomedin hypothesis was conceived, a number of important discoveries have allowed investigators to modify the concept. Originally somatic growth was thought to be controlled by pituitary GH and mediated by circulating insulin-like growth factor-I (IGF-I, somatomedin C) expressed exclusively by the liver. With the discovery that IGF-I is produced by most, if not all, tissues, the role of autocrine/paracrine IGF-I vs. the circulating form has been hotly debated. Recent experiments using transgenic and gene-deletion technologies have attempted to answer these questions. In the liverspecific igf-1 gene-deleted mouse model, postnatal growth and development are normal despite the marked reduction in circulating IGF-I and IGF-binding protein levels; free IGF-I levels are normal. Thus, the normal postnatal growth and development in these animals may be due to normal free IGF-I levels (from as yet unidentified sources), although the role of autocrine/paracrine IGF-I has yet to be determined.

Insulin receptor substrate-1-mediated enhancement of growth hormone-induced mitogen-activated protein kinase activation

Interaction of GH with the cell-surface GH receptor (GHR) causes activation of the GHR-associated tyrosine kinase, JAK2, and consequent triggering of signaling cascades including the STAT, Ras/Raf/MEK1/MAP kinase, and insulin receptor substrate-1(IRS-1)/PI3kinase pathways. We previously showed that IRS- and GHR-deficient 32D cells that stably express the rabbit GHR and rat IRS-1 (32D-rbGHR-IRS-1) exhibited markedly enhanced GH-induced proliferation and MAP kinase (ERK1 and ERK2) activation compared with cells expressing only the GHR (32D-rbGHR). We now examine biochemical mechanism(s) by which IRS-1 augments GH-induced MAP kinase activation. Time-course experiments revealed a similarly transient (maximal at 15 min) GH-induced ERK1 and ERK2 activation in both 32D-rbGHR and 32D-rbGHR-IRS-1 cells, but, consistent with our prior findings, substantially greater activation was seen in the IRS-1-containing cells. In both cells, GH-induced MAP kinase activation was markedly blunted by the MEK1 inhibitor, PD98059, but not by the PKC inhibitor, GF109203X. Interestingly, pretreatment with the PI3K inhibitor, wortmannin (EC50 approximately 10 nM), significantly reduced GH-induced MAP kinase activation in both 32D-rbGHR and 32D-rbGHR-IRS-1 cells. This same pattern in both cells of IRS-1-dependent augmentation and IRS-1-independent wortmannin sensitivity was also observed for GH-induced activation of Akt and MEK1 (using state-specific antibody blotting for both), despite the lack of difference in GHR, JAK2, SHP-2, p85, Akt, Ras, Raf-1, MEK1, ERK1, or ERK2 abundance between the two cells. A different PI3K inhibitor, LY294002 (50 microM), substantially inhibited (roughly 72%) GH-induced MAP kinase activation in 32D-rbGHR-IRS-1 cells, but only marginally (and statistically insignificantly) inhibited GH-induced MAP kinase activation in 32D-rbGHR cells. Because GH-induced Akt activation was completely inhibited in both cells by the same concentration of LY294002, these findings indicate that the wortmannin sensitivity of both the IRS-1-independent and -dependent GH-induced MAP kinase activation may reflect the activity of another wortmannin-sensitive target(s) in addition to PI3K in mediation of GH-induced MAP kinase activation in these cells. Notably, GH-induced STAT5 tyrosine phosphorylation, unlike Akt or MAPK activation, did not differ between the cells. Finally, while GH promoted accumulation of activated Ras in both cells, both basal and GH-induced activated Ras levels were greater in cells expressing IRS-1 than in 32D-rbGHR cells. These data indicate that while GH induces tyrosine phosphorylation of STAT5 and activation of the Ras/Raf/MEK1/MAPK and PI3K pathways, IRS-1 expression augments the latter two more than the former.

Prolactin stimulation of tyrosyl phosphorylation of Shc proteins in Nb(2) lymphoma cells, but not mammary tissues

Prolactin (PRL) stimulates lactogenesis in mammary cells and mitogenesis in a variety of cell types including Nb(2) cells. Studies indicate that a different composite of signaling pathways is involved in the PRL stimulation of mitogenesis as compared to lactogenesis. In the present studies, PRL is shown to stimulate the tyrosyl phosphorylation of all three isoforms of Shc proteins in Nb(2) cells (mitogenesis), but not in the mammary gland. Maximal phosphorylation of the Shc proteins is expressed between 10 and 15 min after a 50-ng/ml PRL treatment. In addition, there is an increased association between the Grb2 protein and Shc proteins upon PRL stimulation. However, no increased association between JAK2 and Shc proteins was observed in either the Nb(2) cells or mammary tissues.

Regulators of growth hormone signaling

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

Janus kinase 2-dependent activation of p38 mitogen-activated protein kinase by growth hormone. Resultant transcriptional activation of ATF-2 and CHOP, cytoskeletal re-organization and mitogenesis

We demonstrate here that p38 mitogen-activated protein (MAP) kinase is activated in response to cellular stimulation by human GH (hGH) in Chinese hamster ovary cells stably transfected with GH receptor cDNA. This activation requires the proline-rich box 1 region of the GH receptor required for JAK2 association and is prevented by pretreatment of cells with the JAK2-specific inhibitor AG490. ATF-2 is both phosphorylated and transcriptionally activated by hGH, and its transcriptional activation also requires the proline-rich box 1 region of the GH receptor. Expression of wild type JAK2 can further enhance hGH-induced ATF-2-, CHOP-, and Elk-1-mediated transcriptional activation, whereas pretreatment with AG490 is inhibitory. Use of either specific pharmacological inhibitors or transient transfection of cells with p38alpha MAP kinase cDNA or a dominant negative variant demonstrated that hGH-stimulated transcriptional activation of ATF-2 and CHOP, but not Elk-1, is regulated by p38 MAP kinase. Both the p38 MAP kinase and p44/42 MAP kinase are critical for hGH-stimulated mitogenesis, whereas only p38 MAP kinase is required for hGH-induced actin cytoskeletal re-organization. p38 MAP kinase is therefore an important regulator in coordinating the pleiotropic effects of GH.

Growth hormone-induced alteration in ErbB-2 phosphorylation status in 3T3-F442A fibroblasts

The growth hormone receptor (GHR), a cytokine receptor superfamily member, requires the JAK2 tyrosine kinase for signaling. We now examine functional interactions between growth hormone (GH) and epidermal growth factor (EGF) in 3T3-F442A fibroblasts. Although EGF enhanced ErbB-2 tyrosine phosphorylation, GH, while causing retardation of its migration on SDS-polyacrylamide gel electrophoresis, decreased ErbB-2's tyrosine phosphorylation. GH-induced retardation was reversed by treatment of anti-ErbB-2 precipitates with both alkaline phosphatase and protein phosphatase 2A, suggesting that GH induced serine/threonine phosphorylation of ErbB-2. Both GH-induced shift in ErbB-2 migration and GH-induced MAP kinase activation were unaffected by a protein kinase C inhibitor but were blocked by the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase 1 (MEK1) inhibitor, PD98059. Notably, leukemia inhibitory factor, but not interferon-gamma, also promoted ErbB-2 shift and mitogen-activated protein kinase activation. Cotreatment with EGF and GH versus EGF alone resulted in a 35% decline in acute ErbB-2 tyrosine 1248 autophosphorylation, a marked decline (approximately 50%) in DNA synthesis, and substantially decreased cyclin D1 expression. We conclude that in 3T3-F442A cells, 1) the GH-induced decrease in ErbB-2 tyrosine phosphorylation correlates with MEK1/mitogen-activated protein kinase activity and 2) GH antagonizes EGF-induced DNA synthesis and cyclin D1 expression in a pattern consistent with its alteration in ErbB-2 phosphorylation status.

Transgenic models of growth hormone action

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

Cytoplasmic and nuclear cytokine receptor complexes

Much of our understanding on how hormones and cytokines transmit their message into the cell is based on the receptor activation at the plasma membrane. Many experimental in vitro models have established the paradigm for cytokine action based upon such activation of their cell surface receptor. The signaling from the plasma membrane activated cytokine receptor is driven to the nucleus by a rapid ricochet of protein phosphorylation, ultimately integrated as a differentiative, proliferative, or transcriptional message. The Janus kinase (JAK)--signal transducers and activators of transcription (STAT) pathway that was first thought to be cytokine receptor specific now appears to be activated by other noncytokine receptors. Also, evidence is accumulating showing that cytokines modulate the signal transduction machinery of the tyrosine kinase receptors and that of the heterotrimeric guanosine triphosphate (GTP)-binding protein-coupled receptors. Thus cytokine receptor signaling has become much more complex than originally hypothesized, challenging the established model of specificity of the action of a given cytokine. This review is focused on another level of complexity emerging within cytokine receptor superfamily signaling. Over the past 10 years, data from different laboratories have shown that cytokines and their receptors localize to intracellular compartments including the nucleus, and, in some cases, biological responses have been correlated with this unexpected location, raising the possibility that cytokines act as their own messenger through inter-actions with nuclear proteins. Thus, the interplay between cytokine receptor engagement and cellular signaling turns out to be more dynamic than originally suspected. The mechanisms and regulations of intracellular translocation of the cytokines, their receptors, and their signaling proteins are discussed in the context that such compartmentalization provides some of the specificity of the responses mediated by each cytokine.

Insulin receptor substrate-1 enhances growth hormone-induced proliferation

GH exerts a variety of metabolic and growth-promoting effects. GH induces activation of the GH receptor (GHR)-associated cytoplasmic tyrosine kinase, JAK2, resulting in tyrosine phosphorylation of the GHR and activation of STAT (signal transducer and activator of transcription), Ras-mitogen-activated protein kinase, and phosphoinositol 3-kinase signaling pathways, among others. GH-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS) proteins has been demonstrated in vitro and in vivo. IRS-1 is a multiply phosphorylated cytoplasmic docking protein involved in metabolic and proliferative signaling by insulin, IL-4, and other cytokines, but the physiological role of IRS-1 in GH signaling is unknown. In this study, as noted by others, we detected in murine 3T3-F442A pre-adipocytes GH-dependent tyrosine phosphorylation of IRS-1 and specific GH-induced coimmunoprecipitation with JAK2 of a tyrosine phosphoprotein consistent with IRS-1. We further examined this interaction by in vitro affinity precipitation experiments with glutathione-S-transferase fusion proteins incorporating regions of rat IRS-1 and, as a source of JAK2, extracts of 3T3-F442A cells. Fusion proteins containing amino-terminal regions of IRS-1 that include the pleckstrin homology, phosphotyrosine-binding, and Shc and IRS-1 NPXY-binding domains, but not those containing other IRS-1 regions or glutathione-S-transferase alone, bound JAK2 from cell extracts. Tyrosine-phosphorylated JAK2 resulting from GH stimulation was included in the amino-terminal IRS-1 fusion precipitates; however, neither tyrosine phosphorylation of JAK2 nor treatment of cells with GH before extraction was necessary for the specific JAK2-IRS-1 interaction to be detected. In contrast, in this assay, specific insulin receptor association with the IRS-1 phosphotyrosine-binding, and Shc and IRS-1 NPXY-binding domains was insulin and phosphotyrosine dependent, as previously shown. To test for significance of IRS-1 with regard to GH signaling, IRS- and GHR-deficient 32D cells were stably reconstituted with the rabbit (r) GHR, either alone (32D-rGHR) or with IRS-1 (32D-rGHR-IRS-1). As assayed by three independent methods, GH induced proliferation in 32D-rGHR cells, even in the absence of transfected IRS-1. Notably, however, GH-induced proliferation was markedly enhanced in cells expressing IRS-1. Similarly, GH-induced mitogen-activated protein kinase activation was significantly augmented in IRS-1-expressing cells relative to that in cells harboring no IRS-1. These results indicate that IRS-1 enhances GH-induced proliferative signaling.

Growth hormone-dependent differentiation of 3T3-F442A preadipocytes requires Janus kinase/signal transducer and activator of transcription but not mitogen-activated protein kinase or p70 S6 kinase signaling

The signals mediating growth hormone (GH)-dependent differentiation of 3T3-F442A preadipocytes under serum-free conditions have been studied. GH priming of cells was required before the induction of terminal differentiation by a combination of epidermal growth factor, tri-iodothyronine, and insulin. Cellular depletion of Janus kinase-2 (JAK-2) using antisense oligodeoxynucleotides (ODNs) prevented GH-stimulated JAK-2 and signal transducer and activator of transcription (STAT)-5 tyrosine phosphorylation and severely attenuated the ability of GH to promote differentiation. Although p42(MAPK)/p44(MAPK) mitogen-activated protein kinases were activated during GH priming, treatment of cells with PD 098059, which prevented activation of these kinases, did not block GH priming. However, antisense ODN-mediated depletion of mitogen-activated protein kinases from the cells showed that their expression was necessary for terminal differentiation. Similarly, although p70(s6k) was activated during GH priming, pretreatment of cells with rapamycin, which prevented the activation of p70(s6k), had no effect on GH priming. However, rapamycin did partially block epidermal growth factor, tri-iodothyronine, and insulin-stimulated terminal differentiation. By contrast, cellular depletion of STAT-5 with antisense ODNs completely abolished the ability of GH to promote differentiation. These results indicate that JAK-2, acting specifically via STAT-5, is necessary for GH-dependent differentiation of 3T3-F442A preadipocytes. Activation of p42(MAPK)/p44(MAPK) and p70(s6k) is not essential for the promotion of differentiation by GH, although these signals are required for GH-independent terminal differentiation.

Cross-talk between janus kinase-signal transducer and activator of transcription (JAK-STAT) and peroxisome proliferator-activated receptor-alpha (PPARalpha) signaling pathways. Growth hormone inhibition of pparalpha transcriptional activity mediated by stat5b

Hepatic peroxisome proliferation induced by structurally diverse non-genotoxic carcinogens is mediated by the nuclear receptor peroxisome proliferator-activated receptor (PPARalpha) and can be inhibited by growth hormone (GH). GH-stimulated Janus kinase-signal transducer and activator of transcription 5b (JAK2/STAT5b) signaling and the PPAR activation pathway were reconstituted in COS-1 cells to investigate the mechanism for this GH inhibitory effect. Activation of STAT5b signaling by either GH or prolactin inhibited, by up to 80-85%, ligand-induced, PPARalpha-dependent reporter gene transcription. GH failed to inhibit 15-deoxy-Delta12, 14-prostaglandin-J2-stimulated gene transcription mediated by an endogenous COS-1 PPAR-related receptor. GH inhibition of PPARalpha activity required GH receptor and STAT5b and was not observed using GH-activated STAT1 in place of STAT5b. GH inhibition was not blocked by the mitogen-activated protein kinase pathway inhibitor PD98059. STAT5b-PPARalpha protein-protein interactions could not be detected by anti-STAT5b supershift analysis of PPARalpha-DNA complexes. The GH inhibitory effect required the tyrosine phosphorylation site (Tyr-699) of STAT5b, an intact STAT5b DNA binding domain, and the presence of a COOH-terminal trans-activation domain. Moreover, GH inhibition was reversed by a COOH-terminal-truncated, dominant-negative STAT5b mutant. STAT5b must thus be nuclear and transcriptionally active to mediate GH inhibition of PPARalpha activity, suggesting an indirect inhibition mechanism, such as competition for an essential PPARalpha coactivator or STAT5b-dependent synthesis of a more proximal PPARalpha inhibitor. The cross-talk between STAT5b and PPARalpha signaling pathways established by these findings provides new insight into the mechanisms of hormonal and cytokine regulation of hepatic peroxisome proliferation.

Insulin produces a growth hormone-like increase in intracellular free calcium concentration in okadaic acid-treated adipocytes

In vivo, GH and insulin usually produce opposing effects on carbohydrate and lipid metabolism in adipocytes, even though their signal transduction pathways overlap. However, when added to rat adipocytes that have been made GH deficient, GH briefly produces responses that are qualitatively like those of insulin. Subsequently, GH induces refractoriness to this acute insulin-like response, in a sense restricting its effects to a unique subset of possible physiological actions. Okadaic acid is an inhibitor of type I and IIa phosphoprotein phosphatases and affects glucose metabolism in fat cells in a manner that is reminiscent of GH. Okadaic acid initially mimics the actions of insulin, and subsequently, even after it has been removed by thorough washing, blunts the ability of adipocytes to accelerate glucose metabolism in response to insulin or GH. Because refractoriness to the insulin-like effect of GH is associated with GH-induced increases in intracellular free calcium concentrations ([Ca2+]i), we examined the effects of insulin on [Ca2+]i in okadaic acid-treated adipocytes. Adipocytes were incubated with 0.25 microM okadaic acid for 1 h, washed, and reincubated without okadaic acid for 2 h before measurement of [Ca2+]i using fura-2 as a calcium indicator. Neither GH (500 ng/ml) nor insulin (100 microU/ml) affected [Ca2+]i in cells in which glucose metabolism was stimulated, but both hormones rapidly increased [Ca2+]i in adipocytes that were refractory to insulin-like stimulation. The characteristics of the increase in [Ca2+]i produced by insulin were identical to those previously reported for GH. The effect of insulin was mimicked by the dihydropyridine calcium channel activator BayK 5552 or depolarization of the cell membrane with 30 mM KCl and was blocked by the dihydropyridine calcium channel blocker, nimodipine (100 nM), implicating activation of voltage-sensitive L-type Ca2+ channels. The increase in [Ca2+]i was also mimicked by sn-1,2-dioctanoylglycerol and blocked by inhibitors of protein kinase C (staurosporine, chelerythrine chloride, and calphostin), and D609, an inhibitor of phospholipase C, as reported for GH. Acquisition of the ability to increase [Ca2+]i in response to insulin required a lag period of at least 2 h after removal of okadaic acid and was prevented by inhibitors of RNA and protein synthesis. Adipocytes that were incubated with inhibitors of protein kinase A (KT-5720), or protein kinase C (staurosporine) along with okadaic acid also failed to increase [Ca2+]i in response to insulin. Conversely, adipocytes that were incubated with dibutyryl cAMP, methylisobutyl xanthine, or phorbol ester instead of okadaic acid increased [Ca2+]i when treated with insulin 2 h later. These results suggest that phosphorylated substrates of protein kinases A and C may mediate the transcriptional event(s) that enable adipocytes to activate L-type Ca2+ channels in response to insulin. Blockade of protein kinases A or C or removal of calcium from the incubation medium did not restore the ability of okadaic acid-treated adipocytes to increase glucose metabolism in response to insulin, nor did pretreatment of adipocytes with dibutyryl cAMP or phorbol ester decrease insulin-induced stimulation of glucose metabolism. The failure of insulin to increase glucose metabolism in okadaic acid-treated adipocytes thus cannot be ascribed to the increase in [Ca2+]i. These findings indicate that just as GH can produce an insulin-like response, so too can insulin produce a GH-like response, and highlight the need to understand how specificity of hormone action is achieved in cells that respond to different hormones that share elements of their transduction pathways.

Growth hormone stimulates phosphorylation and activation of elk-1 and expression of c-fos, egr-1, and junB through activation of extracellular signal-regulated kinases 1 and 2

Growth hormone (GH), a major regulator of normal body growth and metabolism, regulates cellular gene expression. The transcription factors Elk-1 and Serum Response Factor are necessary for GH-stimulated transcription of c-fos through the Serum Response Element (SRE). GH stimulates the serine phosphorylation of Elk-1, thereby enabling Elk-1 to mediate transcriptional activation. The contribution of the Ras/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway to Elk-1-mediated transcriptional activation of the c-fos SRE in response to GH was examined. The MEK inhibitor PD098059 attenuated GH-induced expression of the endogenous SRE-regulated genes c-fos, egr-1, and junB as well as transcriptional activation mediated by the c-fos promoter. The MEK inhibitor blocked GH-stimulated activation of MEK, phosphorylation of ERK1/ERK2, and MAP kinase activity in 3T3-F442A cells. Blocking MEK activation prevented GH-induced phosphorylation of Elk-1, as well as the ability of Elk-1 to mediate transcriptional activation in response to GH. Overexpression of dominant-negative Ras or the ERK-specific phosphatase, mitogen-activated protein kinase phosphatase-1, blocked the Ras/MEK/ERK pathway and abrogated GH-induced phosphorylation of Elk-1. GH failed to stimulate phosphorylation or activation of Jun N-terminal kinase under the conditions used. GH slightly increased p38-mediated mitogen-activated protein kinase-activated protein (MAPKAP) kinase-2 activity, but the p38 inhibitor SB203580 did not attenuate GH-promoted Elk-1 phosphorylation. Wortmannin, which inhibited GH-induced ERK phosphorylation, also attenuated transcriptional activation of c-fos by GH. Taken together, these data suggest that GH-dependent activation of the Ras/MEK/ERK pathway and subsequent serine phosphorylation of Elk-1 contribute to GH-stimulated c-fos expression through the SRE.

Molecular mechanisms involved in GLUT4 translocation in muscle during insulin and contraction stimulation

Studies in mammalian cells have established the existence of numerous intracellular signaling cascades that are critical intermediates in the regulation of various biological functions. Over the past few years considerable research has shown that many of these signaling proteins are expressed in skeletal muscle. However, the detailed mechanisms involved in the regulation of glucose transporter (GLUT4) translocation from intracellular compartments to the cell surface membrane in response to insulin and contractions in skeletal muscle are not well understood. In the present essay we report three different approaches to unravel the GLUT4 translocation mechanism: 1. specific pertubation of the insulin and/or contraction signaling pathways; 2. characterization of the protein composition of GLUT4-containing vesicles with the expectation that knowledge of the constituent proteins of the vesicles may help in understanding their trafficking; 3. degree of co-immunolocalization of the GLUT4 glucose transporters with other membrane marker proteins assessed by immunofluorescense and electron microscopy.