Growth hormone rapidly activates rat serine protease inhibitor 2.1 gene transcription and induces a DNA-binding activity distinct from those of Stat1, -3, and -4

Study of goldfish (Carassius auratus) growth hormone structure–function relationship by domain swapping

Using goldfish as a model, the structure-function relationship of goldfish growth hormone was studied using the strategy of homologous domain swapping. Chimeric mutants were constructed by exchanging homologous regions between goldfish growth hormone (gfGH II) and goldfish prolactin (gfPRL) with their cloned complementary DNAs. Six mutants, with their domain-swapped, were generated to have different combinations of three target regions, including the helix a, helix d and the large section in between these helices (possess the helices b, c and other random coiled regions). After expression in E. coli and refolding, these mutants were characterized by using competitive receptor binding assay (RRA) and growth hormone responding promoter activation assay. The different activity profiles of mutants in Spi 2.1 gene promoter assays from that in RRA shows that, for gfGH, receptor binding dose not confer receptor signal activations. When either helices a or d of gfGH was maintained with other helices replaced by their gfPRL counterparts, both receptor binding and hence gene activation activities are reduced. In mutants with helices b and c in gfGH maintained, containing the gfGH middle section, and helices a and d swapped with gfPRL, the had reduced RRA activities but the promoter activation activities retained. In conclusion, as in the case of human GH, the gfGH molecule possesses two functional sites: one of them is composed of discontinuous epitopes located on the target regions of this study and is for receptor binding; another site is located on the middle section of the molecule that helices a and d are not involved, and it is for activation of GH receptor and intracellular signals.

In vivo regulation of growth hormone-stimulated gene transcription by STAT5b

The long-term effects of growth hormone (GH) are mediated through coordinated changes in gene expression that are the outcome of interactions between hormone-activated signal transduction pathways and specific feedback loops. Recent studies in mice have implicated the transcription factor STAT5b as part of the GH-regulated somatic growth pathway, because mice lacking this protein showed diminished growth rates. To assess the role of Stat5b in GH-stimulated gene expression, we have delivered modified versions of the protein to the liver of pituitary-deficient male rats by quantitative adenovirus-mediated gene transfer. In pilot studies in cell culture, both constitutive-active and dominant-negative STAT5b showed appropriate binding properties toward a specific DNA response element. After in vivo expression, neither protein prevented nuclear accumulation of STATs 1 and 3 in the liver. Dominant-negative STAT5b completely inhibited GH-stimulated transcription of genes encoding the growth-promoting proteins IGF-I, IGF-binding protein-3 (IGFBP-3), and acid-labile subunit (ALS), which comprise the major circulating IGF-I complex, and blocked expression of the GH inhibitors SOCS-1, SOCS-2, and CIS, but had little effect on induction of SOCS-3. Constitutive-active STAT5b stimulated robust transcription of IGF-I, ALS, and IGFBP-3 in the absence of hormone but did little to modify GH-mediated activation of SOCS family genes. An adenovirus encoding EGFP was without effect. These results, in addition to establishing STAT5b as one of the key agents of GH-stimulated gene transcription, demonstrate the feasibility of using in vivo gene transfer to target and dissect the functions of distinct components of complex hormone-activated signal transduction pathways.

Differences in the Hepatic Signal Transcription Pathway and Cytokine Expression Between Thermal Injury and Sepsis

Inflammation and catabolism in response to trauma, surgery, critical illness or bacteria lead to a compromise of essential organs, which can lead to prolonged clinical stay and even death. Mediators responsible for catabolism were thought to be proinflammatory cytokines, but recently the focus has shifted to signal transduction. The purpose of the present study was to determine differences between two pathophysiologic states, sepsis and thermal injury, in signal transduction and cytokine expression and thus define the importance of the signal transcription pathway. Rats were randomly divided to either receive lipopolysaccharide (3 mg/kg body weight or a 30% total body surface area burn) or they received no treatment and served as controls. Animals were sacrificed 1, 2, 5, and 7 days postinsult and serum and liver harvested for analysis. A thermal injury appeared to have a slow release and expression of signal transcription factors and cytokines and a sepsis showed a rapid increase of mediators and also a fast decrease. The changes in cytokine profiles after burn, particularly interleukin-1beta and macrophage inhibitory factor, appear to be mediated by C/EBP-beta and STAT-3, whereas after the induction of a sepsis, tumor necrosis factor and interleukin-6 are mainly mediated by STAT-5. Based on our findings we suggest that the pathophysiologic state of a thermal injury is not comparable with sepsis in association with signal transcription factors and the differences in intracellular and extracellular signaling therefore opens new ideas for therapeutic options.

Acute control of insulin-like growth factor-I gene transcription by growth hormone through Stat5b

Many of the effects of growth hormone (GH) are mediated by insulin-like growth factor-I (IGF-I), a secreted peptide whose gene transcription is induced by GH by unknown mechanisms. Recent studies in mice have implicated Stat5b as part of a GH-regulated somatic growth pathway, because mice lacking this transcription factor show diminished growth rates and a decline in serum IGF-I levels. To test the role of Stat5b in GH-stimulated IGF-I gene expression, we have delivered modified versions of the protein to pituitary-deficient male rats by quantitative adenovirus-mediated gene transfer. In pilot studies in cell culture, both constitutively active and dominant-negative Stat5b appropriately regulated transcription from a GH-responsive Stat5-dependent reporter gene. After in vivo expression, neither protein impaired GH-induced activation of cytoplasmic signaling pathways or blocked nuclear accumulation of Stats 1 and 3 in the liver, the major site of IGF-I production. Dominant-negative Stat5b completely prevented GH-stimulated IGF-I gene transcription, whereas constitutively active Stat5b led to robust IGF-I gene expression in the absence of hormone. An adenovirus encoding enhanced green fluorescent protein was without effect. Similar results were seen with the GH-responsive Stat5b-dependent Spi 2.1 gene, whereas GH-stimulated c-fos transcription was minimally altered. These results establish Stat5b as a key component of GH-stimulated IGF-I gene transcription, and they demonstrate the feasibility of using in vivo gene transfer to target distinct components of hormone-activated signaling pathways.

Recombinant human growth hormone modulates the hepatic acute-phase response and P-selectin in burned rats

The purpose of this study was to determine the effect of recombinant human growth hormone (rhGH) on serum constitutive proteins, cytokines, P-selectin, and insulin-like growth factor (IGF-1) in the thermally injured rats.Sprague-Dawley rats (64 males) were given 30% total body surface area full thickness scald burn. They were randomly divided to receive either 2.5mg/kg per day im rhGH or saline (control). Rats were sacrified on postburn days 1, 2, 5, and 7, and serum constitutive proteins, cytokines, P-selectin, and IGF-1 levels were measured.Serum IGF-1 levels were increased on days 2, 5, or 7 after burn in rhGH-treated rats compared with controls (P<0.001, <0.01 and <0.001, respectively). Serum transferrin and albumin levels were increased on days 7 after burn in rhGH-treated rats compared with controls (P<0.05). The cytokines increased after thermal injury. The rhGH decreased serum levels of tumor necrosis factor-alpha on postburn days 1 compared with controls (P<0.001). Serum levels of interleukin-1 were decreased on days 1 and 2 after burn in rhGH treated rats compared with controls (P<0.001, <0.01, respectively). Rats receiving rhGH showed significantly increased P-selectin levels at 5 and 7 postburn days compared with controls (P<0.001). Our data indicate that rhGH, given after thermal injury, increased albumin, transferrin, IGF-1, and P-selectin levels and decreased serum tumor necrosis factor-alpha and interleukin-1 levels.

Constitutive activation of JAK-STAT3 signaling by BRCA1 in human prostate cancer cells

Germ-line mutations of the breast cancer susceptibility gene 1 (BRCA1) confer a high risk for breast and ovarian cancer in women and prostate cancer in men. The BRCA1 protein contributes to cell proliferation, cell cycle regulation, DNA repair and apoptosis; however, the mechanisms underlying these functions of BRCA1 remain largely unknown. Here, we showed that, in Du-145 human prostate cancer cells, enhanced expression of BRCA1 resulted in constitutive activation of signal transducer and activator transcription factor 3 (STAT3) tyrosine and serine phosphorylation. Moreover, Janus kinase 1 (JAK1) and JAK2, the upstream activators of STAT3, were also activated by BRCA1. Immunoprecipitation assay showed that BRCA1 interacted with JAK1 and JAK2. Blocking STAT3 activation using antisense oligonucleotides significantly inhibited cell proliferation and triggered apoptosis in Du-145 cells with enhanced expression of BRCA1. These findings indicate that BRCA1 interacts with the components of the JAK-STAT signaling cascade and modulates its activation, which may provide a new critical survival signal for the growth of breast, ovarian and prostate cancers in the presence of normal BRCA1.

Inhibition of growth hormone action in models of inflammation

Growth hormone (GH) action is attenuated during the hepatic acute-phase response (APR). To understand this attenuation, we asked whether GH and cytokine-signaling pathways intersect during an APR. In hypophysectomized rats treated with lipopolysaccharide (LPS), accumulation of activated signal transducer and transcription activator 5 (Stat5) in hepatic nuclei in response to GH and its binding to a GH response element (GHRE) from the serine protease inhibitor (Spi) 2.1 promoter are diminished in a time-dependent manner. Similarly, accumulation of activated Stat3 in hepatic nuclei in response to LPS and its binding to a high-affinity sis-inducible element (SIE) are also diminished by the simultaneous administration of GH. In functional assays with primary hepatocytes, LPS-stimulated monocyte-conditioned medium (MoCM) inhibits the GH response of Stat5-dependent Spi 2.1 reporter activity but induces Stat3-dependent Spi 2.2 reporter activity, as in an APR. Similar results are obtained when hepatocytes are treated with either tumor necrosis factor-alpha (TNF-alpha) or interleukin (IL)-1beta. TNF-alpha, IL-1beta, and IL-6 also inhibit GH-induced Spi 2.1 mRNA expression in hepatocytes. Thus inhibition of the GH signaling pathway during an APR results in reduced expression of GH-responsive genes.

Recombinant human growth hormone treatment in pediatric burn patients and its role during the hepatic acute phase response

OBJECTIVE

Recombinant human growth hormone (rHGH) has been shown to increase mortality in adult trauma patients; however, little has been reported on its side effects in children. The acute phase response has been suggested to be a contributing factor to trauma mortality. Therefore, the purpose of this study was to examine the effects of exogenous rHGH on the acute phase response in pediatric bum patients.

DESIGN

Prospective, randomized, double-blind study.

SETTING

Shriners Hospital for Children.

PATIENTS

Thermally injured pediatric patients, ranging in age from 0.1 to 16 yrs.

INTERVENTIONS

Twenty-eight thermally injured children received either 0.2 mg/kg/day of rHGH or saline (placebo) within 3 days of admission and for at least 25 days.

MEASUREMENTS AND MAIN RESULTS

Measurements were patient demographics, incidence of sepsis, inhalation injury, mortality, serum constitutive proteins, acute phase proteins, proinflammatory cytokines and insulin-like growth factor-I (IGF-I), insulin-like growth factor binding protein (IGFBP)-1, and IGFBP-3. No differences could be demonstrated in age, gender, burn size, incidence in sepsis (20% vs. 26%), inhalation injury (46% vs. 27%), or mortality (8% vs. 7%) between those receiving rHGH or placebo. Serum IGF-I and IGFBP-3 increased with rHGH treatment, whereas serum IGFBP-1 decreased compared with placebo (p < .05). Burned children treated with rHGH required significantly less albumin substitution to maintain normal levels compared with placebo (p < .05). Those receiving rHGH demonstrated a decrease in serum C-reactive protein and serum amyloid-A and an increase in serum retinol-binding protein compared with placebo (p < .05). rHGH decreased serum tumor necrosis factor-alpha and interleukin (IL)-1beta, whereas no changes were found for serum IL-1alpha, IL-6, and IL-10 compared with placebo (p < .05). Free fatty acids were elevated in burned children who received rHGH (p < .05).

CONCLUSION

Data indicate that rHGH does not increase mortality. rHGH decreased acute phase proteins, tumor necrosis factor-alpha, and IL-1beta, which is associated with increases in constitutive hepatic proteins and IGF-I.

Hepatocyte growth factor modulates the hepatic acute-phase response in thermally injured rats

OBJECTIVE

Hepatocyte growth factor (HGF) has been shown to modulate the acute-phase response in vitro. The specific in vivo role of HGF in this multifactorial response, however, remains unknown. This study examines the effects of exogenous HGF on the acute-phase response in thermally injured rats.

DESIGN

Prospective, randomized, laboratory study.

SETTINGS

Shriners Hospital for Children and University of Texas Medical Branch laboratories.

SUBJECTS

Fifty-six male Sprague-Dawley rats (weight range, 300-325 g).

INTERVENTION

Animals received a 60% total body surface area third-degree scald burn and were randomly divided to receive either 400 microg/kg/day i.v. HGF or saline (control).

MEASUREMENTS AND MAIN RESULTS

Serum acute-phase proteins, cytokines, and insulin-like growth factor (IGF)-I concentrations, as well as liver weight, protein and triglyceride content, IGF-I concentrations, and cytokine gene expression were measured 1, 2, 5, or 7 days after burn. Serum albumin was increased on days 2, 5, and 7 after burn, and transferrin was increased on day 7 after burn in HGF-treated rats compared with controls (p<.05). HGF increased alpha2-macroglobulin concentrations on postburn days 2, 5, and 7 compared with controls (p<.05). Serum interleukin-6 and tumor necrosis factor-alpha were significantly higher within 2 days of burn in rats treated with HGF (p<.05). HGF increased the hepatic gene expression of tumor necrosis factor-alpha compared with controls (p<.05). Serum IGF-I decreased in rats receiving HGF 1, 2, and 5 days after burn, whereas liver IGF-I concentrations were higher on days 1 and 7 after burn compared with controls (p<.05). Hepatic protein concentrations were higher in the HGF group compared with controls on postburn days 1, 2, and 7, with a concomitant increase in total liver weight (p<.05). HGF exerted a strong mitogenic effect on hepatocytes 1 and 2 days after thermal injury compared with controls (p<.05).

CONCLUSIONS

These findings suggest that HGF modulates the acute-phase response in vivo after burn and causes changes in liver morphology.

Increased expression of insulin-like growth factor-I in serum and liver after recombinant human growth hormone administration in thermally injured rats

BACKGROUND

Recombinant human growth hormone (rhGH) has been shown to modulate the hypermetabolic response and the hepatic acute-phase response after thermal injury. In vitro studies, however, demonstrated that rhGH activates insulin-like growth factor-I (IGF-I) gene transcription and production, suggesting that rhGH may exert some of its effects indirectly through IGF-I stimulation. The purpose of this study was to determine the effects of rhGH on serum and hepatic IGF-I in thermally injured rats.

METHODS

Sprague-Dawley rats (56 males) receiving a 60% TBSA third-degree scald burn were randomly divided to receive either rhGH (2.5 mg/kg/day im) or saline (control). Rats were sacrificed on postburn days 1, 2, 5, and 7 and serum IGF-I, hepatic IGF-I mRNA, and IGF-I protein concentration were measured. The physiologic response to changes in IGF-I levels was evaluated by measuring hepatocyte proliferation, total liver protein concentration, and muscle dry/wet weights.

RESULTS

Serum IGF-I was increased from postburn day 1 through day 7 in rats receiving rhGH compared to controls (P < 0.05). Hepatic IGF-I mRNA and IGF-I protein expression were increased from day 1 to 7 after burn in animals receiving rhGH when compared to controls (P < 0.05). Recombinant hGH increased hepatocyte proliferation at 5 days and total liver protein concentration at 5 and 7 days postburn compared to controls (P < 0.05). Muscle dry/wet weights increased in rats receiving rhGH at 7 days after burn compared to controls (P < 0.05).

SUMMARY

Liver and serum IGF-I levels decreased after a thermal injury. Recombinant hGH attenuated this decrease by stimulating hepatic IGF-I expression. Increases in IGF-I were associated with increases in hepatocyte proliferation and protein concentration in liver and muscle.

CONCLUSION

We suggest that rhGH modulates the hypermetabolic response through IGF-I stimulation in the hepatic parenchyma.

Recombinant human growth hormone alters acute phase reactant proteins, cytokine expression, and liver morphology in burned rats

BACKGROUND

The effects of exogenous recombinant human growth hormone (rhGH) on hepatic acute phase reactant proteins, cytokine expression, and liver morphology were studied in thermally injured rats to define whether rhGH alters the acute phase response.

MATERIALS AND METHODS

Sprague-Dawley rats (56 males) receiving a 60% TBSA third-degree scald burn were randomly divided into two groups to receive either 2.5 mg/kg/day sc rhGH or saline. Rats were sacrificed on Postburn Days 1, 2, 5, and 7. Serum acute phase reactant proteins and cytokines TNF-alpha, IL-1alpha, IL-1beta, and IL-6 were measured. Hepatocyte proliferation, hepatic cytokine gene expression, and liver protein concentrations were determined.

RESULTS

Recombinant hGH increased serum albumin on Days 5 and 7 after burn (P < 0.05). Serum haptoglobin and alpha1-acid glycoprotein levels decreased at 2, 5, and 7 days after burn compared to saline (P < 0.05). In rats treated with rhGH, serum IL-1beta decreased 1 day postburn, while serum TNF-alpha increased 5 days after burn compared to saline (P < 0.05). Serum IL-6 and IL-1alpha did not change. Hepatic RNA levels for TNF-alpha were significantly elevated on Day 1 postburn compared to saline (P < 0. 05). Hepatic protein content increased on Days 2, 5, and 7 postburn compared to saline (P < 0.05). Hepatocyte proliferation in rhGH-treated rats increased on Day 5 after burn (P < 0.05).

CONCLUSION

Data indicate that rhGH alters the hepatic acute phase response by decreasing type I acute phase proteins and modulating IL-1-like cytokine expression. These changes are associated with increased hepatocyte mitosis and serum and total liver protein concentrations.

Definition of a high affinity growth hormone DNA response element

Rat liver contains a growth hormone inducible nuclear factor complex, GHINF, that binds to the growth hormone response element (GHRE) of the serine protease inhibitor (Spi) 2.1 gene. GHINF contains Stat5 and binds to paired gamma-activated sites (GAS) within the GHRE, but poorly to either one alone. By analysis of the sequence of various GAS sites that bind the GHINF complex (based on the GHRE 3' GAS motif), we demonstrate that a 13 nucleotide high affinity DNA recognition sequence (haGHRE) for GHINF complex binding is (ANTTC)C/T(N)A/G(GAA)A/T(A)/T. One copy of the haGHRE will replace the requirement for two GAS elements present in the wild type promoter in supporting a GH response in primary hepatocyte culture. Mutation of the native Spi 2.1 from a paired GAS site to a single haGHRE does not appreciably change its affinity for binding to the GHINF complex, nor does it alter its sensitivity to GH concentration.

Growth hormone suppression of insulin-like growth factor binding protein-1 promoter activity

Hepatic transcription of insulin-like growth factor binding protein-1 (IGFBP-1) is rapidly downregulated by growth hormone (GH) which is also known to induce expression of c-fos and c-jun. Co-expression of c-fos or c-jun in rat hepatocytes, individually or together, suppresses IGFBP-1 promoter activity by approximately 60%. When hepatic nuclear extracts from sham-operated, hypophysectomized (hypox) and GH-treated hypox rats were analyzed by DNase-1 footprinting, differences in the protection pattern were identified in three regions of the IGFBP-1 promoter. F1 corresponding to - 660 to - 640 bp showed acute changes in response to GH administration. In additional regions, F2 and F3, representing - 758 to - 748 bp and - 477 to - 447 bp, respectively, differences were apparent between nuclear extracts from the hypox and sham-operated rats. When F1 and F2 were removed by deletion of the region from - 824 to - 557 bp, the GH response was lost but suppression by co-expression of c-fos and c-jun was preserved. A putative AP-1 binding site was present in the F3 footprint region, however removal of F3 did not affect the GH responsiveness. These data indicate that several distinct sequences, other than the putative AP-1 site are involved in mediating the GH effects on IGFBP-1 transcription.

STAT 5 and NF-Y are involved in expression and growth hormone-mediated sexually dimorphic regulation of cytochrome P450 3A10/lithocholic acid 6beta-hydroxylase

The level of expression of a number of sexually differentiated liver proteins is primarily determined by plasma growth hormone (GH). Adult males have a pulsatile profile of GH release, while females have a relatively steady-state pattern of GH release. An important subset of these sexually differentiated hepatic proteins is certain cytochrome P450s (P450s). CYP3A10/6beta-hydroxylase is a male-specific P450 that catalyzes 6beta-hydroxylation of lithocholic acid, and the pattern of GH secretion is directly responsible for male-specific expression of this gene. The DNA element involved in GH-mediated regulation of CYP3A10/6beta-hydroxylase promoter activity binds a member of the STAT (signal transducers and activators of transcription) family of proteins. In this study we functionally demonstrate that two members of the STAT family, STAT 5a and STAT 5b, mediate GH-dependent regulation of CYP3A10/6beta-hydroxylase promoter activity. Furthermore, a neighboring DNA element binds NF-Y, a transcription factor involved in maintaining high levels of transcription of many genes and known to functionally interact with other factors. In the CYP3A10/6beta-hydroxylase gene, NF-Y also modulates binding of STAT 5, thereby modulating GH-mediated activation of its transcription.

Recombinant human growth hormone (rhGH) downregulates hepatocyte growth factor (HGF) in burns

INTRODUCTION

Hepatocyte growth factor (HGF) and recombinant human growth hormone (rhGH) have both been shown to increase albumin serum concentrations after major injury. However, the effect of rhGH on HGF production after injury is unknown. We postulated that rhGH effects constitutive protein concentrations by inducing HGF expression.

METHODS

In order to determine a dose response, 20 male Sprague-Dawley rats received three different concentrations of rhGH 1, 2.5, and 5 mg/kg and a saline treatment. Rats were sacrificed 10 days after burn and serum albumin and HGF plasma concentrations were measured. Eighty male Sprague-Dawley rats received a 60% TBSA third-degree scald burn injury and were randomly divided into three groups, control, burn plus saline treatment, or burn plus rhGH treatment (2.5 mg/kg SQ qD). Rats were sacrificed 2, 5, 7, and 14 days after burn, and serum albumin, plasma, and hepatic tissue HGF concentrations were determined (ELISA, Institute of Immunology, Tokyo, Japan).

RESULTS

At 10 days postburn serum albumin levels were significantly increased with rhGH 2.5 mg/kg treatment (P < 0.05); however, HGF plasma concentrations were significantly decreased with a dose of 5 mg/kg of rhGH compared to control and rhGH 1.0 mg/kg (P < 0.05). Serum albumin concentrations decreased immediately after burn and remained low until at least the 14th day after injury. RhGH-treated animals had higher levels of albumin on Day 7 after burn (P < 0.05). Plasma HGF levels decreased immediately after burn, but increased after the second day postburn. Beginning on the 5th day after injury, HGF levels in non-rhGH-treated rats were significantly higher compared to those in rhGH-treated rats (P < 0.05). Hepatic tissue HGF concentrations were higher in non-rhGH-treated rats compared to rhGH treated animals 7 days after burn (P < 0.01).

CONCLUSION

Although rhGH treatment improves constitutive protein synthesis, rhGH decreases HGF concentration in a dose-dependent manner. The improvements in constitutive protein concentrations do not occur via a HGF dependent pathway.

Growth hormone stimulates interferon regulatory factor-1 gene expression in the liver

Interferon regulatory factor-1 (IRF-1) is a transcription factor first identified as part of the nuclear response to interferons. IRF-1 has been shown to be activated by many cytokines, including PRL, and has been thought to play a role in PRL-regulated gene expression in several experimental systems, including the Nb2 T lymphoma cell line, where it was first characterized as a PRL-responsive gene. We now find that IRF-1 gene expression is rapidly activated in vivo by both PRL and GH treatment. A single i.p. injection of rat PRL to hypophysectomized female rats caused a transient increase in nascent hepatic nuclear IRF-1 RNA within 15 min of hormone treatment. The rise in IRF-1 transcripts was accompanied by induction of nuclear protein binding to a DNA element from the proximal IRF-1 promoter, as assessed by gel mobility shift assays; this element was shown previously to mediate PRL-activated gene transcription. GH treatment stimulated a greater and more sustained increase in nascent IRF-1 RNA than PRL, leading to accumulation of IRF-1 transcripts for up to 16 h after a single hormone injection. GH also caused a pronounced induction of hepatic nuclear protein binding to the IRF-1 promoter element. Supershift experiments with specific antibodies showed that signal transducer and activator of transcription 1 (STAT1) and to a lesser extent STAT3 were components of the GH-activated protein-DNA complexes. By contrast, these two STATs were not induced in the liver by PRL. Protein binding to the IRF-1 DNA element and IRF-1 gene activation by GH were not blunted by pretreatment with the protein synthesis inhibitor, cycloheximide, indicating that these hormonal effects are primary consequences of GH-activated signal transduction pathways. Our results identify another component of the rapid nuclear response to GH, and support the idea that multiple primary and secondary signaling pathways contribute to the acute actions of GH on gene expression.

The molecular basis of growth hormone action

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

Activation of the signal transducers and activators of transcription signaling pathway by growth hormone (GH) in skin fibroblasts from normal and GH binding protein-positive Laron Syndrome children

We have previously described two families (H and M) with GH binding protein-positive Laron Syndrome (LS), proposed to have one or more post GHR signaling defects. In the present study, we have examined whether the signal transducers and activators of transcription (STAT) pathway is activated by GH in skin fibroblast cultures established from these LS children, to determine the level(s) at which GH insensitivity has occurred. On immunoblots, both normal and LS fibroblasts express JAK2 and STATs 1, 3, and 5. GH induced rapid tyrosine phosphorylation of a protein at approximately 93 kDa in normal fibroblasts, and Western blotting with STAT-specific antibodies revealed STAT5 activation (phosphorylation) by GH. To determine further the identity and the DNA binding characteristics of the STAT proteins that were activated by GH, EMSAs were performed using three DNA elements known to bind STAT proteins; m67, the high affinity c-sis-inducible element (SIE), the interferon response element (IRE), and the lactogenic hormone-responsive region (LHRR). GH failed to induce protein binding to the SIE or IRE in normal skin fibroblasts but did induce the formation of a specific complex with the LHRR. Induction by GH of this LHRR/protein complex, which could be supershifted partially by anti-STAT1 antisera and completely by anti-STAT5 antisera, was transient, maximal between 10 and 30 min and reduced by 60 min. GH also induced distinct LHRR/protein complexes in mouse 3T3-F442A fibroblasts and in human IM-9 lymphocytes, but supershift analysis revealed that these complexes contained STAT5 but not STAT1. Whereas no binding to the LHRR was observed in GH-treated H fibroblasts, GH induced binding to this element in M fibroblasts. These results demonstrate that 1) the JAK-STAT pathway is activated by GH in normal fibroblasts and that STATs 1 and 5 have a role in GH-dependent signaling in these cells; 2) GH activation of DNA/STAT binding is cell type- and species-specific; and 3) GH failed to activate the STAT pathway in H fibroblasts but induced STAT signaling in M fibroblasts, indicating that the site of GH resistance in the latter is likely to be located within another GH signaling pathway. These fibroblast cultures therefore provide unique models with which to further our understanding of the mechanisms of human GH signaling.

CCAAT/enhancer-binding protein delta activates insulin-like growth factor-I gene transcription in osteoblasts. Identification of a novel cyclic AMP signaling pathway in bone

Insulin-like growth factor-I (IGF-I) plays a key role in skeletal growth by stimulating bone cell replication and differentiation. We previously showed that prostaglandin E2 (PGE2) and other cAMP-activating agents enhanced IGF-I gene transcription in cultured primary rat osteoblasts through promoter 1, the major IGF-I promoter, and identified a short segment of the promoter, termed HS3D, that was essential for hormonal regulation of IGF-I gene expression. We now demonstrate that CCAAT/enhancer-binding protein (C/EBP) delta is a major component of a PGE2-stimulated DNA-protein complex involving HS3D and find that C/EBPdelta transactivates IGF-I promoter 1 through this site. Competition gel shift studies first indicated that a core C/EBP half-site (GCAAT) was required for binding of a labeled HS3D oligomer to osteoblast nuclear proteins. Southwestern blotting and UV-cross-linking studies showed that the HS3D probe recognized a approximately 35-kDa nuclear protein, and antibody supershift assays indicated that C/EBPdelta comprised most of the PGE2-activated gel-shifted complex. C/EBPdelta was detected by Western immunoblotting in osteoblast nuclear extracts after treatment of cells with PGE2. An HS3D oligonucleotide competed effectively with a high affinity C/EBP site from the rat albumin gene for binding to osteoblast nuclear proteins. Co-transfection of osteoblast cell cultures with a C/EBPdelta expression plasmid enhanced basal and PGE2-activated IGF-I promoter 1-luciferase activity but did not stimulate a reporter gene lacking an HS3D site. By contrast, an expression plasmid for the related protein, C/EBPbeta, did not alter basal IGF-I gene activity but did increase the response to PGE2. In osteoblasts and in COS-7 cells, C/EBPdelta, but not C/EBPbeta, transactivated a reporter gene containing four tandem copies of HS3D fused to a minimal promoter; neither transcription factor stimulated a gene with four copies of an HS3D mutant that was unable to bind osteoblast nuclear proteins. These results identify C/EBPdelta as a hormonally activated inducer of IGF-I gene transcription in osteoblasts and show that the HS3D element within IGF-I promoter 1 is a high affinity binding site for this protein.

Specificity of transcription enhancement via the STAT responsive element in the serine protease inhibitor 2.1 promoter

The growth hormone regulated serine protease inhibitor (SPI) 2.1 and 2.2 gene promoters have been shown to contain a response element similar to the gamma-interferon activated sequence (GAS) family of signal transducer and activator of transcription (STAT) response elements. We have investigated the STAT and cytokine specificity of the SPI 2.1 STAT responsive element using a luciferase (LUC) reporter construct and a cDNA complementation strategy in the COS 7 cell line. Growth hormone was found to stimulate SPI-LUC reporter gene expression via activation of STAT 5, but not STATs 1 or 3, which indicates that the SPI 2.1 STAT responsive element is STAT 5 specific. In addition to the growth hormone receptor, the receptors for prolactin and erythropoietin enhanced gene transcription via the SPI 2.1 STAT responsive element, which indicates that this element is, on the other hand, not cytokine specific. Activation of STAT 5 was also observed after growth hormone treatment of cells transfected with cDNA expression plasmids for several different truncated growth hormone receptor mutants, although this activation was less efficient than with the wild type receptor. Point mutation of individual tyrosines in the growth hormone receptor intracellular domain to phenylalanines had no significant effect on signal transduction via STAT 5. These data, taken together with results from experiments using the phosphatase inhibitor sodium orthovanadate, suggest that STAT 5 may not have an absolute requirement for specific phosphorylated receptor tyrosine docking sites. That receptor tyrosine residues in a variety of amino acid contexts, or phosphorylated Janus kinase (JAK) 2 alone, can facilitate STAT 5 activation could explain the observed lack of cytokine specificity in STAT 5 activation.

Growth hormone-induced signal tranduction depends on an intact ubiquitin system

The growth hormone receptor (GHR) is a ubiquitinated cell surface protein. Ligand binding and receptor dimerization activate the cytosolic kinase Jak2. This event initiates signal transduction via STAT proteins. Expression of GHR in a Chinese hamster ovary (CHO) cell line, which exhibits a temperature-sensitive defect in ubiquitin conjugation (CHO-ts20), as well as in wild type cells (CHO-E36) has shown that endocytosis of the receptor requires an intact ubiquitin conjugation system (Strous G. J., van Kerkhof, P., Govers, R., Ciechanover A., and Schwartz, A. L. (1996) EMBO J. 15, 3806-3812). We have now examined the requirement for ubiquitin conjugation in growth factor-mediated signal transduction. In CHO-E36 and in CHO-ts20 cells at the permissive temperature, STAT proteins were activated in a growth factor-dependent fashion. However, no activation of STAT proteins was observed at the nonpermissive temperature in CHO-ts20 cells. Neither tyrosine phosphorylation of GHR nor of Jak2 was inhibited at the nonpermissive temperature. When tyrosine phosphorylation was inhibited following treatment with staurosporin, ubiquitination of the receptor proceeded normally. Furthermore, mutation of GHR phenylalanine-327, which prevents GHR endocytosis, inhibited receptor ubiquitination but allowed normal Jak/STAT-mediated signal transduction. Thus, these data provide evidence that the ubiquitin conjugation system is involved in the Jak/STAT signaling pathway, be it not at the initial stage(s) of Jak2 activity.

Contrasting acute in vivo nuclear actions of growth hormone and prolactin

Growth hormone (GH) and prolactin (PRL) exert long-term effects on cellular metabolism, growth, and development through changes in gene expression and protein biosynthesis that are initiated by hormone binding to specific cell-surface receptors. Recent studies have demonstrated that ligand-induced activation of both GH and PRL receptors leads to the tyrosine phosphorylation of multiple intracellular proteins by the identical non-receptor tyrosine kinase, JAK2. We have shown previously that in vivo administration of human recombinant GH rapidly stimulated the inducible transcription factors, Stats1, 3, and 5, and acutely altered gene transcription in the liver. Because human GH can bind to both lactogenic and somatogenic receptors with high affinity, in this study we have addressed the question of specificity of the hormonal response by examining the early nuclear events following a single injection of rat GH or rat PRL to hormone-deficient hypophysectomized female rats. We find that PRL stimulated tyrosine phosphorylation of Stat5, induced nuclear protein binding to the GH-responsive element of the serine protease inhibitor (Spi) 2.1 promoter, and activated Spi 2.1 gene expression. These acute actions of rat PRL were modest compared to the effects of rat GH. GH treatment induced tyrosine phosphorylation of several hepatic nuclear proteins, activated Stats1, 3, and 5, stimulated Spi 2.1 gene expression, and inhibited albumin gene transcription. All of the effects of rat GH paralleled responses to human GH that we have measured previously. Based on these results, it is likely that most of the actions of human GH in the liver are mediated by the GH receptor rather than by the PRL receptor. The diminished response to PRL may be secondary to the high density of short PRL receptor isoforms in the liver, which do not participate effectively in ligand-induced signal transmission.

Growth hormone receptor signalling

The growth hormone (GH) receptor belongs to the superfamily of transmembrane proteins that includes the prolactin (PRL) receptor and a number of cytokine receptors. Two forms exist for the GH receptor: the membrane-bound form is a protein of 620 amino acid residues with a unique transmembrane domain; the GH-binding protein (GHBP), which is a soluble short form, is identical to the extracellular domain of the membrane receptor. In man and many other species, GHBP is believed to result from proteolytic cleavage of the membrane receptor; in human tissues, only one mRNA form of 4.5 kb encoding the full-length receptor has been detected. In rodents, GHBP is encoded by a specific mRNA of 1.2kb. Binding of GH to its receptor results in dimerization of the receptor, phosphorylation of the tyrosine kinase JAK2 and of the receptor, followed by a cascade of protein phosphorylations. Transcription factors belonging to the signal transducers and activators of transcription (STAT) family are involved in the effects of GH on the transcription of genes such as c-fos, serine protease inhibitor Spi 2.1 and beta-casein. GH is able to activate several STAT proteins including STAT1, 3 and 5. The JAK-STAT pathway is a main pathway for GH effects on gene transcription. Other signalling molecules are involved in GH action through different pathways: GH is able to activate mitogen activated protein (MAP) kinases; the hormone can utilize insulin receptor substrate-1 (IRS-1) and induces the association of phosphatidylinositol 3-kinase with IRS-1. Two main functional regions have been defined in the cytoplasmic domain of the GH receptor by testing the activity of mutant forms of the receptor in several systems: Box 1, a proline-rich sequence in the membrane proximal part, is necessary for all GH effects and is probably the region of association with JAK2; the C-terminal region is required for the induction of specific genes. Other molecules involved in the mechanisms of action of GH remain to be identified. As the same signalling pathways are used by many ligands, explanations for the specificity of the cellular effects have to be determined.

Two separate signal transducer and activator of transcription proteins regulate transcription of the serine proteinase inhibitor-3 gene in hepatic cells

The serine proteinase inhibitor (SPI-3) gene expression is transcriptionally regulated by interleukin (IL)-6 and glucocorticoids in hepatic cells. To identify the transcription factors involved in regulation of the SPI-3 promoter-chloramphenicol acetyltransferase constructs we overexpressed Signal Transducer and Activator of Transcription (STAT) proteins (STAT1, STAT3, STAT5B, and STAT6) and CAAT enhancer-binding protein beta. Specific signaling pathways were activated by cointroduced receptors for growth hormone, IL-3, IL-4, or chimeric receptors containing the cytoplasmic domain of gp130. STAT3 and STAT5B induced transcription via the SPI-3 promoter. The STAT5B response was substantially enhanced by truncation of the 5'-flanking region from -1021 to -148. The responsiveness to STAT3 and STAT5B required the STAT binding element at -132 to -124. This element was sufficient to confer regulation onto a heterologous promoter gene construct. In contrast, overexpression of CAAT enhancer-binding protein beta reduced the transcriptional activity of the SPI-3 promoter, presumably by interfering with STAT protein binding to the promoter element. The SPI-3 gene is the first example of an acute phase gene that is responsive to both STAT3 and STAT5B.

Participation of JAK and STAT proteins in growth hormone-induced signaling

The binding of growth hormone leads to dimerization of its receptor, accompanied by phosphorylation and activation of intracellular tyrosine kinases (JAKs) and the latent cytoplasmic transcriptions factors STAT1, STAT3, and STAT5. Both JAK1 and JAK2 are phosphorylated in response to growth hormone in mouse 3T3 F442A and human HT1080 cells. The roles of JAKs in growth hormone signal transduction were examined by using mutant HT1080 cells missing either JAK1 or JAK2. JAK2 is absolutely required for growth hormone-dependent phosphorylation of the receptor, STAT1 and STAT3, JAK1, and the SH2-containing adaptor molecule Shc. In contrast, JAK1 is not required for any of the above functions. These data indicate that JAK2 is both necessary and sufficient for the growth hormone-dependent phosphorylation events required to couple the receptor both to STAT-dependent signaling pathways and to pathways involving Shc. Furthermore, STAT5 is activated by growth hormone in 3T3 F442A cells, but not in HT1080 cells, revealing that the set of STATs activated by growth hormone can vary, possibly contributing to the specificity of the growth hormone response in different cell types.

Unexpected transcriptional signals in normal and mitotically defective cells mediated through cytokine and growth factor receptors

Polypeptide growth factors and cytokines mediate their biochemical functions through their responsive receptors. Known cytokine receptors do not possess intrinsic kinase domains whereas several polypeptide growth factor receptors do. Nevertheless, both classes of ligands are capable of activating sets of overlapping genes. In human epidermoid carcinoma cells, for example, both cytokines and epidermal growth factor (EGF) promote a common transcriptional activation signal through the tyrosine phosphorylation of stat91 (signal transducer and activator of transcription) proteins. The stat family of cytoplasmic proteins also appear to have dual functions. Tyrosine phosphorylated 'stats' are employed for signal transduction and, second, for activation of transcription of several genes. The transcription factor-SIE-DNA binding patterns are now known to be different for EGF and interferon-gamma IFN-gamma-treated cells. Nevertheless, in the active DNA-bound complex, the stat91 polypeptide is a component found in either EGF or INF-gamma-treated extracts. Other stat family members of transcription factors may also be present in the complexes. In this case, tyrosine phosphorylated stat91 polypeptides may form into homodimeric or heterodimeric assemblies with other stat-related transcription factors. We describe a novel stat-related factor, p93, that is found in EGF-treated A431 cell extracts but appears to be absent in bovine fibroblast growth factor (bFGF), IFN-gamma, tumor necrosis factor-alpha (TNF-alpha), and untreated cells. p93 appears to be antigenically related to stat91. p185c-neu+, EGFr+ (M1), and p185c-neu- kinase inactive, EGFr+ (NEN757) expressing cells undergo different mitotic responses to EGF. M1 can respond to EGF mitotically while NEN757 cannot. Both cell lines respond to 10 ng/ml of EGF and also to IFN-gamma in transducing transcriptional activation signals to the nucleus, despite the distinct growth response to EGF. Our work has analyzed the stat pathway in these types of cells and found similar patterns of usage despite the distinct EGF-responsive features. Cytoplasmic nonreceptor tyrosine kinases Jak1 and Jak2 may be involved in the activation of stat91 and other transcription factors in EGF and IFN-gamma signaling pathways. Collectively, these studies suggest that the major EGF-stimulated mitotic growth pathways may not be absolutely linked to the stat91 signaling pathways and that such transcription complexes are more complex than previously reported.

Growth hormone induction of hepatic serine protease inhibitor 2.1 transcription is mediated by a Stat5-related factor binding synergistically to two gamma-activated sites

A growth hormone (GH)-inducible nuclear factor (GHINF) from rat liver has been purified to near homogeneity. On SDS-polyacrylamide gel electrophoresis and UV-cross-linking, a major band of mass approximately 93 kDa and a minor band of approximately 70 kDa are detected in the purified fraction. DNase I footprinting using purified GHINF yields a protected region of -149/-115 on the rat serine protease inhibitor 2.1 (Spi 2.1) promoter encompassed within the growth hormone response element (GHRE). Mutational analysis demonstrated that GHINF binds synergistically to two gamma-interferon-activated sites (GAS) within the GHRE, with the 3' element being the pivotal binding domain. Functional assays show that both GAS elements are necessary for full GH response. GHINF has no immunoreactivity with either a C-terminal Stat1 antibody or an N-terminal Stat3 antibody, while cross-reacting with a C-terminal Stat5 monoclonal antibody. GHINF will bind to two GAS elements from the Stat5 binding region of the beta-casein gene. These studies indicate that GHINF is a Stat5-related factor binding synergistically to two GAS elements to activate Spi 2.1 transcription.

A STAT factor mediates the sexually dimorphic regulation of hepatic cytochrome P450 3A10/lithocholic acid 6 beta-hydroxylase gene expression by growth hormone

Adult male rodents have a pulsatile profile of growth hormone (GH) release, whereas female rodents have a relatively steady-state pattern with uniform, albeit lower levels of GH. The expression of a number of sexually differentiated hepatic proteins is primarily determined by these plasma GH profiles and only secondarily regulated by gonadal hormones. An important subset of these sexually dimorphic proteins is cytochrome P450s. CYP3A10/6 beta-hydroxylase is a cytochrome P450 that catalyzes the 6 beta-hydroxylation of lithocholic acid. CYP3A10/6 beta-hydroxylase is expressed only in male hamsters; however, mimicking the male GH secretion pattern in females induces expression of the gene to male levels. Using chimeric CYP3A10/6 beta-hydroxylase promoter/luciferase reporter genes transfected into hamster primary hepatocytes, we have shown a GH-mediated induction of promoter activity. A combination of 5'-deletion constructs, heterologous promoter constructs, and specific mutagenesis was used to localize the DNA element involved in the GH-mediated regulation of CYP3A10/6 beta-hydroxylase promoter activity, which resembles a STAT binding site. Footprint and gel shift analyses confirmed that the expression of the protein binding to this site is regulated by GH and that the DNA-protein complex can be partially supershifted by anti-STAT-5 antibodies. This protein is 50% more abundant in male than in female hamster livers, is absent in hypophysectomized female livers, and is restored when hypophysectomized females are injected with GH in a manner that masculinizes female hamsters in terms of CYP3A10/6 beta-hydroxylase expression. The system characterized and described here is ideally suited for dissecting the molecular details governing the sexually dimorphic expression of liver-specific genes.

Structure and regulation of the ALS gene

The mouse ALS gene spans at least 6 kb. It contains 2 exons which encode a protein highly homologous to human and rat ALS. It was localized to mouse chromosome 17 by flourescent in situ hybridization. The 5' flanking region lacks a TATA box but contains GC boxes that may be recognised by transcription factors such as Spl. Hepatic ALS mRNA is decreased in rats following hypophysectomy, and restored by stimulated ALS promoter activity in a rat hepatoma cell line, but not in 3T3-F442A mouse preadipocyte fibroblasts, suggesting that utilisation of the ALS promoter is cell-type specific. The rat hepatoma system is a promising system to study the regulation of ALS gene expression, and the signalling pathways of CH regulation.