The molecular basis of growth hormone action

Growth hormone and gastrointestinal malignancy: An intriguing link

Growth hormone (GH) excess is associated with several systemic complications, one of which is the increased risk of neoplastic processes particularly of the gastrointestinal (GI) tract. Among the GI neoplasms, the most reported association is with benign and malignant neoplasms of the colon. In the majority of published literature, an increased incidence of GI neoplasms, both colonic adenomas as well as colorectal carcinoma is reported. However, the studies on colon cancer-specific mortality rate are conflicting with recent studies reporting similar cancer-specific mortality rates in comparison to controls. Many studies have reported an association of colorectal neoplasms with GH levels. Pathogenic mechanisms put forward to explain this association of GH excess and GI neoplasms primarily involve the increased GH-insulin-like growth factor 1 (IGF-1) signaling. Both GH and IGF-1 have proliferative, anti-apoptotic, and angiogenic effects on the systemic tissues leading to cellular proliferation. Other contributing factors to the increased risk of GI neoplasms include slow intestinal transit with a redundant large bowel, altered bile acids, deranged local immune response, shared genetic susceptibility factors and hyperinsulinemia. In view of the increased risk association, most guidelines for the care of acromegaly patients recommend an initial screening colonoscopy. Recommendations for further follow-up colonoscopy differ but broadly, the guidelines agree that it depends on the findings at first colonoscopy and state of remission of GH excess. Regarding the concern about the risk of colorectal cancers in patients receiving recombinant GH therapy, most cohort studies do not show an increased risk.

The wonders of BMP9: From mesenchymal stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism to regenerative medicine

Although bone morphogenetic proteins (BMPs) initially showed effective induction of ectopic bone growth in muscle, it has since been determined that these proteins, as members of the TGF-β superfamily, play a diverse and critical array of biological roles. These roles include regulating skeletal and bone formation, angiogenesis, and development and homeostasis of multiple organ systems. Disruptions of the members of the TGF-β/BMP superfamily result in severe skeletal and extra-skeletal irregularities, suggesting high therapeutic potential from understanding this family of BMP proteins. Although it was once one of the least characterized BMPs, BMP9 has revealed itself to have the highest osteogenic potential across numerous experiments both in vitro and in vivo, with recent studies suggesting that the exceptional potency of BMP9 may result from unique signaling pathways that differentiate it from other BMPs. The effectiveness of BMP9 in inducing bone formation was recently revealed in promising experiments that demonstrated efficacy in the repair of critical sized cranial defects as well as compatibility with bone-inducing bio-implants, revealing the great translational promise of BMP9. Furthermore, emerging evidence indicates that, besides its osteogenic activity, BMP9 exerts a broad range of biological functions, including stem cell differentiation, angiogenesis, neurogenesis, tumorigenesis, and metabolism. This review aims to summarize our current understanding of BMP9 across biology and the body.

Growth problems in children with IBD

Crohn's disease in childhood causes linear growth retardation, which has a substantial effect on management of this disease. By contrast, growth is rarely a problem in children presenting with ulcerative colitis. Depending on how growth failure is defined, approximately one-third of children with Crohn's disease have growth retardation at diagnosis. Although corticosteroids can suppress growth, decreased height at diagnosis demonstrates that this finding is a consequence of the disease and not merely an adverse effect of treatment. Both inflammation and undernutrition contribute to decreased height velocity. Increased cytokine production acts both on the hepatic expression of insulin-like growth factor 1 (IGF-1) and at chondrocytes of the growth plates of long bones. Growth hormone insensitivity caused by deranged immune function is a major mechanism in growth retardation. Resolution of inflammation is the cornerstone of treatment, but current studies on growth hormone and IGF-1 might yield therapies for those children whose inflammation is refractory to treatment.

BMP signaling in mesenchymal stem cell differentiation and bone formation

Bone morphogenetic proteins (BMPs) are members of the TGF-β superfamily and have diverse functions during development and organogenesis. BMPs play a major role in skeletal development and bone formation, and disruptions in BMP signaling cause a variety of skeletal and extraskeletal anomalies. Several knockout models have provided insight into the mechanisms responsible for these phenotypes. Proper bone formation requires the differentiation of osteoblasts from mesenchymal stem cell (MSC) precursors, a process mediated in part by BMP signaling. Multiple BMPs, including BMP2, BMP6, BMP7 and BMP9, promote osteoblastic differentiation of MSCs both in vitro and in vivo. BMP9 is one of the most osteogenic BMPs yet is a poorly characterized member of the BMP family. Several studies demonstrate that the mechanisms controlling BMP9-mediated osteogenesis differ from other osteogenic BMPs, but little is known about these specific mechanisms. Several pathways critical to BMP9-mediated osteogenesis are also important in the differentiation of other cell lineages, including adipocytes and chondrocytes. BMP9 has also demonstrated translational promise in spinal fusion and bone fracture repair. This review will summarize our current knowledge of BMP-mediated osteogenesis, with a focus on BMP9, by presenting recently completed work which may help us to further elucidate these pathways.

Mathematical modelling to restore circulating IGF-1 concentrations in children with Crohn's disease-induced growth failure: a pharmacokinetic study

OBJECTIVES

Children with Crohn's disease grow poorly, and inflammation depresses the response of insulin-like growth factor-1 (IGF-1) to growth hormone. Correcting the inflammation normalises growth velocity; however, removing inflammation cannot be achieved in all children. Our lack of understanding of IGF-1 kinetics has hampered its use, particularly as high IGF-1 concentrations over long periods may predispose to colon cancer. We hypothesised that mathematical modelling of IGF-1 would define dosing regimes that return IGF-1 concentrations into the normal range, without reaching values that risk cancer.

DESIGN

Pharmacokinetic intervention study.

SETTING

Tertiary paediatric gastroenterology unit.

PARTICIPANTS

8 children (M:F; 4:4) entered the study. All completed: 5 South Asian British; 2 White British; 1 African British.

INCLUSION CRITERIA

Children over 10 years with active Crohn's disease (C reactive protein >10 mg/l or erythrocyte sedimentation rate >25 mm/h) and height velocity <-2 SD score.

EXCLUSION CRITERIA

closed epiphyses; corticosteroids within 3 months; neoplasia or known hypersensitivity to recombinant human IGF-1 (rhIGF-1).

INTERVENTIONS

Subcutaneous rhIGF-1 (120 μg/kg) per dose over two admissions: the first as a single dose and the second as twice daily doses over 5 days.

PRIMARY OUTCOME

Significant increase in circulating IGF-1.

SECONDARY OUTCOMES

Incidence of side effects of IGF-1. A mathematical model of circulating IGF-1 (Ac) was developed to include parameters of endogenous synthesis (Ksyn); exogenous uptake (Ka) from the subcutaneous dose (As): and IGF-1 clearance: where dAc/dt=Ksyn - Kout×Ac+Ka×As.

RESULTS

Subcutaneous IGF-1 increased concentrations, which were maintained on twice daily doses. In covariate analysis, disease activity reduced Ksyn (p<0.001). Optimal dosing was derived from least squares regression fitted to a dataset of 384 Crohn's patients, with model parameters assigned by simulation.

CONCLUSIONS

By using age, weight and disease activity scaling in IGF-1 dosing, over 95% of children will have normalised IGF-1 concentrations below +2.5 SDs of the normal population mean, a level not associated with cancer risk.

Growth hormone synergizes with BMP9 in osteogenic differentiation by activating the JAK/STAT/IGF1 pathway in murine multilineage cells

Growth hormone (GH) is usually released by somatotrophs in the anterior pituitary in response to the GH-releasing hormone and plays an important role in skeleton development and postnatal growth. However, it is unclear if extrapituitary GH exerts any effect on murine multilineage cells (MMCs). MMCs are multipotent progenitors that give rise to several lineages, including bone, cartilage, and fat. We have identified bone morphogenic protein 9 (BMP9) as one of the most osteogenic BMPs in MMCs by regulating a distinct set of downstream mediators. In this study, we find that GH is one of the most significantly upregulated genes by BMP9 in mouse MMCs through expression-profiling analysis. We confirm that GH is a direct early target of and upregulated by BMP9 signaling. Exogenous GH synergizes with BMP9 on inducing early and late osteogenic markers in MMCs. Furthermore, BMP9 and GH costimulation leads to a significant expansion of growth plate in cultured limb explants. Although GH alone does not induce de novo bone formation in an ectopic bone formation model, BMP9 and GH costimulated MMCs form more mature bone, which can be inhibited by silencing GH expression. The synergistic osteogenic activity between BMP9 and GH can be significantly blunted by JAK/STAT inhibitors, leading to a decrease in GH-regulated insulin-like growth factor 1 (IGF1) expression in MMCs. Our results strongly suggest that BMP9 may effectively regulate extrapituitary GH expression in MMCs. Thus, it is conceivable that the BMP9-GH-IGF axis may be exploited as an innovative strategy to enhance osteogenesis in regenerative medicine.

Human milk proteins: an interactomics and updated functional overview

Milk and milk fractions are characterized by a wide array of proteins, whose concentration spans across several orders of magnitude. By exploiting a combined approach based on functional gene ontology enrichment (FatiGO/Babelomics), hierarchical clustering, and pathway and network analyses, we merged data from literature dealing with protein-oriented studies on human milk. A total of 285 entries defined a nonredundant list upon comparison with the Ingenuity Knowledge Base from the Ingenuity Pathway Analysis software. Results were compared with an inventory of bovine milk proteins gathered from dedicated proteomic studies. A protein core of 106 proteins was found, with most of the entries associated to three main biological functions, namely nutrient transport/lipid metabolism, concretization of the immune system response and cellular proliferation processes. Our analyses confirm and emphasize that the biological role of the human milk proteins is not only limited to the provision of external nutrients and defense molecules against pathogens to the suckling but also to the direct stimulation of the growth of neonate tissues/organs and to the development of a proper independent immune system, both through the induction of a number of molecular cascades associated with cell proliferation/differentiation. The latter aspects were previously investigated by single-molecule dedicated studies, missing the holistic view that results from our analysis.

The bovine milk proteome: cherishing, nourishing and fostering molecular complexity. An interactomics and functional overview

Bovine milk represents an essential source of nutrients for lactating calves and a key raw material for human food preparations. A wealth of data are present in the literature dealing with massive proteomic analyses of milk fractions and independent targeted studies on specific groups of proteins, such as caseins, globulins, hormones and cytokines. In this study, we merged data from previous investigations to compile an exhaustive list of 573 non-redundant annotated protein entries. This inventory was exploited for integrated in silico studies, including functional GO term enrichment (FatiGO/Babelomics), multiple pathway and network analyses. As expected, most of the milk proteins were grouped under pathways/networks/ontologies referring to nutrient transport, lipid metabolism and objectification of the immune system response. Notably enough, another functional family was observed as the most statistically significant one, which included proteins involved in the induction of cellular proliferation processes as well as in anatomical and haematological system development. Although the latter function for bovine milk proteins has long been postulated, studies reported so far mainly focused on a handful of molecules and missed the whole overview resulting from an integrated holistic analysis. A preliminary map of the bovine milk proteins interactome was also built up, which will be refined in future as result of the widespread use of quantitative methods in protein interaction studies and consequent reduction of false-positives within associated databases.

Human growth hormone-transferrin fusion protein for oral delivery in hypophysectomized rats

Transferrin (Tf)-based recombinant fusion protein approach was investigated to achieve oral delivery for human growth hormone (hGH). Plasmid constructs expressing the fusion proteins were established by fusing coding sequences of both hGH and Tf in frame. Fusion proteins were produced in serum free media by transient transfection of human embryonic kidney HEK293 cells. The SDS-PAGE analysis of conditioned media showed that fusion proteins expressed at high purity with a 100 kDa molecular weight; the Western blot analysis with anti-hGH and anti-Tf antibodies verified the identity of fusion proteins. The Nb2 cell proliferation and Caco-2 cell Tf receptor (TfR) binding assays demonstrated that fusion proteins retained bioactivity of both hGH and Tf, respectively. A helical linker was inserted as spacer between hGH- and Tf-domain to enhance the bioactivity and the yield of the fusion protein. Two fusion proteins, hGH-Tf (GT) and hGH-(H4)(2)-Tf (GHT) were obtained and assessed in hGH-deficient hypophysectomized rats for in vivo biological activity. Results from seven-day subcutaneous dosing (1.25mg/kg/day) demonstrated that both GT and GHT fusion proteins were bioactive in vivo, comparable to native hGH. However, only the GHT, but not GT, fusion protein promoted a modest but statistically significant weight gain after oral dosing with 12.5mg/kg/day.

Transmission ratio distortion at the growth hormone gene (GH1) in bovine preimplantation embryos: An in vitro culture-induced phenomenon?

The growth hormone gene (GH1) and its polypeptide product (GH) have a crucial role in reproduction, embryogenesis and general development. A polymorphism present in the fifth exon of the bovine GH1 gene (GH1 p.Leu127Val) has been associated with GH release and milk production in cattle. The objective of the present study was to examine the genotype frequencies of the GH1 p.Leu127Val polymorphism in bovine blastocysts produced in vitro and in vivo to determine if allelic variation of the GH1 gene affects embryo development and survival. A heterozygous (p.Leu127/Val127) sire was used for in vitro fertilization of oocytes of unknown maternal genotype (n = 104) and known maternal genotype (n = 115). PCR amplification and genotyping of the GH1 gene from Day 8 blastocysts derived from these fertilized oocytes demonstrated that there was significant over-representation from the expected Mendelian ratio of GH1 p.Leu127/Leu127 homozygotes from oocytes of known maternal genotype (P = 0.006). Contrary to this, analysis of in vivo-produced bovine blastocysts of known parental GH1 genotype (n = 69) did not reveal an overrepresentation of GH1 p.Leu127/Leu127 homozygotes. These results suggest that developing in vitro-produced embryos are exposed to a selection process, probably due to a less favorable culture environment, that acts to increase the number of GH1 p.Leu127/Leu127 homozygotes, thereby giving rise to the observed transmission ratio distortion (TRD) of GH1 genotypes when compared to in vivo produced embryos.

Acromegaly: re-thinking the cancer risk

Acromegaly is characterized by sustained elevation of circulating growth hormone (GH) and insulin-like growth factor I (IGF-I), and is clearly associated with increased morbidity and overall mortality mainly due to cardiovascular, metabolic, and respiratory diseases. Although cancer-related mortality varies widely amongst retroperspective studies, it appears to be consistently elevated mainly in patients with uncontrolled disease. We review individual tumor types including neoplasms of the colon, breast, prostate, and thyroid where in vitro, animal studies, and studies in non-acromegalic cancer patients have established a role for the GH/IGF-I axis in tumor progression and possibly initiation. We highlight deficiencies in data in acromegalic patients where the evidence is less convincing. Instead, we explore the hypothesis that acromegaly, independent of hormone secretion, is a disease that heralds genetic and/or epigenetic alterations predisposing to cancer risk elsewhere.

Growth Hormone (GH) receptor C.1319 G>T polymorphism, but not exon 3 retention or deletion is associated with better first-year growth response to GH therapy in patients with GH deficiency

We investigated possible influences of single nucleotide polymorphisms (SNPs) on first-year growth velocity in response to growth hormone (GH) therapy in GH-deficient (GHD) children. We recruited a total of 154 GHD prepubertal children who had undergone GH therapy for 1 y. To exclude the possibility that the genotype/allele variants influenced the height of GHD patients, we studied the same gene polymorphisms in 208 familial short stature (FSS) patients and 100 normal control individuals. In the present study, the first-year growth velocities of GHD patients treated with GH were measured and then compared with the allelic frequencies of various SNP of genes involved in the GH-insulin-like growth factor-I (IGF-I) axis. Only c.1319 G>T of the GH receptor (GHR) gene showed significant correlation with first-year growth velocity (p = 0.02). However, the genetic frequency of the c.1319 G>T polymorphism of GHD did not correlate with FSS and normal controls. Therefore, the c.1319 G>T polymorphism does not influence the height of individuals but can affect the therapeutic efficacy of GH in GHD patients. Moreover, the GHR c.1319 T allele showed higher transcriptional activity and stronger signal transducers and activators of transcription (STAT)-5 Tyr694 phosphorylation. Based on these findings, we conclude that the GHRc.1319 T allele is associated with the therapeutic efficacy of GH replacement therapy.

Growth hormone increases Stat5 and Stat1 expression in lactating goat mammary gland: a specific effect compared to milking frequency

In ruminants, both milking frequency and exogenous GH treatment affect milk production. In a previous report, we showed that the modulation of milk yield due to variations in milking frequency and GH treatment was associated with variations in mammary cell numbers. The aim of this study was to clarify the different mechanisms governing the effects of GH treatment and milking frequency on signal transducer and activator of transcription (Stat) expression and activation, and on the expression of genes involved in mammary cell differentiation. Six Saanen goats in late lactation were milked once daily from one half-udder and thrice daily from the other half-udder for 23 days. At the same time, the goats were divided into two groups: GH-treated versus control group. After slaughter of the goats, soluble mammary proteins and RNA were extracted from half-udder samples. Levels of Stat5, Stat3 and Stat1 proteins and the Stat activation by phosphorylation were analysed by Western blot. The amounts of Stat5 protein and mRNA were significantly elevated by GH treatment in all half-udders (milked once or thrice daily). Positive Stat5 immunoreactivity was principally localised in the nuclei of epithelial cells, with heterogeneous intensity between cells. No significant changes in Stat5 protein phosphorylation levels were observed. Furthermore, GH significantly increased Stat1 protein levels, without modifying the level of Stat1 tyrosine phosphorylation, and tended to reduce the abundance of Stat3 protein. In contrast, milking frequency failed to modify Stat gene expression, protein level and phosphorylation. Using Northern blot, we showed that levels of kappa casein and prolactin receptor mRNA were not affected by the treatments. These observations suggest that GH probably acts specifically on mammary cells by regulating the expression of Stat1, 3 and 5. In contrast, milking frequency does not act through this regulatory pathway.

Growth factors in inflammatory bowel disease: the actions and interactions of growth hormone and insulin-like growth factor-I

Multiple growth hormones (GHs) and factors are relevant to inflammatory bowel disease (IBD) due to their trophic effects on epithelial cells to promote mucosal integrity, renewal, and repair, on mesenchymal cells to promote wound healing, and on intestinal immune cells to modulate inflammation. The anabolic effects of GHs and factors outside the intestine are relevant to minimizing nutritional insufficiency, catabolic state, and the inability to maintain body weight due to inflammation-induced malabsorption. GHs and factors can, however, have a dual role, whereby trophic effects can be beneficial but, if excessive, can promote complications including the increased risk of intestinal tumors/adenocarcinoma and fibrosis. This review focuses on GH and insulin-like growth factor (IGF-I), for which evidence suggests such a dual role may exist. The actions of GH and IGF-I on the healthy intestine are compared with effects during intestinal inflammation or associated complications. Interactions between these growth factors and others relevant to IBD are considered. The role of the newly discovered suppressors of cytokine signaling proteins, which may dictate the balance between beneficial and excessive actions of GH and IGF-I, is also addressed.

The growth hormone-binding protein is a location-dependent cytokine receptor transcriptional enhancer

In the rat, a growth hormone-binding protein (GHBP) exists that is derived from the growth hormone (GH) receptor gene by an alternative mRNA splicing mechanism such that the transmembrane and intracellular domains of the GH receptor are replaced by a hydrophilic carboxyl terminus. In isolation, the GHBP is inactive, although it does compete with the receptor for ligand binding in the extracellular space and therefore inhibits the cellular response to GH. The GHBP is also located intracellularly and is translocated to the nucleus upon ligand stimulation. We show here that endogenously produced GHBP, in contrast to exogenous GHBP, was able to enhance the STAT5-mediated transcriptional response to GH. Interestingly, when the GHBP was targeted constitutively to the nucleus by the addition of the nuclear localization sequence of the SV40 large T antigen, greater enhancement of STAT5-mediated transcription was obtained. The transcriptional enhancement did not require GH per se and was not specific to the GH receptor, since similar enhancement of STAT5-mediated transcription by nuclear localized GHBP was obtained with specific ligand stimulation of both prolactin and erythropoietin receptors. Thus, the GHBP exerts divergent effects on STAT5-mediated transcription depending on its cellular location. The use of a soluble cytokine receptor as a location-dependent transcriptional enhancer and the ligand-independent involvement of the extracellular domain of a polypeptide ligand receptor in intracellular signal transduction provide additional novel mechanisms of transcriptional control.

Persistent neurogenesis in the teleost retina: evidence for regulation by the growth-hormone/insulin-like growth factor-I axis

Based on results from previous studies (J. Comp. Neurol. 394 (1998) 386, 395), it was hypothesized that the persistent neurogenesis in the retina of teleost fish is modulated by insulin-like growth factor-I (IGF-I), which, in turn, is regulated by growth hormone (GH). Two approaches were undertaken to test this hypothesis. First, a variety of techniques were used to determine if IGF-I and the IGF-I receptor (IGF-IR) are expressed in the retina. Second, GH was injected into animals to stimulate IGF-I synthesis in target tissues, and IGF-I expression and cell proliferation in the retina was quantitatively assayed. Reverse transcriptase-polymerase chain reaction, screening a retinal cDNA library and Northern analysis showed that genes encoding IGF-I and IGF-IR are expressed in the retina of goldfish. In situ hybridization showed that IGF-IR is expressed by retinal progenitors and all differentiated retinal neurons. Intraperitoneal injections of GH elevate IGF-I mRNA levels in the liver, brain and retina and produce a dose-dependent change in the proliferation of stem cells and progenitors in the retina. These data indicate that the principal components of the IGF-I signaling cascade are present in the retinas of teleosts, and we suggest these elements mediate the persistent, growth-associated neurogenesis in this tissue.

The somatostatin analog octreotide inhibits GH-stimulated, but not IGF-I-stimulated, bone growth in hypophysectomized rats

IGF-I mediates growth-promoting actions of GH. In the present study we investigated whether the somatostatin analog octreotide blunts the stimulatory effects of GH and/or IGF-I on bone growth in hypophysectomized rats infused for 6 d with vehicle, GH, or IGF-I. We found that octreotide significantly suppressed the GH-induced rise in liver IGF-I mRNA (-27%) and peptide (-32%) and the serum IGF-I level (-26%) and concomitantly inhibited GH-stimulated, but not IGF-I-stimulated, body weight gain (-31%), tibial epiphyseal width (-14%), and bone growth rate (-24%). Furthermore, octreotide significantly reduced the GH-induced increase in the number of IGF-I immunoreactive chondrocytes in all layers (except in the upper hypertrophic zone) of the tibial growth plate cartilage (P < 0.0001 for stem cell and proliferative zone; P < 0.0005 for lower hypertrophic zone). These findings demonstrate that octreotide does not interfere with IGF-I action, but does interfere with local GH-stimulated IGF-I production in the growth plate. Thus, besides inhibiting pituitary GH secretion, octreotide exerts inhibitory peripheral effects on GH-stimulated longitudinal bone growth.

Cloning and characterisation of the GH gene from the common dolphin (Delphinus delphis)

The sequence of growth hormone (GH) is generally strongly conserved in mammals, but episodes of rapid change occurred during the evolution of primates and artiodactyls, when the rate of GH evolution apparently increased at least 50-fold. As a result, the sequences of human and ruminant GHs differ substantially from those of other non-primate GHs. Recent molecular studies have suggested that cetaceans are closely related to artiodactyls and may be deeply nested within the artiodactyl phylogenetic tree. To extend the knowledge of GH in Cetartiodactyla (Artiodactyla plus Cetacea), we have cloned and characterised a single GH gene from the common dolphin (Delphinus delphis), using genomic DNA and a polymerase chain reaction technique. As in other mammals, the dolphin GH gene comprises five exons and four introns. The deduced sequence for the mature dolphin GH differs from that of pig at two residues only, showing that the apparent burst of rapid evolution of GH occurred largely after the separation of cetaceans and ruminants.

The growth hormone-insulin-like growth factor-I axis and colorectal cancer

The growth hormone (GH)-insulin-like growth factor (IGF)-I axis is an important modulator of growth and development, but in addition to their classical role as endocrine hormones, its components also regulate a wide range of biological functions through paracrine and autocrine mechanisms. Their potent mitogenic and anti-apoptotic effects play a critical role in the regulation of rapidly renewing epithelial cell populations such as those found in the colon. Recent evidence suggests an association between inappropriate regulation of the GH-IGF-I axis and the development of colorectal cancer. However, the molecular mechanisms and signalling pathways responsible are only beginning to be unravelled, as are the relative contributions of the endocrine and autocrine or paracrine effects.

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.

Neuropeptide Y has a central inhibitory action on the hypothalamic-pituitary-thyroid axis

Recent evidence suggests that neuropeptide Y (NPY), originating in neurons in the hypothalamic arcuate nucleus, is an important mediator of the effects of leptin on the central nervous system. As these NPY neurons innervate hypophysiotropic neurons in the hypothalamic paraventricular nucleus (PVN) that produce the tripeptide, TRH, we raised the possibility that NPY may be responsible for resetting of the hypothalamic-pituitary-thyroid (HPT) axis during fasting. To test this hypothesis, the effects of intracerebroventricularly administered NPY on circulating thyroid hormone levels and proTRH messenger RNA in the PVN were studied by RIA and in situ hybridization histochemistry, respectively. NPY administration suppressed circulating levels of thyroid hormone (T(3) and T(4)) and resulted in an inappropriately normal or low TSH. These alterations were associated with a significant suppression of proTRH messenger RNA in the PVN, indicating that NPY infusion had resulted in a state of central hypothyroidism. Similar observations were made in NPY-infused animals pair fed to the vehicle-treated controls. These data are reminiscent of the effect of fasting on the thyroid axis and indicate that NPY may play a major role in the inhibition of HPT axis during fasting.

Control of gene expression by growth hormone in liver: key role of a network of transcription factors

Growth hormone (GH) controls gene expression in liver. Recent work suggests that this can result in part from the stimulation by GH of the synthesis of liver-specific transcription factors, one of which is HNF-6. The liver-specific factors HNF-4 and C/EBP alpha respectively stimulate and inhibit transcription of the hnf 6 gene. Upon GH stimulation, the affinity of HNF-4 for the hnf 6 promoter is increased and the binding of C/EBP alpha is decreased. GH therefore controls hnf 6 by a combination of stimulatory and derepressive mechanisms. On the other hand, HNF-6 stimulates transcription of the hnf 3beta and hnf 4 genes, the stimulation of hnf 4 resulting most likely from the GH-induced increase in HNF-6 concentration. We conclude that in liver GH is likely to control the synthesis of a whole set of proteins whose genes are regulated by a GH-sensitive network of transcription factors, which regulate each other by feed-back and autoregulatory loops.