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

Liver impact of growth hormone (GH) intermittent treatment during the growth period in mice

Liver impact of prolonged GH-treatment given to non-GH-deficient growing mice between the third and eighth week of life was evaluated in both sexes. Tissues were collected 6 h after last dose or four weeks later. Somatometric, biochemical, histological, immunohistochemical, RT-qPCR and immunoblotting determinations were performed. Five-week GH intermittent administration induced body weight gain and body and bone length increase, augmented organ weight, higher hepatocellular size and proliferation, and increased liver IGF1 gene expression. Phosphorylation of signaling mediators and expression of GH-induced proliferation-related genes was decreased in GH-treated mice liver 6h after last injection, reflecting active sensitization/desensitization cycles. In females, GH elicited EGFR expression, associated to higher EGF-induced STAT3/5 phosphorylation. Four weeks after treatment, increased organ weight concomitant to body weight gain was still observed, whereas hepatocyte enlargement reverted. However, basal signaling for critical mediators was lower in GH-treated animals and in male controls compared to female ones, suggesting signaling declination.

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

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

Differential effects of androgens and estrogens over cellular GH sensitivity in HEPG2 cells

Testosterone and estrogen concentrations progressively increase during puberty, and in association with growth hormone (GH), lead to the increase in height velocity known as the pubertal growth spurt. Very limited information is available however, regarding the possible effects of sex steroids over GH cellular sensitivity.

OBJECTIVE

To investigate the effects of different concentrations of testosterone, estradiol and dihydrotestosterone over the GH intracellular signaling pathway.

METHODS

We evaluated the effects of these sex steroids on the nuclear phosphorylation of STAT5b and IGF-1 expression, in HEPG2 human hepatoma cells. In addition, we studied whether Tamoxifen (TAM), can modulate these effects.

RESULTS

The highest concentration of T tested (10 ng/mL) co-incubated with a fixed concentration of GH (40 ng/mL) increased nuclear STAT5b phosphorylation compared with GH alone (1.34 ± 0.2 vs 0.6 ± 0.09 AU; *p < 0.05), as well as IGF-1 expression (0.6 ± 0.03 vs 0.32 ± 0.05 AU; *p < 0.05). This effect was not observed with lower concentrations of T tested (1 and 5 ng/mL). A similar increase in nuclear STAT5b phosphorylation was observed with the lowest concentration of E₂ tested (20 pg/mL), co-incubated with the same fixed concentration of GH (3.6 ± 0.5 vs 1.28 ± 0.33 AU; *p < 0.05). This effect was also associated with an increase in IGF-1 expression (0.73 ± 0.02 vs 0.39 ± 0.04 AU; *p < 0.05). These results were not observed with higher concentrations of E₂ tested (75 and 200 pg/mL). DHT at concentrations of 0.1, 0.25 and 0.5 ng/mL, co-stimulated with GH, did not change cytoplasmic STAT5b phosphorylation, nuclear STAT5b or IGF-1 expression. In addition, the co-incubation of TAM with the highest concentration of T tested (10 ng/mL) and GH (40 ng/mL) did not change cytoplasmic, nuclear pSTAT5 levels or IGF-1 expression.

CONCLUSIONS

T and E₂ potentiate the GH signaling pathway in a concentration-dependent fashion. The observation that the non-aromatizable androgen dihydrotestosterone does not stimulate this pathway, and that the effects of T are blocked with TAM, suggests that the effects of T over the GH signaling pathway appear to be mediated by estrogen.

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.

Regulation of gene expression by growth hormone

Growth hormone (GH) plays a pivotal role in many physiological processes in humans, and in other mammalian and non-mammalian vertebrate species, through actions on somatic growth, tissue development and repair, and intermediary metabolism. This review will focus on mechanisms of GH actions on gene expression, primarily from the perspective of the genes that encode proteins stimulated by GH to regulate somatic growth, especially insulin-like growth factor 1 (IGF-I), but also others that are induced or repressed by GH. Topics to be discussed will include a brief overview of GH-mediated signal transduction pathways and how these cascades alter the functions of responsive transcription factors, with a specific focus on STAT5B, a key member of the signal transducers and activators of transcription family, characterization of essential GH-regulated genes, and elucidation of mechanisms of their regulation from biochemical, genetic, and genomic perspectives.

Growth plate alterations in chronic kidney disease

Growth retardation is a major feature of chronic kidney disease (CKD) of onset in infants or children and is associated with increased morbidity and mortality. Several factors have been shown to play a causal role in the growth impairment of CKD. All these factors interfere with growth by disturbing the normal physiology of the growth plate of long bones. To facilitate the understanding of the pathogenesis of growth impairment in CKD, this review discusses cellular and molecular alterations of the growth plate during uremia, including structural and dynamic changes of chondrocytes, alterations in their process of maturation and hypertrophy, and disturbances in the growth hormone signaling pathway.

Bromodomain and Extraterminal Inhibition by JQ1 Produces Divergent Transcriptional Regulation of Suppressors of Cytokine Signaling Genes in Adipocytes

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway has cell-specific functions. Suppressors of cytokine signaling (SOCS) proteins are negative-feedback regulators of JAK-STAT signaling. STAT5 plays a significant role in adipocyte development and function, and bromodomain and extraterminal (BET) proteins may be involved in STAT5 transcriptional activity. We treated 3T3-L1 adipocytes with the BET inhibitor JQ1 and observed that growth hormone (GH)-induced expression of 2 STAT5 target genes from the SOCS family, Socs3 and Cish, were inversely regulated (increased and decreased, respectively) by BET inhibition. Chromatin immunoprecipitation analyses revealed that changes in STAT5 binding did not correlate with gene expression changes. GH promoted the recruitment of the BET protein BRD2 to the Cish, but not Socs3, promoter. JQ1 treatment ablated this effect as well as the GH-induced binding of ribonucleic acid polymerase II (RNA Pol II) to the Cish transcription start site. BRD2 knockdown also suppressed GH induction of Cish, further supporting the role of BRD2 in Cish transcriptional activation. In contrast, JQ1 increased the binding of activated Pol II to the Socs3 coding region, suggesting enhanced messenger RNA (mRNA) elongation. Our finding that JQ1 transiently reduced the interaction between the positive transcription elongation factor (P-TEFb) and its inhibitor hexamethylene bis-acetamide inducible 1 (HEXIM1) is consistent with a previously described off-target effect of JQ1, whereby P-TEFb becomes more available to be recruited by genes that do not depend on BET proteins for activating transcription. These results demonstrate substantially different transcriptional regulation of Socs3 and Cish and suggest distinct roles in adipocytes for these 2 closely related proteins.

Insulin-like growth factor (IGF) signaling in T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer, characterized by an uncontrolled expansion and accumulation of T-cell progenitors. During leukemic progression, immature T cells grow abnormally and occupy the bone marrow compartment, thereby interfering with the production of normal blood cells. Pediatric T-ALL is curable with intensive chemotherapy, but there are significant, long-term side effects and ~20% of patients suffer relapse for which there are limited treatment options. Adult T-ALL in contrast is largely incurable and refractory/relapsed disease is common despite multi-agent chemotherapy (5-year overall survival of ~40%), and thus new therapeutic targets are needed. We have reported previously on the role of insulin-like growth factor (IGF) signaling in T-ALL, and shown that it exerts potent phenotypes in both leukemia stem cell and bulk tumor cell populations. Modulators of IGF signaling may thus prove useful in improving outcomes in patients with T-ALL. In this review, we summarize the most recent findings relating to IGF signaling in T-ALL and outline therapeutic options using clinically relevant IGF signaling modulators.

Characterizing the complexity of Australian marsupial insulin-like growth factor 1 genes

Insulin-like growth factor 1 (IGF1) actions are essential for somatic growth and tissue repair. IGF1 gene regulation is controlled by many inputs, with growth hormone playing a major role. In most mammals, the 6-exon IGF1/Igf1 gene produces multiple transcripts via independent activity of its promoters plus alternative RNA splicing and differential polyadenylation. Here, by analyzing public genomic and RNA-sequencing repositories, I have characterized three Australian marsupial IGF1 genes. Koala, Tasmanian devil, and wallaby IGF1 are more complicated than other mammals, as they contain up to 11 exons, and encode multiple mRNAs and predicted protein precursors, including potentially novel isoforms. Moreover, just two of multiple growth hormone-stimulated transcriptional enhancers found in other IGF1/Igf1 loci are detected in these species. These observations define Australian marsupial IGF1 genes and demonstrate that comprehensive interrogation of genomic and RNA-sequencing resources is an effective strategy for characterizing genes and gene expression in otherwise experimentally intractable organisms.

Insulinlike Growth Factor 1 Gene Variation in Vertebrates

IGF1-a small, single-chain, secreted peptide in mammals-is essential for normal somatic growth and is involved in a variety of other physiological and pathophysiological processes. IGF1 expression appears to be controlled by several different signaling mechanisms in mammals, with GH playing a key role by activating an inducible transcriptional pathway via the Jak2 protein kinase and the Stat5b transcription factor. Here, to understand aspects of Igf1 gene regulation over a substantially longer timeline than is discernible in mammals, Igf1 genes have been examined in 21 different nonmammalian vertebrates representing five different classes and ranging over ∼500 million years of evolutionary history. Parts of vertebrate Igf1 genes resemble components found in mammals. Conserved exons encoding the mature IGF1 protein are detected in all 21 species studied and are separated by a large intron, as seen in mammals; the single promoter contains putative regulatory elements that are similar to those functionally mapped in human IGF1 promoter 1. In contrast, GH-activated Stat5b-binding enhancers found in mammalian IGF1 loci are completely absent, there is no homolog of promoter 2 or exon 2 in any nonmammalian vertebrate, and different types of "extra" exons not present in mammals are found in birds, reptiles, and teleosts. These data collectively define properties of Igf1 genes and IGF1 proteins that were likely present in the earliest vertebrates and support the contention that common structural and regulatory features in Igf1 genes have a long evolutionary history.

Mecasermin in Insulin Receptor-Related Severe Insulin Resistance Syndromes: Case Report and Review of the Literature

Mutations in the insulin receptor (INSR) gene underlie rare severe INSR-related insulin resistance syndromes (SIR), including insulin resistance type A, Rabson–Mendenhall syndrome and Donohue syndrome (DS), with DS representing the most severe form of insulin resistance. Treatment of these cases is challenging, with the majority of DS patients dying within the first two years of life. rhIGF-I (mecasermin) has been reported to improve metabolic control and increase lifespan in DS patients. A case report and literature review were completed. We present a case involving a male patient with DS, harbouring a homozygous mutation in the INSR gene (c.591delC). Initial rhIGF-I application via BID (twice daily) injection was unsatisfactory, but continuous subcutaneous rhIGF-I infusion via an insulin pump improved weight development and diabetes control (HbA1c decreased from 10 to 7.6%). However, our patient died at 22 months of age during the course of a respiratory infection in in Libya. Currently available data in the literature comprising more than 30 treated patients worldwide seem to support a trial of rhIGF-I in SIR. rhIGF-I represents a treatment option for challenging SIR cases, but careful consideration of the therapeutic benefits and the burden of the disease is warranted. Continuous application via pump might be advantageous compared to single injections.

Inflammation and age-associated skeletal muscle deterioration (sarcopaenia)

Ageing is accompanied by chronic inflammatory responses due to elevated circulatory inflammatory cytokine production. Several inflammatory cytokines have been shown to be responsible for a decrease in muscle mass. However, little is known about the possible relationship between inflammation and sarcopaenia. This review aims to summarise the existing evidence about inflammation and sarcopaenia. Sarcopaenia is defined as an age-related decrease of muscle mass and/or muscle strength; it is caused by multiple factors, such as skeletal muscle atrophy, neuromuscular junction degeneration, hormone imbalance, cytokine imbalance, protein synthesis and proteolysis. Several inflammatory cytokines have been considered to promote muscle loss; C-reactive protein levels are significantly upregulated in sarcopaenia and sarcopenic obesity, and high levels of interleukin-6 are associated with reduced muscle mass and muscle strength (the administration of interleukin-6 could lead to a reduction in muscle mass). Up-regulation of tumour necrosis factor-α expression is also related to the development of sarcopaenia. Signalling pathways, such as protein kinase B/mammalian target of rapamycin, Janus kinase/signal transducer and activator of transcription-5 and signal transducer and activator of transcription 3 signalling, involved in muscle metabolism are regulated by insulin-like growth factor-1, tumour necrosis factor-α and interleukin-6 respectively. In conclusion, the inflammatory cytokines produced during chronic inflammation due to ageing, may influence their respective related pathways, thus leading to age-related muscle deterioration.

The translational potential of this article

This review can provide more information for sarcopaenia medicine research in terms of anti-inflammation therapy.

Cryptochromes regulate IGF-1 production and signaling through control of JAK2-dependent STAT5B phosphorylation

The circadian clock regulates IGF-1 production and signaling through proteins called cryptochromes, which regulate the activity of transcriptional factor STAT5B and control mouse body and organ size.

Insulin-like growth factor (IGF) signaling plays an important role in cell growth and proliferation and is implicated in regulation of cancer, metabolism, and aging. Here we report that IGF-1 level in blood and IGF-1 signaling demonstrates circadian rhythms. Circadian control occurs through cryptochromes (CRYs)—transcriptional repressors and components of the circadian clock. IGF-1 rhythms are disrupted in Cry-deficient mice, and IGF-1 level is reduced by 80% in these mice, which leads to reduced IGF signaling. In agreement, Cry-deficient mice have reduced body (∼30% reduction) and organ size. Down-regulation of IGF-1 upon Cry deficiency correlates with reduced Igf-1 mRNA expression in the liver and skeletal muscles. Igf-1 transcription is regulated through growth hormone–induced, JAK2 kinase–mediated phosphorylation of transcriptional factor STAT5B. The phosphorylation of STAT5B on the JAK2-dependent Y699 site is significantly reduced in the liver and skeletal muscles of Cry-deficient mice. At the same time, phosphorylation of JAK2 kinase was not reduced upon Cry deficiency, which places CRY activity downstream from JAK2. Thus CRYs link the circadian clock and JAK-STAT signaling through control of STAT5B phosphorylation, which provides the mechanism for circadian rhythms in IGF signaling in vivo.

Mitogenic signaling pathways in the liver of growth hormone (GH)-overexpressing mice during the growth period

Growth hormone (GH) is a pleiotropic hormone that triggers STATs, ERK1/2 and Akt signaling, related to cell growth and proliferation. Transgenic mice overexpressing GH present increased body size, with a disproportionate liver enlargement due to hypertrophy and hyperplasia of the hepatocytes. We had described enhanced mitogenic signaling in liver of young adult transgenic mice. We now evaluate the activation of these signaling cascades during the growth period and relate them to the morphological alterations found. Signaling mediators, cell cycle regulators and transcription factors involved in cellular growth in the liver of GH-overexpressing growing mice were assessed by immunoblotting, RT-qPCR and immunohistochemistry. Hepatocyte enlargement can be seen as early as 2-weeks of age in GH-overexpressing animals, although it is more pronounced in young adults. Levels of cell cycle mediators PCNA and cyclin D1, and transcription factor c-Jun increase with age in transgenic mice with no changes in normal mice, whereas c-Myc levels are higher in 2-week-old transgenic animals and cyclin E levels decline with age for both genotypes. STAT3, Akt and GSK3 present higher activation in the adult transgenic mice than in the growing animals, while for c-Src and mTOR, phosphorylation in GH-overexpressing mice is higher than in control siblings at 4 and 9 weeks of age. No significant changes are observed for ERK1/2, neither by age or genotype. Thus, the majority of the mitogenic signaling pathways are gradually up-regulated in the liver of GH-transgenic mice, giving rise to the hepatic morphological changes these mice exhibit.

Identifying growth hormone-regulated enhancers in the Igf1 locus

Growth hormone (GH) plays a central role in regulating somatic growth and in controlling multiple physiological processes in humans and other vertebrates. A key agent in many GH actions is the secreted peptide, IGF-I. As established previously, GH stimulates IGF-I gene expression via the Stat5b transcription factor, leading to production of IGF-I mRNAs and proteins. However, the precise mechanisms by which GH-activated Stat5b promotes IGF-I gene transcription have not been defined. Unlike other GH-regulated genes, there are no Stat5b sites near either of the two IGF-I gene promoters. Although dispersed GH-activated Stat5b binding elements have been mapped in rodent Igf1 gene chromatin, it is unknown how these distal sites might function as potential transcriptional enhancers. Here we have addressed mechanisms of regulation of IGF-I gene transcription by GH by generating cell lines in which the rat Igf1 chromosomal locus has been incorporated into the mouse genome. Using these cells we find that physiological levels of GH rapidly and potently activate Igf1 gene transcription while stimulating physical interactions in chromatin between inducible Stat5b-binding elements and the Igf1 promoters. We have thus developed a robust experimental platform for elucidating how dispersed transcriptional enhancers control Igf1 gene expression under different biological conditions.

Differential effects of STAT proteins on growth hormone-mediated IGF-I gene expression

Growth hormone (GH) plays a key role regulating somatic growth and in controlling metabolism and other physiological processes in humans and other animal species. GH acts by binding to the extracellular part of its transmembrane receptor, leading to induction of multiple intracellular signal transduction pathways that culminate in changes in gene and protein expression. A key agent in GH-stimulated growth is the latent transcription factor signal transducer and activator of transcription (STAT) 5B, one of four STAT proteins induced by the GH receptor in cultured cells and in vivo. As shown by genetic and biochemical studies, GH-activated STAT5B promotes transcription of the gene encoding the critical growth peptide, insulin-like growth factor-I (IGF-I), and natural null mutations of STAT5B in humans lead to growth failure accompanied by diminished IGF-I expression. Here we have examined the possibility that other GH-activated STATs can enhance IGF-I gene transcription, and thus potentially contribute to GH-regulated somatic growth. We find that human STAT5A is nearly identical to STAT5B in its biochemical and functional responses to GH but that STAT1 and STAT3 show a weaker profile of in vitro binding to STAT DNA elements from the IGF-I gene than STAT5B, and are less potent inducers of gene transcription through these elements. Taken together, our results offer a molecular explanation for why STAT5B is a key in vivo mediator of GH-activated IGF-I gene transcription and thus of GH-regulated somatic growth.

Reciprocal occupancy of BCL6 and STAT5 on Growth Hormone target genes: contrasting transcriptional outcomes and promoter-specific roles of p300 and HDAC3

Expression of the Growth Hormone (GH)-stimulated gene Socs2 (Suppressor of Cytokine Signaling 2) is mediated by the transcription activator STAT5 (Signal Transducer and Activator of Transcription 5) and the transcription repressor BCL6 (B-Cell Lymphoma 6). ChIP-Sequencing identified Cish (Cytokine-Inducible SH2-containing protein) and Bcl6 as having similar patterns of reciprocal occupancy by BCL6 and STAT5 in response to GH, though GH stimulates Cish and inhibits Bcl6 expression. The co-activator p300 occupied Socs2, Cish and Bcl6 promoters, and enhanced STAT5-mediated activation of Socs2 and Cish. In contrast, on Bcl6, p300 functioned as a repressor and inhibited in conjunction with STAT5 or BCL6. The co-repressor HDAC3 (Histone deacetylase 3) inhibited the Socs2, Cish and Bcl6 promoters in the presence of STAT5. Thus transcriptional outcomes on GH-regulated genes occupied by BCL6 and STAT5 are determined in a promoter-specific fashion by co-regulatory proteins which mediate the distinction between activating and repressive transcription factors.

The GH-IGF-SST system in hepatocellular carcinoma: biological and molecular pathogenetic mechanisms and therapeutic targets

Hepatocellular carcinoma (HCC) is the sixth most common malignancy worldwide. Different signalling pathways have been identified to be implicated in the pathogenesis of HCC; among these, GH, IGF and somatostatin (SST) pathways have emerged as some of the major pathways implicated in the development of HCC. Physiologically, GH-IGF-SST system plays a crucial role in liver growth and development since GH induces IGF1 and IGF2 secretion and the expression of their receptors, involved in hepatocytes cell proliferation, differentiation and metabolism. On the other hand, somatostatin receptors (SSTRs) are exclusively present on the biliary tract. Importantly, the GH-IGF-SST system components have been indicated as regulators of hepatocarcinogenesis. Reduction of GH binding affinity to GH receptor, decreased serum IGF1 and increased serum IGF2 production, overexpression of IGF1 receptor, loss of function of IGF2 receptor and appearance of SSTRs are frequently observed in human HCC. In particular, recently, many studies have evaluated the correlation between increased levels of IGF1 receptors and liver diseases and the oncogenic role of IGF2 and its involvement in angiogenesis, migration and, consequently, in tumour progression. SST directly or indirectly influences tumour growth and development through the inhibition of cell proliferation and secretion and induction of apoptosis, even though SST role in hepatocarcinogenesis is still opened to argument. This review addresses the present evidences suggesting a role of the GH-IGF-SST system in the development and progression of HCC, and describes the therapeutic perspectives, based on the targeting of GH-IGF-SST system, which have been hypothesised and experimented in HCC.

Modulation of the somatotropic axis in periparturient dairy cows

This review focuses on modulation of growth hormone (GH) and its downstream actions on periparturient dairy cows undergoing physiological and metabolic adaptations. During the periparturient period, cows experience a negative energy balance implicating that the feed intake does not meet the total energy demand for the onset of lactation. To regulate this metabolic condition, key hormones of somatotropic axis such as GH, IGF-I and insulin must coordinate adaptations required for the preservation of metabolic homeostasis. The hepatic GHR1A transcript and GHR protein are reduced at parturition, but recovers on postpartum. However, plasma IGF-I concentration remains low even though hepatic abundance of the GHR and IGF-I mRNA return to pre-calving value. This might be caused by alternation in IGFBPs and ALS genes, which consequently affect the plasma IGF-I stability. Plasma insulin level declines in a parallel manner with the decrease in plasma IGF-I after parturition. Increased GH stimulates the lipolytic effects and hepatic glucose synthesis to meet the energy requirement for mammary lactose synthesis, suggesting that GH antagonizes insulin-dependent glucose uptake and attenuates insulin action to decrease gluconeogenesis.

Minireview: mechanisms of growth hormone-mediated gene regulation

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

Body size regulation and insulin-like growth factor signaling

How animals achieve their specific body size is a fundamental, but still largely unresolved, biological question. Over the past decades, studies on the insect model system have provided some important insights into the process of body size determination and highlighted the importance of insulin/insulin-like growth factor signaling. Fat body, the Drosophila counterpart of liver and adipose tissue, senses nutrient availability and controls larval growth rate by modulating peripheral insulin signaling. Similarly, insulin-like growth factor I produced from liver and muscle promotes postnatal body growth in mammals. Organismal growth is tightly coupled with the process of sexual maturation wherein the sex steroid hormone attenuates body growth. This review summarizes some important findings from Drosophila and mammalian studies that shed light on the general mechanism of animal size determination.

Biochemical characterization of diverse Stat5b-binding enhancers that mediate growth hormone-activated insulin-like growth factor-I gene transcription

Many of the biological effects of growth hormone (GH) are mediated by insulin-like growth factor I (IGF-I), a 70-amino acid secreted peptide whose gene expression is rapidly induced by GH via the Stat5b transcription factor. We previously identified multiple evolutionarily conserved GH-activated chromosomal binding domains for Stat5b within the rat Igf1 locus, and proposed that they could regulate IGF-I gene activity. Here we investigate the biochemical and functional characteristics of these putative long-range transcriptional enhancers. Each element contained 2 or 3 individual Stat5b recognition sequences that could bind Stat5b in vitro, but with affinities that varied over a >100-fold range. Full transcriptional responsiveness to GH required that all Stat5b sites be intact within an individual enhancer. Replacement of a single lower-affinity Stat5b sequence with a higher-affinity one increased in vitro binding of Stat5b, and boosted transcriptional potency of the entire element to GH. As enhanced transcriptional activity involved changes in only one or two nucleotides within an enhancer DNA segment, there appears to be remarkable specificity and sensitivity in the ability of Stat5b to transform DNA binding activity into transcriptional function. Stat5b was able to stimulate the transcriptional activity of two enhancers in the absence of GH, indicating that individual Stat5b-regulated elements possess distinct functional features. We conclude that combinatorial interplay among multiple Stat5b-binding response elements with distinguishable biochemical properties is responsible for highly regulated control of IGF-I gene activity by GH.

Opposing regulation of PROX1 by interleukin-3 receptor and NOTCH directs differential host cell fate reprogramming by Kaposi sarcoma herpes virus

Lymphatic endothelial cells (LECs) are differentiated from blood vascular endothelial cells (BECs) during embryogenesis and this physiological cell fate specification is controlled by PROX1, the master regulator for lymphatic development. When Kaposi sarcoma herpes virus (KSHV) infects host cells, it activates the otherwise silenced embryonic endothelial differentiation program and reprograms their cell fates. Interestingly, previous studies demonstrated that KSHV drives BECs to acquire a partial lymphatic phenotype by upregulating PROX1 (forward reprogramming), but stimulates LECs to regain some BEC-signature genes by downregulating PROX1 (reverse reprogramming). Despite the significance of this KSHV-induced bidirectional cell fate reprogramming in KS pathogenesis, its underlying molecular mechanism remains undefined. Here, we report that IL3 receptor alpha (IL3Rα) and NOTCH play integral roles in the host cell type-specific regulation of PROX1 by KSHV. In BECs, KSHV upregulates IL3Rα and phosphorylates STAT5, which binds and activates the PROX1 promoter. In LECs, however, PROX1 was rather downregulated by KSHV-induced NOTCH signal via HEY1, which binds and represses the PROX1 promoter. Moreover, PROX1 was found to be required to maintain HEY1 expression in LECs, establishing a reciprocal regulation between PROX1 and HEY1. Upon co-activation of IL3Rα and NOTCH, PROX1 was upregulated in BECs, but downregulated in LECs. Together, our study provides the molecular mechanism underlying the cell type-specific endothelial fate reprogramming by KSHV.

Kaposi's sarcoma (KS) is one of the most common neoplasms in HIV-positive individuals and organ transplant recipients. KS-associated herpes virus (KSHV), also known as human herpes virus (HHV)-8, has been identified as the causative agent and infects endothelial cells to form KS. Importantly, we and others have discovered that when KSHV infects endothelial cells of blood vessels, it reprograms host cells to resemble endothelial cells in lymphatic vessels. On the other hand, when KSHV infects endothelial cells in lymphatic vessels, the virus directs the host cells to partially obtain the phenotypes of blood vessel endothelial cells. These host cell reprogramming represent abnormal pathological processes, which are not as complete as the physiological process occurring during embryonic development. Currently, it is not clear how and why this cancer causing virus modifies the fate of its host cells. In this study, we aimed to dissect the molecular mechanism underlying the virus-induced host cell fate reprogramming and found two important cellular signaling pathways, interleukin-3 and Notch, playing key roles in the pathological events. Our current study provides a better understanding of KS tumorigenesis with a potential implication in a new KS therapy.

Growth hormone stimulates transcription of the fibroblast growth factor 21 gene in the liver through the signal transducer and activator of transcription 5

Fibroblast growth factor 21 (FGF21) is a recently discovered metabolic regulator. Interestingly, FGF21 is also known to inhibit Janus kinase 2 (JAK2)-signal transducer and activator of transcription 5 (STAT5) signaling from the GH receptor in the liver, where FGF21 mRNA is predominantly expressed. In this study, we tested the hypothesis that FGF21 gene expression in the liver is controlled by GH through STAT5. We found that GH injection to cattle increased FGF21 mRNA expression in the liver. Mapped by a 5'-rapid amplification of cDNA ends assay, transcription of the FGF21 gene in the bovine liver was mainly initiated from a nucleotide 24 bp downstream of a TATA box. The bovine FGF21 promoter contains three putative STAT5-binding sites. EMSA confirmed the ability of them to bind to liver STAT5 protein from GH-injected cattle. Chromatin immunoprecipitation assays demonstrated that GH administration increased the binding of STAT5 to the FGF21 promoter in the liver. Cotransfection analyses showed that GH induced reporter gene expression from the FGF21 promoter in a STAT5-dependent manner. GH also stimulated FGF21 mRNA expression in cultured mouse hepatocytes. These data together indicate that GH directly stimulates FGF21 gene transcription in the liver, at least in part, through STAT5. This finding, together with the fact that FGF21 inhibits GH-induced JAK2-STAT5 signaling in the liver, suggests a novel negative feedback loop that prevents excessive JAK2-STAT5 signaling from the GH receptor in the liver.

Comparison of mRNA for IGFs and their binding proteins in the oviduct during the peri-oestrous period between dairy heifers and lactating cows

The oviduct provides the environment to support gamete maturation, fertilisation and early embryo development. As there is a high incidence of early embryonic death in lactating dairy cows, this study compared expression of IGF family members in the oviduct between lactating Holstein-Friesian dairy cows (n=16, 81±2.4 days in milk) and nulliparous heifers (n=16, age 1.6±0.07 years) at three stages of the oestrous cycle: A) newly selected dominant follicle in the luteal phase, B) follicular phase before the LH surge and C) pre-ovulatory phase after the LH surge. Expression ofIGF1,IGF2, IGF binding protein 2 (IGFBP2),IGFBP3andIGFBP6mRNA was determined in the ampulla of the oviduct. Oviduct side (ipsilateral or contralateral) with respect to the dominant follicle did not affect gene expression. Expression ofIGF1and all threeIGFBPsincreased significantly between the luteal and the pre-ovulatory phases, with no further significant alteration post-LH surge. Concentrations of circulating IGF1 were higher in heifers than in cows, as was the mRNA expression ofIGF1,IGFBP3andIGFBP6. The pre-LH surge rise inIGFBP2mRNA was only observed in heifers.IGF2expression was not influenced by either age or stage of cycle. These three IGFBPs are generally considered to inhibit IGF action. These results indicate tight regulation of IGF bioavailability in the oviductal environment around oestrus, with pronounced differences between cows and heifers, which are likely to influence early embryonic development. Further studies are required to assess the implications for embryo survival.

Defining the epigenetic actions of growth hormone: acute chromatin changes accompany GH-activated gene transcription

Many of the long-term physiological effects of GH require hormone-mediated changes in gene expression. The transcription factor signal transducer and activator of transcription 5b (Stat5b) plays a critical role in the actions of GH on growth and metabolism by regulating a large number of GH-dependent genes by incompletely understood mechanisms. Here we have assessed the impact of GH-initiated and Stat5b-mediated signaling on the chromatin landscape of hormone-regulated genes in the liver of pituitary-deficient young adult male rats. In the absence of GH there was minimal ongoing transcription at the Socs2, Cish, Igfals, and Spi 2.1 promoters, minimal occupancy of Stat5b at proximal promoter sites, and relatively closed chromatin, as evidenced by low levels of core histone acetylation. In contrast, transcriptionally silent Igf1 promoter 1 appeared poised to be activated, based on binding of coactivators p300 and Med1/Trap220, high levels of histone acetylation, and the presence of RNA polymerase II. GH treatment led to a 8- to 20-fold rise in transcriptional activity of all five genes within 30-60 min and was accompanied by binding of Stat5b to the proximal Socs2, Cish, Igfals, and Spi 2.1 promoters and to seven distal Igf1 Stat5b elements, by enhanced histone acetylation at all five promoters, by recruitment of RNA polymerase II to the Socs2, Cish, Igfals, and Spi 2.1 promoters, and by loss of the transcriptional repressor Bcl6 from Socs2, Cish, and Igfals Stat5b sites, but not from two Igf1 Stat5b domains. We conclude that GH actions induce rapid and dramatic changes in hepatic chromatin at target promoters and propose that the chromatin signature of Igf1 differs from other GH-and Stat5b-dependent genes.

Uremia attenuates growth hormone-stimulated insulin-like growth factor-1 expression, a process worsened by inflammation

Growth hormone (GH) resistance is common in uremia and together with resistance to insulin-like growth factor-1 (IGF-1) contributes to uremic growth retardation and muscle wasting. Previously, we found decreased GH-stimulated janus-kinase 2-signal transducers and activators of transcription 5 (STAT5) phosphorylation and nuclear translocation in uremia; however, it is unclear whether there are more distal defects. Therefore, we tested whether the binding of phosphorylated STAT5b to DNA is intact in uremia. Using uremic rats we found that in addition to impaired hepatic STAT5b phosphorylation, the binding of available phospho-STAT5b to DNA is decreased thus contributing to impaired IGF-1 gene expression. As sepsis-induced inflammation causes a loss of body protein and as Gram-negative infections are relatively common in uremia, we also characterized mechanisms in which acute inflammation might contribute to GH resistance in uremia. Endotoxin-induced inflammation markedly increased the resistance to GH-mediated STAT5b signaling, and further decreased STAT5b binding to DNA and IGF-1 gene expression. These perturbations appear to be related to increased cytokine expression. Thus, our findings indicate that hepatic resistance to GH-induced IGF-1 expression in uremia arises due to defects in STAT5b phosphorylation and its impaired binding to DNA, processes further aggravated by inflammation.

Ames dwarf (Prop1(df)/Prop1(df)) mice display increased sensitivity of the major GH-signaling pathways in liver and skeletal muscle

CONTEXT

Growth hormone (GH) is an anabolic hormone that regulates growth and metabolism. Ames dwarf mice are natural mutants for Prop1, with impaired development of anterior pituitary and undetectable levels of circulating GH, prolactin and TSH. They constitute an endocrine model of life-long GH-deficiency. The main signaling cascades activated by GH binding to its receptor are the JAK2/STATs, PI-3K/Akt and the MAPK Erk1/2 pathways.

OBJECTIVES

We have previously reported that GH-induced STAT5 activation was higher in Ames dwarf mice liver compared to non-dwarf controls. The aim of this study was to evaluate the principal components of the main GH-signaling pathways under GH-deficiency in liver and skeletal muscle, another GH-target tissue.

METHODS

Ames dwarf mice and their non-dwarf siblings were assessed. Animals were injected i.p. with GH or saline 15min before tissue removal. Protein content and phosphorylation of signaling mediators were determined by immunoblotting of tissue solubilizates.

RESULTS

GH was able to induce STAT5 and STAT3 tyrosine phosphorylation in both liver and muscle, but the response was higher for Ames dwarf mice than for non-dwarf controls. When Erk1/2 activation was assessed in liver, only dwarf mice showed GH-induced phosphorylation, while in muscle no response to the hormone was found in either genotype. GH-induced Akt phosphorylation at Ser473 in liver was only detected in dwarf mice. In skeletal muscle, both normal and dwarf mice responded to a GH stimulus, although dwarf mice presented higher GH activation levels. The phosphorylation of GSK-3, a substrate of Akt, increased upon hormone stimulation only in dwarf mice in both tissues. In contrast, no differences in the phosphorylation of mTOR, another substrate of Akt, were observed after GH stimulus, either in normal or dwarf mice in liver, while we were unable to determine mTOR in muscle. Protein content of GH-receptor and of the signaling mediators studied did not vary between normal and dwarf animals in the assessed tissues.

CONCLUSION

These results show that several components of the main GH-signaling pathways exhibit enhanced sensitivity to the hormone in liver and muscle of Ames dwarf mice.

GH modulates hepatic epidermal growth factor signaling in the mouse

Epidermal growth factor (EGF) is a key regulator of cell survival and proliferation involved in the pathogenesis and progression of different types of cancer. The EGF receptor (EGFR) is activated by binding of the specific ligand but also by transactivation triggered by different growth factors including GH. Chronically, elevated GH levels have been associated with the progression of hepatocellular carcinoma. Considering EGF and GH involvement in cell proliferation and their signaling crosstalk, the objective of the present study was to analyze GH modulatory effects on EGF signaling in liver. For this purpose, GH receptor-knockout (GHR-KO) and GH-overexpressing transgenic mice were used. EGFR content was significantly decreased in GHR-KO mice. Consequently, EGF-induced phosphorylation of EGFR, AKT, ERK1/2, STAT3, and STAT5 was significantly decreased in these mice. In contrast, EGFR content as well as its basal tyrosine phosphorylation was increased in transgenic mice overexpressing GH. However, EGF stimulation caused similar levels of EGFR, AKT, and ERK1/2 phosphorylation in normal and transgenic mice, while EGF induction of STAT3 and STAT5 phosphorylation was inhibited in the transgenic mice. Desensitization of the STATs was related to decreased association of these proteins to the EGFR and increased association between STAT5 and the tyrosine phosphatase SH2-containing phosphatase-2. While GHR knockout is associated with diminished expression of the EGFR and a concomitant decrease in EGF signaling, GH overexpression results in EGFR overexpression with different effects depending on the signaling pathway analyzed: AKT and ERK1/2 pathways are induced by EGF, while STAT3 and STAT5 activation is heterologously desensitized.

Pressure-mediated hypertrophy and mechanical stretch induces IL-1 release and subsequent IGF-1 generation to maintain compensative hypertrophy by affecting Akt and JNK pathways

RATIONALE

It has been reported that interleukin (IL)-1 is associated with pathological cardiac remodeling and LV dilatation, whereas IL-1beta has also been shown to induce cardiomyocyte hypertrophy. Thus, the role of IL-1 in the heart remains to be determined.

OBJECTIVE

We studied the role of hypertrophy signal-mediated IL-1beta/insulin-like growth factor (IGF)-1 production in regulating the progression from compensative pressure-mediated hypertrophy to heart failure.

METHODS AND RESULTS

Pressure overload was performed by aortic banding in IL-1beta-deficient mice. Primarily cultured cardiac fibroblasts (CFs) and cardiac myocytes (CMs) were exposed to cyclic stretch. Heart weight, myocyte size, and left ventricular ejection fraction were significantly lower in IL-1beta-deficient mice (20%, 23% and 27%, respectively) than in the wild type 30 days after aortic banding, whereas interstitial fibrosis was markedly augmented. DNA microarray analysis revealed that IGF-1 mRNA level was markedly (approximately 50%) decreased in the IL-1beta-deficient hypertrophied heart. Stretch of CFs, rather than CMs, abundantly induced the generation of IL-1beta and IGF-1, whereas such IGF-1 induction was markedly decreased in IL-1beta-deficient CFs. IL-1beta released by stretch is at a low level unable to induce IL-6 but sufficient to stimulate IGF-1 production. Promoter analysis showed that stretch-mediated IL-1beta activates JAK/STAT to transcriptionally regulate the IGF-1 gene. IL-1beta deficiency markedly increased c-Jun N-terminal kinase (JNK) and caspase-3 activities and enhanced myocyte apoptosis and fibrosis, whereas replacement of IGF-1 or JNK inhibitor restored them.

CONCLUSIONS

We demonstrate for the first time that pressure-mediated hypertrophy and mechanical stretch generates a subinflammatory low level of IL-1beta, which constitutively causes IGF-1 production to maintain adaptable compensation hypertrophy and inhibit interstitial fibrosis.

Computational and functional analysis of growth hormone (GH)-regulated genes identifies the transcriptional repressor B-cell lymphoma 6 (Bc16) as a participant in GH-regulated transcription

For insight into transcriptional mechanisms mediating physiological responses to GH, data mining was performed on a profile of GH-regulated genes induced or inhibited at different times in highly responsive 3T3-F442A adipocytes. Gene set enrichment analysis indicated that GH-regulated genes are enriched in pathways including phosphoinositide and insulin signaling and suggested that suppressor of cytokine signaling 2 (SOCS2) and phosphoinositide 3' kinase regulatory subunit p85alpha (Pik3r1) are important targets. Model-based Chinese restaurant clustering identified a group of genes highly regulated by GH at times consistent with its key physiological actions. This cluster included IGF-I, phosphoinositide 3' kinase p85alpha, SOCS2, and cytokine-inducible SH2-containing protein. It also contains the most strongly repressed gene in the profile, B cell lymphoma 6 (Bcl6), a transcriptional repressor. Quantitative real-time PCR verified the strong decrease in Bcl6 mRNA after GH treatment and induction of the other genes in the cluster. Transcriptional network analysis of the genes implicated signal transducer and activator of transcription (Stat) 5 as hub regulating the most responsive genes, Igf1, Socs2, Cish, and Bcl6. Transcriptional activation analysis demonstrated that Bcl6 inhibits SOCS2-luciferase and blunts its stimulation by GH. Occupancy of endogenous Bcl6 on SOCS2 DNA decreased after GH treatment, whereas occupancy of Stat5 increased concomitantly. Thus, GH-mediated inhibition of Bcl6 expression may reverse the repression of SOCS2 and facilitate SOCS2 activation by GH. Together these analyses identify Bcl6 as a participant in GH-regulated gene expression and suggest an interplay between the repressor Bcl6 and the activator Stat5 in regulating genes, which contribute to GH responses.

Endotoxin-induced growth hormone resistance in skeletal muscle

Inflammation-induced skeletal muscle wasting is a serious clinical problem and arises in part because of resistance to GH-stimulated IGF-I expression. Although it is established that in the liver, resistance develops because of impaired signaling through the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) transduction pathway, together with a more distal defect in STAT5 DNA-binding activity, the situation in skeletal muscle is unclear. Accordingly, we set out to characterize the mechanisms behind the skeletal muscle resistance to GH in rats with acute inflammation induced by endotoxin. Endotoxin caused significant declines in GH-stimulated STAT5a/b phosphorylation and IGF-I gene expression, and this occurred despite a lack of change in signaling protein levels or phosphorylation of JAK2. In whole muscle, GH-stimulated phospho-STAT5a/b levels were reduced by half, and in the nucleus, phospho-STAT5b levels were similarly reduced. Furthermore, the binding of phosphorylated STAT5b to DNA was reduced and to a similar extent to the reduction in nuclear phosphorylated STAT5b. Interestingly, GH-induced androgen receptor gene expression was also suppressed. Thus, it appears that skeletal muscle resistance to GH-stimulated IGF-I expression in acute endotoxemia arises from a defect in STAT5b signaling, with a proportionate reduction in STAT5b DNA binding. Finally, it appears that resistance to GH-induced androgen receptor expression also develops and, together with the attenuated GH-induced IGF-I expression, likely plays an important role in the muscle wasting that arises in endotoxin-induced inflammation.

Insulin-like growth factor and epidermal growth factor treatment: new approaches to protecting steatotic livers against ischemia-reperfusion injury

Hepatic steatosis is a major risk factor in ischemia-reperfusion (I/R). IGF-binding proteins (IGFBPs) modulate IGF-I action by transporting circulating IGF-I to its sites of action. Epidermal growth factor (EGF) stimulates IGF-I synthesis in vitro. We examined the effect of IGF-I and EGF treatment, separately or in combination, on the vulnerability of steatotic livers to I/R. Our results indicated that I/R impaired IGF-I synthesis only in steatotic livers. Only when a high dose of IGF-I (400 microg/kg) was given to obese animals did they show high circulating IGF-I:IGFBP levels, increased hepatic IGF-I levels, and protection against damage. In lean animals, a dose of 100 microg/kg IGF-I protected nonsteatotic livers. Our results indicated that the combined administration of IGF-I and EGF resulted in hepatic injury parameters in both liver types similar to that obtained by IGF-I and EGF separately. IGF-I increased egf expression in both liver types. The beneficial role of EGF on hepatic I/R injury may be attributable to p38 inhibition in nonsteatotic livers and to PPAR gamma overexpression in steatotic livers. In conclusion, IGF-I and EGF may constitute new pharmacological strategies to reduce the inherent susceptibility of steatotic livers to I/R injury.

Regulation of STARS and its downstream targets suggest a novel pathway involved in human skeletal muscle hypertrophy and atrophy

Skeletal muscle atrophy is a severe consequence of ageing, neurological disorders and chronic disease. Identifying the intracellular signalling pathways controlling changes in skeletal muscle size and function is vital for the future development of potential therapeutic interventions. Striated activator of Rho signalling (STARS), an actin-binding protein, has been implicated in rodent cardiac hypertrophy; however its role in human skeletal muscle has not been determined. This study aimed to establish if STARS, as well as its downstream signalling targets, RhoA, myocardin-related transcription factors A and B (MRTF-A/B) and serum response factor (SRF), were increased and decreased respectively, in human quadriceps muscle biopsies taken after 8 weeks of both hypertrophy-stimulating resistance training and atrophy-stimulating de-training. The mRNA levels of the SRF target genes involved in muscle structure, function and growth, such as alpha-actin, myosin heavy chain IIa (MHCIIa) and insulin-like growth factor-1 (IGF-1), were also measured. Following resistance training, STARS, MRTF-A, MRTF-B, SRF, alpha-actin, MHCIIa and IGF-1 mRNA, as well as RhoA and nuclear SRF protein levels were all significantly increased by between 1.25- and 3.6-fold. Following the de-training period all measured targets, except for RhoA, which remained elevated, returned to base-line. Our results show that the STARS signalling pathway is responsive to changes in skeletal muscle loading and appears to play a role in both human skeletal muscle hypertrophy and atrophy.

Low-dose growth hormone is cardioprotective in uremia

Growth hormone (GH) is required to maintain normal cardiac structure and function and has a positive effect on cardiac remodeling in experimental and possibly human disease. Cardiac resistance to GH develops in the uremic state, perhaps predisposing to the characteristic cardiomyopathy associated with uremia. It was hypothesized that administration of low-dosage GH may have a salutary effect on the cardiac remodeling process in uremia, but because high levels of GH have adverse cardiac effects, administration of high-dosage GH may worsen uremic cardiomyopathy. In rats with chronic renal failure, quantitative cardiac morphology revealed a decrease in total capillary length and capillary length density and an increase in mean intercapillary distance and fibroblast volume density evident. Low-dosage GH prevented these changes. Collagen and TGF-beta immunostaining, increased in chronic renal failure, were also reduced by GH, suggesting a mechanism for its salutary action. Low-dosage GH also prevented thickening of the carotid artery but did not affect aortic pathology. In contrast, high-dosage GH worsened several of these variables. These results suggest that low-dosage GH may benefit the heart and possibly the carotid arteries in chronic renal failure.

Inhibition of growth hormone signaling by the fasting-induced hormone FGF21

Starvation blocks the actions of growth hormone (GH) and inhibits growth through mechanisms that are not well understood. In this report, we demonstrate that fibroblast growth factor 21 (FGF21), a hormone induced by fasting, causes GH resistance. In liver, FGF21 reduces concentrations of the active form of signal transducer and activator of transcription 5 (STAT5), a major mediator of GH actions, and causes corresponding decreases in the expression of its target genes, including insulin-like growth factor 1 (IGF-1). FGF21 also induces hepatic expression of IGF-1 binding protein 1 and suppressor of cytokine signaling 2, which blunt GH signaling. Chronic exposure to FGF21 markedly inhibits growth in mice. These data suggest a central role for FGF21 in inhibiting growth as part of its broader role in inducing the adaptive response to starvation.

Human sarcopenia reveals an increase in SOCS-3 and myostatin and a reduced efficiency of Akt phosphorylation

Age-related skeletal muscle sarcopenia is linked with increases in falls, fractures, and death and therefore has important socioeconomic consequences. The molecular mechanisms controlling age-related muscle loss in humans are not well understood, but are likely to involve multiple signaling pathways. This study investigated the regulation of several genes and proteins involved in the activation of key signaling pathways promoting muscle hypertrophy, including GH/STAT5, IGF-1/Akt/GSK-3beta/4E-BP1, and muscle atrophy, including TNFalpha/SOCS-3 and Akt/FKHR/atrogene, in muscle biopsies from 13 young (20 +/- 0.2 years) and 16 older (70 +/- 0.3 years) males. In the older males compared to the young subjects, muscle fiber cross-sectional area was reduced by 40-45% in the type II muscle fibers. TNFalpha and SOCS-3 were increased by 2.8 and 1.5 fold, respectively. Growth hormone receptor protein (GHR) and IGF-1 mRNA were decreased by 45%. Total Akt, but not phosphorylated Akt, was increased by 2.5 fold, which corresponded to a 30% reduction in the efficiency of Akt phosphorylation in the older subjects. Phosphorylated and total GSK-3beta were increased by 1.5 and 1.8 fold, respectively, while 4E-BP1 levels were not changed. Nuclear FKHR and FKHRL1 were decreased by 73 and 50%, respectively, with no changes in their atrophy target genes, atrogin-1 and MuRF1. Myostatin mRNA and protein levels were significantly elevated by 2 and 1.4 fold. Human sarcopenia may be linked to a reduction in the activity or sensitivity of anabolic signaling proteins such as GHR, IGF-1, and Akt. TNFalpha, SOCS-3, and myostatin are potential candidates influencing this anabolic perturbation.

Overexpression of Scg5 increases enzymatic activity of PCSK2 and is inversely correlated with body weight in congenic mice

Background

The identification of novel genes is critical to understanding the molecular basis of body weight. Towards this goal, we have identified secretogranin V (Scg5; also referred to as Sgne1), as a candidate gene for growth traits.

Results

Through a combination of DNA microarray analysis and quantitative PCR we identified a strong expression quantitative trait locus (eQTL) regulating Scg5 expression in two mouse chromosome 2 congenic strains and three additional F2 intercrosses. More importantly, the eQTL was coincident with a body weight QTL in congenic mice and Scg5 expression was negatively correlated with body weight in two of the F2 intercrosses. Analysis of haplotype blocks and genomic sequencing of Scg5 in high (C3H/HeJ, DBA/2J, BALB/cByJ, CAST/EiJ) and low (C57BL/6J) expressing strains revealed mutations unique to C57BL/6J and possibly responsible for the difference in mRNA abundance. To evaluate the functional consequence of Scg5 overexpression we measured the pituitary levels of 7B2 protein and PCSK2 activity and found both to be increased. In spite of this increase, the level of pituitary α-MSH, a PCSK2 processing product, was unaltered.

Conclusion

Together, these data support a role for Scg5 in the modulation of body weight.

Vitamin D3 cannot revert desensitization of growth hormone (GH)-induced STAT5-signaling in GH-overexpressing mice non-calcemic tissues

Growth hormone (GH) binding to a membrane receptor dimer triggers multiple intracellular signaling pathways. Signal transducers and activators of transcription are the most relevant of these pathways for GH action. GH also activates several inhibitory mechanisms, particularly suppressors of cytokine signaling (SOCS/CIS) proteins. GH-overexpressing mice exhibit hepatic desensitization of the JAK2/STAT5 GH-signaling pathway, associated with an increased abundance of CIS. Vitamin D3 has been shown to inhibit GH-induced expression of CIS and SOCS-3 and therefore prolong GH signaling in osteoblast-like cells. The purpose of the present study is to determine if vitamin D3 could attenuate CIS expression in GH-overexpressing mice, and consequently allow GH JAK2/STAT5 signaling in GH-responsive tissues in these animals. The abundance of CIS, SOCS-2, SOCS-3, STAT5b and GHR, as well as STAT5b tyrosine phosphorylation after a GH stimulus, were measured in liver and muscle of GHRH-transgenic mice treated with 1alpha,25-dihydroxyvitamin D3 for 7 days. This treatment did not diminish CIS expression in GH-overexpressing mice tissues, nor did the content of SOCS-2 and SOCS-3 significantly vary. GH-induced STAT5b phosphorylation levels were similar to basal values in transgenic mice liver treated with or without vitamin D; the refractoriness to GH was also present in muscle. Therefore, treatment with vitamin D was not sufficient to revert STAT5 GH signaling desensitization in non-calcemic tissues in GH-overexpressing mice.

The growth hormone-insulin-like growth factor-I axis in chronic kidney disease

Growth hormone (GH) and insulin-like growth factor-I (IGF-I) are important physiologic regulators of growth, body composition, and kidney function. Perturbations in the GH-IGF-I axis are responsible for many important complications seen in chronic kidney disease (CKD), such as growth retardation and cachectic wasting, as well as disease progression. Recent evidence suggests that CKD is characterized by abnormalities in GH and IGF-I signal transduction and the interaction of these pathways with those that involve other molecules such as ghrelin, myostatin, and the suppressor of cytokine signaling (SOCS) family. Further understanding of GH/IGF pathophysiology in CKD may lead to the development of therapeutic strategies for these devastating complications, which are associated with high rates of mortality and morbidity.

Interrelationships between negative energy balance (NEB) and IGF regulation in liver of lactating dairy cows

In dairy cows, negative energy balance (NEB) during the early post-partum period is associated with major alterations in the growth hormone-insulin-like growth factor (GH-IGF) axis. Since the liver mediates nutrient partitioning during lactation, we aimed to determine how NEB alters the endocrine regulation of the insulin-like growth factor (IGF) system by investigating the expression of IGF family members and related steroid receptors. On the second day of lactation, cows were allocated to one of two treatments designed to produce mild (MNEB) or severe NEB (SNEB). MNEB cows (n=5) were fed ad lib grass silage supplemented with concentrate and milked x1 daily and SNEB cows (n=6) were restricted in dietary intake and milked x3 daily. Energy balance (EB) status was monitored until the second week of lactation when plasma and liver samples revealed a markedly divergent metabolic profile. At this time, plasma protein and hepatic mRNA for IGF-I was reduced in SNEB cows compared with MNEB cows. Both levels of expression correlated highly when data from all animals was pooled (r=0.963; P<0.01). SNEB cows also exhibited reduced hepatic expression for transcripts encoding IGF-1R, IGF-2R, IGF binding proteins (IGFBPs) -3, -4, -5, -6, acid labile subunit, and receptors for oestrogen (ERalpha) and growth hormone (total GHR and 1A variant), while the expression of IGFBP-2 was elevated. Expression of mRNA for IGF-II, IGFBP-1 and receptors for insulin (A/B) and glucocorticoid (alpha) was unaffected by EB. Results demonstrate that SNEB affects hepatic synthesis of IGF-I, and other components known to modulate the bioavailability and stability of circulating IGF-I.

Negative energy balance in dairy cows is associated with specific changes in IGF-binding protein expression in the oviduct

Negative energy balance (NEB) during early lactation in dairy cows leads to an altered metabolic state that has major effects on the production of IGF family members. Low IGF-I concentrations are associated with poor fertility and therefore we aimed to determine whether NEB exerts a direct effect on IGF expression in the postpartum oviduct. Multiparous Holstein cows were allocated to two treatments (each n=6) designed using differential feeding and milking regimes to produce either mild NEB (MNEB) or severe NEB (SNEB). Animals were slaughtered in week 2 of lactation when divergent metabolic profiles were evident. Oviducts were collected for RNA analysis by real-time RT-PCR and in situ hybridisation. Quantitative measures in oviduct gene expression were obtained for all members of the IGF family (IGF-I/II, IGF-binding proteins (IGFBP) 1-6 and receptors for IGF types 1 and 2), insulin A/B, GH, glucocorticoid and oestrogen alpha/beta. Expression of IGFBP-2 and IGFBP-6 (both of which have a high affinity for IGF-II) was decreased in SNEB relative to MNEB (P<0.05). No other gene was altered by NEB, but IGF-II, IGFBP-3, IGFBP-5 and IGFBP-6 all showed differential expression in different regions of the oviduct. These results indicate that, in addition to low circulating IGF-I after calving, NEB may also influence IGF availability in the oviduct indirectly through changes in specific IGFBP expression. It is possible that the predicted increased signalling by IGF-II may perturb embryo development, contributing to the high rates of embryonic mortality in dairy cows.

Loss of signal transducer and activator of transcription 5 leads to hepatosteatosis and impaired liver regeneration

Growth hormone controls many facets of a cell's biology through the transcription factors Stat5a and Stat5b (Stat5). However, whole body deletion of these genes from the mouse does not provide portentous information on cell-specific cytokine signaling. To explore liver-specific functions of Stat5, the entire Stat5 locus was deleted in hepatocytes using Cre-mediated recombination. Notably, Stat5-mutant mice developed fatty livers and displayed impaired proliferation of hepatocytes upon partial hepatectomy (PHx). Loss of Stat5 led to molecular consequences beyond the reduced expression of Stat5 target genes, such as those encoding suppressor of cytokine signaling 2 (SOCS2), Cish, and insulin-like growth factor 1 (IGF-1). In particular, circulating growth hormone levels were increased and correlated with insulin resistance and increased insulin levels. Aberrant growth hormone (GH)-induced activation of the transcription factors Stat1 and Stat3 was observed in mutant livers. To test whether some of the defects observed in liver-specific Stat5 deficient mice were due to aberrant Stat1 expression and activation, we generated Stat1(-/-) mice with a hepatocyte-specific deletion of Stat5. Concomitant loss of both Stat5 and Stat1 restored cell proliferation upon PHx but did not reverse fatty liver development. Thus the molecular underpinnings of some defects observed in the absence of Stat5 are the consequence of a deregulated activation of other signal transducers and activators of transcription (STAT) family members.

CONCLUSION

Aberrant cytokine-Stat5 signaling in hepatocytes alters their physiology through increased activity of Stat1 and Stat3. Such cross-talk between different pathways could add to the complexity of syndromes observed in disease.

A molecular basis for the sexually dimorphic response to growth hormone

Once reserved solely for the treatment of short stature, the now readily available recombinant GH has expanded the use of the hormone to include the treatment of cardiovascular, renal, muscular, skeletal, immunological, psychosocial, and metabolic abnormalities associated with GH deficiency. There are also proposals for the widespread use of the hormone to ameliorate or reverse aging. However, this extensive use of GH has revealed intrinsic sexual dimorphisms in which females are considerably less responsive to the therapeutic regimen than are males. Dynamic changes in the Janus kinase-2 (Jak2)/signal transducers and activators of transcription (Stat5B) signaling pathway [as determined by transducer activation, Stat5B binding to the GH-responsive promoter of the CYP2C11 gene, and expression levels of the suppressors of cytokine signaling family (Socs2, Socs3, and Cis)] were examined in male and female rat-derived primary hepatocyte cultures exposed to the masculine-like episodic GH profile. We report that the cellular actions of GH normally mediated by activation of the Jak2/Stat5B pathway are suppressed in female cells possibly due to an inherent overexpression of Cis, a member of the suppressors of cytokine signaling family that normally down-regulates the Jak2/Stat5B pathway.

Signal Transducer and Activator of Transcription (Stat) 5b-Mediated Inhibition of Insulin-Like Growth Factor Binding Protein-1 Gene Transcription: A Mechanism for Repression of Gene Expression by Growth Hormone

GH plays a central role in controlling somatic growth, tissue regeneration, and intermediary metabolism in most vertebrate species through mechanisms dependent on the regulation of gene expression. Recent studies using transcript profiling have identified large cohorts of genes whose expression is induced by GH. Other results have demonstrated that signal transducer and activator of transcription (Stat) 5b, a latent transcription factor activated by the GH receptor-associated protein kinase, Jak2, is a key agent in the GH-stimulated gene activation that leads to somatic growth. By contrast, little is known about the steps through which GH-initiated signaling pathways reduce gene expression. Here we show that Stat5b plays a critical role in the GH-regulated inhibition of IGF binding protein-1 gene transcription by impairing the actions of the FoxO1 transcription factor on the IGF binding protein-1 promoter. Additional observations using transcript profiling in the liver indicate that Stat5b may be a general mediator of GH-initiated gene repression. Our results provide a model for understanding how GH may simultaneously stimulate and inhibit the expression of different cohorts of genes via the same transcription factor, potentially explaining how GH action leads to integrated biological responses in the whole organism.

Endotoxin attenuates growth hormone-induced hepatic insulin-like growth factor I expression by inhibiting JAK2/STAT5 signal transduction and STAT5b DNA binding

Gram-negative sepsis with release of endotoxin is a frequent cause of cachexia that develops partly because of resistance to growth hormone (GH) with reduced insulin-like growth factor-I (IGF-I) expression. We set out to more fully characterize the mechanisms for the resistance and to determine whether in addition to a defect in the janus kinase 2 (JAK2)-signal transducer and activator of transcription (STAT) 5b pathway, required for GH-induced IGF-I expression, there might also be a more distal defect. Conscious rats were given endotoxin and studied 4 h later. In liver of these animals, GH-induced JAK2 and STAT5 phosphorylation was impaired and appeared to be caused, at least in part, by a marked increase in hepatic tumor necrosis factor-alpha and interleukin-6 mRNA expression accompanied by elevated levels of inhibitors of GH signaling, namely cytokine-inducible suppressors of cytokine signaling-1 and -3 and cytokine-inducible SH2 protein (CIS). Nuclear phosphorylated STAT5b levels were significantly depressed to 61% of the control values and represent a potential cause of the reduced GH-induced IGF-I expression. In addition, binding of phosphorylated STAT5b to DNA was reduced to an even greater extent and averaged 17% of the normal control value. This provides a further explanation for the impaired IGF-I gene transcription. Interestingly, when endotoxin-treated rats were treated with GH, there was a marked increase in proinflammatory cytokine gene expression in the liver. If such a response were to occur in humans, this might provide a partial explanation for the adverse effect of GH treatment reported in critically ill patients.