Gene regulation by growth hormone

The growth hormone receptor interacts with transcriptional regulator HMGN1 upon GH-induced nuclear translocation

Growth hormone (GH) actions are mediated through binding to its cell-surface receptor, the GH receptor (GHR), with consequent activation of downstream signalling. However, nuclear GHR localisation has also been observed and is associated with increased cancer cell proliferation. Here we investigated the functional implications of nuclear translocation of the GHR in the human endometrial cancer cell-line, RL95-2, and human mammary epithelial cell-line, MCF-10A. We found that following GH treatment, the GHR rapidly translocates to the nucleus, with maximal localisation at 5-10 min. Combined immunoprecipitation-mass spectrometry analysis of RL95-2 whole cell lysates identified 40 novel GHR binding partners, including the transcriptional regulator, HMGN1. Moreover, microarray analysis demonstrated that the gene targets of HMGN1 were differentially expressed following GH treatment, and co-immunoprecipitation showed that HMGN1 associates with the GHR in the nucleus. Therefore, our results suggest that GHR nuclear translocation might mediate GH actions via interaction with chromatin factors that then drive changes in specific downstream transcriptional programs.

The Role of Chicken Prolactin, Growth Hormone and Their Receptors in the Immune System

Prolactin (PRL) and growth hormone (GH) exhibit important roles in the immune system maintenance. In poultry, PRL mainly plays its roles in nesting, hatching, and reproduction, while GH is primarily responding to body weight, fat formation and feed conversion. In this review, we attempt to provide a critical overview of the relationship between PRL and GH, PRLR and GHR, and the immune response of poultry. We also propose a hypothesis that PRL, GH and their receptors might be used by viruses as viral receptors. This may provide new insights into the pathogenesis of viral infection and host immune response.

PCB126 induced toxic actions on liver energy metabolism is mediated by AhR in rats

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor involved in the regulation of biological responses to more planar aromatic hydrocarbons, like TCDD. We previously described the sequence of events following exposure of male rats to a dioxin-like polychlorinated biphenyl (PCB) congener, 3,3',4,4',5-pentachlorobiphenyl (PCB126), that binds avidly to the AhR and causes various types of toxicity including metabolic syndrome, fatty liver, and disruption of energy homeostasis. The purpose of this study was, to investigate the role of AhR to mediate those toxic manifestations following sub-acute exposure to PCB126 and to examine possible sex differences in effects. For this goal, we created an AhR knockout (AhR-KO) model using CRISPR/Cas9. Comparison was made to the wild type (WT) male and female Holtzman Sprague Dawley rats. Rats were injected with a single IP dose of corn oil vehicle or 5 μmol/kg PCB126 in corn oil and necropsied after 28 days. PCB126 caused significant weight loss, reduced relative thymus weights, and increased relative liver weights in WT male and female rats, but not in AhR-KO rats. Similarly, significant pathologic changes were visible which included necrosis and regeneration in female rats, micro- and macro-vesicular hepatocellular vacuolation in males, and a paucity of glycogen in livers of both sexes in WT rats only. Hypoglycemia and lower IGF1, and reduced serum non-esterified fatty acids (NEFAs) were found in serum of both sexes of WT rats, low serum cholesterol levels only in the females, and no changes in AhR-KO rats. The expression of genes encoding enzymes related to xenobiotic metabolism (e.g. CYP1A1), gluconeogenesis, glycogenolysis, and fatty acid oxidation were unaffected in the AhR-KO rats following PCB126 exposure as opposed to WT rats where expression was significantly upregulated (PPARα, females only) or downregulated suggesting a disrupted energy homeostasis. Interestingly, Acox2, Hmgcs, G6Pase and Pc were affected in both sexes, the gluconeogenesis and glucose transporter genes Pck1, Glut2, Sds, and Crem only in male WT-PCB rats. These results show the essential role of the AhR in glycogenolysis, gluconeogenesis, and fatty acid oxidation, i.e. in the regulation of energy production and homeostasis, but also demonstrate a significant difference in the effects of PCB126 in males verses females, suggesting higher vulnerability of glucose homeostasis in males and more changes in fatty acid/lipid homeostasis in females. These differences in effects, which may apply to more/all AhR agonists, should be further analyzed to identify health risks to specific groups of highly exposed human populations.

Growth hormone promotes hepatic gluconeogenesis by enhancing BTG2-YY1 signaling pathway

Growth hormone (GH) is one of the critical factors in maintaining glucose metabolism. B-cell translocation gene 2 (BTG2) and yin yang 1 (YY1) are key regulators of diverse metabolic processes. In this study, we investigated the link between GH and BTG2–YY1 signaling pathway in glucose metabolism. GH treatment elevated the expression of hepatic Btg2 and Yy1 in primary mouse hepatocytes and mouse livers. Glucose production in primary mouse hepatocytes and serum blood glucose levels were increased during GH exposure. Overexpression of hepatic Btg2 and Yy1 induced key gluconeogenic enzymes phosphoenolpyruvate carboxykinase 1 (PCK1) and glucose-6 phosphatase (G6PC) as well as glucose production in primary mouse hepatocytes, whereas this phenomenon was markedly diminished by knockdown of Btg2 and Yy1. Here, we identified the YY1-binding site on the Pck1 and G6pc gene promoters using reporter assays and point mutation analysis. The regulation of hepatic gluconeogenic genes induced by GH treatment was clearly linked with YY1 recruitment on gluconeogenic gene promoters. Overall, this study demonstrates that BTG2 and YY1 are novel regulators of GH-dependent regulation of hepatic gluconeogenic genes and glucose production. BTG2 and YY1 may be crucial therapeutic targets to intervene in metabolic dysfunction in response to the GH-dependent signaling pathway.

Prenatal nutrition supplementation and growth biomarkers in preadolescent Bangladeshi children: A birth cohort study

Little is known about the usefulness of biomarkers to study the influence of prenatal nutrition supplementation in improving child growth. Anthropometry is not always straightforward to understand how nutrition might impact growth, especially in settings with high rates of malnutrition and infections. We examined the effects of prenatal supplementation on growth and growth biomarkers and the relationship between anthropometric measures and growth biomarkers of children at 4.5 and 9 years of age. Children were enrolled from a longitudinal cohort, where mothers were randomized into daily supplementation with either early‐food (≤9 gestation week [GW]) or usual‐food (~20 GW) (608 kcal 6 days/week); they were further randomized to receive 30‐mg or 60‐mg iron with 400‐μg folic acid, or multiple micronutrients (MM) in rural Bangladesh. Anthropometric data were collected from mothers at GW8 and children at 4.5 (n = 640) and 9 years (n = 536). Fasting blood was collected from children at each age. Early‐food supplementation showed reduced stunting and underweight at 4.5 and 9 years age respectively compared to usual‐food. Prenatal supplementations did not have any effect on growth biomarkers except for STAT5b expression which was lower in the early‐food compared to the usual‐food group (β = −0.21; 95 CI% = −0.36, −0.07). Plasma concentrations of 25‐hydroxy vitamin D and calcium were both inversely associated with weight‐for‐age and body mass index‐for‐age Z‐scores at 9 years, particularly in early‐food and MM groups. Although there was minimal effect on child growth by prenatal supplementations, the associations of biomarkers with anthropometric indices were predominantly driven by timing of food or MM supplementations.

Insulin-like growth factor-1 attenuates oxidative stress-induced hepatocyte premature senescence in liver fibrogenesis via regulating nuclear p53-progerin interaction

Stress-induced premature senescence (SIPS), a state of cell growth arrest due to various stimuli, is implicated in the pathogeneses of hepatic fibrogenesis. Progerin, a permanently farnesylated mutant lamin A protein, likely leads to premature senescence to influent liver diseases. The previous reports showed that activation of insulin-like growth factor-1 (IGF-1) signaling could enhance cell longevity and attenuate liver fibrosis. However, the underlying mechanisms about hepatocyte premature senility in liver fibrosis, and how IGF-1 regulates cell premature aging and fibrogenesis, remain poorly understood. In the present study, we found the augment of hepatocyte oxidation and premature aging, along with the decrease of plasm IGF-1 level in patients with liver fibrosis and CCl₄-induced liver injury rat models. Nevertheless, IGF-1 gene transfer to CCl₄ rats to overexpress intrahepatic IGF-1 relieved hepatocyte oxidative stress and premature senescence, which was likely mediated by the p53/progerin pathway, to improve hepatic steatosis and fibrogenesis. In vitro, H₂O₂ caused abnormal accumulation of progerin in nuclear and activation of nuclear p53–progerin interaction to trigger primary rat hepatocyte premature senescence through the p21-independent pathway; while these effects were rescued by prolonged exogenous IGF-1 or the IGF-1 adenovirus vector. Furthermore, the IGF-1 adenovirus vector, transfected to H₂O₂-treated hepatocytes, reversed oxidative stress-induced premature senescence via enhancing cytoplasmic AKT1–p53 interaction and subsequently inhibiting nuclear p53–progerin interaction. Consequently, our data illuminate a novel role of IGF-1 in regulating stress-induced hepatocyte premature senescence in liver fibrosis: prolonged IGF-1 relieves oxidative stress-initiated hepatocyte premature senescence via inhibition of nuclear p53–progerin interaction to ameliorate hepatic steatosis and fibrogenesis.

Insulin-Like Growth Factor (IGF) System in Liver Diseases

Hepatocyte differentiation, proliferation, and apoptosis are affected by growth factors produced in liver. Insulin-like growth factor 1 and 2 (IGF1 and IGF2) act in response to growth hormone (GH). Other IGF family components include at least six binding proteins (IGFBP1 to 6), manifested by both IGFs develop due to interaction through the type 1 receptor (IGF1R). The data based on animal models and/or in vitro studies suggest the role of IGF system components in cellular aspects of hepatocarcinogenesis (cell cycle progression, uncontrolled proliferation, cell survival, migration, inhibition of apoptosis, protein synthesis and cell growth), and show that systemic IGF1 administration can reduce fibrosis and ameliorate general liver function. In epidemiologic and clinicopathological studies on chronic liver disease (CLD), lowered serum levels, decreased tissue expression of IGF1, elevated production of IGF1R and variable IGF2 expression has been noted, from the start of preneoplastic alterations up to the developed hepatocellular carcinoma (HCC) stage. These changes result in well-known clinical symptoms of IGF1 deficiency. This review summarized the current data of the complex role of IGF system components in the most common CLD (nonalcoholic fatty liver disease, cirrhosis, and hepatocellular carcinoma). Better recognition and understanding of this system can contribute to discovery of new and improved versions of current preventive and therapeutic actions in CLD.

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.

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.

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.

Deletion of muscle GRP94 impairs both muscle and body growth by inhibiting local IGF production

Insulin-like growth factors (IGFs) are critical for development and growth of skeletal muscles, but because several tissues produce IGFs, it is not clear which source is necessary or sufficient for muscle growth. Because it is critical for production of both IGF-I and IGF-II, we ablated glucose-regulated protein 94 (GRP94) in murine striated muscle to test the necessity of local IGFs for normal muscle growth. These mice exhibited smaller skeletal muscles with diminished IGF contents but with normal contractile function and no apparent endoplasmic reticulum stress response. This result shows that muscles rely on GRP94 primarily to support local production of IGFs, a pool that is necessary for normal muscle growth. In addition, body weights were ∼30% smaller than those of littermate controls, and circulating IGF-I also decreased significantly, yet glucose homeostasis was maintained with little disruption to the growth hormone pathway. The growth defect was complemented on administration of recombinant IGF-I. Thus, unlike liver production of IGF-I, muscle IGF-I is necessary not only locally but also globally for whole-body growth.

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.

Growth hormone promotes hair cell regeneration in the zebrafish (Danio rerio) inner ear following acoustic trauma

Background

Previous microarray analysis showed that growth hormone (GH) was significantly upregulated following acoustic trauma in the zebrafish (Danio rerio) ear suggesting that GH may play an important role in the process of auditory hair cell regeneration. Our objective was to examine the effects of exogenous and endogenous GH on zebrafish inner ear epithelia following acoustic trauma.

Methodology/Principal Findings

We induced auditory hair cell damage by exposing zebrafish to acoustic overstimulation. Fish were then injected intraperitoneally with either carp GH or buffer, and placed in a recovery tank for either one or two days. Phalloidin-, bromodeoxyuridine (BrdU)-, and TUNEL-labeling were used to examine hair cell densities, cell proliferation, and apoptosis, respectively. Two days post-trauma, saccular hair cell densities in GH-treated fish were similar to that of baseline controls, whereas buffer-injected fish showed significantly reduced densities of hair cell bundles. Cell proliferation was greater and apoptosis reduced in the saccules, lagenae, and utricles of GH-treated fish one day following trauma compared to controls. Fluorescent in situ hybridization (FISH) was used to examine the localization of GH mRNA in the zebrafish ear. At one day post-trauma, GH mRNA expression appeared to be localized perinuclearly around erythrocytes in the blood vessels of the inner ear epithelia. In order to examine the effects of endogenous GH on the process of cell proliferation in the ear, a GH antagonist was injected into zebrafish immediately following acoustic trauma, resulting in significantly decreased cell proliferation one day post-trauma in all three zebrafish inner ear end organs.

Conclusions/Significance

Our results show that exogenous GH promotes post-trauma auditory hair cell regeneration in the zebrafish ear through stimulating proliferation and suppressing apoptosis, and that endogenous GH signals are present in the zebrafish ear during the process of auditory hair cell regeneration.

The effects of type 1 IGF receptor inhibition in a mouse model of diabetic kidney disease

OBJECTIVE

We have recently shown increased sensitivity to IGF-I induced signal transduction in kidneys of diabetic mice. Accordingly we investigated the effects of PQ401, a novel diarylurea compound that inhibits IGF1R autophosphorylation in type I diabetes.

METHODS

Control (C) and Diabetic (D) mice were administered PQ401 (CP, DP) or vehicle (C, D) for 3weeks.

RESULTS

CP animals showed a decrease in renal phosphorylated (p-)AKT and p-IGF1R. However, PQ401 had no effect on diabetic state (hyperglycemia, weight loss) or renal disease parameters (hypertrophy, hyperfiltration and albuminuria). Type IV collagen as well as TGF-β mRNA increased in DP and D compared to C. In the CP group renal hypertrophy with fat accumulation in proximal tubuli and increased renal IGF-I, collagen IV and TGF-β mRNA were seen.

CONCLUSIONS

IGF1R inhibition by PQ401 exerted no significant effects on diabetic kidney disease parameters, arguing against a role for IGF-I in the pathogenesis of diabetic kidney disease. However, PQ401 affects normal kidneys, inducing renal hypertrophy as well as collagen and fat accumulation, with increased renal IGF-I mRNA, suggestive of a damage-regeneration process. Therefore, this diarylurea compound is not beneficial in early diabetic kidney disease. Its potential deleterious effects on kidney tissue need to be further investigated.

C/EBPβ mediates growth hormone-regulated expression of multiple target genes

Regulation of c-Fos transcription by GH is mediated by CCAAT/enhancer binding protein β (C/EBPβ). This study examines the role of C/EBPβ in mediating GH activation of other early response genes, including Cyr61, Btg2, Socs3, Zfp36, and Socs1. C/EBPβ depletion using short hairpin RNA impaired responsiveness of these genes to GH, as seen for c-Fos. Rescue with wild-type C/EBPβ led to GH-dependent recruitment of the coactivator p300 to the c-Fos promoter. In contrast, rescue with C/EBPβ mutated at the ERK phosphorylation site at T188 failed to induce GH-dependent recruitment of p300, indicating that ERK-mediated phosphorylation of C/EBPβ at T188 is required for GH-induced recruitment of p300 to c-Fos. GH also induced the occupancy of phosphorylated C/EBPβ and p300 on Cyr61, Btg2, and Socs3 at predicted C/EBP-cAMP response element-binding protein motifs in their promoters. Consistent with a role for ERKs in GH-induced expression of these genes, treatment with U0126 to block ERK phosphorylation inhibited their GH-induced expression. In contrast, GH-dependent expression of Zfp36 and Socs1 was not inhibited by U0126. Thus, induction of multiple early response genes by GH in 3T3-F442A cells is mediated by C/EBPβ. A subset of these genes is regulated similarly to c-Fos, through a mechanism involving GH-stimulated ERK 1/2 activation, phosphorylation of C/EBPβ, and recruitment of p300. Overall, these studies suggest that C/EBPβ, like the signal transducer and activator of transcription proteins, regulates multiple genes in response to GH.

Identification of differentially expressed genes in the kidneys of growth hormone transgenic mice

OBJECTIVE

Bovine growth hormone (bGH) transgenic mice develop severe kidney damage. This damage may be due, at least in part, to changes in gene expression. Identification of genes with altered expression in the bGH kidney may identify mechanisms leading to damage in this system that may also be relevant to other models of kidney damage.

DESIGN

cDNA subtraction libraries, northern blot analyses, microarray analyses and real-time reverse transcription polymerase chain reaction (RT/PCR) assays were used to identify and verify specific genes exhibiting differential RNA expression between kidneys of bGH mice and their non-transgenic (NT) littermates.

RESULTS

Immunoglobulins were the vast majority of genes identified by the cDNA subtractions and the microarray analyses as being up-regulated in bGH. Several glycoprotein genes and inflammation-related genes also showed increased RNA expression in the bGH kidney. In contrast, only a few genes were identified as being significantly down-regulated in the bGH kidney. The most notable decrease in RNA expression was for the gene encoding kidney androgen-regulated protein.

CONCLUSIONS

A number of genes were identified as being differentially expressed in the bGH kidney. Inclusion of two groups, immunoglobulins and inflammation-related genes, suggests a role of the immune system in bGH kidney damage.

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.

Dispersed Chromosomal Stat5b-binding elements mediate growth hormone-activated insulin-like growth factor-I gene transcription

The growth hormone (GH)-insulin-like growth factor-I (IGF-I) axis regulates somatic growth during childhood and orchestrates tissue repair throughout the life span. Recently described inactivating mutations in Stat5b in humans with impaired growth have focused attention on this transcription factor as a key agent linking GH-stimulated signals to IGF-I gene expression, and several putative Stat5b sites have been identified in the IGF-I gene. Here, we define and characterize potential GH- and Stat5b-activated chromosomal enhancers that can regulate IGF-I gene transcription. Of 89 recognizable Stat5 sequences in 200 kb centering on the rat IGF-I gene, 22 resided within conserved regions and/or were identical among different species. Only 15 of these sites, organized into 7 distinct domains, were found to bind Stat5b by quantitative chromatin immunoprecipitation assays in liver chromatin of rats, but only after acute GH treatment. These sites could bind Stat5b in vitro, and individual domains could mediate GH- and Stat5b-stimulated IGF-I promoter activity in cultured cells. Further analyses revealed that four Stat5b domains possessed chromatin signatures of enhancers, including binding of co-activators p300 and Med1, and RNA polymerase II. These modifications preceded GH-stimulated recruitment of Stat5b, as did lysine 4 monomethylation of histone H3, which was enriched in 6/7 Stat5b-binding elements. In contrast, histone acetylation was induced by GH but was limited to Stat5b binding domains found within the IGF-I transcription unit. We conclude that GH stimulates recruitment of Stat5b to multiple dispersed regions within the igf1 locus, including several with properties consistent with long range transcriptional enhancers that collectively regulate GH-activated IGF-I gene transcription.