Intermittent plasma growth hormone triggers tyrosine phosphorylation and nuclear translocation of a liver-expressed, Stat 5-related DNA binding protein. Proposed role as an intracellular regulator of male-specific liver gene transcription

Food deprivation reduces sensitivity of liver Igf1 synthesis pathways to growth hormone in juvenile gopher rockfish (Sebastes carnatus)

Growth hormone (Gh) regulates growth in part by stimulating the liver to synthesize and release insulin-like growth factor-1 (Igf1), which then promotes somatic growth. However, for fish experiencing food limitation, elevated blood Gh can occur even with low circulating Igf1 and slow growth, suggesting that nutritional stress can alter the sensitivity of liver Igf1 synthesis pathways to Gh. Here, we examined how recent feeding experience affected Gh regulation of liver Igf1 synthesis pathways in juvenile gopher rockfish (Sebastes carnatus) to illuminate mechanisms underlying the nutritional modulation of Igf1 production. Juvenile gopher rockfish were maintained under conditions of feeding or complete food deprivation (fasting) for 14 d and then treated with recombinant sea bream (Sparus aurata) Gh or saline control. Gh upregulated hepatic igf1 mRNA levels in fed fish but not in fasted fish. The liver of fasted rockfish also showed a lower relative abundance of gene transcripts encoding teleost Gh receptors 1 (ghr1) and 2 (ghr2), as well as reduced protein levels of phosphorylated janus tyrosine kinase 2 (pJak2) and signal transducer and activator of transcription 5 (pStat5), which function to induce igf1 gene transcription following Gh binding to Gh receptors. Relative hepatic mRNA levels for suppressors of cytokine signaling (Socs) genes socs2, socs3a, and socs3b were also lower in fasted rockfish. Socs2 can suppress Gh activation of Jak2/Stat5, and fasting-related variation in socs expression may reflect modulated inhibitory control of igf1 gene transcription. Fasted rockfish also had elevated liver mRNA abundances for lipolytic hormone-sensitive lipase 1 (hsl1) and Igf binding proteins igfbp1a, -1b and -3a, reduced liver mRNAs encoding igfbp2b and an Igfbp acid labile subunit-like (igfals) gene, and higher transcript abundances for Igf1 receptors igf1ra and igf1rb in skeletal muscle. Together, these findings suggest that food deprivation impacts liver Igf1 responsiveness to Gh via multiple mechanisms that include a downregulation of hepatic Gh receptors, modulation of the intracellular Jak2/Stat5 transduction pathway, and possible shifts in Socs-inhibitory control of igf1 gene transcription, while also demonstrating that these changes occur in concert with shifts in liver Igfbp expression and muscle Gh/Igf1 signaling pathway components.

Plasma growth hormone pulses induce male-biased pulsatile chromatin opening and epigenetic regulation in adult mouse liver

Sex differences in plasma growth hormone (GH) profiles, pulsatile in males and persistent in females, regulate sex differences in hepatic STAT5 activation linked to sex differences in gene expression and liver disease susceptibility, but little is understood about the fundamental underlying, GH pattern-dependent regulatory mechanisms. Here, DNase-I hypersensitivity site (DHS) analysis of liver chromatin accessibility in a cohort of 18 individual male mice established that the endogenous male rhythm of plasma GH pulse-stimulated liver STAT5 activation induces dynamic, repeated cycles of chromatin opening and closing at several thousand liver DHS and comprises a novel mechanism conferring male bias to liver chromatin accessibility. Strikingly, a single physiological replacement dose of GH given to hypophysectomized male mice restored, within 30 min, liver STAT5 activity and chromatin accessibility at 83% of the dynamic, pituitary hormone-dependent male-biased DHS. Sex-dependent transcription factor binding patterns and chromatin state analysis identified key genomic and epigenetic features distinguishing this dynamic, STAT5-driven mechanism of male-biased chromatin opening from a second GH-dependent mechanism operative at static male-biased DHS, which are constitutively open in male liver. Dynamic but not static male-biased DHS adopt a bivalent-like epigenetic state in female liver, as do static female-biased DHS in male liver, albeit using distinct repressive histone marks in each sex, namely, H3K9me3 at male-biased DHS in female liver and H3K27me3 at female-biased DHS in male liver. Moreover, sex-biased H3K36me3 marks are uniquely enriched at static sex-biased DHS, which may serve to keep these sex-dependent hepatocyte enhancers free of H3K27me3 repressive marks and thus constitutively open. Pulsatile chromatin opening stimulated by endogenous, physiological hormone pulses is thus one of two distinct GH-determined mechanisms for establishing widespread sex differences in hepatic chromatin accessibility and epigenetic regulation, both closely linked to sex-biased gene transcription and the sexual dimorphism of liver function.

Plasma Growth Hormone Pulses Induce Male-biased Pulsatile Chromatin Opening and Epigenetic Regulation in Adult Mouse Liver

Sex-differences in plasma growth hormone (GH) profiles, pulsatile in males and persistent in females, regulate sex differences in hepatic STAT5 activation linked to sex differences in gene expression and liver disease susceptibility, but little is understood about the fundamental underlying, GH pattern-dependent regulatory mechanisms. Here, DNase hypersensitivity site (DHS) analysis of liver chromatin accessibility in a cohort of 18 individual male mice established that the endogenous male rhythm of plasma GH pulse-stimulated liver STAT5 activation induces dynamic, repeated cycles of chromatin opening and closing at several thousand liver DHS and comprises a novel mechanism conferring male bias to liver chromatin accessibility. Strikingly, a single physiological replacement dose of GH given to hypophysectomized male mice restored, within 30 min, liver STAT5 activity and chromatin accessibility at 83% of the pituitary hormone-dependent dynamic male-biased DHS. Sex-dependent transcription factor binding patterns and chromatin state analysis identified key genomic and epigenetic features distinguishing this dynamic, STAT5-driven mechanism of male-biased chromatin opening from a second GH-dependent mechanism operative at static male-biased DHS, which are constitutively open in male liver. Dynamic but not static male-biased DHS adopt a bivalent-like epigenetic state in female liver, as do static female-biased DHS in male liver, albeit using distinct repressive histone marks in each sex, namely, H3K27me3 at female-biased DHS in male liver, and H3K9me3 at male-biased DHS in female liver. Moreover, sex-biased H3K36me3 marks are uniquely enriched at static sex-biased DHS, which may serve to keep these sex-dependent hepatocyte enhancers free of H3K27me3 repressive marks and thus constitutively open. Pulsatile chromatin opening stimulated by endogenous, physiological hormone pulses is thus one of two distinct GH-determined mechanisms for establishing widespread sex differences in hepatic chromatin accessibility and epigenetic regulation, both closely linked to sex-biased gene transcription and the sexual dimorphism of liver function.

Sex-biased gene expression across mammalian organ development and evolution

Sexually dimorphic traits are common among mammals and are specified during development through the deployment of sex-specific genetic programs. Because little is known about these programs, we investigated them using a resource of gene expression profiles in males and females throughout the development of five organs in five mammals (human, mouse, rat, rabbit, and opossum) and a bird (chicken). We found that sex-biased gene expression varied considerably across organs and species and was often cell-type specific. Sex differences increased abruptly around sexual maturity instead of increasing gradually during organ development. Finally, sex-biased gene expression evolved rapidly at the gene level, with differences between organs in the evolutionary mechanisms used, but more slowly at the cellular level, with the same cell types being sexually dimorphic across species.

Developmental Adaptive Immune Defects Associated with STAT5B Deficiency in Three Young Siblings

Patients with rare homozygous mutations in signal transducer and activator of transcription 5B (STAT5B) develop immunodeficiency resulting in chronic eczema, chronic infections, autoimmunity, and chronic lung disease. STAT5B-deficient patients are typically diagnosed in the teenage years, limiting our understanding of the development of associated phenotypic immune abnormalities. We report the first detailed chronological account of post-natal immune dysfunction associated with STAT5B deficiency in humans. Annual immunophenotyping of three siblings carrying a novel homozygous nonsense mutation in STAT5B was carried out over 4 years between the ages of 7 months to 8 years. All three siblings demonstrated consistent B cell hyperactivity including elevated IgE levels and autoantibody production, associated with diagnoses of atopy and autoimmunity. Total T cell levels in each sibling remained normal, with regulatory T cells decreasing in the oldest sibling. Interestingly, a skewing toward memory T cells was identified, with the greatest changes in CD8+ effector memory T cells. These results suggest an importance of STAT5B in B cell function and naïve versus memory T cell survival. Progressive dysregulation of FOXP3+ regulatory T cells and CD8+ memory T cell subsets reveal a crucial role of STAT5B in T cell homeostasis. The early diagnosis and focused immune evaluations of these three young STAT5B-deficient siblings support an important role of STAT5B in adaptive immune development and function.

Sex Differences and the Neuroendocrine Regulation of Seasonal Reproduction by Supplementary Environmental Cues

Seasonal rhythms in reproduction are conserved across nature and optimize the timing of breeding to environmental conditions favorable for offspring and parent survival. The primary predictive cue for timing seasonal breeding is photoperiod. Supplementary cues, such as food availability, social signals, and temperature, fine-tune the timing of reproduction. Male and female animals show differences in the sensory detection, neural integration, and physiological responses to the same supplementary cue. The neuroendocrine regulation of sex-specific integration of predictive and supplementary cues is not well characterized. Recent findings indicate that epigenetic modifications underlie the organization of sex differences in the brain. It has also become apparent that deoxyribonucleic acid methylation and chromatin modifications play an important role in the regulation and timing of seasonal rhythms. This article will highlight evidence for sex-specific responses to supplementary cues using data collected from birds and mammals. We will then emphasize that supplementary cues are integrated in a sex-dependent manner due to the neuroendocrine differences established and maintained by the organizational and activational effects of reproductive sex hormones. We will then discuss how epigenetic processes involved in reproduction provide a novel link between early-life organizational effects in the brain and sex differences in the response to supplementary cues.

Establishment and analysis of a mouse model that regulates sex-related differences in liver drug metabolism

The adult liver performs many metabolic functions for maintaining homeostasis. There are several sex differences in liver function and disease pathogenesis. One important function of the liver is drug metabolism, where cytochrome p450s (CYPs) in hepatocytes are the main enzymes involved. The toxicity of various drugs and chemicals differs with sex due to differences in hepatocytic CYP expression. However, the molecular mechanism regulating sex-related differences in drug metabolism remains unknown. In this study, we identified transcriptional regulator B-cell lymphoma 6 (Bcl6) as an important factor in sex-biased differential CYP expression. Microarray analysis of livers derived from liver-specific Bcl6-knockout mice showed that Bcl6 is required for sex-biased CYP expression patterns in the liver. Additionally, quantitative PCR analysis revealed that hepatocytic expression of male-biased genes, such as Cyp2d9, Cyp2u1, Cyp4a12a/12b, and Cyp7b1, in liver-specific Bcl6-knockout male mice significantly decreased to levels similar to those observed in wild-type female mice. Conversely, hepatocytic expression of female-biased genes, such as Cyp2a4/2a5, Cyp2b9, Cyp3a41, and Cyp17a1, significantly increased in liver-specific Bcl6-knockout male mice. Deletion of Bcl6 caused female-like expression of CYPs in male livers. These results suggest that Bcl6 is a key regulator of sex-related differential regulation of drug metabolism. Moreover, serum sex hormone levels and fertility did not change in liver-specific, Bcl6-knockout mice. Hepatocytic Bcl6 regulates sex-related differential CYP expression in the liver without changing the sex of the whole body. Thus, this mouse model is useful for analyzing liver-specific sex-dependent regulation of drug metabolism and pathogenesis.

Hepatic growth hormone - JAK2 - STAT5 signalling: Metabolic function, non-alcoholic fatty liver disease and hepatocellular carcinoma progression

The rising prevalence of obesity came along with an increase in associated metabolic disorders in Western countries. Non-alcoholic fatty liver disease (NAFLD) represents the hepatic manifestation of the metabolic syndrome and is linked to primary stages of liver cancer development. Growth hormone (GH) regulates various vital processes such as energy supply and cellular regeneration. In addition, GH regulates various aspects of liver physiology through activating the Janus kinase (JAK) 2- signal transducer and activator of transcription (STAT) 5 pathway. Consequently, disrupted GH - JAK2 - STAT5 signaling in the liver alters hepatic lipid metabolism and is associated with NAFLD development in humans and mouse models. Interestingly, while STAT5 as well as JAK2 deficiency correlates with hepatic lipid accumulation, recent studies suggest that these proteins have unique ambivalent functions in chronic liver disease progression and tumorigenesis. In this review, we focus on the consequences of altered GH - JAK2 - STAT5 signaling for hepatic lipid metabolism and liver cancer development with an emphasis on lessons learned from genetic knockout models.

Functional Roles of Sex-Biased, Growth Hormone-Regulated MicroRNAs miR-1948 and miR-802 in Young Adult Mouse Liver

Sex-specific temporal patterns of pituitary growth hormone (GH) secretion determine the sex-biased transcription of hundreds of genes in the liver and impart important sex differences in liver physiology, metabolism, and disease. Sex differences in hepatic gene expression vary widely, ranging from less than twofold to >1000-fold in the mouse. Here, we use small RNA sequencing to discover 24 sex-biased mouse liver microRNAs (miRNAs), and then investigate the roles of two of these miRNAs in GH-regulated liver sex differences. Studies in prepubertal and young adult mice, and in mice in which pituitary hormones are ablated or where sex-specific hepatic GH signaling is dysregulated, demonstrated that the male-biased miR-1948 and the female-biased miR-802 are both regulated by sex-specific pituitary GH secretory patterns, acquire sex specificity at puberty, and are dependent on the GH-activated transcription factor STAT5 for their sex-specific expression. Both miRNAs are within genomic regions characterized by sex-biased chromatin accessibility. miR-1948, an uncharacterized miRNA, has essential features for correct Drosha/Dicer processing, generates a bona fide mature miRNA with strong strand bias for the 5p arm, and is bound by Argonaute in liver tissue, as is miR-802. In vivo studies using inhibitory locked nucleic acid sequences revealed that miR-1948-5p preferentially represses female-biased messenger RNAs (mRNAs) and induces male-biased mRNAs in male liver; conversely, miR-802-5p preferentially represses male-biased mRNAs and increases levels of female-biased mRNAs in female liver. Cytochrome P450 mRNAs were strongly enriched as targets of both miRNAs. Thus, miR-1948-5p and miR-802-5p are functional components of the GH regulatory network that shapes sex-differential gene expression in mouse liver.

Smooth Muscle-Specific BCL6+/- Knockout Abrogates Sex Bias in Chronic Hypoxia-Induced Pulmonary Arterial Hypertension in Mice

The "estrogen paradox" in pulmonary arterial hypertension (PAH) refers to observations that while there is a higher incidence of idiopathic PAH in women, rodent models of PAH show male dominance and estrogens are protective. To explain these differences, we previously proposed the neuroendocrine-STAT5-BCL6 hypothesis anchored in the sex-biased and species-specific patterns of growth hormone (GH) secretion by the pituitary, the targeting of the hypothalamus by estrogens to feminize GH secretion patterns, and the role of the transcription factors STAT5a/b and BCL6 as downstream mediators of this patterned GH-driven sex bias. As a test of this hypothesis, we previously reported that vascular smooth muscle cell- (SMC-) specific deletion of the STAT5a/b locus abrogated the male-dominant sex bias in the chronic hypoxia model of PAH in mice. In the present study, we confirmed reduced BCL6 expression in pulmonary arterial (PA) segments in both male and female SMC:STAT5a/b-/- mice. In order to test the proposed contribution of BCL6 to sex bias in PAH, we developed mice with SMC-specific deletion of BCL6+/- by crossing SM22α-Cre mice with BCL6-floxed mice and investigated sex bias in these mutant mice in the chronic hypoxia model of PAH. We observed that the male-bias observed in wild-type- (wt-) SM22α-Cre-positive mice was abrogated in the SMC:BCL6+/- knockouts-both males and females showed equivalent enhancement of indices of PAH. The new data confirm BCL6 as a contributor to the sex-bias phenotype observed in hypoxic PAH in mice and support the neuroendocrine-STAT5-BCL6 hypothesis of sex bias in this experimental model of vascular disease.

Activation of Male Liver Chromatin Accessibility and STAT5-Dependent Gene Transcription by Plasma Growth Hormone Pulses

Sex differences in pituitary growth hormone (GH) secretion (pulsatile in males vs near continuous/persistent in females) impart sex-dependent expression to hundreds of genes in adult mouse liver. Signal transducer and activator of transcription (STAT) 5, a GH-activated transcription factor that is essential for liver sexual dimorphism, is dynamically activated in direct response to each male plasma GH pulse. However, the impact of GH-induced STAT5 pulses on liver chromatin accessibility and downstream transcriptional events is unknown. In this study, we investigated the impact of a single pulse of GH given to hypophysectomized mice on local liver chromatin accessibility (DNase hypersensitive site analysis), transcription rates (heterogeneous nuclear RNA analysis), and gene expression (quantitative polymerase chain reaction and RNA sequencing) determined 30, 90, or 240 minutes later. The STAT5-dependent but sex-independent early GH response genes Igf1 and Cish showed rapid, GH pulse-induced increases in chromatin accessibility and gene transcription, reversing the effects of hypophysectomy. Rapid increases in liver chromatin accessibility and transcriptional activity were also induced in hypophysectomized male mice for some (Ces2b, Ugt2b38) but not for other liver STAT5-dependent male-biased genes (Cyp7b1). Moreover, in pituitary-intact male mice, Igf1, Cish, Ces2b, and Ugt2b38 all showed remarkable cycles of chromatin opening and closing, as well as associated cycles of induced gene transcription, which closely followed each endogenous pulse of liver STAT5 activity. Thus, the endogenous rhythms of male plasma GH pulsation dynamically open and then close liver chromatin at discrete, localized regulatory sites in temporal association with transcriptional activation of Igf1, Cish, and a subset of STAT5-dependent male-biased genes.

Disruption of STAT5b-Regulated Sexual Dimorphism of the Liver Transcriptome by Diverse Factors Is a Common Event

Signal transducer and activator of transcription 5b (STAT5b) is a growth hormone (GH)-activated transcription factor and a master regulator of sexually dimorphic gene expression in the liver. Disruption of the GH hypothalamo-pituitary-liver axis controlling STAT5b activation can lead to metabolic dysregulation, steatosis, and liver cancer. Computational approaches were developed to identify factors that disrupt STAT5b function in a mouse liver gene expression compendium. A biomarker comprised of 144 STAT5b-dependent genes was derived using comparisons between wild-type male and wild-type female mice and between STAT5b-null and wild-type mice. Correlations between the STAT5b biomarker gene set and a test set comprised of expression datasets (biosets) with known effects on STAT5b function were evaluated using a rank-based test (the Running Fisher algorithm). Using a similarity p-value ≤ 10(-4), the test achieved a balanced accuracy of 99% and 97% for detection of STAT5b activation or STAT5b suppression, respectively. The STAT5b biomarker gene set was then used to identify factors that activate (masculinize) or suppress (feminize) STAT5b function in an annotated mouse liver and primary hepatocyte gene expression compendium of ~1,850 datasets. Disruption of GH-regulated STAT5b is a common phenomenon in liver in vivo, with 5% and 29% of the male datasets, and 11% and 13% of the female datasets, associated with masculinization or feminization, respectively. As expected, liver STAT5b activation/masculinization occurred at puberty and suppression/feminization occurred during aging and in mutant mice with defects in GH signaling. A total of 70 genes were identified that have effects on STAT5b activation in genetic models in which the gene was inactivated or overexpressed. Other factors that affected liver STAT5b function were shown to include fasting, caloric restriction and infections. Together, these findings identify diverse factors that perturb the hypothalamo-pituitary-liver GH axis and disrupt GH-dependent STAT5b activation in mouse liver.

Chemical and Hormonal Effects on STAT5b-Dependent Sexual Dimorphism of the Liver Transcriptome

The growth hormone (GH)-activated transcription factor signal transducer and activator of transcription 5b (STAT5b) is a key regulator of sexually dimorphic gene expression in the liver. Suppression of hepatic STAT5b signaling is associated with lipid metabolic dysfunction leading to steatosis and liver cancer. In the companion publication, a STAT5b biomarker gene set was identified and used in a rank-based test to predict both increases and decreases in liver STAT5b activation status/function with high (≥ 97%) accuracy. Here, this computational approach was used to identify chemicals and hormones that activate (masculinize) or suppress (feminize) STAT5b function in a large, annotated mouse liver and primary hepatocyte gene expression compendium. Exposure to dihydrotestosterone and thyroid hormone caused liver masculinization, whereas glucocorticoids, fibroblast growth factor 15, and angiotensin II caused liver feminization. In mouse models of diabetes and obesity, liver feminization was consistently observed and was at least partially reversed by leptin or resveratrol exposure. Chemical-induced feminization of male mouse liver gene expression profiles was a relatively frequent phenomenon: of 156 gene expression biosets from chemically-treated male mice, 29% showed feminization of liver STAT5b function, while <1% showed masculinization. Most (93%) of the biosets that exhibited feminization of male liver were also associated with activation of one or more xenobiotic-responsive receptors, most commonly constitutive activated receptor (CAR) or peroxisome proliferator-activated receptor alpha (PPARα). Feminization was consistently associated with increased expression of peroxisome proliferator-activated receptor gamma (Pparg) but not other lipogenic transcription factors linked to steatosis. GH-activated STAT5b signaling in mouse liver is thus commonly altered by diverse chemicals, and provides a linkage between chemical exposure and dysregulated gene expression associated with adverse effects on the liver.

Zinc Fingers and Homeoboxes 2 (Zhx2) Regulates Sexually Dimorphic Cyp Gene Expression in the Adult Mouse Liver

The mammalian cytochrome P450 (Cyp) gene family encodes a large number of structurally related enzymes that catalyze a variety of metabolic and detoxification reactions. The liver is the primary site of Cyp expression in terms of expression levels and number of expressed genes, consistent with this organ's essential role in metabolism of endogenous and xenobiotic compounds. Many Cyp genes exhibit sexually dimorphic expression. For example, Cyp2a4 is expressed significantly higher in the adult liver of female mice compared to male mice. An exception to this pattern is seen in BALB/cJ mice, where male hepatic Cyp2a4 mRNA levels are substantially elevated compared to male mice of other strains. The Zinc fingers and homeoboxes 2 (Zhx2) protein governs the silencing of several genes in the postnatal liver, including α-fetoprotein, H19, and glypican 3. Zhx2 also regulates numerous hepatic genes that govern lipid homeostasis. We previously showed that the Zhx2 gene is mutated in BALB/cJ mice, which led us to consider whether elevated male hepatic Cyp2a4 levels in this strain are due to this Zhx2 mutation. Using mice with a conditional Zhx2 deletion, we show here that the absence of Zhx2 in hepatocytes results in increased Cyp2a4 expression in adult male liver. We extend this finding to show that additional Cyp genes are disregulated in the absence of Zhx2. We also show that mRNA levels of Cyp2a4 and several other female-biased Cyp genes are increased, and male-biased Cyp4a12 is decreased in mouse liver tumors. These data indicate that Zhx2 is a novel regulator of sex-biased Cyp gene expression in the normal and diseased liver.

Hepatic Long Intergenic Noncoding RNAs: High Promoter Conservation and Dynamic, Sex-Dependent Transcriptional Regulation by Growth Hormone

Long intergenic noncoding RNAs (lincRNAs) are increasingly recognized as key chromatin regulators, yet few studies have characterized lincRNAs in a single tissue under diverse conditions. Here, we analyzed 45 mouse liver RNA sequencing (RNA-Seq) data sets collected under diverse conditions to systematically characterize 4,961 liver lincRNAs, 59% of them novel, with regard to gene structures, species conservation, chromatin accessibility, transcription factor binding, and epigenetic states. To investigate the potential for functionality, we focused on the responses of the liver lincRNAs to growth hormone stimulation, which imparts clinically relevant sex differences to hepatic metabolism and liver disease susceptibility. Sex-biased expression characterized 247 liver lincRNAs, with many being nuclear RNA enriched and regulated by growth hormone. The sex-biased lincRNA genes are enriched for nearby and correspondingly sex-biased accessible chromatin regions, as well as sex-biased binding sites for growth hormone-regulated transcriptional activators (STAT5, hepatocyte nuclear factor 6 [HNF6], FOXA1, and FOXA2) and transcriptional repressors (CUX2 and BCL6). Repression of female-specific lincRNAs in male liver, but not that of male-specific lincRNAs in female liver, was associated with enrichment of H3K27me3-associated inactive states and poised (bivalent) enhancer states. Strikingly, we found that liver-specific lincRNA gene promoters are more highly species conserved and have a significantly higher frequency of proximal binding by liver transcription factors than liver-specific protein-coding gene promoters. Orthologs for many liver lincRNAs were identified in one or more supraprimates, including two rat lincRNAs showing the same growth hormone-regulated, sex-biased expression as their mouse counterparts. This integrative analysis of liver lincRNA chromatin states, transcription factor occupancy, and growth hormone regulation provides novel insights into the expression of sex-specific lincRNAs and their potential for regulation of sex differences in liver physiology and disease.

Hypothesis: Neuroendocrine Mechanisms (Hypothalamus-Growth Hormone-STAT5 Axis) Contribute to Sex Bias in Pulmonary Hypertension

Pulmonary hypertension (PH) is a disease with high morbidity and mortality. The prevalence of idiopathic pulmonary arterial hypertension (IPAH) and hereditary pulmonary arterial hypertension (HPAH) is approximately two- to four-fold higher in women than in men. Paradoxically, there is an opposite male bias in typical rodent models of PH (chronic hypoxia or monocrotaline); in these models, administration of estrogenic compounds (for example, estradiol-17β [E2]) is protective. Further complexities are observed in humans ingesting anorexigens (female bias) and in rodent models, such as after hypoxia plus SU5416/Sugen (little sex bias) or involving serotonin transporter overexpression or dexfenfluramine administration (female bias). These complexities in sex bias in PH remain incompletely understood. We recently discovered that conditional deletion of signal transducer and activator of transcription 5a/b (STAT5a/b) in vascular smooth muscle cells abrogated the male bias in PH in hypoxic mice and that late-stage obliterative lesions in patients of both sexes with IPAH and HPAH showed reduced STAT5a/b, reduced Tyr-P-STAT5 and reduced B-cell lymphoma 6 protein (BCL6). In trying to understand the significance of these observations, we realized that there existed a well-characterized E2-sensitive central neuroendocrine mechanism of sex bias, studied over the last 40 years, that, at its peripheral end, culminated in species-specific male ("pulsatile") versus female ("more continuous") temporal patterns of circulating growth hormone (GH) levels leading to male versus female patterned activation of STAT5a/b in peripheral tissues and thus sex-biased expression of hundreds of genes. In this report, we consider the contribution of this neuroendocrine mechanism (hypothalamus-GH-STAT5) in the generation of sex bias in different PH situations.

STAT5a/b contribute to sex bias in vascular disease: A neuroendocrine perspective

Previous studies have elucidated a neuroendocrine mechanism consisting of the hypothalamus (growth hormone releasing hormone, GHRH) - pituitary (growth hormone, GH) - STAT5a/b axis that underlies sex-biased gene expression in the liver. It is now established that male vs female patterned secretion of GHRH, and thus of circulating GH levels ("pulsatile" vs "more continuous" respectively), leading to differently patterned activation of PY-STAT5a/b in hepatocytes results in sex-biased gene expression of cohorts of hundreds of downstream genes. This review outlines new data in support of a STAT5a/b-based mechanism of sex bias in the vascular disease pulmonary hypertension (PH). Puzzling observations in PH include its 2-4-fold higher prevalence in women but a male-dominance in many rodent models, and, paradoxically, inhibition of PH development by estrogens in such models. We observed that conditional deletion of STAT5a/b in vascular smooth muscle cells (SMC) in mice converted the male-dominant model of chronic hypoxia-induced PH into a female-dominant phenotype. In human idiopathic PH, there was reduced STAT5a/b and PY-STAT5 in cells in late-stage obliterative pulmonary arterial lesions in both men and women. A juxtaposition of the prior liver data with the newer PH-related data drew attention to the hypothalamus-GH-STAT5 axis, which is the major target of estrogens at the level of the hypothalamus. This hypothesis explains many of the puzzling aspects of sex bias in PH in humans and rodent models. The extension of STAT5-anchored mechanisms of sex bias to vascular disease emphasizes the contribution of central neuroendocrine processes in generating sexual dimorphism in different tissues and cell types.

Cross Talk Between GH-Regulated Transcription Factors HNF6 and CUX2 in Adult Mouse Liver

Hepatocyte-enriched nuclear factor (HNF)6 and CUX2 are GH and STAT5-regulated homeobox transcription factors. CUX2 shows female-specific expression and contributes to liver sex differences by repressing many male-biased genes and inducing many female-biased genes, whereas HNF6 is expressed at similar levels in male and female liver. In cell-based transfection studies, CUX2 inhibited HNF6 transcriptional regulation of the sex-specific gene promoters CYP2C11 and CYP2C12, blocking HNF6 repression of CYP2C11 and HNF6 activation of CYP2C12. These inhibitory actions of CUX2 can be explained by competition for HNF6 DNA binding, as demonstrated by in vitro EMSA analysis and validated in vivo by global analysis of the HNF6 cistrome. Approximately 40 000 HNF6-binding sites were identified in mouse liver chromatin, including several thousand sites showing significant sex differences in HNF6 binding. These sex-biased HNF6-binding sites showed strong enrichment for correspondingly sex-biased DNase hypersensitive sites and for proximity to genes showing local sex-biased chromatin marks and a corresponding sex-biased expression. Further, approximately 90% of the genome-wide binding sites for CUX2 were also bound by HNF6. These HNF6/CUX2 common binding sites were enriched for genomic regions more accessible in male than in female mouse liver chromatin and showed strongest enrichment for male-biased genes, suggesting CUX2 displacement of HNF6 as a mechanism to explain the observed CUX2 repression of male-biased genes in female liver. HNF6 binding was sex independent at a majority of its binding sites, and HNF6 peaks were frequently associated with cobinding by multiple other liver transcription factors, consistent with HNF6 playing a global regulatory role in both male and female liver.

Deletion of STAT5a/b in vascular smooth muscle abrogates the male bias in hypoxic pulmonary hypertension in mice: implications in the human disease

Chronic hypoxia typically elicits pulmonary hypertension (PH) in mice with a male-dominant phenotype. There is an opposite-sex bias in human PH, with a higher prevalence in women, but greater survival (the "estrogen paradox"). We investigated the involvement of the STAT5a/b species, previously established to mediate sexual dimorphism in other contexts, in the sex bias in PH. Mice with heterozygous or homozygous deletions of the STAT5a/b locus in vascular smooth muscle cells (SMCs) were generated in crosses between STAT5a/b(fl/fl) and transgelin (SM22α)-Cre(+/+) parents. Wild-type (wt) males subjected to chronic hypoxia showed significant PH and pulmonary arterial remodeling, with wt females showing minimal changes (a male-dominant phenotype). However, in conditional STAT5(+/-) or STAT5(-/-) mice, hypoxic females showed the severest manifestations of PH (a female-dominant phenotype). Immunofluorescence studies on human lung sections showed that obliterative pulmonary arterial lesions in patients with idiopathic pulmonary arterial hypertension (IPAH) or hereditary pulmonary arterial hypertension (HPAH), both male and female, overall had reduced STAT5a/b, reduced PY-STAT5 and reduced endoplasmic reticulum (ER) GTPase atlastin-3 (ATL3). Studies of SMCs and endothelial cell (EC) lines derived from vessels isolated from lungs of male and female IPAH patients and controls revealed instances of coordinate reductions in STAT5a, STAT5b and ATL3 in IPAH-derived cells, including SMCs and ECs from the same patient. Taken together, these data provide the first definitive evidence for a contribution of STAT5a/b to the sex bias in PH in the hypoxic mouse and implicate reduced STAT5 in the pathogenesis of the human disease.

Genetic background determines if Stat5b suppresses or enhances murine hepatocarcinogenesis

Murine hepatocarcinogenesis requires growth hormone (GH). To determine if the GH-responsive transcription factor STAT5b (signal transducer and activator of transcription 5b) is also required, we compared the hepatic gene expression profiles of global Stat5b null mice to cancer-resistant mice mutant in the GH pathway-GH-deficient little and androgen receptor-null Tfm males. We found a high degree of overlap among Tfm, little, and Stat5b null males. The liver cancer susceptibility of global Stat5b null mice was assessed on three distinct genetic backgrounds: BALB/cJ (BALB), C57BL/6J (B6), and C3H/HeJ (C3H). The effect of Stat5b on hepatocarcinogenesis depended on the genetic background. B6 Stat5b null congenic males and females developed 2.4 times as many tumors as wild-type (WT) controls (P < 0.002) and the tumors were larger (P < 0.003). In BALB/c congenics, loss of STAT5b had no effect on either sex. C3H Stat5b null congenic males and females were resistant to liver cancer, developing 2.7- and 6-fold fewer tumors, respectively (P < 0.02, 0.01). These results provide the first example of a single gene behaving as both oncogene and tumor suppressor in a given tissue, depending only on the endogenous modifier alleles carried by different genetic backgrounds.

Age- and sex-dependent DNA hypomethylation controlled by growth hormone in mouse liver

In mammals, differences in liver function and aging have been observed between sexes; however, the epigenetic mechanisms underlying such differences remain largely unexplored. In this study, we investigated sex- and age-dependent DNA methylation status in the mouse liver. We analyzed 90 known sex-differentially expressed genes, and identified sex-dependent methylation in Zfp809, Hsd3b5, Treh, Cxcl11, Cyp17a1, and Nnmt genes. After 4 weeks of age, we noted the gradual establishment of sex-dependent hypomethylation in each of these genes in either males or females. The exposure of male mice to female-like growth hormone (GH) profile repressed male-predominant hypomethylation and promoted female-predominant hypomethylation. The occurrence of age-dependent hypomethylation, including at loci for which we also observed sex-dependent changes in DNA methylation, was accompanied by the downregulation of DNMT3A/B. In addition, we found that age-dependent hypomethylation was promoted through liver regeneration induced by partial hepatectomy, suggesting that DNMT activities were not enough to retain methylation levels. In conclusion, our results demonstrate that sex-dependent GH profiles influence the age-progressive hypomethylation under decreased DNMT3A/B levels in certain regions of the genome.

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.

Hepatic growth hormone resistance after acute injury

Severe injury and infection are often followed by accelerated protein catabolism and acute insulin resistance. This results in several effects that complicate and prolong recovery, including weakness, immobility, impaired wound healing, and organ dysfunction. Recent studies have demonstrated the development of GH resistance during severe inflammation, providing a potential mechanism for the protein loss that follows injury and infection. To understand this GH resistance, we recently developed a murine model of acute injury. Mice were subjected to soft-tissue injury, alone or combined with hemorrhage, and injected iv with GH 30, 60, or 90 minutes later. Hepatic GH signaling was measured via Western analysis. GH-induced signal transducer and activator of transcription 5 phosphorylation was decreased immediately after completion of the trauma procedure, and at 30 and 60 minutes, but further decreased by 90 minutes after trauma. Combined trauma and hemorrhage resulted in severely decreased GH-induced signal transducer and activator of transcription 5 phosphorylation compared with trauma alone, and this was true at all time points studied. Western analysis revealed an apparent decrease in the molecular weight of the hepatic GH receptor (GHR) after trauma and hemorrhage, but not trauma alone. Additional studies determined that the hemorrhage-induced decrease in receptor size was not due to changes in GHR N-linked glycosylation. These results suggest that GH sensitivity is rapidly impaired after acute injury and that trauma combined with hemorrhage results in a more severe form of GH resistance resulting from alteration or inactivation of hepatic GHR.

Osteoblast-restricted Disruption of the Growth Hormone Receptor in Mice Results in Sexually Dimorphic Skeletal Phenotypes

Growth hormone (GH) exerts profound anabolic actions during postnatal skeletal development, in part, through stimulating the production of insulin-like growth factor-1 (IGF-1) in liver and skeletal tissues. To examine the requirement for the GH receptor (GHR) in osteoblast function in bone, we used Cre-LoxP methods to disrupt the GHR from osteoblasts, both in vitro and in vivo. Disruption of GHR from primary calvarial osteoblasts in vitro abolished GH-induced signaling, as assessed by JAK2/STAT5 phosphorylation, and abrogated GH-induced proliferative and anti-apoptotic actions. Osteoblasts lacking GHR exhibited reduced IGF-1-induced Erk and Akt phosphorylation and attenuated IGF-1-induced proliferation and anti-apoptotic action. In addition, differentiation was modestly impaired in osteoblasts lacking GHR, as demonstrated by reduced alkaline phosphatase staining and calcium deposition. In order to determine the requirement for the GHR in bone in vivo, we generated mice lacking the GHR specifically in osteoblasts (ΔGHR), which were born at the expected Mendelian frequency, had a normal life span and were of normal size. Three week-old, female ΔGHR mice had significantly reduced osteoblast numbers, consistent with the in vitro data. By six weeks of age however, female ΔGHR mice demonstrated a marked increase in osteoblasts, although mineralization was impaired; a phenotype similar to that observed previously in mice lacking IGF-1R specifically in osteoblasts. The most striking phenotype occurred in male mice however, where disruption of the GHR from osteoblasts resulted in a "feminization" of bone geometry in 16 week-old mice, as observed by μCT. These results demonstrate that the GHR is required for normal postnatal bone development in both sexes. GH appears to serve a primary function in modulating local IGF-1 action. However, the changes in bone geometry observed in male ΔGHR mice suggest that, in addition to facilitating IGF-1 action, GH may function to a greater extent than previously appreciated in establishing the sexual dimorphism of the skeleton.

Growth hormone STAT5-mediated signaling and its modulation in mice liver during the growth period

Postnatal growth exhibits two instances of rapid growth in mice: the first is perinatal and independent of growth hormone (GH), the second is peripuberal and GH-dependent. Signal transducer and activator of transcription 5b (STAT5b) is the main GH-signaling mediator and it is related to IGF1 synthesis and somatic growth. The aim of this work was to assess differential STAT5 sensitivity to GH during the growth period in mouse liver of both sexes. Three representative ages were selected: 1-week-old animals, in the GH-independent phase of growth; 2.5-week-old mice, at the onset of the GH-dependent phase of growth; and 9-week-old young adults. GH-signaling mediators were assessed by immunoblotting, quantitative RT-PCR and immunohistochemistry. GH-induced STAT5 phosphorylation is low at one-week and maximal at 2.5-weeks of age when compared to young adults, accompanied by higher protein content at the onset of growth. Suppressor CIS and phosphatase PTP1B exhibit high levels in one-week animals, which gradually decline, while SOCS2 and SOCS3 display higher levels at adulthood. Nuclear phosphorylated STAT5 is low in one-week animals while in 2.5-week animals it is similar to 9-week control; expression of SOCS3, an early response GH-target gene, mimics this pattern. STAT5 coactivators glucocorticoid receptor (GR) and hepatic nuclear factor 1 (HNF1) abundance is higher in adulthood. Therefore, GH-induced STAT5 signaling presents age-dependent activity in liver, with its maximum coinciding with the onset of GH-dependent phase of growth, accompanied by an age-dependent variation of modulating factors. This work contributes to elucidate the molecular mechanisms implicated in GH responsiveness during growth.

Impact of CUX2 on the female mouse liver transcriptome: activation of female-biased genes and repression of male-biased genes

The growth hormone-regulated transcription factors STAT5 and BCL6 coordinately regulate sex differences in mouse liver, primarily through effects in male liver, where male-biased genes are upregulated and many female-biased genes are actively repressed. Here we investigated whether CUX2, a highly female-specific liver transcription factor, contributes to an analogous regulatory network in female liver. Adenoviral overexpression of CUX2 in male liver induced 36% of female-biased genes and repressed 35% of male-biased genes. In female liver, CUX2 small interfering RNA (siRNA) preferentially induced genes repressed by adenovirus expressing CUX2 (adeno-CUX2) in male liver, and it preferentially repressed genes induced by adeno-CUX2 in male liver. CUX2 binding in female liver chromatin was enriched at sites of male-biased DNase hypersensitivity and at genomic regions showing male-enriched STAT5 binding. CUX2 binding was also enriched near genes repressed by adeno-CUX2 in male liver or induced by CUX2 siRNA in female liver but not at genes induced by adeno-CUX2, indicating that CUX2 binding is preferentially associated with gene repression. Nevertheless, direct CUX2 binding was seen at several highly female-specific genes that were positively regulated by CUX2, including A1bg, Cyp2b9, Cyp3a44, Tox, and Trim24. CUX2 expression and chromatin binding were high in immature male liver, where repression of adult male-biased genes and expression of adult female-biased genes are common, suggesting that the downregulation of CUX2 in male liver at puberty contributes to the developmental changes establishing adult patterns of sex-specific gene expression.

The influence of estrogens on the biological and therapeutic actions of growth hormone in the liver

GH is main regulator of body growth and composition, somatic development, intermediate metabolism and gender-dependent dimorphism in mammals. The liver is a direct target of estrogens because it expresses estrogen receptors which are connected with development, lipid metabolism and insulin sensitivity, hepatic carcinogenesis, protection from drug-induced toxicity and fertility. In addition, estrogens can modulate GH actions in liver by acting centrally, regulating pituitary GH secretion, and, peripherally, by modulating GHR-JAK2-STAT5 signalling pathway. Therefore, the interactions of estrogens with GH actions in liver are biologically and clinically relevant because disruption of GH signaling may cause alterations of its endocrine, metabolic, and gender differentiated functions and it could be linked to dramatic impact in liver physiology during development as well as in adulthood. Finally, the interplay of estrogens with GH is relevant because physiological roles these hormones have in human, and the widespread exposition of estrogen or estrogen-related compounds in human. This review highlights the importance of these hormones in liver physiology as well as how estrogens modulate GH actions in liver which will help to improve the clinical use of these hormones.

Dynamic, sex-differential STAT5 and BCL6 binding to sex-biased, growth hormone-regulated genes in adult mouse liver

Sex-dependent pituitary growth hormone (GH) secretory patterns determine the sex-biased expression of >1,000 genes in mouse and rat liver, affecting lipid and drug metabolism, inflammation, and disease. A fundamental biological question is how robust differential expression can be achieved for hundreds of sex-biased genes simply based on the GH input signal pattern: pulsatile GH stimulation in males versus near-continuous GH exposure in females. STAT5 is an essential transcriptional mediator of the sex-dependent effects of GH in the liver, but the mechanisms that underlie its sex-dependent actions are obscure. Here we elucidate the dynamic, sex-dependent binding of STAT5 and the GH/STAT5-regulated repressor BCL6 to mouse liver chromatin genome wide, revealing a counteractive interplay between these two regulators of sex differences in liver gene expression. Our findings establish a close correlation between sex-dependent STAT5 binding and sex-biased target gene expression. Moreover, sex-dependent STAT5 binding correlated positively with sex-biased DNase hypersensitivity and H3-K4me1 and H3-K4me3 (activating) marks, correlated negatively with sex-biased H3-K27me3 (repressive) marks, and was associated with sex-differentially enriched motifs for HNF6/CDP factors. Importantly, BCL6 binding was preferentially associated with repression of female-biased STAT5 targets in male liver. Furthermore, BCL6 and STAT5 common targets but not BCL6 unique targets showed strong enrichment for lipid and drug metabolism. These findings provide a comprehensive, genome-wide view of the mechanisms whereby these two GH-regulated transcription factors establish and maintain sex differences affecting liver physiology and disease. The approaches used here to characterize sex-dependent STAT5 and BCL6 binding can be applied to other condition-specific regulatory factors and binding sites and their interplay with cooperative chromatin binding factors.

Child health, developmental plasticity, and epigenetic programming

Plasticity in developmental programming has evolved in order to provide the best chances of survival and reproductive success to the organism under changing environments. Environmental conditions that are experienced in early life can profoundly influence human biology and long-term health. Developmental origins of health and disease and life-history transitions are purported to use placental, nutritional, and endocrine cues for setting long-term biological, mental, and behavioral strategies in response to local ecological and/or social conditions. The window of developmental plasticity extends from preconception to early childhood and involves epigenetic responses to environmental changes, which exert their effects during life-history phase transitions. These epigenetic responses influence development, cell- and tissue-specific gene expression, and sexual dimorphism, and, in exceptional cases, could be transmitted transgenerationally. Translational epigenetic research in child health is a reiterative process that ranges from research in the basic sciences, preclinical research, and pediatric clinical research. Identifying the epigenetic consequences of fetal programming creates potential applications in clinical practice: the development of epigenetic biomarkers for early diagnosis of disease, the ability to identify susceptible individuals at risk for adult diseases, and the development of novel preventive and curative measures that are based on diet and/or novel epigenetic drugs.

Effects of a growth hormone-releasing hormone analog on endogenous GH pulsatility and insulin sensitivity in healthy men

CONTEXT AND OBJECTIVE

Strategies to augment pulsatile GH may be beneficial in patients with excess visceral adiposity, in whom GH secretion is reduced. The objective of this study was to determine the effects of a novel GHRH (GHRH(1-44)) analog, tesamorelin, on endogenous GH pulsatility and insulin sensitivity in healthy men.

DESIGN, PARTICIPANTS, AND INTERVENTION

Thirteen males (mean age 45 ± 3 yr and body mass index 27.3 ± 1.2 kg/m(2)) received tesamorelin 2 mg sc once daily for 2 wk, with assessment made at baseline, after 2 wk of treatment, and after 2 wk of withdrawal.

OUTCOME MEASURES

The primary end point was change in mean overnight GH as determined by overnight frequent sampling. Secondary end points included insulin-stimulated glucose uptake as measured by euglycemic hyperinsulinemic clamp; IGF-I; and GH secretion parameters, including pulse area, pulse frequency, and basal secretion.

RESULTS

Tesamorelin treatment increased mean overnight GH (change +0.5 ± 0.1 μg/liter, P = 0.004), average log(10) GH peak area (change +0.4 ± 0.1 log(10) μg/liter, P = 0.001), and basal GH secretion (change +0.008 ± 0.003 μg/liter · min, P = 0.008). IGF-I increased by 181 ± 22 μg/liter (P < 0.0001). Neither fasting glucose (P = 0.93) nor insulin-stimulated glucose uptake (P = 0.61) was significantly affected by tesamorelin.

CONCLUSIONS

Once-daily short-term treatment with a GHRH(1-44) analog, tesamorelin, augments basal and pulsatile GH secretion. Moreover, although tesamorelin significantly increases IGF-I, peripheral insulin-stimulated glucose uptake appears to be preserved.

Encoding and decoding mechanisms of pulsatile hormone secretion

Ultradian pulsatile hormone secretion underlies the activity of most neuroendocrine systems, including the hypothalamic-pituitary adrenal (HPA) and gonadal (HPG) axes, and this pulsatile mode of signalling permits the encoding of information through both amplitude and frequency modulation. In the HPA axis, glucocorticoid pulse amplitude increases in anticipation of waking, and, in the HPG axis, changing gonadotrophin-releasing hormone pulse frequency is the primary means by which the body alters its reproductive status during development (i.e. puberty). The prevalence of hormone pulsatility raises two crucial questions: how are ultradian pulses encoded (or generated) by these systems, and how are these pulses decoded (or interpreted) at their target sites? We have looked at mechanisms within the HPA axis responsible for encoding the pulsatile mode of glucocorticoid signalling that we observe in vivo. We review evidence regarding the 'hypothalamic pulse generator' hypothesis, and describe an alternative model for pulse generation, which involves steroid feedback-dependent endogenous rhythmic activity throughout the HPA axis. We consider the decoding of hormone pulsatility by taking the HPG axis as a model system and focussing on molecular mechanisms of frequency decoding by pituitary gonadotrophs.

Disrupted corticosterone pulsatile patterns attenuate responsiveness to glucocorticoid signaling in rat brain

Chronically elevated circulating glucocorticoid levels are although to enhance vulnerability to psychopathology. Here we hypothesized that such sustained glucocorticoid levels, disturbing corticosterone pulsatility, attenuate glucocorticoid receptor signaling and target gene responsiveness to an acute challenge in the rat brain. Rats were implanted with vehicle or 40 or 100% corticosterone pellets known to flatten ultradian and circadian rhythmicity while maintaining daily average levels or mimic pathological conditions. Additionally, recovery from constant exposure was studied in groups that had the pellet removed 24 h prior to the challenge. Molecular markers for receptor responsiveness (receptor levels, nuclear translocation, promoter occupancy, and target gene expression) to an acute challenge mimicking the stress response (3 mg/kg ip) were studied in the hippocampal area. Implantation of 40 and 100% corticosterone pellets dose-dependently down-regulated glucocorticoid receptor and attenuated mineralocorticoid receptor and glucocorticoid receptor translocation to the acute challenge. Interestingly, whereas target gene Gilz expression to the challenge was already attenuated by tonic daily average levels (40%), Sgk-1 was affected only after constant high corticosterone exposure (100%), indicating altered receptor responsiveness due to treatment. Washout of 100% corticosterone recovered all molecular markers (partial), whereas removal of the 40% corticosterone pellet still attenuated responsiveness to the challenge. We propose that corticosteroid pulsatility is crucial in maintaining normal responsiveness to glucocorticoids. Whereas the results with 100% corticosterone are likely attributed to receptor saturation, subtle changes in the pattern of exposure (40%) induces changes at least as severe for glucocorticoid signaling as overt hypercorticism, suggesting an underlying mechanism sensitive to the pattern of hormone exposure.

Intrinsic sex differences in the early growth hormone responsiveness of sex-specific genes in mouse liver

Sex differences in liver gene expression are dictated by sex differences in circulating GH profiles. Presently, the pituitary hormone dependence of mouse liver gene expression was investigated on a global scale to discover sex-specific early GH response genes that could contribute to sex-specific regulation of downstream GH targets and to ascertain whether intrinsic sex differences characterize hepatic responses to plasma GH stimulation. Global RNA expression analysis identified two distinct classes of sex-specific mouse liver genes: genes subject to positive regulation (class I) and genes subject to negative regulation by pituitary hormones (class II). Genes activated or repressed in hypophysectomized (Hypox) mouse liver within 30-90 min of GH pulse treatment at a physiological dose were identified as putative direct targets of GH action (early response genes). Intrinsic sex differences in the GH responsiveness of a subset of these early response genes were observed. Notably, 45 male-specific genes, including five encoding transcriptional regulators that may mediate downstream sex-specific transcriptional responses, were induced by GH within 30 min in Hypox male but not Hypox female mouse liver. The early GH response genes were enriched in 29 male-specific targets of the transcription factor myocyte enhancer factor 2, whose activation in hepatic stellate cells is associated with liver fibrosis leading to hepatocellular carcinoma, a male-predominant disease. Thus, the rapid activation by GH pulses of certain sex-specific genes is modulated by intrinsic sex-specific factors, which may be associated with prior hormone exposure (epigenetic mechanisms) or genetic factors that are pituitary-independent, and could contribute to sex differences in predisposition to liver cancer or other hepatic patho-physiologies.

Origin of ultradian pulsatility in the hypothalamic-pituitary-adrenal axis

The hypothalamic–pituitary–adrenal (HPA) axis is a neuroendocrine system that regulates the circulating levels of vital glucocorticoid hormones. The activity of the HPA axis is characterized not only by a classic circadian rhythm, but also by an ultradian pattern of discrete pulsatile release of glucocorticoids. A number of psychiatric and metabolic diseases are associated with changes in glucocorticoid pulsatility, and it is now clear that glucocorticoid responsive genes respond to these rapid fluctuations in a biologically meaningful way. Theoretical modelling has enabled us to identify and explore potential mechanisms underlying the ultradian activity in this axis, which to date have not been identified successfully. We demonstrate that the combination of delay with feed-forward and feedback loops in the pituitary–adrenal system is sufficient to give rise to ultradian pulsatility in the absence of an ultradian source from a supra-pituitary site. Moreover, our model enables us to predict the different patterns of glucocorticoid release mediated by changes in hypophysial-portal corticotrophin-releasing hormone levels, with results that parallel our experimental in vivo data.

Male-specific hepatic Bcl6: growth hormone-induced block of transcription elongation in females and binding to target genes inversely coordinated with STAT5

The transcriptional repressor Bcl6 is a male-specific rat liver gene product and one of 24 early GH-response genes encoding DNA-binding proteins. Presently, the sex specificity of Bcl6 was shown to emerge at puberty, when hepatic Bcl6 mRNA was induced in males and repressed in females by the female plasma GH profile. Hepatic Bcl6 mRNA was increased to near-normal male levels in hypophysectomized females and was extinguished in intact males given a continuous GH infusion (female-like GH pattern). Bcl6 was also repressed in adult male somatostatin-deficient mice, where plasma GH profiles are female like. Hepatic Bcl6 RNA was rapidly down-regulated by GH pulse treatment, both in hypophysectomized male rats and in primary rat hepatocytes. Bcl6 was substantially induced in female mice deficient in hepatic signal transducer and activator of transcription (STAT)5a/STAT5b, suggesting that these STAT transcriptional mediators of GH signaling repress Bcl6. Indeed, STAT5 was bound to Bcl6 STAT5-binding region-B, previously associated with Bcl6 repression, in both male and female liver chromatin. STAT5 also bound to Bcl6 region-A in male chromatin but only during a plasma GH pulse. Analysis of primary transcripts (heterogeneous nuclear RNA) across the Bcl6 gene revealed a novel mechanism of GH-dependent sex specificity, with two apparent blocks in Bcl6 transcription elongation seen in female liver and in continuous GH-treated male liver, one early in intron 4 and one in exon 5, which together reduced transcription beyond exon 5 more than 300-fold. Finally, Bcl6 was bound to a subset of STAT5-binding sites in male liver chromatin, including a Socs2 STAT5-binding site where Bcl6 binding increased substantially between plasma GH pulses, i.e. when STAT5 binding was low. Bcl6 and STAT5 binding are thus inversely coordinated by the endogenous pulses of pituitary GH release, suggesting this male-specific transcriptional repressor modulates hepatic GH signaling to select STAT5 target genes.

Sex differences in the expression of hepatic drug metabolizing enzymes

Sex differences in pharmacokinetics and pharmacodynamics characterize many drugs and contribute to individual differences in drug efficacy and toxicity. Sex-based differences in drug metabolism are the primary cause of sex-dependent pharmacokinetics and reflect underlying sex differences in the expression of hepatic enzymes active in the metabolism of drugs, steroids, fatty acids and environmental chemicals, including cytochromes P450 (P450s), sulfotransferases, glutathione transferases, and UDP-glucuronosyltransferases. Studies in the rat and mouse liver models have identified more than 1000 genes whose expression is sex-dependent; together, these genes impart substantial sexual dimorphism to liver metabolic function and pathophysiology. Sex differences in drug metabolism and pharmacokinetics also occur in humans and are due in part to the female-predominant expression of CYP3A4, the most important P450 catalyst of drug metabolism in human liver. The sexually dimorphic expression of P450s and other liver-expressed genes is regulated by the temporal pattern of plasma growth hormone (GH) release by the pituitary gland, which shows significant sex differences. These differences are most pronounced in rats and mice, where plasma GH profiles are highly pulsatile (intermittent) in male animals versus more frequent (nearly continuous) in female animals. This review discusses key features of the cell signaling and molecular regulatory mechanisms by which these sex-dependent plasma GH patterns impart sex specificity to the liver. Moreover, the essential role proposed for the GH-activated transcription factor signal transducer and activator of transcription (STAT) 5b, and for hepatic nuclear factor (HNF) 4alpha, as mediators of the sex-dependent effects of GH on the liver, is evaluated. Together, these studies of the cellular, molecular, and gene regulatory mechanisms that underlie sex-based differences in liver gene expression have provided novel insights into the physiological regulation of both xenobiotic and endobiotic metabolism.

Dynamic in vivo binding of STAT5 to growth hormone-regulated genes in intact rat liver. Sex-specific binding at low- but not high-affinity STAT5 sites

Phylogenetic footprinting was used to predict functional transcription factor binding sites (TFBS) for signal transducer and activator of transcription (STAT) 5, a GH-activated transcription factor, in the GH-responsive genes IGF-I, SOCS2, and HNF6. Each gene, including upstream (100 kb) and downstream regions (25 kb), was aligned across four species and searched for conserved STAT5-binding sites using TFBS matrices. Predicted sites were classified as paired or single and whether or not they matched the STAT5 consensus sequence TTCN(3)GAA. Fifty-seven of the predicted genomic regions were assayed by chromatin immunoprecipitation from male rat liver with high STAT5 activity. STAT5 binding was enriched (up to 24-fold) at eight genomic regions of IGF-I, including three novel regions in the second intron, and at four regions of SOCS2, including three novel upstream sites. STAT5 binding to HNF6 was modestly enriched (up to 3-fold) at one consensus site and two novel, nonconsensus sites. Overall, 14 of 17 identified sites were paired STAT5 sites. STAT5 binding to these sites was dynamic in male rat liver, cycling on and off in response to each plasma GH pulse. Moreover, sex-specific STAT5 binding was apparent; in female rat liver, where nuclear STAT5 activity is generally low, STAT5 binding to IGF-I and SOCS2 was limited to high-affinity sites. Analysis of the verified STAT5 binding sites indicated that STAT5 TFBS matrix 459 in combination with a STAT5 consensus sequence was the best predictor of STAT5 binding to these three genes. Using these criteria, multiple novel STAT5 binding sites were identified and then verified in several other GH-inducible genes, including MUP genes, where male-specific gene expression was associated with male-specific STAT5 binding to multiple low-affinity STAT5 sites.

Characterization of hepatic sexual dimorphism in Alb-DsRed2 transgenic rats

We previously created the Alb-DsRed2 transgenic (Tg) rat that specifically expresses the red fluorescent protein, DsRed2, in the liver. Herein, we demonstrate that the DsRed2 expression is sexually dimorphic and exhibits a male-specific pattern. The profiling of sexual dimorphism in DsRed2 expression during pre-pubertal development was investigated using an in vivo fluorescent imaging analysis. The DsRed2 expression decreased gradually in both sexes until 28days after birth. While DsRed2 expression was not persistent in the female liver, the male hepatic expression increased again at 35days. Sexual dimorphic DsRed2 expression did not change in gonadectomized male and female Tg-rats. However, female hepatic DsRed2 was induced 72h after the hypophysectomy. Hepatocytes isolated from the female Tg-rats also revealed DsRed2 induction by 96h in culture. These results suggest that the pituitary hormone suppresses the female hepatic DsRed2 expression causing the sexual dimorphism of DsRed2 expression.

Regulation of human CYP2C18 and CYP2C19 in transgenic mice: influence of castration, testosterone, and growth hormone

The hormonal regulation of human CYP2C18 and CYP2C19, which are expressed in a male-specific manner in liver and kidney in a mouse transgenic model, was examined. The influence of prepubertal castration in male mice and testosterone treatment of female mice was investigated, as was the effect of continuous administration of growth hormone (GH) to transgenic males. Prepubertal castration of transgenic male mice suppressed the expression of CYP2C18 and CYP2C19 in liver and kidney to female levels, whereas expression was increased for the endogenous female-specific mouse hepatic genes Cyp2c37, Cyp2c38, Cyp2c39, and Cyp2c40. Testosterone treatment of female mice increased CYP2C18 and CYP2C19 expression in kidney, and to a lesser extent in liver, but was without effect in brain or small intestine, where gene expression was not gender-dependent. Continuous GH treatment of transgenic males for 7 days suppressed hepatic expression of CYP2C19 (>90% decrease) and CYP2C18 ( approximately 50% decrease) but had minimal effect on the expression of these genes in kidney, brain, or small intestine. Under these conditions, continuous GH induced all four female-specific mouse liver Cyp2c genes in males to normal female levels. These studies indicate that the human CYP2C18 and CYP2C19 genes contain regulatory elements that respond to the endogenous mouse hormonal profiles, with androgen being the primary regulator of male-specific expression in kidney, whereas the androgen-dependent pituitary GH secretory pattern is the primary regulator of male-specific expression in liver in a manner that is similar to the regulation of the endogenous gender-specific hepatic genes.

Mechanism of gender-divergent UDP-glucuronosyltransferase mRNA expression in mouse liver and kidney

UDP-glucuronosyltransferases (UGTs) catalyze the addition of glucuronic acid to endo- and xenobiotics, increasing hydrophilicity and enhancing elimination. Gender-divergent glucuronidation rates are observed in humans and rats, and gender differences in UGT mRNA levels have been observed in rodents. The purpose of this study was to establish the hormonal regulation of gender-dependent Ugt mRNA expression in mouse liver and kidney. Therefore, three mouse models were used to characterize the involvement of sex hormones and gender-specific growth hormone (GH) secretion patterns, including 1) hypophysectomized mice treated with male- or female-pattern GH, testosterone, or 17beta-estradiol; 2) GH releasing hormone receptor-deficient little (lit/lit) mice treated with male- or female-pattern GH; and 3) gonadectomized mice treated with testosterone or 17beta-estradiol. Messenger RNA expression of mouse Ugt isozymes was determined by the branched DNA assay. In C57BL/6 mice, male-predominant expression of Ugt2b1 and Ugt2b38 was observed in liver and kidney, respectively. Female-predominant expression was observed for Ugt1a1 and Ugt1a5 in liver and Ugt1a2 in kidney. In liver, regulation of Ugt1a1 and Ugt1a5 expression was attributed to repression of Ugt mRNA by male-pattern GH secretion. Conversely, regulation of Ugt2b1 expression in liver was attributed to male-pattern GH secretion. In kidney, regulation of Ugt2b38 expression was attributed to inductive effects by testosterone. Conversely, Ugt1a2 expression in kidney was negatively regulated by testosterone. In conclusion, gender differences in mouse Ugt mRNA expression were influenced by male-pattern GH secretion in liver, whereas gender differences were regulated by the effects of androgens in kidney.

Sex and the Liver – A Journey Through Five Decades

Metabolism of steroids and drugs in rodents is sexually differentiated. The reason for this turned out to be the sexually differentiated growth hormone (GH) secretory pattern regulating the expression of a number of hepatic cytochrome P-450 genes. Although not fully resolved, it is clear that several signaling pathways and transcription factors are involved in mediating the effects of GH. It may be argued that such a well-controlled physiological system should have an important biological role and we speculate that the demands of a robust hepatic steroid metabolism during pregnancy has led to the development of this sexually differentiated hypothalamo-pituitary-liver axis.

Sex-specific early growth hormone response genes in rat liver

Pituitary GH-secretory profiles are sex dependent and regulate the sexually dimorphic expression of a large number of genes in the liver. The slow response of many sex-specific liver genes to changes in plasma GH status suggests that GH acts in the liver via both direct and indirect mechanisms organized in a hierarchical regulatory network. Presently, genome-wide liver transcription profiling was conducted to elucidate the global impact of pituitary hormone ablation on the sex specificity of rat liver gene expression and to identify sex-specific genes that respond rapidly to GH as candidates for direct targets of GH action. Hypophysectomy abolished the sex specificity of approximately 90% of 1032 sex-dependent genes, consistent with the dominant role of pituitary GH in regulating liver sexual dimorphism. Two major classes of sex-specific genes were identified: genes that were down-regulated after hypophysectomy and may be subject to positive GH regulation (461 class I genes), and genes that were up-regulated after hypophysectomy and may be subject to negative GH regulation (224 class II genes). Fifty class I sex-specific genes were induced, and 38 class II sex-specific genes were suppressed within 90 min of a physiological GH pulse, suggesting they are primary GH response genes. A further 71 sex-specific genes responded after a second GH treatment and may correspond to secondary response genes. Twenty four DNA-binding proteins were identified as early GH response genes, of which 15 were induced and nine were suppressed by GH. Five of these 24 genes displayed sex-specific expression, consistent with a hierarchical transcriptional network controlling sex-specific liver gene expression. Class II male-specific genes, such as Cyp2a2 and Cyp2c13, were down-regulated within 30 min of GH pulse treatment, as determined by heterogeneous nuclear RNA analysis, suggesting that transcription of these genes is restricted to the GH-free interpulse period in adult male rat liver. We conclude that GH acts via both positive and negative regulatory mechanisms to establish and maintain the sex specificity of liver gene expression.

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.

Differential expression of cytochrome P450 genes between bromadiolone-resistant and anticoagulant-susceptible Norway rats: a possible role for pharmacokinetics in bromadiolone resistance

BACKGROUND

Anticoagulant resistance in Norway rats, Rattus norvegicus (Berk.), has been suggested to be conferred by mutations in the VKORC1 gene, encoding the target protein of anticoagulant rodenticides. Other factors, e.g. pharmacokinetics, may also contribute to resistance, however. To examine the involvement of pharmacokinetics in bromadiolone resistance in male and female rats, liver expression profiles of seven cytochrome P450 genes from a Danish bromadiolone-resistant rat strain (with an Y139C-VKORC1 mutation) were compared with profiles from an anticoagulant-susceptible strain.

RESULTS

In the presence of bromadiolone, the Cyp2e1, Cyp2c13, Cyp3a2 and Cyp3a3 genes were significantly overexpressed, while Cyp2c12 expression was suppressed in resistant female rats compared with susceptible females. Relative to susceptible males, resistant males showed significant overexpression of the Cyp2a1, Cyp2e1, Cyp3a2 and Cyp3a3 genes. On exposure to bromadiolone, females had higher Cyp2e1 expression than males, which possibly explains why female rats are generally more tolerant to anticoagulants than male rats.

CONCLUSION

Results suggest that bromadiolone resistance in a Danish strain of Norway rats involves enhanced anticoagulant metabolism catalysed by cytochrome P450-2e1, -3a2 and -3a3. This pharmacokinetically based bromadiolone resistance is to some extent sex differentiated, as female resistance furthermore seems to involve overexpression of cytochrome P450-2c13 and suppression of P450-2c12, whereas male resistance appears to involve P450-2a1 overexpression.

Stat5 as a diagnostic marker for leukemia

The Jak-Stat-Socs pathway is an important component of cytokine receptor signaling. Not surprisingly, perturbation of this pathway is implicated in diseases of hematopoietic and immune origin, including leukemia, lymphoma and immune deficiencies. This review examines the role of a key component of this pathway, Stat5. This has been shown to be activated in a variety of leukemias and myeloproliferative disorders, including downstream of a range of key oncogenes where it has been shown to play an important role in mediating their effects. Therefore, Stat5 represents a useful pan-leukemia/myeloproliferative disorder diagnostic marker and key therapeutic end point, as well as representing an attractive therapeutic target for these disorders.

Mode of Growth Hormone Action in Osteoblasts

Growth hormone (GH) affects bone size and mass in part through stimulating insulin-like growth factor type 1 (IGF-1) production in liver and bone. Whether GH acts independent of IGF-1 in bone remains unclear. To define the mode of GH action in bone, we have used a Cre/loxP system in which the type 1 IGF-1 receptor (Igf1r) has been disrupted specifically in osteoblasts in vitro and in vivo. Calvarial osteoblasts from mice homozygous for the floxed IGF-1R allele (IGF-1R(flox/flox)) were infected with adenoviral vectors expressing Cre. Disruption of IGF-1R mRNA (>90%) was accompanied by near elimination of IGF-1R protein but retention of GHR protein. GH-induced STAT5 activation was consistently greater in osteoblasts with an intact IGF-1R. Osteoblasts lacking IGF-1R retained GH-induced ERK and Akt phosphorylation and GH-stimulated IGF-1 and IGFBP-3 mRNA expression. GH-induced osteoblast proliferation was abolished by Cre-mediated disruption of the IGF-1R or co-incubation of cells with an IGF-1-neutralizing antibody. By contrast, GH inhibited apoptosis in osteoblasts lacking the IGF-1R. To examine the effects of GH on osteoblasts in vivo, mice wild type for the IGF-1R treated with GH subcutaneously for 7 days showed a doubling in the number of osteoblasts lining trabecular bone, whereas osteoblast numbers in similarly treated mice lacking the IGF-1R in osteoblasts were not significantly affected. These results indicate that although direct IGF-1R-independent actions of GH on osteoblast apoptosis can be demonstrated in vitro, IGF-1R is required for anabolic effects of GH in osteoblasts in vivo.

Hormonal and nutritional regulation of alternative CD36 transcripts in rat liver--a role for growth hormone in alternative exon usage

Background

CD36 is a multiligand receptor involved in various metabolic pathways, including cellular uptake of long-chain fatty acids. Defect function or expression of CD36 can result in dyslipidemia or insulin resistance. We have previously shown that CD36 expression is female-predominant in rat liver. In the present study, hormonal and nutritional regulation of hepatic CD36 expression was examined in male and female rats. Since alternative transcription start sites have been described in murine and human Cd36, we investigated whether alternative CD36 transcripts are differentially regulated in rat liver during these conditions.

Results

Sequence information of the rat Cd36 5'-UTR was extended, showing that the gene structure of Cd36 in rat is similar to that previously described in mouse with at least two alternative first exons. The rat Cd36 exon 1a promoter was sequenced and found to be highly similar to murine and human Cd36. We show that alternative first exon usage is involved in the female-predominant expression of CD36 in rat liver and during certain hormonal states that induce CD36 mRNA abundance. Estrogen treatment or continuous infusion of growth hormone (GH) in male rats induced CD36 expression preferentially through the exon 1a promoter. Old age was associated with increased CD36 expression in male rats, albeit without any preferential first exon usage. Intermittent GH treatment in old male rats reversed this effect. Mild starvation (12 hours without food) reduced CD36 expression in female liver, whereas its expression was increased in skeletal muscle.

Conclusion

The results obtained in this study confirm and extend our previous observation that GH is an important regulator of hepatic CD36, and depending on the mode of treatment (continuous or intermittent) the gene might be either induced or repressed. We suggest that the effects of continuous GH secretion in females (which is stimulatory) and intermittent GH secretion in males (which is inhibitory) explains the sex-different expression of this gene. Furthermore, a female-specific repression of hepatic CD36 in response to food deprivation was found, which was in contrast to a stimulatory effect in skeletal muscle. This demonstrates a tissue-specific regulation of Cd36.