Developmental action of estrogen receptor-alpha feminizes the growth hormone-Stat5b pathway and expression of Cyp2a4 and Cyp2d9 genes in mouse liver

Genome-wide identification of STATs and analysis of their role in sex determination in Pacific oysters (Crassostrea gigas)

STAT (signal transducer and activator of the transcription) proteins, are a group of highly conserved transcription factors and fundamental components of the JAK-STAT signaling pathway. They play crucial roles in a variety of biological processes, such as immunity, proliferation, differentiation, and growth. However, little information is known regarding their role in gonad development and sex determination in mollusks. In this study, we identified 3 STAT genes in Pacific Oyster Crassostrea gigas. Phylogenetic analysis showed that STATs from mollusks were highly conserved, and most of them had four identical motif regions, except for the STAT1 and STAT3 predicted sequences from Crassostrea hongkongensis. Tissue expression analysis indicated CgSTAT1 had a high expression level in most tissues, while CgSTAT3 had a low expression level in most tissues. Expression analysis of early developmental stages showed CgSTAT1 had a higher expression level from egg to D shaped larva and a lower expression level in subsequent stages. In contrast CgSTAT1, CgSTAT2 had a reverse expression pattern. Expression analysis of different developmental stages of diploid gonads indicated that CgSTAT1 had a higher expression level at the S1 and S3 stages relative to the S2 stage in females, while in males the S3 stage had a higher expression than than the S2 stage. The expression level of CgSTAT1 between diploids and triploids in females differed significantly, but there were no significant differences in males. Expression of CgSTAT2 differed significantly between diploid and triploid males. These data suggest an important role for STATs in sex differentiation in diploid and triploid oysters. Our study is the first to explore the role of STATs in sex differentiation and gonadal development in oysters, and will help us better understand the molecular mechanisms of sex differentiation in shellfish.

Contrasting effects of whole-body and hepatocyte-specific deletion of the RNA polymerase III repressor Maf1 in the mouse

MAF1 is a nutrient-sensitive, TORC1-regulated repressor of RNA polymerase III (Pol III). MAF1 downregulation leads to increased lipogenesis in Drosophila melanogaster, Caenorhabditis elegans, and mice. However, Maf1 -/- mice are lean as increased lipogenesis is counterbalanced by futile pre-tRNA synthesis and degradation, resulting in increased energy expenditure. We compared Chow-fed Maf1 -/- mice with Chow- or High Fat (HF)-fed Maf1 hep-/- mice that lack MAF1 specifically in hepatocytes. Unlike Maf1 -/- mice, Maf1 hep-/- mice become heavier and fattier than control mice with old age and much earlier under a HF diet. Liver ChIPseq, RNAseq and proteomics analyses indicate increased Pol III occupancy at Pol III genes, very few differences in mRNA accumulation, and protein accumulation changes consistent with increased lipogenesis. Futile pre-tRNA synthesis and degradation in the liver, as likely occurs in Maf1 hep-/- mice, thus seems insufficient to counteract increased lipogenesis. Indeed, RNAseq and metabolite profiling indicate that liver phenotypes of Maf1 -/- mice are strongly influenced by systemic inter-organ communication. Among common changes in the three phenotypically distinct cohorts, Angiogenin downregulation is likely linked to increased Pol III occupancy of tRNA genes in the Angiogenin promoter.

Estrogen Receptor-α Suppresses Liver Carcinogenesis and Establishes Sex-Specific Gene Expression

Estrogen protects females from hepatocellular carcinoma (HCC). To determine whether this protection is mediated by classic estrogen receptors, we tested HCC susceptibility in estrogen receptor-deficient mice. In contrast to a previous study, we found that diethylnitrosamine induces hepatocarcinogenesis to a significantly greater extent when females lack Esr1, which encodes Estrogen Receptor-α. Relative to wild-type littermates, Esr1 knockout females developed 9-fold more tumors. Deficiency of Esr2, which encodes Estrogen Receptor-β, did not affect liver carcinogenesis in females. Using microarrays and QPCR to examine estrogen receptor effects on hepatic gene expression patterns, we found that germline Esr1 deficiency resulted in the masculinization of gene expression in the female liver. Six of the most dysregulated genes have previously been implicated in HCC. In contrast, Esr1 deletion specifically in hepatocytes of Esr1 conditional null female mice (in which Cre was expressed from the albumin promoter) resulted in the maintenance of female-specific liver gene expression. Wild-type adult females lacking ovarian estrogen due to ovariectomy, which is known to make females susceptible to HCC, also maintained female-specific expression in the liver of females. These studies indicate that Esr1 mediates liver cancer risk, and its control of sex-specific liver gene expression involves cells other than hepatocytes.

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.

Sexual dimorphisms in zonal gene expression in mouse liver

Many of the metabolic functions of the liver are localized either in the pericentral region (zone 3) or in the periportal region (zone 1). However, a systematic analysis of the heterogeneity and sexual dimorphism of gene expression in the liver is lacking. Our objective was to obtain sections of intact tissue from zone 1 and zone 3 from both male and female mouse liver, and to measure the patterns of gene expression in these sections. Zone 1 and zone 3 areas were isolated by laser capture microdissection of liver sections, total RNA was isolated and microarray analysis was conducted using Agilent Whole Mouse Genome oligo arrays. To investigate functional characteristics as well as upstream regulators of specific gene lists, we used Ingenuity Pathway Analysis. We identified more than 925 genes in zone 1 and more than 450 genes in zone 3 of both male and female mice. Sexual dimorphism in metabolic functions was present in zone 1 but not zone 3. In zone 1, canonical pathways related to gluconeogenesis were male predominant, while canonical pathways related to hepatic progenitor cells were female predominant. In addition, we also analyzed the upstream regulators of zone-specific genes. SREBF1 was male-specific in zone 1, while TRIM24 was female-specific in zone 3. These results demonstrate the heterogeneity and sexually dimorphic differences in gene expression in the liver.

Hepatocellular alterations and dysregulation of oncogenic pathways in the liver of transgenic mice overexpressing growth hormone

Growth hormone (GH) overexpression throughout life in transgenic mice is associated with the development of liver tumors at old ages. The preneoplastic pathology observed in the liver of young adult GH-overexpressing mice is similar to that present in humans at high risk of hepatic cancer. To elucidate the molecular pathogenesis underlying the pro-oncogenic liver pathology induced by prolonged exposure to elevated GH levels, the activation and expression of several components of signal transduction pathways that have been implicated in hepatocellular carcinogenesis were evaluated in the liver of young adult GH-transgenic mice. In addition, males and females were analyzed in parallel in order to evaluate sexual dimorphism. Transgenic mice from both sexes exhibited hepatocyte hypertrophy with enlarged nuclear size and exacerbated hepatocellular proliferation, which were higher in males. Dysregulation of several oncogenic pathways was observed in the liver of GH-overexpressing transgenic mice. Many signaling mediators and effectors were upregulated in transgenic mice compared with normal controls, including Akt2, NFκB, GSK3β, β-catenin, cyclin D1, cyclin E, c-myc, c-jun and c-fos. The molecular alterations described did not exhibit sexual dimorphism in transgenic mice except for higher gene expression and nuclear localization of cyclin D1 in males. We conclude that prolonged exposure to GH induces in the liver alterations in signaling pathways involved in cell growth, proliferation and survival that resemble those found in many human tumors.

The role for estrogen receptor-alpha and prolactin receptor in sex-dependent DEN-induced liver tumorigenesis

Mice treated neonatally with diethylnitrosamine (DEN) develop liver tumors in a male-dominant manner, reflecting the male bias in human hepatocellular carcinoma. Evidence suggests that estrogen, androgen, prolactin (PRL) and growth hormone (GH) modify liver tumorigenesis. We determined the roles of estrogen receptor-α (ERα) and prolactin receptor (PRLR) using receptor null mice, ERαKO (C57Bl/6J) and PRLR-KO (129Ola-X-C57BL/6), in the neonatal-DEN model of liver tumorigenesis. In both mouse strains, females had reduced tumorigenesis compared with males (P < 0.01), regardless of ERα or PRLR status. Tumorigenesis was not affected by ovariectomy in C57Bl/6J mice but it was increased by ovariectomy in the mixed strain, 129Ola-X-C57BL/6, regardless of PRLR status. ERαKO males had 47% fewer tumors than ERα wild-type males (P < 0.01). On the other hand, estradiol treatment protected against tumorigenesis in males only in the presence of ERα. As evidenced by liver gene expression, lack of ERα did not alter the pattern of GH secretion in males but resulted in the male GH pattern in females. These observations indicate that ERα is not required for lower tumorigenesis in females, but it is required for the protective effects of exogenously delivered estradiol. Unexpectedly, the results indicate that ERα plays a role in promotion of liver tumors in males. In addition, it can be concluded that sex differences in liver tumorigenesis cannot be explained by the sexually dimorphic pattern of GH secretion. The results also rule out PRL as the mediator of the protective effect of the ovaries.

Sex differences in nuclear receptor-regulated liver metabolic pathways

Liver metabolism is markedly sex-dimorphic; accordingly, the prevalence of liver diseases is different between sexes. The superfamily of nuclear receptors (NRs) governs the proper expression of key liver metabolism genes by sensing lipid-soluble hormones and dietary lipids. When the expression of those genes is deregulated, disease development is favored. However, we lack a comprehensive picture of the differences between NR actions in males and females. Here, we reviewed explorative studies that assessed NR functions in both sexes, and we propose a first map of sex-dimorphic NR expression in the liver. Our analysis suggested that NRs in the female liver exhibited cross-talk with more liver-protective potential than NRs in male liver. This study provides empirical support to the hypothesis that women are more resilient to some liver diseases than men, based on a more compensative NR network. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Disrupting actions of bisphenol A and malachite green on growth hormone receptor gene expression and signal transduction in seabream

Environmental estrogen could mimic natural estrogens thereby disrupting the endocrine systems of human and animals. The actions of such endocrine disruptors have been studied mainly on reproduction and development. However, estrogen could also affect the somatotropic axis via multiple targets such as growth hormone (GH). In the present study, two endocrine disruptors were chosen to investigate their effects on the expression level and signal transduction of growth hormone receptor (GHR) in fish. Using real-time PCR, it was found that exposure to both the estrogenic (bisphenol A) and anti-estrogenic (malachite green) compounds could attenuate the expression levels of GHR1 and GHR2 in black seabream (Acanthopagrus schlegeli) hepatocytes. The expression level of IGF-I, the downstream effector of GHR activation in the liver, was decreased by bisphenol A but not by malachite green. Luciferase reporter assay of the beta-casein promoter was used to monitor GHR signaling in transfected cells. In the fish liver cell line Hepa-T1, both GHR1 and GHR2 signaling were attenuated by bisphenol A and malachite green. This attenuation could only occur in the presence of estrogen receptor, indicating that these agents probably produce their actions via the estrogen receptor. Results of the present study demonstrated that estrogenic or anti-estrogenic compounds could down-regulate the somatotropic axis in fish by affecting both the gene expression and signaling of GHR. In view of the increasing prevalence of these compounds in the environment, the impact on fish growth and development both in the wild and in aquaculture would be considerable.

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.

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.

Sexually dimorphic gene expression in mammalian somatic tissue

BACKGROUND

The sexually dimorphic differentiation of the bipotential gonad into testis or ovary initiates the sexually dimorphic development of mammals and leads to divergent hormone concentrations between the sexes throughout life. However, despite the fact that anatomic and hormonal differences between the sexes are well described, only a few studies have investigated the manifestation of these differences at the transcriptional level in mammalian somatic tissue.

OBJECTIVE

This review focuses on basic regulatory mechanisms of sex-specific gene expression and examines recent gene expression profiling studies to outline basic differences between the sexes at the transcriptome level in somatic tissues.

METHODS

To identify gene expression profiling studies addressing sexually dimorphic gene expression, the PubMed database was searched using the terms sex and dimorp and gene expression not drosophila not elegans. Abstracts of all identified publications were screened for studies explicitly using microarrays to identify sex differences in somatic tissues of rodents or humans. The search was restricted to English-language articles published in the past 5 years. Reference lists of identified articles as well as microarray databases (Gene Expression Omnibus and ArrayExpress) were also used.

RESULTS

The application of microarray technology has enabled the systematic assessment of sex-biased gene expression on the transcriptome level, indicating that the regulatory pathways underlying sexual differentiation give rise to extensive differences in somatic gene expression across organisms.

CONCLUSION

Sustainable annotation of sex-biased gene expression provides a key to understanding basic physiological differences between healthy males and females as well as those with diseases.

Hepatic transcriptional networks induced by exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) serves as a prototype for a range of environmental toxicants and as a pharmacologic probe to study signal transduction by the aryl hydrocarbon receptor (AHR). Despite a detailed understanding of how TCDD exposure leads to the transcriptional up-regulation of cytochrome P450-dependent monooxygenases, we know little about how compounds like TCDD lead to a variety of AHR-dependent toxic end points such as liver pathology, terata, thymic involution, and cancer. Using an acute exposure protocol and the toxic response of the mouse liver as a model system, we have begun a detailed microarray analysis to describe the transcriptional changes that occur after various TCDD doses and treatment times. Through correlation analysis of time- and dose-dependent toxicological end points, we are able to identify coordinately responsive transcriptional events that can be defined as primary transcriptional events and downstream events that may represent mechanistically linked sequelae or that have potential as biomarkers of toxicity.

Characterization of three growth hormone-responsive transcription factors preferentially expressed in adult female liver

Plasma GH profiles regulate the sexually dimorphic expression of cytochromes P450 and many other genes in rat and mouse liver; however, the proximal transcriptional regulators of these genes are unknown. Presently, we characterize three liver transcription factors that are expressed in adult female rat and mouse liver at levels up to 16-fold [thymus high-mobility group box protein (Tox)], 73-fold [tripartite motif-containing 24 (Trim24)/transcription initiation factor-1alpha (TIF1alpha)], and 125-fold [cut-like 2 (Cutl2)/cut homeobox 2 (Cux2)] higher than in adult males, depending on the strain and species, with Tox expression only detected in mice. In rats, these sex differences first emerged at puberty, when the high prepubertal expression of Cutl2 and Trim24 was extinguished in males but was further increased in females. Rat hepatic expression of Cutl2 and Trim24 was abolished by hypophysectomy and, in the case of Cutl2, was restored to near-female levels by continuous GH replacement. Cutl2 and Trim24 were increased to female-like levels in livers of intact male rats and mice treated with GH continuously (female GH pattern), whereas Tox expression reached only about 40% of adult female levels. Expression of all three genes was also elevated to normal female levels or higher in male mice whose plasma GH profile was feminized secondary to somatostatin gene disruption. Cutl2 and Trim24 both responded to GH infusion in mice within 10-24 h and Tox within 4 d, as compared with at least 4-7 d required for the induced expression of several continuous GH-regulated cytochromes P450 and other female-specific hepatic genes. Cutl2, Trim24, and Tox were substantially up-regulated in livers of male mice deficient in either of two transcription factors implicated in GH regulation of liver sex specificity, namely, signal transducer and activator of transcription 5b (STAT5b) and hepatocyte nuclear factor 4alpha (HNF4alpha), with sex-specific expression being substantially reduced or lost in mice deficient in either nuclear factor. Cutl2 and Trim24 both display transcriptional repressor activity and could thus contribute to the loss of GH-regulated, male-specific liver gene expression seen in male mice deficient in STAT5b or HNF4alpha. Binding sites for Cutl1, whose DNA-binding specificity is close to that of Cutl2, were statistically overrepresented in STAT5b-dependent male-specific mouse genes, lending support to this hypothesis.

Loss of sexually dimorphic liver gene expression upon hepatocyte-specific deletion of Stat5a-Stat5b locus

Hepatocyte-specific, albumin-Cre recombinase-mediated deletion of the entire mouse Stat5a-Stat5b locus was carried out to evaluate the role of signal transducer and activator of transcription 5a and 5b (STAT5ab) in the sex-dependent transcriptional actions of GH in the liver. The resultant hepatocyte STAT5ab-deficient mice were fertile, and unlike global STAT5b-deficient male mice, postnatal body weight gain was normal, despite a 50% decrease in serum IGF-I. Whole-liver STAT5ab RNA decreased by approximately 65-85%, and residual STAT5 immunostaining was observed in a minority of the hepatocytes, indicating incomplete excision by Cre-recombinase. Quantitative PCR analysis of 20 sexually dimorphic, liver-expressed genes revealed significant down-regulation of 10 of 11 male-specific genes in livers of male hepatocyte STAT5ab-deficient mice. Class I female-specific liver genes were markedly up-regulated (de-repressed), whereas the expression of class II female genes, belonging to the Cyp3a subfamily, was unaffected by the loss of hepatocyte STAT5ab. STAT5ab is thus required in the liver for positive regulation of male-specific genes and for negative regulation of a subset of female-specific genes. Continuous GH infusion strongly induced (>500-fold) the class II female gene Cyp3a16 in both wild-type and hepatocyte STAT5ab-deficient male mice, indicating sex-specific transcriptional regulation by GH that is STAT5ab independent. In contrast, hepatocyte STAT5ab deficiency abolished the strong suppression of the male-specific Cyp2d9 by continuous GH seen in control mouse liver. Analysis of global STAT5a-deficient mice indicated no essential requirement of STAT5a for expression of these sex-specific liver Cyp genes. Thus, the major loss of liver sexual dimorphism in hepatocyte STAT5ab-deficient mice can primarily be attributed to the loss of STAT5b.

Transplacental exposure to inorganic arsenic at a hepatocarcinogenic dose induces fetal gene expression changes in mice indicative of aberrant estrogen signaling and disrupted steroid metabolism

Exposure to inorganic arsenic in utero in C3H mice produces hepatocellular carcinoma in male offspring when they reach adulthood. To help define the molecular events associated with the fetal onset of arsenic hepatocarcinogenesis, pregnant C3H mice were given drinking water containing 0 (control) or 85 ppm arsenic from day 8 to 18 of gestation. At the end of the arsenic exposure period, male fetal livers were removed and RNA isolated for microarray analysis using 22K oligo chips. Arsenic exposure in utero produced significant (p<0.001) alterations in expression of 187 genes, with approximately 25% of aberrantly expressed genes related to either estrogen signaling or steroid metabolism. Real-time RT-PCR on selected genes confirmed these changes. Various genes controlled by estrogen, including X-inactive-specific transcript, anterior gradient-2, trefoil factor-1, CRP-ductin, ghrelin, and small proline-rich protein-2A, were dramatically over-expressed. Estrogen-regulated genes including cytokeratin 1-19 and Cyp2a4 were over-expressed, although Cyp3a25 was suppressed. Several genes involved with steroid metabolism also showed remarkable expression changes, including increased expression of 17beta-hydroxysteroid dehydrogenase-7 (HSD17beta7; involved in estradiol production) and decreased expression of HSD17beta5 (involved in testosterone production). The expression of key genes important in methionine metabolism, such as methionine adenosyltransferase-1a, betaine-homocysteine methyltransferase and thioether S-methyltransferase, were suppressed. Thus, exposure of mouse fetus to inorganic arsenic during a critical period in development significantly alters the expression of various genes encoding estrogen signaling and steroid or methionine metabolism. These alterations could disrupt genetic programming at the very early life stage, which could impact tumor formation much later in adulthood.

Signalling cross-talk between hepatocyte nuclear factor 4alpha and growth-hormone-activated STAT5b

In the present study, we have characterized signalling cross-talk between STAT5b (signal transducer and activator of transcription 5b) and HNF4alpha (hepatocyte nuclear factor 4alpha), two major regulators of sex-dependent gene expression in the liver. In a HepG2 liver cell model, HNF4alpha strongly inhibited beta-casein and ntcp (Na+/taurocholate cotransporting polypeptide) promoter activity stimulated by GH (growth hormone)-activated STAT5b, but had no effect on interferon-gamma-stimulated STAT1 transcriptional activity. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4alpha towards the ApoCIII (apolipoprotein CIII) promoter. The inhibitory effect of HNF4alpha on STAT5b transcription was associated with the inhibition of GH-stimulated STAT5b tyrosine phosphorylation and nuclear translocation. The short-chain fatty acid, butyrate, reversed STAT5b transcriptional inhibition by HNF4alpha, but did not reverse the inhibition of STAT5b tyrosine phosphorylation. HNF4alpha inhibition of STAT5b tyrosine phosphorylation was not reversed by pervanadate or by dominant-negative phosphotyrosine phosphatase 1B, suggesting that it does not result from an increase in STAT5b dephosphorylation. Rather, HNF4alpha blocked GH-stimulated tyrosine phosphorylation of JAK2 (Janus kinase 2), a STAT5b tyrosine kinase. Thus STAT5b and HNF4alpha exhibit bi-directional cross-talk that may augment HNF4alpha-dependent gene transcription while inhibiting STAT5b transcriptional activity via the inhibitory effects of HNF4alpha on JAK2 phosphorylation, which leads to inhibition of STAT5b signalling initiated by the GH receptor at the cell surface.

Regulation of constitutive mouse hepatic cytochromes P450 and growth hormone signaling components by 3-methylcholanthrene

3-Methylcholanthrene (MC) activates the aryl hydrocarbon receptor and increases expression of cytochrome P450 (P450) enzymes such as CYP1A1. MC also decreases expression of CYP2C11, the major hepatic P450 in male rats that is regulated by pulsatile growth hormone (GH) secretion via a pathway partially dependent on signal transducer and activator of transcription 5b (STAT5b). If disruption of this GH signaling pathway is important for MC's ability to suppress CYP2C11 transcription, we hypothesize that MC suppresses other male-specific genes (e.g., mouse Cyp2d9) regulated by pulsatile GH with STAT5b dependence. We examined the time course of MC's effects on hepatic P450s and GH signaling components in male C57BL/6 mice. P450 content, heme content, and NADPH P450 oxidoreductase activity were induced 2.3-, 1.8-, and 1.3-fold, respectively, by MC. MC dramatically induced CYP1A1 mRNA, protein, and catalytic activity. MC caused a 42% decrease in CYP2D9 protein, a 28% decrease in CYP2D9 mRNA, and a 27% decrease in testosterone 16alpha-hydroxylation activity. MC caused a pronounced decrease in CYP3A protein; however, there was no apparent change in testosterone 6beta-hydroxylation activity, and changes in mRNA levels for CYP3A forms were relatively small. Expression of GH receptor and major urinary protein 2, a gene regulated by GH with STAT5b dependence, was decreased by MC at the mRNA level. These results show that MC suppresses mouse Cyp2d9, a pulsatile GH- and STAT5b-dependent male-specific gene, via a pretranslational mechanism that may involve disrupted GH signaling. Mouse CYP3A protein levels are dramatically decreased by MC via a mechanism that is not yet understood.

Sex-Dependent Liver Gene Expression Is Extensive and Largely Dependent upon Signal Transducer and Activator of Transcription 5b (STAT5b): STAT5b-Dependent Activation of Male Genes and Repression of Female Genes Revealed by Microarray Analysis

Sexual dimorphism in mammalian liver contributes to sex differences in physiology, homeostasis, and steroid and foreign compound metabolism. Many sex-dependent liver genes are regulated by sex differences in pituitary GH secretion, with the transcription factor, signal transducer and activator of transcription (STAT5b), proposed to mediate signaling by the pulsatile, male plasma GH profile. Presently, a large-scale gene expression study was conducted using male and female mice, wild type and Stat5b inactivated, to characterize sex differences in liver gene expression and their dependence on STAT5b. The relative abundance of individual liver RNAs was determined for each sex-genotype combination by competitive hybridization to 23,574-feature oligonucleotide microarrays. Significant sex differences in hepatic expression were seen for 1603 mouse genes. Of 850 genes showing higher expression in males, 767 (90%) were down-regulated in STAT5b-deficient males. Moreover, of 753 genes showing female-predominant expression, 461 (61%) were up-regulated in STAT5b-deficient males. In contrast, approximately 90% of the sex-dependent genes were unaffected by STAT5b deficiency in females. Thus: 1) STAT5b is essential for sex-dependent liver gene expression, a characteristic of approximately 1600 mouse genes (4% of the genome); 2) male-predominant liver gene expression requires STAT5b, or STAT5b-dependent factors, which act in a positive manner; and 3) many female-predominant liver genes are repressed in males in a STAT5b-dependent manner. Several of the STAT5b-dependent male genes encode transcriptional repressors; these may include direct STAT5b targets that repress female-predominant genes in male liver. Several female-predominant repressors are elevated in STAT5b-deficient males; these may contribute to the major loss of male gene expression seen in the absence of STAT5b.

Gender-specific induction of cytochrome P450s in nonylphenol-treated FVB/NJ mice

Nonylphenol (NP) is a breakdown product of nonylphenol ethoxylates, which are used in a variety of industrial, agricultural, household cleaning, and beauty products. NP is one of the most commonly found toxicants in the United States and Europe and is considered a toxicant of concern because of its long half-life. NP is an environmental estrogen that also activates the pregnane X-receptor (PXR) and in turn induces P450s. No study to date has examined the gender-specific effects of NP on hepatic P450 expression. We provided NP at 0, 50 or 75 mg/kg/day for 7 days to male and female FVB/NJ mice and compared their P450 expression profiles. Q-PCR was performed on hepatic cDNA using primers to several CYP isoforms regulated by PXR or its relative, the constitutive androstane receptor (CAR). In female mice, NP induced Cyp2b10 and Cyp2b13, and downregulated the female-specific P450s, Cyp3a41 and Cyp3a44. In contrast, male mice treated with NP showed increased expression of Cyp2a4, Cyp2b9, and Cyp2b10. Western blots confirmed induction of Cyp2b subfamily members in both males and females. Consistent with the Q-PCR data, Western blots showed dose-dependent downregulation of Cyp3a only in females and induction of Cyp2a only in males. The overall increase in female-predominant P450s in males (Cyp2a4, 2b9) and the decrease in female-predominant P450s in females (Cyp3a41, 3a44) suggest that NP is in part feminizing the P450 profile in males and masculinizing the P450 profile in females. Testosterone hydroxylation was also altered in a gender-specific manner, as testosterone 16alpha-hydroxylase activity was only induced in NP-treated males. In contrast, NP-treated females demonstrated a greater propensity for metabolizing zoxazolamine probably due to greater Cyp2b induction in females. In conclusion, NP causes gender-specific P450 induction and therefore exposure to NP may cause distinct pharmacological and toxicological effects in males compared to females.

Regulation of mouse hepatic CYP2D9 mRNA expression by growth and adrenal hormones

The constitutive expression of CYP2D9 is sexually dimorphic, namely, strong in males, but diminutive in females. Repetition of mimic growth hormone (GH) secretion pattern impressively returned the mRNA expression level to that in intact mice: the GH secretion pattern's regulation of CYP2D9 mRNA expression has been predominantly disrupted by exogenous GH-administration. The extensive decline of CYP2D9 mRNA expression becoming a sexually non-specific P450 in 9-week-old male mice exposed as neonates to monosodium L-glutamate (MSG) suggested that the male GH secretion pattern is a key to the regulation of male-specific CYP2D9 mRNA expression in adult mice. Dexamethasone (Dex) showed possibility to induce CYP2D9 mRNA expression in adult MSG-neonatally treated mice of either sex. However, the antagonism was observed by co-administration of Dex and GH in the males. Dex-administration in adrenalectomized mice significantly elevated CYP2D9 mRNA expression levels. These findings suggest that an adrenal hormone participates in the regulatory mechanism of CYP2D9 mRNA expression in association with GH.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

Urogenital carcinogenesis in female CD1 mice induced by in utero arsenic exposure is exacerbated by postnatal diethylstilbestrol treatment

Transplacental inorganic arsenic carcinogenicity, together with postnatal exposure to diethylstilbestrol or tamoxifen, was studied. Pregnant CD1 mice received 85 ppm arsenic in the drinking water from gestation days 8 to 18 and were allowed to give birth. Groups (n = 35) of female offspring were injected s.c. on postpartum days 1 through 5 with diethylstilbestrol (2 microg/pup/d) or tamoxifen (10 microg/pup/d) and observed for 90 weeks. Arsenic alone induced some urogenital system tumors, including mostly benign tumors of the ovary and uterus, and adrenal adenoma. Diethylstilbestrol alone induced some tumors (primarily cervical) but when given after in utero arsenic, it greatly enhanced urogenital tumor incidence, multiplicity, and progression. For instance, compared with the incidence of urogenital malignancies in the control (0%), arsenic alone (9%), and diethylstilbestrol alone (21%) groups, arsenic plus diethylstilbestrol acted synergistically, inducing a 48% incidence of malignant urogenital tumors. Of the urogenital tumors induced by arsenic plus diethylstilbestrol, 80% were malignant, and 55% were multiple site. Arsenic plus diethylstilbestrol increased ovarian, uterine, and vaginal tumors, and urinary bladder proliferative lesions, including three transitional cell carcinomas. Tamoxifen alone did not increase urogenital tumors or affect arsenic-induced neoplasia but did increase arsenic-induced uroepithelial proliferative lesions. Uterine and bladder carcinoma induced by arsenic plus diethylstilbestrol greatly overexpressed estrogen receptor-alpha (ER-alpha) and pS2, an estrogen-regulated gene. In neonatal uteri, prenatal arsenic increased ER-alpha expression and enhanced estrogen-related gene expression induced by postnatal diethylstilbestrol. Thus, arsenic acts with estrogens to enhance production of female mouse urogenital cancers.

Growth hormone regulation of sex-dependent liver gene expression

The liver is a primary target for the action of GH, a pituitary protein hormone that regulates a broad range of physiological processes, including long bone growth, fatty acid oxidation, glucose uptake, and hepatic steroid and foreign compound metabolism. GH exerts sex-dependent effects on the liver in many species, with many hepatic genes, most notably genes coding for cytochrome P450 (CYP) enzymes, being transcribed in a sex-dependent manner. Sex differences in CYP expression are most striking in rats and mice (up to 500-fold male-female differences), but are also seen, albeit to a much smaller degree, in humans, where they are an important determinant of the sex dependence of hepatic drug and steroid metabolism. This article examines the mechanisms whereby GH, via its sex-dependent temporal patterns of pituitary release, activates intracellular signaling leading to the sexually dimorphic transcription of CYPs and other liver-expressed genes. Recent findings implicating the GH-regulated transcription factor STAT5b (signal transducer and activator of transcription 5b), hepatocyte nuclear factors 3beta, 4alpha and 6, and sex differences in DNA methylation and chromatin structure in the sex-dependent actions of GH are reviewed, and current mechanistic models are evaluated.

Modified expression of cytochrome P450 mRNAs by growth hormone in mouse liver

The expression of eight mouse hepatic cytochrome P450s (P450s) genes was investigated at the mRNA level in relation with the pattern of growth hormone (GH) administration. The constitutive expression of five sex-dependent P450s was sexually dimorphic, namely female>male for CYP2A4, CYP2B9, CYP2B10, and CYP3A41, and male>female for CYP2D9. In mice neonatally treated with monosodium L-glutamate to produce GH-deficiency, GH was found to be an essential factor with GH archetype as a determinant in the regulatory mechanism of hepatic CYP2D9 and CYP3A41 expression, and GH was shown to be a repressive factor for the constitutive expression in females. Implantation with micro-osmotic pump containing GH (to yield a constant release of GH to mimic the plasma GH profile in females) to male mice increased CYP2A4, CYP2B9, CYP2B10, and CYP3A41, but decreased CYP2D9, expression to female levels, while conversely, twice-daily administration of GH (to produce the so-called male pattern of plasma GH levels) to female mice resulted in the repression of female-specific, CYP2B9 and CYP3A41, as well as female-predominant, CYP2A4 and CYP2B10, expression, and induction of male-specific CYP2D9 expression. Thus, the sex-dependent plasma GH profile (referred to hereafter as the GH archetype) was a decisive factor for the expression of sex-specific P450 genes in adult mouse liver. On the other hand, the regulation of CYP1A2, CYP2C29, and CYP3A11 expression was either sex-independent or GH archetype-independent, considering the comparable levels between sexes of the constitutive expression and GH-inducible expression of these isoforms. Moreover, the observations suggested for the first time that the expression of CYP2B9 and CYP2A4 was not entirely GH-independent, but rather involved an imprinting GH-related factor that participated in the regulatory mechanism of P450 expression in females.

Codependence of growth hormone-responsive, sexually dimorphic hepatic gene expression on signal transducer and activator of transcription 5b and hepatic nuclear factor 4alpha

Targeted disruption of the signal transducer and activator of transcription 5b gene (STAT5b) leads to decreased expression in male mouse liver of a male-predominant cytochrome (Cyp) 2d protein, whereas female-predominant Cyp2b proteins are increased. Presently, we characterize the effects of STAT5b deficiency on 15 specific, individual Cyp RNAs and other sexually dimorphic liver gene products. All seven male-specific RNAs investigated were decreased to normal female levels in STAT5b-deficient male liver, whereas five of eight female-specific RNAs, designated class I female genes, were increased in expression up to 200-fold or more. STAT5b deficiency had a much more modest effect on the expression of these genes in females. Hypophysectomy and GH replacement studies demonstrated positive GH pulse regulation of all seven male RNAs and negative GH pulse regulation of class I, but not class II, female RNAs in wild-type, but not in STAT5b-deficient, male mice. A majority of the sex-specific genes responded in parallel to the loss of STAT5b and the loss of hepatocyte nuclear factor 4alpha, indicating that both transcription factors are essential and suggesting they may coregulate sexually dimorphic liver gene expression. Continuous GH treatment of intact male mice, which overrides the endogenous male, pulsatile plasma GH pattern, down-regulated all seven male RNAs and induced expression of the five class I female RNAs within 4-7 d; however, induction of class II female RNAs was delayed until d 7-14. Given the slow responses of all 15 genes to changes in plasma GH status, GH regulation of sex-specific Cyp expression is proposed to be indirect and mediated by STAT5b- and hepatocyte nuclear factor 4alpha-dependent factors that may include repressors of female-specific Cyps and other targets of GH action.

Estrogen receptor-β mediates male-female differences in the development of pressure overload hypertrophy

The goal of this study was to determine the role of estrogen receptor subtypes in the development of pressure overload hypertrophy in mice. Epidemiological studies have suggested gender differences in the development of hypertrophy and heart disease, but the mechanism and the role of estrogen receptor subtypes are not established. We performed transverse aortic constriction (TAC) and sham operations in male and female wild-type (WT) mice and mice lacking functional estrogen receptor-α [α-estrogen receptor knockout (α-ERKO)] and mice lacking estrogen receptor-β (β-ERKO). Body, heart, and lung weights were measured 2 wk postsurgery. WT male mice subjected to TAC showed a 64% increase in the heart weight-to-body weight ratio (HW/BW) compared with sham, and WT males have increased lung weight at 2 wk. WT female mice subjected to TAC showed a 31% increase in HW/BW compared with sham, which was significantly less than their male counterparts and with no evidence of heart failure. α-ERKO females developed HW/BW nearly identical to that seen in WT littermate females in response to TAC, indicating that estrogen receptor-α is not essential for the attenuation of hypertrophy observed in WT females. In contrast, β-ERKO females responded to TAC with a significantly greater increase in HW/BW than WT littermate females. β-ERKO females have lower expression of lipoprotein lipase at baseline than WT or α-ERKO females. These data suggest an important role for estrogen receptor-β in attenuating the hypertrophic response to pressure overload in females.

Role of Hepatocyte Nuclear Factors in Transcriptional Regulation of Male-specific CYP2A2

Cytochrome P450 2A2 (CYP2A2) is an adult male-specific rat liver steroid hydroxylase whose sex-dependent expression is regulated at the transcriptional level by sexually dimorphic pituitary growth hormone (GH) secretory patterns. In contrast to CYP2C11 and other male-specific, plasma GH pulse-inducible liver genes, CYP2A2 is highly expressed in hypophysectomized rat liver, despite the absence of GH stimulation. CYP2A2 promoter fragments 0.9-6.2 kb long exhibited unusually high basal promoter activity when transfected into the liver cell line HepG2. A further approximately 2.5-fold increase in activity was obtained by cotransfection of hepatocyte nuclear factor (HNF) 3gamma or HNF4alpha. CYP2A2 promoter activity was inhibited approximately 85% by transfection of HNF3beta or HNF6, both of which are more highly expressed in female than male liver and can strongly trans-activate the female-specific CYP2C12 promoter. The male GH pulse-activated transcription factor STAT5b had no effect on CYP2A2 promoter activity, either alone or in combination with HNF3gamma and HNF4alpha, consistent with the GH pulse-independence of CYP2A2 expression. By contrast, STAT5b synergistically enhanced the transcriptional activity of HNF4alpha toward two other male-specific liver target genes, Cyp2d9 and CYP8B1. Furthermore, STAT5b in combination with the HNF4alpha coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha strongly enhanced the transcriptional activity of HNF4alpha toward CYP8B1 but not toward CYP2A2. These findings support the hypothesis that sex-dependent HNFs contribute to the sexually dimorphic expression of CYP2A2 and other liver CYPs and highlight the ability of STAT5b to act in concert with HNF4alpha to regulate select male-specific liver CYP genes.

Coactivators in Gene Regulation by STAT5

Signal transducer and activator of transcription 5 (STAT5) is a member of the STAT family of transcription factors that relay the effect of diverse cytokines, hormones, and growth factors by regulating the transcription of distinct target genes. This function is emphasized by its crucial role in the development of the mammary gland and the hematopoietic system. Cytokine receptor-associated Janus kinases (JAKs) induce dimerization, nuclear translocation, and DNA binding through tyrosine phosphorylation of STAT5. STAT5 regulates the expression of cytokine target genes by binding to gamma interferon-activated sequence (GAS) motifs. Transcriptional activation requires the contact of STAT5 to coactivators and components of the transcription machinery. Another important point in transcriptional activation is the cooperation with other transcription factors that bind in close vicinity to the target gene promoters and enhancers. Their concerted action can result in an enhanced binding to the promoters or in cooperative recruitment of coactivators. In addition, cross-talk with other signaling pathways as well as secondary modifications of STAT5 have been described to affect transactivation function.

Suppression of female-specific murine Cyp2b9 gene expression by growth or glucocorticoid hormones

CYP2B9 is a constitutively and female-specifically expressed P450 isoform in mouse livers. Hypophysectomy-induced CYP2B9 mRNA expression in males to a level similar to that in females, while the operation did not affect females. Twice-daily injection of growth hormone (GH), which mimics the male pattern of GH secretion, significantly repressed hypophysectomy-induced mRNA expression in males. The same treatment completely suppressed expression in intact females. Treatments with synthetic glucocorticoid dexamethasone (DEX) suppressed expression of CYP2B9 mRNA in intact females, but not in GH-treated and un-treated hypophysectomized females. In primary cultured mouse hepatocytes, CYP2B9 mRNA expression was concentration-dependently suppressed by natural glucocorticoids such as hydrocortisone and corticosterone as well as by DEX. Glucocorticoid-mediated suppression was partially inhibited by RU486, a potent antiglucocorticoid. In contrast, RU486 by itself suppressed expression of CYP2B9 mRNA. These observations suggest that the sexually dimorphic expression of CYP2B9 is partly due to suppression by the masculine plasma GH profile and by glucocorticoid hormones.

Multihormonal regulation of hepatic sinusoidal Ntcp gene expression

Bile acids are efficiently removed from sinusoidal blood by a number of transporters including the Na+-taurocholate-cotransporting polypeptide (Ntcp). Na+-dependent bile salt uptake, as well as Ntcp, are expressed twofold higher in male compared with female rat livers. Also, estrogen administration to male rats decreases Ntcp expression. The aims of this study were to determine the hormonal mechanism(s) responsible for this sexually dimorphic expression of Ntcp. We examined castrated and hypophysectomized rats of both sexes. Sex steroid hormones, growth hormone, thyroid, and glucocorticoids were administered, and livers were examined for changes in Ntcp messenger RNA (mRNA). Ntcp mRNA and protein content were selectively increased in males. Estradiol selectively decreased Ntcp expression in males, whereas ovariectomy increased Ntcp in females, confirming the importance of estrogens in regulating Ntcp. Hypophysectomy decreased Ntcp mRNA levels in males and prevented estrogen administration from decreasing Ntcp, indicating the importance of pituitary hormones. Although constant infusion of growth hormone to intact males reduced Ntcp, its replacement alone after hypophysectomy did not restore the sex differences. In contrast, thyroid hormone and corticosterone increased Ntcp mRNA in hypophysectomized rats. Sex differences in Ntcp mRNA levels were produced only when the female pattern of growth hormone was administered to animals also receiving thyroid and corticosterone. Thyroid and dexamethasone also increased Ntcp mRNA in isolated rat hepatocytes, whereas growth hormone decreased Ntcp. These findings demonstrate the essential role that pituitary hormones play in the sexually dimorphic control of Ntcp expression in adult rat liver and in the mediation of estrogen effects.

Androgens and bone

Loss of estrogens or androgens increases the rate of bone remodeling by removing restraining effects on osteoblastogenesis and osteoclastogenesis, and also causes a focal imbalance between resorption and formation by prolonging the lifespan of osteoclasts and shortening the lifespan of osteoblasts. Conversely, androgens, as well as estrogens, maintain cancellous bone mass and integrity, regardless of age or sex. Although androgens, via the androgen receptor (AR), and estrogens, via the estrogen receptors (ERs), can exert these effects, their relative contribution remains uncertain. Recent studies suggest that androgen action on cancellous bone depends on (local) aromatization of androgens into estrogens. However, at least in rodents, androgen action on cancellous bone can be directly mediated via AR activation, even in the absence of ERs. Androgens also increase cortical bone size via stimulation of both longitudinal and radial growth. First, androgens, like estrogens, have a biphasic effect on endochondral bone formation: at the start of puberty, sex steroids stimulate endochondral bone formation, whereas they induce epiphyseal closure at the end of puberty. Androgen action on the growth plate is, however, clearly mediated via aromatization in estrogens and interaction with ERalpha. Androgens increase radial growth, whereas estrogens decrease periosteal bone formation. This effect of androgens may be important because bone strength in males seems to be determined by relatively higher periosteal bone formation and, therefore, greater bone dimensions, relative to muscle mass at older age. Experiments in mice again suggest that both the AR and ERalpha pathways are involved in androgen action on radial bone growth. ERbeta may mediate growth-limiting effects of estrogens in the female but does not seem to be involved in the regulation of bone size in males. In conclusion, androgens may protect men against osteoporosis via maintenance of cancellous bone mass and expansion of cortical bone. Such androgen action on bone is mediated by the AR and ERalpha.

Transgenic growth hormone mice exposed to lifetime constant illumination: gender-specific effects

Photoperiod affects most of the features altered in transgenic growth hormone (TG) mice, and laboratory rats and mice retain some sensitivity to photoperiod. We examined growth, feeding, longevity, and reproduction of TG mice and normal control mice (Mus musculus L., 1758) in 12 h light : 12 h dark (LD) and 24 h light (LL) photoperiods. Sexual dichotomy in growth and hepatic gene expression are considered to require gender-specific patterns of growth hormone secretion that are absent in TG mice. Regardless, in the LD photoperiod mature TG females were 82.8% (46.8 g) of the mass of TG males (56.5 g, p < 0.05), whereas control mice showed no size dichotomy (≈33 g). Mature masses of TG males and of control mice of either gender were unaffected by the LL photoperiod. TG females, however, reached a mature mass 92% (50.9 g) of that of mature TG males in the LL photoperiod, attenuating the sexual size dichotomy expressed in the LD photoperiod. Growth of females was slower than that of males, even in the control group. TG females in the LL photoperiod expressed faster growth, higher reproduction, and greater mean longevity than TG females in the LD photoperiod. Differences in age-related feeding associated with gender and photoperiod reflected differential growth rates. Females grew more slowly and ate more than males of similar age because they were smaller (i.e., had lower growth efficiencies). The LL photoperiod improved the energy balance of TG females. Possible mechanisms mediating such gender-specific effects are explored.

Sexually dimorphic P450 gene expression in liver-specific hepatocyte nuclear factor 4alpha-deficient mice

Hepatocyte nuclear factor (HNF) 4alpha is a liver-enriched nuclear receptor that plays a critical role in regulating the expression of numerous hepatic genes, including members of the cytochrome P450 (CYP) superfamily, several of which are expressed in a sex-dependent manner. Presently, we use a liver-specific Hnf4alpha-deficient mouse model to investigate the role of HNF4alpha in regulating liver-enriched transcription factors and sexually dimorphic Cyps in liver in vivo. Real-time PCR analysis of RNA isolated from livers of wild-type and Hnf4alpha-deficient mice revealed the following: 1) HNF4alpha exerts both positive regulation (Hnfalpha, C/ebpalpha, and C/ebpbeta) and negative regulation (Hnf3alpha and the HNF4alpha coactivator Pgc-1alpha) on liver transcription factor expression; 2) a strong dependence on HNF4alpha characterizes several male-predominant Cyps (2d9 and 8b1), female-predominant Cyps (2b10, 2b13, 3a41, and 3a44) and Cyps, whose expression is sex independent (3a11, 3a25); 3) HNF4alpha confers a unique, positive regulation of two male-expressed genes (Cyp4a12 and GSTpi) and a negative regulation of several female-predominant genes (Cyp2a4, Cyp2b9, Hnf3beta, and Hnf6), both of which are manifest in male but not female mouse liver. These trends were confirmed at the protein level by Western blot analysis using antibodies raised to Cyp2a, Cyp2b, and Cyp3a family members. Thus, HNF4alpha is an essential player in the complex regulatory network of liver-enriched transcription factors and the sexually dimorphic mouse Cyp genes that they regulate. HNF4alpha is proposed to contribute to the sex specificity of liver gene expression by positively regulating a subset of male-specific Cyp genes while concomitantly inhibiting the expression of certain female-specific Cyps and liver transcription factors, by mechanisms that are operative in male, but not female, mouse liver.

Estradiol represses prolactin-induced expression of Na+/taurocholate cotransporting polypeptide in liver cells through estrogen receptor-alpha and signal transducers and activators of transcription 5a

Na(+)/taurocholate cotransporting polypeptide (ntcp) mediates the uptake of bile salts from plasma across the basolateral domain of the hepatocyte. We have demonstrated that ntcp expression can be induced by prolactin (PRL) and placental lactogen via the PRL receptor and signal transducers and activators of transcription (Stat)5a pathway. However, elevated levels of placental lactogen do not increase the expression of ntcp in pregnant rats. Because plasma estradiol (E(2)) levels are also elevated in pregnancy, we investigated the inhibitory effects of E(2) on PRL-induced ntcp activation. E(2) treatment inhibited the PRL-induced increase in liver ntcp mRNA to the same levels as in rats treated with E(2) alone. Estrogen receptor-alpha (ERalpha) mRNA and protein expression in liver were increased 2.6-fold and 2.2-fold, respectively, in pregnancy relative to controls. In HepG2 cells, E(2) repressed PRL-induced ntcp reporter gene expression in a dose-dependent manner in the presence of cotransfected ERalpha. The ERalpha antagonist ICI 182,780 reversed E(2)-induced repression, indicating specificity of inhibition by E(2). Overexpression of coactivator p300 did not reverse the inhibitory effects of E(2) and ERalpha. Western and gel shift analysis revealed that E(2)-bound ERalpha decreased the tyrosine phosphorylation and DNA-binding activity of Stat5a, indicating that the inhibitory effect of E(2) was mediated, at least in part, by interfering with PRL-mediated signal transduction. The present studies demonstrate the physiological significance of cross-talk between ERalpha and Stat5a in liver, in which both proteins are expressed. These data also establish a novel mechanism by which expression of ntcp, an important hepatic bile acid transporter, can be regulated by multiple hormones.

Estrogen signaling in livers of male mice with hepatocellular carcinoma induced by exposure to arsenic in utero

BACKGROUND

Exposure of pregnant mice to inorganic arsenic induces a spectrum of tumors, including hepatocellular carcinoma (HCC), in their adult offspring similar to that induced by exposing adult mice to estrogenic compounds. To investigate whether arsenic exposure in utero causes altered estrogen signaling, we examined expression of estrogen receptor-alpha (ER-alpha), cyclin D1 (an estrogen-responsive hepatic oncogene), and several cytochrome P450 genes (with sexually dimorphic liver expression patterns) in livers from adult male mice with in utero arsenic-induced HCC.

METHODS

Quantitative real-time reverse transcription-polymerase chain reaction was used to evaluate gene expression in livers of adult male mice that had (i.e., exposed mice; n = 8) or had not (i.e., control mice; n = 5) been exposed to arsenic in utero. DNA methylation status of portions of the ER-alpha and cyclin D1 gene promoters in liver tissue was measured using methylation-specific polymerase chain reaction. Statistical tests were two-sided.

RESULTS

ER-alpha mRNA levels were 3.1-fold (95% confidence interval [CI] = 2.0-fold to 4.3-fold) higher in livers of exposed mice than in those of control mice, and cyclin D1 levels were 3.0-fold (95% CI = 1.7-fold to 4.3-fold) higher. Exposed mice showed a feminized expression pattern of several cytochrome P450 genes, expressing the female-dominant CYP2A4 (P =.017 versus control) and CYP2B9 (P<.001) genes at 8.7 and 10.5 times, respectively, the level in control mice and expressing the male-dominant CYP7B1 at approximately one-fourth the level in control mice(P =.0012). Exposed mice exhibited reduced (by approximately 90%) methylation of the ER-alpha gene promoter in liver DNA as compared with control mice; the cyclin D1 gene promoter was not methylated in either exposed or control mice.

CONCLUSION

Altered estrogen signaling may play a role in induction of HCC by arsenic exposure in utero. Specifically, overexpression of ER-alpha, potentially through promoter region hypomethylation, in livers of such mice may be linked to the hepatocarcinogenicity of arsenic.

Regulation of mouse renal CYP2J5 expression by sex hormones

Mouse CYP2J5 is abundant in kidney and active in the metabolism of arachidonic acid to epoxyeicosatrienoic acids. Western blots of microsomes prepared from mouse kidneys demonstrate that after puberty, CYP2J5 protein is present at higher levels in male mice than in female mice. Northern analysis reveals that CYP2J5 transcripts are more abundant in adult male versus female kidneys, indicating that gender differences in renal CYP2J5 expression are regulated at a pretranslational level. Castration of male mice results in decreased renal CYP2J5 expression, and treatment of castrated male mice or female mice with 5alpha-dihydrotestosterone increases expression to levels that approximate those in intact male mice. In contrast, treatment of ovariectomized female mice or castrated male mice with 17beta-estradiol causes a further reduction in CYP2J5 expression. Growth hormone-deficient (lit/lit) mice respond similarly to castration and 5alpha-dihydrotestosterone treatment, indicating that the androgen effects are not mediated by alterations in the growth hormone secretory pattern. Mice that lack a functional androgen receptor (Tfm hemizygous) have reduced levels of renal CYP2J5 and do not respond to 5alpha-dihydrotestosterone treatment. Similarly, wild-type male mice treated with flutamide, an androgen antagonist, exhibit reduced renal CYP2J5 levels. Female estrogen receptor-alpha knockout (alphaERKO) mice, which are known to have elevated circulating testosterone levels, have significantly increased renal CYP2J5 expression compared with wild-type female mice, and these differences are abrogated by ovariectomy or treatment with flutamide. Based on these data, we conclude that the renal expression of CYP2J5 is up-regulated by androgen and down-regulated by estrogen.

Down-regulation of STAT5b transcriptional activity by ligand-activated peroxisome proliferator-activated receptor (PPAR) alpha and PPARgamma

The nuclear receptor peroxisome proliferator-activated receptor (PPAR) is activated by a diverse group of acidic ligands, including many peroxisome proliferator chemicals present in the environment. Janus tyrosine kinase-signal transducer and activator of transcription (JAK-STAT) signaling is activated by multiple cytokines and hormones and leads to the translocation of dimerized STAT proteins to the nucleus where they activate transcription of target genes. Previous studies have shown that growth hormone (GH)-activated STAT5b can inhibit PPAR-regulated transcription. Here, we show that this inhibitory cross-talk is mutual, and that GH-induced, STAT5b-dependent beta-casein promoter-luciferase reporter gene transcription can be inhibited up to approximately 80% by ligand-activated PPARalpha or PPARgamma. Dose-response experiments showed a direct relationship between the extent of PPAR activation and the degree of inhibition of STAT5-regulated transcription. PPAR did not block STAT5b tyrosine phosphorylation or inhibit DNA-binding activity. Both PPARs inhibited the transcriptional activity of a constitutively active STAT5b mutant, indicating that inhibition occurs downstream of the GH-stimulated STAT5 activation step. Transcriptionally inactive, dominant-negative PPAR mutants did not block STAT5b inhibition by wild-type PPAR, indicating that PPAR target gene transcription is not required. PPARalpha retained its STAT5b inhibitory activity in the presence of the histone deacetylase inhibitor trichostatin, indicating that enhanced histone deacetylase recruitment does not contribute to STAT5b inhibition. PPARalpha lacking the ligand-independent AF-1 trans-activation domain failed to inhibit STAT5b, highlighting the importance of the AF-1 region in STAT5-PPAR inhibitory cross-talk. These findings demonstrate the bidirectionality of cross-talk between the PPAR and STAT pathways and provide a mechanism whereby exposure to environmental chemical activators of PPAR can suppress expression of GH target genes.

The 2001 Veylien Henderson Award of the Society of Toxicology of Canada. Positive and negative transcriptional regulation of cytochromes P450 by polycyclic aromatic hydrocarbons

Most responses to aromatic hydrocarbons such as 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin are mediated by the aromatic hydrocarbon receptor (AHR). The AHR regulates induction of drug-metabolizing enzymes such as cytochrome P450 1A1. However, the expression of several genes of biological significance is decreased by these chemicals. We are examining the mechanisms by which aromatic hydrocarbons suppress constitutive hepatic cytochromes P450, especially the male-specific rat liver cytochrome P450 2C11 (CYP2C11), which is regulated by pulsatile growth hormone (GH) secretion. Aromatic hydrocarbons suppress CYP2C11 via a transcriptional mechanism both in vivo and in cultured hepatocytes, and the AHR appears to be involved; however, studies of protein-DNA interactions and reporter genes driven by the CYP2C11 5'-flanking region have not provided a definitive mechanism for this response. MC attenuates the ability of GH to stimulate hepatic CYP2C11 expression in hypophysectomized (hypx) male rats, and this prompted studies of effects of aromatic hydrocarbons on hepatic GH signaling pathways as a novel aspect of endocrine disruption. Our studies with hypx rats also suggest that the hepatic AHR protein is regulated by a pituitary factor(s). The goal of these molecular mechanistic studies is to improve our understanding of how environmental contaminants modulate the expression of genes coding for xenobiotic- and hormone-metabolizing enzymes.

Regulation of the expression of two female-predominant CYP3A mRNAs (CYP3A41 and CYP3A44) in mouse liver by sex and growth hormones

A second female-predominant murine CYP3A, CYP3A44, was isolated from liver and its mRNA expression was compared with that of the previously described CYP3A41. The expression of CYP3A44 was relatively constant after birth in females, whereas it gradually declined in males after 5 weeks of age. The expression of CYP3A41 increased with age in females after 3 weeks of age, whereas it gradually declined in males after 5 weeks of age. Hypophysectomy and growth hormone replacement indicated that expression of both CYP3A mRNAs in females was dependent on the feminine plasma growth hormone profile. Estradiol induced the expression of both mRNAs and the effect was dependent on the presence of the pituitary gland. These observations suggest that endocrine control of expression might be similar, but not identical, for two female-predominant CYP3A mRNAs.

Estrogen-dependent regulation of the expression of hepatic Cyp2b and 3a isoforms: assessment using aromatase-deficient mice

The role of estrogen in the expression and induction of hepatic Cyp2b and Cyp3a isoforms was studied using mice [Ar (-/-) mice] lacking aromatase, a key enzyme for estrogen biosynthesis. The expression of P450s was determined by reverse transcription-polymerase chain reaction, immunoblotting, and measuring testosterone 6beta- and 16alpha-hydroxylase activity as markers. Basic expression of Cyp3a11 mRNA and protein was seen in both sexes of Ar (+/+) mice. Disruption of the aromatase gene caused an increase in the expression of Cyp3a11 protein, although the mRNA level remained unchanged. Female-specific Cyp3a41 disappeared in Ar (-/-) mice, and this could not be reversed by administration of exogenous beta-estradiol to adult knockout mice. The constitutive expression of female-specific Cyp2b9 also disappeared on disrupting the aromatase gene. However, in clear contrast to Cyp3a41, some individual Ar (-/-) mice exhibited expression of this form following treatment with exogenous beta-estradiol. Disruption of the aromatase gene had no effect on PB-mediated induction of Cyp2b10 or on the noninducible nature of Cyp2b9, Cyp3a11, and Cyp3a41. These results suggest that (1) Cyp3a11 is suppressed by estrogen; (2) the expression of female-specific Cyp3a41 is programmed by neonatal and/or infantile exposure to estrogen; (3) maintenance of the expression of female-specific Cyp2b9 requires estrogen in adults; and (4) endogenous estrogen plays little, if any, role in the mechanism by which PB induces Cyp2b10.

Inhibitory cross-talk between STAT5b and liver nuclear factor HNF3beta: impact on the regulation of growth hormone pulse-stimulated, male-specific liver cytochrome P-450 gene expression

STAT5b is repeatedly activated in rodent liver by the male pattern of intermittent plasma growth hormone (GH) stimulation and is required to maintain the GH pulse-regulated, male-specific pattern of liver gene expression. We presently investigate the interactions between STAT5b and hepatocyte-enriched nuclear factors (HNFs) that contribute to regulation of GH pulse-inducible, male-specific liver cytochrome P-450 (CYP) genes. STAT5 binding sites were identified in the 5'-flank of the adult male-expressed genes CYP2A2 (nucleotides -2255 to -2247), CYP4A2 (nucleotides -1872 to -1864), and CYP2C11 (nucleotides -1150 to -1142). STAT5-DNA complexes were formed by each CYP sequence with nuclear extract from GH pulse-activated male, but not female, rat liver. The CYP2C11 STAT5 site, which is flanked by HNF3 consensus sequences, conferred STAT5b-inducible reporter gene activity in GH-treated HepG2 cells. trans-Activation of the intact CYP2C11 promoter (1.8-kilobase 5'-flank) was strongly induced by the liver nuclear factors HNF1alpha and HNF3beta but, unexpectedly, was inhibited by GH-activated STAT5b. This STAT5b inhibitory effect could be reversed by HNF1alpha and reflects a functional antagonism between STAT5b and HNF3beta, as evidenced by the inhibition of HNF3beta DNA binding and transcriptional activity by STAT5b. HNF3beta, in turn, inhibited STAT5b by a novel mechanism that leads to suppression of GH-inducible STAT5b tyrosine phosphorylation, DNA binding activity, and transcriptional activity. The potential for GH-activated STAT5b to stimulate male-specific liver CYP expression can thus be modulated by HNF3beta, highlighting the complex interrelationship between STAT5b and liver transcription factors controlling expression of GH-regulated CYP genes.

Responses of insulin-like growth factor (IGF)-I and IGF-binding proteins to nutritional status in peroxisome proliferator-activated receptor-alpha knockout mice

Peroxisome proliferator-activated receptor alpha (PPARalpha) plays a central role in glucose and lipid homeostasis. Mice lacking PPARalpha(-/-) have a sexually dimorphic phenotype. We have characterized the IGF system in wild type and PPARalpha-/- mice. In normal mice fasting IGF-I and the IGFBP-3 ternary complex were 2-fold higher in males than in females. PPARalpha influenced the IGF/IGFBP response to feeding, particularly in males. Compared to wild type, male PPARalpha-/- mice had 40% lower total fasting IGF-I concentrations, decreased ALS and less IGFBP-3 ternary complex formation, but within 4 h of refeeding there was an increase in IGF-I and IGFBP-3 ternary complex to values similar to controls. Circulating IGFBP protease activity was induced in male PPARalpha-/- mice during refeeding. IGFBP-1 and insulin concentrations were higher in males than females, and were increased by PPARalpha knockout, suggesting significant hepatic insulin resistance. We speculate that gender differences in the IGF system contribute to the PPARalpha-/- phenotype.

Cell localization, physiology, and nongenomic actions of estrogen receptors

The existence of binding proteins for the female sex steroid, 17beta-estradiol, has been known for almost 50 years. Presently, two estrogen receptors (ERs), ER-alpha and ER-beta, have been cloned in mammals, and they are expressed in many cell types of metazoans. ERs act primarily as nuclear transcription factors, and this effect is enhanced by ligand binding. Emerging data have identified a separate pool of receptors for this steroid in the plasma membrane, but the mechanisms of action and cellular functions of these proteins are just beginning to be defined. In this review, the known details of the nuclear and plasma membrane ER functions will be discussed. A particular focus will be to define the signaling pathways from the membrane that lead to important cell physiology effects of estrogen. The potential interactions of membrane ER with other local proteins will also be discussed, and the unique but often complementary roles of the receptor pools will be highlighted. These details may be of additional relevance to other steroid receptors, since there is evidence of their existence in the cell membrane.

Somatostatin is required for masculinization of growth hormone-regulated hepatic gene expression but not of somatic growth

Pulsatile growth hormone (GH) secretion differs between males and females and regulates the sex-specific expression of cytochrome P450s in liver. Sex steroids influence the secretory dynamics of GH, but the neuroendocrine mechanisms have not been conclusively established. Because periventricular hypothalamic somatostatin (SST) expression is greater in males than in females, we generated knockout (Smst(-/-)) mice to investigate whether SST peptides are necessary for sexually differentiated GH secretion and action. Despite marked increases in nadir and median plasma GH levels in both sexes of Smst(-/-) compared with Smst(+/+) mice, the mutant mice had growth curves identical to their sibling controls and retained a normal sexual dimorphism in weight and length. In contrast, the liver of male Smst(-/-) mice was feminized, resulting in an identical profile of GH-regulated hepatic mRNAs between male and female mutants. Male Smst(-/-) mice show higher expression of two SST receptors in the hypothalamus and pituitary than do females. These data indicate that SST is required to masculinize the ultradian GH rhythm by suppressing interpulse GH levels. In the absence of SST, male and female mice exhibit similarly altered plasma GH profiles that eliminate sexually dimorphic liver function but do not affect dimorphic growth.

Regulation of cytochrome P450 (CYP) genes by nuclear receptors

Members of the nuclear-receptor superfamily mediate crucial physiological functions by regulating the synthesis of their target genes. Nuclear receptors are usually activated by ligand binding. Cytochrome P450 (CYP) isoforms often catalyse both formation and degradation of these ligands. CYPs also metabolize many exogenous compounds, some of which may act as activators of nuclear receptors and disruptors of endocrine and cellular homoeostasis. This review summarizes recent findings that indicate that major classes of CYP genes are selectively regulated by certain ligand-activated nuclear receptors, thus creating tightly controlled networks.