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

Roles of cytochrome P450 enzymes in pharmacology and toxicology: Past, present, and future

The development of the cytochrome P450 (P450) field has been remarkable in the areas of pharmacology and toxicology, particularly in drug development. Today it is possible to use the knowledge base and relatively straightforward assays to make intelligent predictions about drug disposition prior to human dosing. Much is known about the structures, regulation, chemistry of catalysis, and the substrate and inhibitor specificity of human P450s. Many aspects of drug-drug interactions and side effects can be understood in terms of P450s. This knowledge has also been useful in pharmacy practice, as well as in the pharmaceutical industry and medical practice. However, there are still basic and practical questions to address regarding P450s and their roles in pharmacology and toxicology. Another aspect is the discovery of drugs that inhibit P450 to treat diseases.

Relationships between maternal perfluoroalkyl substance levels, polymorphisms of receptor genes, and adverse birth outcomes in the Hokkaido birth cohort study, Japan

We assessed the associations between perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA) levels in third trimester maternal serum, the maternal genotypes of genes encoding nuclear receptors, and birth outcomes. We studied a prospective birth cohort of healthy pregnant Japanese women (n = 372) recruited in Sapporo between July 2002 and October 2005. We analyzed PFOS and PFOA levels using liquid chromatography-tandem mass spectrometry and analyzed 13 single nucleotide polymorphisms (SNPs) of proliferator-activated receptor alpha, gamma, gamma coactivator 1A, delta, constitutive androstane receptor, liver X receptor alpha, and beta (LXRB) using real-time polymerase reaction (PCR). We employed multiple linear regression models to establish the influences of log₁₀-transformed PFOS and PFOA levels and maternal genotypes on birth size. In female infants, we identified interactions between PFOS levels, the maternal genotype of LXRB (rs1405655), and birth weight. The estimated mean changes in birth weight in response to PFOS levels, the maternal genotype LXRB (rs1405655)-TC/CC (compared to TT), and their interactions were -502.9 g (95 % confidence interval [CI] = -247.3, -758.5 g), -526.3 g (95 % CI = -200.7, -852.0 g), and 662.1 g (95 % CI = 221.0, 1,103.2 g; p_int = 0.003), respectively. Interactions between PFOS levels and the maternal genotype of LXRB (rs1405655) also significantly affected birth chest circumference and the Ponderal index (p_int = 0.037 and 0.005, respectively). Thus, interactions between PFOS levels and the maternal genotype of LXRB (rs1405655) affects birth sizes in female infants. We found that certain SNPs modify the effects of PFOS levels on birth size.

Sex differences in the association between perfluoroalkyl acids and liver function in US adolescents: Analyses of NHANES 2013-2016

Perfluoroalkyl acids (PFAAs) are persistent in the environment, highly bio-accumulative in the body, and likely hepatotoxic in humans. There is evidence of sex-specific physiological responses to PFAA exposure. However, epidemiological studies seldom stratify the analyses by sex. Given the high prevalence of liver disease in general population adolescents, this study was designed to determine whether or not there is association between exposure to PFAAs and biomarkers of liver function in adolescent participants of the 2013-2016 National Health and Nutrition Examination Survey, and whether or not such association is sex-specific. Multivariate linear regressions were performed to examine the association between single PFAAs [perfluorooctane sulfonic acid (PFOS); linear form of perfluorooctanoic acid (PFOA); perfluorohexane sulfonic acid (PFHxS); perfluorononanoic acid (PFNA)], and biomarkers of liver function - gamma glutamyltransferase (GGT), aspartate aminotransferase (AST), alanine aminotransferase (ALT), and total bilirubin. Multivariate logistic regressions were performed to estimate adjusted odd ratios (aOR) of elevated ALT, AST and GGT. The study results show that, in females, there was a positive association of the highest PFOA quartile with increased ALT, AST and GGT, and the highest PFNA quartile with increased ALT and AST. Conversely, in male adolescents there was an association of the highest linear PFOA quartile with decreased ALT, and the highest PFNA quartile with ALT and AST. Females had higher odds of clinically-defined elevated ALT with increased PFOA (aOR = 1.79; 95% CI: 1.05, 3.04) or PFNA (aOR = 2.28; 95% CI: 1.08, 2.28), whereas males had decreased odds of clinically-defined elevated ALT with increased n-PFOA (aOR = 0.43; 95% CI: 0.20, 0.93) or PFNA (aOR = 0.5; 95% CI: 0.28, 0.89). In conclusion, there were sex differences in the association between serum PFAA levels and biomarkers of liver function. These results may provide support for analyzing sex-based adverse effects of PFAAs.

Arid1a Has Context-Dependent Oncogenic and Tumor Suppressor Functions in Liver Cancer

ARID1A, an SWI/SNF chromatin-remodeling gene, is commonly mutated in cancer and hypothesized to be tumor suppressive. In some hepatocellular carcinoma patients, ARID1A was highly expressed in primary tumors but not in metastatic lesions, suggesting that ARID1A can be lost after initiation. Mice with liver-specific homozygous or heterozygous Arid1a loss were resistant to tumor initiation while ARID1A overexpression accelerated initiation. In contrast, homozygous or heterozygous Arid1a loss in established tumors accelerated progression and metastasis. Mechanistically, gain of Arid1a function promoted initiation by increasing CYP450-mediated oxidative stress, while loss of Arid1a within tumors decreased chromatin accessibility and reduced transcription of genes associated with migration, invasion, and metastasis. In summary, ARID1A has context-dependent tumor-suppressive and oncogenic roles in cancer.

Compensatory changes in CYP expression in three different toxicology mouse models: CAR-null, Cyp3a-null, and Cyp2b9/10/13-null mice

Targeted mutant models are common in mechanistic toxicology experiments investigating the absorption, metabolism, distribution, or elimination (ADME) of chemicals from individuals. Key models include those for xenosensing transcription factors and cytochrome P450s (CYP). Here we investigated changes in transcript levels, protein expression, and steroid hydroxylation of several xenobiotic detoxifying CYPs in constitutive androstane receptor (CAR)-null and two CYP-null mouse models that have subfamily members regulated by CAR; the Cyp3a-null and a newly described Cyp2b9/10/13-null mouse model. Compensatory changes in CYP expression that occur in these models may also occur in polymorphic humans, or may complicate interpretation of ADME studies performed using these models. The loss of CAR causes significant changes in several CYPs probably due to loss of CAR-mediated constitutive regulation of these CYPs. Expression and activity changes include significant repression of Cyp2a and Cyp2b members with corresponding drops in 6α- and 16β-testosterone hydroxylase activity. Further, the ratio of 6α-/15α-hydroxylase activity, a biomarker of sexual dimorphism in the liver, indicates masculinization of female CAR-null mice, suggesting a role for CAR in the regulation of sexually dimorphic liver CYP profiles. The loss of Cyp3a causes fewer changes than CAR. Nevertheless, there are compensatory changes including gender-specific increases in Cyp2a and Cyp2b. Cyp2a and Cyp2b were down-regulated in CAR-null mice, suggesting activation of CAR and potentially PXR following loss of the Cyp3a members. However, the loss of Cyp2b causes few changes in hepatic CYP transcript levels and almost no significant compensatory changes in protein expression or activity with the possible exception of 6α-hydroxylase activity. This lack of a compensatory response in the Cyp2b9/10/13-null mice is probably due to low CYP2B hepatic expression, especially in male mice. Overall, compensatory and regulatory CYP changes followed the order CAR-null > Cyp3a-null > Cyp2b-null mice.

Chemical and in vitro biological information to predict mouse liver toxicity using recursive random forests

In this study, recursive random forests were used to build classification models for mouse liver toxicity. The mouse liver toxicity endpoint (67 toxic and 166 non-toxic) was a composition of four in vivo chronic systemic and carcinogenic toxicity endpoints (non-proliferative, neoplastic, proliferative and gross pathology). A multiple under-sampling approach and a shifted classification threshold of 0.288 (non-toxic < 0.288 and toxic ≥ 0.288) were used to cope with the unbalanced data. Our study showed that recursive random forests are very efficient in variable selection and for the development of predictive in silico models. Generally, over 95% redundant descriptors could be reduced from modelling for all the chemical, biological and hybrid models in this study. The predictive performance of chemical models (CCR of 0.73) is comparable with hybrid model performance (CCR of 0.74). Descriptors related to the octanol-water partition coefficient are vital for model performance. The in vitro endpoint of CYP2A2 played a key role in the development and interpretation of hybrid models. Identifying high-throughput screening assays relevant to liver toxicity would be key for improving in silico models of liver toxicity.

Sexually dimorphic actions of glucocorticoids: beyond chromosomes and sex hormones

Sexual dimorphism is a well-documented phenomenon that is observed at all levels of the animal kingdom. Historically, sex hormones (testosterone and estrogen) have been implicated as key players in a wide array of pathologies displaying sexual dimorphism in their etiology and progression. While these hormones clearly contribute to sexually dimorphic diseases, other factors may be involved in this phenomenon as well. In particular, the stress hormone cortisol exerts differential effects in both males and females. The underlying molecular basis for the sexually dimorphic actions of glucocorticoids is unknown but clearly important to understand, since synthetic glucocorticoids are the most widely prescribed medication for the treatment of chronic inflammatory diseases and hematological cancers in humans.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

The KRAB zinc finger protein RSL1 regulates sex- and tissue-specific promoter methylation and dynamic hormone-responsive chromatin configuration

Over 400 Krüppel-associated box zinc finger proteins (KRAB-ZFPs) are encoded in mammalian genomes. While KRAB-ZFPs strongly repress transcription in vitro, little is known about their biological function or gene targets in vivo. Regulator of sex limitation 1 (Rsl1), one of the first KRAB-Zfp genes assigned a physiological role, accentuates sex-biased liver gene expression, most dramatically for mouse sex-limited protein (Slp), which provides an in vivo reporter of KRAB-ZFP function. Slp is induced in males in the liver and kidney by growth hormone (GH) and androgen, respectively. In the liver but not kidney, the Rsl1 genotype correlates with methylation of a CpG dinucleotide in the Slp promoter that is demethylated at puberty. RSL1 binds 2 kb upstream of the Slp promoter, both in vitro and in vivo, within an enhancer containing response elements for STAT5b. Chromatin immunoprecipitation (ChIP) assays demonstrate that RSL1 recruits KAP1/TRIM28, the corepressor for KRAB action in vitro, to this enhancer. Slp induction requires rapid cycling of STAT5b in chromatin. Remarkably, RSL1 simultaneously binds adjacent to STAT5b with a reciprocal binding pattern that limits hormonal response. These experiments demonstrate a surprisingly dynamic interplay between a hormonal activator, STAT5b, and a KRAB-ZFP repressor and provide unique insights into KRAB-ZFP epigenetic mechanisms.

Growth hormone-STAT5 regulation of growth, hepatocellular carcinoma, and liver metabolism

The liver is a primary target of growth hormone (GH). GH signals are mediated by the transcription factor signal transducer and activator of transcription 5 (STAT5). Here, we focus on recent discoveries about the role of GH-STAT5 signaling in hepatic physiology and pathophysiology. We discuss roles of the GH-STAT5 axis in body growth, lipid metabolism, and the cell cycle pertaining to hepatosteatosis, fibrosis, and hepatocellular carcinoma. Finally, we discuss recent discoveries about the role of GH-STAT5 in sex-specific gene expression and bile acid, steroid, and drug metabolism.

Sexually dimorphic actions of glucocorticoids provide a link to inflammatory diseases with gender differences in prevalence

Males and females show differences in the prevalence of many major diseases that have important inflammatory components to their etiology. These gender-specific diseases, which include autoimmune diseases, hepatocellular carcinoma, diabetes, and osteoporosis, are largely considered to reflect the actions of sex hormones on the susceptibility to inflammatory stimuli. However, inflammation reflects a balance between pro- and anti-inflammatory signals, and investigation of gender-specific responses to the latter has been neglected. Glucocorticoids are the primary physiological anti-inflammatory hormones in mammals, and synthetic derivatives of these hormones are prescribed as anti-inflammatory agents, irrespective of patient gender. We explored the possibility that sexually dimorphic actions of glucocorticoid regulation of gene expression may contribute to the dimorphic basis of inflammatory disease by evaluating the rat liver, a classic glucocorticoid-responsive organ. Surprisingly, glucocorticoid administration expanded the set of hepatic sexually dimorphic genes. Eight distinct patterns of glucocorticoid-regulated gene expression were identified, which included sex-specific genes. Our experiments also defined specific genes with altered expression in response to glucocorticoid treatment in both sexes, but in opposite directions. Pathway analysis identified sex-specific glucocorticoid-regulated gene expression in several canonical pathways involved in susceptibility to and progression of diseases with gender differences in prevalence. Moreover, a comparison of the number of genes involved in inflammatory disorders between sexes revealed 84 additional glucocorticoid-responsive genes in the male, suggesting that the anti-inflammatory actions of glucocorticoids are more effective in males. These gender-specific actions of glucocorticoids in liver were substantiated in vivo with a sepsis model of systemic inflammation.

Gender differences in drug toxicity

Clinical data suggest that gender dimorphic profiles are emerging in terms of both drug efficacy and adverse drug reactions (ADRs). With an increasing emphasis on individualised therapies and the need to prevent drug attrition there is a compelling need to understand the molecular basis for gender dimorphic profiles in ADRs and the consequences. Classes of agents exhibiting gender-based variation in pharmaceutical efficacy and toxicity include anaesthetics, HIV-1 therapies and antiarrhythmic drugs. Body weight differences are often cited as a reason for differences in drug pharmacokinetics and subsequent toxicity. However, some studies accounted for these factors and still found significance suggesting that dosage versus body weight does not explain the outcome. Here, we present an overview of current understanding of gender-specific drug toxicity and present rational molecular explanations for these adverse events. There is mounting evidence in support of hormonal effects underpinning the majority of the ADR differences observed between the sexes.

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.

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.

Global analysis of in vivo Foxa2-binding sites in mouse adult liver using massively parallel sequencing

Foxa2 (HNF3β) is a one of three, closely related transcription factors that are critical to the development and function of the mouse liver. We have used chromatin immunoprecipitation and massively parallel Illumina 1G sequencing (ChIP–Seq) to create a genome-wide profile of in vivo Foxa2-binding sites in the adult liver. More than 65% of the ∼11.5 k genomic sites associated with Foxa2 binding, mapped to extended gene regions of annotated genes, while more than 30% of intragenic sites were located within first introns. 20.5% of all sites were further than 50 kb from any annotated gene, suggesting an association with novel gene regions. QPCR analysis demonstrated a strong positive correlation between peak height and fold enrichment for Foxa2-binding sites. We measured the relationship between Foxa2 and liver gene expression by overlapping Foxa2-binding sites with a SAGE transcriptome profile, and found that 43.5% of genes expressed in the liver were also associated with Foxa2 binding. We also identified potential Foxa2-interacting transcription factors whose motifs were enriched near Foxa2-binding sites. Our comprehensive results for in vivo Foxa2-binding sites in the mouse liver will contribute to resolving transcriptional regulatory networks that are important for adult liver function.

Sexual dimorphism in non-Mendelian inheritance

There is accumulating evidence for nongenetic transgenerational inheritance with conspicuous marked sexual dimorphism for both the modes of transmission and the effects. Given the critical spatiotemporal windows, the role of the sex chromosomes, the regulatory pathways underlying sexual differentiation during gonad and brain development, and other developmental processes, as well as the lifelong impact of sex hormones, it is not surprising that most of the common diseases, which often take root in early development, display some degree of sex bias. The flexibility of epigenetic marks may make it possible for environmental and nutritional factors, or endocrine disruptors to alter-during a particular spatiotemporal window in a sex-specific manner-the sex-specific methylation or demethylation of specific CpGs and histone/chromatin modifications underlying sex-specific expression of a substantial proportion of genes. Thus, finely tuned developmental program aspects, specific to one sex, may be more sensitive to specific environmental challenges, particularly during developmental programming and gametogenesis, but also throughout the individual's life under the influence of sex steroid hormones. This review highlights the importance of studying both sexes in epidemiologic protocols or dietary interventions both in humans and in experimental models in animals.

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.

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 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.

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.

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.