Sex-specific, growth hormone-regulated transcription of the cytochrome P450 2C11 and 2C12 genes

Growth hormone activates X-box binding protein 1 in a sexually dimorphic manner through the extracellular signal-regulated protein kinase and CCAAT/enhancer-binding protein β pathway in rat liver

Growth hormone (GH) has multiple physiological roles, acting on many organs. In order to investigate its roles in rat liver, we tried to identify novel genes whose transcription was regulated by GH. We identified X-box binding protein 1 (Xbp1) as a candidate gene. XBP1 is a key transcription factor activated in response to endoplasmic reticulum (ER) stress. The purpose of this study was to investigate the mode of action of GH on XBP1, including the relation with ER stress, sex-dependent expression of the mRNA, and the signaling pathway. Intravenous administration of GH rapidly and transiently increased Xbp1 mRNA in hypophysectomized rat livers. Neither phosphorylated inositol-requiring-1α (IRE1α) nor phosphorylated PKR-like ER kinase (PERK) increased, suggesting that Xbp1 expression is induced by an ER stress-independent mechanism. The active form of XBP1(S) protein was increased by GH administration and was followed by an increased ER-associated dnaJ protein 4 (ERdj4) mRNA level. XBP1(S) protein levels were predominantly identified in male rat livers with variations among individuals similar to those of phosphorylated signal transducer and activator of transcription 5B (STAT5B), suggesting that XBP1(S) protein levels are regulated by the sex-dependent secretary pattern of GH. The GH signaling pathway to induce Xbp1 mRNA was examined in rat hepatoma H4IIE cells. GH induced the phosphorylation of CCAAT/enhancer-binding protein β (C/EBPβ) following extracellular signal-regulated protein kinase (ERK) phosphorylation. Taken together, the results indicated that XBP1 is activated by GH in rat liver in a sexually dimorphic manner via ERK and C/EBPβ pathway.

Feminization of Male Mouse Liver by Persistent Growth Hormone Stimulation: Activation of Sex-Biased Transcriptional Networks and Dynamic Changes in Chromatin States

Sex-dependent pituitary growth hormone (GH) secretory profiles-pulsatile in males and persistent in females-regulate the sex-biased, STAT5-dependent expression of hundreds of genes in mouse liver, imparting sex differences in hepatic drug/lipid metabolism and disease risk. Here, we examine transcriptional and epigenetic changes induced by continuous GH infusion (cGH) in male mice, which rapidly feminizes the temporal profile of liver STAT5 activity. cGH repressed 86% of male-biased genes and induced 68% of female-biased genes within 4 days; however, several highly female-specific genes showed weak or no feminization, even after 14 days of cGH treatment. Female-biased genes already in an active chromatin state in male liver generally showed early cGH responses; genes in an inactive chromatin state often responded late. Early cGH-responsive genes included those encoding two GH/STAT5-regulated transcriptional repressors: male-biased BCL6, which was repressed, and female-specific CUX2, which was induced. Male-biased genes activated by STAT5 and/or repressed by CUX2 were enriched for early cGH repression. Female-biased BCL6 targets were enriched for early cGH derepression. Changes in sex-specific chromatin accessibility and histone modifications accompanied these cGH-induced sex-biased gene expression changes. Thus, the temporal, sex-biased gene responses to persistent GH stimulation are dictated by GH/STAT5-regulated transcription factors arranged in a hierarchical network and by the dynamics of changes in sex-biased epigenetic states.

Novel revelation of warfarin resistant mechanism in roof rats (Rattus rattus) using pharmacokinetic/pharmacodynamic analysis

Roof rats (Rattus rattus) live mainly in human habitats. Heavy use of rodenticides, such as warfarin, has led to the development of drug resistance, making pest control difficult. There have been many reports regarding mutations of vitamin K epoxide reductase (VKOR), the target enzyme of warfarin, in resistant rats. However, it has been suggested there are other mechanisms of warfarin resistance. To confirm these possibilities, closed colonies of warfarin-susceptible roof rats (S) and resistant rats from Tokyo (R) were established, and the pharmacokinetics/pharmacodynamics of warfarin in rats from both colonies was investigated. R rats had low levels of warfarin in serum and high clearance activity. These rats can rapidly metabolize warfarin by hydroxylation. The levels of accumulation in the organs were lower than those of S rats. R rats administered warfarin showed high expression levels of CYP2B, 2C, and 3A, which play roles in warfarin hydroxylation, and may explain the high clearance ability of R rats. The mechanism of warfarin resistance in roof rats from Tokyo involved not only mutation of VKOR but also high clearance ability due to high levels of CYP2B, 2C and 3A expression possibly induced by warfarin.

The role of the dorsal noradrenergic pathway of the brain (locus coeruleus) in the regulation of liver cytochrome P450 activity

Our previous study conducted after intracerebroventricular DSP-4 injection showed an important stimulating role of a brain noradrenergic system in the neuroendocrine regulation of liver cytochrome P450 (CYP) expression. The aim of the present research was to study involvement of the dorsal noradrenergic pathway of the brain (originating from the locus coeruleus) in the expression of liver cytochrome P450. The experiment was carried out on male Wistar rats. Local injection of 6-hydroxydopamine to the locus coeruleus selectively decreased noradrenaline level in the brain (e.g. in the hypothalamus). The serum concentration of the growth hormone rose, while that of the thyroid hormones or corticosterone remained unchanged. A comparative study into cytochrome P450 isoform activity revealed significant increases in the activity of liver CYP2C11 and CYP3A after administration of 6-hydroxydopamine. The observed increase in the activity of CYP2C11 positively correlated with that in CYP protein level, while the enhanced activity of CYP3A was not accompanied with a simultaneous change in the enzyme protein. A 5-day-injection of noradrenaline into the lateral ventricles produced opposite effects on the CYP isoforms. It is concluded that damage to or activation of the dorsal noradrenergic innervation of the periventricular nucleus of the hypothalamus containing somatostatin (a growth hormone release-inhibiting factor) may be responsible for the changes observed in the activity of isoforms CYP2C11 and CYP3A that are regulated by the growth hormone. The obtained results indicate that the dorsal noradrenergic pathway plays an inhibitory (but not a crucial) role in the neuroendocrine regulation of cytochrome P450.

Evidence that 20-HETE contributes to the development of acute and delayed cerebral vasospasm

Recent studies have indicated that arachidonic acid (AA) is metabolized by the cytochrome P450 4A (CYP4A) enzymes in cerebral arteries to produce 20-hydroxyeicosatetraenoic acid (20-HETE) and that this compound has effects on cerebral vascular tone that mimic those seen following subarachnoid hemorrhage (SAH). In this regard, 20-HETE is a potent constrictor of cerebral arteries that decreases the open state probability of Ca(2+)-activated K(+) channels through activation of protein kinase C (PKC). It increases the sensitivity of the contractile apparatus to Ca(2+) by activating PKC and rho kinase. The formation of 20-HETE is stimulated by angiotensin II (AII), endothelin, adenosine triphosphate (ATP) and serotonin, and inhibited by NO, CO and superoxide radicals. Inhibitors of the formation of 20-HETE block the myogenic response of cerebral arterioles to elevations in transmural pressure in vitro and autoregulation of cerebral blood flow (CBF) in vivo. 20-HETE also plays an important role in modulating the cerebral vascular responses to vasodilators (NO and CO) and vasoconstrictors (AII, endothelin, serotonin). Recent studies have indicated that the levels of 20-HETE in cerebrospinal fluid (CSF) increase in rats, dogs and human patients following SAH and that inhibitors of the synthesis of 20-HETE prevent the acute fall in CBF in rats and reverse delayed vasospasm in both dogs and rats. This review examines the evidence that an elevation in the production of 20-HETE contributes to the initial fall in CBF following SAH and the later development of delayed vasospasm.

Oxytocin-stimulated NFAT transcriptional activation in human myometrial cells

Oxytocin (OXT) is a peptide hormone that binds the OXT receptor on myometrial cells, initiating an intracellular signaling cascade, resulting in accumulation of intracellular calcium and smooth muscle contraction. In other systems, an elevation of intracellular Ca(2+) stimulates nuclear translocation of the transcription factor, nuclear factor of activated T cells (NFAT), which is transcriptionally active in arterial and ileal smooth muscle. Here we have investigated the role of NFAT in the mechanism of action of OXT. Human myometrial cells expressed all five NFAT isoforms (NFATC1-C4 and -5). Myometrial cells were transduced with a recombinant adenovirus expressing a NFATC1-EFP reporter, and a semi-automated imaging system was used to monitor effects of OXT on reporter localization in live cells. OXT induced a concentration-dependent nuclear translocation of NFATC1-EFP in a reversible manner, which was inhibited by OXT antagonists and calcineurin inhibitors. Pulsatile stimulation with OXT caused intermittent, pulse-frequency-dependent, nuclear translocation of NFATC1-EFP, which was more efficient than sustained stimulation. OXT induced nuclear translocation of endogenous NFAT that was transcriptionally active, because OXT stimulated activity of a NFAT-response element-luciferase reporter and induced calcineurin-NFAT dependent expression of RGS2, RCAN1, and PTGS2 (COX2) mRNA. Furthermore, OXT-dependent transcription was dependent on protein neosynthesis; cycloheximide abolished RGS2 transcription but augmented RCAN1 and COX2 transcriptional readouts. This study identifies a novel signaling mechanism within the myometrium, whereby calcineurin-NFAT signaling mediates OXT-induced transcriptional activity. Furthermore, we show NFATC1-EFP is responsive to pulses of OXT, a mechanism by which myometrial cells could decode OXT pulse frequency.

Simultaneous alterations of brain and plasma serotonin concentrations and liver cytochrome P450 in rats fed on a tryptophan-free diet

Our previous study suggested involvement of the brain serotonergic system in the regulation of liver cytochrome P450 (CYP). The aim of the present study was to demonstrate simultaneous responsiveness of liver CYP and the peripheral and brain serotonergic systems to a tryptophan deficient diet during three days and one or three weeks of ingestion. The concentrations of serotonin, noradrenaline, dopamine and their metabolites were measured in blood plasma, the hypothalamus and brain stem of male rats. The enzyme activity and protein levels in the liver were determined for isoforms CYP1A, CYP2A, CYP2B, CYP2C6, CYP2C11, CYP2D and CYP3A. A three-day tryptophan-free diet increased serotonin content in the hypothalamus (but not in the brain stem or plasma). After one week, the level of serotonin was not changed in the brain, but was markedly increased in the plasma. A three week tryptophan restriction significantly reduced the concentration of serotonin in the brain and plasma. Changes in CYP2C6 and CYP2C11 (an increase and a decrease, respectively) were maintained throughout the experiment, while those found in other CYP isoforms varied, which usually resulted in a gradual increase in the enzyme activity within three weeks. The observed alterations in liver CYPs suggest involvement of both central and peripheral serotonin in the regulation of liver CYP expression whose mechanism is discussed. In conclusion, a deficit in tryptophan in the diet may be responsible for very serious food-cytochrome P450 and food-drug metabolism interactions. Interactions of this type may also refer to drugs acting via serotonergic system.

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

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

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

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

Sex and the Liver – A Journey Through Five Decades

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

Focus on the intermediate state: immature mRNA of cytochromes P450--methods and insights

The scattered and limited data on hnRNAs (pre-mRNAs) of cytochromes P450 (CYP) are compiled and discussed for the first time. The methods for determination and quantification of hnRNAs are compared. In most cases, CYP hnRNA levels were determined as a parameter of transcriptional activity. It is known, however, that some CYPs, in particular CYP2E1, are in addition specifically and extensively regulated by post-transcriptional processes. Obviously, these processes also influence the processing of CYP hnRNAs so that their levels cannot be considered a mere result of transcription. The underlying mechanisms of post-transcriptional CYP hnRNA and mRNA regulation are not well understood. It is our aim therefore to bring together available data on CYP hnRNA and to discuss them in the light of recent advances in knowledge concerning pre-mRNA processing and interactions between RNA and low molecular weight interfering RNAs. By doing this, we hope to drive research in a direction which appears promising in providing some long-awaited answers with respect to mechanisms of post-transcriptional CYP regulation.

Coordinated changes in xenobiotic metabolizing enzyme gene expression in aging male rats

In order to gain insight into the effects of aging on susceptibility to environmental toxins, we characterized the expression of xenobiotic metabolizing enzymes (XMEs) from the livers of male F344 and Brown Norway (BN) rats across the adult lifespan. Using full-genome Affymetrix arrays, principal component analysis showed a clear age-dependent separation between young and old animals in both rat strains. Out of 1135 or 1435 genes altered between the old and young groups in the F344 or BN rats, 7 or 3% were XMEs and included members of the phase I, II, and III classes of genes. There was a 20 or 32% overlap in the gene expression profile between the two strains for F344 or BN, respectively. Lipid, ergosterol, alcohol, and fatty acid metabolism genes were also altered with age in both strains. Some of the genes altered by age exhibited a gender-dependent expression pattern in young adult rats, suggesting an increasingly feminized pattern of gene expression with age in male rats. To examine transcriptional responses across lifespan after challenge with a xenobiotic compound, BN rats were exposed to toluene by oral gavage. Toluene exposure decreased the expression of glutathione synthetase, and dramatically increased the number of phase III genes being downregulated. The expression of CYP2B2 and glutathione-S-transferase decreased with age but increased in all age groups after toluene exposure. Decreased ability to detoxify and transport chemicals out of the body with age could result in increased susceptibility to some classes of chemicals in the aging population.

Growth hormone (GH) secretion, GH-dependent gene expression, and sexually dimorphic body growth in young rats with chronic renal failure

Chronic renal disease results in growth failure in children. This study sought to determine the influences of early renal failure on body growth, growth hormone (GH) secretion, and GH-dependent hepatic gene expression. Neonatal animals were subjected to five-sixth nephrectomy (Nephr) and monitored during growth. Sham-operated male (Sham) and female (Fem) rats served as controls. Whereas Nephr of adult animals causes renal insufficiency, neonatal nephrectomy leads to frank renal failure. In male Nephr compared with Sham animals, GH half-life and GH pulse frequency increased by 1.55- and 1.33-fold, respectively, and GH secretory-burst size decreased by 80%. Approximate entropy analysis quantified more disorderly patterns of GH secretion in Nephr animals, which differed from Sham males, but not from Fem rats. Expression of liver P450 CYP2C11 mRNA, which is dependent upon the male GH pattern, became undetectable, whereas expression of liver P450 CYP2C12 mRNA, which is dependent upon the female GH pattern, increased multifold. Renal failure in young rats abrogates the male pattern of GH pulsatility, abolishes the sexual dimorphism of body weight gain, and induces a female pattern of hepatic gene expression. These data raise the possibility that disruption of pulsatile GH secretion contributes to the growth failure of renal disease.

Implementation of genetics to personalize medicine

BACKGROUND

We stand on the verge of integrating individual genetic and genomic information into health care provision and maintenance to improve health, increase efficiency, and decrease costs. We are beginning to integrate information on inherited susceptibility, gene expression, and predicted pharmacogenomic response to refine our medical management.

OBJECTIVE

This article reviews the current utility of genetics and genomics in a wide array of clinical circumstances, considers the future applications, and defines some of the obstacles and potential solutions to clinical integration of genomic medicine.

METHODS

Using the search terms genetics, genomics, pharmacogenomics, newborn screening, long QT syndrome, BRCA1/BRCA2, maturity onset diabetes of youth, diabetes, hemochromatosis, coronary artery disease, copy number changes, genetic discrimination, and genetic education, the PubMed database was searched from January 2000 to March 2007 to identify pertinent articles. Search results were restricted to English-language and human studies.

RESULTS

Several areas of medicine have begun to incorporate genetics into clinical practice, including newborn screening and breast cancer risk stratification and treatment. Molecular genetic tests are, and will increasingly become, available for inherited arrhythmias, diabetes, cancer, coronary artery disease, and pharmacogenomics. However, there are many barriers to implementation, including the cost of testing, the genetic literacy of patients and health care providers, and concerns about genetic discrimination.

CONCLUSION

Genetics and genomics will be increasingly utilized in every field of medicine; however, health care providers and patients must have realistic expectations about its predictive power and current limitations.

Multiple mechanisms of growth hormone-regulated gene transcription

Diverse physiological actions of growth hormone (GH) are mediated by changes in gene transcription. Transcription can be regulated at several levels, including post-translational modification of transcription factors, and formation of multiprotein complexes involving transcription factors, co-regulators and additional nuclear proteins; these serve as targets for regulation by hormones and signaling pathways. Evidence that GH regulates transcription at multiple levels is exemplified by analysis of the proto-oncogene c-fos. Among the GH-regulated transcription factors on c-fos, C/EBPbeta appears to be key, since depletion of C/EBPbeta by RNA interference blocks the stimulation of c-fos by GH. The phosphorylation state of C/EBPbeta and its ability to activate transcription are regulated by GH through MAPK and PI3K/Akt-mediated signaling cascades. The acetylation of C/EBPbeta also contributes to its ability to activate c-fos transcription. These and other post-translational modifications of C/EBPbeta appear to be integrated for regulation of transcription by GH. The formation of nuclear proteins into complexes associated with DNA-bound transcription factors is also regulated by GH. Both C/EBPbeta and the co-activator p300 are recruited to c-fos in response to GH, altering c-fos promoter activation. In addition, GH rapidly induces spatio-temporal re-localization of C/EBPbeta within the nucleus. Thus, GH-regulated gene transcription mediated by C/EBPbeta reflects the integration of diverse mechanisms including post-translational modifications, modulation of protein complexes associated with DNA and re-localization of gene regulatory proteins. Similar integration involving other transcription factors, including Stats, appears to be a feature of regulation by GH of other gene targets.

Cytochrome P450 enzymes: Central players in cardiovascular health and disease

Cardiovascular disease (CVD) is a human health crisis that remains the leading cause of death worldwide. The cytochrome P450 (CYP) class of enzymes are key metabolizers of both xenobiotics and endobiotics. Many CYP enzyme families have been identified in the heart, endothelium and smooth muscle of blood vessels. Furthermore, mounting evidence points to the role of endogenous CYP metabolites, such as epoxyeicosatrienoic acids (EETs), hydroxyeicosatetraenoic acids (HETEs), prostacyclin (PGI(2)), aldosterone, and sex hormones, in the maintenance of cardiovascular health. Emerging science and the development of genetic screening have provided us with information on the differences in CYP expression among populations and groups of individuals. With this information, a link between CYP expression and activity and CVD, such as hypertension, coronary artery disease (CAD), myocardial infarction, heart failure, stroke, and cardiomyopathy and arrhythmias, has been established. In fact many currently used therapeutic modalities in CVD owe their therapeutic efficacy to their effect on CYP metabolites. Thus, the evidence for the involvement of CYP in CVD is numerous. Concentrating on treatment modalities that target the CYP pathway makes ethical sense for the affected individuals and decreases the socioeconomic burden of this disease. However, more research is needed to allow the integration of this information into a clinical setting.

The biological actions of dehydroepiandrosterone involves multiple receptors

Dehydroepiandrosterone has been thought to have physiological functions other than as an androgen precursor. The previous studies performed have demonstrated a number of biological effects in rodents, such as amelioration of disease in diabetic, chemical carcinogenesis, and obesity models. To date, activation of the peroxisome proliferators activated receptor alpha, pregnane X receptor, and estrogen receptor by DHEA and its metabolites have been demonstrated. Several membrane-associated receptors have also been elucidated leading to additional mechanisms by which DHEA may exert its biological effects. This review will provide an overview of the receptor multiplicity involved in the biological activity of this sterol.

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.

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.

Sexual dimorphism in mammalian gene expression

Males and females have obvious phenotypic differences; they also exhibit differences related to health, life span, cognitive abilities and have different responses to diseases such as anemia, coronary heart disease, hypertension and renal dysfunction. Although the anatomical, hormonal and chemical differences between the sexes are well known, there are few molecular descriptors for gender-specific physiological traits and health risks. Recent studies using microarrays and other methods have made significant progress towards elucidating the molecular differences between mammalian sexes in a variety of tissues and towards identifying the transcription factors that regulate sex-biased gene expression. These findings are providing new insights into the molecular and genetic differences that dictate the different behaviors and physiologies of mammalian sexes.

Novel Gender-Related Regulation of CYP2C12 Gene Expression in Rats

The expression of CYP2C12 by GH occurs in female but not in male rat livers. Direct injection of the CYP2C12 promoter-luciferase gene into male rat livers showed that the CYP2C12 promoter was active in both male and female rats. Thus, to further examine one or more factors that regulate the gender-related expression of CYP2C12, male rats were treated with trichostatin A, a specific inhibitor of histone deacetylase capable of condensing the chromatin structure. Interestingly, the expression of CYP2C12 by GH was seen even in the livers of male rats, indicating that histone deacetylase contributes to the suppression of CYP2C12 expression in male rats. Deoxyribonuclease I hypersensitive assay using nuclei from the livers of male or female rats revealed that the chromatin structure of the CYP2C12 gene was gender specific: a hypersensitive site at a position −4.2 kb containing GH-responsive element that bound to signal transducer and activator of transcription 5 (STAT5), termed as HS (hypersensitive site) 1, was specific for female rat livers, whereas a hypersensitive site at a position −3 kb, designated as HSm (male-specific hypersensitive site), was characteristic of male rat livers. A −3425/−3275 region within HSm functioned as a negative regulatory region, when the region was inserted in front of simian virus 40 promoter. Gel shift assay demonstrated that both CCAAT/enhancer-binding protein α and β bound to the −3425/−3275 region. Based on these results, we conclude that the gender-related expression of the CYP2C12 gene results from the inaccessibility of to STAT5 to the GH-responsive element by chromatin condensation seen in male rat livers, and from the presence of the male-specific HSm that acts as a silencer.

Sexual Dimorphism of Rat Liver Nuclear Proteins

Many genes are expressed in mammalian liver in a sexually dimorphic manner. DNA microarray analysis has shown that growth hormone (GH) and its sex-dependent pattern of pituitary secretion play a major role in establishing the sexually dimorphic patterns of liver gene expression. However, GH may exert effects on protein post-translational modification and nuclear localization that are not reflected at the mRNA level. To investigate these potential effects of GH, we used two-dimensional gel electrophoresis followed by LC-MS/MS to: 1) identify rat liver nuclear proteins whose abundance or state of post-translational modification displays sex-dependent differences; and 2) determine the role of the plasma GH profile in establishing these differences. Nuclear extracts prepared from livers of individual male (n=9) and female (n=5) adult rats, and from males given GH by continuous infusion for 7 days to feminize liver gene expression (n=5 rats), were resolved by two-dimensional electrophoresis. Image analysis of SYPRO Ruby-stained gels revealed 165 sexually dimorphic protein spots that differ in normalized volume between male and female groups by >1.5-fold at p<0.05. Sixty of these proteins exhibited female-like changes in spot abundance following continuous GH treatment. Comparison of male and GH-treated male groups revealed 130 proteins that displayed >1.5-fold differences in abundance, with 60 of these GH-responsive spots being sexually dimorphic. Thus, GH plays an important role in establishing the sex-dependent differences in liver nuclear protein content. Twenty-eight of the sexually dimorphic and/or GH-regulated protein spots were identified by LC-MS/MS. Proteins identified include regucalcin, nuclear factor 45, and heterogeneous nuclear ribonucleoproteins A3, D-like, and K, in addition to proteins such as GST, normally associated with cytosolic extracts but also reported to be localized in the nucleus.

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.

Regulation of CYP2C11 by dehydroepiandrosterone and peroxisome proliferators: identification of the negative regulatory region of the gene

Treatment of rats with peroxisome proliferators is known to affect gene expression, including suppression of CYP2C11. The current study examined the mechanism of negative regulation of CYP2C11, comparing the effects of a classic peroxisome proliferator, nafenopin, with those of the steroid dehydroepiandrosterone (DHEA). In vivo dose-response experiments for DHEA were carried out with rats. Only the highest dose of DHEA in the diet (0.45%), a dose previously shown to produce peroxisome proliferation, caused suppression of CYP2C11 expression. Lower doses of DHEA (0.012 to 0.20% in diet) had little effect on CYP2C11 expression. In HepG2 cells, negative regulation of a CYP2C11 reporter gene by nafenopin required coexpression of PPARalpha, whereas negative regulation by DHEA did not. Deletion analysis revealed that the responsive region for both DHEA and nafenopin was between -108 and -60 relative to the transcription start site. Mutations in several putative transcription factor binding sites in the 5'-flanking region of CYP2C11 were produced. A mutation at -121 bp significantly diminished basal expression of CYP2C11 but did not affect negative regulation by DHEA or nafenopin. A mutation at -75 bp had only a small effect on basal expression but completely abolished negative regulation by DHEA and nafenopin. Gel shift experiments indicated that PPARalpha/RXRalpha heterodimers do not bind DNA in this region. Therefore, the sequence at -75 bp of CYP2C11 is necessary for negative regulation by both DHEA and nafenopin. However, the upstream events leading to suppression at this site must differ for DHEA and nafenopin.

Inactivation of the hepatic cytochrome P450 system by conditional deletion of hepatic cytochrome P450 reductase

Cytochrome P450 (CYP) monooxygenases catalyze the oxidation of a large number of endogenous compounds and the majority of ingested environmental chemicals, leading to their elimination and often to their metabolic activation to toxic products. This enzyme system therefore provides our primary defense against xenobiotics and is a major determinant in the therapeutic efficacy of pharmacological agents. To evaluate the importance of hepatic P450s in normal homeostasis, drug pharmacology, and chemical toxicity, we have conditionally deleted the essential electron transfer protein, NADH:ferrihemoprotein reductase (EC, cytochrome P450 reductase, CPR) in the liver, resulting in essentially complete ablation of hepatic microsomal P450 activity. Hepatic CPR-null mice could no longer break down cholesterol because of their inability to produce bile acids, and whereas hepatic lipid levels were significantly increased, circulating levels of cholesterol and triglycerides were severely reduced. Loss of hepatic P450 activity resulted in a 5-fold increase in P450 protein, indicating the existence of a negative feedback pathway regulating P450 expression. Profound changes in the in vivo metabolism of pentobarbital and acetaminophen indicated that extrahepatic metabolism does not play a major role in the disposition of these compounds. Hepatic CPR-null mice developed normally and were able to breed, indicating that hepatic microsomal P450-mediated steroid hormone metabolism is not essential for fertility, demonstrating that a major evolutionary role for hepatic P450s is to protect mammals from their environment.

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.

Suppression of cytochrome P450 2C11 by aromatic hydrocarbons: mechanistic insights from studies of the 5'-flanking region of the CYP2C11 gene

The aromatic hydrocarbon receptor (AHR) functions as a ligand-activated transcription factor that mediates responses to aromatic hydrocarbons (AHs). Induction of cytochrome p450 1A1 (CYP1A1) is the most fully characterized response and is mediated by binding of the activated AHR complex to dioxin-responsive elements (DREs) located in the 5'-flanking region of the gene. In contrast to CYP1A1 induction, several other genes including the rat male-specific constitutive hepatic CYP2C11 are suppressed by AHs. Our aim was to determine whether CYP2C11 suppression by AHs is mediated by the AHR via interaction with DRE-like sequences. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppressed CYP2C11 mRNA in primary rat hepatocytes without altering the mRNA half-life. We identified five regions in the CYP2C11 5'-flank containing the DRE invariant core; electrophoretic gel retardation assays showed that at least one of these DREs is a potential binding site for the AHR. To test the function of the CYP2C11-DREs, Hepa-1, BRL 5637, and HepG2 cells were transfected with reporter constructs containing regions of the CYP2C11 5'-flank and promoter. No decrease in luciferase activity was found following TCDD treatment. In primary rat hepatocytes, the luciferase reporter vectors were suppressed by interleukin-1 beta but not by TCDD. In vitro footprinting showed protein binding at several sites in the CYP2C11 5'-flank, but the pattern was not altered by in vivo 3-methylcholanthrene treatment. These studies imply that AHs down-regulate CYP2C11 by a negative transcriptional mechanism that is not simply due to AHR binding to an identified DRE-like sequence and that is distinct from that used by inflammatory cytokines.

Cloning and characterization of the 5'-flanking region of the canine growth hormone gene

The growth hormone (GH) gene is expressed in a variety of tissues outside the pituitary, including the mammary gland. GH expression in the mammary gland is stimulated by progestins. The local synthesis of mammary GH may provide a highly proliferative environment within the mammary gland that may contribute to the development or progression of mammary tumours. To elucidate the mechanism regulating mammary GH expression, we cloned the 5'-flanking region of the canine GH gene using inverse polymerase chain reaction. Gel-shift experiments showed that several sequences in the 5'-flanking region of the GH gene bind mammary nuclear proteins and may be involved in basal and progesterone-induced mammary GH expression. Sequence analysis and comparison with the GH promoters of human, rat, and mouse genes revealed a number of shared binding sites for transcription factors such as Pit-1, which is involved in pituitary GH expression, and for factors involved in the differentiation of lymphoid cells. Moreover, a putative binding site for the progesterone receptor (PR) was identified in all promoters, indicating that the progestin-induced expression of GH in mammary tissue is most probably a direct effect of activated PRs on the GH gene promoter and that this may occur in various species.

Sexual dimorphic expression of ADH in rat liver: importance of the hypothalamic-pituitary-liver axis

Hepatic alcohol dehydrogenase (ADH) activity is higher in female than in male rats. Although sex steroids, thyroid, and growth hormone (GH) have been shown to regulate hepatic ADH, the mechanism(s) for sexual dimorphic expression is unclear. We tested the possibility that the GH secretory pattern determined differential expression of ADH. Gonadectomized and hypophysectomized male and female rats were examined. Hepatic ADH activity was 2.1-fold greater in females. Because protein and mRNA content were also 1.7- and 2.4-fold greater, results indicated that activity differences were due to pretranslational mechanisms. Estradiol increased ADH selectively in males, and testosterone selectively decreased activity and mRNA levels in females. Effect of sex steroids on ADH was lost after hypophysectomy; infusion of GH in males increased ADH to basal female levels, supporting a role of the pituitary-liver axis. However, GH and L-thyroxine (T4) replacements alone in hypophysectomized rats did not restore dimorphic differences for either ADH activity or mRNA levels. On the other hand, T4 in combination with intermittent administration of GH reduced ADH activity and mRNA to basal male values, whereas T4 plus GH infusion replicated female levels. These results indicate that the intermittent male pattern of GH secretion combined with T4 is the principal determinant of low ADH activity in male liver.

Stimulation of hepatic signal transducer and activator of transcription 5b by GH is not altered by 3-methylcholanthrene

We are investigating the mechanisms by which aromatic hydrocarbons, such as 3-methylcholanthrene (MC), suppress hepatic cytochrome P450 2C11 (CYP2C11) gene expression. CYP2C11 is an enzyme expressed in the liver of male rats and is regulated by a pulsatile pattern of GH secretion. We have previously shown that MC attenuates the stimulatory effect of GH on CYP2C11 expression in hypophysectomized male rats. In follow-up studies we evaluated the effect of MC on GH-stimulated signal transducer and activator of transcription 5b (STAT5b) phosphorylation, nuclear translocation, and DNA-binding activity. GH-stimulated increases in hepatic nuclear STAT5b and phospho-STAT5b levels were not different between groups of hypophysectomized rats receiving MC or vehicle. This observation was corroborated at the DNA-binding level by EMSA. We also measured GH-induced STAT5b activation in the H4IIE rat hepatoma cell line. STAT5b DNA-binding activity detected in GH-treated cells was not affected by MC. Immunocytochemistry experiments revealed no effect of MC on GH-stimulated STAT5b nuclear translocation in H4IIE cells. These in vivo and in vitro data suggest that interference with GH-stimulated STAT5b activation does not constitute a mechanism by which MC attenuates the stimulatory effect of GH on CYP2C11 gene expression.

Post-transcriptional regulation of hepatic NADPH-cytochrome P450 reductase by thyroid hormone: independent effects on poly(A) tail length and mRNA stability

Thyroid hormone [triiodothyronine (T3)] positively regulates NADPH cytochrome P450 reductase (P450R) mRNA expression in rat liver, with P450R transcription initiation being a key regulated step. T3 is presently shown to have significant post-transcriptional effects on P450R expression. T3 increased the size of cytoplasmic P450R mRNA by approximately 105 nucleotides 12 h after T3 treatment, followed by a return to basal levels at 24 h. Primer extension analysis and Northern hybridization with 5'-untranslated region probes revealed no change in P450R mRNA 5' structure with T3 treatment. By contrast, RNase H analysis revealed a transient, T3-induced increase in P450R mRNA poly(A) tail, from approximately 100 to approximately 205 A. This increase in P450R polyadenylation, detectable in the nucleus 8 h after T3 treatment and in the cytoplasm at 12 h, was transient and was reversed by 16 h, when the T3-induced accumulation of cytoplasmic P450R mRNA was near maximal. Actinomycin D blocked the increase in P450R poly(A) tail and the induction of P450R mRNA, indicating a requirement for ongoing gene transcription for both T3 responses. T3 treatment destabilized P450R mRNA in rat liver in vivo, as shown by the T3-dependent 6-fold decrease in cytoplasmic P450R mRNA half-life, from a basal value of >or=16 h in uninduced liver to approximately 2.5 h, measured 24 h after T3 administration. These findings demonstrate that T3 increases nuclear polyadenylation of P450R RNA as a transient, early regulatory response and that this response is temporally dissociated from the subsequent decrease in cytoplasmic P450R mRNA stability.

P-450 metabolites of arachidonic acid in the control of cardiovascular function

Recent studies have indicated that arachidonic acid is primarily metabolized by cytochrome P-450 (CYP) enzymes in the brain, lung, kidney, and peripheral vasculature to 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) and that these compounds play critical roles in the regulation of renal, pulmonary, and cardiac function and vascular tone. EETs are endothelium-derived vasodilators that hyperpolarize vascular smooth muscle (VSM) cells by activating K(+) channels. 20-HETE is a vasoconstrictor produced in VSM cells that reduces the open-state probability of Ca(2+)-activated K(+) channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal, cerebral, and skeletal muscle arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. They also block tubuloglomerular feedback responses in vivo and the vasoconstrictor response to elevations in tissue PO(2) both in vivo and in vitro. The formation of 20-HETE in VSM is stimulated by angiotensin II and endothelin and is inhibited by nitric oxide (NO) and carbon monoxide (CO). Blockade of the formation of 20-HETE attenuates the vascular responses to angiotensin II, endothelin, norepinephrine, NO, and CO. In the kidney, EETs and 20-HETE are produced in the proximal tubule and the thick ascending loop of Henle. They regulate Na(+) transport in these nephron segments. 20-HETE also contributes to the mitogenic effects of a variety of growth factors in VSM, renal epithelial, and mesangial cells. The production of EETs and 20-HETE is altered in experimental and genetic models of hypertension, diabetes, uremia, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of this pathway in the control of cardiovascular function, it is likely that CYP metabolites of arachidonic acid contribute to the changes in renal function and vascular tone associated with some of these conditions and that drugs that modify the formation and/or actions of EETs and 20-HETE may have therapeutic benefits.

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.

Temporal relationship between the sexually dimorphic spontaneous GH secretory profiles and hepatic STAT5 activity

STAT5 transduces transcriptional responses to GH in liver and other tissues and is proposed to mediate the sexually dimorphic effects of plasma GH secretory profiles on rodent liver gene expression. Previous studies have suggested that STAT5 undergoes repeated activation in direct response to successive GH pulses in adult male rats, with STAT5 activation being desensitized in females by their more persistent pattern of GH exposure. These findings, however, were based on in vitro studies or single blood samples analyzed for GH in vivo. In view of the highly pulsatile nature of rat GH secretion, we presently examined these hypotheses by concurrent monitoring of spontaneous GH secretory profiles and hepatic STAT5 activity in conscious, free-moving adult male and female rats. Rats were killed at times associated with spontaneous peaks or troughs of the GH rhythm; livers were removed and analyzed for STAT5 DNA-binding activity. In males, liver STAT5 activity was highest during the initial phase (15-60 min) of a GH secretory episode (mean +/- SE relative STAT5 activity = 86.5 +/- 11.4; plasma GH = 146.7 +/- 22.4 ng/ml) and was significantly lower (P < 0.01) during the downswing of a pulse, 45-75 min after the GH peak (STAT5 = 26.1 +/- 1.7; GH = 33.3 +/- 13.1 ng/ml), consistent with a time-dependent down-regulation of GH signaling to STAT5. The lowest STAT5 activity was observed during the subsequent GH trough period (STAT5 = 3.6 +/- 1.1; GH = 2.6 +/- 0.1 ng/ml). In females, liver STAT5 activity was significantly lower (P < 0.05) than peak male levels during the initial phase of a GH secretory burst (STAT5 = 35.1 +/- 15.9; GH = 68.1 +/- 31.6 ng/ml) although similar to that of males during a plasma GH nadir (STAT5 = 11.0 +/- 2.6; GH = 8.4 +/- 2.2 ng/ml). We conclude that: 1) liver STAT5 is repeatedly activated by successive, spontaneous GH secretory episodes in intact adult male rats at approximately 3- to 3.5-h intervals; 2) time-dependent down-regulation of GH signaling to hepatic STAT5 in vivo begins by 45 min after GH peak stimulation; and 3) the lower level of liver STAT5 activation seen in adult female rats, compared with males, is a consequence of the sex-dependent differences in GH secretory patterns that characterize these animals (i.e. lower-amplitude GH pulses and lack of prolonged interpulse nadir of GH in the feminine, compared with masculine profile).

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.

Transcriptional induction of hepatic NADPH: cytochrome P450 oxidoreductase by thyroid hormone

Studies were carried out to elucidate the mechanism whereby thyroid hormone (T3) induces NADPH:cytochrome P450 oxidoreductase (P450R) mRNA in rat liver in vivo. Northern blot analysis revealed that T3 treatment increases unspliced liver nuclear P450R RNA 4-fold within 8 h and that this induction precedes the induction of mature, cytoplasmic P450R RNA. Unspliced nuclear P450R RNA was suppressed below basal levels 24 h after T3 treatment, despite the continued presence of elevated circulating T3 levels. To determine whether the T3-stimulated increase in nuclear P450R RNA reflects an increase in P450R transcription initiation, nuclear run-on transcription assays were carried out. T3 induced a 6- to 8-fold increase in P450R transcription rate within 12 h, sufficient to account for the observed increase in nuclear P450R precursor RNA, followed by a decrease back to basal transcription levels at 24 h, consistent with the nuclear RNA profile. Similar transcriptional increases were observed in nuclear run-on transcription studies using hybridization probes corresponding to nine different fragments of the P450R gene, spanning exon 2 to exon 16. Thus, P450R transcription initiation, not transcription elongation, is the T3-regulated event. Similar results were obtained during short (5 min) compared with long (45 min) nuclear run-on transcription assays, suggesting that changes in nuclear RNA processing or regulated degradation do not contribute to the overall RNA induction. This finding was confirmed by the ability of the RNA polymerase inhibitor actinomycin D, administered in vivo, to block T3 induction of P450R transcriptional activity. We conclude that P450R transcription, rather than nuclear RNA processing or mRNA stabilization, is the primary mechanism whereby T3 induces hepatic P450R mRNA.

Synergistic action of hepatocyte nuclear factors 3 and 6 on CYP2C12 gene expression and suppression by growth hormone-activated STAT5b. Proposed model for female specific expression of CYP2C12 in adult rat liver

Growth hormone (GH) exerts sexually dimorphic effects on liver gene transcription through its sex-dependent temporal pattern of pituitary hormone secretion. CYP2C12 encodes a female-specific rat liver P450 steroid hydroxylase whose expression is activated by continuous GH stimulation of hepatocytes. Presently, we investigated the role of liver-enriched and GH-regulated transcription factors in the activation of CYP2C12 gene expression in GH-stimulated liver cells. Transcription of a CYP2C12 promoter-luciferase reporter gene in transfected HepG2 cells was activated 15-40-fold by the liver-enriched hepatocyte nuclear factor (HNF) 3 alpha, HNF3 beta, and HNF6. Synergistic interactions leading to an approximately 300-fold activation of the promoter by HNF3 beta in combination with HNF6 were observed. 5'-Deletion analysis localized the HNF6 response to a single 5'-proximal 96-nucleotide segment. By contrast, the stimulatory effects of HNF3 alpha and HNF3 beta were attributable to five distinct regions within the 1.6-kilobase CYP2C12 proximal promoter. GH activation of the signal transducer and transcriptional activator STAT5b, which proceeds efficiently in male but not female rat liver, inhibited CYP2C12 promoter activation by HNF3 beta and HNF6, despite the absence of a classical STAT5-binding site. The female-specific pattern of CYP2C12 expression is thus proposed to reflect the positive synergistic action in female liver of liver-enriched and GH-regulated transcription factors, such as HNF3 beta and HNF6, coupled with a dominant inhibitory effect of GH-activated STAT5b that is manifest in males.

Plasma growth hormone pulse activation of hepatic JAK-STAT5 signaling: developmental regulation and role in male-specific liver gene expression

The intracellular signaling molecule STAT5 is activated in rat liver by the intermittent male plasma GH pattern to a 10-fold higher level than by the more continuous pattern of plasma GH stimulation seen in females. Individual adult male rats are presently shown to exhibit large differences in liver STAT5 DNA-binding activity, which correlates with the presence of significant levels of GH in plasma at the time of liver excision. Examination of STAT5 activity as a function of postnatal development revealed that these intermittent pulses of liver STAT5 activity are first observed at 5 weeks of age, when plasma GH pulsation first begins and expression of male-specific, GH pulse-activated liver genes, including CYP2C11, first occurs. Prepubertal rats exhibited low liver STAT5 activity, likely a consequence of the absence of high plasma GH pulses in these animals. Proteins required for GH activation of STAT5 are expressed in liver before puberty, and correspondingly, STAT5 can be precociously activated by exogenous administration of GH pulses given to 2-week-old rats, albeit with a lower sensitivity to GH than is seen in hypophysectomized adult rats. However, this precocious activation of STAT5, via twice daily administration of GH for 7 days, did not lead to CYP2C11 expression or masculinization of hepatic enzyme profiles, unlike in GH pulse-stimulated hypophysectomized adult rats. Based on these findings we conclude: 1) liver STAT5 is repeatedly activated in adult male rats in direct response to the intermittent pattern of plasma GH stimulation; 2) the developmental onset of this STAT5 activation pattern supports the proposed requirement of STAT5 transcriptional activity for male-specific, GH pulse-regulated hepatic gene expression; and 3) the activation of STAT5 is, by itself, not sufficient to impart the adult male pattern of liver gene expression, suggesting a requirement for additional liver factors that are absent in prepubertal rats.

Cell biology of cytochrome P-450 in the liver

Cytochromes P-450 (P-450) are members of a multigene superfamily of hemoproteins consisting the microsomal monooxygenase system with NADPH P-450 reductase (reductase) and/or reducing equivalents. Expression of many P-450 isoforms in hepatocytes is shown to be regulated at the level of transcription through interaction between cis-acting elements in the genes and DNA-binding (transacting) factors. Some isoforms of the CYP1A, 2B, 2E, and 3A subfamilies are regulated at the posttranscriptional level. For the topology of P-450 and reductase molecules in ER membrane of hepatocytes, models from stopped flow analysis and electron spin resonance are proposed. The densities of total P-450 and reductase molecules are revealed to be high enough to support the cluster model, suggesting that about ten P-450 molecules form an aggregate and surround one reductase molecule, and therefore the two enzymes form large micelles. ER proliferation after PB administration, which had been correlated with increase in P-450 level, is shown to be probably independent of the increase in P-450 level. There are considerable discrepancies among results reported on sublobular expression of various P-450 isoforms. Causes of the discrepancies are likely to be differences in experimental conditions of histochemical detection carried out and/or in species, strain, and/or sex.

Transcriptional suppression of cytochrome P450 2C11 gene expression by 3-methylcholanthrene

Aromatic hydrocarbon receptor-mediated transcriptional up-regulation of cytochrome P450 (CYP) enzymes of the CYP1A subfamily by polycyclic aromatic hydrocarbons (PAHs) such as 3-methylcholanthrene (MC) is accompanied by down-regulation of rat hepatic CYP2C11 expression at the catalytic activity, protein, and mRNA levels. To gain insight into the molecular mechanism of this CYP2C11 suppression response, we have used a nuclear run-on assay to assess directly the effect of MC on the hepatic transcription rate of the CYP2C11 gene following in vivo administration of MC to adult male rats. A single intraperitoneal dose of MC (40 mg/kg) caused a 179-fold increase in the rate of CYP1A gene transcription at 6 hr, and the rate of CYP2C11 gene transcription was reduced by 51% at this time point, compared with vehicle controls. By 48 hr after MC treatment, the rates of CYP1A and CYP2C11 gene transcription were no longer significantly different from the corresponding vehicle controls. These results indicate for the first time that the suppression of hepatic CYP2C11 caused by in vivo administration of PAHs to adult male rats is at least partially due to a decrease in the rate of transcription of the CYP2C11 gene.

The role of STAT proteins in growth hormone signaling

Growth hormone (GH) has long been known to be the body's primary regulator of body growth and a regulator of metabolism, yet the mechanisms by which GH regulates the transcription of specific genes required for these processes are just now being delineated. GH binding to its receptor recruits and activates the receptor-associated JAK2 that in turn phosphorylates tyrosines within itself and the GH receptor. These tyrosines form binding sites for a number of signaling proteins, including members of the family of signal transducers and activators of transcription (STAT). Among the known signaling molecules for GH, STAT proteins play a particularly prominent role in the regulation of gene transcription. This paper will review what is currently understood about which STAT proteins are regulated by GH, how they are regulated by GH, the GH-dependent genes they regulate, and discuss current theories about how GH-activated STAT signaling is regulated. Particular attention will be given to the novel role that STAT5 plays in sexually dimorphic gene expression in the liver as determined by the secretory pattern of GH and the role of STAT5 in body growth. Oncogene (2000).

Pulsatility of growth hormone (GH) signalling in liver cells: role of the JAK-STAT5b pathway in GH action

The intracellular signalling molecule and transcriptional activator STAT5b is a key mediator of the effects of intermittent plasma growth hormone (GH) pulses on the male-specific pattern of liver gene expression and pubertal body growth rates in rodents. Experiments with Stat5b gene-knockout mice have revealed that these GH-regulated, male-specific phenotypes are a direct consequence of GH pulse-dependent STAT5b activation and that loss of function of STAT5b cannot be compensated for by the closely related signalling molecule STAT5a. Physiological plasma GH pulses are required to obtain the high levels of activated STAT5b seen in the livers of males, and down-regulation of the GH receptor (GHR)-JAK-STAT5b pathway in hepatocytes exposed to GH in a near-continuous fashion underlies the low level of liver STAT5b activity that is characteristic of adult female rats. Termination of nuclear STAT5b signalling occurs at the conclusion of a plasma GH pulse, with STAT5b deactivation catalysed by a tyrosine phosphatase. In males, termination of the intracellular signalling stimulated by a plasma GH pulse is proposed to be additionally facilitated by GH-STAT5b-inducible SOCS-CIS proteins, which block the further activation of STAT5b by binding to and inhibiting the action of the GHR-JAK2 complex via multiple mechanisms. In this manner, the liver cell is rendered temporarily unresponsive to further GH-signalling events. SOCS-CIS proteins synthesized in liver cells stimulated continuously with GH may also contribute to the apparent down-regulation of STAT5b signalling that is observed in the female rat liver.