Role of transcription factors in the age-dependent regulation of the androgen receptor gene in rat liver

The Influence of Sex Hormones in Liver Function and Disease

The liver performs a multitude of bodily functions, whilst retaining the ability to regenerate damaged tissue. In this review, we discuss sex steroid biology, regulation of mammalian liver physiology and the development of new model systems to improve our understanding of liver biology in health and disease. A major risk factor for the development of liver disease is hepatic fibrosis. Key drivers of this process are metabolic dysfunction and pathologic activation of the immune system. Although non-alcoholic fatty liver disease (NAFLD) is largely regarded as benign, it does progress to non-alcoholic steatohepatitis in a subset of patients, increasing their risk of developing cirrhosis and hepatocellular carcinoma. NAFLD susceptibility varies across the population, with obesity and insulin resistance playing a strong role in the disease development. Additionally, sex and age have been identified as important risk factors. In addition to the regulation of liver biochemistry, sex hormones also regulate the immune system, with sexual dimorphism described for both innate and adaptive immune responses. Therefore, sex differences in liver metabolism, immunity and their interplay are important factors to consider when designing, studying and developing therapeutic strategies to treat human liver disease. The purpose of this review is to provide the reader with a general overview of sex steroid biology and their regulation of mammalian liver physiology.

The Postnatal Offspring of Finasteride-Treated Male Rats Shows Hyperglycaemia, Elevated Hepatic Glycogen Storage and Altered GLUT2, IR, and AR Expression in the Liver

Background: A growing body of data indicates that the physiology of the liver is sex-hormone dependent, with some types of liver failure occurring more frequently in males, and some in females. In males, in physiological conditions, testosterone acts via androgen receptors (AR) to increase insulin receptor (IR) expression and glycogen synthesis, and to decrease glucose uptake controlled by liver-specific glucose transporter 2 (GLUT-2). Our previous study indicated that this mechanism may be impaired by finasteride, a popular drug used in urology and dermatology, inhibiting 5α-reductase 2, which converts testosterone (T) into dihydrotestosterone (DHT). Our research has also shown that the offspring of rats exposed to finasteride have an altered T–DHT ratio and show changes in their testes and epididymides. Therefore, the goal of this study was to assess whether the administration of finasteride had an trans-generational effect on (i) GLUT-2 dependent accumulation of glycogen in the liver, (ii) IR and AR expression in the hepatocytes of male rat offspring, (iii) a relation between serum T and DHT levels and the expression of GLUT2, IR, and AR mRNAs, (iv) a serum glucose level and it correlation with GLUT-2 mRNA. Methods: The study was conducted on the liver (an androgen-dependent organ) from 7, 14, 21, 28, and 90-day old Wistar male rats (F1:Fin) born by females fertilized by finasteride-treated rats. The control group was the offspring (F1:Control) of untreated Wistar parents. In the histological sections of liver the Periodic Acid Schiff (PAS) staining (to visualize glycogen) and IHC (to detect GLUT-2, IR, and AR) were performed. The liver homogenates were used in qRT-PCR to assess GLUT2, IR, and AR mRNA expression. The percentage of PAS-positive glycogen areas were correlated with the immunoexpression of GLUT-2, serum levels of T and DHT were correlated with GLUT-2, IR, and AR transcript levels, and serum glucose concentration was correlated with the age of animals and with the GLUT-2 mRNA by Spearman’s rank correlation coefficients. Results: In each age group of F1:Fin rats, the accumulation of glycogen was elevated but did not correlate with changes in GLUT-2 expression. The levels of GLUT-2, IR, and AR transcripts and their immunoreactivity statistically significantly decreased in F1:Fin animals. In F1:Fin rats the serum levels of T and DHT negatively correlated with androgen receptor mRNA. The animals from F1:Fin group have statistically elevated level of glucose. Additionally, in adult F1:Fin rats, steatosis was observed in the liver (see Appendix A). Conclusions: It seems that treating male adult rats with finasteride causes changes in the carbohydrate metabolism in the liver of their offspring. This can lead to improper hepatic energy homeostasis or even hyperglycaemia, insulin resistance, as well as some symptoms of metabolic syndrome and liver steatosis.

Tumor Suppressive Maspin-Sensitized Prostate Cancer to Drug Treatment Through Negative Regulating Androgen Receptor Expression

Overactivation of androgen receptor (AR)-mediated signal has been extensively implicated in prostate cancer (CaP) development, progression, and recurrence, which makes it an attractive therapeutic target. Meanwhile, as an endogenous inhibitor of histone deacetylase 1 (HDAC 1), tumor-suppressive mammary serine protease inhibitor (maspin) was reported to sensitize drug-induced apoptosis with a better therapeutic outcome in CaP, but the relationship between AR and maspin remains unclear. In the current study, treatment of 5'-Aza or MS-275/enzalutamide induced poly (ADP-ribose) polymerase (PARP) cleavage and p-H2A.X in CaP cells with an increase of maspin expression but a decrease of AR. Then, treatment with protease inhibitor MG132 did not rescue the above drug-induced loss of AR. In addition, modulation of maspin expression by gene recombinant or siRNA technology showed an inverse correlation between expression of maspin and AR, consequently affecting the AR-regulated downstream gene transcription (e.g., NKX3.1 and TMPRSS2). Bioinformatics analysis of the data extracted from the National Center for Biotechnology Information Gene Expression Omnibus (NCBI GEO) database also revealed an inverse correlation between low maspin expression and high AR level in advanced CaP. Furthermore, chromatin immunoprecipitation (ChIP) assay using anti-maspin antibody identified that a portion of AR promoter sequence was co-precipitated and presented in the immunoprecipitated complex. Finally, maspin-mediated repression of AR was induced by treatment of MS-275, which promoted enzalutamide treatment efficacy with decrease of prostate-specific antigen (PSA) expression in LNCaP and 22RV1 cells. Taken together, the data not only demonstrated maspin-mediated repression of AR to augment drug anti-tumor activity but also provided in-depth support for combination of HDAC inhibitors with AR antagonist in CaP therapy.

Hepatic actions of androgens in the regulation of metabolism

PURPOSE OF REVIEW

The purpose of this review is to summarize recent findings on hepatic actions of androgens in the regulation of protein, lipid and glucose metabolism. The rationale for liver-targeted testosterone use will be provided.

RECENT FINDINGS

Liver-targeted testosterone administration, via the oral route, induces protein anabolic effect by reducing the rate of protein oxidation to a similar extent to that of systemic testosterone administration. Recent evidence indicates that testosterone exerts whole-body anabolic effect through inhibition of nitrogen loss via the hepatic urea cycle. Several hepatic effects of androgens, particularly on glucose metabolism, are direct and take place before any changes in body composition occur. This includes an increase in insulin secretion and sensitivity, and reduction in hepatic glucose output by testosterone. Furthermore, lack of testosterone in the liver exacerbates diet-induced impairment in glucose metabolism. In the liver, androgens induce the full spectrum of metabolic changes through interaction with growth hormone or aromatization to estradiol.

SUMMARY

Liver-targeted testosterone therapy may open up a new approach to achieve whole-body anabolism without systemic side-effects. Aromatizable androgens may be superior to nonaromatizable androgens in inducing a complex spectrum of direct, estrogen-mediated and other hormone-mediated effects of androgens.

Sex Hormones and Their Receptors Regulate Liver Energy Homeostasis

The liver is one of the most essential organs involved in the regulation of energy homeostasis. Hepatic steatosis, a major manifestation of metabolic syndrome, is associated with imbalance between lipid formation and breakdown, glucose production and catabolism, and cholesterol synthesis and secretion. Epidemiological studies show sex difference in the prevalence in fatty liver disease and suggest that sex hormones may play vital roles in regulating hepatic steatosis. In this review, we summarize current literature and discuss the role of estrogens and androgens and the mechanisms through which estrogen receptors and androgen receptors regulate lipid and glucose metabolism in the liver. In females, estradiol regulates liver metabolism via estrogen receptors by decreasing lipogenesis, gluconeogenesis, and fatty acid uptake, while enhancing lipolysis, cholesterol secretion, and glucose catabolism. In males, testosterone works via androgen receptors to increase insulin receptor expression and glycogen synthesis, decrease glucose uptake and lipogenesis, and promote cholesterol storage in the liver. These recent integrated concepts suggest that sex hormone receptors could be potential promising targets for the prevention of hepatic steatosis.

Loss of androgen receptor in aging and oxidative stress through Myb protooncoprotein-regulated reciprocal chromatin dynamics of p53 and poly(ADP-ribose) polymerase PARP-1

Poly(ADP-ribosyl)ation of transcription factors and coregulators, mediated by the poly(ADP-ribose) polymerase PARP-1, has been emerging as an important epigenetic mechanism that controls transcriptional dynamics in response to diverse intra- and extracellular signals. PARP-1 activity is also implicated in the regulation of mammalian lifespan. Herein we show that transcriptional down-regulation of androgen receptor (AR) in the aging rat liver and in oxidatively stressed hepatoma cells involves exchange of a PARP-1-associated, p/CAF-containing coactivator assembly for a p53-interacting, Groucho/TLE1-, and mSin3A-included corepressor complex at an age- and oxidant-responsive DNA element (age-dependent factor (ADF) element) in the AR promoter. The coregulator switch is mediated by B-Myb and c-Myb, which bind to the ADF element and physically associate with PARP-1 and the tumor suppressor p53. Heterogeneous nuclear ribonucleoprotein K, residing at the ADF element in association with PARP-1, may serve a platform role in stabilizing the activating complex. PARP-1 coactivated B-Myb- and c-Myb-mediated transactivation of the AR promoter, and p53 antagonized the B-Myb/c-Myb-induced AR promoter activation. PARP-1, heterogeneous nuclear ribonucleoprotein K, B-Myb, and c-Myb each serves as a positive regulator of cellular AR content, whereas p53 negatively regulates AR expression. Our results identify a shared, PARP-1-regulated sensing mechanism that coordinates transcriptional repression of AR during aging and in response to oxidative stress. This study may provide insights as to how advancing age and intracellular redox balance might influence androgen-regulated physiology.

Age-dependent differential expression and activity of rat liver cytosolic inorganic pyrophosphatase gene

Besides epigenetic factors, the genetic make-up and differential gene expression not only determines aging and disease susceptibility but also the functional activity of cells in an individual. Analysis of a variety of mammalian tissues revealed that the age-associated differentially expressed genes mainly belong to inflammation, stress, and metabolism. Intracellular PPi is a by-product of multiple biosynthetic reactions and its hydrolysis by cytosolic inorganic pyrophosphatase (iPPase) has long been considered as an important homeostatic mechanism favoring biosynthesis. In this paper we report an age-associated increase ( approximately 2-fold) in the expression of rat liver cytosolic iPPase gene by differential display PCR and northern blot analysis. Expression profiling of iPPase by RNA slot blot analysis in several other tissues revealed no significant change with aging. A comparative spectrophotometric and in-gel analysis of iPPase activity in whole cell lysate (WCL) of liver, brain, skeletal muscle, heart, spleen and kidney exhibited that liver of old rats (24 months ) has approximately 2-fold more activity than the adult (4 months) ones and also its activity is highest among the tissues. The specificity of iPPase activity in the spectrophotometric assay and in-gel analysis was confirmed by specific iPPase inhibitors like CaCl(2) and NaF.

Cloning, expression, and functional analysis of rat liver cytosolic inorganic pyrophosphatase gene and characterization of its functional promoter

Inorganic pyrophosphate (PPi) is formed in several metabolic processes and its hydrolysis by the ubiquitously expressed enzyme inorganic pyrophosphatase (iPPase) is essential for the reactions to proceed in the direction of biosynthesis. Recently, we have reported differential expression and activity of cytosolic iPPase in rat liver with aging. In this article we report the cloning of the coding region of rat liver cytosolic iPPase gene in a bacterial expression vector, its expression, purification, and functional analysis by in-gel enzyme assay. SDS-PAGE and Western blot analysis of this expressed protein revealed that its molecular weight (MW) is approximately 33 kDa, while in-gel assay showed that it is functionally active just as the liver cytosolic iPPase. We have determined the genomic organization of this gene by genome blast approach. We have also cloned and characterized its proximal approximate 1 kb functional promoter (-1009 to +82) by transient transfection and luciferase assay of different 5'-deleted iPPase promoter-luciferase constructs and also established its transcription start site by primer extension analysis, along with protein-DNA interaction studies for a few putative transcription factor binding sites.

The redox-sensitive DNA binding sites responsible for age-related downregulation of SMP30 by ERK pathway and reversal by calorie restriction

It was recently found that age-related changes in SMP30 expression can be modulated by antioxidative action. In the current study, the modulation of SMP30 gene expression was explored by (a) antioxidative calorie restriction (CR), (b) proinflammatory lipopolysaccharide (LPS), in aged rat, (c) oxidative stress promoter, tert-butylhydroperoxide (t-BHP)-injected mouse, and (d) t-BHP-treated Ac2F cells. Utilizing EMSA, particular attention was given to the binding activity of unidentified transcription factor in sites 3 and 5 that are located in -800 bp of the SMP30 promoter. Results showed that CR prevented the age-related decrease in SMP30 expression, and also showed that SMP30 gene expression and binding activities of sites 3 and 5 decreased with treatments of t-BHP or LPS. These findings were confirmed by the antioxidant NAC and ERK-specific inhibitor PD098059 that blunted decreased SMP30 gene expression and binding activity of sites 3 and 5 by t-BHP in Ac2F cell system. Our data strongly indicate that the SMP30 transcriptional process is redox-sensitive and its modulation occurs at DNA binding sites 3 and 5 in the promoter region. Perhaps a more significant finding of the present study is that the downregulation of SMP30 is likely involved in ERK signal pathway.

Hyperthermia-induced proteasome inhibition and loss of androgen receptor expression in human prostate cancer cells

Prostate cancer is the second leading cause of death in men in western countries and is usually treated by surgery and/or radiotherapy. More recently, hyperthermia has been introduced into clinical trials investigating a possible effect in the first-line treatment of prostate cancer. However, the molecular mechanisms of hyperthermia are not completely understood. In this study, we investigated the effects of hyperthermia on proteasome function and its significance for signal transduction, cell death and androgen receptor (AR) expression in PC-3, LnCaP, and DU-145 human and TRAMP-C2 murine prostate cancer cells. Hyperthermia caused apoptosis and radiosensitization and decreased 26S proteasome activity in all three human cell lines to about 40% of untreated control cells. 20S proteasome activity was not affected by heat. Heat treatment inhibited constitutive and radiation-induced activation of nuclear factor kappaB caused by stabilization of IkappaB. Although stabilization of AR by proteasome inhibitors has been reported previously, AR protein levels in LnCaP cells decreased dramatically after heat. Our data suggest that inhibition of proteasome function and dependent signal transduction pathways might be a major molecular mechanisms of heat-induced apoptosis and radiosensitization. Hyperthermia abrogates AR expression in androgen-dependent cells and might thus promote malignant progression of prostate cancer.

TRANSACTIVATION OF GLUCOCORTICOID-INDUCIBLE RAT ARYL SULFOTRANSFERASE (SULT1A1) GENE TRANSCRIPTION

The purpose of the current study was to establish the role of the glucocorticoid receptor (GR) and androgen receptor (AR) transcription factors in the transactivation of rat aryl sulfotransferase (SULT1A1) gene transcription and to identify the functional hormone-responsive element(s) in the SULT1A1 gene. A cis-acting inverted repeat with three intervening bases (IR3) was identified in the 5'-flanking of the SULT1A1 gene that mediates the transactivation of SULT1A1 gene transcription by both the GR and AR. CV-1 cells were cotransfected with SULT1A1-luciferase reporter plasmids and either wild-type or mutant GR or AR expression constructs. In cotransfectants expressing the wild-type GR, treatment with triamcinolone acetonide produced an approximately 4- to 6-fold induction of luciferase activity in IR3-containing SULT1A1 reporter plasmids. IR3-containing SULT1A1 reporter constructs were also activated by treatment with the synthetic androgen R1881 in cells cotransfected with wild-type but not mutant AR. In primary cultured rat hepatocytes, androgen-inducible expression of IR3-containing SULT1A1 reporter plasmids required cotransfection with AR expression plasmid. Targeted disruption of the SULT1A1 IR3 by mutation of a conserved GT sequence in the 3' half-site of the element ablated GR and AR responsiveness. These results indicate that a proximal IR3 element in the 5'-flanking region of the rat SULT1A1 gene is sufficient for the transactivation of SULT1A1 gene transcription by the GR and AR, and that relative to the GR, functional AR activity is reduced in primary cultured rat hepatocytes.

Androgen receptor-mediated inhibition of cutaneous wound healing

Impaired wound healing states in the elderly lead to substantial morbidity, mortality, and a cost to the US Health Services of over $9 billion per annum. In addition to intrinsic aging per se causing delayed healing, studies have suggested marked sex-differences in wound repair. We report that castration of male mice results in a striking acceleration of local cutaneous wound healing, and is associated with a reduced inflammatory response and increased hair growth. Using a hairless mouse model, we have demonstrated that testosterone reduction stimulates the healing response not through hair follicle epithelial/mesenchymal cell proliferation, but directly via effects on wound cell populations. We suggest that endogenous testosterone inhibits the cutaneous wound healing response in males and is associated with an enhanced inflammatory response. The mechanisms underlying the observed effects involve a direct upregulation of proinflammatory cytokine expression by macrophages in response to testosterone. Blockade of androgen action systemically, via receptor antagonism, accelerates healing significantly, suggesting a specific target for future therapeutic intervention in impaired wound healing states in elderly males.

Loss of androgen receptor transcriptional activity at the G(1)/S transition

Androgens are essential for the differentiation, growth, and maintenance of male-specific organs. The effects of androgens in cells are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily of transcription factors. Recently, transient transfection studies have shown that overexpression of cell cycle regulatory proteins affects the transcriptional activity of the AR. In this report, we characterize the transcriptional activity of endogenous AR through the cell cycle. We demonstrate that in G0, AR enhances transcription from an integrated steroid-responsive mouse mammary tumor virus promoter and also from an integrated androgen-specific probasin promoter. This activity is strongly reduced or abolished at the G(1)/S boundary. In S phase, the receptor regains activity, indicating that there is a transient regulatory event that inactivates the AR at the G(1)/S transition. This regulation is specific for the AR, since the related glucocorticoid receptor is transcriptionally active at the G(1)/S boundary. Not all of the effects of androgens are blocked, however, since androgens retain the ability to increase AR protein levels. The transcriptional inactivity of the AR at the G(1)/S junction coincides with a decrease in AR protein level, although activity can be partly rescued without an increase in receptor. Inhibition of histone deacetylases brings about this partial restoration of AR activity at the G(1)/S boundary, demonstrating the involvement of acetylation pathways in the cell cycle regulation of AR transcriptional activity. Finally, a model is proposed that explains the inactivity of the AR at the G(1)/S transition by integrating receptor levels, the action of cell cycle regulators, and the contribution of histone acetyltransferase-containing coactivators.

Impacts of transcriptional regulation on aging and senescence

The genetic makeup of the organism appears to dictate the species-specific rate of aging and the maximum life-span potential. The genotype is converted to phenotype through transcriptional and translational regulation. A group of gene regulatory proteins (transcription factors) play critical roles in controlling the rates of transcription of specific genes by directly interacting with regulatory sequences at gene promoters. Here, we review the basic mechanism of transcriptional control and the role of a number of transcription factors whose level and/or activity alter with age. Among these age-dependent transcription factors, many are involved in the regulation of stress and inflammatory responses and are subjected to functional alterations by reactive oxygen species (ROSs). A progressive rise of oxidative stress, impaired ability to cope with stressful stimuli and prolongation of the inflammatory response are some of the hallmarks of the senescent phenotype. Results published to date are supportive of the concept that a species-specific program of the temporal regulation of genes with additional modulation by a number of epigenetic factors, mediates the age-dependent deterioration of physiological functions and development of the senescent phenotype.

Oxidation of zinc finger transcription factors: physiological consequences

Redox-sensitive cysteine residues are present in the interaction domains of many protein complexes. There are examples in all of the major categories of transcription factors, including basic region, leucine zipper, helix-loop-helix, and zinc finger. Zinc finger structures require at least two zinc-coordinated cysteine sulfhydryl groups, and oxidation or alkylation of these can eliminate DNA-binding and transcriptional functions. We review here the evidence for oxidation of zinc finger cysteines, the pathways and reactive oxygen intermediates involved, and the functional and physiological consequences of these reactions. Despite skepticism that the strongly reducing intracellular environment would permit significant oxidation of cysteine residues within zinc finger transcription factors, there is compelling evidence that oxidation occurs both in vitro and in vivo. Early reports demonstrating reversible oxidation of zinc-coordinated cysteines with loss of binding function in vitro were shown to reflect accurately the changes in intact cells, and these in turn have been shown to correlate with physiological changes. In particular, the accumulation of oxidized Spl zinc fingers during aging, and estrogen receptors in tamoxifen-resistant breast cancers are dramatic examples of what may be a general sensitivity of zinc finger factors to changes in the redox state of the cell.

The budding yeast, Saccharomyces cerevisiae, as a model for aging research: a critical review

In this review we discuss the yeast as a paradigm for the study of aging. The budding yeast Saccharomyces cerevisiae, which can proliferate in both haploid and diploid states, has been used extensively in aging research. The budding yeast divides asymmetrically to form a 'mother' cell and a bud. Two major approaches, 'budding life span' and 'stationary phase' have been used to determine 'senescence' and 'life span' in yeast. Discrepancies observed in metabolic behavior and longevity between cells studied by these two systems raise questions of how 'life span' in yeast is defined and measured. Added to this variability in experimental approach and results is the variety of yeast strains with different genetic make up used as 'wild type' and experimental organisms. Another problematic genetic point in the published studies on yeast is the use of both diploid and haploid strains. We discuss the inherent, advantageous attributes that make the yeast an attractive choice for modern biological research as well as certain pitfalls in the choice of this model for the study of aging. The significance of the purported roles of the Sir2 gene, histone deacetylases, gene silencing, rDNA circles and stress genes in determination of yeast 'life span' and aging is evaluated. The relationship between cultivation conditions and longevity are assessed. Discrepancies between the yeast and mammalian systems with regard to aging are pointed out. We discuss unresolved problems concerning the suitability of the budding yeast for the study of basic aging phenomena.

Identification of novel sequence-specific nuclear factors interacting with mouse senescence marker protein-30 gene promoter

Senescence marker protein-30 (SMP30) gene expression declines during aging in mouse liver. SMP30 also plays a role as Ca(2+)-binding protein localized in cytosol of hepatocytes. To elucidate the molecular mechanism of regulation of SMP30 gene expression we have cloned its gene promoter and carried out DNA-protein interaction analyses by DNase I footprinting and electrophoretic mobility shift assay. We have identified a total of eight nuclear factor binding sites within 0.8 kb upstream of transcription start site. Three of these sites are novel DNA sequences with no homology to the existing transcription factor binding site database. Interaction of nuclear factors to these novel cognate sites are DNA sequence-specific. The other five sites correspond to binding sites of known transcription factors, Sp1, AP2, CCAAT box, Lyf-1, and GATA-1. Coordinated orchestration of these factors may contribute to regulation of SMP30 gene expression.