Hormonal discrimination among transcription start sites of aspartate aminotransferase

Transcriptional regulation of N-acetylglutamate synthase

The urea cycle converts toxic ammonia to urea within the liver of mammals. At least 6 enzymes are required for ureagenesis, which correlates with dietary protein intake. The transcription of urea cycle genes is, at least in part, regulated by glucocorticoid and glucagon hormone signaling pathways. N-acetylglutamate synthase (NAGS) produces a unique cofactor, N-acetylglutamate (NAG), that is essential for the catalytic function of the first and rate-limiting enzyme of ureagenesis, carbamyl phosphate synthetase 1 (CPS1). However, despite the important role of NAGS in ammonia removal, little is known about the mechanisms of its regulation. We identified two regions of high conservation upstream of the translation start of the NAGS gene. Reporter assays confirmed that these regions represent promoter and enhancer and that the enhancer is tissue specific. Within the promoter, we identified multiple transcription start sites that differed between liver and small intestine. Several transcription factor binding motifs were conserved within the promoter and enhancer regions while a TATA-box motif was absent. DNA-protein pull-down assays and chromatin immunoprecipitation confirmed binding of Sp1 and CREB, but not C/EBP in the promoter and HNF-1 and NF-Y, but not SMAD3 or AP-2 in the enhancer. The functional importance of these motifs was demonstrated by decreased transcription of reporter constructs following mutagenesis of each motif. The presented data strongly suggest that Sp1, CREB, HNF-1, and NF-Y, that are known to be responsive to hormones and diet, regulate NAGS transcription. This provides molecular mechanism of regulation of ureagenesis in response to hormonal and dietary changes.

Regulatory sequence responsible for insulin destabilization of cytochrome P450 2B1 (CYP2B1) mRNA

Diabetes has been reported to increase CYP2E1 (cytochrome P450) and CYP2B1 expression at both the mRNA and protein levels in rat livers. This increase has been attributed to mRNA stabilization and can be reversed by daily insulin treatment. In a previous study, we showed that this hormone directly down-regulates CYP2E1 and 2B1 expression through a post-transcriptional mechanism in rat hepatoma cell lines. We then aimed to identify the molecular mechanisms involved in this regulation. We first identified a 16-mer sequence that we later showed to be the actual functional target of insulin on the rat CYP2E1 mRNA. Similar work was performed with CYP2B1. We first investigated the presence of mRNA-protein interactions. Using cytoplasmic proteins of Fao cells treated or not with insulin (0.1 microM) and the full-length CYP2B1 mRNA as a probe, a major CYP2B1 RNA-protein complex was observed with RNase T1 protection experiments. With the use of different CYP2B1 mRNA probes and by means of competition experiments with antisense oligonucleotides, a protein fixation site was located on a 16-nt sequence in the 5' part of the coding region. This sequence has a hairpin loop structure, shows 80% sequence identity with a structure previously identified on CYP2E1 and is also responsible for the post-transcriptional effects of insulin on this mRNA. Protein(s) bound to both CYP2B1 and CYP2E1 sequences are cytosolic and have an apparent molecular mass of 60 kDa. The protein(s) that bind(s) to both these sequences and the insulin transduction signal involved in this regulation remain(s) to identified.

Induction of glucokinase mRNA by dietary phenolic compounds in rat liver cells in vitro

Diabetes and its complications, including oxidative stress, are major reasons for medical intervention and one of the most frequent causes of death in developed countries. Several lines of data suggest that the use of certain dietary polyphenolic compounds may alter glucose metabolism, thus decreasing the risk for type 2 diabetes. In this paper, we present the effect of phenolic acids (caffeic, chlorogenic, rosmarinic, and ferulic) and extracts from Smallanthus sonchifolius and Prunella vulgaris on glucose production in rat hepatocytes and on glucokinase, glucose-6-phosphatase, and phosphoenol-pyruvate carboxykinase mRNA expression in rat hepatoma Fao cells. The phenolics at 500 microM and after 1 h incubation lowered glucose production via both gluconeogenesis (10 mM alanine or dihydroxyacetone as precursors) and glycogenolysis compared with metformin. Most of the phenolics increased the level of glucokinase mRNA after 24 h in the same way as insulin (10(-7) M).

Identification of a necessary element for Toxoplasma gondii SAG1 gene expression

SAG1 codes for the stage-specific major surface antigen P30 of Toxoplasma gondii (T. gondii) tachyzoites. Six tandemly repeated, conserved 27 bp cassettes in the region from -231 to -70 bp were previously confirmed to be essential for high-level expression of SAG1 and serve as a positioning element directing the initiation of transcription. We demonstrate here that an element located between +19 and +28 bp is necessary for SAG1 gene expression by using deletion mutagenesis analysis and electrophoresis mobility shift assay (EMSA). This will provide an insight into the regulatory mechanisms of SAG1 gene expression.

Characterization of the mitochondrial aconitase promoter and the identification of transcription factor binding

BACKGROUND

Mitochondrial (m) aconitase plays an important role in the unique pathway of citrate accumulation in prostate epithelial cells through its limited activity. In the current study, we characterized the human m-aconitase gene promoter.

METHODS

A 1,411-bp 5'-flanking fragment of the human m-aconitase gene was cloned, followed by 5' serial deletion analysis of promoter activity. Transcriptional start sties and transcription factors bound to the promoter were identified by 5' RACE, DNA pull-down assay and transcription factor array analysis.

RESULTS

Two transcriptional start sites were identified. The promoter fragment pulled down 15 transcription factors, some without consensus sequences in the promoter. Deletion of one Sp1 site eliminated all promoter activity.

CONCLUSIONS

The m-aconitase promoter is contained in a 153-bp 5' fragment lacking a TATA or CAAT sequence. Sp1 binding to a specific Sp1 site is required for promoter activity while other transcription factors are recruited through protein-protein interactions.

Opposite regulation of the rat and human cytosolic aspartate aminotransferase genes by fibrates

Fenofibrate, a peroxisome proliferator-activated receptor alpha (PPARalpha) activator, increases the expression of the cytosolic aspartate aminotransferase (cAspAT) gene in human liver cells, which may partially explain the increase of this enzyme in the serum of individuals undergoing fenofibrate treatment. Conversely, in rodents, fenofibrate represses the expression of the cAspAT gene. We compared the mechanisms of fenofibrate action in human and rat hepatoma cells. Transfection assays of the wild-type and mutated rat promoters in Fao and H4IIEC3 cells established a critical role for sequences similar to nuclear receptor responsive elements in the -404/-366 bp region. Nuclear proteins bound to these sequences and the amounts of protein bound were decreased by fenofibrate treatment, probably accounting for the decreased gene expression. Pharmacological studies confirmed the involvement of PPARalpha. However, this receptor did not bind directly to these sequences. The human promoter was cloned and the regulatory region localized between -2663/-706 bp. Co-transfection assays suggested that, in humans, the PPARalpha was also involved in the increase in expression of the cAspAT gene due to fibrates, without the presence of a canonical PPAR responsive element.

Identification of a 16-Nucleotide Sequence That Mediates Post-transcriptional Regulation of Rat CYP2E1 by Insulin

Insulin directly down-regulates the gene expression of the rat CYP2E1 by altering its mRNA stability (De Waziers, I., Garlatti, M., Bouguet, J., Beaune, P. H., and Barouki, R. (1995) Mol. Pharmacol. 47, 474-479). Because the regulation of CYP mRNA stability was poorly understood, the molecular mechanisms involved in this regulation in the rat hepatoma H4IIEC3 cell line were studied. By using RNase T1 protection methods, the formation of a major CYP2E1 RNA-protein complex was observed. By competition experiments, the binding site of this complex was located on a 16-nucleotide sequence in the 5'-proximal region of the CYP2E1-coding sequence. Insulin did not modify the binding pattern of proteins to this sequence. and transfections of expression vectors or antisense oligonucleotides were undertaken to demonstrate the actual functionality of the 16-mer sequence. The insertion of this sequence in a luciferase gene was sufficient to render the chimeric mRNA sensitive to insulin. Furthermore, transfection of H4IIEC3 cells with antisense oligonucleotide complementary to this sequence blocked the insulin effect on the CYP2E1 mRNA expression, i.e. its rapid degradation. All these results demonstrate that this 16-nucleotide sequence is implicated in the CYP2E1 post-transcriptional regulation by insulin.

Genomic structure of proton-coupled oligopeptide transporter hPEPT1 and pH-sensing regulatory splice variant

Proton-coupled oligopeptide transporter PEPT1 facilitates the transport of dipeptides and peptoid drugs (including antibiotics) across the cell membranes of endothelial and epithelial cells. Substrate transport by the proton symport is driven by pH gradients, while the profile of pH sensitivity is regulated by a closely related protein, hPEPT1-RF. We investigated the genomic structure of hPEPT1 and hPEPT1-RF. Analysis of the high-throughput genomic sequence (HTGS) database revealed that hPEPT1 and hPEPT1-RF are splice variants encoded by the same gene located in chromosome 13, consisting of 24 exons. hPEPT1 is encoded by 23 exons and hPEPT1-RF by 6 exons. Coding sequences of hPEPT1-RF share 3 exons completely and 2 exons partially with hPEPT1. The genomic organization of hPEPT1 shows high similarity with its mouse orthologue. Exon-intron boundaries occur mostly in the loops connecting transmembrane segments (TMSs), suggesting a modular gene structure reflecting the TMS-loop repeat units in hPEPT1. The putative promoter region of hPEPT1 contains TATA boxes and GC-rich regions and a potential insulin responsive element.

Fenofibrate modifies transaminase gene expression via a peroxisome proliferator activated receptor alpha-dependent pathway

Fibrates modify the expression of genes implicated in lipoprotein and fatty acid metabolism via the peroxisome proliferator-activated receptor alpha(PPARalpha), leading to reductions in serum triglycerides and cholesterol. The expression of certain genes regulated by PPARalpha have been shown to be modified in a species dependent manner. Aspartate aminotransferase (AspAT or GOT) and alanine aminotransferase (AlaAT or GPT) are enzymes involved in intermediate metabolism in all cells and in hepatic gluconeogenesis. These enzymes are also widely used as serum markers of possible tissue damage. This study investigated whether fenofibrate could modify the expression of liver AspAT and/or AlaAT and thus possibly alter transaminase levels independently of a cytotoxic effect. In human Hep G2 cells, fenofibrate increased cytosolic AspAT (cAspAT) activity by 40% and AlaAT activity by 100%, as well as both mRNAs. Nuclear run on assays showed that this effect was, at least in part, transcriptional. Increases in mRNA were also observed in human hepatocyte cultures at concentrations of the drug attained in patients. In C57BL/6 mice, fenofibrate decreased cAspAT and cAlaAT mRNA, while these effects were abolished in PPARalpha knock-out mice. In conclusion, fenofibrate has been shown to modify cAspAT and AlaAT gene expression in a species and PPARalpha dependent manner. This is the first demonstration that cAspAT and AlaAT activities may be pharmacologically altered, independently of a toxic phenomenon.

Molecular cloning and gene expression analysis of PSP94 (prostate secretory protein of 94 amino acids) in primates

Prostate secretory protein of 94 amino acids (PSP94) has shown the potential to be a diagnostic biomarker and a therapeutic agent for prostate cancer. Primates have been the main animal models for studying the biology of this molecule. We have cloned and analyzed the cDNA and promoter region of PSP94 from baboon (Papio anubis). Sequence divergence among baboon, monkey, pig, and human, in both the exons and 5'-flanking region indicates rapid evolution of the PSP94 gene. There are conserved steroid hormone response elements (SHRE) in the promoter region of all three primate species. Multiple, alternative transcripts starting near these SHREs and upstream to the TATA box were identified by reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of 5'-cDNA ends (5' RACE) in primate prostatic tissues. This differential transcription initiation may be linked to androgen regulation of PSP94 gene expression. PSP94 transcripts were detected by RT-PCR in a wide variety of mucus-secreting tissues. However, the alternative transcripts were found only in the prostate. The distribution of the PSP94 protein in baboon secretory tissues was also examined by Western blot analysis using a polyclonal antibody against the human homolog. A positive immunoreactive band was detected, but it was weak, due probably to epitope divergence between the two species. In all young, healthy primate animals tested, the level of immunoreactive PSP94 in prostate tissues was lower than expected. In addition, RT-PCR combined with Southern blot analysis on prostate tissues in these animals failed to detect the PSP57 mRNA produced by alternative splicing of PSP94 primary transcript. These observations can be explained by the sexual immaturity and incomplete prostate development in these young primates. This explanation was supported by histological examination of their prostate during PSP94 immunohistochemistry.

Contribution of a nuclear factor 1 binding site to the glucocorticoid regulation of the cytosolic aspartate aminotransferase gene promoter

Two regions of the cAspAT gene promoter mediate the glucocorticoid regulation of this gene in the Fao hepatoma cell line. The proximal region was localized by deletion studies and stable transfections in the Fao cells to the sequence -553/-398. This region includes the glucocorticoid-responsive element (GRE) A sequence, which consists of two overlapping GREs and which can mediate the glucocorticoid regulation of a heterologous promoter. DNase I footprinting studies have shown that a site 80 base pairs upstream of the GRE A was protected by liver and brain nuclear extracts (site P8). The binding was displaced by an excess of an oligonucleotide containing a typical NF1 binding site and by NF1-specific antibodies. In electrophoretic mobility shift assay using the P8 oligonucleotide as a probe, several complexes were formed. Most complexes were common to liver and brain but were less abundant when testis extracts were used. At least one complex was specific to the liver. All complexes, with the exception of two, were competed for by the NF1 oligonucleotide. Furthermore, the sequence of the P8 site showed a 7/9-base pair homology with a typical NF1 site. A mutation of the P8 site, which prevents the binding of NF1-like proteins to it, considerably decreases the regulation of the cAspAT promoter fragment by glucocorticoids. Surprisingly, the basal activity of the mutant promoter was increased 2-fold. Thus, the regulation of the cAspAT gene promoter is mediated by a regulatory unit comprising the GRE A and a NF1 binding site.

Androgen modulation of multiple transcription start sites of the mitochondrial aspartate aminotransferase gene in rat prostate

Mitochondrial aspartate aminotransferase (mAAT) is one of two key enzymes in the pathway of citrate production in prostate. Expression of mAAT is modulated by testosterone and prolactin in prostate. We cloned the promoter and 5'-flanking region of the rat mAAT gene and sequenced 2.0 kilobases of the DNA. This fragment contains the 5'-regulatory promoter region that lacks a TATA and a CCAAT box but is G+C rich. The 5'-upstream flanking region contains sequences that have high homology with the consensus glucocorticoid response element/androgen response element (ARE) and a reported ARE sequence that is different from the consensus sequence. Functional transcription studies showed that a 481-base region containing the two ARE sequences was sufficient for androgen-regulated gene expression. There are multiple transcription start sites that are regulated by testosterone in prostate. In liver, on the other hand, castration did not affect transcription from any of the start sites. Therefore, these data provide evidence that transcriptional regulation of the rat pmAAT gene occurs through an ARE located in the 5'-region. In addition, not only is gene expression modulated by testosterone, but the effect of testosterone on transcription is cell specific.

Acinar zonation of the hormonal regulation of cytosolic aspartate aminotransferase in the liver

The zonation of the expression and regulation of the cytosolic aspartate aminotransferase (cAspAT) mRNAs in the liver acinus was investigated in diabetic and/or adrenalectomized rats. Dexamethasone increased cAspAT activity two- to threefold alone and up to sixfold in combination with streptozotocin-induced diabetes. Northern blot analysis showed that the cAspAT mRNAs were increased by those treatments; the effect of streptozotocin was reversed by the administration of insulin. In situ hybridization experiments showed that basal cAspAT mRNAs were uniformly distributed within the liver acinus. However, cAspAT mRNAs were induced by glucocorticoids specifically in the periportal zone and by streptozotocin in a larger area including the periportal and intermediary zone. The alpha 2u-globulin mRNAs which are specifically expressed in the perivenous hepatocytes are also induced by glucocorticoids in this zone, suggesting that the specific regulation of the cAspAT gene by glucocorticoids in the periportal zone is not due to the absence of functional glucocorticoid receptors in the other zones. We conclude that the regulation of the cAspAT housekeeping gene is zone specific in the liver. Furthermore, this zonation depends on the gene and on the type of hormonal or pharmacological treatment.

Modulation of the agonist activity of antisteroids by a novel cis-acting element

The amount of agonist activity displayed by the antiglucocorticoid dexamethasone mesylate (Dex-Mes) for the induction of tyrosine aminotransferase (TAT) in rat hepatoma cells is greater than for glutamine synthetase and varies over a period of weeks. This variation, which has been reproduced over a period of 40 h by changing the density of the cells, suggests the involvement of a trans-acting factor. The target of this proposed trans-acting factor has now been localized to the region between -3.9 to -2.9 of the rat TAT gene from experiments with cells that were stably transfected with hybrid TAT/CAT constructs. Deletion experiments with transiently transfected TAT/tk promoter/CAT constructs revealed that this entire activity could be conveyed by a 21 bp sequence of the TAT gene. Gel shift experiments support the binding of a factor(s) to this 21 bp sequence. Thus the activity of the antagonist Dex-Mes is relatively independent of steroid structure and is largely determined by the further interactions of a trans-acting factor with the cis-acting sequence. We call this novel sequence a glucocorticoid modulatory element. A model is advanced which accounts for almost all of the results concerning TAT induction by glucocorticoids. This same model may also be useful in explaining why the amount of agonist activity of most antisteroids varies, even for different genes within the same cell.

Concepts of citrate production and secretion by prostate: 2. Hormonal relationships in normal and neoplastic prostate

A unique and major function of prostate secretory epithelial cells is to synthesize, accumulate, and secrete extraordinarily high levels of citrate. This function is regulated by testosterone and by prolactin. Concepts of the mechanisms of hormonal regulation are presented. The relationship of testosterone and prolactin to the origin and homologies of different prostate cell lines is described. The metabolic differentiation of citrate and non-citrate producing prostate secretory epithelial cells is discussed. Concepts of the pathogenesis of prostatic neoplasms are presented based on hormonal, metabolic, and homologous relationships associated with citrate production. Characterization of normal and neoplastic secretory epithelial cells by their citrate function is emphasized. The urgency and necessity for research relating to all aspects of prostate citrate production in normal and pathological prostate are emphasized.