Analysis of the upstream elements of the xenobiotic compound-inducible and positionally regulated glutathione S-transferase Ya gene

Antioxidant response elements: Discovery, classes, regulation and potential applications

Exposure to antioxidants and xenobiotics triggers the expression of a myriad of genes encoding antioxidant proteins, detoxifying enzymes, and xenobiotic transporters to offer protection against oxidative stress. This articulated universal mechanism is regulated through the cis-acting elements in an array of Nrf2 target genes called antioxidant response elements (AREs), which play a critical role in redox homeostasis. Though the Keap1/Nrf2/ARE system involves many players, AREs hold the key in transcriptional regulation of cytoprotective genes. ARE-mediated reporter constructs have been widely used, including xenobiotics profiling and Nrf2 activator screening. The complexity of AREs is brought by the presence of other regulatory elements within the AREs. The diversity in the ARE sequences not only bring regulatory selectivity of diverse transcription factors, but also confer functional complexity in the Keap1/Nrf2/ARE pathway. The different transcription factors either homodimerize or heterodimerize to bind the AREs. Depending on the nature of partners, they may activate or suppress the transcription. Attention is required for deeper mechanistic understanding of ARE-mediated gene regulation. The computational methods of identification and analysis of AREs are still in their infancy. Investigations are required to know whether epigenetics mechanism plays a role in the regulation of genes mediated through AREs. The polymorphisms in the AREs leading to oxidative stress related diseases are warranted. A thorough understanding of AREs will pave the way for the development of therapeutic agents against cancer, neurodegenerative, cardiovascular, metabolic and other diseases with oxidative stress.

How do nutritional antioxidants really work: nucleophilic tone and para-hormesis versus free radical scavenging in vivo

We present arguments for an evolution in our understanding of how antioxidants in fruits and vegetables exert their health-protective effects. There is much epidemiological evidence for disease prevention by dietary antioxidants and chemical evidence that such compounds react in one-electron reactions with free radicals in vitro. Nonetheless, kinetic constraints indicate that in vivo scavenging of radicals is ineffective in antioxidant defense. Instead, enzymatic removal of nonradical electrophiles, such as hydroperoxides, in two-electron redox reactions is the major antioxidant mechanism. Furthermore, we propose that a major mechanism of action for nutritional antioxidants is the paradoxical oxidative activation of the Nrf2 (NF-E2-related factor 2) signaling pathway, which maintains protective oxidoreductases and their nucleophilic substrates. This maintenance of "nucleophilic tone," by a mechanism that can be called "para-hormesis," provides a means for regulating physiological nontoxic concentrations of the nonradical oxidant electrophiles that boost antioxidant enzymes, and damage removal and repair systems (for proteins, lipids, and DNA), at the optimal levels consistent with good health.

Alterations in hepatic mRNA expression of phase II enzymes and xenobiotic transporters after targeted disruption of hepatocyte nuclear factor 4 alpha

Hepatocyte nuclear factor 4 alpha (HNF4a) is a liver-enriched master regulator of liver function. HNF4a is important in regulating hepatic expression of certain cytochrome P450s. The purpose of this study was to use mice lacking HNF4a expression in liver (HNF4a-HNull) to elucidate the role of HNF4a in regulating hepatic expression of phase II enzymes and transporters in mice. Compared with male wild-type mice, HNF4a-HNull male mouse livers had (1) markedly lower messenger RNAs (mRNAs) encoding the uptake transporters sodium taurocholate cotransporting polypeptide, organic anion transporting polypeptide (Oatp) 1a1, Oatp2b1, organic anion transporter 2, sodium phosphate cotransporter type 1, sulfate anion transporter 1, sodium-dependent vitamin C transporter 1, the phase II enzymes Uridine 5'-diphospho (UDP)-glucuronosyltransferase (Ugt) 2a3, Ugt2b1, Ugt3a1, Ugt3a2, sulfotransferase (Sult) 1a1, Sult1b1, Sult5a1, the efflux transporters multidrug resistance-associated protein (Mrp) 6, and multidrug and toxin extrusion 1; (2) moderately lower mRNAs encoding Oatp1b2, organic cation transporter (Oct) 1, Ugt1a5, Ugt1a9, glutathione S-transferase (Gst) m4, Gstm6, and breast cancer resistance protein; but (3) higher mRNAs encoding Oatp1a4, Octn2, Ugt1a1, Sult1e1, Sult2a2, Gsta4, Gstm1-m3, multidrug resistance protein (Mdr) 1a, Mrp3, and Mrp4. Hepatic signaling of nuclear factor E2-related factor 2 and pregnane X receptor appear to be activated in HNF4a-HNull mice. In conclusion, HNF4a deficiency markedly alters hepatic mRNA expression of a large number of phase II enzymes and transporters, probably because of the loss of HNF4a, which is a transactivator and a determinant of gender-specific expression and/or adaptive activation of signaling pathways important in hepatic regulation of these phase II enzymes and transporters.

Regulation of UGT1A1 and HNF1 transcription factor gene expression by DNA methylation in colon cancer cells

Background

UDP-glucuronosyltransferase 1A1 (UGT1A1) is a pivotal enzyme involved in metabolism of SN-38, the active metabolite of irinotecan commonly used to treat metastatic colorectal cancer. We previously demonstrated aberrant methylation of specific CpG dinucleotides in UGT1A1-negative cells, and revealed that methylation state of the UGT1A1 5'-flanking sequence is negatively correlated with gene transcription. Interestingly, one of these CpG dinucleotides (CpG -4) is found close to a HNF1 response element (HRE), known to be involved in activation of UGT1A1 gene expression, and within an upstream stimulating factor (USF) binding site.

Results

Gel retardation assays revealed that methylation of CpG-4 directly affect the interaction of USF1/2 with its cognate sequence without altering the binding for HNF1-alpha. Luciferase assays sustained a role for USF1/2 and HNF1-alpha in UGT1A1 regulation in colon cancer cells. Based on the differential expression profiles of HNF1A gene in colon cell lines, we also assessed whether methylation affects its expression. In agreement with the presence of CpG islands in the HNF1A promoter, treatments of UGT1A1-negative HCT116 colon cancer cells with a DNA methyltransferase inhibitor restore HNF1A gene expression, as observed for UGT1A1.

Conclusions

This study reveals that basal UGT1A1 expression in colon cells is positively regulated by HNF1-alpha and USF, and negatively regulated by DNA methylation. Besides, DNA methylation of HNF1A could also play an important role in regulating additional cellular drug metabolism and transporter pathways. This process may contribute to determine local inactivation of drugs such as the anticancer agent SN-38 by glucuronidation and define tumoral response.

A systems biology perspective on Nrf2-mediated antioxidant response

Cells in vivo are constantly exposed to reactive oxygen species (ROS) generated endogenously and exogenously. To defend against the deleterious consequences of ROS, cells contain multiple antioxidant enzymes expressed in various cellular compartments to scavenge these toxic species. Under oxidative stresses, these antioxidant enzymes are upregulated to restore redox homeostasis. Such an adaptive response results from the activation of a redox-sensitive gene regulatory network mediated by nuclear factor E2-related factor 2. To more completely understand how the redox control system is designed by nature to meet homeostatic goals, we have examined the network from a systems perspective using engineering approaches. As with man-made control devices, the redox control system can be decomposed into distinct functional modules, including transducer, controller, actuator, and plant. Cells achieve specific performance objectives by utilizing nested feedback loops, feedforward control, and ultrasensitive signaling motifs, etc. Given that endogenously generated ROS are also used as signaling molecules, our analysis suggests a novel mode of action to explain oxidative stress-induced pathological conditions and diseases. Specifically, by adaptively upregulating antioxidant enzymes, oxidative stress may inadvertently attenuate ROS signals that mediate physiological processes, resulting in aberrations of cellular functions and adverse consequences. Lastly, by simultaneously considering the two competing cellular tasks-adaptive antioxidant defense and ROS signaling-we re-examine the premise that dietary antioxidant supplements is generally beneficial to human health. Our analysis highlights some possible adverse effects of these widely consumed antioxidants.

Phase I to II cross-induction of xenobiotic metabolizing enzymes: a feedforward control mechanism for potential hormetic responses

Hormetic responses to xenobiotic exposure likely occur as a result of overcompensation by the homeostatic control systems operating in biological organisms. However, the mechanisms underlying overcompensation that leads to hormesis are still unclear. A well-known homeostatic circuit in the cell is the gene induction network comprising phase I, II and III metabolizing enzymes, which are responsible for xenobiotic detoxification, and in many cases, bioactivation. By formulating a differential equation-based computational model, we investigated in this study whether hormesis can arise from the operation of this gene/enzyme network. The model consists of two feedback and one feedforward controls. With the phase I negative feedback control, xenobiotic X activates nuclear receptors to induce cytochrome P450 enzyme, which bioactivates X into a reactive metabolite X'. With the phase II negative feedback control, X' activates transcription factor Nrf2 to induce phase II enzymes such as glutathione S-transferase and glutamate cysteine ligase, etc., which participate in a set of reactions that lead to the metabolism of X' into a less toxic conjugate X''. The feedforward control involves phase I to II cross-induction, in which the parent chemical X can also induce phase II enzymes directly through the nuclear receptor and indirectly through transcriptionally upregulating Nrf2. As a result of the active feedforward control, a steady-state hormetic relationship readily arises between the concentrations of the reactive metabolite X' and the extracellular parent chemical X to which the cell is exposed. The shape of dose-response evolves over time from initially monotonically increasing to J-shaped at the final steady state-a temporal sequence consistent with adaptation-mediated hormesis. The magnitude of the hormetic response is enhanced by increases in the feedforward gain, but attenuated by increases in the bioactivation or phase II feedback loop gains. Our study suggests a possibly common mechanism for the hormetic responses observed with many mutagens/carcinogens whose activities require bioactivation by phase I enzymes. Feedforward control, often operating in combination with negative feedback regulation in a homeostatic system, may be a general control theme responsible for steady-state hormesis.

Regulation of the rat glutathione S-transferase A2 gene by glucocorticoids: crosstalk through C/EBPs

Regulation of the rat glutathione S-transferase A2 (GSTA2) gene by glucocorticoids is biphasic in its concentration dependence to glucocorticoids, with concentrations of 10-100 nM repressing gene activity (GR-dependent), and concentrations above 1 microM increasing transactivation (PXR-dependent) in adult rat hepatocytes or transient transfection assays. Over-expression of either C/EBP alpha or beta negatively regulates basal and inducible expression of a 1.65 Kb GSTA2 luciferase reporter, and synergizes the response to glucocorticoids (GC). C/EBP responsive elements have been identified in the GSTA2 5'-flanking sequence, associated with the palindrominic Glucocorticoid Responsive Element (GRE), the Ah receptor response elements, and the antioxidant response element. In reporters lacking the palindromic GRE, negative regulation by GC is observed only when C/EBP alpha is co-expressed. Co-transfection of C/EBP alpha/beta induced gene expression of the GSTA2 XRE reporter, but negatively regulated the GSTA2 ARE-reporter. In contrast, the ARE from the rat NAD(P)H quinone oxidoreductase gene was induced by co-transfection of C/EBPs, but was still negatively regulated by GC. PXR-induction of the GSTA2 reporter was partially ablated by co-transfection of C/EBP alpha and enhanced by co-transfection of C/EBPbeta. We conclude that C/EBP alpha and beta are involved in GC-dependent repression of GSTA2 gene expression and ARE sequences that bind C/EBPs appears to be critical for these responses.

PAS domains: internal sensors of oxygen, redox potential, and light

PAS domains are newly recognized signaling domains that are widely distributed in proteins from members of the Archaea and Bacteria and from fungi, plants, insects, and vertebrates. They function as input modules in proteins that sense oxygen, redox potential, light, and some other stimuli. Specificity in sensing arises, in part, from different cofactors that may be associated with the PAS fold. Transduction of redox signals may be a common mechanistic theme in many different PAS domains. PAS proteins are always located intracellularly but may monitor the external as well as the internal environment. One way in which prokaryotic PAS proteins sense the environment is by detecting changes in the electron transport system. This serves as an early warning system for any reduction in cellular energy levels. Human PAS proteins include hypoxia-inducible factors and voltage-sensitive ion channels; other PAS proteins are integral components of circadian clocks. Although PAS domains were only recently identified, the signaling functions with which they are associated have long been recognized as fundamental properties of living cells.

Ischaemia and reperfusion injury of rat liver increases expression of glutathione S-transferase A1/A2 in zone 3 of the hepatic lobule

Effects of ischaemia-reperfusion injury (I/R) of liver on expression of rat glutathione S-transferase (rGST) isoenzymes that metabolize products of oxidative stress were examined. Rats underwent lobar liver ischaemia for 30 min followed by reperfusion. In ischaemic lobes, rGSTA1/A2 transcript levels increased significantly 12 h after I/R (2.94-fold) and protein levels increased significantly at 24 h (1.45-fold); increased transcript levels were also observed in nonischaemic lobes (1.78-fold). Superoxide dismutase prevented I/R and the increases in transcript and protein levels in ischaemic and non-ischaemic lobes. By in-situ hybridization, increases in transcript levels at 6 h were present in zones 2 and 3 of the ischaemic lobes and peaked at 12 h (2.5-fold zone 2, 4.5-fold zone 3). Significant increases in transcript levels also were observed at 24 h in zones 2 (2.0-fold) and 3 (2.9-fold) of non-ischaemic lobes. Nuclear run-off assays showed a 1.8-fold increase in rGSTA1/A2 transcription rates in ischaemic lobes at 3 h. We conclude that I/R causes increased rGSTA1/A2 expression in the zone of the hepatic lobule most susceptible to oxidative injury and that this expression may be an important defence against injury.

Characterization of the rat glutathione S-transferase Yc2 subunit gene, GSTA5: identification of a putative antioxidant-responsive element in the 5'-flanking region of rat GSTA5 that may mediate chemoprotection against aflatoxin B1

We have isolated and characterized genomic DNA encoding the rat glutathione S-transferase Yc2 subunit. This protein is now referred to as rGSTA5 and is noteworthy because of its high activity towards aflatoxin B1-8,9-epoxide, its marked inducibility by chemoprotectors, its sex-specific regulation, and its over-expression in hepatoma and preneoplastic nodules. The rGSTA5 gene, which was isolated on two overlapping bacteriophage lambda clones, is approx. 12 kb in length and, unlike other class Alpha genes described to date, it comprises six exons. The transcription start site has been identified 228 bp upstream from the ATG translational initiation codon, and is situated 51 bp downstream from a consensus TATA-box. Deletion analysis, using luciferase reporter constructs, has shown that the region between -177 bp and +65 bp from the transcriptional start site contains a functional promoter. Computer-assisted analysis of the upstream sequence has indicated the presence of an antioxidant-responsive element (ARE), and several elements thought to be required for tissue-specific expression of the enzyme. In addition, several putative oestrogen-responsive half sites were observed in both upstream and intronic sequences.

Dioxin receptor and C/EBP regulate the function of the glutathione S-transferase Ya gene xenobiotic response element

The rat glutathione S-transferase Ya gene xenobiotic response element (XRE) has both constitutive and xenobiotic-inducible activity. We present evidence that the XRE is regulated by both the constitutive C/EBP transcription factor and the xenobiotic-activated dioxin receptor. A ligand-activated XRE-binding protein was shown to be dioxin receptor by specific antibody immunodepletion and binding of highly purified receptor. Identification of C/EBP alpha as the constitutive binding protein was demonstrated by competition with a C/EBP binding site, protein-DNA cross-linking to determine the molecular weight of the constitutive protein(s), specific antibody immunodepletion, and binding of purified bacterially expressed C/EBP alpha. Mutational analysis of the XRE revealed that the constitutive factor (C/EBP alpha) shares a nearly identical overlapping binding site with the dioxin receptor. In functional testing of the putative C/EBP-XRE interaction, cotransfected C/EBP alpha activated an XRE test promoter in the non-xenobiotic-responsive HeLa cell line. Unexpectedly, cotransfected C/EBP alpha had no effect on basal activity but significantly increased the xenobiotic response of the XRE test promoter in the xenobiotic-responsive, C/EBP-positive HepG2 cell line. Furthermore, inhibition of C/EBP-binding protein(s) in HepG2 cells by transfection of C/EBP oligonucleotides suppressed the xenobiotic response. These results suggest that C/EBP alpha and dioxin receptor recognize the same DNA sequence element and that transcriptional regulation can occur by cooperative interactions between these two transcription factors.

Dioxin receptor and C/EBP regulate the function of the glutathione S-transferase Ya gene xenobiotic response element

The rat glutathione S-transferase Ya gene xenobiotic response element (XRE) has both constitutive and xenobiotic-inducible activity. We present evidence that the XRE is regulated by both the constitutive C/EBP transcription factor and the xenobiotic-activated dioxin receptor. A ligand-activated XRE-binding protein was shown to be dioxin receptor by specific antibody immunodepletion and binding of highly purified receptor. Identification of C/EBP alpha as the constitutive binding protein was demonstrated by competition with a C/EBP binding site, protein-DNA cross-linking to determine the molecular weight of the constitutive protein(s), specific antibody immunodepletion, and binding of purified bacterially expressed C/EBP alpha. Mutational analysis of the XRE revealed that the constitutive factor (C/EBP alpha) shares a nearly identical overlapping binding site with the dioxin receptor. In functional testing of the putative C/EBP-XRE interaction, cotransfected C/EBP alpha activated an XRE test promoter in the non-xenobiotic-responsive HeLa cell line. Unexpectedly, cotransfected C/EBP alpha had no effect on basal activity but significantly increased the xenobiotic response of the XRE test promoter in the xenobiotic-responsive, C/EBP-positive HepG2 cell line. Furthermore, inhibition of C/EBP-binding protein(s) in HepG2 cells by transfection of C/EBP oligonucleotides suppressed the xenobiotic response. These results suggest that C/EBP alpha and dioxin receptor recognize the same DNA sequence element and that transcriptional regulation can occur by cooperative interactions between these two transcription factors.

Ligand-dependent recruitment of the Arnt coregulator determines DNA recognition by the dioxin receptor

The intracellular basic region/helix-loop-helix (bHLH) dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and functions as a ligand-activated DNA binding protein directly interacting with target genes by binding to dioxin response elements. Here we show that the partially purified, ligand-bound receptor alone could not bind target DNA. In contrast, DNA binding by the receptor could be induced by addition of a cytosolic auxiliary activity which functionally and biochemically corresponded to the bHLH factor Arnt. While Arnt exhibited no detectable affinity for the dioxin response element in the absence of the dioxin receptor, it strongly promoted the DNA binding function of the ligand-activated but not the ligand-free receptor forms. Arnt also functionally reconstituted in vitro the DNA binding activity of a mutant, nuclear translocation-deficient dioxin receptor phenotype in cytosolic extracts from a dioxin-resistant hepatoma cell line. Importantly, coimmunoprecipitation experiments showed that Arnt physically interacted in solution with the ligand-activated dioxin receptor but failed to heterodimerize with the ligand-free, hsp90-associated receptor form. Mutational analysis suggested that the functional interaction between these two factors occurred via the bHLH motif of Arnt. These data suggest that dioxin receptor activity is governed by a complex pattern of combinatorial regulation involving repression by hsp90 and then by ligand-dependent recruitment of the positive coregulator Arnt. The dioxin receptor system also provides the first example of signal-controlled dimerization of bHLH factors.

Constitutive and inducible profile of glutathione S-transferase subunits in biliary epithelial cells and hepatocytes isolated from rat liver

The constitutive and inducible cytosolic glutathione S-transferase (EC 2.5.1.18) subunit compositions of parenchymal cells (hepatocytes) and biliary epithelial cells (BEC) from rat liver have been quantitatively analysed using reverse-phase h.p.l.c. Hepatocytes, analysed in the absence of non-parenchymal cells, expressed constitutively the following subunits, in order of their concentration: 3, 4, 2, 1a, 1b, 8, 6 and 10. BEC express constitutively only four of the GST subunits expressed by hepatocytes and these are, in order of their concentration: subunits 2, 7, 4 and 3. Notable differences from hepatocytes are that BEC completely lack the Alpha-class subunits 1a and 1b that are major subunits in hepatocytes, Mu-class subunits make up a very low proportion of the total, and the Pi-class subunit 7 is a major subunit in BEC, whereas it is essentially absent from hepatocytes. For the first time, the effects of the inducing agents phenobarbitone (PB), beta-naphthoflavone (beta-NF) and ethoxyquin (EQ) have been characterized in a comprehensive and quantitative manner in both cell types. PB, beta-NF and EQ increased total GST protein in hepatocytes by approx. 2-fold, 3-fold and 4-fold respectively. Subunits significantly induced in hepatocytes were (in order of fold-induction): by PB, 1b > 8 > 3 > 2 > 4; by beta-NF, 1b > 8 > 2 > 3 > 4; and by EQ, 7 > 1b > 10 > 8 > 3 > 2 > 1a > 4. In BEC, neither PB nor beta-NF had significant effects on the total amount of GST protein, although PB did significantly induce subunit 3 at the expense of other subunits. EQ increased total GST protein nearly 5-fold in BEC, subunits 7 and 3 being induced dramatically above constitutive levels.

Ligand-dependent recruitment of the Arnt coregulator determines DNA recognition by the dioxin receptor

The intracellular basic region/helix-loop-helix (bHLH) dioxin receptor mediates signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) and functions as a ligand-activated DNA binding protein directly interacting with target genes by binding to dioxin response elements. Here we show that the partially purified, ligand-bound receptor alone could not bind target DNA. In contrast, DNA binding by the receptor could be induced by addition of a cytosolic auxiliary activity which functionally and biochemically corresponded to the bHLH factor Arnt. While Arnt exhibited no detectable affinity for the dioxin response element in the absence of the dioxin receptor, it strongly promoted the DNA binding function of the ligand-activated but not the ligand-free receptor forms. Arnt also functionally reconstituted in vitro the DNA binding activity of a mutant, nuclear translocation-deficient dioxin receptor phenotype in cytosolic extracts from a dioxin-resistant hepatoma cell line. Importantly, coimmunoprecipitation experiments showed that Arnt physically interacted in solution with the ligand-activated dioxin receptor but failed to heterodimerize with the ligand-free, hsp90-associated receptor form. Mutational analysis suggested that the functional interaction between these two factors occurred via the bHLH motif of Arnt. These data suggest that dioxin receptor activity is governed by a complex pattern of combinatorial regulation involving repression by hsp90 and then by ligand-dependent recruitment of the positive coregulator Arnt. The dioxin receptor system also provides the first example of signal-controlled dimerization of bHLH factors.

Cross-coupling of signal transduction pathways: the dioxin receptor mediates induction of cytochrome P-450IA1 expression via a protein kinase C-dependent mechanism

Signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) is mediated by the intracellular dioxin receptor which, in its dioxin-activated state, regulates transcription of target genes encoding drug-metabolizing enzymes, such as cytochrome P-450IA1 and glutathione S-transferase Ya. Exposure of the dioxin receptor to dioxin leads to an apparent translocation of the receptor to the nucleus in vivo and to a rapid conversion of the receptor from a latent, non-DNA-binding form to a species that binds to dioxin-responsive positive control elements in vitro. This DNA-binding form of receptor appears to be a heterodimeric complex with the helix-loop-helix factor Arnt. In this study, we show that activation of the cytochrome P-450IA1 gene and minimal dioxin-responsive reporter constructs by the dioxin receptor was inhibited following prolonged treatment of human keratinocytes with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Inhibition of the receptor-mediated activation response was also achieved by treatment of the cells with a number of protein kinase inhibitors, one of which, calphostin C, shows selectivity for protein kinase C. Taken together, these data suggest that protein kinase C-dependent phosphorylation may play an essential role in the dioxin signaling pathway. This hypothesis is supported by the observation that pretreatment of the cells with 12-O-tetradecanoylphorbol-13-acetate inhibited the DNA-binding activity of the dioxin receptor in vivo. In vivo, the dioxin receptor was found to be a phosphoprotein. In vitro, dephosphorylation of the ligand-activated, heteromeric dioxin receptor form or dephosphorylation of the individual ligand-binding and Arnt receptor subunits inhibited the xenobiotic response element-binding activity. Moreover, dephosphorylation experiments with the individual receptor subunits prior to assembly of the xenobiotic response element-binding receptor form indicated that phosphorylation seemed to be important for the DNA-binding activity per se of the receptor, whereas Arnt appeared to require phosphorylation to interact with the receptor. Finally, a protein kinase C inhibitor-sensitive cytosolic catalytic activity that could restore the DNA-binding activity of the dephosphorylated dioxin receptor form was identified.

Cross-coupling of signal transduction pathways: the dioxin receptor mediates induction of cytochrome P-450IA1 expression via a protein kinase C-dependent mechanism

Signal transduction by dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) is mediated by the intracellular dioxin receptor which, in its dioxin-activated state, regulates transcription of target genes encoding drug-metabolizing enzymes, such as cytochrome P-450IA1 and glutathione S-transferase Ya. Exposure of the dioxin receptor to dioxin leads to an apparent translocation of the receptor to the nucleus in vivo and to a rapid conversion of the receptor from a latent, non-DNA-binding form to a species that binds to dioxin-responsive positive control elements in vitro. This DNA-binding form of receptor appears to be a heterodimeric complex with the helix-loop-helix factor Arnt. In this study, we show that activation of the cytochrome P-450IA1 gene and minimal dioxin-responsive reporter constructs by the dioxin receptor was inhibited following prolonged treatment of human keratinocytes with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Inhibition of the receptor-mediated activation response was also achieved by treatment of the cells with a number of protein kinase inhibitors, one of which, calphostin C, shows selectivity for protein kinase C. Taken together, these data suggest that protein kinase C-dependent phosphorylation may play an essential role in the dioxin signaling pathway. This hypothesis is supported by the observation that pretreatment of the cells with 12-O-tetradecanoylphorbol-13-acetate inhibited the DNA-binding activity of the dioxin receptor in vivo. In vivo, the dioxin receptor was found to be a phosphoprotein. In vitro, dephosphorylation of the ligand-activated, heteromeric dioxin receptor form or dephosphorylation of the individual ligand-binding and Arnt receptor subunits inhibited the xenobiotic response element-binding activity. Moreover, dephosphorylation experiments with the individual receptor subunits prior to assembly of the xenobiotic response element-binding receptor form indicated that phosphorylation seemed to be important for the DNA-binding activity per se of the receptor, whereas Arnt appeared to require phosphorylation to interact with the receptor. Finally, a protein kinase C inhibitor-sensitive cytosolic catalytic activity that could restore the DNA-binding activity of the dephosphorylated dioxin receptor form was identified.

Cloning of a mu-class glutathione S-transferase gene and identification of the glucocorticoid regulatory domains in its 5' flanking sequence

The expression of a mu-class glutathione S-transferase gene (hGSTYBX) isolated from hamster smooth muscle tumor cells (DDT1 MF-2) is transcriptionally up-regulated by glucocorticoids, and this hormonal regulation is dependent upon protein synthesis. To study the mechanism of regulation, we have cloned and sequenced hGSTYBX genomic DNA including its 5' flanking region. The hGSTYBX gene contains nine exons dispersed over a 6.3-kilobase region. When linked to a chloramphenicol acetyltransferase (CAT) reporter gene, the 5' flanking region was able to direct transcription of the reporter gene. With 5' deletion studies, we have localized the major glucocorticoid-inducible regulatory element between nucleotides -353 and -239. Within this region no classic glucocorticoid response element (TGTTCT) was identified, but four potential helix-loop-helix binding domains are embedded in two 16-base-pair repeats. Another glucocorticoid regulatory domain has been localized between nucleotides -239 and -136. Cycloheximide blocks glucocorticoid-induced transcription of both the -353CAT and -239CAT reporter genes (nucleotides -447 to -12 and nucleotides -239 to -12 of hGSTYBX, respectively, ligated to a CAT reporter gene); therefore, our observations support previous results suggesting that hGSTYBX induction by glucocorticoids is a secondary response.

Liver cells contain constitutive DNase I-hypersensitive sites at the xenobiotic response elements 1 and 2 (XRE1 and -2) of the rat cytochrome P-450IA1 gene and a constitutive, nuclear XRE-binding factor that is distinct from the dioxin receptor

Dioxin stimulates transcription from the cytochrome P-450IA1 promoter by interaction with the intracellular dioxin receptor. Upon binding of ligand, the receptor is converted to a form which specifically interacts in vitro with two dioxin-responsive positive control elements located in close proximity to each other about 1 kb upstream of the rat cytochrome P-450IA1 gene transcription start point. In rat liver, the cytochrome P-450IA1 gene is marked at the chromatin level by two DNase I-hypersensitive sites that map to the location of the response elements and exist prior to induction of transcription by the dioxin receptor ligand beta-naphthoflavone. In addition, a DNase I-hypersensitive site is detected near the transcription initiation site and is altered in nuclease sensitivity by induction. The presence of the constitutive DNase I-hypersensitive sites at the dioxin response elements correlates with the presence of a constitutive, labile factor which specifically recognizes these elements in vitro. This factor appears to be distinct from the dioxin receptor, which is observed only in nuclear extract from treated cells. In conclusion, these data suggest that a certain protein-DNA architecture may be maintained at the response elements at different stages of gene expression.

Liver cells contain constitutive DNase I-hypersensitive sites at the xenobiotic response elements 1 and 2 (XRE1 and -2) of the rat cytochrome P-450IA1 gene and a constitutive, nuclear XRE-binding factor that is distinct from the dioxin receptor

Dioxin stimulates transcription from the cytochrome P-450IA1 promoter by interaction with the intracellular dioxin receptor. Upon binding of ligand, the receptor is converted to a form which specifically interacts in vitro with two dioxin-responsive positive control elements located in close proximity to each other about 1 kb upstream of the rat cytochrome P-450IA1 gene transcription start point. In rat liver, the cytochrome P-450IA1 gene is marked at the chromatin level by two DNase I-hypersensitive sites that map to the location of the response elements and exist prior to induction of transcription by the dioxin receptor ligand beta-naphthoflavone. In addition, a DNase I-hypersensitive site is detected near the transcription initiation site and is altered in nuclease sensitivity by induction. The presence of the constitutive DNase I-hypersensitive sites at the dioxin response elements correlates with the presence of a constitutive, labile factor which specifically recognizes these elements in vitro. This factor appears to be distinct from the dioxin receptor, which is observed only in nuclear extract from treated cells. In conclusion, these data suggest that a certain protein-DNA architecture may be maintained at the response elements at different stages of gene expression.