Oncogenic conversion of Ets affects redox regulation in-vivo and in-vitro

The expression of ELK transcription factors in adult DRG: Novel isoforms, antisense transcripts and upregulation by nerve damage

ELK transcription factors are known to be expressed in a number of regions in the nervous system. We show by RT-PCR that the previously described Elk1, Elk3/Elk3b/Elk3c and Elk4 mRNAs are expressed in adult dorsal root ganglia (DRG), together with the novel alternatively spliced isoforms Elk1b, Elk3d and Elk4c/Elk4d/Elk4e. These isoforms are also expressed in brain, heart, kidney and testis. In contrast to Elk3 protein, the novel Elk3d isoform is cytoplasmic, fails to bind ETS binding sites and yet can activate transcription by an indirect mechanism. The Elk3 and Elk4 genes are overlapped by co-expressed Pctk2 (Cdk17) and Mfsd4 genes, respectively, with the potential formation of Elk3/Pctaire2 and Elk4/Mfsd4 sense-antisense mRNA heteroduplexes. After peripheral nerve injury the Elk3 mRNA isoforms are each upregulated approximately 2.3-fold in DRG (P<0.005), whereas the natural antisense Pctaire2 isoforms show only a small increase (21%, P<0.01) and Elk1 and Elk4 mRNAs are unchanged.

Nuclear translocation of haeme oxygenase-1 is associated to prostate cancer

The role of oxidative stress in prostate cancer has been increasingly recognised. Acute and chronic inflammations generate reactive oxygen species that result in damage to cellular structures. Haeme oxygenase-1 (HO-1) has cytoprotective effects against oxidative damage. We hypothesise that modulation of HO-1 expression may be involved in the process of prostate carcinogenesis and prostate cancer progression. We thus studied HO-1 expression and localisation in 85 samples of organ-confined primary prostate cancer obtained via radical prostatectomy (Gleason grades 4–9) and in 39 specimens of benign prostatic hyperplasia (BPH). We assessed HO-1 expression by immunohistochemical staining. No significant difference was observed in the cytoplasmic positive reactivity among tumours (84%), non-neoplastic surrounding parenchyma (89%), or BPH samples (87%) (P=0.53). Haeme oxygenase-1 immunostaining was detected in the nuclei of prostate cancer cells in 55 of 85 (65%) patients but less often in non-neoplastic surrounding parenchyma (30 of 85, 35%) or in BPH (9 of 39, 23%) (P<0.0001). Immunocytochemical and western blot analysis showed HO-1 only in the cytoplasmic compartment of PC3 and LNCaP prostate cancer cell lines. Treatment with hemin, a well-known specific inducer of HO-1, led to clear nuclear localisation of HO-1 in both cell lines and highly induced HO-1 expression in both cellular compartments. These findings have demonstrated, for the first time, that HO-1 expression and nuclear localisation can define a new subgroup of prostate cancer primary tumours and that the modulation of HO-1 expression and its nuclear translocation could represent new avenues for therapy.

Oxidative Stress Induces ADAM9 Protein Expression in Human Prostate Cancer Cells

The ADAM (a disintegrin and metalloprotease) family is a group of transmembrane proteins containing cell adhesive and proteolytic functional domains. Microarray analysis detected elevated ADAM9 during the transition of human LNCaP prostate cancer cells from an androgen-dependent to an androgen-independent and metastatic state. Using a prostate tissue array (N = 200), the levels of ADAM9 protein expression were also elevated in malignant as compared with benign prostate tissues. ADAM9 protein expression was found in 43% of benign glands with light staining and 87% of malignant glands with increasing intensity of staining. We found that ADAM9 mRNA and protein expressions were elevated on exposure of human prostate cancer cells to stress conditions such as cell crowding, hypoxia, and hydrogen peroxide. We uncovered an ADAM9-like protein, which is predominantly induced together with the ADAM9 protein by a brief exposure of prostate cancer cells to hydrogen peroxide. Induction of ADAM9 protein in LNCaP or C4-2 cells can be completely abrogated by the administration of an antioxidant, ebselen, or genetic transfer of a hydrogen peroxide degradative enzyme, catalase, suggesting that reactive oxygen species (ROS) are a common mediator. The induction of ADAM9 by stress can be inhibited by both actinomycin D and cycloheximide through increased gene transcription and protein synthesis. In conclusion, intracellular ROS and/or hydrogen peroxide, generated by cell stress, regulate ADAM9 expression. ADAM9 could be responsible for supporting prostate cancer cell survival and progression. By decreasing ADAM9 expression, we observed apoptotic cell death in prostate cancer cells.

Lycopene

Oxidative stress is now recognized as an important etiological factor in the causation of several chronic diseases including cancer, cardiovascular diseases, osteoporosis, and diabetes. Antioxidants play an important role in mitigating the damaging effects of oxidative stress on cells. Lycopene, a carotenoid antioxidant, has received considerable scientific interest in recent years. Epidemiological, tissue culture, and animal studies provide convincing evidence supporting the role of lycopene in the prevention of chronic diseases. Human intervention studies are now being conducted to validate epidemiological observations and to understand the mechanisms of action of lycopene in disease prevention. To obtain a better understanding of the role of lycopene in human health, this chapter reviews the most recent information pertaining to its chemistry, bioavailability, metabolism, role in the prevention of prostate cancer and cancer of other target organs, its role in cardiovascular diseases, osteoporosis, hypertension, and male infertility. A discussion of the most relevant molecular markers of cancer is also included as a guide to future researchers in this area. The chapter concludes by reviewing global intake levels of lycopene, suggested levels of intake, and future research directions.

Oxidative stress and cyclooxygenase activity in prostate carcinogenesis: targets for chemopreventive strategies

Over the last decade, epidemiological, experimental and clinical studies have implicated oxidative stress in the development and progression of prostate cancer. Oxidative stress may be linked to the effects of androgens, anti-oxidant systems and the pre-malignant condition, high-grade prostatic intraepithelial neoplasia. Cyclooxygenase-2 activity has been linked with prostate carcinogenesis. Evidence suggests that oxidative stress and cyclo-oxygenase-2 activity may be mechanistically linked. Agents such as anti-oxidants and cyclo-oxgenase-2 inhibitors may be of value in the chemoprevention of prostate cancer. The feasibility of intervention with such agents will depend on the development and validation of biomarkers for clinical trials, particularly markers of oxidative damage caused by reactive oxygen species (ROS). A greater understanding of the molecular events associated with oxidative stress will enhance the development of such biomarkers and should result in better strategies for the chemoprevention of prostate cancer.

The role of cyclooxygenase-2 inhibition for the prevention and treatment of prostate carcinoma

Experimental and epidemiologic studies have demonstrated that nonsteroidal antiinflammatory drugs (NSAIDs) are effective in the prevention of human cancers. Nonsteroidal antiinflammatory drugs inhibit the cyclooxygenase (COX) enzyme that functions to convert arachidonic acid to prostaglandins (PGs). Cyclooxygenase-2, a key COX isoenzyme, is rapidly induced in response to inflammatory stimuli, growth factors, cytokines, and promoters of neoplastic growth. Cyclooxygenase-2-catalyzed reactions may be involved in carcinogenesis via 2 distinct mechanisms: (1). DNA damage and (2). PG-mediated effects. Reactions mediated by COX-2 form reactive oxygen species that can directly induce the oxidation of DNA or instigate the bioactivation of carcinogens. Prostaglandin E2, a byproduct of COX-2-mediated arachidonic acid metabolism, exhibits several biologic actions that have been shown to promote tumorigenesis and tumor progression. These actions include increased cell proliferation, promotion of angiogenesis, and the elevated expression of the antiapoptotic protein Bcl-2. In addition, PGE2 decreases natural killer cell activity and alters immune surveillance. In vitro experimental studies find that COX-2 inhibitors decrease cellular proliferation, increase apoptosis, and modulate genes involved in cell cycle regulation. Evidence from animal studies supports a role for NSAIDs in prostate cancer (CaP) prevention. Population-based studies have observed a reduced incidence of CaP among men using NSAIDs. Because CaP evolves slowly and rarely strikes men before the sixth or seventh decade of life, any strategy to delay or lengthen the time to development of clinically evident CaP, such as chemoprevention strategies, would greatly impact the natural history of this disease. Recent progress and critical analyses in the roles of COX-2 inhibition on prostate carcinogenesis and CaP prevention will be presented.

Effects of hydrogen peroxide on bovine leukemia virus expression

Several activators of bovine leukemia virus (BLV) expression, including lipopolysaccharides, phorbol esters and calcium ionophores, are known to generate reactive oxygen species (ROS). Therefore the influence of H2O2 on BLV expression in two BLV producing cell lines was investigated. The effect of H2O2 on BLV expression is apparently dose-dependent. Incubation of FLK/BLV cells with low concentrations of H2O2 (2.5 to 10 microM) induced a marked enhancement of BLV p24 synthesis and an activation of the long terminal repeat (LTR). Higher concentrations resulted in a decrease of proliferation, induction of apoptosis and in a decrease of BLV synthesis. Furthermore, in both cell lines H2O2 treatment led to the activation of NF-kappaB. Pretreatment of cells with antioxidants abrogated the H2O2-induced BLV expression. Taken together, our findings suggest that oxidative stress stimulates BLV expression via activation of NF-kappaB, raising the possibility that biological sources of H2O2, such as stimulated phagocytes, may influence BLV expression.

Antioxidant dietary supplements: Rationale and current status as chemopreventive agents for prostate cancer

Epidemiologic data suggest that the environment is responsible for most prostate cancers (PCA). One major mechanism by which the environment can influence carcinogenesis is oxidative damage. This refers to the generation of reactive oxygen species (ROS) that then damage important biomolecules, including DNA, protein, and lipids. Experimental observations suggest that oxidative damage is associated with PCA. These include: a) the association of PCA and dietary fat consumption (a major substrate for oxidative stress), b) oxidative biomarker data (suggesting increased oxidative stress among patients with PCA), c) ubiquitous defects in the glutathione-s-transferase pi pathway (a major endogenous antioxidant mechanism), and d) evidence that androgens (an important promoter of PCA growth) work in part via generation of ROS. Perhaps the best indirect evidence for oxidative stress comes from randomized double-blind prevention trials of antioxidants. Vitamin E and selenium have both been shown to reduce prostate cancer incidence. Although PCA prevention was not the primary endpoint of these studies, the statistical likelihood that both would prove beneficial by chance alone is 1 in 400. These data suggest that antioxidants may be beneficial in preventing PCA. Further research including randomized trials is warranted.

Carotenoids and chronic diseases

Chronic diseases such as cancer and cardiovascular diseases are the major causes of deaths in North America. Dietary intake of fruits and vegetables has been suggested to have protective effects against such chronic diseases. Carotenoids are important plant pigments which are thought to contribute towards the beneficial effects of fruit and vegetable consumption. This review focuses on the role of carotenoids and particularly lycopene in chronic diseases.

Gene expression and the thiol redox state

The intracellular redox status is a tightly regulated parameter which provides the cell with an optimal ability to counteract the highly oxidizing extracellular environment. Intracellular redox homeostasis is regulated by thiol-containing molecules, such as glutathione and thioredoxin. Essential cellular functions, such as gene expression, are influenced by the balance between pro- and antioxidant conditions. The mechanism by which the transcription of specific eukaryotic genes is redox regulated is complex, however, recent findings suggest that redox-sensitive transcription factors play an essential role in this process. This review is focused on the recent knowledge concerning some eukaryotic transcription factors, whose activation and DNA binding is controlled by the thiol redox status of the cell.

Thioredoxin in the endocrine response to stress

Adaptation to stress evokes a variety of biological responses, including activation of the hypothalamic--pituitary--adrenal (HPA) axis and synthesis of a panel of stress-response proteins at cellular levels: for example, expression of thioredoxin (TRX) is significantly induced under oxidative conditions. Glucocorticoids, as a peripheral effector of the HPA axis, exert their action via interaction with a ligand-inducible transcription factor glucocorticoid receptor (GR). However, how these stress responses coordinately regulate cellular metabolism is still unknown. We demonstrate that either antisense TRX expression or cellular treatment with H2O2 negatively modulates GR function and decreases glucocorticoid-inducible gene expression. Impaired cellular response to glucocorticoids is rescued by overexpression of TRX, most probably through the functional replenishment of the GR. Moreover, not only the ligand binding domain but the DNA binding domain of the GR is also suggested to be a direct target of TRX. Together, we propose that cellular glucocorticoid responsiveness is coordinately modulated by redox state and TRX level, suggesting that cross-talk between neuro-endocrine control of stress responses and cellular antioxidant systems may be essential for mammalian adaptation processes.

Roles of reactive oxygen species: signaling and regulation of cellular functions

Reactive oxygen species (ROS) are the side products (H2O2, O2.-, and OH.) of general metabolism and are also produced specifically by the NADPH oxidase system in most cell types. Cells have a very efficient antioxidant defense to counteract the toxic effect of ROS. The physiological significance of ROS is that ROS at low concentrations are able to mediate cellular functions through the same steps of intracellular signaling, which are activated by natural stimuli. Moreover, a variety of natural stimuli act through the intracellular formation of ROS that change the intracellular redox state (oxidation-reduction). Thus, the redox state is a part of intracellular signaling. As such, ROS are now considered signal molecules at nontoxic concentrations. Progress has been achieved in studying the oxidative activation of gene transcription in animal cells and bacteria. Changes in the redox state of intracellular thiols are considered to be an important mechanism that regulates cell functions.

Tumor promotion by hydrogen peroxide in rat liver epithelial cells

Reactive oxygen species, including H2O2, play an important role in the tumor promotion process. Using an in vitro model of tumor promotion involving the rat liver epithelial oval cell line T51B, the tumor promoting activity of H2O2 in N-methyl-N'-nitro-N-nitrosoguanidine-initiated cells was studied. In this assay system, the promoting effect of H2O2 is evidenced by the formation of colonies in soft agar, appearance of foci in monolayer culture, disruption of gap junction communication (GJC) in foci areas and growth at higher saturation densities. H2O2 preferentially induced the expression of c-fos, c-jun, c-myc and egr-1, while JunB and JunD levels remained almost unchanged. H2O2 also induced hyperphosphorylation of Cx43 and disruption of GJC. The effects of H2O2 on tumor promotion, induction of immediate early (IE) genes and disruption of GJC are blocked by antioxidants. These results suggest that H2O2 acts as a tumor promoter in rat liver non-neoplastic epithelial cells and that the induction of IE genes and disruption of GJC are two possible targets of H2O2 during the tumor promotion process.

Direct association with thioredoxin allows redox regulation of glucocorticoid receptor function

The glucocorticoid receptor (GR) is considered to belong to a class of transcription factors, the functions of which are exposed to redox regulation. We have recently demonstrated that thioredoxin (TRX), a cellular reducing catalyst, plays an important role in restoration of GR function in vivo under oxidative conditions. Although both the ligand binding domain and other domains of the GR have been suggested to be modulated by TRX, the molecular mechanism of the interaction is largely unknown. In the present study, we hypothesized that the DNA binding domain (DBD) of the GR, which is highly conserved among the nuclear receptors, is also responsible for communication with TRX in vivo. Mammalian two-hybrid assay and glutathione S-transferase pull-down assay revealed the direct association between TRX and the GR DBD. Moreover, analysis of subcellular localization of TRX and the chimeric protein harboring herpes simplex viral protein 16 transactivation domain and the GR DBD indicated that the interaction might take place in the nucleus under oxidative conditions. Together these observations indicate that TRX, via a direct association with the conserved DBD motif, may represent a key mediator operating in interplay between cellular redox signaling and nuclear receptor-mediated signal transduction.

Redox regulation of the glucocorticoid receptor

Redox regulation is currently considered as a mode of signal transduction for coordinated regulation of a variety of cellular processes. The transcriptional regulation of gene expression is also influenced by cellular redox state, most possibly through the oxido-reductive modification of transcription factors. The glucocorticoid receptor belongs to a nuclear receptor superfamily and acts as a ligand-dependent transcription factor. We demonstrate that the glucocorticoid receptor function is regulated via redox-dependent mechanisms at multiple levels. Moreover, it is suggested that redox regulation of the receptor function is one of dynamic cellular responses to environmental stimuli and plays an important role in orchestrated crosstalk between central and peripheral stress responses.

Role of far upstream repressor elements controlling proto-Ha-ras gene transcription

The far upstream region of the rat Ha-ras gene has been characterized to determine whether possible repressor sequences may control the low level of Ha-ras gene transcription from its TATA-less, GC-rich strong promoter. The chloramphenicol acetyl transferase (CAT) gene under the control of the 3.8-kb Ha-ras upstream promoter was minimally expressed in HeLa cells. Surprisingly, CAT gene expression was increased by the deletion of a 0.7-kb BglII fragment containing non-coding exon minus 2 and TATA box promoter elements located 1.7 kb upstream of the GC-rich strong promoter. Far upstream (CA)25 repeats also appeared to repress Ha-ras gene activity. Sequences within the 0.7-kb BglII fragment suppressed CAT gene expression when placed upstream of a heterologous thymidine kinase (tk) gene promoter. Repressor activity was further localized to a 160-bp AvrII-BglII sub-fragment. Gel shift assays identified two sequence-specific DNA binding proteins. The results demonstrated for the first time that far upstream repressor sequences control normal transcription of the Ha-ras proto-oncogene.

Purification, molecular cloning, and characterization of TRP32, a novel thioredoxin-related mammalian protein of 32 kDa

We purified a protein of 32 kDa from human thymoma HPB-ALL cells that was co-purified with a catalytic fragment of MST (mammalian STE-20-like), a kinase of the STE20 family, which is proteolytically activated by caspase in apoptosis (Lee, K.-K., Murakawa, M., Nishida, E., Tsubuki, S., Kawashima, S., Sakamaki, K., and Yonehara, S. (1998) Oncogene 16, in press). Molecular cloning of the gene encoding this 32-kDa protein (TRP32) reveals that it is a novel protein of 289 amino acid residues and contains an NH2-terminal thioredoxin domain with a conserved thioredoxin active site. The human and mouse TRP32 proteins have 99% homology, and the thioredoxin domains are completely identical. The thioredoxin domain of TRP32 has thioredoxin-like reducing activity, which can reduce the interchain disulfide bridges of insulin in vitro. However, the thioredoxin domain of TRP32 is more sensitive to oxidation than human thioredoxin. Northern blot analysis showed that TRP32 is expressed in all human tissues. Expression of TRP32 was also confirmed in all mammalian cell lines tested by Western blot analysis using anti-TRP32 monoclonal antibody. Subcellular fractionation and immunostaining analysis showed TRP32 is cytoplasmic protein. These findings suggest that TRP32 is a novel cytoplasmic regulator of the redox state in higher eukaryotes.

Identification of redox-sensitive cysteines in GA-binding protein-alpha that regulate DNA binding and heterodimerization

The transcription factor GA-binding protein (GABP) is composed of two subunits, GABPalpha and GABPbeta. The DNA-binding subunit, GABPalpha, is a member of the Ets family of transcription factors, characterized by the conserved Ets-domain that mediates DNA binding and associates with GABPbeta, which lacks a discernible DNA binding domain, through ankyrin repeats in the NH2 terminus of GABPbeta. We previously demonstrated that GABP is subject to redox regulation in vitro and in vivo through four COOH-terminal cysteines in GABPalpha. To determine the roles of individual cysteines in GABP redox regulation, we generated a series of serine substitution mutants by site-directed mutagenesis and identified three redox-sensitive cysteine residues in GABPalpha (Cys388, Cys401, and Cys421). Sulfhydryl modification of Cys388 and Cys401 inhibits DNA binding by GABPalpha, whereas, modification of Cys421 has no effect on GABPalpha DNA binding but inhibits dimerization with GABPbeta. The positions of Cys388 and Cys401 within the known Ets-domain structure suggest two very different mechanisms for redox regulation of DNA binding. Sulfhydryl modification of Cys388 could directly interfere with DNA binding or might alter the positioning of the DNA-binding helix 3. Modification of Cys401 may inhibit DNA binding through stabilization of an inhibitory helix similar to that described in the Ets-1 protein. Thus, GABP is regulated through at least two redox-sensitive activities, DNA binding and heterodimerization.

Redox regulation of genes that protect against carcinogens

Most carcinogens require activation to electrophilic metabolites or species that generate reactive oxygen in order to initiate the tumorigenic process. These reactive intermediates can, in turn, be detoxified by endogenous enzyme systems that and in the protection of cells from either toxic or mutagenic product formation. The levels of many of these enzymes are elevated by numerous compounds found in the diet, or by antioxidants. Recent evidence describes the mechanism for this induction of carcinogen detoxication enzymes to be regulated at the transcriptional level. Nuclear transcription factors bound to sites common among these carcinogen detoxication genes are activated by as yet unknown signal transduction pathways. The activity of these nuclear transcription factors are modulated by pro- and antioxidant reagents, suggesting that a redox-sensitive component governs the induction of enzymes involved in carcinogen metabolism. In this review, evidence for the redox regulation of the genes encoding carcinogen detoxication enzymes is presented. Evidence is also presented suggesting the participation of nuclear factor kappa B (NF-kappa B), mitogen-activated protein (MAP) kinase, and basic leucine zipper (bZIP) proteins and their activation pathways in this induction.

Redox regulation of the DNA binding activity in transcription factor PEBP2. The roles of two conserved cysteine residues

Transcription factor PEBP2/CBF consists of a DNA binding subunit, alpha, and a regulatory subunit, beta. The alpha subunit has an evolutionarily conserved 128-amino acid region termed "Runt domain" that is responsible for both DNA binding and heterodimerization with the beta subunit. The Runt domain in all mammalian submembers of the alpha subunit contains two conserved cysteine residues, and its DNA binding activity undergoes redox regulation. To investigate the mechanism of this redox regulation, we performed site-directed mutagenesis of the two conserved cysteines in the Runt domain of the mouse PEBP2alphaA homolog. Substitution of Cys-115 to serine resulted in a partially impaired DNA binding, which remained highly sensitive to a thiol-oxidizing reagent, diamide. Conversely, the corresponding substitution of Cys-124 caused an increased DNA binding concomitant with an increased resistance to diamide. In contrast, substitution of either cysteine to aspartate was destructive to DNA binding to marked extents. These results have revealed that both Cys-115 and Cys-124 are responsible for the redox regulation in their own ways with low and high oxidizabilities, respectively. We have also found that two cellular thiol-reactive proteins, thioredoxin and Ref-1, work effectively and synergistically for activation of the Runt domain. Interestingly, the beta subunit further enhanced the activation by these proteins and reciprocally prevented the oxidative inactivation by diamide. These findings collectively suggest the possibility that the Runt domain's function in vivo could be dynamically regulated by the redox mechanism with Trx, Ref-1, and the beta subunit as key modulators.

AP-1 transcriptional activity is regulated by a direct association between thioredoxin and Ref-1

Thioredoxin (TRX) is a pleiotropic cellular factor that has thiol-mediated redox activity and is important in regulation of cellular processes, including proliferation, apoptosis, and gene expression. The activity of several transcription factors is posttranslationally altered by redox modification(s) of specific cysteine residue(s). One such factor is nuclear factor (NF)-kappa B, whose DNA-binding activity is markedly augmented by TRX treatment in vitro. Similarly, the DNA-binding activity of activator protein 1 (AP-1) is modified by a DNA repair enzyme, redox factor 1 (Ref-1), which is identical to a DNA repair enzyme, AP endonuclease. Ref-1 activity is in turn modulated by various redox-active compounds, including TRX. We here report the molecular cascade of redox regulation of AP-1 mediated by TRX and Ref-1. Phorbol 12-myristate 13 acetate efficiently translocated TRX into the HeLa cell nucleus where Ref-1 preexists. This process seems to be essential for AP-1 activation by redox modification because co-overexpression of TRX and Ref-1 in COS-7 cells potentiated AP-1 activity only after TRX was transported into the nucleus by phorbol 12-myristate 13 acetate treatment. To prove the direct active site-mediated association between TRX and Ref-1, we generated a series of substitution-mutant cysteine residues of TRX. In both an in vitro diamide-induced cross-linking study and an in vivo mammalian two-hybrid assay we proved that TRX can associate directly with Ref-1 in the nucleus; also, we demonstrated the requirement of cysteine residues in the TRX catalytic center for the potentiation of AP-1 activity. This report presents an example of a cascade in cellular redox regulation.

Functional dissection of the alpha and beta subunits of transcription factor PEBP2 and the redox susceptibility of its DNA binding activity

The mouse transcription factor PEBP2 is a heterodimer of two subunits: a DNA binding subunit alpha and its partner subunit beta. The alpha subunit shares a region of high homology, termed the Runt domain, with the products of the Drosophila melanogaster segmentation gene runt and the human acute myeloid leukemia-related gene AML1. To study the molecular basis for the DNA binding and heterodimerization functions of this factor, we constructed series of deletions of the alpha and beta subunits and examined their activities by electrophoretic mobility shift and affinity column assays. The minimal functional region of the alpha subunit for DNA binding and dimerization was shown to coincide with the Runt domain. On the other hand, the region of the beta subunit required for heterodimerization was localized to the N-terminal 135 amino acids. Furthermore, it was found that the DNA binding activity of the Runt domain is regulated by a reduction/oxidization (redox) mechanism and that its reductively activated state, which is extremely labile, is stabilized by the beta subunit. These findings add a new layer to the mechanism and significance of the regulatory interplay between the two subunits of PEBP2.

Redox regulation of GA-binding protein-alpha DNA binding activity

We have investigated the reduction/oxidation (redox) regulation of the heteromeric transcription factor GA-binding protein (GABP). GABP, also known as nuclear respiratory factor 2, regulates the expression of nuclear encoded mitochondrial proteins involved in oxidative phosphorylation, including cytochrome c oxidase subunits IV and Vb, as well as the expression of mitochondrial transcription factor 1. GABP is composed of two subunits, the Ets-related GABP-alpha, which mediates specific DNA binding, and GABP-beta, which forms heterodimers and heterotetramers on DNA sequences containing the PEA3/Ets motif ((C/A)GGA(A/T)(G/A)). We demonstrate here that GABP DNA binding activity and GABP-dependent gene expression in 3T3 cells are inhibited by pro-oxidant conditions. DNA binding of recombinant GABP-alpha was activated by chemical reduction (dithiothreitol) and by thioredoxin; however, GSSG inhibited GABP DNA binding activity. Treatment of GABP-alpha, but not GABP-beta1, with sulfhydryl-alkylating agents also inhibited GABP DNA binding activity. Our results suggest that GABP DNA binding activity is redox-regulated in vivo, possibly by thioredoxin-mediated reduction and by GSSG-mediated oxidation of the GABP-alpha subunit. The regulation of GABP (nuclear respiratory factor 2) DNA binding activity by cellular redox changes provides an important link between mitochondrial and nuclear gene expression and the redox state of the cell.

The basic region/helix-loop-helix/leucine repeat transcription factor USF interferes with Ras transformation

Upstream stimulatory factor (USF) is a transcription factor of the basic region/helix-loop-helix/leucine repeat family. It shares the same DNA-binding sequence as the myc oncogene. Based on the three-dimensional structures, its DNA-binding domain is structurally related to that of Max, the partner of Myc. In addition, USF can form heterodimers with a related factor, Fos-interacting protein/upstream stimulatory factor 2 (FIP/USF2), which has been shown to directly interact with Fos. In view of the provocative relationship of USF with other factors involved in cell proliferation, we investigated whether USF could also play a role in cellular growth control. In this study, we report that USF is not an oncogene, but interferes with Ras-driven transformation. This inhibitory effect is independent of USF transactivating domains, but requires its DNA-binding activity. However, the minimal USF DNA-binding domain does not display this inhibitory effect, and even slightly enhances Ras transformation. On the basis of these data, we propose that USF may play an important role in the control of cell growth and proliferation, through both binding to promoter sequences and specific protein/protein interactions.

A conserved cysteine residue in the runt homology domain of AML1 is required for the DNA binding ability and the transforming activity on fibroblasts

The AML1 gene encodes DNA-binding proteins that contain the runt homology domain and is found at the breakpoints of t(8;21), t(3;21), and t(12;21) translocations associated with myelogenous leukemias. AML1 heterodimerizes with PEBP2beta/CBFbeta, resulting in the enhanced affinity with DNA. The runt homology domain is responsible for binding with DNA and heterodimerizing with PEBP2beta/CBFbeta. AML1 is suggested to perform a pivotal role in myeloid cell differentiation, whereas it can cause neoplastic transformation when overexpressed in fibroblasts. In this study, we demonstrated that the reducing reagent, dithiothreitol (DTT), markedly enhances the DNA binding of AML1 expressed in COS7 cells. Oxidation by diamide or modification by N-ethylmaleimide of the free sulfhydryl residues inhibited the interaction of AML1 with DNA. The diamide effect was reversible with excess of DTT, whereas DTT could not restore the DNA binding of AML1 treated with N-ethylmaleimide. Site-directed mutagenesis of the amino acid residue 72, a highly conserved cysteine in the runt homology domain of AML1, to serine almost completely abolished DNA binding without altering the interaction with PEBP2beta/CBFbeta. This substitution also impaired transactivation through the consensus DNA sequence and transformation of fibroblasts induced by AML1b. These data indicate an essential role of the conserved cysteine residue in DNA binding of AML1, and it is possible that the redox state of AML1 could contribute to the regulation of its function.

Characterization of the cooperative function of inhibitory sequences in Ets-1

DNA binding by the eukaryotic transcription factor Ets-1 is negatively regulated by an intramolecular mechanism. Quantitative binding assays compared the DNA-binding activities of native Ets-1, three deletion mutants, and three tryptic fragments. Ets-1 and activated Ets-1 polypeptides differed in DNA-binding affinity as much as 23-fold. Inhibition was mediated by two regions flanking the minimal DNA-binding domain. Both regions regulated affinity by enhancing dissociation of the protein-DNA complex. Three lines of evidence indicated that inhibition requires cooperative interaction between the two regions: first, the two inhibitory regions acted through a common mechanism; second, neither region functioned independently of the other; finally, mutation of the C-terminal inhibitory region altered the conformation of the N-terminal inhibitory region. In addition, partial proteolysis detected an identical altered conformation in the N-terminal inhibitory region of Ets-1 bound to DNA. This finding suggested that repression is transiently disrupted during DNA binding. These results provide evidence that the two inhibitory regions of Ets-1 are structurally, as well as functionally, coupled. In addition, conformational change is shown to be a critical component of the inhibition mechanism. A cooperative, allosteric model of autoinhibition is described. Autoinhibition of Ets-1 could be relieved by either protein partner(s) or posttranslational modifications.

Redox regulation of transcriptional activators

Transcription factors/activators are a group of proteins that bind to specific consensus sequences (cis elements) in the promoter regions of downstream target/effector genes and transactivate or repress effector gene expression. The up- or downregulation of effector genes will ultimately lead to many biological changes such as proliferation, growth suppression, differentiation, or senescence. Transcription factors are subject to transcriptional and posttranslational regulation. This review will focus on the redox (reduction/oxidation) regulation of transcription factors/activators with emphasis on p53, AP-1, and NF-kappa B. The redox regulation of transcriptional activators occurs through highly conserved cysteine residues in the DNA binding domains of these proteins. In vitro studies have shown that reducing environments increase, while oxidizing conditions inhibit sequence-specific DNA binding of these transcriptional activators. When intact cells have been used for study, a more complex regulation has been observed. Reduction/oxidation can either up- or downregulate DNA binding and/or transactivation activities in transcriptional activator-dependent as well as cell type-dependent manners. In general, reductants decrease p53 and NF-kappa B activities but dramatically activate AP-1 activity. Oxidants, on the other hand, greatly activate NF-kappa B activity. Furthermore, redox-induced biochemical alterations sometimes lead to change in the biological functions of these proteins. Therefore, differential regulation of these transcriptional activators, which in turn, regulate many target/effector genes, may provide an additional mechanism by which small antioxidant molecules play protective roles in anticancer and antiaging processes. Better understanding of the mechanism of redox regulation, particularly in vivo, will have an important impact on drug discovery for chemoprevention and therapy of human disease such as cancer.

Superoxide and hydrogen peroxide in relation to mammalian cell proliferation

A wide variety of normal and malignant cell types generate and release superoxide or hydrogen peroxide in vitro either in response to specific cytokine/growth factor stimulus or constitutively in the case of tumour cells. These species at submicromolar levels appear to act as novel intra and intercellular "messengers" capable of promoting growth responses in culture. The mechanisms may involve direct interaction with specific receptors or oxidation of growth signal transduction molecules such as protein kinases, protein phosphatases, transcription factors, or transcription factor inhibitors. It is also possible that hydrogen peroxide may modulate the redox state and activity of these important signal transduction proteins indirectly through changes in cellular levels of GSH and GSSG. Critical balances appear to exist in relation to cell proliferation on one hand and lipid peroxidation and cell death on the other. Progression to a more prooxidant state whilst initially leading to enhanced proliferative responses results subsequently in increased cell death.

Redox signalling and the control of cell growth and death

Cells maintain a reduced intracellular state in the face of a highly oxidizing extracellular environment. Redox signalling pathways provide a link between external stimuli, through the flavoenzyme-mediated NADPH-dependent reduction of intracellular peptide thiols, such as glutathione, thioredoxin, glutaredoxin, and redox factor-1, to the posttranslational redox modification of certain intracellular proteins. This can affect the proteins' correct folding, assembly into multimeric complexes, enzymatic activity, and their binding as transcription factors to specific DNA sequences. Such changes have been linked to altered cell growth and death.

A residue of the ETS domain mutated in the v-ets oncogene is essential for the DNA-binding and transactivating properties of the ETS-1 and ETS-2 proteins

The c-ets-1 locus encodes two transcription factors, p54c-ets-1 and p68c-ets-1 that recognize purine-rich motifs. The v-ets oncogene of the avian retrovirus E26 differs from its cellular progenitor p68c-ets-1 by two amino acid substitutions (alanine 285 and isoleucine 445 in c-ets-1 both substituted by valine in v-ets, mutations A and B respectively) and its carboxy-terminal end (mutation C). The B mutation affects a well conserved residue in the carboxy-terminal 85 amino acids, ETS DNA-binding domain. To address the biological relevance of the B mutation found between v-ets and c-ets-1, we have randomly mutagenized isoleucine 445 of p68c-ets-1 by polymerase chain reaction. Using in vitro gel mobility shift assays, we show that this residue is crucial for the binding properties of c-ets-1 since the 12 mutations we have generated at this position, all diminish or even abolish the binding, to the 'optimized' Ets-1 binding site (EBS), of 35 kDa proteins corresponding to the 311 carboxy-terminal residues of c-ets-1. Among them, substitutions of isoleucine to glutamic acid, glycine or proline have the highest inhibitory effects. Similar results were obtained when the same mutations were introduced either in full-length p68c-ets-1 protein or into a carboxy-terminal polypeptide of 109 amino acids encompassing the ETS-domain which has previously been shown to display a very high binding activity as compared with the full-length protein. Consistent with the in vitro results, point mutations in p68c-ets-1 that decrease binding activity to EBS abrogate its ability to transactivate reporter plasmids carrying either the TPA Oncogene Response Unit of the Polyoma virus enhancer (TORU) or a sequence derived from the HTLV-1 LTR. Furthermore, as this isoleucine residue is rather well-conserved within the ETS gene family, we show that mutation of the corresponding isoleucine of c-ets-2 into glycine also abrogates its DNA-binding and hence, transactivating properties. Thus, the v-ets B mutation highlights the isoleucine 445 as an essential amino acid of the c-ets-1 and c-ets-2 DNA-binding domains.

Endogenously generated active oxygen species and cellular glutathione levels in relation to BHK-21 cell proliferation

In BHK-21 cells (baby hamster kidney fibroblasts) cellularly generated active oxygen species such as hydrogen peroxide and superoxide appear to be important growth regulatory signals as judged from the growth inhibitory effects of catalase, superoxide dismutase and superoxide dismutase mimics. These active oxygen species may contribute a novel redox system of regulatory control superimposed upon established growth signal pathways. This may be achieved by direct oxidative modification of cell regulatory proteins such as transcription factors or protein kinases or indirectly through, for example alterations in levels of glutathione (GSH). This latter possibility is suggested from observations that catalase, or superoxide dismutase treatment of BHK-21 cells brings about increased cellular levels of GSH. However during the normal growth phase cellular levels of GSH actually decline although this effect can be partly reversed by N-acetylcysteine and by mercaptosuccinate which also impair proliferation of these cells.

Characterization of baculovirus recombinant wild-type p53. Dimerization of p53 is required for high-affinity DNA binding and cysteine oxidation inhibits p53 DNA binding

A high-yield, rapid and non-denaturing purification protocol for baculovirus recombinant wild-type p53 is described. Gel-filtration chromatography and chemical cross-linking experiments indicated that purified p53 assembles into multimeric forms ranging from tetramer to higher oligomers. A gel-mobility-shift assay and protein-DNA cross-linking studies demonstrated that purified baculovirus recombinant p53 binds to consensus DNA target as a dimer but that additional p53 molecules may then associate with the preformed p53-dimer-DNA complexes to form larger p53 DNA complexes. These observations suggest that the p53 tetramers and higher oligomers that form the minimal p53 association in solution dissociate upon DNA binding to form p53 dimer-DNA complexes. Binding of the mAB PAb 421 to the oligomerization-promoting domain on p53 stimulated sequentially formation of both p53-dimer-DNA and larger p53-DNA complexes. This observation suggests that factors may exist in vivo that could participate in the formation and the stabilization of the various p53-DNA complexes. Further characterization of the purified p53 revealed that the protein possesses highly reactive cysteine residues. We show that intrachain disulfide bonds form within the purified p53 molecules during storage in the absence of reducing agent. Zn2+ binding to p53 protect sulfhydryl groups from oxidation. Cysteine oxidation by intramolecular disulfide-bond formation did not modify the wild-type immunoreactive phenotype of the p53 protein but totally inhibited its DNA-binding activities. The oxidation of the p53 cysteine residues was also observed for nuclear p53 in baculovirus-infected insect cells. The redox status of the nuclear p53 regulates its DNA-binding activity in vitro confirming the essential role of the reduced state of cysteine residues in p53 for detectable DNA-binding activity.

Epitope mapping of human ETS1 monoclonal antibody

The epitope for E44 monoclonal antibody (mAb) was mapped using mutated ETS1 proteins lacking different carboxy-terminal regions and by the employment of synthetic oligopeptides spanning the epitope region. This epitope lies around Arg211 of the human ETS1 protein since substitution of Arg211 by Gln211 in the epitope region results in the loss of recognition of the mouse ETS1 protein by E44 mAb. Substitution of Leu214 by valine214 in the epitope region (as is found in the chicken ETS1 and viral Ets proteins) does not alter the capacity of the E44 mAb to recognize this antigen. Taken together, these results suggest that a specific ionic interaction is able to play a pivotal role in the recognition of the ETS1 protein by the E44 mAb.

Cellularly generated active oxygen species and HeLa cell proliferation

In HeLa cells evidence is provided that active oxygen species such as hydrogen peroxide and superoxide at low levels are important growth regulatory signals. They may constitute a novel regulatory redox system of control superimposed upon the established cell growth signal transduction pathways. Whilst for example hydrogen peroxide can be added exogenously to elicit growth responses in these cells, it is clear that cellularly generated superoxide and hydrogen peroxide are important. Experiments with superoxide dismutase, superoxide dismutase mimics and inhibitors of both superoxide dismutase and xanthine oxidase suggest that superoxide generated intracellularly and superoxide released extracellularly are both relevant to growth control in HeLa cells.