Redox activation of Fos-Jun DNA binding activity is mediated by a DNA repair enzyme

Repression of gene expression by oxidative stress

Gene expression is modulated by both physiological signals (hormones, cytokines, etc.) and environmental stimuli (physical parameters, xenobiotics, etc.). Oxidative stress appears to be a key pleiotropic modulator which may be involved in either pathway. Indeed, reactive oxygen species (ROS) have been described as second messengers for several growth factors and cytokines, but have also been shown to rise following cellular insults such as xenobiotic metabolism or enzymic deficiency. Extensive studies on the induction of stress-response genes by oxidative stress have been reported. In contrast, owing to the historical focus on gene induction, less attention has been paid to gene repression by ROS. However, a growing number of studies have shown that moderate (i.e. non-cytotoxic) oxidative stress specifically down-regulates the expression of various genes. In this review, we describe the alteration of several physiological functions resulting from oxidative-stress-mediated inhibition of gene transcription. We will then focus on the repressive oxidative modulation of various transcription factors elicited by ROS.

Characterization of Bacillus subtilis ExoA protein: a multifunctional DNA-repair enzyme similar to the Escherichia coli exonuclease III

To discover the physiological role of the Bacillus subtilis ExoA protein, which is similar in amino acid sequence to Escherichia coli exonuclease III, an exoA::Cm disruption was constructed in the chromosomal DNA of B. subtilis. There was no clear difference in tolerance to hydrogen peroxide and alkylating agents between the disruptant and the wild type strain. An expression plasmid of the ExoA in E. coli was constructed by inserting the exoA gene into the expression vector pKP1500. The purified ExoA was used to clarify enzymatic characterizations using synthetic DNA oligomers as substrates. A DNA oligomer containing a 1', 2'-dideoxyribose residue as an AP site, a DNA-RNA chimera oligomer, and a 3' end 32P-labeled oligomer were synthesized. It has been shown that the ExoA has AP endonuclease, 3'-5' exonuclease, ribonuclease H, and 3'-phosphomonoesterase activities. Thus, it has been confirmed that ExoA is a multifunctional DNA-repair enzyme in B. subtilis that is very similar to E. coli exonuclease III except that ExoA has lower 3'-5' exonuclease activity than that of E. coli exonuclease III.

Redox regulation of c-Jun DNA binding by reversible S-glutathiolation

Redox control of the transcription factor c-Jun maps to a single cysteine in its DNA binding domain. However, the nature of the oxidized state of this cysteine and, thus, the potential molecular mechanisms accounting for the redox regulation of c-Jun DNA binding remain unclear. To address this issue, we have analyzed the purified recombinant c-Jun DNA binding domain for redox-dependent thiol modifications and concomitant changes in DNA binding activity. We show that changes in the ratio of reduced to oxidized glutathione provide the potential to oxidize c-Jun sulfhydryls by mechanisms that include both protein disulfide formation and S-glutathiolation. We provide evidence that S-glutathiolation, which is specifically targeted to the cysteine residue located in the DNA binding site of the protein, may account for the reversible redox regulation of c-Jun DNA binding. Furthermore, based on a molecular model of the S-glutathiolated protein, we discuss the structural elements facilitating S-glutathiolation and how this modification interferes with DNA binding. Given the structural similarities between the positively charged cysteine-containing DNA binding motif of c-Jun and the DNA binding site of related oxidant-sensitive transcriptional activators, the unprecedented phenomenon of redox-triggered S-thiolation of a transcription factor described in this report suggests a novel role for protein thiolation in the redox control of transcription.

Post-translational regulation of AP-1 transcription factor DNA-binding activity in the rat conceptus

Activator protein-1 (AP-1) transcription factor DNA binding is induced during transient oxidative stress in the midorganogenesis rat conceptus in culture. L-2-Oxothiazolidine-4-carboxylate (OTC), a cysteine prodrug, prevented oxidative stress and the induction of AP-1 binding activity in the embryo but not in the yolk sac. Because AP-1 activity may be a significant determinant of developmental outcome after insult, we investigated the regulation of AP-1 activity in the conceptus. Supershift assays indicated that basal AP-1 binding in the embryo was due primarily to JunD, whereas in the yolk sac c-Jun and JunD were important. Under oxidative stress, c-Fos and c-Jun contributed to the AP-1 binding in the embryo; in the yolk sac, a c-Fos-shifted complex emerged. OTC protection from oxidative stress did not change the AP-1 composition, suggesting that increased AP-1 activity was due to post-translational modifications. Changes in AP-1 activity in embryos under oxidative stress or with OTC protection were not the result of alterations in the net phosphorylation state of Fos or Jun proteins or of changes in activities of the extracellular signal-regulated kinases 1 and 2 or stress-activated protein kinases. However, immunodepletion of redox factor 1 (Ref-1), a nuclear factor that promotes AP-1 binding, eliminated AP-1 activity from embryonic nuclear extracts under both basal and oxidative stress conditions. Therefore, Ref-1 plays a critical role in regulating AP-1 activity in the conceptus; it is plausible that Ref-1-mediated modulation of the AP-1 stress response is a determinant of embryonic fate.

Cys9, Cys104 and Cys207 of simian virus 40 Vp1 are essential for inter-pentamer disulfide-linkage and stabilization in cell-free lysates

Previous studies have implicated disulfide bonds between Vp1 molecules in the stabilization of the simian virus 40 (SV40) capsid. To identify the cysteine residues involved in intermolecular disulfide interactions, systematic oligo-directed mutagenesis of cysteine codons to serine codons was initiated. Wild-type and mutant Vp1 proteins were produced in rabbit reticulocyte lysates and were allowed to interact post-translationally. Disulfide-linked Vp1 complexes were assessed via non-reducing SDS-PAGE and via sucrose-gradient sedimentation. Wild-type Vp1 forms 7S pentamers followed by 12S disulfide-linked multi-pentameric complexes in cell-free lysates. Mutagenesis of all seven cysteine codons abolished Vp1 12S complexes, but did not affect pentamer formation. A quadruple Vp1 mutant at Cys49, Cys87, Cys254 and Cys267 continued to form 12S complexes, whereas the major products of the Cys9, Cys104 and Cys207 triple mutant Vp1 were 7S pentamers. Single and double mutant Vp1 proteins at the three cysteines affected continued to form 12S complexes, but to a lesser extent. Thus, inter-pentamer disulfide bonds at Cys9, Cys104 and Cys207 are essential and sufficient for stabilization of Vp1 complexes in cell-free lysates. These results are in agreement with previous structural studies of SV40 that implicated the same three residues in disulfide linkage in the capsid. Possible parameters for the involvement of the three cysteines are discussed.

Thyroid hormone regulates reelin and dab1 expression during brain development

The reelin and dab1 genes are necessary for appropriate neuronal migration and lamination during brain development. Since these processes are controlled by thyroid hormone, we studied the effect of thyroid hormone deprivation and administration on the expression of reelin and dab1. As shown by Northern analysis, in situ hybridization, and immunohistochemistry studies, hypothyroid rats expressed decreased levels of reelin RNA and protein during the perinatal period [embryonic day 18 (E18) and postnatal day 0 (P0)]. The effect was evident in Cajal-Retzius cells of cortex layer I, as well as in layers V/VI, hippocampus, and granular neurons of the cerebellum. At later ages, however, Reelin was more abundant in the cortex, hippocampus, cerebellum, and olfactory bulb of hypothyroid rats (P5), and no differences were detected at P15. Conversely, Dab1 levels were higher at P0, and lower at P5 in hypothyroid animals. In line with these results, reelin RNA and protein levels were higher in cultured hippocampal slices from P0 control rats compared to those from hypothyroid animals. Significantly, thyroid-dependent regulation of reelin and dab1 was confirmed in vivo and in vitro by hormone treatment of hypothyroid rats and organotypic cultures, respectively. In both cases, thyroid hormone led to an increase in reelin expression. Our data suggest that the effects of thyroid hormone on neuronal migration may be in part mediated through the control of reelin and dab1 expression during brain ontogenesis.

Transcriptional activation of apurinic/apyrimidinic endonuclease (Ape, Ref-1) by oxidative stress requires CREB

Apurinic/apyrimidinic endonuclease (APE alias Ref-1) is a multifunctional enzyme involved in DNA repair and redox regulation of transcription factors (e.g., AP-1). It also acts as a repressor of its own and other genes. Recently, it was shown that the level of APE mRNA and protein is enhanced upon treatment of cells with oxidative agents, such as hydrogen peroxide (H(2)O(2)), which gives rise to an adaptive response of cells to oxidative stress. Induction of APE is due to APE promoter activation. To elucidate the mechanism of transcriptional activation of APE by oxidative agents, we introduced mutations into the cloned human APE promoter and checked its activity in transient transfection assays. Here we demonstrate that mutational inactivation of a CREB binding site (CRE) present within the promoter completely abolished APE promoter activation by H(2)O(2), indicating that CREB is required for APE induction. The CRE element in the context of the APE promoter sequence binds c-Jun and ATF-2, which was shown in gel retardation experiments. Under conditions of induction of APE by H(2)O(2), the expression of c-Jun was significantly enhanced, which supports the view that induction of c-Jun is involved in signaling leading to APE promoter activation by oxidative stress.

Improved immunodetection of nuclear antigens after sodium dodecyl sulfate treatment of formaldehyde-fixed cells

Immunostaining techniques are commonly employed to determine the temporal and spatial patterns of cellular components in in situ preparations of cells and tissues. Usually, cells are formalin-fixed, permeabilized with nonionic detergents, and probed with specific antibodies. The incorporation of a sodium dodecyl sulfate (SDS) treatment after chemical crosslinking has been shown to improve the immunodetection of some cytosolic and cell surface antigens. By incorporating an SDS treatment after crosslinking, we report a significant improvement in the detection of two nuclear antigens (i.e.) the DNA binding proteins apurinic/apyrimidinic endonuclease and DNA polymerase-beta) and bromodeoxyuridine-tagged DNA by indirect immunofluorescence of whole cells. In bromodeoxyuridine-tagged DNA, the improvement in detection after an SDS treatment was observed only after long incorporation protocols (>48 hr) and, interestingly, it was more pronounced in cultured human foreskin keratinocytes than in bovine aorta endothelial cells. In addition, the SDS treatment proved in these studies to be superior to the standard Triton X-100 permeabilization. SDS thus provides a potential means to visualize previously undetectable or poorly detectable nuclear antigens.

Identification of thioredoxin-binding protein-2/vitamin D(3) up-regulated protein 1 as a negative regulator of thioredoxin function and expression

Recent works have shown the importance of reduction/oxidation (redox) regulation in various biological phenomena. Thioredoxin (TRX) is one of the major components of the thiol reducing system and plays multiple roles in cellular processes such as proliferation, apoptosis, and gene expression. To investigate the molecular mechanism of TRX action, we used a yeast two-hybrid system to identify TRX-binding proteins. One of the candidates, designated as thioredoxin-binding protein-2 (TBP-2), was identical to vitamin D(3) up-regulated protein 1 (VDUP1). The association of TRX with TBP-2/VDUP1 was observed in vitro and in vivo. TBP-2/VDUP1 bound to reduced TRX but not to oxidized TRX nor to mutant TRX, in which two redox active cysteine residues are substituted by serine. Thus, the catalytic center of TRX seems to be important for the interaction. Insulin reducing activity of TRX was inhibited by the addition of recombinant TBP-2/VDUP1 protein in vitro. In COS-7 and HEK293 cells transiently transfected with TBP-2/VDUP1 expression vector, decrease of insulin reducing activity of TRX and diminishment of TRX expression was observed. These results suggested that TBP-2/VDUP1 serves as a negative regulator of the biological function and expression of TRX. Treatment of HL-60 cells with 1alpha, 25-dihydroxyvitamin D(3) caused an increase of TBP-2/VDUP1 expression and down-regulation of the expression and the reducing activity of TRX. Therefore, the TRX-TBP-2/VDUP1 interaction may be an important redox regulatory mechanism in cellular processes, including differentiation of myeloid and macrophage lineages.

Nuclear exclusion of transcription factors associated with apoptosis in developing nervous tissue

Programmed cell death in the form of apoptosis involves a network of metabolic events and may be triggered by a variety of stimuli in distinct cells. The nervous system contains several neuron and glial cell types, and developmental events are strongly dependent on selective cell interactions. Retinal explants have been used as a model to investigate apoptosis in nervous tissue. This preparation maintains the structural complexity and cell interactions similar to the retina in situ, and contains cells in all stages of development. We review the finding of nuclear exclusion of several transcription factors during apoptosis in retinal cells. The data reviewed in this paper suggest a link between apoptosis and a failure in the nucleo-cytoplasmic partition of transcription factors. It is argued that the nuclear exclusion of transcription factors may be an integral component of apoptosis both in the nervous system and in other types of cells and tissues.

Human alveolar macrophages are markedly deficient in REF-1 and AP-1 DNA binding activity

Although many functions of human alveolar macrophages are altered compared with their precursor cell, the blood monocyte (monocyte), the reason(s) for these functional changes have not been determined. We recently reported that human alveolar macrophages do not express AP-1 DNA binding activity (Monick, M. M., Carter, A. B., Gudmundsson, G., Geist, L. J., and Hunninghake, G. W. (1998) Am. J. Physiol. 275, L389-L397). To determine why alveolar macrophages do not express AP-1 DNA binding activity, we first showed that there was not a decrease in expression of the FOS and JUN proteins that make up the AP-1 complex. There was, however, a significant difference in the amounts of the nuclear protein, REF-1 (which regulates AP-1 DNA binding by altering the redox status of FOS and JUN proteins), in alveolar macrophages compared with monocytes. In addition, in vitro differentiation of monocytes to a macrophage-like cell resulted in decreased amounts of REF-1. Finally, addition of REF-1 from activated monocytes to alveolar macrophage nuclear proteins resulted in a marked increase in AP-1 DNA binding. These studies strongly suggest that the process of differentiation of monocytes into alveolar macrophages is associated with a loss of REF-1 and AP-1 activity. This observation may explain, in part, some of the functional differences observed for alveolar macrophages compared with monocytes.

Redox factor-1 (Ref-1) mediates the activation of AP-1 in HeLa and NIH 3T3 cells in response to heat shock

The early response genes, c-Fos and c-Jun, are induced by environmental stress and are thought to modulate injury processes via the induction of AP-1-dependent target genes. AP-1 activation is thought to be regulated by changes in intracellular oxidation/reduction reactions involving the redox factor-1 (Ref-1) protein. In this study, NIH 3T3 and HeLa cells were used to determine whether heat shock induces the AP-1 transcription factor via signaling pathways involving Ref-1. Reverse transcriptase-polymerase chain reaction analysis and immunoblotting demonstrated that c-Fos and c-Jun were induced 2-10 h following heat shock, and this induction was accompanied by an increase in AP-1 DNA binding. Electrophoretic mobility shift assay extracts immunodepleted of Ref-1 protein demonstrated that the increase in AP-1 DNA-binding activity following heating was dependent upon the presence of Ref-1 and that Ref-1 regulates inducible, but not basal, AP-1 DNA-binding activity. This was confirmed by the restoration of heat-inducible DNA binding upon addition of Ref-1 to immunodepleted extracts. The ability of Ref-1 from heated cells to stimulate AP-1 DNA binding was abolished by chemical oxidation and restored by chemical reduction. These results indicate that heat shock activates c-Fos/c-Jun gene expression and AP-1 DNA binding and suggests that redox-sensitive signal transduction pathways involving Ref-1 may mediate heat-induced alterations in AP-1 activation.

Determination of human DNA polymerase utilization for the repair of a model ionizing radiation-induced DNA strand break lesion in a defined vector substrate

Human DNA polymerase and DNA ligase utilization for the repair of a major class of ionizing radiation-induced DNA lesion [DNA single-strand breaks containing 3'-phosphoglycolate (3'-PG)] was examined using a novel, chemically defined vector substrate containing a single, site-specific 3'-PG single-strand break lesion. In addition, the major human AP endonuclease, HAP1 (also known as APE1, APEX, Ref-1), was tested to determine if it was involved in initiating repair of 3'-PG-containing single-strand break lesions. DNA polymerase beta was found to be the primary polymerase responsible for nucleotide incorporation at the lesion site following excision of the 3'-PG blocking group. However, DNA polymerase delta/straightepsilon was also capable of nucleotide incorporation at the lesion site following 3'-PG excision. In addition, repair reactions catalyzed by DNA polymerase beta were found to be most effective in the presence of DNA ligase III, while those catalyzed by DNA polymerase delta/straightepsilon appeared to be more effective in the presence of DNA ligase I. Also, it was demonstrated that the repair initiating 3'-PG excision reaction was not dependent upon HAP1 activity, as judged by inhibition of HAP1 with neutralizing HAP1-specific polyclonal antibody.

Expression levels of the DNA repair enzyme HAP1 do not correlate with the radiosensitivities of human or HAP1-transfected rat cell lines

Apurinic/apyrimidinic (AP) sites in DNA are potentially lethal and mutagenic. They can arise spontaneously or following DNA damage from reactive oxygen species or alkylating agents, and they constitute a significant product of DNA damage following cellular exposure to ionizing radiation. The major AP endonuclease responsible for initiating the repair of these and other DNA lesions in human cells is HAP1, which also possesses a redox function. We have determined the cellular levels of this enzyme in 11 human tumour and fibroblast cell lines in relation to clonogenic survival following ionizing radiation. Cellular HAP1 levels and surviving fraction at 2 Gy (SF2) varied five- and tenfold respectively. However, no correlation was found between these two parameters following exposure to gamma-irradiation at low (1.1 cGy per min) or high (108 cGy per min) dose rates. To examine this further, wild-type and mutant versions of HAP1 were overexpressed, using an inducible HAP1 cDNA expression vector system, in the rat C6 glioma cell line which has low endogenous AP endonuclease activity. Induction of wild-type HAP1 expression caused a > fivefold increase in the capacity of cellular extracts to cleave an oligonucleotide substrate containing a single abasic site, but increased expression did not confer increased resistance to gamma-irradiation at high- or low-dose rates, or to the methylating agent methyl methanesulphonate (MMS). Expression in C6 cell lines of mutant forms of HAP1 deleted for either the redox activator or DNA repair functions displayed no apparent titrational or dominant negative effects. These studies suggest that the levels of endogenous AP endonuclease activities in the various cell lines examined are not limiting for efficient repair in cells following exposure to ionizing radiation or MMS. This contrasts with the correlation we have found between HAP1 levels and radiosensitivity in cervix carcinomas (Herring et al (1998) Br J Cancer 78: 1128-1133), indicating that HAP1 levels in this case assume a critical survival role and hence that established cell lines might not be a suitable model for such studies.

Intracellular oxidation/reduction status in the regulation of transcription factors NF-kappaB and AP-1

The eukaryotic cell contains a multitude of pathways coupling environmental stimuli to the specific regulation of gene expression. Two early response transcriptional complexes, NF-kappaB and AP-1, appear to respond to environmental stressors by inducing the expression of response specific downstream genes. Both are well-characterized transcriptional regulatory factors that are induced by a wide variety of seemingly unrelated exogenous and endogenous agents and serve important roles in cell growth and differentiation, immunity, inflammation, and other preprogrammed cellular genetic processes. The activities of NF-kappaB and AP-1 are also affected following exposure to chemicals, drugs, or other agents that appear to alter the cellular oxidation/reduction (redox) status. From these observations, it has been suggested that changes in cellular oxidation/reduction status, communicated via a series of cellular redox-sensitive signaling circuitry employing metal- and thiol-containing proteins, serve as common mechanisms linking environmental stressors to adaptive cellular responses. As such, these transcription factors are ideal paradigms to study the mechanism and possible physiological significance of early response genes in the cellular response to changes in cellular redox status. In this article we summarize the evidence suggesting that cellular redox regulates these transcription factors.

Nitric oxide inhibits c-Jun DNA binding by specifically targeted S-glutathionylation

This study addresses potential molecular mechanisms underlying the inhibition of the transcription factor c-Jun by nitric oxide. We show that in the presence of the physiological sulfhydryl glutathione nitric oxide modifies the two cysteine residues contained in the DNA binding module of c-Jun in a selective and distinct way. Although nitric oxide induced the formation of an intermolecular disulfide bridge between cysteine residues in the leucine zipper site of c-Jun monomers, this same radical directed the covalent incorporation of stoichiometric amounts of glutathione to a single conserved cysteine residue in the DNA-binding site of the protein. We found that covalent dimerization of c-Jun apparently did not affect its DNA binding activity, whereas the formation of a mixed disulfide with glutathione correlated well with the inhibition of transcription factor binding to DNA. Furthermore, we provide experimental evidence that nitric oxide-induced S-glutathionylation and inhibition of c-Jun involves the formation of S-nitrosoglutathione. In conclusion, our results support the reversible formation of a mixed disulfide between glutathione and c-Jun as a potential mechanism by which nitrosative stress may be transduced into a functional response at the level of transcription.

The requirement of both intracellular reactive oxygen species and intracellular calcium elevation for the induction of heparin-binding EGF-like growth factor in vascular endothelial cells and smooth muscle cells

Heparin-binding EGF-like growth factor (HB-EGF), which is a potent mitogen for vascular smooth muscle cells (SMC) and fibroblasts, has been reported to be strongly implicated in atherosclerosis and wound healing. HB-EGF mRNA is known to be induced by thrombin, angiotensin-II, basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), and HB-EGF itself in SMC. In vascular endothelial cells (EC), its mRNA is induced by tumor necrosis factor-alpha and interleukin-1beta. Only phorbol 12-myristate 13-acetate is a common inducer for HB-EGF mRNA. The present study shows that calcium ionophore A23187 also induced HB-EGF mRNA in both SMC and in EC and that both intracellular reactive oxygen species (ROS) and an increase in calcium levels were essential for the induction of this growth factor mRNA. While HB-EGF caused an increase in both intracellular ROS and calcium in SMC, it increased only calcium, but not the intracellular ROS in EC. When the intracellular ROS was elevated by treatment with hydrogen peroxide (H2O2) or by depletion of glutathione by buthionine sulfoxamine, both HB-EGF and thrombin were observed to upregulate HB-EGF mRNA in EC. These data suggest that H2O2, produced by activated leukocytes in inflammatory lesions, upregulates HB-EGF mRNA by cooperating with thrombin, angiotensin-II, and the above growth factors. Since activated macrophages under the EC are thought to elevate the ROS in neighboring EC, this mechanism might play a major role in the progression of atherosclerosis and for wound healing.

Regulation of gene expression by reactive oxygen

Reactive oxygen intermediates are produced in all aerobic organisms during respiration and exist in the cell in a balance with biochemical antioxidants. Excess reactive oxygen resulting from exposure to environmental oxidants, toxicants, and heavy metals perturbs cellular redox balance and disrupts normal biological functions. The resulting imbalance may be detrimental to the organism and contribute to the pathogenesis of disease and aging. To counteract the oxidant effects and to restore a state of redox balance, cells must reset critical homeostatic parameters. Changes associated with oxidative damage and with restoration of cellular homeostasis often lead to activation or silencing of genes encoding regulatory transcription factors, antioxidant defense enzymes, and structural proteins. In this review, we examine the sources and generation of free radicals and oxidative stress in biological systems and the mechanisms used by reactive oxygen to modulate signal transduction cascades and redirect gene expression.

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.

Hematopoietic Growth Factors Signal Through the Formation of Reactive Oxygen Species

Hematopoietic growth factors (HGFs) stimulate growth, differentiation, and prevent apoptosis of progenitor cells. Each growth factor has a specific cell surface receptor, which activates both unique and shared signal transduction pathways. We found that several HGFs, including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), steel factor (SF), and thrombopoietin (TPO) induce a rapid increase in reactive oxygen species (ROS) in quiescent cells. In an effort to understand the potential biochemical and biological consequences of increased ROS in these cells, we exposed growth factor-deprived cells to hydrogen peroxide (H2O2) at concentrations that increased intracellular ROS. H2O2 induced a dose-dependent increase in tyrosine phosphorylation, including increased tyrosine phosphorylation of the GM-CSF receptor beta chain (βc), STAT5, and other signaling proteins. H2O2 also induced expression of the early response gene c-FOS, and G1- to S-phase transition, but not S- to G2/M-phase transition of MO7e cells. The cell permeable antioxidant pyrrolidine dithiocarbamate (PDTC) decreased the intracellular levels of ROS and inhibited tyrosine phosphorylation induced by GM-CSF in MO7e cells, suggesting that ROS generation plays an important role in GM-CSF signaling. Consistent with this notion, PDTC and two other antioxidants, N-acetyl cysteine and 2-mercaptoethanol, reduced growth and viability of MO7e cells. These results suggest that generation of ROS in response to HGFs may contribute to downstream signaling events, especially those involving tyrosine phosphorylation.

Hematopoietic Growth Factors Signal Through the Formation of Reactive Oxygen Species

Hematopoietic growth factors (HGFs) stimulate growth, differentiation, and prevent apoptosis of progenitor cells. Each growth factor has a specific cell surface receptor, which activates both unique and shared signal transduction pathways. We found that several HGFs, including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), steel factor (SF), and thrombopoietin (TPO) induce a rapid increase in reactive oxygen species (ROS) in quiescent cells. In an effort to understand the potential biochemical and biological consequences of increased ROS in these cells, we exposed growth factor-deprived cells to hydrogen peroxide (H2O2) at concentrations that increased intracellular ROS. H2O2 induced a dose-dependent increase in tyrosine phosphorylation, including increased tyrosine phosphorylation of the GM-CSF receptor beta chain (βc), STAT5, and other signaling proteins. H2O2 also induced expression of the early response gene c-FOS, and G1- to S-phase transition, but not S- to G2/M-phase transition of MO7e cells. The cell permeable antioxidant pyrrolidine dithiocarbamate (PDTC) decreased the intracellular levels of ROS and inhibited tyrosine phosphorylation induced by GM-CSF in MO7e cells, suggesting that ROS generation plays an important role in GM-CSF signaling. Consistent with this notion, PDTC and two other antioxidants, N-acetyl cysteine and 2-mercaptoethanol, reduced growth and viability of MO7e cells. These results suggest that generation of ROS in response to HGFs may contribute to downstream signaling events, especially those involving tyrosine phosphorylation.

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.

Requirement for the Xrcc1 DNA base excision repair gene during early mouse development

Surveillance and repair of DNA damage are essential for maintaining the integrity of the genetic information that is needed for normal development. Several multienzyme pathways, including the excision repair of damaged or missing bases, carry out DNA repair in mammals. We determined the developmental role of the X-ray cross-complementing (Xrcc)-1 gene, which is central to base excision repair, by generating a targeted mutation in mice. Heterozygous matings produced Xrcc1-/- embryos at early developmental stages, but not Xrcc1-/- late-stage fetuses or pups. Histology showed that mutant (Xrcc1-/-) embryos arrested at embryonic day (E) 6.5 and by E7.5 were morphologically abnormal. The most severe abnormalities observed in mutant embryos were in embryonic tissues, which showed increased cell death in the epiblast and an altered morphology in the visceral embryonic endoderm. Extraembryonic tissues appeared relatively normal at E6.5-7.5. Even without exposure to DNA-damaging agents, mutant embryos showed increased levels of unrepaired DNA strand breaks in the egg cylinder compared with normal embryos. Xrcc1-/- cell lines derived from mutant embryos were hypersensitive to mutagen-induced DNA damage. Xrcc1 mutant embryos that were also made homozygous for a null mutation in Trp53 underwent developmental arrest after only slightly further development, thus revealing a Trp53-independent mechanism of embryo lethality. These results show that an intact base excision repair pathway is essential for normal early postimplantation mouse development and implicate an endogenous source of DNA damage in the lethal phenotype of embryos lacking this repair capacity.

DT-diaphorase activity in NSCLC and SCLC cell lines: a role for fos/jun regulation

To assess the potential differential lung tumour expression of NAD(P)H:quinone reductase (NQO1), the human (h) NQO1 promoter was characterized in gene transfer studies. A deletion panel of 5' flanking hNQO1 promoter constructs was made and tested in transient transfection assays in NSCLC and SCLC cell lines. The largest hNQO1 construct (-1539/+115) containing the antioxidant response element (ARE), exhibited robust levels of reporter activity in the NSCLC (H460, H520, and A549) cell lines and expression was over 12 to 77-fold higher than the minimal (-259/+115) promoter construct. In contrast, there was little difference in promoter activity between the largest and minimal promoter construct in the SCLC (H146, H82 and H187) cell lines. Deletion of the sites for NFkappaB and AP-2 and the XRE did not significantly affect hNQO1 promoter activity in either the NSCLC or SCLC cell lines. Robust promoter activity in NSCLC lines was mediated by a 359 bp segment of the proximal promoter that contained a canonical AP-1 binding site, TGACTCAG, within the ARE. Gel supershift assays with various specific Fos/Jun antibodies identified Fra1, Fra2 and Jun B binding activity in NSCLC cells to a promoter fragment (-477 to -438) spanning the AP-1 site, whereas SCLC do not appear to express functional Fra or Jun B. These results suggest a possible role for AP-1 activity in the differential expression of hNQO1 in NSCLC.

Neuronal death and survival in two models of hypoxic-ischemic brain damage

Two unilateral hypoxic-ischemia (HI) models (moderate and severe) in immature rat brain have been used to investigate the role of various transcription factors and related proteins in delayed neuronal death and survival. The moderate HI model results in an apoptotic-like neuronal death in selectively vulnerable regions of the brain while the more severe HI injury consistently produces widespread necrosis resulting in infarction, with some necrosis resistant cell populations showing evidence of an apoptotic type death. In susceptible regions undergoing an apoptotic-like death there was not only a prolonged induction of the immediate early genes, c-jun, c-fos and nur77, but also of possible target genes amyloid precursor protein (APP751) and CPP32. In contrast, increased levels of BDNF, phosphorylated CREB and PGHS-2 were found in cells resistant to the moderate HI insult suggesting that these proteins either alone or in combination may be of importance in the process of neuroprotection. An additional feature of both the moderate and severe brain insults was the rapid activation and/or proliferation of glial cells (microglia and astrocytes) in and around the site of damage. The glial response following HI was associated with an upregulation of both the CCAAT-enhancer binding protein alpha (microglia only) and NFkappaB transcription factors.

Intragenic suppression of an active site mutation in the human apurinic/apyrimidinic endonuclease

The apurinic/apyrimidinic endonucleases (APE) contain several highly conserved sequence motifs. The glutamic acid residue in a consensus motif, LQE96TK98 in human APE (hAPE-1), is crucial because of its role in coordinating Mg2+, an essential cofactor. Random mutagenesis of the inactive E96A mutant cDNA, followed by phenotypic screening in Escherichia coli, led to isolation of an intragenic suppressor with a second site mutation, K98R. Although the Km of the suppressor mutant was about sixfold higher than that of the wild-type enzyme, their kcat values were similar for AP endonuclease activity. These results suggest that the E96A mutation affects only the DNA-binding step, but not the catalytic step of the enzyme. The 3' DNA phosphoesterase activities of the wild-type and the suppressor mutant were also comparable. No global change of the protein conformation is induced by the single or double mutations, but a local perturbation in the structural environment of tryptophan residues may be induced by the K98R mutation. The wild-type and suppressor mutant proteins have similar Mg2+ requirement for activity. These results suggest a minor perturbation in conformation of the suppressor mutant enabling an unidentified Asp or Glu residue to substitute for Glu96 in positioning Mg2+ during catalysis. The possibility that Asp70 is such a residue, based on its observed proximity to the metal-binding site in the wild-type protein, was excluded by site-specific mutation studies. It thus appears that another acidic residue coordinates with Mg2+ in the mutant protein. These results suggest a rather flexible conformation of the region surrounding the metal binding site in hAPE-1 which is not obvious from the X-ray crystallographic structure.

Apoptosis in developing retinal tissue

The mechanisms of apoptosis are strongly dependent on cell-cell interactions typical of organized tissues. Experimental studies of apoptosis using a histotypical preparation of retinal explants are reported in the present article. We found that various characteristics of apoptosis are selectively associated with retinal cell death depending on cell type, stage of maturation, and means of induction of apoptosis. Among these were: (1) the requirements of protein synthesis; (2) the role of cAMP; (3) the expression of certain apoptosis-associated proteins; and (4) the sensitivity to excitotoxicity, modulation of protein phosphatases and calcium mobilization. Dividing cells undergo apoptosis in response to several inducers in specific phases of the cell cycle, and in distinct regions within their pathway of interkinetic nuclear migration. Recent post-mitotic cells are selectively sensitive to apoptosis induced by blockade of protein synthesis, while both proliferating and differentiated cells are more resistant. We also studied the association of several proteins, some of which play critical roles in the cell cycle, with both differentiation and apoptosis in the retinal tissue. Detection of cell cycle markers did not support the hypothesis that retinal cells re-enter the cell cycle on their pathway to apoptosis, although some proteins associated with cell proliferation re-appeared in degenerating cells. The transcription factors c-Jun, c-Fos and c-Myc were found associated with apoptosis in retinal cells, but their sub-cellular location in apoptotic bodies is not consistent with their canonical functions in the control of gene expression. The bifunctional redox factor/AP endonuclease Ref-1 and the transcription factor Max are associated with progressive cell differentiation, and both are down-regulated during cell death in the retina. The data suggest that Ref-1 and Max may normally function as negative modulators of retinal apoptosis. The results indicate that nuclear exclusion of transcription factors and other important control proteins is a hallmark of retinal apoptosis. Histotypical explants may be a choice preparation for the experimental analysis of the mechanisms of apoptosis, in the context both of cell-cell interactions and of the dynamic behavior of developing cells within the organized retinal tissue.

JunB forms the majority of the AP-1 complex and is a target for redox regulation by receptor tyrosine kinase and G protein-coupled receptor agonists in smooth muscle cells

To understand the role of redox-sensitive mechanisms in vascular smooth muscle cell (VSMC) growth, we have studied the effect of N-acetylcysteine (NAC), a thiol antioxidant, and diphenyleneiodonium (DPI), a potent NADH/NADPH oxidase inhibitor, on serum-, platelet-derived growth factor BB-, and thrombin-induced ERK2, JNK1, and p38 mitogen-activated protein (MAP) kinase activation; c-Fos, c-Jun, and JunB expression; and DNA synthesis. Both NAC and DPI completely inhibited agonist-induced AP-1 activity and DNA synthesis in VSMC. On the contrary, these compounds had differential effects on agonist-induced ERK2, JNK1, and p38 MAP kinase activation and c-Fos, c-Jun, and JunB expression. NAC inhibited agonist-induced ERK2, JNK1, and p38 MAP kinase activation and c-Fos, c-Jun, and JunB expression except for platelet-derived growth factor BB-induced ERK2 activation. In contrast, DPI only inhibited agonist-induced p38 MAP kinase activation and c-Fos and JunB expression. Antibody supershift assays indicated the presence of c-Fos and JunB in the AP-1 complex formed in response to all three agonists. In addition, cotransfection of VSMC with expression plasmids for c-Fos and members of the Jun family along with the AP-1-dependent reporter gene revealed that AP-1 with c-Fos and JunB composition exhibited a higher transactivating activity than AP-1 with other compositions tested. All three agonists significantly stimulated reactive oxygen species production, and this effect was inhibited by both NAC and DPI. Together, these results strongly suggest a role for redox-sensitive mechanisms in agonist-induced ERK2, JNK1, and p38 MAP kinase activation; c-Fos, c-Jun, and JunB expression; AP-1 activity; and DNA synthesis in VSMC. These results also suggest a role for NADH/NADPH oxidase activity in some subset of early signaling events such as p38 MAP kinase activation and c-Fos and JunB induction, which appear to be important in agonist-induced AP-1 activity and DNA synthesis in VSMC.

Metabolic oxidative stress activates signal transduction and gene expression during glucose deprivation in human tumor cells

The mechanism of glucose deprivation-induced activation of Lyn kinase (Lyn), c-Jun N-terminal kinase 1 (JNK1) and increased expression of basic fibroblast growth factor (bFGF) and c-Myc was investigated in MCF-7/ADR adriamycin-resistant human breast carcinoma cells. Glucose deprivation significantly increased steady state levels of oxidized glutathione content (GSSG) and intracellular prooxidants (presumably hydroperoxides) as well as caused the activation of Lyn, JNK1, and the accumulation of bFGF and c-Myc mRNA. The suppression of GSSG accumulation and prooxidant production by treatment with the thiol antioxidant, N-acetylcysteine, also suppressed all the increases in kinase activation and gene expression observed during glucose deprivation. In addition, glucose deprivation was shown to induce oxidative stress in IMR90 SV40 transformed human fibroblasts, indicating that this phenomena is not limited to the MCF-7/ADR cell line. These and previous observations from our laboratory show that glucose deprivation-induced oxidative stress in MCF-7/ADR cells activates signal transduction involving Lyn, JNK1, and mitogen activated protein kinases (ERK1/ERK2) which results in increased bFGF and c-Myc mRNA accumulation. These results provide support for the hypothesis that alterations in intracellular oxidation/reduction reactions link changes in glycolytic metabolism to signal transduction and gene expression in these human tumor cells.

Elevation of antioxidant potency in the brain of mice by low-dose gamma-ray irradiation and its effect on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced brain damage

The elevation of endogenous thiol-related antioxidants and free radical scavenging enzymes in the brain of C57BL/6 female mice after low-dose gamma-ray irradiation and its inhibitory effect on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced brain damage were investigated. The brain level of the reduced form of glutathione (GSH) increased soon after irradiation with 50 cGy of gamma-rays, reached a maximum at 3 h post-treatment, and remained elevated until 12 h. Thioredoxin (TRX) was also transiently increased after irradiation. The activities of free radical scavenging enzymes, including Cu/Zn-superoxide dismutase, catalase and glutathione peroxidase, were significantly induced after irradiation as well. Cerebral malondialdehyde was remarkably elevated by MPTP treatment, and this elevation was suppressed by pre-irradiation (50 cGy). The contents of GSH and TRX were significantly decreased by MPTP treatment in comparison with those of the control group. These reductions both seemed to be attenuated by pre-irradiation with gamma-rays. These results suggest that low-dose gamma-ray irradiation induces endogenous antioxidative potency in the brain of mice and might be effective for the prevention and/or therapy of various reactive oxygen species-related neurodegenerative disorders, such as Parkinson's disease and Alzheimer's disease.

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.

Phosphorylation of the DNA repair protein APE/REF-1 by CKII affects redox regulation of AP-1

The DNA repair protein apurinic endonuclease (APE/Ref-1) exerts several physiological functions such as cleavage of apurinic/apyrimidinic sites and redox regulation of the transcription factor AP-1, whose activation is part of the cellular response to DNA damaging treatments. Here we demonstrate that APE/Ref-1 is phosphorylated by casein kinase II (CKII). This was shown for both the recombinant APE/Ref-1 protein (Km=0.55 mM) and for APE/Ref-1 expressed in COS cells. Phosphorylation of APE/Ref-1 did not alter the repair activity of the enzyme, whereas it stimulated its redox capability towards AP-1, thus promoting DNA binding activity of AP-1. Inhibition of CKII mediated phosphorylation of APE/Ref-1 blocked mutagen-stimulated increase in AP-1 binding. It also abrogated the induction of c-Jun protein and rendered cells more sensitive to induced DNA damage. Thus, phosphorylation of APE/Ref-1 appears to be involved in regulating the different physiological activities of the enzyme. CKII mediated phosphorylation of APE/Ref-1 and concomitant increase in AP-1 binding activity appears to be a novel mechanism of cellular stress response, forcing transcription of AP-1 target gene(s) the product(s) of which may exert protective function.

Redox regulation of cellular signalling

Extracellular stimuli elicit a variety of responses, such as cell proliferation and differentiation, through the cellular signalling system. Binding of growth factors to the respective receptor leads to the activation of receptor tyrosine kinases, which in turn stimulate downstream signalling systems such as mitogen-activated protein (MAP) kinases, phospholipase Cgamma (PLCgamma) and phosphatidylinositol 3-kinase. These biochemical reactions finally reach the nucleus, resulting in gene expression mediated by the activation of several transcription factors. Recent studies have revealed that cellular signalling pathways are regulated by the intracellular redox state. Generation of reactive oxygen species (ROS), such as H2O2, leads to the activation of protein tyrosine kinases followed by the stimulation of downstream signalling systems including MAP kinase and PLCgamma. The activation of PLCgamma by oxidative radical stress elevates the cellular Ca2+ levels by flux from the intracellular Ca2+ pool and from the extracellular space. Such reactions in the upstream signalling cascade, in concert, result in the activation of several transcription factors. On the other hand, reductants generally suppress the upstream signalling cascade resulting in the suppression of transcription factors. However, it is well known that cysteine residues in a reduced state are essential for the activity of many transcription factors. In fact, in vitro, oxidation of NFkappaB results in its activation, whereas reductants promote its activity. Thus, cellular signalling pathways are generally subjected to dual redox regulation in which redox has opposite effects on upstream signalling systems and downstream transcription factors. Not only are the cellular signalling pathways subjected to redox regulation, but also the signalling systems regulate the cellular redox state. When cells are activated by extracellular stimuli, the cells produce ROS, which in turn stimulate other cellular signalling pathways, indicating that ROS act as second messengers. It is thus evident that there is cross talk between the cellular signalling system and the cellular redox state. Cell death and life also are subjected to such dual redox regulation and cross talk. Death signals induce apoptosis through the activation of caspases in the cells. Oxidative radical stress induces the activation of caspases, whereas the oxidation of caspases results in their inactivation. Furthermore, some cell-death signals induce the production of ROS in the cells, and the ROS produced in turn stimulate the cell-death machinery. All this evidence shows that the cell's fate is determined by cross talk between the cellular signalling pathways and the cellular redox state through a complicated regulation mechanism.

Thioredoxin activation of iron regulatory proteins. Redox regulation of RNA binding after exposure to nitric oxide

Iron regulatory proteins (IRP1 and IRP2) are redox-sensitive RNA-binding proteins that modulate the expression of several genes encoding key proteins of iron metabolism. IRP1 can also exist as an aconitase containing a [4Fe-4S] cluster bound to three cysteines at the active site. We previously showed that biosynthesis of nitric oxide (NO) induces the transition of IRP1 from aconitase to apoprotein able to bind RNA. This switch is also observed when cytosolic extracts are exposed to NO donors. However, the activation of IRP1 under these conditions is far from maximal. In this study we examined the capacity of physiological reducing systems to cooperate with NO in the activation of IRP1. Cytosolic extracts from the macrophage cell line RAW 264.7 or purified IRP1 were incubated with NO donors and subsequently exposed to glutathione or to thioredoxin (Trx), a strong protein disulfide reductase. Trx was the most effective, inducing a 2-6-fold enhancement of the RNA binding activity of NO-treated IRP1. Furthermore, the effect of NO on IRP1 from cytosolic extracts was abolished in the presence of anti-Trx antibodies. We also studied the combined effect of NO and Trx on IRP2, which exhibits constitutive RNA binding activity. We observed an inhibition of IRP2 activity following exposure to NO donors which was restored by Trx. Collectively, these results point to a crucial role of Trx as a modulator of IRP activity in situations of NO production.

APE/Ref-1 responses to oxidative stress in aged rats

Chronic oxidative stress has been hypothesized to be a major contributor to the aging process. The continued exposure to reactive oxygen species (ROS) generated by oxidative metabolism or environmental sources can damage critical cellular structures and be responsible for some age-related pathology. The exposure of rodents to 100% oxygen, isobaric hyperoxia, increases ambient ROS levels and significantly increases apoptosis in brain. The deleterious effects of ROS also include increased lipid peroxidation, protein oxidation, and DNA damage. Although differences in the relative amounts of oxidative stress in young and old brains have been observed, the mechanisms responsible for impaired aging-associated DNA repair processes have not been characterized. We measured DNA levels of the DNA repair enzyme apurinic/apyrimidinic endonuclease (APE/Ref-1) protein by Western blot analysis in the brains of young (3-month) and old (30-month) male rats exposed to isobaric hyperoxia. Given that APE/Ref-1 is the rate-limiting enzyme in the repair pathway of apurinic/apyrimidinic sites generated in DNA by oxidative damage, we assumed that APE/Ref-1 protein levels were a good reflection of ongoing DNA base excision repair. Isobaric hyperoxia stimulated APE/Ref-1 expression in the hippocampus and basal forebrain of young rats experiencing 100% oxygen for 6 hr, while aged rats showed no significant changes in APE/Ref-1 protein levels in all brain areas at any time tested (0-48 hr) after hyperoxia. Differences in the stress-induced levels of expression of DNA repair enzymes may contribute to apoptotic increases and pathology associated with the aging process.

Expression and subcellular localization of human AP endonuclease 1 (HAP1/Ref-1) protein: a basis for its role in human disease

AIMS

Human AP endonuclease 1 (HAP1) plays a major role in the repair of apurinic/apyrimidinic (AP) sites in cellular DNA by catalysing hydrolytic cleavage of the phosphodiester backbone 5' to the site. HAP1 is also known to be a potent reduction-oxidation (redox) factor, regulating the binding activity of a number of transcription factors. The purpose of the present study was to examine the expression of HAP-1 in a wide range of human tissues.

METHODS AND RESULTS

Using a recently developed specific rabbit polyclonal antibody, we performed immunohistochemistry on paraffin-embedded tissue material. Nuclear staining was detected in crypt cells of the small and large intestine, epithelial cells of breast ducts, basal cells of the skin, alveolar cells of the lung, lymphocytes of the marginal zone of the spleen, in the surface epithelium and stromal cells of the ovary and the transitional epithelium of the bladder. Unexpectedly for a presumed nuclear protein, the staining pattern in some cell populations was mainly cytoplasmic (e.g. superficial cells of gastrointestinal tract, Langerhans cells, Leydig cells and spermatocytes, epithelium of the prostate glands), or both cytoplasmic and nuclear (e.g. epithelial cells of thymus, follicular thyroid cells, parietal cells of the stomach, glandular epithelial cells of the cervix, epithelial cells of exocrine pancreas).

CONCLUSION

This differential expression in a wide spectrum of cells is indicative of a potential multifunctional action of HAP1, not necessarily restricted to a role in the nucleus.

Mutation in P0, a dual function ribosomal protein/apurinic/apyrimidinic endonuclease, modifies gene expression and position effect variegation in Drosophila

In a search for modifiers of gene expression with the white eye color gene as a target, a third chromosomal P-element insertion mutant l(3)01544 has been identified that exhibits a strong pigment increase in a white-apricot background. Molecular analysis shows that the P-element insertion is found in the first intron of the gene surrounding the insertion site. Sequencing both the cDNA and genomic fragments revealed that the identified gene is identical to one encoding ribosomal protein P0/apurinic/apyrimidinic endonuclease. The P-element-induced mutation, l(3)01544, affects the steady-state level of white transcripts and transcripts of some other genes. In addition, l(3)01544 suppresses the variegated phenotypes of In(1)wm4h and In(1)y3P, suggesting a potential involvement of the P0 protein in modifying position effect variegation. The revertant generated by the precise excision of the P element has lost all mutant phenotypes. Recent work revealed that Drosophila ribosomal protein P0 contains an apurinic/apyrimidinic endonuclease activity. Our results suggest that this multifunctional protein is also involved in regulation of gene expression in Drosophila.

Activation of the JAK-STAT pathway by reactive oxygen species

Reactive oxygen species (ROS) play an important role in the pathogenesis of many human diseases, including the acute respiratory distress syndrome, Parkinson's disease, pulmonary fibrosis, and Alzheimer's disease. In mammalian cells, several genes known to be induced during the immediate early response to growth factors, including the protooncogenes c-fos and c-myc, have also been shown to be induced by ROS. We show that members of the STAT family of transcription factors, including STAT1 and STAT3, are activated in fibroblasts and A-431 carcinoma cells in response to H2O2. This activation occurs within 5 min, can be inhibited by antioxidants, and does not require protein synthesis. STAT activation in these cell lines is oxidant specific and does not occur in response to superoxide- or nitric oxide-generating stimuli. Buthionine sulfoximine, which depletes intracellular glutathione, also activates the STAT pathway. Moreover, H2O2 stimulates the activity of the known STAT kinases JAK2 and TYK2. Activation of STATs by platelet-derived growth factor (PDGF) is significantly inhibited by N-acetyl-L-cysteine and diphenylene iodonium, indicating that ROS production contributes to STAT activation in response to PDGF. These findings indicate that the JAK-STAT pathway responds to intracellular ROS and that PDGF uses ROS as a second messenger to regulate STAT activation.

Inhibition of tumor necrosis factor-alpha- and interleukin-1-induced endothelial E-selectin expression by thiol-modifying agents

The expression of endothelial-leukocyte adhesion molecules has been postulated to be regulated by redox-sensitive events. Tumor necrosis factor-alpha (TNF-alpha)- and interleukin-1 (IL-1)-induced E-selectin expression was analyzed after pretreating human umbilical vein endothelial cells with different thiol-modifying agents, ie, diamide, phenylarsine oxide, N-ethylmaleimide, and diethyl maleate. E-selectin protein expression was quantified by indirect immunofluorescence. All compounds suppressed the cytokine-induced E-selectin expression in a concentration-dependent manner, whereas the antioxidant N-acetylcysteine showed no effect. The inhibitory effect of diamide (100 micromol/L, 1 hour) was reversible within 6 hours when the cells were allowed to recover before application of cytokines. Reversibility was strongly delayed when cells were deprived of glutathione by buthionine sulfoximine pretreatment. Glutathione depletion alone did not influence cytokine-induced E-selectin expression. Analysis of cellular glutathione status showed a 3-fold increase in oxidized glutathione after diamide treatment. Monochlorobimane labeling also revealed a decrease in total cellular thiols. During recovery, the glutathione status was restored within 1 hour, whereas total thiol content and E-selectin expression needed at least 6 hours to return to baseline. Complete inhibition of E-selectin expression by the vicinal thiol blocker phenylarsine oxide (0.5 micromol/L) was reversed by dithiols like dithiothreitol or dimercaptopropanol, but not by the monothiol 2-mercaptoethanol. These data suggest that proteins with essential thiols, most probably vicinal thiols. are involved in the IL-1- and TNF-alpha-mediated induction of E-selectin. These thiols must be in the reduced state; oxidation or other modification thereof attenuates or abolishes the cells' response to the cytokines.

Heterogeneous repair of N-methylpurines at the nucleotide level in normal human cells

Base excision repair rates of dimethyl sulfate-induced 3-methyladenine and 7-methylguanine adducts were measured at nucleotide resolution along the PGK1 gene in normal human fibroblasts. Rates of 7-methylguanine repair showed a 30-fold dependence on nucleotide position, while position-dependent repair rates of 3-methyladenine varied only sixfold. Slow excision rates for 7-methylguanine bases afforded the opportunity to study their excision in vitro as a model for base excision repair. A two-component in vitro excision system, composed of human N-methylpurine-DNA glycosylase (MPG protein) and dimethyl sulfate-damaged DNA manifested sequence context-dependent rate differences for 7-methylguanine of up to 185-fold from position to position. This in vitro system reproduced both the global repair rate, and for the PGK1 coding region, the position-dependent repair patterns observed in cells. The equivalence of in vivo repair and in vitro excision data indicates that removal of 7-methylguanine by the MPG protein is the rate-limiting step in base excision repair of this lesion. DNA "repair rate footprints" associated with DNA glycosylase accessibility were observed only in a region with bound transcription factors. The "repair rate footprints" represent a rare chromatin component of 7-meG base excision repair otherwise dominated by sequence-context dependence. Comparison of in vivo repair rates to in vitro rates for 3-methyladenine, however, shows that the rate-limiting step determining position-dependent repair for this adduct is at one of the post-DNA glycosylase stages. In conclusion, this study demonstrates that a comparison of sequence context-dependent in vitro reaction rates to in vivo position-dependent repair rates permits the identification of steps responsible for position-dependent repair. Such analysis is now feasible for the different steps and adducts repaired via the base excision repair pathway.

Dynamics of the interaction of human apurinic endonuclease (Ape1) with its substrate and product

We investigated the interaction dynamics of human abasic endonuclease, the Ape1 protein (also called Ref1, Hap1, or Apex), with its DNA substrate and incised product using electrophoretic assays and site-specific amino acid substitutions. Changing aspartate 283 to alanine (D283A) left 10% residual activity, contrary to a previous report, but complementation of repair-deficient bacteria by the D283A Ape1 protein was consistent with its activity in vitro. The D308A, D283/D308A double mutant, and histidine 309 to asparagine proteins had 22, 1, and approximately 0. 02% of wild-type Ape1 activity, respectively. Despite this range of enzymatic activities, all the mutant proteins had near-wild-type binding affinity specific for DNA containing a synthetic abasic site. Thus, substrate recognition and cleavage are genetically separable steps. Both the wild-type and mutant Ape1 proteins bound strongly to the enzyme incision product, an incised abasic site, which suggested that Ape1 might exhibit product inhibition. The use of human DNA polymerase beta to increase Ape1 activity by eliminating the incision product supports this conclusion. Notably, the complexes of the D283A, D308A, and D283A/D308A double mutant proteins with both intact and incised abasic DNA were significantly more stable than complexes containing wild-type Ape1, which may contribute to the lower turnover numbers of the mutant enzymes. Wild-type Ape1 protein bound tightly to DNA containing a one-nucleotide gap but not to DNA with a nick, consistent with the proposal that substrate recognition by Ape1 involves a space bracketed by duplex DNA, rather than mere flexibility of the DNA.

Ref-1 controls pax-8 DNA-binding activity

Redox potential controls the DNA-binding activity of several transcription factors. In some cases, the regulation of DNA-binding activity by the redox state is mediated by the Ref-1 nuclear protein. In this study, we demonstrate that Ref-1 is able to induce "in vitro" the DNA-binding activity of the Pax-8 paired domain. In co-transfection experiments, Ref-1 increases the Pax-8 activating effect on thyroglobulin promoter. Moreover, immunoreactivity data suggest that, in nuclear extracts of thyroid cells, the levels of Ref-1 correlate with the amounts of reduced Pax-8. Therefore, the regulation of the Pax-8 DNA-binding activity by redox potential, that we have demonstrated occurring "in vitro", could represent a means to control "in vivo" the function of Pax proteins. Alignment of the Paired domains sequences present in the Protein Data Bank demonstrates a strong conservation of Cys residues, suggesting that the redox regulation of the Paired domain DNA-binding activity is widely conserved along phylogenesis.

Excision of C-4'-oxidized deoxyribose lesions from double-stranded DNA by human apurinic/apyrimidinic endonuclease (Ape1 protein) and DNA polymerase beta

Oxidative damage to DNA deoxyribose generates oxidized abasic sites (OAS) that may constitute one-third of ionizing radiation damage. The antitumor drug bleomycin produces exclusively OAS in the form of C-4-keto-C-1-aldehydes in unbroken DNA strands and 3'-phosphoglycolate esters terminating strand breaks. We investigated whether two human DNA repair enzymes can mediate OAS excision in vitro: Ape1 protein (the main human abasic endonuclease (also called Hap1, Apex, or Ref1)) and DNA polymerase beta, which carries out both the abasic excision and the resynthesis steps. We used a duplex oligonucleotide substrate with one main target for bleomycin-induced damage. Ape1 catalyzed effective incision at the C-4-keto-C-1-aldehyde sites at a rate that may be only a few-fold lower than incision of hydrolytic abasic sites at the same location. Consistent with several previous studies, Ape1 hydrolyzed 3'-phosphoglycolates 25-fold more slowly than C-4-keto-C-1-aldehydes. DNA polymerase beta excised the 5'-terminal OAS formed by Ape1 incision at a rate similar to its removal of unmodified abasic residues. Polymerase beta-mediated excision of 5'-terminal OAS was stimulated by Ape1 as it is for unmodified abasic sites. Escherichia coli Fpg (MutM) protein also excised 5'-terminal OAS, but in our hands, the RecJ protein did not. These observations help define mammalian pathways of OAS repair, point to interactions that might coordinate functional steps, and suggest that still unknown factors may contribute to removal of 3'-phosphoglycolate esters.

A murine AP-endonuclease gene-targeted deficiency with post-implantation embryonic progression and ionizing radiation sensitivity

Apurinic/apyrimidinic endonuclease (here designated APE/REF) carries out repair incision at abasic or single-strand break damages in mammals. This multifunctional protein also has putative role(s) as a cysteine 'reducing factor' (REF) in cell-stress transcriptional responses. To assess the significance of APE/REF for embryonic teratogenesis we constructed a more precisely targeted Ape/Ref-deficient genotype in mice. Ape/Ref gene replacement in ES cells eliminated the potential of APE/REF protein synthesis while retaining the Ape/Ref bi-directional promoter that avoided potential inactivation of an upstream gene. Chimeric animals crossed into Tac:N:NIHS-BC produced germline transmission. Homozygous null Ape/Ref-embryos exhibited successful implantation and nearly normal developmental progression until embryonic day 7.5 followed by morphogenetic failure and adsorption of embryos by day 9.5. We characterized the cellular events proceeding to embryonic lethality and examined ionizing radiation sensitivity of pre-implantation Ape/Ref-null embryos. After intermating of heterozygotes, Mendelian numbers of putative Ape/Ref-null progeny embryos at day 6.5 displayed a several-fold elevation of pycnotic, fragmenting cell nuclei within the embryo proper-the epiblast. Increased cell-nucleus degeneration occurred within epiblast cells while mitosis continued and before obvious morphogenetic disruption. Mitogenic response to epiblast cell death, if any, was ineffective for replacement of lost cells. Extra-embryonic yolk sac, a trophectoderm derived lineage retained normal appearance to day 9. Explanted homozygous Ape/Ref-null blastocysts displayed increased sensitivity to gamma-irradiation, most likely a manifestation of APE/REF incision defect. Our study establishes that this new Ape/Ref deficiency genotype is definitely capable of post-implantation developmental progression to the onset of gastrulation. Function(s) of APE/REF in base damage incision and also conceivably in mitogenic responses towards epiblast cell death are critical for transit through the gastrulation stage of embryonic growth and development.

Localization of glutathione and induction of glutathione synthesis-related proteins in mouse brain by low doses of gamma-rays

First, we determined the cerebral localization of reduced glutathione (GSH) in normal mice by means of autoradiography using 99mTc-meso-hexamethyl propylene oxime. A highly specific localization of GSH in the cerebellum and hippocampus was observed. Secondly, we measured the elevation of GSH level in the brain after low-dose gamma-irradiation. The cerebral GSH levels increased soon after irradiation with 50 cGy of gamma-rays, reaching a maximum at 3 h post-treatment, then remaining significantly higher than that of the non-irradiated control until 12 h and returning to the control level by 24 h. Thirdly, we examined the induction of the activities and the mRNAs of proteins involved in the synthesis and regeneration of GSH in the brain of mice subjected to low-dose gamma-ray irradiation. The level of mRNA for gamma-glutamylcysteine synthetase was significantly increased at 0.5 h, and remained high until 2 h post-irradiation (50 cGy). The level was transiently lowered to the non-irradiated control level at 3 h and slightly increased again after 6 h post-irradiation. gamma-Glutamylcysteine synthetase activity was significantly increased 3 h after irradiation, and remained high up to 24 h post-irradiation. As for glutathione reductase, the mRNA level was increased at 0.5 h, and peaked strongly at 2 h, while the enzyme activity was significantly increased at 6 h after irradiation, and continued to increase up to 24 h. The level of mRNA for thioredoxin, which contributes to GSH biosynthesis by supplying cysteine to the de novo pathway, peaked between 0.5 h and 2 h post-irradiation, and rapidly declined thereafter. The content of thioredoxin showed a transient decrease immediately after irradiation, but was then remarkably elevated, reaching a maximum at 3 h, and thereafter declining sharply. These results indicate that the increase in endogenous GSH in mouse brain soon after low-dose gamma-ray irradiation is a consequence of the induction of GSH synthesis-related proteins and occurs via both the de novo synthesis and the regeneration pathways.

Control of genes by mammalian retroposons

Available data on possible genetic impacts of mammalian retroposons are reviewed. Most important is the growing number of established examples showing the involvement of retroposons in modulation of expression of protein-coding genes transcribed by RNA polymerase II (Pol II). Retroposons contain conserved blocks of nucleotide sequence for binding of some important Pol II transcription factors as well as sequences involved in regulation of stability of mRNA. Moreover, these mobile genes provide short regions of sequence homology for illegitimate recombinations, leading to diverse genome rearrangements during evolution. Therefore, mammalian retroposons representing a significant fraction of noncoding DNA cannot be considered at present as junk DNA but as important genetic symbionts driving the evolution of regulatory networks controlling gene expression.

Identification of APN2, the Saccharomyces cerevisiae homolog of the major human AP endonuclease HAP1, and its role in the repair of abasic sites

Abasic (AP) sites arise in DNA through spontaneous base loss and enzymatic removal of damaged bases. APN1 encodes the major AP-endonuclease of Saccharomyces cerevisiae. Human HAP1 (REF1) encodes the major AP endonuclease which, in addition to its role in DNA repair, functions as a redox regulatory protein. We identify APN2, the yeast homolog of HAP1 and provide evidence that Apn1 and Apn2 represent alternate pathways for repairing AP sites. The apn1Delta apn2Delta strain displays a highly elevated level of MMS-induced mutagenesis, which is dependent on the REV3, REV7, and REV1 genes. Our findings indicate that AP sites are highly cytotoxic and mutagenic in eukaryotes, and that the REV3, REV7-encoded DNA polymerase zeta mediates the mutagenic bypass of AP sites.

Redox potential controls the structure and DNA binding activity of the paired domain

Pax proteins are transcriptional regulators controlling a variety of cell fates during animal development. This role depends on the intact function of the paired (Prd) domain that is able to recognize specific DNA sequences. The Prd domain is composed of two distinct helix-turn-helix subdomains, PAI and RED. Molecular functions of Pax proteins are subjected to different levels of regulation involving both pre-translational and post-translational mechanisms. By using Pax-5 and Pax-8 recombinant proteins, we demonstrate that the binding activity of the Prd domain is regulated through the oxidation/reduction of conserved cysteine residues. Mass spectrometry analysis and mutagenesis experiments demonstrate that the redox regulation is accomplished through the reversible formation of an intramolecular disulfide bridge involving the cysteines present in the PAI subdomain, whereas the RED subdomain appears quite insensitive to redox potential. Circular dichroism experiments indicate that only the reduced form of the Prd domain is able to undergo the proper conformational change necessary for sequence-specific DNA binding. Nuclear extracts from different cell lines contain an activity that is able to reduce the Paired domain and, therefore, to control the DNA binding activity of this protein. Immunodepletion of nuclear extracts demonstrate that the protein Ref-1 contributes to the redox regulation of the Prd DNA binding activity. Given the modular nature of the Prd domain and the independent DNA binding specificity of the PAI and RED subdomains, we propose that this control mechanism should be involved in "switching" among different DNA sequences and therefore different target genes.

Transgenic systems in studies on genotoxicity of alkylating agents: critical lesions, thresholds and defense mechanisms

Transgenic systems, both cell lines and mice with gain or loss of function, are being used in order to modulate the expression of DNA repair proteins, thus allowing to assess their contribution to the defense against genotoxic mutagens and carcinogens. In this review, questions have been addressed concerning the use of transgenic systems in elucidating critical primary DNA lesions, their conversion into genotoxic endpoints, low-dose effects, and the relative contribution of individual cellular functions in defense. It has been shown that the repair protein alkyltransferase (MGMT) is decisive for protection against methylating and chloroethylating compounds. Protection pertains also to tumor formation, as revealed by the response of MGMT transgenic and knockout mice. Overexpression of genes involved in base excision repair (N-methylpurine-DNA glycosylase, apurinic endonuclease, DNA polymerase beta) is in most cases not beneficial in increasing the protection level, whereas their down-modulation or inactivation increases cellular sensitivity. This indicates that non-repaired base N-alkylation lesions and/or repair intermediates possess genotoxic potential. Modulation of mismatch repair and poly(ADP)ribosyl transferase has also been shown to affect the cellular response to alkylating agents. Furthermore, the role of Fos, Jun and p53 in cellular defense against alkylating mutagens is discussed.