Regulation of DNA polymerase beta by the LMP1 oncoprotein of EBV through the nuclear factor-kappaB pathway

An assay for DNA polymerase β lyase inhibitors that engage the catalytic nucleophile for binding

DNA polymerase β (Pol β) repairs cellular DNA damage. When such damage is inflicted upon the DNA in tumor cells treated with DNA targeted antitumor agents, Pol β thus diminishes their efficacy. Accordingly, this enzyme has long been a target for antitumor therapy. Although numerous inhibitors of the lyase activity of the enzyme have been reported, none has yet proven adequate for development as a therapeutic agent. In the present study, we developed a new strategy to identify lyase inhibitors that critically engage the lyase active site primary nucleophile Lys72 as part of the binding interface. This involves a parallel evaluation of the effect of the inhibitors on the wild-type DNA polymerase β (Pol β) and Pol β modified with a lysine analogue at position 72. A model panel of five structurally diverse lyase inhibitors identified in our previous studies (only one of which has been published) with unknown modes of binding were used for testing, and one compound, cis-9,10-epoxyoctadecanoic acid, was found to have the desired characteristics. This finding was further corroborated by in silico docking, demonstrating that the predominant mode of binding of the inhibitor involves an important electrostatic interaction between the oxygen atom of the epoxy group and N^ε of the main catalytic nucleophile, Lys72. The strategy, which is designed to identify compounds that engage certain structural elements of the target enzyme, could find broader application for identification of ligands with predetermined sites of binding.

Involvement of p38MAPK-ATF2 signaling pathway in alternariol induced DNA polymerase β expression

Base excision repair (BER) systems are important for maintaining the integrity of genomes in mammalian cells. Aberrant DNA bases or broken single strands can be repaired by BER. Consequently, DNA lesions, which may be caused by cancer and aging, have a close association with BER procedure. DNA polymerase β (polβ) is a critical BER enzyme that can excise 5'-sugar phosphate prior to adding a nucleotide in the gap by its function as a DNA polymerase in the BER process. However, DNA polβ is an error-prone DNA polymerase, and overexpressing polβ increases the cellular spontaneous mutation rate. DNA polβ overexpression has been identified in various human tumors, which implies that DNA polβ overexpression has a close association with tumorigenesis. The present study showed that alternariol (AOH), a secondary product of a fungus that is found in grains and fruits, could cause DNA damage to NIH3T3 cells in a single cell gel electrophoresis, and that 2, 10 and 20 µM AOH induced DNA polβ overexpression in a dose-dependent manner. In the process, the level of phosphorylation of mitogen-activated protein kinase 14 (p38) mitogen-activated protein kinase (MAPK) and activating transcription factor 2 (ATF2) was increased. In addition, SB203580, a p38MAPK inhibitor, resulted in decreased DNA polβ expression. Small hairpin RNA-p38MAPK had the same effect; notably, DNA polβ expression was downregulated in p38MAPK knockdown cells. These data suggest that the p38MAPK-ATF2 signaling pathway may be involved in DNA polβ expression induced by AOH.

RelA and RelB cross-talk and function in Epstein-Barr virus transformed B cells

In this study, we determined the respective roles of RelA and RelB NF-κB subunits in Epstein-Barr virus (EBV)-transformed B cells. Using different EBV-immortalized B-cell models, we showed that only RelA activation increased both survival and cell growth. RelB activity was induced secondarily to RelA activation and repressed RelA DNA binding by trapping the p50 subunit. Reciprocally, RelA activation repressed RelB activity by increasing expression of its inhibitor p100. To search for such reciprocal inhibition at the transcriptional level, we studied gene expression profiles of our RelA and RelB regulatable cellular models. Ten RelA-induced genes and one RelB-regulated gene, ARNTL2, were repressed by RelB and RelA, respectively. Apart from this gene, RelB signature was included in that of RelA Functional groups of RelA-regulated genes were for control of energy metabolism, genetic instability, protection against apoptosis, cell cycle and immune response. Additional functions coregulated by RelA and/or RelB were autophagy and plasma cell differentiation. Altogether, these results demonstrate a cross-inhibition between RelA and RelB and suggest that, in fine, RelB was subordinated to RelA. In the view of future drug development, RelA appeared to be pivotal in both classical and alternative activation pathways, at least in EBV-transformed B cells.

Transcriptional regulation of human DNA repair genes following genotoxic stress: trigger mechanisms, inducible responses and genotoxic adaptation

DNA repair is the first barrier in the defense against genotoxic stress. In recent years, mechanisms that recognize DNA damage and activate DNA repair functions through transcriptional upregulation and post-translational modification were the focus of intensive research. Most DNA repair pathways are complex, involving many proteins working in discrete consecutive steps. Therefore, their balanced expression is important for avoiding erroneous repair that might result from excessive base removal and DNA cleavage. Amelioration of DNA repair requires both a fine-tuned system of lesion recognition and transcription factors that regulate repair genes in a balanced way. Transcriptional upregulation of DNA repair genes by genotoxic stress is counteracted by DNA damage that blocks transcription. Therefore, induction of DNA repair resulting in an adaptive response is only visible through a narrow window of dose. Here, we review transcriptional regulation of DNA repair genes in normal and cancer cells and describe mechanisms of promoter activation following genotoxic exposures through environmental carcinogens and anticancer drugs. The data available to date indicate that 25 DNA repair genes are subject to regulation following genotoxic stress in rodent and human cells, but for only a few of them, the data are solid as to the mechanism, homeostatic regulation and involvement in an adaptive response to genotoxic stress.

B7-H1, which represses EBV-immortalized B cell killing by autologous T and NK cells, is oppositely regulated by c-Myc and EBV latency III program at both mRNA and secretory lysosome levels

EBV-immortalized B cells induce a complex immune response such that the virus persists as a clinically silent infection for the lifetime of the infected host. B7-H1, also called PD-L1, is a cosignaling molecule of the B7 family that can inhibit activated T cell effectors by interaction with its receptor PD-1. In this work, we have studied the dependence of B7-H1 on NF-κB and c-Myc, the two main transcription factors in EBV latency III proliferating B cells, on various lymphoblastoid and Burkitt lymphoma cell lines, some of them being inducible or not for the EBV latency III program and/or for c-Myc. We found that B7-H1 repressed killing of EBV-immortalized B cells by their autologous T and NK cells. At the mRNA level, NF-κB was a weak inducer whereas c-Myc was a strong repressor of B7-H1 expression, an effect mediated by STAT1 inhibition. At the protein level, B7-H1 molecules were stored in both degradative and unconventional secretory lysosomes. Surface membrane B7-H1 molecules were constitutively internalized and proteolyzed in lysosomes. The EBV latency III program increased the amounts of B7-H1-containing secretory lysosomes and their export to the surface membrane. By repressing actin polymerization, c-Myc blocked secretory lysosome migration and B7-H1 surface membrane export. In addition to B7-H1, various immunoregulatory molecules participating in the immunological synapse are stored in secretory lysosomes. By playing on actin polymerization, c-Myc could thus globally regulate the immunogenicity of transformed B cells, acting on export of secretory lysosomes to plasma membrane.

NF-κB regulates DNA double-strand break repair in conjunction with BRCA1-CtIP complexes

NF-κB is involved in immune responses, inflammation, oncogenesis, cell proliferation and apoptosis. Even though NF-κB can be activated by DNA damage via Ataxia telangiectasia-mutated (ATM) signalling, little was known about an involvement in DNA repair. In this work, we dissected distinct DNA double-strand break (DSB) repair mechanisms revealing a stimulatory role of NF-κB in homologous recombination (HR). This effect was independent of chromatin context, cell cycle distribution or cross-talk with p53. It was not mediated by the transcriptional NF-κB targets Bcl2, BAX or Ku70, known for their dual roles in apoptosis and DSB repair. A contribution by Bcl-xL was abrogated when caspases were inhibited. Notably, HR induction by NF-κB required the targets ATM and BRCA2. Additionally, we provide evidence that NF-κB interacts with CtIP-BRCA1 complexes and promotes BRCA1 stabilization, and thereby contributes to HR induction. Immunofluorescence analysis revealed accelerated formation of replication protein A (RPA) and Rad51 foci upon NF-κB activation indicating HR stimulation through DSB resection by the interacting CtIP-BRCA1 complex and Rad51 filament formation. Taken together, these results define multiple NF-κB-dependent mechanisms regulating HR induction, and thereby providing a novel intriguing explanation for both NF-κB-mediated resistance to chemo- and radiotherapies as well as for the sensitization by pharmaceutical intervention of NF-κB activation.

Aberrant expression of alternative DNA polymerases: a source of mutator phenotype as well as replicative stress in cancer

The cell life span depends on a subtle equilibrium between the accurate duplication of the genomic DNA and less stringent DNA transactions which allow cells to tolerate mutations associated with DNA damage. The physiological role of the alternative, specialized or TLS (translesion synthesis) DNA polymerases could be to favor the necessary "flexibility" of the replication machinery, by allowing DNA replication to occur even in the presence of blocking DNA damage. As these alternative DNA polymerases are inaccurate when replicating undamaged DNA, the regulation of their expression needs to be carefully controlled. Evidence in the literature supports that dysregulation of these error-prone enzymes contributes to the acquisition of a mutator phenotype that, along with defective cell cycle control or other genome stability pathways, could be a motor for accelerated tumor progression.

Gastrointestinal hyperplasia with altered expression of DNA polymerase beta

Background

Altered expression of DNA polymerase β (Pol β) has been documented in a large percentage of human tumors. However, tumor prevalence or predisposition resulting from Pol β over-expression has not yet been evaluated in a mouse model.

Methodology/Principal Findings

We have recently developed a novel transgenic mouse model that over-expresses Pol β. These mice present with an elevated incidence of spontaneous histologic lesions, including cataracts, hyperplasia of Brunner's gland and mucosal hyperplasia in the duodenum. In addition, osteogenic tumors in mice tails, such as osteoma and osteosarcoma were detected. This is the first report of elevated tumor incidence in a mouse model of Pol β over-expression. These findings prompted an evaluation of human gastrointestinal tumors with regard to Pol β expression. We observed elevated expression of Pol β in stomach adenomas and thyroid follicular carcinomas, but reduced Pol β expression in esophageal adenocarcinomas and squamous carcinomas.

Conclusions/Significance

These data support the hypothesis that balanced and proficient base excision repair protein expression and base excision repair capacity is required for genome stability and protection from hyperplasia and tumor formation.