Mutagenesis of the HMGB (high-mobility group B) protein Cmb1 (cytosine-mismatch binding 1) of Schizosaccharomyces pombe: effects on recognition of DNA mismatches and damage

Downregulation of vimentin expression increased drug resistance in ovarian cancer cells

Cisplatin and other platinum-based drugs have been widely used in the treatment of ovarian cancer, but most patients acquire the drug resistance that greatly compromises the efficacy of drugs. Understanding the mechanism of drug resistance is important for finding new therapeutic approaches. In the present study, we found that the expression of vimentin was downregulated in drug-resistant ovarian cancer cell lines A2780-DR and HO-8910 as compared to their respective control cells. Overexpression of vimentin in A2780-DR cells markedly increased their sensitivity to cisplatin, whereas knockdown of vimentin in A2780, HO-8910-PM and HO-8910 cells increased the resistance to cisplatin, demonstrating that vimentin silencing enhanced cisplatin resistance in ovarian cancer cells. Quantitative proteomic analysis identified 95 differentially expressed proteins between the vimentin silenced A2780 cells (A2780-VIM-KN) and the control cells, in which downregulation of endocytic proteins and the upregulation of exocytotic proteins CHMP2B and PDZK1 were proposed to contribute the decreased cisplatin accumulation in vimentin knockdown cells. Silencing of vimentin induced upregulation of cancer stem cell markers and both A2780-DR and A2780-VIM-KN cells were more facile to form spheroids than control cells under serum-free culture condition. Our results also revealed that vimentin knockdown increased the 14-3-3 mediated retention of Cdc25C in the cytoplasm, leading to inactivation of Cdk1 and the prolonged G2 phase arrest that allowed the longer period of time for cells to repair cisplatin-damaged DNA. Taken together, we demonstrated that vimentin silencing enhanced cells' resistance to cisplatin via prolonged G2 arrest and increased exocytosis, suggesting that vimentin is a potential target for treatment of drug resistant ovarian cancer.

Pre-activation of the genome integrity checkpoint increases DNA damage tolerance

The genome integrity checkpoint is a conserved signaling pathway that is regulated in yeast by the Mec1 (homologous to human ATR) and Rad53 (homologous to human Chk1) kinases. The pathway coordinates a multifaceted response that allows cells to cope with DNA damage and DNA replication stress. The full activation of the checkpoint blocks origin firing, stabilizes replication forks, activates DNA repair proteins and may lead to senescence or apoptosisin higher eukaryotes. We have recently demonstrated that endogenous replication stress can activate the genome integrity checkpoint in budding yeast at a low level that does not go so far as to interfere with cell cycle progression, but it does activate DNA damage-inducible proteins. Here we demonstrate that the low level pre-activation of the checkpoint, either by endogenous replication stress or by the nucleotide-depleting drug hydroxyurea, can increase damage tolerance to multiple DNA-damaging agents. These results may provide new strategies for using the checkpoint to protect normal cells from genotoxic stress.

The yeast high mobility group protein HMO2, a subunit of the chromatin-remodeling complex INO80, binds DNA ends

DNA damage is a common hazard that all cells have to combat. Saccharomyces cerevisiae HMO2 is a high mobility group protein (HMGB) that is a component of the chromatin-remodeling complex INO80, which is involved in double strand break (DSB) repair. We show here using DNA end-joining and exonuclease protection assays that HMO2 binds preferentially to DNA ends. While HMO2 binds DNA with both blunt and cohesive ends, the sequence of a single stranded overhang significantly affects binding, supporting the conclusion that HMO2 recognizes features at DNA ends. Analysis of the effect of duplex length on the ability of HMO2 to protect DNA from exonucleolytic cleavage suggests that more than one HMO2 must assemble at each DNA end. HMO2 binds supercoiled DNA with higher affinity than linear DNA and has a preference for DNA with lesions such as pairs of tandem mismatches; however, comparison of DNA constructs of increasing length suggests that HMO2 may not bind stably as a monomer to distorted DNA. The remarkable ability of HMO2 to protect DNA from exonucleolytic cleavage, combined with reports that HMO2 arrives early at DNA DSBs, suggests that HMO2 may play a role in DSB repair beyond INO80 recruitment.

Suppression of a DNA polymerase delta mutation by the absence of the high mobility group protein Hmo1 in Saccharomyces cerevisiae

The deletion of the gene encoding the high mobility group protein Hmo1 suppresses the growth retardation of the DNA pol delta mutation, pol3-14, at the restrictive temperature. pol3-14 mutant cells undergo cell cycle arrest, and hmo1Delta alleviates the arrest permitting continual division of the double mutant. Bypass of cell cycle control occurs with an increased rate of mutation. Both pol3-14 and hmo1Delta are mutators and their combination provokes a synergistic rate of CAN1 mutations. RAD18 controls branches of DNA repair pathways and its deletion also suppresses pol3 mutations. Comparing hmo1Delta and rad18Delta suppression of pol3-14 shows that while both require the presence of RAD52-mediated repair, their suppression is independent in that both can suppress in the presence of the other. We conclude that hmo1Delta suppression of pol3-14 occurs by a mechanism whereby normal controls on DNA integrity are breached and lesions flow into RAD52-mediated repair and error-prone pathways.

DNA interstrand cross-link repair in Saccharomyces cerevisiae

DNA interstrand cross-links (ICL) present a formidable challenge to the cellular DNA repair apparatus. For Escherichia coli, a pathway which combines nucleotide excision repair (NER) and homologous recombination repair (HRR) to eliminate ICL has been characterized in detail, both genetically and biochemically. Mechanisms of ICL repair in eukaryotes have proved more difficult to define, primarily as a result of the fact that several pathways appear compete for ICL repair intermediates, and also because these competing activities are regulated in the cell cycle. The budding yeast Saccharomyces cerevisiae has proven a powerful tool for dissecting ICL repair. Important roles for NER, HRR and postreplication/translesion synthesis pathways have all been identified. Here we review, with reference to similarities and differences in higher eukaryotes, what has been discovered to date concerning ICL repair in this simple eukaryote.

The role of chromatin proteins in DNA damage recognition and repair

The structure of chromatin is the major factor determining the rate and efficiency of DNA repair. Chromatin remodeling events such as rearrangement of nucleosomes and higher order chromatin structures are indispensable features of repair processes. During the last decade numerous chromatin proteins have been identified that preferentially bind to different types of DNA damage. The HMGB proteins, which preferentially interact with DNA intrastrand crosslinks induced by cisplatin, are the archetypal example of such proteins. Several hypothetical models have been proposed describing the role of such damage-binding chromatin proteins. The damage shielding model postulates that binding of chromatin proteins to damaged DNA might disturb damage recognition by repair factors and impair its removal. Alternatively, the damage-recognition/signaling model proposes that the binding of specific chromatin proteins to damaged DNA could serve as a hallmark to be recognized by repair proteins. Additionally, the binding of specific chromatin proteins to damaged DNA could induce chromatin remodeling at the damage site and indirectly affect its repair. This paper aims to critically review current experimental data in relation to such possible roles of chromatin proteins.

Cellular processing of platinum anticancer drugs

Cisplatin, carboplatin and oxaliplatin are platinum-based drugs that are widely used in cancer chemotherapy. Platinum-DNA adducts, which are formed following uptake of the drug into the nucleus of cells, activate several cellular processes that mediate the cytotoxicity of these platinum drugs. This review focuses on recently discovered cellular pathways that are activated in response to cisplatin, including those involved in regulating drug uptake, the signalling of DNA damage, cell-cycle checkpoints and arrest, DNA repair and cell death. Such knowledge of the cellular processing of cisplatin adducts with DNA provides valuable clues for the rational design of more efficient platinum-based drugs as well as the development of new therapeutic strategies.