Mammalian endo-exonuclease activity and its level in various radiation sensitive cell lines

Silencing of endo-exonuclease expression sensitizes mouse B16F10 melanoma cells to DNA damaging agents

We previously identified an endo-exonuclease that is highly expressed in cancer cells and plays an important role in DSB repair mechanisms. A small molecular compound pentamidine, which specifically inhibited nuclease activity of the isolated endo-exonuclease from yeast as well as from mammalian cells, was capable of sensitizing tumor cells to DNA damaging agents. In this study, we investigated the effect of precisely silencing the endo-exonuclease expression by small interfering RNA (siRNA) upon treatment with a variety of DNA damaging agents in mouse B16F10 melanoma cells. A maximum of 3.6 to approximately 4-fold reduction in endo-exonuclease mRNA expression was achieved, over a period of 48-72 h of post transfection with a concomitant reduction in protein expression (approximately 4-5 fold), resulting in a substantial reduction (approximately 45-50%) of the corresponding nuclease activity. Suppressed endo-exonuclease expression conferred significant decrease in cell survival, ranging from approximately 30 to approximately 50% cell killing, in presence of DNA damaging drugs methyl methane sulfonate (MMS), cisplatin, 5-fluoro uracil (5-FU) and gamma-irradiation but not at varying dosages of ultra violet (UV) radiation. The data strongly support a role for the endo-exonuclease in repairing DNA damages, induced by MMS, cisplatin, 5-FU and gamma irradiation but not by UV radiation. The results presented in this study suggest that the endo-exonuclease siRNA could be useful as a therapeutic tool in targeting the endo-exonuclease in cancer therapy.

Inhibitory effects of caraway (Carum carvi L.) and its component on N-methyl-N'-nitro-N-nitrosoguanidine-induced mutagenicity

To elucidate the mechanism of antimutagenicity of caraway, we examined the effects of caraway seed extract on N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced mutagenesis in DNA methyltransferase-deficient Salmonella typhimurium strains, O6-methylguanine DNA adduct formation, and thiol content in S. typhimurium cells. MNNG was highly mutagenic for ogt- strains YG7104 (ogt- ada+) and YG7108 (ogt- ada-), and it showed slightly higher mutagenicity in strain YG7100 (ogt+ ada-) than in strains TA100 and TA1535. Hot water extract of caraway seeds inhibited MNNG-induced mutation only in the ogt+ strains. In the presence of caraway extract, O6-methylguanine DNA adducts in strain YG7100 were decreased in proportion to the decrease of MNNG-induced mutagenesis. Although MNNG is known to degrade in the presence of thiols to produce methyl cation which can react with DNA, caraway had no effect on cellular concentrations of acid-soluble thiols. These results indicate that caraway does not directly inactivate MNNG and that Ogt-O6-methylguanine-DNA methyltransferase may be involved in the antimutagenic activity of caraway.

DNA repair protein: endo-exonuclease as a new frontier in cancer therapy

DNA repair mechanisms are essential for cellular survival in mammals. A rapid repair of DNA breaks ensures faster growth of normal cells as well as cancer cells, making DNA repair machinery, a potential therapeutic target. Although efficiency of these repair processes substantially decrease the efficacy of cancer chemotherapies that target DNA, compromised DNA repair contributes to mutagenesis and genomic instability leading to carcinogenesis. Thus, an ideal target in DNA repair mechanisms would be one that specifically kills the rapidly dividing cancer cells without further mutagenesis and does not affect normal cells. Endo-exonucleases play a pivotal role in nucleolytic processing of DNA ends in different DNA repair mechanisms especially in homologous recombination repair (HRR) which mainly repairs damaged DNA in S and G2 phases of the cell cycle in rapidly dividing cells. HRR machinery has also been implicated in cell signaling and regulatory functions in response to DNA damage that is essential for cell viability in mammalian cells where as the predominant nonhomologous end-joining pathway is constitutive. Although HRR is thought to be involved at other stages of the cell cycle, it is predominant in growing phases (S and G2) of the cell cycle. The faster growing cells are believed to carryout more HRR in replicative stages of the cell cycle where homologous DNA is available for HRR. Targeting endo-exonucleases specifically involved in HRR will make the normal cells less prone to mutagenesis, rendering the fast growing tumor cells more susceptible to DNA-damaging agents, used in cancer chemotherapy.

The DNA double-stranded break repair protein endo-exonuclease as a therapeutic target for cancer

DNA repair mechanisms are crucial for the maintenance of genomic stability and are emerging as potential therapeutic targets for cancer. In this study, we report that the endo-exonuclease, a protein involved in the recombination repair process of the DNA double-stranded break pathway, is overexpressed in a variety of cancer cells and could represent an effective target for developing anticancer drugs. We identify a dicationic diarylfuran, pentamidine, which has been used clinically to treat opportunistic infections and is an inhibitor of the endo-exonuclease as determined by enzyme kinetic assay. In clonogenic and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays as well as in the in vivo Lewis lung carcinoma mouse tumor model, pentamidine is shown to possess the ability to selectively kill cancer cells. The LD50 of pentamidine on cancer cells maintained in vitro is correlated with the endo-exonuclease enzyme activity. Tumor cell that has been treated with pentamidine is reduced in the endo-exonuclease as compared with the untreated control. Furthermore, pentamidine synergistically potentiates the cytotoxic effect of DNA strand break and cross-link-inducing agents such as mitomycin C, etoposide, and cisplatin. In addition, we used the small interfering RNA for the mouse homologue of the endo-exonuclease to down-regulate the level of endo-exonuclease in the mouse myeloma cell line B16F10. Down-regulation of the endo-exonuclease sensitizes the cell to 5-fluorouracil. These studies suggested the endo-exonuclease enzyme as a novel potential therapeutic target for cancer.

Transient expression of Saccharomyces cerevisiae endo-exonuclease NUD1 gene increases the frequency of extrachromosomal homologous recombination in mouse Ltk- fibroblasts

Endo-exonucleases (EEs) are nucleolytic enzymes which have been shown to participate in the processes of DNA repair and recombination in eukaryotes. Recently, we have demonstrated that transient expression of Saccharomyces cerevisiae EE NUD1 gene in HeLa cells increased the resistance of the latter to ionizing radiation and cisplatin, suggesting the involvement of the NUD1 gene product in the recombination repair of double-strand breaks (DSB). Here, we report that transient expression of NUD1 results in up to 62% increase in the frequency of homologous recombination between two co-transfected linear plasmids in mouse Ltk- cells.

Heterogeneity of in vitro radiosensitivity in human bladder cancer cells

Human bladder cancer is often heterogeneous containing biologically different populations. Radiotherapy plus chemotherapy is the most common treatment for invasive disease. However few studies have investigated the role of heterogeneity in determining radiosensitivity. The radiation sensitivities of a parent human bladder cancer cell line (UCRU-BL-17CL) and nine cloned cell lines derived from it were determined. These cloned cell lines were previously shown to exhibit different biological characteristics when grown in nude mice. Radiation sensitivity was determined using both MTT and clonogenic assays. The radiobiological parameters, alpha,beta, and surviving fractions at 2 Gy and 8 Gy from the linear-quadratic model, were used to assess radiation sensitivity in the statistical analyses. The nine clones differed in radiosensitivity by both assays. By MTT, but not by the clonogenic assay, their radiation sensitivities were relatively consistent within each of the three biological groups (non-tumorigenic, tumorigenic, invasive); invasive clones were more sensitive than those of the non-tumorigenic and the tumorigenic groups for all the three-test criteria. The heterogeneity exhibited by this cell line may explain some of the variations in the clinical responses seen in the radiation treatment of invasive bladder cancer.

The effect of the Saccharomyces cerevisiae endo-exonuclease NUD1 gene expression on the resistance of HeLa cells to DNA-damaging agents

HeLa cells transiently transfected with a mammalian expression DNA vector expressing the Saccharomyces cerevisiae endo-exonuclease (EE) NUD1 gene have exhibited changes in cell survival frequencies after treatment with different DNA-damaging agents as compared to HeLa cells transfected with a control plasmid. The NUD1-transfected cells showed a dose-dependent increase in sensitivity to UV irradiation resulting in up to 58% decrease in cell survival. In response to gamma-irradiation NUD1 transfected cells featured an increased survival at doses equal to and greater than 2.0 Gy, reaching a maximum enhancement in survival frequency of 17%. At the same time, the NUD1-transfectants featured an increase in resistance to 0.25 microM-0.5 microM cis-platin (up to 58% increase in cell survival) and 1.0 mM EMS (11% increase). At higher concentrations of EMS NUD1 expression resulted in a decreased cell survival of the transfected cells (17% decrease for 2.5 mM EMS). No difference in cell survival frequencies between the NUD1-transfectants and the controls was observed after treatment with different concentrations of chlorambucil and mechlorethamine. These results suggest possible roles played by EEs in different DNA repair pathways--being stimulatory for the repair of certain types of DNA lesions, such as double strand breaks (DSBs), and interfering with the endogenous DNA repair systems for the repair of other types of lesions. Furthermore, these results also provide additional indirect evidence for the role of EEs in homologous recombination.

Endo-exonuclease of human leukaemic cells: evidence for a role in apoptosis

Inactive forms of endo-exonuclease, activated in vitro by treatment with trypsin, have been identified in human leukaemic CEM and MOLT-4 cells. They comprise over 95% of the total single-strand DNase activity in nuclei and are mainly bound to chromatin and the nuclear matrix. The activated enzyme had Mg2+(Mn2+)-dependent, Ca(2+)-stimulated activities with single- and double-strand DNAs and RNA (polyriboadenylic acid) and other properties characteristic of endo-exonucleases previously described. At least twice as much inactive endo-exonuclease has also been localised in extranuclear compartments of CEM and MOLT-4 cells, 85% bound to the membranes of the endoplasmic reticulum and 15% free in the cytosol. The soluble cytosolic trypsin-activatable endo-exonuclease was immunoprecipitated by antibodies raised independently to both Neurospora and monkey CV-1 cell endo-exonucleases. The free and bound enzymes of both nuclear and extranuclear compartments also cross-reacted on immunoblots with the antibody raised to Neurospora endo-exonuclease to reveal multiple polypeptides ranging in size from 18 to 145 kDa, many of which exhibited activity on DNA gels. The major species bound to the chromatin/matrix were in the 55–63 kDa range. Limited proteolysis of the large polypeptides to those of 18 to 46 kDa accompanied spontaneous chromatin DNA fragmentation to form DNA “ladders' in an isolated nuclei/cytosol system. When the leukaemic cells were treated in culture with either etoposide or podophyllotoxin to induce apoptosis, the largest polypeptides disappeared and smaller endo-exonuclease-related polypeptides of 18 to 46 kDa were detected in the nuclear extracts. The appearance of these polypeptides also correlated with extensive chromatin DNA fragmentation. In addition, there were correlations between the depletion of the major 55–63 kDa species bound to the membranes of the endoplasmic reticulum, depletion of the extranuclear trypsin-activatable activity and the onset and extent of chromatin DNA fragmentation in both cell lines. The extranuclear 55–63 kDa species may be precursors of the chromatin/matrix bound endo-exonuclease. The results indicate that endo-exonuclease plays a role in chromatin DNA degradation in mammalian cells during apoptosis.