Chemical protection and cell-cycle effects on radiation-induced mutagenesis

Cell cycle dependence of ionizing radiation-induced DNA deletions and antioxidant radioprotection in Saccharomyces cerevisiae

The yeast DEL assay is an effective method for measuring intrachromosomal recombination events resulting in DNA deletions that when occurring in mammalian cells are often associated with genomic instability and carcinogenesis. Here we used the DEL assay to measure gamma-ray-induced DNA deletions throughout different phases of yeast culture growth. Whereas yeast survival differed by only up to twofold throughout the yeast growth phase, proliferating cells in lag and early exponential growth phases were tenfold more sensitive to ionizing radiation-induced DNA deletions than cells in stationary phase. Radiation-induced DNA deletion potential was found to correlate directly with the fraction of cells in S/G(2) phase. The ability of the antioxidants l-ascorbic acid and DMSO to protect against radiation-induced DNA deletions was also measured within the different phases of yeast culture growth. Yeast cells in lag and early exponential growth phases were uniquely protected by antioxidant treatment, whereas nondividing cells in stationary phase could not be protected against the induction of DNA deletions. These results are compared with those from mammalian cell studies, and the implications for radiation-induced carcinogenesis and radioprotection are discussed.

Flow cytometric detection of apoptotic bone marrow cells with fractional DNA content after application of WR-2721, cyclophosphamide, cisplatin, and exposure of mice to gamma rays

Little is known about the mechanisms of apoptosis triggered in normal cells of the haemopoietic system by the aminothiol WR-2721 (Amifostine), chemotherapeutic drugs, and ionizing radiation; thus, the present study was undertaken to evaluate the effects of WR-2721, cyclophosphamide (CP), cisplatin (CDDP), and 60Co gamma rays on induction of apoptotic DNA degradation in bone marrow cells. Adult male Swiss mice were treated with WR-2721 (400 mg/kg b.wt.), CP (200 mg/kg b.wt.), and CDDP (10 mg/kg b.wt.), and exposed to 6 Gy 60Co gamma rays. Alterations in the number of apoptotic cells with fractional DNA content and also the cell cycle position of the non-apoptotic cells were determined in the bone marrow at 7 and 24 hours after treatment of mice with these agents, using flow cytometric assay of the controlled extraction of low-MW DNA from apoptotic cells. The chemotherapeutic drugs CP and CDDP and 60Co gamma rays triggered apoptosis and affected the cell cycle position of the non-apoptotic cells in the mouse bone marrow. The pretreatment of mice with WR-2721 resulted in the modulatory action of the aminothiol on induction of apoptotic cell death and changes in the cell cycle distribution of the non-apoptotic cells caused by the DNA-damaging agents. The patterns of changes in the frequency of apoptotic cells and the cell cycle position of the non-apoptotic cells, observed in the bone marrow, were dependent on the agent(s) applied and the time interval after application of the drug(s) and exposure of mice to gamma rays. Understanding of the mechanisms responsible for triggering of apoptotic cell death and disturbing of the cell cycle by the DNA-damaging agents, and modulation of the apoptotic and cell cycle pathways by the aminothiol WR-2721, can lead to more effective therapy and chemo- and radio-protection of normal cells.

The cytoprotective aminothiol WR1065 activates p21waf-1 and down regulates cell cycle progression through a p53-dependent pathway

The phosphoaminothiol WR1065, the active metabolite of the pro-drug amifostine (WR2721), protects cultured cells and tissues against cytotoxic exposure to radiation or chemotherapeutic agents. We show here that WR1065 and the pro-drug WR2721 activate the p53 tumor suppressor protein and induce the expression of the cyclin-dependent kinase inhibitor p21waf-1 in the breast cancer cell line MCF-7, and in the mouse fibroblast cell line balb/c 3T3. Using two MCF-7 derived cell lines, MN1 and MDD2, we show that induction of p21waf-1 is detectable in MN1 (expressing a functional p53) but not in MDD2 (p53 disabled). These effects are observed at concentrations of WR1065 (0.5 to 1 mM) identical to those required to protect against cytotoxicity by hydrogen peroxide. Induction of p53 is not prevented by addition of aminoguanidine, an inhibitor of Cu-dependent amine-oxidases which blocks the extra-cellular degradation of WR1065 into toxic metabolites. Moreover, spermidine, a natural polyamine structurally related to amifostine, does not activate p53. Induction of p53 by WR1065 results in a delay in the G1/S transition in MCF-7 and MN-1 cells, but not in the p53 disabled cells MDD2. These data indicate that WR1065, a polyamine analog with thiol anti-oxidant properties, activates a cell cycle check-point involving p53.

Chromosome terminal deletion formation in Chinese hamster ovary cells

To investigate the fate of unrejoined DNA double-strand breaks, the frequency of 60Co gamma-ray- and restriction-enzyme-induced terminal chromosome deletions, a marker of unrejoined breaks, was determined in CHO-K1 and in xrs-5 cells. The xrs-5 cell is a DNA double-strand break repair-deficient derivative of CHO-K1. Terminal deletion frequency was small in both CHO-K1 and xrs-5 cells when cells were irradiated or treated with restriction enzyme while in the G1 phase of the cell cycle. In contrast, previous studies have shown that treatment of cells in G2 leads to large deletion frequencies, especially in xrs-5 cells. Cell cycle analyses show large G2 blocks in irradiated xrs-5 cells with only partial recovery over a 24-96-h period. These results suggest that most CHO cells with unrejoined breaks are blocked in G2 and, therefore, do not contribute to chromosome mutation frequencies. The small frequencies of terminal deletions that are found in these cells may reflect either an inefficiency in the G2 checkpoint mechanism or, perhaps, a modification of broken ends that allows passage through G2.