Dimethyl sulfoxide protects against thermal and epithermal neutron-induced cell death and mutagenesis of Chinese hamster ovary (CHO) cells

Changes of Cell Membrane Permeability Induced by DMSO and Ethanol in Suspension Cultures of Taxus Cuspidata

The changes of cell membrane permeability caused by dimethyl sulphoxide (DMSO) and ethanol, two commonly used solvents in study of water-insoluble elicitors, were investigated in suspension cultures of Taxus cuspidata. The extracellular medium became alkalinized in the case of DMSO while the medium pH fluctuated upon the addition of ethanol. When the content of DMSO or ethanol was larger than 2% (v/v), the concentration of intracellular malonyl dialdehyde (MDA) increased remarkably at day 5 compared to that of the control, while that of the extracellular MDA less changed at a DMSO content of below 2% (v/v) and increased rapidly within 15 min at a DMSO content of 4% (v/v). The electrical conductivity (EC) decreased slightly when DMSO content was below 2% (v/v) but increased markedly at day 5 when DMSO content reached 4% (v/v). EC less varied when the content of ethanol was below 0.4% (v/v) but changed obviously when the ethanol content was larger than 1% (v/v). The cell membrane integrity hardly broke in the case of small concentration of DMSO (below 1%, v/v), but the presence of even small amount of ethanol (0.4%, v/v) caused cell membrane integrity lost partly, especially long time contact. It is thus concluded that DMSO is a more suitable solvent for water-insoluble elicitors compared to ethanol especially at low concentration levels.

Different contributions of the indirect effects of gamma-rays on the cytotoxicity in M10 and XRCC4 transfected M10 cells

The protective effects of dimethyl sulfoxide (DMSO) against cell killing by (137)Cs gamma-rays were investigated in XRCC4-deficient cell line M10, XRCC4-complemented M10 and the parental mouse leukemia cell line L5178Y. Cell survival was determined by the colony-forming ability. M10 cells were more sensitive to gamma-ray-induced cell death than L5178Y and complemented M10 cells. Cell survival was increased in both M10 and L5178Y in the presence of DMSO. However, estimation of the DMSO-protectable fraction revealed a smaller protectable fraction for M10 cells than for L5178Y cells, indicating that indirect effects contributed in a smaller extent to the cytotoxicity in M10 than that in L5178Y. This effect is due to XRCC4 deficiency, since transfection of XRCC4 cDNA into M10 cells restored the radioprotective effects of DMSO to the level seen in L5178Y. In M10 cells, the killing effects of high LET radiation (Auger electrons from (125)I-antipyrine, carbon ions with an LET of 166 keV microm(-1)) were similar to those of low LET radiation ((137)Cs gamma-rays, characteristic X-rays from (125)I-bovine serum albumin). We discuss that lethal lesions produced by indirect actions in L5178Y and XRCC4-complemented M10 cells may differ, at least in part, from DNA double-strand breaks repairable by non-homologous end joining.

A Bystander Effect Observed in Boron Neutron Capture Therapy: A Study of the Induction of Mutations in the HPRT Locus

PURPOSE

To investigate bystander mutagenic effects induced by alpha-particles during boron neutron capture therapy, we mixed cells that were electroporated with borocaptate sodium (BSH), which led to the accumulation of (10)B inside the cells, and cells that did not contain the boron compound. The BSH-containing cells were irradiated with alpha-particles produced by the 10B(n,alpha)7Li reaction, whereas cells without boron were affected only by the 1H(n,gamma)2H and 14N(n,rho)14C reactions.

METHODS AND MATERIALS

The lethality and mutagenicity measured by the frequency of mutations induced in the hypoxanthine-guanine phosphoribosyltransferase locus were examined in Chinese hamster ovary cells irradiated with neutrons (Kyoto University Research Reactor: 5 MW). Neutron irradiation of 1:1 mixtures of cells with and without BSH resulted in a survival fraction of 0.1, and the cells that did not contain BSH made up 99.4% of the resulting cell population. The molecular structures of the mutations were determined using multiplex polymerase chain reactions.

RESULTS

Because of the bystander effect, the frequency of mutations increased in the cells located nearby the BSH-containing cells compared with control cells. Molecular structural analysis indicated that most of the mutations induced by the bystander effect were point mutations and that the frequencies of total and partial deletions induced by the bystander effect were less than those induced by the original neutron irradiation.

CONCLUSION

These results suggested that in boron neutron capture therapy, the mutations caused by the bystander effect and those caused by the original neutron irradiation are induced by different mechanisms.

Ascorbic acid reduced mutagenicity at the HPRT locus in CHO cells against thermal neutron radiation

We investigated the biological effects of the long-lived radicals induced following neutron irradiation. It has been reported that radiation-induced long-lived radicals were scavenged by post-irradiation treatment of ascorbic acid (Koyama, 1998). We studied the effects of ascorbic acid acting as a long-lived radical scavenger on cell killing and mutagenicity in Chinese hamster ovary cells against thermal neutrons produced at the Kyoto University Research reactor. Ascorbic acid was added to cells 30 min after neutron irradiation and removed 150 min after irradiation. The biological end point of cell survival was measured by colony formation assay. The mutagenicity was measured by the mutant frequency in the HPRT locus. The post-irradiation treatment of ascorbic acid did not alter the cell killing effect of neutron radiation. However, the mutagenicity was decreased, especially when the cells were irradiated with boron. Our results suggested that ascorbic acid scavenged long-lived radicals effectively and caused apparent protective effects against mutagenicity of boron neutron capture therapy.

Mutagenic effect of borocaptate sodium and boronophenylalanine in neutron capture therapy

PURPOSE

To investigate the mutagenic effect in boron neutron capture therapy (BNCT), Chinese hamster ovary cells were incubated with 10 B-enriched borocaptate sodium (BSH) or para-boronophenylalanine (BPA) before exposure to thermal neutrons, and the occurrence of mutations at the hypoxanthine-guanine phosphoribosyl transferase (HPRT) locus was measured.

METHODS AND MATERIALS

BSH or BPA was added to cells 20 h or 2 h before irradiation and removed before irradiation. Cells were irradiated with thermal neutrons. The biologic end point of cell survival was measured by colony formation assay. The mutagenicity was calculated from the mutation frequency at the HPRT locus.

RESULTS

The mutagenicity of BSH and BPA was similar to that of 10B boric acid when the cells were irradiated with neutrons at an isosurvival dose after 2-h preincubation. Preincubation with BSH for 20 h, compared with preincubation for just 2 h, had no effect on either cytotoxicity or mutagenicity in BNCT. However, with BPA, 20-h preincubation, compared with 2-h preincubation, caused an increase in the cell killing effect, but a decrease in the mutagenic effect of the BNCT.

CONCLUSION

After 20-h incubation, BPA was less mutagenic than BSH. The mutagenic study of electroporated BPA or BSH revealed a reduced mutagenicity. These results suggest that the retention of these boron compounds in the cells causes a more accurate assault on the cell and lessens the chance of misrepair after neutron irradiation.