An in vivo-in vitro alkaline DNA unwinding assay for hepatic DNA damage: comparison with the alkaline sucrose gradient centrifugation technique

DNA strand breaks in liver for four aliphatic epoxides in mice

Four aliphatic epoxides, 1-naphthyl glycidyl ether (NGE), 1-naphthylpropylene oxide (NPO), 4-nitrophenyl glycidyl ether (NPGE), 3,3,3-trichloropropylene oxide (TCPO) and two of their precursors, 1-allylnaphthalene (AN) and 3,3,3-trichloropropylene (TCP), were selected for DNA strand-break analysis in liver in vivo with mice. The four epoxides selected were among the most mutagenic aliphatic epoxides in our previous structure-mutagenicity studies with the Ames test and had been evaluated for their in vivo genotoxicity as measured by sister-chromatid exchange (SCE) and chromosome aberrations (CA). A significant increase in the percentage of unwound DNA was observed at a 4-h exposure time for all the compounds at high doses except for AN. TCPO, the least genotoxic compound in bone marrow, had the greatest liver toxicity after 1-h exposure while NGE showed the most toxicity after 6 h. As might be expected from their corresponding epoxides, AN but not TCP exhibited significant SCE activity in the bone marrow of mice. This study reemphasizes the importance of evaluating the stability of direct-acting alkylating agents in comparing test results and in establishing the relative order of genotoxicity for such compounds.

S1-nuclease enhancement of the ethidium bromide binding assay of drug-induced DNA interstrand crosslinking in human brain tumor cells

A modification, using S1-nuclease, of a simple and sensitive fluorometric assay with ethidium bromide was developed for the measurement of cellular DNA interstrand crosslinking induced by bifunctional alkylators. Cells are lysed and treated with proteinase K and sodium dodecyl sulfate followed by extensive dialysis to yield intact high-molecular-weight DNA, free of contaminating proteins, on which the crosslink assay is then performed. The assay depends on the differential binding of ethidium bromide to single- and double-stranded DNA. Because of the higher ethidium bromide binding capacity of double-stranded DNA, the fluorescence retained after a heating/cooling cycle is directly proportional to the drug-induced cellular DNA interstrand crosslinking. We demonstrate that the sensitivity of this assay can be increased up to fourfold by including an S1-nuclease digestion step. This modified technique is simple and suited to the quantitation of low levels of DNA-interstrand crosslinking in cells.

In vivo sister chromatid exchange induced by 1,2-dichloroethane on bone marrow cells of mice

In vivo sister chromatid exchange induced by 1,2-dichloroethane (DCE) was studied in bone marrow cells of mice after acute treatment for 24 hr. With the exception of the lowest concentration (0.5 mg/kg), each treated series exhibited a statistically significant increase in SCEs when compared with the control.

A search for DNA alterations in the aging mammalian genome: an experimental strategy

In order to experimentally test the hypothesis that at the basal level senescence is caused by instabilities in DNA, techniques are required that allow the sensitive detection, quantification and characterization of DNA damage and DNA sequence changes in various organs and tissues of naturally aging mammals. In this article a strategy is presented that should allow the analysis of DNA damage metabolism in aging mammals from the original changes in the chemical structure of DNA (DNA damages) via their processing (DNA repair) to the molecular endpoints in terms of gene mutations and DNA rearrangements. With respect to the detection of DNA damage, a short overview is provided of recently emerged biochemical and immunochemical methods that can be applied immediately to study the spectrum of DNA damages in various organs and tissues of aging animals or humans. By contrast, before one is able to study low frequency changes in DNA sequence organization in somatic tissues, formidable problems have yet to be overcome. This is mainly due to the fact that the scale of highly advanced recombinant DNA techniques recently developed is almost totally devoted to the analysis of heritable genetic factors. In order to be able to study low frequency changes in DNA sequence organization in somatic tissues, we have initiated experimental approaches based on recently emerged shuttle vector technology in combination with transgenic mice, and on new electrophoretic separation principles. These approaches and their potentialities for testing somatic mutation theories of aging are discussed.

Rapid analysis of DNA strand breaks in soft tissues

A technique was developed to measure rapidly DNA strand breaks in soft tissues. This method measured the rate of alkaline unwinding of DNA, which was proportional to strand breakage. Alkaline unwinding of DNA was done by treating tissue homogenates with NaOH. Single-stranded DNA was removed by extraction with aqueous phenol. DNA unwinding was quantitated by measuring the remaining double-stranded DNA. Using the described technique, a dose-effect relationship was observed between N-nitrosodimethylamine (NDMA) and alkaline unwinding of mouse liver DNA.

An investigation of the role of microsomal oxidative metabolism in the in vivo genotoxicity of 1,2-dichloroethane

In vitro studies have demonstrated that two different metabolic pathways, glutathione conjugation mediated by the glutathione S-transferases and microsomal oxidation, may be involved in the genotoxicity and carcinogenicity of 1,2-dichloroethane (DCE). To evaluate the importance of microsomal oxidative metabolism in the bioactivation of DCE in vivo, male B6C3F1 mice were pretreated with piperonyl butoxide (PIB), an inhibitor of microsomal oxidative metabolism, and the effect of this pretreatment on the extent of hepatic DNA damage produced by DCE was determined 4 hr after DCE administration. The in vivo genotoxicity of 2-chloroethanol, a product of the microsomal oxidative metabolism of DCE, was also investigated. Hepatic DNA damage was measured with a sensitive, alkaline DNA unwinding assay for the presence of single-strand breaks and alkali-labile lesions in DNA. Pretreatment of mice with PIB to inhibit microsomal oxidative metabolism significantly potentiated the hepatic DNA damage observed 4 hr after a single, 200-mg/kg, ip dose of DCE. Treatment of mice with single, ip doses of 2-chloroethanol as high as 1.2 mmol/kg failed to produce any evidence of single-strand breaks and/or alkali-labile lesions in hepatic DNA. When diethyl maleate (DEM) was used to deplete hepatic glutathione levels prior to administration of 2-chloroethanol, the acute hepatotoxicity of 2-chloroethanol was potentiated but again there was no evidence of hepatic damage. These results indicate that microsomal, oxidative metabolism of DCE to 2-chloroethanol and/or 2 chloroacetaldehyde is not responsible for the hepatic DNA damage observed in these studies after DCE administration.