Inhibition of radiation-induced DNA-protein cross-link repair by glutathione depletion with L-buthionine sulfoximine.

Cells depleted of their glutathione (GSH) by treatment with L-buthionine sulfoximine (BSO) are more sensitive to ionizing radiation and chemotherapeutic agents. To assess the effects of GSH depletion on repair of radiation-induced DNA damage, we have determined DNA-protein cross-links (DPC) in A549 cells by a nitrocellulose filter binding assay. Untreated A549 cells have a low level of DPC (0.7%), whereas cells incubated with BSO for 92 hours have an elevated level of DPC (1.5%). The dose response for production of radiation-induced DPC is approximately 0.4% DNA bound per 10 Gy for both BSO-treated and non-pretreated cells. Cells not exposed to BSO repair 85% of the radiation-induced DPC in 4 hours. Less repair (55%) is observed for BSO-treated cells incubated for 4 hours postirradiation in conditioned medium, and DPC repair is nearly completely blocked if GSH-depleted cells are given fresh medium during the 4-hour repair interval. The DPC repair is not influenced by the pH of the media between 6.6 and 7.4. Data indicate that intracellular GSH regulates the level of a sulfhydryl-containing repair enzyme, and that other, as yet undefined, nutrients may further influence the steady-state level of DPC.

Enhancement in the aerobic toxicity of misonidazole and SR-2508 by buthionine sulfoximine and 4-hydroxypyrazole: the role of hydrogen peroxide.

Chronic aerobic exposure of A549 human lung carcinoma cell cultures to 0.1 mM L-buthionine-S,R-sulfoximine and 1 mM misonidazole, or 1 mM SR-2508 results in inhibition of cell growth and decreased clonogenic survival. These patterns are not apparent with the individual drug treatments. Both parameters demonstrate maximum toxicity after 72 hr in culture, which parallels the time required to deplete A549 cells of glutathione with 0.1 mM L-BSO under these growth conditions. Toxicity appears to be related to hydrogen peroxide-produced during 1 electron reduction of the nitro compounds in the presence of oxygen. A549 cells have a lowered capacity to reduce peroxide due to the effect of thiol depletion on the enzymes GSH-peroxidase and GSH-S-transferase, which require the tripeptide as a substrate. The addition of catalase, another important enzyme involved in peroxide reduction, partially reverses the observed toxicity. 4-Hydroxypyrazole, which inhibits endogenous catalase activity, causes an increase in the observed cytotoxicity. Similar effects of L-BSO and 4-hydroxypyrazole are seen for toxicity due to hydrogen peroxide being added directly to cell cultures.