Accumulation of DNA strand breaks and methotrexate cytotoxicity.

There was a progressive formation of strand breaks in mature DNA of Ehrlich ascites tumor cells that were treated with methotrexate. Cells were labeled with [14C]thymidine before incubation with methotrexate, and DNA strand breaks were measured by alkaline and by neutral filter elution methods. DNA single-strand breaks accumulated in a linear fashion as a function of time during the first 10 hr of incubation with 2 microM methotrexate. Thereafter, the accumulation of DNA strand breaks deviated from linearity because of progressive cell death. The extent of DNA strand breakage in cells that had been incubated with methotrexate for 24 hr was as high as in cells that had been irradiated with 300 rads. DNA strand breaks persisted in cells that were incubated, after exposure to methotrexate, in medium containing thymidine, hypoxanthine, and nonessential amino acids, indicating that these strand breaks were poorly repaired. Cell death commenced after 10 hr of incubation with methotrexate and continued during the following 3-4 days. These findings suggest that cell death was due to a lethal accumulation of DNA strand breaks. The formation of DNA strand breaks is probably due to inefficient DNA repair, resulting from the inhibition of syntheses of thymidylate and of purine nucleotides. The accumulation of DNA strand breaks was minimal in growth-arrested cells, which are resistant to methotrexate toxicity.