DNA damage repair in quiescent murine mammary carcinoma cells in culture.

Murine mammary carcinoma cells (line 67) were grown in unfed cultures for up to 9 days. In cultures (day 2-3) in which cells were proliferatively active and in day 3-5 (transition) cells, a large fraction of nuclear DNA was retained on polycarbonate filters when assayed by the alkaline filter elution technique. In contrast, the fraction of DNA retained on filters was significantly reduced for nonproliferating (Q, quiescent) cells from unfed 7-9 day cultures. The increase in endogenous DNA breaks followed both the decrease in proliferative state and clonogenicity in these cells. When day 7 Q cells were refed these endogenous DNA breaks were removed with a half-time of about 2.5 h. When the cells were exposed to X-irradiation and the integrity of their nuclear DNA measured by the alkaline filter elution assay, as much as a 2-fold greater frequency of radiation-induced DNA breaks was produced in Q versus P cells. DNA breaks were also removed from irradiated Q cells at a rate which was 0.23 that observed in P cells. We suggest that the depressed capacity for DNA damage removal in Q cells is responsible for their greater radiosensitivity, and the impaired DNA damage repair is probably due to a reduced level of energy sources in these unfed Q cell cultures.