Cell killing and the accumulation of breaks in the DNA of HEp-2 cells incubated in the presence of hydroxyurea.

Thymidine incorporation by HEp-2 cells was reduced 10-fold by 2.5 mM hydroxyurea (HU). The DNA in lysates from untreated cells sedimented through alkaline sucrose gradients as an aggregate plus a smaller component which could be chased into the aggregate. Only small DNA (ca. 47 S) was synthesized in nuclei when cells were incubated with HU. HU inhibition was reversible up-to 24 hr. Removal of inhibitor was followed by the conversion of small DNA into rapidly sedimenting material. After 24 hr incubation with HU, an increasing percentage of the small DNA failed to join the ag gregate. At about this time, fragmentation of native DNA molecules was observed, and there was a decrease in viability as measured by colony formation. The accumulation of double-strand breaks in DNA from cells incubated in the pres ence of HU presumably represents irreparable lethal damage.