Breakage of DNA and Alterations in Folded Genomes by Inducers of Differentiation in Friend Erythroleukemic Cells

Friend erythroleukemia cells are induced to differentiate when treated with dimethyl sulfoxide (DMSO) and a number of other structurally unrelated agents, some of which are also known to alter the structure of DNA. In order to determine whether DMSO was responsible for DNA damage, the properties of [³H]thymidine-labeled DNA from untreated cells and cells treated with DMSO were compared on alkaline sucrose gradients. Criteria for DNA degradation were: (a) an increase in the radioactivity of slowly sedimenting approximately 17S DNA, (b) a decrease in the amount of radioactivity in the heavy, major approximately 140S peak, and (c) a shift in the position of the heavy peak to a lower S value. Increasing damage was found with increasing doses of DMSO. The earliest change, which was detected at 3.5 hr, was an increase in radioactivity in the 17S peak. Therefore, breaks in DNA were detected prior to the stimulation of globin messenger RNA and hemoglobin synthesis. Other potent inducers, such as butyrate, caused similar DNA degradation. Agents that cause breaks in DNA (X-ray, bleomycin, and UV) also were found to stimulate partial erythrodifferentiation in these cells. “Folded genomes” were prepared from 24-hr control and DMSO-treated cultures labeled with either [³H]-or [¹⁴C]thymidine, respectively. Cosedimentation of these labeled folded genomes revealed a decrease of approximately 10% in the S value of the DMSO-treated samples as compared to the control. Reverse labeling, in which control samples contained ¹⁴C and the DMSO samples contained ³H, gave similar results. These changes are consistent with the accumulation of single-stranded nicks in the DNA of the folded genomes. Therefore, single-stranded scission in DNA may be an early step in the control of differentiation.