Exposure of certain cell lines to the differentiation-inducing agent N-methylformamide (NMF) enhances their radiosensitivity. As part of an attempt to elucidate the mechanism of NMF-induced radiosensitization, we examined the effects of NMF on chromatin structure, as reflected by changes in DNA-protein cross-links (DPCs) and the chromatin protein/DNA ratio, in two cell lines, clone A and HCA-1. Both lines form a better-differentiated phenotype upon exposure to NMF, yet only clone A is radiosensitized. Ionizing radiation induced DPCs in a linear manner beginning at about 10 Gy and continuing to at least 50 Gy in both cell types. NMF treatment of HCA-1 cells did not affect the background level of DPCs, but it enhanced the formation of radiation-induced DPCs at each dose tested. In clone A cells, NMF exposure elevated the DPC background level more than two-fold, and modified radiation-induced DPCs. The dose response for radiation-induced DPCs in NMF-treated clone A cells consisted of a linear increase up to 12.5 Gy, which was greater than in untreated cells, followed by a plateau level of DPCs out to 50 Gy, the highest dose tested. NMF treatment of clone A, but not HCA-1, cells also increased the chromatin protein/DNA ratio by about 30–35%. In clone A cells, the increases in DPC background level and chromatin protein/DNA ratio as a function of NMF exposure time followed a pattern similar to that of the enhancement of radiosensitivity. These data suggested that modifications of chromatin structure, not involved in differentiation, may be associated with the radiosensitizing actions of NMF.
This work is supported by Grant CA-06294 from the National Institutes of Health.