Combination therapy with 5-fluorouracil and IFN-α2a induces a nonrandom increase in DNA fragments of less than 3 megabases in HT29 colon carcinoma cells

We have used pulsed-field gel electrophoresis to examine 5-fluorouracil (5FU)-induced DNA double-strand breaks (DSBs), both with and without modulation by IFN-α2a (IFNα), in HT29 human colon adenocarcinoma cells. Although 24-h treatment with either 10 μM 5FU or 500 units/ml IFNα did not result in significant DNA fragmentation, the combination of 5FU + IFNα resulted in a significant increase in DNA DSBs versus either drug alone (P < 0.05). The pattern of fragmentation induced by treatment with 5FU + IFNα was compared to that induced by γ-radiation, which generates lesions at random sites, digestion with NotI restriction endonuclease, which cleaves at the specific sequence 5'... GCGGCCGC... 3', and HhaI restriction endonuclease, which cleaves at the specific sequence 5'... GCGC... 3'. 5FU + IFNα resulted in a specific pattern characterized by the accumulation of fragments of <3 Mb in the absence of fragments of >3 Mb, which differed from that of γ- radiation and restriction endonuclease digestion. Because neither morphological nor DNA fragmentation characteristic of apoptosis was observed after 5FU + IFNα treatment, the nonrandom pattern of DSBs that was observed did not appear to be the result of the initiation of programmed cell death within these cells.