DNA Damage and Cellular Stress Responses The Incorporation of 5-Fluorouracil into RNA Affects the Ribonucleolytic Activity of the Exosome Subunit Rrp6

5-Fluorouracil (5FU) is a fluoropyrimidine used for the treatment of solid tumors. 5FU is a precursor of dTTP and UTP during biogenesis, and it interferes with both DNA and RNA metabolism. The RNA exosome, a multisubunit complex with ribonucleolytic activity, has been identified as one of the targets of 5FU in yeast. Studies in human cells have shown that the catalytic subunit of the nuclear exosome, Rrp6, is specifically targeted. Here, we have investigated the direct effect of 5FU on the activity of Rrp6 in Drosophila S2 cells, and we have identified two aspects of Rrp6 function that are altered by 5FU. First, gel filtration analysis revealed that the repertoire of multimolecular complexes that contain Rrp6 is modified by exposure to 5FU, which is consistent with the proposal that incorporation of 5FU into RNA leads to the sequestration of Rrp6 in ribonucleoprotein complexes. Second, the incorporation of 5FU into RNA renders the RNA less susceptible to degradation by Rrp6, as shown by Rrp6 activity assays in vitro. Our results imply that aberrant transcripts synthesized in 5FU-treated cells cannot be turned over efficiently by the surveillance machinery. Together with previous results on the mechanisms of action of 5FU, our findings suggest that the cytotoxicity of 5FU at the RNA level is the result of at least three different effects: the increased levels of retroviral transcripts with mutagenic potential, the reduced synthesis of ribosomes, and the inhibition of the nuclear RNA surveillance pathways. Drugs that reinforce any of these effects may boost the cytotoxicity of 5FU. Mol Cancer Res; 9(3); 332–40. 2011 AACR