Effects of cytotoxic drugs on translocation of nucleolar RNA helicase RH-II/Gu.

Some cytotoxic drugs cause translocation of nucleophosmin/B23 and other nucleolar proteins to the nucleoplasm. The present study shows that these drugs caused a similar translocation of RH-II/Gu, a nucleolar RNA helicase. Other nucleolar proteins including p120, UBF, RNA polymerase I large subunit, fibrillarin, p40, and Ren-1 did not translocate. A 2-h treatment of MCF-7 breast cancer cells with 0.008 or 0.16 microM actinomycin D resulted in translocation of RH-II/Gu to the nucleoplasm; these effects were not reversed by 100 microM guanosine. The effects of 0.008 microM actinomycin D, but not 0.16 microM actinomycin D, on the translocation of RH-II/Gu were reversed when the drug was removed. However, the effects of 0.008 or 0.16 microM actinomycin D on the translocation of nucleophosmin/B23 were not reversible. The translocation effects of 50 microM toyocamycin on RH-II/Gu were reversed when the drug was replaced with fresh medium. RH-II/Gu mostly relocalized to the nucleoli within 15 min after toyocamycin was withdrawn; only partial relocalization of nucleophosmin/B23 occurred 40 h after removal of the drug. The effects of toyocamycin were not blocked by 100 microM guanosine. Mycophenolic acid (50 microM, 2-h treatment) caused partial translocation of RH-II/Gu; this effect was slowly reversed upon drug removal and was inhibited by 100 microM guanosine, in a manner similar to the effects of mycophenolic acid on the localization of nucleophosmin/B23. This study shows similarities and differences in the drug-induced translocation and relocalization of RH-II/Gu and nucleophosmin/B23. Analysis of translocation of specific nucleolar proteins may offer a quantitative approach to assessment of potency and duration of effects of cytotoxic agents.