INHIBITION OF TOPOISOMERASE II-ALPHA ACTIVITY IN CHO K1 CELLS BY 2-[(AMINOPROPYL)AMINO]ETHANETHIOL (WR-1065)

The aminothiol 2-[(aminopropyl)amino]ethanethiol (WR-1065) is the acti ve thiol of the clinically studied radioprotective agent S-2-(3-aminop ropylamino)ethylphosphorothioic acid (WR-2721). WR-1065 is an effectiv e radiation protector when it is administered 30 min prior to exposure of Chinese hamster ovary K1 cells to radiation (i.e., a dose modifica tion factor of 1.4) at a concentration of 4 mM. Under these exposure c onditions, topoisomerase (Topo) I and II alpha activities and associat ed protein contents-were measured in cells of the K1 cell line using t he DNA relaxation assay, the P4 unknotting assay and immunoblotting, r espectively. WR-1065 was ineffective in modifying Topo I activity, but it did reduce Topo II alpha activity by an average of 50%. The magnit ude of Topo II alpha protein content, however, was not affected by the se exposure conditions. The effects on the cell cycle were monitored b y the method of flow cytometry. Exposure of cells to 4 mM WR-1065 for up to 6 h resulted in a build-up of cells in the G(2)/M-phase compartm ent. However, under these conditions and in contrast to Topo II inhibi tors used in chemotherapy, WR-1065 is an effective radioprotective age nt capable of protecting against both radiation-induced cell lethality and mutagenesis. One of several mechanisms of action attributed to am inothiol compounds such as WR-1065 has been their ability to affect en dogenous enzymatic reactions involved in DNA synthesis and repair and progression of cells through the phases of the cell cycle. These resul ts are consistent with such a proposed mechanism and demonstrate in pa rticular a modifying effect by WR-1065 on Topo II, which is involved i n DNA synthesis.