Topoisomerase II binds importin α isoforms and exportin/CRM1 but does not shuttle between the nucleus and cytoplasm in proliferating cells

Resistance to anticancer drugs that target DNA topoisomerase II (topo II) isoforms α and/or β is associated with decreased nuclear and increased cytoplasmic topo IIα. Earlier studies have confirmed that functional nuclear localization and export signal sequences (NLS and NES) are present in both isoforms. In this study, we show that topo II α and β bind and are imported into the nucleus by importin α1, α3, and α5 in conjunction with importin β. Topo IIα also binds exportin/CRM1 in vitro. However, wild-type topo IIα has only been observed in the cytoplasm of cells that are entering plateau phase growth. This suggests that topo IIα may shuttle between the nucleus and the cytoplasm with the equilibrium towards the nucleus in proliferating cells but towards the cytoplasm in plateau phase cells. The CRM1 inhibitor Leptomycin B increases the nuclear localization of GFP-tagged topo IIα with a mutant NLS, suggesting that its export is being inhibited. However, homokaryon shuttling experiments indicate that fluorescence-tagged wild-type topo II α and β proteins do not shuttle in proliferating Cos-1 or HeLa cells. We conclude that topo II α and β nuclear export is inhibited in proliferating cells so that these proteins do not shuttle.