Mechanism responsible for inhibitory effect of indirubin 3′-oxime on anticancer agent-induced YB-1 nuclear translocation in HepG2 human hepatocellular carcinoma cells

YB-1 nuclear translocation/accumulation caused by anticancer agents leads to malignant transformation. Nuclear import of YB-1 requires a nuclear localization signal (YB-NLS). Previously, we identified five nucleocytoplasmic-shuttling proteins as YB-NLS binding proteins, and showed that they co-accumulate in the nucleus with YB-1 in response to treatment with actinomycin D. In addition, another group reported that transportin-1 is the molecule responsible for YB-1 nuclear translocation, binding to a region (PY-NLS) consistent with the YB-NLS. Recently, we found that indirubin 3′-oxime inhibits the nuclear localization of YB-1 in HepG2 cells and increases their sensitivity to actinomycin D. Here, we found that YB-1 nuclear translocation is dependent on the cellular mRNA level and that indirubin 3′-oxime inhibits the interaction between YB-1 and transportin-1. Interestingly, in cells showing inhibition of actinomycin D-induced YB-1 nuclear translocation by the compound, the YB-NLS-binding proteins as well as transportin-1 and its cargos were imported to the nucleus. Furthermore, the compound inhibited nuclear localization of the GFP-conjugated full-length YB-1 but not that of GFP-conjugated YB-NLS. These results indicate that indirubin 3′-oxime is a specific inhibitor of anticancer agent-induced YB-1 nuclear translocation, interacting with YB-1 itself in a region other than the YB-NLS/PY-NLS. This compound would increase the efficacy of cancer therapy.