Human multiple myeloma cells are sensitized to topoisomerase II inhibitors by CRM1 inhibition.

Topoisomerase IIalpha (topo IIalpha) is exported from the nucleus of human myeloma cells by a CRM1-dependent mechanism at cellular densities similar to those found in patient bone marrow. When topo IIalpha is trafficked to the cytoplasm, it is not in contact with the DNA; thus, topo IIalpha inhibitors are unable to induce DNA-cleavable complexes and cell death. Using a CRM1 inhibitor or a CRM1-specific small interfering RNA (siRNA), we were able to block nuclear export of topo IIalpha as shown by immunofluorescence microscopy. Human myeloma cell lines and patient myeloma cells isolated from bone marrow were treated with a CRM1 inhibitor or CRM1-specific siRNA and exposed to doxorubicin or etoposide at high cell densities. CRM1-treated cell lines or myeloma patient cells were 4-fold more sensitive to topo II poisons as determined by an activated caspase assay. Normal cells were not significantly affected by CRM1-topo II inhibitor combination treatment. Cell death was correlated with increased DNA double-strand breaks as shown by the comet assay. Band depletion assays of CRM1 inhibitor-exposed myeloma cells showed increased topo IIalpha covalently bound to DNA. Topo IIalpha knockdown by a topo IIalpha-specific siRNA abrogated the CRM1-topo II therapy synergistic effect. These results suggest that blocking topo IIalpha nuclear export sensitizes myeloma cells to topo II inhibitors. This method of sensitizing myeloma cells suggests a new therapeutic approach to multiple myeloma.