The ubiquitin-conjugating enzyme E2-EPF is overexpressed in primary breast cancer and modulates sensitivity to topoisomerase II inhibition

We identified the ubiquitin- conjugating enzyme E2EPF mRNA as differentially expressed in breast tumors relative to normal tissues and performed studies to elucidate its putative role in cancer. We demonstrated that overexpression of E-2-EPF protein correlated with estrogen receptor (ER) negativity in breast cancer specimens and that its expression is cell cycle regulated, suggesting a potential function for E-2- EPF in cell cycle progression. However, reduction of E2EPF protein levels by > 80% using RNAi had no significant effects on the proliferation of HeLa cervical cancer cells or ER- MDA-MB-231 or MDA- MB-453 breast cancer cells. Because E-2-EPF protein levels were elevated during the G(2)/ M phase of the cell cycle and because E-2-EPF mRNA in patient tumors was frequently coexpressed with genes involved in cell cycle control, spindle assembly, and mitotic surveillance, the possibility that E-2-EPF might have a function in the cellular response to agents that induce a G(2) checkpoint or an M checkpoint was investigated. E-2-EPF knockdown sensitized HeLa cells to the topoisomerase (topo) II inhibitors etoposide and doxorubicin and also increased topo IIA protein levels. These data suggest that combined administration of topo II directed drugs and E-2-EPF inhibitors may enhance their clinical effectiveness.