EDD enhances cell survival and cisplatin resistance and is a therapeutic target for epithelial ovarian cancer

The E3 ubiquitin ligase EDD is overexpressed in recurrent, platinum-resistant ovarian cancers, suggesting a role in tumor survival and/or platinum resistance. EDD knockdown by small interfering RNA (siRNA) induced apoptosis in A2780ip2, OVCAR5 and ES-2 ovarian cancer cells, correlating with loss of the prosurvival protein myeloid cell leukemia sequence 1 (Mcl-1) through a glycogen synthase kinase 3 beta-independent mechanism. SiRNA to EDD or Mcl-1 induced comparable levels of apoptosis in A2780ip2 and ES-2 cells. Stable overexpression of Mcl-1 protected cells from apoptosis following EDD knockdown, accompanied by a loss of endogenous, but not exogenous, Mcl-1 protein, suggesting that EDD regulated Mcl-1 synthesis. Indeed, EDD knockdown induced a 1.87-fold decrease in Mcl-1 messenger RNA and EDD transfection enhanced murine Mcl-1 promoter-driven luciferase expression 5-fold. To separate EDD survival and potential cisplatin resistance functions, we generated EDD shRNA stable cell lines that could survive initial EDD knockdown and showed that these cells were 4-to 21-fold more sensitive to cisplatin. Moreover, transient EDD overexpression in COS-7 cells was sufficient to promote cisplatin resistance 2.4-fold, dependent upon its E3 ligase activity. In vivo, mouse intraperitoneal ES-2 and A2780ip2 xenograft experiments showed that mice treated with EDD siRNA by nanoliposomal delivery [1,2-dioleoyl-sn-glycero-3-phophatidylcholine (DOPC)] and cisplatin had significantly less tumor burden than those treated with control siRNA/DOPC alone (ES-2, 77.9% reduction, P = 0.004; A2780ip2, 75.9% reduction, P = 0.042) or control siRNA/DOPC with cisplatin in ES-2 (64.4% reduction, P = 0.035), with a trend in A2780ip2 (60.3% reduction, P = 0.168). These results identify EDD as a dual regulator of cell survival and cisplatin resistance and suggest that EDD is a therapeutic target for ovarian cancer.