Reduction of 9-nitrocamptothecin-triggered apoptosis in DU-145 human prostate cancer cells by ectopic expression of 14-3-3ζ

A promising family of anticancer agents, the camptothecins, is noted for their ability to induce apoptosis specifically in malignant cells. However, a major obstacle for successful cancer treatment by these and other chemotherapeutic agents is the intrinsic or acquired resistance to drug treatment. Resistance to 9NC(6), a camptothecin derivative, has been modeled in vitro using a human prostate cancer cell line. To elucidate the mechanism for acquired 9NC resistance, we have used a subtractive cloning approach to identify genes whose altered expression level is reflective of 9NC resistance or susceptibility. Differential gene expression was compared between wild-type human prostate cancer cell line, DU-145, and a 9NC-resistant subline, RC I. Results were confirmed by Northern and Western blot analyses. In this report, we focus on one gene, 14-3-3xi. An expression vector of a full-length myc-epitope-tagged 14-3-3xi cDNA was constructed and used for transfection into DU-145 cells. We consistently observed that 14-3-3xi message and protein levels were dramatically increased in 9NC resistant cells. The expression levels of other 14-3-3 family members were unaffected. Strikingly, ectopic overexpression of 14-3-3xi in wild-type 9NC-susceptible prostate cancer cells decreased 9NC-induced apoptosis. Our results suggest a novel direct or indirect role of 14-3-3 in mediating resistance of DU-145 cells to the topoisomerase I inhibitor, 9NC. We are currently exploring whether this represents a more general pathway for drug resistance as well.