Inhibition of base excision repair by natamycin suppresses prostate cancer cell proliferation

Prostate cancer (PCa) progression is characterized by increased expression and transcriptional activity of the androgen receptor (AR). In the advanced stages of prostate cancer, AR significantly upregulates the expression of genes involved in DNA repair. Upregulation of expression for base excision repair (BER) related genes is associated with poor patient survival. Thus, inhibition of the BER pathway may prove to be an effective therapy for prostate cancer. Using a high throughput BER capacity screening assay, we sought to identify BER inhibitors that can synergize with castration therapy. An FDA-approved drug library was screened to identify inhibitors of BER using a fluorescence-based assay suitable for HTS. A gel-based secondary assay confirmed the reduction of BER capacity by compounds identified in the primary screen. Five compounds were then selected for further testing in the independently derived, androgen-dependent prostate cancer cell lines, LNCaP and LAPC4, and in the nonmalignant prostate derived cell lines PNT1A and RWPE1. Further analysis led to the identification of a lead compound, natamycin, as an effective inhibitor of key BER enzymes DNA polymerase β (pol β) and DNA Ligase I (LIG I). Natamycin significantly inhibited proliferation of PCa cells in an androgen depleted environment at 1 μM concentration, however, growth inhibition did not occur with nonmalignant prostate cell lines, suggesting that BER inhibition may improve efficacy of the castration therapies.