Purpose: Bisphosphonates are potent inhibitors of bone resorption. In vitro studies show that zolendronic acid inhibits prostate cancer cell growth by activating apoptosis. We investigated whether zolendronic acid also inhibits prostate cancer cell growth by autophagy (type II programmed cell death). Materials and Methods: We investigated the induction of autophagy in the PC-3, DU-145, LNCaP and CRW22Rv1 cell lines upon zolendronic acid treatment. LC3-II protein formation was detected by Western blot. LC3-II incorporation into autophagosomes was detected by immunofluorescence staining. Acidic organelle formation was detected by acridine orange staining. Rescue experiments using an apoptosis inhibitor and/or an autophagy inhibitor were performed by MTT assay. Results: Autophagy induction was detected by formation of the LC3-II protein after exposure to 100 μM zolendronic acid. LC3-II and caspase-3 processing was detected 6 days after treatment. Acidic organelles were detectable by acridine orange staining and immunofluorescence showed round-up and condensed staining of LC3-II, suggesting autophagosome formation in the cytoplasm during autophagic cell death. Cell growth was rescued only by administering an apoptosis and autophagy inhibitor during zolendronic acid treatment, indicating that zolendronic acid induces prostate cancer death by apoptotic and autophagic cell death. Conclusions: To our knowledge we report the first study showing that zolendronic acid markedly inhibits human prostate cancer cell growth through autophagic cell death. Zolendronic acid shows its anticancer activity via apoptosis and autophagy. These findings can potentially contribute to the beneficial use of zolendronic acid for prostate cancer treatment.
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