The SERCA pump as a therapeutic target: Making a "smart bomb" for prostate cancer

Prostate cancer is uniformly fatal once it has spread outside of the prostate gland. Prostate cancers have a remarkably low proliferative rate, which may in part explain their relative unresponsiveness to conventional antiproliferative chemotherapy. New therapies for prostate cancer that activate proliferation independent cell death are therefore needed. The endoplasmic reticulum (ER) has emerged as an organelle that plays a major role in cell signaling pathways, cellular response to stress and cellular activation of apoptosis. In this review, the SERCA pump is identified as an ER protein whose normal function is required by all cells and represents a potential therapeutic target for cancer therapy. Sustained SERCA inhibition by agents such as thapsigargin results in activation of ER-stress response and simultaneous activation of apoptotic pathways within the ER and the mitochondria. Due to the SERCA pump's critical role in normal cellular metabolism, agents like thapsigargin directed toward inhibiting SERCA function would likely produce significant toxicity to normal cells and, therefore, must be selectively targeted to cancer sites. The cytotoxicity of thapsigargin can be attenuated, however by coupling to a targeting peptide to produce an inactive prodrug that is only activated by prostate cancer specific proteases such as the serine protease prostate-specific antigen (PSA). PSA-activated thapsigargin prodrugs have been characterized that are selectively toxic to PSA-producing prostate cancer cells in vitro and in vivo. These prodrugs are currently undergoing pre-clinical evaluation as potential targeted therapy for prostate cancer.