Actinomycin D and gemcitabine synergistically sensitize androgen-independent prostate cancer cells to Apo2L/TRAIL-mediated apoptosis

The cytotoxic efficacy and kinetics involved in sensitization of Apo2L/TRAI L-resistant, androgen-independent prostate cancer cells to Apo2L/TRAIL or t umor necrosis factor-alpha or Fas ligand-mediated apoptosis were tested usi ng subclinical concentrations of actinomycin D, paclitaxel, cisplatinum, ge mcitabine, and radiation in CL-1, LNCaP, DU-145, and PC3 prostate cancer ce ll lines. CL-1 cells expressed all four Apo2L/TRAIL receptors and were resi stant to Apo2L/TRAIL-mediated apoptosis (1-5,000 ng/mL) and to the sensitiz ers when given alone. Pretreatment with actinomycin D followed by Apo2L/TRA IL or tumor necrosis factor-a or and-Fas CH-11 monoclonal antibody, but not in the reverse order, induced apoptosis. in all cell lines. Synergistic se nsitization in CL-1 cells was shown also with gemcitabine but not with cisp latinum, VP-16, paclitaxel, or radiation. Incubating the Apo2L/TRAIL-resist ant CL-1, LNCaP, DU-145, and PC3 cell lines with 100 ng/mL actinomycin D fo r 4 hours followed by Apo2L/TRAIL for 24 hours resulted in 45.4 +/- 10.3%, 58.8 +/-3.6%, 53.4 +/-1.4%, and 84.2 +/-8.4% apoptosis, respectively. Prolo nging the sensitization time to 24 hours followed by 20 hours of incubation with Apo2L/TRAIL further enhanced the killing activity against CL-1 cells to 89 +/-1% (Delta =60%, synergistic ratio = 3.1). This killing has a bipha sic pattern that was contributed to by apoptosis (83%) and necrosis (17%) a t 10 hours (peak) and 40% and 60%, respectively, at 20 hours. These results suggest that prostate cancer cells' resistance to Apo2L/TRAIL-mediated apo ptosis can be reversed and synergy is achieved by sensitization of tumor ce lls with subclinical concentrations of actinomycin D or gemcitabine and may be useful clinically for the treatment of metastatic hormone- and drug-ref ractory prostate cancer.