Induction of ER stress is critical in Shikonin induced effects in DU-145 and PC-3 prostate cancer cells (A) Effect of Solubrinal or BAPTA on ER stress protein expression in DU-145 and PC-3 cells. Protein expression of ER-stress was examined in pre-treated with BAPTA or Salubrinal in DU-145 and PC-3 cells, and western blot experiments were performed as described previously. (B) Inhibition of intracellular calcium or calpain reveses shikonin induced inhibition of cell viability. Cell viability was quantified using the CCK-8 assay in BAPTA or Calpeptin pretreated DU-145 and PC-3 cells. Data is expressed in means ± SEM and represents the results of three independent experiments.