Cell Death and Survival Inhibition of PP2A Activity Confers a TRAIL-Sensitive Phenotype during Malignant Transformation

TRAIL is a promising anticancer agent because it induces apoptosis in the majority of human cancer cells but spares the normal cells. To determine the mechanistic nature of how normal cells acquire a TRAIL-sensitive phenotype during the process ofmalignant transformation, an experimental cell systemwas developed by sequential introduction of human telomerase reverse transcriptase and SV40 T antigens (large and small) into normal human prostatic epithelial cells (PrEC). This model system demonstrated that inhibition of protein phosphatase 2A (PP2A), either by SV40 small T antigen, okadaic acid, Calyculin A, or PP2A catalytic subunit siRNA, sensitized normal human PrEC and immortalized cells to TRAIL-induced apoptosis.Moreover, sensitization occurred during the premalignant period of tumorigenesis and PP2A exerted its antiapoptotic activity by negatively regulating c-Fos/ AP-1. In addition, low-dose okadaic acid treatment sensitized TRAIL-resistant cancer cells to TRAIL, suggesting that PP2A inhibitors could be used as an enhancer of apoptosis induced byTRAIL orTRAIL-like agents. These data indicate that downregulation of PP2A activity is a critical step for normal cells to acquire a TRAIL-sensitive phenotype during tumorigenesis and that the level of PP2A activity may foretell cellular sensitivity to TRAIL-