Phenethyl isothiocyanate (PEITC) is a promising cancer chemopreventive agent but the mechanism of its anticancer effect is not fully understood. We now demonstrate, for the first time, that PEITC treatment triggers Atg5-dependent autophagic and apoptotic cell death in human prostate cancer cells. Exposure of PC-3 (androgen-independent, p53 null) and LNCaP (androgen-responsive, wild-type p53) human prostate cancer cells to PEITC resulted in several specific features characteristic of autophagy including appearance of membranous vacuoles, formation of acidic vesicular organelles, and cleavage and recruitment of microtubule-associated protein 1 light chain 3 (LC3) to autophagosomes. A normal human prostate epithelial cell line (PrEC) was markedly more resistant towards PEITC-mediated cleavage and recruitment of LC3 compared with prostate cancer cells. Even though PEITC treatment suppressed activating phosphorylations of Akt and mTOR, which are implicated in regulation of autophagy by different stimuli, processing and recruitment of LC3 was only partially/marginally reversed by ectopic expression of constitutively active Akt or overexpression of mTOR positive regulator Rheb. The PEITC-mediated apoptotic DNA fragmentation was significantly attenuated in the presence of a pharmacological inhibitor of autophagy (3-methyl adenine). Transient transfection of LNCaP and PC-3 cells with Atg5-specific siRNA conferred significant protection against PEITC-mediated autophagy as well as apoptotic DNA fragmentation. Xenograft model using PC-3 cells and C. elegans expressing lgg-1:GFP fusion protein provided evidence for occurrence of PEITC-induced autophagy in vivo. In conclusion, the present study indicates that Atg5 plays an important role in regulation of PEITC-induced autophagic and apoptotic cell death.
Keywords: Phenethyl isothiocyanate, Autophagy, Chemoprevention