Abstract 4856: Zinc-induced autophagy in prostate cancer cells is associated with cell death

Abstract

Abstract Loss of cellular zinc accumulation is one of the characteristic features of prostate cancer, compared to the high zinc level in normal prostate epithelial cells. We previously demonstrated that zinc exposure induces apoptosis in human prostate malignant cells via a mitochondrial pathway in both in vitro and in vivo models, suggesting an important role of zinc homeostasis in growth control of prostate cancer. However, it remains unclear whether the apoptotic effect of zinc via the targeting of mitochondrial function is the only mechanism leading to growth inhibition in prostate cancer. Recent progress in autophagy indicates that this evolutionarily conserved catabolic process mediates the degradation of cellular components including damaged mitochondria and could promote either cell survival or cell death. The goal of this study was to ascertain the zinc effect on autophagy and its relationship to prostate cancer cell growth. Zinc induced autophagy was characterized in human prostate cancer PC-3 cells by Western blot analysis of LC-3, sub-cellular localization of ectopically expressed GFP-LC3, and transmission electron microscopy (TEM). Reactive oxygen species (ROS) production and mitochondrial membrane potential (MMP) were measured by fluorescence labeling and FACS analysis. Cell viability was assessed by MTT assay. Our results showed that zinc (15µM) significantly increased the conversion of LC3-I to LC3-II in a time-dependent manner (2.5 fold increase at 24 hrs), and induced the formation of punctuate GFP-LC3 structures in the cytoplasm as revealed by a fluorescence microscopy. Blockade of autophagy flux by inhibiting autolysosomal degradation using a thiol proteinase inhibitor, E-64d, greatly augmented zinc-induced LC3-II conversion. Electron microscopy observation revealed typical morphologic ultra-structures for autophagy including the formation of autophagosomes and autophagosome-related cytoplasmic structures. Inhibition of class III PI3K by 3-MA (3-methyladenine) attenuated zinc-induced autophagy and the cell death as quantified by MTT assay. Furthermore, zinc treatment led to a striking increase in ROS production and alteration in MMP, which were then abrogated by anti-oxidative N-acetyl-cysteine (NAC) associated with a significant suppression of zinc-induced autophagy and cell death. Our data indicate that zinc exposure induces autophagy in prostate malignant cells and leads to autophagy-related cell death, which is most likely due to direct targeting of mitochondrial function by zinc. Thus, we have provided evidence that the inhibitory role of zinc in prostate cancer cell growth may operate through the convergence of apoptosis and autophagy pathways, both of which are downstream from mitochondrial stress signaling leading to nuclear degradation and cellular metabolic alteration. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4856.