| Summary: | The intestinal protozoan parasite, <i>Giardia duodenalis</i>, infects a large number of people in the world annually.<i> Giardia</i> infection has been considered a negative effect on intestinal epithelial cell growth, while the underlying mechanisms remain to be explored. Here we evaluated reactive oxygen species (ROS) production and apoptotic events in <i>Giardia</i> trophozoites-stimulated Caco-2 cells via fluorescence microscopy, transmission electron microscopy, flow cytometry, western blot, and cell counting kit-8 analyses. The results showed that <i>Giardia</i> trophozoite treatment could induce lactate dehydrogenase release and Caco-2 cell apoptosis. The ROS levels were increased post treatment. The observed typical characteristics of mitochondria damage include significant swelling and degeneration of matrix and cristae. After trophozoite treatment, the level of Bax protein expression was increased, while Bcl-2 protein decreased. Trophozoite stimulation also led to reduction of mitochondrial membrane potential and release of cytochrome c from the mitochondria to the cytoplasm, and this process was accompanied by activation of caspase-9 and caspase-3 and poly (ADP-ribose) polymerase 1 cleavage. Pretreatment with N-acetyl-L-cysteine, a ROS inhibitor, reversed <i>G. duodenalis</i>-induced Caco-2 cell apoptosis. Taken together, we indicated that <i>G. duodenalis </i>could induce Caco-2 cell apoptosis through a ROS- and mitochondria-mediated caspase-dependent pathway. This study furthers our understanding of the cellular mechanism of the interaction between <i>Giardia</i> trophozoites and host cells.
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