It has been well established that
Clostridium difficile toxin A (TcdA) induces cell death in human epithelial cells. However, the mechanism of TcdA-induced cell death remains to be fully characterized. Here, we show that TcdA induces dose-dependent cell death in ovarian
carcinoma and colonic
carcinoma cell lines. TcdA-mediated cell death, as well as
caspase 8 and
caspase 3 activation, were specifically abrogated by anti-toxin
antibodies. Although
caspase 8 and
caspase 3 were activated by TcdA in OVCAR3 ovarian
carcinoma and T84
colonic cancer cells, pancaspase and
caspase 8, 3, and 9 inhibitors did not block TcdA-induced cell death. In contrast,
tumor necrosis factor-related apoptosis-inducing
ligand-induced cell death was nearly completely blocked by
caspase inhibitors in OVCAR3 cells. In these cells, TcdA induces the mitochondrial pathway of apoptosis, as demonstrated by changes in mitochondrial outer membrane permeabilization (MOMP). Furthermore, overexpression of the antiapoptotic
proteins Bcl-2 and Bcl-X(L) significantly inhibited TcdA-induced cell death, as well as TcdA-induced MOMP. Conversely,
small interfering RNA-mediated inhibition of Bcl-X(L) in TcdA-resistant SKOV3ip1 cells enhanced TcdA-induced cell death. Overexpression of the antiapoptotic
proteins Bcl-2 and Bcl-X(L) in T84 cells also inhibited TcdA-induced cell death. Altogether, our data demonstrate that TcdA induces cell death in both ovarian and
colonic cancer cells preferentially via the mitochondrial pathway of apoptosis by a
death receptor-independent and a
caspase-independent mechanism. This process is regulated by antiapoptotic members of the Bcl-2 family.