Immunosuppressant FTY720 induces apoptosis by direct induction of permeability transition and release of cytochrome c from mitochondria

FTY720 has immunosuppressive activity in experimental organ transplantation and shows a prompt and protracted decrease of blood T lymphocytes upon ora l administration. The blood lymphocyte decrease in vivo was mainly a result of FTY720-induced apoptosis, However, this apoptotic mechanism is not well understood. We examined the mechanism of FTY720-induced apoptosis in lymph oma. Western blotting and fluorescent caspase-specific substrate revealed t hat caspase-3 is involved in FTY720-induced apoptosis, whereas caspase-1 is not. Apoptotic cell death was inhibited by the pan-caspase inhibitor, Z-VA D-FMK, suggesting that caspase activation is essential for FTY720-induced a poptosis, FTY720 reduced mitochondrial transmembrane potential and released cytochrome c from the mitochondria of intact cells as well as in a cell-fr ee system even in the presence of Z-VAD-FMK. As these mitochondrial reactio ns occurred before caspase activation, we concluded that FTY720 directly in fluences mitochondrial functions. The inhibition of mitochondrial permeabil ity transition by Bcl-2 overexpression or by chemical inhibitors prevented all apoptotic events occurring in intact cells and in a cell-free system. M oreover, using a cell-free system, FTY720 did not directly affect isolated nuclei or cytosol, These results indicate that FTY720 directly affects mito chondria and triggers permeability transition to induce further apoptotic e vents.