Apoptosis of CTLL-2 cells induced by an immunosuppressant, ISP-I, is caspase-3-like protease-independent

In our previous study, the sphingosine-like immunosuppressant ISP-1 was sho wn to induce apoptosis in the mouse cytotoxic T cell line CTLL-8, In this s tudy, we characterized the ISP-1-induced apoptotic pathway, Although caspas e-3-like protease activity increases concomitantly with ISP-1-induced apopt osis in CTLL-2 cells, the apoptosis is not inhibited by caspase-3-like prot ease inhibitors, i.e. DEVD-cho and z-DEVD-fmk. In contrast, sphingosine-ind uced apoptosis in CTLL-2 cells is caspase-2-like protease-dependent. A casp ase inhibitor with broad specificity, z-VAD-fmk, protects cells from apopto sis induced by ISP-1, indicating that TSP-l-induced apoptosis is dependent on caspase(s) other than caspase-3, Overexpression of Bcl-2 or Bcl-xL, supp resses the apoptosis induced by ISP-1, although sphingosine-induced apoptos is is not efficiently inhibited by Bcl-2, Finally, ISP-1-induced mitochondr ial depolarization, which is thought to be a checkpoint dividing the apopto tic pathway into upstream and downstream stages, is not inhibited by DEVD-c ho, but is inhibited by z-VAD-fmk, These data suggest that a pathway depend ent on caspase(s) other than caspase-3 is involved upstream of mitochondria l depolarization in ISP-1-induced apoptosis.