The marine
alkaloid ascididemin (ASC) was shown to exert cytotoxicity even against multidrug-resistant
cancer cells. Here, we address the signaling pathways utilized by ASC to trigger apoptosis in Jurkat
leukemia T cells. We show that ASC (0.5-20 microM) induces a mitochondrial pathway that requires the activation of the initiator
caspase-2 upstream of mitochondria. ASC-triggered apoptosis occurred independent of CD95, but required
mitochondrial dysfunction. The activation of
caspase-2 was shown to precede the processing of
caspase-8, -9 and -3. The specific
caspase-2 inhibitor zVDVADfmk abrogated ASC-induced DNA fragmentation almost completely. Overexpression of Bcl-x(L) blocked
caspase-8 but not
caspase-2 processing. Conversely,
caspase-2 inhibition strongly reduced
caspase-9 activation. As a possible link between
caspase-2 and
mitochondrial dysfunction, Bid was found to be cleaved by ASC. In addition, JNK was activated by ASC upstream of mitochondria via
reactive oxygen species. The specific JNK inhibitor
SP600125 partially inhibited
caspase-2 and -9 processing as well as
cytochrome c release and DNA fragmentation indicating that JNK contributes to, but is not necessary for ASC-mediated apoptosis. Thus, ASC triggers a pathway in which early activation of
caspase-2 provides a possible link between its
DNA-damaging activity and the induction of
mitochondrial dysfunction. The activation of JNK contributes to this signaling upstream of mitochondria.