Protease activation in apoptosis induced by MAL

The proteolytic caspase cascade plays a central role in the signaling and e xecution steps of apoptosis. This study investigated the activation of diff erent caspases in apoptosis induced by MAL (a folding variant of human alph a-lactalbumin) isolated from human milk Our results show that the caspase-3 -like enzymes, and to a lesser extent the caspase-6-like enzymes, were acti vated in Jurkat and A549 cells exposed to MAL. Activated caspases subsequen tly cleaved several protein substrates, including PARP, lamin B, and alpha- fodrin. A broad-range caspase inhibitor, zVAD-fmk, blocked the caspase acti vation, the cleavage of proteins, and DNA fragmentation, indicating an impo rtant role for caspase activation in MAL-induced apoptosis. Since an antago nistic anti-CD9B receptor antibody, ZB4, did not influence the MAL-induced billing, we conclude that this process does not involve the CD95-mediated p athway. While MAL did not directly activate caspases in the cytosol, it col ocalized with mitochondria and induced the release of cytochrome c. Thus, t hese results demonstrate that caspases are activated and involved in apopto sis induced by MAL and that direct interaction of MAL with mitochondria lea ds to the release of cytochrome c, suggesting that this release is an impor tant step in the initiation and/or amplification of the caspase cascade in these cells. (C) 1999 Academic Press.