Granulysin induces cathepsin B release from lysosomes of target tumor cells to attack mitochondria through processing of bid leading to necroptosis

Granulysin is a killer effector molecule localized in cytolytic granules of human NK and CTL cells. Granulysin exhibits broad antimicrobial activity and potent cytotoxic action against tumor cells. However, the molecular mechanism of granulysin-induced tumor lysis is poorly understood. In this study, we found that granulysin causes a novel cell death termed necroptosis. Granulysin can target lysosomes of target tumor cells and induce partial release of lysosomal contents into the cytosol. Relocalized lysosomal cathepsin B can process Bid to active tBid to cause cytochrome c and apoptosis-activating factor release from mitochondria. Cathepsin B silencing and Bid or Bax/Bak deficiency resists granulysin-induced cytochrome c and apoptosis-activating factor release and is less susceptible to cytolysis against target tumor cells. The Journal of Immunology, 2009, 182: 6993–7000. G ranule exocytosis-mediated cytolysis plays a major role inthe innate and adaptive immunity against intracellularpathogens and tumor cells. Granulysin (GNLY)3 is colo-calized in the cytotoxic granules along with a pore-forming protein, perforin (1, 2). GNLY has two major forms, 15 and 9 kDa. The 9-kDa form is processed from the larger precursor 15-kDa form and only the 9-kDa GNLY exerts cytolytic activity (3). GNLY is a member of the saposin-like protein family of lipid-binding proteins that includes amoebapores, saposins, and NK lysin (4–6). GNLY exhibits broad-spectrum antimicrobial activity against both Gram-positive and-neg