CATHEPSIN B-MEDIATED ACTIVATION OF THE PROINFLAMMATORY CASPASE-11

Members of the caspase (CASP) family of cysteine proteases can be subd ivided in proapoptotic caspases and proinflammatory caspases. Whereas the apical activation pathways for the caspases that are involved in t he execution of the apoptotic process are beginning to be understood, the pathways that lead to the activation of proinflammatory caspases a re still largely unknown. Analysis of subcellular fractions for their ability to process and activate several caspases in vitro led to the i dentification of lysosomes as the source for a protease that could pro teolytically activate the proinflammatory CASP-11. Although this lysos omal activity was sensitive to caspase inhibitors, affinity purificati on with the biotinylated broad spectrum caspase inhibitor z-VAD,fmk re vealed the CASP-11 activating protease as cathepsin B. Activation of C ASP-11 by cathepsin B as well as its sensitivity to several caspase in hibitors was further confirmed with purified proteases. Similar to the role of mitochondrial factors in the activation of proapoptotic caspa ses, our results suggest a potential role for lysosomes and cathepsin B as activators of specific proinflammatory caspases. In addition, the aspecific inhibition of cathepsin B by so-called specific caspase inh ibitors implicates that results obtained with these inhibitors should be interpreted with care. (C) 1998 Academic Press.