Abstract
Recent studies have linked necrotic cell death and proteolysis of inflammatory proteins to the adaptive immune response mediated by the lysosome-destabilizing adjuvants, alum and Leu-Leu-OMe (LLOMe). However, the mechanism by which lysosome-destabilizing agents trigger necrosis and proteolysis of inflammatory proteins is poorly understood. The proteasome is a cellular complex that has been shown to regulate both necrotic cell death and proteolysis of inflammatory proteins. We found that the peptide aldehyde proteasome inhibitors, MG115 and MG132, block lysosome rupture, degradation of inflammatory proteins and necrotic cell death mediated by the lysosome-destabilizing peptide LLOMe. However, non-aldehyde proteasome inhibitors failed to prevent LLOMe-induced cell death suggesting that aldehyde proteasome inhibitors triggered a pleotropic effect. We have previously shown that cathepsin C controls lysosome rupture, necrotic cell death and the adaptive immune response mediated by LLOMe. Using recombinant cathepsin C, we found that aldehyde proteasome inhibitors directly block cathepsin C, which presumably prevents LLOMe toxicity. The cathepsin B inhibitor CA-074-Me also blocks lysosome rupture and necrotic cell death mediated by a wide range of necrosis inducers, including LLOMe. Using cathepsin-deficient cells and recombinant cathepsins, we demonstrate that the cathepsins B and C are not required for the CA-074-Me block of necrotic cell death. Taken together, our findings demonstrate that lysosome-destabilizing adjuvants trigger an early proteolytic cascade, involving cathepsin C and a CA-074-Me-dependent protease. Identification of these early events leading to lysosome rupture will be crucial in our understanding of processes controlling necrotic cell death and immune responses mediated by lysosome-destabilizing adjuvants.
Highlights
While research on programmed cell death has focused mainly on apoptosis, recent studies have highlighted the relevance of necrotic cell death in many biological and immunological processes
Taken together our findings indicated that aldehyde proteasome inhibitors and CA-074-Me target distinct proteins involved in a proteolytic cascade preceding lysosome rupture and necrotic cell death mediated by lysosomedisrupting adjuvants
Recent studies have demonstrated that necrotic cell death mediated by lysosome-destabilizing agents induces a strong adaptive immune response [9,11,28]
Summary
While research on programmed cell death has focused mainly on apoptosis, recent studies have highlighted the relevance of necrotic cell death in many biological and immunological processes. While necrosis was originally considered a traumatic disregulated process caused by direct chemical or radiologic insult [15], recent studies indicate that necrotic cell death is, like apoptosis, a highly regulated process with inducer-specific checkpoints [7,16]. The second form of necrotic cell death, necroptosis, is induced by specific death receptors, such as TNF-a and Trail, in the presence of caspase inhibitors [22,23]. Recent studies indicate that lysosome-destabilizing agents mediate a third form of programmed necrosis, termed as lysosome-mediated necrosis (LMN) [9,24,25,26,27]. Inducers of LMN include alum, silica crystals, cholesterol crystals, amyloid proteins, and the dipeptide methyl ester Leu-Leu-OMe (LLOMe) [9,24,28]
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