Chemical Tools Based on the Tetrapeptide Sequence of IL-18 Reveals Shared Specificities between Inflammatory and Apoptotic Initiator Caspases.

Caspases are a family of cysteine proteases that act as molecular scissors to cleave substrates and regulate biological processes such as programmed cell death and inflammation. Extensive efforts have been made to identify caspase substrates and to determine factors that dictate substrate specificity. We recently discovered that that the human inflammatory caspases (caspases-1, -4, and -5) cleave the cytokines IL-1β and IL-18 in a sequence-dependent manner. Here, we report the development of a new peptide-based probe and inhibitor based on the tetrapeptide sequence of IL-18 (LESD). We found that this inhibitor was most selective and potent at inhibiting caspase-8 activity (IC(50) = 50 nM). We also discovered that our LESD-based inhibitor is more potent than the currently used z-IETD-FMK inhibitor that is thought to be the most selective and potent inhibitor of caspase-8. Accordingly, we demonstrate that the LESD based inhibitor prevents caspase-8 activation during Yersinia pseudotuberculosis infection in primary bone-marrow derived macrophages. Furthermore, we characterize the selectivity and potency of currently known substrates and inhibitors for the apoptotic and inflammatory caspases using the same activity units of each caspase. Our findings reveal that VX-765, a known caspase-1 inhibitor, also inhibits caspase-8 (IC(50) = 1 μM) and even when specificities are shared, the caspases have different efficiencies and potencies for shared substrates and inhibitors. Altogether, we report the development of new tools that will facilitate the study of caspases and their roles in biology.