Calcium-Independent Phospholipase A2β Is Dispensable in Inflammasome Activation and Its Inhibition by Bromoenol Lactone

Abstract

Calcium-independent phospholipase A₂ (iPLA₂) has been suggested to play an important role in the activation of caspase-1 induced by lipopolysaccharides (LPS). Here, we used pharmacological and genetic approaches to study the role of iPLA₂ in the activation of caspase-1. Bromoenol lactone (BEL), an inhibitor that was originally used to support a role for iPLA₂ in the secretion of IL-1β, prevented caspase-1 activation induced by LPS and ATP as described, and also activation triggered by Salmonella infection and cytosolic flagellin, which rely on the Nlrc4 inflammasome. Analysis of BEL enantiomers showed that the S-BEL form was more effective than R-BEL in inhibiting the inflammasome, suggesting a role for iPLA₂β. However, caspase-1 activation and IL-1β secretion and their inhibition by BEL were unimpaired in macrophages deficient in iPLA₂β. BEL was originally identified as an inhibitor of serine proteases. Consistent with the latter, the serine proteases inhibitors TPCK, TLCK and AAF-cmk prevented the activation of the Nlrc4 and Nlrp3 inflammasomes while pan-cathepsin inhibitors were ineffective. These results indicate that iPLA₂β is not critical for caspase-1 activation as currently proposed. Instead, the results suggest that serine protease(s) targeted by BEL may play a critical role in the activation of the inflammasome triggered by microbial stimuli.