Follistatin‐like protein 1 enhances NLRP3 inflammasome‐mediated IL‐1β secretion from monocytes and macrophages

Follistatin-like protein 1 (FSTL-1) is overexpressed in a number of inflammatory conditions characterized by elevated IL-1β. Here we found that FSTL-1 serum concentration was increased three-fold in patients with bacterial sepsis and four-fold following administration of endotoxin (LPS) to mice. To test the contribution of FSTL-1 to IL-1β secretion, wild-type and FSTL-1-deficient mice were injected with LPS. While LPS induced IL-1β in the sera of wild-type mice, it was low or undetectable in FSTL-1-deficient mice. Monocytes/macrophages, a key source of IL-1β, do not normally express FSTL-1. However, FSTL-1 was found in tissue macrophages after injection of LPS into mouse footpads, demonstrating that macrophages are capable of taking up FSTL-1 at sites of inflammation. In vitro, intracellular FSTL-1 localized to the mitochondria. FSTL-1 activated the mitochondrial electron transport chain, increased the production of ATP (a key activator of the NLRP3 inflammasome) and IL-1β secretion. FSTL-1 also enhanced transcription of the NLRP3 and pro-caspase-1 genes, two components of the NLRP3 inflammasome. Adenovirus-mediated overexpression of FSTL-1 in mouse paws led to activation of the inflammasome complex and local secretion of IL-1β and IL-1β-related proinflammatory cytokines. These results suggest that FSTL-1 may act on the NLRP3 inflammasome to promote IL-1β secretion from monocytes/macrophages.