Adipose triglyceride lipase suppresses noncanonical inflammasome by hydrolyzing LPS.

Intracellular recognition of lipopolysaccharide (LPS) by mouse caspase-11 or human caspase-4 is a vital event for the activation of the noncanonical inflammasome. Whether negative regulators are involved in intracellular LPS sensing is still elusive. Here we show that adipose triglyceride lipase (ATGL) is a negative regulator of the noncanonical inflammasome. Through screening for genes participating in the noncanonical inflammasome, ATGL is identified as a negative player for intracellular LPS signaling. ATGL binds LPS and catalyzes the removal of the acylated side chains that contain ester bonds. LPS with under-acylated side chains no longer activates the inflammatory caspases. Cells with ATGL deficiency exhibit enhanced immune responses when encountering intracellular LPS, including an elevated secretion of interleukin-1ß, decreased cell viability and increased cell cytotoxicity. Moreover, ATGL-deficient mice show exacerbated responses to endotoxin challenges. Our results uncover that ATGL degrades cytosolic LPS to suppress noncanonical inflammasome activation.

The orphan receptor Nur77 binds cytoplasmic LPS to activate the non-canonical NLRP3 inflammasome.

Intracellular sensing of lipopolysaccharide (LPS) by murine caspase-11 or human caspase-4 initiates a protease cascade, termed the non-canonical inflammasome, that results in gasdermin D (GSDMD) processing and subsequent NLRP3 inflammasome activation. In an effort aimed at identifying additional sensors for intracellular LPS by biochemical screening, we identified the nuclear orphan receptor Nur77 as an LPS-binding protein in macrophage lysates. Nr4a1-/- macrophages exhibited impaired activation of the NLRP3 inflammasome, but not caspase-11, in response to LPS. Biochemical mapping revealed that Nur77 bound LPS directly through a domain in its C terminus. Yeast two-hybrid assays identified NLRP3 as a binding partner for Nur77. The association between Nur77 and NLRP3 required the presence of LPS and dsDNA. The source of dsDNA was the mitochondria, requiring the formation of gasdermin-D pores. In vivo, Nur77 deficiency ameliorated host response to endotoxins. Thus, Nur77 functions as an intracellular LPS sensor, binding mitochondrial DNA and LPS to activate the non-canonical NLRP3 inflammasome.

The neuropeptide CGRP enters the macrophage cytosol to suppress the NLRP3 inflammasome during pulmonary infection.

The NLRP3 inflammasome plays an essential role in resistance to bacterial infection. The nervous system secretes multiple neuropeptides affecting the nervous system as well as immune cells. The precise impact of the neuropeptide CGRP on NLRP3 inflammasome activation is still unclear. Here, we show that CGRP negatively regulates the antibacterial process of host cells. CGRP prevents NLRP3 inflammasome activation and reduces mature IL-1ß secretion. Following NLRP3 inflammasome stimulation that triggers endosome leakage, CGRP internalized to endosomal compartments is released into the cell cytosol. Cytosolic CGRP binds directly to NLRP3 and dismantles the NLRP3-NEK7 complex, which is crucial for NLRP3 inflammasome activation. CGRP administration exacerbates bacterial infection, while the treatment with a CGRP antagonist has the opposite effect. Our study uncovers a unique role of CGRP in inhibiting inflammasome activation during infections, which might shed new light on antibacterial therapies in the future.