Article

Differential Regulation of Caspase-1 Activation, Pyroptosis, and Autophagy via Ipaf and ASC in Shigella-Infected Macrophages

Division of Bacterial Pathogenesis, Graduate School of Medicine, University of the Ryukyus, Okinawa, Japan.
PLoS Pathogens (Impact Factor: 8.06). 09/2007; 3(8):e111. DOI: 10.1371/journal.ppat.0030111
Source: PubMed

ABSTRACT Author Summary

Shigella are bacterial pathogens that are the cause of bacillary dysentery known as shigellosis. A crucial aspect of the propensity of Shigella to cause diseases lies in its ability to invade the cytoplasm of epithelial cells as well as macrophages. The bacterial invasion of macrophages induces pyroptosis, the proinflammatory cell death associated with caspase-1 activation. Activated caspase-1 then cleaves and activates prointerleukin (proIL)-1β and proIL-18, which are proinflammatory cytokines involved in host inflammatory responses. However, the precise mechanisms of caspase-1 activation induced by Shigella infection remain poorly understood. Ipaf, a cytosolic pattern-recognition receptor of the nucleotide-binding oligomerization domain (NOD)-like receptor (NLR) family, is a crucial host factor that activates caspase-1 through the sensing of flagellin produced by some bacteria, such as Salmonella or Legionella. We discovered that Ipaf and the adaptor protein ASC are required for caspase-1 activation induced by non-flagellated Shigella infection. Thus, Ipaf and ASC mediate caspase-1 activation by sensing an unknown bacterial factor, but not flagellin. Autophagy, a cellular system for eliminating intracellular pathogens, was dramatically enhanced in Shigella-infected macrophages by the absence of caspase-1 or Ipaf, but not ASC. The inhibition of autophagy promoted Shigella-induced cell death, suggesting that autophagy protects infected macrophages from pyroptosis. This study provides evidence that in Shigella-infected macrophages, autophagy is inhibited by Ipaf and caspase-1, but positively regulated by ASC, providing a novel function for NLR proteins in bacterial–host interactions.

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