NLRP3 regulates a non-canonical platform for caspase-8 activation during epithelial cell apoptosis

• Published in: Cell death and differentiation 2016-08, Vol.23 (8), p.1331-1346
• Main Authors: Chung, H, Vilaysane, A, Lau, A, Stahl, M, Morampudi, V, Bondzi-Simpson, A, Platnich, J M, Bracey, N A, French, M-C, Beck, P L, Chun, J, Vallance, B A, Muruve, D A
• Format: Article
• Language: English
• Subjects: 13/1; 13/106; 14; 14/19; 631/250/1933; 64/60; Adenosine Triphosphate - pharmacology; Animals; Apoptosis; Apoptosis - drug effects; Biochemistry; Biomedical and Life Sciences; Caspase 1 - genetics; Caspase 1 - metabolism; Caspase 8 - metabolism; Caspases - genetics; Caspases - metabolism; Cell Biology; Cell Cycle Analysis; Cell death; Cells, Cultured; Chronic illnesses; Cycloheximide - toxicity; Epithelial Cells - cytology; Epithelial Cells - drug effects; Epithelial Cells - metabolism; Homeostasis; Humans; Inflammasomes - metabolism; Interleukin-1beta - analysis; Interleukin-1beta - metabolism; Kidney Tubules, Proximal - cytology; Leukocytes; Life Sciences; Macrophages - cytology; Macrophages - drug effects; Macrophages - metabolism; Mice; Mice, Inbred C57BL; Mice, Knockout; Nigericin - pharmacology; NLR Family, Pyrin Domain-Containing 3 Protein - deficiency; NLR Family, Pyrin Domain-Containing 3 Protein - genetics; NLR Family, Pyrin Domain-Containing 3 Protein - metabolism; Original Paper; Proteins; Stem Cells; Tumor Necrosis Factor-alpha - toxicity; Tumor necrosis factor-TNF
• ISSN: 1350-9047; 1476-5403
• DOI: 10.1038/cdd.2016.14

Nod-like receptor, pyrin containing 3 (NLRP3) is characterized primarily as a canonical caspase-1 activating inflammasome in macrophages. NLRP3 is also expressed in the epithelium of the kidney and gut; however, its function remains largely undefined. Primary mouse tubular epithelial cells (TEC) lacking Nlrp3 displayed reduced apoptosis downstream of the tumor necrosis factor (TNF) receptor and CD95. TECs were identified as type II apoptotic cells that activated caspase-8, tBid and mitochondrial apoptosis via caspase-9, responses that were reduced in Nlrp3−/− cells. The activation of caspase-8 during extrinsic apoptosis induced by TNF α /cycloheximide (TNF α /CHX) was dependent on adaptor protein apoptosis-associated speck-like protein containing a CARD (ASC) and completely independent of caspase-1 or caspase-11. TECs and primary human proximal tubular epithelial cells (HPTC) did not activate a canonical inflammasome, caspase-1, or IL-1 β secretion in response to TNF α /CHX or NLRP3-dependent triggers, such as ATP or nigericin. In cell fractionation studies and by confocal microscopy, NLRP3 colocalized with ASC and caspase-8 in speck-like complexes at the mitochondria during apoptosis. The formation of NLRP3/ASC/caspase-8 specks in response to TNF α /CHX was downstream of TNFR signaling and dependent on potassium efflux. Epithelial ASC specks were present in enteroids undergoing apoptosis and in the injured tubules of wild-type but not Nlrp3−/− or ASC−/− mice following ureteric unilateral obstruction in vivo . These data show that NLRP3 and ASC form a conserved non-canonical platform for caspase-8 activation, independent of the inflammasome that regulates apoptosis within epithelial cells.