title: Cholesterol crystals activate the NLRP3 inflammasome in human macrophages: a novel link between cholesterol metabolism and inflammation.
abstract: BACKGROUND
  Chronic inflammation of the arterial wall is a key element in the pathogenesis of atherosclerosis, yet the factors that trigger and sustain the inflammation remain elusive. Inflammasomes are cytoplasmic caspase-1-activating protein complexes that promote maturation and secretion of the proinflammatory cytokines interleukin(IL)-1beta and IL-18. The most intensively studied inflammasome, NLRP3 inflammasome, is activated by diverse substances, including crystalline and particulate materials. As cholesterol crystals are abundant in atherosclerotic lesions, and IL-1beta has been linked to atherogenesis, we explored the possibility that cholesterol crystals promote inflammation by activating the inflammasome pathway.
  
  
  RESULTS
  Here we show that human macrophages avidly phagocytose cholesterol crystals and store the ingested cholesterol as cholesteryl esters. Importantly, cholesterol crystals induced dose-dependent secretion of mature IL-1beta from human monocytes and macrophages. The cholesterol crystal-induced secretion of IL-1beta was caspase-1-dependent, suggesting the involvement of an inflammasome-mediated pathway. Silencing of the NLRP3 receptor, the crucial component in NLRP3 inflammasome, completely abolished crystal-induced IL-1beta secretion, thus identifying NLRP3 inflammasome as the cholesterol crystal-responsive element in macrophages. The crystals were shown to induce leakage of the lysosomal protease cathepsin B into the cytoplasm and inhibition of this enzyme reduced cholesterol crystal-induced IL-1beta secretion, suggesting that NLRP3 inflammasome activation occurred via lysosomal destabilization.
  
  
  CONCLUSIONS
  The cholesterol crystal-induced inflammasome activation in macrophages may represent an important link between cholesterol metabolism and inflammation in atherosclerotic lesions.
authors.affiliation: Wihuri Research Institute, Helsinki, Finland. kristiina.rajamaki@helsinki.fi
authors.forename: Kristiina
authors.initials: K
authors.lastname: Rajamäki
authors.forename: Jani
authors.initials: J
authors.lastname: Lappalainen
authors.forename: Katariina
authors.initials: K
authors.lastname: Oörni
authors.forename: Elina
authors.initials: E
authors.lastname: Välimäki
authors.forename: Sampsa
authors.initials: S
authors.lastname: Matikainen
authors.forename: Petri T
authors.initials: PT
authors.lastname: Kovanen
authors.forename: Kari K
authors.initials: KK
authors.lastname: Eklund
mesh_terms: D015153:Blotting, Western; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D020170:Caspase 1 / Q000378:metabolism; D061945:Caspase Inhibitors; D002401:Cathepsin B / Q000378:metabolism; D002478:Cells, Cultured; D002784:Cholesterol / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology*; D002855:Chromatography, Thin Layer; D015638:Cytochalasin D / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism; D006801:Humans; D007249:Inflammation / Q000235:genetics / Q000276:immunology*; D053583:Interleukin-1beta / Q000378:metabolism; D008070:Lipopolysaccharides / Q000494:pharmacology; D008247:Lysosomes / Q000187:drug effects / Q000378:metabolism; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism*; D018613:Microscopy, Confocal; D009000:Monocytes / Q000187:drug effects / Q000378:metabolism; D000071199:NLR Family, Pyrin Domain-Containing 3 Protein; D010587:Phagocytosis / Q000187:drug effects / Q000502:physiology; D011188:Potassium / Q000378:metabolism; D034741:RNA, Small Interfering / Q000235:genetics / Q000502:physiology; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D002352:Carrier Proteins; D061945:Caspase Inhibitors; D053583:Interleukin-1beta; D008070:Lipopolysaccharides; D000071199:NLR Family, Pyrin Domain-Containing 3 Protein; C453881:NLRP3 protein, human; D034741:RNA, Small Interfering; D015638:Cytochalasin D; D002784:Cholesterol; D002401:Cathepsin B; D020170:Caspase 1; D011188:Potassium