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pubmed:abstractText |
Serum amyloid A (SAA) is an acute-phase protein, the serum levels of which can increase up to 1000-fold during inflammation. SAA has a pathogenic role in amyloid A-type amyloidosis, and increased serum levels of SAA correlate with the risk for cardiovascular diseases. IL-1? is a key proinflammatory cytokine, and its secretion is strictly controlled by the inflammasomes. We studied the role of SAA in the regulation of IL-1? production and activation of the inflammasome cascade in human and mouse macrophages, as well as in THP-1 cells. SAA could provide a signal for the induction of pro-IL-1? expression and for inflammasome activation, resulting in secretion of mature IL-1?. Blocking TLR2 and TLR4 attenuated SAA-induced expression of IL1B, whereas inhibition of caspase-1 and the ATP receptor P2X(7) abrogated the release of mature IL-1?. NLRP3 inflammasome consists of the NLRP3 receptor and the adaptor protein apoptosis-associated speck-like protein containing CARD (a caspase-recruitment domain) (ASC). SAA-mediated IL-1? secretion was markedly reduced in ASC(-/-) macrophages, and silencing NLRP3 decreased IL-1? secretion, confirming NLRP3 as the SAA-responsive inflammasome. Inflammasome activation was dependent on cathepsin B activity, but it was not associated with lysosomal destabilization. SAA also induced secretion of cathepsin B and ASC. In conclusion, SAA can induce the expression of pro-IL-1? and activation of the NLRP3 inflammasome via P2X(7) receptor and a cathepsin B-sensitive pathway. Thus, during systemic inflammation, SAA may promote the production of IL-1? in tissues. Furthermore, the SAA-induced secretion of active cathepsin B may lead to extracellular processing of SAA and, thus, potentially to the development of amyloid A amyloidosis.
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