Enhanced p62 expression through impaired proteasomal degradation is involved in caspase-1 activation in monosodium urate crystal-induced interleukin-1β expression

Abstract

Objective: Evidence for the role of autophagy in the regulation of inflammation, especially IL-1β expression in response to monosodium urate (MSU) crystals, is presented. This study investigated the role of p62, a selective autophagy receptor in autophagy, in IL-1β production in MSU crystal-induced inflammation. Methods: IL-1β, TNF-α and IL-6 mRNA expression was measured by quantitative real-time PCR (qRT-PCR). Autophagy-related molecules such as p62, Cullin-3, microtubule-associated protein 1 light-chain 3 (LC3) I/II, ubiquitin, caspase-1 and mitogen-activated protein kinase (MAPK)-related proteins were measured by immunoblotting. Small interfering RNAs (siRNAs) for Atg16L1, IL-1β and p62 were used to silence each target gene. Results: MSU crystals accelerate the process of autophagosome formation and also induce impairment of proteasomal degradation, resulting in p62 accumulation in autophagy. Enhanced p62 accumulation by MSU crystals leads to IL-1β expression through activation of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), but not p38, of the MAPK pathway and is also involved in activation of caspase-1 in inflammasomes. Impaired autophagosome formation by Atg16L1 siRNA significantly amplified p62 levels, thereby producing enhanced inflammatory responses, including overexpression of IL-1β under stimulation of MSU crystals. IL-1β also induces p62 protein, and blocking IL-1β under stimulation of MSU crystals greatly reduced p62 levels. Conclusion: This study demonstrates that enhanced p62 expression through impaired proteasomal degradation by MSU crystals plays a crucial role in caspase-1 activation in MSU crystal-induced IL-1β production. p62 is required for activation of inflammasomes during acute inflammation in gout. © The Author 2014. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved.