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