Enhanced p62 Is Responsible for Mitochondrial Pathway-Dependent Apoptosis and Interleukin-1β Production at the Early Phase by Monosodium Urate Crystals in Murine Macrophage

The aim of this study was to clarify the role of p62-dependent mitochondrial apoptosis in the initiation of monosodium urate (MSU) crystal-induced inflammation in macrophages. The induction of mitochondrial apoptosis in RAW 264.7 murine macrophages by MSU crystals was measured using western blotting and quantitative real-time polymerase chain reaction for Bax, caspase-3, caspase-9, or PARP1, and by flow cytometric analysis. Immunoprecipitation and western blotting was applied to detect ubiquitination of p62, TRAF6, and caspase-9. Mitochondrial apoptosis, reactive oxygen species (ROS) generation, and cell proliferation were assessed in cells transfected with p62 small interfering RNA (siRNA). Treatment of RAW 264.7 cells with MSU crystals induced activation of Bax, caspase-3, caspase-9, and PARP1 at the early phase, in addition to enhancing IL-1β expression, but these findings were attenuated at the late phase. MSU crystals induced ubiquitination of p62, followed by ubiquitination of TRAF6 and caspase-9, which were significantly reversed by ascorbic acid. RAW 264.7 cells transfected with p62 siRNA showed attenuated expression of Bax, caspase-3, caspase-9, and PARP1, decreased ROS and IL-1β production, and increased cell proliferation, compared to controls. The antioxidant ascorbic acid inhibited p62, caspase-9, and IL-1β expression increased by MSU crystals. p62 may be a crucial mediator for the mitochondrial apoptosis pathway in MSU crystal-induced inflammation, which is linked to the acute inflammatory response during the early phase of gout.