The Sequestosome 1/p62 Attenuates Cytokine Gene Expression in Activated Macrophages by Inhibiting IFN Regulatory Factor 8 and TNF Receptor-Associated Factor 6/NF-κB Activity

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Abstract
Sequestosome 1/p62 (p62) is a scaffold/adaptor protein with multiple functions implicated for neuronal and bone diseases. It carries a ubiquitin binding domain through which it mediates proteasome-dependent proteolysis. In addition, p62 is reported to regulate NF-κB activity in some cells. To date, however, the role of p62 in innate immunity has not been fully elucidated. In this study, we report that IFN-γ plus TLR signaling stimulates late expression of p62 in murine macrophages. Overexpression of p62 inhibited expression of multiple cytokines, IL-12p40, TNF-α, IL-1β, IL-6, and IFN-β, whereas p62 underexpression by small hairpin RNA markedly elevated their expression, indicating that p62 is a broad negative regulator of cytokine expression in stimulated macrophages. We show that p62 interacts with IFN regulatory factor 8 and Ro52, the transcription factor and ubiquitin E3 ligase that are important for IL-12p40 expression. This interaction, detectable at a late stage in stimulated macrophages, led to increased polyubiquitination and destabilization of IFN regulatory factor 8. We also show that upon macrophage stimulation, p62 binds to TNFR-associated factor 6, another E3 ligase important for NF-κB activation, but later this interaction was replaced by the recruitment of the deubiquitinating enzyme, cylindromatosis, an inhibitor of NF-κB activity. Recruitment of cylindromatosis coincided with reduced TNFR-associated factor 6 autoubiquitination and lower NF-κB activation. Our results indicate that p62 orchestrates orderly regulation of ubiquitin modification processes in macrophages to ensure attenuation of cytokine transcription postactivation. Together, p62 may provide a mechanism by which to control excessive inflammatory responses after macrophage activation.