PU.1 and IRF8 modulate the activation of NLRP3 inflammasome via regulating the monocyte-specific expression of the NLRP3 and its component genes

Abstract

NLRP3 inflammasome plays crucial roles in the host defense against pathogens by activating Caspase-1, which catalyzes the processing of immature IL-1β and IL-18 into a biologically active form. NLRP3 expression is restricted in monocytic lineages such as monocytes, macrophages, and dendritic cells, thus its expression must be strictly regulated. However, the regulatory mechanism of lineage-specific transcription of the NLRP3 gene remains largely unknown. In this study, we investigated the molecular mechanisms of cell-type specific NLRP3 expression. RT-PCR revealed that the NLRP3 gene was dominantly driven from the distal promoter in a human monocytic cell line, THP-1. By reporter assay, we identified several cis-acting elements including an Ets/IRF composite element (EICE), which is frequently associated with monocytic lineage-specific gene regulation, in the distal promoter of the human NLRP3 gene. EMSA and ChIP assay demonstrated that transcription factors, PU.1 and IRF8, both of which are known to play essential roles in the development of monocytic lineage, cooperatively bind to the EICE. The expression of NLRP3 was significantly decreased by the siRNA-mediated knockdown of PU.1 and/or IRF8. In addition, the knockdown of PU.1 and/or IRF8 down-regulated the expression of ASC and Caspase-1, which form inflammasome complex with NLRP3. Furthermore, we found that the silencing of PU.1 and IRF8 dramatically abrogated the processing of pro-IL-1β. These results indicate that PU.1 and IRF8 are involved in the monocytic lineage-specific NLRP3 expression by binding to the EICE site within the distal promoter.