The cytoplasmic scaffolding protein PCNA as a new regulator of activation of the classical NRLP3 inflammasome

Key components of innate immune responses, the NLRP3 sensor, ASC adaptor and Caspase-1 effector form the NLRP3 inflammasome, whose assembly leads to both inflammatory cytokines secretion and cell death. Its priming and activation are regulated by numerous proteins including the serine/threonine kinase NEK7. PCNA is a nuclear scaffolding protein essential for DNA replication. In neutrophils, which are differentiated cells deprived of proliferative capacity, PCNA is exclusively cytoplasmic and regulates neutrophil functions through interactions with different protein partners. Among them, pro-Caspases-3/8/9/10 interact with PCNA to regulate apoptosis. We therefore hypothesize that cytoplasmic PCNA could serve as a regulatory platform for the NLRP3 inflammasome via its interaction with its components. In PMA-differentiated THP-1 macrophages, PCNA relocalized mainly in the cytoplasm. Furthermore, PCNA scaffold inhibitors inhibited IL-1{beta} secretion, pro-Caspase-1 cleavage and pyroptosis-activating gasdermin D cleavage, induced by either nigericin, monosodium urate crystals or Escherichia coli bacteria. Nigericin-stimulation resulted in cytoplasmic PCNA binding to NEK7 and potentially NLRP3, and the p21 peptide, the highest competitive inhibitor of PCNA partner binding, disrupted PCNA-NEK7 interaction. Finally, use of bone marrow-derived macrophages from p21 knockout mice suggested that the cyclin-dependent kinase inhibitor p21 might be a natural repressor of the NLRP3 inflammasome. This work provides a proof-of-principle for the potential of disrupting PCNA interactions to regulate both canonical and non-canonical NLRP3 inflammasomes.