Entamoeba histolytica activation of caspase-1 degrades cullin that attenuates NF-κB dependent signaling from macrophages

Abstract

While Entamoeba histolytica (Eh)-induced pro-inflammatory responses are critical in disease pathogenesis, the downstream signaling pathways that subsequently dampens inflammation and the immune response remains unclear. Eh in contact with macrophages suppresses NF-κB signaling while favoring NLRP3-dependent pro-inflammatory cytokine production by an unknown mechanism. Cullin-1 and cullin-5 (cullin-1/5) assembled into a multi-subunit RING E3 ubiquitin ligase complex are substrates for neddylation that regulates the ubiquitination pathway important in NF-κB activity and pro-inflammatory cytokine production. In this study, we showed that upon live Eh contact with human macrophages, cullin-1/4A/4B/5 but not cullin-2/3, were degraded within 10 minutes. Similar degradation of cullin-1/5 were observed from colonic epithelial cells and proximal colonic loops tissues of mice inoculated with live Eh. Degradation of cullin-1/5 was dependent on Eh-induced activation of caspase-1 via the NLRP3 inflammasome. Unlike cullin-4B, the degradation of cullin-4A was partially dependent on caspase-1 and was inhibited with a pan caspase inhibitor. Cullin-1/5 degradation was dependent on Eh cysteine proteinases EhCP-A1 and EhCP-A4, but not EhCP-A5, based on pharmacological inhibition of the cysteine proteinases and EhCP-A5 deficient parasites. siRNA silencing of cullin-1/5 decreased the phosphorylation of pIκ-Bα in response to Eh and LPS stimulation and downregulated NF-κB-dependent TNF-α mRNA expression and TNF-α and MCP-1 pro-inflammatory cytokine production. These results unravel a unique outside-in strategy employed by Eh to attenuate NF-κB-dependent proinflammatory responses via NLRP3 activation of caspase-1 that degraded cullin-1/5 from macrophages.