RNF31 mediated ubiquitination of A20 aggravates inflammation and hepatocyte apoptosis through the TLR4/MyD88/NF-κB signaling pathway

Abstract

Inflammatory cytokine storm is one of the main pathogenesis of acute liver injury, and accumulating evidence suggests that the E3 ubiquitin ligase ring finger protein 31 (RNF31) plays an important regulatory role in the activation of inflammatory pathways. We found that RNF31 expression was up-regulated in lipopolysaccharide (LPS)-treated HL-7702 cells. Western blotting results showed decreased expression of RNF31 and total ubiquitinated proteins after transfection of si-RNF31. The results of MTT assay indicated that cell viability was enhanced. Flow cytometry analysis showed that cell apoptosis and ROS content was decreased, and ELISA assay results exhibited that the inflammatory factors secretion was reduced. Interestingly, A20 protein expression was inhibited as RNF31 expression was upregulated. On this basis, we performed co-immunoprecipitation assays and found that RNF31 could interact with A20. Actinomycin tracing and proteasome inhibition experiments showed that RNF31 degrades A20 through the proteasome pathway. Furthermore, overexpression of A20 enhanced cell viability, reduced apoptosis, and inhibited ROS generation and inflammatory factor secretion. Mechanistic studies revealed that RNF31 was able to degrade A20, which affected the inflammatory response and hepatocyte apoptosis mediated by the toll like receptor 4 (TLR4)/myeloid differentiation factor88 (MyD88)/nuclear transcription factor-κB (NF-κB) signaling pathway. Moreover, knockdown of RNF31 attenuated the inflammatory response induced by d-Gal/LPS in mice with acute liver injury. In conclusion, RNF31 degrades A20 by ubiquitination and activates the TLR4/MyD88/NF-κB signaling pathway to aggravate acute liver injury.

Keywords: A20 protein; Inflammation; Liver injury; Ring finger protein 31; The TLR4/MyD88/NF-κB signaling pathway; Ubiquitination.