Glycyrrhizic Acid Attenuates the Inflammatory Response After Spinal Cord Injury by Inhibiting High Mobility Group Box-1 Protein Through the p38/Jun N-Terminal Kinase Signaling Pathway

Neuroinflammation is an important secondary aggravating factor in spinal cord injury (SCI). Inhibition of the inflammatory response is critical for SCI treatment. Glycyrrhizic acid (GA) is an anti-inflammatory drug, but its utility for SCI is unclear. This study aimed to evaluate the effects of GA on inflammation after SCI and the underlying mechanism. Cell counting kit-8 assays were performed to assess the viability of highly aggressively proliferating immortalized cells that had been treated with lipopolysaccharide (LPS) and/or GA. Reverse transcription quantitative polymerase chain reaction and Western blotting were performed to assess expression of high mobility group box-1 protein (HMGB1), ionized calcium binding adaptor molecule 1, and inflammatory factors in vitro and in vivo. GA (100 mg/kg) was intraperitoneally injected into rats. Anti-inflammatory effects of GA were analyzed in SCI tissues. p38/Jun N-terminal kinase signaling pathway proteins were analyzed by Western blotting. Cell counting kit-8 assay results showed that treatment with 100 ng/mL LPS for 12 hours was optimal. After LPS treatment, highly aggressively proliferating immortalized cells were activated; messenger RNA expression levels of HMGB1 and inflammatory factors were increased. GA significantly inhibited LPS-induced HMGB1 expression and inflammatory responses, as determined by reverse transcription quantitative polymerase chain reaction and Western blotting. Transfection with an HMGB1-overexpression plasmid reversed the anti-inflammatory effects of GA. In addition, intraperitoneal injection of GA (100 mg/kg) into rats for 3 days significantly reduced expression levels of HMGB1 and inflammatory factors after SCI in vivo. GA reduced phosphorylation, but not levels, of p38 and Jun N-terminal kinase proteins. GA attenuates the inflammatory response after SCI by inhibiting HMGB1 through the p38/JNK signaling pathway and thus has therapeutic potential for SCI.