Glycyrrhizin ameliorates inflammatory pain by inhibiting microglial activation-mediated inflammatory response via blockage of the HMGB1-TLR4-NF-kB pathway

Highlights: • Glycyrrhizin (Gly) represses LPS-induced microglial activation without cell toxicity. • Glycyrrhizin dose-dependently inhibits LPS-triggered inflammatory response. • Elevation of HMGB1 reverses glycyrrhizin-inhibited inflammatory response in microglia. • HMGB1-mediated TLR4-NF-κB activation account for gly-restrained inflammation. • Gly alleviates CFA-induced pain hypersensitivity, inflammation, microglia activation. Chronic inflammatory pain is a severe clinical problem that greatly affects patients' quality of life and causes huge economic burden. Microglia-mediated neuroinflammation exerts critical roles in the pathogenic progression of inflammatory pain. Recent evidence corroborates the anti-inflammatory and neuroprotective efficacy of glycyrrhizin; however, its function in inflammatory pain remains poorly elucidated. In the present study, glycyrrhizin suppressed LPS-induced activation of microglial cell BV2 by inhibiting NO production and expression of microglial marker IBA-1. Intriguingly, LPS-induced high expression and generation of inflammatory cytokines (i.e., IL-6, TNF-α and IL-1β) was notably reversed by glycyrrhizin pre-treatment. Mechanistic analysis confirmed that high expression of high-mobility group box 1 (HMGB1) in LPS-activated microglia was inhibited following glycyrrhizin. More importantly, restoring HMGB1 expression by recombinant adenovirus vector of Ad-HMGB1 counteracted glycyrrhizin-restrained inflammatory response in microglia upon LPS stimulation. Furthermore, glycyrrhizin dampened the activation of subsequent TLR4-NF-κB pathway in LPS-stimulated microglia, which was abrogated by HMGB1 elevation. Furthermore, blocking this pathway by si-TLR4 transfection reversed the effects of HMGB1 overexpression on the inhibitor roles of glycyrrhizin in microglia-triggered inflammation. Additionally, glycyrrhizin administration also alleviated CFA-evoked mechanical allodynia and thermal hyperalgesia in inflammatory pain model of mice, concomitant with suppression in inflammatory response and microglial activation. Simultaneously, elevation of HMGB1, TLR4 and p65-NF-κB protein expression induced by CFA injection was also abrogated after glycyrrhizin. Accordingly, this study reveal that glycyrrhizin may act as a promising therapeutic avenue for the treatment of inflammatory pain.