Objective:To investigate the role and underlying mechanisms of inhibiting high mobility group box-1 (HMGB1) in the expression of matrix metalloproteinase-9 (MMP-9) in spinal cord astrocytes (AS) in rats after spinal cord injury (SCI).Methods:After an SCI model was established in Sprague-Dawley (SD) rats using a modified Allen's Weight-Dropping method and ethyl pyruvate (EP) or glycyrrhizin (GL) was used to inhibit the effect of HMGB1, the rats were divided into a sham group, an SCI group, an SCI+EP (50 mg/kg) group, and an SCI+GL (100 mg/kg) group. The expression levels of glial fibrillary acid protein (GFAP) and MMP-9 in spinal cord AS were observed. After the spinal cord AS in SD rats was cultured and incubated by the oxygen-glucose deprivation/reoxygenation (OGD/R) procedure, the expression of MMP-9 protein was detected at 6 h/R 6 h, 12 h, 24 h, and 48 h after OGD. The time point with the highest expression was chosen in the subsequent experiments as an OGD/R group. HMGB1 was inhibited by HMGB1 shRNA or EP to observe the effect of HMGB1 on the expression of MMP-9 protein in AS treated with OGD/R. Then, toll-like receptor 4 (TLR4) inhibitor, TIR-domain-containing adaptor inducing interferon- β (TRIF) inhibitor, and nuclear factor-kappa B (NF- κB) inhibitor were used to investigate the effects of TLR4/TRIF/NF- κB signaling pathway during the regulation of HMGB1 on MMP-9 in vitro. Results:Western blot showed that the expression of MMP-9 protein in the spinal cord was significantly increased in rats at 1 d after SCI, and the expression of MMP-9 protein in the SCI+EP group and the SCI+GL group was significantly lower than that in the SCI group ( P<0.001). Immunofluorescence showed that GFAP and MMP-9 proteins were co-localized in the spinal cord after SCI, and the expression of GFAP and MMP-9 proteins in the SCI+EP and SCI+GL groups was significantly lower than that in the SCI group ( P<0.05). Since the expression of MMP-9 protein in the spinal cord AS cultured in vitro was significantly higher in the OGD 6h/R 12h group than that in the normal group and the OGD 6h/R 6h, 24, and 48 h groups, the OGD 6h/R 12h was taken as the OGD/R group. The MMP-9 protein expression in AS in the OGD/R+HMGB1 shRNA group and the OGD/R+EP group was significantly lower than that in the OGD/R group ( P<0.001). In the cultured AS, moreover, inhibiting TLR4, TRIF, and NF- κB reduced MMP-9 protein expression after OGD 6 h/R 12 h when compared with that in the OGD/R group ( P<0.001). Conclusions:HMGB1 inhibition may result in a reduction in MMP-9 expression both in the spinal cord AS in SCI rats and in AS after OGD/R treatment in vitro. HMGB1 may regulate MMP-9 protein expression in AS after OGD/R treatment via the TLR4/TRIF/NF- κB signal pathway.