The Protective Effect Of 18β-glycyrrhetinic Acid On Staphylococcus Aureus Pneumonia And The Associated Mechanisms

Objective: To investigate the protective effect of 18β-glycyrrhetinic acid(18β-GA)on Staphylococcus aureus(SA)pneumonia and its related molecular mechanisms.Method: C57/BL6 female mice with 6-8 weeks old were divided into three groups.Mice were treated with PBS as the control group,mice were intracheally instilled with SA(1 × 108 CFU)in the infection group,and animals were pretreated with18β-GA(50 mg/kg)2 h before SA instillation in the intervention group.Survival rates in every group were observed.Histological changes in the lung and liver tissue were examined.The number of neutrophils was counted and the expression of inflammatory cytokines in bronchial alveolar lavage fluid(BALF)was detected by enzyme-linked immunosorbent assay(ELISA).The levels of inflammatory cytokines were detected by RT-q PCR,bacterial burden was examined by CFU,and high mobility protein 1(HMGB1)level was determined by Western Blot in lung tissue.The effect of anti-HMGB1(30μg)on survival rates of mice with SA pneumonia was also evaluated.Macrophages RAW264.7 and hepatocytes Hep G2 were respectively pretreated with 18β-GA(20μM),then stimulated with SA,The levels of inflammatory cytokines were detected by ELISA in culture supernatant of RAW264.7 cells.The expression of signaling molecules,including P38/p-P38 MAPK,JNK/p-JNK MAPK,ERK/p-ERK MAPK,and P65/p-P65 NF-κB,was examined by Western Blot in RAW264.7 cells.The activity of alanine aminotransferase(ALT)and aspartate aminotransferase(AST)in culture supernatant of Hep G2 cells was determined.Results: Compared with SA-infected mice,18β-GA pretreatment group showed alleviated the pathological damage in lung and liver tissue,markedly reduced the number of neutrophils(P<0.05)and decreased the expression of the inflammatory cytokines including TNF-α,IL-1β,and IL-6 in BALF(P<0.05),and reduced bacterial load in lung homogenate(P<0.05).18β-GA also decreased HMGB1 expression in lung tissue after SA infection.Furthermore,neutralizing antibody of HMGB1 can improve the survival rate of infected mice(P<0.05).In vitro experimental results indicated 18β-GA can reduce inflammatory cytokine expression by inhibiting the phosphorylation of P65 NF-κB in RAW264.7 cells and decrease ALT levels in Hep G2 cells(P<0.05).18β-GA was also found to directly inhibit the growth of SA.Conclusion: 18β-GA plays a protective effect on SA-induced pneumonia and secondary liver injury in mice,by decreasing inflammatory cytokine production and reducing HMGB1 expression.