Glycyrrhizin (GL) modulates HMGB1/miR-222-associated M2bMφ polarization and improves host antibacterial resistance of γ-irradiated mice against gut bacteria-associated sepsis

Abstract M2bMφ demonstrated in the mesenteric lymph nodes (MLNs) of whole body γ-irradiated mice (WBI-mice) have been characterized as responsible cells for the increased susceptibility of them to gut bacteria-associated sepsis. In our previous studies, MLN-Mφ from WBI-mice treated with GL have been characterized as non-M2bMφ. In this study, we determined how GL suppresses the γ-irradiation-associated M2bMφ polarization. WBI-mice were treated with GL (10 mg/kg, i.p., twice a day) 8 and 9 days post-irradiation. The antibacterial resistance of WBI-mice to Enterococcus faecalis oral infection (a model of sepsis caused by bacterial translocation) was evaluated by a bacterial growth in organs. F4/80+ cells (Mφ) were isolated from NMLs of GL-treated WBI-mice 10 days post-irradiation, and analyzed for the expression of miR-222 and GAS5 by real-time PCR. Serum HMGB1 level was measured by ELISA. In the results, the pathogen vigorously grew in various organs of WBI-mice, while the bacteria did not grow in organs of the same mice treated with GL. HMGB1, increased in sera of WBI-mice 2–15 days post-irradiation, stimulated the expression of miR-222 with a function to degrade GAS5 RNA (a factor to control the M2bMφ polarization). However, miR-222 was not expressed in the same Mφ cultures supplemented with GL. GL has been shown to bind to HMGB1 directly and inhibit its cytokine activity. The reduction of GAS5 and the M2bMφ polarization were not demonstrated in Mφ from WBI-mice treated with GL. These results suggest that GL improves host antibacterial resistance of WBI-mice against gut bacteria-associated sepsis through the modulation of HMGB1/miR-222-associated M2bMφ polarization.