High-mobility Group Box 1 Contributes to Hypoxic-Ischemic Brain Damage by Facilitating Imbalance of Microglial Polarization through RAGE-PI3K/Akt Pathway in Neonatal Rats.

High mobility group box 1 (HMGB1) is a damage-associated molecular pattern integral for hypoxic-ischemic brain damage (HIBD) in neonatal rats since it regulates the phenotypic polarization of microglia, as depicted in our previous studies. Since this mechanism is not clear, this study establishes an oxygen-glucose deprivation (OGD) model of highly aggressively proliferating immortalized microglia while modulating the expression of HMGB1 by plasmid transfection. The M1/M2 microglial phenotype and receptor for advanced glycation end products-phosphoinositide 3-kinase/Akt (RAGE-PI3K/Akt) activation were evaluated, showing that HMGB1 promoted the polarization of microglia to the M1 phenotype under OGD conditions. Meanwhile, RAGE, which is the main receptor of HMGB1, was activated, and phosphorylation of PI3K/Akt was upregulated. However, knockdown or inhibition of HMGB1 can weaken the activation of RAGE and phosphorylation of PI3K/Akt. The inhibition of HMGB1 or RAGE-PI3K/Akt attenuated microglial polarization to the M1 phenotype and promoted M2 microglial polarization instead, reducing the release of pro-inflammatory factors. In the neonatal HIBD rat model, the RAGE-PI3K/Akt pathway was activated seven days after hypoxic-ischemic (HI) exposure, and the activation was partly inhibited after pretreatment with the HMGB1 inhibitor. Concurrently, inhibition of the HMGB1-RAGE-PI3K/Akt pathway alleviated neuronal damage in the hippocampus. These findings verified that HMGB1 could lead to an imbalance in M1/M2 microglial polarization through activation of the RAGE-PI3K/Akt signaling pathway under OGD conditions. Obstructing this pathway may attenuate the imbalanced polarization of microglia, enabling its utilization as a therapeutic strategy against brain injury in HIBD.