Inhibition of the TLR4/RAGE pathway by clearance of extracellular HMGB1 is a potential therapeutic target for radiation-damaged salivary glands

INTRODUCTION: We recently developed a new therapy using effective-mononuclear cells (E-MNCs) and demonstrated its efficacy in treating radiation-damaged salivary glands (SGs). The activity of E-MNCs in part involves constituent immunoregulatory -CD11b/macrophage scavenger receptor 1(Msr1)-positive-M2 macrophages, which exert anti-inflammatory and tissue-regenerating effects via phagocytic clearance of extracellular high mobility group box 1 (HMGB1). Focusing on the phenomena, this study investigated significance of regulating the HMGB1/toll-like receptor 4 (TLR4)/receptor for advanced glycation end products (RAGE) signaling pathway in the treatment of SG dysfunction caused by radiation damage. METHODS: E-MNCs were transplanted into radiation-damaged mice SGs, and changes of TLR4/RAGE expression were observed. Furthermore, the activation of downstream signals was investigated in both intact SGs and cultured SG epithelial cells after irradiation. Subsequently, TLR4-knock-out (KO) mice were employed to examine how HMGB1/TLR4/RAGE signaling affected damage progression. RESULTS: Expression of both TLR4 and RAGE was diminished in ductal cells and macrophages/vascular endothelial cells of damaged SGs with E-MNC transplantation, respectively. Meanwhile, expression of TLR2/4 and RAGE in damaged SGs markedly increased in association with extracellular HMGB1 accumulation. Downstream signals were activated, and intranuclear localization of phospho-nuclear factor-kappa B (p-NF-KB) in ductal cells and production of IL-6, tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ) were observed. Additionally, culture supernatant of irradiated cultured SG epithelial cells contained damaged associated molecular pattern (DAMP)/senescence-associated secretory phenotype (SASP) factors. Treatment of cultured SG epithelial cells with this supernatant activated TLR4 signaling pathway and induced cellular senescence. In TLR4-KO mice, onset of radiogenic SG dysfunction was markedly delayed. However, TLR2/RAGE signalings were alternatively activated, and SG function was impaired. CONCLUSIONS: Clearance of DAMPs such as HMGB1 may attenuate sterile inflammation in damaged SGs via suppression of the TLR4/RAGE signaling pathway. This cellular mechanism may have significant implications for the development of future cell-based regenerative therapies.