Isoliquiritigenin Exhibits Anti-Inflammatory Responses in Acute Lung Injury by Covalently Binding to the Myeloid Differentiation Protein-2 Domain.

Acute lung injury (ALI), a systemic inflammatory response with high morbidity, lacks effective pharmacological therapies. Myeloid differentiation protein-2 (MD2) has emerged as a promising therapeutic target for ALI. Herein, we aimed to evaluate the ability of isoliquiritigenin (ISL), a natural flavonoid found in licorice as a novel MD2 inhibitor, to inhibit lipopolysaccharide (LPS)-induced ALI. We established a mouse ALI model and a RAW 264.7 cell injury model through LPS administration. Then, lung injury was assessed through histopathological examination, and the effects of ISL were evaluated using immunofluorescence, western blotting, reverse transcription-quantitative polymerase chain reaction, flow cytometry, and enzyme-linked immunosorbent assays. In addition, the interaction between ISL and MD2 was investigated through co-immunoprecipitation and LPS displacement assays. Molecular docking and liquid chromatography/mass spectrometry analyses were employed to predict the ISL-binding domain of MD2. We found that ISL covalently bound to the Cysteine 133 residue of MD2, disrupting the formation of the LPS/MD2/toll-like receptor 4 complex, and ISL significantly suppressed proinflammatory cytokine production and reactive oxygen species generation in LPS-induced RAW264.7 cells. Moreover, ISL significantly alleviated lung injury in LPS-induced mice, reducing pulmonary microvascular permeability, inflammatory cell infiltration, and inflammatory cytokine expression. The underlying mechanism of ISL involved the inhibition of nuclear factor kappa B and the p38 mitogen-activated protein kinase pathway. Our findings supported that MD2 is the direct target of ISL in mediating its anti-inflammatory response in vivo and in vitro, and it holds potential as a therapeutic candidate for treating ALI and other inflammatory diseases.