Spermidine mitigates acute lung injury through activation of the Keap1-Nrf2-ARE pathway

Acute lung injury (ALI) is a critical pulmonary disorder precipitated by infection or trauma, characterized by heightened inflammation and oxidative stress. The nuclear factor erythroid 2-related factor 2 (Nrf2) is a pivotal transcription factor that regulates these processes and may act as a significant modulator in ALI. Spermidine, a naturally occurring polyamine with antioxidant and anti-inflammatory properties, is postulated to mitigate ALI by activating Nrf2. This study aims to evaluate the potential of spermidine in attenuating lipopolysaccharide (LPS)-induced ALI and to elucidate the underlying mechanisms involved. An ALI mouse model was generated via intraperitoneal administration of LPS, with spermidine provided as an intervention. Additional experimental groups were treated with either an Nrf2 activator (4-OI) or an inhibitor (ML385). Real-time polymerase chain reaction (PCR) and Western blot analyses were employed to quantify the expression levels of Keap1, Nrf2, inflammatory cytokines (TNF-α, IL-18, IL-1β), and NF-κB in lung tissues and macrophages. To further explore the molecular mechanisms, inhibitors of PI3K and MAPK pathways were utilized. Spermidine increased Nrf2 expression and nuclear translocation in LPS-stimulated macrophages and lung tissues. In vivo, spermidine improved respiratory function, alleviated histopathological lung injury, enhanced antioxidant capacity, and attenuated inflammatory responses. Pharmacological modulation of Nrf2 altered these protective effects, supporting the involvement of Nrf2 signaling. Spermidine also altered PI3K and ERK/MAPK signaling, while inhibition of these pathways produced distinct changes in Nrf2, Keap1, and HO-1 expression. Spermidine alleviates LPS-induced ALI and enhances Nrf2 activation while modulating Keap1-Nrf2-ARE signaling. PI3K and ERK/MAPK signaling may participate in spermidine-mediated regulation of Nrf2, although their precise hierarchical relationship remains to be clarified.