Isorhamnetin alleviates pathological damage in influenza A virus strain PR8-induced pneumonia by activating the Nrf2/HO-1 pathway and suppressing apoptosis

Abstract Background Influenza A viruses (IAVs) are the major pathogens associated with respiratory infections which can result in extensive pathological damage in lungs and serious complications. Isorhamnetin, an abundant natural flavonoid in fruits and medicinal plants, has recently been shown to have strong antioxidative, anti-inflammatory, and antiviral effects. Objective This study investigated the pharmacological effects of isorhamnetin on viral pneumonia and explored the underlying mechanisms by in vivo and in vitro experiments. Materials and methods In the present study, the protective effect of isorhamnetin against IAV was evaluated by the cytopathogenic effect assay, cell counting kit-8 assay, real-time polymerase chain reaction, and immunofluorescence assay in vitro . Then the pathological damage associated with pneumonia was examined by calculating the pulmonary index and performing micro-CT and hematoxylin-eosin staining in vivo . Thereafter, the related protein or gene levels of factors in the mitogen-activated protein kinase (MAPK) and nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase-1 (HO-1) signaling pathways were determined by Western blot and immunofluorescence staining. Results Isorhamnetin exerted significant anti-influenza effects and inhibited the expression of viral RNA in A549 cells, counteracting oxidative stress and apoptosis by suppressing the production of reactive oxygen species and caspase-3. The in vivo experiment results showed that isorhamnetin (20 and 40 mg/kg) caused a significant decrease in the pulmonary index, ameliorated pathological damage in the lung tissue, decreased viral load and NA activity, and reduced cytokines and nuclear factors. Furthermore, isorhamnetin could counteract the B cell lymphoma-2/B cell lymphoma-2–associated X protein (Bax) imbalance induced by PR8, suppress activation of the MAPK pathway, and upregulate the expression of Nrf2 and HO-1. Conclusions Isorhamnetin can protect against viral pneumonia by activating the Nrf2/HO-1 pathway and suppressing the MAPK pathway. This study deciphers the pharmacological mechanism of isorhamnetin in alleviating pathological damage in viral pneumonia and provides rationale for the application of isorhamnetin in influenza treatment.