Anti-inflammatory effects of Allium cepa L. peel extracts via inhibition of JAK-STAT pathway in LPS-stimulated RAW264.7 cells

Ethnopharmacological relevance: Allium cepa L. (A. cepa) is one of the oldest cultivated plants in the world. A. cepa has been used in traditional folk medicine to treat inflammatory disease in several regions, such as Palestine and Serbia. A. cepa peel has a higher content of flavonoids, such as quercetin, than the edible parts. These flavonoids alleviate inflammatory diseases. However, the anti-inflammatory effects of A. cepa peel extract-obtained using various extraction methods-and their underlying mechanisms require further investigation.

Aim of the study: Although research to find safe anti-inflammatory substances in various natural products has been actively conducted for many years, it is important to continue identifying potential anti-inflammatory effects in natural materials. The purpose of this study was to investigate the ethnopharmacological properties of the A. cepa peel extract, whose efficacy when obtained through different extraction methods and underlying action mechanisms is not well known. The present study specifically aimed to observe the anti-inflammatory effects of the A. cepa peel extracts obtained using various extraction methods and the related detailed mechanisms of A. cepa peel extracts in lipopolysaccharide (LPS)-induced RAW264.7 cells.

Materials and methods: The total flavonoid content of the A. cepa peel extracts was determined the diethylene glycol colorimetric method and measured using a calibration curve prepared using quercetin as a standard solution. The antioxidant activity was evaluated using the ABTS assay, and cytotoxicity was measured using the MTT assay. NO production was measured using Griess reagent. Protein levels were measured by western blotting, and mRNA expression was measured by RT-qPCR. Secreted cytokines were analyzed using ELISA or cytokine arrays. In the GSE160086 dataset, we calculated Z-scores for individual genes of interest and displayed using a heat map.

Results: Of the three A. cepa peel extracts obtained using different extraction methods, the A. cepa peel 50% EtOH extract (AP50E) was the most effective at inhibiting LPS-induced nitric oxide (NO) and inducible nitric oxide synthase (iNOS). Furthermore, AP50E significantly reduced the levels of pro-inflammation cytokines interleukin (IL)-1α, IL-1β, IL-6, and IL-27. Additionally, AP50E directly inhibited the Janus kinase-signaling transducer and activator of transcription (JAK-STAT) pathway.

Conclusions: These results showed that AP50E exhibited an anti-inflammatory effect in LPS-induced RAW264.7 mouse macrophages by directly inhibiting JAK-STAT signaling. Based on these findings, we propose AP50E as a potential candidate for the development of preventive or therapeutic agents against inflammatory diseases.