Flavonoid Quercetin Enhances Nuclear factor erythroid 2-related factor 2 (Nrf2) Pathway Activation and Reduces Inflammatory Cytokines in Asthmatic Airway Epithelial cells

Asthma is a chronic disease characterized by airway inflammation, oxidative stress, and bronchial hyperresponsiveness. Quercetin, a safe and well-tolerated flavonoid, reduces airway inflammation and has antioxidant effects, which are partly modulated by activating the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway. This dual anti-inflammatory and antioxidant pharmacological effect makes this compound a potentially effective therapeutic agent. We used primary human bronchial epithelial cells (HBECs) from asthmatic and healthy controls to evaluate whether quercetin modulates Nrf2 expression and reduces inflammatory cytokines. Differentiated ciliated and mucus-producing HBECs were maintained for 21 days at the air-liquid interface (ALI), then were pretreated for 18 hours with 25 µM quercetin. Following a washout with phosphate-buffered saline, cells were exposed to IL-13 (10 ng/mL) for 3 hours. Cell culture supernatants were collected, and a cytokine panel was measured. Additionally, bulk RNA-seq differential expression testing was performed, where a significant between-group difference was defined by a false discovery rate (FDR) < 0.05 and an absolute value of the log2 fold change > 0.5 Results: Human airway epithelial cells treated with quercetin showed a significant increase in Nrf2 protein levels compared to untreated cells (p=0.008). In addition, quercetin treatment was associated with a reduction in TNF-α expression in asthmatic cells. Although this decrease did not reach statistical significance, the observed trend may suggest a potential anti-inflammatory effect worth further investigation. Moreover, compared to control, quercetin significantly upregulated the gene expression of the γ-glutamate-cysteine ligase catalytic subunit (GCLC) subunit and NAD(P)H quinone oxidoreductase-1 (NQO1) (p=0.009 and p=0.04 respectively) in cultured HBECs. This study suggests that quercetin may be a promising therapeutic agent to improve health outcomes in asthma by activating the Nrf2 pathway to reduce oxidative stress and pro-inflammatory cytokines in airway epithelium, which warrants further mechanistic and clinical investigation.