6-Hydroxyrubiadin Regulates TNF-α-Induced MUC5AC Gene Expression via Affecting NF-κB and MAPK Signaling Pathways in Human Airway Epithelial Cells

In this study, we aimed to investigate whether 6-hydroxyrubiadin (6-HR), a compound derived from the root of Rubia akane Nakai, exerts a regulatory effect on the gene expression of MUC5AC mucin under conditions mimicking pulmonary inflammation in human airway epithelial cells. Confluent NCI-H292 cells were pretreated with 6-HR for 30 min, followed by stimulation with tumor necrosis factor (TNF)-α for 24 h. MUC5AC mRNA expression and glycoprotein production were measured using reverse transcription polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA), respectively. Further, we examined the effect of 6-HR on TNF-α-induced activation of the nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways via immunoblot analysis. The results were as follows: (1) 6-HR significantly reduced both the production of MUC5AC glycoprotein and MUC5AC mRNA expression in NCI-H292 cells. (2) 6-HR inhibited TNF-α-induced NF-κB activation through a transforming growth factor-β-activated kinase 1 (TAK1)-dependent mechanism. (3) 6-HR also inhibited TAK1-dependent MAPK signaling pathways activated by TNF-α, resulting in the downregulation of MUC5AC mucin production in NCI-H292 cells. These findings suggest that 6-HR regulates the gene expression of MUC5AC mucin by directly acting on airway epithelial cells through modulation of the MAPK and NF-κB signaling pathways.