Betulin, an Anti-Inflammatory Triterpenoid Compound, Regulates MUC5AC Mucin Gene Expression through NF-kB Signaling in Human Airway Epithelial Cells

Betulin is a triterpenoid natural product contained in several medicinal plants including Betulae Cortex. These medicinal plants have been used for controlling diverse inflammatory diseases in folk medicine and betulin showed anti-inflammatory, antioxidative, and anticancer activities. In this study, we tried to examine whether betulin exerts a regulative effect on the gene expression of MUC5AC mucin under the status simulating a pulmonary inflammation, in human airway epithelial cells. Confluent NCI-H292 cells were pretreated with betulin for 30 min and then stimulated with phorbol 12-myristate 13-acetate (PMA) for 24 h or the indicated periods. The MUC5AC mucin mRNA expression and mucin glycoprotein production were measured by reverse transcription - polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA), respectively. To elucidate the action mechanism of betulin, effect of betulin on PMA-induced nuclear factor kappa B (NF-kB) signaling pathway was also investigated by western blot analysis. The results were as follows: 1) Betulin significantly suppressed the production of MUC5AC mucin glycoprotein and down-regulated MUC5AC mRNA expression induced by PMA in NCI-H292 cells. 2) Betulin inhibited NF-kappa B activation stimulated by PMA. Suppression of inhibitory kappa B kinase (IKK) by betulin led to the inhibition of the phosphorylation and degradation of inhibitory kappa B alpha (I kappa B alpha), and the nuclear translocation of NF-kappa B p65. This, in turn, led to the down-regulation of MUC5AC glycoprotein production in NCI-H292 cells. These results suggest betulin inhibits the gene expression of mucin through regulation of NF-kB signaling pathway, in human airway epithelial cells.