Effect of 7,4′-dihydroxyflavone (7,4′-DHF) and glycyrrhizin on phorbol 12-myristate 13-acetate (PMA)-induced mucin 5AC (MUC5AC) protein secretion. Serum-deprived NCI-H292 cells were pretreated with compounds for 30 min and then stimulated with PMA (10 ng/mL) for 24 h. MUC5AC protein secretion in supernatants was measured by ELISA. (A) Concentration-dependent effect of 7, 4′-DHF on PMA-induced MUC5AC protein secretion. (B) Concentration-dependent effect of glycyrrhizin on PMA-induced MUC5AC protein secretion. Each represents a mean ± SD of three to eight independent experiments in comparison with that of control set at 1. ** p < 0.01, versus PMA stimulation, nontreatment group. (C) Cell viability of 7,4′-DHF-pretreated PMA-stimulated NCI-H292 cells. The cell viability percentage was calculated by comparison with the medium alone group, which was set at 100%.