S100A9 activates the MAPK pathway. (A,B) P38, JNK, ERK, and their phosphorylation expression levels in transfected A549 and SPC-A1 cells were determined by western blotting. (C,D) A549 and SPC-A1 cells were transfected with or without LV-S100A9 and pretreated without or with SB203580 (0.5 μM or 5 μM for 24 hours), respectively. Expression levels of p-P38 and P38 were determined by western blotting; *P < 0.05; **P < 0.01. (E,F) A549 and SPC-A1 cells were transfected with or without LV-S100A9 and pretreated without or with PD98059 (1 μM or 10 μM for 24 hours), respectively; expression levels of p-ERK1/2 and ERK1/2 were determined by western blotting; *P < 0.05.