Activation of mitogen-activated protein kinases and AP-1 by polysaccharide isolated from the radix of Platycodon grandiflorum in RAW 264.7 cells

Abstract

The root of Platycodon grandiflorum has been widely used for the treatment of various diseases in oriental medicine. Our previous study showed that the PG, a polysaccharide isolated from P. grandiflorum, activates macrophages via Toll-like receptor 4 (TLR4). However, the associated biological mechanisms are not fully understood. To elucidate the molecular mechanism responsible for the macrophage activation, we investigated the effect of PG on the activity of mitogen-activated protein kinases (MAPKs) and activator protein-1 (AP-1) in RAW 264.7 cells, a murine macrophage cell line. Treatment of RAW 264.7 cells with PG produced a marked induction of AP-1 DNA binding activity. Moreover, all three MAPKs were activated by PG, and PG-induced activation of MAPKs was abrogated by the treatment of PD98059, curcumin, and SB203580, specific inhibitors of MEK-1/2, stress-activated protein kinases/jun N-terminal kinase (SAPK/JNK), and p38 MAP kianse, respectively. The induction of AP-1 DNA binding activity by PG was also inhibited by these MAPK inhibitors. Moreover, supershift analysis identified that JunB and Fra-1 are major components involved in the PG-mediated induction of AP-1 DNA binding. Additionally, curcumin and SB203580 suppressed PG-induced production of nitric oxide (NO) and tumor necrosis factor-α (TNF-α), whereas PD98059 showed an inhibitory effect only on the TNF-α production. Taken together, these results suggest that macrophage activation by PG is mediated, at least in part, by MAPKs and AP-1.