Identification of intracellular activation mechanism of rhamnogalacturonan-I type polysaccharide purified from Panax ginseng leaves in macrophages and roles of component sugar chains on activity

This study aimed to investigate the mechanisms underlying intracellular signaling pathways in macrophages in relation to the structural features of rhamnogalacturonan (RG) I-type polysaccharide (PGEP-I) purified from Panax ginseng leaves. For this investigation, we used several specific inhibitors and antibodies against mitogen-activated protein kinase (MAPK), nuclear factor-kappa B (NF-κB), and pattern recognition receptors (PRRs). Furthermore, we investigated the roles of component sugar chains on immunostimulating activity through a sequential enzymatic and chemical degradation steps. We found that PGEP-I effectively induced the phosphorylation of several MAPK- and NF-κB-related proteins, such as p38, cJun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and p65. Particularly, immunocytochemistry analysis confirmed the PGEP-I-induced translocation of p65 into the nucleus. Furthermore, the breakdown of PGEP-I side chains and main chain during sequential enzymatic and chemical degradation reduced the PGEP-I-induced macrophage cytokine secretion activity. IL-6, TNF-α, and NO secreted by macrophages are associated with several signaling pathway proteins such as ERK, JNK, and NF-κB and several PRRs such as dectin-1, CD11b, CD14, TLR2, TLR4, and SR. Thus, these findings suggest that PGEP-I exerts potent macrophage-activating effects, which can be attributed to its typical RG-I structure comprising arabinan, type II arabinogalactan, and rhamnose-galacturonic acid repeating units in the main chain.