Ginsenosides Rg1 and Re act as adjuvant via TLR4 signaling pathway

Previous studies have demonstrated that ginsenosides Rg1 and Re extracted from the root of Panax ginseng C.A. Meyer have adjuvant properties. However, the molecular mechanisms behind their adjuvant activities remain unclear. In the present study, we first investigated the adjuvant effect of Rg1 and Re on the immune responses to a model antigen ovalbumin (OVA) in C3H/HeB mice as well as in C3H/HeJ mice carrying a defective toll-like receptor-4 (TLR4) gene, and then evaluated Rg1 and Re for their stimulation of phosphorylation of nuclear factor-kappa B (NF-κB) p65 in the macrophages from above two different strains of mice. In addition, Rg1 and Re were also evaluated for their induction of NF-κB in RAW-Blue™ cells. The results showed that Rg1 and Re had adjuvant activities in stimulating IgG, splenocyte proliferation, and mRNA expression of cytokines IL-4, IL-10, IL-12 and IFN-γ as well as transcription factors GATA-3 and T-bet by splenocytes in C3H/HeB mice but not in C3H/HeJ mice. Rg1 and Re induced phosphorylation of NF-κB p65 at Ser536 in macrophages from C3H/HeB mice but not from C3H/HeJ mice. Both Rg1 and Re induced expression of NF-κB in RAW-Blue™ cells. These results suggested that TLR4 signaling pathway is involved in the adjuvant activities of Rg1 and Re. Nevertheless, pretreatment with anti-TLR4 antibody suppressed the Re- but not Rg1-induced expression of NF-κB, indicating that Rg1 may trigger both extracellular and intracellular TLR4 by passing through the cell membrane while Re only activate extracellular TLR4 as it fails to enter inside of the cells to stimulate intracellular TLR4.