Bioassay-guided isolation of anti-inflammatory components from the root of Rehmannia glutinosa and its underlying mechanism via inhibition of iNOS pathway

Ethnopharmacological relevance: The root of Rehmannia glutinosa (RR) is commonly used to reduce inflammation in various traditional Chinese herbal formulae; however, little is known regarding its active component(s).

Aim of study: The objective of the present study was to examine the active component(s) responsible for the anti-inflammatory activity of RR via anti-nitric oxide production assay-guided fractionation; and the underlying anti-inflammatory mechanism of action of such component(s) was further investigated.

Materials and methods: Anti-nitric oxide (NO) activities with lipopolysaccharides (LPS)-stimulated RAW264.7 murine macrophages was used as screening platform. Gene, protein and inflammatory mediators' expression were also studied using Real-Time PCR, western blotting and ELISA, respectively.

Results: Using anti-NO assay-guided fractionation, sub-fraction C3 (from 31.25 to 62.5 μg/ml, p=0.001 to 0.01) possessed 100-fold more potent anti-inflammatory effect than that of the aqueous extract of RR. Characterization of C3 showed that the anti-inflammatory effect could be partly due to the presence of rehmapicrogenin, which could significantly inhibit NO production (p<0.001). C3 was further demonstrated in blocking inflammation by inhibiting gene (p<0.001) and protein expression of inducible NO Synthase (iNOS) dose-dependently. Besides, C3 also significantly inhibited the production of prostaglandin E(2) (p<0.001 to 0.01), IL-6 (p<0.001 to 0.05) and COX-2 (p<0.05).

Conclusions: Rehmapicrogenin was, for the first time, shown to possess nitric oxide inhibitory activities. Bioassay-guided fractionation demonstrated that rehmapicrogenin-containing subfraction C3 exhibited potent anti-inflammatory effect by inhibiting iNOS, COX-2 and IL-6, while rehmapicrogenin was only partially responsible for the anti-inflammatory effect of RR.