Soulieoside U, a new cycloartane triterpene glycoside from Actaea vaginata

Abstract One new cycloartane triterpene bisdesmoside, soulieoside U, was isolated from the rhizomes of Actaea vaginata. Its structure was elucidated by extensive analysis of the NMR and MS data. Soulieoside U was evaluated for cytotoxic activities against three human cancer cell lines. Graphical Abstract


Introduction
Actaea vaginata (Maxim.) J. Compton, commonly known as Souliea vaginata (Maxim.) Franch, is a traditional treatment for conjunctivitis and pharyngitis (Institute of Botany, Chinese Academy of Sciences & Institute of Materia Medica, Chinese Academy of Medical Sciences 1979). Previously, a number of cycloartane triterpenoids have been reported from S. vaginata (Takao et al. 1985;Sakurai et al. 1986;Inoue et al. 1990;Sakurai et al. 1993;Zhou et al. 2004;Zhou, Yang, Wu, et al. 2005;Zhou, Yang, Tu, and Zou 2006). Recently, we have discovered a series of cycloartane triterpene glycosides (Wu, Zhang, et al. 2016;Wu, Zhu, et al. 2016;Wu, Li, et al. 2017;Wu, Liu, et al. 2017;Wu, Yang, et al. 2017;Zou et al. 2017aZou et al. , 2017bZhang et al. 2019). As a continuation of our search for new anticancer agents from this plant, a new cycloartane triterpene bisdesmoside, named soulieoside U, was isolated from the ethanolic extract of the rhizomes of A. vaginata ( Figure 1). Soulieoside U was tested for cytotoxicity against A549, HT-29 and MDA-MB231cancer cell lines.  Figure 1. Chemical structure of soulieoside U isolated from the rhizomes of A. vaginata. J ¼ 7.8 Hz, H-1 0 ) and d H 5.29 (d, J ¼ 7.8 Hz, H-1 00 ) in the downfield region. The sugar was identified as xylose and glucose by acid hydrolysis followed by comparison with an authentic sample by TLC. The 13 C APT NMR spectrum of soulieoside U exhibited 43 carbon resonances including a methylene carbon of a cyclopropane ring at d C 31.3 (C-19), one oxymethylene carbon at d C 67.0 (C-18), two oxygenated methine carbons at d C 88.6 (C-3) and 73.5 (C-16), three oxyquaternary carbons at d C 87.7 (C-20), 114.1 (C-24) and 80.9 (C-25), and one carbonyl group at d C 170.6. All carbon-bound protons were assigned based on HSQC and 1 H-1 H correlation spectra. The 1 H and 13 C NMR spectra (Supplementary material Table S1) of soulieoside U showed close similarity to those of beesioside L (Ju et al. 2002) except that the C-25 (d C 80.9) signal of soulieoside U was apparently downfield shifted by 8.1 ppm showing that the glucose group was attached at C-25 in soulieoside U. The HMBC experiment (Supplementary material Figure S1) was performed to establish the location of the functional groups and the full structure of soulieoside U. The glucose residue was attached at C-25 due to the long-range correlation between the glucopyranosyl anomeric proton at d H 5.29 (1H, d, J ¼ 7.8 Hz) and C-25 (d C 80.9). The HMBC spectrum of soulieoside U showed long-range correlations between H-1 0 and C-3, between H-17 and C-15 (d C 43.4), C-16 (d C 73.5), C-20 (d C 87.7); between H-16 and C-20, the carbonyl group (d C 170.6); and between H-15 and C-30 (d C 22.6). The HMBC experiment also showed the presence of key correlations between H-19 and C-9, C-10; between H-18 and C-12, C-13, C-14, C-17, C-24; and between C-24 and H-22, H-23, H-26, H-27. On the Dreiding model of soulieoside U, if the configuration of C-20 is R, C-24 should be R, and if C-20 is S, C-24 should also be S (Ju et al. 2002). The coupling constant of J 16 , 17 (10.2 Hz) suggested that 16-OAc was in b configuration, which was also supported by a cross-peak between H-16 and H-17 in the NOESY spectrum. The NOE observed between H-17/CH 3 -21, and OAc-16/H-22b/ H-23b suggested the configuration at C-20 and C-24 be S. NOEs were also detected between H 3 -30/H-16; H-3/H-5. The D-configuration of the xylose and glucose units was confirmed using GC-MS analysis after acidic hydrolysis of soulieoside U following derivatisation with L-cysteine methyl ester and silylation (Wu, Zhang, et al. 2016). Thus, the structure of compound soulieoside U was concluded as (20S,24S)-16b-acetoxy-18,24;20,24-diepoxy-9,19-cyclanostane-3b, 25-diol-3-O-b-D-xylopyranosyl-25-O-b-D-glucopyranoside ( Figure 1).

Results and discussion
The cytotoxicity of soulieoside U was assayed against three human cancer cell lines (A549, HT-29 and MDA-MB231) using the MTT method with 5-FU as the positive control. The results showed that soulieoside U exhibited weak cytotoxic activity against all the tested cell lines with IC 50 values of 23.6 À 45.7 lM (Supplementary material  Table S2).

Acid hydrolysis of soulieoside U
Soulieoside U (2 mg) was heated in 2 mL of 2 M trifluoroacetic acid at 95 C for 2 h, followed by usual workup (Wu, Zhang, et al. 2016). The presence of D-xylose and D-glucose was confirmed by comparison of its retention time with that of standard samples.

Cytotoxic assays
The cytotoxicity of soulieoside U was assessed against A549, HT-29 and MDA-MB231 human cancer cell lines by the MTT method as described earlier (Wu, Liu, et al. 2017).

Conclusion
In conclusion, we described one new cycloartane triterpene glycoside, soulieoside U isolated from the rhizomes of A. vaginata. Soulieoside U showed weak cytotoxic activity against all the tested cell lines with IC 50 values of 23.6 À 45.7 lM.

Disclosure statement
No potential conflict of interest was reported by the authors.