Phytochemicals and biological activities of Ligularia species

Introduction

The genus Ligularia has been taxonomically placed in the Compositae (tribe Senecioneae) with more than 27 species used as folk remedies¹. The systematic and in-depth phytochemical investigations on Ligularia species have resulted in hundreds of secondary metabolites with various skeletons and interesting biological activities have been discovered from this genus. The application of some Ligularia species in traditional medicines has been in period. For example, L. sagitta possess efficacies of relieving phlegm and cough, invigorating circulation of blood, soothing pain, and particularly curing rheumatoid arthritis². L. fischeri has been used as a folk medicine for the treatment of coughs, inflammations, jaundice, scarlet fever, rheumatoidal arthritis, and hepatic diseases³. L. veitchiana was reported for the treatment of influenza, cough, ulcer and pulmonary tuberculosis⁴. L. lapathifolia has been used to treat cough and inflammation⁵. Furthermore, L. sibirica and L. hodgsoni are used as herbal remedies to treat bronchitis, cough, asthma, and phthisis⁶.

Searching for bioactive molecules from nature source has always been our interest^7-12. In the past years, some Ligularia species, such as L. virgaurea spp. oligocephala⁹, L. myriocephala¹³, and L. fischeri¹⁴, have been investigated in our lab from the viewpoint of phytochemistry. The promising results stimulated our interest in Ligularia species as a source of substances with chemical and biological diversity. Here we review the state of the art in the phytochemical investigation and biological activity evaluation of Ligulariaspecies in recent years (1990.1~2011.6).

1 Chemical Constituents

1.1 Sesquiterpenoids

As the major chemical constituents, there are 289 sesquiterpenoids reviewed. These sesquiterpenoids comprise eremophilane-type (1-1 to 1-210), bisabolane-type (1-211 to 1-242), oplopane-type (1-243 to 1-248), guaiane and pseudoguaiane types (1-249 to 1-253), eudesmane type (1-254 to 1-258), and other skeleton types (1-259 to 1-267) as well as dimers (1-268 to 1-289). The names and corresponding plant sources of these sesquiterpenoids were listed in Table 1^{1-6, 9, 13-99}.

1.1.1 Eremophilane Sesquiterpenoids

Of the 368 secondary metabolites reviewed in this paper, there are 210 eremophilane sesquiterpenoids (1-1 to 1-210). Consequently, the eremophilane sesquiterpenoid is the most common phytochemical type. Thus, the taxonomic significance of eremophilane sesquiterpenoids for the genus Ligularia needs further study in future. Most of these eremophilane sesquiterpenoids were obtained in the form of lactones, and they can be divided into five groups from the structural viewpoint: a) eremophilane-12, 8-olides (1-1 to 1-77); b) eremophilane-12, 8(14, 6α)-diolides (1-78 to 1-99); c) eremophilane-14, 6α-olides (1-100 to 1-104); d) furaneremophilane sesquiterpenoids (1-105 to 1-129); e) other eremophilane sesquiterpenoids (1-130 to 1-210).

1.1.1.1 Eremophilane-12, 8-olides

Eremophilane-12, 8-olides (1-1 to 1-77) are the most popular eremophilane lactones. Compounds 1-1 to 1-36 are eremophilane-12, 8α-olides, while compounds 1-37 to 1-57 are eremophilane-12, 8β-olides. Of the structures 1-1 to 1-77, H_a/b-6, H-8, and H-10 were always substituted by various substitutions, such as OH, OAc, OAng, OMe, and OEt. In some cases (1-5, 1-8, 1-12, 1-13, 1-16 to 1-18, 1-22, 1-24 to 1-26, and 1-31 to 1-35), an epoxy group has been formed between C-1 and C-10. Furthermore, a double bond is often constructed between C-8 and C-9 (1-58 to 1-73) or C-7 and C-8 (1-74 to 1-77).

1.1.1.2 Eremophilane-12, 8(14, 6α)-diolides

Of such structures (1-78 to 1-99), an interesting phenomenon is that all H-6 protons are β-oriented. In addition, the H-8 protons are often substituted by OH, OMe, or OEt, while a double bond is often constructed between C-8 and C-9 in some cases (1-91 to 1-94 and 1-99).

1.1.1.3 Eremophilane-14, 6α-olides

As that of 1-78 to 1-99, the H-6 protons in structures 1-100 to 1-104 are all β-oriented. This phenomenon may show some relationships with the biosynthetic pathway of 14, 6α-olide moiety.

1.1.1.4 Furan-eremophilane Sesquiterpenoids

All of structures 1-105 to 1-129 possess a furan ring. Due to the sructural similarity, these compounds are put in one group in this review. Their most obvious structural characteristic is that the C-6 positions always possess various substitutions, such as OH, OAc, and OAng.

1.1.1.5 Other Eremophilane Sesquiterpenoids

Besides above structures, there are still 81 eremophilane-type sesquiterpenoids (1-130–1-210) covered here. Compounds 1-133 to 1-136 are isolated from the roots of L. fischeri in our lab^{14, 54}, and 1-135 and 1-136 are obtained as a pair of epimers and their atructures have been confirmed by single-crystal Xray diffraction analysis¹⁴. Compound 1-137 is obtained as a sesquiterpenoid lactam³, which is rarely discovered from nature source. The structures 1-200 to 1-210 represent a rare carbon skeleton, and the probable biosynthetic pathway of such skeleton is proposed^{77, 78}.

1.1.2 Bisabolane Sesquiterpenoids

Bisabolane sesquiterpenoids 1-211 to 1-242 and their corresponding plant sources were indicated in Table 1. Among them, H_a/b-1, H-2, H_a/b-8, and H_a/b-10 are always substituted by OH or OAng. Furthermore, there is often an epoxy group formed between C-3 and C-4 (1-213, 1-214, 1-218, 1-219, 1-227 to 1-231, and 1-235) or between C-10 and C-11 (1-214, 1-215, 1-217 to 1-220, 1-223, 1-226 to 1-229, 1-233, and 1-235). In some cases (1-237 to 1-242), ring A is often oxygenated to benzonic moiety.

1.1.3 Oplopane Sesquiterpenoids

The six oplopane-type sesquiterpenoids 1-243 to 1-248, listed in Table 1, were all isolated from the roots of L. narynensis. Considering the structural characteristics, the C-3, C-4, C-8, and C-9 positions often possess various substitutions. Furthermore, in all of these structures, there is an epoxy group posited between C-11 and C-12.

1.1.4 Other Sesquiterpenoids

Besides the above main sesquiterpenoids types, there were still guaiane-types 1-249 to 1-251, pseudoguaiane-types 1-252 and 1-253, and eudesmanetypes 1-254 to 1-258, as well as other types 1-259 to 1-267 being reviewed. Their names and corresponding plant sources were detailed in Table 1.

Compound 1-262, possessing a new carbon skeleton, was discovered from L. virgaurea spp. oligocephala⁹. The lactones 1-263 and 1-264 were obtained as a pair of isomers, and their structres were determined using extensive spectroscopic methods⁹². The novel structures 1-265 and 1-266 were obtained as sesquiterpenoid-coumarin dimers⁹³, which are rarely discovered from nature source.

1.1.5 Sesquiterpenoid Dimers

The 22 sesquiterpenoid dimers 1-268 to 1-289 have been indicated in Table 1. Of these, structures 1-268 to 1-282 share the C-C linkage pattern, while 1-283 to 1-289 share the C-O-C linkage pattern^{1, 3, 31, 38, 44, 53, 57, 63, 77, 95-99}.

1.2 Monoterpenoids and Diterpenoids

The structures of monoterpenoids 2-1 and 2-2 and diterpenoids 2-3 to 2-5 were provided, and their names and plant sources were listed in Table 2^{15, 62, 88, 100}. Of them, structure 2-3 was isolated as a C₁₉-diterpenoid carbon skeleton from L. sagitta, and its structure was further confirmed using single-crystal X-ray diffraction method⁶².

1.3 Triterpenoids

The ten triterpenoids 3-1 to 3-10 mainly comprise oleane and norursane types. Their names and corresponding plant sources were indicated in Table 3^{40, 47, 100-105}. Of them, compounds 3-3 and 3-4 were obtained as triterpenoid saponins from L. veitchiana^{102, 103}. Compounds 3-5 and 3-6 were isolated from L. intermedia in the form of 3, 4-seco-oleanolic triterpene acids¹⁰⁴. Compounds 3-7 and 3-8 are norursane-type triterpenoids and were isolated from L. tongolensis⁴⁷.

1.4 Others

Besides the above terpenoid constituents, there were still one steroid (4-1), 11 alkaloids (4-2 to 4-12), two flavonoids (4-13 and 4-14), and three lignans (4-15 to 4-17), as well as other secondary metabolites, possessing various skeletons (4-18 to 4-64), highlighted in Table 4^{4, 6, 39, 66, 80, 83, 89, 105-120}. Among these structures, 4-49 and 4-50, 4-51 and 4-52, 4-53 and 4-54 were isolated as racemates from L. stenocephala, which were further comfirmed by the chiral HPLC analysis¹²⁰.

2 Biological Activities

2.1 Antibacterial Activity

In 2003, Li et al reported that eremophilane sesquiterpenoid lactones 1-20 and 1-51, isolated from L. sagitta, showed antibacterial activity against Staphylococcus aureus, Bacillus subtilis and Escherichia coli according to the paper-disk method²⁶. In 2009, another eremophilane sesquiterpenoid lactone 1-41, from L. hodgsonii, was reported showing weak antibacterial activity against Bacillus subtilis with the MIC of 128 μg/mL37.

2.2 Cytotoxic Activity

In 2006, Wu et al reported the cytotoxic activities of eremoligularin (1-130) and bieremoligularolide (1-268). The result revealed that bieremoligularolide (1-268) showed strong cytotoxicities: IC₅₀ = 5.5, 16.1, and 8.9 μM against HL-60, SMMC-7721, and HeLa cells, respectively. However, eremoligularin (1-130) showed no cytotoxicity against the above three cells (IC₅₀ > 100 μM)¹⁵. In addition, in 2008, an eremophilane sesquiterpenoid 1-152, obtained from L. veitchiana, was reported to exhibited significant inhibiting activities on the growth of lung-cancer (A549) and stomachcancer (BCG823) cell lines, with IC₅₀ values of 10.27 (A549) and 31.14 (BCG823) μg/mL, respectively⁶⁷. While in 2010, an bisabolane sesquiterpene 1-241 was found showing significant cytotoxicity against human lung carcinoma (A-549), human breast adenocarcinoma (MCF-7), epidermoid carcinoma of the nasopharynx (KB), and vincristine-resistant nasopharyngeal (KBVIN) cell lines, with EC₅₀ values of 3.4 (A549), 0.8 (MCF-7), 1.0 (KB), and 0.9 (KBVIN) μg/mL, respectively⁷⁹.

2.3 Protein Tyrosine Phosphatase Inhibitory Activity

In 2009, an eremophilane lactone 1-43, from the roots of L. fischeri, was evaluated for the inhibitory activity against protein tyrosine phosphatase (PTP1B) in vivo by Deng et al²³. The experiment data indicated moderate inhibitory activity with IC₅₀ = 1.34 μM.

2.4 Insecticidal and Antifeedant Activities

The plant L. caloxantha has been used as a folk medicine in the Naxi nationality in Yunnan province for years. In 2005, a phytochemical investigeation on the roots and rhizomes of this plant by Li et al led to the isolation of two benzofuran compounds, euparin (4-61) and 6-methoxy-euparin (4-62)¹²¹. The bioactivity assay revealed that both of the two compounds showed significant insecticidal and antifeedant activities. This conclusion may provide an explanation why L. caloxantha is used as a folk anti-insect agent.

2.5 Antihepatotoxicity and Antioxidative Activity

It has been reported that the MeOH extract (LFS) of L. fischeri var. spiciformis and its active component, 3, 4-dicaffeoylquinic acid (DCQA) (4-63), showed significant antihepatotoxicity, the action mechanism of which was investigated by Choi et al in 2004¹²². The result showed that both LFS and DCQA resultantly prevented hepatotoxicity via antioxidative mechanism. Thus, it was proposed that antihepatotoxicity of LFS was based on the antioxidative action of DCQA.

2.6 Antithrombotic and Anticoagulating Activity

In 2008, Yoon et al reported that the leaf extract of L. stenocephala showed the highest anti-platelet aggregating activity. The active fraction inhibited the platelet aggregation up to above 80% and its blood coagulating time also showed similar effect to aspirin (0.2 μg/mL), known as an antithrombus compound. An activity-guided seperation resulted in two antithrombus active compounds as 3, 4-dicaffeoylquinic acid (4-63) and 3, 5-dicaffeoylquinic acid (4-64). A further assay showed that the two active compounds has not only antiplatelet aggregating activity, but also has anticoagulating activity¹²³.

3 Conclusions

This review summarized the secondary metabolites reported from Ligularia species as well as their biological activities in recent decades. These conclusions indicate that Ligularia species may be a rich source of natural products with chemical and biological diversity.

Notes

Acknowledgments

This work was supported by the Important Directional Project of the Chinese Academy of Sciences (No. KSCX2-EW-R-15) and the National Natural Science Foundation of China (No. 21075127).