Triterpenoids From Alisma Species: Phytochemistry, Structure Modification, and Bioactivities

Plants from Alisma species belong to the genus of Alisma Linn. in Alismataceae family. The tubers of A. orientale (Sam.) Juzep, also known as Ze Xie in Chinese and Takusha in Japanese, have been used in traditional medicine for a long history. Triterpenoids are the main secondary metabolites isolated from Alisma species, and reported with various bioactive properties, including anticancer, lipid-regulating, anti-inflammatory, antibacterial, antiviral and diuretic activities. In this brief review, we aimed to summarize the phytochemical and pharmacological characteristics of triterpenoids found in Alisma, and discuss their structure modification to enhance cytotoxicity as well.


INTRODUCTION
Plants from the genus of Alisma Linn. (Alismataceae) are widely distributed in temperate regions and subtropics of the northern hemisphere, belonging to 11 species. Six species were found in China and Asia, including A. canaliculatum, A. gramineum, A. nanum, A. orientale, A. plantago-aquatica and A. lanceolatum (Flora of China Committee, 1992). The tubers of A. orientale, known as Ze Xie in Chinese or Takusha in Japanese, have been used as diuretic and detumescent medications for a long history (Chinese Pharmacopoeia Commission, 2015). It is also used to treat obesity, diabetes and hyperlipidemia nowadays.
Phytochemical studies have revealed that triterpenoids are dominant components in tubers of Alisma plants. A total of 118 triterpenoids have been isolated and identified from Alisma species so far. Most of them contain protostane tetracyclic aglycones, whereas glycosides are rarely found in other plants. These triterpenoids have been considered as chemotaxonomic markers of the genus (Zhao et al., 2007). A small amount of other kinds of compounds have also been isolated from A. orientale, including diterpenoids, sesquiterpenoids, polysaccharides, phytosterols, amino acids, flavonoids and fatty acids . The presence of triterpenoids attributes to the bioactivities of A. orientales (Tian et al., 2014;Shu et al., 2016), such as alisol A 24-acatate (2), and alisol B 23-acetate (47) (Choi et al., 2019).
This paper aims to systematically review triterpenoids from Alisma species, involving their phytochemical characteristics, biosynthesis, bioactivities and structure modification.

TRITERPENOIDS
Starting from 1968, triterpenoids have been isolated from Alisma genus successively (Murata et al., 1968). All these compounds contain protostane tetracyclic skeleton with the structural characteristics of trans-fusions for A/B, B/C and C/D rings, α-methyl submitted at C-8, β-methyl at C-10, βmethyl at C-14 and side chain at C-17. At present there are 101 protostane triterpenoids, 12 nor-protostanes, and 5 secoprotostanes reported from Alisma. According to the changes of side chains submitted at C-17, protostane triterpenoids from Alisma are divided into four classes, including open aliphatic chains, epoxy aliphatic chains, spiro hydrocarbon at C-17, and epoxy at C-16, C-23 or C-16, C-24. The individual triterpenoids were detailed in Table 1.
Except for epoxy ring, tetrahydrofuran ring from C-20 to C-24 (74, 75) and seven-membered peroxic ring from C-20 to C-25 (76) are also existed in the side chains at C-17.
Frontiers in Chemistry | www.frontiersin.org   Zhao et al., 2007 Anticancer Activities Recently, the experiments in vitro highlight that alisols induce apoptosis and autophagy in human tumor cells, such as lung cancer , ovarian cancer , and prostate cancer (Huang et al., 2006) (59) and alisol A 24-acetate (2) have weaker inhibitory activities against all the tested cancer cells with ED 50 values in the range of 10∼20 µg/ml, while alisol B (46) exhibits significant effect on SK-OV3, B16-F10, and HT1080 with ED 50 values of 7.5, 7.5, and 4.9 µg/ml, respectively (Lee et al., 2001).  Moreover, alisol F 24-acetate (93) and alisol B 23-acetate (47) are found inducing cell apoptosis via inhibiting P-glycoprotein mediation and reversing the multidrug resistance in cancer cell lines (Wang et al., 2004;Hyuga et al., 2012;Pan et al., 2016). Alisol B (46) targets on Ca 2+ -ATP enzymes in the sarcoplasmic reticulum or endoplasmic reticulum to induce autophagy of cancer cells (Law et al., 2010). This compound can also induce cell apoptosis by inhibiting the invasion and metastasis of SGC7901 cells (Xu et al., 2009).
Alisol B 23-acetate (47) can inhibit the proliferation of PC-3 prostate cancer (Huang et al., 2006), and induce the apoptosis of lung cancer A549 and NCI-H292 cells through the mitochondrial caspase pathway . Alisol B 23-acetate (47)  obviously inhibits the proliferation, migration and invasion of ovarian cancer cell lines and induces accumulation of the G1 phase in a concentration-dependent manner. The protein levels of cleaved poly ADP-ribose polymerase (PARP) and the ratio of Bax/Bcl-2 are up-regulated, while the levels of CDK4, CDK6 and cyclin D1 are down-regulated after alisol B 23-acetate (47) treatment. Moreover, it can up-regulate the expression levels of IRE1α and Bip, and down regulate MMP-2 and MMP-9 in a doseand time-dependent manner . However, current studies of Alisma triterpenoids are limited into drug screening in vitro, and their anticancer activities need to be validated in vivo.
Alisol A (1), alisol A 24-acetate (2) and alisol B (46) can decrease TG level in plasma by improving lipoprotein lipase (LPL) activity . The effects of alisols with epoxy aliphatic chain at C-17 on LPL are stronger than those with an open aliohatic chain at C-17. Hydroxyl groups submitted at C-14, C-22, C-28, C-30, and an acetyl group at C-29 are necessary for lipid-regulation action of alisols.

Antiviral
Studies have shown that alisols from A. orientale exhibit obvious anti-hepatitis b virus effect (Jiang et al., 2006). Alisol F (92) and alisol F 24-acetate (93) significantly inhibit the secretion of HBV surface antigen with an IC 50 value of 7.7 and 0.6 µM, and HBVe antigen secretion with an IC 50 value of 5.1 and 8.5 µM, respectively. A series of derivatives of alisol A (1) obtained after structural modification also showed potential effect (Zhang et al., 2008(Zhang et al., , 2009).
Four hydroxyl groups of alisol A (1) are usually the target sites for modification by reacting with acetic anhydride in N, N ′ -dicyclohexylcarbodiimide and 4-dimethylamnopyridine. Alisol A (1) can also dehydrate by SOCl 2 in the presence of anhydrous pyridine. The assessments of anti-hepatitis B virus (HBV) activities suggest alisol A (1) analogs with acetoxyl groups at C-11, C-23, C-24 or the epoxy ring at C-13 and C-17 increase the effects on HBV. Dehydration at C-25/C-26 enhances its sensitivity on HBV (Zhang et al., 2008(Zhang et al., , 2009. Biotransformation of alisol A (1) also derives a series of active compound by several bacteria strains, such as C. elegans AS 3.2028 and P. janthinellum AS 3.510. Alisol A (1) can inhibit the proliferation of HCE-2 cells on the IC 50 of 99.65 ± 2.81 µM . The activity screening results reveal hydroxylation at C-7 and C-12 increases the inhibiting effects of alisol A (1) on human carboxylesterase 2 (IC 50 values of 7.39 ± 1.21 and 3.73 ± 0.76 µM) and the acetyl group at C-23 or C-24 also increases its inhibition effect on HCE-2 cells (IC 50 values of 3.78 ± 0.21 and 6.11 ± 0.46 µM).
Taken together, epoxidation at C-13 and C-17, hydroxylation at C-23, C-7/C-12, amination at C-3, and dehydration at C-25/C-26 contribute to the activities of protostane tetracyclic skeleton of A. orientale, including anticancer activity, antihepatitis B virtus, and the inhibiting activity on human carboxylesterase 2.

CONCLUSION
The present work systematically summarized the information concerning the phytochemistry, bioactivities and structure modification of triterpenoids in Alisma species. To date, more than 100 protostane-type terterpenoids have been isolated and identified. Alisols are reported with anticancer, lipid-regulating, anti-inflammatory, antibacterial, and antiviral activities. Structure modification might contribute to the investigation of the therapeutic potential of alisols.

AUTHOR CONTRIBUTIONS
MJ designed the review and was responsible for the study conception. PW and MJ wrote the paper. PW, TS, and RS contributed to summarizing the phytochemistry and structure modification studies on triterpenoids. MH, RW, and JL contributed to summarizing the bioactivity studies on triterpenoids.

FUNDING
This work was supported by the National major new drug creation science and technology major special support project (Nos. 2018ZX09735-002 and 2018YFC1707904) and the Natural Science Foundation of Tianjin (No. 18JCZDJC97700).