Triterpenoids

Naturally occurring dimeric triterpenoids: Occurrence, chemistry and bioactivities

The triterpenes represent one of the most reported subclasses of specialized metabolites from the plant kingdom. They play a key role in the protection of plants and their metabolism in addition to displaying a high structural diversity and large scale of biological activities. The scaffold can undergo several reactions like oxidation or substitution at different positions of the skeleton leading to the formation of several types of compounds. More specifically, triterpene dimer is a small group of compounds found in nature (from plants precisely). Until 2021, the chemical and pharmacological works reported in the literature indicated the identification of 90 natural dimeric triterpenes and 11 synthetic derivatives from 19 plants species and very few of them have been biologically evaluated for their antibacterial, antioxidant, antiproliferative or molluscicide activities. This review aims to compile the literature on the occurrence, chemistry and biological activities of the triterpenoid dimers. To attend this goal, a literature survey has been done in a number of online libraries including Scifinder, PubMed, Web of Science and Google Scholar using keywords terpene, triterpene, dimer, celastroloid without language restriction. This paper provides the easiest access to the information on triterpene dimers for readers and researchers in view to enhancing the continuity of research works on this topic.

Dammarane-Type Triterpenoids from the Roots of Rhus chinensis and Their Preventive Effects on Zebrafish Heart Failure and Thrombosis

Eight new dammarane-type triterpenoids (1-8), together with a related known analogue (9), were isolated from the roots of Rhus chinensis, a traditional Chinese medicine for treating coronary artery heart disease, guided by LC-MS analysis. Their structures were elucidated based on extensive spectroscopic analysis and quantum chemical calculations. Notably, compounds 1-7 and 9 possess an unusual 17α-side chain, and 1-4, 6, and 9 contain an uncommon 3-methyl-5,6-dihydro-2H-pyran-2-one moiety in the side chain. Compounds 1-5 and 9 have a 3,19-hemiketal bridge in the A ring. In an in vivo bioassay, 1, 2, and 4-6 exhibited significant preventive effects on zebrafish heart failure at 0.5 μg/mL, improving heart dilatation, venous congestion, cardiac output, blood flow velocity, and heart rate. Compound 5, displaying the most promising heart failure preventive activities, showed even better effects on increasing cardiac output (72%) and blood flow velocity (83%) than six first-line heart failure therapeutic drugs. Moreover, 1, 2, and 6 prevented the formation of thrombosis in zebrafish at 0.5 μg/mL. The present investigation suggests that the new dammarane triterpenoids might be partially responsible for the utility of R. chinensis in treating coronary artery heart disease.

Nimbolide induces apoptosis in human nasopharyngeal cancer cells

Nasopharyngeal carcinoma (NPC), a tumor arising from epithelial cells that cover the surface and line the nasopharynx, is a rare malignancy worldwide but is prevalent in certain geographical areas, such as Southern Asia (Taiwan, Hong Kong, Singapore, Malaysia, and Southern China) and North Africa. Despite advances in diagnostic techniques and improvements in treatment modalities, the prognosis of NPC remains poor. Therefore, an effective chemotherapy regimen that enhances tumor sensitivity to chemotherapeutics is urgently required. Nimbolide, derived from Azadirachta indica, has a wide range of beneficial effects, including anti-inflammatory and anticancer properties. The present study evaluated the antitumor activity of nimbolide in NPC cells and its underlying mechanisms. Our results revealed that the treatment of HONE-1 cells with nimbolide potently inhibited cell viability. Moreover, nimbolide led to cell cycle arrest, which subsequently activated caspase-3, -8, and -9 and poly (ADP-ribose) polymerase to induce cell apoptosis. Moreover, nimbolide induced Bik, Bax, and t-Bid expression in HONE-1 cells. The results indicated that nimbolide induces apoptosis through the modulation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathways. Nimbolide induces apoptosis in human NPC cells and is a potential chemopreventive agent against NPC proliferation. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 2085-2092, 2017.

Computational studies on the cyclization of squalene to the steroids and hopenes

A review of computational studies of the related biosyntheses of steroids and hopenes reported during the last two decades is presented.

Pleiocarpa pycnantha leaves and its triterpenes induce apoptotic cell death in Caco-2 cells in vitro

Background

Recently, we reported that the crude fractions and pure triterpenes; ursolic acid (C1), 27-E and 27-Z p-coumaric esters of ursolic acid (C2, C3), together with a new triterpene 2,3-seco-taraxer-14-en-2,3-lactone [pycanocarpine (C4)] and its hydrolysed derivative - (2,3-seco-taraxen-4-hydroxy-14-en-2-oic acid) [pycanocarpene (C5)] from Pleiocarpa pycnantha leaves inhibit cell proliferation. However, there has not been any specific report on the use of Pleiocarpa pycnantha leaves and its constituents to kill colorectal adenocarcinoma cancer CaCo-2 cells.

We performed in vitro study to evaluate the cytotoxic properties of the ethanolic extract of P. pycnanthaP, compounds C2 and C3. A preliminary study of the potential mechanisms were also undertaken.

Methods

Cell viability was measured by WST-1 assay. The Apoptosis level was evaluated by staining with APOPercentage™ dye and the induction of caspases 3/7 and 9 using Caspase-Glo® assays.

Results

The exposure of an ethanolic extract from the leaves of P. pycnantha (0.1–1000 μg/ml) and the isolated compounds C2 and C3 (6,25–100 μg/ml) to human colorectal cancer cells reduced the cell viability with an IC₅₀ > 100, 40.9, 36.3 μg/ml for P, C2 and C3 respectively, after 24 h of incubation. The APOPercentage^TM assay also showed a considerable increase in the percentage of apoptotic cells after 24 h; (25–38 % for P, 5–23 % for C2 and 6–47 % for C3). Caspase 3 was also activated which is a hallmark of apoptosis.

Conclusion

These findings suggest that the P. pycnantha and the isolated compounds induce cell apoptosis in human colorectal adenocarcinoma cells. A further study with other cell lines is also recommended.

Triterpenes as potentially cytotoxic compounds

Triterpenes are compounds of natural origin, which have numerously biological activities: anti-cancer properties, anti-inflammatory, anti-oxidative, anti-viral, anti-bacterial and anti-fungal. These substances can be isolated from plants, animals or fungi. Nowadays, when neoplasms are main cause of death, triterpenes can become an alternative method for treating cancer because of their cytotoxic properties and chemopreventive activities.

The concerted nature of the cyclization of squalene oxide to the protosterol cation

Concerted A-C ring formation: A concerted, but highly asynchronous, pathway was identified for the formation of rings A-C in the biosynthetic conversion of squalene oxide to the prosterol cation, with ring B being formed in the required boat conformation.

Lanostanoids from fungi: a group of potential anticancer compounds

Lanostanes are a group of tetracyclic triterpenoids derived from lanosterol. They have relevant biological and pharmacological properties, such as their cytotoxic effects via induction of apoptosis. This review compiles the most relevant lanostanoids studied from 2000 to 2011, principally those isolated from Ganoderma lucidum and other related fungi, such as Poria cocos, Laetiporus sulphureus, Inonotus obliquus, Antrodia camphorata, Daedalea dickinsii, and Elfvingia applanata, which have great potential as anticancer agents because of their cytotoxic or apoptotic effects. The compounds were selected on the basis of their proapoptotic mechanisms, through their ability to modify transcriptional activities via nuclear factors or genes and the activation or inhibition of pro- or antiapoptotic proteins; studies based only on their cytotoxicity were excluded from this review in the absence of complementary studies on their mechanisms of action. A total of 81 compounds from Ganoderma lucidum and other species from this genus are included, as well as 96 compounds isolated from other fungi, principally Poria cocos. Some of these compounds were found to arrest the cell cycle in the G1 phase, increase levels of p53 and Bax, or inhibit the phosphorylation of Erk1/2 or the activation of NF-κB and AP-1. Other lanostanes have inhibitory effects on the growth of androgen prostate carcinoma through increasing the expression of p21, which activates the tumor suppressor protein p53, while other compounds have been shown to selectively inhibit topo II activity without affecting topo I. General considerations concerning the chemical structure-biological activities of these compounds are also discussed.

Triterpenoid modulation of IL-17 and Nrf-2 expression ameliorates neuroinflammation and promotes remyelination in autoimmune encephalomyelitis

Inflammatory cytokines and endogenous anti-oxidants are variables affecting disease progression in multiple sclerosis (MS). Here we demonstrate the dual capacity of triterpenoids to simultaneously repress production of IL-17 and other pro-inflammatory mediators while exerting neuroprotective effects directly through Nrf2-dependent induction of anti-oxidant genes. Derivatives of the natural triterpene oleanolic acid, namely CDDO-trifluoroethyl-amide (CDDO-TFEA), completely suppressed disease in a murine model of MS, experimental autoimmune encephalomyelitis (EAE), by inhibiting Th1 and Th17 mRNA and cytokine production. Encephalitogenic T cells recovered from treated mice were hypo-responsive to myelin antigen and failed to adoptively transfer the disease. Microarray analyses showed significant suppression of pro-inflammatory transcripts with concomitant induction of anti-inflammatory genes including Ptgds and Hsd11b1. Finally, triterpenoids induced oligodendrocyte maturation in vitro and enhanced myelin repair in an LPC-induced non-inflammatory model of demyelination in vivo. These results demonstrate the unique potential of triterpenoid derivatives for the treatment of neuroinflammatory disorders such as MS.

Antinociceptive action of limonexic acid obtained from Raulinoa echinata

The antinociceptive effect of the limonexic acid isolate of Raulinoa echinata Cowan in four models of pain in mice is described. When evaluated against acetic acid-induced abdominal constrictions, limonexic acid (10, 30 and 60 mg kg−1, i.p.) produced dose-related inhibition of the number of constrictions, with a mean ID50 value of 43 (2.3–79) μmol kg−1, and was more potent than some standard drugs. In the formalin test, limonexic acid inhibited both the first and second phases of formalin-induced pain. Furthermore, the effect was more pronounced in the second phase, with a mean ID50 value of 13.66 (9.35–19.61) μmol kg−1, and had a pharmacological profile that was similar to standard drugs such as acetaminophen and acetyl salicylic acid. Limonexic acid also produced dose-related inhibition of glutamate- and capsaicin-induced pain, with mean ID50 values of 11.67 (8.51–16.0) μmol kg−1 and 47.17 (36.51–60.93) μmol kg−1, respectively. The mechanism of action is not completely understood, but seems to involve direct interaction with the GABAergic and nitroxidergic pathways.

Silylene-mediated ring contraction of homoallylic ethers to form allylic silanes

(-)-Isopulegol derivatives undergo a ring contraction under silylene-mediated conditions to provide cyclopentane products. Silylene transfer to other homoallylic ethers did not provide the ring contraction products. Allylic silane products were elaborated to determine the stereochemical course of the ring contraction reaction. A mechanism for the transformation is proposed.

Synthesis and structure–activity relationship study of cytotoxic germanicane- and lupane-type 3β-O-monodesmosidic saponins starting from betulin

Germanicane-type triterpenes allobetulin (3) and 28-oxoallobetulin (4) can be obtained by the Wagner-Meerwein rearrangement of the more available lupane-type triterpenes betulin (1) and betulinic acid (2), respectively. The medical uses of betulinic acid (2) and its derivatives are limited because of their poor hydrosolubility and pharmacokinetics properties. In order to overcome this major problem, we synthesized and studied the in vitro anticancer activity of a series of 3beta-O-monodesmosidic saponins derived from betulin (14-16), betulinic acid (20-22), allobetulin (23-28) and 28-oxoallobetulin (29-34) based on six different natural sugar residues (d-glucose, l-rhamnose, d-arabinose, d-galactose, d-mannose and d-xylose). This structure-activity relationship study confirmed that betulinic acid saponins are generally better in vitro anticancer agents than those derived from betulin with the exception of betulin 3beta-O-alpha-d-mannopyranoside (15) which exerted a potent cytotoxic activity against lung carcinoma (A-549) and colorectal adenocarcinoma (DLD-1) human cell lines with IC(50) ranging from 7.3 to 10.1mumol/L. Furthermore, although the synthesis of novel germanicane-type saponins was carried out with success, the bioactivity measured for these glycosides was not as high as we anticipated since only the 3beta-O-beta-d-glucopyranoside and 3beta-O-beta-d-galactopyranoside of allobetulin (23,24) showed moderate anticancer activity (IC(50) 30-40 micromol/L).

The discovery of a class of novel HIV-1 maturation inhibitors and their potential in the therapy of HIV

Although HIV infection is now primarily treated with reverse transcriptase and protease inhibitors, HIV therapy must look toward new drugs with novel mechanism(s) of action to both improve efficacy and address the growing problem of drug resistance. Using natural products as a source of biologically active compounds, our drug discovery program has successfully optimised the natural product betulinic acid to the first-in-class maturation inhibitor 3-O-(3',3'-dimethylsuccinyl)-betulinic acid (DSB). DSB's unique viral target has been identified as a late step in Gag processing. Specifically, it inhibits the cleavage of the capsid precursor, CA-SP1, resulting in a block to the processing of mature capsid protein leading to a defect in viral core condensation. DSB represents a unique class of anti-HIV compounds that inhibit virus maturation and provide additional opportunities for anti-HIV therapy. In this review, the discovery of DSB and its mode of action are summarised. Anti-AIDS Agents part 64. For part 63 in the series, see YU D, LEE KH: Recent progress and prospects on plant-derived anti-HIV agents and analogs. In: Medicinal Chemistry of Bioactive Natural Products. XT Liang, WS Fang (Eds), Wiley, New York, USA (2005) (In Press).