Progress in the Studies of Triterpene Glycosides From Sea Cucumbers (Holothuroidea, Echinodermata) Between 2017 and 2021

Structural diversity of triterpene glycosides produced by sea cucumbers or holothurians (Holothuroidea, Echinodermata) is extremely high, although all of them are either lanostane derivatives or, rarely, products of their molecular rearrangements. The majority of them are holostane derivatives possessing an 18(20)-lanostane lactone as aglycone. They contain carbohydrate chains consisting of one to six monosaccharide units including sulfated ones. The glycosides demonstrate interesting biological activities, mainly caused by membranolytic action, namely cytotoxic, ichthyotoxic, antifungal, and hemolytic properties, as well as a series of additional effects at sub-toxic doses, including immunomodulatory, and cancer preventive. This review summarizes the literature data concerning structures and biological activities of all the new triterpene glycosides isolated from sea cucumbers during 2017 to 2021.

Structure-Activity Relationships of Holothuroid's Triterpene Glycosides and Some In Silico Insights Obtained by Molecular Dynamics Study on the Mechanisms of Their Membranolytic Action

The article describes the structure-activity relationships (SAR) for a broad series of sea cucumber glycosides on different tumor cell lines and erythrocytes, and an in silico modulation of the interaction of selected glycosides from the sea cucumber Eupentacta fraudatrix with model erythrocyte membranes using full-atom molecular dynamics (MD) simulations. The in silico approach revealed that the glycosides bound to the membrane surface mainly through hydrophobic interactions and hydrogen bonds. The mode of such interactions depends on the aglycone structure, including the side chain structural peculiarities, and varies to a great extent. Two different mechanisms of glycoside/membrane interactions were discovered. The first one was realized through the pore formation (by cucumariosides A₁ (40) and A₈ (44)), preceded by bonding of the glycosides with membrane sphingomyelin, phospholipids, and cholesterol. Noncovalent intermolecular interactions inside multimolecular membrane complexes and their stoichiometry differed for 40 and 44. The second mechanism was realized by cucumarioside A₂ (59) through the formation of phospholipid and cholesterol clusters in the outer and inner membrane leaflets, correspondingly. Noticeably, the glycoside/phospholipid interactions were more favorable compared to the glycoside/cholesterol interactions, but the glycoside possessed an agglomerating action towards the cholesterol molecules from the inner membrane leaflet. In silicosimulations of the interactions of cucumarioside A₇ (45) with model membrane demonstrated only slight interactions with phospholipid polar heads and the absence of glycoside/cholesterol interactions. This fact correlated well with very low experimental hemolytic activity of this substance. The observed peculiarities of membranotropic action are in good agreement with the corresponding experimental data on hemolytic activity of the investigated compounds in vitro.

Unusual Structures and Cytotoxicities of Chitonoidosides A, A1, B, C, D, and E, Six Triterpene Glycosides from the Far Eastern Sea Cucumber Psolus chitonoides

Six new triterpene tetra-, penta- and hexaosides, chitonoidosides A (1), A₁ (2), B (3), C (4), D (5), and E (6), containing one or two sulfate groups, have been isolated from the Far-Eastern sea cucumber Psolus chitonoides, collected near Bering Island (Commander Islands) from the depth of 100–150 m. Three of the isolated compounds (1, 3 and 6) are characterized by the unusual aglycone of new type having 18(20)-ether bond and lacking a lactone in contrast with wide spread holostane derivatives. Another unexpected finding is 3-O-methylxylose residue as a terminal unit in the carbohydrate chains of chitonoidosides B (3), C (4), and E (6), which has never been found before in the glycosides from holothurians belonging to the Psolidae family. Moreover, this monosaccharide is sulfated in the compound 4 into unprecedented 3-O-methylxylose 4-O-sulfate residue. Chitonoidoside C (4) is characterized by tetrasaccharide moiety lacking a part of the bottom semi-chain, but having disaccharide fragment attached to C-4 of Xyl1. Such architecture is not common in sea cucumber glycosides. Cytotoxic activities of the compounds 1–5 against mouse and human erythrocytes and human cancer cell lines: adenocarcinoma HeLa, colorectal adenocarcinoma DLD-1, and leukemia promyeloblast HL-60 cells were studied. The cytotoxic effect of chitonoidoside d (5) was the most significant in this series due to the presence of pentasaccharide disulfated sugar chain in combination with holostane aglycone. Surprisingly, the glycosides 1 and 3, comprising the new aglycone without γ-lactone, demonstrated similar activity to the known compounds with holostane aglycones. Chitonoidoside C (4) was less cytotoxic due to the different architecture of the carbohydrate chain compared to the other glycosides and probably due to the presence of a sulfate group at C-4 in 3-O-MeXyl4.

Marine natural products

This review covers the literature published in 2019 for marine natural products (MNPs), with 719 citations (701 for the period January to December 2019) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1490 in 440 papers for 2019), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. Methods used to study marine fungi and their chemical diversity have also been discussed.

Structures and Bioactivities of Quadrangularisosides A, A1, B, B1, B2, C, C1, D, D1-D4, and E from the Sea Cucumber Colochirus quadrangularis: The First Discovery of the Glycosides, Sulfated by C-4 of the Terminal 3-O-Methylglucose Residue. Synergetic Effect on Colony Formation of Tumor HT-29 Cells of these Glycosides with Radioactive Irradiation

Thirteen new mono-, di-, and trisulfated triterpene glycosides, quadrangularisosides A-D₄ (1-13) have been isolated from the sea cucumber Colochirus quadrangularis, which was collected in Vietnamese waters. The structures of these glycosides were established by 2D NMR spectroscopy and HR-ESI (High Resolution Electrospray Ionization) mass spectrometry. The novel carbohydrate moieties of quadrangularisosides D-D₄ (8-12), belonging to the group D, and quadrangularisoside E (13) contain three sulfate groups, with one of them occupying an unusual position-at C(4) of terminal 3-O-methylglucose residue. Quadrangularisosides A (1) and D₃ (11) as well as quadrangularisosides A₁ (2) and D₄ (12) are characterized by the new aglycones having 25-hydroperoxyl or 24-hydroperoxyl groups in their side chains, respectively. The cytotoxic activities of compounds 1-13 against mouse neuroblastoma Neuro 2a, normal epithelial JB-6 cells, erythrocytes, and human colorectal adenocarcinoma HT-29 cells were studied. All the compounds were rather strong hemolytics. The structural features that most affect the bioactivity of the glycosides are the presence of hydroperoxy groups in the side chains and the quantity of sulfate groups. The membranolytic activity of monosulfated quadrangularisosides of group A (1, 2) against Neuro 2a, JB-6 cells, and erythrocytes was relatively weak due to the availability of the hydroperoxyl group, whereas trisulfated quadrangularisosides D₃ (11) and D₄ (12) with the same aglycones as 1, 2 were the least active compounds in the series due to the combination of these two structural peculiarities. The erythrocytes were more sensitive to the action of the glycosides than Neuro 2a or JB-6 cells, but the structure-activity relationships observed for glycosides 1-13 were similar in the three cell lines investigated. The compounds 3-5, 8, and 9 effectively suppressed the cell viability of HT-29 cells. Quadrangularisosides A₁ (2), C (6), C₁ (7), and E (13) possessed strong inhibitory activity on colony formation in HT-29 cells. Due to the synergic effects of these glycosides (0.02 μM) and radioactive irradiation (1 Gy), a decreasing of number of colonies was detected. Glycosides 1, 3, and 9 enhanced the effect of radiation by about 30%.

Structures and Bioactivities of Psolusosides B1, B2, J, K, L, M, N, O, P, and Q from the Sea Cucumber Psolus fabricii. The First Finding of Tetrasulfated Marine Low Molecular Weight Metabolites

Ten new di-, tri- and tetrasulfated triterpene glycosides, psolusosides B₁ (1), B₂ (2), J (3), K (4), L (5), M (6), N (7), O (8), P (9), and Q (10), were isolated from the sea cucumber Psolus fabricii collected in the Sea of Okhotsk near the Kurile Islands. Structures of these glycosides were established by two-dimensional (2D) NMR spectroscopy and HR-ESI mass-spectrometry. It is particularly interesting that highly polar compounds 9 and 10 contain four sulfate groups in their carbohydrate moieties, including two sulfates in the same terminal glucose residue. Glycoside 2 has an unusual non-holostane aglycone with 18(16)-lactone and a unique 7,8-epoxy fragment. Cytotoxic activities of compounds 1-10 against several mouse cell lines such as Ehrlich ascites carcinoma cells, neuroblastoma Neuro 2A, normal epithelial JB-6 cells, and erythrocytes were quite different depending both on structural peculiarities of these glycosides and the type of cells subjected to their actions. Psolusoside L (5), pentaoside, with three sulfate groups at C-6 of two glucose and one 3-O-methylglucose residue and holostane aglycone, is the most active compound in the series. The presence of a sulfate group at C-2 of the terminal glucose residue attached to C-4 of the first (xylose) residue significantly decreases activities of the corresponding glycosides. Psolusosides of group B (1, 2, and known psolusoside B) are inactive in all tests due to the presence of non-holostane aglycones and tetrasaccharide-branched sugar chains sulfated by C-2 of Glc4.

Structures and Bioactivities of Six New Triterpene Glycosides, Psolusosides E, F, G, H, H1, and I and the Corrected Structure of Psolusoside B from the Sea Cucumber Psolus fabricii

Seven sulfated triterpene glycosides, psolusosides B (1), E (2), F (3), G (4), H (5), H₁ (6), and I (7), along with earlier known psolusoside A and colochiroside D have been isolated from the sea cucumber Psolus fabricii collected in the Sea of Okhotsk. Herein, the structure of psolusoside B (1), elucidated by us in 1989 as a monosulfated tetraoside, has been revised with application of modern NMR and particularly MS data and proved to be a disulfated tetraoside. The structures of other glycosides were elucidated by 2D NMR spectroscopy and HR-ESI mass-spectrometry. Psolusosides E (2), F (3), and G (4) contain holostane aglycones identical to each other and differ in their sugar compositions and the quantity and position of sulfate groups in linear tetrasaccharide carbohydrate moieties. Psolusosides H (5) and H₁ (6) are characterized by an unusual sulfated trisaccharide carbohydrate moiety with the glucose as the second sugar unit. Psolusoside I (7) has an unprecedented branched tetrasaccharide disulfated carbohydrate moiety with the xylose unit in the second position of the chain. The cytotoxic activities of the compounds 2-7 against several mouse cell lines-ascite form of Ehrlich carcinoma, neuroblastoma Neuro 2A, normal epithelial JB-6 cells, and erythrocytes-were quite different, at that hemolytic effects of the tested compounds were higher than their cytotoxicity against other cells, especially against the ascites of Ehrlich carcinoma. Interestingly, psolusoside G (4) was not cytotoxic against normal JB-6 cells but demonstrated high activity against Neuro 2A cells. The cytotoxic activity against human colorectal adenocarcinoma HT-29 cells and the influence on the colony formation and growth of HT-29 cells of compounds 1-3, 5-7 and psolusoside A was checked. The highest inhibitory activities were demonstrated by psolusosides E (2) and F (3).