Glycoconjugates play significant roles in biological systems and are used in medicine,
for example as vaccines. Glycoproteins, peptidoglycans, lipopolysaccharides, and other
biopolymer glycoconjugates are responsible for cellular interactions, including cell–cell
recognition and the binding of cells to the intercellular matrix. These molecules
perform signal, antigenic and transport functions, and participate in the formation
of receptors and other important membrane and blood constituents. Due to the negative
charges of some sulfated glycoconjugates and the binding of water, they are critical
for maintaining the physical status of connective tissue [1]. Low molecular weight
glycoconjugates, such as triterpene and steroidal glycosides [2,3,4], glycolipids,
are also well known as molecules playing important internal and exterior roles. Diverse
glycoconjugates show a very wide spectrum of biological activities, including defensive,
cytotoxic, antitumor, immunomodulatory, and antioxidant properties. The water environment
requires high solubility for signal or anti-predatory exometabolites of marine organisms,
and carbohydrate moieties provide this property. All these peculiarities explain the
very wide diversity of glycoconjugates of marine origin, including those presented
in the current Special Issue.
Sterol and sphingolipid glycoconjugates, which are widespread, but insufficiently
studied metabolites of microalgae, were discussed by Stonik and Stonik in the review
article. Glycosylated sterols play important biological roles in microalgae and show
different beneficial properties useful for medicine and healthy food. Dietary sterols
and their glycoconjugates of microalgae enter into marine invertebrates through food
chains and may be converted into 7(8)-unsaturated sterols and their derivatives, such
as polyhydroxylated sterols and, probably, glycosides of starfishes and sea cucumbers.
The knowledge of microalgal glycosphingolipids still remains poor, despite intensive
investigations. Some of them are important for their interactions with pathogens and
may induce apoptosis in microalgae. They also participate in the termination of microalgal
blooms [5].
In their experimental article, Galasso et al. have discussed the obtaining and properties
of a water-soluble bioactive fraction isolated from the toxic dinoflagellate Alexandrium
minutem. This substance is a glycoprotein with a molecular weight above 20 kDa. It
demonstrates specific antiproliferative activity (IC50 = 0.4 µg/mL) against the A549
cell line (the human lung adenocarcinoma cells). Moreover, the glycoconjugate did
not reveal a cytotoxicity against human normal lung fibroblasts (WI38), but induced
cell death, triggered by mitochondrial autophagy (mitophagy) in tumor cells. No mitophagic
events were activated by it in normal WI38 cells [6].
Kostetsky et al. from the Far East Federal University (Vladivostok, Russia) have compared
the fatty acid composition and thermal transitions of membrane lipids from green macroalgae
Ulva lactuca, collected in the Sea of Japan and the Adriatic Sea. The adaptation to
a warmer climatic zone was accompanied by a significant decrease in the ratio between
unsaturated and saturated fatty acids (UFA/SFA) in membrane lipids. The decreasing
ratio n-3/n-6 polyunsaturated fatty acids (PUFAs) was found in extra-plastidial lipids
and only in the major glycolipid, non-lamellar monogalactosyldiacylglycerol. The opposite
thermotropic behavior of non-lamellar and lamellar glycolipids can contribute to the
maintenance of the highly dynamic structure of thylakoid membranes [7].
The role of marine and freshwater lectins, compounds specifically recognizing carbohydrate
ligands, as anticancer agents, has been discussed by Italian and American scientists
in the review “Antitumor Potential of Marine and Freshwater Lectins,” of Catanazaro
et al. The co-authors have concluded that lectins from aquatic organism, demonstrating
a great variety of inhibitory effects against tumor cells and triggering apoptosis
and other forms of cell death, promise a bright future for such lectins in anticancer
research. Some these lectins are able to enhance the antineoplastic activity of common
antitumor medications. In the majority of cases, the lectins can distinguish normal
and transformed cells and even different types of tumor cells due to their ability
to recognize glycosylated patterns of various cell types. The lectins tested on animals
revealed optimal antitumor activities at negligible toxicity [8].
In their experimental work, Chinese scientists (Tao Wu et al.) from Zhejiang Sci-Tech
University, Hangzhou, have reported that a gene encoding marine sponge Aphrocallistes
vastus C-type lectin (AVL) was inserted into an oncolytic vaccinia virus vector (oncoVV)
to form a recombinant virus oncoVV-AVL. In vivo experiments showed that oncoVV-AVL
induces a significant antitumor effect in colorectal cancer and liver cancer mouse
models. These findings open the possibility of using the virus, containing marine
lectin AVL, in oncolytic viral therapies [9].
One more lectin, OXYL, a type-2 LacNAc-binding 14 kDa lectin belonging to the C1qDC
family, was isolated and characterized from the feather star Anneissia japonica (Echinodermata,
Crinoidea) by an international team (Hasan, et al.) led by Professor Yasuhiro Ozeki
(Japan). The structural studies of this lectin, carried out by Edman degradation,
revealed its N-terminal region, including the first 40 amino acids of the mature protein.
Its functional characteristics have been studied in detail. The lectin function relates
to immunity in the organism-producer. It caused a strong aggregation of bacterial
cells, but did not act on their growth. The lectin also displayed remarkable LacNAc
recognition-dependent anti-biofilm activity. The authors supposed that, due to its
novel primary structure and unique activities, this lectin may be estimated as a living
fossil, revealing the structural and functional diversification of metazoan lectins
[10].
New low-molecular-weight glycoconjugated compounds of echinoderms were isolated from
different species and studied by scientists from G.B. Elyakov Pacific Institute of
Bioorganic Chemistry, PIBOC (Russian Federation). For example, three new steroidal
glycosides, anthenosides V–X, along with seven previously known anthenosides, E, G,
J, K, S1, S4, and S6, have been isolated from the extract of tropical starfish Anthenea
aspera by Malyarenko et al. It is of interest that anthenoside V contains a rare 5α-cholest-8(14)-en-3α,7β,16α-trihydroxysteroidal
nucleus. All the investigated compounds at nontoxic concentrations inhibit the colony
formation of human melanoma RPMI-7951, breast cancer T-47D, and colorectal carcinoma
HT-29 cell lines. The mixture of anthenosides J and K possesses significant anticancer
activity and induces apoptosis of HT-29 cells. The mechanism of the proapoptotic action
of this mixture was found to be associated with the regulation of anti- and proapoptotic
protein expression, followed by the activation of initiator and effector caspases
[11].
Metabolomic studies concerning the localization of polar steroids, including their
glycoconjugates, in different body components of the Far Eastern starfish Lethasterias
fusca, have been carried out using nanoflow liquid chromatography/mass spectrometry
with captive spray ionization (nLC/CSI–QTOF–MS) by Popov et al. The assumptions concerning
the digestive function of polyhydroxysteroids and their derivatives and the protective
role of asterosaponins in starfish were in good agreement with the obtained results.
The highest level of polar steroids and their glycoconjugates was found in the stomach.
Asterosaponins were found in all other body components; the main share of free polyhydroxysteroids
and related glycosides were located in the pyloric caeca [12].
Seven new sulfated triterpene glycosides, psolusosides B, E, F, G, H, H1, and I, and
the earlier-known psolusoside A and colochiroside D, have been isolated by Sichenko
et al. from the sea cucumber Psolus fabricii, collected in the Sea of Okhotsk. The
structure of psolusoside B has been revised using modern 2D-NMR and HR-ESIMS procedures
and re-established as a disulfated tetraoside. The structures of other glycosides
were elucidated in the same manner. Cytotoxic activities of psolusosides E, F, G,
H, H1, and Iagainst the mouse ascite Ehrlich carcinoma cells, erythrocytes, neuroblastoma
Neuro 2A and normal epithelial JB-6 cells were quite different, while the hemolytic
action of the tested compounds was higher than their cytotoxicity against other cells,
particularly against the Ehrlich ascites carcinoma. Psolusoside G was not cytotoxic
against normal JB-6 cells, but revealed 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 by psolusosides B, E, F, H, H1,
and I, along with psolusoside A as a control, were examined. The highest inhibitory
activities were shown for psolusosides E and F [13].
Additionally, 10 new di-, tri- and tetrasulfated triterpene glycosides, psolusosides
B1, B2, J, K, L, M, N, O, P, and Q, have been also isolated from the sea cucumber
Psolus fabricii by the same team. The cytotoxic activities of these psolusosides on
several mouse cell lines, including Ehrlich ascites carcinoma cells, neuroblastoma
Neuro 2A, normal epithelial JB-6 cells, and erythrocytes, were quite different depending
both on the structural peculiarities of these glycosides and the type of cells. The
most interesting finding was that psolusosides P and Q contain four sulfate groups
in their carbohydrate chains. [14]. The presence of four sulfate groups is extremely
rare in low molecular weight metabolites. However, examples of substances having four
and even more sulfate groups were earlier reported in the studies on another class
of metabolites, namely on polysulfated sterol dimers, hamigerols A and B, from the
Mediterranean sponge Hamigera hamigera [15]. Moreover, axinelloside A, an unprecedented,
highly sulfated lipooligosaccharide from the marine sponge Axinella infundibula, contains
19 sufate groups attached to 12 sugars [16]. Nevertheless, the finding of tetrasulfated
derivative is also unique, particularly in relation to echinoderm metabolites.
Yunmei Chen et al. from Gingdao Ocean University of China have discussed an in vivo
affect of glycosaminoglycan AHG from the edible sea cucumber Apostichopus japonicus
on hyperglycemia in the liver of insulin-resistant mice. The obtained results demonstrated
that AHG supplementation significantly decreased body weight, level of blood glucose
and content of serum insulin in a dose-dependent manner in the mice. The protein levels
and gene expression of gluconeogenesis rate-limiting enzymes G6Pase and PEPCK were
significantly decreased. Although the total expression of IRS1, Akt, and AMPK in the
insulin-resistant liver was not affected by AHG supplementation, the phosphorylations
of IRS1, Akt, and AMPK were clearly increased by AHG treatment. The authors of this
article concluded that AHG could be a promising candidate for the development of an
antihyperglycemic agent [17].
Several reports in this Issue concern the bioactive compounds of microorganisms. Zhuravleva
et al. from PIBOC (Vladivostok, Russian Federation) have described in their article
10 new diterpene glycosides, virescenosides Z9–Z18 isolated from a marine strain of
the fungus Acremonium striatisporum KMM 4401 associated with the sea cucumber Eupentacta
fraudatrix. Glycosides of this class bear rare monosaccharide units such as altrose.
Moreover, virescenosides Z12–Z16 are monosides containing unique methyl esters of
altruronic acid. The carbohydrate moiety of virescenoside Z18 was found to be a methylester
of mannuronic acid. The effects of some glycosides and aglycons on urease activity
and the regulation of ROS and NO production in murine macrophages, stimulated with
lipopolysaccharide (LPC), were also studied [18].
A lipopolysaccharide (LPS), including the O-specific polysaccharide (O-antigen) of
Aeromonas veronii bv. sobria strain K557, serogroup O6, isolated from the carp Cyprinus
carpio during an outbreak of motile aeromonad septicemia (MAS) on a Polish fish farm,
has been immunochemically studied by Dworaczek et al. Freshwater C. carpio is a common
inhabitant of the bays and lagoons of Southern part of the Baltic Sea. The O-polysaccharide
was obtained by acid hydrolysis of the LPS and studied by chemical transformations
and spectral methods including 1H and 13C NMR spectroscopy. The O-antigen comprises
two O-polysaccharides, both containing a unique sugar, 4-amino-4,6-dideoxy-l-mannose
(N-acetyl-l-perosamine, l-Rhap4NAc). Western blotting and an enzyme-linked immunosorbent
assay (ELISA) revealed that the cross-reactivity between the LPS of A. veronii bv.
Sobria K557 and the A. hydrophila JCM3968 O6 antiserum, and vice versa, is caused
by the occurrence of common disaccharides, whereas an additional →4)-α-D-GalpNAc-associated
epitope defines the specificity of the O6 reference antiserum. This investigation
provides additional knowledge of the immunospecificity of Aeromonas bacteria and seems
to be significant for epidemiological studies and for finding the routes of transmission
of pathogenicity [19].
The diversity of marine glycoconjugate sources and their chemical structures, described
in this Special Issue, is impressive. Two articles concern microalgal metabolites
such as steroid and sphingoid glycoconjugates, and a glycoprotein from a sea cucumber
with interesting biological activities, respectively [5,6]. One article discusses
the fatty acid composition and thermotropic behavior of glycolipids and other membrane
lipids of green macrophyte Ulva lactuca [7]. Three articles cover lectin subjects
[8,9,10]. One review article analyzes the results and perspectives of marine and freshwater
lectins’ application in experimental oncology and the therapy of oncological diseases;
another article describes the use of a sponge lectin in the construction of a recombinant
virus. The third article concerns the function of the immunity of a lectin in producing
this compound crinoid. Two articles concern steroid glycosides from starfish [11,12],
and two others concern triterpene glycosides from sea cucumbers [13,14]. One article
describes the effect of a glycosaminoglycan from the sea cucumber Apostichopus japonicus
on hyperglycemia in the liver of insulin-resistant mice [17]. One article concerns
the isolation of 10 new triterpene glycosides from a fungus associated with a sea
cucumber [18]. The article by Dworaczek et al. characterizes the O-specific polysaccharide
(O-antigen) of a bacterial pathogen of common carp by chemical and immunochemical
methods [19]. In total, the Special Issue comprises 13 articles, including two reviews.
It is interesting that six articles are about glycoconjugates from echinoderms and
one article concerns the glycoconjugates of a microorganism associated with an echinoderm,
i.e., the subject of more than half of the articles, is linked with echinoderms that
reveals significant the biomedical potential of this group of marine invertebrates.
In conclusion, the editors are very appreciative to all authors that contributed their
excellent works to our Special Issue and wish them new and exciting discoveries.