Glycosylated Natural Products From Marine Microbes

A growing body of evidence indicates that glycosylated natural products have become vital platforms for the development of many existing first-line drugs. This review covers 205 new glycosides over the last 22 years (1997–2018), from marine microbes, including bacteria, cyanobacteria, and fungi. Herein, we discuss the structures and biological activities of these compounds, as well as the details of their source organisms.


INTRODUCTION
Sugars are ubiquitous in nature and have a multitude of functions, ranging from serving as a simple source of energy to contributing to molecular-recognition scaffolds that are critical to the interactions/communication among a wide array of biomolecules, cells, tissues, and organisms (Gantt et al., 2011). Not only do sugars work alone in the processes of life, but also they play an important role by combing with secondary metabolites. For instance, glycolipids are carbohydrate-attached lipids, which are widely distributed throughout organisms and involved in the biosynthesis of glycoproteins and serve as ligands for toxins, lectins, bacteria, and viruses. In addition, sugars are also attached to the anomeric carbon of a non-sugar moiety via a glycosidic linkage, such as quinones, lactones, peptides, terpenoids, and alkaloids etc., performed by more than 80 families of glycosyl transferases and those secondary metabolites derive multiple drugs, such as gentamycin, vancomycin, bleomycin, and erythromycin etc. (Grynkiewicz et al., 2008;Yu et al., 2012). Although some glycosides are simply attached to saccharides and saccharide parts in which glycosides are mostly inactive in terms of activity, sometimes they are crucial for overall effects, such as the improvement of a drug's pharmacokinetics and/or dose-limiting toxicities and the improvement for a drug's solubility and selective/non-selective uptake into cells/organs of interest (Gantt et al., 2011;Yu et al., 2012).
Oceans cover more than 70% of the Earth's surface and host considerable diversity of species. Approximately 30,000 marine natural products had already been identified by the end of 2017 (Jimenez, 2018). The roles of marine natural products in biomedical research and drug development are significant and promising. Many marine natural products have been in clinical stages and the interest in marine natural products is increasing every year . Among these compounds, seven structural types of approved therapeutic agents are considered derivatives of marine natural products, including two nucleosides-the anticancer cytarabine (ara-C, FDA-approved in 1969) and the antiviral vidarabine (ara-A, FDA-approved in 1976)-derived from two natural arabinonucleosides (Figure 1) (Dyshlovoy and Honecker, 2018). Hence, glycosides have served as a validated platform for the development of many existing front-line drugs (Blanchard and Thorson, 2006). Given the vital role of glycosides in drug discovery, this review provides a comprehensive overview of the structures and biological activities of 205 glycosides (discovered 1997-2018) from marine-sourced bacteria, cyanobacteria, and fungi, along with the details of their source organisms.
Further chemical investigation of the actinomycete Saccharothrix espanaensis An 113, associated with the marine mollusk Anadara broughtoni, led to the isolation of two angucyclines saccharothrixmicine A (12) and B (13, Figure 2). Bioassay results indicated that the saccharothrixmicinecontaining fraction exhibited activity toward Candida albicans and Xanthomonas sp. pv. Badrii (Kalinovskaya et al., 2008(Kalinovskaya et al., , 2010. Based on bioassay-guided analyses and the detection of genes encoding for the biosynthesis of secondary metabolites, the marine Streptomyces sp. strain HB202, which was isolated from the sponge Halichondria panacea, showed profound antibiotic activity and yielded a benz[α]anthracene derivative called mayamycin (14, Figure 2). This compound exhibited potent activity against several human cancer cell lines (IC 50 0.15-0.33 µM) and inhibited growth of a number of bacteria including antibiotic-resistant strains (IC 50 0.31-31.2 µM) (Schneemann et al., 2010).
Guided by a biochemical induction assay, two dimeric diazobenzofluorene glycosides, lomaiviticins A-B (34-35, Figure 3), were isolated from the halophilic actinomycete LL-37I366, which was found to be a new species, Micromonospora lomaivitiensis. Both showed potent DNA-damaging activity at a minimum induction concentration ≤0.1 ng/spot and lomaiviticin A (34) exhibited cleaved double-stranded DNA under reducing conditions. In an assay against a number of cancer cell lines, lomaiviticin A (34) also possessed a unique cytotoxicity profile with IC 50 values ranging from 0.01 to 98 ng/ml as compared to those of known DNA-damaging drugs, such as adriamycin and mitomycin C. Both lomaiviticins A-B (34-35) also exhibited potent antibiotic activity against S. aureus and E. faecium (He et al., 2001). Continuous searching for benzo [b]fluorene led to the discovery of nenestatin A (36, Figure 3) produced from the deep sea-derived Micromonospora echinospora SCSIO 04089. Comparative bioinformatic analysis has indicated a high similarity of nenestatin A (36) and lomaiviticin gene clusters and has led to elucidation of similar biosynthetic pathways, including a conserved set of enzymes for the formation of a diazo group .

Alkaloids
Twelve indolocarbazoles 68-70, 71-76, and 77-79 (Figure 6) were isolated from the marine-derived Streptomyces sp. A68, Streptomyces sp. DT-A61, and Streptomyces sp. A65, respectively. Bioactivity testing showed that these indolocarbazoles had cytotoxic activities toward PC-3 cell lines with IC 50 values of 0.8-41.3 µM. In addition, most of these indolocarbazoles also showed potent kinase inhibitory activities against protein kinase C alpha (PKCα), Roh associated protein kinase 2 (ROCK2), Bruton's tyrosine kinase (BTK), and apoptosis signal-regulating kinase 1 (AKS1). For instance, compound 7 displayed a notable inhibitory effect against ROCK2 with an IC 50 value of 5.7 nM, which was similar to that of the positive control, staurosporine (IC 50 = 7.8 nM). Structure-activity relationships for this set of indolocarbazoles suggested that when the sugar, connected with the K252c unit, was similar to that of staurosporine, the compound would be more effective than those without sugar moiety or those with only a single attachment of the sugar to the aromatic aglycone Wang J. N. et al., 2018;Zhou et al., 2018).
Two antitumor pyranone glycosides, PM050511 (97) and PM0060431 (98, Figure 7), along with their aglycones PM050463 and PM060054, were obtained from the marine-derived Streptomyces albus, POR-04-15-053. Bioassay testing suggested that compounds 97-98 showed excellent cytotoxicity against three human tumor cell lines with GI 50 values in the range of 0.24-2.69 µM (Schleissner et al., 2011). A cytotoxic piericidin derivative, glucopiericidin C (99, Figure 7), was isolated from the marine-derived Streptomyces species B8112 and showed a concentration-dependent cytotoxicity toward a panel of 36 human tumor cell lines with an IC 50 value of 2.0 µM (mean IC 70 =4.2 µM), in addition to the same antibacterial activity as glucopiericidin A (Shaaban et al., 2011). One flavonoid derivative, flavoside A (100, Figure 7), was produced from the EtOAc extract of the culture broth of the sea urchin (Anthocidaris crassispina)-derived actinobacterium, Streptomyces sp. HD01 (Guo et al., 2019). According to the HPLC-UV profile, the Streptomyces sp. CMN-62 isolated from an unidentified sponge sample was selected for its chemical investigation and produced two anthranilate-containing alkaloids, anthranosides A-B (101-102, Figure 7; Che et al., 2018). Chemical analysis of these actinomycete strains using LC/MS identified a Streptomyces sp. SNM31 and led to a metabolite, mohangic acid E (103, Figure 7), which was the first glycosylated compound discovered in the paminoacetophenonic acid family and exhibited good quinonereductase induction activity at a concentration of 20 µM (Bae et al., 2016).
Bioassay-guided investigation of the marine cyanobacterium Lyngbya sp., collected in Okinawa Prefecture, led to an 18membered macrolide glycoside, biselyngbyaside (110, Figure 8). Biselyngbyaside (110) exhibited broad-spectrum cytotoxicity in a panel of human tumor cell lines and likely inhibited cancer cell proliferation through a mechanism indicated by COMPARE analyses (Teruya et al., 2009). Chemical investigation of the marine cyanobacterium Lyngbya sp., collected from the Tokunoshima Island, Japan, led to three new analogs of biselyngbyaside (110), biselyngbyasides B-D (111-113, Figure 8). Biselyngbyaside B (111) was shown to induce apoptosis in HeLa S 3 cells and HL60 cells. Further investigation of this activity in HeLa S 3 cells indicated that apoptosis is likely mediated through increasing cytosolic Ca 2+ concentrations (Morita et al., 2012).
The dimeric macrolide xylopyranoside, cocosolide (114, Figure 8), was obtained from the marine cyanobacterium preliminarily identified as Symploca sp. and reduced IL-2 production without significantly affecting cell viability. Comparison of the activities of analogs indicated the importance of sugars and dimeric structures to the target recognition and engagement process (Gunasekera et al., 2016). Bioassay-guided fractionation of the extract of Leptolyngbya sp., collected from the coast of Itoman City in the Okinawa Prefecture (Japan), led to the separation of two macrolactones, leptolyngbyolides A-B (115-116, Figure 8), both of which showed strong growth inhibition against HeLa S 3 cells with IC 50 values of 0.1 and 0.16 µM, respectively. In addition, structure-activity relationships suggested that the sugar moiety did not affect growth-inhibitory activity (Cui et al., 2017).
The polycavernoside analog, polycavernoside D (117, Figure 9), was isolated from a red-colored Okeania sp. and had moderate activity against the human lung carcinoma cell line H-460 (EC 50 = 2.5 µM). Importantly, polycavernoside D (117) was obtained from the Atlantic, whereas polycavernosides previously isolated were derived from the Western Pacific, suggesting that these toxins occur over a much wider geographical range than originally thought (Navarro et al., 2015). Two glycosylated swinholides, ankaraholides A-B (118-119, Figure 9), were produced by the cyanobacterium, Geitlerinema sp., from a Madagascar field collection. Bioassay testing indicated that ankaraholide A (118) inhibited proliferation (IC 50 values) in NCI-H460 (119 nM), Neuro-2a (262 nM), and MDA-MB-435 (8.9 nM) cell lines (Andrianasolo et al., 2005). Under bioassay-guided separation in combination with the MS2-based molecular-networking dereplication tool, Frontiers in Chemistry | www.frontiersin.org nine glycosylated swinholide-type compounds, samholides A-I (120-128, Figure 9), were separated from the American Samoan marine cyanobacterium cf. Phormidium sp. All of these samholides showed potential activities against the human lung cancer cell line H-460 with IC 50 values ranging from 170 to 910 nM. Comparison of the activities of these samholides suggested that the sugar and glycericacid units played important roles in enhancing the cytotoxic activity (Tao et al., 2018).

Alkaloids
Chemical investigation of the crude organic extract of L. majuscula from Puerto Rico resulted in the quinoline alkaloid 142 (Figure 10), the geometry of which was established as (E) by 1 H-13 C coupling constant measurements from HSQMBC NMR experiments (Nogle and Gerwick, 2003). Except for the mooreaside A (141), the marine cyanobacterium M. producens also yielded two nucleoside derivatives, 3-acetyl-2 ′ -deoxyuridine (143) and 3-phenylethyl-2 ′ -deoxyuridine (144, Figure 10), both of which showed moderate activity toward the MCF-7 cancer cell line with IC 50 values of 18.2 and 22.8 µM, respectively .

CONCLUSIONS
According to an estimation, ∼70% of global drug leads derive directly from natural products, many of which are glycosylated metabolites (Thorson et al., 2001). Chemical investigation for 205 glycosides of the last 22 years  suggests that these compounds are classified as quinones, macrocyclic lactones, esters, lipids, terpenoids, alkaloids, peptides, and other classes. Macrocyclic lactones and quinone glycosides comprise roughly 42% of all these compounds (Figure 15A), and bacteria were the main source of new glycosides at 50% (104/205) (Figure 15B). Given the importance of glycoprotein to many biological processes, although peptide glycosides only account for 1% of these compounds, the peptide glycosides have a considerable potential for the discovery of drug leads.
In this review, the bioactivities of 129 glycosides were summarized in Tables S1-S3. In measured activities for these compounds, more than 50% (Figure 15C) display antitumor and antimicrobial activities, some of which also possess strong cytotoxicity. For example, IB-00208 (16) had a strong antibiotic activity against Gram-positive organisms with MIC values ranging from 0.09 to 1.4 nM, and lomaiviticin A (34) had a unique cytotoxicity profile against cancer cell lines with IC 50 values ranging from 0.01 to 98 ng/ml. At present, two FDA-approved marine drugs, ara-C and ara-A, are antitumor and antiviral nucleosides, which are consistent with the main activity summarized in this review. This suggests that antitumor and antimicrobial drugs may be the main research direction for marine natural products. In addition, some glycosides also exhibited enzyme-inhibitory, antioxidative, DNA-damaging, anti-inflammatory, and anti-plasmodial activities. The recent indepth study of glycosides has revealed their dynamic potential as therapeutic agents in the treatment of different disorders. Based on these findings, it may be possible to discover and develop glycosides with higher selectivities and efficacies.

AUTHOR CONTRIBUTIONS
KL and XZ designed and elaborated the manuscript. JC, ZS, BY, XZ, and YL added valuable comments. XZ, JH, and HT critically revised and improved the manuscript. All authors read and approved the final version of the manuscript.