A Review Study on Macrolides Isolated from Cyanobacteria

Cyanobacteria are rich sources of structurally-diverse molecules with promising pharmacological activities. Marine cyanobacteria have been proven to be true producers of some significant bioactive metabolites from marine invertebrates. Macrolides are a class of bioactive compounds isolated from marine organisms, including marine microorganisms in particular. The structural characteristics of macrolides from cyanobacteria mainly manifest in the diversity of carbon skeletons, complexes of chlorinated thiazole-containing molecules and complex spatial configuration. In the present work, we systematically reviewed the structures and pharmacological activities of macrolides from cyanobacteria. Our data would help establish an effective support system for the discovery and development of cyanobacterium-derived macrolides.


Introduction
Cyanobacteria, also known as blue-green algae, including cyanobacteria from terrestrial, freshwater and marine ecosystems, are a group of ancient photosynthetic prokaryotes. As defensive chemicals, structurally-diverse secondary metabolites from cyanobacteria have been proven to greatly contribute to successful survival and reproduction of cyanobacteria in changing, complex and diverse environments during the long-lasting evolutionary process [1]. At present, hundreds of compounds with important bioactivities have been isolated from terrestrial or marine cyanobacteria [2]. Macrolides are a class of important bioactive compounds, which are commonly found in marine organisms, including cyanobacteria [3]. Some marine macrolides are promising candidates for future applications in medicine. For example, bryostatin-1 shows potent antitumor activity in phase I cancer clinical trials [4]. Macrolide antibiotics, such as erythromycin and polyene macrolides, have been employed for widespread application of severe bacterial infections [5]. Structurally-diverse macrolides from cyanobacteria often contain unique and unusual substituents, including chlorinated residues, thiazole residues [6] or pyran residues [7]. Macrolides usually exhibit potent antitumor or antibacterial activities [8]. In addition, cyanobacteria have great potentials as sustainable sources for production of bioactive macrolides because of their rapid growth, genetic tractability and cultivable property [2]. Although cyanobacteria possess cultivable properties similar to those of microorganisms, cyanobacteria have attracted far less attention than microorganisms.
In the present review paper, we systematically summarized the structures and bioactivities of macrolides isolated from cyanobacteria, and over 50 references were cited. Up to the end of 2016, a total of 64 macrolide compounds have been isolated from cyanobacteria, including 49 macrolides from marine cyanobacteria and 15 macrolides from terrestrial cyanobacteria, most of which are mainly from mainly from Lyngbya, Scytonema and Oscillatoria. It has been reported that most of these cyanobacterium-derived macrolides possess several noticeable bioactivities, including antitumor, antibacterial, antimalarial and toxicity to animals. This review summarizes new macrolides derived from cyanobacteria, providing useful information in the further discovery of novel cyanobacterial macrolides.
Like a cytotoxic biselyngbyaside-related macrolide, biselyngbyolide A (23) was isolated from the marine cyanobacterium Lyngbya sp. harvested from Tokunoshima Island, Japan. Biselyngbyolide A (23) shows strong cytotoxicity against HeLa S3 cells and HL60 cells with IC50 values of 0.22 and 0.027 μM, respectively [18]. Biselyngbyolide B (24) was also isolated from the same strain of Lyngbya sp. and displays significant inhibitory effects on growth of HeLa S3 cells and HL60 cells (IC50 values of 0.028 and 0.0027 μM, respectively, using thapsigargin as a positive control drug). Moreover, biselyngbyolide B (24) has 3-100-fold more potent apoptosis-inducing activity than biselyngbyaside (17) [16,19]. Another distinct class of 18-membered ring glycoside macrolides has been isolated from the cyanobacterial genus Lyngbya (Figure 4). Biselyngbyaside (17) was discovered through a bioassay-guided screening for cytotoxic compounds from cyanobacterium Lyngbya sp. collected from Okinawa Prefecture, Japan. Biselyngbyaside (17) shows a broad spectrum of cytotoxicity against human solid tumor cell lines, especially for HeLa S 3 cells with an IC 50 value of 0.1 µg/mL [15], and its total synthesis was completed [35]. Extensive efforts toward finding cytotoxic natural products have resulted in the isolation of three analogs of biselyngbyaside (17), named biselyngbyasides B-D (18)(19)(20), from the marine cyanobacterium Lyngbya sp. Biselyngbyaside B (18) displays significant cytotoxicity against HeLa S 3 and HL60 cells (IC 50 values of 3.5 and 0.82 µM, respectively, using thapsigargin as a positive control drug). In addition, biselyngbyasides B-D (18)(19)(20) induced apoptosis of cancer cells by inhibiting calcium influx into the endoplasmic reticulum and increasing the concentration of intracellular calcium [16]. Two analogs of biselyngbyaside (17), biselyngbyasides E (21) and F (22), were isolated from the marine cyanobacterium Lyngbya sp. collected from Ishigaki Island, Japan. In vitro cell cytotoxicity assays showed that biselyngbyaside E (21) has higher cytotoxicity against HeLa and HL60 cells (IC 50 values of 0.19 and 0.071 µM, respectively) than biselyngbyaside F (22) (IC 50 values of 3.1 and 0.66 µM, respectively). Based on the trisubstituted olefin geometry, the presence and absence of the sugar moiety are crucial for the biological activities [17].
A rare 40-membered macrolactone, nuiapolide (51), was isolated from Niihau marine cyanobacterium. As a polyhydroxylated macrolide, nuiapolide (51) contains a rare tert-butyl carbinol residue, and it displays anti-chemotactic activity against Jurkat cells and cancerous T lymphocytes and can trigger a predominant G2/M phase shift in the cell cycle [32].
A rare 40-membered macrolactone, nuiapolide (51), was isolated from Niihau marine cyanobacterium. As a polyhydroxylated macrolide, nuiapolide (51) contains a rare tert-butyl carbinol residue, and it displays anti-chemotactic activity against Jurkat cells and cancerous T lymphocytes and can trigger a predominant G2/M phase shift in the cell cycle [32].

Antibacterial Activity
Some macrolides, such as erythromycin and azithromycin, have shown excellent antibacterial activity and are widely used in clinical practice of various types of bacterial infections [44]. Some macrolides from cyanobacteria also show good antibacterial activities. Cyanobacterium-derived macrolides with antimicrobial properties are listed in Table 2.
A bioactive marcolide, 7-OMe-scytophycin B (52), was identified from a culture of a marine cyanobacterium and was found to exhibit antifungal activity against Candida albicans HAMbI 484 and Candida guilliermondii HAMBI 257 with MIC values of 0.40 and 0.80 mM and IC50 values of 0.19 and 0.23 mM, respectively [45]. Two 40-membered macrolactones, amantelides A,B (53,54), are composed of a 1,3-diol and contiguous 1,5-diol units and a tert-butyl substituent. These compounds were isolated from a Guam cyanobacterium belonging to the family Oscillatoriales ( Figure 10). Amantelide A (53) shows a broad spectrum of inhibitory effects on the growth of both eukaryotic and prokaryotic cells. The growth of the fungi Lindra thalassiae and Fusarium sp. is completely

Antibacterial Activity
Some macrolides, such as erythromycin and azithromycin, have shown excellent antibacterial activity and are widely used in clinical practice of various types of bacterial infections [44]. Some macrolides from cyanobacteria also show good antibacterial activities. Cyanobacterium-derived macrolides with antimicrobial properties are listed in Table 2.

Antibacterial Activity
Some macrolides, such as erythromycin and azithromycin, have shown excellent antibacterial activity and are widely used in clinical practice of various types of bacterial infections [44]. Some macrolides from cyanobacteria also show good antibacterial activities. Cyanobacterium-derived macrolides with antimicrobial properties are listed in Table 2.
A bioactive marcolide, 7-OMe-scytophycin B (52), was identified from a culture of a marine cyanobacterium and was found to exhibit antifungal activity against Candida albicans HAMbI 484 and Candida guilliermondii HAMBI 257 with MIC values of 0.40 and 0.80 mM and IC50 values of 0.19 and 0.23 mM, respectively [45]. Two 40-membered macrolactones, amantelides A,B (53,54), are composed of a 1,3-diol and contiguous 1,5-diol units and a tert-butyl substituent. These compounds were isolated from a Guam cyanobacterium belonging to the family Oscillatoriales ( Figure 10). Amantelide A (53) shows a broad spectrum of inhibitory effects on the growth of both eukaryotic and prokaryotic cells. The growth of the fungi Lindra thalassiae and Fusarium sp. is completely inhibited when the concentration of amantelide A (53) is 62.5 μg/mL. When the concentration of amantelide B (54) is 6.25 μg/mL, the growth of the fungus Dendryphiella salina is completely inhibited [46].

Effects of Cyanobacterium-Derived Macrolides on Animals
Toxin-producing cyanobacterial blooms are a potential health risk for other living organisms, including humans [47]. Cyanobacterium-derived macrolides show toxicity to animals, such as brine shrimp and mice. The effects of cyanobacterium-derived macrolides on fauna are described in Table 3. The cytotoxic macrolactone, lyngbyabellin A (1), exhibits potent toxicity to mice in vivo trials (lethal dose of 2.4 to 8.0 mg/kg; sublethal dose of 1.2 to 1.5 mg/kg) [6]. Tolytoxin (38) is highly toxic to mice with a sublethal dose (ip) of 1.5 mg /kg [24]. A 14-membered macrolide, sanctolide A (48), shows high toxicity toward the brine shrimp with an LC 50 value of 23.5 µM [28]. A 10-membered ring macrolide, gloeolactone (55), was isolated from the cyanobacterium Gloeotrichia sp., harvested in Clark Canyon Reservoir ( Figure 11). Gloeolactone (55) exhibits weak toxicity to brine shrimp. All brine shrimps are dead when the concentration of gloeolactone (55) is 125 µg/mL [48]. Phormidolide (56) was isolated from the marine cyanobacterium Phormidium sp. cultured in Indonesia and was found to exhibit very high toxicity (LC 50 value of 1.5 µM) in the brine shrimp test [49]. Cyanolide A (57) can be used as a new, potent molluscicidal agent to effectively control the spread of schistosomiasis [50]. Its total synthesis has been accomplished [51].   A symmetrical macrolide dimer, cyanolide A (57), was obtained from the marine cyanobacterium Lyngbya bouillonii collected from Papua New Guinea. Cyanolide A (57) displays potent molluscicidal activity against the snail vector Biomphalaria glabrata with an LC 50 value of 1.2 µM. Cyanolide A (57) can be used as a new, potent molluscicidal agent to effectively control the spread of schistosomiasis [50]. Its total synthesis has been accomplished [51].

Other Bioactivity
Cyanobacterium-derived macrolides with rich chemical diversity show various important bioactivities ( Table 4). The macrolide biselyngbyaside (17), isolated from the marine cyanobacterium Lyngbya sp., has been investigated for its effects on osteoclast differentiation and function. Biselyngbyaside (17) inhibits RANKL-induced osteoclastogenesis by inhibiting the expression of c-Fos and NFATc1 in mouse monocytic RAW264 cells. Therefore, biselyngbyaside (17) is a potentially promising compound with therapeutic and preventive activities against bone-lytic diseases [52]. A toxic cyanobacterial macrolide, debromoaplysiatoxin (39), has been found to cause severe cutaneous inflammation in humans and other animals after topical application [25]. A rare 40-membered polyhydroxy macrolide, bastimolide A (50), exhibits high selectivity and antimalarial activity against four drug-resistant malaria parasite strains, including TM90-C2A, TM90-C2B, W2 and TM91-C235, with IC 50 values of 80, 90, 140 and 270 nM, respectively. It has been proven that bastimolide A (50) is a potentially promising antimalarial lead compound with high selectivity and antimalarial activity against drug-resistant strains [31]. Malyngolide dimer (58) was isolated from the marine cyanobacterium Lyngbya majuscula collected from Panama and was shown to exhibit moderate antimalarial activity against chloroquine-resistant Plasmodium falciparum (W2) with an IC 50 value of 19 µM [53].
A novel SIRT2-selective inhibitor, tanikolide dimer (59), was isolated from marine cyanobacterium Lyngbya majuscula collected from Madagascar, and it possesses a symmetrical dimer, which has been elucidated by comparison of the natural and synthetic stereoisomers using chiral GC-MS ( Figure 12). The tanikolide dimer (59) is a potent and selective SIRT2 inhibitor with an IC 50 value of 176 nM [54]. structure ensure the functionality of cocosolide (61). In addition, the total synthesis of cocosolide (61) has been accomplished [7].
Three novel nitrogen-containing macrolides, laingolide (62) [57], laingolide A (63) and madangolide (64) [58], have been identified from the marine cyanobacterium Lyngbya bouillonii harvested in Laing Island, Papua-New Guinea (Figure 12). The structures of these macrolides (62-64) contain a lactone ring of 15, 15 and 17 members, respectively [58].  An unusually stabilized neuroactive macrolide, palmyrolide A(60), was isolated, via an assay-based screening program for new neuroactive compounds from cyanobacteria Leptolyngbya cf. and Oscillatoria spp. harvested in Palmyra Atoll. Palmyrolide A (60) contains a rare N-methyl enamide and an intriguing tert-butyl group, and it can potently inhibit Ca 2+ oscillations in murine cerebrocortical neuronal cells with an IC 50 value of 3.70 µM. Moreover, palmyrolide A (60) can significantly block the sodium channel activity of neuro-2a cells (IC 50 value of 5.2 µM) without appreciable cytotoxicity. The above intriguing experimental results suggest that palmyrolide A (60) could be a promising drug candidate for further pharmacological exploration [55], and its total synthesis has been completed [56].
A dimeric macrolide, cocosolide (61), was isolated from the marine cyanobacterium Symploca sp. from Guam, and it strongly inhibits IL-2 production in both T-cell receptor-dependent and independent manners. Both the presence of the sugar moiety and the integrity of the dimeric structure ensure the functionality of cocosolide (61). In addition, the total synthesis of cocosolide (61) has been accomplished [7].

Conclusions
Cyanobacteria are rich sources of various natural products with unprecedented pharmacological and biological activities. Up to the end of 2016, a total of 64 macrolide compounds have been isolated from cyanobacteria, including 49 macrolides from marine cyanobacteria and 15 macrolides from terrestrial cyanobacteria. More than half of the cyanobacterium-derived macrolides, a total of 36 compounds, were isolated from the cyanobacterial genus Lyngbya species, particularly from Lyngbya majuscula. Most of these cyanobacterium-derived macrolides possess several noticeable bioactivities, including antitumor, antibacterial and antimalarial. The overwhelming majority of cyanobacteria derived macrolides (1-51) display in vitro antitumor activity. Secondary metabolites of cyanobacteria are widely evaluated for their antitumor effects since many metabolites of cyanobacteria have exhibited potent antitumor activities. Some of these macrolides, including tolytoxin (38), bastimolide A (50) and tanikolide dimer (59), exhibited surprisingly strong bioactivity, thus representing potential new drug lead compounds, which are worthy of further research on synthesis and pharmacological activity. The total synthesis of 10 bioactive macrolides, such as cocosolide, has been achieved with a great deal of efforts. The research on the total synthesis of macrolides will promote pharmacologic research and create new opportunities to undertake research in drug discovery, medicine design and large-scale manufacturing. At present, three scholars, including Luesch, Moore and Gerwick, have greatly contributed to the discovery of new macrolides from cyanobacteria. Cyanobacteria have great potentials as sustainable sources for the production of bioactive metabolites because of their rapid growth, genetic tractability and cultivable property. Although cyanobacteria possess the cultivable properties similar to those of microorganisms, cyanobacteria have attracted far less attention than microorganisms. More efforts should be devoted to improving the production of bioactive metabolites in cyanobacteria via cultivation design, metabolic engineering together with efficient isolation. In addition, the programs for drug discovery from cyanobacteria, including the Panama International Cooperative Biodiversity Group (ICGB) program, might facilitate and enhance drug discovery from cyanobacteria. A systematic review on macrolides from cyanobacteria would help establish an effective support system for the discovery and development of cyanobacterium-derived macrolides, and such a support system could also facilitate collection, purification and identification of bioactive macrolides, leading to improve bioactivity assay, synthesis, data analysis and information technology.