Published papers

Polycavernoside E (1), a new polycavernoside analog, was isolated from a marine Okeania sp. cyanobacterium. The relative configuration was elucidated primarily by analyzing the two dimensional nuclear magnetism resonance (2D NMR) data. The absolute configuration was clarified by comparing the electronic circular dichroism (ECD) data of 1 with those of known analogs. Polycavernoside E (1) exhibited moderate antitrypanosomal activity against Trypanosoma brucei rhodesiense. Furthermore, the isolation of polycavernoside E (1) from marine cyanobacteria provides additional evidence that marine cyanobacteria, and not red algae, are responsible for the biosynthesis of polycavernosides.

Two linear lipopeptides mabuniamide B (1) and C (2) were isolated from an Okeania sp. marine cyanobacterium collected from the coast of Okinawa. Their structures were elucidated by spectroscopic analyses, and their absolute configurations of amino acid residues were determined using Marfey's analysis of the hydrolysates of 1 and 2. In addition, 1 and 2 inhibited uric acid production in cultured AML12 cells.

Human African trypanosomiasis (HAT) is one of the most lethal of the neglected tropical diseases. While the discovery of a novel antitrypanosomal drug is highly desired, the creation of a superior lead compound is challenging. Herein we report ukabamide (1), which was isolated from a marine Moorena sp. cyanobacterium, as a selective and potent antitrypanosomal lipopeptide (IC50 = 34 ± 18 nM against Trypanosoma brucei rhodesiense). Its structure was elucidated by spectroscopic analyses and degradation reactions and confirmed by total synthesis. In addition, we prepared two modified analogs and revealed the importance of the fatty acid chain length for biological activity. These findings may provide design guidelines for an antitrypanosomal drug.

Ouhanamide (1), a new linear lipopeptide consisting of a long-chain fatty acid with a β-branched methyl group, was isolated from a marine Okeania sp. cyanobacterium. The overall structure was elucidated by a combination of various spectroscopic analyses, degradation reactions, and derivatizations. Ouhanamide (1) showed selective antiparasitic activity (IC50 = 1.2 ± 0.1 μM against Trypanosoma brucei rhodesiense and IC50 = 4.2 ± 0.9 μM against Plasmodium falciparum) without cytotoxicity at 10 μM against HeLa cells.

Sealgamide (1), a new antitrypanosomal lipopeptide, was isolated from a marine Okeania sp. cyanobacterium. Elucidation of its structure was challenging due to signal overlap caused by conspicuous rotamers but was ultimately achieved through partial hydrolysis and subsequent spectroscopic analyses. Sealgamide (1) exhibited moderate antitrypanosomal activity against Trypanosoma brucei rhodesiense (IC50 2.9 μM) while showing no antimalarial activity (IC50 > 25 μM) or cytotoxicity (IC50 > 30 μM). Furthermore, we discovered that an artificial C-terminal methyl ester analogue (2) exhibited notably enhanced antiparasitic activity.

Shimojilide (1), a new cyclic lipopeptide, was isolated from a marine cyanobacterium Okeania sp. Its structure was elucidated by NMR and chemical derivatization, revealing a peptide-polyketide hybrid with a skipped diene and β-branched methyl group. Shimojilide (1) exhibited potent and selective antiproliferative activity with a unique JFCR39 fingerprint that poorly correlates with those of known anticancer agents, suggesting a distinct mechanism of action. Shimojilide (1) represents a promising lead for anticancer drug discovery.

Abstract Iezoside (1a) is a novel, potent sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) inhibitor from marine cyanobacterium. This paper describes the synthesis of comprehensive iezoside (1a) analogs containing C18/19 diastereomers, simplified analogs without the peptide unit, and aglycones. Evaluations of the antiproliferative activities against cancer cells and SERCA inhibitory activities of the synthesized analogs revealed how the absolute configurations at C18/19, peptide, and the sugar unit contribute to each bioactivity.Ca2+ ATPase (SERCA) inhibitor, Structure-activity relationship, Marine natural products

In this era of advanced technologies, is there still wisdom to be gleaned from nature? The answer is an unequivocal “Yes”. Discovering new natural products from nature, determining their structures, and revealing their biological activities ‐ this classical natural product chemistry is still a catalyst for innovation in a wide range of fields, including the expansion of chemical space, elucidation of life phenomena, and drug discovery. In this account, we describe a comprehensive bioorganic study of iezoside (1a), the novel marine natural product we recently discovered, including its discovery, synthesis, mode of action, and structure‐activity relationships.

Here, we report wajeepeptin (1), a new cyclic depsipeptide isolated from a marine Moorena sp. cyanobacterium. The structure was elucidated by a combination of spectroscopic analyses, X-ray diffraction analysis, and degradation reactions. Wajeepeptin (1) showed moderate cytotoxicity (IC50 = 3.7 μM against HeLa cells) and potent antitrypanosomal activity (IC50 = 0.73 ± 0.14 μM against Trypanosoma brucei rhodesiense).

Overweight and obesity are pathological conditions characterized by the accumulation of excessive fat. Adipokines, which are secreted by hypertrophied adipocytes, are known to increase the risk of various diseases. Therefore, identifying compounds that promote lipolysis in adipocytes is a promising way to prevent hypertrophy of these cells. Although numerous bioactive compounds have been isolated from marine cyanobacteria, reports on compounds exhibiting such activity remain limited. In this study, we report the discovery of zampamide (1), a linear lipopeptide that stimulates lipolysis in 3T3-L1 adipocytes. It was isolated from a marine cyanobacterium of the genus Caldora collected in Okinawa, Japan. The planar structure of 1 was elucidated primarily using two-dimensional NMR and mass spectrometry. The absolute configurations of the amino acid residues in 1 were determined after acid hydrolysis, followed by Marfey's analysis of the resulting hydrolysates. Additionally, the absolute configuration of the fatty acid moiety was determined by derivatization. Zampamide (1) increased the amount of free glycerol in a dose-dependent manner, demonstrating its lipolysis-promoting activity. These findings suggest that marine cyanobacteria are a valuable source of novel lipolysis-promoting agents for obese and overweight patients.

Although cancer cells often harbor supernumerary centrosomes, they form pseudo-bipolar spindles via centrosome clustering, instead of lethal multipolar spindles, and thus avoid cell death. Kinesin-14 HSET/KIFC1 is a crucial protein involved in centrosome clustering. Accordingly, a compound that targets HSET could potentially inhibit cancer cell proliferation in a targeted manner. Here, we report three natural compounds derived from Solidago altissima that restored the growth of fission yeast cells exhibiting lethal HSET overproduction (positive screening), namely solidagonic acid (SA) (1), kolavenic acid analog (KAA: a stereo isomer at C-9 and C-10 of 6β-tigloyloxykolavenic acid) (2), and kolavenic acid (KA) (3). All three compounds suppressed fission yeast cell death and enabled reversion of the mitotic spindles from a monopolar to bipolar morphology. Compound 2, which exerted the strongest activity against HSET-overproducing yeast cells, also inhibited centrosome clustering in MDA-MB-231 human breast adenocarcinoma cells, which contained large numbers of supernumerary centrosomes. These natural compounds may be useful as bioprobes in studies of HSET function. Moreover, compound 2 is a prime contender in the development of novel agents for cancer treatment.

Fractionation of ethyl acetate extract obtained by culturing the endophytic fungus Pseudocosmospora sp. Bm-1-1 resulted in the isolation of four new meroterpenoids, cosmosporin A (1), 6-carboxy-cosmosporin A (2), rel(6aS,10aR)-Delta(9)-tetrahydrocannabiorcolic acid B (3) and 8'-hydroxy-cannabiorcichromenic acid (4), in addition to four known compounds 5-8. Structures were elucidated by spectral analysis, as well as by directly comparing the spectral data of new compounds with those of known compounds. Cannabiorcichromenic acid (5), decarboxy-cannabiorcichromenic acid (6), and re/-(6aS,10aR)-decarboxy-Delta(9)-tetrahydrocannabiorcolic acid B (8) restored growth of a Saccharomyces cerevisiae mutant strain involving Ca2+ signal transduction. Furthermore, compounds 3 and 8 had cytotoxic activity against HL60 cells (3: IC50 = 24.1 mu M and 8: IC50, 1.6 mu M).

Four new compounds, namely, nectriapyrones A (2) and B (3), nectriaquinone B (5), and zythiostromic acid C (8), were isolated from the brown rice culture of Nectria pseudotrichia 120-1NP together with four known compounds (1, 4, 6, and 7). To the best of our knowledge, this is the first report of 4 from a natural source. Their structures were determined on the basis of 1D/2D-NMR spectroscopy and HRESITOFMS data. In addition, the absolute configuration of secondary alcohols in 8 were determined using modified Mosher's ester method. All isolated compounds were evaluated for their antimicrobials activity, phytotoxicity, and cytotoxicity.

The total synthesis of acetylene-containing lipopeptides odookeanynes A (1) and B (2), isolated from the marine cyanobacterium Okeania sp., has been achieved. A biological evaluation using synthetic odookeanynes revealed their potent antitrypanosomal activity. This report not only describes a synthetic pathway for structurally related compounds for SAR studies but also offers new insights into their potential as antiparasitic agents.

Many human cancer cells contain more than two centrosomes, yet these cancer cells can form pseudo-bipolar spindles through the mechanism, called centrosome clustering, and survive, instead of committing lethal multipolar mitoses. Kinesin-14/HSET, a minus end-directed motor, plays a crucial role in centrosome clustering. Accordingly, HSET is deemed to be a promising chemotherapeutic target to selectively kill cancer cells. Recently, three HSET inhibitors (AZ82, CW069 and SR31527) have been reported, but their specificity and efficacy have not been evaluated rigorously. This downside partly stems from the lack of robust systems for the assessment of these drugs. Yeasts and filamentous fungi provide not only powerful models for basic and applied biology but also versatile tools for drug discovery and evaluation. Here we show that these three inhibitors on their own are cytotoxic to fission yeast, suggesting that they have off-targets in vivo except for kinesin-14. Nonetheless, intriguingly, AZ82 can neutralize otherwise toxic overproduced HSET; this includes a substantial reduction in the percentage of HSET-driven abnormal mitotic cells and partial suppression of its lethality. SR31527 also displays modest neutralizing activity, while we do not detect such activity in CW069. As an experimental proof-of-principle study, we have treated HSET-overproducing fission yeast cells with extracts prepared from various plant species and found activities that rescue HSET-driven lethality in those from Chamaecyparis pisifera and Toxicodendron trichocarpum. This methodology of protein overproduction in fission yeast, therefore, provides a convenient, functional assay system by which to screen for not only selective human kinesin-14 inhibitors but also those against other molecules of interest.

Amebiasis is an important cause of morbidity and mortality worldwide, and caused by infection with the protozoan parasite Entamoeba histolytica. Metronidazole is currently the first-line drug despite adverse effects and concerns on the emergence of drug resistance. Fumagillin, a fungal metabolite from Aspergillus fumigatus, and its structurally related natural and synthetic compounds have been previously explored as potential anti-angiogenesis inhibitors for cancers, anti-microbial, and anti-obese compounds. Although fumagillin was used for human amebiasis in clinical trials in 1950s, the mode of action of fumagillin remains elusive until now. In this report, we showed that fumagillin covalently binds to methionine aminopeptidase 2 (MetAP2) and non-covalently but abundantly binds to patatin family phospholipase A (PLA). Susceptibility against fumagillin of the amebic strains in which expression of E. histolytica MetAP2 (EhMetAP2) gene was silenced increased compared to control strain. Conversely, overexpression of EhMetAP2 mutants that harbors amino acid substitutions responsible for resistance to ovalicin, a fumagillin analog, in human MetAP2, also resulted in decrease in fumagillin susceptibility. In contrast, neither gene silencing nor overexpression of E. histolytica PLA (EhPLA) affected fumagillin susceptibility. These data suggest that EhPLA is not essential and not the target of fumagillin for its amebicidal activity. Taken together, our data have demonstrated that EhMetAP2 is the primary target for amebicidal activity of fumagillin, and EhMetAP2 represents a rational explorable target for the development of alternative therapeutic agents against amebiasis.

Iheyamides A (1), B (2), and C (3), new linear peptides, were isolated from a marine Dapis sp. cyanobacterium. Their structures were elucidated by spectroscopic analyses and degradation reactions. Iheyamide A (1) showed moderate antitrypanosomal activities against Trypanosoma brucei rhodesiense and Trypanosoma brucei brucei (IC50 = 1.5 μM), but the other two analogues, iheyamides B (2) and C (3), did not (IC50 > 20 μM, respectively). The structure-activity relationship clarified that an isopropyl-O-Me-pyrrolinone moiety was necessary for the antitrypanosomal activity. Furthermore, the cytotoxicity of 1 against normal human cells, WI-38, was 10 times weaker than its antitrypanosomal activity (IC50 = 18 μM).

Plant parts and extracts that are rich in bioactive substances with allelopathic potential can be explored as a possible alternative to herbicides for natural weed control in sustainable agriculture. In the present study, we investigated the allelopathic potential of Marsdenia tenacissima leaves and its active substances. Aqueous methanol extracts of M. tenacissima showed significant inhibitory activities against the growth of lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), and barnyard grass (Echinochloa crusgalli (L.) Beauv.). The extracts were purified through various chromatography steps, and one active substance was isolated and determined by spectral data to be a novel compound, assigned as steroidal glycoside 3 (8-dehydroxy-11β-O-acetyl-12β-O-tigloyl-17β-marsdenin). Steroidal glycoside 3 significantly inhibited the seedling growth of cress at a concentration of 0.03 mM. The concentrations needed for 50% growth inhibition of the cress shoots and roots were 0.25 and 0.03 mM, respectively. These results suggest that steroidal glycoside 3 may be responsible for the allelopathy of M. tenacissima leaves.

Iheyamide A ( 1 ) is an antitrypanosomal linear peptide isolated from aDapissp. marine cyanobacterium by our group in 2020, and based on structure-activity relationships of its natural analogues, the C-terminal pyrrolinone moiety has been identified as the phamacophore for its antiparasitic activity. Further, we isolated this pyrrolinone moiety by itself as a new natural product from the marine cyanobacterium and named it iheyanone ( 2 ). As expected, iheyanone ( 2 ) showed antitrypanosomal activity, but its potency was weaker than iheyamide A ( 1 ). To clarify more detailed structure-activity relationships, we completed a total synthesis of iheyamide A ( 1 ) along with iheyanone ( 2 ) and evaluated the antitrypanosomal activities of several synthetic intermediates. As a result, we found that the longer the peptide chain, the stronger the antitrypanosomal activity. As iheyamide A ( 1 ) showed selective toxicity againstTrypanosoma brucei rhodesiense, these findings can provide design guidelines for antitrypanosomal drugs.