Total synthesis, stereochemical assignment, and biological evaluation of opantimycin A and analogues thereof.

Opantimycin A, a rare antimycin-class antibiotic without the macrolide core, was isolated from Streptomyces sp. RK88-1355 in 2017. In this study, we explored the total synthesis and stereochemical assignment of opantimycin A. The synthesis of all potential diastereomers has been accomplished via traceless Staudinger ligation. A comparison of the spectroscopic data of the synthesized compounds with that reported for the natural product confirmed that the absolute configuration of the natural product was (14S,17R,21R). Two analogous compounds were prepared, where the Dhb ((Z)-dehydrobutyrine) moiety was replaced with Dha (dehydroalanine) or ΔVal moieties, respectively. The inhibitory activities of these synthetic compounds against the production of the anti-inflammatory cytokine IL-6 were evaluated, and two potential candidates for further development as anti-inflammatory agents were identified.

Catalytic Enantioselective Synthesis of a Pyrrolizidine-Alkaloid-Inspired Compound Collection with Antiplasmodial Activity.

A novel enantioselective approach to the synthesis of a compound collection inspired by natural pyrrolizidine alkaloids was developed, employing an enantioselectively catalyzed 1,3-dipolar cycloaddition as the key step. The cycloadducts were obtained with excellent enantio- and diastereoselectivity. Biological evaluation of the resulting compound collection revealed that the compound class has multiple bioactivities, including activity against Plasmodium falciparum 3D7 and inhibition of Hedgehog signaling.

Mitochondrial uncoupler exerts a synthetic lethal effect against ß-catenin mutant tumor cells.

The wingless/int-1 (Wnt) signal transduction pathway plays a central role in cell proliferation, survival, differentiation and apoptosis. When ß-catenin: a component of the Wnt pathway, is mutated into an active form, cell growth signaling is hyperactive and drives oncogenesis. As ß-catenin is mutated in a wide variety of tumors, including up to 10% of all sporadic colon carcinomas and 20% of hepatocellular carcinomas, it has been considered a promising target for therapeutic interventions. Therefore, we screened an in-house natural product library for compounds that exhibited synthetic lethality towards ß-catenin mutations and isolated nonactin, an antibiotic mitochondrial uncoupler, as a hit compound. Nonactin, as well as other mitochondrial uncouplers, induced apoptosis selectively in ß-catenin mutated tumor cells. Significant tumor regression was observed in the ß-catenin mutant HCT 116 xenograft model, but not in the ß-catenin wild type A375 xenograft model, in response to daily administration of nonactin in vivo. Furthermore, we found that expression of an active mutant form of ß-catenin induced a decrease in the glycolysis rate. Taken together, our results demonstrate that tumor cells with mutated ß-catenin depend on mitochondrial oxidative phosphorylation for survival. Therefore, they undergo apoptosis in response to mitochondrial dysfunction following the addition of mitochondrial uncouplers, such as nonactin. These results suggest that targeting mitochondria is a potential chemotherapeutic strategy for tumor cells that harbor ß-catenin mutations.

Identification of a molecular target of a novel fungal metabolite, pyrrolizilactone, by phenotypic profiling systems.

In the course of screening our microbial metabolite fraction library, we identified a novel pyrrolizidinone compound, pyrrolizilactone. In this study, we report the identification and characterization of a molecular target for pyrrolizilactone by using two phenotypic profiling systems. Cell morphology-based profiling analysis using an imaging cytometer (MorphoBase) classified pyrrolizilactone as a proteasome inhibitor. Consistently, proteome-based profiling analysis using 2D difference gel electrophoresis (DIGE; ChemProteoBase) also demonstrated that pyrrolizilactone is associated with proteasome inhibition. On the basis of these predictions, we determined that pyrrolizilactone is a novel type of proteasome inhibitor inhibiting the trypsin-like activity of the proteasome.

Identification of a dihydroorotate dehydrogenase inhibitor that inhibits cancer cell growth by proteomic profiling.

Dihydroorotate dehydrogenase (DHODH) is a central enzyme of the de novo pyrimidine biosynthesis pathway and is a promising drug target for the treatment of cancer and autoimmune diseases. This study presents the identification of a potent DHODH inhibitor by proteomic profiling. Cell-based screening revealed that NPD723, which is reduced to H-006 in cells, strongly induces myeloid differentiation and inhibits cell growth in HL-60 cells. H-006 also suppressed the growth of various cancer cells. Proteomic profiling of NPD723-treated cells in ChemProteoBase showed that NPD723 was clustered with DHODH inhibitors. H-006 potently inhibited human DHODH activity in vitro, whereas NPD723 was approximately 400 times less active than H-006. H-006-induced cell death was rescued by the addition of the DHODH product orotic acid. Moreover, metabolome analysis revealed that H-006 treatment promotes marked accumulation of the DHODH substrate dihydroorotic acid. These results suggest that NPD723 is reduced in cells to its active metabolite H-006, which then targets DHODH and suppresses cancer cell growth. Thus, H-006-related drugs represent a potentially powerful treatment for cancer and other diseases.

cis-Decalin-containing tetramic acids as inhibitors of insect steroidogenic glutathione S-transferase Noppera-bo.

Decalin-containing tetramic acid is a bioactive scaffold primarily produced by filamentous fungi. The structural diversity of this group of compounds is generated by characteristic enzymes of fungal biosynthetic pathways, including polyketide synthase/nonribosomal peptide synthetase hybrid enzymes and decalin synthase, which are responsible for the construction of a linear polyenoyl tetramic acid structure and stereoselective decalin formation via the intramolecular Diels-Alder reaction, respectively. Compounds that differed only in the decalin configuration were collected from genetically engineered mutants derived from decalin-containing tetramic acid-producing fungi and used for a structure-activity relationship study. Our evaluation of biological activities, such as cytotoxicity against several cancer cell lines and antibacterial, antifungal, antimalarial, and mitochondrial inhibitory activities, demonstrated that the activity for each assay varies depending on the decalin configurations. In addition to these known biological activities, we revealed that the compounds showed inhibitory activity against the insect steroidogenic glutathione S-transferase Noppera-bo. Engineering the decalin configurations would be useful not only to find derivatives with better biological activities but also to discover overlooked biological activities.

Identification of a Small Molecule That Enhances Ferroptosis via Inhibition of Ferroptosis Suppressor Protein 1 (FSP1).

Glutathione peroxidase 4 (GPX4) is an intracellular enzyme that oxidizes glutathione while reducing lipid peroxides and is a promising target for cancer therapy. To date, several GPX4 inhibitors have been reported to exhibit cytotoxicity against cancer cells. However, some cancer cells are less sensitive to the known GPX4 inhibitors. This study aimed to explore compounds showing synergistic effects with GPX4 inhibitors. We screened a chemical library and identified a compound named NPD4928, whose cytotoxicity was enhanced in the presence of a GPX4 inhibitor. Furthermore, we identified ferroptosis suppressor protein 1 as its target protein. The results indicate that NPD4928 enhanced the sensitivity of various cancer cells to GPX4 inhibitors, suggesting that the combination might have therapeutic potential via the induction of ferroptosis.

Controlled lipid ß-oxidation and carnitine biosynthesis by a vitamin D metabolite.

Lactone-vitamin D3 is a major metabolite of vitamin D3, a lipophilic vitamin biosynthesized in numerous life forms by sunlight exposure. Although lactone-vitamin D3 was discovered 40 years ago, its biological role remains largely unknown. Chemical biological analysis of its photoaffinity probe identified the hydroxyacyl-CoA dehydrogenase trifunctional multienzyme complex subunit alpha (HADHA), a mitochondrial enzyme that catalyzes ß-oxidation of long-chain fatty acids, as its selective binding protein. Intriguingly, the interaction of lactone-vitamin D3 with HADHA does not affect the HADHA enzymatic activity but instead limits biosynthesis of carnitine, an endogenous metabolite required for the transport of fatty acids into the mitochondria for ß-oxidation. Lactone-vitamin D3 dissociates the protein-protein interaction of HADHA with trimethyllysine dioxygenase (TMLD), thereby impairing the TMLD enzyme activity essential in carnitine biosynthesis. These findings suggest a heretofore undescribed role of lactone-vitamin D3 in lipid ß-oxidation and carnitine biosynthesis, and possibly in sunlight-dependent shifts of lipid metabolism in animals.

N-Acetyl-α-hydroxy-ß-oxotryptamine, a racemic natural product isolated from Streptomyces sp. 80H647.

N-acetyl-α-hydroxy-ß-oxotryptamine (1) along with N-acetyl-ß-oxotryptamine (2) and pimprinine (3) were isolated from the culture broth of Streptomyces sp. 80H647. Compound 1 was found to be a racemate via X-ray diffraction analysis and the enantiomers were successfully purified by chiral-phase HPLC. The absolute configuration was assigned by comparison of the calculated and experimental ECD spectra. The α-hydroxy moiety of 1 was vital for cytotoxicity against different cancer cell lines.

Glyoxalase I disruption and external carbonyl stress impair mitochondrial function in human induced pluripotent stem cells and derived neurons.

Carbonyl stress, a specific form of oxidative stress, is reported to be involved in the pathophysiology of schizophrenia; however, little is known regarding the underlying mechanism. Here, we found that disruption of GLO1, the gene encoding a major catabolic enzyme scavenging the carbonyl group, increases vulnerability to external carbonyl stress, leading to abnormal phenotypes in human induced pluripotent stem cells (hiPSCs). The viability of GLO1 knockout (KO)-hiPSCs decreased and activity of caspase-3 was increased upon addition of methylglyoxal (MGO), a reactive carbonyl compound. In the GLO1 KO-hiPSC-derived neurons, MGO administration impaired neurite extension and cell migration. Further, accumulation of methylglyoxal-derived hydroimidazolone (MG-H1; a derivative of MGO)-modified proteins was detected in isolated mitochondria. Mitochondrial dysfunction, including diminished membrane potential and dampened respiratory function, was observed in the GLO1 KO-hiPSCs and derived neurons after addition of MGO and hence might be the mechanism underlying the effects of carbonyl stress. The susceptibility to MGO was partially rescued by the administration of pyridoxamine, a carbonyl scavenger. Our observations can be used for designing an intervention strategy for diseases, particularly those induced by enhanced carbonyl stress or oxidative stress.

Identification of a Small-Molecule Glucose Transporter Inhibitor, Glutipyran, That Inhibits Cancer Cell Growth.

Cancer cells reprogram their metabolism to survive and grow. Small-molecule inhibitors targeting cancer are useful for studying its metabolic pathways and functions and for developing anticancer drugs. Here, we discovered that glutipyran and its derivatives inhibit glycolytic activity and cell growth in human pancreatic cancer cells. According to proteomic profiling of glutipyran-treated cells using our ChemProteoBase, glutipyran was clustered within the group of endoplasmic reticulum (ER) stress inducers that included glycolysis inhibitors. Glutipyran inhibited glucose uptake and suppressed the growth of various cancer cells, including A431 cells that express glucose transporter class I (GLUT1) and DLD-1 GLUT1 knockout cells. When cotreated with the mitochondrial respiration inhibitor metformin, glutipyran exhibited a synergistic antiproliferative effect. Metabolome analysis revealed that glutipyran markedly decreased most metabolites of the glycolytic pathway and the pentose phosphate pathway. Glutipyran significantly suppressed tumor growth in a xenograft mouse model of pancreatic cancer. These results suggest that glutipyran acts as a broad-spectrum GLUT inhibitor and reduces cancer cell growth.

Identification of Dihydroorotate Dehydrogenase Inhibitors─Indoluidins─That Inhibit Cancer Cell Growth.

Dihydroorotate dehydrogenase (DHODH) catalyzes the rate-limiting step in de novo pyrimidine biosynthesis and is a promising cancer treatment target. This study reports the identification of indoluidin D and its derivatives as inhibitors of DHODH. Cell-based phenotypic screening revealed that indoluidin D promoted myeloid differentiation and inhibited the proliferation of acute promyelocytic leukemia HL-60 cells. Indoluidin D also suppressed cell growth in various other types of cancer cells. Cancer cell sensitivity profiling with JFCR39 and proteomic profiling with ChemProteoBase revealed that indoluidin D is a DHODH inhibitor. Indoluidin D inhibited human DHODH activity in vitro; the DHODH reaction product orotic acid rescued indoluidin D-induced cell differentiation. We synthesized several indoluidin D diastereomer derivatives and demonstrated that stereochemistry was vital to their molecular activity. The indoluidin D derivative indoluidin E showed similar activity to its parent compound and suppressed tumor growth in a murine lung cancer xenograft model. Hence, indoluidin D and its derivatives selectively inhibit DHODH and suppress cancer cell growth.

Inhibition of mitochondrial complex I by the novel compound FSL0260 enhances high salinity-stress tolerance in Arabidopsis thaliana.

Chemical priming is an attractive and promising approach to improve abiotic stress tolerance in a broad variety of plant species. We screened the RIKEN Natural Products Depository (NPDepo) chemical library and identified a novel compound, FSL0260, enhancing salinity-stress tolerance in Arabidopsis thaliana and rice. Through transcriptome analysis using A. thaliana seedlings, treatment of FSL0260 elevated an alternative respiration pathway in mitochondria that modulates accumulation of reactive oxygen species (ROS). From comparison analysis, we realized that the alternative respiration pathway was induced by treatment of known mitochondrial inhibitors. We confirmed that known inhibitors of mitochondrial complex I, such as rotenone and piericidin A, also enhanced salt-stress tolerance in Arabidopsis. We demonstrated that FSL0260 binds to complex I of the mitochondrial electron transport chain and inhibits its activity, suggesting that inhibition of mitochondrial complex I activates an alternative respiration pathway resulting in reduction of ROS accumulation and enhancement of tolerance to salinity in plants. Furthermore, FSL0260 preferentially inhibited plant mitochondrial complex I rather than a mammalian complex, implying that FSL0260 has a potential to be an agent for improving salt-stress tolerance in agriculture that is low toxicity to humans.

Deamino-hydroxy-phoslactomycin B, a biosynthetic precursor of phoslactomycin, induces myeloid differentiation in HL-60 cells.

During the screening for novel differentiation inducers, we found that a culture broth of Streptomyces sp. HK-803 induced myeloid differentiation of HL-60 cells. The active substance was identified as deamino-hydroxy-phoslactomycin B (HPLM) by mass spectrometry, and synthesized HPLM also induced the differentiation of HL-60 cells. HPLM showed greater inhibition of protein phosphatase 2A (PP2A) activity than phoslactomycin B (PLMB); however, PLMB and okadaic acid did not induce differentiation. Moreover, treatment with ATRA and 1alpha, 25(OH)2D3 induced retinoic acid receptor-beta and 1alpha, 25(OH)2D3 24-hydroxylase, respectively, whereas HPLM did not, suggesting that HPLM is a novel differentiation inducer.

YO-001A, a new antifungal agent produced by Streptomyces sp. YO15-A001.

A new antifungal compound YO-001A was found from the culture broth of Streptomyces sp. YO15-A001, which was isolated from a soil sample collected in Toyama Prefecture. YO-001A was identified through morphological changes-based screening of the rice blast fungus, Pyricularia oryzae (P. oryzae). YO-001A is a new 26-membered macrolide of the oligomycin family, which exhibits potent antifungal activity against P. oryzae with an IC50 of 0.012 µM by disrupting mitochondrial respiration via inhibition of the FOF1-ATPase activity.

Bioenergetic and proteomic profiling to screen small molecule inhibitors that target cancer metabolisms.

Cancer cells can reprogram their metabolic machinery to survive. This altered metabolism, which is distinct from the metabolism of normal cells, is thought to be a possible target for the development of new cancer therapies. In this study, we constructed a screening system that focuses on bioenergetic profiles (specifically oxygen consumption rate and extracellular acidification rate) and characteristic proteomic changes. Thus, small molecules that target cancer-specific metabolism were investigated. We screened the chemical library of RIKEN Natural Products Depository (NPDepo) and found that unantimycin A, which was recently isolated from the fraction library of microbial metabolites, and NPL40330, which is derived from a chemical library, inhibit mitochondrial respiration. Furthermore, we developed an in vitro reconstitution assay method for mitochondrial electron transport chain using semi-intact cells with specific substrates for each complex of the mitochondrial electron transport chain. Our findings revealed that NPL40330 and unantimycin A target mitochondrial complexes I and III, respectively.

A novel inhibitor of tumorspheres reveals the activation of the serine biosynthetic pathway upon mitochondrial inhibition.

Differences in the metabolism of cancer cells or cancer stem cells (CSCs) as compared to normal cells have provided avenues to safely target cancers. To discover metabolic inhibitors of CSCs, we performed alkaline phosphatase- and tumoursphere-based drug screening using induced cancer stem cell-like cells. From the screening of a RIKEN NPDepo chemical library, we discovered NPD2381 as a novel and selective cancer-stemness inhibitor that targets mitochondrial metabolism. Using our ChemProteoBase profiling, we found that NPD2381 increases the expression of enzymes within the serine biosynthesis pathway. We also found a role for serine in protecting cancer cells from mitochondrial inhibitors. Our results suggest the existence of a compensatory mechanism to increase the level of intracellular serine in response to mitochondrial inhibitors.

Structure and biological activity of Metarhizin C, a stereoisomer of BR-050 from Tolypocladium album RK17-F0007.

Metarhizin C, a stereoisomer of BR-050 was isolated from a fungus Tolypocladium album RK17-F0007 through a screening program to search for new antitumor compounds. A structure of the isomer was determined by spectroscopic methods including detailed analysis of NOESY correlation and mass spectrometry, and found to be identical to that of 3-desacylmetarhizin A with the absolute structure. Previously, it had been isolated by Kikuchi et al and proposed as BR-050 including the stereo-structure. However, detailed analysis for the newly isolated isomer confirmed that 3-desacylmetarhizin A was not identical to BR-050. Therefore, we assigned it metarhizin C as a new BR-050 isomer. Metarhizin C showed selective cytotoxicity against osteosarcoma MG-63 cells in a glucose independent condition with IC50 value of 0.79 µg/ml, while > 30 µg/ml of IC50 value in a normal condition, and inhibited a mitochondrial respiration.

Discovery of the novel autophagy inhibitor aumitin that targets mitochondrial complex I.

Macroautophagy is a conserved eukaryotic process for degradation of cellular components in response to lack of nutrients. It is involved in the development of diseases, notably cancer and neurological disorders including Parkinson's disease. Small molecule autophagy modulators have proven to be valuable tools to dissect and interrogate this crucial metabolic pathway and are in high demand. Phenotypic screening for autophagy inhibitors led to the discovery of the novel autophagy inhibitor aumitin. Target identification and confirmation revealed that aumitin inhibits mitochondrial respiration by targeting complex I. We show that inhibition of autophagy by impairment of mitochondrial respiration is general for several mitochondrial inhibitors that target different mitochondrial complexes. Our findings highlight the importance of mitochondrial respiration for autophagy regulation.

Proteomic profiling reveals that collismycin A is an iron chelator.

Collismycin A (CMA), a microbial product, has anti-proliferative activity against cancer cells, but the mechanism of its action remains unknown. Here, we report the identification of the molecular target of CMA by ChemProteoBase, a proteome-based approach for drug target identification. ChemProteoBase profiling showed that CMA is closely clustered with di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone, an iron chelator. CMA bound to both Fe(II) and Fe(III) ions and formed a 2:1 chelator-iron complex with a redox-inactive center. CMA-induced cell growth inhibition was completely canceled by Fe(II) and Fe(III) ions, but not by other metal ions such as Zn(II) or Cu(II). Proteomic and transcriptomic analyses showed that CMA affects the glycolytic pathway due to the accumulation of HIF-1α. These results suggest that CMA acts as a specific iron chelator, leading to the inhibition of cancer cell growth.