In vitro and in vivo antimicrobial activity of TS2037, a novel aminoglycoside antibiotic.

To overcome serious methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa infections, we synthesized TS2037, 5,4″-diepi-arbekacin, a novel aminoglycoside antibiotic, and evaluated its biological properties. TS2037 showed broad-range, as well as robust antibacterial activities against Gram-positive and Gram-negative bacteria. The MIC50 and MIC90 of TS2037 against clinical isolates of MRSA (n = 54) were both 0.25 µg/mL, and no resistant strain was observed. The MIC50 and MIC90 of TS2037 against clinical isolates of P. aeruginosa (n = 54) were 1 and 4 µg/mL, respectively. TS2037 and arbekacin, anti-MRSA aminoglycoside, were more stable against AAC(6')-APH(2″), aminoglycoside-6'-N-acetyltransferase and 2″-O-phosphotransferase, produced by resistant S. aureus than gentamicin. Therapeutic efficacies of TS2037 in the mouse models of systemic infection with MRSA were superior to those of arbekacin, vancomycin, and linezolid. The efficacy of TS2037 against systemic infection caused by P. aeruginosa producing AAC(6')-II was superior to those of arbekacin and amikacin. In the nephrotoxicity risk screening, the release of free N-acetyl-ß-D-glucosaminidase from the kidney epithelial cell line after treatment with TS2037 at 2.5 and 5.0 µM were 2.0 and 2.1 (U/L), respectively, which were about two times higher than those of arbekacin. In conclusion, TS2037 exhibited the most potent antibacterial activity among aminoglycosides tested against both MRSA and P. aeruginosa in vitro and in vivo, although its nephrotoxicity risk remains to be improved.

Intervenolin, a new antitumor compound with anti-Helicobacter pylori activity, from Nocardia sp. ML96-86F2.

Because stromal cells can regulate the growth and metastasis of tumor cells, a compound that modulates the interaction between the stromal cells and the tumor cells can control the tumor progression. In the course of our screening for such a compound, we have isolated a new compound, intervenolin, from the culture broth of Nocardia sp. ML96-86F2. Intervenolin inhibits the growth of human gastric and colorectal cancer cell lines in the coculture with the respective organ-derived stromal cells more strongly than that of the cancer cells cultured alone. Intervenolin shows antitumor effect against a xenograft model of human colorectal cancer cells in vivo. Furthermore, intervenolin exerts selective anti-Helicobacter pylori effect.

In vivo imaging of proteasome inhibition using a proteasome-sensitive fluorescent reporter.

A proteasome degrades numerous regulatory proteins that are critical for tumor growth and is therefore recognized as a promising anticancer target. Determining proteasome activity in the tumors of mice bearing xenografts is essential for the development of novel proteasome inhibitors. We developed a system for in vivo imaging of proteasome inhibition in the tumors of living mice, using a proteasome-sensitive fluorescent reporter, ZsProSensor-1. This reporter consists of a green fluorescent protein, ZsGreen, fused to mouse ornithine decarboxylase, which is degraded by the proteasome without being ubiquitinated. In stably transfected cells expressing ZsProSensor-1, the fluorescent reporter was rapidly degraded under steady-state conditions, whereas it was stabilized in the presence of proteasome inhibitors. Subcutaneous inoculation of the transfected cells into nude mice resulted in tumor formation. When the proteasome inhibitor bortezomib was intravenously administered to mice bearing these tumors, the ZsProSensor-1 protein accumulated in the tumors and emitted a fluorescent signal in a dose-dependent manner. Robust fluorescence was sustained for 3 days and then gradually decreased to baseline levels within 15 days. Intravenous administration of bortezomib also showed potent antitumor activity. In contrast, oral administration of bortezomib did not result in fluorescent protein accumulation in tumors or exhibit any antitumor activity. These results indicate that in vivo imaging using the ZsProSensor-1 fluorescent protein can be used as an indicator of antitumor activity and will be a powerful tool for the development of novel proteasome inhibitors.

Increased ABCB1 expression in TP-110-resistant RPMI-8226 cells.

TP-110, a novel proteasome inhibitor, has been found to possess potent growth inhibition in human multiple myeloma cells. To enhance its therapeutic effects, we established TP-110-resistant RPMI-8226 (RPMI-8226/TP-110) cells and elucidated their resistance mechanisms. The IC50 value for TP-110 cytotoxicity in the RPMI-8226/TP-110 cells was about 10-fold higher than that of the parental sensitive cells. The RPMI-8226/TP-110 cells exhibited distinct drug resistance to other proteasome inhibitors. Furthermore, they showed high cross-resistance to the cytotoxic effects of doxorubicin, etoposide, taxol, and vincristine. P-glycoprotein (MDR1), encoded by ABCB1, was elevated in the RPMI-8226/TP-110 cells, and the MDR1 inhibitor verapamil overcame their resistance to TP-110. The results of DNA microarray and RT-PCR suggested that the expression of ABCB1 is significantly elevated in RPMI-8226/TP-110 cells. This indicates that resistance in RPMI-8226/TP-110 cells is involved in the expression of P-glycoprotein, a drug-efflux pump.

Structure-activity relationship of boronic acid derivatives of tyropeptin: proteasome inhibitors.

The structure-activity relationship of the boronic acid derivatives of tyropeptin, a proteasome inhibitor, was studied. Based on the structure of a previously reported boronate analog of tyropeptin (2), 41 derivatives, which have varying substructure at the N-terminal acyl moiety and P2 position, were synthesized. Among them, 3-phenoxyphenylacetamide 6 and 3-fluoro picolinamide 22 displayed the most potent inhibitory activity toward chymotryptic activity of proteasome and cytotoxicity, respectively. The replacement of the isopropyl group in the P2 side chain to H or Me had negligible effects on the biological activities examined in this study.

NBRI16716A, a new antitumor compound against human prostate cancer cells, produced by Perisporiopsis melioloides Mer-f16716.

Prostate stroma can regulate the growth and metastasis of prostate cancer through the tumor-stromal cell interactions. Thus, small molecules that modulate the tumor-stromal cell interactions will have a chance to become new antitumor drugs. In the course of our screening of the modulators, we isolated three new natural compounds, NBRI16716A (1), NBRI16716B (2) and NBRI16716C (3), from the fermentation broth of Perisporiopsis melioloides Mer-f16716, although compound 2 was already reported as a chemical degradation product of isotriornicin. Compounds 1 and 2 inhibited the growth of human prostate cancer DU-145 cells in the coculture with human prostate stromal cells (PrSCs) more strongly than that of DU-145 cells alone. Furthermore, both compounds showed antitumor effect against xenograft models of DU-145 cells and PrSCs in vivo.

NBRI17671, a new antitumor compound, produced by Acremonium sp. CR17671.

The interaction between the receptor for advanced glycation end-product (RAGE) and amphoterin has an important role in tumor growth and metastasis. Because the abrogation of the interaction results in the inhibition of the tumor growth and metastasis, we designed a screening system for an inhibitor of the interaction between RAGE and amphoterin. In the course of our screening of the inhibitor, we isolated a novel natural compound NBRI17671 (1) from the fermentation broth of Acremonium sp. CR17671. We also modified 1 into a more active NBRI17671al (2). Although 1 at 50 g ml(-1) weakly inhibited binding of various cells to amphoterin, 2 at 50 g ml(-1) inhibited it by >50% of control. Compound 2 effectively inhibited the tumor growth of glioma and lung tumor xenografts in mice at 25 mg kg(-1). Furthermore, 2 was found to downregulate mitogen-activated protein kinase (MAPK) activity in the tumor cells.

Mitochondrial inhibitors show preferential cytotoxicity to human pancreatic cancer PANC-1 cells under glucose-deprived conditions.

Large areas of tumor are nutrient-starved and hypoxic due to a disorganized vascular system. Therefore, we screened small molecules to identify cytotoxic agents that function preferentially in nutrient-starved conditions. We found that efrapeptin F had preferential cytotoxicity to nutrient-deprived cells compared with nutrient-sufficient cells. Because efrapeptin F acts as a mitochondrial complex V inhibitor, we examined whether inhibitors of complex I, II, III, and V function as cytotoxic agents preferentially in nutrient-deprived cells. Interestingly, these inhibitors showed preferential cytotoxicity to nutrient-deprived cells and caused cell death under glucose-limiting conditions, irrespective of the presence or absence of amino acids and/or serum. In addition, these inhibitors were preferentially cytotoxic to nutrient-deprived cells even under hypoxic conditions. Further, efrapeptin F showed antitumor activity in vivo. These data indicate that mitochondrial inhibitors show preferential cytotoxicity to cancer cells under glucose-limiting conditions, and these inhibitors offer a promising strategy for anticancer therapeutic.

Rubratoxin A specifically and potently inhibits protein phosphatase 2A and suppresses cancer metastasis.

Although cytostatin analog protein phosphatase 2A (PP2A)-specific inhibitors are promising candidates of a new type of anticancer drug, their development has been hindered because of their liability. To find new classes of PP2A-specific inhibitors, we conducted a screening with microbial metabolites and found that rubratoxin A, a classical mycotoxin, is a highly specific and potent inhibitor of the enzyme. While rubratoxin A inhibits PP2A at Ki = 28.7 nm, it hardly inhibited any other phosphatases examined. Rubratoxin B, a close analog, also specifically but weakly inhibits PP2A at Ki = 3.1 microM. The inhibition of intracellular PP2A in cultured cells is obviously observed with 20 microM rubratoxin A treatment for 3 h, inducing the overphosphorylation in PP2A substrate proteins. Although rubratoxins and cytostatin differ in the apparent structures, these compounds share similarities in the structures in detail and PP2A-binding manners. Rubratoxin A showed higher suppression of tumor metastasis and reduction of the primary tumor volume than cytostatin in mouse experiments. As a successor of cytostatin analogs, rubratoxin A should be a good compound leading to the development of antitumor drugs targeting PP2A.

Leucinostatin A inhibits prostate cancer growth through reduction of insulin-like growth factor-I expression in prostate stromal cells.

Targeting stroma in tumor tissues is an attractive new strategy for cancer treatment. We developed in vitro coculture system, in which the growth of human prostate cancer DU-145 cells is stimulated by prostate stromal cells (PrSC) through insulin-like growth factor I (IGF-I). Using this system, we have been searching for small molecules that inhibit tumor growth through modulation of tumor-stromal cell interactions. As a result, we have found that leucinostatins and atpenins, natural antifungal antibiotics, inhibit the growth of DU-145 cells cocultured with PrSC more strongly than that of DU-145 cells alone. In this study we examined the antitumor effects of these small molecules in vitro and in vivo. When DU-145 cells were coinoculated with PrSC subcutaneously in nude mice, leucinostatin A was found to significantly suppress the tumor growth more than atpenin B. The antitumor effect of leucinostatin A in vivo was not obtained against the tumors of DU-145 cells alone. RT-PCR experiments revealed that leucinostatin A specifically inhibited IGF-I expression in PrSC without effect on expressions of other IGF axis molecules. Leucinostatins and atpenins are known to abrogate mitochondrial functions. However, when we used mitochondrial DNA-depleted, pseudo-rho(0) cells, we found that one of leucinostain A actions certainly depended on mitochondrial function, but it actually inhibited the growth of DU-145 cells more strongly in coculture with pseudo-rho(0) PrSC and reduced IGF-I expression in pseudo-rho(0) PrSC. Taken together, our results suggested that leucinostatin A inhibited prostate cancer cell growth through reduction of IGF-I expression in PrSC.

TP-110, a new proteasome inhibitor, down-regulates IAPs in human multiple myeloma cells.

TP-110, a new proteasome inhibitor, has previously shown potent growth inhibition in various tumor cell lines. In this study, the mechanism of TP-110-induced apoptosis is investigated in a human multiple myeloma cell line. Treatment with TP-110 for 24 h in vitro induced apoptosis in multiple myeloma cell line RPMI8226. Although the expression of Bcl-2, Bcl-xL and Bax was not affected by the treatment of TP-110, cleavage of Bid and release of cytochrome c were enhanced. Interestingly, TP-110 reduced the intrinsic inhibitor of apoptosis proteins (IAPs), cIAP-1 and XIAP, that suppress executioner caspases. The reduction of IAPs was observed not only by TP-110, but also by another proteasome inhibitor, MG-132. These results indicate that proteasome inhibitors reduce the level of IAPs and that the apoptosis induced by TP-110 is correlated with the level of IAPs in leukemia cell lines. Additionally, a reduction of cIAP-1 and XIAP by TP-110 contributes to the sensitization of Fas-mediated apoptosis. Taken together, the alteration of the apoptosis regulatory proteins by a proteasome inhibitor induces apoptosis in tumor cells.

Synthesis of boronic acid derivatives of tyropeptin: proteasome inhibitors.

Boronic acid derivatives of tyropeptin were synthesized with TP-110 as the lead compound. Due to the lability of the aminoboronic acid moiety, careful design of the deprotection and coupling sequence was required. Liquid-liquid partition chromatography was found to be a powerful tool for purification of compounds of this class. The obtained derivatives showed potent inhibitory activities against the human 20S proteasome in vitro.

New atpenins, NBRI23477 A and B, inhibit the growth of human prostate cancer cells.

The growth and metastasis of prostate cancer are regulated by prostate stroma through the tumor-stromal cell interactions. Small molecules that modulate the tumor-stromal cell interactions will be new anticancer drugs. In the course of our screening of the modulators, we isolated two new atpenins, NBRI23477 A (4) and B (5), from the fermentation broth of Penicillium atramentosum PF1420. Compounds 4 and 5 as well as atpenin A4 (1), A5 (2) and B (3) inhibited the growth of human prostate cancer DU-145 cells in the coculture with human prostate stromal cells more strongly than that of DU-145 cells alone.

Ceramidastin, a novel bacterial ceramidase inhibitor, produced by Penicillium sp. Mer-f17067.

Decrease of ceramide in the skin is one of the aggravating factors of atopic dermatitis. The skin is often infected by ceramidase-producing bacteria, such as Pseudomonas aeruginosa. The bacterial ceramidase then degrades ceramide in the skin. To develop anti-atopic dermatitis drugs, we searched for ceramidase inhibitors, which led to the discovery of ceramidastin, a novel inhibitor of bacterial ceramidase, from the culture broth of Penicillium sp. Mer-f17067. Ceramidastin inhibited the bacterial ceramidase with an IC(50) value of 6.25 microg ml(-1). Here we describe the isolation, physicochemical properties, structure determination and biological activity of ceramidastin.

Inhibitors of insulin-like growth factor-1 receptor tyrosine kinase are preferentially cytotoxic to nutrient-deprived pancreatic cancer cells.

Chronic deprivation of nutrients is rare in normal tissues, however large areas of tumor are nutrient-starved and hypoxic due to a disorganized vascular system. Some cancers show an inherent ability to tolerate severe growth conditions. Therefore, we screened chemical compounds to identify cytotoxic agents that function preferentially in nutrient-deprived conditions. We found that AG1024, a specific inhibitor of insulin-like growth factor-1 receptor tyrosine kinase (IGF-1R), showed preferential cytotoxicity to human pancreatic cancer cells in nutrient-deprived conditions relative to cells in nutrient-sufficient conditions. The cytotoxicity of I-OMe-AG538 (another specific inhibitor of IGF-1R kinase) was also enhanced in nutrient-deprived cells. In addition, AG1024 and I-OMe-AG538 potently inhibited IGF-1R activation to nutrient-deprived cells. In contrast, conventional chemotherapeutic drugs, as well as inhibitors of PDGFR and EGFR kinases, elicited weak cytotoxicity. These data indicate that nutrient-deprived human pancreatic cancer cells have increased sensitivity to inhibition of IGF-1R activation. IGF-1R inhibitors offer a promising strategy for anticancer therapeutic approaches that are oriented toward tumor microenvironment.

Phthoxazolin A inhibits prostate cancer growth by modulating tumor-stromal cell interactions.

Because stroma in tumor tissues can promote prostate cancer development, modulation of tumor-stromal cell interactions may represent an attractive new strategy for cancer treatment. Here, we report that phthoxazolin A and its analog inthomycin B inhibit the growth of human prostate cancer DU-145 cells by modulating tumor-stromal cell interactions. Using an in vitro coculture system, in which prostate cancer cell growth is upregulated by prostate stromal cells (PrSC), we found that phthoxazolin A and inthomycin B strongly inhibited the growth of DU-145 cells when in coculture with PrSC compared to DU-145 cells cultured alone. Although PrSC consist of both fibroblasts and myofibroblasts, phthoxazolin A and inthomycin B inhibited the expression of smooth muscle alpha-actin, a myofibroblast marker, without affecting vimentin and beta-actin expression. Because myofibroblasts secrete various factors that can promote tumor cell growth, we examined whether the inhibitory compounds affected the secretion of such factors from PrSC. Proteomic analysis and reverse transcription-polymerase chain reaction revealed that phthoxazolin A and inthomycin B inhibited the expression of several insulin-like growth factor binding proteins and insulin-like growth factor (IGF)-I by PrSC. Transforming growth factor-beta1 increased myofibroblast numbers and IGF-I levels in PrSC. Phthoxazolin A inhibited transforming growth factor-beta1 activity without altering phosphorylation of the downstream molecule smad2. Furthermore, conditioned medium from phthoxazolin A-treated PrSC failed to increase the phosphorylation of IGF-IR and Akt in DU-145 cells. Taken together, our results suggested that phthoxazolin A acts as a small-molecule modulator of tumor-stromal cell interactions that can indirectly suppress prostate cancer cell growth through inhibition of IGF-I production by PrSC.

A new terrein glucoside, a novel inhibitor of angiogenin secretion in tumor angiogenesis.

Angiogenesis is a critical step for the tumor therapy. Many angiogenic factors are involved in the tumor angiogenesis. In the course of our screening for inhibitors of angiogenin secretion, one of angiogenic factors, we have isolated a new terrein glucoside (1) and terrein (2) from the fermentation broth of fungal strain Aspergillus sp. PF1381. The structure and absolute stereochemistry of 1 was determined to be (4S,5R)-5-[(alpha-D-glucopyranosyl)oxy]-4-hydroxy-3-(E-1-propenyl)-2-cyclopenten-1-one on the basis of spectral and enzymatic analyses. Compounds 1 and 2 equally inhibited angiogenin secretion from androgen-dependent prostate cancer cells, LNCaP-CR, with IC50 values of 13 microM. However, both compounds did not affect VEGF secretion, another angiogenic factor. Furthermore, both compounds inhibited tube formation of human umbilical vein endothelial cells (HUVEC). These results suggested that 1 and 2 act as angiogenesis inhibitors through the inhibition of angiogenin secretion.