Three-Dimensional Structures of OSW-1 and Its Congener

Natural-product-based fluorescent probes: recent advances and applications

Fluorescent probes are attractive tools for biology, drug discovery, disease diagnosis, and environmental analysis. In bioimaging, these easy-to-operate and inexpensive probes can be used to detect biological substances, obtain detailed cell images, track in vivo biochemical reactions, and monitor disease biomarkers without damaging biological samples. Over the last few decades, natural products have attracted extensive research interest owing to their great potential as recognition units for state-of-the-art fluorescent probes. This review describes representative natural-product-based fluorescent probes and recent discoveries, with a particular focus on fluorescent bioimaging and biochemical studies.

Galtonosides A-E: Antiproliferative and Antiplasmodial Cholestane Glycosides from Galtonia regalis

An extract of Galtonia regalis from the Natural Products Discovery Institute showed moderate antiplasmodial activity, with an IC50 value less than 1.25 μg/mL. The two known cholestane glycosides 1 and 2 and the five new cholestane glycosides galtonosides A-E (3-7) were isolated after bioassay-directed fractionation. The structures of the new compounds were determined by interpretation of their NMR and mass spectra. Among these compounds, galtonoside B (4) displayed the most potent antiplasmodial activity, with an IC50 value of 0.214 μM against the drug-resistant Dd2 strain of Plasmodium falciparum.

Interactions of OSW-1 with Lipid Bilayers in Comparison with Digitonin and Soyasaponin

OSW-1, a unique steroidal saponin isolated from the bulbs of Ornithogalum saundersiae, has potent cell-growth inhibition activity. In this study, we conducted fluorescence measurements and microscopic observations using palmitoyloleoylphosphatidylcholine (POPC)-cholesterol (Chol) bilayers to evaluate the membrane-binding affinity of OSW-1 in comparison with another steroidal saponin, digitonin, and the triterpenoid saponin, soyasaponin Bb(I). The membrane activities of these saponins were evaluated using calcein leakage assays and fitted to the binding isotherm by changing the ratios of saponin-lipids. Digitonin showed the highest binding affinity for the POPC-Chol membrane (K_app = 0.38 μM^-1) and the strongest membrane disruptivity in the bound saponin-lipid ratio at the point of 50% calcein leakage (r₅₀ = 0.47) occurrence. OSW-1 showed slightly lower activity (K_app = 0.31 μM^-1; r₅₀ = 0.78), and the soyasaponin was the lowest in the membrane affinity and the calcein leakage activity (K_app = 0.017 μM^-1; r₅₀ = 1.66). The effect of OSW-1 was further assessed using confocal microscopy in an experiment utilizing DiI and rhodamine 6G as the fluorescence probes. The addition of 30 μM OSW-1 induced inward membrane curvature in some giant unilamellar vesicles (GUVs). At the higher OSW-1 concentration (58 μM, r₅₀ = 0.78) where the 50% calcein leakage was observed, the morphology of some GUVs became elongated. With digitonin at the corresponding concentration (35 μM, r₅₀ = 0.47), membrane disruption and formation of large aggregates in aqueous solution were observed, probably due to a detergent-type mechanism. These saponins, including OSW-1, required Chol to exhibit their potent membrane activity although their mechanisms are thought to be different. At the effective concentration, OSW-1 preferably binds to the bilayers without prominent disruption of vesicles and exerts its activity through the formation of saponin-Chol complexes, probably resulting in membrane permeabilization.

Dissecting the structural basis for the intracellular delivery of OSW-1 by fluorescent probes

The structural basis for the intracellular delivery of OSW-1 is investigated using fluorescent derivatives of OSW-1 and its closely related congeners. Despite the large differences in activity, all the fluorescent probes are found to translocate across the plasma membrane to the ER and Golgi apparatus. This observation suggests that the glycosylated cholestane moiety plays an important role in the cell internalization and intracellular localization property of OSW-1.

New insights into structure-activity relationship of ipomoeassin F from its bioisosteric 5-oxa/aza analogues

Ipomoeassin F, a plant-derived macrolide, exhibited single-digit nanomolar growth inhibition activity against many cancer cell lines. In this report, a series of 5-oxa/aza analogues was prepared and screened for cytotoxicity. Replacement of 5-CH2 with O/NH simplified the synthesis and led to only a small activity loss. N-methylation almost completely restored the potency. Further studies with additional 5-oxa analogues suggested, for the first time, that size and flexibility of the ring also significantly influence the bioactivity of ipomoeassin F.

Sterol-recognition ability and membrane-disrupting activity of Ornithogalum saponin OSW-1 and usual 3-O-glycosyl saponins

OSW-1 is a structurally unique steroidal saponin isolated from the bulbs of Ornithogalum saundersiae, and has exhibited highly potent and selective cytotoxicity in tumor cell lines. This study aimed to investigate the molecular mechanism for the membrane-permeabilizing activity of OSW-1 in comparison with those of other saponins by using various spectroscopic approaches. The membrane effects and hemolytic activity of OSW-1 were markedly enhanced in the presence of membrane cholesterol. Binding affinity measurements using fluorescent cholestatrienol and solid-state NMR spectroscopy of a 3-d-cholesterol probe suggested that OSW-1 interacts with membrane cholesterol without forming large aggregates while 3-O-glycosyl saponin, digitonin, forms cholesterol-containing aggregates. The results suggest that OSW-1/cholesterol interaction is likely to cause membrane permeabilization and pore formation without destroying the whole membrane integrity, which could partly be responsible for its highly potent cell toxicity.

Synthesis of a fluorescent photoaffinity probe of OSW-1 by site-selective acylation of an inactive congener and biological evaluation

A novel fluorescent photoaffinity probe of OSW-1 was prepared in two steps from a naturally occurring inactive congener by a sequential site-selective acylation strategy using Me₂SnCl₂. It displayed highly potent anticancer activity and a similar intracellular localization property to that of a fluorescently-tagged OSW-1, thereby demonstrating its potential utility in live cell studies.

Fluorescent analog of OSW-1 and its cellular localization

OSW-1 is a steroidal saponin, which has emerged as an attractive anticancer agent with highly cancer cell selective activity. A fluorescent analog was prepared from the natural product to analyze its cellular uptake and localization. We found that the fluorescent analog is rapidly internalized into cells and is primarily distributed in endoplasmic reticulum and Golgi apparatus.

Synthesis of three OSW-1 analogs with maltose side chains bearing different protection groups

In order to simplify the synthesis of OSW-1's disaccharide side chain and explore the structure-activity relationship of OSW-1, three 16α-O-maltose OSW-1 analogs carrying three maltose side chains bearing different protections were designed and synthesized.