Direct interaction of Cucurbitacin E isolated from Alsomitra macrocarpa to actin filament

Recent Advances in the Application of Cucurbitacins as Anticancer Agents

Cucurbitacins are tetracyclic triterpenoid secondary metabolites, widely distributed in the Cucurbitaceae family. These bitter-tasting compounds act primarily as defense mechanisms against external injuries, and thus against herbivores, and furthermore, they have also found use in folk medicine in the treatment of various diseases. Many studies have acknowledged significant biological activities of cucurbitacins, such as antioxidant and anti-inflammatory activities, antimicrobial properties, or antitumor potential. Overall, cucurbitacins have the ability to inhibit cell proliferation and induce apoptosis in various cancer cell lines. Both in vitro and in vivo studies were performed to evaluate the anticancer activity of varied cucurbitacins. Cucurbitacins offer a promising avenue for future cancer treatment strategies, and their diverse mechanisms of action make them attractive candidates for further investigation. The aim of the present study is to shed light on the chemical diversity of this group of compounds by providing the sources of origin of selected compounds and their chemical structure, as well as insight into their anticancer potential. In addition, within this paper molecular targets for cucurbitacins and signalling pathways important for cancer cell proliferation and/or survival that are affected by the described class of compounds have been presented.

Cucurbitacins: A Focus on Cucurbitacin E As A Natural Product and Their Biological Activities

For the last years, different types of cucurbitacins have been extracted from various species of Cucurbitaceae family. For this review, all related papers were accumulated by searching electronic databases in the English language, including PubMed, Scopus, and Google Scholar. The keywords of cucurbitacin, cucumber anticancer therapy, cytotoxic effects, chemotherapy, and inhibitor effect were searched until February 2020. According to the result of this review, cucurbitacin E as a tetracyclic triterpenes compound, has been exhibited cell cycle arrest, anti-inflammatory and anticancer activities. It showed tumor proliferation prevention, induction of apoptosis or synergistically acts with other established antitumor compounds and cytokines throughout many molecular mechanisms. In a function-structure association manner, cucurbitacin E can inhibit Janus kinas2 (JAK2) phosphorylation, the signal transducer activator of transcription 3 (STAT3) and subsequently block these pathways, which seems to be the main mechanism of its activity. Future studies could target its detection in uninvestigated sources, subsequently its derivatives to improve their anticancer activity.

Beneficial and Deleterious Impact of a Nutritional Supplementation for Inhibition of Proliferation of Neuroblastoma in Culture

Neuroblastoma, a cancer of the sympathetic nervous system, primarily affects infants and children ≤10 yr of age. High-risk neuroblastoma is associated with low survival rates and increased risks of treatment-related side-effects. Therefore, effective treatments that increase survival and reduce adverse side-effects are crucial. Cucurbitacin E (CucE), a nutritional supplement shown to have potential as an alternative to chemotherapy, was investigated for potential impact on neuroblastoma alone and in combination with the standard chemotherapeutic agent, paclitaxel, (PAC). CucE and PAC each inhibited proliferation of murine neuroblastoma cells in culture. Combined treatment with CucE and PAC also induced morphological differentiation. However, both differentiation and antiproliferative effects were reversible. Consequently, while nutritional supplementation represents a potential therapeutic approach toward treatment of cancer, certain nutritional/chemotherapeutic combinations may induce transient rather than permanent effects. Transient inhibition of proliferation by nutritional supplementation could inadvertently protect carcinogenic cells from toxicity otherwise induced by a chemotherapeutic agent. Combinatorial treatments involving nutritional supplements should therefore be utilized with caution.

Cucurbitacins: elucidation of their interactions with the cytoskeleton

Cucurbitacins, a class of toxic tetracyclic triterpenoids in Cucurbitaceae, modulate many molecular targets. Here we investigated the interactions of cucurbitacin B, E and I with cytoskeletal proteins such as microtubule and actin filaments. The effects of cucurbitacin B, E and I on microtubules and actin filaments were studied in living cells (Hela and U2OS) and in vitro using GFP markers, immunofluorescence staining and in vitro tubulin polymerization assay. Cucurbitacin B, E and I apparently affected microtubule structures in living cells and cucurbitacin E inhibited tubulin polymerization in vitro with IC50 value of 566.91 ± 113.5 µM. Cucurbitacin E did not affect the nucleation but inhibited the growth phase and steady state during microtubule assembly in vitro. In addition, cucurbitacin B, E and I all altered mitotic spindles and induced the cell cycle arrest at G2/M phase. Moreover, they all showed potent effects on actin cytoskeleton by affecting actin filaments through the depolymerization and aggregation. The interactions of cucubitacin B, E and I with microtubules and actin filaments present new insights into their modes of action.

The effect of red light and far-red light conditions on secondary metabolism in agarwood

BACKGROUND

Agarwood, a heartwood derived from Aquilaria trees, is a valuable commodity that has seen prevalent use among many cultures. In particular, it is widely used in herbal medicine and many compounds in agarwood are known to exhibit medicinal properties. Although there exists much research into medicinal herbs and extraction of high value compounds, few have focused on increasing the quantity of target compounds through stimulation of its related pathways in this species.

RESULTS

In this study, we observed that cucurbitacin yield can be increased through the use of different light conditions to stimulate related pathways and conducted three types of high-throughput sequencing experiments in order to study the effect of light conditions on secondary metabolism in agarwood. We constructed genome-wide profiles of RNA expression, small RNA, and DNA methylation under red light and far-red light conditions. With these profiles, we identified a set of small RNA which potentially regulates gene expression via the RNA-directed DNA methylation pathway.

CONCLUSIONS

We demonstrate that light conditions can be used to stimulate pathways related to secondary metabolism, increasing the yield of cucurbitacins. The genome-wide expression and methylation profiles from our study provide insight into the effect of light on gene expression for secondary metabolism in agarwood and provide compelling new candidates towards the study of functional secondary metabolic components.

Cucurbitacin E has neuroprotective properties and autophagic modulating activities on dopaminergic neurons

Natural molecules are under intensive study for their potential as preventive and/or adjuvant therapies for neurodegenerative disorders such as Parkinson's disease (PD). We evaluated the neuroprotective potential of cucurbitacin E (CuE), a tetracyclic triterpenoid phytosterol extracted from the Ecballium elaterium (Cucurbitaceae), using a known cellular model of PD, NGF-differentiated PC12. In our postmitotic experimental paradigm, neuronal cells were treated with the parkinsonian toxin 1-methyl-4-phenylpyridinium (MPP⁺) to provoke significant cellular damage and apoptosis or with the potent N,N-diethyldithiocarbamate (DDC) to induce superoxide (O₂^•−) production, and CuE was administered prior to and during the neurotoxic treatment. We measured cellular death and reactive oxygen species to evaluate the antioxidant and antiapoptotic properties of CuE. In addition, we analyzed cellular macroautophagy, a bulk degradation process involving the lysosomal pathway. CuE showed neuroprotective effects on MPP⁺-induced cell death. However, CuE failed to rescue neuronal cells from oxidative stress induced by MPP⁺ or DDC. Microscopy and western blot data show an intriguing involvement of CuE in maintaining lysosomal distribution and decreasing autophagy flux. Altogether, these data indicate that CuE decreases neuronal death and autophagic flux in a postmitotic cellular model of PD.

Identification of cucurbitacins and assembly of a draft genome for Aquilaria agallocha

BACKGROUND

Agarwood is derived from Aquilaria trees, the trade of which has come under strict control with a listing in Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora. Many secondary metabolites of agarwood are known to have medicinal value to humans, including compounds that have been shown to elicit sedative effects and exhibit anti-cancer properties. However, little is known about the genome, transcriptome, and the biosynthetic pathways responsible for producing such secondary metabolites in agarwood.

RESULTS

In this study, we present a draft genome and a putative pathway for cucurbitacin E and I, compounds with known medicinal value, from in vitro Aquilaria agallocha agarwood. DNA and RNA data are utilized to annotate many genes and protein functions in the draft genome. The expression changes for cucurbitacin E and I are shown to be consistent with known responses of A. agallocha to biotic stress and a set of homologous genes in Arabidopsis thaliana related to cucurbitacin bio-synthesis is presented and validated through qRT-PCR.

CONCLUSIONS

This study is the first attempt to identify cucurbitacin E and I from in vitro agarwood and the first draft genome for any species of Aquilaria. The results of this study will aid in future investigations of secondary metabolite pathways in Aquilaria and other non-model medicinal plants.

VASP activation via the Gα13/RhoA/PKA pathway mediates cucurbitacin-B-induced actin aggregation and cofilin-actin rod formation

Cucurbitacin B (CuB), a potent antineoplastic agent of cucurbitacin triterpenoids, induces rapid disruption of actin cytoskeleton and aberrant cell cycle inhibiting carcinogenesis. However, the underlying molecular mechanism of such anticancer effects remains incompletely understood. In this study, we showed that CuB treatment rapidly induced vasodilator-stimulated phosphoprotein (VASP) phosphorylation (i.e. activation) at the Ser157 residue and generated VASP clumps which were co-localized with amorphous actin aggregates prior to the formation of highly-ordered cofilin-actin rods in melanoma cells. Knockdown of VASP or inhibition of VASP activation using PKA-specific inhibitor H89 suppressed CuB-induced VASP activation, actin aggregation and cofilin-actin rod formation. The VASP activation was mediated by cAMP-independent PKA activation as CuB decreased the levels of cAMP while MDL12330A, an inhibitor of adenylyl cyclase, had weak effect on VASP activation. Knockdown of either Gα13 or RhoA not only suppressed VASP activation, but also ameliorated CuB-induced actin aggregation and abrogated cofilin-actin rod formation. Collectively, our studies highlighted that the CuB-induced actin aggregation and cofilin-actin rod formation was mediated via the Gα13/RhoA/PKA/VASP pathway.

Actin polymerization mediated by Babesia gibsoni aldolase is required for parasite invasion

Host cell invasion by apicomplexan parasites driven by gliding motility and empowered by actin-based movement is essential for parasite survival and pathogenicity. The parasites share a conserved invasion process: actin-based motility led by the coordination of adhesin-cytoskeleton via aldolase. A number of studies of host cell invasion in the Plasmodium species and Toxoplasma gondii have been performed. However, the mechanisms of host cell invasion by Babesia species have not yet been studied. Here, we show that Babesia gibsoni aldolase (BgALD) forms a complex with B. gibsoni thrombospondin-related anonymous protein (BgTRAP) and B. gibsoni actin (BgACT), depending on tryptophan-734 (W-734) in BgTRAP. In addition, actin polymerization is mediated by BgALD. Moreover, cytochalasin D, which disrupts actin polymerization, suppressed B. gibsoni parasite growth and inhibited the host cell invasion by parasites, indicating that actin dynamics are essential for erythrocyte invasion by B. gibsoni. This study is the first molecular approach to determine the invasion mechanisms of Babesia species.

Biological activities and potential molecular targets of cucurbitacins: a focus on cancer

Cucurbitacin and its derivatives (cucurbitacins) are a class of highly oxidized tetracyclic triterpenoids. They are widely distributed in the plant kingdom, where they act as heterologous chemical pheromones that protect plants from external biological insults. Their bioactivities first attracted attention in the 1960s. Documented data demonstrate that cucurbitacins possess strong pharmacological properties, such as antitumor, anti-inflammatory, and hepatoprotective effects, etc. Several molecular targets for cucurbitacins have been discovered, such as fibrous-actin, signal transducer and activator of transcription 3, cyclooxygenase-2, etc. The present study summarizes the achievements of the 50 years of research on cucurbitacins. The aim was to systematically analyze their bioactivities with an emphasis on their anticancer effects. Research and development has shed new insight into the beneficial properties of these compounds.

The natural product cucurbitacin E inhibits depolymerization of actin filaments

Although small molecule actin modulators have been widely used as research tools, only one cell-permeable small molecule inhibitor of actin depolymerization (jasplakinolide) is commercially available. We report that the natural product cucurbitacin E inhibits actin depolymerization and show that its mechanism of action is different from jasplakinolide. In assays using pure fluorescently labeled actin, cucurbitacin E specifically affects depolymerization without affecting polymerization. It inhibits actin depolymerization at substoichiometric concentrations up to 1:6 cucurbitacin E:actin. Cucurbitacin E specifically binds to filamentous actin (F-actin) forming a covalent bond at residue Cys257, but not to monomeric actin (G-actin). On the basis of its compatibility with phalloidin staining, we show that cucurbitacin E occupies a different binding site on actin filaments. Using loss of fluorescence after localized photoactivation, we found that cucurbitacin E inhibits actin depolymerization in live cells. Cucurbitacin E is a widely available plant-derived natural product, making it a useful tool to study actin dynamics in cells and actin-based processes such as cytokinesis.

Cucurbitacin E Induces G(2)/M Phase Arrest through STAT3/p53/p21 Signaling and Provokes Apoptosis via Fas/CD95 and Mitochondria-Dependent Pathways in Human Bladder Cancer T24 Cells

Cucurbitacin E, a tetracyclic triterpenes compound extracted from cucurbitaceous plants, has been shown to exhibit anticancer and anti-inflammatory activities. The purpose of this study was to elucidate whether cucurbitacin E promotes cell cycle arrest and induces apoptosis in T24 cells and further to explore the underlying molecular mechanisms. The effects of cucurbitacin E on T24 cell's growth and accompanied morphological changes were examined by MTT assay and a phase-contrast microscope. DNA content, mitochondrial membrane potential (ΔΨ_m) and annexin V/PI staining were determined by flow cytometry. The protein levels were measured by Western blotting. Our results demonstrated that cucurbitacin E-induced G₂/M arrest was associated with a marked increase in the levels of p53, p21 and a decrease in phospho-signal transducer and activator of transcription 3 (STAT3), cyclin-dependent kinase 1 (CDK1) and cyclin B. Cucurbitacin E-triggered apoptosis was accompanied with up-regulation of Fas/CD95, truncated BID (t-BID) and a loss of ΔΨ_m, resulting in the releases of cytochrome c, apoptotic protease activating factor 1 (Apaf-1) and apoptosis-inducing factor (AIF), and sequential activation of caspase-8, caspase-9, and caspase-3. Our findings provided the first evidence that STAT3/p53/p21 signaling, Fas/CD95 and mitochondria-dependent pathways play critical roles in cucurbitacin E-induced G₂/M phase arrest and apoptosis of T24 cells.

Cucurbitacin B induces rapid depletion of the G-actin pool through reactive oxygen species-dependent actin aggregation in melanoma cells

Cucurbitacin B (CuB), a triterpenoid compound isolated from Cucurbitaceae plants, has been reported as a promising anti-cancer agent, yet its action mechanism is still controversial. In this study, we explored the potential mechanism of CuB in murine B16F10 melanoma cells. Anti-proliferation and anti-invasion effects were assessed in cultured cells, and in vivo anti-tumor activity was evaluated in a murine subcutaneous melanoma model. Flow cytometry was adopted to analyze cell cycle distribution and reactive oxygen species (ROS) levels. Actin levels were determined by western blot analysis, and the profiles of differential expressed proteins were identified by a quantitative proteomic approach. The results showed that CuB exerted inhibitory effects on cell proliferation, colony formation, as well as migration and invasion potential of the melanoma cells. The growth of subcutaneous melanoma was significantly inhibited in mice treated with CuB when compared with control group. Furthermore, CuB treatment caused rapid cell membrane blebbing and deformation, and induced G(2)/M-phase arrest and formation of multiploid cells. Notably, the G-actin pool was rapidly depleted and actin aggregates were formed quickly after CuB treatment. A number of cytoskeleton-regulatory proteins were differentially regulated. Blockage of ROS production significantly reduced the G-actin depletion ability and the anti-tumor activity of CuB. These findings indicate that CuB induces rapid depletion of the G-actin pool through ROS-dependent actin aggregation in melanoma cells, which may at least partly account for its anti-tumor activity.

Cucurbitacin IIa: a novel class of anti-cancer drug inducing non-reversible actin aggregation and inhibiting survivin independent of JAK2/STAT3 phosphorylation

Background:

Cucurbitacin (Cuc) and triterpene-derived natural products exhibit anti-cancer potential in addition to their conspicuous anti-bacterial and anti-inflammatory activity. Recently, inhibition of Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) signaling was shown to underlie the effects of Cuc family on inducing cell death in cancer.

Method:

We purified Cuc IIa, the active component from the medicinal plant Hemsleya amalils Diels, which shows different structural modifications from other Cuc derivatives. We investigated the mechanisms of its inhibitory effects on cancer cells in vitro and tumour growth in vivo.

Results:

Cuc IIa induced the irreversible clustering of filamentous actin and arrested cell cycle by the increases in G2/M populations. Cuc IIa resulted in the reduced phospho-Histone H3 and markedly increased cleavage of poly-(ADP-ribose) polymerase or PARP, immediate upstream of DNA breakdown as the result of caspase activation, consistent with mitotic blockage-induced cell death. However, unlike other Cuc members, Cuc IIa did not suppress JAK2/STAT3 phosphorylation or alter phosphorylation of mitogen-activated protein kinases. Instead, the expression of the cell cycle-regulated Inhibitor of Apoptosis Protein (IAP) survivin was reduced. Introducing oncoprotein δ-catenin, which increased survivin expression and suppressed small GTPase RhoA, reduced efficacy of Cuc IIa to induce cell death. Supporting the effects of Cuc IIa on actin cytoskeletal signaling, RhoA phosphorylation was reduced suggesting its increased activity.

Conclusion:

Cuc IIa is a novel class of anti-cancer drug in suppression of cancer cell expansion by disrupting the actin cytoskeleton and directing the cell to undergo PARP-mediated apoptosis through the inhibition of survivin downstream of JAK2/STAT3.

Cucurbitacin I inhibits cell motility by indirectly interfering with actin dynamics

BACKGROUND

Cucurbitacins are plant natural products that inhibit activation of the Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) pathway by an unknown mechanism. They are also known to cause changes in the organization of the actin cytoskeleton.

METHODOLOGY/PRINCIPAL FINDINGS

We show that cucurbitacin I potently inhibits the migration of Madin-Darby canine kidney (MDCK) cell sheets during wound closure, as well as the random motility of B16-F1 mouse melanoma cells, but has no effect on movement of Dictyostelium discoideum amoebae. Upon treatment of MDCK or B16-F1 cells with cucurbitacin I, there is a very rapid cessation of motility and gradual accumulation of filamentous actin aggregates. The cellular effect of the compound is similar to that observed when cells are treated with the actin filament-stabilizing agent jasplakinolide. However, we found that, unlike jasplakinolide or phallacidin, cucurbitacin I does not directly stabilize actin filaments. In in vitro actin depolymerization experiments, cucurbitacin I had no effect on the rate of actin filament disassembly at the nanomolar concentrations that inhibit cell migration. At elevated concentrations, the depolymerization rate was also unaffected, although there was a delay in the initiation of depolymerization. Therefore, cucurbitacin I targets some factor involved in cellular actin dynamics other than actin itself. Two candidate proteins that play roles in actin depolymerization are the actin-severing proteins cofilin and gelsolin. Cucurbitacin I possesses electrophilic reactivity that may lead to chemical modification of its target protein, as suggested by structure-activity relationship data. However, mass spectrometry revealed no evidence for modification of purified cofilin or gelsolin by cucurbitacin I.

CONCLUSIONS/SIGNIFICANCE

Cucurbitacin I results in accumulation of actin filaments in cells by a unique indirect mechanism. Furthermore, the proximal target of cucurbitacin I relevant to cell migration is unlikely to be the same one involved in activation of the JAK2/STAT3 pathway.

Jasplakinolide: an actin-specific reagent that promotes actin polymerization

Jasplakinolide, a cyclo-depsipeptide is a commonly used actin filament polymerizing and stabilizing drug. The substance has originally been isolated from a marine sponge, and can now be synthesized and has become commercially available. This, together with the benefit that jasplakinolide is membrane permeable has made it a commonly used tool in cell biology, when actin filament stabilization or polymerization has to be achieved. This may either be the case in studies on morphogenesis, motility, organelle movement, or when apoptosis has to be induced. Its use as a potent anticancer drug is discussed. The direct action on actin filaments may have further consequences in golgi body and membrane raft protein organization. In this chapter, the visualization of jasplaklinolide effects by different fluorescent and transmission electron microscopic methods is described. As competitive binding capacities of jasplakinolide and phalloidin make the detection of actin filaments by fluorescently labeled phalloidin problematic, alternatives are given here.

Inhibitory effect of cucurbitacin E on pancreatic cancer cells growth via STAT3 signaling

PURPOSE

Pancreatic cancer has been a serious disease worldwide for its high mortality. Cucurbitacin E is a member of triterpenoid family isolated from plants showing antiproliferative activity on various cancer cells. In this study, we have explored whether cucurbitacin E also has an anti-tumor effect on pancreatic cancer cells.

METHODS

Human pancreatic cancer cells PANC-1 were used to explore the effect and possible mechanisms of cucurbitacin E on cell cycle progression, apoptosis and proliferation.

RESULTS

Cucurbitacin E has inhibited the growth of PANC-1 cells in a dose- and time-dependent manner, and has caused accumulation of cells at the G(2)/M phase as well as apoptosis. Western blotting also showed that cucurbitacin E treatment can inhibit STAT3 phosphorylation while upregulate p53 expression.

CONCLUSIONS

Our results suggested that cucurbitacin E may be an effective regimen for the chemotherapy of pancreatic cancer.

Actin-aggregating cucurbitacins from Physocarpus capitatus

Bioassay-guided fractionation of Physocarpus capitatus yielded two new cucurbitacins (3 and 4) along with the known cucurbitacin F (1) and dihydrocucurbitacin F (2). Preliminary mechanism of action studies indicate that the cucurbitacins cause actin aggregates and inhibit cell division.