Sesquiterpene lactones as drugs with multiple targets in cancer treatment: focus on parthenolide

Discovery of Small Molecules That Induce Lysosomal Cell Death in Cancer Cell Lines Using an Image-Based Screening Platform

The lysosomal cell death (LCD) pathway is a caspase 3-independent cell death pathway that has been suggested as a possible target for cancer therapy, making the development of sensitive and specific high-throughput (HT) assays to identify LCD inducers highly desirable. In this study, we report a two-step HT screening platform to reliably identify such molecules. First, using a robust HT primary screen based on propidium iodide uptake, we identified compounds that kill through nonapoptotic pathways. A phenotypic image-based assay using a galectin-3 (Gal-3) reporter was then used to further classify hits based on lysosomal permeabilization, a hallmark of LCD. The identification of permeabilized lysosomes in our image-based assay is not affected by changes in the lysosomal pH, thus resolving an important limitation in currently used methods. We have validated our platform in a screen by identifying 24 LCD inducers, some previously known to induce LCD. Although most LCD inducers were cationic amphiphilic drugs (CADs), we have also identified a non-CAD LCD inducer, which is of great interest in the field. Our data also gave new insights into the biology of LCD, suggesting that lysosomal accumulation and acid sphingomyelinase inhibition are not sufficient or necessary for the induction of LCD. Overall, our results demonstrate a robust HT platform to identify novel LCD inducers that will also be very useful for gaining deeper insights into the molecular mechanism of LCD induction.

Chemoenzymatic synthesis and antileukemic activity of novel C9- and C14-functionalized parthenolide analogs

Parthenolide is a naturally occurring terpene with promising anticancer properties, particularly in the context of acute myeloid leukemia (AML). Optimization of this natural product has been challenged by limited opportunities for the late-stage functionalization of this molecule without affecting the pharmacologically important α-methylene-γ-lactone moiety. Here, we report the further development and application of a chemoenzymatic strategy to afford a series of new analogs of parthenolide functionalized at the aliphatic positions C9 and C14. Several of these compounds were determined to be able to kill leukemia cells and patient-derived primary AML specimens with improved activity compared to parthenolide, exhibiting LC50 values in the low micromolar range. These studies demonstrate that different O-H functionalization chemistries can be applied to elaborate the parthenolide scaffold and that modifications at the C9 or C14 position can effectively enhance the antileukemic properties of this natural product. The C9-functionalized analogs 22a and 25b were identified as the most interesting compounds in terms of antileukemic potency and selectivity toward AML versus healthy blood cells.

Simple Preparation of Rhodococcus erythropolis DSM 44534 as Biocatalyst to Oxidize Diols into the Optically Active Lactones

In the current study, we present a green toolbox to produce ecological compounds like lactone moiety. Rhodococcus erythropolis DSM 44534 cells have been used to oxidize both decane-1,4-diol () and decane-1,5-diol () into the corresponding γ- () and δ-decalactones () with yield of 80% and enantiomeric excess (ee) = 75% and ee = 90%, respectively. Among oxidation of meso diols, (-)-(1S,5R)-cis-3-oxabicyclo[4.3.0]non-7-en-2-one (5a) with 56% yield and ee = 76% as well as (-)-(2R,3S)-cis-endo-3-oxabicyclo[2.2.1]dec-7-en-2-one (6a) with 100% yield and ee = 90% were formed. It is worth mentioning that R. erythropolis DSM 44534 grew in a mineral medium containing ethanol as the sole source of energy and carbon Chirality 28:623-627, 2016. © 2016 Wiley Periodicals, Inc.

Platinum-catalyzed cycloisomerizations of a common enyne: a divergent entry to cyclopropane sesquiterpenoids. Formal synthesis of sarcandralactone A

A common enyne scaffold, resembling the structures of natural elemanes was found to be an excellent substrate for highly regioselective cycloisomerizations to produce diverse cyclopropane sesquiterpenoids. Platinum-catalysis was utilized to produce either lindenane or myliol cores, found in natural products, starting from enyne acetate 10 and its corresponding allene 12 respectively. Based on this concept, a second generation strategy allows the formal synthesis of sarcandralactone A.

Cyclic AMP efflux inhibitors as potential therapeutic agents for leukemia

Apoptotic evasion is a hallmark of cancer. We propose that some cancers may evade cell death by regulating 3'-5'-cyclic adenosine monophosphate (cAMP), which is associated with pro-apoptotic signaling. We hypothesize that leukemic cells possess mechanisms that efflux cAMP from the cytoplasm, thus protecting them from apoptosis. Accordingly, cAMP efflux inhibition should result in: cAMP accumulation, activation of cAMP-dependent downstream signaling, viability loss, and apoptosis. We developed a novel assay to assess cAMP efflux and performed screens to identify inhibitors. In an acute myeloid leukemia (AML) model, several identified compounds reduced cAMP efflux, appropriately modulated pathways that are responsive to cAMP elevation (cAMP-responsive element-binding protein phosphorylation, and deactivation of Very Late Antigen-4 integrin), and induced mitochondrial depolarization and caspase activation. Blocking adenylyl cyclase activity was sufficient to reduce effects of the most potent compounds. These compounds also decreased cAMP efflux and viability of B-lineage acute lymphoblastic leukemia (B-ALL) cell lines and primary patient samples, but not of normal primary peripheral blood mononuclear cells. Our data suggest that cAMP efflux is a functional feature that could be therapeutically targeted in leukemia. Furthermore, because some of the identified drugs are currently used for treating other illnesses, this work creates an opportunity for repurposing.

Targeting Thioredoxin Reductase by Parthenolide Contributes to Inducing Apoptosis of HeLa Cells

Parthenolide (PTL), a major active sesquiterpene lactone from the herbal plant Tanacetum parthenium, has been applied in traditional Chinese medicine for centuries. Although PTL demonstrates potent anticancer efficacy in numerous types of malignant cells, the cellular targets of PTL have not been well defined. We reported here that PTL interacts with both cytosolic thioredoxin reductase (TrxR1) and mitochondrial thioredoxin reductase (TrxR2), two ubiquitous selenocysteine-containing antioxidant enzymes, to elicit reactive oxygen species-mediated apoptosis in HeLa cells. PTL selectively targets the selenocysteine residue in TrxR1 to inhibit the enzyme function, and further shifts the enzyme to an NADPH oxidase to generate superoxide anions, leading to reactive oxygen species accumulation and oxidized thioredoxin. Under the conditions of inhibition of TrxRs in cells, PTL does not cause significant alteration of cellular thiol homeostasis, supporting selective target of TrxRs by PTL. Importantly, overexpression of functional TrxR1 or Trx1 confers protection, whereas knockdown of the enzymes sensitizes cells to PTL treatment. Targeting TrxRs by PTL thus discloses an unprecedented mechanism underlying the biological activity of PTL, and provides deep insights to understand the action of PTL in treatment of cancer.

Dehydroleucodine, a Sesquiterpene Lactone from Gynoxys verrucosa, Demonstrates Cytotoxic Activity against Human Leukemia Cells

The sesquiterpene lactones dehydroleucodine (1) and leucodine (2) were isolated from Gynoxys verrucosa, a species used in traditional medicine in southern Ecuador. The activity of these compounds was determined against eight acute myeloid leukemia (AML) cell lines and compared with their activity against normal peripheral blood mononuclear cells. Compound 1 showed cytotoxic activity against the tested cell lines, with LD50 values between 5.0 and 18.9 μM. Compound 2 was inactive against all of the tested cell lines, demonstrating that the exocyclic methylene in the lactone ring is required for cytotoxic activity. Importantly, compound 1 induced less toxicity to normal blood cells than to AML cell lines and was active against human AML cell samples from five patients, with an average LD50 of 9.4 μM. Mechanistic assays suggest that compound 1 has a similar mechanism of action to parthenolide (3). Although these compounds have significant structural differences, their lipophilic surface signatures show striking similarities.

Metabolomic Profile of the Genus Inula

Plants have a long history as therapeutics in the treatment of human diseases and have been used as source of medicines for ages. Searching for new biologically active natural products, many plants and herbs are screened for natural products with pharmacological activities. In this field, the genus Inula, which comprises more than 100 species, several of them being used in traditional medicine, is very important, especially due to the finding that several of the isolated pure secondary metabolites proved to possess important biological activities. Inula species have been reported as rich sources of sesquiterpene lactones, including eudesmanes, germacranes, guaianes, and dimeric structures, and since 2006 ca. 400 secondary metabolites, including more than 100 new natural products, some of them with relevant pharmacological activities, have been identified. Herein, we critically compile and update the information regarding the types of secondary metabolites found in the genus Inula and the progress in their isolation.

Investigation of the Antiproliferative Properties of Natural Sesquiterpenes from Artemisia asiatica and Onopordum acanthium on HL-60 Cells in Vitro

Plants and plant extracts play a crucial role in the research into novel antineoplastic agents. Four sesquiterpene lactones, artecanin (1), 3β-chloro-4α,10α-dihydroxy-1α,2α-epoxy-5α,7αH-guaia-11(13)-en-12,6α-olide (2), iso-seco-tanapartholide 3-O-methyl ether (3) and 4β,15-dihydro-3-dehydrozaluzanin C (4), were isolated from two traditionally used Asteraceae species (Onopordum acanthium and Artemisia asiatica). When tested for antiproliferative action on HL-60 leukemia cells, these compounds exhibited reasonable IC₅₀ values in the range 3.6–13.5 μM. Treatment with the tested compounds resulted in a cell cycle disturbance characterized by increases in the G1 and G2/M populations, while there was a decrease in the S phase. Additionally, 1–3 elicited increases in the hypodiploid (subG1) population. The compounds elicited concentration-dependent chromatin condensation and disruption of the membrane integrity, as revealed by Hoechst 33258–propidium staining. Treatment for 24 h resulted in significant increases in activity of caspases-3 and -9, indicating that the tested sesquiterpenes induced the mitochondrial pathway of apoptosis. The proapoptotic properties of the sesquiterpene lactones were additionally demonstrated withannexin V staining. Compounds 1 and 2 increased the Bax/Bcl-2 expression and decreased the expressions of CDK1 and cyclin B2, as determined at the mRNA level by means of RT-PCR. These experimental results indicate that sesquiterpene lactones may be regarded as potential starting structures for the development of novel anticancer agents.

Molecular mechanism of apoptosis induction by Gaillardin, a sesquiterpene lactone, in breast cancer cell lines : Gaillardin-induced apoptosis in breast cancer cell lines

Medicinal plant extracts have been widely used for cancer treatment. Gaillardin is a natural sesquiterpene lactone that has recently been reported to have anticancer properties. The ability to induce apoptosis is an important property of a candidate anticancer drug, which discriminates between anticancer drugs and toxic compounds. The current study was therefore carried out to address the issue if Gaillardin is able to induce apoptosis in the breast cancer cell lines MCF-7 and MDA-MB-468 and to determine the underlying mechanism of its anticancer effects. Apoptosis induction by Gaillardin treatment was confirmed by annexin V-FITC/PI staining, and caspase-3,-6, and-9 activation. Using Western blot analysis, we found that Gaillardin upregulated the pro-apoptotic protein Bax and p53 and downregulated the anti-apoptotic protein Bcl-2. Moreover, the apoptotic effect of Gaillardin was also related to ROS production and loss of mitochondrial membrane potential (ΔΨm). Taken together, these results demonstrate that Gaillardin can inhibit proliferation of breast cancer cells via inducing mitochondrial apoptotic pathway and therefore, might be a promising molecule in cancer chemoprevention or chemotherapy.

Metal-catalyzed cycloisomerization as a powerful tool in the synthesis of complex sesquiterpenoids

Metal-catalyzed cycloisomerization reactions can be regarded as “artificial sesquiterpenoid cyclase” tools in modern organic synthesis, and their progress is reviewed.

Anti-inflammatory sesquiterpenes from Costus speciosus rhizomes

ETHNOPHARMACOLOGICAL RELEVANCE

Costus speciosus (Koen ex. Retz.) Sm. (crepe ginger, family Costaceae) is an ornamental plant used in traditional medicine for the treatment of inflammation, rheumatism, bronchitis, fever, headache, asthma, flatulence, constipation, helminthiasis, leprosy, skin diseases, hiccough, anemia, as well as burning sensation on urination.

AIM OF THE STUDY

The present study is designed to isolate and identify the active compounds from C. speciosus rhizomes and measure their anti-inflammatory activities.

MATERIALS AND METHODS

The n-hexane-CHCl3 soluble fraction of the MeOH extract of C. speciosus rhizomes has been subjected to a repeated column chromatography, including normal silica gel and RP-18 column to give eight compounds. The structures of these compounds were established by UV, IR, 1D ((1)H and (13)C), and 2D ((1)H-(1)H COSY, NOESY, HSQC, and HMBC) NMR experiments and HRESIMS data. In addition, the anti-inflammatory activity of compounds 1-8 was evaluated by measuring the levels IL-6, IL-1β, TNF-α, COX-2, lipoxgenase-5, and PGE2 using enzyme-linked immunosorbent assay.

RESULTS

The n-hexane-CHCl3 soluble fraction afforded a new eudesmane acid, specioic acid (8), along with seven known compounds, 22,23-dihydrospinasterone (1), dehydrodihydrocostus lactone (mokko lactone) (2), dehydrocostus lactone (3), stigmasterol (4), arbusculin A (5), santamarine (douglanin) (6), and reynosin (7). Compounds 1, 4, and 5-7 were isolated for the first time C. speciosus. Compounds 1-4 displayed potent anti-inflammatory activity, while 7 and 8 showed moderate activity. Compounds 1-8 exhibited a concentration-related decrease in the levels of IL-1β, IL-6, TNF-α, PGE2, lipoxgenase-5, and COX-2. Compounds 5 and 6 did not significantly decrease levels of different cytokines, PGE2, lipoxgenase-5, and COX-2 from PHA treatment at 1 µM. However, all tested compounds significantly decreased cytokines, PGE2, lipoxgenase-5, and COX-2 levels at concentration 100 µM. It is noteworthy that compounds 1-4 had the highest activity, where it lowered levels of cytokines, PGE2, lipoxgenase-5, and COX-2 to the extent that was no statistical difference from the control group. Thus, they decreased proinflammatory cytokines (IL-1β, IL-6, and TNF-α) with decreased level of the target enzymes (COX-2 and lipoxgenase-5) and subsequent reduction of its inflammatory product (PGE2).

CONCLUSION

Good anti-inflammatory activities exhibited of the isolated compounds from C. speciosus corroborate the usefulness of this plant in the traditional treatment of inflammation and related symptoms.

Detyrosinated microtubules modulate mechanotransduction in heart and skeletal muscle

In striated muscle, X-ROS is the mechanotransduction pathway by which mechanical stress transduced by the microtubule network elicits reactive oxygen species. X-ROS tunes Ca(2+) signalling in healthy muscle, but in diseases such as Duchenne muscular dystrophy (DMD), microtubule alterations drive elevated X-ROS, disrupting Ca(2+) homeostasis and impairing function. Here we show that detyrosination, a post-translational modification of α-tubulin, influences X-ROS signalling, contraction speed and cytoskeletal mechanics. In the mdx mouse model of DMD, the pharmacological reduction of detyrosination in vitro ablates aberrant X-ROS and Ca(2+) signalling, and in vivo it protects against hallmarks of DMD, including workload-induced arrhythmias and contraction-induced injury in skeletal muscle. We conclude that detyrosinated microtubules increase cytoskeletal stiffness and mechanotransduction in striated muscle and that targeting this post-translational modification may have broad therapeutic potential in muscular dystrophies.

The sesquiterpene lactone polymatin B from Smallanthus sonchifolius induces different cell death mechanisms in three cancer cell lines

A 8β-angeloyloxy-9α-hydroxy-14-oxo-acanthospermolide and five known melampolide sesquiterpene lactones (uvedalin, enhydrin, polymatin B, sonchifolin, and fluctuanin) were isolated from the leaves of Smallanthus sonchifolius. The compounds were identified by 1D-, 2D-NMR, HRMS, IR and UV analyses. In vitro cytotoxicity assays (MTT) showed that these sesquiterpene lactones display poor cytotoxic effects on peripheral blood mononuclear cells (PBMC) of healthy human subjects, whereas a strong cytotoxicity was observed in leukemia and pancreas cancer cells. For the mechanism of action of polymatin B, oxidative stress seems to be involved. Interestingly, reactive oxygen species (ROS) formation mainly induced different effects: apoptosis in CCRF-CEM cells, necroptosis in CEM-ADR5000 cells through induction of RIP1K, neither apoptosis nor necroptosis in MIA-PaCa-2 cells. Additionally, cells also died partly by necrosis.

Syntheses and Biological Evaluation of Costunolide, Parthenolide, and Their Fluorinated Analogues

Inspired by the biosynthesis of sesquiterpene lactones (SLs), herein we report the asymmetric total synthesis of the germacrane ring (24). The synthetic strategy features a selective aldol reaction between β,γ-unsaturated chiral sulfonylamide 15a and aldehyde 13, as well as the intramolecular α-alkylation of sulfone 21 to construct a 10-membered carbocylic ring. The key intermediate 24 can be used to prepare the natural products costunolide and parthenolide (PTL), which are the key precursors for transformation into other SLs. Furthermore, the described synthetic sequences are amenable to the total synthesis of SL analogues, such as trifluoromethylated analogues 32 and 45. Analogues 32 and 45 maintained high activities against a series of cancer cell lines compared to their parent PTL and costunolide, respectively. In addition, 32 showed enhanced tolerance to acidic media compared with PTL. To our surprise, PTL and 32 showed comparable half-lives in rat plasma and in the presence of human liver microsomes.

Synthesis and antileukemic activities of C1-C10-modified parthenolide analogues

Parthenolide (PTL) is a sesquiterpene lactone natural product with anti-proliferative activity to cancer cells. Selective eradication of leukemic stem cells (LSCs) over healthy hematopoietic stem cells (HSCs) by PTL has been demonstrated in previous studies, which suggests PTL and related molecules may be useful for targeting LSCs. Eradication of LSCs is required for curative therapy. Chemical optimizations of PTL to improve potency and pharmacokinetic parameters have focused largely on the α-methylene-γ-butyrolactone, which is essential for activity. Conversely, we evaluated modifications to the C1-C10 olefin and benchmarked new inhibitors to PTL with respect to inhibitory potency across a panel of cancer cell lines, ability to target drug-resistant acute myeloid leukemia (AML) cells, efficacy for inhibiting clonal growth of AML cells, toxicity to healthy bone marrow cells, and efficiency for promoting intracellular reactive oxygen species (ROS) levels. Cyclopropane 4 was found to possess less toxicity to healthy bone marrow cells, enhanced potency for the induction of cellular ROS, and similar broad-spectrum anti-proliferative activity to cancer cells in comparison to PTL.

Synergistic effects of the sesquiterpene lactone, EPD, with cisplatin and paclitaxel in ovarian cancer cells

BACKGROUND

Ovarian cancer remains still the leading cause of death of gynecological malignancy, in spite of first-line chemotherapy with cisplatin and paclitaxel. Although initial response is favorably, relapses are common and prognosis for women with advanced disease stays poor. Therefore efficacious approaches are needed.

METHODS

Previously, an anti-cancer agent, EPD exhibited potent cytotoxic effects towards ovarian cancer and not towards normal cells. Cell viability and cell cycle analysis studies were performed with EPD, in combination with cisplatin and/or paclitaxel, using the ovarian carcinoma cell lines: SK-OV-3, OVCAR-3, JC, JC-pl and normal fibroblasts. Cell viability was measured using Presto Blue and cell cycle analysis using a flow cytometer. Apoptosis was measured in JC and JC-pl , using the caspase 3 assay kit.

RESULTS

In JC-pl, SK-OV-3 and JC, synergistic interactions between either EPD and cisplatin or EPD and paclitaxel were observed. For the first time the effects of EPD on the cell cycle of ovarian cancer cells and normal cells was studied. EPD and combinations of EPD with cisplatin and/ or paclitaxel showed cell cycle arrest in the G2/M phase. The combination of EPD and cisplatin showed a significant synergistic effect in cell line JC-pl, while EPD with paclitaxel showed synergistic interaction in JC. Additionally, synergistic drug combinations showed increased apoptosis.

CONCLUSIONS

Our results showed a synergistic effect of EPD and cisplatin in an ovarian drug resistant cell line as well as a synergistic effect of EPD and paclitaxel in two other ovarian cell lines. These results might enhance clinical efficacy, compared to the existing regimen of paclitaxel and cisplatin.

Inhibition of AMPK/autophagy potentiates parthenolide-induced apoptosis in human breast cancer cells

Parthenolide is the main bioactive component in feverfew, a common used herbal medicine, and has been extensively studied in relation to its anti-cancer properties. However there have been very few in-depth studies of the activities of this compound at the molecular level. Here, we showed that parthenolide increased reactive oxygen species (ROS), induced cell death, activated AMPK and autophagy, and led to M phase cell cycle arrest in breast cancer cells. Removal of ROS inhibited all parthenolide-associated events, such as cell death, AMPK activation, autophagy induction, and cell cycle arrest. Blockade of autophagy relieved cell cycle arrest, whereas inhibition of AMPK activity significantly repressed the induction of both autophagy and cell cycle arrest. These observations clearly showed that parthenolide-driven ROS activated AMPK-autophagy pathway. Furthermore, inhibition of either AMPK or autophagy significantly potentiated parthenolide-induced apoptosis. Therefore, our results show that parthenolide activates both apoptosis pathway and AMPK-autophagy survival pathway through the generation of ROS, and that suppression of AMPK or autophagy can potentially enhance the anti-cancer effect of parthenolide on breast cancer cells.

Systemic deterrence of aphid probing and feeding by novel β-damascone analogues

β-Damascone appeared a weak attractant close to not active to Myzus persicae, but modifications of its structure caused the avoidance of treated leaves by aphids during settling and reluctance to probe in simple choice- and no-choice experiments in previous studies. Here, the electrical penetration graph (EPG) technique, which allows monitoring of aphid probing within plant tissues, was applied to explore the biological background and localisation in plant tissues of the deterrent activities of β-damascone and its analogues. Activity of β-damascone and β-damascone-derived compounds depended on their substituents, which was manifested in the variation in the potency of the behavioural effect and differences in aphid probing phases that were affected. β-Damascone appeared a behaviourally inactive compound. The moderately active β-damascone ester affected aphid activities only during the phloem phase. The highly active deterrents-dihydro-β-damascol, β-damascone acetate, δ-bromo-γ-lactone, and unsaturated γ-lactone-affected pre-phloem and phloem aphid probing activities. The most effective structural modification that evoked the strongest negative response from M. persicae was the transformation of β-damascone into δ-bromo-γ-lactone. The behavioural effect of this transformation was demonstrated in frequent interruption of probing in peripheral tissues, which caused repeated failures in finding sieve elements, and reduction in the ingestion time during the phloem phase in favour of watery salivation. The inhibition of aphid probing at both the pre-phloem and phloem levels reveals the passage of the compounds studied through the plant surface and their distribution within plant tissues in a systemic way, which may reduce the risk of the transmission of non-persistent and persistent viruses.

Divergent pathways to furosesquiterpenes: first total syntheses of (+)-zedoarol and (Rac)-gweicurculactone

Divergent transformation of a non-natural elemane to furo-guaiane and furo-cadinane cores.

Sesquiterpene Lactones from Artemisia Genus: Biological Activities and Methods of Analysis

Sesquiterpene lactones are a large group of natural compounds, found primarily in plants of Asteraceae family, with over 5000 structures reported to date. Within this family, genus Artemisia is very well represented, having approximately 500 species characterized by the presence of eudesmanolides and guaianolides, especially highly oxygenated ones, and rarely of germacranolides. Sesquiterpene lactones exhibit a wide range of biological activities, such as antitumor, anti-inflammatory, analgesic, antiulcer, antibacterial, antifungal, antiviral, antiparasitic, and insect deterrent. Many of the biological activities are attributed to the α-methylene-γ-lactone group in their molecule which reacts through a Michael-addition with free sulfhydryl or amino groups in proteins and alkylates them. Due to the fact that most sesquiterpene lactones are thermolabile, less volatile compounds, they present no specific chromophores in the molecule and are sensitive to acidic and basic mediums, and their identification and quantification represent a difficult task for the analyst. Another problematic aspect is represented by the complexity of vegetal samples, which may contain compounds that can interfere with the analysis. Therefore, this paper proposes an overview of the methods used for the identification and quantification of sesquiterpene lactones found in Artemisia genus, as well as the optimal conditions for their extraction and separation.

Targeting apoptosis pathways in cancer and perspectives with natural compounds from mother nature

Although the incidences are increasing day after day, scientists and researchers taken individually or by research group are trying to fight against cancer by several ways and also by different approaches and techniques. Sesquiterpenes, flavonoids, alkaloids, diterpenoids, and polyphenolic represent a large and diverse group of naturally occurring compounds found in a variety of fruits, vegetables, and medicinal plants with various anticancer properties. In this review, our aim is to give our perspective on the current status of the natural compounds belonging to these groups and discuss their natural sources, their anticancer activity, their molecular targets, and their mechanism of actions with specific emphasis on apoptosis pathways, which may help the further design and conduct of preclinical and clinical trials. Unlike pharmaceutical drugs, the selected natural compounds induce apoptosis by targeting multiple cellular signaling pathways including transcription factors, growth factors, tumor cell survival factors, inflammatory cytokines, protein kinases, and angiogenesis that are frequently deregulated in cancers and suggest that their simultaneous targeting by these compounds could result in efficacious and selective killing of cancer cells. This review suggests that they provide a novel opportunity for treatment of cancer, but clinical trials are still required to further validate them in cancer chemotherapy.

The genus Inula and their metabolites: from ethnopharmacological to medicinal uses

ETHNOPHARMACOLOGICAL RELEVANCE

The genus Inula comprises more than one hundred species widespread in temperate regions of Europe and Asia. Uses of this genus as herbal medicines have been first recorded by the Greek and Roman ancient physicians. In the Chinese Pharmacopoeia, from the 20 Inula spp. distributed in China, three are used as Traditional Chinese medicines, named Tumuxiang, Xuanfuhua and Jinfeicao. These medicines are used as expectorants, antitussives, diaphoretics, antiemetics, and bactericides. Moreover, Inula helenium L. which is mentioned in Minoan, Mycenaean, Egyptian/Assyrian pharmacotherapy and Chilandar Medical Codex, is good to treat neoplasm, wound, freckles and dandruff. Many other Inula spp. are used in Ayurvedic and Tibetan traditional medicinal systems for the treatment of diseases such as bronchitis, diabetes, fever, hypertension and several types of inflammation. This review is a critical evaluation of the published data on the more relevant ethnopharmacological and medicinal uses of Inula spp. and on their metabolites biological activities. This study allows the identification of the ethnopharmacological knowledge of this genus and will provide insight into the emerging pharmacological applications of Inula spp. facilitating the prioritirization of future investigations. The corroboration of the ethnopharmacological applications described in the literature with proved biological activities of Inula spp. secondary metabolites will also be explored.

MATERIALS AND METHODS

The major scientific databases including ScienceDirect, Medline, Scopus and Web of Science were queried for information on the genus Inula using various keyword combinations, more than 180 papers and patents related to the genus Inula were consulted. The International Plant Name Index was also used to confirm the species names.

RESULTS

Although the benefits of Inula spp. are known for centuries, there are insufficient scientific studies to certify it. Most of the patents are registered by Chinese researchers, proving the traditional use of these plants in their country. Although a total of sixteen Inula species were reported in the literature to have ethnopharmacological applications, the species Inula cappa (Buch.-Ham. ex D.Don) DC., Inula racemosa Hook.f., Inula viscosa (L.) Aiton [actually the accepted name is Dittrichia viscosa (L.) Greuter], Inula helenium, Inula britannica L. and Inula japonica Thunb. are the most frequently cited ones since their ethnopharmacological applications are vast. They are used to treat a large spectrum of disorders, mainly respiratory, digestive, inflammatory, dermatological, cancer and microbial diseases. Fifteen Inula spp. crude extracts were investigated and showed interesting biological activities. From these, only 7 involved extracts of the reported spp. used in traditional medicine and 6 of these were studied to isolate the bioactive compounds. Furthermore, 90 bioactive compounds were isolated from 16 Inula spp. The characteristic compounds of the genus, sesquiterpene lactones, are involved in a network of biological effects, and in consequence, the majority of the experimental studies are focused on these products, especially on their cytotoxic and anti-inflammatory activities. The review shows the chemical composition of the genus Inula and presents the pharmacological effects proved by in vitro and in vivo experiments, namely the cytotoxic, anti-inflammatory (with focus on nitric oxide, arachidonic acid and NF-κB pathways), antimicrobial, antidiabetic and insecticidal activities.

CONCLUSIONS

Although there are ca. 100 species in the genus Inula, only a few species have been investigated so far. Eight of the sixteen Inula spp. with ethnopharmacological application had been subjected to biological evaluations and/or phytochemical studies. Despite Inula royleana DC. and Inula obtusifolia A. Kerner are being used in traditional medicine, as far as we are aware, these species were not subjected to phytochemical or pharmacological studies. The biological activities exhibited by the compounds isolated from Inula spp., mainly anti-inflammatory and cytotoxic, support some of the described ethnopharmacological applications. Sesquiterpene lactone derivatives were identified as the most studied class, being britannilactone derivatives the most active ones and present high potential as anti-inflammatory drugs, although, their pharmacological effects, dose-response relationship and toxicological investigations to assess potential for acute or chronic adverse effects should be further investigated. The experimental results are promising, but the precise mechanism of action, the compound or extract toxicity, and the dose to be administrated for an optimal effect need to be investigated. Also human trials (some preclinical studies proved to be remarkable) should be further investigated. The genus Inula comprises species useful not only in medicine but also in other domains which makes it a high value-added plant.

The oxygen radicals involved in the toxicity induced by parthenolide in MDA-MB-231 cells

It has been shown that the sesquiterpene lactone parthenolide lowers the viability of MDA-MB-231 breast cancer cells, in correlation with oxidative stress. The present report examined the different radical species produced during parthenolide treatment and their possible role in the toxicity caused by the drug. Time course experiments showed that in the first phase of treatment (0-8 h), and in particular in the first 3 h, parthenolide induced dichlorofluorescein (DCF) signal in a large percentage of cells, while dihydroethidium (DHE) signal was not stimulated. Since the effect on DCF signal was suppressed by apocynin and diphenyleneiodonium (DPI), two inhibitors of NADPH oxidase (NOX), we suggest that parthenolide rapidly stimulated NOX activity with production of superoxide anion (O2•-), which was converted by superoxide dismutase 1 (SOD1) into hydrogen peroxide (H2O2). In the second phase of treatment (8-16 h), parthenolide increased the number of positive cells to DHE signal. Since this event was not prevented by apocynin and DPI and was associated with positivity of cells to MitoSox Red, a fluorochrome used to detect mitochondrial production of O2•-, we suggest that parthenolide induced production of O2•- at the mitochondrial level independently by NOX activity in the second phase of treatment. Finally, in this phase, most cells became positive to hydroxyphenyl fluorescein (HPF) signal, a fluorescent probe to detect highly reactive oxygen species (hROS), such as hydroxyl radical and peroxynitrite. Therefore, parthenolide between 8-16 h of treatment induced generation of O2•- and hROS, in close correlation with a marked reduction in cell viability.

Natural compounds as potential treatments of NF2-deficient schwannoma and meningioma: cucurbitacin D and goyazensolide

HYPOTHESIS

Cucurbitacin D and goyazensolide, 2 plant-derived natural compounds, possess potent growth-inhibitory activity in schwannoma and meningioma cells.

BACKGROUND

Currently, no FDA-approved drugs are available for neurofibromatosis type 2 (NF2)-associated schwannomas and meningiomas. Selected natural compounds with antineoplastic activity, such as cucurbitacin D and goyazensolide, may be developed as potential treatments for these tumors.

METHODS

The Nf2-deficient mouse schwannoma Sch10545 and human benign meningioma Ben-Men-1 cells were treated with various concentrations of cucurbitacin D and goyazensolide. The effect on cell proliferation was determined using resazurin assays. Flow cytometry was used to assess the cell cycle profiles. Western blot analysis was performed to investigate the expression of various signaling molecules related to the cell cycle and the AKT pathway.

RESULTS

Cucurbitacin D inhibited proliferation of Sch10545 cells (IC50 ∼ 0.75 μM) and Ben-Men-1 cells (IC50 ∼0.2 μM). Goyazensolide also reduced cell proliferation of Sch10545 cells (IC50 ∼0.9 μM) and Ben-Men-1 cells (IC50 ∼1 μM). The G2/M population increased in both Sch10545 and Ben-Men-1 cells treated with cucurbitacin D or goyazensolide around the IC50. Cucurbitacin and goyazensolide substantially reduced the levels of cyclins E and A in treated Sch10545 and Ben-Men-1 cells. Cucurbitacin D also inhibited cyclin B, phospho-AKT and phospho-PRAS40 expression. In addition, goyazensolide reduced the levels of phospho-AKT and NFκB and increased the expression of pro-apoptotic Bim in Sch10545 and Ben-Men-1 cells.

CONCLUSION

Both cucurbitacin D and goyazensolide effectively inhibit proliferation of NF2-deficient schwannoma and meningioma cells, suggesting that these natural compounds should be further evaluated as potential treatments for NF2-related tumors.

Elucidation and in planta reconstitution of the parthenolide biosynthetic pathway

Parthenolide, the main bioactive compound of the medicinal plant feverfew (Tanacetum parthenium), is a promising anti-cancer drug. However, the biosynthetic pathway of parthenolide has not been elucidated yet. Here we report on the isolation and characterization of all the genes from feverfew that are required for the biosynthesis of parthenolide, using a combination of 454 sequencing of a feverfew glandular trichome cDNA library, co-expression analysis and metabolomics. When parthenolide biosynthesis was reconstituted by transient co-expression of all pathway genes in Nicotiana benthamiana, up to 1.4μgg(-1) parthenolide was produced, mostly present as cysteine and glutathione conjugates. These relatively polar conjugates were highly active against colon cancer cells, with only slightly lower activity than free parthenolide. In addition to these biosynthetic genes, another gene encoding a costunolide and parthenolide 3β-hydroxylase was identified opening up further options to improve the water solubility of parthenolide and therefore its potential as a drug.

Discovery of potent parthenolide-based antileukemic agents enabled by late-stage P450-mediated C-H functionalization

The sesquiterpene lactone parthenolide has recently attracted considerable attention owing to its promising antitumor properties, in particular in the context of stem-cell cancers including leukemia. Yet, the lack of viable synthetic routes for re-elaborating this complex natural product has represented a fundamental obstacle toward further optimization of its pharmacological properties. Here, we demonstrate how this challenge could be addressed via selective, late-stage sp(3) C-H bond functionalization mediated by P450 catalysts with tailored site-selectivity. Taking advantage of our recently introduced tools for high-throughput P450 fingerprinting and fingerprint-driven P450 reactivity prediction, we evolved P450 variants useful for carrying out the highly regioselective hydroxylation of two aliphatic sites (C9 and C14) in parthenolide carbocyclic backbone. By chemoenzymatic synthesis, a panel of novel C9- and C14-modified parthenolide analogs were generated in order to gain initial structure-activity insights on these previously inaccessible sites of the molecule. Notably, some of these compounds were found to possess significantly improved antileukemic potency against primary acute myeloid leukemia cells, while exhibiting low toxicity against normal mature and progenitor hematopoietic cells. By identifying two 'hot spots' for improving the anticancer properties of parthenolide, this study highlights the potential of P450-mediated C-H functionalization as an enabling, new strategy for the late-stage manipulation of bioactive natural product scaffolds.

Phytoagents for cancer management: regulation of nucleic acid oxidation, ROS, and related mechanisms

Accumulation of oxidized nucleic acids causes genomic instability leading to senescence, apoptosis, and tumorigenesis. Phytoagents are known to reduce the risk of cancer development; whether such effects are through regulating the extent of nucleic acid oxidation remains unclear. Here, we outlined the role of reactive oxygen species in nucleic acid oxidation as a driving force in cancer progression. The consequential relationship between genome instability and cancer progression highlights the importance of modulation of cellular redox level in cancer management. Current epidemiological and experimental evidence demonstrate the effects and modes of action of phytoagents in nucleic acid oxidation and provide rationales for the use of phytoagents as chemopreventive or therapeutic agents. Vitamins and various phytoagents antagonize carcinogen-triggered oxidative stress by scavenging free radicals and/or activating endogenous defence systems such as Nrf2-regulated antioxidant genes or pathways. Moreover, metal ion chelation by phytoagents helps to attenuate oxidative DNA damage caused by transition metal ions. Besides, the prooxidant effects of some phytoagents pose selective cytotoxicity on cancer cells and shed light on a new strategy of cancer therapy. The “double-edged sword” role of phytoagents as redox regulators in nucleic acid oxidation and their possible roles in cancer prevention or therapy are discussed in this review.

Sesquiterpene lactones from Ferula oopoda and their cytotoxic properties

Two new sesquiterpene lactones, namely feruhodin A and feruhodin B, together with six known compounds, daucoeudesmanolactone, dehydrooopodin, oopodin, badkhysin, 7-demethylplastochromenol, and scoparone, were isolated from the roots of Ferula oopoda. The structures of these compounds were elucidated by 1D and 2D NMR techniques as well as high-resolution mass spectrometry. Cytotoxic effects of these compounds were evaluated against two human cancer cell lines including MCF7and K562 using the Alamar blue assay. The results showed that dehydrooopodin possessed significant cytotoxic effects with IC₅₀ values of 15 and 5 μM against MCF7 and K562, respectively.

Autophagy-dependent metabolic reprogramming sensitizes TSC2-deficient cells to the antimetabolite 6-aminonicotinamide

The mammalian target of rapamycin complex 1 (mTORC1) is hyperactive in many human cancers and in tuberous sclerosis complex (TSC). Autophagy, a key mTORC1-targeted process, is a critical determinant of metabolic homeostasis. Metabolomic profiling was performed to elucidate the cellular consequences of autophagy dysregulation under conditions of hyperactive mTORC1. It was discovered that TSC2-null cells have distinctive autophagy-dependent pentose phosphate pathway (PPP) alterations. This was accompanied by enhanced glucose uptake and utilization, decreased mitochondrial oxygen consumption, and increased mitochondrial reactive oxygen species (ROS) production. Importantly, these findings revealed that the PPP is a key autophagy-dependent compensatory metabolic mechanism. Furthermore, PPP inhibition with 6-aminonicotinamide (6-AN) in combination with autophagy inhibition suppressed proliferation and prompted the activation of NF-κB and CASP1 in TSC2-deficient, but not TSC2-proficient cells. These data demonstrate that TSC2-deficient cells can be therapeutically targeted, without mTORC1 inhibitors, by focusing on their metabolic vulnerabilities.

IMPLICATIONS

This study provides proof-of-concept that therapeutic targeting of diseases with hyperactive mTORC1 can be achieved without the application of mTORC1 inhibitors.

Targeting aberrant glutathione metabolism to eradicate human acute myelogenous leukemia cells

The development of strategies to eradicate primary human acute myelogenous leukemia (AML) cells is a major challenge to the leukemia research field. In particular, primitive leukemia cells, often termed leukemia stem cells, are typically refractory to many forms of therapy. To investigate improved strategies for targeting of human AML cells we compared the molecular mechanisms regulating oxidative state in primitive (CD34(+)) leukemic versus normal specimens. Our data indicate that CD34(+) AML cells have elevated expression of multiple glutathione pathway regulatory proteins, presumably as a mechanism to compensate for increased oxidative stress in leukemic cells. Consistent with this observation, CD34(+) AML cells have lower levels of reduced glutathione and increased levels of oxidized glutathione compared with normal CD34(+) cells. These findings led us to hypothesize that AML cells will be hypersensitive to inhibition of glutathione metabolism. To test this premise, we identified compounds such as parthenolide (PTL) or piperlongumine that induce almost complete glutathione depletion and severe cell death in CD34(+) AML cells. Importantly, these compounds only induce limited and transient glutathione depletion as well as significantly less toxicity in normal CD34(+) cells. We further determined that PTL perturbs glutathione homeostasis by a multifactorial mechanism, which includes inhibiting key glutathione metabolic enzymes (GCLC and GPX1), as well as direct depletion of glutathione. These findings demonstrate that primitive leukemia cells are uniquely sensitive to agents that target aberrant glutathione metabolism, an intrinsic property of primary human AML cells.

Parthenolide enhances dacarbazine activity against melanoma cells

Dacarbazine induces a clinical response only in 15% of melanoma patients. New treatment strategies may involve combinations of drugs with different modes of action to target the tumor heterogeneity. We aimed to determine whether the combined treatment of heterogeneous melanoma cell populations in vitro with the alkylating agent dacarbazine and the nuclear factor-κB inhibitor parthenolide could be more effective than either drug alone. A panel of melanoma cell lines, including highly heterogeneous populations derived from surgical specimens, was treated with dacarbazine and parthenolide. The effect of drugs on the viable cell number was examined using an acid phosphatase activity assay, and the combination effect was determined by median-effect analysis. Cell death and cell-cycle arrest were assessed by flow cytometry. Gene expression was measured by real-time PCR and changes in the protein levels were evaluated by western blotting. Secretion of vascular endothelial growth factor and interleukin-8 was determined using an enzyme-linked immunosorbent assay. The self-renewing capacity was assessed using a clonogenic assay. Dacarbazine was less effective in heterogeneous melanoma populations than in the A375 cell line. Parthenolide and dacarbazine synergistically reduced the viable cell numbers. Both drugs induced cell-cycle arrest and apoptotic cell death. Importantly, parthenolide abrogated the baseline and dacarbazine-induced vascular endothelial growth factor secretion from melanoma cells in heterogeneous populations, whereas interleukin-8 secretion was not significantly affected by either drug. Parthenolide eradicated melanoma cells with self-renewing capacity also in cultures simultaneously treated with dacarbazine. The combination of parthenolide and dacarbazine might be considered as a new therapeutic modality against metastatic melanoma.

Targeting apoptosis pathways in cancer with alantolactone and isoalantolactone

Alantolactone and isoalantolactone, main bioactive compounds that are present in many medicinal plants such as Inula helenium, L. Inula japonica, Aucklandia lappa, Inula racemosa, and Radix inulae, have been found to have various pharmacological actions including anti-inflammatory, antimicrobial, and anticancer properties, with no significant toxicity. Recently, the anticancer activity of alantolactone and isoalantolactone has been extensively investigated. Here, our aim is to review their natural sources and their anticancer activity with specific emphasis on mechanism of actions, by which these compounds act on apoptosis pathways. Based on the literature and also on our previous results, alantolactone and isoalantolactone induce apoptosis by targeting multiple cellular signaling pathways that are frequently deregulated in cancers and suggest that their simultaneous targeting by these compounds could result in efficacious and selective killing of cancer cells. This review suggests that alantolactone and isoalantolactone are potential promising anticancer candidates, but additional studies and clinical trials are required to determine their specific intracellular sites of actions and derivative targets in order to fully understand the mechanisms of therapeutic effects to further validate in cancer chemotherapy.

Parthenolide from Parthenium integrifolium reduces tumor burden and alleviate cachexia symptoms in the murine CT-26 model of colorectal carcinoma

Excessive elaboration of proinflammatory cytokines are involved in cachexia-related hypercatabolism. Parthenolide, as a potential anti-inflammatory active agent, could effectively inhibit nuclear factor-kappa B, and has the potential for the treatment of cancer cachexia. In this study, the cancer cachexia model was established by subcutaneous transplantation CT26 tumor fragment. Parthenolide or placebo was intraperitoneally given daily from the next day. Parthenolide treatment could effectively preserve the body weight, improve the mass of gastrocnemius and tibialis anterior muscles, and alleviate tumor burden. Sizes of muscle fibers and myosin heavy chain were also increasing. The serum proinflammatory cytokine TNF-α level was lower than placebo treatment mice measure by ELISA. To investigate the possible mechanism, MuRF1 and Fbx32 was subjected to Western blot analysis and expression of MURF1 was inhibited in gastrocnemius muscle. Collectively, parthenolide treatment could effectively alleviate tumor burden of cachexia, preserve the body weight and improve skeletal muscle characteristics.