The synthetic caged garcinia xanthone cluvenone induces cell stress and apoptosis and has immune modulatory activity

Iron-Sulfur Clusters: A Key Factor of Regulated Cell Death in Cancer

Iron-sulfur clusters are ancient cofactors that play crucial roles in myriad cellular functions. Recent studies have shown that iron-sulfur clusters are closely related to the mechanisms of multiple cell death modalities. In addition, numerous previous studies have demonstrated that iron-sulfur clusters play an important role in the development and treatment of cancer. This review first summarizes the close association of iron-sulfur clusters with cell death modalities such as ferroptosis, cuprotosis, PANoptosis, and apoptosis and their potential role in cancer activation and drug resistance. This review hopes to generate new cancer therapy ideas and overcome drug resistance by modulating iron-sulfur clusters.

Mitochondrial iron-sulfur clusters: Structure, function, and an emerging role in vascular biology

Iron-sulfur (Fe-S) clusters are essential cofactors most commonly known for their role mediating electron transfer within the mitochondrial respiratory chain. The Fe-S cluster pathways that function within the respiratory complexes are highly conserved between bacteria and the mitochondria of eukaryotic cells. Within the electron transport chain, Fe-S clusters play a critical role in transporting electrons through Complexes I, II and III to cytochrome c, before subsequent transfer to molecular oxygen. Fe-S clusters are also among the binding sites of classical mitochondrial inhibitors, such as rotenone, and play an important role in the production of mitochondrial reactive oxygen species (ROS). Mitochondrial Fe-S clusters also play a critical role in the pathogenesis of disease. High levels of ROS produced at these sites can cause cell injury or death, however, when produced at low levels can serve as signaling molecules. For example, Ndufs2, a Complex I subunit containing an Fe-S center, N2, has recently been identified as a redox-sensitive oxygen sensor, mediating homeostatic oxygen-sensing in the pulmonary vasculature and carotid body. Fe-S clusters are emerging as transcriptionally-regulated mediators in disease and play a crucial role in normal physiology, offering potential new therapeutic targets for diseases including malaria, diabetes, and cancer.

Targeting the tumor immune microenvironment with "nutraceuticals": From bench to clinical trials

The occurrence of immune effector cells in the tissue microenvironment during neoplastic progression is critical in determining tumor growth outcomes. On the other hand, tumors may also avoid immune system-mediated elimination by recruiting immunosuppressive leukocytes and soluble factors, which coordinate a tumor microenvironment that counteracts the efficiency of the antitumor immune response. Checkpoint inhibitor therapy results have indicated a way forward via activation of the immune system against cancer. Widespread evidence has shown that different compounds in foods, when administered as purified substances, can act as immunomodulators in humans and animals. Although there is no universally accepted definition of nutraceuticals, the term identifies a wide category of natural compounds that may impact health and disease statuses and includes purified substances from natural sources, plant extracts, dietary supplements, vitamins, phytonutrients, and various products with combinations of functional ingredients. In this review, we summarize the current knowledge on the immunomodulatory effects of nutraceuticals with a special focus on the cancer microenvironment, highlighting the conceptual benefits or drawbacks and subtle cell-specific effects of nutraceuticals for envisioning future therapies employing nutraceuticals as chemoadjuvants.

Influence of New Synthetic Xanthones on the Proliferation and Migration Potential of Cancer Cell Lines In Vitro

BACKGROUND

Natural plant metabolites and their semisynthetic derivatives have been used for years in cancer therapy. Xanthones are oxygenated heterocyclic compounds produced as secondary metabolites by higher plants, fungi or lichens. Xanthone core may serve as a template in the synthesis of many derivatives that have broad biological activities.

OBJECTIVE

This study synthesized a series of 17 new xanthones, and their anticancer potential was also evaluated.

METHODS

The anticancer potential was evaluated in vitro using a highly invasive T24 cancer cell line. Direct cytotoxic effects of the xanthones were established by IC50 estimation based on XTT assay.

RESULTS

5 compounds of the total 17 showed significant cytotoxicity toward the studied cancer cultures and were submitted to further detailed analysis, including studies examining their influence on gelatinase A and B expression, as well as on the cancer cells migration and adhesion to an extracellular matrix. These analyses were carried out on five human tumor cell lines: A2780 (ovarian cancer), A549 (lung cancer), HeLa (cervical cancer), Hep G2 (liver cancer), and T24 (urinary bladder cancer). All the compounds, especially 4, showed promising anticancer activity: they exhibited significant cytotoxicity towards all the evaluated cell lines, including MCF-7 breast cancer, and hindered migration-motility activity of cancer cells demonstrating more potent activity than α-mangostin which served as a reference xanthone.

CONCLUSION

These results suggest that our xanthone derivatives may be further analyzed in order to include them in cancer treatment protocols.

Synthetic Chiral Derivatives of Xanthones: Biological Activities and Enantioselectivity Studies

Many naturally occurring xanthones are chiral and present a wide range of biological and pharmacological activities. Some of them have been exhaustively studied and subsequently, obtained by synthesis. In order to obtain libraries of compounds for structure activity relationship (SAR) studies as well as to improve the biological activity, new bioactive analogues and derivatives inspired in natural prototypes were synthetized. Bioactive natural xanthones compromise a large structural multiplicity of compounds, including a diversity of chiral derivatives. Thus, recently an exponential interest in synthetic chiral derivatives of xanthones (CDXs) has been witnessed. The synthetic methodologies can afford structures that otherwise could not be reached within the natural products for biological activity and SAR studies. Another reason that justifies this trend is that both enantiomers can be obtained by using appropriate synthetic pathways, allowing the possibility to perform enantioselectivity studies. In this work, a literature review of synthetic CDXs is presented. The structures, the approaches used for their synthesis and the biological activities are described, emphasizing the enantioselectivity studies.

Fe-S Clusters Emerging as Targets of Therapeutic Drugs

Fe-S centers exhibit strong electronic plasticity, which is of importance for insuring fine redox tuning of protein biological properties. In accordance, Fe-S clusters are also highly sensitive to oxidation and can be very easily altered in vivo by different drugs, either directly or indirectly due to catabolic by-products, such as nitric oxide species (NOS) or reactive oxygen species (ROS). In case of metal ions, Fe-S cluster alteration might be the result of metal liganding to the coordinating sulfur atoms, as suggested for copper. Several drugs presented through this review are either capable of direct interaction with Fe-S clusters or of secondary Fe-S clusters alteration following ROS or NOS production. Reactions leading to Fe-S cluster disruption are also reported. Due to the recent interest and progress in Fe-S biology, it is very likely that an increasing number of drugs already used in clinics will emerge as molecules interfering with Fe-S centers in the near future. Targeting Fe-S centers could also become a promising strategy for drug development.

Contribution of reactive oxygen species to the anticancer activity of aminoalkanol derivatives of xanthone

Reactive oxygen species (ROS) are critically involved in the action of anticancer agents. In this study, we investigated the role of ROS in the anticancer mechanism of new aminoalkanol derivatives of xanthone. Most xanthones used in the study displayed significant pro-oxidant effects similar to those of gambogic acid, one of the most active anticancer xanthones. The pro-oxidant activity of our xanthones was shown both directly (by determination of ROS induction, effects on the levels of intracellular antioxidants, and expression of antioxidant enzymes) and indirectly by demonstrating that the overexpression of manganese superoxide dismutase decreases ROS-mediated cell senescence. We also observed that mitochondrial dysfunction and cellular apoptosis enhancement correlated with xanthone-induced oxidative stress. Finally, we showed that the use of the antioxidant N-acetyl-L-cysteine partly reversed these effects of aminoalkanol xanthones. Our results demonstrated that novel aminoalkanol xanthones mediated their anticancer activity primarily through ROS elevation and enhanced oxidative stress, which led to mitochondrial cell death stimulation; this mechanism was similar to the activity of gambogic acid.

Caged Garcinia Xanthones, a Novel Chemical Scaffold with Potent Antimalarial Activity

Caged Garcinia xanthones (CGXs) constitute a family of natural products that are produced by tropical/subtropical trees of the genus Garcinia CGXs have a unique chemical architecture, defined by the presence of a caged scaffold at the C ring of a xanthone moiety, and exhibit a broad range of biological activities. Here we show that synthetic CGXs exhibit antimalarial activity against Plasmodium falciparum, the causative parasite of human malaria, at the intraerythrocytic stages. Their activity can be substantially improved by attaching a triphenylphosphonium group at the A ring of the caged xanthone. Specifically, CR135 and CR142 were found to be highly effective antimalarial inhibitors, with 50% effective concentrations as low as ∼10 nM. CGXs affect malaria parasites at multiple intraerythrocytic stages, with mature stages (trophozoites and schizonts) being more vulnerable than immature rings. Within hours of CGX treatment, malaria parasites display distinct morphological changes, significant reduction of parasitemia (the percentage of infected red blood cells), and aberrant mitochondrial fragmentation. CGXs do not, however, target the mitochondrial electron transport chain, the target of the drug atovaquone and several preclinical candidates. CGXs are cytotoxic to human HEK293 cells at the low micromolar level, which results in a therapeutic window of around 150-fold for the lead compounds. In summary, we show that CGXs are potent antimalarial compounds with structures distinct from those of previously reported antimalarial inhibitors. Our results highlight the potential to further develop Garcinia natural product derivatives as novel antimalarial agents.

The Fe-S cluster-containing NEET proteins mitoNEET and NAF-1 as chemotherapeutic targets in breast cancer

Identification of novel drug targets and chemotherapeutic agents is a high priority in the fight against cancer. Here, we report that MAD-28, a designed cluvenone (CLV) derivative, binds to and destabilizes two members of a unique class of mitochondrial and endoplasmic reticulum (ER) 2Fe-2S proteins, mitoNEET (mNT) and nutrient-deprivation autophagy factor-1 (NAF-1), recently implicated in cancer cell proliferation. Docking analysis of MAD-28 to mNT/NAF-1 revealed that in contrast to CLV, which formed a hydrogen bond network that stabilized the 2Fe-2S clusters of these proteins, MAD-28 broke the coordinative bond between the His ligand and the cluster's Fe of mNT/NAF-1. Analysis of MAD-28 performed with control (Michigan Cancer Foundation; MCF-10A) and malignant (M.D. Anderson-metastatic breast; MDA-MB-231 or MCF-7) human epithelial breast cells revealed that MAD-28 had a high specificity in the selective killing of cancer cells, without any apparent effects on normal breast cells. MAD-28 was found to target the mitochondria of cancer cells and displayed a surprising similarity in its effects to the effects of mNT/NAF-1 shRNA suppression in cancer cells, causing a decrease in respiration and mitochondrial membrane potential, as well as an increase in mitochondrial iron content and glycolysis. As expected, if the NEET proteins are targets of MAD-28, cancer cells with suppressed levels of NAF-1 or mNT were less susceptible to the drug. Taken together, our results suggest that NEET proteins are a novel class of drug targets in the chemotherapeutic treatment of breast cancer, and that MAD-28 can now be used as a template for rational drug design for NEET Fe-S cluster-destabilizing anticancer drugs.

Garcinia vilersiana bark extract activates the Nrf2/HO-1 signaling pathway in RAW264.7 cells

Garcinia vilersiana is a traditional medicinal plant in Vietnam. The petroleum ether extract of stem bark of Garcinia vilersiana (GVE) was prepared to evaluate its potential to activate Nrf2, a transcription factor of antioxidant and detoxifying enzymes. Exposure of mouse macrophage RAW264.7 cells to GVE (0.625-2.5 µg/ml) resulted in a significant activation of Nrf2, as evaluated by nuclear accumulation of this transcription factor, and increased antioxidant response element (ARE) binding activity in a time- and concentration-dependent manner. As a result, GVE caused ARE-dependent up-regulation of heme oxygenase-1 (HO-1) in the cells. These results suggest that GVE contains components that have the ability to activate the Nrf2/ARE/HO-1 signaling pathway, leading to cellular protection.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

2-deprenyl-rheediaxanthone B isolated from Metaxya rostrata induces active cell death in colorectal tumor cells

Metaxya rostrata C. Presl (Metaxyaceae) is a common tree fern in Central and South America that is used for the treatment of intestinal ulcers and tumours in ethnic medicine. Using a bioactivity-guided strategy 2-deprenyl-rheediaxanthone B (XB) has been isolated as one of the active principles in this plant. XB induced loss of cell viability in colorectal cancer cell lines at IC50 concentrations of 11-23 µM. This was caused by both accumulation of cells in the G2- and S-phase as well as by induction of active cell death in a time and concentration-dependent manner. Cells exposed to XB were incapable of undergoing regular mitosis due to down-regulation of FoxM1 and absence of chromosome condensation. The apoptosis-related proteins Bcl2 and Bclxl were up-regulated so that Caspase 3 was not activated and classical apoptosis was not observed. However, XB triggered damage pathways down-stream of ATR and activated Caspase 2 causing cell death by a mechanism similar to mitotic catastrophe. Our observations are the first to show the cytotoxic activity of 2-deprenyl-rheediaxanthone B and indicate that XB is an interesting new lead compound for cancer therapy that merits further development.

Subcellular localization and activity of gambogic acid

The natural product gambogic acid (GA) has shown significant potential as an anticancer agent as it is able to induce apoptosis in multiple tumor cell lines, including multidrug-resistant cell lines, as well as displaying antitumor activity in animal models. Despite the fact that GA has entered phase I clinical trials, the primary cellular target and mode of action of this compound remain unclear, although many proteins have been shown to be affected by it. By thorough analysis of several cellular organelles, at both the morphological and functional levels, we demonstrate that the primary effect of GA is at the mitochondria. We found that GA induces mitochondrial damage within minutes of incubation at low-micromolar concentrations. Moreover, a fluorescent derivative of GA was able to localize specifically to the mitochondria and was displaced from these organelles after competition with unlabeled GA. These findings indicate that GA directly targets the mitochondria to induce the intrinsic pathway of apoptosis, and thus represents a new member of the mitocans.

Anticancer drugs from traditional toxic Chinese medicines

Many anticancer drugs are obtained from natural sources. Nature produces a variety of toxic compounds, which are often used as anticancer drugs. Up to now, there are at least 120 species of poisonous botanicals, animals and minerals, of which more than half have been found to possess significant anticancer properties. In spite of their clinical toxicity, they exhibit pharmacological effects and have been used as important traditional Chinese medicines for the different stages of cancer. The article reviews many structures such as alkaloids of Camptotheca acuminata, Catharanthus roseus and Cephalotaxus fortunei, lignans of Dysosma versipellis and Podophyllum emodi, ketones of Garcinia hanburyi, terpenoids of Mylabris and Ginkgo biloba, diterpenoids of Tripterygium wilfordii, Euphorbia fischeriana, Euphorbia lathyris, Euphorbia kansui, Daphne genkwa, Pseudolarix kaempferi and Brucea javanica, triterpenoids of Melia toosendan, steroids of Periploca sepium, Paris polyphylla and Venenum Bufonis, and arsenic compounds including Arsenicum and Realgar. By comparing their related phytochemistry, toxic effects and the recent advances in understanding the mechanisms of action, this review puts forward some ideals and examples about how to increase antitumour activity and/or reduce the side effects experienced with Chinese medicine.

Cluvenone induces apoptosis via a direct target in mitochondria: a possible mechanism to circumvent chemo-resistance?

The synthetic caged Garcinia xanthone, cluvenone, has potent and selective cytotoxicity against numerous cancer cell lines including those that are multi-drug resistant. The direct target of this structurally and functionally unique agent is unknown and that of the parent natural product, gambogic acid (GA), presently in clinical trials, is not yet entirely clear. For the first time, using fluorescently labeled GA (GA-Bodipy), we determined that GA-Bodipy localized in mitochondria and was effectively displaced by cluvenone in competition experiments indicating that the direct target of cluvenone resided in mitochondria and was shared by GA. In agreement with these findings, treatment of HeLa cells with cluvenone or GA resulted in disruption of mitochondrial morphology within 4 h. Furthermore, experiments using the potential sensitive JC-1 dye demonstrated that cells treated with 1 μM cluvenone for 1 h had significant loss of MMP compared to control cells. Examination of Cyt c levels in leukemia cells treated with 1 μM cluvenone resulted in a 4-fold increase in levels of both cytosolic and mitochondrial Cyt c. In agreement with Cyt c release, caspase 9 activity was increased 2.6-fold after treatment of cells for 5 h with 1 μM cluvenone. Remarkably, the caspase-9 inhibitor, Z-LEHD-FMK, blocked cluvenone-induced apoptosis in a dose-dependent manner with apoptosis being completely blocked by 10 μM of the inhibitor. In conclusion, cluvenone, an agent with potent cytotoxicity against multi-drug resistant tumor cells, has direct targets in mitochondria thus setting precedence for drug discovery efforts against these targets in the treatment of refractory cancers.