Gambogic acid inhibits the catalytic activity of human topoisomerase IIalpha by binding to its ATPase domain

Anti-cancer natural products isolated from chinese medicinal herbs

In recent years, a number of natural products isolated from Chinese herbs have been found to inhibit proliferation, induce apoptosis, suppress angiogenesis, retard metastasis and enhance chemotherapy, exhibiting anti-cancer potential both in vitro and in vivo. This article summarizes recent advances in in vitro and in vivo research on the anti-cancer effects and related mechanisms of some promising natural products. These natural products are also reviewed for their therapeutic potentials, including flavonoids (gambogic acid, curcumin, wogonin and silibinin), alkaloids (berberine), terpenes (artemisinin, β-elemene, oridonin, triptolide, and ursolic acid), quinones (shikonin and emodin) and saponins (ginsenoside Rg3), which are isolated from Chinese medicinal herbs. In particular, the discovery of the new use of artemisinin derivatives as excellent anti-cancer drugs is also reviewed.

Anti-cancer natural products isolated from chinese medicinal herbs

In recent years, a number of natural products isolated from Chinese herbs have been found to inhibit proliferation, induce apoptosis, suppress angiogenesis, retard metastasis and enhance chemotherapy, exhibiting anti-cancer potential both in vitro and in vivo. This article summarizes recent advances in in vitro and in vivo research on the anti-cancer effects and related mechanisms of some promising natural products. These natural products are also reviewed for their therapeutic potentials, including flavonoids (gambogic acid, curcumin, wogonin and silibinin), alkaloids (berberine), terpenes (artemisinin, β-elemene, oridonin, triptolide, and ursolic acid), quinones (shikonin and emodin) and saponins (ginsenoside Rg₃), which are isolated from Chinese medicinal herbs. In particular, the discovery of the new use of artemisinin derivatives as excellent anti-cancer drugs is also reviewed.

N-fused imidazoles as novel anticancer agents that inhibit catalytic activity of topoisomerase IIα and induce apoptosis in G1/S phase

On the basis of structures of known topoisomerase II catalytic inhibitors and initial molecular docking studies, bicyclic N-fused aminoimidazoles were predicted as potential topoisomerase II inhibitors. They were synthesized by multicomponent reactions and evaluated against human topoisomerase IIα (hTopoIIα) in decatenation, relaxation, cleavage complex, and DNA intercalation in vitro assays. Among 31 compounds of eight different bicyclic scaffolds, it was found that imidazopyridine, imidazopyrazole, and imidazopyrazine with suitable substituents exhibited potent inhibition of catalytic activity of hTopoIIα while not showing DNA intercalation. Molecular docking studies and molecular dynamics (MD) simulation analysis, ATPase-kinetics and ATP-dependent plasmid relaxation assay revealed the catalytic mode of inhibition of the title compounds plausibly by blocking the ATP-binding site. N-Fused aminoimidazoles showed potent anticancer activities in kidney and breast cancer cell lines, low toxicity to normal cells, relatively higher potency compared to etoposide and 5-fluorouracil in kidney cancer cell lines, and potent inhibition in cell migration. These compounds were found to exert apoptotic effect in G1/S phase.

Gambogic acid-induced degradation of mutant p53 is mediated by proteasome and related to CHIP

As an oncoprotein, mutant p53 is a potential tumor-specific target for cancer therapy. Most mutated forms of the protein are largely accumulated in cancer cells due to their increased stability. In the present study, we demonstrate that mutant p53 protein stability is regulated by gambogic acid (GA). Following GA exposure, protein levels of mutant p53 decreased while the mRNA levels were not affected in MDA-MB-435 cells, which indicate that GA down-regulates mutant p53 at post-transcription level. Co-treatment with GA and cycloheximide, a protein synthesis inhibitor, induced a decrease of half-life of mutant p53 protein. These findings indicated that the reduction of mutant p53 by GA was due to the destabilization and degradation of the protein. Furthermore, inhibition of proteasome activity by MG132 blocked GA-induced down-regulation of mutant p53, causing mutant p53 accumulation in detergent-insoluble cellular fractions. Further studies revealed that mutant p53 was ubiquitinated and it was chaperones related ubiquitin ligase carboxy terminus of Hsp70-interacting protein (CHIP) rather than MDM2 involved in the degradation of mutant p53. In addition, GA prevented Hsp90/mutant p53 complex formation and enhanced interaction of mutant p53 with Hsp70. Depletion of CHIP stabilized mutant p53 in GA treated cells. In conclusion, mutant p53 may be down-regulated by GA through chaperones-assisted ubiquitin/proteasome degradation pathway in cancer cells.

Gambogic acid inhibits STAT3 phosphorylation through activation of protein tyrosine phosphatase SHP-1: potential role in proliferation and apoptosis

The transcription factor, STAT3, is associated with proliferation, survival, and metastasis of cancer cells. We investigated whether gambogic acid (GA), a xanthone derived from the resin of traditional Chinese medicine, Garcinia hanburyi (mangosteen), can regulate the STAT3 pathway, leading to suppression of growth and sensitization of cancer cells. We found that GA induced apoptosis in human multiple myeloma cells that correlated with the inhibition of both constitutive and inducible STAT3 activation. STAT3 phosphorylation at both tyrosine residue 705 and serine residue 727 was inhibited by GA. STAT3 suppression was mediated through the inhibition of activation of the protein tyrosine kinases Janus-activated kinase 1 (JAK1) and JAK2. Treatment with the protein tyrosine phosphatase (PTP) inhibitor pervanadate reversed the GA-induced downregulation of STAT3, suggesting the involvement of a PTP. We also found that GA induced the expression of the PTP SHP-1. Deletion of the SHP-1 gene by siRNA suppressed the ability of GA to inhibit STAT3 activation and to induce apoptosis, suggesting the critical role of SHP-1 in its action. Moreover, GA downregulated the expression of STAT3-regulated antiapoptotic (Bcl-2, Bcl-xL, and Mcl-1), proliferative (cyclin D1), and angiogenic (VEGF) proteins, and this correlated with suppression of proliferation and induction of apoptosis. Overall, these results suggest that GA blocks STAT3 activation, leading to suppression of tumor cell proliferation and induction of apoptosis.

Absolute configuration of (-)-gambogic acid, an antitumor agent

(-)-Gambogic acid (1), a biologically active "caged xanthone" from gamboge, the dried resin of Garcinia hanburyi, is of interest as a potential anticancer agent. The planar structure of (-)-gambogic acid has been determined previously by analysis of its detailed NMR data and confirmed by single-crystal X-ray diffraction, with the absolute configuration at C-13 deduced as R through a series of chemical degradations. Using (-)-morellic acid (2), an analogue of (-)-gambogic acid, as a model compound, the 5R, 7S, 10aS, 13R, 27S absolute configuration of (-)-gambogic acid was determined for the first time by comparison of physical and spectroscopic data, especially experimental and calculated electronic circular dichroism.

MFTZ-1 reduces constitutive and inducible HIF-1α accumulation and VEGF secretion independent of its topoisomerase II inhibition

The macrolide compound MFTZ-1 has been identified as a novel topoisomerase II (Top2) inhibitor with potent in vitro and in vivo anti-tumour activities. In this study, we further examined the effects of MFTZ-1 on hypoxia-inducible factor-1α (HIF-1α) accumulation, vascular endothelial growth factor (VEGF) secretion and angiogenesis. MFTZ-1 reduced HIF-1α accumulation driven by hypoxia or growth factors in human cancer cells. Mechanistic studies revealed that MFTZ-1 did not affect the degradation of HIF-1α protein or the level of HIF-1α mRNA. By contrast, MFTZ-1 apparently inhibited constitutive and inducible activation of both phosphatidylinositol-3-kinase (PI3K)-Akt and p42/p44 mitogen-activated protein kinase (MAPK) pathways. Further studies revealed that MFTZ-1 abrogated the HIF-1α-driven increase in VEGF mRNA and protein secretion. MFTZ-1 also lowered the basal level of VEGF secretion. The results reveal an important feature that MFTZ-1 can reduce constitutive, HIF-1α-independent VEGF secretion and concurrently antagonize inducible, HIF-1α-dependent VEGF secretion. Moreover, MFTZ-1 disrupted tube formation of human umbilical vein endothelial cells (HUVECs) stimulated by hypoxia with low-concentration serum or by serum at normoxia, and inhibited HUVECs migration at normoxia. MFTZ-1 also prevented microvessel outgrowth from rat aortic ring. These data reflect the potent anti-angiogenesis of MFTZ-1 under different conditions. Furthermore, using specific small interfering RNA targeting Top2α or Top2-defective HL60/MX2 cells, we showed that MFTZ-1 affected HIF-1α accumulation and HUVECs tube formation irrelevant to its Top2 inhibition. Taken together, our data collectively reveal that MFTZ-1 reduces constitutive and inducible HIF-1α accumulation and VEGF secretion possibly via PI3K-Akt and MAPK pathways, eliciting anti-angiogenesis independently of its Top2 inhibition.

Studies on chemical modification and biology of a natural product, gambogic acid (III): determination of the essential pharmacophore for biological activity

Caged 4-oxa-tricyclo[4.3.1.0(3,7)]dec-2-one structural motifs are found in Garcinia natural products that demonstrate anti-tumor activity. Gambogic acid (GA, 1), the most abundant caged Garcinia xanthones, has been reported to be a promising anti-cancer agent. To identify the essential pharmacophore for its anti-tumor activity, a series of GA analogues that address potential key structural features for biological activity were synthesized, among which compound 11a displayed comparable in vitro anti-tumor activity as GA. Mechanistic studies on 11a determined that the compound induces apoptosis as well as arrests the G2/M phase of the cell cycle in HepG2 cells. The determination of the essential part of the scaffold found in GA to maintain anti-tumor effects, and the SAR based on the caged pharmacophore are reported and will provide key information for future anti-cancer drug development studies.

Organoplatinum(II) complexes with nucleobase motifs as inhibitors of human topoisomerase II catalytic activity

Platinum(II) complexes bearing acetylide ligands containing nucleobase motifs are prepared and their impact on human topoisomerase II (TopoII) is evaluated. Both platinum(II) complexes [Pt(II)(C^N^N)(C≡CCH₂R)] (1a-c) and [Pt(II)(tBu₃terpy)(C≡CCH₂R)](+) (2a-c) (C^N^N=6-phenyl-2,2'-bipyridyl, tBu₃terpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridyl, and R=(a) adenine, (b) thymine, and (c) 2-amino-6-chloropurine) are stable in aqueous solutions for 48 hours at room temperature. The binding constants (K) for the platinum(II) complexes towards calf thymus DNA are in the order of 10⁵ dm³ mol⁻¹ as estimated by using UV/Vis absorption spectroscopy. Of the complexes examined, only complexes 1a-c are found to behave as intercalators. Both complexes 1a-c and 2a-c inhibit TopoII-induced relaxation of supercoiled DNA, while 2c is the most potent TopoII inhibitors among the tested compounds. Inhibition of DNA relaxation is detected at nanomolar concentrations of 2c. All of the platinum(II) complexes are cytotoxic to human cancer cells with IC₅₀ values of 0.5-13.7 μM, while they are less toxic against normal cells CCD-19 Lu.

Synthesis, biological evaluation, and molecular docking study of 3-(3'-heteroatom substituted-2'-hydroxy-1'-propyloxy) xanthone analogues as novel topoisomerase IIα catalytic inhibitor

Epoxide ring-opened xanthone derivatives were synthesized and tested for their topoisomerase inhibitory activity and cytotoxicity. Most of the compounds showed topo IIα specific inhibitory activity. To clarify the mechanism of action of these compounds, the most potent compound (compound 14) of the synthesized analogues was further studied by testing its ATPase inhibitory activity and through molecular docking experiments. The results showed that the topo IIα inhibitory activity of compound 14 was inversely proportional to ATP concentration. In the ATPase inhibitory test, ATP hydrolysis was reduced less efficiently by compound 14 (28.5±4.6%) than novobiocin (60.4±8.1%). Molecular docking study revealed compound 14 to have a stable binding pattern to the ATP-binding domain of human topo II.

Metabolism of gambogic acid in rats: a rare intestinal metabolic pathway responsible for its final disposition

Gambogic acid (GA) is a promising natural anticancer candidate. Although the anticancer activity of GA has been well demonstrated, information regarding the metabolic fate of GA is limited. Previous studies suggested that GA is mainly excreted into intestinal tract in rats through bile after intravenous administration, whereas only traces appeared in the feces, suggesting that GA is metabolized extensively in the intestine. However, there has been no report about the intestinal metabolism of GA either in animals or humans. In this study, large amounts of two sulfonic acid metabolites of GA were found in the feces samples of rats after intravenous administration, and their structures were identified as 10-α sulfonic acid GA and 10-β sulfonic acid GA by comparison of the retention times and spectral data with those of synthesized reference substances using liquid chromatography-diode array detector-tandem mass spectrometry. This rare intestinal metabolic pathway mainly involves Michael addition of the sulfite ion to the 9,10 carbon-carbon double bond of α,β-unsaturated ketone. In addition, a more detailed metabolic profile in rats is proposed, according to the results of in vitro and in vivo studies. It was found that GA can be metabolized by a variety of routes, including monooxidation, hydration, glutathionylation, glucuronidation, and glucosidation in the liver of rats. These findings provide information on the major metabolic soft spot of GA in the intestine and liver of rats, which is not only useful in the future human metabolic study of this compound but also of value in the metabolic studies of GA analogs.

Riccardin D, a novel macrocyclic bisbibenzyl, induces apoptosis of human leukemia cells by targeting DNA topoisomerase II

We studied the effect of riccardin D, a macrocyclic bisbibenzyl, which was isolated from the Chinese liverwort plant, on human leukemia cells and the underlying molecular mechanism. Riccardin D had a significant antiproliferative effect on human leukemia cell lines HL-60, K562 and its multidrug resistant (MDR) counterpart K562/A02 cells, but showed no effect on the topoisomerase-II-deficient HL-60/MX2 cells, as measured by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. The pBR322 DNA relaxation assay revealed that riccardin D selectively inhibited the activity of topoisomerase II (topo II). The suppression of topo II activity by riccardin D was stronger than that of etoposide, a known topo II inhibitor. After treatment with riccardin D, nuclear extracts of leukemia K562 and K562/A02 cells left the majority of pBR322 DNA in a supercoiled form. Further examination showed that riccardin D effectively induced HL-60, K562 and K562/A02 apoptosis as evidenced by externalization of phosphatidylserine and formation of DNA ladder fragments. The activation of cytochrome c, caspase-9, caspase-3 and cleaved poly ADP-ribose polymerase (PARP) was also enhanced, as estimated by Western blot analysis. By contrast, riccardin D was unable to induce apoptosis in the topoisomerase-II-deficient HL-60/MX2 cells, indicating that the induction of apoptosis by riccardin D was due to the inhibition of topo II activity. In addition, riccardin D was able to significantly decrease P-glycoprotein (P-gp) expression in K562/A02 cells. Taken together, our data demonstrate that riccardin D is a novel DNA topo II inhibitor which can induce apoptosis of human leukemia cells and that it has therapeutic potential for both regular and MDR strains of leukemia cells.

Chemistry and biology of the caged Garcinia xanthones

Natural products have been a great source of many small molecule drugs for various diseases. In spite of recent advances in biochemical engineering and fermentation technologies that allow us to explore microorganisms and the marine environment as alternative sources of drugs, more than 70 % of the current small molecule therapeutics derive their structures from plants used in traditional medicine. Natural-product-based drug discovery relies heavily on advances made in the sciences of biology and chemistry. Whereas biology aims to investigate the mode of action of a natural product, chemistry aims to overcome challenges related to its supply, bioactivity, and target selectivity. This review summarizes the explorations of the caged Garcinia xanthones, a family of plant metabolites that possess a unique chemical structure, potent bioactivities, and a promising pharmacology for drug design and development.

Proteasome-inhibitory and cytotoxic constituents of Garcinia lateriflora: absolute configuration of caged xanthones

A new biflavonoid (1), a new xanthone enantiomer (2), five new caged xanthones (3-7), and several known compounds were isolated from the stem bark of Garcinia lateriflora, collected in Indonesia. The structures of the new compounds were determined by analysis of spectroscopic data, and the absolute configuration of the caged xanthones was shown for the first time at carbons 5, 7, 8, 8a, 10a, and 27, by analysis of COSY and NOESY NMR and ECD spectra. The biflavonoids exhibited proteasome inhibitory activity, and the known compound, morelloflavone (8) was found to have the greatest potency (IC(50) = 1.3 muM). The caged xanthones were cytotoxic towards HT-29 cells, with the known compound, morellic acid (10) being the most active (ED(50) = 0.36 muM). However, when tested in an in vivo hollow fiber assay, it was inactive at the highest dose tested (20 mg/kg).

Gambogic acid triggers DNA damage signaling that induces p53/p21(Waf1/CIP1) activation through the ATR-Chk1 pathway

Gambogic acid (GA) has been wildly studied to show potent anti-tumor effects in vivo and in vitro. We have confirmed that GA stabilized and activated p53 through down-regulating the expression of MDM2 in variety of cancer cell lines. However, GA-induced p53 activation could be partially reversed by caffeine, a PI3k inhibitor. Therefore, questions of whether GA induces post-translational modifications of p53 and subsequent activation of p53; and if that is the case, which upstream signaling pathway(s) is (are) responsible for that are proposed. Here, the relationship between p53 activation and its post-translational modifications was investigated in the human cancer cell lines HepG2 and A549 in response to GA or adriamycin treatment. GA induces p53 phosphorylation at sites Ser15 and Ser20 in a concentration- or time-dependent way, which was a direct result of DNA damage, as gamma-HA2X foci and 'comet' DNA fragments were detected. GA induces p53 phosphorylation through activation of an ATM- and Rad3-related pathway, and GA-induced phosphorylation of Chk1 is also involved. Upon treatment with GA, ATR activation is clearly associated with p53 phosphorylation, as well as activation of its target gene p21(Waf/CIP1). Furthermore, we found the dephosphorylation of Cdk1 at Thr161 induced by GA was abrogated, followed by a remarkable disruption of G2/M arrest when the cells were pre-incubated with caffeine. Interestingly, the sensitivity to caffeine enhanced the cytotoxicity of GA as well. Taken together, these data showed an important role of the DNA damage response mediated by ATR-Chk1 in p53/p21(Waf/CIP1) activation and downstream G2/M arrest during GA treatment.

An oxidative analogue of gambogic acid-induced apoptosis of human hepatocellular carcinoma cell line HepG2 is involved in its anticancer activity in vitro

The objective of this study was to investigate the apoptosis-inducing effect of an oxidative analogue of gambogic acid (GA) on the human hepatocellular carcinoma cell line HepG2 and explore the related molecular mechanisms. HepG2 cells were treated with the analogue of GA and the growth inhibition was analysed by MTT assay. The morphological changes in cells were observed under an inverted light microscope and a fluorescence microscope. In addition, both the cell-cycle arrest and the apoptosis rate were detected by flow cytometry. Western blot was used to evaluate the alteration of protein expression. The viability of HepG2 cells was markedly inhibited in a concentration-dependent manner and obvious morphological changes were confirmed, including condensed chromatin and reduced volume. Increased percentage of apoptotic cells was displayed and altered expression level of several apoptosis-associated proteins, P53, Bcl-2, Bax and pro-caspase-3, was obtained. The newly synthesized analogue of GA exhibited potential anticancer activity, induced remarkable apoptosis in HepG2 cells, probably through the intrinsic mitochondrial pathway, and promised to be a new candidate for future cancer therapy.

Mechanisms of gambogic acid-induced apoptosis in non-small cell lung cancer cells in relation to transferrin receptors

Gambogic acid (GA) is one of the important active ingredients of gamboge. Our study examined the expression of transferrin receptors (TFR) on the cell surface of human lung SPC-A1 and SK-MES-1 cells and measured their GA-induced apoptosis rate. The results showed that SPC-A1 cells with a higher TFR expression were more sensitive at the same GA concentrations. To examine its distribution in cultured cells and study the mechanisms of apoptosis, we labeled GA with a (125)I tracer and examined the expression of apoptosis-related proteins. we found that GA uptake into SPC-A1 cells was higher than into SK-MES-1 cells; apoptosis-related proteins Caspase 2, Caspase 9, Caspase 10, Bax and p53 were involved in GA-induced apoptosis. We conclude that GA has an apoptosis-promoting effect on non small cell lung cancer cells. In clinical practice, the histopathological quantitation of TFR expression levels in tumor tissues may become a predictor of the sensitivity of patients' tumors to GA treatment.

Recent cancer drug development with xanthone structures

Objectives

Xanthones are simple three-membered ring compounds that are mainly found as secondary metabolites in higher plants and microorganisms. Xanthones have very diverse biological profiles, including antihypertensive, antioxidative, antithrombotic and anticancer activity, depending on their diverse structures, which are modified by substituents on the ring system. Although several reviews have already been published on xanthone compounds, few of them have focused on the anticancer activity of xanthone derivatives. In this review we briefly summarize natural and synthetic xanthone compounds which have potential as anticancer drugs.

Key findings

The interesting structural scaffold and pharmacological importance of xanthone derivatives have led many scientists to isolate or synthesize these compounds as novel drug candidates. In the past, extensive research has been conducted to obtain xanthone derivatives from natural resources as well as through synthetic chemistry. Xanthones interact with various pharmacological targets based on the different substituents on the core ring. The anticancer activities of xanthones are also dramatically altered by the ring substituents and their positions.

Summary

The biological activities of synthetic xanthone derivatives depend on the various substituents and their position. Study of the biological mechanism of action of xanthone analogues, however, has not been conducted extensively compared to the diversity of xanthone compounds. Elucidation of the exact biological target of xanthone compounds will provide better opportunities for these compounds to be developed as potent anticancer drugs. At the same time, modification of natural xanthone derivatives aimed at specific targets is capable of expanding the biological spectrum of xanthone compounds.

Comparison of electron capture-atmospheric pressure chemical ionization and electrospray ionization for the analysis of gambogic acid and its main circulating metabolite in dog plasma

Gambogic acid (GA), a promising anticancer candidate, is a polyprenylated xanthone abundant in the resin of Garcinia morella and Garcinia hanburyi. Electron capture-atmospheric pressure chemical ionization (EC- APCI) and electrospray ionization (ESI) techniques, both in the negative ion mode, were evaluated regarding ionization, fragmentation patterns and sensitivity for simultaneous liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of GA and its main circulating metabolite, 10-hydroxygambogic acid (10-OHGA) in dog plasma. Both analytes underwent extensive in-source fragmentation in EC-APCI, which was not desirable for reliable quantification of these analytes, whereas the substitution of ESI for EC-APCI almost eliminated the source instability of both analytes. Negative ion ESI was, therefore, chosen for the development of an LC-MS/MS method for simultaneous determination of these analytes. After protein precipitation by acetonitrile, all analytes were separated on a Luna C18 HST column (50 x 2.0 mm i.d., 2.5 microm) with a mobile phase of 20 mmol L(-1) ammonium acetate water solution containing 0.2% acetic acid:acetonitrile (18:82, v/v). The detection was performed on a tandem mass spectrometer using selective reaction monitoring mode. Calibration curves were linear over the range of 10-6000 ng mL(-1) for GA and 3-2000 ng mL(-1) for 10-OHGA. The method was successfully applied to the pharmacokinetics study of GA injection in six beagle dogs.

Echinoside A, a new marine-derived anticancer saponin, targets topoisomerase2alpha by unique interference with its DNA binding and catalytic cycle

BACKGROUND

Echinoside A was isolated from sea cucumber. This study demonstrates its anticancer effects and its mechanisms of action.

MATERIALS AND METHODS

Anticancer effects of echinoside A were evaluated in vitro and in vivo. TUNEL and DNA fragmentation assays were applied to examine its ability to induce apoptosis. A series of biochemical assays were applied to investigate the inhibition of echinoside A on topoisomerase2alpha (Top2alpha). Molecular docking analyses were used to demonstrate the direct interaction between echinoside A and Top2alpha.

RESULTS

Echinoside A inhibited the growth of tumors in mouse models and human prostate carcinoma xenografts in nude mouse models. Echinoside A shows the unique characteristics of inhibiting the noncovalent binding of Top2alpha to DNA by competing with DNA for the DNA-binding domain of the enzyme and of interfering predominantly with the Top2alpha-mediated prestrand passage cleavage/religation equilibrium over with the poststrand passage one. These features distinguish echinoside A from other known Top2alpha inhibitors. As a result, echinoside A induced DNA double-strand breaks in a Top2-dependent manner.

CONCLUSION

Echinoside A targets Top2alpha by unique interference with the binding of Top2 to DNA and by imparing the Top2-mediated DNA cleavage and religation, exerting potent in vitro and in vivo antitumor activities.

Gambogic acid reduced bcl-2 expression via p53 in human breast MCF-7 cancer cells

PURPOSE

In this study, we investigated the correlation between p53 and bcl-2 in gambogic acid (GA)-induced apoptosis.

METHOD

MTT assay was employed to evaluate MCF-7 cell viability after GA treatment. Cell morphological changes were observed follow-up under the inverted microscope after GA withdrawal. To observe the cell apoptosis, DAPI staining was used. Meanwhile, p53 small interfering RNA (si-RNA) was adopted to knock p53 down. All expressions of p53 and bcl-2 were evaluated by Western blot analysis.

RESULTS

MTT assay showed that GA inhibited MCF-7 cell growth effectively in a time-dependent manner. With 0.5 h GA treatment, p53 was significantly increased, whereas bcl-2 was reduced potently with 6 h GA treatment. After GA withdrawal, p53 expressions were maintained in high levels. Furthermore, bcl-2 decreasing was attenuated after co-treatment with PFT alpha, a p53 transcription blocker. Same result was observed after p53 knock-down by p53 si-RNA.

CONCLUSIONS

Gambogic acid induced human breast cancer cells MCF-7 apoptosis by reducing bcl-2 expression via p53.

Discovery of a new class of catalytic topoisomerase II inhibitors targeting the ATP-binding site by structure based design. Part I

Topoisomerase II is a validated target in oncology. Among the different ways of blocking the function of this enzyme, inhibiting its ATPase activity has been relatively less investigated. In an effort to identify topoisomerase II inhibitors of a novel type, exerting their action by this mechanism, we have designed a purine inhibitor scaffold targeting the ATP-binding site of the enzyme. Searching the Novartis compound collection for molecules containing this purine motif has allowed the identification of two micromolar hits providing access to a new class of catalytic topoisomerase II inhibitors.

Molecular targets of nutraceuticals derived from dietary spices: potential role in suppression of inflammation and tumorigenesis

Despite the fact cancer is primarily a preventable disease, recent statistics indicate cancer will become the number one killer worldwide in 2010. Since certain cancers are more prevalent in the people of some countries than others, suggests the role of lifestyle. For instance cancer incidence among people from the Indian subcontinent, where most spices are consumed, is much lower than that in the Western World. Spices have been consumed for centuries for a variety of purposes-as flavoring agents, colorants, and preservatives. However, there is increasing evidence for the importance of plant-based foods in regular diet to lowering the risk of most chronic diseases, so spices are now emerging as more than just flavor aids, but as agents that can not only prevent but may even treat disease. In this article, we discuss the role of 41 common dietary spices with over 182 spice-derived nutraceuticals for their effects against different stages of tumorigenesis. Besides suppressing inflammatory pathways, spice-derived nutraceuticals can suppress survival, proliferation, invasion, and angiogenesis of tumor cells. We discuss how spice-derived nutraceuticals mediate such diverse effects and what their molecular targets are. Overall our review suggests "adding spice to your life" may serve as a healthy and delicious way to ward off cancer and other chronic diseases.

GA3, a new gambogic acid derivative, exhibits potent antitumor activities in vitro via apoptosis-involved mechanisms

AIM

Gambogic acid (GA) is the major active ingredient of gamboge, which is secreted from a Chinese traditional medicine, Garcinia hanburyi, which possesses potent antitumor activity. GA3, a new GA derivative, has been shown to possess better water solubility than GA. The aim of the present study was to examine the antitumor activity of GA3 and the mechanism underlying it.

METHODS

The growth inhibition of cancer cell lines induced by GA3 was assessed using the SRB assay. DAPI staining, flow cytometry, a DNA fragment assay, and Western blot analysis were used to study the apoptotic mechanisms of GA3.

RESULTS

GA3 displayed wide cytotoxicity in diversified human cancer cell lines with a mean IC(50) value of 2.15 micromol/L. GA3 was also effective against multidrug resistant cells, with an average resistance factor (RF) that was much lower than that of the reference drug, doxorubicin. Mechanistic studies revealed that GA3-induced apoptosis in HL-60 cells proceeded via both extrinsic and intrinsic pathways, with caspase-8 functioning upstream of caspase-9. In addition, GA3-driven apoptotic events were associated with up-regulation of Bax, down-regulation of Bcl-2 and cleavage of Bid. Moreover, GA3 triggered cytochrome c release from the mitochondria, in particular bypassing the involvement of the mitochondrial membrane potential.

CONCLUSION

Better solubility and a potential anti-MDR activity, combined with a comparable antitumor efficacy, make GA3 a potential drug candidate in cancer therapy that deserves further investigation.

Enhanced cytotoxicity of an anti-transferrin receptor IgG3-avidin fusion protein in combination with gambogic acid against human malignant hematopoietic cells: functional relevance of iron, the receptor, and reactive oxygen species

The human transferrin receptor (hTfR) is a target for cancer immunotherapy due to its overexpression on the surface of cancer cells. We previously developed an antibody-avidin fusion protein that targets hTfR (anti-hTfR IgG3-Av) and exhibits intrinsic cytotoxicity against certain malignant cells. Gambogic acid (GA), a drug that also binds hTfR, induces cytotoxicity in several malignant cell lines. We now report that anti-hTfR IgG3-Av and GA induce cytotoxicity in a new broader panel of hematopoietic malignant cell lines. Our results show that the effect of anti-hTfR IgG3-Av is iron-dependent whereas that of GA is iron-independent in all cells tested. In addition, we observed that GA exerts a TfR-independent cytotoxicity. We also found that GA increases the generation of reactive oxygen species that may play a role in the cytotoxicity induced by this drug. Additive cytotoxicity was observed by simultaneous combination treatment with these drugs and synergy by using anti-hTfR IgG3-Av as a chemosensitizing agent. In addition, we found a concentration of GA that is toxic to malignant hematopoietic cells but not to human hematopoietic progenitor cells. Our results suggest that these two compounds may be effective, alone or in combination, for the treatment of human hematopoietic malignancies.

Bioassay guided discovery of apoptosis inducers from gamboge by high-speed counter-current chromatography and high-pressure liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry

A screening system, composed of high-speed counter-current chromatography and high-pressure liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry, was established to find bioactive lead compound. This system succeeded in discovering apoptosis inducers from gamboge, the resin of Garcinia hanburyi. High-speed counter-current chromatography was used to provide well-separated fractions for bioassay and the resulted active fractions were rapidly identified using high-pressure liquid chromatography/electrospray ionization quadrupole time-of-flight mass spectrometry. The solvent system of n-hexane/ethyl acetate/methanol/water was optimized to the ratio of 7:3:7:3 (v/v/v/v) by a K value analysis. As a result, two active fractions were obtained. They showed apoptosis inducing effects as potent as that of taxol (500 nM) at the concentration of 1 microg/ml. Gambogenic acid (72.1%) and epimeric isogambogic acids (25.3%) were identified in one of the fractions. The other active fraction mainly contained two epimeric mixtures, gambogic acids (68.7%) and gambogoic acids (26.9%). Among them, gambogenic acid, without epimerization, has priority to be lead compound.

Gambogic acid mediates apoptosis as a p53 inducer through down-regulation of mdm2 in wild-type p53-expressing cancer cells

Gambogic acid (GA) is a natural product with potent apoptotic activity. Here, we showed that GA broadly inhibited the growth of cancer cells that expressed wild-type p53 as determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazol-iumbromide assay, (3)H-thymidine incorporation analysis, and an in vivo mouse xenograft model. GA induced massive cell apoptosis as judged by Annexin V and propidium iodide dual-staining experiments. Furthermore, we found that GA partially induced cancer cell growth inhibition in a p53-dependent manner because cell survival could be restored after endogenous p53 was attenuated by p53 transcriptional repressor pifithrin-alpha or p53 small interfering RNA. Interestingly, GA had no influence on p53 mRNA synthesis but dramatically enhanced its protein expression. This unique observation could be accounted for by the down-regulation of mdm2 at both mRNA and protein levels. It is concluded that GA enhances p53 protein level through inhibition of mdm2 expression and thereby hampers p53 harboring tumor growth.

Anti-invasive effect of gambogic acid in MDA-MB-231 human breast carcinoma cells

Gambogic acid (GA) has been known to have antitumor activity in vitro and in vivo. In the present study, we investigated the anti-invasive effects of GA in MDA-MB-231 human breast carcinoma cells. The results indicated that GA significantly inhibited the adhesion, migration, and invasion of the cells in vitro tested by the heterotypic adhesion assay, wound migration assay, and chamber invasion assay. Results of Western blotting and immunocytochemistry analysis showed that GA could suppress the expressions of matrix metalloproteinase 2 (MMP-2) and 9 (MMP-9) in MDA-MB-231 cells. Furthermore, gelatin zymography revealed that GA decreased the activities of MMP-2 and MMP-9. Additionally, GA exerted an inhibitory effect on the phosphorylation of ERK1/2 and JNK, while it had no effect on p38. Taken together, our results demonstrated the anti-invasive property of GA for the first time and indicated it could serve as a promising drug for the treatment of cancer metastasis.

Involvement of matrix metalloproteinase 2 and 9 in gambogic acid induced suppression of MDA-MB-435 human breast carcinoma cell lung metastasis

Cancer cell invasion is one of the crucial events in local spreading, growth, and metastasis of tumors. The present study investigated the antiinvasive and antimetastatic action of gambogic acid (GA) in MDA-MB-435 human breast carcinoma cells. GA caused a concentration-dependent suppression of cell invasion through Matrigel and significantly inhibited lung metastases of the cells transplanted in vivo. The potent effects of GA have been attributed to its ability to reduce the expression of matrix metalloproteinases (MMP) 2 and 9 in vitro and in vivo both at the protein and mRNA levels, which were associated with protein kinase C (PKC) signaling pathway as supported by the diminished antiinvasive effect of GA in the presence of specific activator of the pathway. Collectively, our data demonstrated that GA exhibited antiinvasion properties on highly invasive cancer cells via PKC mediated MMP-2/9 expression inhibition. This indicated that GA can be served as a potential novel therapeutic candidate for the treatment of cancer metastasis.

Gambogic acid is an antagonist of antiapoptotic Bcl-2 family proteins

The natural product gambogic acid (GA) has been reported to have cytotoxic activity against tumor cells in culture and was identified as an active compound in a cell-based high-throughput screening assay for activators of caspases, proteases involved in apoptosis. Using the antiapoptotic Bcl-2 family protein, Bfl-1, as a target for screening of a library of natural products, we identified GA as a competitive inhibitor that displaced BH3 peptides from Bfl-1 in a fluorescence polarization assay. Analysis of competition for BH3 peptide binding revealed that GA inhibits all six human Bcl-2 family proteins to various extents, with Mcl-1 and Bcl-B the most potently inhibited [concentrations required for 50% inhibition (IC(50)), < 1 micromol/L]. Competition for BH3 peptide binding was also confirmed using a time-resolved fluorescence resonance energy transfer assay. GA functionally inhibited the antiapoptotic Bcl-2 family proteins as shown by experiments using isolated mitochondria in which recombinant purified Bcl-2 family proteins suppress SMAC release in vitro, showing that GA neutralizes their suppressive effects on mitochondria in a concentration-dependent manner. GA killed tumor cell lines via an apoptotic mechanism, whereas analogues of GA with greatly reduced potency at BH3 peptide displacement showed little or no cytotoxic activity. However, GA retained cytotoxic activity against bax-/-bak-/- cells in which antiapoptotic Bcl-2 family proteins lack a cytoprotective phenotype, implying that GA also has additional targets that contribute to its cytotoxic mechanism. Altogether, the findings suggest that suppression of antiapoptotic Bcl-2 family proteins may be among the cytotoxic mechanisms by which GA kills tumor cells.

Microtubule depolymerization and phosphorylation of c-Jun N-terminal kinase-1 and p38 were involved in gambogic acid induced cell cycle arrest and apoptosis in human breast carcinoma MCF-7 cells

Gambogic acid (GA), an ingredient isolated from Garcinia hanburyi, has potent anticancer activity both in vitro and in vivo. In the present study, we examined the effects of GA on intracellular microtubules and reconstituted microtubules in vitro. Immunofluorescence microscopy revealed that 2.5 muM GA caused microtubule cytoskeleton disruption and microtubule depolymerization in human breast carcinoma MCF-7 cells, thereby reducing the amount of polymer form of tubulin and increasing the amount of monomer form of tubulin. We further confirmed that GA could depolymerize microtubule associated protein (MAP)-free microtubules and MAP-rich microtubules in vitro. Thus we suggested that GA-induced G2/M phase cell cycle arrest may be attributed to its depolymerization of microtubules. We also revealed that phosphorylation levels of p38 and c-Jun N-terminal kinase-1 (JNK-1) were increased markedly by GA, resulting in apoptosis of MCF-7 cells. Taken together, our results suggested that GA depolymerized microtubules and elevated the phosphorylation levels of JNK1 and p38, which caused G2/M cell cycle arrest and apoptosis in MCF-7 cells.