Related Articles

Background

Boswellic acids mixture of triterpenic acids obtained from the oleo gum resin of Boswellia serrata and known for its effectiveness in the treatment of chronic inflammatory disease including peritumor edema. Boswellic acids have been extensively studied for a number of activities including anti inflammatory, antitumor, immunomodulatory, and inflammatory bowel diseases. The present study describes the antimicrobial activities of boswellic acid molecules against oral cavity pathogens. Acetyl-11-keto-β-boswellic acid (AKBA), which exhibited the most potent antibacterial activity, was further evaluated in time kill studies, mutation prevention frequency, postantibiotic effect (PAE) and biofilm susceptibility assay against oral cavity pathogens.

Findings

AKBA exhibited an inhibitory effect on all the oral cavity pathogens tested (MIC of 2-4 μg/ml). It exhibited concentration dependent killing of Streptococcus mutans ATCC 25175 up to 8 × MIC and also prevented the emergence of mutants of S.mutans ATCC 25175 at 8× MIC. AKBA demonstrated postantibiotic effect (PAE) of 5.7 ± 0.1 h at 2 × MIC. Furthermore, AKBA inhibited the formation of biofilms generated by S.mutans and Actinomyces viscosus and also reduced the preformed biofilms by these bacteria.

Conclusions

AKBA can be useful compound for the development of antibacterial agent against oral pathogens and it has great potential for use in mouthwash for preventing and treating oral infections.

Activation of signal transducers and activators of transcription (STAT)-3 factors has been linked with survival, proliferation, chemoresistance and angiogenesis of tumor cells, including human multiple myeloma (MM). Thus agents that can suppress STAT3 activation have potential as cancer therapeutics. In our search for such agents, we identified acetyl-11-keto-β-boswellic acid (AKBA), originally isolated from Boswellia serrata. Our results show that AKBA inhibited constitutive STAT3 activation in human MM cells. AKBA suppressed IL-6-induced STAT3 activation, and the inhibition was reversible. The phosphorylation of both Jak 2 and Src, constituents of the STAT3 pathway, was inhibited by AKBA. Interestingly, treatment of cells with pervanadate suppressed AKBA’s effect to inhibit the phosphorylation of STAT3, thus suggesting the involvement of a protein tyrosine phosphatase. We found that AKBA induced Src homology region 2 domain-containing phosphatase 1 (SHP-1), which may account for its role in dephosphorylation of STAT3. Moreover, deletion of SHP-1 gene by SiRNA abolished the ability of AKBA to inhibit STAT3 activation. The inhibition of STAT3 activation by AKBA led to the suppression of gene products involved in proliferation (cyclin D1), survival (Bcl-2, Bcl-xL and Mcl-1), and angiogenesis (VEGF). This affect correlated with the inhibition of proliferation and apoptosis in MM cells. Consistent with these results, overexpression of constitutive active STAT3 significantly reduced the AKBA induced apoptosis. Overall, our results suggest that AKBA is a novel inhibitor of STAT3 activation and has potential in the treatment of cancer.

Ninety percent of cancer-mediated deaths are due to metastasis of the tumor, but the mechanisms controlling metastasis remain poorly understood. Thus, no therapy targeting this process has yet been approved. Chemokines and their receptors are mediators of chronic inflammation and have been linked to the metastasis of numerous cancers. More recently, the CXC chemokine receptor 4 (CXCR4) has emerged as a key mediator of tumor metastasis; therefore, identification of inhibitors of this receptor has the potential to abrogate metastasis. In this report, we demonstrate that acetyl-11-keto-β-boswellic acid (AKBA), a component of the therapeutic plant Boswellia serrata, can downregulate CXCR4 expression in pancreatic cancer cells. The reduction in CXCR4 induced by this terpenoid was found to be cell-type specific, as its expression was also abrogated in leukemia, myeloma, and breast cancer cell lines. Neither proteasome inhibitors nor lysosomal stabilization could prevent the AKBA-induced reduction in CXCR4 expression, and downregulation occurred at the transcriptional level. Suppression of CXCR4 by AKBA was accompanied by the inhibition of pancreatic cancer cell invasion, which is induced by CXCL12, the ligand for CXCR4. In addition, abrogation of the expression of chemokine receptor by AKBA was found in human pancreatic tissues from orthotopic animal model. AKBA also abolished breast tumor cell invasion, and this effect correlated with the disappearance of both the CXCR4 mRNA and CXCR4 protein. Overall, our results show that AKBA is a novel inhibitor of CXCR4 expression and, thus, has the potential to suppress the invasion and metastasis of cancer cells.

Background

Boswellic acids are pentacyclic triterpenes, which are produced in plants belonging to the genus Boswellia. Boswellic acids appear in the resin exudates of the plant and it makes up 25-35% of the resin. β-boswellic acid, 11-keto-β-boswellic acid and acetyl-11-keto-β-boswellic acid have been implicated in apoptosis of cancer cells, particularly that of brain tumors and cells affected by leukemia or colon cancer. These molecules are also associated with potent antimicrobial activities. The present study describes the antimicrobial activities of boswellic acid molecules against 112 pathogenic bacterial isolates including ATCC strains. Acetyl-11-keto-β-boswellic acid (AKBA), which exhibited the most potent antibacterial activity, was further evaluated in time kill studies, postantibiotic effect (PAE) and biofilm susceptibility assay. The mechanism of action of AKBA was investigated by propidium iodide uptake, leakage of 260 and 280 nm absorbing material assays.

Results

AKBA was found to be the most active compound showing an MIC range of 2-8 μg/ml against the entire gram positive bacterial pathogens tested. It exhibited concentration dependent killing of Staphylococcus aureus ATCC 29213 up to 8 × MIC and also demonstrated postantibiotic effect (PAE) of 4.8 h at 2 × MIC. Furthermore, AKBA inhibited the formation of biofilms generated by S. aureus and Staphylococcus epidermidis and also reduced the preformed biofilms by these bacteria. Increased uptake of propidium iodide and leakage of 260 and 280 nm absorbing material by AKBA treated cells of S aureus indicating that the antibacterial mode of action of AKBA probably occurred via disruption of microbial membrane structure.

Conclusions

This study supported the potential use of AKBA in treating S. aureus infections. AKBA can be further exploited to evolve potential lead compounds in the discovery of new anti-Gram-positive and anti-biofilm agents.

STAT3 activation has been associated with survival, proliferation and invasion of various human cancers. Whether betulinic acid, a pentacyclic triterpene, can modulates the STAT3 pathway, was investigated in human multiple myeloma (MM) cells. We found that betulinic acid inhibited constitutive activation of STAT3, Src kinase, JAK1 and JAK2. Pervanadate reversed the betulinic acid -induced down regulation of STAT3 activation, suggesting the involvement of a protein tyrosine phosphatase (PTP). Furthermore, betulinic acid induced the expression of the PTP SHP-1 and silencing of the SHP-1 gene abolished the ability of betulinic acid to inhibit STAT3 activation and rescues betulinic acid-induced cell death. Betulinic acid also downregulated the expression of STAT3-regulated gene products such as bcl-xL, bcl-2, cyclin D1, and survivin. This correlated with an increase in apoptosis as indicated by an increase in the sub-G1 cell population and an increase in caspase-3–induced PARP cleavage. Consistent with these results, over expression of constitutive active STAT3 significantly reduced the betulinic acid-induced apoptosis. Betulinic acid also enhanced the apoptosis induced by thalidomide (from 10% to 55%) and bortezomib (from 5% to 70%) in MM cells. Overall, our results suggest that betulinic acid down regulates STAT3 activation through upregulation of SHP-1 and this may have potential in sensitization of STAT3 over expressing tumors to chemotherapeutic agents.

Inflammation, although first characterized by Cornelius Celsus, a physician in first Century Rome, it was Rudolf Virchow, a German physician in nineteenth century who suggested a link between inflammation and cancer, cardiovascular diseases, diabetes, pulmonary diseases, neurological diseases and other chronic diseases. Extensive research within last three decades has confirmed these observations and identified the molecular basis for most chronic diseases and for the associated inflammation. The transcription factor, Nuclear Factor-kappaB (NF-κB) that controls over 500 different gene products, has emerged as major mediator of inflammation. Thus agents that can inhibit NF-κB and diminish chronic inflammation have potential to prevent or delay the onset of the chronic diseases and further even treat them. In an attempt to identify novel anti-inflammatory agents which are safe and effective, in contrast to high throughput screen, we have turned to “reverse pharmacology” or “bed to benchside” approach. We found that Ayurveda, a science of long life, almost 6000 years old, can serve as a “goldmine” for novel anti-inflammatory agents used for centuries to treat chronic diseases. The current review is an attempt to provide description of various Ayurvedic plants currently used for treatment, their active chemical components, and the inflammatory pathways that they inhibit.

Background

Boswellic acid is a type of triterpenoids with antiinflammatory and antiproliferative properties. Sphingomyelin metabolism generates multiple lipid signals affecting cell proliferation, inflammation, and apoptosis. Upregulation of acid sphingomyelinase (SMase) has been found in several inflammation-related diseases such as inflammatory bowel diseases, atherosclerosis, and diabetes.

Methods

The present study is to examine the effect of 3-acetyl-11-keto-β-boswellic acids (AKBA), a potent boswellic acid, on acid SMase activity and expression in intestinal cells. Both transformed Caco-2 cells and non-transformed Int407 cells were incubated with AKBA. After incubation, the change of acid SMase activity was assayed biochemically, the enzyme protein was examined by Western blot, and acid SMase mRNA was quantified by qPCR.

Results

We found that AKBA decreased acid SMase activity in both intestinal cell lines in dose and time dependent manners without affecting the secretion of the enzyme to the cell culture medium. The effect of AKBA was more effective in the fetal bovine serum-free culture medium. Among different types of boswellic acid, AKBA was the most potent one. The inhibitory effect on acid SMase activity occurred only in the intact cells but not in cell-free extract in the test tubes. At low concentration, AKBA only decreased the acid SMase activity but not the quantity of the enzyme protein. However, at high concentration, AKBA decreased both the mass of acid SMase protein and the mRNA levels of acid SMase in the cells, as demonstrated by Western blot and qPCR, respectively. Under the concentrations decreasing acid SMase activity, AKBA significantly inhibited cell proliferation.

Conclusion

We identified a novel inhibitory effect of boswellic acids on acid SMase expression, which may have implications in human diseases and health.

Background and the purpose of the study

Boswellia carterii have been used in traditional medicine for many years for management different gastrointestinal disorders. In this study, we wish to report urease inhibitory activity of four isolated compound of boswellic acid derivative.

Methods

4 pentacyclic triterpenoid acids were isolated from Boswellia carterii and identified by NMR and Mass spectroscopic analysis (compounds 1, 3-O-acetyl-9,11-dehydro-β-boswellic acid; 2, 3-O-acetyl-11-hydroxy-β-boswellic acid; 3. 3-O- acetyl-11-keto-β-boswellic acid and 4, 11-keto-β-boswellic acid. Their inhibitory activity on Jack bean urease were evaluated. Docking and pharmacophore analysis using AutoDock 4.2 and Ligandscout 3.03 programs were also performed to explain possible mechanism of interaction between isolated compounds and urease enzyme.

Results

It was found that compound 1 has the strongest inhibitory activity against Jack bean urease (IC50 = 6.27 ± 0.03 μM), compared with thiourea as a standard inhibitor (IC50 = 21.1 ± 0.3 μM).

Conclusion

The inhibition potency is probably due to the formation of appropriate hydrogen bonds and hydrophobic interactions between the investigated compounds and urease enzyme active site and confirms its traditional usage.

Synthetic analogues of naturally occurring triterpenoids; glycyrrhetinic acid, arjunolic acid and boswellic acids, by modification of A-ring with a cyano- and enone- functionalities, have been reported. A novel method of synthesis of α-cyanoenones from isoxazoles is reported. Bio-assays using primary mouse macrophages and tumor cell lines indicate potent anti-inflammatory and cytotoxic activities associated with cyanoenones of boswellic acid and glycyrrhetinic acid.

Avicins, a class of electrophilic triterpenoids with pro-apoptotic, anti-inflammatory and antioxidant properties, have been shown to induce redox-dependant post-translational modification of cysteine residues to regulate protein function. Based on (a) the cross-talk that occurs between redox and phosphorylation processes, and (b) the role of Stat3 in the process of apoptosis and carcinogenesis, we chose to study the effects of avicins on the processes of phosphorylation/dephosphorylation in Stat3. Avicins dephosphorylate Stat3 in a variety of human tumor cell lines, leading to a decrease in the transcriptional activity of Stat3. The expression of Stat3-regulated proteins such as c-myc, cyclin D1, Bcl2, survivin and VEGF were reduced in response to avicin treatment. Underlying avicin-induced dephosphorylation of Stat3 was dephosphorylation of JAKs, as well as activation of protein phosphatase-1. Downregulation of both Stat3 activity and expression of Stat 3-controlled pro-survival proteins, contributes to the induction of apoptosis in avicin treated tumor cells. Based on the role of Stat3 in inflammation and wounding, and the in vivo inhibition of VEGF by avicins in a mouse skin carcinogenesis model, it is likely that avicin-induced inhibition of Stat3 activity results in the suppression of the pro-inflammatory and pro-oxidant stromal environment of tumors. Activation of PP-1, which also acts as a cellular economizer, combined with the redox regulation by avicins, can aid in redirecting metabolism from growth promoting anabolic to energy sparing pathways.

Background

Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that regulates various cellular processes such as cell survival, angiogenesis and proliferation. In the present study, we examined that betulinic acid (BA), a triterpene from the bark of white birch, had the inhibitory effects on hypoxia-mediated activation of STAT3 in androgen independent human prostate cancer PC-3 cells.

Methodology/Principal Findings

BA inhibited the protein expression and the transcriptional activities of hypoxia-inducible factor-1α (HIF-1α) under hypoxic condition. Consistently, BA blocked hypoxia-induced phosphorylation, DNA binding activity and nuclear accumulation of STAT3. In addition, BA significantly reduced cellular and secreted levels of vascular endothelial growth factor (VEGF), a critical angiogenic factor and a target gene of STAT3 induced under hypoxia. Furthermore, BA prevented in vitro capillary tube formation in human umbilical vein endothelial cells (HUVECs) maintained in conditioned medium of hypoxic PC-3 cells, implying anti-angiogenic activity of BA under hypoxic condition. Of note, chromatin immunoprecipitation (ChiP) assay revealed that BA inhibited binding of HIF-1α and STAT3 to VEGF promoter. Furthermore, silencing STAT3 using siRNA transfection effectively enhanced the reduced VEGF production induced by BA treatment under hypoxia.

Conclusions/Significance

Taken together, our results suggest that BA has anti-angiogenic activity by disturbing the binding of HIF-1α and STAT3 to the VEGF promoter in hypoxic PC-3 cells.

Stress response pathways allow cells to sense and respond to environmental changes and adverse pathophysiological states. Pharmacological modulation of cellular stress pathways has implications in the treatment of human diseases, including neurodegenerative disorders, cardiovascular disease, and cancer. The quinone methide triterpene celastrol, derived from a traditional Chinese medicinal herb, has numerous pharmacological properties, and it is a potent activator of the mammalian heat shock transcription factor HSF1. However, its mode of action and spectrum of cellular targets are poorly understood. We show here that celastrol activates Hsf1 in Saccharomyces cerevisiae at a similar effective concentration seen in mammalian cells. Transcriptional profiling revealed that celastrol treatment induces a battery of oxidant defense genes in addition to heat shock genes. Celastrol activated the yeast Yap1 oxidant defense transcription factor via the carboxy-terminal redox center that responds to electrophilic compounds. Antioxidant response genes were likewise induced in mammalian cells, demonstrating that the activation of two major cell stress pathways by celastrol is conserved. We report that celastrol's biological effects, including inhibition of glucocorticoid receptor activity, can be blocked by the addition of excess free thiol, suggesting a chemical mechanism for biological activity based on modification of key reactive thiols by this natural product.

Pancreatic cancer (PaCa) is one of the most lethal cancers, with an estimated 5-year survival of <5% even when patients are given the best treatment available. In addition, these treatments are often toxic and expensive, thus new agents which are safe, affordable and effective are urgently needed. We describe here the results of our study with acetyl-11-keto-β-boswellic acid (AKBA), an agent obtained from an Ayurvedic medicine, gum resin of Boswellia serrata. Whether AKBA has an activity against human PaCa, was examined in in vitro models and in an orthotopic nude mouse model of PaCa. We found that AKBA inhibited the proliferation of four different PaCa cell lines (AsPC-1, PANC-28, and MIA PaCa-2 with K-Ras and p53 mutations, and BxPC-3 with wild-type K-Ras and p53 mutation). These effects correlated with an inhibition of constitutively active NF-κB and suppression of NF-κB regulating gene expression. AKBA also induced apoptosis, and sensitized the cells to apoptotic effects of gemcitabine. In the orthotopic nude mouse model of PaCa, p.o. administration of AKBA alone (100 mg/kg) significantly inhibited the tumor growth; this activity was enhanced by gemcitabine. In addition, AKBA inhibited the metastasis of the PaCa to spleen, liver, and lungs. This correlated with decreases in Ki-67, a biomarker of proliferation, and CD31, a biomarker of microvessel density, in the tumor tissue. AKBA produced significant decreases in the expression of NF-κB regulating genes in the tissues. Immunohistochemical analysis also showed AKBA downregulated the expression of COX-2, MMP-9, CXCR4, and VEGF in the tissues. Overall these results demonstrate that AKBA can suppress the growth and metastasis of human pancreatic tumors in an orthotopic nude mouse model that correlates with modulation of multiple targets.

Avicins, a family of triterpene electrophiles originally identified as potent inhibitors of tumor cell growth, have been shown to be pleiotropic compounds that also possess antioxidant, anti-mutagenic, and anti-inflammatory activities. We previously showed that Jurkat cells, which express a high level of Fas, are very sensitive to treatment with avicins. Thus, we hypothesized that avicins may induce cell apoptosis by activation of the Fas pathway. By using a series of cell lines deficient in cell death receptors, we demonstrated that upon avicin D treatment, Fas translocates to the cholesterol- and sphingolipid-enriched membrane microdomains known as lipid rafts. In the lipid rafts, Fas interacts with Fas-associated death domain (FADD) and Caspase-8 to form death-inducing signaling complex (DISC) and thus mediates cell apoptosis. Interfering with lipid raft organization by using a cholesterol-depleting compound, methyl-β-cyclodextrin, not only prevents the clustering of Fas and its DISC complex but also reduces the sensitivity of the cells to avicin D. Avicin D activates Fas pathways independent of the association between extracellular Fas ligands and Fas receptors. A deficiency in Fas and its downstream signaling molecules leads to the resistance of the cells to avicin D treatment. Taken together, our results demonstrate that avicin D triggers the redistribution of Fas in the membrane lipid rafts, where Fas activates receptor-mediated cell death.

Betulinic acid is a widely available plant-derived triterpene which is reported to possess selective cytotoxic activity against cancer cells of neuroectodermal origin and leukemia. However, the potential of betulinic acid as an antiproliferative and cytotoxic agent on vascular smooth muscle (VSMC) is still unclear. This study was carried out to demonstrate the antiproliferative and cytotoxic effect of betulinic acid on VSMCs using 3-[4,5-dimethylthizol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, flow cytometry cell cycle assay, BrdU proliferation assay, acridine orange/propidium iodide staining, and comet assay. Result from MTT and BrdU assays indicated that betulinic acid was able to inhibit the growth and proliferation of VSMCs in a dose-dependent manner with IC50 of 3.8 μg/mL significantly (P < 0.05). Nevertheless, betulinic acid exhibited G1 cell cycle arrest in flow cytometry cell cycle profiling and low level of DNA damage against VSMC in acridine orange/propidium iodide and comet assay after 24 h of treatment. In conclusion, betulinic acid induced G1 cell cycle arrest and dose-dependent DNA damage on VSMC.

Background

Gambogic acid (GA) is a major active ingredient of gamboge, a widely used traditional Chinese medicine that has been reported to be a potent cytotoxic agent against some malignant tumors. Many studies have shown that the NF-kappa B signaling pathway plays an important role in anti-apoptosis and the drug resistance of tumor cells during chemotherapy. In this study, the effects and mechanisms of GA and the NF-kappa B inhibitor celastrol on oral cancer cells were investigated.

Methods

Three human oral squamous cell carcinoma cell lines, Tca8113, TSCC and NT, were treated with GA alone, celastrol alone or GA plus celastrol. Cytotoxicity was assessed by MTT assay. The rate of apoptosis was examined with annexin V/PI staining as well as transmission electronic microscopy in Tca8113 cells. The level of constitutive NF-kappa B activity in oral squamous cell carcinoma cell lines was determined by immunofluorescence assays and nuclear extracts and electrophoretic mobility shift assays (EMSAs) in vitro. To further investigate the role of NF-kappa B activity in GA and celastrol treatment in oral squamous cell carcinoma, we used the dominant negative mutant SR-IκBα to inhibit NF-kappa B activity and to observe its influence on the effect of GA.

Results

The results showed that GA could inhibit the proliferation and induce the apoptosis of the oral squamous cell carcinoma cell lines and that the NF-kappa B pathway was simultaneously activated by GA treatment. The minimal cytotoxic dose of celastrol was able to effectively suppress the GA-induced NF-kappa B pathway activation. Following the combined treatment with GA and the minimal cytotoxic dose of celastrol or the dominant negative mutant SR-IκBα, proliferation was significantly inhibited, and the apoptotic rate of Tca8113 cells was significantly increased.

Conclusion

The combination of GA and celastrol has a synergistic antitumor effect. The effect can be primarily attributed to apoptosis induced by a decrease in NF-kappa B pathway activation. The NF-kappa B signaling pathway plays an important role in this process. Therefore, combining GA and celastrol may be a promising modality for treating oral squamous cell carcinoma.

STAT2 is an essential transcription factor in the type I interferon (IFN-α/β) signal transduction pathway and known for its role in mediating antiviral immunity and cell growth inhibition. Unlike other members of the STAT family, IFNs are the only cytokines known to date that can activate STAT2. Given the inflammatory and antiproliferative dual nature of IFNs, we hypothesized that STAT2 prevents inflammation-induced colorectal and skin carcinogenesis by altering the inflammatory immune response. Contrary to our hypothesis, deletion of STAT2 inhibited AOM/DSS-induced colorectal carcinogenesis as measured by prolonged survival, lower adenoma incidence, smaller polyps, and less chronic inflammation. STAT2 deficiency also inhibited DMBA/TPA-induced skin carcinogenesis as indicated by reduced papilloma multiplicity. A potential mechanism by which STAT2 promotes carcinogenesis is through activation of pro-inflammatory mediators. Deletion of STAT2 decreased AOM/DSS-induced expression and release of pro-inflammatory mediators such as interleukin 6 and CCL2 and decreased interleukin-6 release from skin carcinoma cells, which then decreased STAT3 activation. Our findings identify STAT2 as a novel contributor to colorectal and skin carcinogenesis that may act to increase the gene expression and secretion of pro-inflammatory mediators, which in turn activate the oncogenic STAT3 signaling pathway.

Purpose

Celastrol is an active ingredient of traditional herbal medicine and has recently been identified as a potent natural proteasome inhibitor. In this study, we evaluated the radiosensitizing potential of Celastrol in the human prostate cancer PC-3 model.

Materials and Methods

Clonogenic assays were performed to determine the radiosensitizing effect of Celastrol. Apoptosis was examined by flow cytometry using Annexin V and propidium iodide staining and by a caspase-3 activation assay. DNA damage processing was examined by immunofluorescent staining and Western blot for phosphorylated H2AX (γH2AX). PC-3 xenograft model in athymic nude mouse was exployed for in vivo efficacy of Celastrol in combination with radiation. The tumor samples were also analyzed for apoptosis and angiogenesis.

Results

Celastrol sensitized PC-3 cells to ionizing radiation (IR) in a dose- and schedule-dependent manner, where pretreatment of Celastrol for 1 h followed by IR achieved maximum radiosensitization. Celastrol significantly prolonged the presence of IR-induced γH2AX and increased IR-induced apoptosis. Celastrol combined with fractionated radiation significantly inhibited PC-3 tumor growth in vivo without obvious systemic toxicity. The combination treatment increased γH2AX levels and apoptosis, induced cleavage of poly(ADP-ribose)polymerase (PARP) and Mcl-1, as well as reduced angiogenesis in vivo, as compared with either treatment alone.

Conclusions

Celastrol sensitizes PC-3 cells to radiation both in vitro and in vivo, by impairing DNA damage processing as well as by augmenting apoptosis. Celastrol may represent a promising new adjuvant regime for the treatment of hormone-refractory prostate cancer.

Avicins, a family of plant-derived triterpenoids, have been shown to possess pro-apoptotic, anti-mutagenic and anti-inflammatory properties in mammalian cells. Through thiol binding, avicins can also mediate antioxidant defense. Accumulating evidence uncovered during the past several years suggests that avicins induce tumor cell death via multiple mechanisms. This review will focus on recent studies that provide insights into the cellular and molecular processes and pathways by which avicins induce tumor cell death, including the canonical intrinsic mitochondrial and the Fas-mediated apoptosis cascades as well as autophagy-associated non-apoptotic programmed cell death.

Avicins, a family of apoptotic triterpene electrophiles, are known to regulate cellular metabolism and energy homeostasis, by targeting the mitochondria. Having evolved from “ancient hopanoids,” avicins bear a structural resemblance with glucocorticoids (GCs), which are the endogenous regulators of metabolism and energy balance. These structural and functional similarities prompted us to compare the mode of action of avicin D with dexamethasone (Dex), a prototypical GC. Using cold competition assay, we show that Avicin D competes with Dex for binding to the GC receptor (GR), leading to its nuclear translocation. In contrast to Dex, avicin-induced nuclear translocation of GR does not result in transcriptional activation of GC-dependent genes. Instead we observe a decrease in the expression of GC-dependent metabolic proteins such as PEPCK and FASN. However, like Dex, avicin D treatment does induce a transrepressive effect on the pro-inflammatory transcription factor NF-κB. While avicin's ability to inhibit NF-κB and its downstream targets appear to be GR-dependent, its pro-apoptotic effects were independent of GR expression. Using various deletion mutants of GR, we demonstrate the requirement of both the DNA and ligand binding domains of GR in mediating avicin D's transrepressive effects. Modeling of avicin-GR interaction revealed that avicin molecule binds only to the antagonist confirmation of GR. These findings suggest that avicin D has properties of being a selective GR modulator that separates transactivation from transrepression. Since the gene-activating properties of GR are mainly linked to its metabolic effects, and the negative interference with the activity of transcription factors to its anti-inflammatory and immune suppressive effects, the identification of such a dissociated GR ligand could have great potential for therapeutic use.

The proteasome is a multicatalytic protease complex that degrades most endogenous proteins including misfolded or damaged proteins to ensure normal cellular function. The ubiquitin-proteasome degradation pathway plays an essential role in multiuple cellular processes, including cell cycle progression, proliferation, apoptosis and angiogenesis. It has been shown that human cancer cells are more sensitive to proteasome inhibition than normal cells, indicating that a proteasome inhibitor could be used as a novel anticancer drug. Indeed, this idea has been supported by the encouraging results of the clinical trials using the proteasome inhibitor Bortezomib (Velcade, PS-341), a drug approved by the US Food and Drug Administration (FDA). Several natural compounds, including the microbial metabolite lactacystin, green tea polyphenols, and traditional medicinal triterpenes, have been shown to be potent proteasome inhibitors. These findings suggest the potential use of natural proteasome inhibitors as not only chemopreventive and chemotherapeutic agents, but also tumor sensitizers to conventional radiotherapy and chemotherapy. In this review, we will summarize the structure and biological activities of the proteasome and several natural compounds with proteasome inhibitory activity, and will discuss the potential use of these compounds for the prevention and treatment of human cancers.

The role of angiogenesis in tumor growth and metastasis is well established. Identification of small molecule that blocks tumor angiogenesis and is safe and affordable has been a challenge in drug development. In this study, we demonstrated that acetyl-11-keto-β-boswellic acid (AKBA), an active component from an Ayurvedic medicinal plant (Boswellia serrata), could strongly inhibit tumor angiogenesis. AKBA suppressed tumor growth in the human prostate tumor xenograft mice treated daily (10 mg/kg of AKBA) after solid tumors reached about 100 mm3 (n=5). The inhibitory effect of AKBA on tumor growth was well correlated with suppression of angiogenesis. When examined for the molecular mechanism, we found that AKBA significantly inhibited blood vessel formation in the Matrigel plug assay in mice and effectively and suppressed vascular endothelial growth factor (VEGF)-induced microvessel sprouting in rat aortic ring assay ex vivo. Furthermore, AKBA inhibited VEGF-induced cell proliferation, chemotactic motility, and the formation of capillary-like structures from primary cultured human umbilical vascular endothelial cells (HUVECs) in a dose-dependent manner. Western blot analysis and in vitro kinase assay revealed that AKBA suppressed VEGF-induced phosphorylation of VEGF receptor 2 kinase (KDR/Flk-1) with IC50 of 1.68 μmol/L. Specifically, AKBA suppressed the downstream protein kinases of VEGFR2, including Src family kinase, focal adhesion kinase, extracellular signal-related kinase, AKT, mTOR, and ribosomal protein S6 kinase. Our findings suggest that AKBA potently inhibits human prostate tumor growth through inhibition of angiogenesis induced by VEGFR2 signaling pathways.

Epiceanothic acid (1) is a naturally occurring, but very rare pentacyclic triterpene with a unique pentacyclic triterpene (PT) structure. An efficient synthesis of 1 starting from betulin (3) has been accomplished in 12 steps with a total yield of 10% in our study. Compound 1 and selected synthetic intermediates were further evaluated as anti-HIV-1 agents, inhibitors of glycogen phosphorylase (GP), and cytotoxic agents. Compound 1 exhibited moderate HIV-1 inhibition. Most importantly, compound 5, with an opened A-ring, showed significant GP inhibitory activity with an IC50 of 0.21 μM, suggesting a potential for development as an anti-diabetic agent. On the other hand, compound 12, with a closed A-ring, showed potent cytotoxicity against A549 and MCF-7 human tumor cell lines, with IC50 values of 0.89 and 0.33 μM, respectively. These results suggest that the A-ring of PTs is an important pharmacophore that could be modified to involve different biological activities.

Gastrointestinal cancers are frequently associated with chronic inflammation and excessive secretion of IL-6 family cytokines, which promote tumorigenesis through persistent activation of the GP130/JAK/STAT3 pathway. Although tumor progression can be prevented by genetic ablation of Stat3 in mice, this transcription factor remains a challenging therapeutic target with a paucity of clinically approved inhibitors. Here, we uncovered parallel and excessive activation of mTOR complex 1 (mTORC1) alongside STAT3 in human intestinal-type gastric cancers (IGCs). Furthermore, in a preclinical mouse model of IGC, GP130 ligand administration simultaneously activated mTORC1/S6 kinase and STAT3 signaling. We therefore investigated whether mTORC1 activation was required for inflammation-associated gastrointestinal tumorigenesis. Strikingly, the mTORC1-specific inhibitor RAD001 potently suppressed initiation and progression of both murine IGC and colitis-associated colon cancer. The therapeutic effect of RAD001 was associated with reduced tumor vascularization and cell proliferation but occurred independently of STAT3 activity. We analyzed the mechanism of GP130-mediated mTORC1 activation in cells and mice and revealed a requirement for JAK and PI3K activity but not for GP130 tyrosine phosphorylation or STAT3. Our results suggest that GP130-dependent activation of the druggable PI3K/mTORC1 pathway is required for inflammation-associated gastrointestinal tumorigenesis. These findings advocate clinical application of PI3K/mTORC1 inhibitors for the treatment of corresponding human malignancies.

Background

Interstitial pulmonary fibrosis is characterized by an altered cellular composition of the alveolar region with excessive deposition of collagen. Lung inflammation is also common in pulmonary fibrosis. This study aims to test the inhibition of 5-lipooxygenase (5-LOX) by boswellic acid (BA) extract in an experimental model of pulmonary fibrosis using bleomycin (BL).

Methods

Boswellic acid extract (1 g/kg) was force-fed to rats seven days prior to administration of BL or gamma irradiation or both. BL (0.15 U/rat) in 25 μl of 0.9% normal saline (NS) or 0.9% NS alone was administered intratracheally. Rats were exposed to two fractionated doses of gamma irradiation (0.5 Gy/dose/week) with a gamma cell-40 (Cesium-137 irradiation units, Canada) during the last two weeks of the experiment. BA was administered during BL or irradiation treatment or both. After the animals were sacrificed, bronchoalveolar lavage was performed; lungs were weighed and processed separately for biochemical and histological studies.

Results

In rats treated with BL, levels of transforming growth factor-β1 (TGF-β1) and tumor necrosis factor-α (TNF-α) were significantly elevated (P = 0.05 and P = 0.005). Hydroxyproline was highly and extensively expressed. Immunoreactive compounds were abundantly expressed, represented in the levels of macrophages infiltrate, accumulation of eosinophils and neutrophils in the lung as well as the aggregation of fibroblasts in the fibrotic area. The levels of lipoxygenase enzyme activity were significantly increased (P = 0.005). Antioxidant activities measured in BL-treated rats deteriorated, coupled with the elevation of both levels of plasma lipid peroxide (LP) content and bronchoalveolar lavage lactate dehydrogenase activity. BA-treated rats had reduced number of macrophages, (P = 0.01), neutrophils in bronchoalveolar lavage (P = 0.01) and protein (P = 0.0001). Moreover, the hydroxyproline content was significantly lowered in BA-treated rats (P = 0.005). BA extract inhibited the TGF-ß induced fibrosis (P = 0.01) and 5-LOX activity levels (P = 0.005).

Histologically, BA reduced the number of infiltrating cells, ameliorated the destruction of lung architecture and attenuated lung fibrosis.

Conclusion

BA attenuates the BL-induced injury response in rats, such as collagen accumulation, airway dysfunction and injury. This study suggests that the blocking of 5-LOX may prevent the progression of fibrosis.