Beta-phenylethyl isothiocyanate mediated apoptosis; contribution of Bax and the mitochondrial death pathway

Harnessing the Power of Traditional Chinese Medicine in Cancer Treatment: The Role of Nanocarriers

For centuries, traditional Chinese medicine (TCM) has had certain advantages in the treatment of tumors. However, due to their poor water solubility, low bioavailability and potential toxicity, their effective delivery to target sites can be a major challenge. Nanocarriers based on the active ingredients of TCM, such as liposomes, polymer nanoparticles, inorganic nanoparticles, and organic/inorganic nanohybrids, are a promising strategy to improve the delivery of TCM, resulting in higher therapeutic outcomes and fewer side effects. Therefore, this article intends to review the application of Chinese medicine nano preparation in tumor. Firstly, we introduce the classification and synthesis of nanometer preparations of Chinese medicine. The second part mainly introduces the different responses of TCM nano-preparations in the course of treatment to introduce how TCM nano-preparations play a role in anti-tumor therapy. The third part focuses on Different response modes of Chinese medicine nano preparations in tumor therapy. The fourth part elucidates the application of Chinese medicine nano preparations in the treatment of cancer. Finally, the research direction to be explored in related fields is put forward.

Tumor-targeted hyaluronic acid-based oxidative stress nanoamplifier with ROS generation and GSH depletion for antitumor therapy

Tumor cells with innate oxidative stress are more susceptible to exogenous ROS-mediated oxidative damage than normal cells. However, the generated ROS could be scavenged by the overexpressed GSH in cancer cells, thus causing greatly restricted efficiency of ROS-mediated antitumor therapy. Herein, using cinnamaldehyde (CA) as a ROS generator while β-phenethyl isothiocyanate (PEITC) as a GSH scavenger, we designed a tumor-targeted oxidative stress nanoamplifier to elevate intracellular ROS level and synchronously suppress antioxidant systems, for thorough redox imbalance and effective tumor cells killing. First, an amphiphilic acid-sensitive cinnamaldehyde-modified hyaluronic acid conjugates (HA-CA) were synthesized, which could self-assemble into nano-assembly in aqueous media via strong hydrophobic interaction and π-π stacking. Then, aromatic PEITC was appropriately encapsulated into HA-CA nano-assembly to obtain HA-CA/PEITC nanoparticles. Through enhanced permeability retention (EPR) effect and specific CD44 receptor-mediated endocytosis, HA-CA/PEITC nanoparticles could accumulate in tumor tissues and successfully release CA and PEITC under acidic lysosomal environment. Both in vitro and in vivo results showed that the nanoparticles could efficiently boost oxidative stress of tumor cells via generating ROS and depleting GSH, and finally achieve superior antitumor efficacy. This nanoamplifier with good biosafety provides a potential strategy to augment ROS generation and suppress GSH for enhanced oxidation therapy.

Nanodelivery of natural isothiocyanates as a cancer therapeutic

Natural isothiocyanates (ITCs) are phytochemicals abundant in cruciferous vegetables with the general structure, R-NCS. They are bioactive organosulfur compounds derived from the hydrolysis of glucosinolates by myrosinase. A significant number of isothiocyanates have been isolated from different plant sources that include broccoli, Brussels sprouts, cabbage, cauliflower, kale, mustard, wasabi, and watercress. Several ITCs have been demonstrated to possess significant pharmacological properties including: antioxidant, anti-inflammatory, anti-cancer and antimicrobial activities. Due to their chemopreventive effects on many types of cancer, ITCs have been regarded as a promising anti-cancer therapeutic agent without major toxicity concerns. However, their clinical application has been hindered by several factors including their low aqueous solubility, low bioavailability, instability as well as their hormetic effect. Moreover, the typical dietary uptake of ITCs consumed for promotion of good health may be far from their bioactive (or cytotoxic) dose necessary for cancer prevention and/or treatment. Nanotechnology is one of best options to attain enhanced efficacy and minimize hormetic effect for ITCs. Nanoformulation of ITCs leads to enhance stability of ITCs in plasma and emphasize on their chemopreventive effects. This review provides a summary of the potential bioactivities of ITCs, their mechanisms of action for the prevention and treatment of cancer, as well as the recent research progress in their nanodelivery strategies to enhance solubility, bioavailability, and anti-cancer efficacy.

Potentiating photodynamic therapy of ICG-loaded nanoparticles by depleting GSH with PEITC

Photodynamic therapy (PDT) is a clinically approved cancer treatment which utilizes reactive oxygen species (ROS) to eradicate cancer cells. But the high concentration of GSH inside tumor cells can neutralize the generated ROS during PDT, resulting in an insufficient therapeutic effect. To address this issue, we combined ICG-loaded nanoparticles with PEITC for potent PDT. ICG encapsulated in novel hydroxyethyl starch-oleic acid conjugate (HES-OA) nanoparticles (∼50 nm) exhibited excellent stability and efficient singlet oxygen generation under laser irradiation, promoted cellular uptake, and enhanced tumor accumulation, whilst PEITC depleted intracellular GSH significantly. As a result, PDT based on ICG-loaded NPs combined with PEITC synergistically suppressed cancer cells both in vitro and in vivo. Potentiating ICG-loaded NPs with PEITC represents a novel and efficient strategy to enhance PDT efficacy.

Mitochondrial dysfunction in cancer chemoprevention by phytochemicals from dietary and medicinal plants

Cancer chemoprevention, a scientific term coined by Dr. Sporn in the late seventies, implies use of natural or synthetic chemicals to block, delay or reverse carcinogenesis. Phytochemicals derived from edible and medicinal plants have been studied rather extensively for cancer chemoprevention using preclinical models in the past few decades. Nevertheless, some of these agents (e.g., isothiocyanates from cruciferous vegetables like broccoli and watercress) have already entered into clinical investigations. Examples of widely studied and highly promising phytochemicals from edible and medicinal plants include cruciferous vegetable constituents (phenethyl isothiocyanate, benzyl isothiocyanate, and sulforaphane), withaferin A (WA) derived from a medicinal plant (Withania somnifera) used heavily in Asia, and an oriental medicine plant component honokiol (HNK). An interesting feature of these structurally-diverse phytochemicals is that they target mitochondria to provoke cancer cell-selective death program. Mechanisms underlying cell death induction by commonly studied phytochemicals have been discussed rather extensively and thus are not covered in this review article. Instead, the primary focus of this perspective is to discuss experimental evidence pointing to mitochondrial dysfunction in cancer chemoprevention by promising phytochemicals.

Advances in molecular signaling mechanisms of β-phenethyl isothiocyanate antitumor effects

β-Phenethyl isothiocyanate (PEITC) is an important phytochemical from cruciferous vegetables and is being evaluated for chemotherapeutic activity in early phase clinical trials. Moreover, studies in cell culture and in animals found that the anticarcinogenic activities of PEITC involved all the major stages of tumor growth: initiation, promotion, and progression. A number of mechanisms have been proposed for the chemopreventive activities of this compound. Here, we focus on the major molecular signaling pathways for the anticancer activities of PEITC. These include (1) activation of apoptosis pathways; (2) induction of cell cycle arrest; and (3) inhibition of the survival pathways. Furthermore, we also discussed the regulation of drug-metabolizing enzymes, including cytochrome P450s, metabolizing enzymes, and multidrug resistance.

Phenethyl isothiocyanate promotes immune responses in normal BALB/c mice, inhibits murine leukemia WEHI-3 cells, and stimulates immunomodulations in vivo

Enhanced cruciferous vegetable consumption is associated with the reduction of cancer incidence as shown in epidemiological studies. Phenethyl isothiocyanate (PEITC), one of the important compounds in cruciferous vegetables, has been shown to induce apoptosis in many types of human cancer cell lines, but there is no available information addressing the effects on normal and leukemia mice in vivo. The purpose of this study is to focus on the in vivo effects of PEITC on immune responses of normal and WEHI-3 leukemia BALB/c mice in vivo. Influences of PEITC on BALB/c mice after intraperitoneal (i.p.) injection with WEHI-3 cells and normal mice were investigated. In normal BALB/c mice, PEITC did not affect the body weight when compared to the olive oil treated animals. Moreover, PEITC promoted phagocytosis by macrophages from peripheral blood mononuclear cells (PBMC) and peritoneal cavity, increased the levels of CD11b and Mac-3, decreased the level of CD19 and promoted natural killer (NK) cell cytotoxic activity, but it did not alter the level of CD3. Also, PEITC enhanced T cell proliferation after concanavalin A (Con A) stimulation. Otherwise, PEITC increased the body weight, but decreased the weight of liver and spleen as compared to the olive oil-treated WEHI-3 leukemia mice. PEITC also increased the level of CD19, decreased the levels of CD3 and Mac-3 rather than influence in the level of CD11b, suggesting that the differentiation of the precursor of macrophages and T cells was inhibited, but the differentiation of the precursor of B cells was promoted in leukemia mice. Furthermore, PEITC enhanced phagocytosis by monocytes and macrophages from PBMC and peritoneal cavity, and also promoted the NK cell cytotoxic activity in comparison with the group of leukemia mice. Based on these observations, the biological properties of PEITC can promote immune responses in normal and WEHI-3 leukemia mice in vivo. © 2011 Wiley Periodicals, Inc. Environ Toxicol, 2013.

Cancer chemoprevention with dietary isothiocyanates mature for clinical translational research

Inverse association between dietary intake of cruciferous vegetables and cancer risk observed in population-based case-control studies is partly attributable to structurally simple but mechanistically complex phytochemicals with an isothiocyanate (-N=C=S) functional group. Cancer protective role for dietary isothiocyanates (ITCs) is substantiated by preclinical studies in rodent models. A common feature of many naturally occurring ITCs relates to their ability to cause growth arrest and cell death selectively in cancer cells. At the same time, evidence continues to accumulate to suggest that even subtle change in chemical structure of the ITCs can have a profound effect on their activity and mechanism of action. Existing mechanistic paradigm stipulates that ITCs may not only prevent cancer initiation by altering carcinogen metabolism but also inhibit post-initiation cancer development by suppressing many processes relevant to tumor progression, including cellular proliferation, neoangiogenesis, epithelial-mesenchymal transition, and self-renewal of cancer stem cells. Moreover, the ITCs are known to suppress diverse oncogenic signaling pathways often hyperactive in human cancers (e.g. nuclear factor-κB, hormone receptors, signal transducer and activator of transcription 3) to elicit cancer chemopreventive response. However, more recent studies highlight potential adverse effect of Notch activation by ITCs on their ability to inhibit migration of cancer cells. Mechanisms underlying ITC-mediated modulation of carcinogen metabolism, growth arrest, and cell death have been reviewed extensively. This article provides a perspective on bench-cage-bedside evidence supporting cancer chemopreventive role for some of the most promising ITCs. Structure-activity relationship and mechanistic complexity in the context of cancer chemoprevention with ITCs is also highlighted.

Biological targets of isothiocyanates

BACKGROUND

Isothiocyanates are phytochemicals with a broad array of effects in biological systems. Bioactivity includes the stimulation of cellular antioxidant systems, induction of apoptosis and interference with cytokine production and activity. Epidemiological evidence and experimental studies indicate that naturally occurring isothiocyanates and synthetic derivatives have anti-cancer and anti-inflammatory properties.

SCOPE OF REVIEW

This review focuses on the molecular targets of isothiocyanates, and how target modification translates into a biological response.

MAJOR CONCLUSIONS

Isothiocyanates may mediate their effects via direct protein modification or indirectly by disruption of redox homeostasis and increased thiol oxidation. Some target proteins have been identified, but in-depth searches with new techniques are needed to reveal novel targets. Site-directed mutagenesis and isothiocyanate structure-activity relationships will assist in determining the biological significance of specific modifications.

GENERAL SIGNIFICANCE

Target identification is important for rational drug design and exploiting the therapeutic potential of isothiocyanates. It also provides insight into the diverse pathways that these compounds regulate.

Impaired PI3K/Akt signal pathway and hepatocellular injury in high-fat fed rats

AIM: To determine whether mitochondrial dysfunction resulting from high-fat diet is related to impairment of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt, also known as PKB) pathway.

METHODS: Rat models of nonalcoholic fatty liver were established by high-fat diet feeding. The expression of total and phosphorylated P13K and Akt proteins in hepatocytes was determined by Western blotting. Degree of fat accumulation in liver was measured by hepatic triglyceride. Mitochondrial number and size were determined using quantitative morphometric analysis under transmission electron microscope. The permeability of the outer mitochondrial membrane was assessed by determining the potential gradient across this membrane.

RESULTS: After Wistar rats were fed with high-fat diet for 16 wk, their hepatocytes displayed an accumulation of fat (103.1 ± 12.6 vs 421.5 ± 19.7, P < 0.01), deformed mitochondria (9.0% ± 4.3% vs 83.0% ± 10.9%, P < 0.05), and a reduction in the mitochondrial membrane potential (389.385% ± 18.612% vs 249.121% ± 13.526%, P < 0.05). In addition, the expression of the phosphorylated P13K and Akt proteins in hepatocytes was reduced, as was the expression of the anti-apoptotic protein Bcl-2, while expression of the pro-apoptotic protein caspase-3 was increased. When animals were treated with pharmacological inhibitors of P13K or Akt, instead of high-fat diet, a similar pattern of hepatocellular fat accumulation, mitochondrial impairment, and change in the levels of PI3K, Akt, Bcl-2 was observed.

CONCLUSION: High-fat diet appears to inhibit the PI3K/Akt signaling pathway, which may lead to hepatocellular injury through activation of the mitochondrial membrane pathway of apoptosis.

Influence of fermentation conditions on glucosinolates, ascorbigen, and ascorbic acid content in white cabbage (Brassica oleracea var. capitata cv. Taler) cultivated in different seasons

The content of glucosinolates (GLS), ascorbigen, and ascorbic acid in white cabbage (Brassica oleracea var. capitata cv. Taler) cultivated in different seasons (summer and winter) was determined, before and after spontaneous and starter-induced fermentation. Different salt concentrations (0.5% NaCl or 1.5% NaCl) were used for sauerkraut production. Glucoiberin, sinigrin, and glucobrassicin were dominating in raw white cabbage cultivated either in winter or summer seasons. Ascorbigen precursor, glucobrassicin, was found higher in cabbage cultivated in winter (2.54 micromol/g dw) than those grown in summer (1.83 micromol/g dw). Cabbage fermented for 7 d was found to contain only traces of some GLS irrespective of the fermentation conditions used. Ascorbigen synthesis occurred during white cabbage fermentation. Brining cabbage at low salt concentration (0.5% NaCl) improved ascorbigen content in sauerkraut after 7 d of fermentation at 25 degrees C. The highest ascorbigen concentration was observed in low-sodium (0.5% NaCl) sauerkraut produced from cabbage cultivated in winter submitted to either natural (109.0 micromol/100 g dw) or starter-induced fermentation (108.3 and 104.6 micromol/100 g dw in cabbages fermented by L. plantarum and L. mesenteroides, respectively). Ascorbic acid content was found higher in cabbage cultivated in summer and fermentation process led to significant reductions. Therefore, the selection of cabbages with high glucobrassicin content and the production of low-sodium sauerkrauts may provide enhanced health benefits towards prevention of chronic diseases.

MTBITC mediates cell cycle arrest and apoptosis induction in human HepG2 cells despite its rapid degradation kinetics in the in vitro model

Despite the great variety of structure homologous, experimental research on the cancer preventive properties of isothiocyanates (ITCs) is limited to only a fractional amount thereof so far. Especially the degradation of these compounds in the experimental system has not been investigated so far. In this study, we investigated the effect of 4-methylthiobutyl isothiocyante (MTBITC) on the proliferation of human hepatoma (HepG2) cells and underlying mechanisms. A concentration and time-dependent reduction in proliferation activity could be observed in cells treated with MTBITC exceeding 10 microM. At these concentrations MTBITC-induced apoptosis in HepG2 cells could be observed by internucleosomal DNA fragmentation, flow cytometry analysis, and the detection of single-stranded apoptotic DNA. In all the three assays, clear apoptotic events were present after 6-hr exposure to MTBITC. Apoptosis induction was accompanied by a time-dependent arrest of HepG2 cells at the G2/M phase of the cell cycle. This study shows for the first time the inhibitory potency of MTBITC on metabolically competent hepatoma cells, whereas the loss of reduced glutathione and its impact on mitochondria seem to be the major processes involved in the initiation and execution of the apoptotic cell death. The results of this study also showed that irrespective of the intense degradation kinetics of MTBITC, the strong cytostatic effect of the ITC was not markedly affected by it and suggests that although ITCs are only present at maximum concentrations in a living system for a rather short time, this might be sufficient to exert their therapeutic effects.

Phytochemicals as potential chemopreventive and chemotherapeutic agents in hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is the fifth commonest malignancy worldwide and the incidence is rising. Surgery, including transplantation resection, is currently the most effective treatment for HCC; however, recurrence rates are high and long-term survival is poor. Identifying novel chemopreventive and chemotherapeutic agents and targeting them to patients at high risk of developing HCC or following curative treatment may go some way towards improving prognosis. This review examines current knowledge regarding the chemopreventive and chemotherapeutic potential of phytochemicals in heptocarcinogenesis. Both in-vitro and animal studies demonstrate that several phytochemicals, including curcumin, resveratrol, green tea catechins, oltipraz and silibinin, possess promising chemopreventive and chemotherapeutic properties. Despite this, very few clinical trials have been performed. Problems regarding validation of biomarkers, agent delivery, side effects and patient selection are barriers that need to be overcome to determine the potential of such agents in clinical practice.

Cancer chemoprevention: a radical perspective

Cancer chemopreventive agents block the transformation of normal cells and/or suppress the promotion of premalignant cells to malignant cells. Certain agents may achieve these objectives by modulating xenobiotic biotransformation, protecting cellular elements from oxidative damage, or promoting a more differentiated phenotype in target cells. Conversely, various cancer chemopreventive agents can encourage apoptosis in premalignant and malignant cells in vivo and/or in vitro, which is conceivably another anticancer mechanism. Furthermore, it is evident that many of these apoptogenic agents function as prooxidants in vitro. The constitutive intracellular redox environment dictates a cell's response to an agent that alters this environment. Thus, it is highly probable that normal cells, through adaption, could acquire resistance to transformation via exposure to a chemopreventive agent that promotes oxidative stress or disrupts the normal redox tone of these cells. In contrast, transformed cells, which typically endure an oxidizing intracellular environment, would ultimately succumb to apoptosis due to an uncontrollable production of reactive oxygen species caused by the same agent. Here, we provide evidence to support the hypothesis that reactive oxygen species and cellular redox tone are exploitable targets in cancer chemoprevention via the stimulation of cytoprotection in normal cells and/or the induction of apoptosis in transformed cells.

Covalent binding to tubulin by isothiocyanates. A mechanism of cell growth arrest and apoptosis

Isothiocyanates (ITCs) found in cruciferous vegetables, including benzyl-ITC (BITC), phenethyl-ITC (PEITC), and sulforaphane (SFN), inhibit carcinogenesis in animal models and induce apoptosis and cell cycle arrest in various cell types. The biochemical mechanisms of cell growth inhibition by ITCs are not fully understood. Our recent study showed that ITC binding to intracellular proteins may be an important initiating event for the induction of apoptosis. However, the specific protein target(s) and molecular mechanisms were not identified. In this study, two-dimensional gel electrophoresis of human lung cancer A549 cells treated with radiolabeled PEITC and SFN revealed that tubulin may be a major in vivo binding target for ITC. We examined whether binding to tubulin by ITCs could lead to cell growth arrest. The proliferation of A549 cells was significantly reduced by ITCs, with relative activities of BITC > PEITC > SFN. All three ITCs also induced mitotic arrest and apoptosis with the same order of activity. We found that ITCs disrupted microtubule polymerization in vitro and in vivo with the same order of potency. Mass spectrometry demonstrated that cysteines in tubulin were covalently modified by ITCs. Ellman assay results indicated that the modification levels follow the same order, BITC > PEITC > SFN. Together, these results support the notion that tubulin is a target of ITCs and that ITC-tubulin interaction can lead to downstream growth inhibition. This is the first study directly linking tubulin-ITC adduct formation to cell growth inhibition.

The isothiocyanate class of bioactive nutrients covalently inhibit the MEKK1 protein kinase

BACKGROUND

Dietary isothiocyanates (ITCs) are electrophilic compounds that have diverse biological activities including induction of apoptosis and effects on cell cycle. They protect against experimental carcinogenesis in animals, an activity believed to result from the transcriptional induction of "Phase 2" enzymes. The molecular mechanism of action of ITCs is unknown. Since ITCs are electrophiles capable of reacting with sulfhydryl groups on amino acids, we hypothesized that ITCs induce their biological effects through covalent modification of proteins, leading to changes in cell regulatory events. We previously demonstrated that stress-signaling kinase pathways are inhibited by other electrophilic compounds such as menadione. We therefore tested the effects of nutritional ITCs on MEKK1, an upstream regulator of the SAPK/JNK signal transduction pathway.

METHODS

The activity of MEKK1 expressed in cells was monitored using in vitro kinase assays to measure changes in catalytic activity. The activity of endogenous MEKK1, immunopurified from ITC treated and untreated LnCAP cells was also measured by in vitro kinase assay. A novel labeling and affinity reagent for detection of protein modification by ITCs was synthesized and used in competition assays to monitor direct modification of MEKK1 by ITC. Finally, immunoblots with phospho-specific antibodies were used to measure the activity of MAPK protein kinases.

RESULTS

ITCs inhibited the MEKK1 protein kinase in a manner dependent on a specific cysteine residue in the ATP binding pocket. Inhibition of MEKK1 catalytic activity was due to direct, covalent and irreversible modification of the MEKK1 protein itself. In addition, ITCs inhibited the catalytic activity of endogenous MEKK1. This correlated with inhibition of the downstream target of MEKK1 activity, i.e. the SAPK/JNK kinase. This inhibition was specific to SAPK, as parallel MAPK pathways were unaffected.

CONCLUSION

These results demonstrate that MEKK1 is directly modified and inhibited by ITCs, and that this correlates with inhibition of downstream activation of SAPK. These results support the conclusion that ITCs may carry out many of their actions by directly targeting important cell regulatory proteins.

Comparison of growth inhibition profiles and mechanisms of apoptosis induction in human colon cancer cell lines by isothiocyanates and indoles from Brassicaceae

The isothiocyanates sulforaphane and PEITC (beta-phenethyl isothiocyanate) as well as the indoles indole-3-carbinol and its condensation product 3,3'-diindolylmethane are known to inhibit cancer cell proliferation and induce apoptosis. In this study, we compared the cell growth inhibitory potential of the four compounds on the p53 wild type human colon cancer cell line 40-16 (p53(+/+)) and its p53 knockout derivative 379.2 (p53(-/-)) (both derived from HCT116). Using sulforhodamin B staining to assess cell proliferation, we found that the isothiocyanates were strongly cytotoxic, whereas the indoles inhibited cell growth in a cytostatic manner. Half-maximal inhibitory concentrations of all four compounds in both cell lines ranged from 5-15 microM after 24, 48 and 72 h of treatment. Apoptosis induction was analyzed by immunoblotting of poly(ADP-ribose)polymerase (PARP). Treatment with sulforaphane (15 microM), PEITC (10 microM), indole-3-carbinol (10 microM) and 3,3'-diindolylmethane (10 microM) induced PARP cleavage after 24 and 48 h in both 40-16 and the 379.2 cell lines, suggestive of a p53-independent mechanism of apoptosis induction. In cultured 40-16 cells, activation of caspase-9 and -7 detected by Western blotting indicated involvement of the mitochondrial pathway. We detected time- and concentration-dependent changes in protein expression of anti-apoptotic Bcl-x(L) as well as pro-apoptotic Bax and Bak proteins. Of note is that for sulforaphane only, ratios of pro- to anti-apoptotic Bcl-2 family protein levels directly correlated with apoptosis induction measured by PARP cleavage. Taken together, we demonstrated that the glucosinolate breakdown products investigated in this study have distinct profiles of cell growth inhibition, potential to induce p53-independent apoptosis and to modulate Bcl-2 family protein expression in human colon cancer cell lines.

Phenethyl Isothiocyanate Triggers Apoptosis in Jurkat Cells Made Resistant by the Overexpression of Bcl-2

Isothiocyanates are a class of naturally occurring chemopreventive agents known to be effective at triggering apoptosis. In this study, we show that whereas overexpression of the oncoprotein Bcl-2 renders Jurkat T-lymphoma cells resistant to a range of cytotoxic agents, phenethyl isothiocyanate is able to overcome the inhibitory action of Bcl-2 and trigger apoptosis. A 50-fold increase in Bcl-2 expression shifted the dose-response curve, with an increase in the phenethyl isothiocyanate LD(50) from 7 to 15 micromol/L, but there was still a complete loss in cell viability at doses in excess of 20 micromol/L. At these concentrations, cytotoxicity was strongly associated with caspase activation, phosphatidylserine exposure, and morphologic changes characteristic of apoptosis. Cytotoxicity was inhibited by treatment of the cells with a broad-spectrum caspase inhibitor. A structure-activity analysis showed that the phenethyl and benzyl isothiocyanates were most effective at triggering apoptosis in cells overexpressing Bcl-2 whereas phenyl isothiocyanate and benzyl thiocyanate had no proapoptotic activity. Allyl isothiocyanate also had limited efficacy despite its ability to trigger apoptosis in the parental Jurkat cell line. From this information, we propose that isothiocyanates modify a key cysteine residue in an apoptosis regulatory protein and that the aromatic side chain facilitates access to the target site. An in-depth investigation of the cellular targets of the aromatic isothiocyanates is warranted.

Assessment of the Anti-Genotoxic, Anti-Proliferative, and Anti-Metastatic Potential of Crude Watercress Extract in Human Colon Cancer Cells

Although it is known to be a rich source of the putative anti-cancer chemicals isothiocyanates, watercress has not been extensively studied for its cancer preventing properties. The aim of this study was to investigate the potential chemoprotective effects of crude watercress extract toward three important stages in the carcinogenic process, namely initiation, proliferation, and metastasis (invasion) using established in vitro models. HT29 cells were used to investigate the protective effects of the extract on DNA damage and the cell cycle. The extract was not genotoxic but inhibited DNA damage induced by two of the three genotoxins used, namely hydrogen peroxide and fecal water, indicating the potential to inhibit initiation. It also caused an accumulation of cells in the S phase of the cell cycle indicating (possible) cell cycle delay at this stage. The extract was shown to significantly inhibit invasion of HT115 cells through matrigel. Component analysis was also carried out in an attempt to determine the major phytochemicals present in both watercress leaves and the crude extract. In conclusion, the watercress extract proved to be significantly protective against the three stages of the carcinogenesis process investigated.

Vegetable-derived isothiocyanates: anti-proliferative activity and mechanism of action

Many isothiocyanates (ITC), which are available to human subjects mainly through consumption of cruciferous vegetables, demonstrate strong cancer-preventive activity in animal models. Human studies also show an inverse association between consumption of ITC and risk of cancer in several organs. Whereas earlier studies primarily focused on the ability of ITC to inhibit carcinogen-activating enzymes and induce carcinogen-detoxifying enzymes, more recent investigations have shown that ITC inhibit the proliferation of tumour cells both in vitro and in vivo by inducing apoptosis and arresting cell cycle progression. ITC cause acute cellular stress, which may be the initiating event for these effects. These findings shed new light on the mechanism of action of ITC and indicate that ITC may be useful both as cancer-preventive and therapeutic agents. ITC activate caspase 9-mediated apoptosis, apparently resulting from mitochondrial damage, and also activate caspase 8, but the mechanism remains to be defined. Cell cycle arrest caused by ITC occurs mainly in the G2/M phase, and both the G2 and M phases are targetted; critical G2-phase regulators, including cyclin B1, cell division cycle (Cdc) 2 and Cdc25C, are down regulated or inhibited, and tubulin polymerization and spindle assembly are disrupted. Moreover, ITC are metabolized in vivo through the mercapturic acid pathway, giving rise to thiol conjugates (dithiocarbamates). Studies show that these dithiocarbamates are similar to their parent ITC in exerting anti-proliferative activity. Taken together, dietary ITC are highly-promising anti-cancer agents, capable of targetting multiple cellular components that are important for tumour cell survival and proliferation.

Protective effects of Asian green vegetables against oxidant induced cytotoxicity

AIM

To evaluate the antioxidant and phase II detoxification enzyme inducing ability of green leaf vegetables consumed in Asia.

METHODS

The antioxidant properties of six commonly consumed Asian vegetables were determined using the ABTS, DPPH, deoxyribose, PR bleaching and iron- ascorbate induced lipid peroxidation assay. Induce of phase II detoxification enzymes was also determined for each respective vegetable extract. Protection against authentic ONOO- and HOCl mediated cytotoxicity in human colon HCT116 cells was determined using the MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrasodium bromide) viability assay.

RESULTS

All of the extracts derived from green leaf vegetables exhibited antioxidant properties, while also having cytoprotective effects against ONOO- and HOCl mediated cytotoxicity. In addition, evaluation of the phase II enzyme inducing ability of each extract, as assessed by quinone reductase and glutathione-S-transferase activities, showed significant variation between the vegetables analyzed.

CONCLUSION

Green leaf vegetables are potential sources of antioxidants and phase II detoxification enzyme inducers in the Asian diet. It is likely that consumption of such vegetables is a major source of beneficial phytochemical constituents that may protect against colonic damage.

Effect of isothiocyanates on nuclear accumulation of NF-kappaB, Nrf2, and thioredoxin in caco-2 cells

Early effects (only 1 h of exposure) of three isothiocyanates (benzyl, phenylethyl, and sulforaphane) on nuclear accumulation of thioredoxin, APE/Ref-1, and transcription factors NF-kappaB and Nrf2, as well as production of reactive oxygen species (ROS) and reduced glutathione levels were examined in human adenocarcinoma Caco-2 cells. Nuclear increase of NF-kappaB, Nrf2, and thioredoxin contents was observed in all isothiocyanate-treated cells, whereas the nuclear Ref-1 and cytoplasmic Keap1 contents were not changed. Sulforaphane was the most potent inducer of Nrf2 nuclear accumulation (10 microM, 1.9-fold) and NF-kappaB nuclear accumulation at higher concentration (25 microM, 6.3-fold). In contrast, benzyl isothiocyanate induced more thioredoxin nuclear accumulation (10 microM, 2.9-fold), increased production of ROS, and gave the greatest induction of thioredoxin reductase 1 mRNA (10 microM, 10.2-fold), whereas phenylethyl isothiocyanate was more potent in the depletion of reduced glutathione levels. These results show that different individual isothiocyanates may possess some different activities in nuclear accumulation of thioredoxin, NF-kappaB, Nrf2, and production of ROS.

Mitochondria are the primary target in isothiocyanate-induced apoptosis in human bladder cancer cells

Many isothiocysanates (ITC) are promising cancer-preventive agents, and induction of apoptosis is one of their underlying mechanisms of action. We recently found that caspase-9 was preferentially activated over other initiator caspases in human bladder cancer UM-UC-3 cells. We report here that caspase-9 activation is the major step leading to ITC-induced apoptosis in this cell line. More importantly, our results show that caspase-9 activation by the ITCs may result primarily from mitochondrial damage. Four common naturally occurring ITCs were studied, including allyl ITC, benzyl ITC (BITC), phenethyl ITC (PEITC), and sulforaphane. BITC and PEITC showed more potent mitochondria-damaging ability than the other two ITCs, correlating well with their stronger apoptosis-inducing potentials. Furthermore, BITC and PEITC damaged both the outer and inner mitochondrial membranes. Use of isolated mitochondria allowed us to establish that ITCs, and more importantly their major intracellular derivatives (glutathione conjugates) at concentrations that are readily achievable in cells, damage mitochondria, leading to the collapse of mitochondrial trans-membrane potential and release of cytochrome c. The mitochondria-damaging potencies of the ITCs correlate well with their lipophilicities. Bcl-2 family members are known to influence the stability of mitochondrial membrane. Our results show that the ITCs caused phosphorylation of Bcl-2, induced mitochondrial translocation of Bak, and disrupted the association of Bcl-xl with both Bak and Bax in mitochondrial membrane, indicating that ITC-induced mitochondrial damage results at least in part from modulation of select Bcl-2 family members.

Mitochondria: A novel target for the chemoprevention of cancer

The mitochondria have emerged as a novel target for anticancer chemotherapy. This tenet is based on the observations that several conventional and experimental chemotherapeutic agents promote the permeabilization of mitochondrial membranes in cancerous cells to initiate the release of apoptogenic mitochondrial proteins. This ability to engage mitochondrial-mediated apoptosis directly using chemotherapy may be responsible for overcoming aberrant apoptosis regulatory mechanisms commonly encountered in cancerous cells. Interestingly, several putative cancer chemopreventive agents also possess the ability to trigger apoptosis in transformed, premalignant, or malignant cells in vitro via mitochondrial membrane permeabilization. This process may occur through the regulation of Bcl-2 family members, or by the induction of the mitochondrial permeability transition. Thus, by exploiting endogenous mitochondrial-mediated apoptosis-inducing mechanisms, certain chemopreventive agents may be able to block the progression of premalignant cells to malignant cells or the dissemination of malignant cells to distant organ sites as means of modulating carcinogenesis in vivo. This review will examine cancer chemoprevention with respect to apoptosis, carcinogenesis, and the proapoptotic activity of various chemopreventive agents observed in vitro. In doing so, I will construct a paradigm supporting the notion that the mitochondria are a novel target for the chemoprevention of cancer.

Hydrogen sulfide protects colon cancer cells from chemopreventative agent β-phenylethyl isothiocyanate induced apoptosis

AIM: Hydrogen sulfide (H₂S) is a prominent gaseous constituent of the gastrointestinal (GI) tract with known cytotoxic properties. Endogenous concentrations of H₂S are reported to range between 0.2-3.4 mmol/L in the GI tract of mice and humans. Considering such high levels we speculate that, at non-toxic concentrations, H₂S may interact with chemical agents and alter the response of colonic epithelium cells to such compounds. The GI tract is a major site for the absorption of phytochemical constituents such as isothiocyanates, flavonoids, and carotenoids, with each group having a role in the prevention of human diseases such as colon cancer. The chemopreventative properties of the phytochemical agent β-phenyethyl isothiocyanate (PEITC) are well recognized. However, little is currently known about the physiological or biochemical factors present in the GI tract that may influence the biological properties of ITCs. The current study was undertaken to determine the effects of H₂S on PEITC mediated apoptosis in colon cancer cells.

METHODS: Induction of apoptosis by PEITC in human colon cancer HCT116 cells was assessed using classic apoptotic markers namely SubG1 population analysis, caspase-3 like activity and nuclear fragmentation and condensation coupled with the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrasodium bromide] viability assay and LDH leakage.

RESULTS: PEITC significantly induced apoptosis in HCT116 cells as assessed by SubG1 population formation, nuclear condensation, LDH leakage and caspase-3 activity after 24 h, these data being significant from control groups (P < 0.01). In contrast, co-treatment of cells with physiological concentrations of H₂S (0.1-1 mmol/L) prevented PEITC mediated apoptosis as assessed using the parameters described.

CONCLUSION: PEITC effectively induced cell death in the human adenocarcinoma cell line HCT116 in vitro through classic apoptotic mechanisms. However, in the presence of H₂S, apoptosis was abolished. These data suggest that H₂S may play a significant role in the response of colonic epithelial cells to beneficial as well as toxic agents present within the GI tract.

?-Phenylethyl isothiocyanate mediated apoptosis: A proteomic investigation of early apoptotic protein changes

beta-Phenylethyl isothiocyanate (PEITC) is a promising chemopreventative agent found in abundance in watercress (Rorripa nasturtium aquaticum) as its glucosinolate precursor. In the present investigation, we sought to determine the early changes in protein expression that contribute to the mechanism(s) of PEITC-mediated apoptosis in the human hepatoma HepG2 cell line. Such data may invariably identify new molecular targets of PEITC, contributing to a greater understanding of the mechanism(s) by which isothiocyanates mediate apoptotic cascades. Using two-dimensional difference gel electrophoresis we determined the changes in global protein expression between control (0.01% dimethyl sulfoxide) and PEITC (IC50 approximately 20 microM) treated cells after 3 and 6 h, such time points being used to circumvent the effects of caspase mediate proteolysis. Comparison between PEITC treated cells with their respective controls showed that 17 protein spots were differentially expressed. Fourteen of these spots, representing 9 unique proteins, were successfully identified using matrix-assisted laser desorption / ionization-time of flight (MALDI-TOF) and MALDI tandem time of flight (TOF/TOF) mass spectrometry. We observed significant shifts in isoelectric points on two-dimensional electrophoresis gels in heat shock 27 kDa protein (HSP27), macrophage migration inhibition factor and heterogeneous nuclear ribonucleoprotein K (hnRNP K) indicating that these proteins are probably involved in protein phosphorylation. Indeed, hnRNP K was determined to be phosphorylated on key tyrosine residues as assessed by using antiphosphotyrosine antibodies. In separate experiments we also showed that c-myc is up-regulated in PEITC treated cells, and since hnRNP K is reported to induce overexpression of c-myc, we proposed that PEITC-induced apoptosis may involve a c-myc dependent apoptotic pathway in HepG2 cells.