Cellular thiols status and cell death in the effect of green tea polyphenols in Ehrlich ascites tumor cells

Synergistic Anticancer Activities of Natural Substances in Human Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is highly resistant to currently available chemotherapeutic agents. The clinical outcome of HCC treatment remains unsatisfactory. Therefore, new effective and well-tolerated therapy strategies are needed. Natural products are excellent sources for the development of new medications for disease treatment. Recently, we and other researchers have suggested that the combined effect of natural products may improve the effect of chemotherapy treatments against the proliferation of cancer cells. In addition, many combination treatments with natural products augmented intracellular reactive oxygen species (ROS). In this review we will demonstrate the synergistic anticancer effects of a combination of natural products with chemotherapeutic agents or natural products against human HCC and provide new insight into the development of novel combination therapies against HCC.

Growth inhibition and pro-apoptotic activity of violacein in Ehrlich ascites tumor

The continuing threat to biodiversity lends urgency to the need of identification of sustainable source of natural products. This is not so much trouble if there is a microbial source of the compound. Herein, violacein, a natural indolic pigment extracted from Chromobacterium violaceum, was evaluated for its antitumoral potential against the Ehrlich ascites tumor (EAT) in vivo and in vitro. Evaluation of violacein cytotoxicity using different endpoints indicated that EAT cells were twofold (IC(50)=5.0 microM) more sensitive to the compound than normal human peripheral blood lymphocytes. In vitro studies indicated that violacein cytotoxicity to EAT cells is mediated by a rapid (8-12h) production of reactive oxygen species (ROS) and a decrease in intracellular GSH levels, probably due to oxidative stress. Additionally, apoptosis was primarily induced, as demonstrated by an increase in Annexin-V positive cells, concurrently with increased levels of DNA fragmentation and increased caspase-2, caspase-9 and caspase-3 activities up to 4.5-, 6.0- and 5.5-fold, respectively, after 72 h of treatment. Moreover, doses of 0.1 and 1.0 microg kg(-1) violacein, administered intraperitoneally (i.p.) to EAT-bearing mice throughout the lifespan of the animals significantly inhibited tumor growth and increased survival of mice. In view of these results, a 35-day toxicity study was conducted in vivo. Complete hematology, biochemistry (ALT, AST and creatinine levels) and histopathological analysis of liver and kidney indicated that daily doses of violacein up to 1000 microg kg(-1) for 35 days are well tolerated and did not cause hematotoxicity nor renal or hepatotoxicity when administered i.p. to mice. Altogether, these results indicate that violacein causes oxidative stress and an imbalance in the antioxidant defense machinery of cells culminating in apoptotic cell death. Furthermore, this is the first report of its antitumor activity in vivo, which occurs in the absence of toxicity to major organs.

Vimang (Mangifera indica L. extract) induces permeability transition in isolated mitochondria, closely reproducing the effect of mangiferin, Vimang's main component

Mitochondrial permeability transition (MPT) is a Ca(2+)-dependent, cyclosporin A (CsA)-sensitive, non-selective inner membrane permeabilization process. It is often associated with apoptotic cell death, and is induced by a wide range of agents or conditions, usually involving reactive oxygen species (ROS). In this study, we demonstrated that Mangifera indica L. extract (Vimang), in the presence of 20 microM Ca(2+), induces MPT in isolated rat liver mitochondria, assessed as CsA-sensitive mitochondrial swelling, closely reproducing the same effect of mangiferin, the main component of the extract, as well as MPT-linked processes like oxidation of membrane protein thiols, mitochondrial membrane potential dissipation and Ca(2+) release from organelles. The flavonoid catechin, the second main component of Vimang, also induces MPT, although to a lesser extent; the minor, but still representative Vimang extract components, gallic and benzoic acids, show respectively, low and high MPT inducing abilities. Nevertheless, following exposure to H(2)O(2)/horseradish peroxidase, the visible spectra of these compounds does not present the same changes previously reported for mangiferin. It is concluded that Vimang-induced MPT closely reproduces mangiferin effects, and proposed that this xanthone is the main agent responsible for the extract's MPT inducing ability, by the action on mitochondrial membrane protein thiols of products arising as a consequence of the mangiferin's antioxidant activity. While this effect would oppose the beneficial effect of Vimang's antioxidant activity, it could nevertheless benefit cells exposed to over-production of ROS as occurring in cancer cells, in which triggering of MPT-mediated apoptosis may represent an important defense mechanism to their host.

Modulation of light-induced skin tumors by N -acetylcysteine and/or ascorbic acid in hairless mice

The light emitted by halogen quartz bulbs contains a broad spectrum of UV wavelengths, is strongly genotoxic and is a potent inducer of skin tumors in hairless mice. By using a UVC filter, this light mimics solar radiation and induces a variety of genomic and transcriptional alterations in mouse skin. UV-related carcinogenesis involves depletion of antioxidants and glutathione in skin cells. On this basis, we evaluated modulation of carcinogenicity of UVC-filtered halogen lamps in SKH-1 hairless mice by the antioxidants N-acetyl-l-cysteine (NAC) and ascorbic acid (AsA). Both agents were given in the drinking water, either individually or in combination. The earliest skin lesions were detected after 300 days' exposure to light and became confluent in a number of mice after 480 days. NAC administration prolonged the latency time by 90 days. Moreover, NAC considerably and significantly decreased both incidence and multiplicity of light-induced skin tumors, prevented the occurrence of malignant lesions (squamocellular carcinomas) and reduced the tumor size. In contrast, AsA, which may behave as a prooxidant rather than an antioxidant, increased the multiplicity of total skin tumors, carcinomas in situ and squamocellular carcinomas. Co-administration of NAC with AsA significantly attenuated the negative effect of AsA, presumably due to the ability of this thiol to maintain a reduced environment. Therefore, in agreement with our previous in vitro findings, oral NAC is able to attenuate the detrimental effects of AsA.

Green tea extract inhibits angiogenesis of human umbilical vein endothelial cells through reduction of expression of VEGF receptors

Epidemiological and animal studies have indicated that consumption of green tea is associated with a reduced risk of developing certain forms of cancer. However, the inhibitory mechanism of green tea in angiogenesis, an important process in tumor growth, has not been well established. In the present study, green tea extract (GTE) was tested for its ability to inhibit cell viability, cell proliferation, cell cycle dynamics, vascular endothelial growth factor (VEGF) and expression of VEGF receptors fms-like tyrosine kinase (Flt-1) and fetal liver kinase-1/Kinase insert domain containing receptor (Flk-1/KDR) in vitro using human umbilical vein endothelial cells (HUVECs). GTE in culture media did not affect cell viability but significantly reduced cell proliferation dose-dependently and caused a dose-dependent accumulation of cells in the G1 phase. The decrease of the expression of Flt-1 and KDR/Flk-1 in HUVEC by GTE was detected with immunohistochemical and Western blotting methods. These results suggest that GTE may have preventive effects on tumor angiogenesis and metastasis through reduction of expression of VEGF receptors.

Tea catechin synergies in inhibition of cancer cell proliferation and of a cancer specific cell surface oxidase (ECTO-NOX)

The anticancer properties of tea catechins are most frequently attributed to the principal catechin (-)-epigallocatechin-3-gallate (EGCg). Efficacy was evaluated using growth of cultured HeLa cells and inhibition of the enzymatic activity of a putative cell surface tea target enzyme, a cancer-associated cell surface-located NADH oxidase (ECTO-NOX) designated tNOX. The amounts of EGCg required to inhibit by both criteria was reduced 10 times by combination with inactive catechins such as (-)-epicatechin (EC), (-)-epigallocatechin (EGC) or (-)-epicatechin-3-gallate (ECG). Various synthetic mixtures based on purified catechins and decaffeinated tea extracts treated enzymatically to reduce the ester bond-containing catechins varying in EGCg content from 0.065 to 40% were of comparable efficacy to decaffeinated green tea extracts as long as EGCg was present and the ratio of total catechins to EGCg + EGC was about 1.5. Such mixtures appear to offer potential cancer protection and therapeutic advantages over those of EGCg alone through lowered toxicity of the mixture to normal cells and for more efficient blood delivery of orally-administered catechins to a tumour site.

Effect of whey protein isolate on intracellular glutathione and oxidant-induced cell death in human prostate epithelial cells

Cysteine is the rate-limiting amino acid for synthesis of the ubiquitous antioxidant glutathione (GSH). Bovine whey proteins are rich in cystine, the disulfide form of the amino acid cysteine. The objective of this study was to determine whether enzymatically hydrolyzed whey protein isolate (WPI) could increase intracellular GSH concentrations and protect against oxidant-induced cell death in a human prostate epithelial cell line (designated RWPE-1). Treatment of RWPE-1 cells with hydrolyzed WPI (500 microg/ml) significantly increased intracellular GSH by 64%, compared with control cells receiving no hydrolyzed WPI (P<0.05). A similar increase in GSH was observed with N-acetylcysteine (500 microM), a cysteine-donating compound known to elevate intracellular GSH. In contrast, treatment with hydrolyzed sodium caseinate (500 microg/ml), a cystine-poor protein source, did not significantly elevate intracellular GSH. Hydrolyzed WPI (500 microg/ml) significantly protected RWPE-1 cells from oxidant-induced cell death, compared with controls receiving no WPI (P<0.05). The results of this study indicate that WPI can increase GSH synthesis and protect against oxidant-induced cell death in human prostate cells.

Cellular thiol status-dependent inhibition of tumor cell growth via modulation of retinoblastoma protein phosphorylation by (-)-epigallocatechin

Tea polyphenols have been shown to inhibit tumor cell growth, but there is limited information on their effects on cell signaling and cell cycle control pathways. We have shown the involvement of such mechanisms as activation of mitogenic activated protein kinases, decreases in ornithine decarboxylase activity and in cellular thiol levels, elicitation of mitochondrial cytochrome c release, and activation of caspases by the green tea galloyl polyphenol, epigallocatechin (EGC). In the current study, we sought to determine how EGC alters cell cycle and its related control factors in its growth inhibitory effect in Ehrlich ascites tumor cells. The significant finding here is that EGC caused a dose-dependent accumulation of cells in the G1 phase and a decrease in the phosphorylation of the retinoblastoma (Rb) protein, which was also in a cellular thiol-dependent manner. The involvement of a cellular thiol-dependent modulation in Rb phosphorylation leading to the regulation of tumor cell growth by a green tea polyphenol is a novel observation, to the best of our knowledge.

Structure-activity relationships for inhibition of human 5alpha-reductases by polyphenols

The enzyme steroid 5 alpha-reductase (EC 1.3.99.5) catalyzes the NADPH-dependent reduction of the double bond of a variety of 3-oxo-Delta(4) steroids including the conversion of testosterone to 5 alpha-dihydrotestosterone. In humans, 5 alpha-reductase activity is critical for certain aspects of male sexual differentiation, and may be involved in the development of benign prostatic hyperplasia, alopecia, hirsutism, and prostate cancer. Certain natural products contain components that are inhibitors of 5 alpha-reductase, such as the green tea catechin (-)-epigallocatechin gallate (EGCG). EGCG shows potent inhibition in cell-free but not in whole-cell assays of 5 alpha-reductase. Replacement of the gallate ester in EGCG with long-chain fatty acids produced potent 5 alpha-reductase inhibitors that were active in both cell-free and whole-cell assay systems. Other flavonoids that were potent inhibitors of the type 1 5alpha-reductase include myricetin, quercitin, baicalein, and fisetin. Biochanin A, daidzein, genistein, and kaempferol were much better inhibitors of the type 2 than the type 1 isozyme. Several other natural and synthetic polyphenolic compounds were more effective inhibitors of the type 1 than the type 2 isozyme, including alizarin, anthrarobin, gossypol, nordihydroguaiaretic acid, caffeic acid phenethyl ester, and octyl and dodecyl gallates. The presence of a catechol group was characteristic of almost all inhibitors that showed selectivity for the type 1 isozyme of 5 alpha-reductase. Since some of these compounds are consumed as part of the normal diet or in supplements, they have the potential to inhibit 5 alpha-reductase activity, which may be useful for the prevention or treatment of androgen-dependent disorders. However, these compounds also may adversely affect male sexual differentiation.

Mechanisms of N-acetylcysteine in the prevention of DNA damage and cancer, with special reference to smoking-related end-points

Although smoking cessation is the primary goal for the control of cancer and other smoking-related diseases, chemoprevention provides a complementary approach applicable to high risk individuals such as current smokers and ex-smokers. The thiol N-acetylcysteine (NAC) works per se in the extracellular environment, and is a precursor of intracellular cysteine and glutathione (GSH). Almost 40 years of experience in the prophylaxis and therapy of a variety of clinical conditions, mostly involving GSH depletion and alterations of the redox status, have established the safety of this drug, even at very high doses and for long-term treatments. A number of studies performed since 1984 have indicated that NAC has the potential to prevent cancer and other mutation-related diseases. N-Acetylcysteine has an impressive array of mechanisms and protective effects towards DNA damage and carcinogenesis, which are related to its nucleophilicity, antioxidant activity, modulation of metabolism, effects in mitochondria, decrease of the biologically effective dose of carcinogens, modulation of DNA repair, inhibition of genotoxicity and cell transformation, modulation of gene expression and signal transduction pathways, regulation of cell survival and apoptosis, anti-inflammatory activity, anti-angiogenetic activity, immunological effects, inhibition of progression to malignancy, influence on cell cycle progression, inhibition of pre-neoplastic and neoplastic lesions, inhibition of invasion and metastasis, and protection towards adverse effects of other chemopreventive agents or chemotherapeutical agents. These mechanisms are herein reviewed and commented on with special reference to smoking-related end-points, as evaluated in in vitro test systems, experimental animals and clinical trials. It is important that all protective effects of NAC were observed under a range of conditions produced by a variety of treatments or imbalances of homeostasis. However, our recent data show that, at least in mouse lung, under physiological conditions NAC does not alter per se the expression of multiple genes detected by cDNA array technology. On the whole, there is overwhelming evidence that NAC has the ability to modulate a variety of DNA damage- and cancer-related end-points.

Growth inhibitory effect of green tea extract in Ehrlich ascites tumor cells involves cytochrome c release and caspase activation

We reported previously that the mechanism by which Green tea extract (GTE) elicited growth-inhibitory effects in Ehrlich ascites tumor cells involved a decrease in ornithine decarboxylase (ODC) activity and in cell viability. Decrease in ODC activity has been associated with apoptotic cell death and we therefore studied changes in cytochrome c release and caspase activation, which characterize apoptosis. GTE caused a dose- and time-dependent increase in caspase-3-like protease activation, preceded by a release of cytochrome c from the mitochondria. Inhibiting the activation of caspase-3 with acetyl-Asp-Glu-Val-Asp-alpha-aldehyde (caspase inhibitor) caused a reversal in the effect on cell viability.

Growth inhibitory effect of green tea extract and (-)-epigallocatechin in Ehrlich ascites tumor cells involves a cellular thiol-dependent activation of mitogenic-activated protein kinases

The effect of green tea extract (GTE) in Ehrlich ascites tumor cells (EATC) was studied with respect to changes in the intracellular kinase system involving mitogen-activated protein kinases (MAPKs) and cellular thiol. We have previously shown a reduction in viability of EATC and tyrosine phosphorylation of 42 and 45 kDa proteins by GTE and its polyphenolic component, Epigallocatechin (EGC) (D.O. Kennedy, S. Nishimura, T. Hasuma, Y. Yoshihisa, S. Otani, I. Matsui-Yuasa, Involvement of protein tyrosine phosphorylation in the effect of green tea polyphenols on Ehrlich ascites tumor cells in vitro, Chem. Biol. Interact. 110 (1998) 159-172). Furthermore, GTE and EGC significantly decreased both cellular non-protein and protein sulfhydryl levels in EATC, but replenishing thiol stores with N-acetylcysteine (NAC) caused a recovery in cell viability, and therefore SH groups were identified as a novel target of green tea cytotoxicity (D.O. Kennedy, M. Matsumoto, A. Kojima, I. Matsui-Yuasa, Cellular thiol status and cell death in the effect of green tea polyphenols in Ehrlich ascites tumor cells, Chem. Biol. Interact. 122 (1999) 59-71). In this study, we have observed the stimulation of three forms of MAPK, namely ERK1/2, JNK/SAPK and p38, by EGC, which were dose and time-dependent. These MAPK stimulations were found to be cellular thiol status-dependent events as NAC reversed these stimulations. Furthermore, inhibition of the p38 MAPK pathway using the p38 inhibitor SB203580 caused a marked dose-dependent reduction in the decrease in cell viability caused by EGC treatment. Inhibiting the Erk1/2 MAPK pathway using the MEK inhibitor PD098059 caused a slight change in the decrease in cell viability by EGC. These may suggest that the cytotoxicity associated with EGC was more associated with the other MAPKs than with ERK1/2. This may be the first study of its kind providing a novel evidence of a role for different forms of MAPKs in the antitumor effect of green tea polyphenols, especially EGC, in Ehrlich ascites tumor cells.